Improved Mechanical and Thermal Properties of Polypropylene

Nov 6, 2013 - Here, we first prepared plasticized starch using diethanolamine as a reactive plasticizer and then fabricated polypropylene (PP)/starch ...
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Improved Mechanical and Thermal Properties of Polypropylene Blends Based on Diethanolamine-Plasticized Corn Starch via in Situ Reactive Compatibilization Lina Liu, Youming Yu,* and Ping’an Song* Department of Materials, College of Engineering, Zhejiang Agriculture & Forestry University, Hangzhou 311300, China ABSTRACT: Starch-based polymer blends are able to alleviate the environmental concerns. However, they usually suffer poor mechanical properties. Here, we first prepared plasticized starch using diethanolamine as a reactive plasticizer and then fabricated polypropylene (PP)/starch blends using maleic anhydride-grafted PP (MAPP) as the compatibilizer. It is found that the size of the starch domains gradually reduced with increasing MAPP loading, indicating significantly improved interfacial compatibility. Adding 15 wt % MAPP can basically retain the tensile strength and considerably enhance Young’s modulus, the bending strength, and the modulus of polymer blends relative to PP and its blends without compatibilization. Additionally, the glass transition temperature of PP is increased by 4.9 °C for the PP blend with 30 wt % MAPP. These largely enhanced mechanical properties are mainly attributed to improved interfacial adhesion. Our work provides a facile, effective strategy for reducing the environmental impact of the use of nonbiodegradable plastics.

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INTRODUCTION Polypropylene (PP), one of the cost-advantaged commodity polyolefins, is widely used in automobile, electrical equipment, furniture, and package applications for its excellent properties.1 However, PP derived from limited petroleum resources is not biodegradable, which may cause environmental disaster without proper disposal.2 Traditional pyrolysis of plastic wastes such as PP in landfill sites may produce carcinogens such as dioxin and greenhouse gases such as carbon dioxide.3,4 Besides, the process also consumes much energy. Therefore, it is rather urgent to develop more fully biodegradable plastics to reduce their environmental impact. Many attempts have used sustainable and renewable materials to prepare semibiodegradable PP blends to reduce its environmental footprint.5,6 Starch, one of the most abundant and inexpensive renewable natural polymers, can be degraded by microorganisms and thus can be used to prepare bioplastics and biodegradable polymer blends.7−12 However, dry granular starch, by itself, cannot be directly melt-processed because of strong hydrogen-bonding interactions and needs to be plasticized before melt blending with thermal plastics.13 Recently, Li et al. found that ethanolamine, diethanolamine (DEA), triethanolamine, ethylene glycol, and glycerol all are able to act as plasticizers for starch.14 Glycerol is widely employed to reduce the strong intermolecular hydrogenbonding interactions of starch and improve its processing property, but its presence usually significantly leads to a decrease in the mechanical properties of the resultant materials because of the plasticization effect.15−17 Moreover, the mechanical properties of starch-based PP composites are rather poor because of their polarity differences, leading to thermodynamic immiscibility. In situ reactive compatibilization is one of the mostly effective methods to improve the compatibilization of immiscible polymer blends during the extrusion process.18 In the case of PP, two approaches are commonly used in the reactive extrusion © 2013 American Chemical Society

owing to the lack of reactive groups in PP chains. One approach is the “two-step” process, in which both polymers are first functionalized and then melt-blended in the extruder. The other one is a “one-step” process, where an active compatibilizer is directly added to react with both polymers to form a stronger interface during the extrusion process, which has proven to be much more cost-effective.19 In the present study, we here attempted to use DEA as the reactive plasticizer for starch instead of traditional glycerol because of its higher reactivity. Maleic anhydride-grafted PP (MAPP) was also adopted as the compatibilizer for PP and starch to produce high-performance semibiodegradable polymer composites. In situ reactive compatibilization would be expected to considerably improve the dispersion of starch plasticized with DEA and the interfacial adhesion between starch and the PP matrix. The morphology and mechanical, thermal, and crystallinity properties of starch and PP blends are investigated in detail. Our results strongly showed that the mechanical and thermal properties of PP/starch blends are significantly improved.

