Ageing processes of biomedical polymers in the body - Durability and Reliability of Medical Polymers - page 169

Ê ˆ

k

RT
E

R RT

[8.4] log

Ê Ê Ê Ê

Á

ˆ

=

Á Á

Ë Ë Ë ¯

k

Ë Ë

k

¯ ref act re

k re

Hukins et al. (2008) show that, substituting f ¢ = 2 and D T = 10 °C (the
10-degree rule), into equation [8.4], gives an activation energy of 10 R /log e 2,
which if substituted into equation [8.3] yields a value for f that is identical
to the predictions of f with the empirical 10-degree rule.
Hukins et al. (2008) identifi ed that the main weakness of equation [8.1]
is that it assumes that increasing the temperature by 10 °C doubles the rate
of ageing and they went on to suggest that this can be overcome if E act
for the ageing process is calculated using the fi rst-order chemical reaction
approach. However, it may not always be feasible or practicable to adopt
this approach. Furthermore, both equation [8.1] and the fi rst-order chemical
kinetics approach are not valid if the ambient temperature is increased to
a value that initiates other physical or chemical processes in the material
that are unlikely to be involved in normal ageing processes (Hukins et al. ,
2008). For example, it has been suggested that equation [8.1] is only valid
for T < 60 °C (Hemmerich, 1998); it has also been recommended that the
maximum value of T should be 70 °C (ASTM-WK4863, 2005).
Hukins et al. (2008) conclude that rather than assuming that a temperature
increment of 10 °C increases polymer reaction rates by a factor of two, it
may be more useful to identify that if a temperature increment q were to
increase a given polymer reaction rate n times, then an elevated temperature
would increase the rate of ageing by a factor of n DT/q . Using the fi rst-order
chemical kinetics approach this would then correspond to an activation
energy of qR /log e n , which is a better approach to use because it is based on
experimental measurements specifi c to the process being investigated rather
than an empirical observation (Hukins et al. , 2008; Verdu et al. , 2007).

￿ ￿ ￿ ￿ ￿ ￿

8.4.2 Applications of accelerated ageing in polymers
Several studies investigated the accelerated ageing of silicone elastomers.
A study by Ghanbari-Siahkali et al. (2005), where silicone elastomers were
maintained in water at a temperature of 100 °C for two years, showed that
the surface chemistry of the materials were signifi cantly modifi ed and there
was some modifi cation of the surface layer, although only to a thickness of
about 100 mm. In another study by Konkle et al. (1947), silicone elastomers
that were aged at 150 °C for 50 days retained their mechanical properties.
Patel and Skinner (2001) maintained silicones in a moist inert gas atmosphere
at temperatures of up to 190 °C and reported the loss of volatile products
and some softening.
Several studies (Dootz et al. , 1993; dootz et al. , 1994; Wagner et al. ,
1995; Yu et al. , 1980) investigated the effect of light (using a xenon source to