Lightweight and Strong Cellulose Materials Made from Aqueous

Article pubs.acs.org/Biomac

Lightweight and Strong Cellulose Materials Made from Aqueous Foams Stabilized by Nanofibrillated Cellulose Nicholas T. Cervin,*,† Linnéa Andersson,‡ Jovice Boon Sing Ng,‡ Pontus Olin,§ Lennart Bergström,‡ and Lars Wågberg*,†,§ †

Wallenberg Wood Science Center, KTH Royal Institute of Technology, Teknikringen 56, 10044 Stockholm, Sweden Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16c, 10691 Stockholm, Sweden § Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, 10044 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: A lightweight and strong porous cellulose material has been prepared by drying aqueous foams stabilized with surface-modified nanofibrillated cellulose (NFC). This material differs from other dry, particle stabilized foams in that renewable cellulose is used as stabilizing particles. Confocal microscopy and high speed video imaging show that the octylamine-coated, rod-shaped NFC nanoparticles residing at the air−liquid interface prevent the air bubbles from collapsing or coalescing. Stable wet foams can be achieved at solids content around 1% by weight. Careful removal of the water results in a cellulosebased material with a porosity of 98% and a density of 30 mg cm−3. These porous cellulose materials have a higher Young’s modulus than porous cellulose materials made from freeze-drying, at comparable densities, and have a compressive energy absorption of 56 kJ m−3 at 80% strain. Measurement with the aid of an autoporosimeter revealed that most pores are in the range of 300 to 500 μm.

1. INTRODUCTION Due to the increased need for renewable materials, it is highly motivated to replace petroleum-based polymers with polymers from renewable resources. Cellulose has a particularly good potential, being the most abundant renewable natural polymer on earth, with a crystalline structure and with methods available for preparing large volumes on an industrial scale. Cellulose chains with β-(1−4)-D-glucopyranose repeating units are packed in trees into long nanofibrils (NFC), approximately 4 nm in width and 500−1000 nm in length.1 Development during the last 10 years has resulted in energy efficient processes to liberate nanofibrils from cellulose fibers and the so prepared nanomaterial has received great interest as being part of replacement material for petroleum-based structures and is currently being considered as construction units for nanoscale materials engineering.2−5 One very interesting area is packaging materials where lightness and high structural strength are important and, since the fibrils have very good mechanical properties, that is, high stiffness and high stress at break, this material is interesting to use. Light-weight materials suitable for © 2012 American Chemical Society

insulation, absorption, and packaging are today mainly consisting of plastic foams made from polystyrene and polyurethane.6 These plastic foams are cheap and functional, but they are not environment-friendly since they are produced from a nonrenewable resource and they are not biodegradable. Replacement of these materials with renewable materials based on cellulose fibrils would be of both scientific and commercial interest even though there exists biodegradable foams from polylactic acid (PLA)7 and starch.8 Methods already exist for producing porous cellulose materials, including freeze-drying2,9,10 and supercritical carbon dioxide drying.11,12 These methods can however be questioned when large quantities are needed. If instead NFC could be used as stabilizing agent in a technique similar to Pickering emulsions, it should be possible to prepare low density cellulose materials with good mechanical properties. This Received: November 12, 2012 Revised: December 17, 2012 Published: December 19, 2012 503

dx.doi.org/10.1021/bm301755u | Biomacromolecules 2013, 14, 503−511

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procedure40 but using a carboxymethylation41 pretreatment of the fibers. The never-dried fibers were first dispersed in deionized water at 10000 revolutions in an ordinary laboratory reslusher. The fibers were then solvent-changed to ethanol by washing the fibers in ethanol four times with an intermediate filtration step. The fibers were impregnated for 30 min with a solution of 10 g of monochloroacetic acid in 500 mL of isopropanol and were then added in portions to a solution of NaOH, methanol, and isopropanol that had been heated to just below its boiling temperature in a reaction vessel fitted with a condenser. This carboxymethylation reaction was allowed to continue for 1 h. Following this carboxymethylation step, the fibers were filtered and washed first with deionized water, then with acetic acid (0.1 M) and finally with deionized water. The fibers were then impregnated with NaHCO3 solution (4 wt %) for 60 min to convert the carboxyl groups to their sodium form. Finally, the fibers were washed with deionized water and drained on a Büchner funnel. After this pretreatment, the fibers were homogenized using a highpressure fluidizer (Microfluidizer M-110EH, Microfluidics Corp.) equipped with two chambers of different sizes connected in series (200 and 100 μm). Homogenization was achieved with a single pass at a fiber concentration of 2 wt % and an operating pressure of 1650 bar. The charge density of the NFC was 647 μequiv/g. 2.2.2. Preparation of NFC-Stabilized Foams. NFC-stabilized foams were prepared by adding octylamine (pH = 9, assuming all groups protonated) to NFC. The dispersion was mixed using an Ultra Turrax mixer for 10 min at 8000 rpm and for another 10 min at 13500 rpm. The resulting NFC-octylamine mixture was foamed with a stainless steel milk beater (Severin model no: SM 9669) for 10 min (Figure 1).

