Self-healing silicone elastomers

Abstract

A cross-linkable composition, preferably an additive manufacturing composition, comprising a mixture of at least a silicone-based vitrimer prepolymer and a precursor compound, and optionally silica and / or a photoinitiator, wherein the silicone-based vitrimer prepolymer has terminal thiol moieties and is the reaction product between an alkenyl-functionalized silicone, and a vitrimeric monomer, in a solvent, wherein the solvent is preferably an organic solvent, via a thiol-ene reaction, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound, and preferably is a silicone- based compound.

Description

[0001] TITLE

[0002] SELF-HEALING SILICONE ELASTOMERS

[0003] TECHNICAL FIELD

[0004] The present invention relates to a process for producing a cross-linkable composition that is able to self-heal, and which is particularly suitable for additive manufacturing, by mixing at least a silicone-based vitrimer prepolymer with a precursor compound. The present invention further relates to the cross-linkable composition and its use in a process of additive manufacturing, in particular a process of direct ink writing or light-based printingl.

[0005] PRIOR ART

[0006] P. Wang, L. Yang, M. Sun, Z. Yang, S. Guo, G. Gao, L. Xu, D. Ji, W. Cao, J. Zhu Self- Healable and Transparent Elastomers Based on Dual Reversible Network Macromol. Mater. Eng. 2021 , 306, 2000621 discloses a process for the production of silicon elastomer with good mechanical properties and self-healing ability by introducing polyborosiloxane (PBS) into self-healing polydimethylsiloxane (HPDMS) networks.

[0007] Y. Zhao, T. Qin, C. Jiang, J. Li, Y. Xiong, S. Liu, J. Qin, X. Shi, G. Zhang, "Boronic ester- based vitrimeric methylvinyl silicone elastomer with “solid-liquid” feature and ratedependent mechanical performance", Polymer, Volume 265, 2023, 125545 discloses a process to produce a vitrimeric silicone elastomer by one-step peroxide-induced thiol-ene click reaction between thiol-terminated boronic ester and vinyl groups of commercial methylvinyl silicone rubber (MVQ) via reactive processing in the melt.

[0008] Huang, Y.; Yan, J.; Wang, D.; Feng, S.; Zhou, C. Construction of Self-Healing Disulfide- Linked Silicone Elastomers by Thiol Oxidation Coupling Reaction, Polymers ,2021, 13, 3729, discloses self-healing disulfide-linked silicone elastomers (SEs) based on thiol- terminated sulfur-containing heterochain polysiloxanes (P-SHs) and three thiolcontaining crosslinkers, including pentaerythritol tetrakis( -mercaptopropionate) (PETMP), octa(3- mercaptopropyl)silsesquioxane (POSS-SH), and poly[(mercaptopropyl)methylsiloxane] (PMMS), via thiol oxidation coupling reactions.

[0009] CN114539488A discloses a self-healing organosilicon photosensitive resin for 3D printing, characterized in that the components of the photosensitive resin are 50 to 80 parts of organosilicon prepolymer containing a vitrimeric moiety based on boric acid ester, 10 to 30 parts of an acrylate-based diluent, 0.1 to 5 part of UV photoinitiator and 0.01 to 0.5 parts of a pigment; wherein the organosilicon prepolymer containing borate bond is the reaction product of first reacting aminopropyl polydimethylsiloxane with 3,4-trihydroxybenzaldehyde, and a subsequent reaction of the obtained intermediate product with 4-(2- methylpropenyl)carbonyloxybenzeneboronic acid.

[0010] Self-healing polymers are attractive materials because of their ability to recover their structure after physical damage, such as for example after scuffing, scratching and even after cutting, provided that the resulting sections are brought back into contact.

[0011] In some self-healing polymers, the polymer networks are cross-linked via reversible covalent bonds that are particularly interesting due to the possibility to achieve intrinsic self- healing capabilities. In particular, it has been found that silicone-based vitrimers are enticing materials in the context of self-healing polymers because their glass transition temperature (Tg) is much lower than room temperature, which results in viscoelastic properties that depend primarily on the rate of degenerate exchange reaction at their topology-freezing transition (Tv) rather than on chain mobility (Tg). It has become possible to synthesize a broad range of silicone-based vitrimers, as discussed above, which could be tuned with respect to the topology-freezing transition (Tv), as well as their viscoelastic properties and self-healing response.

[0012] In general, additive manufacturing allows the manufacture of objects having complex geometries from polymers without the need for providing costly metal molds (e.g. injection molding) or without the restrictions of subtractive manufacturing (e.g. machining). However, additive manufacturing imparts its own requirements on the material being used as the "ink" in additive manufacturing, more so when it is not possible to simply heat and fuse the polymer material for extrusion, as is the case for example in fused filament deposition of thermoplastic polymers. For instance, in the case of radiation-cured elastomeric polymers, such as silicone-based elastomeric polymers, the rheological profile of a potential ink must be adjusted to ensure the right amount of shear-thinning and the right amount of stability at rest, so that the ink may be extruded conveniently, and that the printed "green" object may be cross-linked before it collapses.

[0013] In order to provide an additive manufacturing ink that comprises silicone-based vitrimers, there is thus a need to reconcile the requirements that are needed for an additive manufacturing ink in terms of rheology at room temperature while at the same time providing an ink that can be cured, preferably via radiation, into an elastomeric material that has self- healing properties.

