EP4017904A1

EP4017904A1 - Improved thermocurable moulding process

Info

Publication number: EP4017904A1
Application number: EP20754739.9A
Authority: EP
Inventors: Nicholas VERGE
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2019-08-21
Filing date: 2020-08-11
Publication date: 2022-06-29
Also published as: WO2021032537A1; GB201911998D0

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 to 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. In one embodiment the invention is directed at reducing the in mould moulding cycle time to be no more than 100 seconds per cycle, especially in reducing the cycle time required for the production of a moulding from tapes of the thermocurable material in automated moulding processes. This is achieved by the production of mouldings with an initial cure 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. The invention is particularly useful in moulding and in curing of tapes of thermocurable material, even more so in tapes of fibre reinforced thermocurable material.

Typical thermocurable resins are liquids at ambient temperature, and although they are reactive materials they are selected to be largely inactive at ambient temperature so that they do not cure or 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. The cure of the resin is initiated at a certain temperature (known as the onset of cure) by a curing agent initiator, and the speed of cure can be increased by the use of a cure accelerator.

Within this application the term “cure initiator” is a component comprising a curative which is adapted to initiate or promote curing. A curative is a curing agent or a curing accelerator or a combination of the two. Curing accelerators are chemical compounds which enhance the polymerisation reaction (or “curing”) and curing agents are chemical compounds which initiate the polymerisation reaction of a polymerisable resin.

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 reinforcement resin matrix formulation. The reinforcement 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.

Epoxy resins are cured with curatives for the epoxy resin which control the temperature at which the resin will cure and the speed at which it will cure, and these are selected according to the nature of the epoxy resin and the product to be produced. The curatives are incorporated into the epoxy resin prior to moulding to form a curable resin mixture, and in the case of fibre reinforced resins prior to impregnation of the fibre with the resin.

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 230,000 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 two component mixed epoxy formulations of resin and curative which need to be prepared in situ at the moulding location to prevent premature curing. The formulation is rapidly injected into a fibrous preform 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 the resin composition 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 composition may or may not be mixed with the fibrous reinforcement. 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 matrix formulations 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 when mixed with epoxy resin they have the problem that they show an early on-set of curing leading to short outlife. Such curatives therefore cannot be used as a single-component epoxy resin composition which is manufactured and then delivered at the point of use, because these compositions would thicken, gel and cure in transit or in storage. 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 , however the epoxy formulations of these patents are slow curing.

Curatives 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 of this document 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.

WO 2016/087935 also discloses the use of clathrates based on various carboxylic acids in combination with imidazole cure initiators 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 reducing 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 discloses the production of 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.

During the moulding cycle the resin composition is heated to the curing temperature and then maintained at the curing temperature. Pressure is also usually applied during the moulding cycle. During an initial phase of the cycle the viscosity of the resin decreases as it heats up and the viscosity then increases back to and beyond the initial viscosity as the curing takes effect. As the pressure is applied when the viscosity is low there is a risk with conventional curatives that the curatives may be forced to the surface of the composition or right out of the composition.

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 one of the accompanying claims.

In an embodiment, there is provided a moulding process comprising a. providing an uncured thermocurable resin system comprising a resin component and a cure initiator for reacting with said resin component, the cure initiator being provided on the surface of the resin component and the cure initiator comprising a curative, the cure initiator preferably being in particulate form; b. applying pressure to drive the curative into the resin component to trigger cure whereby the thermocurable resin cures to at least 95% cure with an initial cured Tg of at least 120°C in no more than 5 minutes.

By a “cure initiator” we mean a component that comprises a curative but effectively immobilises the curative so that it does not substantially interact with a thermocurable resin with which the cure initiator is in contact under ambient conditions, but that allows the curative to interact with the thermocurable resin to facilitate curing thereof under selected conditions, such as selected pressures and/or temperatures.