process is similar to the Pickering emulsion technique,13 but the nonpolar phase in this case is air. Pickering emulsion technology has been used for more than a century to stabilize high-energy interfaces,13−18 but only recently has this concept been exploited for the preparation of ultrastable wet foams16,17,19−23 and for the production of a wide range of low-density, porous materials.24−28 When particles are partially lyophobic or hydrophobic, they attach to the gas− liquid interface, because it is energetically favorable for particles to reside at the gas−liquid interface and replace part of the high-energy solid−liquid area by a lower energy solid−gas area. From the point of stability, the particles should preferably attach to the interface with a contact angle of approximately 90°.16 This contact angle is ultimately determined by the balance between the gas−liquid, gas−solid, and solid−liquid interfacial tensions. In contrast to surfactants, particles tend to adsorb strongly at interfaces due to the high adsorption energy,16,22 and particle-stabilized foams exhibit outstanding stability compared to surfactant-based systems. Coalescence is hindered by the steric repulsion between the attached particles and the particles form a layer at the interface that strongly resists shrinkage and expansion of bubbles, minimizing Ostwald ripening, and creating long-lasting stable foams.29 Interestingly, previous work has suggested that anisotropic particles with a high aspect ratio are more efficient than spherical particles for stabilizing emulsions and foams due to their higher surface coverage and the possibility of forming intertwined networks of high mechanical stability.30,31 Indeed, several studies on, for example, polymer microrods32 and cellulose17 have shown that rod-shaped particles often produce more stable particle-stabilized emulsions and foams than spherical particles. Although work on particle-stabilized emulsions using nanosized rod-shaped particles, primarily involving the use of carbon nanotubes 33,34 and also NFC,35−37 have been published, studies on foam stabilization using NFC are lacking. The objective of this work was a strong porous NFC foam, prepared from surface modified cellulose nanofibrils, and to study the influence of NFC on foam stability. Efforts were made to preserve the aqueous foam structure in a dry state and to characterize the material and its mechanical properties and in this way establish a base for a new route for the presentation of micro- and nanoporous cellulose foams.

2. EXPERIMENTAL SECTION 2.1. Materials. In the manufacture of NFC, a commercial sulphite softwood dissolving pulp (Domsjö Dissolving Plus; Domsjö Fabriker AB, Domsjö, Sweden) from 60% Norwegian spruce (Picea abies) and 40% Scots pine (Pinus sylvestris) with a hemicellulose content of 4.5% and a lignin content of 0.6% was used as previously described.38 The pulp was thoroughly washed with deionized water and used in its never-dried form. Polished silicon wafers, with a natural silicon oxide layer of about 4 nm,39 were obtained from MEMC Electronic Materials SpA (Novara, Italy) and used as substrates for the preparation of model cellulose surfaces. Polyethyleneimine (PEI) was used to anchor NFC to the silicon wafers and had a molecular weight of 60 kDa, according to the supplier. It was delivered as a 50% aqueous solution from Acros Organics, U.S.A. The polymer was used without further purification. Octylamine (99%) was purchased from Sigma Aldrich and used without further purification. 2.2. Methods. 2.2.1. Preparation of NFC. The NFC was prepared at Innventia AB, Stockholm, Sweden, with the aid of a high-pressure homogenization technique similar to a previously described

Figure 1. Schematic description of the different steps for the preparation of NFC-stabilized foams: (a) NFC-gel (2 wt % in aqueous solution); (b) octylamine mixed with the NFC-gel in an Ultra Turrax mixer; (c) octylamine attaches to the NFC due to electrostatic adsorption; (d) air bubbles are created by a beater and covered with the modified NFC; (e) aqueous foam stabilized by NFC; (f) the wet foam is dried at ambient conditions and a porous cellulose material is formed, as shown in Figure 2. The aqueous NFC-stabilized foam was poured into a Büchner funnel with a filter paper (Munktell grade 3) to drain the excess of water before the foam was allowed to dry at ambient atmosphere. To speed up the drying process, the foams were also dried in an oven at 60 °C with a perforated aluminum cup over the foam to increase the moisture content in the immediate surroundings and to minimize the convection. 504


