Biodegradability of Common Polymers

FIGURE 6.15 Gas evolution data (filled symbols) plotted as percent mineralization for the biodegradation of bleached paperboard packaging material in a respirometer. Soil media (70 wt% humidity) with sewage sludge inoculum was used. Also included is a plot of the data (open symbols) as suggested by Equation 6.2. Source Reproduced with permission from Andrady and Song (1999). 

For bleached paperboard for instance, Andrady et al. found A (0.14-0.18)days (Andrady and Song, 1999). 

In practice, 1(X)% of a plastic material will not mineralize and a small residue remains in the environment. These may include fillers, catalyst residues, and recalcitrant additives. It is important to ensure that these and their reaction products are non-toxic and do not harm soil organisms or affect plant growth. 

Slow biodegradation of common polymers such as PE, PET, or PP does occur in nature (Shah et al., 2008 Tsao et al., 1993) but at rates that are several orders of magnitude lower than that observed with biopolymers such as cellulose and chitosan or biodegradable synthetic polymers such as PCL. These rates are too low to have any significant impact on litter management, and conventional plastics as are not 

FIGURE 6.16 Electron micrographs (a-c) showing the diversity of microbial flora on polyolefin 

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The biodegradation of synthetic polymers by microorganisms and enzymes is much more common than most earlier anticipations. Several major factors affecting the biodegradation of synthetic polymers are ... 

Compared with more common plastics used as packaging materials, the compound does have some disadvantages, such as a high water vapour permeability and limited heat resistance, losing dimensional stability at about 70°C. It is also substantially more expensive than the high-tonnage polyolefins. Last but not least its biodegradability means that it must be used in applications that will have completed their function within a few months of the manufacture of the polymer compound. 

Although it seems obvious that there is a coimection between the natural origin of a polymer and its biodegradabihty this is one of the most common misunderstandings with respect to biopolymers. Biodegradabihty is a function of the chemical structure of a molecule and there is no dependence on its origin. This is the reason why synthetic, man-made polymers can also be biodegradable if their structure obeys certain rules. 

Vinyl chloride (VC) is the monomer from which the common polymer, polyvinyl chloride, is produced. Hence, VC is produced on a very large scale. Not surprisingly, occasional releases of the rather toxic VC to the environment occur. What initial biodegradation product would you expect from VC if it were released into an oxic environment ... 

Lactic acid is commonly found, which contributes to its wide use in food and food-related industries. It also has the potential for production of biodegradable and biocompatible polymers. These products have been proven to be environmentally friendly alternatives to biodegradable plastics derived from petrochemical materials (Zhang, Jin, and Kelly, 2007). Lactic acid is slightly lipid soluble and diffuses slowly through the cell membrane. As a result of this, the disruption of the cell pH is not its main mode of inhibition (Gravesen et al., 2004). 

But, both polymers, PHB and PLA, lead to conflicts in the context of assigning them as biodegradable polymers, because these are naturally occurring polymers, which like PHB were evolved in natural material cycles. However, in the case of PLA only the monomer, L-2-hydroxypropionic acid, (l-Lactic acid) can be found in the natural material cycle. On the other hand, both of them can be generated on a natural nonpetrochemical basis, with renewable resources, and they are degradable. This common denominator should justify the discussion of both polymers herein and makes a comparison meaningful. 

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