Analysis and solution of silicone structural sealant bonding problems.
1. Overview
As a kind of external enclosure and decorative structure, glass curtain wall has been widely used in the field of architecture for their advantages of transparency and beauty. Glass curtain wall can be divided into frame-supported glass curtain wall, point-supported glass curtain wall, and full glass curtain wall according to the fixing method of glass panel; and frame-supported glass curtain wall can be divided into exposed frame glass curtain wall, hidden frame glass curtain wall and semi-hidden frame according to the form of glass curtain wall. The metal frame components of the hidden frame glass curtain wall are not exposed on the outer surface of the panel at all, while the semi-hidden frame glass curtain wall is that the vertical or transverse components of the metal frame are exposed on the outer surface of the panel. The glass curtain wall with a hidden frame uses silicone structural sealant to bond the glass and the sub-frame, and the load of the curtain wall is mainly borne by the sealant; the glass curtain wall with a semi-hidden frame adopts silicone structure in the horizontal or vertical direction (or horizontal and vertical together). The sealant bonds the glass to the sub-frame and bears a certain load of the curtain wall. Therefore, the performance of structural sealants plays a vital role in the safety of hidden frame and semi-hidden frame glass curtain walls.
However, as a semi-finished product, the final use performance of the sealant not only depends on the material itself but also has a close relationship with the joint design and construction quality. In the design and construction related to structural sealants, some situations may occur that cause bonding problems between the sealant and the substrate. These bonding problems may be poor overall bonding, poor bonding in a small area, such as a little point or line non-stick, or unstable bonding performance. If it is poor bonding as a whole, it will be dealt with carefully after it is found in actual construction, and the cause is easier to investigate; while a small area of poor bonding is easy to be ignored. However, the bonding problem of this kind of small-area silicone structural sealant will have potential safety hazards during long-term use, and will seriously affect the safety of hidden frame and semi-hidden frame glass curtain walls under extremely harsh conditions, so everyone needs to pay attention.
2. Problem analysis and solution
The cause of the bonding problem of silicone structural sealant is closely related to the material, design, construction, and other processes of the sealant. Different materials have different specific construction techniques, and it is necessary to confirm the construction process through the adhesion test in advance. The quality of the construction process will have a direct impact on the bonding of the sealant, and the design of the sealant, the injection environment, and the curing environment will also have an impact on the bonding.
2.1 The adhesion test
According to the relevant international industry standard "Technical Specifications for Glass Curtain Wall Engineering": before using silicone structural sealant, it should be tested by an approved testing agency for compatibility and peel adhesion with the materials in contact with it. If the adhesion test is not done in advance before construction, or the method recommended by the adhesion test is not strictly followed, it may cause the adhesion problem of the sealant.
With the advancement of science and technology, the surface treatment technology of aluminum alloy profiles is becoming more and more diverse. Therefore, poor bonding may occur with conventional construction procedures. At this time, a new construction process can be adopted for the adhesion test, such as adding a primer application process, etc., to solve the problem of poor adhesion, as shown in Figure 1. It should be noted that after the adhesion test is completed, the construction should be carried out in strict accordance with the construction process recommended by the adhesion test. In addition, the substrate used in the adhesion test must be consistent with the substrate used on site. If not, the results of the adhesion test are not referenced, and the adhesion test needs to be repeated.
Figure 1 Comparison of debonding (left) and good adhesion after primer coating (right) in a certain project
2.2 Causes and solutions of construction process problems
There are two common causes of sealant bonding problems related to the construction process: (1) Improper cleaning or priming fluid operation; (2) Improper operation of the sealant injection process.
2.2.1 Improper operation of cleaning or priming fluid
During the construction process, the method of cleaning or priming liquid shall comply with the construction process. Cleaning should be carried out in strict accordance with the "secondary rag method", that is, wipe it once with a piece of white cotton cloth with the cleaning agent, and immediately wipe it again in the same direction with another piece of dry white cotton cloth. If there is an obvious oil stain on the bonding surface, xylene can be used as a cleaning agent to enhance the cleaning effect. After the cleaning solvent evaporates, the primer should be applied in time. The chemical activity of the primer is extremely strong, and it is very easy to fail due to hydrolysis. Therefore, after the primer is applied, the glue injection should be carried out within the time specified by the manufacturer. After the specified time, the operation of cleaning and priming should be repeated. Some primers have requirements for the thickness of the brush, too thick or too thin will affect the bonding effect; some primers are relatively loose (as shown in Figure 2 and Figure 3). For specific construction, the manufacturer should be consulted in advance to use them differently.
