Clay Aerogel Composites

Ind. Eng. Chem. Res. 2008, 47, 615-619

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Biologically Based Fiber-Reinforced/Clay Aerogel Composites Katherine Finlay, Matthew D. Gawryla, and David A. Schiraldi* Department of Macromolecular Science and Engineering, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7202

Ultralow-density clay aerogels are formed via the freeze-drying of clay hydrogels. Although unmodified clay aerogels exhibit generally poor mechanical properties, the incorporation of short-cut natural fibers forms a woven-like structure, which significantly increases the mechanical properties of these materials. Incorporation of a matrix polymer into the aerogel/fiber composites further enhances compressive strengths and moduli. A woven-like structure composed of clay aerogel “warp” and fiber “weft” materials seems to be responsible for the enhancements in the material properties in each case examined. Introduction Aerogels, which are ultralow-density materials that are produced from gels via the replacement of solvent with air, can typically exhibit bulk densities of e0.1 g/cm3. The preparation of inorganic aerogels was first described by Kissler, who slowly exchanged the water in aqueous silica with volatile organic solvents, which were subsequently removed to produce lowdensity materials.1,2 A similar process was described for the conversion of aqueous clay gels to fabriclike aerogels by MacKenzie.3 van Olphen4 proposed a “house of cards” structure for the clay aerogels that were produced via the freeze-drying of clay hydrogels; these structural studies were expanded by several other authors, who also broadened the processing techniques to include supercritical drying.5-9 In our own work, we have demonstrated a robust freeze-drying process for converting common smectite clays, such as sodium montmorillonite and bentonite, to aerogels with bulk densities in the range of 0.02-0.10 g/cm3.10 One difficulty with all of these aerogel materials is that they are relatively fragile, exhibiting mechanical properties similar to those of balls of cotton fibers, and, hence, are easily crushed or irreversibly damaged under low stress levels. The introduction of matrix polymers, such as poly(vinyl alcohol) (PVOH) can convert the aerogels to more mechanically robust composites, and such polymer/clay aerogel composites can exhibit environmentally responsive properties.11,12 These polymer/aerogel composites show promise as alternatives to foamed polymers; they contain in excess of 90 vol % air and reflect the thermal/mechanical properties of the matrix polymers themselves.13 Despite these favorable properties, further enhancement of mechanical properties of clay aerogel-based composites would increase the range of applications open for these materials. In the present work, the reinforcement of clay aerogels and clay aerogel/polymer composites with natural fiber materials is described. The opportunity to combine biologically based fibers, domestic clay, and polymers (especially those that are bio-based and/or biodegradable) could lead to environmentally benign materials that are suitable for a wide range of end uses. Experimental Section Materials. Sodium montmorillonite (PGW, Nanocor), PVOH (with a number-based molecular weight of Mn ≈ 108 000, * To whom correspondence should be addressed. Tel.: 216-3684243. Fax: 216-368-4202. E-mail address: [email protected].

