BOPP film preparation process 2

(6) Longitudinal stretching (MDO). MDO is to fully preheat the thick sheet to 130°C-1356°C, and use the longitudinal tension generated by the fast roller and the slow roller to complete it in an instant. The speed ratio of the fast roller and the slow roller is called the longitudinal stretch ratio. MDO elongates and thins the thick sheet, forcing the molecules to align along the stretching direction to improve the longitudinal properties of the product. MDO is generally carried out with a stretch ratio of 5-6 times and a process condition of 130°C to 135°C. If the temperature is too low, there will be cold drawing, large transverse shrinkage, and large turbidity; if the temperature is too high, the roll will stick and the longitudinal strength will decrease.

Transverse stretch (TDO)

After the longitudinal stretch sheet is further preheated, the two sides of the film are firmly clamped with chain clips, and the chain rails are opened in balance to widen and thin the film, and then heat-set to obtain good transverse performance. TD0 is generally carried out at 160°C-165°C and a stretch ratio of 7 to 8 times. TDO is a process that is easier to pull and break the membrane. There are many factors for membrane breakage. In addition to the poor quality of the thick film mentioned above, improper horizontal drawing process can also cause membrane breakage. temperature

If the temperature is too low, there will be a loud cracking sound when the membrane is broken; if the temperature is too high, the membrane will become soft, and the membrane will be broken slowly and silently, which is easier to distinguish.

(8) Edge trimming and edge material circulation. For the film produced by TDO, there are about 100mm thick sheets with uneven stretching on both sides (including the part clamped by the chain clip), which need to be cut off before winding. About 12% of the side strips are cut and pulverized, and sent back to the raw material hopper with the air duct to be mixed and recycled with new PP materials to improve film formation rate and reduce raw material consumption.

(9) Film thickness measurement. The thickness of the trimmed film is monitored by a ray thickness gauge before being rolled, and an automatic recorder is used to scan the vertical and horizontal comprehensive thickness. The operator judges whether the production process is normal according to the scanning curve. Combining thickness measurement and scanning of thick slices to determine whether it is necessary to fine-tune the lip opening to ensure product thickness tolerance.

(10) Corona treatment. PP is a non-polar polymer. The surface wetting tension of untreated BOPP film is only 28mN/m, which has poor adsorption to post-processing materials such as printing ink and glue coating, and its application is limited. Corona treatment is to use a high-frequency high-voltage generator to generate uniform corona discharge between the electrode and the treatment roller. The surface of the film is subjected to corona to form polar groups such as carbonyl groups, and the surface wetting tension is increased to above 38mNm. There are different opinions on the mechanism of corona treatment, which will not be discussed here. But it should be pointed out that corona treatment is an important process of BOPP processing, and we must try to ensure this quality index of the product in actual work.

The post-processing of BOPP film, such as printing, gluing, and metal plating, requires the surface wetting tension after corona treatment to be ≥38mN/m. Experiments have proved that after electricity treatment, the surface osmotic tension of the film is not a constant in the first month, but a variable value that decays with time. The first two weeks attenuate quickly, and the surface osmotic tension decays to 4mNm~6mNm after three weeks of treatment. , which will remain basically unchanged in the future.