selected morphological and functional properties of extruded acetylated starch–cellulose foams

Bioresource Technology 97 (2006) 1716–1726

Selected morphological and functional properties of extrudedacetylated starch–cellulose foams q

Junjie Guan, Milford A. Hanna *

Department of Biological Systems Engineering and Industrial Agricultural Products Center, University of Nebraska—Lincoln,

Lincoln, NE 68583-0730, USA

Received 26 February 2004; received in revised form 9 September 2004; accepted 11 September 2004

Abstract

Starch acetates with degrees of substitution (DS) of 1.68 and 2.3 were extruded with 10%, 20% and 30% (w/w) cellulose and 20%(w/w) ethanol in a twin screw extruder at 150, 160 and 170 �C barrel temperatures and 170, 200 and 230 rpm screw speeds. X-raydiffractogram (XRD), differential scanning calormetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were used toanalyze the morphological properties of extruded foams. A central composite response surface design was applied to analyze theeffects of starch type, cellulose content, barrel temperature and screw speed on specific mechanical energy requirement of extrudingfoams and the radial expansion ratio and compressibility of the extruded foams. XRD showed losses of DS starch and cellulosecrystallinity and the formation of new complexes. FTIR spectra revealed that functional groups and chemical bonds were main-tained after extrusion. Melting temperatures changed significantly when higher DS starch acetate was used. Cellulose content, barreltemperature and screw speed showed significant effects on thermal, physical and mechanical properties of extruded foams and thespecific mechanical energy requirement.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Acetylated starch; Cellulose; Extrusion; Morphology

1. Introduction

Recently, there has been considerable interest in theutilization of starch as a biodegradable plastic material.Next to cellulose, starch is the most abundant renew-able polysaccharide in nature (Rutenberg and Solarek,1984). More and more packaging industries are usingstarch-based polymers as the major component in

0960-8524/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2004.09.017

q This study was conducted at the Industrial Agricultural ProductsCenter, University of Nebraska, Lincoln, NE, USA.

* Corresponding author. Address: Department of Food Science andTechnology, Industrial Agricultural Products Center, University ofNebraska—Lincoln, 208 L.W. Chase Hall, Lincoln, NE 68583-0730,USA. Tel.: +1 402 472 1634; fax: +1 402 472 6338.

E-mail address: [email protected] (M.A. Hanna).

manufacturing loose-fill packaging materials (Larson,1989). Starch-based loose-fill packaging materials havegood mechanical properties, are readily biodegradablein soil and sell at competitive prices. However, nativestarch loose-fill foams suffer from a lack of moistureresistance and abrasion resistance (Guan and Hanna,2003). They collapse when in contact with water or inan atmosphere with high relative humidity (Guan andHanna, 2003). The integrity of starch granules aredestroyed by melting the crystalline region, long-chainmolecules are trimmed and shorter straight chain seg-ments of polysaccharides form due to the high tempera-ture, high shear and high pressure during extrusion(Davison et al., 1984a). The short-chain molecules arereassociated/plasticized after exiting the extruder nozzlein the presence of water (plasticizer) and a pressure

mailto:[email protected]

J. Guan, M.A. Hanna / Bioresource Technology 97 (2006) 1716–1726 1717

drop. The reassociation is formed by hydrogen bonds(Davison et al., 1984b). When extruded starch contactsmoisture/water, the highly polarized solution attacksthe hydrogen bonds, reducing bond forces and finallysignificantly decreasing the functional properties of theloose-fill foams (Guan and Hanna, 2004c).

For packaging purposes, low moisture absorption ispreferred. One possible solution to this problem is theuse of modified starches. Several research projects havebeen conducted using acetylated starch as the maincomponent in preparing loose-fill packaging materials(Miladinov and Hanna, 1999; Guan et al., 2004a,b,c).Technologies for producing acetylated starch have beenknown for more than 100 years. Researchers (Whistlerand Hilbert, 1944; Smith and Tuschhoff, 1960; Jettenet al., 1980; Ostergard, 1988; Betancur et al., 1997) pre-pared starch esters/acetylated starch by reacting organicacid anhydride or vinyl esters with starches. Starch is apolymer of D-glucose with each glucose unit havingthree free hydroxyl groups. All or parts of these hydroxygroups were substituted with acetyl groups throughchemical reaction so as to manipulate the hydrolysisproperty of starch. It has been reported that acetylationof hydroxyl groups of starch, to increase hydrophobic-ity, is one approach to increase the water resistance ofstarch. Compared with acetylated starch in food uses,acetylated starch in industrial uses requires a highdegree of substitution (DS). High DS (>1.5) starch hashigh hydrophobicity, which is a favorable property ofloose-fill packaging material. However, preparing highDS starch requires more chemicals and longer reactiontimes, resulting in higher cost. Therefore, it is necessaryto determine the relation of DS of starch and func-tional properties of extruded acetylated starch loose-fillfoams.

