This invention relates to stabilizing the viscosity of molten vinyl chloride polymers. This invention further relates to a means for extending the useful processing time of molten vinyl chloride polymers.
Polyvinyl chloride and most vinyl chloride copolymers are thermoplastic materials which melt and flow under the proper conditions of heat and pressure and can thereby be formed into useful articles. Conventional shaping techniques include calendering, injection and compression molding and extrusion. The temperatures employed to shape vinyl chloride polymers using these techniques are conventionally 200.degree. C. and above. Maintenance of a substantially constant melt viscosity throughout the forming operation is critical if the articles produced are to exhibit uniform size, weight and other physical characteristics at high output rates. Variations in melt viscosity in the blow molding of bottles, for example, will result in bottles of varying wall thickness. In pipe extrusion, variations in wall thickness with weakening of the pipe results. If the viscosity decreases, extrusion rates will decrease resulting in increased costs; if the viscosity increases, more power will be required to operate the equipment. With some polymers, such as those based on vinyl chloride, the increased viscosity will lead to increased frictional heat and consequent degradation of the polymer. This degradation may be evident as changes in color, brittleness and in extreme cases, the polymer will cease to move out of the equipment, back pressure will increase and the equipment may be damaged. It is an object of this invention to provide polymer additives which control the melt viscosity of thermoplastic materials during processing. These additives may also improve other properties such as color. In addition, physical properties of the finished articles may be more uniform.
It has now been found that certain inorganic and organic polysulfides containing linear chains of three or more sulfur atoms are effective melt viscosity control agents for vinyl chloride polymers. Various elements or organic radicals are bonded to the two terminal sulfur atoms of the chain. These additional atoms or groups may impart other desirable properties to the vinyl chloride polymer.