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Ind. Eng. Chem. Res. 2008, 47, 9896–9902
Structure and Properties of Tough Thermoplastic Starch Modified with Polyurethane Microparticles Qiangxian Wu,*,† Zhengshun Wu,† Huafeng Tian,‡ Yu Zhang,† and Shuilian Cai† Green Polymer Laboratory, College of Chemistry, Huazhong Normal UniVersity, Wuhan, China 430079 and Polymer Science Department, College of Chemistry, Wuhan UniVersity, Wuhan, China 430072
Aged thermoplastic starch (TPS), plasticized with hydrophilic plasticizers such as glycerol, shows brittle property. It is critical to develop tough TPS without using hydrophilic plasticizers. In this work, polyurethane prepolymer (PUP) was synthesized and mixed reactively with TPS in an intensive mixer to prepare modified TPS. Structural and morphological analyses showed that polyurethane (PU) microparticles were formed in situ and dynamically cross-linked to the starch matrix through urethane linkages. The modified TPS without hydrophilic plasticizer become tough. The elastic polyol soft segments in PU played the role of impact modifier, improving the toughness of the modified TPS. Almost 100% of PU was cross-linked to starch, indicating high efficiency of the modification. Formation of multifunctional PU microparticles was essential to achieve the high reaction efficiency. The dynamically cross-linking modification is a novel, green, and efficient method for preparing tough TPS. Introduction Starch, one of the main polysaccharides in the world, has been paid much attention because of its biodegradability and abundance as a renewable resource.1 It has been widely used as a raw material for biodegradable plastics.2 Generally, plastics prepared from starch containing low amounts of water are often brittle.3,4 To reduce the brittleness, starch was plasticized with hydrophilic plasticizers such as glycerol and melted for preparing thermoplastic starch (TPS).5 However, after aging at ambient conditions for several months, glycerol-plasticized TPS showed a brittle behavior due to migration of glycerol from starch matrix.6,7 The reason for easy migration of glycerol plasticizer is that it interacts with starch by noncovalent hydrogen bonding, resulting in phase separation at ambient conditions.8 One effective way of preventing migration of plasticizers is to graft or link flexible impact modifier to starch with covalent bonds which will result in modified starch with ductile property. Within the current available modifiers, isocyanate groups have high activity to react with the hydroxyl group of starch. Thus, polyurethane prepolymer (PUP) bearing isocyanate groups has often been used to toughen starch.9-13 Flexible polyol soft segments in polyurethane (PU) linked to starch through urethane linkage have functioned as impact modifier. However, most of these modifications were conducted in organic solvents,9-11 resulting in serious environmental pollution. Although some of the modifications were carried out in bulk12,13 in which starch granules were directly filled into PUP, the obtained material was thermoset. The problem in thermoset is that they cannot be melted or dissolved, thus limiting their applications. In summary, it is essential to modify starch using PUP in an environmentally friendly way for preparing ductile thermoplastic starch. In our recent patent work14 PUP was dynamically cross-linked to starch in a water system in an intensive mixer. The modified starch obviously became ductile without using hydrophilic plasticizers and was still a thermoplastic, showing its potential * To whom correspondence should be addressed. E-mail:
[email protected]. † Huazhong Normal University. ‡ Wuhan University.
