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The selection of sealant mainly considers the following 6 aspects
1.Compatibility with substrates
Due to the curing principle and main components, some sealants are corrosive to some substrates and are incompatible. For example, the commonly used one-component room temperature curing adhesive sealing silicone rubber selection, one-component silicone rubber is a silicone adhesive that can be cured at room temperature. In an open environment, it undergoes a cross-linking reaction under the action of moisture to form a rubber elastomer. According to the type of small molecules released, it is divided into dealcohol type, deacetone type, deacidification type and deketoxime type. Among them, the deketoxime type product will corrode copper and hard PC in a closed environment, so special attention should be paid to it during selection.
The sealing depth is also a factor to be paid attention to. One-component room temperature curing adhesive sealing silicone rubber is cured by moisture in the air. The principle is that the glue reacts with moisture in the air to cure. After the glue is applied, the surface glue will cure first, and air is difficult to penetrate, which in turn affects the reaction of the deep glue. Therefore, the depth of glue needs to be considered when selecting. When single-component room temperature curing adhesive sealing silicone rubber cures from the surface to the inside, usually under the conditions of 25°C and 50% RH, the surface drying time is generally 10~30 minutes, and 3~4mm can be cured in 24 hours; it usually takes 7 days to achieve good physical properties. This sealing depth generally meets the design requirements. If the sealing depth exceeds 5mm, it is best to use a two-component reactive sealant.
3.Temperature resistance of sealant
Taking the commonly used single-component room temperature curing adhesive sealing silicone rubber as an example, the temperature range of dealcohol type silicone is -40℃ to 200℃, and the long-term use temperature does not exceed 150℃. The temperature range of special high temperature resistant deoxime type silicone is -40℃ to 250℃, and the long-term use temperature does not exceed 180℃. Special attention should be paid to the fact that the temperature resistance of the glue is greatly related to whether the colloid is completely cured. If it is not completely cured, heating will produce bubbles, cracks, smoke and other undesirable phenomena.
4.Process operability
When selecting sealants, not only important curing process conditions such as curing temperature and time in the technical specifications should be considered, but also the actual product and production equipment conditions of the curing principle should be combined: when the curing thickness of the colloid is greater than 6mm, it is recommended to use two times of gluing; increasing temperature and humidity can accelerate the curing speed of the product, but the temperature should not exceed 50℃, and increasing humidity is better than increasing temperature. When selecting sealants, the convenience and cost of the production line should also be considered. Two-component sealing (potting) glue needs to be prepared, mixed, and degassed before use, and an additional workstation may be required. Single-component sealants or two-component sealants with mixing tubes are more convenient to use and may have lower overall costs.
Different substrates have different adhesion properties due to different surface energy, roughness and other factors. The sealing property of the sealant at the interface is mainly achieved by bonding the substrates together, and the bonding force is the guarantee of the sealing effect. When selecting, it is necessary to determine whether the adhesion between the glue and the substrate can meet the requirements. If the adhesion is not up to standard, the sealant needs to be replaced. If there is no other choice, a primer can be used. The following table lists the adhesion of some substrates for reference.
6.Reliability of sealant
The reliability of sealant includes the reliability of the sealing interface and the reliability of the colloid itself. A clear conclusion can only be drawn after the reliability test evaluation. It is also possible to make a preliminary evaluation of the reliability of the sealant based on its characteristics when selecting. For example, epoxy sealant has high hardness, poor resistance to hot and cold cycles, and is prone to cracking. Silicone sealant performs better. Whether it is high temperature, high humidity or hot and cold cycles, silicone can tolerate it well, but the adhesion of silicone sealant is slightly poor, and the interface is prone to damage.