Another way to reduce the cost of preparing loose-fillpackaging material is blending acetylated starch withcellulosic materials. Compared to acetylated starch, cel-lulosic materials such as wood fiber, oat fiber, cellulose,corncob fiber and wheat and rice straw are more eco-nomical. Studies blended wood fiber, oat fiber, celluloseand corncob fiber with acetylated starch (Guan andHanna, 2003, 2004a,b). The extruded foams had optimalfunctional properties, indicating blending cellulosicmaterials in acetylated starch would be practical andinteresting to develop starch-based loose-fill packagingmaterials.

However, when acetylated starch and cellulose areextruded, phase separation may occur because of thehydrophobicity of acetylated starch and hydrophilicitycellulose. The objectives of this research were to evaluatethe effects of the degree of substitution of acetylatedstarch, the level of added cellulose, barrel temperatureand screw speed on the mechanical energy requirementsduring extrusion, and the radial expansion ratio andcompressibility of the extruded foams.

2. Experimentation

2.1. Materials

Acetylated starches, with DS of 1.68 and 2.3, wereprepared from 70% amylose cornstarch. HylonTM VII(70% amylose starch) was purchased from AmericanMaize Products Co. (Hammond, IN). The alpha cellu-lose was purchased from Sigma Chemical Co., St Louis,MO. The decomposition temperature of cellulose is 260–270 �C and its density, in a natural state, is between 200and 500 kg/m3. Talc (magnesium silicate) was purchasedfrom Barret Minerals, Inc. (Dillon, MT). The talc had amedian particle size of 1.2 lm and bulk density of120 kg/m3. Denatured ethanol was purchased fromFisher Scientific, Inc. (Fair Lawn, NJ). Acetic anhydridewas purchased from Vopak Inc., Dallas, TX. Sodiumhydroxide (50% solution) was purchased from HarcrosChemicals Inc., Kansas City, KS.

2.2. Starch acetylation

High amylose corn starch was dried in the walk-indryer at 50 �C for 48 h. To begin the acetylation process,110.0 kg acetic anhydride were placed in a steam-jack-eted reactor with a rotating self-wiping paddle. Subse-quently, 45.45 kg 70% amylose starch was added intothe reactor with 5 min of continuous mixing. Finally, a5 kg NaOH solution was added while mixing. The reac-tion times were 2.0 h and 3.0 h to obtain degree of sub-stitutions of 1.68 and 2.3, respectively. The temperatureof the reactor jacket was maintained at 123 �C. Afterselected time intervals, the reaction was stopped byquickly adding 200 L of cold water to the reactor. ThepH value was adjusted to 5.0 by washing with tap waterbefore drying at 50 �C to a moisture content of 4%(w.b.). The starch was ground in a Standard modelNo. 3 Wiley mill (Arthur H. Thomas Co. Philadel-phia, PA) to pass through a 5 mm opening sieve.Three batches of same DS starches were prepared asreplications.

2.3. Blend preparation

The acetylated starfches and cellulose were dried in amechanical convection oven (GCA Corp., Chicago IL)at 105 �C for 1 h, and then cooled in a desiccator for1 h to ensure they were moisture-free before being usedin sample preparation (Fang and Hanna, 2000). Talcwas added to all samples at a 5% level (w/w). Talc func-tioned as a nucleating agent to ensure uniformity of thecells. Different amounts of the prescribed cellulose (10%,20% and 30%) and 20% (w.b.) ethanol were added to thestarch, and mixed in a Hobart mixer (Model C-100,Hobart Corp., Troy, OH) for 5 min. Samples were thensealed in plastic containers for 24 h at room temperature

1718 J. Guan, M.A. Hanna / Bioresource Technology 97 (2006) 1716–1726

(25 �C) to allow ethanol to be completely absorbed bythe blends.

2.4. Extrusion

A twin-screw extruder (DR-2027-K13, C.W. Brab-ender, Inc., S. Hackensack, NJ) with a manufacturerpre-designed co-rotating mixing screws (Model CTSE-V, C.W. Brabender, Inc., S. Hackensack, NJ) was usedto conduct extrusions. The conical screws had diametersdecreasing from 43 to 28 mm along their length of365 mm from the feed end to the exit end. On eachscrew, there was a mixing section, in which small por-tions of the screw flight were cut away. The mixing sec-tion enhanced the mixing action and also increased theresidence time of the sample in the barrel. The tempera-ture at the feeding section of the barrel was maintainedat room temperature (�25 �C) while the other two bar-rels sections and the die were varied from 150 to 170 �C.Screw speeds varied from 170 to 230 rev/min. A 3-mmdiameter die nozzle was used to produce cylindricalextrudates. The extruder was controlled by a Plasti-Cor-der (Type FE 2000, C.W. Brabender, Inc., S. Hacken-sack, NJ). An adjustable rotating knife located rightnext to the nozzle, was used to cut the extrudates into20 mm lengths. Extrusion data were recorded for sub-sequent analyses.

