GB2521636A - Improvements in or relating to fibre reinforced composites - Google Patents

Abstract

A method of encapsulating a substrate with a resin comprises (I) forming a dispersion comprising (i) an uncured curable resin, (ii) a curing agent, (iii) a substrate, and (iv) a dispersant, (II) reacting said curing agent and curable resin to cure said curable resin and thereby encapsulate said substrate. In preferred embodiments, the dispersant is a solid dispersant, the substrate comprises particles or veils of thermoplastic and the curable resin and curing agent are epoxy resin and amine respectively. The preferred resultant particles are thermoplastics encapsulated in reactive thermoset resin which improves the interface with thermosetting resins into which it is intended to be incorporated. The thus produced particles may be used as toughening elements in fibre reinforce composites.

Description

IMPROVEMENTS IN OR RELATING TO FIBRE REINFORCED COMPOSITES

The present invention relates to a dispersion for producing a resin encapsulated substrate, a resin encapsulated substrate, and a method of encapsulating a substrate with a resin, particularly but not exclusively a thermoset resin encapsulated thermoplastic substrate.

S BACKGROUND

Composite laminar structures are strong and light-weight. Their use is well known and they are frequently used in applications such as automotive, aerospace, sporting goods and marine applications.

Typically, composite materials are manufactured by stacking layers of a fibrous reinforcement material which is preimpregnated with a curable resin material (prepreg). The resin material is then cured by heating the stack whilst it is being compressed. This causes the resin to flow to consolidate the fibrous stack, and then to subsequently cure. This results in an integral laminar composite structure.

Composite materials can also be formed by arranging successive layers of dry fibrous material into a mould and then infusing a curable resin into the mould. The resin wets out the fibres of the dry material before subsequently being cured. This process is known as resin transfer moulding (RTM).

Both methods result in a composite material with a laminar structure having a series of layers of impregnated fibrous reinforcement. Between these layers is a resin rich layer distinguished by an absence of reinforcement fibres. These layers are referred to as the interleaf or interlayer.

The toughness of the composite can be improved by adding tougheners to the interlayer. These may be in the form of veils or particles.

Thermoplastic tougheners function by introducing a second phase into the thermoset matrix. The thermoplastic polymers typically have a lower glass transition temperature (Tg) than the thermoset matrix. These regions of lower Tg act as sites for that affect the normal crack propagation.

Furthermore, the thermoplastic particles typically have a lower Young's modulus than the thermoset matrix; this discontinuity produces regions of increased stress concentration around the particle when the material is subjected to a load. This results in shear yielding which remains localised to the perimeter of the particles and increases the energy absorption of the material during deformation.

The addition of thermoplastics to matrix resins can significantly increase matrix viscosity. During processing it is often necessary to heat the resin to reduce its viscosity to allow complete impregnation of the reinforcement fibres. If this heat is excessive it can cause insoluble thermoplastic particles to melt. Melting causes the particles to lose their dimensional stability affecting their ability to add toughness to the final prepreg. This therefore limits the processing temperature that can be used for impregnation and reduces the rate at which a prepreg containing thermoplastic tougheners can be produced.

Thermoplastic particles are not typically used in composite materials made by RTM because their flow is impeded by the reinforcement fibres and they become unevenly distributed across the mould. Instead a common way of toughening infused composite materials is to place a thermoplastic non-woven fabric in the form of a veil between the fibre layers in the mould before the resin is infused. During cure the infused resin can cause the veil to melt and lose its structure. This can result in an undesirable reduction of toughening.

The addition of thermoplastic tougheners to composite materials also results in an undesirable decrease of electrical conductivity. Electrical conductivity is a desirable characteristic of a composite material used in aerospace and wind energy applications for example. It is particularly important for a composite material to possess a suitable level of conductivity in applications that may be vulnerable to lightning strike.

The present invention aims to overcome the above described problems and/or to provide improvements generally.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a composition, a process for producing a composition, and a composite material as defined in any one of the accompanying claims.

The present invention is a composition comprising an encapsulated material, wherein the encapsulated material comprises a substrate that is encapsulated by a cross-linked polymer. In an embodiment of the invention the composition further comprises a thermoset resin and a curing agent and the encapsulation typically comprises a reaction product of the thermoset resin and the curing agent. In a preferred embodiment of the invention, the substrate comprises a rubber or thermoplastic.

In an embodiment of the present invention, the cross-linked polymer encapsulation has reactive groups on its surface derived from either the curing agent or the resin or both. In an embodiment of the present invention, the thermoset resin is an epoxy resin and the curing agent is an amine, and the surface reactive groups are epoxy or amine groups, or a combination of both.

The presence of reactive epoxy groups on or in the encapsulation of the present invention can be determined by combining with a curing agent (e.g. 4,4'-DDS) and performing differential scanning calorimetry (DSC) on the composition. Likewise, if the encapsulating cross-linked polymer exhibits a surplus of amine groups then it should be combined with an epoxy resin (e.g. [13581 or MY0610).

Encapsulation of the present invention will exhibit exothermic behavior when reacted with either a curative or resin, this can be detected using scanning differential calorimetry. Therefore, the term reactive refers to the presence of unreacted groups derived from either the resin or curing agent on or in the cross-linked polymer encapsulation.

In a preferred embodiment of the present invention, the substrate is a polyamide. In an alternative embodiment the substrate is a rubber such as any synthetic (e.g. CTBN) or a natural rubber.

Embodiments of the present invention may also include ceramic, thermoset or metallic substrates.

The substrate may take any form, preferably it is either in the form of a particle or structured layer e.g. a veil.

