The selection and formulation optimization of the powder coating system for electro-oil statins

China Coatings Online News Information
Abstract: This paper mainly focuses on the use of electro-oil statin environment, research suitable for electro-oil statin powder coatings, including system selection and resin and additive selection, and then formula optimization;
electric oil heaters are now popular on the market. This type of oil is based on thermal oil as a medium, and electric wire as a heat source heating oil, and heat transmission to a metal shell equipped with thermal oil for heat transmission and heating purposes. In order to ensure the rust resistance and decorative properties of metal housing requirements, generally coated paint or powder coating. The advantages of coating paint is easy to paint, can use the traditional brushing or spraying process for construction, and the bottom layer of anti-rust primer and then apply heat-resistant paint, coating the comprehensive performance of good; The use of powder coating can be applied at once to achieve the required coating thickness, high labor productivity, in addition to avoid organic solvents brought about by environmental pollution and harm to the health of operators, but also can save a large number of organic solvents, to prevent the waste of limited resources. Because the surface temperature used in general can reach about 115 degrees C under normal conditions, so usually used for steam or hot water heating radiating sheet powder coating is not able to meet this requirement, even ordinary heat-resistant powder coating can not meet this requirement, need to develop a new type of high temperature-resistant powder coating for electro-oil statin.
general electro-oil statin with high temperature powder coating requirements, preferably in the environment of 200 degrees C, continuous work for 24 hours, coating film preservation performance is good, no yellowing phenomenon, color difference E≤1.0. When the panel and soleplate are installed on the whole machine, at 1.15 times the rated power, continuous work for 200 hours, requiring the coating film without color change yellowing, foaming, de-layering, odor and other issues. Because in unusual circumstances, the temperature of the oil statins reaches 150%, in order to meet these conditions, the heat-resistant temperature requirements of the coating are considered to reach ≥180% of the conditions of the powder coating. In addition, considering that electrolytine has more opportunities for direct contact with people's used items and skin such as hands, this powder coating should be non-toxic and does not contain toxic heavy metals such as lead, cadmium and chromium.
also from the coating convenience considerations can not only be coated with high-voltage electrostectrulation powder coating method, but also with friction electrostectrectrulation powder coating method for coating. From the consideration of reducing the cost of coating, the thickness of the coating film from the past 60 to 80m gradually to the saving type of 50 to 60m of the medium-thin coating thickness of the coating.1, test part
1.1 raw material
polyester 50/50/polyester 60140/polyester 70/30;
polyester 1 s/polyester 2 s/polyester 3 s, polyester 4 s;
Epoxy 1 s/epoxy resin 2 s/epoxy resin 3 s/epoxy resin 4 s/epoxy resin 5 s/epoxy resin 6 s/epoxy resin 7 s;
titanium white powder 1 s/titanium white powder 2 s/titanium white powder 3 s/titanium white powder 4 s;
antioxidant 1 s, antioxidant 2 s/antioxidant 3 s.
1.2 test
test equipment: extruder, electrostectorective spray gun, oven.
1.3 test method
according to the formula weighing raw materials, mixed evenly, extruded with a twin screw extruder, crushed after the high-speed shredder, over 180 mesh standard sieve, and then using electrostectrical spraying, curing time of 200/10min, and then according to table 1 performance indicators for various tests.2, results and discussion
the main reasons for the yellowing of the coating lies in the following two aspects: First, the polymer degradation produces carbon carbon double bond, when the number of carbon double bonds reached more than 10 yellow change; The yellowing and mechanical properties of electro-oil-statin powder coating belong to a normal thermooxygen aging phenomenon of polymers, but there are obvious differences in the aging speed of different polymers, so it is very important to choose resins suitable for electro-oil-statin powder coatings.
2.1 resin selection
2.1.1 polyester resin selection
electro-oiltin powder coating generally choose epoxy / polyester system, so mixed polyester selection is an important part of raw material selection. Mixed polyester is generally made from polycarbonate, acid and polyol shrinkage and addition, polyester resin is generally colorless or pale yellow transparent crystals, softening point is 95-115 degrees, acid value of 68 to 80mgKOH/g, glass transition temperature is greater than 50 degrees C, this resin can make polyester resin / epoxy resin .
