Manufacturing technology of the hottest heat seala

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Manufacturing process of heat sealable BOPET film

biaxially oriented pet (BOPET) film does not have heat sealing performance. In order to be used in heat sealing occasions, it usually needs to carry out cumbersome and wasteful compounding process with other resins, which greatly limits its application field. However, this problem can be solved by using copolymerized PET resin and three-layer coextrusion

BOPET film has excellent comprehensive properties, such as good transparency, low haze, high gloss, high strength, scratch resistance, folding resistance, excellent barrier, etc. it is widely used in printing, aluminum plating, composite packaging, card protection, metallization, insulating materials, photosensitive materials, tape base and other fields. However, because ordinary PET resin is a crystalline polymer, it has a greater tendency to crystallize after hot stretching. If it is heat sealed, it will produce serious shrinkage deformation. Therefore, BOPET film produced with ordinary pet (hereinafter referred to as ordinary BOPET film) does not have heat sealing performance. In order to solve this problem, the method of compounding ordinary BOPET film with PE film or CPP film is usually adopted, but this not only consumes labor and materials, but also increases the cost of raw materials, which greatly limits the application of BOPET film. The following typical application examples illustrate this limitation of ordinary BOPET film

● although ordinary BOPET film has the properties of high transparency, high brightness, high strength, barrier and scratch required by plastic film for cigarette packaging, and even far exceeds BOPP film, it lacks the heat sealing performance required for rapid heat sealing packaging of cigarettes. Therefore, ordinary BOPET film cannot expand the application field of smoke film, and can only give up the lucrative market of smoke film

● in the food packaging industry, because ordinary BOPET film does not have heat sealing performance, processors have to adopt the method of combining BO6, front, back, left and right moving stage pet with CPP or PE film to solve the problem of packaging sealing. For example, bopet/adhesive/cpp or bopet/adhesive/pe or bopet/al/adhesive/cpp and other composite methods are adopted

● for the application in card protection, ordinary BOPET film also has no heat sealing property, so it is necessary to melt and extrude hot melt adhesive (such as EVA) through extruder, coat and compound it on the surface of BOPET film, and then heat seal it by heating and pressing

in order to eliminate this cumbersome and wasteful composite process and further expand the application field of BOPET film, researchers have developed a copolymerization modification method for ordinary PET resin to make it heat sealable

as we all know, ordinary PET resin is a crystalline polymer formed by heating polycondensation of terephthalic acid (PTA) and ethylene glycol (eg) under the action of catalyst. In order to destroy or weaken its crystallization performance, the third or even the fourth component can be used to copolymerize with it to form amorphous pet copolymer, which has heat sealing performance. The third component selected can be dibasic acid or diol for ternary copolymerization, or these two monomers can be selected at the same time for Quaternary copolymerization. PET produced by copolymerization modification with dibasic acid is called APET; PET produced by copolymerization of diols is called PETG. For example, Eastman company of the United States uses terephthalic acid, ethylene glycol and cyclohexanedimethanol (CHDM) for terpolymerization. By controlling the amount of cyclohexanedimethanol, PETG or pctg can be prepared respectively, which are amorphous pet copolymers. Compared with ordinary BOPET film, BOPET film made of this amorphous pet copolymer not only retains the original advantages and solves the problem of heat sealing, but also has the characteristics of low melting point and high shrinkage, which greatly expands the application range of BOPET film. See Table 1 for the comparison of the main properties of this film with BOPP and CPP films

the processing method of heat sealable BOPET film is basically the same as that of ordinary BOPET film. It is prepared by melt extrusion (single-layer extrusion or multi-layer coextrusion), tape casting, longitudinal stretching, transverse stretching, traction winding, slitting and other processes. It can be processed into single-layer film, but it is more processed into three-layer film, and its structure is a/b/c structure. Among them, layers A and C are the surface layer, and layer B is the core layer. One of the surface layers is the copolymerized pet heat sealing layer, which can account for 10% - 20% of the whole film thickness, depending on the requirements of heat sealing strength. Generally, the heat sealing strength range of heat sealable BOPET film is 5~8n/15mm