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2.2.3. Foam Stability Test. Foam stability tests were conducted by foaming NFC and octylamine with different concentrations of octylamine. Octylamine with a volume of 30 mL and a concentration of 0.8 or 2.4 g L−1 corresponds to one-third of the NFC charge and all the NFC charges, respectively, at pH = 9, assuming full protonation of octylamine and dissociation of the NFC charges. The foam was then transferred to vials as shown in Figure 4 and the height of the bubble structure was determined over a period of 10 days (Figure 4). 2.2.4. Confocal Microscopy. To visualize the location of NFC at the air−water interface, a series of experiments were conducted where fluorescently labeled NFC and confocal microscopy were used. To label NFC, 100 mL of aqueous NFC dispersion with a concentration of 1.2 g NFC L−1 and a pH of 4−5 was reacted with 4.8 mg of the condensation agent 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC). The dye was then added, 2 mg of 4-(N,Ndimethylaminosulfonyl)-7-piperazino-2,1,3-benzoxadiazole (DBDPZ), and the color of the mixture changed to orange. The NFCdispersion was left to equilibrate overnight and was then dialyzed against deionized water. The DBD-PZ-tagged NFC was then used to prepare NFC-stabilized foams according to the procedure described above. An inverted Zeiss Axiovert Observer.Z1 microscope equipped with a LSM 5 Exciter scanner was used for confocal laser scanning microscopy imaging. A diode 405−25 nm laser was used together with a long-pass 420 nm filter to image the bubbles stabilized by the (DBDPZ)-tagged cellulose. A 10×/0.45 NA objective lens was used for all imaging, the pinhole was fully opened and profiles were stored as 8 or 12 bit line scans with a resolution of 512 × 512 pixels representing an area of 146.2 × 146.2 μm. 2.2.5. Observations of Bubble Coalescence. Two solutions were prepared, one with octylamine-treated NFC at a charge ratio of 1:1 and with a concentration of 130 mg L−1 and the other with octylamine alone at the same concentration, that is, 130 mg L−1. Small vessels (1.8 mL) were filled with one of the solutions and a bubble of air was introduced with a glass pipet. The vessel was then placed in a TestTube Rotator from Labinco and rotated for 10 min to allow the bubble to adsorb the stabilizing agent. Two bubbles of the same stabilizing agent were then transferred in succession to a test tube with Milli-Q water and recorded with a high speed camera upon contact at the top of the test tube (see Supporting Information). The high speed camera was of model IDT N4S3: sensor, CMOS Polaris II; objective, Pentax Cosmicar 50 mm/F1.4; shutter speed, 41 μs; frame rate, 3000 fps (frames per second); illumination, IDT 7 LED 40 mm middle ring. 2.2.6. Compression Testing. Samples from different parts of the porous material were extracted with a sharp razor blade. The porous material had a circular shape, as can be seen in Figure 6. Three samples were prepared, and sample 2 was selected from the middle part of the material and samples 1 and 3 were selected from the side opposite to sample 2, near the edge of the material (see Supporting Information). The samples were 10 × 10 mm in area and the heights of samples 1, 2, and 3 were, respectively, 15.7, 14.8, and 14.0 mm. The compression test was performed with an Instron 5566 universal testing machine using Instron compression plates (T1223−1021) with a diameter of 50 mm, in a conditioned room at 23 °C and 50% relative humidity. A 500 N load cell was used with a compression rate of 10% of the original sample thickness per min. The final strain was chosen to 80% of the original sample height to evaluate the material behavior over a large deformation interval. Each sample was conditioned at 23 °C and 50% relative humidity for 24 h before being tested according to ISO 844:2007(E). The energy absorbed by the material was calculated as the area below the stress−strain curve between 0 and 80% strain. 2.2.7. Automatic Pore Volume Distribution (APVD) Measurements. A TRI/Autoporosimeter version 2008−12 (TRI/Princeton, Princeton, U.S.A.)42 was used to measure the cumulative pore volume distribution of the foams using hexadecane as liquid. The membrane cutoff 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 500 to 5 μm. The pore radius

corresponding to a given chamber gas pressure was calculated according to eq 1

ΔP =

2γ cos θ r

(1)

