The cause and solution of the formation of snail patterns in exterior coatings

With peeling, cracking, blistering, flowering, fading and other paint film disease, the problem of rain marks on the exterior paint is one of the most common paint film diseases in architectural coatings, is a special manifestation of discolored hair flowers, will seriously affect the appearance of the building. Although rain marks on exterior walls are extremely common, systematic studies of rain marks on outer walls are rarely reported, and there is no clear definition of rain marks in coating terminology. There are many kinds of rain marks on the outer wall, the most common types of rain marks on the outer wall are bright spots, white rain marks, contaminated rain marks and calcium spots. Many of the outer wall rain marks look very similar, such as bright spots of rain marks, water white rain marks and calcium spots of rain marks from the outside to see are white, but the nature is completely different. Therefore, when dealing with complaints of rain marks on the outer wall, we must first determine the type of rain marks, and then the medicine for the disease, in order to correctly solve the problem.The most common four types of external wall rain marks in the water white rain marks, contaminated rain marks and calcium spot rain marks are essentially determined by the paint's water-resistant whiteness, pollution resistance and alkaline resistance, the relevant research reports are more, due to space constraints this article is not analyzed and discussed. The research report of bright spot rain marks is less, so this paper focuses on the bright spot rain marks of the outer wall paint, and discusses the causes of the formation of bright spots, multiple occasions, evaluation methods, influence factors, formula optimization measures, complaint handling methods and so on.The snail print of the outer wall paint (bright spots rain marks)bright spots rain marks refers to the paint film by rain or dew wet after the paint film water-soluble organic matter is dissolved by water and brought out, accumulated on the surface of the paint film to form a vertical strip of light and a little sticky traces, as shown in Figure I, because of the appearance of a snot, so often referred to in the country as "runny nose." Foreign countries often refer to bright spots as snail marks (snail trail), in order to be in line with international standards are collectively referred to as snail patterns, because it is very much like snails after climbing left bright spots, as shown in Figure 2. This strip of bright spots causes different glosses on the surface of the lacquer film, and changes in the gloss lead to differences in color perceived by the vision.the formation mechanism of snail grain It is well known that the outer wall latex paint contains some water-soluble and difficult to volatile higher molecular weight organic matter and polymer organic matter. These hydrophobic substances usually have emulsifiers and protective adhesives brought in emulsions, polyphosphates and acrylic dispersants, thickeners, moisturizers and dispersants and moisturizers brought by slurry in latex paint, etc., the use of these hydrophobic substances in water-based coating formulations is necessary, and after the coating is completely cured, these hydrophobic substances are well combined in the paint film, so it is not easy to dissolve quickly from the dry film. Eventually it may be slowly dissolved by rainwater, which not only does not harm the paint film but also further improves the water resistance of the coating film.Generally speaking, the outer wall latex paint according to the type and dosage of emulsion, film additives, solvents and other formulations, in 23 degrees C and relative humidity of 50% of the conditions of maintenance drying time needs at least three to five days; Because many latex paints do take so long to fully cure the film. Under the adverse conditions of low temperature and high humidity, the curing time of paint film should be further extended. For this reason, the outer wall latex paint is at high risk within one month of completion of construction.The outer wall latex paint is not completely cured into a film, especially after the completion of construction 2 to 24 hours encountered low temperature high humidity weather, coating surface resulting in condensation or drizzle latex paint paint film hydrophoto-like substances are very easy to migrate to the paint film surface, in the wall to form a water bead rich in hydrophotic substances, water beads in the Under the role of gravity to flow down to form a waterline hanging on the wall, when the moisture is completely volatile after the hydrophoto-organic matter can not be volatile, rich in the surface of the paint film, it forms a bright and somewhat sticky traces, equivalent to uneven hood light, because different glosses lead to different colors resulting in the appearance of the coating film flower defects. Color is the sum of all reflected visible light, a sint when the gloss of the surface changes, it gives people a different color feeling, people with color matching experience must know that the gloss of different paint color is a very headache, it is for this reason plurometer designed gloss compensation function, is used to minimize the color matching time on the effect of color.The outer wall latex paint even if not completely dry cured into a film when encountering large rain usually does not produce snail patterns, because although water-soluble substances will dissolve, but will be washed away by rain, will not