ARTICLE pubs.acs.org/IECR
Tough Thermoplastic Starch Modified with Polyurethane Microparticles: The Effects of Processing Temperatures Qiangxian Wu,* Xiaoxia Chen, Yu Zhang, Zhengshun Wu, and Yan Huang Green Polymer Lab, Polymer Science Department, College of Chemistry, Huazhong Normal University, Luoyu Road 152, Wuhan City, Hubei Province 430079, People’s Republic of China ABSTRACT: It is critical to prepare modified starch material with high toughness and hydrophobicity in an environmentally friendly way. In this work, polyurethane prepolymer (PUP) was synthesized and mixed reactively with thermoplastic starches (TPS) in an intensive mixer to prepare modified TPS at various processing temperatures. The modified TPS materials were characterized. Results showed that the toughness and hydrophobicity of the modified TPS were significantly improved as compared to pure starch material, due to the strong interaction between starch and elastic polyurethane microparticles. Modified TPS processed at different temperatures (60-90 °C) showed similar morphologies, thermal behaviors, and tensile properties, indicating that structure/ properties of the modified TPS were independent of the given processing temperatures. The formation of microparticles with a high amount of NCO groups and high reaction activity of each NCO group played important roles to improve the reaction efficiency, which weakened the dependence of the modification on the given processing temperatures.
’ INTRODUCTION Environmental concern over the use of traditional petroleumbased polymers has stimulated the development of polymers from renewable resources as an alternative.1 Starch, one of the important renewable resources, is a naturally occurring biopolymer and of low cost.2 However, the applications of pure starch materials were limited because of their water sensitivity and brittle property.3 Chemical modification has been one feasible way for the valueadded applications of starch. At present, the methods for starch chemical modification included esterification,4 etherification,5 crosslinking,6 oxidation,7 grafting,8 carboxymethylation,9 benzylation,10 and so on. Generally, modified starch becomes brittle and stiff11 due to the degradation of starch. Although starch plasticized with glycerol became flexible, the plasticizer tended to leach out of starch matrix, and thus the material still showed a brittle behavior after being aging.12 In addition, high amounts of wastewater or organic solvents are used in the traditional process of the above starch modifications, resulting in serious environment pollution problems. Therefore, it is important to prepare tough TPS without an aid of hydrophilic plasticizers and in an environmentally friendly way. Reactive extrusion is one green way to prepare chemically modified TPS because those reactions were carried out in melt state and no organic solvents were used.13-18 However, the reaction efficiency of modifiers used was low because of short reaction residence time and so on. Therefore, it is critical to improve the reaction efficiency for the green reactive extrusion. Polyurethane prepolymer (PUP) bearing isocyanate groups has often been used to toughen starch.19 Yet environmental pollution was the main problem in these chemical modifications due to the use of organic solvents,20 and the final product was thermoset instead of thermoplastic.6 In our previous works,21,22 PUP, water, and starch were reactively mixed in an intensive mixer or an extruder for preparing chemically modified TPS. PU microparticles with multifunctional groups were in situ formed and reacted with starch. The reaction efficiency of PU r 2011 American Chemical Society
microparticles was almost 100% because of the special structure of the PU microparticles with larger amounts of isocyanate groups, which significantly improved the reaction probability of PU to starch. The urethane linkages between PU microparticle and starch matrix obviously improved the compatibility of the two polymers. The tough and hydrophobic properties of the modified TPS were also obviously enhanced. Therefore, the idea of the formation of microparticles with multifunctional groups was our alternative for solving the low reaction efficiency problem during reactive extrusion or banbury mixing. Generally, the reaction temperature was an important factor to prepare the novel ductile TPS. The effect of the reaction temperature on the structure/properties of the modified starch was still not studied in our previous work. In this work, the objective was to investigate the effect of the reaction temperature on the structure/ properties relationship of the modified TPS. PU-modified starches processed at various reaction temperatures were first prepared, and the obtained modified starches were characterized to clear the relationship between processing conditions, structure, and properties of the material. To prepare green materials, natural castor oil was used as PU raw material in this study.
