A method for chemical and physical modification of oriented pulp fibre sheets

doi:10.21203/rs.3.rs-1246825/v1

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A method for chemical and physical modification of oriented pulp fibre sheets

https://doi.org/10.21203/rs.3.rs-1246825/v1

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Wood pulp fibres are promising reinforcements for biocomposites due to their renewable resource origin and mechanical properties. An oriented and dense fibre reinforcement structure is beneficial for biocomposite properties. We present a method of modifying fibres (e.g. to increase strain to failure) in pre-formed oriented high-density paper structures intended for biocomposites or as hot-pressed fibre materials. Mildly delignified, well-preserved holocellulose fibres from softwood are used. Cold alkali treatment (hemicellulose removal) and mercerisation (conversion to cellulose II) were carried out successfully on oriented fibre sheets. Controlled anisotropy and sheet density are achieved from untreated and straight fibres in the sheet formation step. High mechanical properties and increased ductility of mercerised sheets were observed, which may be valuable for hot-pressed fibre materials (E ≈ 7.1 GPa, strength of 108 MPa and strain to failure of 5.3%) and biocomposites. In contrast, modified wood pulp fibres were difficult to orient, resulting in higher sheet porosity and weak interfibre bonding.

Biocomposites

Cold alkaline extraction

Mercerisation

Cellulose II

hot-pressed fibers

Biocomposites and fibre materials based on plant fibres are an alternative to non-biodegradable plastics and glass fibre composites due to their renewable resource origin and good mechanical properties (Faruk et al. 2012; George et al. 2001; Niskanen 2012). To take advantage of these properties, plant fibres have been modified to improve cell wall accessibility and fibre/polymer interface adhesion (Li et al. 2007; Saheb, Jog 1999). Here, we suggest a method for chemical and physical modification of dense wood fibre network reinforcements (dense paper) and we are investigating effects from mercerization.

Modifying natural fibres by simple NaOH-treatment changes the fibre/matrix interface and can induce structural changes for increased fibre ductility (Faruk et al. 2012; Goda et al. 2006; Gomes et al. 2004; Nakagaito, Yano 2008). The effect of alkali treatment varies depending on the fibres and process conditions (Hashim et al. 2012). Another fibre modification method is cell wall bulking by acetylation. Increased degree of acetylation decreases the moisture adsorption of fibres (Bledzki et al. 2008; Bledzki, Gassan 1999; Rowell 2004) and improves the wet strength of composites (Joffre et al. 2017).

In addition to chemical modification, the effects of densification and fibre orientation have been explored. Reduced porosity (higher fibre volume fraction) improves network strength in fibre sheets and facilitates high fibre volume fraction in the final biocomposite (Bajpai 2018; Niskanen 2012). Moreover, aligning fibres in a preferential direction during sheet formation improves mechanical performance along the direction of fibre alignment compared to randomly oriented fibres (Dufresne 2008; Niskanen 2012). There is a need for further biocomposite investigations on eco-friendly chemical modification methods for wood fibres combined with well-organised wood fibre reinforcements. Most often, wood fibres are first modified by wet chemistry and then used in composites melt-processing (e.g. Bledzki, Gassan 1999). It often means that the amount of liquids and chemicals used are high and that multiple processing steps are needed, where detrimental mechanical fibre damage is often induced.

We propose an eco-friendly protocol for chemical/physical fibre modification in sheet form using an oriented fibre network, thus achieving higher mechanical performance of composites and fibre materials in the predominant fibre direction. The protocol allows better control of fibre organisation and sheet density. Alkali treatment is used as an example for modification. First, well-preserved holocellulose fibres (high hemicellulose content, almost no lignin) were obtained via a single step peracetic acid (PAA) delignification (Westin et al. 2021). Secondly, the holocellulose fibres were used as starting material for cold alkali extraction and mercerisation processes, using two different protocols as shown in Fig. 1. One protocol (1) consisted of cold alkali extraction or mercerisation of wet holocellulose fibres, here referred to as pulp steeping, followed by washing. A dynamic sheet former was then used in a second stage to produce fibre sheets with a high degree of preferred orientation. In the alternative protocol (2), holocellulose fibres were assembled in a dynamic sheet former, dried, and used as starting material for cold alkali extraction or mercerisation, a process referred to as sheet steeping. Additionally, some samples from the sheet steeping protocol were subjected to hot pressing to test possible sheet property improvements by densification. Finally, sheets from the two protocols were compared for mechanical properties of the fibre “mat”, intended for hot-pressed fibre materials or as reinforcement in polymer matrix composites.

