Curable epoxy resin composition

Abstract

A curable epoxy resin composition comprises a curable epoxy resin, a reactive diluent; and a non-reactive diluent. The reactive diluent is a diglycidyl ether having a structure of: (I) where R1 is an alkyl or alkoxyalkyl group, and R1 includes a branched moiety and / or a cyclic moiety. The non-reactive diluent has a viscosity in the range 20 to 30 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm. The inclusion of the reactive diluent and the non-reactive diluent in the composition may allow a cured resin having an increased glass transition temperature to be obtained. Also provided are a kit comprising the curable epoxy resin and a hardener; a method of forming a cured epoxy resin using the kit; a cured epoxy resin obtainable by the method; and a combination comprising the reactive diluent and non-reactive diluent.

Description

[0001] Curable epoxy resin composition

[0002] Background

[0003] The present disclosure relates to a curable epoxy resin composition, and to the use of such a composition. The curable epoxy resin composition may in particular be used as the resin component of a two-component epoxy resin system, further comprising a hardener component comprising a curing agent system for curing the epoxy resin after the resin component and the hardener component have been mixed together.

[0004] It is well known to use epoxy resin systems, particularly in combination with fibre-reinforced resin composite materials, for the manufacture of structural parts in a variety of industrial sectors. For example, epoxy resin systems are used for the manufacture of composite materials by, for example infusion or lamination, as well as for adhering components together.

[0005] Many formulations for two-component amine-cured epoxy systems currently exist on the market. Such two-component amine-cured epoxy systems comprise a resin component and a hardener component. The resin component comprising a curable epoxy resin. The hardener component comprises a curing agent for curing the epoxy resin after the resin component and the hardener component have been mixed together.

[0006] Pure curable epoxy resins may be too viscous to allow the resin to be mixed easily with the hardener component. For example, a pure bisphenol A liquid epoxy resin may have a viscosity of about 10,800 mPa.s (108 P) as measured at 25 °C. Commercial epoxy resin compositions therefore typically include a diluent to reduce the viscosity of the resin.

[0007] One diluent used in commercial epoxy resin systems is C12-C14 alcohol monoglycidyl ether, commonly abbreviated to "AMGE" or "AGE". AMGE is a mixture of 2- (dodecoxymethyl)oxirane; 2-(tetradecoxymethyl)oxirane; and 2-(tridecoxymethyl)oxirane. Although an effective and widely-used diluent, AMGE is reprotoxic and may damage fertility or the unborn child (GHS Hazard Statement H360F). There is therefore a need for a replacement for AMGE.

[0008] Summary

[0009] In one aspect, there is provided a curable epoxy resin composition comprising a curable epoxy resin, a reactive diluent; and a non-reactive diluent. The reactive diluent is a diglycidyl ether having a structure of: where R1is an alkyl or alkoxyalkyl group; and R1includes a branched moiety and / or a cyclic moiety. The non-reactive diluent has a viscosity in the range 20 to 30 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm. The combination of diluents provides a workable viscosity while avoiding the use of AMGE. When the composition is cured, the resultant cured resin may have improved physical properties, and in particular an improved glass transition temperature Tg2.

[0010] A related aspect provides a kit comprising the curable epoxy resin composition as defined above, and a hardener for curing the curable epoxy resin composition. The curable epoxy resin composition and the hardener are separated. Combining the components of the kit cures the epoxy resin composition. The kit is useful for, for example, forming bonds between two surfaces.

[0011] Another aspect provides a combination of diluents for a curable epoxy resin composition. The combination comprises a reactive diluent and a non-reactive diluent. The reactive diluent is a diglycidyl ether having a structure of: where R1is an alkyl or alkoxyalkyl group; and R1includes a branched moiety and / or a cyclic moiety. The non-reactive diluent has a viscosity in the range 20 to 30 mPa.s as measured at a temperature of 20 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm. The combination may be useful as an alternative to AMGE or other such diluents. The combination is useful for preparing a curable epoxy resin composition as defined herein. The diluents may improve one or more mechanical properties of the resin after curing, for example an improved Tg2.

[0012] A further aspect provides a method of forming a cured epoxy resin. The method comprises mixing a curable epoxy resin composition with a hardener to obtain a mixture, and curing the mixture. The combination of diluents may improve one or more physical properties of the cured resin, for example Tg2.

[0013] Still another aspect provides a cured epoxy resin obtainable by the method. Without wishing to be bound by theory, it is believed that the cured epoxy resin comprises a matrix comprising a polymer or oligomer, and a non-reactive diluent dispersed in the matrix, wherein the polymer or oligomer comprises a residue of the curable epoxy resin; a residue of the reactive diluent; and a residue of the curing agent. The cured epoxy resin may have an improved Tg2, for example.

[0014] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Nor is the claimed subject matter limited to implementations that solve any or all of the disadvantages noted herein. Brief Description of the Drawings

[0015] To assist understanding of embodiments of the present disclosure and to show how such embodiments may be put into effect, reference is made, by way of example only, to the accompanying drawings in which:

[0016] Fig. 1 is a plot showing glass transition temperatures of cured epoxy resins containing various different diluents, as discussed in Example 1.

[0017] Detailed Description

[0018] The verb 'to comprise' is used herein as shorthand for 'to include or to consist of'. In other words, although the verb 'to comprise' is intended to be an open term, the replacement of this term with the closed term 'to consist of' is explicitly contemplated, particularly where used in connection with chemical compositions.

[0019] Unless expressly stated otherwise, viscosity values reported herein are measured at 25 °C using a Brookfield CAP-2000+ viscometer with a cone 1 spindle operating at a spindle speed of 50 rpm. Viscosity values are measured after 15 s.

