Extruded starch-lignin foams

Abstract

Extruded starch foams are well known as biodegradable alternatives to foamed polystyrene packaging materials. Extruded foams of unmodified starch replacing 1% to 20% of the starch with kraft lignin were prepared. At 10% lignin, there are no deleterious effects on foam density, morphology, compressive strength, or resiliency as compared to a starch extruded foam, yet the foam retains its integrity after immersion for 24 hours in water. At 20% lignin there is a decrease in compressive strength and resiliency. Addition of cellulose fibers restore the mechanical properties but with an increase in density.

Claims

1. An expanded cellular structure, comprising: a mixture of: chemically unmodified starch consisting essentially of unmodified amylose and unmodified amylopectin; about 9-18% by weight lignin; and about 5-10% by weight cellulose fibers, wherein a uniform cell structure is distributed throughout the expanded cellular structure, and the expanded cellular structure has a unit density of about 36-61 kg/m.sup.3, a resiliency of about 56% to 72%, a compressive strength of at least 0.16 MPa, and retains structural integrity after 1 hour of aqueous immersion.

2. The expanded cellular structure according to claim 1, wherein the chemically unmodified starch comprises approximately 70% by weight unmodified amylose.

3. The expanded cellular structure according to claim 1, produced by a process comprising extruding the mixture of chemically unmodified starch, cellulose fibers, and lignin, under heat and pressure.

4. The expanded cellular structure according to claim 1, wherein the expanded cellular structure has a density of about 36-39 kg/m.sup.3.

5. The expanded cellular structure according to claim 1, wherein the expanded cellular structure has a compressive strength of 0.16 to 0.18 MPa.

6. The expanded cellular structure according to claim 1, wherein the mixture comprises 9-10% by weight lignin, wherein the expanded cellular structure has a unit density of less than about 36-39 kg/m.sup.3, and a resiliency of about 63-72%.

7. The expanded cellular structure according to claim 1, wherein the lignin is chemically unmodified.

8. The expanded cellular structure according to claim 1, wherein the mixture comprises about 10-19% by weight lignin, wherein the expanded cellular structure is configured to remain intact after immersion in water for longer than 24 hours.

9. The expanded cellular structure according to claim 8, wherein the expanded cellular structure has: a unit density of about 39 kg/m.sup.3, a resiliency of about 63%, and a compressive strength of about 0.18 MPa.

10. The expanded cellular structure according to claim 1, further comprising at least one filler which does not chemically interact with the chemically unmodified starch.

11. The expanded cellular structure according to claim 1, further comprising about 0.5% by weight of a nucleating agent to cause the mixture comprising chemically unmodified starch, lignin, and cellulose fibers to produce a uniform foam within a heated extruder.

12. The expanded cellular structure according to claim 1, wherein the expanded cellular structure remains intact after immersion in water for longer than 24 hours.

13. A method of forming the expanded cellular structure according to claim 1, comprising: mixing between about 9-18% by weight lignin, about 5-10% by weight cellulose fibers, and about 99-80% by weight chemically unmodified starch, of the combined weight of the lignin and the chemically unmodified starch consisting essentially of chemically unmodified amylose and chemically unmodified amylopectin, in an aqueous medium; and extruding the mixture under heat and pressure to form the expanded cellular structure having a uniform cell structure distributed throughout the expanded foam, having a unit density of about 36-61 kg/m.sup.3, a resiliency of about 56% to 72%, a compressive strength of at least 0.16 MPa, and water resistance to retain structural integrity after 1 hour of aqueous immersion.

14. An expanded cellular structure formed by a process comprising: mixing chemically unmodified starch consisting essentially of chemically unmodified amylose and chemically unmodified amylopectin, about 5-10% by weight cellulose fibers, and about 9-18% by weight lignin in an aqueous medium, and extruding the mixture under sufficient heat and pressure to yield an expanded cellular structure, wherein a uniform cell structure is distributed throughout the expanded cellular structure, having a density of about 36-61 kg/m.sup.3, a resiliency of about 56% to 72%, a compressive strength of at least 0.16 MPa, and sufficient water resistance to retain structural integrity after 1 hour of aqueous immersion.

