Moulding materials comprising epoxy resin materials are widely used for the production of high quality lightweight structural components. One of the problems associated with epoxy resin based mouldings is that, for a lot of applications, it is not possible to produce an epoxy moulding with a high quality cosmetic surface directly in the mould. Therefore additional surface treatments such as coating are necessary to arrive at the desired cosmetic surface quality. High cosmetic quality surfaces are however relatively easy to produce directly in the mould using polyester moulding materials, as there are a wide number of excellent polyester in mould coatings (generally known as gel coats) available. However, polyester materials technology is not as good mechanically as epoxy materials technology. Polyester mouldings are generally heavier than epoxy mouldings for mouldings with similar mechanical properties.

In the past, attempts were made to combine different types of moulding materials, but the combined moulding materials had an unsatisfactory bond. For example, polyester gelcoats were combined with epoxy laminate systems. The formed bond between the gelcoat and the laminate was however unreliable and mechanically weak. This rendered the application of combined polyester laminates and epoxy laminates unsuitable for many applications.

Since the chemistries of the various resin systems are essentially different, the resin materials are both chemically and mechanically incompatible, thus strong bonds between the respective resin material can not be readily

formed. This is also observed in combined epoxy and polyester resin systems, for example in the bond between an epoxy laminate and a polyester gelcoat. The bond between polyester resin systems and epoxy resin systems may be affected by a phenomenon known as air inhibition. Without wishing to be bound by any theory, we believe that the oxygen in the air interferes with the free radical cross linking mechanism of the polyester resin. This results in a thin layer of uncured polyester resin material which is situated on the surface of the polyester laminate or gel coat. If an epoxy laminate or epoxy system is applied to this surface, the epoxy resin will not satisfactorily adhere to the surface. As the surface is only partially cured due to the air inhibition, the bond between the polyester laminate or polyester gelcoat and the epoxy laminate or epoxy system is weak.

Another problem that arises particularly in combined epoxy laminates and polyester laminates or polyester gelcoats is that the cured polyester laminate or gelcoat has a higher shrinkage than the cured epoxy laminate. This causes interfacial stress between the laminates which results in loss of bonding between the laminates. A further problem of the combined laminates is that air can be trapped between the laminates which again results in poor bonding.

GB-A-1 291 160 (Hauk Manufacturing Company) discloses a process for moulding laminated articles composed of an elastomeric surface layer and a backing layer of a reinforced synthetic resin. Generally, elastomers are chemically classified as non-polar and completely saturated substances. Synthetic resins which have a highly polar chemical structure, such as polyester resins, are difficult to adhere directly to layers of elastomer because the chemical dissimilarity of the resin and the elastomer prevents the formation of strong interfacial bonds. The elastomeric surface layer is bonded to the backing layer by

applying a coating of elastomer to the surface layer, depositing bonding fibres to the coating which is part cured, rolling the fibres to remove entrapped air and to form a substantially continuous fibrous coating near the surface of the elastomer coating, and applying a layer of catalysed synthetic resin backing followed by the backing layer.

The method as disclosed in GB-A-1 291 160 is laborious since it requires the various layers to be applied separately in order to bond the elastomer coating and the synthetic resin backing. Also, any entrapped air must be removed. If entrapped air is not removed from between the interfaces of the various layers, this affects the quality of the bond. Air inhibition or oxidation of the surface of the elastomer layer still occurs as air interferes with the elastomer surface. The mechanical properties of the backing layer and the elastomer layer further cause interfacial stresses which affect the bond between the layers.

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

According to the invention there are provided a bonding material, a laminate structure and a bonding method according to any of the accompanying claims.

In an embodiment of the invention there is provided a bonding material adapted to improve bonding of a first moulding material to a further moulding material, said first and further moulding material when combined having unsatisfactory bonding properties, wherein said bonding material is located between said moulding materials to form a bonding layer, and said bonding material comprises a bonding resin material and a bonding reinforcement material.

The moulding materials comprise resin materials which may comprise different polymerisation chemistries which prevent the moulding materials from forming strong bonds between

said moulding materials. The bond formed between the first moulding material and the further moulding material in the absence of said bonding material is therefore weaker than the bond formed between the respective moulding materials by the bonding material. The bonding material thus greatly improves the bond between the respective moulding materials.