2.5. Experimental design

The experimental design was a split-plot, with DSstarch type as the whole plot factor and cellulose con-tents, barrel temperature and screw speed as the splitplot factors. The whole plot used a completely random-ized design with two blocks (blocked by starch type) andresponse surfaces were applied to split plots. Responsesurface methodology (RSM) was used to determine theeffects of cellulose content, barrel temperature and screwspeed on the specific mechanical energy requirement,radial expansion ratio and compressibility of the acety-lated starch foams. The central composite experimentaldesign, described by Lee and Han (1997) for three vari-ables with three levels of each variable, was used. Thethree independent variable levels used were selectedbased on preliminary experiments. All treatments wereperformed in random order and data were analyzedusing a response surface regression procedure. The gen-eralized regression model was

Y ¼ b0 þ b1X 1 þ b2X 2 þ b3X 3 þ b12X 1X 2 þ b13X 1X 3

þ b23X 2X 3 þ b11X 21 þ b22X 2

2 þ b33X 23;

where Y is the response, X1 the cellulose content, X2 thebarrel temperature, X3 the screw speed, b0 the interceptand bn the regression coefficient. For each response,three-dimensional plots were produced from regression

equations by holding two variables fixed. Design-ExpertVersion 6 (Stat-Ease Co., Minneapolis, MN) was usedto conduct the statistical analyses and surface plotting.

2.6. Morphological properties of starch–cellulose foams

2.6.1. Differential scanning calorimetry (DSC)

DSC analyses were conducted on all raw materialsand selected extrudates to study the thermal propertiesof foams including glass transition and melting temper-atures. A Perkin-Elmer DSC 7 differential calorimeter(Perkin Elmer, Wilton, CT) was used to analyze thethermal properties of native starch, DS starches andextruded blends. The instrument was first calibratedwith indium and purged with nitrogen gas at 80 ml/min. About 10 mg of sample were sealed in an alumi-num pan, allowed to equilibrate to 25 �C, and scannedfrom 25 to 220 �C at a constant heating rate of 10 �C/min following with slow cooling and rescanning up to220 �C with 10 �C/min. Three replications were scannedfor each sample.

2.6.2. X-ray diffraction

Crystallinity determines the flexibility of the molecu-lar chains of polymers. Polymers with higher crystallinityhave better mechanical strength. X-ray diffractogramswere used to observe the crystallinity of both the rawmaterials and the extruded composites. DS starchesand extruded blends were dried at 40 �C to constantmoisture in a vacuum oven prior to X-ray scanning. X-ray diffractograms were obtained with a Rigaku ModelD/Max-B X-ray diffractometer (Brandt Instruments,Inc., Slidell, LA) with Bragg-Brentano parafocusinggeometry, a diffracted beam monochromater and a con-ventional copper target X-ray tube set to 40 kV and30 mA. The X-ray source was CuKa radiation com-posed of CuKa1 of 1.5405 A and CuKa2 of 1.5443 A.The weighted average of the two was CuKaavg of

1.54184 A. Data were collected from 2h of 4� to 35� (hbeing the angle of diffraction) with a step width of0.02� and step time of 0.4 s. The value of 2h for each iden-tifiable peak on the diffractograms was estimated andcrystal d-spacings were calculated using Bragg’s law.

2.6.3. Fourier transform infrared (FTIR)

FTIR spectroscopy was conducted with a NicoletAvatar 360 (Thermo Electron Co., Woburn, MA). AnAnalect diffuse/specula reflectance apparatus was used.The acquisition parameters were 128 scans at 4 cm�1

resolution. To prepare the samples, 1 mg of the samples(extruded DS starch–cellulose foams, DS starches, cellu-lose) combined with 20 mg of KBr were ground to finepowders. The powders were placed in the oven at130 �C for 1 h to remove any moisture from the airand stored overnight storage in dessicators at room tem-perature (25 �C). Before running the samples, a back-


ground spectrum of ground pure KBr was collected.Subsequently, a small amount of each dried powderwas placed in the diffuse reflectance sample holder anddata were collected.

2.7. Specific mechanical energy requirement (SME)

SME is defined as the total input of mechanicalenergy per unit dry weight of extrudate. SME was deter-mined as described by Bhatnagar and Hanna (1994).Extruded materials were collected for 30 s and dried.SME (Wh/kg) was calculated as:

SME ¼ 2� pðn=60Þ � sMFR

;

where n is the screw speed (rev/min); s the torque (Nm);and MFR the mass flow rate (kg/h).

