Ultrahigh molecular weight polyethylene (UHMWPE) composites are known. U.S. Pat. Nos. 4,097,447 (Howard), 4,126,647 (Howard), 4,151,126 (Adelman and Howard), 4,187,210 (Howard) and 4,104,243 (Howard) disclose homogeneous composites of polyolefins, including UHMWPE, with a variety of finely-divided particulate inorganic filler compounds, including alumina, calcium carbonate, kaolinite clay, mica and conductive carbon, and organic filler materials such as polyacrylonitrile, wherein the composites comprise at least about 25% by weight of filler. The aforementioned patents also disclose methods of preparing composites which permit relatively high loadings of filler without sacrifice of essential physical properties. In these processes ethylene is polymerized onto the surface of the filler particles such that the resultant composite is substantially free of polymer-free filler and of filler-free polymer. U.S. Pat. Nos. 4,330,573 and 4,234,659 disclose composites wherein a polyolefin having a molecular weight not less than 300,000 is polymerized onto a solid porous inorganic carrier material to provide composites wherein the inorganic carrier content is 50 to 99.5% by weight.
One important advantage of these mineral/UHMWPE composites containing high levels of filler is that they can be fabricated into dense, homogeneous articles by the procedures of cold-pressing and free-sintering. By "cold-pressing" is meant forming an article under pressure at a temperature below the melting point of the polymer. By "free-sintering" is meant that no restraint is applied on the article during the hot-fusing (sintering) cycle.
Unfilled particulate UHMWPE in contrast cannot be fabricated into high density articles by this procedure unless its morphology and particle size meet stringent requirements. U.S. Pat. Nos. 3,847,888 and 3,975,481 disclose UHMWPE powders which are cold-pressable and free-sinterable at temperatures well above the crystalline melting point of the polymer, the powders having particles of less than 100 microns mean particle size and a size distribution function of less than 0.80. The required fine powders were prepared by extensive grinding of coarse virgin polymer, a procedure known to degrade molecular weight and related properties of UHMWPE. K. S. Han et al., J. Macromol. Sci.-Phys., B19(3), 313 (1981) disclose that polyethylene morphology is at least as important as particle size in determining sinterability and that only UHMWPE powders having a fibrous morphology provide good mechanical properties after sintering. Han et al., also disclose sinterable composites of UHMWPE with fibrous fillers, glass and graphite.
Japanese Kokai 51-21910 describes a porous sintered product suitable for printer rollers prepared by sintering mixtures containing 50-95% of UHMWPE having an average particle size of 3-6 microns and an apparent specific gravity of 0.30-0.40 g/cc, and 5-50% of UHMWPE having an average particle size of 100-300 microns and an apparent specific gravity of 0.15-0.20 g/cc. The sintered product is highly porous with pore sizes of 15 to over 40 microns. This reference also discloses that the more finely powdered UHMWPE component is easily sinterable alone while the coarser component is not sinterable. The component UHMWPEs were prepared by a low-pressure, Ziegler-catalyzed process, but UHMWPE synthesized by other techniques are said to be suitable for preparing sinterable mixtures.
U.S. Pat. No. 4,972,035 discloses UHMWPE fine powders having an average particle diameter of about 1-80 microns at least 20 weight percent of which passes through a 350 Tyler mesh screen. The products have bulk densities in the range of 0.1-0.5 g/ml and are said to provide molded articles having excellent physical properties. The UHMWPE powders are prepared by polymerizing ethylene in a dispersion of a special Ziegler catalyst comprising a titanium compound supported on magnesium chloride particles plus an organoaluminum co-catalyst. The requisite powder is formed by either shearing the UHMWPE product or by shearing the supported Ziegler catalyst to fine powder prior to polymerization. While the present invention employs a Ziegler type catalyst, the polymerization is homogeneous such that all catalyst components are dissolved in the polymerization solvent.
U.S. Pat. No. 4,983,693 discloses a process for preparing UHMWPE by polymerizing ethylene with a supported Ziegler catalyst comprising titanium tetrachloride supported on a special alumina and an aluminum alkyl, e.g. triethylaluminum. A hydrocarbon solvent such as n-hexane is used. The special alumina support has a surface area of 200-400 m.sup.2 /g, pore volume of 1.5-3.5 ml/g and 85% of the pores exceed 100 .ANG. in size. The molar ratio of Al/Ti in the catalyst is in the range of 2.5/1 to 80/1. The UHMWPE products have a bulk density of at leas 0.3 g/ml and 85% of the particles are 13-32 microns in size. No information on processability of the products is provided.
V. Krishnamurthy et al., Poly. Eng. Sci., 29 (8), 564 (1989) disclose that problems of long process time and non-uniform heating in the sintering of UHMWPE powders can be overcome to some extent by employing a conductive iron filler and sintering by induction heating. Filler level was 10 volume percent, sufficient to completely coat all polymer particles with iron. Unexamined Japanese Kokai Patent Number 60-177047 discloses sinterable compositions comprising 100 parts by weight of UHMWPE, 5-100 parts by weight of an inorganic fiber of selected length and aspect ratio, and 0.005-0.2 parts by weight of an organic peroxide.
The references discussed herein suggest that highly-filled UHMWPE composites are readily processed without any significant loss of physical properties. While some references teach a limited processability of lightly filled or unfilled UHMWPE, it is apparent that such efforts have not been highly successful. Therefore there exists a strongly felt need for unfilled UHMWPE and lightly-filled UHMWPE composites that are as readily processable into articles by cold-pressing and free-sintering as is highly-filled UHMWPE, said articles exhibiting properties which equal or exceed those of articles prepared by extrusion or molding at high temperatures.
It is an object of the present invention to provide composites of UHMWPE and low percentages of filler compound that unexpectedly meet the aforementioned requirements. It is a further object of the present invention to provide non-fibrous, particulate, unfilled UHMWPE having a large surface area and that also unexpectedly meet the aforementioned requirements. A feature of the present invention is the provision for articles manufactured with the composites and UHMWPE herein using the well-accepted techniques of cold-pressing and free-sintering. An advantage of the present invention is the provision for dense, sintered articles exhibiting properties common to melt-processed UHMWPE. These and other objects, features and advantages will become readily apparent upon having reference to the description of the invention herein.