Discussion on the mechanism and application of ant

2022-07-23
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On the mechanism and application of defoamers (Part 2)

v. some practical tests for defoamers the efficacy of defoamers must be determined in the laboratory first. It is not advisable to convert the laboratory test results to production scale without further testing. The test method developed in our work, combined with the conditions of coating manufacturing and construction process, is feasible to evaluate the performance of defoamer. Different test methods can be selected according to different construction conditions (such as brushing, spraying, dipping, pressing "query/j", printing, etc.)

1. Mixing test

for our users, the simplest test method is mixing test. Under certain conditions, a turbine blade is used to infiltrate air into the coating system to be tested with defoamer. After mixing, immediately pour the material into the measuring bottle with marking scale, and then weigh it. Through the comparison of weight differences, determine the amount of air infiltrated into the system, so as to infer the effect of defoamer. The test also shows how much air exists in the form of spherical bubbles in the system

another test method is used to detect the number of macrobubbles. Under certain conditions, air is infiltrated into the system, and then the height of materials with the same weight is measured through a measuring cylinder to evaluate the effect of defoamer. This method can guide us to select the capacity ratio of mixing container in practical work

many defoamers can also cause surface defects of the paint film, such as shrinkage, etc. like the problem of foam, it is not the wish of all our ink producers. This can only be done through a large number of experiments and experiments before batch use

2. Flow coating test

flow coating test can be used to evaluate the compatibility and defoaming effect between defoamer and the coating system being tested. Immediately pour the sample after stirring test onto a piece of polyester film. The polyester film shall be placed on a frame with a vertical inclination of 25 degrees in advance. Carefully observe the flow coated test piece to see if there are bubbles on it. The method can also evaluate the defoaming effect of defoamer by observing the dry film

in some cases, the amount of antifoaming agent selected through mixing test and flow coating test is extremely high, which is not enough to explain the problem and give evaluation to those substrates whose surfaces are not very smooth and contain air. When determining the optimal dosage of defoamer, the end use of the product should be fully considered

3. Roll coating test

the method of roll coating test is very close to the actual construction conditions of the coating, so that the effects of different defoamers can be distinguished. The addition amount can even be tested. During the roll coating test, use a porous sponge as a pad roller, and apply 40g of paint on the substrate without adsorption. It can be a special test card or other standardized test card. The total coating area is 500cm2. Before each test, wet the roller with the same amount of water to ensure the reproducibility of each test. The wet film coating rate is about 300 g/m2

after the film is dried, compare the experimental paper jam and evaluate the surface defects of the test plate. If the test panel is backlit in the standard light source and the bubbles showing as light spots are carefully observed, it is easier to distinguish the advantages and disadvantages of defoamers

the above mixing test, flow coating test and roller coating test are all helpful to test the effect and performance of defoamer, so as to determine the best defoamer type and dosage suitable for the product, so as to meet the requirements of defoamer for comprehensive factors such as substrate, system, printing or construction conditions

VI. the problem of "too many bubbles" in water-based ink and its solutions

we discussed the mechanism of bubble generation earlier. In fact, the phenomenon of bubble generation in water-based ink is particularly easy to occur when the open ink supply mode is used to print a large area of the field. When the circulating closed ink supply system is used, this disadvantage can be described as very small

the reason why water-based inks are prone to generate bubbles actually depends on its composition: water-based inks are generally composed of insoluble pigments, water-soluble or water-dispersible resins, surfactants, water, a small amount of ethanol and other additives. They belong to an alkaline system, that is, a saponification system, which is similar to soapy water or detergent. They are very easy to generate bubbles in the case of frequent turnover

conventional methods to reduce or eliminate bubbles include adding defoamer, controlling viscosity and pH value, mechanical factors, etc

adding defoamers is a direct and effective method, but it is worth noting that most of the defoamers produced by domestic manufacturers use silica gel. After excessive addition, it is easy to make the ink particles coarser, resulting in pinholes in the field patterns of printed products, small dents, turtle cracks and other defects in flexible plates. Therefore, the amount of defoamers added is very important, and it is best to keep it at about 0.5% to 1.5%. As for other aspects of defoamers, they have been described above

control the viscosity and pH value well because the ink is easy to produce bubbles when it is too thick or over diluted. The reason why the ink thickens is that the contact area between the ink bucket and the air in the open ink supply system is large, and the ammonia or amine volatile components in the ink volatilize and escape, making the alkaline system slightly out of balance. At this time, the most intuitive performance is that the ink viscosity increases and the pH value decreases. The method adopted is to add a stabilizer with a pH value of 11 every 30 to 40 minutes to supplement the alkaline ingredients

as for machinery, the front ink baffle can be used to appropriately increase the printing speed, so that bubbles are not easy to transfer to the embossing roller. Relevant policies can be formulated to support the implementation and utilization of new material industry. Make full use of the circulating pump to supply ink. The lower layer of ink in the ink bucket is relatively stable with light bubbles, which generally float on the surface of the ink and will not enter the ink chamber with the ink delivery pipe. Adding a front ink baffle can form a relatively stable semi closed ink area near the inner wrinkle roll of the open ink bucket, and keep the constantly generated bubbles out of this area, reducing the probability of bubbles approaching the wrinkle roll. Properly increase the vehicle speed to increase the pressure in the semi enclosed area and effectively limit the time of bubbles. These two functions are just consistent 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 pieces of waste cardboard into the ink flowing along both sides of the front baffle. At this time, the cardboard must be perpendicular to the direction of flowing ink. The trick is to control the insertion depth of the side ink baffle, which is neither too deep nor too shallow, at the boundary between the bubble layer and the ink layer

many printing houses also use manual ink skimming. When the machine is shut down, the ink in the ink bucket stops turning, and the bubbles originally wrapped in the ink float to the surface of the ink. At this time, you can use cardboard and other tools to skim the bubbles and reduce the times of adding defoamer, which also plays a role in maintaining the printing adaptability of the ink and reducing the depression of the flexo

VII. Summary

the function of defoamer is to inhibit and eliminate the generation of bubbles; Generally speaking, when the viscosity of water-based ink is too high and the pH value is low, or when the printing machine runs faster, it is easy to produce bubbles. If the amount of bubbles is relatively large, dirty versions will appear. Water based inks generally use alkali soluble binder, which is an alkali solution of acid resin, which is an aqueous solution similar to soap. Because the water-soluble binder has the characteristics of soap, foam will be generated in the ink bucket due to the mixing effect of the ink bucket roller, and the production of foam will lead to the quality problems of printed matter. Therefore, defoamer or antifoam agent must be used for water-based inks to control the generation of foam

antifoam agent can break or inhibit the foam that has been produced, and antifoam agent can prevent or inhibit the formation of foam. Antifoam and antifoam agents are generally water-soluble materials such as silicon, modified fatty acids, and alcohols such as ethyl ethanol. The anti foaming effect is usually related to the aging degree of the ink. The newly prepared ink is less likely to produce foam than the previously prepared ink

when using defoamer, attention should be paid to the mixing ratio of defoamer and water. Generally, it is recommended to use a ratio of about 3:1, and it is recommended to use a spray gun to spray on the ink surface in the ink bucket tank. Attention should be paid to the amount of antifoam agent or defoamer, otherwise high yield point and thixotropic rheological problems of the ink will be caused, causing local expansion of the printing plate to produce fish eyes and pits

source: ink information

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