WO2014001537A1

WO2014001537A1 - Prepregs for manufacturing composite materials

Info

Publication number: WO2014001537A1
Application number: PCT/EP2013/063700
Authority: WO
Inventors: Ben CREASER; Paul Spencer
Original assignee: Gurit (Uk) Ltd
Priority date: 2012-06-29
Filing date: 2013-06-28
Publication date: 2014-01-03
Also published as: GB201211576D0; GB2503503A; GB2503503B

Abstract

A prepreg comprising a fibrous material contacting a polymerisable epoxide resin system, the epoxide resin system comprising at least one epoxide resin and a curing agent for the at least one epoxide resin, the curing agent comprising dicyandiamide having a particle size d(0.5) of from 1 to 15 microns in an amount of from 0.75K to less than 3.3K parts per hundred by weight based on the weight of the at least one epoxide resin wherein K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

Description

PREPREGS FOR MANUFACTURING COMPOSITE MATERIALS

The present invention relates to a prepreg for manufacturing a composite material. The present invention also relates to a method of producing a moulded fibre-reinforced composite material using such a prepreg.

Glass and carbon fibre reinforced epoxy composites offer excellent thermal and mechanical properties. They are commonly offered in "pre-impregnated" formats, in which fibres and resin are pre-combined, such as conventional fully or partially impregnated "prepregs" and a particular dry fibre layer/resin layer prepreg format disclosed in EP-A- 1128958 and sold by Gurit (UK) Limited under the registered trade mark SPRINT. Such prepregs allow convenient manufacture of laminates.

One of the constraining factors with using epoxide systems is that the heat of reaction during cure is highly exothermic. This limits the laminate thickness that is possible to avoid thermal degradation or distortion of both the cured article and the mould or tool used in the article's manufacture. There is therefore a desire to reduce the heat release associated with curing to enable the construction of larger laminates. In doing so this offers opportunities to use composite materials in a wider range of applications. And realise larger structures than previously possible.

Another advantage of lower heat release is that components can be cured more rapidly without reaching the maximum temperature limit of the tooling system. This offers reduced manufacturing times for components bringing cost-savings and increased productivity to the end-user of the material.

The term "prepreg" is generally recognised in the art of fibre-reinforced resin composite materials to describe a blend of continuous high strength fibres (e.g. of carbon fibre, glass fibre, or other known fibre materials) combined with a heat hardenable mixture of resins, in particular thermoset resins, and, where required, hardeners. The fibres may originally be present either as woven fabrics or optionally angled directional fibre arrays which have the resin applied to them in a solid or semi-solid state. The degree and nature of the impregnation of resin, and hardener, into the fibres may vary. The resin, and hardener, may selectively be fully impregnated into the fibres; coated onto one side onto the fibres; partially impregnated into the fibres; or sandwiched between opposed dry fibre layers such that the outer surfaces of the prepreg are free of resin, as disclosed in EP-A-1 128958. Such prepregs having opposed dry fibre outer surfaces are easy and clean to cut, stack and react to give a low void content and optimum performance for the fibre and resin materials used in them.

Prepregs can be readily distinguished by those skilled in the art from the manufacture of composite materials made directly from continuous fabrics or discontinuous fibres and liquid resins applied by brush, roller, spray or any other similar method to produce low fibre content "wet lay-up" products. These have an important role in composite manufacture but generally have less than optimum properties with lower fibre contents than are necessary for applications needing the highest possible performance. The liquid resin materials are usually undesirably sticky, difficult to control accurately, and because strongly smelling volatile reactive diluents are often used, require continuous high levels of cleanliness and expensive extraction and recovery facilities in the workplace.

Prepregs can also be readily distinguished by those skilled in the art from the SMC (Sheet Moulding Compounds) or DMC (Dough Moulding Compounds) which are rapid processing materials, in sheet or dough like form, using discontinuous or random fibres and large amounts of mineral fillers combined with fast curing resins. These are cured quickly in relatively thin section between metal moulds to make tough, thin walled cases for many applications including electronic equipment and the like. They are very useful materials in the applications they fulfil but cannot be considered in any physical sense optimised structural composites.

Where necessary to prevent adjacent prepreg surfaces from inadvertently adhering to themselves when presented to the customer for use, or to prevent contamination in the workshop, they may be interleaved on one or both sides with a polyethylene film or alternative release materials.

Typically, prepregs may have nearly or exactly the correct amount of resin in them, matched to the respective fibre content. Once air has been removed from a prepreg assembly by the application of a vacuum, the resin flows under the influence of heat and pressure to fill all the spaces between the fibres. After a heat reaction, called the "curing process" for the resin, the prepreg assembly yields a near or completely void free laminate, the desideratum in a composite fibre-reinforced resin laminate.

Where excess resin is present this must be removed by a variety of techniques well understood by composite material processors to yield void free laminates but in general this is to be avoided where possible as it involves ancillary materials, labour and extra cost.

Such high strength composites have become increasingly used in a wide variety of applications in general industry since their debut in aerospace and some sports goods applications in the early 1970s.

As the applications and volumes of prepregs have multiplied, the fibres and resins from which they have been made have been modified to make them easier and cheaper to buy and use to maximise the performance and volume of products that can beneficially made from them.

However, the composite materials industry has now reached a point where further significant improvements to these fibre and/or resin materials needs to be made so that they may be processed more readily, speedily and cheaply to widen the range of items that can benefit from their properties and can be produced from them more economically.

Aerospace structural composite parts are frequently made from prepregs that are based on resins offering high glass transition temperatures (Tgs) to give large margins of safety should they be exposed to high temperatures, or to very high humidity for long periods of time leading to water saturation of the resins and a lowering of these Tgs but still to acceptable levels. Consequently they tend to use formulations with a high degree of cross linking which results from using resins with a high reactive group content and a consequent very high heat evolution during cure. This heat evolution must be rigorously controlled by careful processing to avoid excessive temperature rise or damage will result to the composite part.

