Nonisothermal Melting and Crystallization of Foamed Crosslinked Polyethylene

Abstract

Foamed crosslinked specimens of low density polyethylene are produced by hot mould injection moulding. Dicumylperoxide, as crosslinking agent and azodicarbonamide as blowing agent, were used. The three components are blended and granulated in advance in an extruder-granulator at temperatures of 100-125°C. The contents of blowing and crosslinking agents in the composites studied are 5-15 and 0.08-0.6, respectively, in percents by weight. The ability for maximum foaming was studied by determining the overall density of the specimens. The crosslinking degree was calculated from the equilibrium strain value defined at temperature 140°C and pressure stress 0.045 MPa. The same parameter was estimated also by the melt flow index. The LDPE morphological parameters were determined on the basis of DSC-graphics obtained by a Perkin-Elmer DSC-7 differential scanning calorimeter at heating and cooling rates of 10°C/min. The activation energy for transport through phase boundary crystal melt, the surface energy of the crystal, the long period and the degree of the crystallinity were calculated by computer programs using formulae of non-isothermal melting and crystallization kinetics theory.

It is found that the overall density of the specimens increases with the increase of the crosslinking agent concentration in the range of 0.08-0.6% by wt. At low concentrations of dicumylperoxide, the increase of the blowing agent concentration results in a decrease of the density, while at high crosslinking agent concentration the opposite trend is observed. The crosslinking degree of the foamed polyethylene increases with the increase of the concentrations of dicumylperoxide and azodicarbonamide, and the flowability of the foamed polyethylene melt decreases correspondingly. The crosslinking of the foamed LDPE affects the non-isothermal phase transitions. With the increase of the crosslinking degree, the phase transition temperatures and the degree of crystallinity decrease. The values obtained for the energies of activation for transport through the phase boundary, the surface energy of the crystal and the long period are comparable with the values obtained by other authors.