Process for producing polymer thickener for lubricating greases

Abstract

The present invention relates to a process for the production of a lubricating grease comprising a "polymer" thickener derived from monomers comprising at least one ethylene group. The invention further relates to a lubricating grease comprising a lubricating base oil and a polymer thickener, derived from monomers comprising at least one ethylene group, preferably (meth)acrylates. The invention further relates to a use of such a polymer thickener in the preparation of lubricating grease compositions, preferably with highly flexible and easily adjustable properties and high yield point.

Description

[0001] The present invention relates to a process for the production of a lubricating grease comprising a "polymer" thickener derived from monomers comprising at least one ethylene group.

[0002] Lubricating greases are made of a lubricating base oil incorporated into a network that acts like a sponge. This network, known as thickener, forms a three dimensional structure in which the oil is contained. The lubricating properties of the grease are due to the oil and additives in this network, while other properties depend on the type and quality of the thickener. During use of the lubricating grease, the oil bleeds out of the oil / thickener-structure onto the lubricated surfaces, e.g. of a bearing, thereby providing the lubricating effect. The oil bleeding characteristics at the service temperature of the lubricant grease composition (i.e. the running temperature of the bearing, as well as the "start-up" temperature) are therefore critical for obtaining the lubricating effect of the composition.

[0003] The thickener network containing the oil influences properties of the grease, such as the grade of consistency of the grease, the temperature at which the oil begins to separate from the thickener (Dropping point), the ability of the grease to withstand pressure while maintaining performance (Load carrying capacity), the grease maintaining its consistency during repeated and rapid movements and separation of the oil from the grease (oil separation).

[0004] In current lubricating greases, such as soap-thickened greases, the oil bleeding characteristics are strongly dependent on temperature. In low temperature applications (such as in windmills) the bleeding of the oil of conventional greases is often so low that oil starvation occurs, i.e. no sufficient oil is released from the grease composition. US 4,392,967 discloses a continuous process for producing soap-thickened greases.

[0005] The production process for polymer-thickened lubricating greases according to the state of the art typically employs the following steps: (a) preparing a polymeric thickener; (b) mixing the obtained thickener composition with one or more lubricating base oils.

[0006] This process however is not suitable for all sorts of polymers being incorporated as thickeners into a lubricating base oil in order to obtain a lubricating grease composition due to failure of creating a homogenous suspension and / or forming a gel from a three-dimensional network from the polymer-thickener incorporating the lubricating base oil.

[0007] Furthermore, polymer-thickened grease compositions sometimes do not show an acceptable mechanical stability and consistency. Poor mechanical stability leads to a collapse of the grease structure upon shearing, resulting in an undue grease leakage and undesirable reduction of grease life. Such drawbacks are not present in the case of compositions according to the invention.

[0008] The aim of this work therefore was to develop an alternative process for the production of polymer-thickened greases. US 4 243 768 A and CN 110 964 593 B disclose batch processes for the production of polymer-thickened greases.

[0009] It was surprisingly found, that lubricating greases can be produced, preferably in-situ, by a process comprising the steps: a) providing a composition comprising lubricating base oil, polymerization initiator and a polymerizable compound comprising monomers with at least one ethylene group, preferably (meth)acrylate monomers; b) polymerizing the polymerizable compound of the composition provided in step a), wherein the process is a continuous process.

[0010] By polymerization of the polymerizable compound in step b), preferably by free radical polymerization, a polymer is obtained which acts as a thickener since it forms a three-dimensional network, in which the lubricating base oil is incorporated, a gel or oleogel. Accordingly, by the above process a lubricating grease is obtained. If a three-dimensional network, in particular a gel, is formed and a grease is obtained, also depends on the polarity of thickener and respective oil. Thus, the polarity of the monomers with at least one ethylene group and the base oil are preferably to be chosen accordingly.

[0011] An advantage of the process according to the invention is the in-situ production of the thickener and the lubricating grease which allows a fast, efficient, one-step production of the grease (homogenization optional) at room temperature. With the process according to the invention thickeners may be employed derived from monomers, for which a subsequent addition to a lubricating oil according to the state of the art would not lead to the formation of a grease. The lubricating greases obtained by the process according to the invention have highly flexible and easily adjustable properties such as adjustable grease hardness (NLGI 1 to 3), adjustable oil separation and high yield point.