[0014] SUMMARY OF THE INVENTION

[0015] The present invention provides a process for producing a cross-linkable composition, in particular an additive manufacturing composition, preferably for direct ink writing, that is shear-thinning and is self-supporting at rest, at room temperature, and which may be crosslinked to provide an elastomeric material that has self-healing properties.

[0016] It is a first object of the present invention to provide a process for producing a cross-linkable composition, preferably an additive manufacturing composition, comprising the step of mixing at least a silicone-based vitrimer prepolymer with a precursor compound, wherein the silicone-based vitrimer prepolymer has terminal thiol moieties and is the reaction product between an alkenyl-functionalized silicone having the formula (I) R°-[(SiO(R2)(R3)]n- R1, where both R° and R1are, independently of each other, an alkenyl moiety, preferably are the same alkenyl moiety, and a vitrimeric monomer comprising at least one vitrimer moiety V and comprising at least, or exactly, two thiol moieties, in a solvent, wherein the solvent is preferably an organic solvent, via a thiol-ene reaction, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound.

[0017] It is a second object of the present invention to provide a cross-linkable composition, preferably an additive manufacturing composition, comprising a mixture of at least a silicone-based vitrimer prepolymer and a precursor compound, and optionally silica and / or a photoinitiator.

[0018] It is a third object of the present invention to provide an elastomeric material obtained by crosslinking, preferably via radiation, more preferably via UV radiation, of the cross-linkable composition according to the second object of the present invention.

[0019] It is a fourth object of the present invention to provide a use of the cross-linkable composition for the manufacture of an object, wherein the object comprises an elastomeric material obtained by crosslinking the cross-linkable composition.

[0020] Further embodiments of the invention are laid down in the dependent claims.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

[0023] Fig. 1 shows, on the left, an illustration of a dogbone being cut in half by a razorblade. On the right, the strength recovery, expressed in %, in dependence of time in minutes of contact between the cut and re-contacted surfaces is graphically shown for both a dogbone made of the cross-linked elastomer of the invention and a dogbone made of silicone elastomer. After 24h of contact between the cut surfaces, the dogbone made of the cross-linked elastomer of the invention recovers fully (upper data points, "100% dynamic covalent bonds") while the dogbone made of silicone elastomer does not (lower data points, "100% permanent covalent bonds").

[0024] DESCRIPTION OF PREFERRED EMBODIMENTS

[0025] It is a first object of the present invention to provide a process for producing a cross-linkable composition, preferably an additive manufacturing composition, comprising the step of mixing at least a silicone-based vitrimer prepolymer with a precursor compound, wherein the silicone-based vitrimer prepolymer has terminal thiol moieties and is the reaction product between an alkenyl-functionalized silicone having the formula (I) R°-[(SiO(R2)(R3)]n- R1, where both R° and R1are, independently of each other, an alkenyl moiety, preferably are the same alkenyl moiety, and a vitrimeric monomer comprising at least one vitrimer moiety V and comprising at least, or exactly, two thiol moieties, in a solvent, wherein the solvent is preferably an organic solvent, via a thiol-ene reaction, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound. In the context of the present invention, the term "cross-linking" refers to the formation of covalent bonds between individual molecules in a composition, which may be brought about by radiation, heat, pressure change in pH or crosslinking agents, and which results in a network of interlinked molecules.

[0026] The present invention provides a process for producing a cross-linkable composition. It is understood that the cross-linkable composition may be shaped prior to being cross-linked into any desirable object. Suitable shaping methods by which the cross-linkable composition may be shaped are, for example, moulding methods such as mould casting or injection moulding. Other shaping methods by which the cross-linkable composition may be shaped are extrusion methods, in particular layer-by-layer, along a predefined path, until an object is formed, as is useful in direct ink writing. In a preferred embodiment of the invention according to the first object, the cross-linkable composition is a cross-linkable additive manufacturing composition, which may be used in additive manufacturing methods such as stereolithography, binder jetting, direct ink writing, and others.

[0027] The present invention provides a process for producing a cross-linkable composition comprising the step of mixing at least a silicone-based vitrimer prepolymer with a precursor compound. It is understood that the mixing at least a silicone-based vitrimer prepolymer with a precursor compound, and other components such as silica, as the case may be, may be achieved via any mixing method, such as manual mixing or mixing in a dedicated mixing device. Suitable mixing devices may be, for example, planetary mixers, ribbon mixers, compounding mixers, twin screw mixers, and such. A person skilled in the art will know how to achieve mixing of the at least one silicone-based vitrimer prepolymer with a precursor compound. For example, a suitable setting for a planetary mixer is 3 minutes at 2200 rpm.

[0028] The silicone-based vitrimer prepolymer suitable for the process for producing a crosslinkable composition according to the present invention has terminal thiol moieties. Upon cross-linking, terminal thiol (-SH) moieties of the silicone-based vitrimer prepolymer react, via thiol-ene reaction, with alkenyl groups of the precursor compound having at least three alkenyl moieties per molecule of precursor compound, forming a covalent bond. Thus, an elastomeric material can be formed, in which at least three alkenyl moieties of a precursor compound molecule are preferably connected to at least three thiol moieties of three different silicone-based vitrimer prepolymer molecules, which in turn are preferably connected to at least three alkenyl moieties of three different precursor compound molecules via their other thiol moieties, and so on.

