Time‐dependent rheological behavior of low‐density polyethylene white color masterbatches under dynamic stress field

The time‐dependent behavior of low‐density polyethylene (LDPE) white color masterbatches (WCMBs), which were concentrated suspensions filled with titanium dioxide (TiO₂), was found using dynamic stress rheometer. The viscosity first decreased slightly with time then continuously increased with time, and T_g(δ) (δ was the angle of loss) decreased with time, which meant the time‐dependent behavior of the elastic contribution was more pronounced than that of the viscous contribution. The higher the experimental frequency and temperature, the more pronounced the viscosity increase. However, the higher experimental stress did not lead to pronounced viscosity increase, which was attributed to the existence of small defects at higher stress. The 30 wt % of TiO₂ content was critical to obvious time‐dependent behavior. The viscosity increase with time was related to the formation of a hard shell around the melt sample during the test. It was verified by thermogravimetric analysis that the TiO₂ concentration at the outer surface was higher than that at the core of the sample and, because the outer surface contained more TiO₂, a hard shell was formed, which impeded further deformation of the sample. This was completely different from the other reported systems with time‐dependent behavior. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2793–2799, 2002