The encapsulated material is produced by providing a dispersion comprising a dispersant, a resin, a curing agent and a substrate, and reacting the resin and curing agent to deposit a cross linked polymer on the surface of the substrate. In an embodiment of the present invention the dispersant is polypropylene glycol. In an embodiment the thermoset resin is an epoxy and the curing agent is an amine. Preferably the substrate is a thermoplastic particle or veil.

As discussed above, the high temperatures used during processing or cure of a composite material can cause additives to melt and lose their shape. This impairs their function so the processing speed is reduced to allow impregnation at a lower temperature to prevent melting. Encapsulation of the additives provides dimensional stability so that when an additive melts its shape is maintained. This is particularly relevant where the additive has a low melting point, e.g. PA 12, or the matrix has a high cure temperature, e.g. epoxy, BMI or polyimide resins. Furthermore, additives with exceptionally low melting points which melt or dissolve too readily to be considered for use in conventional composite materials may be encapsulated in accordance with the present invention to preserve their structure to enable their use in such applications.

The thermoset matrices typically used in composite materials differ in their chemistry to the thermoplastic additives used to toughen such materials; so following cure they exist in two separate phases. This results in a weak interface between the two different phases. Whilst in part, this interface is responsible for improving toughness, it is also attributed to degradation of other properties of the laminate. One object of the present invention is to improve the interface between the thermoplastic toughener and the thermoset matrix to reduce this degradation of properties, whilst providing an additive with a different Tg and Young's modulus to give a beneficial toughening effect.

Embodiments of the present invention provide an additive to a resin matrix that is encapsulated with a cross linked polymer, where the cross linked poly is the same as, or compatible with, the chemistry of the host matrix resin. The reactive groups present on the surface of the cross linked polymer can be controlled by adjusting the composition of the dispersion. Reactive groups on the surface of the encapsulation improve the interface between the encapsulating polymer and the host matrix. Therefore, embodiments of the present invention allow for control of the interface to achieve optimum properties by altering the type and number of reactive groups present on the surface of the material.

The use of additives, in particular thermoplastic tougheners, can also result in an undesirable decrease of matrix conductivity, this effect is also increased following the melting of certain additives. In embodiments of the present invention cross linked polymer encapsulation may comprise a conductive additive such as carbon black, carbon nanoparticles and/or combinations thereof. The conductive additive may comprise an average diameter in the range of from 0.0005 to 1 pm, preferably from 0.001 to 0.5 pm, more preferably from 0.005 to 0.2 pm, even more preferably from 0.01 to 0.1 m and/or combinations of the aforesaid ranges. This can be achieved by adding the conductive component to the dispersion with the reactants to be incorporated into the encapsulation. Thus a toughener can be produced that provides improved conductivity without the viscosity increase that occurs when a separate toughener and conductivity particle is added to the matrix. This also has the benefit that conductivity is added directly to the surface of the toughener.

Tougheners significantly reduce the conductivity of a matrix therefore it is beneficial to provide a conductive around the surface of toughener.

SPECIFIC DESCRIPTION

Particular embodiments of the invention are now described by way of example only.

The composition of cross-linked reactive polymer that forms the encapsulation of the present invention is the reaction product of at least one thermosetting resin and at least one curing agent in the presence of a dispersant, where the reaction conditions (e.g. reaction temperature and reaction time, amongst others) allow for the cross-linked reactive polymer to form on the surface of the substrate encapsulating it without the need for surfactants and with little or no dispersant bound to the encapsulated material.

The encapsulated material of the present invention can be produced by polymerizing a thermoset resin with a curing agent, in a dispersant which contains the substrate to be coated. The reaction can proceed without stirring to produce cured or partially cured cross linked polymer formed on the surface of the substrate. The parameters that potentially have an influence on the encapsulation (e.g. its thickness, surface chemistry, Tg, etc.) include the concentration of the monomers dissolved (expressed as a weight percent of the monomer); the amine/epoxy molar ratio; the reaction temperature and time; the dispersant, the substrate material and the chemical structure of the reactants (e.g. the curing agent and resin). Reactants refers to both the thermoplastic resin and the curing agent.

The reaction between the resin and curative may take place at any temperature up to the boiling point of the dispersant. Preferably the reaction takes place at reaction temperatures between 20°C and 200°C, more preferably between 100°C and 190°C, and more preferably still between 150°C and 180°C. Preferably, the reaction time is between 3 and 7 hours when heated with an oven or similar.

The reaction time depends on the temperature, the amine/epoxy molar ratio; the dispersant, the use of a catalyst and the chemical structure of the epoxy resins and the curing agent. In an embodiment of the present invention the reaction is performed using microwave heating. When heated with an oven, hot plate or similar the reaction time is preferably no greater than 24 hours, or more preferably no greater than 7 hours, or more preferably still no greater than 5 hours, most preferably between 30 minutes and 7 hours when heated with a microwave and/or combinations of the above.

Preferably the reaction time when heated by a microwave at the reaction temperature is in the range of from 1 second to 3 hours, or more preferably between 10 seconds and 1 hour, or more preferably still between 30 seconds and 20 minutes.

To produce an encapsulated particle with the majority of groups derived from the resin component on the surface, the dispersion will preferably contain a weight ratio of resin to curative between 10:1 and 1.01:1, or more preferably between 5:1 and 2:1.To produce an encapsulated particle with the majority of groups derived from the curing agent on the surface, the dispersion will preferably contain a weight ratio of resin to curative between 1:10 and 1:1.01, or more preferably between 1:5 and 1:2. Alternatively a stoichiometric ratio can be used to produce an even number of surface reactive groups derived from the resin and curative.