[0020] All boiling points reported herein are measured at 1 atm (101,325 Pa).

[0021] All densities reported herein are measured at a temperature of 20 °C.

[0022] "Cutting power" is the ability of a diluent to reduce viscosity of a resin with which the diluent is mixed.

[0023] The glass transition temperature, Tg, is the temperature at which a cured epoxy resin undergoes a transition from a glassy state to a rubbery state. The transition is associated with abrupt changes in the physical properties of the resin. For example, the stiffness of the resin is dramatically reduced above the glass transition temperature. It is therefore desirable for the Tg of a resin to be as high as possible. The glass transition temperature may be measured using differential scanning calorimetry, "DSC". The scan is performed from 25°C to 260°C at 10°C / min, in accordance with test standard ISO 11357-1:2023. Examples of suitable instruments include TA Instruments Q20 and Mettler Toledo DSC12e.

[0024] The glass transition temperatures reported herein are Tg2 values.

[0025] It has been found that certain combinations of diluents may have cutting powers that closely approximate those of AMGE. Cured resins comprising the diluents may have good mechanical properties, and may have improved high temperature performance in comparison with AMGE-containing resins.

[0026] More specifically, there is provided a curable epoxy resin composition, comprising a curable epoxy resin, a reactive diluent; and a non-reactive diluent. The reactive diluent is a diglycidyl ether having a structure of:

[0027] R1is an alkyl or alkoxyalkyl group; and R1includes a branched moiety and / or a cyclic moiety. The non-reactive diluent has a viscosity in the range 20 to 30 mPa.s (i.e. 20 to 30 cP) as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm.

[0028] As used herein, the term "diluent combination" refers collectively to the non-reactive diluent and the reactive diluent. The composition may be free of other diluents. The diluent combination is useful as a replacement for AMGE. The compositions provided herein may be free of AMGE.

[0029] The nature of the non-reactive diluent is not particularly limited provided that the non- reactive diluent has a viscosity in the range 20 to 30 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm.

[0030] A "diluent" is a compound that serves to reduce the viscosity of the curable epoxy resin composition, thereby allowing easier handling of the resin. A "non-reactive" diluent is a diluent which is free of epoxide groups. The non-reactive diluent may, for example, be a solvent and / or a plasticizer.

[0031] The non-reactive diluent may be benzyl alcohol:

[0032] Benzyl alcohol has a viscosity of 24 mPa.s (i.e. 24 cP) as measured at 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm.

[0033] The Examples set out hereinbelow demonstrate that the combination of benzyl alcohol with a reactive diluent as defined herein may provide a curable epoxy resin composition having an appropriate viscosity. When the composition is cured, the resulting cured resin may have an improved glass transition temperature.

[0034] Without wishing to be bound by theory, it is believed that non-reactive diluents having physical properties similar to those of benzyl alcohol may behave similarly to benzyl alcohol in curable epoxy resin compositions.

[0035] For instance, the non-reactive diluent may have a density in the range 0.9 to 1.1 g cm'3as measured at a temperature of 20 °C. Alternatively or additionally, the non-reactive diluent may have a boiling point in the range 150 to 250 °C, optionally 200 °C to 210 °C. For example, the non-reactive diluent may have a boiling point which is approximately equal to that of benzyl alcohol, i.e. about 205 °C. The non-reactive diluent may be a small molecule, and may have a molecular weight in the range 75 to 150 g mol-1, optionally 80 to 120 g mol-1.

[0036] The non-reactive diluent may be an accelerator. For example, the non-reactive diluent may be an alcohol.

[0037] The non-reactive diluent may be an optionally-substituted 6-membered carbocycle. The non- reactive diluent may for example have a structure of: wherein: a dotted line represents a bond selected from a single bond and a double bond; and each R4is independently absent or selected from:

[0038] H, a Cl to C3 alkyl group, and a Cl to C3 hydroxyalkyl group; optionally wherein exactly one R4is a Cl to C3 hydroxyalkyl group.

[0039] The non-reactive diluent may be an aromatic diluent, optionally an aromatic alcohol.

[0040] Examples of aromatic diluents include those of formula: wherein:

[0041] R' is a Cl to C3 hydroxyalkyl group; and each RBis independently selected from H and a Cl to C3 alkyl group. Optionally, at least 3 of the RBgroups are H.

[0042] The most preferred non-reactive diluent is benzyl alcohol.

[0043] The reactive diluent has a structure of:

[0044] R1is an alkyl or alkoxyalkyl group; and R1includes a branched moiety and / or a cyclic moiety. A reactive diluent is 'reactive' in the sense of including epoxide groups. When the curable epoxy resin composition is cured, the reactive diluent participates in the curing reaction. The polyepoxide product includes a residue of the reactive diluent: in other words, the polyepoxide includes monomer units derived from the reactive diluent.

[0045] The number of carbon atoms and the number and size of the branched and / or cyclic moieties in the R1group are not particularly limited and may be selected as appropriate provided that the reactive diluent has an adequate cutting power. For example, the R1group may be selected such that the reactive diluent has a viscosity in the range 30 to 120 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm.

[0046] Optionally, the reactive diluent may have a viscosity in the range 40 to 110 mPa.s. Further optionally, the reactive diluent may have a viscosity in the range 40 to 55 mPa.s or 50 to 100 mPa.s.