In practice, sometimes in order to improve the film's heat resistance, weather resistance, toughness, horizontal fluidity and reduce costs, it is necessary to make the necessary adjustments to the amount of epoxy resins and polyester resins in polyester/epoxy systems, other commonly used polyester resins according to polyester, epoxy ratio is divided into: 50/50, 60/40, 70/30, in the project experiments to compare the performance of various performance, as shown in Table 2.
available from the table above, 70/30 polyester resins are better yellow-resistant than the other two polyester, which may be due to the proportion of 70/30 epoxy resins and we know that, in general, polyester resins have better anti-yellowing properties than epoxy resins. On the other hand, compared with the 50/50 curing system, 70/30 polyester has a higher molecular weight, because of its good toughness, to ensure the mechanical properties of the coating after long baking.
2.12 Epoxy Resin Selection
From the selection process of polyester resins, it can be seen that the anti-yellowing properties of the coating are mainly determined by the formulation of epoxy resins with the weakest yellowing resistance, so the screening of epoxy resins is the most critical factor.
At present, epoxy resin according to its structure is roughly divided into BPA epoxy resin, line phenolic eoxygen resin, phthalates epoxy resin, bisphenol F epoxy resin, bisphenol type epoxy resin, hydrogenated BPA epoxy resin, silicone modified epoxy resin seven, can be applied to powder coating epoxy resin usually has ordinary bisphenol A epoxy resin, phenolic epoxy resin.
phenolic epoxy resin because the molecule contains both biphenol A phenol and phenol aldehyde components, there are multiple epoxy groups, with high cross-link density, high coating hardness, good wear resistance and so on, the formation of a smooth coating surface, but the introduction of phenolic structure makes the coating yellowing, so it is not suitable as raw materials for electrolytic powder coating.
BPA type A epoxy resin structure as shown in Figure 3 Bisphenol A epoxy resin structural units give the resin the following functions: epoxy and hydroxyl-given resin reactive, so that the resin curing has a strong cohesion and bonding force; The base group helps to improve the immersion and adhesion force, the ether health and C-C bond make the large molecules flexible, the benzene ring gives the polymer heat resistance and rigidity, the isopropyl also gives the large molecule a certain rigidity, the key energy of one C-O key is high, thus improving alkaline resistance. Therefore, the molecular structure of Bisphenol A epoxy resin determines its performance with the following characteristics:
is a thermoplastic resin, but has thermosteresic properties, with a variety of curing agents, catalysts and additives to form a variety of high-performance curing substances, almost meet the requirements of powder use;
resin process is good, curing basically does not produce small molecular volatiles, can be low-pressure molding, can be dissolved in a variety of solvents, which makes the coating curing to
minimize the appearance of pinholes;
have good corrosion resistance and electrical properties, and
have certain toughness and heat resistance.
So, we choose bisphenol A epoxy resin, commonly used in domestic (1, 3, 4, 6, 7) import (2, 5) its production process is divided into: one-step method and two-step method, for different types of BPA-type epoxy resin and polyester resin with performance analysis, such as Table 3.
can be clearly seen from the table above, epoxy resin produced by two-step method generally has a higher epoxy equivalent. The coating film resistance to yellowing obtained by high epoxy e8 epoxy resin is good, but it has a disadvantage; Therefore, epoxy resins with high epoxy ethylaxels are not suitable for electro-oil powder coatings. By comparing the suitable epoxy equivalent epoxy resin (1 to 4 s), it can be found that the two-step method is better than the one-step method to obtain the resin yellowing performance. This may be because the two-step method results in a more concentrated distribution of resin molecular weight, of which the low molecular weight part is also less, thereby improving the performance of yellowing resistance.
2.2 Choice of titanium dioxide
Because the coating of electro-oil is generally light and dark, dark color due to its dark color, the yellowing of the coating can be covered, so the color change is not obvious. When light colors such as white and gray paint, yellow color changes can not be covered, color changes are more easily detected. In addition to the above resin selection, titanium dioxide screening is also necessary.
