Common knowledge of coating: analysis of main defects of heavy-duty coating after coating

Core tip: after spraying of heavy-duty anticorrosive coating, all kinds of defects formed by the film will seriously affect the coating quality, even the product quality In addition to the quality of the coating itself, most of the defects of the coating are caused by incorrect coating construction "Three paint, seven paint." Therefore, in the coating construction, we must enhance the quality awareness, standardize the process and technical parameters, and improve the operation skills Once any abnormality is found, the construction shall be stopped immediately, the causes shall be analyzed, and corresponding measures shall be taken to reduce the occurrence of quality accidents This paper analyzes the main types of film defects and causes the film Application scope and advantages of roller ceramic coating Recommended introduction: the surface of ceramic rubber plate used for roller ceramic coating is cast into durable rubber plate by hundreds of independent small ceramic pieces Each ceramic piece has raised characteristics Under general conveyor belt pressure, thousands of unique cast bumps can produce positive traction, prevent slipping and extend the service life of conveyor belt roller At the same time, the bottom layer adopts high-quality rubber, which has strong elasticity and can play a good impact resistance role The scope of application of roller ceramic coating is one three six 83857180 (the same as V) 1 Roller coating in extremely harsh working environment 2 The belt joint is not suitable for fixing with iron buckle; 3 It is suitable for leather Coating home news: after heavy-duty anticorrosive coating spraying, various defects of paint film formation will seriously affect coating quality, even product quality In addition to the quality of the coating itself, most of the defects of the coating are caused by incorrect coating construction "Three paint, seven paint." Therefore, in the coating construction, we must enhance the quality awareness, standardize the process and technical parameters, and improve the operation skills Once any abnormality is found, the construction shall be stopped immediately, the causes shall be analyzed, and corresponding measures shall be taken to reduce the occurrence of quality accidents This paper analyzes the main types of film defects, the main causes of film defects, and puts forward the corresponding solutions, aiming to provide a reference for the coating construction 1 Sagging refers to the dropping or flowing of the paint film vertically down the surface after spraying In severe cases, the coating moves downward like a tear The phenomenon that the upper part of the coating is exposed and the lower part of the coating is crowded together is formed at the defect place Causes: from the perspective of coating technology, the main causes of film sagging are as follows: ① the dry film thickness obtained by one-time spraying exceeds the specified value ② More diluents are added to the coating ③ Improper spraying operation Take airless spray painting technology as an example: the nozzle is too close to the steel surface (< 30 cm), the spray gun moves too slowly, and the nozzle aperture is too large ④ The substrate temperature of steel plate is too high After the coating is sprayed on, the bottom coating is dried immediately to make the coating on it flow ⑤ After the two-component coating is mixed according to the proportion, the mixing is uneven, or after adding diluent, the mixing is uneven and sufficient, which will form sagging ⑥ The temperature of the construction environment is too low, or the air exchange rate of the spraying room is not enough, so that the solvent in the coating cannot be discharged in time, the drying and curing are not timely, and the sagging is easy to form ⑦ After the two-component coating has exceeded the mixed service life, the operator attempts to use it by adding diluent At this time, the coating will also form sagging Preventive measures: the main way to prevent the film from sagging is to operate and construct according to the technical instructions provided by the coating manufacturer ① When spraying based on the structure of the surface to be coated and the type of coating, the nozzle with proper aperture and fog amplitude shall be selected as the main part, and the pump pressure shall be adjusted as the auxiliary part to strictly control the amount of diluent added Frequently use the wet film tester to check the film thickness, control the wet film thickness or reduce the overspray ② Improve incorrect spraying technique and operation skill ③ Two component coating mixing or diluent mixing shall be fully stirred ④ According to the environmental temperature and the characteristics of different coatings, the mixing and maturing time should be mastered correctly ⑤ If the two-component coating exceeds the mixed service life, it is strictly prohibited to continue to use ⑥ After coating, during the drying and curing period of the coating, continue to ensure that the temperature and air exchange in the spraying room are sufficient Repair measures: ① the super thick coating deposited by sagging will eventually crack and fall off In the process of spraying construction, if any sagging is found, the coating shall be resumed immediately Remove sagging with a paintbrush and smooth the coating If there is film on the surface of the coating, it can not be treated with paint brush, otherwise, it will get worse ② After drying and curing, the coating can be modified as appropriate After flattening, or sanding with sanding paper or fine sanding paper, the original paint shall be used for repairing or recoating 2 Pinhole pinhole refers to the pinpoint like pinhole on the surface of paint film, which is the most taboo defect in painting It is formed because the air bubbles mixed in the paint film break and cannot flow flat before the paint film is dried and solidified Cause of formation: ① too thick paint film Under the condition of airless painting technology, the pressure of painting is high, the aperture of nozzle is too large, and after the film is too thick, air remains in the film and pinholes are produced ② In the process of air spraying, the oil-water separator fails, the air is not filtered, or the moisture is brought into the paint during spraying, or it is stirred violently for a long time, which will cause pinholes ③ There are many voids on the surface of metal spraying zinc, aluminum layer or inorganic zinc rich primer If the epoxy micaceous iron intermediate coat is directly sprayed, the air in the voids will be intercepted, resulting in pinholes ④ In high temperature environment and steel surface temperature high painting construction ⑤ Excessive diluent or non matching diluent is added to the coating ⑥ In some automatic painting devices, the paint heater is not turned on as required Preventive measures: ① avoid construction in high