During the construction of steel anti-corrosion coatings, it is often found that the coating blisters are caused by various reasons. Therefore, different coating defects such as shrinkage cavities, pinholes, and fish eyes are caused. The problem of coating blistering has not only affected the protective effect of the coating film, but also greatly affected the decorative effect of the coating. The article "Cause Analysis and Control Measures of Blistering Defects in Anti-corrosion Coatings" by Fan Zhiyong and other scholars mainly elaborates on the characteristics and classification of blisters, the causes of blistering, and control measures.
Characteristics and Classification of Bubbles
Blistering can be caused by a variety of coating maladies. A common feature of the blistering area is that the blisters first develop where the film adhesion is weakest. Bubbles vary in size and exist in single or group form, so they can be divided into bubbles and foams according to their shape. When the bubble is broken, the bubble may contain liquid, steam, other gases or crystals, but it may also be a dry bubble. According to the substances contained in the bubble, it can be divided into air bubbles, water bubbles and slag-added bubbles. Blistering usually occurs during coating curing or coating aging. In this sense, blistering can be divided into two types: expansion blistering and corrosion blistering. In any case, the result of blistering may be that the size of the bubbles becomes larger and aggregates together to form large delamination areas of the coating film, resulting in surface defects, resulting in a decrease in the corrosion protection effect of the coating, or even disappearing. If blistering occurs in the primer (the first coating that contacts the steel surface), it may also cause a potential difference on the steel surface, resulting in a primary battery with a large cathode and a small anode that accelerates electrochemical corrosion, causing pitting corrosion. .
cause of blistering
1. Moist steam foaming
Bubble formation Although there may be a variety of causes in the first place, the most common cause is the presence of moisture vapor or other vapors such as air or solvent vapors in the coating. Concentrated areas of paint delamination (i.e., areas of blistering) may occur at the coating-substrate interface or between coats in multi-coat systems. Blistering usually occurs first as the coating is heated in the morning sun and then in the cold and humid conditions of the night, when the steam inside the coating expands due to the increase in temperature . The size of the bubbles depends on the adhesion , the pressure of the liquid or gas in the bubble, and the degree of stretching and reverse jacking of the paint film in order to maintain a balance with the adhesion of the paint substrate . There are several possible reasons for such blistering:
(1) Soluble pigments in paints. Certain pigments are able to , or even draw moisture into the film for hygroscopicity. When used in a multi- coating system, after the next coat is cured, moisture vapor may evaporate from the pigment to form blisters in the next coat due to temperature rise and other reasons.
(2) Soluble chemical salts. If the surface of the substrate or coating is not properly prepared, soluble chemical salts are retained on the substrate or between coats after the subsequent coat is applied, and penetration will occur , drawing more moisture into the coating, resulting in overcoating. The layer is bubbly.
(3) Other pollutants. The presence of contaminants such as oil, wax or dust on the substrate or on the surface of the coating to be overcoated can lead to poor adhesion of subsequent coats. Moisture vapor will enter such areas through the coating and remain there . When the steam expands or evaporates due to temperature changes, etc., the moisture is likely to take advantage of the weak points of poor adhesion and form bubbly.
2. Solvent volatilization, coating film curing and foaming
The way the paint forms a protective coating will affect the surface state of the coating. The paint manufacturing industry generally believes that paint curing can basically be divided into two categories: non-conversion type and conversion type. The coatings of these two curing methods will cause blistering under certain conditions .
(1) Non-conversion coatings. The solvent in the non-conversion coating is released as the coating cures and the temperature rises . When the wet film is formed by coating, there are still some volatile solvents, and other solvents have been volatilized into the atmosphere during the coating process. Other coatings Inappropriate volatilization of the solvent in the medium can also cause solvent residues. The retention capacity of solvents is not only related to the volatilization speed of solvents and external conditions , but also related to their interaction with solutes and the adsorption of fillers. Residual solvents in the paint act like plasticizers and soften the film. Since the solvent has a certain sensitivity , the residual solvent can increase the water absorption of the coating film and the transfer of moisture in the coating. If the coating itself is not bonded firmly, its water resistance will decrease , which will cause loss of adhesion and cause the coating to blister.
(2) Conversion coatings. During the curing process of the conversion coating , the surface temperature of the steel surface may be lower than the ambient temperature, resulting in the rapid curing of the surface coating to form a film, and the solvent or thinner volatilized during the curing reaction of the underlying coating is ejected from the coating surface. bubbly.
3. Chemical reaction foaming
Improper coating matching, chemical substances produced by the reaction between the remaining and the paint can also cause blistering.
(1) Incompatibility of the coating. Sometimes when choosing a primer it is not known what topcoat will be used. If an unsuitable primer is used (eg: alkyd universal primer) and then overcoated with a high performance paint, the system may not have the necessary adhesion or physical properties to provide suitable adhesion for the high performance paint, resulting in Solvents in some high-performance coatings can cause the primer to pull away from the substrate by bulging, blistering.
