WO1997043339A1

WO1997043339A1 - Toughened vinyl ester resins

Info

Publication number: WO1997043339A1
Application number: PCT/AU1997/000290
Authority: WO
Inventors: Peter John Martyn Burchill
Original assignee: Crc For Polymers Pty. Ltd.
Priority date: 1996-05-10
Filing date: 1997-05-09
Publication date: 1997-11-20
Also published as: AUPN978796A0

Abstract

The present invention relates to a vinyl ester resin composition with increased fracture toughness. The resin comprises a vinyl ester resin and a core/shell polymer wherein the core/shell polymer is dispersed throughout the vinyl ester resin. The incorporation of the core/shell polymer in the vinyl ester resin increases the fracture toughness of the vinyl ester resin about threefold at levels of 5 pphr. There is also described a process for toughening vinyl ester resins comprising dispersing a core/shell polymer in a vinyl ester resin and subsequently curing said vinyl ester resin. There is also described articles produced from these toughened vinyl ester resins.

Description

TOUGHENED VINYL ESTER RESINS

The present invention relates to a vinyl ester polymer composition. In particular the present invention relates to a toughened vinyl ester polymer composition and to a process for preparing a toughened vinyl ester polymer composition.

Vinyl ester resins, or epoxy acrylate-based resins as they are alternatively known, are employed in a variety of industrial applications which range from primary structures and containers to adhesives. Vinyl ester resins have also been used in dentistry. Vinyl ester resins demonstrate significantly better chemical resistance than the less expensive polyester resins, particularly in aqueous environments. Vinyl ester resins also allow flexibility in the controlling of the cure process which allows a wide variety of applications from very rapid cure coatings to thick sections which require prolonged cure schedules. Vinyl ester resins are a well known class of thermosetting resin which have been used for many years and were introduced commercially in 1965. There have been a number of attempts to improve the toughness of vinyl ester resins with varying degrees of success.

A consequence of the conventional methods of toughening vinyl ester resins, using reactive liquid polymers is that the glass transition temperature (T_g) is reduced which results in the maximum use temperature of the resin being lowered. The incoφoration of a carboxy terminated reactive liquid polymer (RLP) in the synthesis of the vinyl ester resin is believed to reduce the crosslink density of the polymer, increasing its flexibility. However, the increase in fracture toughness of vinyl ester resins incoφorating carboxy terminated reactive liquid polymers in their synthesis has a corresponding and undesirable effect of lowering the heat distortion temperature relative to the base resin.

Vinyl ester resins have a variety of applications and are preferred to polyester resins when greater environmental resistance required. For example: copper refining tanks made from polymer concrete; glass resin composites; filament wound composite tubes on oil rigs; chemical reactors and holding tanks; hulls and superstructures of marine vessels; and barrier layers in swimming pools. We have now found that the dispersion of core/shell polymers into vinyl ester resins results in significant improvements in the fracture toughness of the cured vinyl ester resins. Accordingly, there is provided a polymer resin comprising a vinyl ester resin and a core/shell polymer wherein said core/shell polymer is dispersed throughout the vinyl ester resin.

In a second aspect there is provided a process for increasing the fracture toughness of a vinyl ester resin comprising dispersing a core/shell polymer in a vinyl ester resin and subsequently curing said vinyl ester resin.

In a third aspect there is provided a moulded, laminated, cast or gel-coated article produced from the polymer resin of the present invention. Additionally there is provided a moulded, laminated, cast or gel-coated article produced by the process of the present invention.

The use of core/shell polymers in accordance with the present invention is suitable for increasing the fracture toughness of a wide variety of vinyl ester polymers. Vinyl ester resins are generally produced by a reaction between epoxy compounds and acrylic or methacrylic acid. The most widely known and used vinyl ester resin is produced from the diglycidyl ether of bisphenol-A (DGEBA) and methacrylic acid. Typically, an amine salt or triphenylphosphine is used to catalyse the reaction at a temperature of about 120°C for 4 to 5 hours. 90 to 95 % of the methacrylic acid reacts to form the monomer. Styrene may be added to reduce the viscosity during synthesis. Alternatively, glycidyl methacrylate may be reacted with a multifunctional phenol to form a vinyl ester resin.

