Polyhydroxyalkanoates as Packaging Materials: Current Applications and Future Prospects - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 198

properties of conventional, synthetic chemicals. For example, addition of
P(3HB) to polyvinyl alcohol (PVOH) can lead to a significant improvement
in its barrier properties. Thellen recently reported the use of a multilayer
film strategy, where PVOH formed a core and PHA formed an outer
skin layer. The barrier property of oxygen transmission rate (OTR) for this
multilayered film was decreased by 55% at a relative humidity (RH) of 0%
and showed a tremendous enhancement of 95% at a relative humidity of
60%. 60
Using PHAs as an additive is only one strategy adopted to modify the water
barrier properties of a material. A coating of P(3HB) on cellulose-based
cardboard demonstrated significant improvements to its resistance to water
uptake; a 90% reduction in moisture absorption was measured when the
cardboard was coated with films containing 20% (w/w) P(3HB). 61
Optimisation of the loading (wt%) of PHA additive is critical in ensuring
maximum barrier conditions are obtained; non-optimisation can also lead to
substantially decreased barrier properties, even lower than that exhibited by
the unblended ff-polymer that the PHA additive is attempting to improve.
The addition of P(3HB) to low density polyethylene (LDPE) was investigated
by Pankova and co-authors for the purposes of drug delivery and
biodegradation studies. 62 The authors determined that an optimal amount
of 4.0 wt% addition of P(3HB) to LDPE produced a reciprocal water
permeability of 7.3 10 9 (cm 2 h mm Hg)/(g cm). However, when the P(3HB)
content was increased to 10 wt%, the reciprocal water permeability
decreased significantly to 2.5 10 9 (cm 2 h mm Hg)/(g cm); this compares to
3.5 10 9 (cm 2 h mm Hg)/(g cm) for P(3HB) homopolymer. 62

d n 2 r 4 n g | 3

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8.6.6 Degradation and Antimicrobial Properties
There are a wealth of studies on the degradation of PHAs, methodology of
degradation and the microbial interaction with the polymer. The overall
conclusion of these studies is that PHAs will degrade in practically any
natural environment when exposed to a microbial community, although the
rates of this degradation are dependent upon the environmental conditions.
In this section, we demonstrate the potential importance of these properties
in the development of plastic packaging using a few articles that deal directly
with the topic of preventing microorganisms from crossing or establishing
colonies on PHA-based packaging surfaces. Similarly, the antimicrobial
properties directly correlate with the degradation rates and behaviour of
PHA-based packaging systems. 63
Unlike petroleum-based polymers, PHAs are broken down in the en-
vironment and the rate of degradation is influenced by factors such as pH,
moisture, temperature, the molecular weight of both the individual's
monomer units and the chain lengths as well as crystallinity and the absence
or presence of antimicrobials within the PHA. The degradation of PHA
packaging is considered to be the natural consumption of the polymer by
biological microorganisms to produce CO 2 and H 2 O.