chemical modification of cassava starch for degradable polyethylene sheets

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Chemical modification of cassava starch for degradable polyethylene sheets Suda Kiatkamjornwong a, *, Prodepan Thakeow b , Manit Sonsuk c a Department of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University, 254 Phyathai Road, Bangkok 10330, Thailand b Multidisciplinary Program of Petrochemistry and Polymer Science, Chulalongkorn University, 254 Phyathai Road, Bangkok 10330, Thailand c The Office of Atomic Energy for Peace, Ministry of Science, Technology and Environment, Vibhavadee Rugsit Road, Chatchuchak Road, Bangkok 10900, Thailand Received 20 February 2001; accepted 28 March 2001 Abstract Cassava starch was chemically modified by radiation grafting with acrylic acid to obtain cassava starch graft poly(acrylic acid), which was further modified by esterification and etherification with poly(ethylene glycol) 4000 and propylene oxide, respectively. The modified product was characterized by NMR spectroscopy and contact angle measurement. The blends of LDPE with EBS wax had properties similar to the LDPE blends with the modified starch in terms of surface wettability, tensile properties, and hardness, but with a much better degradation in soil due to the much higher water absorption. This article describes the chemical modifications of hydrophilic cassava starch to become partially hydrophobic, which was then used for blending with LDPE sheets for evaluations of mechanical, thermal and degradation properties. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cassava starch; Radiation grafting; LDPE sheets; Esterification; Etherification; Ethylene-bis-stearamide; Contact angle; Degradation; Mechanical and thermal properties 1. Introduction Synthetic plastics have become the major new materi- als for everyday life. Much of this growth has taken place at the expense of traditional materials, such as steel, alu- minum, paper and glass. Synthetic polymers, such as polystyrene (PS), polypropylene (PP), and polyethylene (PE), are widely used for food packaging or food service items, the biomedical field, and agriculture. Because they are easily produced, convenient, cheap, and long lasting among other properties. It is almost inevitable that they will continue to play an essential part in the distribution of food and other commodities in spite of the concern about their resistance to biodegradation. This situation leads to the growing problem of pollution. Although these inert polymers can be degraded by natural surroundings, but the degradation process takes very long time. There- fore, there has been an increasing interest in the develop- ment of biodegradable polymers; for example, the synthesis of biodegradable polymers such as poly(3- hydroxy butyrate) or PHB and poly(3-hydroxy valerate) or PHV [1] and the incorporation of natural products such as starch into polymers [2]. Polyethylene is one of the most dominant packaging materials, creating the real problems in the disposal of one-trip packaging. There have been many attempts to make polyethylene become easily degradable. A popular method is the use of starch as natural filler in poly- ethylene. When exposed to a soil environment, the starch component is consumed by microorganisms, leading to increase porosity, void formation, and the loss of integrity of the plastic matrix. The plastic matrix will be broken down into smaller particles. However, including such a component can lead to poor mechan- ical properties such as low tensile strength and low per- centage of strain. These properties may result from immiscibility between starch and polyethylene, because of their difference in hydrophobicity/hydrophilicity. We attempt to modify the hydrophilic property of starch to become more hydrophobic, and study the properties of the plastic composite sheets from the modified starch with low-density polyethylene (LDPE) in comparison with LDPE blended with ethylene-bis-stearamide (EBS) wax. 0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00100-8 Polymer Degradation and Stability 73 (2001) 363–375 www.elsevier.nl/locate/polydegstab * Corresponding author. Tel.: +66-2218-557576; fax: +66-254- 6530; +66-225-3021. E-mail address: [email protected] (S. Kiatkamjornwong).

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Chemical Modification of Cassava Starch

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  • Chemical modication of cassava starch for degradablepolyethylene sheets

    Suda Kiatkamjornwonga,*, Prodepan Thakeowb, Manit Sonsukc

    aDepartment of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University, 254 Phyathai Road, Bangkok 10330, ThailandbMultidisciplinary Program of Petrochemistry and Polymer Science, Chulalongkorn University, 254 Phyathai Road, Bangkok 10330, Thailand

    cThe Oce of Atomic Energy for Peace, Ministry of Science, Technology and Environment, Vibhavadee Rugsit Road,

    Chatchuchak Road, Bangkok 10900, Thailand

    Received 20 February 2001; accepted 28 March 2001

    Abstract

    Cassava starch was chemically modied by radiation grafting with acrylic acid to obtain cassava starch graft poly(acrylic acid),which was further modied by esterication and etherication with poly(ethylene glycol) 4000 and propylene oxide, respectively.

