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Preparation and Characterization of Thermoplastic Starch Mixed with Waterborne Polyurethane Qiangxian Wu and Lina Zhang* Department of Chemistry, Wuhan University, Wuhan 430072, China
A polyester-type waterborne polyurethane (WPU) was synthesized with 2,4-tolylene diisocyanate (TDI), poly-1,4-butylene glycol adipate (PBA), and 2,2-bis-(hydroxylmethyl) propionic acid (DMPA). A series of compression-molded sheets coded as STPU were prepared by mixing thermoplastic starch (TPS) with the desired amount of WPU. The properties and structure of the sheets TPS, STPU, and WPU were investigated by tensile tests, wide X-ray diffraction patterns (WXRDPs), differential scanning calorimetry, infrared spectroscopy, ultraviolet spectroscopy, and scanning electron microscopy. The results showed that the tensile strength, breaking elongation, and water resistance of the STPU sheets were all higher than that of TPS, when the WPU content was in the range from 5 to 30 wt %. The STPU sheets were found to have higher crystallinity than WPU and amorphous TPS and slightly lower light transimitance than TPS, suggesting that partially recrystallized starch formed. The WPU plays an important role in the formation of new morphology and in the enhancement of the mechanical properties of the STPU sheet. The compression-molded STPU sheets provided a potential application in the field of biodegradable materials. Introduction In the past decades, starch has attracted considerable attention because of its low cost, availability from renewable resources and excellent biodegradability,1,2 and it has been used in agricultural mulch,3-5 industrial foam,6 and food packaging.7 In 1972, Griffin was the first to use a screw extruder to prepare biodegradable starch-based polyethylene products, such as mulch in agriculture.8 However, this blend showed that it is only partial biodegradable.9 In an effort to make fully biodegradable starch-based materials, much research has been done recently. Preparation of casting starch films based on edible food and degradable materials have been studied.10-13 Moreover, several efforts have been made on the study of thermoplastic starch (TPS), including its composition,14,15 molecular mass,16 plasticizers,17-21 and blends.23 Processing techniques for TPS mainly include extrusion, kneading, pressure, and injection molding, and the recent studies for TPS focus on its plasticizers such as water19 and glycerol.20,21 Polyurethane (PU) materials have been generally used in the automobile, paint, furniture, and textile industries24 and also in biomass foam incorporated with renewable raw materials.25 Waterborne polyurethane (WPU), as a nontoxic and nonflammable material, has been abundantly used as an environmental coating26,27 and an adhesive,28 and it is endowed with excellent properties. In particular, polyester-type WPU not only has excellent mechanical properties, but also has a hydrolytic character, which makes WPU easy to degrade29,30 and, therefore, friendly to the environment. WPU has a potential miscibility with starch because both have hydrophilic groups. Therefore, it is possible to mix WPU with starch to improve the mechanical properties of TPS. In addition, we have successfully prepared biodegradable and water-resistant regenerated * Correspondence to: L. Zhang. E-mail: public.wh.hb.cn. Fax: +86-27-87882661.
lnzhang@
cellulose films coated with polyurethane/chitosan31 or polyurethane/nitrocellulose.32 Furthermore, polyurethane blends with natural polymers were shown to be easily degraded and metabolized by to be microorganisms, leading to the production of CO2, H2O, glucose, aromatic ether, glucopyranose derivatives, and nitrate.33 The miscibility, structure, and properties of blend membranes from cellulose with other polymers, such as casein34 and alginate,35 were investigated in our laboratory. The results indicated that their mechanical strength, thermostability, and light transmittance were higher than those of regenerated cellulose film. This suggests that blending is a simple and useful method for improving properties of natural polymers. In this work, an attempt was made to mix thermoplastic starch and WPU to prepare new thermoplastic materials, which can be used in medicine and food field. A polyester-based WPU was mixed with thermoplastic starch and compression-molded with a heating press to obtain an improved TPS. The effect of the WPU content on the mechanical properties, water resistance, thermal stability, and morphology of the TPS/WPU mixture was investigated by tensile tests, wide X-ray diffraction patterns (WXRDPs), differential scanning calorimetry (DSC), infrared spectroscopy (IR), ultraviolet spectroscopy (UV), and scanning electron microscopy (SEM). Experiment Materials. Commercial 2,4-tolylene diisocyanate (TDI, supplied by Shanghai Chemical Co., China) was vacuumdried at 80 °C for 2 h and used as hard segments. Poly1,4-butylene glycol adipate (PBA2000, Mw ) 2150, supplied by Zhangjing Polyurethane Factory, China) was vacuum-dried at 105 °C for 5 h, and used as soft segments. 2,2-Bis-(hydroxylmethyl) propionic acid (DMPA, supplied by Chengdu Polyurethane Co., China), used as the chain extender and anionic center, was vacuum-dried at 110 °C for 2 h. Triethylanine (TEA) and acetone, used as the neutralizing reagent and
10.1021/ie000582t CCC: $20.00 © 2001 American Chemical Society Published on Web 12/20/2000
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Figure 1. Sketch map of mold used.
