Further understanding the response mechanism of lignin content to bonding properties of lignocellulosic fibers by their deformation behavior

Abstract

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.

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1 Introduction

In recent years, lignocellulosic fiber derived from abundant plant resources has been paid more attention in many application fields for its many inherent advantages, such as low specific gravity, low cost, recyclability and biodegradability.^1–4 The properties of lignocellulosic fiber will limit its application ranges and proportions and, in particular, they will have a direct effect on the properties of fiber-containing end-uses. For example, for high-value applications in biocomposites, the lignocellulosic fibers can be used to substitute some synthetic fibers for structural reinforcement in biocomposites; and for the papermaking industry, one of the biggest fiber-consuming industries, the inter-fiber bonding strength and bulk properties of lignin-rich fibers are quite important for the properties of paperboard products. Much work has been conducted in characterizing the properties of fibers and improving the properties of lignocellulosic fiber-based materials.^5–8 However, there was a limited literature taking an in-depth look at the response mechanism of lignin content in lignocellulosic fibers to their inter-fiber bonding properties by using fiber deformation behavior theory.

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

Fig. 1 Schematic diagram of lignocellulosic fiber structures with different lignin contents. (a) Fiber with a high lignin content and compact structure; (b) fiber with a low lignin content and loose structure.

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.

2 Experimental

2.1 Materials

An unbleached, never-dried pine thermo-mechanical pulp (TMP) was used, provided by a commercial pulp mill in China. TMP fibers, as a kind of mechanical pulp, are lignin-enriched fibers and closer to natural fibers, compared with other kinds of fiber materials. Therefore, lignocellulosic fibers with different lignin contents from mechanical pulp fibers could be obtained by different treatment levels to meet the experimental requirements. During the production process of TMP, the extractives and hemicellulose of pine fibers were almost removed, and two major compositions (cellulose and lignin) were maintained. Thus, the effects of hemicellulose and extractives of lignocellulosic fibers on their properties were not considered in this study, and only the effects of lignin content were discussed. The TMP fiber was produced with hot disintegration (800-1, Lab tech, Canada) according to TAPPI standard method T262 sp-96 (1996) to remove the latency (fiber twists, rolls and modules), prior to fractionating the pulp fibers by using a Bauer-McNett fiber classifier (TMI 8901-5, USA) according to TAPPI standard method T233 cm-95 (1995), and the fiber fraction R30 was used as the raw material in this study.

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.

2.2 Fiber characterization

The total lignin content of each lignocellulosic fiber sample was determined as the sum of Klason lignin (acid-insoluble lignin) content and acid-soluble lignin content in accordance with TAPPI standard methods T222 om-02 and T13 wd-74, respectively.

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:

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 Determination of wet fiber deformability

The wet fiber flexibility was measured with the method developed by Steadman and Luner.¹⁶ Pulp fibers (0.1 g o.d. weight) were suspended to 0.01% consistency and drained onto a piece of filter cloth (200-mesh) by a TAPPI standard handsheet former (Lab Tech, Canada). Then the filter cloth with a very thin fiber sheet covered with two press felts was placed onto six glass sides which had been fixed with stainless steel wires (diameter, 35 μm) in advance and then it was pressed at 340 kPa by a standard handsheet press (no. 2571-1, KRK, Japan) for 5 min. After pressing, the slides were removed and air-dried at room temperature for 8 h. The dried fibers were imaged by using a light optical microscope with a CCD camera, and only the fibers almost perpendicular to the wires were imaged. The free span length (the length of fibers not in contact with the slide) and the fiber width were measured from the fiber images, using the software accompanying the instrument. The wet fiber flexibility was calculated from eqn (3) as follows,where d is the deflection height (wire diameter, 35 μm) (m); L is the free span length (m); and q is the pressing load, N m⁻¹, calculated from the pressure and the fiber width (m).

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.

2.4 Handsheet preparation and testing

The handsheets with a basic weight of 60 g m⁻² were made from lignocellulosic fibers with different lignin contents by using a laboratory sheet former (Lab Tech, Canada) and an automatic sheet press (no. 2571-1, KRK, Japan), according to TAPPI standard method T205 sp-95. The properties of the fiber network, including light-scattering coefficient, tensile index and zero-span tensile index were tested by an Elrepho Spectrophotometer (L&W SE070, Sweden), a tensile tester (L&W 004, Sweden) and a Z-span tester (PULMAC 2400, USA), respectively, in accordance with TAPPI standard method T220 sp-96.

2.5 Relative bonded area of fiber network

The relative bonded area (RBA) of the fiber network (the handsheets) was determined based on the nitrogen absorption method.^18,19 Briefly, lignocellulosic fibers (0.3 g o.d. weight) were suspended in 100 mL distilled water, and then the water was replaced by ethanol. After that, the fibers were vacuum dried for 24 h and then the dried fibers and the sheets were heated at 105 °C for 8 h before measurement. The specific surface areas of the fibers and the sheets were measured by a physical and chemical adsorption instrument (NOVA3200e, Quantachrome, USA). The RBA was calculated from eqn (4) as follows,where RBA is the relative bonded area (%), S_t is the specific surface area of dry fibers (g m⁻²), and S_u is the specific surface area of the sheets (g m⁻²).