15. The expanded cellular structure according to claim 14, further comprising at least one nucleating agent in an amount of about 0.5% by weight to form a uniform expanded foam.

16. The expanded cellular structure according to claim 14, wherein the lignin is chemically unmodified.

17. The expanded cellular structure according to claim 14, wherein the expanded cellular structure remains intact after immersion in water for longer than 24 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-1D (Prior Art) show SEM images of (a) starch, (b) starch-lignin, (c) starch, and (d) starch-lignin.

(2) FIG. 2 (Prior Art) shows powder X-ray diffraction patterns of foams of starch and starch-lignin.

(3) FIG. 3 (Prior Art) shows water absorption in compression molded starch and starch-lignin.

(4) FIG. 4 shows unit densities of the samples described in Table 1.

(5) FIGS. 5A-5E show low resolution (25) cross-section SEM images of samples with increasing lignin content: (A) 0% (sample 1), (B) 1% (sample 8), (C) 5% (sample 12), (D) 10% (sample 16), (E) 20% (sample 19).

(6) FIGS. 6A-6F show cross-section SEM images (25) showing the effect on cell structure of adding cellulose fibers (5% cellulose, left; 10% cellulose right): (A) sample 4, (B) sample 6, (C) sample 10, (D) sample 18, (E) sample 14, (F) sample 21.

(7) FIGS. 7A-7D show cross-section SEM images (500) showing the effect of cellulose fibers on cell wall thickness (left 0% cellulose; right 10% cellulose). (A) sample 1, (B) sample 7, (C) sample 16, (D) sample 18.

(8) FIG. 8 shows resiliencies of the samples described in Table 1. The dark shaded bars are for samples containing 20% lignin.

(9) FIG. 9 shows compressive strengths of the samples described in Table 1. The dark shaded bars are those of samples containing 10% cellulose fibers.

(10) FIG. 10 shows compressive strength as a function of density.

(11) FIG. 11 shows a ln-ln plot of the weight of water absorbed (ml) relative to the dry weight (m2) versus time in seconds; sample 16 (.box-tangle-solidup.), sample 18 (.square-solid.), sample 21 (.diamond-solid.).

(12) FIG. 12 shows a ln-ln plot of the weight of water absorbed (ml) per unit surface area (cm2) versus time in seconds; sample 16 (.box-tangle-solidup.), sample 18 (.square-solid.), sample 21 (.diamond-solid.).

(13) FIG. 13 shows the mass of water absorbed per unit surface area as a function of time, as expressed in Equation (1); sample 16 (.box-tangle-solidup.), sample 18 (.square-solid.), sample 21 (.diamond-solid.); lines show fitted values.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(14) Materials and Sample Preparation

(15) Hylon VII cornstarch (approximately 70% amylose) was purchased from National Starch and Chemical Company, Bridgewater, N.J.

(16) Indulin AT lignin (kraft pine lignin) was donated by MeadWestvaco, Charleston, S.C.

(17) Norwegian talc was purchased from Zeneca Bioproducts.

(18) Cellulose fibers were obtained from Sigma Aldrich Cat. No. C6288.

(19) Ammonium hydroxide was purchased from Sigma-Aldrich, St. Louis, Mo.

(20) Lignin is soluble in aqueous solution only at high pH. In studies of starch-lignin cast films (Stevens et al., 2007), ammonium hydroxide was used to raise the pH of the casting solution and was found to be a requirement for obtaining viable films. Preparing starch-lignin by extrusion, on the other hand, had no significant high-pH requirement. In the present study, samples were prepared both with and without ammonium hydroxide. When used, ammonium hydroxide was added as a 30% aqueous solution in an amount equal to the estimated stoichiometric amount of hydroxyl (OH) protons in pine kraft lignin, 2.0 mL per 10 g lignin (McCarthy and Islam, 2000). The ammonium hydroxide is taken up by the starch-lignin mixture quickly, so that after extrusion there is no ammonia odor. Talc was added at 0.5% (w/w) as a nucleating agent.

(21) Sample compositions are shown in Table 1. Compositions are based on total dry weight of starch, lignin, and cellulose. Samples were prepared for extrusion in 1.5 kg batches. The dry ingredients were mixed manually. Water and, when used, ammonium hydroxide solution were added and the mixture again mixed manually.

