GB2250710A - Fibre-reinforced elastomeric sheet - Google Patents

Abstract

In a process for the manufacture of fibre-reinforced elastomeric sheet suitable for use as gasket material, a laminate of non-woven fibrous fabric eg. glass tissue, impregnated with a suspension of inorganic filler such as calcium carbonate in an aqueous dispersion of curable elastomer substantially free from organic elastomer-solvent (such as toluene), is cured under heat and pressure to obtain sheet of compression at least 4%, recovery at least 40% and defined stress retention. The process yields a product much less anistropic than that obtained from the well known "it" calendering process, which utilises free reinforcing fibre. The process is also advantageous in that, avoiding the use of organic solvent for the elastomer, it also avoids pollution of the environment.

Description

Fibre-reinforced Elastomeric Sheet This invention relates to fibre-reinforced elastomeric sheet, especially sheet suitable for use in the manufacture of gaskets.

Such sheet can be produced by the "it" calendering process, which involves the preparation of a dough from a mixture of reinforcing fibre eg. glass fibres, filler eg. barytes, and a solution of curable elastomer in an organic solvent, eg toluene. Examples of the elastomer are: nitrile butadiene rubber, styrene butadiene rubber, natural rubber or polychloroprene.The dough is fed to the nip of a calender consisting of a large heated roller or "bowl" and a smaller cooled bowl. The nip is gradually widened, allowing a sheet to build up on the heated bowl, and the organic solvent evaporates from it as the bowl rotates.However, owing to the high pressures prevailing at the nip there is a tendency for fibres to be aligned in the direction of rotation of the bowls, typically giving a ratio of 2:1 (as inferred from measurement of the tensile strength of the sheet produced) for fibrillated fibres such as asbestos and aramid (polyaromaticamide), but as much as 5:1 for non-fibrillated fibres such as glass. Such anisotropy is sometimes undesirable in the resulting elastomeric sheet product.

A method of making a less anisotropic product is described in our UK patent application 9022933.7. In the process there described for the manufacture of fibre-reinforced elastomeric sheet suitable for use as gasketing material, a laminate of non-oven fibrous fabric impregnated with a suspension of inorganic filler in an organic solvent solution of curable elastomer is cured under heat and pressure to obtain sheet of compression at least 4%, recovery at least 40% and stress retention at least 17.5 MPa at a thickness of 1.5 mm and temperature of 3000 C.

Our earlier application acknowledges that there have previously been proposed, eg in GB-A-2 147 8duo, manufacturing processes which comprise impregnation, lamination and curing steps as set out above, but these steps have been so designed as to yield, from the materials subjected to them, composites which are essentially rigid, lacking the compression,recovery and stress retention which are essential for utility as gasket material.

According to the present invention, the process of our earlier application is modified in that, instead of using for impregnation of the laminate of non-woven fibrous fabric an organic solvent solution of curable elastomer, an aqueous dispersion of the elastomer is employed. By use of a system which is substantially free from organic elastomer-solvent, pollution of the atmosphere is much reduced. The inorganic filler which is to be distributed in the fibrous fabric is, of course, suspended in the aqueous dispersion of elastomer.

Preferably, the fabric employed in the process of the invention is made of temperature-resistant fibre, such as carbon, poly (aromaticamide) eg. Kevlar (trade mark), oras is preferred - glass fibre. If a glass tissue is employed as the fabric, it may be one whose fibres are bonded with adhesive or mechanically bonded.

The lamination step is conveniently carried out at a temperature in the range 100 - 1800 C and a pressure in the range 1 to 25 MPa.

The elastomer employed may be any of those already referred to. The process is preferably carried out with proportions of ingredients such as to obtain a product with an elastomer content less than 20% by weight.

The step of impregnation may be carried out in two or more stages. Of these the first may utilise as impregnant an aqueous elastomer dispersion which is free from filler or is of relatively low weight ratio filler:elastomer, while a subsequent stage utilises as impregnant an aqueous elastomer dispersion which is of high weight ratio filler: elastomer.

Alternatively, the same impregnant may be used in each stage. Multi-step impregnation facilitates encapsulation of the fibre with the elastomer, and yet limits the total elastomer content so as to achieve satisfactory high-temperature load-bearing capacity in the product, as measured by stress retention.

Generally the stress retention of the elastomeric sheet product (British Standard BS1832) will be at least 17.5 MPa at a thickness of 1.Smm and temperature of 3000 C; and the process of the invention can, as shown in the Example later, readily be operated to obtain sheet of stress retention greater than 20MPa at 1.5mm thickness.

This ability to generate high stress retention is a specially valuable feature.

The filler employed may be any of those conventional in this field, such as calcium carbonate, barytes (barium sulphate), mica, talc, graphite, exfoliated vermiculite, chlorite, milled glass fibre or wollastonite.

