EP3898792A1

EP3898792A1 - Improved thermocurable moulding process

Info

Publication number: EP3898792A1
Application number: EP19828749.2A
Authority: EP
Inventors: Nicholas VERGE
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2018-12-20
Filing date: 2019-12-19
Publication date: 2021-10-27
Also published as: WO2020127855A1; GB2580087A; GB2580087B; GB201820910D0

Abstract

A fast cure cycle for thermocurable resins coupled with long storage stability is achieved by applying a curative containing particulate cure initiator to the surface of a layer of the thermocurable resin and by employing conditions of temperature and pressure that drive the curative into the resin to cause rapid cure of the resin.

Description

IMPROVED THERMOCURABLE MOULDING PROCESS

INTRODUCTION

The present invention relates to moulding materials, to a moulding process and to mouldings made by such a process. In particular the invention relates to the moulding of thermocurable resins to produce articles such as aerospace and automobile components, sporting goods such as skis and wind turbine components.

Typical thermocurable resin formulations also known as formulated resin systems contain one or more resin components, and one or more curatives. The curative contains a curing agent and may optionally also contain cure accelerators. Most formulated resin systems are liquids at ambient temperature (25 °C).

Although they are reactive materials they are selected to be largely inactive at ambient temperature so that they have a very low degree of cure for extended periods of time at ambient temperature. This period of time is sometimes known as the outlife of the resin formulation and a long outlife is important for storage and transportation. Often outlife can be further extended by storing the formulated resin systems at sub-ambient temperatures as this further reduces the rate of the cure reaction thereby extending outlife. However, sub ambient temperature storage is expensive and inefficient.

In conventional resin formulations, the onset of cure temperature of the resin formulation is defined as the temperature of cure for which the formulated system shows the desired accelerated cure and the final cured formulation has the desired properties including its desired glass transition temperature. Often the manufacturer of the system will specify the onset of cure temperature.

The speed of cure can be further accelerated by the use of a cure accelerator within the formulation. Within this application the term “curative” refers to a curing agent, a cure accelerator or a combination of the two.

In conventional resin formulations speed of cure and outlife are both compromised to provide a suitable trade off between these properties.

BACKGROUND

Composite materials are produced in many forms. Moulding compounds generally comprise a fibrous material in a chopped, isotropic or quasi-isotropic form in combination with a resin matrix formulation. The resin matrix formulations in these materials may be uncured or partially cured. In certain applications particularly automated applications the moulding compound is supplied to the mould as tapes which are laid up on top of each other and adjacent to each other and subsequently integrally cured to form a fibre reinforced article.

Resin matrix formulations can be selected from a wide range of polymerisable components and additives. Common polymerisable components comprise epoxies, polyesters, vinylester, polyisocyanates, and phenolics. Formulations containing these components are generally referred to as epoxy, polyester, vinylester, polyisocyanate and phenolic formulations respectively.

Epoxy resin formulations are widely used in composite materials. The epoxy components in these formulations are selected from a wide range of epoxy containing materials according to the cure cycle to be employed and the nature of the finished article to be produced. Epoxy resins can be solid, liquid or semi-solid and are characterised by their functionality and epoxy equivalent weight. The functionality of an epoxy resin is the number of reactive epoxy sites per molecule that are available to react and cure to form the cured structure. For example, a bisphenol-A epoxy resin has a functionality of 2, while certain glycidyl amines can have a functionality of more than 4. The reactivity of an epoxy resin is indicated by its epoxy equivalent weight (EEW), the lower the EEW the higher the reactivity. The EEW is the weight of epoxy resin material in grams containing 1 gram/mol of epoxy groups.

The properties required of a composite material are that when cured it has the required glass transition temperature (Tg), and also has the required mechanical properties according to the use to which it is to be put. In certain applications it is important that the Tg is retained under damp or humid conditions.

Curatives in epoxy formulations are selected to control onset of cure, outlife, cure speed, glass transition temperature and mechanical performance of the cure formulation. The curatives are incorporated into the epoxy resin formulations which are then applied to a fibrous reinforcement material to form a moulding material such as a moulding compound, a preimpregnated reinforcement (prepreg).

The curing of composite materials to support high volume manufacturing rates requires very short cure cycles. A cure cycle time of product in the mould of 100 seconds can provide for a rate of manufacture of about 230000 mouldings per mould per year providing the unload and re- loading time is no more than an additional 30 seconds and there is 95% utilisation. It is desirable to use thermosetting materials for structural components as they have superior mechanical performance and creep resistance compared to thermoplastics. However for these applications, the thermosetting matrix must have an initial cured Tg that is high enough to allow rapid demoulding of the moulded article at the cure temperature in order to achieve the required unloading time that provides the short unload and re-load time. Additionally a higher cured Tg capability enables curing at higher cure temperature; higher cure temperature will enable shorter cure cycles as reactivity increases with temperature.

