functional properties of extruded acetylated starch–cellulose foams

Functional Properties of Extruded AcetylatedStarch–Cellulose Foams

Junjie Guan,1Kent M. Eskridge,

2and Milford A. Hanna

1,3

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 at165�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 wholeplot design to test the differences among the acetylated starches and the effects of cellulose andethanol. 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 onbulk density. Highest (RER) was obtained from 20% ethanol content. Extrudates containing

12% cellulose had the highest bulk density and the highest compressibility. Higher cellulosecontents required more specific mechanical energy.

KEY WORDS: Acetylated starch; extrusion; cellulose.

INTRODUCTION

There has been much recent interest in the utili-zation of starch as a biodegradable plastic material.Next to cellulose, starch is the second most abundantrenewable polysaccharide in nature [1]. More andmore packaging industries are using starch-basedpolymers as the major component in manufacturingloose-fill packaging materials. Starch-based loose-fillpackaging materials have good mechanical properties,are readily biodegrade in soil and sell at competitiveprices [2]. However, native starch loose-fill foamssuffer from a lack of moisture resistance and abrasionresistance. They collapse when in contact withwater orin an atmosphere with high relative humidity. Theintegrity of starch granules are destroyed by melting

the crystalline region, long-chain molecules are trim-med and shorter straight chain segments of polysac-charides form due to the high temperature, high shearand high pressure during extrusion [3]. The short-chain molecules are reassociated/plasticized afterexiting the extruder nozzle in presence of water (plas-ticizer) and pressure drop [3]. The reassociation isformed by hydrogen bonds. When extruded starchcontacts moisture/water, the highly polarized solutionattacks the hydrogen bonds, reducing bond forces andfinally significantly decreasing the functional proper-ties of the loose-fill foams. For packaging purposes,low moisture absorption is preferred. One possiblesolution to this problem is the use ofmodified starches.Several research projects have been conducted usingacetylated starch as the main component in preparingloose-fill packaging materials [4–6]. Technologies forproducing acetylated starch have been known formore than 100 years. Researchers [7–11] preparedstarch esters/acetylated starch by reacting organic acidanhydride or vinyl esters with starches. Starch is apolymer of D-glucose with each glucose unit having

1 University of Nebraska-Lincoln, Industrial Agricultural Products

Center, 208 L.W. Chase Hall, Lincoln, NE 68583-0730.2 University of Nebraska-Lincoln, Department of Statistics, 103

Miller Hall, Lincoln, NE 68583-0712.3 To whom all correspondence should be addressed. E-mail:

[email protected]

Journal of Polymers and the Environment, Vol. 12, No. 3, July 2004 (� 2004)

1131566-2543/04/0700-0113/0 � 2004 Plenum Publishing Corporation

three free hydroxyl groups. All or parts of these hy-droxyl groups were substituted with acetyl groupsthrough chemical reaction so as to manipulate thehydrolysis property of starch. It has been reported thatacetylation of hydroxyl groups of starch, to increasehydrophobicity, is one approach to increase the waterresistance of starch [12]. Compared with acetylatedstarch in food uses, acetylated starch in industrial usesrequires a high degree of substitution (DS). High DS(DS > 1.5) starch has higher hydrophobicity, which isa favorable property of loose-fill packaging material.However, preparing high DS starch requires morechemicals and longer reaction time, resulting in highermaterial cost. Therefore, it is necessary to determinethe relation of DS of starch and functional propertiesof extruded acetylated starch loose-fill foams.

Another way to reduce the cost of preparingloose-fill packaging material is blending acetylatedstarch with cellulosic materials. Compared to acety-lated starch, cellulosic materials such as wood fiber,oat fiber, cellulose, corncob fiber, wheat and ricestraw are cheaper [1]. Guan et al. [5, 6] and [13]blended wood fiber, oat fiber, cellulose and corncobfiber with acetylated starch. The extruded foams hadgood functional properties, indicating blending cel-lulosic materials in acetylated starch would be prac-tical and interesting to develop starch-based loose-fillpackaging materials.

The objectives of this research were to evaluatethe effects of the DS of acetylated starch, the level ofadded cellulose and the level of added ethanol on thefunctional properties and the mechanical energyrequirements of various extruded foams. Functionalproperties were radial expansion ratio (RER), bulkdensity, and compressibility. Mechanical energyrequirements were determined to evaluate the processof preparing extruded foams from different DSacetylated starches.

