Strong, Water-Durable, and Wet-Resilient Cellulose Nanofibril

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Strong, Water-Durable and Wet-Resilient Cellulose Nanofibril-Stabilized Foams from Oven Drying Nicholas Tchang Cervin, Erik Johansson, Per A. Larsson, and Lars Wagberg ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00924 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 14, 2016

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ACS Applied Materials & Interfaces

Strong, Water-Durable and Wet-Resilient Cellulose Nanofibril-Stabilized Foams from Oven Drying Nicholas Tchang Cervin,*, a, b Erik Johansson, c Per A. Larsson,b, d and Lars Wågberga, b

a

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

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden c

Cellutech AB, SE-114 28, Stockholm, Sweden

d

BiMaC Innovation, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden

*Corresponding author, [email protected]

Abstract Porous materials from cellulose nanofibrils (CNFs) have been prepared using Pickering foams from aqueous dispersions. Stable wet foams were first produced using surface-modified CNFs as stabilizing particles. These foams were then dried in an oven on a liquid-filled porous ceramic frit to better maintain the homogeneous pore structure of the foam after drying. The cell structure was studied by scanning electron microscopy and liquid porosimetry, the mechanical properties were studied by compression testing and the liquid absorption capacity was determined both with liquid porosimetry and by soaking in water. By controlling the charge density of the CNFs, it was possible to prepare dry foams with different densities, the lowest density being 6 kg m3 , i.e. a porosity of 99.6%. For a foam with a density of 200 kg m-3, the Young´s

modulus in compression was 50 MPa and the energy absorption to 70% strain was 2.3 MJ m-3. The use of chemically modified CNFs made it possible to prepare cross-linked foams with water-durable and wet-resilient properties. These foams absorbed liquid up to 34 times their own weight and were able to release this liquid under compression and to reabsorb the same amount when the pressure was released.

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Keywords Cellulose nanofibrils, particle stabilized foams, Pickering emulsions, Pickering foams, porosity, water absorption

Introduction Porous materials and lightweight foams are of general interest in a diversity of applications due to their low density and high specific surface area. By utilizing their high bulk it is possible to prepare light and stiff components such as sandwich panels and large portable structures, and by further utilizing their open structure it is possible to tailor liquid-absorbent materials that are able to store and distribute large amounts of liquids under a high mechanical load.1-3 The low thermal conductivity of these porous materials also promotes their use in different thermal insulation applications and the comparably low compression stiffness makes the foams ideal, for example, for a wide range of cushioning applications, e.g. elastomeric foams for seating applications.1, 4 The strength and large compressive strain of these foams also make them attractive for energy-absorbing applications.1 Today, different types of synthetic polymer foams dominate in the commercial field of foams,2 and it is of special interest to replace petroleum-based polymers with polymers from renewable and biodegradable resources. In this respect, cellulose nanofibrils (CNFs) are very interesting since cellulose is the most abundant natural polymer on Earth. In addition, CNFs have been shown to be an excellent material for the preparation of Pickering foams due to their excellent mechanical properties and importantly also their high aspect ratio.5-6 The preparation of these foams is rather simple, and, due to their excellent wet-stability6, the foams are also promising for preparation in large volumes on an industrial scale. Porous cellulose materials from CNFs, such as aerogels, are today made mainly by processes including freeze drying3, 7-8 or the use of supercritical carbon dioxide.9-11 These methods are however batchwise and mainly suitable for small-scale production. It is therefore desirable to develop a method more suitable for scale-up. In this respect, the Pickering foam technique8-10 is appealing, provided that the water associated with the foams can be removed in an efficient way. The technique is closely related to Pickering emulsion technology where solid particles are used to stabilize the oil–water interface.9-10 The advantage of using colloids instead of surfactants is the large gain in free energy when a colloid is adsorbed at the air/water interface compared with when single surfactant molecules are adsorbed at the interface.12-13 However, it was only recently shown that partially hydrophobic particles can accumulate at the gas– liquid interface and stabilize air bubbles in dilute surfactant-free suspensions.14-20 This pioneering work was mainly performed using inorganic particles, but polymeric rods have also been reported to stabilize foams in a very efficient way, due to their high aspect ratio.21-22 Our previous investigations with CNF-stabilized foams have shown that it is possible to dry wet-stabilized foams into dry porous materials.6 In the present work, the drying technique has been further refined to resolve the problems related to the formation of cavities inside the dry material, and the present results show that by drying the wet foam on a water-filled porous 2 ACS Paragon Plus Environment

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ceramic frit, a more uniform pore structure is created, without the risk of forming large cavities inside the foam. We also present a new method for chemical crosslinking of the CNFs to produce water-durable and wet-resilient foams. This is of great importance since the material can absorb liquid, be squeezed to release the liquid and then reabsorb the same amount of liquid.

