GB2211266A

GB2211266A - Anti-static tubing

Info

Publication number: GB2211266A
Application number: GB8829371A
Authority: GB
Inventors: Harry Pentney; Arthur Clive Angood
Original assignee: SHRINEMARK Ltd
Current assignee: SHRINEMARK Ltd
Priority date: 1987-12-16
Filing date: 1988-12-16
Publication date: 1989-06-28
Anticipated expiration: 2008-12-16
Also published as: GB2211266B; GB8729316D0; GB8829371D0

Abstract

An anti-static tube or duct is formed from woven reinforcing strands embedded in a thermoplastic or thermosetting matrix, such that the strands constitute between 20 and 80% by volume of the tube wall, each strand being formed from a number of fibres. Between 5 and 100% of the strands incorporate an array of conductive or partially conductive filaments. The material for the filaments has a volume resistivity of between 10<-8> to 10<2> ohm metres and the individual monofilaments have an end to end resistance of 10<-1> to 10<10> ohms per metre. The conductive filaments may comprise metallic fibres, graphite, oxidized acrylic fibres doped silicon carbide, nickel coated graphite, carbon coated glass or "epitropic" fibres ie, organic fibres having carbon particles incorporated in the surface thereof. The reinforcing fibres may be of aramid, carbon, glass polyester, polyether ether ketones or polythene. The matrix may be poly-phenylene sulphide, polyether ether ketone, polypropylene, polyurethane or fluorinated thermoplastics.

Description

"Improvements relating to tube formation" This invention relates to the fabrication of tubes and ducts from composite materials in such a way that any static charge that is built up within the pipe by, for example, the flow of a liquid or slurry can be discharged safely to earth. Typical applications in which this invention would be of use would include lightweight semi-flexible tubes for use in aircraft fuel systems, and chemical plant.

Fuel ducts are typically made from lightweight aluminium or titanium alloy tubes, coupled together with flexible couplings. These tubes are inherently stiff and do not deflect without permanent deformation. If these tubes are fitted in the wings of aircraft, flexible couplings have to be fitted to allow for movement. The use of a semi-flexible composite duct would enable longer lengths of tube to be fitted and would deflect in use with the wing without permanent deformation. The net result would be fewer couplings, reduced installation costs and considerable weight savings. With the advent of composite panels and whole wings of composite materials metal fuel ducts pose an increasingly serious problem as they provide a preferential path for electrical energy when for example the aircraft is subjected to a lightning strike.The discharge of this electrical energy via a fuel duct poses a serious problem because of the risk of explosion and catastrophic failure of the wing. Therefore in a composite wing structure the use of composite isolation tubes or a composite fuel duct is essential. One further difficulty associated with composite fuel ducts is that during refuelling or defuelling a static charge can build up within the fuel duct. If this static charge is not discharged the conditions are correct for a potential explosion.

It is an object of this invention to provide a tube or duct formed from a composite material which allows for the discharge of any static charge built up within the tube or duct.

Accordingly this invention provides a tube or duct formed from woven reinforcing strands embedded in a compatible thermoplastic or thermosetting matrix, such that the strands constitute between 20 and 80% by volume of the tube wall, each strand being formed from a number of fibres, and 5 to 100% of the strands, evenly dispersed throughout the matrix, incorporate an array of conductive or partially conductive filaments, whereby the surface resistivity of the tube lies between 1013 and 10 5 ohms per square, whilst the resistance of the whole tube lies within the range of 103 to 1010 ohms per metre, utilising a material for the filaments having a volume resistivity of between 10 s to 102 ohm metres, the individual monofilaments having an end to end resistance of 10 s to 1010 ohms per metre.

It is to be understood that the use of the term "woven" is meant to encompass not only conventional weaving processes but also knitting or braiding or even the formation of knitted or woven tapes or non-woven mats formed from continuous elements or chopped fibres embedded within the matrix. The latter approach is particularly useful as it enables conductive paths to be incorporated into the finished duct in a reproducible fashion. It is a particular benefit of the braiding process that these conductive paths are symmetrically distributed not only along the duct but also through the wall thickness. A preferred braiding angle is 10 to 850. Ideally, at the intersections of the conductive filaments, there is good electrical contact between these filaments, not only for filaments in the same layer but also for filaments in one layer and any adjacent layer.

