Compatibilization of Starch–Polyester Blends Using Reactive Extrusion

Compatibilization of Starch–Polyester Blends Using Reactive Extrusion

R.B. Maliger, S.A. McGlashan, P.J. Halley, L.G. Matthew Centre for High Performance Polymers (CHPP), Division of Chemical Engineering, University of Queensland, St. Lucia, Brisbane, Queensland-4072, Australia

Maleic anhydride (MA) and dicumyl peroxide (DCP) were used as crosslinking agent and initiator respectively for blending starch and a biodegradable synthetic aliphatic polyester using reactive extrusion. Blends were characterized using dynamic mechanical and thermal analysis (DMTA). Optical micrographs of the blends revealed that in the optimized blend, starch was evenly dispersed in the polym polymer er matri matrix. x. Opt Optimized imized blends exhibited exhibited bett better er tensile properties than the uncompatibilized blends. Xray pho photoe toelec lectro tron n sp spect ectros roscop copy y sup suppor ported ted th the e pro pro-posed structure for the starch–polyester complex. Variation ati on in th the e com compos positi itions ons of cro cross sslin linkin king g age agent nt and initiator had an impact on the properties and color of the blends. POLYM. ENG. SCI., 46:248–263, 2006. © 2006 Society of Plastics Engineers

INTRODUCTION Plastics obtained from petrochemical sources generally have lifetimes of hundreds of years when buried in typical solid waste sites, and thus have posed significant environmental threats [1]. These plastics have contributed 17–25% of the volume of waste being land filled [2]. Hence, recently, much research has concentrated on the development of envir environmen onmental-fr tal-friendly iendly biode biodegradab gradable le plastics obtain obtained ed from various natural renewable resources including starch, cellulose, poly(hydroxyalkanoates), and poly(lactic acid). Three main classes of biodegradable polymers have been recognized. The first class consists of synthetic polymers source sou rced d fro from m non nonren renewa ewable ble res resour ources ces wit with h vul vulner nerabl ablee groups grou ps susceptible to hydr hydrolysis olysis attack by microb microbes. es. The second class of materials is composed of naturally-occurring rin g bac bacteri terial al pol polyme ymers rs suc such h as pol polyhy yhydro droxyb xybute uterat rates es (PHB),, polyh (PHB) polyhydro ydroxyval xyvalerates erates (PHV (PHV), ), and polyh polyhydrox ydroxyybuterates-co buter ates-co-valer -valerates ates (PHB/V (PHB/V). ). Althou Although gh polyh polyhydro ydroxyalxyalkanoates are truly renewable and biodegradable, the determinatio min ation n of pro process cessing ing par paramet ameters ers and hig high h pro produc duction tion

[email protected] Correspondence to: Peter J. Halley; e-mail: [email protected] Contract grant sponsor: Plantic™ Technologies, Melbourne. DOI 10.1002/pen.20479 Publis Pub lished hed onl online ine in Wil Wiley ey Int InterS erScien cience ce (ww (www.i w.inter nterscie science. nce.wil wiley. ey. com). © 2006 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2006

costss ar cost aree th thee co conc ncer erni ning ng fa facto ctors rs [1 [1]. ]. Th Thee th thir ird d cl clas asss of materia mate rials ls is bio biodeg degrad radabl ablee pol polyme ymers rs fro from m ren renewab ewable le resour so urce cess su such ch as sta starc rch, h, cel cellu lulo lose se,, an and d po poly lylac lactic tic ac acid id.. Starch-based thermoplastics are inexpensive, but generally possess high viscosities and poor melt properties that make them difficult to process. As a result, products made from starch are often brittle and water-sensitive [3]. To alleviate these the se pro problem blems, s, star starch ch pol polyme ymers rs are oft often en ble blende nded d with more high performance synthetic polymers. Of course, in the case of starch-polyester blend systems one must also factor in the higher cost of the polyester, and aim to increase the percentage of the lower cost starch in the blend while maintaining the desired mechanical properties and performance of the polymer product. Thus to achieve this costperformanc perfo rmancee optim optimization ization,, much must be under understood stood and optimized about the individual polymers and the blending process.

STRUCTURE, MORPHOLOGY, COMPOSITION, AND PROPERTIES OF STARCH Recent NMR studies suggest that there are at least three distinct components in wheat starch granules [4]. They are as follows: (i) highly crystalline regions formed from double-helical starch chains, (ii) solid-like regions formed from amylose-lipi amylos e-lipid d inclus inclusions ions comple complexes, xes, and (iii) comple completely tely amorphous regions associated with branching regions of the amylopectin components of starch and possibly the lipidfree amylose. Starch granules essentially consists of linear poly-(1 3 4)--D-gluc -glucose, ose, amylose, and the branched molecule, amylopectin, where the linear chains are connected throug thr ough h (1 3 6)- linkage linkages. s. Whe When n sta starch rch gra granul nules es are observed under polarized light, a characteristic dark cross is seen se en,, wh whic ich h ha hass led to re rega gard rd th thee gr gran anul ules es as di dist stor orted ted sphaerocrystals [5]. The proportion of amylopectin ranges from fro m 95% in the low amy amylos losee or wax waxy y star starche chess thr throug ough h 70–75% in normal starches to 30% in some high amylose starches [6, 7]. Hydroxyl groups cause starch to generally behave as an alcohol during chemical reactions. This property of starch is important when considering reactive blending of starch with

synthetic polymers. The presence of such large number of OH gro OH groups ups aff afford ordss star starch ch hyd hydrop rophili hilicc pro proper perties ties and theref the refore ore add addss affi affinit nity y for moi moistu sture re and dis disper persab sabilit ility y in water [8]. However, hydrophilicity is undesirable in many plastic packaging applications and hence it is a major limitation in using starch as a homopolymer.