2.8. Functional properties of starch–cellulose foams

Radial expansion ratio (RER) was calculated bydividing the mean cross sectional area of the extrudatesby the cross sectional area of the die nozzle. Each meanvalue was the average of ten measurements.

An Instron universal testing machine (Model 5566,Instron Engineering Corp., Canton, MA) was used tomeasure compressibility of foamed extrudates. The20 mm long extrudates were placed on a flat plate withcareful alignment of cut surfaces so that edges were per-pendicular to the axis of the extrudate sample (directionof extrusion). The extrudates were subsequently com-pressed once to 80% of their original diameter at a load-ing rate of 1 cm/min using another flat plate. The force(kN) divided by the sample density (kg/m3) was reportedas compressibility (kN kg�1 m3). Compressibility foreach sample was measured five times and reported asan average.

Table 1Thermal properties of selected DS 1.68 starch–cellulose and DS 2.3 starch–c

Cellulose (%) Barrel temperatu

DS 1.68 starch acetate – –DS 2.3 starch acetate – –

DS 1.68 starch acetate foamc 10 150DS 1.68 starch acetate foam 10 160DS 1.68 starch acetate foam 20 160DS 1.68 starch acetate foam 30 160DS 1.68 starch acetate foam 10 170

DS 2.3 starch acetate foamc 10 150DS 2.3 starch acetate foam 10 160DS 2.3 starch acetate foam 20 160DS 2.3 starch acetate foam 30 160DS 2.3 starch acetate foam 10 170

a Glass transition temperature, �C.b Melting temperature, �C.c Selected acetylated starch foam with formulation of cellulose content, bad All the Tg and Tm were significantly different (P < 0.05) in a standard t-t

3. Results and discussion

3.1. Morphological properties of starch–cellulose foams

3.1.1. Differential scanning calorimetry (DSC)

Thermal properties of the DS starch–cellulose foamswere significantly (P < 0.05) affected by the starch type,cellulose content, barrel temperature and screw speed(Table 1). Glass transition temperature (Tg) and meltingtemperature (Tm) are two important thermal propertiesof starch-based products. For the extruded starchfoams, they were strongly related to blend composition,barrel temperature and screw speed. Both Tg and Tm

increased when higher DS starch was blended in,because starch chains movement was more restrictedwhen more large functional groups (acetyl) wereattached to the pyranosyl rings. Tg and Tm decreasedwhen cellulose content increased in the blends. Alphacellulose had a Tm of 136.24 �C (Table 1, Guan andHanna, 2003) while the DS starches had Tm higher than200 �C (Miladinov and Hanna, 1999; Guan and Hanna,2004c). Therefore, Tg and Tm decreased as cellulose con-tent increased. As barrel temperature and screw speedincreased, Tg and Tm increased initially and thendecreased when barrel temperature and screw speedwere higher than 160 �C and 200 rpm, respectively.Barrel temperature was an index of the thermal energyapplied to the blends while screw speed was an indica-tion of the mechanical energy applied to the blends. Inthe barrel, starch chains were realigned and depolymer-ized by addition of thermal and mechanical energies.When another phase (cellulose) was blended in, bothphases tended to reach a point that both molecularchains were depolymerized and packed as close as possi-ble by inter-chain hydrogen bondings. At low tempera-ture and screw speed, thermal and mechanical energiespartially melted and depolymerized starch and cellulose.

ellulose foams

re (�C) Screw speed (rpm) Tga Tm

b

– 158.34 216.22– 169.22 231.83

170 122.49d 140.22200 139.56 152.56200 136.89 149.46200 132.11 146.45230 119.34 137.42

170 139.33 163.34200 152.67 189.09200 148.88 180.98200 145.29 176.43230 136.72 149.33

rrel temperature and screw speed, respectively.est.

Fig. 2. X-ray diffractograms of DS 2.3 starch–cellulose foams.


As temperature and screw speed increased, melting anddepolymerization reached optimums and both phasesformed the highest possible inter-chain hydrogen bon-dings, resulting in high Tg and Tm. When temperatureand screw speed were increased beyond the optimums,Tg and Tm decreased because of the over depolymeriza-tions of starch and cellulose.

