Steel drum coating abnormality and prevention method (1)
Steel drum coating abnormality and prevention method (1)
Shanghai Xugong Coating Technology Co., Ltd. Xu Guoxing
One or the other abnormality sometimes occurs when storing or using steel drum paint. Since most of our barrel factories have no inspection measures after purchasing the paint, once we find that the abnormal phenomenon is not clear, the quality of the paint is not good, or it is improperly used or stored. In this way, it is impossible to take corresponding measures to remedy, resulting in poor quality of the steel drum coating or waste of paint.
This article discusses the abnormal phenomenon in the use of coatings in the steel drum industry, and hopes to help the steel drums.
First, the paint is not normal phenomenon and prevention method
(1) The cause of turbidity or gelation of paint after opening the barrel and its prevention method
The turbidity of the coating is mainly for pigment-free, transparent coatings (such as varnish, cooked oil, etc.). The main problems encountered with our steel drums are varnishes (including nitro varnishes, novolacs, alkyd varnishes, amino varnishes, epoxy varnishes, etc.), so varnishes are used to illustrate this problem.
Normal varnish should be transparent, which causes turbidity. In addition to poor manufacturing quality, there are several main reasons:
1. Low storage temperature, high humidity and temperature difference. Since some varnish contains a small amount of impurities such as moisture and wax, these impurities are dissolved in the varnish at a higher temperature, and are easily precipitated when exposed to low temperatures, thereby causing varnish turbidity. Therefore, the storage temperature should not be too low (if the conditions are met, the storage temperature of all coatings is preferably between 15 ° C and 28 ° C), especially in the northern part of the country, it is best to place the paint bucket on the wooden frame to make the coating The bucket does not touch the ground. In addition, large humidity or large changes in temperature difference can also cause varnish turbidity. The varnish is slightly turbid and can generally be used as usual. When the turbidity is severe, the water may be removed by heating, or the water may be separated by a centrifugal method.
2. The diluent is not used properly. Various varnishes are generally solutions of the resin in a solvent. Different resins require different solvents. Its storage stability is mainly determined by its solubility in the solvent. If an improper solvent is used to dilute the varnish, the solubility of the resin will decrease, causing turbid turbidity.
In general, the nitro varnish should be a mixture of butanol, benzene (or toluene), butyl acetate, ethyl acetate, etc.; the novolac should be diluted with solvent gasoline or xylene; For acid varnish, use a mixture of xylene, solvent gasoline No. 200 and turpentine as a diluent (the alkyd varnish used on the spot can be diluted with solvent gasoline or xylene No. 200); the amino baking varnish should be treated with butanol and The mixture of xylene is diluted; the epoxy varnish is diluted with a mixture of xylene and cyclohexanone.
3. The varnish quality is not good or the storage period is too long.
1 Resin problem: The resin in the coating is generally chemically synthesized. Resin of poorly synthesized resin during storage may continue to undergo a slow chain growth reaction, resulting in an increase in molecular weight and a decrease in solubility, resulting in turbid turbidity. If it is slightly turbid, a small amount of the corresponding diluent can be added. If the coating is turbid after 10% of the weight of the coating, the barrel coating can no longer be used.
2 desiccant problem. Generally, the normal temperature drying varnish contains a desiccant such as manganese naphthenate, cobalt naphthenate, zinc naphthenate or lead naphthenate. When there is too much lead naphthenate, the varnish will be cloudy during storage. The prevention method is also solved by adding a diluent.
4. Turbidity can also occur when mixed with different properties of varnish.
(2) Reasons for coating crust after opening the barrel and methods for preventing the same
Coating crust is a common phenomenon, mainly in oil-based or resin coatings containing tung oil. This type of coating forms a crust when it comes into contact with air. In addition, too many drill-type driers in the coating are also easy to crust.
After the coating is crusted, if the skin layer is thin, it is easily broken into small thin skin pieces and mixed into the coating, which affects the surface quality of the coating film; if the skin is thick, although it can be easily removed, the unit consumption of the coating increases.
