Chemically assisted degradation of polymers

Chemically assisted degradation of polymers is a type of polymer degradation that involves a change of the polymer properties due to a chemical reaction with the polymer’s surroundings. There are many different types of possible chemical reactions causing degradation however most of these reactions result in the breaking of double bonds within the polymer structure.

Examples of chemically assisted degradation

Degradation of rubber by ozone

One common example of chemically assisted degradation is the degradation of rubber by ozone particles. Ozone is a naturally occurring atmospheric molecule that is produced by electric discharge or through a reaction of Oxygen with solar radiation. Ozone is also produced with atmospheric pollutants reacted with ultraviolet radiation. For a reaction to occur, ozone concentrations only have to be as low as 3-5 parts per hundred million (pphm) and when these concentrations are reached, a reaction occurs with a thin surface layer (5 x10-7 metres) of the material. The ozone molecules react with the rubber which in most cases is unsaturated (contains double bonds), however a reaction will still occur in saturated polymers (those containing only single bonds). When reaction occurs, scission of the polymer chain (breaking of double covalent bonds) takes place forming decomposition products:

Chain scission increases with the presence of active Hydrogen molecules (for example, in water) as well as acids and alcohols. Along with this type of reaction, cross linking and side branch formations also occur by an activation of the double bond and these make the rubber material more brittle. Due to the increase in brittleness due to the chemical reactions, cracks form in areas of high stress. As propagation of these cracks increases, new surfaces are opened for degradation to occur.

Degradation of poly(vinyl) chloride (PVC)

Degradation can also occur as a result of the formation, and then breakage of double bonds, such as solvolysis in PVC(Peacock). Solvolysis occurs when a Carbon-X bond, with X representing a halogen, is broken. This occurs in PVC in the presence of an acid species. Active Hydrogen atoms will remove a Chlorine atom from the polymer molecule, forming Hydrochloric acid (HCl). The HCl produced may then cause dechlorination of adjacent Carbon atoms. The dechlorinated Carbon atoms then tend to form double bonds, which can be attacked and broken by ozone, just like the degradation of rubbers described above.

Degradation of polyester

Degradation of polyster may occur without the presence of the acidic catalyst that causes degradation of PVC. During hydrolysis water acts as the reactive catalyst instead of the acid. It causes degradation mainly at high temperature and pressure during processing.

In this process the water molecule will attack the C-O ester bond, splitting the polymer in half. The water molecule will then dissociate, with one Hydrogen atom forming a carboxylic acid group on the Carbon atom with the double bonded Oxygen, while the remaining atoms form an alcohol on the other chain end. These reactive products may also cause further degradation of the polymer chain. This chain scission lowers average molecular weight of the polymer, decreasing the number and strength of intermolecular bonds as well as the degree of entanglement. This will increase chain mobility, decreasing strength of the polymer and increasing deformation at low stresses.

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.

References

Cheremisinoff, P 1989, Handbook of Polymer Science and Technology, M Dekker, New York.
Weidner, S, Kuhn, G, Freiedrich, J, Schroder, H 1996, "Plasmaoxidative and Chemical Degradation of Poly(ethylene terephthalate) Studied by Matrix-assisted Laser Desorption/Ionisation Mass Spectrometry",Rapid Communications in Mass Spectrometry, Vol. 10, No. 1, pp. 40–46
Peacock, A, Calhoun, A 2006, Polymer Chemistry Properties and Applications, Hanser Gardner Publications Inc., Munich.
Mitra, S, Ghanbari-Siahkali, A, Amdal, K 2006, "A novel method for monitoring chemical degradation of crosslinked rubber by stress relaxation under tension", Polymer Degradation and Stability, Vol. 91, no. 10, pp. 2520–2526

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