At the moment of vigorous development of the paint industry, the variety of coatings is becoming more and more abundant, the grade continues to improve, and people's expectations for their protection and decoration are also rising. In this context, the surface state of the coating has become the focus of attention, and the existence of bubbles is like a "rat droppings", which seriously damages the appearance of the coating film, causing many shortcomings such as shrinkage holes, pinholes, and fisheyes of the coating film, which not only weakens the protective function of the coating film, but also significantly damages its decorative effect.
Characteristics and classification of bubbles
In the production of coatings, bubbles are an interfering factor that affects the quality of products, and their characteristics are diverse. The bubble can contain liquid, steam, gas or crystal, and the size is affected by the adhesion between the coating and the substrate, the pressure in the bubble and the stretch of the coating film. According to their morphology, bubbles can be divided into bubbles and foams. Vesicles are small vesicles of a single spherical microscopic air dispersed in a highly viscous medium, commonly found in solvent-based coatings; Foam appears in the foaming medium, which is composed of a large number of bubble denses separated by a liquid film, and the foaming of water-based coatings mostly belongs to this category.
The cause of the formation of air bubbles
Improper surface treatment of the substrate
Contamination of impurities – wax, oil, dust, soluble salts and other substances on the surface of the substrate, as well as the presence of moisture, water droplets, volatile liquids, or rust and moisture remaining in the recesses of the workpiece, will cause the coating film to dry and adhere to the substrate in these areas. Moisture accumulates in areas of low adhesion to form a liquid or vapor phase, and the vapor pressure generated by thermal expansion becomes a key factor in the formation of bubbles when the temperature changes. In a hot and humid environment, vapor pressure and osmotic pressure act together to further aggravate the formation of bubbles.
Chemical reaction - uncleaned rust, oxide scale, etc. will react with some substances in the coating, or water, gas, and corrosive medium infiltrated from the microscopic channel of the coating film to form gas, coupled with the influence of ambient temperature and humidity, to promote the coating to blister. For example, the iron ions in rust react with acids in the paint to produce hydrogen, which forms bubbles.
The gases dissolved in the paint are released as the temperature increases
About 10% of the volatile solvent remains in the wet film formed by coating, and improper volatilization of solvents in the paint will cause more solvent residues. The solvent retention capacity is affected by the volatilization rate, external conditions, interaction with solutes, and adsorption performance of the filler. Residual solvents, like plasticizers, increase the absorption of water by the coating film and the transfer of moisture in the coating. If the coating itself is not firmly bonded, the water resistance will be reduced, and the adhesion will be lost, which will eventually lead to the blistering of the coating. At high temperatures, solvent evaporation accelerates, and the solubility of gases originally dissolved in the paint decreases, resulting in a large amount of escaping to form bubbles.
Residual gases from porous substrates
The solvent is easy to make the coating film blister on the porous substrate (such as cement and wood), especially when the organic coating is applied to the inorganic substrate and the surface temperature is high, the bubble formation speed is extremely fast, and it is easy to crack and expose the substrate. This is because there is gas left in the pores of the porous substrate, and the solvent in the coating displaces the gas when it enters the pores, and at the same time, the solvent volatilizes itself and produces gas, which together leads to the rapid formation of bubbles. However, this type of bubble can be solved by applying a penetrating primer, which has a low viscosity, thin film formation, and fast drying, which can fully penetrate into the pores of the substrate and block the gas escape channel.
Gases or liquids are adsorbed on the surface of pigments or fillers
Soluble fillers in coatings are at the root of many bubble ills. The soluble pigment absorbs the moisture through the coating and turns into a solution, and the water is pulled to the concentrated solution based on osmosis. The same can happen if soluble pigments contaminate any surface between substrates or coatings. As long as the paint contains soluble substances, there is a high probability of blistering. For example, some pigments containing soluble salts can attract and dissolve water in a humid environment, creating osmotic pressure that causes moisture to accumulate in the area, which in turn causes bubbles.
Control of coating blistering
Production management
In the production management of coatings, in order to effectively prevent and control the problem of bubbles, it is necessary to start from many aspects. The first is the control of the moisture content of raw materials, strictly controlling the moisture content of pigments, fillers and solvents, such as using drying and other means to make pigments and fillers fully dry and put into production, so as to avoid moisture damaging the stability of the coating system and causing bubbles. Secondly, the primer formula design is optimized to avoid soluble fillers as much as possible to reduce the risk of bubbles caused by soluble substances from the root. In addition, we pay attention to the balance of solvent volatilization, and carefully adjust the solvent formula through experiments to make different solvents volatilize in an orderly manner during the drying process of the coating, so as to prevent excessive solvent residue from causing air bubbles. Finally, for the high humidity environment, the binder with low permeation is reasonably selected to enhance the ability of the coating film to resist water penetration and improve its anti-foaming performance.
Product control
Strictly control the coating parameters, accurately control the coating thickness to reduce water permeability, control the surface roughness, and prevent leakage and cavities caused by roughness. After the substrate is pretreated, the surface should be clean, and the impurities such as electrolytes, dust, and grease that affect the adhesion should be eliminated, and the adhesion between the coating film and the substrate should be enhanced. In the use and storage of products, they should be kept away from high temperature and high humidity environment, and direct contact with water should be reduced. When painting porous substrates, apply a sealing primer with good permeability to remove the remaining air and solvent in the pores and prevent the formation of bubbles.
summary
The causes of coating blistering are complex, involving the selection of raw materials for coating production, formula design, as well as climatic conditions, operation specifications, substrate treatment and other links in the construction process. Only in the process of production and use, take effective measures and strictly control various factors, can we avoid bubbles to a large extent, obtain coatings with excellent decorative and protective properties, meet people's increasingly stringent performance requirements for coatings, and promote the continuous development of the coating industry in the direction of high quality.
