IL33222A - Moulding compositions - Google Patents

Description

Moulding compositionB ROHM AND HAAS COMPANY Ci3 63 This invention is concerned with moulding compounds arid resin compositions, polymerisable to thermoset products, which are suitable for incorporation therein. The invention is also concerned with moulded products formed therefrom.- The invention provides a liquid polymeri.zable resin composition suitable for use in moulding compounds having reduced shrinkage when polymerized under the influence of heat and pressure, theresin composition containing (A) an ethylenically unsaturated crosslinkable polyester, (B) an ■ ethylenically unsaturated monomer copolymerizable with-polyester (A) to effect crosslinking thereof, and (C) a thermoplastic polymer, characterized in that (1) the ethyl-: enically unsaturated polyester has a molecular- weight to double bond factor of 142 to 215, (2) the thermoplastic polymer comprises an addition polymer and contains 0.1 to 5.0 per' cent by weight (calculated as hereinbefore described) of acid groups and is soluble in (B) or mixtures of (A) plus (3), (3) the composition contains, by weight based on the total weight of (A), ^B) and (C), 20 to .80% of polyester (A), 20 to 80% of monomer -(B) and 1 to 25% by weight. of polymer (G) and (4) the resin composition is. polymerizable to form a non-homogeneous polymeric system.

The invention also .'provides moulding compounds containing such resin compositions-, which, moulding compounds may be thickened- for example with metal oxides or hydroxides, or may contain other ingredients such as a filler, mould-release agent, reinforcing material, catalyst, polymerisatio stabiliser or other conventional moulding compound ingredient. The moulding compounds of the invention may be compression-moulded, injection-moulded, or transfer-moulded under the usual conditions of heat and pressure to form useful shapes.

Fibrous moulding compositions based on unsaturated polyester resin systems are widely used for moulding automobile body parts, furniture, appliance housing and many other applications. Common thermoset reinforced moulding compositions include pre-mix moulding compounds and pourable resin compositions utilized on preformed fibrous shapes or tailored fibrous mat. For the purposes of this specification all these techniques will be referred to as "wet-moulding compositions". This term relates to the fact that the moulding compositions are either tacky, sticky, or pourable before and during the moulding process. Pre-mix moulding compositions are commonly based on an unsaturated polyester resin system, fibrous reinforcement such as fibre glass, fillers or extenders, free radical catalyst for polymerization of the resin system and other additives such as release agents, pigments, and the like. The pre-mix compounds are mixed to form a dough-like composition which is charged directly to compression, injection, and transfer moulding equipment. Moulding techniques which utilize preformed fibrous reinforcement, tailored fibrous mat and tailored cloth reinforcement generally utilize a pourable moulding composition based on unsaturated polyester resin systems, fillers or extenders, free radical s As described in our co-pending Israeli- Patent Appln. no 28351 filed 19th July, 1967, a mixture of thermoplastic polymer, certain unsaturated polyesters and unsaturated monomer for cross!inking the polyester can eliminate polymerization shrinkage and thus offer outstanding surface characteristics of moulded parts, when used in wet moulding, compositions. However, the use of the compositions of the invention described in that application in bulk · moulding compounds and sheet moulding compounds .does not yield the same advantages. In fact, v.'hen those compositions are used in bulk moulding compounds and sheet moulding compounds;, the physical characteristics, including the' surface characteristics, are generally poor.

The resin compositions of the invention unexpectedly can provide low shrinkage during polymerization in -bulk and sheet moulding compounds. · The ethylenically unsaturated polyesters of the resin composi ions of the invention may be- prepared as-the poly-condensation product of a, β-ethylenically' unsaturated dicarboxylic acids or anhydrides, saturated dicarboxylic acids or anhydrides and dihycric alcohols - ,"' ■ 215. Preferred is the degree of unsaturation greater than/?**; 147 but less than 186. Examples of polyesters of particular interest are poly (propylene fumarate)', poly ( ethylene/propylen fumarate), pol ( ethylene/diethylene fumarate), poly ( dipropyle fumarate ),' poly ( propylene/dlpropylene fumarate) and. polyCpro-pylene isophthalate/fumarate ) . These polyesters may ■ be prepared with either maleic anhydride or fumaric acid.