Very fast cure at lower temperature can be achieved with multi-component mixed epoxy formulations of resin and curative which need to be prepared in situ at the moulding location to prevent premature curing. These formulations are often injected into a fibrous preform following preparation of the formulation in a process known as resin transfer molding (RTM). However this requires additional mixing and metering equipment which increases the complexity and therefore the occurrence of failures which can be costly in high volume production environments. In addition, RTM processes require the construction in an additional prior step, of a dry fibrous preform. This dry preform can be time consuming to produce and difficult to position accurately into a mould which may be complex shaped.

Therefore moulding materials that comprise a formulated resin system matrix and that can exist with several weeks of latency or outlife without the need for refrigeration are advantageous for composite parts manufacture. The resin system matrix may or may not be mixed with a fibrous reinforcement to form a moulding material or compound. For certain applications the moulding material may comprise the fibre reinforcement and the resin matrix stored as reels in the form of a tape which is ready for transport and use. Such moulding materials can be fed, cut, oriented and stacked as required in automated processes allowing easy placement into a mould for curing.

In formulated matrix systems for composite moulding materials, imidazole based curatives are widely used as they react readily with epoxy resins to form a cured epoxy resin matrix. These curatives are very reactive so they compromise outlife and also have a low temperature on-set of curing. Attempts have been made to increase the temperature at which these curatives start to react with the epoxy resin. See for example US Patents 4,931 ,528 and 5,001 ,211 the epoxy formulations of these patents are however slow curing.

Cure initiators have been devised in the form of a clathrate of a host compound and an imidazole curative which do not release the imidazole until certain conditions are applied hence addressing the outlife problem. For example, US 20120088920 discloses a curable resin composition using a clathrate component of an isophthalic acid-based host compound and an imidazole as guest compound in which the curing reaction is suppressed at low temperature. Figure 21 shows that these systems have cure times of several minutes. US 20100179250 is concerned with improving the storage stability and retaining the flowability of sealants based on epoxy resins when sealing and uses clathrates based on carboxylic acids and imidazole compounds and WO 2016/087935 also discloses the use of clathrates based on various carboxylic acids in combination with imidazole curatives together with dicyandiamide and at least one aromatic urea and the aromatic urea and the clathrate function as latent, heat activated cure accelerators for the epoxy resin with the urea reducing the initial onset temperature for cure and the clathrate reduces the final temperature of cure to result in a cure degree of at least 50% after heating at 163°C for 5 minutes. Taiwanese patent 576473 B produces clathrates by recrystallisation. In all instances the curative is premixed as a component of the resin matrix. A problem that arises with this premixing is that the mixing needs to be performed at low temperatures to prevent premature curing which can slow down the operation. Furthermore since the curatives are highly active chemicals it is necessary to employ strict safety procedures during their use.

Also, the flow properties of the curative during moulding may be different from the flow properties of the other resin components in the resin formulation. This can result in a separation or delocalisation of the curative which in turn can affect the cured properties of the resin formulation.

The present invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.

SUMMARY OF THE INVENTION

According to the invention there is provided a process and an article as defined in any of the accompanying claims.

One object of the present inventions is to provide a moulding process involving a curable resin composition, wherein the composition has excellent storage stability. The process has enhanced rapid curing characteristics to produce mouldings which have an initial cured Tg that allows for rapid demoulding to provide a cured product having excellent mechanical properties.

The invention therefore provides a moulding process comprising a. providing an uncured thermocurable resin system comprising a resin component and a particulate cure initiator for reacting with said resin component, the particulate cure initiator being provided on the surface of the resin component and the cure initiator comprising a curative;

b. applying moulding conditions to release the curative into the resin component to trigger cure, and

wherein the cure initiator is inert in relation to the resin component until the moulding conditions are applied.

In the present process, preferably the thermocurable resin cures to at least 95% cure with an initial cured Tg at the moulding temperature of at least 120°C in no more than 5 minutes.

The cure initiator may be provided externally on the surface of the resin component.

The cure initiator may release the curative in response to the moulding conditions, the moulding conditions comprising an increased temperature above ambient temperature (25°C) and/or increasing the pressure above atmospheric pressure.

The curative that is comprised within the particular cure initiator located on the surface of the resin component may be selected so that it will react with the resin and initiate the cure at the moulding temperature upon its release from the cure initiator. Alternatively it may be a material that expresses the curative at or above a certain temperature known as the release temperature at which the curative is released to react with the resin. The curative may be selected so that the release temperature may be the desired onset of cure temperature.

