Plastic film plays a huge role in industry, agriculture and other areas and has become an integral part of production and life. Plastic films include low density polyethylene (LDPE) film, polypropylene (PP) film, high density polyethylene (HDPE) film, PVC film, etc. LDPE film is mainly used in field of packaging film, agricultural film, heavy packaging film and medical packaging film, among which packaging film dominates LDPE film.
1. What is LDPE material
Let's first understand classification of PE: Depending on different polymerization and production processes of PE, density and molecular structure of PE will be different. According to density of PE, it can be divided into HDPE, LDPE, LLDPE. (Linear Low Density Polyethylene), VLDPE (Very Low Density Polyethylene), ULDPE (Ultra Low Density Polyethylene Film). Among them, LDPE has become main raw material in packaging industry due to its good forming and processing characteristics, easy film forming, excellent optical and mechanical properties.
LDPE is usually produced by free radical polymerization under high pressure (100-300MPa), high temperature (190-210°C) and peroxide catalysis, and product density ranges from 0.91g/cm3 to 0.93g/cm3. . The reason for lower density of LDPE is that during polymerization reaction, a chain transfer reaction occurs, and many branched chains of different lengths are formed on molecular chain, which prevents ordered arrangement of molecular chains.
In general, more branched chains, wider molecular weight distribution and lower melting point will be. The advantage of LDPE film is that it has good toughness, and due to its low crystallinity, it exhibits high transparency and low haze.
2. LDPE film production process
Blow molding method and tape casting method are most important technological methods for processing LDPE film. The blow molding method is more common. LDPE blow molding film is divided into three types: upward blowing method, downward blowing method and flat blowing method. The upward blowing method is most widely used and makes it possible to obtain films with a thickness of 10 to 300 µm.
The specific production process of LDPE blown film: firstly, LDPE resin is melted and plasticized by an extruder, and extruded in extruder to form a tube-like film; secondly, film tube is expanded by compressed air, and then it is clamped by traction roller at same time. Stretch film and quickly stretch it, keep cooling and shaping in air during stretching process, and finally pull roller is pressed and twisted to lift plastic film.
Third, main factors affecting performance of LDPE film
3.1 Extrusion temperature
In melt extrusion process, it is necessary to select appropriate polymer extrusion temperature and make sure that temperature of die is same, and temperature of core mold should be as constant as possible.
Table 3-1 Effect of extrusion temperature on LDPE film
3.2 FrostWhite line
The frost line refers to line separating LDPE melt from a viscous-liquid state to a highly elastic state. In film blow molding process, resin is in a molten state when it is extruded from die, and transparency is high.
Table 3-2 Effect of frost line on LDPE film
3.3 Expansion factor and draft factor
The inflation factor refers to ratio of final diameter of film after winding to diameter of matrix ring. A low degree of inflation often leads to a decrease in transverse mechanical properties of film, a high degree of inflation increases transverse strength of film.
The draw ratio refers to ratio between speed of film traction roller and speed of melt extrusion. As stretch ratio increases, longitudinal mechanical properties of film will also increase and thickness will decrease, so adjusting stretch ratio is an important method for controlling film thickness.
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