GB1583465A - Moulded glass fibre-reinforced resin products and their production - Google Patents

Abstract

10 to 50 parts by weight of a copolymer, 10 to 50 % by weight of acrylonitrile and 90 to 50 % by weight of an aromatic vinyl hydrocarbon are dissolved in 90 to 50 parts by weight of a monomer chosen from the group comprising acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters and mixtures thereof, in order to provide a resin syrup. Glass fibres are then impregnated with the resin syrup obtained and the resin syrup with which the glass fibres have been impregnated is cured.

Description

(54) MOULDED GLASS FIBRE-REINFORCED RESIN PRODUCTS AND THEIR PRODUCTION (71) We, NITTO BOSEKI CO. LTD., a Japanese company, of No. 1 Aza Higashi, Gonome, Fukushima-shi, Fukushima, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: Moulded sheet-like articles obtained by impregnating glass fibres with a resin syrup composed mainly of methyl methacrylate, and hardening the impregnated resin syrup, have frequently been utilised outdoors. The technique of obtaining a transparent moulded resinimpregnated glass fibre article by causing the refractive index of the resin to correspond to that of the glass forming the glass fibres, is known. One method of this type comprises copolymerizing methyl methacrylate and a vinyl aromatic hydrocarbon. The resultant copolymer has a refractive index of from 1.49 to 1.60 and it is thus possible to adjust the refractive index of the copolymer to that of the glass fibres, as described in, for example, R.B.

Beevers, Trans. Faraday Soc.. 58 (1962) 1465. However, since the rate of reaction of the vinyl aromatic hydrocarbon is slow. the copolymerisation does not proceed easily. and the viscous solution or resin syrup obtained by partial polymerisation has a long hardening time.

According to the present invention, in an article comprising resin-impregnated glass fibres, the resin being a hardened resin syrup, the resin syrup is prepared by dissolving 10 to 50 parts by weight of a copolymer in 90 to 50 parts by weight of a monomer, in which the copolymer is of 10 to 50% by weight of acrylonitrile and 90 to 50% by weight of a vinyl aromatic hydrocarbon. and in which the monomer is selected from acrylic acid, acrylic acid esters, methacrylic acid. methacrylic acid esters and mixtures thereof.

According to the present invention. the correspondence of the refractive index of the resin with that of the glass fibres can be achieved by using a resin syrup which is obtained by dissolving a copolymer having a high refractive index in the monomer. In order to harden the resin syrup with which the glass fibres are impregnated within a short time, a so-called polymeric syrup. obtained by dissolving the copolymer in the monomer, is used. Since the copolymer is an acrylonitrile-modified vinyl aromatic hydrocarbon polymer of which a typical example is an acrylonitrile/styrene copolymer, the solubility parameter (calculated from the Small equation) of the polymer is made equal to, or larger than. the solubility parameter of the acrylic acid. the acrylic acid ester. the methacrylic acid or the methacrylic acid ester. This prevents a scattering of light caused by differences in the refractive index duc to a phase separation between the dissolved polymer and the polymer obtained by polymerizing the monomer in the resin syrup. Hence. transparent glass fibre-reinforced resin products can be obtained.

In order to produce moulded products of this invention having especially good transparency. it is necessary to use polymers having a calculated solubility parameter. as described hereinabove. of at least 9.30. It has been confirmed. however, that transparency of a degree required for plastic panels used in greenhouses can be achieved also by polymers having a calculated solubility parameter of about 9.25. The upper limit for the calculated solubility parameter is not critical. but when the chain length of the acrylonitrile unit in the copolymer is too long, an intramolecular cyclisation takes'place in the hardening step, which leads to problems in practice.

The calculated solubility parameter of an acrylonitrile/styrene copolymer, which is a typical example of the acrylonitrile/vinyl aromatic hydrocarbon copolymer used in this invention, is 9.25, 9.35 and 9.63, respectively, when the acrylonitrile content therein is 10, 20 and 30% by weight. Thus, the solubility parameter of the copolymer tends to increase as the content of acrylonitrile increases. Sufficient transparency can be achieved when the acrylonitrile content is about 10% by weight.

If the acrylonitrile content of the copolymer exceeds 50% by weight, the sequence of the chain length of the acrylonitrile unit increases, and intramolecular cyclisation occurs in the hardening stcp performed after impregnating the glass fibers with the resin syrup, thus impairing the transparency of the final product. Moreover, the styrene content of the copolymer naturally decreases, and the refractive index of the polymer can be adjusted only within a narrow range. Thus, it is difficult to adjust the refractive index of the polymer when more than 50% by weight acrylonitrile is used.

