Comparison of test methods for water vapor permeab

Comparison of test methods for water vapor transmission performance of building exterior wall coatings

comparison of test methods for water vapor transmission performance of building exterior wall coatings

November 12, 2018

1 preface external wall coatings have been exposed to the natural environment for a long time, in addition to the door structure is required to adapt to relatively large tension, and have good decoration, water resistance and weather resistance. At the same time, as a part of the external wall insulation system, it should bear the importance of building energy conservation. Water is the primary culprit in destroying the thermal insulation effect of building exterior walls. In the building structure, water in the form of solid water, liquid water and gaseous water will be reported to Wujin District Environmental Protection Bureau for relocation through diffusion, leakage, icing and other phenomena. Among them, people pay more attention to the failure of external wall insulation caused by liquid water leakage. However, the water vapor permeability of external wall coating also affects the efficacy of external wall insulation system. Chinese coating

2 overview of water vapor permeability of exterior wall coating due to the temperature difference inside and outside the building exterior wall insulation system, the water vapor partial pressure on the side with higher temperature is larger. Under the action of water vapor partial pressure difference, water vapor molecules migrate and penetrate from the high-pressure side through the exterior wall coating to the low-pressure side. This process is called water vapor penetration. In cold areas or winter, the temperature inside the outer wall of the building is generally higher than that outside, and the water vapor migrates outward from the inner layer of the building. If the moisture permeability of the outer wall coating is poor, which prevents the water vapor from being discharged outward, it will not only affect the appearance color, but also produce stress, making the coating bubble and fall off. At the same time, because the water vapor is enriched in the interior of the wall, it is easy to cause condensation in the interior of the wall, thus affecting the thermal This is not only because the control must be based on the detection of input information

jg/t "expanded polystyrene board thin plastered external wall external insulation system" clearly stipulates that the water vapor wet flow density of the thin plastered external wall insulation system should not be less than 0.85g/(M2 · h). According to the water vapor wet flow density of the external wall insulation system increases from the inner material to the outer material, so the water vapor wet flow density of the external wall coating should be much higher than 0.85g/(M2 · h), so as to ensure the normal ventilation of the entire external wall insulation system. The "wet flow density of water vapor" mentioned in the standard refers to the mass of vapor phase water flowing through the unit area in unit time. Together with "moisture permeability" and "water vapor transmittance", they are professional terms to describe the permeability of water vapor, but the calculation methods of these three indicators are slightly different in different standards

3 Introduction and comparison of test methods for water vapor transmission performance of exterior wall coatings with the development of science and technology, most of the exterior wall coatings on the market have certain water vapor transmission performance, but whether they meet the use standard still needs further test verification. At present, there are two main standards on the test method of water vapor transmission performance of exterior wall coatings: gb/t "test method of water vapor transmission performance of building materials" and jg/t "determination and classification of water vapor transmission rate of exterior wall coatings". Next, the two standards will be compared and introduced from four aspects: scope of application, test principle, test instruments and materials, and sample preparation

3.1 scope of application

gb/t 17146 is a national standard, which stipulates the wet flow density and moisture permeability test methods of various building materials used for sheet and plate heat insulation, waterproof and moisture insulation, decoration and other purposes. Jg/t 309 is the industry standard of construction industry, which specifically stipulates the measurement method of water vapor transmission of exterior wall coatings used on porous substrates such as bricks, stones and concrete in industrial and civil buildings. Generally, industry standards are professional and technical standards, which are complementary to national standards. For exterior wall coatings that meet the scope of application of the above two standards, the water vapor permeability test can be carried out according to one of them

3.2 test principle

jg/t 309's test principle is to fill a quantitative saturated solution of ammonium dihydrogen phosphate in the test cup to rapidly increase the relative humidity of the air above the test cup to 93%. The test cup is closed with a film like sample to be tested or a substrate coated with a porous structure of the sample to be tested. Place the test cup in a space where the temperature and relative humidity can be controlled to maintain a certain water vapor concentration difference on both sides of the sample. Weigh the test cup at an appropriate time interval. When the change of mass is in direct proportion to the change of time interval, the water vapor transmittance can be measured through the change of mass. Since this method is the result calculated through the reduction of the mass of the test cup, it can also be called the "weight reduction method" test method. In contrast, gb/t 17146 provides two different methods: desiccant method and water method. Water method, namely weight reduction method; For the desiccant method, the sample packaging is the same as the water method, but if the desiccant is placed in the test disk (i.e. the "test cup" in jg/t 309), the relative humidity inside can be regarded as 0. Place the test disk in a space with controllable temperature and humidity and weigh it at a fixed time. Due to the existence of humidity difference on both sides of the sample, the weight of the test disk will continue to increase. Measure the weight of the increased test disk to calculate the water vapor transmission. This method is also called "weight increase method". Theoretically, both methods can obtain correct test data. However, in the actual test process of the weight gain method, the drying capacity of the desiccant continues to decline, and the humidity difference on both sides of the sample changes, which is easy to cause the error of the test data. In the weight reduction method, the test cup is filled with some solution or distilled water to keep the interior at a high humidity level, while the outside of the test cup can be stabilized in a low humidity environment by using a constant temperature and humidity instrument, which can realize automatic control of the constant humidity difference, which is conducive to the accurate conduct of the test. Therefore, the following mainly compares the test differences of the weight reduction method in the two standards