In a preferred embodiment the dispersion contains between 0.001 and 30 weight percent of substrate. More preferably between 0.01 and 10, and more preferably still between 0.02 and 8. In a preferred embodiment the dispersion contains a ratio of substrate to reactants of from 20:1 to 0.01:1, or more preferably from 10:1 to 1:1, and more preferably still from 5:1 to 2:1.

The embodiments of the present invention can be produced by adding the resin and the curing agent to the dispersion such that they have a combined concentration in the dispersion of 0.01 to 50 weight percent (wt %) based on the total weight of the dispersion) formed by the dispersant, resin, the curing agent and substrate. Preferably, the resin and the curing agent have a concentration in the dispersion of 0.05 to 30 weight percent based on the total weight of the dispersion) most preferably between 0.1 and 10 weight percent) most preferably, the epoxy resins and the amine curing agent combined have a concentration in the dispersion of from ito 4 weight percent, and/or combinations of the aforesaid ranges. The epoxy resin and the amine curing agent can be dissolved individually or together in the dispersion. The reaction is allowed to proceed at a rate of reaction which can be adjusted by means of the reaction temperature. The thickness of encapsulation can be influenced by the relative quantities of reactants to substrate and the duration of the reaction. The Tg can be influenced by the selection of reactants as well as the temperature at which the reaction takes place.

In a preferred embodiment of the present invention, the substrate is a thermoplastic material. An encapsulated material of the present invention containing the substrate may be incorporated into an interleaf/interlayer of a dry fibre preform layup or prepreg assembly to improve toughness in the finished composite material. Any thermoplastic substance which is commonly used as a toughener in composite materials is therefore suitable as a substrate in the present invention. Such toughening agents are polymers which can be in the form of homopolymers, copolymers, block copolymers, graft copolymers, or terpolymers. The thermoplastic toughening agents are thermoplastic resins having single or multiple bonds selected from carbon-carbon bonds) carbon-oxygen bonds, carbon-nitrogen bonds, silicon-oxygen bonds, and carbon-sulphur bonds. One or more repeat units may be present in the polymer which incorporate the following moieties into either the main polymer backbone or to side chains pendant to the main polymer backbone: amide moieties) imide moieties, ester moieties, ether moieties, carbonate moieties) urethane moieties, thioether moieties, sulphone moieties and carbonyl moieties. The thermoplastic polymer may be either crystalline or amorphous or partially crystalline.

The thermoplastic polymer may be selected from: polyamides (PA: PA 6, PA 12) PA 11, PA 6-6, PA 6- 10, PAlO-b, PA 6-12, etc.), copolyamides (C0PA -polyamide copolymers of one or more of the aforesaid polyamides), block ether or ester polyamides (PEBAX, PEBA), polyphthalamide (PPA), polyesters) (polyethylene terephthalate -PET-, polybutylene terephthalate -PBT etc. ), copolyesters (CoPE), thermoplastic polyurethanes (TPU), polyacetals (POM, etc.), polyolefins (PP, HOPE, LDPE, LLDPE etc.)) polyethersulfones (PES), polysulfones (PSU etc.), polyphenylene sulfones (PPSU etc.), polyetheretherketones (PEEK), polyetherketoneketone (PEKK), poly (phenylene sulfide) (PPS), or polyetherimides (PEI), thermoplastic polyimides, liquid crystal polymers (LCP), phenoxys, block copolymers such as styrene-butadiene-methylmethacrylate copolymers (SBM), methylmethacrylate-butyl acrylate -methylmethacrylate copolymers (MAM), polyurethanes, epoxies, and/or combinations or mixtures of any of the aforesaid thermoplastic polymers.

S

Polyamides and copolyamides are the preferred thermoplastic materials for the thermoplastic polymer. Particles made from polyamides and copolyamides have been a used as interleaf toughening particles in the past (See U.S. Patent No 7754322 and published U.S. Patent Application No. 2010/0178487A1, all incorporated by reference herein). Polyamides come in a variety of types, such as polycaprolactam (PA 6), polylaurolactam (PA 12), copolymers of PA 6 and PA 12, as well as PA 10 and 11. Any of the polyamides that are suitable for making particles that are used to toughen laminate interleaf zones are also suitable The thermoplastic polymer substrate may be in the form of a fibre, particle or a structured layer (e.g. scrim or veil). Where a thermoplastic resin particle is used as the substrate, the particle shape may be spherical, non-spherical, porous, spicular, whisker-like or in the shape of flakes. A spherical substrate is preferred because it is easier to impregnate into the matrix resin of a prepreg as it does not lower flow properties of the matrix resin. Preferably the particulate is of the nano or micro scale, in particular between 1 nm and 0.1mm. Advantageously the present invention may be used to coat porous substrates, for example, Orgasol or PEEK particles. Porous particles when used as the substrate become completely encapsulated by the cross linked polymer. This eliminates the introduction of voids that occurs in the resin matrix when porous substrates are included.

Where a thermoplastic resin fibre is used as the substrate, long or short fibers can be used. In case of short fibre, a method in which short fibers are used in the same way as particles as shown in J P- 02-69566A (herein incorporated by reference), or a method in which short fibers are used after being processed into a mat is possible. In case of long fiber, a method in which long fibers are arranged in parallel on a prepreg surface as shown in JP-04-292634A (herein incorporated by reference), or a method in which they are arranged randomly as shown in W094/016003 (herein incorporated by reference) is possible. Fibres can be processed into sheet-like base materials such as a woven fabric as shown in JP-H02-32843A (herein incorporated by reference), a non-woven fabric as shown in W094016003A, or a knitted fabric, a short fiber chip, a chopped strand, a milled fiber, or a method in which short fibers are made into a spun yarn and arranged in parallel or random, or processed into a woven fabric or a knitted fabric can also be employed.