solvent, respectively, were immersed in 4 Å molecular sieves for more than a week to become dehydrated before use. DMPA, TEA, and acetone were of analytical grade. Cornstarch containing 23.5% amylose and 75% amylopectin was purchased from Wuhan Starch Co. (China) and vacuum-dried at 105 °C for 5 h before use. Preparation of Waterborne Polyurethane. Anionic waterborne polyurethane (WPU) was synthesized according to the acetone process reported by Yang et al.36 The ratio of NCO/OH in this case was 1.75. A 500mL four-necked flask was fitted with a thermometer, a stirrer, and an inlet and outlet of dry nitrogen. PBA2000 (70.0 g) and DMPA (4.4 g) were added to the flask and stirred at 120 °C for 15 min to easily dissolve the DMPA and then cooled to 85 °C. TDI (19.8 g) were introduced into the flask under dry nitrogen with a funnel, and the reaction was carried out for 2-3 h until the NCO group content reached a given value, which was determined by dibutylamine back-titration.37 When the product was cooled to 52 °C, 80 mL of acetone was added to reduce the viscosity of the prepolymer and then neutralized with 3.3 g of TEA for 30 min. Finally, 290 g of distilled water was added quickly with vigorous stirring, thus forming WPU after 20 min. After being stored in room temperature for a week, the aqueous mixture was concentrated to 60% solid content at 30 °C with a rotary vacuum-evaporator to remove residual acetone. The viscosity of the WPU emulsion was measured by a rotational viscosimeter (NDJ-4, Shanghai Balance Factory) to be 52 MPas. Compression Molding. A heat press device was made by our laboratory, and the sketch map is shown in Figure 1. The press device was equipped with a mold, a pair of steel blocks, whose temperature was controlled, and a lab jack with a pressure meter. Starch was mixed with desired amounts of waterborne polyurethane (WPU), namely, 5, 10, 15, 20, 25, and 30 wt %, a series of TPS/ WPU mixtures were prepared and coded STPU5, STPU10, STPU15, STPU20, STPU25, and STPU30,
respectively. The pure thermoplastic starch was coded TPS. Finally, water was added to adjust the water content to 33 wt %. Every premix was crushed with a mortar and pestle for 30 min and then sealed and stored in refrigerator at 5 °C for 2 days. A pure casting film of WPU was prepared according to our previous work38 and coded WPU. An 8-g sample of starch or a mixture of starch and WPU was placed into the mold and covered with polished stainless steel plate on both sides, and silicon oil was sprayed on the stainless steel plates as a mold oil. The mold was placed between the steel blocks, whose temperature was controlled at 155 °C, and then the pressure was quickly increased from 0.5 to 15 MPa for 1 min and kept for 5 min at 155 °C. After the compression molding, the mold was cooled to room temperature with a fan at a rate of 10 °C/min. A transparent sheet was released from the mold and stored at room temperature for a month at a relative humidity of 65%, in accordance with ISO/R483-1966 (E). Characterization. The wide X-ray diffraction patterns (WXRDPs) were recorded with an X-ray diffractometer (D/MAX-1200, Rigaku Denki, Japan) and with Cu KR radiation (λ ) 1.5405 × 10-10 m) at 40 kV and 30 mA in the range of 2θ ) 6-40°. The degree of crystallinity (χc) was calculated according to a previously reported method.39 Infrared (IR) spectra of the sheets on KBr disks were recorded with an FTIR 3000 spectrometer (Shimadzu Co.). The light transmittance of the sheets with a thickness of 0.35 mm was measured in the wavelength range of 400-800 nm by using an UV160A Spectroscope (Shimadzu Co.) according to the Beer-Lambert law to give percent light transmittance. Sections of the sheets were studied with the aid of a scanning electron microscope (S-570 SEM, Hitachi). The sheets were frozen in liquid nitrogen and then fractured and coated with gold prior to the SEM experiments. The differential scanning calorimetry (DSC) analysis of the samples was performed with a thermal analyzer (Perkin-Elmer DSC-2C, Norwalk, CT) under nitrogen atmosphere from -30 to 150 °C at a heating rate of 10 °C/min. Measurement of Mechanical Properties. The mechanical properties of the sheets were measured with a tensile tester (CMT6503, Shenzhen SANS Test Machine Co. Ltd., China) according to ISO6239-1986 (E) with tensile speed 5 mm/min to determine the tensile strength (σb) and breaking elongation (b). The sheets were immersed in water at room temperature for 1 h and then the water resistance was measured.31 The water resistance (R) of the sheets was evaluated from the σb(dry) value in the dry state and the σb(wet) value in the wet state by the following equation:
