Drum coating and environmental protection (1)
Coatings are a wide range of fine chemicals used in various sectors of daily life, national economy and national defense development. They have developed very rapidly. The variety extends from general purpose coatings with only decorative and protective functions to the ability to change various surfaces Performance special special coating. At present, the variety of paints has reached nearly a thousand kinds, and the world's annual coating output has exceeded 20 million tons. The development of coatings used by our steel barrel industry has also been rapid. We have developed from the beginning of the use of metal universal coatings to the outer coating and inner coating of steel drums with various functions. The amount of paint used each year has also grown to thousands of tons.
With the enhancement of people's awareness of environmental protection, countries around the world have successively enacted regulations to limit the emission of organic substances, and promote the production and coating of paints in the direction of resource conservation, energy conservation, and pollution-free development.
First, the relationship between steel bucket coating and environmental protection
For environmental pollution, the coating of paint is far more serious than the production of paint. Imagine that the solids content of our steel drum coatings is mostly below 50% (except for powder coatings), and the solid content of PVF coatings is only about 24%. Most of the rest are organic vapors that run to the atmosphere during the film curing process. The lower the solids content, the more serious the pollution of the atmosphere.
In fact, for the steel barrel industry itself, no one wants to put the paint he bought on the fly and let it fly to the sky, turning it into a polluted environment. While reducing environmental pollution, it also reduces the unit consumption of steel drum coatings. Therefore, the relationship between steel drum painting and environmental protection can be said to be related to benefits and harm.
Second, steel barrel coating environmental pollution
In steel drum coating, pre-treatment and different coatings and different construction methods have different environmental pollution.
1, pre-treatment of environmental pollution
In the pre-treatment process, in order to remove oil stains and rust on the drum surface, a clean surface with a phosphating film suitable for coating is obtained to generate a large amount of surfactant, acid, alkali, salt, and various Auxiliaries and heavy metal ions in wastewater. In each of the pre-treatment processes only, the amount of wastewater generated per drum can be as high as 30 to 50 liters.
2. Different paints cause different pollution to the environment
The pollution of the paint to the environment is mainly represented by the pollution of the organic volatiles to the atmosphere. The solid content of the paint and the contained organic volatile matter have become the main indicators of our research on environmental pollution.
a, steel drum outer coating
The most widely used outer coating of domestic steel drums is amino baking varnish, followed by phenolic aldehyde lacquer or phenolic enamel enamel, alkyd enamel and nitro-lacquer, and some barrel mills use powder coatings.
The average solid content of domestically produced amino baking varnish is about 45%, and the specific dry solid content of X-516 steel drum is about 48%. Their main organic volatiles are toluene, xylene, butanol, etc. -516 baking paint has a low benzene content.
The phenolic blend paint has a high solids content, generally more than 60%, and its organic volatiles are mainly benzenes and No. 200 solvent gasoline.
The alkyd enamel has an average solids content of more than 45% and its main organic volatiles are xylene and 200# solvent gasoline.
The nitrocellulose lacquer appears to be very thick and has a high viscosity, but its solid content is not high. There are three kinds of nitrocellulose paints that can be used on steel drums. Generally, the solid content is between 30 and 40%. Their main organic volatiles are It is acetate, ketone, butanol and benzene. Since they need to incorporate more diluent than any other paint during use, the nitrocellulose paint during construction is the most environmentally polluting relative to other drum paints.
The least polluting environment is the powder coating, which is 100% solids. The only thing that will volatilize into the atmosphere at the time of baking is the resin and the small amount of the additive when it decomposes at high temperature.
b , drum coating
At present, the domestic steel barrel industry is mainly used in three major categories: epoxy, PVF and phenolic.
Epoxy coating is the earliest use, but also the best cost-effective domestic and foreign steel drum industry is still commonly used in traditional coatings. The domestic production of epoxy coatings has a solid content of about 36%, organic volatiles are mainly xylene, cyclohexanone, alcohols and esters; X-622 steel drums have a solids content of around 40% for epoxy interior coatings. Its organic volatiles already contain no xylene.
PVF coating viscosity is much larger than other coatings, in fact, its solid content is only about 24%. The organic volatiles are mainly dimethyl phthalate, propylene carbonate and the like.
Phenolic steel drum coating is currently only X-362 type, its viscosity looks very low, only about 20 seconds (coating -4 cups 25 °C), but its solid content is also up to about 40%, the main organic volatiles Ketone alcohols.
In addition, the only patented product in steel drum coatings in China and the two-component normal-temperature and quick-drying X-891 paint (which is divided into electrogalvanized steel barrels Transparent exterior paint, steel barrel surface baking paint anti-corrosion anti-stripping anti-stripping paint transparent lightening coating, open steel barrel coating three anti-growth transparent mirror coating, colored steel drum coating anti-corrosion coating, etc.) also increased rapidly, this kind of The solid content of the coating is about 40%, and the organic volatiles are mainly acetates.
3. Different effects of different construction methods on the environment
As mentioned above, for environmental pollution, especially atmospheric pollution, paint construction is far more serious than paint production. During the construction process, all organic solvents that account for more than 50% of the total weight of the solvent-based paint should be volatilized. The amount of solvent used in coating production in the atmosphere is less than 0.25% of the amount of solvent used in coating production. Although paint production also generates waste, the total amount of all wastes does not exceed 3% of the paint production.
In the coating construction process, according to different construction methods, but the loss of the paint can account for 20 to 80% of the amount of paint used. In areas where the industry is relatively concentrated, the organic volatiles in the coating construction process account for more than 27% of the total hydrocarbon emissions in the area.
China's steel barrel industry is currently the main coating methods are artificial brushing, air spray, high pressure airless spray, electrostatic spraying, spin-coating and so on. Among them, the method of painting with a minimum amount of scattered paint mist or even no flying paint during construction is to be considered electrostatic spraying and manual brush painting, and the spray mist is the most important for construction spraying. After air spraying, the amount of paint that eventually forms a coating on the surface of steel drums only accounts for less than 30% of the total amount of paint used for the initial construction, while the organic volatiles that go to the atmosphere alone account for more than 50% and the remaining 20%. The left and right paints are not brought into the atmosphere by organic vapours in the form of paint mist, and they are polluted by the waste residue.
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