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    Home > Coatings News > Coating Technologies > [Popularization of Coating Society] Causes and control methods of air bubbles in paint

    [Popularization of Coating Society] Causes and control methods of air bubbles in paint

    • Last Update: 2021-04-18
    • Source: Internet
    • Author: User
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    With the rapid development of the coating industry, the varieties and grades of coatings continue to upgrade, and people have put forward higher requirements for the protective and decorative properties of the coatings.
    Therefore, the surface condition of the coatings is very interesting.

    The presence of air bubbles seriously affects the appearance of the coating film, and often causes the coating film shrinkage, pinholes, defects, fish eyes and other defects.

    Therefore, the problem of coating bubbles has not only affected the protective effect of the coating film, but also greatly affected the decorative effect of the coating film.

    (Paint porosity) Characteristics and classification of bubbles In the paint production process, bubbles appear as an interference factor, causing surface defects in the application of the coating product, which will damage the appearance and also affect the corrosion resistance and weather resistance of the coating film.

    Bubbles have sizes, and the bubbles can contain liquid, steam, other gases or crystals.

    The size of the bubble mostly depends on the adhesion of the coating to the surface of the substrate, the pressure of the liquid or gas in the bubble, and the extent to which the coating film is stretched and reversed in order to maintain a balance with the adhesion of the coating substrate.

    According to its shape, bubbles can be divided into two types: bubble and foam.

    Bubble refers to a single spherical tiny air bubbles dispersed in a high-viscosity medium.

    Most of the bubbles produced by solvent-based coatings are of this type.

    Foam occurs in a medium that is prone to foaming.

    When this type of medium is dispersed, due to the high filling density of air, a large number of bubbles are often produced.
    Because the density of air and liquid are very different, the bubbles will rise to the liquid surface quickly, forming a small amount of liquid that depends on the liquid film.
    Open bubbles in a dense body.

    Water-based paint blisters are mostly of this type.

    (Bubble on the metal surface after coating) The cause of the blistering is usually accompanied by the aging process of the coating.
    In this sense, blistering can be divided into two types: expansion blistering and corrosion blistering.

    The blistering of the coating film usually has the following reasons.

    PART1.
    Improper surface treatment of substrates Oxides, soluble salts and other substances on the surface of the substrate are the inducing points for coating blistering. The surface of the substrate is contaminated with wax, oil, dust, soluble salt, etc.
    , or there is moisture, water droplets, volatile liquids on the surface of the substrate, or there is rust and moisture in the recesses of the workpiece during surface treatment, causing the coating to dry Later, the coating in these areas is not firmly attached to the surface of the substrate, and moisture accumulates in low adhesion areas, causing liquid or gas phase to accumulate in these areas.

    When the temperature changes, the vapor pressure generated by the thermal expansion is the main reason for the formation of bubbles.

    If it is in a hot and humid environment, the dual effects of vapor pressure and osmotic pressure will aggravate the generation of bubbles.

    Unremoved rust, oxide scale, etc.
    react with certain substances in the coating or water, gas, and corrosive media seeping from the microscopic channels of the coating film to generate gas.
    In addition to the influence of ambient temperature and humidity, it will also cause the coating to start.
    bubble.

    PART2.
    The gas in the paint is released as the temperature rises.
    In the wet film formed by painting, there are still about 10% of the volatile solvent, the other about 90% of the solvent has been volatilized into the atmosphere during the painting process, and some solvents in the paint Inappropriate volatilization can also cause residue and form bubbles.

    The retention capacity of the solvent is not only related to the volatilization rate of the solvent and external conditions, but also to the interaction of their solvents and the adsorption of the filler.

    The solvent remaining in the paint is like a plasticizer, which will soften the coating film.

    Because the solvent has a certain sensitivity to water, the residual solvent can increase the absorption of water by the coating film and the transfer of moisture in the coating.

    If the adhesion of the paint itself is not strong, its water resistance will decrease, which will cause the loss of adhesion and cause the coating to foam.

    PART3.
    Residual gas solvents on porous substrates can also cause the coating film to foam on porous substrates (such as cement and wood).

    Generally, organic coatings are coated on inorganic substrates, and blistering is prone to occur when the surface temperature is high.

    At this time, the bubble generation speed is extremely fast, and it is easy to break and expose the substrate.

    Such bubbles can be eliminated by applying a penetrating primer.

    Penetrating primers usually have low viscosity, thin films, can dry quickly, and have enough time to penetrate.

    Avoiding high temperature time construction can also avoid such bubbles.

    PART4.
    Adsorption of gas or liquid on the surface of pigments or fillers.
    Soluble fillers in coatings are the cause of many bubble defects.

    The soluble pigment absorbs the moisture that penetrates the coating and becomes a solution.

    Due to osmosis, water will be drawn through the coating to the concentrated solution.

    In addition, if the soluble pigment contaminates any surface between the substrate or the coating, the same phenomenon will occur.

    Therefore, as long as the coating contains soluble substances, blistering is inevitable.

    After understanding the causes of drumming and pinholes, find a suitable solution.

    (Coating bulging) To solve the problem from two aspects: strengthening construction management and correct painting method of coating 1.
    Avoid bad weather such as high temperature and humidity during construction, keep the surface humidity of the substrate above 3℃, and the relative humidity not higher than 85%; 2 , The substrate is treated thoroughly, the surface is kept clean, and there is no residual electrolyte, dust, oil and other impurities that affect the adhesion of the paint; 3.
    (Key point) When brushing the paint, use multiple thin coats to increase the brushing time and make the interlayer solvent sufficient Volatile, reducing the residual amount; 4.
    Brushing with cross method to ensure uniform thickness of the paint film;
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