[0029] Likewise, the silicone-based vitrimer prepolymer is the reaction product between an alkenyl- functionalized silicone and the vitrimeric monomer, via thiol-ene reaction. Upon their reaction, terminal thiol (-SH) moieties of the vitrimeric monomer react with terminal alkenyl groups of the alkenyl-functionalized silicone forming a covalent bond. Because both vitrimeric monomer and alkenyl-functionalized silicone provide only terminal reaction partners (-SH and vinyl / allyl), the reaction concatenates these building blocks in an alternating fashion into a linear molecule. It has been found that a linear silicone-based vitrimer prepolymer provides mobility to the prepolymer and results in high flexibility in the elastomeric material.

[0030] The alkenyl-functionalized silicone having the formula (I) R°-[(SiO(R2)(R3)]n-R1, where both R° and R1are, independently of each other, an alkenyl moiety, preferably are the same alkenyl moiety, can be adjusted in length by including more or less -[(SiO(R2)(R3)]- repeat units. It is understood that in principle, the length of the alkenyl-functionalized silicone is not particularly limited to given range, and n may be 1 , 2, 3, 4, 5 or in excess thereof. In a preferred embodiment, n is chosen such that the alkenyl-functionalized silicone is liquid at standard conditions. The chemical nature of R2and R3is not particularly restricted, and either R2and R3may be an alkyl or aryl, independently of each other. In some embodiments, R° and R1may both be methyl (-CH3), or may be phenyl (-CeHs) or benzyl (-CH2-C6H5).

[0031] The vitrimeric monomer comprising at least one vitrimer moiety V and comprising at least, or exactly, two thiol moieties, confers the self-healing properties to the cross-linkable composition according to the present invention, even after being cross-linked. The vitrimer moiety allows for the dynamic formation and cleaving of the covalent vitrimeric bond in the silicone-based vitrimer prepolymer when incorporated into the cross-linkable composition, even when cross-linked, which allows for self-healing. It is understood that when crosslinked, the cross-linkable composition according to the present invention includes a dynamic network of dyamic covalent bonds whose the rate of dynamic formation and cleaving can be influenced by temperature. A higher temperature increases the rate of dynamic formation and cleaving, and thus, the rate of self-healing in the cross-linked composition. Stated alternatively, the vitrimer monomer has the formula SH-V-SH, where V is the vitrimer moiety and SH is thiol.

[0032] Vitrimer moieties are known in the art, and may incorporate dynamic covalent bonds that are based on ester, carbamate, carbonate, carbamide, acetal, imine, vinylogous urethane, diketo-enamine, thioether, triazolium, pyridinium, anilinium, sulfonium, dioxaborolane, boroxine, silyl ether, or disulfide bonds.

[0033] In a preferred embodiment, the vitrimer moiety V is based on an ester bond, and preferably is a dioxaborolane moiety. A dioxaborolane moiety is capable of transesterification and formed by an ester of a boronic acid and a diol.

[0034] It is a first object of the present invention to provide a process for producing a cross-linkable composition, the alkenyl-functionalized silicone is combined with the vitrimeric monomer in a solvent, preferably an organic solvent, to form the silicone-based vitrimer prepolymer via thiol-ene reaction between the alkenyl-functionalized silicone and the vitrimeric monomer.

[0035] The solvent should be chosen such that the vitrimeric monomer may be dissolved in the solvent, as the vitrimeric monomer is generally first dissolved in the solvent and the alkenyl- functionalized silicone is added after dissolution of the vitrimeric monomer for the thiol-ene reaction between vitrimeric monomer and alkenyl-functionalized silicone in the liquid phase.

[0036] The solvent may preferably be an organic solvent that is non polar and / or aprotic, such as alkane compounds such as pentane, hexane, and heptane and aromatic compounds such as benzene, toluene, and xylene. In a preferred embodiment the solvent is toluene.

[0037] In a preferred embodiment of the process according to the first object of the invention, the solvent is removed after the thiol-ene reaction between the vitrimeric monomer and the alkenyl-functionalized silicone, to form the silicone-based vitrimer prepolymer, is completed. Thus, an essentially solvent-free silicone-based vitrimer prepolymer is achieved, which comprises less than 1% by weight of solvent.

[0038] In a preferred embodiment of the process according to the first object of the invention, the precursor compound is a silicone-based compound, preferably comprising repeat units having the formula (II) — [SiO(R4)(R5)]— , where at least one of R4and R5is, independently from the other, an alkenyl moiety and preferably is a vinyl moiety. In one embodiment, R4and R4is an alkenyl moiety and R5is an alkyl moiety. When the precursor compound is a silicone-based compound, the bulk of the resulting cross-linked material will be silicone- based. Silicone compounds are well-known and commercially available, and the silicone- based compound may be a homopolymer of a repeat unit having the formula (II) or may be a copolymer of a repeat unit having the formula (II) with different repeat unit having the formula (II).

[0039] In a preferred embodiment of the process according to the first object of the invention, either the silicone-based vitrimer prepolymer is a linear silicone-based vitrimer prepolymer, or the silicone-based compound is a linear silicone-based compound, or both the silicone-based vitrimer prepolymer and silicone-based compound are a linear silicone-based vitrimer prepolymer, and a linear silicone-based compound, respectively. As is apparent, a linear prepolymer and / or linear precursor compound provide for more flexibility on the molecular level of the cross-linked material than a branched prepolymer and / or precursor compound.

