Improvements in or relating to self-reinforced plastics

This invention relates to a process for the manufacture of composite plastics materials, in particular a process for the manufacture of so-called self-reinforced plastics materials, and to materials produced by that process.

Fibre-reinforced plastics materials are well-known. Such materials are examples of plastics composites, and comprise a polymer matrix that is reinforced, most commonly, with fibres. The polymer matrix can have many different forms, but may be an epoxy, vinylester or polyester thermosetting plastic or a thermoplastic such as polypropylene or nylon, and the reinforcing fibres are most often glass fibres (also called fibreglass), carbon fibres or so-called aramid fibres. The benefits of such composite materials reside in the combination of the properties of the polymer matrix and the reinforcing fibre. In particular, the fibre component improves the strength and stiffness of the material. As a result, fibre-reinforced plastics, and in particular glass-reinforced plastics (GRP) are used in a wide range of applications, such as boatbuilding, building components, pipework, and many others.

Composite materials such as GRP offer many advantages, but their use is not without disadvantages. For instance, reinforcement with glass fibre may lead to a substantial increase in weight, and recycling of the material may not be easy, due to the presence of both the polymer matrix and the glass fibre. The impact absorption properties of the materials may also not be as good as may be desired, and the presence of abrasive fibres may lead to problems with handling and tool wear.

More recently-proposed composite materials that overcome certain of these disadvantages are those known as self-reinforced plastics. Such materials are a family of materials in which the polymer matrix is reinforced with fibres of the same type of material, eg a polypropylene matrix reinforced with polypropylene fibres. Drawing of the polymer into fibres or tapes aligns the molecules and improves the mechanical properties, and these fibres are then used to reinforce a matrix of the

same polymer. Self-reinforced plastics are generally lighter in weight than corresponding glass fibre-reinforced materials, as well as being easier to recycle and to handle. Such materials nonetheless have stiffness and strength values typically five times those of conventional non-reinforced plastics.

Hitherto, self-reinforced plastics materials have been formed mainly of polypropylene, and two main techniques have been used for the production of such materials. These are termed hot-compaction and co-extrusion. In the former process, heat and pressure are applied to a stack of fibre fabrics. The process conditions are controlled in such a way that the surfaces (but only the surfaces) of the fibres melt to form the continuous matrix phase of the composite. In the co- extrusion process, a thin coating of a polypropylene-polyethylene copolymer is co- extruded with fibres of polypropylene. The surface coating melts upon application of heat and pressure, again forming the continuous matrix phase.

There has now been devised an improved process for the production of self- reinforced plastics, which process offers certain advantages over the prior art, and in particular facilitates the production of self-reinforced plastics materials based on different polymers to those that have hitherto been produced.

According to a first aspect of the invention, there is provided a process for the production of a self-reinforced plastics material, which process comprises

(a) forming a mixture of a first grade of a plastics material and a second grade of the plastics material, wherein at least the second grade of plastics material is in the form of elongated elements, and the first grade of material is selectively heatable, and

(b) treating the mixture so formed such that the first grade of plastics material is fused to form a continuous phase, whilst the elongated elements of the second grade of plastics material remain intact or substantially intact, said elements being dispersed in the continuous phase.

In one particular embodiment of the invention, the treatment applied in step (b) of the method involves the application of radiation, eg microwave or radiofrequency

radiation, that is absorbed by the first grade of material, thereby causing the first grade of the plastic material to melt, whilst the second grade remains solid.

The use of microwave energy to heat polymer systems is known, for instance from WO-A-2007/140469, WO-A-2007/143015, WO-A-2007/143018 and

WO-A-2007/143019. These documents describe how thermoplastic compositions may include microwave-sensitive regions, such that exposing the composition to microwave energy results in heating of those regions, with the heat passing to other regions of the composition through conduction and the like. This is said to lead to rapid and controllable heating of the composition so that it can be processed. However, these documents do not disclose the concept of selectively heating one grade of a polymer material so that a continuous matrix is formed, within which intact elongated elements of a second grade of the same material are dispersed.

The direct product of the process according to the invention may be a finished or substantially finished component. In such embodiments, the admixture is treated in a suitable mould such that the material adopts the desired form simultaneously with fusion of the first grade of plastics material. By a "substantially finished" component in this context is meant a component that has the desired final form, subject to the carrying out of finishing operations, such as trimming, after removal from the mould.

