Application of the hottest shrink plastic film in

2022-08-23
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Application of heat shrinkable plastic film in packaging

the requirement of heat shrinkable plastic film is to be stable at room temperature, shrink when heated (above the glass transition temperature), and it is ideal to have more than 50% heat shrinkage in one direction. The characteristics of heat shrinkable plastic film packaging are: body fitting and transparent, reflecting the image of goods; Tight packing, good dispersion resistance; Rain proof, moisture-proof and mildew proof; It has no recoverability and certain anti-counterfeiting function. Heat shrinkable plastic films are often used in the packaging of convenience food, beverages, electronic appliances, metal products, etc., especially shrink labels

in addition to being used as shrink labels, heat shrinkable plastic films have also been used in the outer packaging of daily-use goods in recent years. Because it can not only protect the packaging items from impact, rain, moisture and rust, but also make the products win users with beautifully printed outer packaging. At the same time, it can well show the good image of the manufacturer. At present, more and more packaging manufacturers use printing shrink film to replace the traditional transparent film. Pingmei Guoneng 10gwh high specific energy lithium ion battery project has a total investment of 5billion yuan, because the printing shrinkage film can improve the appearance grade of products, is conducive to the advertising of products, and can make the trademark brand have a deep impression on consumers

heat shrinkable plastic films are generally processed from amorphous plastics, such as polystyrene, polyvinyl chloride, PVDC, etc. Polystyrene (PS) shrink film has low strength and is not resistant to impact, so it is rarely used; Polyvinyl chloride (PVC) is not conducive to recycling and does not meet the requirements of environmental protection. In foreign countries, especially in Europe, polyvinyl chloride (PVC) plastic film has been prohibited from being used in the field of packaging, especially in food packaging

polyester (PET) heat shrinkable film is a new type of heat shrinkable packaging material. Due to its easy recycling, non-toxic, tasteless, good mechanical properties, especially in line with environmental protection, polyester (PET) has become an ideal substitute to replace polyvinyl chloride (PVC) heat shrinkable film in developed countries

copolymerization modification of heat shrinkable polyester (PET) film

polyester (PET) film is a crystalline material, and the heat shrinkage rate of ordinary polyester film can only be less than 30% after special processing. In order to obtain polyester film with high thermal shrinkage, it must be modified. In other words, in order to prepare polyester film with high thermal shrinkage, polyethylene terephthalate (PET), a common polyester, needs to be copolymerized. The highest thermal shrinkage of the copolymerized PET film can be as high as 70%

ordinary polyester is generally prepared by esterification and polycondensation of terephthalic acid (PTA) and ethylene glycol (eg), which belongs to crystalline polymer (strictly speaking, it is a polymer with coexistence of crystalline region and amorphous region). The so-called copolymerization modification is to introduce the third or even the fourth component to participate in the copolymerization in addition to the two main components of terephthalic acid (PTA) and ethylene glycol (eg), in order to generate an asymmetric molecular structure and form an amorphous pet copolymer

the introduced third or even fourth monomer can be dibasic acid or diol. Among them, dicarboxylic acids include isophthalic acid, malonic acid, succinic acid, adipic acid, sebacic acid, etc; Diols include neopentyl glycol, propylene glycol, diethylene glycol, 1,4 cyclohexane dimethyl alcohol, etc. For example, the pet copolymer prepared by copolymerization modification with the friction acid between the binary carboxyl reducing tool and the workpiece is called APET; Pet copolymer prepared by copolymerization modification with glycol is called PETG

among the third monomers introduced above, the most commonly used dicarboxylic acid is isophthalic acid (IPA). The addition of IPA can change the symmetrical tight structure of polyester and destroy the regularity of macromolecular chain, so as to reduce the force between macromolecules and make the molecular structure of polyester more flexible. At the same time, due to the introduction of IPA, it is difficult for polyester to nucleate and crystallize. With the increase of the amount of IPA, APET copolymer transits from partial crystallization to amorphous polymer. Because the crystallization ability of this modified polyester APET decreases and the amorphous region becomes larger, it can be used to manufacture high shrinkage films. The recommended dosage of IPA is about 20%

the third monomer introduced can also be diol. The most commonly used diol is 1,4 cyclohexanedimethanol (CHDM). In the process of polyester copolymerization, the addition of CHDM will have a great impact on the change of TG, TM and crystallization rate of polyester. With the increase of CHDM content, the melting point of copolyester PETG decreased, the glass transition temperature increased, and the copolymer became amorphous. However, the addition amount of 1.4 cyclohexanedimethanol (CHDM) must be controlled within an appropriate range. Generally, the recommended addition amount of CHDM is 30 ~ 40%. This kind of PETG modified with diol can not only be used to prepare high shrinkage films, but also can be used to produce heat sealing films, high transparent films, etc., which are widely used

