GB2125722A - Electrically conductive reinforced plastics structures - Google Patents

Abstract

An electrically conductive fibre reinforced plastics structure e.g. a pipe section, is formed by impregnating a reinforcement fabric 15, comprising a non-conductive base fabric loaded with electrically conductive particulate carbon, with an uncured synthetic resin and moulding the structure prior to curing of the resin. The reinforcement fabric is commercially available employing a base fabric of non-woven polyester and other materials may be used. The density of carbon loading and consolidation of reinforcement in the structure vary its conductivity. A structure so made exhibits bulk resistivity in the range 10<3>-10<9> Ohm-cms which includes the range 10<5>-10<7> Ohm-cms preferred for the dissipation of electrostatic charge. The uncured synthetic resin is applied by a nozzle 12 to the fabric 15 on an expandable mandrel 11. Other moulding techniques may be used. <IMAGE>

Description

SPECIFICATION Electrically conductive reinforced plastics structures This invention relates to moulded structures of reinforced plastics material and in particular to such moulded structures which exhibit electrical conductivity.

Many situations exist which call for a relatively light rigid moulded structure for which plastics materials, such as synthetic resins are, or can be, reinforced to provide suitable strength-to-weight characteristics. In some situations it is desirable, or necessary, for the moulding to have electrical conductivity whereby any static electrical charge built up on or in the moulding is dissipated through the material to a suitable earthing point.

It is known where good structural strength or high strength-to-weight ratio are required, to use plastics material reinforced with carbon (such as graphite) in the form of yarns or tows or woven into a fabric but such material is expensive. It is known also to utilise the electrically conductive properties of such carbon reinforced structures. However such material is expensive and can only be justified where strength-weight ratio is paramount and enables a light structure to be formed regardless of cost.

Unfortunately, the carbon structures, e.g.

graphite, offering maximum strength are not best suited for providing electrical conductivity and steps have to be taken to optimise both of these requirements. British patent application no. 2,089,851 describes a woven fabric of graphite and metallic fibres to satisfy both requirements, said fabric being formed as is virtually a requirement for the brittle nature of carbon fibre products, in a form pre-impregnated with resin, known as a prepreg", for handling purposes.

It will be appreciated that the cost of the raw materials from which the graphite fibres are formed, the formation of the graphite fibres, weaving of the fabric and formation of the prepreg result in an expensive composition, acceptable only for specialised structures.

There are many situations where great structural strength for minimum weight is not required, that is, where less strength or greater weight can be tolerated, and the cost saving over a number of units is significant.

The form of, and nature of, materials used depend upon the precise circumstances of each use. Examples of such uses will help to illustrate the extent of variation in applicability for such mouldings.

One application for such a moulded structure with which this invention is concerned is a pipe for the transport of inflammable materials and/or for use in an inflammable or explosive atmosphere.

The latter conditions may be experienced in mines or other underground workings and the practice has hitherto been to use an earthed metal pipe with obvious weight and handling disadvantages.

Pipes formed of plastics materials are now nearly always formed by extrusion and a number of solutions have been proposed for introducing sufficient electrical conductivity into pipes so formed to enable a static dispersent property to be established.

The most common modification proposed is to load the plastics material prior to extrusion with a conductive filler material such as particulate carbon. However the proportion of filler which can be added before the pipe material becomes structurally weakened is insufficient to introduce a practicable level of conductivity.

British patent specification no. 1,578,467 discloses a construction which in some ways mitigates this problem. A pipe section formed by extrusion is coated on its outer surface with a synthetic thermoset resin containing conductive carbon particles of sufficient density to conduct electrostatic charges from the outer pipe surface. A feature of the arrangement is the continuation of the coating onto an inner surface portion forming a socket end of each pipe section by which electrical conductivity can be established with an outer spigot portion of an adjacent section. This arrangement frees the carbon loaded resin from structural stresses but there are still drawbacks to its use under the above conditions.It is possible, for mechanical impact damage to the pipe to break the conductive coating and exposing a recess of the nonconductive base material upon which an electro-static charge may form, recreating the hazard the pipe construction is intended to avoid.

Other proposals for discharging static electricity in pipes have concentrated on the effects of friction between the contents and the inner wall. British patent specification no.

1,430,604 relates to an extruded pipe of p.t.f.e. in which strips loaded with particulate carbon are incorporated as discrete conductive paths. An alternative proposal is put forward in British patent specification no. 1,439,630 in the form of a flexible textile fabric loaded with particulate carbon to make it conductive.

