The Influence of Papermaking Process on the Properties of Chinese Handmade Bamboo Paper

2.1 Materials

Six types of bamboo paper were selected for the following experiments: Renhua paper, Changting paper1 (uncooked), Changting paper2 (cooked), Panxian paper, Fuyang paper, and Liancheng paper. The first two are processed using cold digestion, while the latter four are processed using hot digestion. The general characteristics of the paper samples and their main production processes are shown in Tables 1 and 2 respectively.

Micrographs of each sample are shown in Figure 1. Fibers of all samples are straight, smooth, with pointed ends and many impurities, which is consistent with the fiber morphology of moso bamboo.

Figure 1:

Micrographs of the morphology of paper samples. (a) Renhua paper, (b) Changting paper1, (c) Changting paper2, (d) Panxian paper, (e) Fuyang paper, (f) Lianchen paper.

2.2 Experimental Instruments

CS-411 spectrophotometer (Hangzhou Caipu Technology Co., Ltd.); CL200+ pen-type residual chlorine pH meter (Shanghai Sanxin Instrument Factory); ZB-L vertical computer tensile tester (Hangzhou Zhibang Automation Technology Co., Ltd.); ZB-NZ135A folding endurance tester (Hangzhou Zhibang Automation Technology Co., Ltd.); 101 type blast drying oven (Beijing Kewei Yongxing Instrument Co., Ltd.); ALPHA type Attenuated total reflectance infrared spectroscopy (Bruker Corporation); SmartLab SE type X-ray diffractometer (Rigaku Corporation); Phenom XL desktop scanning electron microscope (Thermo Fisher Scientific).

2.3 Experimental Methods

3.1 Tensile Index

The tensile index refers to the tensile strength of a paper sample under the conditions specified in standard test methods, expressed in terms of unit width and unit basis weight. It is one of the important indicators for measuring the physical properties of paper. There are three factors that affect the tensile index of paper: the strength properties of the fibers themselves, the bonding strength between fibers, and the arrangement of fibers (Li et al. 2014).

Before measuring the tensile index, the composition was determined with reference to GB/T 2677.10-1995, TAPPI T 223 cm⁻²³ and ultraviolet acetyl bromide method (Moreira-Vilar et al. 2014). The degree of beating of the paper sample was determined according to ISO 5267-1:1999. From Table 3, it can be seen that the beating degrees of each sample were approximately the same. The tensile index of the Panxian paper is the highest (Figure 2), because the lignin in the raw material is mostly removed through the cooking process, which exposes more hydroxyl groups on the fibers and makes the fibers more tightly bound through hydrogen bonding, resulting in a higher tensile index. The tensile index of the cold digestion (uncooked) Renhua paper is the lowest, because the Renhua paper has not been cooked and the lignin content in the raw material is high, which fills the fibers and weakens the binding force between them, resulting in a decrease of its tensile index. Comparing the tensile index of Changting paper1 and Changting paper2 under the same post-fermentation process and without bleaching, it can be seen that the tensile index of Changting paper1 made by the cold digestion (uncooked) method is higher than that of Changting paper2 made by the hot digestion (cooked) method. The reason for this phenomenon is that during the long-term strong alkali cooking process, cellulose is prone to alkaline hydrolysis and peeling reactions. Alkaline hydrolysis breaks the glycosidic bonds of cellulose macromolecular chains, and peeling reactions cause the reduction of terminal groups on cellulose macromolecules, resulting in fiber damage and affecting fiber strength, causing a decrease in tensile index (Yan et al. 2018). In addition, from the perspective of error analysis, the uniformity of Changting paper2 is also poor.

Figure 2:

Tensile index of paper samples during dry heat aging; the initial tensile strength data are plotted at 0 days aging time.

Except for Fuyang paper, which shows a significant downward trend, the tensile index of other paper samples shows an initial increase and later decrease during the aging process (Figure 2). This is because during the initial stage of aging, the moisture in the paper decreases, and the fibers that originally combined with water molecules to form hydrogen bonds turn to combine with internal fibers to form hydrogen bonds, enhancing the tensile index of the paper. In addition, during the aging process, cellulose continuously degrades, the strength of the fibers themselves decreases, the cellulose chain becomes shorter, and the interwoven force between fibers increases. In the early stages of aging, the increase in tensile index caused by the strengthening of the binding force between fibers exceeds the decrease in tensile index caused by the decrease in fiber strength, so the tensile index shows an upward trend during the early stages of aging. In the later stages of aging, the decrease in tensile index caused by the degradation of cellulose itself exceeds the increase in tensile index caused by the strengthening of the binding force between fibers, so the tensile index shows a downward trend. Fuyang paper may have a faster degradation rate due to its initial low pH and rapid pH drop during the aging process, resulting in a decrease in tensile index during the early stages of aging (Tian et al. 2017).

