Exploring the surface treatment before steel drum coating (2)

Exploring the surface treatment before steel drum coating (2)

Third, the process and equipment for surface treatment before steel drum painting

The cleaning of the steel drum is not only the function of the cleaning agent, but also a complete cleaning process and the cleaning equipment. High-efficiency cleaning agents and cleaning equipment plus a reasonable cleaning process are the guarantees for good cleaning results.

(1) Requirements for steel drum cleaning process and equipment

When selecting the cleaning process and cleaning equipment for steel drums, we must first consider the following aspects:

1, reliability

The steel drum cleaning process and equipment required to be selected have a stable cleaning quality and can achieve the cleaning degree required by the process.

2. Impact on steel drums

It is required that the damage to the steel drum during the cleaning process should be as small as possible and no new secondary pollution can be caused to the steel drum.

3. Conducive to the protection of the natural environment

The steel drum cleaning process and equipment are required to prevent or minimize damage to the natural environment caused by cleaning waste, noise, and exhaust gas.

4, efficiency

The production process and equipment required for steel drum cleaning are selected according to the specific conditions of the enterprise, and the production process and equipment with high production efficiency and low labor intensity are selected as much as possible.

5, a good working environment

The process and equipment required for steel drum cleaning can maintain a good working environment, so that the health and safety of the production workers are guaranteed.

6. Economical

It is required to use cleaning processes and equipment that can achieve the cleaning degree of the surface of the steel drum as low as possible.

(2) Process and characteristics of steel drum cleaning

The surface treatment of steel drums before painting is usually carried out by three methods of dipping, spraying and hand wiping. Each of the three cleaning methods has its own characteristics. The specific cleaning method is determined according to the specific conditions of the enterprise and the production batch. Its characteristics are:

1. Characteristics of immersion cleaning method

In this way, the outer surface of the steel drum is cleaned, and a high alkalinity solution can be used. It is not easy to form a foam, so it is allowed to contain more surfactant. The equipment has simple structure, low investment and convenient maintenance. However, the processing time is longer, the temperature is higher, and the production efficiency is lower. Not suitable for mass production.

2, the characteristics of the spray cleaning method

It has a strong mechanical effect and a good cleaning effect. The solution concentration and treatment temperature can be lower. Steel drums also have a short processing time and high production efficiency, which is suitable for mass production. The phosphating film crystallizes finely. It is easy to produce foam and requires low foaming surfactant. However, the equipment is more complicated, the equipment investment is large, and the maintenance is difficult.

3, the characteristics of manual wiping cleaning method

This method is simple and flexible, and is not limited by environmental conditions and production site conditions. The lye concentration and operating temperature should not be too high. However, the production efficiency is low and the labor intensity of the workers is large. Not suitable for high volume automated operations. In addition, when working, care should be taken to prevent lye from damaging the skin and cornea.

The surface treatment before the steel drum is coated mainly includes three parts: degreasing, descaling and phosphating. Below we discuss separately.

(3) Degreasing cleaning of the surface of the steel drum

The degreasing cleaning of the surface of the steel drum is also called degreasing cleaning. The degreasing cleaning of the surface of the steel drum is usually carried out by using an alkaline solution, an organic solvent, a surfactant, a solvent or a mixed solution as a cleaning agent, and a cleaning method such as dipping, spraying and wiping.

1, alkaline elution grease

(1) Mechanism of cleaning the steel drum lye

The lye degreasing cleaning is mainly based on the functions of emulsification, dispersion, dissolution and mechanical action. The oil on the surface of the steel drum is generally mineral oil, which is a non-saponified neutral oil. It cannot react with the alkali. In the cleaning process, the strong alkali is used to dissociate the oil from the surface of the steel drum, and then dissociated. The oil is dispersed stably by the gel of the colloidal base.

There is no need to worry about the corrosion of the steel on the surface of the steel drum by the alkali. Because the steel has excellent alkali resistance, the alkali cleaning agent is widely used for the degreasing cleaning of the steel drum.

