GASKET PAPER

Abstract: A non-asbestos gasket paper comprises, by weight, 4-15 % aramid fibre pulp, 4-10 % polymeric binder, 60-90 % silicate mineral and 2-15 % inorganic binder.

Description: Gasket Paper This invention relates to papers particularly, but not exclusively, suitable for use in fluid sealing applications such as cylinder head gaskets. It is known to make gasket papers from non-asbestos formulations based on other fibres such as glass fibres and/or mineral fibres and including a minor proportion of cellulose as a web-forming agent. For example, GB-A-2138854 and 2138855 disclose cellulose-containing compositions using ball clay as a filler. GB-A-2250302 discloses a gasket material which is cellulose-free and which includes a carefully selected mixture of calcined china clay and ball clay as filler. Whilst these non-asbestos products containing ball clay have good performance in non- critical applications, it has been observed that their behaviour at elevated temperatures is not adequate as regards stress relaxation (or creep), particularly when tested over an extended period of time to simulate actual use. Stress relaxation or creep results in a loss of loading in a bolted joint and can lead to gasket failure, so that in temperature critical applications, better stress retention is very desirable. It has now been discovered that replacement of the ball clay component by a layered kaolinitic clay or a fibrous chain silicate, or by a mixture of these results in improved stress relaxation performance. According to the present invention, a method of manufacture of a paper stock for dewatering to paper, comprising by weight 4-15% aramid fibre pulp, 60-90% silicate mineral, 4-10% polymeric binder and 2-15% inorganic binder, comprises the steps of mixing the aramid fibres in water to produce a slurry of about 2% solids content by weight, diluting the pulp with water, adding the silicate mineral to the mixture, agitating the mixture, further diluting the mixture with water to produce a slurry of about 4% solids content by weight, adding a 10% solution of papermakers alum such that the dry content is about 1% of the total composition, agitating the mixture, adding to the mixture the inorganic binder, adding to the mixture the polymeric binder having a solids content of about 50% by weight, waiting until the polymeric binder is fully dispersed and adding a further quantity of the papermakers alum solution until the supernatant liquid becomes clear. According to a further aspect of the invention, at least some of the silicate mineral is constituted by attapulgite. The inorganic binder is preferably colloidal silica. The polymeric material is preferably nitrile rubber. A particularly preferred paper produced according to the inventive method comprises by weight, 4-8% aramid fibre pulp, 5-8% polymeric binder, 75-90% silicate mineral and 4-10% colloidal silica. It will be appreciated that in the present context "aramid" is a reference to polyaromatic amide material. The formulations of the present invention may be used in at least two different ways. Firstly, the use of calcined china clay in combination with colloidal silica lends itself to the product of a paper which can be impregnated with a silicone or other resin such as poiybutadiene in order to enhance resistance to and seaiabilty against fluids such as water-antifreeze mixtures and oil. Alternatively, by including attapulgite it is possible to produce a paper which swells when exposed to water. Such papers can be used without a resin impregnation treatment. The invention therefore includes a process for producing gaskets from the paper of this invention, both with and without a post-treatment with a resin impregnant. Surprisingly, it has been found that by eliminating ball clay (previously regarded as a critical ingredient) in favour of the formulations of the present invention, it is possible to achieve improved stress relaxation performance at elevated temperatures, without using inorganic fibres. In order that the invention be better understood, preferred embodiments of it will now be described by way of example with reference to the following Examples. In the interests of clarity, the stress relaxation performance was determined by a method based on ASTM test F1276, the samples being exposed to 300°C for 22 hours. It will be appreciated that the latter is significantly longer than some other tests, but investigation reveals that stress relaxation is appreciably higher after longer exposures, which more closely approximate actual use. Thus testing was carried out by bonding the test paper to both sides of a plain steel core, blanking out annular samples of inside diameter 14.7mm and outside diameter 34.5mm. These were then tested, again based on ASTM F1276, by applying an initial stress of 58.6 MPa. After 22 hours at 300°C the residual stress was measured, the stress relaxation calculated and then normalised to a paper thickness of 1.0mm, in the usual way. This procedure was used throughout the following Examples, including tests on paper made according to prior art. Example 1 A paper was made having the following composition: % by dry weight Fibrillated Aramid Fibre Pulp 8 Calcined China Clay 76 Nitrile Rubber 6 Colloidal Silica 10 Stock Preparation The aramid fibres were dispersed in water to give a slurry of around 2% solids content by weight. This pulp had a freeness of 50°SR. The pulp was transferred to a mixing vessel and further diluted with water at 40°C. The calcined china clay was added and the mixture agitated. Further water was added to give a slurry having a solids content of approximately 4% by weight. A 10% solution of papermakers alum was added such that the dry content was approximately 1 % of the total composition. The mixture was agitated for 2 minutes before adding the colloidal silica as a 30% solids content suspension. The mixture was agitated for a further 5 minutes and nitrile rubber added in latex form, having a solids content of around 50%. The nitrile rubber latex was diluted 5:1 with water before adding to the mix. When fully dispersed the latex was then caused to precipitate onto the fibres and fillers by the addition of a further quantity of papermakers alum solution until the supernatant liquid became clear. Paper Manufacture A paper was produced from the above stock by the conventional technique of dewatering on a wire mesh, pressing and drying, a polyacrylamide flocculant was used to aid processing. The paper was subsequently calendered to the required density using a conventional 2-bowl calender. The resulting paper had the following properties. Thickness 0.83mm Substance 920gm 2 Density 1100kgm '3 Tensile Strength 4.2 MPa Compression at 34.5 MPa 14.3% Stress Relaxation 24.8% A conventional paper made according to GB 2250302 showed a stress relaxation of 42% by the same test method. In additional to the above properties the ability of the paper to seal against a mixture of 50% water and 50% antifreeze (w/w) was measured. A sealing stress of 10.3 MPa was applied to an annular sample of the paper and the internal pressure of the water/antifreeze mixture increased in steps of 1 bar. Each pressure was held for a period of 5 minutes and the pressure at which leakage occurred was noted. Samples of the above paper were found to leak at an internal pressure of 2 bar. It was found that the sealing performance of the paper could be dramatically improved by impregnation of the paper with a silicone resin, such that a fluid pressure of 10 bar was sealed. Example 2 A paper was prepared largely as described above from the following formulation. % by dry weight Fibrillated Aramid Fibre Pulp 4 Calcined China Clay 30 Attapulgite 50 Colloidal Silica 10 Nitrile Rubber 6 The paper had the following properties: Thickness 0.65mm Substance 860gm '2 Density 1330kgm '3 Tensile Strength 7.0 MPa Compression at 34.5 MPa 15.3% Stress Relaxation 29.8% A sealing test was carried out as described above and the paper was found to seal an internal pressure of 10 bar without detectable leakage at a sealing stress of 10.3 MPa. The paper also sealed 10 bar fluid pressure at a reduced sealing stress of 3.4 MPa.