’ EXPERIMENTAL SECTION Materials. Castor oil and 4,40 -methylene diphenyl diisocyanate (MDI, 98%) were purchased from Sigma-Aldrich Fine Chemicals (St. Louis, MO). Corn starch (CS, amylose, 23-26 wt %; moisture, 12 wt %) was obtained from Wuhan Corn Starch Co. Ltd. (Wuhan, China) and used without any further pretreatments. Tetrahydrofuran (THF, 99%) was purchased from the Shanghai Zhenxing No. 1 Received: May 24, 2010 Accepted: December 3, 2010 Revised: October 8, 2010 Published: January 18, 2011 2008
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Table 1. Formulations and Tensile Properties of Molded TPS Sheets Tested properties of molded sheetsc
formulations for modification
a
moisture content (%)
σb (MPa)
24.0
9.6 ( 0.7
43.7 ( 3.1
2.7 ( 0.4
29.5
10.1 ( 1.3
33.7 ( 0.1
13.0 ( 1.2
9.5
29.5
10.7 ( 0.2
33.0 ( 0.9
13.3 ( 1.2
9.5
29.5
10.1 ( 0.3
35.7 ( 1.6
10.6 ( 1.5
9.5
29.5
10.2 ( 1.2
31.6 ( 0.4
14.8 ( 1.5
mixing temperature (°C)a
anhydrous starch (g)
PUP (g)
water plasticizer (g)b
TPS
90
60.0
0.0
CPT90
90
52.7
9.5
CPT80
80
52.7
CPT70
70
52.7
CPT60
60
52.7
sample
ɛb (%)
Mixing temperature in an intensive mixer. b Total weights of added water and moisture in samples. c Molded sheets subjected for tensile test.
Chemical Plant (Shanghai, China). CCl3D was purchased from J & K Chemical Ltd. 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 250 mL three-necked flask fitted with a stirrer, a vacuum outlet, and a temperature sensor. The system was heated to 110 °C and degassed for 30 min to remove the moisture in oil bath. After 30 min, the temperature of castor oil in the flask was decreased to 60 °C, and then MDI (125.2 g) was introduced into the flask. The translucent mixture in the flask quickly became clear. The mixture was stirred vigorously and reacted at 85 °C under vacuum for 1 h until the NCO groups content reached a given value (the theory value of the NCO content of PUP (7.49%)), determined by dibutylamine back-titration.23 Finally, clear and yellow PUP was obtained. Preparation of Modified TPS. Corn starch, PUP, and water (added water and the moisture content of starch) were charged into an intensive mixer with two rotates (SU-70, 70 mL, Changzhou Suyan Science and Technology Co., Ltd. Changzhou, China) and mixed reactively at 60, 70, 80, and 90 °C, respectively. If the processing temperature was higher than 95 °C, the water in TPS melt would evaporate quickly, resulting in high motor torque. The processing temperature should be higher than 60 °C for the gelatinization of starch. The weight percentage of added PU to dry modified TPS was controlled to be 20%. The mixing speed was 100 rpm. The weight parts of each raw material are shown in Table 1. After 20 min, a white modified thermoplastic starch was obtained. Preparation of Sample Sheets by Compression-Molding. The wet modified starch was compression-molded in a hot press (R-3201 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 the dumbbelllike sheet (5A type) according to GB/T1040-2006. The length of the dumbbell-like sheet was 75 mm, and the width of narrow section was 4 mm. The sheet from modified starch mixed at 60 °C was designated as CPT60. The “CP” represents corn starch modified with polyurethane, and the “T60” represents the temperature of 60 °C. With the same procedure, the sheets of CPT70, CPT80, and CPT90 were prepared. The formulations of the four CPTs are shown in Table 1. TPS sheet without addition of polyurethane was prepared, and its formation is also shown in Table 1. Hydrogen Nuclear Magnetic Resonance (H NMR). 1H NMR spectra were recorded using a nuclear magnetic resonance (NMR) spectrometer (model Mercury-Plus 600, Varian, Inc., CO). Chemical shifts (δ) were given in ppm, and tetramethylsilane was used as a standard. Samples were dissolved into CCl3D in an NMR tube and then subjected to 1H NMR analysis.
