Photohardenable compositions and methods for use in volumetric three-dimensional printing

Abstract

The inclusion of additives in a photohardenable composition useful in volumetric three-dimensional printing can decrease unfavorable distortion caused by refractive index inhomogeneities during polymerization. Photohardenable compositions including such additives and methods including such photohardenable compositions are disclosed.

Description

[0001] PHOTOHARDENABLE COMPOSITIONS AND METHODS FOR USE IN

[0002] VOLUMETRIC THREE-DIMENSIONAL PRINTING

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims priority to U.S. Provisional Patent Application No. 63 / 625,829 of Quadratic 3D, Inc. filed January 26, 2024, and U.S, Provisional Application No. 63 / 626,892 of Quadratic 3D, Inc. filed January 30, 2024, each of which is hereby incorporated herein by reference in its entirety for all purposes.

[0005] TECHNICAL FIELD OF THE INVENTION

[0006] The present invention relates to the technical field of volumetric three-dimensional printing.

[0007] BRIEF SUMMARY OF THE INVENTION

[0008] The present invention includes compositions and methods for use in forming three- dimensional objects in a volume of a photohardenable composition.

[0009] In accordance with one aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the photohardenable composition comprising: a photohardenable resin component,; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a polymerization rate reduction additive wherein photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

[0010] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0011] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component,; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a polymerization rate reduction additive wherein photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition; (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and

[0012] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the polymerization rate reduction additive slows polymerization to a rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

[0013] In accordance with another aspect of the present invention, there is provided photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon excitation.

[0014] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0015] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon excitation;

[0016] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and

[0017] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the comonomer in the photohardenable composition slows polymerization to a rate that is lower than if the comonomer is not included in the photohardenable composition.

[0018] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon exposure to excitation.

[0019] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0020] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon exposure to excitation;

[0021] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and

[0022] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein inclusion of the chain transfer agent in the photohardenable composition slows polymerization to a rate that is lower than if the chain transfer agent is not included in the photohardenable composition.

[0023] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing with reduced light sheet deflection, the photohardenable composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and an additive for reducing deflection of a light sheet at a selected location in a volume of the photohardenable composition during formation of a three-dimensional object in the volume.

[0024] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0025] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and an additive for reducing deflection of a light sheet at a selected location in a volume of the photohardenable composition during formation of a three-dimensional object in the volume;

[0026] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and

[0027] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein light sheet deflection during formation of the three-dimensional object in the photohardenable composition is less than without inclusion of the light sheet deflection reduction additive.

[0028] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object.

[0029] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising: (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object;

[0030] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and

[0031] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three- dimensional object is less than without inclusion of the comonomer.

[0032] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising photohardenable composition for use in volumetric three- dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object.

[0033] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0034] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three- dimensional object; (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and

[0035] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three- dimensional object is less than without inclusion of the chain transfer agent.

[0036] Formation of one or more three-dimensional objects in a volume of a photohardenable composition may also be referred to herein as “printing”.

[0037] A photohardenable composition may also be referred to herein as a “resin” or “printing resin”.

[0038] A three-dimensional object may also be referred to herein as a “three-dimensional part” or a “part”.

[0039] The foregoing, and other aspects and embodiments described herein and contemplated by this disclosure all constitute embodiments of the present invention.

[0040] It should be appreciated by those persons having ordinary skill in the art(s) to which the present invention relates that any of the features described herein in respect of any particular aspect and / or embodiment of the present invention can be combined with one or more of any of the other features of any other aspects and / or embodiments of the present invention described herein, with modifications as appropriate to ensure compatibility of the combinations. Such combinations are considered to be part of the present invention contemplated by this disclosure.

[0041] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

[0042] Other embodiments will be apparent to those skilled in the art from consideration of the description and drawings, from the claims, and from practice of the invention disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In the drawings,

[0044] FIG. 1 schematically depicts the top view of an example of a volumetric 3D printer including a detector set-up for measuring light sheet deflection system.

[0045] FIGS. 2A, 3A, 4A, 5A, and 6A are graphs of green-state modulus as a function of exposure time for various examples.

[0046] FIGS 2B, 3B, 4B, 5B, and 6B are graphs of green-state modulus as a function of light sheet deflection (LSD) for various examples.

[0047] The attached figures are simplified representations presented for purposes of illustration only; the actual structures may differ in numerous respects, particularly including the relative scale of the articles depicted and aspects thereof.

[0048] For a better understanding of the present invention, together with other advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

[0049] DETAILED DESCRIPTION OF THE INVENTION

[0050] Various aspects and embodiments of the present inventions will be further described in the following detailed description.

[0051] The present invention includes compositions and methods for use in forming three- dimensional objects in a volume of a photohardenable composition.

[0052] Polymerization or cross-linking reactions, particularly those involving radical polymerization, can produce heat and / or increase the density of a photohardenable composition, which can cause the refractive index of the photohardenable composition to vary temporally and / or spatially. In volumetric 3D printing, refractive index inhomogeneities can be detrimental because they typically distort the incident light. Such distortion can lead to a decrease in the accuracy of the object being printed and / or to optical artifacts that cause undesirable features, including, but not limited to, for example, surface roughness.

[0053] Such distortion, when a light sheet is included in printing, is referred to herein as “light sheet deflection (LSD)”.

[0054] In accordance with the present invention, it has been found that the inclusion of one or more additives disclosed herein in a photohardenable composition useful in volumetric three-dimensional printing can decrease unfavorable distortion caused by refractive index inhomogeneities during polymerization. While not wishing to be bound by theory, it is believed that the inclusion of one or more of such additives operates in the photohardenable compositions of the invention by slowing polymerization kinetics such that polymerization occurs further away from the incident excitation light, and thus changes in refractive index related to polymerization occur farther away from the location of the excitation light, and the excitation light is therefore allowed to transit a more homogeneous index of refraction resin and remain minimally disturbed.

[0055] Three elementary steps are present in all radical chain polymerization processes: initiation (eq 1), propagation (eq 2), and termination (eq 3). Light causes initiation. A density increase and heat are produced by propagation. Thus, if propagation is slow relative to initiation, then most of the refractive index change resulting from density increase / heat will occur away from the light sheet. This will lead to lower LSD and thus fewer artifacts. initation •

[0056] (1) Init + Mon - ► Init-Mon

[0057] , propagation ,

[0058] (2) Init-Mon + n x Mon - ► Pol termination

[0059] (3) Pol * - Pol

[0060] It is believed that a preferred additive satisfies three criteria:

[0061] 1) Decreases polymerization rate without decreasing initiation rate or increasing irreversible termination rate.

[0062] A slower initiation or a faster termination is undesirable because it decreases conversion for a given amount of light. This leads to not only slower prints, but also weaker “green” parts.

[0063] 2) Enables target green state conversion to be reached within an acceptable time.

[0064] The “acceptable time” will be dictated by the rate of diffusion of propagating species, as diffusion outside of the irradiated region will result in a loss of resolution. This is resin and part dependent. For example, the slower diffusion in a more viscous resin would make it less important to have a rapid rate of polymerization. 3) Displays desired effect at low loading. < 5% by mass, but < 0.5% is preferred and < 0.1% is especially preferred.

[0065] We classify additives based on their mechanism of action into the three classes shown in Scheme 1. The common feature of all additives is their rapid interception of growing chains (radicals) to slow down (delay) the overall polymerization rate. A suitable comonomer (Scheme la) rapidly produces a new radical that then slowly reacts with another monomer to continue propagation. A suitable chain transfer agent (Scheme lb) rapidly produces a dead polymer chain and a new radical that then slowly reacts with monomer to initiate a new propagating chain. If re-initiation is not slow, then polymerization may still be delayed due to suppression / delay of the Trommsdorff effect (since re-initiating species are more mobile). A suitable ‘‘reversible terminator” (Scheme 1c) rapidly caps growing chains in a reversible process. This decreases their concentration and thus lowers the rate of propagation. Note that any references to rate in this paragraph are relative to the rate of propagation in the absence of the additive.

[0066] Scheme 1

[0067] (a) A = comonomer

[0068] (b) A = chain transfer agent

[0069] Mon

[0070] Pol Pol A-Mon dead (new hain) chain)

[0071] (c) A = reversible terminator Pol

[0072] In accordance with one aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the photohardenable composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a polymerization rate reduction additive wherein photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

[0073] Photohardenable resin components, photoinitiators, optional coinitiators, optional rheology modifiers, and other optional additives are described below.

[0074] Inclusion of a polymerization rate reduction additive in a photohardenable composition slows polymerization of the composition during printing. With such alteration of the polymerization kinetics, polymerization can occur farther away from the incident light. It is believed that the occurrence of polymerization removed from the incident excitation light, especially in the form of a light sheet, can reduce distortion due to changes in refractive index caused by increases in heat and density from polymerization.

[0075] A preferred polymerization rate reduction additive will decrease the polymerization rate without decreasing the initiation rate induced by light or increasing the irreversible termination rate.

[0076] Preferably, photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition while not necessarily reducing the efficiency of the polymerization (number of repeat units polymerized per radical).

[0077] Preferably, a polymerization rate reduction additive does not strongly absorb light at wavelengths employed in printing.

[0078] Examples of polymerization rate reduction additives include comonomers, chain transfer agents, and reversible terminators.

[0079] Examples of preferred comonomer include styrene, 4-cyanostyrene, substituted and unsubstituted styrenic monomers, and divinylbenzene.

[0080] Other comonomers that may be suitable as polymerization rate reduction additives may be identified using the screening protocol set forth below.

[0081] Examples of preferred chain transfer agents include RAFT agents, 2-cyano-2-propyl benzodithioate, and 4-methylpent-l-ene-2,4-diyl)dibenzene. Other chain transfer agents that may be suitable as polymerization rate reduction additives may be identified using the screening protocol set forth below.

[0082] A polymerization rate reduction additive is preferably included in the photohardenable composition at a loading less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0083] A measurable quantity that can be useful with volumetric 3D printing using light sheet excitation as an indication of a reduced polymerization rate is reduced light sheet deflection.

[0084] A protocol for determining light sheet deflection is discussed below.

[0085] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0086] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a polymerization rate reduction additive wherein photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition;

[0087] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0088] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the polymerization rate reduction additive slows polymerization to a rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition. In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate -based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon excitation.

[0089] Photohardenable resin components, photoinitiators, optional coinitiators, optional rheology modifiers, and other optional additives are described below.

[0090] Examples of preferred comonomer include styrene, 4-cyanostyrene, substituted and unsubstituted styrenic polymers, and divinylbenzene.

[0091] Other comonomers that may be suitable as an additive to slow the polymerization rate of the photohardenable composition may be identified using the screening protocol set forth below.

[0092] A measurable quantity that can be useful with volumetric 3D printing using light sheet excitation as an indication of a reduced polymerization rate is reduced light sheet deflection.

[0093] A protocol for determining light sheet deflection is discussed below.

[0094] Preferably the effective amount of the comonomer falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0095] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0096] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon excitation;

[0097] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0098] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the comonomer in the photohardenable composition slows polymerization to a rate that is lower than if the comonomer is not included in the photohardenable composition.

[0099] Preferably the effective amount of the comonomer falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0100] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate -based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon exposure to excitation.

[0101] Photohardenable resin components, photoinitiators, optional coinitiators, optional rheology modifiers, and other optional additives are described below.

[0102] Examples of preferred chain transfer agents include RAFT agents, 2-cyano-2-propyl benzodithioate, and 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0103] Other chain transfer agents that may be suitable as polymerization rate reduction additives may be identified using the screening protocol set forth below. A measurable quantity that can be useful with volumetric 3D printing using light sheet excitation as an indication of a reduced polymerization rate is reduced light sheet deflection.

[0104] A protocol for determining light sheet deflection is discussed below.

[0105] Preferably the effective amount of the chain transfer agent falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0106] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0107] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon exposure to excitation;

[0108] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0109] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein inclusion of the chain transfer agent in the photohardenable composition slows polymerization to a rate that is lower than if the chain transfer agent is not included in the photohardenable composition.

[0110] Preferably, the effective amount of the chain transfer agent falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0111] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing with reduced light sheet deflection, the photohardenable composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dualwavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and an additive for reducing deflection of a light sheet at a selected location in a volume of the photohardenable composition during formation of a three-dimensional object in the volume.

[0112] The additive for reducing deflection of the light sheet is preferably included in the photohardenable composition in an amount effective to reduce deflection of the light sheet during printing. Preferably the additive includes those described herein for reducing light sheet deflection.

[0113] Photohardenable resin components, photoinitiators, optional coinitiators, optional rheology modifiers, and other optional additives are described below.