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EXPERIMENTAL SECTION Materials. Commercially available corn starch was purchased from Hangzhou Mailiang Food Co., Ltd. Polypropylene (PP) is a copolymer of propylene (90% by mole ratio) and ethylene (10% by mole ratio) with Mw and Mn of 300000 and 63000, respectively. Maleic anhydride-grafted PP (MAPP; MA content 1 wt %) was obtained from Shanghai Rizhisheng Co., Ltd. Diethanolamine (DEA) was an analytical agent and was used without further purification prior to use. Received: Revised: Accepted: Published: 16232

August 2, 2013 October 21, 2013 October 24, 2013 November 6, 2013 dx.doi.org/10.1021/ie4025243 | Ind. Eng. Chem. Res. 2013, 52, 16232−16238

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Blend Preparation. PP/corn starch composites were prepared via a one-step melt-blending method. Prior to blending, 20 g of DEA and 80 g of corn starch were manually grinded for 20 min to fabricate plasticized starch. Both PP and its starch-based blends were fabricated via melt compounding using a Thermo Haake Torque rheometer at 180 °C for 10 min with a rotor speed of 60 rpm for each sample, with the experimental formulation listed in Table 1. Table 1. Formulation of Semibiodegradable PP/Starch Blends Prepared in This Work run

PP (wt %)

PP-g-MA (wt %)

corn starch (wt %)

DEA (wt %)

PP PCS PDS PDPS-5 PDPS-10 PDPS-15 PDPS-30

100 70 70 65 60 55 40

0 0 0 5 10 15 30

0 30 24 24 24 24 24

0 0 6 6 6 6 6

Characterizations. Scanning electron microscopy (SEM) images were recorded on a S4800 Scanning electron microscope (FEI, Japan) at an accelerating voltage of 5 kV. Tensile tests were performed with a CMT-6000 Electric Universal Tester (Shenzhen SANS, China) at 25 ± 2 °C according to an ASTM 638 standard, and the data reported here were the means of quintuplicate experiments. Flexural measurements were also carried out on a CMT-6000 Electric Universal Tester with a span of 40 mm (transversal area of 13 × 3 mm) according to an ASTM D 790 standard, and the data reported here were the means of quintuplicate experiments. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vector 22 FTIR spectrometer using the thin films of polymer blends. Dynamic mechanical analysis (DMA) was carried out using a PerkinElmer DMA 8000 analyzer in the dual-cantilever bending mode. The temperature dependence of the storage modulus (E′), loss modulus (E″), and tan δ was measured at a frequency of 1 Hz. The strain amplitude was 0.05%, and the heating rate was 3 °C/min in the temperature range −50 to +150 °C. The thermal stability of the samples was determined by thermogravimetric analysis (TGA) using a TA SDTQ600 (TA Instruments) at a heating rate of 20 °C under a nitrogen atmosphere, from room temperature to 700 °C. Differential scanning calorimetry (DSC) measurements were done on a DSC Q200 instrument (TA Instruments) under a nitrogen atmosphere (flow rate 50 mL/min) using approximately 11 mg samples. The thermal properties of neat PP and PP/starch blends were investigated by initially heating the samples at 10 °C/min from room temperature (25 °C) to 190 °C with an isothermal holding at 190 °C for 5 min to destroy the previous thermal histories. Thereafter, cooling was carried out at the same rate. Enthalpies of melting and degree of crystallization were calculated using TA Universal software.

Figure 1. SEM images of the fracture surfaces of PP/starch blends: (A) PCS, (B) PDS, (C) PDPS-10, and (D) PDPS-30, with a scale bar of 40 μm; (A1) PCS, (B1) PDS, (C1) PDPS-10, and (D1) PDPS-30 at higher magnification, with a scale bar of 5.0 μm.

