Fast-cure pre-preg

Abstract

A laminar pre-preg of fibre-reinforced curable composite material. The pre-preg includes at least one layer of reinforcing fibres impregnated with a curable resin containing one or more curable thermosetting resin(s) and at least one liquid curative, wherein the curable resin exhibits a cure conversion of at least 95% when cured at a cure temperature in the range of from about 100° C. to about 160° C. and the cure cycle has a duration of no more than 10 minutes, and the glass transition temperature (Tg) of the curable resin when cured is in the range of from about 130° C. to about 165° C.

Claims

1. A laminar pre-preg of fibre-reinforced curable composite material, wherein said pre-preg comprises at least one layer of reinforcing fibres impregnated with a curable resin comprising (i) one or more multifunctional epoxy resin(s) having a functionality of greater than two, (ii) one or more difunctional epoxy resin(s), (iii) at least one liquid curative, and (iv) a dicyandiamide in solid or particulate form, wherein: the at least one liquid curative is selected from substituted imidazoles, wherein the substituent groups of said substituted imidazoles are or comprise alkyl and/or aryl substituent groups; the reinforcing fibres are in the form of tows having a tow size of at least 12,000 filaments per tow and said layer of reinforcing fibres has an areal weight of at least 300 g/m.sup.2; the curable resin exhibits a cure conversion of at least 95% when cured at a cure temperature in the range of from about 100° C. to about 160° C. wherein the cure cycle has a duration of no more than 10 minutes; and the glass transition temperature (Tg) of the curable resin when cured is in the range of from about 130° C. to about 165° C.

2. A pre-preg according to claim 1, wherein said liquid curative is selected from: 1-cyanoethyl-2-ethyl-4-methylimidazole, 2-ethyl-4-methylimidazole, 1-cyanoethyl-2-methylimidazole, 1-methyl imidazole, 4-methyl imidazole, 1-benzyl-2-phenylimidazole, 1-benzyl-2-methylimidazole, 1-(3-aminopropyl)imidazole, 1,2-dimethylimidazole, 1-cyanoethyl-2-undecylimidazole, and 4,5-bis[(2-cyanoethoxy)methyl]-2-phenyl-1H-imidazole-1-propiononitrile.

3. A pre-preg according to claim 1, wherein the substituted imidazole is present in the curable resin in an amount such that there is 1 mole of substituted imidazole per 10-25 moles of epoxy groups.

4. A pre-preg according to claim 1, wherein dicyandiamide is present in the curable resin in an amount of from about 1 wt % to about 20 wt %, based on the total weight of the curable resin.

5. A pre-preg according to claim 1, the one or more of difunctional epoxy resin(s) is/are selected from: diglycidyl ether of bisphenol A; and diglycidyl ether of bisphenol F.

6. A pre-preg according to claim 5, wherein the one or more multifunctional epoxy resin(s) is/are selected from: triglycidyl aminophenol; and tetraglycidyl diamino diphenyl methane (TGDDM).

7. A pre-preg according to claim 1, wherein the curable resin further comprises one or more organic polybasic acid(s) or organic polybasic acid anhydride(s).

8. A pre-preg according to claim 7 wherein the curable resin comprises 1 mole of liquid curative per 0.5 to 1.5 mole of acid group of the organic polybasic acid or anhydride.

9. A pre-preg according to claim 1, wherein said reinforcing fibres are carbon fibres.

10. A pre-preg according to claim 1, wherein the reinforcing fibres are glass fibres and exhibit a roving or yarn with a tex of at least 68.

11. A pre-preg according to claim 1, wherein the curable resin does not contain a cure inhibitor which is or comprises boric acid, a Lewis acid derivative of boron, a mineral acid having a nucleophilicity value (n) of greater than zero and less than 2.5, or an organic acid having a pKa value of from 1 to 3, or a mixture of two or more of said cure inhibitors.

