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Selection of silicone weatherproof sealant
Date：2020-10-22 09:40:00
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Now the curtain wall design advocates the principle of energy efficiency, so the sealing effect of the wall also has higher requirements. The sealing effect is mainly related to the correct application of sealant in curtain wall.
2. Surface cleaning and primer application
3 glue temperature
1. Several choices of weather resistant application of silicone adhesive
In the weatherproof application of sealants, it is necessary to pay attention to the selection of appropriate sealants, the width depth ratio of sealants and the selection of backing materials.
1.1 selection of proper sealant
In addition to the good adhesion between the silicone sealant and the material in contact, the silicone sealant also needs enough displacement capacity to meet the design requirements. Taking a 2000 mm × 1000 mm aluminum plate as an example, its thermal expansion coefficient is about 23.2 × 10 - 6 ℃ - 1. When the temperature difference of material surface is 70 ℃, the displacement of aluminum plate caused by temperature difference can be calculated according to the formula.
Displacement caused by thermal expansion and cold contraction = length of aluminum plate × coefficient of thermal expansion of aluminum material × temperature difference (1)
= 2000 ×23.2 ×10 - 6 ×70
= 3.25 (mm)
If the designed width of expansion joint and displacement caused by thermal expansion and cold contraction are known, the displacement capacity of sealant required can be calculated according to formula (2).
The minimum gap width = thermal displacement × (100 / displacement capacity of sealant) (2) assuming that the width of expansion joint designed in this example is 6.5mm, then: 6.5mm = 3.25mm × 100 / displacement capacity of sealant, the displacement capacity of silica gel required is ± 50%. The silicone sealant must have at least one ± 50% displacement capacity for the design. In practical application, the construction error of glue joint should be considered, which is decided by the designer.
1.2 selection of sealant width depth ratio
The width to depth ratio of sealant is generally recommended to be 2:1. The minimum width and depth of sealant are 6 mm and the maximum glue depth is not more than 12 mm. In some cases, 1:1 is acceptable, but the aspect ratio should not be less than 1. This is because when the depth is greater than the width, the displacement capacity of the sealant will decrease, and it can not reach the displacement capacity estimated by the original design. When large displacement occurs, the adhesive stress will increase and fall off.
1.3 selection of backing materials
The function of the backing material can not only effectively control the application depth of the sealant and increase the contact surface with the base material (only for the use of circular backing material), but also prevent the occurrence of three side adhesion phenomenon, so as to avoid possible cohesive failure. The choice of backing material must pay attention to the compatibility and non adhesion of the sealant used. Generally, the compatible backing materials are closed cell polyethylene or open cell polyurethane. The size of the backing material should be 10% ～ 25% larger than the width of the gap, so as to hold the glue seam and not to sink when scraping. When installing the backing material, try to avoid scratching the surface of the backing material, so as to avoid the so-called deflation phenomenon.
2. Surface cleaning and primer application
In addition to the above factors, the other most easily overlooked but has a decisive impact on the success or failure of sealant is the material surface. Whether it is aluminum or glass or other materials, in the process of production, transportation, storage and on-site installation, the surface will be more or less stained with some unclean things, such as oil stains, mold release agents, dust, water vapor and fingerprints, etc. if the silicone sealant is not cleaned up, it will often cause the failure of adhesion in the future. Therefore, the surface must be cleaned before gluing.
Surface cleaning can be divided into non porous and porous according to the material.
2.1 non porous materials
The commonly used method is called "two-stage wiping method". Use a clean, lint free white cloth dampened with solvent to wipe the surface, and then use another clean, lint free white cloth to completely wipe off the solvent and impurities on the surface. The white cloth needs to be changed frequently. Generally speaking, different materials can choose the appropriate solvent to clean to achieve better cleaning effect. For example, IPA (isopropanol), MEK (methyl ethyl ketone) and xylene (xylene) are commonly used for glass; MEK and xylene are commonly used for aluminum cleaning.
2.2 porous materials
Granite and marble porous stone can be cleaned by wire brush, high-pressure water column or solvent (such as MEK) which is very volatile. For cement precast board, steel wire brush or high-pressure water column can be used to clean the surface. These substrates must be completely dry after cleaning.
In addition to surface cleaning work, some in order to increase the adhesion between sealant and material, often use primer. Primer is an adhesion promoter, the thinner it is used, the better. If necessary, a clean cloth can be used to wipe off the excess primer. After the primer, you need to wait for it to dry before applying sealant, but not too long.
3 glue temperature
Because of the special chemical structure of silicone, the use temperature of silicone sealant is very wide, which can reach - 18 ~ - 40 ℃. However, it is suggested that the outdoor application temperature range of sealant should be 5-50 ℃, because 5 ℃ is close to the dew point temperature (4.4 ℃). Near this temperature value, the surface of the base material is easy to form dew that can not be seen by the naked eye. At this time, the adhesive will be directly hit on the condensation, which will often affect the adhesion. When the temperature is above 50 ℃, the adhesive property will be affected, The adhesive is easy to produce bubbles on the surface of the substrate, resulting in reduced adhesion or dry sealant. Of course, at - 18 ～ - 40 ℃, glue can also be applied, but the premise is that the surface of the substrate must be dry. The material surface can be dried by heating bulb, hot blower, etc.
In addition, avoid applying glue at high or low temperature. This is because when the glue is applied at low temperature, such as - 7 ℃, the interface is in the widest state. With the increase of temperature, the interface will continue to shrink, and the sealant will always be in the compression state. On the contrary, when the temperature is high, such as 50 ℃, the joint is in the minimum state. With the decrease of temperature, the sealant will always be in the tensile state. With the change of seasons, this cycle makes the sealant always move in one direction. As a result, the internal stress of sealant will be very large, which may exceed the displacement capacity of design and cause cohesive or adhesive damage. The correct way is to apply glue in the middle of the temperature, such as 21 ℃. At this time, with the increase and decrease of temperature, the sealant will go through compression and extension cycles along different directions, so as to have good interface sealing effect.
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