Gelation and Fusion Characteristics of PVC Resins in Plastisols by Determination of Their Viscoelasticity

Abstract

Abstract A poly(vinyl chloride) (PVC) plastisol is a suspension of fine particles of PVC dispersion resin in a plasticizer. The particle size ranges typically from about 15 to 0.2 µm and smaller. The PVC particles in the plastisol consist of the primary particles, which are products of polymerization, and of the agglomerates of the primary particles, which result from the spray drying and subsequent grinding. The commercial applications of plastisols are coatings, films, sheets, foams and rotational castings, where the plastisol is first spread on substrates, and then heated to gel and fuse. PVC dispersion resins are used to produce a variety of formulations, including those for automotive applications, coated fabrics and resilient flooring. Requirements for the plastisols are many; for example, good air release, viscosity stability, fine particle size, foamability and good heat stability. In general, the processability requirements emphasize the rheological behavior at room temperature and the gelation-fusion behavior, the latter being the subject of this paper. When a PVC plastisol is heated, the resin particles are penetrated by the plasticizer and become swollen. Consequently, the system becomes a dry, coherent mass; at this point the material has not attained its ultimate strength. This stage is called gelation. Upon continued heating to a higher temperature (e.g., 195°C) the microcrystallites of PVC melt and the polymer molecules fuse together more uniformly. Upon cooling, the material exhibits the intrinsic strength of a plasticized PVC. The rates of gelation and fusion are important and are often critical aspects of the processing of the plastisol. For example, the rate of gelation controls the residence time needed to build the wall thickness desired in slush molded boots or hot dip molded pieces. Gel rate also affects the uniformity of rotocast parts and the penetration of plastisol into fabric in cast coating operations. Fusion rate and the rate of decrease in melt viscosity after fusion affect uniformity of cells in chemically blown foams. Therefore, many commercial grades of PVC dispersion resins are offered, each having a different rate of gelation, fusion and ultimate properties.