How to avoid the occurrence of orange peel in powder coatings

coating appearance is becoming increasingly important in new equipment manufacturing (OEM) coatingsTherefore, one of the main objectives of the coating industry is to optimize the performance of the coating according to the final requirements of the user, which also includes the surface of satisfactionSurface conditions influence influence on color, gloss, fog shadow and surface structureGloss and image clarity are often used to control the appearance of the coatingHowever, the use of a high gloss coating, the surface fluctuations also affect the appearance of the entire coating film, while the gloss measurement can not control the effect of fluctuations, this effect is also known as "orange peel." Theorange peel or micro-fluctuation is a corrugated structure with a size between 0.1mm and 10mmOn high-gloss coated surfaces, wave-like, light and dark areas can be seenTwo different levels of fluctuation sit can be distinguished: long fluctuations, also known as orange peels, which are observed at intervals of 2 to 3 distances, and volatility, which is called short fluctuations or micro-fluctuations, which are observed at a distance of about 50cmshould point out that sometimes designed to cover the surface defects of the substrate or to obtain a special coating surface appearance is designed with certain degrees of volatility or ripple structureThethus "orange peel" can be defined as a "wave-like structure of a high-gloss surface" that gives the paint layer surface a patterned, unflattened visual appearanceThe control of the visual appearance (gloss, fog shadow, leveling orange peel) of the powder coating filmis very important, especially when the components of different coatings are assembledthis paper briefly introduces the effect of rheumatosis on membrane flow and surface appearance during the filmforming process of powder coatingsThis paper discusses how to avoid the formation of orange peel, and how to judge and compare the phenomenon of orange peel and other aspectsfactors that affect the flow and appearance of the coating in powder coatings
the phase changes in industrial coatings and powder coatings in the preparation and film formation process are unique The lack of solvents to wet and improve film flow makes powder coatings more difficult to remove surface defects than liquid coatings Although the main components of the two are similar, compared to liquid coatings, thermoset powder coatings based on very different mechanisms.
Powder coatings are a solvent-free system During preparation, pigments and other components are dispersed and partially wrapped in low-molecular solid resins by molten mix Powder coatings are used by air to transfer the powder to the bottom of the material (powder suspended in the air) and then by an electrical charge to attach it to the substrate Heating at a predetermined temperature melts the powder particles, gathers together (concentrated) flow (film) and then levels the surface through a viscous liquid phase) and finally chemically cross-linked to form a high molecular weight coating which is the film-forming process of powder coating The factors that affect the flow and appearance of the coating film the film-forming process can be divided into three stages of molten polyconcline forming coating film leveling the most important factors to control the melting knot velocity at a given temperature are the melting point of the resin, the viscosity of the molten powder particles and the size of the powder particles In order to achieve optimal flow effect, the melting junction should be completed as quickly as possible in order to have a long time to complete the leveling phase The use of curing agents shortens the time required to be available for flow and leveling, so that coating films formed by extremely active powders often appear in orange peels The key factors that affect the flow and leveling of the coating film are the melting viscosity of the resin, the surface tension of the system and the thickness of the film The switching melt viscosity depends in particular on the curing temperature, curing speed and heating rate The factors mentioned above, together with the particle size distribution and film thickness, are usually determined by the required coating performance, the objects being coated and the construction conditions of the powder The power of flow and leveling powder when sprayed comes from the surface tension of the system, which has also been mentioned earlier The opposite effect of this force and the gravitational pull of the molecules applied to the coating film results in greater resistance to flow and leveling, such as the higher the melting viscosity The reforest ingress of surface tension and the difference between the gravitational pull between molecules therefore determines the degree to which the coating film is leveled for very well-mobile coatings, it is clear that the surface tension of the system should be as high as possible and the melting viscosity as low as possible This can be achieved by adding additives that increase the surface tension of the system and using resins with low molecular weight and low melting points The coating sidonted based on the above conditions can have excellent fluidity but its high surface tension will cause the shrink hole and the flow hanging and poor corner coating due to the lower meltvisor In practice, the surface tension and melting viscosity of the system are controlled within a specific range so that a qualified coating surface appearance can be