Powder coating and its construction precautions
Powder coating refers to a new type of solvent-free 100% solid powder coating. It is solvent-free, non-polluting, recyclable, environmentally friendly, saves energy and resources, reduces labor intensity and has high mechanical strength of the coating film.
Powder coating is a completely different form from the general coating, it exists in the state of microfine powder. It is called powder coating because no solvent is used. The main characteristics of powder coatings are: harmless, high efficiency, resource saving and environmental friendly.
It has two major categories: thermoplastic powder coatings and thermosetting powder coatings.
Thermoplastic powder coatings are composed of thermoplastic resins, pigments, fillers, plasticizers and stabilizers. Thermoplastic powder coatings include: polyethylene, polypropylene, polyester, polyvinyl chloride, chlorinated polyether, polyamide system, cellulose system, polyester system.
Thermosetting powder coatings are composed of thermosetting resins, curing agents, pigments, fillers and additives, etc. Thermosetting powder coatings include: epoxy resin system, polyester system, acrylic resin system.
Disadvantages: uneven powder on the edges and corners, cured film defects are difficult to cover, high curing conditions.
There are many classification standards, and the names of various types of powders on the market vary. According to the nature of film-forming substances are divided into two categories: thermoplastic powder coatings and thermosetting powder coatings. Or the appearance of film-forming substances is divided into: matte type, high-gloss type, art type, etc. It can also be divided into two categories according to the use environment: indoor type and outdoor type.
Although there are not as many varieties of powder coatings as solvent-based coatings, there are many polymer resins that can be used as powder coatings. In general, they can be divided into two categories: thermosetting and thermoplastic.
- Thermosetting powder coatings
Thermosetting powder coating refers to the thermosetting resin as the film-forming material, adding the cross-linking reaction of the curing agent after heating can form a hard texture of insoluble and non-melting coating. The coating will not soften at higher temperature like thermoplastic coating, but only decompose. As the resin used in thermosetting powder coating is a low polymerization degree of prepolymer, low molecular weight, so the leveling of the coating is better, with better decorative, and low molecular weight of the prepolymer after curing, can form a network of cross-linked macromolecules, so the coating has good corrosion resistance and mechanical properties. Therefore, the development of thermosetting powder coatings is particularly rapid.
1.1 Epoxy powder coating
Because of the excellent adhesion with metal, anti-corrosion, hardness, flexibility and impact strength, so is the first application of thermosetting powder coatings in the species. Epoxy powder coating is formulated by epoxy resin (Epoxy Resin), curing agent (curing agent), pigment (pigment), filler (filler) and other additives (assistant) are composed. The contribution of these components to the performance of the formed powder coating is mutual constraints and influence, a suitable formula, in fact, is the result of the coordination of various components.
1.2 Polyester powder coatings
Polyester powder coatings have unique properties compared with other types of powder coatings. Performance in weather resistance, UV rotational performance than epoxy resin. In addition, because polyester resin with polar groups, so the powder rate is higher than epoxy resin, baking process is not easy to yellowing, high gloss, good leveling, rich film, light color and other characteristics, and thus has a very good decorative properties. Generally, it is mostly used in refrigerator, washing machine, vacuum cleaner, instrument shell, bicycle, furniture, etc.
1.3 Acrylic powder coating
There are two kinds of acrylic resin powder coatings: thermoplastic and thermosetting. The biggest advantage of thermosetting acrylic resin powder coatings is excellent weather resistance, color retention, pollution resistance, strong metal adhesion, excellent film appearance, and suitable for decorative powder coatings.
- Thermoplastic powder coatings
Thermoplastic powder coating is the emergence of 1950, it melts under the spraying temperature, and solidifies into a film when cooling. Due to the simple processing and spraying method, powder coatings only need to be heated and melted, leveled, cooled or extracted to solidify into a film, without the need for complex curing devices. Most of the raw materials used are common polymers in the market, and most of the conditions can meet the requirements of the use of performance. However, there are some shortcomings, such as high melting temperatures, low coloration levels, and poor adhesion to metal surfaces. Nevertheless, the commonly used thermoplastic powder coatings still exhibit some unique properties, including polyolefin powder coatings with excellent solvent resistance; polyvinylidene fluoride coatings with outstanding weatherability; polyamide with excellent abrasion resistance; polyvinyl chloride with a good price/performance ratio; thermoplastic polyester powder coatings with beautiful appearance, high artistry and other advantages. These characteristics make the thermoplastic powder coatings in the coatings market occupies a large proportion.
2.1 Polyvinyl Chloride Powder
It is one of the cheapest polymers produced on an industrial scale. It has excellent solvent resistance, good corrosion resistance to water and acid, impact resistance, salt spray resistance, protection against food contamination and high insulation strength for electrostatic spraying. It is mainly used for coating metal mesh panels, steel furniture, chemical equipment, etc.
