Based on the degradation mechanism of silicone rubbers
[37e39], the decomposition of methyl groups attached to silicon
atoms leads to generation of silanol groups, which can be further
cross-linked by the formation of new SieOeSi bonds through
condensation reaction. Accordingly, the increased degree of crosslinking
can result in the increase in elastic modulus. Fig 6a displays
the variation of elastic modulus of silicone rubbers with
different hardness after exposed to simulated solutions for a period
of times. As expected, all the samples exhibited slight increase in
elastic modulus with the increase in exposure time. While the
hardness of silicone rubbers increased, the increase in elastic
modulus under the same conditions became less pronounced. For
example, after exposure to simulated solution for 30 days, the
elastic modulus for silicone rubber with hardness of 30 increased
by 45.9% whereas only 8.5% increase was observed for silicone
rubber with hardness of 50, indicating that silicone rubbers became
more durable in simulated solutions with the increase in hardness.
This phenomenon could be attributed to the increased degree of
cross-linking with the increase in hardness of silicone rubbers. Fig
6b shows the variation of elastic modulus of silicone rubber with
hardness of 40 in different solutions. It can be seen that the change
of elastic modulus of the tested material exposed to simulated solution
is slightly smaller than that exposed to acetic solution and is
much smaller than that exposed to strong acid solution. This could
be attributed to the possible dissolution of silicone rubbers in high
acidic solutions, particularly with the presence of Fþ in solutions.