For a long time, engaged in powder coating as well as related fields such as raw materials, equipment, with the help of colleagues in the low temperature curing powder coatings has made considerable progress. Now pure epoxy system sand grain effect curing products can do 130 ℃ / 15 minutes. Plane highlights the epoxy curing system can do 140 ℃ / 15 minutes. But epoxy yellowing resistance and weather resistance is poor, cannot be used for outdoor use. Compared with epoxy system better resistance to yellowing of the polyester/epoxy hybrid system sand grain products can do 135 ℃ / 15 minutes, the plane specular products can do 150 ℃ / 15 minutes curing (flow leveling is a bit poor). Pure polyester system sand grain products can do 140 ℃ / 15 minutes solidification, plane highlights curing can be 160 ℃ / 15 minutes. But pure polyester system planar low light products is still very difficult.
To realize temperature curing powder coatings has great difficulty in technology. First of all, powder coating curing system is a kind of latent curing system at low temperature, low temperature high reactivity, if the system is bound to affect the powder powder extrusion and storage stability, on the other hand the powder coating of resin and curing agent are is the solid high softening point, melt viscosity at low temperature is higher, the low temperature curing, coating flow flat hard, affect the surface effect. If the low softening point of resin and curing agent can reduce the melt viscosity, but the storage stability of powder, need low temperature cold storage, bring inconvenience to use. Therefore, seeking a suitable powder store there are both high softening point and proper reactivity of resin and curing agent is the key to the preparation of low temperature curing powder coatings.
Low temperature curing powder coatings development direction
To reduce the curing temperature of powder coatings, the following from the main body resin, curing agent, catalyst, infrared curing, uv curing, spray drying method of manufacture and free radicals reaction curing powder coatings manufacturing powder coating for discussion.
Thermoplastic or thermosetting powder coating whether, film-forming temperature of around 180-200 ℃, long curing time (10 ~ 20 min), which limits its can only be used for heat resistant base material, such as metals and time-consuming, relatively large energy consumption. Every 10 ℃ lower curing temperature about 10% energy saving, to save energy to reduce costs, expand the range of coating powder coating, better with solvent-based coating line, powder coating must be to develop in the direction of low temperature type. By lowering the resin itself to melting temperature, viscosity, softening point and increase the functional group increase the crosslinking degree of the resin, additives are added, apply the appropriate catalyst and other means to realize low temperature curing powder coatings. Reduce the curing temperature of powder coating, not only can increase the production rate of automatic production line and improve the production efficiency, save energy, and make the application range of the powder coating greatly increased.
Decided to powder coating performance is the key to the matrix resin, to realize low temperature curing powder coatings, has now developed the unsaturated polyester, unsaturated acrylic resin, polyurethane acrylate resin, vinyl ether resin, etc. These new (Allnex) and DSM (DSM) have corresponding products. Unsaturated resin is UV curing or free radical thermal curing powder coatings main film-forming material, is to determine the main components of the coatings properties and film performance. To realize low temperature curing, on the one hand, resin can give good storage stability, powder powder shall be under the condition of 40 ℃ can be stored 3 ~ 6 months without agglomeration; On the other hand, the raw materials used shall be at low temperature (100 ℃ or less) under a low melt viscosity in order to keep the coating has good liquidity in the process of solidification. This requires that the selected resin glass transition temperature (Tg) should be in 50 ~ 70 ℃ (at least in more than 40 ℃), average molecular weight of 1000 ~ 4000, and to narrow molecular weight distribution. To get this resin is not easy, Tg above 50 ℃ resin melt is difficult to control, because C = C double bond at 80 ℃ can begin aggregation, and under 80 ℃, the viscosity is too high and difficult to handle. Commonly used method to reduce resin melt temperature is semicrystalline resin synthesis, join the crystalline compound, or amorphous oligomer. By dendritic polymer structure design, synthesis and hyperbranched semicrystalline polymer preparation of low temperature curing unsaturated resin is a kind of feasible method.