By a “curative” we mean a component suitable for use in promoting curing of a thermocurable resin system, so that the thermocurable resin cures to at least 95% cure with an initial cured Tg of at least 120°C in no more than 5 minutes. The curative may comprise a curing agent, a cure accelerator or both. The curative is particularly useful for curing during the formation of laminates, particularly in stacks formed from multiple layers of reinforcement fibres impregnated with curable resin. By “promoting curing” we mean beneficially affecting curing in any way, such as by acting as the sole curing agent, i.e. by causing curing to take place under specific conditions; acting as a co-curing component to modify the effect of one or more other curing agents; affecting the conditions required for curing to take place and/or the time for curing to complete, for example by causing curing to begin only when a certain temperature is reached; and/or reducing the time to reach peak exotherm and/or maximum cure to be achieved; or affecting the properties of the cured resin.

The cure initiators used in the present invention are preferably solid at ambient conditions. Preferably, in the present invention, the cure initiator is provided on the surface of the resin component as a veil, fibre, mat, film and/or sheet, but more preferably it is provided in particulate form, for example as a powder or as granules.

The cured Tg of the cured thermocurable resins formed in the method of the present invention may be tested by any conventional method, such as the standard method ASTM D7028. Generally, the cured materials will be cooled before testing, however, as the Tg does not change significantly during cooling, the Tg measured for the cooled material can be assumed to be the same as the initial cured Tg, i.e. the Tg at the moulding temperature at the end of the curing step.

The cure initiator that is provided on the surface of the resin may be selected so that it will react with the resin and initiate the cure at the moulding temperature. Alternatively, it may be a material that releases 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 chosen so that the release temperature may be the desired onset of cure temperature.

One object of the present invention is to provide a moulding process involving a curable epoxy resin composition, wherein the composition has excellent storage stability, and the process has enhanced rapid curing characteristics to produce mouldings which have initial cured glass transition temperatures (Tg) that are sufficiently high to allow for rapid demoulding at the moulding temperature to provide a cured product having excellent mechanical properties.

In an embodiment, the cure initiator may comprise a thermoplastic matrix. The curative may be released from the cure initiator at a temperature and/or pressure sufficient to cause the cure initiator to reach or exceed the melting point and/or softening point of the thermoplastic matrix.

The present invention therefore provides a cure initiator comprising a temperature and/or pressure dependent releasable curative which enables the triggered cure of a formulated resin matrix by virtue of its release of the curative at desired condition(s).

Release or expression of the curative allows the curative to react with one or more other components in the resin system.

The release temperature may be selected by selecting the melt point or softening point of the thermoplastic polymer matrix and/or the melt point or softening point of the curative.

In a preferred embodiment, the curative is selected from at least one curing agent component, at least one accelerator component and/or a combination of the aforesaid components.

In a preferred embodiment, the thermoplastic polymer matrix is a solid at 25°C and at atmospheric pressure.

The present invention further provides a method of forming a cure initiator according to the present invention comprising dissolving or dispersing a thermosetting resin curative in a molten thermoplastic polymer or a solution of a thermoplastic polymer and forming a film, filaments of fibres therefrom. The present invention further provides a curable assembly comprising at least one layer of prepreg comprising a layer of fibrous reinforcement material at least partially impregnated with a thermosetting resin, and further comprising at least one cure initiator according to the present invention, wherein the thermosetting resin curing agent of the cure initiator is suitable to promote curing of the thermosetting resin of the prepreg.

Because the thermosetting curing agent is dispersed within the matrix of the thermoplastic polymer in the cure initiator of the present invention, it will generally not be released until the polymer matrix is disrupted in some way, such as by melting or softening due to heating and/or pressurizing. Therefore, the curative will not significantly interact with a thermosetting resin with which the cure initiator is placed in contact until the temperature is raised to a sufficient degree to cause such disruption to begin.

Selection of an appropriate thermoplastic resin therefore allows the release of the thermosetting resin curing agent, and therefore the start of curing, to be delayed until a selected temperature and/or pressure is reached. The controlled release of the curative can also have beneficial effects on the curing process, for example, the time to reach peak exotherm and/or maximum cure temperature can be reduced, resulting in more controlled curing and/or a reduction in the total heat input required for curing. Thus, the cure promoting components of the present invention provide a method of safely and efficiently storing and delivering reactivity to a composite structure.