in some applications. The structure of the novel material had not been determined, and the mechanism of forming the tough TPS should be investigated in a scientific way. In this work, PUP was synthesized from castor oil polyol, and then the PUP was reactively mixed with raw starch as well as water in an intensive mixer for preparing modified TPS. The modified TPS has been characterized. The aim of this work is to study the structure/properties relationship of the modified TPS and mechanism of forming the modified TPS. Castor oil polyol was used because that it falls in the category of renewable natural resource. Experimental Section Materials. Castor oil and 4,4′-methylenedi-p-phenyl diisocyanate (MDI, 98%) were purchased from Sigma-Aldrich Fine Chemicals (St. Louis, MO). Corn starch (amylose, 23-26 wt %; moisture, 12 wt %) was obtained from Wuhan Corn Starch Co. Ltd. (Wuhan, China) and used without any further pretreatment. Synthesis of PUP. The molar number ratio of isocyanate to hydroxyl group (NCO/OH) was 2.0. Castor oil (151.5 g) was charged into a 500 mL three-necked flask fitted with a stirrer operating at a speed of 300 rpm, an inlet, and an outlet. The system was dried in vacuum (2 mmHg) at 110 °C to remove the moisture in the castor oil. After 30 min the temperature of castor oil in the flask was decreased to 70 °C, and then MDI (109.2 g) was charged into the flask under nitrogen atmosphere. Translucent mixture in the flask quickly became clear. Fifteen minutes after addition of MDI the mixture was stirred vigorously and reacted at 87 °C for 1 h. Finally, clear and yellow PUP was obtained. The Brookfield viscosity of the PUP was 25200 cps at 60 °C with 100 rpm of spindle speed and a No. 29 spindle. Preparation of Modified TPS. Corn starch, PUP, and water (added water and the moisture content of starch) were charged into an intensive mixer (SU-70, Changzhou Suyan Science and Technology Co., Ltd. Changzhou City, Jiansu Province, China) and reactively mixed at 95 °C with a stirrer speed of 100 rpm. The weight content of each material is listed in Table 1. After 25 min a white modified thermoplastic starch was obtained. The modified starch was equilibrated in a sealed plastic bag for 1
10.1021/ie801005w CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9897 Table 1. Formulations of Starch Modification and Tensile Properties of Molded TPS Sheets properties of molded sheetsc
formulations for modification
c
samples
PU content (wt %)a
CS CP10 CP20 CP40
0 10 20 40
Anhydrous starch(g) 100 100 100 100
PUP (g)
Water plasticizer (g)b
Moisture content %
σb (MPa)
b (%)
0 11.1 25.0 66.7
57.0 42.0 54.0 96.6
10.8 ( 0.0 9.4 ( 0.1 8.4 ( 0.2 6.7 ( 0.2
49.6 ( 3.3 62.0 ( 2.6 43.0 ( 2.4 40.1 ( 2.3
4.2 ( 1.6 11.0 ( 3.7 21.7 ( 2.2 40.3 ( 3.0
a Percentage of PUP weight to the total weight of anhydrous starch and PUP. b Weight parts of added water and weight parts of moisture in samples. Molded sheets subjected for tensile test.
day before use. The starches modified with 10, 20, and 40 wt % of PUP are designated CP10, CP20, and CP40, respectively. CP represents corn starch modified with polyurethane. The number is the weight percentage of polyurethane (PU) to the total polyurethane and anhydrous starch. Without addition of PUP, native corn starch was also processed and assigned CS. The formulations of CS, CP10, CP20, and CP40 are shown in Table 1. Preparation of Sample Sheets by Compression Molding. Wet modified starch was compression molded in a hot press (R-5001 model, Wuhan Qien Science & Technology Co., Ltd. Wuhan, China) equipped with a water cooling system. The molding time, temperature, and pressure were 5 min, 100 °C, and 20 MPa, respectively. The wet sheets were cut into strips. The strips were equilibrated at 60% RH at least 3 weeks to obtain dry starch strips with a dimensions of 0.2-0.3 mm × 10 mm × 100 mm. As an experimental control, liquid PUP was poured into a mold and cured at ambient conditions for 30 days to prepare castor oil-based polyurethane (COPU) sheet. Moisture Content Measurement. Small parts (about 1 g) were cut from the molded starch sheets, weighed, and dried at 110 °C. After 12 h the sample was weighed again to calculate the moisture content in the sample moisture content (%) ) (W1-W2) ⁄ W1 × 100
(1)
