Discussion on low temperature curing powder coating.

China Coatings Network
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Powder Coatings
Although it has the advantages of four "E" (Economical-Economic, Environmental-Environmental, Efficient-Efficient, Excellent Performance-Excellent Performance), some of its weaknesses are also evident. If the curing conditions are more demanding, the curing temperature is generally between 180 and 200 degrees C. If we can reduce the curing temperature can reduce energy consumption, is the need for environmental protection, energy conservation and emission reduction. High curing temperature, in addition to leading to high energy consumption, some can not withstand high temperature substrates, such as plastics, wood, such as the application of restrictions, greatly affecting the expansion of powder
coatings
applications. From the point of view of further improving efficiency, powder coatings that can often be cured at low temperature can greatly shorten the curing time and improve production efficiency while keeping the temperature constant. Therefore, the realization of low-temperature curing of powder coatings has become one of the development directions of the powder coating industry.
a long time, engaged in powder coatings and related supporting fields such as raw materials, equipment, the efforts of colleagues in powder coatings in low-temperature curing has made considerable progress. Now the pure epoxy system sand effect products can be done 130 degrees C / 15 minutes curing. The flat high-gloss epoxy system can be cured in 140 degrees C/15 minutes. However, epoxy has poor yellowing and weather resistance and cannot be used outdoors. Compared to the epoxy system yellow-resistant polyester/ epoxy mixed system sand products can also do 135 degrees C / 15 minutes, flat highlight products can do 150 degrees C / 15 minutes curing (slightly worse level). Pure polyester system sand products can be cured at 140 degrees C/15 minutes, and flat highlights can be cured at 160 degrees C/15 minutes. However, pure polyester system plane low-light products are still very difficult.
it is technically difficult to achieve ultra-low temperature curing of powder coatings. First of all, the curing system of powder coating is a low-temperature latent curing system, if the system has a high low-temperature reaction activity, it is bound to affect the powder extrusion and storage stability, on the other hand, the resin and curing agent used in powder coating are higher softening point of solids, melting viscosity is high at low temperature, at low temperature curing, the coating film is difficult to level, affecting the surface effect. If the use of resins with lower softening points and curing agents can reduce melt viscosity, but the storage stability of the powder is poor, the need for low-temperature refrigeration preservation, to use a lot of inconvenience. Therefore, the search for a resin and curing agent that can be suitable for powder storage with both high softening point and suitable reaction activity is the key to the preparation of low temperature curing powder coating.
To reduce the curing temperature of powder coatings, the following discussions are made in terms of main resin, curing agent, catalyst, infrared photo-curing, ultraviolet curing, spray drying method manufacturing powder coating and free-based reaction curing.
powder coatings, whether thermoplastic or thermosteroidal, film temperature is around 180-200 degrees C, curing time is long (10-20min), which limits it can only be used for metals and other heat-resistant substrates, and time-saving, relatively large energy consumption. For each reduction of 10 degrees C curing temperature saving about 10%, in order to save energy and reduce costs, expand the coating range of powder coating, better with solvent-based coating line, powder coating must be developed in the direction of low temperature curing type. The low temperature curing of powder coating can be achieved by reducing the melting temperature, viscosity, softening point, increasing the function group of the resin to improve cross-linking, adding appropriate additives, applying the appropriate catalyst and so on. Reducing the curing temperature of powder coatings can not only speed up the production speed and increase the production efficiency of automatic production lines, save energy, but also greatly increase the application range of powder coatings.
the key to determine the performance of powder coatings is the substrate resin, in order to achieve low-temperature cured powder coatings, has developed unsaturated polyester type, unsaturated acrylates resin type, polyurethane acrylic resin type, vinyl ether resin type. Both Allnex and DSM have products. Unsaturated resin is the main
film-forming substance
of UV-cured or free-form thermo-cured powder coatings, and is the main component that determines the properties and coating properties of coatings. In order to achieve low temperature curing, on the one hand, the resin is required to give the powder good storage stability, the powder must be stored at 40 degrees C conditions for 3 to 6 months without claging, on the other hand, the raw materials used at a lower temperature (e.g. 100 degrees C or lower) with a lower melting viscosity to ensure that the coating in the curing process has good flow. This requires that the glass temperature (Tg) of the selected resin should be 50 to 70 degrees C (at least above 40 degrees C), the average molecular weight is 1000 to 4000, and the molecular weight distribution is narrow. To obtain such a resin is not easy, Tg above 50 degrees C resin melting is difficult to control, because the C-C double bond can start polymerization at 80 degrees C, and below 80 degrees C its viscosity is too high to handle. A common way to reduce the melting temperature of resins is to synthesize semicrystalline resins, add crystalline compounds, or amorphous lymers. Through the design of polymer structure, it is also a feasible method to prepare low-temperature cured unsaturated resins for synthetic branch-like and overspending semi-crystalline polymers.
