Discussion on the application of the hottest defoa

2022-08-19
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Discussion on the application of defoamer in solvent based wood coatings (Part 2)

5 the action mechanism of defoamer

defoamer is used to avoid the formation of bubbles or eliminate the formed bubbles as soon as possible, so defoamer generally should have the following three properties: ① selective incompatibility; ② Have a high distribution capacity; ③ Have low surface tension. Defoamer must have a certain solubility in the coating before it can migrate to the air interface. The compatibility is too good to migrate the inlet bubble wall, but into the whole coating system. At this time, the defoaming effect is minimal. However, if the solubility of defoamer is too small, it will lead to paint film defects such as shrinkage, fog shadow, etc. An appropriate defoamer should find a "balance point" between compatibility and incompatibility

the defoamer must be combined with the stable surfactant layer and penetrate into the bubble film bimolecular membrane. Then it must spread quickly and pass through the cracked surfactant layer. Compared with the previously stable interlayer surfactant, the elasticity of this film has been significantly reduced. This instability leads to the cracking of the molecular membrane, thus defoaming. Defoamer has low surface tension and can defoaming. Defoamers are used to eliminate bubbles in coatings. Sometimes it is helpful to distinguish "defoaming" from "defoaming". First, bubbles must reach the surface. This process is called defoaming, and then bubbles break on the surface, which is called defoaming. The function of defoamer is to improve the rising speed of bubbles. These additives make the adjacent bubbles gather and merge with each other, and then because the volume becomes larger, the speed of moving to the surface is accelerated. The defoamer must play this role effectively in the whole coating. On the contrary, defoamers are only effective on the surface, and they only destroy bubbles rising to the surface. In fact, it is impossible to clearly define the working mechanism of an auxiliary agent as defoaming or defoaming, and it may be more appropriate to use defoaming agent in some occasions. As a defoamer, the feasible chemical structure should be some molecules with low surface tension, such as mineral oil, silicone and its modified compounds, silicone free polymeric compounds, fluorocarbon compounds, etc

6 test and evaluation of defoamer

the most important role of defoamer is defoaming, defoaming and breaking bubbles, so as to achieve the purpose of eliminating bubbles in the production process, and it is easy to eliminate bubbles in the construction. There are several methods to test the effect of defoamer. The fastest and easiest method is to put the same amount of the same resin into the test tube, add different kinds of defoamers with the same dose, and after shaking for the same time, observe the speed of bubble rise and rupture, and record the time. The one with the shortest time is the best. At the same time, it can also add different amounts of the same defoamer, record the rising and breaking time of bubbles, and test the best amount and cost performance. Therefore, combined with the cooperation of other additives, the best amount can be obtained. However, this method can only be used as a preliminary test, and it is finally determined that it should be tested in the paint formulation system, because its formulation involves resins, pigments and fillers, solvents, and other additives when the sensor is subjected to external tension or pressure, which will affect the behavior of bubbles. Add different defoamers or different amounts of the same defoamer to the coating formulation system (when other factors remain unchanged), and select an effective defoamer by observing the elimination of bubbles in production and construction spray (brush) board and performance test. In fact, there is also the most critical point: the evaluation of defoaming agent cannot take the effect of the newly prepared coating after construction as the final judgment, otherwise it will cause problems. After the coating is prepared, the rapid storage method should be adopted, that is, the coating should be stored in a constant temperature oven at 60 ℃ for 1 month, and then it should be taken out for construction, and the surface effect should be observed. If it still has defoaming effect and the construction does not produce paint film defects, the effect is the same as that just prepared, then it can be explained that this defoaming agent is effective. Or through natural storage, test the surface effect of defoaming and construction after half a year or one year. Because many defoamers will lose their original defoaming function after storage for a period of time, and even produce paint film defects

it can be seen that the stability of defoamer in the system is particularly important. In solvent based wood coatings, there may be defects due to the poor selection of defoamers, such as the gloss reduction caused by the poor compatibility of Defoamers in gloss varnish, the haze of paint film in gloss (Matt) varnish, which affects the transparency, some produce shrinkage, some affect the adhesion between layers, some even cannot defoaming, etc., so the evaluation of defoamers should be made after comprehensive and comprehensive testing, otherwise it will cause adverse consequences, As a result, the paint is neither decorative nor protective

7 selection and application of defoamer

in solvent based wood coatings, various factors should be considered when selecting defoamer

