Impingement mixing and its effect on the microstructure of RIM polyurethanes

The effect of impingement mixing on the microstructures developed during a reaction injection molding process in a thermoplastic urethane system was investigated. The polyurethane studied was a 4,4′‐diphenylmethane diisocyanate/l,4‐butane diol/poly(propyleneoxide) end‐capped with poly(ethy1eneoxide) polyol system with a 5/4/1 molar ratio. Three different impingement mixing levels ranging from Re = 80 to Re = 210 were implemented by a laboratory RIM machine. The samples were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), optical microscopy, and transmission electron microscopy (TEM). Morphologies of hard‐segment globules, hard‐segment spherulites, and soft‐segment rich matrix were observed. The multiple DSC endotherms were related to different crystalline structures. A higher level of mixing was found to increase the molecular weight and produce more paracrystalline structures. A lower mixing level produced better phase separation and spherulitic structures. High temperature annealing caused transurethanization, which may allow molecular rearrangement and change the crystalline structures.