## 3 Results and Discussion
### Sizing Degree and Contact Angle  
Figure 4 compares the Stockigt sizing degree and the contact angle of the disposable toilet cover sheets based on different levels of freeness and AKD addition. The Stockigt sizing degree measures how long it takes for a specific solution to penetrate the paper. A higher sizing degree indicates that the paper takes longer to absorb the solution, making it more water-resistant. On the other hand, the contact angle is a direct measure of the wettability of the material surface. A high contact angle (above 90\({}^{\circ}\)) typically indicates that the surface is hydrophobic (water repellent), whereas a low contact angle suggests that the surface is hydrophilic (water attractive).  
Figure 3: The SEM image of the surface structure of the cover sheet made from refined pulp fibers.  
The cover seat must immediately resist the absorption or penetration of moisture from the toilet seat surface but must not maintain this resistance for too long after being submerged in water within the plumbing system. Without a sizing agent, liquid readily penetrated the cover sheets irrespective of the freeness levels like the toilet paper in Figure 4. Thus the sizing degree and contact angle for the toilet paper could not be measured due to fast absorption. As AKD was added, the time it took for the liquid to penetrate the cover sheet began to lengthen, and when more than 0.4% AKD was added, the penetration time took over 100 s [30; 31]. Therefore, to alleviate users' discomfort caused by moisture absorption in the cover sheet, adding 0.2% AKD or less was found to be ideal to sufficiently delay absorption or penetration. This is because AKD forms covalent bonds with the cellulose fibers, creating a durable hydrophobic layer. Several studies have also shown that AKD can retain its sizing efficiency for several years under normal storage conditions, except when exposed to extreme environmental conditions [32; 33; 34; 35].  
The contact angles of disposable toilet cover sheets made of disintegrated pulp fibers were lower (85-95\({}^{\circ}\)) than those made from refined pulp fibers [36; 37]. The difference in contact angles between cover sheets made from disintegrated and refined pulp fibers could also be linked to fixing AKD onto the cellulose fibers. AKD, which imparts hydrophobicity to paper, more significantly adhered to the refined pulp fibers with more fibrils, resulting in a greater contact angle [38]. Variations in AKD retention levels between the two types of pulp fibers could affect the surface wettability and hydrophobicity of the cover sheets, leading to differences in contact angles.  
In conclusion, the disposable toilet cover sheets prepared from disintegrated pulp fibers displayed lower contact angles, signifying a slightly greater affinity for water interaction than those from refined pulp fibers. This attribute suggests that they may be better suited for disposal through a toilet system when compared to cover sheets made from refined pulp fibers. The enhanced water interaction implies easier disintegration in water, as shown in Figure 2, potentially leading to more efficient disposal and reduced environmental impact. At the same time, the cover sheet with only 0.2% AKD will not readily absorb moisture from the toilet cover surface, contributing to lowering the discomfort of toilet users caused by contact with moisture. When the cover sheet is flushed down the toilet, AKD is considered relatively safe for the environment when used in controlled amounts. This can be confirmed by the fact that there have been no reports indicating that discarded paper and paperboard in the soil hurt the soil environment.