If the concentration of the acrylonitrile/vinyl aromatic hydrocarbon copolymer in the resin syrup used in this invention is more than 50% by weight, the viscosity of the resin syrup is high, and the syrup is difficult to handle. Preparation of a resin syrup which has too high a concentration should, therefore, be avoided. In order, however, to adjust the refractive index of the hardened resin product to near the refractive index (1.51 to 1.55) of the glass fibres and to avoid non-transparent glass fibre-reinforced resin products caused by the difference between the refractive index of the glass fibres and that of the resin impregnated therein, the vinyl aromatic hydrocarbon added to adjust the refractive index of the resin syrup must be present in an amount of at least 5 % by weight in the resin syrup. It has been ascertained that in view of the amount of the vinyl aromatic hydrocarbon required, the concentration of the acrylonitrile/ vinyl aromatic hydrocarbon copolymer in the resin syrup should be at least 10% by weight. It has been found that, for example, a resin syrup obtained by dissolving an acrylonitrile/styrene (50:50 monomer ratio by weight) copolymer in a concentration of about 10% (the concentration of styrene being about 5% by weight) in methyl methacrylate has a refractive index of about 1.505. and that a glass fibre-reinforced resin product obtained by using such a resin syrup has a transparency which is suitable for practical applications such as for use as plastic panels in greenhouses.

A resin syrup of a 50% by weight solution (the highest viscosity from the standpoint of handling the resin syrup. with a suitable viscosity ranging from about 3 to about 7 poises at the temperature of impregnation) of an acrylonitrile/styrene (10:90 monomer ratio by weight) copolymer has a refractive index of about 1.535, and a resin syrup composed of a 50% by weight solution of an acrylonitrile/styrene (50:50 monomer ratio by weight) copolymer has a refractive index of about 1.5 16. Since glass fibres having a relatively low refractive index near 1.510 are used in the art as reinforcing materials for moulded articles of this kind, hardened resin products of a high refractive index of the degree intended by the present invention can be obtained. even if the amount of the vinyl aromatic hydrocarbon is confined within a range where the handling of the resin syrup is casts namely even if the concentration of the acrylonitrile/vinyl aromatic hydrocarbon copolymer in the resin syrup is limited to not more than 50% by weight.

Suitable vinyl aromatic hydrocarbons which can be used in this invention are aromatic hydrocarbons in which one vinyl group is directly bonded to the aromatic ring. and which is copolymerisable with acrylonitrile. Preferred vinyl aromatic hydrocarbons for use in this invention are styrene (most prcfcrred) vinyltoluene. vinylxylene and a-methylstyrene.

The monomer for dissolving the acrylonitrile/vinyl aromatic hydrocarbon copolymer may be. for example. acrylic acid. ethyl acrylate. hydroxyethyl acrylate. methyacrylic acid, butyl methaciylate or methyl methacrylate. These monomers can be used either individually or as admixtures of two or more thereof. Of these, methyl methacrylate is most suitable for practical application.

As described above. according to this invention. glass fibres are impregnated with a resin syrup obtained by dissolving an acrylonitrile/vinyl aromatic hydrocarbon copolymer in the monomer. Thus. the impregnated resin includes the vinyl aromatic hydrocarbon in a polymeric state. which does not retard the hardening of the resin syrup. Hence, the rate of hardening of the resin syrup after the impregnation can be sufficiently lower than when using a conventional monolllel-prepolymerised resin syrup. This greatly increases the productivity of the process in commercial operations.

Suitable glass fibres which can be used in this invention are those of the so-called "chemical glass" including a substantial amount of alkali metal oxides with a relatively low refractive index near 1 .51 (). The strands of the glass fibres which are used in this invention preferably contain about 2()() filaments having a fibre diameter of about 4.5 ,am and are cut into a size of 2 inches for case of handling.

The hardening time for the resin syrup with which the glass fibres are impregnated can, of course, be shortened further by using means generally used for promoting the hardening of resins, for example, by increasing the reactivity of the resin by adding a catalyst such as benzoyl peroxide, acetyl peroxide or t-butyl peroxypivalate, which can be employed in a small amount to harden the resin syrup, to the composition, by raising the temperature to about 50 to about 80"C, or by using a chain transfer agent. A chain transfer agent can reduce the molecular weight of the resulting polymer, and as a result, the viscosity of the resin syrup does not increase. Thus, it is possible to prepare a syrup having a relatively low viscosity which contains a high content of polymer. The hardening time can be decreased still further by using a polyfunctional monomer to promote a gel effect, and positively performing a threedimensional cross-linking reaction. A shorter hardening time is preferred.

