Liquid photohardenable composition for volumetric additive manufacturing

Abstract

A liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope,, wherein the liquid photohardenable composition exhibits non-Newtonian rheological behavior and a light transmittance greater than 90% for a 5 centimeter (cm) pathlength. Preferably such transmissivity remains above 90% for at least 10 days after preparation. Preferably the liquid photohardenable composition exhibiting non-Newton rheological behavior has a yield stress greater than or equal to about 5 Pascals at 50 degrees Celsius, Methods of forming an object in a volume of a liquid photohardenable composition of the invention and a method for screening thixotropes for inclusion in a liquid photohardenable composition are also disclosed.

Description

[0001] LIQUID PHOTOHARDENABLE COMPOSITION FOR VOLUMETRIC ADDITIVE MANUFACTURING

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Patent Application No. 63 / 561,748 filed on 06 March 2024, which application is hereby incorporated herein by reference in its entirety for all purposes.

[0004] TECHNICAL FIELD OF THE INVENTION

[0005] The present relates to the technical field of 3D-printing and related methods.

[0006] BRIEF SUMMARY OF THE INVENTION

[0007] The present invention includes a liquid photohardenable composition for use in forming an object in a volume of the liquid photohardenable composition that has a yield stress above 5 Pascals at 50 degrees Celsius and a transmissivity above 90% for a 5 centimeter pathlength through the liquid photohardenable composition after preparation. Preferably such transmissivity remains above 90% for at least 10 days after preparation. The present invention also includes methods of forming an object in a volume of a liquid photohardenable composition described herein. The present invention further includes a method for screening thixotropes for inclusion in a liquid photohardenable composition for use in volumetric printing.

[0008] In accordance with one aspect of the present invention, there is provided a liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius after preparation and a light transmittance greater than 90% for a 5 cm pathlength after preparation. Preferably such transmissivity remains above 90% for at least 10 days after preparation.

[0009] In accordance with another aspect of the present invention, there is provided a method for screening thixotropes for inclusion in a photohardenable composition for use in volumetric printing, the method comprising: a. identifying one or more thixotrope candidates as a candidate for inclusion in the photohardenable composition; b. preparing one or more screening samples, each screening sample including a photohardenable resin component, a dual-wavelength photoinitiator, and a selected amount of a thixo trope candidate; and c. measuring yield stress and light transmittance of the one or more screening samples and comparing the measured yield stress and light transmittance against initial target values selected therefor.

[0010] Preferred initial screening target values include yield stress (at least about 5Pa measured at 50 degrees Celsius) and light transmission (greater than 90% transmission through 5 centimeters of resin).

[0011] The method can optionally further comprise: d. identifying screening samples with measured yield stress and transmittance values that are within selected tolerances for the initial targets and remeasuring the transmittance of the identified samples against the initial target transmittance value after a selected time period.

[0012] The selected time period can be used to assess the shelf life of the liquid photohardenable composition and whether it would meet a targeted shelf life, e.g., at least 10 days, for the composition. Other targeted shelf lives may be useful or desirable.

[0013] In accordance with another aspect of the present invention, there is provided a method of forming an object in a volume of a liquid photohardenable composition, the method comprising: a. providing the volume including a liquid photohardenable composition in accordance with the present invention, b. directing one or more excitation wavelengths to a selected location within the volume of the liquid photohardenable composition to induce a crosslinking or polymerization reaction in the liquid photohardenable composition at the selected location to at least partially form the object; and c. optionally repeating step b. one or more times to partially or fully form the object, wherein a repeated step b. comprises irradiating the liquid photohardenable composition at one or more selected locations that are the same as or different from one or more previous selected locations in the volume. In accordance with another aspect of the present invention, there is provided a method of forming an object in a volume of a liquid photohardenable composition, the method comprising: a. providing the volume of the liquid photohardenable composition including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about5 Pa at 50 degrees Celsius and a light transmittance that is greater than 90% for a 5 cm pathlength; b. simultaneously or sequentially irradiating one or more selected locations within the volume of the liquid photohardenable composition with light having a first wavelength and light having a second wavelength, wherein light having the first wavelength and light having the second wavelength activate the dual-wavelength photoinitiator at the one or more selected locations to induce a crosslinking or polymerization reaction in the liquid photohardenable composition at the intersection of the first and second wavelengths at the one or more selected locations within the volume to at least partially form the object; and c. optionally repeating step b. one or more times to partially or fully form the object, wherein the one or more selected locations in a repeated step b. are the same as or different from one or more previous selected locations in the volume.

[0014] Preferably the liquid photohardenable composition exhibits light transmittance that remains above 90% for at least ten days after preparation.

[0015] The foregoing, and other aspects and embodiments described herein and contemplated by this disclosure all constitute embodiments of the present invention.

[0016] 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. 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.

[0017] 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.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In the drawings,

[0020] FIG. 1 depicts an example of a set up for determining transmittance for a 5 centimeter pathlength.

[0021] 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.

[0022] DETAILED DESCRIPTION OF THE INVENTION

[0023] Various aspects and embodiments of the present inventions will be further described in the following detailed description.

[0024] The present invention includes a liquid photohardenable composition for use in forming an object in a volume of the liquid photohardenable composition that has a yield stress above 5 Pascals (Pa) at 50 degrees Celsius and a transmissivity above 90% for a 5 centimeter pathlength through the liquid photohardenable composition after preparation. Preferably such transmissivity remains above 90% for at least 10 days after preparation. The present invention also includes methods of forming an object in a volume of a liquid photohardenable composition described herein. The present invention further includes a method for screening thixotropes for inclusion in a liquid photohardenable composition for use in volumetric printing.

[0025] The advantages of use of a printing resin exhibiting non-Newtonian rheological behavior with yield stress in volumetric 3D printing has been recognized for immobilizing the object being printed during formation for improving dimensional accuracy of the printed object and for eliminating needs for including physical support structures and / or a support substrate during printing. However, the inclusion of additives such as thixotropes in a printing resin to achieve such rheological behavior can result in a printing resin that is not clear, e.g., a printing resin that is hazy or has reduced light transmittance. A non-clear printing resin can be detrimental to achieving feature resolution and / or dimensional accuracy in the object being printed.

[0026] Transmission of light is important in volumetric printing because, unlike layerwise additive manufacturing (e.g., stereolithography (SLA) printing) where curing occurs in a very thin layer of uncured resin disposed on a previously cured layer or support surface, in volumetric printing the curing happens within the volume. In volumetric printing, the light transits a much larger distance through uncured resin, e.g., in some cases 5 or 10 centimeters or more, before reaching the selected location where curing occurs. During this transmission of light through the printing resin, if the printing resin is hazy or has reduced transmissivity, a number of issues can result. For example, the power of light delivered to the curing region will be reduced since it is a function of depth in the resin of the selected curing location. Also, the crispness of the image formed by the light can suffer if the light is scattering instead of transmitting. In such case, print resolution and dimensional accuracy of the printed object can suffer. Therefore, it is important in volumetric printing resins to reduce, and preferably eliminate, conditions that are detrimental to light transmissivity of the liquid resin. It is also desirable for a printing resin to maintain clarity after preparation to enable ease of use.

[0027] Addressing the combination of non-movement of objects during printing and the clarity of the printing resin is particularly important when volumetrically printing large (non-micro- sized) objects since larger objects weigh more due to their size and also produce more heat in the curing step (which can increase object density) relative to smaller (microsized) objects and excitation light has to travel through larger volumes of printing resin before reaching the selected location(s) at which hardening or polymerization is to occur. For small (micro-sized) objects, movement of objects during formation and resin can have a negligible effect on object resolution and dimensional accuracy due to the small size (and low weight) of the object and the shorter distance traveled by excitation light in a smaller printing volume as well as the shorter print times associated with production of smaller objects which result in less time for the object to translate in the resin.