The acid number of the polyester may vary greatly 'while still allowing the advantages of the compositions of this invention to be obtained. Some slight dibasic acid termination is highly desirable. Polyesters with acid numbers of approximately 1 are still effective, particularly when the polyesters are of high molecular weight. However, handling considerations of the moulding compounds and the' physical characteristics of the moulded part generally indicate that a polyester with an acid number in the range of 5 to 100 has the more general utility. Acid numbers in the range of 10 to 70 are most preferred.. The molecular weight of the polyester is not critical and may vary over- a considerable range. Polyesters most useful in the practice of the invention may have molecular weights ranging from about 500 to 5,000. Preferred is the range of about 700 to about 2,000.

As willj be seen from this description the polyesters useful in the uncured composition of the invention are substantially linear polyesters', the polyesters may contain some branching however although they should not be cross-linked to any substantial degree. .

The second essential constituent of the compositions of this invention is an acid functional addition polymer-. The acid functional polymers must be soluble in the monomer system or in the monomer/polyester mixture. It is not necessary . however that the ' resulting ' uncured polyester/monomer/polymer mixture form a homogeneous liquid and in some instances two distinct liquid phases may be formed. Upon polymerisation and cure of the mixture of the polymers of this invention whether the mixture is homogeneous . or not, the polyesters and the monomer system polymerise and cure to a non-homogeneou product, i.e. to an at least partially immiscible, incompatible, or optionally heterogeneous product. Particularly successful results may be attained when the unsaturated . polyester and the monomer system cured with the. acid functional polymer present form a cured composition which is grossly heterogeneous when examined microscopically, for example by reflected light at 40-60X taking the form of a distinct multi-phase structure.

Ethylenically unsaturated monomers useful in preparation of the acid functional polymer Include. (C^ to Clg) -alkyl methacrylate s and (C. to C-g)-alkyl acrylates for example when the alkyl group is methyl, ethyl, n-propyl, isopropyl, n-butyl isobutyl, 2-ethylhexyl or stearylj .cyclic methacrylates and acrylates wherein the cyclic group is cyclohexyl, benzyl-, a bicyclic group such as isobornyl, bornyl, fenchyl or isofenchyl; mono-vinyl aromatic compounds such as styrene, substituted styrene such as a-methylstyrene , vinyItoluene , tert.- butylstyrene , halogen-substituted styrene such as chloro-styrene or dichlorostyrene ; acrylonitrile or meth-acrylonitrile ; mixtures of vinyl chloride and vinyl acetate (and other monomers used in minor quantities which do not affect the basic function of the polymer) . Oclluloao acetate butyratc and oollulooo aootato propionate may aloo bo uood. Preferred are copolymers of (C^ to alkyl-methacrylates and acrylates with incorporated acid functionality. Most preferred are the copolymers of methyl methacrylate and aikyl acrylates with incorporated acid functionality.

