How to ensure the quality of the coating on the surface of the steel drum (1)

Xin Qiaojuan

The quality of the surface coating of the steel drum depends not only on the surface quality of the coating and the steel drum itself, but also on the whole process of the coating formation (including the coating process, machinery and working environment). Proper selection of reasonable coating technology is a necessary condition to improve the surface coating quality of steel drums.

First, the main purpose of the surface coating of steel drum

Painting refers to the process of spraying a coating onto the surface of a surface treated steel drum and drying it into a film. The coating process generally consists of three basic steps: surface treatment (including surface cleaning and chemical treatment), spraying and drying of the steel drum before painting.

The coating is an organic substance in a flowing or powder state. It is applied to the surface of the steel drum to dry and solidify to form a film which is evenly covered and adhered well to the surface of the steel drum.

First, the surface of the steel drum is painted to protect the steel drum from external corrosion. Because the steel drum is in use, the surface of the steel drum is eroded due to external rain, sunlight, weathering and various media. Thereby the surface of the steel drum is rusted and even rusted. According to statistics, about 2% to 5% of steel drums are scrapped every year due to corrosion. The coating is coated with a coating with certain moisture resistance, water resistance, weather resistance, oil resistance and chemical resistance on the surface of the steel drum to protect the steel drum and prolong its service life. .

Secondly, the surface coating of the steel drum is to package the steel drum (decorative effect). Using the color diversity of the paint and the technology of spraying construction, the steel drum can be painted according to the user's requirements, habits, hobbies and market demand to obtain decorative coatings of different colors, lusters, patterns, etc., thereby increasing the attachment of the steel drum. Value, expand the scope of application of steel drums.

Second, the process requirements of steel drum surface coating

In order to ensure the quality of the steel drum coating, it is necessary to consider not only the surface quality of the steel drum, but also the use, performance and construction process of the coating itself, because the performance and function of the coating are reflected by the coating. Therefore, the choice of coating variety and reasonable coating process and painting equipment, working environment are mutually promoting and mutually restrictive. So you must do the following:

1. Requirements for coating performance of steel drum surface coating

The purpose of the surface coating of the steel drum is to protect it and decorate it. Its specific requirements for coatings are bright color, strong adhesion and smooth surface. In order to meet these conditions, the performance and use of the coating must be carefully analyzed. Because each type of coating has its special properties and advantages and disadvantages, it should be developed to avoid weaknesses, and correctly select the type of coating and coating system. At present, the coating methods used in the national steel drum industry are different, and some use electrostatic spraying ( Spraying and painting), some use high pressure airless spraying. In the past two years, due to the increasing number of two-color barrels and three-color barrels required by users, manufacturers using high-pressure airless spraying will also increase.

2. Develop a reasonable steel barrel coating process

According to the use of steel drums and the quality requirements of the coating, as well as the actual situation of each barrel company, a scientific, advanced and reasonable coating process shall be formulated, the contents of which shall include: coating type, detailed coating process and Technical conditions, equipment and tools used, quality standards and testing, acceptance criteria and methods, etc., in order to facilitate the operator to select the type of coating according to the process requirements, and fine operation according to the process requirements to ensure the appearance quality of the steel drum.

3. Strictly carry out surface pretreatment before steel drum coating

Steel drum raw materials are exposed to air during production and transportation. Due to the action of oxygen and water in the air and possibly SO2, CO2, H2S, etc., corrosion is easy to occur; foreign matter, such as scale, is easily adhered during transportation. Oil stains, processing debris, dust, etc. In addition, in the production process, the steel drum is also likely to cause dirt or damage on the surface of the steel drum, and it is also easy to cause corrosion of the metal surface of the steel drum, thereby causing the quality of the steel drum to be lowered. Therefore, the surface treatment must be carried out before the steel drum is coated to eliminate the unfavorable factors affecting the quality of the steel drum coating, thereby improving the coating quality of the steel drum, which is very important for the steel drum.