3.1.2. X-ray diffraction

The X-ray diffractograms of DS 1.68-cellulose foams,DS 2.3-cellulose foams and nonextruded samples arepresented in Figs. 1 and 2. DS 1.68 starch had peaksat 9 and 20. Alpha cellulose had only one strong peakat 23. However, X-ray pattern of DS 1.68 starch–cellu-lose foam was significantly different than these for DS1.68 starch and alpha cellulose. It had sharp peaks atabout 9 and 29. Also, there was a strong and board peakwas at 22.5. That peak had a different intensity whenblend composition and extrusion conditions were chan-ged. With low cellulose content (10%), low barrel tem-perature (150 �C) and low screw speed (170 rpm), therewas a strong and broad peak. This was due to thecrystallinity of extruded DS starch. From previousDSC analyses, cellulose melted at about 136 �C whileDS starches melted at over 200 �C. At 150 �C and170 rpm, most of the thermal and mechanical energieswere consumed to melt DS 1.68 starch, resulting in a pre-

Fig. 1. X-ray diffractograms of DS 1.68 starch–cellulose foams.

dominantly DS starch system. As temperature and screwspeed increased, this peak became sharper (10% cellu-lose, 170 �C barrel temperature and 230 rpm screwspeed) due to the higher crystallinity resulting from hightemperature and screw speed. Meanwhile, when cellulosecontent increased, the DS starch predominantly systembecame more balanced between starch and cellulose.The peak subsequently exhibited less intensity and fol-lowed more closely the X-ray pattern of cellulose.

DS 2.3 starch had one strong peak at 8.8� (Fig. 2).Two sharp signature peaks were shown at about 9�and 29� for DS 2.3 starch–cellulose foams. The changingX-ray pattern from low cellulose content to high cellu-lose content at low temperature and low screw speedwas similar to the DS 1.68 starch–cellulose foams. How-ever, at high screw speed, DS 2.3 starch–cellulose foam(10% cellulose, 170 �C barrel temperature and 230 rpmscrew speed) had only a weak peak at 23. This wasprobably due to the high degree of depolymerizationof both starch and cellulose as a result of thermalenergy, mechanical energy and hardness of the starchagglomerates (Guan et al., 2005).

The difference in crystallinities between foamsextruded from DS 1.68 and DS 2.3 starch acetates weredue to the hardness of starch agglomerate the DS of thestarch acetates. Starch acetate with higher DS hadharder agglomerates (Guan and Hanna, 2004a). Duringextrusion, shear contributed to the depolymerization of

500150025003500

Wavenumbers (cm-1)

Tra

nsm

itta

nce

DS 1.68 starch acetate

Alpha cellulose

10%, 150 °C, 170 rpm

30%, 170 °C, 170 rpm

10%, 170 °C, 230 rpm

Fig. 3. FTIR spectroscopy of DS 1.68, DS 2.3 starch acetates, alphacellulose, and DS 1.68 starch–cellulose foams.


both starch and cellulose. DS 1.68 starch acetate hadlower DS value and had lower hardness. Also, due tolow DS, it had similar characteristics, such as crystallin-ity, to its native form, resulting in high crystallinity ofextruded foams. The DS 1.68 starch acetate wasmore readily degraded thermally than DS 2.3 starch ace-tate. However, DS 2.3 starch acetate agglomerates wereharder and less crystalline/more amorphous (Guan andHanna, 2004a). Therefore, mechanical degradation wasthe predominant effect on both starch and cellulose.

3.1.3. Fourier transform infrared spectroscopy (FTIR)

Representative FTIR spectra in the range of 4000–400 cm�1 for DS 1.68 starch, DS 2.3 starch and alphacellulose are shown in Fig. 3. DS 1.68 and DS 2.3starches had similar characteristic peaks, which weresummarized in Table 2 with corresponding stretches.

Table 2Data obtained from FTIR spectrogram for DS starches, cellulose, DS 1.68 sta

DS 1.68 and DS 2.3 starches Cellulose DS

3537(b) and 3553(b) – 3473319(b) and 3302(b) 3281(b) 3292921(bs) 2883(s) 2891739(s) and 1734(s) 1728(b) 1731369(s) and 1380 1374 1361233(s) and 1238(s) 1238 1221026(s) and 1042 1064(b) 102

b—Board, s—sharp.

Alpha cellulose had similar overall stretches as the DSstarches. The selected FTIR spectra for DS 1.68starch–cellulose and DS 2.3 starch–cellulose foams areshown in Figs. 3 and 4, respectively. All foams main-tained the major functional groups and chain structure.Starch ester structure (1300–1050 cm�1) also was main-tained. Because of these, the new crystalline forms (pro-ven by X-ray diffractograms) had structures similar tothe DS starches, even when blended with another phase(cellulose). But these newly formed crystalline structurescontained shorter molecular chains as known in DSCthermographs.

