GB2282600A - Epoxy resin composition - Google Patents

Description

EPOXY RESIN COMPOSITIONS This invention relates to epoxy resin compositions and more particularly to transparent epoxy resin compositions suitable for use in lenses and other applications.

Plastic lenses are becoming increasingly important in applications ranging from ophthalmic lenses to automotive headlamp lenses. Plastics have a number of advantages over the conventionally used glass in these applications, including their improved impact resistance and hence safety, reduced specific gravity which is important in large lens constructions and relatively facile processing. Because glass is difficult to process, complex optics tend to be made by grinding, which is expensive. Plastics flow sufficiently during processing to allow sharp steps to be produced on the surface of the lens so that complex optics can be made in one step. Plastics do have some disadvantages compared to glass, however. They generally have poor scratch resistance, limited thermal stability and poor weathering (light, oxygen) compared to glass.

The types of plastic which are most commonly used in lens applications are thermoplastics such as polycarbonate and acrylic polymers. Polycarbonates have the higher heat resistance of the two types, but are still limited to about 1150C for the normal grades.

Thermoplastics such as polysulphones are capable of withstanding higher use temperatures, but a substantial cost penalty is incurred. Other specific disadvantages of thermoplastic materials are their poor solvent resistance and their tendency to creep over extended periods at elevated temperatures. Only a very few thermosetting polymers have been described for use in lenses, yet thermosets have much improved solvent and creep resistance relative to thermoplastics. In addition, they can often be used at significantly higher continuous use temperatures.

By far the most common type of plastic material in use in lens applications is the thermoplastics. Low price transparent commodity thermoplastics such as polystyrene, PVC and ABS are often used in clear rigid packaging applications. The slightly higher technology acrylics and the polycarbonates find use also in lens applications as well as transparent containers. The higher temperature, higher priced polymers such as polysulphone, polyethersulphone, amorphous nylons, etc are used in specialist lens applications as well as other areas. The technology of transparent thermoplastics is fully described in a Business Communications Company Inc.

report, no. P-053R, 'Transparent Plastics: Broadening the Base of Materials and Applications', December 1990.

Previous reports on use of thermosetting materials in lenses are much scarcer. PPG Industries (Plastics Engineering, 1986, 42(10), 7) report a casting resin (HIRI) with a refractive index (RI) of 1.56 [the RI of glass is about 1.53 - high refractive index is an advantage in lens applications because the higher the RI, the lower the lens curvature required for equivalent optical power). PPG also produce a commercial allyl thermosetting resin, CR-39, which finds use in lens applications.

Specialised epoxy-based resin materials with potential uses in lenses have been the subject of patent applications. In JP 04,292,611 (Nippon Steel), epoxy acrylate moulding materials containing the fluorene moiety have been described. These are claimed to have a heat distortion temperature of 1700C, light transmittance of 91.5% and an RI of 1.63. JP 04,359,012 (Hitachi Chemical Co.) describes transparent epoxy resins made by curing epoxies with alicyclic anhydride hardeners such as hexahydrophthalic anhydride accelerated with 2ethyl-4-methylimidazole in the presence of a cyclic P=O compound. A glass transition temperature (Tg) of 126"C and light transmission of 85% are described for these compositions.

Transparent epoxy casting resins are described as a component in glass-plastic, laminated ophthalmic lenses in US Patent 5,116,684 (Corning Inc.), as well as in US Patent 5,223,862 (Corning Inc.) where their use as monofocal lenses is also disclosed. The epoxy resin in the first of these patents is preferably a cycloaliphatic epoxy resin and/or an aromatic resin such as DGEBA cured by an anhydride hardener such as hexahydrophthalic anhydride, an active hydrogen source such as propylene glycol and a tin octoate accelerator. The optical properties are provided primarily by the cycloaliphatic resin, while the DGEBA increases the refractive index.

Catalytic curing agents such as BF3 may also be used.

