ARTICLE pubs.acs.org/IECR
Tough Thermoplastic Starch Modified with Polyurethane Microparticles: The Effects of NCO Content in Prepolymers Yu Zhang, Yan Huang, Xiaoxia Chen, Zhengshun Wu, and Qiangxian Wu* Key Laboratory of Pesticide & Chemical Biology of the Ministry of Education, College of Chemistry, Huazhong Normal University, Wuhan, People’s Republic of China, 430079 ABSTRACT: In this work, polyurethane prepolymers (PUPs) with various NCO content proportions were synthesized and mixed reactively with a starchwater mixture to prepare modified thermoplastic starch (TPS) in an environmentally friendly way. The effects of polyurethane NCO content on the structure and/or properties of the modified TPS sheets were investigated. Results revealed that the reaction ratio of PUPs used for preparing the modified TPS was 99.8%, indicating that the PUPs were almost crosslinked to the starch matrix. With an increase of NCO content in PUP, the tensile strength of the modified TPS increased from 19.3 to 37.4 MPa, and the elongation from 1.7% to 17.2%. As the PUP’s NCO content increased, the amount of urethane linkage between PU microparticles and the starch matrix would increase. In addition, the compatibility of the two polymers was also improved resulting in the improvement of tensile properties. The urethane linkage between PU and starch therefore played an important role in improving the toughness of the modified TPS.
’ INTRODUCTION Biopolymers derived from renewable resources have become the focus of public interest due to environmental protection and sustainable development principles.1,2 Starch, being an important set of biopolymers, has been considered as one of most promising candidate materials due to its biodegradability, derivability, availability, and low cost.35 Starch can be processed into a thermoplastic material in the presence of plasticizers;6 however, thermoplastic starch (TPS) is brittle.7 According to our data, TPS with a large amount of plasticizers, such as glycerol, still becomes brittle after being equilibrated at ambient conditions for several months. Modification is one way to improve the toughness of TPS. Polyurethane prepolymer (PUP) has been used to toughen starch owing to its isocyanate groups that have high reactivity with the hydroxyl groups of starch.812 A large amount of organic solvents or water was used in these modifications, however, causing serious environmental pollution.810 For example, starch was initially dissolved in dimethyl sulphoxide (DMSO) and then modified using hexane diisocyanate (HDI). It is therefore necessary to use an environmentally friendly mechanism for preparing ductile thermoplastic starch. In our recent work,13,14 starch, water, and PUPs were mixed reactively in an extruder to prepare tough TPS. PUPs were dynamically cross-linked to a starch matrix through urethane linkages in a water system. The PU component as rubber phase thus effectively toughened the brittle starch matrix.14 The TPS was toughened and the modification was also conducted in a green way. PUPs and soy byproduct were extruded reactively for preparing biodegradable composites.15 The soy particles were dispersed in a PU cross-linked continuous phase, and a thermoset was formed. The thermoset could not be dispersed in solvents nor melted for thermoplastic processing. In our work,13,14 a proper amount of water as plasticizer was used in situ to prepare a natural r 2011 American Chemical Society
polymer-based thermoplastic, and elastic PU microparticles were dispersed in a TPS continuous phase. The final extruded products became tough and hydrophobic thermoplastics instead of thermosets. The novel thermoplastic could thus be extruded or dispersed in water, implying a potential in the paper sizing industry among other applications. PUPs were also dispersed into water to obtain a polyurethane aqueous dispersion and the dispersion was blended with starch for preparing biodegradable TPS.16 The NCO groups of PUPs had been consumed by water, however, before blending with starch, and the urethane linkages between starch and PU could not be formed, resulting in weak compatibility between the two polymeric systems. In our works,14 the formation of urethane linkages between starch and PU microparticles was essential to obviously enhance the compatibility or the tensile properties of TPS. Obviously, the NCO content of PUPs is an important factor in preparing the novel ductile TPS with our new method. The objective of this work is to therefore study the effects of NCO content in PUPs on the properties of TPS. Polyurethane prepolymers with various NCO content levels were synthesized and mixed reactively with a starch-water mixture to prepare modified TPS. The modified TPS was then characterized to analyze the relationship of structure and/or properties of the materials. To prepare green materials, natural castor oil was used as the PU raw material in this study.
