Photocurable optically clear adhesive formulations

Abstract

The invention provides photopolymerizable adhesive compositions for bonding or coating surfaces in optical devices. The clear and low viscosity compositions are based on monomeric acrylates mixed with polymeric siloxane urethane acrylates and, after photocuring, provide transparent and low refractive index adhesive layer with good mechanical properties.

Description

[0001] PHOTOCURABLE OPTICALLY CLEAR ADHESIVE FORMULATIONS

[0002] Field of the Invention

[0003] The present invention relates to the field of transparent and low refractive index photopolymerizable adhesives for optical components, devices and systems. More specifically, the invention relates to photocurable adhesive formulations based on siloxane-urethanes for bonding or coating surfaces in optical components, devices and systems.

[0004] Background of the Invention

[0005] Photocurable low refractive index coatings and adhesives, and more specifically photocurable liquid optically clear lamination adhesives and coatings, are key components of modern optical systems. Examples of applications include electronic displays, back-light units, thinner integrated displays, flexible displays, curved displays, optical touch screens, Virtual Reality and Augmented Reality head-sets, smart glasses, auto-stereoscopic 3-D displays, OLED displays, and nanotech (non ITO) transparent conductors. Further examples of bonding applications are bonding of lenses in eyewear / glasses, bonding of films in billboards, bonding and cladding between safety glass layers or bonding and cladding between a polymeric layer and a glass ply, bonding of optical elements (e.g. lenses, prisms, light-guides), bonding of photonic elements (e.g. bonding an optical fiber to a ferule), bonding of an optical fiber or a fiber array to a photonic or a silicon photonic chip, bonding of elements in a lighting system, bonding of films in a screen of a 3-D movie theatre, and bonding of bio-photonic elements. An important application of the low refractive index adhesive is its use as a cladding layer or a coating in optical fibers, and especially in high numerical aperture specialty optical fibers. Another important application is recoating splices, and use in fiber optic components, such as combiners, splitters, cladding light strippers, and FBG components.

[0006] Most low refractive index adhesives and coating materials have been based on fluoro polymers and monomers, named PFAS (perfluoroalkyl substances), but there is a concern about their use, because of their environmental endurance. As their use is becoming more and more restricted due to environmental regulations, there is an urgent need of alternative materials. It is therefore an object of this invention to provide PFAS-free adhesive formulations, for bonding or coating, exhibiting a sufficiently low refractive index.

[0007] It is another object of this invention to provide a siloxane-urethane based low-refractive index adhesive exhibiting a sufficient bonding strength.

[0008] It is also an object of this invention to provide an optically clear photocurable coating formulation based on siloxane-urethanes, exhibiting a high adhesive strength.

[0009] It is a further object of this invention to provide an optically clear photopolymerizable adhesive and coating formulation based on siloxane-urethanes, exhibiting a sufficiently low working viscosity.

[0010] It is a still further object of the invention to provide a photopolymerizable adhesive formulation based on siloxane-urethanes, exhibiting high lamination strength for a number of different substrates, such as glass, quartz, silicon, polycarbonate, PET, PMMA, or polypropylene.

[0011] It is still another object of the invention to provide a photopolymerizable coating formulation based on siloxane-urethanes, exhibiting suitable adhesive strength and mechanical properties, for a number of different substrates, such as glass, quartz, silicon, polycarbonate, PET, PMMA, or polypropylene.

[0012] The invention aims at providing a process for producing a clear photocurable composition based on siloxane-urethane, exhibiting a high adhesive strength, a low viscosity, and a low refractive index.

[0013] The invention further aims at providing a clear photocurable formulation, for bonding or coating, that is ready-to-use as is, without the need for mixing two components before application. Other objects and advantages of the present invention will appear as the description proceeds.

[0014] Summary of the Invention

[0015] This invention provides an optically clear photopolymerizable adhesive composition comprising i) polysiloxane urethane comprising (meth)acrylate moieties; ii) at least one (meth)acrylate monomer; and iii) at least one free radical initiator. Said polysiloxane urethane comprising (meth)acrylate moieties (also called acrylated polysiloxane urethane), preferably being polysiloxane urethane comprising two terminal acrylate moieties, comprises polysiloxane blocks of formula

[0016] -O-R7-(Si(Me2)O)m-R7-O- where Me is methyl, R7is a bivalent radical comprising C1-C9 alkylene or alkylene-alkoxy, and m is an integer from 2 to 100, for example from 5 to 80. The adhesive composition usually comprises a number of different blocks of said structure differing either in R7or in m or in both; a sample of the adhesive composition according to the invention comprises a number of molecules differing in the number and the type of said blocks; one molecule may comprise more than one type of block. The term "(meth)acrylate" is used in cases where both acrylates and methacrylates are to be included or considered.

[0017] Said bivalent radicals may comprise for example alkyl, alkoxy, or alkyl-alkoxy, or alkoxyalkyl, such as for example (CH2)3, (CH2)3-O-(CH2)2, (CH2)2-O-(CH2)3, (CH2)3-O- C(CH2CH3)(CH2OH)2. Said polysiloxane blocks are interspersed between urethane moieties -U- of formula

[0018] -C(=O)NH-R3-NH(C=O)- wherein R3 is a bivalent radical selected from C4-C14 aliphatic, alicyclic, or aromatic hydrocarbon moieties. In some embodiments said bivalent radical R3 is selected from 5- yl-l-ylmethyl-l,3,3-trimethylcyclohexane, 1,6-hexanediyl, 1,4-butanediyl, 1,3- bis(methyene)cyclohexane, dicyclohexylmethane-4,4'-diyl, l,3-bis(2-yl-2-propyl)- benzene, 2,2,4-trimethylhexane diyl, and 2,4,4-trimethylhexanediyl.

[0019] The acrylated polysiloxane urethane in the adhesive composition of the invention comprises polysiloxane blocks of the above structure separated by said urethane moieties of the above structure, the number of said blocks in the molecules of said acrylated polysiloxane urethane being in average from 1 to 40, such as in some embodiments of the invention from 3 to 30 or from 5 to 10; in other embodiments, the number of the blocks is in average from 1 to 3, for example 2 or less, or in some embodiments about 1. When relating to "average" values, weighted arithmetic mean is meant, as the sample usually comprises a set of molecular weights of a certain distribution.

[0020] The molecules of said acrylated polysiloxane urethane are terminated or capped by an acrylate moiety R1 of formula

[0021] CH2=C(R2)-C(=O)-O-R4-O- wherein R2 is selected from H and CH3, and R4 is selected from C2-C14 hydrocarbon bivalent radicals, optionally further comprising up to 4 oxygen atoms. Said R4 is usually C2-C14 aliphatic or alicyclic hydrocarbon; alternatively, R4 may comprise additional acrylate moieties. Said R4 is in some preferred embodiments selected from ethyl acrylate 2-yl, propyl acrylate 2-yl, propyl acrylate 3-yl, butyl acrylate 2-yl, butyl acrylate

[0022] 3-yl, butyl acrylate 4-yl, ethyl methacrylate 2-yl, propyl methacrylate 2-yl, propyl methacrylate 3-yl, butyl methacrylate 2-yl, butyl methacrylate 3-yl, butyl methacrylate

[0023] 4-yl, l-acryloyloxy-3-yl adamantane, l-methacryloyloxy-3-yl adamantane, 2-

[0024] ((acryloyloxy)methyl)-l,3-diyl, and diacrylate-2-(methylene)propane-2-yl.

[0025] Said acrylate monomer in a photopolymerizable adhesive composition of the invention is selected from acrylates of formula

[0026] CH2=C(R5)-C(=O)-O-R6 wherein R5 is selected from H or C1-C5 hydrocarbon moiety optionally comprising one oxygen atom, R5 being in some preferred embodiments H or CH3, and wherein R6 is selected from C1-C14 hydrocarbon moieties comprising up to three oxygen atoms and up to one nitrogen atom. In some embodiments of the invention, R6 is selected from alkyl, alkyleneoxy, cycloalkyl, or heterocycloalkyl. In some preferred embodiments of the photopolymerizable composition according to the invention, said (meth)acrylate monomer is selected from isobornyl acrylate, methyl 2-[(allyloxy)methyl]acrylate, 3- ethyl-3-oxetanylmethyl acrylate, (2-ethyl-2-methyl-l,3-dioxolane-4-yl)-methyl acrylate, 2-morpholinoethyl acrylate, 4-tert-butylcyclohexyl acrylate, hexyl acrylate, tetrahydrofurfuryl acrylate, isobutyl acrylate, tert-butyl acrylate, cyclohexyl acrylate, hexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl-acrylate, decyl acrylate, isodecyl acrylate, dicyclopentanyl acrylate, dodecyl acrylate, adamantan-l-yl acrylate, 2-methyladamantan-l-yl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, 3,3,5- trimethylcyclohexylacrylate, and 2[[(butylamino)carbonyl]oxy]ethyl acrylate, and their corresponding methacrylate analogues.

[0027] Said free radical initiator in the photopolymerizable coating or adhesive composition of the invention may be a photoinitiator active for a wavelength above 250 nm, for example above 300 nm, selected from the group consisting of 1- hydroxycyclohexyphenyl ketone, 2-hydroxy-2-methyl propiophenone, methylbenzoyl formate, diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide, phenyl bis(2,4,6- trimethylbenzoyl)phosphine oxide, and benzyl dimethyl ketal, or it may be a thermal initiator selected from the group consisting of benzoyl peroxide, tert-butylperoxide, cumene hydroperoxide, tert-butyl peroxybenzoate, cyclohexanone peroxide, tert-butyl hydroperoxide, and 2,2'-azobisisobutyronitrile.

[0028] In a preferred embodiment, the invention provides a photopolymerizable adhesive composition comprising, beside at least one (meth)acrylate monomer and at least one free radical initiator, at least one (meth)acrylated polysiloxane urethane of Formula 1 Rl-U-{O-R7-(Si(Me2)O)m-R7-O-U}p-Rl where U is a urethane moiety having formula

[0029] -C(=O)NH-R3-NH(C=O)- as defined above, R1 is an acrylate moiety of formula CH2=C(R2)-C(=O)-O-R4-O- as defined above, Me is methyl, R7is a bivalent radical selected from C1-C9 alkylene or alkylene-alkoxy, m is selected from integers between 2 and 100, such as from 5 to 80, and p is an integer of from 1 to 40. In some embodiments of the invention, p is preferably from 3 to 30, such as from 5 to 10; in other embodiments of the invention, p is preferably 1 to 3, for example 2 or less, or in some embodiments about 1. As samples of the photopolymerizable adhesive compositions according to the invention comprise molecules differing in the values of parameters m and p, the mentioned preferred values relate to average values. If two types of siloxane blocks are present in one molecule, let us say A and B, the siloxane blocks will be interspersed randomly in the molecule of the (meth)acrylated polysiloxane urethane, and various random combinations of A and B will be present among the molecules of the sample. Thus the acrylated polysiloxane urethane of the photopolymerizable composition according to the invention may comprise in its molecules a number of blocks having the structure -O-R7-(Si(Me2)O)m-R7-O-U- and differing in R7 or m or both. One molecule of the composition may comprise two different blocks.

[0030] The photopolymerizable composition according to the invention exhibits a viscosity of up to 30 000 cP, such as up 25 000 cP, for example up to 20 000 or up to 15 000 or up to 10 000. The photopolymerizable composition according to the invention exhibits a refractive index of up to 1.49 at 589 nm and 25°C. In a preferred embodiment, the curable adhesive composition of the invention exhibits a viscosity of up to 10 000 cP. The transparent and low refractive index, photopolymerizable adhesive composition of the invention exhibits in some preferred embodiments a refractive index of up to 1.460 at 589 nm and 25°C, such as up to 1.450, or up to 1.440, or up to 1.430, or up to 1.420 at 589 nm and 25°C. In some preferred embodiments, the photopolymerizable adhesive composition of the invention has a refractive index 1.43 or less, such as 1.42 or less, for example about 1.41 at 589 nm and 25°C.