(22) TABLE-US-00001 TABLE 1 Sample Compositions Sample Starch (%) Lignin (%) Cellulose (%) NH.sub.4OH (mL) 1 100.0 0.0 0.0 0 2 100.0 0.0 0.0 3.0 3 100.0 0.0 0.0 30.0 4 95.0 0.0 5.0 0 5 95.0 0.0 5.0 3.0 6 90.0 0.0 10.0 0 7 90.0 0.0 10.0 30.0 8 99.0 1.0 0.0 0 9 99.0 1.0 0.0 3.0 10 94.0 1.0 5.0 0 11 94.0 1.0 5.0 3.0 12 95.0 5.0 0.0 0 13 95.0 5.0 0.0 15.0 14 90.0 5.0 5.0 0 15 90.0 5.0 5.0 15.0 16 90.0 10.0 0.0 0 17 90.0 10.0 0.0 30.0 18 81.0 9.0 10.0 0 19 80.0 20.0 0.0 0 20 80.0 20.0 0.0 60.0 21 72.0 18.0 10.0 0

Example 1

(23) Several samples of unmodified high amylose cornstarch containing varying amounts of lignin (Table 1) were extruded. Mixed powders were fed using a loss-in-weight feeder into a ZSK 30 Werner and Pfleiderer twin-screw extruder comprised of 14 barrel sections (including the feed throat) and with temperature control zones. Total feed rates were approximately 120 g/min and varied slightly with formulation. The screw speed was 150 rpm. Starch and lignin powders were mixed with 0.5% talc (w/w) as a nucleating agent and, in some examples, cellulose fibers, and fed using a loss-in-weight feeder into barrel Section 1. Three dispersive mixing sections were located in barrel Sections 5, 6, and 7, followed by a series of four distributive mixing sections separated by conveying elements. These four sections were each comprised of two forwarding kneading blocks, a neutral (non-forwarding) kneading block, and a reverse kneading block. The final barrel section was comprised of conveying elements of narrow pitch. A temperature profile of 40/65/95/120/130/130/95/95 C. was used. Water was added to maintain a total moisture content of approximately 17%. Extrudates were cut at the die face with an attached motorized chopper operating at 600 rpm.

(24) The expanded products were collected and evaluated. Results are shown in Table 2. At 20% lignin there is no significant change in unit density, but there are significant decreases in resiliency and compressive strength, indicating brittleness.

(25) TABLE-US-00002 TABLE 2 Compressive Unit Density Resiliency strength Sample Material (kg/m.sup.3) (%) (MPa) Hylon VII 33.8 3.0 66 4 0.18 0.02 Hylon VII, 1% lignin 32.5 1.3 66 4 0.14 0.02 Hylon VII, 5% lignin 31.5 0.9 72 4 0.16 0.01 Hylon VII, 10% lignin 38.9 3.7 63 4 0.18 0.02 Hylon VII, 20% lignin 31.0 0.9 38 13 0.10 0.01

Example 2

(26) Additional samples of high amylose starch and lignin were prepared with added cellulose fibers to prepare expanded foam products using the same procedure as Example 1. Results are shown in Table 3.

(27) TABLE-US-00003 TABLE 3 Compressive Unit Density Resiliency strength Sample Material (kg/m.sup.3) (%) (MPa) Hylon VII 33.8 3.0 66 4 0.18 0.02 Hylon VII, 5% cellulose 39.0 1.7 68 3 0.19 0.01 Hylon VII, 10% cellulose 55.1 3.9 69 5 0.24 0.02 Hylon VII, 1% lignin, 35.8 1.5 61 4 0.18 0.02 5% cellulose Hylon VII, 5% lignin, 36.9 1.5 67 3 0.16 0.01 5% cellulose Hylon VII, 9% lignin, 54.7 4.7 63 4 0.26 0.02 10% cellulose Hylon VII, 18% lignin, 61.3 3.5 56 5 0.32 0.05 10% cellulose

(28) 10% cellulose fibers restores the compressive strength at high lignin content, but with a significant increase in density. 5% cellulose fibers have little effect.

(29) Samples of Hylon VII starch containing 9-20% lignin remained intact after immersion in water for 24 h. Hylon VII, and samples containing 5% lignin or less, disintegrated in water after 30 s. Parameters describing water absorption are shown in Table 4.

