GB2531051A

GB2531051A - Carbon fibre-containing prepregs

Info

Publication number: GB2531051A
Application number: GB1417877.6A
Authority: GB
Inventors: John Spencer Paul; Edward Creaser Benjamin
Original assignee: Gurit UK Ltd
Current assignee: Gurit UK Ltd
Priority date: 2014-10-09
Filing date: 2014-10-09
Publication date: 2016-04-13
Anticipated expiration: 2034-10-09
Also published as: EP3204445A1; US20170306110A1; US10259917B2; WO2016055581A1; EP3204445B1; GB201417877D0; GB2531051B

Abstract

A prepreg comprises at least one layer of carbon fibres and a curable thermosetting resin system, the curable thermosetting resin system at least partly impregnating the at least one layer of carbon fibres, wherein the curable thermosetting resin system comprises: a curable thermosetting resin including at least one epoxide group, the curable thermosetting resin having an epoxy equivalent weight of from 140 to 180 g/eq; a curing agent for curing the curable thermosetting resin, wherein the curing agent is present in the liquid phase and includes a cyanamide reactive group; and a rheology modifier for the curable thermosetting resin system, wherein the rheology modifier comprises at least one of a thermoplastic resin and an inorganic particulate thickener or a mixture thereof. The preferred at thermosetting resin is a mixture of a tetrafunctional epoxy resin (epoxy equivalent 105-125 g/eq), a first epoxy novolac resin (epoxy equivalent 190-235 g/eq) and a second epoxy novolac resin (epoxy equivalent 1705-190 g/eq). The use of a curing agent which stays in the liquid phase a yields a set resin without visible particles which can ruin the visual appeal of the set product.

Description

The present invention relates to prepregs comprising at least cn.e layer of carbon fibres and a curable thermosetting resin system. to a method of producing carbon fibre reinforced resin matrix composite material and to the use of a curing system in a prepreg comprising at least one layer of carbon fibres and a curable thermosetting resin system.

Carbon fibre composite materials are commonly employed in high cost! high performance applications where lightweight structures are required. For many applications, coupled with the high mechanics! perIhnnance of carbon fibre composite materials is the unique visual appearance of cation fibre composite laminates, in particular those using woven cathon fabrics. Such an aesthetic appearance of carbon fibres is highly desirable, and carbon fibre composite materials find, many applications in high-value markets such as automotive components, luxury yachts and consumer electronics where the visual appearance of the carbon frbres as well as the technical perfonixance of the composite material is important to the user/consumer. For these so-called "cosmetic" applications of carbon fibre composite materials, the visual aspect of the cured laminate needs to he of high quality and free from imperfections such as discontinuous fIbres, particle contamination and surthce effects, for example pin holes, fish eyes, blisters, etc..

Historically for the manufacture of cosmetic quality laminates, carbon fibre prepregs are used, The prèpregs comprise at least one layer of carbon fibres and a curable thermosetting resin system, typically art epoxy resin. The iherinosetting resin in these prepregs is tyDicaily cured using dicyandiamide-based catalysis. Dicyandiamirle is added as a powder to the curable resin.

The soluhility of dicyandiamide in commonly employed resin chemistries is generally poor.

i'hetefore, after cure, residual particles of dicyandiamide are frequently observed in areas of high resin content. To reduce this effect, the prepreg user is required to use low resin content prepregs and careffil processing. These both introduce disadvantageous aspects to the component manufacturc and do not fully mitigate the defects described.

Whilst ii has been historically possible to employ liquid curing compounds such as imidazoles in order to achieve clear cured resins, such curing compounds have technical disadvantages such as short latency (leading to significantly reduced shelf Life of the uncured prepreg), increased cost, and. depending on the chemistry used, often reduced thermal-mechanical properties and panel clarity, exhibited by excessive resin colour and. opacity.

I'here is therefore currently a need in the art for a carbon fibre prepreg which includes a thercnosetting resin system which not only provides a high quality processing, with a high latency of the curing system and a controlled curing, hut also provides a combination of high quality thermoinechanical properties and visual appearance.

WO-A2Ol 2/113878 and WOA20i 2./113878 disclose liquid curing agents for thermosetting resins.

Ihe present invention aims to provide a carbon fibre prepreg which can readily he used in "cosmetic" applications and which can exhibit highly clear cured resin matrices, allowing a high quality visual appearance of the carbon fibres, with zero visible catalyst particles.

The present invention aims to provide a carbon fibre prepreg which can provide a high quality visual appearance of the carbon flbres, combined with good resin processing and high latency.