O

STARCH–POLYMER BLENDS Most synthetic polymers are incom incompatible patible with starch and it is difficult to add starch to synthetic polymer systems in large quantities without dramatically reducing reducing perfo perforrmance properties [9]. To enhance the compatibility of starch and synthetic polymers, polymers, a reactiv reactivee funct functional ional group such as maleic anhydride (MA) can be added to the synthetic polymer [10]. In these blends, the interfacial tension is lowered to generate a small phase size and strong interfacial adhesion to transmit applied force effectively between the componentt phases [11]. Thus, compa ponen compatibiliza tibilization tion through in situ formation of compatibilizer in polymer blends has become increa inc reasin singly gly imp import ortant ant and an alte alterna rnativ tivee to rep replace lace the method of adding block or graft copolymers separately [12]. Synthetic polymers having functional groups such as carboxylic acid, anhydride, epoxy, urethane, or oxazoline can react with the O OH OH group in starch to form a blend with stable morphology. These reactive groups form a chemical or physical bond with hydroxyl or carboxyl groups in natural polymers such as starch [13]. The role of a compatibilizer is to control the properties of multip mul tiphas hasee blen blends ds not sim simply ply by con conver vertin ting g imm immisci iscible ble blends into fully miscible blends, but by controlling the size of the phase domains of immiscible blends. Effective compatibil pat ibilizer izerss mus mustt be loc located ated at the int interf erface ace bet betwee ween n the phase domains of the immiscible blend. Most importantly, the degree of compatibilization in a particular system depends on the reactivity of the compatibilizer used. It has been found that a compatibilizer is most effective when its section sec tionss are of hig higher her molecular molecular weig weight ht tha than n the cor correrespondi spo nding ng ble blend nd com compon ponents ents [14 [14]. ]. Sev Severa erall the theori ories es hav havee been bee n pro propos posed ed to exp explain lain the rol rolee of com compat patibil ibilizer izers, s, of which two mechanisms are considered plausible. The first mechanism is thermodynamic in nature in that the compatibilizer reduces the interfacial tension between the phases. Thee se Th seco cond nd me mech chan anism ism is ki kine netic tic in na natu ture re in th that at th thee presence of the compatibilizer at the interface reduces the agglomeration of domains by steric stabilization. However it is often unclear which of these mechanisms dominates the reduction in the particle size [14], and hence, the mechanism of compatibilization is still a debatable issue. Several researchers have showed that dissimilar blends could be compatibilized using different techniques. Bacon and Farmer [15] showed that MA reacts with natural rubber in presence of benzoyl peroxide and results in the addition of MA to the double bonds and to the  -methylene groups of the polymer. Alder et al. [16] have confirmed the addition reaction of MA with rubber in solid phase and in solution. Mani, Bhattacharya, and Tang [12] have used initiators such DOI 10.1002/pen

as dicumyl peroxide (DCP), benzoyl peroxide, and di- tert butyl peroxide (DBP) for grafting biodegradable polybutylene succinate (PBS) with starch in presence of MA as a crosslinking agent. Sailaja and Chanda [17] have used MAgrafted polyethylene (PE) for preparing PE-starch blends in presence of benzoyl peroxide as initiator. Ceric ammonium nitrate (CAN) is anothe anotherr initiato initiatorr common commonly ly used during reactive grafting. Luftor et al. [18] studied the kinetics of graft polymerization of acrylonitrile onto sago starch using CAN as initiator. These works suggested the use of other crosslinkin cross linking g agents such as phtha phthalic lic anhyd anhydride ride and dodec dodecyl yl succinic anhydride in the presence of initiators to facilitate grafting reactions. Of all the methods used to achieve compatibilization in starch–polyester blends, reactive extrusion (REX) is a wellutilized technique as it combines the traditionally separated chemical processes (polymer synthesis and/or modification) and extrusion (melting, blending, structuring, devolatilization, and shaping) into a single process carried out in the extruder [19]. Using REX, modified polymer can be obtained in a ready-to-use form at the die. Unmodified starch-based thermoplastics generally have higher viscosities and poor melt properties than traditional synthe syn thetic tic pol polyme ymers rs tha thatt mak makee the them m dif difficu ficult lt to pro proces cess. s. Also, starch and synthetic polymers are generally thermodynamically dissimilar in nature, and hence are incompatible unless a compatibilizer is used. In REX of crosslinking dissimilar polymers, free-radical initiation plays a predominant role. However in the extruder, mechanochemistry, by itself, is not powerful enough for free-radical generation in such grafting reactions. Hence, in this study, we have attempted to graft MA to a biodegradable polyester in presence of a free-radical initiator during stage-one of extrusion, and further crosslinked MA-grafted polyester to starch during a second extrusion stage. We used different temperature profiles during the REX process. Should the compatibilized blendss exhib blend exhibit it impro improved ved mechan mechanical ical prop properties, erties, determination tio n of int interf erfacia aciall tens tension ion and con contro trollin lling g the int interf erfacia aciall properties of such blends will lead us on the path to developing well-controlled biodegradable blends with tailorable mechanical properties.

MATERIALS EnPol®, a bio biodeg degrad radabl ablee ther thermop moplast lastic ic pol polyes yester ter,, was obtained from IRe Chemical, Korea. Two types of starch with different levels of amylopectin-to-amylose ratios were used in a preset blend ratio in all formulations. The first was a low-amylose common waxy wheat starch and the other a high-a hig h-amyl mylose ose,, che chemica mically lly-mo -modifi dified ed maiz maizee sta starch rch (hy (hy-droxypropylated starch). The second has a lower gelatinization temperature than the first and is able to withstand higher processing temperatures. Both starches were sourced from Penford (Australia). These starches were blended together with plasticizers in accordance with the patents [20, 21] held by the Cooperative Research Centre for International tio nal Foo Food d Man Manufa ufactu cture re and Pack Packagi aging ng Scie Science nce and a POLYMER ENGINEERING AND SCIENCE—2006

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TABLE 1. 1.

Blends prepared prepared using using twin-screw twin-screw extrud extruder. er.

TABLE TABL E 2. extrusion.

S (wt%)

PEst (wt%)

MA (wt%)

DCP (wt%)

40 40 40 40 40 40 40 40 40 40 40 40 40 40 40

60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

0.0 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.5 1.5 1.5

0.0 0.3 0.5 0.8 1.2 0.0 0.3 0.5 0.8 1.2 0.0 0.3 0.5 0.8 1.2

spin-off spin-o ff com compan pany, y, Plan Plantic™ tic™ Tec Techno hnolog logies, ies, Mel Melbou bourne rne,, Australia. Maleic anhydride (MA) (98.06%), obtained from ICN Biomedicals, USA, was used as a crosslinking agent, and Dicumyl peroxide (DCP, 99%), obtained from Sigma– Aldrich, was used as a free-radical initiator. Inert nitrogen gas was used to prevent the effect of moisture on grafting during the first stage of extrusion. Blend Preparation

A batch of Starch Patent Formulation (SPF) was prepared using a granulator. The composition of SPF to EnPol wass mai wa maint ntain ained ed at 40 40:6 :60 0 (w (wt%) t%) in all th thee bl blen ends ds.. Th Thee different blends prepared using a laboratory scale PRISM corotating twin-screw extruder (length to diameter ratio of 40:1 and screw diameter of 16 mm) are shown in Table 1.

