Selection of sealant and joint design

01. Introduction

Why are there multiple types of sealants?

Three steps can be taken to ensure that the sealant achieves its ideal design lifespan. Firstly, design the adhesive joint based on experience. Then determine the performance indicators that the sealant needs to achieve, and choose the appropriate sealant to meet these performance requirements. The introduction of these functions in this material can help in selecting the appropriate sealant correctly.

The correct selection of a sealant must consider the function of the sealant, the substrate of the sealant, the environment in contact with the sealant, and the required mechanical properties that the sealant can achieve. The two main functions of joint sealant are to provide weather resistant sealing and structural sealing. This requires consideration of the sealant's joint displacement ability, tensile strength, and hardness, as well as the following standards: temperature range, chemical medium, UV radiation, and engineering life cycle. In terms of substrate, the adhesion and compatibility of the adhesive to it must be considered, including auxiliary materials such as support rods and double-sided tape strips.

02. Design of sealant joints

The ideal adhesive joint should be filled evenly with sealant and have good adhesion with the substrate on both sides. Poor bonding performance of sealant can lead to adhesive failure, unreasonable sealant design can cause sealant cracking, and harsh environmental weather can cause sealant hardening and cracking. Organic silicone sealant has a particularly long service life due to its inorganic chemical composition and better resistance to UV aging compared to other sealants.

03. Support rod

The adhesive joint must have a bottom surface to ensure that the sealant can fill the joint well, have sufficient contact with the substrates on both sides, and maintain the correct shape of the joint. In most weather resistant sealing applications, we recommend using closed cell polyethylene support rods. The sealant does not stick to polyethylene, which avoids the formation of three sided bonding. The support rod contains compressed air, which ensures that it will not deform. It will also not slide in the glue seam during glue application; At the same time, it will not carry any moisture. When pressing in the support rod, it is necessary to use non sharp facilities to avoid puncturing its surface and releasing the air inside, which can easily cause the sealant to bubble and be damaged. The diameter of the support rod must be 25% to 50% wider than the adhesive seam. Other kinds of support rods belong to open cell foam, which can carry moisture and solidify sealant just like a sponge; When the elastic sealant solidifies, the presence of water vapor is not good. When applying the sealant, it causes an increase in the amount of sealant used, changes the shape of the rubber strip, affects the full contact between the adhesive and the substrate, and thus leads to adhesive failure.

04. Three sided bonding

Three sided bonding is widely used in embedded panel structures, such as aluminum plate hybrid structures and GRC door frame structures. As the displacement of the sealant changes, the shape of the sealant changes (becoming concave in the middle), but the volume does not change. Therefore, they must be able to move within the adhesive seam while maintaining good adhesion to the substrate. Figure 1 shows that three sided bonding reduces the ability of the sealant to change shape, resulting in increased pressure on the sealant and causing it to break. Three sided bonding will prevent the sealant from sliding out of the third surface of the adhesive seam, requiring all displacement to occur between the two substrate surfaces and the third bottom surface between the adhesive seams. By using a suitable adhesive to break the rubber strip, the sealant only adheres to the two sides of the joint. This way, when the sealant is affected by external forces and changes shape, it can slide in the joint and maintain the groove shape.

05. Weather resistant seal

When sealant is applied to fill joints in two or more frame structures, weather resistant sealing occurs. The sealant must adhere to the frame structure. When the frame structure moves, the sealant provides displacement deformation to the moving surface of the joint, while preventing the passage of heat, dust, water vapor, and bacteria. The sealant does not have a structural fixing effect on the frame structure.

06. Seam displacement

The reason for seam displacement is:;

1. Changes with temperature (thermal motion)

2. Changes with moisture content

3. Building load (compression caused by adding floors to high-rise buildings or changes in furniture in existing buildings)

4. Building treatment

5. External environmental forces (wind, earthquake, etc.)

The sealant must be able to change with the joint and provide corresponding displacement and deformation capabilities. Displacement deformation usually occurs cyclically, such as thermal motion occurring with daily temperature fluctuations, and large cycles occurring with the alternation of hot and cold seasons each year.

