The temporal appearance of the dyed paper with different Amur cork tree colorant concentrations is shown in Fig. 1. The undiluted Amur cork tree dyed paper showed a bright yellow color. It became lighter with a decrease in Amur cork tree concentration. The effect of color changes on paper samples under dry-heat aging is shown in Fig. 2. The color changes with different Amur cork tree concentrations were considerable during aging. The initial bright yellow color for P-A0 before aging became dark brown after dry-heat aging for 50 days. The color difference values (△E) could indicate that the influence of aging was more significant. The △E values of all samples are shown in Figure 3. With dry-heat aging time, the color change of undyed paper could not be detected (△E <3) even after 50 days. On the other hand, the Amur cork tree dyed paper became brownish after only 5 days. The △E increased quickly with dry-heat aging times for the Amur cork tree-dyed papers. The higher the Amur cork tree concentration was, the greater the color change was. The dying mechanism of Amur cork tree-dyed paper is due to its major color component, berberine, a kind of cationic alkaloid charged positively with protons, forming a chemical bond with the hydroxyl groups of the paper fibers through nitrogen. However, yellow berberine turns slowly under heating to deep red–brown–black crude products, including berberrurine [28–31]. In this experiment, all of the dyed paper samples changed to brown to various degrees. The color of the P-A0 sample changed to a slightly red–brown–black color after dry-heat aging for 50 d, which meant that berberine decomposed to some degree during this aging time. With the decrease in the Amur cork tree concentration, the color did not change much, which meant that berberine did not decompose easily when the Amur cork tree concentration was not as high. These results corresponded with the description that when dyeing paper with Amur cork tree, one should not add too much colorant or the color will darken after a long period of time .
Colorant fading is associated with changes in the characteristics, such as pH, mechanical properties and morphology. Mechanical properties are an important direct indication of polymer properties. The mechanical properties of handmade paper is different on the orientation along the transverse direction (TD) and longitudinal direction (LD). The average values of tensile strength and folding endurance of different concentrations of Amur cork tree-dyed paper were tested, and the results are shown in Fig. 4. The values of tensile strength and folding endurance index reflect the detailed structure of the paper. After dying, the tensile strength and folding endurance of the original paper increased greatly. As the Amur cork tree concentration decreased, the mechanical properties of the dyed papers decreased gradually. However, even the tensile strength and folding endurance of P-A10 were both slightly higher than those of undyed paper.
During the aging process, paper fibers can be depolymerized and lead to a loss of mechanical properties. Two types of mechanical properties, tensile strength and folding endurance, were measured in this study and are shown in Figs. 5 and 6. The results showed that the mechanical properties of different samples decreased linearly with increasing aging time. Apparently, the patterns of the change for undyed paper and Amur cork tree-dyed papers were similar, at least in the dry-heat aging time range studied in this work. Even though the thermal stability was not sufficient, samples with a suitable concentration of Amur cork had higher thermal stability than other samples.
The morphologies of the samples during dry heating could be elucidated by SEM. Representative SEM images of the pure paper and dyed papers with different Amur cork tree concentrations before and after being subjected to dry heat for 80 days are shown in Fig. 7. The results showed that the fibers were deposited by toning particle materials after dyeing. Agglomeration of toning particles was clearly observed on the surface of P-A0 and P-A1. The agglomerated color component materials clump together in bundles, do not form chemical bonds with the paper fibers and might decompose easily. Figure 7 shows that all samples were destroyed to various degrees after thermal aging for 80 days. Broken fibers were observed clearly on the surface of the undyed paper. Broken fibers and large holes could be observed on the P-A0 and P-A1 surfaces after aging for 80 d (P-A0-80 and P-A1-80), while only some small holes were observed on the P-A2-80 surface, which meant that P-A2 was not destroyed as much as the other paper samples. The representative SEM images of the dyed samples revealed that colorant materials on the surfaces of P-A2 and P-A5 had better dispersibility than those of P-A0 and P-A1. The serious agglomeration of the Amur cork tree illustrated the importance of the colorant material concentration for good dispersion.
The paper’s pH is considered one of the most important factors determining the paper’s stability toward natural and accelerated aging . The pH of different Amur cork tree concentrations of colorant dyes showed that Amur cork tree colorant dyes were weakly acidic (Fig. 8). Because the undyed paper (P) demonstrated weak alkalinity (pH 7.8), Amur cork tree-dyed papers demonstrated weak acidity to weak alkalinity, which might be ascribed to the presence of alkaline materials in the paper during the papermaking process. Under dry-heat aging conditions, the pH of all samples decreased (shown in Fig. 9). According to Fig. 9, the decrease in pH for P-A0 and P-A1 was greater than that for other samples and resulted in a pH below 6. These results indicated that there should be some acid produced, which corresponded with Cheun’s results . The decomposition of berberine produced weakly acidic materials, which accelerated the degradation of paper under weakly acidic conditions and led to a mechanical decrease. However, for P-A2, no serious agglomeration of toning particles was observed by SEM. The electrovalent bonds between paper fibers and colorant dyes prevented colorant decomposition. For P-A5, there were not enough colorant dyes to form chemical bonds between the paper fibers, so P-A5 had lower mechanical properties than the other samples.
The aging of paper under dry-heat conditions involves several main processes, namely, heat absorption, chemical bond cleavage and formation of oligomer fragments. Therefore, the factors that affect the thermal aging tendency of paper would control the degradation of paper. In the case of Amur cork tree-dyed papers, when the colorant materials dispersed on the paper surface, electrovalent bonds could be formed easily between paper fibers and colorant dyes to prevent paper fiber depolymeration. On the other hand, the agglomerated colorant dyes could be decomposed under thermal conditions, and the produced weak acid would trigger the depolymeration of paper fibers.
The DP was primarily dependent on the molecular chain length [27, 33]. The DP results of the paper samples were determined and are shown in Fig. 10. Interestingly, the DP of P-A2 and P-A5 was slightly higher than that of the undyed paper before aging. However, the DP of P-A0 and P-A1 was slightly lower than that of undyed paper. These results further indicated that the dispersion of colorant materials on paper was very important to the properties of the dyed paper. The DP decreased greatly when the paper samples were subjected to dry-heat aging. The decrease in DP was more pronounced at higher Amur cork tree concentrations than at lower Amur cork tree concentrations. After dry-heat aging for 50 days, the DP of P-A0 was below half the original values.