Epoxy compounds which may be used to vinyl ester resins include the wide variety of commercial epoxy resins which are commercially available. Epoxy resins are characterised by a 3-membered ring known as the epoxy, epoxide, o one or ethoxylene group:

Commercial epoxy resins generally contain aliphatic, cycloaliphatic, or aromatic backbones. Vinyl ester resins which have been formulated for high temperature use, such as cresol novolac based vinyl ester resins, are generally quite brittle. High temperature vinyl ester resins may also be toughened by the addition of a core shell polymer in accordance with the present invention.

Internally modified vinyl ester resins are also suitable for use in the present invention. We have found that internally modified vinyl ester resins also show improved fracture toughness when blended with core/shell polymers in accordance with the present invention.

The simplicity of the synthesis of vinyl ester resins enables considerable structural variation in the cured vinyl ester resin which allows considerable scope for controlling a variety of properties of the cured resin. For example the viscosity and the heat distortion temperature may be controlled. As described in the background, a reactive liquid polymer (RLP) may be used to replace some of the methacrylic acid in the synthesis to increase the toughness of the vinyl ester polymer. In commercial toughened vinyl ester resins an RLP such as carboxy terminated butadiene-acrylonitrile copolymer may be used. These toughened vinyl ester resins may also be further toughened by the use of core/shell polymers in accordance with the present invention. Alternatively, adding bisphenol-A has the effect of separating the methacrylate terminal groups and an expected increase in toughness. Other additives such as maleic anhydride may also be added. Maleic anhydride, for instance, is used to improve the environmental resistance further by increasing the crosslink density in the cured resin.

Core/shell polymers which are dispersed in vinyl ester resins in accordance with the present invention are generally produced by controlled emulsion polymerisation during which the composition of the monomer feed is changed in order to achieve a desired compositional variation over the structure of the core/shell polymer. While many core/shell polymers having a variety of properties are available, the core/shell polymers suitable for use in the present invention typically have a core which is rubbery at ambient conditions and is produced by polymerizing such monomers as butadiene and alkyl acrylates. By "rubbery at ambient conditions" it will be understood that the core of the core/shell polymer has a T_g which is lower than the ambient temperature. While not wishing to be bound by theory, it is believed that the glass transition temperature (T_g ) is desirably less than 20°C more desirably less than 0°C, and even more desirably less than -30°C. The shell of the core/shell polymer is preferably selected such that there is good adhesion between the core/shell polymer and the vinyl ester matrix of the cured resin. The adhesion of the core/shell polymer to the vinyl ester resin may be improved by the addition of functional groups, such as glycidyl, to the polymer of the shell. The shell is preferably a glassy polymer at ambient conditions, and is produced from polymerizing such monomers as styrene and methylmethacrylate. Often other monomers such as acrylic acid, methacrylic acid, glycidyl methacrylate, 2-hydroxyethylmethylacrylate, alkylmethacrylate and 2- hydroxypropylmethacrylate are added in the reaction to form the shell in order to improve the interfacial bond with the vinyl ester resin in which they are mixed.

The core/shell polymers for use in the present invention desirably have a core content in the range of from 40 to 90% by weight and a corresponding shell content in the range of from 10 to 60% by weight. Core/shell polymers do not necessarily have distinct transition from core to shell as they may be produced by a multistage seed emulsion polymerisation method in which the core is formed about the seed and the monomer feed to the emulsion polymerisation mixture is altered to vary the composition of the forming polymer so as to produce the shell. The change of monomer feed may be gradual, resulting in a transitional region from core to shell or it may be discrete, whereby the transitional region is substantially reduced.

A wide variety of core/shell polymers are effective in increasing the fracture toughness of vinyl ester polymers. It is preferred that the core/shell polymers have a particle size less than 2μm.

We have found that preferably the core/shell polymer is added to the vinyl ester resin in an amount of up to about 20 pph of resin (by weight). It is preferred that the core/shell polyrr be present in the resin in an amount in the range of from 1 to 20 pph of resin (by weight) ai. more preferably 5 to 10 pph of resin (by weight).

Preferred core/shell polymers include those consisting of polymerised: butadiene, methyl methacrylate and styrene; butadiene, alkyl methacrylate, alkyl acrylate; butadiene, styrene, alkyl acrylate, alkyl methacrylate and methacrylic acid; butadiene, styrene, alkyl acrylate, alkyl methacrylate, methacrylic acid and low molecular weight polyethylene (as flow modifier); butyl acrylate and methyl methacrylate; alkyl methacrylate, butadiene and styrene; alkyl acrylate, alkyl methacrylate and glycidylmethacrylate; and alkylacrylate and alkylmethacrylate.