’ EXPERIMENTAL SECTION Materials. Castor oil and 4,40 -methylenedi-p-phenyl diisocyanate
(MDI, 98%) were purchased from Sigma-Aldrich Fine Chemicals Received: February 15, 2011 Accepted: September 27, 2011 Revised: September 13, 2011 Published: September 27, 2011 11906
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Figure 1. Schematic diagram of degrading modified TPS using enzyme. S and P represent supernatant and precipitate, SPU and DS represent separated PU and degraded starch, respectively. HTA represents high-temperature amylase.
(St. Louis, MO, USA). Cornstarch (amylose: 2326 wt %; moisture: 13 wt %) was obtained from Wuhan Corn Starch Co., Ltd. (Wuhan, China) and used without any further pretreatments. High-temperature amylase (activity: 20000u/mL) and α-Amylase (activity: 100000u/g) were obtained from Jienuo Enzyme Co., Ltd., (Zaozhuang, China) and glycerol (AR, 98%) was obtained from Shanghai Zhanyun Chemical Co., Ltd. (Shanghai, China). Synthesis of PUPs. The molar number ratio of isocyanate to hydroxyl (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 a vacuum at 110 °C to remove the castor oil’s moisture. After 30 min, the temperature of the castor oil in the flask was reduced to 60 °C. MDI (109.2 g) was then charged into the flask. The translucent mixture in the flask quickly became clear. Five minutes after the addition of MDI, the mixture was stirred vigorously and reacted at 87 °C for 1 h. Yellow PUPs were obtained as a result. The NCO content level of the PUPs was 7.0%, which was measured with the di-n-butylamine method.17 The 7.0% NCO content PUPs were then mixed with castor oil at 60 °C to adjust their NCO content levels. A series of PUPs with various NCO content levels—6.0%, 5.8%, 5.6%, 5.0%, 4.6% and 3.5%—were prepared and used immediately for further preparing modified TPS. The PUPs were poured into a mold and cured at ambient conditions for 30 days to prepare a castor oil-based polyurethane (COPU) sheet. Preparation of Modified TPS. Anhydrous cornstarch (45.8 g), PUPs (11.5 g, NCO %: 7.0), and water (15.8 g, 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, China) and mixed reactively at 90 °C with a stirrer speed of 100 rpm. After 20 min, a white modified TPS was obtained. The modified starch was equilibrated in a sealed plastic bag for 1 day before use. The starch modified with the PUPs containing 7.0% NCO content was designated CP-7.0. (The “CP” stands for cornstarch modified with polyurethane, the number 7.0 represents the weight percentage of NCO groups to the PUPs used for modifying cornstarch.) With the same procedure, PUPs containing 6.0, 5.8, 5.6, 5.0, 4.6 and 3.5% NCO content were used to prepare CP-6.0, CP-5.8, CP-5.6, CP-5.0, CP-4.6, and CP-3.5, respectively. The weight of PU to the total weight of dry starch and PUPs in all modified starches was controlled to be 20 wt %. Without addition of PUP, native cornstarch was also processed and assigned as CS. Cornstarch plasticized with 20% glycerol was designated as CS-G, and the prepared method was the same with modified starch. Preparation of TPS Sheets by Compression-Molding. Wet modified starch was compression-molded using 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, 95 °C, and 40 MPa, respectively. The wet sheets were cut into a dumbbell-like sheet (5A type) according to GB/T1040-2006. The length of the dumbbell-like sheet was 75 mm, with the narrow section’s width being 4 mm, and the thickness was about 0.2 mm. The sheets were equilibrated at 60% RH for at least 2 weeks. Degradation of TPS. It is necessary to separate the PU component linked to starch for clarifying the composition of the modified starch. The starch matrix was therefore first degraded into water-soluble materials through the interaction of a starch enzyme, and the PU component was thus separated out. The detailed procedure is as