Polymer Degradation: Causes and Effects, Lecture notes of Chemistry

Download Polymer Degradation: Causes and Effects and more Lecture notes Chemistry in PDF only on Docsity! Polymer Degradation Definition: 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 production, use, disposal into the environment and recycling. 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 inhibit or promote degradation. For instance, polymers stabilizers ensure plastic items are produced with the desired properties, extend their useful lifespans, and facilitate their recycling. Conversely, biodegradable additives accelerate the degradation of plastic waste by improving its biodegradability. Some forms of plastic recycling can involve the complete degradation of a polymer back into monomers or other chemicals. In general, the effects of heat, light, air and water are the most significant factors in the degradation of plastic polymers. The major chemical changes are oxidation and chain scission, leading to a reduction in the molecular weight and degree of polymerization of the polymer. These changes affect physical properties like strength, malleability, melt flow index, appearance and colour. The changes in properties are often termed "aging". Susceptibility: Plastics exist in huge variety, however several types of commodity polymer dominate global production: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET, PETE), polystyrene (PS), polycarbonate (PC), and poly(methyl methacrylate) (PMMA). The degradation of these materials is of primary importance as they account for most plastic waste. These plastics are all thermoplastics and are more susceptible to degradation than equivalent thermosets, as those are more thoroughly cross-linked. The majority (PP, PE, PVC, PS and PMMA) are addition polymers with all-carbon backbones that are more resistant to most types of degradation. PET and PC are condensation polymers which contain carbonyl groups more susceptible to hydrolysis and UV-attack. Degradation during Processing 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 will oxidize under these conditions, but even in the absence of air, these temperatures are sufficient to cause thermal degradation in some materials. The molten polymer also experiences significant shear stress during extrusion and moulding, which is sufficient to snap the polymer chains. Unlike many other forms of degradation, the effects of melt-processing degrade the entire bulk of the polymer, rather than just the surface layers. This degradation introduces chemical weak points into the polymer, particularly in the form of hydroperoxides, which become initiation sites for further degradation during the object's lifetime. Polymers are often subject to more than one round of melt-processing, which can cumulatively advance degradation. Virgin plastic typically undergoes compounding (masterbatching) 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 molding 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. 1) 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).Reactions follow the general autoxidation mechanism, leading to the formation of organic peroxides and carbonyls. The addition of antioxidants may inhibit such processes. 2) Thermal degradation: Heating polymers to a sufficiently high temperature can cause damaging chemical changes, even in the absence of oxygen. This usually starts with chain scission, generating free radicals, which primarily engage in disproportionation and crosslinking. PVC is the most thermally sensitive common polymer, with major degradation occurring from ~250 °C (480 °F) onwards;[4] other polymers degrade at higher temperatures. 3) Thermo-mechanical degradation: 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- Polymers, which are not fully saturated, are vulnerable to attack by ozone. This gas exists naturally in the atmosphere but is also formed by nitrogen oxides released in vehicle exhaust pollution. Many common elastomers (rubbers) are affected, with natural rubber, polybutadiene, styrene-butadiene rubber and NBR being most sensitive to degradation. The ozonolysis reaction results in immediate chain scission. Ozone cracks in products under tension are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, and fuel leakage and fire may follow. The problem of ozone cracking can be prevented by adding antiozonants. 4) Biological degradation: 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 common plastics biodegrade very slowly, sometimes to the extent that they are considered non-biodegradable. As polymers are ordinarily too large to be absorbed by microbes, biodegradation initially relies on secreted extracellular enzymes to reduce the polymers to manageable chain-lengths. This requires the polymers bare functional groups the enzymes can 'recognise', such as ester or amide groups. Long-chain polymers with all-carbon backbones like polyolefin’s, polystyrene and PVC will not degrade by biological action alone and must first be oxidized to create chemical groups which the enzymes can attack. 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. 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 monomers, which can then be used to generate fresh, un-degraded plastic. In theory, this chemical (or feedstock) recycling offers infinite recyclability, but it is also more expensive and can have a higher carbon footprint because of its energy costs. Mechanical recycling, where the plastic is simply remelted and reformed, is more common, although this usually results in a lower-quality product. Alternatively, plastic may simply be burnt as a fuel in a waste-to-energy process. 1) Remelting: Thermoplastic polymers like polyolefin’s can be remelted and reformed into new items. This approach is referred to as mechanical recycling and is usually the simplest and most economical form of recovery.[33] Post-consumer plastic will usually already bare a degree of degradation. Another round of melt-processing will exacerbate this, with the result being that mechanically recycled plastic will usually have poorer mechanical properties than virgin plastic.[36] Degradation can be enhanced by high concentrations of hydroperoxides, cross- contamination between different types of plastic and by additives present within the plastic. Technologies developed to enhance the biodegradation of plastic can also conflict with its recycling, with oxo-biodegradable additives, consisting of metallic salts of iron, magnesium, nickel, and cobalt, increasing the rate of thermal degradation. Depending on the polymer in question, an amount of virgin material may be added to maintain the quality of the product. 2) Thermal depolymerisation & Pyrolysis: As polymers approach their ceiling temperature, thermal degradation gives way to complete decomposition. Certain polymers like PTFE, polystyrene and PMMA undergo depolymerization to give their starting monomers, whereas others like polyethylene undergo pyrolysis, with random chain scission giving a mixture of volatile products. Where monomers are obtained, they can be converted back into new plastic (chemical or feedstock recycling), whereas pyrolysis products are used as a type of synthetic fuel (energy recycling). In practice, even very efficient depolymerisation to monomers tends to see some competitive pyrolysis. Thermoset polymers may also be converted in this way, for instance, in tyre recycling. 3) Chemical depolymerisation: Condensation polymers baring cleavable groups such as esters and amides can also be completely depolymerised by hydrolysis or solvolysis. This can be a purely chemical process but may also be promoted by enzymes. Such technologies are less well developed than those of thermal depolymerisation, but have the potential for lower energy costs. Thus far, polyethylene terephthalate has been the most heavily studied polymer.[46] Alternatively, waste plastic may be converted into other valuable chemicals (not necessarily monomers) by microbial action. Stabilisers: Hindered amine light stabilizers (HALS) stabilise against weathering by scavenging free radicals that are produced by photo-oxidation of the polymer matrix. UV-absorbers stabilise against weathering by absorbing ultraviolet light and converting it into heat. Antioxidants stabilise the polymer by terminating the chain reaction because of the absorption of UV light from sunlight. The chain reaction initiated by photo-oxidation leads to cessation of crosslinking of the polymers and degradation of the property of polymers. Antioxidants are used to protect from thermal degradation. Detection: Degradation can be detected before serious cracks are seen in a product using infrared spectroscopy. In particular, proxy-species and carbonyl groups formed by photo-oxidation have distinct absorption bands. .