[0012] The process can additionally comprise the steps: c) adding additive / s to the composition in step a), or as formed in steps b) or d), preferably step b); and / or d) homogenising the composition as formed in step b) or c).

[0013] If additives are added in step c) the additives are preferably selected from the group consisting of anti-wear additives, anti-corrosion additives, extreme pressure additives, solid additives, pourpoint depressants, anti-oxidants, detergents, colorants and viscosity index improvers.

[0014] The monomers comprising at least one ethylene group of the polymerizable compound preferably have 1 to 10 ethylene groups, preferably 1 to 6, more preferably 2 to 4.

[0015] Preferably, at least 10 wt% of the monomers, based on the total amount of monomers in the composition of step a), have at least 2 ethylene groups, more preferably 3 or more, most preferably 3 to 6. In an especially preferred embodiment the monomers in the composition of step a), have at least 2 ethylene groups, more preferably 2 to 10, most preferably 3 to 6.

[0016] Preferably, the at least one ethylene group / s is / are part of an acrylate / s, preferably methacrylate / s.

[0017] Examples for suitable (meth)acrylate monomers according to the invention are monofunctional monomer such as 2-hydroxyphenylbenzene acrylate, 2- (hydroxymethyl) tetrahydrofuran acrylate, 4-methyl acrylate, isobornyl acrylate, Amino diacetic acid acrylate, and various derivatives thereof. As the bifunctional monomer, ethylene glycol diacrylates, diethylene glycol diacrylates, propylene glycol diacrylates, glycerol diacrylates, tripropylene glycol diacrylate, 1,6-hexane diol diacrylate, tricyclodecane dimethanol diacrylate, dipropylene glycol diacrylate, glycol diacrylate, neophenyl glycol diacrylate, ethylene glycol dimethacrylate 1,3-propanediol diacrylates, 4-cyclohexanediol diacrylates, 1,4-benzenediol diacrylates, 1,3-butanediol diacrylates, 1,5-pentanediol diacrylates, neopentyl glycol diacrylates, the bis-acrylates of polyethylene glycols of molecular weight 200-4000, α,ω-polycaprolactonediol diacrylates, and derivatives thereof, and trifunctional and polyfunctional monomer may be glycerol triacrylates, mannitol hexaacrylates, dipentaerythritol hexaacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, penta trimethylolpropane triacrylate, trimethylolpropane triacrylate, dipentaerythritol hexaacrylate, pentaerythritol tetracrylates, pentaerythritol triacrylates, 1,1,1-trimethylolpropane triacrylates, pentaerythritol tetraacrylates, triethylene glycol diacrylates, hexamethylenediol diacrylate, or mixtures thereof, preferably methacrylate derivatives thereof and the like.

[0018] If (meth)acrylate monomers are used as polymerizable compound, the (meth)acrylate monomers preferably have a functionality (number of acrylate groups) of 1 to 10, preferably 1 to 6, more preferably 2 to 4.

[0019] In a preferred embodiment, at least 10 wt% of the monomers, based on the total amount of polymerizable compound in the composition of step a), have a functionality of at least two, preferably are di-, tri-, tetra-, penta- and / or hexa (meth)acrylates.

[0020] The composition provided in step a) preferably comprises 1 wt% to 35 wt%, preferably 2 wt% to 25 wt%, more preferably 5 wt% to 10 wt% polymerizable compound, preferably consisting of monomers with at least one ethylene group, preferably (meth)acrylates based on the total amount of the composition.

[0021] The composition provided in step a) preferably comprises 55 wt% to 99 wt%, preferably 65 wt% to 90 wt%, more preferably 80 wt% to 89 wt% lubricating base oil based on the total amount of the composition.

[0022] Basically any oil with a lubricating effect may be used in the composition provided in step a) and the lubricating grease as lubricating base oil. The lubricating base oil preferably is a mineral oil, a natural oil, synthetic oil, more preferably a natural oil or synthetic oil or mixtures thereof.

[0023] Examples for mineral oils naphthenic oils, paraffinic oils, aromatic oils or white oils.

