Fast Preparation Procedure for Large, Flat Cellulose and Cellulose

Aug 10, 2010 - The present study is thus significant because a rapid preparation procedure for large, flat, smooth, and optically transparent cellulos...
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Biomacromolecules 2010, 11, 2195–2198

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Fast Preparation Procedure for Large, Flat Cellulose and Cellulose/Inorganic Nanopaper Structures Houssine Sehaqui,† Andong Liu,† Qi Zhou,‡,§ and Lars A. Berglund*,†,§ Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, School of Biotechnology, Royal Institute of Technology, AlbaNova University Centre, SE-106 91 Stockholm, Sweden, Wallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received May 5, 2010; Revised Manuscript Received July 6, 2010

Nanostructured materials are difficult to prepare rapidly and as large structures. The present study is thus significant because a rapid preparation procedure for large, flat, smooth, and optically transparent cellulose nanopaper structures is developed using a semiautomatic sheet former. Cellulose/inorganic hybrid nanopaper is also produced. The preparation procedure is compared with other approaches, and the nanopaper structures are tested in uniaxial tensile tests. Optical transparency and high tensile strength are demonstrated in 200 mm diameter nanopaper sheets, indicating well-dispersed nanofibrils. The preparation time is 1 h for a typical nanopaper thickness of 60 µm. In addition, the application of the nanopaper-making strategy to cellulose/inorganic hybrids demonstrates the potential for “green” processing of new types of nanostructured functional materials.

Introduction Cellulose nanofibers are one of the most interesting nanoscale biomacromolecular building blocks that are available in nature. Their typical widths are in the range of 5-20 nm and their lengths may exceed 5 µm; thus, the length/width ratio (aspect ratio) is higher than 250. This high aspect-ratio nanostructure is advantageous and contributes high mechanical performance to nanofiber networks and composite materials. The cellulose crystal consists of stiff, close-packed extended cellulose chains with strong intermolecular forces so that Young’s modulus exceeds 100 GPa in the chain direction and thermal expansion in the same direction is negligible.1,2 If severe acid hydrolysis is used, the resulting fibrous celluloses are highly crystalline rods and are termed microcrystalline cellulose,3 nanocrystals,4 or cellulose whiskers,5 depending on origin and dimensions. The elastic modulus of tunicate cellulose whiskers was about 143 GPa, as measured by using a Raman spectroscopic technique.6 A similar elastic modulus value was obtained for single tunicate cellulose microfibrils with a width of 20.3 nm and a thickness of 8.4 nm by a three-point bending test using an atomic force microscopy (AFM) cantilever.7 The elastic modulus of a commercial microfibrillated cellulose fibril with a diameter of about 180 nm was 84 ( 23 GPa as measured by the AFM technique.8 This value is lower than those of the crystal regions of cellulose due to the fact that longer cellulose nanofibers are more flexible and consist with imperfect crystallites and regions of disordered yet still extended molecules. Cellulose nanofibers show many characteristics of the crystal and are truly biomacromolecular high-performance nanofibers. The nanofibers can be obtained from bacteria,9,10 from plant sources such as algae,11 from parenchyma primary cell wall sources such as sugar beet or potato,12 and obviously from traditional cellulosic plant fibers exemplified by hemp and flax.13 * To whom correspondence should be addressed. Tel.: +46-8-7908118. Fax: +46-8-7906166. E-mail: [email protected]. † Department of Fibre and Polymer Technology. ‡ School of Biotechnology. § Wallenberg Wood Science Center.

Wood pulp offers particular industrial interest as a high-quality source of cellulose nanofibers and fast market introduction because the infrastructure for large-scale harvesting and processing is already present.14-16 To reduce the net nonrenewable energy requirements during disintegration of cellulose nanofibers from wood, enzymatic,16 and chemical17 pretreatment procedures have been used, which are followed by mechanical disintegration of fibrous wood pulp water suspensions. The resulting nanofibers prepared by this route are frequently termed microfibrillated cellulose (MFC), because the smallest cellulose building block in the plant cell wall is termed a microfibril by plant physiologists. Researchers typically start with cellulose nanofibers in the form of a water suspension. This suspension is essentially a hydrocolloid of suspended nanofibers and shows gelation at concentrations as low as 1 or 2%.18 Mimicking papermaking processes, a cellulose nanofiber suspension can be used to prepare films simply by filtering the suspension to obtain a wet gel and allow time for evaporation of the water. Nanofibers are mechanically entangled in the wet gel, and as water evaporates, capillary forces provide attraction between individual nanofibers. As they come close enough, secondary attraction forces, including hydrogen bonding, develop between cellulose nanofibers. The resulting nanopaper structures show an interesting combination of Young’s modulus, tensile strength, and toughness.19 In addition, optical transparency,20 low thermal expansion,21,22 and excellent oxygen barrier characteristics23 have been reported. Diverse applications have been proposed, such as loud-speaker membranes,24 foldable substrate for electronic displays,21,25 battery membranes,26 and numerous biomedical applications.27 Several preparation procedures for cellulose nanopaper have been described in the literature, for example, filtration of cellulose nanofiber suspension followed by oven drying19,28 or hot pressing15,20 of the wet gel, air drying of the cellulose nanofiber suspension,29,30 and utilization of a dynamic sheet former.31 Our first procedure involves the use of a laboratory filtration setup that allows preparation of small samples with a diameter of 72 mm.19 If the wet gel obtained by this method is oven-dried, it takes