In the context of this invention, a curative is a compound which is adapted to initiate or advance a polymerisation reaction of a polymerisable resin. The term curative includes (cure) accelerators which are chemical compounds which enhance the polymerisation reaction (or“curing”) and curing agents which are chemical compounds for initiating the polymerisation reaction of a polymerisable resin.

The curative may include a curing agent, an accelerator or both of these materials in order to cure in an acceptably short time. As discussed in the background section, in conventional resin formulations the presence of such a combination can result in premature curing of the resin before moulding resulting in an unacceptably short outlife. Accordingly in the present invention at least part of the curative system is not present in the resin matrix formulation or during the initial impregnation of fibrous material by a resin matrix. This allows greater flexibility in the conditions used in both of these aspects of the process.

In an embodiment the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the release temperature is above the softening point of the cure initiator.

In another embodiment, the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the release temperature is above the dissolution temperature of the cure initiator in the resin component.

In a further embodiment of the invention, the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the release temperature is above the melting point of the cure initiator.

In another embodiment, the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the curative is released upon applying pressure to the cure initiator which exceeds atmospheric pressure. The pressure may be in the range of from 10 to 400 bar (10⁴ to 4x 10⁵ hPa), preferably from 20 to 250 bar (.2 x 10⁵ to 2.5x 10⁵ hPa) or from 100 to 250 bar (10⁵ to 2.5 x 10⁵ hPa), more preferably from 150 to 200 bar (1.5 x 10⁵ to 2 x 10⁵ hPa).

In one embodiment the invention is directed at reducing the in-mould moulding cycle time to be no more than 100 seconds per cycle. This is useful in reducing the cycle time required for the production of a moulding from tapes containing thermocurable material in automated moulding processes.

This is achieved by the production of mouldings with an initial cured Tg that enables the moulding to be removed from the mould immediately following moulding. The invention also allows moulding materials to be storage stable as indicated by an outlife (time in which there is no curing at ambient temperature) of several weeks or longer. In the present invention no curing takes place as the cure initiator is inert unless the moulding conditions (trigger temperature, trigger pressure or both) are present. The invention is particularly useful for the moulding and curing of tapes of the thermocurable material even more so with tapes of fibre reinforced thermocurable material. The cure initiator is provided on the surface of the thermocurable resin system or composition as a particulate solid which may be in the form of powder or granules. The cure initiator may also be provided in the form of a film or layer.

The particulate solid is preferably in the form of a powder and we prefer that it is a powder of average D90 particle size in the range 1 to 60 pm preferably 5 to 50 pm and more preferably 8 to 30 pm. Where the cure initiator is applied to a moving web or tape of the resin matrix system which may contain fibrous reinforcement, we prefer that it is applied in an amount from 5 to 15 g/m², preferably 6 to 12 g/m² of the web or tape.

This invention allows the resin materials which may or may not contain fibrous reinforcement to be stored for several weeks without significant cure and the resin can be then cured in a moulding step in about 100 seconds by heating at moulding temperatures between 120°C and 220°C and which delivers a cured Tg in the range of from 120 to 200°C, typically from 125 to 150°C more typically 130°C to 140°C to allow rapid demoulding.

The cured Tg of the moulding is measured in accordance with ASTM D7028 (Glass Transition Temperature (DMA Tg) of Polymer Matrix Composites by Dynamic Mechanical Analysis (DMA) ) and the retained or wet Tg is measured following isothermal curing at 170°C for 2 minutes of the neat resin formulation and exposing the cured formulation to water at 70°C for 14 days, and then measuring the Tg of the sample using the same measurement standard ASTM D7028.

The heat released during the curing reaction is related to the total heat for fully curing and can be measured using Dynamic Scanning Calorimetry as follows. A reference resin sample is heated from 10°C to 250°C at 10°C/min rate to full cure (100%) and the generated heat DHί is recorded. The degree of cure of a particular resin sample of the same composition as the reference resin sample can then be measured by curing the sample to the desired temperature and at the desired rate and for the desired time by heating the sample at these conditions and measuring the heat AHe generated by this cure reaction. The degree of cure (Cure %) is then defined by:

Cure %=[(AHi-AHe)/AHi]x100 [%]

where DHί is the heat generated by the uncured resin heated from 10°C up to fully cured at 250°C. and AHe the heat generated by the certain degree cured resin heated up to a desired temperature and rate. In a preferred embodiment the curing initiator is a clathrate in which the curative is the guest component and the host component is selected so that the release of the curative from its host may occur at moulding conditions which may comprise a desired temperature and/or pressure which are above ambient conditions. In a preferred clathrate, the bonds between the guest and host components are broken to release the curative under the moulding conditions. In this way the guest component that is the curative is released from the clathrate under the selected moulding conditions. Preferably, the curative is selected so that it will cure the resin to produce an initial cured Tg of at least 120°C in no more than 5 minutes.

