GB2346827A - Composite structures - Google Patents

Description

COMPOSITE STRUCTURES This invention relates to composite structures and, in particular, to a method of manufacturing rigid lightweight structures usable for a variety of applications, for example in manufacturing components for furniture or interior cladding.

In recent years, much development has taken place in the area of manufacturing lightweight panels and other structures using fibre resin composites. Although these enable the production of lightweight structures of complex shape, their manufacture is labour-intensive and, in particular, requires the initial production of an appropriate mould or former having a highly finished surface against which a structure of resin and reinforcing fibre is built up. Disadvantageous with this approach is that mould costs are high, separating the mould from the structure can be difficult, and it is difficult satisfactorily to optimise resin use.

We have now found that by the use of an ordered sequence of process steps, it is possible to construct rigid, lightweight composite structures using combinations of foam materials, reinforcing fibre or fabric materials, and resin, and to do so without the need to construct dimensionally accurate high surface finish moulds.

Indeed, in some cases, no mould is needed at all, or at most one or a few support templates which may be easily and inexpensively constructed.

In accordance generally with the invention, there is provided a method of making a rigid composite structure which comprises the steps of: assembling a plurality of components formed of rigid or stiff foam plastics material to form an underlying structure; applying a curable liquid resin composition to at least part of the surface of the underlying structure; overlaying the parts of the surface to which the resin composition has been applied with reinforcing cloth and/or a layer of reinforcing fibre; applying a parting fabric over the reinforcing cloth or fibre; applying an absorbent layer over the parting fabric; subjecting the assembly of object, reinforcing cloth or fibre, parting fabric and absorbent layer to pressure to enable the liquid composition to permeate through the reinforcing cloth or fibre and the parting fabric and reach the absorbent layer; causing or allowing the resin composition to cure; and removing the parting fabric and absorbent layer together with any resin contained therein from the assembly to leave the rigid composite structure.

This approach provides major advantages in terms of manufacturing outlay and, in particular, avoids the use of expensive pre-formed moulds around which fibre reinforced composite material is assembled and which must then be separated from the composite once the reinforcing resin has been cured. Additionally, it enables the ratio of resin to fibre in the cured parts of the structure to approach the desired optimum.

Broadly speaking, in order to construct a composite structure in accordance with the method of the invention, the first step is to break the desired underlying structure down into a number of components, each of which can conveniently be formed from a foam material, for example a rigid polyvinyl chloride or polystyrene foam material. Sheets, slabs, blocks and the like can be cut to shape and assembled together, preferably by adhesion, to form the underlying structure, i. e. the basic shape for the composite structure. This basic shape will not be particularly strong and naturally cannot be any stronger than the individual foam materials of which it is made, which are generally not very strong.

By following the method of the invention, a rigid strengthening exterior shell is effectively applied to the basic foam underlying structure to form an effectively integrated hard surface which effectively provides a strong, light, rigid shell-like structure, the interior of which still essentially retains the basic properties of a low density stiff foam.

It should be observed that the underlying structure, while it may be formed entirely of components of foam material, does not need to be formed of such materials.

Indeed, foams, although they possess excellent properties such as low density giving rise to lightweight products, have the disadvantage that it is difficult to secure fixings to them. Accordingly, it is sometimes desirable to incorporate in the basic foam structure, for example a metal plate or strip of wood, both enabling conventional fixing means such as screws or pins to be used for secure fixing of the final composite structure. conventional fixing means.

Once the underlying structure has been assembled, the next step is to provide it with a strong external reinforced skin. As noted above, this is done by carrying out a series of steps, the first of which is the application of a curable liquid resin composition to the exterior of at least part of the structure. This may penetrate some distance into the foam, depending upon the structure of the foam itself, the viscosity of the liquid composition, and the compatibility between the two, but generally it is desirable to put quite a lot of liquid resin composition on the exterior of the underlying structure, since some of it will have to be removed again from the surface of the foam, as explained below.

The next step, once the liquid resin composition has been applied, is to overlay the surface with reinforcing fibre material. This preferably takes the form of the application to the surface of an inorganic fibre layer, conveniently in the form of a woven cloth, tape or braid. The fibres from which this is made are preferably inorganic fibres, for example glass fibre, other mineral wool or the like fibres, or carbon fibre. Such fibres are widely available commercially for a variety of uses including forming fibre resin composites. Two or more different types of fibre may be used to cover different parts of the same underlying structure.

The already present still sticky surface of the curable liquid resin composition means that the cloth or fibre can be easily and quickly laid on to the surface of the underlying foam structure and smoothed down without difficulty.