[0040] In a preferred embodiment of the process according to the first object of the invention, the silicone-based vitrimer prepolymer has the formula (V), SH-[V-S-CH2-CH2-[(SiO(R2)(R3)]nCH2- CH2-S-V-S]mH, where V is the vitrimer moiety, SH is thiol, and R2and R3may be an alkyl such as methyl or ethyl, or aryl such as phenyl or benzyl, independently of each other, and may preferably both be methyl (-CH3), or may be phenyl (-CeHs) or benzyl (-CH2-C6H5). In formula (V), n may be 1 , 2, 3, 4, 5 or in excess thereof. In formula (V), m may be 1 , 2, 3, 4, 5 or in excess thereof.

[0041] In a preferred embodiment of the process according to the first object of the invention, in the formula (I) R°-[(SiO(R2)(R3)]n-R1, both R° and R1are, independently of each other, a vinyl or allyl moiety, and more preferebaly are both a vinyl moiety. In formula (I), n may be 1 , 2, 3, 4, 5 or in excess thereof.

[0042] In a preferred embodiment of the process according to the first object of the invention, in the formula (I) R°-[(SiO(R2)(R3)]n-R1, R2and R3are, independently of each other, an alkyl moiety, preferably the same alkyl moiety, where the alkyl moiety is in particular an alkyl moiety chosen from methyl, ethyl, n-propyl, i-propy, n-butyl or i-butyl or t-butyl, and more preferably are methyl. In formula (I), n may be 1 , 2, 3, 4, 5 or in excess thereof.

[0043] In a preferred embodiment of the process according to the first object of the invention, the weight average molecular mass of the alkenyl-functionalized silicone ranges from 5 000 to 30 000 g / mol, preferably from 6 000 to 12 000 g / mol, and more preferably the weight average molecular mass of the alkenyl-functionalized silicone ranges from 5 000 to 30 000 g / mol, preferably from 6 000 to 12 000 g / mol, when R2and R3are methyl and R° and R1are vinyl. It has been found that molecular mass contributes to increasing both stress and strain at break, and thus, these parameters can be influenced by the choice of alkenyl- functionalized silicone.

[0044] In a preferred embodiment of the process according to the first object of the invention, the process according to any preceding claims, wherein the vitrimeric monomer comprises a vitrimeric moiety capable of, disulfide exchange, imine metathesis, silyl ether metathesis, a vinylogous urethane or transesterification, preferably is a vitrimeric monomer comprising a vitrimeric moiety capable of transesterification and comprising a boronic acid diester.

[0045] In a preferred embodiment of the process according to the first object of the invention, the precursor compound is a silicone-based compound comprising repeat units having the formula (II) — [SiO(R4)(R5)]— , where R4is an alkyl moiety, preferably a methyl or ethyl moiety and where R5is an alkenyl moiety, preferably a vinyl or allyl moiety, more preferably where R4is an methyl and where R5is an vinyl moiety.

[0046] In a preferred embodiment of the process according to the first object of the invention, the precursor compound is a silicone-based compound comprising both repeat units according to formula (II) and according to formula (III) -[SiO(R6)(R7)]-, where R6or R7are, independently of each other, an alkyl moiety and in particular are methyl or ethyl, and where preferably R6or R7are the same, and in particular are methyl or ethyl, more preferably where R6or R7are R4is methyl and where R4is a methyl moiety and where R5is an vinyl moiety.

[0047] In a preferred embodiment of the process according to the first object of the invention, the cross-linkable composition further comprises silica particles, preferably in an amount of between 5 and 50 pph, preferably in an amount of 25 and 40 pph by weight, based on the combined weight of the silicone-based vitrimer prepolymer and precursor compound. The silica particles may preferably be mixed into the mixture of silicone-based vitrimer prepolymer and precursor compound that is obtained when mixing the silicone-based vitrimer prepolymer and precursor compound.

[0048] In a preferred embodiment of the process according to the first object of the invention, the cross-linkable composition further comprises a photoinitiator. Suitable photintiators are molecules that will form radicals upon exposure to radiation, such as UV radiation, and which then initiate the cross-linking of the cross-linkable composition. A suitable photoinitiator is benzoyl peroxide or 2,2-dimethoxy-2-phenylacetophenone (DMPA).

[0049] In a preferred embodiment of the process according to the first object of the invention, the cross-linkable composition is a cross-linkable direct ink writing composition. The crosslinkable direct ink writing composition is suitable to be used in a direct ink writing device.

[0050] In a preferred embodiment of the process according to the first object of the invention, the cross-linkable composition comprises the silicone-based vitrimer prepolymer in an amount of 40 to 50 weight percent and the silicone-based polymer in an amount of 15 to 30 weight percent, based on the total weight of the cross-linkable composition.

[0051] In a preferred embodiment of the process according to the first object of the invention, the cross-linkable composition is cross-linkable via radiation, in particular UV radiation. It has bee found that the cross-linkable composition can be cross-linked to various degrees of cross-linking when exposed to more or less radiation. More radiation leads to more crosslinking, less radiation to less cross-linking. It has been found that the, the cross-linkable composition that is cross-linkable via radiation, even when comprising a photoinitator, has a long shelf-life and can be stored for more than 3 months prior to use.

[0052] In a preferred embodiment of the process according to the first object of the invention, the number of allyl or vinyl moieties per molecule of the precursor compound is from 20 to 30 per molecule. While the number of allyl or vinyl moieties is not particularly restricted, provided that 3 or more ally or vinyl moieties per molecule are present, it has been found that also 20 to 30 ally or vinyl, an in particular vinyl moieties, per molecule provide a crosslinkable composition that is well-suited for direct ink writing.