    The modied product was characterized by NMR spectroscopy and contact angle measurement. The blends of LDPE with EBSwax had properties similar to the LDPE blends with the modied starch in terms of surface wettability, tensile properties, andhardness, but with a much better degradation in soil due to the much higher water absorption. This article describes the chemical

    modications of hydrophilic cassava starch to become partially hydrophobic, which was then used for blending with LDPE sheetsfor evaluations of mechanical, thermal and degradation properties. # 2001 Elsevier Science Ltd. All rights reserved.

    Keywords: Cassava starch; Radiation grafting; LDPE sheets; Esterication; Etherication; Ethylene-bis-stearamide; Contact angle; Degradation;

    Mechanical and thermal properties

    1. Introduction

    Synthetic plastics have become the major new materi-als for everyday life. Much of this growth has taken placeat the expense of traditional materials, such as steel, alu-minum, paper and glass. Synthetic polymers, such aspolystyrene (PS), polypropylene (PP), and polyethylene(PE), are widely used for food packaging or food serviceitems, the biomedical eld, and agriculture. Because theyare easily produced, convenient, cheap, and long lastingamong other properties. It is almost inevitable that theywill continue to play an essential part in the distributionof food and other commodities in spite of the concernabout their resistance to biodegradation. This situationleads to the growing problem of pollution. Although theseinert polymers can be degraded by natural surroundings,but the degradation process takes very long time. There-fore, there has been an increasing interest in the develop-ment of biodegradable polymers; for example, the

    synthesis of biodegradable polymers such as poly(3-hydroxy butyrate) or PHB and poly(3-hydroxy valerate)or PHV [1] and the incorporation of natural productssuch as starch into polymers [2].Polyethylene is one of the most dominant packaging

    materials, creating the real problems in the disposal ofone-trip packaging. There have been many attempts tomake polyethylene become easily degradable. A popularmethod is the use of starch as natural ller in poly-ethylene. When exposed to a soil environment, thestarch component is consumed by microorganisms,leading to increase porosity, void formation, and theloss of integrity of the plastic matrix. The plastic matrixwill be broken down into smaller particles. However,including such a component can lead to poor mechan-ical properties such as low tensile strength and low per-centage of strain. These properties may result fromimmiscibility between starch and polyethylene, becauseof their dierence in hydrophobicity/hydrophilicity. Weattempt to modify the hydrophilic property of starch tobecome more hydrophobic, and study the properties ofthe plastic composite sheets from the modied starch withlow-density polyethylene (LDPE) in comparison withLDPE blended with ethylene-bis-stearamide (EBS) wax.

    0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.PI I : S0141-3910(01 )00100-8

    Polymer Degradation and Stability 73 (2001) 363375

    www.elsevier.nl/locate/polydegstab

    * Corresponding author. Tel.: +66-2218-557576; fax: +66-254-

    6530; +66-225-3021.

    E-mail address: [email protected] (S. Kiatkamjornwong).

  • 2. Experimental

    2.1. Graft copolymerization

    Cassava starch used in this study was supplied fromThai Wah Public Co., Ltd. (Bangkok, Thailand). Cas-sava starch-g-poly(acrylic acid) was prepared by -rayirradiation graft copolymerization of gelatinized cassavastarch in the presence of acrylic acid monomer (ThaiMitsui Chemical Co.). The gelatinization was conductedby stirring 20 g of cassava starch in distilled water at853C for 1 h. After cooling, the gelatinized cassavastarch was mixed with acrylic acid solution [20 g of acrylicacid and 2% w w1 of maleic acid (Fluka, Buchs, Swit-zerland) in 50 ml of distilled water], stirred at room tem-perature for 45 min. Every step in this procedure wasperformed under a dry nitrogen atmosphere. The mix-ture was then transferred to an aluminum tube andpurged with nitrogen gas again for 5 min. The tube wastightly closed with a lid and paran lm and then irra-diated by a gamma-ray irradiator (Gamma beam 650Unit, Serial No.18R, Nordian International, Canada) ata dose rate of 2 kGy h1 to a total dose of 10 kGy. Thecharacterization of the obtained graft copolymer wasreported in terms of %homopolymer, %add-on,%conversion, %grafting eciency, and %grafting ratioafter extraction of the homopolymer with methanol(Merck, Darmstadt, Germany) in a Soxhlet extractorfor 24 h. Acid hydrolysis with 65% perchloric acid(Carlo Erba, Milan, Italy) was carried out after thecopolymer was swollen in glacial acetic acid (BDH,Poole, UK) for 1 h.

