Blends and Additives - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 144

observed in accelerated testing, can be monitored by FTIR analysis and the
appearance of absorption in the carbonyl region [12].
Additives to stabilize typically work on one of three principles. Most
commonly, they are radical scavengers. They react with a radical to form
a non-reactive species. Another class of additives sacrificially reacts with
oxygen. A third type will work by sequestering metals that can catalyze the
degradation. Recognize that in the process of forming a plastic article, the
polymer has been subjected to high heat and shear. Often the polymer has been
manufactured in a thermal process. Typically it is then extruded with other
additives. In the extrusion process it is subjected to a high enough heat to allow
the polymer to flow. Also the turning of the screw causes localized frictional
heating. Finally, the polymer is then melt-fabricated to form the final article.
In each instance, there is an opportunity to break bonds and form radicals.
A common type of radical scavenger is a hindered phenol, such as buty-
lated hydroxy toluene (BHT). A radical can react with BHT by removing
a hydrogen atom and forming a new phenolic radical. However this radical
is sterically hindered from further reactions by the two t-butyl groups and
therefore the propagation steps terminate with the formation of this stable
radical. Antioxidants that act as radical scavengers are sometimes referred to
as primary antioxidants.

H

R

O

O

+RH

BHT

BHT and similar compounds are effective antioxidants and they have been
used in polymers and also in food. However, in polymers, the BHT has enough
volatility that it can come out of the polymer. When a material migrates out of
the polymer, it is said to “plate out.” This can happen during the molding step
and BHT can plate out onto the mold as a powder. It can also happen gradu-
ally on storage and over time the article develops a white powder of BHT on
the surface. The tendency to plate out also depends upon the solubility of the
additive in the polymer. Plate-out is undesirable and therefore less volatile
analogues of BHT are typically used. One common example is octadecyl
3,5-di-t-butyl-4-hydroxyhydrocinnamate, originally marketed by Ciba-Geigy
Corp. as Irganox ® 1076. As this and other additives came off patent and
other companies began producing the additives, many of them continued with
the same numbers, so this can be referred to as antioxidant 1076 or AO 1076.
Other common antioxidants are AO 1010 and AO 1098, but there are many