99.7% hydrolyzed, and a polydispersity index of PDI ≈ 1.7; Polysciences, Warrington, PA), and microcrystalline cellulose powder (Aldrich) were used as received. Bio-based fibers (e.g., silk and hemp, from Aurora Silk, Portland, OR), as well as soy silk, bamboo top, INGEO, ramie, and silk latte (from Mielke’s Fiber Arts, LLC) were mechanically cut into lengths of ∼2 mm, prior to incorporation into the aerogels. Deionized water was prepared using a Barnstead ROpure low-pressure, reverseosmosis system. Aerogel Preparation. A quantity of 50.0 mL of deionized water and 2.50 g sodium montmorillonite were placed into a 100-mL beaker, and to that, the desired quantity (0-2.5 g) of chopped fibers were added. The mixture was stirred by hand, to wet the clay, and then was mixed thoroughly in a Waring model MC2 mini laboratory blender for ∼1 min. The resulting clay hydrogel mixtures were poured into 5-dram cylindrical polystyrene vials (Fisher Scientific). These forms were immediately placed into an ethanol/solid carbon dioxide bath to freeze their contents. After the samples were completely frozen, they were removed from the ethanol/dry ice bath and placed in a freezer at a temperature of -12 °C for storage. Samples were placed in a VirTis AdVantage EL-85 freeze dryer, where a high vacuum was applied to sublime the ice and reveal the aerogel structures of the samples. Cylindrical test pieces for compression testing, measuring ∼20 mm in both height and diameter, were cut from each sample, using a band saw. Several small samples were prepared for scanning electron microscopy (SEM) analysis by sputter-coating them with palladium. For those composites that incorporate polymer, 25 mL of a 5% PVOH solution were added slowly to 27.5 g of a 10% clay gel; fibers were then added to this mixture and the freeze-drying process was performed as described previously. Characterization. Compression testing was conducted on the prepared test pieces (measuring 20 mm in height and 20 mm in diameter), using an Instron model 5565 Universal Testing Machine that was fitted with a 1 kN load cell. These tests were performed at a constant strain rate of 1 mm/min and were stopped when the 1 kN load limit (machine limit) was reached; three samples from three separate batches were tested for each composition of interest. All of the samples in this study were sufficiently robust to be handled during testing. The specific number of replicate samples tested in each case is given in the data tables as n. The sputter-coated samples were imaged using a Phillips XL-ESEM SEM microscope. Analysis. Various calculations were performed using the Microsoft Excel program. From the masses and initial dimen-

10.1021/ie0705406 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/21/2007

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Figure 1. Comparison of various fiber lengths and conformation (scale bar ) 1 mm). Table 1. Summary of Biologically Based Fibers Investigated in This Study fiber

chemical nature

fiber diameter

source

soy silk hemp silk INGEO ramie silk latte bamboo top

protein cellulosic protein protein cellulosic protein cellulosic

∼20 µm, monodisperse ∼20 µm, polydisperse ∼10 µm, monodisperse ∼20 µm, monodisperse ∼20 µm, polydisperse ∼25 µm, monodisperse ∼30 µm, polydisperse

manmade, from soy beans naturally occurring, plant-based naturally occurring, animal-based manmade, from corn naturally occurring, plant-based manmade, from milk naturally occurring, plant-based

sions of the compression test pieces, density values were calculated. For each polymer loading, density values were averaged, and the standard deviation and the standard error were calculated. The density for the samples without polymer was plotted against fiber loading, and a linear curve fit was applied. (Because of the inherent compressibility of polymer-free samples, individual densities are not listed in the data tables.) The load-displacement data from each compression test was converted to stress-strain data, and a modulus was calculated from the linear-elastic region of the stress strain curve. For each fiber loading, modulus values were averaged, and the standard deviation was calculated. Stress-strain data were plotted for each test piece, and examples of these curves for each fiber loading were overlaid for comparison. Results and Discussion The chemical natures of the various biologically based fibers, evaluated as modifiers to clay aerogels, are given in Table 1. The manmade fibers and silk seem to have almost monodisperse diameters, whereas the three plant-based natural fibers are varied in diameter. Variation in diameter alone does not seem to affect the ability of a fiber to reinforce the aerogels, as can be seen by hemp being a superior clay aerogel-reinforcing agent over both ramie and bamboo tops. Silk has a diameter almost half that of the other fibers and, therefore, has approximately four times as much fiber length per unit weight than any of the other composites. It is interesting to note that, although there is almost four times more silk length in the composites, they do not