where W1 and W2 are the weight of starch sheets before drying and after drying, respectively. Tensile Test. The samples cut from molded sheets were aged at 60% RH and room temperature for 2 months, and then the mechanical properties were measured using a tensile tester (CMT6503, Shenzhen SANS Test Machine Co. Ltd.) according to ASTM D 882-81 with a strain rate of 50 mm/min. The distance between the two clamps was 50 mm. Strength at break (σb, MPa) and elongation at break (b, %) of the sheets were recorded. Four duplications were made. Fourier Transform Infrared Spectroscopy (FTIR). A FTIR spectrometer (Spectrum One, Perkin-Elmer, Massachusetts) at room temperature was used here for characterization studies. Starch was compression molded into wet thin films, and the films were equilibrated at ambient conditions (60% RH) for 1 day to prepare dry thin film (thickness 30 µm). All spectra of thin films were recorded in transmission mode at a resolution of 4 cm-1 with accumulation of 5 scans. Soxhlet Extraction. Small pieces (diameter < 1 mm) cut from the molded starch sheets were dried at 105 °C in an oven for 24 h and then weighed (W1). The dried pieces were extracted in a Soxhlet extractor with toluene as solvent for 24 h to remove unreacted polyurethane. The extracted samples were exposed in a hood and then placed into an oven at 105 °C for 16 h. The extracted and dried samples were weighed (W2) to calculate the reaction ratio as reaction ratio ) (WL⁄WI) × 100 ) [1-(W1-W2) ⁄ WL] × 100 (2)
where WL is the weight of the PU linked to starch and WI is the weight of the PUP added into starch. Four duplications were carried out. Dynamic Mechanical Analysis (DMA). DMA was carried out using a dynamic mechanical analyzer (DMA Q800, TA Instruments, Delaware) in the single cantilever mode. Samples were investigated from -110 to 140 °C at a heating rate of 3 °C/min. A variable-amplitude, sinusoidal tensile stress (frequency ) 1 Hz) was applied to the samples to produce a sinusoidal strain of (30 µm amplitude. The temperature of the peak of the tan δ curve (T tan δ) was taken as the Tg of the samples. Two duplications were done. Viscosity Measurement. The viscosity of PUP was measured using a Brookfield digital viscometer (model DV-II, Brookfield Engineering Laboratories INC. Stoughton, Massachusetts). Viscosity data were recorded after the samples were equilibrated at 60 °C for 20 min. Spindle speed was 100 rpm, and the spindle no. was 29. Emission Scanning Electron Microscopy (ESEM). A ESEM with energy-dispersive X-ray analysis system (ESEMEDX) (FEI, Quanta 200 FEG, Netherlands) was used to observe the cross-sections of fractured samples. Each sample was frozen using liquid nitrogen and then fractured using tweezers to produce cross-sections. The cross-sections were coated with gold and then used for ESEM observation. To study the effect of solvents on the surface morphology of the modified TPS, the cross-sections of the modified TPS sheet were immersed in toluene at ambient conditions for 24 h to selectively extract soluble PU, and the treated sheet was dried for ESEM observation. With the same treatment procedure, water-treated sheet was prepared and used for ESEM scanning. X-ray Diffraction. Sheets were measured by wide-angle X-ray diffraction (WAXRD) (Y-2000 Dandong radiative instrument group Co. Ltd., China). For irradiation the Cu KR line, λ ) 0.1542 nm was applied (cathode at 30 kV and 20 mA), and scattering was recorded in the range of 2θ ) 2-40°. Thermal Gravimetric Analysis (TGA). Testing was conducted using a thermal gravimetric analyzer (STA 449 C, NETZSCH Instruments Inc. MA). Approximately 10 mg of the sample cut from the sheet was equilibrated at ambient conditions and then subjected to heating from 30 to 500 °C at a rate of 20 °C/min in a nitrogen atmosphere. Results and Discussion Structure. FTIR spectra of CS, CP20, and PUP are shown in Figure 1. The -CH2- bending absorption of starch occurred at 1643 cm-1, which was overlapped by absorption of moisture in CS. For the castor oil component in PUP (Figure 1, PUP), the CdO stretching absorption of the ester groups occurred at 1744 cm-1.15 The peak at 1726 cm-1 was attributed to the “free” urethane groups and the shoulder peak at 1704 cm-1 to the hydrogen-bonded urethane groups.16 Compared with the spectra of CP20 and PUP, the profile of the band from 1703 to 1744
9898 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 1. FTIR spectra of CS, CP20, and PUP.