Gel Permeation Chromatography (GPC). Molecular weight and molecular weight distribution were measured by gel permeation chromatography (GPC, Agilent 1100, U.S.). The mobile phase was tetrahydrofuran (THF) at a rate of 1.0 mL min-1, and the column temperature was maintained at 35 °C. During the experiments, the pump pressure of GPC was recorded. We used polystyrene standard with 10 different molecular weights from 2000 to 5 000 000 g mol-1. All samples (1 wt %) were prepared in THF and used for GPC analysis. Tensile Test. The molded sheets were equilibrated at 60% RH and room temperature for 2 weeks, and then the mechanical properties were measured using a tensile tester (6P-TS 2000S, Shenzhen Gaopin Test Machine Co. Ltd., China) according to ASTM D 882-81 with a strain rate of 12.5 mm min-1. The distance between the two clamps was 40 mm. Strength at break (σb, MPa) and elongation at break (ɛb, %) of the sheets were recorded. Four specimens of each composition were made. Moisture Content. Small parts (about 1 g) were cut from the molded starch sheets, weighed, and dried at 110 °C. After 5 h, the sample was weighed again to calculate the moisture content in the sample: ð1Þ moisture content ð%Þ ¼ ðW 1 - W 2 Þ=W 1 100
where W1 and W2 are the weight of starch sheets before drying and after drying, respectively. Emission Scanning Electron Microscopy (ESEM). An 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. Viscosity Measurements. A rotary viscosimeter analyzer (DV-IIþPRO LV, Brookfield Instruments, U.S.) was used to determine viscosity properties of starch emulsion. Before the test, castor oil was used to calibrate the instrument. Starch aqueous suspension (6 wt %) was prepared, and the starch suspension was cooked at 60 °C for 1 h. The Brookfield viscosity of the sample was measured with 100 rpm of spindle speed and a No. S61 spindle. The viscosity of starch dispersion samples was also tested at 70, 80, and 90 °C, while the spindle speed was 100, 20, and 5 rpm, respectively. According to the same method, the viscosity of TPS was tested at 60, 70, 80, and 90 °C, while the spindle speed was 100 rpm. For modified starch, a suspension (6 wt %) was prepared, heated, and strongly stirred until samples were almost dissolved in water, filtered with gauze, and the obtained modified starch dispersion was tested at 60, 70, 80, and 90 °C while the spindle speed was 100 rpm with a No.S62 spindle. X-ray Diffraction (XRD). Sheets were evaluated by X-ray diffraction (XRD) (X’Pert PRO, PANalytical B.V. Instruments, 2009
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Figure 1. 1HNMR spectrum of CO, PUP, and MDI.
Holand). For irradiation of 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 from 5° to 40°. Differential Scanning Calorimetry (DSC). DSC experiments (model Q100, TA Instruments, DE, U.S.) were conducted in a nitrogen atmosphere. The sample was packed into an aluminum pan and sealed. Indium was used for the calibration of the instrument. Prior to the test, the specimens were heated at a rate of 20 °C min-1 from room temperature to 130 °C to remove any thermal history and then cooled to -50 °C at a rate of 20 °C min-1. Glass transition temperatures of PU component in CPTs (Tg, the midpoint of glass transition) were determined by scanning from -50 to 150 °C at a rate of 20 °C min-1. Three specimens of each composition were carried out. Contact Angle (CA). Surface hydrophobicity and wettability of sheets were estimated from the contact angle (CA) measurement with a contact angle meter (OCA20, Contact Angle System, Germany). A testing liquid, water, was used in this work. Five microliters of testing liquid was deposited on the solid sheet surface. The contact angles were measured with a CCD camera and processed using an image analysis video card, which calculated the contact angle automatically using an image analysis setup.
’ RESULTS AND DISCUSSION Structure. Figure 1 shows the 1H NMR spectrum of MDI, CO, and PUP. It can be seen that the spectrum of PUP was basically overlaid by the spectra of CO and of the PUP, which is consistent with the literature.24 In the spectrum of PUP, the signal of the urethane proton (NHCOO) was observed at 6.62-6.65 ppm, and a peak was observed at 4.85 ppm assignable to methine protons of the carbamate (CHOCO) shifted by methine proton connected to hydroxyl of castor oil at 3.6 ppm. The result indicated that the hydroxyl groups of castor oil were reacted with the isocyanate group of MDI, further implying the formation of urethane bond. However, it is difficult to use NMR to analyze the nonsoluble modified TPS. This confirmation using FTIR has been done in our previous work.22 The absorption peaks at 1729 and 1512 cm-1 were assigned to the absorption of urethane linkages of modified TPS. GPC chromatograms of PUP and MDI are shown in Figure 2. There were three peaks in this chromatogram. The first peak at 9.0 mL of elution volume corresponded to unreacted MDI, which was consistent with the peak of MDI. The result implied that it was necessary to decrease the amount of free MDI in PUP in future work. The second peak (Mw = 2600 g mol-1) was assigned to linear PUP, because PUP was prepared by one of
Figure 2. GPC chromatogram of PUP and MDI.