[0114] Inclusion of an additive for reducing light sheet deflection in a photohardenable composition reduces distortion from refractive index changes during printing. With such reduction, printed part quality can be improved. Such improvements can include, e.g., improvements in part accuracy, improvements in surface quality, and the like. Reduced light sheet deflection can also be indicative of slower polymerization kinetics whereby polymerization can occur farther away from the incident light. As discussed above, it is believed that the occurrence of polymerization removed from the incident excitation light, especially in the form of a light sheet, can reduce distortion due to heat and density increases produced by polymerization.

[0115] A preferred additive for reducing light sheet deflection will decrease the polymerization rate without decreasing the initiation rate or increasing the irreversible termination rate.

[0116] Preferably, an additive for reducing light sheet deflection does not strongly absorb light at wavelengths employed in printing. Examples of additives for reducing light sheet deflection include comonomers and chain transfer agents.

[0117] Examples of preferred comonomer include styrene, 4-cyanostyrene, substituted and unsubstituted styrenic polymers, and divinylbenzene.

[0118] Other comonomers that may be suitable as additives for reducing light sheet deflection may be identified using the screening protocol set forth below.

[0119] Examples of preferred chain transfer agents include RAFT agents, 2-cyano-2-propyl benzodithioate, and 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0120] Other chain transfer agents that may be suitable as additives for reducing light sheet deflection may be identified using the screening protocol set forth below.

[0121] An additive for reducing light sheet deflection is preferably included in the photohardenable composition at a loading less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0122] A protocol for determining light sheet deflection is discussed below.

[0123] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0124] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and an additive for reducing deflection of a light sheet at a selected location in a volume of the photohardenable composition during formation of a three-dimensional object in the volume;

[0125] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein light sheet deflection during formation of the three-dimensional object in the photohardenable composition is less than without inclusion of the light sheet deflection reduction additive.

[0126] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate -based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object.

[0127] Photohardenable resin components, photoinitiators, optional coinitiators, optional rheology modifiers, and other optional additives are described below.

[0128] Examples of preferred comonomer include styrene, 4-cyanostyrene, substituted and unsubstituted styrenic polymers, and divinylbenzene.

[0129] Other comonomers that may be suitable as an additive to reduce distortion from refractive index changes during printing a three-dimensional object may be identified using the screening protocol set forth below.

[0130] Reductions in distortion from refractive index changes can be measured by reduced light sheet deflection.

[0131] A protocol for determining light sheet deflection is discussed below.

[0132] Preferably the effective amount of the comonomer falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent. In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0133] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object;

[0134] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0135] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three- dimensional object is less than without inclusion of the comonomer.

[0136] Preferably the effective amount of the comonomer falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0137] In accordance with another aspect of the present invention, there is provided a photohardenable composition for use in volumetric three-dimensional printing, the composition comprising photohardenable composition for use in volumetric three- dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three- dimensional object.

[0138] Photohardenable resin components, photoinitiators, optional coinitiators, optional rheology modifiers, and other optional additives are described below.

[0139] Examples of preferred chain transfer agents include RAFT agents, 2-cyano-2-propyl benzodithioate, and 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0140] Other chain transfer agents that may be suitable as an additive to reduce distortion from refractive index changes during printing a three-dimensional object may be identified using the screening protocol set forth below.

[0141] Reductions in distortion from refractive index changes can be measured by reduced light sheet deflection.

[0142] A protocol for determining light sheet deflection is discussed below.

[0143] Preferably the effective amount of the chain transfer agent falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0144] In accordance with another aspect of the present invention, there is provided a method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0145] (a) providing the volume of the photohardenable composition, wherein the photohardenable composition comprises a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object;

[0146] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three- dimensional object is less than without inclusion of the chain transfer agent.

[0147] Preferably the effective amount of the chain transfer agent falls within a loading range that is less than about 5 weight percent based on the weight of the photohardenable composition, more preferably less than about 0.5 weight percent, and most preferably less than 0.1 weight percent.

[0148] EXAMPLES

[0149] The following are provided as examples and not limitations, wherein a number of modifications of the exemplified compositions and processes are contemplated and within the scope of the present invention.

[0150] Pre-screening of additives

[0151] Initial criteria used to down- select potential additives are (1) the mechanistic considerations discussed elsewhere in this application and (2) the absorption properties of the additives. Additives that strongly absorb visible light at the wavelength employed in the printing are not preferred due to attenuation effects and potential side reactions (note: this is highly dependent on additive loading).

[0152] A centrifuged, bubble-free cuvette containing a candidate photohardenable composition is placed in a setup that features two intersecting, approximately collimated coherent light sources. The wavelengths and irradiances are chosen to be similar to those described for the light sheet and projector in the printing experiments below. The light sources are orthogonal to one another and intersect at the center of the 1 cm x 1 cm square cuvette. Time taken to form the polymerized spot is noted and serves as a proxy for formation of a part in the printer (“voxel time”). Two to three measurements of each are taken and averaged. In the present work, the primary metric used for down- selecting additives (and their loadings) was the ratio of “voxel times” of a composition with and without an additive. In the case where this value is close to 1, the additive had a non-detectable effect on cure kinetics and is not of further interest at that loading. In the case where this value is greater than 2, loading may be too high. The ratios for the additives assessed herein were typically in the range of 1.3 to 1.7.

[0153] Preparation of resin-filled cuvettes

[0154] Resin 1, the baseline acrylate resin, was prepared as follows. To a Flacktek speedmixing container was added Genomer 4259 (350 g), 1,6-hexanediol diacrylate (15.4 g), a l.OOmg / g solution of dual-wavelength photo switchable photoinitiator AE32 in 1,6- hexanediol diacrylate (7.70 g), and methyldiethanolamine (3.85 g). The contents were speedmixed for 5 minutes at 1200 rpm. To the same container was then added Rheobyk-430 (7.70 g), then the contents were speedmixed again for 5 minutes at 1200 rpm.

[0155] Dual-wavelength photo switchable photoinitiator AE32 was prepared substantially as described in the Example section of International Application No. PCT / US2023 / 022172 of Quadratic 3D, Inc. filed May 13, 2023, for the photoswitchable photoinitiator having the same designation.

[0156] Resins 2-6, containing the additives shown in Table 1, were prepared by addition of the additive to Resin 1 during the cuvette-filling procedure described below.

[0157] Table 1

[0158] A batch of 10 resin-filled cuvettes was prepared for each of Resins 1-6 as follows. Resin 1 (50 g) was added to a 100 mL round-bottom flask containing a large, football-shaped stir bar. The open flask was placed on a heating block pre-set to 95C. The stirring rate was set to 300 rpm. Stirring was inefficient until the viscosity lowered from the rising temperature. A thermometer was submersed into the resin to monitor the internal temperature. Once the internal temperature reached 70-75C, the additive in the mass percentage shown in Table 1 was carefully added via syringe. The stirring was carefully monitored to ensure proper mixing. Once the internal temperature reached 80-85C, the flask was removed from the heating block. The hot resin was immediately transferred to 10 x 1 cm cuvettes with a blunt tipped, large gauge needle. The cuvettes were centrifuged at 4000 rpm for 3 min to remove bubbles. Each cuvette was carefully inspected, and smudges were removed with a nonabrasive wipe (Pec Pad). The cuvettes were incubated in a pre-heated oven (50C) for > 2 h before printing.

[0159] Printing of 3 mm test cubes

[0160] For each of Resins 1-6, a series of 3 mm cubes were printed at the exposure times shown in Table 2. For each run, a 1 cm cuvette, prepared as described above, was placed in a holder on a heated (50C), motorized stage. Green laser light (532 nm CW diode laser, 20 W operating power) was used to illuminate a digital micromirror device (Texas Instruments) to form a pattern which is projected into the cuvette along the z axis to produce a pattern of approximately 0.45 W / cm2 of green light. Violet light (405 nm CW diode laser, 50-365 mW operating power) was used to form a light sheet that passes through the cuvette orthogonally to the projected pattern to illuminate a single x-y plane of approximately 200 micron thickness. The irradiance of the light sheet was approximately 1.0 W / cm2. The stage was advanced in increments of 28 microns at one of the exposure times shown in Table 2, with the 405 nm light forming a light sheet and the green light pattern changing at each advancement corresponding to computer generated slices of a three-dimensional object (here, a 3 mm cube). In regions where there is simultaneous or nearly simultaneous exposure to both wavelengths of light, the photohardenable composition was hardened. In this manner, a three- dimensional solid object was formed without displacement (e.g., sinking or drifting) and without need for support structures or attachment to a build platform due to the high zero shear viscosity or yield stress of the non-Newtonian photohardenable composition.

[0161] Light Sheet Deflection Measurements

[0162] During each print described above, the transmitted light sheet was imaged on a 5 x 5 mm grid situated approximately 300 mm from the exit using a detector set-up as described below. FIG. 1 depicts a top-view of a schematic representation of an example of a volumetric 3D printing system including a detector for determining light sheet deflection in formation of a three-dimensional object in the volumetric 3D printer. Polymerization occurs at a selected location in a volume of a photohardenable composition, which selected location is exposed to light sheet illumination and a projected selected optical image. Preferably such exposure occurs at an intersection of the light sheet and projected selected optical image. More preferably, the light sheet and projected selected optical image intersect in a coplanar manner.

[0163] The depicted example includes a volumetric 3D printer. The depicted printer includes a container 21 for including a volume of a photohardenable composition, light sheet optics 22 for directing a light sheet 201 to a selected location in the volume of the photohardenable composition, projection optics 23 for projecting a selected optical image 200 to the selected location in the volume to form one or more three-dimensional objects in a volume of the photohardenable composition (the depicted example includes one three- dimensional object 24). In the depicted example, printing includes exposure of a series of selected locations in the volume to a light sheet and a sequential series of selected projected optical images. The depicted example also includes a light sheet generator 25 in optical communication with the light sheet optics 22 and a projector 26 in optical communication with the projection optics 23. The depicted example also includes a controller 27 for controlling the printer.

[0164] The depicted example also includes a detector for determining light sheet deflection of the light sheet at a selected location. The depicted detector includes an imaging screen 28 for receiving an image of the light sheet deflection (LSD) 30 and an imaging unit 29 for recording the light sheet deflection. The screen 28 is located at a given distance (d) from the side of the container through which the light sheet exits the container.

[0165] The light sheet deflection data in the Examples was generated with a detector including an imaging device and a screen. The imaging device used was a Logitech C920X webcam. The screen that was used was a 5 mm x 5 mm paper grid mounted flush on an anodized aluminum sheet secured perpendicular to the light sheet path and connected to the optical table supporting the volumetric 3D printing system. The imaging device was placed a given distance, here 180 mm, from the screen, between the light sheet exit face of the container and the screen, facing the screen. A long pass 460 nm filter was used with the Logitech C920X webcam to reduce light sheet brightness while allowing the imaging device to view the grid pattern on which the light sheet deflection line is projected. As mentioned above, the screen is positioned a given distance, here 300 mm, from the exit face of the container. The “Logitech capture” setting for the Logitech C920X webcam was used to collect the videos.

[0166] Preferably the images collected are rectified by well-known techniques used in image processing. This can be useful in a system when the camera is positioned askew to the screen where the light sheet lands. Preferably, it is desirable for the light sheet to land vertically on the pixel grid for analysis purpose.

[0167] All measurements were made with the camera and screen at the same positions relative to each other and the light sheet exit side of the container positions to prevent deviations based on changes in the set-up to avoid inconsistencies in data due to changes in the set up. While LSD measurements change in relation to changes in d., the angle of the light sheet deflection in the photohardenable composition will be consistent without regard to the physical dimensions of the set-up for determining the distance of the deflection.

[0168] Information concerning other examples of volumetric 3D printing systems including a detector for use in determining light sheet deflection during formation of a three- dimensional object include U.S. Provisional Patent Application No. 63 / 625,817 of Quadratic 3D, Inc. filed January 26, 2024, U.S. Provisional Patent Application No. 63 / 704,520 of Quadratic 3D, Inc. filed October 7, 2024, each of which is hereby incorporated herein by reference in its entirety.

[0169] For each print, the maximum horizonal displacement (in mm) from the initial, nondeflected light sheet is shown in following Table 2.

[0170] Table 2

[0171]

[0172] Cube isolation and mechanical testing

[0173] For isolation of the 3 mm cubes, processing was kept to a minimum to preserve the mechanical integrity. The contents of a cuvette were poured onto a sheet of aluminum foil and most of the resin was gently removed from the surface with the aid of a Kimwipe. Compression testing was carried out on a TA Instruments Discovery DMA850 equipped with Compression Clamp. At ambient temperature (-22C), each isolated 3 mm cube was compressed from 0% to 15% strain at a rate of 15% strain per minute. The sampling rate was 10 points per second. The reported modulus values (Table 2) were calculated from the stress / strain curve by a linear fit of the data points between 5% and 10% strain. The calculated slope was interpreted as the compressive modulus of the green state cube.

