# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 4 Preparation and Properties of Sealing Material  
### Preparation  
Although SAP material has the ability of fast sealing, in overburden isolation grouting filling engineering, SAP can only enter the filling area through grouting drilling and subsequently spread to the fracture channel. Additionally, SAP expands rapidly upon encountering water. In actual filling projects, SAP swells completely before entering the fracture channel, preventing the swollen colloid from penetrating the fractures where the slurry is being applied. Consequently, the SAP material becomes ineffective at sealing large open fractures. Moreover, the bulkiness of the colloid that accumulates at the bottom of the borehole significantly hinders the diffusion capacity of the filling slurry, which may result in adverse outcomes, such as the sealing of the filled borehole. Therefore, it is not possible to add SAP directly to the filling slurry for fracture sealing.  
To solve the problem of the rapid expansion of SAP upon contact with water, which prevents the direct use of SAP for fracture sealing, this study used the air suspension method, which utilizes fluidized bed film coating technology, to apply a protective layer to SAP particles, thereby creating a sealing capsule designed for managing the slurry used in coal mine overlay isolation grouting. This external film coating serves as a shell membrane that isolates the SAP material from direct contact with water during the initial mixing with the slurry, ensuring that the SAP remains protected until needed. However, this protective film is not permanent; when the sealing capsule containing the SAP material reaches the slurry diffusion channel in the fracture, the protective shell membrane ruptures. This allows the SAP to rapidly expand upon contact with water, achieving the initial sealing of the slurry as intended.  
Figure 8: Variation curve of flowability of SAP material mixed with fly ash slurry. (**a**) Particle size of 20 mesh. (**b**) Particle size of 40 mesh. (**c**) Particle size of 60 mesh.  
Figure 9 shows the laminating process, where the SAP material particles (the capsule core) are suspended in the air. A wall material solution was sprayed into the fluidized bed, causing the capsule core to roll in a suspended state. As the wall solution was sprayed onto the surface of the particles, the solvent evaporated, allowing the wall material to adhere to the particles. Through a continuous cycle of upward and downward motion, a protective layer gradually formed around the capsule core. This was followed by a drying phase, resulting in the formation of microcapsules.  
Some of the SAP overlay requirements are as follows:  
1. SAP raw material with core sizes of 20, 40, and 60 mesh;
2. The ratio of the coated wall material to the capsule material is 20%, 30%, and 50%.  
To measure the percentage of SAP in the sealing material in relation to the content of the paraffin wall material, the \"overlay ratio\" is defined as follows:  
\[F=l_{\text{m}}/s_{\text{m}} \tag{1}\]  
where \(f\) is the overlay ratio, \(l_{\text{m}}\) is the mass of paraffin (g), and \(s_{\text{m}}\) is the mass of SAP (g).