castor oil and three of MDI. The third peak that appeared as a shoulder in the chromatogram was related to a higher molecular weight component. This type of composition that resulted from the reaction between MDI and polyol is attributed to the difference between the activity of NCO groups present in the reaction system, which was reported by Sheikh et al.25 During the experiments, PUP solution was clear. However, the PUP solution resulted in relatively high pump pressure, implying that the PUP could not be completely dissolved into THF. The result indicated that a small amount of cross-linked PUP component may exist in the PUP. Morphology. Figure 3 shows SEM images of the crosssections of the CPT60 and CPT90 sheets. The cross-section of CPT90 was rough as shown in Figure 3a. According to our previous SEM-EDX analysis,22 the continuous area marked with “Δ” in Figure 3a was assigned to starch matrix, and the island-like area marked with “O” was associated with PU-rich dispersion phase. Usually, the compatibility between a hydrophobic polymer and a hydrophilic polymer is poor due to weak interphase adhesion.26 In this case, PU component was smoothly distributed in the continuous phase of starch. CPT60 showed the same morphology as CPT90, suggesting that the compatible structure can be developed at 90 °C as well as 60 °C. With the existence of water as chain extender, PUP microparticles were in situ formed.27 The special structure of the PU microparticles with larger amounts of isocyanate groups significantly improved the reaction probability of PU to starch, resulting in a higher efficiency of the reaction, and thus formed compatible morphology for CPT60 at low mixing temperature (60 °C). The image also showed that there was microphase separation in the CPT sample, which increased the toughness of the starch material. Chen et al. reported that PUP was dispersed into water to obtain a polyurethane-urea aqueous dispersion, and the dispersion was blended with starch for preparing biodegradable TPS.28 However, the NCO groups of PUP had been consumed by water before mixing with starch, and the urethane linkages between starch and PU could not be formed, resulting in limited compatibility between the two polymers. In our works,22 the formation of urethane linkages between starch and PU microparticles was essential to significantly improve the compatibility or the tensile properties of TPS. PUP and soy materials were also extruded reactively and molded for preparing biodegradable composites.29 The soy particles were dispersed in PU cross-linked continuous phase, and the final product was a thermoset, which could not be dispersed in solvent or melted again for secondary processing. As compared to Chen et al.’s report, our works21,22 have two 2010
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Figure 4. XRD patterns of CS, TPS, and CPT90.
Table 2. Viscosity of CS, TPS, and CPT90 at Different Temperatures viscosity value (cp) at different temperatures sample
Figure 3. SEM images of CPT60 and CPT90: (a) 90 °C (5 kV, 6000); (b) 60 °C (5 kV, 10 000).
