1992 Effect of Starch Granule Size on Physical Properties of Starch.filled

7/28/2019 1992 Effect of Starch Granule Size on Physical Properties of Starch.filled

1/7

Biotechnol. hog. 1992, 8, 51-57 5 1

Effect of Starch Granule Size on Physical Properties of Starch-FilledPolyethylene Film+

Seung-taik Lim and Jay-lin Jane*

Department of Food Science and Human Nutrition, Center for Crops Utilization Research, Iowa State University,Ames, Iowa 50011

Shyamala Rajagopalan and Paul A. Seib

Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas 66506

Physical properties of blown films (25-60-pm thickness) from compounded mixturesof linear low-density polyethylene (LLDPE) an d star ch were investigated. As starc hcontent increased, tensile streng th, percent elongation, an d ight transmittance decreasedan d film thickness increased. Among the tested films, small-particle corn sta rch (2-pmaverage diameter) film had t he highest elongation rat e and tensile and yield streng th(560%, 3.15 kg/mm2, and 1.07 kg/mm2, respectively, at 15% starch content). Potatostar ch (35-pm average diameter) film had th e lowest values (508%, 1.52 kg/mm2, an d0.55 kg/mm2, espectively, at 15 % star ch content). Pot ato starch-LLDPE film had the

highest light transmittance an d film thickness; small-particle corn starch had the lowest.Tensile and yield strength of the films had strong negative correlations with averagesta rch granule diameter ( R = -0.99 and -0.94, respectively). Film thickness and lighttransmittance were linearly correlated with starch granule size ( R = 0.93 and 0.87,respectively). Using small-particle corn starc h substantially increased incorporatedsta rch level in th e film while maintaining the film quality.

Introduction

With growing concern about environmental pollution,the accumulation of plastic waste needs immediateresolution. Biodegradable plastics have been intensivelystudied in recent years (e.g., Evangelistaet al., 1991; Gage,1990; Otey et al., 1987) and have been commercializedinto various products such as garbage bags, compostingyard-waste bags, grocery bags, and agricultural mulches.Commercial biodegradable films are generally manufac-tured from low-density polyethylene with degradativeadditives such as starch and prooxidants (Gage, 1990).Griffin (1974) has suggested naturally available starch asa biodegradable filler satisfying thermal stability andminimum interference with melt-flow properties of mostmanufacturing applications. He reported that , among anumber of different starch sources (including rice, maize,arrowroot, wheat, bean, and potato), polyhedral starchessuch as rice and corn starch, preferably a mixture of th etwo, were suitable as a dry filler in plastic films (Griffin,1977a,b). Additionally, he proposed using unsaturatedfatty acids and their derivatives to enhance the degra-dative oxidation of plastic films.

Starch incorporation produces a plastic film with aporous structure, which enhances the accessibility of t heplastic molecules to oxygen and microorganisms (Griffin,1974). Degradation of starch-filled polyethylene films hasbeen reported by many researchers (Lee et al., 1991; Gouldet al., 1990; Cole, 1990; Gage, 1990). Lee e t al. (1991)

t Journal Paper No. 5-14512 of the Iowa Agriculture and HomeEconomics Experim ent Sta tion , Ames, IA. Proje ct No. 2863.Presented a t the 201st National Meeting of t he Am erican ChemicalSociety in Atlanta, GA.

* Corresponding au thor.

Table I. Polyethylene and Starches of Different Sizes.

compoundlinear low -density polyethylenebcorn starchC

small-particle corn starch ewheat starch8large granule wheat starc hsmall granule wheat starchrice starchpotato starch

abbreviationused in the text

LLDPEcsSPCwsLW SswsRSPS

average granulediameter, pm

14.3d

2.d16.5d22.046.5h5.5d

35.Pa All starch es were v acuum-dried a t 100 OCto


2/7

I

52

h

NE

>Y

v

c'

m

v )

9

Ez

35

3 0

2 5.5 1

-

-

-2. 0 1

70

60

5 0

40

3 0

2 0

, .-- 5 0 5 10 15 2 0 2 5

(b)I I I I

Ym-5.6633X+594.858T-

-

-

-

--

600

5?