The tensile strength of the paper samples is also related to their lignin content, since lignin is chemically active and more easily oxidized than cellulose. However, studies have shown that within a certain range, lignin can act as an antioxidant to protect cellulose during aging significantly (Małachowska et al. 2020). Although the initial pH value of Changting paper1 and Renhua paper is lower than Fuyang paper, their lignin content is higher than Fuyang paper and this might explain that the tensile index of Changting paper and Renhua paper has not decreased.

3.2 Folding Endurance

Folding endurance refers to the logarithm (base 10) of the number of double folds required for a paper sample to break under a certain tensile force under specified conditions. It is one of the important indicators for measuring the basic physical strength of paper. The factors affecting paper folding endurance mainly include fiber strength, flexibility, fiber length, and fiber bonding force.

The thickness of several types of paper selected for this experiment is uneven, and the initial folding endurance cannot be used as a basis for comparison, so it will not be discussed here.

Figure 3 shows the change in folding endurance of paper samples during dry heat aging. All paper samples showed a significant downward trend during aging, with a significant decrease in folding endurance after 60 days of aging.

Figure 3:

The change of folding endurance after dry heat aging of paper samples.

The decrease in the folding endurance of unbleached paper, such as Changting paper2 (5.95 %), Panxian paper (7.61 %), and Fuyang paper (9.03 %) is relatively small. The decrease in the folding endurance of uncooked paper, such as Renhua paper (30.07 %) and Changting paper1 (38.44 %) is significantly larger. The decrease in the folding endurance of bleached Liancheng paper (56.08 %) is the most pronounced. The reason for the above results is that in unbleached paper, cooking removes some lignin from the pulp, resulting in a low lignin content. Since the pulp is not chemically bleached, the loss of fiber strength is relatively small, so the folding endurance remains relatively stable during aging. Liancheng paper has been cooked and bleached with chemical reagents. Although the lignin has been removed more thoroughly, its removal has caused serious damage to the cellulose, resulting in the largest decrease in folding endurance during aging.

3.3 Changes of pH

The pH is the most critical factor affecting the long-term behavior of paper. Under acidic conditions, hydrogen ions act on the oxygen atoms on the β-1,4 glycosidic bonds of cellulose, causing protonation reactions and breaking them, resulting in shortened cellulose macromolecular chains and decreased degree of polymerization. At the same time, acidic substances are generated during the hydrolysis process of cellulose and hemicellulose, which accelerate the acidification of paper. As the concentration of hydrogen ions increases, the concentration of hydronium ions produced also gradually increases, further reducing the stability of β-1,4 glycosidic bonds of cellulose, ultimately leading to more glycosidic bond breaks, causing paper to become brittle and reducing its shelf life (Zervos 2007).

Figure 4 shows the initial surface pH and the change in pH after dry heat aging of paper samples. The initial pH value of the hot digestion method paper samples is generally higher than that of the cold digestion method paper samples. Among them, the pH of Liancheng paper is the highest. After undergoing a caustic soda cooking process, Liancheng paper is further bleached with natural bleaching and bleaching powder, which removes more thoroughly the non-fiber impurities lignin in the raw material, making it less likely to be converted into acidic substances later on due to hydrolysis and oxidation. Therefore, the pH is the highest. Although the Panxian paper has not undergone bleaching, it has undergone multiple weak alkali cooking processes in the early stage, especially the first lime cooking process that lasted for one month. Through multiple weak alkali cooking processes, most of the lignin impurities are removed, so the pH is also high. Similarly, the pH values of Changting paper2 and Fuyang paper produced by the hot digestion method are lower due to the fact that although these two types of paper have undergone cooking processes, during the long-term fermentation process in the later stage, microbial fermentation produces acidic substances, resulting in a lower pH value in the paper.

Figure 4:

The change of pH during dry heat aging of paper samples.