[0031] The invention provides an optically clear low-refractive index photocurable adhesive or adhesive formulation for use in optical systems. The bonding or coating formulation, after photocuring, exhibits desired mechanical properties. In some embodiments, the adhesive formulations after photocuring exhibit an elastic modulus of from 0.5 to 1000, such as from 2 to 1000, for example from 2 to 500, such as from 2 to 250 N / mm2, for example from 5 to 200 N / mm2, for example from 5 to lOOand a tensile strength of 0.2 to 25 N / mm2, for example from 1 to 10 N / mm2. In a particularly preferred embodiment, the invention provides an optically clear photopolymerizable composition for use as a bonding agent, or a coating agent, or cladding agent, for surfaces in optical systems. The invention provides an optically clear photopolymerizable composition exhibiting a low refraction index and a low working viscosity, for use in bonding or coating surfaces in optical components, devices and systems.

[0032] In a preferred embodiment, the invention relates to a photopolymerizable bonding and coating composition which, after photocuring, provides an optically clear layer exhibiting a low refractive index and suitable mechanical properties.

[0033] The invention relates to a method of preparing an optically clear and low-refractive index, photopolymerizable adhesive composition for bonding, coating or cladding surfaces in optical components, devices and systems, comprising steps of i) providing components a) to g): a) at least one polysiloxane comprising terminal-hydroxyl groups of formula

[0034] HO-R7-(Si(Me2)O)m-R7-OH where Me is methyl, R7is a bivalent radical comprising C1-C9 alkylene or alkylene-alkoxy, and m is an integer from 2 to 100, such as from 4 to 90 or from 5 to 80 or from 6 to 70; b) at least one hydroxy(meth)acrylate, preferably selected from hydroxyalkylacrylates and hydroxyalkyl(meth)acrylates, wherein the molar ratio of the hydroxyl groups provided by said hydroxyl terminated polysiloxane and said hydroxy(meth)acrylate is less than 40, said hydroxy(meth)acrylate preferably having formula

[0035] CH2=C(R2)-C(=O)-O-R4-OH wherein R2 is selected from H and CH3, and R4 is selected from C2-Ci4 hydrocarbons optionally further comprising up to 4 oxygen atoms; c) at least one polyisocyanate, such as diisocyanate of formula

[0036] O=C=N-R3-N=C=O wherein R3 is a bivalent radical selected from C4-C14 aliphatic, alicyclic, or aromatic hydrocarbon moieties, and -N=C=O is isocyanate group; d) at least one polymerization catalyst for urethane formation; e) at least one (meth)acrylate monomer without hydroxyl function; f) at least one free radical initiator, for example a photoinitiator active for a wavelength above 250 nm; and optionally g) additives selected from solvents, chain extenders, crosslinkers, adhesion promoters, additional (meth)acrylates, additional siloxanes, additional catalysts or initiators; and ii) allowing said isocyanate groups to react with said hydroxyl groups provided by hydroxyl terminated polysiloxane and hydroxyalkyl(meth)acrylates until the urethane- forming reaction is complete, at a temperature of from 20°C to 120°C, such as from 40°C to 70°C, while optionally externally heating the mixture if the reaction heat does not suffice; thereby obtaining a photocurable adhesive formulation exhibiting suitable optical properties, mechanical properties, and viscosity. In a preferred embodiment, the urethane-forming reaction and the consumption of the isocyanate being advantageously monitored by FTIR.

[0037] Said (meth)acrylate monomer usually comprises from 1 to 50% (w / w) of said polysiloxane urethane composition, such as from 2 to 40% or from 3 to 30% or from 4 to 20%. In some preferred embodiments, said monomer comprises from 1 to 25% (w / w) of said polysiloxane urethane composition, such as from 2 to 20%. In some embodiments, said component e) comprises 1-5% (w / w) of said component a); in other embodiments said component e) comprises 4-10% or 8-15% or 12-25% or 20-35% or 30- 50% (w / w) of said component a).

[0038] In one embodiment, the invention provides a method of preparing an optically clear and low-refractive index, photopolymerizable adhesive composition for bonding surfaces in optical components, devices and systems, wherein the molar ratio of the hydroxyl groups provided by said hydroxyl terminated polysiloxane and said hydroxy(meth)acrylate is from 3 to 30, for example from 6 to 10. Said step ii) may comprise admixing said components a), c), and d) in the reaction mixture simultaneously; said component b) being admixed either simultaneously or with a time delay. The remaining components may be added to the formulation in any order at any stage.

[0039] In another preferred embodiment, the invention provides a method of preparing an optically clear and low-refractive index, photopolymerizable composition for coating surfaces in optical components, devices and systems, wherein said molar ratio of the hydroxyl groups provided by said hydroxyl terminated polysiloxane and said hydroxy(meth)acrylate is 3 or less, usually 2 or less, for example 1, and wherein said step ii) comprises admixing said components a), c), and d) in the reaction mixture simultaneously, while said component b) is preferably admixed with a time delay. Said delay allows the isocyanate to preferably react with the polysiloxane diol, and the extent of said delay is determined in accordance with the employed molar ratios of the components and with the monitoring FTIR data. The remaining components may be added to the formulation in any order at any stage.

[0040] In some embodiments of the method of making the adhesive formulation according to the invention, said hydroxy(meth)acrylate is selected from 2-hydroxyethyl acrylate, 2- hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 3- hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2- hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutyl methacrylate, 1- acryloyloxy-3-hydroxyadamantane, l-methacryloyloxy-3-hydroxyadamantane, 2- hydroxy-3-phenoxypropyl acrylate, and pentaerythritol triacrylate. In a preferred embodiment of the invention, said hydroxy(meth)acrylate is hydroxyethyl acrylate. In another preferred embodiment, said hydroxy(meth)acrylate is 2-hydroxypropyl methacrylate. In another preferred embodiment, said hydroxy(meth)acrylate is 3- hydroxypropyl methacrylate. In another preferred embodiment, said hydroxy(meth)acrylate is a mixture of 2-hydroxypropyl methacrylate and 3- hydroxypropyl methacrylate.

[0041] In some embodiments of the method according to the invention, said diisocyanate is selected from isophorone diisocyanate, 1,6-hexanediisocyanate, norbornane-2,5- diisocyanate, 1,4-butanediisocyanate, l,3-bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane-4, 4' -diisocyanate, l,3-bis(2-isocyanato-2-propyl)benzene, 2,2,4- trimethylhexamethylene diisocyanate, tetramethyl-m-xylylene diisocyanate, and 2,4,4- trimethylhexamethylene diisocyanate. In a preferred embodiment of the invention, said diisocyanate is selected from hexamethylene diisocyanate or isophorone diisocyanate. Said additives may comprise, among others, adhesion promotors, including silane (meth)acrylates, such as (meth)acrylated trialkoxysilanes, for example (3- trimethoxysilyl)-propyl) acrylate. The additives may comprise, for example, 3- (triethoxysilyl)-propyl methacrylate (methacryloxypropyl trimethoxysilane, MAOTS), 3- (triisopropoxysilyl)-propyl methacrylate; 3-(tripropoxysilyl)-propyl methacrylate; 3- (trimethoxysilyl)-propyl acrylate; 3-(triethoxysilyl)-propyl acrylate; 3-(triisopropoxysilyl)- propyl acrylate; 3-(tripropoxysilyl)-propyl acrylate. The additives may comprise silanes such as Silmer TMS Di-10, Di-50, Di-400, and Silmer TMS C50.

[0042] In some preferred embodiments of the method of preparing the adhesive composition according to the invention, said acrylate monomer without hydroxyl function is selected from isobornyl acrylate, methyl 2-[(allyloxy)methyl]acrylate, 3-ethyl-3-oxetanylmethyl acrylate, 3-ethyl-3-oxetanylmethyl methacrylate, (2-ethyl-2-methyl-l,3-dioxolane-4-yl)- methyl acrylate, 2-morpholinoethyl acrylate, 4-tert-butylcyclohexyl acrylate, trimethylcyclohexyl acrylate, hexyl acrylate, tetra hydrofurfury I acrylate, isobutyl acrylate, tertbutyl acrylate, cyclohexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl-acrylate, decyl acrylate, isodecyl acrylate, lauryl acrylate, dicyclopentanyl acrylate, dodecyl acrylate, adamantan-l-yl acrylate, and 2-methyladamantan-l-yl acrylate. In a preferred embodiment of the invention, said acrylate monomer without hydroxyl function is selected from methyl 2-[(allyloxy)methyl] acrylate, isobornyl acrylate, isobornyl methacrylate, tert-butyl acrylate, or a mixture thereof.

[0043] The invention provides a method of bonding together two surfaces in an optical system, the method comprising i) applying to at least one of the surfaces at least one adhesive composition as described above, the composition serving as a photocurable optically clear adhesive; ii) assembling the surfaces to be bonded; and iii) irradiating the assembled surfaces with a sufficient dose of irradiation of a wavelength above 250 nm to ensure full photopolymerization of the applied adhesive composition; thereby obtaining two surfaces strongly bonded with an optically clear adhesive, the adhesion exhibiting the peel strength of at least 50 g / cm, such as at least 100 g / cm, for example at least 200 g / cm or at least 400 g / cm, when measured as a 90° peel adhesion on glass. The invention further provides a method of coating a surface in an optical system, the method comprising i) applying to said surface at least one adhesive composition as described above, serving as a photocurable optically clear adhesive; and ii) irradiating the surface with a sufficient dose of irradiation of a wavelength above 250 nm to ensure full photopolymerization of the applied composition; thereby obtaining a surface coated with an optically clear, low-refractive, and mechanically suitable layer. The adhesion exhibits the peel strength of at least 40 g / cm, such as at least 100 g / cm, for example at least 200 g / cm, when measured as a 90° peel adhesion on glass.

[0044] Detailed Description of the Invention

[0045] The explosive development of optoelectronics, including for example displays and optical fibers, has resulted in demands for new engineering materials for bonding, coating and cladding, exhibiting suitable optical and mechanical properties, such as including for example transparent and low-refractive index adhesives. In order to achieve a lower refractive index in adhesive formulations, polyurethanes based on dihydroxyalkyl-siloxanes have been considered by the present inventors. The disclosed ready-to-use photocurable formulations comprise a special mixture of reactants, combining suitable acrylated monomers and acrylated polymers, providing a surprising combination of low refractive index, low working viscosity, high adhesive strength and desired mechanical properties, usable for all bonding and coating applications in modern optoelectronics.

[0046] It has been found that reacting a polysiloxane diol in a reaction mixture with diisocyanate, hydroxy(meth)acrylate and a polymerization catalyst for urethane formation and mixing the reaction products with suitable acrylate monomers and photoinitiators provides a photocurable adhesive formulation exhibiting an excellent working viscosity, transparency, refractive index, and adhesive strength. Moreover, the formulation of the invention is such that it is ready-to-use as is, without the requirement for mixing two parts prior to their use, in contrast to many known adhesives; the formulations consist of 100% active components and are not solvent based. In a preferred embodiment, the invention provides an optically clear and low-refractive index, photopolymerizable bonding composition for use in optical devices and systems, by reacting a polysiloxane diol, a diisocyanate, a hydroxy(meth)acrylate, and a polymerization catalyst, wherein the molar ratio of the hydroxyl groups provided by said diol and by said hydroxy(meth)acrylate is from 0.5 to 50, usually from 1 to 25.

[0047] In some preferred embodiments, the invention provides an optically clear and low- refractive index, photopolymerizable composition for bonding surfaces in optical devices and systems, by reacting a polysiloxane diol, a diisocyanate, a hydroxy(meth)acrylate, and a polymerization catalyst, wherein the molar ratio of the hydroxyl groups provided by said diol and by said hydroxy(meth)acrylate is from 1 to 25, usually from 2 to 14, such as from 5 to 10.

[0048] In some preferred embodiments, the invention provides an optically clear and low- refractive index, photopolymerizable composition for bonding surfaces in optical devices and systems, by reacting a polysiloxane diol, a diisocyanate, a hydroxy(meth)acrylate, and a polymerization catalyst, wherein the molar ratio of the hydroxyl groups provided by said diol and by said hydroxy(meth)acrylate is from 1 to 4, usually from 1 to 3, such as from 1 to 2.

