On the water permeability and water vapor permeabi

Talking about the water permeability and water vapor permeability of exterior wall coating film

talking about the water permeability and water vapor permeability of exterior wall coating film

December 29, 2018

about the water permeability and water vapor permeability of coating film

exterior wall coating used in buildings has two main functions: decorative function and protective function. For the protection function, it refers to that the coating film of the paint can resist the erosion of rain, snow, ultraviolet light, acid rain and dust on the outer wall. Among these protective functions, one of the main functions is to prevent water from penetrating the coating and entering the wall substrate, so as to prevent water from harming the wall structure and thermal insulation system of the external wall of the building. Therefore, the impermeability of the coating film has become one of its main properties

if the external water seeps into the wall, the water in the pores and cracks of the wall will freeze in the severe cold climate, and its volume will expand by 9%. Squeezing the material will cause the weak parts to be eroded and peeled. If the building structure is reinforced concrete, the intrusion of water will cause corrosion of reinforcement and reduce the section. The volume of the corroded body of the reinforcement will expand by times, causing the expansion of small cracks in the concrete, and even the collapse of the protective layer. These will be strict "I believe that after 5 (1) 0 years of development, the integrity, durability and safety of the building structure will be seriously affected. For the external wall insulation system, the invasion of water will improve the thermal conductivity of the insulation material and reduce the insulation effect. Therefore, improving the water permeability of the external wall coating film is one of the main properties of high-quality external wall coating.

we know that the denser the coating film, the smaller the micro pores, the lower the water permeability of the coating film. If the coating has no pores that can let water pass through, water cannot enter the wall through the coating

is it true that the smaller the permeability of the exterior wall coating film, the better

when the indoor temperature is higher than the outdoor ambient temperature, the indoor water vapor will diffuse and penetrate through the wall to the outdoor with lower temperature. If the coating film of exterior wall paint is very dense, its penetration positive force to water vapor must be very large, and the water vapor is difficult to volatilize outward through the coating film, and it will stay inside the wall. Therefore, the permeability of the exterior wall coating is poor, and the diffusion of water vapor is blocked, resulting in the accumulation of moisture in the interior of the wall or the coating interface, resulting in stress. Humidity changes cause the imbalance of internal and external pressure of the wall, resulting in condensation on the wall surface, bubbling and falling off of the coating, resulting in the destruction of the wall structure. The moisture content of the masonry wall matrix gradually increases, resulting in condensate accumulation, which has an adverse impact on the thermal and structural performance of the wall. This internal condensation or icing phenomenon is more harmful, causing material softening, strength reduction, veneer falling off and other phenomena. Especially for the outer coating of the external thermal insulation system of the external wall, if the permeability of the film is poor, the moisture cannot diffuse outward, and remains in the thermal insulation material, the thermal conductivity of the thermal insulation material increases, which will eventually affect the thermal insulation effect and safety and stability of the whole thermal insulation system

the exterior wall adopts coating as the finishing layer. In order to prevent rainwater and snow from penetrating through the exterior wall coating, causing moisture in the interior of the wall structure and affecting the building structure. 1 some enterprises have developed flame retardant PP products that can meet the requirements of household appliance enterprises for integrity, durability and safety. The water permeability of the exterior wall coating film should be low and should have good waterproof performance. At the same time, in order to prevent the water vapor inside the wall from diffusing outdoors in time, staying inside the wall to condense or even freeze, it is also required that the outer wall coating film has a small water vapor penetration resistance, that is, to make the water vapor easy to penetrate. Therefore, the ideal exterior wall coating should not only have good waterproof performance, but also have good water vapor permeability. Any unilateral emphasis on the waterproof or permeability of the film is one-sided, and both must be taken into account

the water resistance (low water permeability) of the film is closely related to the water absorption of the film. The absorption and permeation of water is carried out through the capillary effect in the film. The lower the porosity, the lower the water absorption. At the same time, the permeability of the film is also closely related to the porosity. The greater the porosity, the greater the permeability

since the water permeability of lotion membrane is closely related to its own water absorption, we first discuss the water absorption of lotion membrane. There are many factors affecting the water absorption of lotion membrane:

1, chemical composition and polymer polarity

2. Types and quantities of water-soluble salts and emulsifiers (surrounded by particles to form osmotic pressure)

3. Type and quantity of water swelling additives, such as protective colloids

4, particle size

5. Glass transition temperature of polymer

6, film thickness

7. The membrane is dry

8, temperature

9. Salt content and pH value of water

the polarity of monomer has an important influence on the water absorption of lotion membrane. Hydrophilic functional groups (such as carboxyl groups) dissolve in water, which will improve water absorption. The general rule is that the more hydrophilic the basic polymer itself, the higher the water absorption under the same other conditions. This can be seen from the water absorption of a series of Polyacrylate Polymers at the same glass transition temperature. When the chain length decreases, the polarity of the soft acrylate monomer increases (EHA Ba EA in the system), and the water absorption of the lotion film increases significantly. Similarly, the same is true when the hard monomer styrene is replaced by the more hydrophilic methyl methacrylate (Fig. 5)

the particle size of lotion also affects the water absorption of lotion membrane itself. In terms of the effect of particle size on long-term water resistance, the degree of 24h water absorption is unbalanced (see Figure 6). The membranes of coarse lotion often exhibit rapid water permeability due to relatively poor film formation. The film of particle size fine lotion becomes a more continuous film, and the water absorption is slow. Due to the high barrier to the filtration of water-soluble components, high final water absorption value is often obtained after long-term storage