The even dispersal of conductive or partially conductive filaments (within the strands) throughout the matrix ensures that there will be an even response throughout the length and breadth of the tube to the build up of a static charge, with the charge being discharged safely to earth through an earth point to which the tube will be connected. The filaments may be formed from conductive, semiconductive or super conductive materials either in the form of homogeneous fibres, for example metallic fibres, pure graphite, oxidised acrylic fibres or doped silicon carbide fibres, or as coatings on other fibres which may be either conductive or non conductive, for example nickel coated graphite fibres or carbon coated glass fibres.Another possibility is for the filaments to be formed as a yarn or tow containing, in any proportion, organic fibres in which carbon particles have been incorporated into the surface, namely so-called "epitropic" fibres. Nonlinear materials could be used as a semiconducting element, if desired.

In the preferred arrangement between 4 to 100%, ideally 20 and 80%, of the strands will incorporate a conductive filament. The conductive filament in any of the strands can be either a monofilament, or a yarn in which any proportion of conductive filaments have been included by a conventional textile handling process.

Furthermore, where each strand incorporates a semiconductive filament, this will ideally comprise between 20 and 80% of the semiconductive material. It is also preferred that the resistance of the tube should lie within the range of 104 to 1010 ideally between 105 to 3.5 x 102 ohms per metre and that the surface resistance of the tube lies within the range 10 3 to 1013 ohms per square. Preferably also the conductive material used will have a volume resistivity of between 10 8 and 102 ohm metres.

Examples of reinforcing fibres would include the aramid fibres, carbon or glass fibres or other conventional reinforcing fibres and possibly high modulus polyester and polyether ether ketones, polyethylene or other high strength and high modulus fibres. Examples of suitable matrix materials could include poly-phenylene sulphide, polyether ether ketone,polypropylene, polyurethane, and the fluorinated thermoplastics. It is also possible to use other thermoplastic or thermosetting matrix materials. It is envisaged that the fibres will typically have a diameter of 3 to 40 microns, although fibres forming the semiconductive element may need to be way in excess of this diameter, for example more than 40 microns, in order to meet the electrical requirements, and withstand processing without premature fracture. Obviously there is a limiting lower size.

For example, if copper was contemplated as forming the semiconductive element, for a typical tube only one copper fibre would be possible having a diameter of a quarter of a micron which is too small to manufacture.

Furthermore fibres of such small size would vapourise as a consequence of the energy developed in the fibre, thus causing holes to be blown in the matrix.

Since the tube or duct is contemplated for use in the transmission of fuel or other liquids, it is important that the interior of the tube or duct should have good fluid resistance. thus it may be necessary to provide an integral inner liner (which will not delaminate from the rest of the tube). Thus the liner could be made of the same material as the matrix of the tube. The liner would need to be sufficiently conductive that it can transmit any static to the outer layers of the tube or duct. In the same way the matrix and liner material should ideally fall between the range of a partial conductor and a partial resistor.

The present invention enables the electrical resistance characteristics of the tube to be matched to the total mass of conductor available to carry the current, thereby reducing the tendency for good conductors such as copper to vapourise when exposed to large sudden influxes of energy as a consequence of the low surface area of these fibres which is needed to obtain the desired resistance characteristics.

It should be appreciated that the term "tubes or ducts" is used in a general sense and is intended to cover such items as manifolds and junctions used in liquid supply systems.

Example In a typical embodiment of this invention a thermoplastic composite tube was manufactured based on the following composition.

Thermoplastic resin polyphenylene sulphide Reinforcing fibres glass fibres Conducting fibres stainless steel fibres 80 strands of a PPS/glass continuous prepregnated fibre were braided together, in two equal individual isoltropic layers. Six individual stainless steel fibres were wrapped around each of 20 of the strands in each layer before braiding. This equates to a tube in which 50% of the strands incorporate a conducting element. The final braided preform was consolidated to give a continuous fibre reinforced thermoplastic composite tube which was also electrically conducting.

The conducting element namely the stainless steel fibres formed a continous conducting network, not only from one end of the tube to the other, but also from the inside of the tube to the outside of the tube.

The stainless steel fibres used had the following electrical characteristics.

Resistivity 70 x 10 8 ohm m End to end resistance 600 ohm per m The composite tube had a measured point to point resistance of 75 ohm, with a standard deviation of 25 ohms. These measurements were independent of the separation of the electrodes, the orientation of the electrodes with respect to each other, and also applied when the electrodes were placed one on the outside and the other on the inside of the tub.

