Drum coating abnormal phenomenon and prevention method (2)
Shanghai Xugong Coating Technology Co., Ltd. Xu Guoxing
(B) Some paints do not dry after coating for a long time, the reasons for total stickiness and its prevention methods
After some paints are applied, the drying time of the paint film exceeds the specified standard and it does not work. This phenomenon is called slow drying. Some paints are dry for a few days. When the finger touches the paint film, there is still sticky finger, or when the paint film is dry, it is not sticky. After a long time, it is softened and sticky. This phenomenon is called back sticking. The two morbidities of slow-drying and stick-backing occur almost at the same time. The reasons are:
1 The paint itself is of poor quality. The resin content of the coating formulation is low, or the solvent used is a poorly volatile solvent or a non-drying and semi-drying oil; in addition, the paint that is stored for a long time or the paint that is made of tung oil with a low tempering temperature , It will also make the film slow and sticky. Therefore, paint users must purchase the paint after the paint must be tested according to the standard.
2 coating is too thick. Most of the single-component normal-temperature white dry paints are formed by oxidation drying. If the coating is too thick, oxidation is limited to the surface. In this way, after the film is formed on the surface, the solvent of the underlying layer cannot be evaporated, and the coating film is not sufficiently dried.
3 Mixing different types of paint or using an excessive amount of driers can cause the surface conjunctiva to close, making the inner layer dry slowly and even stick back.
4 The construction environment is poor. The coating is affected by cold, rain, smoke, acids, alkalis or moisture; it is painted in damp, airy rooms; during rainy and foggy days, a large amount of moisture accumulates on the work piece; during the cold season When slow-drying coatings are applied, no suitable amount of driers are added to the coatings, which can cause slow drying or sticking of the coating.
5 improper surface treatment. The residual oil, acid, alkali, salt and other debris on the surface of steel drums, especially old steel drums, have not been completely eliminated. If the primer is not dried, applying a layer of paint, etc. will cause slow-drying or partial stickiness, and some will not even dry for a long time.
6 improper diluent. The use of kerosene, diesel, etc. as a diluent may also occur.
To prevent the paint from slow drying, it is necessary to select a high-quality paint that is suitable for the above-mentioned circumstances. The paint should be thin and thick; different types of paint cannot be mixed; the surface of the drum must be pre-treated to remove dirt before painting. And moisture; primer is not dry and can not be painted second coating: For long-term recoating of the coating must use solvent or thinner to wash off all the coating, and then re-painted. Do not cover the new paint without treatment, so as to avoid unnecessary trouble.
(3) Causes of bite bottom when coating the second pass and methods for preventing it
In the coating, sometimes a short time after the second coating is applied, the first time the coating film will automatically expand, shift, shrink, wrinkle, bulge, and even peeling. This phenomenon is biting the bottom. There are generally 3 types of causes for bite bottom:
(1) The surface of oil-based paints such as oils and fats, alkyds, etc., if it encounters a coating containing a strong solvent, especially when the top coat is very thick, the bottom coat can easily be swollen by the solvent in the top coat. muster.
(2) When the perchloroethylene type paint is not yet dry, add a layer of the same kind of paint; the nitro- or perchloroethylene type paint may also be applied before the short oil is transferred to the dry alkyd primer. Bite at the end.
3 When steel drums are to be primed, primers and topcoats are required. If the bottom and finish are not matched, bite bottoming may occur. Even if it is a matching primer and finish, it should wait for the primer to dry before applying the finish.
In order to avoid biting the bottom and improve the adhesion of the coating, attention should be paid to the compatibility between the bottom and the top coating, and it is best to paint with the same kind of paint and thinner. When the conditions are not met, the alkyd paint can be mixed with the amino paint; the phenolic paint, the ester glue paint, the calcium ester paint can be combined with the oily paint blend, the clear oil and the thick paint; the lacquer and the perchloric paint, except In addition to supporting use, it can also be used in the lower layers of various slow-drying coatings. In addition, pay attention to the time between two layers of coating and the appropriate solvent. It is not easy to bite the bottom.
