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Bayer Corporation Develops User-Friendly Systems For Polyurethane Elastomers With Low Compression Sets

Bayer MaterialScience AG : 12 April, 2001  (Company News)
Bayer Corporation researchers have developed user-friendly formulation methods to produce polyurethane elastomers with excellent compression set properties (resistance to deformation under compressive loads), as well as good overall physical properties, for use in such applications as gaskets, sealing rings and load-bearing pads.
Ashok Sarpeshkar, Ph.D., a Development Scientist in the Elastomers Business Group of Bayer's Polyurethanes Division, presented a paper at the Epoxy Resin Formulators Spring 2001 Conference, held April 1-3 in Toronto, Ontario, Canada, describing the methods and how they can be a cost-effective alternative to hot-cast elastomeric systems, especially when very superior physical properties are not needed.

Co-authors of the paper, 'Designing Polyurethane Elastomers with Low Compression Sets,' are Peter H. Markusch, Ph.D., Manager in the Elastomers Business Group, and Robert L. Cline, Research Chemist, of Bayer.

'Cast solid polyurethanes are elastomers of choice when superior mechanical properties coupled with very low compression sets are desired,' Sarpeshkar said. 'However, their processing involves solid prepolymers that have to be melted at high temperatures, and the mix ratio of prepolymer to extender is typically 10:1, requiring precise metering of the two components,' he continued.

Additionally, these systems have very short pot lives and require a strict regimen of post-cure to achieve optimum compression set and other physical properties.

'In an effort to produce user-friendly, two-component systems, we have developed two formulation methods based on liquid raw materials that can be processed at ambient temperature in more favorable ratios,' Sarpeshkar explained. These methods use more cost-effective raw materials, such as polymeric diphenylmethane diisocyanate (pMDI). The resulting two- component systems have long pot lives and, often, do not require post-curing to achieve optimum compression set, he added.

In one method, low compression set is achieved when the formulator reacts a polyol blend with an average functionality greater than 2.0 with a difunctional liquified MDI at 25 degrees Celsius. This method produced polyurethane elastomers with compression sets under 10 percent.

The other method achieves crosslinking from the isocyanate component. Bayer found that formulators can produce polyurethane elastomers with compression sets ranging from 22 to 4 percent by reacting polyisocyanates with a functionality higher than 2.0 with polyol blends that are difunctional in nature.

'In both methods, we also found that the addition of a 2,4' MDI-rich product to the isocyanate component had a profound effect on the compressive properties of the elastomer,' Sarpeshkar said. 'In our experiment, we were able to reduce the compression set of a polyurethane elastomer to 4 percent. This phenomenon has not beenpreviously reported,' he said.
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