Influence of the Processes Parameters on the Properties of The ...

Chapter 1.
Polylactide Based Bio-Materials
from anaerobic digestion is biogas, consisting <strong>of</strong> mostly methane and carbon dioxide, with trace gasses such
as ammonia and hydrogen sulphide. Often, <strong>the</strong> complete digestion will require several different types <strong>of</strong>
bacteria where one type partially processes <strong>the</strong> waste to a point where ano<strong>the</strong>r bacterium strain takes over.
Most biodegradation <strong>of</strong> solid waste in landfill occurs under anaerobic conditions by design because it is
typically much slower than aerobic degradation. In <strong>the</strong> absence <strong>of</strong> oxygen, <strong>the</strong> reaction <strong>of</strong> degradation is
given by equation 1.2:
C Polymer → CO 2 + CH 4 + H 2 O + + C Residue + C Biomass (1.2)
Complete biodegradation occurs when no residue remains, and complete mineralisation is
established when <strong>the</strong> original substrate (C Polymer in this example) is completely converted into gaseous
products and salts. However, mineralisation is a very slow process under natural conditions because some <strong>of</strong>
<strong>the</strong> polymer undergoing biodegradation will initially be turned into biomass [Bastioli, 2005]. Most
biodegradable substances come from plant and animal matter, or from artificial materials that are very
similar in molecular structure to <strong>the</strong>se naturally occurring substances. As <strong>the</strong> naturally occurring substances
evolved, micro-organisms also evolved to use <strong>the</strong> substances as a food source, carbon in particular, used as a
building block for life-sustaining compounds. Simple sugars are readily absorbed into <strong>the</strong> cell to be
metabolized. However, larger and more complex molecules such as starches, proteins and cellulose, require
enzymes and acids to reduce <strong>the</strong>ir size enough to be absorbed. Living organisms have developed <strong>the</strong> ability
to secrete specific digestive compounds so as to best utilize <strong>the</strong> available food supply. For example, <strong>the</strong>
enzyme amylase, found in human saliva, is used to breakdown long-chain starch molecules into smaller
simple sugars.
1.3.2 Biodegradable Polymer Materials
Currently available degradable polymer materials can be broken down into two main groups:
Polyester polymers,
Synergistic and hybrid polymers.
1.3.2.1 Biodegradable Polyesters
Biodegradable polyesters which do not contain six-carbon rings are known as aliphatic polyesters.
They will typically react with moisture at elevated temperatures to breakdown <strong>the</strong> long polymer chains. This
process, called chemical hydrolysis, reduces <strong>the</strong> higher molecular weight polymer to much smaller
hydrocarbon compounds. The resulting molecules can <strong>the</strong>n be absorbed by microorganisms and metabolized
for energy. Since it is a chemical reaction, <strong>the</strong> hydrolysis occurs at a much higher rate than one would expect
for a purely biological process, and as a result, relatively quick degradation is observed.
Aliphatic polyesters have attracted interest as biodegradable plastic materials; however <strong>the</strong>y
typically have poor physical and mechanical properties like strength, flexibility, heat resistance, etc. [Chen
et al., 2008]. Some common biodegradable polyester polymers in commercial use include
poly(caprolactone), poly(glycolic acid) and poly(butylene succinate) (cf. chemical formula reported on
Figure 1.1). Although expensive to make, <strong>the</strong>se biodegradable polymers are ideal for use in specialized, high
margin applications such as medical devices (e.g. dissolving, drug delivery systems, tissue engineering
scaffolds and bone repair etc.) [Ikada and Tsuji, 1999].
Ano<strong>the</strong>r well known aliphatic polyester is poly(lactic acid). PLA is a syn<strong>the</strong>tic polymer made
from fermented sugars extracted primarily from food crops such as corn, beets or sugarcane. The resulting
lactic acid monomer is chemically processed and <strong>the</strong>n polymerized, in <strong>the</strong> presence <strong>of</strong> a metal catalyst, to
form <strong>the</strong> high molecular weight plastic material. Like petroleum-based biodegradable polyesters, PLA has
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