WO2025062112A1

WO2025062112A1 - Thermally curable resin blend and its use in composite materials

Info

Publication number: WO2025062112A1
Application number: PCT/GB2024/052243
Authority: WO
Inventors: Mark Whiter
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2023-09-18
Filing date: 2024-08-29
Publication date: 2025-03-27
Anticipated expiration: 2026-03-18
Also published as: GB202314183D0; GB2637459A

Abstract

A thermally curable resin blend, comprising a) from 50 to 90 wt% cycloaliphatic curable epoxy resin having an epoxy equivalent weight of greater than 350, b) from 2 to 25 wt% of a tri- or tetra- functional epoxy reactive diluent having an epoxy equivalent weight of less than 200, and c) a curing agent; a process of thermally curing such a resin blend, the cured resin, a prepreg impregnated with the curable resin blend, and a composite material obtainable by curing such a prepreg.

Description

Thermally Curable Resin Blend and its Use in Composite Materials

Technical Field

The present invention relates to a thermally curable resin blend, particularly for use in prepregs, comprising a structural layer of fibres, which when cured is UV-stable and has convenient mechanical properties for structural applications.

Background

Curable resin systems are widely known and have a wide range of uses in a variety of technical fields. These systems function by reaction between resin molecules and curing agents. Upon activation, e.g. by mixing together (for so-called two-component systems) or by heating (for so-called one-component systems), functional groups on the curing agent react with functional groups on the resin molecule to form an extended polymeric network, which is the process known as curing.

Such one-component systems are typically viscous liquid or semi-solids at room temperature and only become flowable (i.e. pourable with a low viscosity) at an elevated temperature, e.g. from 60°C to 100°C, when required for use. As the curing agent and the resin are together in the same material, the curing agent must be selected to have relatively low reactivity. The low reactivity must also be maintained at the increased temperature when the composition becomes a flowable liquid. However, when raised to a higher temperature for curing, the reactivity is required to be conveniently high, so that curing times are not excessive. There is also a risk of a runaway exothermic reaction occurring at higher temperatures, which must be avoided to prevent degradation of the resin. Thus, one-component systems are significantly more challenging to formulate than two-component systems.

The resulting thermally cured resin has physical properties which are largely or entirely dictated by the choice of resin, the choice of curing agent and the curing regime employed. A wide variety of chemical and physical properties can be obtained by altering one or more of these variables. Common curable resins include the epoxy, isocyanate and acid anhydride resins. Epoxy resins are widely used and well-known as effective resin in a wide range of applications. The most common form of epoxy resin are based on aromatic epoxy systems, such as the bisphenol glycidyl family of epoxy resins (e.g. the widely used Bisphenol-A epoxy). Such aromatic epoxy resins provide excellent strength and toughness properties, making them ideal for use in the manufacture of a structure, especially as the matrix in a composite material.

Prepregs, comprising a fibre or fabric arrangement impregnated with thermosetting one- component curable resin such as epoxy resin, are widely used in the generation of such composite materials. The curable resin may be combined with the fibres or fabric in various ways. The curable resin may be tacked to the surface of the fibrous material, however more usually it partially or completely impregnates the interstices between the fibres. Such prepregs may sometimes be referred to as semipregs or towpregs, depending on the degree of impregnation of curable resin, but are considered to fall within the more general term of prepreg in the context of the present invention.

Such prepregs require the resin to be a one-component system, so that it can impregnate the fibres for later thermal curing.

Once manufactured, typically a number of plies of such prepregs are “laid-up” as desired and the resulting prepreg stack, i.e. a laminate or preform, is cured, typically by exposure to elevated temperatures, to produce a cured composite structure.

However, aromatic epoxy resins are known to suffer from discolouration when exposed to sunlight, and especially UV light, due to the reactivity of the aromatic groups.

In applications where exposure to sunlight, and especially UV light, cannot be avoided, aliphatic or cycloaliphatic epoxy resin systems may be considered, due to the fact that they are known not to discolour when exposed to UV light. However, suitable aliphatic epoxy resins are generally low viscosity liquids at room temperature, making them unsuitable for prepreg manufacture, which requires a viscous liquid for impregnation of the fibres. Additionally, due to their lower reactivity, aliphatic epoxy resins are known to suffer from longer cure times and result in a cured resin with a relatively low Tg, which is especially the case for low temperature curing regimes such as less than 100°C. For this reason aliphatic and cycloaliphatic epoxy resins tend to be formulated as two-component systems, again making them unsuitable for use in prepregs.