the water vapor permeability and water permeability of lotion membrane are interrelated. The test conducted by kossmann and schwartzanzh in accordance with DIN 53615 proved that the water vapor transmittance of the film of styrene acrylic lotion was lower than that of pure acrylic lotion. The test cycle is to immerse the film in water for 24h, then take it out and dry it at 50 ℃ for 48h. The film is dried at room temperature for several days in advance (keep constant weight). The dry film thickness is about 500 microns. For all lotion, the water vapor transmission rate of the membrane decreases with the increase of the number of test cycles. When the number of soaking cycles increases, the water vapor transmission rate of pure acrylic lotion membrane decreases, while that of styrene acrylic lotion membrane decreases slightly

in the above test, when the film is put into water for a period of time and dried for a period of time, it is observed that the water vapor transmittance of the film decreases, in fact, the water absorption of the film also decreases. It can be considered that the water-soluble components of the film are leached out, and the quality of the film is improved. In addition, when water passes through the micro pores of the film, it will combine with hydrophilic Suan groups to swell the film and reduce the water passing through. Of course, the passing capacity of water vapor is also reduced accordingly

the data in Figure 8 shows that for these polymers with almost the same minimum film-forming temperature, when the water vapor resistance of the film increases, the water absorption decreases. Pure acrylic copolymer has higher water absorption and water vapor permeability than styrene acrylic polymer. When the copolymer becomes more hydrophobic, the water vapor permeability and water absorption will decrease at the same time. Trying to reduce the water absorption often makes the water vapor transmission worse

the above description is only part of the lotion film. For exterior wall coatings, in addition to lotion, there are various kinds of pigments and fillers with different particle sizes, various additives, PVC with different formulas, etc. There are many factors that affect the water absorption and water vapor transmission of the coating film

German Ku ̈ Nzel proposed a theory of exterior wall protection. Only when the test values of air permeability and water permeability (water absorption) and their comprehensive calculation reach a suitable value, the coating can have superior protection function. In Europe, the water vapor diffusion resistance of the material layer is usually described by its diffusion equivalent air layer thickness SD (s= thickness, d= diffusion) (see EN ISO 12572). The SD value describes the thickness of the air layer (still air) with the same diffusion resistance as the measured material layer. The thickness of the air layer (still air) with the same diffusion resistance of the Kunzel outer wall. Ku ̈ Requirements of nzel exterior wall protection theory:

water absorption coefficient of coating w ≤ 0.5kg/(M2 · H0 · 5)

the equivalent static air layer thickness of air permeability SD ≤ 2m

and it is required that w · SD ≤ 0.1kg/(m · H0 · 5)

the comprehensive result of these three requirements, the water repellency and air permeability of the coating must fall within the shadow area of Figure 9 in order to have a better protection function

Table 1 shows the test results of water vapor transmission, equivalent static air layer thickness SD and water absorption of coatings with different lotion, PVC and film thickness. Lotion styrene acrylic 2a and styrene acrylic 2B have the same monomer components, different emulsifiers are used in the production process, and different neutralizers are used after the polymerization reaction. The dispersion phenylpropyl 2a is neutralized with ammonia, while phenylpropyl 2B contains sodium hydroxide

it is the task of coating technicians to balance these two contradictory properties and produce an external wall finishing coating with appropriate comprehensive performance and balanced waterproof and breathable performance

the waterproof (water absorption) and water vapor transmission of the exterior wall coating film are important performance indicators of the coating. These two properties complement each other and contradict each other. We cannot emphasize one aspect unilaterally. Jg/t general technical requirements for building exterior wall coatings includes two index requirements: water absorption and water vapor transmission. Gb50574 unified technical code for the application of wall materials requires that the coating with waterproof and breathable performance be selected for the exterior finishing coating used for external and internal insulation walls. Gb/t 9755 synthetic resin lotion exterior wall coatings adopts the tangent pendulum force measuring mechanism in the 2014 revised edition, which adds the waterproof index "water permeability" for the first time, and sets different water permeability indexes from qualified products, first-class products and excellent products. It can be considered that gb/t advocates that the lower the water permeability of the coating is, the better, but does not put forward requirements for the water vapor transmission rate of the coating at the same time. The lower the water permeability of the coating, the lower the water vapor permeability will be. Too low water vapor permeability will make the water vapor permeability resistance of the coating too high, affecting the comprehensive performance of the exterior wall coating. From this point of view, gb/t has a one-sided guiding role in this regard, and it is suggested that standard makers revise it

Research on the test method of water permeability

the examination and measurement of the waterproof property and water vapor transmission rate of the outer wall coating film need a reasonable test method. International standards and foreign advanced standards, such as ISO, ASTM, DIN, etc., have developed test method standards for the testing of these two properties. Because the waterproof performance of the film (low water permeability) is closely related to the water absorption of the film, these test methods and standards all test the waterproof performance of the film based on water absorption. Referring to these methods and standards, China has also formulated jg/t "determination and classification of water absorption of exterior wall coatings" (modified by en: 2008 "paints and varnishes - coating materials and coating systems - external masonry and concrete - Part 3: Determination of liquid water permeability") and jg/t "determination and classification of water vapor transmission of exterior wall coatings" (modified by ISO: 1999)

gb/t "synthetic resin lotion exterior wall coating" standard uses water permeability index as the standard of coating film