follows: CP-5.8 (6 g) and water (150 mL) were charged into a 250 mL three-necked flask fitted with a stirrer operating at a speed of 300 rpm. The temperature of the system was increased to 95 °C within 10 min to obtain starch dispersion. High-temperature amylase (0.2 g) was added into the flask at 95 °C to partially degrade the starch. After 1 h from the addition of high-temperature amylase, the temperature of the system was decreased to 60 °C within 10 min, and the pH value of the dispersion was adjusted to 4. The α-amylase (0.2 g) was then added. After 2 h from the addition of the α-amylase, the viscosity of the dispersion decreased sharply. The dispersion contained only hydrophobic PU precipitate and degraded starch (DS) at this time. The dispersion was separated into a degraded starch solution and a precipitate in a centrifuge machine (Shanghai Anting Scientific Instrument Factory). The degraded starch solution was concentrated and freeze-dried to prepare degraded starch. Owing to the existence of enzyme proteins in the precipitate, the precipitate was separated into a PU precipitate and a protein solution using an aqueous NaOH solution. The PU precipitate was washed and freeze-dried to obtain separated PU (SPU). The procedure is shown in Figure 1. Preparation of PU Powder. The COPU sheet was milled into PU powder in a polymer grinder (GP-00001 model, Wuhan Qien Science & Technology Co., Ltd., Wuhan, China) through the use of liquid nitrogen. The PU powder was filtered using a screen with 40 meshes. Reaction ratio. A quantitative method was used to analyze the reaction ratio of the modified starch. CP-4.6 (5 g) and water (250 g) were cooked in a beaker (500 mL) at 95 °C for 1 h to obtain a starch dispersion, and then butyl acetate (20 g) was added to separate the unreacted PU components of the CP20. After being mixed for 30 min, the aqueous system was allowed to stand at 5 °C for 12 h to obtain a clear butyl acetate layer and a water layer. To take images for the separation of the butyl acetate layer and the water layer conveniently, the water/butyl acetate solvent mixture (20 mL) 11907
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was placed into a glass tube for image recording using a digital camera. The water layer was mostly removed from the beaker using a pipet, and the residue in the beaker was washed four times with a large amount of water. The washed residue (containing unreacted PU) was concentrated, dried, and weighed. As a control, anhydrous unmodified starch (4 g) was extracted using the above water/butyl acetate solvent mixture. To measure the efficiency of this analysis method, COPU power (1 g) and anhydrous unmodified starch (4 g) were mixed and extracted using the same procedure as that for CP-4.6. The “CS-SR” and “CP4.6-SR” represent the solid residue in the butyl acetate layer of the CS and CP-4.6 samples, respectively. The reaction ratio of modified starch was calculated as follows: reaction ratio ð%Þ ¼ 100 ðWL W 1 Þ=W 2 100 where WL represents the weight of solid residue in the butyl acetate layer for modified starches, W1 is the weight of solid residue in the butyl acetate layer for anhydrous unmodified starches, W2 is the weight of PU used to modify the starch in theory (1 g). Three duplications were carried out. Moisture Content Measurement. Small parts (about 1 g) were cut from the molded starch sheets, weighed, and dried at 110 °C. After 6 h, the sample was weighed again to calculate the moisture content in the sample: moisture content ð%Þ ¼ ðW 1 W 2 Þ=W 1 100 where W1 and W2 were the weight of the 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 weeks, and then the mechanical properties were measured using a tensile tester (6PTS 2000S, Shenzhen GaoPin Test Machine Co. Ltd.) with a strain rate of 12.5 mm/min. The distance between the two clamps was 40 mm. Strength