Journal of Polymers and the Environment, Vol. 12, No.

3, July 2004 ( 2004)

Functional Properties of Extruded Acetylated

Starch–Cellulose Foams

Junjie Guan,1 Kent M. Eskridge,2 and Milford A. Hanna1,3

Acetylated starches, with degrees of substitution (DS) of 2, 2.5 and 3, were blended with 3%,
7.5% and 12% a-cellulose and 14%, 17% or 20% (d.b.) ethanol and twin-screw extruded at
165
C barrel temperature and 225 rpm screw speed. A response surface methodology exper-
imental design was applied to the sub-plot and a completely randomized design to the whole
plot design to test the differences among the acetylated starches and the effects of cellulose and
ethanol. DS, cellulose and ethanol contents significantly affected the functional properties and
specific mechanical energy requirement. DS had positive effects on radial expansion ratio
(RER), compressibility and specific mechanical energy requirement and a negative effect on
bulk density. Highest (RER) was obtained from 20% ethanol content. Extrudates containing
12% cellulose had the highest bulk density and the highest compressibility. Higher cellulose
contents required more specific mechanical energy.

KEY WORDS: Acetylated starch; extrusion; cellulose.

INTRODUCTION the crystalline region, long-chain molecules are trim-

med and shorter straight chain segments of polysac-
There has been much recent interest in the utili-
charides form due to the high temperature, high shear
zation of starch as a biodegradable plastic material.
and high pressure during extrusion [3]. The short-
Next to cellulose, starch is the second most abundant
chain molecules are reassociated/plasticized after
renewable polysaccharide in nature [1]. More and
exiting the extruder nozzle in presence of water (plas-
more packaging industries are using starch-based
ticizer) and pressure drop [3]. The reassociation is
polymers as the major component in manufacturing
formed by hydrogen bonds. When extruded starch
loose-fill packaging materials. Starch-based loose-fill
contacts moisture/water, the highly polarized solution
packaging materials have good mechanical properties,
attacks the hydrogen bonds, reducing bond forces and
are readily biodegrade in soil and sell at competitive
finally significantly decreasing the functional proper-
prices [2]. However, native starch loose-fill foams
ties of the loose-fill foams. For packaging purposes,
suffer from a lack of moisture resistance and abrasion
low moisture absorption is preferred. One possible
resistance. They collapse when in contact with water or
solution to this problem is the use of modified starches.
in an atmosphere with high relative humidity. The
Several research projects have been conducted using
integrity of starch granules are destroyed by melting
acetylated starch as the main component in preparing
loose-fill packaging materials [4–6]. Technologies for
1
University of Nebraska-Lincoln, Industrial Agricultural Products producing acetylated starch have been known for
Center, 208 L.W. Chase Hall, Lincoln, NE 68583-0730. more than 100 years. Researchers [7–11] prepared
2
University of Nebraska-Lincoln, Department of Statistics, 103
Miller Hall, Lincoln, NE 68583-0712.
starch esters/acetylated starch by reacting organic acid
3
To whom all correspondence should be addressed. E-mail: anhydride or vinyl esters with starches. Starch is a
mhanna1@unl.edu polymer of D-glucose with each glucose unit having
113
1566-2543/04/0700-0113/0  2004 Plenum Publishing Corporation
114 Guan, Eskridge, and Hanna

