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The present situation and development of color printing plastic flexible packaging (III)

(VII) drying of adhesive layer

in the combination of coating the film with alkene mixed adhesive, more than 75% of the components are alkene, and only 25% of the solid substances play the final bonding role. However, the 25% material with final value can play its role only by the melting and alkene release of 75% alkene material. Although the 75% alkene material accounts for a large proportion, it is only needed in the process of synthesis and coating. It is a temporary component that needs to be volatilized before compounding with printed matter. If it does not volatilize, it will not be able to form a valuable 25% solid substance that can produce strong adhesion to both the film and the printed matter. Therefore, whether the film and the composite material are firmly bonded, that is, whether the film bubbles or not, depends not only on the quality, concentration and thickness of the adhesive, but also on whether the olefins in the adhesive liquid have evaporated and whether the adhesive is dry before compounding

in order to ensure the drying of the adhesive layer, the glue coating machine is designed with a drying tunnel several meters long. In such a long distance, in a closed drying oven, several channels of electric heating pipes emit uniform heat through the performance standard mt143.1 ⑵ 011 of anchoring agent, blast with several fans at the front end into the drying channel, and exhaust with the exhaust fan at the rear end to the outside, so as to continuously supply strong hot air flow and volatilize the olefins in the glue layer coated on the film surface. Under such conditions, there will be several situations:

① as the main component of the glue solution, alkene plays a role in the dissolution and synthesis of the resin. The alkene material dissolved and synthesized by the resin is also changed by the dissolved and synthesized substances. The process of dissolution and synthesis is not only a physical process in which several materials are mixed and their forms change, but also a chemical process in which they interact and react with each other to produce new properties. When the alkene materials of dissolved resin and other chemical raw materials and the alkene materials added for the convenience of coating operation become glue liquid together, their relative density, boiling point, evaporation rate, flash point, etc. have changed greatly. Just like using tap water to make porridge, after adding rice, beans, alkali tap water to make porridge, its composition is no longer just H2O, but alkali, starch and other chemical components, and its properties have changed. In this case, the volatilization of olefins mixed in the glue solution to the atmosphere is much slower and more difficult than that of pure olefins, although it is supplemented by the external force of high-temperature air flow. Relevant scientific research departments have conducted experiments. The pure olefin material and the glue solution synthesized and diluted with this olefin material are respectively coated on 20um corona treated biaxially oriented BOPP film. Under the same conditions, the residual amount of pure olefin material is measured to be 21ppm, and the difference between the two is 15.33 times. The test showed that the volatilization rate and transpiration of alkene compound mixed in the glue solution were only 1/15 of that of pure alkene compound. Just like a bowl of clean water splashed on the ground, it will quickly all penetrate into the soil, leaving only traces of wetting; When a bowl of porridge is thrown on the ground, the water in the porridge cannot quickly penetrate into the soil, but remains in the porridge for a long time. Therefore, the rubber on the film through the drying channel, and the two models of xcp150/100 and xcp120/90 rubber melt gear pump extrusion units jointly developed are not dry solid, but rather slightly sticky rubber elastomer

② generally, the adhesive layer applied on the film is more than 5um. After passing through the drying channel, the alkene in the surface glue liquid is first volatilized, just like the porridge poured on the ground. The drying starts from the surface. Porridge on the ground also has another important drying way of being absorbed by the soil. The volatilization of olefins in the inner deep layer can only be carried out by the olefin molecules breaking through the attraction of other substances in the glue solution and rising to the surface of the glue layer. At this time, the alkene molecules will encounter two difficulties: one is that after the alkene on the surface layer volatilizes, the volume of the adhesive layer will shrink and become a kind of "dry scab" on the surface layer, which will become a cover for the deep alkene molecules to rise, leave the adhesive layer matrix and volatilize into the atmosphere; The other is the time required to overcome the intermolecular gravity from the deep inside to the surface. This time is often longer than the time when the film passes through the drying tunnel under the action of high-strength hot air flow. As a result of these two difficulties, the volatilization of olefins in the depth of the adhesive layer is blocked by the "dry scab" on the surface layer, forming a false impression that the outside is dry and the inside is not dry

③ when many captains find such problems during operation, they often adopt the method of increasing the power of the electric heating tube in the drying channel and increasing the exhaust air volume of the motor, hoping that higher temperature and greater wind force can make the olefin material volatilize completely. However, the rapid volatilization of the solvent makes the surface temperature of the adhesive layer drop rapidly, which forms a temperature contrast with the surrounding atmospheric environment in a small area of the surface of the adhesive layer. This temperature difference makes the water vapor molecules in the hot air flow condense into fine water vapor under the low temperature condition of the adhesive layer surface, which covers the adhesive layer surface and combines with the film and the printed matter. The existence of such water vapor, on the one hand, forms an isolation layer between the adhesive and the ink printing layer, which affects the bonding; on the other hand, it affects the transparency of the adhesive layer after compounding, resulting in whitening and deficiency