The acid functionality of the polymer may be incorporated by any convenient known method. The acid functionality may be imparted by, for example, carboxy-lie acids, phosphonic acids, phosphoric acids or sulfonic acids. Preferred is the carboxylic acid functionality. The acid functionality may be incorporated after the polymer is formed although it is preferred that the acid functionality be incorporated by using an acid functional unsaturated' monomer as a component in the monomer system used to prepare the polymer. The acid functional monomer used depends on the strength of the acidity desired, the chemical thickener to be used, the speed and quality of the thickening desired, the amount of the polymer utilized, the reactivity characteristics of the acid functional monomer and the general quality of the moulding compound desired. Typical acid func- 33222/2 tional monomers that are easily copolymerizable with the comonomers of this invention include acrylic acid, meth- acrylic acid,' methacryloxyacetic acid, acryloxyacetic acid, methacryloxypropionic acid, methylenemalonic acid, a-chloroacrylic acid, itaconic acid, monomethyl itaconate, a-methylene- a«-methylglutaric acid, p-vinyl-' benzoic acid, β-methacryloxyethylphosphonic acid, β -methacryloxyethylphosphoric acid, β -methacryloxy- ■ ethylsulfonic acid, and β -sulfatoethyl methacrylate · Somewhat less reactive monomers for use during copoly- merisation include ethacrylic acid, a-alkylacrylic ' acids, crotonic acid, cinnamic acid, maleic acid, fum- aric acid, a-cyanoacrylic acid, monovinyl succinic acid, a-carbomethoxyvinylphosphonic acid, p-vinyl- benzenephosphonic acid, a-carbomethoxyvinylphbsphoric : acid, p-vinylbenzenephosphorous acid , vinylsulfonic: acid , .„.oc.par¾0met.ho¾^inylsulfonic^ cid ,p-vinylbenzene- ·'. . sulfonic acid and the like. There are many other acid functional monomers which may be used to prepare the 'acid functional: polymer. Many of these acid functional monomers are relativel lens .reactive in copolymerization and are not listed for that reason-only. · · ', Thus acid functional thermoplastic polymers useful in accordance with the present invention include , the polymers of ,' methyl methacrylate a.nd acrylic acid polymers of methyl, methacrylate and methacrylic acid polymers of methyl methacrylate, ethyl acrylat.e and acrylic acid, polymers of methyl methacrylate, ethyl acrylate and methacrylic acid, polymers of methyl methacrylate , · butyl acrylate and acrylic acid, polymers of styrene , acrylonitrile ' and acrylic acid, polymers of styrene, methyl methacrylate and · .',,'. a'crylijc acid, polymers of methyl methacrylate ,,and methacryl- , : ', oxyacetic acid, polymers of methyl methacrylate and 'acryloxyacetic ; acid, polymers of methyl methacrylate and methacryloxypropionic acid, polymers of methyl methacrylate and ά-chloroacrylic acid,'.-. f polymers of methyl methacrylate and p-vinylbenzoic acid,' poly- ; '; ,,: mers of methyl methacrylate and β-methacryloxyeth.ylphosphonic '; ,' acid or p-methacryloxyethylphosphoric acid, polymers of methyl';./''.' methacrylate.;.and p-methacryloxyethyl sulfonic acid and- polymers ■' '.' of methyl methacrylate and β-auifatoethyl methacrylate. · The molecular weight of the thermoplastic acid functional polymers is not particularly critical. It may vary over a wide range from about 5»000 to 10,000,000.

The structure of the polymer may be essentially linear or may be extensively branched. The preferred molecular weight range is 25,000 to 500,000.

The third essential constituent of the resin corn-positionsof this invention is the monomer system that cures rapidly with the ethylenically unsaturated poly- either ester. The monomer system/with or without the polyester present should dissolve the acid functional polymer. The monomer system must be copolymer!zable with the unsaturated polyester to develop a cross-linked, thermoset structure. This monomer system may, for example, be styrene, or a substituted styrene such as vinyl toluene or tert.-butyl styrene. Other ethylenically unsaturated monomers which may be used in combination with the monomers above in quantities less than 0% include, for example, lower (C£ to CZj_)-alk l esters of acrylic and methacrylic acids, a -methylstyrene , cyclic acrylates and methacrylates such as cyclohexyl methacrylate and acrylate, benzyl methacrylate and acrylate, bicyclic methacrylates and acrylates such as isobornyl methacrylate and acrylate, halogenated styrenes such as chloro-styrene, dichlorostyrene , 1,3-butanediol dimethacrylate , diallyl phthalate.

The ratio of the components of the resin composition chemical thickener, fillers and extenders , fibrous reinforcement, free radical catalyst, polymerization stabilizer, release agent, compound any other components ·> generally useful in moulding/ The bulk moulding compounds and sheet moulding compounds are either prepared at the moulding site or may be pre-pared by an outside supplier and shipped to the moulder, who might use them many months after they were first prepared.