three free hydroxyl groups. All or parts of these hy- Hylon VII (70% amylose starch) was purchased
droxyl groups were substituted with acetyl groups from American Maize Products Co. (Hammond, IN).
through chemical reaction so as to manipulate the The a-cellulose was purchased from Sigma Chemical
hydrolysis property of starch. It has been reported that Co., (St. Louis, MO). The decomposition tempera-
acetylation of hydroxyl groups of starch, to increase ture of a-cellulose is 260–270
C and its density, in a
hydrophobicity, is one approach to increase the water natural state, is between 200–500 kg/m3. Talc (mag-
resistance of starch [12]. Compared with acetylated nesium silicate) was purchased from Barret Minerals,
starch in food uses, acetylated starch in industrial uses Inc. (Dillon, MT). The talc had a median particle size
requires a high degree of substitution (DS). High DS of 1.2 lm and bulk density of 120 kg/m3. Denatured
(DS > 1.5) starch has higher hydrophobicity, which is ethanol was purchased from Fisher Scientific, Inc.
a favorable property of loose-fill packaging material. (Fair Lawn, NJ). Acetic anhydride was purchased
However, preparing high DS starch requires more from Vopak Inc., (Dallas TX). Sodium hydroxide
chemicals and longer reaction time, resulting in higher (50% solution) was purchased from Harcros Chemi-
material cost. Therefore, it is necessary to determine cals Inc., (Kansas City, KS).
the relation of DS of starch and functional properties
of extruded acetylated starch loose-fill foams.
Another way to reduce the cost of preparing Starch Acetylation
loose-fill packaging material is blending acetylated
High amylose corn starch was dried in the walk-
starch with cellulosic materials. Compared to acety-
in dryer at 50
C for 48 h. To begin the acetylation
lated starch, cellulosic materials such as wood fiber,
process, acetic anhydride was placed in a steam-
oat fiber, cellulose, corncob fiber, wheat and rice
jacketed reactor with a rotating self-wiping paddle.
straw are cheaper [1]. Guan et al. [5, 6] and [13]
Then, 70% amylose starch was added into the reactor
blended wood fiber, oat fiber, cellulose and corncob
with 5 min of continuous mixing. Finally, NaOH
fiber with acetylated starch. The extruded foams had
solution was added while mixing. The chemicals ad-
good functional properties, indicating blending cel-
ded to the reactions and reaction times were sum-
lulosic materials in acetylated starch would be prac-
marized in Table I. The temperature of the reactor
tical and interesting to develop starch-based loose-fill
jacket was maintained at 123
C. After various times,
packaging materials.
the reaction was stopped by quickly adding 200 L of
The objectives of this research were to evaluate
cold water to the reactor. The pH value was adjusted
the effects of the DS of acetylated starch, the level of
to 5.0 by washing with tap water before drying
added cellulose and the level of added ethanol on the
at 50
C in a walk-in dryer to a moisture content of
functional properties and the mechanical energy
4% (w.b.). The starch was ground in a Standard
requirements of various extruded foams. Functional
model No. 3 Wiley mill (Arthur H. Thomas Co.
properties were radial expansion ratio (RER), bulk
Philadelphia, PA) to pass through a 5 mm opening
density, and compressibility. Mechanical energy
sieve. Three batches of same DS starches were
requirements were determined to evaluate the process
prepared as replications.
of preparing extruded foams from different DS
acetylated starches.
Blend Preparation
MATERIALS AND METHODS The acetylated starches and cellulose were dried
in a mechanical convection oven (GCA Corp., Chi-
Materials
cago IL) at 105
C for 1 h, and then cooled in a
Acetylated starches, with DS of 2, 2.5 and 3, desiccator for 1 h to ensure they were moisture-free
were prepared from 70% amylose cornstarch. before being used in sample preparation [2]. Talc

Table I. Experimental parameters for preparation of acetylated starches

DS Starch (kg) Acetate anhydride (kg) NaOH (kg) Reaction time (h)
2.0 45.45 110.0 5.0 2.0
2.5 45.45 110.0 5.0 3.0
3.0 22.45 81.0 4.4 5.0
Functional Properties of Extruded Acetylated Starch–Cellulose Foams 115

