Environmental Engineering Reference
In-Depth Information
PLA{O(CH
3
)CHCO-}
n
isasemi-crystalline polymerthatcanbemadefrom
bio-derived raw materials. Its properties are comparable to PS and even
approaches those of PET (Scaffaro et al., 2011). PLA-degrading
microorganismsarenotwidelydistributedinnatureandsoilbiodegradation
rates are slow (Ohkita and Lee, 2006) but PLA is rapidly compostable
compared to PCL or poly-3-hydroxybutyrate-
co
-3-hydroxyvalerate (PHBV).
PLAresinisnowinsmall-scalecommercialproductionandislikelytobethe
leadingcompostableresinavailableinthenearterm.Itisusedindisposable
single-use items such as food service items, yogurt cups, water bottles, and
thermoformed boxes. The claimed superior environmental benefit of using
biodegradable PLA over a conventional resin such as PP in food packaging,
however, is not always obvious (Hermann et al., 2011). Poly(glycolic acid)
(PGA) is a biodegradable polymer that is less hydrophobic than PLA. Both
copolymers and blends of PGA with PLA are used in medical applications as
the metabolites they produce on degradation are nontoxic.
Poly(hydroxyalkanoates) are polyesters synthesized by microorganisms
grown under special conditions. They are biopolymers similar to cellulose
or chitin, but treated separately because they are produced only by stressed
organisms and because of their potential as a commercial biodegradable
plastic. For instance, PHB {-O(CH
3
)CHCH
2
CO-}
n
is a natural polymer
produced in high yield by many species of bacteria grown under special
culture conditions. Its properties are similar to those of PP (Mooney, 2009).
Up to 10% of soil microbial colonies can degrade PHB at ambient
temperatures and the polymer is readily compostable as well. A popular
biodegradable copolymer is PHBV with 12-72 mol% of hydroxyl valerate.
In soil media seeded with compost (mixed microbial environment), 90%
biodegradation can be expected in 10-22 months at 25°C, when ASTM D
5988-03 was followed (Arcos-Hernandez et al., 2012; Kim et al., 1999).
Films of PHBV exposed to soil undergo ready biodegradation and show
surface colonization by microbial species and pit and crevice formation on
surface (Sang et al., 2002).
However, under anaerobic conditions (such as in a landfill), PHBV does not
break down readily (Ishigaki et al., 2004).
Figure 6.12
shows the weight loss
versus duration of soil exposure for PHB and PHBV film and pellet samples
at two Russian exposure sites and illustrates the ease of biodegradation of
the biopolymer PHBV and PHB.