at break (σb, MPa) and elongation at break (εb, %) of the sheets were recorded. Five duplications were carried out. Fourier Transform Infrared Spectroscopy (FTIR). An FTIR spectrometer (Avator 360, Nicolet, Massachusetts, USA) at room temperature was used. Test samples were pulverized with KBr and pressed into transparent disks for analysis. All sample spectra were recorded in transmission mode at a resolution of 4 cm1 with accumulation of eight scans. Elemental Analysis. Because a nitrogen atom exists in the PU component instead of a starch macromolecule, the nitrogen content in PU is appropriated to the PU content in the modified TPS. An elemental analyzer was therefore used to measure nitrogen content in the sample, and the PU content can be calculated on the basis of the nitrogen content data. Separated PU (SPU) powder as shown in Figure 1 was dried at 60 °C for 24 h, and subjected to elemental analysis using an element analyzer (Vario, EL111, Germany). The nitrogen content to calculate the purification of polyurethane was determined as follows: purification degree ð%Þ ¼ N ðSPUÞ =N ðCOPUÞ 100 where N(SPU) and N(COPU) represent the N content of SPU-5.8 and COPU, respectively. Emission Scanning Electron Microscopy (ESEM). An ESEM (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
Figure 2. FTIR spectra of CS, DS-5.8, SPU-5.8, CP-5.8, CP-6.0, CP-7.0, and COPU.
cross sections. The cross sections were coated with gold and then used for ESEM observation. Malvern Laser Particle Analysis (MLPA). Anhydrous starch (1 g) was dispersed in boiling water (99 g) and the hot dispersion was intensively mixed in a kitchen mixer for 1 min. The particle size distribution of the dispersion at 25 °C was analyzed using a Malvern Laser particle analyzer (ZEN3690, Malvern Instruments Ltd., UK). Viscosity Measurements. A rotary viscosimeter analyzer (DV-II+PRO LV, Brookfield, U.S.) was used to determine viscosity properties of the starch suspension. Before the test, castor oil was used to calibrate the instrument. An aqueous suspension of starch (6%, w/w) was prepared, and the starch suspension was cooked at 60 °C with stirring. The Brookfield viscosity of the sample was measured with a No. S61 spindle at a spindle speed of 100 rpm. The viscosity of dispersion samples was also tested at 70, 80, and 90 °C, respectively. The viscosity of CS dispersion at 90 °C was measured with a spindle speed of 20 rpm and a No. S62 spindle. X-ray Diffraction (XRD). Sheets were measured with wideangle X-ray diffraction (WAXRD) (Y-2000 Dandong radiative instrument group LTD CO., China). For irradiation, the Cu Kα line was applied (λ at 0.1542 nm, cathode at 30 kV and 20 mA), and scattering was recorded as 2θ in the range from 2 to 40°.
’ RESULTS AND DISCUSSION Structure. FTIR spectra of CS, CP-5.8, CP-6.0, CP-7.0, PUP, degraded starch (DS), and separated PU (SPU) are shown in Figure 2. The peak for PUP at 2270 cm1 was attributed to the absorption of the NCO groups. The peak for modified starches at 2270 cm1 disappeared, implying complete consumption of the NCO groups. As compared with the spectrum of CS, a new peak for CP-5.8 at 1725 cm1 was assigned to absorption of urethane linkages.14,18 The spectra profiles for CP-6.0, CP-5.8, and CP-7.0 were almost the same, suggesting a similar composition for the three samples. The CH2 bending absorption of DS-5.8 occurred at 1650 cm1, which was the same as that of CS. In addition the peak profile of DS was similar to that of CS, indicating that their composition was similar except for their 11908
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Figure 4. WAXRD patterns of TPS, CP-5.8, and COPU. Figure 3. FTIR spectra of CS-SR and CP4.6-SR. The “CS-SR” was the solid residue in butyl acetate layer of the CS sample, and the “CP4.6-SR” was the solid residue in butyl acetate layer of the CP-4.6 sample.