[0024] In a preferred embodiment the lubricating base oil is a natural oil selected from animal oil or plant oil, preferably a plant oil, more preferably selected from the group consisting of castor oil, palm oil, coconut oil, corn oil, cottonseed oil, peanut oil, olive oil, rapeseed oil, soybean oil or sunflower seed oil, and mixtures thereof.

[0025] In another preferred embodiment the lubricating base oil is a synthetic oil, preferably selected from the group consisting of poly-α-olefins (PAO), including metallocene poly-α-olefins, polyisobutylene (PIB), poylesters, polyethers, perfluorpolyethers (PFPE), polyalkylene glycols (PAG), silicone based oils, and mixtures thereof.

[0026] Synthetic base oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as oligomerized, polymerized, and interpolymerized olefins [e.g., polybutylenes, polypropylenes, propylene, isobutylene copolymers, chlorinated polylactenes, poly(1-hexenes), poly(1-octenes), etc., and mixtures thereof]; alkylbenzenes [e.g., polybutylenes, polypropylenes, propylene, isobutylene copolymers, chlorinated polylactenes, poly(1-hexenes), poly(1-octenes), etc., and mixtures thereof]; alkylbenzenes [e.g., dodecyl-benzenes, tetradecylbenzenes, dinonyl-benzenes, di(2-ethylhexyl)benzene, etc.]; polyphenyls [e.g., biphenyls, terphenyls, alkylated polyphenyls, etc.]; and alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs, and homologs thereof, and the like. The preferred synthetic oils are oligomers of a-olefins, particularly oligomers of 1-decene, also known as polyalpha olefins or PAO's.

[0027] Synthetic base oils also include alkylene oxide polymers, interpolymers, copolymers, and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc. This class of synthetic oils is exemplified by: polyoxyalkylene polymers prepared by polymerization of ethylene oxide or propylene oxide; the alkyl and aryl ethers of these polyoxyalkylene polymers (e.g., methylpolyisopropylene glycol ether having an average molecular weight of 1000, diphenyl ether of polypropylene glycol having a molecular weight of 100-1500); and mono- and poly-carboxylic esters thereof (e.g., the acetic acid esters, mixed C3 -C8 fatty acid esters, and C12 oxo acid diester of tetraethylene glycol).

[0028] Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, subric acid, sebasic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic acid, alkylmalonic acids, alkenyl malonic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoethers, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, diisobutyl adipate, di(2-ethylhexyl) sebacate, din-hexyl fumarate, dioctyl sebacate, diisooctyl phthalate, diisooctyl azelate, diisooctyl adipate, diisodecyl azelate, didecyl phthalate, diisodecyl adipate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebasic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid, and the like. A preferred type of oil from this class of synthetic oils are adipates of C4 to C12 alcohols.

[0029] Esters useful as synthetic base oils also include those made from C5 to C12 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane pentaerythritol, dipentaerythritol, tripentaerythritol, and the like.

[0030] Silicon-based oils (such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxysiloxane oils and silicate oils) comprise another useful class of synthetic lubricating oils. These oils include tetra-ethyl silicate, tetraisopropyl silicate, tetra-(2-ethylhexyl) silicate, tetra-(4-methyl-2-ethylhexyl) silicate, tetra-(p-tert-butylphenyl) silicate, hexa-(4-methyl-2-pentoxy)-disiloxane, poly(methyl)-siloxanes and poly (methylphenyl) siloxanes, and the like. Other synthetic lubricating oils include liquid esters of phosphorus containing acids (e.g., tricresyl phosphate, trioctylphosphate, and diethyl ester of decylphosphonic acid), polymeric tetra-hydrofurans, poly-α-olefins, and the like.

[0031] The composition provided in step a) preferably is provided as a layer, preferably a layer with a thickness of at most 5 cm, more preferably a layer with a thickness of at most 2 cm, even more preferably a layer with a thickness of at most 0.5 cm, most preferably a layer with a thickness of at most 0.1 cm.

[0032] Since the lubricating grease is produced in-situ from the composition provided in step a) in the process according to the invention, the process is a continuous process.

