CA2160489A1

CA2160489A1 - Process for preparing a fiber-reinforced composite and molded articles made therefrom

Info

Publication number: CA2160489A1
Application number: CA002160489A
Authority: CA
Inventors: David A. Campanella; Chih-Pin G. Hsu
Original assignee: Individual
Current assignee: Cook Composites and Polymers Co
Priority date: 1994-03-23
Filing date: 1995-03-21
Publication date: 1995-09-28
Also published as: NO953599D0; EP0700419A1; FI954166A; FI954166A0; ES2141372T3; NO953599L; AU2110495A; JPH10511124A; KR960701952A; KR100346678B1; WO1995025771A1; US5900311A; DE69513462T2; DE69513462D1; PL310968A1; EP0700419B1; ATE186935T1; CN1123555A

Abstract

A vacuum-assisted transfer molding process for preparing a fiber-reinforced thermosetting polyester composite, the said composite comprising reinforcing fiber in excess of 30 wt %, based upon the weight of the composite, in a thermoset polyester matrix. The said thermoset polyester matrix is prepared from a one-phase matrix precursor comprising, in weight percent based upon the weight of the matrix precursor, from: (a) 20 to 60 % of an unsaturated polyester resin with a molecular weight/double bond factor between 150 and 190; (b) 30 to 70 % of a reactive monomer; (c) 1 to 25% of a thermoplastic polymer which is miscible in a blend of the polyester resin and the reactive monomer; and (d) an initiating amount of a free radical initiator. Application to the manufacture of molded articles, such as boat hulls or truck body panels, comprising a composite made according to the above process, a gel coat and a skin laminate.

