Liquid monomer compositions

Abstract

A liquid monomer composition comprising [i] 20-90 wt% of one or more monofunctional curable monomers selected from monofunctional (meth)acrylate and (meth)acrylamide having a weight in the range 140-4

Description

FIELD OF THE INVENTION This application relates in general to liquid compositions comprising (meth)acrylate monomers that are suitable for use in the formation of curable coatings. The application relates in particular to novel monomer liquid compositions that are suitable for use in two-part acrylic compositions that may be used to prepare cosmetic fingernail coatings. The application further relates to methods forming polymeric structures on substrates using the using the monomer liquid composition of the invention together with polymer powders. The application further provides a novel additive for use in monomer liquid compositions. BACKGROUND The cosmetic enhancement of the fingernails and toenails is a major sector in the fashion and beauty industry, with an estimated global market size in excess of 20 billion USD in 2022. A wide range of products has been developed to enhance both the appearance of nails and the physical properties of nails, particularly to add length and strength to nails. Nail enhancement compositions fall into two general product categories. Nail polishes (also known as lacquers) are used for decorative purposes, in particular to provide colour and shine to the nails. Artificial nails are products that are used to provide structural enhancements to the nails, in particular to modify the strength and / or shape of the nails, although they may also include decorative enhancements, such as colorants and coatings. The term “acrylic nails” is used to refer to a specific type of nail enhancement product that involves forming a cured acrylate polymer coating on the nail. Acrylic nails are formed using a two-part composition comprising a liquid monomer component and a polymer powder component and are consequently also known in the art as “liquid and powder” formulations. These two components are combined to form a curable paste which is applied to the nail and cured to form a durable nail coating. Curing is typically by way of a thermal initiator contained within the polymer powder component, optionally in combination with a catalyst from the liquid component. The polymer powder component typically comprises beads of poly(meth)acrylate polymers or co-polymers, typically based on poly(methylmethacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and copolymers of methylmethacrylate (MMA) and ethylmethacrylate (EMA). An initiator is generally coated on and / or embedded within the polymer beads. The polymer beads may also contain a colorant. The liquid component contains reactive monomers that function both to solubilise the polymers from the powder component and then to cure to form a hardened polymer coating. The liquid component generally comprises EMA and optionally one or more additives, such as aromatic amine catalysts that react with the initiator contained in the polymer powder to promote the formation of free radicals. For a number of reasons, EMA is essentially the only liquid monomer component widely in use in the nail industry. The use of MMA in monomer liquids has been discontinued for many years due to serious health concerns, with symptoms of exposure including hypersensitivity, asthmatic reactions, local neurological symptoms and skin reactions. Other alternatives to EMA with the necessary combination of low toxicity and the required chemical properties (particularly solubilising power and reactivity) have not been identified. The use of ethylmethacrylate as a monomer liquid in acrylic nail formulations is itself is coming under increasing scrutiny for a number of reasons. Exposure to EMA is associated with allergic reactions, including dermatological and respiratory reactions. Occupational exposure resulting in sensitisation is a particular issue for nail technicians working in nail salons. EMA is also a volatile and odorous chemical and therefore its use creates an unpleasant working environment for nail technicians and is off-putting for their customers. In addition, the flammability of EMA creates an additional hazard. There is accordingly a need in the art for a replacement monomer liquid that can be used as an EMA equivalent without the need for modification to the conventional polymer powders that are used in acrylic nail formulations. Such a replacement would desirably need to have low toxicity, low allergenicity, low or no odour and low or no flammability. SUMMARY OF INVENTION In a first aspect, the invention provides a liquid monomer composition comprising: (i) from 20 to 90 wt% of one or more monofunctional curable monomers selected from monofunctional (meth)acrylate and (meth)acrylamide monomers having a molecular weight in the range from 140 to 400; (ii) from 5 to 40 wt% of one or more alcohol solvents selected from C2 to C4 alkyl alcohols; and (iii) from 5 to 40 wt% of one or more carbonate solvents selected from C3-C11 alkylene carbonates and mixtures thereof. In a second aspect, the invention provides a kit of parts comprising: (i) a liquid monomer composition according to the first aspect of the invention; and (ii) an acrylic polymer powder. In a third aspect, the invention provides a method of forming a polymeric structure on a substrate; the method comprising the steps of: (i) combining an acrylic polymer powder with a liquid monomer composition according to the first aspect of the invention to form a reactive dispersion; and (ii) applying the reactive dispersion to a substrate and allowing it to cure. In a fourth aspect, the invention provides a compound of the formula (I): R1xC{CH2OC(O)-R2-S-(CH2CHR3)-C(O)O-[R4-OC(O)-R5-C(O)O-]zR6-OC(O)CR^^ (I) wherein: x is from 0 to 2 and x+y = 4; z is from 0 to 2; each R1 is independently H or a Ci to C4 alkyl group; each R2 is independently 1,2-ethylenyl, 