Polyurethane chemistry

Polyethers, polyesters, HR foam and Stylex

The manufacture of soft polyurethane foam is based on highly reactive diisocyanate. In so-called polyaddition reactions, isocyanate (OCN-R-NCO) builds a polymer together with the other main raw material, polyol (HO-R-OH), water (H2O) and some intermediate agents.

Modern polyurethane chemistry uses diisocyanates or polyisocyanates and macromolecular compounds containing OH groups, so-called polyols, as well as additives and adjuvants, such as catalysts, expansive materials, emulsifying agents, stabilisers, fillers, and colouring agents.

When the reaction between isocyanate and polyol leads to the creation of urethane, urea as well as carbon dioxide that gives rise to foaming is obtained through the reaction between isocyanate and water, after an unstable intermediate compound. Urea then reacts with new isocyanate groups building polyurethane polymer.

When the polymer or plastic reactions are in balance with the gas reaction, the result is polyurethane foam. Depending on the type of polyol, i.e. ether or ester polyol, the result is ether or ester foam. Polyurethane foam of HR type is manufactured by using modified special polyol, and in the manufacture of rubbery foams, both the polyols and isocyanate have been specially designed for this type of foam. By adding more water, more gas is created, which results in a light foam. The amount of water can be used to regulate the specific weight of polyurethane foam to the desired range.

Previously, CFC compounds that boil in low temperatures were used, in addition to water, for decreasing the specific weight, but already at the beginning of the 90s their use was terminated due to the negative environmental impact of the compounds. Different additives and adjuvants are also used in the manufacture of polyurethane foam. These include catalysts used for steering the reaction, stabilisers, which regulate the cell size and prevent sinking, fire and mould protectors as well as colouring agents.

The blowing of the foam takes place in a special mould machine. Paper is fed to the bottom and sides of a moving conveyor. This creates a moving paper mould, which is filled with a liquid, reactive raw materials mixture, which increases in volume 30 to 50-fold in just the first few metres. The finished polyurethane foam contains 95 to 98 volume percent air, depending on the specific weight, while the rest is polyurethane plastic. After blowing, some slow reactions still take place in the warm preform, and therefore the foam must wait until the following day for cutting, the next step in the process.