Temp Te mper erat atur uree (° (°C) C) pr profi ofiles les fo forr st stag agee-1 1 an and d st stag agee-2 2 of

Die Zone9 Zone8 Zone7 Zone6 Zone5 Zone6 Zone5 Zone4 Zone3 Zone2 Zone1

Stage-1

Stage-2

100 120 160 180 180 180 180 180 180 180 150 120

100 120 160 180 180 180 180 180 180 180 150 120

speed spee d of 45 rp rpm m wa wass us used ed he here re.. A co cont ntin inuo uous us flo flow w of nitrogen was maintained with the help of an inlet device duri du ring ng th thee fir first st sta stage ge of ex extr trus usio ion. n. Th Thee to torq rque ue an and d di diee pressure were monitored using a torque-meter and pressuretransducer, respectively. The temperature between mixing and transportation zones was maintained at 180°C to facilitate perox peroxide-in ide-initiated itiated free-r free-radical adical genera generation. tion. The extrudate obtained from the first stage of extrusion was pelletized using a pelletizer and stored in a humidi humidifier fier at 60% RH until further furth er use. The sec second ond ste step p was com compou poundi nding ng of the pel pelleti letized zed MA-grafted polyester with SPF. Here, a screw speed of 70 rpm and a feed rate of 0.45 kg/h were used. The temperature between mixing and transportation zones was maintained at 145°C. The extruded strands were stored in a humidifier at 60% RH.

ANALYSIS Grafting Procedure

The grafting reactions were carried out in a laboratory scale PRISM co-rotating twin-screw extruder with a barrel length to diameter ratio of 40:1, a screw diameter of 16 mm and eight heating zones. The composition of SPF and the polyester was maintained at 40:60 (wt%) in all the blends. The first step was the preparation of maleated polyester. From initial trials we found that the color of the extrudate varied as the composition of MA was increased (0.5% MA, white; 1% MA, light-pink; 1.5% MA, dark-pink; 2% MA, brown; 3% MA, brownish-black; 5% MA, black). Maleated polyesters with MA concentration of more than 2% were found to be unsuitable for compounding with starch. Therefore, after optimizing the process, three different compositions of MA (0.5, 1, 1.5%) and four different compositions of DCP (0.3, 0.5, 0.8, 1.2%) were selected. The temperature profiles for stage-1 and stage-2 of extrusion are shown in Table 2. EnPol was dried under vacuum for 24 h prior to the day of extrusion. MA and DCP were used in their powder form. A mixture of polyester, MA, and DCP was introduced using a mechanical feeder at a feed rate of 0.38 kg/h. Screw 250


Tensile Testing

The strands of different blends were subjected to tensile testing using an Instron Universal tensile testing machine (model 5584). A 5 KN load cell was used for determining important properties such as stress at break, strain at break, Young’s modulus, and stress and strain at maximum load. A grip separation of 50 mm, crosshead speed of 5 mm/min, and a sensitivity factor of 20% were adopted. All testings were done according to the procedure outlined in ASTM test method D-638. Dynamic Mechanical and Thermal Analysis

A Dynamic Mechanical and Thermal Analysis (DMTA) instrument (model IV) from Rheometric Scientific, Piscataway, NJ was used to study the effect of crosslinking and phase separation on the thermal properties of starch–EnPol blends ble nds.. A tem temper peratu ature re swe sweep ep test was con conduc ducted ted wit with h a temperature range of 30–85°C, a frequency of 1 Hz, soak time of 10 sec, and a fixed strain of 0.1%. The extruded DOI 10.1002/pen

samples were sample-pressed to a size of 100  10  1 mm3 and subjected to DMTA. X-ray Photoelectron Spectroscopy

Compression-molded films (thickness 1 mm) of six different blends were subjected to X-ray photoelectron spectroscopy (XPS) using a PHI model 560 XPS/SAM/SIMS I multite mul titechn chniqu iquee sur surface face ana analys lysis is sys system tem inc incorp orpora oratin ting g a model 25–270 AR electron energy analyzer. Mg K  X-rays (1253.6 eV) generated using 400 W (15 kV, 27 mA) was used to prod produce uce photoelectrons photoelectrons from the sample surf surface. ace. Survey (wide) scans, at analyzer pass energy of 100 eV, over 1000 eV were taken, followed by multiplex (narrow) scan sc anss of C 1s 1s,, O 1s 1s,, an and d Si 2p energ energy y le leve vels ls at hi high gh resolution using a pass energy of 25 eV. Curve fittings of C (1s) and O (1s) were carved out using Resident PHI V6.0 curve-fitting software to establish the relative percentage of different functional groups.

pus BH-2 model and a lens of the type Splan 10PL were used. A scale of 10 m was adopted for all the samples. The blends were pressed into films of thickness of 10  m using a sample press device. The thin sections were stained with iodine/KI solution before obtaining images under the microscope at a magnification of 10. This method was used to dif differ ferenti entiate ate star starch ch fro from m EnP EnPol ol in the opt optical ical micr microographs.

CROSSLINKING MECHANISM Before examining the study’s main results, it is instructive to discuss hypothesized reaction mechanism. The proposed crosslinking mechanism for our starch-polyester system is described by the scheme below. Consider the structure of EnPol.

Differential Scanning Calorimetry

(1)

A TA Instrument modulated DSC (TA2920) was used to dete de term rmin inee th thee th ther erma mall tr tran ansi sitio tions ns of sta starc rch– h–po poly lyes ester ter blends. The sample size was 10–15 mg, with heating and cooling rates of 10 and 20°C min 1, respectively.

Consider b  4. Th Ther eree ar aree th thre reee st step epss in invo volv lved ed in th thee crosslinking of starch to the polyester. They are initiation, propagation, and termination.

Optical Microscopy (OM)

Initiation

Analysis of the morphology of the blends was performed using an optical microscope. An optical microscope Olym-

Step 1. Formation of EnPol microradical

(2) Step 3. The EnPol microradical microradical unde undergoes rgoes -scission under high temperature conditions ( 150°C).

(3) DOI 10.1002/pen


251

Step 4. RO could further react with {2} to form another microradical.

(4)

Propagation

Step 5. The free radical associated chain attaches itself to MA (double-bond cleavage)

(5)

Step 6. Microradical {3} formed in step 4 could participate in the grafting process, resulting in the formation of a stable complex shown below.

(6)

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DOI 10.1002/pen

Step 7. During Durin g stage 2 of extru extrusion sion where starch is compo compounded unded with MA-grafted MA-grafted EnPol, C-6 atom of starch [12] reacts with the anhydride group of MA to form starch–MA–polyester complex.

(7)

The following starch–MA–polyester complex can also be expected.

DOI 10.1002/pen


253

(8)

Termination

Step 8. The reaction could be terminated in two ways; combination (coupling) or disproportionation. The possible termination steps are given below. Scheme 1(a) of Coupling

(9) Scheme 1(b) of Coupling:

(10)

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DOI 10.1002/pen

Scheme 2(a) of Disproportionation:

(11) Scheme 2(b) of Disproportionation:

(12) It has been found that polystyrene terminates predominantly by combin combination, ation, whereas poly( poly(methyl methyl methacrylate) methacrylate) terminates by disproportionation at polymerization temperatures higher than 60°C, and partly by each mechanism at lower temperatures [22]. Major studies on termination reaction schemes have been completed with linear-chain polymers, but no published research is available on the termination mechanism of starch–polyester crosslinking.

