COMPOSITION BASED ON POLYTETRAFLUOROETHYLENE NOT CONTAINING WATER, SUITABLE FOR OBTAINING A SELF-LUBRICATING LAYER ON POROUS BRONZE BEARINGS

Abstract: A composition based on PTFE, in the form of a paste, obtained by mixing a PTFE powder with mineral fillers selected from PbO, Pb powder, aluminum silicates, graphite, MoS₂, a resin of high adhesive properties, in particular an epoxy resin; solvents with good wetting properties for the solid ingredients and vaseline in order to impart to the paste the suitable de­ gree of viscosity, said paste being applicable in a very thin layer to porous bronze bearings or supports, and suited for forming a self-lubricating layer, after a thermal treatment of said PTFE on said bearings.

Description: An object of the present invention is to provide a PTFE-based composition, in the form of a paste, suitable for obtaining a thin self-lubricating layer on a porous bronze bearing. The composite material thus obtained is used in the production of sliding-friction main bearings working in the absence of lubricating greases or oils. By this method there may be obtained bushings, flanged sleeves, thrust bearings, sliding strips and slippage surfaces in general, particularly used in applications where the maintenance is difficult and where any presence of lubricants is not provided. Moreover they may work in corrosive surroundings, within a wide temperature range from -200 to 260°C and they are of small size. There are known composite materials useful for the manufacture of supporting sliding friction bearings, working under dry conditions and consisting of: a metal support, preferably of steel; a thin porous bronze layer firmly fixed to the metal support, preferably obtained by a sintering process; a self-lubricating layer, coating the porous bronze and firmly anchored to the same, having a low friction coefficient and consisting of a resin with a binding function, said resin being filled with suitable materials. US-A-4 394 275 discloses composite materials of the above described type, in which the resinous coating layer consists of polyvinylidenefluoride filled with lead metal in a powdery form and optionally with a minor quantity (max. 30 % by weight) of PTFE. Coatings of this kind are suitable only when the working temperature of the manufactured article is maintained within rather low values. In fact, at the melting temperature of polyvinylidenefluoride, there would occur a drastic reduction of the wear resistance. EP-A-163 937 describes a process using a PTFE paste which comprises an aqueous dispersion of the polymer and suitable mineral charges. The presence of water requires a very complex application process for the paste by the use of auxiliary filtering paper sheets, a polyester fabric for the partial elimination of the water before the sintering. The PTFE-based composition according to this invention is suitable for supplying a coating layer displaying excellent self-lubricating properties as well as an excellent resistance to the high working temperatures, to which the manufactured article may be exposed. Said composition is prepared in the form of a paste suitable for being applied by means of automatic mechanical systems, such as spreading rollers and the like, onto the sintered bronze layer, and subsequently subjected to a thermal heat treatment under conventional temperature conditions, in order to sinter the PTFE. The composition according to this invention does not contain water but comprises the following essential components: a) polytetrafluoroethylene in the form of a dry powder comprising particles with a granulometric size range of from 1 to 8 µm, preferably below 4 µm, said granulometry being also achievable by size reduction of the granules of the PTFE starting material during the processing of the mixture in a roller mill. The PTFE must have a molecular weight of from 300 000 to 500 000 and a melt flow index of from 0.1 to 0.3, measured at 380°C according to ASTM 1238 with P = 22 kg, a nozzle with an inside diameter of 1 mm and a height of 8 mm. The amount of PTFE in the end composition is 30 - 60 % by weight. PTFE as used in the present invention is preferably obtained by a dispersion or emulsion process or by cocoagulation; b) mineral charges with good self-lubricating properties selected from MoS₂, graphite, PbO, Pb metal powder, aluminum silicate. The granulometry of the filler substances is preferably comprised within the range of from 4 to 5 µm. The total amount of charges or fillers in the end composition is 10 to 25 % by weight. The charges must also confer good thermal conductivity to the coating composition obtained in order to transfer the heat of friction developed during the operation, to the support. For this purpose it is preferred that the graphite be present in a quantity of from 1 to 5 % by weight; c) a resin with the function of ensuring a high adhesion of the coating layer covering the porous bronze substrate, combined with good flexibility, particularly in view of a possible mechanical bending of the bonded (laminated) composite material during the production of manufactured articles. The resin must, moreover, resist to the high temperature requried for the thermal heat treatment during the sintering of the PTFE. Resins suitable for that purpose are for example, epoxy , silicone, and polyamide resins. The quantity of resin should be kept within the necessary limits, in general from 2 to 8 %, preferably 3 to 5 % by weight, based on the total weight of the composition. The resin is, introduced into the formulation suitably diluted or dissolved in a volatile solvent, preferably having a boiling point of from 75 to 120°C, in order to be easily evaporated before the thermal sintering treatment; d) a liquid having a good wetting power with regard to PTFE and also to the charges or fillers selected from chlorinated hydrocarbon solvents, sufficiently volatile (boiling point from 50 to 150°C). Preferred liquids for this purpose are perchloroethylene and methylene chloride. The quantity of liquid generally is from 10 to 20 % by weight, and is determined depending on the desired viscosity of the paste; e) a paraffinic hydrocarbon having a viscosity of from 10 to 20 centistokes (10·10⁻⁶ to 20·10⁻⁶ m²/s) at 40°C. The quantity of this substance is adjusted with reference to the viscosity desired for the final paste, and in general is from 10 to 50 %. The mixing of the above indicated ingredients is carried out by means of conventional equipment. One starts with mixing the powdery materials (PTFE and the mineral charges or fillers), whereafter there is admixed resin c), dissolved in a solvent, the wetting liquid d) and the paraffinic hydrocarbon e). The thorough mixing of the components may be preferably carried out in a roller mill. The paste is then applied, by means of a mechanical spreading system, for instance by means of a roller spreading device, on the porous bronze substrate supported by the metal sheet (in general made of steel). Subsequently a preliminary thermal treatment is carried out in order to eliminate the volatile components, followed by a thermal treatment at high temperatures in order to achieve the sintering of the PTFE according to conventional procedures. In this way, there is obtained a covering layer based on filled PTFE, perfectly anchored to the porous bronze layer, and having an excellent wear resistance, excellent self-lubricating properties, capable of resisting to high operational temperatures of the sliding-friction type support. The PTFE-based covering layer, in general, has a thickness of from 25 to 30 µm. The composite material thus obtained may subsequently be machined and then subjected to bending, molding, cutting, in order to obtain finished manufactured articles such as bushings, slideways and guide shoes. It has been found that the flexibility and mechanical resistance of the PTFE-based layer is better when it is applied in more layers, i.e., a first layer of 3 to 5 µm, the subsequent layers with a thickness of 10 to 15 µm, up to the final thickness. In the following there is described, for purely illustrative purposes, the preparation of the paste according to this invention, and its application onto the metal substrate of steel/porous bronze. EXAMPLE Having the following components, the following composition has been prepared: The powdery products, i.e. the first four components, were mixed together in dry state. Subsequently first the epoxy resin was admixed, dissolved in methylethylketone and then the perchloroethylene, while continuing the mixing until the added perchloroethylene had wetted all the components. At last there was added the vaseline, thereby obtaining a paste with the desired consistency. The thus obtained paste was then passed through a tricylindrical refiner for grinding and homogenization, this operation being repeated at least two times. Thereby was obtained a paste ready for use, with spreading characteristics suitable for coating the porous support. The viscosity of the thus obtained paste amounted to 110 mPa·s, it may, however, be adjusted by adding vaseline oil with reference to the technique adopted for the application to the substrate. By spreading, there was first applied a layer of about 3 to 5 µm thickness, and then there were applied successive layers of 10 to 15 µm thickness each, until a total thickness of 25 to 30 µm is reached. In this way flaws in the sintering process, due to the critical thickness of the coating are avoided. The sintering on a laboratory scale (30 × 70 mm laminations) was carried out at 400°C for 7 minutes, in a static oven. The test pieces were subjected to wear-tests and to a determination of the friction coefficient. There were found excellent values.

