Heat shrinkable film is a kind of thermoplastic film which is oriented in the longitudinal and transverse direction of the film in the process of production and contracted by heat in the process of use. The thermal shrinkage of the film was applied as early as 1936. Initially, it was mainly used to shrink and pack perishable food. Nowadays, thermal shrinkage technology has been developed to pack almost all kinds of commodities with plastic shrinkage film, which has a wide range of applications in food and beverage, electronic products, automotive products, daily chemicals, stationery, medicine and other fields.
The production process of heat shrinkable film is divided into two methods: split casting method and blown film method. Casting method: Take the production process of BOPET thermal shrinkage label as an example, its production process is first drying the PET copolymer resin, then melting extrusion in the extruder, through the die/cold drum casting piece, then heating the casting piece to a suitable temperature range above the glass transition temperature and below the melting temperature, and under the action of external force, unidirectional or bi-directional pulling of a certain number of times. Extension, usually requires 3.5 to 4.0 times the transverse stretching.
The blowing method of heat shrinkable film is generally divided into single bubble method, double bubble method and three bubble method. The common blown film production process belongs to the single-bubble process. Because the blown and stretched ratio of the film in the longitudinal and horizontal direction can not be accurately controlled, and the blown ratio is generally about 2-3, the thermal shrinkage rate of the film is not high, so the products produced by one-bubble process are mainly used in the packaging of product logistics, such as beer beverage, daily electronic products and so on. The double bubble method is mainly used in the production of POF films. Because the film is blown twice, the tensile ratio of the film can be controlled vertically and horizontally, and the tensile ratio is high. The shrinkage of the film can reach more than 70% by controlling the technological formula. The main application fields of POF film are in some small package products. With the application of heat shrinkable film in food, especially in cold meat, the production technology of three-bubble heat shrinkable film came into being. Fresh meat products require high thermal shrinkage stability for film packaging, low temperature for thermal shrinkage and good barrier performance, so ordinary double-bubble method can not produce products that meet the requirements. The production technology, technical difficulties and market situation of three-bubble heat shrinkable bags are described emphatically.
Production technology and principle of three-bubble heat shrinkable bag. The three-bubble process is generally as follows:
Material additives--mixing uniformity--melt extrusion--multi-layer die extrusion--membrane embryo(one bubble) -- cooling setting--water bath heating--hot air preheating--second blowing(two bubbles) -- cooling setting(three bubbles) -- rotary winding--cross-linking--printing--bag-making--packaging into storage
The key control points are as follows:
1) One-bubble extrusion: The embryo tube was extruded by flat extrusion and blow-down method, at which time the polymer was in a melting state.
2) Bubble inflation: After co-extrusion with multi-layer dies, the tube is cooled by a water jacket in a vacuum tube immediately, then heated to a hot water bath or heater, and stretched longitudinally above the glass transition temperature. Then the tube is blown horizontally by compressed air. The ratio of blow to stretch is about 3.5:1.
There are two important concepts here: traction ratio and inflation ratio.
Traction ratio: refers to the ratio between the traction speed of the film and the extrusion speed of the tube embryo. The traction ratio is the longitudinal drawing ratio, which makes the film have directional effect in the traction direction. With the increase of traction ratio, the longitudinal tensile strength will increase, and the thickness of the film will become thinner. However, if the traction ratio is too large, the thickness of the film will be difficult to control, and may even break the film, resulting in film breakage.
Blow ratio: refers to the ratio between the diameter of the vesicle after blown and the diameter of the unblown tube. The blown ratio is the transverse expansion multiple of the film, which is actually the transverse stretching of the film. The stretching will have a certain degree of orientation effect on the plastic molecules, and the blown ratio will increase, so that the transverse tensile strength of the film will be improved. However, the blowing ratio can not be too large or too small, too large can easily cause unstable membrane bubbles, and the membrane bubbles are prone to wrinkle; too small, the shrinkage rate of finished products is not enough. Therefore, the blowing ratio should be properly matched with the traction ratio.
The physical principle of the two-bubble process is that when the polymer is in a high elastic state, its tensile orientation is oriented, and then the polymer is quenched below the glass transition temperature, the molecular orientation is frozen. When the product is heated in the packaging process, the stress relaxation is caused by the thermal movement of the molecule, and the polymer is separated and restored to its original state, resulting in shrinkage.
3) Three-bubble setting: The film after secondary blowing needs to be finalized and then rolled up. Shrinkage occurs even when the film is stored at room temperature, so the setting treatment is very important.
4) Heat shrinkable films often need to be crosslinked. Generally, electron radiation crosslinking is used, which can be divided into online crosslinking and offline crosslinking. The properties of the cross-linked film can be greatly improved. The mechanical properties, environmental stress cracking resistance, chemical corrosion resistance, creep resistance and electrical properties of the film can be significantly improved, and the temperature resistance level can be significantly improved. This process is one of the key process control points in the production of heat-shrinkable bags.
5) The key difference between the three-bubble process and the ordinary process is that the polymer is stretched vertically and horizontally above the melting point when the one-bubble process blows up, while the three-bubble process controls the tube temperature above the glass transition temperature and below the melting point, and then stretches and blows up.
Selection of raw materials for three-bubble heat shrinkable film. Because the three-bubble shrinkable film is produced by bottom-blown water-cooled process, the raw material is usually blown film level. In the process of heat shrinkage after packaging, hot water, air shower or infrared heating are usually used for 3-5 seconds. In order to reduce the influence of temperature on the surface of cold meat, the thermal shrinkage temperature of the film is required to be low. Therefore, polyethylene, metallocene linear polyethylene and EVA with lower density should be selected to achieve more than 40% thermal shrinkage at 90 C or even lower temperature. At the same time, we should consider that the film has good body-fitted properties, large shrinkage rate and low shrinkage force, so as to avoid the extrusion of blood and water.
