Zhiqiang Lia,
Hongjie Zhang*ab,
Xin Wanga,
Fengshan Zhangb and
Xiaoliang Lib
aTianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, P. R. China. E-mail: hongjiezhang@tust.edu.cn; Fax: +86-22-6060-2510; Tel: +86-22-6060-1988
bShandong Huatai Paper Industry Co. Ltd., Huatai Group, Dongying 257335, P. R. China
First published on 31st October 2016
Lignocellulosic fiber has been increasingly used in many fields. The properties of fiber-based materials are affected significantly by the lignin content in lignocellulosic fibers. In this study, lignocellulosic fibers with different lignin contents were investigated by using the wet fiber deformation behavior and the related response mechanism of lignin content to the inter-fiber bonding properties, and other properties were discussed for improving the high-value applications of lignocellulosic fibers. The results showed the deformation behavior of wet lignocellulosic fibers, including wet fiber flexibility and collapsibility (aspect ratio) which increased from 0.516 × 1012 to 5.454 × 1012 N−1 m−2 and 1.616 to 3.652, respectively, when the lignin content decreased from 24.28% to 2.67%. As a result, the inter-fiber bonding properties of lignocellulosic fibers were enhanced. For instance, the relative bonded area increased from 15.64% to 43.76% and the bonding strength index increased from 3.597 N m g−1 to 84.065 N m g−1 with the increase in fiber deformability. Consequently, a more compact fiber network could be formed, showing a significant decrease in the bulk property. Therefore, the contradiction between physical strength and bulk properties of the fiber network could be further revealed by the wet fiber deformation behavior, which was influenced significantly by the lignin content in lignocellulosic fibers.
Lignocellulosic fiber material, which has a cylinder-like structure with a hollow lumen and a solid fiber cell wall, is essentially composed of cellulose, hemicellulose and lignin. Generally, cellulose is mainly organized in thin crystalline microfibrils which are the backbone of the fiber cell wall through hydrogen bonds, while hemicellulose and lignin serve as a special “filler” or “adhesive” filling between microfibrils,9,10 as shown in Fig. 1. Wet fiber deformability is one of the most significant fiber properties, which has important effects on fiber-based end-uses. This refers to the ability of lignocellulosic fibers to deform one another in the fiber network, which makes better contacts between fibers.5,11 Due to their special structure and chemical compositions, the lignocellulosic fibers will deform during the formation of the fiber matrix, generally showing in two ways, fiber bending and lumen collapse. Therefore, the wet fiber deformability is usually characterized by wet fiber flexibility and collapsibility.12 All forms of fiber deformability are interrelated and dependent mainly on fiber morphology properties (fiber cell wall thickness and lumen area), the fiber cell wall structure and its chemical composition. In general, it is easier for the fibers with relatively smaller ratio of fiber cell wall thickness to lumen width, smaller microfibril angle of the secondary layer and more carbohydrate content to be deformed. Therefore, the chemical composition of lignocellulosic fibers plays an important role in fiber deformation behavior. In particular, the presence of lignin will limit the deformability of fibers significantly, because it fills the gaps between microfibrils and makes them “stick” together.12 In addition, the presence of hemicellulose, such as xylan, is able to take up water, resulting in an improvement in the swelling capability of fibers, which will have a positive effect on the deformability of fibers. During the chemical separation and bleaching processes of lignocellulosic fibers, there is almost a delignification reaction, somewhat lowering the amount of hemicellulose, and the lignin content will decrease significantly, which contributes to a loose cell wall structure and the fibers (mainly cellulose) become more deformable. Compared with mechanical pulp fibers, which have a higher lignin content, chemical pulp fibers deform more easily during the process of fiber matrix formation.13
It is traditionally considered that wet fiber deformability has significant effects on each physical and optical property of fiber-based materials; no matter whether the fiber raw material is softwood or hardwood because of their similar cylinder-like structures and chemical composition. More deformable fibers respond better to Campbell's forces and the fiber network becomes more compact with a greater number and area of the inter-fiber bonds,12,14,15 and hence a decrease in light scattering coefficient could be observed. Steadman and Luner16 reported that the apparent density of the fiber network increased as the fiber flexibility increased, and further suggested that for a particularly fines-free pulp, the average fiber flexibility could be measured by using sheet apparent density. Recently, Tao17 developed a method to determine the relative bonded area of the fiber network by using apparent sheet density and flexibility, which further revealed the relationship between fiber deformability and apparent density of the fiber matrix. The fiber deformability is also important to the tensile strength of paper sheets. Paavilainen12 reported that the tensile strength property was enhanced with an increase in fiber flexibility. Paavilainen attributed this to an increase in the number and area of inter-fiber bonds, but did not actually determine the inter-fiber bonding area and bonding strength of the fiber network. However, limited literature could be found which used the fiber deformation behavior to investigate the relationship between lignin content and inter-fiber bonding properties of lignocellulosic fibers.
