Foaming Phenomenon and Reasons 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, and become one of the main means to beautify the urban landscape. Curtain wall industry has also developed rapidly. Now curtain wall forms are also diverse, and materials are emerging in an endless stream, which injects new development blood into the curtain wall industry. Among them, people are more and more concerned about the quality of curtain wall sealant, because sealant is one of the indispensable materials for building curtain wall, which has the functions of leak proof, waterproof, vibration proof, sound insulation and heat insulation. Silicone sealant has become the preferred sealing material for building curtain wall because of its good anti-ultraviolet ability, resilience and excellent adhesion to common curtain wall materials. With the decrease of large area concealed frame glass curtain wall, metal plate curtain wall and stone curtain wall, the proportion of silicone weatherproof sealant has been increasing, gradually exceeding the silicone structural sealant, becoming the largest amount of silicone sealant used in building curtain wall.
I. Reasons for Manufacturers'Varieties
Many brands of silicone sealants have experienced similar blistering phenomenon, which is not a quality problem after careful testing and repeated replenishment confirmation. Silicone sealant has two types: alcohol sealant and ketoxime sealant. The methanol contained in alcohol sealant during curing process releases gas (methanol begins to volatilize at about 50 degrees), especially when exposed to direct sunlight and high temperature. It is very difficult to breathe through the metal window frames of concrete. In addition, the higher temperature and humidity in summer, the curing will be faster. The gas released by the glue can only run out from the completely cured glue layer, and there will be bubbles of different sizes on the cured glue strip.


In the construction process, we should choose different types of silicon tunnel weather-resistant sealant according to different conditions. For example, I have this problem in the curtain wall project of Zhongshan City Traffic Police Brigade completed in 2009. Baiyun's SS611 and SS811 weather-resistant sealants were used in this project, but in summer construction, SS611 sealant foaming phenomenon was found and SS811 did not exist. Later, the reasons were found through manufacturers. Baiyun brand's SS611 and SS811 are two types of weather-resistant sealants under general conditions. Their application ranges are basically the same as the product characteristics. However, SS611 is dealcoholic, SS811/911 is deketone oxime, and SS611 may bulge in the case of low humidity (< 50% RH) or large diurnal temperature difference (> 15 C), but they do not affect the seam.  Sealing performance.
II. Construction Reasons


1. Air is wrapped in the gel.
Silicone sealant is generally used to complete the injection work in the construction site, and the working conditions are poor. Workers stand on scaffolding to inject glue, the uniformity of the glue is not easy to grasp, and the air is easily wrapped in the glue seam. After gluing, the surface of the sealant is scraped and pressed, and the finish is smooth. The pads in the glue seams usually use closed polyethylene foam rods, and the air in the glue seams is not easy to overflow. When the air is compressed, the air is compressed, and the outward tension is generated. The sealant seams are generally wider, and the thickness is relatively small, providing weather resistant glue with high displacement capability. The hardness is low and the adhesive is soft before curing.  The swelling tension of compressed air bubbles up the seam slowly.  This phenomenon is more likely to occur in the application of sealants that can provide displacement capacity of seams above (+25%). On the contrary, some sealants with lower allowable displacement have higher hardness and are thicker before curing. The swelling tension of compressed air wrapped in the seam does not easily prop up the rubber to foam. Therefore, the more weatherproof adhesives with high displacement ability can be provided, the more attention should be paid to the uniformity of the adhesives when they are injected.  Generally, vertical seam glue can be applied from bottom to top, which can overcome some foaming phenomena. More importantly, it is the proficiency and responsibility of the operator.


2. Workers'Injection Habits
In glue injection construction, we often find that when workers glue glue, many people like to use soap water, detergent water, washing powder water and other sprayed glue joints, and then use hand to smooth, to ensure that the glue does not stick to hands, but did not expect the water to be absorbed by the foam strip, that is, the water is dry, as long as there is soap, detergent and laundry on the foam strip. Powder will foam when it is sized in the sun.


3. Reasons for External Construction Conditions
1. Wet Conditions
When working outdoors, we often encounter rainfall, especially in the summer in the south, it rains almost every day. If it rains, the interface will be directly glued before it is wet. When the sun shines on the bonding substrates and seams, the temperature in the interface will rise, and the moisture in the seams will evaporate, which will make the unsolidified and softer seams come up. Bubbles, sometimes wet with polyethylene foam mats, also cause blister. Therefore, construction can not blindly seek progress, until the interface is dry, then gel injection, can avoid the above situation.

2. Solar exposure conditions
Silicone sealant can not be exposed to the sun before it is cured. Especially when it is just injected, it will cause honeycomb bubbles in the seam, which will cause the external bulge of the seam. This situation will be more obvious when the alcohol-type silicone sealant is used. In the hot summer construction, this situation is not easy to overcome. In order to avoid the sun exposure, some foaming phenomena can be overcome by injecting glue in the morning on the West or north side of the building and in the afternoon on the east or south side of the building. After a period of solidification, the surface of the gel has crusted. When the sun is re-exposed, the gel has a certain resistance and can overcome some foaming phenomena.