10.1021/bm100490s  2010 American Chemical Society Published on Web 08/10/2010

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Figure 1. Preparation procedure of large and smooth MFC nanopaper structures using a semiautomatic sheet former (Rapid-Ko¨then).

several days for the preparation. The resulting sample often shows wrinkles at the edge due to significant moisture concentration gradients and the corresponding nonuniform drying process. Hot pressing of the wet nanofiber gel at high pressure reduces the drying time considerably,20 but significant experience needs to be developed to find appropriate conditions. Casting in a Petri dish has also been used for the preparation of cellulose nanopaper12,30 but is an extremely time-consuming method. Syverud et al. reported on the use of a dynamic sheet former for the preparation of MFC nanopaper with some level of nanofiber orientation.31 A disadvantage was that nanofibers were lost through the wire. Nanopaper structures are of great interest as reinforcement in biocomposites, as gas barriers, membranes, filters, and films for use in high-technology devices, including biomedical applications. Rapid preparation of large and flat nanopaper structures of high surface smoothness and optical transparency is therefore important in order to facilitate development of new applications. Furthermore, the hydrocolloid nature of the nanofiber suspension suggests inclusion of inorganic particles, as has been done by mixing water-soluble polymers with exfoliated nanoscale silicate platelets from montmorrilonite.32 The suspension is filtrated and dried so that a nanocomposite film is formed.32 The present procedure is applied to cellulose nanopaper but also to prepare cellulose/inorganic hybrid nanopaper sheets. The present communication reports on the procedure using a semiautomatic sheet former (Rapid Ko¨then-Rycobelgroup). Flat nanopaper structures with a diameter of 200 mm and a thickness of 60 µm were prepared in about 1 h. The nanopaper structure and properties are compared with results from other preparation methods.

suspension was degassed for 10 min with a water vacuum pump. The filtration of the degassed suspension was done in a semiautomatic sheet former (Rapid-Ko¨then) under vacuum (Figure 1, step 1). It is likely that other types of handsheet preparation equipment would function as well. The suspension was poured into a hollow cylinder containing a metallic sieve at the bottom (pore size, 110 µm). On top of the sieve, a nitrocellulose ester filter membrane (Millipore DAWP29325) with 0.65 µm pore size was placed. The filtration time of the 0.2 wt % suspension depended on the final thickness of the nanopaper and was around 45 min for a 60 µm thick nanopaper. After filtration, a strong gel is formed on top of the filter membrane. The gel “cake” is peeled from the membrane and stacked first between two woven metal cloths (aperture width, 80 µm; wire diameter, 50 µm) and then two paper carrier boards (Figure 1, step 2). This package was placed in the sheet dryer for 10 min at 93 °C and a vacuum of about 70 mbar (Figure 1, step 3). Preparation of Nanopaper: Other Methods. Filtration + oven drying: a 0.2 wt % MFC suspension was prepared as described above; it was then degassed and vacuum filtrated on a glass filter funnel (7.2 cm in diameter) using a 0.65 µm pore size filter membrane (DVPP, Millipore). After filtration, the wet gel was stacked between woven metal cloths and Whatman No. 1 filter paper and dried in an oven at 55 °C for 48 h at a pressure of about 25 kPa. Filtration + hot pressing: filtration was performed as described above; the wet gel (7.2 cm in diameter) was stacked between woven