2.2 Polyethylene powder coating
It has excellent anti-corrosion properties, chemical resistance and excellent electrical insulation and UV radiation resistance. The disadvantage is that the mechanical strength is not high and the adhesion to the substrate is poor. It can be used for chemical pool tank, impeller, pump, pipe inner wall, instrument shell, metal plate, refrigerator inner mesh plate, auto parts, etc.
2.3 Nylon powder coating
Nylon (Nylon) is also known as polyamide, because the N atom of the chlorine group on the molecular chain and the hydrogen atom on the adjacent chain segment are easy to form hydrogen bonds, so the melting point of polyamide resin is generally higher. Nylon has the advantages of high mechanical strength, impact resistance, hardness, wear resistance and small coefficient of friction, low dust absorption, etc. It can be used for parts with special requirements. For example, it is used for water pump impellers, textile machinery parts, starting piston parts of diesel engines, impellers of motor sailboat propellers, automobile wheels, motorcycle brackets, agricultural machinery, construction and sports equipment.
In addition, because of nylon’s resistance to salt water and inert to mold, bacteria, is very suitable for manufacturing dipped in seawater or contact with seawater coating, while nylon powder coatings are non-toxic, tasteless, not eroded by mold, will not promote the growth of bacteria, very suitable for spraying food industry parts, drinking water pipes and food packaging, etc.
2.4 Fluorine Resin Powder Coating
There are many kinds of fluoropolymers that can prepare powder coatings, such as polytetrafluoroethylene (PTFE), polytrifluoroethylene (PTFCE), polyvinylidene fluoride (PVDF), etc. PTFE has a melting point as high as 327℃ and can be used for a long time in the range of -250~250℃. In addition, it has excellent corrosion resistance, even in aqua regia, excellent dielectric properties, very low coefficient of friction and self-lubrication, so it is known as the “King of Plastics”. It is widely used in petroleum, chemical anticorrosive coating, sealing, bearing lubricating material, electronic and electrical materials, ship launching guide and non-stick pan coating, etc.
The price of polytrifluoroethylene is cheaper than PTFE, the processing temperature can be reduced, and the coating can be used below 130℃ for a long time. The resistance to alkali and hydrogen fluoride corrosion is better than that of acid-resistant enamel, and the resistance to hydrochloric acid, dilute sulfuric acid, hydrogen chloride and chlorine corrosion is better than that of stainless steel equipment. It has been widely used in chemical plants, pesticide plants, pharmaceutical plants, detergent plants and other anti-corrosion equipment.
The biggest advantage of polyvinylidene fluoride powder coating is excellent weather resistance, high degradation resistance in outdoor exposure, and does not absorb dust, it is easy to maintain the original gloss. 5 Chlorinated polyether powder coating. Chlorinated polyether has excellent chemical stability, coating film on a variety of acids, alkalis and solvents have good corrosion resistance, anti-dissolution properties, chemical stability second only to polytetrafluoroethylene, mechanical and friction properties are also very good. Chlorinated polyether powder coating is mainly used in chemical equipment, pipeline lining, instrumentation equipment shell, etc.. The disadvantage is that it has poor adhesion to metal. By adding additives can improve the adhesion with metal.
2.5 Emulsion powder coating
Emulsion powder is made by spray drying emulsion, mostly vinyl acetate copolymer. The paint produced with latex powder is called latex powder coating. It is the most environmental friendly paint in wall coating.
1、The product does not contain toxicity, solvent and volatile toxic substances, so there is no poisoning, no fire, no “three waste” emissions and other public hazards, in full compliance with the requirements of national environmental protection law.
2、High utilization rate of raw materials, some well-known brands of powder suppliers to produce powder, its over spray powder can be recycled, the highest utilization rate can even reach more than 99%.
3、After pre-treatment, one-time construction, no primer, you can get enough thickness of the coating film, easy to achieve automated operation, high production efficiency, can reduce costs.
4、Dense coating, good adhesion, impact strength and toughness, high edge coverage, excellent chemical resistance and electrical insulation properties.
5, powder coating storage, transportation safety and convenience.
Powder coating construction process and requirements: the so-called powder electrostatic spraying is the use of high-voltage electrostatic corona electric field principle. In the head of the spray gun metal guide on the negative high-voltage, the workpiece being sprayed grounded to form a positive electrode, so that the gun and the workpiece between the formation of a strong electrostatic electric field.
When the compressed air as a carrier gas, the powder coating from the powder supply barrel through the powder tube to the gun’s guide rod, due to the guide rod connected to the high voltage negative corona discharge, in the vicinity of its dense negative charge, so that the powder with a negative charge, and into the electric field strength of the electrostatic field is very high, in the electrostatic force and the dual role of the carrier gas, the powder evenly fly to the grounded workpiece surface to form a uniform thickness of the powder layer The powder is then heated and cured into a durable coating film. Coating construction process Pre-treatment a dry remove water a spraying a check a baking a check a finished product.
