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Selection and effect of special additives for powder coatings

In order to prevent the powder coating pinhole produced the formulation system, usually add a called benzoin additives to overcome this defect. The chemical name of benzoin and called two phenyl ethyl ketone, is white or pale yellow crystalline powder. It can also be used as a very effective degassing agent in epoxy system. But in the benzoin light or white in the system is very easy to cause yellowing, especially more serious in baking conditions, this is because the industrial grade will be called a benzoin benzil material, the material is very sensitive to temperature, is the main material yellowing, therefore people are constantly looking for new alternatives to overcome the inherent defects of benzoin, but so far all of these alternatives are not economic performance or other characteristics, so the benzoin is still the most main degassing agent, usually in white or light colored, benzoin and other systems in order to achieve the coordinated use of degassing agent better application result.
People looking for a substitute of benzoin for many years, the earliest in 1976 Japan Jiro Yoshikawa paint company explored in many compounds, such as bisphenol A, phenyl acetyl salicylic acid, benzoin to prevent air bubbles in the coating. In 1992 DSM Jan Schippers found that the crystalline amide wax in Primid system can prevent the generation of pinhole without yellowing, it is difficult to do especially benzoin, in the baking conditions. In 1992 Stanislaw Skora developed a  bisphenoxy propanol to prevent the powder coating pinhole and does not cause yellowing, adding amount of 0.5~3%. However, the additive also has defects, easy to produce "tears" phenomenon, in the drying channel condensation when there is a drop of liquid, high baking temperature is also easy to cause the film Red tendency. In 1993 the world water, high solids and powder coating workshop, Velsicol William Arendt published a 1.4 CHDM dibenzoate can be used as mixed powder coating degassing agent used in the thesis, the article pointed out that the degassing agent has good degassing effect and does not produce yellowing. Wilkinson and Philips in 1993 also invented a kind of thermoplastic polymer can replace benzoin used in the Primid system, at the same time that the degassing effect and differences in other systems in the. The United States Air Products and non yellowing degassing agent products provide the commodity brand is Surfynol Pzoo is a kind of propargyl alcohols, also belong to fine hydrogenated castor oil derivatives, some PE also has certain effect of degassing wax.
But it needs to be pointed out is the mechanism of the degassing benzoin does not have a unified explanation, but from the experimental point of view that benzoin escaped from film in the initial curing 6~8 minutes with benzoin gas escape, leaving a film for the accumulation of free volume diffusion. In order to obtain better effects of degassing, the best is selected according to the processing characteristics of different degassing agent system may be appropriate, benzoin and other degassing agent combination makes up will have a better effect.
A year to become more and more popular for special degassing machine for porous substrates, because for those who Aluminum Alloy, galvanized steel and cast iron substrate such as ordinary degassing machine can not play a positive role, so it is necessary to use the degassing machine with special functions to solve. Hefei Ketai KT961, KT962 and KT418DG etc.. Have a good effect on the porous base material containing wax based special degassing machine, its mechanism is this: molten wax with low surface tension at the melting temperature of the temperature reached after the degassing agent penetrate into pores in the workpiece gap in air discharged in vitro, with low melt viscosity in melting process of degassing the powder agent in the air of the overflow coating powder so as to achieve the effects of degassing. Because of the different composition of the degassing agent, the processing conditions and effects of the workpiece should be considered in the selection. Such as KT961 has a relatively low melting point, so it is more suitable for those thicker workpiece. The KT962 with a fast heat transfer effect of the degassing effect is better, can also be mixed use. For the micronized degassing agent can also be used to solve some difficult problems of the late dry mixing. KT418DG is a non yellowing inhibitor for transparent powder or Primid.
In the selection of degassing agent need to choose according to the specific application of the system, such as yellow high requirements to minimize the use amount of benzoic, and for degassing agent containing wax also need to consider the negative effects of excessive import from the general strength of the manufacturers will give out the corresponding application instruction.

