Preserving Cellulose Structure: Delignified Wood Fibers for Paper

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Preserving Cellulose Structure – Delignified Wood Fibers for Paper Structures of High Strength and Transparency Xuan Yang, fredrik berthold, and Lars A. Berglund Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00585 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Preserving Cellulose Structure – Delignified Wood Fibers for Paper Structures of High Strength and Transparency Xuan Yang†, Fredrik Berthold‡, and Lars A. Berglund†* †

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

‡

RISE - Research Institutes of Sweden, Mäster Samuelsgatan 60, SE-11121 Stockholm, Sweden

KEYWORDS. pulp, holocellulose fiber, hemicellulose, peracetic acid, hot-pressing, mechanical strength, optical transparency

ABSTRACT.

To expand the use of renewable materials, paper products with superior

mechanical and optical properties are needed. Although beating, bleaching and additives are known to improve industrially produced Kraft pulp papers, properties are limited by the quality of the fibers. While the use of nanocellulose has been shown to significantly increase paper properties, the current cost associated with their production has limited their industrial relevance. Here, using a simple mild peracetic acid (PAA) delignification process on spruce, we produce hemicellulose-rich holocellulose fibers (28.8 wt. %) with high intrinsic strength (1200 MPa for fibers with microfibrillar angle smaller than 6°). We show that PAA treatment causes less

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cellulose/hemicellulose degradation and better preserves cellulose nanostructure in comparison to conventional Kraft pulping. High-density holocellulose papers with superior mechanical properties (Young’s modulus of 18 GPa and ultimate strength of 195 MPa) are manufactured using a water-based hot-pressing process, without the use of beating or additives. We propose that the preserved hemicelluloses act as “glue” in the interfiber region, improving both mechanical and optical properties of papers.

Holocellulose fibers may be affordable and

applicable candidates for making special paper/composites where high mechanical performance and/or optical transmittance are of interest.

INTRODUCTION The problem of waste in the form of synthetic polymers is enormous.1,2 Out of the 8.3 billion metric tons of plastics produced since the 1950’s, it is estimated that as much as 6,6 billion metric tons are still present in the form of deposits or as waste in nature.3 Since the consequences of this are largely unknown, such a gigantic experiment is not acceptable in a sustainable society. Cellulosic materials are of obvious interest as replacement for non-biodegradable plastics, since cellulose is obtained from biobased resources in the form of plants, and is both biodegradable and recyclable in unmodified form.4–6 Nanocellulose is a “new” form of cellulose and a very promising component for new materials,7–10 for instance as strong, transparent, gas-barrier films11,12 or as reinforcement in biocomposites with properties matching those of semi-structural polymer composites.13 Still, there are significant processing challenges associated with nanoscale fibrils. The viscosity of hydrocolloidal cellulose nanofibril (CNF) suspensions reaches

high levels at

low

concentrations14 and gelation may take place at only 0.125% CNF content.10 In the present


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study, the potential to use nanostructurally tailored wood fibers is investigated as an alternative to wood CNF materials. Wood fibers may have advantages to CNF nanocellulose from a cost and processing point of view. There are already large-scale industrial processes for cellulosic wood fiber materials. Wood fibers subjected to the industrially important Kraft process,15 are delignified and the native biopolymer

constituents

including

cellulose,

lignin

(small

remaining

amount)

and

hemicelluloses,16–18 are thermochemically and mechanically degraded. In addition, during drying, much of the native nanostructure in the wood fiber cell wall is destroyed due to “hornification”. This is a form of cocrystallization or fusion of cellulose nanofibrils in the wood fiber.19,20 The nanostructurally separated fibrils in the native wood cell wall are agglomerating to form larger entities and the original nanostructure of the wood cell wall becomes more coarse. The hemicellulose phase, which coats and separates individual fibrils, is removed or redistributed so that cellulose aggregates are formed.20,21 The Kraft process for cellulosic wood fibers is designed to disintegrate macroscopic wood chips into discrete fibers.15 Lignin, but also hemicelluloses, are degraded into small, watersoluble molecules that can be easily washed away. The celluloses nanofibrils in the native cell wall are unfortunately also degraded so that average molar mass of the polymer is reduced. The Kraft process removes lignin, as intended, but also removes other carbohydrates, resulting in a low yield.15,22 An additional bleaching step is often required, in order to remove lignin chromophores for the purpose of high-quality white paper. Beating and additive are also used to improve the paper performance,15,23 but properties are limited by the quality of the fibers to a large extent. It is apparent that the Kraft process, even if mechanical fiber damage is not considered, is severely degrading the remaining cellulose and hemicelluloses of the wood fiber.


