Discussion on the mechanism and application of defoamer (2)
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The effectiveness of defoamers must first be measured in the laboratory. Transferring laboratory test results to production-scale applications without further testing is not advisable. The test methods we have developed in our work, combined with the conditions of paint manufacturing and construction process, are feasible for the evaluation of defoamers. According to different construction conditions (such as brushing, spraying, dipping, printing, etc.), different test methods can be selected.
1. Stirring test For our users, the simplest test method is the stirring test. Under certain conditions, a turbine blade is used to infiltrate the air into the paint system to be tested with a defoamer. Immediately after stirring, the material is poured into a measuring flask with a marking scale, and then weighed. By comparing the difference in weight, the amount of air infiltrated in the system is determined, and the defoamer effect is inferred. The test also shows how much air is present in the system in the form of spherical bubbles.
Another test method is to detect the number of macrobubbles. Under certain conditions, the air is infiltrated into the system, and then the height of the same weight material is measured by a measuring cylinder to evaluate the effectiveness of the defoamer. This method can guide us in selecting the mixing vessel capacity ratio in actual work.
Many defoamers also cause paint film surface defects such as shrinkage holes. It is the same as foam problem, which is unwilling to be seen by all our ink producers. It can only be used in batches before and after mass production. Experiment and test.
2. Flow coating test Flow coating test can be used to evaluate the compatibility and defoaming effect of defoamer with the coating system being tested. The agitated test sample was immediately poured onto a piece of polyester film. The polyester film was previously placed on a frame that was vertically inclined at an angle of 25 degrees. Observe the flow-coated test strips carefully and see if any air bubbles are present on the top. This method can also evaluate the defoaming effect of the defoamer by observing the dry film.
In some cases, the extreme amount of defoamer selected through the agitation test and the flow coating test is insufficient for those surfaces that are not very smooth, and the air-containing substrate is insufficient to illustrate the problem and give evaluation. When deciding on the optimum amount of defoamer, full consideration should be given to the end use of the product.
3, roller coating test roller coating test method is very close to the actual coating conditions, so that can distinguish the role of different defoamers. It can even test its added amount. During the roll coating test, a porous sponge pad was used to coat 40 g of paint on non-absorbent substrates, either as a dedicated test card or as a standardized test paper jam. The total coating area was 500 cm2. Before each test, the rollers were wetted with an equal amount of water to ensure the reproducibility of each test. The wet film coating rate is about 300 g/m2.
After the film was dried, the test paperboard was compared to evaluate the surface defects of the test panel. If the standard light source, the test panel for backlight inspection, carefully observed as light spots of air bubbles, which makes it easier to distinguish the merits of defoamer performance.
The above stirring test, flow coating test, and roll coating test all contribute to the effectiveness and performance of the defoaming agent, so as to determine the optimum type and amount of the defoamer suitable for the product, so as to satisfy the requirements such as the substrate and the system. , printing or construction conditions and other comprehensive factors on the defoamer requirements.
Six, water-based ink "bubble more" failure and solution
We previously discussed the mechanism of bubble generation. In fact, the phenomenon of bubble generation in water-based inks is particularly likely to occur when open-type ink supply is used to print a large area on the ground. When a closed-cycle ink supply system is used, this drawback can be described in negligible quantities.
The reason why water-based ink is prone to generate bubbles depends on its composition. Water-based inks generally consist of insoluble pigments, water-soluble or water-dispersible resins, surfactants, water, a small amount of ethanol, and other additives, and are basic systems. That is, a saponification system, similar to soapy water or detergent, can easily generate bubbles when it is frequently turned.
The conventional methods for reducing or eliminating bubbles are: adding defoamers, controlling viscosity and pH, mechanical factors, and the like.
Adding a defoamer is a direct and effective method, but it is worth noting that: most of the defoamers produced by domestic manufacturers use silica gel, and after adding too much, it is easy to make the ink particles thicker, causing pinholes in the printed field patterns. Due to the appearance of small depressions, cracks, and other defects in the flexible plate material, the amount of added material is very important, and it is best to maintain it at around 0.5% to 1.5%. As for other aspects of the antifoaming agent, the foregoing has been described.
The viscosity and pH are controlled because the ink is prone to bubbles when it is too thick or over-dilution. The reason why the ink is thicker is because the contact area between the ink fountain and the air in the open type ink supply system is large, and volatile components such as ammonia or amines in the ink volatilize and escape, making the alkaline system slightly misaligned, and the most intuitive performance at this time. It is the viscosity of the ink that increases and the pH drops. The method used was to add a stabilizer with a pH of 11 every 30 to 40 minutes to supplement the alkaline ingredients.
As for the mechanical aspects, it is possible to use a frontal ink shield to properly increase the printing speed so that bubbles do not easily transfer to the anilox roll. Make full use of the circulation pump to supply ink, the lower layer of the ink fountain is relatively stable, and the bubbles are lighter. Generally, it floats on the surface of the ink and does not enter the ink chamber with the ink tube. The addition of a positive baffle plate can create a relatively stable semi-enclosed ink area near the anilox roller in the open ink fountain, and keep the continuously generated bubbles out of this area, reducing the chance of bubbles approaching the anilox roller. Appropriately increasing the speed of the vehicle will increase the pressure in this semi-enclosed area and effectively limit the time for the existence of bubbles. These two functions just coincide with the advantages of the closed ink supply system. Therefore, it is also a very scientific and effective method.
In fact, there is a simpler way. After adding two self-made ink baffles, insert two waste cardboard into the ink flowing along the two sides of the front baffle, and the paperboard must be perpendicular to the direction of the flowing ink. . The è¯€çª is the depth at which the side stripper is inserted, neither too deep nor too shallow, controlled at the boundary between the bubble layer and the ink layer.
Many printers also use artificial ink. When the machine stops, the ink in the ink fountain stops turning. The bubbles that were originally wrapped in the ink float to the surface of the ink. At this time, bubbles can be scraped away with a tool such as cardboard to reduce the number of times the defoamer is added, and the ink is kept printed. Adaptability and reducing the appearance of depressions in the flexo version also have a role.
Seventh, summary
The defoamer's function is to suppress and eliminate the generation of air bubbles; generally speaking, when the viscosity of the water-based ink is too high, the PH value is low, or when the printing machine runs at a high speed, air bubbles are likely to be generated, if the amount of generated air bubbles Bigger version will appear dirty. Water-based inks generally use alkali-soluble binder. This binder is an alkaline solution of acidic resin. It is an aqueous solution similar to soap. Because the water-soluble binder has soap characteristics, it is due to the stirring effect of the ink fountain roller in the ink fountain. Foam can occur, and the generation of foam can cause print quality problems. Therefore, defoamers or anti-foaming agents must be used to control the foaming of water-based inks.
Defoamers can break or suppress the foam that has been produced, while antifoams prevent or suppress the formation of foam. Antifoams and defoamers are generally water-soluble materials such as silicon, modified fatty acids, and alcohols such as ethyl alcohol. The antifoam effect is usually related to the age of the ink, and the newly formulated ink is less likely to foam when used than previously formulated inks.
The use of defoamers should pay attention to the mixing ratio of defoamer and water. Generally, a ratio of about 3:1 is recommended. It is recommended that the spray gun be used to spray the ink on the surface of the ink tank. The amount of antifoam or defoamer should be taken into account. Should not be too much, otherwise cause the ink high yield point and thixotropy rheological problems, so that the local expansion of the printing plate produces fish eye and pit defects.
Source: Ink Information