2.6 Bonding strength index of fiber network

Eqn (5), developed from the Page equation,²⁰ was used to determine the bonding strength index of the fiber network, as follows,where B is the bonding strength index of the fiber network (N m g⁻¹), T is the tensile index of the handsheet (N m g⁻¹), Z is the zero-span tensile index (N m g⁻¹), P is the perimeter of the fiber cross-section (m), L is the fiber length (m), c is the fiber coarseness (g/100 m fiber), b is the specific bonding strength per unit bonded area (N m⁻²), and RBA is the relative bonded area.

3 Results and discussion

3.1 Morphology analysis of different lignocellulosic fibers

As an “adhesive” filling in the lignocellulosic materials, lignin can be removed during the chemical separation and bleaching processes, usually accompanied by the degradation of carbohydrates, which causes damage to the fibers. In this study, chlorine dioxide was chosen to treat the lignocellulosic fiber samples, since it has the most selectivity to lignin removal and has only a slight effect on the degradation of carbohydrates.^21,22

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

Table 1 Characteristics of lignocellulosic fibers with different lignin contents

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

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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.

3.2 Responses of lignin content to wet fiber deformability

The deformability of wet lignocellulosic fibers is one of the most fundamental fiber properties, which characterizes the ability of pulp fibers to deform one another in the fiber network.⁵ The delignification progress loosened the fiber wall structure, and what happened to the fiber cell wall changed the wet fiber deformability. We used wet fiber flexibility and collapsibility to quantify the deformability of wet fibers.

3.2.1 Wet fiber flexibility. The flexibility of wet lignocellulosic fibers is defined as the reciprocal of the product of the elastic modulus (E) and the moment of inertia (I) of a single fiber, which is based on treating the fiber as a beam subject to pure elastic deformation.^14,26 The value of wet fiber flexibility can be an indicator of its deformability. Fibers with high flexibility tend to be easier to bend in the fiber network, showing a good deformability. Several methods have been reported to measure the flexibility of wet fibers.^{13,16,26–28} In this study, Steadman and Luner's method was used to determine wet fiber flexibility, which was considered to be the most convenient method and the most similar to the way that fibers behave in a real fiber network.¹²

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.

Fig. 2 Response of lignin content to wet fiber deformability. (a) Wet fiber flexibility and collapsibility (AR); (b) relative deformability.

3.2.2 Collapsibility. During the formation of the fiber network, fibers will be subjected to pressing, which leads to the part or full collapse behavior of fibers. Collapsibility was used to characterize this type of fiber deformation. The extent of fiber collapse was determined by the aspect ratio (the ratio of the longest diameter to the shortest diameter of the fiber cross-section).^13,29

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.

3.3 Influence of fiber deformability on inter-fiber bonding strength

The interaction among lignocellulosic fibers during dewatering of the fiber network is significantly affected by the fibers' morphological characteristics. The removal of lignin loosened the fiber cell wall structure, so more water molecules could go inside between the microfibrils. And thus the wet fiber deformability increased, which had a substantial influence on the inter-fiber bonding properties of lignocellulosic fiber-based materials, as shown in Fig. 3.

Fig. 3 Effect of wet fiber deformability on inter-fiber bonding properties.

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.

Table 2 Physical properties of handsheets with different lignin contents

Lignin content (%)	Light-scattering coefficient (m^2 kg^−1)	Zero-span tensile index (N m g^−1)	Apparent density (cm^3 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

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3.4 Influence of lignin content on physical properties of lignocellulosic fiber material

Lignocellulosic fiber materials usually have poor physical strength properties with the presence of lignin in the fibers.^31,32 Physical strength can be enhanced by the removal of lignin, because fibers with lower lignin content are more deformable, resulting in an increase in inter-fiber bonding properties, as discussed above. The physical properties of handsheets with different lignin contents are listed in Table 2. As can be seen in Table 2, the treatment for reducing the fiber lignin content tended to increase the tensile index and zero-span tensile index, as well as the apparent density, and to decrease the light scattering coefficient of the handsheets. The reason for the reduction in the light scattering coefficient obtained by delignification was the increase in inter-fiber bonding ability³³ resulting from the rise in fiber deformability, as discussed above, and as seen in Fig. 3. The increase in zero-span tensile index was mainly due to the increase in the inter-fiber bonding strength.

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.

Fig. 4 Effect of fiber deformability on tensile index and bulk property of handsheets.

4 Conclusions

Delignifying the lignocellulosic fibers by sodium chlorite in acidic conditions could loosen the fiber cell wall significantly and maintain fiber morphology, such as a decreased coarseness and an increased water retention value, with almost the same fiber length and width. A decrease in the lignin content of lignocellulosic fibers from 24.28% to 2.67% contributed to a variation in wet fiber deformability, including the wet fiber flexibility which increased from 0.516 × 10¹² to 5.454 × 10¹² N⁻¹ m⁻² and collapsibility (aspect ratio) which increased from 1.616 to 3.652, which resulted in the inter-fiber bonding strength index of the fiber network increasing from 3.597 N m g⁻¹ to 84.065 N m g⁻¹ because it promoted an improvement in the relative bonded area of the fiber matrix, and thus a more compact fiber network was formed. This was due to an increase in wet fiber deformability caused by a decrease in lignin content leading to an increase in the tensile strength and a decrease in the bulk property. Hence, the tensile strength and the bulk property of a fiber network could be balanced to some degree by adjusting the wet fiber deformability.

Acknowledgements

The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 31670588; 31370577).

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