P

v

z

5z

W9

50 0

- 5 0 5 10 15 20 25

STARCH CONCENTRATION (%)

Figure 1. Tensile strength (a) and percent elongation (b ) ofLLDPE films containing corn starch at various concentrations(w/w). Data were averages of 10 eplicates.

Materials and MethodsBoth large and small wheat starch granules were isolated

according to a modified procedure of Bathgate and Palmer

(1972). A nylon screen (10-pm mesh diameter) was usedto sieve small granules from an aqueous wheat starch slurry.The mesh was attached to th e end of a glass column (40X 6 cm) inside a 2-L beaker. A magnetic stir bar wasplaced on the mesh inside the column.

Wheat s tarch slurry (25 g in 400 mL of distilled water)was transferred into the column and stirred continuouslyto promote granule passage through the mesh. Distilledwater was added unti l the passing filtrate cleared. Thefiltrate was centrifuged (5875g, 6 min), and the precipitatedsmall wheat starch granules were dried in an oven (40-45"C, 24 h).

The residue collected on the mesh was suspended inwater (1.5 L), and th e large starch granules were allowedto sett le (25 "C, 15 min). The upper water layer containing

small granules was discarded. This process was repeatednine times. Final sediment was collected by centrifugation(5875g, 6 min) and dried (40-45 "C, 24 h). Small-particlecorn starch was prepared by an acid hydrolysis of cornstarch followed by a ball-milling, according to the pro-cedure of Jane et al. (1991).

Film Preparation. Corn starch was added to LLDPEat 0,4,7,10,15, and 20% by weight to examine th e effectsof starch content on film properties. For comparisonamong the starches, 7 and 15% starch was incorporatedinto LLDPE films.

Starch and LLDPE (total weight of 200 g) werecompounded with a Brabender PL 2000 Plasti-Corderdrive (Hackensack, NJ) and a Brabender twin screw mixer

10 ' I I I I I I- 5 0 5 10 15 20 25

STARCH CONCENTRATION (%)

Figure 2. Film thickness (a) and light transmittance (b) of LL-DPE films containing corn starch at various concentrations. Datawere averages of 14 and 6 replicates, respectively, for filmthickness and light transmittance tests.

Table 11. Tensile Strength of LLDPEFilms Filled withVarious Starches at 7% and 15%.

starch contentstarchvarietycswsLW SswsRSPSSPC

7 % 15%2.89 f 0.203.29 0.372.90 f 0.253.64 f 0.433.76 f 0.322.03 f 0.213.91 f 0.20

2.46 i .272.48 i .162.21 i .102.81 f .112.89 f 0.221.52 i .103.15 i .28

Data reported were averages of 10 replicates. LLDPE filmswithout starch had an average tensile strength of 4.70 i 0.12 kg/mmz.

(Model 15-02-000). The mixer was equipped with twocounterrotating screws (42-mm diameter X 9.6-mm lead)having an interrupted mixing zone (Model 15-02-001) and

with a pelletizing die (Model 15-20-000). The extruderbarrel was thermocontrolled a t three consecutive heatingzones (175,185, and 190 OC, in the direction toward thedie). The die was maintained at 195 "C. Two strands ofcompounded starch-plastic mixture were extruded throughth e die nozzles (3.2-mm diameter) a t 20 rpm, air-cooledin a cooling trough (152 cm X 12.7 em X 12.7 em), andpelletized (Brabender pelletizer, Model 12-72-000). Themixture of 20% starch was compounded twice to achieveuniform starch-LLDPE mixing. The compounded ma-terial was stored in a sealed container within a desiccatorto prevent atmospheric moisture absorption.

Film-blowing was conducted with a 3.2-cm Brabendersingle-screw extruder (Model 125-25 HC) and a side-


3/7

Bbtechml. Rag... 1992, Vol. 8, No. 1

15% STARCH 1

53

Figure 3. Scanning electron micrographsof isolated small B-granulesof wheat starch con taminated with A-granules (left) and largeA-granulesof wheat starch (right). Scalebars represent 10 pm .

