Ultra high molecular weight polyethylene may be abbreviated UHMW PE, UHMW-PE, UHMW/PE, UHMW P/E, UHMW-HDPE, U.H.M.W.P.E., Ultra-high, UH, or UHMW. UHMWPE's monomer is ethylene. It is a linear polyolefin. Its weight average molecular weight is over 3 million. UHMWPE's average molecular weight is greater than 3,100,000 as defined by ASTM D4020 standard specification for UHMWPE. This resin with a molecular weight 3,100,000 + is well above the molecular weight of most typical polymers which are between 10,000 and 1,000,000 thus earning the name Ultra High Molecular Weight Polyethylene. The molecular weight of UHMWPE must be approximated using the solution viscosity procedure found in ASTM D4020. Commercially available resins are available from 3-6 million molecular weight. Resin in the 3.1 - 4.5 range have the best combination of impact strength and abrasion resistance. Because the resin does not flow when melted, it normally is compression-molded into thick sheets or ram-extruded.
There are several trade names for UHMWPE such as the Spiratex company's "ULTREX".
Some Properties are:
- Excellent abrasion resistance
- Excellent impact strength (among the highest of plastics)
- Superior chemical resistance even to the most aggressive chemicals. Greater chemical resistance than HDPE, polyurethane, steel, polyacetal and nylon.
- Water repellent highly resistance to hydrolysis
- Biologically inert
- Excellent dielectric & insulting properties
- Stress crack resistant
UHMW is a semi-crystalline thermoplastic, with a continuous use temperature range of up to 180°F (82°C). It has a low difficulty to machine. It has extermely high impact resistance and good corrosion resistance. It has extremely low moisture absorption. It has superior abrasion resistance and superior noise abatement. It has a low coefficient of friction.
Some Modified Versions Through Additives
The base resin of UHMWPE can be modified through the addition of additives to alter performance properties to suit custom applications. A few are:
- An enhanced conductivity UHMWPE achieved by adding a superconductive carbon black which colors the material black and lowers the surface resistivity range to approximately 105 - 109. The dispersed superconductive additive allows electron movement across the part itself. The product color is black and the material is not FDA complaint. This is the most effective option for dealing with conductivity and static charges.
- Ultraviolet-stabilized UHMWPE. UHMWPE can be attacked by ultraviolet (UV) radiation. The attack is known as UV degradation, and can be a problem in products highly exposed to sunlight or in some cases artificial sources. Continuous exposure for a long period of time is a more serious problem than intermittent exposure, since the attack is dependent on the extent and degree of exposure. Examples of UV degradation on natural UHMWPE include chalking, a yellow discoloration and cracking. In severe cases, complete product disintegration can occur. The attack can be detected before serious cracks are seen in a product using infrared spectroscopy. UV attack by sunlight can be slowed or prevented by dispersing UV stabilizers in the polymer prior to extruding. The most effective methods for protecting plastic polymers for UV attack is the addition of specific carbon blacks which when added in sufficient quantities absorb the UV radiation and give it off as heat; protecting the polymer chain. Properly loaded carbon black can give 10+ years of protection from normal exposure. Simply coloring the part black doesn't mean the product willbe optimally protected from UV attack.
- Antistatic additives have the ability to reduce the buildup of static electricity generally caused by the triboelectric effect. The additive leaches to the surface or the material and absorbs moisture from the air. The molecules of an antistatic additive have both hydrophilic and hydrophobic areas, similar to those of a surfactant; the hydrophobic side interacts with the surface of the material, while the hydrophilic side interacts with the air moisture and binds the water molecules. The effectiveness of this option is dependent on the humidity of the environment as well as the leaching of the additive. This material when used in accordance with 21 CFR 177.1520, the applicable FDA polyolefin regulation, meets the requirements of 2.1 and 2.2 of that regulation for food contact applications.
- Lubricated UHMWPE has supplementary lubricity in the form of a continuously regenerating surface film. This effect is achieved by incorporating proprietary additives into the base UHMWPE polymer which are extruded under high pressure. This homogeneous blend of an ultra-high molecular weight polyethylene polymer and normally solid lubricants with a grease yield products that are self-lubricating, slowly releasing lubricant at temperatures developed in ordinary use.The extruded product exhibits excellent dimensional stability, good wear resistance, and a surface which is not oily to the touch. The character of the additives that produce the lubricating film are such that dimensional changes, as the surface film is displaced, are inconsequential and other desirable properties are not unfavorably affected. The standard color for this material is gray when used in accordance with 21 CFR 177.1520, the applicable FDA polyolefin regulation, meets the requirements of 2.1 and 2.2 of that regulation for food contact applications.
Common Applications
High strength, chemical resistance, and lubricity make UHMWPE ideal for gears, slides, rollers, and other industrial parts. It also is used to make artificial hip joints. In fiber form, UHMWPE's linear structure exhibits liquid crystal properties that are useful in reinforcing composite structures. The fiber also can be woven into lightweight ultrastrong fabrics.
UHMWPE is commonly used in the material handling industry including food contact surfaces as it is acceptable under FDA and USDA guidelines. It is used in lead-acid battery separators, medical implants, filters and a wide variety of other applications.