Technology progress of coil powder coating paint
Pre-coated coil can be used in building internal and external wall panels, and there are broad prospects in the appliance, automotive, metal furniture and other industries. From the 1980s, China began to introduce and absorb foreign technology, especially in recent years due to the building materials market and the automotive electronics market costs and environmental requirements,a large number of domestic coil powder coating production line launched.
The powder coating is known for its high efficiency and environmental protection,China has become the world’s largest powder coating market.The typical powder coating line speed of 10m/min, but attention to the extent of this curing cycle,more and more close to the saturation point.The new breakthrough for traditional powder beginning to emerge,including medium density fiberboard,plastic parts,heat-sensitive components pre-assembled,such as electric motors,pneumatic compression springs and other coating.
The powder coating on the coil has a larger space, such as perforation and relief printing metal; high film thickness, pattern coating; In addition, hardness, flexibility, scratch and chemical resistance can be improved.Precoating the membrane in the production efficiency and quality, environmental aspects have a greater advantage than the coated membrane in the traditional way.
The traditional powder coating process does not meet the requirements of high speed,may need to use the gun to overlap more than 50,but has basically reached its limit.Therefore,we must adopt a new coating technology to adapt to the needs of the coil coating development.
UV, IR and EB curing cycle is very short, and infrared technology enables the powder curing within the 60s, and EB curing technology in the 20s, UV technology can make the powder a few seconds curing. How to match the formation of high-speed coating line with these forms of curing, wire-speed to reach 100m/min, or higher, is a focus for researchers.
A. Electrostatic spraying
With the usual electrostatic powder spraying, according to the width of the coil and wire-speed to determine the number and arrangement of the spray gun. Of gas heating in the usual way, the coil of wire speed can only reach l520m/min further increase wire speed, powder coating the substrate high-speed mobile was taken away, the deposition efficiency of only 40% -50%; and gun layout-intensive, electrostatic coating film thickness is difficult to control. Also prone to other coating defects, such as pitting, orange peel. Now a focus for researchers in radiation curing instead of gas heat curing.
2 powder cloud technology
As we all know, the substrate wire speed the faster, more air moves.And electrostatic spray gun “point source compared to the MSC company” line source “can generate 1,000 times stronger than the electrostatic spray gun powder source, which makes the powder to penetrate the membrane in the fast wire-speed airflow layer become possible .
Powder cloud cover four areas: two substrate moving forward, two reverse, shown in Figure 1. Highlight the advantages of this technology are: the area to brush evenly distributed powder cloud density and charge the amount of static electricity and the thickness of the coating powder particle size and substrate wire-speed control. Usual thickness 10 ~ 130μm, powder deposition rate is an average of more than 93%. And according to different requirements for spraying single or double. Change color with the traditional liquid coating is almost time for about 30min. Different from the contact roll coating, powder cloud technology more suitable for coating pre-stamping, embossing coil; and in the requirements of the three-dimensional-effect paint has unparalleled advantages, such as a sand grain, hammer.
Similar to the above process, the phosphate capsule of the powder was from the upper part of the form of a nozzle mist spray down the amount of air through the ejector suction volume and convection nozzle to regulate the concentration of the powder cloud. Cloud of powder generated by the corona needle electrode panel located on both sides of the ion charged, studies show that: the coating thickness and the load voltage and powder discharge rate.
3 EMI technology
DSM’s EMB technology (electromagnetic brush technology) stems from the principle of copying and laser printing. Shown in Figure 2, the powder particles and carrier particles with a strong mix, this carrier particles are polytetrafluoroethylene (Teflon) or a similar polymer coating. In the mixing process, the powder particles are charged with the carrier particle friction, and they adhere to the carrier. Mixed roll of this mixture was transferred to a medial installation of a fixed magnet rotating drum on the other side of the plate for the ground state. Magnet within the carrier beads carrying the powder particles in a magnetic field to form a chain, the chain is called the adhesion to the drum surface of the magnetic brush, brush the magnetic length determines the rotating drum and a fixed fixed a long knife, that is, the distance between the scraper. Electrostatic field applied between the rotating drum shell and light sensors, the adhesion of powder particles in the membrane, shown in Figure 3. The amount of the powder particles depends on the electrostatic field strength, when electrostatic force is greater than the Coulomb force between the powder particles and the carrier, the powder particles will be deposited to adjust the thickness of the coating by adjusting the size of the electrostatic field.
