Low density polyethylene (LDPE) was crosslinked with dicumyl peroxide (DCP) by gas curing method. In order to prevent the formation of microvoids, crosslinking reactions were carried out under 50 psi pressure.
The reaction kinetics of the crosslinking process were evaluated by dynamic differential scanning calorimetry (DSC) method (measuring exothermic heat of reaction during crosslinking process). The kinetic equation obtained was
$\frac{dx}{dt}=1.5521\times10^{12}\exp(-30.5kcalmol^{-1}/RT)(1-x)^{0.5}$.
An empirical correlation was obtained between the conversion x and the soluble fraction (sol %) in the crosslinked polymer. Thus one can actually calculate the expected sol % of a specific crosslinking process if the reaction temperature, time, DCP concentration and $\bar{M}_n$/polydispersity of LDPE are known.
The degree of crosslinking, density, crystallinity and mechanical properties of the crosslinked polymers were also studied. The degree of crosslinking, density and crystallinity of the crosslinked polymers decreased with increasing DCP concentration. The elongation at break and tensile strength were found to be influenced by the combined effect of crosslink concentration and crystallinity change. The elongation at break reaches maximum at 1.0 PHR DCP concentration. The tensile strength reaches maximum at 1.0 PHR DCP concentration and decreases as the crosslinked polymer becomes brittle at higher crosslink concentration. The melting point depression was also noted as the degree of crosslinking increased and an empirical equation was developed correlating the $T_m$ and the crosslink initiator concentration.
