Biomedical Engineering Reference
In-Depth Information
are recorded in this way and the numbers are accu-
mulated to achieve a full population data set.
In the manufacture of the compound, the fibers
are introduced to the polymer as chopped fibers,
typically with an initial length of 6 mm (as will be
described in Section 3.4.1 ), whereas the fiber length
distribution observed in injection-molded parts is as
illustrated in Fig. 3.14 and cumulatively in Fig. 3.15 .
These data show that the majority of fibers fall
below about 400 m m (0.4 mm) in length, so they are
much shorter than their initial starting length of
6 mm. This reduction in average fiber length in
injection-molded parts occurs as a result of fiber
attrition during processing at both the compounding
and injection molding stages. Both the processes
involve shear mixing and significant flow of material
under high pressure, which breaks the fibers into the
fragments recorded in the fiber length analysis. The
reinforcing efficiency of these short fibers is rela-
tively low compared with continuous fiber
composites, but, nevertheless, still an important
level of mechanical strength and stiffness results
from their addition.
The maximum amount of short carbon fibers that
can be added to a polymer and still retain the ability
to process by molding and extrusion is around 30%
by weight. Compounds with greater amounts of fiber
show diminishing returns in terms of mechanical
property improvement and also they are more diffi-
cult to process, needing higher temperatures and
pressures to fill the mold.
Considering the mechanical properties of short
fiber-reinforced PEEK, Fig. 3.16 illustrates the
tensile strength of a 30% by weight short CFR PEEK-
OPTIMA compound compared with standard
unfilled PEEK-OPTIMA, UHMWPE (polyethylene),
and an “average” cortical bone. It is clear that both
PEEK-OPTIMA materials are superior to UHMWPE
and to cortical bone with CF PEEK-OPTIMA
compound displaying a tensile strength approxi-
mately three times that of bone. This is important
because it means that high-strength (relative to bone)
implants can be manufactured from these materials,
using their ease of processing by injection molding
and machining.
Stiffness (elastic modulus) is illustrated in Fig. 3.17
for these samematerials, but now some typical metallic
and ceramic implant materials are also included. It is
clearly seen that all the metallic materials and the
alumina ceramic are very stiff in comparison with
cortical bone (by a factor of 10 e 20 times) and
Figure 3.12 Relationship between fiber length, fiber
volume fraction, and processing.
according to their processing history. Figure 3.13 is
a micrograph of fibers removed from short CFR
PEEK by acid digestion. This process involves
heating the compounded blend of carbon fibers and
PEEK polymer (as granules) in boiling sulfuric acid
for several hours, which dissolves the polymer and
leaves behind a mat of fibers. The fibers are washed
with water, and small samples picked from the mat
are distributed onto a white tile or glass plate for
observation and analysis. This is typically achieved
using an automated image analysis system, which,
using a camera connected via a computer, measures
the length and diameter for each fiber in the sample
and then plots a population distribution classed
according to length. Typically a large number of tiles
Figure 3.13 Carbon fibers removed from injection-
molded short CFR PEEK by acid digestion.
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