[0028] While the liquid photohardenable compositions and methods of the present invention are useful to print micro and non-micro- sized objects, liquid photohardenable compositions and methods of the present invention are particularly advantageous for use in volumetric printing non-micro sized objects having at least one dimension of at least 3 centimeters for reasons discussed above. Examples of application areas including objects having at least one dimension greater than or equal to about 3 centimeters include, but are not limited to, include orthodontic and other dental applications such as aligners, night guards, retainers, and optical elements including, but not limited to, ophthalmic lenses, which can include typical sizes that include at least one dimension having a size in a range from about 31 mm to about 60 mm.

[0029] Printing resin clarity can be additionally important for printing objects for use in an application or end-use that has a clarity specification, such as, but not limited to, printing of optical elements or clear dental appliances.

[0030] In accordance with one aspect of the present invention, there is provided a liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius after preparation and a light transmittance greater than 90% for a 5 cm pathlength.

[0031] Preferably such transmissivity remains above 90% for at least 10 days after preparation.

[0032] Measuring the yield stress after preparation is carried out after the thixotrope has set, e.g., after an amount of time for permitting the composition to stabilize or reach viscosity equilibrium.

[0033] A transmittance greater than 95% is desirable. A transmittance in a range from 98.6% to 99.0% has been obtained and is very desirable. A transmittance that approaches even closer to 100% is most desirable. Due to a decrease in transmittance as a result of the presence of the thixotrope, a transmittance of 100% is not expected.

[0034] A yield stress greater than about 5 Pa, e.g., greater than 10 Pa is desirable. A yield stress greater than 20 Pa can result in printed parts for which any residual liquid photohardenable composition adhered to and / or trapped with crevices, hollows, or other features of the printed part can be difficult to wash off or otherwise remove. For cleaning purposes, for parts with significant crevices or hollows, a yield stress greater than 5 Pa up to 20 Pa can be useful. In other cases where a printed part has very few crevices or hollows, even higher yield stresses can be useful.

[0035] A liquid photohardenable composition in accordance with the present invention is particularly advantageous for use in printing an object suspended in a volume of the composition during formation of the object wherein the object has at least one dimension that is at least 3 cm.

[0036] In another aspect of the present invention, there is provided a method for screening thixotropes for inclusion in a photohardenable composition for use in volumetric printing, the method comprising: a. identifying one or more thixotrope candidates as a candidate for inclusion in the photohardenable composition; b. preparing one or more screening samples, each screening sample including a photohardenable resin component, a dual-wavelength photoinitiator, and a selected amount of a thixotrope candidate; and c. measuring yield stress and light transmittance of the one or more screening samples and comparing the measured yield stress and light transmittance against initial target values selected therefor.

[0037] Preferred initial screening target values include a yield stress that is greater than or equal to about 5Pa measured at 50 degrees Celsius and light transmission greater than 90% transmission through 5 centimeters of resin.

[0038] The method can optionally further comprise: d. identifying screening samples with measured yield stress and transmittance values that are within selected tolerances for the initial targets and remeasuring the transmittance of the identified samples against the initial target light transmittance value after a selected time period.

[0039] For example, to determine a possible ten day shelf life, the identified light transmittance of the identified screening sample is remeasured against the initial light transmittance target ten (10) days from the preparation of the screening sample. If the remeasured light transmittance remains within the selected tolerance for the initial light transmittance target at least 10 days after preparation, it could be considered as having a shelf-life of at least that time period.

[0040] Other targeted shelf lives may be useful or desirable.

[0041] The selected time period can be set to assess the shelf life of the liquid photohardenable composition and whether it would meet a targeted shelf life for the composition. Optionally the light transmittance can be remeasured again after time periods of longer lengths to gain additional information about the potential shelf life of the liquid photohardenable composition represented by the screening sample.

[0042] Alternatively, the initial light transmittance target can be light transmission greater than 90% through 5 centimeters of resin 10 days from the time the composition is prepared.

[0043] A photohardenable resin component for inclusion in a liquid photohardenable composition described herein can comprise one or more resins (e.g., a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing) that are photohardenable by exposure to light in the presence of a dual-wavelength photoinitiator. Examples of photohardenable resin components useful for inclusion in the liquid photohardenable composition include ethylenically unsaturated compounds and, more specifically, a polyethylenically unsaturated compound. These compounds include monomers, oligomers, and pre-polymers 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.

[0044] Preferred examples include, but are not limited to, a urethane acrylate or a urethane methacrylate -based oligomers. Commercially available examples include, but are not limited to, the following: BRC4421, BRC4421M, BR5825I30, , BR541MB, BR843D, BR443D, BR741MD1, BR741, BR952, etc. from Bomar; Genomer 4247, Genomer 4259, Genomer 7244, etc. from RAHN; CN9009, CN1964, CN1968, CN1970, etc. from Sartomer.

[0045] 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 the liquid photohardenable composition of the present invention.

[0046] Aliphatic urethane acrylates, aliphatic urethane methacrylates, and mixtures including one or more aliphatic urethane acrylates and / or one or more aliphatic urethane methacrylates may also be desirable for use as a photohardenable resin component for inclusion in the liquid photohardenable composition described herein.

[0047] Mixtures including multifunctional acrylate monomers, such as dipentaerythritol pentaacrylate (e.g., SR399 from Sartomer), and mono-, di-, or multi-functional urethane acrylates or / and mono-, di-, or multi-functional urethane methacrylates can also be used.

[0048] A photohardenable resin component including other mixtures including one or more resin components can also be useful.

[0049] It is desirable that the photohardenable resin component included in a liquid photohardenable composition described herein be selected to achieve an optically transparent medium, which is important in processes in which light, e.g., excitation light, is directed into the composition or light.

[0050] Examples of 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.

[0051] Other examples of photohardenable resin components include a di- or multifunctional oligomer with (meth)acrylate, vinylester, vinylcarbonate, and a backbone of the polymers bearing urethane, urea, amide, bisphenols, epoxies, carbonates, ethers, and esters.

[0052] Liquid photohardenable compositions including a dual-wavelength photoinitiator enable volumetric additive manufacturing with fast and precise curing, superior green and final state hardness, tunable strength, modulus, and elongation, and optionally no leachable ingredients. The liquid photohardenable composition of the present invention may also include blends of different photohardenable resin components.

[0053] For example, a blend of different photohardenable resin components may comprise one or more difunctional oligomers, e.g., 20-95 wt% based on the total weight of the polymerizable composition, including vinylesters, vinylcarbonates, methacrylates, and / or acrylates functionalities; one or more multi-functional (functionality > 2) reactive diluents, e.g., 1-20 wt% based on the total weight of the polymerizable composition, wherein a reactive diluent preferably includes a methacrylate, acrylate, vinylester, and / or vinylcarbonate group functionalities.

[0054] Examples of difunctional oligomers include difunctional oligomers including a backbone of the polymers bearing urethane, urea, amide, anhydride, bisphenols, epoxies, carbonates, ethers, and esters; difunctional oligomers including optional reactive or blocked functional groups in the backbone, such as hydroxyl, carbonyl, amino, azide, epoxy, isocyanate groups.

[0055] Examples of multi-functional reactive diluents include diluents including a functionality of 2, 3, 4, 5, or 6, can be 1,6-hexanediol di(meth)acrylate, ditrimethylolpropane tetraacrylate, 1,3 -butanediol di(meth)acrylate, bisphenol A (ethoxylate) di(meth)acrylate, bisphenol A diglycidildi(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, tri(ethylene glycol) divinyl ether, di(ethylene glycol) divinyl ether, N,N'- Methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, Di(trimethylolpropane) tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta- or hexa- (meth)acrylate, tris(2-acryloyloxyethyl) Isocyanurate, 1,12-dodecanediol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, poly(ethylene glycol) di(meth)acrylate with a molecular range of 180-1,000 Da, and the mixture thereof.

[0056] Examples of compositional ranges for the photohardenable resin component in a liquid 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.