where γ is the liquid−gas surface tension of the liquid used, in this case hexadecane (27 mN m−1), θ is the liquid−solid contact angle (full wetting, cos θ = 1, is assumed), ΔP is the difference between the chamber gas pressure and atmospheric pressure, and r is the pore radius. 2.2.8. Cellulose Model Surfaces. Polished silicon wafers were used as substrates for preparation of the cellulose surfaces. The wafers were cleaned by rinsing with a water/ethanol/water sequence. Any contamination was removed by treatment for 3 min in a plasma oven (PDC-002, Harrick Scientific Inc. U.S.A.) operating at 30 W under reduced air pressure. PEI was used to attach the cellulose to the silicon wafer. The substrate was dipped into a 1 g L−1 PEI solution at a pH of 7.5 for 10 min and then rinsed with Milli-Q-water and dried in nitrogen. The same substrate was then dipped into a 1 g L−1 NFC solution at a pH of 9 for 10 min followed by rinsing in Milli-Q-water and dried in nitrogen. After these two steps, the substrate was dipped into octylamine with a concentration of 1 g L−1 in aqueous solution (critical micelle concentration for octylamine is 22.8 g L−1)43 at a pH of 9 for 10 min and then rinsed in Milli-Q-water and dried in a flow of nitrogen gas. 2.2.9. Contact Angle Measurement. A CAM 200 (KSV Instruments Ltd., Helsinki, Finland) contact angle goniometer was used for static measurements of the contact angle of water on surface-modified NFC. The software calculates the contact angle on the basis of a numerical solution of the full Young−Laplace equation. Measurements were made at 23 °C and 50% relative humidity with Milli-Q water. The contact angle was determined at three different positions on each sample. The values reported were taken after the contact angle had reached a stable value, typically less than 10 s after deposition of the droplet and the size of the drop was 5 μL in all cases. 2.2.10. Field Emission Scanning Electron Microscope (FE-SEM). To study the microstructure of the NFC-stabilized foam and to clarify whether the NFC formed a densely packed film, the material was studied with a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) to obtain secondary electron images. The material was cut into small pieces with a sharp razor blade to fit onto a metal stub which was mounted with colloidal graphite paint and then coated with a 9 nm thick gold/palladium layer using a Cressington 208HR high resolution sputter coater. 2.2.11. Antek. Octylamine was added to the NFC dispersion in an amount ranging from 10 to 100% of the total NFC charge. The suspension was foamed and then filtrated through a 200 nm filter to get rid of all the NFC. Nitrogen elemental analysis on the remaining solution was then performed with the aid of an ANTEK MultiTek by PAC, Houston, Texas, U.S.A. In this procedure the sample is first incinerated in an oxygen lean atmosphere creating NO from the nitrogen in the sample. This is then exposed to O3, which creates exited NO2, and when this returns to its equilibrium state, a light quantum is emitted and detected by a photomultiplier tube. From a calibration curve, it is then possible to determine the correct nitrogen content in the sample.

3. RESULTS 3.1. Preparation of Foams and Foam Stability. By drying aqueous foams stabilized by surface modified NFC, it was possible to obtain porous cellulose materials as can be seen in Figure 2. Different amounts of octylamine were added to the NFC to evaluate the degree of surface modification that resulted in the best foam stability (Figures 3 and 4) defined as the situation when no foam/liquid separation could be detected even after 10 days. The graph shows that the most stable foams were achieved when the amount of added octylamine was 35 505


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Table 1. Adsorbed Amount of Octylamine to the NFC at Different Charge Ratios of Octylamine and NFC

Figure 2. Images showing NFC-stabilized foam upon drying in room temperature (23 °C and 29% relative humidity): (a) wet foam; (b) dry foam (porous material).

charge ratio octylamine/NFC (%)

equilibrium concentration (mg/L)

adsorbed octylamine per gram NFC (mg/g)

error estimate (%)

10 20 33 50 75 100

5.54 7.84 17.84 22.97 47.45 88.67

5.16 13.56 17.51 30.50 32.85 18.43

±3 ±2 ±2 ±6 ±3 ±0.3

mixing NFC dispersion with octylamine it was possible to lower the surface energy of the NFC, as was shown by a change in the contact angle for water on the NFC from 20 to 40° after octylamine had been deposited on the cellulose. These contact angle measurements were performed with flat cellulose model surfaces, as described in the Experimental Section. By decreasing the surface energy, it was possible to adsorb NFC to the air−water interface and the adsorption energy can be calculated according to eq 2 G = πr 2γLG(1 ± cos θ )2

