Silicone Sealant has been widely used in the field of building waterproof sealing because of its excellent resistance to high and low temperature, excellent weather aging resistance and good bonding properties. However, when it is applied to the building curtain wall, the color change of silicone sealant is often encountered. The application technology engineers of Silicon Treasure Technology conducted an in-depth analysis of the color changes of silicone sealant in two practical projects, and summarized the reasons leading to the discoloring of silicone sealant. 1. Introduction As we all know, the design life of the building is generally more than 50 years, and the materials required to be used must also have a long service life. Because silicone sealant has excellent weather aging resistance and high and low temperature resistance, extensive adhesion, good physical and mechanical properties, it has been used for decades in the adhesive sealing of various curtain walls and doors and Windows, and has made outstanding contributions to the reduction of energy consumption and sealing and waterproof of new buildings. Silicone sealant in the process of use, often appear bond failure, cracking, discoloration and other problems. Bonding failure and silicone sealant cracking will lead to the failure of the waterproof seal of the building, which has been paid attention to by everyone, so before the project starts, the construction side will carry out the adhesion test on the corresponding bonding substrate, and the construction process is becoming more and more standardized. However, the discoloration problem of silicone sealant is still a more difficult thing at present, including silicone sealant and rubber strip incompatible discoloration, silicone sealant and foreign body contact discoloration and environmental factors caused by silicone sealant discoloration. 2. Silicone sealant is incompatible with adhesive strips In the building curtain wall industry, when the adhesive sealing material or the adhesive sealing material and the substrate are in contact with each other, the performance that does not produce harmful physical and chemical reactions with each other is defined as compatible. At present, the most used rubber strip in construction is EPDM (EPDM) rubber strip, but many years of actual cases show that most EPDM rubber strips are incompatible with silicone sealant. The incompatibility phenomenon is specifically manifested as the discoloration of light-colored silicone weather-resistant sealant, or even the failure of silicone sealant to bond with the substrate, which seriously damages the beauty of the curtain wall and the safety of waterproof sealing. This discoloration phenomenon cannot be simply understood as a quality problem with silicone sealant or strips, but is caused by the incompatibility of the two materials. Case 1: The main structure of a building is a reinforced concrete frame structure, and the outer protection structure adopts a semi-hidden frame hollow glass curtain wall. The main height is 101.3m and the area is about 10000m2. Grey silicone sealant is used for vertical joints and black silicone sealant for horizontal joints. The building began the construction of the external wall silicone sealant in March 2014, and in June 2014, the surface of the gray silicone sealant appeared yellow, while the surface color of the black silicone sealant did not change significantly (see Figure 1). Figure 1 Discoloration of silicone sealant
After the on-site rubber cutting inspection, Silbao technology application technical engineers found that the silicone sealant buried under the EPDM adhesive strip, the initial determination of the local yellow color of gray silicone sealant is caused by the incompatibility of sealant and EPDM adhesive strip, gray rubber color is light, the sensory change is obvious, while black rubber color is deep, the sensory is not obvious. Then the EPDM tape sample was taken from the field and brought back to the inspection center (affiliated to the "National Enterprise Technology Center" of Sibao Technology) for compatibility test and verification. The experimental results are as follows: Figure 2 Compatibility test between silicone sealant and EPDM strip: (a) blank comparison; (b) EPDM tape test
As can be seen from Figure 2, after 48 hours of accelerated aging by the UV box, the surface of the silicone sealant in contact with the EPDM rubber strip shows obvious color changes, further indicating that the EPDM rubber strip used in the project is incompatible with the silicone sealant. In practical engineering applications, direct contact between the EPDM strip and silicone sealant must be avoided. In order to solve the problem of discoloration of silicone sealant in engineering, silicone application technical engineers try to repair it in the following ways in the application experiment: cleaning the silicone sealant surface and removing the discoloration layer of silicone sealant. 2.1 Silicone sealant surface cleaning scheme FIG. 3 Cleaning effects of different solvents
The surface of the discolored silicone sealant retrieved from the site was cleaned with different solvents, and the results were shown in Figure 3. As can be seen from Figure 3, most solvents have no obvious effect, only the use of anhydrous ethanol and 70% ethanol scrubbing surface can make the surface color of the sealant lighter, but can not completely eliminate yellowing (see Table 1). In the cleaning must be wiped several times to have an effect, this operation is very difficult to perform on the project. It is indicated that it is not feasible to treat the discolored silicone weather-resistant adhesive by cleaning method. Table 1 Cleaning effects of different cleaning solvents
| Cleaning solvent | Soapy water | xylene | Mineral oil | Ethyl acetate | Anhydrous ethanol | 70% ethanol |
| effect | no | no | no | no | Not obvious | Not obvious |
2.2 Cut off the discolored layer silicone sealant solution Silicon treasure application technology engineers will be taken from the site has been discolored silicone sealant from the middle of the section, found that the section still maintains the color of silicone sealant. In order to verify the actual effect of the method of cutting off the discolored layer of silicone sealant, UV box was used to accelerate aging treatment of the non-discolored sealant layer, and the treatment results were shown in FIG. 4. FIG. 4 Comparison of ultraviolet accelerated aging of the discolored layer of silicone sealant: (a) the newly cut silicone sealant; (b) Ultraviolet radiation for 2 days