[0057] In addition to a photohardenable resin component, the liquid photohardenable composition includes a thixotrope. A thixotrope can be in the form of a solid, a predissolved liquid, or a mixture thereof. Examples of a thixotropes (which may also be referred to herein as rheology modifiers) include, for example and without limitation, urea derivatives and modified urea compounds, including but not limited to urea modified polyurethanes such as Rheobyk 410, urea modified polyamides such as Rheobyk 430, a solutions of a modified urea such as Rheobyk-D-410, Rheobyk D-411, Rheobyk D-415, Rheobyk D-420 and Rheobyk 7410-ET 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, fumed silica, 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 / swellable 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. When a thixotrope comprising a silica is used, it can be desirable to also include a booster, such as, for example, polyhydroxycarboxylic acid amides, where booster refers to additives that serve to further increase the thixotropic behavior of a fumed metal oxide. Commercially available polyhydroxycarboxylic acid amides include, but not limited to, Rheobyk 405. Examples of commercial silicas include, but are not limited to, Aerosil 200 (a hydrophilic fumed silica with a specific surface area of 200 m2 / g), Aerosil 300 (a hydrophilic fumed silica with a specific surface area of 300 m2 / g), and Aerosil 380 (a hydrophilic fumed silica with a specific surface area of 380 m2 / g). Thixotropes that can be preferred include urea modified polyamides such as Rheobyk 430 and fumed silicon oxides such as Aerosil 200 and the other Aerosil silicas listed above. 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.

[0058] 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.

[0059] Metal oxides, including, but not limited to, silicas that have been surface-treated to impart dispersibility characteristics compatible with a liquid photohardenable composition described herein may be desirable for use as thixotropes.

[0060] A thixotrope or rheology modifier is preferably included in a liquid photohardenable composition in accordance with the present invention in an amount that is effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength. Preferably the light transmittance remains greater than 90% for at least 10 days following preparation of the liquid photohardenable composition.

[0061] More preferably, the thixotrope is included in a liquid photohardenable composition in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength. Preferably the light transmittance remains greater than 90% for at least 10 days following preparation of the liquid photohardenable composition.

[0062] Examples of ranges of amounts of thixotrope or rheology modifier within which an effective amount can fall include, for example, but without limitation, from about of 0.05 weight percent to about 10 weight percent, from about 0.05 weight percent to about 5 weight percent, from about 0.1 weight percent to about 10 weight percent, from about 0.1 to about 5 weight percent of the composition. Other ranges within which the effective amount falls may also be determined.

[0063] A thixotrope can be selected and an effective amount thereof for inclusion in a liquid photohardenable composition in accordance with the present invention can be determined, for example, by the screening method described herein.

[0064] More preferably the position of the object in the volume of the liquid photohardenable composition remains at a fixed position during formation of the object and light transmittance is greater than 95%, and most preferably in a range from 98.6% to 99.0%. A transmittance that approaches even closer to 100% is most desirable. However, due to a decrease in transmittance as a result of the presence of the thixotrope, a transmittance of 100% is not expected

[0065] In addition to a photohardenable resin component and thixotrope, the liquid photohardenable composition includes a dual-wavelength photoinitiator.

[0066] A dual-wavelength photoinitiator can be selected taking into account its suitability for the mechanism to be used to initiate polymerization as well as its suitability for and / or compatibility with the resin to be polymerized.

[0067] Preferred dual-wavelength photoinitiators 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 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. (Dualwavelength photoinitiators including such photochromic properties are also referred to herein as “photoswitchable photoinitiators”.)

[0068] Several considerations in selecting a particular photoswitchable photoinitiator for inclusion in a liquid 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 in the monomer solution, the stability of the photoswitchable photoinitiator and the reduction and oxidation potentials of the second form of the photoswitchable photoinitiator.

[0069] Preferred photoswitchable 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 photoswitchable 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 (preferably, e.g., an iodonium or sulfonium salt)).

[0070] Liquid photohardenable compositions including a photoswitchable photoinitiator are particularly desirable for use in forming three-dimensional objects in a volume. 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.

[0071] Information concerning photohardenable compositions, photoswitchable photoinitiators, and printing 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.

[0072] 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., and International Application No. PCT / US2023 / 022170 of Quadratic 3D, Inc. filed May 13, 2023, each of the foregoing applications being hereby incorporated herein by reference in its entirety.

[0073] Additional examples of photoswitchable photoinitiators suitable for inclusion in a liquid 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.

[0074] 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, which is hereby incorporated herein by reference in its entirety.

[0075] Examples of compositional ranges for a dual-wavelength photoinitiator in a liquid 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. weight percent of the photohardenable resin component.

[0076] A liquid photohardenable composition described herein can further optionally include one or more additives. Examples of such optional additives include, but are not limited to, a coinitiator, one or more diluents, a second light activated photoinitiator, a filler, a defoamer, a stabilizer, a thermally activated radical initiator, a solvent, a sensitizer, and a colorant. Additives are preferably selected so that they do not undergo unwanted reactions with other components or additives that may be included in a liquid photohardenable compositions.

[0077] A liquid photohardenable composition can optionally include one or more coinitiators. (A coinitiator can also be referred to as a synergist). Optionally, one or more coinitiators can be included.

[0078] Inclusion of one or more coinitiators can be desirable when the photoinitiator comprises a dual-wavelength or photoswitchable photoinitiator.

[0079] Suitable coinitiators include coinitiators which are reducing agents, oxidizing agents, or hydrogen donating compounds.

[0080] Examples of coinitiators that may be useful can be selected from among those known in the art. Examples include, but are not limited to, tertiary amines, onium salts (preferably, e.g., iodonium or sulfonium salts), and organoborate salts. lodonium salts may also be used in combination with a borate salt or an iodonium salt. A combination of an iodonium salt or a sulfonium salt with a tertiary amine can be preferred. A particular nonlimiting example of a preferred onium salt includes bis-(4-t-butylphenyl)-iodonium hexafluorophosphate. Examples of other useful electron donating coinitiators are discussed by Eaton, D. F., "Dye Sensitized Photopolymerization", Advances in Photochemistry, Vol. 13, pp 427-486.

[0081] 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.

[0082] Certain other tertiary amines are also useful coinitiators including triethylamine, triethanolamine, N-methyldiethanolamine, 2-ethyl-4-(dimethylamino)benzoate, 2- ethylhexyl-4-(dimethylamino)benzoate, etc.

[0083] 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.

[0084] Representative examples of aryl groups represented by Ra-Rdinclude benzyl. 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.

[0085] Additional examples of coinitiators include a curable monomer with (meth)acrylate groups, a cured polymer with tertiary amine groups, a small molecule with tertiary amine groups. Examples include, but are not limited to, Sartomer CN3715US, CN3705, CN374, [2-(dimethylamino)ethyl methacrylate], Allnex Ebecryl Pl 15, [2-ethylhexyl 4- (dimethylamino)benzoate] , (2-Mercaptobenzoxazole), A-methy Idiethanolam i ne, triethanolamine, etc.

[0086] Examples of compositional ranges for a coinitiator when optionally included in a liquid 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.

[0087] 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.

[0088] Optionally an additive comprising a reactive diluent comprising a monofunctional or multifunctional diluent can be included in the liquid photohardenable composition. Examples include, but are not limited to, 1-adamantyl (meth)acrylate, methyl 2- ((allyloxy)methyl)acrylate, trimethylolpropane formal (meth)acrylate, isobonyl (meth)acrylate, (hydroxyethyl) (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4- hydroxybutyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, (2- dimethylaminoethyl) (meth)acrylate, and a mixture thereof. Commercially available examples include, but are not limited to, SR421A(3,3,5 Trimethylcyclohexyl Methacrylate), CTFA ((5-Ethyl-l,3-dioxan-5-yl)methyl Acrylate), and AOMA™ (Methyl 2- (allyloxy methyl) aery late). Examples of multi-functional reactive diluents include, but are not limited to, diluents have a functionality of 2, 3, 4, 5, or 6, can be 1,6-hexanediol di(meth)acrylate, di-trimethylolpropane tetraacrylate, 1,3 -butanediol di(meth)acrylate, bisphenol A (ethoxylate) di(meth)acrylate, bisphenol A diglycidildi(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, tri(ethylene glycol) divinyl ether, di(ethylene glycol) divinyl ether, N,N'- Methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, Di(trimethylolpropane) tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta- or hexa- (meth)acrylate, tris(2-acryloyloxyethyl) Isocyanurate, 1,12-dodecanediol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, poly(ethylene glycol) di(meth)acrylate with a molecular range of 180-1,000 Da, and the mixture thereof.

