This article studied the dripping behavior of eight polymers under UL94 vertical test conditions. The results suggested two different dripping behaviors: Type I, dripping with uniform- and small-sized drops with the short first dripping time, and Type II, dripping with irregular- and large-sized drops with the long first dripping time. Polymers of Type I dripping had dominant decomposition mechanism of random-chain scission, low activation energy of viscous flow, and high ratio of effective heat of combustion to heat of gasification. Otherwise, Type II dripping dominates. The surface tension at ambient temperature and the melt flow index at processing temperatures were not as important to dripping as expected. It was found that talc could convert the dripping of low-density polyethylene from Type I to Type II at a critical talc loading level of 20 wt%, which was ascribed to the reduction in the burning rate and the formation of an integral residue. Finally, a generalized model was presented, and a derived correlation showed that the drop mass was a power law function of the dripping time.
The physical and chemical effects of ultrasound on polypropylene (PP) melts in extrusion were investigated. By applying ultrasound vibration to the entrance of the die, apparent pressure and viscosity of PP can be obviously decreased under the appropriate ultrasound power. Ultrasound has both physical and chemical effects on the polymer melt. In our study with specific polymer and ultrasound system, we determined that the chemical effect makes up 35-40% of the total effect of ultrasound on the apparent viscosity reduction of PP melts at most of the studied intensities. The physical effect plays a more important role in the ultrasound-applied extrusion than the chemical effect. This chemical effect is an irreversible and permanent change in molecule weight and the molecular-weight distribution due to ultrasound. As the ultrasound intensity increases, the molecular weight of PP reduces and its molecular-weight distribution becomes narrower; the orientation of PP molecules along the flow direction reduces (in melt state) and the crystallinity of PP samples (in solid state) decreases by applying the ultrasound vibration. Ultrasound vibration increases the motion of molecular chains and makes them more disorder; it also affects the relaxation process of polymer melts by shortening the relaxation time of chain segments, leading to weakening the elastic effect and decreasing the extruding swell ratios. All the factors discussed above reduce the non-Newtonian flow characteristics of the polymer melt and result in the viscosity drop of the polymer melt in extrusion.
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