When comparing urethane materials for sealing applications the global market has made it difficult to differentiate at times. Some things should be a given, but are they?
Let’s start with the basics. Urethane is used for seal material because it is tough. Fluid power applications today see operating pressure routinely at 3000 psi and often 5000 psi and above in some heavy duty applications (even higher than that but not as universal). Extended wear characteristics with abrasion resistance, that will extend the life of the seal, are typical of a good urethane material designed for sealing. Notice I said “good” urethane… there’s junk out there too.
But even among the good urethanes there are characteristics that can sometimes bewilder. Not to turn this into a chemistry lesson, thermoplastic polyurethanes (TPU) are produced with many formulations to get the desired physical properties. One of the primary elements to getting desired results however is the type of TPU being produced. In our world there are three fundamental TPU materials used for dynamic sealing and that’s where I’m going with this. The market is changing and the same old urethane may not be the same old urethane or is becoming obsolete in favor of an upgraded formulation.
What are they…
- MDI: Diphenylmethane diisocyanate
- Has been the prominent material in North America for years. Good wear resistance, Descent resilience and elasticity and good temperature range. Typically the lowest part cost of the three but limited in extreme conditions.
- PPDI: p-phenylene diisocyanate
- Claim to fame is high temperature and resilience. Excellent rebound characteristics and performs in operating conditions to 275F. Physical Property tensile strength to 8000psi.
- TODI: Diphenyldiisocyanate
- Perhaps the least understood because its largest volumes are produced outside the USA. Predominantly Europe and Asia. Great compression set characteristics and published low temp capabilities as low as -80C (that’s -115F to you and me). But, less tensile strength than other material at 5000 psi
I’m not trying to make any claims here but the reality is that the largest producers of TPU seals are working with formulations to enhance performance, life and cost effectiveness. Some materials are better suited for o-rings, some for lip seals. It’s not a one size fits all commodity any more – if it ever was. Many compounds are being discontinued or reformulated and new ones are coming into the market.
Let this serve as a reminder to understand the material you are using and its true characteristics. There’s a reason some work better than others in given conditions.