07. Width of sealant strip

Determine the ideal width of the adhesive joint according to the following calculation. Generally speaking, the width of elastic sealant filling should not be less than 6mm or greater than 30mm. If the joint is too large, it is difficult to repair the adhesive surface. The adhesive thickness should not be less than 6mm, usually half of the width (e.g. 6mm deep for a 12mm width). However, the adhesive thickness usually does not need to be greater than 10mm (e.g. 10mm deep for a 25mm wide adhesive seam, which is sufficient to meet the requirements). The width of the sealant application controls the ideal displacement ability that the sealant can provide, but it does not cause the sealant to break. The wider the seam, the greater the required displacement capacity. Therefore, the total seam displacement must be calculated before determining the ideal seam adhesive width. The displacement caused by thermal changes is easy to calculate; But at the same time, other forms of displacement deformation must be calculated, and both calculations require calculating the ideal joint size. Firstly, calculate the possible displacement deformation, and then derive the joint width.

08. Calculation of seam displacement

Calculate the possible deformation of the substrate using the following formula:

M＝(MT-T)×S×L

In the formula, M represents the deformation of the substrate in millimeters.

MT represents the temperature that the substrate may reach, including temperature increases caused by radiation and temperature changes. At a temperature of 40 ℃, the surface temperature of black aluminum may reach over 80 ℃. T represents the low temperature ℃ that the substrate surface may reach, including temperature changes caused by cold winds. S represents the thermal expansion coefficient of the substrate, and the expansion coefficients of typical substances are shown in the table below. L represents the length m of the substrate.

There are two cycles of seam displacement: the daily cycle caused by temperature differences between day and night, and the annual cycle caused by temperature differences between winter and summer. The sealant is required to be able to meet these displacement cyclic changes after many years without losing its elasticity, while also being able to meet these displacement changes. If it is required to meet the possible large and small displacement changes, calculate the actual joint size required for the sealant.

09. Seam width

After calculating the possible displacement and deformation of the substrate, the ideal joint size can be calculated using the following formula:

W＝(100×M)/S

In the formula, W represents the ideal seam width in millimeters.

M represents the possible ideal seam displacement.

S represents the allowable displacement and deformation of the sealant, expressed as a percentage, which can be provided by the product specifications.

The maximum possible displacement of the joint can be calculated in advance based on the deformation range of the substrate. If the substrate expands, the joint will become smaller and the joint adhesive will be compressed; If the substrate shrinks, the adhesive seam will become larger. The displacement and deformation around the joint are often different, for example, the bottom of a vertical plate mechanically fixed at the bottom will not move, and the deformation caused by heat will cause the substrate to deform upwards. In (Figure 2), the substrate is supported at the bottom, so all deformations related to the Y-axis direction will have an impact on the horizontal direction. Although the deformation and displacement in the x-axis direction each account for 50%, the displacement and deformation between substrate A and B can reach 100%. The fixed displacement between substrate B and the wall only accounts for 50% of the horizontal displacement, and it will move in the horizontal direction with the displacement and deformation of the wall. When the size of each panel is different, it is necessary to consider that they will cause different displacement deformations on the joints; If the materials on both sides of the joint are different, such as in glass and aluminum curtain walls, the displacement and deformation effects on the joint need to be calculated separately; Each different deformation causes the deformation of the entire joint displacement. Displacement deformation may also be caused by other reasons, such as tilting of suspension plates when installing fixed or mobile devices on buildings, or adding floors to structures. Porous materials such as concrete and bricks expand and shrink due to differences in moisture content. All of these changes should be considered for their impact on joint deformation. Usually, it should also be considered that different construction times may result in certain deviations. During summer construction, the substrate expands due to heat, causing the joints to become smaller and extending the working life of the sealant; In winter, the glue seam will widen and the working life of the glue will be shortened. The above formula only calculates the ideal seam width, and the performance of the sealant and the use of the substrate can also affect the actual seam width. We require a width of no less than 6mm. There is also an ideal seam width. The expansion sealant has limitations on the application time and curing time of the sealant. For silicone adhesive, the ideal seam width is 40-50mm.

10. Depth of sealant strip/depth of adhesive seam

The reasonable application of the substrate and the performance of the sealant will affect the ideal depth of the joint.