Claims: A PTFE-based composition not containing water which comprises the following components in parts by weight per 100 parts of the mixture: a) PTFE in the form of a dry powder,having a granulometric distribution of from 1 µm to 8 µm, a molecular weight of from 300 000 to 500 000 and a melt flow index of from 0.1 to 0.3, measured at 350°C according to ASTM 1238, in an amount of from 30 to 60 parts by weight; b) mineral charges selected from MoS₂, PbO, Pb powdery metal, graphite, aluminum silicates with a granulometry of from 4 to 5 µm, in a total amount of from 10 to 25 parts by weight; c) a resin having adhesive properties selected from epoxy, silicone, polyamide resins, in an amount of from 2 to 8 parts by weight, dissolved in a volatile solvent; d) a liquid having good wetting power with respect to the PTFE and the charges, selected from chlorinated hydrocarbon solvents having a boiling point of from 50 to 150°C, in an amount of from 10 to 20 parts by weight; e) predominantly aliphatic paraffinic hydrocarbons having a viscosity of from 10 to 20 centistokes (from 10·10⁻⁶ to 20·10⁻⁶ m²/s) at 40°C in an amount of from 10 to 50 parts by weight. A composition according to claim 1, characterized in that the resin having adhesive properties contains graphite in a quantity of from 1 to 5 parts by weight. A composition according to claim 1 or 2, characterized in that the resin having adhesive properties is an epoxy resin used in an amount of from 3 to 5 parts by weight. A composition according to any one of claims 1 to 3, characterized in that the liquid with wetting properties is selected from perchloroethylene and methylene chloride. A composition according to any one of claims 1 to 4, characterized in that the paraffinic hydrocarbon is vaseline having a viscosity of 15 centistokes (15·10⁻⁶ m²/s) at 40°C. The use of a composition according to any one of claims 1 to 5 for forming a covering layer with self-lubricating properties on a porous bronze substrate by thermal sintering.

Inventors: CATALDO, FEDERICO, MARCORA, GIOVANNI, DR.
Applicants: AUSIMONT S.P.A.
Application year: 1987
Main classification: C08L27
IPC codes: C08K3, C08L27, C09D127, C08K13, F16C33, C08L101