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    Home > Coatings News > Paints and Coatings Market > How to avoid the occurrence of orange peel in powder coatings

    How to avoid the occurrence of orange peel in powder coatings

    • Last Update: 2020-03-25
    • Source: Internet
    • Author: User
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    In new equipment manufacturing (OEM) coating, coating appearance becomes more and more important Therefore, one of the main objectives of the coating industry is to achieve the best coating performance according to the final requirements of users, which also includes the satisfaction of surface appearance The visual effect is influenced by color, luster, haze and surface structure Gloss and image clarity are often used to control the appearance of the coating However, even if the coating with high glossiness is used, the fluctuation of the surface will also affect the appearance of the whole coating At the same time, the gloss measurement can not control the fluctuation of the visual effect, which is also known as "orange peel" Orange peel or microwave action is a corrugated structure with a size of 0.1mm-10 mm In the high gloss coating surface, people can see wavy, bright and dark areas Two different levels of waves can be distinguished: long waves, also known as orange peel, which can be observed at a distance of 2-3; short waves or microwave waves, which can be observed at a distance of about 50cm It should be pointed out that a certain degree of fluctuation or corrugated structure is sometimes designed for the purpose of covering the surface defects of the substrate or obtaining a special surface appearance of the coating Therefore, "orange peel" can be defined as "wavy structure of high gloss surface", which makes the surface of paint layer produce speckle and unsmoothed visual appearance It is very important to control the visual appearance (gloss, haze, leveling orange peel) of the powder coating film, especially in the assembly of parts sprayed in different fields In this paper, the influence of the flow force on the film flow and surface appearance during the film formation of powder coatings is briefly introduced This paper discusses how to avoid the formation of orange peel and how to judge and compare the phenomenon of orange peel Factors affecting the flow and appearance of coating film in powder coating the phase change of industrial coating and powder coating in the process of preparation and film forming is unique Due to the lack of solvent to wet and improve the fluidity of the coating, it is more difficult for powder coating to remove surface defects than liquid coating Although their main components are similar, they are based on very different mechanisms compared with liquid coatings and thermosetting powder coatings, which are solvent-free homogeneous systems In the process of preparation, pigment and other components are dispersed and partially wrapped in low molecular solid resin by melting and mixing The use of powder coating is to transfer the powder to the substrate through air (the powder is suspended in the air) and then attach it to the substrate through electric charge Heating at a predetermined temperature causes the powder particles to melt, gather together, flow (film formation), level the surface through a viscous liquid phase during this period, and finally form a high molecular weight film through chemical crosslinking, which is the film formation process of the powder coating Factors affecting the film flow and appearance the film forming process can be divided into three stages: melting and coalescence to form film leveling the most important factors to control the melting and coalescence speed at a given temperature are the melting point of resin, the viscosity of molten powder particles and the size of powder particles In order to achieve the best flow effect, the fusion and coalescence should be completed as soon as possible so as to have a long time to complete the leveling stage The use of curing agents shortens the time required for flow and leveling, so the coating formed by the most active powder often presents orange peel The key factors affecting the film flow and leveling are the melt viscosity of the resin, the surface tension of the system and the film thickness In turn, the melt viscosity depends on the curing temperature, curing speed and heating rate The factors mentioned above, together with particle size distribution and film thickness, are usually determined by the required film properties, coated objects and powder construction conditions The power of flow and leveling during powder spraying comes from the surface tension of the system, which has also been mentioned before This force is opposite to the intermolecular attraction applied to the film, which results in the higher resistance to flow and level, for example, the higher the melting viscosity Therefore, the difference between surface tension and intermolecular gravity determines the level of film For coatings with good fluidity, it is obvious that the surface tension of the system should be as high as possible and the melting viscosity should be as low as possible These can be achieved by adding additives that can improve the surface tension of the system and using resins with low molecular weight and low melting point The coating prepared according to the above conditions can have excellent fluidity, but due to its high surface tension, it will cause shrinkage cavity, and due to its low melting viscosity, it will produce sagging and poor corner coating In practical work, the surface tension and melt viscosity of the system are controlled within a specific range so as to obtain a qualified surface appearance of the film See Fig 2 for the