[0174] A simple and direct test was employed to assess each additive, as summarized in Table 2 and FIGS. 2A-6B. A series of 3 mm cubes was printed at different exposure times. Maximum LSD was measured for each print. The compressive modulus of the as-printed cube (“green-state”) was determined using dynamic mechanical analysis (DMA). This modulus value is useful since it can be indicative of the processibility of the printed part. Higher modulus will translate to less damage during washing and handling. The observed tradeoff between modulus and LSD is clear: an increase in modulus invariably comes at the expense of an increase in LSD. A preferred additive is one that improves this unfavorable modulus / LSD tradeoff.

[0175] Prints from a base all-acrylate resin (black dots in all of FIGS. 2A-6B) serve as a baseline for comparison of additives. This resin provides cubes with high green-state moduli, but at the expense of high LSD. For example, for a cube with a 1.8 MPa modulus, an LSD of 25 mm was observed. This modulus describes a green state of good mechanical integrity, which can minimize defects introduced during a full processing effort; however, the LSD also indicates an uncontrolled print that would likely result in artifacts and loss of resolution. We quantify this tradeoff herein by the slope of the simple linear regression of the data. For the all-acrylate resin tested, the regression indicates that it costs 1 mm of LSD to increase modulus by only 0.093 MPa.

[0176] All tested additives were effective at delaying polymerization, as indicated by the generally lower LSD values and longer exposure times. Three of the additives provided markedly better modulus / LSD tradeoffs: 0.01% 2-cyano-2-propyl benzodithioate (a commercial chain transfer agent); 0.2% divinylbenzene (a difunctional comonomer); or 0.2% 4-cyanostyrene (a monofunctional comonomer). These additives in an otherwise identical acrylate resin dramatically increase the slope of the LSD / modulus curve as determined by a simple linear regression. This regression indicates that it costs 1 mm of LSD to increase modulus by 0.27, 0.28, and 0.20 MPa for 2-cyano-2-propyl benzodithioate, divinylbenzene, and 4-cyanostyrene, respectively. Thus, the tradeoff is 2-3x better than for the analogous “additive-free” all-acrylate resin described above.

[0177] It should be noted that the linear fit is of poor quality for styrene, 2-cyanostyrene, and 4-methylpent-l-ene-2,4-diyl)dibenzene. In these cases, an exponential relationship (not shown) better fits the data. Such a relationship would indicate a better LSD / modulus tradeoff than that represented by the linear fit.

[0178] Preferably a photohardenable composition including an additive in accordance with the present invention can facilitate obtaining a higher modulus part directly from the printer with similar or reduced amount of light sheet deflection during printing.

[0179] A photohardenable composition includes a photohardenable resin component and a photoinitiator.

[0180] A photohardenable composition preferred for use in the various aspects of the present invention preferably includes a photohardenable resin component and a dual wavelength photoinitiator.

[0181] Dual wavelength photoinitiators for inclusion in a photohardenable composition for use in the present invention preferably possess photochromic properties and can be converted to a second form upon irradiation with light of a first wavelength, which second form can be converted to back to the first form upon irradiation with light of a second wavelength, the process of cycling between these forms capable of inducing a crosslinking or polymerization reaction in the photohardenable resin component. The conversion of the photoswitchable photoinitiators described herein to a second form of the molecule (e.g., an isomer thereof) is preferably a reversible photochemical structural change. (Dual wavelength photoinitiators including such photochromic properties are also referred to herein as “photoswitchable photoinitiators”.)

[0182] Several considerations in selecting a particular photoswitchable photoinitiator for inclusion in a photohardenable composition or method in accordance with the present invention include, by way of example, but not limited to, the absorption spectra and Amax of the molecule and its second forms, the solubility of the photoswitchable photoinitiator in the photohardenable resin component, the photoinitiation sensitivity of the first and second forms of the photoswitchable photoinitiator, the amount of initial concentration of the second form 1 in the monomer solution, the stability of the photo switchable photoinitiator and the reduction and oxidation potentials of the second form of the photo switchable photoinitiator.

[0183] Preferred photo switchable photoinitiators include, but are not limited to, photochromic molecules, (e.g., but not limited to, a benzo spiropyran molecule, a naphthopyran molecule, a spironaphthoxazine molecule, a diarylethene molecule) which photochromic molecules can more preferably include one or more functional groups attached thereto. Such photochromic molecules can undergo a reversible intramolecular transformation forming an active form of the molecule by irradiation (photochromic). Such preferred photo switchable photoinitiators, e.g., in the case of benzospiropyrans, naphthopyrans, and spironaphthoxazines, can function by light activated opening of the photoswitchable photoinitiator to form the activated form upon exposure to a first wavelength. In the case of diarylethenes, the activation process instead involves a ringclosing. The colored form may subsequently absorb light of a different second wavelength which may cause it to revert to the first state. From the first uncolored state it can be excited again to the colored state by the first wavelength, the process of cycling in between these states being capable of subsequently induce photoinitiation, either alone or in combination with a coinitiator (e.g., amine, thiol, organoborate compounds, onium salts).

[0184] Preferred photohardenable compositions including a photoswitchable photoinitiator are particularly suitable for use in the methods of the present invention for forming three- dimensional objects. The photoswitchable photoinitiator molecule in its initial form and the photoinitiator molecule in its activated second form can have sufficiently distinct absorption spectra that once the initial form of the molecule is activated form, the activated form absorbs in a wavelength region where the initial form is minimally absorbing. In this way, the activated form can be independently excited with the second wavelength without causing unintended excitation of the initial form by the second wavelength. The second wavelength can cause more rapid cycling of the photoswitchable photoinitiator than in the presence of the first wavelength alone, this more rapid cycling causing increased rate of radical formation and inducing desired hardening of the photohardenable resin at the intersection of the two colors of light.

[0185] Information concerning photohardenable compositions and photoswitchable photoinitiators that may be useful in connection with the various aspects of the present invention includes International Application No. PCT / US2022 / 037491, filed July 18, 2022, of Quadratic 3D, Inc., International Application No. PCT / US2022 / 042179, filed August 31, 2022, of Quadratic 3D, Inc., International Application No. PCT / US2022 / 042183, filed August 31, 2022, of Quadratic 3D, Inc., International Application No. PCT / US2022 / 042186, filed August 31, 2022, of Quadratic 3D, Inc., International Application No. PCT / US2023 / 022172 of Quadratic 3D, Inc. filed May 13, 2023, International Application No. PCT / US2023 / 022170 of Quadratic 3D, Inc. filed May 13, 2023, and International Application No. PCT / US2023 / 022173 of Quadratic 3D, Inc. filed May 13, 2023, each of the foregoing applications being hereby incorporated herein by reference in its entirety.

[0186] Additional examples of photoswitchable photoinitiators suitable for inclusion in a photohardenable composition useful in the present invention are described in International Application No. PCT / US2023 / 022170 of Quadratic 3D, Inc. filed May 13, 2023, and International Application No. PCT / US2023 / 022173 of Quadratic 3D, Inc. filed May 13, 2023, each of which is hereby incorporated herein by reference in its entirety.

[0187] Examples of preferred dual wavelength photoinitiators for use in the methods of the present invention include substituted or unsubstituted P-type photochromic molecules. Examples of such preferred photoinitiators include, but are not limited to, substituted or unsubstituted diarylethene molecules. See, for example, International Application No. PCT / US2023 / 022172 of Quadratic 3D, Inc. filed May 13, 2023.

[0188] Examples of compositional ranges for a dual wavelength photoinitiator in a photohardenable composition in accordance with the present invention include, but are not limited to, about 0.0001 to about 0.5, including, for example, but not limited to, e.g., about 0.0001 to about 0.1, about 0.0001 to about 0.05, about 0.0001 to about 0.01, about 0.0001 to about 0.009, about 0.0001 to about 0.005, from about 0.0001 to about 0.0025, etc.

[0189] A photohardenable resin component suitable for inclusion in a photohardenable composition can comprise any resin (e.g., a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing) that is photohardenable by exposure to light in the presence of a photoinitiator. Examples of photohardenable resin components useful for inclusion in a photohardenable composition include ethylenically unsaturated compounds and, more specifically, a polyethylenically unsaturated compounds. These compounds include both monomers having one or more ethylenically unsaturated groups, such as vinyl or allyl groups, and polymers having terminal or pendant ethylenic unsaturation. Such compounds are well known in the art and include, but are not limited to, acrylic and methacrylic esters of polyhydric alcohols such as trimethylolpropane, pentaerythritol, and the like; and acrylate or methacrylate terminated epoxy resins, acrylate or methacrylate terminated polyesters, etc. Representative examples include, but are not limited to, ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane triacrylate (TMPTA), pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hydroxypentacrylate (DPHPA), hexanediol- 1,6-dimethacrylate, and diethyleneglycol dimethacrylate. Preferred examples include, but are not limited to, a urethane acrylate or a urethane methacrylate.

[0190] A photohardenable resin component can optionally comprise one or more multifunctional acrylate monomers. Dipentaerythritol pentaacrylate, a pentafunctional acrylic monomer available from Sartomer as SR399 is an example of a photohardenable resin component that may be desirable for inclusion in photohardenable composition of the present invention.

[0191] Aliphatic urethane acrylates may also be desirable for use as a photohardenable resin component for inclusion in a photohardenable composition described herein.

[0192] Mixtures of multifunctional acrylate monomers, such as dipentaerythritol pentaacrylate (e.g., SR399 from Sartomer), and aliphatic urethane acrylates can also be used.

[0193] A photohardenable resin component including other mixtures including one or more resin components can also be useful.

[0194] Preferably, the photohardenable resin component included in a photohardenable composition is selected to achieve an optically transparent medium, which is desirable in processes in which light, e.g., excitation light, is directed into the composition or light.

[0195] Examples of particularly preferred photohardenable resin components include, but are not limited to, free-radical-polymerizable resins, cross -linkable resins, multifunctional acrylate monomers, methacrylates, aliphatic urethane acrylates, and the like.

[0196] Examples of compositional ranges for a photohardenable resin component in a photohardenable composition in accordance with the present invention include, but are not limited to, about 10 to 99.9999 parts by weight (based on 100 parts total). The weight percent of the photohardenable resin component can be less than 10 weight percent, e.g., less than five weight percent, less than 3 weight percent, less than 2 weight percent, or one weight percent or less, in some cases such as printing of hydrogels where the remainder of the resin is then comprised of non-reactive components that are suspended within the final photohardened resin.

[0197] Optionally a solvent, preferably, for example, but not limited to, an acrylamide monomer or an acrylate monomer, can be further included in a composition described herein for mixing the photoswitchable photoinitiator in the photohardenable resin component. Other suitable solvents may also be used.

[0198] A photohardenable composition can optionally include a coinitiator. (A coinitiator can also be referred to as a synergist). Optionally, one or more coinitiators can be included.

[0199] Suitable coinitiators include coinitiators which are reducing agents, oxidizing agents, or hydrogen donating compounds.

[0200] Examples of coinitiators that may be useful can be selected from among those known in the art and, more particularly, tertiary amines and organoborate salts. lodonium salts may also be useful, particularly in combination with a borate salt. In certain embodiments, an iodonium salt may also be included in combination with a tertiary amine. Examples of other useful electron donating coinitiators are discussed by Eaton, D. F., "Dye Sensitized Photopolymerization", Advances in Photochemistry, Vol. 13, pp 427-486.

[0201] Representative examples of N,N-dialkylanilines useful in the present invention as coinitiators include 4-cyano-N,N-dimethylaniline, 4-acetyl-N,N-dimethylaniline, 4-bromo- N,N-dimethylaniline, 4-methyl-N, N-dimethylaniline, 4-ethoxy-N,N-dimethylaniline, N,N- dimethylthioanicidine, 4-amino-N, N-dimethylaniline, 3-hydroxy-N, N-dimethylaniline, N,N,N,'N, -tetramethyl- 1 ,4-dianiline, 4-acetamido-N, N-dimethylaniline, 2,6-diethyl-N,N- dimethylaniline, N,N,2,4,6-pentanethylaniline (PMA) and p-t-butyl-N, N-dimethylaniline.

[0202] Certain other tertiary amines are also useful coinitiators including triethylamine, triethanolamine, N-methyldiethanolamine, 2-ethyl-4-(dimethylamino)benzoate, 2- ethylhexyl-4-(dimethylamino)benzoate, etc.