The pH of each paper sample showed a downward trend as the aging time increased. In addition, it can also be seen from Figure 3 that the decrease in pH of the hot digestion method papers is generally smaller than that of the cold digestion method papers. For example, the decrease in pH of the Panxian paper and Changting paper2 is only 0.34 and 0.52 respectively, which is far lower than the decrease in pH of the Renhua paper and Changting paper1, which is 1.09 and 1.03 respectively. This is because the Panxian paper has undergone multistage cooking with lime, soda ash, and caustic soda, while the Changting paper2 has undergone strong alkali cooking. During the above processes, lignin impurities in the raw materials are effectively removed, so the paper is not prone to acidification during subsequent aging processes. The reason for the higher pH decrease in Fuyang paper is that weak alkaline lime is used as a cooking agent, which fails to effectively remove lignin. In addition, acidic substances are produced during post-fermentation, so it is prone to acidification during subsequent aging processes, resulting in a low pH of the paper.

3.4 Degree of Polymerization

The main component of paper is cellulose, which is a high molecular weight polymer formed by the connection of d-glucopyranose groups through β-1,4 glycosidic bonds. The number of glucose units that make up the cellulose molecule is called the degree of polymerization (DP).

The initial degree of polymerization and the change in degree of polymerization after dry heat aging of paper samples is shown in Figure 5. There are significant differences in the degree of polymerization between each paper sample. Comparing the degree of polymerization of Renhua paper and Panxian paper, it can be stated that although one is a cold digestion method paper and the other a hot digestion method paper, their degrees of polymerization are almost the same; Renhua paper and Changting paper both use the cold digestion method in their production processes, but Changting paper was chemically bleached during the production process, which has caused significant damage to the fiber raw materials and therefore it has a lower degree of polymerization. The same situation also applies to Liancheng paper; Fuyang paper has a low degree of polymerization due to the use of water immersion fermentation, which results in a high residual microbial content in the paper which adversely affects its long-term stability.

Figure 5:

The change of degree of polymerization during dry heat aging of paper samples.

Furthermore, the degree of polymerization of all paper samples has significantly decreased after aging. Overall, multiple cooking, strong alkali cooking, and bleaching contributed to the decrease in paper polymerization.

The pH level significantly affects the degree of polymerization in paper. During the aging process of paper, H⁺ ions can break the glycosidic bonds between cellulose glucose units, but they are not consumed. Instead, they accumulate over time, leading to a decrease in the degree of polymerization of the paper. This conclusion can be drawn by comparing the changes in the degree of polymerization of the paper samples in Figure 5 with the changes in pH in Figure 4.

The degree of polymerization is the number of glucose units that make up the cellulose molecule. The higher the degree of polymerization, the longer the cellulose molecular chain and the longer the fiber. Therefore, the degree of polymerization has a strong influence on the mechanical properties of paper. Combining Figure 5 with Figure 3, it can be seen that as the degree of polymerization of each paper sample decreases, the corresponding paper sample folding endurance also decreases.

3.5 Optical Properties

The whiteness (D65 brightness) of paper refers to the ability of paper to fully reflect light after being illuminated, expressed as a percentage. The whiteness of paper is greatly affected by its lignin content. Lignin is a high-molecular-weight polymer formed by the interconnection of three phenylpropanoid units through ether and carbon-carbon bonds, forming a three-dimensional network structure. There are active groups such as aromatic groups, phenolic hydroxyl groups, and alcohol hydroxyl groups in the molecular structure of lignin. These groups are chemically active and prone to oxidation, reduction, hydrolysis, and other reactions that damage the fibers. In addition, lignin contains chromophore groups such as cyclic conjugated groups, i.e., cinnamaldehyde groups and cinnamic acid, as well as carbonyl structures such as α-carbonyl groups (Hon and Glasser 1979). Therefore, generally, the higher the lignin content of paper, the lower its whiteness.

The initial whiteness of Liancheng (62.57 %) and Panxian paper (46.37 %) is higher. This is because Panxian paper uses multiple cooking processes, while Liancheng paper is processed via hot digestion method and undergoes natural and chemical bleaching, removing most of the lignin in the raw material, resulting in a low lignin content and high whiteness. After caustic soda cooking, the lignin in Changting paper2 (41.72 %) is partially removed, resulting in a higher whiteness than Changting paper1 (37.44 %). Although Fuyang paper (29.46 %) has undergone cooking processes, the lime cooking process is a weak alkali and the time is short, so the lignin is not effectively removed. During the subsequent long fermentation process, new chromogenic groups are introduced, resulting in a low whiteness. Although Renhua paper (45.34 %) has not undergone cooking and bleaching, it can remove some lignin during the long lime immersion process, resulting in a moderate whiteness.

