Quasiprepolymer technique

Quasiprepolymers are obtained in a similar way to the prepolymers, with the difference that the reaction between oligo-polyol and the isocyanates is developed in the presence of a large excess of isocyanate. Quasiprepolymers are a mixture of prepolymers and free isocyanates (around 16-32% free isocyanates)  

Quasiprepolymers are frequently used to transform a solid isocyanate, (e.g., pure MDI), into a liquid, and are used in flexible PU foams, in microcellular elastomers and in other PU applications. 

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In polyurethane fabrication, some special techniques are used, such as prepolymer technique, quasiprepolymer technique and one shot technique. 

The high exotherm and the number of feeds have led to the popularity of the quasiprepolymer variation of the one-shot technique. Quasiprepolymers are a version of the standard prepolymer method, where some of the polyol is reacted with an excess of diisocyanate. This is later reacted with the remainder of the polyols, including the chain extender and some catalysts. The advantage of this scheme is that the exotherm is spread between the preparation of the quasiprepolymer and the final cured part. In MD1 systems, BDO is often used as the curative. As the molecular weight of BDOs is low, the amount added is low. By dissolving it in the polyol, a more even mix ratio can be obtained. 

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