GB2523298A - Moulding materials - Google Patents

Abstract

A method of making a polymer matrix composite comprising the steps of assembling a preform of one or more composite elements each comprising a fibrous reinforcement material, a resin matrix or a combination thereof; followed by processing of the lay-up to cure, wherein at least one dimension of the preform following assembly prior to the processing step is within 5%, preferably 1% of the cured lay-up. The present invention relates to methods of producing mouldings comprising reinforcement fabrics and reinforcement resin and to polymer-matrix composite (PMC) materials, especially PMC materials with high volume fractions of structural fibres produced by impregnation of the reinforcement with a curable or otherwise viscous polymer. In one embodiment, the present invention provides a method for manufacturing thick net-shape continuous-fibre textile preforms from carbon reinforcement. These performs may be bent and draped over complex moulds, thereby reducing labour intensive activities such as trimming, taping, layering fabric. In another embodiment, the present invention provides a method of designing and manufacturing directional thick, net-shape, drapable reinforcement performs. In a further embodiment, the present invention provides a method of optimizing yarn paths for enhanced coverage of the mould. In an embodiment, the present invention provides a novel textile architecture where yarn spacings of a textile may vary across the textile.

Description

MOULDING MATERIALS

INTRODUCTION

The present invention relates to methods of producing mouldings comprising reinforcement fabrics and reinforcement resin and to polymer-matrix composite (PMC) materials, especially PMC materials with high volume fractions of structural fibres produced by impregnation of the reinforcement with a curable or otherwise viscous polymer.

BACKGROUND

Existing textiles are unable to produce flat reinforcement fabrics that are thick, net-shape, drapable onto doubly-curved surfaces, and made from continuous yarns oriented in a way to promote coverage, resin flow and/or structural stiffness. Existing textiles generally deform to a greater or lesser extent thereby compromising their mechanical performance.

Existing reinforcement fabrics are generally pieces cut out from continuous roll of thin flat fabric that are draped individually onto one another, and sometimes adhered to one another using spray adhesives or mesh adhesives or other types of binders in a process generally known as preforming Rolls of thin, flat fabrics have been obtained by one of the following process: weaving of continuous yarns (plain weaves, twill weaves, satin weaves and/or any other pattern of woven fabrics), knitting of continuous yarns (any type of warp-knitting and/or weft-knitting), braiding of continuous yarns (any type of biaxial, triaxial or tubular braiding), stitching of parallel continuous yarns using a secondary thread, or any other process where continuous yarns are combined into thin flat reinforcements.

The group of manufacturing methods known as Liquid Composite Moulding (LCM) is widely used for Polymer Matrix Composite (PMC) production.

Liquid composite mouding (LCM) or iquid resin infusion (LRi) is a process used to manufacture fiber-reinforced composite structures and components for use in a range of dfferent industhes indudng the aerospace, transport, electronics, and building and leisure industries The general concept in LRI technoogy invoves infusing resins into a fiber reinforcement, fabric or a pre-shaped fibrous reinforcement ("preform") by pacing the material or preform into a mod (two-component mold or single-sided mold) and then injecting resin under high pressure (or ambient pressure) into the mold cavity or vacuum bag sealed single-sided mold, The resin infuses into the material or preform resulLng in a fiber-reinforced composite structure. LCM technology is especially usefu in manufacturing complex-shaped structures which are otherwise difficult to manufacture using conventional technoogies.

Alternatively, the perform may be manufactured from textiles which are preimpregnated.

Although these processes are versatile and cost-effective, they are extremely dependent on the availability of textile preforms offering good quality and consistency, realised through well-controlled fibre volume fractions, fibre orientations and thicknesses as well as the absence of defects such as out-of-plane deformations and inter-fibre gaps. Preform quality is itself largely determined by the draping operation.

SUMMARY OF THE INVENTION

According to the invention there is provided a method, a preform and a moulding assembly as defined in any one of the accompanying claims.

The preform may be preimpregnated to form a near net-shape moulding material.

Alternatively, the preform is unimpregnated but may be infused to form a near net shape moulding material.

In one embodiment, the present invention provides a method for manufacturing thick net-shape continuous-fibre textile preforms from carbon reinforcement. These performs may be bent and draped over complex moulds, thereby reducing labour intensive activities such as trimming, taping, layering fabric.

In another embodiment, the present invention provides a method of designing and manufacturing directional thick, net-shape, drapable reinforcement preforms, the method comprising the steps of: designing yarn patterns so that a flat preform can be produced which can be draped in a simple manual operation onto a mould defined in 3D.

In a further embodiment, the present invention provides a method of optimizing yarn paths for enhanced coverage of the mould, by using a model that tests different yarn path curvatures based on a Monte-Carlo optimisation method until a set of curvatures that maximises or minimises a parameter of interest or a combination of parameters of interest is identified.

In an embodiment, the present invention provides a novel textile architecture where yarn spacings of a textile may vary across the textile. These variable-length textiles can be custom tailored for specific PMC applications, offering advantages over conventional constant-lengths textiles such as a larger surface area covered by a single piece of textile, lower values of in-plane shear, and controlled fibre orientations. The variable-length textiles can be optimised manually or using model algorithms based on Monte Carlo methods which are implemented in the model.

We have discovered that the models for draping textiles also apply to resin impregnated textiles and fibers such as prepregs.

SPECIFIC DESCRIPTION

The term composite identifies a class of engineered materials typically made from two constituent materials characterised by different physical and mechanical properties. The constituent materials can be polymers, metals or ceramics. Once combined, they offer performance advantages beyond those of the constituents. In one embodiment, the present invention provides polymer matrix composites (PMCs) made from carbon or glass fibres, encased in a polymer resin. Described below are aspects of manufacturing PMCs where the reinforcement is a textile. These PMCs are widely used in the aerospace, automotive, nautical and recreational goods industries.

The basic principle of moulding processes for PMC production consists in creating a lay up of structural fibres, known as a preform, the volume and shape of which correspond generally to the volume and shape of the PMC part to be made. The structural fibres may be dry or they may be impregnated with resin. The operation of creating a lay-up assembly of fibres is commonly known as preforming.

