Ageing processes of biomedical polymers in the body - Durability and Reliability of Medical Polymers - page 170

mimic natural sunlight) and increased temperature (exposure to air at a high
relative humidity at 43 °C for 600-900 h) on maxillofacial implant materials
and soft denture liners. Using equation [8.1] the harshest of these conditions
corresponds to 57 days of ageing at 37 °C. It was reported (Dootz et al. , 1993;
dootz et al. , 1994; Wagner et al. , 1995; Yu et al. , 1980) that the tensile strength
of some silicones was unaffected but that of the others decreased, whereas
the tensile strength of poly(vinylchloride) (PVC) and poly(phosphazine)
did not change considerably, however, a PU showed considerable signs
of degradation. Yu et al. (1980) found that the tear resistance of silicones
did not decrease as much for silicones as for PVC or PU; although the tear
resistance of one silicone was found to increase. Furthermore, Dootz et al.
(1993) found that the tear resistance of plasticised polymers increased and
this was attributed to increased polymerisation and/or loss of plasticisers.
In the second study by Dootz et al. (1994), the hardness of two out of the
three silicones increased with ageing. Wagner et al. (1995) found that the
viscoelastic properties of silicones and a poly(phosphazine) were affected
by exposure to increased temperature. Furthermore, in two other studies
(Craig et al. , 1980; Goldberg et al. , 1978), accelerated ageing degraded the
mechanical properties of simple polyurethans, used for maxillofacial implants
and dental liners.
Other ageing studies (Mahomed et al. , 2010b; Saber-Sheikh et al. , 1999)
concluded that accelerated ageing of silicone is not likely to cause the silicone
to degrade or greatly affect its viscoelastic properties or change its appearance.
Wilson and Tomlin (1969) concluded that the appearance of two silicones
for dental liners, Molloplast-B and Silastic 390, did not change after being
immersed in water at 37 °C for six months.
Kennan et al. (1997) investigated the effect of accelerated ageing of medical-
grade silicones at 100 °C for 45 h in saline solution. Using equation [8.1],
these conditions simulate ageing at 37 °C, for approximately 59 days. The
tensile strength remained unaffected, but there was a change in the contact
angle of a liquid drop on the surface of the silicones before and after heat
treatment, which suggested a change in surface properties. Furthermore, in
a more recent study (Leslie et al. , 2008), it was reported that maintaining at
least some silicones in saline solution at an elevated temperature may reduce
their mechanical strength.
Simmons et al. (2006) investigated the shelf-life of Elast-Eon™ (AorTech,
Melbourne, Australia), a PU with silicone segments which has been suggested
as a possible material for making flexible finger and wrist joints (Shepherd
and Johnstone, 2005). For the specimens that underwent accelerated ageing
at 70 °C for two weeks (using equation [8.1], this corresponds to 138 days
of ageing at 37 °C) and were sterilised by g-irradiation or with ethylene
oxide, it was reported that accelerated ageing showed no signs of degradation
or surface cracking of Elast-Eon™ but led to microcrack formation in an

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