Durable Stability of Silicone Structural Adhesive and Quality Control Measures of Glass Curtain Wall

Glass curtain wall is widely used because of its beautiful appearance, energy saving, easy maintenance and many other advantages. The glass curtain wall is a structural exterior wall assembled on the basis of a single element of the glass curtain wall sealed by adhesive bonding. The glass unit is elastically connected with the metal frame by silicone structural sealant. The structural sealant plays a structural bonding role and transmits and bears the stress of the main structure. Therefore, the durability and stability of silicone structural sealant are directly related to the safety of the curtain wall. Once the structural adhesive bonding failure, it will lead to the failure of the entire curtain wall system, and even produce breathable water leakage, glass falling off and other accidents, resulting in extremely adverse social impact. With the increasing of glass curtain wall engineering, the quality problems of glass curtain wall engineering caused by structural adhesive are becoming more and more prominent. This paper analyzes the typical quality problems of glass curtain wall from the perspective of silicone structural sealant, and puts forward the corresponding quality control measures of glass curtain wall by expounding the product quality control project requirements set by the new standard of construction industry "silicone structural sealant products for building curtain wall" (hereinafter referred to as "new standard").

1. Quality problems of glass curtain wall project

Maintenance and Service Life of 1.1 Building Curtain Wall

JGJ 102 "glass curtain wall engineering technical specifications" clearly pointed out that the design life of glass curtain wall is generally not less than 25 years, silicone structural sealant as one of its main constituent materials, and plays a very key role, the service life should be at least 25 years. However, at present, the silicone structural sealants sold in the domestic market are tested in accordance with the national standard GB 16776-2005 "Structural Silicone Sealants for Construction" [1] to control product quality. Manufacturers can only provide 10 years of quality assurance based on this standard, which obviously cannot meet the minimum requirements for the design and service life of glass curtain walls. Therefore, as the key material of the glass curtain wall, the structural adhesive has become the "short board", that is, the service life of the structural adhesive determines the service life of the curtain wall. In addition, once the structural adhesive has been used for more than 10 years, the curtain wall needs to be repaired and rebuilt. For more and more high-rise and super high-rise building curtain walls today, the maintenance will be particularly complicated and the cost will be very high. According to statistics of relevant technicians, if you want to replace a piece of glass at an altitude of 100 meters, the cost will be as high as hundreds of thousands. The construction industry has drawn up a new standard "Silicone Structural Sealant for Building Curtain Walls" based on advanced international standards, which was approved in December 2014. The standard clearly requires a minimum service life of 25 years for silicone structural sealant products for building curtain walls, so as to ensure the quality stability and safety of building curtain walls. The minimum service life of 25 years is matched with the requirements of the design service life of glass curtain wall in JGJ 102 "Technical Specification for Glass Curtain Wall Engineering" [2], which solves the maintenance and reconstruction problems caused by the aging of structural adhesive after a period of use of glass curtain wall.

Effect of 1.2 Environmental Factors on Aging of Structural Adhesive

Silicone structural sealant is a polymer material. In the actual application process of curtain wall, it is inevitably affected by the surrounding environmental factors, such as: rain, ultraviolet light, high temperature, high humidity, severe cold, and coastal, saline-alkali salt spray environment and Factors such as acid mist environment caused by environmental pollution will accelerate the aging of the sealant, in the application process, discoloration, blistering, cracking and even embrittlement, cracking, debonding and other phenomena occur, and the service life is reduced, which seriously affects the safety of curtain wall engineering.

Effect of 1.3 complex stress on structural adhesive aging

The shape of the glass curtain wall is becoming more and more novel and unique, and the new design form of high-rise buildings is constantly emerging, which also puts forward higher requirements for the performance of the structural sealant. In the process of applying structural sealant to glass curtain wall, it bears a variety of complex external forces:(1) wind load in horizontal positive and negative directions;(2) Shear force in vertical direction;(3) Tearing force caused by stress concentration due to structural adhesive defects;(4) Mechanical fatigue stress caused by dynamic loads such as cyclic wind load and vibration;(5) Stress of creep of structural adhesive caused by long-term glass self-weight load. These complex external forces make the structural sealant in a dynamic deformation state that cannot be observed by the naked eye for a long time, and accelerate the aging of the structural sealant.

