Stress-strain cures of cellulose substrates with and without welding treatment were shown in Fig. 2. The initial cellulose strips from pine displayed a strength of 22.5 MPa, however, the values of its welded strips were showed to be a certain decrease from 0.1 MPa (welded by 10 % ZnCl2 solvent) to 16.4 MPa (welded by 40 % ZnCl2 solvent). In the meantime, the elongation values of the welded strips were 7.7% (10% ZnCl2 solvent), 8.3% (20% ZnCl2 solvent), 4.4% (30% ZnCl2 solvent), and 2.2% (10% ZnCl2 solvent), respectively, which were lower than that of the initial one (7.3%).
Through the observation, it was found that the bonding area was strong enough that no breakage was found. As a matter of fact, the real reason that was resulted in the reduction of tensile strength was the shrinks of external dimensions which was near by the welding area (signed as infiltration area), as shown in Fig. 2. This experimental phenomenon was consistent with that in the study of Ferreira’s (Ferreira et al. 2015). It is well known that the exchange of solvent with non-solvent leads to a desolvation of the cellulose molecules, and to the reformation of their intra- and inter-molecular hydrogen bonds (Medronho and Lindman 2015). Furthermore, these exchanges of molecules in usually let to dimensional shrinks of cellulose gel during the regeneration, causing the structure unevenness of whole materials, as shown in Fig. 3 of welding process. Therefore, due to this change of the external structure, the stress force was concentrated in these shrinks, resulting in the fracture of strips. The uniformity of whole materials were hardly to maintain after the regional welding, thus, many studies were focused on the whole substrate treatment through surface selective dissolution or dissolved cellulose and micro-/nano-cellulose composition (Fujisawa et al.; Huber et al. 2012; Isobe et al. 2012; Yousefi et al. 2015; Zhang et al. 2016; Khakalo et al. 2019).
It was clear that the infiltration of chemicals affected the uniformity of cellulose substrate near its welding area, and brought about the reduction of mechanical property. Due to the excellent hydrophilicity of cellulose, it is scarcely possible to eliminate the permeable diffusion of aqueous solvent among cellulosic matrix (Bai et al. 2017). It seems that the unnecessary infiltration was weaken as the usage of chemicals reduction. Conventionally, ZnCl2 solvent needs relatively high concentration (at least 65%, w/w) to form ZnCl2·nH2O system in order to dissolve cellulose (Fischer and Thümmler 2010). Based on our previous study, however, the initial concentration of ZnCl2 can be reduced to 45%, in this case, whether the dissolution of cellulose mainly depended upon the amount of water evaporation (Zhang et al. 2019). Hence, it is possible to reduce the metal -salt dosage in the welding process under the same mechanism.
In the current study, high concentration (or dosage) of ZnCl2 solvent brings a remarkable wrinkle on the experimental substrates, resulting in dimensional changes (the translucent parts from C and D in Fig. 2b) nearby the superimposed parts. Because this chemical was infiltrated out of the regional welding region with water, making their cellulose swollen and dissolved. Theoretically, the shrink of cellulose substrate was also occurred in the welding region, however, due to the compaction of multilayer substrate made this effect inapparent. Therefore, it was important to regulate the dosage of chemical solvent to avoid the un-necessary swelling or dissolving.
From Fig. 2, it was observed that the tensile strength of cellulose strips (treated by 10% ZnCl2) obtained a relatively close value (22.4 MPa) to that of the pristine one (22.5 MPa). Hence, the mechanical strength of strip was barely affected during the wielding, implying that the structure of an un-welding part of the cellulose strip was relatively intact. This result was confirmed that low dosage chemicals have more advantages on cellulose-strip connection compared with the high dosage. Therefore, the subsequent stage of this study was focused on the effect of adhesive using 10% ZnCl2 solvent (w/w).
To understand the bonding effect of cellulose strips via ZnCl2 aqueous solution, detailed microstructures of the welding, diffusion, and pristine areas were investigated by SEM images in Fig. 4. It was observed from Fig. 4 that the surface of welding area was compact, and gaps between fibers was filled with regenerated cellulose. The part of regenerated cellulose was acted as a glue that connected the strips closely like one integral matrix, as shown in Fig. 4 of the cross-section view. It was demonstrated that the new chain-chain association was established between the strip’s surface after regeneration. It is well known that the dissolution was conducive to establish the new hydrogen bonding, however, the Coulomb forces, van der Waals interactions, and hydrophobic interactions of cellulose were also promoted after the dissolution (Medronho et al. 2012). Hence, the mechanical strength of a bonding area between strips was stronger than that of a single strip.
