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The present invention provides a two component polyurethane system with excellent wetting properties for casting a supported or unsupported, expanded polytetrafluoroethylene structure. It consists essentially of a polyisocyanate component having an isocyanate functionality of 2.2 or less and a polyo

The present invention provides a two component polyurethane system with excellent wetting properties for casting a supported or unsupported, expanded polytetrafluoroethylene structure. It consists essentially of a polyisocyanate component having an isocyanate functionality of 2.2 or less and a polyol blend having an overall hydroxyl functionality of 2.2 or less and a viscosity of less than 500 cps., preferably 200 cps. or less. The polyol blend consists essentially of (i) a linear polyether or polyester (about 200-2500 molecular weight), (ii) a diol or triol (less than 200 molecular weight), and (iii) a catalyst in an amount sufficient to provide gelling at greater than 2 minutes and less than 10 minutes. The isocyanate to hydroxyl ratio is between 0.95:1 and 1.10:1. In some cases, it is desirable to include selected siloxane surfactants when polyol blends with the higher viscosities, i.e., about 200-500 cps., are used. The casting system is useful for making membrane filters.

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A two component polyurethane composition for casting a supported or unsupported, expanded polytetrafluoroethylene structure, the polyurethane system being capable of wetting the polytetrafluoroethylene structure in about two minutes or less, the system consisting essentially of: Component A which is

A two component polyurethane composition for casting a supported or unsupported, expanded polytetrafluoroethylene structure, the polyurethane system being capable of wetting the polytetrafluoroethylene structure in about two minutes or less, the system consisting essentially of: Component A which is a polyisocyanate having an isocyanate functionality of 2.2 or less; and Component B which is a polyol blend having a total hydroxyl functionality of 2.2 or less and having a viscosity of less than 200 cps., which consists essentially of: (i) a linear polyether or polyester diol having a molecular weight of about 200-2500, (ii) a diol or triol having a molecular weight of less than 200, and (iii) a catalyst for the isocyanate-hydroxyl reaction to form the polyurethane, the catalyst being present in an amount sufficient to provide gelling at greater than about 2 minutes and less than about 10 minutes; with the amounts of Component A and Component B which are mixed to form the polyurethane being sufficient to provide a molar equivalent isocyanate to hydroxyl ratio between about 0.95:1 and about 1.10:1.