WO2021/133972 discloses the use of a blend of cycloaliphatic epoxy resins and epoxyamine adducts for use as a one-component curable resin system suitable for use in a prepreg.

The development of a one-component thermally curable resin system that is stable in sunlight, and especially UV light, and can be used in composite material manufacture, would therefore be highly desirable.

Summary of Invention

In a first aspect, the invention relates to a thermally curable resin blend comprising a) from 50 to 90 wt% cycloaliphatic curable epoxy resin having an epoxy equivalent weight of greater than 350; b) from 2 to 25 wt% of a tri- or tetra- functional epoxy reactive diluent having an epoxy equivalent weight of less than 200; and c) a curing agent.

Such a blend has been found to be stable at low temperatures and surprisingly reactive at higher temperatures. Additionally, when cured, the resins are UV-stable and have desirable mechanical properties for composite material manufacture, e.g. having a glass transition temperature (Tg) in excess of 100°C, making them excellent for use as structural resins within a composite material.

Due to its high equivalent epoxy weight, the cycloaliphatic curable epoxy resin is a solid at 25°C. However, due to its low equivalent epoxy weight, the tri- or tetra-functional epoxy reactive diluent ensures that the resulting blend is flowable at 25°C e.g. a viscous liquid having a viscosity of around 30,000 Pas at 50°C and 10 Pas at 100°C.

All viscosities herein are measured with 20mm parallel plates, 50 to 160°C @ 2°C/min, strain 1 %, frequency 1%. The blend preferably comprises from 60 to 85 wt% of the cycloaliphatic curable epoxy resin.

The blend preferably comprises from 3 to 15 wt% of the tri- or tetra-functional epoxy reactive diluent.

The cycloaliphatic curable epoxy resins have an epoxy equivalent weight (EEW), (i.e. number of grams of resin for 1 mole of epoxy groups) of greater than 350, more preferably greater than 400, more preferably greater than 450 or even greater than 500, or even greater than 600.

The cycloaliphatic curable epoxy resins generally have a high molecular weight, and as such do not have a measurable viscosity at 25°C, as they will be solid or semi-solid at this temperature. Preferably they do not have a measurable viscosity at a temperature of 30°C, preferably 40°C, more preferably 50°C, or even 60°C, due to them remaining in solid or semi-solid form.

A preferred family of cycloaliphatic curable epoxy resins have the general structure:

Which is available as EPALLOY™ 5001 (EEW 200 to 220, n 0.16 to 0.30), Kukdo ST- 3000 (EEW 220 to 240; n 0.30 to 0.43) at low molecular weights, but are however too low a molecular weight for use in the present invention. Suitable examples of higher molecular weight cycloaliphatic epoxies for use in the present invention include ST- 5080™ (EEW 550 to 650; n 2.53 to 3.20) and ST-4000D™ (EEW 600 to 750; n 2.86 to 3.88).

The epoxy reactive diluent is a low molecular weight reactive epoxy, so that when blended with the solid cycloaliphatic epoxy provides a blend that has a measurable viscosity at room temperature and is also reactive at higher temperatures. Preferred epoxy reactive diluents have a central methyl or ethyl group, attached to which is three or four epoxy groups, i.e. tri- or tetra- functional. The high density of epoxy groups provides the desired reactivity at elevated temperatures.

The epoxy reactive diluent has an epoxy equivalent weight (EEW), (i.e. number of grams of resin for 1 mole of epoxy groups) of less than 200, preferably less than 170, more preferably less than 140.

The epoxy reactive diluent generally has a low molecular weight, and as such has a measurable liquid viscosity at 25°C, preferably from 0.01 to 2 Pas, more preferably from 0.1 to 1 Pas.

A preferred family of tri-functional epoxy reactive diluents have the general formula:

where R¹, R², R³, R⁵ is C_nH2n, R⁴ and R⁶ is C_nH2n+i and

A preferred tri-functional reactive diluent has the following structure, known as trimethylolpropane triglycidylether:

CH

Clb

Another preferred tri-functional reactive diluent has the following structure, which is a triglycidyl ether of glycerol.

A preferred family of tetra-functional epoxy reactive diluents have the general structure:

or

where R¹, R², R³, R⁵ is C_nH2n, R⁴ and R⁶ is C_nH2n+i and

The blend also comprises a curing agent, which is required in order to provide the active hydrogen groups to react with the active epoxy groups.

