Polymer Films

Polymer Films

 

continuous layers of polymers up to 0.2–0.3 mm thick. Thicker layers of polymeric materials are called sheets. Polymer films are made from natural, artificial, and synthetic polymers. The first group includes films made from proteins, natural rubber, and cellulose. The most common film in this group is cellophane. A second, larger group consists of films made of artificial polymers, or products of the chemical treatment of natural polymers. This group includes films produced from cellulose esters, as well as from natural rubber that has undergone prior hydrochlorination. Polymer films made of synthetic polymers make up the largest group. The most common representatives of this group are films based on polyolefins, polyvinyl chloride, polyamides, polyvinylidene chloride, polystyrene, polyethylene terephthalate, and polyimides.

The major industrial methods for the production of polymer films are extrusion of a polymer melt, casting of a solution of a polymer on a polished metal surface (in some cases, the solution of the polymer is introduced into a precipitation vat), casting a dispersion of the polymer on a polished surface, and calendering.

Extrusion of a polymer melt is suitable in cases in which the materials to be treated do not undergo thermal degradation upon transition to the state of viscous flow. Most synthetic polymers are made into films by this method. Extruders with an annular or slit head are used for extrusion. In the former case, the polymer melt is extruded in the form of a tube, which is inflated

Table 1. Some physicomechanical and electrical properties of polymer films
  Tensile strength (MN/m2 [kgf/cm2])Relative elongation upon rupture (%)Tear strength (g)Tangent of dielectric loss angle at 106HzDielectric permeability at 106 HzElectric strength (MV/m, or kV/mm)
*For a film 50 microns thick †For a film 25 microns thick
Polyethylene      
 low-density........10–21 (100–210)100–700100–5000.00032.230–60
 high-density........17–43 (170–430)10–65015–3000.00052.330–60
Polyvinyl chloride      
 rigid ............49–70 (490–700)2510–7000.006–0.172.8–3.117–54
 flexible ..........10–40 (100–400)150–50060–1,4000.04–0.143.3–4.545
Biaxially oriented polystyrene55–85 (550–850)3–4050.00052.4–2.7100
Polyamide-6 (Nylon-6) ….65–125 (650–1,250)250–55050–900.0253.450–60*
Polyethylene terephthalate .140–210 (1,400–2,100)70–12012–270.0163.0300†
Polytetrafluoroethylene …10–28 (100–280)100–35010–1000.00022.0–2.125–40
Cellulose triacetate ….65–110 (650–1,100)10–404–10†0.0333.3150
Unlacquered cellophane …50–125 (500–1,250)10–502–203.280–100
Table 2. Resistance of polymer films to various actions1
  Strong acidsStrong alkaliesGreasesOrganic solventsWater absorption in 24 hr (%)Resistance to sunlightHeat resistance (°C)Cold resistance (°C)
1Conventional designations: (+ +) very good, (+) good, (±) moderate, (–) poor, (– –) very poor 2Lacquer-coating may be nonresistant
Polyethylene        
 low-density............+ ++ ++<0.01– to +80–90–57
 high-density ...........+ ++ +++0– to +120–46
Polyvinyl chloride        
 rigid................+ ++ +++0+65–93
 flexible ..............++++0+65–93–46
Biaxially oriented        
 polystyrene ...........++ ++0.04–0.0680–95–56 to –70
Polyamide-6 (Nylon-6) ......– –+ ++ ++ +9.5– to +90–200–70
Polyethylene terephthalate ....+++ ++ +0.8± to + +150–60
Polytetrafluoroethylene......+ ++ ++ ++ +0.005+ +260–90
Cellulose triacetate ........ + +2.4–4.5+ +150–200
Lacquer-coated cellophane ...++245–115+130–18

by compressed air, leading to a biaxial orientation of the film. The inflated-tube process, or blown-bubble extrusion process, is the most efficient and economical method for the production of polymer films. The slot-die method, or slot-extrusion method, makes possible the formation of unoriented (isotropic), uniaxially oriented, and biaxially oriented polymer films, which in some cases are subsequently smoothed by rolls. This method is preferred in cases in which a film of uniform thickness with a high-quality surface is required. Films made from crystallizing polymers, such as polyethylene terephthalate, undergo crystallization after orientation; this greatly increases the strength of the film.