The cure initiator of the present invention can also provide numerous additional advantages. For example, thermosetting resins and/or prepregs containing the cure intiator can be stored longer, and the need for special storage conditions, such as refrigeration is reduced or can be omitted.

Furthermore, because the cure initiators of the present invention are solid at 25°C, they can be handled relatively easily under ambient conditions, and can therefore be easily used when laying up layers of prepregs and/or dry reinforcement fibres prior to curing.

Similarly, because the curatives are dispersed in the cure initiator they are much safer to handle than powdered resin, and do not require the use of special conditions for handling, such as forced ventilation or protective equipment. In addition, the cure initiators of the present invention can be easily placed in the most appropriate locations during the laying up, allowing increased amounts of curative and/or alternative or additional curatives to be used in certain locations within a lay-up, to modify or improve the curing at selected locations. The cure initiators of the present invention also encourage exothermic enhancement early in the heat-up phase of cure, and therefore reduce, or even totally remove, the possibility of an unmanageable exothermic overshoot at peak exotherm.

Yet further, any additional components such as thermoplastic polymers contained in the cure initiators of the invention may themselves provide beneficial effects to the cured material, such as improved toughness and/or improved interlaminar shear strength.

The choice of the thermoplastic polymer used in the cure initiator of the present invention may depend on many factors; the main requirements for such materials being that they form solid films, filaments or fibres at 25°C and at atmospheric pressure and that they maintain a thermosetting resin curative in the polymer matrix but release the thermosetting resin curative under suitable conditions (for example, by melting or softening at a suitable temperature) and/or use of external pressure. The selection of suitable materials may also take into account the intended use of the cure initiator, the thermosetting resin with which the curative is to be used and the conditions that are to be used to cure the resin, particularly the curing temperature.

It is preferred that the thermoplastic polymer has a melting point and/or softening point in the range of 40°C to 400°C, preferably from 50 to 250°C and more preferably from 50 to 180°C, all at atmospheric pressure. Examples of suitable polymers include aliphatic polyesters such as polycaprolactones

The cure initiator may be exposed to pressure 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 to 25 MPa), more preferably from 150 to 200 bar (15 to 20 MPa).

In an embodiment of the present invention the thermoplastic polymer is a polyester, preferably an aliphatic polyester such as a polycaprolactone polymer. The polycaprolactone polymer preferably has a molecular weight of from 10,000 to 100,000 g/mol, more preferably from 25,000 to 75,000 g/mol. In certain embodiments of the present invention, the thermoplastic polymer may comprise a mixture of two or more different thermoplastic materials to provide a range of properties.

The thermosetting resin curing agents used in the cure initiator may be selected from any materials promoting curing of thermosetting resins, including sole curatives, co-curatives, cure accelerators or any other materials that beneficially affect curing. Stoichiometric thermosetting resin curatives may be used in the cure initiators of the present invention; but particularly useful curatives include non-stoichiometric curing agents, i.e. curing agents that have a catalytic effect on curing rather than forming a component of the cure complex and/or cure accelerators, and which can act to accelerate the curing promoted by a curative already present in the curable thermosetting resin composition. The selection of a particular curative will depend on many factors, including the intended use of the cure initiator, the thermosetting resin to be cured, and the nature of the thermoplastic polymer. Preferably the thermosetting resin curatives are solid at 25°C, and more preferably have a melting point and/or softening point in the range of 40°C to 400°C, even more preferably from 50 to 250°C, and most preferably from 50 to 180°C.

