As with most polymers, in order to achieve the desired properties the molecules will have to "grow" in size. For this purpose are used molecules that contain more than one reactive group: they are used with 2 or more NCO groups. in this way the molecule will have 2 or more reactive points where it will grow and increase in size. A polyurethane is a molecule formed of multiple urethane groups, each resulting from the reaction between a hydroxyl and an isocyanate.

The isocyanates commonly used in the production of prepolymers for the agglomeration of cork are TDI and MDI, whereas in the production of dispersions for the bonding of discs IPDI is usually used.