gather on the surface of the paint film, not easy to form snail patterns; Snail-printed multiple occasions the outer wall latex paint must have hydromassive substances, so any external wall latex paint has the possibility of producing snail-print; Snail patterns of exterior paint are usually easy to occur in the following situations: 1. Climate conditions: low temperature high humidity season of the outer wall latex paint drying speed is slow, when encountering heavy fog, the surface of the outer wall dew, the most likely to produce serious snail patterns, because at this time latex paint film water resistance is poor, hydromassive substances are easy Dissolved by dew, and because of the atmospheric humidity, surface dew for a long time can not be volatile, paint film soaked in dew for a long time can be a large number of water solutions extracted, the sun comes out after the moisture volatile, hydro-hydro matter in the paint film surface presumptive, leaving a strong shiny white traces, very ugly. Light rain is another climatic condition produced by the outer wall snail pattern, at which time the rainwater extracts the water solution from the paint film and leaves it on the surface of the paint film in the form of water drops or waterlines, and the water evaporates to form a snail pattern or bright spot.。 2. Time: Snail grain is most likely to occur within 24 hours after construction, with the extension of time, paint film curing more fully, water-soluble substances are not easily dissolved out, the possibility of snail grain will be lower and lower.。 3. Paint varieties: oily exterior paint because there is no hydromassive substances, generally do not produce snail patterns. Low-light (or silk) outer wall latex paint, that is, emulsion content of 30-40% of the outer wall latex paint is most likely to appear snail pattern, while matte exterior paint (general emulsion content between 20-30%,) half-gloss and high-gloss exterior paint (general emulsion content of more than 40%) is relatively difficult to occur snail pattern. In China's ordinary outer wall paint emulsion content is generally not high, the chance of snail pattern is relatively low, but the outer wall elastic latex paint emulsion content is generally between 35-40%, is the most likely to appear snail pattern of the outer wall paint. 。 4. Geography: Due to the low temperature and humidity of snails in easy-to-dew and the geographical conditions along the river and coast, the winter, early spring and mid-autumn festivals in east China have low temperature and high humidity, and are closer to rivers, rivers, lakes, seas, and have a year-round tradition of uninterrupted construction of external wall coatings, In particular, the year did not have the habit of catching up, all factors are highly in line with the conditions produced by snail patterns, so the East China region has the most complaints of bright spots and rain marks, followed by South China and Southwest China, because of more rain, humidity is also greater, Northeast, North China, and Northwest China due to low air humidity, winter does not work, the probability of occurrence is relatively small.。 5. Color range: white and light color due to the high brightness of the color, even if there are rain marks are not very conspicuous, and because of no or less color paste, water solution is relatively small, so it is not easy to appear snail pattern. Dark color because of the addition of more color slurry, hydro-hydromassive substances are relatively more, water resistance is worse, and the appearance of snail patterns after the color change is more obvious, so it is easier to more hair. Because the human eye is the most sensitive to gray, dark gray is the most likely to produce snail-print complaints, for example, two cases in Figure I are darker gray.Snail-print assessment method There is no anti-snail pattern performance detection method in the relevant standards at home and abroad, the national standard "GB/T 31815-2015 building exterior surface self-cleaning paint" in the "outdoor rain stain test" is used to detect the external wall paint anti-pollution rain mark performance, can not be used to evaluate the anti-snail stain performance of the outer wall coating. Now the industry uses the following two methods to evaluate: 1. Direct method: latex paint after drying 24 hours, will be deionized water with wool brush coated on the surface of the paint film, so that water on the surface of the paint film formed a strip, waiting for the water to evaporate dry after visual snail pattern severity.。 2. Condensation method: set the appropriate conditions for the surface of the paint film to condense (the temperature of the substrate is lower than the dew point temperature), and form a strip of rain dew hanging on the paint film, waiting for the water to evaporate and dry after visually measuring the snail grain severity. Dew conditions can be set up to refer to the "HG/T4560-2013 paint anti-dew performance test method" standard, but can not be fully tested according to this standard, because the standard is by collecting the weight of dew per unit of time to evaluate the paint's dew resistance, and can not assess the paint's ability to resist snail patterns.two assessment methods have their advantages and disadvantages. The direct method is simple and convenient, easy to operate, does not require special instruments, so more is used. The condition setting of the dew method is more difficult, there is no standard instrument to buy, the operation is more cumbersome, the use of less, but if the condition setting is appropriate, the dew method can simulate the scene produced by the real snail grain, the evaluation effect will be better. Anti-snail pattern assessment multi-select medium and dark gray outer wall latex paint, because as mentioned earlier, medium and dark gray is the most likely to produce snail pattern, the evaluation effect is the best. Snail pattern influence factors affect the outer wall latex paint produces snail pattern factors are many, summarized from the source of A water solution, B water solution dissolved channel (water resistance), C anti-dew ability and D visual sensitivity and other four aspects to consider comprehensively.。 