The CPP film is a polypropylene (PP) film produced by a cast extrusion process. This kind of film is different from BOPP (bidirectional polypropylene) film and belongs to non-oriented film. It has the characteristics of good transparency, high gloss, good stiffness, good moisture resistance, excellent heat resistance and easy heat sealing. After printing and compounding. , bag making, widely used in food, medicine, household paper, clothing, daily necessities, industrial packaging.
CPP film has the characteristics of good transparency, high gloss, good stiffness, good moisture resistance, excellent heat resistance and easy heat sealing. It can be divided into general CPP film, aluminum plated CPP film and cooking grade CPP according to different purposes. Film and the like. CPP film has been printed and bagged, and is suitable for clothing, knitwear and flower bags, document and photo album films, food packaging and metallized films suitable for barrier packaging and decoration.
In addition, there is an increasing demand for food packaging, candy packaging (twisting film), pharmaceutical packaging (infusion bag), self-adhesive tape, business card holder, ring folder, and the like.
CPP is excellent in heat resistance. Since the PP softening point is about 140 ° C, such films can be used in the fields of hot filling, retort pouch, aseptic packaging and the like. Plus acid, alkali and grease resistance, making it the material of choice for bread packaging or laminates. It is safe to contact with food, has excellent demonstration performance, does not affect the flavor of the food, and can choose different grades of resin to obtain the desired characteristics.
Key points of CPP cast film production process
The T-type head is one of the key equipments for production. The design of the machine head should make the material flow out evenly along the entire lip width. There is no dead angle in the internal flow passage of the machine head, and the material mold has a uniform temperature. Many factors including behavior. To use precision machining heads, it is common to reduce the manifold hanger type head. The surface of the chill roll shall be finished, the surface roughness shall be no more than 0.15mm, the rotation speed shall be stable, and the dynamic balance performance shall be good to avoid longitudinal thickness fluctuation. Film thickness is monitored using a beta or infrared thickness gauge to achieve satisfactory thickness tolerances. To produce a qualified cast film, it is necessary not only to adjust the process on the raw material, but also to master the processing conditions.
The most important influence on the film properties is the temperature. As the temperature of the resin increases, 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. A suitable temperature is 230 to 250 °C. The chiller upper wind knife forms a thin layer of air on the surface of the film and the chill roll to evenly cool the film, thus maintaining high speed production. The adjustment of the air knife must be appropriate. If the air volume is too large or the angle is improper, the thickness of the film may be unstable or not applied to the roller, causing wrinkles or patterns to affect the appearance quality. As the temperature of the chill roll increases, the stiffness of the film increases and the haze increases.
If the internal additive of the raw material is deposited on the surface of the cooling roller, it must be shut down to avoid affecting the appearance quality of the film. The cast film is relatively soft, and the pressure and tension must be adjusted according to the thickness of the film, the production speed, and the like when winding. Otherwise, ripples will affect the flatness. The tension selection depends on the tensile strength of the product. Generally, the larger the winding tension, the less the reeling and the deviation of the reel after the coiling, but the corrugation is easy to occur when the winding is started, which affects the flatness of the winding. On the contrary, the take-up tension is small, and the starting effect is good, but the more the roll is, the more likely the film is loose and the deviation occurs. Therefore, the tension should be moderate and the tension should be constant.
Process characteristics of multi-layer co-extruded cast film
In order to improve film performance, reduce cost, and meet users’ various uses and high performance requirements, multi-layer composite film has developed rapidly, especially in developed countries with relatively high living standards, environmental protection, and prolonged food shelf life and quality. Multi-layer copolymerized cast film is also a kind of multi-layer film, which has changed the problems and disadvantages of CPP film products with single performance and unable to meet various market requirements.
1. General-purpose type: Multi-layer copolymerized cast film can be used according to different uses and designs, such as bread packaging, clothing (especially underwear, pants) packaging, fruit packaging, etc. for automatic packaging machines, or for BOPP after printing. The membrane is compounded into a BOPP/CPP two-layer film for packaging of clothes and dry foods (such as instant noodle bags, bowl lids, etc.), and the general-purpose structure is copolymerized PP/homopoly PP/copolymerized PP or homopolymerized.
