Sun Shunjie (Beijing Technology Center, Guangdong Longhu Sci. & Tech. Company Limited, Beijing 100076)
Abstract:This paper mainly studies the materials and manufacturing technology of water-borne dry powder architectural coatings and studies the inflfl uence of pigments, re-dispersible binder powder and functional additives on coatings property. The manufacture and application technologies are simple. The product is of excellent covering power, scrubbing resistance and water resistance. Because the coatings are completely water-dispersible, the performances of convenient transportation, good environment adaptability and convenient application are all possessed.
In recent years, China’s architectural coatings have developed rapidly, and product research and development have also developed in the direction of water-based, high solid content, low VOC. As an important type of water-based coatings, latex paints have an absolute share in the coatings market, especially the home improvement market, due to their convenient use, easy toning, and good water resistance. But because it is an aqueous dispersion system, most of the solid content is 45% to 55%, and the rest are moisture and volatile substances, which causes a great waste for packaging and transportation. In addition, taking into account factors such as emulsion film formation and winter antifreeze, film forming additives, ethylene glycol, ammonia and other additives are generally added to latex paints, becoming the main source of VOCs and causing environmental pollution. Compared with latex paints, dry powder architectural coatings have the advantages of convenient production, zero VOC, and convenient packaging and transportation, and have gradually attracted everyone’s attention.
In the field of dry powder architectural coatings, Japan started earlier in this area and applied for related patents in 1980. China ’s research in this area started around the 1990s. Due to the limitations of the raw materials at that time, the development was relatively slow. Most of them used polyethanol products as the main film-forming substance. The product varieties mainly include imitation porcelain coatings, interior wall putties, and other products Generally low-grade, its water resistance and weather resistance are poor, and it cannot meet the requirements of exterior wall decoration. In recent years, due to the emergence of redispersible rubber powder and various new building material additives, the quality of dry powder architectural coatings has greatly developed, but there are still problems such as many bubbles, poor leveling, difficult dispersion, and poor color stability. Compared with latex paints, there is still a certain gap, so it is of great significance to develop a dry powder coating for construction, which is convenient in construction, good in water resistance, stable in storage, and close in performance to latex paints.
1 Formulation design of water-based dry powder architectural coatings
Like latex paint, the main raw materials for dry powder architectural coatings are film-forming substances, pigments, fillers and functional additives. Among them, the film-forming substances are mainly redispersible emulsion powders. It is a polymer polymer emulsion that is spray-dried and subsequently treated. Into a powdery thermoplastic resin. Common types of redispersible powders on the market today are: vinyl acetate and ethylene copolymerized rubber powder (Vac / E), ethylene and vinyl chloride and lauric acid terpolymerized rubber powder (E / VC / VL), vinyl acetate Terpolymer copolymerized with ethylene and higher fatty acid vinyl esters (Vac / E / VeoVa). At present, acrylic latex powder has also appeared in the market, and its weather resistance and water resistance have been greatly improved. As the main film-forming substance for dry powder architectural coatings, latex powders should have a certain degree of whiteness, better fluidity, better adhesion properties and good dispersibility. In addition, they should have better anti-caking functions during storage to avoid Agglomerated with other powders. Generally speaking, the greater the amount of redispersible latex powder added, the higher the water resistance, scrub resistance, and weather resistance of the coating, but considering the cost factor, the amount is generally 10% to 20%. There are many types of pigments and fillers that can be used in coatings. Different pigments and fillers have a greater impact on the gloss and weatherability of coatings. Among them, titanium dioxide is the most excellent white pigment, which has the advantages of strong hiding power, high brightness, and easy dispersion. Other colored pigments such as iron oxide series have good weather resistance, low oil absorption and easy dispersion. Fillers used in coatings, common types are heavy calcium, wollastonite powder, mica, talc, kaolin, dolomite and so on.
Dry powder architectural coatings have higher requirements on the performance of additives. The first requirement is powder materials. In addition, the additives are required to be easy to disperse and have certain fluidity. Common additives include dispersants, defoamers, lignocellulosic fibers, cellulose ethers, thixotropic lubricants, and the like. In addition, in order to improve product performance, special additives such as powdered silicone hydrophobing agent and antibacterial agent can be added to improve the water resistance and antibacterial property of the coating.
Dispersant, also called wetting dispersant, is a key raw material in the production process of dry powder architectural coatings. The dispersant itself should have efficient dispersibility and good fluidity. The dispersion efficiency will directly affect many of the key properties of the coating, such as the amount of water added to the coating, viscosity and storage stability, and workability. Especially in colored systems, the efficiency of the dispersant will affect the color spreadability, uniformity of the coating, and whether or not the color is flooding. There are many types of dispersants. According to the ingredients, there are mainly polyacrylates and polyphosphates. However, most of the products currently on the market are liquids, powder dispersants are relatively small, and most are imported products. Defoamer is also a key raw material, and its type and addition amount will affect the appearance of the coating film. Good defoamers have strong defoaming ability and long-lasting efficiency. Common types of defoamers include organically modified polysiloxanes, higher fatty alcohols, and mineral oils. In order to obtain better workability and storage stability, a suitable thickener needs to be added. Common thickeners are cellulose ether, bentonite and thixotropic lubricant, which can make the coating have higher viscosity in low shear state, obtain better storage stability, and reduce brush stagnation during normal construction. Reduce splashes. The addition of wood fiber can extend the open time of the coating, improve the workability and prevent the initial cracking of the coating film.