[0047] The molar mass of the reactive diluent may be varied by varying the nature of the R1group. The R1group may be selected such that the reactive diluent has an epoxy equivalent value in the range 125 to 425 g / eq. For example, the reactive diluent may have an epoxy equivalent value in the range 150 to 175 g / eq or 380 to 410 g / eq. The epoxy equivalent value is the number of grams of epoxy resin required to provide 1 mole of epoxy groups. Since the reactive diluents used herein have two epoxy groups, the epoxy equivalent value is equal to the molar mass of the reactive diluent divided by two.

[0048] The reactive diluent may have a structure of: wherein:

[0049] L1and L2are each independently absent or selected from a Cl to C3 alkyl group; and R1Ais a C4, C5, C6 or C7 cycloalkyl group.

[0050] As demonstrated in the Examples, the combination of a reactive diluent according to the above structure with a non-reactive diluent may have a cutting power which is similar to that of AMGE, and / or may increase the glass transition temperature of the cured resin.

[0051] R1Amay be a cyclohexyl group.

[0052] L1and L2may each be methylene groups (i.e., -CH2-).

[0053] The preferred reactive diluent in the above class is the diglycidyl ether of cyclohexane dimethanol:

[0054] This compound is commercially available from Huntsman Corporation (UK) Ltd, United Kingdom, under the trade name Araldite (RTM) DY-C.

[0055] Alternatively, R1may include a branched moiety. In such examples, the reactive diluent may have a number average molecular weight in the range 600 to 900 g mol-1, optionally 750 to 850 g mol-1.

[0056] The reactive diluent may have a structure selected from: wherein: p is in the range 5 to 15; n is 0, 1, 2, or 3; m is 0, 1, 2, or 3; and

[0057] R3is a Cl to C3 alkyl group. p may be in the range 8 to 14, and is optionally 11 to 12.

[0058] For example, the reactive diluent may be a poly(oxypropylene) diglycidyl ether: where p is as defined above.

[0059] A poly(oxypropylene) diglycidyl ether having a degree of polymerisation p in the range 11 to 12 is commercially available from Huntsman Corporation (UK) Ltd, United Kingdom, underthe trade name Araldite (RTM) DY3601.

[0060] The ratio of non-reactive diluent to reactive diluent is not particularly limited and may be selected as appropriate. As will be demonstrated in Example 3 below, the ratio may be varied to tune the properties of the curable epoxy resin composition. Increasing the proportion of non-reactive diluent may reduce the viscosity of the curable epoxy resin composition more effectively, but may reduce mechanical and / or thermal performance. Typically, the ratio of non-reactive diluent to reactive diluent by weight is in the range 1:4 to 3:2. In other words, the diluent combination may comprise 20 % to 60 % of non-reactive diluent by weight, based on the total weight of diluents.

[0061] Optionally, the ratio of non-reactive diluent to reactive diluent may be 2:3 to 3:2 by weight. In other words, the diluent combination may comprise 40 % to 60 % of non-reactive diluent by weight, based on the total weight of diluents.

[0062] Further optionally, the ratio of non-reactive diluent to reactive diluent by weight may be in the range 9:11 to 11:9. In other words, the diluent combination may comprise 45 % to 55 % of non-reactive diluent by weight, based on the total weight of diluents.

[0063] In particular, the ratio of non-reactive diluent to reactive diluent by weight may be about 1:1. A 1:1 ratio was found to provide a good level of cutting power while maintaining good mechanical and thermal properties.

[0064] The total amount of diluents present in the composition is not particularly limited and may be selected as appropriate. The reactive diluent and the non-reactive diluent are present in the composition in a combined amount of 2.5 to 25 % by weight. Optionally, the reactive diluent and the non-reactive diluent may be present in the composition in a combined amount of 5 to 15 % by weight, based on the total weight of the composition.

[0065] Further optionally, the reactive diluent and the non-reactive diluent may be present in the composition in a combined amount of 8 to 12 % by weight, based on the total weight of the composition. In particular, the reactive diluent and the non-reactive diluent may be present in the composition in a combined amount of about 10 % by weight of the composition. The Examples hereinbelow show that unexpectedly high Tg2 values were achieved when approximately 10 % of total diluent was used.

[0066] The nature of the curable epoxy resin is not particularly limited, and may be selected as appropriate. A 'curable' epoxy resin is a resin which includes at least one epoxide group and is capable of undergoing polymerisation and / or crosslinking to form a polyepoxide. The polymerisation and optional cross-linking are referred to as 'curing'. The polyepoxide product may be referred to as a 'cured' epoxy resin.

[0067] The curable epoxy resin may comprise or consist of epoxy monomers. The curable epoxy resin may be a liquid epoxy resin, "LER". A 'liquid' epoxy resin is in the liquid state at 21 °C and 1 atm.

[0068] The term 'curable' epoxy resin further encompasses a so-called 'B-staged' resin. A B-staged resin is a partially-cured epoxy resin, which includes epoxy groups capable of undergoing further polymerisation and / or cross-linking to form a fully cured resin. A curable epoxy composition including a B-staged resin typically further comprises a latent curing agent, such as dicyandiamide or imidazole.

[0069] The curable epoxy resin may, for example, comprise a curable bisphenol-based resin. A "bisphenol based" resin, sometimes referred to as simply a "bisphenol resin", is a derivative of a bisphenol compound which includes at least one reactive epoxide group. Most typically, a bisphenol-based resin is a diglycidyl ether of a bisphenol compound. A bisphenol-based resin does not necessarily include any phenol groups.

[0070] The curable epoxy resin may for example comprise bisphenol-A diglycidyl ether, bisphenol-F diglycidyl ether, or a mixture thereof.

