旋转模塑过程中PVC溶胶物理凝胶化转变的有限元分析

FINITE ELEMENT ANALYSIS OF THE PHYSICAL GELATION PROCESS OF PVC PLASTISOL DURING ROTATIONAL MOLDING

采用有限元法,数值模拟了成型过程中镍制模具和PVC材料的非稳态温度场,进而得到了PVC材料结构参数的时间演变和空间分布规律,再由结构-性能关系获得了PVC材料的溶胶黏度及其演变特点,在此基础上比较分析了不同增塑剂对PVC材料物理凝胶化转变过程的影响.结果表明,由于热阻不同,靠近模具壁的PVC溶胶升温速率较大,首先发生凝胶化转变;加热过程中,PVC材料的黏度首先降低至最小值,凝胶化转变开始后,PVC树脂和增塑剂之间的相互作用增强,材料黏度迅速升高;不同增塑剂对材料结构参数和溶胶黏度的演变规律有着不同影响.

The physical gelation of PVC plastisol is an important procedure in rotational molding processes,at the early stage of which the variation characteristics of plastisol viscosity determine its distribution on the mold surface,then affect the quality of products.On the basis of the unsteady temperature fields of the nickel mould and PVC material simulated by finite element method,the evolution and distribution of PVC material structures are calculated according to a theoretical model fitting well with experimental results,and then the plastisol viscosity and its evolution characteristics are obtained by the material structure-performance relationship.Hence the effect of different plasticizers on the physical gelation process of PVC materials is compared and analyzed.The results show the that PVC plastisol close to a mold surface,gets faster heating rate and earlier gel transition because of its smaller heat resistance.During the heating process,the viscosity of the PVC material is first reduced to a minimum,then increases rapidly due to the start of the gel transition.The micro mechanism of the gelation process is that the solution of PVC in the plasticizer and the swelling of PVC particles by the plasticizer are both strengthened,while the macro picture is a sharp increase in viscosity which makes the material stop flowing.Different plasticizers have different impacts on the evolution of material structure and plastisol viscosity,which is mainly due to their different lengths of chains.For the rotational molding process,the PVC thickness distribution depends on three important factors appeared during the physical gelation process of PVC plastisol： the minimum viscosity determined by the plastisol,the rate of the gelation process and the gelation temperature.