Other sources of hydroxyl groups (e.g. polyols) may be used and these also affect the RI. The amount of accelerator used is important and is preferably limited to between 0.01 and 0.1% of the total mix to permit an acceptable pot-life. Other Lewis acids such as zinc octoate may be used, as well as Lewis bases. The curing schedule is also important and gelation needs to take place at less than 1000C for at least two hours, with final curing at more than 1500C for at least two hours.

The related US Patent 5,223,862 describes in more detail the use of the aforementioned preferred resin compositions as plastic lens members, particularly in a laminated multifocal lens structure with glass. Further curing agents for the epoxy resins are described. These are aromatic anhydrides, aromatic diamines, thioamides and thioamines. Refractive index enhancing additives are selected from alkyl or aromatic diols or thiols and transition metal alkoxides. Phthalic anhydride is a preferred curing agent and propylene glycol is the preferred diol. The epoxy cure cycles exemplified are long, a total curing time of about twenty hours being common.

Although not mentioned in the prior art, the thermosetting epoxy compositions described above have a number of drawbacks. The long total cure times compared to the rapid cycle times of competitive thermoplastic injection moulded lenses represent a significant disadvantage. Some of the specialised epoxy resins are expensive, therefore limiting the use of such products in many industrial applications. Cycloaliphatic epoxy resins are also expensive and, when cured, lead to relatively brittle products, although this may not be critical in laminated applications. Cured resin products produced using aromatic diamines or imidazole accelerators are expected to show some colouration unless special steps are taken to purify these reactants. In addition, their colour stability is likely to be relatively poor, particularly when used at elevated temperatures.

A need exists for transparent and colourless thermosetting resin compositions which overcome these drawbacks. Our invention provides for such an epoxy resin which is relatively cheap for use in industrial lens applications. Initial colour and colour stability are better than compositions containing aromatic curing agents and the products have improved impact resistance.

An object of this invention is to prepare epoxy resin compositions which are cost-competitive with polycarbonate thermoplastic resins, particularly for use in industrial lighting applications. The cured resin formulations should have acceptable impact resistance, be fast curing, have low colour and good transparency and be suitable for use at higher continuous operating temperatures than polycarbonates and acrylics.

This invention provides epoxy resin compositions which are colourless, transparent, have high glass transition temperatures ( > 1400C, preferably > 1500C) and can be cured by rapid moulding techniques such as thermoset injection moulding or resin transfer moulding.

The invention provides a curable epoxy resin composition, comprising a curable epoxy resin and a curing agent selected from (a) diamine curing agents which include at least one aliphatic, heterocyclic or aromatic ring, and which have two amine groups each independently bonded to an aliphatic group or incorporated in a heterocyclic ring; (b) linear polyanhydride curing agents; and (c) cycloaliphatic anhydride curing agents, wherein the epoxy resin and the curing agent are chosen so as to provide, on curing, a colourless or low colour product with a Tg of > 1400C, preferably > 1500C.

The invention also provides a method of preparing a cured epoxy resin composition, which comprises curing a curable epoxy resin composition as defined above.

The maximum possible transparency of the final product can be calculated from well known theoretical considerations. Taking the case of an uncoated product material transmitting perpendicularly incident light in air, the quantity of light reflected from each surface (interface) is dependent on the refractive index of the product material; such that for a product of refractive index 1.5, the theoretical maximum transmission is 92.2% and for a product of refractive index 1.6, the theoretical maximum transmission is 89.7%. Real (nontheoretical) materials have lower levels of transmission than those calculated. The epoxy products of this invention generally possess transmission levels > 90% of the theoretical maximum.

The level of colour in the product materials is determined by the presence or absence of absorption bands in the visible region of the electromagnetic spectrum. The epoxy products have no absorptions in the 400 - 800nm wavelength range. A second indicator of colour such as the ASTM D1925 "yellowness" index may also be employed. These secondary measures are both less precise and can also be achieved by colour compensation methods which mask the original colouration (eg. by the use of blue masterbatch to remove yellowness). Such approaches also lead to lower overall transmission properties.