[0071] Bisphenol-A diglycidyl ether has a structure of: Bisphenol-F diglycidyl ether has a structure of:

[0072] Optionally, the curable epoxy resin may comprise or consist of a mixture of bisphenol-A diglycidyl ether and bisphenol-F diglycidyl ether. In such examples, the weight ratio of bisphenol-A diglycidyl ether to bisphenol-F diglycidyl ether may be in the range 0.8:1 to 0.9:1.

[0073] The curable epoxy resin composition described herein may have a viscosity in the range 1000 to 2000 mPa.s (i.e. 10 to 20 P), optionally 1500 to 2000 mPa.s (i.e 15 to 20 P), as measured at a temperature of 25 °C using a Brookfield CAP-2000+ viscometer with a cone 1 spindle operating at a spindle speed of 50 rpm. Viscosity may be adjusted by selecting the nature of the resin, the nature of the diluents, and the amount of diluents present.

[0074] The curable epoxy resin composition may optionally further include one or more additives. Examples of additives include defoamers; fillers; pigments; fire-retardant additives; antisettling agents; and the like.

[0075] The curable epoxy resin composition may be a latent curing epoxy resin composition, further comprising a latent curing agent such as dicyandiamide or imidazole.

[0076] Alternatively, the curable epoxy resin composition may be mixed with a hardener immediately before use. To that end, there is provided a kit comprising a curable epoxy resin composition as defined herein; and a hardener for curing the curable epoxy resin composition. In the kit, the curable epoxy resin composition and the hardener are separated. For example, the curable epoxy resin composition and the hardener may be supplied in two different containers, or in respective different compartments or chambers of a single container. In use, the hardener is mixed with the curable epoxy resin composition to initiate the curing reaction. The kit may be described as a two-component epoxy resin system.

[0077] As used herein, the term 'curing agent' refers to an active ingredient which causes the curing reaction; and a 'hardener' is a composition which comprises one or more curing agents. The hardener may consist of the curing agent(s), or may comprise a curing agent and one or more additives.

[0078] The nature of the curing agent is not particularly limited and may be selected as appropriate. For example, the hardener may comprise an amine curing agent, optionally a cycloaliphatic amine curing agent.

[0079] One example of a cycloaliphatic curing agent is isophorone diamine, which has a structure of:

[0080] Another aspect provides a method of forming a cured epoxy resin. The method comprises mixing a curable epoxy resin composition as defined herein with a hardener to obtain a mixture; and curing the mixture.

[0081] The ratio at which the curable epoxy resin composition and the hardener are mixed is not particularly limited and may be selected as appropriate. For example, wherein 20 to 30 parts by weight of the hardener may be mixed with 100 parts by weight of the curable epoxy resin composition.

[0082] The curing may take place at a temperature in the range 60 to 80 °C, optionally about 70 °C.

[0083] The curing reaction is an exothermic reaction. External heating may also be applied. The kits provided herein are useful as adhesives. The cured epoxy resin may be formed in contact with two surfaces, thereby bonding the two surfaces. Alternatively or additionally, the kits may be used in the manufacture of composite materials, for example by infusion or lamination.

[0084] As will be appreciated, in variants where the curable epoxy resin composition includes a latent curing agent, the curing method may instead comprise activating the latent curing agent. The activation technique may be selected as appropriate depending on the nature of the latent curing agent, and may for example comprise heating the composition and / or exposing the composition to ultraviolet light.

[0085] Still another aspect provides a product comprising a cured epoxy resin obtainable by the methods provided herein. The cured epoxy resin may comprise a matrix comprising a polymer or oligomer, the polymer or oligomer comprising a residue of the curable epoxy resin; a residue of the reactive diluent; and a residue of the curing agent.

[0086] The curing temperature of the epoxy resin may in some examples approach or even exceed the boiling point of the non-reactive diluent. However, without wishing to be bound by theory, it is believed that at least some of the non-reactive diluent remains trapped in the matrix. The cured epoxy resin may be a solid solution of the non-reactive diluent in a polymer matrix.

[0087] Still another aspect provides a combination of diluents for a curable epoxy resin composition. The combination comprises a reactive diluent as defined hereinabove, and a non-reactive diluent as defined hereinabove. The combination may be free of resins, but may further comprise one or more additives, such as a pigment or a filler.

[0088] One example provides a combination of benzyl alcohol and the diglycidyl ether of cyclohexane dimethanol. The ratio of the benzyl alcohol to the diglycidyl ether of cyclohexane dimethanol may be in the range 9:11 to 11:9, or may be about 1:1. Another example provides a combination of benzyl alcohol and a poly(oxypropylene) diglycidyl ether having a structure of: where p is in the range 8 to 14, optionally 11 to 12. The ratio of the benzyl alcohol to the poly(oxypropylene) diglycidyl ether may be in the range 9:11 to 11:9, or may be about 1:1.

[0089] The combination may have a cutting power which is similar to that of AMGE, and may be useful as a replacement for AMGE. The combination is especially useful for preparing a curable epoxy resin composition as described hereinabove, and curing such a resin may yield a product with a high glass transition temperature.

[0090] Examples

[0091] Example 1: effects of various diluents on properties of a bisphenol-A liquid epoxy resin

[0092] The effects of various different diluents were investigated by combining a commercially- available high purity bisphenol-A liquid epoxy resin ("LER") with the diluents.

[0093] The resin consisted essentially of bisphenol A diglycidyl ether.