1,3-propylenyl, or 1,4-butylenyl; each R3 is independently H or methyl; each R4 and R5 is independently selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl; each R6 is independently a selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl, wherein R6 is optionally substituted by a hydroxyl group; each R7 is independently H or methyl. The compound of the fourth aspect of the invention is useful as an additive in the liquid monomer compositions of the first aspect of the invention. When brought into contact with a polymer powder comprising a free-radical initiator, it supresses oxygen inhibition and is subsequently incorporated into the polymer that is formed by the curing of the curable monomer component. DETAILED DESCRIPTION The first aspect of the invention provides a liquid monomer composition comprising: (i) from 20 to 90 wt% of one or more monofunctional curable monomers selected from monofunctional (meth)acrylate and (meth)acrylamide monomers having a molecular weight in the range from 140 to 400; (ii) from 5 to 40 wt% of one or more alcohol solvents selected from C2 to C4 alkyl alcohols; and (iii) from 5 to 40 wt% of one or more carbonate solvents selected from and C3-C11 alkylene carbonates and mixtures thereof. The liquid composition of the invention provides a liquid monomer composition which can be used as a substitute for EMA monomer liquids in two-part curable formulations that further comprise a polymer powder component. Such formulations are widely used in the cosmetic nail industry but may also find applications, for example, as bone cements or dental fillers. When used in two- part cosmetic nail formulations (also known as “acrylic” or “liquid and powder” formulations), the liquid monomer composition can be used as a direct replacement for conventional EMA monomer liquids without any substantial difference in performance and therefore without the need for retraining of nail technicians or modification of the methods they use. Prior efforts to identify replacements for EMA monomer liquids has focused on the use of alternative (meth)acrylate monomers as direct replacements for EMA. The inventor has arrived at the present invention after a detailed study of the chemical functionality of EMA in liquid and powder formulations. While the primary purpose of the EMA is obviously as a reactive monomer that is curable via free-radical initiation to form a poly(ethylmethacrylate) polymer, the inventor has identified that EMA in fact has multiple functions in liquid and powder formulations. In particular, EMA is an excellent solvent for the (meth)acrylate polymers found in the polymer powders of conventional liquid and powder systems. Therefore, when the EMA liquid and polymer powder are mixed, the EMA monomer functions initially as a solvent which dissolves any free-radical initiator disposed on the surface of polymer powder. It also starts to dissolve the polymer powder itself, thereby also releasing any free-radical initiator contained within the polymer powder. Decomposition of the free-radical initiator initiates the early stages of free-radical polymerisation, with retardation of the reaction by chain termination reactions and the effect of oxygen in neutralising the radicals. Dissolution of the polymer in the EMA liquid also increases the viscosity of the polymer solution the in the EMA monomer to the point where the Trommsdorf effect operates. This is an effect where increased viscosity of the polymerizing system disfavours termination reactions resulting in acceleration of the overall rate of reaction. The EMA therefore fulfils at least the functions of both reactive monomer and solvent for the polymer powder. Research efforts into alternatives to EMA liquids has focused on identifying alternative monomers that can substitute both of these functions of EMA. These efforts have been largely unsuccessful. One reason for this is that alternative monomers of higher boiling point are much poorer solvents for the polymer powder. A second reason is that (meth)acrylate monomers are prepared in the presence of polymerisation inhibitors to prevent unwanted polymerisation reactions during production. The relatively low-boiling EMA is usually distilled prior to use, so that any inhibitors included during the manufacture of the monomer is removed and replaced by a minimum quantity of inhibitor (typically ca. 5 to 40 ppm). However, alternative monomers of higher boiling point contain much higher levels of inhibitor than EMA liquids (typically in excess of 100 ppm), since distillation is impractical or impossible. The increased inhibitor content of these monomers therefore adds to oxygen inhibition due to the radical scavenging effect of atmospheric oxygen. As a consequence of reduced solubilising power and higher inhibitor content, the performance of these alternative monomers in liquid and powder formulations is significantly worse than EMA. In particular, the formulations are slow to cure and tend to form a thick, tacky inhibition layer at the surface that is exposed to air. The present invention is founded on the recognition by the inventor that not all of the EMA in conventional EMA liquids is reacted. A portion of the EMA evaporates during use and a portion of the EMA remains unreacted in the final cured composition. This means that a portion of the EMA is contributing to the solvating properties of the monomer liquid but is not ultimately participating in a polymerisation reaction. The inventor has recognised that this provides the possibility to substitute at least a portion of the monomer with a non-reactive solvent and, in this way, to boost the solvating properties of the monomer liquid even when the monomer component itself is a less effective solvent than EMA. This opens up the possibility to use a range of alternative monomers that avoid some or all of the disadvantages associated with EMA. It also allows for an optimised solvent system to be used, in which the choice of