Two, the application of light stabilizers and antioxidants in powder coatings
Powder coating by high temperature baking or sun exposure will occur after aging, yellowing phenomenon has seriously affected the appearance and performance of our products demand, in order to prevent or reduce the occurrence of this trend, we usually use the addition of antioxidant or light stabilizers to realize, although there are many factors affecting the film aging, such as resin, pigments, additives, coating quality and type of formulation design, production process, the change of temperature and the composition of the atmosphere, humidity and other natural factors, but the application of suitable additives is to reduce the occurrence of this trend.
Mechanism and application characteristics of 1 antioxidants
The aging mechanism of polymer is described in detail in the chapter of "outdoor powder coating crosslinking agent and its application". In fact, is a kind of antioxidant to inhibit or delay the oxidation and degradation of polymer material. Many varieties of antioxidant effect, are not the same, if according to the mechanism of classification it can be divided into two categories, a class called chain-breaking antioxidant, the emergence of such antioxidants and free radicals in the polymer chain reaction interrupted growth also known as primary antioxidants, another class called preventive antioxidant, this kind of antioxidants can generate free radicals and inhibit or slow initiation process, also known as auxiliary antioxidant. The Lord has hindered phenolic antioxidant, secondary aromatic amine, a preventive antioxidant phosphite, two dithiocarbamate metal salts, so in the practical application of how to select antioxidants?
Colour change. Discoloration problem antioxidant is one of the important factors we consider. General phenolic antioxidants no pollution, can be used for colorless or light color system and the discoloration of aromatic amine and strong pollution.
Volatile. High molecular weight antioxidants for low volatile, different varieties of antioxidants also have great differences. Therefore, the choice should be based on the selection of different varieties.
Solubility. The ideal antioxidant is high solubility in polymer solubility in Yan in other medium and low factors depending on the structure and types of antioxidants and temperature etc..
The structure of the polymer. It is necessary to point out that different polymer structures have different antioxidant capacity in antioxidant must take into account the differences.
The effect of heat. The effect of heat is extremely important, and the temperature increases about 10 times per hour. At 100 C, the oxidation rate will be about 256 times higher than that at room temperature (20 degrees C). Oxidation at high temperature is a very important factor. Must choose the high temperature performance good antioxidant. Hindered phenolic antioxidants in the high temperature performance is poor, and the two kind is acridine tetrahydroquinoline special use value at high temperature.
The effect of metal ions. Multivalent metal ions such as copper, iron, manganese and other trace elements will accelerate the oxidation of the polymer, so the metal ion deactivators to resist.
Such as N- and salicylic -N '- salicylhydrazone is an inhibitor of copper.