Production of epoxy resins which are colourless or have low colour results from minimising the factors which lead to colour. These factors include the presence of charge transfer complexes and oxidation products.

The type of epoxy resin is the first consideration. High temperature epoxy resins often contain tertiary nitrogen groups which have a tendency to lead to oxidation and formation of highly coloured impurities. These products are frequently also too expensive for commercial, non-aerospace products. These glycidyl amine epoxy materials are therefore best avoided. Glycidyl ethers such as the commonly used DGEBA are less prone to oxidation and are available in high quality, low colour grades. Epoxy novolacs, which are also glycidyl ethers, may be used alone or in conjunction with other resins. Cycloaliphatic epoxy resins also have low colour, but they have the disadvantage of giving brittle products when cured and are also expensive. The preferred epoxy resins are glycidyl ethers.

The selection of curing agent is at least as critical as the choice of epoxy resin. In addition to controlling the colour and colour stability of the cured product, the curing agent plays an important role in determining the processing characteristics of the resin formulation. For high volume applications such as industrial lenses, fast cure and rapid throughput are essential. Aromatic diamines are among the most common epoxy curing agents. They almost invariably lead to coloured products, unless special steps are taken to purify the diamine reactant to remove oxidation impurities which can lead to colour formation. The products will have a tendency to form charge transfer complexes which are usually highly coloured. In addition, the presence of an aromatic ring linked nitrogen atom will lead to poor thermo-oxidative stability and discolouration of the product with time.

The curing rate of aromatic diamines tends to be low.

Aliphatic and cycloaliphatic curing agents, on the other hand, lead to rapid cure and colourless or low colour products. The presence of the aliphatic or cycloaliphatic groups reduces the interactions which lead to charge transfer complex formation. The products also have a reduced tendency to oxidise in use due to the nonaromatic nitrogen linkage. Imidazole accelerators used commonly with anhydride curing agents also lead to colour in the cured resin products for similar reasons to those outlined above for the aromatic diamines. Preferred accelerators should contain no groups capable of producing colour through interaction with other components of the system or the intrinsic behaviour of the accelerator itself. For this reason, zinc carboxylates are preferred. The Zn- ion produces no colour and can become involved in no colour-producing redox reactions.Similarly, the aliphatic carboxylate groups do not lead to any colour formation.

Epoxy resins are selected from the range of good quality, low colour (e.g. with Gardner number < 1 or AHPA number < 250) resins available commercially.

These include high purity DGEBA (diglycidyl ether of bisphenol-A) resins with epoxy equivalent weights in the range 170-6000, and preferably 170-250, and high purity epoxy novolacs. Cycloaliphatic epoxy resins may also be used if the curing agent is an anhydride as in (b) and (c) above, but their use is generally limited because of higher cost and poor impact resistance of the cured resins.

The diamine curing agents are preferably those where the amine groups are independently bonded to an aliphatic or cycloaliphatic group. Cycloaliphatic diamine curing agents may be selected from the group shown in Table 1, preferred curing agents being isophorone diamine (IPD) and bis(4aminocyclohexyl)methane (PACM). The proportion of curing agent to epoxy resin is generally in the range 120% of that required for stoichimetric cure.

Anhydride curing agents may be selected from those shown in Table 2, preferred anhydrides being hexahydrophthalic anhydride (HHPA) and methylhexahydrophthalic anhydride (MHHPA). Zinc salts are preferred as accelerator, especially zinc stearate or zinc octoate.

In Tables 1 and 2 substituted analogues are included. The optional substituents are, for example, selected from Ci4 alkyl and halogen.

Reactive diluents may include low viscosity glycidyl, diglycidyl and polyglycidyl ethers, polyols such as diols and triols and epoxidised unsaturated materials. The reactive diluents may serve a number of purposes, such as modifying the viscosity of the resin formulation, improving the impact properties of the cured product a-3 modifying the refractive index of the cured product. Often there is a trade-off in the form of a reduction in ultimate Tg.