[0089] Optionally, a liquid photohardenable composition can further include a second 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 second light-activated photoinitiator can be desirable in connection with optional post-processing that includes, for example, a postcuring step involving exposure of the printed object to UV light after printing. Examples include, but are not limited to, Omnirad 184. When a second light activated photoinitator is further included in a liquid 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, from about 0.005-0.5, etc.

[0090] Examples of liquid photohardenable compositions include, but are not limited to, compositions which fall within the following compositional ranges in parts by weight (based on 100 parts total):

[0091] Photohardenable component* - about 89.5 to about 99.998 monomer and / or oligomer, preferably one or more monomers and / oroligomers include one or more vinylester, vinylcarbonate, methacrylate, and / or acrylate functionalities.

[0092] Photoswitchable photoinitiator - about 0.001 to about 0.5, preferably a photo switchable photoinitiator, more preferably including a substituted or unsubsubstitued diarylethene photochromic moiety.

[0093] Thixotrope - an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three- dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength preferably comprising urea derivatives, modified urea compounds, solutions of a modified urea, or fumed metal oxides.

[0094] Coinitiator, preferably a tertiary amine, more preferably a combination including a tertiary amine and an onium salt (preferably, e.g., an iodonium or sulfonium salt), in an amount of 0.5-20 ;

[0095] Optionally one or more reactive diluents in an amount of 0-35.

[0096] Optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5.

[0097] Optionally one or more polymeric or non-poly meric fillers in an amount of 0.1-10.

[0098] Optionally a non-reactive diluent in an amount of 0.1-10.

[0099] * The lower value in the range for the photohardenable component may be reduced taking into account any amounts of other optional additives that may be included in the photohardenable composition. A liquid photohardenable composition can optionally include one or more fillers, which can include a combination of one or more fillers. A filler can be a polymer, nanoparticle, pre-dissolved / dispersed solution / suspension, and can perform different functions. Examples include halogenated or non-halogenated fire retardants, dispersants, toughener, anti-microbial agents, antioxidants, antistatic agents, lubricants, anti-foam agents, wetting agents, matting agents, colorants dyes, pigments, adhesion promoters, etc.

[0100] A filler 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 liquid 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.

[0101] 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 sodalime 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 liquid photohardenable composition.

[0102] Another example of an additive that can optionally be included in a liquid 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.

[0103] Another example of an additive that can optionally be included in a liquid 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.

[0104] Another example of an optional additive includes a thermally activated radical initiator in a liquid photohardenable composition. Thermally activated radical initiator examples include but are not limited to 2,2'-azobis(2-methylpropionitrile), 1,1'- 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, peroxy disulfate salts.

[0105] 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.

[0106] A liquid photohardenable composition can optionally include one or more sensitizers. Optionally, one or more sensitizers can be included.

[0107] 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, alkoxyketocoumarins, Esacure 3644, and the like.

[0108] Examples of compositional ranges for a sensitizer when optionally included in a liquid 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.

[0109] Optionally, a composition described herein can include one or more coinitiators and one or more sensitizers.

[0110] Unless otherwise indicated, specified weight percents are based on the total weight of the liquid photohardenable composition.

[0111] A liquid photohardenable composition in accordance with the invention can be prepared using known or conventional procedures.

[0112] Any component or additive included in a liquid photohardenable composition can be a single component or additive or a mixture of two or more of components or additives.

[0113] The selection of a photohardenable resin component, the amount of photoinitiator, and, when applicable, a coinitiator, a thixotrope, and any optional additive(s), included in liquid photohardenable compositions will vary with the particular intended end-use of the part to be printed, the emission characteristics of the exposure sources, the development procedures, the physical properties desired in the hardened product and other factors.

[0114] A liquid photohardenable composition preferably displays non-Newtonian rheological behavior. Such rheological behavior can facilitate forming an object in a volume of a liquid 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 nonNewtonian 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.

[0115] As discussed above, preferably the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pascals at 50 degrees Celsius.

[0116] Non-Newtonian rheological behavior can be imparted to a liquid photohardenable composition described herein by including one or more photohardenable resin 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 one or more thixotropes in the liquid photohardenable composition. As discussed above, the thixotrope(s) is(are) preferably included in a liquid photohardenable composition in an amount effective to both: impart non-Newtonian rheological behavior thereto and provide a liquid photohardenable composition having a light transmittance greater than 90% along a 5 centimeter (cm) pathlength for at least 10 days following preparation thereof.

[0117] A liquid 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 liquid 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.

[0118] 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.)

[0119] FIG. 1 provides an example of a setup for measuring the transmission of a liquid photohardenable composition described herein. The transmission (which may also be referred to herein as light transmission, transmittance, or light transmittance) is measured by shining 10 mW of collimated 638 nm light from a 1 Watt laser beam 10 along a 5 cm pathlength 12 through a sample of liquid photohardenable composition contained in a quartz cuvette 15. The transmitted light 16 is measured with a 9.7 x 9.7 mm active area silicon photodiode power sensor (Thorlabs, S121C) 18 positioned 15 cm from the side of the cuvette through which the transmitted light exits the cuvette. (The 5 cm and 15 cm lengths depicted in the FIG. are not to scale.) A transmission greater than 90% 10 days after preparation is indicative of a high transmission printing resin with acceptable stability.

[0120] EXAMPLES

[0121] The examples provided herein 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.

[0122] Example 1

[0123] An example of a liquid photohardenable composition including a high molecular weight, urea modified, medium polarity polyamide thixotrope is detailed below. The liquid photohardenable composition includes a diarylethene photo switchable photoinitiator designated by AE32. Photoswitchable 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. The photoswitchable photoinitiator was pre-dissolved in 1,6-hexanediol diacrylate (HDD A) for ease of incorporation therein.

[0124] The composition of the liquid photohardenable composition of Example 1 is set forth below:

[0125] * The AE32 concentration is 20 ppm.

[0126] Preparation of the Liquid Photohardenable Composition of Example 1,

[0127] 1. In a 3-L metal container, HDDA (105 g) was charged, followed by adding Genomer 4247 (Rahn, 810 g) and 4259 (Rahn, 555 g). AE32 / HDDA photoswitchable photoinitiator-premix (30 g), MDEA synergist (15 g), and Omnirad 184 (0.3 g) were also charged. The mixture was stirred with an overhead stirrer at 100 rpm and a heating pad to heat the internal temperature above 50C for 10 minutes. The mixing speed increased to 250 rpm and Rheobyk 430 thixotrope (7.5 g) was added and then stirred for another 10 min.

[0128] 2. The liquid photohardenable compositions were then hand-poured into cuvettes after going through a paint filter. The poured liquid photohardenable composition in the cuvettes was then placed in a convection oven and a thermal cycle was applied to remove the bubbles and activate the thixotrope. Specifically, the liquid photohardenable compositions were slowly heated to 90C for 1-2 hours in the oven, then slowly cooled down to 50C (-1 h) for further conditioning and printing. The liquid photohardenable compositions can be kept at 50C for printing for an extended period.

[0129] Measurement of the as-prepared liquid photohardenable composition of Example 1

[0130] Tensile) properties (ASTM D638) of the fully-cured liquid photohardenable composition of

[0131] Example 1

[0132] Transmittance and measurement

[0133] Transmittance measurement was made using the setup shown in FIG. 1. A collimated laser beam from a 1 Watt 638 nm laser (Civil Laser) was used to deliver 10 mW of collimated light that was transmitted through 5 centimeters of liquid photohardenable composition (a 5 cm pathlength) contained in a quartz cuvette. The laser beam is aimed at a power meter. When haze scatters the light from its path, the amount of power transmitted to the detector is reduced. We report the percentage in transmitted light compared to a control sample that contains all resin ingredients except the thixotrope. Thixotrope incorporation to bear the required yield stress for our printing led to a decrease in transmittance. The transmission of Example 1, as prepared, was measured as described above. The measured transmission of the as prepared composition was between 98.6-99.0%.