In other embodiments, the direct product of the process according to the invention is a composite material that can be subjected to further processing in order to manufacture a finished article. Such a composite material most commonly takes the form of a sheet that can later be formed into a finished component, eg by any of a number of moulding techniques that are known per se. In such cases, the plastics material will generally need to be a thermoplastic material.

The first grade of plastics material and the second grade of plastics material are two forms of the same polymer. Clearly, the two grades are not identical, but they will belong to the same class of polymer materials. Generally, the similarity

between them should be such that they are physically and chemically compatible and such that they would be recognized by those skilled in the art as being of the same polymer class. The first grade of plastics material may differ from the first in that the first grade may include additives that render it selectively heatable. Such additives may be incorporated into the second grade of plastics material, eg by being encapsulated within that material, or may intimately mixed with, or bound to, that material. In other embodiments, the first grade of plastics material may be a chemically modified form of the polymer. For instance, the polymer may be modified by the introduction of polar groups that render the material sensitive to microwave radiation.

The first grade of plastics material is "selectively heatable". By this is meant that the first grade selectively or preferentially absorbs applied radiation, leading to heating of the first grade more rapidly and/or to a higher temperature than the second grade.

The elongated elements of the second grade of plastics material remain "intact or substantially intact" while the first grade is fused to form a continuous phase. By "intact or substantially intact" is meant that the elongated elements of the second grade of material retain their integrity sufficiently that they perform their reinforcing function within the finished product.

The second grade of plastics material used in the invention is necessarily in the form of elongated elements. Such elements may be fibres. Generally, suitable fibres will be high tenacity variants (highly orientated to provide high strength and stiffness in the finished product). The fibres may be continuous, ie with a length that is comparable to the dimensions of the product, or the fibres may be shorter, eg chopped fibres with a length in the range 3-25mm. Continuous fibres may be used in the form of a textile fabric, providing higher levels of reinforcement. Shorter fibres may give lower levels of reinforcement, but may provide greater flowability of the material during moulding. Individual fibres typically have a diameter in the range 10-1000μm, most commonly in the range 10-50μm, eg about 20μm. The fibres may be characterised by a tex or dtex value (weight in

grams per 1000m or 10000m length respectively). The fibres may have a dtex value within a wide possible range, but typically the dtex of the fibres will be in the range 2 to 50, more commonly 2 to 10, eg about 5.

In other embodiments of the invention, the elongated elements are tapes of the second grade of plastics material. Typically, such tapes will have widths of the order of 1 -10mm, most commonly 1 -5mm, eg about 2mm. The tapes typically have a thickness of 10-250μm, more commonly 10-100μm, eg about 50μm. Such tapes will most commonly be used in the form of woven sheets.

The first grade of plastics material may be used in various different forms, but is preferably also in the form of elongated elements, such as fibres.

Where both the first and the second grades of plastic material are used in the form of fibres, the fibres of the two grades of plastic material are preferably comingled, ie filaments of the two grades of plastic material are intimately mixed to form a yarn containing both types of filament. Comingling of the fibres may be brought about by various means that are known in the art, but is most preferably brought about by air-mingling, ie by the application of compressed air to the mixture of the two types of fibre. Typically, the comingled yarn may have tex value in the range 50-1200, most commonly 100-500, eg about 200.

Comingled yarns comprising a first grade of a plastics material and a second, selectively-heatable grade of the plastics material are believed to be novel, and represent a further aspect of the invention. In another, related aspect of the invention, there is provided a fabric comprising, eg woven from, such a yarn.

Most preferably, the comingled yarns comprising the first and second grades of plastics material (or separate yarns comprising each grade of the material) are woven into a fabric-like material. A single layer of the fabric-like material may be laid within a suitable mould and heated and compressed to form the desired finished component. More commonly, however, a plurality of such layers are

assembled into a stack. The stack may be assembled outside the mould and then placed in the mould, but more commonly the layers are built up within the mould.

In other embodiments of the invention, the first grade of plastics material may be used in a form other than fibres. For instance, the first grade may be used as a particulate or powder that is sprinkled on a bed of the fibrous second grade, or is intimately mixed with the material of the second grade, eg by being impregnated into, or distributed through, the bed of second grade material. Alternatively, the first grade may be used in the form of a film of material.

In a yet further embodiment of the invention, quantities of the first grade of material may be caused to adhere to elongated elements of the second grade of material. For instance, the first grade of material in particulate form may be applied to fibres or yarns of the second grade of material and subjected to heat treatment in such a way that the particulate first grade of material is fused to the fibres or yarns. The fibres or yarns may then be woven into a fabric.

