Biomedical Engineering Reference
In-Depth Information
Precise conditions are required for industrial processing PHA polymers. For ex-
ample, Biomer (Germany) has recommended to process PHA polymers in two-step
melt-state transformation, for Biomer products in particular: first, melting the product
when its introduction into the equipment and, second, to carry out a gradual decrease
of temperature in the next zones until the die: 185°C in zone 1 down to 150°C in zone
4 and 145°C in the die for Biomer grade P226 (PHB polymer). As of hydroxyvalerate
units incorporated into PHB polymer, the process temperatures were lower than that of
PHB ones and was kept below 170°C. The temperature of the barrel at the bottom sec-
tion and of the injection molder (155°C) was kept below the melting point of PHBV
(at around 157°C). Bhardwaj and co-workers presented the fabrication of eco-friendly
green composites from PHBV with 13 mol% valerate units (Zeneca Bio Products from
Biomer) by co-rotating twin-screw microcompounder extrusion (Bhardwaj, 2006).
The temperature profile used for a perpendicular barrel at top, center, and bottom sec-
tions are 155°C, 160°C, and 155°C, respectively. The lower melting point of PHBV
improves the melt stability and broadens the processing window of this polymer.
Therefore, melt processing should be limited to a narrow temperature window and
alternative processing technologies can be developed, such as solid-state processing,
in order to minimize thermal degradation.
Solid-state Processing
Depending on the combination of pressures with temperatures used, the solid-state
processing can resemble to powder sintering (metallurgy process) or to hot-press
moulding. The solid-state processing consists in transform polymer powders at very
high pressures and elevated temperatures but below the melting point.
A two-step solid-state transformation by sintering powders has been described by
Lüpke and co-workers (Lüpke, 1998) and its principles are also applicable to other
PHA polymers than PHB. The authors estimated that the ultimate mechanical proper-
ties depend substantially on the initial molecular weight of polymer as well as on the
processing temperature. A slight decrease of crystallinity was also reported.
Chen and co-workers (Chen, 2007) reported the preparation of PHBV/hydroxy-
apatite nanocomposites by using hot-press molding for thermal and physical tests in
order to consider potential medical applications.
transport Properties methods
From published reports and studies, we can note that some technical procedures de-
pend on the background of scientific teams and available apparatus for sorption and
permeation measurements (Alexandre, 2009; Erkske PEASC, 2006; Follain, 2010;
JAPS, 1999a, 1999b; Marais, 1999; Miguel Yoon JAPS 77, 2000). In addition, the use
of different equipments and dissimilar conditions for the measurements complicated
the comparison of the different results obtained.
Water Vapor Sorption
Water sorption isotherm is useful for predicting water sorption properties of polymer
and allowed to provide little insight into the interaction between water molecules and
polymer tested.
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