Rotational moulding of metallocene polyethylenes

This paper compares the rotomouldability and mechanical properties of conventional polyethylenes with metallocene polymers having a similar melt flowrate or the same density. The rotomouldability has been evaluated in terms of bubble removal, cycle times, and heat stability during the moulding process. The mechanical properties, such as tensile strength and peak impact energy, were measured on parts produced using a variety of moulding conditions. It was found that the metallocene polymers display greater toughness if subjected to the same moulding conditions as the conventional polymers. Conversely, the same toughness can be obtained in both materials if shorter cycle times are used for the metallocenes. This behaviour has been linked to the relaxation behaviour of the molecules in the solid state. Other material parameters, such as the nature of the comonomer, the long chain branching density, and the crystalline morphology were studied by NMR, DSC, and polarising light microscopy. The differences between the molecular structures of the two types of polyethylene can be used to explain why the metallocene materials possess many rheological and other characteristics that are desirable in rotational moulding.