(30) TABLE-US-00004 TABLE 4 Water absorbed Water absorbed in 24 h in 24 h Sample Material (g/g) (g/cm.sup.2) Hylon VII Sample disintegrates Sample disintegrates Hylon VII, 5% lignin Sample disintegrates Sample disintegrates Hylon VII, 10% lignin 3.65 0.041 Hylon VII, 9% lignin, 3.13 0.044 10% cellulose Hylon VII, 18% lignin, 2.62 0.039 10% cellullose

(31) The data indicate that extruded foams prepared with unmodified high amylose starch to which lignin has been added at a level of 9-18% have significant water resistance. After 24 h immersed in water they remain intact, in contrast to extruded 100% unmodified starch foams and foams containing 5% lignin and 95% unmodified starch, which disintegrate in less than a minute. Moreover, foam material containing 10% lignin, 90% unmodified starch, and no cellulose has approximately the same water resistance as foam material containing 9% lignin and 10% cellulose fibers and foam material containing 18% lignin and 10% cellulose fibers. Neither increasing the lignin content to 18% nor adding 10% cellulose fibers significantly increases water resistance beyond the resistance of foam material containing 10% lignin and no cellulose fibers.

(32) Table 5 shows previously reported results on disintegration time in water together with results for the present invention. The present invention provides water resistance to starch-based foams.

(33) TABLE-US-00005 TABLE 5 Disintegration time following immersion Composition in water References Hylon VII <0.2 min (high-amylose starch) Hylon VII ether + styrene 1-35 min U.S. Pat. No. 5,043,196 acrylate resins Hylon VII ether + flour 0.2-17 min U.S. Pat. No. 5,554,660 proprionate Eco-Foam ~1 min U.S. Pat. No. 5,854,345 (National Starch) Clean Green (Clean Green) ~2 min 20 sec U.S. Pat. No. 5,854,345 Enpak (DuPont) ~2 min U.S. Pat. No. 5,854,345 Starch + polyester >30 min U.S. Pat. No. 5,854,345 Hylon VII, 10% lignin >24 hr present invention Hylon VII, 9% lignin, >24 hr present invention 10% cellulose Hylon VII, 18% lignin, >24 hr present invention 10% cellulose .sup.a PO, propylene oxide .sup.b 3.9% styrene acrylate resin A, B, or C .sup.c PVA, polyvinyl alcohol

Example 3(Comparative)

(34) This example illustrates examining the effect of adding ammonium hydroxide to the extrusion formulations in this invention. Lignin is soluble in aqueous solution only at high pH and ammonium hydroxide is required when casting starch-lignin films in order to increase the compatibility of starch and lignin. Ammonium hydroxide was added as a 30% aqueous solution in an amount equal to 2.0 mL per 10 g lignin. The ammonium hydroxide is taken up by the starch-lignin mixture quickly so that after extrusion there is no ammonia odor. As seen in the results of Table 6, compared with Tables 2 and 3, the addition of ammonium hydroxide has no significant effect on the properties of the extruded foams.

(35) TABLE-US-00006 TABLE 6 Compressive Unit Density Resiliency strength Sample Material (kg/m.sup.3) (%) (MPa) Hylon VII, 1% lignin, 33.8 1.2 66 3 0.15 0.01 NH.sub.4OH Hylon VII, 1% lignin, 36.4 2.0 70 5 0.20 0.02 5% cellulose, NH.sub.4OH Hylon VII, 5% lignin, 32.2 1.3 66 3 0.16 0.02 NH.sub.4OH Hylon VII, 5% lignin, 37.1 1.7 66 3 0.17 0.01 5% cellulose, NH.sub.4OH Hylon VII, 10% lignin, 33.2 1.9 67 4 0.16 0.01 NH.sub.4OH Hylon VII, 20% lignin, 30.6 1.1 33 6 0.10 0.02 NH.sub.4OH

Example 4

(36) Sample Characterization

(37) Unit Density

(38) Unit density is the weight-to-volume ratio of an individual specimen; it is a measure of the reduction in density of the solid material that results from the expansion process. The volume of a specimen was determined by measuring the weight of glass beads it displaced.

