Environmental Engineering Reference
In-Depth Information
6.4.2 Biodegradable Plastics
Biodegradable polymers are broadly classified into two groups: biopolymers
and other (bio-derived and fossil-fuel derived) polymers. The first includes
well-known examples of biopolymers such as cellulose and chitin that
rapidly break down in most environments. It is their rapid biodegradation
that is responsible for the removal of waste biomass (by mineralization)
from environment. Biodegradable polymers are not used widely as
packaging materials though the potential for their use clearly exists. The
second class includes conventional polyesters such as PCL, those derived
from bio-based raw material such as PLA, and bacterial polyesters such
as poly(hydroxyl butyrate) (PHB). Aliphatic polyesters are susceptible to
hydrolysis by lipolytic enzymes commonly secreted by microorganisms
(Tokiwa et al., 1976). These enzymes are widely distributed in different
environments (Mergaert and Swings, 1996). The populations of aliphatic
polymer-degrading microorganisms in different ecosystems was found to be
generally in the following order: PHB = PCL > PBS > PLA (Tokiwa et al.,
2009). Three crucial factors that facilitate high rates of biodegradation of
these polymers are the low average molecular weight,
Mn
, low degree of
crystallinity (Tsuji and Miyauchi, 2001), and the lack of (or minimal) side
chains in their structure (molecules with side chains are relatively more
difficult to be assimilated by microorganisms (Tokiwa et al., 1976). The
amorphous matrix with a looser structure is relatively easier to biodegrade
because of better accessibility to enzymes compared to the crystalline
domains. Also, higher the melting point Tm (°C) of the polymer, lower will
be its biodegradability. Some examples of common biodegradable plastics
are introduced below.
PCL {-OCH
2
CH
2
CH
2
CH
2
CH
2
CO-}
n
) is a partially-crystalline polyester
that is biodegraded by microbial lipases and esterases. The plastic is made
from petrochemical feedstocks. It has too low a melting point (60°C) to
be useful in any packaging applications. Higher aliphatic polyesters such
as poly(butylene succinate) (PBS) (-O(CH
2
)
4
OOC(CH
2
)
2
CO-)
n
and
poly(ethylene succinate) (PES) (-O(CH
2
)
2
OOC(CH
2
)
2
CO-)
n
are also
biodegradable at a rate that depends on environmental factors (Kasuya et
al., 1997). They have higher melting points of 112-114°C and 103-106°C,
respectively, and the properties compare well to those of polyolefins. As
succinic acid can be derived from plant sources, the polysuccinates can be
potentially a bio-based polymer.
