Novel, Cellulose-Based, Lightweight, Wet-Resilient Materials with

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Novel cellulose-based lightweight, wet-resilient materials with tunable porosity, density and strength Veronica Lopez Duran, Johan Erlandsson, Lars Wågberg, and Per A. Larsson ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01165 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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ACS Sustainable Chemistry & Engineering

Novel cellulose-based lightweight, wet-resilient materials with tunable porosity, density and strength Verónica López Durán†,‡,*, Johan Erlandsson†, Lars Wågberg†,‡, Per A. Larsson†,‡*.

†Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56–58, SE-10044 Stockholm, Sweden ‡BiMaC Innovation, KTH Royal Institute of Technology, Teknikringen 56–58, SE-10044, Sweden

Corresponding authors: [email protected], [email protected], Phone: +46(0)87908296, Fax: +46(0) 879096166.

Keywords: ambient drying, cellulose, chemical modification, chlorite oxidation, light-weight material, periodate oxidation.

Abstract Highly porous materials with low density were developed from chemically modified cellulose fibres using solvent-exchange and air drying. Periodate oxidation was initially performed to introduce aldehydes into the cellulose chain, which were then further oxidised to carboxyl groups by chlorite oxidation. Low-density materials were finally achieved by a second periodate oxidation under which the fibres self-assembled into porous fibrous networks. Following a solvent exchange to acetone, these networks could be air dried without shrinkage. The properties of the materials were tuned by mechanical mixing with a high intensity mixer for different times prior to the second periodate oxidation, which resulted in porosities between 94.4 and 96.3% (i.e. densities between 54 and 82 kg/m3). The compressive strength of the materials was between 400 and 1600 kPa in the dry state and between 20 and 50 kPa in the wet state. It was also observed that in the wet state the fibre networks could be compressed up to 80% while still being able to recover their shape. These networks are highly interesting for use in different types of absorption products and since they also have a high wet integrity they can be modified with physical methods for different high-value added end-use applications.

Introduction Nowadays there is a great interest in developing more sustainable materials from biopolymers. Among the different biopolymers found in nature, cellulose is one of the polymers that has attracted great attention due to its abundance and the well-developed processes for its isolation from different raw 1 ACS Paragon Plus Environment

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materials. Chemically, cellulose is a linear chain of linked anhydroglucose units organized in a complex hierarchical structure. It can be sourced at a fairly low cost and due to the organization of the cellulose molecules in long and slender, nano-sized fibril aggregates with a very high aspect ratio, the cellulose material provides high stiffness, high strength and low density1,2. There is great interest in developing porous light-weight materials3 from cellulose since these can be used in advanced applications such as in biomedical scaffolds4, thermal insulation5, oil and water absorbents6,7 and energy storage devices8. Sehaqui et al.9 produced highly porous materials from TEMPO-oxidised cellulose nanofibrils (CNFs). With this procedure the samples were freeze-dried from tert-butanol and dry materials were achieved with porosities of 92.8–99.0 % and strengths of 3–238 kPa. Cai et al.10 prepared highly porous materials by gelation from aqueous sodium hydroxide/urea solutions followed by drying with supercritical CO2. More recently, Erlandsson, et al.11 prepared chemically cross-linked aerogels by oxidising carboxymethylated CNFs with sodium periodate, followed by freezing at -18 °C. The aerogels were then thawed and solvent exchanged before being dried at room temperature. Another approach to produce light-weight materials utilised recycled fibres from newspaper Nguyen et al.12,13. The fibres were dissolved in a solution of sodium hydroxide and urea, which was then frozen to allow gelation, followed by solvent exchange to allow coagulation. After coagulation there was a solvent exchange back to water and the material was freeze dried. Cervin et al.14 prepared porous materials from CNFs by making Pickering foams using octylamine to tune the surface chemistry of the CNFs. The foam was produced by vigorous mixing where air was forced into the dispersion followed by drying of the wet foam at 60 °C. The dried foams became water stable after the introduction of aldehyde groups, and absorbed 34 times their own weight of water and had porosities between 87 and 99.6%. Following a similar procedure, Liu et al.15 prepared Pickering foams by combining different ratios of wood fibres, microfibrillated cellulose, and sodium dodecyl sulfate as surfactant. Agar was used as a secondary liquid to reinforce the network and polyamide epichlorohydrin was added as wet strength additive. The foams had densities between 90.8 and 92.1 % and compressive strengths between 3.5 and 13.8 kPa.