RESULTS RESUL TS AND DISC DISCUSSIO USSION N Twin Screw Extrusion

The twin-scre twin-screw w extr extrude uderr her heree was used as a rea reactiv ctivee extruder to combine peroxide-initiated grafting reaction and conven con ventio tional nal ext extrus rusion ion into a sin single gle pro proces cess. s. The scr screw ew configuration, torque, rotational speed, and mass flow rate are the impor important tant terms in determ determining ining Specific Mechanical Mechanical Energy (SME), which is given as SME



M d / m.

(13)

Where, SME  Specific mechanical energy (J/kg), Md  Torque (Nm)   Rotational speed of screw (s1), m  Mass flow rate (kg/s). The greater the torque in the extruder the higher the bulk viscosity of the system. Thus, for crosslinked systems, the SME required should be more. The calculated SMEs for different blends are given in Table 3. During blending of starch with polyesters in presence of a crosslinking crosslinkin g agent and initiator, the anhydrid anhydridee functional group DOI 10.1002/pen

could react with O OH OH group of starch to form ester linkages [23]. Hence, in this reactive extrusion (REX), inter-crosslinked polymer chains are expected, and as a result, the torque generated and hence the SME of a compatibilized blend should be higher than the SME of uncompatibilized blends. It is evident from Table 3 that the SME of each compatibilized blend is higher hig her tha than n th thee SM SME E of the un uncom compat patibi ibiliz lized ed ble blend nd (4 (40S 0S,, 60PEst). It can be observed that the SME variation in blends containing 1% MA is greater than in other blends. In case of blends containing 0.5% and 1.5% MA, the variation in SME was negligible. This indicates that blends containing 1% MA had hig higher her mic micro rorad radica icall gen genera eratio tion n and bet better ter cro crossl sslink inking ing tha than n in other blends. This could have increased the bulk viscosity and SME of system.

TABLE 3. 3.

Specific mechanical mechanical energy energy of differ different ent blends. blends.

BLEND 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S,

SME (kJ/kg)

60PEst 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst,

0.5MA 0.5MA, 0.3DCP 0.5MA, 0.5DCP 0.5MA, 0.8DCP 0.5MA, 1.2DCP 1MA 1MA, 0.3DCP 1MA, 0.5DCP 1MA, 0.8DCP 1MA, 1.2DCP 1.5MA 1.5MA, 0.3DCP 1.5MA, 0.5DCP 1.5MA, 0.8DCP 1.5MA, 1.2DCP

152.98 198.53 535.23 560.42 579.31 540.81 214.35 271.66 353.23 413.23 598.97 348.94 453.37 454.46 452.53 458.9


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TABLE 4. 4.

Blend 40S, 60PEst 40S, 60PEst, 0.5MA, 0DCP 40S, 60PEst, 0.5MA, 0.3DCP 40S, 60PEst, 0.5MA, 0.5DCP 40S, 60PEst, 0.5MA, 0.8DCP 40S, 60PEst, 0.5MA, 1.2DCP 40S, 60PEst, 1MA 40S, 60PEst, 1MA, 0.3DCP 40S, 60PEst, 1MA, 0.5DCP 40S, 60PEst, 1MA, 0.8DCP 40S, 60PEst, 1MA, 1.2DCP 40S, 60PEst, 1.5MA 40S,60PEst, 1.5MA, 0.3DCP 40S, 60PEst, 1.5MA, 0.5DCP 40S, 60PEst, 1.5MA, 0.8DCP 40S, 60PEst, 1.5MA, 1.2DCP

Tensilee properties Tensil properties of differen differentt blends.

Tensile stress at maximum load (Mpa)

Strain at break (%)

Young’s modulus (5% Tangent) (MPa)

14.68  0.95 15.70  1.22 20.11  0.9 19.89  1.78 18.63  1.75 17.55  1.55 12.54  4.42 10.5  3.33 15.71  0.96 18.92  1.04 16.05  1.71 17.61  0.54 19.8  2.78 18.27  1.56 19.35  1.61 18.98  1.58

78.68  9.58 80.82  4.38 75.18  8.74 78.68  11.52 92.38  8.65 50.24  10.19 31.06  2.42 150.12  15.15 111.24  12.98 257.35  37.69 165.73  38.61 35.34  3.62 40.86  4.81 36.06  5.51 44.08  7.85 43.02  4.96

36.77  6.02 38.57  12.48 86.87  13.22 79.87  1.10 62.87  15.55 48.38  19.83 51.74  21.79 30.88  9.94 43.07  9.09 51.59  6.89 44.07  5.88 82.64  20.33 88.31  11.63 80.86  5.18 74.78  27.36 79.99  20.28

Tensilee Testing Analysis Tensil

Three important parameters were considered for tensile testing test ing ana analys lysis, is, nam namely ely ten tensile sile str stress ess at max maximu imum m loa load, d, stress str ess at bre break, ak, and You Young’ ng’ss mod modulu uluss (ta (tange ngent nt 5%) 5%).. The effect of the concentration of MA and DCP on these parameters was determined and is shown in Table 4. The blends at 0.5% MA exhibited better tensile properties in terms of Young’s modulus and stress at break, than the unmodified blend, but showed little or no improvement in elongation at break. The blends with 1% MA had moderately higher Young’s modulus and stress at break values than the unmodified blend. They also showed much higher elonga elo ngatio tion n at bre break ak tha than n the unc uncomp ompatib atibiliz ilized ed blen blend d and indeed ind eed the oth other er rea reactiv ctively ely extr extrude uded d ble blends nds.. All ble blends nds containing 1.5% MA had lower elongation at break than the uncompatibilized blend. The Young’s modulus and stress at break were increased over the uncompatibilized system. In comparing between the compatibilized blends, blends containing 0.5% MA and 1.5% MA had higher Young’s modulus ul us th than an th thee bl blen ends ds co cont ntain ainin ing g 1% MA MA.. Ho Howe weve verr th thee blends at 1% MA had greatly improved elongation at break values than 0.5% and 1.5% MA blends. Mani et al. [23] observed that starch blends that encountered ter ed hig higher her spe specific cific mech mechani anical cal ene energy rgy had hig high h ten tensile sile strength. Our results indicated that only few starch blends with higher specific mechanical energy had better tensile streng str ength. th. It is hen hence ce imp import ortant ant to con consid sider er the eff effects ects of interfacial properties and maleation on the tensile properties of starch–polyester blends. Bhatt Bh attac acha hary ryaa et al. [2 [24] 4] no noti ticed ced th that at th thee ad addi ditio tion n of compatibilizers (styrene MA copolymer and ethylene–propylene–g–maleic anhydride copolymer) had a profound effect on the tensile properties of the starch blend (60% starch, 40% compatibilizer). However, those blends exhibited poor elongation at break. Avella et al. [25] have shown that an increase increa se in the composition of starch and preco precompatib mpatibilizer ilizer decreases both tensile strength and elongation at break, but 256


