Prepolymer quasiprepolymers

Polyurethane prepolymers are supplied in a variety of sizes, from small 1-kg cans up to 1-ton IBC containers. The available isocyanate (%NCO) level should be marked on every container of conventional prepolymers. Quasiprepolymers do not have individual batch levels supplied, as the overall reaction levels are factory adjusted. 

The reactivity of aliphatic diisocyanates is low in comparison to aromatic isocyanates. It is a problem in the manufacturing stage when using the prepolymer route. Quasiprepolymers and one-shot reactions require the correct choice of curative and catalyst for the system to work. 

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. 

Prepolymers are normally produced with a mole ratio of approximately two moles of diisocyanate to one mole of polyol. If the ratio is very much larger (e.g., four to one), the resultant product is called a quasiprepolymer. 

The preparation of prepolymers and quasiprepolymers allows the production of polyurethane parts by component manufacturers without the large capital outlay required to produce materials from the basic raw materials. The production of any prepolymer requires a good understanding of the chemistry involved. The final quality of the polyurethane product is dependent on the initial control of the chemistry of the system and would be expensive for small operators to carry. 

Polyurethane systems can be produced to have near-equal or easy mixing component ratios (by either weight or volume). The prepolymers in this type of system are called quasiprepolymers. Quasiprepolymers are normally taken as having at least a fourfold molar excess of isocyanate in the isocyanate side of the system. The other side contains the remainder of the polyol, curative, and any catalysts. 

The production of solid polyurethane parts of the correct engineering quality requires the conversion of either the prepolymer or quasiprepolymer to a solid material. The grade and chemistry of the material must be carefully considered in order to obtain a material that can be reproducibly processed and that has the correct final properties. The correct application of heat also must be used to obtain the best product. 

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

Most urethane adhesives are based on urethane prepolymers. A prepolymer is made by reacting an excess of diisocyanate with a polyol to yield an isocyanate-terminated urethane as shown in Fig. 11. Prepolymers may have exeess isocyanate present ( quasiprepolymers ) or they may be made in a 2 1 stoichiometric ratio to minimize the amount of free isocyanate monomer present. Most moisture-cured prepolymers are based on 2 1 stoichiometric ratios. Two-component adhesives generally are based on quasi-prepolymers, which use the excess isocyanate to react with either chain extenders present in the other component or with the substrate surface. 

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