In this study, the response mechanisms of lignin content to the inter-fiber bonding properties of lignocellulosic fibers were studied by using their fiber deformation behavior. The lignin content of unbleached softwood mechanical pulp fibers was varied by sodium chlorite treatment in acid conditions (actually delignification by chlorine dioxide which is the most selective to lignin removal). The wet fiber deformability, including wet fiber flexibility and collapsibility, was analyzed by using widely used methods. The responses of lignin content to wet fiber deformability, and the influences of wet fiber deformability on the inter-fiber bonding and physical properties of the fiber matrix were investigated. The main objective of this study was to give a further understanding of lignin removal for strengthening lignocellulosic fiber-based materials and for weakening the bulk property in terms of fiber deformation behavior, and to further improve the high-value applications of lignocellulosic fibers.
The removal of lignin was performed on 20 g (o.d. weight) of pulp fibers with a pulp consistency of 3% using a 2 L conical flask at 75 °C, pH 3.5–4 in a water bath for an hour, with 1% sodium chlorite added to obtain chlorine dioxide. Then the same amount of sodium chlorite was added for another delignification stage under the same conditions. Different delignification levels of pulp fibers were obtained by applying the treatment stages 2, 4, or 6 times respectively. By using the delignification method of chlorine dioxide, lignin could be removed selectively and carbohydrates received little damage compared with other chlorine-free methods. Then a series of lignocellulosic fiber samples with different lignin contents were obtained.
The fiber morphology characteristics were measured with a Fiber Tester (L&W 921, Sweden), including average fiber length, average fiber width, shape factor, kinks and fines content. The fiber curl was calculated by using the shape factor from eqn (1) as follows:
![]() | (1) |
Water retention value (WRV) measurement was based on the TAPPI method um-256 procedure. Approximately 0.15 g (o.d. weight) of fiber sample at 0.1% consistency was drained onto a 100-mesh screen in a cylinder, which was placed in a centrifuge (3–16 PK, Sigma) at 2500 rpm for 20 min to remove the free water. After centrifuging, the sample was weighed to produce the wet weight, and then it was oven dried at 105 °C for 8 h, and weighed to produce the dry weight. The WRV was calculated from eqn (2) as follows,
![]() | (2) |
![]() | (3) |
The collapsibility of the fibers was defined as the aspect ratio (AR, the ratio of the longest diameter to the shortest diameter of the fiber cross-section), which was determined by a method in accordance with that of Paavilainen.12 The dried sheet sample was embedded in the frozen embedding medium, and sliced to obtain the cross-section of the handsheet by a freezing ultramicrotome (Shandon Cryotome FE/FSE, Thermo Scientific, USA). Then the prepared samples were observed by a scanning electron microscope (SEM, JEOL JSM-IT300, Japan) to obtain cross-sectional images. Only the complete cross-sections of the fibers were used to calculate the collapsibility (AR) with the image processing toolbox in Image-Pro Plus 6.0 (Media Cybernetics, Inc.). Approximately 30 effective data points were used to calculate the wet fiber flexibility and collapsibility (AR), respectively.
