Polyethylene melt cast film

Figure 3.15. Two images of a thin melt cast film of high density polyethylene the region is 200 x 250//m. The left hand image was taken in crossed polars. The radial Maltese cross is due to the extinction position. The spherulites in this material have dark circumferential bands. The crystals twist as they grow, and their orientation in these bands has the optic axis perpendicular to the specimen plane. The right hand image is the same area when a first order red plate is also used. The blue and yellow colors show that the spherulites are negative. (See color Insert.)...

The polyethylene samples examined are shown in Table III slowly cooled or quenched from melt, original monofilament, annealed, over drawn, cold drawn, single crystal, cast film, extended chain crystal, etc. The sample-probe distance can be chosen from TO to 260 mm. The setting angle is defined as the angle between the molecular plane and the be plane according to Bunn (1 1) as shown in Figure 12. 

Figure 13. Cell dimensions plotted against the lattice distortion parameter of polyethylene (40) ((O) melt crystallized, A (9) melt crystallized, B (Is) single crystal ( ) cast film high pressure crystallized, C)...

A new approach was proposed for making effective helmets which could replace the former British army steel helmet. Essentially the new helmet used modified phenolic resins reinforced with nylon, and the crown cap inside was thermoformed from polyethylene. Formerly the crown cap was attached to the steel by rivets—not an appropriate method for fixing polyethylene to reinforced plastics. Instead a method was developed with a hot-melt adhesive based on ethylene-vinyl acetate copolymers cast as film on release paper. For assembly, the cast film is cut in advance to match the intricate shape required and activated by heat to bond under light pressure subsequently, a further heat activation is employed to fix the crown cap in place (Figure 52 illustrates this). 

Figure 10.35. Temperature distributions before and after a 1.77 inch inside diameter six element Kenics HEM mixer performing thermal homogenization of polyethylene melt. The apparent viscosity of polyethylene used in the test was 11,000 poises. A homogeneous melt stream was obtained using a Kenics Mixer of six elements. It was found that thermal homogenization in the Kenics Mixer is independent of the initial radial temperature profiles and the size of the unit. A radial thermal gradient reduction from 100°F to less than 1 °F was obtained in a PVC cast film production. In general, the unit delivers a polymer melt stream with less than a 3°F radial temperature gradient.

Cast film is made by extruding the melt (usually polyethylene) through a large die equal in size to the width of the film being cast. The material is extruded as a thin sheet onto a mirrored surface chiU roll, and it is then drawn down further by other rolls. In addition to the die, which deposits the hot melt directly onto the chill roll, an air blast is often used at the chiU roll to help eliminate air entrapment and force the melt onto the chill roU. 

In the examination of potential applications for these unique materials that possess a wide variety of properties depending on copolymer composition, the Dow group examined finished articles formed by injection and blow molding, blown and cast film and melt extrusion. Potential applications for these new materials would be as substitute materials for flexible PVC, styrenic block copolymers, ethylene/vinyl acetate copolymers and ethylene/propylene-based elastomers. These new ethylene/styrene copolymers once again demonstrate that new catalyst technology creates new markets and applications for the polyethylene industry by competing with materials outside of the polyethylene product mix. 

The drawing illustrates the important features necessary for the cast film process which include (a) the melting of the polyethylene onto a heated roll, (b) a stretching zone where the molten polyethylene film thickness is reduced to a thinner gauge by moving the take-up roll at a faster rate than the take-off roll, and (c) the cooling tank where the polyethylene material solidifies to a thin film. 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

For some operations, the chill roll method does not provide rapid enough cooling. In that case, a water-filled quench tank may be used for cooling and solidifying the plastic, as shown in Fig. 7.5. After solidification, the film is dried, trimmed, and rolled up. Drying may be accomplished by evaporation alone, or air jets, heated rolls, or radiant heat maybe used. The film characteristics are controlled by the die dimensions, extrusion rate, melt temperature, drawdown, and water temperature. This method used to be widely used for polyethylene and polypropylene, but is now much less common, since chill roll casting can provide better control over optical properties and thickness. 

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