the rate of chemical reactions from a physical and chemical point of view can be represented by the Arrhenius formula, k-Aexp-Ea/RT (exponential). k is the rate constant, R is the Molar gas constant, T is thermodynamic temperature, Ea is the oscic active energy, A is the pre-factor (also known as the frequency factor), and it is also commonly used in its other form: lnk-lnA-Ea/RT (ddissostic). In order to improve the reaction rate under low temperature curing, the chemical reaction structure can be analyzed. It can be seen that Ea is an important factor for the surface active energy, which refers to the minimum energy required by the reactant molecules to reach the active molecule in a chemical reaction. The chemical reaction rate is closely related to the size of its active energy, the lower the activity energy, the faster the reaction rate, so reducing the activity can effectively promote the reaction. The promoter promotes some otherwise slow chemical reactions by reducing the activity energy (in fact, by changing the reaction pathway).
In order to make the promoter better promote chemical reaction, for solid compounds with good resin compatibleity and low melting point (80 to 120 degrees C), good dispersion can be obtained due to extrusion, melting can be added to the powdering Co-extrusion, while solids or other liquid compounds with poor compatible melting points are habitually dispersed by so-called masterbatch, which pre-adds them to molten carriers (e.g. epoxy resins, polyaexual resins). Obviously this is helpful for the mixing uniformity of additives. Depending on the nature of the cross-linked curing system, the dual cyanide-cured epoxy system is catalyzed with methazole, methazine, cyclocondium, cyclocondium, BF3 complex, epoxy/polyester mixing system and polyester/TGIC The system uses compounds such as methylene, methylene, seasonal ammonium, phosphorus, , and polyurethane (PU) systems use organotin compounds such as diclones, tin noctic acid and tin ditamine.
Mimi, 2-methyl-methane, 2-benzodiamin, 2-isopropyl mide, 2-propylamine and a small number containing long-chain replacement groups such as eleven or seventeen alkyl, which are mainly used as a reaction promoter or catalyst, methamphetamine curing agent is a class of highly active curing agents, in a short period of time at medium temperature can make the ring Oxygen resin curing, so its single-part system consisting of epoxy resin storage period is short, it must be chemically modified, in its molecules to introduce a larger subsort to form a spatial bit resistance of Mimi derivatives, or with the transition metal Cu, Ni, Co, Zn and other inororable salt reaction to generate the corresponding mide salt complex, in order to become a certain storage period at room temperature latent curing agent. There is less research on the latent curing agent of mides in China, and more research on the foreign market. Japan's First Industrial Pharmaceutical Co., Ltd. will be a variety of methane and toluene diocyanate (TDI), isofolone diocyanate (IPDI), six times methyl diocyanate (HDI) reaction into a closed product, weakening the activity of the amine base on the methane ring, has a longer life, when the temperature rises to more than 100 degrees C closed action lifted, Mimi reactive solidified epoxy resin.
Luisic acid-amine polymer is a class of effective epoxy resin latent curing agent, by BF3, AlCl3, ZnCl2, PF3 and other Luisic acid and beramine or midamine formation of the adhesive. As a curing agent of epoxy resin, this kind of ensemble is quite stable at room temperature, while at 120 degrees C, epoxy resin is fast cured, of which the most studied is the boron teflon-amine ensemble.
microcapsules epoxy resin latent curing agent is actually the use of physical methods to room temperature two-group curing agent using a fine oil drop film wrap, the formation of microcapsules will cure the curing reaction activity of the curing agent temporarily closed, through heating, pressurization and other conditions to break the capsule, release curing agent, so that epoxy resin curing. Microcapsules epoxy latent curing agent film-forming agents include cellulose, gelatin, polyethylene alcohol, polyester, etc., due to strict preparation process requirements, the thickness of the capsule film on storage, transportation and use will bring different degrees of impact.
infrared is a high-energy, high-density radiation heating technology, infrared according to its wavelength is usually divided into near-infrared (0.75 to 2.0 m), medium-infrared (2.0 to 4.0 m) and far-infrared (4.0 to 1000 m). Near-infrared, medium-infrared can make the coating film, coating both heating, infrared curing actual or thermal curing, is the use of thermal energy generated by infrared to achieve the energy required for the curing reaction. Infrared curing is characterized by fast heating, while energy can be concentrated in the surface coating, so high efficiency. The same curing effect can be achieved with a lower curing temperature than a common hot air furnace. Powder coating on medium fiber plates (MDF) has been successfully cured with infrared. MDF is a thermal substrate, heating speed, thus solving the problem that the substrate is not easy to overheat, and the internal strength of the substrate is not lost, saving time and space.