[0053] In a preferred embodiment of the process according to the first object of the invention, when the precursor compound is a silicone-based compound, the silicone-based compound is trialkylsiloxy-terminated, and more preferably is trimethylsiloxy or triethylsiloxy terminated and / or the silicone-based compound is a copolymer of (vinylmethylsiloxane)- (dimethylsiloxane).

[0054] In a preferred embodiment of the process according to the first object of the invention, the vitrimeric monomer is present in a molar excess with respect to the alkenyl-functionalized silicone, preferably in a molar excess of at least 10%, preferably of at least 50% and between 50% and 100%. When in molar excess, the alkenyl of the alkenyl-functionalized silicon reacts with the overabundant amount of thiols of the vitrimeric monomer to yield a silicone-based prepolymer that has no unreacted alkenyl moieties but only displays available thiol moieties at its end, which are then available for reaction with the alkenyl moieties of precursor compound. Without wishing to be bound to a certain theory, it is believed that any unreacted vitrimeric monomer may still react with alkenyl moieties of the precursor compound to additionally cross-link the precursor compound molecules.

[0055] In a preferred embodiment of the process according to the first object of the invention, the molar ratio between the thiol moieties of the silicone-based prepolymer and the vinyl moieties of the silicone-based compound is in excess of 1 , and preferably in excess of 2 or even in excess of 3 equivalents. In another preferred embodiment of the process according to the first object of the invention, the molar ratio between the thiol moieties of the silicone- based prepolymer and the vinyl moieties of the silicone-based compound is preferably of approximately 2.

[0056] In a preferred embodiment of the process according to the first object of the invention, the alkenyl-functionalized silicone is a vinyl-terminated polydimethylsiloxane and / or wherein the vitrimeric monomer is (1 ,4-phenylenebis(1 ,3,2-dioxaborolane-2,4-diyl))dimethanethiol, and / or wherein the silicone-based polymer is trimethylsiloxy-terminated (vinylmethylsiloxane)-(dimethylsiloxane) copolymer and / or the solvent is toluene and / or the silica is fumed hydrophobic silica.

[0057] In a preferred embodiment of the process according to the first object of the invention, the silicone-based vitrimer prepolymer has a viscosity of 5 000 to 100 000 mPas at 25°C, when measured on a stress-controlled compact rheometer (Anton Paar MCR 302 rheometer) using a sandblasted parallel plate geometry (PP25) with a 1 mm gap under steady-shear conditions and increasing the applied shear rate from 0.1 to 100 s-1. It has been found that when the viscosity is in the aforementioned range, the prepolymer is particularly suitable for use in the production of the cross-linkable composition.

[0058] In a preferred embodiment of the process according to the first object of the invention, the cross-linkable composition has a zero shear viscosity of 106to 109mPas at 25°C when measured on a stress-controlled compact rheometer (Anton Paar MCR 302 rheometer) using a sandblasted parallel plate geometry (PP25) with a 1 mm gap. It has been found that when the viscosity is in the aforementioned range, in particular when incorporating silica, the cross-linkable composition is particularly suitable as a direct ink writing composition, since it is self-supporting after the ink is extruded into the desired shape.

[0059] In a preferred embodiment of the process according to the first object of the invention, the a storage modulus (at low strain values) of 30000 to 80000 mPa at 25°C and / or a loss modulus of 10000 to 20000 mPa at 25°C when measured on a stress-controlled compact rheometer (Anton Paar MCR 302 rheometer) using a sandblasted parallel plate geometry (PP25) with a 1 mm gap.

[0060] It is a second object of the present invention to provide a cross-linkable composition, preferably an additive manufacturing composition, more preferably a direct ink writing composition, comprising a mixture of at least a silicone-based vitrimer prepolymer and a precursor compound, and optionally silica and / or a photoinitiator, wherein the silicone- based vitrimer prepolymer has terminal thiol moieties and is the reaction product between an alkenyl-functionalized silicone having the formula (I) R°-[(SiO(R2)(R3)]n-R1, where both R° and R1are, independently of each other, an alkenyl moiety, preferably are the same alkenyl moiety, and a vitrimeric monomer comprising at least one vitrimer moiety V and comprising at least, or exactly, two thiol moieties, in a solvent, wherein the solvent is preferably an organic solvent, via a thiol-ene reaction, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound, and preferably is a silicone-based compound, preferably comprising repeat units having the formula (II) — [SiO(R4)(R5)]— , where at least one of R4and R5is, independently from the other, an alkenyl moiety and preferably is a vinyl moiety.

[0061] It is understood that the embodiments of the first object apply equally to the cross-linkable composition according to the second object of the present invention, and vice-versa. That is, embodiments that relate to the chemical nature, ratios, amounts, physico-chemical- properties and such, of the constituent materials, apply just as well to the cross-linkable composition.

[0062] In a preferred embodiment of the cross-linkable composition according to the second object of the invention, the cross-linkable composition comprises a mixture of at least a silicone- based vitrimer prepolymer and a precursor compound, wherein the silicone-based vitrimer prepolymer has formula (IV): starting repeating unit where n is may be 1 , 2, 3, 4, 5 or in excess thereof, preferably between 1 and 100, and m may be 1 , 2, 3, 4, 5 or in excess thereof, is preferably between 1 and 100, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound, and preferably is a silicone-based compound, preferably comprising repeat units having the formula (II) — [SiO(R4)(R5)]— , where at least one of R4and R5is, independently from the other, an alkenyl moiety and preferably is a vinyl moiety.