[0049] In other preferred embodiments, the invention provides an optically clear and low- refractive index, photopolymerizable composition for coating or cladding in optical devices and systems, by reacting a polysiloxane diol, a diisocyanate, a hydroxy(meth)acrylate, and a polymerization catalyst, wherein the molar ratio of the hydroxyl groups provided by said diol and by said hydroxy(meth)acrylate is from about 1 to about 3, usually from 1 to 2.

[0050] In one embodiment of the invention, a polysiloxane diol and hydroxyethyl acrylate were mixed in such a ratio that the hydroxyl groups provided by said diol and by said acrylate were in a ratio between 6 and 9.5, and were reacted with hexamethylene diisocyanate, providing acrylated polysiloxane-urethane which, in mixture with monomeric acrylates and a photoinitiator, formed a superior photopolymerizable bonding composition. The clear composition exhibited a viscosity between 7000 and 10 000 cP and a refraction index between 1.417 and 1.419. The adhesion after photocuring, measured as Peel 90° was about 200 g / cm, and the refraction index after photocuring was about 1.423 (see, examples 8 and 19 in Table 1).

[0051] In one preferred embodiment, the invention provides an optically clear and low- refractive index, photopolymerizable composition for coating surfaces in optical components, devices and systems, by reacting a polysiloxane diol with diisocyanate and a polymerization catalyst in a first stage, while adding hydroxy(meth)acrylate with a delay in a second stage, the ratio of hydroxyl provided by the diol and by the hydroxy(meth)acrylate being less than 3, usually 2 or less. Said delay allows isocyanate to preferably react with the polysiloxane diol, and the extent of said delay is determined by a skilled person in accordance with the employed molar ratios of the components and with the monitoring FTIR data. The delay may be for example from 15 minutes to 120 minutes, such as from 30 to 60.

[0052] The formulations of the invention exhibit high lamination strength for a number of different substrates such as glass, quartz, silicon, silicon oxide, indium-tin oxide, polycarbonate, PET, PMMA, polypropylene, and others.

[0053] In some embodiments of the invention, a small amount of branching agent is added into the reaction mixture with said polysiloxane diol. The branching agent might be a triisocyanate, such as biuret or isocyanurate trimers, or a triol such as trimethylolpropane or carbinol derivatives of polydimethylsiloxane (functionality of >2). In some embodiments of the invention, the branching possibility resides in the used monomer acrylates, for example when using trimethylolpropane-triacrylate or pentaerythritol tetraacrylate.

[0054] In some embodiments of the invention, a small amount of a chain extender may be added into the reaction mixture to increase the size of the polymer, resulting in better physicochemical properties of the composition such as elastic modulus and tensile strength. Non-limiting examples of typical chain extenders are small aliphatic or cyclo- aliphatic diols such as 1,2-ethanediol, 1,4-butanediol, 1,6-hexanediol; 1,4- cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 4,8-bis(hydroxymethyl)- tricyclo[5,2,l,02'6]-decane (all isomers). In a preferred embodiment, 1,4-butanediol is employed. Typical amounts of chain extender are in the range of 0.1% to 10% of the molar quantity of polysiloxane diol hydroxyl groups. In some embodiments the amount of chain extender ranges from 0.5% to 5%. In a preferred embodiment the amount of chain extender ranges from 1% to 3%.

[0055] In some embodiments of the invention, compounds containing two or more (meth)acrylate groups may be added to the formulations. These compounds may increase the elastic modulus and tensile strength of the formulations through the formation of cross-linked structures upon polymerization. Non-limiting examples of crosslinking compounds include 1,6-hexanediol diacrylate, 1,4-butanediol diacrylate, ethyleneglycol-dimethacrylate, 1,6-hexanediol dimethacylate, 1,4-butanediol dimethacrylate, tris(2-hydroxyethyl) isocyanurate triacrylate, trimethylolpropane triacrylate, and diacrylate of a bis(hydroxyalkyl)-polysiloxane. In a preferred embodiment, diacrylate of a bis(hydroxyalkyl)-polysiloxane (such as X-22-2445) is employed.

[0056] The terms optically clear, clear, and transparent describe a transmittance for visible and infrared light of at least 90%. The term "low refractive index" as used herein refers to a value of up to 1.48, preferably up to 1.47, such as up to 1.46, or up to 1.45, or up to 1.44 or up to 1.43, which is relatively low when compared to most polymers, that exhibit values of 1.50 to 1.70. In some embodiments, the term "low refractive index" may refer to a value of up to 1.427 or up to 1.426 or up to 1.425. The term "photopolymerizable" or "photocurable" as used herein refers to a material able to be polymerized under influence of radiation of a wavelength above 250 nm, usually between 300 and 420 nm. The term "(meth)acrylate" is used in cases where both acrylates and methacrylates are to be included or considered.

[0057] In a preferred embodiment of the invention, a polyisocyanate such as diisocyanate and a polymerization catalyst for urethane formation are reacted with a bis(hydroxyalkylene)polysiloxane and a hydroxy(meth)acrylate, until the urethane- forming reaction is essentially complete, said bis(hydroxyalkylene)polysiloxane comprising for example dihydroxyalkylene or dihydroxy-alkylene-alkoxy polysiloxane, such as for example dihydroxypropylene polydimethylsiloxane, said catalyst being for example an organo-tin compound such as dibutyltin diacetate, dibutyltin dilaurate, dibutyltin oxide, dibutyltin dioctanoate, dibutyltin bis-acetylacetone, dibutyltin dibenzoate, and said hydroxy (meth)acrylate may be selected for example from 2- hydroxy-ethyl-acrylate, 2-hydroxy-ethyl-methacrylate, 2-hydroxy-propyl-acrylate, 2- hydroxy-propyl-methacrylate, 2-hydroxy-butyl-acrylate, 3-hydroxy-butyl-acrylate, 4- hydroxy-butyl-acrylate, and pentaerythritol triacrylate. Said hydroxy (meth)acrylate may comprise a mixture of hydroxy(meth)acrylates. The mixture of the above components including dihydroxy polysiloxane, polyisocyanate, polymerization catalyst and hydroxy (meth)acrylate is combined, before or after said reaction, with at least one acrylate monomer and a photoinitiator. Preferred photoinitiators are active in the UV range of 250-400 nm and may include 1-hydroxycyclohexyphenyl ketone, 2-hydroxy-2-methyl propiophenone, methylbenzoyl formate, diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide, phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide, or benzyl dimethyl ketal. Additional photoinitiators can be found in the relevant literature, for example in "Industrial Photoinitiators" by W.A. Green. Besides tin compounds, additional nonlimiting examples of metal catalysts for the urethane formation include metalcontaining complexes like bis-acetylacetone complexes, wherein the metal is selected from the group comprising titanium, aluminum, bismuth, zinc, zirconium, antimony, manganese, cobalt, iron. Nonlimiting examples of metal-free catalysts for urethane synthesis are commercially available tertiary nitrogen-bases such as TBD, DABCO, DMAP and DBU.

[0058] In some embodiments, a blend of two photoinitiators with different responses to different wavelengths is used. This is advantageous when a semi-final product is coated on one type of a sheet substrate and then covered with a release liner to be used for lamination at another place and another time and with a different UV wavelength. The flow of the viscous non-polymerized adhesive may create a problem which can be solved by including a small level of a photoinitiator that is activated at one wavelength, e.g., above 350 nm, but its low level prevents the ability of the polymer to fully crosslink and thereby allows removal of the release liner and lamination to another substrate. The low level of cross-linking will allow proper flow and wetting of the second substrate. A second photoinitiator which can be activated at a different wavelength, e.g., only below 350 nm, is introduced at a level to fully crosslink the laminate in a second irradiation process by a UV source emitting at the second wavelength, e.g. within 250- 350 nm. Other irradiation-based processes for accomplishing polymerization of the formulations are two-photon polymerization and electron-beam polymerization.

[0059] Free radical thermal initiators include compounds such as benzoyl peroxide, tertbutylperoxide, cumene hydroperoxide, tert-butyl peroxybenzoate, cyclohexanone peroxide, tertbutyl hydroperoxide, 2,2'-azobisisobutyronitrile and the like.

[0060] Reacting said mixture of the components is preferably carried out at a temperature of from about 20°C to about 120°C, and more preferably from 40°C to about 70°C, resulting in the formation of polysiloxane urethanes comprising acrylate moieties. In some embodiments, the reactants are mixed at 20-25°C and the temperature of the reaction mixture will rise without the need for external heating due to the exothermic nature of the urethane-forming reaction. At least one photoinitiator active for a wavelength above 250 nm, and at least one acrylate monomer without a hydroxyl group are added either to said components before said reaction or to the reaction mixture after the urethane-forming reaction is complete, resulting in a photocurable adhesive composition of low viscosity and low refractive index, the composition exhibiting excellent coating properties and bonding strength, as well as desired mechanical properties. When surfaces are to be bonded or coated in the manufacture of optical systems, the polymerization reaction is initiated by light of a wavelength preferably from 300 nm to 420 nm and comprises both the acrylate moieties of said acrylated polysiloxane urethanes and of said acrylate monomers in the coating or adhesive composition of the invention.

[0061] The invention thus provides a clear and low refractive index adhesive composition for optical systems and a method for producing it. In some embodiments of the invention, said bis(hydroxyalkylene)polysiloxane, for example selected from bis(hydroxypropyl) polysiloxanes, has an average molecular weight from about 200 to about 10 000, such as from 1500 to 6000, or from 400-5500. Said bis(hydroxyalkylene)polysiloxane may be selected, for example, from bis(hydroxyalkyl)polysiloxanes and bis(hydroxyalkylalkoxy) polysiloxanes.

[0062] Polyisocyanate may be selected from isophorone diisocyanate, 1,6-hexanediisocyanate, 1,4-butanediisocyanate, l,3-bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane- 4, 4' -diisocyanate, l,3-bis(2-isocyanato-2-propyl)benzene, 2,2,4-trimethylhexa methylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, tetramethyl-m- xylylene diisocyanate, and mixtures thereof.

[0063] Acrylate or methacrylate monomers, that are comprised in the adhesive composition of the invention together with said photoinitiators, may be selected from isobornyl acrylate, methyl 2-[(allyloxy)-methyl]acrylate, 3-ethyl-3-oxetanylmethyl acrylate, 3- ethyl-3-oxetanylmethyl methacrylate, (2-ethyl-2-methyl-l,3-dioxolane-4-yl)-methyl acrylate, 2-morpholinoethyl acrylate, 4-tert-butylcyclohexyl acrylate, trimethylcyclohexyl acrylate, hexyl acrylate, tetrahydrofurfuryl acrylate, isobutyl acrylate, tertbutyl acrylate, cyclohexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl acrylate, decyl acrylate, isodecyl acrylate, lauryl acrylate, dicyclopentanyl acrylate, dodecyl acrylate, adamantan-l-yl acrylate, 2-methyladamantan-l-yl acrylate, and their corresponding methacrylates, or a mixture of two or more (meth)acrylates. If said (meth)acrylate monomers are added after the formation of the polysiloxane urethanes derivatized with acrylate moieties, said monomers may be selected also from (meth)acrylates with a hydroxyl group. Where isomeric structures such as regioisomers, stereo-isomers, diastereomers, optical isomers are possible, the invention comprises both pure isomers and mixtures of isomers. Said acrylate or methacrylate monomers comprise from 1 to 50% (w / w) of the composition, in some embodiments from 1 to 5%, in other embodiments from 4 to 10%, or from 8 to 15% or from 12 to 25% or from 20 to 35% or from 30 to 50%.