Where a structured thermoplastic layer is used as a substrate, the layer must be in a physical form that allows it to be substituted in place of particles in the interleaf zone. In particular, the layers must be sufficiently thin to fit within the interleaf zone and the density of the layer must be such that the appropriate amount or concentration of thermoplastic material is present in the interleaf zone to impart the desired amount of damage tolerance. Interleaf zones in cured high strength structural laminates typically have a thickness that ranges from 10 to 100 microns. Preferred interleaf zones range in mean thickness from 15 microns to 50 microns.

The density of the structured thermoplastic layer must be such that it provides the desired amount (concentration) of thermoplastic toughener to the interleaf zone. The required density for the layer is directly dependent upon the thickness of the layer being used. The thinner the layer, the denser the layer must be in order to provide the same concentration of thermoplastic toughener in the interleaf zone. The density of the structured thermoplastic layer should be such that it provides a structured layer that has an areal weight of ito 20 g/m2 (gsm) for layers that range from 0.5 to 50 microns in thickness. For preferred thermoplastic layers that are 2 to 35 microns thick, it is preferred that the density of the thermoplastic layer be such that the areal weight of the layer is from 2 gsm to gsm. For layers that are from 3 to 20 microns thick, the density of the layer should be such that the areal weight of the layer is from 2 gsm to8 gsm.

Structured thermoplastic polymer layers having the required combination of thickness and areal weight are available commercially in the form of spunlaced and random fibrous veils. An exemplary lightweight (4 gsm) fibrous veil is available as 128004 Nylon veil from Protechnic (Cernay, France), this veil is made from randomly oriented PA 12 fibres. Another suitable nylon veil is 128D06 nylon veil, which is a 6 gsm PA 12 fibrous veil that is also available from Protechnic.

The substrate may also comprise rubber particles, which are well known for use as a toughening agent. Particulate rubbers employed in the practice of this invention may be characterized as comprising rubber particles, including carboxylated rubber, and more particularly as being a finely-divided, cross-linked rubber. Core shell rubber particles for example those in Kaneka MX717 are particularly suited for use as the substrate. Nanoscale rubbers are incompatible with PES toughened matrices. Advantageously, if such particles are used as the core material in accordance with the present invention, the coating of the present invention improves their compatibility in a PES toughened matrix.

The substrate may also be a carbon fibre or similar structural fibre. The structural fibre when used as the substrate, may be continuous or discontinuous. The fibre may be advantageously coated to improve its interface with the resin matrix. For example carbon fibre when used as a structural fibre with BMI resin matrix has a poor interface between the two; this can result in microcracking. By encapsulating the carbon fibre with a BM I the interface can be improved reducing the microcracking.

Hollow microspheres may also be used as the substrate. Hollow microspheres are frequently used within composite materials as an additive to reduce the density. During processing of a resin containing microsphere, a low shear mixing is required to prevent breakage of the fragile micro particles. By encapsulating the microspheres the cross linked polymer shell increases the particles' ability to withstand shear during mixing. This means that faster mixing rates and conventional equipment may be used, which reduces processing costs. Suitable hollow microspheres are commercially available in a variety of sizes, materials, and properties. Examples of some existing hollow microspheres useable with the present invention are available from 3MTM and Zeelan Industries, Inc. under the trade names 3M'TM Scotchlight'M glass bubbles, 3M'TM Scotchlight'M glass bubbles floated series, 3MTM Z-Light'M Spheres microspheres, and 3MTM Zeeospheres'M microspheres.

The microspheres can be made from a variety of materials, for example glass, ceramic, or plastic.

The dispersant can be either a neat solvent or a mixture of solvents, as long as the solubility parameters of the dispersant can be matched to those of the resin and curing agent so as to provide a phase separation of the cross-linked polymer onto the substrate. For the various embodiments, a variety of dispersants can be used in the dispersion polymerization of the present disclosure. For example, the dispersant can be selected from the group consisting of polyethers (e.g. polypropylene glycol (PPG) and/or polyisobutylene ether), poly(oxypropylene), polybutylene oxide, aliphatic ketone, polyether glycol (PEG), cyclic ketone such as cyclohexane and/or cyclohexanone, and combinations thereof. Preferably, the dispersant is polypropylene glycol. Preferably the dispersant has a molecular weight between 400 and 10,000 g/mol, or between 600 to 5000 g/mol, or between 800 to 1500 g/mol, or between 900-1200 g/mol, and/or combinations of the aforesaid ranges.

For the various embodiments, a nonsolvent can also be used with the dispersant. Examples of suitable nonsolvents include, but are not limited to, alkenes (either aliphatic (dodecane) or cyclic), aromatic alkene, orthopthalates, alkyl azelates, other alkyl capped-esters and ethers, and combinations thereof.

In a preferred embodiment of the invention the thermosetting resin is an epoxy resin. A wide variety of epoxy resins are useful for the purpose of the present disclosure. Epoxy resins are organic materials having an average of at least 1.5, generally at least 2, reactive 1,2-epoxy groups per molecule. These epoxy resins can have an average of up to 6, preferably up to 4, most preferably up to 3, reactive 1)2-epoxy groups per molecule. These epoxy resins can be monomeric or polymeric) saturated or unsaturated, aliphatic, cyclo-aliphatic, aromatic or heterocyclic and may be substituted, if desired, with other substituents in addition to the epoxy groups. e.g. hydroxyl groups, alkoxyl groups or halogen atoms.

