Films polyethylene

Polyethylene films as packaging ma terial plastic squeeze bottles are molded from high density poly ethylene... 

Materials that typify thermoresponsive behavior are polyethylene—poly (ethylene glycol) copolymers that are used to functionalize the surfaces of polyethylene films (smart surfaces) (20). When the copolymer is immersed in water, the poly(ethylene glycol) functionaUties at the surfaces have solvation behavior similar to poly(ethylene glycol) itself. The abiUty to design a smart surface in these cases is based on the observed behavior of inverse temperature-dependent solubiUty of poly(alkene oxide)s in water. The behavior is used to produce surface-modified polymers that reversibly change their hydrophilicity and solvation with changes in temperatures. Similar behaviors have been observed as a function of changes in pH (21—24). 

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

Table 6 shows the sales estimates for principal film and sheet products for the year 1990 (14). Low density polyethylene films dominate the market in volume, followed by polystyrene and the vinyls. High density polyethylene, poly(ethylene terephthalate), and polypropylene are close in market share and complete the primary products. A number of specialty resins are used to produce 25,000—100,000 t of film or sheet, and then there are a large number of high priced, high performance materials that serve niche markets. The original clear film product, ceUophane, has faUen to about 25,000 t in the United States, with only one domestic producer. Table 7 Hsts some of the principal film and sheet material manufacturers in the United States.

Linear Low Density Polyethylene. Films from linear low density polyethylene (LLDPE) resias have 75% higher tensile strength, 50% higher elongation-to-break strength, and a slightly higher but broader heat-seal initiation temperature than do films from LDPE. Impact and puncture resistance are also improved over LDPE. Water-vapor and gas-permeation properties are similar to those of LDPE films. 

Liaear low density polyethylene films are used ia many of the same packagiag appHcations as LDPE. The greater film extensibiUty permits the printing of small bags by one or more of 800 American companies. 

Flexographic Inks. Fluorescent toners such as the Radiant GF, Lawter HVT, and Day-Glo HM and HMS Series toners are used in flexographic ink formulations. These products are soluble in blends of alcohol (80%) and ester solvents (20%) and are compatible with modifying materials such as nitroceUulose resins and acryHc solution polymers. Flexographic inks of this type are used most commonly to print products such as ceUophane and polyethylene film for packaging, and also to print paper products such as gift wrap and price labels. 

Attempts have been made to use cold-set adhesives in the cormgating operation, such as poly(vinyl acetate) and modified, precooked starch formulations, but these have not achieved any appreciable degree of commercial acceptance (20). The use of a polyethylene film appHed to the inside surface of the linerboard facing, which serves as a hot-melt cormgator adhesive, has achieved some commercial usage. However, its use is limited to the small, specialty product niche of fast-food hamburger cartons (see Olefin polymers, polyethylene). 

Table 17. Physical Properties of a Multilayer Barrier Film and a Polyethylene Film ...

Figures 4 and 5 show how the diffusion coefficient and solubility coefficient vary for a series of linear esters in low density polyethylene film. The trends are generally tme for other permeants in other films. As the size of the permeant increases, the diffusion coefficient decreases and the solubility coefficient increases. Since the increase in solubility coefficient is larger than the decrease in the diffusion coefficient, the permeability actually increases as the permeant size increases.

Fig. 4. Diffusion coefficient at 30°C of esters in a low density polyethylene film (18).

Free polyethylene film 1.03 Gross-weigh 1.10 -30 to Granules, sharp 1.01 1.01 1.03 1.02... 

Polyethylene film bags are closed by heat-seahng together the face and back of the bag. The closing unit consists of a pair of belts that support the top of the bag and guide it through a heated section that fuses the face and back. This is followed by a coohng section. 

This is a material produced from the impregnation of glass fibre-mat (fibreglass, which is in the form of dry sheet, is commonly known as chopped stranded mat (CSM)) or rovings, with a liquid and unsaturated polyester resin, which thickens chemically to a dry sheet form. The total mix is sandwiched between polyethylene films and then roller-pressed to impregnate and consolidate it. 

The chemical thickening enables the material to be handled after the polyethylene film has been removed before moulding. 

The widespread use of plastics film for printing paper appears to be only a remote possibility in the near future. There is, however, the prospect of the establishment of polyethylene film for use in service manuals, maps and other printed articles which are required to be resistant to water, oils and other liquids. 

Figure 10.13. Methods of producing polyethylene film (a) tubular process using air cooling (b) flat film process using water bath cooling (c) flat film process using chill...

Figure 10.14. Effect of freeze-line distance and other operating variables on the haze of low-density polyethylene film. (Reproduced by permission of ICI)...

Considerable amounts of polyethylene film are produced using coaxial extrusion processes in which two or more melt streams are combined in the die to produce extruded film of two or more layers of plastics materials. Layers in such a composite may be included, for example, to improve barrier properties, to enhance sealability or even simply to act as an adhesive between dissimilar layers. 

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