Strip coating processing technology for aluminum profiles

1 Introduction

Aluminum coating organic coating surface treatment process will inevitably appear unqualified coating quality products, such as electrophoretic coating defects often occur in the film low, yellowing, particles, bubbles and other defects, powder electrostatic spray often appear pinholes, bubbles Defects such as granules, strands, and cross-coloring, if they are scrapped, will inevitably affect the yield and delivery timeliness, and thus cause unnecessary losses. Therefore, more and more aluminum companies are concerned about how to remove unqualified coatings on aluminum profiles and recycle substrates for rework. In addition, in order to ensure the electrical conductivity of the workpiece during the electrostatic spraying process, the spray coating tool is also subjected to a peeling treatment of the organic coating.
Demoulding basic requirements:
1, can not have any damage to the original aluminum alloy workpiece, including the appearance of size, material composition, surface hardness, finish, roughness are not allowed to have the slightest change. This means that only the membrane can be removed and the body cannot be injured;
2. The film is clean and free from imprints. The surface of the aluminum material cannot have starpoint film traces, that is, it must be removed.

Aluminum stripping organic coating methods can be roughly divided into three categories: chemical methods, pyrolysis methods and mechanical methods. The chemical release method uses an etchant, a solvent or a combination of the two; the pyrolysis method utilizes a high temperature furnace, an open flame, a hot fluidized bed, a molten salt bath, and a laser to perform the release treatment; a mechanical release method uses a high pressure water, an abrasive medium , Brush, Mill, Scraper, Cutting Machine, Low Temperature Treatment, etc.

2 Chemical stripping

The working principle of the chemical release agent is to soften or dissolve the coating film, to break the connection between the coating film and the substrate, and then the loose coating film is mechanically removed. Chemical release agents can be classified according to operating temperature (hot or cold) and can be classified according to their composition, such as corrosives (acidic or basic), solvents or corrosive/solvent combinations. The corrosive agent stripper generally uses a heating process, and the stripping temperature reaches 80 to 100°C.

Solvent-based chemical release agents are cold-processed and operate at or near room temperature. Solvent-based chemical release agents are usually liquids obtained by mixing solvents such as chlorinated hydrocarbons, ketones, esters, alcohols, and benzenes. The use of a solvent to osmotically swell the overcoat layer can effectively remove various aluminum surface-treated coatings, and it is easy to directly peel the coating film or peel off the coating film. The release of the release agent is achieved through a series of physical and chemical processes such as dissolution, infiltration, swelling, exfoliation, and reaction.

There are many chemical release agents that are currently commercially available. The advantages of chemical stripping agents are the variety of formulations and the ability to remove highly durable coatings such as acrylics, epoxies, polyesters, and polyurethanes with minimal impact on the aluminum substrate. The disadvantages are potential health and environmental hazards related to the use of corrosive materials, chlorine-containing solvents, and flammable solvents.

Corrosive release agents are usually acidic and are strippers composed of strong acids such as concentrated sulfuric acid, hydrochloric acid, phosphoric acid, and nitric acid. Concentrated hydrochloric acid and nitric acid are easily volatile to produce acid mist and corrode the aluminum base material. Concentrated phosphoric acid stripping takes a long time and has a corrosive effect on the substrate. Therefore, the above three acids are used less frequently. Concentrated sulfuric acid and aluminum, iron and other metals passivation reaction, the corrosion of aluminum is very small, while the organic matter has a strong dehydration, carbonization and sulfonation to make it soluble in water, so concentrated sulfuric acid is often used as an acidic release agent .

Because solvent-based mold release agents are cold, ie, operate at or near room temperature, they are slower than hot, corrosive release agents and take longer to coat. Therefore, a combination of release agent came into being. The combined release agent formulation uses corrosive materials and solvents that combine the advantages of both. They can remove the most durable and chemical resistant coatings. They can operate at or near room temperature, operate at a temperature lower than the boiling point of the solvent, and have a speed of action between the corrosive release agent and the solvent-based release agent. Domestic products have been listed, the use of good results.