(2) Types and properties of common lye cleaning agents for steel drum cleaning

a, sodium hydroxide, also known as caustic soda, is a strong alkaline compound. When cleaning, it mainly acts to dissociate oil. It does not have the buffering effect of alkaline salt, and it has no erosion effect on steel drum at normal temperature and medium temperature, and can produce slow corrosion under high temperature. Sodium hydroxide is cheap, but it has a strong corrosive effect on human skin, so you should pay attention to it when handling it.

b, sodium carbonate, also known as soda ash, is an inexpensive alkaline salt. It has a buffering effect and does not erode steel drums like strong alkali. Sodium bicarbonate (also known as soda) and trisodium hydrogencarbonate are weaker in basicity, but they can react with calcium ions in water to form soluble salts, which have certain properties for hard water. Softening effect.

c. Silicates include sodium metasilicate, sodium orthosilicate and water glass. The silicic acid formed during the hydrolysis of the silicate is insoluble in water, and is suspended in the bath in a colloidal state to disperse the solid dirt. The alkalinity of sodium orthosilicate is close to caustic soda. Because of the strong colloidality of the silicic acid contained in it, it has the advantage of dispersing and retaining the dirt, so it is often used together with caustic soda in the cleaning of steel drums. The general characteristic of silicate is that the lye is excellent in wetting and permeability, and it maintains the dispersibility of the soil and the hard water resistance, and has a certain anti-corrosion effect on the surface of the steel drum.

d. Phosphate used as an alkali cleaning agent includes trisodium phosphate, tetrasodium pyrophosphate and sodium tripolyphosphate. Phosphate has a more pronounced dispersion which disperses large particulate soils into small particles resembling colloidal particles. Tetrasodium pyrophosphate also has a surface active effect. Phosphate can also be used as a sequestering agent, which is combined with calcium and magnesium ions in hard water to be removed as a water-insoluble salt. The common characteristic of phosphate is to inhibit the corrosion of the metal surface of steel drums. However, due to its low alkalinity and high price in alkalis, it is rarely used as a cleaning agent by steel drum manufacturers, but often with low price. The strong base is used together.

(3) Requirements for alkaline cleaners in steel drum cleaning

a, the cleaning agent should have high detergency, it is conducive to the high quality and high added value of the steel drum surface spraying, and the stability of the steel drum cleaning during cleaning is high.

b. The amount of cleaning fluid used in the cleaning of the steel drum should be small, which can reduce the load of wastewater treatment.

c. When cleaning, the lye cleaning agent should be the least toxic and easy to decompose, and the waste liquid is easy to handle.

d. The cost of the cleaning agent is low, and the corrosion of the steel drum is small.

(4) Alkali washing process commonly used in steel drums

The alkaline washing process commonly used in steel drums is immersion cleaning and spray cleaning.

a, immersion cleaning

The steel drum is immersed in the cleaning tank to make the whole surface contact with the alkali liquid. In order to improve the degreasing efficiency, the cleaning time can be heated and extended, and the flow speed of the alkali washing liquid can also be accelerated, such as stirring with a mixer and circulating a pump.

b, spray cleaning

The spray cleaning device is used to spray the alkaline cleaning liquid onto the surface of the steel drum for cleaning. The spray cleaning has the functions of saponification and emulsification of the alkali washing agent, and also has a spray-impact effect. The greater the injection pressure, the greater the proportion of mechanical detergency in degreasing.

(5) Influencing factors and common problems of lye during steel drum cleaning

a, the composition of the lye

The composition of lye is the main factor affecting the quality of steel drum degreasing. Alkali degreasing relies on saponification, wetting, emulsifying, dispersing, etc. to remove oil stains. Because a single material does not have these properties at the same time, a combination of materials is required. The recipe number is not strictly required, but the match is appropriate for the best overall performance.

b, the concentration of lye

The high concentration of the alkali solution is favorable for the saponification reaction, and the higher the concentration of the alkali liquid, the lower the surface tension between the oil stain and the solution, and the oil stain is easily emulsified. Practice has shown that when the pH of the solution is below 8.3, the solution hardly degreases.