Experimental Materials Cellulose fibers A never-dried commercial sulphite softwood dissolving pulp (Domsjö Dissolving Plus; Aditya Birla Domsjö, Domsjö, Sweden) was used as raw material for CNF production. The pulp was composed of 60% Norwegian spruce (Picea abies) and 40% Scots pine (Pinus sylvestris). The hemicellulose content was 4.5% and the residual amount of lignin was 0.6%. Chemicals TEMPO (2,2,6,6-Tetramethyl-1-piperidinyloxy) (99%), sodium bromide (99%) and sodium hypochlorite (14% active chlorine) used for oxidation of the cellulose prior to homogenization to CNFs were purchased from Sigma Aldrich, Alfa Aesar and VWR Chemicals, respectively. Octylamine (99%) and Triton X-100 (10% in water) for foam preparation were purchased from Sigma Aldrich. Sodium (meta)periodate (98%) for the cross-linking of the foams was purchased from Alfa Aesar, and hydroxylamine hydrochloride (99%) for the determination of carbonyl content was purchased from Sigma-Aldrich. All other chemicals, such as sodium hydroxide and hydrochloric acid, were of analytical grade. All chemicals were used as received without further purification. Cellulose nanofibrils CNFs with different charge densities were prepared according to a previously described method.23 TEMPO (0.32 g) and sodium bromide (2.0 g) were added to suspensions of fibers (20 g) in Milli-Q water (2 L), and the suspensions were adjusted to pH 10. Sodium hypochlorite adjusted to pH 10 was then added to the continuously stirred suspension; 20, 60 or 100 mmol of sodium hypochlorite was added to produce TEMPO-oxidized fibers with charge densities of approximately 300, 815 or 1400 µeq/g. The oxidation lowers the pH and it was therefore carefully readjusted to pH 10 by adding sodium hydroxide (1 M) until no pH change could be detected. All reactions were performed at room temperature. The TEMPO-oxidised fibers were then converted to CNFs using a high-pressure fluidizer (Microfluidizer M-110EH, Microfluidics Corp.). Homogenization was performed by a single pass through a 400 and a 200 µm chamber connected in series and six passes through a 200 and 100 µm chamber connected in series at a fiber concentration of 1.0 wt% and operating pressures of 1000 and 1650 bar respectively for the two chamber combinations.



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Methods Cross-linking of cellulose nanofibrils To cross-link the foam during drying, aldehydes were introduced prior to foam formation by periodate oxidation of the CNFs (using the 815 µeq/g CNFs), as schematically shown in Figure 1.24-27 Under high-intensity mixing (10 000 rpm) using an Ultra Turrax, 0.35 g sodium periodate per gram of cellulose, i.e. an amount stoichiometrically corresponding to an oxidation of about 25% of all C2–C3 bonds in the material, was added to a 16 g/L CNF aqueous suspension. After 24 h the reaction was stopped by dialysis against deionized water using dialysis tubing with a cut-off of 8 kDa.

a)

b)

c)