12. A process for the production of a moulded article from a plurality of pre-pregs comprising the steps of: (a) providing a mould; (b) disposing a laminar pre-preg into or onto said mould; (c) repeating step (b) at least once to dispose one or more further pre-pregs into or onto said mould; and (d) curing the plurality of pre-pregs; wherein said laminar pre-preg is a fibre-reinforced curable composite material as defined in claim 1.

13. A process according to claim 12, wherein curing is effected while the pre-pregs are compressed in a mould cavity.

14. A process according to claim 12 wherein curing is conducted at a cure temperature in the range of from about 100° C. to about 160° C., and wherein the plurality of pre-pregs is held at said cure temperature for a duration of no more than 10 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the plot obtained for an embodiment (Example 2) in the Dynamic Mechanical Analysis test method disclosed hereinbelow for measurement of Tg; the storage modulus is shown as plot (a); the tan delta is shown as plot (b) and the loss modulus is shown as plot (c).

(2) FIGS. 2-13 show corresponding plots for Examples 3, 4, 6, 7, 9, 10, 12, 13, 15, 16, 18, and 19, repsectively.

(3) FIGS. 14-17 show corresponding plots for Comparative Examples 1-4, respectively.

MEASUREMENT METHODS

(4) The pre-pregs described herein were characterised as follows.

(5) Viscosity The viscosity of the resins was measured, at 21° C. unless otherwise stated, by following ASTM D4440-15: using a 25 mm diameter parallel plate, with a strain of 1%, a frequency of 1 Hz and a gap of 500 μm.

(6) Glass Transition Temperature

(7) The glass transition temperature, T.sub.g, of the cured resins was measured by Dynamic Mechanical Analysis (DMA) using a dynamic mechanical analyser (TA Instruments Q800) under flexural oscillation mode according to ASTM 7028-07, with a heating rate of 5° C./min and without purge gas. The thermocouple in the TA Instruments Q800 equipment remained in its fixed position. The dimensions of the sample were 58±5×10±1×1.75±0.75 mm (Length×Wdth×Thickness). The Tg reported herein is the intercept of the two tangent lines (i.e. the Lines “A” and “B” referred to in ASTM 7028-07) from the plot of storage modulus on a linear scale vs. temperature.

(8) The uncured glass transition temperature, T.sub.g, of the resins was measured by differential scanning calorimetry (DSC) at a heating rate of 10° C. per minute, according to ISO 11357-2:2013, on a Metler Toledo DSC 822e instrument.

(9) Cure Conversion

(10) Differential Scanning calorimetry (DSC) was utilized to determine the cure conversion under a given set of cure conditions, substantially in accordance with ISO-11357-5:2013. The residual enthalpy (remaining heat of reaction) detected during the DSC measurement is correlated to the total enthalpy (heat evolved) of the curing reaction. DSC measurements are performed by heating a sample from 30° C. to a temperature that is sufficient to capture the entire curing reaction (225° C. is sufficient for the resins of interest described herein) at a heating rate of 10° C./min. The sample size is about 5-10 mg. The cure conversion (%) is calculated as:

(11) $\mathrm{cure}\mathrm{conversion}\left(\%\right)=\frac{\left(\Delta \mathrm{Hi}-\Delta \mathrm{He}\right)}{\Delta \mathrm{Hi}}\times 100$

(12) wherein:

(13) ΔHi is the enthalpy generated by the uncured test sample during heating from 30° C. to 225° C.; and

(14) ΔHe is the enthalpy generated by a cured sample during the heating scan of heated from 30° C. to 225° C.

(15) The invention is further illustrated with reference to the following non-limiting examples.

(16) Epoxy Equivalent Weight (EEW)

(17) Epoxy equivalent weight is assessed according to ISO3001:1999.

EXAMPLES

(18) Pre-pregs were prepared using the components presented in Table 1 below.