In the case of dry fibres, once the fibres are in position within a mould, the preform is infused with a curable liquid polymer or resin and the fibres are impregnated in an operation called resin infusion. Many different liquid moulding processes exist including resin transfer moulding (RTM), RTM-light, vacuum assisted resin transfer 2 moulding (VARTM), resin film infusion (REI), Seeman Composites Resin Infusion Molding Process (SCRIMPTM) and QuickstepTM. The RTM process uses a mould made of at least two stiff parts, forming a closed cavity which has the shape and dimensions of the PMC component to be made.

During pad manufacture one half of the mould is covered or draped with pieces of dry textile reinforcement, introduced along a prescribed sequence of orientations and layering.

Alternatively, the preforming can be carried out on a separate tool, with the preform transported in the actual mould afterwards. Once the preform is ready the mould is closed and resin is injected through carefully chosen injection pods; the thickness of the gap separating the mould parts ensures predictable component thickness. The orientation of the fibres and volume fraction throughout the component are largely determined when the preform is made, so precision in preform construction is of paramount importance. RTM-light is very similar to RTM with the exception that it uses one rigid mould half and one thin shell mould half made of composite material. This thin and relatively flexible mould half limits the resin injection pressures that can be used; resin flow in RTM-light is often driven by vacuum.

VARTM is a further evolution of RTM where the top mould is replaced by a series of breather cloths, a perforated film and an air-tight membrane which compresses the preform when a vacuum is drawn for resin infusion. Similarly to RTM, the preform construction largely determines how the fibres are oriented in the PMC component, while the compaction and vacuum level will have an effect on the final fibre volume fraction and PMC component thickness. RFI is very similar to VARTM with the difference of having a film of semi-cured resin incorporated in the layup, which will melt when heated and infuse into the textile reinforcement, primarily through the thickness. The resin film replaces the liquid resin which is drawn in by vacuum in the VARTM process. The patented SCRIMPTM process is also very similar to the VARTM process with the addition of a patented flow medium which assists the flow of resin. While the process does offer advantages in terms of resin infusion over VARTM, it is still largely dependent on the preform construction in order to produce high quality PMC components.

The QuickstepTM process resembles both VARTM and RTM; while the preform is held within a flexible membrane much like in VARTM, a surrounding liquid can exert positive pressure on the preform much like the mould halves in RTM. However, unlike both RTM and VARTM, the curing process is activated by heat transferred via a heat transfer fluid (HTF); RTM may depend on heat transferred from the mould. Again, a high quality PMC component can only be manufactured by using a high quality preform.

Preforms may also be formed from fibrous reinforcement layers which are preimpregnated with a reinforcement resin (prepreg).

The performance, integrity and reproducibility of PMC parts manufactured using any of the above moulding processes are highly dependent on the quality of the preform. High preform quality refers primarily to the absence, or the presence in a limited amount, of defects such as inter-yarn gaps and wrinkles in the textile reinforcement, and also to the consistency of the volume fraction vf at different scales throughout the preform. The edges of a high quality preform should be devoid of fraying. Finally, preform quality refers to whether preform thickness and fibre orientation are as specified. The thickness and fibre orientation directly impact the mass and structural performance of the finished PMC component while any gaps and wrinkles may lead to major manufacturing problems as well as premature failure. The vf is defined as the percentage of the total volume of the PMC part which is occupied by the reinforcement. Typical values of vf range from 55% to 70% depending on the reinforcement and manufacturing process used. A PMC part with a low of vf will benefit fully from the mechanical properties of the reinforcement, but will be heavier due to the excess resin.

Conversely, a PMC part with a vf which is too high may not be consolidated due to a lack of resin, resulting in voids and very poor structural properties. The fibre orientation is the direction along which fibres impart their strength and stiffness to the PMC part. Fibre orientations which diverge from design specifications or are out-of-plane will yield a PMC component lacking the desired design stiffness and/or strength. Lastly, the thickness of the finished PMC component is nearly identical to the thickness of the compacted preform; thus it is imperative that the preform thickness corresponds to the desired thickness of the finished PMC component.

Textile reinforcements be it dry or resin impregnated and used in creating preforms generally come as rolled stock, which needs to be cut, draped and compacted upon preform manufacturing. Draping involves placing layered pieces of dry textile reinforcement onto the mould surface while compacting involves applying pressure, either positively or through vacuum, to the dry fibre bed in order to reduce its thickness and ensure a higher fibre volume fraction. The draping operation should be repeated to attain the desired preform thickness, while the compaction operation may be done in between draping operations or only once at the end. Both the draping and compaction of the textile reinforcement deform the textile. Possible deformation modes for the textile include fibre stretching, fibre straightening, interfibre slip, tow buckling and in-plane shear amongst others. Whilst these deformations enable the draping of high vf preforms, they can also lead to defects and low preform quality if badly applied or controlled.

S

In-plane shear is especially important to the preforming process as it is the deformation mode that enables flat sheets of textile reinforcements to conform to doubly-curved surfaces.

As such, it accounts for the vast majority of preform deformation. In-plane shear is introduced during the draping operation; it is limitations on in-plane shear that dictate the surface area that can be covered by a textile reinforcement as it is draped. All textile reinforcements are limited in the amount of in-plane shear deformation that they can withstand before deforming out-of-plane through wrinkling and buckling. Out-of-plane deformations of dry reinforcements before infusion are particularly damaging to PMC parts because they may result in major difficulties for resin injection or major defects in the part upon infusion. In the RTM process, out-of-plane deformations could result in the inability to close the mould due to areas of increased thickness. The most critical consequence of out-of-plane deformation is the near complete loss of structural properties due to fibres which are no longer aligned with applied loads, therefore little or no load is transferred to the fibres.

To effectively avoid the above defects and component deficiencies, it is important to drape the textile correctly, which can be made possible with accurate predictive simulations of the draping operation.

The design of preforms and parts, as well as their manufacturing, can be engineered by simulating the draping operation using models to detect areas of high in-plane shear, or of low fibre volume fraction, and the presence of potential out-of-plane deformation zones. The idea of simulating the draping of a 2D textile reinforcement onto a double curvature surface defined in 3D has been pursued and methods have been elaborated towards this purpose, including kinematic algorithms, finite element (FE) simulations and hybrid kinematic energy methods.