Description

WO 95/25771 ~ 1 ~ O ~ 8 9 PCT/EP95/01042 PROCESS FOR PREPARING A FIBER-REINFORCED COMPOSITE
AND MOLDED ARTICLES MADE THEREFROM.
' This invention relates to fiber-reinforced th ermosetting polyester composites. In one aspect, the invention relates to such 5 composites made by a vacuum-assisted transfer mol~ling technique while in another aspect, the invention relates to such composites made from low or zero shrinkage polyester resin systems. The physical strength of the composites of this invention is much greater and their surface appearance much smoother than those of 10 simil~r composites made from collv~ ion~l hand lay-up, spray-up, or resin transfer molding techniques. The composites of this invention can serve as the cosmetic surface of molded articles, e.g. a boat hull or a truck body panel without the fini.chin~ steps of sanding and poli.~hin~.
Composite materials are known to have the advantages of high strength, light weight, design flexibility, dimensional stability, corrosion resistance, parts consoli~l~tion, better fini.chinp~, and low tooling cost over traditional construction materials such as metal, ceramics, and wood. Fiber-reinforced thermosetting polyester 2 0 composites are widely used in many applications, e.g. marine, automotive, transportation, electrical, construction, consumer and industrial goods, etc. Compared to the composites made from other types of thermosetting resins such as vinyl ester, epoxy, and polyamide, thermosetting polyester composites have the advantages 2 5 of lower material cost and easy material h~n-lling during processing Therefore, unsaturated polyester resins are the materials of choice for most of the fiber-reinforced thermosetting composites in applications in which the working ellvilullment of the composite is not very harsh.
Fiber-reinforced thermosetting polyester composites usually consist of reinforcing fibers, either in chopped or continuous form, embedded in a matrix of one or more unsaturated polyester resins.
In the form~tion of the matrix, the lm~t1lrated polyester resin is blended typically with (l) one or more monomers capable of cros~linking with the vinyl groups in the polyester, (2) one or more free-radical initi~tors, and possibly (3) various other additives which WO 95/25771 ~ 8q PCT/EP95/01042 impart desired characteristics to the matrix upon cure or which will improve the processing and/or curing properties of resin. This curable blend of componçnts is known as the matrix precursor.
The physical and chemical properties of the composite, such 5 as its physical strength, physical modulus, flexibility, and heat distortion temperature, can be controlled by appropriate selection of the ingredients in the m~nllf~cture of lln.c~tllrated polyester resin, or the crosslinkin~ monomers, free-radical initiators, flbers, and other additives used in the preparation of composite.
Various processing methods can be applied to produce fiber-reinforced thermosetting polyester composites. The hand lay-up and spray-up processes are the most common practices in the manufacture of large and complex composite parts, such as boat hulls and truck body r~nel.~. Continuous or chopped fiber mats are 15 impregnated with and engulfed in a matrix resin, and the resin is cured without additional heat or pressure. The typical fiber reinforcement (e.g. glass fiber) content of a composite made by these techniques is only about 20 to about 40 % by weight, based on the weight of the cured composite. Therefore, the physical strength 20 (as measured by any one of a number of different tests) of these composites is typically not very great and if greater physical strength is desired for a particular applicaWon, then a thicker composite is usually required (the physical strength of a composite being a function of the fiber content of the composite and its 25 thickness). Moreover, the surface appearance of the finished part made with these methods may vary widely from part to part depending on various factors, e.g. processing con lition~, the nature of the thermosetting resin, and the like.
Thermosetting polyester composites with better physical 30 strength and/or consistent surface appearance can be produced by other types of manufacturing techniques, such as fil~mçnt win-ling.
compression molding, transfer molding, injection molding, and pultrusion. These techniques can produce parts with very high fiber content, ~ypically from about 50 to about 70 % by weight, based on 35 the weight of the cured composite. How~vel, the nature of these processes, and in some the added tooling and operational costs, WO 95/25771 2 1~ ~ ~ 8 ~ PCT/EP95/01042 plevellt their use in the manufacture of very large and complex parts such as boat hulls and truck body panels.
With the introduction of vacuum-assisted transfer molding techniques as described in US-A-4,902,215 very large, complex and physically strong composites can be manufactured with relatively low tooling and operational costs . Broadly spe~kin E, in such techniques a flexible sheet, liner, or bag is used to cover a single cavity mold which contains the dry or wet fiber lay-up. In accordance with the former, the edges of the flexible sheet are clamped ~inst the mold to form an envelope and seal the member, a catalyzed liquid resin is generally introduced into the envelope, or bag interior, to wet the flber, and a vacuum is applied to the bag interior via a vacuum line to collapse the flexible sheet against the fiber and surface of the mold, while the plastic wetted fiber is pressed and cured to form the fiber reinforced plastic structure.
Resin fumes from the process are ~revellted from esc~ping into the ambient work space. The apparatus disclosed in US-A-4,902,215 is specifically designed for the production of fiber-reinforced plastic structures having high reinforc~ment-to-resin ratios.
2 0 Hc wevel, because a composite made by vacuum-assisted transfer mol~ling has a very high fiber co~tent, the cosmetic surface appearance of the composite is more sensitive to shrinkage that naturally occurs during the cure of a thermosetting polyester resin.
Si~nific~nt fiber pattern print through can be observed, sometimes even though both a skin l~min~te and a gel coat are further applied to the surface of the composite construction. Correction of this problem by sanding and poli~hing after the composite is made requires considerable effort which undermines, or even may elimin~te, the savings in operating and material costs otherwise 3 0 gained from using a vacuum-assisted technique.
The composite industry holds a continuing interest in the development of a method which therefore is the main purpose of the present invention, for the manufacture of a fiber-reinforced thermosetting polyester composite that possesses both great physical strength (relative to a composite made from a traditional hand lay-up and spray-up method) and a smoot-h- surface appearance.