(19) TABLE-US-00001 TABLE 1 Materials Material Trade Name Chemical Description Supplier Araldite ® EPN1138 Epoxy phenol novolak resin Huntsman Araldite ® LY1556 Diglycidyl ether of bisphenol A Huntsman (liquid) Araldite ® GT7071 Diglycidyl ether of bisphenol A Huntsman (solid) Araldite ® ECN1273 Epoxy cresol novolak resin Huntsman Araldite ® MY9512 Tetra-functional epoxy based on Huntsman methylene dianiline Dyhard ® DF50EP 50% Dicyandiamide dispersed in a Alzchem 50% Bisphenol-A epoxy resin Dyhard ® UR505 Substituted urea catalyst Alzchem Imicure ® EMI-24 2-Ethyl-4-methylimidazole Air Curing Agent 90% Products Curimid ® CN 1-(Cyanoethyl)-2-ethyl-4- PCI methylimidazole synthesis WS17321A Black Pigment dispersed in West & Diglycidyl ether of bisphenol A Senior (liquid) Succinic Acid Butanedioic Acid DSM

Example 1

(20) An epoxy resin formulation according to Table 2 below was prepared by blending the first three ingredients listed at 150° C. until all the Araldite® GT7071 had dissolved. At 50° C. the remaining two ingredients were added and mixed until uniform.

(21) TABLE-US-00002 TABLE 2 Epoxy resin formulation of Example 1 Ingredient % Araldite EPN1138 44.6 Araldite LY1556 19.6 Araldite GT7071 24.1 Dyhard DF50EP 9.0 1-(Cyanoethyl)-2-ethyl-4-methylimidazole 2.7

(22) The resin formulation of Table 2 exhibited an EEW (based only on the epoxy components) of 220 g/mol). The resin was analysed by DSC (differential scanning calorimetry), which demonstrated that the resin exhibits 95% cure conversion after 5 minutes at 150° C.; 97% cure conversion after 10 minutes at 150° C.; and 99% cure conversion after 5 minutes at 160° C.

Example 2

(23) A prepreg was prepared from the epoxy resin formulation of Example 1 as follows. A carbon fibre fabric (2×2 twill weave; areal weight of 200 gsm) manufactured from a 3k fibre (HTA40-E13-3K available from Toho Tenax®) was impregnated with the epoxy resin formulation of Example 1 to obtain a prepreg with a resin content of 38%. The prepreg was used to make a 10-ply 0/90° laminate which was then subjected to a 3 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 145° C. (see FIG. 1).

(24) In FIG. 1, the storage modulus is shown as plot (a); the tan delta is shown as plot (b) and the loss modulus is shown as plot (c).

Example 3

(25) A prepreg was prepared from the epoxy resin formulation of Example 1. A carbon fibre fabric (2×2 twill weave; areal weight of 660 gsm) manufactured from a 12k fibre (T700SC-12k-60E available from Toray) was impregnated with the epoxy resin formulation of Example 1 to obtain a prepreg with a resin content of 38%. The prepreg was used to make a 3 ply 0°/90° laminate which was then subjected to a 3 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 143° C. (see FIG. 2).

Example 4

(26) A prepreg was prepared from the epoxy resin formulation of Example 1. A carbon fibre fabric (+/−45° non-crimp fabric; areal weight of 600 gsm) manufactured from a 24k fibre (STS40-F13-24k available from Toho Tenax) was impregnated with the epoxy resin formulation of Example 1 to obtain a prepreg with a resin content of 38%. The prepreg was used to make a 4 ply laminate which was then subjected to a 3 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was cut into DMA specimens in such a way that the fibres were oriented at 0° and 90° to its length, which were then tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 146° C. (see FIG. 3).

Example 5

(27) An epoxy resin formulation according to Table 3 below was prepared by blending the first three ingredients listed at 150° C. until all the Araldite® GT7071 had dissolved. At 50° C. the remaining ingredient was added and mixed until uniform.

(28) TABLE-US-00003 TABLE 2 Epoxy resin formulation Ingredient % Araldite EPN1138 48.1 Araldite LY1556 22.1 Araldite GT7071 26.0 1-(Cyanoethyl)-2-ethyl-4-methylimidazole 3.8

(29) The resin formulation of Table 3 exhibited an EEW (based only on the epoxy components) of 221 g/mol. The resin was analysed by DSC, which demonstrated that the resin exhibits 98% cure conversion after 5 minutes at 150° C.; 98% cure conversion after 10 minutes at 150° C.; and 99% cure conversion after 5 minutes at 160° C.

