Uses of Waste Starch - The Economic Utilisation of Food Co-Products - page 77

filler into polyethylene matrix. 88,89 Fillers can enhance different properties of
the polymer including strength, stiffness, permeability and cost. However, poor
adhesion between fillers and polymer matrix can lead to undesirable conse-
quences, such as decreased tensile strength and elongation. 90 Starch and
polyethylene are incompatible, therefore the filler or the polymer has to be
modified to improve the adhesion between the two components. 91 Maleic
anhydride or acrylic acid have been used as compatibilisers and grafted to
polyethylene chains in order to enhance its compatibility with starch. 92
Modification of starch with n-octenyl succinic anhydride was found to
increase the compatibility of starch and polyethylene, resulting in improved
tensile strength of the composite. 93
However, blending starch with nondegradable polymers only yields partially
biodegradable products. In order to prepare completely biodegradable starch-
based composites starch is typically blended with aliphatic polyesters, polyvinyl
alcohol (PVA) or other biopolymers.

d n 9 r 3 n g | 0

5.3.8.3 Starch Blends with Synthetic Degradable Polymers
Synthetic degradable polyesters that have been blended with starch include
poly(hydroxyl alkanoates) (PHAs), poly(lactic acid) (PLA) and polycapro-
lactone (PCL). PHAs are naturally occurring polyesters produced by
bacteria. 94 One of the most studied PHAs is poly(3-hydroxybutyrate) (PHB). 95
The primary aim of the inclusion of starch into PHB resins was to improve the
properties of the composites and lower the production cost. The resin produced
from PHB/starch ratio 70/30 (wt./wt.) present a cost-effective material with
enhanced mechanical properties compared to those of virgin PHB. 96
Poly(lactic acid) is a biodegradable thermoplastic polymer of lactic acid that
can be obtained through waste starch/ carbohydrate fermentation (as discussed
previously in Section 5.2.7). 84 Starch and PLA are immiscible due to the poor
interfacial attraction between starch granules and PLA matrix. Compatibilisers
such as maleic anhydride or methylenediphenyldiisocyanate (MDI) are
therefore needed in the composite to maintain the mechanical properties of the
product. 97,98
Starch has also been blended with petroleum-derived biodegradable
polymers such as PCL and PVA. The association of PCL with thermoplastic
starch (TPS) has been studied and the dimensional stability and hydrophobicity
of the blends was improved significantly compared to those of native TPS,
despite the phase separation of the blend. 99 The use of high amylose starch as a
filler in PCL-based materials gives better mechanical properties than other
starches owing to its small granular size, allowing good dispersion in PCL
matrix. 100
PVA and starch are compatible and the addition of PVA into the starch
matrix has demonstrated enhanced tensile strength and elongation. 101
However, the fact that both starch and PVA are hydrophilic limits their
applications. Methods that have been attempted to enhance the water

3 .