3.3 test instruments and materials

3.3.1 test containers and sealing methods

jg/t 309 requires the use of a test cup made of corrosion-resistant materials with certain strength, such as glass, polyethylene, aluminum, etc., with a diameter of 81mm and a test area of at least 50cm2. The sample and cup mouth are effectively sealed with a mechanical clamping device or screw device with an annular cover and a suitable sealing ring (see Figure 1). Molten wax can also be used as sealant. Compared with the standard test vessel provided by jg/t 309, gb/t17146 puts forward the basic requirements of the test vessel. On this basis, a variety of test disc designs are listed according to the thickness and moisture permeability of the sample. The test disk should be large and shallow, with a disk diameter of at least 60mm. The thicker the sample is, the larger the disk diameter is waiting for approval. For thicker and larger samples, aluminum foil or metal materials should be used to make a mask to cover the part of the sample beyond the pan opening, so that the pan opening area is basically equal to the test area, so as to prevent water vapor from escaping from the edge and the parts that should not be exposed. For samples prone to shrinkage or warpage, a column ring with flange shall be set outside the disc opening, the inner side shall be coated with sealant, the sample shall be fixed in the middle position in the column ring of the test disc, and finally the aluminum foil shall be used to surround and stick to the bottom of the flange and column ring to complete the sealing, as shown in Figure 2. In addition to sealing with sealant, gb/t17146 also allows gasket sealing

3.3.2 relative humidity inside and outside the container

jg/t 309, the test cup contains a saturated solution composed of 1665g ammonium dihydrogen phosphate and 4500g water, which can make the relative humidity of the air above the test cup reach 93%, while the test cup should be in a room or box with a temperature of (23 ± 2) ℃ and a relative humidity of (50 ± 5)%, and the wind speed should be less than 0.3m/s. In gb/t17146 water method test, distilled water is placed on the sample plate, the air humidity above the test plate is regarded as 100%, the surrounding environment can be selected between 21 ℃ ~ 23 ℃, and the relative humidity is generally maintained at (50 ± 2)%

3.4 sample preparation

jg/t 309 is an industry standard for exterior wall coatings. Therefore, for different coating types, such as thin coating Waterborne Coatings, solvent based coatings, elastic building coatings and thick paste coatings (inelastic), the substrate type, coating amount, preparation method and curing time that should be selected are described in detail, as shown in Table 1. While gb/t 17146 focuses more on the introduction of the macro nature of the sample, such as thickness uniformity, representativeness of the sample, sampling thickness, etc

preparation of test samples

4 taking latex paint as an example, jg/t 309 water vapor transmission test method is introduced in detail. In order to introduce the water vapor transmission test method of exterior wall paint in detail, and understand the water vapor transmission performance of commercial paint, according to jg/t "determination and classification of water vapor transmission of exterior wall paint", the water vapor transmission rate of a brand of latex paint is tested

4.1 test instrument

w3/031 water vapor transmission tester produced by Jinan Languang Electromechanical Technology Co., Ltd

4.2 test principle

weight reduction test, under a certain temperature, a specific humidity difference is formed on both sides of the sample, and the water vapor enters the dry side through the sample sealed at the mouth of the test cup. By measuring the change of the weight of the test cup with time, the water vapor transmittance and other parameters of the sample are calculated

4.3 sample preparation

4.3.1 substrate

emulsion paint is a water-based coating, and the coating made of it is a non self-supporting coating. According to the requirements of Table 1, porous substrate is required. Jg/t309 stipulates that the water vapor transmission rate of porous substrate should be greater than 240g/(M2 · d). Therefore, porous non-woven substrate is selected for this test, as shown in Figure 3. The test shows that the water vapor transmission rate is 435.21g/(M2 · d), which meets the requirements of the standard

non woven fabric substrate

4.3.2 preparation of samples

the emulsion paint was diluted according to the product instructions and evenly coated on the non-woven fabric substrate. According to the requirements of jg/t309, "coating dry film thickness ≥ 80 μ m. Brush coating, at least 2 coats, with an interval of 6h ", and the coating thickness of the sample after coating is 147.6 μ m. The average gram weight is 309.62g/m2. Curing for 14 days after coating, the temperature and humidity of the curing environment are (23 ± 2) ℃, (50 ± 5)%

4.3.2 sample state adjustment

after curing, the sample shall be subjected to three cycles of aging test under the following conditions: at the temperature of (23 ± 2) ℃, immerse in the third level water specified in gb/t for 24h, and the coating shall face down when immersing in water and ensure complete immersion. Then, dry the sample at (50 ± 2) ℃ for 24h. After the aging test, store the samples under standard curing conditions for 48h before the test. If the coating bulges and cracks, the sample shall be prepared again

4.4 test

cut the sample into three samples with a diameter of 74mm, seal them in the test cup containing (200 ± 5) g saturated ammonium dihydrogen phosphate solution in the form of coating face up, and put the three test cups on the test frame in the test chamber of the instrument. Because the instrument adopts automatic control intermittent weighing measurement, the test can be started by setting a certain weighing time interval, test temperature of 23 ℃, and test humidity of 55%. When the water vapor penetration reaches the equilibrium state, the test will automatically stop, and the results will be automatically stored and displayed on the supporting computer screen

4.5 result calculation

4.5.1 water vapor transmission rate of porous substrate coated with paint

according to the calculation method of water vapor transmission rate VI of porous substrate coated with paint in jg/t309 (1):

calculation method of water vapor transmission rate VI

where: VI - water vapor transmission rate of sample [g/(M2 · D)]; M1, M2 - total mass at time T1 and T2 respectively (mg); T1, T2 - time (H); Ai - surface area of the sample (cm2)

the water vapor transmittance measured by three samples can be obtained