15% STARCH

i1

0

CS WS LWS SWS RS PS SPC

L.5

Y

w s2.5

v,W

Y=

-0.05X+3.18R = -0.99

LWS2. 0

1.5

1.0 ' I I 1 I I I I 10 5 10 15 20 25 30 35 40

AVC. GRANULE OIAM. (um )

Figure 4. Tensile strengthof starch-LLDPE films (a) and linear

correlation with the average granule diam etersof starch (b). Datawere averagesof 10 replicates.

feeding vertical blown film die (Model 05-74-000). Thebarrel temperature was set a t four zones (170, 180, 185,and 190 "C, in the direction toward the die) and the dietemperature a t 200 "C. The screw speed was 15 rpm, witha compression ratio of 3:l. The blow-up ratio of the filmwas 4.0 f 0.1.

Physical Property Tests. An Instron Model 4502testing system (Park Ridge, IL) was used to test tensilestrength a t break, yield strength, and percent elongationat break in the machine direction according to ASTMD882-83. Ten specimens were tested for each treatment(n = 10). Film thickness was measured randomly a t 14

(b)1.2 I I I I I I I I

I0 spc15% STARCH

Y = -0.01x+1.00n 1.0

E"E

2

9

Y

0.8c'v,

w* 0.6

0.4 I I I I I I I I I0 5 10 15 2 0 2 5 30 35 40

AVC. GRANULE D IM . (um )

Figure 5. Yield strength of starch-LLDPE films (a) and linear

correlation with th e average granule diametersof starch (b).Datawere averagesof 10 replicates.

points (n = 14) with a Fowler Digitrix Mark 2 micrometer(Chicago, IL).

Light transmittance of the starch-LLDPE films wasmeasured with a Beckman Model DU-50 spectrophotom-eter (Fullerton, CA )a t 650 nm according to a modificationof ASTM D1003. Th e film was placed perpendicular tothe light path, and the average transmittance was obtainedfrom six measurements (n = 6). Regression analyses ofthe analytical da ta with average granule size and incor-poration percentage were obtained with SigmaPlot (IBM-PC and compatibles version 4.0, December 1989).


4/7

54 Bbtechnol. Rug., 1992, Vol. 8, No. 1

z

c0

$1

50

so0

0CS WS LWS SWS RS PS SPC

15% STARCHY -1.24X+555

R = -0.69

50 00 5 10 15 20 25 30 35 40

AVG. GRANULED I N . (um)

Figure 6. Percent elongation of starch-LLDPE films (a) andlinear correlation with the average granule diamete rs of starch(b). Data were averages of 10 replicates.

Results and Discussion

1. Effect of Starch Content on Film Properties.The control LLDPE film had an average tensile strengthof 4.7 kg/mm2. A 4% corn starch incorporation caused adramatic reduction to 3.5 kg/mm2 (Figure la ). Thestrength reduction became less substantial a t high starchconcentrations. Regression analysis revealed tha t th erelation between starch concentration (0-20% ) and tensilestrength of the film fitted a second-order equation (R2=0.956, Figure la). Percent elongation of the film alsodecreased with starch concentration, showing a strongnegative linear correlation (R 2 = 0.980, Figure lb). Instarch incorporation of greater than 20 % , he film tendedto possess ai r bubbles, and the blown extrusion becamedifficult to control.

Reduction in tensile strength and in percent elongationof the film by incorporating starch has been reported byseveral researchers (Griffin, 1977a,b; Gage, 1990; Evan-gelista et al., 1991). Because covalent linkage betweenstarch and polyethylene was not likely formed during t heprocessing, starch incorporation produced discontinuityin the film matrix. As starch concentration and discon-tinuity increased, it became difficult to blow intact thinfilms.

Starch also made the film thicker (Figure 2a). Filmthickness corresponded with a second-order equation (R 2= 0.979). Corn starch granular diameter ranged between5 and 30 pm (Snyder, 1984). When the starch wasincorporated into pure LLDPE film (22pm), the thicknessincreased in the area of pure LLDPE, as well as wherestarch granules were accommodated. As the film starch

Figure 7. Light micrographs of LL DP E films containing 15%corn starch (top) or small-p article corn starch (bottom ) tretchedby tensile force.

load and distribution increased, the number of bumpedgranules increased, as did the measured thickness.