The first moulding material may comprise a reinforcement resin material and/or a reinforcement material and/or a gelcoat and/or a surface resin material or combinations thereof. The further moulding material may also comprise a reinforcement resin material and/or a reinforcement material and/or a gelcoat and/or a surface resin material or combinations thereof.

The first moulding material may comprise an epoxy resin material and the further moulding material may comprise a polyester resin material, alternatively, the first moulding material may comprise an epoxy resin material and the further moulding material may comprise a vinylester resin material, or alternatively, the first moulding material may comprise a vinylester resin material and the further moulding material may comprise a polyester resin material.

The bonding resin material may comprise an epoxy resin material. The first moulding material may comprise an epoxy resin material and the further moulding material may comprise a polyester resin material. The further moulding material may also comprise a gel coat. Said gel coat may comprise said polyester resin material.

In another embodiment of the inventions there is provided a bonding material suitable for improving the adherence of an epoxy laminate to a polyester moulding material, said bonding material comprising a layer of a bonding resin material. Said bonding resin material may comprise an epoxy resin material and said laminate layer may comprise an epoxy resin material.

In another embodiment of the inventions, the bonding

material may comprise one or more layers of a reinforcement material. The reinforcement material may be fibrous and may be woven or non-woven. The reinforcement material may be provided on the bonding resin material. In a preferred embodiment the bonding resin material is in the form of a bonding resin film. The film may be applied interleaved between layers of said reinforcement material. For most applications, a single bonding resin film layer covered on each\ side by a layer of a reinforcement material is sufficient for providing a strong bond between the respective moulding materials. The reinforcement material may be partially impregnated with the bonding resin material or the reinforcement material may be dry. The reinforcement material may be conjoined to the surface of the bonding resin material such that the reinforcement material is substantially unimpregnated with the resin material.

In a further embodiment of the inventions, the reinforcement material may be in a woven or a non-woven form. The reinforcement material facilitates the manufacturing and handling of the bonding material. The weight of the bonding resin material may vary from 50 g/ml to 800 g/rril, preferably the weight of the bonding resin material is 250 g/m2.

In yet another embodiment of the inventions, the reinforcement material may comprise a ventilating structure to allow gases to pass out of the laminate structure during processing of said bonding material. Gas entrapment in the laminate structure can lead to voids at the interface between the bonding material and the gel coat and the interface between the bonding material and the laminate structure. The ventilating structure enables these gases to escape via the dry reinforcement layer. This results in a stronger bond between the interfaces of the bonding material and the moulding materials, and improved environmental resistance due to the absence of voids at the interfaces.

The ventilating structure further prevents air inhibition on the surface of the polyester moulding material or polyester gelcoat. The ventilating structure may be formed by the reinforcement material. During processing, the laminate structure comprising the bonding layer may be evacuated. In this way all air is removed from the interface between the bonding layer and the moulding materials thus preventing air inhibition and/or oxidation of the interface.

The laminate structure formed by the moulding materials and the bonding material may be cured by applying heat to the structure. Upon heating the material, the resin viscosity drops. This causes the bonding resin to reach its flow point which enables it to completely wet-out the bonding material including the interfaces of the bonding material and the respective moulding materials whereby intra-and interlaminar gases such as entrapped air can conveniently escape via the venting structure. During processing of the material, vacuum pressure may be applied to the laminate structure. The evacuation of the structure also prevents entrapment of inter-and intralaminar gases between the bonding material and the respective moulding material and it prevents air inhibition on the external surface of the further moulding material. During processing the bonding layer bonds with the surrounding moulding materials.

The viscosity of a resin material is strongly affected by temperature. Upon heating the material, the resin viscosity drops dramatically, allowing it to flow around the reinforcement material (flow-point). However, as the resin material is heated beyond a certain point (activation temperature), the catalysts within it begin to react and the cross-linking reaction of the molecules accelerates. The progressive polymerisation increases the viscosity of the resin material until it has passed a point where it will not flow at all ("no flow point"). The reaction then proceeds to

full cure. Within this application, if reference is made to the flow properties of any of the resin materials herein described, and more in particular the viscosity of a resin material, it is herein referred to the flow properties of the resin during processing, up to the point in time when the resin material reaches its no flow point.

In an embodiment, the bonding resin material may comprise pigments. The pigments may be applied to colour the bonding material. In combination with the coloured bonding material this enables the application of a thinner gel coat layer to arrive at the same colour intensity at the surface of the moulding in comparison with a thick gel coat layer.

