What is polyurethane foam?
Polyurethanes can be defined as polymers which contain urethane groups NH CO O in the main polymer chain. Several types of polyurethanes PU are commercially significant and can be classified, Flexible foams, Rigid foams, Elastomers, Fibres, Moulding compositions, Surface coatings and Adhesives.
The first commercial polyurethane foams were developed in 1950 by Bayer in Germany, and were based on polyester resins to produce flexible polyester foams. In the late 1950’s several American companies developed polyether based foams. These foams offered a wider range of properties than polyester foams, and at lower cost. Polyurethane foams are produced by forming a polyurethane based polymer concurrently with a gas evolution process. Provided these two processes are balanced, bubbles of gas are trapped in the polymer matrix as it is formed and a cellular product results.
The balancing of the two reactions is essential for the formation of satisfactory foams. If gas production is too rapid the foam initially rises well but then collapses because polymerisation has not advanced sufficiently to give a polymer matrix with sufficient strength to retain the gas. If polymerisation is to fast the foam does not rise adequately because the foam gels and sets before blowing / gassing is complete leading to foam shrinkage.
The formation of polyurethane foams is based on the reactions of isocyanates with active hydrogen containing groups. Isocyanates are compounds having one or more of the highly reactive isocyanate group ( –N==C==O ). In a system containing a diisocyanate, a polyol and water two principal reactions occur simultaneously as follows:
1) Diisocyanate + Polyol = Polyurethane
2) Diisocyanate + Water = Carbon dioxide + heat
The carbon dioxide generated in Reaction 2 behaves as a blowing agent for foam formation. Adjusting the amount of water in a foam formulation will result in the density of the foam produced being altered. Foams having a low water level will produce high density foams whilst high water levels will result in foams having a lower density. The isocyanate – water reaction is an exothermic reaction ( ie. gives off heat ) and there is a limit to the amount of water that can safely be used alone in foam formulations so that a peak exotherm of 165 Centigrade is not exceeded. This means that for foam densities less than ( 20 – 22Kg/m3 ) auxiliary blowing agents need to be used to achieve densities lower than 20Kg/m3 safely.
Auxiliary blowing agents function by absorbing heat from the exothermic reactions in polyurethane foam formation and provide additional gassing to achieve lower foam densities. Previously the most common type of auxiliary blowing agent used was Chlorofluorocarbons but due to Ozone depletion concerns ( Montreal Protocol ) these were phased out and replaced by other low boiling point solvents eg Methylene chloride. Continuing concerns about the use of low boiling solvents has resulted in them being removed from a lot of foam manufacturer’s formulations. Many foam manufacturers now use liquid Carbon dioxide as a more environmentally friendly way of producing lower density foams.