EP0916001B1

EP0916001B1 - Gasket paper

Info

Publication number: EP0916001B1
Application number: EP97926103A
Authority: EP
Inventors: David Geoffrey Hall; Antony Latkowski
Original assignee: Federal Mogul Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1996-06-27
Filing date: 1997-06-12
Publication date: 2001-05-23
Anticipated expiration: 2017-06-12
Also published as: GB9613456D0; AU713243B2; GB2314569A; AU3099097A; JP2000513053A; WO1997049864A1; BR9709944A; ZA975620B; DE69704947D1; DE69704947T2; EP0916001A1; KR20000015807A; GB2314569B

Description

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. GB-A-2254346 discloses a high temperature gasket material including glass or carbon fibre staples and temperature tolerants. fillers. GB-A-2093474 and EP-A2-0522441 disclose compositions including aramid fibre staple materials.

Whilst some of 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 silicate mineral such as 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 set out in the precharaterising part of claim 1 further characterised by producing a first mixture containing said aramid fibres and silicate mineral prior to adding the inorganic binder thereto and, prior to adding said inorganic binder, adding to said first mixture a 10% solution of papermakers alum such that the dry content is 1% of the total composition and agitating the mixture to prepare the surface of the silicate mineral by said alum to receive the inorganic binder, and, after addition of said inorganic binder, adding said polymeric binder having a solid content of 50% by weight and dispersing said polymeric binder in the mixture without precipitation thereof.

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 according to the inventive method comprises by weight, 4-8% aramid fibre pulp, 5-8% polymeric binder, 75-87% 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 polybutadiene in order to enhance resistance to and sealabilty 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 process of the invention leads to a paper stock which is capable of producing gaskets 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 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 intemal 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

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

producing a mixture in water of aramid fibres in a slurry of 2% solids content by weight, the silicate mineral and the inorganic binder, adding the polymeric binder to the mixture and, when the polymeric binder is fully dispersed, adding papermaker's alum until the polymeric binder is precipitated and the supernatant liquid of the mixture becomes clear, characterised by

producing a first mixture containing said aramid fibres and silicate mineral prior to adding the inorganic binder thereto and, prior to adding said inorganic binder,

adding to said first mixture a 10% solution of papermakers alum such that the dry content is 1% of the total composition and agitating the mixture to prepare the surface of the silicate mineral by said alum to receive the inorganic binder, and,

after addition of said inorganic binder,

adding said polymeric binder having a solid content of 50% by weight and dispersing said polymeric binder in the mixture without precipitation thereof.
A method according to claim 1 characterised by adding at least some of the silicate mineral added as a fibrous chain silicate.
A method according to claim 2 characterised by adding said fibrous chain silicate as attapulgite.
A method according to any of claims 1-3 characterised by adding said silicate mineral as a layered kaolinitic clay.
A method according to claim 4 characterised by calcining the china clay before adding it.
A method according to claim 5 characterised by calcing said china clay at over 800°C.
A method according to any of claims 1-6 characterising by adding said inorganic binder as colloidal silica.
A method according to any preceding claim characterised by adding said polymeric binder as nitrile rubber.
A method according to any preceding claim comprising mixing by weight, 4-8% aramid fibre pulp, 5-8% polymeric binder, 75-87% silicate mineral, 4-10% inorganic binder.