Abstract
The technical requirements of semi-rigid polyurethane foams for automotive instrument panels have become increasingly demanding in recent years. This is predominantly due to the change in design of cars and the consequent impact upon the design of instrument panels. In particular, larger and flatter windscreens have resulted in higher interior temperatures and higher thermal stresses. In addition, some companies have moved towards a closed mould technique which when combined with the requirement to produce longer, more complex instrument panels, has put pressure on foam formulators to provide chemicals with excellent flow characteristics whilst reducing foam demould times. A common problem with the production of instrument panels is the potential staining of the PVC caused or assisted by its reaction with polyurethane foam ingredients.
Industrial Foam Systems has carried out extensive evaluation of the effect of formulation changes on the important processing and discoloration properties of polyurethane foams. In particular, we have determined the effect of these parameters on foam reactivity, flowability, mouldability (including void formation and cell collapse), PVC discoloration, adhesion to PVC and rate of cure. Reactivity and flowability studies utilised an automated rise profile/dynamic viscosity apparatus, a technique used to determine the change in foam viscosity as a function of rise height. This apparatus was found to be particularly useful in evaluating the effect of changes in catalyst type on foam flowability. An assessment of foam mouldability has been carried out by foaming into a complex-shaped mould and determining the extent of void formation, cell collapse and knit-line efficiency under these stringent flow conditions.
The results of our laboratory evaluation have been transferred to production conditions in two types of application. The first is the production of a complex instrument panel manufactured by the closed mould method. The most important processing criteria were good flowability and fast demould times. The second application was an open mould process where foam flowability was less critical but more importance was attached to a very fast demould time and low levels of PVC staining. In both cases very successful production runs have been realised.
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