Materials

Wood chips

Spruce wood chips with a dry content between 40-60% were provided by Högland Såg & Hyvleri AB sawmill, Örnsköldsvik. Bark and knots were excluded manually prior to delignification process. Chips were predominantly from the sapwood region of the logs, where microfibril angle is expected to be higher than average in the log.

Chemicals

The reactants, glacial acetic acid (100%, Merck KGaA), hydrogen peroxide (30%, Merck KgaA), and NaOH (≥98%, VWR) were used without any further purification. Ultrapure Milli-Q H₂O was used for preparation of the NaOH solutions. NaOH solutions were kept in vessels protected from atmospheric CO₂. Deionised H₂O was used for washing the alkali-treated samples.

Methods

Holocellulose pulp preparation

A single step peracetic acid (PAA) delignification process was used to extract holocellulose fibres with minimal lignin content while preserving cellulose and hemicellulose contents and causing minimal mechanical degradation of the wood fibre (Westin et al. 2021). PAA containing liquor was prepared by mixing 30% hydrogen peroxide and 100% acetic acid at a molar ratio of 1:1.9 and kept at room temperature for 24 h to allow equilibration. Wood chips were placed in an autoclave reactor, after which the PAA-containing liquor was added. Impregnation of the wood chips was assured by applying 5 bars of pressurised air to the autoclave. The reaction was then carried out in a heated rotating setup at 70°C for 25 h. At the end of the reaction, the liquor was removed, and the obtained holocellulose pulp washed using deionised water. PAA delignification yields a holocellulose pulp composed of ~23% preserved hemicelluloses and residual lignin content of 1-2 % (Westin et al. 2021).

Preparation of paper sheets

Sheets with preferred fibre orientation were prepared from a pulp suspension at 1 g/L consistency using a dynamic sheet former (Formette, Fibertech AB, Sweden) and a drum rotation speed of 1300 m/min. Wet sheets were compressed for 5 min at a pressure of 0.3 MPa between blotting papers to remove excess water, after which they were dried under mechanical constraint at 85°C temperature for 7 min. The dried sheets with a thickness of 0.1–0.2 mm, were conditioned for at least 24 h at 23°C and 50% RH prior to further processing, testing and measurements. The fibre orientation achieved by using the dynamic sheet former is intended to improve mechanical performance along the direction of the fibre alignment (machine direction, MD) compared to that of a sheet of randomly oriented fibres.

Alkali treatment of holocellulose in pulp and sheet form

For cold alkali extraction, 5 % NaOH at 5°C was used. Mercerisation was carried out using 18% NaOH at room temperature. The used experimental conditions were selected since low NaOH concentration (i.e., 5% NaOH) allows for removal of hemicellulose (Sixta 2006) without causing a transformation into cellulose II (Porro et al. 2007). While the use of 18% NaOH is expected to yield cellulose II almost instantaneously (Albán Reyes et al. 2016; Langan et al. 2001; Nishiyama et al. 2000). The samples were then gently introduced into the container containing lye to minimise the risk of damaging the sample. This step was crucial for the preformed sheet samples.

For pulp steeping, wet holocellulose pulp was used as a starting material at 5% (w/w) holocellulose content. Note that no agitation was used to avoid damage of the well-preserved holocellulose fibres. The first filtrate was taken for Chemical Oxygen Demand (COD) analyses, and the reaction was quickly stopped by washing the alkali samples with deionised H₂O in a Büchner funnel. The samples were then suspended in deionised H₂O for 2h, and their pH was measured to ensure neutrality before paper making.