This level of cross linking with aerospace structural prepregs leads to brittleness which is reduced by the incorporation of significant levels of thermally resistant thermoplastics which in itself leads to high melt viscosities and the need then for high processing pressures. The resins and processes employed tend to be very expensive. For general structural composites, the current state of the art performance prepregs and composite materials made from them, excluding aerospace primary structural parts, largely consist of glass, carbon and aramid fibres in any required combination, usually impregnated with a blend of solid and liquid Bisphenol A epoxy resins of relatively low molecular weight plus a hardener system which only reacts very slowly at room temperatures giving a storage life of several weeks without significant reaction. This hardener is usually a mixture of finely divided dicyandiamide coupled with a latent urea accelerator. These resin combinations will normally give a substantially full cure after reaction at around 120 - 130°C for 1 hour or 12 to 16 hours if cured at 90°C.

These resins are used because they give excellent composite mechanical properties for applications requiring temperature resistance up to the region of 120°C. Most applications in general industry rarely need their best performance above 80°C - 90°C. Current applications include wind turbine blades, leisure and commercial marine, automotive body panels, less critical exterior and most interior aerospace applications, sports goods and the like.

Examples of commercially available prepregs which use lower molecular weight bisphenol and similar epoxy resins and have lower cross link densities than the structural aerospace materials, and typically have a thermal resistance of 100°C - 120°C, include those sold by Hexcel Corporation under the product names M9, M9F, Ml 1, Ml 1.5 and those sold by Gurit (UK) Limited under the product names WE90, WE 1, and WT93.

If attempts are made to cure these prepregs quickly, that is at a temperature of around 100°C - 120°C, the temperature range at which the hardener become very reactive, large amounts of heat of reaction are generated in a short time. Unless the composite sections being produced are very thin and the moulds on which they are made conduct this heat away quickly then the composites can reach damaging and even decomposition temperatures.

In thick laminates, i.e. typically having a thickness of greater than 10 mm, in particular greater than 20 mm, made from standard epoxy formulations and glass fibres, temperatures as high as 250°C - 300°C can easily be achieved. These both damage the composite and often the moulds on which they are made if they are non-metallic. The majority of high performance moulds are usually made from epoxy composites themselves and it would be a great advantage if cheaper and lower temperature resistant tools could be used formed from; vinyl ester composites, or even better the much cheaper wet lay-up polyesters composites or the CNC machinable epoxy and polyurethane tooling blocks and pastes.

This damaging temperature rise must be prevented and this is usually achieved by heating the prepreg stack to a level where the reaction just begins and holding it at that temperature, possibly for several hours, whilst a large proportion of the total reaction slowly takes place and the resultant heat is continuously conducted away thus limiting the temperature rise. The reaction is finally completed with the standard cure cycle of around one hour at 120°C. This step is essential to ensure consistency in thick sections and full cure in thin sections. This two-step cure process is common practice and for the first lower temperature stage is often referred to as an "intermediate dwell".

There are particular practical problems producing large components, such as wind turbine blades, from prepregs. A typical composite laminate usually contains areas of different thickness to meet the strength requirements of a given structure. Foams, wood and honeycomb are also often incorporated to form sandwich structures to lighten the construction by separating the fibre reinforced skins with a lower weight core material.

In order to produce such a mixed thickness laminate the method typically used is first to heat the prepreg lay-up to an intermediate dwell temperature to allow the cure reaction to proceed slowly in the thick sections thus allowing the polymerisation to proceed at a rate where the heat produced in the laminate can both flow towards the mould tool and the opposite vacuum bag face. Heat can then be lost through conduction and then either natural or forced convection. Nevertheless, due to the heat releasing nature of the reaction this still usually results in a tolerable temperature increase above the curing temperature, "the exotherm", in the thicker section during the intermediate dwell but no significant exotherm in the thin sections. Once the exotherm has been controlled in the thicker sections the temperature of the whole lay-up needs to be increased to cure the thin section in a reasonable time.

Without the low temperature intermediate dwell, the rate of heat production would exceed the rate of conduction to the edges of the laminate where it can be lost by normal conduction, convection and radiation. This causes the temperature of the material to rise which in turn leads to a greater rate of reaction producing more heat and a more rapid temperature rise and frequently a large exotherm event. Effectively this may be close to the actual adiabatic temperature rise of which the prepreg is capable. It is not until the reaction rate begins to slow as a significant number of reactive groups have been consumed that the material begins to cool down to the surrounding temperatures.

For example a typical cure for a wind turbine blade using WE90, a DEGBA epoxy prepreg, from Gurit (UK) Limited is likely to have a 1 to 4 hour dwell at 80°C - 90°C to first control the exotherm, which otherwise might become destructive, followed by a 1 hour further cure at 1 15 - 120°C to ensure full reaction in all areas of the laminate. If the thinner sections of the blade were allowed to remain at 80C - 90°C then it would take a further 12 to 16 hours to be certain that full cure had taken place.

Frequently the thickness of the laminates range from 5 to 45 mm for the majority of the blade then increase to 70mm in some designs to accommodate local bolts or other fixings to attach the turbine blade to the hub assembly. It is clear that heat release must be slow otherwise it would cause an uncontrollable exotherm in the thicker sections.

The design and control of the curing process can become complex. For instance this may need to prevent unwanted exothermic heat flow from the medium thickness areas to the thicker sections, which normally heat up more slowly, triggering early exotherm in them before the reaction has taken place in a controlled manner.

These extended curing cycles are naturally not restricted to wind turbine blades but apply to any thicker section components.

Clearly such cure cycles are both time consuming and severely limit production rates making composites from the current prepregs too expensive for many applications which could benefit from them. Production could be increased by utilising more moulds but these can be very expensive and occupy more factory space resulting in even more cost.

The key factor often limiting the cure speed is the mould tool. For both small volume runs and large parts, such as wind turbines, mould tools tend to be constructed from composite materials. The cost of the tooling materials will increase with the temperature performance. Lower temperature cures are preferred, but are not always possible, as they also help reduce the tool stress and can lead to longer tool life. All such composite tools have a low thermal conductivity and hence exacerbate the exotherm event temperature rise problem.

The usual approach in currently known lower exotherm epoxy prepregs to manufacture thick laminate components is to formulate the opposite of a snap curing material - that is materials are formulated to have a broader heat release curve to try to reduce reactivity closer to the cure onset temperature. This provides a temperature window for the component manufacturer to programme an intermediate dwell within the tolerance capability of their heating system to control the initial heat release by holding at a temperature where the reaction proceeds at a slow enough rate to avoid a damaging out of control exotherm. This approach leads to undesirably long cure cycles.