    2.2. Esterication

    Esterication of cassava starch-g-poly(acrylic acid)was performed by a reaction of cassava starch-g-poly(acrylic acid) (30.0 g) with poly(ethylene glycol) 4000(PEG 4000, Fluka, Buchs, Switzerland) (41.75 g) in thepresence of p-toluene sulfonic acid (6.67 g, Fluka,Buchs, Switzerland) for 8 h at 70C. The reaction wascarried out in a ve-necked glass equipped with a stirrer,thermometer, nitrogen gas inlet, condenser, and columnpacked with a molecular sieve. The cassava starch-g-poly(acrylic acid) was rst dispersed in methanol, andthen the catalyst and the PEG 4000 were added insequence. After the time of reaction had elapsed, thereaction mixture was left to cool. The mixture was thenltered by a suction lter, washed with methanol(analysis grade, Merck, Darmstadt, Germany), anddried at 50C in a vacuum oven for 24 h. The obtainedproduct was called esteried cassava starch-g-poly-acrylate. The amount of PEG 4000 in esteried cassavastarch-g-polyacrylate was obtained after hydrolyzingwith 1 N HCl (37% RPE, Carlo Erba, Milan, Italy)for 6 h [3].

    2.3. Etherication

    The esteried cassava starch-g-polyacrylate (50.0 g)was later etheried by reacting with 50 ml of propyleneoxide (Analysis grade, Merck, Darmstadt, Germany) inthe presence of 1.5 g of sodium hydroxide (anhydrouspallet, analysis grade, min assay 98%, Carlo Erba, Mail-man, Italy), 4 g of distilled water, and 100 g of iso-propa-nol (analysis grade, min assay 99.9%, Carlo Erba, Milan,Italy). The reaction mixture was agitated in a closed vesselat 50C for 48 h. After that, the suspension was neu-tralized with acetic acid (BDH, Poole, UK) and ltered ona suction lter. The product was washed with an excessamount of methanol and dried in a vacuum oven for 24 h.This product was called the modied starch [4]. Theamount of hydroxypropyl groups on the modiedstarch was determined spectrophotometrically via acalibration curve, according to the Jones and Riddickmethod [5] using an UVvis-NIR scanning spectrometer(Model UV-3101PC, Shimadzu, Japan).FTIR (Model 1760, Perkin-Elmer, USA) and 1H- and

    13C-NMR (Model DPX-300, Bruker, Switzerland) wereused to verify the reaction products. Thermal propertiesand the contact angle were measured using a ThermalGravimetric Analyzer (Model TGA 7, Perkin-Elmer,USA) and contact angle goniometer (FACE, Kyoto,Japan), respectively.

    2.4. Blending

    An LDPE resin with a melting index of 30.00 g (10min)1 from Thai Petrochemical (Public) Co., Ltd.,Bangkok, Thailand was used for blending. The EBS wax(Aromowax EBS SF1, Chemmin Corporation, Bangkok,Thailand) was used as a dispersing agent. Mixing of thecomponents of LDPE, LDPE/modied starch (MS), andLDPE/starch (ST) blends was accomplished using a two-roll mill (Model LRM110, Lab Tech Engineer, Thailand)at 165170C for 20 min. LDPEwasmixed withmodiedstarch (MS) of 1, 5, 10, or 20% ww1, and 2% ww1 ofEBS wax, respectively. The LDPE/ST blends used forproperty comparisons were also prepared by the samerecipe. The plastic sheets obtained were then cut into smallpieces using a crushing machine (Bosco Engineering Co.,Ltd., Thailand), which were later compressed in the mold(150x150x2.5 mm3) using a compression molding machine(Model LP 20, Lab Tech Engineer, Thailand) at 170C for5 min with a pressure of 6895 kN m2.

    2.5. Contact angle measurement

    The contact angles between the water droplets andpolymer sheets were measured using the contact anglegoniometer, according to ASTM D5946-96. Cassavastarch, cassava starch-g-poly(acrylic acid), esteriedcassava starch-g-polyacrylate, and modied starch

    364 S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375

  • sheets were prepared to form a at pellet. LDPE andLDPE composite sheets were prepared by hot press mold-ing at 170C with a pressure of 6895 kN m2 for 5 min.

    2.6. Mechanical property testing

    Tensile strength and percentage strain were measured,in accordance with ASTM D638-96 using a tensile test-ing machine (Instron model 1011 tensile tester, USA). Acrosshead speed of 500 mm min1 was used. Five spe-cimens were tested for each blend.

    2.7. Morphology property analysis

    A scanning electron microscope (SEM, JSM-6400,Japan) was used to observe the morphology of theblends. The polymer blends were fractured in liquidnitrogen and the fractured surfaces were sputter coatedwith a thin layer of gold before observation.