c, the temperature of the lye

Practice has proved that in the degreasing process of the steel drum, the high temperature of the lye is beneficial to the saponification reaction, and the oil with higher melting point is softened, which is beneficial to the infiltration and emulsification, so the cleaning effect of the high temperature steel drum is good. . In general, the temperature of the solution is between 70 and 100 °C. ,

d, mechanical action

The mechanical action contributes to the removal of oil stain on the surface of the steel drum, and the mechanical action is strong, and the cleaning effect is good. Stir the solution properly during immersion cleaning. Spray cleaning should maintain a certain flushing force on the steel drum, usually between 50kPa and 400kPa.

e, surfactant

The addition of a suitable surfactant to the alkali cleaning solution can reduce the interfacial tension, improve the wettability and emulsifying properties, and improve the cleaning effect. However, surfactants are prone to foaming.

When using immersion cleaning, more cationic and nonionic surfactants can be added without affecting the cleaning operation due to foaming. However, when spray cleaning is used, even if a small amount of surfactant is used, it is often prone to a large amount of foam overflowing out of the tank, which not only contaminates the work site, but also consumes a large amount of the cleaning solution, so that the production cannot be continuously performed.

A foam is formed by dispersing a gas in a liquid and a solid. Foaming depends on the ability of the material to foam and the stability of the bubbles formed. In order to overcome the large amount of foam generated during spray cleaning, the following measures can be taken:

1 Use a low foam surfactant and control its amount.

2 Add defoamer. Silicone oils, higher alcohols, poorly water-soluble benzoic acids and volatile petroleum solvents all have defoaming action.

3 Reduce the structure of the equipment to bring air into the lye to reduce the generation of foam.

f, washing

After the steel drum is degreased and washed in the alkali solution, the water is rinsed to remove the cleaning agent remaining on the surface of the steel drum. Because the residual cleaning agent will affect the adhesion of the coating, and the alkali and salt in the cleaning agent absorb moisture, it will cause early foaming of the coating.

The water washing can be carried out by dipping or spraying. Steel drums are often washed with two stages of water, one with hot water and the other with cold water. The temperature of the hot water should not be too high, generally controlled at around 60 °C. When using the second-stage water washing, which level of hot water is more suitable should be determined according to the specific conditions of the steel drum cleaning. If the cleaning agent used is not easily washed away with cold water, the first stage water washing should be hot water. If the cleaning agent used is easy to wash with cold water or if there are many cleaning agents attached to the surface of the steel drum, it needs to be washed with running water. The first-stage water washing should use cold water to avoid a large amount of heat.

If multi-stage reverse cleaning is used, the quality of washing can be improved, water consumption can be saved and wastewater discharge can be reduced.

After the steel drum is rinsed with water, it should directly enter the rust and phosphating process and then dry. If you do not directly enter the descaling and phosphating process, you should also dry it first. Compressed air can be used to blow off water or water from the surface of the steel drum before drying. Generally, it can be dried at 100 ° C for 1 min to 10 min. If there is yellow rust in the drying process, a small amount of anti-rust corrosion inhibitor, such as triethanolamine, can be added to the water of the last washing to adjust the pH of the water to 8-9, which can effectively prevent the generation of yellow rust.

2, solvent degreasing

The degreasing ability of various oils by means of organic solvents is widely used in the barrel industry. The characteristics of the organic solvent are high in oil removal efficiency, but the inorganic salts and alkalis cannot be washed away. In the coating process, the common organic solvent for degreasing is white alcohol (also known as paint gasoline, boiling point of 100 ~ 200 ° C, flash point of 30 ° C) or cleaning gasoline (boiling point of 80 ~ 120 ° C) and various A chlorine solvent such as trichloroethylene. However, due to the high toxicity of chlorine-containing solvents, it is not suitable for manual cleaning, so most barrel companies use gasoline scrubbing.

Cleaning with gasoline is usually done by hand scrubbing with a brush or rag, and can also be directly immersed in a solvent tank for cleaning. Manual scrubbing is labor intensive and has poor working conditions. Gasoline cleaning stations should have good ventilation and fire protection facilities.

