Patent Publication Number: US-2022235245-A1

Title: Curable compositions with outdoor performances

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to gel coat finish layers. 
     BACKGROUND OF THE INVENTION 
     Light-weight composites are commonly used in manufacturing many items, especially substrates comprising fiber reinforced composites. 
     To obtain a reasonable surface appearance, a coating, often named as “gel coat”, is applied over a supporting substrate comprising fibers. Parts can be produced to have a finish bearing any desired color originally carried by the gel coat. However, attainment of a gel coat surface with a suitably smooth finish remains a challenge. 
     It is desirable to produce a pigmented gel coat, so as to eliminate the need of painting, and to provide good protection against fading due to heat and ultraviolet radiation if the substrate is exposed to sun radiations such as outdoor exposed substrates are. A particular challenge is to produce a surface finish on the gel coat that is both defect free and highly resistant to degradation from exposure to ultraviolet radiation. WO2004/015004 describes gel coat compositions based on aliphatic polyester polyols and aliphatic polyisocyanates. 
     Therefore, it is desirable to improve the surface appearance of the gel coat, and provide a part that will maintain color and high gloss when exposed to outdoor circumstances. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a gel coat suitable as a finish for a composite article. The gel coat layer contains a urethane acrylate resin, and in a preferred embodiment is pigmented. In a preferred embodiment, the gel coat further comprises an ultraviolet inhibitor package and viscosity control agents to control sag and surface appearance. A gel coat composition can be produced that on curing yields a surface having a good surface finish and gloss retention of more than 60% when exposed to UV radiation according to the procedure detailed below. The gel coat preferably provides a suitable surface finish to a supporting substrate. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     The invention provides a radiation curable gel coat composition for fibers reinforced substrates, comprising a gel coat resin and at least one diluent monomer (B), 
     wherein the gel coat resin comprises urethane (meth)acrylate reaction products (A) of a reaction mixture comprising:
         (a) a hydroxy-terminated polyol having a weight average molecular weight of about 200 to about 4000, this polyol being a polycaprolactone polyol or a [(poly)carbonate-(poly)caprolactone]polyol;   (b) a diisocyanate; and   (c) a hydroxyalkyl (meth)acrylate,       

     and wherein both the diluent monomer (B) and the reaction product (A) contain polymerizable ethylenic unsaturated groups, preferably (meth)acrylate groups. 
     Preferably, compound (a) has a weight average molecular weight of about 500 to about 3000. Preferably, a reaction mixture of (a), (b), and (c) contains a molar ratio of about 0.75 to about 1.25 mole (a) to about 1.5 to about 2.5 moles (b) to about 1.5 to about 2.5 moles (c), preferably a molar ratio of about 0.9 to about 1.1 mole (a) to about 1.7 to about 2.2 moles (b) to about 1.7 to about 2.2 moles (c), more preferably a molar ratio of about 0.95 to about 1.05 mole (a) to about 1.7 to about 2 moles (b) to about 1.7 to about 2 moles (c). 
     Preferably, compound (b) comprises an aliphatic diisocyanate and optionally up to 20% of an aromatic diisocyanate, by total weight of the diisocyanate, the aliphatic diisocyanate comprising preferably 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 2,4′-dicyclohexylmethane diisocyanate, and/or 4,4′-dicyclohexylmethane diisocyanate more preferably isophorone diisocyanate (IPDI). 
     Preferably, compound (c) has a structure 
       CH 2 ═C(R 1 )—C(═O)—O—R 2 —OH
 
