Biomedical Engineering Reference
In-Depth Information
PTFE/PDMS can be obtained via a similar process [112]. PTFE chains are
trapped in the PDMS network during casting. Jones et al. [113] also patented
a process where adipic acid and hexane diamine (or
-caprolactam [114])
were added to a silicone latex leading to the formation of a polyamide/PDMS
semi-IPN.
ε
4.2.4
Semi-IPNs
From the considerable number of patents in this category, it appears that
semi-IPNs are of particular commercial interest. Thermoplastic elastomers
(TPE) where the elastomer phase is crosslinked are sometimes called ther-
moplastic vulcanizates (TPV). More specifically, when the elastomer phase is
a silicone one, then the material is called a silicone thermoplastic vulcanizate
(
STPV
) [115]. One can also find the term “TPSiV” used but it is a trade name
and hence is less employed. This growing commercial interest is especially
due to the characteristics of the final material which simultaneously exhibit
the properties of both the thermoplastic and the silicone. Moreover, their pro-
cessability and recycling potential make them of great interest compared to
strict IPNs. As the network formation forces the polymers to entangle, chem-
ical interactions should play a minor role. It has nevertheless been observed
that the chemical nature of the silicone side groups drives the degree of in-
terpenetration [116] and hence the phase separation and the properties of the
final materials. Among the other parameters that drive the final structure are
the crosslink density and method of synthesis, both of which are of prime
importance.
The original use of hydrosilylation [117] in semi-IPN formation is at-
tributed to Arkles [118-120]. His approach was to mix two polysiloxanes with
the thermoplastic. One contained hydride functions and the other vinyl func-
tions. The subsequent introduction of a platinum catalyst induced crosslink-
ing. Depending on the number of hybrid and vinyl functions per chain, either
chain-extension or crosslinking was observed. As pointed out by Arkles [119],
according to theory, these phenomena should be obtained for a 1 : 1 hy-
dride : vinyl group ratio, however, in practice the hydride functions were
consumed by several side reactions so that a higher hydride ratio was re-
quired.
This technology was first commercially applied to polyurethane blend [121]
and patented as Rimplast® (for Reactive Injection Molding), but many poly-
mers have since been blended with polysiloxane thanks to this method:
polyethylene [122], polypropylene [122, 123], polyamide [124-130], poly-
esters [128, 131-133], poly(phenylene ether) [134], fluorocarbons [135] and
many more. Many of them include reinforcing fillers such as fumed silica.
Thesiliconebaseinvolvedcanmoreovercontainreactivegroupssuchasthe
epoxy group [136, 137]. A typical silicone base useful for these blends was de-