Light transmittance of LLDPE films decreased as tarch

concentration increased (Figure 2b) because starch gran-ules shielded the light. Pure LLDPE film had 69% lighttransmittance. Light transmittance decreased inverselyand exponentially as starch concentration increased (R2= 0.990).

2. Effect of Starch Granule Size on the PhysicalProperties of Film. Starch Granule Size. Percentyields for large and small starch granules from native wheatstarch were 29.6% and 15.8% (w/w), respectively. Scan-ning electron micrographs showed tha t the isolated smallspherical B-granules were contaminated with small disk-shaped A-granules (Figure 3, eft), whereas the isolatedlarge A-granules were free of B-granules (Figure 3, ight).Average diameters of small and of large wheat starchgranules were 6.5 and 22.0 pm, espectively (Table I).

The average diameter of the small-particle corn starchwas determined to be 2.0 pm by an image analyzer (TableI) .

Tensile Strength at Break. Tensile strength ofstarch-LLDPE films differed among starch types (TableI1 and Figure 4a). The film containing wheat starch (WS)showed slightly greater tensile strength than did thatcontaining corn starch (CS). When three wheat starches(WS,LWS, and SWS) with different average granule sizeswere compared, tensile strength of the films exhibited aninverse relation to average granule size (Figure 4b). Potatostarch (PS), with the greatest average diameter among allthe tested starches (35 m, Table I) , produced films withthe least tensile strength (2.0 and 1.5 kg/mm2, espectively,


5/7

Biotechnol. Prog,, 1992, Vol. 8 , No. 1

(4

15 % STARCH

h LLDPE - 69 %

r?20

CS WS LWS SWS RS PS SPC

15% STARCH

R = 0.87h

b- 35

I b spc

60

7% STARCH15% STARCH

h

f 5 0 LLDPE- 22 ur n

v

VIVIz5 4 0i

E 30

t-

3

0CS WS LWS SWS RS PS SPC

15% STARCH

R - 0.93fi 5 0

55

0 5 10 15 20 25 30 35 40

AVG. GRANULE DIAM.(um)

Figure 8. Light transmittance of starch-LLDPE films (a) andlinear correlation with the average granule diameters of starch(b). Data were averages of 6 replicates.

0 5 10 15 20 25 30 35 40

AVG. GRANULE D I M . (urn)

Figure 9. Film thickness of starch-LLDPE films (a) and inearcorrelation with the average granule diameters of starch (b). Datawere averages of 14 replicates.

a t 7 and 15% starch contents, Figure 4a). In contrast,small-particle corn starch (SPC), with the smallest averagediameter (2 pm, Table I), produced films with the greatesttensile strength (3.9 and 3.2 kg/mm2). Among th e un-modified starches (CS, WS, RS, an d PS), rice starch (RS)produced the film having the highest tensile strength.There was a negative linear correlation between starchgranule diameter and film tensile streng th (Figure 4b, R= -0.99).

Film containing 1 5 % SPC exhibited greater tensilestrength (3.2 kg/mm2) than did t ha t containing 7 % CS(2.9 kg/mm2) (Figure 4a). This difference suggested t ha tusing small-particle corn starch instead of native cornstarch may raise th e incorporated starch content whilemaintaining the film strength.

Yield Strength. Yield strength of LLDPE filmscontaining starches of different sizes (Figure 5a) followed

a similar trend as tensile strength (Figure 4a). At 1 5%starch content, SPC film exhibited the greatest yieldstrength (1.07 kg/mm2), whereas PS film showed the least(0.55 kg/mm2) (Figure 5a). When starch content was raisedfrom 7 % to 15% in the film, small-particle corn starch(SPC) showed a negligible decrease in yield s treng th (0.01kg/mm2), whereas other starches showed significant drops(0.05-0.17 kg/mm2). Yield strengths of the 15% starchfilms were negatively correlated with th e average sta rchgranule diameters (R = -0.94, Figure 5b).