Compared with the images of welding area, it was clear that the connections by the regenerated cellulose as fillers between fibers was remarkably reduced in the diffused area. Specifically, wrinkles were observed on the surface of fibers, indicating that the regional fibers in this area have been partial dissolved or swollen. Due to the low usage of chemicals, however, zinc chloride/water system did not affect severely the main structure of fibers in diffusion area. Hence, it was indicated that small amount of inorganic metal salt barely damages cellulose fibers, only wrinkling in the surface of fibers. Meanwhile, it was obvious that the morphology of fibers in the rest part of strip were not affected by ZnCl2. The images of Fig. 4 exhibited that the fibers in pristine area were smooth, stiff, and loose; The boundaries among fibers were clear, implying that their connections were the same as the conventional paper, which were mainly dependent on hydrogen bonds and Van der Waals force (Hirn and Schennach 2015).
In order to insight into the crystalline change of the cellulose sheets before and after ZnCl2 adhesion, XRD was used for observation. Figure 5 shows the XRD patterns of the cellulose substrates from different areas of the same strip which was bonded by 10% ZnCl2 solvent. The diffraction curve of cellulose from pristine area was exhibited to be typical characteristic of cellulose I, with the main peaks at approximately 22.5° and broad peaks near16.7° and 14.9°, respectively (Oudiani et al. 2011; French 2014; Garemark et al. 2020). Differently, the diffraction curve of cellulose from welded area was showed to be classical characteristic of cellulose II with the main peaks at approximately 21.0° and 12.0°, respectively (Chen et al. 2020; Zhang et al. 2018). The diffraction cure of cellulose from the diffusion area was more complicated compared with the others, since its pattern was exhibited to be a transition state of crystalline form from I to II (Tang et al. 2021).
Due to the intrusion of zinc ions into crystalline regions and destruction of the hydrogen bonds of the pristine network, it is no doubt that the crystalline type of cellulose was transformed during cellulose dissolution. Cellulose welding treatment, however, was belong to a class of partial dissolution, that is, the reaction was mainly happened in the P wall and parts of S1 regions of fibers (Tang et al. 2021). Therefore, although conspicuous fiber structures were still observed in the paper sheet from Fig. 4, the XRD outcome of the welding area was a typical characteristic of cellulose II. Due to the free diffusion of zinc ions with water molecules in the cellulose substrate, partly zinc ions were moved into the diffusion area, and concentrated during the heating process. Hence, partial dissolution was also happened in this area, however, only surface of few fibers was affected in that the amount of zinc ions was comparatively low. As a matter of fact, most of these fibers in the diffusion region were just swelled. Thus, the crystalline form in the diffusion area exhibited a state of polycrystalline mixture, including both cellulose I and II. More specifically, the XRD pattern of the diffused cellulose exhibited an intensity reduction of the main peaks of cellulose I, meanwhile, an emergence of characteristic peak of cellulose II. Therefore, it was observed that the crystallinity index was decreased gradually in the adhesive strips from 77.44 % for the pristine cellulose to 73.10 % for the diffused cellulose and 68.82 % for the welded cellulose, as displayed in the inserted table of Fig. 5.
The thermal degradation behavior of cellulose from the welded, diffused, and pristine areas in the bonded substrate was observed using TGA and DTG analysis, as shown in Fig. 6. Generally, the initial weight loss before 150 ℃ was due to the moisture evaporation, while the sharp drop of weight in the approximately rang at 250–380 ℃ was associated with decomposition of crystal water and thermal degradation of cellulose backbones (Jia et al. 2017; D’Acierno et al. 2020).
Table 1
Thermal properties of a cellulose strip from the welded, diffused, and pristine areas.
