WO2009087372A2

WO2009087372A2 - Fuel pipes with controlled resistivity and method for producing the same

Info

Publication number: WO2009087372A2
Application number: PCT/GB2009/000026
Authority: WO
Inventors: Scott Roberts; Michael James Dewhirst
Original assignee: Crompton Technology Group Ltd
Priority date: 2008-01-11
Filing date: 2009-01-08
Publication date: 2009-07-16
Also published as: WO2009087372A3; GB0821548D0; GB0800538D0; GB2456367A

Abstract

A glass reinforced composite pipe P, particularly for use in fuel lines of aircraft, and comprising a body B having at least its outer portion O comprising an electrically non- conductive polymeric resin matrix having an electrically non-conductive fibre tow reinforcement, typically glass fibre, and a dispersion of electrically conductive particulate filler, such as carbon black, the resistivity of the outer portion being tailored to between 50 k'O per metre length and 4.0 MO per meter length.

Description

FUEL PIPES WITH CONTROLLED RESISTIVITY

Field This invention is related to fuel pipes or lines, and in particular to fuel pipes for use in aircraft and in particular in the wings of aircraft.

Background of the Invention

Electrically conductive, polymer based composites are becoming increasingly important for use in a variety of electrical, electronic, and aerospace applications. Major reasons for using conductive elastomers and plastics are to dissipate static charges and to manufacture contact switches, connectors, surface heaters, pressure sensitive electrically conductive adhesives, printed circuit boards, electromagnetic interference shielding materials, and various other products ranging from calculators to artificial organs. Conductive polymeric composites can be made by incorporation of specific conductive additives_such as conductive carbon black, carbon fibre, metallic powder, metallic fibre, and intrinsically conductive polymeric powder, e.g. poly-pyrrole or polyaniline. The electrical conductivity of an insulating polymer filled with conducting particles or short fibres increases discontinuously at some specific filler content equivalent to the percolation limit. Evenly distributed small spherical particles display percolation at ~19v/o whereas higher aspect ratio fibres percolate at ~5v/o.

For fuel pipes or lines, particularly when used in aircraft, it is necessary to balance the conflicting electrical requirements needed such that the fuel lines are resistive enough to prevent them offering the preferred path for lightning conduction whilst at the same time being conductive enough to prevent charge build up and sparking though electrical discharge in the dielectric fuel flowing at the tube core. A known example _;of an aircraft fuel pipe is disclosed in EP297 990-A in which the pipe body is formed from a insulating composite material such as glass filled epoxy resin and the inner wall of the pipe is provided with a conductive liner. The pipes are connected through flanged end fittings which are connected to the conductive liner and are grounded to the aircraft wings.

Mechanically the fuel lines are also required to be of low weight with high vibration resistance and good static and fatigue strength in their operational environment. To achieve the mechanical properties, low material densities, reduced length and high longitudinal modulus are all advantageous characteristics. However, for any specific design application the lengths and diameters of the pipes are fixed. A material combination with high specific axial modulus (high longitudinal modulus and low density) is required to produce a pipe system with high vibration frequencies. To achieve this, composite tubes reinforced with high modulus fibres and in particular high modulus carbon fibre reinforced plastics (CFRP) would be the materials of choice. However such pipes are electrically , too conductive having resistance levels of up to a¹ few hundred

Ohms per metre. A lightning strike would be preferentially attracted to the pipe surface and could result in sparking or arcing and ignition of the flammable fuel vapours. Within a large composite fuel tank such as a wing tank the expected voltage drop along the pipes would be <100Vm^'1. Similar glass reinforced pipe sections would be too resistive with many tens of millions of Ohms per metre and would be susceptible to electrical discharge in the fuel. In both cases the fuel would be in danger of igniting.

This invention relates to a means of producing lightweight composites fuel pipes with controlled levels of electrical resistivity intermediate between insulating glass reinforced composites and conductive carbon reinforced composites.

Statements of Invention According to the present invention there is provided a glass reinforced composite pipe, particularly for use in fuel lines of aircraft, and comprising a body having at least its outer portion comprising an electrically non-conductive polymeric resin matrix having an electrically non-conductive fibre tow reinforcement and a dispersion of electrically conductive particulate filler and having a resistivity of between 50 kΩ per metre length and 4.0 MΩ per meter length.