Useful chain transfer agents which can be used in general include alkyl mercaptans such as n-dodecyl mercaptan, isopropyl mercaptan and n-butyl mercaptan, aryl mercaptans such as thiophenol, thiocresol and thionaphthol and sulfur compounds containing an active hydrogen such as thioglycolic acid and the esters thereof. The effective amount of the chain transfer agent is 0.1 to 1.0 part by weight per 100 parts by weight of monomer.

Examples of suitable polyfunctional monomers which can be employed include ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, ethylene glycol diacrylate, trimethylolethane triacrylate, 1,3-butylene dimethacrylate, glycidyl methacrylate, tetrahydrofurfuryl methacrylate, divinylbenzene, triallyl cyanurate and triallyl isocyanurate. Of these compounds, 1,3-butylene dimethacrylate, ethylene dimethacrylate and trimethylolpropane trimethacrylate are especially effective. However, the use of such a polyfunctional monomer as a cross-linking agent is not essential.

In the present invention, the monomer dissolved in the resin syrup can be partially polymerised by adding a small amount of a catalyst such as benzoyl peroxide or azobisisobutyronitrile. The polyfunctional monomer can be employed with equal results before or after the partial polymerisation of the resin syrup.

When the amount of the polyfunctional monomer is up to 5 % by weight, a shortening of the hardening time (depending on the amount thereof) is promoted, and the mechanical characteristics of the hardened product are not adversely affected. If the amount of the polyfunctional monomer exceeds 5% by weight, the hardened product becomes brittle. Hence, it is necessary to restrict the amount of the polyfunctional monomer within 5% by weight based on the monomer used. Generally, when such a cross-linking agent is employed in an amount of about 5% by weight, the hardening time for the monomer-prepolymerized resin syrup, which is generally about 30 minutes in the absence of such a cross-linking agent, can be shortened to about 20 to 22 minutes. It has been confirmed that the hardening time for the polymeric syrup used in this invention, under the same conditions, can be shortened to about 8 to 10 minutes from about 16 minutes by using the polyfunctional monomer.

To obtain good mechanical strength in a glass fibre-reinforced plastic panel in accordance with this invention, the glass fibres are generally employed in an amount of 20 to 30 parts by weight per 100 parts by weight of the resin syrup.

The following experimental Examples and comparative experimental Examples are given to illustrate in detail the synthesis of the resin syrup which forms a basis for the discovery of the process for producing glass fiber-reinforced resin products in accordance with this invention, and the various properties of the hardened products obtained by using 1 part by weight of t-butyl peroxypivalate, as a polymerization initiator, per 100 parts by weight of the resin syrup in comparison with the synthesis of a resin syrup using a conventional method and the properties of the hardened product obtained by curing the resin syrup. Unless otherwise indicated herein, all parts, percents, ratios and the like are by weight.

In the experimental Examples, comparative experimental Examples, Example and comparative Example given hereinafter, the various properties described were measured using the following methods.

Viscosity Average Molecular Weight of Polymer Measured using an Ostwald capillary viscometer using benzene (25"C).as a solvent for the methyl methacrylate/styrene system and dimethylformamide (25 C) as a solvent for the methyl methacrylate/styrene/acrylonitrile system.

Polymerization Conversion Measured by a precipitation method using acetone (good solvent)-methanol (poor solvent).

Viscosity Measured with a BM-type standard viscometer (a product of Tokyo Keiki K.K.) using a No. 2 rotor at a speed of 30 rpm.

Hardening Time The time required to harden (cure) a mixture of 100 parts of each of the resin syrups and 1 part of t-butyl peroxypivalate was measured using a differential scanning calorimeter (DSC, a product of Perkin-Elmer Company) at 650C.

Transparency Evaluated by visual observation with the naked eye.

Light Transmittance Transmittance of light at a wavelength of 350 m,a which was measured using a doublebeam spectrophotometer (a product of Shimadzu Seisakusho K.K.).

Refractive Index Measured at 25 0C using an Abbe refractometer (a product of Shimadzu Seisakusho K.K.).

