## 3 Results and Discussion  
### Disintegration Rate  
The disintegration rate of disposable toilet seat cover sheets is a critical parameter influencing both user comfort and the environmental impact of the product [24]. Figure 2 illustrates how the disintegration rate varies with different levels of freeness and the addition of AKD (alkyl ketene dimer). This study investigates the effects of refining and AKD addition on the disintegration rate, aiming to optimize the balance between product performance and user convenience.  
Figure 1: Simultaneous measurement device for the Stöckigt sizing degree and contact angle: (**a**) automatic sizing tester; (**b**) contact angle on the cover sheet.  
Figure 2: Comparison of disintegration rate of the disposable toilet cover sheets according to varying freeness and AKD addition levels.  
The study examined disposable toilet seat cover sheets prepared with varying levels of pulp freeness and AKD addition. AKD was incorporated into the pulp stock to enhance the hydrophobic properties of the cover sheets, thereby preventing liquids from being readily absorbed and improving user comfort [25]. The disintegration rate was measured as the time taken for the sheets to fully break down in water, compared to standard toilet tissue paper.  
The results indicate that the non-sized cover sheets (without AKD) disintegrated in approximately 23 s, which is about 10 s longer than the disintegration time for standard toilet tissue paper (13 s). The refining process, which subjects the fibers to compression and shear forces, appears to impact the disintegration rate negatively. The morphological changes in pulp fibers during refining enhance inter-fiber bonding, making the cover sheets more resistant to disintegration. Interestingly, the difference in freeness after refining did not significantly affect the disintegration rate of the cover sheets. The addition of AKD was found to be a more significant factor. Increased levels of AKD led to slower disintegration rates, as the cover sheets became more resistant to water absorption [26; 27]. However, when AKD was added to the unrefined pulp stock, there was a slight improvement in the disintegration rate, although this effect was not substantial compared to the impact on refined pulp stocks.  
The findings highlight the complex interplay between fiber morphology, refining, and chemical additives in determining the disintegration rate of disposable toilet cover sheets. Non-sized cover sheets, while quicker to disintegrate, do not provide the same level of user comfort due to their higher water absorption. On the other hand, cover sheets with higher levels of AKD, while more comfortable, disintegrate more slowly, posing potential issues for disposal in toilets.  
As shown in Figure 3, the SEM image displaying the surface structure of the cover sheet made from refined pulp fibers illustrates how the refining process increases inter-fiber bonding [28]. This enhancement contributes to the durability and structural integrity of the cover sheets but comes at the expense of disintegration efficiency [29]. This emphasizes the need for a balanced production approach, where the benefits of refining and AKD addition are carefully weighed against the disintegration requirements.