Example 6

(30) A prepreg was prepared from the epoxy resin formulation of Example 5 as follows. A carbon fibre fabric (2×2 twill weave; areal weight of 200 gsm) manufactured from a 3k fibre (HTA40-E13-3K available from Toho Tenax®) was impregnated with the epoxy resin formulation of Example 1 to obtain a prepreg with a resin content of 38%. The prepreg was used to make a 10-ply 0/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 144° C. (see FIG. 4).

Example 7

(31) A prepreg was prepared from the epoxy resin formulation of Example 5. A carbon fibre fabric (2×2 twill weave; areal weight of 660 gsm) manufactured from a 12k fibre (T700SC-12k-60E available from Toray) was impregnated with the epoxy resin formulation of Example 1 to obtain a prepreg with a resin content of 38%. The prepreg was used to make a 3 ply 0°/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 147° C. (see FIG. 5).

Example 8

(32) An epoxy resin formulation according to Table 4 below was prepared by blending the first three ingredients listed at 150° C. until all the Araldite® GT7071 had dissolved. Separately 1-(Cyanoethyl)-2-ethyl-4-methylimidazole and Succinic Acid were blended at 80° C. until all the Succinic Acid had dissolved.

(33) At 50° C. the two preblends were mixed and the remaining two ingredients were added and mixed until uniform.

(34) TABLE-US-00004 TABLE 4 Epoxy resin formulation Ingredient % Araldite EPN1138 61.3 Araldite LY1556 12.25 Araldite GT7071 14 Dyhard DF50EP 8.75 1-(Cyanoethyl)-2-ethyl-4-methylimidazole 2.65 Succinic Acid 0.95 WS17321A 0.1

(35) The resin formulation of Table 4 exhibited an EEW (based only on the epoxy components) of 201 g/mol. The resin was analysed by DSC, which demonstrated that the resin exhibits 97% cure conversion after 5 minutes at 150° C.; and 99% cure conversion after 5 minutes at 160° C.

Example 9

(36) A prepreg was prepared from the epoxy resin formulation of Example 8 as per Example 6 but using Example 8 resin formulation. The prepreg was used to make a 10-ply 0/90° laminate which was then subjected to a 3 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 155° C. (see FIG. 6).

Example 10

(37) A prepreg was prepared from the epoxy resin formulation of Example 8 as per

(38) Example 7 but using Example 8 resin formulation. The prepreg was used to make a 3 ply 0°/90° laminate which was then subjected to a 3 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 150° C. (see FIG. 7).

Example 11

(39) An epoxy resin formulation according to Table 5 below was prepared by blending the first three ingredients listed at 150° C. until all the Araldite® GT7071 had dissolved. At 50° C. the remaining ingredient was added and mixed until uniform.

(40) TABLE-US-00005 TABLE 5 Epoxy resin formulation Ingredient % Araldite EPN1138 48.5 Araldite LY1556 24.3 Araldite GT7071 24.3 Imicure EMI-24 Curing Agent 90% 2.9

(41) The resin formulation of Table 5 exhibited an EEW (based only on the epoxy components) of 218 g/mol. The resin was analysed by DSC, which demonstrated that the resin exhibits 97% cure conversion after 5 minutes at 150° C.; 98% cure conversion after 10 minutes at 150° C.; and 99% cure conversion after 5 minutes at 160° C.

Example 12

(42) A prepreg was prepared from the epoxy resin formulation of Example 11 as per Example 6 but using Example 11 resin formulation. The prepreg was used to make a 10-ply 0/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 149° C. (see FIG. 8).

Example 13

(43) A prepreg was prepared from the epoxy resin formulation of Example 11 as per Example 7 but using Example 11 resin formulation. The prepreg was used to make a 3 ply 0°/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 148° C. (see FIG. 9).

Example 14

(44) An epoxy resin formulation according to Table 6 below was prepared by blending the first two ingredients listed at 150° C. until all the Araldite® GT7071 had dissolved. At 50° C. the remaining ingredients were added and mixed until uniform.