4. Strict coating process of steel film surface coating

When the paint is evenly sprayed on the surface of a clean steel drum, after drying, a protective film with good performance, namely a paint film, can be obtained. The formation process of the entire paint film can be divided into two stages, a coating process and a dry film forming process.

1 coating process

The coating process is a simple physical process that refers to the process of uniformly coating the coating on the surface of the steel drum to the required thickness. This process is typically done by an automatic sprayer. Although the coating process is simple, it must meet the two major requirements required, namely a certain thickness and uniform coating, and no defects such as leakage coating, sagging and coarse grain defects. The thickness of the coating is an important process parameter during the coating process. The cured coating formed by any coating has a certain gas permeability, which is related to the structural pores of the coating. The pore size of the coating structure is 10-6~10-7mm on average, which is larger than the molecular diameter of gas and liquid, so not only gas can pass, but also general ions and low molecular liquid can pass. In order to maximize the water resistance and gas permeability of the coating, the coating must be of a certain thickness to overlap the structural pores with each other. The minimum thickness at which the coating is normally protected is referred to as the critical thickness. For example, the critical thickness of the steel drum coating process is 0.02 mm, that is, the thickness of the paint film formed by the steel drum half is not less than this critical thickness. Only such a film thickness can protect the steel drum from external corrosion. Different coatings have different critical thicknesses, but the coating thickness specified by the process must be met during the coating process. In addition, the uniformity of coating is also very important. If it is not uniform, the excessively thin places will reduce the corrosion resistance, and if it is too thick, the defects such as sagging and wrinkling will easily occur.

2 Dry film formation process of steel drum surface coating

The process in which the paint is applied to the steel drum is mostly a liquid to solid film (except for dusting), and is called a dry film forming process of the paint. The dry film formation process is a comprehensive physical-chemical process including solvent evaporation, condensation, oxidative polymerization, etc. The whole process has both physical and chemical effects and can be roughly divided into two stages:

a, solvent evaporation stage

The solvent volatilization stage on the upper surface of the steel drum generally occurs in the early stage of drying, and immediately after the coating, a large amount of solvent is volatilized, and the volatilization process is carried out almost on the free surface of the liquid. Due to the volatilization of the solvent, the concentration of the paint film on the surface of the steel drum changes, and the paint loses fluidity from the surface.

b, chemical reaction stage

On the surface of the steel drum, the solvent loses fluidity after volatilization. Under the action of thermal energy, there are some polymer compounds which form a network structure by condensation reaction. Other paint bases are oxidized in the air and polymerized to form a network structure. The polymer compound, this process is a relatively complicated chemical reaction process, but the final generation of polymer compounds with a network structure of varying degrees of polymerization. This polymer compound has good properties, and almost all the protective properties of the film are provided by it.

Therefore, heat and time are essential in the drying process of the paint film coating on the surface of the steel drum. Without sufficient heat and sufficient time, the coating does not get enough chemical reaction energy or the reaction cannot be completed completely. The so-called selection of appropriate drying specifications is to set a reasonable configuration of drying temperature and drying time in order to obtain sufficient heat, so that the paint film can reach the optimal structure for protecting steel drums and decorative steel drums.

5. Strictly monitor the coating quality of steel drums

In order to ensure the coating quality of the surface of the steel drum, it is necessary to have accurate testing instruments and reliable testing methods to monitor every important part of the steel drum painting operation to control the quality of the steel drum coating to meet the prescribed high standards. The inspection of the quality of the steel drum coating includes the detection of the quality of the pre-painting treatment, the inspection of the quality of the coating product itself, the quality monitoring of each process in the coating construction process, and the detection of the quality of the coating film after the coating is completed.

6, timely treatment of steel drum surface coating defects

Defects found in the inspection of the coating process and the final coating properties shall be promptly disposed of, and corresponding measures shall be taken to remedy the coating quality on the surface of the steel drum to achieve the purpose of coating.