Figure 2. Statistic starch domain size of PCS, PDS, PDPS-5, PDPS-10, and PDPS-30.

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RESULTS AND DISCUSSION Morphology. Figure 1 shows a typical image of the fractural surface of PP/starch blends. For PP/starch, starch granules are poorly dispersed in the PP matrix (as shown in Figure 1A), with an average particle size of ∼15 μm (as shown in Figure 2). Some starch granules resided in the PP matrix, while many holes or cavities can also be observed, which can be attributed

to the fall-out of starch particles from the cross section during the preparation of the samples. This can be related with poor interfacial action between a strong polar starch and a nonpolar PP matrix, as reported by other researchers.20 As for PP/DEAplasticized starch, the holes and cavities decrease significantly and a much more compact surface is found (see Figure1B), but 16233

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the size of the starch granules only slightly changes (∼14 μm), which can be due to the plasticization process of DEA. During the plasticization process, small DEA molecules more easily diffuse into corn starch and form new hydrogen bonding with starch macromolecular chains, which allows starch to more easily mix with PP and thus less starch granules will migrate the fractural surface (Figure 3).

starch, which will contribute to reduction of the interfacial tension and strengthening of the interfacial adhesion. To observe the interfaces more clearly, SEM images at higher magnification are also shown in Figure 1. The PP/starch blend (Figure 1A1) displays a poor interface, and some starch granules located loosely in the PP matrix tend to fall under strong shear force and thus generate the apertures observed in the above SEM images. On the starch being plasticized by DEA, the interfaces are slightly improved (Figure 1B1), which helps to prevent the starch granules from falling. The addition of MAPP into the systems remarkably improves the interfacial compatibility (Figure 1C1), indicating the formation of strong interfaces between PP and corn. With the further addition of MAPP, the interfacial compatibility is further improved and the average size of the starch domains is simultaneously reduced, as shown in Figure 1D1. IR Analysis. IR spectrometry was employed to confirm the in situ compatibilization reaction among MAPP, DEA, and starch during the melt-blending process, as shown in Figure 4. For PCS without DEA (Figure 4a), several broad and strong absorption peaks are observed, respectively located at 3100− 3600 cm−1 (O−H stretching from starch), 1723 cm−1 (CO stretching from starch), 1644 cm−1 (intramolecular hydrogen bonding), 1256 cm−1 (−O−C(O)− and C−OH stretching), 1167 cm−1 (C−O−C stretching), and 1080 cm−1 (C−O stretching vibration), which are characteristic peaks for starch.21 After plasticization by DEA (see Figure 4b), a new weak absorption peak appears at 3150 cm−1 attributed to the symmetrical stretching of N−H, and the weak peak intensity is most likely to due to the strong interactions of DEA and starch and the overlapping effects of N−H with the O−H groups. Before one investigates the IR spectra of PDPS systems containing DEA and MAPP, it is very important to study the IR spectrum of MAPP. MAPP (Figure 4f) exhibits two characteristic absorption peaks at 1784 and 1861 cm−1 belonging to the symmetrical and asymmetrical stretching vibrations of anhydride groups,21 while for all PDPS samples, both characteristic peaks of MAPP almost disappear (Figure 4c−e), strongly suggesting that the anhydride groups have been reacted with the N−H and O−H groups of DEA and starch. In addition, a new peak appears at 1695 cm−1 assigned to amide groups, which further confirms the reaction between anhydride groups and secondary N−H groups in DEA. DMA Properties. The storage modulus allows us to evaluate the stiffness of a material. As shown in Figure 5A, compared with neat PP, the addition of corn starch can significantly increase the storage modulus (E′) from 1.5 to 2.0 GPa at 25 °C. However, plasticization of DEA leads to reduction in the storage modulus because of the plastication effect of DEA. The storage modulus values increase consistently with an increase of the MAPP content. When the MAPP content reaches as high as 30 wt %, the storage modulus reaches the highest value (2.2 GPa at 25 °C). The plot of tan δ as a function of the temperature of the blends is shown in Figure 5B. Interestingly, two glass transition temperatures were observed, respectively corresponding to the chain segments of polyethylene (lower Tg) and PP (higher Tg). The addition of starch makes Tg shift to high temperatures, which is attributed to the fact that the presence of starch restricts the chain movements of PP to some extent, while the addition of DEA into the blend decreases Tg to lower temperatures because of the plastification effect. After incorporation of MAPP into the blends, the Tg values are again enhanced to higher temperatures

