New advances in fast or low-temperature curing powder coatings for different substrates.

Introduction:
powder coatings and coating systems are known for their ecological, economical and applicability, reducing VOC emissions and meeting regulatory requirements.
powder coating has now expanded from metal-resistant, high-temperature substrates only to a wider range of coating areas, such as thermal substrates.
most of these areas have previously been coated with technologies that are "ecologically compatible" poor, such as liquid coatings,

. Powder coatings and coating systems also use rapid curing technologies such as coil coating as potential application markets.
In recent years, the powder coating industry is vigorously promoting rapid curing, low temperature baking, peroxide curing, UV (irradiation) curing and other new powder coating technology research and development, and gradually formed a relevant areas of technological expansion of the development trend.new technologies, new products and
are available in a variety of powder coating technologies for new application markets.
1.1 thermo-curing
fast-curing powder coatings can be cured in a short period of high temperature baking, such as coil coating. Conversely, powder coating varieties with long-term low temperature curing are suitable for thermal substrates, such as 25 min×125 degrees C, and near-infrared heating curing is a very fast and effective low temperature curing system.
1.2 polymerization reactions
use different polymer reaction dynamics to meet the need for rapid or low-temperature curing, such as peroxide (heat-triggered) curing and UV curing (irradiated and photo-catalytic).
1.3 Issues to focus on
Powder coating film-forming film by melt viscosity and storage stability limitations, a variety of curing systems have their own inherent characteristics and shortcomings.
(1) thermo-cured ultra-low temperature baking system
(1) extrusion process is not easy to control (resin system gel);
(2) flow and level performance is not good;
(3) There are limited varieties of coatings suitable for medium density fiberboard (MDF) thermal substrates, mostly textured products, which are difficult to achieve the surface effect of liquid coating film,
(4) curing cycle is long, and
(5) is prone to edge cracking and degassing.
(2) near-infrared heating curing (750 nm < near-infrared wavelength <1,500 nm) system
(1) high energy consumption
(30 to 50 times for ordinary infrared curing);
(3) curing cycle is short (1 to 15 s), the coating film is not easy to level;
(4) requires fillers (CaCO3);
(5) moisture evaporation; and
(6) is not suitable for wood composites.
(3) Peroxide curing system
(1) curing reaction degree is low, there may be unresponsive double bond;
(2) free-forming process is heat release reaction, no heat required;
(3) adhesion is generally insufficient, more suitable for inner mold powder coating;
(4) double bond reaction leads to coating film shrinkage;
(5) difficult to obtain the ideal mechanical properties.
(4) UV curing system
(1) difficult to cure thick coating film (>120 m);
(2) dark yellow paint can not be fully cured;
(3) complex work parts can not be all the face that needs to be cured towards UV irradiation direction;
(4) Difficult to design multi-purpose coating lines suitable for work items of different sizes and shapes;
(5) minimum curing temperature is currently at least 100 degrees C or more;
(6) requires special polymer materials with low melting viscosity, such as crystalline or semi-crystalline polymers;
(7) is difficult to obtain high gloss and very low gloss coating film.
thermal substrates and non-conductive materials
thermal substrates are generally non-conductive materials, such as wood, medium density plates, plastics, ceramics, etc., so in electrostitive coating should consider the following issues:
(1) substrate preheat or pre-coated conductive primer;
thermal substrate should be applied to minimize the curing temperature, so the powder coating used should be low temperature curing or radiation curing, the available curing techniques include:
(1) low temperature curing, 125
degrees C×25 min;
the ultra-low temperature baking curing system
low temperature thermo cure system is still growing varieties in the powder coating market. A range of new polyesters are made by adjusting the promoter mixture.
is used epoxy resin or shrinking glycery compounds at a low temperature of 125 degrees C after a long period of baking and curing into a film, mainly used for thermal substrate coating. Experimental polyesters and promoters for ultra-low temperature baking systems are listed in Table 1.
low temperature thermocly curing system is mainly used in MDF coating. In recent years, the MDF industry has developed rapidly and there is a strong demand for solvent-free coatings. MDF itself has certain characteristics and is coated with powder coatings with the following special process conditions.
