Transmission Efficiency of plastic Films Part 1

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TRANSMISSION EFFICIENCY OF PLASTIC FILMS When correct dispersion has been achieved the blend is removed via a valve at the base of the mixing chamber and weighed out into batch tins. If pigment is required it is added at this stage to each separate batch. The batch tins are then emptied into a primary mixer, such as a Ban bury, and treated for about five to ten minutes at 120- 160°C. The combination of heat and the ‘kneading’ action of the Ban bury causes partial gelatin of the mix. This partly gelled material is then passed to a two-roll mill and made to form a sheet round the front roll. This may be fed directly to the calendar but for thin sheet or film a straining process is added to remove any coarse particles. A typical strainer consists of a screw extruder with a filter screen directly in front of the screw. The screen consists of a fine stainless steel woven wire mesh backed up by a medium to coarse mesh for mechanical support, together with a perforated strainer plate. Calendars may consist of from two to five hollow rolls (or bowls) arranged for steam heating or water cooling and are characterized by the number of bowls their arrangement as for example, I, Z or inverted L types. Four roll, inverted L and Z types are the most usual. Very high forces have to be exerted on the rolls in order to squeeze the plastics to a thin film and these cause the rolls to bend giving sheet that is thicker in the middle than at the edges. Z types have the advantage in this respect because bending of the rolls has no effect on the succeeding nip and so improves thickness accuracy. This is because alternative rolls are at right angles rather than in-line. As mentioned earlier, thickness of the film or sheet leaving the calendar is measured using a beta-ray gauge. This gives an average reading of thickness over an area of sheet. Radiation from an isotope such as Thallium 204 is passed through the sheet and then collected in an ionization chamber. The radiation received by the chamber is inversely proportional to the weight per unit area of the material being measured. Advantage of calendaring is that better mixing is obtained. The amount of energy available in a calendaring line is very much more than in an extrusion line, so calendared film is less dependent on the uniformity of the feedstock. The main advantage of extrusion for PVC is the much lower capita costs involved, so that shorter runs can be made at economic rates. 2.6 SOLVENT CASTING: Solvent casting methods are expensive but cellulose nitrate film was made in this way because of the flammable nature of the film. A casting solution is prepared by dissolving cellulose nitrate and camphor (as plasticizer) in a 70:30 ether/alcohol mixture using paddle mixers. After 8—10 h mixing the product is filtered through 16
TRANSMISSION EFFICIENCY OF PLASTIC FILMS cellulose wadding and de-aerated in vacuum at a temperature just under the boiling point of the solvent. The film casting is carried out on an endless belt a smooth surface being obtained by depositing a layer of hard gelatin on a copper support. After being stripped from the casting surface the film is seasoned in a heated drying cabinet where it passes over a zigzag pattern of rollers. After a final passage over chilled rollers to cool the film it is wound into reels. Some cellulose acetate film is produced by solvent casting as well as by extrusion and solution casting has also been used for producing vinyl chloride/vinyl acetate copolymer films. CASTING OF REGENERATED CELLULOSE FILM: Methods of dissolving and then regenerating cellulose date back to 1857 but the one which is the basis for the modern cellulose film was discovered by Cross, Bevin and Beadle in 1892. It was not until 1911, however, that Brandenburg patented the continuous manufacture of cellulose film. Uses for the film were restricted at first because of its high moisture vapor permeability but the development in 1927 of moisture proof grades opened up many new markets and cellulose film is still an important packaging material. The basic raw material cellulose is obtained from wood pulp or cotton linters. After soaking in caustic soda for an hour the excess alkali is pressed out. It is then shredded and allowed to age for 2—3 days and absorbs oxygen from the air. This reduces the length of the cellulose molecule chain and so reduces the viscosity of the solution during the next stage. Here the pulp is transferred to rotating churns and then sprayed with carbon disulphide. The solution is discharged into a tank and dispersed by stirring with dilute caustic soda solution. The resultant solution is known as viscose. 17
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4. ANTI-STATIC ADDITIVES TRANSMISSI
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TRANSMISSION EFFICIENCY OF PLASTIC
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Butyl, GRS, polyurethane, neoprene
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Transmission Efficiency of plastic Films Part 1

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