Flexu ral Strength Measured with a tensile tester marketed under the name "Tensilon" (registered Trade Mark) from Toyo Baldwin Co., Ltd. using a test piece having a width of 20 mm, a span of 50 mm and a thickness of 1 mm.

Tensile Strength Measured with a tensile tester ("Tensilon") using a dumbbell-shaped test piece having a width of 5 mm (central width of 3 mm), a length of 100 mm and a thickness of 1 mm.

Weatherability A test sample was exposed to a weather-ometer (a product of Suga Tester Co., Ltd.) for 400 hours, and then the change in the color of the sample was visually observed.

Experimental Example 1 In accordance with Run No. l shown in Table 1 below, a monomeric mixture of 10 parts by weight of acrylonitrile and 90 parts by weight of styrene, 0.1 part by weight of t-butyl peroxypivalate as a polymerization initiator and 0.6 part by weight of n-dodecyl mercaptan as a chain transfer agent were charged into a reactor, and reacted at 600C to produce a pre-copolymer of acrylonitrile and styrene having a viscosity average molecular weight of 70,000.

To 25 parts by weight of the resulting polymer were added 75 parts by weight of methyl methacrylate and 0.05 part by weight of azobisisobutyronitrile as a polymerization initiator, and these materials were reacted in a reactor to produce a resin syrup having a polymerization conversion of 31 to 33% and a viscosity of 5.5 poises (25"C) for use in this invention.

Then, t-butylperoxypivalate was added in an amount of 1 part by weight per 100 parts by weight of the resin syrup. The mixture was poured into a mold having the shape of a flat plate, and heated at about 65"C for 16 minutes to produce a resin hardened product having a thickness of about I mm.

Repetition of these procedures confirmed that the curing of the resin syrup ended in 15 to 17 minutes.

The resulting resin hardened products were semi-transparent but sufficiently transparent that they could be used as transparent plates in practical application. The product had a light transmittance at 350 ml.L of 82 to 83%, a refractive index of 1.513, a flexural strength of 10 to 12 kg/mm2, and a tensile strength of 5 to 7 kg/mm2.

The proportions of the monomers used to form resin syrups, and the characteristics of the resin hardened products in the above and subsequent experimental examples are'summar- ized in Table 1 below.

Experimental Examples 2 to 4 In Run Nos. 2 to 4 in Table 1, a monomeric mixture of acrylonitrile and styrene in the proportions indicated in Table 1, 0.1 part by weight of t-butyl peroxypivalate as a polymerization initiator and optionally 0.6 part by weight of n-dodecyl mercaptan as a chain transfer agent were charged into a reactor, and reacted at 600C to produce a pre-copolymer of acrylonitrile/sytrene.

To 25 parts by weight of the prepolymer were added 75 parts by weight of methyl methacrylate and optionally 0.05 part by weight of azobisisobutyronitrile as a polymerization initiator. These materials were reacted in a reactor, or allowed to stand as a mixture. Resin syrups having the polymerization conversions and viscosities shown in Table 1 were obtained.

Then, t-butyl peroxypivalate was added in an amount of 1 part by weight per 100 parts by weight of the resin syrup. and the mixture was poured into a mould having the shape of a flat plate. and heated at about 65"C for each of the periods indicated in Table 1 below to produce resin hardened products having a thickness of about 1 mm and very good transparency.

In these Examples. the use of a polyfunctional monomer as a cross-linking agent capable of shortening the hardening time for resin syrups was omitted. However, as described hereinabovc. the joint use of a suitable amount of a polyfunctional monomer can reduce the hardening time (16 to 19 minutes) for the resin syrups to about 8 to 10 minutes.

Table I shows that the mechanical strength characteristics of the hardened products obtained from the resin syrups used in this invention are somewhat better than those of hardened products obtained in the comparative experimental Examples (conventional method) to be given hereinbelow. and that when the hardened products are in the form of a plate, the plates are flexible and thus pliable when bent. Because of this characteristics, plastic panels for greenhouses, which are one important use of plate-like products of this kind outdoors, can be designed with a curvature, and the insertion or fitting operation of the plate-like product by bending the product is made possible. Accordingly, the glass fibrereinforced resin products obtained by the present invention have very good utility in commercial applications.