(titanium dioxide) chemical properties are stable and do not react with most substances in general. In nature, titanium dioxide has three crystals: plate titanium, sharp titanium and redstone. Plate titanium type is unstable crystal type, no industrial utilization value, sharp titanium type (Anatase) referred to as A type, and Rutile type (Rutile) short for R type, both have a stable lattice, is an important white pigment and porcelain glaze, compared with other white pigments have superior whiteness, coloring, masking, weather resistance, heat resistance, and chemical stability.
experiments examined the effects of titanium white powder l, titanium white powder 2, titanium white powder 3, titanium self-powder hiss on coating performance, and the results were shown in Table 4. From the preparation process of titanium dioxide, chlorination method compared to the sulfuric acid method of titanium dioxide powder performance is more superior, such as domestic 2 and imported 3, chlorinated legal system out of almost no iron ions, whiteness is usually very good, sulfuric acid method is uneven, followed by fineness and dispersion, chlorination particle distribution is uniform, finer, dispersion is generally very good, and sulfuric acid method in production will produce a large amount of waste sulfuric acid, causing a lot of damage to the ecological environment. Titanium white powder torn into sharp titanium-type titanium white powder, but its coloring force, weather resistance, low cover rate, and sharp titanium-type titanium white powder will have some impurities, will cause the powder coating particles to produce, so in the actual production of less use, only in the case of the need to reduce costs a small number of additions.
2.3 antioxidants
antioxidants (or thermal stabilizers) are used to prevent yellowing of coatings in over-baking. In general, they are a mixture of spatial resistance-inhibiting antioxidants and hydrolytic organophosphates. Powder coatings can seriously affect the appearance and performance of the product after being exposed to high temperature baking or sunlight, in order to prevent or reduce the occurrence of this trend, usually by adding antioxidants or thermal stabilizers to achieve. Although there are many factors affecting film aging, such as the quality and type of resins, pigments, additives, coating formulation design, production process, temperature, atmosphere, humidity and other natural factors, but the application of suitable additives does reduce the occurrence of this trend.
the use of electrolytin powder coatings are mainly affected by heat. According to the analysis, the oxidation rate of each temperature increase of 10 degrees C is about 1 times. At 100 degrees C, the oxidation rate will be 256 times that of room temperature (20 degrees C). Therefore, oxidation at high temperature is a very important factor. Antioxidants with good high temperature resistance must be selected. Blocked phenol antioxidants perform poorly in high temperature resistance, while hydroxyquine, etc., have special use value at high temperatures.
oxidants are of many varieties and have different efficacy, and their rationale can be divided into two categories: chain-breaking antioxidants, which inhibit the growth of free-based reaction chains produced in polymers, also known as primary antioxidants, and preventive antioxidants, which inhibit or trigger the production of free-based antioxidants in processes. The main antioxidants have blocked phenols, zhongfangamines, auxiliary antioxidants have phosphates, methotrexate metal salts and so on.
can be seen from the table above, the addition of antioxidants has an important effect on the yellowing performance of the coating, the addition of antioxidants can reduce the color difference of about 0.5. The blocked phenol antioxidants added in the experiment worked well, but better results could be obtained by re-matching the antioxidants. This is due to the synergistic effects of phenolic chain termination antioxidants in conjunction with peroxide decomposition agents such as phosphates, which increase the anti-thermal oxidation capacity of polymers. 3, the conclusion
because of the use of electro-oil statin in the process of temperature has a large fluctuation, so the surface coating put forward particularly high requirements, such as excellent baking resistance (yellowing performance), excellent physical and mechanical properties and better corrosion resistance. After the above experiments found that: the use of 70:30 polyester resin and two-step Epoxy resin film-forming system, goldstone-type titanium dioxide powder as a filler, supplemented by blocked phenols and phosphate compound antioxidants, can be made to meet the conditions of use of electro-oil statin thermo-solid powder coating.