[0134] Yield stress measurement

[0135] Dynamic oscillation stress / strain sweep test with TA Instruments Discovery HR20 Rheometer was used to analyze the yield behavior of our liquid photohardenable compositions. A 40mm parallel plate was used for the test. The results were viewed in a double logarithmic plot of the storage modulus (G’) as a function of oscillation stress. The yield stress is the critical stress at which irreversible plastic deformation occurs and the yield stress numbers are taken as the onset value of the storage modulus curves.

[0136] Specifically, oscillation amplitude measurement was used with 10 rad / s angular frequency, logarithmic sweep from 0.01 Pa to 1000 Pa, and measuring 5 points per decade.

[0137] In the formulation of Example 1, we have obtained a yield stress of 5.9 Pa at 50 degrees Celsius, which is sufficient for the successful printing of the designed parts without sinking and sagging.

[0138] The liquid photohardenable composition of Example 1 exhibited high transmittance and also sufficient yield stress without significant change in the base liquid photohardenable composition mechanical properties.

[0139] Example 2

[0140] An example of a liquid photohardenable composition including a thixo trope including a boosted silica is detailed below. The silica is pre-dispersed in isobornyl acrylate (IBOA). The oligomer (BR541MB, a difunctional aliphatic poly ether urethane methacrylate oligomer, available form BOMAR) was pre-diluted in IBOA for lower viscosity for ease of incorporation. The liquid photohardenable composition includes a diarylethene photoswitchable photoinitiator designated by AE32. Photoswitchable 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. The photoswitchable photoinitiator was pre-dissolved in 1,6-hexanediol diacrylate (HDDA) for ease of incorporation therein.

[0141] The composition of the liquid photohardenable composition of Example 2 is set forth below:

[0142] ** The silica concentration is 0.35 wt %.

[0143] *** The AE32 concentration is 33 ppm.

[0144] Preparation of the Liquid Photohardenable Composition of Example 2

[0145] 1. Silica thixotrope (Aerosil 200, 2%) was pre-dispersed in IBOA. For a 300-g predispersion, Aerosil 200 Fumed Silica (Evonik, 6 g) was charged into a 600-mL container, and IBOA (294 g) was added. A high-speed, high-shear metal homogenizer was used to mix the ingredients at 9k- 10k rpm for 10 min and was used directly without additional storage time.

[0146] 2. BR541MB / IBOA 7.5% premixture was prepared in advance. For a -1.9 kg resin premix, IBOA (225 g) and BR541MB (Bomar, 1650 g) were charged into a metal container and mixed with an overhead stirrer stirring at -150 rpm and with a heating pad wrapped outside of the container to heat the internal resin above 45C for 30 min. The mixture was used as is and can be stored for an extended time.

[0147] 3. In a 3-L metal container, BR541MB / IBOA 7.5% premixture (937.5 g) was charged, followed by adding Genomer 4259 (Rahn, 225 g). AE32 / HDDA photo switchable photoinitiator-premix (49.5 g), MDEA synergist (15 g), Omnirad 184 (0.6 g), and booster Rheobyk 405 (5.25 g) was also charged. The mixture was stirred with an overhead stirrer at 150 rpm and a heating pad to heat the internal temperature above 50C for 20 minutes. The silica dispersion (2% Aerosil 200 in IBOA, 262.5 g) was then charged with the mixing speed increased to 200 rpm and stirred for another 20 min after the mixture temperature reached above 50C.

[0148] 4. The resin was then filtered through a paint filter and centrifuged at 4400 rpm for 20 min, before pouring into cuvettes for printing. The liquid photohardenable composition can be maintained at 50C for an extended time.

[0149] Measurement of the as-prepared liquid photohardenable composition of Example 2

[0150] Transmittance and measurement

[0151] Transmittance measurement was made using the setup shown in FIG. 1. A collimated laser beam from a 1 Watt 638 nm laser (Civil Laser) was used to deliver 10 mW of collimated light that was transmitted through 5 centimeters of liquid photohardenable composition (a 5 cm pathlength) contained in a quartz cuvette. The laser beam is aimed at a power meter. When haze scatters the light from its path, the amount of power transmitted to the detector is reduced. We report the percentage in transmitted light compared to a control sample that contains all resin ingredients except the thixotrope. Thixotrope incorporation to bear the required yield stress for our printing led to a decrease in transmittance. The liquid photohardenable composition of Example 2 exhibited high transmittance and also sufficient yield stress without significant change in the base liquid photohardenable composition mechanical properties.

[0152] Yield stress measurement

[0153] Dynamic oscillation stress / strain sweep test with TA Instruments Discovery HR20 Rheometer was used to analyze the yield behavior of our liquid photohardenable compositions. A 40mm parallel plate was used for the test. The results were viewed in a double logarithmic plot of the storage modulus (G’) as a function of oscillation stress. The yield stress is the critical stress at which irreversible plastic deformation occurs and the yield stress numbers are taken as the onset value of the storage modulus curves. Specifically, oscillation amplitude measurement was used with 10 rad / s angular frequency, logarithmic sweep from 0.01 Pa to 1000 Pa, and measuring 5 points per decade.

[0154] In the formulation of Example 2, we have obtained a yield stress of 8.3-9.1 Pa, which is sufficient for the successful printing of the designed parts without sinking and sagging.

[0155] In the formulation of Example 2, we have obtained a yield stress of 8.3-9.1 Pa, which is sufficient for the successful printing of the designed parts without sinking and sagging.

[0156] The liquid photohardenable composition of Example 2 exhibited high transmittance and also sufficient yield stress without significant change in the base liquid photohardenable composition mechanical properties.

[0157] Tensile properties (ASTM D638) of the fully-cured liquid photohardenable composition of

[0158] Example 2

[0159] Example 3

[0160] Thixotrope Screening Example Including Stock Solution A

[0161] Following are the results of screening of a number of thixotropes for use in a liquid photohardenable composition and method in accordance with the present invention.

[0162] Stock Solution A was prepared by mixing together 530 grams urethane dimethacrylate (Genomer 4247), 36.5 grams aliphatic urethane acrylate (Genomer 4259), 69 grams 1 ,6-hexanediol diacrylate (HDD A), 20 grams AE32 stock solution in HDDA, 10 gram methyldiethanolamine (MDEA), and 0.2 grams of 1-Hydroxycyclohexyl-phenyl ketone (Omnirad 184) by stirring at 50 degrees Celsius until homogeneous using an overhead stirrer in a steel container.

[0163] Screening samples were prepared for each of the thixotropes listed below at the specified concentration by placing 50 grams of Stock Solution A along with a listed thixotrope in the specified amount in a speedmix 100 gram cup and speedmixing the screening samples at 3500 rpm for 5 minutes or longer if necessary for full homogeneous incorporation (Flaktek DAC 150.1). Screening samples were then centrifuged, poured into containers and viscosity was monitored at 50 degrees Celsius on the rheometer until it reached equilibrium.. Once equilibrium was reached, the yield stress was then measured, along with the haze value.

[0164] Results are shown in the table below.

[0165] In the above table, ND indicates that a measurement was not made because either the transmittance or yield stress was below the targeted value therefor on day one.

[0166] Based on the above results, a liquid photohardenable composition including a formulation similar to that of the above Stock Solution A and a thixotrope including a urea modified polyamide (Rheobyk 430) at 0.5, 1 or 2%, a thixotrope including a urea_modified polyurethane (Rheobyk 410) at 1%, or a thixotrope including fumed silica (Aerosil 200) at 1% or 2%, exhibits the criterion of yield stress above 5 Pa and greater than 90% light transmission over 5 cm of resin at 50 degrees Celsius that is maintained after 10 days.