temperature environment, add appropriate diluent with slow volatilization (consult the technical service personnel of relevant coating manufacturers), so as to slow down the solidification of paint film surface ② When the surface temperature of the coated object is too high, it is not suitable for construction ③ In the case of high ambient temperature, it is not suitable to apply too thick at one time; it is not suitable to add excessive diluent or non matching diluent into the coating ④ When mixing the paint, try to avoid air mixing If there is air mixed in, it can be properly used for a period of time to let the mixed air out before use ⑤ For recoating on porous and rough base material or primer, pre spray the permeable sealing layer (also known as mist spray) first to drive away the air in the gap, that is, add 15% - 30% of the supporting diluent to the original construction coating, or use the special "sealing coating" to spray once, covering without omission After spraying, there will be a large number of small bubbles on the surface of the coating, but soon after the bubbles burst, the coating will make the small holes closed Wait for 10-20 minutes according to the ambient temperature, and then spray according to the process requirements Repair measures: ① remove one layer of slightly small "half eye" pinhole with sandpaper, dilute it properly with original matching paint after cleaning, and repair one layer with paint brush thinly, so that the paint is easy to fill the pinhole ② For the deep missed pinholes, they must be ground with sanding paper or fine sanding paper until they are basically flat If pinhole is serious, the coating should be removed basically After cleaning, repair or spray again according to the original matching coating ③ If it is a large area of deep pinhole, it is necessary to use fine sand paper to roughen, clean and then spray the sealing layer for construction 3 After the coating is sprayed, small bowl shaped pits are formed on the paint film, and there are often dropwise or strip shaped substances in the center of the pits, with the edge raised The paint film can't be evenly attached and uneven Some of them shrink to zigzag, bead like and speckled like water sprinkled on wax paper Most of them are round with a diameter of 0.1-2.0mm, which makes the paint film damage and expose the bottom layer This is one of the most serious film defects on site It is generally called "shrinking" or "laughing" for the large area of the amorphous, and "shrinking hole" for the round area If there are particles in the round hole, it is called "fish eye" There are epoxy, polyurethane and some solvent volatile coatings which are easy to produce shrinkage cavity Causes: in the coating construction, the main causes of shrinkage are as follows: ① after the A and B components of the two-component coating are mixed, the curing time is not enough, resulting in poor leveling of the coating ② The surface of coated steel is too smooth, such as without sand blasting, oil removal, wax removal, etc., or polluted by organic silicic acid mist ③ The previous coating is too hard or too smooth ④ When the steel surface is degreased and cleaned, the organic solvent is not matched ⑤ Special steel plate For example, in the container industry, we often encounter some special steel plates, whose surface is smooth, hard and rarely rusted Due to a layer of silicone oil (up to 0.9 mm) pressed into the steel surface, there will be even shrinkage holes with larger diameter after painting ⑥ There is water on the steel surface, or there is a large temperature difference between the coated steel surface and the coating Preventive measures: ① before recoating, carefully clean the surface of the coated object ② If it is necessary to apply solvent, it is necessary to select the solvent matching with the coating ③ Before the recoating construction, carefully check the film-forming and drying conditions of the previous coating, especially the surface cleaning conditions ④ For bright, smooth and hard coated surface, it needs to be sanded ⑤ Pay attention to the type and characteristics of steel plate Repair method: ① after the shrinkage of the coating, wipe it with solvent in the wet film state to completely remove the defective film, then polish and clean the surface of the previous coating, and then repaint it according to the original matching coating ② For the dry paint film surface with shrinkage cavity, the top coat can only be recoated after all sand blasting or grinding 4 When the spray particles of the dry spraying heavy-duty anticorrosive coating reach the surface of the steel to be coated, the liquid has become dry, unable to form a continuous coating film In serious cases, it is like sand particles, and the coating adhesion is poor, which will cause serious corrosion Causes of formation: ① during construction, the temperature is too high (> 35 ℃), or the paint is sprayed outdoors, encountering strong wind ② The distance between the spray gun and the steel plate is more than 40 cm The walking gesture of the workers is arc or tilt, and the moving speed is too fast ③ Painting pressure is too high, not matching with nozzle ④ The coating is too thin, or high temperature spray fast drying coating Preventive measures: ① improve the skills of operators and standardize the operation and construction Especially when airless spraying is carried out under the temperature of more than 30 ℃, it must be done that: the minimum pressure (pressure enough to atomize the coating), the distance between the spray gun and the coated object shall be appropriate (30-40 cm), the nozzle with small spray width and the spray gun with large diameter shall be used ② Reasonably arrange the construction sequence, avoid adverse climate (such as strong wind) factors, reduce waste, and properly cover the adjacent parts ③ If spraying fast drying coating, apply appropriate diluent to slow down the evaporation rate Treatment method: on the polluted surface with paint mist flying, it is necessary to scrub, clean, wipe and clean Only after the rough polluted surface is restored to its original flat appearance, can construction continue 5 The crack of the crack coating is a serious condition of the film defects There are fine cracks, crazing and mud cracks in the coating The fine crack is the micro crack on the surface of the coating, but it will not penetrate the substrate; the crack is more serious than the fine crack, and the crack develops to penetrate the coating and expose the substrate; the mud crack causes the coating to be damaged seriously, so that the coating completely loses the adhesion and falls off by itself Causes of formation: ① cracks generally occur when the coating is cured and dry If the temperature difference of the coating changes greatly in the drying process, the cohesion of the inner and outer layers of the coating is inconsistent, which is easy to cause cracking ② The thickness of the first coating film is too thick By convention