It is a very important flexible packaging material with a wide range of applications. BOPP film is colorless, odorless, odorless, non-toxic, and has high tensile strength, impact strength, rigidity, toughness and good transparency. The surface energy of BOPP film is low, and corona treatment is required before gluing or printing. However, after corona treatment, BOPP film has good printing adaptability and can be overprinted to obtain exquisite appearance, so it is often used as the surface layer material of composite film. BOPP film also has shortcomings, such as easy accumulation of static electricity and no heat sealability. On a high-speed production line, BOPP film is prone to static electricity, so it is necessary to install static electricity remover. In order to obtain a heat-sealable BOPP film, heat-sealable resin glue, such as PVDC latex, EVA latex, etc., can be coated on the surface of the BOPP film after corona treatment, solvent glue, or extrusion coating or The method of co-extrusion and compounding produces heat-sealable BOPP film. The film is widely used in the packaging of bread, clothes, shoes and socks, as well as the cover packaging of cigarettes and books. The induced tear strength of BOPP film is improved after stretching, but the secondary tear strength is very low. Therefore, no cuts should be left on both ends of the BOPP film, otherwise the BOPP film will be easily torn during printing and lamination. After BOPP is coated with self-adhesive, sealing tape can be produced, which is a market with a large amount of BOPP.
BOPP film can be produced by tube film method or flat film method. The properties of BOPP films obtained by different processing methods are different. The BOPP film produced by the flat film method has a large stretch ratio (up to 8-10), so the strength is higher than that of the tube film method, and the uniformity of the film thickness is also better.
In order to obtain a better overall performance, it is usually produced by a multi-layer composite method during use. BOPP can be compounded with many different materials to meet the needs of special applications. For example, BOPP can be compounded with LDPE (CPP), PE, PT, PO, PVA, etc. to obtain high gas barrier, moisture barrier, transparency, high and low temperature resistance, cooking resistance and oil resistance. Different composite films can be applied to oily food.
It has the characteristics of good transparency, high gloss, good stiffness, good moisture resistance, excellent heat resistance, and easy heat sealing. CPP film is printed and bagged, and is suitable for: clothing, knitwear and flower packaging bags; document and photo album film; food packaging; and metallized film for barrier packaging and decoration. Potential uses also include: food packaging, candy packaging (twisted film), pharmaceutical packaging (infusion bag), replacing PVC in photo albums, folders and documents, synthetic paper, self-adhesive tape, business card holders, ring folders And standing bag composite material.
CPP has excellent heat resistance. Since the softening point of PP is about 140°C, this type of film can be used in hot filling, retort bags, aseptic packaging and other fields. Coupled with excellent resistance to acid, alkali and grease, it becomes the first choice for bread product packaging or laminate materials. It is safe in contact with food, has excellent performance, does not affect the flavor of the food inside, and can choose different grades of resin to obtain the required characteristics.
As mentioned above, the production process of cast film generally adopts the T-die method. The characteristics of this method are:
(1) The casting method eliminates the film blowing stage of the tube film method, which is easy to drive and has less waste;
(2) In the production by casting method, the chemical molecules are arranged in an orderly manner, so it is beneficial to improve the transparency, gloss and thickness uniformity of the film, and is suitable for advanced packaging;
(3) The casting part adopts electric up and down swing and back and forth movement structure, which is easy to operate;
(4) The corona part is air-cooled and water-cooled, and the product is not easy to deform.
The extruder first melts the raw material resin, and the molten resin is cast on the cooling roller with a smooth surface through the die to quickly cool into a film. After thickness measurement, traction, corona treatment, and flattening, cut off the thicker edge material, unfold and wind it into a film roll again.
The main points of CPP cast film production process
The T-type head is one of the key production equipment. The design of the head should make the material flow out evenly along the entire width of the lip. There is no dead angle in the flow channel inside the head, and the material mold has a uniform temperature. It needs to consider the material rheology. Many factors including behavior. To use precision machining heads, the gradual reduction manifold hanger heads are commonly used. The surface of the cooling roller should be finished, the surface roughness should not be greater than 0.15mm, the speed should be stable, and the dynamic balance performance should be good to avoid longitudinal thickness fluctuations. Use β-ray or infrared thickness gauge to monitor film thickness to achieve satisfactory thickness tolerance. To produce a qualified cast film, not only must the raw material be adjusted, but also the processing conditions must be mastered.
The biggest influence on film performance is 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. The suitable temperature is 230～250℃. The air knife on the cooling roll makes the film and the surface of the cooling roll form a thin air layer, so that the film is uniformly cooled, thereby maintaining high-speed production. The adjustment of the air knife must be appropriate. Excessive air volume or improper angle may make the thickness of the film unstable or not stick to the roll, causing wrinkles or patterns to affect the appearance quality. The temperature of the cooling roll increases, the stiffness of the film increases, and the haze increases.