Based on the above factors, the basic formula design is shown in Table 1.
Table 1 Basic formula of dry powder architectural coating for exterior wall construction In the
white coating formula, the pigment is titanium dioxide. In colored paint, the ratio of m (titanium dioxide): m (iron oxide red) = 4: 1 is used. In order to obtain better hiding power and stability, the filler uses two different powders. Before construction, add part of the water to the container, and then slowly add the powder. The stirring speed is controlled at 750 r / min (close to the speed of the on-site electric mixer). Considering the actual situation of the construction, the stirring time is controlled at 15 min. The remaining water is adjusted to a suitable viscosity.
2 Test Section
2.1 Raw materials and testing equipment
Raw materials: Redispersible latex powder, 5010N, 5044N, silane hydrophobic dry powder building Power A, Germany Wacker; Pigments: titanium dioxide R595, Meriline; iron oxide red pigment 4110, Germany LANXESS; Defoaming agent, AGITAN®P803, MUNZING, Germany; Sodium hexametaphosphate (NaPO3) 6, commercially available; cellulose ether, 250HBR, American Aqulon; wood fiber, BE600 / 30U, German JRS; thixotropic lubricant, OPTIBENT® 602, American ROCKWOOD; filler, domestic.
Test and testing instruments: C84-Ⅱ contrast ratio meter, JTX-Ⅱ scrub resistance tester, stain resistance tester (Shanghai Modern Instrument Corporation), WGG60 digital display gloss meter; high-speed disperser; color difference meter: Minolta D2600; UV light box, power 500 W, Henan Building Materials Institute.
2.2 Coating performance testing method
Water resistance: Carry out according to GB / T 1733-1993 method A.
Alkali resistance: according to GB / T 9265.
Scrub resistance: According to DIN 53778: The coating is irradiated with QUV-B for 200 h before testing, and spray-treated.
UV light stability: After the prepared coating sample is cured for 7 days, it is irradiated in an ultraviolet light box for 100 hours, and after cooling, it is taken out to observe the surface chalking.
Viscosity test: NDJ-1 rotating viscometer test.
3 Test results and analysis
of pigments and fillers The basic properties of pigments and fillers commonly used in coatings are shown in Tables 2 and 3.
In dry powder architectural coatings, pigments and fillers should have a lower absorption to ensure better dispersibility. From the above comparison, it can be seen that rutile titanium dioxide has a lower oil absorption than anatase titanium dioxide in white pigments, is easier to disperse, and has a better refractive index and strong hiding power, so it is more suitable for building dry powder construction. In paint. Compared with iron oxide yellow, iron oxide red has better hiding power and is easier to disperse. According to the above principles, the filler is suitable for the selection of heavy calcium carbonate, dolomite, wollastonite, etc. Although kaolin, talc, etc. can improve the storage stability or leveling of the coating to a certain extent, comprehensive consideration is not suitable for dry powder construction coating in. In addition, different pigment and filler ratios were prepared according to Table 4 to prepare coatings. After a period of outdoor exposure test, the contrast ratio, discoloration, and degree of differentiation were tested. The results are shown in Table 4.
From the above, it can be seen that when the pigment-filler ratio is 3: 7 or more, the coating contrast ratio meets the requirements of superior products, the color difference is small, and the degree of chalking is slight. Therefore, pigment: filler = 3: 7 or more can be selected in the design of coating formula. The filler is usually selected from two or more powders with low oil absorption, such as heavy calcium carbonate, dolomite, wollastonite, etc. In order to ensure the fineness and dispersion of the coating, the fineness of the powder should not be too coarse or too fine. Generally, it is selected in the range of 600 to 1,000 mesh.
3.2 Effect of wetting and dispersing agent on coating performance
In the test, different dispersing and wetting agents were selected. The basic properties are shown in Table 5.
500 g dry powder building was selected, and the mixing ratio was m (titanium dioxide): m (heavy calcium carbonate): m (wollastonite) = 1: 1: 1, the wetting and dispersing agent was mixed with the above powder in advance and uniformly, at high speed Slowly add to the water in the dispersed state to prepare a slurry with 70% solids content. After dispersing for 30 minutes, test the viscosity of the slurry. The results are shown in Figure 1.