Figure 6 shows the changes in color difference after dry heat aging of paper samples. As the aging time increases, the color difference value ΔE of all papers gradually increases.

Figure 6:

Color difference (ΔE) of the paper samples after up to 60 days of dry heat aging.

The color aging resistance of unbleached hot digestion paper is the best. For example, Changting paper2 has a ΔE value of only 1.59 after 60 days of aging, and no visible color change has occurred. Panxian paper also has no visible color change after 40 days of aging, with a ΔE value of 1.34. It is observed that color change occurs after 50 days of aging. After bleaching, the ΔE value of Liancheng paper is greater than 2 after 20 days of aging, because the initial whiteness of paper is high, and a small amount of lignin oxidation can cause color changes during the aging process. The cold digestion Renhua paper and Changting paper1 have a ΔE value greater than 2 after 10 days of aging, because these two types of paper have not undergone cooking and bleaching treatment, and have a high lignin content, which is most likely to turn yellow during the aging process. In general, the lignin content has a great influence on the ΔE value during the aging process of paper samples, and paper samples with high lignin content have a large ΔE value after aging. Although the lignin content of Fuyang paper is higher, it is lower than that of Renhua paper and Changting paper1, and the initial whiteness value is the lowest, so the degree of its color change is not as obvious as that of other papers.

3.6 Chemical Structure Analysis with FTIR Spectroscopy

The absorption peak at 3,330 cm⁻¹ in the infrared spectra is attributed to the stretching vibration of hydrogen bonds OH groups (Senthamaraikannan and Kathiresan 2018; Hajji et al. 2016). As the aging time increases, the intensity of the absorption peak at 3,330 cm⁻¹ gradually decreases in the infrared spectra of Changting paper1, Changting paper2, Fuyang paper, and Liancheng paper (Figure 7). The absorption peak at 2,890 cm⁻¹ is attributed to the stretching vibration of methyl and methylene C–H groups in cellulose and hemicellulose (Reddy et al. 2018). With increasing aging time, the intensity of the absorption peak in the infrared spectra at this position in Changting paper1, Changting paper2, Fuyang paper, and Liancheng paper decreases indicating that the content of –CH₃ and –CH₂ in the paper samples decreases. After aging for 60 days, new absorption peaks appear at 1,725 cm⁻¹ in Panxian paper and Renhua paper, which are attributed to the stretching vibration of non-conjugated C=O in the acetyl group of hemicellulose in the paper samples, indicating that the cellulose in these two paper samples oxidizes and generates C=O groups after aging (Anupama et al. 2010; Sepe et al. 2018). The absorption peak at 1,642 cm⁻¹ is the bending vibration of adsorbed water –OH in the paper samples (Alemdar and Sain 2008). The absorption peak at 1,596 cm⁻¹ is the vibration of the aromatic ring skeleton of lignin, the absorption peak at 1,508 cm⁻¹ is the stretching vibration of C=C in the aromatic ring of lignin, and the absorption peak at 1,455 cm⁻¹ is the –C–H bending vibration of lignin (Sain and Panthapulakkal 2006). The absorption peak at 1,425 cm⁻¹ is the cellulose –CH₂ bending vibration, and the absorption peak at 1,366 cm⁻¹ is the cellulose –C–H bending vibration (Sun et al. 2000). The absorption peak at 1,315 cm⁻¹ is the bending vibration of cellulose –CH₂, which is related to the content of crystalline cellulose type I (Colom and Carrillo 2002). For details of the infrared spectra in the range of 2,000–800 cm⁻¹ please refer to Figure S2.

Figure 7:

ATR-FTIR spectra of paper samples. (a) Renhua paper, (b) Changting paper1, (c) Changting paper2, (d) Panxian paper, (e) Fuyang paper, (f) Liancheng paper.

3.7 Determination of Crystallinity Index with X-Ray Diffraction

The cellulose structure is a crucial factor determining the properties of paper, it consists of two types of aggregate structures: crystalline regions and amorphous regions (Nurain et al. 2012). The relative content of the crystalline regions of cellulose is usually represented by the crystallinity index (CrI) (Kumar et al. 2009). Figure S3 shows the X-ray diffraction patterns of various paper samples before and after aging for different periods of time. It can be seen from the figure that all samples exhibit significant diffraction peaks around 2θ = 22.5° and 2θ = 16°, which are attributed to the 200 and 110 crystal planes of cellulose I (Klemm et al. 2005). Therefore, it can be considered that the crystalline structure of cellulose in bamboo paper is cellulose I.

