Catalysts have a key role in PU production being required to maintain a balance between the reaction of the isocyanate and polyol.
The combination of very complex PU chemistry and diverse processing and moulding conditions make great demands of the catalyst. Its main function is to exploit the diverse reactions to create a product with the desired properties.
There are two main classes of catalyst used in PU production.
Organometallics are used to accelerate the reaction and formation of urethane linkages and hence promote rapid curing.
The most popular organometallic catalysts are tinbutyltin dilaurate and stannous octoate. Tin catalysts are used to catalyse micro cellular elastomers and reaction injected moulded (RIM) systems.
Amines are the other major class of catalysts widely used in the making of PU foams. Some amine catalysts promote crosslinking whilst others assist in controlling the foam's cell structure.
Chain extenders are reactive low molecular weight di-functional compounds such as hydroxyl amines, glycols or diamines and are used to influence the end properties of the PU.
The chain-extender reacts with the isocyanate to affect the hard/soft segment relationship and therefore the modulus and glass transition temperature (Tg) of the polymer. The Tg provides a measure of the polymer's softening point and some indication of the safe upper limit of its working temperature range.
Cellular or foamed PUs are manufactured by using blowing agents to form gas bubbles in the reaction mixture as it polymerises. They are usually low boiling point liquids which are volatilised by the heat generated by the exothermic reaction between the isocyanate and polyol.
Rigid foams yield sufficient exothermic heat from the reaction to allow foam expansion in association with the blowing agent.
Flexible PU foams are usually blown by the C02 generated by the reaction of water and isocyanate (or in association with methylene chloride). Blowing of the foam can also be accomplished by the direct injection of air or gas into the foam. Chloroflurocarbons (CFCs) have been used as blowing agents but their effects on the ozone layer have led to restrictions of their use and they are being replaced by more environmentally acceptable alternatives such as pentane.
Certain end use sectors now take greater account of possible 'worst scenarios' in materials selection.
These considerations will include the effects of smoke and toxic decomposition products on people, property and equipment. PU foams used in furniture are an example which spring to mind. Fire retardancy can be achieved by the addition of fluorine, chlorine, bromine or iodine compounds to the polyol. Solid compounds such as melamine and aluminium trihydrate are also important flame retardants.
Materials and products are continuously evolving and developing and the trends are now to lower smoke and fume generation, and in the much longer term `lower toxicity'. There is an increasing commitment to tougher requirements and in certain sectors of the PU industry this has led to the development of low or halogen free systems.
Many PUs tend to yellow in the light albeit without any adverse affect on the physical properties. To produce coloured PUs pigmented pastes are added to the polyol formulation. The pigments, both inorganic and organic, improve the light stability of PU products.
As with other polymers the use of fillers in PUs will yield products with modified performance. Calcium carbonate and glass fibres are most commonly used. The former primarily to make cheaper formulations, the latter are of growing interest in reaction injection moulding (RIM) technology (see later).