cm-1 was different from each other, indicating that the structure of urethane linkage for the two samples was also different. Compared with the spectra of PUP and CS, the new peaks at 1729 and 1512 cm-1 for CP20 were assigned to absorption of urethane linkages between castor oil component and starch macromolecules. Therefore, the polyurethane components of CP20 were cross-linked to starch matrix by urethane linkages. Morphology. The cross-section of CP10 was rough as shown in Figure 2a. The result indicates that there was microphase separation in CP10 sample. The continuous area as marked by the arrow in Figure 2a was assigned to starch matrix. In Figure 2b there was an appreciable amount of uneven PU microparticles dispersed in the continuous starch matrix marked with “o”. The particle size was around 5 µm. Usually, the compatibility between hydrophobic polymer and hydrophilic polymer was poor due to weak interphase adhesion.1 However, the interphase between PU microparticles and starch matrix was continuous and dense, indicating good compatibility between the hydrophobic PU and hydrophilic starch in our case. Combining with the above FTIR analysis, the good compatibility was contributed to formation of urethane linkage between PU microparticles and starch matrix. In CP20 only urethane linkage in PU had a nitrogen atom, implying the nitrogen content was associated with PU content. As shown in Figure 2c (EDX analysis) the nitrogen content of the dispersion phase marked with “+” was 4.2% and that of the continuous starch matrix marked with “o” was 1%, suggesting rich PU content in dispersed phase and further proving that the dispersion was mainly composed of PU microparticles. The results also indicated that the continuous phase was mainly composed by starch as well as by a small amount of PU. The starch matrix area marked with “o” inflated under high energy beam and high magnification, implying starch matrix had less thermal stability than PU dispersion. With high energy beam scanning on CP20 for a long time starch matrix as shown in Figure 2d inflated into the white area and many cracks were observed. Even under inflation stress the interphase between the white starch matrix and PU dispersion marked with an arrow (Figure 2d) was tight and dense, suggesting good compatibility between starch matrix and PU microparticles. The cross-section of CP20 was immersed in toluene for 24 h, and its SEM image is shown in Figure 2e. PU microparticles were almost not extracted out of the starch matrix, implying that the PU microparticles were cross-linked to starch matrix, and the interphase between the two polymers was strongly
bonded by urethane linkages. In order to study the effect of water on the morphology of CP20, CP20 was immersed into water for 24 h and dried. The cross-section image of the watertreated CP20 is shown in Figure 2f. The surfaces of the starch matrix marked with arrows as shown in Figure 2f were even, indicating that the modified starch could not be dissolved by water. The results implied the improvement of water resistance of the modified starch as compared with raw starch. The schematic diagram describing modified TPS morphology is shown in Scheme 1. Polyurethane microparticles were linked to starch matrix through urethane linkages. The compatibility between hydrophobic polyurethane and hydrophilic starch was improved effectively because of the existence of the urethane linkages. The morphology of the modified TPS was similar to that of poly(styrene-butadiene-styrene) (SBS) copolymers in which spherical and tough butadiene domains are dispersed into brittle polystyrene matrix.17 In our case, the flexible castor oilbased polyurethane microparticles was dispersed into the brittle starch matrix, leading to increased toughness of the TPS. The PU-starch material became tough, but the starch component in the material was fragile regardless of PU content. Preparation of castor oil-based waterborne polyurethane/starch blends have been reported elsewhere.18,19 In these blends, compatibility between the polyurethane component and starch was limited because the NCO groups were almost consumed in water before blending with starch, leading to nonformation of urethane linkages between PU and starch. Starch, glycerol, and thermoplastic polyurethane were used to prepare films by extrusion,20 but the compatibilities between the polyurethane and starch in the starch/polyurethane thermoplastic blends were poor because the interactions between the two components were weak. WAXRD patterns of CS, CP20, and castor oil-based PU (COPU) are shown in Figure 3. There are three peaks on the curve of CS at 13.23°, 19.95°, and 22.22°, which are attributed to the VH-type crystallinity.21 The crystallinity is caused by rapid recrystallization of single-helical structure of amylase during cooling after processing. The three characteristic peaks can also be observed on the curve of CP20 owing to the rich starch component in CP20. The two peaks on the curve of COPU at 18.9° and 20.12° are characteristic of MDI-hard domains crystallinity.22 Mechanical Properties. In the modified TPS PUP crosslinkers increased the molecular weight of starch, leading to an improvement of the strength for starch sheets. At the same time, castor oil soft segments in starch sheets acted as impact
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9899
Figure 2. SEM images of (a) CP10 (3000×), (b) CP20 (5 kv, 6000×), (c) CP20 (EDX analysis, 6000×), (d) CP20 (EDX analysis, 6000×), (e) CP20 (toluene extracted, 3000×), and (f) CP20 (water extracted, 3000×).