The clathrate may comprise a host component (A) and a guest component (B) where (A) is the protecting means and (B) is the curative. The release mechanism such as heating or pressure or both at or above the expression or release temperature comprises a release which affects the interactions between the host component (A) and the guest component (B) without chemically altering the composition of the each of the components. Typical moulding conditions employ a pressure in the range of 10⁵ to 2.5 x 10⁵ kPa (100 to 250 Bar) and a temperature in the range of 100°C to 250°C typically 120 to 200°C, more typically from 140 to 180 °C and the material is selected to have an release temperature under these conditions and the curative is selected to effect the curing in no more than 5 minutes preferably no more than 100 seconds under these conditions.

In such clathrates the interactions between the host component (A) and the guest component (B) are non-covalent bonds usually hydrogen bonds. The guest (B) and host (A) components are then separated through a physical release by destruction of these bonds at the moulding conditions corresponding to the release or expression temperature and/or release or expression pressure. The clathrate preferably has a crystalline structure as can be determined by X-ray.

We have found that clathrates based on a host compound comprising carboxylic acids and/or an esters containing an aromatic group which is linked to the carboxylic group or ester group by a divalent hydrocarbyl group and/or based on phenolphthalin as a host compound containing a curative as a guest compound are particularly suitable for curing resins as described in our copending application GB 1721593.0. We have found that use of such clathrates as cure initiators provides a good combination of cure conditions and enables the resin to have a long time until onset of curing at ambient temperature (known as outlife). Additionally the use of such clathrates in the process of this invention provides cured resins of sufficiently high initial cured glass transition temperature (Tg) to enable rapid demoulding of the moulded article at the cure temperature together with good Tg retention. The guest component (B) of the clathrate may be a curing agent, an accelerator or both materials. When it is a curing agent it is preferably selected from at least one compound selected from the group consisting of a compound represented by formula:

in which Ri represents a hydrogen atom, a C1-C10 alkyl group, an aryl group, an arylalkyl group, or a cyanoethyl group, and R2 to R4 each independently represent a hydrogen atom, a nitro group, a halogen atom, a C1-C20 alkyl group, a C1-C20 alkyl group substituted with a hydroxy group, an aryl group, an arylalkyl group, or a C1-C20 acyl group; and a part with a dashed line represents a single bond or a double bond, and diazabicycloalkanes (DBCA) such as [1 ,8- diazabicyclo[5.4.0]undecene-7, 1 ,4-diazabicyclo[2.2.2]octane and 1 ,5- diazabicyclo[4.3.0]non-5-ene.]

to 2, preferably 0.7 to 1.7, more preferably from 0.9 to 1.5 and more preferably from 0.95 to 1.4 or from 0.95 to 1.1 and/or combinations of these ratios.

In another embodiment, the resin matrix may contain a curing agent and the guest component (B) in the clathrate may be a cure accelerator to enhance the curing reaction of the curative. For example the formulation may comprise an epoxy resin component, and the curative comprises a heat activated hydrazide based curative such as adipic dihydrazide (ADH) or vinyl dihydrazide (VDH) provided with the resin matrix. The guest component (B) may be an imidazole or imidazoline based component which acts as an accelerator in combination with the hydrazide based curative. Other accelerators include urea-based accelerators (or “Urones”) may also be present as component (B) in the curative composition.

The preferred urone is 4,4-methylene diphenylene bis (N, N- dimethylurea) which is present in the composition in an amount relative to the total weight of the composition of from 2 to 20 weight% and more preferably from 3 to 12 weight%, most preferably in an amount of the total weight of the composition of from 4 to 8 weight%.

Where a fibrous material is employed as fibrous reinforcement it may be a woven fabric or a multi-axial fabric to form a prepreg, or it may comprise individual fiber tows for impregnation with the resin composition to form towpregs, or as chopped fibers, short fibers or filaments to form a moulding compound or, as is preferred it may be a tape. The preferred fibrous material is selected from carbon fibre, glass fibre, aramid and mixtures thereof.

The resin matrix used in this invention is storage stable and is capable of fast curing whilst the Tg, the retained Tg and mechanical properties enable use of the cured resin composition in industrial structural applications particularly automotive and aerospace structural components as well as sporting goods and wind turbine components.

In the curing of the thermosetting resin according to this invention the cure initiator may be driven into the resin and activated by the moulding conditions to effect curing of the resin. The activation may comprise a release or expression of the curative such as by heating or pressurizing to the release temperature which affects the interactions between the host means and the guest component such as the curative of a clathrate by chemically altering the composition of one or both of the components.

