Chemistry Reference
In-Depth Information
The same can be accomplished by replacing a small quantity of the methyl groups with vinyl
ones. This can be done by including a small amount of vinyl methyldichlorosilane into the
monomer mix (about 0.1%). The product with a small quantity of pendant vinyl groups cross-
links readily by free-radical mechanism. When portions of the methyl groups on the polysiloxane
backbones are replaced with phenyl structures, the elastomers exhibit particularly good low
temperature properties [
159
].
Room temperature cross-linkable
-s) are prepared
by two techniques. In the first one, chloro siloxanes with functionality larger than two are added to
hydroxyl-terminated prepolymers. The products are subsequently cross-linked by a second addition
of polyfunctional compounds like tetraalkoxysilane in the presence of tin catalysts, like stannous
octoate. Cross-linking occurs at room temperature. This reaction may vary from 10 min to 24 h:
polysiloxane elastomers (commonly called
RTV
O i
OH
HO
Si
OC
2
H
5
O i
O
O
Si
C
2
H
5
O
Si
OC
2
H
5
Si
OC
2
H
5
O i
O
O
Si
O i
H
HO
Si
In the second one, polysiloxanes terminated by hydroxyl groups are either: (1) acetylated, (2)
converted to ketoximes, or (3) etherified. The cross-linking is activated by reaction with atmospheric
moisture:
O
Si
Si
Si
Si
+
O
HO
O
O
O
+
(1)
CH
3
COOH
R
N
Si
+
Si
(2)
Si
Si
O
O
HO
O
O
R
R
+
HON
R
Si
Si
(3)
+
Si
Si
O
OR'
HO
O
+
R'OH
O
The acetate-capped RTV-s exhibit good adhesion to substrates, but the released acetic acid can be
corrosive.
