金属茂熔体鲨鱼皮不稳定性过程中的带电
Electrical charging during the sharkskin instability of a metallocene melt
摘要：
流动不稳定性由于其经济和理论意义而被广泛研究，然而关于挤出过程中聚合物带电的研究成果很少。然而，电荷的产生是聚合物熔体和模壁之间相互作用的特征。在我们的研究中，通过碳化钨模具的茂金属聚乙烯（mPE）的毛细管挤出的特点是通过精确的电测量感谢法拉第桶。没有观察到显著的电荷，因为挤出物表面保持光滑。然而，一旦鲨鱼皮扭曲出现，可测量的电荷被收集（约5 10-8 C/m2）。在喷射或粗熔体破裂（g.m.f）缺陷期间测量更高水平的电荷。这项工作的重点是电荷在鲨鱼皮不稳定性。电荷的变化与表观壁面剪应力的影响，为不同的模具几何形状。该曲线呈现线性增加，紧接着在突峰不稳定性开始之前突然增加。这种突然的充电也对应于鲨鱼皮不稳定性的结束。同样众所周知的是，壁面滑移恰好同时出现，其速度值小于喷射流期间的速度值。我们的研究结果表明，带电可能是一个签名的壁滑。我们还表明，带电曲线可以根据时间-温度叠加原理移动，从而得出聚合物的分子特征也参与这一过程的结论。
英文摘要：
Flow instabilities are widely studied because of their economical and theoretical interest, however few results have been published about the polymer electrification during the extrusion. Nevertheless the generation of the electrical charges is characteristic of the interaction between the polymer melt and the die walls. In our study, the capillary extrusion of a metallocene polyethylene (mPE) through a tungsten carbide die is characterized through accurate electrical measurements thanks a Faraday pail. No significant charges are observed since the extrudate surface remains smooth. However, as soon as the sharkskin distortion appears, measurable charges are collected (around 5 10-8 C/m2). Higher level of charges are measured during the spurt or the gross-melt fracture (g.m.f) defects. This work is focused on the electrical charging during the sharkskin instability. The variation of the electrical charges versus the apparent wall shear stress is investigated for different die geometries. This curve exhibits a linear increase, followed by a sudden growth just before the onset of the spurt instability. This abrupt charging corresponds also to the end of the sharkskin instability. It is also well-known that wall slip appears just at the same time, with smaller velocity values than during spurt flow. Our results indicate that electrification could be a signature of the wall slip. We show also that the electrification curves can be shifted according to the time-temperature superposition principle, leading to the conclusion that molecular features of the polymer are also involved in this process.
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