Sample from
|
TGA: degradation temp (℃)
|
Char yield at 600 ℃
(%)
|
DTG: maximum temperature peaks (℃)
|
T10%
|
T50%
|
Tonset
|
25–150
|
150–200
|
200–400
|
Pristine area
|
286.3
|
341.0
|
275.4
|
26.5
|
72.5
|
192.8
|
333.7
|
Diffused area
|
277.4
|
350.9
|
259.4
|
31.9
|
69.8
|
193.6
|
305.8
|
Welded area
|
246.3
|
391.5
|
230.9
|
37.9
|
nd
|
193.4
|
271.0
|
T10%: 10% weight loss temperature. T50%: 50% weight loss temperature. Tonset: the temperature of degradation occurs. nd: not dected. |
Although the cellulose samples were released adsorbed water at low temperature (< 100 ℃), they might still contain a slight amount of water, which could be noticed from the DTG curve, and the results was shown to be 150-200 ℃ in Table 1. The reason of this phenomenon was due to the strong hydrophilicity of pine cellulose, in the meantime, the dense surface of pine fiber also obstructed the adsorbed water releasing in a certain extent. Thus, when the samples were heated up over 193 ℃, the absolutely dry state of the samples could be obtained (Li et al. 2019). Compared with other samples, the welded cellulose exhibited more thermolability in the low temperature range (< 100 ℃) because there was no prominent weight loss. It was implied that the welded cellulose contained less adsorbed water than the other specimens due to its compact and uniform structure. During the main stage of thermal decomposition, it is well known that the thermal stability of cellulose I is much higher compared with cellulose II due to the changing of crystal form (Xing et al. 2018). According to the above XRD analysis (Figure 5), the pristine area with cellulose I resulted in its higher thermal stability than the welded area with crystal form of cellulose II. Furthermore, the cellulose from the diffused area was a mixed state containing both cellulose I and cellulose II, thus, its thermal stability was between the pristine and welded areas. Therefore, it was observed that the initial decomposition temperature (Tonset) of samples was 230.9 ℃ (welded), 259.4 ℃ (diffused), and 275.4 ℃ (pristine), respectively.
The char weight at 800 ℃ of welded strips, interestingly, was 5.4-11.4 % higher compared with the other samples, as exhibited in Table 1. While, the residual weight of the pristine sample was the lowest (26.5 %) among these samples. Combined with the infiltration of zinc ions in the welding process, it was believed that the change of char weight mainly depended on zinc ions remained in the sheets.
The evidence of zinc ions was appeared in cellulose welding strips as residue was exhibited in the TGA curves (Fig. 6). Thus, it was necessary to identify further the presence of zinc ions in the adhesive cellulose strips, XPS spectra were recorded, as shown in Fig. 7a. The XPS spectrum of welded and diffused area exhibited C 1s, O 1s, and Zn 2p peaks, indicating that the cellulose-Zn composite was obtained (Ma et al. 2016a; Li et al. 2020). Especially, the Zn 2p spectrum showed two peaks (Zn 2p3/2 and Zn 2p1/2 lines) with binding energies of 1023.3 and 1046.4 eV in Fig. 7b (Ma et al. 2016b). The difference of binding energy was 23.4 eV which was closely to the standard value of ZnO (23.0 eV) (Goktas and Goktas 2021). The result indicated that the zinc ions were mainly remained in the cellulose substrate as ZnO. Base on the references, the ZnO can be obtained from ZnCl2 through hydrolysis reactions even at a relatively low temperature, describing by Eqs. (1) and (2) (Yang et al. 2011; Niazi et al. 2020).
ZnCl2 + 2H2O ↔ Zn(OH)2 + 2HCl (1)
ZnCl2 + H2O ↔ ZnO + 2HCl (2)
Due to the washing process, vast majority of ZnCl2 molecules were rinsed into the effluent, however, there were parts of zinc ions was formed as ZnO remained in the cellulose materials for their property of insoluble in the water. Based on the literatures, the binding energy at 532.9 eV was characteristic of the O atom from alcoholic C-OH groups in ZnO-cellulose composite, whereas that at 532.2 eV originated from the O atom of ZnO (Li et al. 2021; Kotsis and Staemmler 2006). In our study, the two peaks at 532.9 and 532.2 eV were combined as one peak at 532.7 eV, shown in Fig. 7c. Thus, it was reconfirmed that there was ZnO substance in the cellulose welding strips. Furthermore, it was verified that there was interaction between ZnO and cellulose base on the Ma’s reaserch (Ma et al. 2016a). Therefore, due to the interaction of cellulose-ZnO and slightly solubility of ZnO in water, it was clearly to understand that the zinc ions was hardly to remove from cellulose bonding strips in washing process.