(39) Unit density is the weight-to-volume ratio of an individual specimen; it is a measure of the reduction in density of the solid material that results from the expansion process. The volume of a specimen was determined by measuring the weight of glass beads it displaced (Hwang and Hayakawa, 1980; Bhatnagar and Hanna, 1995; Tatarka and Cunningham, 1998; Rutledge et al., 2008). The volume of a weighing bottle, with its top surface cut flat, was calibrated with glycerol (V=21.430.04 mL, SD, n=10). The effective density of the glass beads (.sub.gb), defined as the ratio of a given mass of beads to the volume they occupy, was determined by filling the weighing bottle with glass beads (0.5 mm diameter) in four steps, tapping the bottle 40 times to settle the beads after each step. The bottle was then overfilled with glass beads, the excess was removed by drawing a metal flat edge across the top, and the bottle was weighed. .sub.gb=1.5590.003 g/mL(SD, n=10).

(40) To determine the density of a foam specimen, the weighing bottle was one-quarter filled with glass beads and tapped 40 times to settle. A weighed foam specimen was placed on the surface of the glass beads, and the bottle was filled with glass beads in three steps. The bottle was then overfilled, the excess removed, and the bottle weighed. The density of the specimen was calculated from the mass of the displaced glass beads. Three specimens of each composition were measured, with ten measurements of each specimen.

(41) The results of the density measurements are displayed in FIG. 4, in which the dark shaded bars are for samples containing 10% cellulose fibers.

(42) The addition of 20% lignin (samples 19 and 20) does not increase foam density; it has no effect on foam expansion. The addition of 5% cellulose, with or without lignin, also has no effect on density (samples 4, 5, 10, 11, 14, 15).

(43) Samples containing 10% cellulose fibers displayed increased density, whether or not lignin was present (samples 6, 7, 18, 21; shown as dark shaded bars in FIG. 1). The addition of ammonium hydroxide had no significant effect on density.

(44) All samples in the present study had densities in the range of 30-40 kg/m.sup.3 except for samples that contained 10% cellulose fibers, which had densities of 50-65 kg/m.sup.3. A wide range of extruded starch-based foams have been studied in the laboratory. Their densities, depending on formulation and processing, have been in the ranges of 21-40 kg/m.sup.3 (U.S. Pat. No. 5,801,207), 22-30 kg/m.sup.3 (Nabar et al., 2006), 18-30 kg/m.sup.3 (U.S. Pat. No. 5,854,345), and 30-60 kg/m.sup.3 (Bhatnagar and Hanna, 1995). Except for the samples with 10% cellulose fibers, the densities of the compositions studied here are in the same range of density as extruded foams previously studied in the laboratory. The densities of the extruded starch and starch-lignin foams described here are significantly less than those of starch and starch-lignin foams prepared by compression molding (Stevens et al., 2010).

(45) Densities of various commercial starch fills are lower; they have been measured as 23.2 kg/m.sup.3 (Bhatnagar and Hanna, 1995) and 17-23 kg/m.sup.3 (Tatarka and Cunningham, 1998). Densities of commercial foamed polystyrene samples have been reported to be as low as 8.9 kg/m.sup.3 (Bhatnagar and Hanna, 1995), 7.2 kg/m.sup.3 (Tatarka and Cunningham, 1998), 7.9 kg/m.sup.3 (Tatarka and Cunningham, 1998), and 20.3 kg/m.sup.3 (Tatarka and Cunningham, 1998).

(46) Morphology

(47) For scanning electron microscope (SEM) measurements, specimens were fractured in liquid nitrogen, dried, sputter-coated with AuPd, and examined with a Hitachi S-4700 scanning electron microscope.

(48) Effect of Lignin on Cell Size

(49) In FIG. 5, low resolution (25) cross-section SEM images are shown of starch samples containing increasing amounts of lignin (0-20%), but no cellulose fibers. Lignin appears to have no appreciable effect on cell size; cell sizes for all samples are in the range of 0.6-1.0 mm. Lack of contrast makes the location of the dispersed lignin impossible. Nevertheless, the SEM images show that 20% lignin can be incorporated into starch foams without collapse of the foam and with no major reduction in cell size. Stevens et al. (2010) previously found the same result with starch-lignin foams prepared by a compression molding method.