[0094] The diluents investigated were:

[0095] 1) C12-C14 alcohol monoglycidyl ether ("AMGE");

[0096] 2) Araldite (RTM) DY3601, a commercially-available poly(oxypropylene) diglycidyl ether having a degree of polymerisation in the range 11 to 12;

[0097] 3) Araldite (RTM) DY-C, the diglycidyl ether of cyclohexane dimethanol;

[0098] 4) Benzyl alcohol;

[0099] 5) Combination A, a mixture of benzyl alcohol and DY3601 at a ratio of 1:1 by weight;

[0100] 6) Combination B, a mixture of benzyl alcohol and DY-C at a ratio of 1:1 by weight; and 7) Combination C, a mixture of benzyl alcohol and Epodil (RTM) LV5, a commercially- available liquid hydrocarbon diluent supplied by Evonik Industries AG, Germany. According to its technical datasheet, Epodil (RTM) LV5 has a viscosity of 50 mPa.s as measured at 25 °C using a Brookfield RVTD with Spindle 4.

[0101] The viscosities of mixtures of the LER with the diluents were measured at 25 °C, using a Brookfield CAP-2000+ viscometer. Measurements were taken after 15 s. Where not otherwise specified, a cone 1 spindle operating at a spindle speed of 50 rpm was used.

[0102] The results of the viscosity measurements are set out in the table below. For comparison, the viscosity of the LER in the absence of any diluent was 10800 mPa.s (i.e., 108 P). Measurements were recorded in Poise (P) and have been converted to mPa.s.

[0103] NT: not tested.

[0104] A: this measurement was performed using a cone 2 spindle.

[0105] B: a spindle speed of 100 rpm was used for this measurement. c: a spindle speed of 150 rpm was used for this measurement.

[0106] The viscosity of the compositions was found to decrease as the amount of diluent increased. Benzyl alcohol was found to have the greatest cutting power, in other words, produced the largest reductions in viscosity. Diluent combinations A and B had cutting powers which were close to that of AMGE. In particular, at an LER:diluent ratio of 90:10, the cutting power of combination A was almost the same as that of AMGE.

[0107] Each of the epoxy resin compositions was combined with isophorone diamine ("IPD") at a weight ratio in the range 19.4 to 23.3 parts of IPD to 100 parts of epoxy resin composition. A 15 g sample of each of the resultant mixtures was cured in a respective pot lid for 5 hours at 70 °C. The glass transition temperature Tg2 of each sample was then measured by differential scanning calorimetry ("DSC"). The results are reported in the table below and are further shown in Fig. 1.

[0108] It was found that, generally, increasing the amount of diluent reduced Tg2. Benzyl alcohol and Epodil (RTM) LV5 caused the largest reductions in Tg2. The above data further show that combination C produced a larger reduction in Tg2 than combinations A and C.

[0109] At a resin to diluent ratio of 90:10, combination A, which is a mixture of benzyl alcohol and DY3601 at a ratio of 1:1 by weight, provided a Tg2 which is higher than the mean of the results for benzyl alcohol and DY3601 used individually. This demonstrates that the combination of benzyl alcohol and DY3601 is synergistic.

[0110] Diluent combination B, used at resin to diluent ratios of 90:10 and 85:15, provided a cured resin having better high temperature mechanical properties than the comparative AMGE formulation. Example 2: Tecam gel time

[0111] The Tecam gel times of various ones of the 90:10 LER:diluent compositions of Example 1 after were measured at 25 °C. The results are reported in the table below. Tecam gel time provides a measure of the handling time of the composition.

[0112] Combinations A and B, which included benzyl alcohol, gelled more quickly than the comparative compositions lacking benzyl alcohol. Benzyl alcohol accelerates the curing of epoxy resin compositions.

[0113] Gelling times remained within a useful, practical range: a handling time of approximately 2 hours is suitable for many use cases. Gelling times can be adjusted by selecting an appropriate hardener.

[0114] Example 3: effects of ratio of non-reactive diluent to reactive diluent

[0115] The epoxy resin composition comprising high purity bisphenol-A liquid epoxy resin and diluent combination A as defined in Example 1 was investigated further by adjusting the ratio of benzyl alcohol to DY3601. In these tests, a resi diluent ratio of 90:10 was used. Tg2 was measured as described in Example 1. Viscosity was also measured as described in Example 1, at 25 °C using the Brookfield CAP-2000+ viscometer with a cone 1 spindle operating at 50 rpm. Tecam gel time was measured as described in Example 2. The results are tabulated below.

[0116] The data demonstrate that the properties of the epoxy resin composition may be adjusted by modifying the ratio of non-reactive diluent to reactive diluent. Increasing the amount of the non-reactive diluent, benzyl alcohol, reduced the viscosity and gel time of the composition, as well as the Tg2 of the cured resin.

[0117] Example 4: mechanical performance

[0118] Curable epoxy resin formulations were prepared in accordance with the table below.

[0119] Resin composition 1 is a commercially-available resin, marketed under the trade name "Ampreg (TM) 30", supplied by Gurit (UK) Ltd, United Kingdom. Resin composition 2 is a resin of the type described herein. Resin composition 3 is a comparative resin comprising benzyl alcohol as the sole diluent. DY3601 is a commercially-available poly(oxypropylene) diglycidyl ether having a degree of polymerisation in the range 11 to 12.

[0120] BYK-A 525 is a commercially-available defoamer, comprising a polyether-modified methylalykl polysiloxane copolymer dissolved in a mixture of white spirit and methoxypropylacetate.

[0121] The viscosity of resins 2 and 3 were measured at 25 °C using a Brookfield CAP-2000+ with a cone 1 spindle operating at a speed of 50 rpm. The viscosity reading was taken after 15 s. The reported viscosity values for resin 1 are taken from the product's technical data sheet, and were measured using a Brookfield CAP-2000+ with a cone 1 spindle operating at a spindle speed of 50 rpm.