The kinematic or geometric method is essentially a mapping algorithm which predicts the path of yarns on the curved surface from simple geometric assumptions. The assumption made is that the textile reinforcement acts much like a fishnet; a square mesh made of inextensible segments which correspond to the centrelines of the textile's yarns, running in two directions, which are usually orthogonal. The yarns may rotate relative to one another, but yarn stretching and slipping is not permitted. The fishnet may only deform by the rotation of segments around the yarn crossovers. The kinematic method is dependent on the creation of two orthogonal geodesics which cross each other and define the global shape of the fishnet.

It is also possible to simulate the draping of textile reinforcements using finite elements.

Different methods have been established, including a simplified unit cell method which uses 1 D bar elements as shear and tow elements. The tow elements of the unit cell represent the yarns of the textile while the added shear elements represent the resistance to shearing of the textile, which makes it possible to find a minimum strain-energy solution for the textile.

The draping is simulated by finding the configuration in which the total strain-energy of the simulated textile is minimal.

Alternatively, FE simulations can be performed using an elementary pattern. The elementary pattern represents a crossover of perpendicular yarns before deformation, which is repeated in order to create an isoparametric bilinear four-node shell element. Each node in the element has three degrees of freedom to account for the three components of the node's displacement. The directions of the yarns in the elementary pattern correspond to the reference coordinates of the element. Such modelling can be used for simulating draping in a punch and die, where the punch and die are of the desired preform shape. Non-linear kinematics are used in order to calculate the deformation energy for each element, which should be minimised in order to determine the correct configuration.

The hybrid kinematic energy method operates similarly to the kinematic method by mapping the textile to a pin-jointed mesh similar to a fishnet. The segments may rotate relative to each other in order for the mesh to conform to the model surface. However, instead of solving for geometric constraints only, the hyblid kinematic energy method solves for minimal shear deformation energy. Unlike the purely kinematic method, this method does not rely on initially determined geodesics that extend to the boundaries of the surface to cover, even before coverage is actually started. Instead these basic lines are determined as coverage progresses, allowing the simulated textile to deform more freely. The direction of the fibres and the shear deformation of each the fishnet's segments is defined by identifying the configuration in which the deformation energy is minimal. Additionally, the magnitude of the resistance to shear can be defined differently depending on the direction of the shear; this gives the method the ability to simulate a wider variety of textiles, including textiles which have a tendency to shear in one direction with more ease than in the other direction.

Simulation software can accurately predict and display how a textile reinforcement will deform when draped over a 3D surface. These predictions are used for better informing the design of preforms used in liquid moulding processes. This enables the creation of preforms which have significantly less defects such as out of plane deformations, and allows for the manufacturing of preforms with predictable thicknesses, fibre volume fractions and fibre orientations. However, the extent to which a specific textile can deform in order to conform to a surface is fixed and limited by the locking angle of that textile. Therefore there are limits to the surfaces and shapes that can be draped without defect. In conventional textiles the spacing between yarns and between yarn crossovers are constant for a given textile. In one embodiment, the present invention provides a novel textile architecture where the textile's yarn spacing is allowed to vary across the textile. This enables the creation of textile reinforcements which can be optimised for improved drapability onto a specific surface, and/or offer advantages over conventional textile reinforcements such as less in-plane shear or a larger draped area. The optimisation of these variable-length textiles can be done manually or using software. Care should also be taken to ensure that the optimised textiles can be manufactured flat, hence the software probes in-plane shear and vf of the optimised textiles both in the flat state before draping and in the draped state. It is also important to limit the segment length difference from yarn to adjacent yarn, as large differences may lead to high curvature in yarns and impact the feasibility of manufacturing the textile.

When draping the preform, the textile architecture has a large effect on how the textile reinforcement will conform and shear on the surface. Textile reinforcements can be classified in different types according to their manufacturing process and structure, including weaves, knits, braids and stitched textile reinforcements. Such reinforcements are referred to as periodic fabrics and are generally supplied as rolls.

The preforming operation involves the deformation of the textile layers which are then superimposed to create a preform which should be a near net shape fibre bed comprised of only dry reinforcement, with possibly a small amount of binder to hold the textile layers in place. The process and the deformation modes encountered during preforming were discussed in previous sections, with the exception of nesting. Nesting occurs when multiple layers of textile reinforcement are compacted and tows from one layer penetrate between the tows of the adjacent layer. Nesting may reduce the relative motion between textile layers and enable the textile to be compacted more easily.

Another point of practical importance should be mentioned. It is not possible, using current textiles and draping methods, to successfully drape any preform with a single piece of textile, covering the surface of the mould entirely. Generally this can only be done in cases of small parts which are mostly flat or only gently curved. For this reason, multiple pieces of fabric are used within each layer, creating joints in the preform. Several types of joints exist such as lap joints, butt joints and step joints. Only butt and step joints may be used for manufacturing processes such as RTM where the extra thickness of the lap joint makes it impossible in practice for the mould to close on the preform. While commonly used in VARTM applications, lap joints increase the local volume fraction of the preform at the joint seems.

Joints may cause problems by enabling resin to racetrack in the gaps between the textile pieces during the infusion process.

To temporarily affix multiple pieces of textile to the preform mould, adhesives commonly called tackifiers or binders are typically used. Tackifiers, often supplied as a powder, can also be applied between layers in multilayered textiles to simplify the handling of textile pieces in the creation of complex preforms. While the practical advantages to the preforming operation are evident, the use of a tackifier may have adverse effects on the infusion operation. Preform permeability decreases and may become uneven as tackifier concentration is increased unless the tackifier is located within yarns. The tackifier may also interfere with the resin on a chemical basis, therefore care should be taken to ensure compatibility between the tackifier and resin.

The simulation of the draping operation can be performed using geometric principles, using a method called kinematic simulation algorithm. The algorithm and mathematics for kinematic draping are described in detail in for example US 201310269159 which is incorporated herein by reference.