In embodiments in which the first grade of plastics material is not in the form of fibre, but the second grade of material is fibrous, the second grade of material may be formed into a yarn, and the yarn may be woven into a fabric-like material. Stacks of fabric may then be formed or placed in the mould, with first grade material between them, prior to treatment by heating and compression.

The proportion of first grade of plastics material to second grade may vary over quite a wide range, depending principally on the application and the desired properties of the finished product. Typically, the weight ratio of the first grade of material to the second grade is in the range 20:80 to 80:20. For embodiments of the invention involving the use of fabric reinforcements, the weight ratio of first grade to second grade will typically be from 30:70 to 50:50. For embodiments in which the mixture of first and second grades is flowable within the mould, the ratio of first grade of material to second grade of material is typically from 60:40 to 80:20.

In all embodiments of the invention, the mixture of first and second grades of the plastics material, whatever form it takes, eg a fabric woven from comingled yarn, a stack of fabric sheets woven from the second grade of material interspersed with powder or films of the first grade of material, or any other form of mixture, is converted to rigid, solid components by the application of radiation which is selectively or preferentially absorbed by the first grade of the material, so that the first grade of material melts to form a continuous matrix. A compaction pressure may then be applied, and the product then cooled to a temperature below the glass transition temperature of the first grade of material.

In one typical process, a press moulding operation, the mixture of the first and second grades of the plastics material is irradiated with a suitable form of radiation, eg microwaves or radiofrequency, to selectively heat the first grade of material and cause it to melt, and then the mixture is transferred to a press and formed in cold or warm matched tools. A compaction pressure is then applied, and maintained for long enough for the first and second grades of material to adhere and for the plastics material to adopt the desired form. The material is then cooled or allowed to cool, typically by heat being removed through the mould. Generally, pressure will be maintained during cooling and solidification of the product, until the product can be demoulded without distortion. Cooling can be natural cooling or may be forced, eg using a cooling fluid such as water or oil.

Another process, which may be applicable to many embodiments of the invention, is a vacuum consolidation operation, in which the mixture of the first and second grades of plastics material is placed on or in a single-sided tool, covered with a vacuum bag, and air is drawn out by means of a vacuum pump. As a result, a pressure of approximately one atmosphere is applied across the whole surface of the laminate. The assembly is then irradiated with a suitable form of radiation, eg microwaves or radiofrequency, to selectively heat the first grade of material and cause it to melt. Again, the completed laminate is then cooled or allowed to cool.

The process according to the invention may also be applied to other forming methods, eg injection moulding or extrusion.

Treatment of the mixture of the first grade of plastics material and the second grade of plastics material, eg by the application of infra-red or microwave radiation, may take place batchwise or continuously. In a batch process, the product to be treated may be placed within a treatment unit, eg an oven, and treated for a particular length of time. In a continuous process, products may be transported past or through a treatment station, the size of that station and/or the speed of travel of the products determining the duration of treatment.

The treatment of the mixture that leads to fusing of the first grade of plastics material and formation of a continuous phase within which the elongated elements of the second grade of plastic material are dispersed may be brought about in various ways. For example, the treatment may involve exposure of the mixture to microwave energy, infra-red radiation or other forms of electromagnetic radiation, eg radiofrequency radiation. In another alternative, the mixture may be treated by induction heating. The apparatus used to treat the mixture by any of these means may be generally conventional, and of a form that is familiar to those skilled in the art.

The process according to the invention is generally used to produce components with a thickness of less than 30mm, usually less than 10mm, and more typically less than 5mm. The thickness of the components is typically greater than 0.10mm, and is usually greater than 1 mm. Most commonly, the resulting components have a thickness in the range 1 mm to 10mm, more commonly 1 mm to 5mm.

The process according to the invention may be used for the manufacture of a wide range of self-reinforced plastics. The process may therefore be used to produce components that find application in many different fields. In particular, components produced in accordance with the invention may benefit from light weight, recyclability, high impact performance and low cost.

Because components produced in accordance with the invention are not contaminated with dissimilar reinforcing materials, such as glass fibre, they can be more easily recycled, whilst offering comparable strength and stiffness properties and the advantage of reduced weight, compared with corresponding GRP materials.

The process according to the invention may be particularly useful in the production of self-reinforced polypropylene.

The polymers used may alternatively be other polyolefins, eg polyethylene, notably high molecular weight polyethylene.