MATERIALS AND METHODS

Materials

Acetylated starches, with DS of 2, 2.5 and 3,were prepared from 70% amylose cornstarch.

Hylon� VII (70% amylose starch) was purchasedfrom American Maize Products Co. (Hammond, IN).The a-cellulose was purchased from Sigma ChemicalCo., (St. Louis, MO). The decomposition tempera-ture of a-cellulose is 260–270�C and its density, in anatural state, is between 200–500 kg/m3. Talc (mag-nesium silicate) was purchased from Barret Minerals,Inc. (Dillon, MT). The talc had a median particle sizeof 1.2 lm and bulk density of 120 kg/m3. Denaturedethanol was purchased from Fisher Scientific, Inc.(Fair Lawn, NJ). Acetic anhydride was purchasedfrom Vopak Inc., (Dallas TX). Sodium hydroxide(50% solution) was purchased from Harcros Chemi-cals Inc., (Kansas City, KS).

Starch Acetylation

High amylose corn starch was dried in the walk-in dryer at 50�C for 48 h. To begin the acetylationprocess, acetic anhydride was placed in a steam-jacketed reactor with a rotating self-wiping paddle.Then, 70% amylose starch was added into the reactorwith 5 min of continuous mixing. Finally, NaOHsolution was added while mixing. The chemicals ad-ded to the reactions and reaction times were sum-marized in Table I. The temperature of the reactorjacket was maintained at 123�C. After various times,the reaction was stopped by quickly adding 200 L ofcold water to the reactor. The pH value was adjustedto 5.0 by washing with tap water before dryingat 50�C in a walk-in dryer to a moisture content of4% (w.b.). The starch was ground in a Standardmodel No. 3 Wiley mill (Arthur H. Thomas Co.Philadelphia, PA) to pass through a 5 mm openingsieve. Three batches of same DS starches wereprepared as replications.

Blend Preparation

The acetylated starches and cellulose were driedin a mechanical convection oven (GCA Corp., Chi-cago IL) at 105�C for 1 h, and then cooled in adesiccator for 1 h to ensure they were moisture-freebefore 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

114 Guan, Eskridge, and Hanna

was added to all samples at a 5% level (w/w). Talcfunctioned as a nucleating agent to ensure unifor-mity of the cells. Different amounts of the prescribedcellulose (3%, 7.5%, and 12%) and ethanol (14%,17% and 20%) were added to the starch, and mixedin a Hobart mixer (Model C-100, Hobart Corp.,Troy, OH) for 5 min. Samples were then sealed inlow density polyethylene (LDPE) plastic containersfor 24 h at room temperature (25�C) to allow etha-nol completely absorbed by the blends.

Extrusion

A twin-screw extruder (DR-2027-K13,C. W. Brabender, Inc., S. Hackensack, NJ) with amanufacturer pre-designed co-rotating mixing screws(Model CTSE-V, C. W. Brabender, Inc., S. Hacken-sack, NJ) was used to conduct extrusions. The man-ufacturer designed conical screws had diametersdecreasing from 43 mm to 28 mm along their lengthof 365 mm from the feed end to the exit end. On eachscrew, there was a mixing section, in which smallportions of the screw flight were cut away. The mix-ing section enhanced the mixing action and also in-creased the residence time of the sample in the barrel.A 250-rev/min screw speed was used for all extru-sions. The temperature at the feeding section of thebarrel was maintained at room temperature (~25�C)while the other two barrels sections and the die weremaintained at 165�C. A 3-mm diameter die nozzlewas used to produce cylindrical extrudates. The ex-truder was controlled by a Plasti-Corder (Type FE2000, C. W. Brabender, Inc., S. Hackensack, NJ). Anadjustable rotating knife located right next to thenozzle, was used to cut the extrudates into 20 mmlengths. Extrusion data were recorded for subsequentanalyses.

