Chapter 21 - Polyolefin Blends: Rheology, Melt Mixing, and Applications

Abstract

Publisher Summary This chapter outlines the reasons why polyolefins are blended, modified rheologically, or melt mixed. Polyolefins constitute a group ofpolymers of complex macromolecular structure. Typical examples of commercial polyolefin blends are impact-resistant polypropylene, super thin, highly orientable film grades of polyolefins, polyolefins for high-speed cable covering, and blow molding of environmental stress cracking resistant containers. The chapter also highlights the behavior of polyolefin blends in processing. The behavior of a blend during processing and the properties of its products are influenced by mechano-chemical processes and the melt shearing in the processing equipment. Low-density polyethylene (LDPE) used in the manufacture of blends are usually delivered in granulate form prepared by mixing granulates from various batches. A polyolefin blend manufactured from LDPE displays excessively high extrudate swelling and low critical shear rate during processing. Processing leads to transformations in the super molecular structure of the blends. These transformations are divided into three main types—structure changes because of orientation effects, which may be further influenced by the thermal history of the melt; structural changes because of generation of fibrillar and lamellar forms; and redistribution of the blend components because of shearing flow. Properties of micro heterogeneous polyolefin blends display several characteristic features—nonlinear dependence on composition (non-additivity), strong dependence on the degree of orientation and rheothermal treatment, and substantial flexibility for change to suit particular applications using fillers and compatibilizers. The chapter concludes with a review of various analytical techniques that have been used not only for polyolefins but also for other systems as well.