The preferred products of the process of the invention have a compression of at least 5%, preferably 5-20%.

The invention will now be further described with reference to the following Example, in which flexible sheet suitable for use as gasket material for sealing fluids (eg steam, oil, water) at temperatures up to 4500 C and even higher is made from glass tissue.

EXAMPLE 1 The glass tissue employed was a commercially available product manufactured by a wet laying process from chopped ECR glass fibres of length 18 mm and diameter 13pom. The binder was polyester (8% by weight of the tissue). The mass per unit area of the tissue was 71g/m2 Impregnation - stage 1 The tissue was unwound from a roll and was impregnated by passage through a bath containing an aqueous dispersion (a latex) of acrylonitrile butadiene rubber and a conventional curing system, as follows: Nitrile rubber latex 800g Curing system lOOg Added water 1200g The latex (an anionic aqueous dispersion, pH 10.3; average particle size,100 nm) of butadiene-acrylonitrile copolymer (acrylonitrile content, 33%) had a solids content of 47% by weight.The curing system, which was a conventional blend of cross-linking agents dispersed in water, had a solids content of 50% by weight. The latex and the curing system were stirred together for 30 minutes before being diluted with water.

The impregnated tissue was passed through the nip of a pair of rolls at low pressure to remove excess impregnant, and then dried Imprenation - stage 2 The dried tissue impregnated as above (mass/unit area, 77g/m2 ) was passed through an aqueous bath which, in addition to dispersed elastomer and curing system for it, contained calcium carbonate in suspension.The bath composition was: Nitrile rubber latex 1100g Curing system 143g Calcium carbonate aqueous slurry 4800g The aqueous slurry of calcium carbonate had been prepared thus: CaC03 (a commercially available grade with 99.5X < 40 rm; 35% < 2 pm) in an amount of 7095g was added with stirring to water (2400g) in which sodium carbonate (270cc of a 20% by weight aqueous solution) and Dispex N40 (Allied Colloids' sodium polycarboxylate dispersant; 15cc; Dispex is a registered trade mark) had been included. The sodium carbonate maintains a pH above 7 to prevent precipitation of the rubber latex in the bath.

The nitrile rubber latex and the curing system (identical to those used in stage 1) were stirred together for 30 minutes before being added to the aqueous slurry of calcium carbonate.

The impregnated tissue resulting from the above treatment was passed through the nip of a pair of rolls as in stage 1, and dried; and the steps impregnation - removal of excess stage 2 impregnant - drying, were carried out 4 times in all to obtain dry impregnated tissue of mass/unit area 800g/m2 .

Each side of the dry impregnated tissue resulting from stage 2 impregnation was then sprayed with a fine mist of the stage 1 aqueous elastomer dispersion, and the sprayed material was dried. It had a mass/unit area of 803g/m2 . The composition of the impregnated tissue was calculated to be: % drv weight Glass fibre 8.8 Nitrile rubber 12.4 Calcium carbonate 77.0 Curing system 1.7 Four thicknesses of the sprayed dried impregnated tissue superimposed one upon another were then pressed at a temperature of 1200 C for 5 minutes under a pressure of 1200 psi (=8.3 MPa). The resulting sheet was 1.5 mm thick and of density 1850kg/m3 . It had similar properties to conventional "it" calendered sheets but with a much reduced anisotropy, in particular: Compression (ASTM F36) at 34.5 MPa 9.1% Recovery (ASTM F36) 64.7% Stress retention (BS1832) 23.9 MPa Tensile strength A direction 15.0 MPa B direction 13.5 MPa Stress retention was measured according to BS1832 at temperature of 3000 C for 16 hours with an initial stress of 40 MPa.

Claims (9)

1. A process for the manufacture of fibre-reinforced elastomeric sheet suitable for use as gasketing material, in which a laminate of non-woven fibrous fabric impregnated with a suspension of inorganic filler in an aqueous dispersion of curable elastomer substantially free from organic solvent is cured under heat and pressure to obtain sheet of compression at least 4%, recovery at least 40% and stress retention at least 17.5 MPa at a thickness of 1.5mum and temperature of 3000 C

2. A process according to claim 1 in which the fabric is of glass fibre.

3. A process according to claim 2 in which the fabric is a non-woven glass tissue.

4. A process according to claim 3 in which the glass tissue is one whose fibres are bonded together with an adhesive.

5. A process according to any of claims 1-4 in which the sheet product contains less than 20% by weight of elastomer.

6. A process according to any of claims 1-5 in which impregnation of the non-woven fibrous fabric with said suspension of inorganic filler in a solution of curable elastomer is carried out in two or more stages.

7. A process according to any of the preceding claims, in which the sheet product has a stress retention of at least 20 MPa.

8. A process according to claim 1, substantially as described with reference to the Example herein.

9. A gasket made from fibre-reinforced sheet produced according to any of claims 1-8.