![]() | (4) |
![]() | (5) |
The morphology characteristics of lignocellulosic fibers treated to different delignification extents are presented in Table 1. The values of the mean fiber length, width and fines were weighted averages, the kink and coarseness were given directly by a Fiber Tester, and the curl values were determined from the shape factor with eqn (1).23
Lignin content (%) | Length (mm) | Width (μm) | Curl (%) | Kink (mm−1) | Coarseness (μg m−1) | Fines (%) | WRV (%) |
---|---|---|---|---|---|---|---|
24.28 | 2.297 | 40.4 | 10.50 | 0.151 | 257.3 | 0.6 | 135.91 |
13.57 | 2.289 | 39.7 | 12.11 | 0.206 | 245.1 | 0.7 | 143.09 |
7.19 | 2.163 | 39.8 | 15.05 | 0.259 | 217.6 | 0.9 | 171.11 |
2.67 | 2.154 | 39.9 | 14.97 | 0.326 | 191.3 | 1.2 | 219.12 |
With an increase in delignification degree, the total lignin content in the fiber samples decreased from 24.28% to 2.67%, and the fiber length and width were almost unchanged, indicating that the fiber morphology remained almost unchanged during the delignification process with chlorine dioxide in acid conditions. But the curl increased from 10.50% to 14.97% and the kink increased from 0.151 to 0.326 per mm, respectively, which were contrary to the study of B. P. Lin.24 The reason for this is that the fiber we used was produced by hot disintegration prior to delignification, which had already removed the latency (fiber twists, rolls and nodules). And our treatment of fibers with chlorine dioxide was more severe, which caused slight damage to the fibers. The slight rise in fines content could be attributed to the falling of the fiber fragments attached to the fiber surface.
Fiber coarseness is defined as the weight per unit length of the fiber, and it depends on the fiber diameter, cell wall thickness, cell wall density and fiber cross section.25 The morphology of lignocellulosic fibers treated by chlorine dioxide was almost unchanged. However, the coarseness of the treated fibers decreased remarkably from 257.3 μg m−1 to 191.3 μg m−1, which suggested that during the delignification process with chlorine dioxide, the lignin in the fibers dissolved gradually, and the fibers became loose, the density of fiber cell wall decreased and more water molecules could get in between the microfibrils (schematically shown in Fig. 1). This was confirmed by the water retention value (WRV) increasing from 135.91% to 219.12%, as shown in Table 1. These changes in the fibers can have positive effects on the deformability of the fiber wall, and thus the deformability of wet fibers with different lignin contents, as will be discussed in the next section.
The wet fiber flexibility of four treated fiber samples varied with the lignin content, as shown in Fig. 2a. When the total lignin content decreased from 24.28% to 2.67%, the wet fiber flexibility increased from 0.516 × 1012 to 5.454 × 1012 N−1 m−2. The values of wet fiber flexibility we obtained were in the same order of magnitude as those the values from the literature.12,13 As can be seen in Fig. 2a, during the delignification progress, the wet fiber flexibility rose slowly at the beginning, but rose quickly with the continued removal of lignin. This may because the reaction of chlorine dioxide and lignin was from the fiber surface to the interior. For mechanical pulp fibers, much lignin covered the fiber surface. Thus most of the lignin removed first was surface lignin, and then internal lignin of the fiber wall was removed, loosening the fiber wall structure significantly, which resulted in a remarkable increase in wet fiber flexibility.
As shown in Fig. 2a, the collapsibility (AR) had a similar upward trend to the wet fiber flexibility when the total lignin content declined. The reason for the curve rising slowly at first and then quickly was likely due to the wet fiber flexibility that has been discussed above.
Both wet fiber flexibility and collapsibility can quantify the deformability of lignocellulosic fibers. But there were some similarities and differences between them. Firstly, both fiber flexibility and collapsibility were affected by the fiber cross-sectional properties and the elasticity modulus of the fiber wall.12 And the elasticity modulus of the fiber wall is dependent on the fiber cell wall structure and chemical composition.11 Therefore, both the wet fiber flexibility and collapsibility increased and had similar tendencies with the delignification process of the fibers. We assumed that the improvement in fiber deformability during delignification was mainly due to the decrease in the fiber wall elasticity modulus, because the geometry of the fiber cross-section showed almost no change, as shown in Table 1. This is confirmed by the research of Miletzky et al.7 that the elasticity modulus of unbleached and bleached fibers decreased from 31.15 GPa to 7.69 GPa. Secondly, the fiber flexibility mainly emphasizes the bending ability of lignocellulosic fibers in the fiber network, which treats a fiber as a beam subject to pure elastic deformation. However, the collapsibility mainly represents the ability of the fiber lumen to decrease when it suffers significant pressing during the formation of the fiber network.
