Chemistry Reference
In-Depth Information
properties first with optical transparency, weld strength, hot tack and
transport properties being optimised on a second pass.
52
8.6.3 Thermal Properties
The thermal properties are of a vital consideration when selecting a polymer
for packaging. Fortunately, PHAs provide (through diversity of structure and
chemistry) a wide range to select for thermal properties suitable to packaging
needs. Melting temperatures (T
m
)from60to1771C, glass transition tem-
peratures (T
g
)from
50 to 4 1C and thermal degradation temperatures at
highs of 256 to 277 1C are all within the range of PHAs currently being pro-
duced.
53
Despite this variety of thermal properties within the family of PHAs,
it is important to note that P(3HB), the most commonly studied homo-
polymer, exhibits thermal instability during conventional melt processing at
or near its melting point temperature (175 1C) and consequently provides only
a narrow processing window for temperature driven processes.
54
The thermal
degradation of PHB is understood to be due to a random chain scission
process. The proximity of the melting point and thermal degradation tem-
perature of P(3HB) have supported the inclusion of additives to separate these
two temperatures and to better support thermal processing.
d
n
2
r
4
n
g
|
3
8.6.3.1 Additives to PHAs
Carboxyl-terminated butadiene acrylonitrile rubber (CTBA) or 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 crystal-
lisation rate, crystallinity, melting temperature and thermal stability with the
addition of only 1% (w/w) of these additives.
55
The improvement in thermal
stability was proposed to be due to the steric hindrance that the PVP or
CTBN exerted on the organisation of the PHB chains. Similarly, Wang and
co-authors demonstrated how the addition of poly(d,l-lactide) (PDLLA) into
P(3HB-co-3HV) substantially improved the thermal stability of this co-
polymer as measured with TGA and DSC. Furthermore, the addition of PEG
to the blend of P(3HB-co-3HV) and PDLLA had the added benefit of accel-
erating degradation of the composite at room temperature, with a mass loss
of 20% after 30 days.
56
.
8.6.3.2 PHAs as Additives
In an effort to improve PHB's properties, Lim et al. synthesised a copolymer
of P(3HB) with P(3HHx), P(3HB-co-3HHx), using fats and oils.
57
The prop-
erties of P(3HB-co-3HHx) extended the potential application range of pure
PHAs and also provided another copolymer blend ingredient for other bio-
polymers such as PLA. Lim and co-authors demonstrated that increasing
amounts of P(3HB-co-3HHx) melt blended with PLA increased suppression
of PLA crystallization.
57
Despite a lack of chemical interaction, amorphous
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