The modifiers selected may include thermoplastic materials, inorganic fibres and various particulate materials. Thermoplastics are preferred, but also materials such as glass fibres, various silicas and feldspar can be fruitfully employed as modifiers.

Thermoplastics which may be used include high performance acrylics such as reactive or unreactive polymethacrylimide (available commercially from Rohm & BR< Haas and Röhm as Kamax and Pleximid s respectively), amorphous polyarylates, amorphous polyamides, polyimides and polyetherimides (e.g. General Electric's Ultem ), polysulphones and polyethersulphones, poly (phenyleneoxide), polycarbonates including polyestercarbonates and polyalkylene carbonates, styrene-maleic anhydride (SMA) copolymers, styrene/butadiene/styrene copolymers, polyethylene glycol and other amorphous thermoplastics.

Other related materials, such as proprietary core-shell impact modifiers for example (e.g. acrylic-based or styrene-butadiene-based core-shell elastomers sold as Paraloid EXL additives from Rohm & Haas), may also be used. Thermoplastics are added to improve the impact strength of the cured resin and also act as rheology modifiers for certain types of processing such as injection moulding. The behaviour of the thermoplastics during cure is crucial to the successful use of the cured products in optical applications. The modified resin must be both colourless and transparent.

Thermoplastics are commonly added to thermosets such as epoxy resins in order to improve their toughness and impact resistance. In the most common application of this technology, the thermoplastic dissolves in the epoxy resin during cure but becomes increasingly incompatible as the molecular weight of the thermoset increases. At some point during the cure cycle, the thermoplastic phase separates from the epoxy matrix at a rate and time determined by the overall kinetics and thermodynamics of the system. This approach is exemplified in WO 91/02029 to Hitco, where epoxy resins are toughened by polyimides. The nature of the thermoplastic phase is affected by the kinetics and thermodynamics mentioned before, as well as the proportion of thermoplastic present in the formulation.

The three most common situations are (a) discrete thermoplastic particles distributed in an epoxy matrix, (b) discrete epoxy phases distributed in a thermoplastic matrix (phase inversion) and (c) a co-continuous network of the thermoplastic and epoxy phases (via so-called spinodal decomposition). The size of the particles is generally in the range 1 to 10 ssm. Since the wavelength of light is around 0.4 to 0.8 ym, products containing these systems tend to be opaque or translucent. In order to obtain toughened, transparent epoxy systems, one or more criteria need to be satisfied. This is one of the keys to success in one aspect of this invention.

The main routes to obtaining transparency in thermoplastic toughened epoxies are threefold: (1) Production of cured blends which are single phase - the toughening effect obtainable by this route is sometimes limited and Tg may be reduced. (2) Production of cured blends where the phase size is less than the wavelength of light - this can be influenced by altering the cure kinetics. (3) Matching the refractive indices of the cured epoxy and thermoplastic components so that even if phase sizes larger than the wavelength of light occur, no refraction will occur at the phase interface and light will pass through the product without deviation. Our approach is to create conditions where either (1) or (2) is favoured while matching refractive indices as much as possible to minimise adverse effects should we fall short of the aims of (1) and (2).Improving the compatibility of the thermoplastic with the epoxy and optimising the cure conditions are the main routes to (1) and (2).

For applications spanning a wide range of use temperatures, it is possible to optimise matching of cured epoxy and thermoplastic refractive indices over that temperature range.

Following the above, we have made a range of different epoxy resin systems which provide the properties required of an epoxy optical lens. The formulations have gel times < 120 seconds, preferably < 60 seconds, at 20 OOC, making them amenable to rapid processing such as injection or transfer moulding.

The formulations may also include antioxidant(s) and/or W stabiliser(s) to reduce yellowing resulting from prolonged use at elevated temperatures.