Cold alkali extraction via pulp steeping was carried out using a pre-cooled 5% (w/w) NaOH solution, and the cold temperature (5°C) was maintained during the reaction. Reaction time was set to 10-, 200- and 1200-min. Samples were then subjected to alkali wash to remove as much of the hemicellulose as possible. Mercerisation via pulp steeping was carried out using 18% (w/w) NaOH solution at room temperature with reaction times of 10 min and 1200 min. Then, the samples were subjected to alkali wash.

Sheet steeping of dry oriented holocellulose paper sheets (~5% moisture content, MC) was carried out using a 0.4 % (w/v) consistency to ensure that the whole sheet surface was covered with the NaOH solution. Otherwise, the experimental conditions used for cold alkali extraction and mercerisation were the same as those described for pulp steeping. Samples were subjected to alkali washing followed by excess washing with deionised water to ensure neutrality prior to further processing. An overview of the used experimental conditions and setup are shown in Table 1 and illustrated in Fig S1 (Supplementary materials), respectively.

Table 1. Overview of used experimental conditions for cold alkali extraction and mercerisation. Sample from sheet steeping and pulp steeping are labelled as SS and PS, respectively

Samples		T (°C)	[NaOH] (%)	t (min)
Pulp	HC
	AE-1	5	5	10
	AE-2	5	5	200
	AE-3	5	5	1200
	CII-1	23	18	10
	CII-2	23	18	1200
Oriented sheets	HC-O	-	-	-
	AE-1-PS	5	5	10
	AE-1-SS	5	5	10
	CII-1-PS	23	18	10
	CII-1-SS	23	18	10
	CII-2-PS	23	18	1200
	CII-2-SS	23	18	1200

Densification

As described in the paper preparation section, wet sheets were densified by pressing prior to drying. Washed samples from the sheet steeping protocol were similarly pressed with blotters at 0.4 MPa for 5 min, after which they were dried under constraint at room temperature and 50% RH for at least 24 hours prior to mechanical evaluation.

Selected samples from sheet steeping were densified by hot pressing to explore the possibility to further enhance the mechanical performance. Samples were compressed in a continuous press manufactured by IPCO AB, described in detail in (Joelsson et al. 2021). The press is designed with an infrared-heated steel belt carrying the sample into a roll nip pressure point, applying a line pressure across the sample as it passes through. Samples were pressed at a temperature of 105 °C with a line pressure of 27.5 kN/m, approximated to 5.5 MPa, with a speed of 0.5 m/min.

Analysis

Intrinsic viscosity

The intrinsic viscosity of the pulp samples was determined according to the standard method ISO 5351:2010. Pulp samples were weighted, dispersed in water, and dissolved in 1 M cupri-ethylenediamine (CED) solution. The dissolved pulp in CED was then analysed by timing flow through a capillary tube viscometer (Viscomat II) using a shear rate of 200 ± 10 s-1.

Chemical Oxygen Demand

The COD was measured using a Lange LCK 114 COD cuvette tube test (150 – 1000 mg/L O2). The first filtrate sample from the Büchner filtration was diluted with Milli-Q H₂O to a dilution factor of 20. After this, 2 ml of sample was pipetted into the test tube. The sample was then heated in a thermostat at 148 °C for 2h and was then allowed to cool before it was measured in the HACH LANGE DR 2800 spectrophotometer.

Molecular weight distribution

The molecular weight of holocellulose pulp and selected alkali-treated pulp samples was determined by size exclusion chromatography (SEC), using an Agilent PL-GPC 220 with a refractive index (RI) detector. Three 20 µm Mixed-A columns (Polymer lab) were used, one guard column and two 30 cm columns in series. The eluent DMAc containing 0.5 % LiCl was used at a flow-rate of 1 ml/min and at 70°C. For calibration, Pullulan polysaccharide standards (Polymer lab) were used with a molecular weight of 708 000, 344 000, 200 000, 107 000, 47 100, 21 100, 9 600 and 6 100 Da. Samples were subjected to solvent exchange with methanol and DMAc, then dissolved in 8% (w/v) LiCl in DMAc and left for at least 2 days at room temperature with constant stirring. Samples were diluted to a concentration of 1.6% (w/v) and filtered before analysis.