Experimental Section Materials. Microfibrillated cellulose nanofibers (MFC) were prepared from softwood sulphite pulp (Domsjo¨ AB, Sweden) according to a method previously developed.16 The pulp suspension was first subjected to a pretreatment step involving mechanical beating and enzymatic treatment. The pretreated pulp suspension was then mechanically disintegrated by a homogenization process using a Microfluidizer M-110EH (Microfluidics Ind., U.S.A.). A MFC water suspension with concentration of 2.0 wt % was obtained. The clay was a sodium montmorillonite (Cloisite Na+, Southern Clay Products) with a cation-exchange capacity (CEC) of 92 mequiv/ 100 g. Platelet thickness is 1 nm and length is in the 50-1000 nm range with and average dimension of 110 nm according to the manufacturer. A 1.0 wt % clay suspension was prepared by dispersing 10 g of clay in 1 L of deionized water under vigorous stirring. Preparation of Nanopaper by Sheet Former. A total of 80 g of the MFC suspension was diluted with water, followed by mixing at 12000 rpm using an Ultra Turrax mixer (IKA, D125 Basic) for 10 min. The final MFC suspension had a concentration of 0.2 wt %. The

Figure 2. (a) Photograph of a 200 mm diameter cellulose nanopaper structure on top of conventional A4 copy paper. (b) Tensile stress-strain curves of nanopaper structures prepared by different methods.

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Table 1. Comparison of Different Nanopaper Preparation Methodsa preparation method

diameter (mm)

thickness (µm)

preparation time (h)

tensile strength (MPa)

strain to failure (%)

Young’s modulus (GPa)

roughness rms (nm)

T600b (%)

suspension casting filtration + oven drying filtration + hot pressing rapid-Ko¨then rapid-Ko¨then hybrid

80 72 72 200 200

40 45 55 40 45

120-144 48-72 2-3 1-2 1-2

180 (10) 211 (26) 178 (17) 232 (19) 122 (10)

5.9 (0.8) 6.6 (1.5) 6.3 (1.4) 5.0 (1.1) 2.8 (0.3)

10.3 (0.16) 12.1 (0.29) 10.3 (0.31) 13.4 (0.25) 7.4 (0.24)

18.6 (2.8) 26.0 (1.3) 110.5 (10.6) 21.9 (0.8) 47.5 (6.6)

49.7 (1.1) 47.1 (1.5) 19.0 (0.8) 42.0 (1.0) 1.1 (0.02)

a

Values in parentheses are standard errors.

b

Percent transmittance at 600 nm.

metal cloths and Whatman No. 1 filter paper and dried in a hot press at 105 °C for 10 min at a pressure of about 30 MPa. Suspension casting: the 0.2 wt % MFC suspension was degassed and poured in a polystyrene Petri dish and air-dried for about 6 days under atmospheric conditions at 22 °C. MFC/montmorillonite (MMT) nanopaper with 50 wt % MMT was prepared as follows: 1.63 wt % MFC suspension containing 1.5 g of MFC was slowly added to a 1.0 wt % MMT suspension containing 1.5 g of MMT. This suspension was stirred for 24 h and was then further treated by ultrasonication for 30 min. The mixture was vacuumfiltrated and dried in the Rapid Ko¨then. Inorganic-organic hybrid clay nanopaper with a thickness of about 60 µm was obtained. Mechanical Properties. Tensile tests of the cellulose nanopaper were performed using a universal materials testing machine Instron 5566 equipped with a 500 N load cell. Specimen strips prepared using the Rapid-Ko¨then method are of 50 mm in length and 15 mm in width, whereas 50 mm in length and 5 mm in width for nanopaper prepared by other methods. Specimens of thicknesses in the 50-80 µm range were tested at 5 mm/min strain rate under a controlled relative humidity of 50%. A total of 3-4 specimens were tested per material. Young’s modulus was determined at low strain and ultimate strength was determined as the stress at specimen separation. Strain was determined from displacement data from a noncontact laser extensometer. Field-Emission Scanning Electron Microscopy (FE-SEM). The surface texture of the prepared nanopapers was observed by SEM using a Hitachi S-4800 equipped with a cold field emission electron source. The samples were coated with graphite and gold-palladium using Agar HR sputter coaters (total thickness ca. 5 nm). Secondary electron detector was used for capturing images at 1 kV. Atomic Force Microscopy (AFM). Nanoscope IIIa AFM (Picoforce SPM, Veeco, Santa Barbara, CA) was used to characterize the surface roughness of MFC and MFC hybrid nanopapers. All measurements were performed in the tapping mode with a scan rate of 2 Hz/512 dots using standard noncontact silicon cantilevers (RTESP, Veeco, Santa Barbara, CA) with a tip radius of 8 nm and a spring constant of 40 N/m. The roughness values of the prepared films were determined from the height images over a 4 µm2 area and were presented as a rootmean-square (rms) value. No image processing except flattening was undertaken. All measurements were made at 23 °C and 50% relative humidity.

Light Transmittance. Regular light transmittance was measured using a UV-visible spectrophotometer (Cary 1E, Varian Corp.) by placing the specimens on the cuvette port. Percent transmittance at 600 nm (T600) was recorded.

