Main quality indicators and inspection methods for steel drum coatings (2)
Shanghai Xugong Coating Technology Co., Ltd. Xu Guoxing
4, fineness
Fineness refers to the size or uniformity of the dispersion of particles such as pigments and fillers in the paint, calculated in microns. The quality of the coating can affect the gloss, water permeability and storage stability of the coating. Due to the variety, the fineness standards required for various coatings are different. In general, the finer the fineness, the better the gloss, but the too fine and too thick will affect the storage stability. Therefore, we require that the fineness of various coatings should comply with the relevant standards. Currently, the most commonly used measure of fineness is the squeegee fineness meter. It is constructed as a polished tool alloy steel plate. There is a groove on the plate and a tick mark on the side of the groove. The specifications are divided into three types: O-50μm, 0-100μm, and 0-150μm. There is another scraper and both edges are polished. When using, it is preferable to drop the sample to the deepest part of the groove to fill the groove and slightly excess. Then, with both hands, the doctor blade is perpendicular to the flat plate, and is pulled from the deep part of the groove to the shallow part at a relatively constant speed (generally about 3 seconds), so that the sample is filled with the groove and the flat plate is left and left. Immediately (generally no more than 6 seconds), the position of the particles in the groove is uniformly observed with a 30° angle, where the scale line reading is the fineness of the sample. This method is easy to operate, easy to clean, and test speed is fast, but it should be noted that the consistency of the sample should meet the requirements of the product standard to avoid errors during testing.
5. Occlusion
The ability of the coating to be applied to a uniform coating film so that the background color of the surface of the object to be coated is no longer present is called opacity. There are two ways to represent the minimum amount of paint required to cover a unit area (in g/m2) or the minimum wet film thickness (in μm) required to cover the bottom surface.
In China, the method of testing opacity is generally used in the former, namely the weight per unit area method (also called black and white grid method). The instrument is a 100×200mm black and white glass plate composed of 25×25mm black and white small squares. Apply the paint sample evenly to the glass plate, visually measure until the black and white grid is not visible under astigmatism, and weigh the amount of paint used, and then calculate according to the following formula:
X= (Ww)/S×10000
Where: X-covering power (g/m2); W-coating sample and container total weight (g); W-, - total weight of remaining paint and container (g); S--black and white grid glass coating There is a coating area (cm2).
The method is simple to operate, and the black and white glass plate is also convenient to manufacture, but sometimes the end point is not observed, and the leveling property of the paint and the brush marks during the operation have a certain influence on the test result of the hiding power.
The size of the coating hiding power has a great influence on the unit consumption of our steel drum coating, so we must pay attention to the hiding power index of the coating. The general coatings used in our steel drum industry have opacity indicators as follows (g/m2):
Magnetic paint class:
Pearl red 160~200
White 300
Gray 90
Black 40
Yellow 250
Blue 80~150
Green 80~150
Blending paint:
Red 90-160
White 260
Gray 90
Grass green 60
Iron red 50-60
Yellow 90~220
Blue 80~150
Green 80~150
Black 40
Brown 60
Anti-rust paint:
Red Dan 200
Iron red 60~80
Gray 100
(two) coating film
To test the performance of the film, firstly, a good coating sample should be prepared. The sample is well treated and the coating performance can be truly reflected.
l, general preparation method of coating film
(1) Materials:
1 tinplate with a thickness of 0.2-0.3mm and a length and width of 120×50mm. Used to measure impact strength, etc.
2 thickness of 0.2-0.3mm, length and width of 120 × 25mm Guankou iron plate. Used to measure flexibility and so on;
3 thick 0.45-0.55mm, steel tube iron plate with the same length and width. Used to measure adhesion and so on.
(2) Surface treatment: first use 0# sandpaper to polish, remove rust, remove tin plating layer, then wipe with net cloth, then wipe with cotton wool solvent (pine perfume or xylene) until the cotton ball is not stained. And dry it.
