Modification of DE with AKD
The modification conditions of AD and their sizing performance are listed in Table 1, where the weight ratio of AKD wax in AD and cellulosic fibers (oven dry) was set as 0.20%. The paper sheet filled with AD-1~3 all exhibited excellent hydrophobicity. In comparison with AD-1 and AD-3, AD-2 had better performance, which implied that the desired load amount of AKD wax on diatomite contributed to enhancing the efficiency of AKD wax for hydrophobicity of the cellulosic paper. The subsequent work in this paper was also based on the sample of AD-2 (denoted as AD), considering its better performance as a sizing agent.
Table 1
Modification conditions of AD and their sizing performance
Sample
|
AKD Conc.
(g/mL)
|
AKD volume (mL)
|
DE mass
(g)
|
AKD/DE (%)
|
Sizing degree (min)
|
AD-0
|
0.000
|
5
|
2
|
0.0
|
0
|
AD-1
|
0.003
|
5
|
2
|
0.8
|
4 ±0.25 min
|
AD-2
|
0.010
|
5
|
2
|
2.5
|
10 ±0.18min
|
AD-3
|
0.017
|
5
|
2
|
4.3
|
6±0.23min
|
Thermo Gravimetric Analysis of DE, AKD wax, and AD are shown in Fig. 2. It can be seen that the weight of DE roughly became stable with the increasing of the temperature due to its excellent thermal stability, and few impurities existed in it (Zhao et al. 2021). The AKD wax underwent two obvious weight loss steps when heated from 30 to 800°C (Ryu et al. 2020). The weight loss of 5% occurred below 242°C may be attributed to the loss of physically absorbed water on the surface of AKD and the decomposition of AKD with low molecular weight. The weight loss of 95% occurred in the temperate range of 242-500°C owing to the decomposition of AKD with high molecular weight. In contrast, the weight loss of AD was 27% in the temperate range of 30-450°C, which was attributed to the decomposition of AKD. When the temperature was higher than 450°C, the sample tended to be stable, and no further mass loss was observed in the TG curve, which can be attributed to the complete decomposition of AKD.
SEM images of AKD, the diatomite before and after modification of AKD are shown in Fig. 3. Fig. 3b-3c show that DE has a clean surface with micro-pores (Li et al. 2021). It is distinctly observed that AKD was loaded on the surface of DE, as shown in Fig. 3d-3f. The mean particle diameter of DE and AD is approximately 45 µm and 56 µm, respectively, as shown in Fig. 3g-3h. We probably inferred that AKD was successfully loaded on the surface of DE and therefore increased its mean particle diameter.
The FT-IR spectrogram (Fig. 4) showed that the prominent characteristic peak of the DE appearing at 1090 cm−1 is due to Si-O-Si antisymmetric stretching vibrations. The peak at 792 cm−1 could be ascribed to Si-O-Al symmetrical stretching and bending vibration (Yuan et al. 2013). Compared with DE, the AD displayed a new characteristic peak of 2910 cm−1 (Shang et al. 2018). The unique characteristic peak belonged to the symmetric and antisymmetric stretching vibrations of -CH2 in the AKD alkyl chain, demonstrating that AKD had been successfully loaded on the DE.
Application of AD as a filler in papermaking
The adding ratio of AD to cellulosic fibers and the correspondent ratio of AKD to cellulosic fibers in hand sheets are listed in Table 2. When the AD in ADP increased from 0 to 32%, the roughness of the topside/backside increased from 8.78 to 9.27µm and 9.53 to 10.52µm, respectively. It is well known that the roughness of the backside of the hand sheets paper is generally higher than that of the topside because of more filler and fiber fines loss in the backside during the dehydration process of the stock. According to the results in Fig. 5a, the roughness of both the top and back sides increased when adding more AD in hand sheets. Meanwhile, the difference in the roughness between the back and top sides also became more significant. The CA of the hand sheets remarkably increased from 0o to more than 80o when the dosage of AKD in AD increased from 0.05-0.2%, as shown in Fig. 5b, which seemed to suggest there is a minimum amount of AKD to grant hydrophobic properties for cellulosic paper. While the dosage of AKD was more than 0.2%, the CA was further enhanced. It is worth noting that the backside of the hand sheets had a higher CA than the topside regardless of the dosage of AKD, which seemed paradoxical because AD lost more on the backside of the hand sheets. The super-hydrophobic theory may explain it, i.e., the two necessary conditions for the super-hydrophobic surfaces are low surface free energy and microscopic rough surface (Kwon et al. 2009). In this study, under the combined effect of roughness and AD retention of both sides of the hand sheets, the backside had a larger CA than the topside; therefore, the backside was expected to perform better hydrophobicity.