The products may optionally be coated with a commercial polymer hard-coat to improve surface hardness and abrasion resistance.

The main advantages of the invention are:1) Colourless, transparent epoxy products suitable for lens applications are produced from relatively cheap precursors. Other suitable applications include fluid reservoirs, e.g. in cars, medical containers etc.

2) Colour quality and colour stability are better than for epoxy resins prepared from aromatic diamines or imidazole accelerators.

3) The cured resin products have high Tg values, generally in excess of 1400C, preferably above 1500C.

4) The toughness or impact resistance of the cured products can be varied over a wide range, depending on the choice of reactive diluent or thermoplastic toughening additive and on the required application.

5) Cure times are rapid, with gel times less than 120 seconds, preferably less than 60 seconds, at temperatures around 200 or. These formulations are therefore amenable to rapid processing using techniques such as injection moulding or transfer moulding, in contrast to the slower casting method reported in some of the prior art.

6) Refractive indices of the products can be tailored over a wide range, including the range 1.49 - 1.62.

7) Creep resistance and surface hardness are superior to polycarbonate and comparable thermoplastic lens materials.

8) In contrast to US Patent 5,116,684, levels of zinc stearate or zinc octoate accelerator well in excess of 1% may be used with no adverse effect on colour or pot life, and a beneficial effect on total cure time and Tg.

9) The epoxy products allow visible light transmission of greater than 89% and exhibit u.v. cut-offs around 320nm.

10) The solvent resistance of the epoxy products is improved over competitor thermoplastic materials.

Table 1.

Diaminocycloalkanes and substituted analogues.

where m,n 0-8 Aminoalkyl-aminocyclohexanes and substituted analogues.

where n = 1-5 Diaminoalkyl-cyclohexanes and substitued analogues.

where m,n = 1-5 Diaminoalkyl-benzenes and substituted analogues.

where m,n = 1-5 NH2 1 2 2n (CH2)m NH2 Aminoalkylpiperazines and substituted analogues.

where m,n = 1-5 Diaminoalkyl-piperazines and substituted analogues.

where m,n = 1-5 Bis-aminocyclohexanes and substituted analogues.

where X = -0-, -CH2-, -C(CH3)2 -C(CF3)2-, -SO2 Table 2.

Linear polyanhydrides HO[OC(CH2)mCOO]nH where m = 1-5 and substituted analogues. n = 2-20 Alicyclic anhydrides and substituted analogues.

where n = 1-4 Adducts of maleic anhydride and substituted dienes e.g.

Copolymers including maleic anhydride e.g.

Hexahydrophthalic anhydride and substituted analogues Tetrahydrophthalic anhydrides and substituted analogues.

Bis-nadicanhydrylbutene and substituted analogues.

LIST OF ABBREVIATIONS Trade names of epoxy resins:- MY750 Epon 828 DER 332 DEN 431 DER 736 GT 7071 Heloxy 107 (or Hel 107) Epodil 750 ERL 4221 IPD - isophorone diamine PACM - p-aminocyclohexylmethane SMA - styrene/maleic anhydride copolymer PPO - poly(phenylene oxide) SBS - styrene/butadiene/styrene copolymer PEG - poly (ethylene glycol) EXL2600 - core-shell impact modifier MHHPA - methyl hexahydrophthalic anhydride MNA - methyl nodic anhydride Examples Example 1.

This example illustrates the general preparation of epoxy products using cycloaliphatic diamine curing agents. A mixture of 50g MY750 (epoxy monomer) and 14g PACM is prepared and made uniform. This mixture is then de-aerated and cured in a hot mould at 1600C for 30 minutes. The epoxy product has a glass transition temperature (Tg) of 1670C as measured by DMTA (dynamic mechanical thermal analysis), refractive index (nD) of 1.578 and Izod impact strength (ASTM D256) of 1.46 ftlb/in2. This and other illustrative epoxy products prepared by the same general procedure are tabulated below (Table 3).