increases Young’s modulus. In synthetic polymer blends, the addition of a second phase to the polymer matrix usually diminishes the elongation properties at break [26] and in many cases, when 20% of the dispersed minor phase has been added highly deformable matrix materials are transformed into brittle materials [27]. The elongation at break in synthe syn thetic tic pol polyme ymerr ble blends nds is ther therefo efore re con consid sidere ered d to be highly sensitive to the state of the interface. However, we observ obs erved ed a not notabl ablee inc increa rease se in elo elonga ngatio tion n at bre break ak and overall decrease in Young’s modulus in blends containing 1% MA. Krishnan and Narayan [28] have described in their patent that the hydroxyl groups of plasticizers and starch molecules molecu les could interact with compat compatibilizer ibilizers, s, promo promoting ting interfacial interf acial adhesion. They also indicate an acute possib possibility ility of plasticizers acting as stretching agents. If it is assumed that maleation did not occur, then starch plasticizers should have increased elongation at break and interfacial adhesion in al alll bl blen ends ds,, ir irre resp spec ecti tive ve of th thee co comp mpos osit itio ion n of th thee crosslinkin cross linking g agent and the initiator. The obser observation vation that only few maleated starch blends showed improved mechanical properties indicates the importance of optimizing the composition of the crosslinking agent and the initiator. In particular, the blend (40S, 60PEst, 1MA, 0.8DCP) showed the hig highes hestt elo elonga ngatio tion n at bre break ak (25 (257%) 7%).. Thi Thiss cou could ld hav havee resu re sult lted ed fr from om th thee st stro rong ng in inte terf rfac acia iall ad adhe hesi sion on du duee to crosslinking of the two phases, and the greater ability of this interface to withstand higher extension to break. However, blends containing 1.5% MA had relatively higher Young’s modulu mod uluss and low lower er elo elonga ngation tion at bre break. ak. Ide Ideally ally an opt optiimized compatibilized blend here is a compromise between desired mechanical properties and starch composition.

Dynamic Mechanical and Thermal Analysis

Dynamic Mec Dynamic Mechan hanical ical and The Therma rmall Ana Analys lysis is (DM (DMTA) TA) experiments are used to investigate the mechanical behavior of materials, and to obtain information about the relaxation DOI 10.1002/pen

FIG. FI G. 1.

Grap Gr aph h o of f E E  and E  Vs temperature for uncompatibilized blend.

mechanisms that may be correlated with the dynamics and the microstructure of the material [29]. The effect of temperature on storage modulus ( E ), loss modulus ( E ), and loss factor (tan ) at a fixed strain rate (0.1%) was studied here. The storage modulus E  is related with the mechanical energy stored during each load cycle and per unit volume. Loss modulus E  signifies the dissipation of energy as heat during the deformation. The loss factor tan  is equal to E  / E  and is thus sensitive to balance of the dissipated and stored energy of the system and is useful to detect thermomechanical relaxations [30]. uncomp ompatib atibiliz ilized ed blen blend d (40 (40S, S, 60P 60PEst) Est) was E  for the unc found to decrease gradually with increase in temperature, indicating a stiffness loss. Here, a definite glass transition was observed for E  and tan  curves between 35 and 50°C, which should be due to starch. The graphs for E  and tan  are shown in Figure 1. Analysis of compatibilized blends

Graphs of storage modul Graphs modulus us versu versuss temper temperature ature of differ differ-ent blends containing 0.5, 1, 1.5% MA, and various compositions of DCP are shown in Figures 2–4. Although there appears to be no specific trend, the blend (40S, 60PEst, 0.5MA, 0.8DCP) exhibited the highest storage modulus (4.64  108Pa) at 30°C, followed by blends containing 0.5, 0.3, and 1.2% DCP (Fig. 2). It was also DOI 10.1002/pen

FIG. 2. Gra FIG. Graph ph of E E  and tan  Vs temperature for blends containing 0.5% MA { (40S, (40S, 60P 60PEst Est,, 0.5 0.5MA, MA, 0.3 0.3DCP DCP); ); ‚ (40S, (40S, 60P 60PEst Est,, 0.5 0.5MA, MA, 0.5DCP);  (40S, 60PEst, 0.5MA, 0.8DCP);  (40S, 60PEst, 0.5MA 0.5MA,, 1.2DCP)}.

observed that at higher temperatures (80°C), blends containing higher compositions of DCP lost their stiffness more quickly than the blends containing lower compositions of DCP. For the blend (40S, 60PEst, 0.5MA, 0.3DCP), two distinct peaks were observed at 55 and 68°C. The first peak at 55 55°C °C is du duee to th thee ad addi ditio tion n of th thee in inte terf rfac acee mo modi difie fierr (compatibilizer). The other peak at 68°C could be due to the molecular motions within the starch phase. At higher concentrations of DCP, these peaks were found to disappear. When compared with the transition peak in the uncompatibilized blend, it is found that with the addition of compatibilizer the transition peak in compatibilized blends shifted towards higher temperatures. It can be observed from Figure 3 that in all the blends containing 1% MA, a drastic decrease in storage modulus ( E ) is observed. This indicates that as the concentration of MA in the blend is increased from 0.5 to 1% 1%,, th ther eree is a sh shar arp p de decr crea ease se in st stif iffn fness ess.. Th This is is in agreement with the tensile testing results. In all the blends containing 1% MA, a transition peak is observed between 70°C 70 °C an and d 85 85°C °C,, wh which ich is a sh shif iftt by ab abou outt 10 10°C °C wh when en comp co mpar ared ed wit with h th thee tr tran ansi sitio tion n pe peak ak in th thee bl blen end d (4 (40S 0S,, 60PE 60 PEst st,, 0.5 .5M MA, 0. 0.3D 3DCP CP). ). Th This is in indi dica cate tess th that at th thee POLYMER ENGINEERING AND SCIENCE—2006

257

FIG. 3. Gr FIG. Grap aph h of of E temperature ature for blends containing containing 1% E  and tan  Vs temper MA. {— (40S, 60PEst, 1MA);  (40S, 60PEst, 1MA, 0.3DCP 0.3DCP); ); ‚ (40S, 60PEst, 1MA, 0.5DCP);  (40S, 60PEst, 1MA, 0.8DCP);  (40S, 60PEst, 1MA, 1.2DCP)}.