[0064] The clear and low refractive index adhesive composition of the invention, for bonding or coating or cladding, comprises acrylated polysiloxane urethanes obtained by reacting bis(hydroxyalkylene)siloxanes with polyisocyanates and hydroxy (meth)acrylates, wherein said hydroxy (meth)acrylates may be selected from 2-hydroxyethyl acrylate, 2- hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 3- hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2- hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutyl methacrylate, 1- acryloyloxy-3-hydroxyadamantane, l-methacryloyloxy-3-hydroxyadamantane, or a mixture comprising some of the mentioned hydroxylated (meth)acrylates. Where isomeric structures such as regio-isomers, stereo-isomers, diastereomers, optical isomers are possible, the invention comprises both pure isomers and mixtures of isomers.

[0065] It was further found that the molar ratio of hydroxyl groups incorporated in the mixture by said bis(hydroxyalkylene)siloxane and by said hydroxy (meth)acrylates has the principal effect on the adhesive and coating properties. The higher the ratio, the longer the chain length of the acrylated polysiloxane polyurethane formed in the reaction with isocyanates, being reflected by increasing value of parameter p in Formula 1. The longer the chain length, the higher the viscosity of the photopolymerizable composition. The system may be optimized according to the intended application. The molar hydroxyl ratio of less than 3, such as 2 or less, provides the most preferred coating properties. It was also found that addition of suitable free (meth)acrylate(s) to the polyurethane formulation can significantly enhance the adhesive performance of the resulting formulation.

[0066] The invention thus provides a method for preparing a photocurable transparent and low-refractive index adhesive composition, exhibiting strong adhesion and desired mechanical properties, comprising the step of reacting bis(hydroxyalkylene)polysiloxane with a diisocyanate and a hydroxy(meth)acrylate, wherein the ratio of the hydroxyl groups incorporated in the reaction by said polysiloxane and by said hydroxy(meth)acrylate is from 0.5 to 50 such as from 1 to 25 or from 1 to 14 or from 1 to 10 or from 1 to 3 or from 1 to 2, or from 2 to 14 such as from 4 to 11 or from 5 to 10 or from 5 to 9 or from 5 to 8 or from 6 to 9 or from 6 to 8 or from 5 to 7. In a preferred embodiment, the invention provides a method for preparing a photocurable transparent and low-refractive index composition for bonding surfaces in optical systems comprising the step of reacting bis(hydroxyalkylene)polysiloxane with a diisocyanate and a hydroxy(meth)acrylate, wherein the ratio of the hydroxyl groups incorporated in the reaction by said polysiloxane and by said hydroxy(meth)acrylate is in average usually from 2 to 14, such as from 5 to 10.

[0067] In another preferred embodiment, the invention provides a method for preparing a photocurable transparent and low-refractive index composition for coating or cladding applications in optoelectronics comprising the step of reacting bis(hydroxyalkylene)polysiloxane with a diisocyanate and a hydroxy(meth)acrylate, wherein the ratio of the hydroxyl groups incorporated in the reaction by said polysiloxane and by said hydroxy(meth)acrylate is in average 2 or less.

[0068] The optically clear and low refractive index adhesive composition of the invention, comprising acrylated polysiloxane urethanes, and acrylate monomer(s) with photoinitiator(s), exhibits a viscosity of from 500 to 35 000 cP, usually from 1000 to 30 000 cP, such as from 5000 to 15 000 cP or from 1000 to 20 000 cP or from 1000 to 10 000 cP or from about 1000 cP to about 4000 cP.

[0069] In some preferred embodiments, the photopolymerizable and optically clear, low refractive index, adhesive composition of the invention exhibits a viscosity range from about 5000 cP to about 20 000 cP. In some preferred embodiments, the optically clear and low refractive index adhesive composition of the invention exhibits a viscosity of from about 5000 cP to about 10 000 cP. In some preferred embodiments, the clear and low refractive index adhesive composition of the invention exhibits a viscosity of from about 3000 cP to about 7000 cP. In some preferred embodiments, the clear and low refractive index adhesive composition of the invention exhibits a viscosity of from about 2000 cP to about 7000 cP. In some preferred embodiments, the clear and low refractive index adhesive composition of the invention exhibits a viscosity of from about 3000 cP to about 6000 cP. In some preferred embodiments, the clear and low refractive index adhesive composition of the invention exhibits a viscosity of from about 2000 to about 5000 cP. In some preferred embodiments, the clear and low refractive index adhesive composition of the invention exhibits a viscosity of from about 3000 to about 5000 cP.

[0070] The transparent and relatively-low-viscosity adhesive compositions of the invention, comprising acrylated polysiloxane urethanes, acrylate monomers and photoinitiators, exhibit in some embodiments a refractive index of up to 1.49 at 589 nm and 25°C, such as up to 1.48, or up to 1.47, for example up to 1.46, or up to 1.45, or up to 1.44, or up to 1.43 or up to 1.42 or up to 1.41 at 589 nm and 25°C. In a preferred embodiment, the optically transparent composition for bonding or coating exhibits a refractive index of from 1.40 to 1.49 such as from 1.41 to 1.49. In some preferred embodiments of the invention, optically transparent composition for bonding or coating exhibits a refractive index of from 1.40 to 1.47 before polymerization, such as from 1.40 to 1.46 or from 1.41 to 1.46 or from 1.41 to 1.45 or from 1.41 to 1.44, in other embodiments from 1.42 to 1.47, such as from 1.42 to 1.46 or from 1.42 to 1.45 before polymerization. In some embodiments, the adhesive composition exhibits a refractive index of from 1.45 to 1.47 before polymerization. In some preferred embodiments, the adhesive composition exhibits a refractive index of from 1.41 to 1.43 before the photopolymerization, such as from 1.415 to 1.430. After the photopolymerization, in some embodiments, the cured adhesive composition exhibits a refractive index of from 1.41 to 1.49, such as from 1.41 to 1.48 or from 1.41 to 1.47 or from 1.41 to 1.46 or from 1.41 to 1.45, or from 1.42 to 1.48 such as from 1.42 to 1.47, typically from 1.42 to 1.44, for example from 1.421 to 1.431.

[0071] The optically transparent and relatively low-viscosity bonding and coating compositions of the invention, comprising acrylated polysiloxane urethanes, acrylate monomers, additives, and photoinitiators, provide after photocuring excellent mechanical properties. The cured composition according to the invention usually exhibits an elastic modulus of from 0.05 to 1000 N / mm2and a tensile strength of from 0.01 to 25 N / mm2. In some embodiments, said elastic modulus is from 0.05 to 200 N / mm2such as from 5 to 100 N / mm2. In other embodiments, said elastic modulus is 1 to 1000 N / mm2, such as from 2 to 700 N / mm2or from 3 to 500 N / mm2or from 4 to 300 N / mm2or from 5 to 100 N / mm2. In some preferred embodiments, the cured composition according to the invention exhibits a tensile strength of from 0.01 to 25 N / mm2such as from 0.05 to 25 N / mm2, or from 0.5 to 10 N / mm2, such as from 1 to 10 N / mm2.

[0072] The invention provides a method of preparing a formulation to be advantageously used in building optical components, devices and systems, comprising the steps of i) reacting at least one polyisocyanate and a polymerization catalyst for urethane formation with a hydroxy(meth)acrylate and with a polysiloxane diol of formula HO-R7-(Si(Me2)O)m-R7- OH, where R7 is selected from C1-C9 alkylene or alkylene-alkoxy, and wherein m is an integer from 2 to 100, until the urethane-forming reactions are essentially complete, at a temperature of between 20°C and 120°C; and ii) incorporating in the mixture at any stage a photoinitiator, one or more (meth)acrylic monomers, and additives for adjusting the properties of the reaction mixture or the product. The molar ratio of the hydroxyl groups provided by said polysiloxane and said hydroxy(meth)acrylate is from 0.5 to 50 such as from 1 to 25 or from 1 to 14 or from 1 to 10 or from 1 to 3 or from 1 to 2, or from 2 to 14 such as from 4 to 11 or from 5 to 10.

[0073] The invention provides a clear and low-refractive index photocurable adhesive and coating compositions exhibiting suitable viscosities before curing and excellent adhesion and bonding strength after curing, as well as a method for producing the compositions, preferably comprising reacting dihydroxy polysiloxane and hydroxy(meth)acrylate with a diisocyanate and a catalyst such as DBTDA at about 60°C, whereas the photocurability is ensured by the presence of an additional acrylate monomer with a photoinitiator in the final adhesive composition. Said diisocyanate is preferably IPDI or HDI, and said hydroxy(meth)acrylate is preferably hydroxyethyl acrylate or 2-hydroxypropyl methacrylate. Said additional acrylate monomer is preferably isobornyl acrylate, isobornyl methacrylate, methyl 2-[(allyloxy)methyl] acrylate, 2-hydroxy-3- phenoxypropyl acrylate, tert-butyl acrylate, or a mixture of at least two of these, and said photoinitiator is preferably phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide.

[0074] The transparent and low refractive index photopolymerizable adhesive formulations according to the invention exhibit ideal optical and mechanical properties for use in modern optical systems. In a preferred embodiment, the invention aims at using the formulations exhibiting good working viscosity and excellent coating and bonding properties such as adhesion, modulus and tensile strength. In a preferred embodiment, the invention aims at using the formulations exhibiting good working viscosity and excellent bonding strength and / or coating properties, comprising steps of i) reacting a diisocyanate and a urethane-forming catalyst with a dihydroxy polysiloxane and a hydroxy(meth)acrylate, thereby providing urethane polysiloxane polymer chains terminated with acrylate moieties, and ii) curing a thin layer of a mixture comprising said acrylated urethane polysiloxane, an additional acrylate monomer and a photoinitiator with a UV light, thereby providing a bonding or coating layer. In some preferred embodiments, the molar ratio of hydroxyl groups incorporated in step i) by said polysiloxane and by said hydroxy(meth)acrylate is less than 10, in other preferred embodiments said ratio is less than 3. The molar ratio between isocyanate groups incorporated by said diisocyanate and hydroxyl groups incorporated by said polysiloxane in step i) is usually between 1.05 and 1.20.

[0075] The invention relates to a method of bonding surfaces in optical systems, comprising applying to at least one of the surfaces a formulation according to the invention, assembling the surfaces to be bonded in a manner that facilitates bonding upon photoirradiation, and irradiating the assembled surfaces with a sufficient dose of irradiation to ensure full photopolymerization of the applied adhesive composition, resulting in strongly bonding the two surfaces together. The photopolymerization reaction may be performed in an inert atmosphere.

[0076] The invention relates to a method of coating surfaces in optical components, devices and systems, comprising applying to the corresponding surface a formulation according to the invention, and irradiating the surface with a sufficient dose of irradiation to ensure full photopolymerization of the applied adhesive composition, resulting in a desired coating of the component, device or system. The photopolymerization reaction may be performed in an inert atmosphere. One major application is coating of an optical fiber (coating, or cladding). The low refraction index coating is essential for the performance of specialty optical fibers and high numerical aperture optical fibers, especially for coating of the silica core in optical fibers. Additional, nonlimiting, examples of surfaces for coating are glass, quartz, silicon, silicon-oxide, polycarbonate, PET, PMMA, indium-tin oxide, and others. The invention enables bonding and coating of surfaces comprising a broad variety of materials employed in optical systems, such as glass, quartz, silicon, silicon-oxide, semiconductors, polycarbonate, PET, PMMA, polystyrene, polypropylene, etc.

[0077] The invention thus provides lamination layers either for bonding together of two surfaces or for coating a single surface, the layers exhibiting the required mechanical and optical properties; their refractive index being in the range of from 1.40 to 1.48 after curing, whereas the 90° peel test of the cured layer on glass provides at least 25 g / cm, such as at least 50 g / cm, such as at least 100 g / cm, such as at least 300 g / cm, for example at least 500 g / cm, or at least 700 g / cm.

[0078] In some embodiments, two or more adhesive formulations according to the invention may be mixed to advantageously combine different properties of both. For example, a formulation providing a higher cohesive strength may be combined with a formulation providing a higher adhesion; this may also control the peel strength. Of course, also two or more formulations exhibiting different viscosities or refractive indexes may be mixed.

