Wednesday, 9 January 2013

Secondary Chemistry - Superstuffs: Cellulose (part 2)

The nineteenth century dominance of cotton persuaded many chemists to explore the properties of cellulose.  In 1846, Christian Schonbein reacted cotton with a mixture of nitric acid and sulfuric acid.  His product, nitrocellulose, was six times more explosive than gun powder.  Schonbein’s gun-cotton was used for blasting in mines and quarries, as well as in cannon.  Jules Verne imagined it being used to launch a space craft to the Moon.

Alexander Parkes experimented with cellulose reacted with a slightly less violent mixture of nitric acid to make a form of nitrocellulose that was not explosive though still rather flammable.  Parkes mixed his nitrocellulose with camphor to make a hard substance.  In 1872, John Hyatt developed Parkes’ discovery into artificial ivory for snooker and pool balls.  The material was called celluloid.   Celluloid replaced ivory in many products such as combs, handles of cutlery, and accordion keys.    The properties of celluloid could be modified to turn it into a flexible transparent film.  In 1889, the Eastman Kodak Company, among others, started to use it to make photographic film.  It was this development that paved the way for motion picture films.  Celluloid film is unfortunately very flammable and cinema fires were unfortunately a frequent occurrence in the early twentieth century.  Also many early films have been lost because celluloid decomposes after just a few years.
Copyright of Marcel Oosterwijk
It was the 1930s when one solution was found using another derivative of cellulose – cellulose acetate.
Cellulose acetate was first made in 1865 by Paul Schutzenburger by reacting cellulose with acetic anhydride (a pungent, corrosive liquid).  The product was dissolved in acetone.  One of the first uses was in “dope”.  Early aircraft which were made of cloth stretched over a wooden frame.  When the dope was painted over the cloth the acetone evaporated leaving a film of cellulose acetate that made the cloth rigid and airtight.  It took until 1934 before cellulose acetate “safety” film was used in the cinema.  It was less flammable and therefore safer but in the 1980s was replaced by synthetic polyester film.   Cellulose acetate could also be extruded through tiny holes called spinnerets to make a fibre.

In the late nineteenth century the textile industry had two problems.  The supply of cotton could not keep up with demand and people wanted a fibre that looked and felt like silk but was cheap.  Rayon was the answer to both problems.   Cellulose, from wood pulp, was reacted with carbon disulfide (a toxic liquid) in an alkali to produce a solution called “viscose”.  When viscose reacted with dilute sulfuric acid and sodium sulfate the cellulose was reformed and could be extruded to form fibres.  The Courtaulds company began manufacture of rayon in 1905.  It was marketed as artificial silk because the fibres in rayon are smoother than the natural cellulose fibres.   Rayon is still widely used in clothing and soft furnishings (such as curtains).

Viscose can also be mixed with glycerine to make a flexible film.  This product was called cellophane and was first used in 1912 for sweet wrappers.  Later it was used in sticky tape (sellotape) and as a transparent wrapping material.

In the last sixty years, materials derived from cellulose have been replaced by synthetic polymers for many uses but the story of cellulose is not over yet.  Paper has yet to be replaced despite computers and e-books and cotton is still the most popular textile because it absorbs moisture and feels comfortable against the skin.  Cotton is still being improved upon.  In 1987 Courtaulds introduced a new cellulose fibre called Tencel.  It was made by dissolving cellulose in a special solvent then extruding it through spinnerets.  Tencel is softer, stronger and more hardwearing than natural cotton and is used in many clothes today.

The story of cellulose is not over.  It is a renewable resource and biodegradable.  In a future of declining stocks of crude oil for fuel and synthetic polymers, cellulose will continue to be in demand both as a source of energy and as a versatile material.  Surely cellulose deserves the accolade of “superstuff”.

1              Check your clothes – what items are made of cotton or cellulose derivatives such as rayon (or viscose) or Tencel (also known as lyocell)?

2              Find out what things are made of cellulose derivatives such as celluloid, cellophane or cellulose acetate.

3              Explain why newspapers are sometimes referred to as the “old rags”.

4             Discover how cellulose fibres from flax, hemp, sisal, coir and bamboo are used today.

5              Examine the cultivation of elephant grass, willow or poplar as energy crops.  How do they compare with using mature trees as a source of wood for fuel?

6              Find out how to make your own paper from plant material or old clothes (made from cellulose fibres not synthetic polymer fibres).

7              What is the future of cellulose?  Compare the properties and uses of cellulose fibres and derivatives with those of synthetic polymers.

8 (A level) (a) Find out the structure of glucose and cellulose
(b)          Write an equation for the formation of tri-nitrocellulose (gun cotton).
(c)           Explain why guncotton can explode even in the absence of oxygen.

Peter Ellis

Peter Ellis taught science (mainly chemistry) in secondary schools to GCSE and A level for 35 years and was a head of department for twenty years.  He is now a freelance writer of educational materials in science and dabbles in writing fiction.  

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