(3) Coating: Dilute the coating sample to the viscosity as described above and filter it into zero. The coating can then be applied by hand or mechanical spraying. When applying the coating, pay attention to the uniform coating film; the thickness of the coating film, the varnish is about 15μm; the general steel barrel coating is about 18-23μm; the anti-corrosion coating anti-corrosion test coating should be coated with two layers, the total thickness should be more than 80μmo
2, drying time measurement
After the coating is applied to the surface of the object with a certain thickness, it undergoes physical volatilization or chemical oxidative polymerization to form a solid film. The time required for such a process is called drying time. This process is divided into two phases:
(1) Dry surface: When the coating film forms a microfilm layer from a fluid to a surface, it is called a surface dry. There are two methods for measuring the dry time, namely, the blown ball method and the finger touch method. The method of blowing the cotton ball is to put a cotton wool ball on the surface of the coating film, the mouth is 10-15 cm away from the cotton ball, and the cotton ball is lightly blown horizontally. If the cotton ball can be blown away from the surface of the coating film without leaving the cotton thread, it is considered It has been dry and sticky, but no paint sticks to the hand, it is also considered to be dry.
(2) Dry: When the coating film forms a solid film layer from fluid to all, it is called solid dry. The method of measuring the dry time is a filter paper method, a cotton ball method, a blade method, or the like. This article introduces the most convenient JJ ​​film method: the protective film can be used to scrape the coating film on the sample plate, and the bottom layer of the coating film has no sticking phenomenon, that is, the coating film is considered to be practical.
Many steel drums used in the barrel factory are single-component white dry. After the steel drum is coated with these paints, it is necessary to wait until it is ready to roll and stack. Thus, if the drying time of the paint is too long, the site turnover will not be opened, which is not conducive to the continuous production of the steel drum. Therefore, the single-component white dry coating for steel drums preferably has a surface dry time of less than two hours and a dry time of preferably less than eighteen hours.
For dry coatings (paints), the best baking conditions should be determined by experiment. Because the external drying coatings of our steel drums are mostly amino baking varnishes, the general baking broth is baked at 105 ° C ~ 120 ° C for two hours; the fast drying amino baking varnish should also be baked at this temperature for 1 hour; There is also a so-called express dry amino baking varnish, but under the same temperature conditions above, the baking time cannot be shortened too much. Amino baking varnish is not baked, it is also dry, and it is good in vision and feel. The cost of lowering the baking conditions is that the film is not completely cured. Although we seem to feel good or feel good, the performance of the film is greatly reduced. When an old user of the Shanghai Chemical Co., Ltd. once reported that the steel barrel was filled with their chemical products, the steel drum coating was stained with these products (liquid) and wrinkled after a while, while they used to use their steel drums. There has never been such a phenomenon in filling the same product. The factory sent technicians to analyze and then learned that it was the result of incomplete baking. After inspection by the workshop, a batch of steel drums could not be completed due to the output. When the temperature of the drying tunnel had not reached the specified value, the steel drum was baked, and the baking time of two minutes was shortened. As a result, the factory carried out quality education and strictly enforced the system according to the process regulations. No such thing happened in the future.
The design of the drying tunnel of the steel drum factory is mostly within half an hour, and more often between 10 and 20 minutes. (Of course, there are also many barrel mills that are designed to be continuously variable, so that the baking time can be arbitrarily extended, but the time is extended at the expense of production). For this reason, we have to increase the baking temperature. However, one disadvantage of the amino baking varnish is that the temperature is too high; the coating film is easily brittle. After dozens of tests in the central laboratory of Shanghai Chemical Co., Ltd., it was found that the baking temperature of the amino baking varnish was higher than 185 °C and began to be brittle. Therefore, the baking temperature should preferably not exceed 185 °C. The homemade amino baking varnish used in Shanghai Chemical Co., Ltd. is baked at 175 ° C for 10 minutes. Under such baking conditions, the coating film of the steel drum is bright and tough, and various properties of the coating film are good. Of course, the drying type coatings used in each factory have different resin types and different formulations, and the baking conditions will naturally not be the same.