Table 2
The sizing conditions of paper sheets
Sample
|
AD/dry fiber (%)
|
AKD wax/dry fiber (%)
|
ADP-0
|
0
|
0.00
|
ADP-1
|
2
|
0.05
|
ADP-2
|
8
|
0.20
|
ADP-3
|
14
|
0.35
|
ADP-4
|
20
|
0.50
|
ADP-5
|
26
|
0.65
|
ADP-6
|
32
|
0.80
|
Figure 6 shows the retention of AD and corresponding tensile strength of ADP (ADP-2) in the case of various dosages of CPAM. The filler retention of AD in paper sheets gradually increased from 49.29–67.00% as the dosage of CPAM increased. However, the tensile strength of the paper sheet decreased when more AD was retained in the hand sheets because the AD particles occupied the space among the pulp fibers and weakened the hydrogen bonding between the cellulosic fibers (Huang et al. 2013; Kinoshita et al. 2000). Like other commonly used paper fillers, the increase in the amount of AD will also lead to a significant decrease in the tensile strength of cellulosic paper.
Figure 7a and 7c show that the sizing degree and CA of ADP and ASP increased when more CPAM was added. Moreover, AD had better sizing performance than AKD emulsion. At the same time, the CA of ADP was also higher than that of ASP in the case of the same side, i.e., topside versus topside and back side versus back side. Fig. 7b and 7d show that with bentonite dosage increasing from 0.0–0.1%, the CA and sizing degree of both ADP and ASP increased. The CA and sizing degree decreased when its dosage was further supplemented by more than 0.1%. Compared with the mean particle size of AKD in the emulsion, AD had a larger mean particle size, which could facilitate AD to perform better than AKD emulsion under CPAM retention aid and CPAM/ bentonite retention aid system.
Sizing mechanism of AD in the paper sheet
SEM images of CP and ADP are shown in Fig. 8. AD was evenly distributed inside the paper sheet and on its surface, as shown in Fig. 8d-f. Furthermore, some particles with irregular shapes were also observed, which could be some broken diatomite enveloped with AKD. It is evident that the amount of AD used in this study is not capable of thoroughly changing the hydrophilic property of the cellulosic fibers. However, AD can provide numerous hydrophobic sites, which are enough to endow the paper sheets with hydrophobic properties (Hao et al. 2015).
The sticky super-hydrophobicity of rose petals is a typically super-hydrophobic phenomenon found in the natural world (Yu et al. 2017); the water droplets easily adhere to the microstructured surfaces of rose petals (Su et al. 2016; Rahman et al. 2020). An interesting sticky hydrophobicity phenomenon was also observed in this study. Water droplets did not slide or roll even when we turned upside the paper sheet, as shown in Fig. 8g. ADP possessed adhesion with water droplets, attributing to the surface microstructure of the hydrophobic paper sheet. We inferred that the wetting behavior of water droplets on the paper sheet conformed to the Wenzel model (Shirtcliffe et al. 2010), and the droplets were trapped in the grooves, as shown in Fig. 8g. It was challenging to overcome the barrier when the droplets slipped off, and the droplets did not roll. This study provides a facile method to prepare the sticky hydrophobic paper sheet and can further find its application in some non-traditional application fields such as no loss micro-droplet transportation and chemical microreactors (Li et al. 2014; Cheng et al. 2013; Mata-Cruz et al. 2017).
The peak at 1090 cm−1 illustrated the existence of diatomite (Fig. 9). The peak at 2890 cm−1 was related to the symmetric stretching vibration of -CH2, which revealed the presence of cellulose (Li et al. 2018). The absorption peak at 3340 cm−1 was associated with the stretching vibration of -OH. Yan et al. reported that AKD might react with free hydroxyl groups of cellulose to form β-keto ester linkages, endowing paper sheets with hydrophobicity (Yan et al. 2016). However, the formed β-keto ester linkages were not observed in the ADP spectrum, which could be due to the low amount of AKD in the paper sheet. It also implied that the chemical reaction of forming ester linkage during AD sizing might not be indispensable.