Suitable examples include epoxy resins from the reaction of polyphenols and epihalohydrins, poly alcohols and epihalohydrins, amines and epihalohydrins, sulfur-containing compounds and epihalohydrins, polycarboxylic acids and epihalohydrins, polyisocyanates and 2,3 -epoxy-1-propanol (glycidyl) and from epoxidation of olefinically unsaturated compounds.

Preferred epoxy resins are the reaction products of polyphenols and epihalohydrins, of polyalcohols and epihalohydrins or of polycarboxylic acids and epihalohydrins. Mixtures of polyphenols, polyalcohols, amines, sulfur-containing compounds, polycarboxylic acids and/or polyisocyanates can also be reacted with epihalohydrins. Illustrative examples of epoxy resins useful herein are described in The Handbook of Epoxy Resins by H. Lee and K. Nieville, published in 1967 by McGraw-Hill, New York, in appendix 4-1, pages 4-56, which is incorporated herein by reference.

The epoxy resin may comprise an average epoxy equivalent weight (EEW) in the range of from 40 to 2000, preferably from 40 to 1500, more preferably from 40 to 1000, and even more preferably from 50 to 300 and/or combinations of the aforesaid ranges. The average EEW is the average molecular weight of the resin divided by the number of epoxy groups per molecule. The molecular weight is a weight average molecular weight.

For a difunctional epoxy resin, e.g. a bisphenol-A type epoxy resin, the average epoxy equivalent weight is advantageously from about 170 up to about 3000, preferably from about 170 up to about 1500.

Preferred examples of bisphenol A type epoxy resins are those having an average epoxy equivalent weight of from about 170 to about 200. Such resins are commercially available from The Dow Chemical Company, as D.E.R. 330, D.E.R. 331 and D.E.R. 332 epoxy resins. Further preferred examples are resins with higher epoxide equivalent weight, such as D.E.R. 667, D.E.R. 669 and D.E.R.

732 all of which are commercially available from The Dow Chemical Company, or Araldite'' MY0610, MYO600 or MYO51O all of which are available from Huntsman.

Another class of polymeric epoxy resins which are useful for the purpose of the present disclosure includes the epoxy novolac resins. The epoxy novolac resins can be obtained by reacting, preferably in the presence of a basic catalyst, e.g. sodium or potassium hydroxide, an epihalohydrin, such as epichlorohydrin, with the resinous condensate of an aldehyde. e.g. formaldehyde) and either a rnonohydric phenol, e.g. phenol itself, or a polyhydric phenol. Further details concerning the nature and preparation of these epoxy novolac resins can be obtained in Lee, H. and Neville, K. Handbook of Epoxy Resins, McGraw Hill Book Co. New York, 1967, which teaching is included herein by reference.

Other useful epoxy novolac resins include those commercially available from The Dow Chemical S Company as D.E.N. 431, D.E.N. 438 and D.E.N. 439 resins, respectively.

In alternative embodiments any resin from the following classes may also be used as the thermoplastic resin: benzoxazine resins, vinyl ester resins, unsaturated polyester resins, urethane resins, phenol resins, melamine resins, maleimide resins, cyanate resins and urea resins.

In a preferred embodiment of the invention the curing agent is an amine curing agent. A variety of amine curing agents can be used in preparing the cross-linked polymer encapsulation of the present invention. The amine curing agents which may be employed are primarily the multifunctional, preferably di-to hexafunctional, and particularly di-to tetrafunctional primary amines. Examples of such amine curing agents include, but are not limited to, isophorone diamine (IPDA), ethylene diamine, diaminodiphenylsulfones, tetraethyleaniine, 2,4-diaminotoluene (DAT), dianiines and dicyanodiamide (DICY). Mixtures of two or more of the amine curing agents can also be used. Also modified hardeners where amines have been reacted in vast excess with epoxy resin are good candidates as amine curing agents It is also possible to use an accelerator in forming the cross-linked polymer encapsulation of the present disclosure. Such accelerators are known in the art. Suitable accelerators are, for example, arnines, preferably ethylene diamine, diethylene triamine, dicyanopolyamide, triethylene tetraarnine, aminoethyl piperazine, organic acids, e.g. dicarboxylic acids, phenol compounds, imidazole and its derivatives) urea based curing agents e.g. those under the commercial name Uron&' and calcium nitrate. For the various embodiments, the choice of the reaction temperature, the dispersant and the amine curing agent, as provided herein, influence the solubility of the cross-linked polymer. These choices allow for a phase separation of the cross-linked polymer from the dispersant to occur before a significant amount of the dispersant has an opportunity to react with either of the curing agent and/or the thermoplastic resin. For example, with a rapid phase separation of the cross linked polymer due to the choice of reaction temperature, curing agent, and the solubility parameters of the dispersant, the opportunity for the dispersant to react with the epoxy resin can be greatly reduced. In other words, the less solubility the cross-linked reactive polymer has at a given reaction temperature and time) the less likely it is react or interact with the dispersant. It is appreciated that not all dispersants reacts with the epoxy and/or amine groups) where most dispersants do not react at all.

In a preferred embodiment of the present invention, a continuous flow process is used for producing the encapsulated material. This process is also suitable for the production of homogenous thermoplastic cross linked micro particles (without the substrate). Present invention can also be produced by a batch process. The encapsulated material of the present invention can be produced continuously by passing the dispersion through a tube, part of which is heated zone to provide the necessary conditions for the reaction to proceed. This method is particularly suitable where the substrate is a particle. The tube length (i.e. duration in heated zone), flow rate and composition of the dispersion can be optimised to achieve the desired encapsulation. Heating may be achieved by way of an oven or other source of thermal energy, or by microwave radiation.