[0203] Another class of useful coinitiators includes alkyl borate salts such as ammonium salts of borate anions of the formula BRaRbRcRdwherein Ra-Rdare independently selected from the group consisting of alkyl, aryl, alkaryl, allyl, aralkyl, alkenyl, alkynyl, alicyclic and saturated or unsaturated heterocyclic groups. Representative examples of alkyl groups represented by Ra-Rdare methyl (Me), ethyl, propyl, butyl, pentyl, hexyl, octyl, stearyl, etc. The alkyl groups may be substituted, for example, by one or more halogen, cyano, acyloxy, acyl, alkoxy or hydroxy groups. Representative examples of aryl groups represented by Ra- Rdinclude phenyl, naphthyl and substituted aryl groups such as anisyl and alkaryl such as methylphenyl, dimethylphenyl, etc.

[0204] Representative examples of aryl groups represented by Ra-Rdinclude benzyl.

[0205] Representative alicyclic groups include cyclobutyl, cyclopentyl, and cyclohexyl groups. Examples of an alkynyl group aryl propynyl and ethynyl, and examples of alkenyl groups include a vinyl group. Preferably, at least one but not more than three of Ra, Rb, Rc, and Rdis an alkyl group. Each of Ra, Rb, Rc, and Rdcan contain up to 20 carbon atoms, and they typically contain 1 to 7 carbon atoms. More preferably Ra-Rdare a combination of alkyl group(s) and aryl- group(s) or aralkyl group(s) and still more preferably a combination of three aryl groups and one alkyl group, i.e., an alkyltriphenylborate, e.g., but not limited to, a butyltriphenyl borate.

[0206] Examples of compositional ranges for a coinitiator when optionally included in a photohardenable composition in accordance with the present invention include, but are not limited to, about 0.001 to about 10 including, for example, but not limited to, about 0.001 to about 7.5, about 0.001 to about 5, about 0.001 to about 2.5, about 0.001 to about 1, about 0.001 to about 0.5, from about 0.0001 to about 0.25, etc.

[0207] A photohardenable composition can optionally include a sensitizer. Optionally, one or more sensitizers can be included.

[0208] A sensitizer can create the excited state of the photoswitchable photoinitiator via absorbing light and transferring energy to the photoswitchable photoinitiator. For example, a sensitizer can control the sensitivity of the composition and extend the spectral sensitivity of the closed form of the photoswitchable photoinitiator. Useful sensitizers include those known in the art such as acetophenone, benzophenone, 2-acetonaphthone, isopropyl thioxanthone, alkoxy ketocoumarins, Esacure 3644, and the like.

[0209] Examples of compositional ranges for a sensitizer when optionally included in a photohardenable composition in accordance with the present invention include, but are not limited to, about 0.1 to about 0.75, about 0.1 to about 0.5, about 0.1 to about 0.25, etc.

[0210] Optionally, a composition described herein can include one or more coinitiators and one or more sensitizers.

[0211] A photohardenable composition for use in the methods of the present invention preferably display non-Newtonian rheological behavior. Such rheological behavior can facilitate forming an object in a volume of a photohardenable composition upon exposure to at least two different wavelengths of excitation light wherein the object remains at a fixed position or is minimally displaced in the volume of the unhardened composition during formation. Minimal displacement refers to displacement of the object being formed during its formation in the volume that is acceptable for precisely producing the intended part geometry. Such rheological behavior can also facilitate separation of the partially hardened object from the volume in which it is formed upon application of stress. While not wishing to be bound by theory, upon the application of stress, the apparent viscosity of the non-Newtonian composition can drop to a lower value (e.g., the steady shear viscosity) than the static value (e.g., zero shear viscosity or yield stress) allowing the unhardened composition to more easily flow off and separate from the object. Examples of such non-Newtonian rheological behavior include but are not limited to pseudoplastic fluid, yield pseudoplastic, Bingham plastic, Bingham pseudoplastic rheological behavior.

[0212] Non-Newtonian rheological behavior can be imparted to a photohardenable composition by further including one or more reactive components (e.g. urethane acrylate oligomers, urethane methacrylate oligomers, acrylated or methacrylated polyurethanes, acrylated or methacrylated polyurethane-ureas, acrylated or methacrylated polyesters, acrylated or methacrylated polyamides, acrylate- or methacrylate-functional block copolymers, alkenyl- or alkynyl-functional urethane oligomers, alkenyl- or alkynyl- functional polyurethanes, alkenyl- or alkynyl- functional polyurethane-ureas, alkenyl- or alkynyl-functional polyesters, alkenyl- or alkynyl-functional polyamides, alkenyl- or alkynyl- functional block copolymers, thiol-functional urethane oligomers, thiol-functional polyurethanes, thiol-functional polyurethane-ureas, thiol-functional polyesters, thiol- functional polyamides, thiol-functional block copolymers) in the photohardenable resin component and / or by further adding one or more nonreactive additives (e.g., but not limited to, one or more thixotropes and / or rheology modifiers) to the composition. Selection of the one or more of reactive components and the amounts thereof for addition to a photohardenable resin component included in a photohardenable composition to impart non-Newtonian rheological behavior thereto is within the skill of the skilled artisan in the relevant art without undue experimentation. Similarly, selection of nonreactive additives and the amount(s) thereof for addition to the photohardenable composition to impart non-Newtonian rheological behavior thereto is within the skill of the skilled artisan of the relevant art without undue experimentation .

[0213] A photohardenable composition described herein can preferably have a steady shear viscosity, for example, which is less than 30,000 centipoise, less than 20,000 centipoise, less than 10,000 centipoise, less than 5,000 centipoise, less than 1,000 centipoise. (Steady shear viscosity refers to the plateau value of the viscosity achieved with unidirectional constant shear, e.g., the value of the viscosity after the thixotrope network has broken up.) Steady shear viscosities may be measured at ambient (e.g., room temperature), printing temperature, or some other temperature (e.g., elevated or reduced). Measurement at printing temperature may provide advantage in determining the suitability of a photohardenable composition for printing. Preferred steady shear viscosities are less than 30,000 centipoise, more preferably less than 10,000 centipoise, and most preferably less than 1,000 centipoise.

[0214] Steady shear viscosity can be measured under continuous constant-rate shear, such as at shear rates ranging from about 0.00001 s'1to about 1000 s’1.)

[0215] Photohardenable compositions can further include one or more additives. Examples of additives include, but are not limited to, a filler, a thixotrope / rheology modifier, a defoamer, a stabilizer, an oxygen scavenger, a non-reactive solvent diluent, a thermally activated radical initiator, and a colorant. Any additive can be a single additive or a mixture of additives. For example, a thixotrope can comprise a single thixotrope or a mixture of two or more thixotropes. Additional information concerning additi ves and non-limiting examples thereof are provided below.

[0216] Additives are preferably selected so that they do not react with other components or additives that may be included in photohardenable compositions.

[0217] As mentioned above, one example of an additive that may be included in photohardenable compositions includes a filler. Optionally a filler can include a combination of one or more fillers. A fillers can be included in an amount greater than 0 to about 90 weight percent, the amount being determined by the purpose for the filler and the desired end use characteristics for the intended three-dimensional object. Advantageously, fillers may be selected to maintain the optical transparency of the photohardenable composition, e.g., by controlling particle size to be substantially less than the excitation wavelengths or by matching the refractive indices of the filler and matrix to reduce optical scatter.

[0218] Fillers may be used to modify the properties of a hardened photohardenable composition, for example the stiffness, strength, toughness, impact resistance, resistance to creep, resistance to fatigue, mechanical energy return, mechanical loss tangent, glass transition temperature, thermal degradation temperature, thermal conductivity, thermal resistance, moisture uptake, electrical conductivity, static dissipation, dielectric constant and loss tangent, density, refractive index, optical dispersion, opacity to ionizing radiation, and resistance to ionizing radiation. Fillers may also be used to modify the properties of the liquid (e.g., unhardened) photohardenable composition, such as rheological properties such as viscosity and thixotropy and optical properties such as refractive index. Examples of fillers include but are not limited to silica, alumina, zirconia; silicates glasses such as soda-lime glass, borosilicate glass, sodium silicate glass, lead glass, aluminosilicate glass, barium glass, thorium glass, glass ceramics; chalcogenide glasses; glass microspheres and microbubbles; nanoclays such as laponite, montmorillonite, bentonite, kaolinite, hectorite, and halloysite; calcium phosphate minerals such as hydroxyapatite, mineral fillers such as chalk, rock dust, slag dust, fly ash, hydraulic cement, loess, limestone, kaolin, talc, and wollastonite. Examples of particle size ranges include but are not limited to less than 10 microns, less than 1 micron, 10 nm to 500 nm, 10 nm to 90 nm, 40 nm to 70 nm. Smaller particles sizes, in particular sizes less than about 100 nm, may be beneficial to provide high optical clarity of the liquid composition to better facilitate printing. Controlling the particle size distribution, for example monodisperse, bimodal, or trimodal distributions of sizes, may be beneficial to control rheological properties, increase filler weight percent, or modify the properties of a photohardenable composition.

[0219] Other examples of additives that may be included in photohardenable compositions include a thixotropes and rheology modifiers. Thixotropes and rheology modifiers suitable for inclusion in a photohardenable composition described herein include, for example and without limitation, urea derivatives; modified urea compounds such as Rheobyk 410 and Rheobyk-D 410 available from BYK-Chemie GmbH, part of the ALTANA Group; fumed metal oxides (also referred to as pyrogenic metal oxides) including for example, but not limited to, fumed silica, fumed alumina; zirconia; precipitated metal oxides including for example, but not limited to, precipitated silica, precipitated alumina; unmodified and organo- modified phyllosilicate clays; dimer and trimer fatty acids; polyether phosphates; oxidized polyolefins; hybrid oxidized polyolefins with polyamide; alkali soluble / s wellable emulsions; cellulosic ethers; hydrophobically-modified alkali soluble emulsions; hydrophobically- modified ethylene oxide-based urethane; sucrose benzoate; ester terminated polyamides; tertiary amide terminated polyamides; poly alkyleneoxy terminated polyamides; poly ether amides; acrylamidomethyl-substituted cellulose ester polymers; polyethyleneimine; polyurea; organoclays; hydrogenated castor oil; organic base salts of a clay mineral (e.g., montmorillonite) and other silicate-type materials; aluminum, calcium, and zinc salts of fatty acids, such as lauric or stearic acid.

[0220] See U.S. Patent Nos. 6,548,593 of Merz, et al., issued April 15, 2003, and 9,376,602 of Walther, et al. issued June 28, 2016, which are hereby incorporated herein by reference in their entireties, for information relating to urea derivatives that may be useful as thixotropes.

[0221] Thermally reversible gellants such as ester terminated polyamides, tertiary amide terminated polyamides, polyalkyleneoxy terminated polyamides, and polyether amides, and combinations thereof, may be desirable for us as thixotropes. Examples include Crystasense LP1, Crystasense LP2, Crystasense LP3, Crystasense MP, Crystasense HP4, Crystasense HP5, Rheoptima X17, Rheoptima X24, Rheoptima X38, Rheoptima X58, Rheoptima X73, and Rheoptima X84 available from Croda. Crystasense HP-5 is a preferred example of a thixo trope.

[0222] Metal oxides that have been surface-treated to impart dispersibility characteristics compatible with a photohardenable composition described herein may be desirable for use as thixotropes.

[0223] A thixotrope or rheology modifier can be included in a photohardenable composition in an amount, for example, in a range from about 0.05 weight percent to about 15 weight percent, from about 0.5 weight percent to about 15 weight percent, from about 0.5 weight percent to about 10 weight percent from about 1 to about 10 weight percent of the composition. Other amounts may also be determined to be useful.

[0224] A thixotrope or rheology modifier is preferably included in a photohardenable composition in an amount effective to restrict movement of the three-dimensional object or one or more regions thereof in the photohardenable composition during formation.

[0225] More preferably, the thixotrope is included in a photohardenable composition in an amount effective to restrict movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation. Most preferably the position of the object in the volume of the photohardenable composition remains fixed position during formation of the object.

[0226] Optionally, a photohardenable composition can further include a light activated photoinitiator as described in International Application No. PCT / US2023 / 022171 of Quadratic 3D, Inc., filed 13 May 2023, which is hereby incorporated herein by reference in its entirety. Preferably such photoinitiator is not appreciably responsive to light of a first wavelength or second wavelength. Inclusion of a photoinitiator can be desirable in connection with optional post-processing that includes, for example, a post-curing step involving exposure of the printed object to UV light after printing. When a second light activated photoinitator is further included in a photohardenable composition, it can be included, for example, in a compositional range, in part by weight [based on 100 parts total], in a range from about 0.0001 to about 25%, including, for example, but not limited to, about 0.0001 to about 10, about 0.0001 to about 7.5, about 0.0001 to about 5, about 0.0001 to about 2.5, about 0.0001 to about 1, from about 0.0001 to about 0.5, etc. Another example of additives that can be included in a photohardenable composition includes defoamers. A defoamer can be included to aid in removing bubbles introduced during processing and handling. A preferred defoamer is BYK 1798 (a silicone based defoamer) available from BYK-Chemie GmbH, part of the ALTANA Group.