3.2. Specific mechanical energy requirement (SME)

SME requirement is an easy-to-monitor, real-timeindicator of the process inside the extruder (Miladinovand Hanna, 1999). Mechanical energy, in the form ofshear, was converted into thermal energy to melt thecrystalline polymer. At higher energy levels, the longchain molecules were broken to form new covalentbonds in the mixed dough. In the two-phase polymersystem (hydrophilic and hydrophobic), it was importantto form a homogeneously mixed dough to obtain ahighly expanded extrudate. Also, high mechanical prop-erties were obtained when uniform cells were formed.These required the formation of strong covalent bondsbetween the two-phases (polymers) (Guan and Hanna,2004b). During extrusion, mechanical energy changedthe chemical structure of the polymer, especially depo-lymerization of long-chain molecules. In the presenceof plasticizer (ethanol), the depolymerized long-chainstarch molecules had a greater tendency to reform/realign when exiting the die nozzle. Even low moleculeweight polymers had less mechanical strength and lowermelting point, which limited their application. Highexpansion and mechanical properties were achievablewhen these depolymerized long-chain molecules reasso-ciated and formed crystalline regions.

Statistical analyses revealed that SME was signifi-cantly affected by cellulose content, barrel temperatureand screw speed (Table 3). Starch type also significantlyaffected SME (P < 0.0001). Response surface plots of

rch–cellulose foams and DS 2.3 starch–cellulose foams (peaks at cm�1)

starch–cellulose foams Characteristic groups

7–3542(b) O–H stretch1–3330(b) O–H stretch9–2937(bs) –CH3 or –CH3– stretch4–1745(s) –CO–O– stretch9–1374(s) O–H stretch7–1238(s) Two signals for esters0–1042(s) –C–O–C– stretch

500150025003500

Wavenumbers (cm-1)

Tra

nsm

itta

nce

DS 2.3 starch acetate

Alpha cellulose

10%, 150 °C, 170 rpm

30%, 170 °C, 170 rpm

10%, 170 °C, 230 rpm

Fig. 4. FTIR spectroscopy of DS 1.68, DS 2.3 starch acetates, alphacellulose, and DS 2.3 starch–cellulose foams.


SME for DS 1.68 starch–cellulose foams are presentedin Fig. 5. Plots for DS 2.3 starch–cellulose foams weresimilar and not shown. When cellulose content

Table 3Regression equation coefficientsa of second order polynomialsb for two resp

Coefficient SMEc (Wh/kg) RERd (unit les

DS 1.68 2.3 1.68

b0 3135.33 3196.34 �1748.48

Linearb1 5.11*** 7.07*** �2.99***b2 29.77*** 29.45*** 15.79**b3 �45.34*** �44.31*** 5.37**

Quadraticb11 �0.29*** �0.29*** 0.00081b22 �0.30*** �0.30*** �0.038***b33 0.03*** 0.03*** �0.0057***

Cross productb12 0.44** 0.44** 0.01b13 �0.24*** �0.24*** 0.0014b23 0.24*** 0.24*** �0.019***R2 0.9773 0.9773 0.9132

Probability of F <0.0001 <0.0001 <0.0001

a *, **, and *** indicate significance at p < 0.10, 0.05 and 0.01, respectivelb Model on which X1 (cellulose content), X2 (barrel temperature), X3 (screw

b33X 23 þ b12X 1X 2 þ b13X 1X 3 þ b23X 2X 3 þ e.

c Specific mechanical energy.d Radial expansion ratio.e Compressibility.

increased, SME increased. Compared to DS starch,cellulose was a minor constituent in the system. Asreported previously, DS starch and cellulose werehydrophobic and hydrophilic, respectively. When thesetwo materials were blended, increasing the minor con-stituent content tended to increase the viscosity of theblend, resulting in higher mechanical energy require-ment. As barrel temperature increased, SME decreaseddue to decreasing viscosity, while SME increasedsignificantly as screw speed increased. However, in com-bination with the effects of thermal energy (barrel tem-perature), screw speed showed a less significant effect(Fig. 5) on SME at low temperature. This was probablybecause, at low temperature, mechanical and thermalenergies tended to work together to melt and depoly-merize the blend. While at high temperature, blendswere completely melted and the mechanical energy morereadily depolymerized and mixed the melted blends.

Overall, the SME of DS 2.3 starch–cellulose foamswas higher than the SME of DS 1.68 starch–cellulosefoams. This was due to the hardness of DS starchagglomerates in the blends. The higher the DS valueof the starch, the harder were the starch agglomerates(Guan et al., 2005).