The intermediate levels of resistance are particularly important for pressurised aircraft fuel systems where the aircraft wings are constructed of composite materials. The electrieai resistance of the pipe system has to be controlled to semiconductor levels between the low resistance of metals and carbon fibre reinforced plastics and the very high resistance of insulators such as glass-reinforced plastics. The resistance is measured along the length of the tube at the outer surface and through the thickness at the opposite ends of a pipe section. Preferably, the resistivity lies between 150k'Ω and 1.4MΩ and more preferably <1.25 MQ.

The fibre tow may include fibre reinforcements such as E glass, S glass, alumina silicate or polymer fibre which can be impregnated with the matrix resin between individual filaments such that it can be formed into tubes using standard composite fabrication techniques such as filament winding, tape winding, fabric wrapping or resin impregnation techniques .

The particulate filler may include cost, high conductivity carbon blacks, and conductive metal oxides such as antimony tin oxide (ATO) or indium tin oxide (ITO). The conductive particulate filler and be incorporated into a liquid thermosetting resin, for example an epoxy resin, using a simple mixing process. In the case of carbon blacks the mixing process should retain the structure of the carbon black and evenly impregnate between individual filaments in the fibre tow.

The body may further comprise an inner core comprising a non-conductive polymeric resin matrix and an electrically conductive fibre tow reinforcement.

Description of the Drawings

The invention will be described by way of Example and with reference to the accompanying drawings in which:

Fig. 1 is a schematic part cross-section through a fuel pipe/line in accordance with the present invention, Fig. 2 is graph of resistance data for a tube section according to the present invention and

Fig.3 is a graph of resistance vs weight % of carbon black additive.

Detailed Description of the Invention

Figurel shows a structurally efficient design based on a composite pipe (P) made from glass fibre reinforced epoxy having semi-conductive properties, electrically connected to a metallic ferrule (F) via a semi-conductive resistive link (L) at the interface between (P) and (F). This is the subject of our invention.

In an alternative arrangement, where structural considerations are necessary due to the dynamic environment in which the part may be deployed, a hybrid composite pipe (P) can be constructed having a body (B ) comprising two layers, (O) and (C). An electrically conductive structural core (C) is made from carbon fibre reinforced epoxy and a more insulative outer layer (O) is made from glass reinforced epoxy having semi- conductive properties.

The two aluminium alloy end flanges (F) are attached onto outer composite layer (O) though a conductive or semiconductive adhesive similar to the matrix resin in layer (O). Alternatively a more conductive adhesive could be used. This also acts as a seal.

Outer composite Layer (O) is based on glass fibre reinforced epoxy modified to exhibit a semi-conductive property; in this particular case, an epoxy resin matrix comprising S glass reinforcement and carbon black nano-particulate additions. The resistance between the end flanges is governed by the resistive characteristics of layer (O) which is a semiconductor. The resistance between the left outer flange and right inner core is governed by the resistive characteristics across the thickness of the composite tube. The inner bore resistance is governed by the resistive characteristics of the inner composite layer (C).

Material requirements

The outer layer (O) utilises a fibre reinforcement of S2 or E glass with fibre volume fraction of -60%. For hybrid shafts aerospace qualified Tenax HTS 12k fibre can be used in the pipe core. The matrix resin is an epoxy based system - modified with the particulate filler to give the required electrical resistance in the cured composite pipe sections. Fibre tows can be accurately positioned in the pipe wall using filament winding techniques in layers dispersed at typically (±89° / +287+28°) in typical thickness ratio (2:5:5). These angles and ratios can be adjusted to match the required mechanical and thermal expansion characteristics of the pipes.

For the case of a hybrid pipe with an inner carbon fibre reinforced core C, the inner +89° / +28° layers will be reinforced with Tenax HTS or similar carbon fibre.

The impregnation matrix system is based on LY556 / HY917 epoxy (Huntsman) having a Enaco 250 Carbon Black filler with BYK-P 9055 (BYK Chemie) dispersing additive. The matrix components were mixed at 4O⁰C by stirring at 500rpm. The 10% carbon black system consisted of 100 parts by weight (epoxy) resin, 10 pbw (carbon black), 2 pbw dispersant. The lower carbon black matrix systems may be produced by the addition of further quantities of the mixed epoxy resin followed by further stirring.

The S2 glass fibre tows were impregnated with the resin system using standard filament winding procedures. The end fittings will be bonded and sealed in place with either EA 9394 or EA 9395 or if controlled electrical resistance of the bond interface is necessary, with an epoxy based on the composite matrix resin.