Claims: CLAIMS 1. A method of manufacture of a paper stock for dewatering to paper, comprising by weight 4-15% aramid fibre pulp, 60-90% silicate mineral, 4-10% polymeric binder and 2-15% inorganic binder, comprising the steps of mixing the aramid fibres in water to produce a slurry of about 2% solids content by weight diluting the pulp with water adding the silicate mineral to the mixture agitating the mixture further diluting the mixture with water to produce a slurry of about 4% solids content by weight adding a 10% solution of papermakers alum such that the dry content is about 1% of the total composition agitating the mixture adding to the mixture the inorganic binder adding to the mixture the polymeric binder having a solids content of about 50% by weight waiting until the polymeric binder is fully dispersed and adding a further quantity of the papermakers alum solution until the supernatant liquid becomes clear. 2. A method according to claim 1 in which at least some of the silicate mineral added is constituted by a fibrous chain silicate. 3. A method according to claim 2 in which said added fibrous chain silicate is attapulgite. 4. A method according to any of claims 1-3 in which the silicate mineral added comprises a layered kaoiinitic clay. 5. A method according to claim 4 wherein the added clay comprises calcined china clay. 6. A method according to claim 5 wherein said added chain clay has been calcined at over 800°C. 7. A method according to any of claims 1-6 wherein the inorganic binder added is colloidal silica. 8. A method according to any preceding claim wherein the polymeric binder added is nitrile rubber. 9. A method according to any preceding claim comprising by weight 4-8% aramid fibre pulp, 5-8% polymeric binder, 75-90% silicate mineral, 4-10% inorganic binder.

Inventors: HALL DAVID GEOFFREY, LATKOWSKI ANTONY, HALL, DAVID, GEOFFREY, LATKOWSKI, ANTONY
Applicants: HALL DAVID GEOFFREY, LATKOWSKI ANTONY, T & N TECHNOLOGY LTD, HALL, DAVID, GEOFFREY, LATKOWSKI, ANTONY, T & N TECHNOLOGY LIMITED
Application year: 1997
Main classification: D21H13
IPC codes: D21H13, D21H17