There are thus several methods to produce highly porous and light-weight materials from cellulose, especially from CNFs. However, one disadvantage with CNFs is the high cost associated with the production of the material, together with the low solids content of the prepared material. Furthermore, conventional drying methods are expensive and difficult to scale-up. In this study, we present a simple method for preparing highly porous, low-density cellulose-based materials from chemically modified wood fibres. To produce these materials, the fibres were first modified in aqueous media, followed by self-assembling of the fibres and a final air drying. The fact that the fibres were not fibrillated to CNFs means a considerable decrease in the energy needed to produce these light-weight materials. It should also be made clear that the somewhat less environment-friendly chemicals such as sodium periodate or sodium chlorite were used in this study to modify the fibres, but it also needs be taken into 2 ACS Paragon Plus Environment

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consideration that there are effective ways of taking care of these chemicals and their reaction products; sodium periodate can fairly easily be

recycled16–18 and chlorite and other chlorine-

containing molecules formed during the oxidation step can be treated and made harmless with sodium thiosulphate19,20. The materials were characterized in terms of total aldehyde content, charge density, morphology, density and wet and dry mechanical properties in compression. Since no foaming agent or CNFs were used in the fabrication of these light-weight materials, they are hereafter referred to as fibre networks.

Experimental

Materials Softwood kraft fibres (K48) bleached according to a (OO)Q(OP)(ZQ)(PO) sequence were supplied by SCA Forest Products (Östrand pulp mill, Timrå, Sweden). The fibres were disintegrated and washed to remove metal ions and convert anionic charges to their sodium form21. Sodium metaperiodate (99%) was purchased from Alfa Aesar. Sodium carbonate (≥99.5%), hydroxylamine hydrochloride (99%), 2propanol (99.9%), sodium chlorite (80%) and hydrogen peroxide (30 wt% in water) were purchased from Sigma-Aldrich. Acetone (99%) was obtained from VWR chemicals. Standard solutions of sodium hydroxide (0.1 M) and hydrochloric acid (0.01 M) were purchased from Merck Millipore. Deionised water was used throughout the study.

Methods Preparation of light-weight materials: Fibres were suspended to a consistency of 20 g/l with the addition of 6.3%wt of 2-propanol as radical scavenger. The suspension was heated to 50 °C, and 1.35 g NaIO4/g fibre was added to the suspension. The reaction was allowed to proceed for 30 min in the dark. Thereafter, the fibres were thoroughly washed until a conductivity of 5 µS/cm of the washing water was achieved. The fibres were then resuspended to a consistency of 20 g/l with the addition of 2.5 mmol of NaClO2 and 2.5 mmol H2O2 per millimole of aldehyde (determined according to the protocol below). The suspension was acidified with 5 M HCl to pH 5 and allowed to react for 20 h at room temperature. Afterwards the fibres were washed until a conductivity of 5 µS/cm was reached. After being washed, the fibres were suspended to a consistency of 10 g/l and mixed using an IKA Ultra-turrax at 10 000 rpm for 0 to 30 min. Afterwards, 2.7 g NaIO4/g fibre was added to the fibre suspension and allowed to react for 16 h at room temperature, again protected from light. Finally, the light-weight materials were solvent exchanged to acetone for 15 min, followed by washing with water until the conductivity was 5 µS/cm. Finally, the material was solvent exchange to acetone twice, 15 min each time, before it was allowed to air dry at room temperature. The entire procedure is schematically described in Figure 1.