Figure 1: a) Non-modified cellulose, b) periodate-oxidized cellulose, i.e. dialdehyde cellulose, c) a cross-liking reaction between an aldehyde and a neighboring hydroxyl to form a hemiacetal. Foaming and drying of CNF-stabilized foams Octylamine was physically adsorbed on the CNFs to tune their wettability and optimize the adsorption of the CNFs at the air–liquid interface.6 Using a high-speed mixer (UltraTurrax), octylamine equivalent to one third of the total amount of charges of the CNFs (5 wt% of the cellulose content when the charge density of the CNF was 815 µeq/g) was added to the CNF suspension. For the 300 µeq/g foam, this meant 60 g of 1 wt% CNF gel mixed with 1.09 mL octylamine at a concentration of 9.6 g/L. For the 815 µeq/g foam, the corresponding amounts were 60 g of CNF gel and 2,85 mL octylamine, and for the 1400 µeq/g foam 60 g of CNF gel and 4.44 mL octylamine. The mixture was then foamed at 2000 rpm using an overhead stirrer until no further increase in foam volume could be observed. The wet foam was then placed on a porous, water-filled ceramic frit with closed sides. The wet frit had been degassed under vacuum to fill the pores with water and then placed on a supporting metal plate. The foam was then dried at 60 °C in an oven without forced convection. To create a more humid environment, and thereby retard the drying rate, a perforated aluminium cover was placed around the foam. The wet foam was also dried on a homogeneous PTFE plate for comparison. Compressibility testing All the compression tests were performed on foams conditioned at 23 °C and 50% relative humidity in accordance with the ISO 844-2007 standard, using an Instron 5944 universal 4 ACS Paragon Plus Environment

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testing machine equipped with Instron compression plates (T1223-1021) and a 50 N load cell. Test pieces were cut with a razor blade to a square cross-section of approximately 5x5 mm and a height of 10 mm. The compression rate was 10% of the original sample thickness per minute. The final strain was set to 95% of the original sample height in order to be able to evaluate the material behaviour over a large degree of deformation. The energy absorbed by the material was calculated as the area below the stress–strain curve between 0% and 70% strain. Between four and eight specimens were reported for standard deviations. Liquid absorption capacity A TRI/Autoporosimeter (APVD) version 2008-12 (TRI/Princeton, Princeton, USA)28 was used to measure the cumulative pore-volume distribution of the foams using water. The membrane cut-off pore radius was 1.2 µm, which effectively limited the smallest measurable pore radius to about 5 µm. Cumulative pore volume distributions were recorded based on 13 pressure points corresponding to pore radii in the range of 5 to 500 µm. The pore radius corresponding to a given chamber gas pressure was calculated according to the Laplace equation assuming full wetting when water was used with TritonX-100 added to lower the surface tension (30 mN m-1). Determination of carbonyl content A modified version of an earlier established protocol was used to determine the carbonyl content of the oxidized CNFs.29-30 An amount of 0.20 g of periodate-oxidized CNFs, mixed with MilliQ water and adjusted to pH 4, was added to 25 ml of 0.25 M hydroxylamine hydrochloride, also adjusted to pH 4. One molecule of hydroxylamine then reacts with one carbonyl and produces one free proton that consequently reduces the pH of the solution. After 2 h of reaction, sodium hydroxide (0.10 M) was used to titrate the pH back to 4, i.e. where the total amount of sodium hydroxide added corresponded to the total amount of carbonyls in the oxidized CNF. Scanning Electron Microscopy To study the micro-structure of the foams, the specimens were studied with a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) to obtain secondary electron images. The specimens were fixed on a metal stub with colloidal graphite paint and coated with a 6 nm thick gold/palladium layer using a Cressington 208HR High Resolution Sputter Coater. The specimens were cut into square cubes using a sharp scalpel at ambient conditions.



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Results and Discussion Formation and drying of CNF-stabilized cellulose foams Wet foams were achieved by high-speed mixing as earlier described by Cervin et al.5-6 and dry foams were formed by drying at 60 °C. The whole process is schematically described in Figure 2. By controlling the charge density of the CNFs (300, 815 and 1400 µeq/g) it was possible to prepare wet foams with different volumes from a given amount of CNF-gel i.e. foams including different volume fractions of air (Table 1). Anionic charges, in this case carboxyl groups, were introduced onto the CNF surface by oxidizing the CNFs with TEMPO as earlier described by Saito et al. 2007.23 A recent paper5 shows that the higher the charge density of the CNF the thinner was the CNF film formed at the air–water interface during foaming, due to electrostatic repulsion between the CNFs. This is probably also the explanation of why the volume of the foam was greater for more highly charged CNFs. Another possibility is a more efficient liberation of individual CNFs from CNF aggregates, resulting in a lower average diameter of the CNFs and a greater concentration of CNFs at the air–water interface for a given CNF addition. This altogether facilitates formation of wet foams with different stabilities.5 Table 1: Properties of wet and dry foams fabricated from CNFs with different charge densities. Energy absorption was calculated between 0 and 70% compressive strain. The Young’s modulus and energy absorption values relate to compression along the vertical axis. The CNF foam8 was freeze-dried from water, the only difference between the two samples being the density. The freeze-dried CNF aerogel9 was freeze-dried from tert-butanol. Values in parentheses represent standard deviations. CNF-stabilized foams