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In addition, the native nanostructure with ordered cellulose fibrils in an amorphous biopolymer matrix, is significantly altered. Here, the intention is to preserve as much as possible of the nanocomposite nature of the native wood fiber cell wall, while at the same time removing as much lignin as possible. The cellulose fibrils should also be as close to the native state as possible, in order to make full use of their mechanical property potential. Two established, very mild delignification methods are often used in the laboratory, employing either sodium chloriteacetic acid (SC/AA)24,25 or peracetic acid (PAA).26–30 The PAA method has higher selectivity for lignin removal and results in lower reduction of cellulose and hemicellulose contents.25,30 In the present study, the term “holocellulose" is used to describe fibers with almost no lignin but where the cellulose and hemicellulose are largely preserved.31 In previous studies, hemicellulose-rich holocellulose nanofibrils (Holo-CNFs) were obtained through PAA delignification and mechanical disintegration of the wood fibers by a homogenization process.32,33 The Holo-CNFs were unique in terms of their high molar mass and cellulosehemicellulose core-shell structure. Nanopaper films based on these Holo-CNFs have higher optical transmittance and much better mechanical properties compared with most other nanocellulose-based papers. Arola et al. used Kraft pulping to prepare Holo-CNFs that are more stable against flocculation than typical 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized CNFs (TEMPO-CNFs).34 Tanaka et al. obtained Holo-CNFs using a SC/AA treatment.35 These HoloCNFs and their corresponding wet films were less sensitive to electrolyte concentration and pH due to the hemicellulose outer-shell. Gu et al. obtained Holo-CNFs using both an SC/AA treatment and a TEMPO oxidation.36 The resultant Holo-CNFs exhibited higher affinities toward hydrophilic surfaces compared with TEMPO-CNFs.


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PAA-treated holocellulose fibers from wood are highly interesting by themselves as a new type of fibers. While PAA delignification has been studied extensively for biomass treatment, chemical parameter optimization, and use as a bleaching agent,26,27,30 PAA-treated holocellulose fibers do not seem to have been studied at the single fiber level for materials performance purposes. This work is primarily on high-density paper structures of exceptional mechanical properties. In order to separate fiber effects from paper structure effects, the chemical composition, physical appearance, crystallinity, molar mass, charge density, and mechanical properties of PAA-treated holocellulose fibers are also investigated. Compared with Kraft fibers, holocellulose fibers have a better preserved structure and much higher hemicellulose content. Furthermore, ultra-strong, high-density holocellulose paper with a Young’s modulus of 18 GPa and a tensile strength of 195 MPa, was obtained through a simple water-based hot-pressing route. Together with high optical transparency and haze, this high-density holocellulose paper structure is of interest as a material by itself and as reinforcement structure for polymer matrix composites.

EXPERIMENTAL SECTION Holocellulose Fiber Preparation. Holocellulose fibers were obtained using a PAA treatment, adapted with some modifications from a previous work.32 Softwood spruce chips were cut into fine “matchsticks” (~2 mm widths) and soaked in water under vacuum to remove any trapped air. Then the matchsticks were treated with PAA (4 wt. % in water, pH=4.8 before reaction) for 45 min at 85 °C. The chemical loading of PAA was 35 g PAA to 100 g of raw materials (spruce). The reaction vessel (a plastic bottle) was gently agitated every 10 min, with no stirring required. After reaction, the treated matchsticks were drained and treated with a fresh batch of PAA


