Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1995-07-31
1997-09-09
Szekely, Peter A.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525535, 525537, C08L 8100, C08L 2718
Patent
active
056658257
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with a plain bearing material and a method of making same.
A very successful plain bearing material was developed by the Glacier Metal Company. This material is called "DU" and consists essentially of polytetrafluoroethylene with a filler of about 20% by volume of lead. The material forms a surface layer providing a wear resistant surface layer and infiltrates a sintered metallic layer, e.g. of bronze, secured to a steel backing. The polytetrafluoroethylene in the surface layer imparts low friction characteristics and the lead is necessary for the dry wear resistance of the material. DU finds application in plain bushes and thrust washers.
It is desirable to develop a lead free plain bearing material with equivalent or superior properties to DU. Many attempts have been made but without success. Such attempts have concentrated on improving strength by using different fillers in approximately the same proportion as lead. Significant reductions in the proportion of polytetrafluoroethylene was not seen as a feasible option as this was expected to reduce the low friction characteristics and hence the wear resistance. Furthermore, the addition of more filler to the polytetrafluoroethylene was difficult since working the polytetrafluoroethylene to mix it with a filler causes it to fibrillate and become unworkable so that it cannot be successfully infiltrated into sintered metallic material. Indeed, DU has to be applied with considerable care to avoid fibrillation.
It is an object of the present invention to provide a lead free plain bearing material with improved wear resistance.
The invention provides a plain bearing material formed from two interpenetrating networks, one of polytetrafluoroethylene and the other of polyphenylene sulphide.
A bearing material according to the invention is found to have a very greatly improved wear resistance when compared to DU. The wear resistance achieved in tests is ten times better than any previously-known DU substitute not containing lead. Indeed the wear resistance is up to five times better than DU itself. Polyphenylene sulphide has been proposed as a plain bearing material (see GB Patent Specification No. 1,475,295) and that specification suggests the addition of a small amount of polytetrafluoroethylene to improve low friction characteristics. This material would be essentially a polyphenylene sulphide matrix containing particles of polytetrafluoroethylene whereas the material of the invention has interpenetrating polyphenylene sulphide and polytetrafluoroethylene matrices. Bearing materials are also known which have up to about 20% by volume of polyphenylene sulphide mixed into polytetrafluoroethylene. This material is a polytetrafluoroethylene matrix containing polyphenylene sulphide particles. The wear resistances of these materials do not approach that of the invention.
Preferably, the percentage by volume of polyphenylene sulphide is between 45 and 55%. Excellent wear test results are obtained when the percentage is approximately 50. The material may be infiltrated into a sintered metallic layer mounted on a backing plate.
A material according to the invention may be manufactured by encapsulating polytetrafluoroethylene particles in resin, e.g. an acrylic resin, mixing the encapsulated particles with particles of polyphenylene sulphide, applying heat and pressure to cross-link the material and form it into a smooth bearing surface.
The encapsulation of the polytetrafluoroethylene particles prevents them from fibrillating during mixing and application and the acrylic resin can be burnt off without leaving a residue.
In order to facilitate mixing, the polyphenylene sulphide particles may also be encapsulated in resin, e.g. acrylic resin, before mixing with the encapsulated polytetrafluoroethylene.
The mixing of the particles is facilitated if they are mixed as aqueous dispersions.
Alternatively, a material according to the invention may be manufactured by melt compounding the polytetrafluoroethylene and polyphenylene sulphide, extruding the
REFERENCES:
patent: 4172622 (1979-10-01), Baker et al.
patent: 4493917 (1985-01-01), Bailleux et al.
patent: 5159019 (1992-10-01), Yamamoto et al.
patent: 5470901 (1995-11-01), Ishiwari et al.
Davies Glyndwr John
Moisey Philip
Szekely Peter A.
T&N Technology Limited
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