WO 95/ZS771 - - - . PCTIEP95/01042 Such a composite will be a ready candidate for molded parts, especially parts of large size and/or complex shape, requiring both physical attributes.
According to this invention, there is provided a vacuum-assisted transfer molding process for preparing a flber-reinforced thermosetting polyester composite, the said composite comprising reinforcing flber in excess of 30 wt %, based upon the weight of the matrix precursor, from:
a) about 20 to about 60 % of an llns~tllrated polyester resin with a molecular weight/double bond factor between about 150 to about 190;
b) about 30 to about 70 % of a reactive monomer;
c) about 1 to about 25 % of a thermoplastic polymer which is miscible in a blend of the polyester resin and the reactive monomer; and d) an iniff~tin~ amount of a free radical initiator.
The h~l1m~rk of the composites thus prepared is their combination of physical strength (as measured by one or more standard strength tests for composites) and smooth surface profile (as compared to the thermosetting polyester composites made from a typical hand lay-up or spray-up proce$s).
Molded articles in which the composites of this invention are used as a component usually comprise a layer of gel coat, typically 0.25 to about 0.63 mm in thickness, as the surface coating.
Optionally a skin l~min~te, typically from about 0.25 to 0.76 mm in thickness, may be applied behind the gel coat to improve the hydrolytic stability and surface smoothness of the molded article.
The fiber content of the skin l~min~te typically ranges about 25 to about 45 % by weight, and the fiber typically is either in the form of 12 to 50 ~rlm chopped flber or a sheer of a continuous strand flber nat.
The lm~tllrated polyester resins used in the invention are known in the art. Preferred resins are those with a molecular weight/double bond or vinyl group (-C=C-) factor bet~,veen about 155 and about 190, more preferably bet~veen about 155 and about 170, such as described in US-A-3,701,748. These resins are made from WO95/25771 ~ L 8 ~ PCT/EP95/01042 the reaction of at least one polyhydric alcohol with at least an ethylenically unsaturated dicarboxylic acid or its anhydride. The reaction mixture may also include dicyclop~nt~(iiene in order to control the molecular weight of the lm.~tl~rated polyester resin. The unsaturated polyester resin typically has a number average molecular weight in the range from about 500 to about 5,000, preferably in the range from about 700 to about 2,000.
The ethylenically dicarboxylic acid or its anhydride used in the preparation of the unsaturated polyester resin include maleic acid or anhydride, fumaric acid, citraconic acid, mesaconic acid, methyl maleic acid, tetraconic acid and itaconic acid. A minor proportion of ethylenically unsaturated dicarboxylic acid or anhydride p,erelably up to about 30 mole percent, can be replaced by one or more saturated dicarboxylic acids or their anhydrides, such as phthalic acid or anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, succinic acid, adipic acid, sebacic acid, methylsuccinic acid, tetrabromophthalic acid, tetrachlorophthalic acid, hexachloro-endomethylene tetrahydrophth~lic acid, glutaric acid, pimelic acid and dimerized 2 0 fatty acids.
The polyhydric alcohols used in the preparation of the unsaturated polyester resin include saturated aliphatic diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, butylene glycols, neopentyl glycol, 1,3- and 1,4-butane diols, 1,5-pentane diol, 1,6-hexanediol and 2-methyl- 1,3 propanediol. Glycerol, 1,1,1-trimethylolpropane, bisphenol A and its hydrogenated and alcoxylated derivatives may also be used.
The molar ratio of the polyhydric alcohol to the dicarboxylic acid or anhydride in the reaction m~ lre is preferably between about 1.0 and aboutl.2.
The amount of unsaturated polyester resin in the matrix percursor is preferably between about 30 and about 50 percent by weight.
Any reactive monomer that will copolymerize and crosslink with the vinyl groups of the unsaturated polyester resin can be used, WO 9S/25771 ; . . ` .......... - - PCT/EP9S/01042 ~
8 ~

alone or as a mi~llre of monomers, in the practice of this invention.
These monom~rs include such materials as styrene, vinyl toluene, p-methyl styrene, chlorostyrene, t-butyl styrene, divinylether, allyl phth~l~te, diallyl maleate, allyl methacrylate, allyl acetate, N-vinylpyrrolidone, N-vinylcarbazole, dichlorostyrene, dialkyl fumarates and maleates, diallyl phth~l~te, mono- or multifunctional lower (Cl-C8) alkyl esters of acrylic and methacrylic acids such as methyl methacrylate, cyclohexyl (meth)acrylate, ethylene glycol di(meth)acrylate, propyleneglycol di(meth)acrylate, 1,3- and 1,4-butanediol di(meth)acrylates, 1,6- hexanediol di(meth)acrylate, neopentylglycol di(meth)acrylate, diethyleneglycol di(meth)acrylate, dipropyleneglycol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, triethyleneglycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, glycerol di(meth)acrylate, pentaerythritol di(meth)acrylate, and the like. The amount of monomer in the matrix precursor ranges preferably between about 40 to about 60 percent by weight.
The thermoplastic polymers used in the invention as a low profile additive are those that are miscible with the polyester resin 2 0 and reactive monomer such that upon blenrling one with the others, a one-phase matrix precursor is formed. These polymers include polyvlllyl acetate, ~olyester-based polyureth~nes, polycaprolactones, cellulose acetate buty-rate, saturated polyesters and copolymers of alkyl methacrylate(s) in which the alkyl group has from 1 to 4 carbon atoms and of unsaturated monorners bearing at least one hy~ ~yl group, the said copolymers having a molecular weight of between 1,000 and 20,000. Except for the latter class of polymers, the weight average molecular weight of these polymers can range from about 3,000 to about 1,000,000, plefelably from about 5,000 to about 500,000. The amount of thermoplastic polymer present in the matrix precursor ranges preferably between about 5 to about 20 percent by weight.
The viscosity of the matrix precursor, which is an important feature of this invention, is typically in the range from about 0.1 to about 1 Pa.s, ~ relably from about 0.15 to about 0.5 Pa.s at ambient temperature, i.e from about 10C to about 35C.