For the various embodiments, the method may further include phase separating the encapsulated material and the dispersant. For the various embodiments, the encapsulated material can also undergo one more washing(s) so as to remove the dispersant from the encapsulated material surface. This may be especially preferred when it is desired to remove more of the dispersant from the encapsulated material than would be possible by evaporation alone. For example, following the formation of the encapsulated material if it is in particulate form, the dispersant and the encapsulated material can be separated (e.g. by filtration and/or centrifugation followed by decanting). The encapsulated material can then be re-suspended in a washing liquid at room temperature (e.g. 23CC) and separated from the washing liquid (e.g., by filtration followed by decanting). The encapsulated material can be washed more than once. A variety of washing liquids are possible, examples include, but are not limited to, acetone, ethanol) tetrahydrofuran, ketones such as methylethyl ketone, water, detergents, end capped ethers, and combinations thereof.

In an embodiment of the present invention, conductive additives are added to the dispersion. The additives become incorporated into the encapsulation during the reaction, and increase the conductivity of the encapsulation. This is particularly beneficial for composite materials intended for use in aerospace or wind energy applications which may be subject to lightning strikes. Composite materials comprising toughening additives are known to significantly reduce conductivity. Therefore the addition of a conductive encapsulation to a toughening additive will provide a method of improving conductivity of a matrix without significantly adding weight to the composite. Suitable conductive additives include graphite, carbon black, carbon nano tubes and metallic nanoparticles, including silver flakes.

The encapsulated material of the present invention may be incorporated into a resin matrix which is then used to form a prepreg. The resin matrix is made in accordance with standard prepreg matrix processing. In general, various epoxy resins are mixed together at room temperature to form a resin mix to which the encapsulated material is added.

The matrix resin is applied to the fibrous reinforcement in accordance with any of the known prepreg manufacturing techniques. The fibrous reinforcement may be fully or partially impregnated with the matrix resin. In an alternate embodiment, the matrix resin may be applied to the fiber fibrous reinforcement as a separate layer, which is proximal to, and in contact with, the fibrous reinforcement, but does not substantially impregnate the fibrous reinforcement. The prepreg is typically covered on both sides with a protective film and rolled up for storage and shipment at temperatures that are typically kept well below room temperature to avoid premature curing. Any the other known prepreg manufacturing processes and storage/shipping systems may be used if desired.

The fibrous reinforcement of the prepreg may be selected from hybrid or mixed fiber systems that comprise synthetic or natural fibers) or a combination thereof. The fibrous reinforcement may preferably be selected from any suitable material such as fiberglass, carbon or aramid (aromatic polyamide) fibers. The fibrous reinforcement is preferably carbon fibers.

The fibrous reinforcement may comprise cracked (i.e. stretch-broken) or selectively discontinuous fibers) or continuous fibers. It is envisaged that use of cracked or selectively discontinuous fibers may facilitate lay-up of the composite material prior to being fully cured, and improve its capability of being shaped. The fibrous reinforcement may be in a woven, non-crimped, non-woven, unidirectional, or multi-axial textile structure form, such as quasi-isotropic chopped prepreg. The woven form may be selected from a plain, satin, or twill weave style. The non-crimped and multi-axial forms may have a number of plies and fiber orientations. Such styles and forms are well known in the composite reinforcement field, and are commercially available from a number of companies, including Hexcel Reinforcements (Les Avenieres, France).

The prepreg may be in the form of continuous tapes, towpregs, webs, or chopped lengths (chopping and slitting operations may be carried out at any point after impregnation). The prepreg may be an adhesive or surfacing film and may additionally have embedded carriers in various forms both woven, knitted, and non-woven. The prepreg may be fully or only partially impregnated, for example, to facilitate air removal during curing.

The prepreg may be moulded using any of the standard techniques used to form composite parts.

Typically, one or more layers of prepreg are placed in a suitable mould and cured to form the final composite part. The prepreg of the invention may be fully or partially cured using any suitable temperature, pressure, and time conditions known in the art. Typically, the prepreg will be cured in an autoclave at temperatures of between 120°C and 190°C. The composite material may also be cured using a method selected from UV-visible radiation, microwave radiation, electron beam, out of autoclave cure, gamma radiation, or other suitable thermal or non-thermal radiation.

Composite parts made from the improved prepreg of the present invention will find application in making articles such as primary and secondary aerospace structures (wings, fuselages, bulkheads and the like), but will also be useful in many other high performance composite applications including automotive, rail and marine applications where high tensile strength, compressive strength, interlaminar fracture toughness and resistance to impact damage are needed.

In RTM, an infusible structure (or preform) is made from reinforcing fibres and other additives including binders, and the preform is injected or infused with liquid resin and the resin is cured at elevated temperature to form a useable component. It is very difficult to toughen RTM resins because the addition of thermoplastics increases resin viscosity. This can make it impossible to inject the resin into a large part, because the resin begins to cure before the preform is completely filled with resin. Additionally, if the thermoplastic or rubber toughening agent is dispersed into the resin in the form of undissolved particles, these particles are then filtered by the fibre preform, resulting in a concentration gradient of the toughener, or in fact completely blocking further injection/infusion of resin.