     wherein R1 is hydrogen or methyl and R 2  is a C1 to C6 alkylene group or an arylene group, preferably 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl methacrylate, or 3-hydroxypropyl methacrylate, more preferably 2-hydroxyethyl acrylate (HEA). 
     Preferably, the polycaprolactonepolyol comprises a reaction of product of (i) neopentyl glycol, 1,6-hexanediol, or a mixture thereof, and (ii) ε-caprolactone. 
     Preferably, the urethane (meth)acrylate is present in the gel coat resin composition in an amount of about 20% to about 80%, preferably 30 to 70% by weight, of the composition. 
     Preferably, the composition further comprises a pigment. 
     Preferably, the diluent monomer comprises an acrylate monomer, preferably difunctional or trifunctional, more preferably isobornyl(meth)acrylate, hexanediol di(meth)acrylate, trimethylolpropanetri(meth)acrylate or a mixture thereof. 
     The invention also provides a cured gel coat prepared by curing a gel coat composition as defined above. 
     Preferably, the composition contains a photoinitiator and is cured by ultraviolet radiation. 
     Preferably, the composition is cured by electron beam. 
     Preferably, the composition contains a peroxide and is peroxide cured. 
     The invention also provides an article of manufacture having an exterior gel coat prepared by curing a gel coat composition as defined above. 
     The invention also provides a gel coat composition comprising a gel coat resin and at least one diluent monomer, wherein the gel coat resin comprises urethane(meth)acrylate reaction product made by a process comprising the steps of
         (a) preparing a hydroxy-terminated polycaprolactonepolyol, and/or a hydroxy-terminated [(polycaprolactone)(polycarbonate)]polyol having a weight average molecular weight of about 200 to about 4000 by reacting (i) a saturated diol and optional saturated polyol with ε-caprolactone, in sufficient relative amounts of (i) and (ii) to provide terminal hydroxy groups;   (b) adding a hydroxyalkyl (meth)acrylate to the hydroxy-terminated polyol of step (a) to form a prereaction mixture;   (c) then adding a diisocyanate to the prereaction mixture of step (b) to form a reaction mixture; and   (d) maintaining the reaction mixture of step (c) at a sufficient temperature for a sufficient time to react essentially all isocyanate moieties of the diisocyanate and yield the urethane acrylate gel coat resin.       

    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The substrate provides most of the strength of the composite article. The gel coat layer may be pigmented. It is used to provide an aesthetic appearance to the composite article. 
     Composite articles comprising the gel coat of the invention can be prepared by conventional processes. For example, a gel coat composition may be spread across the surface of a mold by any one of a number of conventional techniques, e.g., brushing, hand lay-up, or spraying, and usually as a relatively thick layer, e.g., 0.1 to 0.5 mm, to maximize its weather and wear resistance, and if the molded article is fiber-reinforced, to help mask the fiber reinforcement pattern which can appear through the gel coat due to inherent resin shrinkage that occurs around the fibers during cure. After the gel coat is applied to the surface of the mold, it is at least partially cured. A plastic, optionally fiber-reinforced, then is applied to the partially or fully cured gel coat by any one of a number of conventional techniques, and the resulting laminate structure containing gel coat and substrate are cured. Advantageously, the gel coat cures on the substrate at a temperature of 50° C. or less. The cure is typically obtained through the use of free radical polymerization processes by exposure to UV radiation or EB radiation. The cure can alternatively be obtained by peroxide curing. 
     Gel coats of the invention are based on a specific class of urethane acrylate resins. The main ingredients of the gel coats are resin, pigment paste, diluent monomers, additives, and initiator, each of which will be further discussed below. In a preferred embodiment, the gel coats of the invention retain a gloss of 60-70%, no visible cracks appear and the colors stays consistent during the Xenon accelerated weathering test. 
     The gel coat compositions typically contains from 30-80% of resin, preferably 40-60% resin, based on the total weight of the composition. The gel coat composition may contain pigment. 
     When present, the pigment is typically present as a pigment paste, wherein the pigment paste is in the range of about 2-30% by weight of the total composition. In preferred embodiments, the pigment paste is present at from 5-20% by weight. Diluent monomers can be present in the gel coat composition at a range of about 10% to about 50% by weight of the composition, preferably from about 20% to about 50%. Additives make up the remainder of the composition. Such additives include, without limitation, dispersing agents, defoamers, ultraviolet light stabilizers, thixotropic agents, and the like. In addition, the compositions include up to 3% by weight of a photoinitiator capable of initiating free radical polymerization of the monomers and the resins to cure the resin. 
     In one aspect, the resin of the gel coat is based on a urethane (meth)acrylate resin containing a polyurethane polymer with olefin functionality at the ends of the polymer. Preferred resins contain urethanes, or polyurethanes, end capped with (meth)acrylic based monomers, especially urethanes. Surprisingly, it was found that a gel coat being a reaction product of (a) a polycaprolactone polyol, or [(polycaprolactone)(polycarbonate)]polyol with a weight average molecular weight (M w ) about 200 to about 4000, (b) a diisocyanate, and (c) a hydroxyalkyl (meth)acrylate provides good performances in accelerated outdoor tests. 
     A urethane-(meth)acrylate gel coat resin of the present invention has an idealized structure (1) C-B-A-B-C, (I) 
     wherein (I) is the reaction product of a polycaprolactonepolyol, or 
     [(polycaprolactone)(polycarbonate)]polyol having M w  of about 200 to about 4,000 (A), a diisocyanate (B), and a hydroxyalkyl (meth)acrylate (C). A urethane (meth)acrylate gel coat resin of the present invention is a reaction product of A, B, and C, thus other reactions species generally are present in addition to a resin of idealized structure (I). 
     In accordance with an important feature of the present invention, a present urethane acrylate gel coat resin contains a polycaprolactonepolyol, or [(polycaprolactone)(polycarbonate)]polyol of M w  about 200 to about 4000 that is reacted with a diisocyanate, and the resulting urethane product is end-capped with a hydroxyalkyl (meth)acrylate. The urethane (meth)acrylate resin therefore contains terminal vinyl groups available for free radical polymerization, typically using a photoinitiator or a peroxide catalyst. Another option is to cure the product through EB radiation. 
     The individual ingredients used in the manufacture of a present urethane (meth)acrylate gel coat resin are described in more detail below.
         (a) a polycaprolactonepolyol, or [(polycaprolactone)(polycarbonate)]polyol       

