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

Chapter 1.
Polylactide Based Bio-Materials
many <strong>of</strong> <strong>the</strong> same undesirable mechanical properties, such as low heat deflection temperature. The polymer
is also very brittle and has a low-melt strength leading to difficulty in processing. Consequently, most
commercial applications using PLA require a syn<strong>the</strong>tic rubber and/or acrylic additive to compensate for
<strong>the</strong>se deficiencies.
Figure 1.1: Structures <strong>of</strong> selected biodegradable polymers.
[Gross and Kalra, 2002]
Degradation <strong>of</strong> PLA occurs quickly through a multistep process <strong>of</strong> chemical depolymerization,
followed by dissolution <strong>of</strong> <strong>the</strong> intermediate lactic acid in <strong>the</strong> presence <strong>of</strong> moisture, and <strong>the</strong> absorption into
<strong>the</strong> cells <strong>of</strong> microorganisms with subsequent metabolization [Dunja Manal Abou Zeid, 2001]. Initiation <strong>of</strong>
this chain <strong>of</strong> events typically occurs at elevated temperatures (above heat deflection temperatures), such as
conditions existing in an industrial compost operation. The relatively fast chemical reaction at <strong>the</strong> beginning
<strong>of</strong> <strong>the</strong> chain <strong>of</strong> events explains <strong>the</strong> surprisingly quick degradation <strong>of</strong> polymer in an industrial compost
environment. This mechanism <strong>of</strong> chemical attack followed by cell metabolism does not meet <strong>the</strong> true
definition <strong>of</strong> a biodegradable material.
1.3.2.2 Synergistic or Hybrid Polymers
Synergistic polymers are typically intimate mixtures <strong>of</strong> oil-based and naturally occurring polymers
where <strong>the</strong> two have some chemical affinity for each o<strong>the</strong>r. When mixed, <strong>the</strong>re is intimate contact between
<strong>the</strong> two polymer chains so as to create a homogenous single phase. In o<strong>the</strong>r words, once mixed <strong>the</strong>y could
not be mechanically separated. A good example <strong>of</strong> a commercial, synergistic, biodegradable material is
<strong>the</strong>rmoplastic starch. The key to this blend <strong>of</strong> <strong>the</strong> two natural starch polymers, amylose and amylopectin, and
<strong>the</strong> syn<strong>the</strong>tic polymer, polyvinyl alcohol (PVOH), is <strong>the</strong>ir natural affinity to each o<strong>the</strong>r, due to <strong>the</strong> large
number <strong>of</strong> hydroxyl (OH) groups present in <strong>the</strong> compounds (cf. Figure 1.1). This hybrid can be made into
foamed articles, plastic films or moulded parts such as cutlery.
The intimate mixing <strong>of</strong> <strong>the</strong> natural and syn<strong>the</strong>tic polymers can be taken one step fur<strong>the</strong>r: where <strong>the</strong>
attraction <strong>of</strong> <strong>the</strong> syn<strong>the</strong>tic and natural polymers is enhanced by grafting o<strong>the</strong>r chemically compatible groups
along <strong>the</strong> chains <strong>of</strong> <strong>the</strong> natural and/or syn<strong>the</strong>tic polymers. Initiation <strong>of</strong> <strong>the</strong> process begins with <strong>the</strong> formation
<strong>of</strong> a bio-film on <strong>the</strong> surface <strong>of</strong> <strong>the</strong> polymer, which is facilitated by <strong>the</strong> inclusion <strong>of</strong> <strong>the</strong> compatible natural
polymers. These films <strong>of</strong> microorganisms have been shown to efficiently biodegrade petroleum based
polymers [Seneviratne et al., 2006].
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