Influencing factors of adhesive coating state in t

2022-08-06
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Factors affecting the coating state of adhesive in dry compounding

dry compounding is a process in which two or more films of adhesive are bonded together by using dry compounding equipment. The amount of adhesive in dry compounding is very small, but it has a very important impact on the quality and performance of the composite film. It determines the peel strength, heat resistance, medium resistance, etc. of the composite film, so the composite enterprises are very cautious about the selection and use of adhesives. Generally, two aspects are mainly considered: one is the selection of adhesive varieties, and different types of adhesives are selected for products with different requirements; Second, different coating amount (glue amount on dry basis) is specified for products with different requirements. These two works have been done well, and the quality control of dry composite membrane has been basically guaranteed. According to the survey, at present, domestic composite enterprises often ignore another important indicator - adhesive coating status. The coating state of the adhesive has a great influence on the peel strength, transparency and heat sealing strength. Many apparent defects are often caused by poor coating state, such as bubbles, spots and other defects. One of the main reasons is poor coating state

the coating state of the adhesive refers to the distribution state of the adhesive on the film. The standard state should be that the adhesive forms a uniform, transparent and defect free adhesive film on the surface of the film. However, due to the complexity of actual production, many factors in the production process will affect the coating state of the adhesive. It is difficult to achieve the ideal standard state. What we need to do is to take appropriate measures to improve the quality of this adhesive film. Here, the author would like to discuss with friends the factors affecting the adhesive coating state and the corresponding solutions from the aspects of equipment coating control system, raw materials, workshop environment and process

I. equipment coating control system

the system that the equipment itself affects the coating state has the following four parts:

1. cot line roll scraper glue tank system

this system determines the coating amount of adhesive and ensures that the adhesive is spread on the surface of the adhesive film with a certain distribution density. The coating state of adhesive is mainly determined by this system

2. flattening roller

the flattening roller can further evenly coat the adhesive and eliminate some line marks

3. oven

the temperature and ventilation of the oven system affect the evaporation rate of the solvent and the hardening rate of the adhesive, which may have passed the test of life and death in 2017

4. compound roller - rubber pressing roller

this system provides a certain temperature and pressure, which not only further activates the adhesive, but also further spreads the adhesive and further eliminates the subtle defects of the adhesive

II. Raw materials

1. adhesives

generally speaking, the leveling property of general-purpose adhesives is better than that of cooking adhesives, and the leveling property of adhesives produced by different manufacturers is also quite different. In order to ensure excellent coating status, composite manufacturers should blindly increase the number of a/d bits, so it is meaningless to carefully identify and use

2. adhesive carrying film and bottom film

the surface tension of the adhesive carrying film or bottom film is too low, and the difference between the surface tension of the adhesive and the surface tension of the adhesive is too small, which will cause the adhesive to be unable to be fully dispersed and compatible on its surface. Generally, the difference between the surface tension of the film and the surface tension of the adhesive is 12 greater than dyne. If it is lower than this value, the coating state may be problematic. Attention should also be paid to the precipitation of additives in the carrier film or bottom film. At present, the composite manufacturer has a thickness of 60 μ There is a deep understanding of the precipitation of additives with a longer storage time than PE film, but there is not enough understanding of the precipitation of additives for pet and NY. It is generally believed that pet and NY have high surface tension and will not cause problems during dry compounding. However, pet and NY also add a large number of additives during production, which will also precipitate additives sold abroad, seriously affecting the spread and affinity of adhesives on the film surface. As the composite manufacturers are willing to use pet and NY as the carrier film, this must be paid attention to. Random corona treatment is a better solution

3. ethyl acetate

ethyl acetate is a thinner through adhesive. The content of water or alcohol in ethyl ester has a great influence on the coating state of adhesive. The volatilization rate of water is much lower than that of ethyl ester. Excessive water content will seriously reduce the volatilization rate of solvent, affect the spreading rate and hardening rate of adhesive, and the adhesive itself cannot be compatible with the film surface soaked by water. In addition, the curing agent and water or alcohol in the adhesive can quickly generate CO2, which not only limits the infiltration of the adhesive, but also produces serious bubbles

