Biomedical Engineering Reference
In-Depth Information
6.3 Thermal Stability
The thermal stability of PEEK has been studied
because of its high-temperature industrial applica-
tions and processing conditions. Studies have shown
that thermal degradation occurs in PEEK at temper-
atures between the glass transition and melt transi-
tion, but that temperatures exceeding the processing
temperature of PEEK are needed to produce volatile
degradation products
[9
e
13]
. Hay and Kemmish
[9]
reported that thermal degradation, accompanied by
the generation of volatiles, was difficult to measure
below 427
C. Cole and Casella
[12,13]
studied
thermal degradation in PEEK and CFR-PEEK
composites between 400 and 480
C using Fourier
transform infrared (FTIR) spectroscopy techniques.
No significant difference was found in the thermal
degradation behavior of neat PEEK as compared with
CFR-PEEK composites.
Buggy and Carew
[10,11]
investigated the degra-
dation of flexural properties and crystallinity in
oriented PEEK composite laminates (APC-2)
between 120 and 310
C for up to 76 weeks. At
120
C, below the glass transition temperature for
PEEK, negligible changes in the static and fatigue
properties of the composite were observed
[10]
.At
250
C, mechanical degradation was detected after
16 weeks of thermal aging, whereas aging at 310
C
produced “rapid” degradation
[10]
. Based on these
studies, it is clear that thermal degradation is not
a concern during clinical use of PEEK biomaterials
around 37
C.
Figure 6.1
Effect of up to 2500 h of continuous expo-
sure to pressurized steam (200
C, 14 Bar) on the ulti-
mate tensile strength of PEEK-OPTIMA LT1.
changes to the microstructure of the interphase
region adjacent to the fiber were detected. Using
a combination of nanoindentation and nanoscratch
testing, researchers determined that the characteris-
tics of the interphase were modified only slightly by
the steam sterilization process. Specifically, the width
of the interphase region was found to increase from
approximately 3 to 5
m
m following steam steriliza-
tion
[5]
. The foregoing discussion of chemical
stability, water insolubility, thermal stability, and
minor microstructural rearrangements helps to
explain why, at a macroscopic level, the bulk prop-
erties of PEEK are relatively unaffected by repeated
autoclaving and long-term exposure to steam.
6.5 Radiation Stability:
Implications for Gamma
Sterilization and Postirradiation
Aging
6.4 Steam Sterilization of PEEK
Pressurized steam, also known as autoclaving,
is a common sterilization method for medical devi-
ces and has been investigated for PEEK and its
carbon composites
[5,6]
. Kwarteng and Stark
[6]
,
for example, exposed CFR-PEEK (APC-2/AS-4
composites fabricated by ICI) to over 100 autoclave
cycles. No significant reduction in flexural strength
was observed
[6]
. Similarly, up to 2500 h of contin-
uous exposure to pressurized steam (200
C, 14 Bar)
does not degrade the ultimate tensile strength of
PEEK-OPTIMA LT1 (
Fig. 6.1
).
The microstructure of CFR-PEEK has been
recently characterized in detail following steam
sterilization
[5]
. Although the bulk behavior of the
composite was not affected by autoclaving, minor
Radiation stability is another common concern for
aliphatic polymers, including polyolefins such as
ultrahigh-molecular-weight polyethylene (UHMWPE),
which are susceptible to bond cleavage during irradia-
tion, leading to the generation of long-lived macro-
radicals (often referredtoas“freeradicals”)
[14]
.In
contrast, because of its distinctive aromatic chemical
structure, PEEK displays remarkable resistance to
gamma and electron beam radiation, with
values of
radical formation about two orders of magnitude lower
than aliphatic polymers, such as polyethylene
[15]
.
Furthermore, even though free radicals are generated
during irradiation of PEEK, they rapidly decay, likely
due to recombination reactions made possible by the
G
