Foaming phenomenon and cause of silicone sealant
With the rapid development of China's economy, the application of curtain wall products in many high-rise public buildings will greatly exceed the use of other wall materials, becoming one of the main means of beautifying urban landscapes. The curtain wall industry is also developing rapidly. Now the curtain wall has a variety of forms and materials are emerging, which has injected new blood into the curtain wall industry. Among them, people are more and more concerned about the quality of curtain wall sealants, because sealants are one of the essential materials for building curtain walls. They have the functions of preventing leakage, waterproofing, vibration, sound insulation, and heat insulation. Silicone sealant has become the preferred sealing material for architectural curtain walls due to its good UV resistance, elastic recovery ability and excellent adhesion to common curtain wall materials. As the use of large-area hidden frame glass curtain walls gradually decreases, the use of metal plate curtain walls and stone curtain walls gradually increases, and the application rate of silicone weathering sealants has also increased, gradually exceeding the silicone structural sealants, and becoming building curtain walls. The largest amount of silicone sealant.
There are many reasons for the foaming of silicone weathering sealants during construction, which are relatively complicated. It is difficult to determine the cause of foaming by one method and one result. Many times during the construction of our construction workers, the phenomenon of foaming of the sealant will be found. Since I have been engaged in building curtain walls for 5 years, I have also encountered many problems of foaming of the sealant. Based on personal experience, manufacturers and related information, the reasons for the foaming phenomenon of the sealant are analyzed.
First, the reasons for the variety of manufacturers
Many brands of silicone sealants have experienced similar blistering. After careful testing and repeated replenishment, it is not a quality problem. There are two types of silicone sealants: alcohol-based and ketoxime-based. The alcohol contained in the alcohol-based adhesive will release gas during the curing process (methanol starts to volatilize at about 50 degrees), and the reaction will be more intense when exposed to direct sunlight and high temperature. The concrete metal window frames are difficult to breathe. In addition, in summer, the temperature and humidity are high, the curing will be faster. The gas released by the glue can only run out from the incompletely cured glue layer, and the cured glue strip will appear. Bubbles of different sizes.
In the construction process, we should choose different manufacturers or the same manufacturer of different types of silicone sealant based on different conditions. For example, I had this problem in the curtain wall project of Zhongshan City Traffic Police Brigade, which was completed in 2009. Baiyun's SS611 and SS811 weather-resistant sealants were used on this site, but during the summer construction, SS611 sealant was found to foam and SS811 was not. Later, Find out the reason through the manufacturer. Baiyun brand SS611 and SS811 are two types of weather-resistant sealants under normal conditions. The use range of the two is basically the same as the product characteristics. In the case of lower (<50% RH) or large temperature difference between day and night (> 15 ℃), the glue seam may swell, but it will not affect the sealing performance.
1. Air was trapped during the injection.
Silicone sealant is generally completed at the construction site, the working conditions are poor. Workers stand on the scaffold to inject the glue. The uniformity of the glue injection is not easy to grasp, and the air in the glue seam is easy to be wrapped. After the glue is applied, the surface of the glue seam is scraped and trimmed cleanly. Generally, closed-cell polyethylene foam rods are used for the cushion rods in the glue seam. The air wrapped in the glue seam is not easy to overflow. The external tension, the sealant seam is generally wide, the thickness is small, the weathering glue that provides high displacement capacity, the hardness is low, the glue is very soft before curing. The pressure of the compressed air slowly foams the glue seam. This phenomenon is more likely to occur in sealants that can provide more than ± 25% of the displacement of the glue seam. Conversely, some sealants with lower displacement capacity have higher hardness and are thicker before curing. The tension of the compressed air wrapped in the glue seam does not easily foam the glue. Therefore, the more weather-resistant adhesive that can provide high displacement capacity, the more uniform the injection should be paid more attention to when injecting. Generally, vertical glue seam injection can be applied from bottom to top, which can overcome some foaming phenomena. More important is the proficiency and responsibility of the work operator.