The fabric is fashioned into rings having inter nally radial projections and which are located periodically along the pipe such that the contents flow through them. Charge collected from the contents is conducted by way of electrical conductors to earth. A similar fabric is disclosed in British Patent Specification 966,575.

Clearly these last two described arrange ments offer no surface protection against use in a hazardous atmosphere nor do they provide vide a continuously conductive path from all points of the pipe.

It is an object of the present invention to provide a moulded structure of fibre reinforced plastics material exhibiting electrical conductivity within a range suited for. dissipat- ing electrical charge which can be made more cheaply than hitherto known and which mitigates disadvantages of known structures.

According to one aspect of the present invention a fibre reinforced plastics structure comprises a laminate including at least one reinforcement ply of an electrically non-conductive base fabric material, loaded with particulate electrically conductive material in order to form an electrically conductive reinforcement fabric, impregnated with an uncured resin and moulded to the structural form prior to curing of the resin.

Preferably the base fabric is a non-woven fabric [ of suitable "pore volume" to encourage impregnation by the resin ] . If desired the base fabric may be woven but some of the cost advantages of non woven fabrics are lost with little improvement in mechanical properties of the moulded structure.

Different base materials are available and preferred ones may be selected in accordance with the characteristics required of the finished structure. In keeping with producing a low cost structure the preferred base fabric is polyester but for other properties the base fabric may be, for instance if a degree of resilience is required, a polyamide or may be polypropylene, an aramid or glass.

The electrically conductive loading material may be particulate metal for a low resistivity material but is preferably particulate carbon.

The carbon may be formed as carbon black by incomplete combustion of a gas in a rarified atmosphere in order to achieve desired electrical conductivity of the carbon.

The electrical conductivity of the reinforcement fabric will of course be a function of the carbon type, density of carbon loading and density of the fabric in the structure.

The actual formation of the electrically conductively loaded fabric does not form part of the invention, said material being produced to specification by specialist manufacturers thereof e.g. the 3300 range manufactured by Lantor International Limited and/or in a manner described for instance in the aforementioned Patent Specification Nos. 966,575 or 1,439,630.

Whereas in forming structures using the known carbon fibre material, the materials have to be pre-impregnated with the desired resin in uncured, or slow-curing form during manufacture, and then be subjected to a subsequent moulding and/or curing operation, construction in accordance with the present invention enables a commercially available handlable fabric, to be impregnated only during the moulding operation with any resin chosen to form the finished structure.

Thus apart from the cost factor of using the conductive material which may be of the order of 100 times cheaper than a woven graphite fibre structure, and whose electrical properties can be tailored more readily to particular applications, this freedom from the pre-im pregnated manufacture of the fabric gives greater versatility to its use.

As mentioned the resin may be any of the known synthetic types such as polyester, phe nolic or epoxide chosen for their method of curing or physical properties, such as strength, fire resistance and the like which may be an important factor in the production or use of a moulded structure. The resin may be electrically non-conductive or may be made conductive e.g. by the addition of carbon based fillers, to impart a further degree of electrically conductivity to the structure, the conductivity of the resin being significantly less than that of the reinforced fabric and the amount of filler being insufficient to impair the strength of the resin.

Articles made in accordance with the pre sent invention in which all laminating plies are of the aforementioned base fabric rendered electrically conductive and are caused to make physical contact in moulding exhibit a bulk conductivity, unlike some structures, such as the pipes of the prior art discussed earlier, which exhibit conductivity along predeter mined tracks only.

According to a second aspect of the present invention a method of moulding a fibre rein forced plastics structure comprises applying to a mould plies of reinforcement fabric includ ing at least one of an electrically non-conduc tive base fabric loaded with particulate electri cally conductive material and impregnating the plies with a settable uncured resin applied to the mould either before, with, or after the reinforcement fabric.

The method is particularly applicable to the formation of a pipe section in which the reinforcement is wound onto a rotating man drel and then sprayed with the uncured resin and removed from the mandrel after the resin has cured sufficiently for the section to be self-supporting.

The method may include winding further reinforcement either with electrically conduc tive in filamentary form over, or with, the reinforcement fabric. As stated earlier the pre sent invention enables the resin to be kept separate from the reinforcement until lamina tion/impregnation permitting rapid change of either component to achieve different charac teristics but it does not of course preclude pre impregnation of the conductive reinforcement to form a pre-preg for final moulding and curing of the resin in site.