was added to all samples at a 5% level (w/w). Talc acetylated starch type and cellulose and ethanol
functioned as a nucleating agent to ensure unifor- contents on foam functionalities and specific
mity of the cells. Different amounts of the prescribed mechanical energy requirement:
cellulose (3%, 7.5%, and 12%) and ethanol (14%,
17% and 20%) were added to the starch, and mixed Y ¼ b0 þ b1 X1 þ b2 X2 þ b12 X1 X2 þ b11 X21 þ b22 X22 þ e
in a Hobart mixer (Model C-100, Hobart Corp.,
Troy, OH) for 5 min. Samples were then sealed in Where Y ¼ response (RER, bulk denisty, com-
low density polyethylene (LDPE) plastic containers pressibility, and specific mechanical energy),
for 24 h at room temperature (25
C) to allow etha- X1 ¼ cellulose content, X2 ¼ ethanol content,
nol completely absorbed by the blends. b0 ¼ intercept, bn ¼ regression coefficient and
e ¼ experimental error with normal distribution,
Extrusion mean zero and variance r2. Significance level was
defined as P<0.05 and R2 was used to evaluate
A twin-screw extruder (DR-2027-K13, model fit. For each response, three-dimensional plots
C. W. Brabender, Inc., S. Hackensack, NJ) with a were produced from regression equations by holding
manufacturer pre-designed co-rotating mixing screws two variables fixed from Microsoft Excel. SAS
(Model CTSE-V, C. W. Brabender, Inc., S. Hacken- software [15] was used for statistical calculations and
sack, NJ) was used to conduct extrusions. The man- Microsoft Excel was used for surface graphing.
ufacturer designed conical screws had diameters
decreasing from 43 mm to 28 mm along their length
of 365 mm from the feed end to the exit end. On each Physical Characteristics
screw, there was a mixing section, in which small
Physical characteristics of the foamed materials
portions of the screw flight were cut away. The mix-
including RER and bulk density were determined.
ing section enhanced the mixing action and also in-
RER was calculated by dividing the mean cross sec-
creased the residence time of the sample in the barrel.
tional area of the extrudates by the cross sectional
A 250-rev/min screw speed was used for all extru-
area of the die nozzle. Each mean value was the
sions. The temperature at the feeding section of the
average of ten measurements [16].
barrel was maintained at room temperature (~25
C)
Bulk density (bulk density) of the extrudates was
while the other two barrels sections and the die were
measured using a cylindrical Plexiglas container [2].
maintained at 165
C. A 3-mm diameter die nozzle
The container had a diameter of 160 mm and a height
was used to produce cylindrical extrudates. The ex-
of 160 mm. A funnel having an opening of 160 mm at
truder was controlled by a Plasti-Corder (Type FE
the top and an opening of 64 mm at the bottom was
2000, C. W. Brabender, Inc., S. Hackensack, NJ). An
mounted at a height of 160 mm above the container.
adjustable rotating knife located right next to the
Bulk densities (kg/m3) of the extrudates were calcu-
nozzle, was used to cut the extrudates into 20 mm
lated from the mass of the as-compacted sample di-
lengths. Extrusion data were recorded for subsequent
vided by the volume of the container. Five
analyses.
replications were measured for each sample.

Statistical Design and Analysis

Compressibility
The experimental design was a split-plot with
starch type (DS level) as whole plot factor and the An Instron universal testing machine (Model
cellulose and ethanol contents as the split plot fac- 5566, Instron Engineering Corp., Canton, MA) was
tors. The main plot used a randomized complete used to measure compressibility of foamed extru-
block design with three blocks (blocked by batch) and dates. The 20-mm long extrudates were placed on a
response surfaces were fitted to cellulose and ethanol flat plate with careful aligning cut surfaces so that
factors for each DS level. Analysis of variance was edges were perpendicular to the axis of the extrudate
used to test the influence of acetylated starch type sample (direction of extrusion). Then the extrudate
(DS) on RER, bulk density, compressibility and was compressed once to 80% of its original diameter
specific mechanical energy requirement. For each at a loading rate of 1 cm/min using another flat plat.
starch type, a polynomial regression model [14] was The force (kN) divided by the sample density (kg/
employed to investigate the response surface of m3) was reported as compressibility (kN/kg m3).
116 Guan, Eskridge, and Hanna

