Polyhydroxyalkanoates (PHAs), their Blends, Composites and Nanocomposites: State of the Art, New Challenges and Opportunities - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 11

recognised in the 1980s with the commercial release of Biopol s , thermo-
plastic resins of P(3HB) with various copolymer loadings of (3HV), by Im-
perial Chemical Industries (ICI, now Zeneca). One of the prohibiting factors
against the use of PHAs as a resin for packaging materials is that it is eco-
nomically uncompetitive in the current market compared to fossil fuel
sourced synthetic raw material (ff-polymers). 107,108 In the production of
PHAs, the cost of the carbon substrate represents approximately 50% of the
total production cost. In order to rival current synthetic polymers used in
packaging such as polyethylene (PE), polypropylene (PP) or polystyrene (PS)
for example, PHAs' physical and chemical properties need to be comparable.
The optical properties of plastic packaging, certainly in the case of food
packaging, offer a convenient, lightweight and flexible adaption of pack-
aging technologies for the food industry, reducing the reliance upon glass
and metallic canning. Transparency, a variety of packaging options such as
shrink wrap, a modified atmosphere and printability allow plastic packaging
to be tailored to the type of food to be contained. The thermal properties are
a vital consideration when selecting a polymer for packaging. Fortunately,
PHAs provide (through diversity of structure and chemistry) a wide range of
thermal properties for selection to suit packaging needs. Melting tempera-
tures (T m ) from 60 to 177 1C, glass transition temperatures (T g ) from -50 to
4 1C and thermal degradation temperatures at highs of 256 to 277 1C are
all within the range of the PHAs currently being produced. 109 Carboxyl-
terminated butadiene acrylonitrile rubber (CTBA) and polyvinyl pyrrolidone
(PVP) have been added to PHB in an effort to modify its thermal processing.
Hong et al. report a significant modification of PHB crystallisation rate,
crystallinity, melting temperature and thermal stability with the addition of
only 1% (w/w) of these additives. 110 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. 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 mechanical,
morphology and crystallinity can play a substantial role in determining a
packaging's vapour barrier properties. One particular strategy of improving
PHA packaging barrier properties would be to develop suitable nano-
composites. In particular, nanocomposites incorporating nanoclays of
montmorillonite and kaolinite clays could also substantially improve the
mechanical strength and thermal stability as well as the gas barrier prop-
erties. In a similar strategy as that used to improve the thermal properties of
polymers, PHAs have been used as additives to improve the barrier prop-
erties of conventional, synthetic chemicals. For example, addition of P(3HB)
to polyvinyl alcohol (PVOH) can lead to significant improvement in its bar-
rier properties. The herbicide product was successful delivered over a period
of time with gradual degradation of the PHA packaging and was successful

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