An embodiment of the invention will now be described by way of example with refer ence to the accompanying drawings, in which Figure 1 is a cross-sectional view through a moulding arrangement according to the present invention for the formation of a pipe section, and Figure 2 is a sectional elevation through a pipe section moulded in the arrangement of Fig. 1 Referring to Fig. 1 a pipe moulding machine 10 comprises a cylindrical expandable (or inflatable) mandrel 11 of expanded diameter equal to the desired inside diameter of the pipe section to be formed and a length equal to the length of the pipe section.The mandrel has associated therewith means of applying an uncured resin, shown as a spray nozzle 12 to which is applied a resin such as polyester resin IMPOLEX (RTM) 925 from reservoir 13 and at which the resin is mixed with a curing agent, from reservoir 14. The nozzle 12 is reciprocable along the length of the mandrel to ensure that resin is applied to all parts of its surface upon rotation.

An electrically conductive reinforcement material, stored as a roll 15 rotatable about an axis 16, is drawn off the roll by feed rollers 17 and wound into the mandrel as it rotates.

The reinforcement material comprises a nonwoven polyester base fabric loaded with particulate carbon in order to render it electrically conductive. A suitable fabric is 3305L of the 3300 produced by Lantor International Limited, Bolton, England, having a thickness in the range 0.1 to 0.3mm.

In forming the pipe section a thin layer of resin is sprayed onto the mandrel surface and the reinforcement drawn from the roll 15 by the feed rollers adheres to the resinous mandrel surface and against which it is pressed by a consolidating roller 18. Alternatively, the feed rollers 17 may pinch the fabric so that it wraps onto the mandrel is a compressed state.

As the mandrel rotates further resin is sprayed onto the reinforcement to impregnate the fabric. As each ply of reinforcement is applied, the tension inherent in winding and, or feed roller 17 and, if necessary, the consolidation provided by roller 18 press the plies such that electrical contact is established between them and the density of the conductively linked plies is such that an electrical path exists between any two points within the moulded material. Clearly, at a microscopic level the moulding is not homogeneously conductive i.e. the resin-filled pores of the reinforcement material will be non-conductive but for practical purposes such regions are small and sufficiently close to a conductive region that for contact on a macroscopic level, i.e. by metallic objects, or for dissipation of electrical charges, the material may be considered as homogeneous.Furthermore the reinforcement fabric reaches the surface of the moulding so that it exhibits surface, as well as bulk conductivity.

The density of the particulate carbon, which determines the bulk conductivity, or conversely the resistivity of the pipe structure is a matter of choice and variable both by variation of the density of carbon in the manufactured reinforcement fabric and by varying the density of the reinforcement fabric in the moulded structure.

When the resin is cured sufficiently for the pipe section moulding to be self supporting the mandrel is contacted and the section removed for final curing and/or finishing.

It will be appreciated that the mandrel may be so shaped as to produce pipe section with end portions adapted for joining to co-operating ends of adjacent sections e.g. socket and spigot ends, or flanges. Alternatively, the sections may be machined after curing e.g. by cutting screw threads or other fastening means may be bonded thereto by a compatible resin or other adhesive. The property of bulk conductivity enables such machining to be carried out without special provisions to preserve electrical continuity; for example, a surface coated section would require to be machined before coating and the machining limited to that which permits subsequent coating of the machined parts.

The method of moulding is given as exemplary, there being variations within the moulding techniques themselves. In the arrangement of Fig. 1, for instance the resin may be mixed prior to spraying, applied other than by spraying, or may be applied simultaneously by way of an extensive feed to the whole mandrel length. The form in which the reinforcement fabric is applied is also open to variation. The fabric may be of width equal to the length of the pipe section so that it is rolled spirally as shown from the stock roll onto mandrel. Alternatively the fabric may be of narrower width and wrapped hetically along the length of the mandrel either in overlapping parallel directed layers or in a crossing manner.

Moulding is not limited to the wrapped mandrel form illustrated and described above.

Other conventional techniques may be employed, such as pressure moulding under vacuum, or forming the pipe in continuous lengths by braiding the strips of the reinforcement fabric onto one end of the mandrel and removing the (at least) partially cured pipe section from the other end.

A sectional elevation through a typical pipe section 20 is shown in Fig. 2. This illustrates schematically the lamination of the structure from a plurality of concentric plies 21, each of which is conductive in the plane of the ply and between plies. Fig. 2 also serves to illustrate the effect of physical damage to the pipe wall exemplified by a sharp V-sectioned notch 22.