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solution under the same conditions as the previous PAA treatment. In this way, multiple rounds of PAA treatment were performed until all individual fibers were liberated, which depends on the wood source and size. Kraft Pulp Fiber Preparation. Laboratory unbleached Kraft pulp fibers based on spruce were obtained through a conventional Kraft pulping process (18% EA, 40% sulfidity, 1190 H-factor, 160 °C temperature, and 167 min time), using a pilot-scale forced-flow circulation digester (500 g of wood chips per run). Fibers were separated in a water jet defibrator from NAF (Nordiska Armaturfabriken).37 Unbeaten Kraft fibers, with original fiber structures maintained as intact as possible, were used as reference for the holocellulose fibers. Chemical Composition. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were recorded at room temperature across the frequency range of 4000−600 cm-1 using a Spectrum System 2000 FT-IR spectrometer (Perkin Elmer). Quantitative analysis of different components was conducted as described below. The extractives content was determined by successive extraction of the wood in water first and then in ethanol. The lignin content (Klason lignin) was determined using a TAPPI method (TAPPI T 222 om-02).38 After total hydrolysis, carbohydrate analysis was carried out by determining the monosaccharide content using a Dionex ICS-3000 ion chromatography system (Thermo Fisher Scientific Inc., USA), and the result was listed in Table S1. The detailed Hemicellulose and cellulose contents were then calculated. Glucomannan and xylan are the principle hemicelluloses in spruce. The glucomannan content was calculated based on galactose, glucose, and mannose contents using a 1:3 ratio of glucose:mannose,18,39 while the xylan content was calculated based on xylose and arabinose. X-Ray Diffraction (XRD). XRD measurements were carried out using a PANalytical X'Pert alpha1 Powder Diffractometer (Netherlands) with CuKα radiation generated at 45 kV and 40


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mA. Scans were obtained between 5–50° 2θ in 0.013° steps at a rate of 0.5 second per step. Two different methods were used to calculate the crystallinity index (CI).40 In the first method (Peak Height), CI was calculated from the height ratio between the intensity of the crystalline peak (I200 - IAM) and total intensity (I200). In the second method (Peak Deconvolution), CI was calculated by fitting all individual crystalline peaks and a broad amorphous peak located at approximately 21.5º, until an iteration with an R2 value of 0.997 was reached. Molar Mass. The molar mass was determined using size exclusion chromatography (SEC), as described in literature.41 Samples were activated sufficiently in DI water, and then were subjected to the solvent exchange three times with both methanol and Dimethylacetamide (DMAc). Each sample was added into 8 % LiCl/DMAc. The mixture was stirred gently and left at 4 °C for 5 days. The formed solutions were diluted to the concentration of 0.5 % (wt %). Ultimately, prior to chromatographic characterization, the solutions of cellulose samples were filtered through a 0.45-µm poly (tetrafluorethylene) filter and stored in vials. The SEC system was consisted of a DGU-20A3 degasser (Shimadzu), a LC-20AD liquid chromatography (Shimadzu), a Rheodyne 7725i fixed loop (100 µl) and a RID-10A refractive index detector (Shimadzu). The separation system consisted 23 of a mixed-A 20 µm guard column (7.5 × 50 mm, Polymer Laboratories) and four mixed-A20 µm columns (7.5 × 300 mm, Polymer Laboratories) connected in series. The flow rate was set at 0.5 ml/min. The columns were thermostated at 80 °C and the mobile phase was 0.5%LiCl/DMAc. The linear coefficient of determination (r2) was 0.996 for the curve of pullulan molecular weight versus the elution time. The system and data were controlled and evaluated with LC Solution software (Shimadzu). Preparation of Paper Structures. A water-based two-step hot-pressing procedure was used to prepare the final papers, which is modified from previous studies.42,43 Firstly, wet papersheets (~


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30 wt. %) were prepared from holocellulose suspensions (~0.4 wt. %) using a Finnish Sheet Former. Then each wet papersheet was pressed under 1 MPa at room temperature for 5 min, followed by a second pressing using a 105 °C pre-heated press at 15 MPa for 15 min. A 5 kg weight was placed atop the final holocellulose paper to limit warping during cooling. Kraft paper was obtained through the same procedure starting from Kraft pulp. The density and grammage of the final papers were determined by measuring their thicknesses, dimensions, and masses. Mechanical Properties. The tensile strength and Young’s modulus of a single holocellulose fiber were determined according to the ASTM D3379-75 Standard Test Method.44 Isolated single holocellulose fibers were obtained by freeze drying diluted holocellulose fiber suspension, without any mechanical peeling. Single holocellulose fibers were carefully glued onto a paper frame with a gauge length of 1 mm (Figure S1). Accurate starting gauge length was measured using optical microscopy (Olympus, Japan). Fibers after tensile test were immerged in liquid nitrogen and cut below the fracture surface using razor blade. Then the cross-sectional area (cellwall only excluding lumen space) was measured for every single fiber using by scanning electron microscopy (SEM). The microfibril angle (MFA) of each fiber was obtained by determining the maximum extinction position as angle, when rotating each fiber under polarized microscopy (Leitz Ortholux II, Germany).45 For paper tests, the width and gauge length were set to 5 mm and 25 mm, respectively. Prior to testing, the fibers and papers were conditioned at a relative humidity of 50% and 23 °C for at least 3 days. Mechanical tests were performed using an Instron 5944, which is equipped with a 500 N load cell and a video extensometer, at strain rate of 10% per minute. For the fibers, 80 specimens were tested, and for each type of paper, a minimum of 5 specimens were tested.