In one embodiment, the present invention provides methods for designing, optimising and manufacturing textile performs used in the production of structural polymer-matrix composite (PMC) materials and parts, whereby the preform constitutes the integral reinforcement of the composite material and part. Reinforcement preforms are typically draped onto a mould and combined with a polymer resin supplied in liquid or film form; once combined and solidified these two constituents form a PMC part. Advantages of the performing technology of the present invention include: (1) The preforms are designed and optimised individually for specific PMC parts. (2) The preforms are manufactured as flat sheets to be draped onto moulds in PMC manufacturing processes. (3) Curved yarns are oriented along two selected and/or optimised directions at each point of a preform but these orientations typically vary over the surface of the flat preform, and they change further as the preform is draped onto the mould, aiming at improving coverage of mould surfaces by the dry preforms and structural performance of the PMC parts. (4) The preforms are designed and manufactured net-shape, reducing/removing the need for any tailoring and cutting of reinforcement fabrics.

(5) The preforms are manufactured to required thickness, reducing/removing the need for laminating successive layers of reinforcement fabric.

The stiffness and strength of structural polymer-matrix composite parts stem from the use of strong and stiff reinforcement fibres (typically carbon, graphite or glass, with numerous other types of fibres available) assembled into a reinforcement preform, with the polymer matrix (typically epoxy, with numerous other types of thermoplastic, thermosetting and other polymers available) encapsulating the fibres and used primarily for positioning the fibres in the PMC parts, for protecting the fibres from the environment, and for transferring stresses from the environment to the fibres. Fibres with diameters ranging from about 5 [mu]m to 15 [mu]m are preferred for easy conformability of the reinforcement preforms to curved mould surfaces and effective reinforcing in PMC parts. Handling reinforcement fibres individually in PMC manufacturing operations is not practical, hence fibres are traditionally assembled into textiles of various types such as weaves, braids and others in separate textile manufacturing operations, before PMC manufacturing takes place. Initially, individual fibres are grouped into yarns of a few hundred to a few thousand fibres. Then, traditional textile processes tuned to high levels of control over dimensional accuracy, yarn tension and other manufacturing parameters are used for turning yarns into fabrics that can be handled easily in subsequent PMC manufacturing operations. These fabrics become the reinforcement in PMC parts.

Most traditional textile technologies typically produce textiles-such as weaves or braids-in a continuous form. Woven fabrics are produced in continuous sheets of set width sold as rolls.

A typical woven reinforcement is much thinner than the thickness of a typical PMC part, hence such fabrics are usually referred to as thin (2D') fabrics. Traditional textile reinforcements are not designed for specific PMC parts in terms of outline, thickness or orientations of fibres and yarns.

The shapes of PMC parts relevant to the performing technology according to one embodiment of the present invention fall into the general category typically labelled as shells: parts where one dimension-the thickness-is orders of magnitude smaller than other dimensions. Shells usually are not flat-they feature varying levels of curvature and are defined in the 3D space. An airfoil, the nose cone of an aircraft and a saddle shape are examples of shells. Shells are also defined by a contour or outline. Preparing the reinforcement for such shells typically involves cutting pieces of 2D fabric from a roll and laying them onto the curved surface of a mould. Many superimposed pieces of thin fabric are needed to reach a required part thickness-especially as dry fabrics undergo much compression normal to the shell surface during FMC manufacturing; hence FMC parts are often referred to as laminated: more layers are needed for thicker parts, and inversely. The operation of conforming textile pieces to the surface of a mould is called draping. A stack of superimposed layers of reinforcement fabric pieces draped into shape on a mould is often referred to as a preform. The outline of the individual pieces of fabric cut from the continuous roll of thin fabric should match the outline of the preform to be made as much as possible, so as to limit the number of pieces of fabrics needed to make up the preform. However, the outline of a flat piece of fabric differs from the outline of the same piece once it is draped on a mould and into the final preform, and the draping operation is very delicate because of the nature of textile reinforcements. Therefore, preform manufacturing remains highly manual-many unsuccessful attempts at automation were made-and time-consuming, and its outcome is very variable. Laser projection systems are often used to indicate to workers where and how a given piece of fabric should fall on the mould once draped. Cutting textiles to outlines leads to potential fraying and handling of single layers can result in some disassembling. Positioning layers on top of each other with precision is tremendously difficult. Traditionally this has precluded textile-based PMCs based on dry textile preforms-which are relatively economical-from entering aerospace markets, especially for primary load-bearing structures. The preforming technology according to one embodiment of the present invention removes the limitations and difficulties associated with building preforms from laminated pieces of thin fabric.

A typical 2D fabric is made from two sets of parallel and equidistant yarns (warp yarns and weft yarns in a weave, for example). In a piece of flat fabric coming off the roll these two sets of yarns are perpendicular. However, fabrics have the capability of undergoing large in-plane shear strains whereby the angle between the two sets of yarns changes upon draping, even though yarns within each set remain parallel and equidistant-the distance varies during in-plane shear when measured perpendicularly to the yarns. This can be visualised if a square is drawn on the fabric; as the fabric undergoes in-plane shear the square takes the aspect of a diamond, with the 4 sides maintaining a constant length. This capability for large and reversible strain shear strain is not found in materials such as sheet metal. Point 1 is important because a fabric made of stiff and virtually inextensible reinforcement fibres can be draped on a (doubly) curved surface only because it can undergo large in-plane shear strain. Hence this capability to undergo in-plane shear is essential, with the following 2 limitations: 1) there is a limit shear angle-typically labelled locking angle-beyond which a fabric will not shear but buckle-buckling of fabrics in preform is highly detrimental to PMC performance and should be avoided. Fabrics can shear more or less depending on construction; capability to undergo high shear strain is always desirable. 2) a fabric may be sheared in either of 2 directions away from the original 900 angle separating both sets of yarns; resistance to shear and locking angle may differ for both directions, depending on construction.

A fabric in any given state is characterised by its fibre volume fraction (vf). This value can change in different situations for a given fabric. It can also change from point to point in a piece of fabric and in a preform. The fibre volume fraction is the fraction of the volume circumscribing a fabric, or part of a fabric or preform, that is effectively occupied by fibres.