[0063] It is a third object of the present invention to provide an elastomeric material obtained by crosslinking, preferably via radiation, more preferably via UV radiation, of a cross-linkable composition according to the second object of the present invention. Preferebly, the crosslinkable composition according to the second object of the present invention is formed in to a shape via direct ink writing, prior to crosslinking, preferably via radiation, more preferably via UV radiation, of the cross-linkable composition.

[0064] It is a fourth object of the present invention to provide a use of a cross-linkable composition according to the second object for the manufacture of an object, wherein the object comprises an elastomeric material obtained by crosslinking the cross-linkable composition according to the second object.

[0065] EXPERIMENTAL DATA

[0066] Materials

[0067] Vinyl-terminated polydimethylsiloxane (DMS-V22, 200 cSt) and (7.0-8.0% vinylmethylsiloxane)-dimethylsiloxane copolymer, trimethylsiloxy terminated (VDT-731 , 800-1200 cSt) were purchased from Gelest, Inc. (USA). Thioglycerol (397%), tetrahydrofuran (THF, > 99.9%), toluene (> 99.7%), chloroform-d (99.8%) and molecular sieves (zeolite, 4 ) were acquired from Sigma-Aldrich (USA). 1 ,6-Hexanedithiol (97%) was purchased from AlfaAesar (USA), whereas (1 ,4-Phenylenediboronic acid (98%) was obtained from Combi-Blocks, Inc. (USA). 2,2-Dimethoxy-2-phenylacetophenone (DMPA, 99%) was purchased from Acros Organics (USA). SilcPigTM silicone color pigments were acquired from Smooth-On Inc. (USA), while fumed silica (Aerosil R8200) was purchased from Evonik AG (Germany).

[0068] Synthesis methods

[0069] Synthesis of monomer containing dioxaborolane functional groups.

[0070] The monomer (1 ,4-phenylenebis(1 ,3,2-dioxaborolane-2,4-diyl))dimethanethiol was prepared via condensation of 1 ,4-phenylenediboronic acid with thioglycerol. First, 16.00g (96.48 mmol, 2eq) of 1 ,4-phenylenediboronic acid were added to a 2000ml 2-neck round bottom flask, followed by the slow addition of 1440ml of toluene under stirring until complete dissolution.

[0071] The flask containing this milky solution was then connected a Soxhlet extractor equipped with molecular sieves and slowly warmed up to 60 °C. After 10 minutes, 20.88g (193.04 mmol, 4eq) of thioglycerol were added dropwise while keeping the temperature at 110°C. The solution turned completely clear in a few minutes, and it was left under vigorous magnetic stirring at reflux temperature. After 18h, the reaction solution was filtered and the final product was concentrated by rotary evaporation without further purification to obtain a coarse white powder (28.96g, yield -82%.1H NMR (300MHz, CDCh): 6 7.82 p.p.m (s, 4H), 4.74 (m,2H), 4.49 (m, 2H), 4.17 (m, 2H), 2.81 (m, 4H), 1.50 (dd, J=11, 7.4 Hz, 2H).

[0072] In a 250ml single-neck round-bottom flask, 6.5g (20.9 mmol, 3eq) of (1 ,4- phenylenebis(1 ,3,2-dioxaborolane-2,4-diyl))dimethanethiol were slowly dissolved with 120mg (0.46 mmol) DMPA in 150ml of THF. After complete dissolution of the solids, 66.3g (7.0 mmol, 1eq) of vinyl-terminated polydimethylsiloxane were slowly added under magnetic stirring. The solution was then purged with argon for 3 minutes. The sealed flask was then placed inside a home-built, air-tight UV curing chamber under N2 atmosphere. The solution was kept under magnetic stirring and simultaneously irradiated for 10 minutes using a mercury lamp (Omnicure S2000 UV lamp 200W, 320-500nm) from the side and using a home-built LED system (50W, 405nm) purchased from AliExpress (China) from the bottom. For both UV sources, the measured intensity was >200mW / cm2 . The solution was then concentrated by rotary evaporation to obtain a viscous, milky solution. is of silicone elastomer

[0073] In a 250ml single-neck round-bottom flask, 2.3g (15.3mmol, 2eq) of 1 ,6-hexanedithiol were slowly dissolved with 130mg (0.49 mmol) DMPA in 160ml THF. After both chemicals were dissolved, 73.8g (7.7mmol, 1eq) of vinyl-terminated polydimethylsiloxane were slowly added under stirring. The solution was then purged with argon for 3 minutes. The sealed flask was then placed inside the UV curing chamber and, under magnetic stirring, irradiated simultaneously with the mercury lamp from the side and with the LED system from the bottom for 10 minutes. In both cases, the UV intensity was measured to be >200mW / cm2 . The solution was then concentrated by rotary evaporation to obtain a viscous, transparent solution.

