02 | 2008

Materials
polymer technologies, such as co-polymerization, blending,
modification with additives, and combining materials
or films with different properties.
Further recognition of PLA in specific high-end applications
is currently limited by a number of material properties
that need improvement to meet the material requirements
in these markets:
1. weak structural integrity at elevated temperatures, expressed
as the heat deflection temperature (low HDT),
2. brittleness, i.e., low impact strength,
Figure 1: PLA cup collapsed with hot coffee
3. gas barrier performance, in particular for bottle applications.
The biggest issue is the low heat resistance of PLA. The
material becomes soft and weak upon heating beyond
temperatures of 50-60°C, which causes practical problems
during storage, transportation and use of pellets and
finished articles. When hot coffee is poured into a PLA cup,
if collapses (Fig. 1). Clearly, amorphous – glassy – PLA
loses its structural integrity completely when subjected to
temperatures above its glass transition temperature.
Due to the chiral (see box) nature of lactic acid, several
distinct forms of polylactide exist: poly(L-lactide) (PLLA) is
the product based on L(+) lactic acid or L-lactide, the major
product of Purac. Likewise, polymerization of D-lactide
produces PDLA. Today commercially available PLA grades
are random copolymers of D- and L-lactic acid isomers
with relatively slow nucleation and crystallization rates.
As a result, most PLA materials will be amorphous – i.e.,
glassy and not crystalline – after melt processing. These
materials become sticky and soft at temperatures above
60°C.
Purac allows polymer producers to add value in a new
way by offering L-lactide and D-lactide as solid flakes,
available in bulk quantities from 2009. By combining these
lactides smartly, new PLA grades with tailored physical
properties – like improved heat-stability – can be made by
polymer industry.
• If lactides are combined in the same polymer chain by
a one-pot polymerization of L- and D-lactides, polylactides
with melting temperatures ranging from about
130-180°C can be made. At very low D-isomer content,
semi-crystalline PLLA is obtained, while amorphous,
optically clear PLA is made with D-contents higher than
10-15%.
• Polymerization of only the D-lactide monomer produces
PDLA. This PLA type is the mirror reflection of PLLA
and can be mixed with PLA (co)polymer to improve the
material’s heat resistance according to the stereocomplexation
concept.
22 bioplastics MAGAZINE [02/08] Vol. 3