If you need to coat two layers of paint and require not very high steel drums, you don't have to use a special primer, just apply the same two layers of paint. Be sure to wait until the first layer is dry before recoating the second layer. If the second layer of paint is not very thick, applying the same two layers of paint will not happen.
(4) Causes of many small particles on the surface of the coating after drying and methods for preventing it
The surface of the coating has granular debris, which not only affects the appearance and gloss of the steel drum, but also affects the smoothness of the coating. The main reasons for producing particles are:
1 The gel is produced when the paint is stored, or the paint crust is shaken into pieces, mixed into the paint, and has not been filtered at the time of painting.
2 During the storage process, the paint deteriorates (base material is precipitated, thickened, pigment aggregated, etc.).
3 coating itself quality problems. If the base material and the solvent do not match, there is an insoluble resin; the fillers and pigments are not finely ground and do not reach the specified fineness.
4 The coating site is not clean, dust, wind sand falls on the surface of the un-dried coating film; residual particles or sand on the paint brush remain on the coating film during construction.
5 When spraying, the distance between the spray gun and the steel drum is too far. Before the paint falls on the surface of the steel drum, the solvent in the paint has volatilized, causing the paint to lose its fluidity and form particles.
6 Others, such as spraying tools dirty, too thick coating, spray suddenly change the weather (from the wind) will produce particles.
Based on the above reasons, it is not difficult to know what measures to take to prevent the formation of coating particles. If you do a good job of cleaning the environment and tools, paints should be filtered before use to improve the spraying method. After the particles appear on the film, generally after the film is thoroughly dried, use warm sandpaper to warm the soapy water, carefully level the particles, slide, clean the dust, and then evenly spray (or brush) them again. Kind of paint.
(5) After coating, the causes of pitting (partial shrinkage, formation of spots, and the appearance of bottom layer) on the surface of the coating film and their prevention
The main reason that the coating film generates pits is that the paint is less wettable to the surface of the drum. Poor wetting is generally caused by the following factors:
1 The surface of the steel drum is too smooth (such as cold-rolled steel drums without surface treatment), or the primer is too smooth and the top coat cannot be uniformly attached, so the surface of the cold-rolled steel drum must be treated with oil-phosphorized liquid. Primers that are too smooth to cause burrs are generally available with a hard bristle brush. When the top coat is not yet dry, it is forced to alternately brush on the film alternately or diagonally and alternately several times. The pits disappear naturally. In addition, it is also possible to use a fine sandpaper to add a rough coat of primer, and then to apply the topcoat without producing pits.
2 There are oil, water, acid and alkali stains, wax, etc. on the surface of the steel drum, or a large amount of smoke after painting. For these reasons, the pits need only be removed from the surface of the drum to avoid smoke. If partial pitting has already occurred, a small amount of solvent can be used to wipe off the pitted part of the cotton yarn and re-apply the paint.
3 During the spraying process, the compressed air is not passed through the oil-water separator, so that the water in the air and the oil inside the air compressor are mixed with the paint and sprayed on the surface; or the solvent with extremely fast evaporation (such as acetone) is misused in the spray paint. ), the paint film is too late to level, the solvent has evaporated, and thus produces pitting. In this case, it is better to wash off the solvent when the paint is not dry, and install an oil separator in the air circuit.
4 In the rainy season, on cloudy days or in wet places, the surface of the drums will be exposed to water and pitting will also occur. As long as the construction of sunny days and avoid the drum surface water.
5 Cooling paint on the surface of hot drums. Suddenly the coating heats up, the solvent in the coating rapidly evaporates, and the coating is too late to cause flat spots.
6 The spray gun is too close to the surface of the drum, or the nozzle of the spray gun is too small, and the spraying pressure is too great to damage the coating.
7 In the deployment of two-component coating when the mixing is uneven or after deployment without maturation immediately coated, it is easy to produce pits. In general, when mixing any two-component coating, it should be thoroughly mixed and it should be used after a certain period of time according to the length of the two-component coating curing time. However, the quick-drying two-component steel drum outer coating newly developed by the Shanghai Chemical Cooperage Factory has overcome this shortcoming. It can be used immediately after mixing the two components, and it has been dried after about 10 minutes. It is a glossy coating like enamel.