2. Plastic injection habits of workers
In the glue injection construction, we often find that when workers inject glue, many people like to spray soapy water, detergent water, detergent powder, etc. on the joints of the glue, and then smooth it by hand to ensure that the glue is not sticky, but I did not expect that the flowing water was absorbed by the foam strip, that is, the water was dried. As long as soap, detergent, and detergent are stored on the foam strip, the glue will foam when the glue is applied during the sun.
Reasons for external construction conditions
When working outdoors, rain is often encountered, especially in the summer in the south. It rains almost every day. If the interface is wet and not wet after rain, inject the glue directly. Wait for the sun to irradiate the bonding substrate and glue joints. When the temperature rises, the moisture in the glue seam evaporates, which will foam the uncured, relatively soft glue seam, and sometimes the polyethylene foam cushion rod is wet, which will cause the glue seam to foam. Therefore, do not blindly schedule during construction, wait until the interface is dry, and then inject glue to avoid the above situation.
2. Sun exposure conditions
Silicone sealant can't be exposed to the sun before curing. Especially the sun is exposed immediately after the glue is injected, which will cause honeycomb bubbles to appear inside the glue seam, which will cause the glue seam to swell outside. This is an alcohol type. Silicone sealants are more noticeable when used. This situation is not easy to overcome during hot summer construction. Avoiding sun exposure when injecting glue will overcome some foaming phenomena. The specific method is to inject glue on the west or north side of the building in the morning and inject the glue on the east or south side of the building in the afternoon. After the glue is injected, the glue has been cured for a period of time, and the surface layer has crusted. When the sun is re-sunned, it has a certain resistance and can overcome some foaming phenomena.
3.The surface temperature of the substrate is too high
When the silicone sealant is cured, the temperature of the bonded material cannot exceed 50 ° C. This conclusion has been recognized by most silicone sealant manufacturers. If the glue is injected when the surface temperature of the substrate exceeds 50 ° C, it will cause foaming of the glue seam. When weathering caulking of the metal plate curtain wall, the above situation will be encountered. In the hot summer, when the metal plate is exposed to the sun, especially the surface temperature of the aluminum plate will reach 80 ° C. At this time, the glue injection will easily cause foaming of the glue seams. So you should choose to inject glue in the second half of the afternoon or on cloudy days to improve the above situation.
Fourth, other reasons
Closed-cell high-foam polyethylene foam strips are generally used for the cushion rods in the glue seam. Due to improper compression when placed, the internal airbag ruptures and releases air. When the glue is injected, there will be bubble bulges or Bulging appearance. During my construction, foaming caused by foam strips. When filling the foam strip with closed outer holes in the rubber seam, the foam strip selected is generally 2 to 4 mm larger than the rubber seam. Therefore, the foam strip must be compressed when it is inserted into the rubber seam, and the air in the foam strip will follow. If it is compressed, if the surface of the closed foam strip is in contact with the weatherproof rubber, the surface will slowly leak out. If the foam strip is glued immediately after the foam strip is plugged, the overflowed air will inevitably have not yet The cured weathering glue arches and causes bubbling problems.
The solution to this situation is to cut the foam strip with the cutting side facing inward. The second is to use open-cell foam strips or foamed polyethylene foam strips to overcome the above situation. In another case, weathering rubber generally has more alcohol types. Alcohol weathering rubber will release ethanol when cured, and ethanol reacts with some foam sticks. In addition, strict requirements can also be imposed on the construction process. The first is that the surface of the foam strip must be intact. Do not use hard and sharp objects such as flat shovel to press the foam strip into the glue gap to avoid damage to the surface closed by the foam strip. The second is to put the foam strip into the glue seam in advance if conditions allow, and wait for 1 to 2 hours before applying the glue seal.
Summarizing the various reasons for the foaming of the silicone sealant above, we can know that the procedures and processes of the construction process are the main reasons. This requires that during the construction process, the operator must perform the silicone sealant injection according to relevant specifications, and fully consider the selection of silicone sealant, the injection conditions and environment, and the use of auxiliary materials.