The curative may include a cure initiator, an accelerator or both of these materials. Typically in order to cure a resin in an acceptably short time a combination of both a cure initiator and an accelerator is used. However, the presence of such a combination in a prepared resin matrix formulation 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 a preferred embodiment the curative is a clathrate in which the curative is the guest component and the host component is selected so that 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 moulding conditions when applied. 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 curing initiator is preferably provided on the surface of the thermocurable resin system in solid form, more preferably as a particulate, which may be powder or granules. Powder is particularly preferred, 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. The amount of the cure initiator applied to the surface of the thermocurable resin system will depend on many factors, such as the amount of resin, the concentration of curative in the cure initiator and the relative reactivity of the curative with the resin system. Where fibre is present in the resin system, the amount of the cure initiator applied will also depend on the amount of fibre. Preferably, the amount of cure initiator applied will be from 2 to 30 wt% of the amount of resin. In general, the amount of cure initiator applied to the surface of a resin system will be from 1 to 100 gm², preferably from 5 to 50 gm², more preferably from 5 to 40 gm². However, one skilled in the art will understand how to select values within the above ranges for particular combinations of resin and fibre areal weight (if present). For example, in a prepreg comprising carbon fibre at a fibre areal weight of 150 gm² and a thermocurable resin at 92 gm², the cure initiator may be added at an amount of from 1 to 40 gm², preferably 5 to 25 gm², more preferably 5 to 15 gm².

In the preferred automated moulding process, it is preferred that the particulate curative is applied shortly before the web or tape is introduced into and laid up in the mould.

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 while moulding 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.

In a preferred embodiment moulded articles are produced from tapes of fibre reinforced resins and the invention comprises forming a resin matrix formulation that does not include at least one element of the curative, impregnating a moving web of fibrous material with the resin matrix, slitting the impregnated moving web into tapes, applying the particulate curative to the surface of the tape and laying up the tapes in a mould and moulding the tape under pressure and temperature conditions that cause the resin material to cure. The curative may be applied to the moving web before or after slitting and may be applied before or upon introduction of the resin impregnated fibrous material into the mould.

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.

Where the curative comprises a clathrate, it comprises a host component (A) and a guest component (B) where (A) is the shielding means and (B) is the curative. The shielding means

(A) prevent the curative (B) from reacting until its release from the shielding means in accordance with a release mechanism.

The release mechanism such as heating to at or above the 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.

An alternative release mechanism may comprise an increase in pressure to a level which releases component (B). Release mechanisms may also be combined (such as temperature and pressure) which may coincide with the desired moulding conditions.

Typical moulding conditions employ a pressure in the range of 10 to 25 MPa (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 a 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 temperature. 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 acid and/or an ester 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 co-pending application GB 1721593.0. We have found that use of such clathrates as cure initiators provide a good combination of cure conditions and enable 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 curative (B) of the clathrate may be a curing agent , an accelerator or both and 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.]

The mol ratio of component (A) to component (B) in the clathrate is in the range of from 0.5 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 cure initiator 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 2 to 20 wt% and more preferably 3 to 12 weight% in relation to the total weight of the composition, most preferably in an amount of the total weight of the composition with respect to 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 fibre tows for impregnation with the resin composition to form towpregs, or as chopped fibres, short fibres 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 curative is driven into the resin and activated by the moulding conditions to effect curing of the resin. When the cure initiator is a clathrate the activation may comprise a release such as by heating 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 curative 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 is 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 system is heated under pressure and there is contact between the accelerator and cure initiator and the resin. 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.

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

SPECIFIC DESCRIPTION

Various aspects of the inventions are now described in more detail and by way of example only.

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 WO2014/062531.

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, fibres, filaments and recycled derivatives, and titanium dioxide.

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 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. Rand 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) or2,2-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 preferred. 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.

Cure Initiator or Curing Accelerator

The resin system usually contains a curing accelerator which can be included in the resin matrix or it can be in the curative that is applied to the layer of the resin system, and in this instance it could be that the guest component is a curing accelerator, a cure agent may be further included.

The agent may not be 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 cure 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 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 curing initiator or as a curing accelerator and the other component may be present in the resin matrix so that the guest component may operate as a curative or as a curing accelerator. The component is selected so as to be quickly released from a host component, under the moulding conditions at the release temperature 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 curative 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 invention is illustrated but in no way limited by reference to the following Examples.