Table 1. Elemental Analysis Results for COPU and CP-5.6 elementary weight compositions sample
%C
%H
%O
%N
COPU (theoretical value) COPU (experimental value)
72.7 73.3
8.2 8.4
14.3 13.2
5.0 5.1
SPU-5.6
69.0
8.7
18.2
4.1
molecular weights. There was no absorption of urethane linkage at 1750 cm1 in the spectrum of DS, implying that PU was completely separated from DS and the enzyme method was an efficient way to study the fractionation of modified TPS. FTIR spectra of CS-SR and CP4.6-SR are shown in Figure 3. The band at 1644 cm1 for CS-SR was assigned to the absorption of carbonyl groups in protein,19 which was in accord with the results obtained by Shukla et al., who reported that native cornstarch contained little protein.20,21 The same absorption peak was also observed in CP4.6-SR, and the peak at 1736 cm1 for CP4.6-SR was attributed to the absorption of urethane groups, these indicated CP4.6-SR contained corn protein and free polyurethane components. Elemental analysis data is shown in Table 1. The nitrogen content of COPU (5.1%) as shown in Table 1 was near to its theoretical value (5.0%). This result indicated good accuracy of the elemental analysis. The nitrogen content of SPU-5.8 was 4.1% and the calculated purification degree of SPU was 82%, indicating some polysaccharides may be linked to the PU component in SPU. It was necessary to degrade the modified TPS in optimal conditions to increase the purity of the final SPU product in future work. Average particle sizes of CP-7.0, CP-6.0, CP-5.8, and CS were 976.6 ( 181.6, 586 ( 184.3, 789.1 ( 123.8, and 163 ( 18.0 nm, respectively. As the hydrophobic PU linked to the starch components, the particle sizes of the modified TPS were increased compared to that of CS. With an increase of NCO content from 5.8% to 7.0%, there was no significant difference in particle size for the modified starches, indicating NCO content had no significant effect on particle size changes of the modified starch. The Brookfield viscosity of CS at 60, 70, 80, and 90 °C were 24.4 ( 1.14, 37.6 ( 0.97, 510.1 ( 63.3, and 4355.7 ( 587.4 cP, respectively. From 60 to 90 °C, the viscosity of CS increased from 24.4 to 4355.7 CP. With an increase of temperature, the crystalline structure in CS granules was destroyed,
and the interaction between starch macromolecules and water was improved, and finally the viscosity of CS significantly increased.22 The Brookfield viscosity of CP-7.0 at 60 °C, 70 °C, 80 and 90 °C were 10.4 ( 0.8, 10.6 ( 0.9, 11.4 ( 0.6 and 12.5 ( 0.2 cP, respectively. As the temperature increased, there was no significant change in viscosity for CP-7.0, indicating that the hydrophobicity of CP-7.0 was improved due to the existence of the hydrophobic PU particles. In addition, the starch macromolecules tended to form a cluster structure due to the existence of hydrophobic PU linkages which would reduce the interaction between starch and water. WAXRD patterns of TPS, CP-5.8, and castor oil-based PU (COPU) are shown in Figure 4. The peaks at 14.9°, 18.2°, and 22.5° were associated with the V-type crystallinity of starch in the CP-5.8 sample, which was formed after recrystallization of amylose after the destructurization or plasticization of starch granules.23 The peak distribution was also similar to that of TPS due to the existence of the starch component in CP-5.8. The peak at 21.1° on the COPU curve was associated with the crystallization of MDI-based PU.24 Morphology. SEM images of CP-5.6 are shown in Figure 5. Several microparticles assigned to the PU-rich dispersion are shown, suggesting that PU microparticles were formed during the intensive mixing. As shown in the SEM micrograph, large polyurethane particles were dispersed in the TPS matrix and the compatibility