For example, the hybrid powder coating and isocyanuric acid shrunk glyceride (TGIC) curing of the pure polyester powder coating is 24μm average particle size of the friction charged powder modified in 100m/min, the available 25μm thick coating.
Heidelberg Digital has wire-speed 120m/min improved rotating electromagnetic brush technology used in steel and stainless steel, aluminum coating, there have been several different carriers, such as conductive or insulating carrier. Industrialized fixed magnetic core or a rotating magnetic core coated roller electromagnetic brush technology, these systems include fixed magnetic core conductive electromagnetic brush, a fixed magnetic core insulation electromagnetic brush, rotating magnetic core insulation electromagnetic brush. The last technology, also known as the rotating magnetic brush to improve the system. Almost all of the existing system insulated carrier particles can be coated with insulating layer conductive medium, such as iron particles coated with Teflon ®, or simply use an insulator, such as high dielectric constant magnetic ferrite type. Improved rotating electromagnetic brush Magnetic type ferrite as the carrier, while the traditional system for use with the insulating layer conductive carrier.
Rotating electromagnetic brush technology usually improved with the cylindrical conductive shell and change receptor Antarctic Arctic bar magnet. Magnetic vector in the magnetic field of the roller on the roller forming a continuous chain. This is referred to as “fluff” when connected with Antarctic Arctic carrier chain and vertical color nuclear. Between North and South poles, the magnetic field of the magnetic core and parallel to the color of nuclear carrier chain basic and color nuclear parallel. The outer surface of the roller wheel or color nuclear receptor at the same time movement. When the rotation of the magnetic core, carrier chain along the direction of movement of the receiving body of light thrown. In contrast, the traditional system, due to the presence of a fixed magnetic core, the “fluff” is static. The typical conditions were: powder coating is recommended to join the live agent 1.5pph, and milled into powder, grading into a powder of average particle size is 12.9μm. The mixture also includes 15% of the strontium ferrite, strontium ferrite surface topcoat 0.3pph live agent, mixed in a blender in 1min, powder surface area of 30g / m. Wire-speed, in 120m/min next, on the conductive substrate, the non-conductive substrate and the ferromagnetic-type substrate coating. Conductive substrate, as long as the electromagnetic brush roller and the substrate surface electric field, the powder can be deposited on a grounded conductive substrate. Can be used for non-conductive substrate, the powder itself, the corona charging or in the substrate below or adjacent to embedded electrodes. For the rough surface, easy to retain the substrate of the carrier particles, such as wood and the pattern of plastic, the method can be fired with powder instead of the direct contact of the carrier substrate. For this non-contact or soft contact system, line speed and the distance between the substrate and roller there is a match. For the magnetic-type substrate, a small amount for it is necessary to eliminate the roller and the substrate carrier of the magnetic type.
Rotating electromagnetic brush technology benefits include: improved high sedimentation rate,high-wire-speed, flat film, a wide range of film thickness. Deposition voltage to adjust the thickness of the powder. Fine powder, particle size less than 9μm powder can also be used.
4 TransAPP technology
The Fraunhofer’s TransAPP technology, the use of powder transmission technology instead of gun, shown in Figure 4, to avoid the limitations of the traditional powder coating application speed and film thickness differences.
In this technique, the powder through the loop conveyor transfer to taken off the substrate, the powder particles evenly deposited on the substrate surface, resulting in a more uniform thickness. Moreover, there is no transmission to the powder particles on the substrate does not waste, but with the transfer to the next cycle. This process also applies to non-metallic substrate, wire-speed maximum in 60m/min for NIR curing epoxy polyester hybrid powder coating available 70μm film thickness.
The European market is about 10 coil powder coating line, wire-speed 20m/min, basic coating spray guns and rotary. The MSC powder cloud technology has been in the semi-commercial stage. DSM’s EMB technology is basically in a small pilot stage TransAPP technology has only just completed the trial. Matching powder coating and painting line, usually by well-known companies, such as industry giants such as DuPont, Akzo, Rohm and Haas, and PPG.
Coil powder coating in recent years in China’s development space, with the strengthening of the awareness of environmental protection and cost reduction requirements, powder coating, coil coating is the development trend. Some people predict that the coil coating will usher in the era of the powder coating. But for various reasons, by far not yet a true sense of the powder coil coating line, people’s attention does not. This article focuses on the development trend of the foreign, to put more attention to the expectations of people of insight powder coil coating.