Compressibility for each sample was measured five increased dramatically with increasing ethanol con-
times and reported as an average of the five reading. tent. This increase became less significant with less
cellulose. Ethanol affected RER differently when
cellulose content increased. RER decreased when
Specific Mechanical Energy Requirement (SME)
cellulose content increased at low ethanol content. At
SME is defined as a total input of mechanical high ethanol content, RER increased when cellulose
energy per unit dry weight of extrudate. SME was content decreased. Similar trends were found with DS
determined as described by Bhatnagar and Hanna 3 acetylated starch–cellulose foams (Fig. 1b), except
[17]. Extruded materials were collected for 30 s and that RER firstly increased and then decreased as
dried. SME (Wh/kg) was calculated as ethanol content increased. At low ethanol content,
RER decreased with increasing cellulose content
n while at high ethanol content, RER increased. Cel-
SME ¼ ½2
pð Þ
s=MFR
60 lulose content did not have significant effect on RER
of DS 2.5 acetylated starch–cellulose foams
where n ¼ screw speed (rev/min); s ¼ torque (Table II). DS starch had a significant effect on RER
(N m); and MFR ¼ mass flow rate (kg/h). which increased when higher DS acetylated starch
was blended in (Table III).
RESULTS AND DISCUSSIONS Starch–cellulose dough foamed after exiting the
die nozzle in presence of the blowing agent (ethanol)
Physical Characteristics
and the pressure drop [1]. The degree of foaming/
Expansion is a very important index of loose-fill expansion was closely related to the dough viscosity
foam physical properties. There are two types of and the amount of blowing agent. Higher viscosity
expansion of extruded foams, radial and longitudinal. allowed the dough to elongate more, given sufficient
Radial expansion is more interesting because it more blowing agent. During extrusion, high shear, tem-
obviously represents extruded dough rheological perature and pressure caused the starch to melt.
properties and foaming kinetics than longitudinal Shear further depolymerized the starch, resulting in
expansion [18]. short-chain amorphous polymers. Cellulose also was
The response surface plots for RER of acetylated depolymerized during extrusion in presence of etha-
starch–cellulose foams are shown in Fig. 1. The RER nol [1, 19]. Firstly, ethanol penetrated cellulose ma-
of DS 2 starch–cellulose foams were affected signifi- trix and partially solubilized the hydrogen bonds
cantly by ethanol (blowing agent) and cellulose con- among cellulose chains during the overnight storage.
tents (Fig. 1a). At high cellulose content, RER Then, shear depolymerized the long chain molecules

(a) Y = 20.75 – 1.79 X1 – 1.26 X2 + 0.09 X1X2 (b) Y = – 51.42 – 1.54 X1 + 8.44 X2 + 0.05 X1X2 + 0.05 X12 – 0.25 X22
2
(R = 0.9385) (R2 = 0.7401)

14

13
Y = Radial expansion ratio

Y = Radial expansion ratio

12 20

19
11
18
10
17
9
16
20
8 15 19
7 20 14 18
19 3 17
6 18 4.5
17 6 16
5 16 X = Ethanol content, % 7.5 X2= Ethanol content, %
2 15
3 4.5 15 9
6 7.5 X1= Cellulose content, % 10.5 14
9 14
10.5 12 12
X1= Cellulose content, %

Fig. 1. (a) Effects of cellulose content and ethanol content on RER of DS 2.0 starch acetate–cellulose foams. (b) Effects of cellulose content
and ethanol content on RER of DS 2.5 starch acetate–cellulose foams.
 Functional Properties of Extruded Acetylated Starch–Cellulose Foams 117

[3]. The short segments of both starch and cellulose

1647.56***

Model on which X1 (cellulose content), X2 (ethanol content) is calculated: Y = b0 + b1X1 + b2X2 + b12X1X2 + b11X12 + b22X22 +e. RER, radial expansion ratio; qB bulk density;
)136.52***

0.69**
3.9***
0.7329
0.0002
)7.44**

)0.056
were fully mixed inside the barrel. When exiting the
3.0

nozzle, ethanol evaporated because of sudden tem-

perature and pressure drops. The starch and cellulose
SME (WÆh/kg)

reassociated and formed a starch–cellulose matrix by

1.92***
1047.26***

)74.56***

<0.0001
0.9733
)0.118

hydrogen and covalent bonds. Ethanol functioned as

3.21

0.13
2.5

a plasticizer as well as a blowing agent. When ethanol

was added to the blend, it was more readily absorbed
by cellulose. Therefore, the more cellulose in the
)1.56***
247.08**

38.56**
10.26**

<0.0001
0.9425
)0.038
)0.512

blend, the more ethanol was needed. At low ethanol

2.0

content, most of the ethanol penetrated the cellulose

Table II. Regression Equation Coefficientsa of Second Order Polynomialsb for Two Response Variables

and solubilized the bonds among cellulose chains.