Freeze-dried CNF foam8 n/a n/a

Freeze-dried CNF aerogel9 n/a

CNF charge density (µeq/g) Starting volume (mL) Volume after foaming (mL)

300

815

1400

60

60

60

n/a

n/a

n/a

63

90

150

n/a

n/a

n/a

Volume increase (%) Density dry foam (kg m-3) Porosity (%)

5

50

130

n/a

n/a

n/a

200 (10)

13 (2)

6 (0.7)

7*

12*

14*

87 (0.8)

99.1 (0.1)

99.6 (0.1)

99.5*

99.2*

99.0*

0.2 (0.08)

0.2 (0.03)

0.06*

0.2*

0.03 (0.003)

24 (6)

6,0 (0,1)

7,8*

29,6*

3.2 (0.4)

19 (4)

8 (0.6)

8*

23*

11 (0.8)

Young’s modulus 50 (7) (MPa) Yield strength 3500 (100) (kPa) Energy absorption 2340 (484) (kJ m-3) * No standard deviation reported


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Figure 2: Schematic illustration of the drying of the wet CNF-stabilized foams: a) a CNFstabilized wet aqueous foam with a dry content of approximately 1 wt%, b) dry porous ceramic frit (top) and water-infused porous ceramic frit (bottom) with closed sides, c) wet CNF-stabilized foam on top of a water-infused porous ceramic frit, d) drying at 60 °C with a frit placed on a metal plate to force evaporation in one direction, e) dry CNF foam on top of a dry porous ceramic frit. By placing the wet foam on top of a water-filled porous ceramic frit, it was possible to dry the foam and preserve the cellular structure in the material. Within 36 h, 130 mL of cellulose foam had dried completely, regardless of whether it was dried on a porous ceramic frit or on a solid PTFE plate. It was observed that foams dried on the PTFE plate had cavities and disrupted cells in the center of the foam, in contrast to the outer region, i.e. the region that was first dried (Figure 3). There are naturally different steps in the drying process which will induce different forces causing shrinkage of the foam.31 At first, the evaporation of a liquid from a saturated porous body exposes the solid network and a solid–vapor interface is created with a higher interfacial energy than the solid–liquid interface. Liquid will therefore move from the interior of the wet foam to prevent exposure to air of the solid. This has two consequences: first, liquid tends to flow from the interior along the pressure gradient according to Darcy’s law, and second, the capillary pressure, created when the porous network is exposed to air due to evaporation, is balanced by the build-up of a compressive stress in the network during shrinkage of the foam.31 As long as the rate of transport of liquid is comparable to the evaporation rate, the liquid remains interconnected in the lamellae between the pores. However, as the distance from the exterior to the drying front increases, the capillary pressure gradient decreases and so does the flow rate. Eventually it becomes so slow that the liquid near the outside of the body is isolated in pockets and the flow to the surface will stop.31 Isolated air pockets can then result in water columns with double-curved menisci leading to a capillary pressure that will compress the pore and may ultimately even destroy the pore structure, depending on the integrity and strength of the lamellae between the pores. This is probably the reason why cavities and disrupted lamellae could be seen in the middle of the foam when no saturated frit was used. When the drying instead took place on a 7 ACS Paragon Plus Environment


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water-filled porous ceramic frit, which can act as a water reservoir for the foam and provide a flow of liquid so that the liquid remains interconnected between the pores, cavities and disrupted zones were avoided since isolated air pockets could then be minimized and prevented for a longer time. When the last water was removed, the CNF concentration, and consequently the strength of the foam lamellae, was high enough to resist the capillary forces acting on the solid material.

a)

1 cm b)

1 cm Figure 3: a) Foam with a homogeneous cellular structure dried on water-filled porous frit, b) foam dried on a non-porous plate showing larger cavities. In Figures 4a and 4b, the low density (13 kg m-3) of the dry cellulose foam is demonstrated by placing the foam on a plant leaf. The cross-section of the foam (dried on a water-filled porous ceramic frit) shows the cellular structure and the homogeneity of the foam throughout the material. In Figure 4c, the formability and adaptability of the wet foam are shown by shaping the foam into a complex shape that is preserved throughout the drying step (Figure 4d). It was found that it was easier to shape the foam from the most highly charged CNFs, presumably since it was more aerated and thus contained less water per volume segment. By placing a cylindrical piece of metal in the bottom of the foam during drying it was also possible to create molded dry cellulose foams (Figure 4e).