R ) σb(wet)/σb(dry)
(1)
The water content (WH2O, wt %) in the sheets was measured according to ISO62-1980 (E). WH2O was calculated by applying the formula
WH2O ) [(Ww -Wd)/Wd] × 100
(2)
where Ww is the weight of the wet sheet and Wd is the weight of the dry sheet. The moisture content (Wmc, wt % ) was determined as described in ref 40 and calculated using
Wmc ) [(Wm - Wd)/Wd] × 100
(3)
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Figure 2. Effect of WPU content (WWPU) on the tensile stress at break (σb) and breaking elongation (b) of the STPU sheets.
Figure 3. Effect of WPU content (WWPU) on the water resistance (R) of the STPU sheets.
where Wm stands for the weight of the sheet stored at 75% RH for 1 week and Wd is the weight of the dry sheet. Results and Discussion Effects of WPU Content on Mechanical Properties. The effects of the WPU content on the tensile strength (σb) and breaking elongation (b) of the molded sheets are shown in Figure 2. The tensile strength of WPU was 23 MPa and the breaking elongation was 520%. The tensile strengths of the sheets STPU15STPU30 were higher than those of both TPS and WPU, suggesting an interaction between starch and WPU. The breaking elongations of the STPU sheet were between those of starch and WPU and much higher than that of TPS. The results indicate that the mechanical properties of TPS improved remarkably by mixing with WPU using the heating-press method. The WPU plays an important role in improving the mechanical properties of the STPU sheets. Effects of WPU Content on Water Resistance. Figure 3 shows the effect of the WPU content (WWPU) on the water resistance (R) of the sheets. The water resistance of the STPU sheets increased considerably with increasing WPU content. The R value for the sheet TPS was 0.01, and that for STPU30 was 0.24, 24 times that of the TPS. The latter value was attributed to the excellent water resistance of the WPU component. The effect of the WPU content (WWPU) on the absorbed water (WH2O) is shown in Figure 4. The WH2O value of the TPS sheet was about 317 wt %, significantly higher than the corresponding values for the STPU sheets, which were 70% for STPU10, 34% for STPU20, and 58% for STPU30.
Figure 4. Effect of WPU content (WWPU) on the water content (WH2O) of the STPU sheets.
Figure 5. Moisture content for the STPU sheets as a function of WPU content.
This further supports our claim that mixing the TPS with WPU was beneficial in terms of improving its water resistance. The effect of WPU content on moisture content of the STPU sheets is shown in Figure 5. With increasing WPU content, the moisture content of the STPU sheets exhibited a sharp drop, indicating that polyester-based WPU enhanced substantially the moisture-proofness of the STPU mixtures because of the low moisture content for WPU (0.1%). Structure and Miscibility. The WXRD patterns of the TPS, STPU20, and WPU sheets are displayed in Figure 6. The degrees of crystallinity (χc) of TPS, STPU20, and WPU were 0.18, 0.45, and 0.44, respectively. The TPS material was almost amorphous. The peaks at 19.5° and 13° in the TPS pattern are attributed to V-type crystallinity, formed after recrystallization of amylose after destructurization or plasticization of starch granules.22 The result indicates that partially recrystallized starch formed in the sheet STPU. Three diffraction peaks of WPU at 2θ)13.8°,16.8°, and 21.4° are due to the orderly arrangement of soft segments, and a peak at 24.3° is from hard segments.36 Interestingly, the peak at 16.8° for the STPU20 sheet was higher than the peaks recorded for the other two samples. It has been reported that the B-type crystallinity at 16.8° for TPS is attributed to recrystallization of both amylose and amylopectin.22 The sharp peak at 16.8° for WPU resulted from the orderly arrangement of soft segments.36 It is worth noting that the peak at 24.3° in the STPU20 pattern disappeared, because of the interaction between the -COOH or -NH groups of WPU and the -OH groups of starch. Therefore, the TPS and WPU are miscible. However, in general, the crystallinity of the mixtures with good miscibility is expected
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Figure 6. X-ray diffractograms of the TPS, STPU20, and WPU sheets.
