Silicone and polysulfide type sealant bonding assembly insulating glass has a significant difference in service life

1. Introduction

Architectural insulating glass is a unit component assembled by sealing and bonding the periphery of the partition glass. It is bonded by a sealant to form two internal and external sealing layers to block moisture. The infiltrated moisture is absorbed by the desiccant in the partition frame, and the gas in the cavity continues Drying is the basis of thermal insulation. See Figure 1 for bonding structure and penetration path.

^{Figure 1 The insulating glass bonding structure and penetration path}

The desiccant absorbs the water vapor that penetrates through the sealing layer, and its quality continues to increase until the moisture absorption is saturated, which is characterized by condensation and water condensation on the hollow glass, that is, the end of the functional life of the product. According to the size of the moisture-permeable channel, the water permeability of the sealant, and the moisture absorption capacity of the desiccant, the theoretical calculation of the insulating glass life should be more than 30 years. The factors are improper or incorrect installation of product design, material selection, and operation technology. The edge is sealed and bonded under the action of environmental water, water vapor, ultraviolet radiation and chemical media erosion, wind load, atmospheric pressure, and air pressure difference caused by temperature changes. The body is repeatedly loaded and cyclically deformed, which accelerates the degradation of the sealed bonded body, and the deterioration of the bonding and sealing performance causes early failure.

Tests have shown that accelerated aging tests under conditions such as ultraviolet radiation, high and low temperatures, or high humidity can evaluate the structure and sealant degradation trends, but an effective method for evaluating durability life should be characterized by the change in water absorption quality of molecular sieves under the conditions of use. With its saturated moisture absorption period, this test method has been adopted by the International Organization for Standardization. Studies have shown that the temperature-humidity cyclical change will promote the penetration of moisture, especially under the conditions of continuous high temperature and high humidity, the gas in the cavity expands, and the hardened high modulus edge bond produces ultra-high stress (such as the hollow glass cavity at a temperature of 58 ℃). The internal air pressure can reach 13 kpa), which accelerates the penetration of moisture. Based on many years of field experience and research results, the European standard EN 1279 "Glass for Building-Insulating Glass" adopts the "Test Method for Moisture Permeation Durability of Insulating Glass", which uses the saturated water absorption rate of the desiccant, the initial moisture content and the final moisture content after the test. Calculate the moisture permeability index of insulating glass to characterize the sealing durability of the product. This method has good scientificity and applicability. The European glass bonding assembly structure certification guide confirms that insulating glass that meets the EN 1279 standard and obtains product quality certification will have a 25-year service life.

According to European standards, it is required that after 28 days of high and low-temperature cycle test, the test is continued for 49 days under 58℃ high temperature and high humidity conditions, and the moisture permeability index of insulating glass is not more than 0.20. Taking the accelerated durability verification test of 40 sets of samples by the China National Glass Quality Supervision and Inspection Center as an example shows the effectiveness and applicability of this method. This article focuses on the analysis of the main factors that affect the test results to find out the influence of the type of insulating glass outer sealant on the service life of the insulating glass.

2. Accelerated durability test　　

2.1 Specimen　　 

There are 15 samples in each group, prepared according to the same process conditions and materials, the specification is 510 mm*360 mm, and the dew point of the sample should be ≤-40℃.

2.2. Sample equipment　　

The text box should be able to provide two-stage test conditions:

2.2.1 High and low-temperature cycle stage: the temperature is cyclically raised and lowered from -18°C to 53°C, a temperature cycle every 12h, the heating or cooling rate is 14°C/h, and the cycle continues for 58 times (28 days);

2.2.2 High-temperature test stage: keep the temperature at 58℃ for 7 weeks (49 days)　　 

When the test temperature is higher than 23℃ in the two stages, the relative humidity in the box should be ≥95%.

2.3. Test procedure　　 

Take 4 specimens, and determine the initial moisture content (Ti,%) of the desiccant after decomposition　　

Take 5 specimens and decompose at the end of the 77-day test treatment, and determine the final moisture content of the desiccant (Tf,%);

Take 2 test pieces, take a sample of the desiccant after decomposing, absorb water to constant weight after saturation, then subject to high-temperature treatment at 950℃ or 350℃ for 120min, and measure the standard water absorption rate (Tc,%), that is, the saturated water absorption capacity of the desiccant...

Accelerated durability is expressed by the water penetration index (I), calculated as follows, take the average of the I values of 5 specimens, and round to two decimal places:

^{Formula 1 Water vapor permeability formula of insulating glass}

3. Test results and analysis　　 

The verification test selects 40 sets of samples from various types of enterprises from all over the world, of which the two sealed outer layer sealants are 20 sets of polysulfide type and 19 sets of silicone type. Only one group is sealed with hot-melt sealant.

3.1. Test results　　

The test measured the bonding width of the inner and outer seals of each group of test pieces, and the test determined the moisture permeability index of each group of test pieces. The test results are listed in Table 1.