[0227] Another example of additives that can be included in a photohardenable composition includes a stabilizer. A stabilizer can be included to improve shelf-life of the composition and / or to control the level of cure and / or spatial resolution during printing. An example of preferred stabilizer is TEMPO (2,2,6,6-tetramethylpiperidinooxy free radical available from Sigma- Aldrich). Examples of other stabilizers include, but are not limited to, hindered phenols such as butylated hydroxy toluene; hydroquinone and its derivatives such as hydroquinone methyl ether; hindered amine light stabilizers; alkylated diphenylamines; and phosphite esters.

[0228] Optionally an additive comprising a non-reactive solvent diluent can be included. Examples include, but are not limited to, acetone, amyl acetate, n-butanol, sec-butanol, tertbutanol, butyl acetate, cyclohexanone, decane, dimethylacetamide, dimethylformamide, dimethylsulfoxide, dipropylene glycol, dipropylene glycol methyl ether, ethanol, ethyl acetate, ethylene glycol, glycerol, heptane, isopropanol, isopropyl acetate, methyl ethyl ketone, N-methyl pyrrolidone, propylene carbonate, propylene glycol, propylene glycol diacetate, tetrahydro furan, tripropylene glygol methyl ether, toluene, water, xylenes.

[0229] It may also be desirable to include a thermally activated radical initiator in a photohardenable composition. Thermally activated radical initiator examples include but are not limited to 2,2'-azobis(2-methylpropionitrile), l,l'-azobis(cyclohexanecarbonitrile), 2,2’ - azobis[N-(2-carboxyethyl)-2-methylpropionamidine] n-hydrate, 2,2’-azobis[2-methyl-N-(2- hydroxyethyl)propionamide], organic peroxides, inorganic peroxides, peroxydisulfate salts.

[0230] Unless otherwise indicated, specified weight percents are based on the total weight of the photohardenable composition.

[0231] A photohardenable composition in accordance with the invention can be prepared using known or conventional procedures.

[0232] The nature of a photohardenable resin component, the amount of the dual wavelength, and, when applicable, a coinitiator, a sensitizer, a thermally activated radical initiator, or other optional additive, included in photohardenable compositions will vary with the particular use of the compositions, the emission characteristics of the exposure sources, the development procedures, the physical properties desired in the hardened product and other factors. The methods in accordance with various aspects of the invention can further include post-treatment of the three-dimensional object(s) formed.

[0233] Examples of post-treatments include, but are not limited to, removing the formed three-dimensional object from the container. Following removal from the container, the completed object can be further processed. Examples of further processing include, without limitation, washing, post-curing (e.g., by light, e-beam, heat, non-ionizing radiation, ionizing radiation, time (aging), pressure, humidity, or simultaneous or sequential combinations of techniques), metrology, labelling or tracking (e.g., by barcode, QR code, or RFID tag), freeze- dry processing, critical point drying, and packaging.

[0234] Preferably photo switchable photoinitiators useful in methods for printing 3D objects in accordance with the present invention can absorb first wavelength light from about 300 nm to about 550 nm. Other examples of ranges in which the photoswitchable photoinitiator will absorb first wavelength light include, but are not limited to, from about 350 to about 460 nm, from about 350 to about 455 nm, from about 350 nm to about 445 nm, from about 350 nm to about 410 nm, from about 375 to about 455 nm, from about 375 to about 445 nm, from about 375 nm to about 405 nm. Other examples are described herein. Depending upon the extinction coefficient for the particular photoswitchable photoinitiator, the conversion to the second form can be induced by exposure to any source which emits in this range, e.g., lasers, light emitting diodes, mercury lamps. Filters may be used to limit the output wavelengths. A non-limiting example of filtered light includes filtered emission from a mercury arc lamp, etc. Easers can be preferred sources of radiation for generating radiation of the first wavelength.

[0235] The second form of the photoswitchable photoinitiator will preferably absorb second wavelength light in a range of about 450 nm to about 1000 nm, and from about 450 nm to about 850 nm most typically. Other examples of ranges in which the second form of the photoswitchable photoinitiator will preferably absorb second wavelength light include from about 450 nm to about 700 nm. This form can be activated by the second excitation light to cycle the switch back to the initial form. The process of cycling the switch between the forms may generate free radicals directly or electron transfer or hydrogen abstraction (optionally via electron, hydrogen, or energy transfer to coinitiator(s) in aspects of the invention including one or more coinitiator). For the second excitation, exposures may be accomplished using a laser source, an FED or FED array, the filtered emission from an arc lamp, or other suitable source with emission within the desired wavelength range, argon ion, He-Ne, laser diodes, krypton, frequency-multiplied Nd-YAG, etc. Other light sources may be used, optionally with filters to limit output wavelengths, e.g., light emitting diodes, incandescent lamps, halogen lamps, mercury lamps, arc lamps, etc. Lasers can be preferred sources of radiation for generating radiation of the second wavelength.

[0236] As used herein a wavelength can refer to a wavelength or range of wavelengths.

[0237] Embodiments of inventions described herein including the following:

[0238] Embodiment 1. A photohardenable composition for use in volumetric three- dimensional printing, the photohardenable composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a polymerization rate reduction additive wherein photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

[0239] Embodiment 2. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive is included in the photohardenable composition in an amount effective for slowing photopolymerization upon selective exposure of the photohardenable composition to one or more excitation wavelengths.

[0240] Embodiment 3. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive is included in the photohardenable composition in an amount greater than zero and less than 5 weight percent, preferably greater than zero and less than 0.5 weight percent, more preferably greater than zero and less than 0.1 weight percent.

[0241] Embodiment 4. The photohardenable composition of any one of embodiments 1-3 wherein the slower polymerization is detectable by a lower deflection of a light sheet at a selected location in a volume of the photohardenable composition at which a light sheet intersects with an optical image during formation of a three-dimensional object in a volume of the photohardenable composition than if the polymerization rate reduction additive is not included.

[0242] Embodiment 5. The photohardenable composition of any one of embodiments 1- 3 wherein the polymerization rate reduction additive comprises a comonomer . Embodiment 6. The photohardenable composition of any one of embodiments 1- 3 wherein the polymerization rate reduction additive comprises a comonomer that (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e., cross-propagation) at a slower rate than a unit of the photohardenable component undergoes homo-propagation.

[0243] Embodiment 7. The photohardenable composition of any one of embodiments 1-3 wherein the polymerization rate reduction additive comprises a chain transfer agent.

[0244] Embodiment 8. The photohardenable composition of any one of embodiments 1-3 wherein the polymerization rate reduction additive comprises a chain transfer agent wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

[0245] Embodiment 9. The photohardenable composition of any one of embodiments 1-8 wherein the photoinitiator comprises a photoswitchable photoinitiator.

[0246] Embodiment 10. The photohardenable composition of embodiment 9 further including a coinitiator.

[0247] Embodiment 11. The photohardenable composition of embodiment 9 further including a rheology modifier.

[0248] Embodiment 12. The photohardenable composition of embodiment 9 further including a coinitiator and a rheology modifier.

[0249] Embodiment 13. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises styrene.

[0250] Embodiment 14. The photohardenable composition of embodiment 5 or 6 wherein the polymerization rate reduction additive comprises styrene.

[0251] Embodiment 15. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises 4-cyanostyrene.

[0252] Embodiment 16. The photohardenable composition of embodiment 5 or 6 wherein the polymerization rate reduction additive comprises 4-cyanostyrene. Embodiment 17. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises a styrenic.

[0253] Embodiment 18. The photohardenable composition of embodiment 5 or 6 wherein the polymerization rate reduction additive comprises a styrenic.

[0254] Embodiment 19. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises divinylbenzene.

[0255] Embodiment 20. The photohardenable composition of embodiment 5 or 6 wherein the polymerization rate reduction additive comprises divinylbenzene.

[0256] Embodiment 21. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises a RAFT agent.

[0257] Embodiment 22. The photohardenable composition of embodiment 7 or 8 wherein the polymerization rate reduction additive comprises a RAFT agent.

[0258] Embodiment 23. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises 2-cyano-2-propyl benzodithioate.

[0259] Embodiment 24. The photohardenable composition of embodiment 7 or 8 wherein the polymerization rate reduction additive comprises 2-cyano-2-propyl benzodithioate.

[0260] Embodiment 25. The photohardenable composition of embodiment 1 wherein the polymerization rate reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0261] Embodiment 26. The photohardenable composition of embodiment 7 or 8 wherein the polymerization rate reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0262] Embodiment 27. The photohardenable composition of any one of embodiments 1-3 wherein inclusion of the polymerization rate reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three- dimensional object formed therefrom.

[0263] Embodiment 28. The photohardenable composition of embodiment 5 or 6 wherein inclusion of the polymerization rate reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom. Embodiment 29. The photohardenable composition of embodiment 7 or 8 wherein inclusion of the polymerization rate reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

[0264] Embodiment 30. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0265] (a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of embodiments 1-29;

[0266] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0267] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the polymerization rate reduction additive slows polymerization to a rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

[0268] Embodiment 31. The method of embodiment 30 wherein polymerization is slowed at the intersection of the light sheet with the projected optical image.

[0269] Embodiment 32. The method of embodiment 30 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

[0270] Embodiment 33. A photohardenable composition for use in volumetric three- dimensional printing with reduced light sheet deflection, the photohardenable composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and an additive for reducing deflection of a light sheet at a selected location in a volume of the photohardenable composition during formation of a three-dimensional object in the volume.

[0271] Embodiment 34. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive is included in the photohardenable composition in an amount greater than zero and less than 5 weight percent, preferably greater than zero and less than 0.5 weight percent, more preferably greater than zero and less than 0.1 weight percent.

[0272] Embodiment 35. The photohardenable composition of embodiment 33 wherein light sheet deflection at the intersection of the light sheet with the optical image during the formation of the three-dimensional object in the volume is less than if the light sheet deflection reduction additive is not included in the photohardenable composition.

[0273] Embodiment 36. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive reduces distortion from refractive index changes during formation of the three-dimensional object.

[0274] Embodiment 37. The photohardenable composition of any one of embodiments 33- 36 wherein the light sheet deflection reduction additive comprises a comonomer .

[0275] Embodiment 38. The photohardenable composition of any one of embodiments 33- 36 wherein the light sheet deflection reduction additive comprises a comonomer that (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e. cross-propagation) at a slower rate than a unit of the photohardenable component undergoes homo-propagation.

[0276] Embodiment 39. The photohardenable composition of any one of embodiments 33- 36 wherein the light sheet deflection reduction additive comprises a chain transfer agent.

[0277] Embodiment 40. The photohardenable composition of any one of embodiments 33- 36 wherein the light sheet deflection reduction additive comprises a chain transfer agent wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

[0278] Embodiment 41. The photohardenable composition of any one of embodiments 33- 40 wherein the photoinitiator comprises a photo switchable photoinitiator.

[0279] Embodiment 42. The photohardenable composition of embodiment 41 further including a coinitiator.

[0280] Embodiment 43. The photohardenable composition of embodiment 41 further including a rheology modifier.

[0281] Embodiment 44. The photohardenable composition of embodiment 41 further including a rheology modifier.

[0282] Embodiment 45. The photohardenable composition of embodiment 41 further including a rheology modifier and a coinitiator.

[0283] Embodiment 46. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises styrene.

[0284] Embodiment 47. The photohardenable composition of embodiment 37 or 38 wherein the light sheet deflection reduction additive comprises styrene.

[0285] Embodiment 48. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises 4-cyanostyrene.

[0286] Embodiment 49. The photohardenable composition of embodiment 37 or 38 wherein the light sheet deflection reduction additive comprises 4-cyanostyrene.

[0287] Embodiment 50. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises a styrenic.

[0288] Embodiment 51. The photohardenable composition of embodiment 37 or 38 wherein the light sheet deflection reduction additive comprises a styrenic.

[0289] Embodiment 52. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises divinylbenzene.

[0290] Embodiment 53. The photohardenable composition of embodiment 37 or 38 wherein the light sheet deflection reduction additive comprises divinylbenzene. Embodiment 54. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises a RAFT agent.

[0291] Embodiment 55. The photohardenable composition of embodiment 39 or 40 wherein the light sheet deflection reduction additive comprises a RAFT agent.