The present invention accordingly provides a prepreg comprising at least one layer of carbon fibres and a curable therniosetting resin system, the curable thermosetting resin system at least partly impregnating the at least one layer of carbon fibres, wherein the curable thermosetting resin system comprises: a. a curable thermosetting resin including at least one epoxide group, the curaible thennosetting resin having, an epoxy equivalent weight of from 140 to 180 g/eq; h. a curing agent for curing the curable thermosetting resin, wherein the curing agent is present in the liquid phase and includes a cyanamide reactive group; and c. a rheology modifier for the curable thermosetting resin system, wherein the rheology modifier comprises at least one of a thermoplastic resin and an inorganic particulate thickener or a mixture thereof.

The present invention fl.irther provides a. prepreg comprising at least one layer of carbon fibres and a curahke thermosetting resin system, the curable thermosetting resin system at least partly impregnating the at least one layer of carbon fibres, wherein the curable thennosetting resin system comprises: a. a curable thermosetting resin including at least one epoxide group, the curable thermosetting resin having an epoxy equivalent weight of from 140 to 180 g/eq; a curing agent for curing the curable therrn.osetting resin, wherein the curing agent includes a cyanamide constituent which is presen.t in the liquid phase; and wherein the curing agent is formulated to remain in the liquid phase prior to curing of the themioseuing resin so as to avoid the presence of particles of curing agent as a separate phase within the cured thetmoset resin system, and wherein the thernosetting resin system is formulated w have a minimum viscosity during curing minimum viscosity at a temperature of from 100 to 120 °C, oitionaliy from 105 to 115 °C. further optionally about 110 °C.

Th.e present invention further provides a method of producing carbon fibre reinfhrced resin matrix compostie material, the method comprising the steps of: a. providing a laminated stack of prepregs according to the prcsent invention; and b. heating up the laminated stack of preregs to cause the curable therruosetting resin system to flow and fully impregnate the carbon fibres and to cure to form a cured thermoset resin matrix The present invention further provides the use, in a prepreg comprising at least one layer of carbon fibres and a curable thermosetting resin system, the curable thermosetting resin system at least partly impregnating the at least one layer of carbon fibres and comprising a curable thermosetting resin including at least one epoxide group, the curable thermosetting resin having an epoxy equivalent weight of from 140 to 180 g/eq, of a curing system comprising a liquid phase curing agent which includes a cyanamide reactive group for voiding a visible residue of particles of curing agent in a carbon fibre reinfurced resin matrix composite material produced from the prepreg.

Preferred or optional faatures are defined in the respective dependent claims.

The present invention is at least partly predicated on the finding by the present inventors that using an afternative catályst'curing compound to dicyandiamide, in the form of a liquid curing agent with similar chemical functiona' groups, together with a curable thennosetting resin including at least one epoxide group, the curable therrnosetting resin having an epoxy equivalent weight of from 140 to 1St) g/eq, it is possible to formulate a prepreg resin system with similar handling, curing and thermaI-mechanicai properties as dicyandiarnide based prepregs, yet remove or avoid any presence of insoluble particles, thereby significantly improving the surface finish of the resultant cured. composite material components.

Furthermore, the cured resin can have a high glass transition temperature, Tg, typically above °C. for example about 179 °C Such high Tg cured composite material components are suitable for the manufacture of components which are subjected to high temperature either during subsequent manufacturing processes or during the product lifetime. For example, vehicle body panels formed from the cured composite material components may need to he processed though a high temperature varnish/paint line, and it is essential that the Tg of the cured resin is higher than the processing temperature otherwise the vehicle body panel would become deformed The present invention has particular application to the bnriulation of carbon fibre prepregs where the tinal cured laminate is desired to have a high quality surface fnish, for example tbr unpainted carbon fibre finish components. These are common place in highvalue consumer applications such as niche cars, luxury yachts, and highend consumer electronics The prepregs of the preferred embodiments of the present provide a number of technical advantages over known carbon fibre prepregs, and are formulated to provide not only enhanced mechanical properties but also aesthetic properties as a result of the carbon fibres being visible to the consumer/user through a clear resin rnamx.

in particular, the processing of carbon fibre prepregs of the preferred embodiments can be less sensitive to resin content, tool design and laminate processing as compared to known prepregs.

