Technology and development of coatings in steel drums

Technology and development of coatings in steel drums

[Singapore] David Thomas

[translation] Yang Wenliang

1.0 Preface

Due to the ever-changing petrochemical, food and pharmaceutical markets, steel drum packaging is becoming more and more difficult to adapt. Today's unchanging internal coating specifications and techniques cannot meet the requirements, and effective packaging must meet a series of most basic elements.

Proper selection of internal coatings and the distinction between different contents of the steel drum are critical. Therefore, in order to achieve the correct and effective coating in the steel drum, we must:

1. Correctly understand the type and nature of various internal coatings.

2. Ensure the correct selection of technical specifications and composition of the internal coating.

3. Ensure correct internal coating construction methods and final results.

2.0 Type of paint inside

There is no universal interior coating anywhere. If there is some kind of optimal inner coating, it must be low cost and easy to apply, with good benefits and wide adaptability. It also resists the most irritating acids, bases and all other chemical mixtures and is resistant to odors, colors or other contaminants from outside the contents.

2.1 three series

â—† epoxy based coating

â—†Phenolic base colorless and colored coating

â—†Epoxy-phenolic based colorless and colored coatings

2.2 Five basic performance requirements

All types of inner coatings must meet five performance requirements regardless of the content requirements. They are:

â—†Chemical properties

The inner coating must exhibit chemical reactivity that is not easily responsive to the steel drum itself and its contents.

â—†Adhesion

There must be a strong adhesion between the inner coating and the inner wall of the steel drum.

â—†Flexibility

The inner coating should have a certain flexibility or elasticity, so that the steel drum is not easy to rupture the inner surface during operation.

â—† resistance to permeability

The inner coating should be low enough to resist the slight penetration of the contents.

â—†Convenience of construction

In the manufacturing process of the steel drum, the operator can carry out the internal coating construction more conveniently and can meet the use requirements.

2.3 General performance range:

The manufacturer of the inner coating must provide different types of performance indicators for the inner coating so that the user can understand and properly use the resin-based type of the inner coating.
The general properties of the inner coating are listed below:

3.0 Selection of coatings in steel drums

The basic selection criteria for some coatings for steel drums are listed below. The initial starting point here is to divide the contents of the steel drum into two types: acidic and alkaline products.

3.1 Epoxy based coatings Epoxy based coatings are suitable for use in alkaline products and solvents with low solubility.

Such coatings are more suitable for products such as cosmetics, foods and chemicals with higher pH.

3.2 phenolic based coating

Acidic products and some more soluble solvents, including ketones and alcohols, are more suitable for colorless or colored phenolic coatings. It is also suitable for corrosive products with a slightly lower pH value and chemical solvents with stronger solvency.

3.3 epoxy-phenolic based coating

Generally, paint products made from phenolic materials have less flexibility (elasticity) and can be used as inner coatings for most steel drums. However, it has greater incompatibility with highly corrosive chemicals. However, epoxy-phenolic based coatings made with high technology are the preferred coatings.

Therefore, most coating manufacturers are developing such coatings to solve the problem of insufficient flexibility of epoxy coatings.

This formulation is extremely detailed and requires careful weighing to find the optimum ratio of epoxy resin to phenolic resin for better flexibility and chemical resistance. In addition to the epoxy-phenolic ratio, the type of phenolic and the composition of the epoxy also have better adaptability and good economy.

3.4 Inner paint selection chart

The following chart lists the different properties and characteristics of the three types of coatings for the correct selection and application by the user of the material.

Inner coating type use and performance film forming properties (1) phenolic a) colorless coating

Good adaptability to acidic chemicals

Poor anti-penetration performance

Can be used to coat acidic water-based paint products

Can be used to package food, solvents and low corrosion materials

Low adaptability to alkaline chemicals

The paint film is hard and brittle, and it is easy to crack during transportation and handling.

Non-toxic

Thermosetting

Good adhesion after surface treatment

It is recommended to adjust the coating thickness to improve the hardness and adhesion of the film.

b) colored paint

Low resistance to penetration

Improved performance (depending on the type and amount of pigment)

Same as above (2) epoxy-phenolic a) colored coating

Good chemical adaptability

Most suitable for alkaline products

Good adaptability to weakly acidic chemicals, detergents, food solvents or weakly alkaline solvents

Not suitable for strong acidic products

The coating film has good elasticity and good adhesion, and can resist the bump in the handling.

Non-toxic

Thermosetting resin, the baking process is harsh, the temperature should not be too low, otherwise it will affect the film quality

b) colorless paint

Low resistance to penetration

Lower permeability than pure phenolic coating (3) Epoxy-based color

Good adaptability to chemicals with higher acidity

Bad adaptability to alkaline substances

Has good elasticity and durability

4.0 Inner Coating Application Technology

After selecting the appropriate internal coating, the application of the internal coating is also very important, which affects the final packaging quality. The technical requirements for the application of the inner coating described below must be carefully implemented.

4.1 Steel drum inner surface pretreatment

4.2 Construction method

4.3 Construction process

4.4 Film thickness

4.1 Steel drum surface pretreatment

Before the internal coating, if the surface pretreatment is not performed, the coating film is always weak unless the surface of the steel sheet never rusts and sticks foreign matter.

4.2 Construction method

The construction of the inner coating can basically adopt the conventional spraying method, the airless spraying method, the centrifugal spraying method and the roll coating method. Spraying equipment should be suitable for spraying high viscosity coatings, easy to operate, uniform film coating and high speed. In the case of ensuring a certain degree of coating film, it does not sag and bake without foaming.

4.3 Construction process

The most important construction requirement for internal coatings is temperature control during processing. Epoxy-phenolic coatings are thermosetting Type 5 coatings with a minimum baking temperature of approximately 195 ° C to achieve the correct coating results.

Practice has proved that the epoxy-phenolic paint has the most severe working temperature, and the temperature of the steel drum must reach the peak of 180-240 °C in 10 minutes. The baking temperature of a single type of coating is normal. The first time when the two coatings are '170 ~ 175 ° C, the second coating is feasible after the coating is completely cured, and the temperature and time are the same as the first time.

The use of the internal coating should be carefully carried out according to the construction requirements of the selected coating. If the coating is not completely dry during construction, the construction will not work. It is necessary to regularly check whether the baking time and temperature are correct, and finally whether the adhesion strength of the coating reaches the requirements, whether the elasticity and adaptability of the coating film are good, and the like.

4.5 Film thickness

The thickness requirements of the inner coating are generally proposed by the paint supplier's root coating characteristics, but it is only a recommended range, and the actual processing thickness should also be considered to meet the process requirements of the subsequent processing of the steel drum.

Although reducing the coating thickness of the phenolic coating can effectively reduce the coating cost, it is necessary to prevent the excessive reduction of the coating film quality.

5.0 New developments in internal coatings

In recent years, environmental pollution problems of internal coatings have received increasing attention, and most coatings manufacturers have focused their research on reducing paint contamination.

The development of internal coating products is energy-saving low-temperature curing coatings and water-based coatings that reduce air pollution. Because manufacturers that cause air pollution in some countries are subject to environmental taxes.

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