(485h) Reaction-Diffusion Model Describing Antioxidant Depletion in Polyethylene-Clay Nanocomposites

Polyethylene (PE) pipes are gradually replacing metal pipes in water and drainage transportations, but are inferior to metal pipes in mechanical properties. Many properties of PE are improved by adding clay to form nanocomposites. Long-term degradation of PE properties is primarily due to environmental oxidative degradation which is stabilized by antioxidants (AO). Although clay improves mechanical properties, in its presence AO fails to stabilize. Therefore a better understanding of underlying mechanisms of AO depletion in the nanocomposites can guide formulation of proper AO to ensure their long term durability. This paper presents research on modeling of AO (hindered-phenol) depletion in samples whose thicknesses are about 3 mm. The mathematical model considers reaction kinetic scheme for degradation and stabilization of neat PE and its nanocomposite. The model not only describes the AO depletion with time but also evolution of its profile throughout the depth of sample. Both chemical and physical losses of AO have been considered. Experimental ‘oxidation induction time’ (OIT) data showing variation in depletion with depth through the sample has guided development of the model. In nanocomposite samples there are two zones of AO depletion: a ‘core zone’ and ‘depleted surface zone’. In the ‘core zone’ AO depletion is slower and uniform, and in the ‘depleted surface zone’, AO depletes more rapidly to produce a surface layer which is void of active AO.

In the model, alkyl free radical oxidizes rapidly compared to O₂ diffusion. Due to limited amount of alkyl free radicals present in the samples, all of them are oxidized in time less than 30 days. Once all alkyl free radicals are oxidized, O₂ diffuses into the sample and uniformly distributes throughout it. The role of AO is to stabilize all those oxidized free radicals, and thus stop their propagation. But continuous depletion of AO is brought about by generation of more free radicals. These additional free radicals are generated by decomposition of hydroperoxides due to catalytic effect of clay. The hydroperoxide decomposition keeps on supplying free radicals in a cyclic fashion resulting in continuous depletion of AO.

The surface depletion of AO can be brought about by evaporation of AO to the surrounding air combined with slower rate of diffusion in the nanocomposites. This can also be brought about by even lower diffusivity of AO in highly clay-orientation layer adjacent to sample surfaces.

In addition, the model predicts AO depletion profiles for a number of different experimental conditions: (1) thermal aging in stagnant air; (2) thermal aging in inert atmosphere; (3) initial AO concentration beyond saturation limit; and (4) thermal aging at a different temperature.