[0079] In some embodiments of the invention, the composition is used as a lamination adhesive or as a laminate in electronic displays, back-light units, thinner integrated displays, flexible / curved displays, optical touch screens, Virtual Reality and Augmented Reality head-sets, auto-stereoscopic 3-D displays, OLED displays, and nanotech (non ITO) transparent conductors. In some embodiments of the invention, the composition is used in bonding of lenses in eyewear / glasses, bonding of films in billboards, bonding and cladding between safety glass layers or bonding and cladding between a polymeric layer and a glass ply, bonding of optical elements (e.g. lenses, prisms, light-guides), bonding of photonic elements (e.g. bonding an optical fiber to a ferule), bonding of elements in a lighting system (e.g. bonding an external film over a light guide in a building lighting system), bonding of films in a screen of a 3-D movie theatre, and bonding of bio-photonic elements. In some embodiments of the invention, the composition is used as a tie coat layer or primer layer to improve adhesion of other low refractive index adhesives which otherwise lack the ability to strongly bond to the coated surface. In some embodiments of the invention, the composition is used as a cladding layer which is covered by another external hard coating or by an external film. Alternatively, an external film can be coated with the adhesive described herein and then laminated on top of lightguiding medium or layer.

[0080] Low refractive index optically clear adhesives and coatings are useful in various optical and photonic applications, including electronic displays (e.g. for bonding optical films), specialty optical fibers, fiber-optic components, bio-photonic applications, lighting systems, etc. The use of low refractive index adhesives and coatings enables efficient encapsulation or cladding of the light travelling inside a light-guiding medium.

[0081] In the electronic display industry, some common terms for adhesives are Liquid Optically Clear Adhesives (LOCA), Optically Clear Adhesives (OCA, an adhesive in the form of a film), Optical Lamination Adhesives and Pressure Sensitive Adhesives. The current invention relates to all these forms of adhesives and terms.

[0082] As will be appreciated by the skilled person, the compositions of the invention exhibit surprisingly good adhesive properties, while maintaining a low refractive index. The unique combination of high bond strength and low refractive index of the present invention enables improvements in various applications, including more efficient displays, thinner displays, flexible / curved displays, improved optical touch screens, better Virtual Reality and Augmented Reality head-sets, improved auto-stereoscopic 3- D displays, higher efficiency OLED displays and nanotech (non ITO) transparent conductors.

[0083] The invention will be further described and illustrated by the following examples. Examples

[0084] Abbreviations

[0085] AOMA methyl 2-[(allyloxy)methyl] acrylate

[0086] IBOA isobornyl acrylate

[0087] IBOMA isobornyl methacrylate

[0088] HPMA hydroxypropyl methacrylate

[0089] HEA hydroxyethyl acrylate

[0090] HEMA hydroxyethyl methacrylate

[0091] DBTDA dibutyltin diacetate

[0092] DMDI dicyclohexylmethane-4, 4' -diisocyanate

[0093] FT-IR Fourier-transform infrared spectroscopy

[0094] HDI hexamethylene diisocyanate

[0095] IPDI isophorone diisocyanate

[0096] ITO indium tin oxide

[0097] 1-819 phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide

[0098] MW molecular weight

[0099] OLED organic light-emitting diode

[0100] PET polyethylene terephthalate

[0101] TBA tert-butyl acrylate

[0102] PMMA polymethyl methacrylate

[0103] TMXDI tetramethylxylylene diisocyanate

[0104] TBD triazabicyclodecene (l,5,7-triazabicyclo[4.4.0]dec-5-ene

[0105] DABCO l,4-diazabicyclo[2.2.2]octane

[0106] DMAP 4-dimethylaminopyridine

[0107] DBU l,8-diazabicyclo[5.4.0]undec-7-ene

[0108] MBF methylbenzoylformate

[0109] HPPA 2-hydroxy-3-phenoxypropyl acrylate

[0110] MAOTS methacryloxypropyl trimethoxysilane

[0111] X-22-2445 diacrylate of a bis(hydroxyalkyl)-polysiloxane (EW ca.1600)

[0112] ACMO 4-Acryloylmorpholine

[0113] BD 1,4-butanediol

[0114] 1-819 phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide FTIR Fourier-transform infrared spectroscopy

[0115] Methods

[0116] Peel testing - 90°

[0117] The formulation was sandwiched between a glass-plate and a flexible sheet of UV- transparent polymer (for example polycarbonate). The average layer thickness of obtained samples was 120 microns. The prepared assembly was irradiated with a 1500 mJ / cm2dose of UV-irradiation from a high-pressure mercury lamp (D-bulb). After being kept in the cured assembly at ambient conditions for at least 12 hours, the polymer layer was cut to strips 2 cm wide and 15 cm long. The 90° Peel adhesion was determined using a Shimadzu Autograph AGS-X, using a peel speed of 5 cm / min. The 90° Peel adhesion values (see Table 1 below) are an average of at least 3 samples.

[0118] T-Peel test

[0119] Peel 180° is used in film-to-film and peeling the film 180 deg and parallel to the glass. The formulation was sandwiched (ca 100 microns thickness) between two clean flexible sheets of UV-transparent polymer (for example polycarbonate), and the assembly was irradiated with a 1500 mJ / cm2dose of UV-irradiation from a high-pressure mercury lamp (D-bulb). The obtained sample was cut into pieces of 2 cm width and 15 cm length. Cured samples were kept at ambient conditions for at least 24 hours before conducting the T-Peel test. The T-Peel adhesion was determined using a Shimadzu Autograph AGS- X, using a peel-speed of 5 cm / min.

[0120] Preparation of samples for modulus, tensile strength and adhesion tests

[0121] The formulation was spread evenly over a glass or quartz plate between suitable spacers using a rod, followed by irradiation with a 1500 mJ / cm2dose of UV-irradiation from a high-pressure mercury lamp (D-bulb) under an inert atmosphere. Cured samples were kept at ambient conditions for at least 12 hours before further preparation. The 90° Peel adhesion was determined as described above. For modulus and tensile strength testing, the obtained coating was cut into pieces of 2 cm width and 12 cm length, which were carefully removed from the plate. Modulus and tensile strength were determined using a Shimadzu Autograph AGS-X, using a speed of 5 cm / min. The obtained values are an average of at least 3 samples and are provided in Table 1 below. Materials

[0122] Examples of commercially available polysiloxane diols

[0123] Shin-Etsu: KF-6000 (MW ca 935 g / mol), KF-6001 (MW ca 1900 g / mol), KF-6002 (MW ca 3200 g / mol), KF-6003 (MW ca 5000 g / mol).

[0124] Gelest: DMS-C16 (MW 600-850), DMS-C21 (MW 4500-5500).

[0125] SISIB: OF 1300.

[0126] Siltech Silmer: OH Di-10, Silmer OH Di-50, OHT Di-10, Silmer OHT Di-50, OH Di-100,

[0127] Silmer OH Di-400.

[0128] Samples 1-54

[0129] Example 1

[0130] A solution of photoinitiator (1-819, 0.32 g) in an acrylate monomer (AOMA, 16 g) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.32 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.4 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C, and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. If required, additional acrylate monomer(s) could be added at this stage (also in the following Examples), with further stirring at 60°C until obtaining a homogeneous formulation. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0131] Example 2

[0132] A solution of photoinitiator (1-819, 0.32 g) in a mixture of acrylate monomers (IBOA, 8g; AOMA, 7 g) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (0.9g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 3

[0133] A solution of photoinitiator (1-819, 0.32 g) in acrylate monomers (IBOA, 7.0 g; AOMA, 8.0) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm_1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0134] Example 4

[0135] A solution of photoinitiator (1-819, 0.3 g) in acrylate monomers (IBOA, 4.0 g; AOMA, 11.0) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm_1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0136] Example 5

[0137] A solution of photoinitiator (1-819, 0.3 g) in an acrylate monomer (AOMA, 10 g) was added to a mixture of KF-6003 (60 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.3 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 6

[0138] A solution of photoinitiator (1-819, 0.3 g) in an acrylate monomer (AOMA, 10 g) was added to a mixture of KF-6003 (63 g), DMS-C16 (0.5 g) and hydroxyethyl acrylate (0.3 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0139] Example 7

[0140] A solution of photoinitiator (1-819, 0.3 g) in acrylate monomers (IBOA, 5 g; AOMA, 10 g) was added to a mixture of KF-6003 (63 g), DMS-C16 (0.5 g) and hydroxyethyl acrylate (0.3 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0141] Example 8

[0142] A solution of photoinitiator (1-819, 0.4 g) in an acrylate monomer (AOMA, 15 g) was added to a mixture of KF-6003 (66 g) and hydroxyethyl acrylate (0.3 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for 30-60 more minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 9

[0143] A solution of photoinitiator (1-819, 0.32 g) in AOMA (15 g) was added to a mixture of KF- 6003 (66 g) and hydroxyethyl acrylate (0.32 g). The obtained mixture was heated to 60°C, and diisocyanate (I PDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO- peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0144] Example 10

[0145] A solution of photoinitiator (1-819, 0.3 g) in a mixture of acrylate monomers (IBOA, 5 g; AOMA, 10 g) was added to a mixture of KF-6003 (66 g) and hydroxyethyl acrylate (0.6 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0146] Example 11

[0147] A solution of photoinitiator (1-819, 0.3 g) in a mixture of acrylate monomers (IBOA, 5 g; AOMA, 6 g ) was added to a mixture of KF-6003 (66 g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 12

[0148] A solution of photoinitiator (1-819, 0.3 g) in a mixture of acrylate monomers (IBOA, 4 g; AOMA, 4 g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0149] Example 13

[0150] To the formulation of Example 12, AOMA (2.40 g) was added, and the mixture was stirred until homogeneous.

[0151] Example 14

[0152] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4.4 g; AOMA, 4.4 g) was added to a mixture of KF-6003 (71 g) and hydroxyethyl acrylate (3 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 6 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0153] Example 15

[0154] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 5.8 g; AOMA, 5.8 g) was added to a mixture of KF-6003 (99 g) and hydroxyethyl acrylate (1.8 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 6 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0155] Example 16

[0156] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4.65 g; AOMA, 4.65 g) was added to a mixture of KF-6003 (80 g) and hydroxyethyl acrylate (1 g). The obtained mixture was heated to 60°C, and diisocyanate (I PDI, 4.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0157] Example 17

[0158] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4.4 g; AOMA, 4.4 g) was added to a mixture of KF-6003 (77 g) and hydroxyethyl acrylate (0.8 g). The obtained mixture was heated to 60°C, and diisocyanate (I PDI, 4.1 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0159] Example 18

[0160] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4.25 g; AOMA, 4.25 g) was added to a mixture of KF-6003 (73 g) and hydroxyethyl acrylate (0.6 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.8 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0161] Example 19

[0162] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4.1 g; AOMA, 4.1 g) was added to a mixture of KF-6003 (71 g) and hydroxyethyl acrylate (0.55 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.6 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0163] Example 20

[0164] A solution of photoinitiator (1-819, 0.3 g) in a mixture of acrylate monomers (IBOA, 4 g; AOMA, 4 g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0165] Example 21

[0166] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.9 g; AOMA, 3.9 g) was added to a mixture of KF-6003 (68 g) and hydroxyethyl acrylate (0.4 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0167] Example 22

[0168] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.7 g; AOMA, 3.7 g) was added to a mixture of KF-6003 (65 g) and hydroxyethyl acrylate (0.25 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.1 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0169] Example 23

[0170] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.7 g; AOMA, 3.7 g) was added to a mixture of KF-6003 (64 g) and hydroxyethyl acrylate (0.2 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0171] Example 24

[0172] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.6 g; AOMA, 3.6 g) was added to a mixture of KF-6003 (63.3 g) and hydroxyethyl acrylate (0.2 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 2.9g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0173] Example 25

[0174] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.55 g; AOMA, 3.55 g) was added to a mixture of KF-6003 (62.6 g) and hydroxyethyl acrylate (0.16 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 2.9g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0175] Example 26

[0176] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.55 g; AOMA, 3.55 g) was added to a mixture of KF-6003 (61.8 g) and hydroxyethyl acrylate (0.12 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 2.8g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 27

[0177] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.5 g; AOMA, 3.5 g) was added to a mixture of KF-6003 (61 g) and hydroxyethyl acrylate (0.08 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 2.7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0178] Example 28

[0179] To the formulation of Example 19 (57.6 g) was added AOMA (2.4 g), and the mixture was stirred until homogeneous.