According to the types and shelf life requirements of packaging contents, they are generally divided into heat-insulating shrinkage bags, high heat-insulating shrinkage bags and non-heat-insulating shrinkage bags according to oxygen barrier performance. The barrier material is usually PVDC, EVOH and copolymer nylon. Therefore, the product structure can be divided into four basic categories as follows:
1) Non-obstructive structure: (MPE+EVA)/ LLDPE/ (MPE+EVA)
2) Barrier structure: PA/TI/(MPE+EVA)
3) One of the high barrier structures: PA/EVOH/TI/(MPE+EVA)
4) High barrier structure II: (MPE + EVA) / EVA / PVDC / EVA / (MPE + EVA)
When EVOH and nylon are used as barrier layers, they are usually designed as asymmetric structures, and nylon is designed as outer layers, so that the melting points of inner and outer layers are quite different, which is convenient for bag making. The temperature difference between the inner and outer layers of the symmetrical structure product is small, so it is necessary to cross-link the film to improve the temperature resistance of the film, so it is easy to make bags.
Here we should focus on the cross-linking process. Not all polymer materials can be cross-linked by electron radiation, and the cross-linking and cracking are carried out simultaneously, but there is always one aspect that is the main, so there are two types of polymer: electron radiation cross-linking and cracking. For example, polyethylene and EVA belong to radiation crosslinking substances. The yellowing of PVDC after radiation crosslinking will affect the display effect of products, which is a difficult problem for the industry at present. Some companies solve this problem very well. It is said that online cross-linking is used, while others use offline cross-linking, i.e. cross-linking after coil production is completed. Off-line cross-linking will cause another problem, that is, when thermal shrinkable film is crosslinked by electronic radiation, it will produce stimulating gas - ozone. Ozone can not be discharged because it exists in the bag. Although it meets the requirements of food safety, it is sometimes rejected by customers.
Market prospects of three-bubble heat shrinkable bags. According to relevant reports, meat packaging bags have become the fastest growing soft packaging market at the rate of 20% per year. More than 30% of the world's more than 10 billion fresh meat packages are packaged in bags. It can be predicted that heat shrinkable packaging bags will become a fast-growing soft packaging market. The main advantages of heat shrinkable bags are as follows:
1. Shrinkage helps to reduce redundant materials outside packaging, make packaging more compact, exhibit better effect, and enhance the attractiveness of meat sales;
2. The shrinkage bag has no wrinkles after heat shrinkage, and the inner material is very close to the body, which eliminates the capillary water absorption phenomenon caused by the shrinkage bag, and at the same time, the low shrinkage force reduces the leakage of blood and water to the greatest extent.
3. The barrier layer of heat shrinkable bags effectively prevents oxygen permeation and prolongs the shelf life of meat products.
4. Heat shrinkable bags can effectively prevent the loss of moisture, maintain the original taste of products, and save costs for enterprises.
5. Heat shrinkable packaging products avoid contact with the outside world, avoid secondary pollution, and are safer to eat.
At present, PVDC heat shrinkable bags are mainly used in the packaging of cold meat, and some of them are used in the processing of seafood which needs high barrier. According to the history of cold meat packaging in the United States, we can see the development direction of cold meat packaging in China in the future. After World War II, because of the development of supermarkets, film-packaged cold meat began to appear. It was mainly used in the back stores of supermarkets to prevent the loss of moisture in the storage process of fresh meat after slicing. However, there were still some problems in this kind of packaging, such as the change of meat color, the leakage of packaging and the short shelf life. With the invention and application of PVDC materials, thermal shrinkage packaging materials gradually evolved into barrier shrinkage bags in the next 20 years (1950-1970), which prolonged the shelf life of meat products and enabled manufacturers to adjust production and inventory more flexibly and efficiently. Major fresh-keeping beef suppliers in the United States were pioneers in the use of vacuum shrinkage bags and central packaging plants, which now account for three-quarters of total cattle slaughter, 85% of which are packaged in vacuum shrinkage. Vacuum shrinkage packaging of beef in the United States has accounted for 90% of its total beef production.
Ordinary non-barrier heat-shrinkable bags are mainly used in the packaging of refrigerated beef and mutton products. Because of their good fitness and the ability to effectively prevent the dry consumption of products in cold storage below minus 18 degrees, this product has been widely used in northern areas such as Inner Mongolia. Nylon and EVOH heat shrinkable bags are mainly used in the field of ham and other dry meat products. China's ham consumption is not high, but in Europe, North America and Russia, the use of these packaging is very large.
Heat shrinkable bags are being used more and more widely. A good example is halogen products. China's brine products are basically family workshop-style processing. With the attention of the country and the people to food safety, some food processing factories began to produce low-temperature slow-boiled meat products, and there is a very good market reaction. Low-temperature slow cooking Sous-vide comes from French, which means "under vacuum" - "vacuum cooking". It is based on scientific research to find out the temperature range of protein cells heated and blasted for each food ingredient, so as to calculate the blasting temperature and how long it takes to cook the food best. The best way to "cook" is to put it in a plastic bag that can resist high temperature, and then put it in a vacuum state after exhausting the air, and soak it in a constant temperature hot water for a specified time, so that the food can be slowly cooked in hot water, and the food can release the most sodium glutamate in the most suitable temperature, which is the best state of original taste, texture and nutrition. At present, many food processing factories in China have used heat shrinkable bags to produce low-temperature slow-cooking food. We have reason to believe that the market prospect of heat shrinkable bags will be more and more extensive.
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