Biomedical Engineering Reference
In-Depth Information
In this way, two chains have been linked, and the
reaction will proceed further from the remaining
acetoxy groups. An organometallic tin catalyst is
normally used. The cross-linking reaction requires mois-
ture to diffuse into the material. Accordingly cure will
proceed from the outside surface inward. These materials
are called one-part room temperature vulcanization
(RTV) sealants, but actually require moisture as a second
component. Acetic acid is released as a by-product of the
reaction. Problems resulting from the acid can be over-
come using other cure (cross-linking) systems that
have been developed by replacing the acetoxysilane
RSi(OAc) 3 with oximosilane RSi(ON ΒΌ CR 0 2 ) 3 or alkoxy-
silane RSi(OR 0 ) 3 .
Condensation curing is also used in some two-part
systems where cross-linking starts upon mixing the two
components, e.g., a hydroxy end-blocked polymer and an
alkoxysilane such as tetra- n -propoxysilane (Noll, 1968):
Fig. 3.2.3-1 RTV silicone adhesive.
Because a large excess of silane is used, the probabi-
lity of two different chains reacting with the same
silane molecule is remote. Consequently, all the chains
are end-blocked with two acetoxy functional groups.
The resulting product is still liquid and can be pack-
aged in sealed tubes and cartridges. Upon opening the
acetoxy groups are hydrolyzed by the ambient moisture
to give silanols, which allow further condensation to
occur.
Here, no atmospheric moisture is needed. Usually an
organotin salt is used as catalyst, but it also limits the
stability of the resulting elastomer at high temperatures.
Alcohol is released as a by-product of the reaction,
leading to a slight shrinkage upon cure (0.5 to 1% linear
Search WWH ::




Custom Search