As compared to the use of particulate dicyandiamide curing agent, the "scrap" rate of cosmetic components due to presence of visihk particles can he reduced to zero, which provides a significant cost saving to the manufacture of the composite material components Higher resin content prepregs can he used, if required, as compared to the resin content which could be used thr cosmetic carbon products incorporating particulate dicyandiamide curing agent, the latter tynically and conventionally having a resin contentkept low, at typically c40% by weight relative to the total prepreg weight, to ensure that the presence of any insoluhe curing agent particles was minimised.

The prepreg of the preferred embodiments of the invention is more tolerant to varied processing and cure temperatures than hen using particulate dicyandia.mide curing agent. Typically and conventionally the cure profile of prepregs comprising particulate di.cyandiarnide curing agent for cosmetic applications needed to he carefully controlled to ensure optimal dissolution of any catalyst or accelerator particles present in the resin matrix. The combined epoxide resinicuring agent system employed in the present invention can permit readily controllable curing eondthons to be implemented, ensuring the reliable industrial production of composite materials which exhibit the combination of high quality mechanical and esthetic properties.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a graph showing the relationship between viscosity and time for a thermosetting resin system according to an Example of the present invention; and Figure 2 is a graph showing the relationship between viscosity and temperature fbi.

themiosetting resin rystern according to an Example of the present invention.

in accordance with preferred embodiments of ci e present invention, there is provided a prenreg.

The prepreg comprises at least one layer of carbon fibres and a curable thermosetting resin system.

The carbon fibres may be present in any form known to those skilled in the art, and typically are present in the torin of a woven fabric. Various weave patterns and fabric weights may he employed, in accordance with the common general knowledge of the skilled person. The carbon fibres may have a uniaxial, biaxia or multiaxial fibre oriencation. as is well known to those skilled in the art.

The curable thermosetting resin system at least partly impregnates the at least one layer of carbon fibres. Typically, the curable thermosetting resin system fully impregnates the at least one layer of carbon fibres. Alternatively, a "semi-preg" prepreg s'trucwre may he provided, in which a layer of the resin is adjacent to a layer of carbon fibres. In either prepreg constnjction, during the curing step the resin is heated and is consequently lowered in viscosity so as to flow and thily to wet out the fibres and form, a coherent resin matrix surrounding the fibres prior to curing of the resin.

The curable thennosetting resin system of the preferred embodiments comprises four components: a. a curable thermosetting resin inchiding at least one epoxide group, the curable thernosetting resin having an epoxy equivalent weight of from 140 to 180 g/eq 1. a curing agent for curing the curaNe thermosetting resin, wherein the curing agent is present in the liquid phase and includes a cyanamide reactive group; and c. a rheology modifier for the curable thermosetting resin system, wherein the rheology modifier comprises at least one of a thermoplastic resin and an inorganic particulate thickener or a mixture thereof.

As discussed below, in certain embodiments the rheology modifier may be omitted.

The curable thermosetting resin is typically selected from at least one of an epoxy resin, an epoxy novolac resin, an epoxy cresol novolac resin and an epoxy phenol novolac resin, or a mixture of any two or more thereof. Typically, the curable thermosetting resin is a mixture of at least one tetraftmnctional ox*resin and at least one epoxy novolac resin, The curable therrnosetting resin is preferably a mixture, or blend, of from 30 to 60 wt% of the at east one tetrafi.mnctional epoxy resin and from 70 to 30 wt% of the at least one epoxy novolac resin, each based on the weight of the curable thermosetlirng resin, More preferably, the at least one tetraflinctional epoxy resin has an epoxy equivalent weight of from 105 to 125 g/eq and the at least one epoxy novolac resin comprises a first epoxy novolac resin which has an epoxy equvaient weight of from 190 to 235 g/eq and a second epoxy novolac resin which has an epoxy equivalent weight of from 175 to 190 g/eq. Typically, the first and second. epoxy novolac resins are each present in an amount of from 25 to 30 wt%, each based on the weight of the curable thermosetting resin. the mixture or blend can provide an overall epoxy equivalent weight of from 140 to 180 gleq.

The epoxy resin, and the associated curing agent and accelerator, may be formulated so that the epoxy resin is to be cured at various curing temperatures. A typical curing temperature is °C, although other curing temperatures may he employed. Ihe cured epoxy resin may be formulated to have a desired glass transition temperature, 1g. as known to those skilled in the art, Typically, the epoxy resin has a Tg of from 160 to 190 °C, typicaUy from 175 to 180 °C, for example bouL 179 00, The curable thennosetting resin is typically blended with the rheology modifier, although in some embodiments no such rheology modifier is present in the curable thermosetting resin system. As stated above, the rheology modifier typically comprises at least one of a thermoplastic resin and an inorganic particulate thickener or a mixture thereof The rheology modifier provides the required drape, handhng properties and mechanical properties for the particular application of the prepreg to be moulded. in sonic embodiments, the drape properties niav he provided by the curable thermosetting resin, thereby obviating the need for a rheology modifier.