Suitable pigments or fillers may comprise titanium dioxide and carbon black (carbon particles). In this advantageous embodiment, since the gel coat layer is thinner, problems of shrinkage of the polyester moulding material or the polyester gel coat are absent or significantly reduced. As discussed hereinbefore, in conventional structures, such shrinkage usually causes interfacial stresses at the bond between the bonding material and the gel coat layer.

The bonding resin material may comprise fillers which are suitable for reducing the water permeability of the bonding resin material and for improving the resistance of the bonding material to water and other chemicals. Suitable fillers may comprise mica, aluminium, micro balloons and glass flakes.

In another embodiment of the invention, the bonding material may comprise suitable resin material properties such that during processing of the laminate structure, the bonding resin material is prevented from emerging to the external surface of the further moulding material, in particular to the external surface of a gel coat material.

The resin material properties may comprise the viscosity of the resin material which is selected such that during processing of the bonding material, the minimum viscosity of

the bonding resin material is higher than the minimum viscosity of the gel coat material during processing of the bonding material.

A preferred way of applying the bonding material to a moulding layer is by part curing the layer before the bonding material is applied. In particular, a gelcoat material may preferably be gelled off at room temperature.

This is achieved by partially curing the gelcoat. At this stage, the bonding material may be applied on to the gelcoat material. The viscosity of the gelcoat is then such that the bonding resin material is prevented from migrating away from the bonding material and emerging at the external surface of the gelcoat material. Also, at this stage, no intermixing of the gelcoat resin and the bonding resin can take place.

The skilled man recognises the gelled off stage of the gelcoat by the property of the gelcoat that it is tacked off. This means that when a finger is applied onto the gelcoat, the finger slightly sticks to the gelcoat. Upon release of the finger, no gelcoat sticks to it.

In another embodiment of the invention, the bonding material may comprise a heat cured epoxy resin material. The resin material may be any standard epoxy resin material. The resin material may be selected such that the material is a semi-solid material at an ambient temperature ranging from 10°C to 40°C.

The bonding resin material may also comprise one or more toughening components for toughening the cured resin material, which may be in a concentration of approximately 1 to 30% by weight of the resin material, preferably approximately 12% by weight of the resin material. The toughening material components may comprise CTBN (Carboxy Terminated Butadiene). CTBN is a liquid polymer which can be conveniently blended into the epoxy resin. CTBN can be partially reacted with an epoxy resin. The CTBN material components are liquid polymers upon their application to the

resin material. Processing of the bonding material may be achieved by a suitable catalyst. A suitable catalyst may comprise dicyandiamide, and/or imidazole, and/or Boron trifluoride amine. During processing the bonding material bounds with the moulding material layers.

In a preferred embodiment of the invention, the bonding material may be provided as a preform. In this form, the bonding material is pre-fabricated and comprises a bonding resin material and one or more reinforcement layers. The pre-fabricated material is supplied ready for its application in the mould. One or more reinforcement layers may be at least partially pre-impregnated with the bonding resin material. Alternatively, the reinforcement layer or layers are adhered to the bonding resin material and held in place by the inherent tack of the bonding resin. The reinforcement layers are thus substantially dry and free from resin. The bonding material is preferably supplied to the fabricator on a roll.

In a further embodiment of the inventions there is provided a laminate structure comprising a gel coat layer, an epoxy laminate layer and a bonding layer as herein before described. The bonding layer is preferably arranged between the gel coat layer and the laminate layer as an interleave bonding layer. The gelcoat layer preferably comprises a polyester gel coat material.

In yet another embodiment of the invention, there is provided a method of bonding a gel coat layer to a laminate layer by providing a bonding layer as herein before described between said gel coat layer and said laminate layer.

In another embodiment of the invention, there is provided a method for bonding a polyester gel coat layer to an epoxy laminate layer comprising the steps of providing a bonding material comprising an epoxy resin layer, and partially curing said gel coat layer, said method further

comprising the steps of locating said bonding material in relation to said gel coat layer, locating said epoxy laminate layers in relation to said bonding layer, thereby forming a laminate structure, and processing said laminate structure.

There is thus provided a bonding material, a laminate structure and a bonding method according to embodiments of the inventions.

Historically the bond between a polyester gel coat layer and an epoxy laminate layer has never been very satisfactory. One of the reasons for this is that the two chemistries of the polyester layer and the epoxy layer are radically different and therefore incompatible. This results in poor bonds at the interface between an epoxy resin containing material and a polyester resin containing material.