whether using single-layer extrusion drawing or three-layer co extrusion drawing process to produce heat sealable BOPET film, the technical key is to master the characteristics of pet copolymer resin different from ordinary PET resin, that is, it has amorphous structure, which can be used for tensile test, pressure test, bending test structure, high glass transition temperature and low melting point and softening point. Here, taking three-layer coextrusion to produce BOPET film with a/b/c structure as an example, the film making process is briefly introduced

1. Mix ingredients. Assuming that layer C is a heat sealable layer, in order to make it open and prevent adhesion between films during winding, it is necessary to add a certain proportion of silicon containing chips into the copolymer resin and mix them evenly, in order to make the surface of the film have appropriate roughness and reduce the friction coefficient

2. Drying treatment. Because PET resin molecules contain ester groups, even if there is only a small amount of water at high temperature, it is easy to hydrolyze, which will reduce the molecular weight of pet and deteriorate its quality. At the same time, due to the existence of moisture, a large number of bubbles will be produced in the processing process. Therefore, PET resin must be dried before film making. The drying temperature should be controlled at about 70 ℃, and the drying time of the vacuum drum should not be less than 6h

3. Melt extrusion. The three-layer coextrusion of a/b/c structure requires three extruders, one of which is used as the main extruder to extrude the materials of the core layer (layer B), and the other two are used as auxiliary extruders to extrude the materials of the two surface layers (layer a and layer C). If the surface materials are extruded by exhaust twin-screw extruder, the drying devices of a and C surface layers can be omitted in Huaibei to build an aluminum based new material industrial base. The reason is that this exhaust twin-screw extruder has two exhaust ports, which are connected with the vacuum pump through pipes. Under high vacuum (≤ 0.08mpa), the moisture in pet and the volatile low molecular substances produced in the processing process can be directly discharged from the exhaust port. This method of processing surface resin by exhaust extruder can not only save equipment investment, but also reduce operating costs. The extrusion temperature of copolymer PET resin as C layer should be controlled at 230 ℃ ~270 ℃. The temperature of metering pump, filter and melt pipe shall be controlled at 270 ℃ ~280 ℃

4. Die casting. For the processing of three-layer coextrusion heat sealable BOPET film with a/b/c structure, it is best to choose the die head with the third mock examination cavity. The reason is that the die head design of the three-layer mold cavity can make the two ends of the cast sheet free of heat sealable copolymer PET resin components, so as to avoid adhesion with the chain clamp due to the low softening point of the resin during transverse stretching. At the same time, this is also convenient for the recycling of waste materials. As for the temperature of the die head, it should be controlled at about 270 ℃, and the temperature of the cooling drum should be controlled at about 30 ℃

5. Longitudinal stretching. In order to prevent the hot seal from adhering to the stretching steel roll as a batch, the last few preheating rolls and slow stretching rolls of the longitudinal stretching machine should adopt ceramic rolls or spray "TEFLON" anti sticking layer on the roll surface. The longitudinal tensile temperature is 75 ℃ ~80 ℃, and the tensile multiple is 3~3.5 times. The temperature of the infrared heater used for supplementary heating should be controlled lower and placed on the other side of the heat sealing layer

6. Transverse stretching. The process conditions are basically similar to the transverse stretching of ordinary BOPET. If it is used for horizontal stretching of single-layer heat sealable BOPET, the stretching temperature should be controlled at 90 ℃ ~100 ℃, and the stretching multiple should be 3~3.5 times. The chain clamp must be cooled to prevent pet copolymer resin from sticking to the clamp

7. Traction winding. Corona treatment in this process should be selected on the non heat sealing surface. This is because the heat sealable surface has good printing performance and aluminizing performance without corona treatment

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