The wall of the pipe section which forms the surface within the notch is still formed of the electrically conductive material and it will be appreciated that this represents a significant improvement over pipe sections which are given a conductive surface coating only, such a break in the outer wall therein leaving a relatively large surface area of the notch to collect charge and produce a likely discharge point for an explosive atmosphere to/or from the sharply angled base of the notch.

As stated above the bulk electrical resistivity of the section is a function of the resistivity of the reinforced material and of the consolidation of the reinforcement in the resin structure. Within tolerable manufacturing limits it is possible to produce carbon loaded fabrics enabling a resistivity to be obtained in the range 103-109 Ohm-cm.

For antistatic purposes in hazardous atmospheres a range of resistivities in the range 105-107 Ohm-cm is preferred which is easily met by the presently described carbon-loaded construction.

If desired the resin itself may be made slightly conductive by the addition of carbon or metal-containing fillers, the minimum resistivity obtainable from such filling alone being of the order of 1 Oii Ohm-cm without impairing the strength of the resin but assisting the microscopic distribution of resistive material.

If a lower resistivity than the above range is required the reinforcement material may be manufactured using particulate metal having much greater conductivity than carbon.

As stated the reinforcement fabric extends to the surface of the moulded section and providing a resistive surface conductivity as well as bulk conductivity. With the materials employed in the specific embodiment, namely Lantor 3305L carbon loaded non-woven polyester fabric and Impolex 925 polyester resin produce a pipe having a surface resistivity of the order of 105 Ohms per square.

Apart from the specifically mentioned materials others may be chosen on the basis of special requirements of the finished product.

For example the base fabric may be a polyamide such as nylon which enables a more resilient structure to be made, but is more limited in upper operating temperature than polyester. For structures required to operate at high temperatures phenolic resins may be more suitable at the expense of resilience.

Where superior strength requirement exist an expoxide can be used.

The above description has related principally to a moulded structure in the form of a pipe. It will be appreciated that any form of moulding suitable for formation by conventional fibre reinforced plastics lamination techniques may be formed using the electrically conductive reinforcement fabric in accordance with the present invention.

Furthermore where operating conditions are less rigourous in requiring bulk conductivity, a moulding formed in accordance with the present invention may be combined with a nonconductive structure e.g. as a skin, either during moulding of the structure where all that is required is a change of reinforcement fabric, or by bonding thereto or laminating thereon a covering including the electrically conductive reinforcement as described.

Claims (14)

1. A fibre reinforced plastics structure comprising a laminate including at least one reinforcement ply of an electrically non-conductive base, fabric loaded with particulate electrically conductive material in order to form an electrically conductive reinforcement fabric, impregnated with an uncured resin and moulded to the structural form prior to curing of the resin.

2. A fibre reinforced plastics structure as claimed in claim 1 in which the base fabric is a non-woven fabric.

3. A fibre reinforced plastics structure as claimed in claim 2 in which the fabric is polyester.

4. A fibre reinforced plastics structure as claimed in claim 2 or claim 3 in which the electrically conductive material is carbon.

5. A fibre reinforced plastics structure as claimed in claim 4 in which the electrically conductive reinforcement material is Lantor 3305L.

6. A fibre reinforced plastics structure as claimed in any one of claims 1 to 5 in which the electrically conductive reinforcement material has a thickness in the range 0.1 to 0.3mm.

7. A fibre reinforced plastics structure as claimed in any one of claims 4 to 6 in which the bulk electrical resistivity of the structure is in the range 103-109 ohm-cm.

8. A fibre reinforced plastics structure as claimed in any one of the preceding claims in which the resin is a polyester resin.

9. A fibre reinforced plastics structure as claimed in any one of the preceding claims in which the resin exhibits electrical conductivity.

10. A fibre reinforced plastics structure substantially as herein described with reference to the accompanying drawings.

11. A method of moulding a fibre reinforced plastics structure comprising applying to a mould plies of reinforcement fabric including at least one of an electrically nonconductive base fabric loaded with particulate electrically conductive material and impregnating the plies with a settable uncured synthetic resin applied to the mould either before, with, or after the reinforcement fabric.

12. A method of moulding a structure as claimed in claim 11 in which the reinforcement fabric is supplemented by filamentary reinforcement material.

13. A method of moulding a pipe section comprising winding on a mandrel reinforcement fabric including at least one of an electrically non-conductive base fabric loaded with particulate electrically conductive material and impregnating the plies with a settable uncured synthetic resin applied to the mould either before, with, or after the reinforcement fabric.

14. A method of moulding a fibre reinforced plastics structure substantially as herein described with reference to the accompanying drawings.