1.4 sealant product quality problems

The domestic sealant market is chaotic and the market competition is gradually intensifying. Some sealant companies use unfair competition methods to pursue huge profits, regardless of social responsibility, and manufacture ultra-low-priced products at the expense of product quality. Even at the expense of high-filling, the use of inferior raw materials and other means to entrap users, reduce product prices, and even fake and inferior products to fake brand names, disrupting the market and seriously endangering the healthy development of the industry. The main performance of the following aspects:

(1) sealant filled with white oil, cracking silicone oil and other inferior plasticizers.

White oil is a colorless, odorless white oily long-chain alkane refined from high-pressure hydrogenation of petroleum lubricating oil fractions. It has a low boiling point and is easy to volatilize. The sealant is filled with white oil, which not only greatly reduces the cost of the sealant, but also greatly improves the appearance and extrusion performance of the sealant. Users cannot find quality problems temporarily during use. With the passage of time, the white oil in the sealant gradually volatilizes, causing the sealant itself to harden, become brittle, and even crack. At the same time, the polarity of white oil and butyl sealant is similar. According to the principle of similar compatibility, when butyl sealant meets white oil, it will swell and dissolve, resulting in oil flow, fog, debonding and other phenomena of hollow glass [3].

Cracking silicone oil is silicone oil extracted from cured silicone sealant by high temperature cracking. It has high volatile content, complex chemical composition and various polar substances. If it is added to silicone structural sealant, it will not only have poor compatibility with the main silicone rubber and be volatile, making the silicone structural sealant hard and brittle, but also have adverse reactions with butyl or profiles, resulting in debonding failure of sealant and serious safety risks.

(2) The sealant itself has poor aging resistance. Some manufacturers of the initial performance of the sealant can meet the national (industry) standard requirements, after a period of wind, rain, sun aging, the performance of the sealant has a substantial attenuation, sealant will harden, brittle, in the construction project will appear after the use of serious cracking, water leakage problems.

(3) fake brand name, shoddy. In order to pursue high profits, some manufacturers and project contractors use inferior sealants to counterfeit brand-name products and shoddy products in the middle of the production project, causing serious quality hazards to the curtain wall project. Some enterprises rely on low-price competition to take risks. The final result is a serious decline in the quality of construction projects, making the glass curtain wall a "time bomb" over the city a reality, leaving serious hidden dangers to people's personal safety.

2. Quality control measures

In view of the above problems, it is an important measure to adopt scientific and strict product standard requirements. It is necessary to scientifically set up corresponding testing items, especially the test methods for the aging performance of silicone structural sealants, further improve my country's silicone structural adhesive standards, implement mandatory policies to improve the industry access system, and improve the durability of silicone structural sealants. Stability to ensure that it has a long service life.

The relevant silicone structural adhesive standard abroad is the earliest American ASTM C1184 "Silicone Structural Sealant". China has formulated the GB16776-1997 "Silicone Structural Sealant for Construction" with reference to ASTM standards, and then partially revised it and promulgated the 2005 edition. ETAG 002 "European Technical Certification Guidelines for Structural Sealant Assembly Systems (SSGS)" was promulgated in Europe in 1999 and revised in 2012 [4]. In addition, there are EN15434 in Europe: 2006(2010) "Standard for Architectural Glass-Structural or Anti-ultraviolet Sealant Products (for Structural Sealing Assembly or Exposed Sealing Insulating Glass Units)" [5], in which ETAG002 and EN15434 have basically the same requirements for silicone structural sealant. Corresponding testing items are proposed for environmental, complex stress, quality control and other influencing factors, and strict requirements are put forward for the durability and stability of silicone structural sealant, greatly improve the level of product quality. The new standard "silicone structural sealant for building curtain wall" draws on the ETAG002-2012 of international advanced standards, clearly points out the requirements of the minimum service life of silicone structural sealant products for building curtain wall, and puts forward more scientific and strict requirements, which can provide control measures for the quality problems of building curtain wall.