[0167] Example 4

[0168] Thixotrope Screening Example Including Stock Solution B

[0169] Stock Solution B was prepared by combining 630 grams difunctional aliphatic polyether urethane methacrylate oligomer ((BR541MB available form BOMAR), 176 grams isobomyl acrylate (IBOA), 150 grams aliphatic urethane acrylate (Genomer 4259), 33 grams AE32 solution in 1 ,6-hexanediol diacrylate (HDD A), 10 grams methyldiethanolamine (MDEA), and 4 grams of 1 -Hydro xycyclohexyl-phenyl ketone (Omnirad 184) in a steel container and stirred at 50 degrees Celsius until homogeneous.

[0170] Screening samples were prepared for each of the thixotropes listed below at the specified concentration by placing 50 grams of Stock Solution B along with a listed thixotrope in the specified amount in a speedmix 100 gram cup and speedmixing the screening samples at 3500 rpm for 5 minutes or longer if necessary for full homogeneous incorporation (Flaktek DAC 150.1). Screening samples were then centrifuged, poured into containers and viscosity was monitored at 50 degrees Celsius on the rheometer until it reached equilibrium.. Once equilibrium was reached, the yield stress was then measured, along with the haze value.

[0171]

[0172] In the above table, ND indicates that a measurement was not made because either the transmittance or yield stress was below the targeted value therefor on day one.

[0173] The table identifies the tested photohardenable compositions including thixotrope / concentration pairings and measured results for stability over 10 days and transmittance measured as described above. Two examples (e.g., one including 0.5 weight percent Rheobyk 410, 0.5% and a second including 0.5 weight percent hydrophilic fumed silica with a specific surface area of 200 m2 / g (Aerosil 200) with 0.5% booster) exhibited a yield stress greater than or equal to about 5 Pascals at 50 degrees Celsius and a light transmittance greater than 90% for a 5 centimeter (cm) pathlength for at least 10 days after preparation.

[0174] Transmittance and measurement

[0175] In the Examples 3 and 4, transmittance measurement was made using a setup substantially as shown in FIG. 1. A collimated laser beam from a 1 Watt 638 nm laser (Civil Laser) was used to deliver 10 mW of collimated light that was transmitted through 5 centimeters of liquid photohardenable composition (a 5 cm pathlength) contained in a quartz cuvette. The laser beam is aimed at a power meter. When haze scatters the light from its path, the amount of power transmitted to the detector is reduced.

[0176] Yield stress measurement

[0177] Dynamic oscillation stress / strain sweep test with TA Instruments Discovery HR20 Rheometer was used to analyze the yield behavior of the screening samples in Examples 3 and 4. A 40 mm parallel plate was used for the test. The results were viewed in a double logarithmic plot of the storage modulus (G’) as a function of oscillation stress. The yield stress is the critical stress at which irreversible plastic deformation occurs and the yield stress numbers are taken as the onset value of the storage modulus curves. Specifically, oscillation amplitude measurement was used with 10 rad / s angular frequency, logarithmic sweep from 0.01 Pa to 1000 Pa, and measuring 5 points per decade.

[0178] Preferably the liquid photohardenable composition exhibits light transmittance that remains above 90% for at least ten days after preparation.

[0179] In accordance with another aspect of the present invention, there is provided a method of forming an object in a volume of a liquid photohardenable composition, the method comprising: a. providing the volume including a liquid photohardenable composition in accordance with the present invention, b. directing one or more excitation wavelengths to a selected location within the volume of the liquid photohardenable composition to induce a crosslinking or polymerization reaction in the liquid photohardenable composition at the selected location to at least partially form the object; and c. optionally repeating step b. one or more times to partially or fully form the object, wherein a repeated step b. comprises irradiating the liquid photohardenable composition at one or more selected locations that are the same as or different from one or more previous selected locations in the volume.

[0180] In accordance with another aspect of the present invention, there is provided a method of forming an object in a volume of a liquid photohardenable composition, the method comprising: a. providing the volume including a liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius and a light transmittance greater than 90% for a 5 cm pathlength, preferably wherein the light transmittance of the liquid photohardenable composition remains above 90% » b. simultaneously or sequentially irradiating one or more selected locations within the volume of the liquid photohardenable composition with light having a first wavelength and light having a second wavelength, wherein light having the first wavelength and light having the second wavelength activate the photoswitchable photoinitiator at the one or more selected locations to induce a crosslinking or polymerization reaction in the liquid photohardenable composition at the intersection of the first and second wavelengths at the one or more selected locations within the volume to at least partially form the object; and c. optionally repeating step b. one or more times to partially or fully form the object, wherein a repeated step b. comprises irradiating the liquid photohardenable composition at one or more selected locations that are the same as or different from one or more previous selected locations in the volume.

[0181] In the methods described herein, the volume of the photohardenable composition may be irradiated with 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. When two excitation lights are used, at least one of the excitation lights is directed into the volume desirably can be a light sheet.

[0182] 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.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] 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.

[0187] 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.

[0188] 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.

[0189] 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.

[0190] 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”.)

[0191] 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 non-limitation, lasers, laser diodes, light emitting diodes, light-emitting diodes (LEDs), micro-LED 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.

[0192] 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.

[0193] 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.)

[0194] 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. 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.

[0195] 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.

[0196] Excitation light may be visible light, ultraviolet light, or other suitable forms of electromagnetic radiation.

[0197] In methods described herein, each wavelength is preferably generated by a different light source or different optical projection or other optical systems.

[0198] 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.

[0199] 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).

[0200] 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).

[0201] Other exposure energies may also be determined to be useful.

[0202] 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. 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).

[0203] Other power densities or intensities may also be determined to be useful.

[0204] 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.

[0205] 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.

[0206] It can be desirable for the liquid photohardenable composition to be at a temperature of about 50 degrees Celsius during printing.

[0207] 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.

[0208] 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 Nanostructuration for Optical Surfaces by Holographic Two-Photon-Polymerization”, Int’l 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.

[0209] Other 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.

[0210] The 3D printing methods described herein do 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. The methods additionally facilitate 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.

[0211] 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).

[0212] The methods in accordance with various aspects of the invention can further include post-treatment of the three-dimensional object(s) formed.

[0213] 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. 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. Lasers can be preferred sources of radiation for generating radiation of the first wavelength.

[0214] 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 LED or LED 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.

[0215] As used herein a wavelength can refer to a wavelength or range of wavelengths.

[0216] As used herein, the singular of "a”, “an”, and "the" include plural unless the context clearly dictates otherwise. Thus, for example, reference to a particular component of a photohardenable composition includes reference to one or more of the particular component.

[0217] 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.

[0218] 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 liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius and a light transmittance greater than 90% for a 5 cm pathlength.

2. A liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius and a light transmittance greater than 90% for a 5 cm pathlength for at least 10 days following preparation.

3. The liquid photohardenable composition of claim 1 wherein the liquid photohardenable composition includes the thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength.

4. The liquid photohardenable composition of claim 2 wherein the liquid photohardenable composition includes the thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength for at least 10 days after preparation.

5. The liquid photohardenable composition of claim 1 wherein the photohardenable resin component comprises one or more of an acrylated monomer, an acrylated oligomer, a methacrylated monomer, and a methacrylated oligomer.

6. The liquid photohardenable composition of claim 2 wherein the photohardenable resin component comprises one or more of an acrylated monomer, an acrylated oligomer, a methacrylated monomer, and a methacrylated oligomer.

7. The liquid photohardenable composition of claim 3 wherein the photohardenable resin component comprises one or more of an acrylated monomer, an acrylated oligomer, a methacrylated monomer, and a methacrylated oligomer.

8. The liquid photohardenable composition of claim 4 wherein the photohardenable resin component comprises one or more of an acrylated monomer, an acrylated oligomer, a methacrylated monomer, and a methacrylated oligomer.