Typically, when the rheology modifier is a thermoplastic resin, the rheology modifier includes at least one of a phenoxy resin, an acrylic resin, a polyacrylate resin, a polyacrylomutrile resin, a poiyetherimide resin, a polyketone resin and a polysuiphone resin, or a mixture of any two or more thereof Most typically, the thermoplastic resin is a. phenoxy resin, such as a phenoxy resin available in powder form under the trade name InCheniRez PKT'IP-200 available in commerce from InChem Corporation. Typically the thermoplastic resin of the rheology modifier has a softening point within the range of from 170 to 190 °C, further optionally from to I 85CC, yet rther optionally about 180 C. fOr example as provided by the lnChemRez PKF1P200 product.

The weight ratio of the curable thermosetting resin and the thermoplastic resin of the rhe&ogy modifier is controlled so as to provide the desired drape, handling and mechanical properties to the prepreg. For example, the resin tortion of the curable therinosetting resin system, namely the curahe thermosetting resin and the thermoplastic resin of the rhe&ogy mouifier, may typically include about 90w&% of the base therrnosetting resin, for example epoxy resin.

Typically, the curable therinosetting resin and the thermoplastic resin of the rheology modifier are present in a weight ratio of from 80:20 to 99:1, optionally from 85:15 to 95:5, for example about 90:10.

As well as or instead of a. thennoplastic resin rheology modifier, the rheology modifier may comprise an inorganic particulate thickener, such as fumed silica, Typically, the fumed silica is present in an amount of from 0.5 to I wt%, based on the weight of the curable thermosetting resin system, The rheology modifier may optionally further comprise at least one polyhydroxycarboxylic acid amide as a dispersion and wetting aid fix the fumed silica, the at least one p6lyhydroxycarboxyiic acid arnide being present in an amount of from 10 to 40 wt% of the weight of the fumed silica.

Other inorganic narticulate thickeners are known in the art and may be employed in the invention.

in each embodiment, the composition and amount of the rheology modifier(s) may readily he selected based on the desired drape properties of the prepreg during layup of the prepreg into a mould, and also selected based on the desired flow properties of the curable therrnosetting resin system during the coring step at elevated temperature.

The curing agent used in accordance with the present invention is in the form of a liquid and so has no particles. Furthermore, the curing agent is reactive enough to cure within a desired cure schedule, defined by time and temperature. The curing agent also has latency to provide storage stability tin the prepreg material.

For achieving the desired properties of the cured thermoset resin, the curing agent should be used in a suitable ratio together with the curable thermoset epoxide-containing resin as discussed above. The active hydrogen equivalent weight of the curing agent and. the epoxy equivalent weight of the selected resin are used to detennirie the ideal mix ratio, as known to those skilled in the art. The mix ratio and the components are selected in order to ensure that the material cures within the desired cure schedule and also in order to control the resin flow properties prior to resin gelation during prcpreg resin curing to form the composite material dud ug manufacture.

There are numerous other liquid curing agents that could he used for the purpose of the curing the resin formulation. These include many amines arid imidazoles. 1-lowever, these result in a number of drawbacks for the required application, meaning that they were not selected for use within the resin formulation according to the preferred embodiments of the present invention.

For example, many amines and irnidazoles exhibit poor shelf-life when mixed with resin.

Imidazoles also commoffly result in final products that are very dark in colour, which would he detrimental within a visual quality product. They also frequently cause a reduction in mechanical properties when compared with other curing agents.

Typically, the curing agent comprises cyanarnide of formula NC-NI-li. The cyanamide curing agent may be provided as a liquid curing agent commercially available under the trade name of Dyhard VP Ill by AlzChem AG, Germany. Typically the curing agent is present in an amount offiom (ito 13 wt% based on the weight of the curable thermosetting resin system Typically, in the prepreg the curable thermosetting resin system comprises from 35 to 45 wt% of at least one tetrafunctional epoxy resin which has an epoxy equivalent weight of from 105 t.o 125 glen, from 20 to 30 wt% of a first epoxy novolac resin which has an epoxy equivalent weight of from 190 to 235 g/eq, from 20 to 30 wt% of a second epoxy novolac resin which has an epoxy equivalent weight of from 175 to 190 g.1eq, from 50 to 15 v% of a phenoxy resin, from 8 to 10 wt% of a liquid phase curing agent which includes a eyanamide reactive group, from 0.5 to I wt% of fumed silica and from Di to 0.3 wt% of a polyhydroxycarboxylic acid amide as a dispersion and wetting aid for the fumed silica, the amounts being based upon the weight of the curable thermosetting resin system.