Scheme 1. Morphological Structure of Modified TPS
modifiers, leading to increased elongation and decreased strength. Therefore, the ultimate tensile properties of starch
sheets were affected by the balance of the increased molecular weight of starch and content of castor oil content. The tensile properties of the starch sheets are shown in Table 1. Brittle native starch sheet (Table 1, CS) showed high strength and low elongation, which was in accordance with the results reported by Shogren.23 CP10 powder was dispersed into dimethyl sulfoxide (DMSO) solvent, which is a good solvent for native starch. Results showed that CP10 could not be solved completely into the solvent and only dispersion of CP10 was formed, implying the existence of cross-linking structure of the modified starch. The molecule weight of native starch was increased by the cross-links with the polyurethane microparticles (Figure 2a) for CP10. Hence, the σb of CP10 (62.0 MPa) was higher than that of CS (49.6 MPa) due to the increasing molecular weight
9900 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 3. WAXRD patterns of CS, CP20, and COPU.
of starch in CP10. At the same time, the castor oil as soft segments in CP10 served as an impact modifier, leading to a higher b (11.0%) than that of CS (4.2%). The results showed that 10 wt % of PU could improve the tensile strength as well as elongation of the corn starch materials. The tensile strength of the CP20 (43.0 MPa) was lower than those of CS (49.6 MPa) due to dominance of castor oil as an impact modifier. The same holds true for CP40. Therefore, about 10 wt % of PUP was an effective additive for improving both the strength and elongation of starch sheets. Additives that can improve both the strength and elongation are generally rare in polymer modification. Therefore, the role of the PUP additive whose content at or near 10 wt % will be studied in future work. MDI-modified starch/PLA blends were prepared by Wang et al. 24 They used MDI to increase compatibility of starch and PLA. The modified starch granules prepared by them were still rigid because there were no flexible polyol segments in the polyurethane system. The existence of flexible polyol in our modified starch was essential to toughen the modified TPS. That was one of the reasons why polyurethane prepolymer was used in our work instead of only diisocyanate. The moisture contents of CS, CP10, CP20, and CP40 were reduced significantly with an increase of polyurethane content, indicating polyurethane increased the hydrophobicity of the starch materials. In our case, the flexibility of starch was improved with an increase of polyurethane content and decrease of moisture content. The results suggested that an improvement in the flexibility of the modified starch was due to impact modification of polyurethane instead of external plasticization by water. DMA results of CS, CP10, and CP20 are shown in Figure 4. A broad glass transition for CS sample was observed from 0 to 100 °C. The intensity and position of the glass transition were dependent on the moisture content in the CS sample. The results were in good agreement with that reported by Lu et al., who found that the tan δ peak of TPS ranged from 0 to 110 °C.18 The damping peak at 33.7 °C was assigned to relaxation of the castor oil segments,15 indicating the castor oil segments played an impact modifier role in the modified TPS. The tan δ peak temperature was 33.7 °C for CP20, which was in accord with the results obtained by Lu et al., who reported that the tan δ peak temperature of castor oil-based PU in PU/TPS blend ranged from 29.8 to 56.1 °C depending on PU content.18 The E′ values of CS, CP10, and CP20 at 25 °C were 3.9, 3.1, and 2.6 GPa, respectively. Compared with CS, the modified starches (CP10 and CP20) showed lower storage modulus due to the impact modification of castor oil.
Figure 4. DMA thermograms for CS, CP10, and CP20: (a) storage modulus (E′) and (b) loss tangent (delta).
TGA thermograms of CS, CP20, and COPU are shown in Figure 5. Generally, CS contained 10-13% of moisture depending on the RH of ambient conditions. Therefore, CS showed a broad weight loss transition from 30 to 170 °C due to dehydration, where the maximum in weight loss rate occurred nearly at the boiling temperature of water (100 °C). From 278 to 330 °C CS showed a sharp weight loss transition, which was associated with the thermal decomposition of starch macromolecules.25 The curve profile of CP20 from 30 to 330 °C was similar to that of CS because the two samples had a similar carbon backbone structure. CP20 showed a quicker degradation transition than COPU during the first degradation stage from 278 to 330 °C, implying the starch component had less thermal stability than polyurethane. Therefore, when CP20 was subjected to SEM-EDX scanning at 15 kv and 6000 amplitude the continuous starch phase as shown in Figure 2c and 2d started to degrade first and then inflate. Reaction Ratio. In this work reaction ratio is defined as the weight percentage of the PU linked to starch to total weight of PUP added into starch. In traditional starch modifications,26,27 the reaction ratio of modifier was generally low. However, the reaction ratios of PUP for modified CP20 and CP10 were 99.7 ( 0.2% and 99.8 ( 0.3%, respectively. These results indicated that PUP was almost completely cross-linked to starch matrix, suggesting that modification was efficient and successful. Mechanism of Formation of Tough TPS. The schematic diagram of the modification mechanism is shown in Scheme 2. Crystallized starch granules (Scheme 2a) were plasticized by water and destructured. Starch macromolecules became flexible, resulting in formation of thermoplastic starch (Scheme 2b). Water used as plasticizer for preparing TPS under high shear has been well studied.4,28 As it is known, water is a kind of effective chain extender and cross-linker for preparing waterborne polyurethane microparticles.29,30 Hence, PUP (Scheme 2c) would react with water under high shear to form crosslinked PU microparticles during intensive mixing. The surface
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9901
Figure 5. TGA thermograms of CS, CP20, and COPU.