(2) Surfaces that are specially treated, such as galvanized surfaces, fresh galvanized surfaces may remain acid or alkali that remains on the galvanized surface during passivation treatment , and may react when the surface is subjected to new coating construction , resulting in foaming .
4. Blistering in cathodic protection
Cathodic protection (CP) systems are often used to reinforce coatings for dipping or burial applications. Blistering is usually associated with higher voltages (or overvoltages) near the sacrificial anode , or with the impressed current being set too high. Under these conditions, the blistering was partly due to the generation of hydrogen gas from ultra-high voltages touching the steel substrate . This is usually due to coating defects such as pinholes or low film thickness where the cathodic protection system can focus its energy to damage the coating. For coatings their cathodic disbondment resistance performance should be tested first. This coating should have high adhesion strength and high dielectric strength.
5. Blistering on poor coated surface
Residual gas on porous substrates can also cause blistering of the coating film. Generally, organic coatings are coated on inorganic substrates, and foaming is prone to occur when the surface temperature is high. At this time, the generation speed is extremely fast, and it is easy to rupture and expose the substrate. Such blisters can be eliminated by applying a penetrating primer. Penetrating primers typically have low viscosity and thin films that dry quickly and allow enough time to penetrate. Such bubbles can also be avoided by avoiding high temperature time construction, etc.
涂覆特殊表面状态的涂层时也容易产生起泡现象，如无机锌涂料吸收水分固化后，其表面状态多孔，并附着大量锌盐，如不进行处理直接涂装下度涂层常会因其多孔的表面状态产生大量的气泡。施工人员可施工稀薄涂料（添加大量稀释剂稀释的涂料），能够渗透无机锌涂料多孔表面，这种特殊设计的涂料称作过渡涂层或者防泡层。
6、涂装的不正确操作造成起泡
（1）快干涂料如乙烯和氯化橡胶涂料最易出现起泡。由于采用了不适当的喷涂压力产生喷雾或溶剂逃逸，快干表面使油漆中的空气不能带走，从而形成起泡，此种表面气泡产生较多，容易产生泡沫。
（2）在涂料喷涂施工中由于喷枪离被涂表面太近，而迫使气泡进入涂层也是涂层起泡的一个重要原因。施工人员应保证适当调节喷枪，在距表面的很好的距离施工涂料，确定涂料在表面上充分流动，润湿表面并以多道薄层和重叠 50％的方式施工涂料也能有效地防止起泡。
（3）为使涂料充分固化，所有溶剂需要全部挥发，所以，挥发型涂料不应施工太厚的涂膜。如果施工过厚涂膜，可能会有溶剂残留在涂层内部。残留溶剂处在溶剂挥发后最终变为空隙，形成气泡。
涂层起泡的控制措施
综上所述，According to涂层起泡的原因分析涂层起泡控制可从下面三个方面下手：一是涂料生产过程控制，比如溶剂挥发率或树脂含水率、颜填料吸湿性等；二是涂层设计，比如不同类型涂层的搭配、阴极保护区域内涂料的选择等；三是涂料施工时的控制。
First of all, production management must be strengthened. During the production of coatings , strictly control the water content of the pigments, fillers and solvents used ; in the primer formula, try to avoid the use of soluble fillers; when using solvents, take into account their volatilization balance and avoid excessive solvent residues; in high humidity environments, use Binder with low permeability Secondly, the construction should be carried out according to the design of professional coating designers , or the coating matching system certified by the certification agency should be used as much as possible to avoid coating defects caused by unmatched coating construction. Finally, construction management should be strengthened . Avoid high temperature, humidity and other adverse weather conditions during construction. The temperature of the surface to be coated should be kept above , and the relative humidity should not be higher than 85% . If the coating encounters rain, condensation, frost, etc. before construction, Make it dry before painting the next coat of paint construction should be carried out according to the paint manual under the guidance of the paint company's technical service , and reasonable construction and repair methods should be adopted, paying special attention to the two-component The coating should have a certain aging period, so as to avoid uneven mixing and reaction heat, causing air bubbles; control the thickness of the coating to reduce its water permeability; control the final state of the surface treatment , pay attention to surface roughness and surface cleanliness. If the roughness is too large , leaks and voids will occur, which will inevitably cause air bubbles; the surface be kept clean, and impurities such as electrolytes, dust, grease, etc. that affect the adhesion of the coating should not be left;Avoid high-temperature and high-humidity environments, and reduce the chance of direct contact with water; when painting porous substrates , you should first apply a sealing primer with good permeability or spray an anti-foaming layer to remove residual air and solvents in the pores as much as possible things.
