Hydrated Cellulose Films

films formed from alkaline solutions of cellulose xanthate (viscose) or produced by saponification of finished cellulose acetate film. Hydrated cellulose films are manufactured in industry mainly from viscose (so-called cellophane) by the cellophane, transparent, or dry methods.

The cellophane method is the most widely used. It consists in the production of viscose and the shaping, finishing, and drying of the film. The shaping and later stages are carried out in the same unit—the film machine. The viscose is fed through a plate filter at a uniform rate into a cast-iron spinneret with a slitlike opening. From the spinneret the viscose passes into the precipitating bath (a mixture of sodium-sulfate and sulfuric-acid solutions), where the film is formed. The film then passes through a series of vessels (vats) in which solutions of various reagents for finishing, dyeing, plasticizing, and washing are circulated. Then it is dried and rolled up in reels.

The transparent method consists in the shaping of the film by means of a pouring attachment. Viscose is applied to the surface of a rotating drum about 3 m in diameter, whose lower part is immersed in a precipitating bath. On emerging from the bath the film is unwound from the drum and processed as in the cellophane method. The transparent method makes possible the production of a film of high transparency, without streaks or lines. Among the disadvantages of this method are low productivity and technical difficulties in making the basic equipment.

The dry method is also called the two-bath method, because the coagulation of the xanthate is carried out in an air medium, and the saponification takes place in acid solutions or organic solvents. A thin layer of viscose is applied to a rotating drum, where most of the moisture evaporates and the film forms; the film is then dried slightly on the drum for various lengths of time, depending on its thickness, after which it is then saponified, washed with hot water, and dried.

Hydrated cellulose films are nontoxic, have low vapor and moisture permeability, and are highly resistant to the action of fats and microorganisms. Films made by the dry method have excellent elastic properties. When wet their strength drops by 65-70 percent. The properties of hydrated cellulose films made from viscose depend to a great extent on the method of manufacture.

Hydrated cellulose films are modified to produce high water resistance and reduced vapor and moisture permeability. Modification also facilitates the processing of the films by thermal welding and prevents sticking when they are stored in rolls. The methods of modification used are so-called plying-up (the application of another polymer, such as molten polyethylene, to the film) and lacquering (the application of another polymer in the form of a lacquer).

Lacquered hydrated cellulose film is widely used as a packaging material for fatty dairy and meat products, cleaned fruit, confectionery, cigars, and so on. Ordinary film is used for packaging nonfood goods, as well as industrial products.