solvent(s) may be tailored to achieve a range of desirable objectives, including low odour, low toxicity, and low flammability. The curable component of the monomer liquid of the invention comprises one or more monofunctional (meth)acrylate or (meth)acrylamide monomers having a molecular weight in the range from 140 to 400. Monomers having increased molecular weight are generally less volatile than EMA and therefore provide a solution at least to the problem of malodour, and monomers within this molecular weight range may also be selected for their low toxicity and low flammability without a negative impact on curing performance when used in compositions according to the invention. As used herein, the term “acrylate” used in the context of monomers, relates to curable compounds comprising a moiety of the formula CH2=CH-C(O)O- The term “methacrylate” used in the context of monomers, relates to curable compounds comprising a moiety of the formula CH2=C(CH3)-C(O)O-. The term “(meth)acrylate” is used herein to refer generically to the group of acrylate and methacrylate monomers as well as mixtures thereof. As used herein, the term “acrylamide” used in the context of monomers, relates to curable compounds comprising a moiety of the formula CH2=CH-C(O)N<. The term “methacrylamide” used in the context of monomers, relates to curable compounds comprising a moiety of the formula CH2=C(CH3)-C(O)N<. The term “(meth)acrylamide” is used herein to refer generically to the group of acrylamide and methacrylamide monomers as well as mixtures thereof. As used herein, the term “monofunctional” refers to (meth)acrylate and (meth)acrylamide compounds comprising a single polymerizable double bond (i.e. a single (meth)acrylate and (meth)acrylamide moiety). It does not limit the presence of other functional groups, such as esters and ethers, that do not undergo addition polymerization. The liquid monomer composition of the invention comprises 20 to 90 wt% of the one or more monofunctional curable monomers based on the total liquid monomer composition. More preferably, the liquid monomer composition of the invention comprises from 40 to 85 wt%, more preferably from 45 to 80 wt%, more preferably from 50 to 75 wt% of the one or more monofunctional curable monomers. The one or more monofunctional curable monomers are preferably selected from monofunctional (meth)acrylate and (meth)acrylamide monomers having a molecular weight in the range from 140 to 400, more preferably from 140 to 300, more preferably from 140 to 250, more preferably from 140 to 220, more preferably from 160 to 220, and most preferably from 180 to 210. Preferred monofunctional curable monomers are monofunctional methacrylate monomers, monofunctional methacrylamide monomers or mixtures thereof. More preferably, the monofunctional curable monomers are monofunctional methacrylate monomers. Preferred monofunctional (meth)acrylate monomers are compounds of the formula CH2=CR-C(O)O-R’. wherein R is H or methyl and R’ is selected from C3-C10, linear or cyclic aliphatic hydrocarbyl moieties, optionally comprising one or more heteroatoms. Preferably R is methyl. Preferably, R’ comprises a heterocylic moiety comprising two heteroatoms in the heterocylic ring. Examples of monofunctional monomers that can be used in accordance with the present invention are butyl (meth)acrylate, isobutyl (meth)acrylate, ethylene glycol methyl ether (meth)acrylate, ethylene glycol ethyl ether (meth)acrylate, di(ethylene glycol) methyl ether (meth)acrylate, di(ethylene glycol) ethyl ether (meth)acrylate, tri(ethylene glycol) methyl ether (meth)acrylate, tri(ethylene glycol) ethyl ether (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-norbornyl (meth)acrylate, isobornyl (meth)acrylate, 2-N-morpholinoethyl (meth)acrylate, isodecyl (meth)acrylate, glycerol carbonate (meth)acrylate, isopropylideneglycerol (meth)acrylate, (meth)acryloyl morpholine and mixtures thereof. A preferred group of monofunctional monomers with particularly low toxicity includes ethylene glycol methyl ether methacrylate, ethylene glycol ethyl ether methacrylate, di(ethylene glycol) methyl ether methacrylate, di(ethylene glycol) ethyl ether methacrylate, tri(ethylene glycol) methyl ether methacrylate, tri(ethylene glycol) ethyl ether methacrylate, 2- 2- / V-morpholinoethyl methacrylate, glycerol carbonate (meth)acrylate, isopropylideneglycerol (meth)acrylate, (meth)acryloyl morpholine and mixtures thereof. More preferably, the one or more monofunctional curable monomers are selected from glycerol carbonate (meth)acrylate, isopropylideneglycerol (meth)acrylate, (meth)acryloyl morpholine and mixtures thereof. More preferably, the one or more monofunctional curable monomers selected are selected from glycerol carbonate methacrylate, isopropylideneglycerol methacrylate, methacryloyl morpholine and mixtures thereof. More preferably, the one or more monofunctional curable monomers selected are selected from glycerol carbonate methacrylate, isopropylideneglycerol methacrylate and mixtures thereof. The liquid monomer compositions of the invention optionally include one or more multifunctional curable monomers as cross-linkers. As used herein, the term “multifunctional” refers to compounds comprising two or more polymerizable double bonds that can cross-link polymer chains formed from the monofunctional curable monomers described above. Preferably, the multifunctional curable (meth)acrylate and / or multifunctional curable monomers comprise two or (meth)acrylamide moieties. monomers comprise two or more (e.g. two to four) More preferably, the more (e.g. two to four) (meth)acrylate moieties. The multifunctional monomers are suitable present in an amount from 1 to 15 wt%, more preferably from 2 to 12 wt%, for example from 5 to 10 wt% based on the total liquid monomer composition. Suitable polyfunctional monomers include di(meth)acrylates, tri(methacrylates) and tetra(methacrylates) having a molecular weight in the range from 