Statistical Design and Analysis

The experimental design was a split-plot withstarch type (DS level) as whole plot factor and thecellulose and ethanol contents as the split plot fac-tors. The main plot used a randomized completeblock design with three blocks (blocked by batch) andresponse surfaces were fitted to cellulose and ethanolfactors for each DS level. Analysis of variance wasused to test the influence of acetylated starch type(DS) on RER, bulk density, compressibility andspecific mechanical energy requirement. For eachstarch type, a polynomial regression model [14] wasemployed to investigate the response surface of

acetylated starch type and cellulose and ethanolcontents on foam functionalities and specificmechanical energy requirement:

Y ¼ b0 þ b1X1 þ b2X2 þ b12X1X2 þ b11X21 þ b22X

22 þ e

Where Y ¼ response (RER, bulk denisty, com-pressibility, and specific mechanical energy),X1 ¼ cellulose content, X2 ¼ ethanol content,b0 ¼ intercept, bn ¼ regression coefficient ande ¼ experimental error with normal distribution,mean zero and variance r2. Significance level wasdefined as P<0.05 and R2 was used to evaluatemodel fit. For each response, three-dimensional plotswere produced from regression equations by holdingtwo variables fixed from Microsoft Excel�. SAS�

software [15] was used for statistical calculations andMicrosoft Excel� was used for surface graphing.

Physical Characteristics

Physical characteristics of the foamed materialsincluding RER and bulk density were determined.RER was calculated by dividing the mean cross sec-tional area of the extrudates by the cross sectionalarea of the die nozzle. Each mean value was theaverage of ten measurements [16].

Bulk density (bulk density) of the extrudates wasmeasured using a cylindrical Plexiglas container [2].The container had a diameter of 160 mm and a heightof 160 mm. A funnel having an opening of 160 mm atthe top and an opening of 64 mm at the bottom wasmounted at a height of 160 mm above the container.Bulk densities (kg/m3) of the extrudates were calcu-lated from the mass of the as-compacted sample di-vided by the volume of the container. Fivereplications were measured for each sample.

Compressibility

An Instron universal testing machine (Model5566, Instron Engineering Corp., Canton, MA) wasused to measure compressibility of foamed extru-dates. The 20-mm long extrudates were placed on aflat plate with careful aligning cut surfaces so thatedges were perpendicular to the axis of the extrudatesample (direction of extrusion). Then the extrudatewas compressed once to 80% of its original diameterat a loading rate of 1 cm/min using another flat plat.The force (kN) divided by the sample density (kg/m3) was reported as compressibility (kN/kg m3).

115Functional Properties of Extruded Acetylated Starch–Cellulose Foams

Compressibility for each sample was measured fivetimes and reported as an average of the five reading.

Specific Mechanical Energy Requirement (SME)

SME is defined as a total input of mechanicalenergy per unit dry weight of extrudate. SME wasdetermined as described by Bhatnagar and Hanna[17]. Extruded materials were collected for 30 s anddried. SME (Wh/kg) was calculated as

SME ¼ ½2� pð n60

Þ � s�=MFR

where n ¼ screw speed (rev/min); s ¼ torque(N m); and MFR ¼ mass flow rate (kg/h).

RESULTS AND DISCUSSIONS

Physical Characteristics

Expansion is a very important index of loose-fillfoam physical properties. There are two types ofexpansion of extruded foams, radial and longitudinal.Radial expansion is more interesting because it moreobviously represents extruded dough rheologicalproperties and foaming kinetics than longitudinalexpansion [18].

The response surface plots for RER of acetylatedstarch–cellulose foams are shown in Fig. 1. The RERof DS 2 starch–cellulose foams were affected signifi-cantly by ethanol (blowing agent) and cellulose con-tents (Fig. 1a). At high cellulose content, RER

increased dramatically with increasing ethanol con-tent. This increase became less significant with lesscellulose. Ethanol affected RER differently whencellulose content increased. RER decreased whencellulose content increased at low ethanol content. Athigh ethanol content, RER increased when cellulosecontent decreased. Similar trends were found with DS3 acetylated starch–cellulose foams (Fig. 1b), exceptthat RER firstly increased and then decreased asethanol content increased. At low ethanol content,RER decreased with increasing cellulose contentwhile at high ethanol content, RER increased. Cel-lulose content did not have significant effect on RERof DS 2.5 acetylated starch–cellulose foams(Table II). DS starch had a significant effect on RERwhich increased when higher DS acetylated starchwas blended in (Table III).