perform significantly different than the other materials; this behavior differs from that of typical polymer/fiber composites, wherein the fiber aspect ratio directly determines the level of reinforcement.14 We postulate that fiber diameter would have some role in the level of reinforcement, when comparing otherwise identical reinforcements; we conclude here that this physical parameter is of secondary importance to the chemical nature (and, hence, compatibility) of the fiber/aerogel blend. Example images of the reinforcing fibers used in this study are given in Figures 1 and 2; small differences in linearity and surface roughness can be noted between the materials, although they seem to be relatively equivalent. Each of the fiber additives, when combined with sodium montmorillonite clay (5 wt %/vol), produced aerogel structures when freeze-dried from aqueous gels. An example SEM image of the aerogels, with and without fiber additives, is given in Figure 3. From this SEM image, it can be seen that there is minimal disruption of the structure and the fibers span many supergalleries. Particles in a solidifying melt (in this case, water) will be trapped by the solidification ice front only if the front is traveling at a critical velocity, which is dependent on the size of the particle. The larger the particle, the slower the ice front that is needed to trap the particle with almost no change in the particle position in space. There are many factors that determine the distance a particle will travel before being encapsulated, such as particle buoyancy, size, solution viscosity, and velocity of the solidification

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Figure 2. Comparison of the surfaces of various fibers (scale bar ) 10 µm). Table 2. Preliminary Data on 0.5%-Fiber-Reinforced 5% Clay Aerogels, n ) 2 reinforcing fiber

compressive modulus (kPa)

none silk latte bamboo top ramie INGEO soy silk microcrystalline cellulose hemp

14 ( 8 66 ( 26 29 ( 5 36 ( 3 33 ( 1 90 ( 5 7(1 93 ( 22

The fibers tested in this study are cellulosic and protein-based, and are generally hydrophilic in nature; thus, they have the potential for hydrogen and ionic bonding with the hydrophilic clay surfaces. Silk is a hydrophobic material, although being polyamide-based allows for some level of adhesion with the clay.16

Figure 3. Comparison of a 5% clay aerogel (left) and a 5% clay aerogel that contains 1% hemp fibers (right).

front.15 The fibers in this study seem to be too large to be moved/ oriented by the ice front under these freezing conditions and are thus randomly oriented throughout the structure. SEM images show clay adhering to the fibers in such a way as to suggest an attractive force between them. The samples are subjected to moderate air currents during handling and preparation, such that any clay simply sitting on the fibers would most likely fall from the fiber.

Average compressive moduli values for fiber-reinforced clay aerogels are given in Table 2. As can be seen from these data, each of the fibrous additives (including those based on cellulose) significantly increased the compressive modulus of the clay aerogels, whereas the addition of an equivalent amount of microcrystalline cellulose had a minimal effect. These results suggest that the length scale (∼2 mm) of the fibers has an important role in mechanical property enhancement, very similar to that observed in fiber-reinforced polymer composites.14 The effect of fiber length in composite reinforcement is well-known, but the present work demonstrates that it can be extended to the organic fiber reinforcement of inorganic aerogels as well. Among the reinforcements examined herein, there seems to be no preference for cellulosic or protein fibers. The effect of fiber length is currently being investigated both in native aerogels and polymer-composite aerogels. An example set of compression curves for the soy-silk-reinforced aerogel can be observed in Figure 4. As would be expected, the more fiber that is present, the higher the compressive strength and modulus. The induction

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Figure 4. Example stress-strain curves for 5% clay aerogel reinforced with varying amounts of soy silk fibers. Table 4. Detailed Data on Polymer Composites with the Three Fibers of Interest: (a) Silk, (b) Soy Silk, and (c) Hemp polymer content (wt %/vol %)

density (g/cm3)

compressive modulus (kPa)

compressive strength (kPa)

(a) Silk (In Addition to 5% Clay + 2.5% PVOH) 0 0.07 1600 ( 208 0.5 0.078 3056 ( 229 1 0.084 5562 ( 752 5 0.121 8857 ( 1521 (b) Soy Silk (In Addition to 5% Clay + 2.5% PVOH) 0 0.07 1600 ( 208 0.5 0.079 4026 ( 793 1 0.058 5526 ( 995 5 0.093 8500 ( 596