Particularly preferred core/shell polymers for use in the present invention are core/shell polymers which incoφorate butadiene as a core component. We have found these to be particularly effective in increasing the fracture toughness of vinyl ester resins.

While not wishing to be bound by theory, it is believed that the core/shell polymer provides a dispersed phase which has a lower elastic modulus than the continuous cured vinyl ester resin phase. Furthermore core/shell polymers generally have a relatively small particle size. It is preferred that the particle size of the core/shell polymer is less than 2μm. While not wishing to be bound by theory it is believed that the stress conditions around a crack tip are altered by the dispersed phase and results in shear yielding which absorbs more energy than brittle failure and reduces crack propagation.

The polymer blend of the present invention may be made by simply dispersing the core/shell polymer with the vinyl ester resin by blending. Core/shell polymers are commercially available in the form of core/shell powders and pellets. We have found that powders may simply be dispersed in the vinyl ester resin by blending. We have found that core/shell polymers in pellet form are more difficult to disperse in vinyl ester resins. The core/shell polymer may be swollen in styrene prior to its addition to the vinyl ester resin. It is preferred that the mixture of core/shell polymer and styrene be allowed to equilibrate for at least 24 hours prior to blending to allow easier dispersion of the core/shell polymer into the vinyl ester resin. The mixture of core/shell polymer and vinyl ester resin may be subsequently blended with a high shear mixer in order to disperse the core/shell polymer evenly throughout the vinyl ester resin. Alternatively core/shell polymers may be dispersed in a vinyl ester resin which incoφorates additional styrene. The fracture toughness properties do not appear to differ when either procedure is used. The core/shell polymer is preferably dispersed in the vinyl ester resin by high shear mixing so as to provide good dispersion in a commercially acceptable time frame.

The blended mixture may then be cured by any convenient means to produce a vinyl ester polymer. Vinyl esters are generally cured (or polymerised) by a free radical reaction typically initiated by thermal or catalytic decomposition of peroxides or decomposition of a photoinitiator. We have found that the addition of core/shell polymers to the vinyl ester resins does not significantly effect the curing process, either in terms of rate or extent.

Curing agents typically used for curing vinyl ester resins include peroxy compounds such as cumene hydroperoxide, cumene peroxide, alkylperoxyesters (eg. tertbutylperoxybenzoate) and any of the wide variety of commercially available peroxy compounds. The curing process may be controlled analogously to the control of curing in prior art vinyl ester resin compositions. The use of accelerators and retarders may be advantageous in certain applications. Accelerators suitable for use include aromatic tertiary amines such as dimethyl analine. Retarders include antioxidants (free radical reaction inhibitors), such as hydroquinone, and aliphatic 2,4-diones, such as pentane 2,4 dione.

Catalysts may also be added to the polymer resin so as to promote the curing of the resin. Catalysts, such as transition metal salts (eg. cobalt (II) octoate), promote the cure of the resin by catalytic decomposition of the peroxides to generate free radicals:

Co²⁺ + ROOH ^■- Co³⁺ + RθVθH

Co³⁺ + ROOH «- Co²⁺ + ROO'+ H⁺

The vinyl ester resin produced from the diglycidyl ether of bisphenol-A and methacrylic acid typically produces a brittle highly crosslinked network with a glass transition temperature of about 220°C. These vinyl ester resins are generally polymerised after dilution with other vinyl monomers such as styrene or other acrylates or methacrylates. In dental applications triethylene dimethacrylate has been used as a comonomer. Styrene copolymers of vinyl esters are typically prepared with 40 to 50% styrene by weight. The styrene copolymers typically have a glass transition temperature of about 115°C. The styrene copolymers show small decreases in modulus and glass transition temperature but increases in elongation to break as the styrene content increases. The fracture toughness of these styrene copolymers generally shows no substantial change relative to the fracture toughness of the vinyl ester resin. The present invention significantly improves the fracture toughness of styrene copolymers of vinyl esters.

The polymer blend of the present invention may also incoφorate a variety of additives typical of those generally incoφorated into vinyl ester resins of the prior art. Such additives will include pigments, UV stabilisers, processing aids, fillers such as aggregate for the formation of polymer concrete and fibres for the formation of glass fibre reinforced resin.