Starch–cellulose dough foamed after exiting thedie nozzle in presence of the blowing agent (ethanol)and the pressure drop [1]. The degree of foaming/expansion was closely related to the dough viscosityand the amount of blowing agent. Higher viscosityallowed the dough to elongate more, given sufficientblowing agent. During extrusion, high shear, tem-perature and pressure caused the starch to melt.Shear further depolymerized the starch, resulting inshort-chain amorphous polymers. Cellulose also wasdepolymerized during extrusion in presence of etha-nol [1, 19]. Firstly, ethanol penetrated cellulose ma-trix and partially solubilized the hydrogen bondsamong cellulose chains during the overnight storage.Then, shear depolymerized the long chain molecules

3 4.5 6 7.5 9 10.5 1214

1516

1718

1920

5

6

7

8

9

10

11

12

13

14

Y =

Rad

ial e

xpan

sion

rat

io

X1= Cellulose content, %

X2= Ethanol content, %

Y = 20.75 – 1.79 X1 – 1.26 X2 + 0.09 X1X2

(R2 = 0.9385)

34.5

67.5

910.5

12

14

15

16

17

18

19

20

14

15

16

17

18

19

20

Y =

Rad

ial e

xpan

sion

rat

io



Y = – 51.42 – 1.54 X1 + 8.44 X2 + 0.05 X1X2 + 0.05 X12 – 0.25 X2

2

(R2 = 0.7401) (a) (b)

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.


[3]. The short segments of both starch and cellulosewere fully mixed inside the barrel. When exiting thenozzle, ethanol evaporated because of sudden tem-perature and pressure drops. The starch and cellulosereassociated and formed a starch–cellulose matrix byhydrogen and covalent bonds. Ethanol functioned asa plasticizer as well as a blowing agent. When ethanolwas added to the blend, it was more readily absorbedby cellulose. Therefore, the more cellulose in theblend, the more ethanol was needed. At low ethanolcontent, most of the ethanol penetrated the celluloseand solubilized the bonds among cellulose chains.Insufficient ethanol was left for the further foaming.Since that, high RER foams were obtained with highethanol content and high cellulose content.

Low density is important for packaging materialsfrom a shipping cost standpoint. Bulk density ofacetylated starch–cellulose foams are shown in Fig. 2.DS 2 starch–cellulose foams had the lowest bulkdensity at high ethanol and cellulose contents. At highcellulose content, bulk denisty significantly decreasedwhen ethanol content increased; while at low cellulosecontent bulk density slightly decreased when ethanolcontent was higher then 16.8%. At high ethanol con-tent, bulk density decreased significantly from 33 to22 kg/m3. Bulk density of DS 2.5 starch–cellulosefoams showed similar trends except cellulose contenthad a greater effect on bulk density than ethanolcontent. As ethanol content increased, bulk densityfirstly decreased sharply and then the decrease becameless significantly. The bulk density of DS 2.5 starch–cellulose foams ranged from 11 to 29 kg/m3, in com-pared from 22 to 36 kg/m3 of DS 2-cellulose foams.Cellulose also had a negative effect on bulk density inDS 3 starch–cellulose foams. An opposite trend wasfound in bulk density when increased ethanol con-tent. At high cellulose content, bulk density increasedwith increasing ethanol content. But bulk density de-creased when ethanol content increased at low ethanolcontent. Bulk density of DS 3 starch–cellulose foamsranged from 20 to 24.5 kg/m3. The lowest bulk density(20 kg/m3) was obtained at low ethanol content andhigh cellulose content. DS of acetylated starch had asignificant effect on bulk density because of strongcorrelation between RER and bulk density withhigher DS acetylated starch extruded causing lowerbulk density (Table III).

All the three acetylated starch–cellulose foamshad lower densities at high cellulose content, sug-gesting cellulose was well blended and formed uni-form cell structure with acetylated starches. RER andbulk density are strongly related. The higher the

Table

II.

RegressionEquationCoeffi

cientsaofSecondOrder

PolynomialsbforTwoResponse

Variables

RER

qB(kg/m

3)

COMP(kN/kgm

3)

SME(W

Æh/kg)