The next stage in the manufacturing process is to apply a parting fabric on top of the reinforcing cloth or fibre. Such parting fabrics are well-known and conventionally consist of a close woven nylon fabric which is nevertheless sufficiently permeable to enable liquid resin composition to flow through it under pressure. The weave is sufficiently close, the intrinsic strength of the material sufficiently high, and the incompatibility of the fibre from which the parting fabric is made and the liquid resin composition, or rather the set resin, is such that the parting fabric may be torn away from the cured resin reinforced cloth or fibre surface, as explained below. Close-woven nylon fabrics are suitable for use as the parting fabric.

Thereafter an absorbent layer is applied over the parting fabric. This absorbent layer may be selected from a wide variety of resin composition compatible material, but is conveniently, for example, a non-woven cloth or fleece made from spun polyester.

As noted above, once this assembly has been constructed, it is subjected to pressure which forces the liquid resin composition to permeate through the reinforcing cloth or layer of reinforcing fibre, with excess then passing through the parting fabric and being absorbed in the absorbent layer. Pressure is kept up for a substantial time, for example 6 to 20 hours, to enable the resin to cure. If appropriate, the assembly may be heated to accelerate cure, or otherwise subjected to cure-promoting treatments.

The pressure which needs to be applied to cause the liquid resin composition to impregnate the reinforcing cloth and fibre does not generally need to be particularly large. A particularly preferred way of generating the required pressure is to ease atmospheric pressure on the pressure source. For example, the assembly may be placed on a suitable table, if necessary on a reasonably loosely fitting support, and then a soft air-impermeable sheet, e. g. of rubber or rubbery plastics material is draped over the entire assembly of base reinforcing cloth or fibre, parting fabric and absorbent layer. The edges of the sheet are then sealed down to the table surrounding the assembly and the space between the table and the sheet is evacuated. This causes a pressure of approximately one bar to be exerted across the assembly which is sufficient to produce the desired effects. Alternatively, the entire assembly may be placed in a bag made of rubber or rubbery plastics, the bag sealed and then evacuated to apply atmospheric pressure to all sides of the assembly.

Following the application of pressure and achievement of curing, the excess resin will have been absorbed into the absorbent layer, whereafter, following removal of the air impermeable bag or sheet, the absorbent layer and the parting fabric can be simply peeled away from the remainder of the assembly.

In this way, the whole of the exterior of an underlying structure built up from pieces of foam material may be provided with a strong reinforcing shell. In the case of a typical item which might be produced by the method of the present invention, for example a tray-like structure having a surrounding frame and a thin sheet laid to one side of it, it would be straightforward first to provide one side of the structure with the rigid outer shell, and then turn it over and provide the other side with the same.

The invention is illustrated by way of example with reference to the manufacture of a bodywork table component in accordance with the method. Bodywork tables are well-known articles of commerce and are described, for example, in the following United States patent specifications: 4943041,5335676 and 5479852.

As is evident from those specifications, the conventional materials for the main sections are laminated wood structures, though large metal fabrications or plastics mouldings have also been suggested, as have composite structures. None is particularly light in weight, though, as noted in Specification 4943041, overall light weight is desirable.

In order to assist the explanation of one way of putting the invention into effect which is set out below, this specification is accompanied by some diagrammatic drawings in which: Figure 1 is an exploded view of an assembly of plastics foam components destined to be incorporated in one component of a bodywork table.

Figure 2 is a cross-section on an enlarged scale of the components when assembled together, e. g. by adhesion.

Figure 3 is a partial section through a portion of the unit showing the unit assembled during manufacture in accordance with the present invention.

Referring first to Figure 1, this shows a structure destined to become one end of a bodywork table and consisting of a generally flat sheet portion 1, which will form a table top in use, a number of relatively rigid foam blocks 2 and a wooden cross piece 3 which fits adjacent one of the foam blocks 2. Sheet portion 1 is of foamed high density PVC, while blocks 2 are made of relatively rigid polystyrene foam, conveniently polystyrene foam as used in building work and meeting British standard specification BS 3837 Part II (1990).

Grade E1 of that standard gives satisfactory results.

The basic structure is assembled together from components 1,2 and 3 by gluing them together.

The assembly as shown in Figure 2 in section, resembles an upside down tray having a flat surface formed by component 1 on its upper side and components 2 and 3 forming a downwardly depending skirt on the other.

In order to convert this structure to a lightweight load bearing structure as is necessary for its intended purpose, it is laid on a processing table 4 as shown in Figure 3. There are then sequentially applied to the exposed surfaces of the structure a layer of liquid resin composition, for example by brushing on or spraying on using a hand-held spray gun, a layer of reinforcing fibre 5,6,7, a parting fabric 8 and an absorbent layer 9. As indicated in Figure 3, these are all applied while the basic structure is set on table 4 which has been provided with a loosely fitting wooden support 10 for the sheet 1. Support is necessary as will appear below.

