Heat sealable BOPET film manufacturing process

Biaxially oriented PET (BOPET) films do not have heat sealing properties. In order to be used for heat-seal applications, it usually requires complex and wasteful complex processes with other resins, thus greatly limiting its application area. However, using a copolymerized PET resin and coextrusion through three layers can solve this problem.

BOPET film has excellent comprehensive properties, such as good transparency, low haze, high gloss, high strength, very scratch, folding resistance, excellent barrier properties, etc., widely used in printing, aluminum plating, composite packaging, card protection, metallization , Insulating materials, photosensitive materials, tape bases and other fields. However, since ordinary PET resin is a crystalline polymer, it tends to have a large tendency to crystallize after being heated. If it is subjected to heat sealing, severe shrinkage deformation will occur. Therefore, a BOPET film (hereinafter referred to as an ordinary BOPET film) produced using ordinary PET does not have heat sealing properties. In order to solve this problem, a method of compounding an ordinary BOPET film with a PE film or a CPP film is generally adopted, but this is both labor-consuming and material-costing, thereby greatly limiting the application of the BOPET film. The following typical application examples illustrate this limitation of ordinary BOPET films.

● Although ordinary BOPET film is fully equipped with the characteristics of high transparency, high lightness, high strength, barrier property, and strong scratching required by the plastic film for cigarette packaging, it even exceeds that of BOPP film, but it lacks the need for fast heat-sealed packaging of cigarettes. The heat seal performance. Therefore, ordinary BOPET film can not expand the application of tobacco film, can only give up the lucrative market.

● In the food packaging industry, because ordinary BOPET film does not have heat-sealing properties, processors have to use the method of compounding BOPET with CPP or PE film to solve the problem of package sealing. For example, a combination of BOPET/adhesive/CPP or BOPET/adhesive/PE or BOPET/AL/adhesive/CPP is used.

● For the application of card protection, ordinary BOPET film must be melt-extruded through an extruder, coated on the surface of BOPET film, and then heated and pressed, because of the lack of heat sealing properties. To complete the heat seal.

In order to eliminate this complicated and wasteful compounding process, and further expand the application of BOPET film, researchers have developed a method of copolymerization modification of common PET resin to make it have heat sealability.

It is known that ordinary PET resin is a crystalline high polymer obtained by heating and polycondensation of terephthalic acid (PTA) and ethylene glycol (EG) under the action of a catalyst. In order to destroy or weaken its crystallinity, the third or even the fourth component can be copolymerized with it to produce an amorphous PET copolymer, thereby having heat sealability. The third component selected may be a dibasic acid or a dihydric alcohol and a ternary copolymerization, and the two monomers may also be selected for the quaternary copolymerization. The PET produced by copolymerization modification with a dibasic acid is called APET; the PET produced by copolymerization modification with a dihydric alcohol is called PETG. For example, the United States Eastman Corporation uses terephthalic acid, ethylene glycol and cyclohexane dimethanol (CHDM) terpolymerization reaction, by controlling the amount of cyclohexane dimethanol added, respectively, can be obtained PETG or PCTG They are all amorphous PET copolymers. Compared with ordinary BOPET film, BOPET film made from this amorphous PET copolymer not only maintains its original advantages and solves the problem of heat sealability, but also has the characteristics of low melting point and high shrinkage. Expanded the application of BOPET film. The main properties of the film and BOPP, CPP film are shown in Table 1.

The method for processing heat-sealable BOPET film and ordinary BOPET film is basically the same, and it adopts melt extrusion (single-layer extrusion or multi-layer co-extrusion), cast casting, longitudinal stretching, transverse stretching, traction winding, and centrifugation. Cut and other processes obtained. It can be processed into a monolayer film, but more is processed into a three-layer film with a structure of A/B/C structure. Among them, A, C layer is the surface layer, B layer is the core layer. One of the surface layers is a copolymerized PET heat seal layer. The thickness of this layer can account for 10% to 20% of the total film thickness, which depends on the heat seal strength requirements. In general, the heat sealable BOPET film has a heat seal strength in the range of 5 to 8N/15mm.

Whether it is a single-layer extrusion or a three-layer coextrusion process to produce heat-sealable BOPET films, the key to the technology is to understand the characteristics of PET copolymer resins that distinguish them from common PET resins, ie, have an amorphous structure. , higher glass transition temperature and lower melting point and softening point. In this article, only the three-layer co-extruded BOPET film with A/B/C structure is taken as an example to briefly introduce its film making process.

1, mixing ingredients. Assume that the C layer is a heat sealable layer. In order to make it openable and prevent adhesion between the films during winding, it is necessary to incorporate a certain proportion of silicon-containing chips into the copolymer resin, and the mixture should be uniformly mixed so as to make the system uniform. The resulting film surface has a suitable roughness to reduce the coefficient of friction.

2, dry processing. Because the PET resin molecule contains an ester group, even if only a trace amount of moisture exists at a high temperature, hydrolysis is likely to occur, resulting in a decrease in the molecular weight of PET and a deterioration in quality. At the same time, due to the presence of moisture, a large number of bubbles are generated during processing. Therefore, the PET resin must be dried before the film is processed. The drying temperature should be controlled at about 70°C, and the vacuum drum drying time should be not less than 6h.

3, melt extrusion. The three-layer co-extrusion of the A/B/C structure requires three extruders, one of which is used as a main extruder for the extrusion of the core layer (B layer) materials, and the other two are used as auxiliary extruders. , used to carry out the extrusion of the two surface layers (A and C layers). If the surface layer material is extruded using a vented twin-screw extruder, the two surface layers A and C can be omitted. The reason for this is that the vented twin-screw extruder has two exhaust ports, which are connected to a vacuum pump through a pipe, under high vacuum (≤ 0.08 MPa), moisture in PET and during processing. Volatile low molecular weight can be directly discharged from the exhaust port. This method of processing the surface resin through a vented extruder not only saves equipment investment but also reduces operating costs. The extrusion temperature of the C-layer copolymerized PET resin should be controlled at 230°C to 270°C. The temperature of the metering pump, filter and melt tube should be controlled at 270°C~280°C.

4, die casting. For the three-layer co-extruded heat-sealable BOPET film in the A/B/C structure, it is best to use a die with a three-cavity cavity. The reason is that the die head design of the three-layer cavity allows the two ends of the cast piece not to contain the heat-sealable copolymerized PET resin component, so as to avoid adhesion to the chain clip due to the lower softening point of the resin in the transverse stretching. . At the same time, doing so also facilitates the recycling of scraps. As for the die temperature, it should be controlled at about 270°C, and the cooling drum temperature should be controlled at about 30°C.

5, longitudinal stretching. In order to prevent the heat seal layer from adhering to the stretched steel rolls, the last few preheat rolls and slow stretch rolls of the longitudinal stretcher are preferably coated with a ceramic roll or sprayed with a "Teflon" release layer on the roll surface. The longitudinal stretching temperature is 75° C. to 80° C. and the stretching ratio is 3 to 3.5 times. The temperature of infrared heaters for supplementary heating should be controlled lower and placed on the other side of the heat seal layer.

6, horizontal stretching. The process conditions are basically similar to those of ordinary BOPET. If it is used for horizontal stretching of single-layer heat-sealable BOPET, the stretching temperature should be controlled at 90°C~100°C and the stretching ratio is 3~3.5 times. Chain clips must be reinforced to prevent sticking of the PET copolymer resin.

7, traction winding. The corona treatment in this process should be selected on the non-heat seal surface. This is because the heat sealable surface has good printing performance and aluminum plating performance without corona treatment.

Source of information: pack.cn