(1) mass consistency between batches of MDF
(1) density distribution is stable (approx. 780 kg/m3);
(2) water distribution randomness, from the point of view of conductivity and degassing requirements, MDF moisture should be controlled at 6% to 7%,
(3) size temperature stability.
(2) warm-up requirements (infrared lamp exposure 30 s) substrate temperature between 70 to 80 degrees C.
(3) electrostectrectectectral spray (corona charged /friction charged) control static spray control, otherwise it is difficult to estimate the thickness of the coating film.
the laboratory-prepared polyester resin preparation powder coating, Table 2 to 5 gives the experimental formula and the metal substrate and MDF substrate coating test results.
powder coating preparation is an experimental single screw extruder, extrusion temperature of 80 to 90 degrees C, screw speed of 200 r/min;
2 and 4 are 60/40 polyester, 50/50 polyester/epoxy hybrid systems. The coating properties under different curing conditions were tested separately (Table 3, Table 5).
RER curing technology
near-infrared curing is a process time can be significantly shortened curing technology, curing time only 1 to 15 s, meaning that the energy density is very high, is 30 to 50 times the medium wave infrared light.
between near-infrared technology and conventional hot air/standard infrared curing technology is shown in Figure 1. The curing process of the system utilizes the energy absorption characteristics of the polar group, and CaCO3 is added to the coating formulation.
its curing characteristics give this curing system the advantage of being different from other systems, that is, the coated work piece is not the whole heat, thermal substrate in the curing process of the thermal impact is less. The compact design of the near-infrared curing equipment greatly saves the space required for the production line.
coating and metal pre-coating
's growing greenhouse effect is forcing further reductions in VOC emissions and solvent combustion emissions, powder coatings are a good solution.
Of course, powder coating for coil coating there are still many shortcomings, although after many efforts, coil powder coating problems are still concentrated in the coating process and curing technology, the specific performance in:
(1) line speed can not reach the level of solvent-based coating (200 m/min), can now only reach 60 to 100 m/min;
(2) it is difficult to obtain a thin and uniform coating film,
(3) is subject to the rapid curing system of powder coating, the current induction curing or near-infrared heating curing system, and the long-term stability of
(4) mechanical properties is not ideal.
europe currently has more than 50 coil coating lines, of which 7 to 8 are powder coating. Over the past few years, many companies have attempted to develop new powder coating technologies that can be used in coils, focusing on the following areas:
(1) BHP Australia has developed a "solid block" coating technology, i.e., pre-preparation powder coating pre-forming blocks, and then pressing them on the moving preheat metal strip, thereby mechanically transferring the coating to the metal strip surface. The main disadvantage is that the speed is slow, and the surface coating effect of the metal strip is not ideal.
(2) electromagnetic brush technology was developed by DSM and works in a similar way to photocopiers and laser printers. The powder coating is mixed with the magnetic carrier particles, which are arranged along the magnetic line, so it is called the magnetic brush.
this method is suitable for the preparation of extremely thin powder coating film, the main disadvantage is that the speed is slow, and can not obtain different thickness of coating film through a single construction.
(3) thick film "powder knife" with coating speeds of up to 50 m/min.
(4) powder rotary cup spray.
(5) "Powder Clouds" are MSC (Material Sciences Corp.) and TDC (Terronics Development Corp.). The two companies jointly developed coating technology a few years ago.
powder coating is atomized with a number of dispersion brushes and forced through the electrostitive zone so that it can be adsorbed to a moving metal surface for high-speed coating.
the main disadvantage of the equipment is that the equipment is too expensive, and the process is difficult to control, is not currently available for industrial production. It is easy to obtain a coating film of 55 to 60 m by this method, but it is difficult to reduce the thickness of the coating film.