From the perspective of physical chemistry can be applied to consider the chemical reaction rate Arrhenius formula (Arrhenius), said k = Aexp - Ea/RT (exponential). As the rate constant, k R for molar gas constant, T for thermodynamic temperature, Ea as the apparent activation energy, A is refers to the former factor (also called frequency factor), is also commonly used another form: LNK = lnA - Ea/RT (log). To improve the rate of reaction under low temperature curing, can from the chemical reaction mechanism analysis. It can be seen that the Ea for the apparent activation energy is a very important factor, activation energy is refers to the chemical reaction, the reactant molecules to reach the minimum energy required to activate molecular. Closely related to the size of the chemical reaction rate and activation energy, the lower the activation energy, the faster the reaction rate, thus reduces the activation energy will effectively promote the reaction. Promoter by lowering the activation energy (which is actually decreased by changing the way reaction activation energy) to promote rapidly for some very slow chemical reaction.
In order to make better promote chemical reaction accelerator, good compatibility with resin, have lower melting point (80 ~ 120 ℃) of solid compounds, due to the extrusion can get good dispersity, can melt co-extrusion are joined when milling; And those poor compatibility, high melting point of solid, or other liquid compound, traditionally used so-called Masterbatch (Masterbatch) method, which put them into the molten carrier in advance, such as epoxy resin, poly resin vinegar) in molecular dispersion. Obviously this do for fertilizer mixing uniformity is helpful. Promoter choice depends on the nature of the crosslinking curing system, the dicyandiamide curing epoxy system with imidazole ring, imidazoline, amidine, BF3 complex catalytic, epoxy/polyester mixed system and polyester/TGIC system is using imidazole season, imidazoline, quaternary ammonium and phosphorus, amidine compounds, polyurethane (PU) system using the organotin compounds, such as two lauric acid tin tin, bitterness and dibutyl tin oxide, etc.
Imidazole, 2 - methyl imidazole, 2-2 - phenyl imidazoline, isopropyl imidazole, 2 - propyl imidazole and minority groups containing long chain to replace such as 11 alkyl group or alkyl 17, it mainly as a reaction promoter or catalyst and application of imidazoles curing agent is a kind of highly active firming agent, under the temperature in a short time can make the epoxy resin curing, therefore its single component system with epoxy resin composition storage period is shorter, shall be on the chemical modification, the introduction of large substituent in its molecular form has the space steric hindrance imidazoles derivatives, Cu, Ni, Co or and transition metals, zinc and other inorganic salt reaction to generate the corresponding imidazole salt complex, can be at room temperature has a certain storage period of latent curing agent. Domestic study of imidazoles latent curing agent is less, foreign markets are relatively more. Japans first industrial pharmaceutical co., LTD will be a variety of imidazole with toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), six methyl diisocyanate (HDI) made closed reaction product, weakened the imidazole ring on the activity of the amino, have a long life, when the temperature rises to above 100 ℃ sealing up lifted and recover the active of imidazole curing epoxy resin.
Lewis acid - amine complex is a kind of latent curing agent for epoxy resin effectively, by BF3, AlCl3, ZnCl2, PF3 lewis acid with primary amine or secondary amine to form complex. As the curing agent of epoxy resin, this kind of complex is very stable under normal temperature, and at 120 ℃ is fast curing epoxy resin, is one of the most studied boron trifluoride - amine complex compound.
Microcapsule type latent epoxy resin curing agent is actually two components, using physical method to room temperature curing agent USES the fine droplets membrane package, forming microcapsule curing agent curing reactivity temporarily closed up, by heating and pressure conditions that capsule rupture, releasing the firming agent, so that the epoxy resin curing. Microcapsule kind of latent curing agent for epoxy resin film formers including cellulose, gelatin and poly (vinyl alcohol), polyester, etc, due to the strict preparation technology, the thickness of capsule membrane for storage, transportation and use will bring different degree of influence.
Ir is a kind of high energy density of radiation heating technology, infrared wavelength according to its different is usually divided into near infrared (0.75 ~ 2.0 microns), infrared (2.0 ~ 4.0 microns) and far infrared ray (4.0 ~ 1000 microns). Near infrared, middle infrared can make coating, the coating both heating and infrared curing actual or heat curing, is produced by using infrared thermal energy to achieve the energy for the curing reaction. Infrared curing characteristic is warming faster, at the same time, energy can be focused on the coating surface, and high efficiency. Hot blast stove with general than, can use lower curing temperature to achieve the same effect of solidified. Currently in middle density fiberboard (MDF) use infrared curing powder coating has been successful. MDF is thermosensitive base material, heating speed, so as to solve the problems of base material is not easy to overheating, and base material internal strength from loss, save time and space.