Figure 3. Schematic representation for the possible chemical reactions among MAPP, starch, and DEA.

MAPP holds nonpolar chains and reactive anhydride groups, which enables it to not only well compatilize with the PP matrix but also react with both hydroxyl groups of corn starch and DEA as well as the secondary amine groups in the latter. Upon the addition of MAPP, dispersion of starch is considerably improved with almost no holes, and the sizes of the starch granules are also gradually reduced with increasing MAPP (see Figure 2). After incorporation of 30 wt % MAPP, the size of starch is further decreased to ∼9 μm, clearly indicating significant improvement in the interfacial compatibilization. This interfacial improvement is primarily due to the in situ compatibilization reaction occurring between MAPP- and DEA-plasticized starch, as shown in Figure 3. While the temperature of mixing is raised to 180 °C, MAPP starts to react with DEA or corn starch, forming MAPP-g-DEA or MAPP-g16234

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Figure 4. FTIR spectra of (a) PCS, (b) PDS, (c) PDPS-5, (d) PDPS-10, (e) PDPS-30, and (f) MAPP.

sharply with increasing MAPP content, which, in turn, suggests improved interfacial compatibility between PP and starch, in good accordance with the thicker interface seen in SEM images. Mechanical Properties. Because the interfacial bonding is greatly enhanced because of the presence of MAPP and plasticization of starch, a considerable enhancement in the mechanical performances of the PP matrix will be theoretically expected, as verified by the above DMA results. Figure 6

Figure 6. Representative tensile behavior of neat PP, PCS, PDS, PDPS-5, PDPS-10, and PDPS-30.

presents typical stress−strain plots of PP/starch blends, with the detailed data listed in Table 2. Apparently, the addition of starch can improve the bending strength, the bending modulus, and Young’s modulus but to different extents decrease the yield strength, tensile strength, and elongation at break (∼95%; 650% for the PP matrix) because of the poor interfacial compatibility between PP and starch. Upon plasticization of starch with DEA, the elongation at break of the blends is remarkably restored to 220%, while both the yield strength and tensile strength merely slightly change. Upon the addition of 5 wt % MAPP into the blends, the bending strength, the bending modulus, the yield strength, the tensile strength, and Young’s modulus are increased to 36 MPa, 1.64 GPa, 20 MPa, 22 MPa, and 1.36 GPa, respectively. All of these mechanical parameters are continuously increased with the further loading of MAPP. However, the Young’s modulus values of both PDPS-5 and PDPS-10 are still lower than that of PCS, mainly because of the

Figure 5. Storage modulus (A) and loss factor (B) of PP, PCS, PDS, PDPS-5, PDPS-10, and PDPS-30. Tg is the glass transition temperature, which was determined from the peak temperatures of the tan δ−temperature curves obtained from DMA tests.

because of the improved interfacial adhesion limiting the movement of PP chains. When the MAPP content is increased up to 30 wt %, Tg reaches as high as 23.2 °C. Hamdan et al. found that both E′ and tan δ of the PP/starch blends hardly changed with an increase in the amount of starch because of the poor interfaces.22 In our system, both E′ and Tg increased 16235

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Table 2. Detailed Data Obtained from Bending and Tensile Tests of As-Prepared PP/Starch Blends run PP PCS PDS PDPS-5 PDPS-10 PDPS-15 PDPS-30