<0.0001
0.8531
)7279.36**

Insufficient ethanol was left for the further foaming.

319.2**
157.84

150.64
)486.54
124071**

Since that, high RER foams were obtained with high

3.0

ethanol content and high cellulose content.

Low density is important for packaging materials
COMP (kN/kg m3)

from a shipping cost standpoint. Bulk density of

372.89***
)7960.27***

582.77**

<0.0001
0.8769

acetylated starch–cellulose foams are shown in Fig. 2.

243835***

106.26
)16448**
2.5

DS 2 starch–cellulose foams had the lowest bulk

density at high ethanol and cellulose contents. At high
cellulose content, bulk denisty significantly decreased
238.93***

when ethanol content increased; while at low cellulose

0.6310
)922.95**

)185.35**

)514.51**

0.0036

content bulk density slightly decreased when ethanol

217062**

)15249**
2.0

content was higher then 16.8%. At high ethanol con-

tent, bulk density decreased significantly from 33 to
22 kg/m3. Bulk density of DS 2.5 starch–cellulose
)0.07***

)0.06***
31.58**

0.7829
<0.0001 <0.0001
)0.54**
)0.56**

foams showed similar trends except cellulose content

0.011
3.0

had a greater effect on bulk density than ethanol

content. As ethanol content increased, bulk density
qB (kg/m3)

firstly decreased sharply and then the decrease became

136.93***

0.31***

*, **, and *** indicate significance at P < 0.10, 0.05 and 0.01, respectively.
0.16**

0.058*

0.8335
)12.0***

)0.08**

less significantly. The bulk density of DS 2.5 starch–

2.5

cellulose foams ranged from 11 to 29 kg/m3, in com-

pared from 22 to 36 kg/m3 of DS 2-cellulose foams.
)0.18***

Cellulose also had a negative effect on bulk density in

50.04**

10.11**
2.43**

0.7397
0.0002
)0.29**
0.066

COMP, compressibility; and SME, specific mechanical energy.

2.0

DS 3 starch–cellulose foams. An opposite trend was

found in bulk density when increased ethanol con-
tent. At high cellulose content, bulk density increased
51.42***

8.44***
)1.54***

)0.25***
0.05**

0.05**

0.7401
0.0002

with increasing ethanol content. But bulk density de-

3.0

creased when ethanol content increased at low ethanol

content. Bulk density of DS 3 starch–cellulose foams
ranged from 20 to 24.5 kg/m3. The lowest bulk density
)0.0995*
20.75*** 28.83**

4.26**

0.0096

0.8959
Probability of F <0.0001 <0.0001
RER

0.030
)1.79*** )0.748

(20 kg/m3) was obtained at low ethanol content and

2.5

high cellulose content. DS of acetylated starch had a

significant effect on bulk density because of strong
0.09***

0.9385
)1.26**

correlation between RER and bulk density with

0.011
2.0

0.04

higher DS acetylated starch extruded causing lower

bulk density (Table III).
All the three acetylated starch–cellulose foams
Cross product

had lower densities at high cellulose content, sug-

Coefficient

Quadratic

gesting cellulose was well blended and formed uni-

Linear

form cell structure with acetylated starches. RER and

DS

b12

b11
b22
R2
b0

b1
b2

bulk density are strongly related. The higher the

b
a
118 Guan, Eskridge, and Hanna

Table III. Analysis of Variance of Acetylated Starch Type (whole plot factor) on Functional Properties and Specific Mechanical Energy

RER qB (kg/m3) COMP (kN/kg1Æ m3) SME (Wh/kg)

DF 2 2 2 2
SS 305.396 1316.977 40609550141 171903.738
ME 152.698 658.488 20304775070 85951.864
F-value 369.00 1769.10 2181.82 941.33
Probability of F <0.0001 <0.0001 <0.0001 <0.0001

RER, radial expansion ratio; qB, bulk density; COMP, compressibility; SME, specific mechanical energy; DF, degree of freedom; SS, sum of
square; and ME, mean square.