The production of polymer films by casting a solution of the polymer on a cold or heated polished surface was one of the first industrial methods but is now of limited importance. It is used mainly for films of cellulose and its derivatives, as well as for some films made of synthetic polymers—for example, polyimides, polyvinyl alcohol, and polycarbonate. The method consists of preparation of the solution, casting on the smooth, polished surface of a drum or endless metal belt, and removal of the solvent from the polymer. The resultant film undergoes thermal treatment to remove internal stresses, and also, when necessary, uniaxial or biaxial orientation.

The production technology of polymer films using polymer dispersions is quite similar to the method of casting a polymer solution. The dispersion is usually a colloidal system—for example, a latex—in which the dispersion medium is water and the dispersed phase consists of particles of the polymer. This method is used, in particular, for producing rubber sanitary items. Calendering is used primarily in the production of polyvinyl chloride films.

In most cases, films made from synthetic polymers are superior in physicomechanical and chemical properties to films made from natural and artificial polymers (Tables 1 and 2); their industrial production is continuously expanding.

Polymer films are mainly used as packing materials for food products, consumer goods, and liquid and bulk chemical and petrochemical products, as well as for household purposes. Materials used in the production of packing films include polyethylene, polypropylene, cellulose and its esters, polyvinyl chloride, polystyrene, polyamides, polyesters, and the hydrochloride of natural rubber. Multilayer film-film, film-paper, and film-foil materials, as well as foamed films, have special properties.

Electrically insulating polystyrene, polyolefin, polyethylene terephthalate, polycarbonate, polytetrafluoroethylene, and polyimide films are commonly used for insulating wires and cables, in the production of capacitors, and for slot linings in electrical machines. Polymer films are the base, or substrate, for motion-picture and still film and magnetic tapes for audio and video recording and playback. Biaxially oriented and crystallized cellulose acetate and polyethylene terephthalate films are most suitable for this purpose. Atmosphere-resistant transparent polymer films (polyethylene, polyamide, polyvinyl chloride, and polyethylene terephthalate films, and in some cases, films reinforced by glass fiber or fabric made from synthetic fibers) are used in the production of hothouse frames and roofs and removable protective coverings for shielding plants in open fields from freezing or for creating a microclimate favorable for vegetation.

Waterproofing films are used in construction and in the building of artificial ponds and canals. Ion-exchange films are used for the extraction of substances by electrodialysis, for desalination of water, for purification of organic compounds and their solutions (for example, sugar solutions), and for concentration of solutions and separation and identification of various compounds. Polaroid films are widely used as light filters to prevent motorists from being blinded by the headlights of oncoming vehicles, for various types of signaling systems, and for the production and showing of stereoscopic motion pictures.

Polyolefin films are first in total world production, and polyvinyl chloride films are second. In 1970 in the USA, polyethylene films accounted for more than 62.3 percent of total film production; polyvinyl chloride films, more than 25.1 percent; polypropylene films, 2.4 percent; polyamide films, 0.1 percent; and other films, about 10 percent.

REFERENCES

Kozlov, P. V., and G. I. Braginskii. Khimiia i tekhnologiia polimernykh plenok. Moscow, 1965.
Takahashi, G. Plenki iz polimerov. Leningrad, 1971. (Translated from Japanese.)
Gul’, V. E. Polimernye plenochnye materialy. Moscow, 1972.
Entsiklopedia polimerov, vols. 1–3. Moscow, 1972–73.

V. E. GUL’ and P. V. KOZLOV

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