The cure initiator is forced into the resin formulation from the surface of the resin by the pressures applied to the materials in the mould during the moulding process. Once it is located within the resin it may also be activated by the combination of the pressure and temperature applied in the mould.

We prefer that the curative has a melting temperature that is higher than the cure temperature as this ensures that the curative is retained within the resin during the cure cycle. It is believed that the moulding pressure causes a rapid interaction of the curative and any curative such as an accelerator that may be present in the resin formulation to effect a rapid and uniform cure of the resin particularly when the resin is an epoxy resin. Additionally by the use of this invention the desired outlife of the resin matrix is obtained because curing does not take place until the composition is heated under pressure and there is contact between the accelerator and/or curing agent and the resin component(s). The invention is useful in a host of applications: composite materials, coatings, gel coats, adhesives and laminates. The resin matrix may comprise additional resin components, fillers, and/or impact modifiers.

SPECIFIC DESCRIPTION

Epoxy resin component

When the resin is the preferred epoxy resin the epoxy component may be mono-functional or multifunctional, preferably at least difunctional. In an embodiment, the epoxy resin component (A) may be selected from various conventionally-known polyepoxy compounds. Examples thereof include: aromatic glycidyl ether compounds such as bis(4- hydroxyphenyl)propane diglycidyl ether, bis(4-hydroxy-3,5-dibromophenyl)propane diglycidyl ether, bis(4-hydroxyphenyl)ethane diglycidyl ether, bis(4-hydroxyphenyl)methane diglycidyl ether, resorcinol diglycidyl ether, phloroglucinol triglycidyl ether, trihydroxy biphenyl triglycidyl ether, tetraglycidyl benzophenone, bisresorcinol tetraglycidyl ether, tetramethyl bisphenol A diglycidyl ether, bisphenol C diglycidyl ether, bisphenol hexafluoropropane diglycidyl ether, 1 ,3-bis[1-(2,3-epoxypropoxy)-1-trifluoromethyl-2,2,2-trifluoroethyl]benzene, 1 ,4-bis[1 -(2,3- epoxypropoxy)-1-trifluoromethyl-2,2,2-trifluoromethyl]benzene, 4,4'-bis(2,3- epoxypropoxy)octafluorobiphenyl, and phenolic novolac type bisepoxy compounds; alicyclic polyepoxy compounds such as alicyclic diepoxy acetal, alicyclic diepoxy adipate, alicyclic diepoxy carboxylate, and vinylcyclohexene dioxide; glycidyl ester compounds such as diglycidyl phthalate, diglycidyl tetrahydrophthalate, diglycidyl hexahydrophthalate, dimethylglycidyl phthalate, dimethylglycidyl hexahydrophthalate, diglycidyl-p-oxybenzoate, diglycidylcyclopentane-1 ,3-dicarboxylate, and dimer acid glycidyl ester; glycidyl amine compounds such as diglycidyl aniline, diglycidyl toluidine, triglycidyl aminophenol, tetraglycidyl diaminodiphenyl methane, and diglycidyl tribromoaniline; and heterocyclic epoxy compounds such as diglycidylhydantoin, glycidyl glycidoxyalkylhydantoin, and triglycidyl isocyanurate; and oligomer compounds thereof.

Examples of the liquid epoxy resin include polyalkylene ether type epoxy compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and trimethylolpropane triglycidyl ether; glycidyl ester type epoxy compounds such as dimer acid diglycidyl ester, phthalic acid diglycidyl ester, and tetrahydrophtalic acid diglycidyl ester; and homopolymers of glycidyl (meth)acrylate, allyl glycidyl ether and the like or copolymers of these monomers with other soft unsaturated monomers. In this context, soft unsaturated monomer refers to a monomer which contains a homopolymer which has a glass transition temperature of less than 60°C. Examples of soft unsaturated monomers include methyl acrylate, ethyl acrylate, butyl(meth)acrylate, isobutyl(meth)acrylate, 2- ethylhexyl(meth)acrylate, and lauryl methacrylate.

Clathrate component

A "clathrate" is a compound in which two or more molecules are bound via a bond other than a covalent bond, typically hydrogen bonds and which are crystalline as indicated by X-ray diffraction. For example in a clathrate formed between a host containing a carboxyl or carboxylic acid ester group such as phenolphthalin and a guest component containing nitrogen such as an imidazoline the molecules may be bound together by one of the hydrogens on the nitrogen forming a hydrogen bond with the oxygen of the carboxylate functionality.

In a clathrate a compound which includes another compound or compounds is referred to as the host compound or the encapsulating material and the compound or compounds which is or are included in the host compound is referred to as the guest compound.

Instead of a single guest compound, two or more different guest compounds may be present in the clathrate. In the clathrates used in this invention the guest compounds are preferably amino based curatives such as imidazole compounds or imidazoline compounds as defined above under formula (II). The guest compounds may also include accelerators or a combination of curatives.