           -OH            HO
            
           -C  NH  N = C-
           ||              |
           O              H

The coordination effect. Amine or phenolic antioxidant chain termination and peroxide decomposition agent (e.g. phosphite ester) with the use of polymer can improve the ability of anti oxidation, which is also known as the coordination effect. The coordination effect refers to two or more than two kinds of antioxidants with various effects when using sum total effect is greater than when used alone. But when using several antioxidants also may have a harmful effect, also known as the antagonistic effect of antioxidants, antioxidant effect should be considered when coordination should also consider the antagonistic effect.
The use of antioxidants. The amount of antioxidants depends on the type of polymer. Factors of crosslinking system and antioxidant efficiency, synergistic effect and cost etc.. Most antioxidants have a suitable concentration at an appropriate concentration within the increasing amount of antioxidant capacity increased more than appropriate concentration has adverse effects, should also be considered antioxidant evaporation, extraction, oxidation loss and other factors.
2.UV absorbent and light stabilizer
Ultraviolet absorber (UV absorber) is a type of stable AIDS prevention, in the ultraviolet selectively, endanger the polymer structure before strongly absorb ultraviolet light, and convert them into harmless low-energy radiation, prevent the UV irradiation of polymers.
The photophysical process typical UV absorber is formed by intramolecular hydrogen bond chelate ring to absorb energy photons inhaled off hydrogen bond chelate ring in this process, the light energy is converted to heat release. The stronger hydrogen bond is formed in the UV absorber, the higher the cracking energy of the bond, the more energy it can absorb ultraviolet radiation. The protective effect of ultraviolet absorber depends on the thickness of the coating film is protected, according to Lambert. Bill's Law: E (lambda) = epsilon (lambda) C.D.
ε -The molar absorption wavelength
C - UV absorber concentration
D - film thickness
E - absorbance at lambda wavelength
The same kind of ultraviolet absorber in different concentrations, film thickness, the ability to absorb ultraviolet radiation is also different, the thicker the film, the better the effect of UV absorbers. There are two commonly used UV absorbent benzophenone compounds, benzene and three azole compounds (such as Tinuvin 900).
HALS is a free agent acquisition agent, mainly including piperidine, imidazolidine ketones, Azacycloalkane ketone derivatives, in photooxidation of polymer materials in these compounds by capturing free radicals, decomposition of hydrogen peroxide and quenching of excited state energy to achieve the purpose of anti aging, commonly used in nonferrous system, such as icba-Gergy Tinuvin144 is a kind of material.
The mechanism of light stabilizer with the use of the excellent results are obtained both in plastic, paint, if it can capture free radicals ability of hindered amine light stabilizers (HALS) and with UV absorber can get light stabilization effect is very good, the same HALS antioxidant and phosphite mixed use and its effect synergistic effect.
Charge regulator series
Charge regulator series we simply put them into the friction agent and charge control agent in two parts, the first to talk about the charge control agent, adding antistatic agent or charge regulator in powder coatings can reduce the surface resistance of the powder is deposited on the surface of the powder to work better, but also improved the electrostatic spraying method Ladi effect. As far as electrostatic coating is concerned, a very strong voltage is applied between the workpiece and the electrode of the spray gun and a strong electrostatic field is generated. Due to the applied voltage is high enough to produce CORONA discharge in part gunpoints, CORONA plasma discharge is a cold, it will produce a large number of electrons in the discharge region and is filled between the electrode and the workpiece, these electrons can produce adsorption of negatively charged ions in the air molecules, adsorbed on the powder particles and with the process of spraying deposition in the workpiece on the charged ions will be charged, in fact the efficiency of the powder particles is very low, only about 0.5%, most of the other free ions, they will be as the spraying process adsorbed on the coated surface and accumulated until rejection or discharge, seriously affecting the rate of powder deposition and further the groove on the surface of the powder, and the surface charge regulator can improve balance or by accumulation of free ions coated on the surface of the workpiece, thereby improving the spraying effect, so thick Film coating has also become possible.
The high surface resistance of the polymer surface makes it difficult to discharge the static charge, which can be obtained by.
Increasing the humidity of the processing environment is conducive to the suppression of static charge and the leakage of the charge;
To improve the structure of polymers, such as the introduction of polar or ionic groups to improve the conductivity;
The addition of conductive materials such as metal powder in polymer materials, such as carbon black;
Adding antistatic agent to improve material polarity or hygroscopicity.
The characteristics of the above methods, the moisture on the surface resistance material or volume resistance has great influence, and some methods such as adding metal powder or improving polymer materials not in any applications can be easily obtained, there are limitations, and method of adding antistatic agent of the most simple.
The antistatic agent can be divided into ionic and non-ionic form according to whether it can be ionized. If the hydrophilic ionization is negatively charged anionic, cationic and charged for. If a hydrophilic group of more than two and the ionization after each with different charge is divided into zwitterionic antistatic agent, if not ionization is known as non-ionic antistatic agent, regardless of how the classification of its principle is basically the same, mainly using the surface active characteristics of absorbing moisture in the air, the surfactant has a conductive layer formed of thin polarization, a charge leakage channel. Therefore, most of the antistatic effect depends on the relative humidity of the structure and environment. Compared with the mechanism of moisture absorption, the effect of adding the antistatic agent and reducing the friction coefficient to a certain extent also restrain the generation of electrostatic charge. It should be noted that the amount of antistatic agent and polymer structure and the use of the environment, too much import will lead to the reduction of spray efficiency and even the phenomenon of non adsorption.
Anionic antistatic agents are generally sulfuric acid derivatives, phosphoric acid derivatives and high molecular weight anionic polyacrylic acid, etc..
A cationic quaternary ammonium salt, alkyl imidazoline etc..
Nonionic is fatty acids, alcohols, alkyl phenol epoxy amine adduct, derivatives, in the practical application of the vast majority of antistatic agents are cationic quaternary ammonium salt and anionic sulfonic acid derivatives, especially the amine product will have a catalytic effect on epoxy or lead to baking yellowing, while import too much will lead to decreased charged affect adsorption rate.
Today, the market is more quaternary ammonium salts products, the disadvantage is easy to lead to yellow coating due to a certain degree of acceleration will have a certain impact on advection. But as a good comprehensive performance of products, aliphatic derivatives may be the best option, representative: Noveon stat 308 and Cognis texauart900 and KT308 Hefei Ketai (S) series, KT308 and Noveon S308 are the same type of material, this type of products not only can increase the rate of the powder and reduce the shielding effect but also has promoted the leveling effect, non yellowing and no catalytic activity is another property.
Triboelectric charging additive is another form of product, because between the gun and the workpiece has very strong electric fields, so it is difficult to produce electrostatic coating coating effect is very good on the corner or groove, and the friction charged spraying can solve the problem, because the friction Caqiang (tribo gun) does not produce voltage strong, does not produce a strong electric field on the surface of the workpiece near to prevent charged particles flying into. Spray gun is mainly through friction friction charged powder particles in the pipeline, the resin system with different charging effect is different, so in order to obtain a better rate charged, we add a certain amount of friction agent in the coating formulation or resin in order to improve the friction effect. Nitrogenous compounds can significantly improve the triboelectrification, typical material is hindered amine, such as Hefei Ketai TB88 is the substance, so some pigment even -HAA Schnuffel are effectively friction material. In order to avoid nitrogen additives have accelerated to epoxy system, usually used by yin or amine alcohols as friction agents, light stabilizers such as (HALS) such as Ciba-Geigy Tinuvin144 are usually used as friction agents. Sometimes in order to enhance the friction electrification, adding a small amount of highly dispersed alumina powder in the formulation system to increase the friction electrification effect.