    2.8. Thermal property analysis

    Thermogravimetry analysis was performed on a ther-mogravimetric analyzer (Perkin-Elmer, USA) The sam-ples were heated with a heating rate of 20C min1 in anitrogen atmosphere up to 500 and 700C. The sampleswere dried in a vacuum oven at 65C for 24 h prior tothermal analysis.

    2.9. Water absorption test

    The water absorption test was measured using a plas-tic sheet of 252525 mm3 dimensions according toASTM D570-98. The test specimens were rst dried in avacuum oven for 24 h at 50C, then cooled in a desi-ccator, and immediately weighed. The conditioned spe-cimens were entirely immersed in a container of distilledwater. Each sample was removed from the water con-tainer at a specied interval, wiped with a clean cloth,and consequently weighed. The samples were placedback in water after each measurement. The dierencebetween the saturated weight and the dried weight wascalculated as the water absorption.

    2.10. Soil burial degradation test

    The soil burial test is an outdoor experiment, whichprovides a realistic environment with seasonal changes,less control of soil wetness and temperature, and thepresence of macro-organisms. The test was carried out insoil of Saraburi Province, Thailand from JuneNovem-ber 2000 or 5 months. The plastic sheets were buried at adepth of 79 inches from the surface. After the plasticsheets were removed, their surfaces were wiped withwater. They were then dried at 50C for 24 h in avacuum oven and kept in dark before tensile testing.

    3. Results and discussion

    3.1. Graft copolymerization

    The gelatinized cassava starch and acrylic acid mix-ture was irradiated with -rays at a predetermined doserate of 2 kGy h1 to a total dose of 10 kGy to avoid ahigh amount of homopolymer and to achieve a highgrafting eciency. However, the obtained product wassubjected to extraction of homopolymer of poly(acrylicacid). The graft copolymer of cassava starch-g-poly(acrylic acid) was characterized using FTIR. It can beseen in Fig. 1a and b that the presence of the carbonylpeak at 1727 cm1 in the FTIR spectrum veried thegrafting reaction of poly(acrylic acid) onto gelatinizedcassava starch. The graft copolymer was later hydro-lyzed to characterize the graft copolymer, which gavethe following IR spectra in Fig. 1d, and chemical shiftsin 13C-NMR in Fig. 2a: 41 ppm (methylene carbon), 61,72, 82 and 102 ppm (carbon of the anhydroglucose unit)

    Fig. 1. IR spectra of cassava starch and chemically modied products:

    (a) cassava starch; (b) cassava starch-g-poly(acrylic acid); (c) esteried

    cassava starch-g-polyacrylate; (d) side chain of esteried cassava

    starch-g-polyacrylate; (e) esteried/etheried cassava starch-g-poly-

    acrylate (MS, modied starch).

    S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375 365

  • and 178 ppm (carbonyl carbon of poly(acrylic acid)).According to Table 1, a high eciency of grafting isneeded for further chemical modications. Basically, thetotal dose and dose rate irradiation are importantparameters in any radiation grafting systems. In thedirect irradiation method, the total dose determines thenumber of grafting sites, while the dose rate determinesthe length of the grafted branches. The length of thebranches is also controlled by other factors such as thepresence of chain transfer reactions, the concentrationof the monomer, the reaction temperature, the viscosityof the reaction medium, diusion phenomena and soon. Too high a dose rate would aect the amount ofesterication and the subsequent etherication. Thegrafting reaction of acrylic acid gives a pure graftcopolymer since there is no residual initiator that mightinterfere the subsequent reactions and blending [6].

    3.2. Esterication

    After esterication, the appearance of esteried cas-sava starch-g-polyacrylate was changed from that of arigid, non-sticky, and clear, white powder to the non-rigid, sticky and opaque-white powder. The evidence ofesterication was veried by utilizing FTIR, showing

    the shift of the carbonyl of carboxylic acid groups to thecarbonyl of ester groups from 1727 to 1738 cm1