Another class of plastics material for which the process according to the invention may be used to produce self-reinforced plastics with particularly useful properties is polyester, and especially polyethylene terephthalate (PET).

PET is the most widely used member of the polyester family, and indeed is often referred to simply as "polyester" (particularly when it is used in textile applications).

The present invention is particularly useful for the manufacture of components in self-reinforced PET. PET can be moulded under vacuum, which renders it particularly suitable for the manufacture of large, low-volume parts. In addition, it is easily painted and bonded, and offers significant weight savings compared to GRP mouldings.

PET is obtained by a polycondensation reaction of the monomer normally obtained either by esterification of terephthalic acid (benzene-1 ,4-dicarboxylic acid) with ethylene glycol, or by transesterification of dimethyl terephthalate with ethylene glycol.

A yet further class of polymer that may be treated in accordance with the invention is polyamide. One example of such a polyamide is that known as polyamide-6.

In a preferred aspect, therefore, the invention provides a process as described above for the production of a self-reinforced plastics material selected from the group consisting of polyolefins, eg polypropylene or polyethylene, polyesters and polyamides.

Other plastics materials that may be used include unsubstituted or mono or poly halo-substituted vinyl polymers, polyetherketones and polyacetals.

The first grade of the plastics material is a form of the plastics material that is selectively heatable. In order to render the first grade of plastics material selectively heatable, that grade of material may comprise an additive with suitable radiation-absorbing properties. Additives that may exhibit suitable properties as radiation-absorbing materials include the following:

metal oxides, eg zinc oxide in the form of particles;carbon nanoparticles, eg carbon nanocoils;nanosized barium ferrite;metal particles; silicon carbide; polymethine ;phthalocyanine; conductive powders conductive fibres; carbon black; graphite; calcium silicates; zirconium silicates; zeolite; mica; kaolin; talc; cordierite; organic pigments; inorganic pigments; polymer-compatible organic dyes.

A particular additive that it is presently envisaged will be of utility in the present invention is carbon black.

Additives that may be receptive to microwave radiation include zinc oxide whiskers, carbon nanocoils, barium ferrite, carbon black, metal particles and silicon carbide. Additives that may be receptive to infra-red radiation include carbon black, polymethine and phthalocyanine. Additives that may be receptive to induction heating include conductive powders and conductive fibres.

The amount of radiation-absorbing additive present in the first grade of the plastics material may vary over quite a wide range, depending inter alia on the nature of the additive, the type and power of the radiation that is applied, and the degree of heating required to melt the material. However, the first grade of the plastics material typically comprises from 0.1 % to 50% w/w of additive, more typically from 0.1 % to 5% w/w of add itive .

The impact, weight, temperature and recycling characteristics of self-reinforced plastics means that they are ideally suited to a wide range of applications in almost all industrial sectors. Examples of applications for which components produced in accordance with the present invention may be useful are:

Protection - safety helmets, impact-absorbing pads;

Construction - pipes, roofing materials, door and window frames, screws and hardware; Automotive - manifolds, bumpers, panels;

Electronics - housings, light fixtures, cabinets; Medical devices - implants, external supports; Household - consumer packaging, furniture, toys.

In a yet further aspect of the invention, there is provided a self-reinforced plastic material produced by the process according to the first aspect of the invention.

The invention will be described in greater detail, by way of illustration only, with reference to the following Example.

Example Production of self-reinforced polypropylene (PP)

High tenacity polypropylene (PP) continuous filament yarns are manufactured using additional sodium benzoate to provide enhanced thermal stability and are comingled (mixed) with continuous filament yarns of PP containing 2% carbon black. The yarns are brought together and a jet of compressed air is applied to intimately comingle them in a continuous process. The resultant comingled yarn contains 50% by weight/volume of PP filaments (reinforcement) with the remainder being carbon-modified PP filaments (matrix). The yarn is then woven into a plain weave fabric with equal yarn counts in the warp (0) and weft (90) directions using a standard weaving loom.

The fabric is cut into several pieces and is stacked in loose layers in a microwave oven. Microwave energy is used to heat the matrix filaments to a temperature at which they can be formed, but below the temperature at which the reinforcement fibres are affected. The fabric pack is then transferred to a matched metal mould tool which has a constant temperature of 80 ⁰ C, and which is then closed rapidly under pressure to form the hot material to the required shape.

Once the moulded part has cooled to the temperature of the tool, the press is opened and the part is removed from the press.