9. The liquid photohardenable composition of any one of claims 1-8 wherein the thixotrope comprises a modified urea, a modified polyurea, a modified polyamide, a ureapolyamide, a urea modified polyamide, a urea modified polyurethane, a fumed metal oxide, or a mixture including any one or more of the foregoing.

10. The liquid photohardenable composition of any one of claims 1-8 wherein the thixotrope comprises a fumed metal oxide in combination with a booster compound that causes a further increase in the yield stress compared to the fumed metal oxide without the booster.

11. The liquid photohardenable composition of claim 1 wherein the light transmittance is determined by measuring the amount of lOmW of collimated 638 nm light from a 1 Watt laser that is transmitted along a 5 cm pathlength through a sample of the liquid photohardenable composition contained in a quartz cuvette.

12. The liquid photohardenable composition of claims 2 wherein the light transmittance is determined by measuring the amount of lOmW of collimated 638 nm light from a 1 Watt laser that is transmitted along a 5 cm pathlength through a sample of the liquid photohardenable composition contained in a quartz cuvette.

13. The liquid photohardenable composition of any one of claims 1-8 wherein the photoswitchable photoinitiator comprises a photochromic molecule.

14. The liquid photohardenable composition of claim 9 wherein the photoswitchable photoinitiator comprises a photochromic molecule.

15. The liquid photohardenable composition of claim 10 wherein the photoswitchable photoinitiator comprises a photochromic molecule.

16. The liquid photohardenable composition of claim 13 wherein the liquid photohardenable composition further includes a coinitiator.

17. The liquid photohardenable composition of claim 14 wherein the liquid photohardenable composition further includes a coinitiator.

18. The liquid photohardenable composition of claim 15 wherein the liquid photohardenable composition further includes a coinitiator.

19. The liquid photohardenable composition of claim 1 wherein the liquid photohardenable composition comprises a blend including: a. one or more difunctional oligomers in an amount of 20-95 wt% based on the total weight of the polymerizable composition, wherein an oligomer preferably includes one or more vinylester, vinylcarbonate, methacrylate, and / or acrylate functionalities; b. one or more multi-functional (functionality > 2) reactive diluents in an amount of 1-20 wt% based on the total weight of the polymerizable composition, wherein a reactive diluent preferably includes a methacrylate, acrylate, vinylester, and / or vinylcarbonate group functionalities; c. a thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength; d. a photo switchable photoinitiator in an amount greater than about 10 ppm and less than about 500 ppm based on the total weight of the liquid photohardenable composition; e. a coinitiator, preferably a tertiary amine, more preferably comprising a combination including an tertiary amine and an onium salt (preferably, e.g., an iodonium or sulfonium salt), in an amount of 0.5-20 wt% based on the total weight of the liquid photohardenable composition; f. optionally one or more diluents in an amount of 0-45 wt% based on the total weight of the polymerizable composition; g. optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5 wt% based on the total weight of the polymerizable composition;h. optionally one or more polymeric or non-poly meric fillers in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition; and i. optionally a non-reactive diluent in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition.

20. The liquid photohardenable composition of claim 2 wherein the liquid photohardenable composition comprises a blend including: a. one or more difunctional oligomers in an amount of 20-95 wt% based on the total weight of the polymerizable composition, wherein an oligomer preferably includes one or more vinylester, vinylcarbonate, methacrylate, and / or acrylate functionalities; b. one or more multi-functional (functionality > 2) reactive diluents in an amount of 1-20 wt% based on the total weight of the polymerizable composition, wherein a reactive diluent preferably includes a methacrylate, acrylate, vinylester, and / or vinylcarbonate group; c. a thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength; d. a photo switchable photoinitiator in an amount greater than about 10 ppm and less than about 500 ppm based on the total weight of the liquid photohardenable composition; e. a coinitiator, preferably a tertiary amine, more preferably comprising a combination including a tertiary amine and an onium salt (preferably, e.g., an iodonium or sulfonium salt), in an amount of 0.5-20 wt% based on the total weight of the liquid photohardenable composition; f. optionally one or more diluents in an amount of 0-45 wt% based on the total weight of the polymerizable composition;g. optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5 wt% based on the total weight of the polymerizable composition; h. optionally one or more polymeric or non-poly meric fillers in an amount of 0.1- 10 wt% based on the total weight of the liquid photohardenable composition; and i. optionally a non-reactive diluent in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition.

21. The liquid photohardenable composition of claim 19 or 20 wherein the amount of thixotrope is effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength.

22. The liquid photohardenable composition of claim 1 wherein the transmittance is greater than 95%.

23. The liquid photohardenable composition of claim 1 wherein the transmittance is greater than 98%.

24. The liquid photohardenable composition of claim 1 wherein the transmittance is greater than 99%.

25. The liquid photohardenable composition of claim 2 wherein the transmittance is greater than 95%.

26. The liquid photohardenable composition of claim 2 wherein the transmittance is greater than 98%.

27. The liquid photohardenable composition of claim 2 wherein the transmittance is greater than 99%.

28. The liquid photohardenable composition of claim 1 wherein the yield stress is greater than 10 Pascals.

29. The liquid photohardenable composition of claim 2 wherein the yield stress is greater than 10 Pascals.

30. A method for screening thixotropes for inclusion in a photohardenable composition for use in volumetric printing, the method comprising: a. identifying one or more thixotrope candidates as a candidate for inclusion in the photohardenable composition; b. preparing one or more screening samples, each screening sample including a photohardenable resin component, a dual-wavelength photoinitiator, and a selected amount of a thixotrope candidate; and c. measuring yield stress and light transmittance of the one or more screening samples and comparing the measured yield stress and light transmittance against initial target values selected therefor.

31. The method of claim 30 wherein the initial yield stress target is greater than or equal to about 5 Pa at 50 degrees Celsius after preparation.

32. The method of claim 30 wherein the initial light transmittance target is greater than 90% for a 5 cm pathlength after preparation.

33. The method of claim 32 wherein the initial light transmittance target is greater than 90% for a 5 cm pathlength after preparation.

34. The method of claim 30 further comprising: d. identifying screening samples with measured yield stress and transmittance values that are within selected tolerances for the initial targets and remeasuring the transmittance of the identified samples against the initial target transmittance value after a selected time period.

35. The method of claim 32 further comprising: d. identifying screening samples with measured yield stress and transmittance values that are within selected tolerances for the initial targets and remeasuring the transmittance of the identified samples against the initial target transmittance value after a selected time period.

36. The method of claim 35 wherein the selected time period is a 10 day period from preparation of the screening sample.

37. A method of forming an object in a volume of a liquid photohardenable composition, the method comprising: a. providing the volume including a liquid photohardenable composition comprising the liquid photohardenable composition of any one of claims 1-8, b. directing one or more excitation wavelengths to a selected location within the volume of the liquid photohardenable composition to induce a crosslinking or polymerization reaction in the liquid photohardenable composition at the selected location to at least partially form the object; and c. optionally repeating step b. one or more times to partially or fully form the object, wherein a repeated step b. comprises irradiating the liquid photohardenable composition at one or more selected locations that are the same as or different from one or more previous selected locations in the volume.

38. The method of claim 37 wherein the thixotrope included in the liquid photohardenable composition comprises a modified urea, a modified polyurea, a modified polyamide, a ureapolyamide, a urea modified polyamide, a urea modified polyurethane, a fumed metal oxide, or a mixture including any one or more of the foregoing.

39. The method of claim 37 wherein the thixotrope included in the liquid photohardenable composition comprises a fumed metal oxide in combination with a booster compound that causes a further increase in the yield stress compared to the fumed metal oxide without the booster.