~ WO 95/25771 ~ 1 ~ 0 4 ~ ~ PCT/EP95/01042 .. .

Although not preferred, a certain amount of filler can be added to the matrix precursor. Acceptable fillers include natural or precipitated calcium carbonates, clay, silica, talc, mica, and hydrated all1min~ If present, the amount of filler added to the matrix precursor is ty-pically less than about 10 percent, preferably less than about 5 percent, by weight, based on the weight of the matrix precursor.
The matrix precursor is cured through the action of one or more free radical initiators, such as an organic peroxide compound, e.g. t-butyl perbenzoate, benzoyl peroxide, cumene hydroperoxide, t-butyl peroctoate, methyl ethyl ketone peroxide, a pelu2~y(1lcarbonate, a pelu~yester such as t-butyl pelo~yL~enzoate, t-butyl peloxyoctoate, 2,5- diperoxyoctoate or 2,4- pentanedione peroxide,and others known in the art. The miniml1m amount of such initiator used in an iniff~ting amount, and typical amounts present in the matrix precursor preferably range from about 0.1 to about 3 percent by weight, based ûn the weight of the matrix precursor.
Other materials that can be present in the matrix precursor include polymeri7.~tion inhibitors, accelerators and other types of additives to improve the processing and/or curing properties of the resin, and/or which impart one or more desired features to the composite. These other materials are used in known amounts and in known ways. Hc,wevel, it is preferable for the performance of the 2 5 process of the invention that the matrix precursor is free from any demol~ ng agent.
The gel time of the matrix precursor of this invention will vary with, among others, its composition and the cure conditions, but it is typically between about 5 and about 75 minutes, preferably between about 15 and about 60 minutes, in the absence of he:~Wng during the curing process. This feature is of particular importance for detel~ ng the m~ mllm time allowed for filling the mold.
The vacuum-assisted transfer molding techniques used in the practice of this invention include those described in US-A-3 5 4,902,215, already cited above. These techniques can produce composite materials with a high fiber content, i.e. preferably in WO 95/25771 . - r ~ PCT/EP9S/01042 2 1 1~ 8 ~

excess of 40 percent, typically between about 50 and about 80 percent by weight based on the weight of composite.
The composite of this invention are usually comhined with a layer of gel coat and a layer of skin l~min~te to form a molded part.
5The gel coat is typically 0.25 to 0.63 mm in thickness, and is the surface coating of the molded part. The gel coat provides the fini~hinp~ color and surface profile of the part. Gel coats are well known and various grades are commercially available. The selection of gel coat will depend upon the desired characteristics of the part 10relative to, among other things, weatherability, hydrolytic stability, and surface flni~hing.
The layer of skin l~min~te, typically 0.25 to 0.76 mm in thickness, can be applied behind the gel coat to improve the hydrolytic stability and surface smoothness of the molded part. The 15skin l~min~te provides an extra barrier to the composite from hydrolytic attack during the employment of the part. The skin l~min~e also provides protection to the gel coat from the reaction heat and shrinkage normally incident to the cure of the composite.
Moreover, the thermosetting resins typically used in the 20preparation of the skin l~minzlte usually exhibit better hydrolytic stability than those used in the preparation of the composite.
~x~mples of these resins include vinyl esters, vinyl ester modifled epoxies, and vinyl ester modified unsaturated polyester resins. The typical flber content of a skin l~min~te ranges from about 25 to 25about 45 percent by weight. The fiber used in the skin l~min~te is typically either about 12 to about 51 mm chopped fiber or a sheer of a continuous strand fiber mat.
The main structure of the molded part can also include a core insert. An insert is used in those applic~tio~.s in which weight 30reduction is a factor in the design of the part. The core insert can also serve as a supplement reinforcement material to the composite.
mples of core materials include polyurethane foam, honeycomb structures made from various light weight material, and balsa wood.
The thickness of the core can vary widely, but is typically between 35about 2.5 mm to about 50 mm, the exact ~limen~ion being a filnction WO 95/25771 ~ PCT/EP9S/01042 _ 9 of, among other things, the physical strength and weight requirements of the molded part.
Typically, most of the strength characteristics of the molded part are a function of the strength characteristics of the composite, 5 and these characteristics in turn are a function of the amount and nature of the reinforcing flber. Usually, continuous fiber mats with various weight/area ratios are used in the construction of the composite to provide the desired strength/weight performance to the part. F'x~mples of the various types of reinforcement flbers that 10 can be used in the practice of this invention are glass fibers, boron fibers, carbon fibers, aramid fibers, and other types of natural and synthetic fibers, such as jute, sisal and flax. The typical fiber cont~nt of the composite is preferably between about 50 and about 80 percent by weight.
The composite and the molded part can, and often are, constructed in one operation. First, a gel coat is usually applied to the surface of the mold, at least partially cured, and then a skin l~min~tç is applied over the at least partially cured gel coat. These are open mold operations. Then the fiber reinforcement is applied 20 to the skin l~min~te, the mold closed, and the matrix precursor injected under vacuum. The precursor is then allowed to cure, with or without a heat supplçmçnt, and the part or article demolded.
During the construction of the molded part or article, all reinforced materials, i.e. the composite, skin l~min~te and, perhaps, 2 5 the core insert, are used under dry conditions. As such, these components can be prepared without undue deference to time.
Once prepared, resin is injected into the mold under a vacuum condition through one or more injection paths. The mold fllling time can be controlled by the number of injecting paths and the 3 0 strength of the vacuum. The gel or cure time of is usually about 5 to 10 minutes longer than the fill time. Large, e.g. 30 m by 6 m, parts of complex shape can be made in a single molding process. Because the entire process is under a vacuum condition, the emission of monomers is minimum during the preparation of composites and 3 5 molded parts.

