Cast polypropylene film is a polypropylene film produced by the casting process.

Classification

It can be divided into general CPP (GCPP for short), metalized CPP (MCPP for short) and cooking CPP (RCPP for short) films. It has excellent transparency, uniform thickness, and uniform performance in the vertical and horizontal directions. It is generally used as the inner layer material of the composite film. The thickness of ordinary CPP film is generally between 25 and 50¦Ìm. After being compounded with OPP, it has good transparency, bright surface, and firm feel. This material is generally used in gift packaging bags. This film also has good heat sealing properties. The thickness of cooking-grade CPP film is generally between 60 and 80 ¦Ìm. It can withstand high-temperature cooking at 121¡æ for 30 minutes, has good oil resistance and air tightness, and has high heat sealing strength. The inner layer of general meat packaging uses cooking-grade CPP film.

Development History

Starting

According to industry insiders, in the world packaging material market, the production of cast film is showing a trend of continuous increase, and the cast film industry is entering a new round of investment boom. China's cast film production started in the 1980s. With the continuous development and growth in the past 20 years, it has now become a sunrise industry with considerable production capacity and production level in the packaging industry.

Polypropylene cast film (CPP) is a non-stretched, non-directional flat extruded film produced by melt casting and quenching. Compared with blown film, it is characterized by fast production speed, high output, good film transparency, glossiness, thickness uniformity, and excellent balance of all-directional properties. At the same time, because it is a flat extruded film, subsequent processes such as printing and lamination are extremely convenient, so it is widely used in the packaging of textiles, flowers, food, and daily necessities.

CPP production has two methods: single-layer cast and multi-layer co-extrusion cast. Single-layer film mainly requires the material to have good low-temperature heat sealing performance and flexibility. Multilayer co-extruded cast film can generally be divided into three layers: heat sealing layer, support layer, and corona layer. The material selection is wider than that of single-layer film. Materials that meet the requirements of each layer can be selected separately to give the film different functions and uses. Among them, the heat sealing layer group needs to be heat-sealed, requiring the material to have a low melting point, good hot melt, wide heat sealing temperature, and easy sealing; the support layer plays a supporting role on the film and increases the stiffness of the film; the corona layer needs to be printed or metallized, requiring a moderate surface tension, and strict restrictions on the addition of additives.

After printing and bagging, CPP can be used alone for the outer packaging of food, clothing, toilet paper, flowers, etc. In addition, due to its excellent transparency and low heat sealing temperature, it can also be used as a base film for various composite films, such as composite with PET film, BOPP film, etc., for packaging fast food products, tea, etc.; composite with barrier resins EVOH, PA, PVDC, etc. through adhesives to package foods containing oil or soup. Other important application areas include high-temperature and medium-temperature cooking films, vacuum aluminum-plated films, etc.

With the continuous increase in market demand, the products and materials used in cast film are also being updated. CPP's new product development is mainly focused on ultra-low temperature heat-sealing films, cold-resistant films, high-temperature cooking films, etc. Some films for special purposes are also being developed and applied, such as antistatic films, pesticide films, high-transparency and ultra-soft films, etc., which are gradually being introduced to the market and occupy new application areas.

Development

The development of CPP production scale and extrusion equipment has increased the amount of raw materials used and the range of options, which has led to the development of cast polypropylene special materials in a more refined, specialized and higher-performance direction. Various special materials with more refined division of labor continue to emerge, and each material plays its own role at different levels, giving the film new properties to adapt to different market needs. At the same time, the situation of producing high-performance raw materials specifically for special requirements of users is also emerging, which will bring greater profits to resin producers. At present, the hot spot of special material development is low-temperature heat-sealing materials, and industrially produced butene/propylene random copolymers are new products launched in the market. Compared with conventional ethylene/propylene random copolymer, this product has much higher heat sealing strength at the same heat sealing temperature of 100-112¡æ, and is less prone to frosting and fogging.

