40 Chapter 3
papers were generally weak. There are many ways of measuring paper strength
but Barrow used an instrument called the MIT folding endurance tester (see
below for details of this test).
Barrow’s other approach to the problem was to obtain several pieces of
paper and age them artificially and follow the decrease in strength. The results
reinforced the relationship between initial acidity and loss of strength on ageing.
There is more about artificial ageing later in this chapter.
The loss of strength of paper produced by acidity is produced by acid-
catalysed hydrolysis of the cellulose chains. In ambient RH, cellulose con-
tains several percent of water that may participate in hydrolysis reactions.
The point of scission is the link between the glucose (pyranose) rings. In
paper-conservation science, the degree of polymerisation (DP), i.e.the aver-
age number of monomer units, is commonly used to discuss the molecular
weight. Loss of DP does not produce an effect on the strength immediately;
it has been proposed that the DP has to reach a critical point before loss of
strength becomes apparent. Celluloses of an initially high DP, such as natural
cellulose from cotton and linen, thus have an advantage over other types of an
initially lower DP such as wood celluloses after extraction from wood. Loss
of strength occurs most in amorphous areas as water and acids have difficulty
in obtaining access to crystalline areas. Paper can start life acidic because of
its method of manufacture and ingredients, or it may absorb acidic air pollu-
tants such as sulfur dioxide and nitrogen oxides. Sulfur dioxide is believed to
form sulfuric acid in the paper. Papermaker’s alum produces an acid solution
as it can be thought of as a salt of a weak base (aluminium hydroxide) and a
strong acid (sulfuric acid). Rosin, used in some types of size, also contains
rosin acids.
Alkaline degradation is possible but occurs much less readily than acid
degradation. Degradation in alkaline conditions is of interest to the paper sci-
entist mostly because of the degradation which may occur during analytical
procedures rather than what may happen during the ageing of paper.
4.2 Oxidation of Cellulose
Paper exists in air that contains about 20% of oxygen. Cellulose is capable
of reacting slowly with oxygen in ambient conditions (autoxidation). Such
oxidation reactions involve intermediates called free radicals. In a simple
compound A–B, the single bond between A and B contains two shared elec-
trons, if both go to B then two ions are produced Aand B. If the electrons
are shared between A and B, two free radicals Aand Bare produced because
each has an unpaired electron. Such free radicals can participate in chain
reactions in which the reaction of a free radical with a neutral molecule
creates a product and another free radical (propagation). The process of