[0180] Example 29

[0181] To the formulation of Example 21 (57.6 g) was added AOMA (2.4 g), and the mixture was stirred until homogeneous.

[0182] Example 30

[0183] A solution of photoinitiator (1-819, 0.32 g) in acrylate monomer (AOMA, 2.90 g) was added to a mixture of KF-6003 (59.55 g); DMS-C16 (0.95 g) and hydroxyethyl acrylate (0.32 g). The obtained mixture was heated to 60°C, and diisocyanate (DMDI; 3.8 g) and DBTDA (20 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomer (AOMA, 7.3 g) was added at this stage, with further stirring at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 31

[0184] A solution of photoinitiator (1-819, 0.3 g) in a mixture of acrylate monomers (IBOA, 4 g; AOMA, 4 g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0185] Example 32

[0186] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomer (TBA, 8g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm4). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomer (TBA; 4.3 g) was added at this stage, with further stirring at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0187] Example 33

[0188] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomer (TBA, 8g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm4). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomer (AOMA, 4.3 g) was added at this stage, with further stirring at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0189] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomer (TBA, 8 g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.65 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm4). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0190] Example 35

[0191] A solution of photoinitiator (1-819, 0.4 g) in tert-butyl acrylate (TBA, 8 g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.65 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm4). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomer (TBA, 4.3 g) was added at this stage, with further stirring at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0192] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomers (TBA, 8 g) was added to a mixture of KF-6003 (70 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (HDI, 2.65 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm4). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomers (TBA, 4.3 g; AOMA, 4.3 g) were added at this stage, with further stirring at 6O°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0193] Example 37

[0194] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.75 g, AOMA, 3.75 g) was added to a mixture of KF-6003 (67 g) and hydroxyethyl acrylate (0.4 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomer (TBA, 4 g) was added at this stage, with further stirring at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0195] Example 38

[0196] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3.75 g; AOMA, 3.75 g) was added to a mixture of KF-6003 (67 g) and hydroxyethyl acrylate (0.4 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.3 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. Additional acrylate monomer (AOMA, 4 g) was added at this stage, with further stirring at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0197] Example 39

[0198] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4 g, AOMA, 4 g) was added to a mixture of KF-6003 (47 g), KF-6001 (21 g) and hydroxyethyl acrylate (0.7 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0199] Example 40

[0200] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4 g, AOMA, 4 g) was added to a mixture of KF-6003 (47 g), KF-6001 (22 g) and hydroxyethyl acrylate (0.8 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 5.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0201] Example 41

[0202] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4 g, AOMA, 4 g) was added to a mixture of KF-6003 (47 g), KF-6002 (23 g) and hydroxyethyl acrylate (0.6 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 4.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0203] Example 42

[0204] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomers (IBOA, 8.0 g; AOMA, 10.0) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm_1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0205] Example 43

[0206] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 4 g, AOMA, 5 g) was added to a mixture of KF-6003 (47 g), KF-6002 (23 g) and hydroxyethyl acrylate (0.6 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 4.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0207] Example 44

[0208] A solution of photoinitiator (1-819, 0.4 g) in a mixture of acrylate monomers (IBOA, 3 g, AOMA, 6 g) was added to a mixture of KF-6002 (44 g) and hydroxyethyl acrylate (0.5 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 45

[0209] A solution of photoinitiator (1-819, 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g, AOMA, 3 g) was added to a mixture of KF-6000 (13 g), KF-6001 (26 g) and hydroxyethyl acrylate (1 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0210] Example 46

[0211] A solution of photoinitiator (1-819, 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g, AOMA, 3 g) was added to a mixture of KF-6000 (13 g), KF-6002 (44 g) and hydroxyethyl acrylate (1 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0212] Example 47

[0213] A solution of photoinitiator (1-819, 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g, AOMA, 3 g) was added to a mixture of KF-6000 (13 g), KF-6003 (70g) and hydroxyethyl acrylate (1 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 48

[0214] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomers (IBOA, 8.0 g; AOMA, 11.0) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm_1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0215] Example 49

[0216] A solution of photoinitiator (1-819, 0.4 g) in acrylate monomers (IBOA, 9.0 g; AOMA, 10.0 g) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxyethyl acrylate (0.33 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm_1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0217] Example 50

[0218] A solution of photoinitiator (1-819, 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g; AOMA, 4 g) was added to a mixture of KF-6000 (13 g), KF-6002 (44 g) and hydroxyethyl acrylate (1 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 51

[0219] A solution of photoinitiator (1-819, 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g; AOMA, 3.5 g) was added to a mixture of KF-6000 (13 g), KF-6003 (70g) and hydroxyethyl acrylate (1 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0220] Example 52

[0221] A solution of photoinitiator (1-819; 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g; AOMA, 3.5 g) was added to a mixture of KF-6000 (13 g), KF-6003 (70 g) and hydroxypropyl acrylate (1.1 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI; 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0222] Example 53

[0223] A solution of photoinitiator (1-819; 0.4 g) in acrylate monomers (IBOA, 9.0 g; AOMA, 10.0 g) was added to a mixture of KF-6003 (59.5 g), DMS-C16 (1.0 g) and hydroxybutyl acrylate (0.41 g). The obtained mixture was heated to 60°C, and diisocyanate (IPDI, 3.2 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm_1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 54

[0224] A solution of photoinitiator (1-819; 0.2 g) in a mixture of acrylate monomers (IBOA, 2 g; AOMA, 3.5 g) was added to a mixture of KF-6000 (13 g), KF-6003 (70 g) and hydroxypropyl methacrylate (1.3 g). The obtained mixture was heated to 60°C, and diisocyanate (I RD I; 7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0225] Properties of samples 1-54

[0226] The following table shows the properties of the samples prepared according to the above examples. "Hydroxy ratio" is the molar ratio of the hydroxyl groups comprised in the hydroxy polysiloxane and in the hydroxylated acrylate before starting the reaction. "Rl liquid" is the refractive index of the photopolymerizable composition prepared in the above examples, and "Rl cured" is the refractive index of the compositions prepared in the above examples after irradiation curing. "90° Peel" characterizes the adhesion strength in g / cm (sometimes denoted as gf / cm) of the compositions. "Viscosity" relates to the clear liquid compositions before curing.

[0227] Table 1 Properties of clear photopolymerizable adhesive compositions according to some embodiments of the invention.

[0228] The table shows that compositions prepared in accordance with the invention have sufficiently low refractive index. The compositions according to the preferred embodiments, exhibit desirable viscosities and peel strength. Some effects of the individual reactants on the product properties, as implied by the experimental results, are discussed in the paragraph "Detailed Description of the Invention" above. Samples 55-68

[0229] Example 55

[0230] A mixture of polysiloxane diol KF-6OO1 (63.70 g) and (meth)acrylate IBOMA (3.00 g) was prepared and heated to 60°C. Diisocyanate IPDI (13.77 g) and polymerization catalyst DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, chain extender BD (0.58 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, a solution of photoinitiator 1-819 (0.4 g) in hydroxy (meth)acrylate (HPMA, 8.35 g) was added to the mixture. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO- peak at ca 2260cm -1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional (meth)acrylate monomers (AOMA, 4.0 g; IBOMA, 4.5 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0231] Example 56

[0232] A mixture of KF-6001 (65.08 g), AOMA (5.00 g), and IBOMA (3.50 g) was prepared and heated to 60°C. IPDI (15.31 g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (0.58 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, a solution of 1-819 (0.4 g) in HPMA (10.14 g) was added to the mixture. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cml). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, an additional (meth)acrylate monomer (IBOMA, 1.70 g) was added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 57

[0233] A mixture of polysiloxane diol KF-6OOO (7.42 g), KF-6OO1 (46.11 g) and IBOMA (5.00 g) was prepared and heated to 60°C. IPDI (13.16 g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (0.87g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, a solution of 1-819 (0.4 g) in HPMA (10.05 g) was added to the mixture. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO- peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional (meth)acrylate monomer (IBOMA, 11.20 g) and photoinitiator MBF (2.00 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0234] Example 58

[0235] A mixture of KF-6000 (23.70 g), KF-6001 (28.77 g), 1-819 (0.4 g), (meth)acrylate monomer IBOA (10.00 g) and IBOMA (5.00 g) was prepared and heated to 60°C. IPDI (14.52 g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (0.48 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, hydroxy (meth)acrylate HEA (4.64 g) was added to a mixture. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional (meth)acrylate monomers (IBOA, 4.50; IBOMA, 5.30 g) and HPPA (1.00 g), and MBF (2.00 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0236] Example 59

[0237] A mixture of KF-6000 (26.10 g), KF-6001 (31.68 g), 1-819 (0.4 g), IBOA (10.00 g) and IBOMA (5.00 g) was prepared and heated to 60°C. IPDI (16.12 g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (0.58 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, HEA (5.11 g) was added to the mixture. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional acrylate monomers (IBOA, 16.00; IBOMA, 5.00 g; HPPA, 4.00 g) and MBF (2.50 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0238] Example 60

[0239] A mixture of KF-6001 (12 g), KF-6003 (44 g) and IBOMA (2g) was heated to 60°C, and IPDI (6.7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and for 30 minutes and a solution of 1-819 (0.4 g) in HPMA (4.5 g) was added. The conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 60 minutes. To this mixture, (meth)acrylate MAOTS (1.4 g) was added, and the mixture was stirred for another 30 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0240] Example 61

[0241] A mixture of KF-6001 (12 g), polysiloxane diol KF-6003 (44 g) and IBOA (2g) was heated to 60°C, and IPDI (6.7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and for 30 minutes and a solution of 1-819 (0.4 g) in HPMA (4.5 g) was added. The conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 60 minutes. To this mixture, MAOTS (1.4 g) was added, and the mixture was stirred for another 30 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 62

[0242] A solution of 1-819 (0.8 g) in HPMA (9 g) was added to a mixture of KF-6000 (12 g), KF- 6003 (88 g) and IBOA (7 g). The obtained mixture was heated to 60°C, and IPDI (13.5 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm'1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 60 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0243] Example 63

[0244] A mixture of KF-6003 (88 g) and crosslinking siloxane diacrylate X-22-2445 (12 g) was heated to 60°C, and IPDI (7.8 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C for 30 minutes, and a solution of 1-819 (0.5 g) in HPMA (10 g) was added. The mixture was stirred at 60°C and the reaction was monitored by FT-IR (NCO-peak at ca 2260 cm4). Once all diisocyanate had been consumed (about 90 minutes), the mixture was kept at 60°C for another 30 minutes. The formulation was allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0245] Example 64

[0246] A mixture of KF-6002 (20 g), KF-6003 (44 g) and IBOA (2 g) was heated to 60°C, and IPDI (6.7 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C for 30 minutes, and a solution of 1-819 (0.4 g) in HPMA (13 g) was added. The conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30 minutes. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0247] Example 65

[0248] A mixture of polysiloxane diol DMS-C16 (51.8 g) and (meth)acrylate ACMO (6.0 g) was heated to 60°C, and IPDI (31.7 g) and DBTDA (15mg) were added. The mixture was stirred at 60°C for 30 minutes, and a HEA (16.5 g) was added. The reaction was monitored by FT-IR. Once no more isocyanate signal could be detected by FT-IR, a solution 1-819 (0.5 g) in HPMA (15 g) was added. The mixture was stirred for 10 minutes at 60°C and was allowed to cool to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0249] Example 66

[0250] A mixture of DMS-C16 (11.6 g), KF-6003 (22.0 g) and ACMO (1.8 g) was heated to 60°C, and IPDI (9 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C for 30 minutes, and HEA (4.7 g) was added. The reaction was monitored by FT-IR. Once no more isocyanate signal could be detected by FT-IR, a solution 1-819 (0.5 g) in HPMA (4.9 g) was added. The mixture was stirred for 10 minutes at 60°C and was allowed to cool to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0251] Example 67