Table 3.

Epoxy monomers Diamine Tg nD Izod ( C) (ftlb/in) XY750 50g PACM 14g 167 1.578 1.46 MY750 70g PACM 16.7g 142 1.577 1.84 DER332 50g IPD 12.5g 178 1.571 1.04 DER332 70g PACM 21.4g 177 1.574 0.73 Epon828 60g IPD 13.9g 165 1.572 0.56 DEN431 60g PACM 18.1g 166 1.586 0.55 MY750 50g IPD 13.5g 154 1.562 2.29 Heloxy107 7.5g MY750 50g IPD 14g 159 1.567 1.69 Epodil750 7.5g MY750 50g PACM 16g 158 1.567 0.89 DER736 7.5g DER332 50g IPD 12.2g 157 - 1.82 GT7071 7.5g Example 2. This example illustrates the general preparation of epoxy products as described in article 2 of section 3.2. A mixture of 50g Epon 828 (epoxy monomer), 48g MHHPA and 2g zinc stearate is prepared and the temperature of the mixture raised to 90 C to aid dissolution. The mixture is made uniform and deaerated before being cured in a hot mould at 200 C for 30 minutes. The epoxy product has a Tg of 151 C, nD of 1.545 and Izod of 0.29 ftlb/in notch. This and other illustrative epoxy products prepared by the same general procedure are tabulated below (Table 4).

Table 4.

Epoxy monomer/ Diluent Zinc Tg nD Izod anhydride salt ( C) (ftlb/in2) Epon828 50g - Zinc 151 1.545 0.29 MHHPA 48g stearate MY750 50g - Zinc 149 1.561 0.28 MNA 46g stearate MY750 50g Glycerol Zinc 158 1.534 0.30 MHHPA 52g 5g stearate Epon828 50g Glycerol Zinc 135 1.536 0.58 MHHPA 49g 5g stearate DEN431 50g Glycerol Zinc 136 1.541 0.54 MHHPA 53g 5g stearate Epon828 60g - Zinc 164 - 0.46 MHHPA 50g stearate Epon828 50g - Zinc 156 0.24 MHHPA 41.5g octoate ERL4221 50g Glycerol Zinc 239 1.504 0.16 MHHPA 51g 0.7g octoate Example 3.

This example illustrates the general preparation of epoxy products using cycloaliphatic diamine curing agents and thermoplastic modifier. 50g DER 332 (epoxy monomer) is heated to 900C and a solution of Kamax T-170 in dichloromethane (DCM) is added. The DCM solvent is removed and the mixture cooled. 6g Heloxy 107 (reactive diluent) and 16g IPD are added and the mixture made uniform and de-aerated before being cured in a hot mould at 1600C for 30 minutes. Epoxy product properties are Tg 1550C, nD 1.563 and Izod 1.67 ftlb/in2. This and other illustrative epoxy products prepared by the same general procedure are tabulated below (Table 5).

T--le 5.

Epoxy Diamine Thermo- Tg Izod monomers plastic ( C) (ftlb/in) DER332 50g IPD Kamax 5g 155 1.563 1.67 He1107 16g (T-170) MY750 50g IPD Pleximid 4g 154 1.562 0.71 He1107 7.5g 13.6g (6607-F) DER332 50g IPD Kamax 3.75g 157 1.563 0.64 He1107 7.5g 14.6g (T-150) GT7071 7.5g DER332 50g FACM KAmax 3.75g 171 1.566 0.90 He1107 7.5g 17.8g (T-170) DER332 70g IPD Kamax 10.5g 149 - 0.77 He1107 11g 20.5g (T-170) Example 4.This example illustrates the general preparation of epoxy products as described in article 4 of section 3.2. 50g MY750 is heated to 90 C and a solution of 5g SMA (styrene-maleic anhydride copolymer available from DSM) in DCM is added. The DCM solvent is removed and 44g MHHPA and 2g zinc stearate added. The mixture is made uniform and de-aerated before being cured in a hot mould at 200 C for 30 minutes. Epoxy product properties are Tg 168 C, no 1.543 and Izod 0.52 ftlb/in notch. This and other illustrative epoxy products prepared by the same general procedure are tabulated below (Table 6).

