Heat-shrinkable polyester (PET) film processing and application

Heat-shrinkable plastic film requirements are stable at room temperature, heat (above glass transition temperature), contract and is in more than one direction of 50% heat-shrinkable ideal. Heat-shrinkable plastic film packaging features are: body transparent, reflecting product image; tight packaging, preventing good waterproof, moisture-proof, anti-mildew; no resilience, there is a certain security features. Heat-shrinkable plastic film commonly used in food and beverage, electronics, metal products, such as packaging, in particular shrink labels for the main areas of application.


Heat-shrinkable plastic film than as the shrink label, and in recent years also begun to consumer goods packaging. Because it will enable packaged goods to avoid shocks, waterproof, moisture-proof, rust-proof, and make products for printing exquisite packaging to win customers, and it's good to show the good image of the manufacturer. At present, more and more packaging manufacturers use printed shrink films instead of the traditional transparent film. Because printed shrink films can improve the appearance of the product quality, product advertising, brands can have a deep impression in the minds of consumers.


Heat-shrinkable plastic films generally amorphous plastics processing, such as polystyrene, PVC, PVDC,. Polystyrene (PS) shrink low film strength, impact resistance, it is rarely used, and polyvinyl chloride (PVC) is not conducive to recovery, does not meet the requirements of environmental protection. In other countries, especially in Europe, polyvinyl chloride (PVC) plastic film has been banned in the area of packaging in particular, used in food packaging.


Polyester (PET) heat shrink film is a new type of heat shrink packaging materials. Because it is easy to recycle, no poison, no smell, good mechanical properties, in particular environmental protection features, developed polyethylene terephthalate (PET) has become to replace polyvinyl chloride (PVC) ideal substitute for heat-shrinkable film.


Heat-shrinkable polyester (PET) film of copolymerization


Polyethylene terephthalate (PET) film is a crystalline material, General polyester film obtained by special treatment can only be 30% the following heat shrinkage rate. To get higher rate of heat shrinkable polyester film, it must be modified. In other words, for the preparation of high shrinkage polyester film needs to ordinary polyester or polyethylene terephthalate copolymer modified. Copolymerization modification of PET film its maximum thermal shrinkage rate can be as high as 70%.


Ordinary polyester is composed of purified Terephthalic acid (PTA), ethylene glycol (EG) made through esterification, condensation reactions, belonging to crystalline polymers (strictly speaking is the coexistence of crystalline and non-crystalline polymer). Copolymerization modification is in addition to the so-called purified Terephthalic acid (PTA), ethylene glycol (EG) outside the two main ingredients, then introduced the third or even the fourth component involved in co, aims to produce asymmetrical molecule to form the amorphous PET copolymer.


Third or even forth monomers can be introduced dicarboxylic acids and diols. Among them, there is a benzene dicarboxylic acid, propionic acid, succinic acid, adipic acid, sebacic acid; diols neopentyl glycol, propylene glycol, diethylene glycol, 1.4, such as cyclohexane dimethanol. Dicarboxylic acid (Acidic) copolymer modification, the prepared PET copolymer, as APET; if diols (Glycolic) copolymer modification, the prepared PET copolymer is called PETG.


The introduction of third monomer, dicarboxylic acids are the most frequently used isophthalic acid (IPA). Join IPA can change the polyester symmetrical structure closely, destroy the regularity of macromolecular chains, thereby reducing the forces between the molecules, molecular structure becomes more supple polyester. Also, due to the introduction of IPA, make it difficult for pet nucleating crystallization, and with the increase of the IPA into, APET copolymer transition from partially crystalline to amorphous polymers, due to the decline in the ability of modified polyester APET, amorphous, so they can be used in the manufacture of high-shrink film. Recommended IPA content around 20%.


