Degradability in Polymers<br />By<br />Asst. Lect. Sara Ibrahim <br />Polymer degradation includes all changes in both the chemical structure and physical properties of polymers or polymer-based products that lead to the loss of properties such as tensile strength, color, shape, etc., under the influence of processing conditions, or one or more environmental factors (e.g., heat, light, or exposure to chemicals) (Hawkins, 1984b; Matusinovic and Wilkie, 2014). Such loss of properties can occur by the breakage of polymer molecules, or by polymer fragmentation into pieces that may be small enough to disappear but which are still similar to the original material (Vert, 1992). The loss of properties in a finished product is undesirable and needs to be prevented or delayed, but in other cases the loss of properties at a given rate is desirable as it happens in the production of biodegradable polymers (Niaounakis, 2015).<br />The processes by which polymers can suffer degradation are thermal, mechanical, hydrolitic, chemical, biological, photolitic, ultrasonication, pollution contact, radiolytic, and sludge activation. The amount of time to have a certain degree of degradation depends on the type of polymer, morphology, molecular size, and the conditions to which it is subjected (Allen and Edge, 1992; Jasso-Gastinel et al., 1998; Matusinovic and Wilkie, 2014; White and Turnbull, 1994; Yousif and Haddad, 2013).<br />For many applications, polymer properties should have a longer lifetime than the useful life of the article. To achieve this, stabilizers are added to the formulations that extend the useful life of the polymer (White and Turnbull, 1994).<br />The chemicals that offer protection against ultraviolet radiation are classified according to their mechanism of action (Yousif and Haddad, 2013) in radical scavengers (phenolic-anti-oxidants), ultraviolet absorber (hydroxybenzophenones), light screeners (carbon black), excited state deactivation (transition metal chelates), hydroperoxide decomposers (phosphite esters) (Allen et al., 1985).<br />An important case is related to plasticized polyvinyl chloride (PVC), because it decomposes at processing temperatures and it is necessary to add thermal stabilizers to the formulation (González-Ortiz et al., 2005), such as carboxylates derived from Zn and Cd, that can scavenge released HCl and react with allylic chlorine atoms (Manzoor et al., 1996).<br />The large amount of polymers produced for the manufacture of articles with a relatively short useful life as packaging and for a wide range of other applications, such as diapers, sutures, mulch, drug delivery, etc., has increased the demand for the so-called degradable plastics (Kawai, 1992). Technically all the polymers are degradable but this term is used for polymers capable of being decomposed chemically or biologically.<br />Degradable plastics can be obtained using biopolymers (polymers derived from renewable biomass sources), such as poly(alkyl hydroxyalkanoates), poly(lactic acid), cellulose, and starch, among others. Synthetic degradable polymers, such as polycaprolactone and poly(vinyl alcohol), can also be used to obtain degradable plastics. These can be degraded by abiotic and biodegradation mechanisms (Pitt, 1992; Kawai, 1992). Polymer abiotic degradation occurs by hydrolysis where the first step is the random hydrolytic cleavage of functional groups susceptible to be hydrolyzed without weight loss: for instance, ester and urethane groups. The second stage of degradation is when besides chain cleavage, there is weight loss. This stage begins when the MW is so low that there is the possibility that small oligomers can diffuse from the polymer bulk and catastrophic loss of mechanical properties occurs (Pitt, 1992). Polymer biodegradation happens when a polymeric material is broken down by microorganisms (bacteria, fungi, algae) into natural elements such as water and carbon dioxide (Kawai, 1992).<br />Degradable plastics can be classified as: (1) Environmentally biodegradable biopolymers (films, packaging, mulch) that should have no degradation or a low degree of degradation when in use and accelerated degradation when in the disposal stage; and (2) Biodegradable biopolymers which need to undergo controlled degradation when in application and which are mainly used in the medical sector (e.g., sutures, implants, and drug delivery systems). Degradation of biodegradable materials depends on a variety of factors including chemical structure of the polymer, morphology, processing conditions, form and size of the article, as well as environmental conditions, among other aspects (Niaounakis, 2015). The future of degradable plastics depends on their production at an affordable cost and that their degradation occurs at desired rates.<br />Polymer degradation is a change in the properties of the polymer, such tensile strength, color, shape, and molecular weight, or of a polymer-based product under the influence of one or more environmental factors, such as heat, light, chemicals, or any other applied force. Degradation is often due to a change in the chemical and/or physical structure of the polymer chain, which in turn leads to a decrease in the molecular weight of the polymer. These changes may be undesirable, such as changes during use, or desirable, as in biodegradation or deliberately lowering the molecular weight of a polymer. Such changes occur primarily because of the effect of these factors on the chemical composition of the polymer. The susceptibility of a polymer to degradation depends on its structure. Epoxies and chains containing aromatic functionality are especially susceptible to ultraviolet degradation, while hydrocarbon-based polymers are susceptible to thermal degradation and are often not ideal for high temperature applications.<br />The degradation of polymers to form smaller molecules may proceed by random scission or specific scission. The degradation of polyethylene occurs by random scission—a random breakage of the bonds within the polymer. When heated above 450 C (840°F), polyethylene degrades to form a mixture of hydrocarbon derivatives. Other hydrocarbon polymers, such as poly-α-methylstyrene, undergo specific chain scission with breakage occurring only at the ends, and such polymers depolymerize (unzip) to produce the constituent monomer.<br />While the degradation process represents failure of the polymer to perform in service, the process can be useful from the viewpoints of understanding the structure of a polymer or recycling/reusing the polymer waste to prevent or reduce environmental pollution by conversion to useful hydrocarbon derivatives (Sarker et al., 2011; Abbas and Mohamed, 2015; Pundhir, and Gagneja, 2016). The sorting of polymer waste for recycling purposes may be facilitated by the knowledge of the degradation process and assist in recycling.<br />