Polypropylene casting film
DATE：2022/3/21 8:47:57 / READ： / SOURCE：This station
Polypropylene casting film
The casting method can produce polyolefin casting film, polyolefin fresh-keeping film, polyamide casting film, etc. In general, cast film refers to polyolefin cast film, which can be divided into cast polypropylene (CPP) film and cast polyethylene (CPE) film. CPP film has many characteristics, such as high transparency, good stiffness, low heat sealing temperature, heat resistance, moisture resistance, good barrier, strong adaptability to printing and compounding, smooth surface, cooking resistance, etc. it can be used for food, stationery, groceries and textiles packaging after printing and bag making, and can also be used for packaging all kinds of food after compounding with other films (PP is generally used as the inner and outer materials of composite film), including food, condiment, soup, etc. that need to be heated and sterilized. Compared with PP blown film, CPP film has excellent optical properties and high production efficiency. In developed countries, the output of polypropylene cast film CPP film is higher than that of PP blown film; Compared with biaxially oriented polypropylene (BOPP) film, CPP film has the advantages of simple processing equipment and low cost per unit area, and occupies a place in the field of high-grade packaging film.1. Structure and main raw materials in 1980s, PP casting film was mostly single-layer. In the 1990s, multi-layer composite extrusion has gradually become the mainstream, and now it has developed to 5-layer and 7-layer coextrusion casting. Multilayer coextrusion composite film adopts materials with different properties, which has incomparable advantages and characteristics of single-layer tape casting film. According to the type of use, CPP film is divided into general type, metallized type and cooking resistant type. Different casting films have different requirements for PP raw materials, and there are obvious differences in the properties of different resins. In general, the multilayer composite CPP film is composed of corona treatment layer, support layer and heat sealing layer. The functions of the three layers and the requirements for raw materials are also different. Casting film production line www.handern.com com
2. (1) corona treatment layer in order to improve the printing performance of CPP film, corona treatment must be carried out on the film surface. CPP film must meet the requirements of easy corona treatment and slow decay rate. Corona treatment can increase the surface roughness and surface wetting tension of the polymer, but the larger the surface wetting tension is, the better. Otherwise, the film surface will be brittle due to excessive oxidation, resulting in the decline of mechanical properties. The failure degree of surface wetting tension is directly related to the content of low molecular weight compounds in the resin. The more low molecular weight compounds, the faster the failure of surface wetting tension. It is ideal when the content of wetting agent is 0.8 ~ 1.2x10-3. The basic requirements for corona layer raw materials are: not only have certain anti adhesion, but also control the content of low molecular weight. Casting film production line www.handern.com com
3. (2) the main purpose of supporting is to make the film have certain mechanical properties. The supporting layer must first have excellent rigidity, so the core layer is mostly made of homopolymer; Secondly, the good optical properties (low haze and high gloss) of multilayer co extruded CPP films also depend on the middle layer. Casting film production line www.handern.com com
(3) Firstly, the heat seal of the heat seal layer has good heat sealing performance. It is required that the material has low melting point, good heat fusibility, wide heat sealing temperature and easy sealing. High speed CPP film packaging production line mainly uses binary and ternary copolymers with low heat sealing temperature as heat sealing layer. When a small amount of ethylene is added to PP to form a binary copolymer, its crystallinity decreases, the transparency, impact strength and low temperature resistance of the material increase, and the initial heat sealing temperature of the material decreases. On the basis of binary copolymerization, butene monomer is added for ternary copolymerization, so that the heat sealing temperature of the material is reduced to below 120 ℃, but it still has high gloss, transparency and impact properties.
Special resin shall be selected for the production of these three layers of CPP. Generally, the melt flow rate is required to be 6 ~ 12g / 10min, and the thickness ratio of the three layers is generally 1:3:1. At present, the raw materials used by domestic manufacturers of CP film are mostly imported, mainly from Sandu Huixing company of South Korea, Nordic chemical company, Montell company of the United States and Singapore
4. Low volume There is still a considerable gap in processing and use performance compared with imported raw materials. TPC company, etc. The development and research of CP in China started relatively late, with few types of raw materials and casting film production line www.handern.com com
2. Production process
In the production of CPP film, the prepared materials are first fed by vacuum to the extruder for melting and plasticization, then extruded, cast by T-shaped die, blown and pasted to the quench setting roll by the compressed air ejected by the air knife, and quenched to make it extend smoothly on the roll surface to form a film, then further cooled by the next roll, measured by the thickness measuring device, and then corona treated, cut the edge materials, flattened and rolled by the take-up roll, and the products are accepted and packaged in the warehouse.
In the melt casting process, T-shaped die and quench setting roller are the key equipment for production. The die design should make the materials flow out evenly along the width of the whole die lip, with uniform temperature distribution. The die temperature is generally 230 ~ 250 ℃, and the die temperature for processing homopolymer PP is slightly higher than that for copolymerized pp. the surface of quench setting roller should be subject to finishing and stable speed to avoid temperature fluctuation in the machine direction. The temperature should also be adjusted appropriately, Supercooling or overheating has a great influence on the mechanical properties, transparency and haze of the films. For PP, the temperature of quench setting roller is about 25 ℃; The temperature difference between the head temperature and the quench setting roller is an important parameter to determine the product degree. Increasing the temperature difference can reduce the product degree and improve the transparency, toughness and heat sealing performance, but the tensile strength decreases. An air knife is arranged above the quench setting roller. Under the blowing pressure of the air knife, the molten material immediately clings to the surface of the quench setting roller. Another function of the air knife is to extract low molecular volatiles such as smoothing agent to prevent them from accumulating on the quench setting roller, so as to better ensure the appearance quality of the film. The film is used after cooling β The thickness is monitored by X-ray or infrared thickness gauge in order to control the thickness tolerance. Then, corona treatment was carried out on the film to increase the composite fastness of printing ink or composite adhesive and the film, and the surface wetting tension was controlled above 42mn / m. CPP film is relatively soft. The winding tension must be adjusted according to the film thickness, production speed and other factors. If the tension is too large or too small, it will produce ripples and affect the flatness of the film. In the whole production process, in addition to the important process parameter of temperature, another important parameter is the drawing speed. The larger the drawing speed is, the higher the production efficiency is. However, if it continues to increase after a certain value, it is easy to produce film breaking phenomenon. The stretching speed is also the main factor of molecular orientation. The greater the stretching speed is. The greater the molecular orientation along the mechanical direction, the thickness of the film can also be adjusted by adjusting the tensile speed
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