    (Fig. 1c and d). Moreover, the solid state 13C-NMRspectra shown in Fig. 2b conrm the esterication. Thechemical shift of the carbonyl carbon from 177.9 to175.6 ppm also indicated the conversion of carboxylicgroups into ester groups. The peak at 52 ppm indicatesthe carbon of CO on PEG 4000 chains. The otherchemical shifts are similar to those of the grafted cas-sava starch. In addition, the side chain of the esteriedcassava starch-g-polyacrylate was separated and deter-mined using 1H-NMR as shown in Fig. 3. The peaksfound in 1H-NMR spectra are as follows. The chemicalshifts in cassava starch in D2O: at 3.34.2 ppm (protonsof the anhydroglucose units of starch), 4.64.9 ppm(protons of water molecules), 5.25.6 ppm (equatorialprotons of anhydroglucose units of starch as shown inFig. 3a). The chemical shifts in the side chain of starch-g-poly(acrylic acid) in CDCl3 (Fig. 3b): 1.5 ppm (methy-lene protons), 4.3 ppm (protons of methylene groupshaving their carbon atoms bonded with the oxygenatom), 7.2 ppm (protons of the impurity in CDCl3). Thechemical shifts in the side chain of esteried starch-g-polyacrylate in CDCl3 (Fig. 3c): 1.22.8 ppm (methyleneprotons), 3.43.8 ppm (protons of PEG 4000), 7.27.3ppm (protons of the impurity of CDCl3). The amount ofPEG 4000 on the esteried cassava starch-g-polyacrylate,determined by a gravimetric method of acid hydrolysis,was 15.21.8%. The formation of a carboxylic acid estercan be prepared by treatment of carboxylic acid with analcohol in the presence of an acid catalyst. Althoughp-toluenesulfonic acid was used as an external catalyst,the percentage esterication is relatively low, becauseesterication is a reversible reaction and appreciableconcentrations of both the carboxylic acid and ester arepresent at equilibrium. To obtain a good yield, it isnecessary to force the reaction to completion either byremoving the water formed or using excess reactants. Inaddition, this starch graft poly(acrylic acid) is a macro-molecule with steric hindrance. The yield of the reactionproducts of the starch graft poly(acrylic acid) andpoly(ethylene glycol) 4000 is thus the optimum resultunder the specied conditions.

    Table 1

    Characterization of cassava starch-g-poly(acrylic acid)

    Propertya Value (%)

    Homopolymer content 2.70.2Add-on 24.91.0Conversion 40.00.8Grafting eciency 90.00.2Grafting ratio 33.21.8

    a Prepared by a dose rate of 2 kGy h1 to a total dose of 10 kGywith a starch-to-acrylic acid ratio of 1:1.

    Fig. 2. 13C-NMR spectra of (a) cassava starch-g-poly(acrylic acid); (b)

    esteried cassava-starch-g-polyacrylate; (c) esteried/etheried cassava

    starch-g-polyacrylate.

    366 S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375

  • 3.3. Etherication

    The hydroxypropyl groups on cassava starch wereetheried with propylene oxide in the presence of sodiumas a catalyst. After etherication, the white powder ofesteried cassava starch-g-polyacrylate became yellowishpale. The puried, modied starch was analyzed by IR(Fig. 1e), 13C-NMR (Fig. 2c) and 1H-NMR (Fig. 3d)spectrometry. The presence of a peak at a chemical shiftof 19.96 ppm is indicative of the presence of hydrox-ypropyl groups on the modied starch. The chemicalshift at 52 ppm indicates the presence of CO on PEG4000 chains. Furthermore, the occurrence of a distinctpeak at 1.2 ppm in the 1H-NMR spectrum (Fig. 3d) isattributed to protons of hydroxypropyl groups on themodied starch.The amount of hydroxypropyl groups was determined

    spectrophotometrically, which includes hydrolysis of thehydroxypropyl group to propylene glycol, which in turnis dehydrated to propionaldehyde and to an enolic formof allyl alcohol. These products are measured spectro-photometrically after they are reacted with ninhydrin to

    form a product having a purple color. The value of thehydroxypropyl equivalent groups on the modied starchis found to be 1.30%.When the esteried starch reacts with propylene

    oxide, two dierent starch ethers may result dependingon whether the nucleophilic attack takes place at theprimary or the secondary carbon atom as shown in Eqs.(1) and (2). The major and usual product is the one thatresults from the nucleophilic attack at the least hindered(primary) carbon atom. This relatively low content(1.30%) could be the result of the steric hindrance ofesteried cassava starch-g-polyacrylate, which limits theaccessibility of sodium hydroxide and propylene oxide.Structures I, II, and III show the starch products of

    starch-g-poly(acrylic acid), esteried starch-g-poly-acrylate, and esteried/etheried starch-g-polyacrylate,respectively, resulted from the three modication reac-tions of starch, which represent the possible sites ofesterication and etherication reactions.

    Fig. 3. 1H-NMR spectra of (a) cassava starch; (b) side chain of starch-g-poly(acrylic acid); (c) side chain of esteried cassava starch-g-polyacrylate;

    (d) esteried/etheried cassava starch-g-polyacrylate.