[0252] A mixture of DMS-C16 (11.6 g), KF-6003 (22.0 g) and ACMO (1.8 g) was heated to 60°C, and IPDI (9 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C for 30 minutes, and a solution of 1-819 (0.25 g) in HPMA (10.8 g) was added. The reaction mixture was stirred at 60°C, and conversion of isocyanate was monitored by FT-IR. Upon disappearance of the isocyanate signal in the FT-IR, the mixture was stirred for another 30 minutes at 60°C and was then allowed to cool to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0253] Example 68

[0254] A mixture of DMS-C16 (11.6 g), KF-6003 (22.0 g) and ACMO (1.8 g) was heated to 60°C, and IPDI (9 g) and DBTDA (15 mg) were added. The mixture was stirred at 60°C for 30 minutes, and a solution of 1-819 (0.25 g) in HEMA (13.2 g) was added. The reaction mixture was stirred at 60°C, and conversion of isocyanate was monitored by FT-IR. Upon disappearance of the isocyanate signal in the FT-IR, the mixture was stirred for another 30 minutes at 60°C and was then allowed to cool to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Properties of samples 55-68

[0255] Table 2 shows the properties of the samples prepared according to the above examples. "Hydroxy ratio" is the molar ratio of the hydroxyl groups comprised in the hydroxy polysiloxane and in the hydroxylated acrylate before starting the reaction. "Rl liquid" is the refractive index of the photopolymerizable composition prepared in the above examples, and "Rl cured" is the refractive index of the compositions prepared in the above examples after irradiation curing. "90° Peel" characterizes the adhesion strength in g / cm (sometimes denoted as gf / cm) of the compositions. "Viscosity" relates to the clear liquid compositions before curing. Elastic modulus and tensile strength are further presented.

[0256] Table 2 Properties of clear photopolymerizable coating compositions according to some embodiments of the invention.

[0257] Table 2 shows that compositions prepared in accordance with the invention have the desired optical and mechanical properties. Some effects of the individual reactants on the product properties, as implied by the experimental results, are discussed in the paragraph "Detailed Description of the Invention" above. Samples 69-84

[0258] Example 69

[0259] A solution of 1-819 (0.64 g) in IBOA (8 g) and AOMA (8 g) was added to a mixture of KF- 6003 (136.53 g), 1,4-BD (96 mg) and HEA (0.96 g). The mixture was stirred and heated to 60°C, and IPDI (7.0 g) was added, followed by DBTDA (30 mg). The mixture was stirred at 60°C and the reaction was monitored by FT-IR for conversion of the isocyanate group (2260-70 cm4); upon complete conversion, the mixture was stirred for another 30 minutes at 60oC and allowed to cool to room temperature to give 161 g formulation. As KF-6003 and HEA incorporated about 54 mmol and 8.3 hydroxyl groups, respectively, the molar ratio of the hydroxyl groups provided by hydroxyl terminated polysiloxane and hydroxy(meth)acrylate was about 6.5. The "hydroxy ratios" of other samples are calculated in the same way.

[0260] A solution of 1-819 (0.6 g) in IBOMA (5 g) and AOMA (1 g) was added to a mixture of KF- 6003 (135.25 g), 1,4-BD (144 mg) and HEA (1,29 g). The mixture was stirred and heated to 60°C, and IPDI (7.56 g) was added, followed by DBTDA (30 mg). The mixture was stirred at 60°C and the reaction was monitored by FT-IR for conversion of the isocyanate group (2260-70 cm4). Upon complete conversion of the isocyanate according to FT-IR, the mixture was stirred for another 30 minutes at 60°C and allowed to cool to room temperature. To the obtained mixture was added MAOTS (9 g; 6% w / w), the mixture was stirred for 60 minutes at 50°C and allowed to cool to room temperature giving 159 g of formulation.

[0261] Example 71

[0262] A solution of 1-819 (0.6 g) in IBOMA (5 g) and AOMA (1 g) was added to a mixture of KF- 6003 (135.2 g), HEA (1.3 g) and 1,4-butanediol (140 mg). The resulting solution was heated to 60°C with stirring, and IPDI (7.5 g) and DBTDA (30 mg) were added. The mixture was stirred at 60°C and the conversion of isocyanate was monitored by FT-IR (2260-70 cm4). Once conversion was complete, the mixture was stirred for another 30 minutes at 60°C and allowed to cool to room temperature. To the cooled mixture was added MAOTS (9 g) and the mixture was stirred at 50°C for 1 hour. Cooling to room temperature gave 159 g of formulation.

[0263] Table 3 Properties of clear photopolymerizable lamination adhesive compositions according to some embodiments of the invention.

[0264] Example 72

[0265] A mixture of KF-6OOO (14 g), KF-6OO2 (48 g) and IBOMA (4 g) was heated to 60°C, and IPDI (13.5 g) and DBTDA (30 mg) were added. The mixture was stirred at 60°C for 30 minutes. Then a solution of 1-819 (0.60 g) in IBOMA (8.80 g) and HPMA (8.92 g) was added, and the mixture was stirred further at 60°C. The conversion of isocyanate was monitored by FT-IR (2260-70 cm'1). When conversion of isocyanate was complete according to FT-IR, the mixture was stirred for another 30 minutes at 60°C and then allowed to cool to room temperature. This gave about 97 g of formulation. As KF-6000 and KF-6002 incorporated about 61 mmol hydroxyl groups, and HPMA incorporated about 62 mmol hydroxyl groups, the molar ratio of the hydroxyl groups provided by hydroxyl terminated polysiloxanes and hydroxy(meth)acrylate was about 1.

[0266] Example 73

[0267] IPDI (6.7 g) and DBTDA (30 mg) were added to a hot (60°C) stirred mixture of KF-6000 (7 g), KF-6003 (38 g) and IBOA (2 g). The mixture was heated to 60°C and, and the mixture was stirred 30 minutes at 60°C. To the resulting mixture was added a solution of 1-819 (0.3 g) in AOMA (4.4 g) and IBOA (4.5 g). After stirring at 60°C for another hour, HPMA (4.5 g) was added, and stirring at 60°C was continued for 12 hours, yielding about 67 g formulation.

[0268] Example 74

[0269] A mixture of KF-6000 (7 g), KF-6003 (38 g) and IBOA (2 g) was heated to 60°C, and IPDI (6.7 g ) and DBTDA (20 mg) were added. After stirring for 30 mins at 60°C, a solution of 1-819 (0.3 g) in AOMA (4.40 g) and HPMA (4.5 g) was added and the mixture was stirred at 60°C for 6 hours.

[0270] Example 75

[0271] A mixture of KF-6000 (7 g), KF-6002 (24 g) and IBOA (2 g) was heated to 60°C, and IPDI (6.7 g) and DBTDA (20 mg) were added. After stirring for 30 mins at 60°C, a solution of I- 819 (0.3 g) in AOMA (4.40 g) and HPMA (4.5 g) was added and the mixture was stirred at 60°C for 6 hours.

[0272] Example 76

[0273] A mixture of KF-6002 (20.4 g), KF-6003 (44 g) and IBOMA (2.2 g) was heated to 60°C with stirring. IPDI (6.7 g) and DBTDA (20 mg) were added and the mixture was stirred at 60°C for 30 minutes. To this mixture, a solution of 1-819 (0.45 g) in HPMA (4.5 g) was added. The mixture was stirred at 60°C for a further 6 hours and allowed to cool to room temperature. To this formulation was added MAOTS (4.7 g) and the mixture was stirred at 60°C for 1 hour.

[0274] Example 77

[0275] A mixture of KF-6000 (12 g), KF-6003 (88 g) and IBOA (4 g) was stirred and heated to 60°C, and IPDI (13.5 g) and DBTDA (30 mg) were added. The mixture was stirred for 30 mins at 60°C and a solution of 1-819 (0.6 g) in HPMA (17.5 g) was added. The mixture was stirred at 60°C for 3 hours and allowed to cool to room temperature, giving 135 g of formulation. To this formulation was added IBOA (8 g) and the mixture was stirred at 60°C for 1 hour.

[0276] Example 78

[0277] A mixture of KF-6000 (12 g), KF-6003 (88 g) and IBOA (4 g) was stirred and heated to 60°C, and IPDI (13.5 g) and DBTDA (30 mg) were added. The mixture was stirred for 30 mins at 60°C and a solution of 1-819 (0.6 g) in HPMA (17.5 g) was added. The mixture was stirred at 60°C for 3 hours and allowed to cool to room temperature, giving 135 g of formulation. To this formulation was added ACMO (6 g) and the mixture was stirred at 60°C for 1 hour. Example 79

[0278] A mixture of KF-6000 (12 g), KF-6003 (88 g) and IBOA (4 g) was stirred and heated to 60°C, and IPDI (13.5 g) and DBTDA (30 mg) were added. The mixture was stirred for 30 mins at 60°C and a solution of 1-819 (0.6 g) in HPMA (17.5 g) was added. The mixture was stirred at 60°C for 3 hours and allowed to cool to room temperature, giving 135 g of formulation. To this formulation was added IBOMA (8 g) and the mixture was stirred at 60°C for 1 hour.

[0279] Example 80

[0280] A mixture of KF-6002 (20 g), KF-6003 (44 g) and IBOMA (2 g) was heated to 60°C with stirring. IPDI (6.7 g) and DBTDA (20 mg) were added, and the mixture was stirred at 60°C for 30 minutes. A solution of 1-819 (0.45 g) in HPMA (4.5 g) was added and the mixture was stirred at stirred at 60°C for a further 6 hours. To the above formulation was added MAOTS (4.7 g) and the mixture was stirred at 60°C for 30 minutes.

[0281] Example 81

[0282] A mixture of KF-6003 (85 g) and 1,4-butanediol (62 mg) was heated to 60°C and IPDI (7.9 g) and DBTDA (30 mg) were added. The mixture was stirred at 60°C for 30 minutes and a solution of 1-819 (0.2 g) in HPMA (5 g) was added. The reaction mixture was stirred for 16 hours at 60°C and monitored for the conversion of isocyanate by FT-IR. This gave 150 g of intermediate formulation. To this formulation was added MAOTS (9 g), the mixture was stirred for 30 minutes at 60°C and allowed to cool down to room temperature.

[0283] Table 4 Properties of clear photopolymerizable coating compositions according to some embodiments of the invention. The "hydroxy ratio" values are about 1.

[0284] *Q stands for Quartz and G for Glass

[0285] Example 82

[0286] A mixture of KF-6001 (52.99 g), 1-819 (0.4 g) and IBOA (6.00 g) was prepared and heated to 60°C. IPDI (10.15 g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (0.58 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, HPMA (8.29 g) was added to the mixture. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional acrylate monomers (AOMA, 2.00 g; IBOMA, 13.00 g; HPPA, 5.50 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0287] Example 83

[0288] A mixture of KF-6000 (28.17 g), KF-6001 (36.22 g), 1-819 (0.4 g) and IBOA (10.00 g) was heated to 60°C. IPDI (15.92 g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (0.58 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, HPMA (9.90 g) was added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional acrylate monomers (IBOA, 10.00; AOMA, 3.00 g; HPPA, 8.00 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature. Example 84

[0289] A mixture of KF-6000 (25.28 g), KF-6001 (30.63 g), 1-819 (0.4 g) and IBOA (10.00 g) was prepared and heated to 60°C. IPDI (17.15g) and DBTDA (10 mg) were added, and the mixture was stirred at 60°C for 20 min. Afterward, BD (1.15 g) was introduced, and the mixture was stirred for an additional 20 minutes at the same temperature. Next, HPMA (11.18 g) was added. The mixture was stirred at 60°C and the conversion of the diisocyanate was monitored using FT-IR (NCO-peak at ca 2260 cm1). Once FT-IR indicated that all diisocyanate had been consumed, the mixture was kept at 60°C for another 30-60 minutes. At this stage, additional acrylate monomers (IBOA, 12.60; IBOMA, 5.00 g; HPPA, 4.00 g) and photoinitiator (MBF, 2.50 g) were added, and stirring continued at 60°C until a homogeneous formulation was obtained. The formulation was then allowed to cool down to room temperature. The obtained clear formulation was stored in a dark bottle at room temperature.

