Characterisation and oxidative degradation of a
room-temperature vulcanised poly(dimethylsiloxane) rubber
A.N. Chaudhry, N.C. Billingham*
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QJ, UK
Received 20 September 2000; accepted 30 December 2000
Abstract
A typical room-temperature vulcanised, foamed, silica-filled siloxane rubber has been characterised using FTIR(ATR), SEM,
density measurements and measurements of swelling in toluene vapour. The results show that the foamed PDMS rubber is an open-
cell structure with a void fraction of 50% of the total volume. Long term stability was assessed by degrading the material at high
temperatures (180–200
C) in air. Changes in FTIR(ATR) were not noticeable, except rapid loss of peaks due to the catalyst and its
solvent. The material showed a small, but statistically significant, increase in density and decrease in equilibrium swelling. The most
sensitive method to measure degradation was chemiluminescence (CL), which confirms that the PDMS is extremely stable because
the CL intensity is weak as compared to carbon chain polymers. Removal of oligomers and catalyst residues by toluene extraction
results in the disappearance of initial decay and removal of peroxide residues by treatment with nitric oxide also confers greater
oxidative stability. Ramped chemiluminescence in air shows that the PDMS residue after toluene extraction is more stable than the
virgin material, indicating that degradation is due to small fragments and catalyst solvents rather than the main siloxane network.
# 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Polysiloxane; Degradation; Ageing; Chemiluminescence
1. Introduction
Silicone rubbers are widely used for high temperature
applications [1], because of the high thermal stability of
poly(dimethylsiloxane)s (PDMS) [2] and their resistance
to oxidative degradation. They also have very good low-
temperature performance as well as good non-stick
properties, low toxicity and low chemical reactivity. They
are widely used in electrical insulation, coating and
encapsulation applications because of their outstanding
resistance to climatic and industrial pollution, under
electrical stress and their good tracking behaviour and
resistance to arcing and electrical erosion. They are highly
water repellent with high water repellency recovery capa-
city in polluted atmospheres. Crosslinked PDMS net-
works can be formed in many ways, one of which is room
temperature vulcanisation (RTV). Although RTV sili-
cone elastomers have poorer mechanical properties
compared to heat-cured silicones, they are easier to
make and hence widely used [1].
One important application of PDMS rubbers is in the
form of filled, foamed materials, which are used in many
applications where a resilient, highly-stable foam is
required. These foams are typically produced by reac-
tion of a silica-filled mixture of linear, hydroxy-termi-
nated PDMS and poly(methylsiloxane) with a tetra
alkoxysilane in the presence of a suitable Lewis acid cat-
alyst (typically a tin salt). The network is created by the
coupling of the PDMS hydroxy groups with the alkox-
ysilane, and the foamed structure is created by the
blowing reaction between the silane functional groups
on the PDMS and the silanol groups resulting in the
evolution of hydrogen.
Foamed RTV elastomers are widely regarded as
highly stable materials under conditions of long-term
service use, but it has been shown [3] that PDMS net-
works can undergo rearrangement, especially in the
presence of catalytic impurities, which lead to compres-
sion stress relaxation. Thomas [4] showed that the net-
work rearrangement in peroxide cured PDMS rubbers
at high (250
C) temperatures in closed systems is due to
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0141-3910(01)00139-2
Polymer Degradation and Stability 73 (2001) 505–510
www.elsevier.com/locate/polydegstab
* Corresponding author. Tel.: +44-1273-678313; fax: +44-1273-
677196.
E-mail address: n.c.billingham@sussex.ac.uk (N.C. Billingham).