WO 9S/25771 PCT/EP~5/01042 4~9 The invention is further descAbed by the following examples.
All percentages are by weight unless otherwise indicated.
Measur~.mçnt of Surface Appearance The rating value (ACT Orange Peel St~n-l~rds) are typical industry visual test methods used to describe the surface appearance of an object. A BYK-Gardner wave-scan was used to measure the surface appearance of various test panels. The wave-scan can report the results in both long-term (structure size greater than 0.6 mm) and short-term waviness (structure size less than 0.6 mm). Both long-term and short-term waviness are rated from 0 to 100. The hi~her the number, the more waviness is observed. The long-term and short-term are then mathematically correlated to a surface rating value from 0 to l00. The higher the number, the smoother the surface appears.
h`~r~mr)le 1 A three-component matrix precursor was prepared from an unsaturated polyester, a thermoplastic polymer, and styrene. The unsaturated polyester was prepared by esterifying l.l moles of propylene glycol with 0.83 moles of maleic anhydride and 0.17 moles of isophthalic acid to an acid number of 30. The polyester was then dissolved in styrene to a concentration of 63 %
solids. This unsaturated polyester had an average molecular weight/vinyl group (-C=C-) factor of 165.
The thermoplastic polymer was a polyvinylacetate with a number average molecular weight of ll0,000. This polymer was then dissolved in styrene to a concentration of 17 % solids. 54 parts of the polyester/styrene solution was then blended with 46 parts of the vinyl acetate/styrene solution to yield a liquid, one-phase matrix precursor composition. This precursor composition co~t~ined:
3 o Parts Unsaturated polyester 34 Thermoplastic polymer 8 Styrene 58 The resinous composition was then mixed for 30 minutes to 35 form a homogeneous m~ re. This ml~ lre was catalyzed for cure WO 95/25771 ~ 0 4 ~ ~ PCT/l~P95/01042 with methyl ethyl ketone peroxide initiator. The gel time of this homogeneous m~ re was 45 minutes at 23 C.
A high strength, fiberglass reinforced panel was made on a flat mold at 23 C using the apparatus and technique described in US-A-4,902,215. The flberglass reinforcements consisted of four layers of PPG 2 oz chopped strand mat. The fiber content of the composite was 55 % by weight based on the weight of composite.
On this composite were measured:
- flexural strength according to ASTM D-790: 4,720 MPa - tensile strength according to ASTM D-638: 2,700 MPa The surface appearance properties of this composite were also measured as in~lic~ted above and are reported in the following table.
Example 2 An unsaturated polyester prepared by esterifying 1.1 mole of propylene glycol with 1.0 mole of maleic anhydride to an acid number of 30 was blended with a saturated polyester thermoplastic polymer with a number average molecular weight of 2,500. The unsaturated polyester had an average molecular weight/vinyl group factor of 156. Both resins were dissolved in styrene, the 2 0 composition of the resulting matrix precursor being as follows:
Parts Unsaturated polyester 42 Thermoplastic polymer 12 Styrene 46 The matrix precursor was then catalyzed and molded by the vacuum-assisted method as in F~mple 1. The surface appearance properties of the resulting composite are indicated in the following table.
F~mple 3 (comparative) 3 0 The procedure of ~mple 1 is repeated except that the three-component matrix precursor is replaced by a mixture of 65 parts styrene and 35 parts of a commercial lln.~tllrated polyester resin marketed by COOK COMPOSITES AND POLYMERS. The surface appearance properties of the resulting composite are 3 5 indicated in the following table.