    S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375 367

  • 3.4. Contact angle measurement

    The contact angle measurement gave the resultsshown in Table 2. The contact angle increases in everystep of modication, because of more carbon atoms onthe modied starches. This result indicates that themodied starch (MS) becomes more hydrophobic. Afterblending with LDPE, the contact angles of all plasticcomposite sheets were not signicantly dierent. Oneinteresting point is that the modied cassava starches (the

    esteried starch and the esteried/etheried starch) arestill relatively polar since their contact angles are stillsmaller than 90. The EBS wax, which is ethylene-bis-stearamide with an amine content of 3.0% and watercontent of 0.20% max, has a similar polarity to theesteried/etheried starch, which has 15.25 and 1.30% ofpoly(ethylene glycol) and hydroxypropyl groups, respec-tively. Both the hydroxyl groups of the glycol and pro-pylene oxide are polar like that of the amide, while theethylene moiety in EBS wax is similar to the ethylene

    Table 2

    Contact angles of cassava starch, esteried starch-g-polyacrylate,

    etheried modied starch, EBS wax, and low density polyethylene and

    its blends, and the calculated work of adhesion

    Sample Contact anglea

    ()Wad

    b

    (mN m1)

    Cassava starch Dissolve,

    hydrophilic (0)145.6

    Cassava starch-g-poly(acrylic acid) 77.50.5 88.6Esteried starch-g-polyacrylate 800.3 85.4Etheried modied starch 820.8 82.9EBS wax 931.2 68.5LDPE (without EBS wax) 96.40.9 64.7LDPE (with EBS wax) 941.4 68.0LDPE/MS1 (with EBS wax) 94.81.2 66.7LDPE/MS5 (with EBS wax) 94.91.2 66.6LDPE/MS10 (with EBS wax) 94.80.6 66.7LDPE/MS20 (with EBS wax) 94.40.4 67.2LDPE/MS10 (without EBS wax) 95.91.7 65.3

    a Contact angles of methylene diiodide on the modied starch and

    blends were approximately 0.b Work of adhesion, Wad=LV(1+cos), where LV is the surface

    tension of water (72.8 mN m1).

    368 S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375

  • moiety in the acrylate chains grafted on the starchbackbones.Therefore, LDPE without EBS wax (96.4), LDPE

    with EBS wax (94), and LDPE/MS 10 without EBSwax (95.9) have close contact angles and work ofadhesion. The EBS wax and MS are anticipated to per-form similarly in the LDPE blends.

    3.5. Thermal property analysis

    The thermogravimetric curves are shown in Figs. 4and 5. Cassava starch was stable up to 275C (Fig. 4a).The maximum decomposition rate appeared at 375C.The free water bound with starch molecules was present,although the starch was heated before the analysis. Forthe modied cassava starch (Fig. 4bd), there were twostages of decomposition. The rst was starch moietydecomposition, and the second was poly(acrylic acid)moiety decomposition. Then second decomposition stagestarted above 400C and ended at about 550C, giving anash residue. For LDPE (Fig. 5), the curve was stable up

    to 350C and reached maximum decomposition at505C. After blending with various contents of modiedstarch, the decomposition onset temperature of plasticcomposite sheets was lower than for LDPE sheets (atabout 300C). This was attributed to the decompositionof the modied starch composition. This observationsuggests that the starch or modied starch blendedLDPE plastics consume less thermal energy to start adecomposition process. It also implies that this type ofblended material degrades easily in modied starch orstarch.

    3.6. Morphology of the blended samples

    The plastic sheets of LDPE (Fig. 6a), LDPE/ST10(Fig. 6b), and LDPS/MS10 (Fig. 6c) were selected forobservation of the fractured surface, and their morpholo-

    Fig. 5. TGA and DTA thermograms of (a) LDPE; (b) LDPE/MS1;

    (c) LDPE/MS5; (d) LDPE/MS10; (e) LDPE/MS [LDPE, low density

    polyethylene; MS, modied starch (esteried/etheried starch)].

    Fig. 4. TGA and DGA thermograms of (a) cassava starch; (b) cassava

    starch-g-poly(acrylic acid); (c) esteried cassava starch-g-polyacrylate;

    (d) esteried/etheried cassava starch-g-polyacrylate (modied starch).

    S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375 369

  • gies were compared and illustrated in Fig. 6. Small parti-cles were observed in the starch-lled polyethylene sheet(Fig. 6b). These particles could be the ruptured grains,which were immiscible with LDPE. Fewer particles wereobserved in the modied starch-lled LDPE compositesheets (Fig. 6c). These results indicated that the modi-ed starch improved the compatibility with the LDPEmatrix.