Chemical thickeners commonly used in the art include magnesium oxide, magnesium hydroxide, calcium oxide and calcium hydroxide. We have discovered that many other metal oxides and hydroxides, particularly polyvalent metal oxides and hydroxides and other polyvalent metal compounds capable of reacting with -C00H groups are effective in thickening the resin systems of this invention. The speed and effectiveness of the thickening process vary considerably, depending on the metal oxide or hydroxide. The corrosive tendencies and other characteristics also determine the utility of the thickener employed. The metal hydroxides and oxides which may be used in the moulding compounds of this invention include those of Groups I and II such as beryllium, magnesium, calcium, zinc, strontium, cadmium, copper, barium, lithium, potassium and sodium. Preferred thickening agents are the oxides and hydroxides of Group II metals. Most preferred are calcium hydroxide or oxide and magnesium hydroxide .or oxide.

The bulk and sheet moulding compounds may generally contain a reinforcing agent in the form of fibrous materials, particularly fibreglass. Other reinforcements may be used alone or in combination with fibreglass to obtain special effects of either appearance or physical Other characteristics. Ai*«*i¾ati-ve reinforcements include sisal, asbestos, cotton, organic synthetic fibres such as nylon, polyester, polypropylene, and the like; inorganic fibres such as quartz, beryllium, and other metal fibres. The form and the quantity of ' the reinforcing components will vary greatly, depending upon the physical characteristics desired in the final moulded part and the particular production technique involved. For example, when fibreglass is used in bulk moulding compounds, chopped strand fibreglass of approximately ■ to ¾■ inch in length is preferred. When fibreglass is used in sheet moulding compounds , chopped strands are preferred of approximately 2 inches in length. In the case of sheet moulding compounds, the fibreglass may be in the form of a chopped strand mat bound together by a soluble binder or, as is preferred, in the form of chopped continuous filaments without binder. Other forms of reinforcement may be used with the resin system of this 33222/2 invention such as woven cloth or veil for special effects or increased strength and reinforcement in ..certain areas ■ ■ of the part. In the preferred embodiment of the invention, the reinforcement should be free to flow with the compound to the extremities of the mould. Thus, the preferred length of .the fibre and the exact preferred nature Of the fibre'' depends upon the moulding process and it is often desirable' that the fibreglri.ss not be bound together.

Various types, grades and concentrations of fillers and extenders may generally be added to the mouldin com-' position in order to improve or change the physical characteristics of the mouldin · compound and/or the final cured part. Fillers may, for example, be used in ,. , quantities of 5 to 70% on the weight of the moulding system or compound. The filler content usually included in bulk moulding compounds and. sheet mouldin compounds ranges from about 50 to 300: weight per cent on the weight of the resin' system. Fillers useful with- the reain system of the present invention include clay, talc, calcium. . carbonate, silica, calcium silicate, wood flour, phenolic'. • microballoons , glass beads and spheres, titanium dioxide, . carbon black, and the like. The inclusion of relatively large quantities of filler is useful to obtain the sur- .. face ■ smoothness improvement, to reduce cost, and to im- prove the flow and handling characteristics of both the moulding compounds before cure and the flow characteristics during the moulding and curin process.

Other additives are often necessary in bulk and sheet moulding compounds such as a free radical catalyst to provide as full a cure as possible in a relatively short time . The ■ catalyst is chosen to allow full fill of the mould before gelation and to provide a fast cure after gelation. Preferred are the free radical catalysts which do not degrade until a relatively. high temperature is reached in the heated mould. For example, tert; -butyl perbenzoate is preferred when the moulding-temperature is in the range of 2 5 "to 325°F. However, many other catalysts may be used such as benzoyl peroxide, tert.-butyl peroxide, tert.-butyl peroctoate, di-tert.-butyl peroctoate, cyclohexanone peroxide, lauroyl peroxide, and the like. Also often used are free radical inhibitors to provide sufficient stability of the moulding compound at ambient temperatures before the moulding process. The inhibitors also provide a sufficient length of time for flow within the mould before gelation. Such inhibitors include hyd oquinone , p-benzoquinone, and the like. Also often necessary are release agents to provide fast and efficient release of the moulded part from the surface of the mould after cure. At higher curing temperatures metal stearates are useful such as the stear-ates of zinc, calcium or aluminum. The release agents may be applied as a spray on the mould or incorporated interrially in the bulk or sheet moulding compound. Other 33222/2 release agents which may be used include lecithin and ■.; . mixtures of phosphates such as those marketed under the - . ' Zelec trademark by E.I. DuPont de Nemours.

