3. Coating (Figure 38.3): This process is used in the production of self-supporting films by coating sheet materials such as paper and textiles. The most important materials that are used for the production of coatings are PVC pastes.

The equipment for coating consists of an unwinding station, an expander, coating head, gelling channel, cooling system, and a rewind station. The disk brakes slows down the unwind station, and the rewind station enables the product to be held under tension. A smooth finish to the sheet is provided by the expanders. With the help of the coating head, the PVC paste is spread upon the moving sheet manually or mechanically. Thereafter, a doctor blade is employed to spread the sheet to required thickness. A revolving roller supports the sheet, due to which an exactly defined contact area is maintained. The rolls distribute the escaping paste and form a film. The rotation speeds of the rolls can be adjusted.

Hot air or infrared dryers supply the heat required for gelation of the paste. However, circulating air heating in combination with infrared heaters is used in some cases.

The cooling effect must be high enough so that no adhesion between the individual layers occurs. The cooling is attained by two water – cooled , corrosion – resistant hollow drums.

Figure 38.3 . Coating Unit

a)  Unwinding Station; b) Expander; c)Coating head; d)Gelling Tunnel; e)Cooling Roll; f)Rewind Station

 

4. Foaming : By adding physical or chemical blowing agents to a polymer, foams are generated. Physical blowing agents include gases and low-boiling compounds , whereas ; chemical blowing agents generally are gas – releasing compounds. Only a few materials are employed in the manufacture of foams , such as polystyrene , PVC, LDPE as well as phenolic and polyurethane resins.

The foaming process must begin when the polymer is in a flowable form and allows bubbles to develop. The conditions are fixed, once the bubbles reach the optimum size.