The polymer blends of the present invention may be cured to form articles for use in applications for which vinyl ester resins are well known. We have found that the properties of the polymer blend of the present invention render such blends particularly suitable for the preparation of polymer concrete with the incoφoration of aggregate. Other applications for which the toughened vinyl ester polymer blends of the present invention are particularly suited include tie layers for swimming pools. We have found that the incoφoration of the core shell polymer into the vinyl ester resin does not prevent the resin being formulated with a viscosity such that it is suitable for spray application in a manner typically used for laying up applications such as swimming pools.

We have also found that the cured resin of the present invention advantageously exhibits improved adhesion properties and increased delamination resistance in fibre glass composites. The resistance of the cured resin to water and acid is not significantly affected.

We have found that the addition of a core/shell polymer does not significantly alter the cure behaviour of the vinyl ester resin, i.e. the gellation time is not significantly effected nor the time of further reaction to form the glassy cured resin. Also the glass transition temperature, is not substantially affected by the addition of the core/shell polymer. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The present invention will be further described by the following non limiting examples.

Materials

The vinyl ester resins used include: Hetron 922 (Huntsman Chemical Company Australia Ltd) and Derakane 411-

45 (Dow Chemical Coφ.).

These resins are essentially mixtures of the methacrylate ester of the diglycidylether of bisphenol-A and styrene. Small differences between the products probably arises from the other materials (bisphenol-A and maleic anhydride) also added during manufacture.

Hetron 980 (Huntsman Chemical Company Australia Ltd), a cresol novolac based vinyl ester resin made for high temperature applications.

Derakane 8084 (Dow Chemical Coφoration), a modified version of Derakane

411-45 recommended for applications requiring improved toughness.

The core/shell additives were manufactured by Kureha and by Rohm and Haas and were obtained from Fidene Coφoration, details of their composition are given in Table 1. These additives were in the form of powders or pellets. Table 1

Blending

The core/shell additives were either directly added to the resin or first mixed with styrene before addition to the resin. The mixtures were allowed to equilibrate for at least 24 hours before blending with a high shear mixer-Silverson L4R. The equilibration rendered the core/shell polymers more readily dispersible in the vinyl ester resin. All the additives used increased the viscosity of the resin. Curing

Vinyl ester resins (100 parts) were mixed with cobalt octoate (6% solution in white oil) 0.3 parts and methylethylketone peroxide (40% solution in phthalate ester) 1.5 parts, degassed, poured into a mould, and kept at room temperature to cure for at least 16 hours before post curing. Post curing was carried out at 60°C or at 90°C for 90 minutes.

Toughness

Fracture toughness was measured using double torsion with at least four specimens tested for each material. The double torsion specimen was used to obtain both the strain energy release rate G_jc and the critical stress intensity factor K_lc for slow crack growth, G_lc and K,, from double torsion tests were calculated from:

G - £1 ?^C_ ^lc 2/ ^' da

in which P is load, t_n is the crack width and C is compliance and a is the crack length,

in which v is Poissons ratio, / is the applied load moment arm, W ϊs specimen width, t is specimen thickness, and fc, is a correction term for finite thickness to width ratio.

Comparative Examples A to N

Table 2 contains the fracture properties measured by double torsion of several castings of the vinyl ester resins cured at room temperature (rt) and given different post cure temperatures. Generally unstable stick-slip growth was observed which the initiation (K_lci, G_lci) and arrest values (K_lca, G_lcll) are shown. There was good reproducibility in the measurements, and shows that nominally the same resins made by different manufactures have the same toughness. Two of the castings (*#) post cured at 90°C showed both stick-slip and stable continuous crack growth, the stable values are given in Table 2 and are slightly less than the initiation value for the unstable stick-slip growth. As the post cure temperature was increased and as the resin became more fully crosslinked it became more brittle, at the same time the elastic modulus increased. Table 2

Examples 1-15

Core/shell polymers were added directly to the resin (Derakane 411-45), and Table 3 contains the results obtained for several casting with KCA 102 and 304. The core/shell additive had been mixed in at 5pph, and the blends post cured at 90°C. Although, fracture behaviour was unstable, large increases in values compared to the neat resin were recorded. These results indicate that core/shell polymers with butadiene cores are more effective than those with acrylic cores.