The electrical results obtained indicated that there was good electrical contact not only between electrically conducting fibres within different stands in the same layer, but also between fibres in the two layers of braiding.

Claims

1. A tube or duct formed from woven reinforcing strands embedded in a compatible thermoplastic or thermosetting matrix, such that the strands constitute between 20 and 80% by volume of the tube wall, each strand being formed from a number of fibres, and 5 to 100% of the strands, evenly dispersed throughout the matrix, incorporate an array of conductive or partially conductive filaments, whereby the surface resistivity 13 of the tube lies between 10 and 10 ohms per square, whilst the resistance of the whole tube lies within the range of 103 to 1010 ohms per metre, utilising a material for the filaments having a volume resistivity of between 10 e to 102 ohm metres, the individual monofilaments having an end to end resistance of 10-l to 1010 ohms per metre.

2. A tube or duct according to claim 1, wherein the filaments are single conductive monofilaments, or yarns or tows in which some or all of the individual filaments are conductive.

3. A tube or duct according to claim 1, or claim 2 wherein the woven strands are formed by a braiding process in one or more layers such that the conductive paths are symmetrically distributed not only along the duct, but also through the wall thickness.

4. A tube or duct according to claim 3, wherein the braiding angle is 100 to 850, the braiding angles in any of the individual layers may be the same or different, and that any number of zero angle longitudinal fibres are included, as desired.

5. A tube or duct according to any one of claims 1 to 4, wherein, at the intersections of the conductive filaments, there is good electrical contact between these filaments, not only for filaments in the same layer, but also for filaments in one layer and any adjacent layer.

6. A tube or duct according to any one of claims 1 to 5, wherein the conductive filaments are formed from conductive, semiconductive or super conductive materials either in the form of homogenous fibres, for example metallic fibres, pure graphite, oxidised acrylic fibres or doped silicon carbide fibres, or as coatings on other fibres which may be either conductive or non conductive, for example nickel coated graphite fibres or carbon coated glass fibres.

7. A tube or duct according to any one of claims 1 to 5, wherein the filaments are formed as a yarn or tow containing, in any proportion, organic fibres in which carbon particles have been incorporated into the surface, namely so-called "epitropic" fibres.

8. A tube or duct according to any one of claims 1 to 7, wherein between 4 and 100% of the strands incorporate a conductive filament.

9. A tube or duct according to any one of claims 1 to 8, wherein the conductive filament in any of the strands can be either a monofilament, or a yarn in which any proportion of conductive filaments have been included by a conventional textile handling process.

10. A tube or duct according to any one of claims 1 to 9, wherein each strand incorporates a semiconductive element which comprises between 20 and 80% of the semiconductive material.

11. A tube or duct according to any one of claims 1 to 10, wherein the resistance of the tube lies within the range of 104 to 10(9 ohms per metre, preferably 105 to 3.5 x 108 ohms per metre.

12. A tube or duct according to any one of claims 1 to 1-1, wherein the surface resistance of the tube lies within the range 10 3 to 1013 ohms per square.

13. A tube or duct according to any one of the claims 1 to 12 wherein the conductive material used has a volume resistivity of between 10 2 to 102 ohm metres.

14. A tube or duct according to any one of claims 1 to 13, wherein the reinforcing fibres are selected from the aramid fibres, carbon or glass fibres or other conventional reinforcing fibres and high modulus polyester and polyether ether ketones, polyethylene or other high strength and high modulus fibres.

15. A tube or duct according to any one of claims 1 to 14, wherein the matrix materials are selected from poly-phenylene sulphide, polyether ether ketone, polypropylene, polyurethane, fluorinated thermoplastics or other thermoplastic or thermosetting materials.

16. A tube or duct according to any one of claims 1 to 15, wherein at least the non-conductive fibres have a diameter of 3 to 40 microns.

17. A tube or duct according to claim 16, wherein the fibres forming the strands have a diameter in excess of 40 microns.

18. A tube or duct according to any one of claims 1 to 17, incorporating an integral inner liner which will not delaminate from the rest of the tube.

19. A tube or duct according to claim 18, wherein the liner#is made of the same material as the matrix of the tube.

20. A tube or duct according to claim 1 and substantially as herein described.