Structured layers which are encapsulated in accordance with the present invention are ideally suited for use with dry fibre preforms. They can be used as a substitute for thermoplastic interleaf toughening particles, also provide an effective means for holding unidirectional fibre layers together during handling prior to resin infusion. The encapsulated fibrous veils function both as a temporary holding system for the unidirectional fibres and as thermoplastic toughening agent for the cured laminate. Their encapsulation maintains dimensional stability during the resin cure cycle (i.e. it retains it, preventing it from flowing into the resin).

In the case of RTM assemblies are prepared by applying the encapsulated materials described herein to the dry fibrous material of the preforms. The matrix resin is of a viscosity such that, during the resin injection stage, the resin passes through the membrane into the fibrous material. Similar technologies are described in chapter 9 of "Manufacturing Processes for Advanced Composites", F. C. Campbell, Elsevier, 2004.

The preferred thermoset matrices for RTM processes are epoxy or bismaleimide (BMI) with suitable epoxy examples being HexElow® RTM 6 or RTM 120. A typical BMI matrix is HexFlow® RTM 651.

HexFlow® VRM 34 may be used for Vacuum-assisted Resin Transfer Moulding (VaRTM) applications.

All of the above materials are available from Hexcel Composites, Duxford, UK.

The reinforcement fibres can be selected from any of the following commercially available high performance fibres which may be used alone or in combination:-aramid (e.g KevlarTM), glass, carbon, ceramic, hemp, or polyolefin. Carbon fibres are the preferred material, particularly standard or intermediate modulus fibres of between 3000-24000 filaments per fibre tow. The desirable reinforcement form is a woven or non-crimped textile structure of between 150-1000 g/m2 fibre areal weight. Typical weave styles include plain, satin and twill weaves. Non-crimped or multiaxial reinforcements can have a number of plies and fibre orientations such as +457-45°; 01+451-45; 01+451-45/90. Such styles are well known in the composite reinforcement field and are available from a number of companies including Hexcel Reinforcements, Les Avenieres, France.

Examples

Coating of a structured veil sheets of thermoplastic V800 veil (Protechnic, Cernay, France) were interleafed with 99 sheets of Aerovac A5000RP3 perforated release film was placed in a silicone tray containing the following composition: g Araldite LY1SS6 (Huntsman, Duxford, UK) g IPDA (Isophorone diamine) (BASF, Germany) 1000 ml PPG1200 with average molecular weight of 1200 (Sigma Aldrich, Gillingham, UK) The mixture was placed in an oven at 120T for 4 hours. The veils were washed with MEK and dried.

A veil comprising a reactive cross linked thermoset coating was produced.

The veils were successfully laid up, by interleaving with 24 plies of Hexcel 2096 plain weave carbon fibre in a 0/90 quasi isotropic arrangement. The assembly was then infused with RTM6 (Hexcel, Duxford, UK) and cured with the recommended cure schedule of 2 hours at 180CC to form a cured laminate.

Example Additive 1

Additives of the present invention were formed by creating a dispersion comprising: g Rilsan PAll P (Arkema, France) 1600 ml PPG1200 with average molecular weight of 1200 (Sigma Aldrich, UK) 22.2 g MYO61O (Huntsman, Duxford, UK) 10.2 g Isophorone diamine, (BASF, Germany) The dispersion was placed in a 2 litre glass dish and stirred with an overhead stirrer to disperse the substrate and to aid dissolution of the reactants. The dispersion was heated using a microwave oven (Daewoo KOR-L15) to a temperature of 150°C a. An infrared sensor was used to measure temperature.

The dispersion was allowed to cool to 80°C and then combined with 2.5 L of methylated spirits. This was then filtered under vacuum to dryness. The residue was washed under once each with 1 L of IMS, MEK, and Acetone all under vacuum. The washed residue was then dried in an oven for 12 hours at 50°C. Furthermore an endotherm associated with the melting of the substrate and a corresponding exotherm upon cooling was also observed.

A sample of the heads were analysed for the presence of a reactive coating using a differential scanning calorimeter (DSC). The residue comprising coated particles was combined with 4,4,'-DDS at a ratio of 1:1 and placed in a DSC machine (TA Instruments, Delaware, US). An exothermic event was observed at approximately 175°C demonstrating that the PA 11 particle had been coated with a reactive coating containing a surplus of epoxy groups.

A sample of the coated particles was heated to a temperature greater than the melting point of PAll. The morphology of the coated particles were examined under a microscope and found to be unchanged.

Coated particles were produced by the above process using a dispersion containing lOg Rilsan, lOOmI of PPG, l.4g of IPDA, 3.04g MYOG1O and 0.4wt % C.l. solvent violet 13 dye (The soap kitchen, UK). An uncoated control sample of lOg Rilsan was added to lOOmI of PPG with the same quantity of the dye. Both samples underwent thorough washing with using the method described above. Both the coated PA 11 and uncoated PA 11 were examined under a microscope. The dye had washed off the uncoated PA 11 particles leaving only a trace, whereas after thorough washing a substantial quantity of dye was visible on the coated PA 11 particles. This suggests the dye was incorporated into or onto the coating of the coated PAll particle and not on the uncoated PAll.

Example Carbon black additive Additives of the present invention containing carbon black in the coating were produced by creating a dispersion as described above, with the addition of 20% superconductive carbon black.

Microscope examination indicated the coating had been coloured black as a result, suggesting the conductive carbon black was present in the coating. The black colour persisted even after thorough washing.

