Chemistry Reference
In-Depth Information
can be fabricated immediately, it is common practice to permit the mate-
rial to “recover” or “age” for a few days. If hardening occurs during this
period, some remilling (“refreshening”) will be required.
A variety of processing steps may then be carried out. Examples are
compression molding, injection molding, transfer molding, extrusion, cal-
endering, dispersion coating, and blowing into foams.
104,
107
Although some cures can take place at room temperature, most are car-
ried out at elevated temperatures.
104,
117-120
Conventional electric, gas or
forced-air ovens are used, but presumably microwave heating could be
used as well. There has been some interest in developing polysiloxane
thermoplastic elastomers.
172
Attempts have also been made to model the
processing various types of polysiloxane materials.
173
2.6 ANALYSIS AND TESTING
Infrared and UV spectroscopy are often used to determine the composi-
tion of siloxane copolymers and of mixtures of siloxanes and silicates with
other species.
1,
174
These spectroscopies can also be used to monitor (i)
vinyl groups introduced to facilitate cross-linking, (ii) phenyl groups to
suppress crystallization or to improve radiation resistance, or (iii) silanol
end groups introduced during polymerization and used to determine
number-average molecular weights, or for chemical reactions such as end
linking. Some important absorbances are those for Si-O-Si groups at
1010 cm
-1
, Si(CH
3
)
2
groups at 800 cm
-1
, SiCH
3
groups at 1260 cm
-1
, and
SiH groups at 2200 cm
-1
. Not surprisingly, these methods are used for
quality control in the commercial production of siloxane products.
1, 174
NMR (nuclear magnetic resonance) is used for a variety of purposes,
most of which parallel those used to characterize small-molecule sys-
tems.
1, 174
In addition to
1
H and
13
C NMR,
29
Si NMR is frequently em-
ployed. These methods are used to characterize chemical composition,
structural features, and conformational preferences. NMR is also used to
characterize hybrid inorganic composites, silica-type ceramics, and silox-
ane films.175-176
175 -176
Specific functional groups are also analyzed by chemical methods. For
example, the various chlorosilanes can be hydrolyzed and the resulting
chloride ions determined by titration with silver salts. Si-H groups can be
determined by measurement of the amount of hydrogen gas evolved
during hydrolyses. As a final example, silanol groups can be monitored
through measurement of the amount of methane gas evolved when they
react with methyl Grignard reagent.
1, 174