[0122] Samples of each resin were then combined with a commercially-available hardener at a weight ratio of approximately 25 parts by weight of hardener to 100 parts by weight of resin. The samples were cured for 5 hours at 70 °C. Four different hardeners were investigated: "Ampreg (TM) 3X Fast", "Ampreg (TM) 3X Standard", "Amprex (TM) 3X Slow" and "Ampreg (TM) 3X extra slow".

[0123] Glass transition temperature, Tg2, after curing was measured by differential scanning calorimetry ("DSC") using a 15 g sample cured in a pot lid. Cured samples were analysed using a Differential Scanning Calorimeter (e.g. TA Instruments Q20 or Mettler Toledo DSC12e). The dynamic programme used was from 25°C to 260°C at 10°C / min, in accordance with test standard ISO11357.

[0124] Tensile strength, tensile modulus, and tensile strain were measured in accordance with ISO 527-2:2012(E). Flexural strength, flexural modulus, and flexural strain were measured in accordance with ISO 178:2019.

[0125] Separately, the Tecam gel time of each mixture was measured in water at 25 °C. The results of the measurements are reported in the table below.

[0126]

[0127] The measurements using the "standard" and "slow" hardeners show that resin composition 2 had an improved Tg2 in comparison with composition 1. At the same time, composition 2 had a viscosity which was very similar to that of composition 1.

[0128] Mechanical properties remained within acceptable ranges. Compositions 1 and 2 had similar tensile and flexural strengths. Composition 2 had reduced flexural strain, i.e., deformed less under flexion, than composition 2.

[0129] The results confirmed that handling time, as measured by Tecam gel time, may be adjusted by modifying the hardener. Composition 2 when used with a 'standard' and 'extra slow' hardeners had gel times which were within the specifications for composition 1 with a 'fast' or 'slow' hardener, respectively.

[0130] Accordingly, the diluent combinations provided herein are suitable as a replacement for AMGE in curable epoxy resin compositions.

[0131] Clauses

[0132] The present disclosure provides the following Clauses:

[0133] Clause 1. A curable epoxy resin composition, comprising a curable epoxy resin, a reactive diluent; and a non-reactive diluent; wherein the reactive diluent is a diglycidyl ether having a structure of: wherein:

[0134] R1is an alkyl or alkoxyalkyl group; and

[0135] R1includes a branched moiety and / or a cyclic moiety.

[0136] Clause 2. The composition according to Clause 1, wherein the non-reactive diluent has a viscosity in the range 20 to 30 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm.

[0137] Clause 3. The composition according to Clause 1 or Clause 2, which is a curable liquid epoxy resin composition.

[0138] Clause 4. The composition according to any preceding Clause, wherein the non- reactive diluent has a density in the range 0.9 to 1.1 g cm'3as measured at a temperature of 20 °C.

[0139] Clause 5. The composition according to any preceding Clause, wherein the non- reactive diluent has a boiling point in the range 150 to 250 °C, optionally 200 °C to 210 °C, or of about 205 °C, as measured at a pressure of 1 atm.

[0140] Clause 6. The composition according to any preceding Clause, wherein the non- reactive diluent has a molecular weight in the range 75 to 150 g mol-1, optionally 80 to 120 g mol-1.

[0141] Clause 7. The composition according to any preceding Clause, wherein the non- reactive diluent is an accelerator.

[0142] Clause s. The composition according to Clause 7, wherein the non-reactive diluent is an alcohol.

[0143] Clause 9. The composition according to any preceding Clause, wherein the non- reactive diluent is an optionally-substituted 6-membered carbocycle. Clause 10. The composition according to Clause 9, wherein the non-reactive diluent has a structure of: wherein: a dotted line represents a bond selected from a single bond and a double bond; and each R4is independently absent or selected from:

[0144] H, a Cl to C3 alkyl group, and a Cl to C3 hydroxyalkyl group; optionally wherein exactly one R4is a Cl to C3 hydroxyalkyl group.

[0145] Clause 11. The composition according to any preceding Clause, wherein the non- reactive diluent is an aromatic diluent, optionally an aromatic alcohol.

[0146] Clause 12. The composition according to Clause 11, wherein the aromatic diluent is an aromatic alcohol of formula: wherein:

[0147] RAis a Cl to C3 hydroxyalkyl group; and each RBis independently selected from H and a Cl to C3 alkyl group. 1

[0148] Clause 13. The composition according to Clause 12, wherein at least 3 of the RBgroups are H.

[0149] Clause 14. The composition according to Clause 13, wherein the aromatic diluent is benzyl alcohol:

[0150] Clause 15. The composition according to any preceding Clause, wherein the reactive diluent has a viscosity in the range 30 to 120 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm; optionally wherein the reactive diluent has a viscosity in the range 40 to 110 mPa.s; further optionally wherein the reactive diluent has a viscosity in the range 40 to 55 mPa.s or 50 to 100 mPa.s.

[0151] Clause 16. The composition according to any preceding Clause, wherein the reactive diluent has an epoxy equivalent value in the range 125 to 425 g / eq; optionally wherein the reactive diluent has an epoxy equivalent value in the range 150 to 175 g / eq or 380 to 410 g / eq.

[0152] Clause 17. The composition according to any preceding Clause, wherein the reactive diluent has a structure of: wherein:

[0153] L1and L2are each independently absent or selected from a Cl to C3 alkyl group; and R1Ais a C4, C5, C6 or C7 cycloalkyl group.

[0154] Clause 18. The composition according to Clause 17, wherein R1Ais a cyclohexyl group.

