The development direction of low temperature curing powder coating.

To reduce the curing temperature of powder coatings, the following are discussed 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 thermoolytic, film temperature is around 180-200 degrees C, curing time is long (10-20min), which limits its use only for metals and other heat-resistant substrates, and time-saving, relatively large energy consumption. For each reduction of 10 degrees C curing temperature about 10% energy saving, 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

. 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 resin type, polyurethane acrylic resin type, vinyl ether resin type. Unsaturated resin is the main film-forming substance of UV-cured or free-form thermo-cured powder coating, and it is the main component that determines the properties of coating and coating properties. 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 at 80 degrees C can start polymerization, and 80 degrees C below 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 (ddissomorly). 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 compatible 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-crowding, 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, cycloazine, cyclo, 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.
methane, 2-methyl-methane, 2-benzodialycerine, 2-isopropyl mide, 2-propylamine and a small number containing long-chain replacement groups such as eleven alkyl 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 period of use, when the temperature rises above 100 degrees C closed action lifted, Methane resumes the activity of cured epoxy resin.
Luisic acid-amine polymer is a class of effective epoxy latent curing agent, formed by BF3, AlCl3, ZnCl2, PF3 and other Luisic acid and beramine or midamine formation. As a curing agent of epoxy resin, this kind of ensemble is quite stable at room temperature, and 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, UV curing powder coating photo-curing process has free-form-triggered polymerization and cation-triggered polymerization, both of which have their advantages and disadvantages. 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 wrinkle is obvious and oxygen has a blocking effect on the reaction; 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 acrylic polyester system, unsaturated polyester, polyurethane acrylic system. The biggest feature of UV-cured powder coating 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 be fully leveled and de-bubbled. 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 of the coating structure is complex, can not be directly exposed to UV light to the part so that the radiation is uneven.
non-saturation resin thermo-cured powder coatings are generally composed of unsaturated resins, thermal triggers, leveling agents, fillers and pigments. The curing agent of this unsaturated resin is to break down the heat trigger to produce free-forming free-forming agents when heating the melting state, and in the process of free-forming, the growth chain free-agent seizes an atom from other molecules and terminates it into a stable large molecule, and makes the molecule that loses the atom become a new free-form, 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, and are 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 Fluid Process, developed by Ferro, is to add various components of powder coatings to the high-pressure reactor of mixed blades, fill the kettle with carbon dioxide to critical state, supercritical carbon dioxide fluidizes the various components of the coating and mix them into a uniform state, and then spray through the nozzle into the required granular products, the process advantages are not melted out of the mixing steps, preventing the use of raw materials, can be used in the past.
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 the 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 coating such as pinhole appearance defects, reasonable degassing agent screening is very important, low melting point or low viscosity degassing aid is more conducive to bubbles from the coating.
Conventional anti-light agent in low-temperature curing powder coating formula does not work or the anti-light effect is not obvious, and the stability of the anti-light effect is poor, the reasonable screening of the anti-light agent or can be eliminated at low temperature additives need further development and research.
Low temperature curing powder coating additives in the formulation design, due to the production process conditions are more stringent such as low temperature extrusion, high-speed shearing and other conditions, the choice of additives need to be adaptive to the production process such as low temperature extrusion gel led to formula failure.
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