obtained The effect of surface tension and melt viscosity on coating flow figure 2 It can be seen that too low surface tension or too high melt viscosity will prevent the membrane flow caused the coating film flow is poor and surface tension is too high when the film will appear in the shrink hole Too low melting viscosity will make the physical storage stability of the powder worse when the corner coating is poor and the construction on the facade produces a flow hanging The final surface condition, defects and deficiencies (e.g orange peel fluidity poor shrink hole pinholes) of the powder coating film, which are obviously obtained in the are closely related to each other and are also controlled by the rhei force stomulation involved in phase change during the filmformation process The distribution of powder particle size also affects the surface appearance of the coating film The smaller the particles because of their low thermal capacity of large particles, the melting time is also faster than the short polymer of large particles to form a better surface appearance of coating Large powder particles may have an orange peel effect when the coating film formed longer than the small particles Powder electrostatic construction methods (corona discharge or frictional discharge) are also a factor in the formation of orange peel How to reduce or avoid the orange peel effect promote steam and leveling to reduce or avoid orange peel The system uses lower melt viscosity, extended leveling time during curing, and higher surface tension to improve flow and leveling Controlling the surface tension gradient is an important parameter to reduce the orange peel, while also controlling the surface tension of the coated surface evenly to obtain the smallest surface area in practice often use flow promoters or leveling agents to improve the appearance of the coating film to eliminate orange peel, shrink holes, pinholes and other surface defects Good flow promoters reduce melting viscosity and thus help melt mixing and pigment dispersion improve the wetting of the substrate, and the flow leveling of the coating helps eliminate surface defects and facilitate air release The relationship between the dosage and effect of mobile modifier should be examined Insufficient use can lead to shrinkholes and orange peels, while excessive use can lead to loss of light, fog shadow and problems with re-coating the upper layer Usually the flow modifier is added during premixing They are either the parent material of the resin (the ratio of the resin to the additive is 9/1 to 8/2) or they are adsorbed in powder to the inorganic carrier The amount of this additive in powder coatings is 0.5 to 1.5% (in effective polymers calculated in base materials) but may also work well at low concentrations Polyacrylate resins are most widely used in flow modifiers such as polyacrylates ("Acronal4F"), acrylic-acrylic ethyl hexalate copolymers and acrylic-acrylic hexalate copolymers They can be used in a wide range of concentrations General polyacrylates have little effect on surface tension and they can help the coating form a relatively constant, uniform surface Compared to additives that reduce surface tension (e.g sioxane, etc.), they do not reduce surface tension and can therefore be used to accelerate leveling Additives to reduce surface tension include surfactants, fluorinated alkylests, and siloxane They are very sensitive to how much they join Resting incense is a degassing agent that also has the effect of reducing surface tension and is widely used to improve the surface appearance of powder coating film The the appearance of the orange peel can be visually or using the mechanical method of the shape detector or through ripple scanning to evaluate and compare the appearance of the orange peel The (1) visual inspection method the template in this test is placed under a two-tube fluorescent lamp The template's reflective light source can be obtained by placing the template appropriately Qualitative analysis of the clarity of reflected light visually evaluates the properties of flow and leveling In the case of poor liquidity (orange peel), two fluorescent tubes look blurry and highly liquid products can achieve clear reflections (2) "Form measurement": in this method, the records the surface shape by the offset of a highly sensitive probe This allows for a quick distinction between roughness, orange peel, and fluctuations caused by shrinkholes, pinholes or dirt (3) Byk-Gardiner "Ripple Scan Orange Instrument Measurement" is designed to simulate visual judgment by focusing "ripple scanning" on the surface The instrument is moved more than 10cm on the sample surface to scan the surface of the sample This method uses a laser light source to irradiate the surface of the template from a 60-degree angle and then measure the reflected light at a relatively similar angle The signal detected the most when the beam hits the peak end or trough on the surface, and the signal that is recorded when the beam hits the peak slope Ripple scanning measures the visually visible optical surface finish, while the shape-measurement instrument measures the mechanical shape finish that people can feel The summary
this paper makes a brief discussion on the filmforming mechanism of powder coatings and expounds the conditions of orange peel shape factors affecting the flow and appearance of coating film, as well as the principle of selecting the best film coating and using flow promoters to avoid these defects In addition, the evaluation method of orange peel is briefly introduced