UV curing powder coating (UV curing powder coating) is a new technology that combines traditional powder coating with UV curing technology. The advantages of free-based polymerization reaction are that water has no blocking effect on the system and the curing speed is fast, the disadvantage is that the wrinkle is obvious and oxygen has a blocking effect on the reaction, the advantage of cation-triggered polymerization reaction is that the wrinkle is slight and anaerobic impedance, the disadvantage is that water has a blocking effect on the reaction, the curing time is long and the molecular weight growth is slow. The photopolymer of solid biphenol A epoxy resin and vinyl ether resin can be achieved by cation polymerization, but in most cases the photo-curing of UV powder coatings is still free-based polymerization, such as methyl acrylate polyester system, unsaturated polyester, polyurethane acrylic system. The biggest feature of UV-cured powder coatings is that the process is divided into two distinct stages, and the coating does not occur early curing of the resin during the melting level phase, thus providing sufficient time for the coating to fully level and de-bubble. The use of UV curing can significantly reduce the temperature of the heating and curing process (120 to 140 degrees C), avoid excessive heating of substrates, and open up a wider range of applications of powder coatings such as wood, plastics, paper, thermal alloys and metal components containing thermal parts. However, UV light curing powder coating variety is limited, because:
pigments have some organic pigments do not withstand the direct exposure of UV light, or opaque coloring pigments absorb UV light characteristics, so that the coating film curing is not good;
of the coating film is not easy to cure, such as the shape structure of the coating is complex, can not be directly exposed by UV light to the part and uneven irradiation.
non-saturation resin thermo-cured powder coating is generally composed of unsaturated resin, heat trigger, leveling agent, filler and pigment and other components. The curing agent of this unsaturated resin is to break down the thermal trigger to produce free-form when heating the melting state, and in the process of free-forming, the growth chain free-forming one atom is taken from other molecules and terminated to become a stable large molecule, and the molecule that loses the atom becomes a new free-molecule, and then triggers the unsaturated double bond to continue the new chain growth, so that the polymerization reaction continues, and the resin self-insulates and cures under the action of free-form. The active double-bond density, heat trigger decomposition temperature and dosage in resin have important effects on the preparation and performance of powder coating, which is the basis and key of powder formulation design.
spray drying powder coating is the powder coating after the powder coating slurry after atomization, contact with hot air to make the moisture vaporize rapidly, to obtain a powder coating with uniform atomization and uniform distribution of the size of the mist droplets. The principle of Vedoc Advanced Manufacturing Process, a supercritical fluid method developed by Ferro, is to add various components of the powder coating to the high-pressure reactor of the mixing blade, fill the kettle with carbon dioxide to the critical state, the supercritical carbon dioxide makes the various components of the coating fluidized and mixed into a uniform state, and then sprayed by the nozzle into the required granular products, the process advantages are not melted out of the mixing steps, preventing the use of the past materials.
additives in powder coating formulation is very small, but its role can not be ignored, commonly used additives are leveling agents, degassing agents, anti-lighting agents, wax powders, corner covering modifiers and so on. These additives usually have to be stored steadily in the powder coating in order to function as they should, so the additives used should be well compatible with epoxy, polyester, acrylic and other resins.
In the preparation of low-temperature cured powder coating leveling agent is to reduce the melting surface tensor pressure of powder coating, so that the coating before curing into a film quickly to level, to avoid the production of surface defects such as orange peel and shrink holes, so in order to make a small amount of leveling agent play a full role, the formulation of leveling agent must be fully evenly dispersed, pre-dispersed into the resin carrier of the leveling agent dispersion effect is better, more conducive to its role in the low-temperature melting solidification process.
The purpose of using degassing agents such as sabbath and wax powder is to reduce or eliminate bubbles, in the low temperature curing process can quickly remove bubbles from the coating, to prevent the appearance of coatings such as pinhole appearance defects, reasonable degassing agent screening is very important, low melting point or low viscosity degassing aids are more conducive to bubbles from the coating.
Conventional anti-light agents in low-temperature curing powder coating formulation does not play a role or the anti-light effect is not obvious, and the stability of the anti-light effect is poor, the reasonable screening of anti-light agents or can be eliminated at low temperature additives need further development and research.
low temperature curing powder
coating additives
in formulation design,