Percent Elongation at Break. Reduction in percentelongation is another undesirable aspect of starch incor-poration in plastic films. Standard deviations for eachtreatment (Figure 6a) were greater than those for the

strength test s (Figures 4a and 5a). Corn starch ((2s)-LLDPE f i b s had smaller percent elongations than didWS-LLDPE films. Percent elongation of th e 1 5% starchfilms was also negatively correlated with t he average sizeof starch granules although the R value was relatively small(-0.69).

Elongation of the starch-LLDPE films may relate notonly to starch granule size but also to granule shape.According to Khan and Prud'homme (19871, shape andorientation of filler particles showed significant effects onth e melt rheology of filled thermoplastics. Griffin (1990)demonstrated that wheat starch granules may possessinherent advantages as a filler in plastic films. Both largeand small wheat s tarch granules were readily accommo-dated inside a thin film, because the disk-shaped largegranules oriented with the long axis in the machinedirection of the extruded film.

Among the films containing 15% starch, SPC filmdisplayed th e greatest average percent elongation (560 % ),which was also greater than the corresponding value for7 % CS film (544 % 1 (Figure 6a). Using SPC instead of CSwould allow the film to incorporate more starch withoutreducing the film's tensile properties.

Th e starch-LLDPE films produced voids (air spaces)on the starch granule surface duringtensile testing (Figure7). These voids indicated that there was no linkagebetween starch and polyethylene produced during theprocessing and that discontinuity in the film existed atthe interface of both materials. When LLDPE filmscontaining 15 % CS and those containing 15% SPC werecompared under a light microscope, the CS film exhibited


6/7

Bbtechnol. Prog., 1992, Vol. 8, No. 1

I

c

.,!,k

Figure 10. Scann ing electron micrographs of LLDPE ilms containing 15% corn ( top left), wheat ( top right), potato (bottom left),or small-particle corn (botto m right) starch. Scale bars represent 20 pm.

obvious, large voids (Figure 7, op), whereas the SPC filmexhibited much smaller voids uniformly distributedthroughout the film (Figure 7, bottom). It is plausibleth at the uniformly distributed small particles caused lesssevere discontinuity of the LLDPE matrix. Consequently,the loss of tensile properties was less significant.

Light Transmittance. Pure LLDPE film had a lighttransmittance of 69 % . Film with 15% PS had the highestlight transmittance (42 76 ) among the starch-LLDPE films,whereas 15% SPC film had the lowest (18%) Figure 8).Films containing wheat starches (WS, LWS, and SWS)showed slightly higher transmittance than did thosecontaining CS or RS. Light transmittance of the filmsshowed a positive correlation with the average starchgranule diameter (R = 0.87, Figure 8b), indicating th at afilm containing larger starch granules was more trans-parent.

A plausible explanation of the differences in lighttransmittance is th at a small granular starch sample atthe same weight basis consisted of more particles th an alarge granular starch. Although each individual largestarch granule has a greater area of cross section than a

small granule, the larger number of small granules (whenevenly distributed) would provide a greater total area ofcross section in the film. Therefore, more light wasshielded by the SPC film, and the lowest light transmit-tance was observed.

Film Thickness. Thickness of the starch-LLDPEfilms prepared under t he same conditions ranged from 25to 60 pm (Figure 9a), whereas that of pure LLDPE filmwas 22 pm. The thinnest film was produced with SPC (25and 26 pm, respectively, at 7 and 15% starch contents);the thickest was PS (51 and 60 pm). Small-particle cornstarch (SPC) showed a negligible difference (1 pm) in filmthickness between 7 and 15 % incorporation, whereas otherstarch-LLDPE films differed from 3 to 9 pm (Figure 9a).A strong correlation between starch granule size and filmthickness was observed (R = 0.93, Figure 9b).

Scanning electron micrographs of starch-LLDPE filmscontaining CS, WS, PS, and SPC are shown in Figure 10,top left, top right, bottom left, and bottom right, respec-tively. Th e micrographs indicated substantial protrusionof potato starch granules on the surface of the film (Figure10, bottom left) compared with other starches.