As such the practical use of these prepregs has been limited to the manufacture and rapid cure of thin laminates where the heat can be loss by conduction into the mould tool and radiation and convection from any exposed surfaces.

Latent hardeners can be more easily selected to control the heat release during cure of epoxy resins. For both mechanical performance and processing reasons epoxy resins have to a large extent been the matrix resins of choice for making most high performance composites. It would be highly desirable to have prepregs that have both a long shelf life at room temperature to remove the need for refrigerated storage, and transport. So far this has proved difficult. Typically a 90°C curing prepreg would have a shelf life of 8 weeks at 20°C and a low temperature curing 50-60°C system a shelf life of 1-3 days at 20"C and these prepregs are transported and stored in temperature controlled and sub-ambient conditions.

The majority of these epoxy systems have been based on the readily available Bisphenoi A (4,4' dihydroxyphenyl 2,2 propane) series. This is a homologous series of essentially diglycidyl ethers. They range from the crystalline virtually pure monomer, through flowing liquid resins to semi solids, solids and ultimately to very high molecular weight polymers with almost no epoxy content.

Other epoxy resins based on Bisphenoi F (4,4' dihydroxyphenyl methane) and oligomers of this as well as those based on higher molecular weight polyfunctional novolac resins have also been used. Much the same reasoning below applies to these epoxy resins as well It is standard practice to blend liquid and low molecular weight solid versions from the range to yield mixtures which are fluid enough at safe temperatures when containing the curing agent to enable good impregnation of fabrics and fibres and casting of films, and flexible and tacky enough as prepregs for good processing at shop temperatures, and with good viscosity control for processing into high quality laminates.

In some cases no tack and low flow are required and then a slightly higher proportion of solid resin will be used.

The following calculations demonstrate the current exotherm problem clearly.

Glycidyl epoxy groups of the type found in these resins usually have a heat of polymerisation in the region of 98.4 KJ per mole (23,500 cals per mole).

To increase the final thermal performance it is necessary in most structural aerospace applications to use an epoxy resin with a rigid backbone and a higher epoxy content to increase the final cross link density. These resins with a high epoxy content result in high heat of polymerisation.

Current "state of the art" lower exotherm prepregs are made with a blend of difunctional liquid and solid epoxy resins and have a lower final thermal performance. If there is too much liquid in the formulation they are too tacky to handle and do not have sufficient body to maintain the fibres in place. If there is too much solid resin then they become rigid and brittle. The ratio of liquid to solid epoxy resins in most such prepregs usually falls in the range of 60:40 to 40:60 by weight.

Examples of such lower exotherm epoxy prepregs are sold by Hexcel Corporation under the product names M9, M9F, Mi l, Ml 1.5 and those sold by Gurit (UK) Limited under the product names WE90, WE91 , and WT93 and would have an average heat of polymerisation in the range 230 to 375 KJ/Kg when measured using Differential Scanning Calorimetry (DSC). All of these epoxy prepregs require an indeterminate dwell to allow the cure to first take place at a slow reaction rate to prevent a damaging exotherm in thicker laminates.

To improve productivity and reduce the risk of exotherm damage for new components an increasing trend is to attempt to model the cure cycle dwell times to optimise the curing processing, but even this often leads to only small percentage reductions in the overall cure times. Each newly configured composite material part then requires a new remodelling and optimising process.

To avoid the need for this simulation it would be highly desirable to reduce the exotherm so that any heat generated would be insufficient to damage the mould tool or other materials within the laminate stack to allow a simpler, more tolerant, cure to be used that would negate the need for an intermediate dwell step.

One current approach in prepregs to reducing the curing exotherm is therefore to have a more gradual heat release after the temperature of curing initiation (T onset) has been attained, to give an opportunity to control heat release with a more gradual reaction rate.

Thus there is a major need in the composite materials industry to provide improved, more versatile prepregs that possess a reasonable storage life, are free from strong smelling or significantly volatile materials, have no adverse reactions during storage and use, have good drape and tack for the desired application, have good mechanical and thermal resistance, and can be cured quickly without a damaging exotherm event.

A prepreg with these characteristics would be a major advance for most composite fabrication applications and it is an aim of this invention to provide such a prepreg.

It is accordingly an aim of this invention to provide a prepreg and a method of processing prepregs which at least partially overcome at least some of these significant disadvantages of the existing fibre and/or resin materials currently used to manufacture prepregs.

The present invention provides a prepreg comprising a fibrous material contacting a po!ymerisable epoxide resin system, the epoxide resin system comprising at least one epoxide resin and a curing agent for the at least one epoxide resin, the curing agent comprising dicyandiamide having a particle size d(0.5) of from 1 to 15 microns in an amount of from 0.75K to less than 3.3K parts per hundred by weight based on the weight of the at least one epoxide resin wherein K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

The present invention further provides a fibre-reinforced resin matrix composite material produced from the prepreg according to the invention, the resin matrix comprising a cured epoxy resin produced by curing the epoxide resin system, wherein the resin matrix has at least one of a glass transition temperature Tg of from 100 to 125°C, a compressive stress of from 1 15 to 140Mpa, a tensile stress of from 75 to 90Mpa, and a flexural stress of from 1 10 to 120N/mm².

The present invention further provides a method of manufacturing a structural component composed of a fibre-reinforced resin matrix composite material, the method comprising the steps of:

(a) providing a mould defining a mould surface;

(b) laying-up against the mould surface a stack of the prepregs according to the invention; and

(c) heating the stack to a curing temperature for the epoxide resin system, the heating causing full impregnation of the fibrous material by the epoxide resin system; and

(d) curing the epoxide resin system at the curing temperature to form the fibre-reinforced resin matrix composite material.

The present invention further provides a prepreg comprising a fibrous material comprising carbon fibres having a fibre diameter of 5 to 10 microns and contacting a polymerisable epoxide resin system, the epoxide resin system comprising at least one epoxide resin and a curing agent for the at least one epoxide resin, the curing agent comprising dicyandiamide having a particle size d(0.5) of from 1 to less than 4 microns.