crosslinked points could have been obstructing the conformationa mati onall mob mobilit ility y of the seg segmen ments ts of EnP EnPol, ol, res result ulting ing in further shift of the transition peak. A series of transitions observed in the blends containing 0.3, 0.5, and 0.8% DCP could be due to the side groups and segmental motions of the cross crosslinked linked chains. It is noticeable that all the blends containing 1.5% MA have storage modulus between that of blends containing 1 and 0.5% MA. These blends are found to exhibit similar tensile properties to the blends containing 0.5% MA. Most of these blends exhibit a transition between 50 and 60°C, which wh ich co coul uld d ag again ain be du duee to th thee pr pres esen ence ce of in inter terfa face ce modifier. These blends also showed transition peak between 60 and 70°C, signifying the molecular motion within starch. X-ray Photoelectron Spectroscopy

Although X-ray Photoelectron Spectroscopy (XPS) is a surface analysis technique, it still gives information about the structure of crosslinked polymers. Compression molded films of six blends [(40S, 60PEst), (40S, 60PEst, 0.5MA, 0.5D 0. 5DCP CP), ), (4 (40S 0S,, 60 60PE PEst st,, 1M 1MA) A),, (4 (40S 0S,, 60 60PE PEst, st, 1M 1MA, A, 0.8DCP 0.8 DCP), ), (40 (40S, S, 60P 60PEst, Est, 1.5 1.5MA) MA),, (40 (40S, S, 60P 60PEst, Est, 1.5 1.5MA, MA, 258


FIG. 4. Gra FIG. Graph ph of E E  and tan  Vs temperature for blends containing 1.5% MA. {— (40S, 60PEst, 1.5MA); 224 (40S, 60PEst, 1.5MA, 0.3DCP);  (40S, 60PEst, 1.5MA, 0.5DCP);  (40S, 60PEst, 1.5MA, 0.8DCP);  (40S, 60PEst, 1.5MA, 1.2DCP)}.

0.5DCP)] were washed using hexane to remov 0.5DCP)] removee the surface impuri imp urities ties and the then n exa examin mined ed wit with h XPS XPS.. In the “su “surve rvey y scans” of all the graphs, Si (2p) and Si (2s) peaks were observed in the range of 102 and 106 eV. Peaks at binding energies of 102 and 103.4 eV correspond to Si and methyl (CH3) gro group, up, respectiv respectively. ely. Hence, Hence, it is assu assumed med that the active peaks between 102 and 106 eV were the presence of poly(dimeth poly( dimethylsilo ylsiloxane) xane) [PDMS], which could have been present in the polyester in the form of crystal impurity. In the survey scan, a graph of electron count Vs binding energy (eV)) was plotted. (eV plotted. In all the gra graphs phs,, dis distinc tinctt pea peaks ks cor correresponding to C (1s) and O (1s) were observed in the range 250–300 eV and 500–550 eV, respectively. Then a high resolution scan (called multiplex) of the C (1s) and O (1s) energy levels of the mentioned blends was carried out at a pass energy of 25 eV. It was observed that the ratio of C/O of the blends was approximately 3. According to the proposed structure, the O OH OH group of C-6 of starch attaches itself to the C A O group of MA, resulting in the formation of a carboxylic group ( O COOH). COOH). We determined the compo compositions sitions of O CO C O and O COO COO groups groups in different differ ent blend blendss using curve-fitting curve-fitting softw software are “Resid “Resident ent PHI DOI 10.1002/pen

FIG. 5. Hig FIG. High h res resolu olutio tion n scan scanss of (a) unc uncomp ompatib atibiliz ilized ed blen blend; d; (b) 40S, 60PEst, 60PEst, 0.5 0.5MA, MA, 0.5DCP; 0.5DCP; (c) 40S, 60PEst, 1MA; (d) 40S, 60PEst, 1MA, 0.8DCP; (e) 40S, 60PEst, 1.5MA; (f) 40S, 60PEst, 1.5MA, 0.5DCP.

V6.0”. All the data was charge-corrected at 101.8 eV using Si (2p) as reference for PDMS. The results of the curvefitting are shown in Figure 5. Thee cu Th curv rvee fit fittin ting g re resu sults lts of C (1 (1s) s) en ener ergy gy le leve vels ls of different blends are shown in Table 5. Forr si Fo six x di diff ffer eren entt bl blen ends ds,, th thee fa facto ctorr A1 /A2 (Area (Area of O COO/Area COO/Area of O CO) CO) was determined based on the area of respective peaks in the curve-fitted graphs. It is to be noted

TABLE 5. Blend 40S, 40S, 40S, 40S, 40S, 40S,

60PEst 60PEst, 60PEst, 60PEst, 60PEst, 60PEst,

0.5MA, 0.5DCP 1MA 1MA, 0.8DCP 1.5MA 1.5MA, 0.8DCP

DOI 10.1002/pen

here that XPS does not detect H. It was observed that the composition of O COO COO groups in the blend (40S, 60PEst, 1MA, 0.8DCP) was higher than the composition of O COO COO groups in other blends. The O COO COO peak in the curve-fitted graph is due to the presence of O COO COO groups of EnPol and O COOH COOH gro groups ups,, whi which ch are for formed med due to cro crossl sslink inking ing (referr to mechan (refe mechanism). ism). As crossl crosslinking inking increases, increases, the percentage of O COOH COOH groups in the compa compatibilize tibilized d blend blendss

Curve-fitting Curvefitting results results of C (1s) energy energy levels of different different blends. blends. Area of O COO COO groups ( A1)

Area of O CO CO groups ( A2)

A1/ A2

983 880 1246 1362 872 789

1372 1074 1508 1125 1354 977

0.72 0.82 0.83 1.21 0.64 0.81


259

TABLE 6. The TABLE Therma rmall tran transiti sitions ons for star starch/p ch/poly olyeste esterr unc uncomp ompatib atibiliz ilized ed and compatibilized blends. Blends 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S, 40S,

60PEst 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst, 60PEst,

0.5MA, 0.3DCP 0.5MA, 0.5DCP 0.5MA, 0.8DCP 0.5MA, 1.2DCP 1MA 1MA, 0.3DCP 1MA, 0.5DCP 1MA, 0.8DCP 1MA, 1.2DCP 1.5MA 1.5MA, 0.3DCP 1.5MA, 0.5DCP 1.5MA, 0.8DCP 1.5MA, 1.2DCP

Endotherm T (° (°C) C)

Exot Ex othe herm rm T (°C)