140 to 400. Examples of suitable polyfunctional monomers include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate. Preferably, the polyfunctional monomers are selected from the polymethacrylate monomers, such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol di methacrylate, trimethylolpropane trimethacrylate, and pentaerythritol tetramethacrylate. The liquid monomer compositions of the invention include a solvent mixture comprising one or more alcohol solvents and one or more carbonate solvents. The one or more alcohol solvents function as low toxicity volatile solvents in the compositions of the invention. When used within the preferred ranges disclosed herein, the alcohol component does not confer flammability onto the liquid monomer composition. The one or more alcohol solvents are selected from C2 to C4 alkyl alcohols and are used in an amount in the range from 5 to 40 wt% based on the total liquid monomer composition. More preferably the liquid monomer composition comprises from 5 to 30 wt% of the one or more alcohol solvents, more preferably from 5 to 25 wt% of the one or more alcohol solvents, more preferably from 5 to 20 wt% of the one or more alcohol solvents, more preferably from 5 to 15 wt% of the one or more alcohol solvents, for example around 10 wt% of the one or more alcohol solvents. The alcohol may in principle be selected from any of the C2 to C4 alkyl alcohols, including branched isomers thereof and mixtures thereof. Preferably the alcohol is selected from ethanol and isopropanol. Most preferably the alcohol is ethanol. The liquid monomer compositions of the invention also include one or more alkylene carbonate (i.e. cyclic carbonate) solvents. It has been found that a mixture of alcohol and carbonate solvents is necessary to fully replicate the solvent effect of EMA. Cyclic carbonate solvents are excellent solvents for ester-based materials and therefore can replicate the high solvating power of EMA for (meth)acrylate monomers and (meth)acrylate-based polymer powders. They also have low volatility (and therefore low odour) and very low toxicity. Whereas the alcohol solvent forms a volatile solvent component, the carbonate solvent component functions as a substantially non- volatile solvent. This replicates the effect of unreacted EMA in the final composition. This carbonate component is also found to function as a plasticiser in the final cured composition and therefore may improve the mechanical properties of the cured coating when compared to monomer liquids based on EMA. The one or more carbonate solvents are selected from C3-C11 alkylene carbonates and mixtures thereof, wherein the carbonyl carbon is included in the total number of carbons (for example, propylene carbonate is considered a C4 alkylene carbonate as defined herein). The one or more carbonate solvents are used in an amount in the range from 5 to 40 wt% based on the total liquid monomer composition. More preferably the liquid monomer composition comprises from 5 to 30 wt% of the one or more carbonate solvents, more preferably from 5 to 25 wt% of the one or more carbonate solvents, more preferably from 5 to 20 wt% of the one or more carbonate solvents, more preferably from 5 to 15 wt% of the one or more carbonate solvents, for example around 10 wt% of the one or more carbonate solvents. The one or more carbonate solvents may in principle be selected from any of the C3 to C11 alkylene carbonates, including isomers thereof and mixtures thereof. Preferably, the one or more carbonate solvents are selected from the linear 1,2-carbonates. More preferably the one or more carbonate solvents are selected from ethylene carbonate, 1,2-propylene carbonate, 1,2-butylene carbonate and mixtures thereof. Most preferably the carbonate solvent is 1,2-propylene carbonate. The liquid monomer compositions may optionally comprise one or more additives. Optionally, the liquid monomer compositions may comprise from 0.1 to 10 wt% of one or more low volatility alcohols selected from polyhydric alcohols and glycol ethers. Examples of suitable compounds include ethylene glycol, 1,2-propylene glycol, 1,2-butylene glycol, 1,4-butylene glycol, glycerol, pentaerythritol, diethyleneglycol monomethyl ether, dipropyleneglycol monomethyl ether, diethyleneglycol monoethyl ether and dipropyleneglycol monoethyl ether. Without being bound by theory, it is believed that such compounds function a polar plasticisers in cured acrylic coatings, resulting in coatings that are more flexible and resilient over time. Another group of additives include tertiary amines that are conventionally used as activators for the free radical initiator that is conventionally present in the powder component of a two-part liquid and powder acrylic formulation. Tertiary amines promote the breakdown of free radical initiators, such as benzoyl peroxide, to form radicals and therefore promote faster reaction. Suitable tertiary amines include 4-dimethylaminotoluene and N-4-tolyldiethanolamine. The tertiary amine additives preferably have the formula Ar-NR72, wherein Ar represents a Ce-Cw aromatic group, preferably 4-tolyl, and R7 represents a C1-C4 alkyl or hydroxyalkyl group. The liquid monomer compositions preferably comprise from 0.1 to 3 wt% of tertiary amine additive(s). However, it has also been identified by the inventor that tertiary amines may be inadequate to overcome the elevated level of inhibitors found in (meth)acrylate or (meth)acrylamide monomers having a molecular weight greater than that of EMA, particularly when this is combined with the radical scavenging effect of atmospheric oxygen. It is known that mercaptans are far more effective than amines in suppressing oxygen inhibition of radical polymerisation. However, these compounds are also unstable when formulated with curable monomers. The inventor has therefore developed a novel tetra-(meth)acrylate functional thioether which is able to suppress oxygen inhibition effectively without causing