PhD Candidate: Iftekhar Ahmad
Thesis Title
Mathematical Models to Describe Antioxidant Depletion in Polyethylene-Clay
Nanocomposites under Thermal Aging
Advisor: Richard Cairncross, PhD
Co-Advisor: Grace Hsuan, PhD
Date & Time: Monday, December 1st, 2014; 10:00AM
Location: CBE Conference Room, CAT-267
Abstract:
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. In this research
mathematical models have been developed to describe the underlying mechanisms of antioxidant
(hindered-phenol) depletion and to predict experimentally-measured antioxidant profiles in polyethyleneclay nanocomposites.
The mathematical model uses 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.