[0074] Ink formulations for direct ink writing

[0075] Silicone vitrimer

[0076] In a 300ml Thinky mixer cup (Thinky, USA), 69.23g of silicone vitrimer prepolymer were added to 46.15g of trimethylsiloxy-terminated (7.0-8.0% vinylmethylsiloxane)- dimethylsiloxane copolymer, 225mg DMPA (0.15 wt%) and 300mg (0.2wt%) SilcPig (color: electric pink). The fraction of vinylmethylsiloxane in the (vinylmethylsiloxane)- dimethylsiloxane copolymer was 7-8%. The mass ratio of 2:3 between the silicone vitrimer prepolymer and the vinyl-siloxane copolymer was optimized empirically to maximize the mechanical performance of the silicone vitrimer. After manual mixing for 1 minute, 34.61g (30 parts per hundred of rubber, phr) of silica nanoparticles (Aerosil R8200) were slowly incorporated into the resin formulation and mixed for 3 minutes at 2200rpm in a planetary centrifugal mixer (Thinky Mixer ARE-250) to obtain approximately 150g of 3D-printable silicone-based resin. Silicone elastomer (Comparative)

[0077] In a 300ml Thinky mixer cup, 70.31g of silicone elastomer prepolymer were added to 23.43g of trimethylsiloxy-terminated (7.0-8.0% vinylmethylsiloxane)-dimethylsiloxane copolymer, 225mg DMPA (0.15 wt%) and 300mg (0.2 wt%) SilcPig (color: electric violet). The fraction of vinylmethylsiloxane in the (vinylmethylsiloxane)-dimethylsiloxane copolymer was 7-8%. The mass ratio of 2:3 between the silicone vitrimer prepolymer and the vinyl-siloxane copolymer was optimized empirically to maximize the mechanical performance of the silicone elastomer. After manual mixing for 1 minute, 56.25g (60phr) of silica nanoparticles (Aerosil R8200) were slowly incorporated into the resin formulation and mixed for 3 minutes at 2200rpm in a Thinky mixer to obtain approximately 150g of 3D-printable silicone-based resin.

[0078] 3D-printing and curing

[0079] To 3D-print complex structures via direct ink writing we used 30 cm3syringe barrels (model ADV830BA) equipped with syringe pistons (model ADV830WW) and with a 0.84mm nozzle (model TT 18-rigid). All 3D-printing consumables were purchased from Adhesive Dispensing Ltd., UK. The syringe barrels were filled by hand up to 75% of their full capacity and then centrifuged for 3 minutes before being connected on different Direct-Ink-Writing 3D printers. We used an extrusion-based printer (3D-discovery, RegenHU SA, Switzerland) to print 1.8mm-thick films using distinct ink formulations. Dogbone samples were extracted from these films and used in the self-healing tests. The inks were printed with an applied pressure of approximately 3.5bar at a feed rate of 12mm / s.

[0080] All printed parts were then transferred inside the air-tight UV chamber and placed approximately 5mm away from the UV source. After flushing for 3 minutes with nitrogen, the UV lamp (Omnicure S2000) was turned on and the parts were illuminated for 10 minutes under a slightly positive N2 pressure with a light intensity of approximately 200mW / cm2.

[0081] For the self-healing tests on the silicone vitrimer, printed films were cut into half with a razor blade, immediately brought back into contact and left unloaded for precise time intervals before the mechanical measurements. The time intervals varied broadly from 5 minutes to 30 days. The strength of the samples after this resting period was measured and compared to that of the pristine sample. The results are shown in Figure 1. As can be seen in Figure 1 , a dogbone incorporating the cross-linked elastomer of the invention and which is cut in half, heals back to a intact dogbone, recovering more than half of its initial strength within 5 minutes, and completely recovers its initial strength after 24 hours. In contrast, the dogbone made of silicone elastomer does not recover at all.

[0082] 5

Claims

CLAIMS1. A process for producing a cross-linkable composition, preferably an additive manufacturing composition, comprising the step of mixing at least a silicone- based vitrimer prepolymer with a precursor compound, wherein the silicone-based vitrimer prepolymer has terminal thiol moieties and is the reaction product between an alkenyl-functionalized silicone having the formula (I) R°- [(SiO(R2)(R3)]n-R1, where both R° and R1are, independently of each other, an alkenyl moiety, preferably are the same alkenyl moiety, and a vitrimeric monomer comprising at least one vitrimer moiety V and comprising at least, or exactly, two thiol moieties, in a solvent, wherein the solvent is preferably an organic solvent, via a thiol-ene reaction, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound.

2. The process according to any preceding claims, wherein the precursor compound is a silicone-based compound, preferably comprising repeat units having the formula (II) -[SiO(R4)(R5)]-, where at least one of R4and R5is, independently from the other, an alkenyl moiety and preferably is a vinyl moiety.

3. The process according to any preceding claims, wherein in the formula (I) R°-[(SiO(R2)(R3)]n-R1, both R° and R1are, independently of each other, a vinyl or allyl moiety, and / or R2and R3are independently of each other, an alkyl moiety, preferably the same alkyl moiety, where the alkyl moiety is in particular an alkyl moiety chosen from methyl, ethyl, n-propyl, i-propyl, n- butyl or i-butyl or t-butyl and / or wherein the weight average molecular mass of the alkenyl-functionalized silicone molecular ranges from 5 000 to 30 000 g / mol, preferably from 6 000 to 12 000 g / mol, and / or wherein the vitrimeric monomer is a vitrimeric monomer capable of, disulfide exchange, imine methathesis, silyl ether metathesis, a vinylgous urethane or transesterification, preferably is a vitrimeric monomer capable oftransesterification and comprises a boronic acid ester.

