Rotational moulding of metallocene polyethylenes

This paper compares the rotomouldability and mechanical properties of conve ntional polyethylenes with metallocene polymers having a similar melt flowr ate or the same density. The rotomouldability has been evaluated in terms o f bubble removal, cycle times, and heat stability during the moulding proce ss. The mechanical properties, such as tensile strength and peak impact ene rgy, were measured on parts produced using a variety of moulding conditions . It was found that the metallocene polymers display greater toughness if s ubjected to the same moulding conditions as the conventional polymers. Conv ersely, the same toughness can be obtained in both materials if shorter cyc le times are used for the metallocenes. This behaviour has been linked to t he 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 polar ising light microscopy. The differences between the molecular structures of the two types of polyethylene can be used to explain why the metallocene m aterials possess many rheological and other characteristics that are desira ble in rotational moulding.