Discussion on the reason of silicone sealant foaming in construction

In the construction of silicone sealant, it is difficult to determine one of the reasons for foaming. In this paper, through the experience of our technical personnel in the first line of work, the bubble phenomenon under various construction conditions is analyzed in detail.

1. There is air in the process of glue injection: silicone weatherproof glue is generally injected at the construction site, and the operation conditions are poor. When workers stand on the scaffold to inject glue, the uniformity of glue injection is not easy to master, and air is easily trapped in the glue seam. After gluing, the glue seams are scraped and pressed inside the surface, and the finish is smooth. The pads in the glue seams usually use closed polyethylene foam rods, and the air in the seams is not easy to overflow. After the air is compressed, the air is compressed, and the external tension is generated. The weather resistance is generally wider and the thickness is smaller. The weatherproof glue with high displacement capability is low, and the hardness is very low. The tension of the air slowly blisters the glue seam. This phenomenon is more likely to occur in the application of weather resistant adhesive which can provide more than ± 25% of the adhesive seam displacement capacity. On the contrary, some weathering adhesives with low displacement capacity have higher hardness and thicker before curing. It is not easy to blister the rubber due to the grand tension of compressed air wrapped into the glue seam. Therefore, the more weather resistant adhesive can provide high displacement capacity, the more attention should be paid to the uniformity of injection. In general, the vertical joint can be filled with glue from bottom to top. Some blistering can be overcome. More important is the proficiency and sense of responsibility of the operator.

2. The joint is wet during glue injection: when working outdoors, it often rains, especially in the southern summer. If it rains, the joint will be directly injected if it is wet and not dry. When the sun irradiates the substrate and glue joint, the temperature inside the joint will rise, and the moisture inside the joint will evaporate, which will blister the uncured and soft glue joint, and sometimes polyethylene Foam pad wet will also cause glue seams and blisters. Therefore, the construction can not blindly seek progress, wait until the interface is dry, and then inject glue, which can avoid the above situation.

3. foam rod bleed: the pads inside the glue seams usually use closed foamed polyethylene foam rods. Due to improper extrusion, the internal airbags are released and air is released when placed. When the glue is finished, bubbles will appear or bulge up. The solution to this problem is to cut the foam rods and cut them to the inside. The two is to use open foam rods or foam based polyethylene foam rods to overcome the above situation. In another case, the weather resistance gum is generally more alcohol type. When the curing agent is used, the alcohol type weather resistant adhesive releases ethanol, and ethanol reacts with some foam rods. For example, when a fish NP silicone rubber is used in a construction site, it is bubbling. If you change to Hui fish NP glue, another oxime type weather resistant glue injection, there will be no foaming. In such a situation, the problem is solved by changing foam sticks or changing glue.

4. Exposure to the sun when the glue is not cured: silicone adhesive can not be exposed to the sun before curing, especially when it is exposed to the sun just after the glue injection, which will cause honeycomb bubbles inside the glue joint, leading to the external uplift of the glue seam. This situation will be more obvious when the alcohol type silicone adhesive is used. In the hot summer construction, this situation is not easy to overcome. When injecting glue, avoid exposure to the sun, which will overcome some blistering phenomenon. The specific methods are as follows: injecting glue on the West or north side of the building in the morning, and injecting glue on the east or south side of the building in the afternoon. After glue injection, the glue solidifies for a period of time, and the surface layer has crusted. When it is exposed to the sun again, it has a certain resistance and can overcome some blistering phenomenon.

5. The surface temperature of substrate is too high: when the silicone adhesive is cured, the temperature of the bonding material is not more than 50 . This conclusion has been recognized by most silicone adhesive manufacturers. When the surface temperature of the base material is over 50 , the glue joint will blister, and the above situation will be encountered in the weatherproof caulking of metal panel curtain wall. In the hot summer, when the metal plate is exposed to the sun, especially the surface temperature of aluminum plate will reach 80 , at this time, glue injection is easy to cause blistering. Therefore, we should choose to inject glue in the late afternoon or cloudy day to improve the above situation.

6. The temperature difference between day and night is large, and the thermal expansion and cold contraction of the joint are large: this kind of situation is easy to occur when aluminum curtain wall is injected with glue. Aluminum is a material with larger expansion coefficient. In northern China, the temperature is higher day and night in spring or autumn, and when the sun is directly on the aluminum plate at noon, the surface temperature of aluminum plate can be as high as 60-70 , and the temperature at night will drop to about 10 . Large aluminum plate will have more thermal expansion and cold contraction, and the joint displacement is larger. After the weatherproof sealant is caulked, the actual displacement of the joint is borne by the glue joint, and after curing In the process, excessive and repeated expansion and contraction displacement will cause blistering in the glue joint. Therefore, avoid the maximum direct sunlight at noon to improve some of the above foaming phenomena.

7. Dry climate in spring in northern China: silicone sealant should be polymerized into elastic sealing material with the help of moisture in the air. The temperature in the air is closely related to the curing speed. The climate in northern China is very dry in spring. After the silicone sealant is injected, the curing time is very long. The silicone sealant before curing is easily affected by the external environment, which provides opportunities for various foaming phenomena.