In the production of prepolymers, the extent of the reaction between the polyols and the isocyanates is limited so that the polyurethane molecules do not reach too large sizes and at the end there is still an unreacted isocyanate group. By limiting the molecular size of these polyurethanes it is thus possible to obtain liquid materials that are ideal for use as binders.

Pre-polymers when used as binders will be cured primarily by the action of water. Once the curing process is complete, the isocyanate groups disappear making the prepolymers perfectly harmless materials.