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a)

a)

b)

a)

1 cm

1 cm

c)

d) A

e)

2 cm

2 cm

2 cm

Figure 4: a, b) Images of the cross-section of a porous CNF foam with a density of 13 kg m-3 (99% porosity), c) Wet foam illustrating its formability, d) the same foam as in c) after drying in an oven at 60 °C, e) molded dried foam. Pore structure and mechanical data of CNF-stabilized cellulose foams By oxidizing the CNFs with TEMPO, and thereby introducing carboxyl groups onto the CNF surface, it was possible to prepare dry foams of different densities and different Young’s moduli. It should be pointed out that these foams were dried on a PTFE plate prior to the compression tests, except for the cross-linked foam that was dried on a water-filled porous ceramic frit. Since the Young’s modulus is dependent on the density of the foam, the modulus of the foams is higher for CNFs with lower charge density (Figure 5). As shown in Figure 6 and Figure 7, the dry foam was anisotropic, and had a higher Young’s modulus when compressed in the direction perpendicular to the vertical axis (from the side) than in the vertical direction (from the top), see Figure 2 for illustration, whereas no significant difference in modulus was observed when the foam was compressed in the two directions perpendicular to the vertical axis. Figure 7 shows micrographs of sectioned foams and shows that the pores have a circular shape when viewed from the top, whereas they are more oval when viewed perpendicular to the vertical axis. During drying, the foam does not shrink significantly in the radial direction but it shrinks in the vertical direction.



14 12 3

Compressive stress (MPa )

6 kg/m (vertical) 3 13 kg/m (vertical) 3 200 kg/m (vertical)

10 8 6 4 2 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0

20

40

60

80

Compressive strain (%) Figure 5: Representative stress-strain curves, measured in compression, for CNF foams of different densities showing the durability against deformation. Vertical means that the foam is compressed from the top, see Figure 2. 0.5

Compressive stress (MPa)

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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3

13 kg/m (vertical) 3 13 kg/m (perpendicular) 3 13 kg/m without high density skin (perpendicular) 3 20 kg/m cross-linked (vertical) 3 20 kg/m cross-linked (perpendicular)

0.4 0.3 0.2 0.1 0.0 0

20

40

60

80

100

Compressive strain (%) Figure 6: Representative stress-strain curves, measured in compression, showing anisotropic behavior of the foam, compressed in the vertical direction (from the top) and perpendicular to the vertical direction (from the side), see Figure 2 for illustration. Stress-strain curves also for 10 ACS Paragon Plus Environment

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CNF foam without the high density surface skin, perpendicular to the vertical axis and for cross-linked CNF foam, along the vertical axis and perpendicular to the vertical axis.

Figure 7: SEM images of cross-sections taken a) parallel to the vertical axis of the foam, and b) perpendicular to the vertical axis of the foam. The foam had a density of 13 kg/m3. The fact that the foam shrinks in the vertical direction could explain the formation of the flattened ellipsoidal pores. When the pores are anisotropic and slightly elongated or flattened, the mechanical properties typically depend on the direction of the applied force with a higher modulus in the directions perpendicular to the grain.1 This could explain the higher modulus in the radial direction than in the vertical direction. Another explanation for the anisotropic properties of the foam could be density variations in the foam, where the denser outer layer of the foam gives a higher modulus when compressed perpendicular to the vertical axis. The exact thickness of this outer layer is not defined and the mechanical properties is furthermore very hard to determine experimentally and therefore it was decided to exclude a detailed analysis of the contribution from the different layers for the overall properties of the foams. However, from the SEM images in Figure 8 this outermost layer appears to be 50 µm thick and rather dense. This can explain the anisotropic behavior shown in Figure 6 and Table 2 where the modulus increases when compression is accomplished along the skin whereas the modulus is lower when compression is accomplished perpendicular to the skin. Figure 6 also shows the stress–strain properties for a cross-linked foam. Modification of the 815 µeq/g CNFs by periodate oxidation to create aldehyde groups on the surface of the CNFs (Figure 1), resulting in a carbonyl content of about 1 mmol/g CNF, made it possible to crosslink the foams through covalent bonds between the aldehydes and neighboring hydroxyl groups.31 The modulus is in the same range as for non-cross-linked foams but the energy absorption value is about 50% higher than the value of the non-cross-linked foam (Table 2). A comparison to commercial foams from synthetic polymers, for example expanded polystyrene (EPS), show that they typically have densities around 30 kg m-3 and higher. This means that the data reported here for the CNF-stabilized dry foams can be compared with 11 ACS Paragon Plus Environment