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Figure 1. Schematic protocol describing the preparation of light-weight fibre networks.

Carbonyl content The aldehyde content was measured as the total carbonyl content by titration with hydroxylamine. The fibre suspension and 100 ml of 0.25 M of hydroxylamine were set to pH 4 before they were allowed to react. Thereafter the suspensions were mixed and allowed to react for at least 2 h at room temperature. Afterwards, the fibres were filtered and dried overnight in an oven at 105 °C. The filtrate was titrated back to pH 4 with 0.1 M NaOH and the carbonyl content was calculated from the moles of NaOH used22,23.

Charge density Prior to charge determination, the fibres were set to pH 2 and kept at this pH for 20 min. The fibres were then washed until the conductivity was 5 µS/cm and approximately 0.25 g of fibres were resuspended in 485 ml of water, 10 ml of 0.1 M NaCl and 5 ml of 0.1 M HCl. Thereafter, 0.1 M NaOH was added using a Metrohm 702SM Titrino titrator at a rate of 50 µL/min until a conductivity of ~140 µS/cm was reached. Finally, in order to determine the exact dry mass of fibres used, the fibres were filtered off and dried overnight in an oven at 105 °C. Two measurements were performed per sample.

Scanning electron microscopy A Hitachi S-4800 Field Emission Scanning Electron Microscope (EM) operating at 1 kV was used to capture the structure of the fibre networks. Prior to imaging, the materials were sputtered with a Pt-Pd coating in a 208 HR Cressinton sputter for 30 seconds.


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Brunauer-Emmett-Teller (BET) specific surface area determination: BET surface area was determined by nitrogen physisorption using a Micrometrics ASAP 2020 (Norcross, USA). A sample of 0.2–0.25 g was first dried overnight at 80 °C in a vacuum oven. Thereafter, the sample was placed in the instrument for degassing at 80 °C for 10 h prior to analysis. The analysis was performed in the relative pressure interval 0.05–0.25 at -196 °C.

Density and porosity The density of the materials was calculated by dividing their weight by their volume. The porosity was estimated according to:

(1) Where ρ* is the calculated density of the material and ρc is the density of the cellulose 1500 kg/m3. Mechanical properties The mechanical properties of the fibre networks were evaluated on both dry and wet fibre networks using an Instron 5566 (Norwood, MA, USA) universal testing machine equipped with a 500 N load cell. The samples consisted of cylindrical pieces with a radius of 160 to 240 mm, and a thickness between 9 and 110 mm. The wet samples were soaked in water just before they were placed between two flat compression plates, and compressed at 10%/min up to 80% compression. For the wet samples, the compressive strain was reversed after reaching 80% compression, and retracted at the same rate until 0% compression was reached. Each sample was tested in duplicate. The compressive modulus was calculated from the initial linear section of the compression curves and the yield strength was calculated as the strength reached at the intersection between the tangent to the elastic region and the tangent to of the plateau region.

Results and discussion Chemical modification Cellulose fibres were modified to produce light-weight materials according to Figure 1. The first step was to perform periodate oxidation to selectively cleave some of the C2–C3 bonds of cellulose and partially produce dialdehyde cellulose. Thereafter some of the aldehydes were further oxidized to carboxylic acids with sodium chlorite. After the chlorite oxidation, mechanical mixing for 0 to 30 min at 10 000 rpm was performed, owing to the high charge density of the fibres, in order to partly liberate some fibrils and thereby tune the properties of the material in terms of density, porosity and mechanical strength. After the mixing, 2.7 g NaIO4/g of fibre was added to the fibre suspension and allowed to react for 16 h (without any mixing). During this time, the fibres self-assemble into the shape of the container used, in this case into a cylindrical shape, followed by an isotropic shrinkage. 5 ACS Paragon Plus Environment


