Page 96 - Biodegradable Polyesters

74 4 Synthesis, Properties, and Mathematical Modeling of Biodegradable Aliphatic Polyesters

that have been commercialized include poly(butylene succinate) (PBSu), which
till today is the most important APD polyester, poly(ethylene succinate) (PESu),
poly(butylene adipate) (PBAd), poly(ethylene adipate) (PEAd), and PBSu adipate
copolymers. This is because ethylene and butylene glycols were for a long
time available in the market in higher quantities owing to the production of
poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT)
polyesters. Other aliphatic polyesters such as poly(L-lactide) (PLA), polyglycolide
(PGA), their PLGA copolymers, and polycaprolactone (PCL) are also of high
importance and used extensively in many applications.
The history of aliphatic polyesters begins in the late 1920s when the American
chemist Wallace Carothers and his research group at DuPont began pioneering
work that investigated the synthesis of polyesters from the reaction of aliphatic
diacids with aliphatic diols, in order to prepare polymers appropriate for the pro-
duction of fibers. However, during this attempt, only some soft materials with low
molecular weights and high susceptibility to hydrolytic degradation were pro-
duced. This is because aliphatic monomers were used and the prepared water
could not be removed from the reactor at the temperatures used. Fibers from
polyesters were successfully produced some years later by J. R. Whinfield and
J. T. Dickson after successful synthesis of PETs, which were patented under the
®
®
names Terylene and Dacron . Familiar alipharomatic polyester, namely, PBT was
also synthesized using terephthalic acid or its diester with methanol and butylene
glycol. However, PET and PBT are microbial-resistant polyesters, while aliphatic
polyesters are biodegradable under composting conditions, making them attrac-
tive materials in applications for which biodegradability is important.
Biodegradable aliphatic polyesters are found also in nature as some type
of microorganisms can synthesize aliphatic polyesters such as polyhydrox-
yalkanoates (PHAs) in order to store “energy.” Polyhydroxybutyrate (PHB),
poly(hydroxyl valerate) (PHV), and their copolymers are such examples and they
can be enzymatically produced from certain bacteria by feeding them sugar or
other type of nutrition (alcohols, alkanes, alkenes, etc.). A lot of companies are
producing such polymers commercially by microbial fermentation. However,
their cost is quite high owing to difficulties in extracting and purifying the
polymer from microorganisms.
The market for biodegradable polymers has shown strong growth during the
last 10 years. In 2005, the global biodegradable plastics market tonnage was
estimated at 94 800 tonnes and in 2010 reached or overextended is forecast the
214 400 tonnes, which represents a compound annual growth rate of 17.7% during
the period 2005–2010 [3]. Packaging (including rigid and flexible packaging,
paper coating, and foodservice) consumes about the 39% of the total biodegrad-
able polymer market volumes following by loose-fill packaging (about 24%),
bags and sacks (21%), fibers (9%), and others (7%). PLA is the largest produced
material of biodegradable polyesters with a production of 35 800 tonnes in 2005,
followed by synthetic aliphatic–aromatic copolyesters at 14 000 tonnes. PCL is
a linear semicrystalline, synthetic aliphatic polyester of high importance, which
can be biodegraded by a variety of microorganisms.