destabilisation of the monomers. Due to the (meth)acrylate functionality, this component is incorporated into the cured material following the curing reaction. The liquid monomer composition therefore preferably further comprises a compound of the formula (I): R1xC{CH2OC(O)-R2-S-(CH2CHR3)-C(O)O-[R4-OC(O)-R5-C(O)O-]zR6-OC(O)CR7=CH2}y (I) wherein: x is from 0 to 2 and x+y = 4 z is from 0 to 2 each R1 is independently H or a Ci to C4 alkyl group each R2 is independently 1,2-ethylenyl, 1,3-propylenyl, or 1,4-butylenyl; each R3 is independently H or methyl. each R4 and R5 is independently selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl; each R6 is independently a selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl, wherein R6 is optionally substituted by a hydroxyl group each R7 is independently H or methyl. In the compound of formula (I), x is preferably 0 or 1, more preferably 0. Each R2 is preferably 1,2-ethylenyl. Each R3 is preferably H. Each R4 and each R5 is preferably 1,2-ethylenyl. Each R6 is preferably 2-hydroxy-1,3-propylenyl. Each R7 is preferably methyl. Each z is preferably 1 or 2, more preferably each z is 1. The liquid monomer composition preferably comprises from 0.1 to 5 wt% of the compound of formula (I) based on the total liquid monomer composition. More preferably the liquid monomer composition from 0.5 to 4.5 wt%, more preferably from 1 to 4 wt% more preferably from 1.5 to 3.5 wt% of the compound of formula (I). Another group of additives include colorants, dyes, pigments, optical brighteners and combinations thereof. The liquid monomer composition may optionally comprise from 0.01 to 5 wt% of one or more colorants, dyes, pigments, optical brighteners or any combinations thereof. As used herein, the term “colorant” relates to any substance that can impart colour to the compositions of the invention. The colorant may be a dye or pigment. Suitable dyes include azo dyes, cyanine dyes, anthraquinone dyes, phthalocyanine dyes, naphthoquinone dyes, quinoneimine dyes, methine dyes, azomethine dyes, squarylium dyes, acridine dyes, styryl dyes, coumarin dyes, quinoline dyes and nitro dyes. Suitable pigments include inorganic pigments and organic pigments, such as azo pigments, diazo pigments, acid / base dye pigments, phthalocyanine pigments, quinacridone pigments, and any other polycyclic pigments. As used herein, the term “optical brightener” relates to any substance with fluorescent properties. Another group of additives include fragrances, which may be used to provide an appealing scent when using the compositions of the invention, which contrasts with the malodour associated with EMA liquids. The liquid monomer composition may comprise at least one UV absorber. As used herein, the term “UV absorber” relates to any molecule or substance that can absorb UV radiation, often used to reduce UV-induced degradation (photooxidation) of a material, such as yellowing caused by the degradation of polymeric materials, for example those within acrylic nails. The liquid monomer composition may comprise from 0.1 to 2 wt% of one or more UV absorbers. The UV absorber may be selected from benzotriazoles, cyanoacrylates, hydroxybenzophenones, hydroxyphenyl benzotriazoles, oxanilides, benzotriazoles, rutile titanium oxide, hydroxyphenyl triazines, or benzophenones. An example of a suitable UV absorber is 2-hydroxy-4-octoxybenzophenone. The liquid monomer composition may comprise at least one light stabiliser. As used herein, the term “light stabiliser” relates to a substance or compound that can inhibit the photo-oxidation of polymers through continuous removal of free radicals which are produced by the photo-oxidation of the polymer. In an embodiment, the light stabiliser is an amine light stabiliser. The liquid monomer composition of the present invention can be used in the preparation of artificial nail coatings, as described above, but may also have utility in the preparation of acrylic materials for use in medical applications, such as tooth or bone fillers and cements. As used herein, the term “nail coating” or “artificial nail” relates to a wide variety of products that can be applied to a natural nail to, for example, artificially enhance its appearance. Such nail coatings or artificial nails may be press-on nails, sculpted nails, or acrylic nails, and may cover part of, or all of, a natural nail. Nail coatings or artificial nails may have regions extending beyond the boundary of the nail edge. Such coatings may be produced in a mould or other suitable device to form a cosmetic which may be applied to a nail. In a second aspect, the invention provides a kit of parts comprising: (i) a liquid monomer composition according to the first aspect of the invention; and (ii) an acrylic polymer powder. The provision of such a kit comprising the liquid monomer composition of the present invention and an acrylic polymer powder allows the liquid monomer composition to be used in a liquid and powder, or two-part, composition to form a durable, cross-linked thermoset substance. The components of the kit may be combined to form a reactive dispersion which is then applied to a surface and allowed to cure. In kits of the invention, the acrylic polymer powder may be selected from any commercially acrylic polymer powders, that are conventionally used in the nail industry. As noted above, the liquid monomer composition of the invention advantageously is compatible with conventional polymer powders without modification thereof. In principle, the acrylic polymer powder may be any polymer formed via the addition polymerisation of monomers comprising acrylate and / or methacrylate functional groups. Preferably, the acrylic polymer powder comprises a polymer or copolymer of methyl methacrylate (MMA) and / or ethyl methacrylate (EMA). It will be understood that polymer powders based on MMA and EMA include the polymerised form of MMA and / or EMA and therefore do not involve exposure to MMA and / or EMA monomers