In our invention we have modified the electrical characteristics of the matrix resin in such a way that the S glass fibre tow is impregnated with the matrix resin between individual filaments such that the resulting impregnated tows may be formed into tubes using standard composite fabrication techniques.

The liquid rheological dispersing additive was used to break up agglomerates, stabilize the carbon black and prevent sedimentation, separation or floating. In this way electrical percolation is maintained throughout the matrix at a level of 8-10 % by weight of carbon black addition whilst at the same time maintaining the viscosity of the resin at <1000 cp at 40-50°C as required for filament winding and liquid impregnation. At the same time the good mechanical integrity of the composite is maintained. The resulting fibre reinforced composite pipes exhibit other advantages over metallic pipes, i.e., they are lighter in weight, more resistant to corrosion, more inert and the expansion coefficients can be tailored.

Electrical Resistance

Pipes in accordance with the present invention containing 10% by weight carbon black and having an ID of 20mm and OD 21.4 mm with a fibre angle of 45° were tested for resistance as measured between the end fitting and the opposite end pipe bore and between the two end fittings along the pipe outer surface. The electrical resistance across a typical carbon fibre reinforced filament wound tube from the bore at one end to the outer machined surface of the other end is ~25Ω m^"1 in contrast to a similar glass reinforced composite tube which would be highly insulating at >200MΩm^"J . These values may be affected by the surface absorption of the aviation fuel but since the electrical conductivity is low at typically 50-450 pSm^"1 the effect is expected to be minimal.

In contrast and as is shown in Fig 1, the pipe according to the present invention has a resistance measured using digital volt metre of ~137Ωm^'' for a 1000mm length of pipe. This value does not vary significantly over the operational temperature range.

The resistivity of pipes may be controlled by varying the amount of the carbon black contents as is shown in Fig. 3. Resistance of between 1.4 MΩ and 22 MΩ were achieved between 8-10w/o of carbon black. Lightning impulse voltages of 900V transient have been applied to a hybrid composite pipe section according to the present invention without any signs of sparking between layers within the pipe, or at end fittings.

The thermal expansion coefficients can be controlled by the details of the fibres and angles used in the windings. The thermal expansion coefficient along the pipe axis can be controlled with the fibre angles and thicknesses used in the winding. The estimated longitudinal expansion coefficient is ~10 ppm ⁰C^"1 for S2 glass reinforcement and ~4 ppm ⁰C^"1 for a typical S2/carbon hybrid structure. -

Claims

1. A glass reinforced composite pipe, particularly for use in fuel lines of aircraft, and comprising a body having at least its outer portion comprising an electrically non- conductive polymeric resin matrix having an electrically non-conductive fibre tow reinforcement and a dispersion of electrically conductive particulate filler and having a resistivity of between 50 k'Ω per metre length and 4.0 MΩ per meter length.

2. A composite pipe as claimed in Claim 1 wherein the resistivity lies between 150k'Ω and 1.4MΩ .

3. A composite pipe as claimed in Claim 1 or Claim 2, wherein the resistivity <1.25 MΩ.

4. A composite pipe as claimed in any one of Claims 1 to 3, wherein the fibre tow includes at least one of S glass, E glass, alumina silicate or a polymeric fibre which is impregnated with the matrix resin between individual filaments.

5. A composite pipe as claimed in Claim 4 wherein at least a portion of the reinforcing fibres within the outer portion are helically wound around the longitudinal axis of the pipe .

6. A composite pipe as claimed in any one of Claims 1 to 5, wherein the particulate filler comprises at least one of carbon black, ATO and ITO.

7. A composite pipe as claimed in any one of Claims 1 to 6, wherein the resin matrix comprises epoxy resin.

8. A composite pipe as claimed in any one of Claims 1 to 7, wherein the pipe body further includes an inner core comprising a non conductive resin matrix and an electrically conductive fibre tow reinforcement.

9. A composite pipe as claimed in any one of Claims 1 to 8 and further including end fittings attached to the outer portion of the body by a conductive or semi-conductive adhesive.

10. A method of manufacture of a composite pipe as claimed in Claim 6, wherein the carbon black filler is incorporated into the resin matrix using a simple mixing process, which preserves the structure of the carbon black, and then evenly impregnating the resin between individual filaments in the fibre tow.

1 1. A method as claimed in Claim 10, wherein the fibre tow is formed into tubes using filament winding at selected angles which are adjusted to match the required mechanical and thermal expansion characteristics of the pipe.