7/7

Biotechnol. mug., 1992, Vol. 8 , No. 1

In th e film cross sections, embraced starch granules andthe holes in which starch granules had been lost duringcutting were evident (Figure 10, top panels). The looselyfilled starch granules readily fell apart at the cut edge,leaving the holes visible in the micrographs. The filmcross section containing WS (Figure 10, op right) revealedflat holes in the direction of the film orientation; thisdirection indicates that disk-shaped granules had locatedin that orientation. This finding agrees with Griffin's(1990) observation that WS readily oriented in the machinedirection. The shape and the orientation of the WSgranules are believed to contribute to the smooth surfaceof the film.

Acknowledgment

We thank the Iowa Corn Promotion Board and the IowaDepartment of Economic Development for financialsupport, American Maize-Products Co. and Midwest GrainProducts, Inc., for supplying starch samples, and D. J.Burden for reviewing the manuscript.

Literature CitedBathga te, G. N.; Pa lme r, G.H. A Reassessm ent of the Chem ical

Structure of Barley and Wheat S tarch Granules.Staerlze 1972,24, 336-341.

Cole, M. A. In Agricultural and Synthetic P olymers, Bio-degradability and Utilization;Glass,J. E., Sw ift, G., Ed.; ACSSymposium Series433; American Chemical Society: Wash-

57

ingto-n, DC , 1990, pp 76-95.Eva ngelista , R. L.; Niko lov, Z. L.; Sung, W.; Jane , J.; G elina, R.

J. Effect of ComDoundine and S tarc h Modification on ProD-.erties of Starch-killedLow Density Polyethylene. In d. En'g.Chem. Res. 1991,30,1841-1846.

Gage,P. Degradable Polyethylene Film-the Fact.Tappi J. 1990,IO , 161-169.

Gould, J. M.; Gordon, S. H.; Dex ter, L. B.; Sw anso n, C. L. InAgricultural and S ynthetic P olymers, Biodegradability andUtilization;Glass,J. E. ,Swift, ., Eds; ACS Symposium Series433; Ame rican Chem ical Society: W ashington, DC,1990, pp

Griffin, G. J. L. Biodegradable Fillers in Thermop lastics.Adu.Chem. Ser. 197 4,13 4, 159-170.

Griffin, G. J. L. Biodegradable Synthe tic Resin She et Ma terialContaining Starch and Fa tty Material. U.S. Pate nt4,021,-388,1977a.

Griffin, G. J. L. Synthetic Resin Sheet Material. U S . Paten t4,021,388, 1977b.

G r i f f i , G .J . L . In Wheat is Unique;Pomeranz,Y.,Ed.; AmericanAssociation of Cereal Chem ists: St. Paul, MN ,1990,pp 695-706.

Ja ne , J.; Evangelista, R. L.; Wang, L.; Ra mr atta n,S.; Moore, J.A.; Gelina, R. J. Use of Modified Starches in DegradablePlastics. Corn U tilization Co nference 3Proceedings 1990,4,

Ja ne , J.; Shen , L.; W ang, L.; M aningat, C. C. Prepara tion andPrope rties of Sma ll Particle Co rn Starch.Cereal Chem. 1991,in press.

Khan, S. A.; Prud'homme, R. K. Melt Rheology of FilledThermoplastics. Rev. Chem. Eng. 1987,4,205-269.

Lee, B.; Pometto, A. L., 111; Fratzke, A.; Bailey, T. B., Jr.Biodegradation of Degradable Plastic Polyethylene byPhan-erochaete and Streptomyces Species. App l. Enuiron. Mi-crobiol. 1991, 57 (3), 678-685.

Otey, F. H.; Westhoff, R. P.; Doane,W. M. Starch-Based BlownFilms. 2. Ind. Eng. Chem. Res. 1987,26,1659-1663.

Snyde r, E. M. In Starch Chem istry and Technology; Whistler,R. L., BeMiller,J. N., Paschall, E. F., Eds.; Academic PressInc.: Orlan do, FL ,1984; pp 661-673.

65-75.

1-5.

Accepted October14,1991.

Registry No. PE2045, 26221-73-8;starch, 9005-25-8.

Documents

1992 Effect of Starch Granule Size on Physical Properties of Starch.filled