     The polyol component (a) of a present urethane acrylate gel coat resin preferably has a weight average molecular weight of about 200 to about 4000. These polyols can be made according to prior art. Many different grades are commercially available on the market. 
     Polycaprolactonepolyols are commercialized under the tradenames CAPA from Ingevity or PLACCEL from Daicel Corporation. Mixed polycarbonate/polycaprolactone polyols are available from Covestro under the brandname of Desmophen. Examples are Desmophen C 1100 and 1200. 
     The manufacturing of polycarbonatepolyols is known in the art. They can be prepared from one or more saturated polyols and one or more carbonates and/or phosgene. 
     Non-limiting examples of polyols used to prepare the polycarbonatepolyol include ethylene glycol, diethylene glycol, tri ethylene glycol, propylene glycol, dipropylene glycol, 1,6-hexanediol, 1,3-butylene glycol, 1,4-butylene glycol, neopentyl glycol, cyclohexanedimethanol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,2-ethylbutyl-1,3-propanediol, isopropylidene bis(p-phenyleneoxypropanol-2), a polyethylene or polypropylene glycol, glycerol and trimethylolpropane, ethoxylated, propoxylated or a mixed ethoxylated/propoxylated trimethylolpropane. 
     Non-limiting examples of carbonates are dimethylcarbonate, diethylcarbonate, diphenylcarbonate, ethylenecarbonate or propylenecarbonate. 
     Polycaprolactone polyols and [(polycaprolactone)(polycarbonate)]polyol are preferably prepared from reacting one or more saturated polyol(s) with ε-caprolactone. 
     
       
         
         
             
             
         
       
     
     R1 can be derived from an alkyl polyol or derived from a polycarbonatediol, R2 is —(CH 2 ) 5 — 
     
       
         
         
             
             
         
       
     
     Example of a Polycarbonatediol 
     The polyol and ε-caprolactone, in correct proportions, are interacted under standard esterification procedures to provide the necessary M w , molecular weight distribution, branching, and hydroxy-terminated functionality for use in a present urethane acrylate gel coat resin. 
     R3 and R4 are alkyl rests of the starting diol. They may be the same or different, and present in various ratios in the structure. 
     Polycarbonatediols can also be used to prepare polycarbonatecaprolactonepolyols. 
     Preferred are the alkyl polyols, wherein the alkyl comprises 2-24 carbon atoms, preferably 2-12 carbon atoms, such as 1,6-hexanediol, 1,3-butylene glycol, 1,4-butylene glycol, cyclohexanedimethanol, 1,5-pentanediol, 3-methyl-1,5-pentanediol and 2,2-dimethyl-1,3-propanediol (neopentyl glycol). 
     As used herein, the terms “polyol” is defined as a compound that contains two or more, and typically two to four, hydroxy (OH) groups, or two or more, typically two or three, hydroxy (OH) groups, respectively. 
     Preferably, the polycaprolactonepolyols and [(polycaprolactone)(polycarbonate)]polyol are hydroxy terminated to provide reactive moieties for a subsequent reaction with a diisocyanate. 
     The polycaprolactonepolyols and [(polycaprolactone)(polycarbonate)]polyols typically have a M w  of about 200 to about 4000, more preferably about 400 to about 3500, and most preferably about 500 to about 3000. Accordingly, the polyols have low molecular weight. 
     A polycarbonate polyol can be obtained from reaction of one (OH—R 3 —OH) or 2 different diols (OH—R 1 —OH, OH—R 2 —OH) on a carbonate reactant (RO—C(═O)—OR).
         (b) Diisocyanate       