(a) providing a mould defining a mould surface; (b) laying-up against the mould surface a stack of the prepregs according to the invention; and

(c) heating the stack to a curing temperature for the epoxide resin system, the heating causing full impregnation of the carbon fibres by the epoxide resin system; and

For all of these inventions, preferred features are defined in the dependent claims.

In this specification particle size was evaluated and analyzed in accordance with ISO 13320 using a laser scattering method. The device used was a Malvern Mastersizer 2000 and hexane was used as the dispersant. Ten repeat measurements were taken on each sample which was tested. The particle size is expressed as a mean-based volume particle size in μπι. The results have been represented by their 10th, 50th, and 90th percentiles d (0.1) meaning 10% of the volume distribution of the particles is below this value; d (0.5) meaning 50% of the volume distribution of the particles is below this value; and d (0.9) meaning 90% of the volume distribution of the particles is below this value.

In this specification enthalpy was evaluated using Differential Scanning Calorimetry (Mettler Toledo DSC821e). The dynamic program used was from 25°C to 250°C at 10°C/min followed by a cooling step from 250°C to 25°C at 20°C/min and a final heating step from 25°C to 200°C at 10°C/min a sample size of 5-15mg was used with a nitrogen atomosphere.

In this specification Tg was evaluated using Dynamic Mechanical Analysis (DMA) with TA Instruments Q800 in single cantilever mode. The program used was from 25°C to 200°C at 3°C/min.

In this specification flexural stress was evaluated on non-fibre reinforced resin casts of 4.7mm nominal thickness. Casts were cured for 2hrs at 120°C prior to testing. Samples were tested in accordance to ISO 178 using a test speed of 3mm/min.

In this specification tensile stress was evaluated on non- fibre reinforced resin casts of 4.7mm nominal thickness. Casts were cured for 2hrs at 120°C prior to testing. Samples were tested in accordance to ISO 527-2: 1996 using a test speed of lmm min. In this specification compressive stress was evaluated on non-fibre reinforced resin casts of 4.7mm nominal thickness. Casts were cured for 2hrs at 120°C prior to testing. Samples were tested in accordance to ISO 640 using a test speed of 2mm/min.

The present invention uses dicyandiamide as a curing compound for epoxide resin systems. The present invention is at least partly predicated on the finding by the present inventors that when the dicyandiamide curing compound is used at particularly low amounts in the epoxide resin systems, the exotherm and heat release are reduced, but at a greater rate than any reduction in the thermal and mechanical properties of the resultant cured resin. In other words, the thermal and mechanical properties of the resultant cured resin have been found to be greatly improved at low dicyandiamide content, as compared to what properties would have been expected. Therefore particularly low amounts of dicyandiamide can be employed which (a) significantly and beneficially reduce the exotherm and heat release during the curing process but do not (b) significantly decrease the thermal and mechanical properties of the resultant cured resin. The prior art does not suggest such low dicyandiamide content for particular particle sizes of the curing agent because it was believed that this would provide unacceptably low mechanical and thermal properties. However the inventors have found that unexpectedly that low dicyandiamide content provides the combination of good mechanical properties and good curing processing and cost.

This surprising and unexpected technical effect provides the advantage that the modified epoxy resin system can be employed to make structural composite materials using an improved manufacturing process without compromising on mechanical performance or component cost.

A particular use of a mixture of dicyandiamide and optionally an accelerator, which may comprise a chemically blocked-urone accelerator, has enabled the formulation of a low enthalpy and low exotherm resin system. This system achieves a similar mechanical performance but at reduced cost as compared to some known epoxy resin systems in prepregs. The epoxide resin system can exhibit reduced heat release during cure. This has several advantages and facilitates the use of composite materials to make both larger components than previously possible or alternatively cures components faster, increasing productivity. Epoxy prepreg resins are commonly cured with dicyandiamide. Milled dicyandiamide is typically dispersed into the epoxy resin matrix. Upon heating, the particles melt and dissolve into the resin system becoming available to react and cross-link (or cure the matrix). The melting point of pure dicyandiamide is 210°C. In order to reduce the activation temperature of the curing reaction, dicyandiamide particles are commonly micronised. By doing this it is possible to reduce the activation temperature to approximately 140°C. Therefore, the particle size of the dicyandiamide is proportionally related to reactivity.

In accordance with the invention, using dicyandiamide which has been ground to a micron- sized scale significantly increases the reactivity of the system. As such, a reduced amount of dicyandiamide is required to cure the system. The rate of reaction heat release is delayed using this approach which results in a lower peak temperature and lower heat release during cure. It is important to note that whilst the exotherm rate is reduced, total heat of reaction remains similar to a non-micronized system. This means that Ml cure-conversion of the resin is still achieved during the curing cycle.

This invention is applied to the field of structural epoxide, fibre-reinforced composite materials. An epoxide prepreg resin system consists of a 'semi-solid' epoxy resin to which a solid curing compound, in the form of dicyandiamide, is added in powder form. An additional accelerator, such as a urea or urone compound, is commonly added to reduce the activation energy of the chemical system and allow curing at a lower temperature.

It is known in the art to use particular minimum amounts of dicyandiamide which were set based upon the belief that these were the minimum amounts to achieve the required acceptable thermal-mechanical properties of the cured epoxy resin system.

However, the present invention typically uses approximately half or less of the previously specified minimum amount of dicyandiamide to achieve not only the greatest reduction in heat release but also the maintenance of the required thermal-mechanical properties of the cured epoxy resin system.

This is an unexpected effect. It is known in the industry that reducing the amount of micronised dicyandiamide used reduces the heat release during cure somewhat; however the thermal and mechanical properties of the cross-linked system are significantly reduced to below an unacceptable minimum threshold. This does not occur with the dicyandiamide- containing epoxide resin system used in accordance with the present invention which maintains such thermal and mechanical properties even when lower amounts are used.

Typically, the dicyandiamide particles are coated with a 'flow-aid', typically fumed silica. This serves to coat the solid particles and prevent them agglomerating and forming larger- sized particular agglomerates. Care must be taken when dispersing the dicyandiamide into the epoxide resin system to ensure that the dicyandiamide particles are homogeneously dispersed and stabilised within the epoxide resin system. This is important to prevent flocculation, namely agglomeration of solid particles in a colloid dispersion, during subsequent processing and storage.