differences: (1) the proper amount of water as plasticizer was used to in situ prepare natural polymer-based thermoplastic, and (2) elastic PU microparticles were dispersed in TPS continuous phase. The results showed that the final extruded products became a tough and hydrophobic thermoplastic instead of a thermoset, and thus the novel thermoplastic could be extruded or dispersed in water, implying a potential in paper sizing industry and so on. As compared to traditional starch modification using polyurethane,30 our modification was efficiently carried out in the semisolid state and without using organic solvents, showing some potential in applications. The XRD patterns of the CS, TPS, and CPT90 were obtained in Figure 4. The X-ray patterns of native starch showed strong diffraction peaks at around 15.0°, 17.0°, 18.8°, and 23.0°, respectively. The result was in agreement with that reported by Liu et al.31 The peaks at 19.5° and 13.0° in the TPS pattern were attributed to V-type crystallinity, formed after recrystallization of amylose after being destructurized or plasticization of starch granules.32 It was reported that the B-type crystallinity at 16.8° for TPS was attributed
60 °C
70 °C
80 °C
90 °C
CS
24.4 ( 1.2
37.6 ( 1.0
510.1 ( 63.3
4355.7 ( 87.4
TPS
6.3 ( 1.3
10.8 ( 1.7
12.7 ( 1.8
27.5 ( 0.9
CPT90
4.4 ( 1.0
5.1 ( 0.9
5.8 ( 0.5
7.7 ( 0.4
to recrystallization of both amylose and amylopectin.32 The two characteristic peaks at 17.4° and 20.8° were appeared on the curve of CPT90, due to the crystallinity of PUP.33 Mechanical Properties Analysis. The formulations and tensile properties of molded TPS sheets tested are shown in Table 1. The water weight to the total material weight was about 32.2%, which made starch granules melt at the mixing temperature. The consumed water amount during the modification process was low, and no organic solvent was used as compared to the other methods such as esterification34 and grafting.35 The results implied that our modification method was green and environmentally friendly. However, isocyanate is harmful, and it is better to use safe raw material for preparing the multifunctional microparticles. The reaction ratio of PUP used for modified starch was nearly 100% in our previous study,22 indicating our method was also effective. In addition, the modification can be conducted using a twin-screw extruder,22 showing a potential application in industry. In Table 1, CS showed brittle properties with high strength (σb = 43.7 MPa) and low elongation (ɛb = 2.7 (%)). However, the CPT60 showed lower strength (σb = 31.6 MPa) and higher elongation (ɛb = 14.8 (%)), suggesting that the PU component acted as an impact modifier. Because castor oil as a soft segment was flexible, the PU component showed an elastomer. Therefore, the PU elastomer dispersed in starch matrix and steadily interacted with starch through urethane linkage, leading to an increase in the compatibility between the two phases. The CPT70, CPT80, and CPT90 showed tensile properties similar to those of CPT60, suggesting the mixing temperature ranging from 60 to 90 °C did not obviously affect the tensile properties of the modified sheets. These results suggested that the structure/properties of the modified starch were independent of the given mixing temperature. The conclusion implied that the 2011
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Table 3. CA of Water Drop on the Surface of CS and CPT90 Films at Different Time
Scheme 1. Schematic Diagram for Preparing Tough Thermoplastic Starch Modified with Polyurethane Microparticles
Figure 5. DSC of CS, PUP, CPT60, and CPT90.
NCO groups of PUP showed high reaction activity during the starch modification and the modification also could be effectively carried out even if the reaction temperature decreased from 90 to 60 °C. Viscosity results of CS, TPS, and CPT90 are shown in Table 2. For CS dispersion, a sharp increase in viscosity was observed from 80 to 90 °C, which was associated with the gelatinization of starch granules.36 During the gelatinization of starch, starch crystalline structure was destroyed, starch granules slowly expanded, and the hydrogen-bonding interaction between starch macromolecules and water increased, resulting in an increasing viscosity. The viscosity of TPS dispersion increased with an increase of temperature, which presented a similar viscosity behavior with CS dispersion. However, the viscosity of the TPS dispersion did not significantly enhance from 80 to 90 °C in comparison with that of CS dispersion. This may be due to the degradation of starch under high shear in an intensive mixer.37 Chan et al. reported that starch exhibited the highest peak viscosity after the sodium dodecyl sulfate and sonication treatment.38 This could be due to the effective removal of hydrophobic protein layers that help to increase the swelling power of corn starch. However, in our experiment, the viscosity of CPT90 did not change significantly from 60 to 90 °C because starch macromolecules were partially linked with PU particles and could not easily extend into water. Moreover, the viscosity of modified starch at 90 °C obviously decreased in comparison with that of CS or TPS, because the hydrogen-bond interaction between modified starch and water weakened, and modified starch was difficult to extend into water, which lead to an increased hydrophobic property of the modified starch. The DSC curves for PUP, CS, CPT90, and CPT60 are shown in Figure 5. In our previous DMA study,39 PUP showed a broad damping peak from 0 to 55 °C, which was associated with the