In a preferred embodiment the host compound is at least one compound selected from the group consisting of a carboxylic acid compound represented by a phenolphthalin or the formula (I):

i. the host component (A) being defined by the formula (I):

where n is 0 or 1

Ar is an optionally substituted aryl group

X is independently selected from H, OH, an optionally substituted alkyl group and an optionally substituted aryl group;

Y is independently selected from H, OH, an aryl group, an optionally substituted alkyl group, an optionally substituted aryl group; R is a divalent optionally substituted hydrocarbyl group and

Z is selected from

1) C(=0)0-R’ wherein R’ is selected from hydrogen, an optionally substituted hydrocarbyl group; and

2) a ring structure formed including Y and C in formula I;

or

ii. phenolphthalein.

R and R’ may be independently a linear or branched substituted or unsubstituted, saturated or unsaturated C1-C9 alkyl or aryl hydrocarbyl group and when it is an alkyl group it may be cyclic or heterocyclic.

The "optional substituent" of R and/or R’ may be a halogen atom, a C1-C6 alkyl group, an aryl group, a C1-C6 alkoxy group, a hydroxy group, a carboxy group, a nitro group, an amino group, and an acyl group.

Preferably the host component contains both phenolic and carboxylic acid or ester functionalities both of which are capable of forming clathrates with imidazoles. For example a preferred cost component is 4,4’-bis(4’-hydroxyphenyl)valeric acid (BHPVA) which contains both phenol and carboxylic acid functionalities. Preferably the clathrate is formed with 2-ethyl-4-methylimidazole (2E4MZ).

The host component may also be phenolphtalin (PhPh) which contains bis-phenol and mono-carboxylic acid functionalities, both of which are capable of forming clathrates with imidazoles. Preferably the clathrate is formed with 2-ethyl-4-methylimidazole (2E4MZ).

In another clathrate, the host component may be benzilic acid (BA) which contains phenyl and mono-carboxylic acid functionalities, which is capable of forming clathrates with imidazoles. Again, preferably the clathrate is formed with 2-ethyl-4-methylimidazole (2E4MZ).

In a further clathrate, the host component may be 4-aminophenylacetic acid (APAA) containing aminophenyl and mono-carboxylic acid functionalities, both of which are capable of forming clathrates with imidazoles. Esters of these carboxylic acid based clathrates may also be selected. In preferred embodiments the carboxylic acid or carboxylic ester compound may be selected from phenylacetic acid, 4-aminophenylacetic acid (APAA), , phenolphthalin ® (PhPh), benzilic acid (BA), 2,2-bis(p-hydroxyphenyl)propionic acid (BHPPA), or 4,4-bis(p- hydroxyphenyl)valeric acid (BHPVA) or 2,2-bis(p-hyroxyphenyl) acetic acid (BHPAA) and their alkyl esters preferably Ci to Cg alkyl ester.

Guest Compound

The guest component preferably comprises a curative compound having an amino group. Imidazole-based and/or imidazoline based curative compounds such as those of formula (II) are particularly suitable.

The guest component may be selected from at least one compound selected from the group consisting of a compound represented by formula (II) and/or DBCA.

The structure of the clathrate can be verified by thermal analysis (TGA-DSC, Simultaneous Thermogravimetry & Differential Scanning Calorimetry), an infrared absorption spectrum (IR), an X-ray diffraction pattern, a NMR spectrum, or the like, X-ray diffraction being particularly useful. Further, the composition of the clathrate can be verified by thermal analysis, a ¹H-NMR spectrum, high performance liquid chromatography (HPLC), elementary analysis, or the like.

Curative

As discussed, a formulated resin system or composition usually contains a cure accelerator which can be included in the resin matrix.

In the present invention the guest component is a curative which may be a cure accelerator, or a curing agent or both.

The curing agent which may be contained in addition to the curing accelerator is not particularly limited as long as it is a compound which reacts with an epoxy group in an epoxy resin to cure the epoxy resin. Similarly, a curing accelerator which may be contained in addition to the curing agent is not particularly limited as long as it is a compound which accelerates the above curing reaction.

Any one of conventional cure agents or cure accelerators of epoxy resins can be selected and used as such a curing agent or a cure accelerator, respectively. Examples thereof include amine-based compounds such as aliphatic amines, alicyclic and heterocyclic amines, aromatic amines, and modified amines, imidazole-based compounds, imidazoline- based compounds, amide-based compounds, ester-based compounds, phenol-based compounds, alcohol-based compounds, thiol-based compounds, ether-based compounds, thioether-based compounds, urea-based compounds, thiourea-based compounds, Lewis acid-based compounds, phosphorus-based compounds, acid anhydride-based compounds, onium salt-based compounds, and active silica compound-aluminium complexes.

