What is a polyurethane

Polyurethane products first made their appearance in the market place in the late 1930’s.  This was after an intensive investigation by Dr Otto Bayer who was looking for a synthetic fibre to replace nylon.  Once the basic technology was understood, new applications for foams (1941) and elastomers (1943) were developed.  In the early 1950’s, commercial applications appeared. The variety of raw materials as well as the range of different manufacturing techniques enabled pioneers to make a seemingly endless range of tailor-made products with different physical properties.  These included soft foams, rigid structural and insulation foams, soft and hard elastomers and coatings. Processing techniques include casting, spraying, foaming, reaction injection moulding, injection moulding, extrusion and painting.

Basic chemistry

Polyurethane is a name that is given to resins that contain urethane groups. The urethane groups in the molecule give the end product certain unique properties.

A urethane group is formed when an ISOcyanate and a hydroxyl chemically react together:

The component containing the hydroxyl groups is referred to as the Polyol component and the component containing the ISOcyanate groups is referred to as the ISOcyanate (ISO) component.  There are a large number of raw materials available and this means that polyurethanes are one of the most versatile products on the market.

Most polyurethanes consist of large molecules made by reacting various combinations of the following three basic materials together:

1. Diisocyanate  :  e.g. TDI, MDI ,PPDI  
2. Polyol:  e.g. long chain polyether, polyester or polycaprolactone
3. Crosslinker :  such as a glycol or diamine.

Note  :  Prepolymers are resins where the diisocyanate and polyol have been pre-reacted under carefully controlled conditions.

To make a useful polyurethane, very high molecular weight (macromolecules) must be achieved.  This means that the OH : NCO (polyol : ISO) ratio must be correct and the two components must be mixed thoroughly.  (the components can only react if they are in contact with each other)