UHMW-PE – VERSIONS, USES, LIMITATIONS

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Ultra High Molecular Weight Polyethylene (UHMW) is one the most widely used thermoplastic materials today. It is defined as having a minimum molecular mass (molecule length) of 3.2 million; some resins go as high as 7.5 million (for reference, HDPE = ~1MM, LDPE = ~500,000)

UHMWPE advantages in its virgin form:

• odorless, tasteless, and nontoxic
• highly resistant to corrosive chemicals except oxidizing acids
• almost no moisture absorption
• extremely low coefficient of friction (comparable to PTFE (&#;Teflon®&#;))
• self-lubricating
• excellent abrasion resistance, especially wet (~15X better than carbon steel)
• highest impact resistance of any thermoplastic (IZOD = no break)

Easily modified, this base material is used in applications as diverse as cargo dock impact bumpers to dump truck liners to food equipment to orthopedic joint replacements. Essentially a &#;poor man&#;s Teflon®&#;, it shares similar properties, with some significant limitations.

Like PTFE, UHMW cannot be melt processed, as this breaks down the molecular chain. It is made using compression and heat to &#;fuse&#; the material below its melting point. It is available in compression molded sheets up to 8&#; x 20&#;, and ram-extruded rod, tubes & profile shapes.

The largest industrial markets for UHMW are food processing and heavy equipment, both primarily for bearing & wear components. For such applications, the limitations versus nylon or acetal are:

• Heat Resistance &#; the heat deflection temperature is 116&#;F, compared to ~200&#;F
• Thermal Expansion &#; UHMW changes at 2X the rate of nylon with temperature changes
• Compressive Strength &#; its compressive strength = psi, versus 15,000 psi for nylon &#; UHMW&#;s safe working load is psi maximum
• Inserts &#; since the material is only 25%as strong as nylon, larger / specialty inserts with a foot extension on the bottom (prevents pull-through), and limited torqueing is needed

Where food contact is not an issue, reprocessed UHMW is a cost effective alternative. Using clean regrind mixed with virgin black resin yields a material with the same low friction, low moisture absorption and impact resistance of virgin at lower material cost. Due to the inclusion of the larger regrind particles, it is actually slightly harder and some OEMs prefer it over virgin.

There many modifications of UHMW available, including colors; UV stabilized; lubricant filled (solid, oil or MoS2), ceramic filled; anti-static/conductive; metal/X-ray detectable; high temperature.

This last version needs come clarification. Some mills offer an anti-oxidant filled UHMW touted to withstand 275&#;F &#; HOWEVER -THE HEAT DEFLECTION TEMPERATURE IS THE SAME AS VIRGIN, 116&#;F! What the 275F means is that, where pure virgin UHMW starts to oxidize (turn brown) over time @ ~175&#;F, this version adds ~100&#;F heat resistance before oxidation.

Too much information? Give us a call, the experts at WS HAMPSHIRE can walk you through all the data to find the right material for your needs &#; you&#;re in the right place!

If you are looking for more details, kindly visit UHMWPE Fiber.

Tom Connelly is a self proclaimed &#;street engineer&#; with over 40 years in the plastics industry.

Understanding the difference between HDPE and UHMWPE

HDPE (high-density polyethylene) and UHMWPE (ultra-high-molecular-weight polyethylene) are two types of polyethylene, a thermoplastic polymer that is widely used in many industrial and consumer applications. Both materials have their own unique properties and advantages, but they also have some drawbacks that should be considered when choosing between them.

Pros of HDPE:

• High strength-to-density ratio: HDPE is a strong and durable material that is lightweight and easy to handle. This makes it ideal for a wide range of applications such as pipes, tanks, and containers.
• Chemical resistance: HDPE is resistant to many chemicals and is not affected by most acids and bases. This makes it ideal for use in chemical processing and storage applications.
• Low cost: HDPE is relatively inexpensive compared to other plastics, making it a cost-effective choice for many applications.

Cons of HDPE:

• Low temperature resistance: HDPE becomes brittle at low temperatures and can crack or break if exposed to freezing temperatures for an extended period. This makes it less suitable for use in cold environments.
• Limited UV resistance: Natural HDPE (no additives) can be damaged by prolonged exposure to UV radiation, making it less suitable for outdoor applications where it will be exposed to direct sunlight.

Pros of UHMWPE:

• Extremely high strength: UHMWPE is one of the strongest and toughest plastics available, making it ideal for use in high-impact and high-wear applications.
• Low friction: UHMWPE has a very low coefficient of friction, making it ideal for use in bearings, gears, and other mechanical components that need to move smoothly and easily.
• Excellent chemical resistance: UHMWPE is highly resistant to most chemicals, including acids and bases, making it ideal for use in harsh chemical environments.

Cons of UHMWPE:

• High cost: UHMWPE is more expensive than HDPE, making it a less cost-effective choice for some applications.
• Limited temperature resistance: UHMWPE can become brittle at low temperatures and can lose some of its impact resistance at high temperatures, making it less suitable for use in extreme temperature environments.

In conclusion, HDPE and UHMWPE are both excellent materials with unique properties and advantages, but each has its own specific set of drawbacks. The choice between them will depend on the specific requirements of the application and the trade-offs between cost, strength, chemical resistance and temperature resistance.

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