Epoxy resin curing systems are often characterised by the amine:epoxy ratio, or A:E ratio. This is the ratio of the number of active hydrogen groups to the number of epoxy groups. For conventional epoxy curing systems, an A:E ratio of 1 :1 might provide the most effective curing regime as all the epoxy groups have an active hydrogen group to react with. Suitable curing agents include amines, including aromatic amines, e.g., 1 ,3- diaminobenzene, 1 ,4-diaminobenzene, 4,4'-diamino-diphenylmethane, and the polyaminosulphones, such as 4,4'-diaminodiphenyl sulphone (4,4'-DDS--available from Huntsman), 4-aminophenyl sulphone, and 3,3'- diaminodiphenyl sulphone (3,3'-DDS).

Another class of suitable curing agents are the imidazoles (such as Arador 3123 (available from Huntsman, Duxford, UK)). Another suitable class of curing agents are the anhydrides.

Cure accelerators provide a similar chemical functionality to curing agents, but have a higher reactivity, and so are considered in the art to be a different class of material. However, for the purposes of the present invention, such accelerators are considered to be curing agents. Cure accelerators are usually heat activated and shorten the time taken to cure the resin. Suitable accelerators include substituted ureas, for example the range of materials available under the tradename Dyhard® from AlzChem Group AG, Trostberg, Germany, including UR200, UR300, UR400, UR500, UR600 and UR700, and the range of materials available under the tradename Omicure® from Emerald Performance Materials, Moorefield, New Jersey, USA, including U-24M, U-35M, U-52, U-52M, U-210, U-210M, U-405, U-405M, U-410M and U-415M.

Typically, the resin blend of the present invention includes a small quantity of curing agent, or cure accelerator, e.g. from 0.05 to 6 wt%, preferably from 0.1 to 5 wt%, for example from 2 to 5 wt%, according to the needs of the intended application.

The resin blend may be thermally cured merely by exposing it to an elevated temperature, such as from 80 to 150°C.

Thus, in a second aspect, the invention relates to a process of curing a resin blend as described herein, by exposure to elevated temperature and/or pressure, wherein the time taken to reach 95% conversion at 95°C is less than 140 minutes.

Preferably the time taken to reach 95% conversion at 95°C is less than 120 minutes, preferably less than 100 minutes.

The time to 95% cure is well-known to the person skilled in the art and can be determined by running an uncured sample by an isothermal DSC method to generate an exotherm trace. The DSC software (e.g. either STARe for Mettler Toledo instruments or TRIOS for TA instruments) is then able to calculate the % conversion by determining from the area of the exotherm in the trace.

Preferably the process of curing is carried out at a temperature of less than 100°C.

As discussed, the cured resin provides attractive mechanical properties, particularly for use as a matrix in composite materials.

Thus, in a third aspect, the invention relates to a cured epoxy resin, obtainable by the process of curing the resin blend described herein, wherein the cured epoxy resin has a glass transition temperature (Tg) of greater than 100°C, e.g. from 100 to 120°C, preferably from 100 to 110°C.

The Tg measurement is well-known to the person skilled in the art e.g. by DSC following a test standard (ISO 11357-2). A cured sample is run through a dynamic DSC and the Tg is determined by a small inflection in the trace.

The resulting cured resin is also UV-stable, however, one or more additional UV stabilisers or absorbers may be added to the curable resin blend, if desired. The total amount of UV stabilisers may be in the range of from 0 to 5 wt% based on the total weight of the resin blend.

Examples of such UV-stabilisers include phenolic antioxidants such as butylated hydroxytoluene (BHT); Pentaerythritol Tetrakis(3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate; 2-hydroxy-4-methoxy-benzophenone; 2,4-bis(2,4- dimethylphenyl)-6-(2-hydroxy-4-octyloxyphenyl)-1 ,3,5-triazine; 3,5-di-tert-butyl-4- hydroxybenzoic acid, n-hexadecyl ester; liquid hindered amine light stabiliser (HALS) such as 2-(2H-benzotriazol-2-yl)4,6-ditertpentylphenol, and methyl 1 , 2, 2,6,6- pentamethyl-4-piperidyl ester and 2-[4-[(2-Hydroxy-3-tridecyloxypropyl)oxy]-2- hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1 ,3,5-triazine may also be used as suitable UV stabilisers.