influence of surface tension and melt viscosity on film flow It can be seen in the figure that too low surface tension or too high melting viscosity will prevent the film from flowing, resulting in poor fluidity of the film, and when the surface tension is too high, shrinkage cavity will appear in the film forming process Too low melting viscosity will lead to poor physical storage stability of the powder, poor coating performance of the corners during construction and sagging during construction on the facade In conclusion, it is obvious that the final surface condition, defects and deficiencies (such as pinholes with poor orange peel fluidity) of the powder coating film are closely related to each other and controlled by the rheological force involved in the phase transformation during the film formation process The particle size distribution of the powder also affects the surface appearance of the coating The smaller the particles, the lower the heat capacity of the larger particles, the better the surface appearance of the coating However, the orange peel effect may be caused by the coating formed when the melting time of large powder particles is longer than that of small particles Electrostatic powder construction method (corona discharge or friction discharge) is also a factor that causes orange peel formation How to reduce or avoid orange peel effect? Promoting flow and leveling can reduce or avoid orange peel Low melt viscosity, long leveling time and high surface tension can improve the flow and leveling of the system Controlling the surface tension gradient is an important parameter to reduce the orange peel, and at the same time, controlling the surface tension of the coating to be uniform to obtain the minimum surface area In practice, flow accelerator or leveling agent is often used to improve the appearance of the coating to eliminate surface defects such as orange peel, shrinkage cavity and pinhole The flow accelerator with good performance can reduce the melting viscosity, which is helpful for the melt mixing and pigment dispersion to improve the wettability of the substrate The flow leveling of the coating can help to eliminate the surface defects and facilitate the release of air The relationship between the amount of flow modifier and its effect should be investigated If the dosage is insufficient, it will lead to shrinkage and orange peel, but if the dosage is too much, it will lead to the loss of light, fog and shadow and the problem of adhesion to the upper layer Usually, the flow modifier is added during premixing They can be made into resin masterbatch (the ratio of resin and the additive is 9 / 1-8 / 2) or adsorbed on inorganic carrier in powder form The dosage of the additive in powder coating is 0.5-1.5% (in the effective polymer calculated by the base material), but the effect may be good when the concentration is low Among the flow modifiers, polyacrylate resins are widely used, such as butyl acrylate ("acronal 4f"), ethyl acrylate ethylhexyl acrylate copolymer and butyl acrylate hexyl acrylate copolymer They can be used in a wide range of concentrations Generally, polyacrylates have little effect on the surface tension, which can help the coating to form a relatively constant uniform surface They do not reduce the surface tension compared to additives that reduce the surface tension (such as siloxane, etc.) and can therefore be used to accelerate leveling The additives for reducing surface tension include surfactants, fluorinated alkyl esters and siloxanes They are very sensitive to the amount of addition Benzoin is a kind of degasser which can also reduce the surface tension It is widely used to improve the surface appearance of powder coating Judgment of appearance of orange peel can be evaluated and compared visually or by mechanical method of appearance tester or by ripple scanning (1) visual inspection: in this test, the sample is placed under the double tube fluorescent lamp The reflection light of the template can be obtained by placing the template properly Qualitative analysis of the sharpness of the reflected light allows a visual assessment of the flow and levelling properties In the case of poor fluidity (orange peel), the two fluorescent tubes look fuzzy and unclear, while the products with high fluidity can obtain clear reflection (2) "profile measurement method": in this method, the surface shape is recorded by the offset of a highly sensitive probe Thus, the difference between rough, orange peel caused by shrinkage cavity, pinhole or dirty dirt and fluctuation caused by poor flow can be quickly distinguished (3) BYK gardener "ripple scanning orange peel measuring method" is designed to simulate visual judgment by focusing on the surface of the "ripple scanning" Move the instrument more than 10cm on the sample surface to scan the sample surface In this method, a laser source is used to irradiate the surface of the sample plate at an angle of 60 ° and then measure the reflected light at the same relative angle The detected signal is the largest when the beam hits the peak or valley on the surface, and the recorded signal is the smallest when the beam hits the peak slope The wave scanning measures the optical surface finish that people can see visually, while the shape measuring instrument measures the mechanical surface finish that people can feel In this paper, the film-forming mechanism of powder coating is briefly discussed, and the factors affecting the film flow and appearance, the condition of orange peel appearance, the principle of selecting the best film-forming coating and using flow accelerator to avoid these defects are also described In addition, the evaluation method of orange peel is introduced briefly“
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