Dry compound process is a common compound method. The technological process is: unwinding of facial mask → coating of concave eye rolls → heating and drying of drying tunnel → lamination of compound rollers → winding of finished products → curing of finished products. Due to the fierce competition in the flexible packaging industry, customer requirements are becoming higher and higher. This is especially true for highly transparent packaging. This article analyzes the decrease in transparency of dry composite films , and hopes to help colleagues in the flexible packaging industry.
1. Adhesive itself
The color of the adhesive itself is too dark, such as dark yellow, yellow-red, and sometimes even dark red like soy sauce soup, and there will still be a corresponding color left on the adhesive film, which reduces the transparency. Especially when the large area blank film is compounded. Therefore, when high transparency is required, yellowish or colorless adhesives should be used. At present, most of the composite bags processed in flexible packaging use two-component adhesives (alcohol-soluble adhesives and ester-soluble adhesives), but the use of rubber-type single-component adhesives with non-drying and hot-melt properties is not excluded Mixture; in fact, transparency is a general concept. The best way to judge the transparency of the adhesive itself is to sample the main agent and curing agent according to a certain ratio, and put them in a transparent cup at 50 ℃ for 8-12 hours. Observe through light and draw a comparative conclusion.
2. The effect of glue dust
The glue liquid is mixed with particles such as dust , or there is too much air dust in the workshop environment. After the glue is applied, there is also dust in the hot air blown in the drying tunnel, which is stuck on the glue layer. It is sandwiched between the two base films during compounding, and there are many small dots, causing opacity. This requires the glue filtered through a 140-mesh metal screen, the gum solution to remove insoluble particles, also pay attention to the workshop hygiene; rough ground is not smooth, dust free, less dust in the air, into the drying tunnel A high number of filter screens or other filter materials can be used to remove the dust in the hot air. At the same time, as a packaging company, it should pay full attention to the working environment, such as reducing the number of mobile personnel, hygiene must be in place, and the substrate or composite film to be composited must be placed in a clean place.
3. The influence of the surface tension of the substrate
The surface tension of the film is not high and does not meet the requirements. Glue can not evenly infiltrate it, but shrink. After drying, the adhesive film is not uniform , and the transparency of the dry composite film is not good. Therefore, the surface tension of the dry composite membrane can be as high as 40 ; if the surface tension is not high, it must be treated. Commonly used is electric spark treatment, the purpose of the treatment: to roughen the surface, fluff the surface , and increase the surface roughness. When the adhesive is in contact with its surface, it can produce a good wetting effect.
4. Insufficient fluidity of adhesive, poor flatness
In this case, use a fluid adhesive or use a solvent that improves flatness. But to prevent the problem of odor and odor caused by too much residual solvent, there are two cases of adhesive fluidity difference: {1}; caused by poor quality of the adhesive itself , {2} due to too high working concentration, or high temperature and humidity Due to weather, the solvent volatilizes too quickly, which results in poor leveling . To solve this problem, it is necessary to pay attention to the viscosity of the solvent . When changes occur, corresponding measures should be taken, such as adding thinner. In addition, the use of circulating pumps for glue supply can also reduce the increase in viscosity and improve transparency.
Explanation of glue leveling: small molecular weight, low viscosity and good leveling. The transparency of transparent cooking bags is generally not as good as ordinary packaging bags. This is caused by the difference in leveling properties of different types of glue. The problem of glue fluidity has a lot to do with the design and selection of glue molecules. Now dry laminating machines, most of which use glue to squeeze the roller, is a good improvement, which has improved transparency.
5. The impact of insufficient glue
Insufficient amount of glue, small bubbles in the blank space, causing mottling or opacity. The water content of the solvent should not exceed 0.2%. If it exceeds this index, it will react with the curing agent and consume to reduce the solid content. Not only reduces the peel strength but also affects transparency. The amount of glue is related to the concentration of the glue solution, the depth, shape and arrangement of the coated screen roller. The amount of glue is determined by the actual material structure and use.
6. Poor transparency is also related to the temperature of the drying tunnel during compounding
If the temperature is not properly controlled, the temperature when entering the drying tunnel is too high , and the drying is too fast , so that the solvent on the surface of the glue liquid quickly evaporates, causing a local increase in the temperature of the surface glue and skinning. Then when the heat penetrates deep into the glue layer, the solvent under the film vaporizes and breaks through the glue film to form a ring , which makes the glue layer uneven and opaque. Therefore, the temperature control from low to high should be strictly observed. Let the solvent evaporate slowly from the inside to the outside to avoid the ring phenomenon and improve the transparency.
7. Decreased transparency is also related to the smoothness of the compound rubber roller and the rubberized wire roller
The compound rubber roller must have a smooth surface and no traces or foreign objects. Poor transparency is also related to the mesh depth and shape of the anilox roller, which determines the amount of glue. If the cells are clogged and the gluing is uneven, poor transparency will occur. The solution is to clean the cells until they are clean.
8. Decreased transparency is also related to curing time
Especially the two-component glue performance is more obvious. General curing temperature is controlled at 50--55 degrees; time in 24-48 hours, there are special circumstances time may be longer, because transparency is not a large part of the curing temperature, and time does not match the result. In addition, due to operational errors, it may also lead to poor transparency. If the glue is prepared, it must be completed according to the normal process. First add the solvent to the main agent, stir thoroughly, then add the curing agent, and then mix it before use. Otherwise, the glue will produce crystallization and affect transparency.
In short, there are many factors influencing the phenomenon of poor transparency of dry-type composite films, which must be judged from the actual situation and apparent phenomena and resolved. To make a good product, we must start from many aspects, and take appropriate preventive measures when there is no production.
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