Cooling cast film

Film generated by depositing a layer of liquid plastic onto a surface and stabilizing by evaporating the solvent, by fusing after deposition or by cooling. Cast films are generated from solutions or dispersions. 

One of the requirements of this process is that the melt maintain good contact with the chill roU, ie, air must not pass between the film and the roU. Otherwise, air insulates the plastic and causes it to cool at a rate different from the rest of the plastic and this spoils the appearance of an otherwise satisfactory product. The melt should not emit volatiles, which condense on the chill roU, reduce heat transfer, and mar the film s appearance. The cast film process allows the use of a higher melt temperature than is characteristic of the blown film process. The higher temperature imparts better optical properties. 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

BROWN et al.i95) have given data on the performance of 1.5 m diameter sulphonators and nitrators of 3.4 m3 capacity as used in the dyestuffs industry. The sulphonators were of cast iron and had a wall thickness of 25.4 mm the annular space in the jacket being also 25.4 mm. The agitator of the sulphonator was of the anchor type with a 127 mm clearance at the walls and was driven at 0.67 Hz. The nitrators were fitted with four-bladed propellers of 0.61 m diameter driven at 2 Hz. For cooling, the film coefficient hb for the inside of the vessel was given by ... 

The films prepared by melt prosing are very rapidly cooled to room temperature. Under these conditions no detectable crystallization in the polymer can develop, as is proved by x-ray examination. On the other hand, in the solvent-cast films chain mobility of the polycarbonate is enhanced by the presence of solvent and crystallinity can occur. In some extreme cases the solvent cast films are turbh ana the a ystallmn. can be a. nonstrated by x-ray photograms which showr x-ray patterns characteristic of crystallized polycarbonate. It is assumed that... 

Polypropylene (PP) films were first produced by extrusion casting. Polymer is extruded through a slit or tubular die and quenched by cooling on chill rolls or in a water bath. Cast films can be sealed over a wide range of temperatures and do not shrink in a steam autoclave, Polymers with melt flow rates below 5 dg/min are usually used to maintain the stability of the extra date. Higher clarity films are produced using random copolymers. 

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. 

Precipitation of the cast liquid polymer solution to form the anisotropic membrane can be achieved in several ways, as summarized in Table 3.1. Precipitation by immersion in a bath of water was the technique discovered by Loeb and Souri-rajan, but precipitation can also be caused by absorption of water from a humid atmosphere. A third method is to cast the film as a hot solution. As the cast film cools, a point is reached at which precipitation occurs to form a microporous structure this method is called thermal gelation. Finally, evaporation of one of the solvents in the casting solution can be used to cause precipitation. In this technique the casting solution consists of a polymer dissolved in a mixture of a volatile good solvent and a less volatile nonsolvent (typically water or alcohol). When a film of the solution is cast and allowed to evaporate, the volatile good solvent evaporates first, the film then becomes enriched in the nonvolatile nonsolvent, and finally precipitates. Many combinations of these processes have also been developed. For example, a cast film placed in a humid atmosphere can precipitate partly because of water vapor absorption but also because of evaporation of one of the more volatile components. 

Perhaps the simplest solution-precipitation membrane preparation technique is thermal gelation, in which a film is cast from a hot, one-phase polymer/solvent solution. As the cast him cools, the polymer precipitates, and the solution separates into a polymer matrix phase containing dispersed pores filled with solvent. Because cooling is usually uniform throughout the cast film, the resulting membranes are relatively isotropic microporous structures with pores that can be controlled within 0.1-10 i m. 

Cast film extrusion. In a cast film extrusion process, a thin film is extruded through a slit onto a chilled, highly polished, turning roll where it is quenched from one side. The speed of the roller controls the draw ratio and final film thickness. The film is then sent to a second roller for cooling of the other side. Finally, the film passes through a system of rollers and is wound onto a roll. A typical film casting process is depicted in Figs. 3.53... 

Two types of samples were prepared for detailed morphological examination. Sections of the as-polymerized material were examined directly by WAXS and DSC. Samples also were prepared by casting films from a solution of the polymer in dimethyl formamide (DMF) at 52°C. Solvent was allowed to slowly evaporate, and the films were dried by annealing for 670 hr at 52°C, followed by slow cooling to room temperature. Thin films suitable for electron microscopy were cast from 0.5 wt % polymer in DMF onto clean glass slides, and after 20 hr of annealing at 52°C, floated off on distilled water and mounted on 300-mesh copper grids. 

Cast film is produced by extruding the melt from a slit die and cooling it either by contact with a chill roll or by quenching in a water bath. The most popular process used to produce the flat film is with the chill rolls. Chill roll lines can be arranged in different layouts to meet different requirements. Example is shown in Figure 5.7. Water chill tank or quench film is also a popular process. 

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