Plastics 189
They are made by esterifying cellulose, in the form of either cotton linters or
wood pulp. Cotton linters, the short fibres removed from cotton seeds, are puri-
fied to remove impurities and coloured entities by heating at 130–180°C under
pressure in a 2–5% aqueous solution of sodium hydroxide. The resulting cel-
lulose has a molecular weight in the range 100,000–500,000. Non-cellulosic
material is removed from wood pulp, most frequently via the sulfite process,
which involves treatment with a solution of calcium bisulfite and sulfur diox-
ide. Each of the thousands of repeat units of pure cellulose has an empirical
formula C 6 H 10 O 5 and, despite the abundance of hydroxyl groups, is not water-
soluble due to its high crystallinity and extensive hydrogen bonding.
Reaction Scheme 1 shows that during preparation of cellulose plastics, the
hydroxyl groups on each cellulose monomer ring are replaced by other substituent
groups. To prepare cellulose nitrate, pre-dried cotton linters are treated with con-
centrated nitric and sulfuric acids. Water tends to slow the reaction, so its presence
is limited. Sulfuric acid catalyses the reaction, so it can occur without heating and
under ambient conditions. The product is washed with water to remove the resid-
ual acids; if residues of sulfuric acid are allowed to remain, an explosive reaction
can occur. Remaining water in the cellulose nitrate is displaced by alcohol.
The likelihood of a particular hydroxyl group being replaced is largely
determined by its position in the monomer molecule. Substitution of all three
hydroxyl groups on repeat units of the cellulose molecule results in the creation
of explosive cellulose trinitrate, containing 14.4% nitrogen as shown in Reaction
Scheme 1. Cellulose dinitrates containing between 11% and 13% nitrogen (an
average degree of substitution between 1.9 and 2.7 hydroxyl groups) are useful
for plastics, photographic film bases and lacquers. New cellulose nitrate is a
colourless polymer, which is brittle and tough, but flexible when plasticised
and cast as thin films and sheets. Cellulose nitrate is highly soluble in polar
solvents, such as acetone, ethanol and butyl acetate. It softens at approximately
80–90°C and is thermoplastic, flows at 150°C, and ignites in air at 160°C. The
high flammability of cellulose nitrate was a highly undesirable property in its
applications as coating, doping (tightening and wind-proofing) treatment for
the canvas-covered wings of aircraft used in the First World War and in cine-
matographic films; it resulted in the development of other cellulose esters,
most important of which was cellulose acetate at the start of the 20th century.
Reaction Scheme 1Preparation of cellulose nitrate involves replacing the hydroxyl groups
on each of the many monomer units of cellulose