Taking biaxial stretched polypropylene film as an example for introduction.
Biaxially stretched polypropylene film (BOPP) has the advantages of lightweight, transparency, non toxicity, moisture resistance, and water resistance. It has good thermal stability, chemical stability, and electrical properties. Therefore, it is widely used in packaging such as food, candy, cigarettes, tea, medicine, juice, milk, textiles, as well as in the production of electrical adhesive tapes. In recent years, the production of BOPP has adopted a new technology of seven layer co extrusion, which enables BOPP to be evenly distributed inside the central gas barrier layer containing acrylonitrile butadiene copolymer, effectively blocking the entry of corrosive substances such as oxygen, carbon dioxide, and water vapor, greatly increasing the anti-corrosion ability of the packaged material and extending the storage period of the item. Therefore, people have given this BOPP film with a gas barrier layer the nickname “packaging queen”.
- Main raw materials
BOPP film is mainly made of isotactic PP as the main raw material, combined with additive masterbatch with a mass fraction of 3% to 5% according to different products and uses. The raw material should be a film grade PP homopolymer, which should have the following characteristics: ① The melt flow rate is 1.5g/10min to 4.0g/10min; ② The regularity is between 95% and 98%; ③ Moisture content<0.03%; ④ Ash content<0.2×10. Additives are auxiliary components for film making. Generally, erucic acid amide type lubricants, ethylenediamine type anti-static agents, and anti freezing agents containing silica particles are selected to improve the surface performance of the product. - Main equipment and basic process flow
The equipment is mainly manufactured by professional companies of BOPP automatic production lines. Companies that can provide complete BOPP equipment technology include MIT-SUBISHI and TOSHIIBA; CELLIER; BRUCKNER, LENZING, MARSAL, etc. Among these competitive technologies, Mitsubishi Heavy Industries and Bruckner are more advanced. Mitsubishi Heavy Industries’ technology is characterized by series twin-screw extrusion and multi roll cooling forming; Bruckner is known for its multi machine co extrusion, dual cylinder filtration with large roller water bath quenching, and rational use of additives. Mitsubishi Heavy Industries’ series twin-screw extrusion has good PP plasticization, uniform and stable pressure, small lateral shrinkage of thick sheets, and good thick sheet tolerance. However, the multi roll cooling process conditions are narrow, cooling is slow, and large-scale high-speed production is limited. Brookner’s multi machine co extrusion, using different combinations of extruders and matching die heads, can produce light films and multi-layer films; Adopting a dual cylinder filtration device, capable of replacing filters while producing; By using large roller internal cooling and water bath external cooling, the molten material can quickly pass through the crystallization temperature zone and be suddenly cooled to room temperature, which is conducive to high-speed and large-scale production; By using additives to adjust the formula, the product has high variability and good film performance. But the rapid cooling in the water bath causes shrinkage on both sides of the thick film, which affects the product tolerance. Mitsubishi Heavy Industries and Bruckner represent two different molding processes today, and have achieved coexistence and development. However, regardless of the company’s process technology, the production of BOPP film is carried out according to the following basic process flow, namely PP → extrusion filtration the first mock examination head molding → quench casting → thickness measurement of thick film → longitudinal stretching → transverse stretching → trimming → film thickness measurement → corona treatment → winding → aging treatment → slitting packaging.