    3.7. Mechanical property analysis

    The eects of the starch and modied starch contentson mechanical properties of LDPE composite sheets aredemonstrated for the tensile strength and %strain,respectively, as shown in Fig. 7. For cassava starch- andmodied starch-lled LDPE composite sheets, the sameconclusion regarding tensile strength properties can bereached. The tensile strength of the LDPE compositesheets increased with increasing starch content anddecreased with lowmodied starch content. Furthermore,the tensile strengths of starch-lled LDPE compositesheets became even higher than LDPE sheets (withoutstarch) when the contents of starch were higher than1%. The starch phase possibly enhances the tensilestrength of LDPE composite sheets containing starchand modied starch. Because the tensile strength of theunlled-LDPE was found to be lower than that foreither MS-lled or starch-lled LDPE sheets. However,it tended to be higher when the starch or modiedstarch concentration was up to 20%. The percentage ofstrain decreased with increasing starch and modiedstarch contents in the blends. The addition of starch andmodied starch to LDPE markedly reduced the percen-tage of strain, and a signicant dierence was observedbetween LDPE/ST and LDPE/MS blends. This is becausestarch and modied starch mixed incompatibly withLDPE, leading the LDPE composite sheets to becomebrittle, with low tensile strength and %strain. When com-paring LDPE/MS10 blends with and without EBS wax,their tensile strengths were not signicantly dierent, butthe %strain values were drastically dierent. The decreasein the %strain of LDPE/MS10 that contains EBS waxmay be the result of EBS wax distributed throughoutLDPE matrix, leading to a discontinued phase of LDPE.On the other hand, the modied starch may be acting likea plasticizer.

    Fig. 7. Tensile strength and %strain of LDPE composite sheets.

    Fig. 6. Morphology of blended samples (a) LDPE; (b) LDPE/ST10;

    (c) LDPE/MS10.

    370 S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375

  • 3.8. Morphology of the failure samples

    Fig. 8 elucidates the failure mechanisms of plasticcomposite sheets: the failure surface morphology of thesample subjected to tensile strength at break is used as ahelpful indicator. The LDPE, LDPE/ST10, and LDPE/MS10 sheets were chosen for the tensile property test.For LDPE and LDPE/ST10 sheets (Fig. 8ad), the fail-ure surface morphology of the tensile test at breakseemed to be similar. The surfaces after the tensile fail-ure test showed the presence of cavities in the form of

    ridges and valleys. A ridge is present on one fracturesurface and a corresponding valley on the other. Thefailure mode appeared to be shear tearing, which is anaccepted mode of failure in metals and polymers [7].The observed bril bundles were the result of slow crackgrowth. It was also found that the bril bundle of theLDPE/ST10 blend was shorter than that of the LDPEsheet. For LDPE/MS10 blend (Fig. 8e and f ), therewere fewer valleys that indicated that materials snappedat the point of stress. The presence of the rough surfaceindicated that the failure surface was similar to a planar

    Fig. 8. Failure of starch-lled LDPE sheets under tensile testing at break: (a) LDPE sheet (500); (b) LDPE sheet (3500); (c) LDPE/ST10 (500);(d) LDPE/ST10 (3500); (e) LDPE/MS10 (500); (f) LDPE/MS10 (3500).

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  • fracture surface. The phenomena were evidence of nosevere interfacial failure between starch and LDPEresulting from a faster crack growth.

    3.9. Soil burial degradation test

    In order to evaluate the degradation of LDPE com-posite sheets in a realistic environment, a soil burialexperiment was carried out. After removal, dark spots ofmold growth were observed on the surface of LDPEcomposite sheets. Mechanical property measurement andsurface morphology of the samples were then performed.

    3.10. Mechanical property measurement of soil burialdegradation test

    Figs. 9 and 10 present the tensile strength and thepercentage strain of LDPE, LDPE/ST, and LDPE/MSblends, respectively, after the soil burial test. The resultsshow that both the tensile strength and the percentage ofstrain decreased at a slow rate. This indicates that the soilburial had a weak eect on tensile properties of LDPEsheets. The decline of the tensile strength and%strain wasan indicator of soil burial eciency, especially where largeamounts of starch and modied starch were used. The

    presence of themodiedmoieties of ester and ether groupsled to a decrease in mechanical properties, becausethey could absorb moisture from the surroundings,which could then be attacked by microorganisms such asfungi and bacteria, and resulting in porosity and voids.Then the loss of integrity of the plastic matrix enhancesthe breakdown of the matrix polymer into smaller par-ticles, which could be attacked by other degradationroutes.