The particular technical advantages to a manufacturer of composite materials using the prepregs of the present invention are that the end-user manufacturer can cure composite material components faster, because the component can be heat to higher temperatures, increasing the cure rate, and/or because lower temperature "dwell" stages, used to allow exotherm heat to dissipate during cure, can be avoided. In addition, or alternatively, the present invention can permit the cure of larger, typically thicker, components than previously possible due to the restriction of excessive exothermic heat release during cure. Both of these attributes, namely the processing characteristics and the product characteristics, are restricted by the thermal performance of the tooling being used and the degradation temperature of the prepreg epoxide resin system.

The prepregs of the present invention may have particular application in the manufacture of structural components for use in wind energy, in particular wind turbine blades. When using the prepregs to manufacture thick sections such as root-sections and spar-sections of wind turbine blades, the low-exotherm characteristics of the prepregs of the invention allow faster processing and/or thicker/larger components to be made, in turn realising larger- dimensioned blades using similar processing times.

The prepregs of the present invention may also have particular application in civil engineering, in particular for the manufacture of thick components where high temperature resistant tooling cannot be used, or sections which axe too thick to prevent degradation of material due to high heat release during cure.

The prepregs of the present invention may also have particular application in the manufacture of automotive components. For higher volume manufacture of automotive components, there is a requirement for snap-curing systems where components are cured at rapid cure cycles. Some commercially available prepreg systems for manufacturing automotive components at a fast cure rate are currently very exothermic. This causes problems with some automotive body panel manufacture. The low exotherm of the prepregs of the present invention can prevent damage to automotive panels during the curing cycle. The low heat release systems of the present invention are advantageous to prevent degradation of automotive parts during the manufacturing process, particularly during the cure cycle.

The prepregs of the present invention may also have particular application in the high-volume manufacture of components. The manufacture of composite materials is still, in general highly labour intensive. A low exotherm system enables rapid, "snap-cure" of components so that component manufacture rates can be increased. This in turn allows automated manufacturing processes to be used and for advanced composites to be more cost-competitive with existing high-volume processes such as sheet moulding compounds (SMC), thermoplastic mouldings/injections and metal pressing.

The prepregs of the present invention may also have particular application in the manufacture of marine components. The use of lower heat release prepregs allows marine customers to use lower cost, lower temperature tooling. This would allow production boat manufactures to use existing low Tg tooling with prepregs and for race boats to use cheaper on-off tooling constructions.

The present invention provides a number of advantages over known prepreg technologies. In particular, the present invention uses existing epoxy technology in a modified form, and the existing epoxy technology has proven reliability, fatigue performance and confidence with end-users. Furthermore, lower heat release allows faster curing of components, thereby saving time and money during the manufacturing process. In addition, the lower heat release allows the manufacture of thicker laminates, in turn allowing the use of composite materials to reach wider applications and markets than previously possible. The present invention provides that the use of particles as catalysts produces optically clear resin systems, allowing ease of inspection. It is a common requirement to inspect for areas of fibre in the component which are 'dry' or not fully coated by the resin system and could lead to a failure or reduced mechanical performance. In the resultant cured composite materials, the cured resin matrix is a continuous phase, resulting in improved mechanical performance.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows the relationship between the peak exotherm temperature and time for resins according to Examples of the invention;

Figure 2 shows the relationship between the peak exotherm temperature and dicyandiamide content according to Examples of the invention;

Figure 3 shows the relationship between the exothermic energy release and dicyandiamide content according to Examples of the invention;

Figure 4 shows the relationship between the glass transition temperature and dicyandiamide content according to Examples of the invention;

Figure 5 shows the relationship between the tensile strength and dicyandiamide content according to Examples of the invention;

Figure 6 shows the relationship between the flexural strength and dicyandiamide content according to Examples of the invention;

Figure 7 shows the relationship between the compressive strength and dicyandiamide content according to Examples of the invention; and

Figure 8 shows the relationship between the peak exotherm temperature, exothermic energy release, glass transition temperature and mechanical properties and dicyandiamide content according to Examples of the invention.

In accordance with the present invention, a prepreg comprises a fibrous material contacting a polymerisable epoxide resin system. The fibrous material may be fully or partly impregnated by the polymerisable epoxide resin system. Alternatively, the prepreg may comprise at least one layer of the fibrous material and an adjacent layer of the polymerisable epoxide resin system, for example as disclosed in the Applicant's EP-A-1 128958 discussed above. The epoxide resin system comprises at least one epoxide resin and a curing agent for the at least one epoxide resin. The curing agent comprises dicyandiamide.

The epoxide resin system may further comprise at least one accelerator for the curing agent. Dicyandiamide may be accelerated using a variety of compounds, including both electrophilic and nucleophilic species. The purpose of the accelerator is to lower the net activation energy of the system. As is known in the art, the accelerator may be selected based upon the particular activation energy and enthalpy of the combined epoxide resin and a curing agent in the epoxide resin system. The accelerator may be selected from at least one of a urea compound, a urone compound, a Lewis acid, a Lewis base, a tertiary amine, an imidazole or a boron tri fluoride complex, or a mixture of any of these accelerators. Typically, the accelerator comprises a urea compound and/or a urone compound, which may be chemically blocked as is known in the art.

The epoxy containing materials which may be used as part of this invention include those based on bisphenol A, bisphenol F and oligomers thereof, higher functionality novolacs, amines, amino phenols, esters, unsaturated cycloaliphatic materials, hydantoins and unsaturated oils and mixtures of any of these. Those epoxy resins based on bisphenols A and F, aliphatic and polyfunctional novolacs and mixtures of them are particularly preferred for the majority of applications, any or all of which may be halogenated.

Unreactive diluents, fillers, thixotropes, pigments, core shell particles, surfactants, foaming agents, fire retardants, smoke suppressors, coupling agents and dyes amongst others may be incorporated into the resin system for special properties or effects as required. Unreactive or reactive thermoplastics may be incorporated for flow control, viscosity adjusters or tougheners. A variety of inorganic basic compounds may also be used as adjusters of tack, flow and handling texture.

