Cellulose Fibers and Microcellular Foam Starch Composites

Transcription

1 Cellulose Fibers and Microcellular Foam Starch Composites Richard A. Venditti*, Joel J. Pawlak, Andrew R. Rutledge, Janderson L. Cibils Forest Biomaterials Science and Engineering NC State University, Raleigh, NC

2 Outline Introduction to Starch Microcellular Foams Objective Procedures Results and Discussion Conclusions Further Work Acknowledgements

3 Microcellular Starch Foams SMCF production methods (avoids drying from high surface tension liquids) Freeze Drying rapid freezing, many nucleating sites Multiple Solvent exchange Example: 8% aqueous solution of unmodified wheat starch, equilibrated with 40, 70, 90, 100, 100, 100 % ethanol solutions Supercritical Fluid Extrusions Glenn, et al, J. Agric Food Chem, Venditti and Pawlak and coworkers, 2007 and δ ΔP = P Cell wall water P1 P2 Cell wall 2 P1 = 2γ δ Laplace Equation simplified for liquid between 2 plates. The pressure exerted on the plates separated by a 0.01 micron layer of water is approximately 142 atmospheres.

4 Pore Preserving Drying Dried from Water Starch Aquagel Dried from Ethanol Microcellular Foam

5 SEM of Uncooked Starch and SMCF 100 Brightness (ISO) Glutaraldehyde, % Uncooked Starch 7.5g glutaraldehyde / 100g starch Length scale is 20 microns

6 Microcellular Starch Foams Microcellular starch foams: a starch based porous matrix containing pores ranging from 2 micrometers to sub-micrometer size Cell densities > 10 9 cells per cm3 Specific density reduction of 5 to 98% of matrix material SMCF materials have high specific surface areas (air-solid interfaces). SMCF from CO2: m2/g Calcium Carbonate: 2-6 m2/g TiO2: 8-25 m2/g B Coatings and filler particles made from microcellular C foams are expected to be excellent in their ability to scatter light and be strong opacifying materials Micrographs of SMCF created by freeze drying starch (top) and starch/akd(middle) Also shown is SMCF created by solvent exchange (bottom). B

7 Structure of SMCF Foams Glenn, Agricultural Materials as renewable resources: nonfood and industrial applications, ACS, Beaded Polystyrene Puffed Wheat Freeze Dried Starch 100 microns 20 microns 5 microns SMCF particle formed under shear SMCF formed from molded aquagel

8 Challenges The mechanical properties are poor,especially during processing Moisture properties are such that these materials are very humidity sensitive

9 Objectives Produce tougher SMCF materials via composites with wood fibers Carbonize and reduce the humidity sensitivity of such materials as well as modify other properties, such as chemical inertness Understand the effect of carbonizing temperature on the properties

10 Procedures Production of starch-fiber aquagels Solvent exchange process and drying Carbonization Process Analysis

11 Sample Preparation Procedures 120 s microwave Max Temp 90 C Stirring every 15 s Cooked Starch- Fiber Cooling to form stiff gels 24 hours Temp: 5 C 2X gel weight 7 exchanges 24 hrs Ethanol Exchanges Carbonization Tube Furnace Nitrogen atmosphere 0.5 C/min heat/cool ramp Also: TGA Furnace with Nitrogen ramped at 10C/min

12 Starch Fiber Cooking: Microwave Heating Uncooked 120 seconds 240 seconds

13 Starch Microwave Cooking Temp, C Microwave Time, s

14 Results: Composition Effects 28% AMYLOSE Shape Strength 16% starch - no fiber Mostly clumpy poor drying Good 12% starch - no fiber Warped and clumpy Good 8% starch - no fiber Loss of volume, poor shape Brittle 8% starch - 3% fiber Warped Good 8% starch - 4% fiber Warped Good 8% starch - 5% fiber Good shape but clumpy Good 50% AMYLOSE 10% starch - 3% fiber Maintained shape/smoothness Good 10% starch - 4% fiber Maintained shape/smoothness Good 10% starch - 5% fiber Maintained shape/smoothness Good HIGH AMYLOSE 15% starch - no fiber Cracked upon drying None 12% starch - no fiber Cracked upon drying None 15% starch - 1% fiber Cracked upon drying Brittle 15% starch - 2% fiber Cracked upon drying Poor 15% starch - 3% fiber Maintained shape/smoothness Good 14% starch - 3% fiber Maintained shape/smoothness Good 12% starch - 3% fiber Maintained shape/smoothness Good 12% starch - 4% fiber Maintained shape/smoothness Good 12% starch - 5% fiber Maintained shape/smoothness Good

15 Composition Effects Increased amylopectin improved strength properties of the foams Increased amylose improved the shape smoothness of the molded parts Increased amylose increased the density of molded parts Increased fiber content up to 5% improved the structure (decreased density) and toughness Above 5% fiber content the foams were had an uneven distribution of fibers

16 Composition Effects Increased fiber content up to 5% improved the structure (decreased density) and toughness Above 5% fiber content the foams had an uneven distribution of fibers

17 Procedures: Mechanical Propts Compression testing on a MTS Model Alliance RF300 with 60,000 lb frame and 260 lb Omega load cell. Strain rate of 0.5 mm/min.

18 Heat Treatment/Carbonization of SMCF-Fiber Composites Heat treatment at 200 C produces a wood-like foam material of low density (0.45 g/cc) Produces a low density (0.20 g/cc) porous carbon microstructure Inert, strong, low density (0.20 g/cc) foam upon heating to 350 C and above 200 C Untreated 350 C

19 TGA Results: 50% Amylose with Fiber Starch Fiber

20 Effect of Treatment Temperature on Yield: SMCF-Fiber Composites Yield, % Temperature, C

21 Effect of Treatment Temperature on Density: SMCF-Fiber Composites Density, g/cc Temperature, C

22 Dimensional Changes vs Treatment Temperature: SMCF-Fiber Composites Shrinkage, % X-Y Plane Z-Direction Temperature, C

23 Compression Testing Results Omega Loadcell (N) 400 Typical results for untreated and treated at 200 C. 300 P (Note Y-axis max is 400 N) 200 S [3 ] 100 M B Crosshead (mm) Omega Loadcell (N) S P Typical results for 350 C and higher treatment temperatures [11] (Note Y-axis max is 90 N) 20 M 10 B 0 Y Crosshead (mm)

24 Effect of Treatment Temperature on Mechanical Properties: Increases in viscosity improve pore structure and brightness.

25 Effect of Treatment Temperature on Weight NormalizedMechanical Properties: Increases in viscosity improve pore structure and brightness.

26 Results from study: Particles with a fine porous structure and high brightness can be formed with the described solvent exchange method Crosslinking/Molecular weight found to improve pore structure and optical properties The resulting SMCF particles absorbed water rapidly and lost structure upon wetting Water resistance must be increased

27 Challenges The major challenge still remains to control the wetting properties of the SMCF Blending, derivatization and crosslinking are being explored as approaches to development of water resistance Alternative methods of foam formation including carbon dioxide assisted extrusion are also being explored to modify pore structure

28 ACKNOWLEDGEMENT This research is supported by National Research Initiative Competitive Grant from the USDA Cooperative State Research, Education, and Extension Service Support was also provided by the NCSU and College of Natural Resources Undergraduate Research Program.