For example, assuming a representative thickness for a sheet of fabric, a vf of 50% means that half of the volume within this thickness is occupied by fibres, the rest being occupied by air in a dry fabric and by resin in a composite. A high vf is highly beneficial fol the specific structural properties of PMCs; however vf cannot reach 100%. Textiles can be compacted normal to their plane and this clearly increases vf. vf also increases upon shearing as the yarns get closer, but only if the thickness of the fabric stays constant. Usually thickness does not stay constant upon shearing, so there is no systematic structural advantage for the PMC part to having fabrics highly sheared upon draping.

Woven and braided fabrics are interlaced as yarns cross over each other during textile manufacturing; this is what holds yarns together for these textile manufacturing processes.

But interlacing generally reduces the ability of fabrics to undergo in-plane shear, and it also reduces the primary structural properties of FMCs made from reinforcements featuring interlaced yarns. Other means of manufacturing 2D fabrics from two perpendicular sets of parallel yarns exist, where the yarns are held together by light stitch lines realised by processes including stitching, warp-knitting and others. In this document and much PMC literature such fabrics are termed non-crimp stitched (NCS) 2D fabrics.

Advantages of the preforrning technology according to one embodiment the present invention include: (1) the ability to design, optimise and manufacture individual textile preforms for specific PMC parts of known shape and geometry, promoting easier PMC part manufacturing, higher preform consistency, and better PMC structural behaviour; (2) the ability to design, optimise and manufacture textile preforms as flat, drapable individual pieces of textile as opposed to cutting separate textile pieces from continuous textiles supplied in rolled form; (3) the ability to design, optimise and manufacture textile preforms as flat, drapable individual pieces of flat textile as opposed to preforms that are produced to the final configuration of the part in 3D, so that the same machinery may be used in all cases and preforms can be shipped easily; (4) the ability to design, optimise and manufacture textile preforms featuring two local yarn orientations that vary throughout the preform-both prior to and after draping on a mould, enhancing FMC manufacturing and structural performance; (5) the ability design, optimise and manufacture net-shape textile preforms, reducing/removing the need for cutting and assembling numerous textile pieces; and (6) the ability design, optimise and manufacture textile preforms to the required thickness, reducing/removing the need for laminating and superimposing numerous textile pieces.

Textile preforms manufactured using the technology according to one embodiment of the present invention are flat, hence typically they should undergo draping onto a curved mould defined in 3D. This way of manufacturing distinct, part-specific textile pieces enables at least three important technical features: (1) textile preforms can be built to the required final thickness in a single operation-the resulting thick textile preform is known as 2.5D (thick textile preform in this proposal); (2) textile preforms can be built to the required outline (flat) in a single operation-the resulting thick textile preform is known as net-shape (thick net-shape textile preform in this proposal); (3) there is no limitation whereby flat fabrics feature straight yarns only as yarn paths in the flat fabric (before draping) can be designed as curves defined for a specific PMC part. This enables highly valuable possibilities such as using yarn paths that are more favourable to in-plane shear of the fabric upon draping, or to impregnation of the preform by a resin in PMC manufacturing, or to the structural properties of the PMC part. Such textiles are referred to here as directional thick net-shape textile preforms. It should be noted that it is perfectly possible to make thick net-shape textile preforms featuring straight yarns, and that this might constitute an optimal solution in some cases. But the design manufacturing process defined herein allows yarn directionality. The architecture of the preforms according to one embodiment of the present invention is such that 2 yarn directions can be identified at any point of the fabric, that these directions are not necessarily perpendicular, and that they can vary from point to point in the fabric-both flat and draped.

One notable advantage of the preforming technology according to one embodiment of the present invention is the manufacturing of preforms as flat fabrics to be draped, as opposed to producing fabrics already to final part shape straight from the yarn. This promotes process versatility and simplicity. Whilst it may be possible to lay down individual yarns is space using an industrial robot or highly dedicated machinery, dry yarns laid on anything but horizontal surfaces do not stay in place and should be held using ancillary devices. The preforming technology according to one embodiment of the present invention uses simpler machinery that is versatile, and it produces flat preforms that are easier to wrap and ship.

Another notable advantage of the preforming technology according to one embodiment of the present invention as opposed to existing technologies is that it removes most operations that induce variability and potential damage in preforms, namely the cutting, manual handling and manual draping of individual thin layers. Hence it does offer potential for improved consistency.

The architecture of the preforms according to one embodiment of the present invention is straightforward. Preforms consist of layers of superimposed no-crimp yarns assembled by a stitch. Simpler textile structures are available in thin (2D') form and sold as continuous rolls featuring straight yarns only-such thin fabrics are generally recognised as superior to their woven counterparts and their use is widespread. However, the preforms according to one embodiment of the present invention are very different and superior in many aspects, including but not limited to: 1) the preforms are thick, 2) the preforms are net-shape, and 3) yarns in the preforms can be directional.

In one embodiment, the present invention provides a method for manufacturing the aforementioned directional thick net-shape reinforcement preforms.

In one embodiment, the present invention provides optimizing yarn paths in the flat directional thick net-shape reinforcement preforms, aiming at optimizing mould coverage, resin flow through fabric and/or stiffness of final PMC part.

Most resin infusion systems are inherently brittle, and the viscosity levels necessary to achieve the injection process preclude the use of toughening agents. Said differently, the properties of toughness and ow iiscosity are typicafly mutually exclusive in conventional resin infusion systems. In prepregs, high levels of toughness are generally achieved through the addition of about ten percent (10%) to about thirty percent (30%) by weight of a thermoplastic toughener to the base resin. However, addition of such. tougheners to LRI systems generally results in an unacceptable increase in the viscosity of the resin and/or reduction in resistance of the cured material to solvents. In the specific case of particulate toughener, there may be additional filtering issues in the textile. These limitations render the addition of tougheners conventionaUy added in prepregs generafly unsuitable in conventional LRI applications.

One technology to toughen fiber-reinforced composite structures manufactured by LRI technologies is to integrate the toughener into the preform itself. For example. a soluble toughening fiber may be directly woven into the preform thereby eliminating the need to add toughener into the resin which otherwise would ncrease the viscosity of the resin (rendering it unsuitable for resin infusion). Another example is the use of soluble or insoluble veils comprising of tough.ener used as an interleaf with the reinforcement of the preform.

