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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective Figure 8. Specific surface degradation rates for various plastics, in μm year−1. Vertical columns represent different environmental conditions (L, landfill/compost/soil; M, marine; B, biological; S, sunlight) and plastics types (represented by their resin identification codes). Plastics type 7, “others”, corresponds to various nominally biodegradable plastics. The range and average value for plastics types 1−6 are shown on the right as lines and squares, respectively, as well as for biodegradable “others”. Data points representing degradation rates that were unmeasurably slow are shown on the x-axis. Gray columns represent combinations for which no data were found. essentially invariant with time, we obtain the corresponding equations for mass loss and estimated complete degradation time shown in eqs 9 and 10 1/2 2 − kd(πρh) t] (9) A cylindrical HDPE fiber of volume 2.9 cm3 (r = 2 mm, h = 23 cm, SA = 29 cm2) would require an estimated 465 years for complete degradation. Thus, an HDPE film should degrade completely 260 times faster than a fiber of the same mass and crystallinity, and 1100 times faster than a comparable bead, in the absence of significant fragmentation, crystallization, or shape dependence of the SSDR. The ratios of the initial degradation rates (based on surface area ratios) are 390:3:1 for the film, fiber, and bead, respectively. However, the degradation rates of the fiber and the bead decrease as their radii shrink, so their “average” degradation rates are even lower. In addition, the exceedingly long extrapolations for the fiber and the bead result in errors with a far greater magnitude, in years, than the short extrapolation for the film. Thus, if the relative error in kd is 20%, the film will degrade in 1.8 ± 0.4 years, while the fiber will degrade in 465 ± 100 years and the bead in 2000 ± 400 years. We can also reasonably assume that kd will vary far more in the course of two millennia than it will during the first decade, making the actual error for the bead even larger. The shape-dependent degradation profiles are compared in Figure 7. Some additional points are worth mentioning. First, the surface roughness is unlikely to remain constant over time. Polymers that are melt processed may have initially smooth surfaces. However, as degradation proceeds, the surface will become pitted, and cracks will appear, increasing the surface area and hence the degradation rate. Such cracks can also lead to surface ablation and mass loss due to the release of microplastic fragments. Second, an amorphous polymer (whose surface abundance may or may not be the same as the bulk) will undergo faster degradation than a crystalline polymer, requiring the addition of a scaling factor to eqs 3 and 6 to represent the amorphous surface area fraction (aSA, 0 ≤ a ≤ 1). Once the amorphous polymer is eliminated, degradation in the remaining crystalline regions may be much slower. Furthermore, partial polymer degradation can lead to cross- linking and/or crystallization in the amorphous regions adjacent to crystallites,107 thereby slowing degradation, although this effect cannot be quantified or modeled at this time. Clearly, polymer degradation times in the environment should be tremendously sensitive to the shape and size of the material, in addition to its intrinsic chemical reactivity. Nevertheless, for plastics pieces of different sizes and compositions but with similar aspect ratios, it is possible to compare initial degradation rates via the SSDR, which is inversely proportional to the degradation time for a material whose surface area remains approximately constant. This assumption is discussed further below. Analysis of Reported Degradation Rates. Of the hundreds of published papers screened for this Perspective, only 25 reported all of the information needed to calculate an SSDR (mass loss, sample dimensions, experiment dura- tion).71,79,127−149 The resulting 54 data points are organized in Figure 8. The data are arranged according to plastics type and degradation environment (Landfill/soil/compost, Marine, Biological, or Sunlight). While these categories are not completely orthogonal, they are useful to represent the four major categories of polymer degradation experiments that have been conducted: on land (without exposure to sunlight), in water (in freshwater or seawater with exposure to sunlight), in a lab using enzymes or microbes, or with exposure exclusively to sunlight and air. Data for plastics type 7 (“others”) includes the nominally biodegradable plastics PLA, polyhydroxybuty- rate (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), Mater-Bi and polycaprolactone (PCL). The presence of a filler component (e.g., starch, PLA, etc.) or accelerating https://dx.doi.org/10.1021/acssuschemeng.9b06635 1/2 mt = [(m0) 1m t= j z iy d jz1/2 k πρh dk { (10) 3500 ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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