If the internal additives of the raw material precipitate on the surface of the cooling roller, the machine must be shut down for cleaning to avoid affecting the appearance of the film. The cast film is relatively soft, and the pressure and tension must be adjusted according to factors such as film thickness and production speed during winding. Otherwise, it will produce ripples and affect the flatness. The choice of tension depends on the tensile strength of the product. Generally, the greater the winding tension, the product after winding is less likely to experience roll slack and deviation, but it is prone to ripples at the beginning of the winding, which affects the smoothness of the roll. On the contrary, the winding tension is small and the initial effect is good, but the more the winding is, the more the film is slack and the deviation occurs. Therefore, the tension should be moderate and the tension should be kept constant.
Process characteristics of multi-layer co-extruded cast film:
In order to improve film performance, reduce costs, and meet user requirements for multiple uses and high performance, multilayer composite films have developed rapidly, especially in developed countries where living standards are relatively high, environmental protection is emphasized, and food shelf life and quality are required to be extended. Multi-layer copolymerized cast film is also one of the multi-layer films, which has changed the problems and drawbacks of CPP film products that have single performance and cannot meet the requirements of the market.
1. Universal type: Multi-layer copolymerized cast film can be used for different purposes and designs, such as bread packaging, clothing (especially underwear, pants) packaging, fruit packaging, etc. on automatic packaging machines, or used with BOPP after printing The film is compounded into a BOPP/CPP two-layer film, which is used for packaging of clothing and dry food (such as fast food noodle bags, bowl covers, etc.). The general-purpose structure is copolymer PP/homo PP/co PP or homopolymer.
2. Metallized type: The surface of the product is required to have strong adhesion to the vapor-deposited metal (such as aluminum), and it can still maintain good dimensional stability and rigidity after vapor deposition. The other surface has a lower heat sealing temperature and Higher heat sealing strength, metalized structure is also PP copolymer/homopolymer PP/PP copolymer.
3. Cooking type: Two-layer copolymerized CPP used for cooking, which can withstand cooking and sterilization at 120℃ and 15MPa pressure. It not only maintains the shape and flavor of the internal food, but the film will not crack, peel or bond, and has excellent dimensional stability. It is often compounded with nylon film or polyester film, and is packaged with soup foods, meatballs, and dumplings. For food or processed frozen food before eating, the cooking type three-layer PP film structure is co-PP/co-PP/co-PP.
4. High-temperature cooking type: packaging roasted chicken, roasted ribs and jam, beverages need to be 121-135 ℃ high temperature sterilization three-layer copolymer CPP film, of which copolymer PP requires better performance than the general cooking type copolymer PP. In addition to the three-layer film, there are also five-layer packaging with cast barrier properties. Its structure is: PP/adhesive/PA/adhesive/copolymer PE; PP/adhesive/PA/adhesive/EVA; PP /Adhesive/EVOH/Adhesive/PE; PP/Adhesive/EVOH/Adhesive/EVA; PP/Adhesive/EVOH/Adhesive/PP.
OPP film is a kind of polypropylene film. Because the production process is multi-layer extrusion, it is called co-extrusion oriented polypropylene film (Oriented Polypropylene Film). If there is a process of biaxial stretching in the processing, it is called Biaxially Oriented Polypropylene Film. Compared with the co-extrusion process, the other is called Casting Polypropylene Film, which is relatively thick.
The current applications in packaging include self-adhesive labels, in-mold labels, winding labels and wet glue point labeling. The latter is mainly used in beverage and alcohol labeling abroad. In the past few years, it has gradually been introduced into the Chinese market from Europe, and its application fields include food, chemical, and daily chemical products. In addition, OPP is applied to shrink film like PE and PVC film. The advantages are high transparency, good gloss, high shrinkage and shrinkage stress, but poor heat sealing performance and heat sealing strength, high shrink temperature, and narrow range.
For the relatively saturated and homogeneous daily chemical products in the market, appearance is everything. The first impression determines the purchase behavior of consumers. Shampoo, shower gel, detergent and other products are used in warm and humid bathrooms and kitchens. Labels are required to withstand moisture without falling off. Their squeeze resistance must match the bottle body. At the same time, the transparent bottle body is suitable for adhesives and The transparency of the label surface material puts forward stringent requirements. Compared with paper labels, OPP labels have the advantages of transparency, high strength, moisture resistance, and resistance to falling off. Although the cost is increased, it can get a good label display and use effect. With the development of domestic printing technology and coating technology, the production of self-adhesive film labels and printed film labels is no longer a problem. It is foreseeable that the use of domestic OPP labels will continue to increase.
Since the label itself is made of PP, it can be well combined with the surface of PP/PE containers. Practice has proved that OPP film is currently the best material for in-mold labels. It has been widely used in the European food and daily chemical fields and has gradually spread to In China, more and more users have begun to pay attention to or adopt the in-mold labeling process.