However, in either of these methods, the manufacturing process may he more complicated and costly, in addition to increasing the r!sk of hoVwet performance knockdowns and solvent sensitivity with a polymer based insoluble interleaf. Another technology is the addition of particles to the resin. The amount of particles required to reach a suitable toughness threshold, however, is often high resulting in a viscous resin requiring a very narrow process window that is generally unfavorable for LRI.

A formulation, comprising: (i) at least one base resin; (ii) an amount of particles within a predetermined range in a carrier resin; and (iii) an amount of thermoplastic material within a predetermined range wherein the base resin, the particles and the thermoplastic material are combined to form a modified resin system, the modified resin having an average viscosity below a threshold average viscosity within a predetermined temperature range is herein disclosed.

We have discovered that this formulation either used as a liquid infusion resin or as a resin for preimpregnating textiles, fibres or fabrics as hereinbefore described, is particularly suited as a resin for preforms which are close to their final cured dimensions ("near netshape compositeC).

The formulation may further comprise a curing agent. The curing agent may be an aniline-based amine compound. The base resin may be one of epoxy, bismaleimide, cyanate ester or a combination thereof. The base resin may be a combination of epoxies including at least one di-, UI-or tetra-epoxy. The particles may be one of chemically functionalized or chemically non-functionalized core-shell rubber particles or hollow particles. A material comprising the core may be one of polybutadiene-styrene, polybutadiene or a combination thereof, and a material comprising the shell may be one of silica, polymerized monomers of acrylic acid derivatives containing the acryl group including acrylic and poly(methyl methacrylate) or a combination thereof. In a cured condition, the particles may be substantially uniformly dispersed throughout the modified resin system. The thermoplastic material may be one of phenoxy-based polymers, poly(ether sulfone) polymers, poly(ether ether sutfones), poly(methyl methacrylate) polymers, carboxylterminated butadiene acrylonitille polymers, copolymers thereof, or combinations thereof. The formulation wherein the amount of thermoplastic material is below approximately 30% net weight, preferably below 7%, of the modified resin system. In a cured condition, at least the thermoplastic material phase may separate from the base resin. More particularly, the thermoplastic material phase may separate into aggregate domains from the base resin, each aggregate domain having an island-like morphology. The morphology in a cured article may evolve: (i) during the later stages of a ramp to dwell temperature; or (ii) after a ramp to dwell has been completed during the cure cycle. The amount of particles and the amount of thermoplastic material may be combined in a I to 0.56 ratio. The threshold average viscosity may be less than 5 Poise at a temperature of less than 180°C, more narrowly between 80°C and 130°C.

A composite article, comprising: a structure having a predetermined shape, the structure having a plurality of layers of a fiber-based fabric, the structure having a targeted composite toughness within a predetermined range, wherein the toughness is at least partially imparted by a modified resin system during a process, the modified resin system including: (I) at least one base resin; (ii) an amount of particles within a predetermined range in a carrier resin; and (iii) an amount of thermoplastic material within a predetermined range wherein the base resin, the particles and the thermoplastic material are combined to form the modified resin system, the modified resin having a average viscosity below a threshold average viscosity within a predetermined temperature range is herein disclosed. The modified resin system may further include a curing agent, the curing agent comprising an aniline-based amine compound. The base resin may be one of epoxy, bismaleimide, cyanate ester or a combination thereof. The base resin may include a combination of epoxies including at least one di-, UI-or tetra-epoxy. The particles may be one of core-shell rubber (CSR) particles or hollow particles wherein, when the particles are CSR particles, a material comprising the core is one of polybutadiene-styrene, polybutadiene or a combination thereof, and a material comprising the shell is one of silica, polymerized monomers of acrylic acid derivatives containing the acryl group including acrylic and poly(methyl methacrylate) or a combination thereof. In a cured condition, the particles may be substantially uniformly dispersed throughout the modified resin system. The thermoplastic material may be one of phenoxy-based polymers, poly(ether sulfone) polymers, poly(ether ether sulfones), polymerized monomers of acrylic acid derivatives containing the acryl group including acrylic and poly(methyl methacrylate) polymers, carboxylterminated butadiene acrylonitrile polymers, copolymers thereof, or combinations thereof. The amount of thermoplastic material is below approximately 30% net weight, preferably below 7% net weight of the modified resin system. \Mth the base resin in a partially cured or gel-like state, the thermoplastic material may separate into aggregate domains from the base resin, each aggregate domain having an island-like morphology. The amount of particles and the amount of thermoplastic material may be combined in a I to 0.56 ratio. The structure may exhibit a high level of microcrack resistance. The threshold average viscosity may be less than 5 Poise at a temperature of less than 180°C, more narrowly between 80°C to 130°C.

The fiber-based fabric may be comprised of reinforcing fibers of a material selected from the group consisting of organic polymer, inorganic polymer, carbon, glass, inorganic oxide, carbide, ceramic, metal or a combination thereof. The process may be a liquid resin infusion manufactunng process. a prepreg manufacturing process or a resin ifim infusion process.

A formulation, comprising: (i) a base resin comprising at least one epoxy; (ii) a curing agent; (iii) an amount of thermoplastic material; and (iv) an amount of core-shell particles wherein the base resin, the curing agent, the thermoplastic material and the particles are combined to fonii the modified resin system, the modified resin having an amount of thermoplastic material of less 30% net weight, preferably less than 7% net weight, of the total weight of the modified resin system is herein disclosed.

With the base resin n a partially cured or gel-like state, the thermoplastic material phase may separate into aggregate domains from the base resin. The amount of particles and the amount of thermoplastic material may be combined in a Ito 0.56 ratio. With the base resin in a partiafly cured, gel-like, cured or vitrified state the particles are substantially uniformly thspersed throughout the moddied resin system. The modfied resn system may have an average viscosity of less than 5 Poise at a temperature of less than 180°C, more narrowly between 80°C and 130°C. With the base resin in a cured or vitrified condition, the theniioplastic material may separate into aggregate domains from the base resin, each aggregate domain having an island-iike morphology The morphology in a cured article may evolve (i) during the later stages of a ramp to dwell temperature or (ii) after a ramp to dwell has been completed during the cure cycle. A manufacturing process, comprising: (i) preparing a preform; (ii) laying the preform within a mold; (iii) heating the mold to a predetermined temperature; and (iv) injecting a resin wherein the resin is a modified resin, the modified resin system comprising a combination of: (i) at least one base resin; (ii) a curing agent;(Hi) an amount of particles within a predetermined range in a carrier resin; and (iv) an amount of thermoplastic material within a predetermined range wherein the amount of thermoplastic material of the rnodihed resin S less than 30% net weight, preferably less than 7% net weight, of the total weight of the modified resin system is herein disclosed.