Figure 2 The uneven application of primer in a certain project leads to partial sealant (left)
Figure 3 The uneven application of the primer in a certain project caused the sealant to be non-stick (right)
It should be noted that the process of cleaning and priming will also be affected by the construction environment. For example, the air in the low latitude regions is very humid, and the surface of the substrate is easy to get damp. At this time, it is necessary to pay attention to the glue injection as soon as possible after cleaning and primer coating to avoid the failure of the primer solution due to hydrolysis.
2.2.2 Improper operation of the injection process
When using the two-component structural sealant, attention should be paid to mixing components A and B evenly, and the mixing ratio should be within the range recommended by the manufacturer, which can be confirmed by butterfly test and pull-off test respectively. When changing the barrel, pay attention to sufficient exhaust. If the exhaust is not sufficient, a large number of dense air bubbles will be mixed in the part of the sealant that is punched out. At this time, the bonding effect between the sealant and the base material (aluminum, glass, etc.) is very poor, and the adhesive strip is pulled by hand, and the adhesive strip is separated from the base material; at the same time, a layer of black is often left on the base material. print. As shown in Figure 4 and Figure 5.
Figure 4 The sealant with air bubbles is poorly bonded to the glass (left)
Figure 5 The sealant with air bubbles is poorly bonded to aluminum and the rubber strip with air bubbles (right)
For the reasons of construction technology, the training of workers, especially new workers, can be strengthened, and the equipment should be maintained regularly; construction should be carried out in strict accordance with the requirements of construction technology, and quality control tests should be done during the construction process, and complete construction operations and quality assurance should be done. Control test records.
2.3 Design reasons and solutions
When using two-component structural sealants, it should be noted that there are two reasons for the mixing of components A and B: (1) The size of the sealant joint is too large or the width-to-thickness ratio is too large; (2) The sealing design of the sealant joint.
LIJIANG Glass recommends that glass curtain wall processing practitioners carefully understand the difference between two-component secondary adhesives, as well as the relevant knowledge and industry experience of automatic sealing, and try to use the automatic insulating glass sealing robot lines for two-component sealing between pieces of insulating glasses.
2.3.1 The size of the glue joint is too large or the width-to-thickness ratio is too large
The hidden frame glass curtain wall must strictly abide by the "Technical Specifications for Glass Curtain Wall Engineering" in the design process, and the design of sealant joints should be calculated according to the formula given in the specification. The width of the structural sealant should not be less than 7mm, and the thickness of the structural sealant should not be less than 6mm; the width of the structural sealant should be greater than the thickness of the structural sealant, but not more than twice the thickness; the thickness of the structural sealant should not be greater than 12mm. In the actual sealant application process, due to the large size of the curtain wall unit in some projects, the width of the two-component structural sealant is more than twice the thickness, but it is still not suitable to be more than three times the thickness. In the design and calculation of actual engineering projects, the calculation results may be too large and exceed the requirements of the specification, for example, if the width of the single-component structural sealant is greater than or equal to 24mm, the width of the two-component structural sealant is greater than or equal to 36mm, or the thickness is greater than or equal to 12mm. Another situation is that the width and thickness of the sealant do not exceed the specification requirements, but the width-to-thickness ratio is too large (single-component exceeds 2:1, two-component exceeds 3:1) o For a single-component sealant, it is too large A small glue joint size or an excessively large width-to-thickness ratio will seriously affect the curing process of the sealant, greatly prolonging the curing time and even causing the sealant to not cure for a long time; Excessive glue joint size or excessive width-to-thickness ratio will affect the release and volatilization of small molecules inside the sealant, thereby affecting the curing process of the sealant. Whether it is a one-component structural sealant or a two-component structural sealant, insufficient internal curing will cause bonding problems; and the insufficiently cured sealant, its bonding strength, elasticity, and other properties cannot meet the expected values.
The one-component structural sealant used in a certain project has a glue injection width of about 33mm and a thickness of 5mm. The project reported that nearly a month after the glue was applied, the inside of the sealant had not been fully cured; when the plate was moved, the sealant and the substrate had local poor adhesion, and there was also separation between the cured and uncured sealant inside the colloid. As shown in Figure 6.
Figure 6 The glue joint of a certain project is too wide and the aspect ratio is too large
This project is a typical case of unreasonable design. Not only the width exceeds the specification requirement (>24mm), but also the width-to-thickness ratio is very large, reaching 33:5 (6.6:1), and there are obvious curing problems in practical applications. Therefore, when designing, we must pay attention to the size of the sealant, and the size and width-thickness ratio of the glue joint should meet the requirements of the sealant manufacturer. If the calculation fails, it is necessary to confirm with the sealant manufacturer and obtain the manufacturer's approval.