     The diisocyanate component (b) of a present urethane acrylate gel coat resin is an aliphatic diisocyanate. The diisocyanate component optionally can contain up to about 20%, and preferably up to about 10%, by total weight of the diisocyanate, of an aromatic diisocyanate. The identity of the aliphatic diisocyanate is not limited, and any commercially available commercial or synthetic diisocyanate can be used in the manufacture of a urethane acrylate gel coat resin of the present invention. 
     Non-limiting examples of aliphatic diisocyanates include 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 1,4-cyclohexane diisocyanate, 2,4′-dicyclohexylmethane diisocyanate, 4,4′-dicyclohexylmethane diisocyanate, 1,3-bis-(isocyanatomethyl)cyclohexane, 1,4-bis(isocyanatomethyl)cyclohexane, 
     tetramethylxylylene diisocyanate, 1,11-diisocyanatoundecane, 1,12-diisocyanatododecane, 2,2,4-trimethyl-1,6-diisocyanatohexane, 2,4,4-trimethyl-1,6-diisocyanatohexane, 1,2-bis(isocyanatomethyl)cyclobutane, hexahydro-2,4-diisocyanatotoluene, hexhydro-2,6-diisocyanatotoluene, 1-isocyanato-2-isocyanatomethyl cyclopentane, I-isocyanato-3-isocyanatomethyl-3,5,5-trimethyl cyclohexane, I-isocyanato-4-isocyanatomethyl-I-methyl cyclohexane, 1-isocyanato-3-isocyanatomethyl-1-methyl cyclohexane, and mixtures thereof. A preferred aliphatic diisocyanate is isophorone diisocyanate. 
     Non-limiting examples of optional aromatic diisocyanates includes toluene 2,4-diisocyanate, toluene 2,6-diisocyanate, 4,4′-methylene diphenyl 
     diisocyanate, 2,4′-methylene diphenyl diisocyanate, polymeric methylene diphenyl diisocyanate, p-phenylene diisocyanate, naphthalene-1,5-diisocyanate, and mixtures thereof.
         (c) Hydroxyalkyl (meth)acrylate       

     The hydroxyalkyl (meth)acrylate component (c) of a present urethane acrylate gel coat resin is preferably a mono hydroxyalkyl ester of an α,β-unsaturated acid, or anhydride thereof. Suitable α,/β-unsaturated acids include a monocarboxylic acid such as, but not limited to, acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, β-methylacrylic acid (crotonic acid), -phenylacrylic acid and mixtures thereof. As used throughout this specification, the term “(meth)acrylate” is an abbreviation for acrylate and/or methacrylate. 
     A preferred acrylate monomer containing a hydroxy group is a hydroxyalkyl (meth)acrylate having the following structure: 
     
       
         
         
             
             
         
       
     
