29 октября, 2015 
Pokraskin Nitrocellulose, also known as cellulose nitrate, was the workhorse thermoplastic binder to produce fast-drying lacquers for many decades. However, its very low solids (~20 %) at spraying viscosity was the main reason for the steep decline of its market share with inception of legislation pertaining to the reduction in VOC emissions.
Nitrocellulose is manufactured by nitration of cellulose by reacting it with a mixture of nitric acid and sulfuric acid in the presence of water. The degree of substitution is the important consideration of the product for its use in coatings. The degree of substitution is
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normally expressed as nitrogen content, and different grades are listed in Table 2.11. Nitration of approximately two hydroxyl groups per cellulose unit is useful for coating applications, while a higher level of substitution results in more unstable material that is used as an explosive (gun cotton). Even coating grade nitrocellulose has the tendency to ignite readily if allowed to dry out in finely divided form. Therefore, it is invariably supplied as alcohol wetted fibers (30 to 35 %) and requires special care during handling, storage and transportation. It must not be stored in dry form, and any source of heat, sparks or static electricity should be avoided. It is quite common practice to store it away from other raw materials.
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Table 2.11: Grades of nitrocellulose for the coating industry
AM — partly soluble In alcohol A and AM grades are obviously soluble In esters, ketones and glycol ethers, too. |
A second important criterion for classification of nitrocellulose is its average MW, which is more conveniently expressed as the viscosity of its solutions. Based on this, it is classified as low viscosity, medium viscosity, and high viscosity grades.
Rapid drying is one of the most important characteristics of nitrocellulose-based coatings. Nitrocellulose alone produces hard and brittle films, and therefore, nitrocellulose-based coatings generally are formulated with plasticizers to improve flexibility and adhesion as well as compatibility with other resins. They are also blended with other polymers such as hard resins and non-drying alkyd resins, to name a couple. On the limiting side, nitrocellulose-based lacquers have generally poor resistance to chemicals and poor durability.
Use of nitrocellulose-based coatings has declined considerably in the United States and Europe because of their low solids at application viscosity and hence high VOC content. However, they are still used
in parts of the world where environmental regulations are not very stringent.
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