The method of removing the crust is simply filtering. The method of preventing skinning is to cover a surface of the coating with a slow-releasing solvent, such as turpentine oil or a high-boiling coal tar solvent, so that the surface of the coating is isolated from the air, and then stirred evenly when used.
(3) The reason why the paint is thickened after opening the barrel and the prevention method thereof
Thickening of the coating means that the viscosity of the coating increases during storage, but it does not deteriorate completely. The main reasons are:
1. The coating container is not well sealed, causing the solvent in the coating to evaporate.
2, the problem of the paint itself, such as the newly produced tanning material, the official paint price is too high and the pigment is alkaline, so that the chemical reaction will continue in the storage process, so that the paint thickens.
3. As described above, if the synthetic resin in the coating does not have a reactive group or has not been terminated, the reaction will continue during storage to thicken the coating.
4. Asphalt paint is also thickened with solvent gasoline No. 200 as a diluent, so xylene should be used as a diluent.
If the paint is not thick enough and can be evenly integrated after dilution with thinner, it can still be used. If it has become too thick, or has become a block, it can no longer be used. Therefore, it is best not to store the paint for too long.
(4) Reasons for the swelling of the paint and its prevention method
The coating swells somewhat like the third problem, which is an increase in viscosity. But what is different from the third point is that the swelling is a large increase in the viscosity of the coating during storage, and finally becomes a thick slurry, even in the form of a gel.
The paint in this state is analyzed. If it is false, the original viscosity can be restored by strong stirring.
False thickness occurs mainly in high pigment coatings. During storage, the pigment particles and the macromolecules in the coating base attract each other due to the difference in potential, and gradually form a gel-like integral. This state is unstable and will be destroyed by strong shearing force, so it can be recovered by strong agitation. This false thick phenomenon is not a deterioration of the paint but a thixotropic change.
If the paint in this state does not return to its original viscosity by vigorous agitation, the coating may have been livered or gelled.
Paints and base-based pigments with higher acid value during storage will undergo saponification reaction to form a hard gel, just like pig liver, so it is called liver. Once the liver is produced, it is not easy to recover, and the paint in which this happens is preferably disposed of. If it must be used, it can only be used as a primer when the liver is mildly added and the diluent is evenly mixed.
If the synthetic resin in the coating is over-polymerized or the coating is heated, the base of the coating and the pigment particles will condense in the entire gelatinous form, called gelation. Paint in this case can only be scrapped.
(5) Reasons for precipitation after the paint is opened and its prevention method
The precipitation of the paint is mainly for the paint. We know that the pigment in the coating is not dissolved in the solvent of the coating but dispersed in the solvent. The downward gravity of the pigment dispersed in the paint is relatively balanced by the buoyancy and interfacial tension of the paint, but the balance is not permanent. Once the surrounding environment changes, the stable conditions are affected, and the pigment is affected. Will precipitate down. The main reason is:
1. The storage temperature is too high. If the storage temperature exceeds 28 ° C for a long time, the viscosity of the coating decreases, and the suspension properties of the pigment in the coating are destroyed, and precipitation occurs. Therefore, the storage temperature of the general coating is preferably below 28 °C.
2. Storage time is too long. As mentioned above, the suspension balance of the pigment in the coating is not permanent. Over time, the pigment is slowly affected by the influence of gravity, and eventually settles into a large amount of precipitate. Therefore, during the storage process, it is best to shake the paint bucket several times every few days and change the orientation (horizontal, inverted) o
3. There is a problem with the quality of the paint itself. If the pigment particles in the coating are too thick, the viscosity of the coating is too low, the quality of the resin in the coating is not good, etc., so after the purchase of the coating, it should be tested according to the coating standard.
4. Too much diluent is added and the precipitation is accelerated upon storage.
For the precipitated coating, if it can be re-uniformed by stirring, it can still be used; if it has become a hard block, the upper paint can be used in a small amount of the same paint, and the hard part can only be discarded.