2. luster
The so-called gloss is the ability of the surface of the coating film to reflect the light projected on it in one direction. The greater the amount of light reflected, the higher the gloss. The gloss of the film is an important indicator of the decorative properties of the coating.
Gloss measurements are generally performed using an optical gloss meter. Glossy readings are available directly on the optometrist. At present, the gloss of the coating is measured, and the photoelectric gloss meter used generally adopts an incident angle of 45° or 60°. The gloss of the coating (compared to the gloss of the standard board) measured according to the specified 60° photoelectric gloss meter is classified as follows:
70% or more is high gloss; 30-70% is semi-gloss or medium gloss; 6%-30% is eggshell light; 2%-6% is flat-shell light; 2% is flat .
A high-gloss coating film of more than 70% can be measured by a 20° photoelectric gloss meter; on the contrary, for a low-gloss coating film, a photoelectric gloss meter having a large human angle (for example, 75°) is more advantageous. Therefore, it has been used to measure the gloss of the coating film by using a multi-angle photoelectric gloss meter (O°, 20°, 45°, 60°, 75°) and a living angle optical gloss meter (0°-85°). An instrument can be used for a variety of purposes, increasing the scope of the test.
The determination of gloss is not necessarily required for our steel drum industry. Because the effect of the gloss of the film on the decorative performance can be judged by visual. Only for certain special users. If testing is required, a glass plate having a thickness of 2-3 mm and a length and a width of 90 × 20 mm, respectively, is required, and a film sample plate is formed on the glass plate. Moreover, the measurements need to be made at three different locations on the sample, and the readings need to be accurate to 1%. The difference between each measurement point reading and the average value should not be greater than 5% of the average value, and finally the average of the three points.
4. Adhesion
When the coating is applied to the surface of the object to be coated, the coating and the underlayer are bonded together by physical and chemical forces. This ability is called adhesion. This adhesion is caused by the combination of polar groups (such as hydroxyl groups, carboxyl groups, etc.) of the polymer in the coating film with the polar groups on the surface of the coated object, and therefore, All factors will lead to a decrease in adhesion of the coating film. Imagine if the surface of the object to be coated is oily, ash, or rust to isolate the coating from the object to be coated, how can we talk about adhesion? Therefore, the surface condition of the object to be coated is very important. This is why our steel drums require pre-treatment before coating.
Adhesion is one of the most important properties for evaluating coating films. A coating has poor adhesion, so there is no other performance of the coating.
It is difficult to actually measure the adhesion between the coating film and the object to be coated, so it can only be measured by indirect means. The measured adhesion value also includes some other bonding properties. Test methods such as impact strength, flexibility, and Kleemann hardness can also indirectly exhibit adhesion of the coating film.
There are two methods for testing adhesion: a comprehensive measurement method and a peeling measurement method. Our steel drum industry is more convenient to adopt the grid method in the comprehensive measurement method.
The adhesion test is carried out by the grid method. The tools, methods and grades to be achieved are specified in the GB325-91 "Packaging Container Steel Barrel" standard and are illustrated. According to the provisions of GB325-91 "Packing Container Steel Barrel", Shanghai Chemical Co., Ltd. has made a frame guide and a single-edged knife. It has been proved by practice that this test tool is convenient and practical, and each factory can fully implement it. Since each barrel factory has GB325-91 "packaging container steel drum" standard, the adhesion test is not much to say.
5. Shock proof
The impact resistance test measures the ability of the coating film to resist external impact, and also shows the elasticity of the coating film and the adhesion to the bottom plate, and the degree of deformation of the coating film under high-speed load.
Tested by a coating impact tester, the weight of 1 kg is dropped on the coating film without causing the maximum height of the coating film destruction, and the unit is kg. Cm.