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SEM. The nanostructure of the single fibers and holocellulose papers were observed by a field emission scanning electron microscope (FE-SEM, Hitachi S-4300, Japan) after palladium sputtering (30 seconds to give a ca. 5 nm palladium conductive layer; sputter coater from Cressington 208HR, UK). Optical Properties. Brightness of the papers was determined using a uv-vis spectrophotometer (UV-2550, Shimadzu, Japan) at the wavelength of 457 nm, following ISO 2470 standard. Papers based on fully bleached softwood pulps were used as reference samples. These pulps were supplied by Södra (Sweden), and the papers are obtained through the same hot-pressing method described before. Transmittance and haze were measured using an integrating sphere, equipped with a high brightness light source, whose spectrum spanned the UV to near-IR wavelengths (170−2100 nm; EQ-99 from Energetiq Technology Inc.).46,47

RESULTS AND DISCUSSION The processing routes from spruce, to either holocellulose fibers or Kraft fibers and final highdensity paper structures are depicted schematically in Figure 1. Holocellulose fibers were obtained through a mild PAA delignification process, see experimental section. To keep the pH and PAA concentration stable throughout the reaction, multiple rounds of treatment were undertaken until individual fibers were disintegrated. No mechanical stirring was used in order to minimize fiber damage. Neither holocellulose nor Kraft fibers were subjected to beating in order to minimize fiber damage.


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Figure 1. Schematic representation of the processing route from spruce wood to holocellulose fibers or Kraft fibers, and their corresponding high-density paper structures after a two-step pressing process. The scale bar in all photographs is 10 mm.

Chemical Composition and Yield The chemical compositions of the spruce, holocelluose fibers and Kraft fibers are presented in Table 1. Beginning from 100 g of “virgin” spruce sticks, both PAA and Kraft methods removed most of the lignin (1.5 g and 4.5 g remaining after the PAA and Kraft processes, respectively, out of initial lignin content of 21.0 g). Most of the cellulose was preserved (44.3 g for PAA and 45.2 g for Kraft, out of 46.6 g). Significant differences were observed with respect to hemicellulose content. Hemicellulose xylan was largely preserved in both PAA and Kraft methods (5.9 g for PAA and 5.2 g for Kraft, out of initial 6.8 g). Half of the glucomannan was preserved after PAA treatment (12.3 g remaining out of 21.1 g), and much less after Kraft treatment (4.4 g remaining).


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Thus, the higher fiber yield (64%) for the PAA method compared with the Kraft method (59%) is mainly attributed to improved hemicellulose preservation. It is noted that compared with other PAA treatment processes,27–30 the present study uses lower PAA concentration but a larger number of delignification rounds in order to better preserve carbohydrates. In alkaline Kraft pulping, a majority of glucomannan is lost through primary peeling, while xylan is preserved due to “stopping” reactions that proceed via the formation of meta-saccharinic acid end groups to stabilize the chains against further alkaline peeling.22 In the PAA delignification process, acid-form PAA (pH 4.8) mainly acts as an electrophile and will therefore not easily react with the aliphatic hydroxyl groups of most carbohydrates.27 Moreover, the PAA treatment will oxidize the reducing-end groups of carbohydrate polymers to generate carboxylates.27,30 ATR-FTIR spectra (Figure 2) support these changes in chemical composition. Compared with “virgin” spruce, Kraft fiber peaks at 1245 cm-1 (C-O stretching vibration of acyl group in lignin and hemicellulose), 1515 cm-1 (aromatic ring vibrations) and 1730 cm-1 (C=O stretching in hemicellulose)48 decrease or even disappear, while the peak at 1320 cm-1 (C-H deformation in cellulose and hemicellulose) is strengthened. This is due to the removal of lignin and hemicellulose, resulting in an increase in the relative cellulose content. For holocellulose fibers, the peaks at 1245 cm-1 and 1730 cm-1 are still visible after treatment due to better preservation of hemicellulose. The increased intensity of the peak at 1730 cm-1 is due to the newly formed carboxylates from PAA oxidation. The final holocellulose fibers contain lignin, cellulose, and hemicellulose at relative amounts of 2.3%, 69.2%, and 28.8%, respectively. Kraft fibers contain lignin, cellulose, and hemicellulose at contents of 7.6%, 76.2%, and 16.2%, respectively (Table 1). Due to higher lignin content, Kraft fibers were yellowish in color, whereas holocellulose fibers were bright