3 Pyrolysis and film removal

The pyrolysis stripping equipment includes an open flame, a high temperature furnace, a fluidized bed, and a molten salt bath. At high temperatures, most organic coatings can be pyrolyzed in a relatively short period of time. The advantage is that the film removal speed is fast and thorough. The disadvantage is that it consumes a lot of energy and damages some substrates, and it is not applicable to large aluminum products such as aluminum profiles and can only be used for small aluminum workpieces.

High temperature furnace stripping is the partial pyrolysis of organic coatings in a high temperature and low oxygen atmosphere. The first stage will produce volatile organic compounds, leaving behind carbon and inorganic compounds; the second stage will burn the carbon left on the aluminum at high temperatures and excess oxygen to form carbon dioxide. The inert pigments and fillers remaining on the substrate afterwards are removed mechanically. In order to meet air quality standards, an afterburner is placed in the high-temperature furnace exhaust to burn off residual organic volatile compounds to form carbon dioxide and water. In more efficient systems, heat exchangers can recycle energy.
Hot fluidized bed stripping is the use of a pyrolysis process in a fluidized bed, and the medium suspended in the fluidized bed is hot sand. The fluidizing gas can be oxygen deficient or oxygen-enriched, depending on the process requirements. This closed loop system has dust collectors, afterburners and heat exchangers. Organic volatile compounds are converted into carbon dioxide and water after the burner is burned, and the discharge is up to standard. Heat exchangers increase thermal efficiency.

Molten salt bath stripping utilizes patented fusion, oxidation, and inorganic salt bath processes. The coated part is soaked in a salt bath for 5 to 10 minutes, which is related to the formulation of the salt and the composition of the coating film. The gas discharged from the circulatory system is treated to meet air quality standards. A number of patented molten salt baths have been developed that work at lower temperatures to perform release treatment on certain aluminum products.

Laser stripping is a high-tech method that uses laser beam energy to pyrolyze organic coatings. The laser beam moves automatically along the workpiece and decomposes the film as it passes through the coating. The process is very slow, and the best film release is on a flat substrate.

4 Mechanical stripping

Mechanical release includes many conventional methods such as sanding, scraping and cutting, shot peening, etc. by hand and power tools. Mechanical stripping also includes high pressure water stripping and low temperature stripping. Mechanical stripping is usually a supplement to other methods to completely remove loose coating residues.

Hand sanding, scraping and cutting are still used in practical work. The tools used in this method include sanding pads, sandpaper wire brushes, doctor blades, screed hammers, rotary wire brushes, and laminated polishing wheels. The advantage is that the film is removed at a high speed, and the disadvantage is that the labor intensity is high.

Sand blasting uses various types of sanding media, which are applied to the workpiece by centrifugal force generated by compressed air or low pressure water. Centrifugal blasting machines use motor-driven wheels to smash media to the surface of the coating. In air blasting, the medium is thrown onto the surface of the workpiece by the compressed air flow. These physical processes are carried out in closed containers and are therefore only suitable for small aluminum products.

The blasting medium in water blasting is delivered in a low pressure stream. The advantage of blasting and stripping is that it can quickly remove the coating from most substrates and meet environmental requirements. The disadvantage is that the workpiece is distorted and worn by the impact of the media. At present, sand blasting and stripping media mainly include sand, steel shots, plastics, ice crystals and carbon dioxide beads.

The traditional sand and steel balls and other sanding media have been used for sandblasting. Because of their high hardness, they are also the most abrasive. Their advantages may be masked by their distortion and wear on the substrate.

High-pressure water stripping utilizes a pressure of 15000 to 50000 Pai to impact the surface of the workpiece and remove the coating. Using a specially designed nozzle, even the most durable coating can be removed by changing parameters such as water pressure, impact angle and residence time. membrane.

5 Concluding remarks

There is no "best" way to remove organic coatings. All of the previously mentioned methods can be effectively stripped. Only after careful consideration of the dimensions of the stripped aluminum profiles, coating properties, operating costs, environmental impacts, and worker safety, can an accurate choice be made.