(2)

where G is the energy of attachment or free energy gained, r is the radius of the particle, γLG is the gas−liquid interfacial tension, and θ is the contact angle for the particle at the interface. By assuming a radius of 10 nm for the NFC, the adsorption energy is calculated to be 302 kTs at room temperature and it was enough to stabilize the air bubbles. The air bubbles stabilized with modified NFC that were introduced with an ordinary milk beater according to the process scheme in Figure 1 increased the volume from 76 to 100 cm3 (approximately 30%) and created stable aqueous foams that could be dried under ambient conditions or in an oven to increase the rate of drying. The drying rate became approximately three times higher when dried in the oven compared to drying at ambient conditions and it took approximately 48 h to dry 28 cm3 of wet foam at 60 °C. It was hence possible to create dry, porous lightweight materials with this process despite the low concentration of solid particles in the original dispersion (Figure 2). 3.2. Surface Accumulation of NFC Prohibiting Coalescence. Because the main concept introduced in the present work was to stabilize air bubbles with octylamine-treated NFC, it is necessary to show that there is indeed an accumulation of NFC around the air bubbles. This was done using confocal

Figure 3. Foam stability, that is, the volume of the foam as a function of time normalized with the foam volume directly after stopping the foaming as a function of adsorbed octylamine. The added amount of octylamine charges ranged from 10 to 100% of the total NFC charge.

mg/g NFC, which corresponds to a 20 and 33% neutralization of the total NFC charge assuming a full dissociation of the charges on NFC and octylamine, respectively. A detailed study reveals that the foam volume was greater with the foam corresponding to 33% and then 20% charge neutralization, which in turn corresponds to 37 and 31 cm3, respectively. The result in Figure 3 also clearly shows that a 10% addition of octylamine is not enough to stabilize the foams. To evaluate the octylamine adsorption to the NFC prior to foaming, measurements on nitrogen content in the solution after foaming were conducted and the results are summarized in Table 1. From this table it is shown that approximately 60% of the octylamine adsorbs to the NFC when octylamine corresponding to 33% of the total NFC charge is used. By

Figure 4. Aqueous foam stability test with octylamine-treated NFC after different times: (a) 0, (b) 3, and (c) 10 days. Foam 1:3 means that the added amount of octylamine charges was equal to one-third of the charges of the NFC, assuming that all the charges in the NFC were dissociated, and foam 1:1 hence means that the amount of octylamine added was equal to the amount of NFC charges. 506


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formed and that many of the lamellas between the pores are closed. The enlarged image in Figure 7c also shows that the fibrils have formed dense foam lamellas between the pores. The use of different NFC concentrations also resulted in dry foams with different densities, as summarized in Table 2. The porosity was calculated from eq 3

microscopy, and the result is shown in Figure 5. The result shows that NFC is located around the air bubbles and that the

⎛ ρ⎞ ϕ = 100⎜⎜1 − a ⎟⎟ ρs ⎠ ⎝

(3)

where ρa and ρs (1.57 g/cm3)44 are the densities of the porous material and NFC, respectively. It took approximately two days for a NFC-stabilized aqueous foam (28 cm3) to dry in an oven at 60 °C without convection drying and with a perforated aluminum cover over the sample to increase the moisture content inside the container, that is, to decrease the driving force for drying the foam and to minimize disturbance from forced convection. The 1.5 wt % foam, dried at ambient conditions without any aluminum cover, became buckled when it was dried. This was prevented when using the aluminum cover. The average pore size of the different foams was determined by APVD, and the results are summarized in Figure 8. They show that the average pore size is in the range of 300−500 μm, which agrees well with the SEM image in Figure 7b. In all cases, the advancing and receding curves coincide, which indicate the lack of bottle necks in the pores that are available for liquid penetration. This is also supported by the SEM image showing spherical pores. The mechanical properties of the dried NFC-stabilized foam were measured in compression (Figure 9) on the 1 wt % NFC foam with a density of 0.05 g cm−3 and a porosity of 96.7%. The Young’s modulus, determined as the slope at low strain, was 437 ± 63 kPa. The compressive energy absorption value was 48 ± 11 kJ m−3 at 80% strain, which is lower but of the same order of magnitude as the value for a cellulose aerogel (68 kJ m−3)45 and for a cellulose foam made by freeze-drying (92 kJ m−3).46

Figure 5. Confocal microscopy image showing air bubbles in water covered with fluorescently labeled octylamine-treated NFC.

amount of used octylamine, 33% of the total NFC charge, was sufficient to cause NFC to accumulate at the air−water interface. To further quantify the stabilizing effect of NFC and to distinguish whether the air bubbles are stabilized by octylamine or by NFC, the interaction between two bubbles was investigated using a high speed camera. Figure 6a shows that

Figure 6. Images from high-speed camera experiments, showing the interaction between two air bubbles: (a) air bubbles stabilized with only octylamine in Milli-Q water coalesced within three seconds; (b) air bubbles stabilized with octylamine-treated NFC in Milli-Q water remained stable for more than 10 min (i.e., the time for analysis).