Pigments and/or dyes for adding colour to the curable resin blend may be included. Examples include red iron oxide, green chromium, carbon black, and titanium oxide. In preferred embodiments, a titanium oxide (TiC>2) white pigment is added to the curable resin blend. Titanium dioxide is commercially available in two crystal structures - anatase and rutile. Rutile TiC>2 pigments are preferred because they scatter light more efficiently, are more stable and are more durable than anatase pigments. Among them, rutile TiC>2 with particle size from 0.2 and 0.3 microns in diameter is most preferred, such as TR-33 available from Jiangxi Tikon Titanium Company, Shanghai, China.

Such pigments may be present at a level of from 0 to 20 wt%, according to the intended application.

The resin blend may also comprise a thermoplastic material which is soluble in the epoxy resin such as polyethersulphone, to improve the toughness of the resin. Exemplary thermoplastic toughening agents/particles include any of the following thermoplastics, either alone or in combination: polyamides, copolyamides, polyimides, aramids, polyketones, polyetheretherketones, polyesters, polyurethanes, polysulphones, polyethersulfones, high performance hydrocarbon polymers, liquid crystal polymers, PTFE, elastomers, and segmented elastomers.

As the resin blend of the present invention is a viscous liquid at room temperature, they are ideal for use in forming prepregs, as they can flow between the interstices between adjacent structural fibres.

Thus, in a fourth aspect, the invention relates to a prepreg comprising a structural layer comprising fibres having interstices therebetween, and comprising a thermally curable resin blend as described herein, impregnated within the structural layer and present within the interstices.

The fibres in the structural layer may have a circular or almost circular cross-section with a diameter in the range of from 3 to 20 pm, preferably from 5 to 12 pm, although the invention applies to a wide variety of fibre types, arrangements and sizes.

The prepreg may be conveniently produced as a continuous web of material, as discussed below, having a length greater than its width, typically much greater. Such prepregs are generally produced as a prepreg roll, the length of which is given by the width of the prepreg. In view of the tacky nature of the prepreg, a polymeric backing sheet is provided to enable the prepreg roll to be unfurled at the point of use. The fibres may be randomly arranged, in the form of a fabric, rovings or be formed from tows of discrete fibres. The fibres may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous fibres.

Suitable woven and non-woven fabrics for use in composites are commercially available from a number of specialist manufacturers including Chomarat Textiles Industries, Esher, Surrey, United Kingdom, Hexcel Reinforcements UK Limited, Narborough, Leicestershire, United Kingdom, and Zhenshi Group Hengshi Fibreglass Fabrics Co., Ltd., Tongxiang Economic Development Zone, Jiaxing Zhejiang, 314500 China. In an embodiment, the fabric is a carbon fibre or glass fibre biaxial non-woven fabric, such as BB200, BB600 or BB1200 or a triaxial non-woven such as LBB1200.

The fibres may be selected from the list consisting of carbon fibres, glass fibres, flax fibres, graphite fibres, metallised polymers and mixtures thereof.

However, the present invention is particularly applicable to glass fibres, because composite materials made from such fibres are generally more prone to visual changes due to UV reactivity. Thus, the present invention provides an even greater benefit to glass fibre composites.

Exemplary layers of carbon fibres are made from HexTow™ carbon fibres, which are available from Hexcel Corporation. Suitable HexTow™ carbon fibres for use in making many unidirectional fibre layers include: IM5 carbon fibres, which are available as 6,000, 12,000 and 24,000 filaments; IM7 carbon fibres, which are available as fibres that contain 6,000 or 12,000 filaments and weigh 0.223 g/m and 0.446 g/m respectively; IM 8- IM 10 carbon fibres, which are available as fibres that contain 12,000 filaments and weigh from 0.446 g/m to 0.324 g/m; and AS7 carbon fibres, which are available in fibres that contain 12,000 and 24,000 filaments and weigh 0.800 g/m and 1.600 g/m respectively. The tows typically have a width of from 3 to 7 mm and are fed for impregnation on equipment employing combs to hold the tows and keep them parallel and unidirectional, as discussed below.

Examples of glass fibre rovings include R25H and HIPER-TEX™ W2020 Rovings from 3B Fibreglass lldefonse Vandammestraat 5-7 B-1560 Hoeilaart, Belgium, or Windstrand® 2000 from Owens Corning Fiberglas Sprl. 166 Chaussee de la Hulpe B- 1170 Brussels Belgium. Examples of flax rovings and woven reinforcements include the Lincore® range of rovings from 300 to 2400 Tex and twill and plain woven fabrics from 220 gsm to 720gsm (FWT2 220, FWT2 360 and FWP2 720) available from Depestele 5 Rue de I'Eglise, 14540 Bourguebus, France.

