Semi-synthetic polymers cellulose nitrate

Preparation of semi-synthetic polymers. Cellulose plastics, particularly cellulose nitrate and acetates, were the most commercially-important semi-synthetics, and have been used to prepare photographic films, textile fibres and lacquers. 

Semi-synthetic plastics are natural polymers which have been treated chemically to modify their properties with the aim of producing physically stable, mouldable products. The Swiss chemist, Schonbein, unwittingly prepared the first semi-synthetic polymer by treating paper (cellulose) with a mixture of nitric and sulphuric acids in 1846. The resulting cellulose nitrate was soluble in organic liquids and was softened by heat. It was highly flammable and only thought to be useful as an explosive, which was named gun cotton . 

It is easier to characterize unknown material if its historical and technological backgrounds are known. The development of semi-synthetic polymers in the second half of the nineteenth century can be followed using patents (Fernandez-Villa and Moya, 2005). Development of cellulose nitrate was prompted by the need to find substitutes for natural materials including tortoiseshell, ivory and ebony, which were very expensive. By 1858, approximately 8 per cent of British patents concerned the synthesis or moulding of semi-synthetics. 

In intermediate stages (the 19th century) polymers made by the modification of natural resins have appeared, the most prominent ones being the cellulose derivatives. Celluloid (obtained by nitration of cellulose) represents the first semi-synthetic polymer. It became useful after compotmding with a plasticizer (mamly camphor) to reduce its brittleness. Many cellulose derivatives are still currently in use (as plastomers, textiles or coatings) but the major development in the 20th century is definitely attributed to many families of synthetic polymers—the era of polymers. 

Cellulose-based plastics, particularly cellulose nitrate and acetates, were the most commercially important semi-synthetics up to the 1940s and were used as the base for photographic film, textile fibres, moulded goods and in lacquers. Naturally occurring polymer cellulose in the form of cotton linters or wood pulp is chemically treated to increase its solubility. Cellulose has a high molecular weight of between 100000 and 500000 and an empirical formula C0H1OO5. Casein-formaldehyde is the only protein-based moulded plastic that achieved commercial success. It is based on cow s milk and is still produced in very small quantities for specialist items such as hand-coloured buttons. 

Traces of metals are present in most polymers as a result of processing in stainless steel reaction vessels, being transported in metal containers and shaping in metal moulds or rolling between metal rollers. Many metal ions catalyze oxidation by accelerating decomposition of hydroperoxides at room temperature. Observation of plastics materials in museums suggests that copper is a highly effective catalyst for semi-synthetic materials such as cellulose nitrate and... 

The chemical industry s interest in polymers dates back to the 19th century. In those days it was a case of synthetically modifying natural polymers with chemical reagents to either improve their properties or produce new materials with desirable characteristics. Notable examples were nitration of cellulose giving the explosive nitrocellulose, production of regenerated cellulose (rayon or artificial silk) via its xanthate derivative, and vulcanization of rubber by heating with sulphur. Manufacture of acetylated cellulose (cellulose acetate or acetate rayon) developed rapidly from 1914 onwards with its use both as a semi-synthetic fibre and as a thermoplastic material for extrusion as a film. 

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