In recent years, with the strengthening of the consumer market, the application field of CPP has been continuously expanded, and its development has been accelerated. So far, there are more than 30 CPP manufacturers in the country, more than 40 production lines, and a total production capacity of about 150,000 to 170,000 tons/year. However, the dependence of special materials for cast film on imported materials is more serious than that of BOPP film and blown film. At present, the annual import volume accounts for about 60% of the total usage. The variety of special materials for CPP in China is relatively single. Only Shanghai Petrochemical Co., Ltd. and Beijing Yanshan Petrochemical Co., Ltd. produce them. They are generally used for the heat sealing layer of composite films. Resins for semi-cooking films, aluminum-plated films, and high-temperature cooking films are relatively scarce. At the same time, the phenomenon of product similarity among Chinese companies is relatively serious. The increase in equipment capacity of similar low- and medium-end products is higher than the increase in demand. The market is relatively weak, which has inhibited the expansion of film production and the rise in prices. In addition, in the past two years, the multi-layer co-extrusion blown film market is in a growth stage, and some varieties of CPP films are subject to competition and impact.

Now, CPP has become an indispensable product in the packaging industry. Manufacturers use new equipment and new materials to continuously expand production scale, improve production efficiency, increase product types, and expand new application areas. It can be expected that with the improvement of China's economic level and consumption level, its production development will surely expand further. At the same time, it should also be noted that cast film still has a situation of partial or full dependence on imports. How to accelerate its localization is also a major problem that the plastic processing and resin production industries need to solve as soon as possible.

Production process

Process characteristics

The CPP process generally adopts the T-die method, which has the following characteristics:

(1) The flat film method omits the film blowing stage of the tube film method, is easy to start, and has less waste;

(2) During the flat film method, the PP molecules are arranged in an orderly manner, which is conducive to improving the transparency, gloss and thickness uniformity of the film, and is suitable for high-end packaging;

(3) There is a special retention groove in the flat film, which can be integrated with the die gap and is easy to adjust.

Schematic diagram of CPP film production process

Key points of CPP film production process

The T-die is one of the key production equipment. The die design should make the material flow out evenly along the entire lip width, without dead corners in the flow channel inside the die, and make the material have a uniform temperature. Many factors including the rheological behavior of the material need to be considered. To use a precision machined die, the commonly used one is the decreasing manifold hanger die. The surface of the cooling roller should be finely processed, with a surface roughness of no more than 0.15 mm, and the rotation speed should be stable to avoid longitudinal thickness fluctuations. Use ¦Â-ray or infrared thickness gauge to monitor the film thickness to achieve a satisfactory thickness tolerance.

To produce qualified CPP film, not only good raw materials are required, but also good processing conditions must be mastered.

The biggest influence on film performance is temperature. As the resin temperature rises, the longitudinal (MD) tensile strength of the film increases, the transparency increases, and the haze gradually decreases, but the transverse (TD) tensile strength of the film decreases. The more suitable temperature is 230-250¡æ. The wind knife on the cooling roller forms a thin air layer between the film and the surface of the cooling roller, so that the film is evenly cooled, thereby maintaining high-speed production. The wind knife must be adjusted appropriately. Excessive air volume or improper angle may make the film thickness unstable or not stick to the roller, causing wrinkles or patterns that affect the appearance quality.

As the temperature of the cooling roller increases, the stiffness of the film increases and the haze increases.

If there are any additives from the raw materials precipitated on the surface of the cooling roller, the machine must be stopped for cleaning to avoid affecting the appearance quality of the film. CPP film is relatively soft, and the pressure and tension must be adjusted according to the film thickness, production speed and other factors when winding. Otherwise, ripples will be generated and affect the flatness. The tension selection should be based on the tensile strength of the product. Generally, the greater the winding tension, the less likely the product will have loose rolls and deviations after winding, but ripples are likely to appear at the beginning of winding, affecting the flatness of the roll. On the contrary, a small winding tension will have a good effect at the beginning, but the more you wind, the more likely the film will loosen and deviate. Therefore, the tension should be moderate and the tension should be kept constant.

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