Silicone sealant is an elastic material formed by vulcanization with hydroxyl-terminated polydimethylsiloxane as the basic polymer, adding a crosslinking agent, reinforcing filler, and other additives. Compared with other sealants (such as polyurethane, acrylic, polysulfide sealant, etc.), silicone sealant has better resistance to ultraviolet light and atmospheric aging due to the larger Si-O bond energy in the molecular structure. It can maintain no cracking or deterioration for 30 years and has ±50% resistance to deformation and displacement in a wide temperature range.
1. The classification of silicone sealants
According to the types of by-products generated by the condensation reaction between the crosslinking agent and hydroxyl-terminated polydimethylsiloxane, silicone sealants can be divided into deacidification type, deketosis type, dealcoholization type, deamidation type, and dehydration type. There are 7 kinds of acetone type, dehydroxylamine type, and deamination amount. Among them, the deacidification type, the deketosis type, and the dealcoholization type are the most widely used, and their main performance characteristics are compared in Table 1.
Table 1 The performance characteristics of commonly used RTV silicone sealants
|Sealant type||Typical by-products||Product performance|
|Deacidification type||MeCOOH||Fast vulcanization, good adhesion||By-product acetic acid is corrosive to metals|
|Deketosis type||MeEtC=NOH||Fast vulcanization, long shelf life, good adhesion||It is weakly corrosive to copper and platinum, and has a slight odor|
|Dealcoholized type||MeOH||Non-corrosive, odorless||Slow vulcanization, short shelf life, poor adhesion|
As shown in Table 1, although the deoxidized silicone sealant has the advantages of fast vulcanization speed and good adhesion, the corrosion of the by-products to the substrate limits its application; Fast speed, long shelf life, good adhesion, is currently the most widely used silicone sealant; dealcoholized silicone sealant released alcohol substances during vulcanization has no corrosive effect on the substrate, but its curing speed is slow Poor storage performance.
2.The common problems in the application of silicone sealants
In the actual application process, even a silicone sealant with good performance may have various problems such as poor bonding effect, oil leakage, discoloration, air bubbles, and drumming. The following analyzes some common problems that may occur during the application of silicone sealants.
2.1 Incorrect selection of sealing flat
Different types of silicone sealants have different performance characteristics and application fields. When the sealant is not selected correctly, it is difficult to achieve the sealing effect we expect. Therefore, it is very important to choose the correct silicone sealant from the standard level.
1) The selection of structural adhesives for glass curtain walls refers to the following standards: "Silicon Structural Sealants for Buildings"; "Silicon Structural Sealants for Building Curtain Walls"; "Guidelines for Technical Review of Structural Sealant Assembly Systems"; Adhesive Standards and Specifications", and at the same time, the production enterprise must pass the national "certification of silicone structural adhesive production enterprises".
2) The selection of silicone sealant for insulating glass should refer to the following standards: "Silicone Structural Sealant for Insulating Glass"; "Elastic Sealant for Insulating Glass"; "Elastic Sealant for Insulating Glass for Building Door, Window, and Curtain Wall".
3) The selection of other sealants refers to the following standards: "Silic+.on and Modified Silicone Building Sealants"; "Flame Retardant Sealants for Buildings"; "Moldproof Sealants for Buildings"; "Building Sealants for Stone"; "Sealants for Concrete Building Joints"; "Elastic Sealants for Building Windows".
Different standards put forward requirements for the performance of the sealant according to the field of application. The wrong sealant cannot be used, and the sealant cannot be used instead of the structural adhesive.
2.2 Bonding problem
2.2.1 Chemical reaction between the sealant and the substrate
Acidic silicone sealant has the characteristics of translucency, excellent weather resistance and water resistance, and good adhesion to glass substrates. It is mainly used for bonding and sealing glass tanks, glass doors and windows, and large glass plates. Because it will release acetic acid during the curing process, which has a corrosive effect on metal and concrete surfaces, it cannot be used on the surface of metal, coated glass (including mirrors), laminated glass, cement surfaces, stone, and other substrates, otherwise, it will cause substrates The surface of the material is rusted, oxidized, or even corroded, causing pollution or colloidal bond failure.
The advantages of neutral alcohol-type silicone sealants are that they are odorless and have no pollution to the substrate, but they cannot be used on the surface of glass or aluminum in high-temperature environments. For example, when the outdoor temperature is 35°C, the surface temperature of glass or aluminum can reach 80°C, and the methanol gas released during the curing process of the alcohol-based adhesive cannot be discharged in time (methanol starts to volatilize at around 50°C), which may easily cause foaming.