3.2. Statistical analysis　　

3.2.1. The overall level of accelerated durability of 40 groups of samples

^{Figure 2 Distribution of Water Permeability Index and Probability of Qualification}

The samples come from 40 insulating glass companies, which are representative of the technical level of domestic products. Knowing the type of sealant for each group of test pieces, the bonding width of the outer layer of the test piece and the bonding width of the butyl rubber was measured (Figure 1). The thickness of butyl rubber is difficult to measure, the sealant grade is unknown, and the manufacturing process level is also different. These factors will also affect the test results. To evaluate the accelerated durability performance of the samples, this analysis eliminated three groups (22#, 27#, 38#) of defective samples, and took I as a random variable, and calculated the water permeability index of the remaining 37 groups of samples according to a normal distribution. It shows that the overall mean value is 0.1679, the variance is 0.1044, and the maximum value is 0.55. According to the confidence level of 0.05 (confidence level 95%), the probability of the overall I value ≤ 0.20 is 62% (the area of the shaded part in Figure 2).　　

3.2.2. The influence of the type of external rubber on the value of IThe outer layers of 37 groups of samples were made of polysulfide and silicone-type sealants. Among them, 19 groups of silicone-type samples had an average I value of 0.2063, a variance of 0.1116, a maximum value of 0.55, and a reliability level of 0.05. The test I value was ≤0.20 The probability of I value is 48% (Figure 3); in 18 groups of polysulfide samples, the average value of I is 0.1165, the variance is 0.0789, the maximum value is 0.29, and the probability of I value ≤ 0.20 is 86% (Figure 4). . Comparing Figure 3 to Figure 4, it can be seen that the water permeability index of the two types of samples is significantly different, and the I value estimated at the 95% confidence level is nearly doubled.　　

3.2.3. The influence of bonding width on I value　　 

Increasing the bonding width improves the bearing capacity of the edge bonding and increasing the length of the moisture-permeable path (Figure 1), which is beneficial to reduce the moisture permeability index of the insulating glass. The test result of the bonding size of the test sample is basically within the range specified by the standard. According to the test results, the correlation between the bonding width of the inner and outer sealant and the I value is calculated. The analysis results are as follows.　　

1) The relationship between the bonding width of the outer sealant and the I value 　　 

19 groups of polysulfide type samples, the outer seal bonding width (seal thickness) average 7.11 mm, variance 0.65mm; 18 groups of silicone type samples, average bonding width 6.95 mm, variance 0.83mm. It can be seen that the bonding thickness is comparable. Taking the sample bond thickness as the independent variable to plot the I value, the linear correlation coefficients are 0.3949 and 0.0186, respectively, indicating that the bond thickness is weakly correlated with the I value under the test conditions (Figure 3 and Figure 4).

^{Figure 3 and Figure 4 The relationship between the bonding width of the outer sealant and the I value}

2) The influence of inner sealant bonding width　　 

19 groups of polysulfide type samples, the average bonding width (seal thickness) of butyl rubber was 3.82 mm, and the variance was 0.85 mm; for the 18 groups of silicone type samples, the average bonding width of butyl rubber was 3.50 mm and the variance was 0.84 mm. It can be seen that the bonding size of butyl rubber is at the same level. Analyzing the correlation between the I value of the sample and the bonding thickness of the butyl rubber shows that the I value under the test conditions has a decreasing trend with the increase of the bonding thickness of the butyl rubber. The correlation coefficients are 0.6540 and 0.1141, respectively, which are medium and weak correlations (Figure 5 and Figure 6).

^{Figure 5 and Figure 6 The influence of inner sealant bonding width}

4. Discussion　　

4.1. Silicone rubber molecules have a helical structure with large free space and good gas and water vapor transmission rates. This feature is widely used in gas separation and water vapor separation. Silicone sealant is a condensation type room temperature vulcanized silicone rubber based on hydroxy-terminated polydimethylsiloxane (such as HO(Me2SiO)NH). The main chain is polydimethylsiloxane, and the crosslinking density is low. The vapor transmission rate is higher than that of basic sealants such as polysulfide and polyurethane. The water vapor transmission rate of silicone type sealant is about 20g/m2.d, and polysulfide type sealant is less than 10g/m2.d. The water permeability index of insulating glass is related to the type of external sealant. Under the same conditions, the silicone type is nearly twice as high as the polysulfide type. This test result is closely related to the difference in the moisture permeability of the base polymer.

4.2. The accelerated durability test method based on the change in the water absorption quality of the desiccant is suitable for the durability evaluation and product quality control of insulating glass. The German DIN 1286-1 standard uses this method to classify insulating glass products. Using this test principle to study the performance of air-exposed insulating glass, the results show that the water permeability varies significantly with the type of external rubber under the same conditions (Figure 7). The polysulfide type sample has been exposed for 3 years, and the mass of water penetration is 0.35g. , 5 years is 0.55g, while the silicone-type exposure is 0.80g for 3 years, and  the water penetration is twice the difference. The results show that if the polysulfide type outer sealing insulating glass has a service life of 25 years, the silicone type product has a service life of only 10 years. The significant difference is consistent with the aforementioned test results.

^{Figure 7 Moisture penetration of double-channel sealed insulating glass after atmospheric exposure test}

5. Conclusion　　

1) Accelerated durability test is an effective method to evaluate the sealing quality of insulating glass, and it provides important information for evaluating the service life;

2) Within the standard range, the bonding width has no significant effect on the water permeability of insulating glass;

3) The type of external sealant is a significant factor affecting the permeability of insulating glass. The water permeability index of insulating glass bonded by silicone type sealant is much higher than that of polysulfide type, and the difference in durability is nearly doubled.

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