The use of the resin systems of this invention in bulk and sheet moulding compounds provides moulding systems for the operator and end user with one. or more great advantages. The most significant advantage may be the reduction or elimination of shrinkage which, allows bulk , - ·., and sheet moulding compounds to be used in applications . where size and dimensions are extremely, critical. ; In addition the moulding compounds have physical and/or chemical propertie which make them commercially attractive ■ since j' efficient and '. accurate mouldings can be obtained.. t ·.·· .'··· ·' ··'.,··:... ' ·. '·'.

The resulting moulded parts may offer surface characteristic which essentially duplicate the surface of the mould, whether the mould has a mirror. finish or a special pattern, and painting and coating may be possible without any finishing or sanding steps. The mouldings can successfully contain metal reinforcements, bushings and insertions and may be flexible in design, for example, including great thickness variation or large webs and reinforcing ribs.

The deformation temperature under load of the cured moulding compounds using the resin Example An unsaturated polyester is prepared by esterifying I.05 moles of dipropylene glycol with 1.0 mole of maleic anhydride to an acid number of 52. The polyester is dis-solved in styrene monomer at 65% solids.

Example 5 The resin systems above are used in the following moulding systems: In each case the bulk moulding compound is prepared using the following formulation: Resin (as prepared by Example 1, 2, 3 or 4·) 25 parts Calcium carbonate filler .55 parts ■ inch-length glass fibres 20 parts Calcium hydroxide 0.275 part Tert. -butyl perbenzoate 0.25 part Zinc stearate 1.0 part ; The calcium hydroxide, tert.-butyl perbenzoate and zinc stearate are added to the resin system, which in turn is added to the filler in a sigma blade mixer. The mix is agitated until it becomes a smooth paste, after which the fibre glass is added and mixed Just long enough to provide good dispersion of the fibres. The dough-like mix is removed, placed in a plastic bag or wrapped in film and allowed to age for 4-8 hours. During this time the compound thickens and the surface becomes noticeably less tacky.

The resulting moulding compositions are moulcfed on a tote box compression mould which is kept at $00°F. One thousand and forty-five grams of each of the bulk moulding compounds is charged to the mould and subjected to 50 tons, pressure for 0 seconds.

(A) The bulk moulding compound (BMC-1) based on the resin system of Example 1 mixed easily and is easily handled for placement in the mould. The compound moulds easily and release from the mould is excellen . The surface of the tote box obtained duplicates the mirror surface of the mould almost exactly. Measurements of the surface profile (Smoothness) are obtained by traversing the surface of the moulded part with a specifically modified linear differential transformer and continuously recording the fluctuations. Surface roughness (maximum distance from peak to valley within the scan) is expressed as microinches of waviness or roughness in a ½-inch surface scan. The surface roughness is about 5 in this high surface quality mould, which is as smooth or smoother than is obtained from any known moulding compound. This extraordinary surface is painted, using standard coating procedures for reinforced plastics without any sanding or finishing operations, to provide a finish at least equivalent to that presently obtained with the best quality steel plate or sheet. The following physical characteristics are obtained on the bottom of the tote box: Physical Property Value Flexural strength (psi x 10"^) 13-16 Flexural modulus (psi x 10~6) 1.2-2.0 Tensile strength (psi 10~J) 4—6 Tensile modulus (psi 10~6) 1.3-1.7 Izod notched impact strength (ft. lbs. /inch of notch) 3.5-7 Unnotched Izod impact strength (ft.lbs. /inch of notch) 9-12 (B) When the resins system of Example 2 is used in the bulk moulding compound above, the thickening process is not efficient in that there is tackiness on the surface of the compound. The compounds are not consistent and are unsatisfactory in commercial operations. Upon moulding the compounds prepared from the resin system of Example 2, there is substantial sticking to the mould surface after cure and a mottled appearance of the surface of the moulded part results. The surface of the parts, although flat, has a roughness resulting in a chatter when smoothness is measured. Values of about 100 or greater are obtained.