For the coating film that has generated pitting, it should be washed out with solvent. If it has already dried, it must be polished with emery cloth before recoating.
8 There are quality problems in the paint itself, such as too much silicone oil in the paint, inappropriate high boiling point and low boiling point solvents, etc. This is not a problem that our coating users can solve. If we have this procedure for incoming inspection, we can negotiate with the paint production unit.
(6) The causes of pinholes after the film is dried and methods for preventing it
In the drying process, the coating forms a number of small round circles, shaped like needle-punched holes called pinholes. The diameter of the pinhole is generally small, and the largest is only a few millimeters. Sometimes such pinholes form cones or hemispheres.
In the steel drum industry, pinholes generally occur in lacquers that are sprayed with compressed air or have a low pigment content.
The principle of pinhole formation is similar to that of pitting, which is generally considered to be due to poor wetting on the surface of steel drums.
1 The surface of the steel drum is damp or oily, and the temperature in the construction environment is low.
2 The construction viscosity is too large, and the coating is too short to be placed after stirring, and bubbles remain inside the paint.
3 The improper combination of solvents and the excessive use of low-boiling volatile solvents cause the coating surface to rapidly dry and form a film, and the solvent at the bottom is not easy to escape. In this way, during the entire coating drying process, pinholes are formed as the surface stress of the coating increases and the impact of the solvent vaporization pressure in the bottom coating forms.
4 Some steel drum plants use bituminous paint as a paint inside the drum. If the paint is baked with gasoline as a diluent, serious pinholes will occur.
5 In spray painting (especially gas spraying), the spray pressure is too large, the nozzle diameter is too small, the distance between the spray gun and the steel drum is too close, and the impact on the coating is too large to produce pinholes.
6 There is moisture in the paint and there is dust in the sprayed compressed air.
7 The coating of the drying type coating is too thick to enter the drying tunnel (drying room) too early.
8 Apply lacquer or vinyl chloride paint over 30°C.
Therefore, in addition to the emphasis on pre-treatment, it is also necessary to grasp the nature of the coating and its corresponding construction requirements. In general, small amounts of methyl cyclohexanone or chlorinated paraffin are added to cellulosic coatings; some turpentine or ethyl cellulose are added to ester-based paints; and traces of ethylene glycol butyl ether are added to polyurethane coatings. Prevents pinholes in the coating. For the coating film that has already produced pinholes, except that bituminous paint, polyethylene powder coating, etc. can be used to remove the pinholes by the re-bake method, it is preferable to wash the coating film with a solvent and re-coating it.
(7) Causes and prevention methods of orange peel produced by coating film
Orange peel mostly appears in the coating film of spray coating (especially spraying nitrocellulose paint and other quick-drying paint). It is a kind of semi-circular protrusion appearing on the surface of the coating film, resembling orange peel-like phenomenon.
The main reasons for this are the low boiling point solvents in the paint and too much volatilization. This results in strong convective microcurrents in the stationary liquid coating, which makes the coating convex around the middle and concave in the middle, showing semicircular protrusions. The orange pattern <...\ is not evenly leveled and the surface has dried to form orange peel. In addition, the spray pressure is too high, the viscosity is too high, the nozzle is too small, the distance between the spray gun and the steel drum is improper, the steel drum season (especially the hot rolled steel drum with rust), etc., will cause the coating film to be too late. Produces orange peel.
In the spraying process, a small amount of high-boiling solvent (such as cyclohexanone, etc.) can be added to the paint to prevent the dream from being overshadowed, and the pressure of the spray, the size of the nozzle, and the viscosity of the paint should be paid attention to.
(8) Causes of foaming after coating film is dried and methods for preventing it
Foaming of a coating film refers to irregular irregular projections (bubbles) of uneven size on the surface of the coating film, and the inside of the coating film is filled with liquid or gas. This phenomenon is likely to occur in high humidity environments or after immersion in water and in the sun. Blisters and pinholes are different. After the foaming, the partial coating film will crack, leaving a circular void but no core, while the pinhole is not yet cracked and has a core.
