Research on the problem of shedding of the insulating glass outer sheet of the opening fan of the hidden frame of the glass curtain wall.

1. The Project Overview

A glass curtain wall is located in a glass building in Los Angeles, USA. The building is an office building and it has been 7 years since its completion. The fixed glass of the curtain wall is installed in the open frame, and the opening fan glass is installed in the hidden frame (see Figure 1). The highest elevation of the glass curtain wall is 89m. There are about 700 opening fans in total, and the glass is made of insulating glass. A glass falling incident occurred on the glass curtain wall of this project. The falling glass was the outer piece of insulating glass with a hidden frame opening fan. The outer piece of glass did not shatter before falling and fell off completely. The inner piece of insulating glass was intact, and there was a second seal of insulating glass on the inner piece glue.

^{Figure 1 The schematic diagram of hidden frame opening fan}

^{Figure 2 The schematic diagram of insulating glass sealing structure}

In order to ensure the quality of the insulating glass, the sealing structure of the insulating glass generally adopts a double-channel sealing structure (see Figure 2). LIJIANG Glass, relying on years of experience in automatic sealing coating application of insulating glass sealant, pointed out in the research on insulating glass sealant that a sealant plays a pre-positioning role, and can isolate water vapor, prevent air and inert gas from entering and leaving the insulating glass cavity; The function of the road sealant is to act as an adhesive to bond the glass and the spacer into a insulating glass unit as a whole.

It is preliminarily judged that this project is due to the failure of the secondary sealant of the insulating glass, which causes the outer sheet to fall off. Further inspection and research on the secondary sealant of the insulating glass is required to provide a technical basis for the next step.

2. The formulate a research plan

Generally speaking, secondary sealants can be made of different materials, among which there are three main types: polysulfide sealant, polyurethane sealant and silicone sealant. After analyzing the degumming and falling accident of the insulating glass in the hidden frame curtain wall, LIJIANG Glass believes that the accident is mostly related to the selection of structural bonding materials, bonding size design and construction defects.

This project will carry out research and analysis on the secondary sealant from two aspects of quality inspection and type identification. Quality inspection includes glue injection size, glue injection situation, appearance quality, bonding quality with glass, Shore hardness, elongation at maximum tensile strength; type identification includes infrared spectrum test, elemental analysis, and combustion test.

The inspection and research of the secondary sealant shall be sampled at a ratio of 1% to not less than 5 of the total number of opening fans. The total number of opening fans in this project is 700 pieces. Therefore, 7 pieces of opening fan insulating glass are randomly selected on site according to the principle of uniform distribution. And after disassembling it, check and study the secondary sealant.

3. The inspection results and analysis

3.1 Injection size, injection condition, appearance quality inspection

The 7 pieces of opening fan insulating glass are all 5mm+9A+5mm thick insulating glass. The secondary sealant of the inspected 1#~7# insulating glass is full of glue injection, without cracking or bubbling, but all of them have discoloration and whitish phenomenon (see Figure 3). The discoloration of the sealant indicates that the colloid has undergone physical and chemical reactions, and there may be problems such as decreased adhesion and hardening of the colloid.

^{Figure 3 The discoloration of the secondary sealant}

The width of the secondary sealant for 1#~7# insulating glass is 3.50~4.56mm (see Table 1), all of which are less than 5mm, which does not meet the minimum requirements for the width of the secondary sealant in the literature: the width of the secondary sealant for insulating glass should be N5 mm. The secondary sealant of insulating glass acts as a structural force, bears wind load and gravity load, and its width does not meet the minimum requirements, indicating that there is a problem in the design of the bonding size of the secondary sealant. The insulating glass is a substandard product with insufficient bearing capacity. and other security risks.

Table 1 Measurement results of secondary sealant size

3.2 Inspection results of bonding quality with glass

In order to ensure that the secondary sealant is firmly bonded to the glass substrate, the current national standard stipulates that the failure mode of the secondary sealant when it is damaged under tension should be cohesive failure, that is, the bonding strength between the structural adhesive and the glass substrate should be greater than The bond strength of the colloid itself, if bond failure occurs, the bond failure area should be S10%.

In order to test the bonding quality of the secondary sealant, 7 pieces of insulating glass were decomposed first, and then the peeling test between the secondary sealant and the inner and outer glass was carried out respectively. The results of the peeling test show that the failure mode of the secondary sealant of the 7 pieces of glass and the inner and outer sheets of glass is bonding failure (see Figure 4), and the peeling bond failure area is 100%, indicating that the secondary sealant and the The bonding strength of the glass is much smaller than that of the colloid itself. The secondary sealant fails to bond with the inner and outer glass, and there is no effective bonding connection between the inner and outer glass, and the outer glass may fall off at any time.