As can be seen from Figure 4, the colloid profile irradiated by ultraviolet light can observe obvious yellowing phenomenon 48h. It indicates that some small molecule substances such as plasticizers and release agents in EPDM have completely penetrated into the interior of silicone sealant, and the colloid color turns yellow after ultraviolet irradiation. Therefore, even if the yellowing of the silicone sealant surface at the site is cut off, after a period of sunlight exposure, the new colloidal surface will still yellowing. The above two validation tests show that for discolored silicone sealant, cleaning or cutting off the discolored layer of silicone sealant cannot fundamentally solve the discoloration problem. The glass curtain wall project sealant has been completed, if you want to fundamentally solve the problem of discology of silicone sealant, you can only cut off all silicone sealant, choose a compatible rubber strip to replace the original EPDM rubber strip, and then glue. Such late remediation measures will greatly extend the construction period, and cause waste of materials and labor, and the cost is more than three times that of new projects. 3. Silicone sealant is in contact with foreign bodies In addition to the reasons for discoloration caused by the incompatibility of silicone sealant and rubber strip, some accidental factors can also cause the color change of silicone sealant. Case 2: The main structure of a building is a reinforced concrete frame structure, the outer protective structure is a point-supported insulating glass curtain wall, and the Acid Silicone Sealant is used to seal between the panel elements of the curtain wall. The construction time of silicone sealant was July 2013, and the yellowing phenomenon of silicone sealant appeared in September 2013 (Figure 5). FIG. 5 Yellowing of silicone sealant (a) Panorama; (b) Local map
Sibao application technical engineers to the site of the project field investigation, to observe the discoloration of the site, found that all the cross joints have obvious yellow phenomenon, and spread to the four sides; In the non-cross seam position, several points appear yellow, and all are near the corner position (Figure 6). Figure 6 The silicone sealant appears yellow at the fixed corners
In the design application of the project, the silicone sealant used for the joint will only be in contact with the insulating glass, there is no necessary attachment to contact, and there is no need to perform compatibility testing. Through careful observation of the site, the Silbo application technical engineers found that when installing the point support glass plate on the project, the rubber pad would be stuck in the cross joints and corners for positioning. After the cutting inspection of the discolored parts, the residue of the rubber pad was found under the silicone sealant (Figure 7), indicating that the rubber pad was not cleaned (some of the rubber pad was not even removed) before the rubber construction, and the residue of the rubber pad was in direct contact with the silicone sealant, resulting in pollution and discoloration. FIG. 7 Failure to remove clean rubber pads causes discoloration of silicone sealant
According to the situation of the curtain wall project, it is necessary to thoroughly remove the yellow transparent glue seam and the rubber pad residue that may be in contact with the silicone, and then re-glue the seal. This case illustrates that in actual construction, possible contacts that have not been tested for compatibility with silicone sealant must be cleaned prior to gluing to prevent accidental discoloration of silicone sealant. 4. Solutions 4.1 Silicone sealant and rubber strip incompatible solution The best solution to prevent incompatibility between silicone sealant and adhesive strip on the project is to test the compatibility between silicone sealant and contact material in accordance with Appendix A of GB 16776-2005 "Silicone Structural Sealant for Construction" before the construction of silicone sealant on the project to avoid incompatibility in the actual construction of the curtain wall. In GB 16776-2005, "Silicone structural sealant for construction", it is pointed out that the change of color in the compatibility test is a key indicator that can be used to determine the compatibility of materials, and practice shows that those accessories that will change color in the test will also occur in the actual project. Sibao Technology enterprise technology center covers an area of 3000m2 (the most in the industry), is the only national enterprise technology center in the industry, but also the first national laboratory approved laboratory in the industry, with more than 60 sets of advanced scientific research and testing equipment at home and abroad, under the seven testing laboratories: Aging resistance, environmental simulation, compatibility, mechanics, constant temperature and humidity, infrared electrical properties, appearance and construction performance. According to the project related materials received by the laboratory, the accessories that will come into contact with the silicone sealant will be tested for compatibility before applying, so as to eliminate the problem of incompatibility between the silicone sealant and the adhesive strip. As mentioned above, most EPDM strips are incompatible with silicone sealant, and compatibility testing is required in advance for use on projects. To solve this problem, Sibao National Enterprise Technology Center developed silicone rubber sealing strip (Figure 8) to replace EPDM rubber strip. Compared with EPDM strips, silicone rubber seals have the following advantages: color diversity, service life of more than 50 years, better elasticity, high rebound after thermal aging. Silicone rubber sealing strip and silicone sealant belong to the silicone series of polymer products, the two have very good compatibility, will not occur EPDM caused by the problem of silicone sealant discolcolation, and do not need to carry out compatibility test in advance; And the product has been successfully used in Nantong Country Garden, Chongqing International Metropolis, Luoyang Central Square, Hexin Center and other projects. Figure 8 Silicone rubber seal
4.2 Silicone sealant and foreign body contact incompatible solution The guarantee of the late performance of silicone sealant is closely related to the field construction process, and the non-standard process flow is easy to lead to various problems in the practical application of sealant. Sibao Technology has always put the concept of "responsible for customers" in the first place, and is committed to providing customers with effective overall solutions. Technical staff on-site guidance, technical exchanges, sizing training, and the necessary on-site sampling can cooperate with customers to find problems in construction. For the specific construction process of silicone sealant, please refer to the Application Technical Manual of Silicone Sealant for Construction.
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