Chemistry Reference
In-Depth Information
Fig. 2.13
Polylactic acid synthesis.
vegetable oil cost; this has focused attention onto the
use of used cooking oil for example, but non-uni-
formity, availability and collection issues have pre-
vented commercial use to date.
Use of renewable resources for making polymers
is an area receiving much attention due to the rela-
tive ease of making biodegradable plastics with
useful chemical and physical properties. It is impor-
tant to caution against the perception, however, that
just because a plastic is made from a renewable
resource it is automatically greener than one made
from petroleum. Many petroleum-based polymers
such as polyethylene [32] and polyisoprene are fairly
readily biodegraded; it is the additives (antioxidants)
specifically added to prevent degradation, thus
ensuring a useful life, that are the causes of many
of the environmental problems. As in the case of
biodiesel, one of the main issues preventing growth
of the 'renewable polymers' sector is cost. In many
cases the cost is associated with the relatively small
amount of the required chemical being present in
the crop, entailing high extraction cost and the pro-
duction of large quantities of waste. In these cases a
holistic approach is required with, for example,
waste biomass being used as a fuel.
Recent advances in producing polylactic acid
(PLA) from corn starch have lead to the building of
the first large-scale commercial production unit by
Cargill-Dow [33]. The commercial viability of the
polymer relies on novel processing that can be used
to manipulate the molecular weight, crystallinity and
chain branching, enabling materials with a wide
range of end uses and markets to be made. Potential
applications for PLA include:
The process involves fermentation of unrefined dex-
trose, derived from corn, to give
D
- and
L
-lactic acids,
which are converted to
D
-,
L
- and
meso
-lactides
before polymerisation (Fig. 2.13). By controlling the
D
,
L
and
meso
ratio, together with molecular weight,
polymer properties can be tailored to meet product
specifications [34].
Society in the not too distant future will need
to find viable alternatives to the use of fossil fuels
for energy and probably for the synthesis of many
chemicals. If the solution is to grow our energy, as
opposed to using solar cells for example, then we will
need to face the issues concerned with land usage
[35]. Although there is no real shortage of land on
the planet, there are serious debates as to the viabil-
ity of growing most of our energy needs. These
debates centre on land quality, accessibility, nearness
to population centres, etc.
4.4 Process intensification
When designing a chemical process the engineering
aspects are as important as the chemistry and it is
often the lack of interaction between chemists and
engineers at an early enough stage that results in
processes being developed that are not as green or
efficient as they otherwise could be. In many ways
process intensification can be regarded as the engi-
neering solution to green chemistry problems;
the concept originated in the 1970s as a means of
making large reductions in the cost of processing
systems [36]. Like many cost reduction concepts,
process intensification is concerned mainly with
reducing materials use and energy consumption by
reducing plant footprint and increasing throughput.
Some of the key aspects of process intensification are
shown in Fig. 2.14 [37].
A fuller account of process intensification is pre-
sented elsewhere in the topic but in the context
of materials reduction it is worth mentioning an
• Packaging—PLA can have the processability of
polystyrene and the strength properties of
poly(ethylene terephthalate), with good resistance
to fats and oils.
• Textiles—PLA has good drape, wrinkle and UV-
light-resistance properties.