^{(a) Bonding failure with the outer glass}

^{(b) Bond failure with inner glass}

^{Figure 4 The bond failure between the secondary sealant and the glass}

3.3 Shore hardness test

LIJIANG Glass and others mentioned in the review of structural adhesive testing and evaluation that Shore hardness is used to characterize the softness and hardness of materials, and refers to the ability of materials to resist local deformation such as foreign object intrusion and plastic deformation. As the sealant is used for a long time, its hardness will also change. An important manifestation of the aging failure of the secondary sealant is that the Shore hardness exceeds the range (30~60) stipulated by the current international industry standard.

The Shore hardness test was carried out on the secondary sealant of 7 pieces of insulating glass. The results showed that the Shore hardness was 61~65, and the average value was 63 (see Table 2), all of which were greater than 60, which exceeded the allowable range.
The secondary sealant is aging and hardening, reflecting that its performance has changed, and there is a problem with the bonding and assembly quality of the insulating glass.

Table 2 Shore hardness test results

3.4 Elongation test at maximum tensile strength

In addition to good bonding performance and a certain hardness, the secondary sealant should also have excellent elasticity, and when it is deformed due to dynamic load, it can restore its original shape and performance.

The elongation test at the maximum tensile strength was carried out on the secondary sealant of 7 pieces of insulating glass, and the result was 40%~47%, the average value was 44% (see Table 3), all less than 50%, lower than the current national standard Requirements: Elongation N50% at maximum tensile strength. The elasticity of the secondary sealant decreases, and the deformation ability becomes poor.

Table 3 Elongation test results at maximum tensile strength

3.5 Infrared spectrum test

The infrared spectrum test was carried out on the secondary sealants of 7 insulating glasses. The infrared spectrograms showed that the spectral characteristics of the 7 sealants were consistent. The stretching vibration peak of the C-S bond appeared at the wavelength of 749~743 cm", and the stretching vibration peak of the C-S bond appeared at the wavelength of 1283~1279 cm. The methylene-CH2-vibration peak connected to S appears at J, which is in line with the characteristics of the infrared spectrum of polysulfide rubber. The 7 sealants are all polysulfide rubber. Among them, 1#, 4#, and 7# insulating glass are sealed in two layers The infrared spectrum of the glue is shown in Figure 5.

^{Figure 5 Infrared spectrum of insulating glass secondary sealant}

3.6 Elemental analysis

The elemental composition of some elements in the periodic table after 12 in the sealant was analyzed, and the results are shown in Table 4. It can be seen that the element composition and content of the 7 sealants are basically the same, indicating that they are the same type of sealant. The sealant contains S element, but the Si component cannot be detected. It can be judged that the rubber sample is not a silicone sealant and should be polysulfide rubber, which is consistent with the infrared spectrum test results.

Table 4 Analysis results of main elements / %

3.7 Combustion test

A combustion test was carried out on the sealant, and all 7 sealants had a strong flame when burned, produced black smoke, and produced irritating gas, and the burning ash was in the form of a relatively hard block. Using the known silicone sealant as a comparison, the flame produced by the silicone sealant is weaker, no irritating gas is produced, the smoke is white, the burning ash is white, and it is easy to pulverize. The combustion state and ashes state of the rubber sample are inconsistent with those of the silicone rubber, and the comparison is shown in Figure 6.

^{Figure 6 The comparison of combustion state and ashes state}

3.8 Result Analysis

After inspection, the secondary sealant of the hollow glass of the hidden frame opening fan has changed color, the colloid has hardened and lost its elasticity, the secondary sealant has failed to bond with the inner and outer glass, and the performance of the sealant has seriously declined.

Through infrared spectrum test, elemental analysis, combustion test and other methods to identify the type of colloid, it can be judged that the second sealant of the hidden frame opening fan insulating glass of this project is polysulfide glue. In particular, one case of glass falling is that the double-layer glass used in the opening part uses polysulfide glue.

Polysulfide glue has poor UV resistance. It is generally used for open frame curtain walls. Polysulfide glue is exposed to sunlight for a long time in hidden frame curtain walls. Exposure to radiation will cause problems such as aging and deterioration of bonding quality. 

Therefore, the main reason for the shedding of the outer sheet of the insulating glass in this project is that the secondary sealant uses polysulfide glue. The injection width of the second sealant for insulating glass does not meet the minimum requirement, resulting in insufficient adhesion, which is also one of the reasons for the peeling off of the outer glass.

4. The epilogue

After the quality inspection and type identification of the secondary sealant, the following main conclusions are drawn:

(1) The main reason why the outer sheet of the insulating glass of the hidden frame opening fan falls off is that the secondary sealant uses polysulfide glue, and it needs to be replaced with the insulating glass with silicone glue as the secondary sealant to ensure the safety of the project.

(2) When replacing the glass, it is necessary to pay attention to the width of the second sealant injection should be >5mm.

(3) For insulating glass used in hidden frame curtain walls, simple and fast methods such as combustion tests can be used to identify the type of secondary sealant, so as to prevent the wrong material selection of secondary sealant, eliminate potential safety hazards, and avoid economic costs caused by glass replacement in the later stage.

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