BSa (MPa) 33 38 30 36 39 40 42

± ± ± ± ± ± ±

1 1 2 2 1 2 2

BMa (GPa) 1.22 1.93 1.41 1.64 1.78 1.95 2.20

± ± ± ± ± ± ±

0.10 0.16 0.12 0.15 0.12 0.10 0.15

YSa (MPa) 22 16 15 20 21 22 25

± ± ± ± ± ± ±

0.7 0.6 0.5 0.5 0.3 0.4 0.6

TSa (MPa)

YMa (GPa)

± ± ± ± ± ± ±

0.95 1.49 1.10 1.36 1.48 1.72 1.66

28 17 16 22 24 27 30

0.5 0.6 0.5 0.4 0.3 0.7 0.8

EBa (%) 650 95 220 180 160 83 22

a

BS and BM refer to the bending strength and bending modulus; YS, TS, YM, and EB represent the yield strength, the tensile strength, Young’s modulus, and the elongation at break. The values following ± are errors of the standard deviation.

fact that the presence of MAPP is not able to offset the plasticization effect of DEA on the modulus of polymer blends. With a MAPP content above 15 wt %, both tensile and bending moduli dramatically increase, slightly above those of PCS because of the significantly improved interfacial bonding. When the loading level of MAPP is 30 wt %, these parameters reach the peak values of 42 MPa (by 40%), 2.20 GPa (by 56%), 25 MPa (by 67%), 30 MPa (by 87%), and 1.66 GPa (by 51%), respectively (relative to those of PDS). Because the elongation at break is related with the ductility or toughness of a material, much higher loading of MAPP sharply increases the stiffness and thus decreases the toughness of the blends. Mantia et al.23 used manganese stearate to assist the dispersion of sago starch in PP and found that the tensile strength and elongation at break were decreased by 12.5% and 27%, respectively. For our system, compared with PP/starch blend, the Young’s modulus and tensile strength are improved respectively by 56.3% and 76.5% by using MAPP as an in situ compatilizer and DEA as the plasticizer. Elongation at break can be effectively tuned by adjusting the loading level of DEA and MAPP. Therefore, for our present system fixing the loading of DEA, the optimal addition amount of MAPP should be below 15 wt % to achieve the balanced mechanical properties of the blends. Thermal Properties. The TGA curves of starch, PP, and its starch-based blends in nitrogen conditions are shown in Figure 7, with detailed data summarized in Table 3. Pristine starch starts to decompose at about 190 °C (Tonset, defined as the temperature where 5 wt % mass loss occurs) and undergoes a sharp mass loss at 310 °C (Tmax, defined as the temperature where the maximum weight loss takes place), leaving 16.6 wt % of residue char at 550 °C due to the high-char-forming ability of starch (see Figure 7A). By contrast, neat PP is much more thermally stable, showing Tonset of about 426 °C and Tmax of about 470 °C, but at 500 °C, it almost degrades completely with a char residue of as low as 0.29 wt %. The addition of 30 wt % starch into PP leads to a two-step decomposition. Tonset of PCS is shifted to a much lower temperature of 294 °C, and two Tmax values occur at 328 and 474 °C (∼4 °C higher than that of pure PP), respectively corresponding to the degradation of corn starch and PP, which agrees well with the previous report.25 Incorporating DEA into the blends further decreases Tonset (∼264 °C) and the first Tmax (Tmax1 ∼ 307 °C) because of the lower thermal stability and volatilization of DEA at elevated temperature. However, the second Tmax (Tmax2) belonging to degradation of the PP matrix is further increased up to 476 °C, ca. 6 °C higher than that of the PP matrix, indicating an improved thermal stability due to the thermal protection action of the char residue formed by degradation of starch. However, with the addition of MAPP and its increasing loading level, both Tonset and Tmax values first increase and then decrease

Figure 7. TGA curves of (A) for pristine starch and (B) for neat PP, PP/starch (PCS), PP/DEA/starch (PDS), PP/DEA/starch with different MAPP contents (PDPS) in nitrogen conditions.