3.3. Functional properties of starch–cellulose foams

RER is an important index for extrusion foamingstarch-based products. The higher the RER, the lower

onse variables

s) COMPe (kNkg�1 Æ m3)

2.3 1.68 2.3

�1769.61 �3.05831E+006 �3.07554E+006

�3.08*** 7857.20*** 7771.19***16.00** 35542.53** 35678.25**5.32** 2148.59** 2176.74**

0.00081 �105.70*** �105.70***�0.038*** �120.37*** �120.37***�0.0057*** �12.18*** �12.18***

0.01 �15.44 �15.440.0014 2.98 2.98�0.019*** 17.00 17.000.9132 0.9910 0.9910

<0.0001 <0.0001 <0.0001

y.speed) is calculated: Y ¼ b0 þ b1X 1 þ b2X 2 þ b3X 3 þ b11X 2

1 þ b22X 22þ

658.992

792.479

925.965

1059.45

1192.94

10.00

15.00

20.00

25.00

30.00

150.00

155.00

160.00

165.00

170.00

Cellulose content, %

Temperature, °C

SM

E,

W·h

/kg

658.992

792.479

925.965

1059.45

1192.94

10.00

15.00

20.00

25.00

30.00

150.00

155.00

160.00

165.00

170.00


Screw speed, rpm

SME

,W

·h/k

g

776.901

870.443

963.985

1057.53

1151.07

150.00

155.00

160.00

165.00

170.00

170.00

185.00

200.00

215.00

230.00

Screw sp

eed, rp

m

Screw speed, rpm

SM

E,

W·h

/kg

Fig. 5. Effects of cellulose content, barrel temperature and screw speed on specific mechanical energy requirement (SME) of DS 2.3 starch–cellulosefoams.


the density and the fewer raw materials required. RERof the foams were affected by many factors, such asdough viscosity in the barrel, homogeneity of the blendsand blowing agent content (fixed in this study).

Because of the similar curvature of DS 1.68 starch–cellulose foam surface plots, only response surface plotsfor DS 2.3 starch–cellulose foams are presented inFig. 6. When cellulose content increased, RERdecreased. As reported earlier (Guan et al., 2004b),inter-chain hydrogen bonds probably were formedamong DS starch chains and cellulose chains when theywere melted and depolymerized. When exiting the dienozzle, the strong hydrogen bonds hindered chain sepa-rations, resulting in low RER. When barrel temperatureincreased, RER increased initially and then decreased astemperature continued increasing. Similar effects werefound for increasing screw speed. As mentioned previ-

ously, to obtain high RER in two-phase blends, highdough viscosity and homogenous dough were required.Higher temperatures and screw speeds promoted starchand cellulose melting and depolymerization to form ahomogeneous dough. However, extensive melting anddepolymerization in high temperature and screw speedresulted in overly depolymerized starch and cellulosechains which formed a low viscosity dough and hinderedexpansion.

RER of the DS 2.3 starch–cellulose foams werehigher than DS 1.68 starch–cellulose foams. DS 2.3starch had a higher melting point and had more acetylgroups than the DS 1.68 starch. These contributed thehigher viscosity of the blends and also higher RER.But DS 2.3 starch and cellulose blends may have beenover depolymerized by the high temperature, the highscrew speed and the hardness of starch agglomerates.

26.4643

30.5193

34.5744

38.6294

42.6845

10.00

15.00

20.00

25.00

30.00

150.00

155.00

160.00

165.00

170.00

Cellu

lose conten

t, % Temperature,°C

RE

R

27.7139

31.4297

35.1455

38.8613

42.5771

10.00

15.00

20.00

25.00

30.00

170.00

185.00

200.00

215.00

230.00

Cellulose

content, %

Screw speed, rpm

RE

R

19.2404

24.259

29.2777

34.2963

39.315

150.00

155.00

160.00

165.00

170.00

170.00

185.00

200.00

215.00

230.00

Screw speed, rpm

Tempera

ture,°C

RE

R

Fig. 6. Effects of cellulose content, barrel temperature and screw speed on radial expansion ratio (RER) of DS 2.3 starch–cellulose foams.


Compressibility is an important mechanical propertyof expanded foam. To function as packaging materials,high compressibility is always preferred so as to absorbimpact forces to protect the shipment. Compressibilityresponse surface plots of DS 2.3-cellulose foams areshown in Fig. 6 (Fig. 7). Compressibility increased ascellulose content increased. This agreed with a previousreport (Guan et al., 2004b). With more cellulose blendedin, more hydrogen bonds were formed among starchand cellulose chains making the blend more crystalline.This was in agreement with the X-ray diffractographs.Because of these, compressibility increased. Compress-ibility increased initially and subsequently decreasedwhen barrel temperature and screw speed wereincreased. DS 2.3 starch–cellulose foams had higher

compressibility then DS 1.68 starch–cellulose foamsbecause of the highly packed crystalline structure.