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white and even glossy in appearance (Figure 1). The hemicellulose content was almost twice as high in holocellulose than in Kraft fibers.

Figure 2. ATR-FTIR spectra of spruce, Kraft fibers and Holocellulose fibers.

Table 1. Yield and chemical composition of holocelluose fibers and Kraft fibers based on spruce. Yield Spruce

-

Holocellulose fibers

64%

Kraft fibers

59%

abs (g)* wt& abs (g)* wt& abs(g)* wt&

Extractives

Lignin

Cellulose

4.6 4.6%

21.0 21.0% 1.5 2.3% 4.5 7.6%

46.6 46.6% 44.3 69.2% 45.2 76.2%

-

Hemicellulose Glucomannan 21.1 21.1% 12.3 19.2% 4.4 7.4%

Xylan 6.8 6.8% 5.9 9.2% 5.2 8.8%

* Absolute value (abs) was the weight of each component from 100 g spruce starting material. & Weight percentage (wt), calculated based on the weight of final material.


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Molar Mass, Crystallinity and Charge Density In addition to improved hemicellulose content preservation, PAA delignification also results in less cellulose degradation compared with Kraft pulping.28,29 As shown in Table 2, holocellulose fibers showed higher weight-average molar mass (Mw) of 1.1 × 106 g/mol compared with Kraft fibers (8.1 × 105 g/mol). Moreover, there are significant differences in the molar mass distributions (Figure 3). The molar mass distribution for holocellulose shows two distinct peaks with the low molar mass peak dominated by hemicellulose. This observation was also reported for PAA treated cellulose nanofibrils.32 An Mw of ~ 2 × 104 g/mol for hemicellulose is similar to Mw determined for hemicellulose extracted from spruce.49 In contrast, the molar mass distribution of Kraft fibers is characterized by a broad peak at a lower Mw range for cellulose and with a significantly smaller peak for hemicellulose. The Kraft degradation of cellulose will shift the peak to lower molecular weight, while the hemicellulose peak is weakened due to degradation and removal of hemicellulose. These results imply that degradation is more limited for both cellulose and hemicellulose during PAA delignification process, compared with Kraft pulping.

Table 2. Molar mass, physical dimensions, and charge density of holocellulose fibers and Kraft fibers. Mw × 106 (g/mol)

Crystallinity Peak Height

Peak Deconvolution

Crystallite size (nm)

Length (mm)*

Diameter (µm)&

Charge density (µeq/g)

Holocellulose fibers

1.1

80%

66%

3.8

3.4 ± 1.0

29 ± 6

270 ± 20

Kraft fibers

0.81

81%

66%

4.5

3.1 ± 0.9

27 ± 6

130 ± 10


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* Fiber lengths measured from photographs taken by DSLR camera. Fiber diameters measured from optical microscope images.

&

Figure 3. SEC molar mass distribution curves of holocellulose and Kraft fibers.

Holocellulose fiber and Kraft fiber samples have nearly identical XRD spectra (Figure 4). Calculated crystallinity results are presented in Table 2. Compared to Kraft fibers, holocellulose fibers have similar crystallinity (66%) although they have lower cellulose content (69.2% compared to 76.2%). This means that cellulose fibrils in holocellulose have higher degree of long-range molecular order, suggesting more cellulose degradation occurs during Kraft pulping. It is noted that the estimated crystallinity of 66% in the holocellulose sample with 69% cellulose content cannot be interpreted as having 95% crystalline cellulose. This is because the peak deconvolution method for amorphous fraction estimation is likely to contain some error, which requires further development for holocellulose fiber analysis.40


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Figure 4. XRD curves of Kraft fibers, holocellulose fibers, and corresponding high-density paper structures.