[0033] According to the invention the process is a continuous process. In such a continuous process the composition in step a) is preferably provided in a continuous feed and step b) is taken out alongside this feed, more preferably the feed is passing a light source emitting a wavelength suitable to create a reactive species from a photoinitiator or a heat source emitting heat suitable to create a reactive species from a thermoinitiator in order to induce the polymerization of the polymerizable compound.

[0034] The continuous feed may for example be produced by a conveyer belt, in a three roll mill or in a plug-flow-reactor (PFR).

[0035] In one embodiment the composition in step a) is provided as a layer on a conveyer belt, more preferably passing a light source emitting a wavelength suitable to create a reactive species from a photoinitiator or a heat source emitting heat suitable to create a reactive species from a thermoinitiator.

[0036] Suitable initiators are known to a person skilled in the art and may be selected from the initiators typically used for thermal or UV curing of (meth)acrylates. The composition comprises preferably from about 0.01 wt% to about 1.5 wt% polymerization initiator based on the total amount of the composition.

[0037] In a preferred embodiment the polymerization initiator is a thermal initiator. In such a process step b) polymerizing of the polymerizable compound, is preferably taken out by heating the composition of step a), more preferably polymerizing of the polymerizable compound, is taken out below or equal to 90 °C, most preferably between 50 °C and 90 °C. Thermal initiator is preferably selected from peroxide compounds such as benzoyl peroxide, t-butyl peroctoate and cumene hydroperoxide; and azo compounds such as azoisobutyronitrile and 2,2'-azobis (2-methylbutanenitrile).

[0038] In another embodiment the polymerization initiator is a photoinitiator and in step b) polymerizing of the polymerizable compound is taken out by irradiating the composition of step a) with light at a wavelength suitable to create a reactive species from the photoinitiator.

[0039] The process may also be combined and in step b) polymerizing of the polymerizable compound is taken out by irradiating the composition of step a) with light at a wavelength suitable to create a reactive species from the photoinitiator and by heating the composition of step a), more preferably polymerizing of the polymerizable compound, is taken out below or equal to 90 °C, most preferably between 50 °C and 90 °C. The heating and irradiating of the composition of step a) may be taken out subsequently or at the same time. Preferably step b) polymerizing of the polymerizable compound is taken out by irradiating the composition of step a) with light at a wavelength suitable to create a reactive species from the photoinitiator and further polymerization of the composition of step b) is taken out by heating the composition of step b), preferably below or equal to 90 °C, most preferably between 50 °C and 90 °C.

[0040] In this embodiment the photoinitiator is preferably selected from the group consisting of phenones, acyl-phosphine oxides, xanthones, thioxanthones, quinones, xanthenes, thiazenes, acylsilanes, acylgermanes, acylstannanes, flavonoids, metallocenes, azo compounds, organic peroxides, diaryl iodonium salts, triarylsulfonium salts, and mixtures thereof, preferably from the group consisting of benzoin ethers, benzil ketals, acetophenones, α-dialkoxy acetophenones, α-hydroxy alkylphenones, α-amino alkylphenones, acylphosphine oxides, or mixtures thereof. A suitable example is 2-hydroxy-2-methyl-1-phenylpropan-1-one.

[0041] The mean irradiation time in this process is preferably between 0.5 to 300 seconds, preferably 0.5 to 100 seconds, more preferably 0.5 to 10 seconds, most preferably 0.5 to 5 seconds.

[0042] Suitable reactors for the process according to the invention are for example screw extruder, batch and semi batch reactors, plug flow reactor, micromixer / microreactor, belt reactor or tubular reactor.

[0043] The lubricating grease, preferably according to DIN 51825, and produced by the process according to the invention, comprises: i) lubricating base oil and ii) thickener, wherein the thickener comprises, preferably consists of, at least one polymer derived from monomers comprising at least one ethylene group, preferably (meth)acrylates.

[0044] The lubricating grease may additionally comprise (iv) a polymerization initiator, preferably selected from the group consisting of thermoinitiators, photoinitiators and mixtures thereof. Furthermore, the lubricating grease may optionally comprise additives.

[0045] Lubricating base oil, optional additives and respective polymerization initiators in the composition are as defined above.

[0046] The thickener in the lubricating grease comprises, preferably consists of, at least one polymer derived from monomers comprising at least one ethylene group, preferably (meth)acrylates, as defined above.

