The difference between semi-prepolymer method and prepolymer method is that some polyester polyols or polyether polyols are added to the prepolymer in the form of mixture with chain extender and catalyst.In other words,
(1) the formula of oligomer polyol is divided into two parts, one part and excess diisocyanate reaction synthesis of prepolymer, the other part and chain extender mix, when pouring in.

(2) the generated prepolymer has a high mass fraction of free NCO, generally 0.12 ~ 0.15(12% ~ 15%), so it is often referred to as "quasiprepolymer".
The characteristics of the semi-prepolymer method are:
(1) the viscosity of the prepolymer component is low, can be adjusted to the viscosity of the mixture component and curing agent is similar,
(2) the ratio is also similar (that is, the mixing mass ratio can be 1:1), which not only improves the mixing uniformity, but also improves some properties of the elastomer.
Semi-prepolymer method is mostly used in the production of MDI type casting elastomer, because the viscosity of MDI prepolymer is high, the preparation of semi-prepolymer can reduce the viscosity, at the same time, can also make the commonly used diol chain extender and MDI viscosity and dosage match.In addition, in the MDI casting system, the excessive MDI odor in the prepolymer is small, which is less toxic to operators.Semi-prepolymer method is also suitable for reaction injection molding process.
Generally speaking, although the semi-prepolymer method is not as good as the elastomer obtained from the prepolymer system, the semi-prepolymer method has its advantages in technology:
(1) such as two components viscosity is low, easy to mix evenly;
(2) in most cases, the synthesis of different properties of a series of products, semi-prepolymer can be universal, just another component (chain extender and oligomer polyol) formula can be adjusted appropriately, can greatly shorten the production and processing cycle.