Analysis On Correlation Of Plasticized PVC Materials Slush Molding, Structure And Performance

The instrument panel skins produced by powder slush molding processes are widely used in various types of middle and high-end cars. The optimal design of slush powder and its thermal processing performance can promote the manufacturing localization of high-end cars, and achieve energy saving and emission reduction, In this paper, the slush molding of PVC plastisol is first analyzed. 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, and affect the quality of products. On the basis of the unsteady temperature fields of the nickel mould and PVC material simulated through finite element method by the software COMSOL Multiphysics, the evolution and distribution of PVC material structures are calculated according to a theoretical model fitting well with experimental results, and then the viscosity field 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 that PVC plastisol closer to the 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 the 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 viscosity field, 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. Then both scanning electron microscopy and particle size analyzer is used to observe the surface morphology and particle size distribution of two kinds of slush powders. The compositions of PVC slush powder and the PVC molecular weight are determined by means of gel permeation chromatography (GPC) after its separation and purification. The melt and combination of the powder during the formation processes of the slush molded skins are observed by polarizing microscope and melt flow rate experiments. Through HAAKE rheology studies the structure-performance relationship of slush powder is analyzed. The effect of temperature, feeding quantity and types of powder on the rheological properties is also investigated. Based on that, dynamic mechanical analysis (DMA), stress relaxation experiments and Haward model are then used to study the physical and mechanical properties of the skin. It is found that the sizes of these powders are about150μm. The powder contains abundant plasticizers because of which the glass transition temperature Tg of the skin is quite low. There is a critical temperature above which the melt and combination of the powder accelerate. The melt flow properties of the two kinds of powder have large difference. The powder undergoes complex physical and chemical changes in the HAAKE rheometer, temperature and feeding quantity affect this process significantly. Stress relaxation experiments can eliminate the interference of viscous force. Based on that, the elastic network structure of the skin and its Gaussian modulus can be well represented by Haward model.At last, pyrolysis gas chromatography/mass spectrometry (PGC-MS) is used to analyze the pyrolysis products and contents of the PVC slush powder. Combined with thermal gravimetric analysis (TGA) experiments, the thermal degradation characteristics are studied. The effects of heating rate, atmosphere and slush molding on the pyrolysis of slush powders are discussed. Combined with X-ray fluorescence spectrum experiment (XRF), the degradation kinetics is analyzed. The results indicate that, the thermal degradation process of PVC slush powder can be divided into two stages:the first degradation stage, thermally stable stage and the second degradation stage. The first degradation stage is the main stage with a weight loss of about75%, which is mainly caused by the dehydrochlorination of PVC and the evaporation of plasticizer, the thermal degradation of phthalate plasticizers can also exist and be catalyzed by the presence of HCl. In the range of the slush molding temperature, the stability of the slush powders is fine. The interaction between PVC and plasticizer makes the thermal degradation process more complicated. The heating rate affects greatly the powder pyrolysis, and the TG and DTG curves shift to higher temperatures with the increase of the heating rate, while the ratio of weight loss at each stage remains almost the same. The degradation of slush powder becomes easier in oxygen-containing atmosphere than in nitrogen, with a larger ratio of total weight loss. Compared with slush powder, the stability of the polymer-based skins is improved because that plasticizer can act as a protection coating of the PVC macromolecules. In addition, the physical gel network in the plasticized PVC products resulting from macromolecular chains entanglement and microcrystalline can also affect the stability of the polymer-based skins. The study on the kinetics at the first degradation stage reveals that the activation energy and reaction order are smaller in nitrogen atmosphere than in oxygen-containing atmosphere.