Polyhydroxyalkanoates as Packaging Materials: Current Applications and Future Prospects - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 194

8.6.1 Mechanical Properties
Compared to conventional, synthetically derived plastics, the mechanical
properties of PHA biopolymers are often seen as inferior. 49 The argument for
composite materials, and indeed where the additive is of a nano-dimension,
is proposed as a solution that produces mechanical properties far more
desirable from a packaging point of view. 49 The nano-dimension additives
have the extra advantage of possessing a high surface area and therefore
increasing the matrix and additive interaction. However, the ultimate fate
of these nano-particles has raised concerns and is an area worthy of
greater study.
Further support for nanocomposite blends involving PHA was highlighted
by the study of Ohashi et al. in which they aimed to produce packaging films
by blending P(3HB), polyethylene (PE) and colloidal silica. Blending was
achieved in a single-screw extruder and the mechanical properties proved to
be of great interest. For a small amount of added silica (0.2 to 0.4% where
moles or wt% is not specified by the article), the tensile strength increased
significantly, by 20% in magnitude. 50 Importantly, the improvement in
mechanical properties thanks to nano-composite blending highlights that
properties desired within PHB can often be tuned as needed with additive
polymers such as PE and SiO 2 but ideally the additives should be from re-
newable sources in their own right.

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8.6.2 Optical Properties
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, modified atmosphere and printability allow plastic packaging
to be tailored to the type of food to be contained. Thus, the food's visual
appeal can be enhanced while maintaining preservation against ageing,
degradation and chemical or microbial contamination.
Maintaining optical transparency in packaging is often desirable, espe-
cially from a customer's visual point of view and when observation of the
package's contents is desirable for, for example, food or safety products. The
optical properties of PHA can often be traded for the need to improve barrier
properties over that of pure PHA homopolymer. 51 However, careful screen-
ing and characterisation of certain additives or fillers can produce improved
barrier properties without compromising optical properties. For example,
the addition of keratin derived from waste chicken feathers provided im-
proved barrier properties to P(3HB) and P(HB-co-HV) without diminishing
optical properties with a keratin content of up to 5%. 51
Optical property optimisation has been known to be given an 'elaboration'
or secondary priority status when packaging design has been considered.
A far greater focus has been placed on ensuring good mechanical and barrier

.