An important method of varying the properties of the final polyurethane is by varying the type of di-isocyanate used. Di-isocyanates may be modified in many ways to give products with differing physical and chemical properties.

Polyurethanes are based on one of two polyisocyanates: diphenylmethane diisocyanate (MDI), in which Huntsman Polyurethanes specialises, and toluene diisocyanate (TDI). Both are derived from readily available petrochemical intermediates and are manufactured by well understood and closely defined chemical processes.

The production chemistry of MDI is considerably more complex than that of TDI. However, this complexity gives a significant degree of freedom to the chemical manufacturer, enabling them to modify and optimise grades of MDI to meet specific user needs.

To make polyurethanes as cost-effective as possible, producers of di-isocyanates have developed processes for tailoring the MDI composition to the requirements of the end product. The polyisocyanate mixtures are formulated to offer a range of differing functionalities.