The relative lignin content and relative deformability were obtained by comparison with the untreated sample, which is presented in Fig. 2b. It can be seen that the relative deformability of fiber flexibility was several times greater than the relative deformability of fiber collapsibility for the same relative lignin content. That was because during the determination of wet fiber flexibility, fibers suffered mechanical pressing, which resulted in fiber collapse. Hence, the deformability of wet fibers characterized by flexibility included some fiber collapse deformability. Therefore, the wet fiber flexibility is more representative to characterize wet fiber deformability.
The inter-fiber bonding strength is generally characterized by the bonding strength index, which is evaluated in terms of fiber morphological characteristics, shear bond strength per unit bonding area and the relative bonded area (RBA), as seen in eqn (5). The bonding strength index was obtained from the physical properties of the handsheets (shown in Table 2). The relative bonded area (RBA) is the ratio of bonded area to the total area available for bonding of the fibers in the fiber network,30 which can characterize the inter-fiber bonding area in the fiber network and contribute to the inter-bonding strength directly. As shown in Fig. 3, the RBA and the bonding strength index increased from 15.64% to 43.76% and from 3.60 to 84.07 N m g−1, respectively, with the wet fiber flexibility increasing from 0.516 × 1012 to 5.454 × 1012 N−1 m−2. With the improvement in wet fiber flexibility, the fibers were more deformable, meaning that fibers intertwined with each other easily and the fiber lumen collapsed readily during the formation of the fiber network. Consequently, the relative bonded area (RBA) increased; there was greater possibility for contacts between fibers, which enhanced the inter-fiber bonding strength. A linear relationship between wet fiber flexibility and the bonding strength index was obtained with a slope of 15.70 and regression coefficient of 0.994. Therefore, the deformability (flexibility) of the fibers had a positive effect on inter-fiber bonding strength, and could be an indicator of the inter-fiber bonding properties of the fiber network.
Lignin content (%) | Light-scattering coefficient (m2 kg−1) | Zero-span tensile index (N m g−1) | Apparent density (cm3 g−1) | Tensile index (N m g−1) |
---|---|---|---|---|
24.28 | 25.03 | 107.48 | 0.197 | 3.47 |
13.57 | 24.38 | 119.21 | 0.282 | 16.56 |
7.19 | 20.67 | 145.17 | 0.401 | 35.86 |
2.67 | 19.97 | 165.83 | 0.547 | 54.10 |
The macroscopic strength property of the lignocellulosic fiber network is usually characterized by the tensile index, which depends mainly on the inter-fiber bonding ability of the fiber matrix,34 while apparent density is a representation of the tightness of fibers in the network. As shown in Table 2, the tensile index increased from 3.47 to 54.10 N m g−1, and the apparent density increased from 0.197 to 0.547 g cm−3 with the lignin content decreasing from 24.28% to 2.67%, respectively. The reason for this was that the delignification of fibers improved the deformability of the lignocellulosic fibers. The increase in fiber deformability led to an increase in inter-fiber bonding strength and a more compact fiber network was formed during the sheet consolidation process, which resulted in an increase in tensile index and apparent density of the fiber matrix.
Bulk, which is the reciprocal of the apparent density of the fiber network, is an important parameter with which to characterize the loose extent of the paper or paperboard products. In general, lignocellulosic fiber material with a higher bonding property and higher bulk value is desired in the papermaking industry, because a higher bonding property can provide good physical strength performance and a higher bulk value can provide good printability and stiffness.31 However, for lignocellulosic fiber material, the increase in the physical strength property usually results in a decrease in the bulk value. The contradiction between the two aspects during the delignification progress could be explained by the changing fiber deformability. With lignin removal, fibers were more deformable and bonding more compact, which increased the tensile index and decreased the bulk property of the fiber network, as shown in Fig. 4. With the increase in wet fiber flexibility from 0.516 × 1012 to 5.45 × 1012 N−1 m−2, the tensile index increased from 3.47 to 54.10 N m g−1 and the bulk decreased from 5.08 to 1.83 cm3 g−1. Therefore, in view of future research to balance the decrease in bulk and increase in physical strength of the fiber network, the deformation behavior of lignocellulosic fibers should receive more attention.
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