when used. The polymer powder preferably comprises a free radical initiator on the surface of the polymer powder and / or embedded in the polymer powder. A preferred free radical initiator is benzoyl peroxide since it is activated by body heat. Suitably the particle size of the polymer powder is in the range from 20 to 250 pm, as determined by sieving, i.e. smaller than mesh size 60 (250 pm openings) and larger than mesh size 500 (25 pm openings). The kit generally comprises separate containers for each of the liquid monomer composition and the polymer powder. This will allow the comprising components of the kit to be kept separate until in use. The kit may comprise a mixing container in which the liquid monomer composition and the polymer powder may be combined to form a reactive dispersion. Optionally, the kit may comprise a means for applying the reactive dispersion onto a substrate and / or for manipulating the reactive dispersion after application. A variety of application implements may be contemplated, such as a spatula, a brush, a roller, a syringe, or any other suitable implement for applying the reactive dispersion that may be used in the cosmetics industry or medical field. In a third aspect, the invention provides a method of forming a polymeric structure on a substrate; the method comprising the steps of: (i) combining an acrylic polymer powder with a liquid monomer composition according to the first aspect of the invention to form a reactive dispersion; and (ii) applying the reactive dispersion to a substrate and allowing it to cure. The acrylic polymer powder may be selected from any commercially acrylic polymer powders, that are conventionally used in the nail industry. As noted above, the liquid monomer composition of the invention advantageously is compatible with conventional polymer powders without modification thereof. In principle, the acrylic polymer powder may be any polymer formed via the addition polymerisation of monomers comprising acrylate and / or methacrylate functional groups. Preferably, the acrylic polymer powder comprises a polymer or copolymer of methyl methacrylate (MMA) and / or ethyl methacrylate (EMA). It will be understood that polymer powders based on MMA and EMA include the polymerised form of MMA and / or EMA and therefore do not involve exposure to MMA and / or EMA monomers when used. The polymer powder preferably comprises a free radical initiator on the surface of the polymer powder and / or embedded in the polymer powder. A preferred free radical initiator is benzoyl peroxide since it is activated by body heat. Suitably the particle size of the polymer powder is in the range from 20 to 250 pm, as determined by sieving, i.e. smaller than mesh size 60 (250 pm openings) and larger than mesh size 500 (25 pm openings). The method of the invention is preferably a cosmetic method. More preferably, the method of the invention is preferably a cosmetic method of forming a coating on a nail (i.e. fingernail or toenail). The nail may be a natural human nail or it may be an artificial nail that is applied over a natural nail (e.g. to modify the shape and / or length of the natural nail). In a fourth aspect, the invention provides a compound of the formula (I): R1xC{CH2OC(O)-R2-S-(CH2CHR3)-C(O)O-[R4-OC(O)-R5-C(O)O-]zR6-OC(O)CR7=CH2}y (I) wherein: x is from 0 to 2 and x+y = 4 z is from 0 to 2 each R1 is independently H or a Ci to C4 alkyl group each R2 is independently 1,2-ethylenyl, 1,3-propylenyl, or 1,4-butylenyl; each R3 is independently H or methyl. each R4 and R5 is independently selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 5 1,4-butylenyl; each R6 is independently a selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl, wherein R6 is optionally substituted by a hydroxyl group each R7 is independently H or methyl. 10 In the compound of formula (I), x is preferably 0 or 1, more preferably 0. Each R2 is preferably 1,2-ethylenyl. Each R3 is preferably H. Each R4 and each R5 is preferably 1,2-ethylenyl. Each R6 is preferably 2-hydroxy-1,3-propylenyl. 15 Each R7 is preferably methyl. Each z is preferably 1 or 2, more preferably each z is 1. EXAMPLES The following tables provide examples of liquid monomer compositions within the scope of the present invention. Percentages are by weight unless specified otherwise. The following terms are used: 10 GCMA: Glycerol carbonate methacrylate IPGMA Isopropylideneglycerol methacrylate EGDMA Ethylene glycol dimethacrylate TEGDMA Triethylene glycol dimethacrylate PC Propylene carbonate DPM Dipropylene glycol monomethyl ether DMPT 4-Dimethylaminotoluene TETRA Compound of formula (I), wherein x is 0, each R2 is 1,2-ethylenyl, each R3 is H, each R4 is 1,2-ethylenyl, each R5 is 1,2-ethylenyl, each R6 is 2- hydroxy-1,3-propylenyl, each R7 is methyl, and each z is 1. 15 Lowilite22 UV absorber (2-hydroxy-4-octoxybenzophenone.) Liquid Monomer Composition 1 Chemical Percentage (%) GCMA 44.0 EGDMA 9.0 Ethyl alcohol (denat.) 13.0 PC 20.0 DMPT 0.9 P-T olyldiethanolamine 0.9 TETRA 4.5 Dye / Optical brightener / Fragrance concentrate 6.7 Lowilite 22 1.0 100.0 Liquid Monomer Composition 2 Chemical Percentage (%) IPG MA 52.0 EGDMA 7.0 Ethyl alcohol (denat.) 12 PC 16 Glycerine 2.2 DMPT 0.9 P-T olyldiethanolamine 0.9 TETRA 2.0 Oracet 580 6.0 Lowilite 22 1.0 100.0 Liquid Monomer Composition 3 Chemical Percentage (%) I PG MA 50.0 EGDMA 9.0 Ethyl alcohol (denat.) 12.0 PC 16.0 DMPT 0.9 P-T olyldiethanolamine 0.9 TETRA 4.0 Crystal violet stain 0.2 Oracet 580 stain 6.0 Lowilite 22 1 100.0 Liquid Monomer Composition 4 Chemical Percentage (%) I PG MA 51.0 EGDMA 8.0 TEGDMA 4.0 Ethyl alcohol (denat.) 11.5 PC 16.5 DMPT 0.9 P-T olyldiethanolamine 0.9 Crystal violet stain 0.2 Oracet 580 stain 6.0 Lowilite 22 1.0 100.0 Liquid Monomer Composition 5 Chemical Percentage (%) I PG MA 55.0 EGDMA 7.0 Ethyl alcohol (denat.) 10.0 PC 17.0 DPM 2.0 DMPT 0.9 P-T olyldiethanolamine 0.9 Crystal violet stain 0.2 Oracet 580 stain 6.0 Lowilite 22 1.0 100.0