RER, the less dense are the foams. As DS of the cellulose solubilizing agent for cellulose depolymer-
starches increased, ethanol effects began to change. ization. But which of these two functions acting
Ethanol functioned as blowing agent for foaming and predominantly depended upon the type of starch.

(a) Y = – 50.04 + 2.43 X1 + 10.11 X2 – 0.18 X1X2 – 0.29 X22 (b) Y = 136.93 + 0.16 X1 – 12.0 X2 – 0.08 X12 + 0.31 X22
(R2 = 0.7397) (R2 = 0.8335)

30
37 28
Y = Bulk density, kg/m3

35 26
Y = Bulk density, kg/m3

33 24
31 22
29 20
27 18
16
25
14
23
20 12
321 310 20
19
4.5 19
18 4.8
6 18
7.5 17 6.6
17
9 16 X2 = Ethanol content, % 8.4 16
10.5 15 X1= Cellulose content,% 10.2 15
X1= Cellulose content, % X2= Ethanol content, %
12 14 12 14

(c) Y = 31.58 – 0.54X1 – 0.56 X2 – 0.07 X1X2 – 0.06 X12

(R2 = 0.7829)

25
Y = Bulk density, kg/m3

24

23
22
21
20 20
19
19
18
3 17
4.5
6 16
7.5 15
9
X2= Ethanol content, %
10.5 14
X1= Cellulose content, %
12

Fig. 2. (a) Effects of cellulose content and ethanol content on qB of DS 2.0 starch acetate–cellulose foams. (b) Effects of cellulose content and
ethanol content on qB of DS 2.5 starch acetate–cellulose foams. (c) Effects of cellulose content and ethanol content on qB of DS 3.0 starch
acetate–cellulose foams.
Functional Properties of Extruded Acetylated Starch–Cellulose Foams 119

(a) Y = 217062 – 922.95X 1 – 15249 X2 – 185.35 X1X2 + 238.93 X12 – 514.51 X22 (b) Y = 243835 – 7960.27 X1 – 16448 X2 + 372.89 X12 + 582.77 X22
(R2 = 0.6310) (R2= 0.8769)

130000

Y = Compressibility, kN·kg-1·m3
Y = Compressibility, kN·kg-1·m3

125000
120000
110000
115000
105000 110000
100000 105000
100000
95000
95000
90000 20
19 90000 20
19
85000 18 85000 18
3 17
17 80000 16 X = Ethanol content, %
4.8 3 2
16 4.5 15
6.6 6 7.5 9 14
8.4 15 X2 = Ethanol content, % 10.5 12
X1 = Cellulose content, % 10.2 14 X1= Cellulose content, %
12

Fig. 3. (a) Effects of cellulose content and ethanol content on compressibility of DS 2.0 starch acetate–cellulose foams. (b) Effects of cellulose
content and ethanol content on compressibility of DS 2.5 starch acetate–cellulose foams.

