WO1992008917A1

WO1992008917A1 - Improved gaskets

Info

Publication number: WO1992008917A1
Application number: PCT/GB1991/001997
Authority: WO
Inventors: Philip Ferdani
Original assignee: T&N Technology Limited
Priority date: 1990-11-13
Filing date: 1991-11-13
Publication date: 1992-05-29
Also published as: GB9124116D0; GB9024678D0; EP0513270A1; GB2250024A; JPH05503313A

Abstract

Injection moulded gaskets comprise a thermoplastic elastomer material containing a filler predominantly comprised of plate-like particles, the amount of filler being up to 30 % by weight of the material. The filler is preferably selected from the group comprising kaolin, graphite, mica and talc and the elastomer is preferably a polyester block copolymer predominantly comprised of polybutylene terephthalate.

Description

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Improved gaskets

This invention relates to gaskets at least partly comprised of non-metallic material.

Known non-metallic gasket materials include fibre reinforced thermosett-ing mate-rials, typically fibrous compositions which employ rubber as a binder. More recently, it has been proposed to use thermoplastic elastomers as gasket materials, but although these materials have excellent properties, resistance to progressive permanent deformation under load (herein referred to as "creep") is not one of them. Creep resistance is an important characteristic for a gasket material .

Whilst the introduction of some fibrous material as a reinforcement does improve the resistance to creep, this improvement is only achieved at the expense of mouldabilit . The mouldability of fibre-free thermoplastic elastomer compositions is important, because it enables gasket manufacture by injection moulding, a process which makes optimum use of expensive materials by largely eliminating waste. But fibre reinforced materials have intrinsically higher melt viscosities which adversely affect their flow behaviour, for example in the passageways in moulding tools.

According to this invention, a gasket is injection moulded from a fibre-free thermoplastic elastomer material including a filler in the form of plate-like particles the amount of said filler being ug to 30% by weight of the material. Particularly preferred fillers include kaolin, graphite, mica and talc, although other minerals such as vermiculite may also be useful. Kaolin has proved especially useful in amounts of up to 30 percent by weight. Wherein kaolin is used, it is preferably calcined and surface modified by treatment with an aminosilane. The use of plate-like fillers does not significantly detract from the elastomer flow properties, contrary to experience gained with fibre reinforced elastomers, so that the compositions can be injection moulded without a special mould design. A kaolin content of 20 percent by weight has proved very satisfactory for injection moulding in circumstances where the same thermoplastic elastomer material was rendered virtually unmouldable by the addition of only 10 percent by weight of glass fibres.

The thermoplastic elastomer is preferably a polyester, polyester block copolymers being preferred. For example, a copolymer of polybutylene (polytetramethylene) terepthalate (PBT) with a polyester ether or a polyester glycol may be used, the "hard" segments of the PBT being complemented by the "soft" segments of the other component. It will be appreciated that PBT itself is not an elastomer; the desired blend of hard and soft segments may be achieved by copolymerisation and/or by blending with a polyester elastomer which is itself a block copolymer of PBT. However, for the present purposes a PBT content of at least about 70% by weight is preferred. It should be noted that blends of homopolymers are not generally desirable due to their tendency to undergo phase separation on injection moulding.

It has been found that whilst gaskets made according to the invention by injection moulding are in some ways inferior to fibre reinforced gaskets made from the same thermoplastic elastomer, they are at least equal when used as gaskets. Thus, although their tensile strength properties may well be inferior, their behaviour under compression is very similar to that of the fibre reinforced material. Behaviour under compressive loads is

SUBSTITUTESHEET much more significant than tensile strength for a gasket. However, the performance under injection moulding conditions is vastly superior to that of an equivalent fibre reinforced material for a given gasket configuration and moulding tool. A further advantage of the invention resides in the discovery that the products tend to be very significantly less prone to warpage and distortion than fibre reinforced gaskets of the same material.

In order that the invention be better understood, a preferred embodiment of it will now be described with the aid of the following Examples.

Comparative Example 1

An automotive engine sump gasket was injection moulded from a thermoplastic elastomer constituted by a polyester block copolymer sold under the registered trade mark HYTREL. The product was a flat gasket with good sealing properties, including heat resistance. However, it exhibited a tendency to creep, on prolonged exposure to normal clamping pressures in a test rig.

Comparative Example 2

An attempt was made to produce an identical gasket from the same elastomer material as that of comparative Example 1, but this time compounded to contain about 10 percent by weight . of short glass fibres. In order to satisfactorily injection mould this new composition, it was necessary to modify the tooling, to provide wider flow channels. Even so, it was not easy to produce a satisfactory moulding. The product had significantly better tensile strength than the unreinforced gasket and its resistance to creep was greatly improved. However, its processing by injection moulding was at best difficult and the finished product exhibited unacceptable warpage for use in practical gasket applications.