After the film is dried, place the paint sample on the pillow block with the coating film facing up. The edge of the sample is more than 15mm. The weight is controlled by the control device at a height of 10cm. Press the control button and the weight is free. The ground falls on the punch, lifts the weight, and observes with a 4 times magnifying glass. When the coating film has no cracks, wrinkles and peeling off, the weight of the weight can be increased to 20-50 cm in turn and then measured. Because steel drums are subject to frequent transportation, loading and unloading, rolling, and often subject to frequent collisions and friction, the impact resistance of steel drum coatings should reach 40kg. More than cm. Similarly, steel drum coatings have higher requirements on the wear resistance of the coating film, but because the test wear resistance is more complicated, it is not easy to grasp, and will not be described here. Conditional units can directly purchase the film wear tester. This instrument is to measure the weight loss of the coating film after the coating film is subjected to a predetermined number of grinding times under a certain load.
6. Flexibility
The flexibility of the coating film is that the coating film does not rupture after being bent to a certain extent, so it is also called the bending property of the coating film. If the film is subjected to a prescribed bend without being damaged, the flexibility of the paint is good. Due to the special process of steel drum manufacturing, this performance is especially important for the inner coating of steel drums.
The flexibility of the coating film was measured on a flexibility tester. Specifically, the coated tinplate is bent on a shaft of a certain diameter on the flexibility tester, and is represented by the diameter of the smallest shaft that does not cause damage to the coating after bending. The diameter of the shaft is 1, 2, 3, 5.10.15 mm6.
During the test, the coated tinplate sample with the length and width of 25×120mm and thickness of 0.2-0.3mm mentioned above is coated upwards, placed on the shaft and bent 180°, and the bending time is 2- 3 seconds, then observe with a 4 times magnifying glass to see if there are cracks, texture and peeling. If there is no abnormality, it is qualified,
When the coating film is bent on the shaft, the coating film is elongated, and the smaller the diameter of the bending, the greater the elongation of the outer surface of the coating film, so that the flexibility through the diameter of 1 mm is the best. Our inner coated steel drum is first coated with the inner coating and then rolled up to the bottom of the bucket and the top of the bucket. During the winding process, the coating film bends with the coil of the steel sheet. If the flexibility of the coating film is not good, then it is here. Cracks, netting or peeling occur during the process. Therefore, as the inner coating of the steel drum, the flexibility is required to reach 1 mm. Otherwise, the coating film is liable to be broken when the top and bottom of the steel drum are wound up, and the corrosion resistance of the inner coating is deteriorated.
7. Film thickness test
Film thickness is an important control indicator during the paint inspection process. The same test items can produce different test results at different film thicknesses. Therefore, when the coating film is inspected, the coating film thickness of the template is specified. The performance comparison of the various samples is comparable only at the same film thickness.
(1) The simpler method of measuring the wet film thickness is to use a metal plate that can be carried in a pocket. There are rectangular teeth on both sides, and the upper two planes of the two ends of each tooth are on the same horizontal plane, and the middle teeth are different heights which are successively raised from the horizontal plane. The measuring principle is that when the two faces on the same horizontal surface of both ends press the bottom plate under the wet film, the third face can vertically contact the wet film. Since the third surface and the outer two sides have a height difference, the third surface which first contacts the wet film layer represents the thickness of the wet film, and the value indicated is the thickness value of the wet film.
(2) Determination of dry film thickness
The dry film thickness is measured, and a dry film thickness meter is currently widely used. These instruments are produced in many places. According to the material of the bottom plate, it can be divided into a magnetic thickness gauge and a non-magnetic thickness gauge. Our steel drum industry is made of steel, so we use a magnetic thickness gauge. The magnetic thickness gauge mainly uses the magnetoresistance method to measure the thickness of the coating on the steel floor. It should be noted that some paint varieties will have certain influence on the test results due to the inclusion of metal powder such as iron red and aluminum powder. Alternatively, mechanical means such as a lever micrometer can be used to determine the thickness of the coating on the metal base plate, but due to the limitations of the structure of the lever micrometer itself, it can only be used to measure a smaller area of ​​the test plate. For the measurement of dry film thickness on the construction site, there are many types of pocket thickness gauges at home and abroad. In one case, the test principle is a magnetic field generated by a permanent magnet, which is driven by a change in the test path to deflect the pointer to obtain a film thickness reading. The instrument is simple in structure and easy to carry, but with a slightly lower accuracy and the test results are susceptible to the Earth's magnetic field. The other is a magnetic thickness gauge with a battery that can be carried around.
