Mathematical Models to Describe Antioxidant Depletion in Polyethylene-Clay Nanocomposites under Thermal Aging

Antioxidants are typically added to polyethylene to extend its durability, and recently clay nanoparticles have been blended into polyethylene to improve mechanical properties. However, the clay nanoparticles also accelerate the rate of antioxidant depletion in polyethylene. This thesis presents mathematical models that describe the underlying mechanisms of antioxidant (hindered-phenol) depletion and predict experimentally-measured antioxidant profiles in polyethylene-clay nanocomposites.;The mathematical models use a reaction kinetic scheme that includes free radical initiation and propagation reactions, antioxidant stabilization reactions and free radical termination reactions. In the model, alkyl free radicals oxidize rapidly. The role of antioxidants is to stabilize the oxidized free radicals to hydroperoxides, and interrupt propagation reactions. However, in nanocomposites, continuous depletion of antioxidant is caused by the clay acting as a catalyst to decompose hydroperoxides and regenerate alkyl free radicals. This cyclic hydroperoxide generation and decomposition leads to much faster antioxidant depletion in polyethylene nanocomposites. Phenoxyl radicals of antioxidants generated by stabilization reactions contribute to terminate polymeric free radicals and limit their accumulation.;The model also describes diffusion of antioxidant and oxygen within the samples and loss of antioxidant by evaporation or blooming to the surface of neat polyethylene. In nanocomposite samples there are two zones of antioxidant depletion: a 'flat core zone' and 'depleted skin zone'. In the flat core zone antioxidant depletion is slower and uniform, and in the depleted skin zone, antioxidant depletes more rapidly to produce a skin layer which is void of active antioxidant. The diffusion-reaction model describes this by combining several factors such as variable diffusivity caused by clay orientation at skin and additional generation of alkyl free radicals at the skin layer.;The model also predicts antioxidant depletion profiles for a number of different experimental conditions including aging in air or inert atmosphere that cause different profiles of antioxidant concentration.