Prepreg matrix formulation

The moulding material may be constructed from a cast resin film of the resin formulation and may be combined with a fibrous reinforcement layer. Preferably the resin film impregnates the fibrous reinforcement which may be accomplished by pressing a layer of resin onto the fibrous material or by infusion of the resin into fibrous material within a mould.

The resin formulation is excellent in both storage stability and curing characteristics and can be used for applications which require long term storage of the resin or storage in unconditioned facilities at room temperature.

The cure initiator is applied onto a film of the resin perhaps including fibrous reinforcement and it is forced into the film of the resin by the moulding pressure whilst at the same time the moulding conditions activate the cure initiator to cause rapid cure of the resin.

Where a clathrate component is used the clathrate component can be used as a cure initiator or as a cure accelerator and the other component may be present in the resin component. The guest component may operate as a curing agent or as a cure accelerator. The component is selected so as to be quickly released from a host component, under the moulding conditions which include a temperature and/or pressure which exceeds ambient or storage conditions and if the component is a curing agent, it will undergo a rapid crosslinking reaction with the resin component. If the component is an accelerator, the released curing accelerator acts as a curing catalyst of the curing agent and the resin component, thereby forming a cured formulated resin matrix.

Depending upon the nature of the host compound it can be selected so that it will react with the resin after releasing the guest compound, thereby having an additional effect as a crosslinking agent. This is particularly so when the host compound is a carboxylic acid and can result in the cured resin formulation product having improved flexibility and improved impact resistance and adhesion. The resin matrix formulation used in the present invention can be prepared by uniformly mixing the resin and other additives using a pot mill, a ball mill, a bead mill, a roll mill, a homogenizer, Supermill, Homodisper, a universal mixer, Banbury mixer, a kneader, or the like.

The resin matrix may include other typical additives used in thermosetting resins such as impact modifiers, fillers, antioxidants and the like.

Impact modifiers

The composition resin matrix formulation employed in this invention may comprise an impact modifier. Impact modifiers are widely used to improve the impact strength of cured resin compositions with the aim to compensate their inherent brittleness and crack propagation. Impact modifier may comprise rubber particles such as CTBN rubbers (carboxyl-terminated butadiene-acrylonitrile) or core shell particles which contain a rubber or other elastomeric compound encased in a polymer shell. The advantage of core shell particles over rubber particles is that they have a controlled particle size of the rubber core for effective toughening and the grafted polymer shell ensures adhesion and compatibility with the epoxy resin composition. Examples of such core shell rubbers are disclosed in EP0985692 and in WO 2014062531.

Alternative impact modifiers may include methylacrylate based polymers, polyamides, acrylics, polyacrylates, acrylate copolymers, phenoxy based polymers, and polyethersulphones. Fillers

In addition one or more fillers may be included to enhance the flow properties of the composition. Suitable fillers may comprise talc, microballoons, flock, glass beads, silica, fumed silica, carbon black, fibers, filaments and recycled derivatives, and titanium dioxide.

EXAMPLES

The invention will now be illustrated by way of example only and in no way limited by reference to the following Examples.

A formulated resin matrix system was prepared by mixing the following materials.

i) 40 parts of a novalac resin

ii) 30 parts of a Bisphenol A diglycidyl ether resin

iii) 15 parts of a Bisphenol A resin

iv) 8 parts of adipic acid dihydrazide

v) 7 parts of a urone.

A fabric of unidirectional carbon fibres Grafil™ 34-700WD (Mitsubishi) was impregnated with the formulated resin matrix system to form a moulding material composition comprising a web of 150 g/m² containing 38% of the resin system. The web was cut to form plies of 300 mm by 300 mm.

16 plies were stacked on top of each other in a mould and the stack was cured in a mould at a temperature of 150°C and a pressure of 200 Bar (20 MPa) to produce a panel (referenced herein as Comparative Example 1).

Additional panels including a further Comparative Example 2 and Examples 1 and 2 of the invention were prepared by stacking 16 plies on top of each other in a mould with 9 g/m² of various reactive curing agents placed between the plies and the stack was cured in a mould at a temperature of 150°C and a pressure of 200 Bar to produce a panel.

In the first example of the invention (Example 1), the cure initiator was a clathrate of 4,4”-Bis (4 hydroxy phenyl valeric acid) and 2-Ethyl-4-methyl imidazole (BHPVA-2E4MZ). In the second example of the invention (Example 2) the cure initiator was Aradur 3123 1- ((2- methyl-1 H-imidazol-1-yl) Methyl) naphthalen-2-ol as available from Huntsman having a melting point about 210°C. In Comparative Example 2 , 2-ethyl-4-methyl imidazole (2E4MZ) was used as the cure initiator.