The CrI of the paper samples was calculated using the Segal formula, and the results are shown in Table 4. Overall, the CrI of the hot digestion method paper samples (Changting paper2, Panxian paper, and Liancheng paper) is higher than that of the cold digestion method paper samples (Renhua paper and Changting paper1). This is because the hot digestion method removes some hemicellulose and lignin from the raw material through alkaline cooking, resulting in higher CrI. However, the CrI of Fuyang paper and Renhua paper is similar, because the main purpose of using the hot digestion method for Fuyang paper is to shorten the pulping time. Moreover, the lime soaking time in the early stage is only 14 days, which is much shorter than the cold digestion method paper samples. Therefore, in terms of paper texture, Fuyang paper is inferior to the cold digestion method paper samples. This is also reflected in the fact that the CrI of Fuyang paper is lower than that of other hot digestion method paper samples, but is close to Renhua paper made by the cold digestion method.

Panxian paper, which has the highest initial CrI, shows a downward trend in CrI during the subsequent aging process. During aging, the CrI of Chanting paper1 shows a trend of first increasing after 30 days of aging and then decreasing, while the CrI of all other bamboo papers show a trend of first increasing after 10 days of aging and then decreasing; this result is consistent with the study of Hajji (Hajji et al. 2016). This is because in the early stage of aging, hydrolysis preferentially occurs in the amorphous region, while the crystalline region is densely and neatly arranged, which is more stable to chemical erosion. Since CrI is a ratio, its increase expresses a loss in amorphous regions which are attacked and decrease. The crystalline regions remain unchanged, the amorphous regions become less and therefore, the CrI increases (Liu et al. 2020; Zhao et al. 2007). In the later stage of aging, the degree of damage to the cellulose crystalline region increases, and the crystallinity index decreases, corresponding to the decrease in the absorption peak intensity at 1,315 cm⁻¹ in the infrared spectrum after 60 days of aging (Figure S2).

3.8 Observations in Scanning Electron Microscope Images

Figure 9 and Figure S4 show the scanning electron microscope micrographs of six types of bamboo paper before and after aging. The internal structure of the paper presents a complex network structure before aging, with tightly connected cellulose fibers arranged randomly. The fiber surface is very smooth, and white particles can be seen attached to the fiber surface. In addition, it can be observed that there are white particles attached to the surface of the fiber. Taking Panxian paper as an example, the points with white particles and the points without white particles in the SEM image were selected for EDS element analysis. EDS element analysis showed that the atomic concentration of Ca element at the white particle point was 22.125 %, and Ca element was not detected at the other point (Figure 8 and Table 5). EDS element analysis showed that these white particles are lime particles left over from the lime pickling and cooking process used as raw material.

Figure 8:

EDS element analysis. The sampling position is the position of the blue target in the picture. The white arrow points at the selected white particle visible in the (b) picture, and similarly in picture (a) the white arrow points at the selected area without white particles; (a) and (b) are the same picture.

Figure 9:

Scanning electron microscope images of paper samples. (a) Fuyang paper,(b) Fuyang paper aging for 60 days,(c) Changting paper2,(d) Changting paper2 aging for 60 days,(e) Panxian paper,(f) Panxian paper aging for 60 days.

From the scanning electron microscope image, it can be seen that the fiber morphology of the paper samples has undergone significant changes after aging. The four paper samples of Changting paper1, Fuyang paper, Liancheng paper, and Renhua paper showed severe fiber peeling, with the fiber surface becoming rough and uneven, peeling off from each other, showing a filamentous and flaky structure. As the dry heat aging time increases, even severe holes and fiber breakage appear after 60 days. However, after 60 days of aging, the fiber surface of Changting paper2 and Panxian paper only showed slight peeling, and no fiber breakage occurred. The above results indicate that the fibers of the paper samples have undergone a certain degree of oxidative degradation, which explains why the mechanical properties of Changting paper1, Fuyang paper, Liancheng paper, and Renhua paper decrease significantly after dry heat aging, while the mechanical properties of Changting paper2 and Panxian paper decrease less.