Weather resistant application of silicone sealant

Now the curtain wall design advocates the principle of energy efficiency, which puts forward higher requirements for the sealing effect of the wall. The sealing effect is mainly related to the correct application of sealant in curtain wall. In this paper, the weatherproof application of silicone sealant is briefly introduced, so that colleagues in the construction industry can understand it.

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 gasket 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%. Therefore, silicone sealant with at least ± 50% displacement capacity must be selected for this 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, the commonly used solvents for glass are IPA (isopropanol), MEK (methyl ethyl ketone) and xylene (xylene), etc.; for aluminum materials, MEK and xylene are commonly used for cleaning. Among them, the powder spraying aluminum material

At the same time, according to many years of experience, Xuanwei curtain wall adhesive suggests that the common anodized aluminum and coated glass should be cleaned with a solution composed of 50% IPA and 50% water, which can remove the water-soluble salt impurities brought by the material production process (these impurities will precipitate out of the surface when the material is immersed in water, thus causing damage to the adhesion), so that the adhesion effect is better. In addition, the toxicity of 50% IPA is much lower than that of xylene or MEK. As for plastics, because there are many kinds of solvents, it is better to do a simple test before choosing solvents, so as not to corrode the plastic surface.

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. For the primer products of Dow Corning company, the general time range is about 5 min to 0.5 h after the primer is applied, and the sealant must be applied.

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 ℃.