Comparative Experimental Examples 1 to 3 In Runs Nos. 5 to 7 shown in Table 1 below, a monomeric mixture of methyl methacrylate and each of the vinyl aromatic hydrocarbons indicated in Table I below in the proportions indicated, 0.05 part by weight of azobisisobutyronitrile as a polymerization initiator and 0.6 part by weight of n-dodecyl mercaptan as a chain transfer agent were charged into a reactor, and reacted at 800C to produce a monomeric syrup having a viscosity average molecular weight of 50,000.

Then, t-butylperoxypivalate was added in an amount of 1 part by weight per 100 parts by weight of the monomer-prepolymerized resin syrup. The mixture was poured into a mould having the shape of a flat plate in the same manner as in Experimental Examples 1 to 4, and heated at about 65"C to produce resin hardened products having a thickness of about lmm.

As indicated in Table 1 below, the required hardening time was more than 30 minutes in all of the runs. The flexural strengths and tensile strengths of these products were inferior to those of the products obtained in Experimental Examples 1 to 4.

Comparative Experimental Examples 4 and 5 In Runs Nos. 8 and 9 in Table I below, a prepolymer of styrene was prepared, and mixed with methyl methacrylate in the proportions shown in Table 1 below. The mixture was optionally heated in a reactor and resin syrups were obtained. Then, t-butylperoxypivalate was added in an amount of 1 part by weight per 100 parts by weight of each resin syrup. The mixture was poured into a mould having the shape of a flat plate, and heated at about 65 C to produce resin hardened products having a thickness of about 1 mm.

As can be seen from the results in Table I, the required hardening time could be shortened to about 15 to 20 minutes. However. the resulting resin hardened products all had white cloudy areas therein, and the transparent products intended by the invention could not be formed.

T A B L E 1 Experimental Comparative Experimental Example Example Example Example Example Example Example Example Example 1 2 3 4 1 2 3 4 5 Run No. 1 2 3 4 5 6 7 8 9 Resin Syrup Polymer- Polymer- Polymer- Polymer- Polymer- Polymer Monomer Monomer Monomer Polymerization System Monomer Monomer Monomer Monomer Monomer Monomer Proportions of Constitu- MMA:ST:AN MMA:ST:AN MMA:ST:AN MMA:ST:AN MMA:ST MMA:ST:EA MMA:VT MMA:ST MMA:ST ents of Resin Syrup (wt%) 75:22.5: 75:20:5 75:18.5: 75:17.5: 80:20 60:20:20 80:20 70:30 80:20 2.5 6.5 7.5 Reaction Conditions Acrylonitrile (AN) 10 20 25 30 - - - - Ethyl Acrylate (EA) - - - - - 20 - - Vinyltoluene(VT) - - - - - - 20 - Styrene (ST) 90 80 75 70 20 20 - 100 100 Methyl Methacrylate (MMA) - - - - 80 60 80 - Azobisisobutyronitrile - - - - 0.05 0.05 0.05 0.05 0.05 t-Butylperoxypivalate 0.1 0.1 0.1 0.1 - - - - n-Dodecyl Mercaptan 0.6 0.6 - 0.6 0.6 0.6 0.6 0.6 0.6 Reaction Temperature ( C) 60 60 60 60 80 80 80 100 100 Viscosity Average 70.000 70.000 250.000 70.000 50.000 50.000 50.000 50.000 50.000 Molecular Weight of Polymer (cont'd) Experimental Comparative Experimental Example Example Example Example Example Example Example Example Example 1 2 3 4 1 2 3 4 5 Conditions for Preparing Resin Syrup Prepolymer 25 25 25 25 - - - 30 20 Methyl Methacrylate 75 75 75 75 - - - 70 80 Azobisisobutyronitrile 0.05 0.05 - 0.05 - - - - 0.05 n-Dodecyl Mercaptan - - - - - - - - 0.3 Reaction Temperature ( C) 80 80 - 80 - - - - 80 Properties of Resin Syrup Polymerization Conversion(%) 31-33 30-32 25 29-31 34-36 34-36 34-36 30 34-36 Viscosity (poises at 25 C) 5.5 5.5 6.0 5.5 5.0 4.8 5.2 4.5 5.5 Hardening Time (minutes) 16 17 19 18 34 32 31 19 15 Properties of Hardened Products Transparency Semi- Excellent Excellent Excellent Excellent Excellent Excellent Cloudy Cloudy transpar- with with ent white white areas areas Light Transmittance at 82-83 82-83 82-83 80-81 350 m (%) 82-83 82-83 81-82 - Properties of Hardened Products Refractive Index 1.513 1.511 1.510 1.509 1.512 1.511 1.512 - Flexural Strength (kg/mm2) 10-12 11-13 11-13 12-13 10-12 10-12 9-11 9-11 9-11 Tensile Strength (kg/mm2) 5-7 6-8 6-8 7-8 5-7 5-7 5-7 5-7 5-7 Glass fibres were impregnated with the same resin syrups as produced in Experimental Example 2 and Comparative Experimental Example 1 to obtain resin hardened products.