(C) When the resin s sterns of Examples 3 and 4· are used in the bulk moulding compound above, the surface profile and the smoothness of the moulded part are poor with substantial fibre prominence and roughness. The surface profile ranges from 550 to greater than 800.

Painting the parts with no surface finishing gives the characteristically poor surface appearance associated with fibrous-reinfo ced plastics.

(D) The sheet· moulding compound prepared using the resin system of Example 1 (SMC-1) is essentially tack-free and may be handled easily. The pieces of SMC-1 can be cut and stacked for charging into the heated matched metal die mould. The SMC-1 is cut to an 8-inch by 11-inch size, stacked as necessary to achieve the de- charge sired/weight, and placed in a tote box compression mould at 500°Pi The mould is. closed with a pressure of 50 tons and held for 90 seconds. The part is easily re-moved from the mould and has outstanding surface characteristics as compared with standard sheet moulding compounds. The surface smoothness is about 100 micro-inches. The following physical characteristics are obtained on the bottom of the tote box: Flexural strength (psi x 10~5) 26-28 Flexural modulus (psi x 10"^) 1.5-2.0 Tensile strength (psi x 10""^) 10-1. —6 Tensile modulus (psi x 10-°) 1.4-1.7 Izod notched impact strength (ft. (lbs. /Inch of notch) 8-12 Izod. unnotched impact strength (ft.- :lbs./ inch Of notch) 18-21 The SMC-1 is placed on a large compression mould for a complicated ribbed automobile front body part kept at 300°F. The compression mould is closed on the compound at 500 psi for minutes. The mould fills easily, even to the utmost extremities of the parts.

Surface characteristics are truly outstanding - better than any surface ever obtained using this production mould. There are essentially no voids and the surface smoothness is essentially equivalent to that of stamped steel automobile body parts. There are no sink-marks opposite the thicker ribs and bosses as normally associated with plastics. Without any sanding or surface preparation, the parts are painted with coating systems chosen for good initial adhesion to these automobile body parts* The appearance is superior to that obtained with common reinforced plastic compounds even if they are sanded and finished to provide an acceptable surface.

After ten days' immersion in water at 90°F. , the surface finish is essentially unchanged. The paint adhesion is tested by cutting a narrow "X" with a razor blade on the surface and applying pressure-sensitive cellophane tape with a pencil eraser to obtain the best adhesion possible. The tape is yanked off and the coatings are rated as having excellent adhesion retention.

(E) When the resin system of Example 2 is used in the sheet moulding compound above, the thickening process is inconsistent and relatively ineffective. The surface of the compound after 4-8 hours is tacky. Upon moulding, the parts are of relatively poor quality and the surface characteristics are inferior to SMC-l. The comparison is similar to the difference between BMC-2 and BMC-1. (ϊ When the resin systems of Examples 3 and 4-are used in the sheet moulding compound above, the physical characteristics, in particular, the surface

Claims (22)

·.·..; ,

1. A liquid polymerizable resin composition suitable for use in moulding compounds having reduced shrinkage '. y/hen polymerized, under the influence of■ heat and pressure, the resin composition containing (A) an ethylenically un- ■. saturated cross-linkable polyester, (B).an e'thylenically. ■ unsaturated monomer copolymerizable with polyester (A). to effect crosslinking thereof, and (C) a thermoplastic polymer; characterized in that (1) the ethylenically unsaturated, polyester has molecular, weight to double bond factor ', of 142, to 215, (2) the thermoplastic polymer comprises . an . addition polymer and contains 0.1 to 5.0 per cent by. weight , (calculated as hereinbefore described) of acid groups and is soluble in (B) or mixtures of (A) plus (B), (3) the composition- contains, by weight based on the total weight of (A),. (B) and (C), 20 to 80¾ of polyester (A) , 20 to 80% of monomer (B) and 1 to 25¾ by weight of polymer (C)f and (4) the resin composition is polymcrizable to form a non- homogeneous polymeric system. composition

2. , A resin /as claimed in Claim 1 which, upon, curing, undergoes a volumetric change within the range of 10% expansion by volume to 5% shrinkage by volume..

