Causes and Countermeasures of bubbles in the hotte

2022-08-12
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Causes and Countermeasures of bubbles in dry composite flexible packaging

flexible packaging plays an extremely important role in the packaging and printing industry. Composite packaging made of plastic film is widely used in various fields such as food. The composite bag made of plastic film can meet the packaging needs of different levels because of its low price, high quality, excellent heat sealing and barrier properties, acid resistance, alkali resistance, leakage resistance and other factors. As the competition in the flexible packaging market intensifies, its quality requirements from inside to outside are becoming higher and higher. The appearance of flexible packaging requires good flatness, the color scheme requires authenticity and nature, and the transparency is high. There should be no "pits", "stripes" and so on. Dry compounding refers to coating a layer of solvent adhesive (one component hot-melt adhesive or two-component reactive adhesive) on the plastic film, drying it after removing the solvent through the drying channel of the laminating machine, and bonding it with other composite substrates (plastic film or aluminum foil) to form a composite film under hot pressing. Because it can be combined in the form of two or more layers, it can meet the needs of various commodities. In the actual dry process, various faults are often caused by improper operation or other factors. Bubble problem is one of the common faults. The generation of bubbles will not only affect the appearance quality, but also affect the storage life between the film and the film layer. There are many reasons for bubbles, and the reflection forms of bubbles are also diverse

the generation of bubbles is inseparable from adhesives. Adhesives are divided into single component hot-melt adhesives and erxinda phase I project, with a total investment of 2.8 billion yuan of liquid reactive two-component adhesives. In practical work, the probability of bubbles in one component hot-melt adhesive is much greater than that in two-component adhesive, and if bubbles occur in the finished product, they will not be eliminated. This is because the one component hot-melt adhesive is coated on the cot and then dried in the drying path, and the finished product has no curing time after winding. Accordingly, the molecular weight diffusion force in the glue solution is relatively weakened. Of course, the problem of bubbles in two-component adhesive is not eliminated. When selecting the two-component adhesive solution, we must choose the regular manufacturer with qualified quality, and choose the adhesive with slightly larger molecular weight, uniform molecular weight distribution, excellent solvent release and good leveling performance, which will effectively reduce the probability of bubble generation. Generally speaking, the leveling performance of the general-purpose adhesive is better than that of the cooking adhesive. Of course, for the cooking composite bag with special requirements, we must use a special cooking resistant adhesive. Before formal use, it should be confirmed by test, and the glue solution should be prepared in strict accordance with its instructions. In the actual production summary, the correct method of preparing glue solution can also reduce the generation of bubbles. The correct method is to pour the main agent first, then add ethyl acetate in the normal proportion, stir it evenly, and then pour 1/3 of the curing agent into the proportion while stirring and pouring the curing agent until it is finally uniform. At the same time, it should also be noted that the evenly stirred glue liquid should be clear and transparent, and there are strict requirements for the normal preparation of glue liquid. When the plastic tray is not poured, it must be vacuumed and coated on the upper tray, which are effective methods to directly reduce bubbles. The impact of the plastic liquid poured into the plastic tray must be small to prevent the increase of impact force and the bad coating caused by mixing air into the plastic liquid. For example, use an iron bucket for processing, and weld its liquid flow pipe. The liquid flow pipe is plastic late protection, which will also cause large costs. Set a switch on it, and the function of the switch is to control the flow of glue, and make corresponding adjustments according to the actual situation. The liquid pipe passes into the plastic tray in a "liquid" shape, which not only reduces the impact force, but also prevents air from entering the tray liquid, thereby reducing the probability of bubbles

dry compounding adopts two-component two-liquid reactive adhesive. In order to make the cloth uniform and conducive to infiltration, solvents need to be added for dilution. Diluents include toluene, ethyl acetate, methyl ethyl ketone, etc. toluene is often used only as one component hot melt adhesive. Due to environmental protection problems, toluene is only allowed to be used in petroleum toluene, and coking toluene is strictly prohibited, because there is not only peculiar smell, which affects the content of the product, but also easy to produce bubbles, and toluene is gradually being cancelled; Methyl ethyl ketone is rarely used due to its high price. Generally, ethyl acetate is used as the diluent of two-component adhesive. The diluent shall not contain substances with active groups such as water, alcohol and amine, if the water content exceeds 0 2%, it will have an adverse reaction with the curing agent and produce bubbles. The - NCO group contained in the curing agent has accumulated double bonds formed by heteroatoms, which is very active and easy to react with hydroxyl, amino and other groups containing active hydrogen, affecting the wettability and other properties of the adhesive. The curing agent reacts with water and free acid, and will directly generate carbon dioxide to form bubbles

the volatilization rate of water is much lower than that of ethyl acetate, and the water content exceeds 0 At 2%, the evaporation rate of the solvent will be seriously reduced, and the spreading speed and hardening speed of the adhesive will be affected. Therefore, the purity of ethyl acetate should be above 99%, and the water content should not exceed 0 7 %。 In practical work, if water is mixed, what is the performance on the flexible packaging? How to detect and confirm: it can be judged by the form of bubbles. In the bopp/CPP structure, the bubbles are large and form irregular outward diffusion. If there is no peeling degree in the large bubbles after aging, as if there is no glue, it is easier to distinguish if the inner layer of the composite film is aluminized, because water will directly decompose the aluminum layer. After aging, the aluminum will be decomposed and diffused to the edge. Finally, it is transparent and there is no aluminum layer. Experts often call it "aluminizing solution", so it is said that the glue liquid cannot contain water, and the poor leveling of the adhesive will directly affect the coating effect, and eventually cause bubble failure of the product. The glue with poor leveling property can be observed after being coated on the rubber roller. Of course, it can also be found after being dried in the drying channel. In addition to checking the fluidity with a detector, we should observe from experience:

