In the production and application of CPP, there are two main problems: first, the stretching speed is small, and increasing the stretching speed is easy to break the film. This disadvantage of domestic CPP resin is more obvious; The second reason is that the heat sealing temperature is high and the range is narrow, which does not meet the requirements of high-speed bag making.
In CPP casting, polymer melt is extruded through a slit die and stretched in air to cool on a rapid cooling shaping roller. Between the mouth mold and the rapid cooling shaping roller, there are mainly two phenomena related to tensile flow, which have aroused the interest of researchers: first, the observation of edge thickening and necking phenomenon; Secondly, instability occurs when the critical stretching speed is exceeded, namely stretching resonance or film fracture phenomenon. The so-called edge thickening refers to the thickness of the edge of the cast film being greater than the thickness in the middle, mainly due to the effect of edge stress. This can be overcome by cutting off the edges of the film with a knife, and the edge material can be mixed back without causing an increase in cost. The cause of the second phenomenon is very complex and is currently a research hotspot in polymer casting. The starting point of stretching resonance is related to the relative molecular weight, viscoelasticity, relaxation time, stretching ratio, aspect ratio of the film, and process parameters of the polymer. Almost all research results do not involve experimental content, are mostly limited to numerical simulation, and do not specifically involve a specific polymer.
Due to the fact that CPP films are mostly used in the packaging and food industries, heat sealing is an important step. 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 thin film. In order to reduce the melting point, from a process perspective, the temperature of the cooling roller can be lowered, the undercooling can be increased, and the crystallinity can be reduced. However, this is limited and may cause a decrease in other mechanical properties. Now, from the perspective of raw materials, the use of random copolymers of propylene, ethylene, and butene, through the insertion of ethylene and butene units into the PP molecular chain segments, disrupts the continuity of the propylene unit. The shorter propylene chain segments are conducive to the formation of the Y crystal phase, and the melting point of the Y crystal is much lower than the common alpha crystal, thus achieving the goal of reducing the heat sealing temperature.