2.1 environmental factors on sealant accelerated aging

The new standard sets up testing items such as water-ultraviolet light test, salt spray environmental test, acid fog environmental test, curtain wall cleaner test, etc., and comprehensively considers the influence of various environmental factors on the aging of structural sealant.

Water-ultraviolet light test is combined with the effects of ultraviolet irradiation and exposure to water to test the aging resistance of sealant. The time of water-ultraviolet irradiation test is 1008h, which is much higher than the national standard (300h), and puts forward higher requirements for water quality and ultraviolet light intensity. The salt spray environment test and acid fog environment test respectively use artificial simulation of salt spray environment and acid fog environment conditions to assess the aging resistance of the sealant.

2.2 mechanical factors on the accelerated aging of sealants

In addition to the tensile performance requirements specified in the national standard, the new standard adds shear, tear, mechanical fatigue, and creep testing items, and comprehensively considers the various complex external forces received by the structural sealant during actual use.

Consistency identification of 2.3 sealant products

The new standard adds control items such as thermogravimetric analysis, infrared spectrum analysis, density and hardness to ensure the stability and uniqueness of the product. Density is related to the amount and ratio of sealant, as well as the consistency of the product. Too large or too small hardness will also have some adverse effects on the use of sealant, and the hardness can also reflect whether the product is consistent. Thermogravimetric analysis can quantitatively analyze the content of low boiling point substances in the sealant, and infrared spectroscopy can qualitatively determine the molecular structure of the substance and determine the composition of the substance. The use of thermogravimetric analysis and infrared spectroscopy can ensure product consistency, eliminate the application of counterfeit and inferior products, and effectively determine whether the silicone sealant is mixed with inferior plasticizers (such as white oil, etc.).

Determination of mechanical properties of sealant after aging by 2.4

The new standard uses the retention rate of mechanical properties to measure the performance of the sealant. Tensile cohesiveness under different temperature conditions, tensile cohesiveness after salt spray environment, tensile cohesiveness after acid fog environment, tensile cohesiveness after water and ultraviolet light, tear performance and fatigue performance are all compared with the tensile strength after treatment and the tensile strength under initial conditions, and the ratio (I. e. performance retention rate) ≥ 0.75 is required. This requirement is designed to control the durability of structural adhesive products, that is, structural adhesives are required to withstand various aging factors during long-term use, and still be able to maintain good performance and have a long service life.

For products that meet the requirements of the new standard and ordinary products sold in the domestic market, long-term atmospheric exposure tests and regular sampling tracking tests are carried out to compare and analyze their durability. The results of the test followed continuously for 30 months are shown in Figure 1.

In Figure 1, 1# is a product that meets the new standard, and 2# to 5# are products that only meet the national standard. After 30 months of continuous atmospheric exposure, the tensile bond strength of the products that met the new standard remained basically unchanged, and the elongation at break increased slightly at the initial stage of aging, and stabilized after 3 months; while the tensile bond strength of the other products showed different degrees of attenuation, and the elongation at break decreased. It can be seen that the silicone structural sealant meeting the new standard has good mechanical properties retention ability, excellent durability, and can better ensure the safety and service life of the curtain wall.

3. Conclusion

Structural adhesive plays an important role in glass curtain wall engineering, and its durability and stability are directly related to the safety of glass curtain wall. We should learn from international advanced standards, improve China's silicone structural adhesive standards, implement compulsory policies to improve the industry access system, so as to ensure that silicone structural sealant has a long life in use. At present, the new standard of the construction industry, "Silicone Structural Sealant for Building Curtain Wall", is drawn up on the basis of 25 years of quality assurance for silicone structural sealant, which is consistent with the design service life of the curtain wall structure. The detection method is comprehensive. The set items and judgment methods are more scientific, reasonable and more in line with the actual application environment of silicone structural sealant, and can strictly control the durability and stability of silicone structural sealant, to ensure the quality stability and safety of building curtain wall engineering.

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