On a weight basis, typically the prepregs comprise from 15 to 70 wt% of the curable resin blend, preferably from 20 to 65 wt%, more preferably from 25 to 50 wt% and most preferably from 25 to 40 wt%. On a volume basis, typically the prepregs comprise from 15 to 70 vol% of the curable resin blend, preferably from 20 to 60 vol%, more preferably from 30 to 50 vol% of the curable resin blend.

On a volume basis, typically the prepregs comprise from 45 to 75 vol% of structural fibres, preferably from 55 to 70 vol%.

Resin and fibre content of uncured prepregs which contain carbon fibres are determined in accordance with DIN EN 2559 A (code A). Resin and fibre content of cured composites which contain carbon fibrous material are determined in accordance with DIN EN 2564 A.

The prepregs according to the invention may be manufactured in known manner, e.g. by the process described and illustrated in WO2010/150022, typically in a continuous process involving the passage of many thousands of fibres, forming a structural layer of fibres, through a series of impregnation stages, typically guided by rollers, which act to impregnate resin into the structural layer.

Before the fibres are contacted with the resin blend and reach the impregnation zone they are typically arranged in a plurality of tows of fibres, each tow comprising many thousands of filaments, e.g. 12,000. These tows are mounted on bobbins and are fed initially to a combing unit to ensure even separation of the fibres. The structural layer is typically formed from a plurality of tows of fibres, which are spread out to merge together over spreader bars, prior to impregnation with the resin.

In order to improve handling of the resin it is conventional that it is supported onto a backing material, such as paper. The resin is then fed, typically from a roll, such that it comes into contact with the fibres, the backing material remaining in place on the exterior of the resin and fibre contact region. During the subsequent impregnation process the backing material provides a useful exterior material to apply pressure to, in order to achieve even impregnation of resin.

During this process of impregnation, resin passes between the interstices of the fibres. To facilitate impregnation of the resin into the fibres it is conventional for this to be carried out at an elevated temperature, e.g. from 60 to 120°C preferably from 80 to 100°C, so that the resin viscosity reduces, i.e. to from 1 Pas to 150 Pas, preferably from 6 to 100 Pas, more preferably from 18 to 80 Pas, and even more preferably from 20 to 50 Pas. This is most conveniently achieved by heating the resin and fibres, before impregnation, to the desired temperature, e.g. by passing them through an infra-red heater.

Following impregnation there is typically a cooling step, to reduce the tackiness of the formed prepreg. This may be followed by further treatment stages such as laminating, slitting and separating. Once prepared the prepreg tape may be rolled-up so that it can be stored for a period of time.

When it is desired to manufacture a composite material, a number of such prepregs are typically stacked together, producing a prepreg stack or preform.

A stack of prepreg tape so formed by tape lay-up is typically subsequently cured by exposure to elevated temperature, wherein the thermosetting resin cures to provide the resulting cured composite material. The cure cycles employed for curing prepregs and stacks of prepregs are a balance of temperature and time, taking account the reactivity of the resin and the amount of resin and fibre employed. This may be carried out under elevated pressure in known manner, such as the autoclave techniques. Alternatively or additionally, curing may be carried out close to atmospheric pressure, in the so-called vacuum bag technique.

As will be known to a person skilled in the art, such curing processes are generally exothermic, and so care must be taken to prevent excessive temperatures, which can damage any moulds or cause decomposition of the resin.

Once cured, the prepreg or prepreg stack becomes a composite material, suitable for use in a structural application, for example an aerospace structure. Thus, in a fifth aspect, the invention relates to a cured composite material, obtainable by the process of exposing at least one prepreg according to the fourth aspect to elevated temperature, and optionally elevated pressure, to cure the curable resin blend and thereby produce the cured composite material.

Examples

The following one-component thermally curable epoxy resin formulations were prepared as shown in table 1 below, where the numbers are the weight %. Examples A, B and C are comparative examples, whereas examples 1 and 2 are according to the present invention.

Table 1

For the formulations B, C, 1 and 2 the solid cycloaliphatic epoxy was heated to 95°C until molten followed by blending in any other epoxy resin and/or diluent, until homogenoeous. This was then cooled to 70°C followed by the addition of the curing agent and pigment until homogeneous. For formulation A the aromatic epoxy was heated to 70°C followed by the addition of the curing agent and pigment until homogeneous.