When cellulose blended with DS 2 and DS 2.5 star- obtained at high ethanol content and low cellulose
ches, ethanol functioned as blowing agent more than content. As ethanol content decreased, compressibility
as a cellulose-solubilizing agent as ethanol content decreased, while increasing cellulose content resulted in
increased, resulting in decreased bulk density. But for decreased compressibility. Ethanol content did not
the DS 3 starch–cellulose foams, the starch–cellulose have a significant effect on compressibility of DS 3
blends were subjected to higher shear than the lower starch–cellulose foams. Compressibility decreased
DS starch–cellulose blends due to the harder texture firstly and then increased as cellulose content increased
of DS 3. Because more acetyl groups were substi- in DS 3-cellulose foams. DS had a significant effect on
tuted, stronger covalent bonds formed among starch compressibility of acetylated starch-cellulose foams
chains. Hence, ethanol penetration of the starch (Table III) with compressibility increased as DS of
chains was hindered. Then, cellulose absorbed most acetylated starch increased.
of the ethanol and the chains were fully solubilized. Mechanical properties, such as compressibility,
When high shear applied, they were easily broken are strongly related to the foam formation and the
down to small segments and formed starch–cellulose blended materials matrix formation. Starch–cellulose
matrices. Since these matrices had shorter chains, matrix formation affected the dough rheological
bond forces between starch and cellulose were not properties, resulting in foam brittleness. The better
strong and small cells were broken when exiting the starch–cellulose matrix formed the more com-
nozzle, resulting in higher density. This may have pressible and less brittle were the foams. As men-
been the reason why RER and bulk density were tioned previously, the higher DS, the more acetyl
positively affected by ethanol content at high cellu- groups substituted. With more acetyl groups, the
lose content (Fig. 1b and 2c). greater chance for the starch and cellulose chains to
form covalent bonds. Hence, the dough viscosity in-
creased resulting in a more resilient texture of the
Compressibility
foams. When mechanical forces were applied to the
Compressibility is an important mechanical higher DS starch–cellulose foams, more forces were
property of packaging materials. During shipping, required to destroy the foam structure. On the other
impact forces are absorbed by compressing loose-fill hand, the higher the viscosity of the dough, the better
foams to minimize the damage. Therefore, high it could sustain the ethanol vapor pressure during
mechanical forces absorption is preferred, requiring a foaming. Therefore, higher DS starch–cellulose
high compressibility. Response surface plots of com- foams had lower densities, which also contributed to
pressibility are shown in Fig. 3. DS 2 starch–cellulose higher compressibilities. Because the strong covalent
and DS 3 starch–cellulose foams exhibited similar bond forces hindered ethanol penetration into the
trends in compressibility. High compressibility was starch, it functioned as a blowing agent in DS 3
120 Guan, Eskridge, and Hanna

starch–cellulose foam. Therefore, ethanol did not molecules. At low cellulose content, as ethanol con-
have significant contribution to the dough viscosity. tent increased the cellulose became fully solubilized.
This may have been the reason why ethanol had The residual ethanol was heated up and became
insignificant effect on compressibility. superheat by thermal energy. This consumed part of
the thermal energy, resulting in less thermal degra-
dation of cellulose and acetylated starch. Therefore,
Specific Mechanical Energy Requirement (SME)
more mechanical energy was required to be converted
SME is an easy-to-monitor, real-time indicator of to thermal energy to compensate for this in order to
a process inside the extruder. Mechanical energy, ap- form the well-mixed short-chain molecule. While at
plied to an extruder, mostly is converted to thermal high cellulose content, more ethanol was needed to
energy while some of it is used to break or create new solubilize the cellulose. With less ethanol (14%), more
covalent bonds in the extrudates [4]. Also, reducing the mechanical energy was required to depolymerize the
amount of SME is important to reduce process cost. cellulose. The SME also depended upon the barrel
Similar trends were found in SME for DS 2, DS friction and the viscosity of the material. Viscosity of
2.5 and DS 3 starch–cellulose foams (Table II). Cel- the sample depends upon its molecular weight and
lulose content had a significant effect on SME. As intermolecular interactions [4]. The higher the DS of
cellulose content increased, SME increased. SME the starch was, the more rigid the starch particle was,
increased as ethanol content increased at low cellu- because starch molecules interacted via hydrophobic
lose content. But at high cellulose content, SME de- interactions, and the lower DS starch had more hy-
creased when ethanol content decreased (Fig. 4). The droxyl groups. The availability of hydroxyl groups
DS had a significant impact (P > 0.02) on the SME. facilitated their participation in hydrogen bonds
SME was affected significantly by DS of acetylated when they came in close proximity. When both starch
starch with the higher the DS, the more mechanical and cellulose were depolymerized and in the melted
energy required (Table III). state, they could more readily form strong hydro-
During extrusion, thermal energy combined with phobic covalent bonds, especially when exiting nozzle
mechanical energy melted and broke covalent bonds in the presence of ethanol. Because of the covalent
in the acetylated starch and cellulose. Therefore, bond formation in the barrel, higher DS starch–cel-
before exiting the nozzle, the shorter-chain length and lulose blends had higher viscosities and more SME
melted starch and cellulose were well mixed. It was requirement. For the lower DS starch blends, signif-
obvious that with higher cellulose content in the icant amount of hydroxyl groups were present in the
blend, more mechanical energy was required to melt vicinity of the hydrophobic covalent bonds. These
and depolymerize the covalent-bonded long chain hydroxyl groups had disrupting effects on the
hydrophobic interactions between starch chains and
cellulose chains. The decreases in intermolecular
interactions resulted in lower viscosity and lower
Y = 1647.56 – 7.44 X1– 136.52 X2+ 0.69 X12+ 3.9 X22 SME requirements.
(R2 = 0.7329)

