Manufacturing polyurethane foam

Polyurethanes, thermosetting polymers, occupy a privileged position in industry. They result from the chemical polymerization reaction of an isocyanate with mobile hydrogen-bearing groups (mainly hydroxyl groups), e.g. alcohol functions. To obtain cellular materials, this exothermic reaction must be coupled with a gaseous release (chemical or physical) which allows the creation of overpressurized gas cells within the polymer (thus its expansion) until the polyurethane is completely polymerized. The final properties of polyurethane foam depend on the chemical components (diphenylmethane diisocyanates DMI or toluylene diisocyanates TMI), the blowing agent, the process conditions (temperature, pressure, hygrometry) and the nature of the mold facings (strong skin effect). Thanks to the constant development of new formulations (new monomers, new catalysts and the addition of other substances), polyurethanes can now be manufactured in a wide variety of textures and hardnesses.

However, due to the complexity of the thermo-chemo-rheological couplings taking place during processing, and the sometimes complex geometry of the molds used, process control and optimization is proving difficult. The analytical models found in the literature are inadequate to account for the complexity of these phenomena. Due to the concomitance of mechanisms (chemical reactions, rapid and heterogeneous temperature evolution, strong rheological couplings, 3D flows), only a model including all these elements has a chance of describing this type of process. The aim of this dossier is therefore to set up, from an engineer's point of view, a systematic protocol for identifying the major thermomechanical parameters of cellular polyurethane, and their use in a 3D model, presented here, capable of accounting for the evolution of polyurethane foam properties during the process and predicting, at least on average, the foam's cellular microstructure.