Carbohydrate composition

Carbohydrate analysis was carried out according to SCAN-CM 71:09, briefly described as follows. Samples were subject to two-stage hydrolysis by treatment 72% (w/w) and 2.5 % (w/w) sulfuric acid solution at 30° C and at 120 ° C, respectively. The level of saccharides in the samples were determined using Ion Chromatography (Thermo Fischer Scientific).

Fourier Transform Infrared Spectroscopy

All spectral data were collected by a PerkinElmer Fourier Transform Infrared (FT-IR) spectrometer (Frontier), equipped with a PerkinElmer Spotlight 200 FT-IR Microscope and with an integrated Micro-ATR including a 100 µm Germanium ATR crystal. Spectra collection was obtained using 128 scans, in a spectral region of 4000 – 580 cm^-1, at a 4 cm^-1 spectral resolution. All spectral data were normalised (A=1).

Wide-Angle X-ray Diffraction

Wide-Angle X-ray Diffraction (WAXD) measurement was carried on an Anton Paar’s SAXSpoint 2.0 system (Anton Paar, Graz, Austria) equipped with a Microfocus X-ray source (Cu Kα radiation, wavelength 1.5418 Å) and an Eiger R 1M detector with 75×75 µm pixel size. All samples were measured with a beam size of 500 µm at room temperature. The sample to detector distance was 77 mm. The samples were exposed for 10 minutes. The data reduction was performed by using SAXS analysis software. The crystallinity was calculated based on the amorphous and crystalline area determined from WAXD peaks.

Scanning Electron Microscope

SEM imaging was performed to determine the pulp samples morphologies using a JEOL microscope, model JSM – 6610LV, equipped with an X-ray detector from Oxford instruments. Prior to SEM observations, the pulp samples were air-dried on a specimen holder and coated with a thin layer of carbon. The oriented sheet fibres morphologies were investigated using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan). Prior to imaging, the samples were sputter-coated with Pt/Pd (Cressington 208H, UK).

Paper mechanical testing

Prior to mechanical evaluation, paper sheets were conditioned for a period of at least 24 h at 23°C and 50 % RH. Strips of paper sheets were cut to 10- or 15-mm width and ~65 mm length and tested for tensile properties using a gauge length of 50 mm in a Lorentsson & Wettre paper testing setup at a speed of 100 mm/min.

Characterisation of wood pulp fibre sheets

FT-IR and WAXS are used to characterise cellulose structure after alkali treatment according to Table 1. The FT-IR technique can characterise the two polymorphs of cellulose, cellulose I and cellulose II. As an example, Dinand et al. 2002; Hurtubise, Krassig 1960; Nelson, O’Connor 1964; Oh et al. 2005 showed that the bands at 1430 cm^-1 and 895 cm^-1 could be used to study cellulose crystal structure since cellulose I and cellulose II spectra differ in these two bands. In cellulosic pulp fibres with a significant amount of cellulose I, the characteristic band is at 1430 cm^-1. For samples with more cellulose II, the absorption band at 895 cm^-1 should be more pronounced. In cold alkali treated samples, the band assigned to cellulose I (1430 cm^-1) is present in the AE-1 sample (pulp, 5%NaOH, 10 min, 5°C) (Fig. 2), and the cellulose I form is maintained. For samples CII-2 (pulp, 18%NaOH, 1200 min) and CII-1-SS (oriented sheets, 18%NaOH, 10 min), the band 895 cm^-1 shows increased intensity, see Fig. 2. This confirms that at higher NaOH concentration, 18% (w/w), cellulose I was transformed to cellulose II.

WAXS data are presented in Fig. 3 and Table 2, and it is confirmed that the treatment in 18% NaOH results in conversion from cellulose I to cellulose II. In Fig. 3, the intensities of the lattice plane peaks are apparent by colour coding for the four different materials: the unmodified holocellulose reference (HC), alkali extracted at 5% NaOH (AE-1-SS) and at 18% (CII-1-SS and CII-2-SS), see conditions in Table 1. Intensities of peaks from different planes are quantified in Fig. 3, and estimated crystallinities are in Table 2. It is interesting that the holocellulose reference and the cold alkali sample AE-1-SS show almost identical (200) plane peaks, indicating that the cellulose microfibril structure is undamaged by the 10-minute treatment, although hemicellulose content is reduced (Table 3). In contrast, the CII-1-SS and CII-2-SS samples show not only cellulose II structure ((1-10), (110) and (020) peaks in Fig. 3) but also that the fraction of disordered regions is increased.

