If conditions of polymerization in production of High Density Polyethylene (HDPE) are changed somewhat, the polymer chain can grow to a much longer length yielding ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE (UHMW-PE), often simply called UHMW. An easy way to envisage this material is to think of a plate of cooked spaghetti with the long strands all intertwined. If the spaghetti is cut up, it would closely approximate the molecular appearance of HDPE. If the very long strands of spaghetti had Velcro® on them (simulating the slight bonding between the polymer chains), it would be very difficult to pull them apart. This simplification explains the outstanding abrasion resistance (the difficulty in pulling out particles of the POLYMER) and the great difficulty in melt processing (the difficulty in sliding the POLYMER chains across each other) inherent in UHMW. In addition, physical strength, toughness and chemical resistance are all higher than HDPE. This effect of higher molecular weight on properties explains why CAST NYLON 6 is a stronger, tougher, more abrasion and chemical resistant material than EXTRUDED NYLON 6 and why the same is true for CAST ACRYLIC over EXTRUDED or MELT CALENDARED ACRYLIC. UHMW, with the highest impact of any known THERMOPLASTIC, is a crystalline POLYMER and at its crystalline melting point UHMW becomes almost clear and still possess reasonable tensile strength and seems somewhat rubbery. With all of these special properties, UHMW is an important engineering material. Used often for its abrasion resistance, this property is enhanced by the fact that UHMW has a coefficient of friction second only to that of PTFE, is almost as non-stick as PTFE and has similarly good chemical resistance. With a cost about 1/5 that of PTFE and these very similar characteristics, UHMW is replacing PTFE in applications such as seals, candy mixing and processing equipment, wear strips and wear plates; up to its continuous thermal exposure limit of about 220°F. UHMW is used where abrasion and impact is high such as scrapers, chain guides, conveyor wheels and rollers, idlers, chute and hopper liners and impact pads in mining; slurry doctor blades, filtrate suction blades, bucket and sluice liners, slurry pipe and pipe lining in mineral processing; log drag chain bed liner, chip bin liner, bearings, tree harvester slide plates and conveyor chain wear strips in lumbering; gears, bumper pads, suction box covers and wear plates in pulp and paper; wear shoes, auger screw flight edging and hanger bearings, skimmer blades, mixer blades and impellers, discharge tube and chute liners in municipal waste treatment; and on and on. A variety of materials such as talc, glass-fibres or beads, cross-linking agents or silica are added to UHMW to increase load-bearing capacity with minimal sacrifice of abrasion resistance. UHMW machines very easily to a smooth surface, but in lathe turnings come off as a long string that must be prevented from wrapping around the work piece. Due to unique thermal characteristics, very heavy cuts at high speeds and feeds can be made without the use of coolant. As with HDPE, the very high thermal expansion rate of UHMW makes close tolerance work difficult and uneconomical. UHMW can be butt and spin welded satisfactorily but hot gas welding presents difficulty in obtaining good joint strength as low weight POLYMER, inherent in UHMW, blooms to the surface in heating. As with HDPE, the surface of UHMW must be flame or gas treated in order to use adhesives such as epoxies and urethanes.
Values shown in the above table are averages and there will be variances from lot to lot. After selecting a material based upon these values, you must conduct tests specific to your application to be assured the material suits your needs.
As defined by an AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) task force, the minimum average molecular weight of UHMW is 3.1 MILLION. Materials, particularly sheet and extrusions, produced principally by European manufacturers, include a grade called HIGH MOLECULAR WEIGHT POLYETHYLENE, often simplified to HMW. With a molecular weight of 1/2 to 1 MILLION, HMW is very much easier and cheaper to manufacture and process. HMW has lower physical and impact properties; roughly between those of HDPE and UHMW. In fact, REPRO UHMW, a blend of virgin and reprocessed UHMW, has much higher abrasion resistance and lower coefficient of friction. Next time you are offered cheap UHMW, check to make sure that it is not HMW. If it is and the price is the same as HDPE and you do not need the outstanding properties of UHMW - buy it. If you need something better than HDPE - you will have to pay for quality material.
NOLU-S® is a blend of VIRGIN UHMW with special lubricants designed to reduce friction to the range of TEFLON® and improve on the wear resistance of UHMW, in applications where lubrication is difficult or impossible, by generating a slippery film between the NOLU-S® and the mating part. Also, NOLU-S® is FDA approved.
X-LINKED or CROSS-LINKED UHMW is 60% better than VIRGIN UHMW in sand slurry abrasion tests, has a lower coefficient of friction and UV stabilization for outdoor use. X-LINKED UHMW is typically used for chute and hopper liners handling products such as sand, mineral fines, cement, coal and grain plus baghouse and chain drag lining.
For applications requiring a non-stick and/or slippery surface, UHMW POLYETHYLENE TAPE with PRESSURE SENSITIVE adhesive provides a chemically inert, food approved material with a very low coefficient of friction. Sounds almost like TEFLON® type PTFE material, doesn't it? Except for temperature resistance, where PTFE is serviceable to 450°F, with a high temperature resistant SILICONE adhesive, and UHMW is good to 180°F with an ACRYLIC adhesive, as well as price, with PTFE being 7 to 10 times more than UHMW; you will find the products quite similar. So for room temperature or warm non-stick or low slip applications; save yourself a good buck and save the PTFE products for areas where they can really help you - buy UHMW POLYETHYLENE PRESSURE SENSITIVE TAPE.