A. The source of water solutionThe root cause of snail patterns is that latex paint memory is a large variety of hydromassive substances, so from the source to consider hydromassive substances mainly have film-forming substances - emulsifiers in emulsions and protective colloids; And color paste compatible wetting agent, help pigment, filler dispersion and long-term stable presence of dispersants, as well as color palette containing moisture, dispersants and other hydromassive substances, to prevent pigment, filler subsidion, to provide the appropriate construction performance of the paint rheumor thickener.。 B. Water-soluble solution channel (water resistance) water solution in latex paint is essential, so complete elimination is almost impossible, as long as latex paint is fully dried and completely filmed, latex paint paint film can have good water resistance, at this time the water-soluble solution channel is basically blocked, generally not easy to appear snail pattern. Therefore, to improve the water resistance of the coating film, reduce the dissolved channel of water-soluble substances can effectively reduce the occurrence of snail patterns. The main way to improve the water resistance of the coating film is to improve the water resistance of the film emulsion, the use of water-resistant wetting agents, dispersants and slurry, and the use of other functional additives to improve the water resistance of the coating. 。 C. Anti-dew capacity the surface of the outer wall paint is most likely to produce snail patterns, and at this time snail patterns are also the most serious, so improve the external wall coating anti-dew capacity can reduce the production of snail patterns. The anti-dew ability of the coating is mainly determined by the PVC of the coating, the roughness of the surface and the affinity/hydrophobicity of the coating surface. The higher the PVC of latex paint, the rougher and porous the lacquer film, so that the substrate and the environment can reach the temperature balance faster does not meet the conditions of condensation (substrate temperature is equal to or lower than the ambient air dew point temperature will condense), even if the substrate can absorb dew and achieve the purpose of anti-dew. The greater the roughness of the coating surface at the same PVC, according to the Wenzel model theory of the contact angle, when the contact angle of the liquid is less than 90 degrees on a smooth and ideal surface (water is generally less than 90 degrees at the contact angle of the surface of the latex lacquer coating film), the apparent contact angle on the rough surface will be smaller, i.e. the surface is better moisturized, more difficult to form water beads and dew. That is, the coarser the surface of latex paint, the better the anti-dew performance, water will be evenly moistened on the surface of the coating film spread out, without the formation of water drops and waterline hanging on the surface of the paint film. The affinity/hydrophobicity of the surface of the lacquer film will affect the contact angle of the water, the stronger the hydrophobicity of the surface of the lacquer film, the smaller the contact angle of the water, the easier the water is to be evenly absorbed by the surface of the lacquer film, not easy to form water beads and condensation, the better the anti-dew performance. 。 D. Visual sensitivity the human eye to snail patterns can be considered in terms of color field and gloss. reflects light a little more where there are snail prints, giving the impression that it may feel a little more "white". Whiteness is already high for white and light colors, vision is less sensitive to the feeling of "white" again, not easily noticeable, so the chances of snail patterns are less and lighter; The hydrohydrant on the surface of the varnish film will be absorbed by the rough or even porous surface, the influence on the gloss is small, not easy to occur snail pattern; Limited, so it is not easy to occur snail pattern, low light or silk surface is just dense smooth and low gloss, can no longer absorb hydroporite, hydropyurite after the surface will increase more gloss, the most sensitive reaction, so the most likely to appear snail pattern problem. The following is from the above four aspects to analyze the effects of each component in the outer wall latex paint on snail pattern law: 1. Emulsion From the source of hydromassive substances, soap-free emulsion is best anti-snail pattern, because it does not use or less emulsions and the use of polymerized reaction emulsions with emulsion monogroups, so that the final emulsion does not contain or contain less emulsion. from the dissolved channel of hydro-water, the better the water resistance of the emulsion, the more difficult it is to be extracted by rainwater, the better the anti-snail texture. The order of water resistance of common emulsions from good to bad is generally silicone-propylene emulsion-pure propylene emulsion, study table