Effect of volume loading and surface treament on the thixotropic behavior of polypropylene filled with calcium carbonate

The rheological properties of calcium carbonate-filled polypropylene were e xamined using a Rheometrics dynamic analyzer. The study included steady she ar test, transient stress growth test with sequential deformation history, and two-step dynamic oscillatory shear flow. Thixotropic behavior was obser ved in transient tests for highly filled compounds when volume loading exce eding a critical value at about 20%. The material responses of these viscoe lastic thixotropic materials depend on the duration of shear as well as on the rate of shear. The effects of filler on the rheological behavior of hig hly filled compounds are dominant at low strain rates; however, the effects of activity of the filler are almost negligible at high strain rates becau se of complete breakdown of the filler network. The timescales for structur al changes in filled systems often become long compared with the viscoealst ic time constants of the unfilled melt. The magnitudes of rheological prope rties and the degree of hysteresis appear to increase with increasing volum e loading of filler particles. Conversely, surface treatment of fillers, wh ich presumably reduces interaction between filler particles and the extent of agglomeration, results in major reductions of both rheological propertie s and the degree of hysteresis. The diverse experimental observations are i nterpreted in terms of a system forming a filler network due to weak interp article forces. The thixotropy resulting from breakdown and recovery of the filler network is dependent on the characteristic time of the individual t est.