By selecting the curing agent and its amount and the specific epoxide resin component and its amount, during resin curing, resin flow during the temperature ramp stage of the cure cycle prior to gelation can he readily controlled. Furthermore, the curing can he controlled over a range of cure schedules. In addition, the properties of the final cured epoxy resin in the composite material can be controlled.

For the curing agent within the preferred formulation, it is desirable that it is latent at ambient temperatures, thus allowing good shelf4ife of the mixed., uncured product under ambient condtjons.

When fonnuiating the curable thermosetting resin system, the base epoxide-containing resin and the thermoplastic resin rheology modifier are blended to form the overall resin constituent of the ihrmuiation and the curing agent and inorganic particulate thickener, together with any wetting/dispersion aid, are mixed together to form the catalyst paste. ftc catalyst paste can be either with or without a can-icr. Typically, no carrier is required. The overall resin constituent and the catalyst paste are mixed together in the desired ratio prior to impregnation of the carbon fibres during manufacture of the prepreg. l'he curable thermosetting resin composition is permitted to partly cure to the desired B-stage so as to provide the required drape properties of the resin prior to use in a moulding process to form a composite material. The moulding process is typically conducted under a negative pressure provided by a vacuum, but alternatively may he conducted under an elevated positive pressure in an autoclave.

The present invention further provides a method of producing carbon fibre reinfbrced resin matrix composite material for the prepregs of the invention.

In the method, a laminated stack of prepregs is provided. The stack is typically laid up in or on a mould, so that the resultant composite material has the desired shape, configuration and dimensions. Then the stack of prepregs is the typically subjected to vacuum moulding, in which the prepreg stack is maintained in a vacuum throughout the curing cycle. The vacuum removes interlaminar and intralarninar air so as to reduce the void volume of the resultant moulded composite material product. Alternatively, an autoclave moulding process may he icr employed. Still ifirther, the composite materird product may be moulded by press moulding the multilaminar stack of prepregs.

Each prepreg comprises at least one layer of carbon fibres and a curable therroosetting resin system, The curable thermosetting resin system at kast partly impregnates the at least one layer of carbon fibres. The curable thermosetting resin system is described above, As discussed above, a rheology modifier may optionally he present.

In the mould, the laminated stack of prepregs is heated up to cause the curable themrosetting resin system to flow and filly impregnate the carbon fibres and to cure to form a cured thermoset resin matrix.

The epoxide-containing resin system and the curing agent are formulatcd to remain in the liquid phase prior to curing of the thermosettirkg resin so as to avoid the presence ofparticles of curing agent as a separate phase within the cured therinoset resin system. In addition, the thermosetting resin system is formu'ated to have a minimum viscosity during curing wherein the curing agent is formulated to remain in the liquid phase prior to curing of the thermosetting resin so as to avoid the presence of particles of curing agent as a separate phase within the cured thermoset resin system, and wherein the thermosetting resin system is formulated to have a minimum viscosity at a temperature of from 100 to 120 °C, optionally from 105 to 115 °C, ibrther optionally about 110 °C.

Typically, the thermosetting resin system is formulated to have a i ininiuni viscosity of from 3 to 6 Pa,s at a temperature of from 105 to 115 °C, frr example a udnimum viscosity of from 4 to 5 Pa.s at a temperamre of from 107 to 112 °C. For the heological testing, in this specification the viscosity is measured using the following parameters: TA instruments AR2000EX, instrument in oscillation, 30lS0 °C at I °C./min, controlled strain of ii %.

frequency of 1 Hz, Typically, the thermosetting resin system is forrnuated to have curing reactivity so that the viscosity of the themiosetting resin system is above 10000 Pa,s at a temperature of from 123 t.o 129 °C.

Optionally, during the heating step the laminated stack of prepregs is heated from ambient temperature (typically 20 °C) up to a curing temperature of from 170 to 190 C at a ramp rate of from 03 to 2 °C/rninute and held at the curing temperature for a period of at least 30 minutes.