98.8 99.1 98.4 98.4 97.1 99.9 98.5 98.2 97.5 97.5 99.0 98.9 98.1 98.1 98.2

63.8 49.6 45.9 51.6 52.6 48.4 50.7 48.4 59.2 64.8 50.2 55.9 45.3 44.8 45.1

also increases, which is due to the transfer of O OH OH group of C-6 of starch to the C A O group of MA to form O COOH COOH group. The number of C A O groups in the compatibilized blendss varies with the degre blend degreee of crosslinking. crosslinking. For examp example, le, even in compa compatibilize tibilized d blend blends, s, the prese presence nce of unrea unreacted cted MA increases increases the num number ber of C A O gro groups ups.. Als Also, o, in the blends that did not contain DCP, XPS detected more C A O groups, indicating that there is a possibility of MA remaining in unreacted state or partially reacted state. Differential Scanning Calorimetry

Differential scanning calorimetry was used to evaluate the thermal transition of the blends. To eliminate thermal history, the samples were equilibrated at 30°C, heated to 220°C at 10°C min1, cooled to 70°C at 20°C min1, maintain main tained ed und under er iso isothe therma rmall con conditi ditions ons for 5 min min,, and 1 1 heated to 220°C min at 10°C min . Thermal transitions such as melting and crystallization are of high importance in polymer processing techniques. For the base blend containing 60% polyester and 40% starch based plastic, the melting endotherm occurred at 98.8°C, while the crystallization exotherm occurred at 63.8°C (Ta-

FIG. 6. FIG. 1.5MA.

260

ble 6). For compatibilized starch–polyester blends, no significant change in the endotherm (melting) temperature was observed, but the exotherm (crystallization) temperatures of all the compatibilized blends were lower than the crystallization zati on tem temper peratu ature re of the base ble blend nd (re (refer fer to Tab Table le 6). Hence, crosslinking crosslinking agents do have significant significant effect on the crystallization temperature of starch–polyester blends. Because of -scission of the polyester the chains could form crosslinked regions, resulting in the restriction of polymer chain mobility, which could significantly reduce the degree of crystallinity. However, it is interesting to note that the crystallization temperatures of the two blends at 1.0% MA addition {(40S, 60PEst, 1MA, 0.8DCP) and (40S, 60PEst, 1MA, 1.2DCP)} were higher than the crystallization temperatures of other compatibilized blends. In fact, the greatest increase in exotherm temperature was found in (40S, 60PEst, 1MA, 1.2DCP). The presence of optimum concentration of the compatibilizer could have prevented appreciable mic micelle elle for formati mation on and hen hence ce red reduce uced d the int interf erfacia aciall energy of the blend. As a result, the degree of crystallization could have increased in both phases and at the interface, further leading to an increase in the number of nucleation sites. In the two blends mentioned earlier, the compositions of MA and DCP could hence be in the proximity of their respective optimum concentrations. The crystallization temperatures of other compatibilized blends were low, indicating that the compositions of the crosslinking agent and the initiator in these blends are not optimized. It could also be observed that the crystallization temperature for the uncompatibilized blend (40S, 60PEst) is quite high (63.8°C), and yet the ble blend nd exh exhibit ibited ed poo poorr mech mechanic anical al pro proper perties ties.. The absence of crosslinking agent could promote micelle formation at the interface of the hydrophobic polyester and the hydrophilic starch, thus contributing towards poor mechanical properti properties. es. At the same time time,, the two pha phases ses could crystallize in their domains, resulting in an increase in the crystallization temperature of the blend. Optical Microscopy

The optical micrographs of the uncompatibilized blend and blends that did not contain DCP are shown in Figure 6.

Optica Opt icall mic microg rograp raphs hs of (a) unc uncomp ompatib atibili ilized zed blend; blend; (b) 40S, 60PEst, 60PEst, 1MA ; and (c) 40S 40S,, 60P 60PEst Est,,


DOI 10.1002/pen

FIG. 7. Opt FIG. Optical ical microgr micrograph aphss of (a) 40S 40S,, 60P 60PEst Est,, 0.5 0.5MA, MA, 0.3DCP; 0.3DCP; (b) 40S, 60PEst, 0.5MA, 0.5DCP; (c) 40S, 60PEst, 0.5MA, 0.8DCP; and (d) 40S, 60PEst, 0.5MA, 1.2DCP.

In the uncompatibilized blend (Fig. 6a), due to interfacial tensio ten sion n betw between een the sta starch rch and the pol polyes yester ter pha phases ses,, the starch phase is not homogeneously distributed in the polymer matrix. In the blend {40S, 60PEst, 1MA}, the starch phase could have remained as a separate phase (Fig. 6b), whereas whe reas in the oth other er ble blend nd {40 {40S, S, 60P 60PEst, Est, 1.5MA}, 1.5MA}, the starch phases are finer than the phases of the former blend. This could be due to hydrolysis of starch by MA that is present in excess concentration, leading to the break down of the starch phase (Fig. 6c).

FIG. 8. Optica Opticall micrographs micrographs of (a) (a) 40S, 60PEst, 60PEst, 1MA, 0.3DCP; 0.3DCP; (b) (b) 40S, 60PEst 60P Est,, 1MA 1MA,, 0.5D 0.5DCP; CP; (c) 40S, 60P 60PEst Est,, 1MA 1MA,, 0.8 0.8DCP DCP;; and (d) 40S 40S,, 60PEst,, 1MA, 1.2DCP. 60PEst

Analysis of Blends Containing 1.5% MA. The micrographs of the blends containing 1.5% MA and different compositions of DCP are shown in Figure 9. It can be observed from micrographs of blends containing 1.5% MA and different compositions of DCP that starch phases are concentrated in certain regions of the polymer matrix, and that the cocontinuous phases exhibited at 1% MA, have now been lost. In these blends with higher com-

Analysis of Blends Containing 0.5% MA. The optical micrographs of the blends containing 0.5% MA and different compositions of DCP are shown in Figure 7. It could be observed in the micrographs (Fig. 7) that the starch phase is homogeneously distributed and has formed a cocontinuous phase with the polyester phase. These blends exhibited better mechanical properties than the uncompatibilized blend. Analysis of Blends Containing 1% MA. The optical micrographs of blends containing 1% MA and different compositions of DCP are shown in Figure 8. It can be observed that in the blends with higher DCP content ((40S, 60PEst, 1MA, 0.8DCP) and (40S, 60PEst, 1MA,, 1.2 1MA 1.2DCP DCP)) )) (Fi (Figur guree 8c and d), star starch ch is eve evenly nly dispersed throughout the polymer matrix. This suggests optimum compositions of MA and DCP helps generate polyester es ter mic micro rora radi dical cals, s, pr prom omot otin ing g ef effic ficien ientt cr cros ossl slin inki king ng whereupon more and more starch reacts with the compatibilizer.. This reduces the interf bilizer interfacial acial tension between the two dissimilar phases and promotes adhesion. DOI 10.1002/pen

FIG. 9. Optica Opticall micrographs micrographs of (a) 40S, 40S, 60PEst, 60PEst, 1.5MA, 0.3DCP; 0.3DCP; 40S, 60PEst, 1.5MA, 0.5DCP; (c) 40S, 60PEst, 1.5MA, 0.8DCP; and (d) 40S, 60PEst, 1.5MA, 1.2DCP.