To say the thickness of the layer, it should be pointed out that when we use steel coatings for construction, it is best to adopt a multi-layer thin coating method. Of course, a certain thickness of the coating may be formed at one time or multiple times. However, practical experience has shown that most coatings are applied several times with each thin coating. The coating film formed by coating multiple times is better than the coating film forming the same thickness once, because the thin coating has good drying conditions, the internal stress during drying is small, and the adhesion is strong; the coating is too thick at one time. At the same time, not only the internal stress of drying is large, but also wrinkles and other pathological conditions occur.
Nowadays, most of the barrel mills use a single coat of paint, which is probably not acceptable for users who are not very demanding. At most, it looks a little bad, and it takes a long time to rust. However, if the inner coating of the steel drum is also sprayed once, it may cause problems in the future. Because, firstly, it is impossible to achieve a thickness of 80f/m or more for anti-corrosion requirements in one spray of paint; secondly, after the coating film is formed, it has micropores, and only two or more coating films can apply micropores of the coating film. Misaligned with each other; third, a thicker coating film is formed at a time, and various properties are affected. Therefore, the inner coating of the steel drum must be painted more than two layers. Also, be sure to measure the thickness of the coating to see if it meets the specified requirements.
8. Corrosion resistance
The corrosion resistance of the film includes water resistance, heat resistance, damp heat resistance, salt spray resistance, acid and alkali resistance, gasoline resistance, and lubricating oil resistance. This performance test, for our steel drum industry, can be tested for targeted projects. In particular, the paint inside the steel drum can be tested for the items to be filled in the inner coated steel drum.
The method is to apply a coating on a hemispherical 120 mm long carbon steel rod with a diameter of 10-12 mm, a small hole in the non-spherical shape, and a nylon rope in the small hole. The above method for producing a coating film may be employed, or a dip coating method may be employed. It is required to apply two or more layers, and the total thickness is greater than 80 μm.
The dried coating film stick is immersed in the liquid to be filled in the steel drum. After a certain period of time, it is taken out and rinsed with water, and then the film is checked. In general, the immersion time is more than 3 months, the film is intact, can be considered qualified, can also be immersed in a constant temperature of 55 ° C for two weeks for rapid testing, for reference.
The above is the coating inspection project that Shanghai Chemical Co., Ltd. has been insisting for for several years. Of course, some of the test conditions of the project are not in compliance with the coating test requirements, such as the temperature conditions of the viscosity test, the temperature and humidity conditions of the impact strength test, etc., so the data obtained from these tests cannot be said to be standard. However, in a fixed environment, especially in the case of similar standard test conditions, the test results are quite close to the standard data. Moreover, tests of other test items such as solids content, adhesion, etc. do not have high requirements for environmental conditions.
Third, special steel drum coating standards and inspection methods should be established as soon as possible
All kinds of paints have their own characteristics, such as marine paints, furniture coatings, etc., all have their own performance characteristics, have their own special occasions, so they have their own special standards. Steel drum coatings also have its particularity. Only the roots - according to these specialities, the corresponding standards are established, and the coatings used are subject to the inspection of the specifications to ensure the coating quality of our steel drums. Of course, the test items and test methods of various coatings (such as blending paint, two-component paint, baking paint, etc.) are different. Therefore, for each coating product we use, we should first determine which inspection items and inspection methods are used to truly reflect the performance and quality of the product. Then, according to the purpose of use of the coating and construction conditions combined with the production process of our steel drums, the corresponding reasonable standards and testing methods are formulated.
In short, our steel drum industry should establish its own special standards for steel drum coatings and their inspection methods as soon as possible to better ensure the quality of steel drums.
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