Figure 3 shows a section through one side of the item after it has been assembled, but before the resin is cured. The resin composition is not shown, but clearly visible are areas of reinforcing fibre. These consist of glass fibre cloth 5 and carbon fibre tape or braid 7 which have been laid down on to the resin composition coated surfaces of sheet portion 1 and foam block 2 respectively. Braid 7 also covers the resin composition coated surface of wooden component 3. Bridging the edge of sheet portion 1 is a flat parallel fibre tape of carbon fibres 6, the fibres running parallel to the edges of sheet portion 1.

A nylon close-weave cloth 8 is then laid over the assembly, followed by a non-woven fleece 9 made of spun polyester.

A rubbery cover sheet 20 is then laid over the entire assembly and the edges of sheet 20 then sealed down to the working table 4 outside the area of the assembly, e. g. using a clamped down frame. The cover sheet 20 is provided with a valve which can be connected to a source of suction and once this is actuated, the flexible rubbery cover sheet 20 serves to press the assembly together gently and evenly. The initial air permeability of the absorbent layer 9 enables air to be exhausted from between the table 4 and the rubbery cover sheet 20 across its entire area. Suction is continued and the air pressure forces the assembly together, thus compressing some of the liquid resin composition through the glass fibre cloth or carbon fibre layers 6,7, through the nylon parting fabric 8 and into the absorbent layer 9. This occurs relatively rapidly after suction is first applied, and suction is then maintained while the resin cures.

At the end of curing, the suction is turned off and the rubbery cover sheet 20 removed (it does not usually adhere materially to the absorbent layer 9 unless far too much resin has been applied locally). The nylon parting fabric 8 and overlaying absorbent layer 9 are then peeled away from the remainder of the structure.

The structure may be turned upside down and the process repeated in order to provide a complete outer fibrebased resin reinforced rigid layer entirely surrounding the assembly of components 1,2 and 3.

The resulting structure is rigid, but ultra light in weight. It can be used as a bodywork table main support component, for which purpose it is preferably upholstered using a resilient foam material such as a polyurethane or rubber foam, and with an outer covering, e. g. of a soft polyvinyl chloride fabric composite material. The resilient foam and composite material may be fixed to the rigid composite structure by mechanical means such as staples, but it is preferred to affix by adhesion. Fixture of the overall unit itself to another, e. g. via a hinge, may be made by using a conventional metal hinge fixed to the component via screws penetrating into the wooden member 3.

Claims (11)

1. CLAIMS 1. A method of making a rigid composite structure which comprises the steps of: assembling a plurality of components formed of rigid or stiff foam plastics material to form an underlying structure; applying a curable liquid resin composition to at least part of the surface of the underlying structure; overlaying the parts of the surface to which the resin composition has been applied with reinforcing cloth and/or a layer of reinforcing fibre; applying a parting fabric over the reinforcing cloth or fibre; applying an absorbent layer over the parting fabric; subjecting the assembly of object, reinforcing cloth or fibre, parting fabric and absorbent layer to pressure to enable the liquid composition to permeate through the reinforcing cloth or fibre and the parting fabric and reach the absorbent layer; causing or allowing the resin composition to cure; and removing the parting fabric and absorbent layer together with any resin contained therein from the assembly to leave the rigid composite structure.
2. 2. A method according to Claim 1 wherein the underlying structure includes one or more non-foam structure components forming part thereof.
3. 3. A method according to Claim 1 or 2 wherein the liquid resin composition is applied by brushing.
4. 4. A method according to any one of Claims 1 to 3 wherein the reinforcing cloth and/or fibre is formed of inorganic fibrous material.
5. 5. A method according to Claim 4 wherein the inorganic fibrous material includes one or more of carbon fibre, glass fibre and mineral fibre.
6. 6. A method according to any one of Claims 1 to 5 wherein the parting fabric is a nylon cloth.
7. 7. A method according to any one of Claims 1 to 6 wherein absorbent layer is a spun polyester fleece.
8. 8. A method according to any one of Claims 1 to 7 wherein the pressure applied is atmospheric, and is applied via a flexible air-impermeable elastic sheet material.
9. 9. A method according to Claim 8 wherein the pressure is applied by evacuating the atmosphere from a sealed chamber containing the assembly, one or both sides of the chamber being formed of the elastic sheet material.
10. 10. A method of making a composite structure substantially as hereinbefore described with reference to the accompanying drawings.
11. 11. A composite structure made by the method of any one of Claims 1 to 10.