Figure 4 also shows that the (200) peak for holocellulose fibers is wider than for Kraft fibers. This peak width can be related to the width of the cellulose fibril within the fibers (cellulose crystallite size perpendicular to 002 plane). The cellulose crystallite width of holocellulose fibers and Kraft fibers are calculated to be 3.8 nm and 4.5 nm, respectively.50 It has been suggested that the hemicellulose located in the interfibrillar space will hinder hornification of fibers, in other words, restrict cellulose microfibril agglomeration.19,20 Thus the larger crystallite size of Kraft fibers may be caused by partial hornification during drying, due to lower hemicellulose content. Due to the formation of carboxylates during PAA oxidation,27,30 holocellulose fibers have a charge density of 270 µeq/g, which is roughly twice the charge density of Kraft fibers (130


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µeq/g). The higher charge density of the holocelluose fibers imparts improved colloidal stability, and may reduce the tendency towards fibril agglomeration.

Dimensions and Nanostructure Holocellulose fibers and Kraft fibers have similar lengths and diameter (Table 2). The distribution of lengths and diameters for 200 measurements for each type of fiber are listed in Figure S2. Typical optical and SEM images are presented in Figure 5. As apparent in Figure 5A, holocellulose fibers are straight and rod-like without noticeable kinks or collapsed segments, while Kraft fibers are curved or kinked. Both fibers have not been beaten, and are freeze-dried from never-dried suspension using same procedure. The present laboratory scale Kraft-process involves stronger mechanical treatment, since fibers are separated in a water jet defibrator (see experimental section). SEM images provide a more detailed view of fiber structures. The crosssections of both fibers show a hollow, quasi-cylindrical structure (Figure 5B) with apparent edges, as expected from the cell structure in wood. Both fibers are earlywood tracheids with thin cell walls. Interestingly, the pit structure of the holocellulose fiber is well-preserved, whereas it is collapsed in the Kraft fibers (Figure 5C and 5D). In summary, the data presented indicate that mild PAA treatment under minimal mechanical stirring and agitation results in less fiber damage as compared with lab-scale Kraft pulping.


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Figure 5. Photographic images of fibers (A) and SEM images (B, C and D) of holocellulose fibers and Kraft fibers.

Mechanical Properties of Fibers Determining the mechanical properties of cellulose fibers at a single fiber level is challenging. Isolated fibers with minimum damage must be mounted and tested in a sensitive tensile testing apparatus.51–55 The holocellulose fibers isolated from wood sticks by the PAA treatment can move freely at the single fiber level in aqueous suspension. Freeze drying the diluted suspension, gives access to isolated single holocellulose fibers, which can be fixed to a paper frame (Figure S1). Optical microscopy was used to screen fibers with visible defects. Straight and long fibers were used for tensile testing, with minimum damage to the fibers.52,55 Figure 6 shows the ultimate tensile strength of single holocellulose fibers, and their stress strain curves are shown in Figure S3. It is known that tree selections, earlywood/latewood variations, and microfibril angle (MFA) all impact the tested tensile strength for wood single


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fibers.55–57 In present study, we pick MFA as the classification factor. As MFA decreased from 40° to below 10°, the ultimate tensile stress of the holocellulose fibers increased from 260 MPa to 1200 MPa. The average tensile strength as high as 1200 MPa for the six samples of low microfibril angle is a highly notable result. Great care was taken to determine true cross-sectional area of the wood fiber cell wall. The reported values are similar to typical values for single flax, hemp or ramie fibers with low microfibril angle. Fibers with low MFAs are stiffer due to higher load carried by the more favorably oriented cellulose fibrils.51,55 The majority (75%) of the holocellulose fibers had MFAs in the range of 10–25°, with a corresponding tensile strength range of 670–890 MPa. This is in the higher range among reported results for single softwood pulp fibers (Figure 6).55,56,58–60 It is noted that variation in ultimate tensile strength within each MFA group is due to the limited testing sample size (80 samples, with detailed information in Table S2),57 without the sub-classification of other parameters like earlywood/latewood variation. Previous studies of PAA delignification have mainly focused on the pulping process, delignification efficiency, and corresponding paper properties,25,27–30 whereas the current study is the first to investigate mechanical properties at the single fiber level.


