Plant-made plastic: history
The first ever plastics were derived from plants, not oil, and were the foundations of new materials and industries.
Nowadays, reference to plastics usually refers to materials made from synthetic polymers that are derived from oil reserves. The diversity of materials and products is vast and while Kevlar was invented in the early 1960s by Stephanie Kwolek, many of the others were developed in the first half of the 20th Century. Suprisingly, these early plastics were produced from renewable resources attracting renewed attention, in particular cellulose – a major component of plants and trees.
Photosynthesis and glucose
Plants use photosynthesis to generate energy. The process converts carbon dioxide, water and the pigment chlorophyll, in the presence of sunlight, into glucose.
Two glucose molecules combine within a plant cell in a chemical condensation reaction that joins molecules with an oxygen linkage. A molecule of water produced in the condensation reaction joins the sap. This mechanism is complex and part of the metabolic pathway in the plant. The linked glucose molecules undergo further condensation reactions in the cell wall to form the natural polymer, cellulose.
The thread-like cellulose molecules, also known as fibrillae, correspond to several hundred glucose units in length, making it a polysaccharide. This is the major component of vegetation, and the polymer chains of cellulose are oriented in the direction of the vertical axis in trees which adds strength to the trunk against external mechanical stresses. The closely packed polymer chains are linked through hydrogen bonding and van de Waals forces, making them impenetrable to water.
In the first half of the 19th Century, German-Swiss Chemist, Professor Christian Frederick Schonbein, investigated the reaction between cellulose and nitric acid. It has been said that when carrying out experiments in his kitchen, he spilled acid onto his wife’s cotton apron he was wearing so hung it out to dry. Later, when the dry apron was touched, it exploded violently due to the formation of cellulose nitrate, known as gun cotton.
Strictly speaking, gun cotton is a brittle material. It went on to become an important component of the propellant cordite, which was invented by British scientists Frederick Abel and James Dewar in 1889, and used as a propellant to fire shells by the British Navy in World War I.
Parkesine, celluloid and cinema
Reduction of the amount of nitric acid used in the chemical reaction with cellulose yielded useful products. Alexander Parkes is often credited as the inventor of the first man-made plastic, Parkesine, in 1856. Here, cellulose dinitrate (nitrocellulose) was blended with ethanol and a plasticiser, in this case camphor, to produce a tough material that could be moulded when hot, coloured and polished to produce buttons. Also, cellulose dinitrate is not explosive. In the 19th Century, the required camphor, a cyclic ketone, was obtained from laurel trees, although now it is synthesised.
Parkes exhibited a set of small mouldings made from Parkesine at the 1862 International Exhibition in London. It was described in the exhibition promotion leaflet as a replacement for natural materials, such as ivory and tortoiseshell, which were becoming scarce and expensive.
However, Parkesine was not a commercial success due to the expensive solvent. A printer and later inventor in the USA, John Hyatt Jr, developed an interest in celluloid because he wanted to win a US$10,000 prize offered by the Phelan and Collender company in 1863 for a new synthetic material to make billiard balls. The common use of ivory for the balls had by this time driven the African elephant to the edge of extinction, and an alternative material was desired. He extended the work of Parkes and succeeded in producing a manufacturing process for a mixture of cellulose dinitrate and camphor, to create the highly flammable thermoplastic, celluloid, in 1869.
By this, Hyatt had developed the first commercial process for the production of plastic from cellulose nitrate. One reason for its success was John Hyatt and his younger brother Isaiah created an injection moulding machine in 1872, which allowed bars and sheets of celluloid to be produced that could then be worked into products. Use of injection moulding allowed Hyatt to reduce the amount of solvent used significantly, and apply heat and pressure to shape the plastic.