glass transition of castor oil soft segments. Therefore, the glass transition of PUP ranged from 20 to 50 °C and was associated with the thermal behavior of castor oil soft segment. Because the ΔCP of PUP was weak and the weight percentage of PUP in CPT60 and CPT90 was only 20%, the transitions of PU for CPT60 and CPT90 were too weak to be observed in the DSC curves. Starch contained a little amount of moisture, because water can evaporate with an increase of temperature, which led to an increase of the heat flow of the sample, and thus the DSC curves of CS, CPT90, and CPT60 showed ascending behavior as shown in Figure 5. The profile of the DSC curve for CPT60 was similar to that for CPT90, indicating a similar DSC thermal behavior for the two samples. Typical pictures of water drop on the surfaces at different time are displayed in Table 3. The contact angle of water is one of the basic wetting properties of materials, indicating hydrophilic/ hydrophobic surface properties of materials.40 Usually, the more hydrophilic materials have the lower contact angle. CS had a lower contact angle than that of CPT90, and the angle decreased rapidly after water droplet deposition. The water droplet was totally absorbed by CS films within about 40 s. This behavior indicated the hydrophilic and highly wettable characteristics of CS surface.41 When PU was cross-linked to starch, the contact angle of modified starch was significantly increased. In addition, the evolution of contact angle with time slowed. This indicated that the hydrophobic PU content was responsible for the 2012
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Industrial & Engineering Chemistry Research improvement of hydrophobic characteristic of the modified starch surface. The result about increasing the hydrophobic property for the modified TPS was in agreement with that from viscosity analysis. Schematic of the Formation of Tough TPS. A primary schematic diagram of modification mechanism for preparing tough TPS was discussed in our previous work.22 According to the above GPC analysis, the previous modification schematic was further updated as shown in Scheme 1. In Scheme 1, castor oil (c) and MDI (d) were used to synthesize PUP, which contained cross-linked PUP (e) with multifunctional NCO groups and branched PUP (f). The PUPs were in situ cross-linked into PU microparticles with more multifunctional groups that would significantly improve the reactive efficiency of the PU modifier. The water played the role of chain extender for PUP and plasticizer for TPS.
’ CONCLUSION A series of modified TPS under various processing temperature were successfully prepared. As compared to pure starch material, toughness and hydrophobicity of the modified TPS were significantly improved, due to the strong interaction between starch and elastic polyurethane microparticles. The modified TPS processed at the given temperature (60-90 °C) showed similar morphology, thermal behavior, and tensile properties, indicating that structure/properties of the modified TPS were independent of the given processing temperatures. The abnormal result was because the formation of microparticles with a high amount of NCO groups significantly improved the efficiency of the reaction, and thus weakened the effect of processing temperatures on properties of the modified TPS, which further showed the formation of multifunctional PU microparticles was essential to achieve the high reaction efficiency.
’ AUTHOR INFORMATION Corresponding Author
*Tel./fax: þ86-27-67867953. E-mail:
[email protected].
’ ACKNOWLEDGMENT We appreciate financial support from the National Natural Science Foundation of China under grant no. 50803024, and from the Self-determined Research Funds of CCNU from the College’s Basic Research and Operation of MOE under grant no. 120002040059. ’ REFERENCES (1) 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. (2) Sorokin, A. B.; Kachkarova-Sorokina, S. L.; Donze, C.; Pinel, C.; Gallezot, P. From native starch to hydrophilic and hydrophobic products: a catalytic approach. Top. Catal. 2004, 27, 67–76. (3) Funke, U.; Lindhauer, M. G. Effect of reaction conditions and alkyl chain lengths on the properties of hydroxyalkyl starch ethers. Starch/St€arke 2001, 53, 547–554. (4) Tanaka, H. Starch ester. U.S. Patent 6,617,449, 2003. (5) Zhou, J.; Ren, L. L.; Tong, J.; Xie, L.; Liu, Z. Q. Surface esterification of corn starch films: Reaction with dodecenyl succinic anhydride. Carbohydr. Polym. 2009, 78, 888–893. (6) Ha, S. K.; Broecker, H. C. Characteristics of polyurethanes incorporating starch granules. Polymer 2002, 43, 5227–5234.
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Industrial & Engineering Chemistry Research
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2014
dx.doi.org/10.1021/ie101149q |Ind. Eng. Chem. Res. 2011, 50, 2008–2014
