When I saw this display case – and its contents – at the Museum of Science & Industry, Manchester, I gave something of a wry smile. It shows the basic steps in the production of acetate fibre from pine logs and indicates some of the uses for this most natural of polymers.
You see, cellulose acetate, and various products associated with it, were once a big part of my life. I spent more than 3 years in No. 2 Works Laboratory, at what was then British Celanese, Spondon, Derby. This plant lies to the south and east of the centre of the city, and, although it has shrunk somewhat in size from its peak, it is still a significant production unit for the now-named Celanese Acetate.
The process of converting cellulose to a useable polymer was developed by the Swiss brothers Camille and Henri Dreyfus in Basle, Switzerland in 1905 – they opened their first factory in 1910, which lead to a worldwide adoption of the polymer soon afterwards. Following ‘digestion’ in acetic acid and other chemicals, the cellulose in the wood is converted to cellulose diacetate (this process taking place on a different site to Spondon). When delivered to Spondon, the white acetate flake is mixed with the solvent acetone (the one used as a nail varnish remover), to form a thick, viscous white paste. This is pushed through a flat metal plate, which has been precision-drilled with hundreds of tiny holes. As these polymer ‘strings’ fall through a column of hot air, acetate filaments are formed and are drawn off by powered rollers – the textile so formed was marketed as ‘Dicel’, and used for the production of smooth, ‘drapable’ fabrics with a feel similar to that of silk. Cellulose diacetate is also used as an absorbent material in wound dressings, and as a lining material in suits, for example. You can also form thin, flexible, clear film from cellulose acetate, which can be used to make enclosures for pharmaceuticals, packaging, and other thermoplastic items.
The major problem with acetone/air mixtures is that they can be highly inflammable, even explosive, so the solvent must be recovered as quickly as possible. Fortunately, acetone is highly soluble in water, so the acetone/air mixture is ‘scrubbed’ through water, and put through a large and complex solvent recovery plant. This a) recovers the valuable solvent for re-use, and b) ensures than any water discharged to the local effluent system meets environmental standards, such as colour (in Hazen units) and the Biochemical Oxygen Demand time. The plant also produced cellulose triacetate fibre, which was dissolved in a different solvent – methylene chloride – and a different solvent recovery process, as methylene chloride is both insoluble in water and much denser than it.
The major end-use of cellulose diacetate is, however, less appealing. In the form of a crimped web of fibres – called a tow – it is used to produce billions upon billions of filter tips for cigarettes. Unfortunately, despite the fact that cellulose diacetate can be composted, the estimates for the time a cigarette ‘butt’ to bio-degrade range from 1 month (from the cigarette companies) to up to 15 years (from environmental pressure groups). Whichever estimate you choose, the remains of billions of cigarettes cause a major environmental problem.