4. The process according to any preceding claims, wherein the precursor compound is a silicone-based compound comprising repeat units having the formula (II) -[SiO(R4)(R5)]-, where R4is an alkyl moiety, preferably a methyl or ethyl moiety and where R5is an alkenyl moiety, preferably a vinyl or allyl moiety, and / or wherein the precursor compound is a silicone-based compound comprising repeat units having the formula (III) -[SiO(R6)(R7)]-, where R6or R7are, independently of each other, an alkyl moiety and in particular are methyl or ethyl, and where preferably R6or R7are the same, and in particular are methyl or ethyl.

5. The process according to any preceding claims, wherein the cross-linkable composition further comprises silica particles, preferably in an amount of between 5 and 50 pph, preferably in an amount of 25 and 40 pph by weight, based on the weight of the silicone-based vitrimer prepolymer and precursor compound, and / or wherein the cross-linkable composition further comprises a photoinitiator and / or wherein the cross-linkable composition is a crosslinkable direct ink writing composition.

6. The process according to any preceding claims, wherein the cross-linkable composition comprises the silicone-based vitrimer prepolymer in an amount of 40 to 50 weight percent and the silicone-based polymer in an amount of 15 to 30 weight percent, based on the total weight of the cross-linkable composition and / or wherein the cross-linkable composition is cross-linkable via radiation, in particular UV radiation.

7. The process according to any preceding claims, wherein the number of allyl or vinyl moieties per molecule of the precursor compound is from 20 to 30 per molecule and / or when the precursor compound is a silicone-based compound, the silicone-based compound is trialkylsiloxy-terminated, and more preferably is trimethylsiloxy or triethylsiloxy terminated and / or the silicone-based compound is a copolymer of (vinylmethylsiloxane)- (dimethylsiloxane).

8. The process according to any preceding claims, wherein the vitrimericmonomer is present in a molar excess with respect to the alkenyl- functionalized silicone, preferably in a molar excess of at least 10%, preferably of at least 50% and between 50% and 100%, and / or wherein the molar ratio between the thiol moieties of the silicone-based prepolymer and the vinyl moieties of the silicone-based compound is in excess of 1, preferably of 2.

9. The process according to any preceding claims, wherein the alkenyl- functionalized silicone is a vinyl-terminated polydimethylsiloxane and / or wherein the vitrimeric monomer is (1 ,4-phenylenebis(1 ,3,2-dioxaborolane- 2,4-diyl))dimethanethiol, and / or wherein the silicone-based polymer is trimethylsiloxy-terminated (vinylmethylsiloxane)-(dimethylsiloxane) copolymer and / or the solvent is toluene and / or the silica is fumed hydrophobic silica.

10. The process according to any preceding claims, wherein the silicone-based vitrimer prepolymer has a viscosity of 5 000 to 100 000 mPas at 25°C, when measured on a stress-controlled compact rheometer (Anton Paar MCR 302 rheometer) using a sandblasted parallel plate geometry (PP25) with a 1 mm gap under steady-shear conditions and increasing the applied shear rate from 0.1 to 100 s’1.

11. The process according to any preceding claims, wherein the cross-linkable compound has a zero shear viscosity of 106to 109mPas at 25°C when measured on a stress-controlled compact rheometer (Anton Paar MCR 302 rheometer) using a sandblasted parallel plate geometry (PP25) with a 1 mm gap and / or a storage modulus (at low strain values) of 30000 to 80000 mPa at 25°C and / or a loss modulus of 10000 to 20000 mPa at 25°C when measured on a stress-controlled compact rheometer (Anton Paar MCR 302 rheometer) using a sandblasted parallel plate geometry (PP25) with a 1 mm gap.

12. A cross-linkable composition, preferably an additive manufacturing composition, comprising a mixture of at least a silicone-based vitrimer prepolymer and a precursor compound, and optionally silica and / or a photoinitiator,wherein the silicone-based vitrimer prepolymer has terminal thiol moieties and is the reaction product between an alkenyl-functionalized silicone having the formula (I) R°- [(SiO(R2)(R3)]n-R1, where both R° and R1are, independently of each other, an alkenyl moiety, preferably are the same alkenyl moiety, and a vitrimeric monomer comprising at least one vitrimer moiety V and comprising at least, or exactly, two thiol moieties, in a solvent, wherein the solvent is preferably an organic solvent, via a thiol-ene reaction, and wherein the precursor compound comprises at least three alkenyl moieties per molecule of precursor compound, and preferably is a silicone-based compound, preferably comprising repeat units having the formula (II) - [SiO(R4)(R5)]-, where at least one of R4and R5is, independently from the other, an alkenyl moiety and preferably is a vinyl moiety.

13. A cross-linkable composition, preferably an additive manufacturing composition, comprising a mixture of at least a silicone-based vitrimer prepolymer and a precursor compound, wherein the silicone-based vitrimer prepolymer has formula (IV)starting repeating unit where n is may be 1 , 2, 3, 4, 5 or in excess thereof, and m may be 1 , 2, 3, 4, 5 or in excess thereof, and wherein the precursor comprises at least three alkenyl moieties per molecule of precursor compound, and preferably is a silicone-based compound, preferably comprising repeat units having the formula (II) -[SiO(R4)(R5)]-, where at least one of R4and R5is, independently from the other, an alkenyl moiety and preferably is a vinyl moiety.

14. An elastomeric material obtained by crosslinking, preferably via radiation, more preferably via UV radiation, of a cross-linkable composition according to claim 12 or 13.

15. Use of a cross-linkable composition according to claim 12 or 13 for the manufacture of an object, wherein the object comprises an elastomericmaterial obtained by crosslinking od said cross-linkable composition.

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