Hetron 922 blends were also produced and confirmed these early results. Suφrisingly, one of the additives gave a blend which showed stable crack growth even though the toughness was not as high as found with some of the other additives. Table 3

Examples 16 to 39 Kureha KCA 102 was dispersed in Hetron 922 and gave a significant improvement in toughness. Table 4 shows that, even at a 5% addition of the core/shell polymer toughness is greatly increased over the neat resin. Extra styrene was added at 17 parts and at 45 parts and did not appear to significantly change the fracture toughness of the blends. At this concentration of styrene the fracture of castings was observed to be both stable and unstable, while at greater concentrations of styrene only stable crack growth has been observed. There appeared to be some variability in toughness of castings having the same composition and this is believed to be dependent upon how well the additive had been dispersed. It was also observed that the addition of styrene to the Hetron 922, in the absence of any core/shell polymer, did not substantially affect the fracture toughness of the Hetron 922. It was observed that the effect of post cure temperature on toughness of the blends was similar to that on the neat resin. However with room temperature and 60°C post cure only stable crack growth was recorded. One of the room temperature castings could not be broken in the double torsion test until aging for 3 months had advanced the cure.

The Kureha additive KCA 102 was replaced by EXL2602 having substantially the same composition. Values of K and G obtained using this material were lower than those obtained with KCA 102, but at 5 parts of the core/shell polymer the crack growth was stable. Again with the EXL 2602 the butadiene containing additives appeared to be more effective than those containing an acrylic core. Nevertheless, the acrylic cored materials seem more able to promote stable crack growth. It is believed that this effect may be due to these core/shell polymers being more easily dispersible.

Table 4

Example 40 and Comparative Example O

A resin/fibre composite was made by putting down alternate layers of resin and glass which were then vacuum bagged to compact the layers and reduce the void content. After allowing to cure at room temperature for 16 to 24 hours the composite was post cured at 90°C for 90 minutes (Comparative Example O). Volume fraction of glass in the cured composite was about 55% and the void content generally less than 2%. The glass used was COLAN AR106, plain weave, 630g/m² cloth of which 16 layers were used to give a composite about 9mm thick. The blend used was HETRON 922 blended with 5pph EXL2602 and 17pph styrene cured at rt + 90°C (Example 40).

Double torsion test

# Double cantilever beam test (interlaminar fracture toughness)

The blend diφlayed a greatly increased fracture resistance with a more than two fold increase in the critical stress intensity factor as measured using the double torsion test specimen. The energy required to propagate a crack has been increased about five times. When used as a matrix resin in a fibre glass composite, the energy required to propagate a delamination crack is also increased about five times as shown in the table.

Example 41 and Comparative Example P

HETRON 980 - a cresol novolac based vinyl ester resin made for high temperature use - (100 parts) was well mixed with the core shell polymer EXL2602 (6 parts), then catalysed with a 6% cobalt octoate solution (0.3 parts) and methyl ethyl ketone peroxide solution (1.5 parts) as before. This mixture was degassed, poured into a mould and kept at room temperature for 24 hours before post curing at 120°C for 1.5 hours. Fracture toughness was determined using the double torsion technique and compared with resin cured in the same manner without the EXL2602. Values of K_ic and G_ic for crack growth initiation and arrest are given in the table. Table 5

These results show that even the more brittle, high temperature vinyl ester resins such as Hetron 980 can be toughened through the addition of a core/shell polymer.

Examples 42 and 43 and Comparative Examples Q and R

The T_g of plain and vinyl ester resin toughened with a core/shell polymer has been measured using a Dynamic Mechanical Thermal Analyser (Polymer Laboratory, UK). The temperature corresponding to the maximum in the tan(delta) was taken as the T_g, and some results are given in the Table.

For both HETRON 922 vinyl ester resin and for HETRON 980, the high temperature vinyl ester resin, no significant change in the T_g was recorded on blending with the core/shell polymer.

Table 6

Examples 44 to 49 and Comparative Examples S to U

The toughening of different vinyl ester resins was investigated. Derakane 8084 is a vinyl ester resin modified by the addition of carboxy terminated butadiene acrylonitrile copolymer.