between them was poor. However, small polyurethane particles showed good compatibility with starch. This result was in disagreement with that observed by our previous work.14 This may be because the NCO content in CP-5.6 was low and the interaction between large polyurethane particles and starch was weak. Therefore, the compatibility also decreased with the NCO content decreasing in modified starch. Mechanical Properties. As shown in Table 2, the CS sample showed high strength (43.7 MPa) and low elongation (2.7%), indicating the brittleness of the material. However, with an addition of 20% polyurethane modifier, CP-7.0 showed lower strength (37.4 MPa) and higher elongation (17.2%) as compared to that of CS, implying the PU is a plasticizing agent. With a decrease of NCO content in PUPs from 7.0% to 3.5%, the strength of the modified TPS decreases from 37.4 to 19.3 MPa, and the elongations also decrease from 17.2% to 1.7%, suggesting that the NCO content in PUPs are one main factor in improving the tensile properties of the modified TPS. A series of semi-interpenetrating polymer networks (semi-IPNs) from castor oil-based polyurethane (PU) and benzyl starch (BS) was prepared by Cao et al., who concluded that the elongation of the IPNs would decrease with a decrease of PU content from 100 to 30 wt %.25 With the increasing NCO content, the 11909
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Figure 5. SEM images of CP-5.6. SEM images of (a) CP-5.6 (10.0 KV 3000) and (b) CP-5.6 (10.0 KV 1000).
Table 2. Formulations of Starch Modification and the Tensile Properties of Molded TPS Sheets formulations for modification
properties of molded sheets CS-Water-PUP (g)
σb (Mpa)
εb (%)
moisture (%)
7.0 6.0
45.815.811.5 45.815.811.5
37.4 ( 3.1 33.2 ( 1.0
17.2 ( 1.6 13.1 ( 1.6
10.0 ( 1.2 9.7 ( 1.0
CP-5.8
5.8
45.815.811.5
33.7 ( 0.1
12.9 ( 0.8
10.1 ( 1.9
CP-5.6
5.6
45.815.811.5
31.4 ( 2.1
12.9 ( 1.5
9.4 ( 1.4
CP-5.0
5.0
45.815.811.5
28.8 ( 1.5
12.2 ( 1.1
9.9 ( 0.3
CP-4.6
4.6
45.815.811.5
28.2 ( 1.7
4.3 ( 0.7
9.9 ( 0.8
CP-3.5
3.5
45.815.811.5
19.3 ( 5.0
1.7 ( 0.4
10.1 ( 0.1
CS
0
60.024.00
43.7 ( 3.1
2.7 ( 0.4
9.6 ( 0.7
CS-G
0
45.013.509.8
12.5 ( 1.2
5.0 ( 1.1
12.1 ( 0.8
samples
NCO% in PUP
CP-7.0 CP-6.0
number of urethane linkages between PU microparticles and the starch matrix would increase, thus improving the compatibility of the two polymers. In addition, the PU is an elastomer which can efficiently absorb impact energy.26 Therefore, the toughness of the modified TPS can be improved with increasing compatibility through the addition of a PU component. Both the elongation and strength of CP-3.5 were lower than that of CS, implying that the number of urethane linkages between starch and PU were obviously too small to increase compatibility of the two polymers. After being aged at 60% RH for 20 days, the CS-G sample containing 20% glycerol showed lower strength (12.5 MPa) and higher elongation (5.0%) than the CS showed, indicating glycerol was a plasticizer for starch but deteriorated in strength over time. In one reference,27 NCO groups of polyurethane prepolymer were consumed using a large amount of water for preparing waterborne polyurethane (WPU), and the WPU was blended with starch in an extruder. They found that the tensile properties of starch were also improved due to the existence of hydrogen bonding between the waterborne polyurethane and starch, indicating that interaction or compatibility was important to improve mechanical properties of the blend. In our study, the PUPs were directly mixed with starch melt to form urethane linkages, and the covalent bond provided stronger interaction than the hydrogen bonding reported by Lu et al.,27 and thus the tensile properties were improved significantly in our study.
