The analysis and countermeasure of glass weathering on building curtain wall.

The glass curtain wall is a kind of architectural exterior wall decoration with a novel shape and strong transparency. It has the obvious characteristics of simplicity and brightness of modern buildings, has become the first choice of commercial and landmark buildings, and is favored by architects and developers. The architectural appearance effect is the reference and basis of curtain wall design, and the shadow box design is an important part of the overall design of building curtain walls. The shadow box adopts the method of glass plus backplate insulation layer in structure, and a closed space is formed between the glass and the back plate. To meet the isobaric design, the open-air isobaric hole is often used. According to the statistics of a large number of curtain wall projects at home and abroad, many curtain wall projects located in the tropics and subtropics have weathering phenomena (also known as fogging and mildew) on the inside of the shadow box glass. The fogging phenomenon of the curtain wall glass has seriously affected the architectural appearance effect. At present, there is no other way to replace the glass, and it cannot be eliminated from the root cause. The excessive quantity of foggy glass has generated huge maintenance costs. If the root cause and solution of mildew cannot be found, the curtain wall parties will bear huge losses.

1. The glass fogging and mildew

Figure 1 is a picture of weathering and mildew of shadow box glass after the completion of a project in Hong Kong and Singapore. According to many years of engineering experience and observation, the structural design of the curtain wall shadow box (see Figure 2) adopts the method of opening air holes in the upper part of the shadow box and opening drainage holes in the lower part to solve the problems of isobaric and air convection. When the single-sheet glass is used in the interlayer closed area, the shadow box glass is prone to fog and dew.

^{Figure I The mouldy phenomenon of shadow box glass}

^{Figure 2 The design of shadow box glass}    

Through the analysis of the causes of water vapor in the shadow box cavity of a large number of projects, it can be seen that the air inside the shadow box is connected with the outdoor atmosphere, and the air is slowly converging in the vent hole and drain hole. When the sun shines, the temperature in the cavity will rise much faster than that outside. Due to the size of the vent and drain hole, the airflow exchange will take a long time. At this time, the cavity is in a high temperature and high humidity state, and the dew point temperature controlled by humidity and temperature will rise sharply. When the temperature exceeds the temperature of the inner surface of the glass, fogging and condensation will occur; When the outdoor temperature drops rapidly, the glass temperature will decrease, and fog and condensation will occur below the dew point temperature in the cavity. According to the thermal conductivity and thermal calculation of the glass, the indoor and outdoor temperature of the single glass is equivalent, so the temperature of the inner surface of the glass depends on the outdoor air temperature and heating rate. The analysis results are consistent with the field observation. In the morning, in the evening, or during the sudden change of the weather, fog is often found on the inside of the shadow box glass. Most of the fog disappears after the outdoor temperature rises and stabilizes.

Statistical analysis and observation were made on the data of mildew phenomenon in different projects. When the temperature difference changes greatly in a short time, the single glass is prone to fog and condensation. With the temperature rising, the water droplets gradually evaporate and disappear; When the shadow box glass is a non-coated single layer, it is very easy to appear white spots that will never fade, commonly known as "mildew" or "mildew: the analysis of the" white matter "bonded on the glass surface shows that these" white matter "residues are attached to the glass surface, which is very firm and difficult to clean, and then they are sent to the laboratory for chemical composition analysis after sampling. Under a scanning electron microscope (SEM), the white residue particles are fine and uniform in texture, which can not be distinguished, but they can be determined to be good crystals. With the magnification of SEM, the composition of the crystal is siliceous material, which is consistent with the composition of the sample glass. It can be seen that the white attachment is the weathering phenomenon inside the single-layer glass under high temperature, high humidity, and a rich water environment. The temperature changes in the morning and evening in the tropics and subtropics, the extreme weather, etc. cause the fluctuation of humidity and temperature, and the cycle, plus the promotion of acid gases such as CO₂ and SO₂ in urban industry and automobile exhaust, further accelerate the weathering effect on the inside of the glass.

2. The mouldy mechanism of glass

According to the literature, building glass is soda-lime glass, its main component is SiO₂, followed by Na₂O and CaO, and a small amount of sodium silicate. Sodium silicate is a soluble silicate that is easily soluble in water. When water is attached to the glass surface, the sodium silicate, sodium oxide, and calcium oxide on the surface are hydrolyzed or hydrated to generate sodium hydroxide, calcium hydroxide, and silicic acid or silicon dioxide. The generated lye combines with CO₂ and SO₂ in the air to generate Na₂CO₃, CaCO₃, and H₂0, which are attached to the glass surface. Sodium carbonate has excellent water absorption and is not easy to volatilize. After absorbing water, it delinquents to form an alkaline liquid that corrodes the glass surface. The generated water accelerates the "moisture return" phenomenon of the glass, further enhances the surface weathering rate and produces a point-to-point weathering phenomenon, similar to the biological mildew phenomenon, Commonly known as "mildew II" of glass, when the ambient temperature increases and the humidity decreases, the concentration of lye on the glass surface increases. These relatively high concentrations of lye accelerate the erosion of the glass surface after long-term contact with the glass, and the residue after drying forms "white spots".