Figure 8. IR spectra of the sheets of TPS, STPU20, an uncompressed mixture of TPS and WPU, and WPU.
Figure 7. DSC thermograms of the TPS, STPU20, and WPU sheets. Table 1. Experimental Results of DSC for the TPS, STPU, and WPU Sheets
material
melted crystallinity peak (°C) onset point midpoint endpoint
melting enthalpy (J/g)
STPU20 PU
34 ( 3 35 ( 3
0.71 ( 0.3 4.46 ( 0.6
36 ( 1 40 ( 2
46 ( 4 48 ( 2
to decrease; the recorded increase in χc for the STPU20 sheet in this case implies that the mixture had a certain level of miscibility and was not perfect on a molecular scale. Figure 7 displays some representative DSC thermograms of the TPS, STPU20, and WPU sheets. Their melting points and melting enthalpies are shown in Table 1. The Tg of WPU usually was between -70 and -40 °C.41 The Tg of TPS containing 13.3% water content
Figure 9. Effect of WPU content (WWPU) on the light transmittance of the sheets at 600 nm.
was not obvious on the DSC curve. In addition, the ∆CP of starch is very low. Both STPU20 and WPU exhibited clear endothermic peaks, indicating that they both contained crystalline or semicrystalline material. However, the intensity and position of the peaks for the STPU20 sheet were lower than those for WPU, suggesting miscibility. On the contary, TPS behaved rather as an amorphous material almost without any crystal melting peak. Two small peaks for STPU20 might indicate a crystallization melting of WPU and a recrystallization-melting process for a new crystal peak of the partially recrystallized starch. The melting enthalpy of STPU20 was much lower than that of WPU, resulting from the partial crystallinity of WPU being destroyed
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Figure 10. SEM images of sections of (A) TPS, (B) STPU20, (C) STPU30, and (D) PU.
(see Figure 6), and the melting enthalpy of the recrystallized starch in STPU20 is very low. These results support the conclusion from the WAXDPs. The IR spectra of sheets of TPS, STPU20, a mixture of TPS and WPU (80:20), and WPU are shown in Figure 8. The absorption of the carboxylic groups of WPU at 1225 cm-1 significantly decreased in the IR spectra of STPU10-STPU30, indicating that there is an interaction between the carboxylic groups of WPU and the hydroxyl groups of TPS. The band at around 1736 cm-1 is attributed to the stretching of free urethane carbonyl groups, whereas the band at 1726 cm-1 is assigned to hydrogen-bonded urethane carbonyl groups.42,43 Compared with the uncompressed mixture, the native intensity of the peak for STPU20 at 1726 cm-1 increased. This indicated that some of the free-urethane carbonyl groups of the polyurethane had bonded with hydrogen groups of the starch, suggesting that an intermolecular force occurred between polyurethane and starch in the STPU. The structure analysis is in good agreement with the results from mechanical properties, the WXRDPs, and DSC. Therefore, the WPU plays an important role in forming of a new morphology and enhanceming the mechanical properties of the STPU. Figure 9 shows the light transmittance of the STPU sheets as a function of WPU content. Molded starch TPS exhibited higher light transmittance (95%) than the STPU and the cast starch film prepared and measured before (63%). The light transmittance of STPU5-
STPU30 reached approximately 86%, similar to that of WPU (88%). The results imply that the granular structure of the starch has been utterly deformed or destroyed and a continuous polymeric phase has been formed, which led to the enhanced light transmittance of the STPU. Otherwise, the light transmittance should be more lower due to recrystallization of starch. Therefore, the starch and WPU in the mixtures are miscible. Figure 10 shows SEM images of the sections of the sheets of TPS, STPU20, STPU30, and WPU. The SEM images of all of the samples indicated a homogeneous structure for the sections. For TPS, this is due to high pressure and temperature, resulting in all of the granules of the starch being broken up and formed into a homogeneous phase. The section of WPU was slightly coarse because of microphase segregation between hard and soft segments. The sheets STPU20 (B) and STPU30 (C) were characterized by homogeneous morphology containing TPS and WPU, an assumption supported by the light transmittance experiments shown in Figure 9, which suggest a miscibility between starch and waterborne polyurethane. Conclusions Compression-molded sheets based on mixtures of polyester-based WPU and thermoplastic starch (TPS) were successfully prepared. When the WPU content was in the range of 5-30 wt %, both the tensile strength