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This phenomenon has been observed earlier for other systems11,24, but the mechanism behind the selfassembly and shrinkage upon continued periodate oxidation is still not fully understood and further studies are therefore needed and are in progress. The self-assembled structure was finally solvent exchanged to acetone to lock the structure and avoid collapse during the subsequent drying, during which no measurable further shrinkage was observed. Figure 2 shows that the mixing also affected the final size of the material, i.e. the shrinkage during self-assembly, as the mixing time increased, making it more compact. It should be mentioned that periodate oxidation has no major effect on the water retention value (WRV)25.On the other hand, introduction of charges (chlorite oxidation) is known to increase the WRV. Since a material can be formed without introduction of charges (i.e. periodate oxidation alone), an improvement in WRV is not likely to explain the observations made.

Figure 2. Appearance of the light-weight materials produced with cellulose fibres. From left to right: Sample without mixing, 10 min mixing, 20 min mixing and 30 min mixing.

The degree of modification after periodate oxidation was determined by titration with hydroxylamine. As can be seen in Table 1, no significant amount of aldehydes was present before the chemical modification, but after the first periodate oxidation 1.35 mmol of aldehydes per gram of fibre were formed, which is equivalent to about 11% of the theoretical maximum. The subsequent chlorite oxidation only partially oxidized these aldehydes and 0.54 mmol aldehydes per gram of cellulose remained after the chlorite oxidation step. After the second periodate oxidation, a total of 4.50 mmol of aldehydes per gram of fibre were determined. In total, 5.2 mmol of glucose per gram of cellulose, or about half of all the glucose units in the starting material were modified. For each gram of material, this value is the result of 1.35 mmol of aldehydes produced during the first periodate oxidation and 4.5 mmol of aldehydes determined after the second periodate oxidation minus the aldehydes remaining after chlorite oxidation.

As can be observed in Table 1, the mixing had no significant influence on the amount of aldehydes formed in the second periodate oxidation step. The reference fibres had an initial charge density of about 60 µeq/g. After the first periodate oxidation, the charge density decreased slightly to about 48 6 ACS Paragon Plus Environment

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µeq/g. It has earlier been suggested that this decrease is due to the oxidation of hemicelluloses which are then lost during the washing of the fibres26,27. After chlorite oxidation, the aldehydes were converted to carboxylic acids and the charge density increased to 870 µeq/g of fibre, which is well in agreement with the difference in aldehyde content before and after chlorite oxidation (1.35 mmol/g minus 0.54 mmol/g).

Table 1. Aldehyde content and total charge density after each oxidation step of the fibre network fabrication protocol. Total charge density

Sample

Carbonyl content (mmol/g)

Reference

0.03 ± 0.01

60 ± 3

Periodate oxidation for 30 min

1.35 ± 0.24

48 ± 4

0.54 ± 0.03

870 ± 5

Periodate oxidation for 30 min and chlorite oxidation for 20 h Periodate oxidation for 30 min, chlorite oxidation for 20 h and a second periodate oxidation for 16 h

(µeq/g)

0 min mixing*

4.4 ± 0.3

578 ± 6

10 min mixing*

4.3 ± 0.1

586 ± 10

20 min mixing*

4.4 ± 0.1

571 ± 13

30 min mixing*

4.4 ± 0.2

594 ± 9

* Ultra-turrax mixing time of the fibres before the second periodate oxidation

Morphology of the fibres Figure 3 shows SEM images of the fibre networks. As can be seen, the fibres are randomly oriented and connected in a 3D structure. The fibres are long, i.e. there is no obvious (qualitative) indication that the chemistry has affected the length, and have a diameter of the order of a few tens of micrometres. At higher magnification, the fibres display a fibrillar structure consisting of bundles of nanofibrils. With increased mixing, more and more fibril aggregates were observed. It has been reported that fibrillation results in an increased swelling of the fibres as well an increased specific surface area, which facilitates an increase in joint strength between the individual fibres in a paper28–31. Moreover, Hollertz et al. (2017)28 showed that the addition of carboxymethylated CNFs improves the mechanical properties of handsheets mainly due to an increase in density. However, when carboxymethylated CNFs were further modified with sodium periodate, that is the same chemistry used here, the improvement in mechanical properties were attributed to the formation of hemiacetal crosslinks between aldehydes and alcohols, since the improvement in strength was better than what could be expected from densification alone.