Figure 6. Images for analyzing reaction efficiency.
Reaction Ratio Analysis. Images for analyzing reaction efficiency are shown in Figure 6. The weight of CS-SR and CP4.6-SR were 0.02 ( 0.01 and 0.03 ( 0.01 g, respectively. According to the FTIR analysis (Figure 2.), CP4.6-SR contained corn protein and free polyurethane components. The reaction ratio of PUPs used for preparing CP-4.6 was 99.8 ( 0.1% by our calculation; this result indicated that the PUPs were almost completely cross-linked to the starch matrix, suggesting that the modification was conducted successfully. As shown in Tube 2 11910
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Industrial & Engineering Chemistry Research (a mixture of COPU powder and CS), the weight of the solid residue (0.98 ( 0.02 g) in the butyl acetate layer was near to the added amount of COPU (1 g), indicating that PU powder could be almost completely extracted out of the water medium by use of butyl acetate. Therefore, the method for analyzing the reaction ratio of PUPs was effective.
’ CONCLUSIONS PUPs with various NCO content were prepared and used to prepare novel modified TPS in an environmentally friendly way. The reaction ratio of PUPs used for preparing CP-4.6 was 99.8 ( 0.1%, indicating that PU was almost cross-linked to the starch matrix, suggesting successful modification. With an increase of NCO content, more urethane linkages were formed, and the compatibility of starch and PU was enhanced, which resulted in the improvement of the tensile properties of the modified TPS. Therefore, the NCO content of polyurethane played an important role in improving the mechanical properties of TPS. The viscosity of the modified TPS was obviously lower than that of pure starch owing to the starch macromolecules linked with hydrophobic PU particles. The modified TPS sheet showed higher thermal stability and hydrophobicity than that of pure starch material, due to the strong interaction between starch and polyurethane. Therefore, the urethane linkages between PU and starch were essential to improve the miscibility and properties of the modified TPS materials. ’ AUTHOR INFORMATION Corresponding Author
*Tel./Fax: +86-27-67867953. E-mail: greenpolymerlab@ yahoo.com.
’ ACKNOWLEDGMENT The authors would like to express their appreciation for the financial supports from the National Natural Science Foundation of China under Grant No. 50803024, from the Self-determined Research Funds of CCNU from the Colleges’ basic Research and Operation of MOE under Grant No. 120002040059, and from the Graduate Student Foundation for Self-starting Innovation of CCNU (Grant No. 2009009). ’ REFERENCES (1) Cao, X. D.; Zhang, L. N.; Huang, J.; Yang, G.; Wang, Y. Structureproperties relationship of starch/waterborne polyurethane composites. J. Appl. Polym. Sci. 2003, 90, 3325. (2) Maria, N.; Szczepan, Z.; Monika, S. Environmentally friendly photosensitizer: Starch modified with chlorophyll-type chromophores. Polym. Int. 2007, 56, 635. (3) Landreau, E.; Tighzert, L.; Bliard, C.; Berzin, F.; Lacoste, C. Morphologies and properties of plasticized starch/polyamide compatibilized blends. Eur. Polym. J. 2009, 45, 2609. (4) Liu, P.; Yu, L.; Liu, H. S.; Chen, L.; Li, L. Glass transition temperature of starch studied by a high-speed DSC. Carbohydr. Polym. 2009, 77, 250. (5) Wang, N.; Yu, J. G.; Ma, X. F. Preparation and characterization of thermoplastic starch/PLA blends by one-step reactive extrusion. Polym. Int. 2007, 56, 1440. (6) Chaleata, C. M.; Rowan, P. J. H.; Truss, W. Properties of a plasticised starch blend. Part 1: Influence of moisture content on fracture properties. Carbohydr. Polym. 2008, 71, 535.
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