Foaming of the coating is mainly caused by improper construction, and of course there are environmental factors. mainly:
1 Paint on the surface of steel drums containing moisture. When the coating film is dried, moisture in the interior evaporates and expands to bulge the coating film and form bubbles. You can feel the elasticity by pressing the air bubbles with your fingers. When the pressure is applied, the air bubbles will also expand or break through the surface.
2 The primer is applied to the finish before it is dry. In this way, when the solvents and diluents in the primer are heated and vaporized, they are covered by the coating film of the cover, and the bubbles do not escape.
3 The surface of the steel drum is not very smooth, especially the heat is shifted second, and there are many pits. In the surface treatment, the moisture accumulated in these pits is not completely removed, and the iron matrix in these parts will expand due to localized rust under the coating film to bulge the coating film, or when the moisture expands, the coating film drum will be applied. From.
4 When the coating is applied under strong sunlight, if the coating is too thick, the coating on the surface is dried and filmed by exposure. After the heat is transferred to the inner layer, the solvent in the coating rapidly volatilizes and is blocked, causing the coating film to foam.
In view of the above reasons, the method for preventing blistering of the coating film is to pretreat the surface of the steel drum before the coating is applied, and in particular to remove the moisture on the surface of the steel drum. During construction, it should be avoided to contact the surface of the steel drum with contaminated hands; after the steel drum is painted, it should not be exposed to sunlight or high temperatures. The appropriate paint type should be selected according to the environment in which the paint is used; do not spray or brush the paint. Too thick, if you want to get a thick coating, it should be divided into several brushing; if the coating has existing bubbles, it should be decided according to the disease situation is a partial repair, or re-painting after eradication.
(9) Causes of peeling of coating film on the surface of steel drums and their prevention methods
After the coating is dried, partial peeling occurs, and even if the entire coating film is separated from the surface of the steel drum or separated from the primer, it is called peeling. The reason is:
1 The surface of the steel drum is unclean, stained with oil, dust, moisture, oxide scale, chemicals, etc. The coating is not firmly bonded on the surface of the drum, causing peeling and peeling.
2 The surface of the steel drum is too smooth or improperly surface-treated and no phosphating treatment has been performed.
3 Primer and topcoat are not matched or stained between layers, which may cause large areas of peeling.
4 baking baking type coating, the baking temperature is too high or too long, the coating is too thick.
5 In the double-coating of perchloroethylene paint or polyurethane paint, if the first layer is completely dry and then coated with the second layer, the first layer may be too hard and smooth to make the coating The combination is not good, causing the coating to fall off.
6 poor paint quality. Coatings containing too much rosin or thinner, poor adhesion, prone to film peeling.
As mentioned above, to avoid falling off the coating film of the steel drum, it is necessary to find out the reasons and apply the medicine according to the above analysis. The most important thing is to emphasize the surface treatment before painting. If coating film peeling has already occurred, clean the coating film and repaint it.
(10) Causes of wrinkled skin after dried film and prevention methods
Wrinkle skin appears mainly on the coating of oily paints. During the drying process of the coating film, the coating surface dries faster and forms a conjunctiva due to the difference in drying speed of the coating film between the inner layer and the outer layer, and the contact between the inner layer and the air is blocked, and the drying of the inner coating film is affected. In this way, the tension on the outer surface of the coating film surface is greater than the tension on the contact surface with the inner layer, resulting in the skin layer shrinking upwards, thereby forming wrinkled skin. The specific reasons are as follows:
1 dry fast coating and slow dry coating blend. The quick-drying paint is dried first and forms a coating film, while the slow-drying coating material is slowly formed into a coating film, thereby causing the film to wrinkle.
2 brush coating coating viscosity is too large, the coating is thick, resulting in the inner layer of the coating is too slow to dry the surface, causing wrinkles.
