The cellulose acetate is an organic and synthetic compound that can be obtained in solid form as flakes, flakes or white powder. Its molecular formula is C 76 H 114 O 49. It is manufactured from the raw material obtained from plants: cellulose, which is a homopolysaccharide.
Cellulose acetate was manufactured for the first time in Paris in 1865 by Paul Schützenberger and Laurent Naudin, after acetylating cellulose with acetic anhydride (CH 3 CO – O – COCH 3). Thus they obtained one of the most important cellulose esters of all time.
According to these characteristics, the polymer is destined to the manufacture of plastics for the field of cinematography, photography and in the textile area, where it had its great boom.
It is even used in the automotive and aeronautical industry, as well as being very useful in chemistry and research laboratories in general.
Chemical structure
The upper image represents the structure of cellulose triacetate, one of the acetylated forms of this polymer.
How is this structure explained? It is explained from cellulose, which consists of two pyranous rings of glucoses linked by glucosidic bonds (R – O – R), between carbons 1 (anomeric) and 4.
These glycosidic bonds are of the β 1 -> 4 type; that is, they are in the same plane of the ring with respect to the group -CH 2 OCOCH 3. Therefore, its acetate ester retains the same organic skeleton.
What would happen if the OH groups at the 3-carbons of cellulose triacetate were acetylated? The steric (spatial) tension in its structure would increase. This is because the –OCOCH 3 group would "collide" with neighboring glucose groups and rings.
However, after this reaction, cellulose acetate butyrate is obtained, the product obtained with the highest degree of acetylation and whose polymer is even more flexible.
The explanation for this flexibility is the elimination of the last OH group and, therefore, of the hydrogen bonds between the polymer chains.
In fact, the original cellulose is capable of forming many hydrogen bonds, and the elimination of these is the support that explains the changes in its physicochemical properties after acetylation.
Thus, acetylation occurs first on the less sterically hindered OH groups. As the concentration of acetic anhydride increases, more H groups are replaced.
As a result, although these -OCOCH 3 groups increase the weight of the polymer, their intermolecular interactions are less strong than hydrogen bonds, "flexing" and hardening the cellulose at the same time.
Obtaining
Its manufacture is considered a simple process. Cellulose is extracted from the pulp of wood or cotton, which is subjected to hydrolysis reactions under different conditions of time and temperature.
Cellulose reacts with acetic anhydride in a sulfuric acid medium, which catalyzes the reaction.
In this way, cellulose is degraded and a smaller polymer is obtained that contains 200 to 300 glucose units for each polymer chain, the hydroxyl of cellulose being replaced by acetate groups.
The end result of this reaction is a white solid product, which can have the consistency of powder, flakes, or lumps. From this, the fibers can be made, by passing it through pores or holes in a medium with hot air, evaporating the solvents.
Through these complex processes, various types of cellulose acetate are obtained, depending on the degree of acetylation.
Because cellulose has glucose as its monomeric structural unit -which has 3 OH groups, which are the ones that can be acetylated-, di, tri or even butyrate acetates are obtained. These -OCOCH 3 groups are responsible for some of its properties.
Properties
Cellulose acetate has a melting point of 306 ° C, a density that ranges from 1.27 to 1.34, and has an approximate molecular weight of 1811.699 g / mol.
It is insoluble in various organic components such as acetone, cyclohexanol, ethyl acetate, nitropropane, and ethylene dichloride.
Of the products that contain cellulose acetate, flexibility, hardness, resistance to traction are valued, not being attacked by bacteria or microorganisms and their impermeability to water.
However, the fibers present dimensional changes according to extreme variations in temperature and humidity, although the fibers resist temperatures up to 80 ° C.
Applications
Cellulose acetate finds many uses, including the following:
- Membranes for the manufacture of plastic, paper and cardboard objects. An indirect effect of a chemical additive of cellulose acetate is described when it is in contact with food in its packaging.
- In the health area it is used as membranes with holes the diameter of blood capillaries, embedded in cylindrical devices that fulfill the function of an artificial kidney or hemodialysis equipment.
- Within the art and film industry, when used as thin films for the cinema, photography and magnetic tapes.
- In past times it was used in the textile industry, as fibers to make different fabrics such as rayon, satin, acetate and triacetate. While it was in fashion, it stood out for its low cost, for the brightness and for the beauty it gave to the garments.
- In the automobile industry, for the manufacture of engine and chassis parts for different types of vehicles.
- In the field of aeronautics, to cover the wings of airplanes in times of war.
- It is also widely used in scientific and research laboratories. It is generally used in the manufacture of porous filters, as a support for cellulose acetate membranes to perform the electrophoresis or osmotic exchange run.
- It is used in the manufacture of cigarette filter containers, electrical cables, varnishes and lacquers, among many other uses.
References
- Fischer, S., Thümmler, K., Volkert, B., Hettrich, K., Schmidt, I. and Fischer, K. (2008), Properties and Applications of Cellulose Acetate. Macromol. Symp., 262: 89-96. doi: 10.1002 / masy.200850210.
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