shrinkage mechanism of heat shrinkable film

taking the production process of BOPET film as an example, this movement will cause the test piece to zigzag around the selected circular axis. The production process is to first dry the pet copolymer resin, then add it to the extruder for melting and extrusion, and then cast the sheet through the die/cold drum, and then heat the cast sheet to a suitable temperature range above the glass transition temperature and below the melting temperature, and under the action of external force, For unidirectional or bidirectional stretching of a certain number of times, it is usually required to carry out transverse stretching of 3.5 ~ 4.0 times. The pet macromolecular chain is oriented along the direction of external force by stretching, and then cooled and shaped, so that the oriented PET molecular structure is "frozen" and shaped. The high elastic deformation under the action of external force has the "memory effect" of thermal contraction. When the film with "memory effect" is reheated above the tensile temperature, the frozen macromolecular orientation structure begins to relax, which is macroscopically manifested as the shrinkage of PET film. It is worth mentioning that the thermal shrinkage of this PET film is mainly contributed by the oriented amorphous part. This is why the heat shrinkage of amorphous PET film (APET or PETG) modified by copolymerization is much larger than that of ordinary crystalline PET film. Therefore, by increasing the oriented amorphous region in the film, the thermal shrinkage of the film can be greatly improved. As mentioned above, the thermal shrinkage of ordinary polyester film is only less than 30%, while the thermal shrinkage of copolymerized polyester can be as high as more than 70%

production process of heat shrinkable polyester film

the production process of heat shrinkable polyester film is: amorphous polyester chip (APET or PETG) - vacuum drying - melt extrusion or exhaust twin screw extrusion - quench casting - unidirectional stretching - cooling and shaping - winding, slitting - Inspection and packaging

the production process conditions and effects of heat shrinkable polyester film are briefly introduced as follows:

1. Drying treatment: because polyester macromolecular chain contains ester group, it has hygroscopic tendency. Under the condition of heating, even if there is a small amount of water, it is very easy to hydrolyze. As a result, a large number of bubbles will be produced in the process of molding, affecting normal production; At the same time, due to hydrolysis and degradation, the molecular weight decreased and the quality of pet deteriorated. Therefore, in addition to these advantages, drying treatment must be carried out before melt extrusion processing. It is recommended to use vacuum drum drying, with the drying temperature of 70 ~ 75 ℃ and the drying time of 6h

2. Melt extrusion: the dried copolymer PET resin can be added to the single screw extruder for melt extrusion. The temperature of each section is set as 180 ℃ - 240 ℃ - 260 ℃ - 270 ℃ - 275 ℃, and the melt temperature is about 270 ℃

if the exhaust twin-screw extruder is used for melt extrusion, the vacuum drum drying system can be omitted. Because the exhaust twin-screw extruder is generally equipped with two exhaust ports, which are respectively connected with two vacuum pumping systems. The water contained in PET resin and the low molecular weight produced in the process of melt extrusion can be pumped away by vacuum pump, which achieves the same effect, and can greatly save investment and operation cost

3. Cast sheet: the melt extruded by melt enters the hanger die head through the melt metering pump, filter and melt pipe, and then is cast from the die lip to the cooling drum to form a cast sheet. The cooling water temperature of the cooling drum is controlled at about 30 ℃

4. Unidirectional stretching: high heat shrinkable polyester films often require unidirectional shrinkage, especially transverse heat shrinkage. The preheating temperature of transverse stretching is 90 ~ 100 ℃, the stretching temperature is 105 ~ 110 ℃, and the stretching ratio is 3 ~ 3.5 times. The chain clamp must be strengthened with air cooling, and the clamp temperature should be controlled below 110 ℃ to prevent the copolymer resin from sticking to the clamp. The stretched PET film is immediately cooled without heat setting treatment

5. Winding and slitting: the PET film after unidirectional stretching is wound by thickness measurement and traction, and finally cut, inspected and packaged according to the specifications required by the user, which is the finished product of heat shrinkable film

with the continuous development of China's plastic packaging industry and the continuous improvement of people's living standards, fruit juice, soft drinks and other beverages are mostly packaged in PET plastic bottles with excellent comprehensive performance, good barrier performance, non-toxic and tasteless, and their consumption is very considerable. According to statistics, at present, the consumption of PET beverage bottles in China is at least 250000 tons, and the consumption of pet heat shrinkable label film can reach 20000 ~ 30000 tons. In addition, since China joined the WTO, foreign trade has been expanding. In order to improve the image and value of export commodities, it is also necessary to use high-grade packaging materials such as polyester heat shrinkable film. In short, the application prospect of heat shrinkable PET film is very promising

source: Chinese polymers

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