A resin matrix formulation was prepared by mixing the following materials. i) 40 parts of an epoxy novalac resin ii) 45 parts of a bisphenol A resin iii) 8 parts of adipic acid dihydrazide iv) 7 parts of a urone.

A fabric of unidirectional carbon fibres Grafil™ 34-700WD having a fibre areal weight of 150 gm² (obtained from Mitsubishi) was impregnated with the above defined resin matrix formulation to make a prepreg with 38 w/w resin. The prepreg 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 to produce a panel (comparative example 1).

Additional panels representing Examples 1 and 2 of the invention and further comparative example 2 were prepared by stacking 16 plies on top of each other in a mould with 9 grams per square meter of various cure initiators placed between the plies, and the stacks were cured in a mould at a temperature of 150°C and a pressure of 20 MPa (200 Bar) to produce a panel.

In Example 1 of the invention, the cure initiator was a clathrate of 4,4”-Bis (4 hydroxy phenyl valeric acid) and 2-Ethyl-4-methyl imidazole (BHPVA-2E4MZ). In Example 2 of the invention 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, a conventional curative, 2-phenylimidazole (Curezol 2PZ, available from Evonik), in powder form (50 pm, at room temperature of 25 °C), was used as the cure initiator.

The speed of cure was determined by dielectric analysis in which the curing material is subject to an oscillating electric current and determining how the ions move in the current, the ions became less mobile as the matrix is cured and the results are interpreted to determine the time required for 95% cure.

The initial cured Tgs of the cured materials were measured in accordance with ASTM D7028.

The results were as follows.

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

A marked increase in speed of cure was observed in the process for the cure initiators of Examples 1 and 2. The two Examples of the invention also both had initial Tgs above 120°C.

Claims

1. A moulding process comprising a. Providing an uncured thermocurable resin system comprising a resin component and a cure initiator for reacting with said resin component, the cure initiator being provided on the surface of the resin component and the cure initiator comprising a curative; and b. applying pressure to drive the curative into the resin component to trigger cure; whereby the thermocurable resin cures to at least 95% cure with an initial cured Tg of at least 120°C in no more than 5 minutes.

2. A process according to claim 1, wherein the cure initiator is a solid at ambient conditions.

3. A process according to claim 1 or claim 2, wherein the cure initiator is provided on the surface of the resin component as a veil, fibre, mat, film and/or sheet, more preferably it is provided in particulate form, most preferably as a powder or as granules.

4. A process according to any preceding claim, wherein the curative comprises a curing agent, a cure accelerator or a combination thereof.

5. A process according to any preceding claim, wherein the cure initiator comprises a release temperature above which the cure initiator releases the curative, the process further comprising the step of increasing the temperature of the resin system above the release temperature of the cure initiator.

6. A process according to claim 5, wherein the release temperature is above the softening point of the curative, or the release temperature is above the melting temperature of the curative.

7. 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 and the host component is selected so that the bonds between the guest and host components are broken to release the curative under the moulding conditions.

8. A process according to claim 7, wherein the guest component of the clathrate is a curative agent 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.]

9. 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 .

10. 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.

11. A process according to claim 10, in which the cure initiator is applied in an amount from 1 to 100, preferably from 5 to 50 and most preferably from 5 to 40 grams per square meter of the web or tape.

12. A process according to claim 10 or claim 11 , wherein the cure initiator is applied shortly before the web or tape is introduced into and laid up in the mould.

13. 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 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.

14. A process according to any of the preceding claims in which the moulding conditions employ a pressure in the range of 1 to 40 MPa (10 to 400 Bar), and a temperature in the range of 100°C to 250°C.

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

16. 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 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 1 to 40 MPa (10 to 400 Bar) for less than 100 seconds to produce a moulding with a cured Tg of at least 120°C.

17. A moulded article produced by a process according to any one of Claims 1 to 16.

18. An article according to Claim 17 comprising an automobile component, an aerospace component or a wind turbine component.