261

position of DCP (Fig. 9b, c, and d), numerous microradicals generated gener ated could react among themselves (disp (dispropo roportionrtionation or coupling) and terminate the reaction. By this, the crossli cro sslinki nking ng of the two pha phases ses is hin hinder dered ed for forcin cing g som somee starch phase to remain in unreacted state.

DISCUSSION It is apparent that the mechanical and thermal properties of the compatibilized starch based polymer–polyester polymers are affected by the addition of the compatibilization system. In summary it appears that All blends (0.5, 1.0, and 1.5 wt% MA) showed an improv pr ovem emen entt in Yo Youn ung’ g’ss mo modu dulu luss an and d str stres esss at br brea eak. k. Blends with 1.0% MA showed larger improvements in elongation at break. ● DMTA studies revealed that the blend blendss conta containing ining 1% MA had lower stiffness ( E ) than blends containing 0.5% MA and 1.5% MA. ● XPS ana analys lysis is ind indica icated ted the pre presen sence ce of mor moree O COO COO groups in the blend with 1% MA, thus supporting the proposed propo sed compatibilized compatibilized struct structure ure for the blend blend.. ● The DSC results indicated that the crystallization temperatures were reduced with increasing compatibilizers because of an increased hindrance to crystallization. Crystallization temperatures of the two blends at 1.0% MA addition {(40S, 60PEst, 1MA, 0.8DCP) and (40S, 60PEst, 1MA, 1.2DCP)} had higher than the crystallization temperatures than that of other compatibilized blends. ● Optical micrographs micrographs of compa compatibiliz tibilized ed blend blendss with 1% MA revealed uniform distribution of starch in the polymer matrix, indicating compatibilization between starch and polyester phases ●

Thus optical, DSC, and XPS tests indicates that a reactively compatibilized compatibilized structure is optim optimized ized for the 1.0% MA samples. Interestingly the 1.0% MA samples showed lower elastic modulus or stiffness during DMTA tests, yet showed high young’s modulus, stress at break and elongation at break during tensile testing. This is possibly due to the fact the DMTA tests are linear deformation deformation tests and the tensile tests induce nonlinear deformation. That is, the linear DMTA tests determine a lower linear elastic modulus under small deformations, possibly due to the plasticizing addition of the MA to the system as previously noted [30], and the MA induced crosslinks have little effect. However, under nonlinear tensile testing the deformation is larger and more destructive, and the compatibilized network structure is able to reduce the breaking of tie layers commonly described in basic polymer fracture studies. It is also interesting that the Young’s modulus, stress at break and elongation at break all increase, increa se, unlike normal composite systems, which sacrifice elongation [extensibility] for increases in strength.

an improvement in Young’s modulus and stress at break. Blends Blen ds wit with h 1.0 1.0% % MA sho showed wed larg larger er imp improv roveme ements nts in elongation elong ation at break break.. DMTA studies revealed that the blend blendss containing contai ning 1% MA had lower stiffness stiffness than blend blendss contai containning 0.5% MA and 1.5% MA. The compatibilized blends exhibit exh ibited ed sev severa erall tra transi nsition tion peaks due to the pre presen sence ce of interface modifier, molecular motions within starch phase, and side groups and segmental motions of the crosslinked chains. XPS analysis indicated the presence of more O COO COO grou gr oups ps in th thee co comp mpat atib ibili ilize zed d bl blen end d wi with th 1% MA MA,, th thus us supporting the proposed structure for the blend. The DSC results indicated that the crystallization temperatures were reduced with increasing compatibilizers because of an increased hindrance to crystallization. Crystallization temperature atu ress of th thee two bl blen ends ds at 1. 1.0% 0% MA ad addi ditio tion n {( {(40 40S, S, 60PEst, 60PEs t, 1MA, 0.8DCP) and (40S, 60PEst, 1MA, 1.2DC 1.2DCP)} P)} had higher crystallization crystallization temperatures than that of other compat com patibi ibilize lized d ble blends nds.. We bel believ ievee tha thatt the pre presen sence ce of optimum concentrations of MA and DCP in these blends reduce red ucess app apprec reciab iable le mice micelle lle for formati mation on and inc increa reases ses the number of nucleation sites resulting in an increase in crystallization temperature and interfacial adhesion. Optical micrographs of compatibilized blends with 1% MA revealed uniform distribution of starch in the polymer matrix, indicating cati ng com compat patibi ibiliza lizatio tion n bet betwee ween n star starch ch and pol polyes yester ter phases. In this study study,, compat compatibilizat ibilization ion of starch and biodeg biodegradradable polyester has been achieved using REX, and thus it offers a new direction for enhancing the properties of low cost-b cos t-base ase ren renewa ewable ble bio biodeg degrad radabl ablee pol polyme ymers. rs. Als Also, o, for compatibilized blends the study of interfacial tension (sub ject of future work) is a vital aspect with regards to t o improving the pro proper perties ties of bio biopol polyme ymers. rs. Thi Thiss abi ability lity to tail tailor or biodegradable polymer morphology and properties is crucial if low cost biodegradable polymers are ever to be fully optimized for appropriate performance properties.

REFERENCES 1. R. Cha Chandr ndraa and R. Rus Rustgi tgi,, Polym. Polym. Degra Degrad. d. Stab., 56 56,, 185 (1997). 2. W.M. Doane, Doane, J. Polym. Mater., 11, 229 (1994). 3. S.A. McGlashan and P.J. Halley, “Preparation and Characterisation of Biodegradable Starch-Based Nanocomposite Packaging agi ng Mat Materi erials, als,”” in Polymer Univ iver ersi sity ty of Polymer sympos symposium ium, Un Queensland (2001). 4. P.J. Frazier, Frazier, P. Richmond, and A.M. Donald, Donald, Starch Structure and Functionality, Royal Society of Chemistry, Cambrdige, UK (1997). 5. T. Gal Galliar liard, d, Starch: Wiley: ey: New Starch: Prope Properties rties and Poten Potential tial, Wil York (1987).

CONCLUSIONS

6. R.F. Tester, J.B. South, South, W.R. Morrison, Morrison, and R.P. Ellis, J. Cereal Sci., 13 13,, 113 (1991).

The main conclusions from this work are summarized below. First, all blends (0.5, 1.0, and 1.5 wt% MA) showed

7. R.F. Tester, W.R. Morrison, Morrison, R.P. Ellis, Ellis, J.R. Piggott, and G.R. Batts, J. Cereal Sci., 22 22,, 63 (1995).

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Compatibilization of Starch–Polyester Blends Using Reactive Extrusion

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