Typically, during the heating step the laminated stack of prepregs is heated from ambient temperature up to a curing temperature of from 175 to 185 °C at a ramp rate of from 0.75 to I 3 C/j and held at the curing temperature for a period of at least 30 minutes. Optionally, during the heating step the curable thernosetting resin has a viscosity of from 3 tc 6 Pa.s at a temperature of from 105 to LI 5 °C.

The present invention wifl now be illustrated in greater detail with reference to the following non..hmiting Example. ExaSj

In Examp'e 1, the catalyst paste comprised 90.5 wt% Dyhard Fluid VP1 11 curing agent, 7.5 wt% fiwned silica (a commercially available thmed silica sold as Cábosil MS by Cabot Corporation, USA) and 2 wt% of a poiyhydroxycarhoxy[ic acid amide as a dispersion and wetting aid for the fumed silica (commercially availahe as BYKR 605 from BYKChemie GmbH, Gennany).

ftc catalyst paste was combined with the base resin portion, comprising the epox.y resin and thermoplastic resin rheology modifier, to provide a mix ratio of 100 parts by weight base resin /rheology modifier blend to 10 parts catalyst paste.

The base resin portion comprised a blend of 40 wt% of &ikote 496 (commercially available from Hexion Speciality Chemicals GmbH., Geimany), a tetrafunctional epoxy resin which has an epoxy equivalent weight of 114.9 +/ 4.5 g/eq, 25 wt% of EPON Resin SU*8 (commercially available from Momentive Speciality Chemicals lnc, USA), an epoxy novolac resin which has an epoxy equivalent weight of from 195 to 230 gieq, 25 wt% of D,E.N, 438 (commercially available from The Dow Chemical Company. USA), an epoxy novolac resin which has an epoxy equivalent weight of from 176 to 181 g'eq, and 10 wt% of a phenoxy thermoplastic resin available in commerce from InChem Corporation. under the trade name PKHP200. The weight percentage values are with respect to the base resin portion. The blend had an overall epoxy equivalent weight within the range of from 140 to 180 g/eq.

The resin blend and the catalyst paste were mixed to form the curable epoxy resin composition for incorporation into a carbon fibre prepreg.

The curable resin was subjected to a curing schedule. which simulated a curing schedule to be used for making moulded composite material products using the prepregs, of heating from ambient temperature at a ramp rate of I CC/mm to a curing temperature of 1 80 C and holding at 180 °C for a period of 30 minutes. it was found that there was satisfactory curing within the required time frame, at the required temperatures and also resulted in satisfactory mechanical properties of the resultant cured resin.

Figure 1 shows the relationship between viscosity (yaxis) and time (xaxi.s) during the curing schedule. Figure 2 shows the relationship between viscosity (yaxis) and temperature (xaxis) during the curing schedule.

it may be seen from Figures 1 and 2 that as the temperature is increased, the viscosity of the resin decreases. The reduced resin viscosity permits the resin to flow and fully wet out and impregnate the prepreg carbon fibres. A viscosity minimum was achieved after a period of about 80 minutes at a temperature of about 110 °C, The minimum viscosity was about 4.5 Pa.s.

In other preferred examples. the viscosity minimum may he achieved at 95 to 105 tC and the minimum viscosity may be 6 to 18 Pa,s, using a similar cure schedule to that used in Example 1.

After the viscosity minimum was achieved, the viscosity rapidly increased, which indicates that the resin is rapidly curing. The rapid cure ensures that the mecha.nica properties of the resultant cured resin are enhanced. The viscosity increased to at least 10000 Pa.s at 126 CC. in other preferred examples, the viscosity may be increased to at least 10000 Pa,s in the range of 123 to 129°C The rnunmurn viscosity of the mixed resin/catalyst paste system was sufficiently high to allow the resin to flow less during the early stages of the cure, which would result in less resin being lost from the prepreg. This, in ttìni, improves both the quality of the cured product and reduces the wasted resin.

In comhinaton therefore, the resin system provided- desired rapid cure schedule, to enable the product to cure as required. and resulted in both optimum wet-out/bleed and cured mechanical properties, yet avoiding any visible white residue in the cured resin. The aesthetic appearance of the cured resin was excellent, with high clarity. Furthermore, the cured resin had ahigh Tg of 179°C The resultant carbon fibre prepreg is suitable thrmanufacturing composite material parts for high quality cosmetic applications.

Various modifications to the preferred embodiments of the present invention and to the Example-of the present invention will readily be apparent to those skilled in the art and are encompassed within the scope of the present invention,