Second, the color is not normal in the painting and its prevention method
(1) The reason why the color after painting does not match the template and its prevention method
Paints tend to discolor during storage for the following reasons:
1. The pigment fades. The most notable is the blue or grass green paint with iron blue. Due to insufficient oxygen in the container, iron blue is reduced by fading. After a period of exposure to air, the color can be restored. In this way, as long as it is fully stirred during use, it is brought into contact with air, and the color of the paint will be the same; it is basically consistent with the sample.
2. A multi-color paint made of several color paints, due to the different specific gravity of the pigment, in storage, the ratio is significant. The pigment sinks. Faster, sinking to the bottom layer, the color of the upper and lower layers of the paint is different. After mixing, the color can be restored. Green paint such as iron blue and chrome yellow, sinking chrome yellow for a long time. If it is not stirred evenly during use, the applied color will appear blue, so be sure to stir well when painting.
3. The metal pigment is discolored. A coating containing a metal powder as a pigment loses its vivid luster due to the corrosive action of the free acid in the coating resin on the metal powder, and turns black and green. For example, special metal paint for steel drums, although cheap and excellent in coating quality, when the storage time is more than one month, the beautiful metallic luster will gradually darken, and this change is more obvious after more than 50 days. Therefore, it is best to use this kind of coating when buying, and use it as you like, and it is best to use it for the month, and stir it thoroughly. Now, Shanghai has developed a special metal paint for two-component steel drums with metal pigment paste and paint paint. As long as the two components are thoroughly stirred, they can be used. It doesn't matter if the paint is stored for a long time.
4. Some coating containers should be special. If the container used by the paint factory or the user is improper and stored for too long, some of the components in the coating react with the packaging container, causing the color of the coating to change. The alkyd resin in the alkyd varnish reacts with the iron drum and tends to turn red; the novolac varnish contains loose, oil-saving, and is stored in the iron drum for too long, which is easy to produce reddish brown pigment, which makes the color of the spine deeper; The solvent in the base paint is also highly corrosive to the iron drum, which causes the nitro paint to be brown in color. because! The coating is preferably contained in a plastic bucket or a galvanized bucket, and a small amount can be contained in a porcelain jar or a glass bottle.
(2) Reasons for whitening of coating film and its prevention method
The whitening of the coating film refers to the phenomenon that the coating film appears turbid, the gloss is reduced, or even whitish during the drying process. The phenomenon of whitening mainly occurs in the drying of the coating material (such as nitrocellulose paint, acrylic plastic paint, etc.). After the white coating is whitened, the appearance is greatly affected, and the quality of the coating is greatly reduced.
The main reasons and methods of prevention are as follows:
1. In spray application, moisture is brought into the paint due to failure of the water separator or moisture in the compressed air.
2. In the drying process, the solvent evaporates quickly, so the surface temperature of the coating drops rapidly. If the humidity of the construction environment is large, part of the water vapor in the air is deposited on the surface of the coating film. White mist, causing turbidity and loss of light.
Since the culprits of the film are from water or moisture, the oil-water separator should be inspected regularly, and the coating tools and compressed air should not carry water. Fast-drying coatings are best not to be applied in seasons and in humid or cold weather. If it is necessary to construct, it is best to raise the temperature of the construction environment to above 15 °C-20 °C, and the relative humidity is not more than 75%. In addition, the addition of a slower volatility in the fast drying coating prevents or reduces the whitening of the coating film.
For coatings that have already been whitened, it is best to wash them with solvent and repaint them. Of course, if the user requirements are not high. The coated steel drum can also be baked in a drying oven (oven) and repainted.
(III) Reasons for the loss of light and reflection of the coating film and methods for preventing the same
Loss of coating film means that after coating with light coating (not matt or matt paint), the coating film is dull; coating film reversal means that there is light when the coating is applied to film, but in the short term (hours As a result, the gloss gradually diminishes for several weeks.
