Analysis of Cracking Causes of Silicone Sealant After Curing
There are three forms of silicone sealant cracking: cohesive failure, which is characterized by cracking in the middle of the joint; bond failure, which is characterized by cracking on both sides of the joint; mixed damage, which is characterized by the simultaneous cracking of the joint and cracking on both sides. The following small series explains the causes of cohesive failure and bond failure of the sealant:

The cohesive failure of the sealant usually manifests as a crack in the colloid (called a colloid after the sealant is cured) because the thinnest part is first cracked. Cause Analysis:
1
Sealant quality problem
1. With the continuous precipitation of white oil for a long time, it causes the excessive shrinkage, rigidity and loss of displacement of the joint colloid, and it can not adapt to the displacement of the thermal expansion and contraction of the substrate to cause cracking.
2. The content of cross-linking agent in the formula is small, which is not enough to make the sealant react to form a perfect cross-linked network structure and structural defects, which easily lead to colloid cracking.
3. The sealant is not used within the validity period.


2
Application reasons
1. The joint design is unreasonable, and the joint width is less than 6mm, which is easy to cause cracking.
2. The formation of a large number of bubbles during the sizing process easily leads to colloid cracking.
3. The uneven thickness of the sizing is easy to cause the colloid to crack in a thin place.
4. When the surface temperature of the substrate is too high or too low, the gel is easy to crack after the glue is cured.
5. The sealant is subjected to an external force during the curing process, which tends to cause cracking of the colloid after curing.
6. The large external force or large deformation of the substrate, for example, earthquakes, typhoons, etc., the colloid may crack.
7. When the three-sided bonding phenomenon occurs, the displacement that the sealant can withstand will be limited to within ±15% of the original design displacement, which will easily lead to colloid cracking.
3
Applicability problem
1. The displacement capacity of the selected sealant cannot meet the joint displacement requirements.
2. The modulus of the selected sealant does not meet the joint requirements.
3, the bond is broken.
Adhesive failure means that the sealant is poorly bonded to the substrate, resulting in poor sealing. Usually manifested as the peeling of the substrate and the sealant. Cause Analysis:
1
Applied problem
1. The method of cleaning the surface of the substrate is improper, and the solvent used for cleaning is not suitable;
2. The surface cleaning of the substrate does not meet the requirements of the sealant application, and the surface of the substrate is not volatilized and dried when the sealant is applied;
3. The primer used is improper or the primer has expired before use;
4. The primer on the surface of the substrate is excessively applied, and the surface of the substrate is not volatilized and dried when the sealant is applied;
5. The sealant in the interface of the sealant is not fully compacted during the application process;
6. The contact area between the sealant and the substrate is too small to ensure the adhesion of the sealant to the substrate (the interface design is unreasonable);
7. The sealant is affected by external influences during the curing process, such as wind load, thermal expansion and contraction of the substrate, etc.
8. The ambient temperature during construction is lower than 5 °C, causing condensation and darkening on the surface of the substrate.