Claims

1. A liquid monomer composition comprising:(i) from 20 to 90 wt% of one or more monofunctional curable monomers selected from monofunctional (meth)acrylate and (meth)acrylamide monomers having a molecular weight in the range from 140 to 400;(ii) from 5 to 40 wt% of one or more alcohol solvents selected from C2 to C4 alkyl alcohols; and(iii) from 5 to 40 wt% of one or more carbonate solvents selected from C3-C11 alkylene carbonates and mixtures thereof.

2. A liquid monomer composition according to claim 1, comprising from 40 to 85 wt%, or from 45 to 88 wt%, or from 50 to 75 wt% of the one or more monofunctional curable monomers.

3. A liquid monomer composition according to claim 1 or claim 2, wherein the one or more monofunctional curable monomers have a molecular weight in the range from 140 to 400, or from 140 to 300, or from 140 to 250, or from 140 to 220, or from 160 to 220, or from 180 to 210.

4. A liquid monomer composition according to any preceding claim, wherein the one or more monofunctional curable monomers are selected from monofunctional methacrylate monomers, monofunctional methacrylamide monomers, or a mixture thereof; optionally wherein the curable monomers are selected from monofunctional methacrylate monomers.

5. A liquid monomer composition according to any preceding claim, wherein the one or more monofunctional curable monomers are selected from butyl (meth)acrylate, isobutyl (meth)acrylate, ethylene glycol methyl ether (meth)acrylate, ethylene glycol ethyl ether (meth)acrylate, di(ethylene glycol) methyl ether (meth)acrylate, di(ethylene glycol) ethyl ether (meth)acrylate, tri(ethylene glycol) methyl ether (meth)acrylate, tri(ethylene glycol) ethyl ether (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-norbomyl (meth)acrylate, isobornyl (meth)acrylate, 2-N-morpholinoethyl (meth)acrylate, isodecyl (meth)acrylate, glycerol carbonate (meth)acrylate, isopropylideneglycerol (meth)acrylate, (meth)acryloyl morpholine and mixtures thereof.

6. A liquid monomer composition according to claim 5, wherein the one or more monofunctional curable monomers are selected from glycerol carbonate (meth)acrylate, isopropylideneglycerol (meth)acrylate, (meth)acryloyl morpholine and mixtures thereof.

7. A liquid monomer composition according to claim 6, wherein the one or more monofunctional curable monomers are selected from glycerol carbonate methacrylate, isopropylideneglycerol methacrylate and mixtures thereof.

8. A liquid monomer composition according to any preceding claim, further comprising from 1 to 15 wt%, or from 2 to 12 wt%, or from 5 to 10 wt% of one or more multifunctional curable monomers, preferably wherein the one or more multifunctional curable monomers are selected from di(meth)acrylates, tri (methacrylates) and tetra(methacrylates) having a molecular weight in the range from 140 to 400.

9. A liquid monomer composition according to any preceding claim, comprising from 5 to 30 wt% of the one or more C2 to C4 alkyl alcohol solvents, or from 5 to 25 wt% of the one or more C2 to C4 alkyl alcohol solvents, or from 5 to 20 wt% of the one or more C2 to C4 alkyl alcohol solvents, or from 5 to 15 wt% of the one or more C2 to C4 alkyl alcohol solvents.

10. A liquid monomer composition according to any preceding claim, whereinthe one or more C2 to C4 alkyl alcohol solvents is selected from ethanol and isopropanol or a mixture thereof, or wherein the C2 to C4 alkyl alcohol solvent is ethanol.

11. A liquid monomer composition according to any preceding claim, comprising from 5 to 30 wt% of the one or more carbonate solvents, or from 5 to 25 wt% of the one or more carbonate solvents, or from 5 to 20 wt% of the one ormore carbonate solvents, or from 5 to 15 wt% of the one or more carbonate solvents.