4. DISCUSSION 4.1. NFC-Stabilized Foams and Their Stability. The basic idea in this investigation was to stabilize the interface of air bubbles with the surface-modified NFC in an aqueous suspension. By reducing the surface energy of NFC and decreasing their inherent hydrophilic nature,3 it should be possible to stabilize the air bubbles and this was indeed strongly supported both by confocal microscopy (Figure 5) and with the foam stability measurements (Figures 2 and 3). With the confocal microscopy it was actually possible to reveal the location of the NFC at the air−water interface. The SEM micrographs of the dry foam show spherical pores resembling the NFC-covered air bubbles. Octylamine was chosen because it has been successfully used in other inorganic particle-stabilized foams22 and when it was adsorbed onto NFC, the contact angle between Milli-Q water and NFC/octylamine was increased from 20 to 40°. According to Gauckler et al.,29 this contact angle is still relatively low but they have also showed that contact angles as low as 20° could be related to adhesion energies of the order of 103 kT (k is the Boltzmann constant and T is the absolute temperature) in the case of 100 nm particles. Figures 2 and 5 show, however, that this lowering of the surface energy of the NFC was sufficient to

air bubbles stabilized with only octylamine coalesced within three seconds, whereas Figure 6b shows that air bubbles covered with octylamine-treated NFC could withstand coalescence for longer than 10 min (actually the total time of video recording; see also Supporting Information). The bubbles were probably stable for much longer than this. These results hence support the foam stability measurements and show that the stable foams are actually created by an accumulation of the NFC at the air/water interface. Due to the dimensions of the fibrils, the free energy gain of the adsorbed nanoparticles and, hence, the stability created by the particles, is superior to the stability created by the octylamine. 3.3. Structural Characteristics of the Porous Material. Dry NFC-stabilized foams were produced from a suspension with an NFC concentration ranging from 0.3 to 1.5 wt % and characterized both with SEM and liquid porosimetry. The SEM images in Figure 7b,c show that rather spherical pores are 507


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Figure 7. (a) Dried low-weight NFC-stabilized foam (1 wt %); (b) SEM image of cross section of dried foam (1 wt %); (c) SEM image of a highly dense foam lamella where the organized NFC is shown. Scale bar is 100 nm.

Table 2. Properties of NFC-Stabilized Foams Made from Dispersed NFC of Different Concentrationsa concentration NFC (wt %)

ρ (g cm−3)

porosity (vol %)

0.3 0.6 1 1.5

0.2 0.05 0.03

86.7 96.7 98.0

a

The lowest concentration of NFC lost its continuous porosity and was not recordable. The foams were dried in an oven at 60 °C with a perforated aluminium cover on top to minimize convection drying and to increase moisture content in the atmosphere surrounding the sample.

Figure 9. Stress−strain curves in compression for NFC-stabilized foam (1 wt %). The three samples are taken from different positions in the same dry foam.

In Figure 3, the concentration of octylamine has been changed in relation to a fixed amount of NFC (0.1 wt % NFC), and it is evident that the addition of octylamine corresponding to 33% (assuming that all octylamine are protonated at pH = 9) of the total NFC charge in the suspension gave the best foam stability. One explanation of why this foam is so stable could be the disjoining pressure between the NFC-covered air bubbles. Because NFC is more charged in the systems with lower amounts of adsorbed octylamine, there are more counterions surrounding the air bubbles. This probably leads to an increased disjoining pressure between the bubbles which prevents coalescence and Ostwald ripening. A crucial step when producing these types of foams is to avoid Ostwald ripening (disproportionation) where air diffuses from smaller to larger bubbles.29 A close look at pictures b and c in Figure 4 shows that the bubbles have become somewhat larger after 10 days but that coalescence has to a large extent been avoided. This shows that stabilization with NFC reduces the rate of coalescence of air bubbles in NFC-stabilized foams.