[0290] Table 5 Properties of clear photopolymerizable coating compositions according to some embodiments of the invention.

[0291] While the invention has been described using some specific examples, it is understood by those skilled in the art that many modifications and variations are possible. It is therefore understood that the invention is not intended to be limited in any way, other than by the scope of the appended claims.

Claims

CLAIMS1. A clear photopolymerizable adhesive composition comprising i) at least one polysiloxane urethane comprising (meth)acrylate moieties (acrylated polysiloxane urethane); ii) at least one (meth)acrylate monomer; and iii) at least one free radical initiator.

2. A photopolymerizable composition according to claim 1, wherein said acrylated polysiloxane urethane comprises polysiloxane blocks of formula-O-R7-(Si(Me2)O)m-R7-O- where Me is methyl, R7is a bivalent radical comprising C1-C9 alkylene or alkylenealkoxy, and m is an integer from 2 to 100, said blocks being interspersed between urethane moieties -U- of formula-C(=O)NH-R3-NH(C=O)- wherein R3 is a bivalent radical selected from C4-C14 aliphatic, alicyclic, or aromatic hydrocarbon moieties.

3. A photopolymerizable composition according to claim 2, wherein molecules of said acrylated polysiloxane urethane comprise more blocks of said formula, differing in R7or m from each other.

4. A photopolymerizable composition according to claim 2 or 3, wherein the number of said polysiloxane blocks interspersed between said urethane moieties in the molecules of said acrylated polysiloxane urethane is in average from 1 to 40.

5. A photopolymerizable composition according to any one of claims 2 to 4, wherein the molecules of said acrylated polysiloxane urethane are terminated by an acrylate moiety R1 of formulaCH2=C(R2)-C(=O)-O-R4-O- wherein R2 is selected from H and CH3, and R4 is selected from C2-Ci4 hydrocarbon bivalent radicals, optionally further comprising up to 4 oxygen atoms.

6. A photopolymerizable composition according to any one of claims 1 to 5, wherein said acrylate monomer in a photopolymerizable adhesive composition of the invention is selected from acrylates of formulaCH2=C(R5)-C(=O)-O-R6 wherein R5 is selected from H or C1-C5 hydrocarbon moiety optionally comprising one oxygen atom, and wherein R6 is selected from C1-C14 hydrocarbon moieties comprising up to three oxygen atoms and up to one nitrogen atom.

7. A photopolymerizable composition according to any one of claims 2 to 6, wherein said bivalent radical R3 is selected from 5-yl-l-ylmethyl-l,3,3- trimethylcyclohexane, 1,6-hexanediyl, 1,4-butanediyl, 1,3- bis(methyene)cyclohexane, dicyclohexylmethane-4,4'-diyl, l,3-bis(2-yl-2-propyl)- benzene, 2,2,4-trimethylhexane diyl, and 2,4,4-trimethylhexanediyl.

8. A photopolymerizable composition according to any one of claims 5 to 7, wherein said bivalent radical R4 is selected from ethyl acrylate 2-yl, propyl acrylate 2-yl, propyl acrylate 3-yl, butyl acrylate 2-yl, butyl acrylate 3-yl, butyl acrylate 4-yl, ethyl methacrylate 2-yl, propyl methacrylate 2-yl, propyl methacrylate 3-yl, butyl methacrylate 2-yl, butyl methacrylate 3-yl, butyl methacrylate 4-yl, 1-acryloyloxy- 3-yl adamantane, l-methacryloyloxy-3-yl adamantane, 2-((acryloyloxy)methyl)- 1,3-diyl, and diacrylate-2-(methylene)propane-2-yl.

9. A photopolymerizable composition according to any one of claims 1 to 8, wherein said (meth)acrylate monomer is selected from isobornyl acrylate, methyl 2- [(allyloxy)methyl]acrylate, 3-ethyl-3-oxetanylmethyl acrylate, 3-ethyl-3- oxetanylmethyl methacrylate, (2-ethyl-2-methyl-l,3-dioxolane-4-yl)-methyl acrylate, 2-morpholinoethyl acrylate, 4-tert-butylcyclohexyl acrylate, hexyl acrylate, tetrahydrofurfuryl acrylate, isobutyl acrylate, tert-butyl acrylate, cyclohexyl acrylate, hexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl-acrylate, decyl acrylate, isodecyl acrylate, dicyclopentanyl acrylate, dodecyl acrylate, adamantan-l-yl acrylate, 2-methyladamantan-l-yl acrylate, 2-(2- ethoxyethoxy)ethyl acrylate, 3,3,5-trimethylcyclohexylacrylate, and2[[(butylamino)carbonyl]oxy]ethyl acrylate.

10. A photopolymerizable composition according to any one of claims 1 to 9, wherein said free radical initiator is a photoinitiator active for a wavelength above 250 nm selected from the group consisting of 1-hydroxycyclohexyphenyl ketone, 2- hydroxy-2-methyl propiophenone, methylbenzoyl formate, diphenyl (2,4,6- trimethylbenzoyl)-phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and benzyl dimethyl ketal, or is a thermal initiator selected from the group consisting of benzoyl peroxide, tert-butylperoxide, cumenehydroperoxide, tertbutyl peroxy benzoate, cyclohexanone peroxide, tert-butyl hydroperoxide, and 2,2'-azobisisobutyronitrile.

11. A photopolymerizable composition according to any one of claims 1 to 10, comprising acrylated polysiloxane urethane of Formula 1Rl-U-{O-R7-(Si(Me2)O)m-R7-O-U}p-Rl where U is a urethane moiety and R1 is an acrylate moiety, both as defined above, Me is methyl, R? is a bivalent radical selected from C1-C9 alkylene or alkylenealkoxy, m is selected from integers between 2 and 100, and p is an integer of from 1 to 40.

12. A photopolymerizable composition according to any one of claims 1 to 11, comprising more blocks -O-R7-(Si(Me2)O)m-R7-O-U- differing in R7 or m.

13. A photopolymerizable composition according to claim 11, comprising molecules of Formula 1 in which p is greater than 1, and in which at least two blocks -O-R7-(Si(Me2)O)m-R7-O-U- in one molecule differ in R7 or m.

14. A photopolymerizable composition according to any one of claims 1 to 13 exhibiting a viscosity of up to 30000 cP, for example of up to 15 000 cP.

15. A photopolymerizable composition according to any one of claims 1 to 14 exhibiting a refractive index of up to 1.46, for example up to 1.44 at 589 nm and 25°C.

16. A photopolymerizable composition according to any one of claims 1 to 15 exhibiting, after photocuring, an elastic modulus of from 2 to 250 N / mm2, for example from 5 to 200 N / mm2.

17. A photopolymerizable composition according to any one of claims 1 to 16 exhibiting, after photocuring, a tensile strength of 0.2 to 25 N / mm2, for example from 1 to 10 N / mm2.

18. A photopolymerizable composition according to any one of claims 1 to 17, being a clear low-refractive index photocurable adhesive for use in optical systems.

19. A photopolymerizable composition according to any one of claims 1 to 18, being a clear low-refractive index photocurable adhesive for use in bonding, coating or cladding surfaces in optical components, devices and systems.

20. A method of preparing a clear and low-refractive index, photopolymerizable adhesive composition for bonding, coating or cladding surfaces in optical devices and systems, comprising steps of i) providing components a) to g): a) at least one polysiloxane comprising terminal-hydroxyl groups of formulaHO-R7-(Si(Me2)O)m-R7-OH where Me is methyl, R7is a bivalent radical comprising C1-C9 alkylene or alkylene-alkoxy, and m is an integer from 2 to 100; b) at least one hydroxy(meth)acrylate of formulaCH2=C(R2)-C(=O)-O-R4-OH wherein R2 is selected from H and CH3, and R4 is selected from C2-Ci4 hydrocarbons optionally further comprising up to 4 oxygen atoms; wherein the molar ratio of the hydroxyl groups provided by said hydroxyl terminated polysiloxane and said hydroxy(meth)acrylate is less than 40; c) at least one diisocyanate of formulaO=C=N-R3-N=C=O wherein R3 is a bivalent radical selected from a bivalent radical selected from C4-C14 aliphatic, alicyclic, or aromatic hydrocarbon moieties, and -N=C=O is an isocyanate group; d) at least one polymerization catalyst for urethane formation; e) at least one (meth)acrylate monomer without hydroxyl function;f) at least one free radical initiator, for example a photoinitiator active for a wavelength of above 250 nm; g) additives selected from solvents, chain extenders, crosslinkers, adhesion promoters, additional (meth)acrylates, additional siloxanes, additional catalysts or initiators; and ii) allowing said isocyanate groups to react with said hydroxyl groups provided by hydroxyl terminated polysiloxane and hydroxyalkyl(meth)acrylates, at a temperature of from 20°C to 120°C, such as from 40°C to 70°C, while monitoring the urethane-forming reaction by FTIR, and optionally externally heating the mixture if the reaction heat is not sufficient; thereby obtaining a photocurable adhesive formulation exhibiting suitable optical properties, mechanical properties, and viscosity.

21. A method of preparing a clear and low-refractive index, photopolymerizable adhesive composition for bonding surfaces according to claim 20, wherein said molar ratio of the hydroxyl groups provided by said hydroxyl terminated polysiloxane and said hydroxy(meth)acrylate is from 3 to 30.

22. A method of preparing a clear and low-refractive index, photopolymerizable adhesive composition for coating surfaces according to claim 20, wherein said molar ratio of the hydroxyl groups provided by said hydroxyl terminated polysiloxane and said hydroxy(meth)acrylate is from 1 to 3.

23. A method according to claim 20, wherein said diisocyanate is selected from isophorone diisocyanate, 1,6-hexanediisocyanate, 1,4-butanediisocyanate, 1,3- bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane-4,4'-diisocyanate, 1,3- bis(2-isocyanato-2-propyl)benzene, 2,2,4-trimethylhexamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate.

24. A method according to claim 20, wherein said hydroxy(meth)acrylate is selected from 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 3-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2- hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutyl methacrylate, l-acryloyloxy-3-hydroxyadamantane, 1- methacryloyloxy-3-hydroxyadamantane, 2-hydroxy-3-phenoxypropyl acrylate, and pentaerythritol triacrylate.

25. A method according to claim 20, wherein said acrylate monomer without hydroxyl function is selected from isobornyl acrylate, methyl 2-[(allyloxy)methyl]acrylate, 3-ethyl-3-oxetanylmethyl acrylate, 3-ethyl-3-oxetanylmethyl methacrylate, (2- ethyl-2-methyl-l,3-dioxolane-4-yl)-methyl acrylate, 2-morpholinoethyl acrylate, 4- tert-butylcyclohexyl acrylate, trimethyl cyclohexyl acrylate, hexyl acrylate, tetrahydrofurfuryl acrylate, isobutyl acrylate, tert-butyl acrylate, cyclohexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl-acrylate, decyl acrylate, isodecyl acrylate, dicyclopentanyl acrylate, dodecyl acrylate, adamantan- 1-yl acrylate, and 2-methyladamantan-l-yl acrylate.

26. A method according to claim 20, wherein said acrylate monomer without hydroxyl function comprises 2 to 40% (w / w) of said composition.

27. A method of bonding, coating or cladding a surface in an optical device or system comprising at least steps of i) applying to said surface a formulation according to any one of claims 1 to 19, the formulation serving as a photocurable optically clear adhesive; and ii) irradiating the surface with a sufficient dose of irradiation of a wavelength above 250 nm to ensure full photopolymerization of the applied adhesive composition; thereby obtaining a surface bonded, coated or cladded with an optically clear, low-refractive index, and mechanically suitable layer.

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