The cause of gel spots in the compounding process of aqueous adhesives

In the process of dry compounding, white spots are a common quality problem. Because the waterborne adhesives and solvent-based adhesives have significant differences in the nature and operation of the product, the causes and mechanisms for the occurrence of white spot stains are also different. The issue of white spot in the dry compounding of solvent-based adhesives has been described in many articles. Here, the author summarizes several common causes of spotting of white spots in the process of water-based adhesive dry compounding and communicates with peers.

Insufficient or uneven rubber coating

The white spot produced due to the insufficient amount of glue can be observed when the compound is removed. In the dry composite process, the coated adhesive must be able to fill in the unevenness of the surface of the ink and film in order to avoid appearance defects such as obvious white spots and spots. In the production of lightweight packaging products using water-based adhesive dry compounding, the economical coating weight of aqueous adhesives is 1.8-2.2g/m2 and the working concentration is 42%. (The working concentration of different brands of water-based adhesives may be The difference is that the coating is generally performed with a coating roller having a line number of 180 lines/inch and a cell depth of 32 μm. Due to the large opening degree of the mesh openings on the coating roller, if the surface of the coating roller is worn, it will have a significant effect on the amount of glue applied. Therefore, in the process of dry compounding water-based adhesives, the operator must always check the actual amount of glue applied by the rubberizing roller to avoid the insufficient amount of glue being applied due to excessive abrasion of the rubberized roller.

In addition, the uniformity of the amount of water-based adhesive glue is also a very critical factor. If the coating is not uniform, there will inevitably be a situation where the amount of localized glue is too small, resulting in white spots. Therefore, we must pay special attention to the following two issues: 1 The assembly of the scraper should be straight and it should not be distorted. The angle and pressure of the scraper should be properly adjusted to ensure a uniform scraping effect. 2 During normal production, the scraper should be left and right. Move to avoid abrasion of the applicator roller in a fixed position.

Poor glue leveling

Because the water-based adhesive is not like the solvent-based adhesive, it can achieve better leveling effect under the action of its own gravity and surface tension. Therefore, when applying the aqueous adhesive, it is necessary to use a smoothing roller. The glue is flattened. Smoothing roller rotation speed, pressure, surface cleanliness are related to the leveling effect of water-based adhesive, the direction of rotation of the smoothing roller should be opposite to the composite direction, otherwise it will not achieve smoothing effect, its rotation speed should be slightly higher than the composite speed.

The leveling effect of aqueous adhesives is also affected by the surface tension of the coated film. Because the surface tension of water-based adhesives is significantly higher than that of solvent-based adhesives, the surface tension of the ink on the coating film is required to be not less than 38 mN/m. Otherwise, the water-based adhesives will automatically shrink after coating. Continuous film formation. Therefore, the operator should pay attention to the detection of the surface tension of the coated film.

In addition, if the surface of the coating film is not clean, a material with a low surface tension adheres, etc., the leveling performance of the adhesive will be reduced, and a failure similar to the “shrinkage” of the printing will occur, appearing on the composite film. "Pin point."

Coated roller plugging

In the solvent-based adhesive dry compounding, there are generally two types of plugging of the coating roller in the network. One is that the two-component adhesive is deposited on the corners of the coating roller and can not be transferred out over time. Cross-link curing occurs to make the cells shallower. Second, when high-concentration adhesives are applied in high-temperature environments using high-strand coated rolls, ethyl acetate evaporates too quickly and the glue transfers poorly to clog the cells. It is a bit similar to the “dry plate” phenomenon caused by the organic solvent volatilizing too quickly during the printing process.

In the dry compounding with water-based adhesive, in order to prevent clogging of the coating roller, it is necessary to release the scraper when it is stopped, and make the coating roller soaked in the adhesive to maintain the rotation. If clogging of the rubberizing roller occurs, it must be cleaned with organic solvents such as butanone.

If the shape of the cells of the gluing roller is not suitable, the clogging of the cells is likely to occur, but this clogging occurs gradually. The adhesive roller used in water-based adhesive dry compounding has a shallow cell opening and a large opening, so as to make the contact area between the aqueous adhesive and the film substrate as large as possible, which is favorable for improving the coating effect and drying efficiency. . We generally use a 180-thread/inch square flat embossing roller or a 200-thread/inch four-corner flat-bottom continuous embossing roller, or an electric engraving roller with a small internal angle and a large open groove. It is not recommended to use a quadrangular pyramid. Embossing roller, because of the latter's larger inner corner of the cell, poor release of the glue, and easy to plug the cell, showing the appearance of the film when the composite production is very good, but after producing several kilometers or nearly ten thousand meters White spot spots will appear, and at this time the test will find that the amount of glue has been relatively insufficient.

In addition, when reusing two-component waterborne adhesives, it is important to understand the extent to which they can be used and the proportion of old glue added, because this will not only directly affect the peel strength of the composite product, but also make the waterborne adhesives Poor transferability may even result in blockage of the rubber roller net, resulting in white spots.

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