Environmental Engineering Reference
In-Depth Information
The drawbacks of PLA are overshadowed by its advantages in products with short lifecycles
[38].
3.3. Synthesis
The two general synthetic routes for PLA include the condensation polymerization of
lactic acid and the ring-opening polymerization of lactide. Condensation polymerization
methods have produced high molecular weight PLA through chain extension and, more
recently, through dehydration using azeotropic high-boiling (boiling point > 150°C) aprotic
solvents and vacuum techniques. Polymerization utilizing a ring-opening mechanism is the
preferred method of synthesis, however, and is the basis of the commercial Natureworks
process.
Condensation polymerization has recently become a reliable method for production of
high molecular-weight PLA. Attempts as recently as the mid-1990s to produce high
molecular- weight PLA through simple dehydration without catalysis were unsuccessful,
primarily due to side reactions [40]. To reduce the interfering side reactions, two methods are
currently available. The first, developed by Ajioka et al. in 1995 [40], involves the
dehydration of lactic acid using high-boiling aprotic solvents, such as diphenyl ether, that
form azeotropes with water under vacuum conditions, as well as a catalyst. The second
method involves using a dual catalyst system [41]. Other methods have been attempted with
the goal of producing high molecular- weight PLA directly from lactic acid monomers, but to
date have yielded only moderate molecular weight materials [42, 43].
Chain extension of oligomeric PLA is another approach, within the category of
condensation polymerization, used to produce higher molecular weight PLA. Two basic
methodologies have been developed. The first uses an additive to promote esterification of
two PLA chains into one continuous chain [44, 45] while the other uses a linking agent to
couple two or more chains together. Chain-linking agents such as diisocyanates [46, 47],
thiirane [47], and diacidchlorides [44] are more economical than esterification promoting
agents due to fewer purification steps and the important ability to run the reactions in the
bulk. Problems associated with linking agents, however, include the persistence of unreacted
chain-linking agents, residual metals, and the non-biodegradability of the linking agent, all of
which diminish the environmental advantages of the resultant PLA.
Although the straight dehydration of lactic acid does not produce high molecular-weight
polymers, the process is important in the production of lactide. The amount of lactide
produced is influenced by temperature, pressure, and the types of catalysts added to the
system [48]. Multiple purification methods are used industrially, including Cargill's multiple
reflux controlled columns [11, 12], multi-step recrystallization reactors [49-56], and direct
vapor-phase reaction of lactic acid [57, 58]. Formation of lactide occurs through a back-biting
reaction involving two lactic acid monomers to form the six-member lactide ring. Three types
of lactide can be formed: L-lactide or D-lactide with melting point = 97°C, or Meso-lactide
with melting point = 52°C, depending on the composition of the starting material and extent
of racemization (Figure 10). A fourth type, D,L-lactide, is formed when a racemic
stereocomplex of D- and L-lactide are crystallized together; this form has a much higher
melting point of 126-127°C [16].
Lactide, once formed, can be polymerized by three general mechanisms: anionic [33, 59-
61], cationic [33, 62, 63], and coordination insertion (see Figure 11). Of these, coordination-
insertation is the most prevalent and industrially most important.