     wherein R is hydrogen or methyl, and R is a to C 6  alkylene group or an arylene group. For example, R 2  can be, but is not limited to (—CH—) n , wherein n is 2 to 6, or any other structural isomer of an alkylene group containing three to six carbon atoms, or can be a cyclic C 3 -C 6  alkylene group. R 2  also can be an arylene group like phenylene (i.e., C 6 EU) or naphthylene (i.e., C 10 H 6 ). R 2  optionally can be substituted with relatively non-reactive substituents, like Ci-Cβ alkyl, halo (i.e., Cl, Br, F, and I), phenyl, alkoxy, and aryloxy (i.e., an OR 2  substituent). 
     Specific examples of monomers containing a hydroxy group are the hydroxy(C 1 -C 6 )alkyl (meth)acrylates, e.g., 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl methacrylate, and 2-hydroxypropyl acrylate. 
     The relative amounts of (a), (b), and (c) used in the manufacture of a urethane acrylate gel coat resin of the present invention are sufficient to provide a reaction product having an idealized structure (I). Accordingly, component (a) is used in a molar amount of about 0.75 to about 1.25, and preferably about 0.9 to 1.1 moles; component (b) is used in an amount of 1.5 to about 2.5, and preferably about 1.7 to about 2.2 moles; and component (c) is used in an amount of about 1.5 to about 2.5, and preferably about 1.7 to about 2.2 moles. To achieve the full advantage of the present invention, the mole ratio of (a):(b):(c) is 1:1.7-2:1.75-2. 
     An urethane acrylate gel coat resin of the present invention is manufactured by first preparing the polycaprolactonepolyol or the [(polycaprolactone)(polycarbonate)]polyol. The polycaprolactonepolyol and the [(polycaprolactone)(polycarbonate)]polyol can be prepared from a polyol, predominantly or completely a diol such as a polycarbonate diol, and ε-caprolactone, using standard esterifying condensation conditions. The amounts and relative amounts of polyol and ε-caprolactone are selected, and reaction conditions are used, such that the polycaprolactonepolyol preferably has an M w  of about 200 to about 4000 and is hydroxy terminated. The polycaprolactonepolyol can be saturated or unsaturated. Preferably the polycaprolactone polyol is saturated. 
     The polycaprolactonepolyol or the [(polycaprolactone)(polycarbonate)]polyol then is blended with the hydroxyalkyl (meth)acrylate, followed by addition of the diisocyanate. 
     The resulting reaction leads to a mixture of products, including a species having the idealized structure (I). Structure (I) has terminal acrylate moieties available for polymerization using standard free radical techniques. 
     In embodiments, the composition may comprise a photoinitiator. The curing of the composition can be performed with or without the use of photoinitiators. Typically, the compositions of the invention comprise at least one photoinitiator. Any photoinitiator or mixtures thereof capable of generating free radicals when exposed to radiation may be used. Preferred photoinitiators include benzophenones, benziketals, or acyl phosphine oxides. Preferred photoinitiators are available as Omnirad by IGM), ADDITOL® photoinitiators available from allnex, or Speedcure photoiniators from Lambson Ltd. 
     When present, the amount of photoinitiators in the composition of the second aspect is typically from 0.01 to 10 wt %, preferably from 1 to 8 wt %, more preferably from 3 to 5 wt % relative to the total weight of the composition. 
     In some embodiments, the composition may comprise inhibitors. Examples of suitable inhibitors include but are not limited to phenolic inhibitors such as hydroquinone (HQ), methyl hydroquinone (THQ), tert-butyl hydroquinone (TBHQ), di-tert-butyl hydroquinone (DTBHQ), hydroquinone monomethyl ether (MEHQ), 2,6-di-tert-butyl-4-methylphenol (BHT) phenothiazine (PTZ) and the like. When present, inhibitors are preferably present in an amount up to 0.5 wt %, in particular from 0.0001 to 0.2 wt %, and preferably from 0.01 to 0.1 wt % of the composition. 
     They may also include phosphines, like triphenylphosphine (TPP) and other materials such as tris-nonylphenylphosphite (TNPP) and triphenyl antimony (TPS). 
     In embodiments, the composition may further comprise a UV absorber and/or a hindered amine light stabilizer. The UVAs protect the polymers by absorbing destructive UV radiation, while the HALS material protects by reacting with the free radicals that occur after a high-energy UV photon breaks a chemical bond in a polymer. 
     Examples of UVAs are benzotriazoles such as Tinuvin® 328, Tinuvin® 1130, Tinuvin® 900, Tinuvin® 99-2, and Tinuvin® 384-2, triazines such as Tinuvin® 400, Tinuvin® 405, Tinuvin® 460, Tinuvin® 477, and Tinuvin® 479, and benzophenones such as Tinuvin® 531. 
     Examples of HALS are Tinuvin® 123, Tinuvin® 144, and Tinuvin® 292, 2,2,6,6-tetramethylpiperidine and 2,6-di-tert-butylpiperidine. 
     The sum of UV absorbers and HALS, when present, may be used in an amount of from 0.1 to 5.0, preferably from 0.5 to 3 wt % of the composition. 
     In embodiments, the composition of the second aspect may further include one or more reactive diluents. The reactive diluents are radiation curable unsaturated compounds. They can be used to reduce the viscosity of the composition. 
     Reactive diluents comprise at least one radiation curable unsaturation such as an allyl group, a vinyl group, or a (meth)acryloyl group. Reactive diluents typically are (meth)acrylated monomers. 
     They are for example selected from mono-, di-, tri- and poly (meth)acrylates. Herein, a poly (meth)acrylate is a compound having more than three reactive (meth)acrylate groups. 
     Preferably the viscosity of reactive diluents used is in the range of from 5 mPa·s to 2 Pa·s at a temperature of 25° C. and most preferably it is &lt;500 mPa·s. 
     In embodiments, enough reactive diluents are present in the coating composition to decrease its viscosity below 50 Pa·s, preferably below 40 Pa·s, more preferably below 30 Pa·s. Preferably the reactive diluents used have a number average molecular weight (Mn) in the average range of from 100 to 1000 Daltons, more preferably 200 to 800 Daltons and most preferably 200 to 500 Daltons. Typically, the weight average molecular weight (MW) is at most 1000 Daltons. 
     An example of suitable (meth)acrylated monomer are alkyl (meth)acrylates represented by a formula CH 2 ═C(R 1 )COOC z H 2z+1 , wherein R 1  is a hydrogen atom or a methyl group, and z is an integer of from 1 to 13, wherein C z H 2z+1  may have a straight chain structure or a branched chain structure. Suitable examples of suitable reactive diluents may include but are not limited to: allyl (meth)acrylate, benzyl (meth)acrylate, butoxyethyl (meth)acrylate, butanediol di(meth)acrylate, butoxytriethylene glycol mono (meth)acrylate, t-butylaminoethyl (meth)acrylate, 3-chloro-2-hydroxypropyl (meth)acrylate, 2-cyanoethyl (meth)acrylate, cyclohexyl (meth)acrylate, 2,3-dibromopropyl (meth)acrylate, dicyclopentenyl (meth)acrylate, N, N-diethylaminoethyl (meth)acrylate, N, N-dimethylaminoethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-(2-ethoxyethoxy) ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, glycerol (meth)acrylate, glycidyl (meth)acrylate, heptadecafluorodecyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxy-3-(meth)acryloyloxypropyltrimethylammonium chloride, 2-hydroxypropyl (meth)acrylate, [gamma]—(meth)acryloxypropyltrimethoxysilane, 2-methoxyethyl (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxytriethylene glycol (meth)acrylate, methoxytetraethylene glycol (meth)acrylate, methoxydipropylene glycol (meth)acrylate, methoxylated cyclodecatriene (meth)acrylate, morpholine (meth)acrylate, nonylphenoxypolyethylene glycol (meth)acrylate, nonylphenoxypolypropylene glycol (meth)acrylate, octafluoropentyl (meth)acrylate, phenoxyhydroxypropyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxytetraethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, phenoxy (meth)acrylate, polypropylene glycol (meth)acrylate, sodium 2-sulfonate ethoxy (meth)acrylate, tetrafluoropropyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, trifluoroethyl (meth)acrylate, hexafluoroisopropyl acrylate, vinyl acetate, N-vinyl caprolactam, N-vinylpyrrolidone, dicyclopentadienyl (meth)acrylate, and isobornyl (meth)acrylate (IBO(M)A. Preferred however are monomers with at least 2, more preferably at least 3 polymerizable functional groups such as (meth)acryloyl groups. Examples of poly-unsaturated compounds from this category are trimethylolpropane tri-(meth)acrylate (TMPT(M)A), 1,6-hexanediol di(meth)acrylate (HDD(M)A), glycerol tri-(meth)acrylate, pentaerythritol tri, tetra-(meth)acrylate, pentaerythritol tetra-(meth)acrylate, di-trimethylolpropane tetra-(meth)acrylate, di-pentaerythritol hexa-(meth)acrylate and their (poly) ethoxylated and/or (poly) propoxylated equivalents, as well as mixtures thereof. The acrylated forms hereof are preferred over methacrylated forms. Most preferred are di- and/or tri-acrylates. 
     Reactive diluents e are preferably included in a proportion of 0.01 to 90 wt %, in particular from 0.5 to 75 wt %, in particular from 10 to 60 wt % of the composition. 
     The gel coat compositions may contain an initiator capable of initiating cure of the gel coat by a free radical polymerization mechanism at temperatures of about 50° C. or lower. Preferably, the initiator is capable of initiating cure at room temperature, or about 20-30° C. Generally, the initiator includes both an initiator compound and an activator or promoter. The initiator and activator work in combination to initiate cure at a desired processing temperature. Preferred initiators include various organic peroxides and peracids. Examples of initiators that initiate cure at a temperature of about 50° C. or less include, without limitation, benzoyl peroxide, methyl ethyl ketone hydroperoxide (MEKP), and cumene hydroperoxide. In a preferred embodiment, methyl ethyl ketone hydroperoxide is used in a level of about 1-3% Oxygen. Activators such as cobalt octoate, cobalt 2-ethylhexanoate, and cobalt naphthenate are suitable for working with the methyl ethyl ketone hydrogen peroxide to initiate cure. Non-cobalt containing promoters such as dimethylacetoacetamide may also be used, in a preferred embodiment, the gel coat compositions contain up to 1% of a cobalt containing promoter and up to 1% of a non-cobalt containing promoter such as dimethylacetoacetamide. 
     To prepare the gel coat compositions, the additives may be added in sequence to the resin with stirring. Thereafter the pigment paste may be added. The mixture is mixed thoroughly, filtered and stored in a drum. Thus, the invention provides a gel coat layer comprising a resin suitable for use in relatively low temperature curing processes. 
     The invention has been described above with respect to preferred embodiments. Further non-limiting examples are given in the examples that follow. 
     EXAMPLES 
     The following abbreviations are used in the Examples:
     DBTDL Dibutyltin dilaurate   HEA 2-hydroxyethylacrylate   IPDI isophorone diisocyanate   HALS hindered amine light stabilizer   HDDA 1,6-hexanediol diacrylate   MEHQ:4-methoxyphenol   BHT:2,6-Di-tert-butyl-4-methylphenol   TPO-L:Ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate   Pigment MB:HDDA/EBECRYL 331/Kronos 2160   Additol XL 6507: additives available from allnex   Additol Lambda: additives available from allnex   BYK 346: additive available from Byk   Tinuvin 292: Light stabilizer from BASF   Tinuvin 1130: UV light absorber additive from BASF   Curox 1-300 is Methyl isobutyl ketone peroxide available from United Initiators   Oligoester: polyester polyol made from a polyol and polyacid   Polycaprolactone diol is typically CAPA 2054 from Ingevity. Polycaprolactone diol of MW 550,   OH value 204 mg KOH/g   

