Tubular-film stretching

4 TWO-DIMENSIONAL STRETCHED nLMS 12.4.1 Tubular-film stretehii  

With polymers such as polyvinylidene chloride copolymers and polypropylene the bubble method is carried out in a different way. First a tube is extruded and quenched as rapidly as possible to keep the crystallinity as low as possible. As shown in Fig. 10 the collapsed tube passes through a pair of nip rolls (a) and is then heated to the desired temperature and blown up to a 4-5 times larger diameter before it reaches the next set of nip rolls (b) rotating at a higher speed to provide a longitudinal stretch ratio 1 3-1 4. 

The film can then be fiattened and wound up as a tube or slit to obtain a single sheet. A polyvinylidene chloride film obtained in this way is heat-shrinkable at temperatures above SO C. If a heat-stabilised film is desired, an annealing step is applied by re-inffating the film and leading it through a radiant-heating tunnel (d). The third set of nip rolls (c) rotates at the same rate as the second set, and the air pressure in the bubble prevents the film from shrinking in the transverse direction. This is very effective 

An advantage of this process is that the amorphous tube crystallises rapidly during biaxial stretching. This increases the modulus and therefore also the stress at a faster rate than the thickness decreases. Hence, the balloon will obtain a diameter where the developed tensile stress just balances the internal pressure. This diameter will, of course, depend on several factors, such as temperature, wall thickness and on the stretch ratio in the machine direction, but not on the amount of air. If more air is introduced, the balloon will become only longer, not wider and, hence, the balloon is self-stabilising. 

More or less the same process as used for polyvinylidene chloride film can be applied to polypropylene, which also shows this self-stabilising property. In order to reduce the crystallinity of the unstretched film, internal cooling of the balloon by cold mandrels and (or) external cooling by water sprays is necessary. 

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An important application for polypropylene is film tape. This is made by slitting unoriented film (cast or blown) into tapes 2 or 3.5 mm wide and stretching under heat about seven-fold. With cast film the orientation is more completely monoaxial and there is a tendency for the film to split along its length (fibrillate). Tubular film does not self-split so easily and also has a somewhat softer feel. Such tapes may be woven into sacks and these have... 

Apart from the processes which were briefly discussed here, there are a number of other methods for stretching tubular film, such as octagonal film stretching and stretching over a horseshoe mandrel. These methods are based on similar principles and have been dealt with by Park and Conrad.2o... 

During the stretching of tubular films, as discussed in the previous section, longitudinal and transverse stretching are usually carried out simultaneously. This is also possible in flat-film stretching by means of a tenter frame. 

In film blowing, a tubular film is extruded upwards. It is blown upwards, with air introduced below the die, into a larger tubular film which is then picked up by a pair of nip rolls that seals the bubble (Fig. 3.11 Han, 2007). An external stream of chilled air cools and solidifies the film at a certain point called the freeze line, where the temperature of the film is equal to the melting temperature. A feature of this process is that the film is stretched biaxially, improving mechanical properties. Tangential circumferential stretching depends on blow-np ratio, i.e. the ratio between the tubular film diameter after air introduction and the initial tabular film diameter. This parameter is determined by the pressure level within the bubble. Axial stretching depends... 

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

Film blowing. In film blowing, a tubular cross-section is extruded through an annular die, normally a spiral die, and is drawn and inflated until the freezing line is reached. Beyond this point, the stretching is practically negligible. The process is schematically depicted in Fig. 3.55 [14],... 

Simultaneous Biaxial Orientation. There are two predominate systems available to do this, tubular and flat film. In the tubular process (see Fig. 9), also referred to as the double bubble process, a continuous tube is extruded and quenched. Typically, an interior cooled mandrel is hung from the die inside the tube. The surface of the mandrel may greatly influence the interior surface of the tube. Care must be taken not to impart scratch lines in the melt as it is pulled down over the mandrel. Air pressure in this primary tube is very critical. The melt needs to be held out over the mandrel but not too far away. A water bath on the external side of the tube helps quench the tube rapidly. A nip pulls the tube from the die and acts to isolate the casting bubble from the air pressime in the stretching bubble (27). 

In the fiat film biaxial process there are several methods available to simultaneously stretch a web (28,29). The throughput on the film line can be significantly greater with the fiat film process as compared to the tnbnlar process. With fiat film processes, there is more fiexibiUty in the temperatme profile during the stretching process than that found in the tubular process. This can help attain greater levels of orientation in the film. 

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