[0047] In the lubricating grease obtainable by a process according to the invention the thickener is present in a three-dimensional network, preferably enclosing the base oil, thus forming a gel. The polymer can be a linear polymer or a branched polymer.

[0048] Preferably the thickener is a poly(meth)acrylate, copolymer or homopolymer, preferably a poly(meth)acrylate homopolymer.

[0049] The lubricating grease preferably comprises 1 wt% to 35 wt%, preferably 2 wt% to 25 wt%, more preferably 5 wt% to 10 wt% of the thickener, preferably the poly(meth)acrylate based on the total amount of lubricating grease. The lubricating grease preferably comprises 55 wt% to 99 wt%, preferably 65 wt% to 90 wt%, more preferably 80 wt% to 89 wt% lubricating base oil based on the total amount of lubricating grease. The lubricating grease preferably comprises from about 0.01 wt% to about 1.5 wt% polymerization initiator based on the total amount of lubricating grease.

[0050] The lubricating grease preferably has an NLGI consistency number measured according to ASTM D4950 between 000 and 6, preferably 0 and 5, most preferably 1 and 3.

[0051] The lubricating grease has an oil separation, measured according to DIN 51 817 of 0 % to 40 %, preferably 2 % to 35 %.

[0052] Preferably, the lubricating grease has a dropping point, measure according to IP 396 of above 240 °C.Figures

[0053] Fig. 1Shows the thickener network after extraction. Fig. 2Shows the thickener network after extraction (REM image).Shows data of yield point measurements on samples 1 to 4. Castor oil (Sample 1), rapeseed oil (Sample 2), polyglycol (Sample 3) and synthetic ester (Sample 4). Fig. 3Shows photographs of lubricating greases according to samples 6, 10, 27 and 28. Fig. 4Shows photographs of lubricating greases according to samples 31, 32, 33, 34 and 35. Examples

[0054] Compositions were provided consisting of 5 wt% trimethylolpropantriacrylate, 1 wt% of a blend of 2-Hydroxy-2-methyl-1-phenylpropanone, Bis(2,4,6-Trimethylbenzoyl)-phenylphosphine oxide and Ethyl(2,4,6-Trimethylbenzoyl)-phenyl phosphinate and a 94 wt % lubricating base oil. Several base oils were tested: Castor oil (Sample 1), rapeseed oil (Sample 2), polyglycol (Sample 3) and synthetic ester (Sample 4).

[0055] The lubricating greases were produced from the compositions by irradiating with light. To homogenize the grease, it was rolled using a three-roll mill. Yield Point and Oil separation according to DIN 51817 have been measured for samples 4 and 3 (results see tables 1 and 2, respectively). SRV load level tests were performed (see figure 4).

[0056] Lubricating greases have been prepared according to the process employing UV-curing, as described above, and standard tests for lubricating greases were performed. Results are given in tables 1 and 2. Triacrylate used in these samples was trimethylolpropantriacrylate (BASF). Initiator used in these samples was Omnirad 2022 (IGM Resins). Base oils employed were Synthetic Ester Synative 3345 (BASF), Polyglycol B01 / 20 (Clariant). Table 1: sample 4ParameterMethodResultDrop PointIP 396>240 [°C]ConsistencyDIN ISO 2137NLGI Class 2Oil separationDIN 518175 [%]Grease roll stability testASTM D 1831Δ2 [0,1 mm]Copper corrosionDIN 518111aGrease / Water StaticDIN 51807-1Class 1Water Wash Out (WWO)ASTM D12642 [wt %] Table 2: sample 3 ParameterMethodResultDrop PointIP 396>240 [°C]ConsistencyDIN ISO 2137NLGI Class 2Oil separationDIN 518170 [%]Grease roll stability testASTM D 1831Δ16 [0,1 mm]Copper corrosionDIN 518111aGrease / Water StaticDIN 51807-1Class 1Water Wash Out (WWO)ASTM D126414 [wt %]