Table 2. Crystallinity of untreated holocellulose sheet (HC), alkali extracted (AE) and mercerised sheet steeping samples determined from WAXS curves

Samples	Crystallinity (%)
Holocellulose HC	43.2
AE-1-SS	38.3
CII-1-SS	33.7
CII-2-SS	35.5

The intrinsic viscosity of the pulp steeping samples (alkali treatment of pulp fibres rather than sheets) was measured to determine conditions in cold alkali extraction and mercerisation in which hemicelluloses could be removed without significant cellulose degradation (see Fig. 4). Dissolved pulp viscosity is known to increase when low molar mass molecules (mainly hemicelluloses) are removed through alkali treatment (Sixta 2006). For cold alkali extracted pulp, viscosity reaches its highest value after 10 min treatment (see AE-1 sample). After that, the viscosity decreased. Fast removal of primarily hemicelluloses during the first 10 minutes of cold alkali extraction was supported by SEC data, in accordance with increased viscosity AE-1 (see MWD curve in Fig S1). Results from MWD need to be interpreted with care since full dissolution was not confirmed. Samples treated with higher NaOH concentration show similar behaviour; viscosity increases after 10 min treatment but decreases after prolonged treatment (Fig. 4). Wood cellulose shows almost instantaneous swelling at low temperature, facilitating the extraction of low molar mass fractions (mainly hemicelluloses) (Sixta 2006). The drop in viscosity with longer treatment (samples AE-2, AE-3, and CII-2) is due to reduced cellulose molar mass during prolonged alkali treatment (Golova and Nosova 1973; Knill and Kennedy 2002; Saukkonen et al. 2012; Albán Reyes et al. 2016).

Next, the carbohydrate composition of the holocellulose pulp, cold alkali extracted pulp and mercerised pulp was measured, see Table 3. The cold alkali treatment effectively removed hemicelluloses, especially xylose. The increased viscosity must then be due to the removal of hemicelluloses. In the first 10 minutes of the extraction, the hemicellulose content decreased by 56 %. However, prolonged extraction time (1200 min, sample AE-3) does not necessarily mean a decrease in hemicellulose fraction. On the contrary, data show a slight increase in hemicellulose fraction (w/w), possibly by reduced glucose content from cellulose degradation. Reduced viscosity (see Fig. 4) and increased COD, i.e., dissolved organic material in the filtrate, with prolonged time (see supplementary material Fig S2), are in support of reduced cellulose molar mass and weight fraction due to degradation.

When holocellulose pulp is mercerized, the hemicellulose content is reduced from 210.5 g/kg to only 30.5 g/kg, see sample CII-2, Table 3. The two materials treated for 10 min (AE-1 and CII-1) show very similar levels of hemicelluloses despite NaOH concentration differences. Note that although the same alkali treatment conditions were used for wet pulp and dry oriented fibre sheets, wet pulp shows slightly higher reduction in hemicellulose content than dry sheets, see supplementary material Fig. S3. This is in accordance with results by Spinu et al. 2011.

Table 3. Carbohydrate composition of holocellulose pulp and selected alkali treated pulp. Note that NaOH concentration is much lower for AE samples (5%, 5°C) compared with CII samples (18%, 23°C)

Samples	t (min)	Glucose (g kg^-1)	Hemi- cellulose^a (g kg^-1)	Arabinose (g kg^-1)	Galactose (g kg^-1)	Xylose (g kg^-1)	Mannose (g kg^-1)
HC	0	705	210,5	1	3,1	54,8	114
AE-1	10	842	93,0	0	0,6	6,9	64,3
AE-3	1200	829	99,6	0	1	5,8	69,8
CII-1	10	871	93,463	0,4	1,3	10,5	61,1
CII-2	1200	948	30,5	0	0	4,8	19,3

^aTotal amount of hemicelluloses is expressed as the sum of xylose, arabinose, galactose, and mannose, assuming an addition of glucose at glucose: mannose ratio of 1:3

