Process of Blown Film

Although almost no LDPE, LLDPE or HOPE manufacturing line is equipped with desiccant bed dryers designed to dry down to less than 2S0 ppm of moisture, drying of PLA is necessary before extrusion. In addition, perhaps more importantly, once PLA material is dried, it needs to be conveyed to the extruder with dry air to keep the resin from regaining moisture. All the blending and holding hoppers also need to be sealed to keep a dry environment for the resin. 

A second consideration is the screw design. Most of the commercial lines have extruder screws that have been optimized to run olefin resins at the maximum output for the diameter of the screw. To do this, they need to design for the specific melt flow properties (rheology) of the resin. PLA has a significantly different rheological behavior from olefin materials. It does not shear thin as readily and therefore typically requires additional power input from the drive. This additional power input manifests itself as an excessive increase in melt temperature. Quite often, if the drive has enough power to process PLA, it will be screw speed limited (and hence rate limited) by excessive melt temperature. 

In extrusion blow film process, molten PLA is extruded to form a tube with an annular die. By blowing air through the die head, the tube is inflated into a thin tubular bubble and cooled. The tube is then flattened in the nip rolls and taken up by the winder (Fig. 5.19). The ratio of bubble diameter to the die diameter is called the blow-up-ratio (BUR). BUR ratios of 2 1 -4 1 with the die temperature of 190 - 200 C have been used for extrusion blowing of PLA films. By varying the BUR, screw speed, air pressure, and winder speed, films of different thicknesses (10- 150 pm) and degree of orientation can be achieved [93,94]. Compared to 

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Some recommended conditions for the processing of blown films are listed in Table 6.6. 

Natural and S5mthetic silica, and talc, influence the processing of blown film because of their varying absorption of process additive, as was discussed in various parts of this book. There are many improved additives among synthetic silicas and modified talcs which minimize this effect. 

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