1、General powder coating construction requirements
(1) In order to give full play to the characteristics of powder coating and extend the service life of the coating film.
(2) When spraying, the object to be coated must be completely grounded to increase the efficiency of powder coating.
3）Check immediately after powder spraying, if defects are found, they should be dealt with in time, if defects are found after curing, the scope is small only partially and does not affect the surface decoration of the painted object, the same color powder with acetone dilution can be used to repair, if the scope is large and affects the quality of the surface, then sandpaper sanding, spraying again or use paint stripper to remove the coating, and then re-shoe powder.
(4) recycled powder shall be screened to remove debris, according to a certain proportion of the role of mixing with the new powder.
2, art type powder spraying construction requirements
Fine art powder, with beautiful, three-dimensional, decorative effect, etc., but the construction process requirements are strict.
(1) when spraying powder input air pressure should not be too large, generally controlled at 0.5-1.5 kg/cm2 is good. The air pressure will cause poor clarity of the pattern or produce some numb spots. Electrostatic voltage should not be too high, generally controlled at about 60-70 kV. Voltage is too high, it will make the powder attached to the surface of the workpiece rebound phenomenon appears pockmark. Defects such as poor leveling.
(2) spraying powder to ensure that the thickness of the coating film, generally controlled between 70-100μm can be conducive to the formation of obvious patterns and larger patterns, thin coating film is not obvious patterns, and patterns are also small, while there will be a pockmarked bottom and other defects.
3) Curing must be baked according to the specified temperature and time. If the temperature is too low and time is too short will cause the powder can not be patterned, while the mechanical properties due to incomplete curing is also greatly reduced. In addition, because of the special nature of the production process of art-type pattern powder, with recycled powder re-spray, the pattern will be smaller or inconspicuous changes, it is generally recommended that the art-type powder recycling powder should not be used. To use must also be tested. From the above, although the construction requirements are strict, I believe that as long as the above factors are well grasped in the painting process, the ideal satisfactory effect will be achieved.
1、Healthy and environmental protection
It eliminates the need to add all kinds of liquid chemical additives such as film forming, dispersing, wetting, leveling, anti-corrosion and anti-mildew in order to achieve the coating performance.
2、Convenient transportation and storage
Common coatings contain about 20–50% water or solvent, while powder coatings, neither water nor solvent, completely solid, easy and safe transportation, in addition to water or solvent coatings, when the transport and storage temperature below 0 ℃, often freeze, powder coatings do not have this problem.
3、No need of preservative
In traditional liquid coatings, there is both water and bacterial food, which is easily contaminated by bacteria. Therefore, in order to prevent deterioration, preservatives should be added, while latex powder coating has no bacterial contamination problem and does not need preservatives
4、Smell of paint
Compared with the general traditional paint odor residue, this paint can be dissipated quickly, so it can be said that it can be applied immediately.
The curing condition has some limitations.
The biggest growth area of our powder coating coating market is: marine industry and pipeline industry. The problem is that the impact resistance and moisture absorption need to be improved and enhanced, and the researchers focus their main efforts on the modification with various base materials. Domestic production of pipeline anti-corrosion epoxy powder coating coating in storage stability and coating constructability, compared with foreign high-quality products still have a certain gap.
As foreign automotive powder coating preparation technology has made progress, automotive overcoat, wood and plastic with paint coating are the most dynamic potential market. The automotive industry will be its big consumer, Greece, Turkey and Eastern Europe powder coating coating market growth momentum is good, the Asia-Pacific market potential is greater, has been considered the fastest growing region in the world.
The coatings are prepared by mixing special resins, color fillers, curing agents and other additives in a certain ratio, and then prepared by the process of hot extrusion and crushing through sieves. They are stored and stabilized at room temperature, electrostatic spraying, friction spraying (thermosetting method) or fluidized bed dip coating (thermoplastic method), and then heated and baked to melt and cure to form a flat and bright permanent coating film to achieve the purpose of decoration and corrosion prevention.
Electrostatic spraying (refer to the coating process of powder coating for details)
Surface treatment of coated object → electrostatic powder coating → melting leveling or cross-linking curing → cooling → product
(1) In order to give full play to the characteristics of powder coating and prolong the service life of the coating film, the surface of the object to be coated should be treated strictly beforehand.
(2) When spraying, the coated material should be completely grounded to increase the efficiency of powder coating.
3）For the coated objects with large surface defects, conductive putty should be applied to ensure the flatness and smoothness of the coating film.
4）After spraying, the object should be heated and cured, and the curing condition should be based on the technical index of the powder product, but the curing temperature and time must be fully guaranteed to avoid quality accidents caused by insufficient curing.
(5) immediately after the powder spraying inspection.
(6) recycled powder shall be screened to remove debris, according to a certain proportion of the new powder mixed with the role.
(7) powder supply barrel, powder spraying room and recovery system should avoid its