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those materials. EPSs usually show a linear relationship between elastic modulus and density and an extrapolation of their properties to densities corresponding to CNF foams (13 and 6 kg m-3) result in a Young´s modulus in the same range as the data in the present work.32 Previously reported results on mechanical properties for foams/aerogels made from CNF where the CNFs were freeze dried from water or from tert-butanol clearly demonstrate that the present CNF foams have very similar mechanical properties.8-9

Table 2: Summary of the mechanical properties for CNF-stabilized dry foams, measured in compression, illustrating the anisotropic behavior when compressed along the vertical axis and perpendicular to the vertical axis. Data is also shown for foams without the high density surface skin and for cross-linked foams. Values in parentheses represent standard deviations.

Along the vertical axis Perpendicular to the vertical axis Perpendicular to the vertical axis; foam without high density surface skin Along the vertical axis; cross-linked foam Perpendicular to the vertical axis; cross-linked foam

Density (kg m-3) 13

Young’s modulus (MPa) 0.2 (0.08)

Yield strength (kPa) 24 (6)

Energy absorption (kJ m-3) 19 (4)

13

1.2 (0.3)

44 (10)

33 (6)

10

0.2 (0.09)

18 (2)

34 (5)

20

0.2 (0.01)

13 (2)

19 (2)

20

1.1 (0.3)

96 (10)

70 (8)


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a)

b)

c)

d)

Figure 8: SEM micrographs of foams with a density of 13 kg/m3 a) the surface fraction of a CNF-stabilized foam, b) high-magnification image of the skin seen in a), c) the cellular structure of the dry foam, revealing both open and closed pores, d) high-magnification image of the pore cell wall consisting of sub-micron pores. Liquid Absorption The cross-linking of the foam made the cellulosic materials water durable and water resilient.26 When a 20 kg m-3 foam, dried on a water-filled porous ceramic frit, was fully compressed under water before the initial liquid absorption, it absorbed 34 times its own weight in 10 s when the compressive force was released. This corresponded to 44% of the total volume confined by the outer dimensions of the foam. When the absorption capacity of the same foam was measured after five days in water, it had increased to 38 times its own weight, corresponding to 63% of its total volume (Figure 9a). In contrast to foams made from non-periodate-oxidized CNFs, the foams made from cross-linking periodate-oxidized CNFs could be compressed to release the water, and then able to reabsorb the same amount of water (Figure 10). Without cross-linking, the foams could not be compressed without permanent deformation. If the cross-linked foam was not compressed before the initial absorption of liquid, it absorbed only four times its own weight after 10 s (Figure 9a). The effect of compressing the foam before initial absorption was also investigated by using the APVD equipment. Since the APVD is able to detect absorption only in pores with a radius below 500 µm, the measurements were performed by first absorbing and then desorbing the liquid. Upon 13 ACS Paragon Plus Environment