(45) TABLE-US-00006 TABLE 6 Epoxy resin formulation Ingredient % Araldite LY1556 31.7 Araldite GT7071 40.4 Araldite MY9512 24.1 1-(Cyanoethyl)-2-ethyl-4-methylimidazole 3.8

(46) The resin formulation of Table 6 exhibited an EEW (based only on the epoxy components) of 221 g/mol. The resin was analysed by DSC, which demonstrated that the resin exhibits 99% cure conversion after 5 minutes at 150° C. and 99% cure conversion after 5 minutes at 160° C.

Example 15

(47) A prepreg was prepared from the epoxy resin formulation of Example 14 as per Example 6 but using Example 14 resin formulation. The prepreg was used to make a 10-ply 0/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 145° C. (see FIG. 10).

Example 16

(48) A prepreg was prepared from the epoxy resin formulation of Example 14 as per Example 7 but using Example 14 resin formulation. The prepreg was used to make a 3 ply 0°/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 146° C. (see FIG. 11).

Example 17

(49) An epoxy resin formulation according to Table 6 below was prepared by blending the first two ingredients listed at 150° C. until all the Araldite® GT7071 had dissolved. At 50° C. the remaining ingredients were added and mixed until uniform.

(50) TABLE-US-00007 TABLE 7 Epoxy resin formulation Ingredient % Araldite LY1556 48 Araldite ECN1273 40 Dyhard DF50EP 8.9 1-(Cyanoethyl)-2-ethyl-4-methylimidazole 3.3

(51) The resin formulation of Table 7 exhibited an EEW (based only on the epoxy components) of 234 g/mol. The resin was analysed by DSC, which demonstrated that the resin exhibits 94% cure conversion after 5 minutes at 150° C. and 97% cure conversion after 5 minutes at 160° C.

Example 18

(52) A prepreg was prepared from the epoxy resin formulation of Example 17 as per Example 6 but using Example 17 resin formulation. The prepreg was used to make a 10-ply 0/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 161° C. (see FIG. 12).

Example 19

(53) A prepreg was prepared from the epoxy resin formulation of Example 17 as per Example 7 but using Example 17 resin formulation The prepreg was used to make a 3 ply 0°/90° laminate which was then subjected to a 5 minute cure at a curing temperature of 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 154° C. (see FIG. 13).

Comparative Example 1

(54) The resin formulation presented in Table 8 below was exemplified in WO-2014/096435-A.

(55) TABLE-US-00008 TABLE 8 Compound wt % Description Epoxy resin formulation 77.5 See below DICY 18.0 50% Dicyandiamide in 50% Bisphenol-A epoxy resin Dyhard UR505 4.5 bis urea accelerator Composition Epoxy resin formulation Phenoxy resin 3.9 YP50 supplied by Kukdo Bisphenol-A epoxy resin 59.4 EEW 320, 2-functional Epoxy phenyl novolac, 35.6 EEW 180, 3.6 functional YD PN 638 100.00

(56) The present inventors faithfully reworked the above resin formulation, using the following replacement commercially available components Araldite® EPN1138 for the Epoxy Phenyl novolac, with an EEW of 180 and a functionality of 3.6. A blend of Araldite® LY1556 and Araldite® GT7071 for the Bisphenol-A epoxy resin, with an EEW of 320 and a functionality of 2. Dyhard® DF50EP was used as the 50% dicyandiamide dispersed in a 50% Bisphenol-A epoxy resin Dyhard® UR505 as listed

(57) A prepreg was prepared from this epoxy resin formulation along using the same 200 gsm 3k carbon fibre fabric as that used in Example 2. The prepreg had a resin content of 38%. This prepreg was used to make a 10 ply laminate which was then subjected to a 5 minute cure at 150° C. The curing was performed in a steel-matched die tool that was pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 109° C. (see FIG. 14).

