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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective conditions (e.g., UV pretreatment, thermal pretreatment, or microbial incubation) in each study is noted. Two of the papers used to construct Figure 8 studied the degradation of blends of polypropylene132 and polyethylene149 with a biodegradable filler (5 or 10 wt % starch, respectively). The authors assumed that the filler indirectly increases the degradation rate of the plastic, possibly by causing the surface area of the plastic to increase as the starch component degrades. The problems of inferring plastics degradation rates in this way are discussed further below. Data for PET, PP, and PS degradation are sparse. Finally, only one report for PVC degradation met our criteria, regardless of the environment, and it found no measurable degradation after 32 years.148 Considering only the nonzero values, the SSDR values for all plastics types, regardless of environment, vary over several orders of magnitude. Overall, the average reported values for accelerated degradation in each environmental condition are slightly higher than their nonaccelerated counterparts, as expected, although the differences may not be statistically significant. For accelerated degradations (filled circles, Figure 8), involving polymer pretreatment or a filler, the rates still vary by about an order of magnitude in most cases. The variation is especially noticeable for LDPE decomposing on land, where the range is a factor of 50. The highest reported accelerated SSDR for LDPE, 83 μm year−1, is for a blend with 20 wt % PLA decomposing under composting conditions (37 °C).144 The lowest reported accelerated SSDR for LDPE is 3.7 μm year−1, measured after addition of P. aeruginosa to the soil. Interestingly, the SSDR for the LDPE/PLA blend (83 μm year−1) is higher than the SSDR for pure PLA (21 μm year−1),138 both measured in composting conditions at 37 °C. However, the durations of the two experiments differed (28 days for the LDPE/PLA blend, compared to 365 days for the pure PLA). The discrepancy could also be due to differences in crystallinity, since blending can increase the volume fraction of amorphous regions, which show higher degradation rates.57 Extrapolated degradation rates for plastics blended with degradable fillers (e.g., starch) assume constant degradation rates, but this may be highly inaccurate. Such fillers are typically degraded by microorganisms first. Once the readily accessible filler is consumed, the remaining plastics degrades much more slowly through a combination of environmental degradation (e.g., photo-oxidative, hydrolysis, etc.) and microbial action.150 The degradation should therefore be described as a multiphase kinetic process.151 The variable durations and rates of these phases depend on the dimensions of the material, type and concentration of the filler, degradation environment and conditions, etc. Abiotic degra- dation of plastics may be enhanced relative to the unfilled polymer due to greatly increased surface area after removal of the filler.150 Despite the convenience of shorter time scales, great caution should be used when interpreting degradation kinetics of blended polymers, and they should not be used to infer mechanisms and degradation rates for pure polymers. Although biodegradable plastics such as PHB and PLA show large average SSDRs in compost and landfill conditions (59 and 21 μm year−1, respectively), their degradation in marine environments is significantly slower and may even be comparable to the degradation rates of their petrochemical counterparts. For example, the average SSDR of PLA in the marine environment, 7.5 μm year−1, is similar to the averages for HDPE (4.3 μm year−1) and LDPE (15 μm year−1). Although PLA and other “biodegradable” plastics are expected to fully degrade in industrial composting conditions (≥ 60 °C, and moist), the temperature in marine environments rarely exceeds 20 °C and therefore lacks the thermal energy for depolymerization. In soil conditions, however, the average SSDR of PLA is notably higher (21 μm year−1) than the corresponding values for petrochemical-based plastics (e.g., HDPE, 1.0 μm year−1). Temperatures in landfills have been reported to reach 80−100 °C,152 which is sufficient to degrade plastics like PLA as long as moisture is present. Extrapolated Degradation Times. Two methods have been widely used to estimate polymer lifetimes, defined here as the time required for complete degradation (>99% loss of the initial polymer mass): (1) Arrhenius extrapolation of accelerated aging result, and (2) extrapolation based on initial rates measured under environmentally relevant conditions. Both make important and often poorly justified assumptions which limit their validity. The first method assumes that degradation rate constants have Arrhenius-like temperature dependences.153 A polymer lifetime at ambient temperature is extrapolated from the faster degradation rates that are more readily measured at higher temperatures, ca. 25−200 °C. For example, the lifetime of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) at 25 °C in distilled water was predicted to be 300 d, based on extrapolation of decreases in tensile strength154,155 and molecular weight measured at temperatures of 25−50 °C.156 However, lifetime estimation by Arrhenius extrapolation assumes that the same degradation mechanism is operative at all relevant temperatures. For PE and PP, the Arrhenius plots are nonlinear,157−159 suggesting a change in mechanism and/or rate-determining step with temperature.160 Arrhenius extrapolation is less useful for estimating biodegradation rates, because enzymatic degradation processes take place only under biologically relevant reaction conditions; they usually do not occur at elevated temperatures. Therefore, biodegradation lifetimes are generally predicted by the second method, using initial degradation rates obtained over prolonged measurement times instead.148,161 For instance, the degradation rates of LDPE−starch blends were calculated by measuring weight loss as a function of time over a period of 125 days in composting conditions, and this rate was used to predict the complete degradation time using a linear extrapolation.120 However, since relatively rapid biodegrada- tion of the starch component should be followed by much slower degradation of the remaining LDPE, extrapolation based on initial measurements could be highly misleading. The accuracy of the method is compromised by the occurrence of several phases of degradation, with very different rates.162 Thus, information on degradation rates for each component in a mixture is necessary to make accurate lifetime estimates. Although extrapolation from initial rates is inaccurate, it is the simplest method to estimate plastics lifetimes in the environment. The approach requires knowledge of the rate law but does not account for dependence on the shape of the material. The shrinking core model (common in TGA studies of phase transformations) assumes that the volume, and hence the surface area, change as the reaction proceeds.163 While it may be more realistic, it does not take into account changes in surface roughness, which may be considerable. Applications of the shrinking core model to plastics degradation have so far focused on high temperature catalytic degradation, which is not directly relevant to plastics degradation in the natural environment.164,165 https://dx.doi.org/10.1021/acssuschemeng.9b06635 3501 ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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