(Q): What are compounding requirements of single screw extruders?

(A): Mixing can, for convenience, be divided into "dispersive mixing" and "distributive mixing". 1. Dispersive mixing: "An operation that reduces the agglomerate size of the minor constituent to its ultimate particle size." 2. Distributive mixing : "An operation that increases the randomness of the spatial distribution of the minor constituent within the major base with no change in the size of the minor particle." The Preparation of a masterbatch (concentrate) is primarily a dispersive mixing operation. The dilution of a masterbatch in natural polymer is a distributive mixing operation. Good dispersion requires mixing conditions quite different from those required for good distribution. Dispersion is achieved at a specialized masterbatch preparation stage where high shear forces are applied. Distributive mixing is achieved by diluting with natural polymer in randomizing "stirring" mixing process. The dilution stage could be a second compounding stage but more often it is carried out during conversion to product by injection molding, film extrusion, bottle blowing, etc. In some cases, both operations are carried out as one within a compounding machine. Single-screw extruders are intrinsically very limited in both their dispersive and distributive mixing abilities but good dispersion can often (but not always) be achieved by using specialized additives, whereas, distributive mixing can, with retrofitted mixing devices, equal any twin-screw compounder.

(Q): What are challenging issues in the polymer processing areas in the industry?

(A): Any success heavily depends on the understanding of the processing for a product and polymer physics within that equipment. The analysis of the melting mechanisms in an extruder or banbury mixer or any other plasticating equipment requires the reliable physical property data for a given polymer resin. For blends of polymers, we need the data for the product as well as its components. A lot is understood about the elements, of mixing, the role of rheology in mixing, the nature of dispersive and distributive mixing. Accurate in-line measurement of shear stress and temperature is very important not only for current processes but also for the future attempt in developing intelligent processing of Polymeric Materials.  

(Q): What are the problems in the processing of recycled PET?

(A): Recently, increasing interest has been focused on the recycling of plastic wastes, especially poly (ethylene terephthealate) (PET) scraps. The main problem faced during processing of recycled PET is degradation resulting from the simultaneous presence of retained moisture and contaminants [adhesives and poly (vinyl chloride)(PVC)]. They latter generate acidic compounds at processing temperatures, catalyzing hydrolytic ester bonds cleavage, leading to lower molecular weights and higher amount of carboxylic end groups. PVC-rich recycled PET reaches very low melt viscosity after less than 10 min. in processing conditions, while virgin PET retained high viscosity even after 30 min.

Courtesy: Mr. P.M. Jariwala, Kolsite Industries