Chemistry Reference
In-Depth Information
operates on a different part of the radical chain oxidation mechanism. Simple
polymer systems, like polyethylene and polypropylene, whose uses were once
severely limited by stability issues, now compete with low-end engineering
thermoplastics because highly functional and synergistic antioxidant combina-
tions now allow them to withstand more abusive environments and have much
longer lifetimes. But it has taken more than simple phenolic antioxidants to
make such a change possible. This transformation required the use of com-
binations of highly effective antioxidants taking advantage of synergistic effects.
12.6.1 Primary antioxidant comparisons
There are literally hundreds of phenolic compounds that have been synthesized
for use as antioxidants in polymer systems. Some of these compounds such as
BHA, BHT, TBHQ and propyl gallate, after rigorous toxicological studies, have
proven very useful in the stabilization of foods. There are literally hundreds of
naturally occurring phenolic compounds that have shown antioxidant activity.
Recently, the `distinction' between natural and synthetic phenolic antioxidants
narrowed a bit with the finding of a fresh water strain of phytoplankton that
produces BHT naturally (Babu and Wu, 2008). Both naturally and synthetically
derived phenolic compounds demonstrate high antioxidant activity and both
classes of materials provide food formulators with benefits. It is not likely that
major advances in the practice of food stabilization will come from the
discovery of additional natural phenolic substances.
12.6.2 Carbon-centered radical scavenging ± a special case
Carbon radical scavengers play an incredibly important role in polymer stabili-
zation. Phenolic compounds do not act as carbon-centered radical scavengers in
either polymers or foods. Hydrogen transfer from a phenolic compound to a
carbon-centered radical cannot compete with the rapid, near diffusion controlled
rate of the reaction of a carbon-centered radical with oxygen. The compounds that
act as carbon-centered radical scavengers in plastics include the hindered amine
light stabilizers (HALS), hydroxylamines and certain benzofuranone derivatives.
HALS and hydroxylamines will be discussed in a separate section, below.
Benzofuranones are a new class of carbon-centered radical scavenger being
used to improve polymer stability. The mechanism by which they act is shown
in Fig. 12.21 (Schwarzenbach et al., 2001). The `active' carbon-hydrogen bond
is tertiary and doubly benzylic, and the radical formed upon abstraction of the
hydrogen is stabilized by resonance into both of the aromatic rings. The
benzofuranones seem to be active only at high temperatures and are most
effective under low oxygen partial pressure conditions. They appear to have no
antioxidant effect under ambient conditions. Given these properties, benzo-
furanones find their major use in the polymer extrusion process, where oxygen
concentrations are low and high temperatures and shear forces create carbon-
centered radical species. Benzofuranones are not used in food applications, but
Search WWH ::
Custom Search