A STUDY OF THE ROTATIONAL MOULDING OF LINEAR LOW DENSITY POLYETHYLENE

The aim of this investigation was to increase understanding of the rotational moulding process by studying the rotational moulding of linear low density polyethylene, and to thus contribute to the optimisation of the process. A test moulding was selected as the standard. It was established that the heating cycle of the moulding can be divided into three well defined phases; mould induction, polymer fusion and melt densification. The processing conditions to give optimal physical properties in the moulding were determined as 1 ± 0.5 minute mould induction time, 5 ± 0.5 minutes fusion time, 7 ± 0.5 minutes densification time and a cooling cycle of 5 ± 0.5 minutes using a water spray. The optimum oven set temperature during the heating cycle was determined as 2200 C. A quantitative study of the effects of oven set temperature and polymer mass on the three phases of the heating cycle was then made. The oven set temperature was found to have the greatest effect on the melt densification phase. Increasing the temperature from 2200 C to 3300 C reduced the melt densification time from 6 ± 0.5 minutes to 1 ± 0.5 minutes. The oven set temperature of 330°C was found to be significant, in that the melt densification time was found to remain constant after this temperature, even for increasing polymer mass. Increasing the oven set temperature from 2200 C to 4400 C reduced the heating cycle from 12 ± 0.5 minutes to 4 ± 0.5 minutes, without loss in yield stress. At 2200 C oven set temperature, the melt densification phase was found to be the longest and hence the rate determining step of the heating cycle. At 4400 C the fusion phase was measured as the longest phase. The fusion time of the moulding was found to be directly proportional to the polymer mass. However, contrary to expectation, the overall heating cycle did not show a linear relationship with increasing polymer mass. During the fusion process particle boundaries were found to be completely eliminated, contrary to some reports that complete boundary elimination is not possible in low density polyethylene. The yield stress at oven set temperatures of 2200 C and 3300 C was found to be unaffected by the presence of bubbles in the moulding wall. However, elongation at break was reduced at the higher temperature. The degree of crystallinity and spherulitic size were found to increase at the higher processing temperature of 4400 C, and a slower cooling rate (achieved by rotating mould in ambient air conditions for 15 ± 0.5 minutes.) A description of polymer fusion processes in a typical heating cycle has been made. A statement of the heat transfer processes in a typical moulding cycle, was made for any future mathematical modelling of the process. The thermal properties of linear low density polyethylene and mould necessary in heat transfer calculations were also established, and the thermal conductivity of the polyethylene measured. A method for measuring the thermal conductivity of poly ethylene powder under steady state conditions was developed. Finally, the isotropy of physical properties of rotational mouldings (in the plane of the wall), until now only assumed but not proven, was established by measuring stress at yield of rotationally moulded samples cut at right angles to each other.