Scheme 2. Schematic Diagram for Preparing Tough Thermoplastic Starch Modified with Polyurethane Microparticles
dissolved in DMSO, and a clear solution is obtained. However, the CP10 sample could not be dissolved into DMSO, and the obtained dispersion was opaque. The results proved that the CP10 sample was not one network because it can be dispersed into DMSO. The opaque dispersion proved that the CP10 contained a large amount of micronetworks. In the modified TPS the PU microparticles were elastic and played an impact modification role for starch matrix, thus improving the breaking elongation of the modified TPS as shown in Table 1. The strength would be increased due to the existence of a partially cross-linked network if there was no impact modifier in the modified TPS. Thus, we successfully prepared tough TPS without using hydrophilic plasticizer such as glycerol. During the intensive mixing starch thermoplastic (Scheme 2b), multifunctional microparticles (Scheme 2d), and final modified TPS (Scheme 2e) were obtained in situ. Conclusions
of each PU microparticle borne much of the NCO groups, and thus, multifunctional microparticles (Scheme 2d) were formed. The high amount of NCO groups of the multifunctional microparticles increased the reaction probability and efficiency of PU linked to starch, resulting in a high reaction ratio (almost 100%) of PU in our work. Usually starch granules were cross-linked with PUP, and final product became only one network under no shear condition.31 The one network-based material could not form melt flow in plastic processing machines and thus was thermoset. In our case, the rotors of the intensive mixer produced high shear which dynamically mixed reaction system. Under the dynamical mixing condition the reaction system could not form only one huge network instead of a large amount of independent micronetworks as shown in Scheme 2e. There was no covalent bond between two independent micronetworks; thus, under the action of plasticizer and high temperature the independent micronetworks formed movable melt flow. Therefore, the final modified starch with the special structure as shown in Scheme 2e could be processed again using plastic processing machines and was a thermoplastic. Raw starch can be completely
Tough TPS without using hydrophilic plasticzers was successfully prepared. In the modified TPS PU microparticles were cross-linked to the starch matrix through urethane linkages. The PU microparticles acted as an impact modifier for the modified TPS and consequently improved the toughness of TPS. The reaction ratio of PU was nearly 100%, implying high reaction efficiency in the given modification process. Formation of multifunctional PU microparticles was essential to achieve the high reaction efficiency. Acknowledgment This work was supported by the National Natural Science Foundation of China under grant no. 50803024. Literature Cited (1) Yu, Y.; Dean, K.; Li, L. Polymer blends and composites from renewable resources. Prog. Polym. Sci. 2006, 31, 576–602. (2) Sarazin, P.; Li, G.; Orts, W. J.; Favis, B. D. Binary and ternary blends of polylactide, polycaprolactone and thermoplastic starch. Polymer 2008, 49, 599–609. (3) Thunwall, M.; Boldizar, A.; Rigdahl, M. Compression molding and tensile properties of thermoplastic potato starch materials. Biomacromolecules 2006, 7, 981–986. (4) Chale′at, C. M.; Halley, P. J.; Truss, R. W. Properties of a plasticised starch blend. Part 1: Influence of moisture content on fracture properties. Carbohydr. Polym. 2008, 71, 535–543.
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ReceiVed for reView June 29, 2008 ReVised manuscript receiVed September 8, 2008 Accepted September 19, 2008 IE801005W