In accordance with the present invention, the dicyandiamide has a particular particle size and is present in a particular amount in the polymerisable epoxide resin system. In this embodiment, the dicyandiamide has a particle size d(0.5) of from 1 to 15 microns, typically from 5 to 12 microns, and/or optionally a particle size d(0.9) of from 10 to 35 microns, typically from 15 to 30 microns. The dicyandiamide is present in an amount of from 0.75K to less than 3.3K parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight (EEW), expressed in g/eq, of the at least one epoxide resin. For example, if the epoxide resin has an epoxy equivalent weight of 287g/eq, then the dicyandiamide is present in an amount of from 0.75 to less than 4 parts per hundred by weight based on the weight of that epoxide resin.

Typically, the dicyandiamide is present in an amount of from 0.75 to 2.8 , optionally from 0.9K to 2.8K, optionally from 0.9K to less than 2.5K, optionally from UK to less than 2.5K, parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

Typically the epoxide resin system has a curing enthalpy of from 85 to 210 J/g.

When the prepreg is processed to form a fibre-reinforced resin matrix composite material produced from the prepreg of the invention, the resin matrix comprises a cured epoxy resin produced by curing the epoxide resin system. The resin matrix preferably has at least one of a glass transition temperature Tg of from 100 to 125°C, a compressive stress of from 115 to 140Mpa, a tensile stress of from 75 to 90Mpa, and a flexural stress of from 110 to 120N/mm² and preferably all of a glass transition temperature Tg of from 100 to 125°C, a compressive stress of from 1 15 to 140Mpa, a tensile stress of from 75 to 90Mpa, and a flexural stress of from 1 10 to 120N/mm².

In accordance with particular embodiment of the present invention, the dicyandiamide has a particular particle size d(0.5) of from 1 to less than 4 microns, typically from 2 to 3 microns, and optionally a particle size d(0.9) of from 1 to 3 microns, typically from 2.25 to 2.75 microns. In this embodiment, the dicyandiamide is present in an amount of from 0.75 to less than 3.3 parts per hundred by weight, typically from 0.75 to 2.8 parts per hundred by weight, more typically from 0.9 to 2.8 parts per hundred by weight, more typically from 0.9 to less than 2.5 parts per hundred by weight, more typically from 1.8 to less than 2.5 parts per hundred by weight, each based on the weight of an epoxide resin having an EEW of 287 g/eq., and with a corresponding coefficient of K as discussed above if the epoxide resin has an EEW o greater than or less than 287 g eq. In accordance with a further aspect of the present invention, there is provided a method of manufacturing a structural component composed of a fibre-reinforced resin matrix composite material. The structural component may be any structural component but in particularly preferred embodiments may be s a root of a wind turbine blade, a spar of a wind turbine blade, or an automotive body panel. The method comprises laying-up a stack of the prepregs according to the invention against a mould surface of a mould. The stack typically has a thickness of from 3 to 100 mm. The mould surface preferably comprises a fibre-reinforced resin matrix composite material, the resin matrix of the mould surface having a glass transition temperature Tg of less than 1 0°C, optionally less than 160°C. Then the stack is heated to a curing temperature for the epoxide resin system in the prepregs. The heating causes full impregnation of the fibrous material by the epoxide resin system. In the subsequent curing step, the epoxide resin system is cured at the curing temperature to form the fibre-reinforced resin matrix composite material. The resultant cured resin matrix has a glass transition temperature Tg of from 100 to 125°C and during the curing step the stack exhibits an exothermic temperature increase to a maximum temperature of less than 190°C, optionally less than 160°C.

The present invention is illustrated further with reference to the following non-limiting examples.

Examples 1 to 9 and Comparative Examples 1 to 3

In these Examples and Comparative Examples, a polymeri sable epoxide resin system was prepared. In each Example, the epoxide resin system comprised liquid and solid DEGBA epoxide resin to give an epoxide resin with an EEW of 287 g/eq. A curing agent comprising dicyandiamide, being a selected one of three different curing agents for Examples 1 to 3, 4 to 6 and 7 to 9 respectively, and 3.18 parts of urea accelerator were fully dispersed into the resin. In the Examples the selected dicyandiamide curing agent was present in an amount of 2.80, 1.92 or 0.92 parts by weight per hundred parts by weight of the epoxy resin (since the EEW of the epoxide resin was 287 g/eq, the coefficient K was 1).

In these Examples a different particle size and distribution was employed for the dicyandiamide curing agent, as summarised in Table 1.

Table 1 Dicyandiamide Amount dicyandiamide d(0.5) d(0.9)

curing agent Trade pbw per 100 parts

Name Epoxide resin (EEW

287g/eq)

Examples Technicure Example 1 - 2.80 2.2 micron 2.5 micron 1 - 3 NanoDicy, Example 2 - 1.92

available from Example 3 - 0.92

AC Catalysts, USA

Examples Technicure D-10, Example 4 - 2.80 8.3 micron 27.2 micron 4 - 6 available from Example 5 - 1.92

AC Catalysts, USA Example 6 - 0.92

Examples Amicure Example 7 - 2,80 1 1.0 30.0 micron 7 - 9 CG1200E, Example 8 - 1.92 micron

available from Example 9 - 0.92

Air Products, USA

The epoxy resin system of each of the Examples was cured under laboratory conditions to model prepreg curing. In particular, rather than measuring the exotherm using a fibre- reinforced prepreg, the system was modelled using adiabatic resin casts simulating a prepreg laminate comprising a stack of 84 plies of 1200g/m² glass fibre prepregs with 43 wt% resin content based on the total prepreg weight.

The curing of the resin alone, absent fibre, was carried out in an insulated mould 30mm long x 12.5mm wide x 10mm deep with a central thermocouple, to give approximate adiabatic conditions to simulate a thick stack of prepreg curing. The resin was cast to form a block 30mm x 12.5mm x 10mm in dimensions and with the temperature being measured at the geometric centre of the block by the central thermocouple

The curing process conditions comprised heating up the epoxy resin system from ambient temperature (25°C) at a heat up rate of 1 °C/min to the curing temperature of 120°C and then maintenance at the curing temperature of 120°C for a period of Ihr. The temperature was measured throughout the curing process and the results are shown in Figure 1.