11. Weather resistant seal

Usually, the depth of the adhesive seam is half the width. If the depth is too large, the deformation of the joint increases, and the surface is prone to unevenness when the sealant is displaced, resulting in excessive pressure deformation and adhesive failure. If the depth of the adhesive seam is too small and the amount of adhesive is insufficient, it may cause cracking due to stress concentration or the formation of bubbles, and even lead to adhesive failure. Solvent based sealants (such as butyl rubber) can also cause shrinkage cracking due to loss of plasticizers. Therefore, the depth of the weather resistant adhesive should not exceed 12mm and the width should not be less than 6mm.

12. Structural sealing

Structural adhesive not only needs to support the load-bearing capacity of the structure, but also must withstand the deformation and displacement of structural components. Therefore, the structural adhesive must have sufficient quantity to fully contact the substrate and bear the weight of the structural unit. The structural bonding area is the portion of sealant that provides structural strength. It refers to the part of the sealant that adheres to the bottom at the correct angle within a certain range of expansion and contraction when the aluminum plate is subjected to external force.

The formula for calculating this part of the sealant is as follows:

W＝((D×5)×P)/S

In the formula, W represents the bonding width m of the structure. D represents the width m of the small edge of the aluminum plate.

P represents the maximum pressure Kpa that the glass can withstand. S represents the maximum pressure Kpa that the sealant is allowed to withstand.

13. Continuous bearing capacity of sealant

The above formula is used to calculate the intermittent bearing capacity of sealant, such as glass. The glass in the aquarium, on the other hand, is subjected to continuous load-bearing pressure, so there are different calculation formulas for it.

14. Shape of sealant

The ideal shape for sealant bonding should be square or rectangular. When the adhesive substrate is displaced, the conical substrate has three or more concentrated pressures. The adhesive failure is caused by the narrow edge of the adhesive seam.

15. Adhesive function/weather resistant sealing

Some non silicone sealants may experience increased hardness over time due to exposure to ultraviolet radiation in the natural environment. Therefore, choosing a long-lasting quality sealant requires careful consideration. Weather resistant sealant needs to be filled in the substrate. It will resist the natural environment such as wind, rain, and dust. So, the sealant must have good stretchability with changes in the size and displacement of the substrate, and be able to fully bond with its related parts.

16. Structural sealing

Structural adhesive is generally used for structural assembly bonding between components, while also addressing the tensile and compressive forces experienced. Therefore, this requires a certain degree of structural force equivalent to the mechanical standard requirements, expressed in terms of modulus and tensile strength. The larger the value, the greater the product strength. Structural adhesive will not deteriorate over time, resulting in poor adhesion between the structural adhesive and the substrate. Common structural assemblies such as glass structure assembly: aquariums, mirror installations, etc.

17. Mechanical performance/displacement capability

It is a technical indicator that uses dynamic displacement capability to represent the cured sealant. When the bonding width in actual operation does not match the value in the design draft, the larger value is usually used.

18. Tensile strength

It refers to the ideal force that the product can withstand when it breaks, with the substrate bonded intact.

19. Hardness

We use Shore hardness to represent it. High hardness sealants can reduce the risk of compression and wear caused by softer sealants. Softer sealants generally have greater elasticity and are suitable for areas where the adhesive seam is prone to displacement.

20. Environmental factors

In the ever-changing natural environment, different types of sealants or the state of the same sealant will have different reactions. The environmental factors affected include: ultraviolet radiation.

21. Chemical reactions

Temperature difference variation (application range of sealant, its adaptation range)

22. Design lifespan of adhesive seams

Adhesive testing is related to all materials, not just structural components. It is very important to conduct adhesive testing on silicone adhesive in order to better utilize materials. It can ensure that the adhesion between the adhesive and the substrate is affected by the different properties of the substrate surface.

23. Stickiness

In order to reduce the adhesion between the sealant and the stone, we recommend conducting a adhesion test between the sealant and the stone sample before using the substrate. It can avoid the adhesion of different properties of stone and glue obtained from different quarries. The testing standard we use is ASTMD2203. This standard is specifically designed to test the adhesion between natural stone and colloids.

24. Compatibility

Incompatibility manifests in discoloration of the adhesive seam or detachment of the adhesive from the substrate. Plastic rubber strips, such as butyl rubber and rubber, are typically incompatible with sealant. It takes three weeks to conduct compatibility testing.