Protection Against Chemically Assisted Degradation
Both physical and chemical barriers can be used to protect a polymer from chemically assisted degradation. A physical barrier must provide continuous protection, must not react with the polymer’s environment, must be flexible so that stretching may occur and must also be able to regenerate (after wear processes). A chemical barrier must be highly reactive with the polymer’s surroundings so that the barrier reacts with the environmental conditions rather than the polymer itself. This barrier involves addition of a material into the polymer blend during fabrication of the polymer. Due to this, the barrier addition must have a suitable solubility, must be economically feasible and must not hinder the production process. For the barrier to be activated, the addition must diffuse to the surface and so a suitable diffusivity is also required. There are four theories on how these types of barriers protect the polymer material:
- Scavenger Theory: the protective layer reacts with the ozone rather than the polymer.
- Protective Film Theory: the protective layer reacts with the polymer producing a thin film on the polymer surface which is inert and can't be penetrated.
- Re-linking Theory: the protective layer causes broken double bonds to be reformed.
- Self Healing Theory: the protective layer reacts with degraded polymer chains to form low molecular weight material which forms an inert film on the surface.
Of these theories, the Scavenger Theory is the most common and most important. However, more than one theory can act at the same time and the theory that takes place depends on the protective materials, the polymer and surrounding environment.
Read more about this topic: Chemically Assisted Degradation Of Polymers
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