Example 3

Next an identical gasket was produced in accordance with the present invention by injection moulding -the same fibre-free elastomer material as was used in comparative Example 1 and to which had been added about 20 percent by weight of finely divided kaolin. The kaolin used was of a calcined and surface modified grade, the latter taking the form of an aminosilane surface treatment. Moulding performance was similar to that of unreinforced material^', but the tensile strength was not as good as that of the glass fibre reinforced material. However, the behaviour under compression, including resistance to creep, was excellent, even after sustained exposure to clamping loads in a test rig. To further illustrate this, the following table compares two grades of glass and kaolin filled HYTREL thermoplastic polyester elastomer when subjected to different tests, including the ASTM test designated F38. The latter test determines the percentage of clamping load lost after 22 hours at 100°C, starting from an initial load of 60 kN. It will be appreciated that ASTM test F38 is in this instance really a measure of creep.

Tensile

Compres- Elong- F38

Tensile sive ation load

Modulus Modulus at loss

Grade Filler % wt (GPA) (GPA) break % %

8238 none - 0.5 0.97 400 52

8238 glass fibre 20% 1.41 2.24 6 27

8238 kaolin 20% 0.63 2.20 31 39

7246 none - 0.29 0.27 28 59

7246 glass fibre 20% 1.05 1.02 8 41

7246 kaolin 20% 0.46 0.83 33 40 1 -

It can be seen that whilst the tensile modulus of kaolin filled material was inferior to that of the glass fibre filled material, the compressive moduli were comparable. The F38 load loss test confirmed this. The behaviour under injection moulding was however very much better, clearly demonstrating the benefit of the invention. Elastomer containing as much as 20 percent by weight of glass fibres was in fact useless for moulding into the form of the test sump gasket mentioned earlier. The F38 test results reflect the fact that in real gaskets, some load loss is to be expected. However, the 52-59 percent determined for unreinforced elastomer was too great for commercial utility in the particular application under investigation.

The invention is further illustrated by the accompanying bar chart Figure 1 in which the effect on compressive modulus of various fillers is shown by comparison with that of an unreinforced thermoplastic elastomer sold under the trade mark HYTRE , grade 8238. It will be seen that kaolin was superior to other materials tested, although finely divided mica ("MICA 200") also performed well.

To further illustrate the behaviour of gaskets made in accordance with the invention, a series of tests were carried out on a range of engines from a given manufacturer. Each test took the form of running the engine with a sample sump gasket through a series of hot/cold cycles. The gasket was monitored for leakage and examined after testing to determine the loss of bolt loading developed.

Engine No 1 was tested with three different sump gaskets.

Gasket A was injection moulded from thermoplastic polyester block copolymer HYTREL grade 8238, with no filler.

Its leakage behavious after only 100 cycles was rated as good, but a bolt load loss of 47% was observed. A test on a different engine showed oil leakage at 400 cycles, with a load loss of 33%.

Gasket B was injection moulded from HYTREL thermoplastic, polyester block copolymer grade 8238, as before, but this time containing 20% by weight of a finely divided calcined kaolin which had been treated with an aminosilane. Its leakage behaviour after 400 cycles was rated as good and the bolt load loss was 28%.

Gasket C was injection moulded from a pure polybutylene terepthalate (PBT) containing 10% by weight of the same kaolin as was used for gasket B. The leakage performance at 400 cycles was again rated as good, but on inspection the gasket had cracked. Its bolt load loss was 36%. The composition used for gasket B was also tested in several other engines, with similar results as regard leakage, no leakage being recorded in any case at 300 cycles and more. It is important to note with a gasket according to the present invention, satisfactory sealing was still achieved after 300-400 cycles, even in cases where a relatively high load loss was observed.

Claims

1. An injection moulded gasket comprising an essentially fibre-free thermoplastic elastomer material including a filler in the form of plate-like particles, the amount of said filler being up to 30 percent by weight.

2. A gasket according to claim 1 wherein said filler is selected from kaolin, graphite, mica and talc.

3. A gasket according to any preceding claim wherein the filler is finely divided kaolin present in an amount from 1O to 20 percent by weighty _s

4. A gasket according to -claim 2 or claim 3 wherein the kaolin is calcined and surface modified with a aminosilane prior to incorporation into the fibre-free thermoplastic elastomer material.

5. A gasket according to any preceding claim wherein said thermoplastic elastomer comprises a thermoplastic polyester.

6. A gasket according to any preceding claim wherein the thermoplastic elastomer is a polyester block copolymer.

7. A gasket according to claim 6 wherein the polyester block copolymer comprises polybutylene terepthalate

? and a polyether ester.

8. A gasket according to claim 6 wherein the polyester block copolymer comprises polybutylene terepthalate and a polyether glycol.

9. A gasket according to claim 7 or claim 8 wherein the polybutylene terepthalate constitutes at least about 70% of the copolymer.

10. An injection moulded gasket comprising a thermoplastic elastomer including finely divided_^ kaolin as a filler, substantially as described and illustrated with reference to Example 3.