Blowout occurs with parts used in dynamic applications, including wheels, tires and rollers. When polyurethane elastomers undergo cyclic loading and unloading, mechanical energy is transformed into heat energy and the elastomer will heat up. If this heat buildup is too great, the polyurethane can melt and the part can experience blowout.

System Choice
Some polyurethane elastomers have inherently better dynamic performance than others. High-performance polyether systems are generally preferred for optimum dynamic performance; however, some systems perform better than others. Many applications use TDI/Mboca/PTMEG elastomers with great success. Products such as Airthane® prepolymers are usually the highest performance materials in this class. Alternative curatives have also been developed which offer improvements over conventional curatives. Consult your supplier for recommendations on the appropriate system.

Elastomer Hardness
With dynamic applications, the greater the cyclic deflection of the elastomer, the greater the heat buildup. Generally, an in-use deflection of 5-10% is acceptable, although a deflection of under 5% is desirable. If the deflection is higher than 5-10%, then the part has a greater chance for blowout. Therefore, specifying a harder elastomer is usually recommended to minimize deflection. However, there are typically other performance attributes that also need to be considered, such as ride comfort or traction, where use of a softer formulation may be desired.

Stoichiometry Effects
With most systems, the dynamic performance of an elastomer is optimized in the 90-95% stoichiometry range. Performance will decline as a formulation moves significantly away from this stoichiometry.

Part Loading
Even when the correct formulation has been used, the loading on a wheel, tire or roller can be uneven, leading to localized meltdown and blowout in the region of maximum load. This happens frequently with rollers when the ends are tightened down to increase the nip pressure. It also happens with tires and wheels that have an improper crown. Proper installation of the part into service is critical. Loading across the face of the part should be as uniform as possible, and crowning of a wheel or roller may be required.

Load/Speed Considerations
Dynamics are a function of load and speed. If either is too great, then the part may not be able to dissipate the heat fast enough to avoid a blowout. This can happen even with a properly designed part. Sometimes it may be necessary to redesign the application. This can be accomplished by redesigning the wheel/roller to have a larger diameter, effectively reducing the speed. The part can also be made wider to distribute the load over a wider area. Also, additional wheels/tires can be added to spread the load. In some instances, the part can be redesigned to more readily dissipate heat away from the polyurethane.

For more information on designing for dynamic applications, request the following technical paper brochures, "Optimization of High-Performance Elastomer Properties," and "Designing PUR Elastomers for Dynamic Applications," by calling our Product Information Center at 800-345-3148 or 610-481-6799, or go to our on-line Product Information Request Form. and enter the above titles into the "Literature" section of the form.

For further information about other common elastomer processing and performance problems, go to our on-line Trouble Shooting Guide, accessible from the navigation at the left of this page.