^{Figure 3 The describes the whole process of a chemical reaction and product interaction.} 

^{Figure 4 The evaporation and condensation circulation inside the shadow box}

With the cyclic fluctuation of temperature day and night, the water on the glass surface continuously condenses, hydrates, evaporates, recondenses, rehydrates, and evaporates. This cyclic evaporation and condensation (see Figure 4), accelerating weathering effect, is the main process and mode of glass mildew. In contrast, the mildew of outdoor glass is not prominent. Its frequent exposure to wind and rain can effectively reduce the concentration of surface lye, which is consistent with the outdoor temperature and humidity, and is not prone to condensation.

3. The simulation test of shadow box glass

To find out the cause of the mildew of the shadow box glass, a simulation sample is set on the roof of a building in Singapore to explore the root cause. Figure 5 shows the test box.

^{Figure 5 The test box}

In this test, 9 parameters are set, including 27 different comparison groups in three directions east, west, and north. The test results of these comparison groups are used to determine the glass configuration, pore size, pore location, and curtain wall orientation, as well as the impact and degree of fogging on the shadow box. At the same time, record the indoor and outdoor temperature and humidity, the temperature and humidity of the inner surface of the glass, and provide data for the dew point temperature and conclusion analysis and judgment. The unit system glass curtain wall is used for the test. The test period is one year and the data collection period is one month. Table 1 shows the relevant parameters of the test comparison group.

Table 1 Relevant parameters of the test comparison group

The test results are as follows:

(1) Comparing the two groups of templates S1/S1A and S2/S2A, it can be observed that there is fog on the inside of the shadow box glass in the morning, at night, or on rainy days, and it gradually disappears after sunny days. It can be seen that the dew point temperature in the shadow box chamber is higher than the outdoor temperature in the morning and evening or in the rainy season, and the size and position of the exhaust hole have limited influence on the fogging inside the glass, especially in a short time.

(2) Compared with the three groups of samples, SI, SIA, S2, S2A, S3, and S3A, it is observed that S3/S3A rarely appears fogging, even if it does, it is later than the first two groups. According to the thermal calculation, the heat transfer coefficient and shading coefficient of the coated glass are far less than that of the clear glass, the humidity and temperature change in the shadow box cavity is lower than that of the clear glass, and the dew point temperature in the cavity is 1~2 ℃ lower than that of the clear glass.

(3) According to the observation of S4 and S4A, compared with single glass, there are very few fogging phenomena, and even if there are fogging phenomena, they also occur around the glass and near the frame. The hollow layer plays the role of isolating and insulating the temperature of the inner and outer sheets, that is, the temperature of the inner sheet glass does not rise and fall rapidly with the outer sheet glass, and mainly depends on the temperature in the shadow box cavity, which is much higher than the dew point temperature in the cavity, and fogging is difficult to occur. According to the experimental results, the maximum temperature in the cavity can reach more than 72 ℃, and the minimum temperature is slightly higher than the indoor air conditioning temperature. The frame is greatly affected by the temperature in the cavity, resulting in occasional fogging on the inside of the adjacent glass, but it is not obvious.

(4) According to the observation of S5, it is difficult to completely dissipate the fog phenomenon after it occurs. During glass assembly or installation, the temperature and humidity in the shadow box are uncontrollable. After sealing, the gas does not convection with the outside world. If fogging occurs, it will become normal, and even cannot be eliminated, as shown in Figure 6. There is an obvious water mist at the edge of the glass, which is difficult to evaporate.

^{Figure 6 Test chamber S5 comparison group sample}

(5) By comparing the three groups of templates SI, SIA, S2, S2A, and S5, it can be seen that the exhaust hole plays a vital role in pressure balance, and gas convection inside and outside the cavity, and is also an important channel for the humidity in the cavity to balance. Realize the consistency of indoor and outdoor humidity, and avoid uncontrolled normal glass fogging.

(6) Compared with S3, S3A, S4, and S4A, energy-saving glass such as coated glass and insulating glass has good thermal performance. The coated glass reduces the heat entering the shadow box cavity and slows down the heating rate. The insulating layer of insulating glass can effectively block the direct transmission of indoor and outdoor glass temperature and slow down the cooling rate of the inner glass surface. At the same time, the coated glass has another important feature: the coating layer can isolate the direct contact between water vapor and the body glass, and cut off the weathering corrosion chain. Figure 7 shows the temperature and humidity change curve in the shadow box of the S1 control group for 6 consecutive days.

^{Figure 7 The temperature and humidity change curve in a shadow box of the S1 control group for 6 consecutive days}

According to the temperature and humidity, the dew point temperature in the shadow box cavity of each recording point can be calculated. The result analysis shows that when the temperature is constant, the humidity changes by 5%, and the dew point temperature changes by about i℃; When the humidity is constant, the temperature changes rc, and the dew point temperature changes by about i ° c. Therefore, reducing the dew point temperature in the shadow box cavity or increasing the temperature inside the glass is the root cause of glass fogging.