The speed of cure was determined by dielectric analysis (DEA) in accordance with ASTM E ASTM E 2038 and E 2039 using a DEA 230 Epsilon from Netzsch. The results were set out in below Table 1.

Table 1. Comparative examples 1 and 2 and Examples 1 and 2

Claims

1. A moulding process comprising

a. providing an uncured thermocurable resin system comprising a resin component and a particulate cure initiator for reacting with said resin component, the particulate cure initiator being provided on the surface of the resin component and the cure initiator comprising a curative; b. applying moulding conditions to release the curative into the resin component to trigger cure wherein the cure initiator is inert in relation to the resin component until the moulding conditions are applied.

2. A process according to claim 1 , wherein the cure initiator releases the curative in response to the moulding conditions, the moulding conditions comprising an increased temperature above ambient temperature (25°C) and/or increasing the pressure above atmospheric pressure.

3. A process according to claim 1 or claim 2, wherein the curative comprises a curing agent, an accelerator and/or a combination thereof.

4. A process according to any of the preceding claims, wherein the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the release temperature is above the softening point of the cure initiator.

5. A process according to any of the preceding claims, wherein the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the release temperature is above the dissolution temperature of the cure initiator in the resin component.

6. A process according to any of the preceding claims, wherein the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the release temperature is above the melting point of the cure initiator.

7. A process according to any of the preceding claims, wherein the cure initiator is a solid at room temperature (25 °C) and at atmospheric pressure and the curative is released upon applying pressure to the cure initiator which exceeds atmospheric pressure.

8. A process according to claim 7, wherein the pressure is in the range of from 10 to 400 bar (1 to 40 MPa), preferably from 20 to 250 bar (2 to 25 MPa) or from 100 to 250 bar (10 MPa to 25 MPa), more preferably from 150 to 200 bar (15 MPa to 20 MPa).

9. A process according to any of the preceding claims, in which the cure initiator is a clathrate having a guest component and a host component in which the curative is the guest component.

10. A process according to claim 9, wherein the host component is selected so that the bonds between the guest and host components are broken to release the curative under the application of pressure above atmospheric pressure and/or increasing the temperature above ambient temperature (25 °C).

11. A process according to any of the preceding claims in which the cure initiator is provided on the surface of the thermocurable resin system as a powder of average D90 particle size in the range of from 1 to 60 pm, preferably from 5 to 50 pm and most preferably from 8 to 30 pm .

12. A process according to any of the preceding claims in which the cure initiator is applied to a moving web or tape of the resin matrix containing fibrous reinforcement which is then placed in a mould.

13. A process according to Claim 12 in which the cure initiator is applied in an amount from 1 to 40 g/m² , preferably from 5 to 25 g/m² and most preferably from 5 to 15 g/m² of the web or tape.

14. A process according to Claim 12 or 13 wherein the cure initiator is applied shortly before the web or tape is introduced into and laid up in the mould.

15. A process according to any of the preceding claims for the production of moulded articles from tapes of fibre reinforced resins comprising forming a resin matrix formulation that does not include at least one element of the cure initiator, providing a moving web of fibrous material and impregnating the moving web with the resin matrix slitting the web into tapes, applying the particulate cure initiator to the surface of the tape and laying up the tapes in a mould and moulding the tapes under pressure and temperature conditions that cause the resin material to cure.

16. A process according to any of the preceding claims in which the moulding conditions employ a pressure in the range of 10⁵ to 2.5 x 10⁵ kPa (100 to 250 Bar) and a temperature in the range of 100°C to 250°C.

17. A process according to any of Claims 9 to 16 wherein the guest component of the clathrate is a curing agent selected from at least one compound selected from the group consisting of a compound represented by formula:

18. A process according to any of the preceding claims wherein the uncured thermocurable resin contains a fibrous material.

19. A process according to any of the preceding claims, wherein the thermocurable resin cures to at least 95% cure with an initial cured Tg at the moulding temperature of at least 120°C in no more than 5 minutes.

20. A moulding process wherein thermocurable resin containing a fibrous material is stored and transported to a moulding location as a web where it is provided with a particulate cure initiator on the surface of the web and then passed to a mould where it is cured at a temperature in the range 100°C to 250°C and a pressure of 10⁵ to 2.5 x 10⁵ kPa (100 to 250 Bar) for less than 100 seconds to produce a moulding with a cured glass transition temperature (Tg) of at least 120°C as determined in accordance with ASTM D7028.

21. A moulded article produced by a process according to any one of Claims 1 to 19.

22. An article according to Claim 222 comprising an automobile component, an aerospace component or a wind turbine component.