Polyurethane as a seal material
The foam in your armchair. The strap on your wristwatch. The wheels on a supermarket shopping trolley and beyond; Polyurethane certainly has a diverse range of uses since its invention almost 85 years ago. Aside from day-to-day products; it is also a highly capable and versatile sealing material – and an option that is often overlooked.
What is Polyurethane?
Is it rubber? Is it plastic? Is it rigid? Is it flexible? The answer to all of those questions is – yes. Polyurethane covers a group of materials; plastic polymers produced by the combination (or synthesis) of di-isocyanates with polyols and a chain extender.
Thermoplastic Polyurethane (TPU) belongs to the Thermoplastic Elastomer (TPE) family. It was originally invented by Otto Bayer in 1937 and further developed during WWII as an alternative to rubber (which was difficult to source at the time). It can be formulated to produce different finished materials with an array of properties suitable for a wide range of applications.
In addition to seals, Polyurethane is the basis of products as diverse as varnish, foam mattresses, roller skate wheels and surfboards. There are literally hundreds of different types of polyurethanes, each made in a slightly different way to suit the demands of the final product.
How are Polyurethane materials manufactured?
In a one-step process, the polyol (a compound containing multiple hydroxyl groups) is mixed with isocyanate (highly reactive low molecular weight chemicals) and chain extender (low molecular weight diols or diamines). The result is a random copolymer with a physically cross-linked irregular molecular structure.
In a two-step process, the polyol and isocyanate are mixed first to produce a pre-polymer. This is then mixed with the chain extender to produce a block copolymer with more regular molecular structure. This can result in improved and more consistent material properties in exchange for a slightly higher production cost.
In both cases, additives can be used to help prevent hydrolysis attack (water absorption), UV or ozone etc. For more rigid Polyurethanes (like the ones we use for sealing products) the mixed liquid material can either be cast into tubes from which seals can be machined on CNC lathes, or cast into bricks which are then chipped, mixed with dye (or other additives), and fed into injection moulding machines.
Why is Polyurethane a good seal material?
When formulated appropriately, Polyurethane yields an impressive set of material properties that make it an ideal material for sealing products. It can be flexible; which allows seals to be assembled into closed grooves, resist large deformation and make them robust enough to survive without damage. In this respect it outperforms PTFE and is at least on a par with rubber.
It has excellent elastic behaviour, recovering almost instantly from deformation; again outperforming PTFE (no springs or rubber energisers needed here to give the initial sealing contact stress) and matching rubber materials. However, unlike rubber it has a lower coefficient of friction, with very high abrasion resistance, tensile strength and stiffness. These properties mean it can often be used without anti-extrusion/back-up rings at considerably higher pressures than even a 90 durometer elastomer O-ring would be capable of.
In abrasion tests, Polyurethane has a quarter of the wear rates shown by typical rubber sealing materials such as NBR (and considerably lower than even filled grades of PTFE). Its tensile and tear strengths are typically 3-5 times higher than rubber seal materials, and although it lacks the chemical resistance and temperature capability of PTFE, it is compatible with mineral oils.
Polyurethane seal materials typically have a general operating temperature range of around -35°C to +110°C, although more specialist grades can remain flexible down to -50°C, and other grades can push the upper limit to around 130°C.
Because it can be injection moulded as well as machined from tubes, it lends itself to production of seals in both high and low quantities, which allows commercial flexibility at both prototype and production stages of a project.
With complex chemistry, care must be taken to ensure sealing fluid resistance and temperature capabilities are always considered (as is the case when selecting any seal material). In many applications though, Polyurethane can often bring physical property and whole-life cost advantages over rubber or PTFE options, therefore bridging the gap in capabilities between these material groups.