ABSTRACT
The oxidative ageing process is governed by a complex multiple-stage radical mechanism. As it is experimentally very difficult to determine the rate of every reaction that contributes to the ageing process, a simplified model has to be developed. For this work a very simple model is used. It takes oxygen, the elastomer (i.e. monomers which are able to react with oxygen) and the anti-ageing agent into account. The model considers the following two reactions:
O elasto e inactive products2 1⎯m r →⎯ , k (1)
O antioxidant inactive products2 2⎯ →⎯ . k (2)
Thus, oxygen reacts with the elastomer and the antioxidant. This scheme leads to the following differential equations for the concentrations of the considered substances due to diffusion and reaction (Atkins & Paula 2006):
o o o m o o aoc D c k c c k c c2 2 2 2 21 2 = Δ − , (3)
1 INTRODUCTION
The majority of rubbery components is subjected to environmental influences. Especially components exposed to air change their behaviour due to chemical ageing processes which are initiated by the diffusion and reaction of oxygen. The primary mechanisms leading to the modification of the material properties are chain scission and the creation of new crosslinks which are caused by a complex multiple-stage radical reaction (Bolland 1949). To slow down the degradation processes, antioxidants are added, which mainly act as free radical chain stoppers (Huntink, Datta, & Noordermeer 2004). In the outer regions with high oxygen concentration the antioxidant is consumed. Due to the evolving concentration gradient, diffusion of the antioxidant occurs. Hence, to simulate the chemical ageing process realistically, diffusion and reaction of oxygen and antioxidants have to be taken into account.
