The main problems of cpp cast film
DATE：2022/2/16 8:43:59 / READ： / SOURCE：This station
The main problems of cpp cast film
In the production and application of CPP, there are two main problems: the first is that the stretching speed is small, and it is easy to break the film when the stretching speed is increased. This disadvantage of domestic CPP resin is obvious; Narrow, does not meet the requirements of high-speed bag making.In the casting of CPP, the polymer melt is extruded through a slot die and drawn in air onto a quenched setting roll for cooling. From the die to the quenching setting roll, the extensional flow problem is mainly involved, during which two phenomena arouse the interest of researchers: one is to observe the phenomenon of edge thickening and necking; Instability, i.e. tensile resonance or film breakage phenomena. The so-called edge thickening means that the thickness of the edge of the cast film is greater than the thickness of the middle, mainly due to the effect of edge stress. This can be overcome by cutting off the edges of the film with a cutter, and the shavings can be backmixed without incurring additional costs. The cause of the second phenomenon is very complicated, and it is also a research hotspot in polymer casting. The starting point of the stretching resonance is related to the relative molecular mass of the polymer, viscoelasticity, relaxation time, stretching ratio, film aspect ratio and process parameters. Almost all the research results do not involve experimental content, and are mostly limited to numerical simulations, and they are not specific to a certain polymer.
Since CPP films are mostly used in the packaging industry and food industry, heat sealing is an important link. In order to improve efficiency, the heat sealing temperature should be reduced as much as possible. The heat sealing temperature corresponds to the melting point of the film. In order to reduce the melting point, from the process point of view, the temperature of the cooling roll can be reduced, the degree of supercooling can be increased, and the degree of crystallinity can be reduced, but this is limited and will cause other mechanical properties to decline. Now, from the perspective of raw materials, random copolymers of propylene, ethylene and butene are used, and ethylene and butene units are inserted into the PP molecular segment, which disrupts the continuity of the propylene unit, while the shorter propylene segment It is beneficial to the formation of the Y crystal phase, and the melting point of the Y crystal is much lower than the common α crystal, so the purpose of reducing the heat sealing temperature is achieved.
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