1 After gluing, if you do not enter the drying channel and stop the machine for inspection, you will find that the spreading property of the glue liquid is poor, the distribution is uneven, and the leveling property is poor, which not only produces bubbles and affects its transparency, but also greatly reduces the appearance quality and composite peel strength of the product. Therefore, it is very important to choose the glue with good leveling property. When the two-component reactive adhesive reels up the finished product, there are tiny bubbles just off the machine section, which generally disappear after aging

2. If the aging time is not complete, tiny bubbles will also be produced. For example, when Pu - 1975a adhesive is used, the aging time is generally 24 - 48 hours, and some products need longer time. The short aging time is also closely related to the good and bad peel strength. It can be seen that the effect of ripening is much better than that of non ripening, and the bubbles finally disappear completely

3. The cause of bubbles is closely related to the residual solvent. The dry-type composite film base material is put into the oven for drying after being glued and coated. The cleaner the solvent volatilization is, the better. The degree of solvent volatilization is closely related to the machine speed, drying temperature and drying air volume. The solvent release of the adhesive determines the drying speed. If it is not well dried, the amount of residual solvent will increase, which will not only produce peculiar smell, but also the solvent is not volatilized completely. With the continuous increase of the winding volume, The gradual accumulation of residual solvent will leave tiny pinholes in the cooled composite material. The drying channel of the dry compounding machine is divided into three areas, namely, evaporation area, hardening area and odor elimination area. The energy consumption in the three areas is almost equal. In actual work, corresponding adjustments should be made according to the actual composite substrate. Generally speaking, the temperature of the evaporation area should be controlled at 50 - 60 ℃, the hardening area should be controlled at 70 - 80 ℃, and the odor should be discharged at 80 - 100 ℃, The temperature increases in a gradient value. It is strictly forbidden to bake the road with high initial temperature and low odor discharge area. This is because too high initial drying temperature is not conducive to the solvent. Please re tension the tensioning wheel and volatilize it from the inside to the outside. Too high temperature can only dry the surface of the base rubber film, which cannot be completely dried, but form a "ring" and eventually produce bubbles

4. The reason for bubbles in flexible packaging composite film is also related to the wire roller. The function of the line roller is to evenly coat the glue liquid in the point on the base film, so the line roller is a very important core part of the coating effect. The amount of coating depends on the depth of the line roller and the working concentration. Usually, the line roller can only be coated on the base film by 1/4-1/6 under the pressure of the glue roller, so this requires the line roller to be kept clean and free of blockage. If the line roller is blocked, poor transfer and uneven coating will occur, Finally, bubbles and other faults will appear in the lamination. The solution is to clean it. There is a special film cleaner. For example, th - 01800 strong film remover produced by Shanghai Shenda Technology Co., Ltd. has a good effect. It does not maintain the hygiene of the line roll. Often, the return to zero difference is a more important factor than the difference in the incoming and return journey. It should be cleaned regularly. When there is no work after work, it should be washed with ethyl ester, and the glue pressure roll should be separated from the line roll. Of course, the line roll is too shallow, which is also prone to be blocked, It can be solved by replacing the line rollers with the same number of lines

5. The generation of bubbles is also related to the environment. In practical work, the temperature and humidity of the environment have a great impact on the effect of composite products. When the environmental temperature is too high, the two-component glue liquid is easy to be muddy and white, and bubbles are also generated. Therefore, it is often emphasized that "how much glue liquid is used, how much is prepared". Muddy white glue liquid makes the coating performance poor. If the ambient temperature is too low, the poor leveling property of the glue liquid will also produce bubbles, especially in winter, it is often found that the glue liquid becomes thicker and the leveling property is poor. In response to this phenomenon, the main agent is cured for 1 - 2 hours first, and then mixed. The leveling property is much better, but someone should be specially guarded and pay attention to safety. When the humidity is too high and exceeds 70%, the coating quality will be significantly affected. Condensation and other phenomena occur at the coating line roller and the pressure roller on the eve of the coating, and water participates in the curing agent, Produce carbon dioxide, affect the amount of sanctions, and produce bubbles. The solution is to use high boiling point solvents to reduce the volatilization rate, reduce the volatilization amount, prevent the temperature from decreasing or increase the heating device, so as to improve the leveling property of the glue liquid. Temperature 23 - 25 ℃, relative humidity 50 - 60% is the best production environment to ensure product quality

6 . There are two kinds of bubbles: regular bubbles are often caused by mechanical reasons or production parts, and the bubble position is just in the same position. For example, bubbles will be generated if the rubber roller is uneven or the hot pressing roller has traces, and it is not smooth. Therefore, smooth rubber roller and fine rubber roller are the basis to ensure that the product is free of bubbles. If the temperature of the hot steel roll is low, the pressure is not enough, and the included angle of the composite film is too large, it will affect and produce bubbles. The temperature control of the hot steel roll should be adjusted according to the actual situation. Generally, it is adjusted at 60 - 80 ℃. If the temperature of the hot steel roll is too low, it is not conducive to the further activation of the glue liquid and produce bubbles. If the pressure is not enough, it can be solved by increasing the pressure, usually at 4 0 - 7. 0Pa adjustment. The angle of the composite film clip is too large, and half air is brought in, which brings bubbles between the film layers, so the clip angle should be moderate, generally at 35 ℃. The generation of irregular bubbles is related to the tension of the material. If the tension is out of balance and relaxes, bubbles will also be generated, so it can be solved by replacing the material

reprinted from: China Packaging News

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