slightly, which demonstrates that the presence of MAPP has limited effects on the thermal properties of PP/starch. Crystallization Behavior. DSC curves of PP and its blends are given in Figure 8, with detailed data listed in Table 4. The degree of crystallinity (χc) can be calculated using the following equation. χc = ΔHm/f ΔHm°

where f is the weight fraction of PP in the blends, ΔHm is the enthalpy of melting, and ΔHm° is the melting enthalpy of 100% crystalline PP, taken as 190 J/g as reported.24 The melting temperature (Tm) of neat PP is found at around 165.0 °C and the crystallization temperature (Tc) at 121.9 °C, displaying a degree of crystallinity (χc) of 29.7%. The addition of starch promotes the crystallization of PP (a χc of 31.2%) to some 16236

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Table 3. Detailed Data Obtained from TGA and DMA Measurements of As-Prepared PP/Starch Blends run

Tonseta (°C)

Tmax1a (°C)

PP PCS PDS PDPS-5 PDPS-10 PDPS-15 PDPS-30 Starch

426 294 264 265 272 253 262 190

470 328 307 309 308 311 308 310

Tmax2a (°C)

chara (wt %)

Tg1a (°C)

Tg2a (°C)

E′a at −30 °C (GPa)

E′ at 25 °C (GPa)

474 476 476 474 476 476

0.290 4.61 3.13 4.94 4.12 4.41 4.02 16.6

−34.5 −31.4 −32.2 −31.1 −30.7

18.2 20.7 18.3 21.3 21.9

2.7 3.5 3.1 3.2 3.4

1.5 2.0 1.7 1.7 1.9

−30.7

23.2

3.8

2.2

a

Tonset and Tmax represent the onset degradation temperature where 5 wt % mass loss occurs and the maximum mass loss temperature where maximum loss rate takes place, which is obtained through the differential peak of the TGA curve. The char value is obtained at 550 °C. Tg and E′ represent the glass transition temperature and storage modulus.

Table 4. Detailed Data of Nonisothermal Crystallization of As-Prepared PP/Starch Blends run

Tma (°C)

ΔHma (J/g)

Tca (°C)

ΔHca (J/g)

xca (%)

PP PCS PDS PDPS-5 PDPS-10 PDPS-15 PDPS-30

165.0 165.3 164.7 164.3 164.2 162.8 165.3

56.7 59.4 62.4 60.5 61.5 67.6 83.0

121.9 122.8 124.2 124.4 124.6 124.3 120.3

56.4 59.1 62.0 59.5 60.9 63.5 83.0

29.7 31.2 32.7 31.5 32.1 33.4 43.7

a Tm and ΔHm refer to the melting temperature and enthalpy of melt, respectively. Tc, ΔHc, and xc represent the crystallization temperature, enthalpy of crystallization, and crystallization degree.

and a Tc of about 120.3 °C, ∼1.6 °C lower than that of the PP matrix. The largely increased χc is most likely ascribed to the nucleating action of more starch granules in the PP matrix due to improved dispersion and decreased domain size, as observed in the above SEM images.

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CONLUSIONS In the present work, we have successfully prepared PP/starch blends with largely enhanced mechanical properties by a onestep reactive compatibilization technology using MAPP as the interfacial agent and DEA as a reactive plasticizer for starch. The addition of MAPP can effectively reduce the sizes of the starch domains in the PP matrix and improve the interfacial bonding between starch and the polymer matrix because of in situ reactions among MAPP, starch, and DEA. Compared with PP, incorporating 30 wt % starch increases the bending modulus from 33 to 38 MPa but considerably reduces the tensile strength from 28 to 17 MPa and the elongation at break from 650% to 95%. When 30 wt % of plasticized starch is added, both the strength and modulus decrease, but the elongation at break greatly increases. Compared with PP/starch blends, incorporating MAPP can significantly enhance the mechanical properties of the blends via in situ reactive compatibilization. The addition of 30 wt % MAPP leads to a comparable or even higher strength (bending strength, 42 MPa; tensile strength, 30 MPa) and modulus (Young’s modulus, 1.66 MPa) relative to pure PP while maintaining an acceptable toughness. A total of 30 wt % MAPP can increase the Tg values of PP by about 4 °C. Our work offers a facile and effective approach for creating advanced biobased polymer blends, which will contribute to reducing the environmental impact of the use of nonbiodegradable plastics.

