These materials are mixed and fluxed in typical rubber machinery used for this purpose, such as a Banbury or Brabender Mixer. The instant invention, which differs from the usual practice by the prestabilization of the PVC, results in greatly increased mill time and reduced color. The ingredients comprising this blend are the same as those in current use; the marked and surprising difference in properties are the result of the process of prestabilization of the PVC.
The above remarks indicate the need in the art for new and improved stabilizer systems for polyvinyl chloride/nitrile rubber compositions. The improved stabilizer systems would impart the desired combinations of properties, including increased stability, as demonstrated by color stability and increased mixing time. An additional benefit of the stabilizer system of this invention would be increased efficiency in terms of time and expense of production of the PVC/NBR blend.
It is therefore an object of this invention to describe a process by which the PVC particulate and nitrile rubber is compounded with stabilizer and/or antioxidant in a high shear dry blending process. The shear should be sufficient to raise the temperature of the PVC particulate/stabilizer mixture to at least about 60° C but no higher than 80° C. The use of this pre-stabilized PVC particulate in the standard process yields a fluxed blend with significantly enhanced thermal stability leading to increased mixing (mill) time and better color stability.