Designing Polyurethane Elastomers with Low Compression Sets

Solid polyurethane elastomers that resist permanent deformation when subjected to compressive loads are widely used in applications such as gaskets, sealing rings, and load-bearing pads. These are usually obtained from high performance elastomers such as Vulkollan® and those based on prepolymers of MDI and TDI. These elastomers are characterized by a high degree of phase separation of the soft and hard segments and show very low compression sets besides their high resilience, abrasion resistance and overall superior mechanical properties. Processing these cast elastomer systems, however, calls for high raw material temperatures (80 – 110°C) and unfavorable mix ratios such as 90:10 of isocyanate to extender, which requires precise metering of the two components. Further, these systems are characterized by very short pot lives and require a strict regimen of post-cure to achieve optimum physical properties. This paper describes one-shot elastomers with very low compression sets that can be processed at ambient temperatures and mix ratios closer to 1:1 or 1:2, which facilitate easy metering and dispensing. The systems have long pot lives and, often, do not need post-curing. These elastomers have been produced by two alternate routes: (1) by reacting 2,4’-rich difunctional MDIs or MDI prepolymers with polyol blends that have an average functionality greater than 2.0, or (2) by reacting polymeric MDIs rich in the 2,4’-MDI isomer, and a functionality greater than 2.0, with polyol blends that have an average functionality of 2.0 or less. Working formulations and physical test data useful to the formulator are presented.