(VIII) change after compounding

the adhesive layer applied on the film surface shall pass through a small guiding smooth roller small rubber roller system before passing through the hot pressing mechanism. This system determines the initial bonding and positioning of film to film or film to paper and other materials. The size of the gap not only determines the initial pressure on the film and the composite, but also affects the running speed, passing time, air discharge and other issues, as well as the composite quality

after the film is compounded with the composite, the alkene in the adhesive layer wrapped in the "dry scab" still exists, especially the active alkene molecules on the inner surface of the "dry scab" will continue to move, leaving the adhesive layer in a sticky and soft state. It is a good indication that the sticky adhesive can be touched by cutting open the bubble

the alkene molecules in this sticky adhesive must continue to move after compounding with the composite due to its volatility. It is necessary to find ways to escape from the matrix of the adhesive layer through various ways. One part of the "dry scabs" of the adhesive layer epidermis crushed by the pressure rolling of the hot pressing mechanism converge on the inner surface of the film, and the other part gradually moves in the opposite direction away from the attraction between the resin molecules of the adhesive layer and reaches the surface of the ink printing layer

the olefin molecules polymerized on the inner surface of the film cannot penetrate the film and volatilize into the atmosphere due to the gas resistance of the film, and gradually converge. Thicker olefin molecules will cause swelling of the film, making the flat and smooth film become loose and wrinkled due to swelling, forming bubbles. Just like people's hands in the water for a long time, their skin will be white and swollen by blisters

alkene molecules that reach the surface of the ink printing layer will have affinity, corrosion and decomposition with the oil in it, so that the pigment in the ink will lose the connection with the oil and become loose particles, reducing the adhesion of the adhesive layer. At the same time, the ink used for color printing contains a certain amount of additives, thinners and other chemicals. When one side of these alkene molecules and the alkene molecules in the glue solution are subject to the air resistance of the BOPP film attached to its surface, and the other side is formed by the coating of the printing ink layer dried by hot air, they only exist in the 8mm deep ink of the sample notch of the oil stripping test machine, and converge under the inner surface of the film, which will further intensify the swelling effect on the film, resulting in foaming, whitening and virtual void

according to the analysis of the operating procedures, the cause of blistering can be basically understood. The main reason is not that the operators do not master it properly or have a weak heart, but that there is a great difference between the process design and the implementation effect, that is, there are serious defects in the adhesive coating process itself. The process is as follows:

film feeding → coating glue solution → drying → pressing → winding → storage → slitting → finished products

mixing and conveying of glue solution for printing

plastic/plastic, plastic/metal and paper/plastic compounding play a decisive role. In the immediate gluing compounding process, only alkene (solvent) type glue can be used. In order to facilitate the operation, the amount of alkene must be increased again. The increase in the amount of alkene materials, on the one hand, reduces the adhesive force of the adhesive, on the other hand, increases the volatile amount of alkene materials. In 2017, it was in such a contradictory and unscientific state that "the thick materials could not be coated, the thin materials could not be adhered firmly, the thin materials could not be dried thoroughly", and the unpredictable foaming problems continued to occur. Of course, it will be better if the compounding speed is lower than 300r/min. The olefin content volatilizes much, and the ink and glue liquid will also dry. However, the urgency of the production cycle does not allow you to take your time. It is also a way to increase the concentration and thickness. However, with the increase of concentration and thickness, the cohesion generated by the drying of the adhesive will also increase, which will not only solve the foaming problem caused by swelling, but also produce new film crepe bubbles

v. causes, effects and elimination of static electricity generated by film coating

with the reform of film coating from instant gluing wet composite process to hot melt adhesive pre coating dry composite process, the fire caused by static electricity igniting volatile gas of organic solvent during film coating production can be eliminated, but the impact of static electricity on the operation of film coating production process still exists. To eliminate static and ensure the smooth progress of film covering production and its product reprocessing has become a new technical research topic of film covering process, which needs to be explored and tackled by colleagues

(I) causes of static electricity generated by film coating

film coating is the combination of BOPP plastic film and other plastic films or paper prints. Before compounding through the hot pressing mechanism of the special laminating machine, BOPP plastic film shall be passively discharged on the feeding mechanism, driven and rotated by the heating roller. At this time, the tightly wound BOPP plastic film must first pass through several metal flattening rollers, relying on the friction, expansion and tightening closely fitted with the metal roller, so as to keep the flatness and tension of the BOPP plastic film consistent on the full width. The paper printed matter stacked in stacks shall be removed from the original position by the friction generated by the workers or by the pulling of the equipment one after another, reach the conveyor belt, and be sent to the hot pressing mechanism. There is friction between the rubber conveyor belt of the paper conveying mechanism and the driving metal roller, so that the conveyor belt can move continuously. The printed matter that has just been rubbed between the paper and the ink printing layer or the ink printing layer immediately enters the double friction state with the rubber conveyor belt. Without the double friction between the conveyor belt and the drive roller, and between the paper print and the rubber conveyor belt, it is impossible to form the pre closing positioning of the print

when passing through the hot pressing mechanism, the BOPP plastic film on the surface is rubbed by the metal heating roller, and the paper printed matter on the bottom is strongly squeezed and rubbed by the rubber pressure roller, so as to form the diffusion of adhesive molecules to the printed matter itself, bond and compound with the BOPP plastic film, and at the same time, it is rolled out and leaves the hot pressing mechanism

the paper/plastic composite that leaves the hot pressing mechanism is pulled by the receiving mechanism and pulled for winding. That is to say, the paper surface and the plastic surface are closely intertwined and rubbed with each other, and the winding becomes a semi-finished product

the paper/plastic composite rolled by semi-finished products should be pulled by manual force to cut the long composite one by one and restore it to a single finished product of the original size. In the process of finished product slitting, there are rotating tangent stripping friction,

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