2.3.2 Seam sealing
When designing structural sealant joints, whether it is a single-component structural sealant or a two-component structural sealant, it should be in contact with the outside air so that the structural sealant can be fully cured and achieve the best performance. During the design, the structural sealant should be avoided to be completely closed, because the complete closure may cause the structural sealant to not be fully cured, and the expected physical properties cannot be obtained. A unit of a project is processed with a two-component structural sealant. When the finished unit product after curing was tapped and inspected, it was found that the hardness of the tapped rubber did not reach 20 (Shore A) after 24 hours of curing, and there was local poor bonding. As shown in Figure 7.
Figure 7 The closed design of a project leads to poor local adhesion of the sealant
Compared with the above phenomena, the control group exposed to the air not only bonded well but also reached a hardness of 34 (Shore A) after 24 hours. For the rubber joints whose hardness does not reach 20 (Shore A) in the single unit, the hardness will continue to increase when exposed to the air after tapping, and the hardness will reach above 35 (Shore A) in 48h~72h, and the bonding effect will also improve.
The main reason for the above-mentioned local non-stickiness and substandard hardness in the unit-unit tapping of this project is the rubber seam sealing design. The interface design of this project is shown in Figure 8 (left). The width of the structural sealant is 22mm and the thickness is 8mm. At this time, the aspect ratio is 2.75:1, which has not exceeded the aspect ratio requirements of conventional designs. But the sub-frame bonded with the sealant and the edge protector on the side are integrally formed profiled aluminum profiles, and the rubber strips that are in contact with the sealant are also in an airtight state. Therefore, the structural sealant joint to be injected is equivalent to three airtight surfaces (the materials are glass, aluminum, and rubber strips) and one surface in contact with the air (the materials are foam rods and weather-resistant adhesives).
Left: Schematic diagram of structural sealant interface
Right: Schematic diagram of actual construction of structural sealant
Figure 8 Schematic diagram of structural sealant closure design
During actual construction, after injecting the two-component structural sealant, the workers will fill the foam rod in a short time and apply the weather-resistant sealant. This operation makes the only surface of the structural sealant in contact with the air closed, and the curing and maintenance of the structural sealant are in relatively closed conditions of use. What's more serious is that in the actual construction process, due to the limitation of the edge protection aluminum profile, the structural sealant is only injected from the side, and the sealant overflowing from the side during the injection process is not removed, almost filling the entire cavity, as shown in Figure 8 (right), resulting in the actual glue injection width of the unit plate of the project reaching about 28mm~38mm, while the thickness is still 8mm, and the width-thickness ratio is nearly 3.51-4.75:1, which exceeds the specification requirements.
This project prolongs the time interval between the injection of structural sealant and the injection of weather-resistant adhesive and removes the sealant overflowing from the side during the injection process, allowing the structural sealant to fully cure in the air before applying weather-resistant sealing Glue, finally obtained better curing and bonding effect.
It can be seen that design and construction are not independent of each other, but influence each other. The design with too large a width-to-thickness ratio not only affects the curing and bonding properties of the sealant itself, but also makes the actual construction very difficult, and it is easy to cause defects such as insufficient glue injection and bubbles; Inject glue from the side, then remove the sealant overflowing from the side, and prolong the time interval between injecting the structural sealant and injecting the weather-resistant adhesive, to obtain a good bonding effect. This design is very good in actual construction. It is troublesome and prone to problems, so LIJIANG Glass is not recommended.
2.4 Reasons and solutions for materials
Since there are many batches of materials (aluminum, glass, sealant, cleaning agent, primer, etc.) used in the actual construction process, it is necessary to pay attention to the possible differences between different batches. At the same time, the environment at the construction site is different from the laboratory environment where the adhesion test is carried out. Therefore, during the construction process, it is necessary to conduct an on-site adhesion test for each batch of materials entering the factory, as shown in Figure 9. In addition, it is also necessary to pay attention to the auxiliary materials that are in contact with the sealant, such as rubber strips, double-sided tapes, rubber gaskets, etc., which may affect the bonding. Certain auxiliary materials may contain excessive plasticizers or plasticizers that are incompatible with the sealant, which may penetrate the silicone structural sealant and cause discoloration, softening, and debonding of the sealant. Therefore, the auxiliary materials also need to be tested for adhesion and compatibility with the sealant in advance.
