Stress relaxation and thermogravimetric studies on room temperature vulcanised polysiloxane rubbers

r Commercial 2, Defence Procurement
Agency, Ash 2b, Mailpoint 88, Ministry of Defence, Abbey Wood,
Bristol BS34 8JH, England.
* Corresponding author.
E-mail address: mogon.patel@awe.co.uk (M. Patel).
are defined by the extent to which it regains its original
dimensions after release from deformation, under the
same environmental conditions as applied during the
deformation stage [3].
Such rubbers are usually prepared using a stannous
2-ethylhexanoate catalyst to induce the crosslinking and
foaming reactions. Previous studies by Patel et al. [4]
have shown that residual tin catalyst fragments in the
rubber may have a significant influence on the important
load bearing characteristics of the material as well
as the low temperature thermal properties. Furthermore,
Stein et al. [5], showed that the tin catalyst, together
with water, produces siloxane bond rearrangement
(cleavage followed by recombination of the siloxane
linkages) resulting in chemical stress relaxation. Polymer
chain ends attached to the rubber network may also
have an important influence on compression set properties
through self diffusion processes. A large number
of chain ends can be generated if the efficiency of the
crosslinking reaction is poor. Consequently, on compression
and collapse of the cell structure, the cell walls
would be brought into contact with each other and selfdiffusion
effects, would lead to entanglements of the free
chains and adhesion between cell walls. Compression set
will occur when the forces of adhesion are sufficient to
prevent disentanglement by normal elastic restoring
forces. This type of compression set may not be permanent
in the accepted sense (as it may recover in time) but
is still cause for concern with regard to the long-term
performance of the rubber.
The main difference between physical and chemical
relaxation processes, in terms of their impact on rubber
compression set properties, is that certain physical processes
such as entanglements, diffusion of chain ends or
cell wall buckling, have the potential to recover with
time whereas all chemical relaxation processes are nonrecoverable.
On removal of load, the recovery in foam
thickness due to physical processes will be rapid followed
by a gradual recovery process which may take
several days. The recovery may also remain incomplete
and require an external stimulus such as heat or solvent
to complete. The aim of the work reported here was to
develop an improved understanding of the nature of the
degradation processes that influence stress relaxation
and to establish a temperature range over which Arrhenius
kinetics apply. Of particular interest was whether
there is a change in the underlying degradation processes
with temperature and whether the mechanism
responsible for stress relaxation also induces other
material property changes.
2. Experimental
2.1. Materials
Foamed S5370 polysiloxane rubber pads were tested
as received from LANL (Los Alamos National
Laboratories). These rubbers were prepared by adding
5% by wt. of stannous 2-ethylhexanoate catalyst to 30 g
of S5370 polysiloxane gum (both supplied by Dow
Corning). The polymer begins expanding and vulcanising
as soon as the catalyst is added as the reaction
readily proceeds at room temperature. For this reason,
20 g of the mix was quickly placed in a perspex mould
having a 2 mm thick cavity and moulded under compression.
The polymer was cured at ambient conditions
for 20 min and post-cured at 120
C for 180 min.
2.2. St