Third monomer can also be introduced by diols. 1.4 is the most commonly used glycol cyclohexane dimethanol (CHDM). Polyester copolymer in the process of joining CHDM on changing polyester TG, TM and crystallization rate will have a big impact. With increase of CHDM content, copolyester PETG drop melting point, glass transition temperature, copolymers become amorphous structure. However, 1.4 cyclohexane dimethanol (CHDM) amount of control in the appropriate range, usually recommend CHDM content for 30~40%. The glycol modification of PETG not only can be used for preparing high-shrink film, can also be used to produce high heat seal film, transparent film, a wide range of uses.


The contraction mechanism of heat-shrinkable film


To BOPET film of production process for cases, its production process is first will PET copolymer resin for dry processing, then joined extrusion machine in the melting extrusion, and through die head/cold drum cast tablets, then will cast tablets heating to glass of temperature above, and melting temperature following of a a appropriate temperature range within, and in forces Xia, for one-way or two-way must multiples of stretch, usually requirements for horizontal 3.5~4.0 times of stretch. Stretch to make the PET molecular chains along the direction of external orientation, and then to cooling, the orientation of molecular structure of PET "freezes" stereotypes. This force under high deformation with heat-shrinkable "memory effect", when the "memory effect" of film and then heated to a temperature above, been frozen the macromolecular orientation structure begins to relax, PET film in macro contract. Worthy of mention is the PET film heat shrinkage is mainly contributed by the amorphous part of orientation. This is why amorphous copolymer modification of PET film (APET/PETG) than ordinary crystalline account of PET film heat shrinkage rate is much larger. Therefore, by increasing orientation in thin films of amorphous can achieve the objective of improved film heat shrinkage rate. As noted earlier, ordinary pet film heat shrinkage rate below 30%, copolymerization modification of polyester thermal shrinkage of up to more than 70%.


Heat-shrinkable polyester film production technology


Heat-shrinkable polyester film production process is:


Heat-shrinkable polyester film production conditions and effects of introduced are as follows:


1. drying process: as the polyester macromolecules containing ester groups in the chain, hygroscopic tendency, in the case of heated, even if there are traces of moisture, they are extremely prone to hydrolysis. As a result, in the course of processing will produce large amounts of bubbles, affecting the normal production and hydrolysis and molecular weight, PET quality deterioration. So before the melt extrusion process, must be dried. Recommended vacuum drum dryer, drying 70~75℃ drying time >6h.


2. melt: after drying process of PET copolymer resins to join single screw extruder with melt extrusion, the temperature is set to: 180-240-260-270-275 , the melting temperature of about 270 degrees.


If you use exhaust type twin screw extruder with melt extrusion, you can save the vacuum drum drying system. Because the exhaust type twin screw extruder set has two vents, which correspond to the two connected to the vacuum system. In the PET resin by vacuum pump the vacuum can be moisture and melt extrusion of low molecular weight compounds produced in the process, achieve the same effect, and can reduce investment and operating costs.


3. casting cast piece: melt melt melt dosing pump, filter, melt pipes into the coat-hanger die from lip mouth coating to form on the cooling drum casting. Cooling drum cooling water temperatures of 30 degrees Celsius.


4. under uniaxial tension: heat shrinkable polyester film often requires a one-way contract, special requirements are horizontal shrink. Horizontal stretch of preheating temperature for 90~100℃ drawing temperature for 105~110℃ draw ratio 3~3.5 times. Chain clamp to strengthen air cooling, control clip temperature is below 110 degrees to prevent resin adhesive clips. Stretched PET film cooling without the need for a heat setting treatment immediately.


5. winding, slitting: after tensile of on-line PET film by thickness, traction roll, final cut according to user specifications, testing, packaging, that is, heat-shrink film products.


With the development of China plastic package industry and the improvement of people's living standards, fruit juice, soft drinks and other beverages are used best performance, better barrier properties, nontoxic and odorless PET plastic bottle packaging, its usage is very considerable. According to statistics, at present, the amount of PET beverage bottles of at least 250,000 tons or more, matching PET shrink label films with amount up to 2~3 million tons. In addition, after China's accession to the WTO, foreign trade is continuously expanding, in order to improve the image and value of goods for export, you must also use polyester heat shrink film packaging materials of higher value, such as. In General, application of heat-shrinkable PET film is very promising!