compressing the foam, the cell structure changed and showed a significant amount of pores below 5 µm, which were not detected in the non-compressed foam (Figure 9 b). The reason for this is most probably that the lamellae of the closed pores are partly cracked and opened during the dry compression, making the pores below 5 µm available for absorption. This explanation was further supported by a cracking sound during the first compression cycle. A cracking of the lamellae would be irreversible and would presumably affect the mechanical properties of a re-dried foam. The macroscopic dimensions upon rehydration and followed by drying at 100°C did not change for both a compressed and a non-compressed foam. This was as expected since the foam was dried from water the first time. The ability to absorb water corresponding to 34 times its own weight in 10 s and to release the water on compression makes the foam a valid alternative to liquid-absorbing aerogels made by freeze-drying.3 The foams have a very high porosity, but from the images it is difficult to determine whether the pores are interconnected or not. This question can at least partly be answered by simply placing a foam in water for a prolonged time to see whether it floats or not, and also by determining the amount of liquid absorbed using the APVD-equipment. When the foam was placed in water, it floated with only a small fraction (approximately 10%) below the surface. When it was forced beneath the surface it quickly surfaced again upon release of the applied force. This indicates that air is trapped inside the foam and that a significant fraction of the pores are closed. When the foams are compressed before exposure to liquid, these closed pores are opened and almost the entire foam would be under water.

40 35 30 25

Compressed gravimetrically 5 days

5

Compressed gravimetrically 10 sec

10

Compressed porosimetry

15

C

D

Noncompressed gravimetrically 10 sec

20

Noncompressed porosimetry

a)

Water absorption (mg liquid/mg foam )

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0

A

B


E

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b)

Cumulative pore volume (mg liquid/mg foam)

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20 Absorption (compressed) Desorption (compressed) Absorption (non-compressed) Desorption (non-compressed)

18 16 14 12 10 8 6 4 2 0 0

100

200

300

400

500

Pore radius (m)

Figure 9: a) Absorption of water by cross-linked CNF foams (20 kg m-3) treated in four different ways: (A) non-compressed foam; initial liquid absorption into pores up to 500 µm measured with APVD, (B) compressed foam; initial liquid absorption into pores up to 500 µm measured with APVD, (C) compressed foam. No pore-size restriction for liquid absorption and an absorption time of 10 s, measured gravimetrically, (D) Compressed foam. No poresize restriction for liquid absorption and an absorption time of five days, measured gravimetrically, E: Non-compressed foam. No pore-size restriction for liquid absorption and an absorption time of 10 seconds, measured gravimetrically. Regarding column d and e, no standard deviation could be reported due to too few measurements. b) Cumulative pore volume and pore-volume distribution, from APVD measurements, of the cross-linked CNF foam (20 kg m-3). The absorption/desorption cycle labelled compressed relates to the sample that was compressed before the liquid absorption. The absorption/desorption half-cycle labelled non-compressed relates to the sample that was not compressed before the liquid absorption.



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b)

a)

1 cm

1 cm

c)

d)

1 cm

1 cm

Figure 10: a) A cross-linked 20 kg m-3 CNF foam showing wet integrity after being soaked in water for 7 days, b) water-swollen foam containing 34 times its own weight of water after 10 s soaking in water, c) demonstration of the wet resilience and the ability to release the liquid from the foam, d) water-swollen foam after compression, reabsorbing the same amount of water as before the compression.

Conclusions Wet CNF-stabilized foams can be oven-dried at 60 °C on a water-filled porous ceramic frit in order to create a more homogeneous pore structure with less cavities and disruptions than a foam dried on a flat and homogeneous PTFE plate. It is also possible to make dry foams with different complex shapes. By controlling the charge density of the CNFs, it was possible to prepare foams with different densities. The foam was anisotropic, as demonstrated by the Young’s modulus and energy absorption capacity, and had values in the same range as earlier porous CNF materials made by freeze-drying. A foam material with a density of 20 kg/m3 had a Young’s modulus of 1.1 MPa and an energy absorption capacity of 70 kJ/m3 at 70% strain in the stiffest direction and 0.2 MPa and 19 kJ/m3 in the less stiff direction. The foam could be chemically cross-linked by introducing aldehyde groups onto the surface of the CNFs, creating a more water-stable and water-resilient foam that could absorb 34 times 16 ACS Paragon Plus Environment

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its own weight of water, be compressed to release the liquid and then reabsorb water to the same amount as in the first cycle.

Acknowledgement The Wallenberg Wood Science Center and Vinnova, through the VINN Excellence Centre BiMaC Innovation, are greatly acknowledged for financial support.

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Graphic for manuscript


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Strong, Water-Durable, and Wet-Resilient Cellulose Nanofibril

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