The time required to cure the resin syrup and the properties of the resulting glass fibrereinforced resin products are described in the following Examples.

Example l part by weight of t-butyl peroxypivalate was added to 100 parts by weight of the same resin syrup as obtained in Experimental Example 2 (MMA 75 : ST 20 : AN 5). After thorough mixing, the mixture was used to impregnate a 2-inch chopped strand of chemical glass fibres having a refractive index of 1.517 with the weight ratio of the glass to the resin syrup being maintained at 1 : 4. By using a spacer to obtain a plastic plate having a uniform thickness of l mm, the impregnated glass fibres were heated at 650C for 17 minutes to cure the resin. Post-curing was subsequently performed at 120"C for 5 minutes to produce a glass fibre-reinforced resin plate.

The characteristics of the resulting resin plate are shown in Table 2 shown below.

Comparative Example l part by weight of t-butyl peroxypivalate was added to 100 parts by weight of the same monomeric syrup (MMA 80 : ST 20) as obtained in Comparative Experimental Example 1, and the materials were thoroughly mixed. The mixture was used to impregnate a 2-inch chopped strand of chemical glass fibres with the weight ratio of the glass to the resin syrup being maintained at 1:4. While being pressed with a spacer having a thickness of 1 mm, the impregnated glass fibers were heated at 650C. More than about 34 minutes were required to cure the resin. Post-curing was performed at 1200C for 5 minutes to produce a glass fibre-reinforced resin plate having the characteristics shown in Table 2 below.

Claims (12)

1. Table 2 Moulded Plate Characteristics of Comparative Moulded Plate Example Example Transparency Excellent Excellent Light Transmittance 82 - 83 82 - 83 at 350 my (9'c') Weatherability No discoloration No discoloration Flexural Strength 13 - 15 12-14 (kg/ nim2) Tensile Strength 8-9 7-9 (kg/ mm 2) WHAT WE CLAIM IS: An article comprising resin-impregnated glass fibres, the resin being a hardened resin syrup prepared by dissolving 10 to 50 parts by weight of a copolymer in 90 to 50 parts by weight of a monomer in which the monomer is of 10 to 50% by weight of acrylonitrile and 90 to 50% % by weight of a vinyl aromatic hydrocarbon. and in which the monomer is selected from acrylic acid. acrylic acid esters, methacrylic acid. methacrylic acid esters and mixtures thereof.
2. 2. An article according to claim I wherein the vinyl aromatic hydrocarbon is selected from styrene. vinyltoluene. vinylxylerie and ot-methylstyrene.
3. 3. An article according to claim I wherein the vinyl aromatic hydrocarbon is styrene.
4. 4. An article according to any preceding claim wherein the monomer is methyl methacry latc.
5. An An article according to any preceding claim wherein the resin syrup additionally comprises a chain transfer agent dissolved therein.
6. 6. An article according to claim 5 wherein the chain transfer agent is selected from alkyl mercaptans. aryl mercaptans and sulfur compounds containing an active hydrogen.
7. 7. An article according to claim 5 or claim 6 wherein the amount of the chain transfer agent is (). l to I .() parts by weight of the monomer.
8. 8. An article according to any preceding claim. wherein the resin syrup additionally comprises a poly-functional monomer dissolved therein.
9. 9. An article accolding to claim 8 whercin the polyfunctional monomer is selected from ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, ethylene glycol diacrylate, trimethylolethane triacrylate, 1,3 butylenedimethacrylate, glycidyl methacrylate, tetrahydrofurfuryl methacrylate, divinylbenzene, triallyl cyanurate and triallyl isocyanurate.
10. 10. An article according to claim 8 or claim 9 wherein the amount of the polyfunctional monomer is up to 5% by weight, based on the weight of the monomer.
11. I I. An article according to claim 1 in which the resin syrup is substantially as described in any of the Experimental Examples.
12. 12. An article according to claim 1 substantially as described in the Example.