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Figure 6. Ultimate tensile stress of single softwood fibers from present work and literatures. Standard deviations are reported as error bars.

Paper Preparation As mentioned, holocellulose fibers have higher hemicellulose content and well-preserved structure, compared to the Kraft pulp fiber reference. It is therefore of interest to investigate fiber effects on the properties of corresponding high-density paper structures. Figure 1 shows the preparation of holocellulose paper using a water-based two-step hot-pressing procedure. Note that the resulting paper structures essentially have random-in-the-plane fiber orientation, and have higher density (lower porosity) than conventional paper or fiber-board.

Mechanical Properties of Papers Figure 7A shows representative stress-strain curves of high-density holocellulose and Kraft paper, and mechanical properties are presented in Table 3. Since both materials fail at similar strains (~2%), it is possible that ultimate fracture is strain-controlled. If the fiber cell wall is


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assumed to have a density of 1500 kg/m3, the densities in Table 4 correspond to porosities of 21% for holocellulose paper and 27% for Kraft paper. Holocellulose paper showed a yield strength of 79 MPa, significantly higher than for Kraft paper (43 MPa). The yield strength in paper fiber networks has been associated with the debonding of individual fibers at fiber-fiber bond sites.61 Holocellulose fibers may have improved fiber-fiber bond properties, due to the high content of hemicellulose, which may act as a bonding agent between fibers. For instance, Wågberg et al have showed that addition of polyelectrolytes as fiber-fiber bonding agents may led to substantial improvements in paper strength.62,63 In nanopaper based on holocellulose CNFs, mechanical properties are also improved compared with low hemicellulose content native CNFs.32 This is possibly also related to strong hemicellulose bonding between CNF nanofibrils and better intrinsic CNF strength properties. The fracture surfaces in Figure 7B offer more information. Firstly, holocellulose fibers are more collapsed (resemble like tagliatelle pasta) than Kraft fibers, which may give better interfiber bonding.15 Secondly, the fracture surface of holocellulose paper shows a clean edge of fractured fibers with short pull-out lengths, while the fracture surface of the Kraft paper shows much longer fibers, in support of long pull-out lengths associated with weaker fiber-fiber bonding. The improved interfiber bonding in holocellulose paper results in better stress transfer between fibers, this leads to higher stress levels and a larger extent of fiber fractures. Holocellulose paper has a Young’s modulus of 18 GPa and an ultimate tensile strength of 195 MPa. Both numbers are roughly twice the values for Kraft fiber paper. One may also note that these numbers are extremely high for wood fiber networks, and are approaching the data for first generation cellulose nanopaper.12 We propose that high intrinsic fiber strength (see fiber strength data, molar mass data and crystallinity data) and hemicellulose-improved fiber-fiber bonding in


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holocellulose paper, contribute to the higher modulus and ultimate tensile strength of the holocellulose papers. Favorable fiber-fiber bonding improves load transfer and explain the higher stress level at a given strain, and explains the high modulus of holocellulose paper. The effect of hornification in the Kraft fiber material cannot be excluded, since hornification reduces the fiberfiber bonding potential, which leads to lower tensile strength.64,65 The somewhat higher porosity in Kraft paper (27% vs 21%) cannot explain the vast difference in modulus reported in Table 3 (18 GPa vs 9 GPa). The mechanical property of the present holocellulose paper is significantly higher than paper prepared from bleached sulphite pulps using a similar hot-pressing route.42 Even after beating, those bleached sulphite pulp paper structures have a Young’s modulus lower than 13 GPa and an ultimate tensile strength lower than 80 MPa. As mentioned, the high modulus and strength of the present holocelluose paper is comparable with paper based on nanofibrillated cellulose.12,32,66–68 For reference purposes, we also report the tensile index and tensile stiffness index parameters used in the paper industry, which define the mechanical properties of papers normalized by industry term “grammage” (weight per surface area). Holocellulose paper has a tensile index of 166 Nm/g and a tensile stiffness index of 15.3 kNm/g, which surpasses PAA-treated sugarcane bagasse pulp paper,28,29 typical softwood pulp paper,69 and even nanocellulose paper.68,70 The high mechanical properties of holocellulose paper is due to higher intrinsic fiber strength (see reported data) and higher hemicellulose content in the wood fibers. Also, paper structures show identical XRD curves compared to corresponding fiber samples (Figure 4), indicating that our water-based hot-pressing route preserves fiber structure. Compared with Kraft pulp fibers, this work suggests a route for strong fiber networks through better preservation of the native wood fiber structure during mild delignification.


