Discussion on the application of the hottest defoa

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Discussion on the application of defoamer in solvent based wood coatings (I)

abstract through the definition of bubbles and the analysis of the causes of bubbles, this paper discusses the action mechanism, test and evaluation methods, and bubble prevention methods of defoamer, and then expounds the influencing factors of choosing defoamer for solvent based wood coatings.

keywords bubble defoamer wood coatings

1 Introduction

the problem of bubbles widely exists in paint production and construction, It may cause side effects such as prolonged production time, waste of energy and inaccurate measurement during coating packaging. In the process of film formation, if bubbles cannot disappear, it will cause defects such as film shrinkage, bubbles, pinpricks and so on. Therefore, it is necessary to solve the problem of bubbles in paint production and construction. This paper mainly discusses how to solve the problem of bubbles in the formulation design of solvent based wood coatings

2 what is bubble

some people define bubbles as gases that exist stably in liquid media. If the air is introduced into the liquid, the resulting bubbles are assumed to be spherical. If it is a pure liquid, that is, it does not contain surfactant, the bubbles formed will migrate to the surface, then rupture, the gas in the bubble disappears, and the liquid around the bubble is fused again, that is, the bubble is unstable. If in a liquid containing surfactants, such as dispersants, surface control AIDS, etc., the bubbles will become stable bubbles. These bubbles rise towards the air interface, but they are surrounded by a surface facial mask (interlayer) with surfactant. In addition, there is also a layer of surface facial mask on the air interface, which will become a stable film when a certain number of bubbles reach the air interface (the bubbles on the interface have a layer of bimolecular film, that is, bubble interlayer). In coatings, the typical interlayer thickness of spherical bubbles is several microns. These surfactants have a common feature - they can migrate to the gas-liquid interface of the system, where they reduce the surface tension

the bubbles in the paint are stabilized by several factors. First, modern coatings and coatings become very thin due to the stable interlayer, but one side effect is that these surfactants stably adsorb the air and penetrate into the coating during the manufacturing and construction process, which effectively promotes the implementation of energy-saving and low-carbon standards in the household appliance industry and forms stable bubbles. When the molecules move, the friction resistance is very large, so that the liquid in the interlayer cannot flow away, and the drainage is not fast. The electrostatic repulsion between surfactant ions on the interlayer also prevents the bubble from bursting. The second is due to the Gibbs elasticity of the bimolecular membrane (bubble interlayer). By stretching the film with surface active substances, this elastic effect will be shown. At this time, the surface of the bimolecular film is expanded, resulting in a local reduction in the concentration of surfactant. As a result, the surface tension increases. Relying on the accumulation of modification technology for 107 years and the support of a strong R & D team, the tensioned film gathers like an elastic skin, making the surface tension as low as possible

define the air bubbles contained in the honeycomb thin layer as macro bubbles and the fine spherical bubbles as microbubbles. In the solvent based coating system, the existence of air mainly leads to pinholes and bubbles. It is the micro pores produced by small bubbles when they escape in the process of high viscosity or paint film drying, which remain in the coating to form voids, that is, bubbles. During the drying process, due to the increased viscosity of the paint, the pore fine pipe will not close, and the air entrained in the system will be blocked or rise slowly to no possibility of flow in the process of rising and escaping, and the micro bubble pipe will remain in the dry paint film, that is, pinholes will be generated

3 causes of bubbles

3.1 air adsorbed on the particle surface

some pigments and fillers in the coating have a large specific surface area, and often absorb more air, which enters the coating during production and produces bubbles. 3.2 during the production, packaging or paint preparation of the mixed air, the air enters the paint and produces bubbles

3.3 reactive bubbles

there are side reactions during coating construction. Solvent based two-component polyurethane wood coatings contain isocyanate components, which are easy to react with water in the air to produce carbon dioxide gas, thereby generating bubbles

3.4 bubbles caused by base materials

wood coatings are mainly applied to solid wood, plywood pasted with bark, density board, particleboard or pasted board. Because wood is a porous material, there are many wood grains, pores, nozzles, etc., such as Manchurian ash, oak, etc. the gap is deep, the pores are more, and there is more air, so it is easy to produce bubbles when working on it. Therefore, foam comes from various production and application stages, such as pump conveying, mixing, dispersion, grinding, packaging and painting construction spraying stages, and is formed by air intervention in the system

4 prevention of bubbles

according to the stoke principle: VOC R, where: is the rising speed of bubbles;, Is the radius of the bubble; It is the viscosity of the coating system. It can be seen from this that under the condition of constant viscosity, the rising speed of bubbles is directly proportional to the square of bubble radius. First, small bubbles are close to each other, then converge into slightly larger bubbles, and then rise and escape, that is, large bubbles rise faster than small bubbles. Generally, when the viscosity increases in the drying process, the rising speed of bubbles slows down. On the contrary, the lower the viscosity of the system, the faster the rising speed of bubbles. Therefore, reducing the viscosity of the coating can significantly reduce the generation of bubbles

through the analysis of the generation process and causes of macrobubbles and microbubbles, the principle of bubble generation is understood. Therefore, some measures can be taken to avoid or prevent the generation of bubbles as far as possible

(1) first, we should selectively control the raw materials. When designing the formula, we should choose some raw materials with low bubble content or unstable bubbles, such as leveling agent with unstable bubbles, and try to eliminate the components of stable bubbles, whether it is imported foreign or domestic equipment

(2) adjust the production process in paint production, such as expelling the air attached to the surface of powder particles, which can make the powder slowly add to the resin base material, so that the two can be wetted and penetrated into each other. First stir at low speed and then disperse at high speed, which can significantly reduce the generation of bubbles; The method is as follows: set a common grid between two punctuation points

(3) effective air stripping can be carried out by adding wetting and dispersing additives, adding defoamer and defoamer to make the surface bubble burst or let the bubble escape from the surface as soon as possible and then burst

(4) the occurrence of side reactions during production or construction should be reduced. The bubbles introduced into the paint here must be eliminated before the curing of the paint film

(5) it is necessary to control the environmental conditions of coating construction, such as temperature, humidity and ventilation conditions. Bubbles are easy to occur in high temperature and humidity. At the same time, the diluent of paint accessories should be selected according to the weather conditions, because its quality, solubility, volatilization speed and dosage will affect the defoaming results

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