Table 7

Example 50 and Comparative Example V

Hetron 922 containing peroxide and cobalt catalyst as herein described before was mixed with a mixture of glass beads of nominal sizes 1.1mm and 0.2mm, poured into a mould and consolidated by vibration and cured at tr and post cured at 90°C to five a product simulating the highly filled resin character of polymer concrete (Comparative Example V). Similarly the polymer blend Hetron 922 mixed with the core/shell additive EXL2602 and catalysed as before was mixed with the same glass mixture to give after cure a toughened polymer concrete (Example 50). Table 8 gives the values of K and G, and of strength and modulus of the test materials clearly showing the benefit of the toughened resin even for highly filled systems. Pot life to give efficient time to carrying out the mising pouring and consolidation may be easily adjusted through the addition of the reatarder pentane 2,4 dione. Table 8

Example 51 and 52

Tables 9 and 10 show that blending the vinyl ester resin - Hetron 922 - with KCA 102 does not degrade the resistance ot the vinyl ester resin towards water or sulfunc acid The loss in flexural propeπies after 770 days immersion in either media at 65°C are similar for piain and blended resin Table 9 shows the flexural propeπies of Hetron 922 in distilled water at 65°C Table 10 shows the flexural properties of Hetron 922 in 20% sulfunc acid at 65°C

Table 9

Table 10

Example 53 and Comparative Example W

Hetron 922 was cured as described above and postcured at 90°C (Comparative Example W). Hetron 922 (lOOpph) with KCA 102 (5pph) and styrene (17pph) was cured (Example 53).

T-Peel joints made and tested according to ASTM D1876 - 'Peel Resistance of Adhesives (T- Peel Test) revealed that a core shell additive will enhance the adhesive properties of a vinyl ester resin.

Table 11

Example 54 and Comparative Examples X and Y

The following table shows the viscosity of the mixture: 5 parts EXL 2602, 17 parts styrene, 100 parts Hetron 922 - (Example 54). The measurements were made with a Carri-Med Rheometer at several shear rates and temperatures. An independent assessment showed that the blend presented no difficulty in spray gun lay-up of a chopped glass-resin composite. Table 12

The blend is compared in the table with a thixotropic resin Hetron 922PAW formulated for use in the spray lay-up application. Although the blend is more viscous at low shear rates, it displays greater shear thinning than the formulated resin. The blend also displays thixotropy, and so addition of thixotropic additives such as silica is unnecessary.

Example 55 Figure 1 shows the probability plots for observing a delamination growth initiation energy of less than or equal to a given value for composites made with Hetron 922 and Hetron 922 toughened with both KCA102 and EXL2602. These equivalent core/shell polymers behaved indistinguishably in the toughening of the composite. The observed offset between the curves clearly indicates the benefit to be gained by toughening the resin as described. Composites with the toughened resin will be much more reliable against failure through determination than when an untoughened resin is used.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A polymer resin comprising a vinyl ester resin and a core/shell polymer wherein said core/shell polymer is dispersed throughout the vinyl ester resin.

2. A polymer resin according to claim 1 wherein the core of the core/shell polymer is rubbery at ambient conditions.

3. A polymer resin according to either claim 1 or claim 2 where the core of the core/shell polymer has a glass transition temperature of less than 20°C.

4. A polymer resin according to any one of claims 1 to 3 wherein the core of the core/shell polymer has a glass transition temperature of less than 0°C.

5. A polymer resin according to any one of claims 1 to 3 wherein the core of the core/shell polymer has a glass transition temperature of less than -30°C.

6. A polymer resin according to any one of claims 1 to 5 wherein the core of the core/shell polymer comprises polymerised butadiene monomers.

7. A polymer resin according to any one of claims 1 to 6 wherein the shell of the core/shell polymer provides good adhesion to the vinyl ester resin.

8. A polymer resin according to any one of claims 1 to 7 wherein the shell of the core/shell polymer is glassy at ambient temperatures.

9. A polymer resin according to any one of claims 1 to 8 wherein the core/shell polymer has a core content in the range of from 40 to 90% by weight and a shell content in the range of from 10 to 60% by weight.

10. A polymer resin according to any one of claims 1 to 9 wherein the core/shell polymer has a particle size of less than 2μm.

11. A polymer resin according to any one of claims 1 to 10 wherein the core/shell polymer is selected from core/shell polymers comprising polymerised: butadiene, methyl methacrylate and styrene; butadiene, alkyl methacrylate, alkyl acrylate; butadiene, styrene, alkyl acrylate, alkyl methacrylate and methacrylic acid; 5 butadiene, styrene, alkyl acrylate, alkyl methacrylate, methacrylic acid and low molecular weight polyethylene (as flow modifier); butyl acrylate and methyl methacrylate; alkyl methacrylate, butadiene and styrene; alkyl acrylate, alkyl methacrylate and glycidylmethacrylate; and alkylacrylate and alkylmethacrylate.