Table 6.

Epoxy resin Zinc Thermo- Tg nd Izod anhydride salt plastic ( C) (ftlb/in MY750 50g Zinc SMA 5g 168 1.543 0.52 MHHPA 44g stearate MY750 50g Zinc PPO 5g 151 1.542 0.44 MHHPA 50g stearate SBS 1.25g SMA l.25g MY750 50g Zinc p(vinyl 5g 158 1.535 0.54 MHHPA 50g stearate -butyral) MY750 50g Zinc PEG 5.3g 140 1.538 0.66 MHHPA 50g stearate (mw=400) MY750 40g Zinc Kamax 4g 151 - 0.40 MHHPA 40g stearate (T-260) Epon828 50g Zinc EXL2600 10g 127 - 4.10 MHHPA 49g stearate ERL4221 24g Zinc EXL2600 20g 165 - 1.49 Epon828 10g octoate DEN431 log MHHPA 47g ERLA221 50g Zinc PEG 10.5g 209 - 1.22 NHHPA 51g octoate (mw=400) ERL4221 50g Zinc PEG 13g 198 1.504 0.95 MHHPA 51g octoate (mw=2000) ERL4221 50g Zinc SMA 15g 220 - 0.23 MHHPA 43g octoate DEN431 50g Zinc PEG 5g 142 - 0.52 MHHPA 53g stearate SMA 10g

Claims (10)

1. CLAIMS: 1. A curable epoxy resin composition, comprising a curable epoxy resin and a curing agent selected from (a) diamine curing agents which include at least one aliphatic, heterocyclic or aromatic ring, and which have two amine groups each independently bonded to an aliphatic group or incorporated in a heterocyclic ring; (b) linear polyanhydride curing agents; and (c) cycloaliphatic anhydride curing agents, wherein the epoxy resin and the curing agent are chosen so as to provide, on curing, a colourless or low colour product with a Tg of > 1400C, preferably > 1500C.
2. 2. A composition according to claim 1, which also contains a reactive diluent to improve impact resistance and/or refractive index, for example a glycidyl ether, a polyol or an epoxidised unsaturated material.
3. 3. A composition according to claim 1 or 2, also including an accelerator for the anhydride curing agent (b) or (c), for example a tin or zinc salt.
4. 4. A composition according to any of claims 1 to 3, also including a modifier to improve the impact resistance of the cured resin, for example a thermoplastic material, inorganic fibers or particulate material.
5. 5. A composition according to claim 4, in which the modifier is an amorphous thermoplastic material.
6. 6. A composition according to any of claims 1 to 5, in which, in the diamine curing agent (a), the aliphatic ring is cyclohexane or two cyclohexane rings joined by a linking group, the heterocyclic group is piperazine, the aromatic ring is benzene, and the aliphatic group is alkylene having 1 to 5 carbon atoms.
7. 7. A composition according to any of claims 1 to 5, in which the cycloaliphatic anhydride curing agent (c) is selected from alicyclic anhydrides, adducts of maleic anhydride and substituted dienes, copolymers including maleic anhydride units, and optionally substituted hexahydrophthalic anhydride, tetrahydrophthalic anhydride and bis-nadicanhydrylbutene.
8. 8. A method of preparing a cured epoxy resin composition, which comprises curing a curable epoxy resin composition according to any of the previous claims.
9. 9. A cured epoxy resin composition prepared from a curable epoxy resin composition according to any of claims 1 to 7 or by a method according to claim 8.
10. 10. Use of a cured epoxy resin composition according to claim 9 for optical applications, in particular for plastic lenses.