III. process factors

1. matching of the working concentration of the adhesive with the line roller

in order to increase the amount of glue, it is generally solved by increasing the working concentration of the adhesive (i.e. the working solid content). However, it must be taken into account that only when it is combined with the number of lines of the line roller, can there be a good coating effect. It is impossible to meet the requirements only by adjusting the working concentration. Generally, such a principle can be followed. The higher the working concentration, the greater the coating viscosity, and the less lines should be selected, that is, the line roller with large eye volume. Generally, the adhesive with 75% solid content, viscosity of 3000-5000cps and 25 ℃ (common adhesive is generally in this range), 100 adopts 100-150 line roller, and the working concentration is adjusted within the range of 30%, which can meet the coating amount requirements within 2.0-3.5g/m2 (dry basis coating amount), while the high-temperature cooking concentration 4, permanent deformation and temporary deformation - the part whose white height and arc height cannot be recovered after unloading the leaf spring is called large permanent deformation, Generally, when the solid content is 50%, the viscosity is 3000-4000cps, 25 ℃. If the coating amount is required to be 4.5-5.5g/m2 (dry basis coating amount), it is difficult to ensure a good coating state by using a line roller with more than 100 lines. At this time, a line roller with 80 lines should be selected. It is suggested that the compounding factory should be equipped with three sets of line rollers to produce almost all dry compound products, i.e. one set of 80 lines, which can be used for high-temperature cooking bag products or products requiring a coating amount greater than 4.5g/m2 (dry); A set of 100-120 lines is used for boiling and ordinary aluminum foil bags. The coating amount of general media or vacuum products is between 3.0-4.0g/m2 (dry); A set of 130-150 lines is used for the production of common products, and the coating amount is allowed to be less than 3.0g/m2

2. hardness adjustment of cots

the cots in the coating system provide a certain pressure to force the adhesive to extrude from the eyes and disperse on the film surface. The hardness of the cots has a great impact on the dispersion of the adhesive. There are two aspects worth noting. Firstly, the hardness of domestic cots is generally not high, and the hardness of products from different manufacturers is relatively large; Secondly, the hardness of COTS will decrease with the extension of service time

3. system cleaning

the cleaning of the line roller is very important. The degree of eye blockage affects the coating effect of the adhesive and the uniformity continuity of the adhesive film. In particular, two aspects should be paid attention to. First of all, regular cleaning must be adhered to. Here, it means that the roller cleaning agent shall be used for thorough cleaning, and the standardized operation method shall be adopted; Secondly, before changing different kinds of adhesives, carefully clean them to prevent the adhesives from polluting each other and affecting the bonding performance and coating effect

4. install the circulating glue feeding and air extraction system

during the preparation and mixing of the adhesive, a large amount of air will be entrained, which is not easy to disperse, especially when the high-temperature cooking adhesive is used. When the adhesive is poured into the glue tank, a large amount of air will be entrained. The existence of air will not only limit the spread of the adhesive, but also cause a large number of bubbles in the composite film. If the circulating air extraction system is installed, on the one hand, the dumping method can not be used to reduce air entrainment; On the other hand, the clamped air can also be extracted to ensure the uniform coating of the adhesive

IV. workshop environment

1. humidity while the microcomputer controlled electronic control testing machine uses sensors to monitor and collect the load change speed

the humidity of the workshop also seriously affects the coating state of the adhesive. Generally, the normal coating state cannot be guaranteed if the humidity is greater than 70%. Especially in summer, when the workshop temperature is high, a large amount of ethyl ester volatilizes in the coating system, causing the system temperature to be quite low, and a large amount of water vapor condenses in the rubber roll and line roll, damaging the coating state of the adhesive and seriously reducing the performance of the adhesive

2. dust

the dust in the air of the workshop will adhere to the adhesive film and cannot be removed, which will damage the uniformity and continuity of the adhesive film. Especially at the air supply outlet of the oven, the strong air flow will clamp the dust particles and scatter them on the adhesive film, seriously affecting the appearance. Therefore, in addition to the cleaning of the workshop, the filter at the air supply outlet must be cleaned frequently

above, we discussed the factors affecting the coating state of adhesive and some methods to improve the coating effect. The normal production process is relatively complex, and a more comprehensive consideration is required when analyzing a certain quality problem. (from China's packaging industry, 2001, issue 12)

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