(50) A significant difference between the morphology of the extruded starch-lignin foams and starch-lignin foams prepared by a compression molding process (Stevens et al., 2010; Tiefenbacher, 1993; Shogren et al., 1998) is the absence, in the present extruded foams, of any significant surface skin. The extruded foams (FIGS. 5A-5E) have a very thin continuous or semi-perforated skin, approximately the thickness of the interior cell walls (see below). The thick skin in compression molded foams may be due to the rapid drying of a starch layer on the hot metal surface, so that it cannot expand. In contrast, there may be enough steam present outside the expanding starch extrudate that it remains viscoelastic and can expand, thus thinning the exterior wall.

(51) The similarity of the SEM images shown in FIGS. 5A-5E is consistent with the similarity in the densities of those samples (FIG. 4); 20% of the starch can be replaced with lignin without affecting the overall cell size or density of extruded foams.

(52) Effect of Cellulose Fibers on Cell Structure

(53) FIG. 6A-6E shows low-resolution (25) cross-section SEM images that display the effect of adding cellulose fibers. The left images (A, C, E) are those of foams containing only 5% cellulose fibers. The cell sizes are approximately the same as those of foams containing no cellulose fibers (FIG. 5A). The absence of any effect of 5% cellulose fibers on cell structure, as displayed in the SEM images, is reflected in the absence of any effect on density (FIG. 4).

(54) The right images in FIG. 5(B, D, F) are those of foams containing 10% cellulose fibers. There is a significant disruption of the cell structure, although the remaining cells are of approximately the same size as those in foams with no cellulose and those with 5% cellulose. The partial cell collapse resulting from adding 10% cellulose fibers is independent of the amount of lignin; the foams shown in FIG. 6(B, D, F) contain 0%, 9%, and 18% lignin, respectively.

(55) The effect of adding 10% cellulose fibers, as shown in the SEM images, is clearly reflected in the increase in unit densities of those foams (FIG. 4). It may be that the open cells created during expansion in the presence of 10% cellulose fibers prevent the foam from continuing the expansion.

(56) Therefore, there is a limit in the amount of cellulose fibers that can be added before introducing a deleterious effect on density.

(57) Effect of Cellulose Fibers on Internal Cell Walls

(58) FIGS. 7A-7D show cross-section SEM images (500) indicating that the addition of 10% cellulose fibers results in thinner cell walls, regardless of the amount of lignin. Images on the left are of samples containing 0% (A) and 10% (C) lignin but no cellulose; the cell wall thickness is in the range of 52 m. Images on the right are of samples containing 0% (B) and 9% (D) lignin and 10% cellulose; the cell wall thickness is approximately 1.5 m. Therefore, the addition of cellulose fibers at 10% increases the density, partially collapses the cell structure, and results in thinner cell walls. In contrast, the walls of the internal cells in starch-lignin compression molded foams are approximately 10 m thick (Stevens et al., 2010).

(59) Mechanical Properties

(60) Compressive Strength and Resiliency

(61) Compressive strength and resiliency were measured with an Instron Model 4500 testing instrument. For resiliency measurements the sample was compressed 3 mm, the probe was lifted for 1 min, followed by recompressing until the probe touched the sample and measuring the distance (d) at which the load started to increase. The percent resiliency was calculated as

(62) $R\left(\%\right)=\frac{3.0-d}{3.0}100$

(63) The results of resiliency measurements are shown in FIG. 8. At lignin contents of 10% or less (samples 1-18), lignin causes no reduction in resiliency. At 20% lignin content (samples 19 and 20) there is a significant decrease in resiliency, which can be restored with the addition of 10% cellulose fibers (sample 21) but with an accompanying increase in density (see above).

(64) Compressive strengths are displayed in FIG. 9. The results are strongly correlated with sample density (FIG. 4). The addition of 10% cellulose fibers increases compressive strength (dark shaded samples) whether or not lignin is present. As with resiliency (FIG. 8), there is a significant decrease in compressive strength at 20% lignin content (samples 19 and 20) which can be restored with the addition of 10% cellulose fibers (sample 21), but with an accompanying increase in density.

