MDI-based quasi-prepolymer
The A-side of most aromatic spray polyureas: MDI partially pre-reacted with a long-chain polyol to tune viscosity, flexibility and mix ratio.
A quasi-prepolymer is made by reacting a large excess of MDI with a smaller amount of a long-chain polyol (or sometimes an amine-terminated resin). The product is a mixture of isocyanate-capped prepolymer chains and remaining free MDI. Typical spray polyurea A-sides land around 14–17% NCO.
Pre-reacting part of the MDI does several jobs at once:
At 15.5% NCO, the equivalent weight is about 271 g/eq (4,202 ÷ 15.5). The prepolymer’s NCO groups react with the B-side’s polyetheramines and chain extenders within seconds to form polyurea. The term “quasi” distinguishes it from a full prepolymer, in which nearly all monomer has reacted and free MDI is very low.
Quasi-prepolymers still contain substantial free MDI and must be handled as diisocyanates: respiratory and skin sensitizers. Protect drums from humidity with desiccant breathers, because moisture forms crystals that block screens and pumps.
A full prepolymer uses close to stoichiometric isocyanate, leaving very little free monomer. A quasi-prepolymer uses a large excess of MDI, so it contains both capped prepolymer chains and significant free MDI.
Equal volumes are easy to meter reliably with matched proportioner pumps. Formulators adjust the A-side % NCO and the B-side equivalent weight to hit that ratio.