DS

2.0

2.5

3.0

2.0

2.5

3.0

2.0

2.5

3.0

2.0

2.5

3.0

Coeffi

cient

b0

20.75***

28.83**

51.42***

50.04**

136.93***

31.58**

217062**

243835***

124071**

247.08**

1047.26***

1647.56***

Linear

b1

)1.79***

)0.748

)1.54***

2.43**

0.16**

)0.54**

)922.95**

)7960.27***

)7279.36**

38.56**

3.21

)7.44**

b2

)1.26**

4.26**

8.44***

10.11**

)12.0***

)0.56**

)15249**

)16448**

)486.54

10.26**

)74.56***

)136.52***

Cross

product

b12

0.09***

0.030

0.05**

)0.18***

0.058*

)0.07***

)185.35**

106.26

157.84

)1.56***

0.13

)0.056

Quadratic

b11

0.011

0.0096

0.05**

0.066

)0.08**

)0.06***

238.93***

372.89***

319.2**

)0.038

)0.118

0.69**

b22

0.04

)0.0995*

)0.25***

)0.29**

0.31***

0.011

)514.51**

582.77**

150.64

)0.512

1.92***

3.9***

R2

0.9385

0.8959

0.7401

0.7397

0.8335

0.7829

0.6310

0.8769

0.8531

0.9425

0.9733

0.7329

ProbabilityofF

<0.0001

<0.0001

0.0002

0.0002

<0.0001

<0.0001

0.0036

<0.0001

<0.0001

<0.0001

<0.0001

0.0002

a*,**,and***indicate

significance

atP<

0.10,0.05and0.01,respectively.

bModelonwhichX1(cellulose

content),X2(ethanolcontent)iscalculated:Y=

b0+

b1X1+

b2X2+

b12X1X2+

b11X12+

b22X22+

e.RER,radialexpansionratio;q B

bulk

density;

COMP,compressibility;andSME,specificmechanicalenergy.


RER, the less dense are the foams. As DS of thestarches increased, ethanol effects began to change.Ethanol functioned as blowing agent for foaming and

cellulose solubilizing agent for cellulose depolymer-ization. But which of these two functions actingpredominantly depended upon the type of starch.

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.

34.5

67.5

9

10.5

12 14

15

16

17

18

1920

212325

27

29

31

33

35

37

Y =

Bul

k de

nsity

, kg/

m3


X2 = Ethanol content, %

Y = – 50.04 + 2.43 X1 + 10.11 X2 – 0.18 X1X2 – 0.29 X22

(R2 = 0.7397)

3

4.8

6.6

8.4

10.2

12 14

15

16

1718

192010

1214161820222426

28

30

Y =

Bul

k de

nsity

, kg/

m3

X1= Cellulose content,% X2= Ethanol content, %

Y = 136.93 + 0.16 X1 – 12.0 X2 – 0.08 X12 + 0.31 X2

2

(R2 = 0.8335)

34.5

67.5

910.5

12

14

15

16

17

18

1920

19

20

21

22

23

24

25

Y =

Bul

k de

nsity

, kg/

m3

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

(R2 = 0.7829)



(c)

(a) (b)

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.


When cellulose blended with DS 2 and DS 2.5 star-ches, ethanol functioned as blowing agent more thanas a cellulose-solubilizing agent as ethanol contentincreased, resulting in decreased bulk density. But forthe DS 3 starch–cellulose foams, the starch–celluloseblends were subjected to higher shear than the lowerDS starch–cellulose blends due to the harder textureof DS 3. Because more acetyl groups were substi-tuted, stronger covalent bonds formed among starchchains. Hence, ethanol penetration of the starchchains was hindered. Then, cellulose absorbed mostof the ethanol and the chains were fully solubilized.When high shear applied, they were easily brokendown to small segments and formed starch–cellulosematrices. Since these matrices had shorter chains,bond forces between starch and cellulose were notstrong and small cells were broken when exitingnozzle, resulting in higher density. This may havebeen the reason why RER and bulk density werepositively affected by ethanol content at high cellu-lose content (Fig. 1b and 2c).

Compressibility

Compressibility is an important mechanicalproperty of packaging materials. During shipping,impact forces are absorbed by compressing loose-fillfoams to minimize the damage. Therefore, highmechanical forces absorption is preferred, requiring ahigh compressibility. Response surface plots of com-pressibility are shown in Fig. 3. DS 2 starch–celluloseand DS 3 starch–cellulose foams exhibited similartrends in compressibility. High compressibility was

obtained at high ethanol content and low cellulosecontent. As ethanol content decreased, compressibilitydecreased,while increasing cellulose content resulted indecreased compressibility. Ethanol content did nothave a significant effect on compressibility of DS 3starch–cellulose foams. Compressibility decreasedfirstly and then increased as cellulose content increasedin DS 3-cellulose foams. DS had a significant effect oncompressibility of acetylated starch-cellulose foams(Table III) with compressibility increased as DS ofacetylated starch increased.

