Cellulose acetate was first prepared in 1865 by Schützenberger, and a manufacturing process developed by Cross and Bevan in 1894. An acetone-soluble polymer was developed by Miles in 1904, employed for ‘safety film’ in 1909, and extensively used for aircraft dope during the First World War. The dope factories were later converted for the spinning of fibres – Celanese.
Cellulose acetate was produced commercially as sheet and rod material in about 1927 and as moulding powder in about 1930. It is a tough thermoplastic available in a full colour range, including transparent. It has a good gloss and was widely used for toys until ABS plastics were developed. Attractive and with a ‘natural feel’ it is still used for tool handles and spectacle frames.
In more detail
Although cellulose acetate was first prepared in 1865 by the French chemist Paul Schützenberger, it was not until 1894 that the first industrial process for its manufacture was patented in the UK by Charles Cross and Edward Bevan. At about the same time, Little in the US made cellulose acetate filaments experimentally, as did Bronnert in Germany. However, this material was essentially cellulose triacetate, a rather intractable polymer, not readily soluble in commonly available solvents. In 1904 George Miles, an American chemist, discovered that if the polymer was partially hydrolysed, it became soluble in acetone.
The Swiss brothers Henri and Camille Dreyfus used this procedure for lacquer and film production at Basle in 1910, and at the outbreak of World War I set up a factory in Spondon, Derbyshire, England to make acetate ‘dope’ for waterproofing and stiffening fabric covered aeroplane wings. They set up a similar plant at Cumberland, Md., for the US army in 1917.
After the war they concentrated their efforts in England, and in 1919 introduced the first acetate yarn Celanese. In 1924, they switched entirely to the US where acetone and acetic anhydride were cheaply available. In the same year, rayon became adopted as a generic term for all ‘artificial silk’.
Cellulose acetate (CA) in combination with plasticizers, such as diethyl and dimethyl phthalate, produces a plastics material which can be heat softened and forced under pressure into a cool mould. This injection moulding process was developed by Dr Arthur Eichengrün of Celonwerke to exploit the moulding properties of the plasticized CA moulding materials he had produced. His first injection moulding machine had a maximum shot weight of about 8 gm provided by a hand-operated plunger mechanism, a far cry from today’s machines, some of which have a maximum moulding size in excess of 100 kg.
CA plastics are tough with deep gloss and high transparency. They possess a ‘feel’ which is different to other plastics and which is often described as more ‘natural’. This may explain why CA has retained its popularity for making items which are handled frequently such as spectacle frames and tool handles, many transparent tool handles are still made from cellulose acetate and its sister material cellulose butyrate. Other items made from CA included combs, fashion accessories, pen barrels and toys, but these are now more likely to be moulded from more modern thermoplastics.
Early spectacle frames were cut from sheet material, mostly in imitation tortoiseshell which was often referred to as ‘optical shell’. Reinforcing nickel wires for the side arms were forced into heat-softened strips of CA sheet. Nowadays, frames are generally moulded into shape – a more economic process. However, certain high-class frames are still made using the old process, especially to achieve special colour effects not possible using injection moulding.
Despite being much less flammable than cellulose nitrate, acetate film did not become established for photographic use until after World War II because of the technical excellence of celluloid and the vested interests of film manufacturers. There was, however, a demand for transparent sheet material in laminated safety glass, especially car windscreens before toughened glass became available for this purpose.
Ironically, cellulose triacetate which was unsuccessful initially, returned to favour when a suitable, relatively non-toxic solvent (dichloromethane ) became available in the 1940s. Since then photographic film has been almost entirely based on cellulose triacetate and Tricel cellulose triacetate fibres were introduced in 1954.
Few plastics can boast such a long pedigree as cellulose acetate and, being made entirely from renewable resources, CA may yet have a long way to run.
Cellulose Acetate admin 2016-12-06T05:35:20+00:00