Why Does Paper
Turn Yellow?
紙為何會變黃？
By Marco Wong
黃俊銘
This article may be useful for chemistry classes, based on the DSE syllabus.
根據 DSE 科目，這篇文章有助化學課程。
One
of the four great inventions of
ancient China, paper, as we know it, made
its first appearance around 105 AD. Since this
revolutionary invention, much of human history
has been recorded on paper; but the frazzled
yellow colour of old books, dated photographs
and ancient documents serves as evidence of
the unforgiving effects of time. However, paper
discolouration is ascribed to more than just time,
as it involves chemical reactions with specific
conditions.
A closer inspection of the composition of
paper i s necessar y in under standing paper
discolouration. Paper manufacture involves the
separation of individual fibres from wood, mixing
of water and wood pulp, colouration (depending
on application) and drying. The main components
of paper are the same as that of wood fibres
– cellulose, hemicellulose and lignin. Cellulose
and hemicellulose are white polysaccharides
(chains of glucose molecules), whereas lignin is
a dark complex organic compound. To produce
white paper, lignin is removed in the pulping and
bleaching process.
In lignin-free paper, discolouration is mainly
attr ibuted to the ox idation of cellulose and
hemicellulose. These compounds do not have
delocalised electrons to absorb visible light. This
means that the electrons are associated within
the electron cloud of a specific atom (metallic
bondi ng, for example, exh ibit s delocal i sed
electrons). However, oxidation of hydroxyls forms
carbonyl and carboxyl ic groups. The newly
fo rmed doub l e - bonded funct i ona l g roups
(C=O and C=C) are chromophores, which are
responsible for giving the colour of the organic
compound. The reaction occurs in the presence
of oxygen, which is also why the exposed edges of
the pages in a book turn yellow more rapidly than
the interior.
Not a l l pape r i s c reated equa l e i t he r.
Newspaper discolours much faster than does
white paper, due to the former having a higher
content of lignin. In addition to increasing the
rigidity of paper, lignin also gives paper a rough
and dark appearance. Removal of lignin in paper
production is an expensive procedure and is
skipped to lower newspaper costs. A study done
in 1996 showed that UV light possesses enough
energy to break down the ether linkage in lignin,
producing phenyl radicals and ketyl radicals.
The lat ter continues to breakdown l ignin to
yield more phenyl radicals. Oxidation of phenyl
radicals produces phenyl acids, giving the aged
newspaper a yellow appearance and a brittle
texture.
Since oxidation and photooxidation are the
main culprits responsible for paper yellowing,
slowing down this process preserves the natural
colour of the paper. The reactions are possible
under the presence of heat, light and oxygen.
Avoiding unnecessary exposure to heat and light
would slow this process down considerably. In
addition, there are certain coatings that claim
to contain chemicals that are able to absorb UV
light to prevent yellowing due to photooxidation,
o r r ed u c i n g ag e n t s , wh i ch p r o t e c t t h e
polysaccharides from being oxidised.