Mechanical properties, such as compressibility,are strongly related to the foam formation and theblended materials matrix formation. Starch–cellulosematrix formation affected the dough rheologicalproperties, resulting in foam brittleness. The betterthe starch–cellulose matrix formed the more com-pressible and less brittle were the foams. As men-tioned previously, the higher DS, the more acetylgroups substituted. With more acetyl groups, thegreater chance for the starch and cellulose chains toform covalent bonds. Hence, the dough viscosity in-creased resulting in a more resilient texture of thefoams. When mechanical forces were applied to thehigher DS starch–cellulose foams, more forces wererequired to destroy the foam structure. On the otherhand, the higher the viscosity of the dough, the betterit could sustain the ethanol vapor pressure duringfoaming. Therefore, higher DS starch–cellulosefoams had lower densities, which also contributed tohigher compressibilities. Because the strong covalentbond forces hindered ethanol penetration into thestarch, it functioned as a blowing agent in DS 3

34.8

6.68.4

10.212

14

15

16

1718

1920

85000

90000

95000

100000

105000

110000

X1 = Cellulose content, %

X2 = Ethanol content, %

Y =

Com

pres

sibi

lity,

kN

·kg-1

· m3

Y = 217062 – 922.95 X 1 – 15249 X2 – 185.35 X1X2 + 238.93 X12 – 514.51 X2

2

(R2 = 0.6310)

3 4.5 6 7.5 9 10.5 1214

1516

1718

1920

80000

85000

90000

95000

100000

105000

110000

115000

120000

125000

130000



Y = 243835 – 7960.27 X1 – 16448 X2 + 372.89 X12 + 582.77 X2

2

= 0.8769)

Y =

Com

pres

sibi

lity,

kN

·kg-1

· m3

(R2(b) (a)

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.


starch–cellulose foam. Therefore, ethanol did nothave significant contribution to the dough viscosity.This may have been the reason why ethanol hadinsignificant effect on compressibility.

Specific Mechanical Energy Requirement (SME)

SME is an easy-to-monitor, real-time indicator ofa process inside the extruder. Mechanical energy, ap-plied to an extruder, mostly is converted to thermalenergy while some of it is used to break or create newcovalent bonds in the extrudates [4]. Also, reducing theamount of SME is important to reduce process cost.

Similar trends were found in SME for DS 2, DS2.5 and DS 3 starch–cellulose foams (Table II). Cel-lulose content had a significant effect on SME. Ascellulose content increased, SME increased. SMEincreased as ethanol content increased at low cellu-lose content. But at high cellulose content, SME de-creased when ethanol content decreased (Fig. 4). TheDS had a significant impact (P > 0.02) on the SME.SME was affected significantly by DS of acetylatedstarch with the higher the DS, the more mechanicalenergy required (Table III).

During extrusion, thermal energy combined withmechanical energy melted and broke covalent bondsin the acetylated starch and cellulose. Therefore,before exiting the nozzle, the shorter-chain length andmelted starch and cellulose were well mixed. It wasobvious that with higher cellulose content in theblend, more mechanical energy was required to meltand depolymerize the covalent-bonded long chain

molecules. At low cellulose content, as ethanol con-tent increased the cellulose became fully solubilized.The residual ethanol was heated up and becamesuperheat by thermal energy. This consumed part ofthe thermal energy, resulting in less thermal degra-dation of cellulose and acetylated starch. Therefore,more mechanical energy was required to be convertedto thermal energy to compensate for this in order toform the well-mixed short-chain molecule. While athigh cellulose content, more ethanol was needed tosolubilize the cellulose. With less ethanol (14%), moremechanical energy was required to depolymerize thecellulose. The SME also depended upon the barrelfriction and the viscosity of the material. Viscosity ofthe sample depends upon its molecular weight andintermolecular interactions [4]. The higher the DS ofthe starch was, the more rigid the starch particle was,because starch molecules interacted via hydrophobicinteractions, and the lower DS starch had more hy-droxyl groups. The availability of hydroxyl groupsfacilitated their participation in hydrogen bondswhen they came in close proximity. When both starchand cellulose were depolymerized and in the meltedstate, they could more readily form strong hydro-phobic covalent bonds, especially when exiting nozzlein the presence of ethanol. Because of the covalentbond formation in the barrel, higher DS starch–cel-lulose blends had higher viscosities and more SMErequirement. For the lower DS starch blends, signif-icant amount of hydroxyl groups were present in thevicinity of the hydrophobic covalent bonds. Thesehydroxyl groups had disrupting effects on thehydrophobic interactions between starch chains andcellulose chains. The decreases in intermolecularinteractions resulted in lower viscosity and lowerSME requirements.