The reasons for the loss of light and the refraction of the coating film are basically the same:
1. Rough surface of the steel drum (such as hot-rolled steel drums, especially untreated with floating rust) can not show the proper brightness after the coating is applied. This can only be solved by proper surface pretreatment (rust removal, phosphating) or by applying a primer.
2. The surface of the steel drum is not clean, and it is accompanied by impurities such as acid, alkali, oil, wax and moisture. This is a matter of common knowledge of painting. Any coating must be cleaned before application.
3. Kerosene or diesel is mixed in the paint. Due to the poor solubility of kerosene and diesel to the coating, it is easy to make the coating thicker, so that the coating film is semi-gloss or matte.
4. Excessive amount of diluent. The content of solids is reduced, the number of paintings is small, and the thickness is not reached, resulting in a dullness of the coating film. For this purpose, the amount of diluent should not exceed 10% of the amount of coating.
5. Some of the coatings contain too much pigment, and when used, they are not evenly mixed. The upper half of the barrel has less pigment, more resin, and light after painting; the remaining half of the barrel has more pigment, less resin, and gloss after application. Worse.
6. Several different performance coatings are mixed. Adding too much moisture-proofing agent to the nitro varnish can also cause loss of light. 7, nitrocellulose paint must be painted on the smooth and smooth bottom layer to be bright. If you only apply 1-2 times, it will not show light.
8. When applying quick-drying coatings (such as acrylic plasticized coatings, nitrocellulose lacquers, etc.), the climate is humid, or the steel drum is moved from the cold to the hot place before the coating film is dried, and the coating film is condensed with water to cause loss of light.
9. If the primer is not dry, the paint finish may also cause loss of light.
10. Using the light-fast abalone coating, the film is exposed to sunlight and will soon lose its luster.
11. Excessive amount of low-boiling diluent in the drying type coating, or too fast into the drying tunnel (drying room) causes the diluent to evaporate too quickly, and the coating film is difficult to lose light. Therefore, the drying type paint generally has to wait for the leveling before it can enter the drying equipment, and it is preferable to use a special diluent.
12. The coating itself has a low resin content and a high pigment content.
After the film is out of light, it should be repainted after drying. Find out the cause of the loss of light, and deal with it in a targeted manner to prevent loss of light and reflection.
(4) Reasons for uneven gloss after drying of the coating film and methods for preventing the same
After the surface coating of the steel drum is dried, sometimes the gloss on the coating film is better, a little worse, or there is a light. These undesirable phenomena are called uneven gloss. The reason is:
1. When the paint is applied, the surface of the steel drum has moisture or moisture, which causes the gloss of the coating film to be uneven after drying.
2. When the thicker alkyd paint is applied, the paint pile is not swiftly opened due to uneven brushing, so that the thick part of the paint is easy to be lustrous, and the gloss of the rest is poor. Therefore, the whole film is coated. Look, it is uneven gloss.
3. When brushing acrylic plastic paint, nitrocellulose paint and other fast-drying paints, the number of times of brushing back on the film is too much, or when it is too late to brush, and the speed of the spray gun is not uniform when spraying, the film will appear after drying. Uneven gloss.
A remedy for uneven gloss. It is generally remedy by recoating a layer of the same varnish. If the gloss is noticeably uneven, apply the fine sandpaper to polish the film and repaint the coating.
(5) The reason why the film appears to be similar to the hoarfrost when the plum is ripe and its prevention method
After a few days or more than ten days of coating, the surface of the coating may sometimes be partially or even covered with a layer of fine particles similar to the maturity of the plum when it matures. This phenomenon is called blooming. The reason is that there is smoke, mildew and moisture in the construction environment. The stronger the water resistance of the coating, the greater the likelihood of blooming in a humid environment. In addition, the addition of a solvent that is very volatile in the coating is also prone to this phenomenon. In addition, some barrel factories prefer to add a drier to the paint to make the paint dry quickly. It is necessary to note that if a cobalt-based drier is added to the alkyd paint, it is most likely to bloom. Therefore, the environmental quality of the painting construction is also very important.