Neutral ketone moon-loss type silicone sealant has good storage stability, no pollution to substrates, wide bonding range, no corrosion to substrates other than brass and yellow platinum, and good overall performance.
2.2.2 The sealant is not compatible with the base material
Figure 1 The interface between the sealant and the substrate is damaged
Silicone sealants bond well to most substrates, but there are also cases of "bad bonding" (Figure 1). In summary, there are mainly two reasons:
1) The sealant does not stick to the substrate itself. Due to the variety of substrate types and their surface treatment methods, any sealant cannot 100% guarantee a good bond with all substrates; the double-layer silicone sealant coating machine for insulating glass produced by LIJIANG Glass can effectively improve the sealing Adhesive bonding efficiency to glass substrates.
2) The construction is not standardized. For example, the surface of the substrate is not clean, the curing environment or curing time does not meet the requirements, the primer is not applied as required, and the ratio of the two components is not equal.
Because of the reasons for the above-mentioned "poor bonding" between the sealant and the substrate, the following three steps can be taken to prevent it:
Step 1 : Do an adhesion test.
For any sealant, the adhesion test must be carried out on the substrate before construction to determine whether a primer is required, and then proceed with construction according to the correct construction method. If necessary, a suitable sealant product should be replaced.
Step 2 : Clean the substrate surface.
Before sizing, the dust, oil, and other dirt on the surface of the substrate must be cleaned according to the prescribed method: clean with a cleaning agent (such as acetone, xylene, methyl ethyl ketone, thinner, 50% isopropanol, etc.). If the adhesion test shows that a primer is required, apply the primer after cleaning and finish sizing within 1 hour to avoid re-contamination.
Step 3 : Ensure sufficient curing time.
The single-component should be cured for at least 7 days, and the double component should be cured for at least 3 days. In the case of low temperature and humidity, the curing time should be extended appropriately.
2.3 Discoloration of silicone sealant
2.3.1 Yellowing of neutral transparent silicone sealant
Compared with conventional silicone sealant products, transparent silicone sealants are more beautiful, but they are also more prone to yellowing during application (Figure 2) o Reasons for yellowing of neutral transparent silicone sealants: mainly Because of the "amine group" in its curing agent, the amine group is easy to cause yellowing; in addition, the use of neutral transparent silicone glue and acid glass glue at the same time may also cause the neutral glue to turn yellow after curing; in some cases Among them, yellowing may also be caused by the sealant being stored for too long or reacting with the substrate.
Figure 2 The yellowing phenomenon of neutral transparent gum 1
2.3.2 Discoloration of silicone sealant
In some projects, the silicone sealant will change color after a period, which seriously affects the aesthetics of the project. There are three main reasons for the discoloration of silicone sealants:
1) Accessory materials such as EPDM strips, neoprene rubber, and asphalt paint are incompatible with silicone adhesives and cause discoloration.
2) Acidic and alkaline substances such as certain cleaning agents, cement slurry, and other corrosive substances cause discoloration of silicone rubber.
3) Moon-deficit silicone adhesive is prone to discoloration when used in an acidic environment. To prevent discoloration of the silicone sealant, the compatibility test of the accessory materials should be carried out before construction, and the glue should be applied after passing the test. In addition, the colloid should be prevented from contacting corrosive substances or being exposed to corrosive environments.
2.4 The silicone sealant has bubbles, drums, cracks, or oil leakage
2.4.1 Neutral silicone sealant is used in the junction of concrete and metal window frames in summer, and many (outer wall) bubbles will appear after curing
Many brands of neutral silicone sealants have similar phenomena to the above. The inspection and repeated experimental verification found that this phenomenon is not caused by the quality of the sealant: on the one hand, it is because the methanol gas released during the curing process of the alcohol-type adhesive is too late to volatilize (methanol begins to volatilize at around 50°), the reaction is stronger when it encounters direct sunlight or high temperature. On the other hand, because concrete is a porous material, the gas volume in the concrete pores will expand dozens of times under the conditions of high temperature and humidity in summer; while the curing of the sealant is from the surface to the inside, and the surface first forms elasticity. After the body, the released gas is wrapped inside, forming bubbles. Therefore, bubbles of different sizes appear on the cured adhesive strip (Figure 3).
Figure 3 There are many bubbles on the surface of the silicone sealant after curing
The appearance of bubbles has seriously affected the quality and appearance of the project. To solve such problems, the following points should be paid attention to:
1) Do a local application test first to observe whether it meets the needs of use (generally, the reaction can be seen 2 to 3 days after tendering).
2) Keep the construction surface clean and dry.