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and the breaking elongation of the STPU sheets were higher than those of TPS. The water resistance, moistureproofness, and breaking elongation of the mixtures significantly increased with an increase in the WPU content. The IR, WXRDP, DSC, and SEM analyses of the prepared samples indicated that interactions between TPS and WPU exist in the STPU sheets, resulting in a certain level of miscibility. The WPU played an important role in improving the mechanical properties and water resistance and enhancing the miscibility. Acknowledgment This work was supported by the National Natural Science Foundation of China (59773026 and 59933070) and the Laboratory of Cellulose and Liginocellulosic Chemistry of the Chinese Academy of Science. Literature Cited (1) Satyanarayama, D.; Chaterji, P. R. Biodegradable polymers: Challenges and strategies. J. Macromol. Sci.sRev. Macromol. Chem. Phys. 1993, 33, 349. (2) Wool, P. R. The science and engineering of polymer composite degradation. In Degradable Polymers; Scott, G., Gilead, D., Eds.; Chapman & Hall: London, 1995; p 207. (3) Arvanitoyannis, I. S.; Psomiadou, E.; Ogawa, H.; Biliaderis, C.; Kawasaki, N.; Nakayama, A. Biodegradable films made from LDPE ethylene acrylic acid (EAA), polycaprolactone (PCL) and wheat starch. Part 3. Staerke/Starch 1997, 49, 306. (4) Arvanitoyannis, I. S.; Biliaderis, C. G.; Ogawa, H.; Kawasaki, N. Biodegradable films made from low-density polyethylene (LDPE), rice starch and potato starch for food packaging applications. Part 1. Carbohydr. Polym. 1998, 36 (2-3), 89. (5) Arvanitoyannis, I. S. Totally and partially biodegradable polymer blends based on natural and synthetic macromolecules: preparation, physical properties and potential. J. Macromol. Sci.s Rev. Macromol. Chem. Phys. 1999, C39 (2), 205. (6) Alfani, R.; Iannace, S.; Nicolais, L. Synthesis and characterization of starch-based polyurethane foams. J. Appl. Polym. Sci. 1998, 68 (5), 739. (7) Guilbert, S.; Gontard, N. Edible and biodegradable food packaging. In Food Packaging Materials; Ackerman, P., Jaegerstad, M., Ohlsson, T., Eds.; Royal Society of Chemistry; Oxford, U.K., 1995, 159. (8) Griffin G. J. L. Biodegradable plastics. Brit. Patent Appl. 23469/72, 1972. (9) Thakore, I. M.; Iyer, S.; Desai, A.; Lele, A.; Devi, S. Morphology, thermomechanical properties, and biodegradability of low-density polyethylene/starch blends. J. Appl. Polym. Sci. 1999, 74 (12), 2791. (10) Arvanitoyannis, I. S.; Psomiadou, E.; Nakayama, A. Edible films made from gelatin, soluble starch and polyols. Part 3. Food Chem. 1997, 60 (4), 593. (11) Arvanitoyannis, I. S.; Biliaderis, C. Physical properties of polyol-plasticized edible films made from sodium caseinate and soluble starch blends. Food Chem. 1998, 62 (3), 3333. (12) Arvanitoyannis, I.; Nakayama, A.; Aiba, S. Edible films made from hydroxypropyl starch and gelatin and plasticized by polyols and water. Carbohydr. Polym. 1998, 36 (2-3), 105. (13) Wolff, I. A.; Davis, H. A. Preparation of film from amylose. Ind. Eng. Chem. 1951, 43 (4), 915. (14) VanSoest, J. J. G.; Borger, D. B. Structure and properties of compression-molded thermoplastic starch materials from normal and high-amylose maize starches. J. Appl. Polym. Sci. 1997, 64, 631. (15) VanSoest, J. J. G.; Essers, P. Influence of amyloseamylopectin ratio on properties of extruded starch plastic sheets. J. Macromol. Sci.sPure Appl. Chem. 1997, A34 (9), 1665. (16) VanSoest, J. J. G.; Benes, K.; De Wit, D. The influence of starch molecular mass on the properties of extruded thermoplastic starch. Polymer 1996, 37 (16), 3543.
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Received for review June 15, 2000 Revised manuscript received September 19, 2000 Accepted October 17, 2000 IE000582T