A fibre network was observed also without mixing (Figure 3a), but the network became denser with mixing. This could be a consequence of the loosening of fibrils at the fibre surface and within the 7 ACS Paragon Plus Environment


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modified fibre wall, which then not only could facilitate formation of stronger fibre–fibre contact points, but possibly also more contact points due to higher flexibility of the fibre wall. At higher magnification (for example Figure 3c3), it can clearly be observed that the fibre wall was not completely collapsed after drying, indicating an increased specific surface area. However, rather surprisingly, a specific surface area no greater than 1.2 m2/g was measured by BET. This value is only about four times larger than a smooth tube of similar dimensions as the fibres, which indicates that the pores seen in the fibre wall under SEM (third column in Figure 3) are closed and not available for gas adsorption, probably due to the procedure used for drying the fibres, despite large efforts to minimise capillary forces and keep the pores interconnected.

Figure 3. SEM images of the fully modified cellulose fibres forming light-weight materials, with and without mixing of the modified fibres. (a1–a3) No Ultra-turrax mixing, (b1–b3) 10 min, (c1-c3) 20 min and d) 30 min of mixing. The left-hand column shows images at 35 times magnification (scale bar


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equal to 1000 µm), the middle column images at 90 times magnification (scale bar equal to 500 µm), and the right-hand column shows images at 3500 times magnification (scale bar equal to 10 µm)

Mechanical properties The density and porosity of the fibre networks was calculated after weighing and measuring their dimensions. As seen in Table 2, without any mixing before the second periodate oxidation step, a density of 54 kg/m3 was obtained (porosity 96.3%), which increased to 82 kg/m3 after 30 min of Ultraturrax mixing (94.4% porosity), suggesting that mixing could be used to tailor the density of the material.

Table 2. Density and porosity of the wet-stable fibre networks. Mixing time (min)

Density (kg/m3)

Porosity (%)

0

54 ± 2

96.3 ± 0.2

10

65 ± 1

95.5 ± 0.1

20

67 ± 1

95.4 ± 0.9

30

82 ± 1

94.4 ± 0.1

The fibre networks displayed a compressive stress–strain behaviour (Figure 4) characteristic of porous open-cell materials, both in the dry and in the wet state. The compression curves show three deformation regimes: the first regime (ɛ ≤ 10%) relates to the elastic properties of the material, the second regime (10% ≤ ɛ ≤ 60%) relates to a continuous collapse at a more or less constant stress, where the cells collapse and opposite sides start to meet (densification stress) and the third regime (ɛ ≥ 60%) is a region where the material becomes stiff and the stress rises steeply due to dense structure32,33. All three deformation regimes were observed in the dry as well as the wet state, although less distinct and naturally at lower stress levels in the wet state, showing that the fibre network structure was greatly plasticized by water34. The structure remained visibly intact in the wet state, even at a compressive strain of 80%, suggesting that the structures were cross-linked, which was further supported by the high shape recovery capacity, seen when the load was released in Figure 4b and in Figure 5. The shape recovery is the result of the fibres being unable to move independently against each other due to inter-fibre crosslinks and the material is therefore able to regain its shape when the stress is released. In a non-crosslinked fibre network, the fibres are able to move more independently of each other and would therefore display permanent deformation and no shape recovery. The crosslinks are presumably due to the formation of hemiacetals in the fibre joints, formed from the aldehydes introduced during periodate oxidation and the native hydroxyl groups in the cellulose chain. The same crosslinking reaction and similar mechanical behaviour have been reported previously for cellulose aerogels prepared from periodate-oxidized CNFs11. An increase in paper wet strength has also been attributed to the formation of hemiacetal crosslinks between periodate-oxidized fibres22,26. 9 ACS Paragon Plus Environment