The SEM micrographs show significant fibre shape effects from alkali treatments (Fig. 5). The untreated holocellulose fibres spontaneously collapse during drying and appear flat, ribbonlike and straight, while the alkali-treated fibres show a more porous network in the dried state with smaller, irregular diameters and shapes. Alkali treated fibres are also more swirled in an irregular manner, rather than straight, see Fig. 5. When comparing cold alkali extracted pulp (B and C) with mercerised pulp (D), the mercerised fibres appear to have a more tubular shape and be more curved and wavy. It is worth noting that (C) samples treated for an extended time (20 h) show rougher fibre surface morphologies, perhaps related to microfibrils. A short treatment time (10 min) of holocellulose wood fibres seems to make fibers more wavy, less flat and, as we know, with lower hemicellulose content.

Pulp or sheet steeping in alkali?

The two methods, pulp steeping and sheet steeping, were compared to consider these routes for subsequent preparation of hot-pressed fibers or biocomposites. Fibre orientation was achieved by a dynamic sheet former. Cold alkali treated and mercerised holocellulose with 10 min treatment times were used as examples of modifications for pulp and sheets. For pulp fibre steeping, the pulp was easy to handle during alkali treatment. The fibres became highly swollen once in contact with the lye. However, in the following oriented sheet preparation step, the handling of wet fibre sheets was difficult. This was especially problematic for the mercerised fibre sheets, which tended to disintegrate during removal from the wire mesh.

Sheet steeping, in contrast, meant that it was straightforward to prepare high quality oriented sheets from holocellulose fibres. The steeping stage of dried sheets was slightly more elaborate than for pulp and required care to prevent damage of submerged holocellulose sheets in the swollen state. Cold alkali extracted paper sheets easily fell apart if mishandled. The mercerised sheets were less sensitive but showed notable shrinkage in contact with the lye (see Fig. 6). The level of shrinkage is in the order of 13-14%. Similar shrinkage was also reported when sheets from bleached kraft pulp and microfibrillated cellulose were treated with 20% NaOH (Nakagaito and Yano 2008). A possible explanation for the observed shrinkage is the longitudinal contraction of microfibrils proposed by Nakano (Nakano et al. 2000; Ishikura and Nakano 2007), and here we observed that fibres themselves also changed shape, possibly due to microfibril shrinkage. No shrinkage was observed for cold alkali treated sheets.

SEM micrographs of the oriented fibre sheets were obtained to investigate the morphological effects on the sheet network from the two protocols (pulp steeping and sheet steeping). The untreated holocellulose fibres are straight, flat and aligned from the dynamic sheet forming step (Fig. 7 A). Sheets made from pulp steeping protocol in Fig. 7 C, E (first alkali treatment, then sheet preparation) have less oriented fibres, also curved. This must be due to changes in fibre shape during alkali treatment (see Fibre properties section, Fig. 5). The straight holocellulose fibres are much easier to orient in the dynamic sheet former than the alkali treated fibres, which are curved and wavy. Materials made via sheet steeping protocol appear to preserve the orientation of the holocellulose sheet, with more straight fibres. It is evident that the degree of fibre orientation is higher in sheet steeping than pulp steeping protocols (see B and C for cold alkali extraction, and D and E for mercerisation in Fig. 7). Furthermore, it is noted that despite the observed shrinkage of mercerised sheets, the fibres appear to broadly maintain their preferred orientation.

Materials were then subjected to physical and mechanical testing. Results are shown in Fig. 8 and summarised in Table 4. Sheets made from untreated holocellulose fibres show excellent properties. The density is 706 kg/m³, anisotropy of longitudinal/transverse strength ~5, tensile strength 172 MPa, and Young’s modulus 16.8 GPa. The reason is better utilization of axial fibre properties in sheets with preferred fibre orientation in one direction. Since the level of anisotropy in properties should correlate with the degree of fibre orientation in the sheet, a higher anisotropy value is desirable. Pulp steeping samples (AE-1-PS and CII-1-PS) have an anisotropy of strength of <2, while sheet steeping samples (AE-1-SS, CII-1-SS) maintain the anisotropy of the untreated sheet at ~5. These results are supported by SEM micrographs in Fig. 7 in that there is less orientation in the fibre network of pulp steeping samples. This reduces tensile properties in the machine direction.

