THE ROLE OF THE

Engineering and Structural Polymers 89
R
Stabilization by labile-hydrogen donating antioxidant (HA)
O O +
stage of this process. They react with the radicals that propagate oxidation, thus
reducing the hydrogen donating antioxidant (HA) shown in Figure 6.3.
The edge of the structure of carbon black is given in Figure 6.4 [8]. Carbon radicals
can migrate along a polymer chain for significant distances [8]. With a carbon
black filler, there is the possibility that radicals will migrate along the chain.
Commercial carbon blacks have between 3 and 4% oxygen combined on the surface
and much of this is believed to be present as phenolic groups. Carbon black can
thus be regarded as a giant phenol and as such should be capable of functioning as a
labile-hydrogen donor [8].
6.8 PolyetHeretHerKetones
A
H
R O
FIgure 6.3 Stabilization by labile hydrogen donating antioxidant.
HO
H
FIgure 6.4 Structure of carbon black.
H
O
O
H
The most widespread polymer currently in use for high-speed dynamic sealing applications
in automotives is polyetheretherketone. These thermoplastic aromatic polymers
are used in the aerospace, electronics, and nuclear industries [9]. These materials have
excellent mechanical- and chemical-resistance properties, which permit polyetheretherketones
to be used in many engineering applications—often in harsh environments
[10]. These polyetheretherketones are the product of the reaction of 1,4-dihydroxybenzene
and 4,4′-difluorobenzophenone [8]. Figure 6.5 shows this reaction.
Various amounts of filler contents are available; typically, 33% glass fiber is used.
T g of 143°C and melting temperature of 342°C, with a continuous use temperature
of 260°C, are observed without thermal degradation [11]. This material dissolves
in concentrated sulfuric acid and is not chemically attacked by water or pressurized
steam. Most importantly, it can be injection molded and is capable of holding
O
H
O
OH
H
O
+
OH
A