It can be seen from Figure 1 that dispersant A, which was originally applied in mortar systems, is not suitable for use in dry powder architectural coating systems. With the increase of the amount, the viscosity of the slurry decreases slightly at the beginning, but increases later. When dispersants B, C, and D are added to the pigment slurry, the viscosity will drop suddenly when it is added to a certain amount, which can be called the critical point amount. The critical point dosage of B is 0.1%, and the critical point dosage of C and D are 0.2%. According to coating dispersion experience, the actual amount of dispersant additive is generally 1.5 to 2 times the critical point.
The effects of dispersants on the storage stability and foaming properties of pigment pastes are shown in Table 6.
It can be seen from Table 6 that different dispersants have different dispersion efficiencies, of which A is not suitable for use in dry powder architectural coatings. After C is added, the pigment slurry has more bubbles and has a certain pungent odor. D efficiency is high, but it is mainly inorganic components, and the stability in the later period is poor. Taken together, B has a good dispersing efficiency, no odor, and good foaming and stability. It is a relatively good dispersant.
3.3 Effect of redispersible emulsion powder on coating performance
Add 5%, 10%, 15%, 20% (coating weight) of 5010N and 5044N, respectively, to test the scrub resistance, gloss, water resistance and alkali resistance of coatings The test results are shown in Figures 2 and 3.
It can be seen from Figures 2 and 3 that as the amount of redispersible powder added increases, the scrub resistance and gloss of the coatings also increase. With the same amount of addition, 5010N has better scrub resistance than 5044N, but the gloss is lower. The Tg of 5010N is 16 ℃, has a glass transition temperature higher than 5044N (-7 ℃), and the coating film is relatively hard, so it has good scrub resistance and stain resistance. In dry powder architectural coatings, wood fibers can be added to improve scrub resistance and storage stability. Because wood fiber can form a special three-dimensional frame structure and capillary action in the coating, it can effectively improve the scrub resistance and initial crack resistance of the coating film. In the water resistance and alkali resistance tests, except for the addition of 5% of the coating, the samples meet the standards. In dry powder architectural coatings, the water resistance of the coating film can be improved by adding silicone dry powder construction. Usually adding 0.1% to 0.3% of silane powder such as WACKER’s Powder A can effectively improve the hydrophobicity and water resistance of the coating film.
3.4 Effect of defoamer, thickener, etc. on coating performance
In the test, the effects of different defoamer additions on the appearance of dry powder architectural coatings were tested. The results are shown in Table 7.
A is a mineral oil defoamer, B is an organic modified polysiloxane defoamer with an active content of 100%. It can be seen in this test that B has a better defoaming effect than A, but is prone to appearance morbidity such as “oil shrinkage”. According to the test results, 0.2% A and 0.2% B can be added in combination, which has better defoaming effect. Types of thickeners that can be used in dry powder architectural coatings include cellulose ether and bentonite. There is currently no polyacrylic acid and associative thickener in powder form on the market. Bentonite has good suspendability, good thickening and thixotropic properties, but the thickening efficiency is not high. However, the modified bentonite can reach the nanometer level, and becomes a white dry powder building with a flaky structure, which can form a transparent colloid with high viscosity in water. Even at very low concentrations, it has excellent thixotropic and yield values. There are many types of cellulose ether. According to the water retention effect, solubility, thickening effect, and splash resistance, hydroxyethyl cellulose ether (HEC) is mostly used. Wood fiber can also be used in dry powder architectural coatings to improve its anti-settling and scrub resistance due to its special three-dimensional structure and “water-locking” function. Table 8 shows the effect of different materials such as cellulose ether on the rheological properties of coatings.
It can be seen from Table 8 that HEC has a greater influence on the viscosity of the coating. With the increase of the amount of HEC, the viscosity of the coating changes greatly, and the scrub resistance is improved. Considering the water resistance of the coating film, when the amount of HEC added is 0.3%, it has a good state. The addition of modified bentonite can greatly improve the thixotropy of the coating, which contributes to the storage stability of the coating, but affects the leveling property of the coating to a certain extent. The addition of wood fibers can increase the viscosity of the coating and effectively improve the scrub resistance.
3.5 Effect of mixing process on coating performance
In the process of using dry powder architectural coatings, taking into account the actual conditions on the site, a handheld electric stirring gun is often used for manual stirring, the rotation speed is about 700-800 r / min, and the stirring time should not exceed 15 minutes. According to the actual stirring observation, after stirring in accordance with the above method, a small amount of particles in the dry powder architectural coating have not been completely dispersed. Prolong the stirring time by 15min or use a fast stirring gun (the rotation speed is about 1 500 r / min) to stir for 15min. The coating can be obtained in a uniform state with good workability. In order to obtain a uniform coating dispersion and improve efficiency under the above conditions, the coating formulation should be further adjusted, such as the use of cellulose ether with lower viscosity and better solubility, etc., which needs further experimental determination.
Through comparative tests on pigments and fillers, redispersible rubber powders, and dispersants and thickeners in functional additives in dry powder architectural coatings, it is possible to prepare products with good hiding power, scrub resistance, and water resistance that meet national standards Products, because dry powder architectural coatings are easy to transport, construction, excellent environmental protection, etc., have broad application prospects.