(6) When a light-colored paint is applied to a dark primer, sometimes dark colors will permeate. The cause and how to prevent it.
The bleed of the coating means that after the topcoat is applied, the primer is softened or dissolved by the topcoat, and the color of the primer penetrates into the topcoat to discolor the topcoat and the appearance is affected. The reasons for bleeding are as follows:
1. Apply a primer when the primer is not dry. For example, when applying blue or green alkyd enamel or blending paint on the dried red phenolic phenolic rust paint, it is easy to make the color of the primer penetrate the surface of the topcoat.
2. The primer is an oily paint, and when the top coat contains a strong solvent, it is easy to dissolve the oil paint to partially bleed the top coat.
3. Before the primer is applied, the dirt on the surface of the steel drum is not removed, and the paint film is bleed.
4. The white topcoat is applied to the red or brown primer, and the topcoat will ooze red or brown, especially the nitrocellulose bleed.
5. Others, such as topcoats on asphalt primers and very slow drying primers, can be lacquered by applying a topcoat to a primer containing an indigo pigment or creosote.
If you want to achieve non-bleeding, first plan to finish the topcoat after the primer is dry. Second, try to avoid the primer and the topcoat combination with a dark primer and a light color. Third, the steel drum must be Fourth, when applying different colors of nitrocellulose lacquer, the amount of diluent should be appropriately reduced in the topcoat, and the coating should be thin to make the coating film dry quickly. Fifth, the volatilization speed is fast. Dilute the topcoat with a thinner that dissolves the underlying coating film. If bleeding has occurred, the bleed portion can be scraped off, sealed with lacquer, and then coated with a layer of paint.
Third, the cause of other abnormal phenomena in the painting process and its prevention methods
(1) Reasons for the sagging phenomenon of the coating film during the painting process and its prevention method
The phenomenon that the coating film flows downward during the painting process is called sagging. It is a pathological condition that affects the appearance of the film, and it appears in the vertical surface, the steel barrel curl and the ring rib. Common sag ills are tear-like and curtain-like.
Tear-like sag is usually found at the edges and corners of the steel drum and the rim. Generally occurs in brush construction. The main reason is that after brushing, the remaining paint on these parts is not cleaned up in time, so that the remaining paint flows to the surface of the coating film to cause tear marks. The remedy for this sag is to gently level the track with a brush or finger when the flow marks are not dry. If the sag is dry, use a knife to gently level the tears, or use a sandpaper to smooth the traces and then cover the lacquer.
Most of the curtain-like sag occurs in the spraying process, mainly in the vertical plane of the object to be coated. Since most of the steel drum industry is to paint the steel drum vertically, this phenomenon mainly occurs on the barrel; if the steel drum is sprayed horizontally, the steel barrel body and the bottom cover may occur. When this happens, it looks like the curtain is drooping, which is very unattractive. The main reason is that the nozzle aperture of the spray gun is too large, the amount of paint is too much, or the distance between the spray gun and the steel drum is too close when spraying, the moving speed of the spray gun is too slow, and the air pressure is too large. In the manual painting process, the coating film is too thick or the coating is too thin, the brush used is small, the bristles are too soft, or the coating itself is slow to dry, and the curtain sag is also generated.
The remedy for curtain sag depends on the actual situation. Usually, when there is such a problem in phenolic, ester, and calcium ester coatings, the sag can be brushed again with a clean brush and turpentine, so that the sag can be dissolved again, and then the curtain is pushed open with a brush. If the sag is dry, you can use the wide scraper) 9 to level the paint and then paint; if the alkyd paint appears sag, use the alkyd paint thinner to wet the curtain, then use the brush to push open and brush flat; The lacquer of the nitrocellulose lacquer can be wiped off with the same solvent and then sprayed; for the nitrocellulose lacquer film that has been dried, the sag can be smoothed with coarse sandpaper (but do not wear the coating film). ), then spray the coating 1-2 times.
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