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starch composites with succinic anhydride
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 2360–2366 www.elsevier.com/locate/compscitech
Properties of biodegradable poly(propylene carbonate)/starch composites with succinic anhydride Xiaofei Ma, Jiugao Yu *, Ang Zhao School of Science, Tianjin University, Tianjin 300072, China Received 12 July 2005; received in revised form 26 October 2005; accepted 25 November 2005 Available online 10 January 2006
Abstract Biodegradable composites of poly(propylene carbonate) (PPC) reinforced with granular cornstarch are prepared in a single screw extruder. The effects of succinic anhydride (SA) on the morphology, thermal properties, as well as mechanical properties of PPC/starch composites, are investigated. Scanning electron microscope (SEM) shows that starch surface becomes coarse, the interface is not clear and compatibility is increased when SA is added. Fourier transform infrared (FT-IR) Spectroscopy reveals that SA can improve the interaction between PPC and starch. Thermogravimetric analysis (TGA) results show that SA leads a significant improvement of thermal stability for PPC/starch composites. Mechanical testing illustrates that SA can increase mechanical properties of PPC/starch composites. The yield stresses of PPC/starch composites without SA and with SA are, respectively, 19.20 and 22.94 MPa. SA enhances the properties of PPC/starch composites, which are all ascribed to the improvement of the interaction between PPC and granular starch at the existence of SA. 2005 Elsevier Ltd. All rights reserved.
1. Introduction Much effort [1–3] has recently been made to develop biodegradable materials because of the worldwide environment problems resulted from petroleum-derived plastics. Many renewable resource-based biopolymers such as starch plastics, cellulose plastics [4], polylactides, polyhydroxyalkanoates (bacterial polyesters) [5], and soy-based plastics [6,7] have been investigated to alternate conventional non-degradable or incompletely degrading synthetic polymers (e.g. polyolefin) in the application scopes of oneoff materials [8]. Poly(propylene carbonate) (PPC) is a biodegradable aliphatic polycarbonate. At the existence of heterogeneous catalyst system, propylene oxide and carbon dioxide produce a polymer involving the regular alternating copolymer PPC [9,10], which can be used as adhesives, solid electrolytes, polyols, photoresists, barrier materials, flexibilizers and plasticizers [11]. However, such materials *
Corresponding author. Tel.: +86 22 27406144; fax: +86 22 27403475 E-mail address: [email protected] (J. Yu).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.11.028
are generally easy to decompose under the influence of heat and catalysts, which is resolved by end-capping using different active agents. [12] And their mechanical properties need to be improved by blending with other polymers to form polymer composites [11]. Starch is one of the promising raw materials for the production of biodegradable plastics [13]. Starch is renewable and biodegradable from a great variety of crops. Since 1970s starch has been incorporated into polyethylene in order to increase the biodegradability. Biodegradable composites of poly(propylene carbonate) (PPC) reinforced with unmodified cornstarch have been studied by Peng et al. [14] and Ge et al. [15]. In this paper, succinic anhydride (SA) as an active agent is introduced to PPC/starch composites. On the one hand, it is expected that SA can end-cap PPC to improve thermal stability. On the other hand, succinic anhydride is prone to react with hydroxyl groups in starch, introduce ester groups into starch and improve the compatibility between PPC matrix and starch. In this paper, the morphology, FT-IR, thermal properties and mechanical properties of PPC/starch composites are investigated. SA increases
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366
the interaction between PPC and starch, which results in the improvement of starch dispersion, thermal stability and mechanical properties in PPC/starch composites. 2. Experimental section
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2.6. Mechanical testing Samples 8 cm · B3 mm in size are cut from the extruded strips. The Testometric AX M350-10KN Materials Testing Machines operates and a crosshead speed of 100 mm/min is used for tensile testing (ISO 1184-1983 standard).
2.1. Materials 3. Results and discussion PPC is obtained from State Key Polymer Physics and Chemistry Laboratory, Changchun Institute of Applied Chemistry, China. Its molecular weight measured by GPC is 130,000 and Mw/Mn = 4.3. Cornstarch (11% moisture) was obtained from Langfang Starch Company (Langfang, Heibei, China). Succinic anhydride (SA) is purchased from Tianjin Chemical Reagent Factory (Tianjin, China). SA is analytical reagent.
3.1. Morphology
The cryo-fractured surfaces of extruded PLA/PPC composites are performed with Scanning Electron Microscope Philips XL-3 (FEI Company, Hillsboro, Oregon, USA), operating at an acceleration voltage of 20 kV. The composites are cooled in liquid nitrogen, and then broken. The fracture surfaces are vacuum coated with gold for SEM.