Figure 5. Example stress-strain curves for 5% clay aerogel reinforced with 2.5% polymer and varying amounts of soy silk fibers. Table 3. Detailed Compressive Modulus Data on the Three Fibers of Interest When Incorporated into 5% Clay Aerogels, n ) 9 Compressive Modulus (kPa) fiber content (wt %)

soy silk

hemp

silk

0 0.5 1 5

14 ( 8 112 ( 14 128 ( 18 473 ( 78

14 ( 8 65 ( 12 179 ( 26 1292 ( 277

14 ( 8 166 ( 20 419 ( 46 1771 ( 327

microcrystalline cellulose 14 ( 8 7(1

period prior to the linear elastic region is likely due to the compression surfaces of the composites not being perfectly parallel. Using the compression data for the fiber-reinforced aerogels as guidance, further experiments were conducted using a water-soluble polymer for further reinforcement. Soy silk, silk, and hemp were chosen as the most promising fiber reinforcements for clay aerogels (their performance at different levels of loading is expanded upon in Table 3), and they then were incorporated into polymer/clay aerogels (2.5 wt %/vol % polymer, and 0-5 wt %/vol % fiber in the initial hydrogel blends). Previous work has demonstrated that there is a change in structure when PVOH is incorporated into the molecular weight range used in this study.17 Fiber/polymer/clay composites exhibit structures similar to those produced without the 2-mm-fiber addition; therefore, the addition of the fibers does not seem to significantly alter the in situ aerogel formation process.

29 73 93 108 29 46 39 42

(c) Hemp (In Addition to 5% Clay + 2.5% PVOH) 0 0.07 1600 ( 208 29 0.5 0.077 2688 ( 207 59 1 0.082 3314 ( 224 74 5 0.101 8560 ( 487 105

A small inflection point is observed in the stress-strain curves (Figure 5) at ∼100 kPa for all samples that contain both fiber and polymer reinforcement. The magnitude of these apparent structural changes seems to increase with fiber content; such inflections are not observed in the many polymer/aerogel composites that we have tested over several years. We attribute this to the fibers separating from the polymer matrix, similar to fiber pullout that is observed in the tensile testing of fiberreinforced composites.14 In this case, a compressive failure mode could involve a loss of threading of the reinforcing fibers as part of the overall compaction of the composites. Further investigation will examine the effect of combining fibers and polymer made from the same material in an effort to eliminate the fiber/polymer separation and increase strength. The incorporation of fibers increases the density slightly but shows a pronounced effect on the modulus. Compressive strength and initial moduli for the fiber/polymer/ clay aerogel composites are given in Table 4. For comparison, 2.5% PVOH/5% clay aerogel composites, without any additional reinforcement, have a compressive modulus of 1600 kPa and a density of 0.07 g/cm3. The addition of biologically based fibers to clay aerogel/PVOH composite materials results in monotonic increases in both the compressive moduli and compressive strengths of these materials, as was the case in the polymerfree clay aerogel/fiber structures. The density of fiber-filled polymer/clay aerogel composites increases with fiber loading, as expected.

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Given the low density and environmentally friendliness of these materials, it is fitting that they be viewed as a replacement for expanded polystyrene foam (EPS). When compared to materials such as EPS (0.01 g/cm3, compressive modulus of 3000 kPa, as measured in this study), it is obvious that density is a major consideration. Although the present clay aerogel materials do not match the density of EPS, they exhibit substantially higher moduli and compressive strength. It is not beyond consideration that further optimization could yield a material with properties very similar to those of EPS, yet with the ability to biodegrade. Conclusions Novel, low-density structures that combine biologically based fibers with clay aerogels were produced in an environmentally benign manner, using water as a solvent, with no additional processing chemicals. Incorporation of these 2-mm-length fibers resulted in a monotonic increase in clay aerogel compressive strengths and moduli, independent of the chemical nature (cellulosic versus proteinaceous) of the fibers. The fiber/ clay aerogel structures seem to be almost woven, with clay layers in the “warp” direction, and reinforcing fibers in the “weft”. The incorporation of biologically based fibers further served to reinforce clay aerogel/polymer composites, increasing compressive moduli and compressive strengths of these materials by as much as 5-fold at 5 wt % fiber loadings, while bulk densities were increased by a factor of