(65) The dependence of mechanical properties on foam density can be described in terms of a power law function (Gibson and Ashby, 1997; Liu et al., 1999, 2003; Christensen, 2000; Roberts and Garbocczi, 2001, 2002a, 2002b; Zimmerman and Bodvarsson, 1989). FIG. 10 shows the compressive strength as a function of density. When the data are fit to a power law function the results shown in Equation 1 are obtained.
Compressive Strength(MPa)=(0.00360.0015)Density(kg/m.sup.3).sup.(1.070.015)(1)

(66) Willett and Shogren (2002) similarly found an exponent of 0.920.12 in their study of starch foams. The simple linearity between compressive strength and density may be the result of the large volume fraction of voids in the present foam samples; i.e., the density of the foams (p) is small compared to the density of the cell walls. If the density of the cell walls is taken to be the density of unfoamed starch (.sub.s), 1500 kg/m.sup.3 (Liu et al., 1999), the volume fraction of solid material in the present samples, =/.sub.s, is quite small and in the range 0.020-0.040.

(67) Water Absorption

(68) Water absorption was first evaluated by immersing a specimen in 100 mL of water and measuring the time it took for the specimen to disintegrate completely. Selected samples which did not disintegrate within 10 minutes were further examined. Water absorption of those samples was measured using an immersion gravimetric method. Specimens were conditioned for 24 h at 50 C., weighed, then immersed in a 23 C. water bath for 22 specified times ranging from 1 s to 24 h. To keep the specimens submerged and maximally exposed to water, the specimens were penetrated with a thin wire attached to a support, which was then immersed in the water and held in place with a clamp. Upon removal from the water bath, excess water was removed with absorbent paper and the specimens were reweighed. Three specimens were measured at each immersion time, for each sample. The weight of absorbed water per unit surface area was calculated using the mass of the specimen, the volume of the specimen determined from its density, and an equivalent sphere model.

(69) Samples containing 9-20% lignin (samples 16-21, Table 1) remained intact, even after being immersed in water for 24 h. Specimens containing no lignin, however, and samples containing up to 5% lignin (samples 1-15) disintegrated within 30 s when immersed in water.

(70) The water absorption of samples 16, 18 and 21 was measured using an immersion gravimetric method (Stevens et al., 2010; ASTM, 2007a; Abacha et al., 2009; Berketis and Tzetzis, 2009). Specimens were conditioned for 24 h at 50 C., weighed, then immersed in a 23 C. water bath for 22 specified times ranging from 1 s to 24 h. To keep the specimens submerged and maximally exposed to water, the specimens were penetrated with a thin wire attached to a support, which was then immersed in the water and held in place with a clamp. Upon removal from the water bath, excess water was removed with absorbent paper and the specimens were reweighed. Three specimens were measured at each immersion time, for each sample.

(71) It had previously been found that starch-lignin foams prepared by compression molding (Stevens et al., 2010), after immersion for more than 1-2 h, were weak and no longer able to support their own weight.

(72) Water absorption was studied in further detail with samples 16, 18, and 21. FIG. 11 shows a ln-ln plot (Masaro and Zhu, 1999; Meinders and von Vliet, 2009) of the weight of water absorbed (m.sub.1) in grams relative to the dry weight (m.sub.2) versus time in seconds. There is an almost immediate absorption of water followed, after approximately 60 s, by the absorption of additional water. The amount of initial water absorbed is lower for sample 21 (18% lignin, 10% cellulose), but after 24 h there is no significant difference in the amount of water absorbed. After 24 h immersed in water, the samples remain intact, in contrast to compression molded starch-lignin foams (Steven et al. 2010).

(73) SEM data (above) indicate that these extruded samples have a semi-perforated skin, and the internal structure of the foam may be a combination of interconnecting and non-connecting cells. The initial immediate water absorption may represent unhindered movement of water through a system of interconnecting cells, which is followed by diffusion through cellular walls into the non-connecting cells, eventually resulting in saturation.

