I used to design products used in chemistry labs and pilot plants. These were subject to harsh treatments: acids, solvents, pressures and high temperatures. These products were made from chemically resistant polymers like polytetrafluoroethylene (PTFE) or polyether ether ketone (PEEK) to resist these harsh treatments.
But one of the problems with these polymers, besides cost, are the complexities associated with moulding due to their high melting points. Sometimes, you want a cheap component that can be easily injection moulded, has a low coefficient of friction and won’t melt when cleaned with a solvent.
Polyethylene (PE) ticks most of these boxes. In fact, it’s resistant to the harshest chemicals: hydrofluoric acid (if you’ve seen Breaking Bad, it’s what they used in the bath). If you were coated in hydrofluoric acid and holding a tub of butter, you would melt away, but the PE butter tub would be left to tell your story.
Surprisingly, PE can resist acid but isn’t great when used with acetone, a solvent commonly used as a cleaning agent in chemistry labs. This led me to discover ultra-high molecular weight polyethylene (UHMWPE – pronounced ‘oohmpee’), also known as PE1000.
What’s so great about UHMWPE?
• It has an amazing strength to weight ratio
• It has the highest impact strength of any thermoplastic presently made
• It has a very low coefficient of friction (when running on stainless steel, for example)
• It is highly resistant to abrasion
• It is self-lubricating
• It is relatively cheap
• It resists most chemicals (including acids, alkalis and other organic solvents)
• It has a relatively low melting point
So, what is UHMWPE used for?
It can be used in simple components such as bearings, but it’s also been used in protective clothing since WWII. However, a primary market for this material is in medical implants. Due to some of the benefits mentioned above, UHMWPE is being used increasingly for orthopaedic and cardiovascular implants.
My colleague, Dr Rhona Sinclair adds:
“In fact, its high wear resistance alongside its structural properties and biocompatibility have led it to be the gold standard material for bearing surfaces of joint replacements.” “Although aseptic loosening of implants has been an issue due to submicron wear particles, this is being addressed through further crosslinking of the polymer, antioxidant additives and selection of sterilisation method.” “Alongside being used as a bulk material, UHMWPE is valuable as a fibre or thin film for applications such as sutures or minimally invasive cardiovascular implants, for example covered stents and valves.”
If you’re planning to use UHMWPE, be warned!
The material is notoriously tricky to machine and I have witnessed first-hand the problems which occur when injection moulders don’t consider its shrinkage properly. If you’re going to use it, make sure your manufacturer has previous experience with it.
Complexities aside, the performance of the material still makes it a fantastic choice for many applications. Its characteristics coupled with its reliability and performance makes UHMWPE my favourite polymer.
So what’s your favourite?
Oliver has broad experience in start-to-finish product development, applying his engineering and design skills to a range of client projects at Team. Oliver has designed products including LED lights, fully automated labelling machines, and pilot-scale chemical reactors.
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