This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada
Polyurethanes (PU) are best known in the form of flexible foams: upholstery mattresses, chemical-resistant coatings, specialty adhesives and sealants, and packaging. It also comes to the rigid forms of insulation for buildings, water heaters, refrigerated transport, and commercial and residential refrigeration as well as for automotive components like seat cushions, acoustic panels, instrument panels.
Cellular or foamed PUs are manufactured by using blowing agents to form gas bubbles in the reaction mixture as it polymerizes. They are usually low boiling point liquids which are volatilized 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.
Therefore polyurethane foam starts with the mixing of two liquids: polyol and diisocyanate. The chemical reaction of polymerization produces heat which will blow up a blowing liquid agent producing the expansion of the volume by several orders of magnitude. The foam which is produced is by two orders of magnitude larger in volume than the initial volume of the liquids which are mixed together. After mixing has occurred inside the tool, the foaming reaction takes usually around one minute to be completed.
Typical questions around the PU foaming process arise around the presence of air inclusions inside the mould during and after the filling process, their transport during the process and subsequently the location of the vents in order to reduce these inclusions. These air pockets are compressed by the foam and will modify the PU expansion according to the extra pressure they apply. They are usually created by obstacles in the path of the PU foaming front.
ESI Group has developed and validated a simulation methodology validated for polyurethane foam expansion. The validation of such models has been achieved within industrial projects in the automotive industry. Polyurethane foaming process can be simulated, namely by accounting for the most important effects of the associated chemical reactions, in particular gas production, heat production and viscosity build-up due to polymerization. The simulation methodology will be presented and illustrated on application cases.
|Date||18th June 2019|