CONCLUSIONS
Y = Specific mechanical energy, Wh/kg

Acetylated starch extruded with cellulose has

600
promising properties as a loose-fill packaging mate-
580 rial. At the same ethanol and cellulose contents,
560 RER, spring index, bulk spring index and com-
540 20
520 19
pressibility increased and unit and bulk density
500 18 decreased as DS of the acetylated starch increased.
480 17 Even specific mechanical energy requirement
460 X = Ethanol content, %
16 2
440 increased when higher DS acetylated starch was
420 15
3
blended in, overall accomplishment of previous
4.5 6 7.5 9 10.5 14
12 mentioned functional properties was more significant.
X1= Cellulose content, % Cellulose content significantly affected the properties
Fig. 4. Effects of cellulose content and ethanol content on specific of all three DS starch–cellulose foams except for the
mechanical energy of DS 2.0 starch acetate–cellulose foams. RER of DS 2.5 starch–cellulose foams. As cellulose
Functional Properties of Extruded Acetylated Starch–Cellulose Foams 121

content increased, higher RER, spring indices, com- 5. J. Guan, Q. Fang and M. A. Hanna (2004) J. Polym Environ.
pressibility and lower densities were achieved. 12(2), 57–63.
6. J. Guan, Q. Fang and M. A. Hanna (2004) Cereal Chem.
Mechanical energy requirement increased when more 81(2), 199–206.
cellulose was extruded with acetylated starch. Etha- 7. R. L. Whistler and G. E. Hilbert (1944) Ind. Eng. Chem. Res.
nol functioned as blowing and cellulose solubilizing 36, 796–803.
8. C. E. Smith and J. V. Tuschhoff (1960) U.S. Patent No.
agents. The more ethanol in the blends, the higher 2,928,828, issued March 15, 1960.
RER, lower spring indices and lower compressibility 9. W. Jetten, E. J. Stamhuis, and G. E. H. Joosten (1980) Starch
were the foams. However, this was a function of the 32, 364–368.
10. K. Östergård, I. Björck, and A. Gunnarsson (1988) Starch 40,
cellulose and ethanol contents in the blends. Sufficient 58–66.
ethanol solubilized both cellulose and acetylated 11. A. D. Betancur, G. L. CheL, and H. E. Cañizares (1997)
starch, while ethanol solublized cellulose more readily J. Agric. Food. Chem. 45, 378–382.
12. M. W. Rutenberg and D. Solarek (1984) Starch derivatives:
than acetylated starch when ethanol content was low. Production and uses. Academic Press, New York.
13. J. Guan and M. A. Hanna (2004) Ind. Crop. Prod. 19, 255–269.
14. R. N. Chavez-Jauregui, Silva, M. E. M. P., and J. A. G. Areas
REFERENCES (2000) J. Food Sci. 65, 1009–1015.
15. SAS Institute (2002) SAS User’s Guide. 8th ed. Cary, NC.
1. J. Guan (2002) M.S. thesis. University of Nebraska-Lincoln, 16. Q. Fang and M. A. Hanna (2000) Trans. ASAE. 43(6),
NE 68503. 1715–1723.
2. Q. Fang and M. A. Hanna (2000) Cereal Chem. 77(6), 17. S. Bhatnagar and M. A. Hanna (1994) Cereal Chem. 71,
779–783. 587–593.
3. J. Guan and M. A. Hanna. (2003) Trans. ASAE. 46(6), 18. Q. Fang (1999) Ph.D dissertation. University of Nebraska-
1613–1624. Lincoln, NE 68503.
4. V. D. Miladinov and M. A. Hanna (1999) Ind. Eng. Chem. 19. J. Guan, Q. Fang, and M. A. Hanna (2004) Carbohydr.
Res. 38(10), 3892–3897. Polym. 47(1), 205–212.