The predeterm!ned temperature of the mold may be 1 10°C. The manufactur!ng process may further comprise ramping a temperature of the mold to 180°C at a rate of less than 10°C per minute, more narrowly, less than 5C per minute. The manufacturing process wherein, when the mold reaches 180°C, the temperature is held for between 90 minutes and minutes. The preform may be sealed within the mold by at east a vacuum bag. An average viscosity of the modified resin system may be less than 5 Poise at a temperature range of less than 180°C. more narrowly between 80°C and 130°C. The preform may be comprised of plurality of layers of fiber-based fabric, The fiber-based fabric may have a structure comprising one of woven fabrics, multi-warp knitted fabrics, non-crimp fabrics, unidirectional fabrics, braided socks and fabrics, narrow fabrics and tapes or fuy-fashioned knit fabrics. The fiber-based fabric may be comprised of reinforring fibers of a material such as organic polymer, inorganic polymer, carbon., glass, inorganic oxide. carbide. ceramic, metal or a combination thereof.

Embodiments of the invention are directed to modified resin systems for use in resin infusion (RI) processes, variations of LRI processes and other suitable processes such as prepreg processes. In one embodiment, the modified resin system inciudes a novel combination of at east one base resin, an amount of particles within a predetermined range and an amount of thermoplastic material within a predetermined range wherein, when combined, the modified resin system has an average viscosity below a threshold average viscosity within a specific temperature range and a high. level ol toughness. The modified resin system may additionally include a curing agent and other suitable components. The modified resin system has been experimentally shown to exhibit a unique, controllable and constant morphology which is substantially or completely responsible for imparting a required toughness and damage resistance to a finished composite article without adversely impacting resin properties such as viscosity, potlife, cure temperature, glass transition temperature or tensile modulus of the modified resin system.

According to embodiments of the invention, a combination of at least one base resin, an amount of particles within a predetermined range and an amount of thermoplastic materiai within a predeiermned range. in addition to other components, may be combined n a:!ore pof' formulation to generate a modified resin system which can be used in Rl/LRI processes or prepreg processes. The modified resin system as formulated according to embodiments of the invention was discovered to have an unexpectedly low viscosity, low reactivity, a high level of toughness (C3JC), among other character!stics. when subjected to nurnertus experimental tests. It is anticipated that the modified resin may also be used in variations of liquid resin infusion processes induding, but not limited to, Resin Infusion with Flexible Tooling (RIFT). Constant Pressure Infusion (CPU. Bulk Resin Infusion (SRI), Controlled Atmospheric Pressure Resin infusion (CAPRI), Resin Transfer Molding (RTM), Seemann Composites Resin Infusion Molding Process (SCRIMPTM). Vacuum-assisted Resin Infusion (VARI), Resin Transfer Injection (RTI) and Vacuum-assisted Resin Transfer Molding (VARTM) as well as other processes used to manufacture composite articles.

In the context of this application, a "resin" is a synthetic polymer compound which begins in a viscous state and hardens with treatment. Resins are used as a structural matrix material in the manufacture of adhesives and composites and are often reinforced with fibers (e.g., glass, evlar, Boron and Carbon). In some embodiments, the base resin may be any one of epoxy, bismaleimide, benzoxazine, cyanate ester, vinyl ester, polyisocyanurates, bismalimide, cyanate ester, phenolic resin or any combination thereof in addition to other suitable resins. In some embodiments, the base resin is an epoxy resin or a combination of epoxy resins. The epoxy resin may be a tetra-, tn-, di-epoxy or combinations of tetra-, til-and/or di-epoxies. Exemplary til-epoxies include higlycidyl p-aminophenol (MY-0510 available from Huntsman Advanced Materials, Inc.) and ARALDITE® (MY-0600 available from Huntsman Advanced Materials, Inc.). An exemplary tetra-epoxy is tetraglycidyl diaminodiphenyl methane (MY-721 available from Huntsman Advanced Materials, Inc.).

Other suitable epoxy resins include bisphenol F epoxy (PY-306 available from Ciba Geigy).

In the context of this application, a "particle" is a polymer-based material having a core-shell or hollow morphology. Core-shell rubber (CSR) particles have the characteristic of having a core comprising of a rubbery material surrounded by an outer shell of glassy material. CSR particles are used as toughening agents when combined with polymeric matrices, e.g..

epoxy resins. In some embodiments, the particles may be any commercially available chemically functionalized or chemically nonfunctionalized CSR particles having a core material of polybutadiene-styrene or polybutadiene and having a shell material of silica or polymerized monomers of acrylic add derivatives containing the acryl group including acrylic and poly(methyl methacrylate). The CSR particles may be supplied in a carrier resin such as tetraglyddyl diaminodiphenyl methane (i.e.. MY-721) and may have a diameter of between about fifty (50) nanometers (run) and about eight hundred (800) nm, in one embodiment; about one-hundred (100) nm. Examples of commercially available CSR particles include, but are not limited to, the Paraloid series of materials (available from Rohm and Haas), MX4I I (polybutadiene-styrene/acrylic) and MX416 (polybutadienelacrylic) (both are dispersions in Huntsman MY721 epoxy resin and are available from Kaneka Corp.); however, any particle exhibiting the CSR or hollow structure as described above may be used in the modified resin systems according to embodiments 01 the invention.

Core-shell particles have been evidenced to toughen LRI systems via a cavitation mechanism in addition to crack pinning or "tear out' mechanisms. In a cavitation mechanism, the rubbery cores of the CSR particles yield under the stress concentrations at a crack tip, resulting in dissipation of energy from the crack front and the formation of voids in the core material.