In the Comparative Examples 1, 2 and 3, Examples 1, 4 and 7 respectively were repeated by using in each case a higher amount of the respective selected dicyandiamide curing agent, so that in Comparative Examples 1, 2 and 3- the respective selected dicyandiamide curing agent was present in an amount of 3,67 parts by weight per hundred parts by weight of the epoxy resin (since the EEW of the epoxide resin was 287 g/eq, the coefficient K was 1). The results of the same temperature measurement are also shown in Figure 1.

It may be seen that temperature at initiation of curing at the curing temperature of 120°C rapidly increased, indicating an exothermic curing reaction. The peak exotherm temperature ranged from 135°C using the lowest amount of curing agent in Examples 3, 6 and 9 (0,92 pbw to 100 parts of epoxide resin) to 169°C when using the higher levels of curing agent in Examples 1,4 and 7 (2.80 pbw) . In Comparative Examples 1, 2 and 3 the peak exotherm temperature was the highest value, 185°C, and higher than in the Examples which all used a lower amount of curing agent.

For each dicyandiamide loading level the individual temperature response of the different particle size materials was similar. For clarification in Figure 1 the results have been grouped and the average thermal response for each dicyandiamide loading is shown. The difference in the peak exotherm can be seen more clearly in Figure 2 which plots the peak exotherm vs. the dicyandiamide loading for each of the different particle sizes. It should be noted that the origin of the ordinate is at a peak temperature value of 120°C. It may be seen that the peak exotherm temperature generally increases with an increase in the amount of dicyandiamide in the resin system.

Furthermore, Figure 3 shows for Examples 1 to 9 and Comparative Examples 1 to 3 the relationship between the exothermic energy released in J/g, measured by differential scanning calorimetry, during the curing process and the amount of dicyandiamide. It should be noted that the origin of the ordinate is at an enthalpy value of 80 J/g. It may be seen that by reducing dicyandiamide to unconventionally low amounts, the enthalpy is dramatically reduced and correlates well to the bulk exotherm temperature as described with reference to Figure 1. The thermal properties of the cured resins of the Examples and Comparative Examples were determined, in particular by performing a dynamic mechanical analysis of the glass transition temperature of the cured resin. The glass transition temperature was evaluated to determine the impact of low dicyandiamide levels on cured system thermal properties and thermal performance. The results are illustrated in Figure 4, showing the relationship between the glass transition temperature of the cured resin and the amount of dicyandiamide for Examples 1 to 9 and Comparative Examples 1 to 3. It should be noted that the origin of the ordinate is at a glass transition temperature value of 100°C. The findings were surprising, showing minimal decrease in the glass transition temperature (Tgl) of the cured resin when the system enthalpy of Figure 3 would imply only partial cure. Furthermore, a glass transition temperature (Tgl) of over 100°C was achievable using only 0.92pph dicyandiamide. That could not have been expected from the state of the art.

The mechanical properties of the cured resins were determined. The following mechanical properties were evaluated to determine the reduction in performance as a result of lowering the dicyandiamide concentrations. It was found that although there is a decreasing trend in strength/performance, the reduction is much less than anticipated and in some cases, within experimental error, largely unchanged. The results are illustrated in Figures 5 to 7.

Figure 5 shows the relationship between tensile strength of the cured resin and the amount of dicyandiamide for Examples 1 to 9 and Comparative Examples 1 to 3. Figure 6 shows the relationship between flexural strength of the cured resin and the amount of dicyandiamide for Examples 1 to 9 and Comparative Examples 1 to 3, Figure 7 shows the relationship between compressive strength of the cured resin and the amount of dicyandiamide for Examples 1 to 9 and Comparative Examples 1 to 3.

For each of Figures 5 to 7 the origin of the ordinate, representing the respective mechanical property, is not at zero. All of these three graphs show that the mechanical properties of the cured resin system do not significantly decrease with decreasing dicyandiamide at the low dicyandiamide content levels employed in the present invention.

The variation in peak exotherm temperature, exothermic heat release, thermal properties and mechanical properties with respect to dicyandiamide amount at the low dicyandiamide contents employed in the present invention for the Examples 1 to 9 are summarised in Figure 8, which also shows the results for the higher dicyandiamide amount in Comparative Examples 1 to 3, Broadly, it may be seen that the mechanical and thermal properties of the cured resin decrease significantly less (have a lower slope) at progressively decreasing dicyandiamide contents than the peak exotherm temperature and exothermic heat release (which consequently have a higher slope) during curing.

By providing that the dicyandiamide is present in an amount of from 0.75 to less than 3.3 parts per hundred by weight, typically from 0.75 to 2.8 parts per hundred by weight, more typically from 0.9 to 2.8 parts per hundred by weight, more typically from 0.9 to less than 2,5 parts per hundred by weight, more typically from 1.8 to less than 2.5 parts per hundred by weight, each based on the weight of an epoxide resin having an EEW of 287 g/eq., the thermal behaviour during curing can be improved, by significantly lowering the peak exotherm on curing and lowering the enthalpy on curing, without significantly worsening the mechanical properties and thermal performance, expressed as the glass transition temperature, of the resultant cured resin.

Accordingly, in accordance with the invention the curing process and cost can be improved significantly without a significant decrease in mechanical properties of the resultant cured resin.

The results are summarised in Table 2,

Table 2

Claims

1. A prepreg comprising a fibrous material contacting a polymeri sable epoxide resin system, the epoxide resin system comprising at least one epoxide resin and a curing agent for the at least one epoxide resin, the curing agent comprising dicyandiamide having a particle size d(0.5) of from 1 to 15 microns in an amount of from 0.75K to less than 3.3K parts per hundred by weight based on the weight of the at least one epoxide resin wherein K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

2. A prepreg according to claim 1 wherein the dicyandiamide has a particle size d(0.5) of from 5 to 12 microns.

3. A prepreg according to claim 1 or claim 2 wherein the dicyandiamide has a particle size d(0.9) of from 10 to 35 microns.