The presence of hemiacetals can be further demonstrated by soaking the fibre networks in alkaline water (pH 12) where the hemiacetal formation is reversed back to an aldehyde and a hydroxyl, and the network easily disintegrates. Together with its shape-recovery capacity, the material was able to absorb 13 times its own weight of water. Nguyen et al. (2014)13 reported aerogels made from recycled fibres with a water absorbency capacity between 15 to 20 times its own weight in water. On the other hand, aerogels prepared from CNFs have been reported to have a water absorbency capacity of up to 210 times its own weight35. However, these were produced by combining freezing and freeze drying.

a)

1750

a)

1500 1250

Compressive stress (kPa)

Compressive stress (kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 min 10 min 20 min 30 min

1000 750 500 250

b) 0 min 10 min 20 min 30 min

80

60

40

20

0

0 0

20

40

60

80

0

20

40

60

80

Compressive strain (%) Compressive strain (%) Figure 4. Stress–strain curves for fibre networks with increasing mechanical treatment in (a) the dry state and (b) the wet state.

Mixing of the fibres before fabrication of the dry networks resulted in materials with a higher mechanical strength, the more extensive the mixing the stronger being the material (Figure 4). This trend was observed for both dry and wet samples. In paper it is commonly observed that an increase in mechanical strength is accompanied by an increase in density. However, as seen in Figure S1 in the supporting information, the increase in mechanical strength in the fibre networks is due not only to a densification of the structure, but appears also to be a result of the increased area of contact in the fibre-to-fibre contact points. The increased contact and higher joint strength presumably stem from the partial fibrillation taking place during the mixing of the fibres, which, not only increase the contact area, but also increase the probability of forming inter-fibre crosslinks. This is further supported by the SEM images (Figure S1 in the supporting information) where the fibres subjected to the two longest mixing times show a film-like structure in the fibre joints. Interestingly, the modulus of the dry networks was unaffected by mechanical mixing for more than 10 min and remained stable at about 1 MPa (Table 3). The modulus of the wet samples was not significantly affected by the mixing, regardless of mixing time. This suggests that it is not the joint that contributes to the stiffness of the wet material but rather the wet properties of the fibre. The improvement in the mechanical properties


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of single fibres after periodate oxidation has been reported by López Durán et al.36, who showed that the modification not only affects the fibre network but also the individual fibres. It has also been shown that fibres with aldehydes and carboxylic acids are very stiff, which could explain the mechanical behaviour of the fibres networks25. There seems to be an optimal degree of mechanical treatment since the sample subjected to 30 min of mixing did not display any improvement in properties compared the material mixed for 20 min, indicating that the treatment starts to break down and weaken the individual fibres, and that at a certain point this weakening is not compensated by the increased area in contact. A further support for fibre weakening can indeed be found in Figure 3 where the fibres subjected to mixing display more kinks and a more irregular shape. At the same time, it is clear that a minimum amount of treatment is required to alter the material properties, since the properties of the 10 min sample did not significantly exceed these of the untreated sample. Only a weak/insignificant increasing trend in yield strength was observed for the dry and wet fibre networks with increasing mechanical treatment, while a significant effect was observed in the energy absorption, which increased continuously with increasing mixing time. Table 3 summarizes the Young’s modulus, the yield strength and energy absorption values of both dry and wet fibre networks. To place our results into the same context as other low-density cellulose materials, the values after 30 min of mixing, for example the Young´s modulus of 1.1 MPa, can be compared with the results of Sehaqui et al.37 who obtained a modulus of 1.36 MPa after preparing aerogels at a concentration of 3.5wt%. However, at higher concentrations of CNFs, the materials prepared by these authors were stiffer than the light-weight fibre networks reported in the present work. For further comparison, Gawryla et al.38 prepared aerogels from montmorillonite nanoplatelets and CNFs from tunicates and the resulting materials had a modulus of up to 788 kPa and a density of 101 kg/m3 when 7.5%wt clay and 3%wt nanofibers were used. The present material is crosslinked and can be compared to crosslinked aerogels prepared from cellulose nanocrystals, which had a compressive stress of up to 100 kPa in air and 50 kPa in water at 95% compression39. In our case, after 30 min of mixing, the wet fibre networks showed a compressive stress of 85 kPa at 80% compression.