    3.11. Surface morphology of samples

    The surface morphologies of LDPE, LDPE/ST10,and LDPE/MS10 sheets were studied in order to followthe changes after soil burial for 2 months. It can be seenin Fig. 11 that the surface of LDPE sheets (Fig. 11b, dand f ) after the soil burial test became rougher, withmore cavities than the control sheet (Fig. 11a, b and e).For LDPE/ST10 and LDPE/MS10 plastic sheets, manyholes throughout the plastic sheets were observed. Thisoccurrence may be caused by microorganisms in the soilthat utilized the carbon sources in starch and modiedstarch as a food source or by the external inuence ofrainfall and underground water leading to leaching ofthe destroyed surface.

    Fig. 9. Tensile strengths of LDPE, LDPE/ST and LDPE/MS blends.

    Fig. 10. Percentages of strain of LDPE, LDPE/ST and LDPE/MS

    blends after the soil burial test.

    372 S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375

  • 3.12. Water absorption test

    The water permeability of modied starch is anothercause of degradation. The synthetic component appearedto be biodegraded by surface absorption of moisture andmicroorganisms, such as fungi and bacteria, becausewater absorption on the material allows the microorgan-isms to grow and utilize the material as a carbon source.Figs. 12 and 13 reveal the results for the %water absorp-tion and water absorption rate. For LDPE, the percentage

    of water absorption is less than 0.5%, indicating thehydrophobicity of LDPE. The water absorption ofstarch- and modied starch-lled LDPE compositesheets increased with increasing modied moiety con-tent. However, the water absorption of the samples ofthis work is still low. It can be observed that the waterabsorption of the modied starch-lled LDPE sheets washigher than that of the unmodied ones. This resultimplied that the former could absorb more water andmicroorganisms in soil water, and thus leading to a greater

    Fig. 11. SEM micrographs of (a) control LDPE sheets; (b) LDPE sheet after 2-month soil burial test; (c) control LDPE/ST10 sheet; (d) LDPE/ST10

    sheet after two-month soil burial test; (e) control LDPE/MS10 sheet; (f) LDPE/MS10 sheet after 2-month soil burial test.

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  • weight increase (with water) and more biodegradation ofthe lled LDPE sheets. The highest water absorption ratewas on the rst day of exposure. After that the waterabsorption rate decreased slowly. This result implied thatthe blends became saturated with water.

    4. Conclusions

    Cassava starch-g-poly(acrylic acid) copolymers wereprepared by a simultaneous irradiation technique with-ray irradiation from a 60Co source. The graft copoly-mers obtained were later modied from being hydro-philic to hydrophobic, and were mixed with LDPE. Thephysical and mechanical properties of the blends werestudied. The graft copolymer was prepared using -rayirradiation at a dose rate of 2 kGy h1 to a total dose of10 kGy, and a ratio of acrylic acid to cassava starch of1:1. The resulting graft copolymer contained 2.7%homopolymer, 24.9% add-on, 40% conversion, 90%grafting eciency, and a 33.2% grafting ratio.The chemical modications were carried out by ester-

    ication and etherication with poly(ethylene glycol)4000 and propylene oxide, respectively, onto the graftedstarch. The amounts of the poly(ethylene glycol) 4000and the hydroxypropyl group on the modied starch

    were found to be 15.25 and 1.30%, respectively. Fouriertransform infrared spectroscopy, 13C-NMR and 1H-NMR spectrometry, and ultraviolet spectroscopy wereused for analysis. This modied starch was mixed withLDPE at various proportions in a two-roll mill. Mechan-ical and hardness properties, thermal properties, degra-dation in soil, and water absorption were investigated. Itwas found that the tensile strength of LDPE compositesheets decreased, compared with the unmodied LDPEsheet, but they showed a positive tendency to increasewith an increasing amount of the modied cassava starch.The hardness increased with an increasing amount of themodied cassava starch, but the resistance to thermaldegradation decreased from 350C (for LDPE) to300C. The degradation of plastic sheets in a soil burialtest took place slowly according to the low water absorp-tion of 2.5%. This work shows that incorporation of themodied starch to LDPE did not signicantly improve themechanical properties of the LDPE composite sheets.Interestingly, this modied starch did not absorb toomuch moisture in the air, but could slightly absorb waterin direct contact by not more than 2.5% by weight,helping to enhance the biodegradability of LDPE plas-tic sheets. The modied starch disperses favorably in theLDPE matrix, which is similar to the dispersing prop-erty of EBS wax.

    Fig. 13. Water absorption rate of LDPE, LDPE/ST and LDPE/MS

    blends after exposure to distilled water.

    Fig. 12. Water absorption of LDPE, LDPE/ST and LDPE/MS blends

    after exposure to distilled water.

    374 S. Kiatkamjornwong et al. / Polymer Degradation and Stability 73 (2001) 363375

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