12. A liquid monomer composition according to any preceding claim, wherein the one or more carbonate solvents are selected from ethylene carbonate 1,2-propylene carbonate, 1,2-butylene carbonate and mixtures thereof, optionally wherein the carbonate solvent is 1,2-propylene carbonate.

13. A liquid monomer composition according to any preceding claim, further comprising from 0.1 to 10 wt% of one or more of ethylene glycol, 1,2-propylene glycol, 1,2-butylene glycol, 1,4-butylene glycol, glycerol, pentaerythritol, diethyleneglycol monomethyl ether, dipropyleneglycol monomethyl ether, diethyleneglycol monoethyl ether and dipropyleneglycol monoethyl ether.

14. A liquid monomer composition according to any preceding claim, comprising from 0.1 to 3 wt% of one or more tertiary amine additives, optionally wherein the tertiary amine additive is selected from the group consisting of 4-dimethylaminotoluene, N-4-tolyldiethanolamine and mixtures thereof.

15. A liquid monomer composition according to any preceding claim, comprising a compound of the formula (I):R1xC{CH2OC(O)-R2-S-(CH2CHR3)-C(O)O-[R4-OC(O)-R5-C(O)O-]zR6-OC(O)CR7=^^(I)wherein:x is from 0 to 2 and x+y = 4z is from 0 to 2each R1 is independently H or a Ci to C4 alkyl groupeach R2 is independently 1,2-ethylenyl, 1,3-propylenyl, or 1,4-butylenyl;each R3 is independently H or methyl.each R4 and R5 is independently selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl;each R6 is independently a selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl, wherein R6 is optionally substituted by a hydroxyl group each R7 is independently H or methyl.

16. A liquid monomer composition according to claim 14, whereinx is 0 or 1, optionally x is 0;each R2 is 1,2-ethylenyl;each R3 is H;each R4 and each R5 is 1,2-ethylenyl;each R6 is 2-hydroxy-1,3-propylenyl;each R7 is methyl;each z is 1 or 2, optionally each z is 1.

17. A liquid monomer composition according to claim 15 or claim 16, comprising from 0.1 to 5 wt%, or from 0.5 to 4.5 wt%, or from 1 to 4 wt%, or from 1.5 to 3.5 wt% of the compound of formula (I), based on the total liquid monomer composition.

18. A kit of parts comprising:(i) a liquid monomer composition according to any preceding claim; and (ii) an acrylic polymer powder.

19. A kit of parts according to claim 18, wherein the acrylic polymer powder comprises a polymer or copolymer of methyl methacrylate (MMA) and / or ethyl methacrylate (EMA).

20. A kit of parts according to claim 18 or claim 19, wherein the acrylic polymer powder comprises a free-radical initiator on the surface of the polymer powder and / or embedded in the polymer powder, optionally wherein the free radical initiator is benzoyl peroxide.

21. A kit of parts according to any of claims 18 to 20, wherein the acrylic polymer powder has a particle size in the range from 20 to 250 pm.

22. A method of forming a polymeric structure on a substrate; the method comprising the steps of:(i) combining an acrylic polymer powder with a liquid monomer composition according to any of claims 1 to 17 to form a reactive dispersion;(ii) applying the reactive dispersion to a substrate and allowing it to cure.

23. A method according to claim 22, wherein the acrylic polymer powder comprises a polymer or copolymer of methyl methacrylate (MMA) and / or ethyl methacrylate (EMA).

24. A method according to claim 22 or claim 23, wherein the acrylic polymer powder comprises a free-radical initiator on the surface of the polymer powder and / or embedded in the polymer powder, optionally wherein the free radical initiator is benzoyl peroxide.

25. A method according to any of claims 22 to 24, wherein the acrylic polymer powder has a particle size in the range from 20 to 250 pm.

26. A method according to any of claims 22 to 25, wherein the method is a cosmetic method.

27. A method according to claim 26, wherein the method is a cosmetic method for forming a coating on a nail28. A method according to claim 27, wherein the nail is a natural human nail or an artificial nail.

29. A compound of the formula (I):R1xC{CH2OC(O)-R2-S-(CH2CHR3)-C(O)O-[R4-OC(O)-R5-C(O)O-]zR6-OC(O)CR7=CH2}y(I)wherein:x is from 0 to 2 and x+y = 4z is from 0 to 2each R1 is independently H or a Ci to C4 alkyl groupeach R2 is independently 1,2-ethylenyl, 1,3-propylenyl, or 1,4-butylenyl;each R3 is independently H or methyl.each R4 and R5 is independently selected from 1,2-ethylenyl, 1,2-5 propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and1,4-butylenyl;each R6 is independently a selected from 1,2-ethylenyl, 1,2-propylenyl, 1,3-propylenyl, 1-2, butylenyl, 2,3-butylenyl, 1,3-butylenyl and 1,4-butylenyl, wherein R6 is optionally substituted by a hydroxyl group10 each R7 is independently H or methyl.

30. A compound according to claim 29, whereinx is 0 or 1, optionally x is 0;each R2 is 1,2-ethylenyl;each R3 is H;15 each R4 and each R5 is 1,2-ethylenyl;each R6 is 2-hydroxy-1,3-propylenyl;each R7 is methyl;each z is 1 or 2, optionally each z is 1.