Comparative additive 1 Rilsan PAll P, Uncoated particles Comparative additive 2 A comparative toughening additive of neat epoxy particles were formed with the same methodology as Example additive 1 with the exception that the PAll particles were omitted, instead a dispersion with the following composition was used: 1600m1 PF'G1200 with average molecular weight of 1200 (Sigma Aldrich, UK) 22.2g MY0610 (Huntsman, Duxford, UK) 1O.2g Isophorone diamine, (BASE, Germany) The dispersion was placed in a 2 L glass dish and stirred with an overhead stirrer. The dispersion was heated using an oven and maintained at a temperature of 150°C for 7 hours.

The dispersion was allowed to cool to 80°C and then combined with 2.SL of methylated spirits. This was then filtered under vacuum to dryness. The residue was washed with 1 Leach of IMS, MEK, and Acetone all under vacuum. The washed residue was then dried in an oven for 12 hours at 50°C Cured resin test samples The additives were combined with a resin composition as follows: 194g LY3581 Epoxy resin (Huntsman, Duxford, UK) 74g 4,4'-DDS curing agent (Huntsman, Duxford, UK) 33g PES (Sumitomo Chemical co Ltd.) 33g Additive of either Example 1 or Comparative Examples 1 or 2 LY3581 was mixed with the PES and heated to 120°C until the PES was dissolved. The additive was then combined with the resin and mixed using a speed mixer. The mixture was cooled to 80°C before the 4,4'-DDS was added, before speed mixing again. The resin composition was poured into a mould and degassed in a vacuum oven to remove any excess air. The resin composition was cured in an autoclave at 6 bar with a cure schedule of 0.5°C per mm to 180°C, it was held at this temperature for 3 hours.

Fracture toughness testing The cured resin was cut into seven test samples for each additive containing example. The cured resin samples were tested in mode 1 for plane strain critical-stress-intensity factor (Kic) and critical strain energy release rate (Gic) at fracture initiation. The test methods were performed in accordance with ASTM standard D 5045-99. Crack area was calculated using Keyence Digital Microscope (Keyence UK Ltd. United Kingdom) and UTHSCSA image tool Software (University of Texas) USA). Results of the each set of seven samples per example were averaged and are recorded

in table 1.

Coated PAll PAll Gic i/rn2 519.30 304.36 Kic MPa.rn°5 1.38 1.08 Table 1 G1 and Kic of coated PAll of the present invention, PAll particles, or neat resin beads each as the additive in a cured resin.

Claims (24)

1. Claims 1. A composition comprising a dispersant having dispersed therein a substrate and a curable polymeric resin, the curable polymeric resin encapsulating the substrate upon cure.
2. 2. A composition according to claim 1, wherein the composition further comprises a solvent for removing the dispersant following curing of the resin.
3. 3. A composition according to claim 1 or 2, wherein the composition comprises a polymeric resin encapsulated substrate following curing of the polymer and removal of the dispersant.
4. 4. A composition according to any of the preceding claims, wherein the curable polymeric resin comprises a thermoset resin component and a curing agent.
5. S. A composition according any of the preceding claims, wherein the cured polymeric resin has reactive groups on its surface derived from either the curing agent or the resin or both.
6. 6. A composition according to any of the preceding claims, wherein the thermoset resin is an epoxy resin and the curing agent is an amine curative.
7. 7. A composition according to any of the preceding claims, wherein the substrate comprises a thermoplastic comprising a polyether sulfone, a polyamide, a PEEK and/or combinations of the aforesaid thermoplastics.
8. 8. A composition according to claims 1 to 6, wherein the substrate is a rubber.
9. 9. A composition according to any one of the preceding claims wherein the substrate is in the form of a particle.
10. 10. A composition according to any one of claims ito 8 wherein the substrate is in the form of a veil
11. 11. A composition according to any one of the preceding claims wherein the dispersant comprises polypropyleneglycol (PPG), or polyethyleneglycol (PEG) and/or combinations of the aforesaid components.
12. 12. A composition according to any one of claims 2 to 11, wherein the solvent comprises acetone, methylethylketone (MEK), toluene, cellulose, formaldehyde, styrene and/or combinations of the aforesaid solvents.
13. 13. A method of producing an encapsulated material comprising a rubber or thermoplastic substrate and a cross linked polymer encapsulation, comprising the steps of: a. forming a dispersion comprising an uncured thermoset resin, a curing agent, a substrate and a dispersant, b. reacting the thermoset resin and curing agent within the dispersant to encapsulate the substrate with a cross-linked polymer encapsulation.
14. 14. A method according to claim 13, wherein the therrnoset resin is an epoxy and the curing agentisanamine.
15. 15. A method according to claim 13 or 14, wherein the substrate is a thermoplastic particle or veil.
16. 16. A method according to claim 13 or 14, wherein the substrate is a rubber particle.
17. 17. A method according to claims 13 to 16, wherein the dispersant is poly propylene gylcol.
18. 18. A method according to claims 13 to 17, wherein the reaction takes place at a temperature between 2OC and 2OOC.
19. 19. A method according to claims 13 to 18, wherein the ratio of substrate to reactants is 20:1 to 0.01:1 by weight.
20. 20. A method according to claims 13 to 19, wherein the ratio of substrate to reactants is from 10:1 to 1:1 by weight.
21. 21. A method according to claims 13 to 20, wherein the dispersion is heated by microwave.
22. 22. A method according to claims 13 to 21, wherein the method of production is a continuous process.
23. 23. A moulding material comprising a fibrous reinforcement and a resin matrix, wherein the resin matrix comprises the encapsulated material of any of claims ito 11.
24. 24. A dry fibre preform comprising a fibrous reinforcement and the encapsulated material of any of claims ito 11 that is subsequently infused with a resin.