Figure 8. Cumulative pore volume for dry NFC-stabilized foams (0.6−1.5 wt % NFC) using hexadecane as liquid. The graph shows both advancing and receding curves and the total amount of liquid absorbed in the material for all pores up to a radius of 500 μm.

attach the NFC at the air−liquid interface so that the air bubbles could be stabilized over several weeks. 508


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based on the presented results were made: concentration of NFC, 10 kg m−3; volume increase upon foaming, 26 m3 air/76 m3 dispersion; radius of formed bubbles, 400 × 10−6 m; volume of one fibril, (5 × 10−9)2 × 1 × 10−6 m3; density of cellulose, 1500 kg/m3. Based on this, it is possible to calculate that the number concentration of NFC at the air−water interface was 1.1 × 1017 m−2 assuming that all fibrils were concentrated at the interface. Assuming a square lattice of nanoparticles at the interface this also means that the average distance between the NFC was 3.0 nm showing that the air−water interface is actually crowded with NFC in this situation. At these concentrations and at these distances it is also known that the NFC, mainly due to their high aspect ratio and high concentration of counterions, are able to form stable gels,40 and it can be suggested that it is the properties of these gels that will preserve the foam during the drying process. It is also obvious from these considerations that a NFC modification that will increase the magnitude of the interaction between the NFC particles during drying, that is, a gel strengthening, will be most beneficial for the preparation of even more robust foams. However, it must be stressed that this simple calculation assumes that all the NFC in the dispersion is located at the interface and this is naturally an oversimplification of the actual situation since the fibrils have a physical dimension that must be considered. From the dimensions of the NFC mentioned earlier, the actual area covered by the NFC can be calculated to be 5.5 × 102 m2/m2 air−water interface. This situation is unrealistic and suggests that there is still a fair amount of NFC free in the aqueous phase. Furthermore, as the NFC concentration is increased at the air−water interface, due to the large gain in free energy of this process, there will be a large tendency for the NFC to form macroscopic gels at the interface. As shown in Figure 7c, the lamellae of the foam are composed of densely packed NFC. This is in accordance with the explanation to the good stability of the foam and it can also explain the good mechanical properties of the dry foam, naturally in combination with the high Young’s modulus of the NFC.48 The structure of the lamellae can both be explained by an initial dense packing of NFC in the foam and by a further reorganization and packing of the NFC during the drying. Because there is a considerable volume decrease of the foam during drying and because foam collapse or Ostwald ripening is small, it is clear that there will be a shrinking of the bubbles and a further packing of the NFC during drying. The reorganization of the NFC during drying might also be the reason why the drying rate and the moisture content is so important because the packing of the long and slender NFC is a slow process. Additives that would speed up this packing process, while maintaining a high gel strength, would be highly desirable and are currently evaluated. The fact that there is a variation in the lamellae thickness can be due either to a merging of several bubbles or to an uneven distribution of the NFC during the foaming. It is not currently clear which process is dominating. There are several implications of these results. First of all it is clear that the accumulation of the surface-modified NFC particles at the air/water interface is sufficient to create a very stable foam of NFC. It is furthermore obvious that the surface accumulation process and the properties of the particlestabilized interface are poorly understood and need more fundamental investigations to identify the critical parameters to best optimize the properties of the prepared foams.

During drying of the foam it is necessary to remove the remaining water without macroscopic foam collapse due to the drying forces created when the water is removed. The results in the present work show that local moisture gradients and convection drying have a profound effect on the properties of the dry, porous material. Earlier investigations have shown that a high moisture content and the absence of forced convection drying promotes uniform moisture removal and minimize drying stress due to moisture gradients within the foam lamellae upon drying.47 When the foam was dried in an oven at 60 °C with convection drying, there was a large collapse of the foam, whereas when the foam was dried under ambient conditions, that is, 20 °C and 50% RH, the foam structure was preserved, as shown by the images in Figure 7. Rapid evaporation at an elevated temperature will hence lead to a rapid movement of the drying front and an uneven moisture distribution, and this can induce sufficiently large drying stresses to collapse the foam structure. This was shown by drying the foam in a convection-free environment with a higher moisture content. By using a convection-free oven at 60 °C and placing a perforated aluminum cup over the sample, the convection is minimized and local moisture gradients are reduced. After drying at these conditions, the foam structure was preserved and the drying rate was increased compared to that achieved during drying at ambient conditions. These results hence indicate that it would be possible to continuously prepare a foam of nanofibrills by forming a stable foam, spreading it on a moving, draining wire and subjecting it to gentle drying to create a low density and strong foam of NFC. 4.2. Properties of the Porous Dry NFC Foam. The APVD curves show the amount of liquid absorbed in the foams. These results can, together with the porosity of the foams, be interpreted with respect to open and closed pores. The foam made from 1.5 wt % NFC absorbed approximately 10 mg hexadecane per mg foam. Knowing the weight of the foam (41.5 mg) and density of hexadecane (0.77 g cm−3), the volume of absorbed liquid in the foam is 539 mm3.The density of the foam is 0.03 g cm−3 (Table 2) and the volume of the foam is then 1380 mm3. With a 98% porosity this results in a free volume of 1355 mm3. This means that approximately 40% of the volume is filled with liquid and that a large fraction of the pores are closed to the liquid. In foams made from 1 wt % NFC approximately 80% of the volume is filled with liquid, and in foams made from 0.6 wt % the amount of filled liquid in the pores is 100%. The fact that the density of the foam decreases with increasing NFC concentration could be explained by the coverage of the air bubbles. It is higher, shown from the APVD results, for higher concentrations allowing for a formation of more or less dense NFC films as the foam dries, and consequently, the foam is more stable and less prone to collapse, giving a greater pore volume and a lower density. For the foams prepared at a lower NFC concentration it can be suggested that this film formation is less pronounced and there will hence be a lower amount of closed pores for these foams and a higher tendency for cell collapse. The results in Figures 3, 5, and 7 show the interesting and intriguing result that a 1% by weight NFC dispersion is able to create a stable foam. A foam that is so stable that relatively strong and dry NFC foams can be prepared upon drying. When trying to understand the mechanism behind all this it is necessary to estimate the concentration of nanofibrils at the air−water interface. To do this, the following assumptions, 509


