Choosing The Right Material For The Job
A common question customers will ask is “What polyurethane product do I use for my application?” That is sometimes an easy answer and sometimes very difficult. With so many different types of applications, practically no two applications are the same. The trick is to figure out what material properties are the most important, while at the same time keeping price and processability in mind.
Polyurethane (PU) elastomers are most often used because of their overall toughness, especially when it comes to abrasion resistance. High cut and tear resistance, good dynamic performance under load (low hysteresis), and ease of processing are other reasons. PU is also very versatile in what type of material form it can take. Various formulations can achieve very low durometer (hardness) similar to a soft rubber or silicone material, but also very hard material like nylon can be cast. This is all while optimizing such properties as compression set, resilience, abrasion resistance, flex fatigue, load bearing capability, solvent resistance, hydrolysis resistance, thermal stability, and UV stability to name a few.
Below is a table with general property info for a given type of polyurethane system.
Aromatic Isocyanate/Polyester Backbone
Excellent physical properties, excellent abrasion resistance, good solvent resistance
Aromatic Isocyanate/PTMEG Polyether Backbone
High resilience, excellent dynamic performance, great physical properties, good abrasion resistance, great hydrolytic stability, excellent low temperature flexibility
Aromatic Isocyanate/PPG Polyether Backbone
Good physical properties, low viscosity (easier to process), great hydrolytic stability
Aliphatic Isocyanate/Polyether Backbone
Great physical properties, excellent thermal stability, excellent light stability
Specialty Isocyanate/Polyether or Polyester
Ultimate toughness with excellent physicals for very demanding applications
Below are a few example applications in which cast PU has excelled and an explanation of why the properties of a particular PU system make it the right choice.
Load bearing capabilities and low hysteresis are the attributes V-Rollers need to succeed and PTMEG-based cast polyurethane offers both of these properties. In addition, cast polyurethane can be formulated for low compression set, which means when a load is placed on a stationary roller for a period of time it will resist forming a flat spot, so that when it starts to rotate again, the motion will be smooth. In high speed applications, the low hysteresis (low heat buildup) of the polyurethane allow it to run faster without getting so hot that it deforms from internal melting. The high resilience, or rebound and low tan delta, which is a measure of energy absorption, show that a PTMEG-based material will perform well dynamically.
Cast polyurethane makes a good material for seals because of its versatility. Seals can be soft (40-50 Shore A) or a little bit more rigid (95 Shore A or higher), depending on the stress that is put on the seal or gasket. A key property needed is good compression set so that the seal won’t take a set when the stress is relieved as well as keeping a tight seal. Good flex fatigue and hysteresis may also be an important property if the seal or gasket is being flexed or compressed cyclically. Also, depending on what type of fluid (gas or liquid) the seal is sealing and the temperature of the application, a specific type of urethane might need to be used.
For applications that need a soft tough urethane, a polyester urethane is typically used, especially in situations where there is exposure to oil or solvents, as is the case many times with seals and gaskets. A polyester polyurethane has excellent flex fatigue, too. In the case where the fluid is air or a water-based material, a polyether polyurethane should be used.
The key for a good bumper pad is resilience and toughness. The main job of the bumper is to be a buffer between two surfaces or objects. A high resilience, or rebound material results in something bouncy so that when the pad is bumped, the energy is given back. On the other end of the spectrum would be a low rebound material, in which the energy is absorbed, similar to a dead blow hammer. Cast polyurethane can do both.
Scraper blades, which are used in many different industries for cleaning and scraping a belt or other objects, see a lot of abrasion and wear. That’s why cast PU is used, with very good abrasion resistance, it outperforms other materials such as steel and rubber, and less down time means higher productivity.
While there is usually a polyurethane type that will work for every application, sometimes the answer is to use a different material since there are limits to polyurethane, namely temperatures above 150°C, hot environments with humidity, or exposure to glycols or amines.