Comparative Example 2

(58) The epoxy resin formulation of Comparative Example 1 was used to prepare a pre-preg with the same 660 gsm 12k carbon fibre fabric as that used in example 3. The pre-preg had a resin content of 38%. This prepreg was used to make a 3-ply laminate which was then subjected to a 5 minute cure at 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 96° C. (see FIG. 15).

(59) The Tg values of the cured pre-pregs of Examples 2 and 3 exhibit a difference of less than 3° C., demonstrating that the resin formulations of the present invention exhibit excellent Tg development regardless of the type of fibrous reinforcement. In particular, there is very little reduction in Tg when the pre-preg comprises a heavy “industrial-grade” fabric. In contrast, the Tg value of the cured pre-preg of Comparative Example 2 exhibits a Tg reduction of 13° C. with the heavier “industrial-grade” fabric, relative to the lighter fabric of Comparative Example 1, demonstrating the inferior Tg development resulting from curative filtration. As noted hereinabove, it is important to ensure the proper development of Tg in the curing process. An inferior Tg is disadvantageous because it limits the suitability of the moulded article in applications where the article is expected to work at elevated temperatures; and also increases the likelihood that the moulded part becomes distorted when removing it from the hot mould.

Comparative Example 3

(60) Table 9 below describes a resin formulation exemplified as “composition 7” in EP-1279688-B1.

(61) TABLE-US-00009 TABLE 9 Composition 7 Epoxy resin A (EEW = 176) parts 76.4 Epoxy resin B (EEW = 1200-1400) parts 23.6 PVF.sup.a parts 3.4 DICY parts 5.0 OMICURE ® U-24.sup.b parts 4.2 Total parts 112.6 Gel time (min.) at 130° C. 3.9 .sup.aThermoplastic PVF powder (Vinylek ™ type K) .sup.bOMICURE ® U-24 is 2,4-toluene bis dimethyl urea

(62) The composition 7 of EP-1279688-B1 was reworked as faithfully as possible using the following replacement commercially available components: Araldite® LY1556 was used as a direct alternative to Epon®828 for Epoxy resin A A blend of Bisphenol-A epoxy resins Araldite® GT6099 (average EEW=2631) and Araldite® GT7071 (average EEW=512) were blended at a ratio of 3:1 to give an the desired EEW to be used as an alternative to Epoxy resin B. Dyhard® DF50EP was used to provide the required loading of dicyandiamide (dicy) which is carried in bisphenol-A type epoxy resin at a concentration of 50%. Omicure U24 as listed

(63) A prepreg was prepared from this epoxy resin formulation along using the same 200 gsm 3k carbon fibre fabric as that used in Example 2. The prepreg had a resin content of 38%. This prepreg was used to make a 10 ply laminate which was then subjected to a 3 minute cure at 150° C. The curing was performed in a steel-matched die tool that was pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07 in the same way as Example 2, and found to exhibit a Tg of about 111° C. (see FIG. 16).

Comparative Example 4

(64) The epoxy resin formulation of Comparative Example 3 was used to prepare a pre-preg with the same 660 gsm 12k carbon fibre fabric as that used in Example 3. The pre-preg had a resin content of 38%. This prepreg was used to make a 3-ply laminate which was then subjected to a 3 minute cure at 150° C. The curing was performed in a steel-matched die tool pre-heated to the curing temperature. The resulting laminate was tested by DMA following ASTM 7028-07, and found to exhibit a Tg of about 58° C. (see FIG. 17).

(65) As noted above, the Tg values of the cured pre-pregs of Examples 2 and 3 exhibit a difference of less than 3° C., demonstrating that the resin formulations of the present invention exhibit excellent Tg development regardless of the type of fibrous reinforcement. In particular, there is very little reduction in Tg when the pre-preg comprises a heavy “industrial-grade” fabric. In contrast, the Tg value of the cured pre-preg of Comparative Example 4 exhibits a Tg reduction of 53° C. with the heavier “industrial-grade” fabric, relative to the lighter fabric of Comparative Example 3, demonstrating the inferior Tg development resulting from curative filtration. As noted hereinabove, it is important to ensure the proper development of Tg in the curing process.