2. Metallization type: The surface of the product is required to have strong adhesion strength to the vapor-deposited metal (such as aluminum), and still maintain good dimensional stability and rigidity after evaporation, and the other surface has a lower heat-sealing temperature and The higher heat seal strength, the metallized structure is also copolymerized PP / homopolymer PP / copolymer PP.
3. Cooking type: Two-layer copolymerized CPP for cooking, which can withstand retort sterilization at 120 ° C and 15 MPa pressure. It not only maintains the shape and flavor of the internal food, but also does not crack, peel or bond the film, and has excellent dimensional stability. It is often compounded with a nylon film or a polyester film. The package contains soup foods and meatballs and dumplings. For food or pre-food processing of frozen food, the three-layer PP film structure of the cooking type is copolymerized PP/copolymerized PP/copolymerized PP.
4, high-temperature cooking type: packaging three-layer copolymerized CPP film of roast chicken, burned pork ribs and jam, beverages at 121 ~ 135 ° C high temperature sterilization, wherein the copolymer PP requirements are better than the general cooking type PP copolymer. In addition to the three-layer film, there is a cast barrier five-layer package with the following structure: PP / adhesive / PA / adhesive / copolymerized PE; PP / adhesive / PA / adhesive / EVA; PP /Adhesive/EVOH/Adhesive/PE; PP/Adhesive/EVOH/Adhesive/EVA; PP/Adhesive/EVOH/Adhesive/PP.
Analysis of CPP film structure and raw material use
The CPP cast film is similar to other types of packaging film structures. At present, the domestic factory produces three-layer co-extruded cast film, and the five-layer structure is less produced. The three-layer structure is: heat seal layer, support layer, and corona layer. Of course, the raw materials required for the production process of the three-layer structure of the cooking film, the aluminized film and the ordinary composite film are certainly different. Xiaobian makes a simple summary analysis:
The main upstream raw material of CPP is PP, but the raw materials used in different membranes are different. PP homopolymer, binary copolymer and ternary copolymer are used together in film production and cooperate with each other. Because the heat sealing layer is to be heat-sealed, the melting point of the material is required to be low, the hot melt is good, the heat sealing temperature is wide, and the sealing is easy; the supporting layer plays a supporting role on the film, increasing the stiffness of the film and reducing Cost; the corona layer should be printed or metallized, and a moderate surface tension is required. The addition of additives to the resin should be strictly limited, such as the corona layer of the aluminized substrate, because the surface of the product is required to be metal. Aluminum has a strong adhesion strength, and it maintains good dimensional stability and rigidity after evaporation, so it is produced with a dedicated aluminized corona.
The future development direction of CPP film
In 2004, the annual CPP capacity of the country was nearly 420,000 tons. In 2016, the production capacity was 1.09 million tons, and the total market demand was about 660,000 tons.
(1) high transparency, good opening performance, high smoothness (COF <0.2 after curing), low heat sealing temperature, etc. to meet the requirements of high-speed bag making and packaging;
(2) Maintain the surface wetting tension of the film for 6 months up to 38 dynes to extend the shelf life of the product.
The important performance indicators of CPP cooking film are heat seal strength, impact strength, composite strength, etc., especially the above indicators after cooking. In addition, the film is odorless or contains very low odor is its development direction.
Aluminized film substrate
(1) Improve the stiffness to improve the quality of the aluminum plating of the product;
(2) improving the adhesion of the aluminum layer after plating;
(3) improving the temperature resistance of the aluminum layer after plating;
(4) Reduce the heat sealing temperature to meet the requirements of high speed automatic packaging;
(5) Improve the opening and smoothness to meet the requirements of high-speed bag making and packaging;
(6) Maintain the surface wetting tension of the aluminum layer to extend the shelf life of the product.
Food packaging bag
CPP film is used as the inner layer special film for food packaging bags. Nowadays, the performance of domestic bag making machines has been relatively stable, and the degree of automation has been continuously improved. Therefore, the demand for CPP film is increasing, and the quality requirements for film are increasing. high. In particular, in order to expand the range of influence of downstream demand, films with special uses are also under development. The excellent heat sealing effect of CPP film also enables us to foresee the broader market prospects of the CPP market in the future. It is only at the same time of rational production, the realization of new packaging technology development, thereby enhancing the overall competitiveness of the industry, and constantly moving towards high-end.