     Example 1 
     Polycaprolactonediol (400 parts) was added into a flask equipped with an agitator, and the mixture was melted. Next, the reactor was sparged using one part air and 2 part nitrogen as a blanket. Next, DBTDL (0.03 parts), BHT (0.05 wt. parts), HEA (150 parts), and IPDI (320 parts) were added to the polyol. The IPDI was added at a rate such that the exothermic reaction was maintained below 80° C. (e.g., over about 30-60 minutes). The reaction was continued for 2 to 3 hours, periodically testing for free isocyanate groups (% NCO). A % NCO of less than 0.3 is preferred. At the completion of the reaction, MEHQ (0.03 wt. parts) and HDDA (150 parts) were added slowly to the urethane acrylate resin at a temperature below 90° C. The resulting mixture was than cooled down while stirring to 60° C. for at least one hour. The resulting product contained about 85% urethane acrylate resin and about 15% HDDA monomer. 
     Example 2* (Comparative) 
     Synthesis of Oligoester 
     Butanediol (84 parts) and adipic acid (100 parts) were added in a reactor equipped with an agitator and the mixture is covered with a nitrogen blanket and heated. Water is removed during the esterification. The reaction is run at a maximum temperature of 215° C. under nitrogen sparging. When the acid number drops below 20 mg KOH/gram catalyst is added (Fascat 4102, 0.03 parts) and the reaction is continued until the acid number is below 1 mg KOH/gram. At the end of the reaction the product is cooled down to 90° C. and drummed off. 
     Synthesis of Urethane Acrylate 
     The urethane acrylate gel coat resin of this example contains a saturated butanediol-adipate. As in Example 1, this product is reacted with IPDI and HEA to produce a urethane polyester copolymer having acrylic unsaturation at the terminal positions. The resin of Example 2 is prepared in a manner essentially identical to Example 1. 
     Example 3: Coating and Curing 
     A composite material (non-woven glass fibre and unsaturated polyester) is coated with a primer by wet spray with 150 g/m 2 . The composition of the primer is shown in the table below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Primer formulation 
                 pbw 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 EBECRYL 524/EBECRYL 113 
                 100 
               
