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domains of P(3HB-co-3HHx) were found in the PLA and it was concluded to
be the cause of a comparatively more ductile PLA with minimal aggregation
of the P(3HB-co-3HHx).
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8.6.4 Sustainability
Plastics derived from biological sources such as the PHA family of polymers
are renowned for their environmentally friendly properties.
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PHAs are
biodegradable and sourced from the bacterial consumption of glucose or
fatty acids; they not only contain zero percent petrochemical monomers
but are a biological polymer created in an aqueous phase at temperatures of
o
40 1C.
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Sustainability consists of environmentally favourable production,
coupled with the environmental recycling of the product. Furthermore, the
economic drive to reduce costs in PHA production has supported the use of
carbon waste sources from agriculture and the sewage treatment system.
Thus, waste carbon is given added value and effectively recycled, further
supporting sustainability.
8.6.5 Barrier Properties
When compared to the other typical ff-polymers of polyethylene (PE) and
polystyrene (PS) used in packaging, the oxygen and water barrier properties
of commercial packaging PHA resins are considered to be naturally of a
superior level (Figure 8.6).
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In order for the plastic packaging to be effective against aroma molecules,
prevention of these molecules, often less than 400 MPa in size, from crossing
the polymer barrier needs to be maintained.
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The vapour pressure exerted
by these small aroma bearing molecules provides an added challenge for the
application of biopolymers in packaging. A packaging's vapour barrier
properties relates to its ability to prevent water vapour from crossing the
polymer packaging boundary. Several factors including mechanics, morpho-
logy and crystallinity can play a substantial role in determining a packaging's
vapour barrier properties.
.
8.6.5.1 Additives to PHAs for Barrier Enhancement
One particular strategy of improving PHA packaging barrier properties
would be to develop suitable nanocomposites. In particular, nanocompo-
sites incorporating nanoclays of montmorillonite and kaolinite clays could
also substantially improve the mechanical strength and thermal stability as
well as the gas barrier properties. The addition of layered silicates or clay-like
materials has also been demonstrated to improve the barrier properties of
PHAs.
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When an organically modified kaolinite and an organically modified
montmorillonite (MMT) were incorporated into P(3HB) at 4 wt% loading, the
oxygen permeability was found to decrease by 43% at 24 1C and a relatively
humidity of 0%.
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