As intended, celluloid replaced ivory in the manufacture of billiard balls, but the material was particularly volatile. Once the level of chemicals was addressed, the material took off and was used to manufacture spectacle frames, fountain pens, dentures and dolls. Celluloid had an important role in the development of cinema as a mass-entertainment industry, as from 1908, purpose-built cinemas started to appear in Britain. But the transparent, flexible celluloid film base was highly flammable and fires were difficult to put out as the material had its own source of oxygen and could burn under water. In 1909, the British government introduced the Cinematograph Act that required local authorities to license premises showing films, and the projector had to be kept in a fire-resistant enclosure.
At the beginning of the 20th Century, newsreels were introduced to the public, which appeared at the start of 1910 when each week, a compilation film on a topical subject was shown to audiences. But the golden age for newsreels was the period between the wars in the 1920s and 1930s, when they were an important source of information for the cinema-going public.
Another type of cellulosic is regenerated cellulose. Towards the end of the 19th Century, textiles were based on natural fibres, for example wool and cotton, as silk from silk worms was appealing but expensive. At the end of the 19th Century, a cheaper, synthetic fibre with the same appeal as silk was to be a popular fabric – viscose rayon. Invented by Charles Cross and Edward Bevan in 1892, the fabric has chains of cellulose polymer closely bound together by hydrogen bonding and van de Waals forces.
In order to obtain a textile fibre from cellulose it is necessary to separate the individual cellulose fibres, dissolve them, then regenerate the cellulose to produce the fibre. During production, cellulose is treated with caustic soda and then reacted with carbon disulphide to form cellulose xanthate solution. The latter is extruded from spinnerets (rotating nozzles) submerged in a bath of sulphuric acid to regenerate the cellulose by precipitation in the form of continuous fibre, namely viscose rayon. Rayon was a very popular artificial fibre before the advent of nylon as it has a pleasant feel, drape and could be dyed easily.
This fabric was used for blouses and curtains and could be blended with other fibres, such as cotton for bed linen, and its utility means it is still available today. However, it creases easily and loses strength when wet. Other blends include using cellulose xanthate solution to prepare cellophane (1918) and a more recent cellulosic textile fibre, Tencel, which was developed in the 1980s as a luxurious alternative to silk.
Cellulose acetate is regenerated cellulose and was invented by Cross and Bevan in the 1890s. Transparent hard films and sheets could be obtained from the solution of cellulose acetate in chloroform and cellulose acetate was used as a substitute for collodion – a solution of cellulose nitrate – for temporary bandages, varnishes and shatterproof glass for windscreens. As a thermoplastic, cellulose acetate could be shaped by pressing a hot sheet into a mould. Unlike cellulose nitrate, cellulose acetate is non-explosive.
Fibres could be spun from cellulose acetate solution and these were known as acetate rayon. Another method to make regenerated cellulose involved dissolving cotton lint or pulp in a solution prepared by dissolving copper hydroxide in aqueous ammonia solution, or by dissolving a cellulose source in a solution of basic copper sulphate in aqueous ammonia to which the solution pH is increased by adding sodium hydroxide. The resulting solution can be spun to cupro-ammonium rayon.
The first artificial fibre based on cellulose was produced by French chemist Comte de Chardonnet in 1884. The fibre, a nitrocellulose cloth, initially sold well and production reached 10,000t per year, but the cloth had a tendency to burst into flames or explode and was deemed undesirable.
This plant-derived material drove advances in industry as well as social and cultural experience. Cellulose is considered to be one of the most abundant substances on the planet. Today, cellulose and its many derivatives are staple elements of cardboards and moisture controlling linings in nappies and sanitation products, and microcrystalline cellulose powders – which are used as fillers in drugs and processed foods. The substance can be broken down into cellulosic ethanol for non-polymer applications such as biofuel.
Continued research into cellulose has informed the development of practical plastics, including cellulose acetate, and blended varieties, including cellulose/polyhydroxyalkanoate, to improve performance, presenting a viable alternative to oil-based materials. Current pressures to better manage natural resources and design for end-of-life suggest that cellulosics have not yet hit their peak, and that there is more to be gained from further research.
Materials World thanks Dr David Segal for his work on this article, which has been adapted for length.