Method of making a pre-impregnated composite material.

FIELD OF THE INVENTION

The present invention pertains to a method of making a pre-impregnated composite material of asbestos fibre weaving. BACKGROUND OF THE INVENTION

For sometime now, asbestos fibres have been used as reinforcements in various plastic resins to form a material, called "composite". Whether it be in the form of brake linings, pipes or covering panels, asbestos fibres have, in most cases, been used in the form of short fibres. With the use of mixers, the asbestos fibres have been dispersed and encapsulated using an adhesive or a resinous matrix. This technology, which is more or less known, is characterized by hazardous environmental conditions and it has therefore become necessary to evaluate a more modern technology with improved results, whether it be at the level of mechanical resistance or at the level of environmental conditions of handling.

OBJECTS OF THE INVENTION

An object of the present invention therefore concerns the use of a composite in the shape of a pre- impregnated material which is essentially constituted of a weaving of asbestos fibres, the mechanical performances of which after moulding or shaping are clearly superior

to the materials which are constituted of short fibres isotropically dispersed in accordance with the previous method. A pre-impregnated material, commonly known as "prepreg", is constituted of an assembly of reinforcing fibres impregnated with a resinous matrix or adhesive, which is maintained in a non-polymerization state or sub- polymerization state prior to being moulded. The fibres are in the shape of continuous strands and woven according to different geometries, such as unidirectional, biaxial or angular.

The pre-impregnated material is maintained in a maturing chamber and then is refrigerated until its use for the next stage, i.e. moulding. The orientation of the fibres in the weaving enables to obtain mechanical properties which vary in accordance with the laminating pattern, and the selection of plastic resin ensures the chemical resistance of the part.

Therefore, the preparation of asbestos fibres in the form "prepreg" enables moulders to have access to the potential of performance of such a fibre without having to be subjected to the inconveniences of air contamination and the inherent risks of the previously known technology of mixing and moulding asbestos fibres.

Also, this new technology enables moulders to obtain a material which has improved mechanical, chemical and thermal properties.

STATEMENT OF THE INVENTION

The present invention therefore relates to a method of making a pre-impregnated composite material of asbestos fibre weaving which comprises the steps of passing a bidirectionally woven asbestos fabric in a resin bath formed between a pair of impregnating rollers to form an impregnated sheet, both faces of which are then covered with a film of plastic material to form a lamination which is then wound onto a roller.

The roll of laminated fabric is matured and refrigerated until it is later used for moulding.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic side representation of an apparatus for carrying out the method of the present invention; and

Figure 2 is a schematic front view of the apparatus shown in figure 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

A first step in the making of an asbestos woven fabric consists in mixing chrysotile fibres of various lengths with organic or synthetic fibres. In a second step, this mixture is introduced in the form of a sandwich in a mixer and it is conveyed to the hopper of a card. The material is then carded in an apparatus having an arrangement of cylinders rotating at varying speeds. These cylinders are covered with many short brushes perpendicular to the axis of the cylinders; this operation removes impurities and brushes off unwanted fibres. The mixture is then separated into bands of various dimensions, called rovings, which are then either wound separately or together with two, three or four strands. A third step in the making of asbestos fabric is the spinning operation during which the rovings pass through a loom which transforms them by torsion into a thread which is resistant to traction. This resistance is also augmented by the addition of a further thread which may be metallic, synthetic or organic. These twisted threads, called strands, are wound and shipped to clients where they are woven. During weaving, the strands are placed on a loom where the cloth is formed by the interweaving of two types of strands: the warp which extends along the length of the cloth, and the weft which

traverses the width of the cloth. This cloth is finally vaporized with an adhesive of phenolic resin. Thereafter, the resin is polymerized at high temperature and the cloth is wound.

