Factors Influencing the Cell Structure of Flexible PU Foam
by Duncan Geddes
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 and a cellular product results with the cells having a polyhedral type structure. Thus when considering the formation of a polymeric foam, several factors such as bubble nucleation, bubble growth and bubble stability are of prime importance.
Gas bubbles in flexible PU foam
The first step in the preparation of a foam is the formation of a gas bubble in a liquid system. The reaction between di-isocyanate and water creates carbon dioxide gas and gives off exothermic heat. When the gas finally reaches a point of super saturation it is given off from the liquid in the form of bubbles.
Bubble size may vary widely and is dependent to an extent on the nucleation of the system. Nucleation can be defined as the entrapment of gas in a semi-fluid system to obtain bubbles, ie, when the bubbles begin to grow nucleation is initiated.
The impact of surfactants
In the case of flexible polyurethane foam manufacture, surfactants are added to the foam formulation to act as stabilisers. Surfactants can be described as substances which provide resilience and stability to thin films and reduce the surface tension of liquids thus allowing easier bubble formation. Surfactants perform several functions:-
- a) Regulate nucleation to give a desired cell size
- b) Control cell opening to obtain stable foam without shrinkage
- c) Reduce surface tension
- d) Stabilise the rising foam and avoid coalescence
- e) Aid emulsification
Stabilisation of the cell walls is one of the most important factors in foam formation, the surfactant prevents the coalescence of rapidly growing cells until the cells have sufficient strength through polymerisation to become self-supporting. A wide variety of surfactants are available that can be used to regulate cell/pore size in the manufacture of polyester and polyether polyurethane foams to give foams with a wide range of cell sizes for various applications, e.g. packaging, acoustic insulation, automotive, textile laminates, filtration, ceramic filters, sponges, seals, paint rollers, polishing foams and foams for medical applications.
As well as the various types of surfactants that can be used to regulate the cell size of polyurethane foam, mechanical factors are also important. These include foam head pressure, mixer speed and design, and extra gas loading of the reaction mixture in the mixing head either through gas loading of the bulk raw materials or injection of gas directly into the mixing head. To determine the cell count of a foam, the number of cells/pores per linear inch or centimetre is measured. The old technique of eyeglass counting is still used by a number of foam manufacturers, however for some highly technical applications that require a foam with a particular cell size or structure, image analysis techniques may be utilised.
By the selection of suitable polyols/resins and isocyanates, suitable surfactants/stabilisers and production machine settings cell sizes from about 8ppi (pores per inch) up to approximately 100ppi can be achieved.