(6) Powder Spray Gun has been successfully applied to low-speed coil coating lines at 20 m/min in Europe. The system sprays powder on the surface of a moving steel plate or aluminum plate, the main disadvantages of which are slow working line speed, easy overspouting, unequidezed coating thickness and slow color change of the system.
(7) "Powder Injection" system has no removable components at all and uses compressed air to apply powder directly to the surface of the moving metal strip. Metal strips can be coated on one or both sides, and the thickness of the coating can be precisely controlled according to customer requirements. It is said that its cost and quality parameters are fully comparable to any kind of coil liquid coating line.
has developed hydroxyl polyesters and promoters for pre-coating and coil coating of metal plates, as seen in Table 6.
the use of laboratory-prepared polyester resin, according to table 7 formula prepared powder coating samples, powdering equipment for a single screw extruder, extrusion temperature of 80 to 90 degrees C, screw speed of 200 r/min.
the powder is sprayed on A-36 aluminum plates, and the test curing conditions on the coil production line are 1 min×290 degrees C or 1.5 min×275 degrees C, and all formula model coatings are qualified for mechanical and chemical properties. The results of laboratory tests are listed in Table 8.
the inner mold powder coating
the inner mold powder coating is applied to the mold before the molding work piece. The coating technique is simple in terms of the complexity of the equipment and control parameters. The coating used is made of unsaturated polyester, and the curing reaction is a two-bond thermal polymerization reaction catalytic for peroxide.
the formulation and construction environment of the inner mold powder coating are the factors that must be considered in order to obtain good coating effect (good appearance and less secondary products).
the powder coating is evenly sprayed on the inner surface of the preheated mold, the work piece is pressurized, the coating reacts with the fiber composite material during the curing process, and eventually becomes a solid surface coating of the fiber composite material surface.
the inner mold powder coating film is between the mold surface and the high viscosity composite material, which covers the entire coating film by flow during the molding process. Many fiber composites can be used to prepare fiber-reinforced plastics, as shown in Table 9.
the thickness of the inner mold powder coating film is 40 to 400 m, which makes up for the surface defects of the composite material. There is a chemical bond between the powder coating film and the composite material, so the adhesion is excellent. Newly developed inner mold powder coating unsaturated polyester is listed in Table 10, and the inner mold powder coating formulation is listed in Table 11.
UV-cured powder coating
. UV curing technology has been successfully applied in low temperature curing. However, the photo-cured powder coating is difficult to obtain excellent coating appearance under low temperature melting conditions, and the adhesion to the substrate is not as good as the thermal curing system. At the current technical level, the coating appearance of the UV curing system can be better than that of the ultra-low temperature baking type.
(1) UV curing coating line of the main performance is:
(1) to achieve the thermal substrate powder coating (temperature does not exceed 120 degrees C), such as wood or composite wood, plastic, pre-assembled work parts of powder coating;
(2) fast curing (2 to 3min, conventional powder coatings need 20 min) ;(3) low energy consumption;
(2) compared to ultra-low temperature thermal curing powder coating, UV cured powder coating has the following advantages (see Figure 2):
(1) reaction speed is fast;
(3) compared to UV-cured liquid coatings, UV-cured powder coatings have the following advantages:
(1) because the material used relative to the molecular quality is high, does not contain active monosomes, so the toxicity to the human body is small, there is no skin irritation problem;
(2) easy to obtain a higher coating thickness,
(3) material utilization rate is very high, over-spraying powder can be recycled,
(4) coating film uniform, good corner covering,
(5) good adhesion to rigid substrate,
(6) will not penetrate the wood texture.
UV curing system powder coating does not have a curing reaction during the melting phase, which improves leveling in order to form a highly flat coating film. This is also one of the advantages of its overheating curing system.
difference between melting leveling and curing of powder coatings can be seen from the DSC figure (see Figure 3), the reaction dynamics of the UV curing system are independent of the heat flow applied to the system, unlike the thermo cure system.
in order to take full advantage of this advantage, it is necessary to develop a new unsaturated resin system
with the following characteristics:
(1) resin reaction activity appropriate to a sufficient amount of light trigger;