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Notes

Porous cellulose materials have also been produced before by, for example, supercritical drying11 or freeze-drying.2,9,10 The density of the material made from supercritical drying (nanopaper) is 640 mg cm−3 and that of materials made from different forms of freeze-drying varies between 7 and 14 mg cm−3.45,46 The NFC-stabilized foam, prepared in the present work, with a density of 30 mg cm−3 is no doubt comparable with these materials and also with expanded polystyrene foam (EPS) with a density between 20 and 640 mg cm−3.49 The porosity of the NFC-stabilized foam, 98%, is also comparable with that of other porous cellulose materials. It is higher than that of nanopaper (40−86%)11 and equal to that of freeze-dried aerogels (93−99%).45,46 The mechanical properties of the dry foams are also clearly comparable with those of other types of NFC foams, as shown in Figure 9. The Young’s modulus of 437 kPa (mean value) is higher than that of cellulose aerogels (199 kPa)45 but lower than that of cellulose foams made by freeze-drying (718 kPa)46 and EPS (6000 kPa).50 There is a slight deviation in the results for sample 2, indicating that there are small variations in mechanical properties of the porous cellulose material although the differences in density for samples 1−3 were not significant (31, 31, and 33 mg cm−3, respectively). By changing the chemical composition of the NFC and the foaming composition, it would be possible to tailor both the structure and mechanical properties of the foams to suite specific enduses.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Wallenberg Wood Science Center for financial support. Professor Emeritus Lars Ö dberg is gratefully acknowledged for valuable discussions. Senior Lecturer Stefan Lindström, now at Linköpings University, is acknowledged for valuable comments on the project and Innventia AB is acknowledged for supplying the NFC.

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5. CONCLUSIONS A dry porous material from NFC has been prepared with a novel foam stabilization procedure that has the potential of being a practical process for preparing low density foams of NFC. This material is new in the way that it is prepared with renewable cellulose as stabilizing particle in contrast to foams stabilized with other types of particles. In this procedure, low concentration NFC dispersions is used to stabilize air bubbles in an aqueous suspension. The lifetime of the aqueous foam is significantly prolonged compared to that of a nonstabilized foam, and the stability of the foam bubbles is such that the foam upon drying yields a porous material with an average pore diameter of 500 μm. In the present case, this material has a density of 30 mg L−1 and a porosity of 98%. The Young’s modulus in compression is 437 ± 63 kPa for a dry foam with a density of 50 mg L−1 and a porosity of 96.7% which is higher than that of other cellulose foams made by freeze-drying but lower than that of polystyrene foams. The compressive energy absorption value is 48 ± 11 kJ m−3 at 80% strain and this is of the same order of magnitude as that of cellulose aerogels (68 kJ m−1)45 and cellulose foams made by freeze-drying (92 kJ m−1).

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ASSOCIATED CONTENT

S Supporting Information *

The resistance to coalescence between two air bubbles with different stabilizing agents was recorded by a high speed camera upon contact. A figure describing the samples for the compression test is also supported. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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*E-mail: [email protected] (N.T.C.), [email protected] (L.W.). 510


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