[0155] Clause 19. The composition according to Clause 17 or Clause 18, wherein L1is a methylene (-CH2-) group and L2is a methylene group.

[0156] Clause 20. The composition according to Clause 19, wherein the reactive diluent is:

[0157] Clause 21. The composition according to Clause 20, wherein the non-reactive diluent is benzyl alcohol, and the reactive diluent is

[0158] Clause 22. The composition according to any of Clauses 1 to 16, wherein R1includes a branched moiety.

[0159] Clause 23. The composition according to Clause 22, wherein the reactive diluent has a number average molecular weight in the range 600 to 900 g mol-1, optionally 750 to 850 g mol-1. Clause 24. The composition according to Clause 22 or Clause 23, wherein the reactive diluent has a structure selected from: wherein: p is in the range 5 to 15; n is 0, 1, 2, or 3; m is 0, 1, 2, or 3; and

[0160] R3is a Cl to C3 alkyl group.

[0161] Clause 25. The composition according to Clause 24, wherein the reactive diluent is a poly(oxypropylene) diglycidyl ether:

[0162] Clause 26. The composition according to Clause 24 or Clause 25, wherein p is in the range 8 to 14, optionally 11 to 12.

[0163] Clause 27. The composition according to Clause 26, wherein the non-reactive diluent is benzyl alcohol, and wherein the reactive diluent is a poly(oxypropylene) diglycidyl ether: p being in the range 11 to 12. Clause 28. The composition according to any preceding Clause, wherein the ratio of non-reactive diluent to reactive diluent by weight is in the range 1:4 to 3:2, optionally 2:3 to 3:2 by weight.

[0164] Clause 29. The composition according to Clause 28, wherein the ratio of non- reactive diluent to reactive diluent by weight is in the range 9:11 to 11:9.

[0165] Clause 30. The combination according to Clause 29, wherein the ratio of non- reactive diluent to reactive diluent by weight is about 1:1.

[0166] Clause 31. The composition according to preceding Clause, wherein the reactive diluent and the non-reactive diluent are present in the composition in a combined amount of 2.5 to 25 % by weight, optionally 5 to 15 % by weight, based on the total weight of the composition.

[0167] Clause 32. The composition according to Clause 31, wherein the reactive diluent and the non-reactive diluent are present in the composition in a combined amount of 8 to 12 % by weight, based on the total weight of the composition.

[0168] Clause 33. The composition according to Clause 32, wherein the reactive diluent and the non-reactive diluent are present in the composition in a combined amount of about 10 % by weight of the composition.

[0169] Clause 34. The composition according to any preceding Clause, wherein the curable epoxy resin comprises a curable bisphenol-based resin.

[0170] Clause 35. The composition according to Clause 34, wherein the curable epoxy resin comprises bisphenol-A diglycidyl ether, bisphenol-F diglycidyl ether, or a mixture thereof. Clause 36. The composition according to Clause 35, wherein the curable epoxy resin comprises a mixture of bisphenol-A diglycidyl ether and bisphenol-F diglycidyl ether, wherein the weight ratio of bisphenol-A diglycidyl ether to bisphenol-F diglycidyl ether is in the range 0.8:1 to 0.9:1.

[0171] Clause 37. The composition according to any preceding claim, wherein the composition has a viscosity in the range 1000 to 2000 mPa.s, optionally 1500 to 2000 mPa.s, as measured at a temperature of 25 °C using a Brookfield CAP-2000+ viscometer with a cone 1 spindle operating at a spindle speed of 50 rpm.

[0172] Clause 38. A kit, comprising: a curable epoxy resin composition as defined in any preceding claim; and a hardener for curing the curable epoxy resin composition; wherein the curable epoxy resin composition and the hardener are separated.

[0173] Clause 39. The kit according to Clause 38, wherein the hardener comprises an amine curing agent.

[0174] Clause 40. The kit according to Clause 39, wherein the amine curing agent comprises a cycloaliphatic amine curing agent.

[0175] Clause 41. The kit according to Clause 40, wherein the cycloaliphatic amine curing agent comprises isophorone diamine.

[0176] Clause 42. A combination of diluents for a curable epoxy resin composition, which combination comprises a reactive diluent as defined in any of Clauses 1 to 30, and a non-reactive diluent as defined in any of Clauses 1 to 30.

[0177] Clause 43. A method of forming a cured epoxy resin, which method comprises: mixing a curable epoxy resin composition as defined in any of Clauses 1 to 37 with a hardener to obtain a mixture; and curing the mixture. Clause 44. The method according to Clause 43, wherein the hardener is as defined in any of Clauses 39 to 41.

[0178] Clause 45. The method according to Clause 43 or Clause 44, wherein 20 to 30 parts by weight of the hardener are mixed with 100 parts by weight of the curable epoxy resin composition.

[0179] Clause 46. The method according to any of Clauses 43 to 45, wherein the mixture is cured at a temperature in the range 60 to 80 °C, optionally about 70 °C.

[0180] Clause 47. The method according to any of Clauses 43 to 46, wherein the cured epoxy resin is formed in contact with two surfaces, thereby bonding the two surfaces.

[0181] Clause 48. A product comprising a cured epoxy resin obtainable by the method according to any of Clauses 43 to 47.

[0182] Clause 49. A cured epoxy resin, comprising: a matrix comprising a polymer or oligomer, and a non-reactive diluent dispersed in the matrix, wherein the polymer or oligomer comprises a residue of a curable epoxy resin; a residue of a reactive diluent; and a residue of a curing agent; and wherein the reactive diluent, the curable epoxy resin, and the non-reactive diluent are as defined in any of Clauses 1 to 37; optionally wherein the curing agent is as defined in any of claims 39 to 41.