               
                   
                 6/4 
                   
               
               
                   
                 XL 6507 
                 0.5 
               
               
                   
                 BYK 346 
                 0.5 
               
               
                   
                 Photoinitiator blend 
                 2 
               
               
                   
                 50/50% wt 
                   
               
               
                   
                 benzophenone/1-hydroxy- 
                   
               
               
                   
                 cyclohexyl-phenyl-ketone 
                   
               
               
                   
                 TPO//TPO-L 
                 2 
               
               
                   
                 Pigment MB 
                 4 
               
               
                   
                 Desmodur 3900 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     EBECRYL 524 and 113 were obtained from allnex. As an alternative EBECRYL 571 and 741 can also be used, each time diluted with EBECRYL 113. 
     This primer was then dried at 60° C. and UV-cured at 6 m/min with a 80 W Ga+Hg UV source. 
     Once cured, the topcoat is applied at a thickness of 20 to 40 gram/m 2  with a roller coater. The three formulations are shown below. 
     
       
         
           
               
               
            
               
                   
               
               
                   
                 Formulations (parts in weight) 
               
            
           
           
               
               
               
               
            
               
                   
                 A 
                 B* 
                 C 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 1 
                 100 
                   
                 100 
               
               
                 Example 2* (Comparative) 
                   
                 100 
                   
               
               
                 1,6-hexanedioldiacrylate 
                 42 
                 42 
                 42 
               
               
                 Tinuvin 292 
                 0.5 
                 0.5 
                 1 
               
               
                 Tinuvin 1130 
                 0.5 
                 0.5 
                 2 
               
               
                 Tego glide 100 
                 0.5 
                 0.5 
                 0.5 
               
               
                 XL 6507 
                 1 
                 1 
                 1 
               
               
                 MBF 
                 4 
                 4 
                 4 
               
               
                 Desmodur 3900 
                 5 
                 5 
                 5 
               
               
                   