WO 95/25771 . PCT/EP95/01042 Example 4 The matrix precursor of F~mrle 1 was tested for surface proflle properties on a gel coated surface to .~im~ te construction of r boat assemblies. The fiberglass reinforced panel design was as follows:
4 layers of 2 oz PPG chopped strand mat 1 layer o,5 mm veil Nico Fibers Gel coated surface on mold The matrix precursor was used and molded by the vacuum-assisted method described in F,~mpl~ 1. The surface appearance properties of the resulting molded article are indicated in the following table.
~,~mple 5 The matrix precursor of F,~r~mple 1 was tested for surface profile properties on a gel coated surface to simulate light weight sections of a boat. The fiberglass reinforced panel design was as follows:
1 layer PPG 1808 cornbin~tic-n mat 1 layer 3/8" balsa core material 3 layers 1.5 oz PPG chopped strand mat 1 layer 0.5 veil Nico ~ibers Gel coated surface of mold The matrix precursor was used and molded by the vacuum-assisted methode described in Example 1. The surface appearance 2 5 properties of the resulting molded article are indicated in the following table.
TA~LE
F,~r~mple Long-term Short-term Surface waviness waviness rating value 2.4 0.3 9.8 2 17.7 22.1 5.6 3 62.2 51.8 1.8 4 6.5 16.0 7.8 0.9 1.5 10.5

Claims

1. A vacuum-assisted transfer molding process for preparing a fiber-reinforced thermosetting polyester composite, the said composite comprising reinforcing fiber in excess of 30 wt %, based upon the weight of the composite, in a thermoset polyester matrix, wherein the said thermoset polyester matrix is prepared from a one-phase matrix precursor comprising, in weight percent based upon the weight of the matrix precursor, from:
a. 20 to 60 % of an unsaturated polyester resin with a molecular weight/double bond factor between 150 and 190;
b. 30 to 70 % of a reactive monomer;
c. 1 to 25 % of a thermoplastic polymer which is miscible in a blend of the polyester resin and the reactive monomer; and d. an initiating amount of a free radical initiator.

2. A process according to Claim 1, wherein the number average molecular weight of the unsaturated polyester resin is from 500 to 5.000.

3. A process according to any of Claims 1 and 2, wherein the weight average molecular weight of the thermoplastic polymer is from 3.000 to 1.000.000.

4. A process according to any of Claims 1 to 3, wherein the thermoplastic polymer is selected from the group consisting of polyvinyl acetate, polyester-based polyurethanes, polycaprolactones, cellulose acetate butyrate, saturated polyesters and copolymers of alkyl methacrylate(s) in which the alkyl group has from 1 to 4 carbon atoms and of unsaturated monomers bearing at least one hydroxyl group, the said copolymers having a molecular weight of between 1,000 and 20,000.

5. A process according to any of Claims 1 to 4, wherein the free radical initiator is an organic peroxide.

6. A process according to any of Claims 1 to 5, wherein the matrix precursor comprises from 0.1 to 3 weight percent initiator.

7. A process according to any of Claims 1 to 6, wherein the matrix precursor further comprises not more than 10 percent by weight, based on the weight of the matrix, of a filler.

8. A process according to any of Claims 1 to 7, wherein the matrix precursor is free from any demoulding agent.

9. A molded article comprising:
a a gel coat, b. a skin laminate, and c. a composite made by a process according to any of Claims 1 to 8.

10. The molded article of Claim 9 further comprising a core insert.

11. The molded article of Claim 10 in the shape of a boat hull or deck section.

12. The molded article of Claim 10 in the shape of a truck body panel.