[0292] Embodiment 56. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises 2-cyano-2-propyl benzodithioate.

[0293] Embodiment 57. The photohardenable composition of embodiment 39 or 40 wherein the light sheet deflection reduction additive comprises 2-cyano-2-propyl benzodithioate.

[0294] Embodiment 58. The photohardenable composition of embodiment 33 wherein the light sheet deflection reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0295] Embodiment 59. The photohardenable composition of embodiment 39 or 40 wherein the light sheet deflection reduction additive comprises 4-methylpent-l-ene-2,4- diyl)dibenzene.

[0296] Embodiment 60. The photohardenable composition of any one of embodiments 33- 36 wherein inclusion of the light sheet deflection reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three- dimensional object formed therefrom.

[0297] Embodiment 61. The photohardenable composition of embodiment 37 or 38 wherein inclusion of the light sheet deflection reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

[0298] Embodiment 62. The photohardenable composition of embodiment 39 or 40 wherein inclusion of the light sheet deflection reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

[0299] Embodiment 63. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0300] (a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of embodiments 33-62; (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0301] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein light sheet deflection during formation of the three-dimensional object in the photohardenable composition is less than without inclusion of the light sheet deflection reduction additive.

[0302] Embodiment 64. The method of embodiment 63 wherein light sheet deflection is reduced at the intersection of the light sheet with the projected optical image.

[0303] Embodiment 65. The method of embodiment 63 wherein the achievable green- state modulus of the object as formed in the volume is substantially unaffected by inclusion of the light sheet deflection reduction additive in the photohardenable composition.

[0304] Embodiment 66. A photohardenable composition for use in volumetric three- dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon excitation.

[0305] Embodiment 67. The photohardenable composition of embodiment 66 wherein the comonomer (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e., cross- propagation) at a slower rate than a unit of the photohardenable component undergoes homopropagation.

[0306] Embodiment 68. The photohardenable composition of embodiment 66 or 67 where excitation comprises exposing a selected location in the photohardenable composition to two intersecting excitation wavelengths.

[0307] Embodiment 69. The photohardenable composition of embodiment 68 wherein polymerization is slowed at the intersection.

[0308] Embodiment 70. The photohardenable composition of embodiment 66 or 67 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0309] Embodiment 71. The photohardenable composition of embodiment 66 or 67 wherein the photoinitiator comprises a photoswitchable photoinitiator.

[0310] Embodiment 72. The photohardenable composition of embodiment 66 or 71 wherein the photohardenable composition further includes a coinitiator.

[0311] Embodiment 73. The photohardenable composition of embodiment 66 or 71 wherein the photohardenable composition further includes a rheology modifier.

[0312] Embodiment 74. The photohardenable composition of embodiment 66 or 71 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

[0313] Embodiment 75. The photohardenable composition of embodiment 66 or 67 wherein the comonomer comprises styrene.

[0314] Embodiment 76. The photohardenable composition of embodiment 66 or 67 wherein the comonomer comprises 4-cyanostyrene.

[0315] Embodiment 77. The photohardenable composition of embodiment 66 or 67 wherein the comonomer comprises a styrenic.

[0316] Embodiment 78. The photohardenable composition of embodiment 66 or 67 wherein the comonomer comprises divinylbenzene.

[0317] Embodiment 79. The photohardenable composition of any one of embodiments 71- 74 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer. Embodiment 80. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0318] (a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of embodiments 66-79;

[0319] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0320] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the comonomer in the photohardenable composition slows polymerization to a rate that is lower than if the comonomer is not included in the photohardenable composition.

[0321] Embodiment 81. The method of embodiment 80 wherein polymerization is slowed at the intersection of the light sheet with the projected optical image.

[0322] Embodiment 82. The method of embodiment 80 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

[0323] Embodiment 83. The method of embodiment 80 wherein the photohardenable composition displays non-Newtonian rheological behavior.

[0324] Embodiment 84. A photohardenable composition for use in volumetric three- dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three- dimensional object.

[0325] Embodiment 85. The photohardenable composition of embodiment 84 wherein the comonomer (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e., crosspropagation) at a slower rate than a unit of the photohardenable component undergoes homopropagation.

[0326] Embodiment 86. The photohardenable composition of embodiment 84 or 85 where excitation comprises exposing a selected location in the photohardenable composition to two intersecting excitation wavelengths.

[0327] Embodiment 87. The photohardenable composition of embodiment 86 wherein distortion is reduced at the intersection.

[0328] Embodiment 88. The photohardenable composition of embodiment 84 or 85 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0329] Embodiment 89. The photohardenable composition of embodiment 84 or 85 wherein the photoinitiator comprises a photoswitchable photoinitiator.

[0330] Embodiment 90. The photohardenable composition of embodiment 84 or 89 wherein the photohardenable composition further includes a coinitiator.

[0331] Embodiment 91. The photohardenable composition of embodiment 84 or 89 wherein the photohardenable composition further includes a rheology modifier.

[0332] Embodiment 92. The photohardenable composition of embodiment 84 or 89 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

[0333] Embodiment 93. The photohardenable composition of embodiment 88 wherein the photohardenable composition further includes a rheology modifier.

[0334] Embodiment 94. The photohardenable composition of embodiment 84 or 85 wherein the comonomer comprises styrene. Embodiment 95. The photohardenable composition of embodiment 84 or 85 wherein the comonomer comprises 4-cyanostyrene.

[0335] Embodiment 96. The photohardenable composition of embodiment 84 or 85 wherein the comonomer comprises a styrenic.

[0336] Embodiment 97. The photohardenable composition of embodiment 84 or 85 wherein the comonomer comprises divinylbenzene.

[0337] Embodiment 98. The photohardenable composition of any one of embodiments 89- 92 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0338] Embodiment 99. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0339] (a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of embodiments 84-98;

[0340] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0341] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three-dimensional object is less than without inclusion of the comonomer.

[0342] Embodiment 100. The method of embodiment 99 wherein distortion is reduced at the intersection of the intersection of the light sheet with the projected optical image.

[0343] Embodiment 101. The method of embodiment 99 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition. Embodiment 102. The method of embodiment 99 wherein the photohardenable composition displays non-Newtonian rheological behavior.

[0344] Embodiment 103. A photohardenable composition for use in volumetric three- dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon exposure to excitation.

[0345] Embodiment 104. The photohardenable composition of embodiment 103 wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

[0346] Embodiment 105. The photohardenable composition of embodiment 103 or 104 where in the exposure to dual wavelength excitation comprises the intersection of two excitation wavelengths in the photohardenable composition.

[0347] Embodiment 106. The photohardenable composition of embodiment 105 wherein polymerization is slowed at the intersection.

[0348] Embodiment 107. The photohardenable composition of embodiment 103 or 104 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0349] Embodiment 108. The photohardenable composition of embodiment 103 or 104 wherein the dual-wavelength photoinitiator comprises a photoswitchable photoinitiator.

[0350] Embodiment 109. The photohardenable composition of embodiment 103 or 108 wherein the photohardenable composition further includes a coinitiator. Embodiment 110. The photohardenable composition of embodiment 103 orl08 wherein the photohardenable composition further includes a rheology modifier.

[0351] Embodiment 111. The photohardenable composition of embodiment 103 or 108 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

[0352] Embodiment 112. The photohardenable composition of embodiment 103 or 104 wherein the chain transfer agent comprises a RAFT agent.

[0353] Embodiment 113. The photohardenable composition of embodiment 103 or 104 wherein the chain transfer agent 2comprises 2-cyano-2-propyl benzodithioate.

[0354] Embodiment 114. The photohardenable composition of embodiment 103 or 104 wherein the chain transfer agent comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0355] Embodiment 115. The photohardenable composition of any one of embodiments 108-111 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0356] Embodiment 116. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0357] (a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of embodiments 103-115;

[0358] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0359] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein inclusion of the chain transfer agent in the photohardenable composition slows polymerization to a rate that is lower than if the chain transfer agent is not included in the photohardenable composition. Embodiment 117. The method of embodiment 116 wherein polymerization is slowed at the intersection of the light sheet with the projected optical image.

[0360] Embodiment 118. The method of embodiment 116 wherein the achievable greenstate modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

[0361] Embodiment 119. The method of embodiment 116 wherein the photohardenable composition displays non-Newtonian rheological behavior.

[0362] Embodiment 120. A photohardenable composition for use in volumetric three- dimensional printing, the composition comprising: a photohardenable resin component, preferably comprising an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer; a photoinitiator, preferably comprising a dual-wavelength photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object.

[0363] Embodiment 121. The photohardenable composition of embodiment 120 wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

[0364] Embodiment 122. The photohardenable composition of embodiment 120 or 121 where excitation comprises exposing a selected location in the photohardenable composition to two intersecting excitation wavelengths.

[0365] Embodiment 123. The photohardenable composition of embodiment 122 wherein distortion is reduced at the intersection.

[0366] Embodiment 124. The photohardenable composition of embodiment 120 or 121 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer. Embodiment 125. The photohardenable composition of embodiment 120 or 121 wherein the photoinitiator comprises a photoswitchable photoinitiator.

[0367] Embodiment 126. The photohardenable composition of embodiment 120 or 125 wherein the photohardenable composition further includes a coinitiator.

[0368] Embodiment 127. The photohardenable composition of embodiment 120 or 125 wherein the photohardenable composition further includes a rheology modifier.

[0369] Embodiment 128. The photohardenable composition of embodiment 120 or 125 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

[0370] Embodiment 129. The photohardenable composition of embodiment 120 or 121 wherein the chain transfer agent comprises a RAFT agent.

[0371] Embodiment 130. The photohardenable composition of embodiment 120 or 121 wherein the chain transfer agent comprises 2-cyano-2-propyl benzodithioate.

[0372] Embodiment 131. The photohardenable composition of embodiment 120 or 121 wherein the chain transfer agent comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

[0373] Embodiment 132. The photohardenable composition of any one of embodiments 125-128 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0374] Embodiment 133. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:

[0375] (a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of embodiments 120-132;

[0376] (b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images, preferably at an intersection of the light sheet and the selected projected optical image at the selected location; and

[0377] (c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three-dimensional object is less than without inclusion of the chain transfer agent.

[0378] Embodiment 134. The method of embodiment 133 wherein distortion is reduced at the intersection of the intersection of the light sheet with the projected optical image.

[0379] Embodiment 135. The method of embodiment 133 wherein the achievable greenstate modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

[0380] Embodiment 136. The method of embodiment 133 wherein the photohardenable composition displays non-Newtonian rheological behavior.

[0381] Embodiment 137. The photohardenable composition of any one of embodiments 1, 33, 66, 84, and 120 wherein the photohardenable resin component comprises a dualwavelength photoinitiator.

[0382] Embodiment 138. The photohardenable composition of any one of embodiments 1, 33, 66, 84, 103, and 120 wherein the photoinitiator comprises a photoswitchable photoinitiator.

[0383] Embodiment 139. The photohardenable composition of embodiment 1 or 33 wherein the photohardenable resin component comprises an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer.

[0384] Embodiment 140. The photohardenable composition of any one of embodiments 1, 33, 66, 84, 103, and 120 wherein the photohardenable resin component comprises an acrylate- based monomer or oligomer and / or a methacrylate based monomer or oligomer and the photoinitiator comprises a photoswitchable photoinitiator.

[0385] Embodiment 141. The photohardenable composition of claim 33 wherein the additive for reducing deflection of the light sheet is included in the photohardenable composition in an amount effective to reduce deflection of the light sheet during printing.

[0386] In methods described herein, the volume of the photohardenable composition may be irradiated with one or more one or more excitation lights to at least partially harden, polymerize, and / or cross-link photohardenable composition to at least partially form the one or more 3D objects in the volume of the photohardenable composition. Preferably at least one of the excitation lights is directed into the volume as a light sheet.

[0387] In methods described herein, selection of excitation lights and wavelengths is preferably made taking into account the photohardening mechanism of a photohardenable composition, For example, for a photohardenable composition that is photohardenable via a mechanism that involves a single wavelength of excitation light, the wavelength of the one or more excitation light projections can be the same.

[0388] In cases in which a photohardenable composition is photohardenable via a mechanism that involves more than wavelengths of excitation light, the excitation lights will be selected to include appropriate wavelengths (or ranges of wavelengths) for the mechanism.