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Table 3. Mechanical properties of holocellulose paper and Kraft paper, with standard deviations reported in parentheses. Thickness (µm)

Density (g/cm3)

Grammage (g/m2)

Yield strength (MPa)

Young’s modulus (GPa)

Ultimate strength (MPa)

Strain at break (%)

Tensile index (Nm/g)

Tensile stiffness index (kNm/g)

Holocellulose paper

170 (5)

1.18 (0.04)

200 (6)

79 (8)

18 (1)

195 (8)

2.0 (0.2)

166 (7)

15.3 (0.8)

Kraft paper

175 (5)

1.09 (0.04)

190 (5)

43 (5)

9.2 (0.4)

102 (3)

2.1 (0.2)

92 (4)

8.5 (0.5)

Figure 7. Stress-strain curves (A) and photographs and SEM images of typical fracture surfaces (B) of holocellulose paper and Kraft paper.

Optical Properties of Papers Figure 8A shows that holocellulose paper is semi-transparent with the words “Holocellulose” visible below the sheet, while Kraft paper is opaque. Moreover, holocellulose paper has a similar visual appearance compared to papers based on fully bleached softwood pulps (Figure S4), and both have the same ISO brightness of 68 ± 1%. In comparison, Kraft paper has an ISO brightness


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Biomacromolecules

of only 16 ± 0.5%. This enables holocellulose paper to function as good substrates for printing or coating. From Figure 8B, the optical transmittance of the holocellulose paper is approximately 55%, which is significant higher than that of the Kraft paper (below 30%). Several factors contribute to the higher light transmittance of the holocellulose papers, including the lower lignin content (lignin absorbs light), higher paper density, as well as the possibility of “soft” hemicellulose to fill the gaps in the fiber network, which means fewer fiber/air interfaces and light scattering sites. Figure 8C shows that both papers have high haze transmittance, approximately 80%, which is higher than for both glass and nanopaper.71,72 Since a higher transmission of haze is preferred in applications such as solar cells, due to the improved light absorption efficiency,72 the holocellulose paper is of interest as an affordable surface layer for optical devices.

Figure 8. A) Photographs of papers based on holocellulose and Kraft fibers placed at top of the words “Holocellulose”; B) Total transmittance, and C) Transmission haze of the two papers.


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CONCLUSION A high-density paper structure from holocellulose softwood fibers is reported to show exceptionally high Young’s modulus (18GPa), yield strength (79MPa) and ultimate strength (195 MPa) at 21% porosity. These data are about twice the values for the Kraft fiber reference. Data are even competitive with some research grades of cellulose nanofibrils (CNF), and certainly with industrial grades of microfibrillated cellulose fibrils (MFC). The ease of processing and low viscosity of wood pulp fiber suspensions, compared with CNF and MFC suspensions, makes the present results interesting also in an industrial perspective. No paper chemistry additives or energy-demanding beating steps are used. After conventional filtration, the wet cake was hotpressed in a simple procedure, in order to reach high-density. Efforts were made to separate effects from intrinsic fiber properties and effects from paper structure (fiber network structure) on mechanical properties. The much higher modulus of the holocellulose paper is most certainly due to less efficient interfiber bonding in the Kraft paper. This is supported by the more collapsed feature for holocellulose fibers after hot-pressing, which will favor more efficient bonding area. The much higher stress level in the holocellulose fiber network at a given strain is probably due to the hemicellulose acting as a bonding agent, which improves interfiber stress transfer. This strong interfiber bonding is also supported by fracture surface appearance, where much shorter pull-out lengths are observed for holocellulose fibers compared with Kraft fibers. The high ultimate strength of high-density holocellulose paper is due not only to favorable interfiber bonding effects, but also to the high intrinsic strength of holocellulose fibers. This is supported by measured strength values for single fibers, as a function of microfibril angle.


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Values were high compared with literature data. The strongest fibers (MFA

Preserving Cellulose Structure: Delignified Wood Fibers for Paper

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