7.1 paint formulation system

the formulation design must be comprehensively considered. From the perspective of composition, there are resins, pigments and fillers, solvents and additives

resins include acrylic resin, alkyd resin, nitrocellulose, etc. the miscibility and defoaming effect of defoamer in them may be different, which should be tested and evaluated according to the above methods

pigments and fillers also have a great impact. For example, when producing transparent primer, two different kinds of domestic and imported zinc stearate are used, which are very different. Imported products are not easy to produce bubbles when dispersed, while domestic products are very easy to produce bubbles. The former can choose a relatively weak defoamer, while the latter needs a strong defoamer to eliminate

in terms of solvent, due to different surface tension, different solubility may have foam stabilizing effect

there are many kinds of additives in the system, including wetting dispersant, anti sedimentation agent, leveling agent, drying agent, etc. How to mix them with defoamer to produce a good overall effect needs to be determined by experiment. In the bright varnish, the compatibility between defoamer and resin should be considered first. If the compatibility is good, the appearance of the paint is clear and transparent, and if the compatibility is not good, it is easy to produce turbidity. On the other hand, defoaming agent should have certain defoaming effect in this system, and its dosage should also be considered, so as to find a balance point between compatibility and incompatibility, so as to achieve the purpose of defoaming and clear appearance. The defoamers that can be selected for this system include silicone polymer defoamer, silicone free polymer defoamer or both. In matte varnish, the compatibility of defoamer is also very important, but this is often ignored in the design of formula, thinking that the appearance of paint cannot be seen. In fact, this is not the case. It must be judged by observing the state of the paint and the curing agent after forming a film. The transparency of the paint can be observed by scraping the glass plate to see whether there is fog or haze. Sometimes when mixing paint, in order to achieve strong defoaming effect, a large amount is added, which often brings another side effect: shrinkage. In the topcoat, if the defoamer selected is improper or too strong, or contains organic silicon, if it is not matched properly, it may lead to poor adhesion between the coating and the bottom layer, so it is also used as a beautiful design element of the vehicle series. Therefore, when designing the formula, we should comprehensively consider the influence of various factors in the system and choose the best combination

7.2 factors of substrate

there are different substrates in wood decoration, such as solid wood, bark, sticker, MDF, plywood, etc., so the coating requirements are also different. Different materials in solid wood also have different requirements, such as Fraxinus mandshurica board, which has deep seams and many holes. At this time, the primer is required to have good defoaming property and fast defoaming. When choosing defoaming agent, its strong defoaming function and rapid defoaming and rapid defoaming ability should be considered. Silicone and polymeric defoaming agent can be used together. Beech wood is relatively easy to defoaming, because its surface is flat and there are few pores. Sticking bark board is similar to solid wood. The pasteboard is different. The paster is very flat and smooth, and the air is not easy to seep down. It is mainly to eliminate the bubbles that the air seeps into the paint during spraying, and finally achieve the purpose of breaking bubbles through defoaming. The MDF is relatively loose and flat, and the coating is easy to infiltrate. In this case, the defoamer that is neither too strong nor too weak should be selected to achieve the goal. The defoaming property of the finish coat should consider the adhesion with the primer, defoaming ability, and it is not easy to shrink. The high-efficiency polyurethane pultrusion process is used to make holes. At this time, the defoamer without organic silicon polymer or silicone modified defoamer can be selected

7.3 adding method of defoamer

due to the different types of defoamers, some are silicone type, some are polymer type, some are mineral oil type, and their relative molecular weight is also different. Some defoamers are easy to disperse into the system, some need high-speed dispersion to enter the system, some can be added before coating production grinding or dispersion, and some can be added later. These should be determined according to the performance introduction, use instructions or system of defoamer products. In fact, the addition method is also very important, because it involves the storage stability and construction effect in the future. This must be clearly indicated in the production process of the product, otherwise the product quality will not be guaranteed

7.4 construction methods

there are usually several construction methods of wood coatings, such as spraying, roller coating, drenching and brushing. The first three are mainly used in furniture manufacturing industry, among which UV wood coating is mainly applied by roller coating or drenching to a certain load. The latter is mainly used in indoor home decoration. These different construction methods have different requirements for the selection of defoamers, and the requirements of brushing are higher than that of spraying. Therefore, the defoamer selected for wood decoration paint should combine the one containing organic silicon and the one without organic silicon to defoaming, defoaming and breaking bubbles. Spray coating is more difficult than roll coating defoaming. It is necessary to consider not only the selection of defoaming agent with certain defoaming property, but also the stability of spray screen. In a word, different defoamers should be selected according to different construction methods or matched and supplemented to meet their requirements

Zeng Jin

(Guangdong Shunde Hongchang Coating Industry Co., Ltd

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