[0389] Preferably methods described herein include irradiation with excitation light from two light sources, such two excitation light sources can be included in separate optical systems. A first system, preferably a light sheet generating system, can generate and direct a light sheet including a first wavelength to a selected location in the volume, and a second optical system, preferably a projection system including a projector and a second light source for projecting an optical image (e.g., a two-dimensional cross-sectional slice of an object to be printed or other two-dimensional image) including a second wavelength of light. Depending on the hardening mechanism of the photohardenable composition being used, the first and second wavelengths can be the same or different. For example, for a photohardenable composition including a dual-wavelength photoinitiator, the first and second wavelength are typically different. The optical image and light sheet may be arranged such that they are projected to intersect or overlap at the selected location, preferably in a coplanar manner. The light sheet and optical image are preferably directed into the volume in directions orthogonal with respect to one another. The optical image is preferably orthogonal to the direction along which it is directed into the volume.

[0390] A light sheet can be constructed by means known in the art including, for example, but not limited to, techniques including a laser and a Powell lens, galvanometer, and / or polygon scanning mirror. Alternatively, one or more LEDs can be used as a light source.

[0391] A light sheet generating system can include a light source of a first excitation light including a first wavelength, preferably a laser, from which a light sheet is generated by a light sheet generator, the light sheet including two major parallel faces that are parallel to the direction in which the light sheet is directed to a selected location in the volume. The light sheet generating system can also preferably include further light sheet optics between the light sheet generator and the container.

[0392] A projection system for generating an optical image can be selected to apply continuous excitation light. An optical system can be selected to apply intermittent excitation light. Intermittent excitation can include random on and off application of light or periodic application of light. Examples of periodic application of light includes pulsing. An optical system can be selected to apply a combination of both continuous excitation light and intermittent light, including, for example, an irradiation step that includes the application of intermittent excitation light that is preceded or followed by irradiation with continuous light.

[0393] A projection system can further include one or more additional components including, but not limited to, one or more translational stages for moving the system or components thereof.

[0394] A projection system can include a projector (e.g., a DMD) and a light source in combination with illumination optics to illuminate the DMD. Such illumination optics can optionally comprise beam conditioning and condenser optics and relay optics. A light source for a second excitation light including a second wavelength illuminates the projection device. A light source comprising a non-pulsed laser or a continuous wave laser can be preferred. A projection system can further include projection optics positioned between the projector and the container. Projection optics can be used for magnifying and projecting a focused optical projection of excitation light into the container. Optionally, prism(s) can be positioned between the projector and the projection optics. A projected image (typically a 2- dimenstional cross-section slice of the object to be printed) is projected to the selected focal plane at the selected location in the volume. The optical image is preferably orthogonal to the direction in which it is projected into the volume.

[0395] Examples of projectors (which may also be referred to as projector devices or projection devices) for use in the methods described herein may include, but are not limited to, a laser projection system, a liquid crystal display (also referred to herein as “LCD”), a spatial light modulator (also referred to herein as “SLM”) (for example, but not limited to, a digital micromirror device (also referred to herein as “DMD”) or a digital light processing device (also referred to herein as “DLP”)), a micro-LED array, a vertical cavity laser array (also referred to herein as “VCL”), a Vertical Cavity Surface Emitting Laser array (also referred to herein as “VCSEL”), a liquid crystal on silicon (also referred to herein as “LCoS”) projector, and a scanning laser system. (Light emitting diode is also referred to herein as “LED”.)

[0396] Examples of light sources of the excitation light that may be suitable for use in various aspects of the present invention including light sources include, by way of example and nonlimitation, lasers, laser diodes, light emitting diodes, light-emitting diodes (LEDs), microLED arrays, vertical cavity lasers (VCLs), and filtered lamps. Such light sources are commercially available and selection of a suitable light source can be readily made by one of ordinary skill in the relevant art. Laser light sources can be preferred.

[0397] Optionally, the excitation light can be temporally and / or spatially modulated. Optionally, the intensity of the excitation light can be modulated. Optionally, source drive modulation can be used to adjust the absolute power of the light beam.

[0398] A configuration of a projector device and light source can optionally further include one or more optical components (e.g., projection optics, illumination optics, lenses, lens systems, mirrors, prisms, etc.)

[0399] A projection system may also be referred to herein as an optical projection system. A projector may also be referred to herein as a projector device.

[0400] Preferably the directions in which the light sheet and optical image are directed to the selected location in the volume are orthogonal to each other with the optical image and light sheet intersecting or overlapping in a coplanar manner.

[0401] In embodiments in which excitation light is projected from two light sources into container, the photohardenable composition is at least partially hardened, polymerized, or cross-linked at the intersection region of the light sheet and optical image from two light sources. By moving the intersection through volume of the photohardenable composition included in the container, a 3D object may be created from photohardenable composition. In such embodiments, the intersection may be moved with respect to container. In other embodiments, the intersection may be fixed, and container may be moved in order to form one or more 3D objects in the container. In other embodiments, both the container and the light sheet can be moved relative to each other.

[0402] Excitation light may be visible light, ultraviolet light, or other suitable forms of electromagnetic radiation.

[0403] In methods described herein, each wavelength is preferably generated by a different light source or different optical projection or other optical systems. The radiation or excitation source is preferably selected to emit radiation at a wavelength or within a range of wavelengths absorbed by the particular photoinitiator, e.g., the photo switchable photoinitiator.

[0404] In methods described herein, exposure energies of excitation light directed into the volume of photohardenable composition may be, without limitation, in a range from about 0.01 to about 100,000 mJ / cm2(inclusive).

[0405] When first and second wavelengths are included in a method or system, examples of power examples of exposure energies for the first wavelength light include exposure energies in a range from about 0.001 to about 1,000 mJ / cm2(inclusive) and examples of exposure energies for the second wavelength light include exposure energies in a range from about 0.01 to about 100,000 mJ / cm2(inclusive).

[0406] Other exposure energies may also be determined to be useful.

[0407] In methods described herein, power densities or intensities of excitation light directed into the volume of photohardenable composition may be, without limitation, in a range from about 0.01 to about 100,000 W / cm2.

[0408] When first and second wavelengths are included in a method or system, examples of power densities for the first wavelength light include power densities in a range from about 0.01 to about 100,000 W / cm2(inclusive) and examples of power densities for the second wavelength light include power densities in a range from about 0.01 to about 100,000 W / cm2(inclusive).

[0409] Other power densities or intensities may also be determined to be useful.

[0410] In the methods described herein, the container optionally may be rotated to provide additional angles of illumination or projection of excitation light into the volume of photohardenable composition contained therein. This can be of assistance in patterning object volumes or surfaces more accurately or it can be used as a means of providing multiple exposure of a given feature from different angles.

[0411] In the method described herein, the container optionally may be stationary while a beam or optical projection of excitation light is being directed into the volume of the printing composition. Alternatively, the position of the container may be translated or moved during excitation while a beam and / or optical projection of excitation is being directed into the volume of the printing composition.

[0412] Information that may be useful in connection with the various aspects of the present inventions includes International Application No. PCT / US2022 / 052157, filed December 7, 2022, of Quadratic 3D, Inc., and International Application No. PCT / US2022 / 039766, filed August 9, 2022, of Quadratic 3D, Inc., each of the foregoing applications being hereby incorporated herein by reference in its entirety.

[0413] Before printing, a digital file of the object or object to be printed is obtained. If the digital file is not of a format that can be used to print the object, the digital file is then converted to a format that can be used to print the object. An example of a typical format that can be used for printing includes, but is not limited to, an STL file. Typically, the STL file is then sliced into two-dimensional layers with use of three-dimensional slicer software and converted into G-Code or a set of machine commands, which facilitates building the object. See B. Redwood, et al., “The 3D Printing Handbook - Technologies, designs applications”, 3D HUBS B.V. 2018.

[0414] Other information concerning optical systems that may be useful in connection with the various aspects of the present inventions includes Texas Instruments Application Report DLPA022-July 2010 entitled “DLP™ System Optics”; Texas Instruments “TI DLRTechnology for 3D Printing - Design scalable high-speed stereolithography [sic] systems using TI DLP technology” 2016; Texas Instruments “DLP6500 0.65 1018p MVSP Type A DMD”, DLP6500, DLPS040A-October 2014 - Revised October 2016; and Y-H Lee, et al., “Fabrication of Periodic 3D Nano structuration for Optical Surfaces by Holographic Two- Photon-Polymerization”, IntT Journal of Information and Electronics Engineering, Vol 6, No. 3, May 2016, each of the foregoing being hereby incorporated herein by reference in its entirety.

[0415] The 3D printing method described herein does not require adhering the object being printed to a fixed substrate (e.g., build plate) at the beginning of the printing process avoiding a post-processing step of separating the printed object from the fixed substrate. It additionally facilitates printing three-dimensional objects in a volume of photopolymerizable liquid without requiring support structures to form a printed object. Post-processing steps of removing support structures and / or removing the printed object from a fixed substrate add labor (e.g., manual removal), waste (discarded support structures), and reduce throughput (a build plate cannot be reused until the printed object is removed), all of which add cost to the process.

[0416] When used as a characteristic of a portion of a container or build chamber, “optically transparent” refers to having high optical transmission to the wavelength of light being used, and “optically flat” refers to being non-distorting (e.g., optical wavefronts entering the portion of the container or build chamber remain largely unaffected).

[0417] As used herein, the singular forms "a", "an" and "the" include plural unless the context clearly dictates otherwise. Thus, for example, reference to a photohardenable resin component includes reference to one or more of such components.

[0418] Applicant specifically incorporates the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the present invention be limited to the specific values recited when defining a range.

[0419] Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the present specification and practice of the present invention disclosed herein. It is intended that the present specification and examples be considered as exemplary only with a true scope and spirit of the invention being indicated by the following claims and equivalents thereof.

Claims

CLAIMS1. A photohardenable composition for use in volumetric three-dimensional printing, the photohardenable composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a polymerization rate reduction additive wherein photopolymerization of the photohardenable composition occurs at a polymerization rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

2. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive is included in the photohardenable composition in an amount effective for slowing photopolymerization upon selective exposure of the photohardenable composition to one or more excitation wavelengths.

3. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive is included in the photohardenable composition in an amount greater than zero and less than 5 weight percent.

4. The photohardenable composition of any one of claims 1-3 wherein the slower polymerization is detectable by a lower deflection of a light sheet at a selected location in a volume of the photohardenable composition at which a light sheet intersects with an optical image during formation of a three-dimensional object in a volume of the photohardenable composition than if the polymerization rate reduction additive is not included.

5. The photohardenable composition of any one of claims 1- 3 wherein the polymerization rate reduction additive comprises a comonomer.

6. The photohardenable composition of any one of claims 1-3 wherein the polymerization rate reduction additive comprises a comonomer that (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e., cross-propagation) at a slower rate than a unit of the photohardenable component undergoes homo-propagation.

7. The photohardenable composition of any one of claims 1-3 wherein the polymerization rate reduction additive comprises a chain transfer agent.

8. The photohardenable composition of any one of claims 1-3 wherein the polymerization rate reduction additive comprises a chain transfer agent wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

9. The photohardenable composition of any one of claims 1-8 wherein the photoinitiator comprises a photo switchable photoinitiator.

10. The photohardenable composition of claim 9 further including a coinitiator.

11. The photohardenable composition of claim 9 further including a rheology modifier.

12. The photohardenable composition of claim 9 further including a coinitiator and a rheology modifier.

13. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises styrene.

14. The photohardenable composition of claim 5 or 6 wherein the polymerization rate reduction additive comprises styrene.

15. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises 4-cyanostyrene.

16. The photohardenable composition of claim 5 or 6 wherein the polymerization rate reduction additive comprises 4-cyanostyrene.

17. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises a styrenic.

18. The photohardenable composition of claim 5 or 6 wherein the polymerization rate reduction additive comprises a styrenic.

19. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises divinylbenzene.

20. The photohardenable composition of claim 5 or 6 wherein the polymerization rate reduction additive comprises divinylbenzene.

21. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises a RAFT agent.

22. The photohardenable composition of claim 7 or 8 wherein the polymerization rate reduction additive comprises a RAFT agent.

23. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises 2-cyano-2-propyl benzodithioate.

24. The photohardenable composition of claim 7 or 8 wherein the polymerization rate reduction additive comprises 2-cyano-2-propyl benzodithioate.

25. The photohardenable composition of claim 1 wherein the polymerization rate reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

26. The photohardenable composition of claim 7 or 8 wherein the polymerization rate reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

27. The photohardenable composition of any one of claims 1-3 wherein inclusion of the polymerization rate reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

28. The photohardenable composition of claim 5 or 6 wherein inclusion of the polymerization rate reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

29. The photohardenable composition of claim 7 or 8 wherein inclusion of the polymerization rate reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

30. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:(a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of claims 1-29;(b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and(c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the polymerization rate reduction additive slows polymerization to a rate that is lower than if the polymerization rate modification additive is not included in the photohardenable composition.

31. The method of claim 30 wherein polymerization is slowed at the intersection of the light sheet with the projected optical image.