3. Cleaning of surfactant aqueous solution and mixed solvent degreasing

Because hydrocarbon solvents such as gasoline used for solvent cleaning are flammable and explosive, and halogenated hydrocarbon solvents are limited by toxic or ozone depleting, safe, non-toxic, mild surfactants or mixed solvents are degreased in steel drums. The new direction.

The surfactant aqueous solution and the mixed solvent are used as a steel drum cleaning agent, and the surface oil is cleaned by the characteristics of low surface tension, good wettability, and strong emulsifying power. With the development of various new synthetic surfactants, especially in the development of non-foaming or less foaming cleaning ability, and biodegradable surfactants, surfactants have been widely used in the cleaning of steel drums.

Surfactant aqueous solutions and mixed solvents have the following characteristics:

1. Under the same conditions, the cleaning ability is stronger than that of the lye cleaning, the oil quality is good, the alkalinity is weak, and the corrosion to the steel drum is small.

2. It can be combined with other steel drum surface treatment processes, such as the process of de-oil cleaning and phosphating two-in-one treatment, which can simplify the process under certain conditions.

3. After the biodegradable surfactant is used, it is beneficial to the sewage treatment of the surface treatment process.

(1), the degreasing mechanism of surfactant

The degreasing mechanism of the surfactant generally consists of the following three stages.

1. Wet the surface of the steel drum to make the oil roll away.

Due to the adhesion of oil stains and the adsorption of trace amounts of air, the surface of the steel drum has a large interfacial tension and is not easily wetted by water. When the surfactant is added to the water, the surfactant is easily adsorbed at the steel drum-water interface and the oil-water interface, reducing the interfacial tension, replacing the gas on the surface of the steel drum, infiltrating into the surface of the steel drum and between the oil and the oil. Between the oil stains, the surface of the steel drum is preferentially wetted, and the oil film is curled into oil beads and leaves the surface of the steel drum.

2. Transfer the oil from the coil to the solution.

The emulsion with emulsification acts at the oil-water interface to form an emulsion with certain stability, or dissolves the water-insoluble oil into the surfactant micelle by the solubilization of the surfactant. in. Both of these methods allow the oil to be removed to be transferred to the solution.

3. The oil that enters the solution is trapped in the solution.

Since the adsorption film surrounding the oil stain point has sufficient mechanical strength to be broken by the collision, the oil stain will not accumulate again, and the dispersed oil stain point is trapped in the solution and will not be deposited on the surface of the steel drum to cause secondary pollution.

(2), surfactant degreasing cleaning

The surfactant-cleaning agent for removing oil is composed of a surfactant, a cleaning aid, and an additive. The surfactant is generally prepared by combining an anionic surfactant having strong detergency and a nonionic surfactant. The cleaning aid is generally an alkaline substance such as tripolyphosphate steel. The purpose is to make the cleaning liquid alkaline to improve the degreasing ability. The surfactant does not cause corrosion to the steel drum, but the ability to remove old viscous oil is poor. .

(3), mixed solvent degreasing cleaning

The mixed solvent used for the degreasing cleaning may be in various forms such as a lipophilic organic solvent and a surfactant mixture, a mixture of an organic solvent and an aqueous alkali solution.

When the lipophilic organic solvent is present in an emulsified or solubilized state in an aqueous surfactant solution, the two synergistically act to make the cleaning ability better. The degreasing ability of the mixed solvent formed by the alcohol or ether hydrophilic organic solvent and the aqueous alkali solution is higher than that of the solvent alone, and the flammability is greatly lowered. In addition, there is an emulsion cleaning agent, which is an emulsifier to make the organic A mixed solution in which a solvent is dispersed in water. The organic solvent in such an emulsified cleaning liquid is usually a petroleum fraction having a boiling point of 60 to 210 °C. Emulsifiers are anionic surfactants and nonionic surfactants. In order to make the emulsifier have a certain solubility in an organic solvent, some water-soluble coupling agents are often added.