Figure 9 Comparison of poor adhesion (left) and good adhesion (right) in the on-site adhesion test of a certain project
2.5 Reasons and solutions for sealant injection environment and maintenance environment
2.5.1 Reasons and solutions for glue injection environment
According to the provisions of Chapter 9 of the "Technical Specifications for Glass Curtain Wall Engineering", the temperature and humidity conditions of the injection environment of silicone structural sealant should comply with the provisions of structural sealant products. The environmental requirements for the use of general silicone structural sealant products are: temperature 10°~40°C, relative humidity 40%~80%Clean environment, no construction when it rains or snows.
Adhesion problems may occur if the application is not performed under the application environment specified by the sealant product. For example, when the temperature is too low, the wettability between the sealant and the substrate surface decreases, which affects the adhesion between the sealant and the substrate, as shown in Figure 10.
Figure 10 Poor construction bonding in a project at low temperature
Therefore, in actual construction, it should be ensured that the construction environment meets the requirements of the sealant brand.
There should be heating in winter in the north, and cooling measures in summer in the south to ensure that the construction temperature is within the range required by the sealant manufacturer. If the user has special circumstances and needs to construct in a lower temperature environment (such as the ambient temperature is slightly lower than 10°C), it is recommended to conduct a small-area glue test before use, and conduct a peel adhesion test to confirm that the sealant is cured, The bonding is good, and the curing time should be extended appropriately according to the situation. If necessary, you can choose to use xylene to wash and prime the liquid to speed up the bonding speed and reduce the risk of poor bonding caused by too low ambient temperature.
2.5.2 Reasons and solutions for environmental conservation
According to the relevant international industry standard "Technical Specifications for Glass Curtain Wall Engineering", the environmental temperature and humidity conditions of silicone structural sealant injection should meet the requirements of structural sealant products. The environmental requirements for the use of general silicone structural sealant products are temperature 10°~40°C, and relative humidity 40%~80%. After the sealant injection is completed, the maintenance process should be in a clean and ventilated environment. 10°C~40°C, humidity kept at 50%~90%. When the temperature and humidity of the maintenance environment cannot meet the requirements, the curing speed and bonding speed of the silicone sealant will be slower than in normal conditions, and even curing and bonding problems will occur. Due to the difference in the curing mechanism of single-component and two-component sealants, the influence of the curing environment on the curing of sealants is more obvious in one-component structural sealants. The curing condition of the one-component sealant has an obvious relationship with the curing time. Under the same environment, the longer the curing time, the higher the curing degree. Although the curing environment has no obvious effect on the curing speed of the two-component sealant, it has a significant impact on the bonding speed of the two-component sealant. The curing process of the two-component sealant and the establishment of the adhesiveness are independent. Even if the two-component sealant is fully cured, it does not mean that the sealant and the substrate have established complete adhesion.
Therefore, when the curing environment cannot meet the requirements, it is necessary to extend the curing time appropriately and pay close attention to the curing and bonding of the sealant through the tapping test. If the sealant is affected by the environment, the bond will be unstable. Then even if the primer is not required according to the results of the adhesion test, the on-site adhesion test can be used to use a suitable primer to enhance the bonding effect.
A project uses a one-component sealant, and the injection depth is 10mm. Under low-temperature conditions, the time required for complete curing maybe 2 to 3 times or even longer than that at normal temperature. In addition, ventilation conditions and air humidity also affect the curing process of single- and two-component sealants. Good ventilation conditions are beneficial to the curing and bonding process of single and two-component. Air humidity that is too low (less than 50%) or too high (above 90%) will adversely affect the curing of the sealant. At this time, quality control should be strengthened and the curing and bonding of the sealant should be closely monitored through the rubber tapping test.
3. Conclusion
(1) Combined with specific cases, this article systematically analyzes the cause of the bonding problem between structural sealants and substrates from the aspects of adhesion test, construction technology, design, materials, sealant injection environment, and maintenance environment, and gives a corresponding solution is given.
(2) During the design process, full consideration should be given to the size, width-to-thickness ratio, and closure of the sealant joint, and if necessary, communicate with the sealant manufacturer for confirmation. During the construction process, the adhesion test should be done in advance, and the recommended construction should be followed. Process construction, and at the same time standardize construction operations such as cleaning, primer coating, and sealant injection.
(3) In the application process of structural sealants, paying attention to the above links can obtain ideal bonding effects and greatly reduce the occurrence of poor bonding problems.
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