Physical Ageing of Carbon Black Filled Rubber

Abstract

Filled rubber materials are employed in a variety of applications, from everyday use like rubber gloves, adhesives, or golf balls to high-performance scenarios like formula one tires, oil rig plants o-rings, or spacecraft seals.  Because of their widespread usage, a lot of effort has been put into the analysis of the different factors influencing their mechanical properties to obtain an accurate prediction of their performance and service life. Temperature arises as one of the main variables influencing the properties of rubber materials, which show brittle and stiff behaviour at low temperatures, and become soft and ductile with increasing temperature. 

Physical ageing refers to the equilibration process taking place after a temperature change when the properties of the material need additional time to shift from their initial state towards their final value at the new temperature. Previous literature indicates that the physical ageing process only happens at temperatures below the glass transition temperature, however, recent experimental results hinted at the physical ageing process taking place at room temperature for filled rubbers. This is a critical finding, since the existing physical ageing models can no longer be used to describe the behaviour of filled rubber, one of the most used products in engineering applications. 

In order to characterise the physical ageing process for filled rubbers at room temperature this research has followed two stages. 

First, experimental testing was conducted to characterise how the physical ageing process affects the mechanical properties by submitting a set of carbon black filled samples to a temperature change at room temperature. Next, their mechanical properties were monitored, showing that up to four days were needed to reach equilibrium at the final temperature. These findings ratify the presence of physical ageing at room temperature for filled rubber and throw new light on its effect on the viscoelastic properties over time.

The second stage focused on the development of a model able to capture the different physical ageing process that takes place for filled rubber. Two contributions were considered to reflect the effect of physical ageing on the mechanical properties, the free volume and the configurational changes. The evolution of the free volume was modelled by updating the Kari model with an additional γ parameter, in order to capture the longer physical ageing times needed to reach equilibrium for filled rubber. The configurational changes were introduced into the model via the α parameter, fitting the storage and loss modulus evolution measured experimentally. 

This research work provides a better understanding of the physical ageing process and its effect on the mechanical properties of carbon black filled reinforced rubber. The experimental findings together with the proposed physical ageing model contribute to a better prediction of the material mechanical properties, potentially leading to a more accurate prediction of their service life and a reduction in rubber waste, number of replacements and material consumption.

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