In the context of this application, a "thermoplastic" is a polymer that is elastic and flexible above a glass transition temperature (Tg). In some embodiments, the thermoplastic material comprises one of phenoxy-based polymers, poiy(ether sulfone) (PES) polymers, poly(ether ether suifones), polyrnerized monomers of acrylic acid derivatives containing the acryl group including acrylic and poly(methyl methacrylate) (EMMA) polymers, carboxyl terminated hutadiene acrylonitrile (CTBN) polymers. copolymers thereof, or combinations thereof.

Representative thermoplastics include, but are not Umited to, KM 180 (avadable from Cytec Industries. nc.), 5003P (avaHable from Sumitomo Corp.), PKHB (InChemRes); however, any thermoplastic or other suitable material (e.g., Nanostrength X, available from Arkerna. nc.) exhibiting a thermaUy driven phase separation from a base resin, more particularly, exhibiting aggregate domains, or an "island-Uke" morphobgy (explained in more detail below), may he used in the modified resin systems according to embodiments of the invention.

An example of a typical mechanism for thermoplastic toughening of composite or resin matrices is crack pinning. Indications of crack pinning mechanisms include tailing behind thermoplastic domains or apparent plastic deformation around such thermoplastic zones originating from a divergent crack front around a thermoplastic rich region and subsequent convergence of the split crack fronts. Another example of a typical toughening mechanism is that of ductile tearing which can be described as a localized plastic deformation upon application of a stress to the material.A curing agenl is a substance or mixture 01 substances added to a polymer composition (e.g., resin) to promote or control the curing reaction. Addition of ouring agent functions to toughen and harden a polymer material by cross-linking of polymer chains. Representative curing agents include, but are not limited to, methylenebis (3-chloro-2, 6 diethylaniline) (MCDEA), 3,3-diaminodiphenyl sulfone (33- DDS), 4,4'-diaminodiphenyl sulfone (4,4-DOS), dicyandiamide (DICY). N-methyl-diethanolamine (MDEA) and 4,4'-methylene-bis-(2-isopropyl-6-methyl-aniline) (MMIPA).

According to embodiments of the invention, the modified resin system may include a thermoplastic which is 7% or less net weight of the modified resin system combined with an amount of CSR particles in a Ito 0.56 ratio of thermoplastic to CSR particles. In one embodiment, the base resin may be a combination of di-, tetra-and tn-epoxies such as PY- 306, MY-0500 and/or MY-0600). In one embodiment, the thermoplastic material may be 5003P and the CSR particles may be MX4I 1 (in MY-721) or MX416 (in MY-721) one-hundred (100) nm particles. A curing agent, such as MCDEA, may be added to the "one pot' resin system to make the resin system curable when heat and/or pressure is/are applied thereto.

The formulation of the present invention. comprises at least one base resin; an amount of particles within a predetermined range in a carrier resin; and an amount of thermoplastic material within a predetermined range wherein the base resin, the particles and the thermoplastic material are combined to form a modified resin system, the modified resin having an average viscosity below a threshold average viscosity within a predetermined temperature range. The threshoM average viscosity of the formulation is ess than 5 Poise at a temperature of ess than 180°C and preferably at a temperature of between 80°C and 4 D flofl JU Li.

Vvlien the formulation is in a cured condition, at least the thermoplastic material is phase separated from the base resin and preferably phase separates into aggregate domains from the base resin, each aggregate domain having an island-Uke morphology. The cure morphology evolves (i) during the ater stages of a ramp to dwell temperature or (ii) after a ramp to dwell has been completed during the cure cycle.

The amount of thermoplastic material in the formulation is below approximately 30% net weight of the modified resin system and preferably below approximately 7% net weight of the modified resin system.

The formulation may include an amount of panides and the amount of thermoplastic matenal combined in a 1 to 0.56 ratioS When the formulation is in a cured condition, the thermoplastic material is phase separated from the base resin and preferably, the thermoplastic material phase separates into aggregate domains from the base resin, each aggregate domain having an island-like morphology.

Further embodments of the present nvention include a manufacturing process, comprising preparing a preform, laying the preform within a mold, heating the mold to a predetermined temperature and injecting a resin wherein the resin is a modified resin, the modified resin system comprising a combination of: (i) at east one base resin: (ii) a curing agent; (iii) an amount of particles withn a predetermined range in a carrier resin; and (iv) an amount of thermoplastic matenal within a predetermined range wherein the amount ci thermoplastic material of the modified resin is less than 30% net weight of the total Neight of the modified resin system.

The above manufacturing process may further be modified wherein the predetermined temperature of the mold is between 90°C and 120°C or more preferably the predetermined temperature of the mold is 110°C.

The manufacturing process may be practiced by ramping a temperature of the mold to 180°C at a rate of up to 5°C per minute or more preferably at a rate of 2°C per minute.

Furthermore, when the mold reaches 180°C, the temperature may be held at this temperature for about 120 minutes.

The manufacturing process may be practiced wherein the preform is a plurality of layers of fiber-based fabric as hereinbefore described. The fiber-based fabric may have a structure comprising one of woven fabrics, multi-warp knitted fabrics, non-crimp fabrics, unidirectional fabrics, braided socks and fabrics, narrow fabrics and tapes or fully-fashioned knit fabrics.

The fiber-based fabric may uthize reinforcing fibers of a material selected from the group consisting of organic polymer, inorganic polymer, carbon, glass, inorganic oxide, carbide, ceramic, metal or a combination thereof.

The preform may be infused with resin or preimpregnated with a resin as hereinbefore described.

Claims (4)

1. CLAIMS1. A method of making a polymer matrix composite comprising the steps of assembling a preform of one or more composite elements each comprising a fibrous reinforcement material, a resin matrix or a combination thereof; followed by processing of the lay-up to cure, wherein at least one dimension of the preform following assembly prior to the processing step is within 5%, preferably 1% of the cured lay-up.
2. 2. The method of claim 1, wherein a composite element is infused with a resin prior to processing of the composite.
3. 3. The method of claim 1 or 2, wherein the composite element is impregnated with a resin prior to processing of the composite.
4. 4. The method of any of the preceding claims, wherein the composite element may be shaped or formed prior to processing of the composite.