The present invention is concerned with a method of making a pre-impregnated composite material that includes such asbestos fibre fabric. Referring to figure 1, it comprises the step of passing the bidirectionally woven asbestos fabric 10 taken from roll 12 in a resin bath 14 formed between a pair of impregnating rollers 16 and 18. The bath contains a plastic resin, such as polyester, vinylester, phenolic, etc., catalysts and other additives. The impregnated fabric 10' thus obtained is covered on both faces thereof with a plastic film 20, 22 mounted on transverse rotatable supports 21 and 23 respectively and is simultaneously wound to form a roll 24 supported on transverse support 25.

Thereafter, the roll of impregnated sheet is stored at ambient temperature for maturing, that is until the viscosity of the resin is sufficiently high to enable it to be properly handled. It is thereafter refrigerated in order to increase its shell life and until it is wished to utilize it in a moulding operation. The shell life of the prepreg is about three to six months.

One type of resin which can be used is a liquid unsaturated polyester of the isophtalic type which is diluted in a styrene monomer. This resin is classified among those which are thermosetting and is largely utilized in cases where corrosion resistance is required.

Another resin is a liquid vinylester which also forms part of the thermosetting resin family. This resin is obtained from an epoxy made from bisphenol A to which an elastomer is added to obtain a flexibilized vinylester resin offering excellent adhesive properties.

Another resin among thermosetting resins is an epoxide having excellent electrical properties which is also made of bisphenol A and epichlorohydrin.

Another semi-solid thermosetting resin is novalac epoxide which combines, in a molecule, the thermal stability of a phenolic resin with the reactivity and the versatility of epoxy resin. This resin offers good chemical resistance at high temperatures.

Another resin is phenolic resin which is a phenol formaldehyde copoly er in liquid form. This thermosetting resin forms part of the resol family which has a ratio of for ol-to-phenol higher than one. This resin has high fire resistance and low smoke emission properties.

The resin bath may also include catalysts.

Perkadox 16 (trade name) may be used; it is a solid catalyst in powder form of the peroxide family. It is used as a curing agent for unsaturated polyester resins and vinylester resins.

Another catalyst is DEH 40, which is a commercial name for a solid hardener in powder form and which is used during moulding operations at high temperature using epoxy resins (DEH 40 is a dicyandiamide accelerated with 2-methylimidazole) .

Another catalyst is BF ₃ MEA which is a solid hardener in the shape of flakes which is used for moulding at high temperature using epoxy resins. Its chemical denomination is boron trifluoride monomethyl amine.

Certain phenolic resins have a catalytic system which is very particular, in that they do not necessitate a catalyst to polymerize during moulding at high temperature. This is due to their ratio of formol-to- phenol being higher than one.

The resin bath used with the present invention may also contain solvents, such as Dowanol PM which is a commercial name given to propylene glycol monomethyl

ether; it is used as a solvent for dissolving the catalyst DEH 40. Also used as a solvent, is dimethylformamide which is used to dissolve DEH 40.

The resin bath may also contain additives such as Aerosil (trade name) , MgO, MgOH, CaO, Ca OH, talc and hollow glass balls. Aerosil is a powder of fine particles of silicium dioxide; it is a thixotropic agent which prevents drainage of the resin on a vertical wall. Magnesium oxide, such as Maglite D (trade name) , is highly reactive; it is a thickening agent used to increase the viscosity of the resin at ambient temperature. Talc is an inert material, in powder form, forming part of the family of silicates; it has high chemical resistance and is used to thicken the resin and reduce the cost of materials. Finally, hollow glass balls are used as reinforcing elements, mainly to reduce the density of the molded parts.

Referring to figures 1 and 2, once the resin is mixed, it is fed from a container 29 to a distribution system that includes pipe lines 26 and 28 connected to a pair of parallel conduit systems 30 and 32 equipped with a series of nozzles 34, 36.

Although the invention has been described above with respect with one specific form, it will be evident to a person skilled in the art that it may be modified

and refined in various ways. It is therefore wished to have it understood that the present invention should not be limited in scope, except by the terms of the following claims.