3 uneven coating paint, resulting in uneven thickness of the coating, especially in steel drum ring ribs, corrugated and curled corners of the paint accumulation in excess, thick skin will wrinkle.
4 When the paint is applied or just after coating, it will be exposed to high temperature and sunlight exposure to dry the paint surface in advance. The interior is too dry to form wrinkled skin.
5 There is a problem with the quality of the paint. Excessive cobalt manganese driers, for example, can also cause wrinkles. Therefore, dry fast coatings and slow dry coatings should not be blended: the viscosity of the coating should not be too large during construction; the coating should be uniform in thickness, with particular attention to the rims of the steel hoops, corrugations, and crimps. The excess paint is collected at the brush, and the coating should not be too thick. After the paint is applied, it must not be exposed to the sun or exposed to high temperatures.
(11) Causes of Cracking of Coatings and Preventive Measures
After coating, due to various factors, the phenomenon of cracking, cracking, and cracking will soon occur, collectively referred to as cracking.
Fine cracking is a phenomenon in which the coating film is not easily recognizable by the naked eye after coating, and fine lines that can be observed with a 10x microscope are required. It is caused by the internal shrinkage force of the coating film exceeding the cohesive force of the coating film itself: it is not detrimental to the aesthetic appearance of the coating film and is the inevitable result of the coating film aging. If this happens soon after painting. That is a morbid condition.
Rough cracking is a noticeable, coarser crack produced by the coating shortly after coating. Sometimes it can even reach the bottom.
Cracking occurs after the coating is applied to the coating and resembles a crack in the turtle's back.
Most of the cracking of the coating film is the result of the aging of the coating film. This is due to the effect of oxidation over a long period of time. If the coating film loses its elasticity, cracking will occur. This is not a pathological condition. However, if the brush cracks soon, it is the coating quality problem. The reason is:
1 If the primer film is too thick, it is not equal to a transparent finish, or a paint with a long oil degree is used as a primer. Covering the finish with a short oil degree will cause cracking due to insufficient elasticity of the top coat. And the thick place shrinks even more, and even exposes the primer.
2 Use short oil paint or epoxy paint as outer coating of steel drum. Because of its poor resistance to ultraviolet radiation, it will crack soon after use.
3 There is air pollution at the steel drum coating site, such as sulfur dioxide and ammonia gas.
4 Paint quality problems, such as adding excessive amounts of volatiles or driers to the paint, can also cause cracking.
The prevention of cracking of the coating should be corrected for the above reasons. If the film has cracked, it should be completely removed and repainted.
(12) Causes of brush marks (brush lines) when hand-brushed and their prevention
In the application of paint, as long as the paint has no quality problems and is properly constructed, the paint will generally be leveled by its surface tension. When this surface tension is affected, leveling occurs and a brush mark appears.
The influencing factors are:
1 The application viscosity of the coating is too high or the diluent used is too volatile.
2 The paint is mixed with water, so that the paint particles in the paint flocculation at the oil-water interface, reducing the paint's fluidity.
3 Use a bristle brush or an oversized paint brush to apply quick-drying paints such as lacquer, acrylic plastic paint, etc.
4 The pigment content in the paint is too high or the pigment is locally condensed.
The phenomenon of brush marks when brushing steel drums is mostly caused by high viscosity of the paint or quick-drying paint. In order to prevent the occurrence of brush marks, the viscosity of the paint can be adjusted appropriately; the paint brush used should be suitable, not too small and not too soft or too hard; for the quick-drying paint, the spraying method is preferably used.
There are many abnormalities in the coating and coating processes that occur during the coating process, but these are the most common problems encountered in our steel barrel industry. Of course, there are many accidental encounters that are not listed, such as powdering, floating color, and the like. Due to limited space and infrequent encounters, it is no longer necessary to write more.
As long as we implement the inspection of incoming paint; surface treatment of steel drums prior to painting; appropriate coating process according to the nature of the paint; environmental considerations when painting, coating devices, paint viscosity, etc., steel drum industry in the paint And abnormalities that occur during painting can certainly be prevented.