The formulations were subjected to isothermal DSC analysis at 95°C to determine the time taken to reach 95% cure conversion. The cured resins were then analysed by dynamic DSC to measure their cured Tg (midpoint), and the results are shown below in table 2.

Table 2

It can be seen that the resin blends according to the invention reach 95% cure conversion much more rapidly than known cycloaliphatic resins, and have a higher resulting Tg.

Formulations A and 1 were added as a film onto the surface of glass composite laminates (acting as substrate) and the laminates were then co-cured.

Both cured laminates were exposed to UVA radiation (0.71 W/m²) for 400 hours. The degree of yellowing of the two laminates was measured as AE (colour change) values when measured using an X-Rite SP60 Spectrophotometer (L*a*b*, D65/10 illuminant/observer, specular component excluded). The measured values are shown below in table 3. Table 3

It can be seen that the UV stability of the resin according to the present invention is far superior to a conventional aliphatic epoxy resin.

Claims

1. A thermally curable resin blend, comprising a) from 50 to 90 wt% cycloaliphatic curable epoxy resin having an epoxy equivalent weight of greater than 350; b) from 2 to 25 wt% of a tri- or tetra- functional epoxy reactive diluent having an epoxy equivalent weight of less than 200; and c) a curing agent.

2. A thermally curable resin blend according to claim 1 , which comprises from 60 to 85 wt% of the cycloaliphatic curable epoxy resin.

3. A thermally curable resin blend according to claims 1 or claim 2, which comprises from 3 to 15 wt% of the tri- or tetra- functional epoxy reactive diluent.

4. A thermally curable resin blend according to any one of the preceding claims, which is flowable and has a measurable viscosity at 25°C.

5. A thermally curable resin blend according to any one of the preceding claims, wherein the cycloaliphatic curable epoxy resin has an epoxy equivalent weight of greater than 400, more preferably greater than 450 or even greater than 500.

6. A thermally curable resin blend according to any one of the preceding claims, wherein the cycloaliphatic curable epoxy resin does not have a measurable viscosity at a temperature of 30°C, preferably 40°C, more preferably 50°C, or even 60°C.

7. A thermally curable resin blend according to any one of the preceding claims, wherein the cycloaliphatic curable epoxy resin, comprises a material having the general structure:

8. A thermally curable resin blend according to any one of the preceding claims, wherein the tri- or tetra- functional epoxy reactive diluent has an epoxy equivalent weight of less than 170, preferably less than 140.

9. A thermally curable resin blend according to any one of the preceding claims, wherein the tri- or tetra- functional epoxy reactive diluent has a viscosity at 25°C of from 0.01 to 2 Pas, more preferably from 0.1 to 1 Pas.

10. A thermally curable resin blend according to any one of the preceding claims, wherein the tri- or tetra-functional epoxy reactive diluent comprises a tri-functional epoxy reactive diluent having the general structure:

where R¹, R², R³, R⁵ is C_nH2n, R⁴ and R⁶ is C_nH2n+i and

11. A thermally curable resin blend according to claim 10, wherein the tri-functional epoxy reactive diluent has the formula:

12. A thermally curable resin blend according to any one of the preceding claims, wherein the tri- or tetra-functional epoxy reactive diluent comprises a tetrafunctional epoxy reactive diluent having the general structure:

13. A process of curing a thermally curable resin blend according to any one of the preceding claims, by exposure to elevated temperature and/or pressure, wherein the time taken to reach 95% conversion at 95°C is less than 140 minutes.

14. A process according to claim 13, wherein the time taken to reach 95% conversion at 95°C is less than 120 minutes, preferably less than 100 minutes.

15. A process according to claim 13 or 14, which is carried out at a temperature of less than 100°C.

16. A cured epoxy resin, obtainable by the process according to any one of claims 13 to 15, which has a Tg of greater than 100°C.

17. A prepreg comprising a structural layer comprising fibres having interstices therebetween, and comprising a thermally curable resin blend according to any one of claims 1 to 12, impregnated within the structural layer and present within the interstices.

18. A cured composite material, obtainable by the process of exposing at least one prepreg according to claim 17 to elevated temperature, and optionally elevated pressure, to cure the thermally curable resin blend and thereby produce the cured composite material.