Flame treatment principle
Flame treatment is a widely used process for surface modification of substrates such as plastics, glass and metal foils to improve their adhesion to liquids such as inks, paints, binders and coatings. Plastics used for flamming are thermoplastics and thermosets. When heated, thermoplastics will gradually soften and melt with the increase of temperature, and will become hard after cooling.At the same time, this process is reversible.
We generally use surface tension or surface energy to measure the ability of the substrate to adsorb liquids, and if the substrate surface energy is at a high level, the liquid will become more wetted on its surface and have good ductility. Making the liquid more wetted on the substrate is the basis for the substrate to adsorb the liquid.
If the liquid is spherical on substrate surface, it indicates the substrate surface tension is lower than that of the liquid and the wettability is poor. If the liquid forms a large area of uniform liquid film on the substrate surface, it indicates that the surface tension is greater than the liquid surface and it has better wettability.
Flame treatment can increase the substrate surface tension so that the bond strength of the material surface and the coating tends to be larger, easier to print, spray and bond.
Flame treatment involves a series of complex physical and chemical reactions.
The physical mechanism of flame treatment: high-temperature flame transfers energy to the grease and impurities on substrate surface, so that it is heated and evaporated, which plays a cleaning role.
The chemical mechanism of flame treatment: the flame contains a large number of ions, has strong oxidation, and oxidizes with the surface of the treated object at high temperature, so that a layer of charged polar functional group is formed on the surface of the treated object, which improves its surface energy and thus also improves its ability to adsorb liquids.
This feature of thermoplastics is exactly what flame spraying needs.Therefore, the plastics currently used for flame spraying are mainly thermoplastics, of which the most widely used is polyamide 1010, that is, nylon 1010, and polyethylene, such as PE polyethylene.
The common feature of thermoplastics is low melting point, for example, nylon 1010 has a melting point of 200-210 degrees, and polyethylene has a melting point of 123-130 degrees. The difference between the ignition point temperature or decomposition temperature of the powder and the melting point temperature is small. Therefore, flame treated plastic spraying must effectively control the flame temperature and the degree of heating. It is necessary to ensure that the heated plastic particles are both in a molten state and below the ignition point or decomposition temperature, otherwise the plastic powder will burn or deteriorate. This is the technical key to plastic flame spraying. The method to solve this problem is to add a layer of air (referred to as aerosol or air curtain) outside the oxygen-acetylene flame of the gas welding gun on the basis of the original gas welding gun, which is the flame directly heating the air, and then let the air heat the plastic powder, so that the required spraying heating temperature can be reached to complete the spraying operation.
The process of plastic spraying
Degreasing and cleaning of the surface to be sprayed -> sanding of the surface to be sprayed -> flame preheating of the surface to be sprayed -> spraying plastic coating(the degreasing and cleaning of the surface should be determined by the specific conditions of the parts, generally divided into coarse and fine cleaning. When cleaning finely, acetone is commonly used for cleaning).
Plastic flame spraying is mainly used to improve the physical, chemical and mechanical properties of the material surface, and also has the role of decorative. At present, this technology has been gradually applied to product manufacturing, equipment maintenance and preparation of anti-corrosion coatings. With the needs of modernization construction in all walks of life and the development of plastic flame spraying technology, the application of plastic flame will become more and more extensive.
