GB1389498A - Composite materials - Google Patents

Abstract

1389498 Wet-laid reinforcing fibrous material L E DINGLE and MEN EVANS 5 Jan 1973 727/73 Addition to 1249291 Headings D2A and D2B A mat of aligned fibres suitable for impregnating with a matrix to give a composite material is made by dispersing the fibres in a liquid, passing the dispersion through a nozzle such that at least 85% of the fibres are aligned within 10‹ of one direction, laying the dispersion on a permeable surface moving relatively to the nozzle at a velocity at least equal that at which the dispersion leaves the nozzle and subjecting the dispersion on the surface to a supra-atmospheric pressure while the opposite side of the permeable surface is at a lower pressure such that the liquid passes through the surface rapidly enough for the fibre alignment to be maintained. The latter pressure may be atmospheric or suction. The fibres may be reinforcing fibres such as chopped carbon, glass or nylon fibres, asbestos, or silicon carbide or silicon nitride whiskers, or the fibres may be intended to modify properties of the composite such as the electrical properties. The permeable surface may be a gauze or cloth belt and may have a mesh size just smaller than the length of the majority of fibres. It may be part of a rotating drum but is preferably a horizontal bed with the nozzle extending the width of the bed. A thick sheet may be built up on the bed by repeated applications of the dispersion by one or more nozzles. Preferably the nozzle is reciprocated above the bed in a manner such that the surface is never stationary relative to the nozzle. A continuous multi-layer sheet may be built up by moving the surface unidirectionally beneath the nozzle at a velocity sufficient for each reciprocation of the nozzle to deposit a layer of fibres which overlap but is staggered from the preceding layer. A circular mat may be formed having a circumferential fibre alignment, by rotating a circular permeable surface below the nozzle which is stationary or reciprocating radially of the surface. The flow of dispersion from the nozzle may be stopped when a reciprocated bed is stationary or a drip tray may catch the dispersion for recirculation The liquid removed from the fibres through the permeable surface may also be recirculated. Pressure may be applied to the surface by enclosing the nozzle and at least part of the surface in an air-tight container continuously supplied with pressurized gas by a pump or from a cylinder. The nozzle may be formed by a slit, e.g. 0.02 to 0.08 inches wide, having inwardly convex walls. The liquid may be a viscous liquid which is water-soluble, water in fine droplet or vapour form being sprayed onto the mat formed on the permeable surface; the liquid may be glycerol, aqueous solutions of glycerol or aqueous solutions of cellulose ethers. Heat may be applied to the permeable surface, preferably as a current of hot air directed onto the fibres, for reducing viscosity of the liquid. The mat may be impregnated with a synthetic resin, e.g. a thermosetting resin such as phenolformaldehyde, epoxy, polyester, or a silicon resin. The resin may be solvent-diluted to about 20% of resin weight, the solvent being removed after impregnation, by drying. The impregnated mat may be stored in an uncured or partially cured state. It may be preconsolidated by a hydraulic press or by passing it between rollers, at a suitable temperature. It may then be cured by hot-pressing at below'200 lbs/ins<SP>2</SP> and may form a composite having a packing density of at least 40% by volume retained by immediate solidification of the resin by cooling or completion of the cure. The mat thickness may be between 0.002 and 0.02 inches. It is flexible enough to conform to chosen shapes, and to be laminated with fibre alignment in the layers in different directions, the, layers being attached to each other by synthetic resin which is then cured. It may be cut into strips which can be made into rocket motor casings by filament winding techniques, or it may be used to form honeycomb structures for air frames. The mat may be impregnated with cold setting resins, and it may be formed from a dispersion of two or more fibre materials. The apparatus shown in the figure comprises an applicator I and a tray 2 mounted in a casing having lower and upper parts 3, 4. The applicator has a slit nozzle 5 and a dispersion supply tube 6, and the tray 2 is adjustably mounted and has a filter bed 8 of 120 mesh nylon gauge and a tube 11 leading to a suction pump. The casing has a drain 13 and a gas inlet tube 14 leading to an air compressor. An O-ring seal 7 permits motor-reciprocation of the applicator 1. In an example asbestos fibres up to # inch are dispersed in aqueous glycerol having a viscosity of about 2 poises, at a concentration of 3 % by weight. The casing is initially pressurized to 2 to 3 lbs/sq ins and the pressure increased during mat formation to a maximum of 20 lbs/sq ins. The dispersion leaves the nozzle at 1¢ ft/sec and the applicator is reciprocated at 2 ft/sec. When the mat has been formed the casing part 4 is removed and the mat sprayed with water mist to wash out the glycerine. A felt of 200 layers is made and impregnated with epoxy resin.