Effect of chemical degradation and formulation stability on viscoelastic relaxation processes

Current accelerated stress relaxation tests indirectly show how a polymer network rearranges (viscoelastic considerations) to reduce the applied stress (thermodynamic considerations). During thermal acceleration however, many different processes occur in addition to the viscoelastic relaxation.

The rate of loss of formulation additives is often assumed to scale linearly with temperature (diffusion/out gassing/leaching), however, this has never been measured. A combination of experimentation and modelling are required to provide a modified age aware prediction.

Certain accelerated ageing test regimes study the chemical ageing of a network by thermo-oxidative degradation. It is known that a combination of polymer scission and / or cross-linking occurs to alter the state of the polymeric network (in addition to chemical modification). It is generally assumed that elastomeric materials stiffen (due to loss of viscous character) and thus the rate of stress relaxation diminishes however, this is not measured. It is therefore advantageous to understand how chemical degradation also directly affects the viscous relaxation process. This would require a combination of experimentation, modelling and also an understanding of formulation changes (i.e. leaching of additives, processing variables, additives, plasticisers) so that the complex behaviour exhibited by a dynamic system evolves due to the combination of physical, viscoelastic and chemical processes which occur through life. It is not fully understood how fillers (e.g. Silica, Boron, etc) alter these ageing processes. This is particularly so with highly filled soft AM materials where potential changes (e.g. hydrolysis, physical rearrangement) at the boron/binder phase could alter relaxation rates (and performance) in a complex way which is not fully understood.

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School of Mechanical Engineering Sciences