4. Discussion on the tin surface and vent hole

4.1 Discussion on the tin surface of the glass

Some researchers put forward the view that the tin surface of the glass can prevent glass mildew. This view is mainly studied from the process of float glass forming. Under the protection of N₂ and H₂, the glass liquid flows into the tin bath, floats on the surface of the tin liquid, and spreads and flattens under the action of gravity and surface tension to form a glass with flat upper and lower surfaces. Doping is inevitable in the process. 2. Oxidizing reaction with tin generates S/+, which reacts with Fe₃+in the contact glass liquid to generate S/+, infiltrates into the glass surface, and drives the Na+in the contact surface into the tin liquid, forming a 10~20 Plm permeable layer, which enhances the chemical stability of the lower surface. Because of this phenomenon, when producing coated glass, the glass factory should distinguish the tin surface in advance to find the coated surface.

For this influencing factor, the sample survey method is used for targeted analysis. According to statistical data, tin surface and air surface exist in moldy glass, and moldy glass has a certain batch nature. According to data comparison, tin surface mildew of glass accounts for less than air surface mildew. It can be inferred from this that the quality improvement of the production process of the original sheet reduces the penetration of tin ions, and the tin surface has a certain effect on glass mildew, but the process is not controllable, and its production process control cannot be quantified, and it cannot be a solution to glass mildew temporarily.

4.2 Discussion on shadow box vent

It has become the mainstream construction practice of curtain wall design to set external ventilation holes in the closed cavity of the shadow box of the unit curtain wall. As for the role and significance of the vent, there are many different views, which can be summed up in two points: first, internal and external isobaric; Second, drain hole of condensate. It can be seen that the phenomenon of glass condensation has a long history. The design idea of water drainage is adopted for structural treatment, and it is not investigated that some glass will be moldy due to weathering. According to the ideal gas state equation pV=nRT, At the maximum temperature (72 ℃), assuming that the volume of gas in the cavity remains unchanged and the amount of gas substance remains unchanged, the pressure in the cavity will increase by nearly 0.2 standard atmospheric pressure, about 20 kPa; if the pressure remains unchanged and the amount of gas substance remains unchanged, the volume of gas will expand by 20%, which is consistent with the observation of some glass "belly bulge" phenomenon. Based on the above analysis, setting a vent can solve the pressure balance problem inside and outside the cavity.

Can air vents solve the problem of glass mildew? From the root of glass mildew, the basic environment of glass weathering reaction is a small amount of water and an alternate dry and wet environment. A large amount of water flows away after diluting the lye, which is not easy to condense on the glass; The dry and wet alternate environment is the key to the residue of glass. The condensed salt produced by alkali corrosion of glass is bound to the glass in the dry environment and gradually forms a dense condensed crystal. At the same time, various urban exhaust gases (CO₂, SO₂, etc.) enter the shadow box cavity through the pores, which accelerates the weathering of glass under high temperatures and high humidity. It can be seen that it is not feasible to set vent holes to solve glass mildew.

5. The conclusion

The moldy phenomenon of float glass has been studied for a long time, especially in automobile, food, and other industries. In the past 10 years, while building curtain walls has become a business card of the city, the problems that were not found in the early stage are gradually exposed. After nearly five years of investigation and research, combined with experiments, observation models, and research results of relevant industries, the following conclusions are given for reference.

(1) Glass mildew is the direct reaction of weathering effect. Improper selection of shadow box glass is easy to produce weathering effect, and urban exhaust gas accelerates weathering effect.

(2) In the design of the shadow box, the vent shall be set. The internal and external air convection of the closed chamber is the basis for solving the indoor and outdoor temperature and humidity balance. Otherwise, the non-circulation of moisture in the chamber will inevitably lead to a water mist that is difficult to dissipate. The uneven pressure will easily lead to the unreasonable suction of outdoor water into the chamber, and it is not conducive to waterproofing.

(3) The design of the shadow box should preferably select single-coated glass. The excellent thermal performance of single-coated glass can reduce the rapid accumulation of heat in the cavity, effectively reduce the dew point temperature in the shadow box cavity, and the coating layer can effectively protect the glass from weathering. The insulating layer of insulating glass and PVB and SGP of laminated glass can delay the cooling rate of the inner side of the glass, help to increase the temperature of the inner side of the glass, and reduce the risk of condensation, but cannot avoid the weathering of the inner side of the glass.

(4) Standardize the assembly and installation process, strictly follow the design drawings to carry out relevant work, and avoid the occurrence of missed openings, hole plugging, hole blocking, and other problems.

(5)The orientation of the curtain wall, the size and location of the air vent, and the curtain wall system have certain effects on the glass mildew, but not prominent. According to engineering experience, the diameter of the vent hole should not be less than 10 mm, and should not be less than 2; For frame systems that cannot be equipped with air vents, single-coated glass or insulating glass should be used.

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