It is known [10, 11, 18] that reactive carbonaceous compounds are present in air
discharges. Formation of reactive carbon species and possible routes to production
of anhydrides are thought to rely on either:
a activated oxygen attack on the methyl group:
R–CH3+O2→R–CH2+OOH
or
b hydroxyl radical attack on the methyl group:
R–CH3+OH→R–CH2+H2O
both of these initiating reactions produce a methylene radical on the resin chain.
The radical reacting with oxygen produces an aldehyde:
R–CH2+O2→R–CH2O2→R–(C=O)–H+OH
the aldehyde reacts with oxygen as follows:
R–(C=O)–H+O2→R–C=O+OOH
Carbonyl and hydroxyl radicals interact as follows:
R–C=O+OH→R–(C=O)–OH
A second interaction with a hydroxyl radical produces another radical as shown:
R–(C=O)–OH+OH→R–(C=O)–O+H2O
Interaction of the two radicals highlighted will produce a linear anhydride, as detected
on the epoxy resin surface after electrical stressing:
R–(C=O)–O+R–C=O→R–(C=O)–O–(C=O)–R
Given that activated oxygen species are less prevalent in a moist atmosphere [11]
and that anhydrides are widespread following stress in a moist atmosphere but less
so in a dry atmosphere, reactionbwas considered the more probable initiating step.
The production of nitrated species following normal air discharges is explained by
the higher levels of nitrogen oxides in a moist atmosphere [11].
Interactions between the radical species involved in development of anhydrides
can also be used to postulate reactions for the production of acids and peracids detected
on the stressed resin surface.
The production of radical species, R, on the bisphenol chain (by removal of the
methyl group) was postulated earlier. Interaction with oxygen and carbon species
allows the following reaction:
R+O2+R–H→RO2+R–H→RO2H+R
Peroxides can thus be formed on the resin surface.