10 12. A polymer resin according to any one of claims 1 to 11 wherein the vinyl ester resin is produced from the diglycidyl ether of bisphenyl-A and methacrylic acid.

13. A polymer resin according to any one of claims 1 to 11 wherein the vinyl ester resin is a cresol novolac based vinyl ester resin.

15

14. A polymer resin according to any one of claims 1 to 13 wherein the vinyl ester resin is an internally modified vinyl ester resin.

15. A polymer resin according to any one of claims 1 to 14 wherein the core/shell 20 polymer is present in the polymer resin in an amount of up to 20 pph of vinyl ester resin (by weight).

16. A polymer resin according to any one of claims 1 to 15 wherein the core/shell polymer is present in the polymer resin in a range of from 1 to 20 pph of vinyl ester

25 resin (by weight).

17. A polymer resin according to any one of ,ιms 1 to 16 wherein the core/shell polymer is present in the polymer resin in a range of from 5 to 10 pph of vinyl ester resin (by weight).

30

18. A process for increasing the fracture toughness of a vinyl ester resin comprising dispersing a core/shell polymer in a vinyl ester resin and subsequently curing said vinyl ester resin.

19. A process according to claim 18 wherein the core of the core/shell polymer is rubbery at ambient conditions.

20. A process according to either claim 18 or claim 19 where the core of the core/shell 5 polymer has a glass transition temperature of less than 20°C.

21. A process according to any one of claims 18 to 20 wherein the core of the core/shell polymer has a glass transition temperature of less than 0°C.

10 22. A process according to any one of claims 18 to 21 wherein the core of the core/shell polymer has a glass transition temperature of less than -30°C.

23. A process according to any one of claims 18 to 22 wherein the core of the core/shell polymer comprises polymerised butadiene monomers.

15

24. A process according to any one of claims 18 to 23 wherein the shell of the core/shell polymer provides good adhesion to the vinyl ester resin.

25. A process according to any one of claims 18 to 24 wherein the shell of the core/shell 20 polymer is glassy at ambient temperatures.

26. A process according to any one of claims 18 to 25 wherein the core/shell polymer has a core content in the range of from 40 to 90% by weight and a shell content in the range of from 10 to 60% by weight.

25

27. A process according to any one of claims 18 to 26 wherein the core/shell polymer has a particle size of less than 2μm.

28. A process according to any one of claims 18 to 27 wherein the core/shell polymer is 30 selected from core/shell polymers comprising polymerised: butadiene, methyl methacrylate and styrene; butadiene, alkyl methacrylate, alkyl acrylate; butadiene, styrene, alkyl acrylate, alkyl methacrylate and methacrylic acid; butadiene, styrene, alkyl acrylate, alkyl methacrylate, methacrylic acid and low molecular weight polyethylene (as flow modifier); butyl acrylate and methyl methacrylate; alkyl methacrylate, butadiene and styrene; alkyl acrylate, alkyl methacrylate and glycidylmethacrylate; and alkylacrylate and alkylmethacrylate.

5 29. A process according to any one of claims 18 to 28 wherein the vinyl ester resin is produced from the diglycidyl ether of bisphenyl-A and methacrylic acid.

30. A process according to any one of claims 18 to 29 wherein the vinyl ester resin is a cresol novolac based vinyl ester resin.

10

31. A process according to any one of claims 18 to 30 wherein the vinyl ester resin is an internally modified vinyl ester resin.

32. A process according to any one of claims 18 to 31 wherein the core/shell polymer is 15 present in the polymer resin in an amount of up to 20 pph of vinyl ester resin (by weight).

33. A process according to any one of claims 18 to 32 wherein the core/shell polymer is present in the polymer resin in a range of from 1 to 20 pph of vinyl ester resin (by 0 weight).

34. A process according to any one of claims 18 to 33 wherein the core/shell polymer is present in the polymer resin in a range of from 5 to 10 pph of vinyl ester resin (by weight).

25

35. A process according to any one of claims ¹8 to 34 wherein the core/shell polymer is in the form of a pellet and said pellet is di: rsed in styrene prior to the dispersion of the core/shell polymer in the vinyl ester ; ,ια.

30 36. A polymer concrete composition comprising a cured polymer resin according to any one of the preceding claims.

37. A tie layer comprising a cured polymer resin according to any one of the preceding claims.

38. A fibre reinforced composite comprising a cured polymer resin according to any one of the preceding claims.