4. Conclusions

Acetylated starch with degrees of substitution of 1.68and 2.3 was blended well with alpha cellulose duringextrusion. X-ray diffractogram showed that both acety-lated starches and cellulose lost crystallinity duringextrusion. Fourier transform infrared spectroscopy con-firmed that there no bond changes occurred during andafter extrusion. Acetylated starches and cellulose main-tained their functional groups. However, type of acety-lated starch, cellulose content, barrel temperature and

75960.5

88442.3

100924

113406

125888

10.00

15.00

20.00

25.00

30.00

150.00

155.00

160.00

165.00

170.00

Cellulose

conten

t, %

Temperature, °C

Com

pres

sibi

lity,

kN·k

g-1·m

3

Com

pres

sibi

lity,

kN·k

g-1·m

3

79445.5

91108.2

102771

114434

126096

10.00

15.00

20.00

25.00

30.00

170.00

185.00

200.00

215.00

230.00


Screw speed, rpm

Com

pres

sibi

lity,

kN·k

g-1·m

3

91174.8

98287.6

105400

112513

119626

150.00

155.00

160.00

165.00

170.00

170.00

185.00

200.00

215.00

230.00

Temperature, °C

Screw speed, rpm

Fig. 7. Effects of cellulose content, barrel temperature and screw speed on compressibility of DS 2.3 starch–cellulose foams.


screw speed had significant effects on the thermal prop-erties, specific mechanical energy requirement, radialexpansion ratio and compressibility of the acetylatedstarch foams.

References

Betancur, A.D., Chel, G.L., Canizares, H.E., 1997. Acetylation andcharacterization of Canavalia ensiformis starch. J. Agric. Food.Chem. 45, 378–382.

Bhatnagar, S., Hanna, M.A., 1994. Extrusion processing conditionsfor amylose-lipid complexing. Cereal Chem. 71, 587–593.

Davison, V.J., Paton, D., Diosady, L.L., Larocque, G., 1984a.Degradation of wheat starch in a single screw extruder: character-istics of extruded starch polymers. J. Food Sci. 49, 453–458.

Davison, V.J., Paton, D., Diosady, L.L., Rubin, L.T., 1984b. A modelfor mechanical degradation of wheat starch in a single-screwextruder. J. Food Sci. 49, 1154–1157.

Fang, Q., Hanna, M.A., 2000. Functional properties of polylactic acidstarch-based loose-fill packaging foams. Cereal Chem. 77 (6), 779–783.

Guan, J., Hanna, M.A., 2003. Post-extrusion steaming on starchacetate foams. Trans. ASAE 46 (6), 1613–1624.

Guan, J., Hanna, M.A., 2004a. Functional properties of extrudedfoam composites of starch acetate and corn cob fiber. Ind. CropsProd. 19 (3), 255–269.

Guan, J., Hanna, M.A., 2004b. Macromolecular characteristics ofstarch acetate extruded with natural fibers. Trans. ASAE 47 (1),205–212.

Guan, J., Hanna, M.A., 2004c. Extruded foams from native starch andacetylated starch. Biomacromolecules 5 (6), 2329–2339.

Guan, J., Fang, Q., Hanna, M.A., 2004a. Functional properties ofextruded starch acetate blends. J. Polym. Environ. 12 (2), 57–63.

Guan, J., Fang, Q., Hanna, M.A., 2004b. Selected functionalproperties of extruded starch acetate-natural fibers foams. CerealChem. 81 (2), 199–206.

Guan, J., Eskridge, K., Hanna, M.A., 2004c. Functional properties ofextruded acetylated starch–cellulose foams. J. Polym. Environ. 12(3).


Guan, J., Eskridge, K., Hanna, M.A., 2005. Acetylated starchpolylactic acid loose-fill packaging materials. Ind. Crops Prod. 22(2), 109–123.

Jetten, W., Stamhuis, E.J., Joosten, G.E.H., 1980. Acetylation ofpotato starch to a low degree of substitution. Starch 32, 364–368.

Larson, M., 1989. Cushioning packages fill more niches than ever.Packaging 35, 39–43.

Lee, C., Han, O., 1997. Optimization of extrusion-process of rice-ISP-file fish mixture by response surface methodology. Food Biotech-nol. 6, 97–103.

Miladinov, V.D., Hanna, M.A., 1999. Physical and molecular prop-erties of starch acetates extruded with water and ethanol. Ind. Eng.Chem. Res. 38 (10), 3892–3897.

Ostergard, K., Bjorck, I., Gunnarsson, A., 1988. A study of native andchemically modified potato starch. Part I: Analysis and enzymicavailability in vitro. Starch 40, 58–66.

Rutenberg, M.W., Solarek, D., 1984. Starch derivatives: productionand uses. In: Whistler, R.L., BeMiller, J.N., Paschall, E.F. (Eds.),Starch Chemistry and Technology, 2nd ed. Academic Press, NewYork, pp. 311–388.

Smith, C.E., Tuschhoff, J.V., 1960. US Patent No. 2,928,828. IssuedMarch 15.

Whistler, R.L., Hilbert, G.E., 1944. Mechanical properties of filmsfrom amylose, amylopectin, and whole starch triacetate. Ind. Eng.Chem. Res. 36, 796–803.

selected morphological and functional properties of extruded acetylated starch–cellulose foams

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