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
Outline
•
•
•
•
•
•
•
Introduction to Starch Microcellular Foams
Objective
Procedures
Results and Discussion
Conclusions
Further Work
Acknowledgements
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
Cell wall
δ
water
P1
Cell wall
ΔP = P2 − P1 =
P2
2γ
δ
Laplace Equation simplified
for liquid between 2 plates.
– Supercritical Fluid Extrusions
– Glenn, et al, J. Agric Food Chem,
2002.
– Venditti and Pawlak and
coworkers, 2007 and 2008.
The pressure exerted on the
plates separated by a 0.01
micron layer of water is
approximately 142
atmospheres.
Pore Preserving Drying
Dried from Water
Starch Aquagel
Dried from
Ethanol
Microcellular Foam
SEM of Uncooked Starch and SMCF
Brightness (ISO)
100
90
80
0
5
10
15
20
25
Glutaraldehyde, %
Uncooked Starch
7.5g glutaraldehyde / 100g starch
Length scale is 20 microns
30
35
Microcellular Starch Foams
•
•
•
•
Microcellular starch foams: a starch based
porous matrix containing pores ranging from
2 micrometers to sub-micrometer size
Cell densities > 109 cells per cm3
Specific density reduction of 5 to 98% of
matrix material
SMCF materials have high specific surface
areas (air-solid interfaces).
B
– SMCF from CO2: 50-150 m2/g
– Calcium Carbonate: 2-6 m2/g
– TiO2: 8-25 m2/g
•
Coatings and filler particles made from
microcellular
foams are expected to be
C
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
Structure of SMCF Foams
Glenn, Agricultural Materials as renewable resources: nonfood and industrial applications, ACS, 1996.
Beaded Polystyrene
Puffed Wheat
Freeze Dried Starch
100 microns
5 microns
20 microns
SMCF particle formed under shear
SMCF formed from molded aquagel
Challenges
• The mechanical properties are
poor,especially during processing
• Moisture properties are such that
these materials are very humidity
sensitive
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
Procedures
•
•
•
•
Production of starch-fiber aquagels
Solvent exchange process and drying
Carbonization Process
Analysis
Sample Preparation Procedures
120 s microwave
Max Temp 90 C
Stirring every 15 s
Cooked
StarchFiber
2X gel weight
7 exchanges
24 hrs
Cooling to
form stiff
gels
24 hours
Temp: 5 C
Ethanol
Exchanges
Carbonization
Tube Furnace
Nitrogen atmosphere
0.5 C/min heat/cool
ramp
Also: TGA Furnace
with Nitrogen
ramped at 10C/min
Starch Fiber Cooking: Microwave Heating
Uncooked
120 seconds
240 seconds
Starch Microwave Cooking
Temp, C
100
80
60
40
20
0
0
50
100
150
200
Microwave Time, s
250
300
Results: Composition Effects
28% AMYLOSE
16% starch - no fiber
12% starch - no fiber
8% starch - no fiber
8% starch - 3% fiber
8% starch - 4% fiber
8% starch - 5% fiber
50% AMYLOSE
10% starch - 3% fiber
10% starch - 4% fiber
10% starch - 5% fiber
HIGH AMYLOSE
15% starch - no fiber
12% starch - no fiber
15% starch - 1% fiber
15% starch - 2% fiber
15% starch - 3% fiber
14% starch - 3% fiber
12% starch - 3% fiber
12% starch - 4% fiber
12% starch - 5% fiber
Shape
Mostly clumpy poor drying
Warped and clumpy
Loss of volume, poor shape
Warped
Warped
Good shape but clumpy
Strength
Good
Good
Brittle
Good
Good
Good
Maintained shape/smoothness
Maintained shape/smoothness
Maintained shape/smoothness
Good
Good
Good
Cracked upon drying
Cracked upon drying
Cracked upon drying
Cracked upon drying
Maintained shape/smoothness
Maintained shape/smoothness
Maintained shape/smoothness
Maintained shape/smoothness
Maintained shape/smoothness
None
None
Brittle
Poor
Good
Good
Good
Good
Good
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
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
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.
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
TGA Results: 50% Amylose with Fiber
Starch
Fiber
Effect of Treatment Temperature on Yield:
SMCF-Fiber Composites
Yield, %
120
100
80
60
40
20
0
0
200
400
Temperature, C
600
800
Density, g/cc
Effect of Treatment Temperature on Density:
SMCF-Fiber Composites
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
200
400
Temperature, C
600
800
Dimensional Changes vs Treatment Temperature:
SMCF-Fiber Composites
Shrinkage, %
40
30
20
X-Y Plane
10
0
‐10
Z-Direction
‐20
1
2
3
Temperature, C
4
5
Compression Testing Results
O m e g a L o a d c e ll ( N )
400
Typical results for untreated
and treated at 200 C.
P
300
200
[3 ]
S
(Note Y-axis max is 400 N)
M
100
B
0
0
1
2
3
4
C ro s s h e a d (m m )
O m e g a L o a d c e ll ( N )
90
P
S
80
70
Typical results for 350 C
and higher treatment
temperatures.
60
50
[1 1 ]
40
30
20
(Note Y-axis max is 90 N)
M
10
B
0
0 .0
Y
1 .0
2 .0
C ro s s h e a d (m m )
3 .0
Effect of Treatment Temperature on
Mechanical Properties:
Increases in viscosity improve pore structure and brightness.
Effect of Treatment Temperature on
Weight NormalizedMechanical
Properties:
Increases in viscosity improve pore structure and brightness.
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
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
ACKNOWLEDGEMENT
This research is supported by National Research Initiative Competitive
Grant 2005-35504-16264 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.