               
            
           
         
       
     
     It was then cured at 6 m/min with first a 80 W Ga+Hg light source followed by a 120 W Hg light source. This resulted in a clear finish that has a high gloss. 
     Example 4: Exposing of the Articles of Example 3 to a Xenon Light 
     The coating of Example 3 has been subjected to xenon weathering testing (including water spray) according to EN ISO 16474-2, method A, cycle 1, for a total testing time of almost 3000 hours. After 738, 1231, 1739, 2239 and 2739 hours of xenon-testing had been completed, the coating has been optically analysed. Up to 2739 hours of testing, no visible defects could be observed and gloss levels at 20 and 60° remained at high level for formulation A and C. The coating was still intact. For formulation B*(comparative) gloss level dropped faster and cracks appeared after 2239 hours. 
     This testifies of the excellent weathering resistance of the coating according this embodiment of the present invention. 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                   
                 Formulations (parts in weight) 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 A 
                 B* 
                 C 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Example 1 
                 100 
                   
                 100 
               
               
                   
                 Example 2* (comparative) 
                   
                 100 
                   
               
               
                   
                 1,6-hexanedioldiacrylate 
                 42 
                 42 
                 42 
               
               
                   
                 Tinuvin 292 
                 0.5 
                 0.5 
                 1 
               
               
                   
                 Tinuvin 1130 
                 0.5 
                 0.5 
                 2 
               
               
                   
                 Tego glide 100 
                 0.5 
                 0.5 
                 0.5 
               
               
                   
                 Additol XL 6507 
                 1 
                 1 
                 1 
               
               
                   
                 MBF 
                 4 
                 4 
                 4 
               
               
                   
                 Desmodur 3900 
                 5 
                 5 
                 5 
               
               
                   
                   
               
            
           
         
       
     
     Testing Results: 
       
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                 Before 
                 738 hours 
                 1231 hours 
                 1739 hours 
                 2239 hours 
                 2739 hours 
               
               
                   
                 Xenon test 
                 Xenon 
                 Xenon 
                 Xenon 
                 Xenon 
                 xenon 
               
               
                   
                 Gloss 
                 Gloss 
                 Gloss 
                 Gloss 
                 Gloss 
                 Gloss 
               
               
                 Formulation 
                 20°/60° 
                 20°/60° 
                 20°/60° 
                 20°/60° 
                 20°/60° 
                 20°/60° 
               
               
                   
               
             
            
               
                 A 
                 83/91 
                 82/90 
                 83/90 
                 83/89 
                 72/87 
                 73/87 
               
               
                 B* 
                 80/89 
                 78/84 
                 76/85 
                 71/84 
                  65/83* 
                   
               
               
                   
                   
                   
                   
                   
                 (Cracks) 
                   
               
               
                 C 
                 81/90 
                 83/89 
                 81/88 
                 74/87 
                 73/87 
                 73/87 
               
               
                   
               
            
           
         
       
     
     Example 5 
     The resin from Example 1 was used in a gel coat formulation indicated in the following table. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                   
                 pbw 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Example 1 
                 60 
               
               
                   
                 HDDA 
                 40 
               
               
                   
                 Tinuvin 1130 
                 2 
               
               
                   
                 Tinuvin 292 
                 1 
               
               
                   
                 Additol Lambda 
                 0.5 
               
               
                   
                 Additol XL 6507 
                 0.5 
               
               
                   
                 Pigment MB 
                 4 
               
               
                   
                 TPO-L 
                 2 
               
               
                   
                   
               
            
           
         
       
     
     This formulation was applied on a PET film using a bar coater with a film thickness of 150 μm and cured at 5 m/min with first a 80 W Ga light source. Then 300 μm of a mixture VIAPAL 4714 BET/52 (Unsaturated polyester available from allnex) with 4 ppw Curox-I 300 (peroxide available from Nurion) was applied using a bar coater. 3 layers of woven glass fiber was applied followed by a second 300 μm layer of the mixture VIAPAL 4714 BET/52 with 4 ppw Curox-I 300 (Methyl isobutyl ketone peroxide available from United Initiators). Finally a second PET foil is applied and the buildup is compressed using a rubber roller. The composite is subsequently cured for 1 hour at 70° C. 
     This procedure resulted in a white, high gloss finish of the composite matrix material. The coating of Example 5 has been subjected to xenon weathering testing (including water spray) according to EN ISO 16474-2, method A, cycle 1, for a total testing time of almost 3000 hours. Also for this formulation an excellent weathering resistance of the coating according was achieved.