40. A method of forming an object in a volume of a liquid photohardenable composition, the method comprising: a. providing the volume including a liquid photohardenable composition for volumetric printing including a photohardenable resin component, a dual-wavelength photoinitiator, and a thixotrope, wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius and a light transmittance greater than 90% for a 5 cm pathlength; b. simultaneously or sequentially irradiating one or more selected locations within the volume of the liquid photohardenable composition with light having a first wavelength and light having a second wavelength, wherein light having the first wavelength and light having the second wavelength activate the photoswitchable photoinitiator at theone or more selected locations to induce a crosslinking or polymerization reaction in the liquid photohardenable composition at the intersection of the first and second wavelengths at the one or more selected locations within the volume to at least partially form the object; and c. optionally repeating step b. one or more times to partially or fully form the object, wherein a repeated step b. comprises irradiating the liquid photohardenable composition at one or more selected locations that are the same as or different from one or more previous selected locations in the volume.

41. The method of claim 40 wherein the liquid photohardenable composition has a yield stress greater than or equal to about 5 Pa at 50 degrees Celsius and a light transmittance greater than 90% for a 5 cm pathlength-for at least 10 days after preparation.

42. The method of claim 40 wherein the liquid photohardenable composition comprises a blend including: a. one or more difunctional oligomers in an amount of 20-95 wt% based on the total weight of the polymerizable composition, wherein an oligomer preferably includes one or more vinylester, vinylcarbonate, methacrylate, and / or acrylate functionalities; b. one or more multi-functional (functionality > 2) reactive diluents in an amount of 1-20 wt% based on the total weight of the polymerizable composition, wherein a reactive diluent preferably includes a methacrylate, acrylate, vinylester, and / or vinylcarbonate group functionalities; c. a thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength; d. a photo switchable photoinitiator in an amount greater than about 10 ppm and less than about 500 ppm based on the total weight of the liquid photohardenable composition; e. a coinitiator, preferably a tertiary amine, more preferably comprising a combination including a tertiary amine and an onium salt (preferably, e.g., aniodonium or sulfonium salt), in an amount of 0.5-20 wt% based on the total weight of the liquid photohardenable composition; f. optionally one or more diluents in an amount of 0-45 wt% based on the total weight of the polymerizable composition; g. optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5 wt% based on the total weight of the polymerizable composition; h. optionally one or more polymeric or non-poly meric fillers in an amount of 0.1- 10 wt% based on the total weight of the liquid photohardenable composition; and i. optionally a non-reactive diluent in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition.

43. The method of any one of claims 40-42 wherein the thixotrope included in the liquid photohardenable composition comprises a modified urea, a modified polyurea, a modified polyamide, a ureapolyamide, a urea modified polyamide, a urea modified polyurethane, a fumed metal oxide, or a mixture including any one or more of the foregoing.

44. The method of any one of claims 40-42 wherein the thixotrope included in the liquid photohardenable composition comprises a fumed metal oxide in combination with a booster compound that causes a further increase in the yield stress compared to the fumed metal oxide without the booster.

45. The method of any one of claims 40-42 wherein the object formed has at least one dimension of at least 3 cm.

46. The method of claim 43 wherein the object formed has at least one dimension of at least 3 cm.

47. The method of claim 44 wherein the object formed has at least one dimension of at least 3 cm.

48. The liquid photohardenable composition of claim 1 wherein the liquid photohardenable composition comprises: a. a photohardenable resin component in an amount from about 89.5 to about 99.998 weight % of the liquid photohardenable composition monomers and / or oligomers;b. a-dual-wavelength photoinitiator in an amount of about 0.001 to about 0.5 weight % of the liquid photohardenable composition; c. a thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength; d. a coinitiator, in an amount of 0.5-20 wt% based on the total weight of the liquid photohardenable composition; e. optionally one or more reactive diluents in an amount of 0-45 wt% based on the total weight of the polymerizable composition; f. optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5 wt% based on the total weight of the polymerizable composition; g. optionally one or more polymeric or non-poly meric fillers in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition; and h. optionally a non-reactive diluent in an amount of 0-10 wt% based on the total weight of the liquid photohardenable composition.

49. The liquid photohardenable composition of claim 2 wherein the liquid photohardenable composition comprises: a. a photohardenable resin component in an amount from about 89.5 to about 99.998 weight % of the liquid photohardenable composition monomers and / or oligomers; b. a-dual-wavelength photoinitiator in an amount of about 0.001 to about 0.5 weight % of the liquid photohardenable composition; c. a thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength;d. a coinitiator, in an amount of 0.5-20 wt% based on the total weight of the liquid photohardenable composition; e. optionally one or more reactive diluents in an amount of 0-45 wt% based on the total weight of the polymerizable composition; f. optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5 wt% based on the total weight of the polymerizable composition; g. optionally one or more polymeric or non-poly meric fillers in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition; and h. optionally a non-reactive diluent in an amount of 0-10 wt% based on the total weight of the liquid photohardenable composition.

50. The liquid photohardenable composition of claim 48 or 49 wherein the photohardenable resin component comprises one or monomers or oligomers including one or more vinylester, vinylcarbonate, methacrylate, and / or acrylate functionalities.

51. The liquid photohardenable composition of claim 48 or 49 wherein the dualwavelength photoinitiator comprises, preferably a photo switchable photoinitiator, more preferably including a substituted or unsubsubstitued diarylethene photochromic moiety.

52. The liquid photohardenable composition of claim 48 or 49 wherein the thixotrope comprises a urea derivative, a modified urea compound, a solution of a modified urea, or a fumed metal oxide.

53. The liquid photohardenable composition of claim 48 or 49 wherein the coinitiator comprises a tertiary amine.

54. The liquid photohardenable composition of claim 48 or 49 wherein the coinitiator comprises a combination of a tertiary amine and an onium salt.

55. The method of claim 40 or 41 wherein the liquid photohardenable composition comprises: a. a photohardenable resin component in an amount from about 89.5 to about 99.998 weight % of the liquid photohardenable composition monomers and / or oligomers; b. a dual-wavelength photoinitiator in an amount of about 0.001 to about 0.5 weight % of the liquid photohardenable composition;c. a thixotrope in an amount effective to achieve (i) a yields stress greater than or equal to about 5 Pascals at 50 degrees Celsius for restricting movement of the three-dimensional object suspended (without contact with a container surface) in the volume of composition during formation, and (ii) a light transmittance greater than 90% for a 5 centimeter (cm) pathlength; d. a coinitiator, in an amount of 0.5-20 wt% based on the total weight of the liquid photohardenable composition; e. optionally one or more reactive diluents in an amount of 0-45 wt% based on the total weight of the polymerizable composition; f. optionally a light activated Norish type I photoinitiator in an amount of 0.005 to 0.5 wt% based on the total weight of the polymerizable composition; g. optionally one or more polymeric or non-poly meric fillers in an amount of 0.1-10 wt% based on the total weight of the liquid photohardenable composition; and h. optionally a non-reactive diluent in an amount of 0-10 wt% based on the total weight of the liquid photohardenable composition.

56. The method of claim 52 wherein the photohardenable resin component comprises one or monomers or oligomers including one or more vinylester, vinylcarbonate, methacrylate, and / or acrylate functionalities.

57. The method of claim 52 wherein the dual-wav elength photoinitiator comprises, preferably a photoswitchable photoinitiator, more preferably including a substituted or unsubsubstitued diarylethene photochromic moiety.58 The method of claim 52 wherein the thixotrope comprises a urea derivative, a modified urea compound, a solution of a modified urea, or a fumed metal oxide?59. The method of claim 52wherein the coinitiator comprises a tertiary amine.

60. The method of claim 52 wherein the coinitiator comprises a combination of a tertiary amine and an onium salt.

61. The method of any one of claims 40-42 wherein the thixotrope included in the liquid photohardenable composition comprises a modified urea, a modified polyurea, a modifiedpolyamide, a ureapolyamide, a urea modified polyamide, a urea modified polyurethane, a fumed metal oxide, or a mixture including any one or more of the foregoing.

62. The method of claim 52 wherein the thixo trope included in the liquid photohardenable composition comprises a fumed metal oxide in combination with a booster compound that causes a further increase in the yield stress compared to the fumed metal oxide without the booster.

63. The method of claim 52 wherein the object formed has at least one dimension of at least 3 cm.

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