[0183] It will be appreciated that the above embodiments have been described by way of example only.

[0184] Other variants or use cases of the disclosed techniques may become apparent to the person skilled in the art once given the disclosure herein. The scope of the disclosure is not limited by the described embodiments but only by the accompanying claims.

Claims

Claims1. A curable epoxy resin composition, comprising a curable epoxy resin, a reactive diluent; and a non-reactive diluent; wherein the reactive diluent is a diglycidyl ether having a structure of:wherein:R1is an alkyl or alkoxyalkyl group; andR1includes a branched moiety and / or a cyclic moiety; and wherein the non-reactive diluent has a viscosity in the range 20 to 30 mPa.s as measured at a temperature of 25 °C using a Brookfield RVT viscometer with a #2 spindle operating at a spindle speed of 100 rpm.

2. The composition according to claim 1, wherein the non-reactive diluent has: a density in the range 0.9 to 1.1 g cm'3as measured at a temperature of 20 °C; and / or a boiling point in the range 150 to 250 °C, optionally 200 °C to 210 °C, or of about 205 °C, as measured at a pressure of 1 atm; and / or a molecular weight in the range 75 to 150 g mol-1, optionally 80 to 120 g mol-1.

3. The composition according to claim 1 or claim 2, wherein the non-reactive diluent has a structure of:wherein: a dotted line represents a bond selected from a single bond and a double bond; and34 each R4is independently absent or selected from:H, a Cl to C3 alkyl group, and a Cl to C3 hydroxyalkyl group; optionally wherein exactly one R4is a Cl to C3 hydroxyalkyl group.

4. The composition according to claim 3, wherein the non-reactive diluent is an aromatic alcohol of formula:wherein:RAis a Cl to C3 hydroxyalkyl group; and each RBis independently selected from H and a Cl to C3 alkyl group.

5. The composition according to claim 4, wherein the non-reactive diluent is benzyl alcohol:

6. The composition according to any preceding claim, wherein the reactive diluent has a structure of:wherein:L1and L2are each independently absent or selected from a Cl to C3 alkyl group; and R1Ais a C4, C5, C6 or C7 cycloalkyl group.

7. The composition according to claim 6, wherein the reactive diluent is:

8. The composition according to claim 7, wherein the non-reactive diluent is benzyl diluent is9. The composition according to any of claims 1 to 5, wherein R1includes a branched moiety.

10. The composition according to claim 9, wherein the reactive diluent has a number average molecular weight in the range 600 to 900 g mol-1, optionally 750 to 850 g mol-1.

11. The composition according to claim 9 or claim 10, wherein the reactive diluent has a structure selected from:wherein: p is in the range 5 to 15; n is 0, 1, 2, or 3; m is 0, 1, 2, or 3; andR3is a Cl to C3 alkyl group.

12. The composition according to claim 11, wherein the reactive diluent is a poly(oxypropylene) diglycidyl ether:

13. The composition according to claim 11 or claim 12, wherein p is in the range 8 to 14, optionally 11 to 12.

14. The composition according to claim 13, wherein the non-reactive diluent is benzyl alcohol, and wherein the reactive diluent is a poly(oxypropylene) diglycidyl ether:p being in the range 11 to 12.

15. The composition according to any preceding claim, wherein the ratio of non-reactive diluent to reactive diluent by weight is in the range 1:4 to 3:2, optionally 2:3 to 3:2, further optionally 9:11 to 11:9.

16. The composition according to preceding claim, wherein the reactive diluent and the non-reactive diluent are present in the composition in a combined amount of 2.5 to 25 % by weight, optionally 5 to 15 % by weight, based on the total weight of the composition.

17. The composition according to claim 16, wherein the reactive diluent and the non- reactive diluent are present in the composition in a combined amount of 8 to 12 % by weight, optionally about 10 % by weight, based on the total weight of the composition.

18. The composition according to any preceding claim, wherein the curable epoxy resin comprises a curable bisphenol-based resin.

19. The composition according to claim 18, wherein the curable epoxy resin comprises bisphenol-A diglycidyl ether, bisphenol-F diglycidyl ether, or a mixture thereof.

20. The composition according to claim 19, wherein the curable epoxy resin comprises a mixture of bisphenol-A diglycidyl ether and bisphenol-F diglycidyl ether, wherein the weight ratio of bisphenol-A diglycidyl ether to bisphenol-F diglycidyl ether is in the range 0.8:1 to 0.9:1.

21. A kit, comprising: a curable epoxy resin composition as defined in any preceding claim; and a hardener for curing the curable epoxy resin composition; wherein the curable epoxy resin composition and the hardener are separated.

22. The kit according to claim 21, wherein the hardener comprises an amine curing agent, optionally a cycloaliphatic amine curing agent, further optionally isophorone diamine.

23. A method of forming a cured epoxy resin, which method comprises: mixing a curable epoxy resin composition as defined in any of claims 1 to 20 with a hardener to obtain a mixture; and curing the mixture.

24. The method according to claim 23, wherein the mixture is cured at a temperature in the range 60 to 80 °C, optionally about 70 °C.

25. A cured epoxy resin, comprising: a matrix comprising a polymer or oligomer, and a non-reactive diluent dispersed in the matrix, wherein the polymer or oligomer comprises a residue of a curable epoxy resin; a residue of a reactive diluent; and a residue of a curing agent; and wherein the reactive diluent, the curable epoxy resin, and the non-reactive diluent are as defined in any of claims 1 to 20.

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