Degradability and Degradation Mechanism

The term biodegradation of plastics is often used for totally different mechanisms of degradation phenomena and it is essential for a rational discussion about biodegradable plastics to differentiate properly between these different mechanisms. 

When plastics come in contact with microorganisms it can cause changes (negative) in the properties of plastics. Typical material parameters which are altered by the microbial action are mechanical properties, (e.g., tensile strength, flexibility), or simply the colour. Biocorrosion is usually an unwanted process which is to be avoided, e.g., by the addition of biocides to the material. The mechanism of biocorrosion is often a selective degradation of one component of the plastic, e.g., a plasticiser, and is accompanied in many cases by other, non-biological mechanisms such as hydrolysis or oxidation. 

Biocorrosion can lead to a complete deterioration of the material to (invisible) particles. 

Polymers for medical applications which are degraded in living tissnes or in the environment of the living body are also called biodegradable, becanse the degradation takes place in a biological environment However, very often degradation mechanisms here are solely abiotic, (e.g., non-enzymically catalysed hydrolysis). Other expressions used in this context 

For that reason microorganisms excrete enzymes into the environment, which can attach to the polymer surface and cleave the polymer chains, as long as the degradation products become short enough to be water soluble (this biological system has been developed by the evolution to use natural polymers or other poorly bio-available substrates for microbes). Then these intermediates can diffuse into the surrounding environment of the plastics, be incorporated into the microbial cells and metabolised there to form biochemical end products such as water and carbon dioxide (and many others). 

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Although there is a substantial body of literature on the degradation of elastomeric materials (Bousquet and Fouassier, 1983 Vondrdiek and Dolezel, 1984), few reports have been pubUshed on degradation and degradation mechanisms in the PEMFC environment (Tan, 2007a,b), and even these papers are all related to the degradation of sihcone seals, partly because these are easily available and have been widely used in the PEMFC sector. Unfortunately, almost all of the reported results are not particularly positive. 

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