Uv-curable powder coatings (UV curing powder coatings for short) is a traditional powder coatings and UV curing technology with the combination of new technology, the light of UV curing powder coatings curing mechanism of free radical polymerization and cationic polymerization, both have their advantages and disadvantages. The advantage of free radical polymerization reaction is water right system in effect and the curing speed, the disadvantage is that crumple and oxygen reaction has obviously polymerization; The advantage of cationic polymerization reaction is crumple mild and anaerobic polymerization phenomenon, disadvantage is the polymerization reaction for water, curing time and slow growth in molecular weight. Bisphenol A epoxy resin and solid state polymerization of vinyl ether resin light can be achieved by cationic polymerization, but the majority of cases, the UV powder coating curing light or by free radical polymerization, such as methyl acrylic polyester system, unsaturated polyester, polyurethane acrylate system. The biggest characteristic of the UV curing powder coatings is process is divided into two distinct phases, flat stage will not occur in the molten coating early curing of the resin, for peace in addition to the bubble fully coating provides plenty of time. Using UV curing can obviously reduce the temperature of the heating and solidifying process (120 ~ 140 ℃), avoid excessive heating of base material, powder coating is more broad application fields, such as wood, plastic, paper, thermal alloy and containing temperature sensitive parts of metal components, etc. But the variety of UV light curing powder coatings co., LTD., because:
Some organic pigments in paint in the resistance to UV light of direct illuminate, or opaque pigments absorption characteristics of UV light, make the coating curing bad;
Deep, coating is not easy to cure, such as the shape of the coating structure is complex, part cannot be UV light direct illuminate that the uneven.
Unsaturated resin thermal curing powder coatings generally made from unsaturated resin, thermal initiator, flow ping agent, fillers and pigments and other components. The curing mechanism of unsaturated resin is thermal initiator decomposition when heated molten state to produce free radicals, in the process of free radicals and growing chain radical end up an atom from other molecules become stable macromolecular, and made out of atoms and become a new free radical, to trigger a new chain unsaturated double bonds continue to increase, to continue the polymerization reaction, resin under the effect of free radicals on the crosslinking curing reaction. Resin in the active double bond density, thermal initiator decomposition temperature and the dosage of powder coating preparation and performance of all have important influence, is the basis of the powder formula design and the key.
Spray drying powder coating is to pass through the atomized powder coating slurry, make contact with the hot air moisture evaporation, quickly get uniform atomization droplets size distribution and uniform powder coating. Development of supercritical fluid method by Ferro company VAMP (Vedoc Advanced Manufacturing Process) is the principle of the mixing various ingredients added to the powder coating blade high pressure reaction kettle, filled carbon dioxide in the kettle to the critical state, supercritical carbon dioxide make the paint of various ingredients fluidization and mixing evenly, and then through the nozzle spray into required size of the product, the technology advantage is without melting extrusion mixing step, prevent gel, enlarge the application range, can be used in the past, hard to use raw materials.
Dosage of additives in the powder coating formulation is very small, but its role is not to be ignored, commonly used additives have flow ping agent, degassing agent, matting agent, wax powder, edge covering modifier etc. These additives are usually reserved to be stable in the powder coating to play its proper function, so the use of additives with epoxy, polyester, acrylic resin has good compatibility, etc.
Flow ping agent in the preparation of low temperature curing powder coatings, main effect is to reduce the molten surface tension of the powder coating, the coating is able to flow quickly before the curing film, avoid the orange peel and causing surface defects such as shrinkage cavity, thus to make a small amount of flow ping agent give full play to the role, formula of uniform flow ping agent must be fully dispersed, advance dispersed into the carrier resin flow ping agent distribution effect is better, more conducive to play a role in the process of low temperature melting solidification.
Selects the degassing agent such as benzoin and wax powder, the purpose is to reduce or eliminate the bubbles, can quickly at low temperature curing process will bubbles emerge from the coating, to prevent the occurrence of such as pinhole coating apparent defects, reasonable degassing agent selection is very important, low melting point and low viscosity degassing agents more conducive to bubbles emerge from the coating.
Conventional matting agent in low temperature curing powder coatings formula doesnt work or extinction effect is not obvious, poor stability and extinction effect, flatting agent can reasonable selection or in low temperature area of auxiliary light still need to further the development of the research.
Low temperature curing powder coatings additives in formula design, because the production process conditions are harsh, such as cold extrusion, high speed shearing, etc., the choice of additives for adaptive production process such as squeeze gel formula failures in low temperature.