The other reasons include the difference in thermal expansion between the layers which results in interfacial stresses, the shrinkage of the cured polyester which results in further interfacial stresses and the problem of air inhibition at the surface of the polyester layer which results in poor chemical bonds. Apart from this, air can be trapped on the interface between the polyester and epoxy layers which results in voids in the cured laminate structure and a poor bond between the layers.

In an embodiment of the invention the bonding material forms an improved bond between an epoxy laminate and a cosmetic polyester in mould coating which is generally known as a polyester gel coat. Without wishing to be bound by any theory, we believe that the bonding of the gel coat and the laminate structure by the bonding material is based on three important principles.

Firstly, the bonding layer comprises a reinforcement layer which is unimpregnated or partially impregnated by the bonding resin material. This allows for intra-laminate and

inter-laminate gases such as air, which may be trapped between the gel coat layer and the bonding layer or the laminate layer and the bonding layer, to be evacuated upon processing of the laminate. In this way gases can be evacuated from the critical interface between the gel coat layer and the bonding layer. The evacuation of air results in a large reduction of voiding which improves the bond between the bonding layer and the polyester gel coat layer.

Secondly, the bonding material reduces the interfacial stresses between the gel coat layer and the laminate layer.

The polyester gel coat layer has a lower modulus of elasticity compared to the modulus of the epoxy laminate layer. The epoxy laminate layer is relatively inelastic in comparison to the gelcoat layer. The interfacial stresses result primarily from the higher shrinkage of the cured polyester layer in comparison to the cured epoxy layer. In use, additional stresses result from the difference in thermal expansion between the layers. If a gel coat layer were adhered directly to an epoxy laminate structure, this would result in significant interfacial stresses which can lead to premature cracking and failure of the coating of the epoxy laminate. The bonding material comprises both flexible properties and tough properties which allow the bonding layer to be elastic enough to absorb the stresses from the polyester gel coat interface. This effectively reduces the stress on the interface which results in an improved performance of the polyester gel coat layer.

The layer of bonding material is tougher than the layers of the epoxy laminate material and the polyester laminate material. The bonding layer comprises toughness properties which make the material resilient and resistant to crack propagation. The layer of bonding material is more flexible and resilient than the epoxy laminate layer and the polyester gel coat layer Thirdly, although we do not wish to be bound by any

theory, we believe that the bonding layer may form chemical bonds between both the epoxy laminate and the polyester gel coat. Conventionally applied ambient cured polyester gel coats do not cure completely on the exposed gel coat surface.

This is due to a phenomenon which is widely known as air inhibition. The oxygen in the air interferes with the free radical cross linking mechanism of the polyester molecules which results in a thin layer of uncured material which is present on the surface of the gel coat layer. If a conventional epoxy system is applied onto this polyester gel coat surface, the epoxy laminate adheres to this surface; but since it is only partially cured due to the air inhibition, the interface bond between the epoxy laminate and the polyester gel coat is weak, which results in a poor bond between the two layers. We believe that the bonding layer forms a good bond to the polyester gel coat layer due to the toughening components which are present in the bonding layer.

The toughening components have a degree of unsaturation because of carbon-carbon double bonds which are in the components. These toughening components can react with the styrene in the polyester resin material of the unsaturated gel coat and form a chemical bond between the polyester gel coat layer and the bonding layer. Since the bonding layer comprises an epoxy resin material this layer also bonds with the epoxy laminate layer.

As discussed, the bonding material reduces the interfacial stresses between the gel coat layer and the laminate layer. The gel coat layer has a relatively low modulus, whereas the epoxy laminate layer has a high elasticity modulus. The gel coat layer also comprises a higher shrinkage than the laminate layer. The bonding material comprises both flexible properties and tough properties which allows the bonding material to marry up the two layers. The required mechanical and chemical properties of the bonding material may be adapted to the mechanical and

chemical properties of both the gel coat layer and the laminate layer. This is achieved by adding additives such as toughening components to the bonding resin material. Also, in the selection of the catalyst, the mechanical properties of the bonding material may be controlled.

The bonding layer comprises an epoxy resin which is preferably applied as a film. The epoxy resin may comprise a standard heat cured epoxy resin material which may be selected so that it is semi-solid at an ambient temperature.