We observe that the choice of alkali treatment protocol affects mechanical performance. Pulp steeping results in lower density and inferior mechanical performance, more noticeable in mercerised samples (Table 4 and Fig. 8). Samples prepared by sheet steeping show higher density and better mechanical performance. The difference in mechanical performance is dramatic; samples prepared by sheet steeping are 8-10 times stronger than those prepared from the pulp. Modulus is also superior for sheet steeped samples, but here the effect is less pronounced. Lower stiffness in mercerised samples is expected since cellulose II is less stiff than cellulose I (Nishino et al. 1995), although interfibre bonding and fibre orientation also are of obvious importance. The superior mechanical strength and modulus of materials from the sheet steeping protocol is due to higher density (higher volume fraction fibers), higher degree of orientation and possibly better interfibre adhesion.

Table 4. Physical and mechanical properties of prepared pulp fiber sheets. Data for oriented sheets are measured along the axis of preferred orientation. Standard deviations are available in parentheses. Note that d - thickness, 𝜌 - density, V_f - fibre volume fraction, 𝜎 - tensile strength, E - Youngs modulus and 𝜀 - strain to failure

Sample^a	Grammage [g/m²]	d [µm]	ρ [kg/m³]	V_f^b [%]	σ [MPa]	E [GPa]	ε [%]	Strength Anisotropy
HC-O	103 (1)	145 (4)	706 (14)	46	172	16.8	2.03	5.4
AE-1-PS	102	194	525	34	16.5	3.0	0.97	1.8
AE-1-SS	103 (1)	132 (2)	784 (21)	51	128	11.8	1.03	4.8
AE-1-SS/HP	80 (2)	105 (3)	765 (14)	49	127 (10)	11.5 (1.1)	1.1 (0.1)	N/A
CII-1-PS	77.8	180	433	28	7.25	1.1	0.66	1.4
CII-1-SS	120 (2.6)	164 (3.1)	733 (25)	47	73.8	2.14	3.69	5
CII-1-SS/HP	115 (3.8)	146 (2.3)	789 (29)	51	108 (7.2)	7.1 (0.3)	5.3 (0.4)	N/A

^a PS indicates sample was prepared from pulp steeping, SS indicates sample was prepared using sheet steeping, HP indicates sample was densified by hot pressing.

^b Fibre volume fraction as calculated based on

Fig. 9 shows stress-strain behaviour for representative materials. A significant increase in strain to failure ( ) is observed for the mercerised materials prepared by sheet steeping, and the behaviour is more non-linear with lower yield stress and more ductile behaviour. This is helpful in that the original brittleness problem of the oriented fibre material is solved. Nakagaito et al previously showed an increased ductility in microfibrillated cellulose (MFC) sheets mercerised by sheet steeping. This effect was also observed in composites reinforced with mercerised MFC or wood pulp sheets (Nakagaito and Yano 2008). Although the experimental conditions were different Gomes et al. reported a doubling in strain to failure from 4% to ~9% for individual curaua fibres treated for 2 h with 15% NaOH solution (Gomes et al. 2004). However, we do not observe this increased ductility in sheets prepared from the mercerised pulp (see Table 4). Possibly, the poorly bonded fibre network (low density and low strength) results in poor interfibre stress transfer; thus, any gain in ductility of the fibres is not reflected in the sheet behaviour.

We next used sheet steeping samples to explore the possibility of further improving mechanical performance by using a continuous hot press (105 °C, ~5.5 MPa) available as an industrial process. In our test with cold alkali treated sheets, no significant effect was observed. The mercerised sheets, however, showed substantial increase in strength (108 vs 74 MPa), modulus (dramatic increase: 7.1GPa vs 2.1GPa) and strain to failure (5.3% vs 3.7%) accompanied by only a slight increase in density and fibre volume fraction, see CII-1-SS/HP in Table 4. The increase in modulus is remarkable, becoming more than 3 times higher than non-densified sheets, although the density is only mariginally increased. The most likely explanation is improved interfibre bonding from hot-pressing.