[0057] Further examples for lubricating greases prepared according to above process employing UV-curing are shown in table 3 below. Table 3:Ex. Monomer 1 Monomer 2 Amount [wt%] Baseoil 1 Baseoil 2 Amount [wt%] NLGI Class 5TMPTA / 5Synthetic Ester946TMPTA / 5.5Polyglycol93.517TMPTA7Castoroil928TMPTA7Rapeseedoil929TMPTA7Complexester9210TMPTA6Siliconoil931-211TMPTA10EO / PO Basestock8912TMPTA10RapeseedoilCastoroil44 / 4413TMPTA10RapeseedoilCastoroil22 / 6614TMPTA10RapeseedoilCastoroil66 / 2215TMPTA7RapeseedoilPolyglycol45 / 4516TMPTAMADDE9 / 1Rapeseedoil8917TMPTAMADDE8 / 2Rapeseedoil8918TMPTAMADDE7 / 3Rapeseedoil8919TMPTA5Synthetic Ester9420HDDA8Rapeseedoil9121HDDA5.5Polyglycol93.522TMPTAHDDA3.25 / 3.25Synthetic Ester92.523DPEHA5Synthetic Ester9424DPEHA5,5Polyglycol93.525PETA5Synthetic Ester9426PETA5.5Polyglycol93.527TMPTMA5Synthetic Ester942-328BAHDDA5 / 5PAO 4089229IBA11PAO 408830TMPTA7.5Gr 1 - Mineraloil91.5 TMPTATrimethylolpropantriacrylate MADDELaurylmethacrylate HDDA1,6-Hexandioldiacrylate DPEHADipentaerythritol hexaacrylatee PETAPentaerythritoltetraacrylate TMPTMATrimethylolpropantrimethacrylate BAButylacrylate IBAIsobornylacrylate

[0058] Further examples for lubricating greases prepared by employing heat-curing are shown in table 4 below. Compositions as depicted in table 4 were provided additionally comprising 1 wt% of Dibenzoylperoxid and the respective lubricating greases were produced from the compositions by heating them to approx.. 85°C. To homogenize the grease, it was rolled using a three-roll mill. Table 4:Ex.MonomerAmount [wt%]BaseoilAmount [wt %]NLGI Class31TMPTA5Synth. Ester94232TMPTA5.5Polyglykol93.51-233TMPTA6Siliconoil93234PETA5Synth. Ester94135TMPTMA5Synth. Ester9400-0

Claims

1. A process for the manufacturing of a lubricating grease, comprising the steps: a) providing a composition comprising lubricating base oil, polymerization initiator and polymerizable compound comprising monomers with at least one ethylene group; b) polymerizing the polymerizable compound, characterized in that the process is a continuous process.

2. The process according to claim 1, characterized in that the monomers are (meth)acrylates.

3. The process according to claim 1 or 2, characterized in that the lubricating base oil is a natural oil, preferably a plant oil, more preferably selected from the group consisting of castor oil, palm oil, coconut oil, corn oil, cottonseed oil, peanut oil, olive oil, rapeseed oil, soybean oil or sunflower seed oil, and mixtures thereof.

4. The process according to claim 1 or 2, characterized in that the lubricating base oil is a synthetic oil, preferably selected from the group consisting of (metallocene) poly-α-olefins (PAO), polyisobutylene (PIB), poylesters, polyethers, perfluorpolyethers (PFPE), polyalkylene glycols (PAG), silicone based oils, and mixtures thereof.

5. The process according to claims 1 to 4, characterized in that the process additionally comprises the steps: c) adding at least one additive to the composition as formed in step b), and / or d) homogenizing the composition formed in step b) or c).

6. The process according to claims 1 to 5, characterized in that the composition provided in step a) comprises 1 wt% to 35 wt%, preferably 2 wt% to 25 wt% polymerizable compound comprising monomers with at least one ethylene group, preferably (meth)acrylates, based on the total amount of the composition.

7. The process according to claims 1 to 6, characterized in that the polymerization initiator is a thermal initiator and that in step b) polymerizing of the polymerizable compound is taken out by heating the composition of step a) to a temperature suitable to create a reactive species from the thermoinitiator.

8. The process according to claims 1 to 7, characterized in that the polymerization initiator is a photoinitiator and that in step b) polymerizing of the polymerizable compound is taken out by irradiating the composition with light at a wavelength suitable to create a reactive species from the photoinitiator.

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