The resin material may have an amount of toughening agent present, preferably approximately 2 to 20% by weight of the resin material and more particularly between 10% and 15% by weight of the resin material. Preferably the amount of toughening agent is around 12% by weight of the resin material. Preferred toughening agents are CTBN and preferred catalysts may comprise accelerated dicyandiamide, an imidazole, or a Boron trifluoride amine complex. Processing of the layers is preferably achieved at temperatures ranging between 50°C and 150°C but would normally be at approximately 85°C. The weight of the resin film may vary between 50 g/m2 to 700 g/m2 but preferably the weight of the resin film may be 250 g/m2. This film may also have a light weight glass carrier material and/or synthetic carrier material present on the external surfaces to facilitate manufacturing and handling of the material. This glass carrier material or reinforcement material can comprise a woven or non-woven material, which can be present on both or one of the surfaces of the resin film material. As discussed, these materials serve to reduce the possibility of air entrapment leading to voids at the interfaces of the bonding material and the gel coat and backing laminate.

It is also possible to incorporate pigments into the bonding layer. These can be used to give a colour to the bonding layer which allows a thinner application of the gel coat material. Suitable pigments may comprise titanium

dioxide or carbon particles.

The bonding resin material may further comprise fillers which may be used to reduce the water permeability of the film and improve its long-term resistance to other chemicals.

Such fillers could comprises mica, aluminum, glass flakes, or combinations thereof.

Embodiments of the inventions will now be described as an example only and with reference to the accompanying drawings in which: Figure 1 presents a cross-sectional, diagrammatic view of a bonding material according to an embodiment of the inventions, and Figure 2 presents a diagrammatic cross-sectional view of a laminate structure comprising the bonding material of Figure 1.

The embodiments are an example only and illustrate the general structural format of the bonding material. The chemical constitution of the resin is as hereinbefore described.

The bonding material or tie coat material 10 comprises a resin film 12 comprising a suitable resin material comprising toughening agents. For specific applications the resin material can also comprise fillers and pigments. The bonding material further comprises reinforcement materials 14,16 which are applied on each side of the resin film 12.

The top surface layer of the resin film 18 comprises a woven reinforcement material 14 and the lower surface of the resin film 19 comprises a non-woven reinforcement material 16.

These reinforcement materials facilitate manufacturing of the bonding material and improve handling of the bonding material. The materials further serve to reduce the possibility of air entrapment which can lead to voids at the interfaces of the bonding material and the gel coat and the bonding material and the laminate. This greatly improves adherence of the bonding material to the gel coat and the

laminate. The reinforcement materials 14,16 can be woven or non-woven.

The bonding material is manufactured by applying a resin film 12 onto the woven carrier 14 and subsequently applying a layer of a non-woven reinforcement material onto the opposite side of resin film 12.

A laminate structure 20 is formed by bonding a polyester gel coat 24 to an epoxy laminate structure 22 by means of bonding material 10. The bonding material 10 is provided between the gel coat 24 and the epoxy laminate 22. A preferred way of applying the bonding material is by applying the gel coat material in the mould and processing the gel coat material such that it partially cures at an ambient temperature. This allows the gel coat to be tacked off, that is to process the gel coat to a state in which the gel coat is not tacky, but at which point it is not completely cured.

The bonding material as herein described provides a significant improvement over a laminate structure comprising a polyester gel coat and an epoxy laminate.

The bonding material 10 as presented in Figs. 1 preferably comprises an epoxy resin film 12 of a weight of 250g/m2. The reinforcement material 14,16 may comprise fibres of only one type or a combination of different types of fibre. Any fibre size may be used. Preferably the fibres range in size from 1 Atm to 100, um and 1 to 4000 tex. In a preferred embodiment, the non-woven material 16 is typically a lightweight fibrous layer of between 1 and 100 g/m2 and the material 14 is between 100 g/m2and 2000 g/m2in weight.

As an example the strength of the bond can be quantified using a cleavage test according to British Standard 5350 part Cl. Without using the bonding material according to the invention a cleavage strength of approximately 2 kN to 3 kN was measured for a conventional polyester gel coat layer and an epoxy laminate bond. When a bonding material as presented in Fig. 1 is applied, the bonding material as herein described having a resin film weight of 250 g/m2 interleaved between the gel coat layer and the laminate layer, a cleavage strength of 5.8 kN newton was measured.