3. A resin composition as claimed in any preceding claim, wherein the unsaturated polymerizable poly-. .. ester comprises the condensation. product of an α, β- ' ethylenically unsaturated diearboxylic acid (or an ester- , forming equivalent) with a dihydric alcohol (or an ester- forming equivalent). ■ .

4. Y ester is the condensation product of a dihydric alcohol with a saturated dicarboxylic acid.

5. A resin ' composition as claimed in .Claim 3 wherein the unsaturated dicarboxylic acid (or ester-forming equivalent) is maleic anhydride or fumaric acid and the dihydric alcohol is propylene glycol, ethylene glycol, 1,3-butanediol, diethylene glycol, neopentyl glycol, or dipropylene glycol.

6. A resin composition as claimed in any preceding claim wherein the unsaturated polymerisable polyester is poly(propylene fumarate).

7. A resin composition as claimed in any preceding claim wherein the unsaturated cross-linkable polyester has an acid number in the range of 5-100.

8. A resin composition as claimed in any preceding claim wherein the monomer is styrene , vinyltoluene or a mixture of styrene or vinyltoluene with tert. -butyl-styrene, methyl methacrylate or chlorostyrene.

9. A resin composition as claimed in any preceding claim wherein the thermoplastic polymer (c) contains units from one or more of methyl methacrylate, ethyl acrylate, methacrylic acid, acrylic acid and styrene monomers.

10. A resin composition as claimed in any preceding claim wherein the thermoplastic polymer has a molecular weight of 25,000 to 500,000.

11. A resin composition as claimed in any preceding claim wherein the thermoplastic polymer contains 0.5 to 3% by weight acid groups (calculated as herein- -≠ before described). resin

12. A/composition as claimed in any preceding claim wherein (a) is present in an amount of from 30-50% by weight , (b) is present in an amount of from 40-65% by weight and (c) is present in an amount of from 10-15% by weight.

13. A resin composition as claimed in Claim 1, substantially as hereinbefore described in any of Examples 1, 6, ft and /&.

14. A bulk or sheet moulding compound comprising a resin composition as claimed in any preceding claim and one or more of the following: chemical thickener, filler, extender, reinforcing agent, free-radical catalyst, polymerisation stabilizer, release agent or other additive known as useful in bulk or sheet moulding compounds.

15. A bulk moulding compound as claimed in Claim 14, comprising a resin composition as claimed in any of Claims 1 to 13 together with a chemical thickener, parti culate filler, and/or fibrous reinforcing agent. compound

16. A bulk or sheet moulding/as claimed in Claim 14 wherein the. chemical thickener is a metal oxide or hydroxide and/or the reinforcing agent is fibre glass

17. A bulk or sheet moulding compound as claimed in any of Claims 14 to 16 wherein the chemical thickener is a Group II metal oxide or hydroxide and/or the reinforcing agent is fibre glass.

18. A bulk or sheet moulding compound as claimed in Glaim 17 wherein the fibre glass is present in an amount of about 5 to 60%.

19. A bulk or sheet moulding compound as claimed in Claim 14- substantially as hereinbefore described in 7 any of Examples 5(A), 5(D), 6, and &i

20. A method of producing a cured low shrink, optionally reinforced article which comprise subjecting to polymerisation a resin composition or a bulk or sheet moulding compound as claimed in any preceding claim.

21. A method as claimed in Claim 20, wherein there is subjected to polymerisation a bulk or sheet moulding compound as claimed in Claim 16.

22. A rigid cured composition obtained by a method as claimed in Claim 2Θ or 21.