Figure 8. Nonisothermal crystallinity curves for neat PP, PCS, PDS, and PDPS with increased MAPP loading at a heating rate of 10 °C/ min.

degree, indicating that starch can act as a nucleating agent for spherulite growth of PP. In the case of PP/starch plasticized with DEA, Tm decreases slightly, but both Tc and χc are further increased to 124.2 °C and 32.7%, respectively. This is probably due to the plasticizing effect of DEA, which makes the PP chains much easier to move and thus accelerates the alignment of polymer chain segments. Interestingly, with increasing MAPP loading level, the crystallization degree of polymer blends experiences first a decrease and then an increase, which should be attributed to the improved dispersion of starch and interfaces between starch and PP. Moreover, the presence of 30 wt % MAPP leads to a degree of crystallinity of as high as 43.7% 16237

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biodiesel and polypropylene blends. J. Therm. Anal. Calorim. 2010, 102, 181. (17) Chang, P. R.; Jian, R.; Zheng, P.; Yu, J.; Ma, X. Preparation and properties of glycerol plasticized-starch (GPS)/cellulose nanoparticle (CN) composites. Carbohydr. Polym. 2010, 79, 301. (18) Rahmat, A. R.; Rahman, W. A. W. A.; Sin, L. T.; Yussuf, A. A. Approaches to improve compatibility of starch filled polymer system: A review. Mater. Sci. Eng., C 2009, 29, 2370. (19) Wang, S. J.; Yu, J. G.; Yu, J. L. Compatible thermoplastic starch/ polyethylene blends by one-step reactive extrusion. Polym. Int. 2005, 54, 279. (20) Rosa, D. S.; Bardi, M. A. G.; Machado, L. D. B.; Dias, D. B.; Silva, L. G. A.; Kodama, Y. Influence of thermoplastic starch plasticized with biodiesel glycerol on thermal properties of PP blends. J. Therm. Anal. Calorim. 2009, 97, 565. (21) Wang, S. J.; Yu, J. G.; Yu, J. L. Preparation and characterization of compatible thermoplastic starch/polyethylene blends. Polym. Degrad. Stab. 2005, 87, 395. (22) Hamdan, S.; Hashim, D. M. A.; Ahmad, M.; Embong, S. Compatibility studies of polypropylene (PP)−sago starch (SS) blends using DMTA. J. Polym. Res. 2000, 7, 237. (23) Mantia, F. P. L.; Morreale, M.; Mohd Ishak, Z. A. Processing and mechanical properties of organic filler−polypropylene composites. J. Appl. Polym. Sci. 2005, 96, 1906. (24) Roy, S. B.; Ramaraj, B.; Shit, S. C.; Nayak, S. K. Polypropylene and potato starch biocomposites: Physicomechanical and thermal properties. J. Appl. Polym. Sci. 2011, 120, 3078. (25) Ye, L.; Friedrich, K. Mode I interlaminar fracture of co-mingled yarn based glass/polypropylene composites. Compos. Sci. Technol. 1993, 46, 187.

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 571 6374 2789. E-mail: [email protected]. *Tel: +86 571 6374 2789. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of the National Science Foundation for Young Scholars of China (Grant 51303162), Scientific Research Foundation of Zhejiang Agriculture & Forestry University (Grant 2351001088), and Commonwealth Project of Science and Technology Agency of Zhejiang Province of China (Grants 2012C22077 and 2013C32073).

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