Results and Discussion Preliminary preparation attempts using conventional metal wire sieves failed in the sense that substantial loss of MFC occurred through the sieve. Filter paper was also tried on top of the metal sieve, but there were difficulties in the separation of nanopaper from the filter paper. The 0.65 µm pore size membrane described in the experimental section was therefore used. The preparation procedure for large and smooth nanopaper structures is illustrated in Figure 1 and described in the Experimental Section. The third step is of particular interest. The vacuum drying under constrained clamping is a very efficient procedure. The vacuum setup provides rapid moisture removal and helps to decrease the moisture concentration gradient in the paper structure. The biaxial clamping prevents shrinkage, which also helps to avoid wrinkling. A smooth and flat nanopaper structure is produced with a high degree of optical transparency (Figure 2a). The tensile stress-strain curves for the nanopaper structures prepared by different methods are shown in Figure 2b. The differences between the different preparation methods and properties of the resulting nanopapers are compared and summarized in Table 1. Suspension casting is the most time-consuming method and results in low mechanical properties. Furthermore, the suspension cast sample suffers from wrinkling. Even slower drying would provide less severe moisture concentration gradients and may reduce the wrinkling problem. For filtration and oven drying, the mechanical properties are much improved, and preparation time is shorter. Filtration + hot pressing significantly reduces preparation time due to the fast drying. Hot-pressing requires substantial pressure and well-aligned plates. Strength and modulus of the hot-pressed nanopaper are in the same range as for suspension cast nanopaper (180 MPa strength, 10.3 GPa modulus), although

Figure 3. Scanning electron micrograph of nanopaper structures prepared by Rapid-Ko¨then: (a) MFC nanopaper surface and (b) hybrid MFC/ montmorrilonite nanopaper surface.

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data are lower than for nanopaper prepared by filtration + oven drying, see Table 1. However, the surface roughness and percent transmittance at 600 nm (T600) of the hot pressed nanopaper are 110 nm and 19%, respectively, which are much lower than those from the previous two methods. The Rapid-Ko¨then method is attractive for nanopaper preparation as filtration and drying are conveniently performed in a single piece of equipment. Furthermore, the nanopaper specimens prepared by this procedure, have the best mechanical properties, see Table 1. The mechanical properties of nanopaper are very sensitive to orientation distribution of the nanofibers. In a fiber network mechanics context, increased out-of-plane orientation will lower the in-plane modulus.19 The high modulus and ultimate strength indicate that the constrained drying leads to good in-plane orientation of the nanofibers and possibly also higher density. Densities were estimated from the weight and volume of the sample determined from measurements of thicknesses and widths. Typical data were above 1400 kg/m3, which is slightly higher than data previously reported using a more accurate method for density estimation.19 The variations in estimated density with preparation method were small, and no correlation could be established between these minor variations in density and mechanical properties. Based on previous experience19 and FE-SEM observations of pores, the porosity is in the 10-20% range. The nature of the porosity is to some extent apparent from Figure 3a. We note that the preparation time is 1 to 2 h (depending on nanopaper thickness), and samples are flat (surface roughness of 21.9 nm) and highly transparent optically (T600 of 42%). In an attempt to increase the applicability of the procedure, exfoliated layered silicate clay platelets (montmorrilonite) were added to the water suspension in 50/50 proportion by weight to cellulose. Also, this suspension was conveniently converted to a nanopaper structure in 1-2 h with a cellulose/montmorrilonite hybrid structure. The surface of the nanopaper is dominated by a dense montmorrilonite coating (see Figure 3b), although an underlying fibrous structure may be discerned. This cellulose/montmorrilonite hybrid nanopaper shows a tensile strength of 122 MPa and a modulus of 7.4 GPa at an inorganic content of 50% by weight.

Conclusions A fast and environmentally friendly water-based preparation method is described for high-quality cellulose nanopaper structures, where the starting point is a hydrocolloid based on cellulose I nanofibers in water. A polymer membrane with very fine pores is placed on top of a metal sieve. Filtration results in a wet gel “cake” of about 30% dry content. The cake is biaxially clamped and subjected to vacuum drying at elevated temperature. The resulting nanopaper has high strength (232 MPa) and modulus (13.4 GPa) and is characterized by substantial optical transparency (T600 of 42%), 200 mm diameter, flatness, and surface smoothness (surface roughness of 21.9 nm). The procedure can also be used to prepare nanopaper structures of cellulose nanofibers combined with inorganic nanoparticles. This was demonstrated for cellulose/montmorrilonite nanopaper with as much as 50% by weight of inorganic content.

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Acknowledgment. We thank Mohamed Eita for measuring the surface roughness of the nanopapers by atomic force microscopy.

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