Table 3. Mechanical properties of the fibre networks. Young’s

Yield

Energy

Young’s

Yield

Energy

Mixing

Modulus

strength

absorption

Modulus

strength

Absorption

time

dry

dry

Dry

wet

wet

Wet

3

(MPa)

(kPa)

(kJ/m )

(kPa)

(kPa)

(kJ/m3)

0 min

0.6±0.02

67±30

148±19

11.3±3.3

0.8±0.4

2.2±0.1

10 min

1.1±0.1

98±5

146±6

11.5±1.3

1.2±0.1

2.7±0.2

20 min

1.0±0.1

67±22

208±7

11.7±2.3

1.8±0

4.1±0.5



30 min

1.1±0.1

120±56

234±22

14.0±9.8

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2.1±0

5.9±0.7

Figure 5. Qualitative demonstration of the shape recovery of the wet fibre networks. a) before any compression, b) under compression and c) after release of the compressive load.

Conclusions A novel method to produce light-weight cellulose-fibre-based materials has been developed. The fibres were first modified by a periodate oxidation of the cellulose molecules to introduce aldehydes. These aldehydes were then further oxidized to carboxylic acids by chlorite oxidation, followed by a mechanical treatment with a high intensity mixer and a subsequent second periodate oxidation during which the fibres self-assemble into an interconnected fibre network. Due to the high charge density provided by the carboxyl groups formed during chlorite oxidation, the fibres are more prone to fibrillation during the high-shear mixing. It was shown that the mixing time could be used for tuning of the properties of the materials in terms of density, porosity and mechanical strength. As the mechanical mixing time increased, the density and the strength of the fibre networks increased, and it is suggested that this is due to an increase in the number of fibre joints and an increased contact zone. However, the modulus was not affected by the mechanical treatment. Owing to the presence of aldehydes, the fibre networks showed a significant wet strength and a capacity to recover their shape after wet compression. The properties of the fibre networks have modulus and strength values similar to those of materials prepared from nanocellulose. In this study chemical modification in aqueous media, followed a solvent exchange and air drying was sufficient to produce this new type of lightweight material with an astonishingly high wet resilience. The protocol is very interesting for scale-up since it does not involve any freeze-drying, freezing or supercritical drying.

Associated content: The following supporting information is available online: SEM image of fibre-fibre joints after 30 min of mixing and density-normalized stress–strain curves for fibre networks

Author information: Verónica López Durán: [email protected] 12 ACS Paragon Plus Environment

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Per A. Larsson: [email protected]

Acknowledgements The authors acknowledge the Swedish Governmental Agency for Innovation Systems, VINNOVA for financial support through BiMaC Innovation Excellence Centre. L. Wågberg also acknowledges the financial support of the Wallenberg Wood Science Center. Johan Erlandsson acknowledges the Swedish Energy Agency for funding through Batterifonden and the MODULIT-project.

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A new generation of cellulose-based lightweight materials has been developed by using chemically modified wood fibres. The reactions were performed under aqueous system and the chemicals can be recycled. 207x120mm (300 x 300 DPI)

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Novel, Cellulose-Based, Lightweight, Wet-Resilient Materials with

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