CONCLUSIONS

Acetylated starch extruded with cellulose haspromising properties as a loose-fill packaging mate-rial. At the same ethanol and cellulose contents,RER, spring index, bulk spring index and com-pressibility increased and unit and bulk densitydecreased as DS of the acetylated starch increased.Even specific mechanical energy requirementincreased when higher DS acetylated starch wasblended in, overall accomplishment of previousmentioned functional properties was more significant.Cellulose content significantly affected the propertiesof all three DS starch–cellulose foams except for theRER of DS 2.5 starch–cellulose foams. As cellulose

3 4.5 6 7.5 9 10.5 1214

15

16

17181920

420440460480500520540560580600

Y =

Spe

cifi

c m

echa

nica

l ene

rgy,

Wh/

kg



Y = 1647.56 – 7.44 X1– 136.52 X2+ 0.69 X12+ 3.9 X22 (R2 = 0.7329)

Fig. 4. Effects of cellulose content and ethanol content on specific

mechanical energy of DS 2.0 starch acetate–cellulose foams.


content increased, higher RER, spring indices, com-pressibility and lower densities were achieved.Mechanical energy requirement increased when morecellulose was extruded with acetylated starch. Etha-nol functioned as blowing and cellulose solubilizingagents. The more ethanol in the blends, the higherRER, lower spring indices and lower compressibilitywere the foams. However, this was a function of thecellulose and ethanol contents in the blends. Sufficientethanol solubilized both cellulose and acetylatedstarch, while ethanol solublized cellulose more readilythan acetylated starch when ethanol content was low.

REFERENCES

1. J. Guan (2002) M.S. thesis. University of Nebraska-Lincoln,NE 68503.

2. Q. Fang and M. A. Hanna (2000) Cereal Chem. 77(6),779–783.

3. J. Guan and M. A. Hanna. (2003) Trans. ASAE. 46(6),1613–1624.

4. V. D. Miladinov and M. A. Hanna (1999) Ind. Eng. Chem.Res. 38(10), 3892–3897.

5. J. Guan, Q. Fang and M. A. Hanna (2004) J. Polym Environ.12(2), 57–63.

6. J. Guan, Q. Fang and M. A. Hanna (2004) Cereal Chem.81(2), 199–206.

7. R. L. Whistler and G. E. Hilbert (1944) Ind. Eng. Chem. Res.36, 796–803.

8. C. E. Smith and J. V. Tuschhoff (1960) U.S. Patent No.2,928,828, issued March 15, 1960.

9. W. Jetten, E. J. Stamhuis, and G. E. H. Joosten (1980) Starch32, 364–368.

10. K. Ostergard, I. Bjorck, and A. Gunnarsson (1988) Starch 40,58–66.

11. A. D. Betancur, G. L. CheL, and H. E. Canizares (1997)J. Agric. Food. Chem. 45, 378–382.

12. M. W. Rutenberg and D. Solarek (1984) Starch derivatives:Production and uses. Academic Press, New York.

13. J. Guan andM. A. Hanna (2004) Ind. Crop. Prod. 19, 255–269.14. R. N. Chavez-Jauregui, Silva, M. E. M. P., and J. A. G. Areas

(2000) J. Food Sci. 65, 1009–1015.15. SAS� Institute (2002) SAS� User’s Guide. 8th ed. Cary, NC.16. Q. Fang and M. A. Hanna (2000) Trans. ASAE. 43(6),

1715–1723.17. S. Bhatnagar and M. A. Hanna (1994) Cereal Chem. 71,

587–593.18. Q. Fang (1999) Ph.D dissertation. University of Nebraska-

Lincoln, NE 68503.19. J. Guan, Q. Fang, and M. A. Hanna (2004) Carbohydr.

Polym. 47(1), 205–212.


functional properties of extruded acetylated starch–cellulose foams

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