4. A prepreg according to claim 3 wherein the dicyandiamide has a particle size d(0.9) of from 15 to 30 microns.

5. A prepreg according to claim 1 wherein the dicyandiamide has a particle size d(0.5) of from 1 to less than 4 microns.

6. A prepreg according to claim 5 wherein the dicyandiamide has a particle size d(0.5) of from 2 to 3 microns.

7. A prepreg according to claim 5 or claim 6 wherein the dicyandiamide has a particle size d(0.9) of from 1 to 3 microns.

8. A prepreg according to claim 7 wherein the dicyandiamide has a particle size d(0.9) of from 2.25 to 2.75 microns.

9. A prepreg according to any foregoing claim wherein the dicyandiamide is in an amount of from 0.75 to 2.8K, optionally from 0.9K to 2.8K, parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

10. A prepreg according to claim 9 wherein the dicyandiamide is in an amount of from 0.9K to less than 2.5K, optionally from 1.8K to less than 2.5K, parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

1 1. A prepreg according to any foregoing claim wherein the epoxide resin system has a curing enthalpy of from 85 to 210 J/g.

12. A prepreg according to any foregoing claim wherein the epoxide resin system further comprises at least one accelerator for the curing agent, the at least one accelerator being selected from a urea compound, a urone compound, a Lewis acid, a Lewis base, a tertiary amine, an imidazole or a boron tri fluoride complex.

13. A prepreg according to claim 12 wherein the at least one accelerator for the curing agent comprises at least one of a urea compound and a urone compound.

14. A prepreg according to any foregoing claim wherein the fibrous material is partly impregnated by the polymerisab!e epoxide resin system.

15. A prepreg according to any foregoing claim which comprises a layer of the fibrous material and an adjacent layer of the polymerisable epoxide resin system.

16. A fibre-reinforced resin matrix composite material produced from the prepreg according to any foregoing claim, the resin matrix comprising a cured epoxy resin produced by curing the epoxide resin system, wherein the resin matrix has at least one of a glass transition temperature Tg of from 100 to 125°C, a compressive stress of from 115 to 140Mpa, a tensile stress of from 75 to 90Mpa, and a flexural stress of from 1 10 to 120N/mm².

17. A fibre-reinforced resin matrix composite material according to claim 16 wherein the resin matrix has a glass transition temperature Tg of from 100 to 125°C, a compressive stress of from 115 to 140Mpa, a tensile stress of from 75 to 90Mpa, and a flexural stress of from 1 10 to 120N/mm².

18. A method of manufacturing a structural component composed of a fibre-reinforced resin matrix composite material, the method comprising the steps of:

(a) providing a mould defining a mould surface;

(b) laying-up against the mould surface a stack of the prepregs according to any one of claims 1 to 15;

(d) curing the epoxide resin system at the curing temperature to form the fibre- reinforced resin matrix composite material.

19. A method according to claim 18 wherein the mould surface comprises a fibre- reinforced resin matrix composite material, the resin matrix of the mould surface having a glass transition temperature Tg of less than 190°C.

20. A method according to claim 18 or claim 1 wherein the cured resin matrix has a glass transition temperature Tg of from 100 to 125°C and during the curing step the stack exhibits an exothermic temperature increase to a maximum temperature of less than 190°C.

21. A method according to any one of claims 18 to 20 wherein the stack has a thickness of from 3 to 100 mm.

22. A method according to any one of claims 18 to 21 wherein the structural component is a root of a wind turbine blade, a spar of a wind turbine blade, or an automotive body panel.

23. A prepreg comprising a fibrous material comprising carbon fibres having a fibre diameter of 5 to 10 microns and contacting a polymeri sable epoxide resin system, the epoxide resin system comprising at least one epoxide resin and a curing agent for the at least one epoxide resin, the curing agent comprising dicyandiamide having a particle size d(0.5) of from 1 to less than 4 microns.

24. A prepreg according to claim 23 wherein the dicyandiamide has a particle size d(0.5) of from 2 to 3 microns.

25. A prepreg according to claim 23 or claim 24 wherein the dicyandiamide has a particle size d(0.9) of from 1 to 3 microns.

26. A prepreg according to claim 25 wherein the dicyandiamide has a particle size d(0.9) of from 2.25 to 2.75 microns.

27. A prepreg according to any one of claims 23 to 26 wherein the dicyandiamide curing agent is present in an amount of from 0.75K to less than 3.3K parts per hundred by weight based on the weight of the at least one epoxide resin wherein K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

28. A prepreg according to claim 27 wherein the dicyandiamide is in an amount of from 0.75 to 2.8 parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

29. A prepreg according to claim 28 wherein the dicyandiamide is in an amount of from 0.9K to 2.8K parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

30. A prepreg according to claim 29 wherein the dicyandiamide is in an amount of from 0.9 to less than 2.5K parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g eq, of the at least one epoxide resin.

31. A prepreg according to claim 30 wherein the dicyandiamide is in an amount of from UK to less than 2.5K parts per hundred by weight based on the weight of the at least one epoxide resin, where K = 287 / the mean epoxy equivalent weight, expressed in g/eq, of the at least one epoxide resin.

32. A prepreg according to any one of claims 23 to 31 wherein the carbon fibres have a Young's Modulus greater than 200 GPa and a tensile strength greater than 2 GPa.

33. A prepreg according to any one of claims 23 to 32 wherein the carbon fibres are partly impregnated by the polymerisable epoxide resin system.

34. A prepreg according to any one of claims 23 to 33 which comprises a layer of the carbon fibres and an adjacent layer of the polymerisable epoxide resin system.

35. A prepreg according to any one of claims 23 to 34 wherein the carbon fibres are mutually spaced by a distance which is larger than the diameter of at least a majority of the dicyandiamide particles.

36. A method of manufacturing a structural component composed of a fibre-reinforced resin matrix composite material, the method comprising the steps of:

(a) providing a mould defining a mould surface;

(b) laying-up against the mould surface a stack of the prepregs according to any one of claims 23 to 35; and

37. A method according to claim 36 wherein the carbon fibres are mutually spaced by a distance which is larger than the diameter of at least a majority of the dicyandiamide particles to cause substantial homogeneous dispersion of the dicyandiamide particles throughout the resin surrounding carbon fibres after full impregnation.