(74) Crank (1975) has solved Fick's diffusion equation for diffusion into a sphere, a model used by others (Zimmerman and Bodvarsson, 1989; Weinstein and Papatolis, 2006). Here we apply that model by adopting an equivalent-sphere approach to treat the irregularly-shaped specimens. The equivalent-sphere approach is commonly used in analyzing hydrodynamic measurements on irregularly-shaped globular proteins (Cantor and Schimmel, 1980). To express the mass of water absorbed per unit surface area, m.sub.1, in g/cm.sup.2, the radius of the equivalent sphere is calculated as

(75) $r=\sqrt[3]{3V/4},$
where V is obtained from the measured specimen mass and density (FIG. 4). The surface area (cm.sup.2) is then calculated as A=4r.sup.2.

(76) FIG. 12 shows a ln-ln plot of the weight of water absorbed (m.sub.1) in grams per unit surface area versus time in seconds. The difference between FIGS. 11 and 12 in the relative position of sample 16 at small times is the result of the lower density of that sample.

(77) Crank (1975) provides the following expression for the amount of water (g) absorbed per unit surface area (cm.sup.2) per unit time (s))
m.sub.1(t)=m.sub.1(){1(6/.sup.2).sub.n=1.sup.(1/n.sup.2)exp(Dn.sup.2.sup.2t/r.sup.2)}(1)
where m.sub.1() is the mass of water absorbed per unit surface area in the limit of long times and D is an effective diffusion constant, in cm.sup.2/sec.

(78) For each sample, the amount of rapidly absorbed water, measured as the average of data points taken from 1 s to 60 s, was subtracted from the later data points and the additional water absorbed, after 60 s, was fit to Equation (1). In the fitting procedure, m.sub.1() was taken as the value at 24 h. r was taken as the average equivalent-sphere radius for the measured specimens. Water absorption parameters are summarized in Table 2. Only D was varied in the fitting procedure. After the fitting, the amount of water initially absorbed was added to the fitted values of m.sub.t. FIG. 13 shows the fitted results for data points taken at times of 60 s and longer.

(79) Table 7 shows the parameters describing water absorption. The measured diffusion constant is pictured here as a measure of diffusion through the walls separating non-connecting cells. That value is not significantly different for the three samples.

(80) TABLE-US-00007 TABLE 7 Parameters describing water absorption. Sample 16 Sample 18 Sample 21 10% lignin 9% lignin 18% lignin Parameter No cellulose 10% cellulose 10% cellulose r, cm 0.85 0.77 0.73 Initial water absorbed 1.05 1.26 0.44 (60 s), g/g Initial water absorbed 0.0116 0.0174 0.0065 (60 s), g/cm.sup.2 Ultimate water absorbed 3.65 3.13 2.62 (24 h), g/g Ultimate water absorbed 0.041 0.044 0.039 (24 h), (g/cm.sup.2) D, 10.sup.6 cm.sup.2/s 7.6 6.7 7.4 .sup.2 of fit, 10.sup.5 1.6 3.8 9.7

(81) The volume of water absorbed after 24 h is less than the free volume available in the foam. If the volume fraction of voids is initially taken to be 0.96 (see above), only 17% of the free volume is filled with water after 24 h. Air presumably gets trapped during absorption, preventing further absorption.

(82) The data indicate that extruded foams prepared with unmodified high-amylose starch to which lignin has been added at a level of 9-18% have significant water resistance. After 24 h immersed in water they remain intact, in contrast to extruded unmodified starch foams and foams containing 5% lignin, which disintegrate in less than a minute (above), and in contrast to starch-lignin foams prepared by compression molding, which lose their integrity after several hours and are no longer able to support their own weight (Stevens et al., 2010).

(83) Moreover, sample 16 (10% lignin, no cellulose) has approximately the same water resistance properties as samples 18 and 21; neither the additional lignin nor the presence of cellulose fibers in those samples significantly increases water resistance.

(84) Baumberger et al. (1998), who studied starch-lignin films, also found that lignin improves water resistance, as long as no plasticizer is used. Stevens et al. (2007) found that if glycerol is used to plasticize cast starch-lignin films, the effect of the glycerol is to reduce or eliminate the hydrophobic effect of lignin.

(85) Various modifications and variations of the described methods, procedures, techniques, and compositions as the concept of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed is not intended to be limited to such specific embodiments. Various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art, or related fields are intended to be within the scope of the following claims.

(86) Each document, patent application or patent publication cited by or referred to in this disclosure is hereby expressly incorporated herein by reference in its entirety.

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