Polymer degradation

Polymer science
Properties Architecture Tacticity Morphology Degradation Phase behavior Mark–Houwink theory UCST LCST Flory–Huggins solution theory Coil–globule transition
Synthesis Chain-growth polymerization Free-radical polymerization Controlled radical polymerization ATRP RAFT Nitroxide-mediated radical polymerization Step-growth polymerization Condensation polymerization Addition polymerization
Classification Functional type Polyolefin Polyethylene Polypropylene Polyisobutylene Polyurethane Polyester Polycarbonate Vinyl polymers PVC PVA PVAc Polystyrene Structure Homopolymer Copolymer Gels Hydrogels Self-healing hydrogels
Characterization GPC FTIR X-ray crystallography DSC NMR TGA DMA Rheology Rheometry Viscometry
Scientists Flory Heeger MacDiarmid Shirakawa Natta Edwards de Gennes Ziegler Staudinger Goodyear Baekeland Hayward Braconnot
Applications Industrial production Extrusion Blow molding Applied coatings Protective Coatings 3D printing Consumer products Tires Whitewalls Cookware and bakeware Bakelite Food Container Vinyl record Kevlar Plastic bottle Plastic bag
v t e

Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling.^[1] The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours.

Technologies have been developed to both inhibit or promote degradation. For instance,

Susceptibility

Plastics exist in huge variety, however several types of

These plastics are all

Thermoplastic polymers (be they virgin or recycled) must be heated until molten to be formed into their final shapes, with processing temperatures anywhere between 150-320 °C (300–600 °F) depending on the polymer.

Polymers are often subject to more than one round of melt-processing, which can cumulatively advance degradation. Virgin plastic typically undergoes compounding to introduce additives such as dyes, pigments and stabilisers. Pelletised material prepared in this may also be pre-dried in an oven to remove trace moisture prior to its final melting and moulding into plastic items. Plastic which is recycled by simple re‑melting (mechanical recycling) will usually display more degradation than fresh material and may have poorer properties as a result.^[3]

Thermal oxidation

Although oxygen levels inside processing equipment are usually low, it cannot be fully excluded and thermal-oxidation will usually take place more readily than degradation that is exclusively thermal (i.e. without air).

Heating polymers to a sufficiently high temperature can cause damaging chemical changes, even in the absence of oxygen. This usually starts with

Molten polymers are non-Newtonian fluids with high viscosities, and the interaction between their thermal and mechanical degradation can be complex. At low temperatures, the polymer-melt is more viscous and more prone to mechanical degradation via shear stress. At higher temperatures, the viscosity is reduced, but thermal degradation is increased. Friction at points of high sheer can also cause localised heating, leading to additional thermal degradation.

Mechanical degradation can be reduced by the addition of lubricants, also referred to as processing aids or flow aids. These can reduce friction against the processing machinery but also between polymer chains, resulting in a decrease in melt-viscosity. Common agents are high-molecular-weight waxes (paraffin wax, wax esters, etc.) or metal stearates (i.e.zinc stearate).

In-service degradation

Most plastic items, like packaging materials, are used briefly and only once. These rarely experience polymer degradation during their service-lives. Other items experience only gradual degradation from the natural environment. Some plastic items, however, can experience long service-lives in aggressive environments, particularly those where they are subject to prolonged heat or chemical attack. Polymer degradation can be significant in these cases and, in practice, is often only held back by the use of advanced

The in-service degradation of mechanical properties is an important aspect which limits the applications of these materials. Polymer degradation caused by in-service degradation can cause life threatening accidents. In 1996, a baby was fed via a Hickman line and suffered an infection, when new connectors were used by a hospital. The reason behind this infection was the cracking and erosion of the pipes from the inner side due to contact with liquid media.^[7]

Chlorine-induced cracking

Polymer degradation by

Photo-oxidation is the combined action of UV-light and oxygen and is the most significant factor in the weathering of plastics.

Polymers with an all-carbon backbone, such as polyolefins, are usually resistant to hydrolysis. Condensation polymers like polyesters,^[31] polyamides, polyurethanes and polycarbonates can be degraded by hydrolysis of their carbonyl groups, to give lower molecular weight molecules. Such reactions are exceedingly slow at ambient temperatures, however, they remain a significant source of degradation for these materials, particularly in the marine environment.^[32] Swelling caused by the absorption of minute amounts of water can also cause environmental stress cracking, which accelerates degradation.

Ozonolysis of rubbers

Polymers, which are not fully

The major appeal of biodegradation is that, in theory, the polymer will be completely consumed in the environment without needing complex waste management and that the products of this will be non-toxic. Most

Oxidation can be caused by melt-processing or weathering in the environment. Oxidation may be intentionally accelerated by the addition of biodegradable additives. These are added to the polymer during compounding to improve the biodegradation of otherwise very resistant plastics. Similarly, biodegradable plastics have been designed which are intrinsically biodegradable, provided they are treated like compost and not just left in a landfill site where degradation is very difficult because of the lack of oxygen and moisture.^[37]

Degradation during recycling

The act of recycling plastic degrades its polymer chains, usually as a result of thermal damage similar to that seen during initial processing. In some cases, this is turned into an advantage by intentionally and completely depolymerising the plastic back into its starting

As polymers approach their

Chemical depolymerisation

Degradation can be detected before serious cracks are seen in a product using infrared spectroscopy.^[53] In particular, peroxy-species and carbonyl groups formed by photo-oxidation have distinct absorption bands.

See also

• Applied spectroscopy
• Forensic polymer engineering
• Environmental stress fracture
• Polymer engineering
• Weather testing of polymers

Bibliography

• Lewis, Peter Rhys, Reynolds, K and Gagg, C, Forensic Materials Engineering: Case studies, CRC Press (2004)
• Ezrin, Meyer, Plastics Failure Guide: Cause and Prevention, Hanser-SPE (1996).
• Wright, David C., Environmental Stress Cracking of Plastics RAPRA (2001).
• Lewis, Peter Rhys, and Gagg, C, Forensic Polymer Engineering: Why polymer products fail in service, Woodhead/CRC Press (2010).

References