HDPE liquefaction: Random chain scission model

Liquefaction of commodity polymers to oils and gases can be used to recover the energy value of these materials. This article reports liquefaction data for high-density polyethylene (HDPE), one of the major plastics in recycled material. Thermal degradation of HDPE to oil-gas mixtures required higher temperatures (450-490°C) than low-density polyethylene (LDPE) (430-460°C) because of fewer chain branching points for HDPE, which are more susceptible to chain scission reactions. The addition of hydrogen (0.1-1.5 MPa) had negligible effect on product distribution. HDPE thermal degradation is consistent with a random chain scission mechanism. Product distributions for degradation at 450°C were modeled assuming random chain scission with a rate constant k(x) dependent on the molecular weight x by a power law model dependence, k(x) = k_b x^b, where k_b is the pseudo-first-order rate constant, and b is the power index of dependence on molecular weight. Degradation rates dropped rapidly after initial breakup of the chains, and 2 sets of coefficients were needed to describe the molecular weight distributions as functions of reaction time. The error in model was about 10%. This model can be used to optimize the production of oils from thermal degradation of HDPE.