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2. Formation of lactide by depolymerization of low M W PLA.
3. Polymerization of lactide ring-opening in the presence of a catalyst to produce
high M W PLA.
Though ROP produces PLA of high M W and without side products, the process is
complicated and time-consuming because additional purification is required, and
this increases the cost of production. In the ROP method, the high purity of lactide
is essential, which is used as a monomer. The impurities in lactide lead to undesirable
effects on the properties of the PLA obtained.
3.4 Azeotropic Dehydration Polycondensation (ADP)
In the DP technique, the biggest issue is the removal of by-product (mainly water)
from the viscous mixture at the end of the process. Azeotropic dehydration is the
technique that is mainly applied to overcome the difficulty of by-product removal.
In this method, water is efficiently removed by using appropriate azeotropic solvents
in a single step, and high M W PLA is produced. The conditions applied for azeotropic
removal of water are usually controlled by manipulation of equilibrium between
polymer and monomer in the organic solvent and at a temperature less than the
melting point of PLA. This avoids the formation of impurities by depolymerization
and racemization. Therefore, the selection of solvent and condition applied is
critical to have the desired properties in the PLA produced. Also, in this approach,
a Soxhlet extractor with molecular sieve (3 Å) inside was mounted simultaneously
with azeotropic solvents to remove traces of water from the refluxed solvent, and a
polymer of M W ~ 30,000 g mol
−1
was obtained [10, 11, 13, 16].
3.5 Solid-State Polymerization (SSP)
This is another technique of polymerization of lactic acid, which mainly goes
through two steps:
1. Melt state to produce oligomer at high temperature (usually between 150 and
200 °C).
2. Solid state to further increase M W at a temperature between melting temperature
(T m ) and glass temperature (T g ) of PLA.
Further, in the second step, the prepolymer (in the form of semi-crystallized powder,
chip, the pellet of fibers) of relatively low M W is usually pulverized and dried
thoroughly before heating, and this leads to heat distribution homogeneously among
the dry particles that produce PLA of high M W . This method produces PLA with
better quality and properties because of the excellent control of side reactions and
avoiding decomposition at low temperatures. However, this approach is time-consuming [17].
M. A. Ali et al.
2. Formation of lactide by depolymerization of low M W PLA.
3. Polymerization of lactide ring-opening in the presence of a catalyst to produce
high M W PLA.
Though ROP produces PLA of high M W and without side products, the process is
complicated and time-consuming because additional purification is required, and
this increases the cost of production. In the ROP method, the high purity of lactide
is essential, which is used as a monomer. The impurities in lactide lead to undesirable
effects on the properties of the PLA obtained.
3.4 Azeotropic Dehydration Polycondensation (ADP)
In the DP technique, the biggest issue is the removal of by-product (mainly water)
from the viscous mixture at the end of the process. Azeotropic dehydration is the
technique that is mainly applied to overcome the difficulty of by-product removal.
In this method, water is efficiently removed by using appropriate azeotropic solvents
in a single step, and high M W PLA is produced. The conditions applied for azeotropic
removal of water are usually controlled by manipulation of equilibrium between
polymer and monomer in the organic solvent and at a temperature less than the
melting point of PLA. This avoids the formation of impurities by depolymerization
and racemization. Therefore, the selection of solvent and condition applied is
critical to have the desired properties in the PLA produced. Also, in this approach,
a Soxhlet extractor with molecular sieve (3 Å) inside was mounted simultaneously
with azeotropic solvents to remove traces of water from the refluxed solvent, and a
polymer of M W ~ 30,000 g mol
−1
was obtained [10, 11, 13, 16].
3.5 Solid-State Polymerization (SSP)
This is another technique of polymerization of lactic acid, which mainly goes
through two steps:
1. Melt state to produce oligomer at high temperature (usually between 150 and
200 °C).
2. Solid state to further increase M W at a temperature between melting temperature
(T m ) and glass temperature (T g ) of PLA.
Further, in the second step, the prepolymer (in the form of semi-crystallized powder,
chip, the pellet of fibers) of relatively low M W is usually pulverized and dried
thoroughly before heating, and this leads to heat distribution homogeneously among
the dry particles that produce PLA of high M W . This method produces PLA with
better quality and properties because of the excellent control of side reactions and
avoiding decomposition at low temperatures. However, this approach is time-consuming [17].
M. A. Ali et al.