The influence of fatty acid coating on the rheological and mechanical properties of thermoplastic polyurethane (TPU)/nano-sized precipitated calcium carbonate (NPCC) composites

TPU was reinforced by two types of NPCC particulates (NPCC201 and 401) through melt compounding. Thermal analysis suggested that all the stearic acid coating on NPCC201 was attached to the particulate surface by forming calcium stearate, whereas the fatty acid coating of NPCC401 was excessive and part of the coating existed as free fatty acids. The tensile modulus of TPU/NPCC composites increased slightly with the addition of NPCC, while the toughness showed a larger increase. TPU/NPCC401 showed a unique rheological behavior which has not been reported before. In dynamic shear, G' increased with particulate loading at low frequencies but decreased at high frequencies. In steady shear, viscosity decreased with NPCC loading and an additional Newtonian plateau was observed at low shear rates. The free fatty acid coating on NPCC401 surface was believed to dissipate into the TPU matrix and generate this unique behavior. These results show the importance of achieving optimal coating. Full coverage of the particulate surface is required to achieve optimal coating effects, but over-coating should be prevented to avoid the deterioration of properties.