Sheet steeping potential

Collectively our results show that the sheet steeping protocol allows physical and chemical modification of fibres without sacrificing good network properties such as density and fibre orientation. The sheet steeping protocol allows for increased fibre orientation and density (fibre volume fraction) in the sheet compared with pulp steeping based on chemically modified fibres. Higher fibre orientation and density (fibre volume fraction) are important for mechanical performance. The high degree of fibre orientation is because of well-preserved, straight holocellulose fibres that facilitate subsequent fibre alignment. For the pulp steeping protocol in contrast, poor alignment of the chemically treated pulps is due to changes in fibre shape, as treated fibres were observed to be more wavy and curved than untreated holocellulose fibres. For sheet steeping, transformation to cellulose II and extraction of hemicelluloses, at comparable levels to pulp steeping, was achieved while maintaining the network structure. The mercerised sheet steeping material showed increased strain to failure in tensile testing. Rapid hot-pressing of mercerised sheets using a scalable belt-press resulted in much improved Young’s modulus, strength and strain to failure; this is an inspiration for further studies of thermoformed “molded fibre” type of materials.

The use of paper sheet reinforcement for biocomposites or for hot-pressed fiber materials, based on holocellulose fibres and mild chemical treatments is rather unexplored, despite advantages, e.g. compared with melt-compounded wood fibre biocomposites. Fibres will have improved intrinsic strength since mechanical, thermal and chemical fibre degradation is relatively low. In addition, fibres can be of very high aspect ratios, straight and have controlled orientation. The present fibre volume fraction of the reinforcement was around 50%, which facilitates the manufacturing of high fibre volume fraction biocomposites.

This approach to fibre modification of pre-formed, oriented wood fibre sheets (high fibre volume fraction paper structures) is promising. Both cold alkali treatment (hemicellulose removal) and mercerisation (conversion from cellulose I to cellulose II) were carried out successfully; the potential of modified sheets as biocomposites reinforcement is encouraging. The wood fibre sheets showed high mechanical integrity, and the increased ductility of mercerised sheets is relevant to load-bearing applications of pulp fiber materials. The increased ductility is not only from the cellulose II structure but also from changes in fibre shape, from straight to more curved and wavy fibre segments.

Acknowledgements

Treesearch Research Infrastructure is acknowledged for their financial support of the WAXD analysis at Research Institutes of Sweden (RISE). This research is supported by the Kempe Foundation. L. A. B acknowledges funding from the Knut and Alice Wallenberg Foundation for the Biocomposites project.

Author information

Note

P-O. W. and D. C. R. F. have contributed equally to this work.

Affiliations

MoRe Research AB (RISE), Box 70, SE-89122 Örnsköldsvik, Sweden

P-O. Westin (0000-0003-3969-3804), D.C. Reyes Forsberg (0000-0002-7064-1060), A. Svedberg, H. Grundberg

Department of Fibre and Polymer Technology, Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

L. A. Berglund (ORCID 0000-0001-5818-2378)

Corresponding authors

Correspondence to Lars A. Berglund ([email protected])

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Editorial decision: Major revisions
02 Mar, 2022
Reviews received at journal
02 Feb, 2022
Reviewers invited by journal
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Editor invited by journal
21 Jan, 2022
Editor assigned by journal
12 Jan, 2022
First submitted to journal
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A method for chemical and physical modification of oriented pulp fibre sheets

Status:

Version 1

Abstract

Figures

Introduction

Experimental

Materials

Wood chips

Chemicals

Methods

Holocellulose pulp preparation

Preparation of paper sheets

Alkali treatment of holocellulose in pulp and sheet form

Densification

Analysis

Intrinsic viscosity

Chemical Oxygen Demand

Molecular weight distribution

Carbohydrate composition

Fourier Transform Infrared Spectroscopy

Wide-Angle X-ray Diffraction

Scanning Electron Microscope

Paper mechanical testing

Results And Discussion

Characterisation of wood pulp fibre sheets

Pulp or sheet steeping in alkali?

Sheet steeping potential

Conclusions

Declarations

Acknowledgements

References

Supplementary Files

Status:

Version 1