32. The method of claim 30 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

33. A photohardenable composition for use in volumetric three-dimensional printing with reduced light sheet deflection, the photohardenable composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and an additive for reducing deflection of a light sheet at a selected location in a volume of the photohardenable composition during formation of a three-dimensional object in the volume.

34. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive is included in the photohardenable composition in an amount greater than zero and less than 5 weight percent.

35. The photohardenable composition of claim 33 wherein light sheet deflection at the intersection of the light sheet with the optical image during the formation of the three- dimensional object in the volume is less than if the light sheet deflection reduction additive is not included in the photohardenable composition.

36. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive reduces distortion from refractive index changes during formation of the three-dimensional object.

37. The photohardenable composition of any one of claims 33-36 wherein the light sheet deflection reduction additive comprises a comonomer.

38. The photohardenable composition of any one of claims 33-36 wherein the light sheet deflection reduction additive comprises a comonomer that (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e. cross-propagation) at a slower rate than a unit of the photohardenable component undergoes homo-propagation.

39. The photohardenable composition of any one of claims 33-36 wherein the light sheet deflection reduction additive comprises a chain transfer agent.

40. The photohardenable composition of any one of claims 33-36 wherein the light sheet deflection reduction additive comprises a chain transfer agent wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

41. The photohardenable composition of any one of claims 33-40 wherein the photoinitiator comprises a photo switchable photoinitiator.

42. The photohardenable composition of claim 41 further including a coinitiator.

43. The photohardenable composition of claim 41 further including a rheology modifier.

44. The photohardenable composition of claim 41 further including a rheology modifier.

45. The photohardenable composition of claim 41 further including a rheology modifier and a coinitiator.

46. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises styrene.

47. The photohardenable composition of claim 37 or 38 wherein the light sheet deflection reduction additive comprises styrene.

48. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises 4-cyanostyrene.

49. The photohardenable composition of claim 37 or 38 wherein the light sheet deflection reduction additive comprises 4-cyanostyrene.

50. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises a styrenic.

51. The photohardenable composition of claim 37 or 38 wherein the light sheet deflection reduction additive comprises a styrenic.

52. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises divinylbenzene.

53. The photohardenable composition of claim 37 or 38 wherein the light sheet deflection reduction additive comprises divinylbenzene.

54. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises a RAFT agent.

55. The photohardenable composition of claim 39 or 40 wherein the light sheet deflection reduction additive comprises a RAFT agent.

56. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises 2-cyano-2-propyl benzodithioate.

57. The photohardenable composition of claim 39 or 40 wherein the light sheet deflection reduction additive comprises 2-cyano-2-propyl benzodithioate.

58. The photohardenable composition of claim 33 wherein the light sheet deflection reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

59. The photohardenable composition of claim 39 or 40 wherein the light sheet deflection reduction additive comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

60. The photohardenable composition of any one of claims 33-36 wherein inclusion of the light sheet deflection reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

61. The photohardenable composition of claim 37 or 38 wherein inclusion of the light sheet deflection reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

62. The photohardenable composition of claim 39 or 40 wherein inclusion of the light sheet deflection reduction additive in the photohardenable composition does not substantially affect the achievable green-state modulus of a three-dimensional object formed therefrom.

63. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:(a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of claims 33- 63;(b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images,; and(c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein light sheet deflection during formation of the three-dimensional object in the photohardenable composition is less than without inclusion of the light sheet deflection reduction additive.

64. The method of claim 63 wherein light sheet deflection is reduced at the intersection of the light sheet with the projected optical image.

65. The method of claim 63 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the light sheet deflection reduction additive in the photohardenable composition.

66. A photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon excitation.

67. The photohardenable composition of claim 66 wherein the comonomer (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e., cross-propagation) at a slower rate than a unit of the photohardenable component undergoes homo-propagation.

68. The photohardenable composition of claim 66 or 67 where excitation comprises exposing a selected location in the photohardenable composition to two intersecting excitation wavelengths.

69. The photohardenable composition of claim 68 wherein polymerization is slowed at the intersection.

70. The photohardenable composition of claim 66 or 67 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

71. The photohardenable composition of claim 66 or 67 wherein the photoinitiator comprises a photo switchable photoinitiator.

72. The photohardenable composition of claim 66 or 71 wherein the photohardenable composition further includes a coinitiator.

73. The photohardenable composition of claim 66 or 71 wherein the photohardenable composition further includes a rheology modifier.

74. The photohardenable composition of claim 66 or 71 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

75. The photohardenable composition of claim 66 or 67 wherein the comonomer comprises styrene.

76. The photohardenable composition of claim 66 or 67 wherein the comonomer comprises 4-cy ano styrene.

77. The photohardenable composition of claim 66 or 67 wherein the comonomer comprises a styrenic.

78. The photohardenable composition of claim 66 or 67 wherein the comonomer comprises divinylbenzene.

79. The photohardenable composition of any one of claims 71-74 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

80. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:(a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of claims 66- 79;(b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and(c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein the inclusion of the comonomer in the photohardenable composition slows polymerization to a rate that is lower than if the comonomer is not included in the photohardenable composition.

81. The method of claim 80 wherein polymerization is slowed at the intersection of the light sheet with the projected optical image.

82. The method of claim 80 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

83. The method of claim 80 wherein the photohardenable composition displays nonNewtonian rheological behavior.

84. A photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a comonomer, the comonomer being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three- dimensional object.

85. The photohardenable composition of claim 84 wherein the comonomer (a) reacts with the photohardenable component in a cross-propagation event at a faster rate than the photohardenable component undergoes homo-propagation and, (b) once incorporated into a chain, adds a monomer of the photohardenable component (i.e., cross-propagation) at a slower rate than a unit of the photohardenable component undergoes homo-propagation.

86. The photohardenable composition of claim 84 or 85 where excitation comprises exposing a selected location in the photohardenable composition to two intersecting excitation wavelengths.

87. The photohardenable composition of claim 86 wherein distortion is reduced at the intersection.

88. The photohardenable composition of claim 84 or 85 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

89. The photohardenable composition of claim 84 or 85 wherein the photoinitiator comprises a photo switchable photoinitiator.

90. The photohardenable composition of claim 84 or 89 wherein the photohardenable composition further includes a coinitiator.

91. The photohardenable composition of claim 84 or 89 wherein the photohardenable composition further includes a rheology modifier.

92. The photohardenable composition of claim 84 or 89 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

93. The photohardenable composition of claim 88 wherein the photohardenable composition further includes a rheology modifier.

94. The photohardenable composition of claim 84 or 85 wherein the comonomer comprises styrene.

95. The photohardenable composition of claim 84 or 85 wherein the comonomer comprises 4-cy ano styrene.

96. The photohardenable composition of claim 84 or 85 wherein the comonomer comprises a styrenic.

97. The photohardenable composition of claim 84 or 85 wherein the comonomer comprises divinylbenzene.

98. The photohardenable composition of any one of claims 89-92 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

99. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:(a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of claims 84- 98;(b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and(c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different froma previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three-dimensional object is less than without inclusion of the comonomer.

100. The method of claim 99 wherein distortion is reduced at the intersection of the intersection of the light sheet with the projected optical image.

101. The method of claim 99 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

102. The method of claim 99 wherein the photohardenable composition displays nonNewtonian rheological behavior.

103. A photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to slow polymerization of the photohardenable composition upon exposure to excitation.

104. The photohardenable composition of claim 103 wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

105. The photohardenable composition of claim 103 or 104 where in the exposure to dual wavelength excitation comprises the intersection of two excitation wavelengths in the photohardenable composition.

106. The photohardenable composition of claim 105 wherein polymerization is slowed at the intersection.

107. The photohardenable composition of claim 103 or 104 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

108. The photohardenable composition of claim 103 or 104 wherein the photoinitiator comprises a dual-wavelength photoinitiator.

109. The photohardenable composition of claim 103 or 108 wherein the photohardenable composition further includes a coinitiator.

110. The photohardenable composition of claim 103 orl08 wherein the photohardenable composition further includes a rheology modifier.

111. The photohardenable composition of claim 103 or 108 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

112. The photohardenable composition of claim 103 or 104 wherein the chain transfer agent comprises a RAFT agent.

113. The photohardenable composition of claim 103 or 104 wherein the chain transfer agent 2comprises 2-cyano-2-propyl benzodithioate.

114. The photohardenable composition of claim 103 or 104 wherein the chain transfer agent comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

115. The photohardenable composition of any one of claims 108-111 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

116. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:(a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of claims 103- 115;(b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and(c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein inclusion of the chain transfer agent in the photohardenable composition slows polymerization to a rate that is lower than if the chain transfer agent is not included in the photohardenable composition.

117. The method of claim 116 wherein polymerization is slowed at the intersection of the light sheet with the projected optical image.

118. The method of claim 116 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

119. The method of claim 116 wherein the photohardenable composition displays nonNewtonian rheological behavior.

120. A photohardenable composition for use in volumetric three-dimensional printing, the composition comprising: a photohardenable resin component; a photoinitiator; optionally a coinitiator; optionally a rheology modifier; and a chain transfer agent, the chain transfer agent being included in the photohardenable composition in an amount effective to reduce distortion from refractive index changes during printing a three-dimensional object.

121. The photohardenable composition of claim 120 wherein (a) the chain transfer agent produces a dead polymer chain at a rate more rapid than the rate of propagation of the photohardenable resin component and (b) a resulting newly created radical initiates new chains at a rate slower than the rate of propagation of the photohardenable resin component.

122. The photohardenable composition of claim 120 or 121 where excitation comprises exposing a selected location in the photohardenable composition to two intersecting excitation wavelengths.

123. The photohardenable composition of claim 122 wherein distortion is reduced at the intersection.

124. The photohardenable composition of claim 120 or 121 wherein the photohardenable resin component comprises an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

125. The photohardenable composition of claim 120 or 121 wherein the photoinitiator comprises a photo switchable photoinitiator.

126. The photohardenable composition of claim 120 or 125 wherein the photohardenable composition further includes a coinitiator.

127. The photohardenable composition of claim 120 or 125 wherein the photohardenable composition further includes a rheology modifier.

128. The photohardenable composition of claim 120 or 125 wherein the photohardenable composition further includes a rheology modifier and a coinitiator.

129. The photohardenable composition of claim 120 or 121 wherein the chain transfer agent comprises a RAFT agent.

130. The photohardenable composition of claim 120 or 121 wherein the chain transfer agent comprises 2-cyano-2-propyl benzodithioate.

131. The photohardenable composition of claim 120 or 121 wherein the chain transfer agent comprises 4-methylpent-l-ene-2,4-diyl)dibenzene.

132. The photohardenable composition of any one of claims 125-128 wherein the photohardenable resin component comprising an acrylate based monomer or oligomer and / or a methacrylate based monomer or oligomer.

133. A method of forming a three-dimensional printed object in a volume of a photohardenable composition, the method comprising:(a) providing the volume of the photohardenable composition, the photohardenable composition comprising the photohardenable composition of any one of claims 120- 132;(b) inducing a photopolymerization or cross-linking reaction at a selected location in the volume upon exposure of the selected location to a light sheet and a selected projected optical images; and(c) optionally repeating step (b) one or more times to partially or fully form the object, wherein for a repeated step (b), the selected location is the same as or different from a previous selected location and the projected optical image is the same as or different from a previous projected optical image, wherein distortion from refractive index changes during printing the three-dimensional object is less than without inclusion of the chain transfer agent.

134. The method of claim 133 wherein distortion is reduced at the intersection of the intersection of the light sheet with the projected optical image.

135. The method of claim 133 wherein the achievable green-state modulus of the object as formed in the volume is substantially unaffected by inclusion of the polymerization rate reduction additive in the photohardenable composition.

136. The method of claim 133 wherein the photohardenable composition displays nonNewtonian rheological behavior.

137. The photohardenable composition of any one of claimsl, 33, 66, 84, and 120 wherein the photohardenable resin component comprises a dual-wavelength photoinitiator.

138. The photohardenable composition of any one of claimsl, 33, 66, 84, 103, and 120 wherein the photoinitiator comprises a photoswitchable photoinitiator.

139. The photohardenable composition of claim 1 or 33 wherein the photohardenable resin component comprises an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer.

140. The photohardenable composition of any one of claims 1, 33, 66, 84, 103, and 120 wherein the photohardenable resin component comprises an acrylate-based monomer or oligomer and / or a methacrylate based monomer or oligomer and the photoinitiator comprises a photoswitchable photoinitiator.

141. The photohardenable composition of claim 33 wherein the additive for reducing deflection of the light sheet is included in the photohardenable composition in an amount effective to reduce deflection of the light sheet during printing.

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