oxidation starts when a free radical reacts with oxygen to make a peroxy radical
(ROO) that can then react further. Some autoxidation reactions could include:
RHO 2 →RHOO (initiation)
RO 2 →ROO (propagation)
ROORH→ROOHR (propagation)
RH may represent any organic compound.
Autoxidation is slow in pure cellulose and it may be considered to take
place at a negligible rate, however, certain conditions will initiate and accel-
erate free radical reactions. If light is absorbed by cellulose or a material
associated with cellulose, e.g.a dye or lignin, the absorbing molecule’s energy
increases greatly. Many things may happen to this energy but one of them is
that the energy can be transferred to another molecule, e.g.if lignin transfers
its energy to cellulose, this is called photosensitisation of the cellulose by the
lignin. The energetic cellulose could split open one of its chemical bonds
producing two free radicals or the energy may be transferred to an oxygen
atom which, then being in a highly energetic state (e.g.singlet oxygen), can
oxidise the cellulose.
Metal ions that are capable of changing their valency, in particular, ions of
copper and iron, are powerful oxidation catalysts as they can initiate oxida-
tion reactions. During a valency change, negatively-charged electrons (e)
are donated or accepted.
One example of the initiation of a chain reaction is:
CuROH →CuROH
Certain pigments and inks, e.g. copper acetate pigments and iron gall ink,
are particularly notorious for their ability to produce metal-catalysed degrad-
ation of paper and damage can be so severe that the area covered by colourant
falls out of the sheet and the area is lost. During industrial oxidative bleach-
ing of paper pulp, the presence of traces of iron and copper can cause exten-
sive damage to the cellulose and measures are taken to avoid this.
Loss of strength of the paper is caused during oxidation as chain scission
is produced. During oxidation, carbonyl ( CO) groups can be formed on
some of the carbon atoms in the cellulose molecule.
Cu Cu e
Fe e Fe
Paper 41