3) When sizing glue in summer, avoid high-temperature periods (above 35 C) and direct sunlight, and it is generally more suitable in the evening.
4) For similar projects, please notify the manufacturer's technical personnel to follow up.2.4.2 Blistering of weather-resistant adhesive for aluminum curtain wallWhen the surface of the sealant is not fully cured, if the width of the glue gap changes greatly, the surface will appear uneven, which is called "bubbling" (Figure 4).
Figure 4 Aluminum curtain wall sealant bubbling phenomenon
Most of the aluminum curtain wall bulging occurs in spring and autumn, mainly due to the expansion and contraction deformation during the curing process of the sealant. The specific heat capacity of aluminum is small, about 0.88x103J/(kg・C), and the thermal expansion coefficient is about 23x10-6/℃ (about 3 times that of glass). Therefore, when the temperature difference between day and night in the environment is high, the thermal expansion and contraction of the aluminum strip and aluminum plate will change greatly, resulting in a large change in the width of the glue joint. Then a drum was formed. To prevent the sealant from bulging, the weather with the small temperature difference between day and night should be selected for construction; sunshade measures can be used to prevent the aluminum curtain wall from bulging; when the bulging has already occurred, the manual silicone sealant glue applicator can be used for secondary glue application to improve.
2.4.3 Silicone glue seam cracking
After the silicone sealant is fully cured, the surface should be a smooth and uniform elastomer. However, cracking of the sealant often occurs after actual construction (Figure 5).
Figure 5 Silicone sealant cracking phenomenon
Cracking of silicone sealant may be caused by the following reasons:
1) The sealant is improperly selected, and the displacement capacity cannot meet the design requirements. For example: according to the design requirements, glue with a displacement capacity of 25 grades is used, but the displacement grade of the sealing chamber used is 12.5E, which cannot meet the actual deformation requirements and leads to cracking.
2) The glue seam design is unreasonable. Excessively wide glue seams will cause difficulty in gluing and poor sealing. This situation can be solved by multiple sealing or automatic sealing solution.
3) The installation of the curtain wall plate is shifted, and the deviation is too large.
4) Before the sealant is fully cured, the joint displacement is too large.
5) The three-sided bonding of the sealant results in limited displacement capacity.
6) The sizing thickness is too thin, resulting in damage to the sealant or bond failure.
2.4.4 Oil leakage
Oil seepage occurs in silicone sealants, which not only affects the appearance of the curtain wall but also increases the difficulty and cost of curtain wall cleaning. Oil leakage is generally due to the addition of inferior white oil plasticizers to silicone sealants. This plasticizer is cheap but has poor compatibility with the sealant system. It will slowly precipitate from the gel after long-term use. Thereby contaminating the surface of the substrate (Figure 6). Therefore, a silicone sealant that does not contain white oil should be selected.
Figure 6 Curtain wall oil seepage pollution phenomenon
2.5 Silicone sealant for insulating glass fails to seal
The insulating glass adopts two seals: the first seal mostly uses butyl hot melt adhesive to prevent the penetration of water vapor; the second seal mostly uses silicone sealant to play the role of sealing and bonding. The main reasons for the failure of the silicone sealant for insulating glass are as follows:
1) The quality of the butyl hot melt adhesive for the first sealant is not good, or it is not compatible with the silicone sealant.
2) 2-way sealing silicone sealant filled with mineral oil. Mineral oil will dissolve the butyl rubber, causing the seal to fail; at the same time, it will stain the glass and affect its appearance (Figure 7).
Figure 7 Oil leakage inside the insulating glass of the hidden frame curtain wall
3) Contact with other mineral oil-filled sealants (such as curtain wall weathering adhesives or door and window sealants).
4) Other factors, such as desiccant or processing technology, etc.
3. The epilogue
Silicone sealants are widely used in the bonding of indoor and outdoor glass, metal, and marble components due to their excellent UV resistance and atmospheric aging resistance, and their ability to resist deformation and displacement within a wide temperature range of ±50% seal. However, the quality problems of the sealant itself, improper selection, or improper operation during the construction process may affect its use effect and bring huge losses to the curtain wall project. Therefore, to ensure the quality of the curtain wall project and achieve a good bonding and sealing effect, it is necessary to strictly control every step from material selection to construction:
first, select a quality-assured silicone sealant, and perform bonding and compatibility Test to ensure that the sealant is well bonded to the base material and compatible with the contact materials;
secondly, the surface of the base material must be cleaned correctly to ensure that the base material is dry before applying glue;
at the same time, a good maintenance environment and sufficient maintenance must be ensured time.