The morphology structure of polymer composites is a very important characteristic because it ultimately determines many properties of the polymer composites, such as thermal stability and mechanical properties. In the present study, starch functioned as the filler because the granular structure of starch is retained after extrusion and is homogeneously dispersed in the PPC matrix (shown in Fig. 1). There are the smooth surface of corn starch and the distinct interfacial appearance between corn starch and PPC. Therefore, their interfacial tension is large and their interfacial adhesion is low, which agrees with recent results of Liu et al. [17] and Sailaja et al. [18] for polyethylene and granular starch composites. The morphology of cryo-fractured surfaces is shown in Fig. 1. At the existence of SA (Fig. 1(c) and (d)), the distinction between corn starch and PPC is not as clear as that of the composites without SA (Fig. 1(a) and (b), and the surface of corn starch becomes coarse. These characteristics are typical of compatibility, suggesting the occurrence of good interaction between starch and PPC. PPC is an aliphatic polycarbonate, which is hydrophobic. Starch is a multi-hydroxyl polymer with three hydroxyl groups per monomer, which is hydrophilic. SA improves the interfacial adhesion between PPC and corn starch, and the improved interfacial adhesion results in increased compatibility. The improved interfacial adhesion attributes to the strong chemical and physical interaction. Besides the end-capped reaction between PPC and SA, the chemical interaction presumably results from reaction of hydroxyl groups in starch with anhydride groups in SA [19] under the extrusion conditions of high temperature and high shear. This reaction makes starch surface become coarse, as shown by arrows in Fig. 1(d). The strong physical interactions occur between PPC and SA modified starch, between SA end-capped PPC and starch, between SA end-capped PPC and SA modified starch. When SA is situated at the interface between starch and PPC and interacted with both, the interfacial tension is reduced and compatibility is increased.
2.5. Thermogravimetric analysis (TGA)
3.2. FT-IR
The thermal properties of the composites are measured with a ZTY-ZP type thermal analyzer. The sample weight varies from 10 to 15 mg. Samples are heated from the room temperature to 500 C at a heating rate of 15 C/min.
The interaction of polymer composites can be identified by the FT-IR spectra. On the basis of the harmonic oscillator model the reduction in force constant f can be represented by [16]
2.2. Composite preparation Starch samples were dried at 115 C for 4 h to eliminate water. PPC is blended (3000 rpm, 2 min) with starch or/and SA by use of High Speed Mixer GH-100Y (made in China). The composites are mixed in the single screw Plastic extruder SJ-25(s) (Axon Australia Pty. Ltd., Australia) with a screw diameter of 30 mm and a length to diameter ratio of 25:1. The temperature profile along the extruder barrel was 110/115/120 C, and the temperature at the die is 90 C. The screw speed is 20 rpm. The die is a round sheet with the diameter 3 mm holes. The optimum content of SA is 1 wt/gross of PPC and starch. The superfluous SA can result the degradation of PPC during the processing. 2.3. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra are obtained at 2 cm1 resolution with BIO-RAD FTS3000 IR Spectrum Scanner. Typically, 64 scans are signal-averaged to reduce spectral noise. The extruded TPS strips are compressed to the transparent slices with the thickness of around 0.2 mm in the Flat Sulfuration Machine, tested by the transmission method [16]. 2.4. Scanning electron microscope (SEM)
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Fig. 1. SEM micrographs of cryo-fractured PPC/starch and PPC/starch/SA composites: (a) PPC/starch (70/30), 500·; (b) PPC/starch (70/30), 2000·; (c) PPC/starch/SA (70/30/1), 500· and (d) PPC/starch/SA (70/30/1), 2000·.
lðm2b m2nb Þ 4p2
ð1Þ
where l = m1m2/(m1 + m2) corresponded to the reduced mass of the oscillator, m the oscillating frequency and f the force constant. The subscripts b and nb denote bonded and non-bonded oscillators, respectively. The reduction of force constant brought about by some interaction is directly related to the frequency (or wave number) shift of stretching vibrations. Thus, the lower the wave number corresponding to absorption peak, the stronger is the hydrogen bond interaction between polymer composites. Fig. 2 shows the spectra of PPC and PPC/starch composites without or with SA at room temperature in the carbonyl stretching region. PPC had a strong carbonyl stretching absorption at about 1750 cm1. With the increase of starch content in PPC/starch composites without SA, the absorption peak shifts towards lower wave number. For 70/30 PPC/starch composite, the wave number corresponding to absorption peak is 1747.5 cm1, about 2 cm1 lower than that for pure PPC. For 50/50 PPC/starch composite, the wave number corresponding to absorption peak is 1745.58 cm1, about 4 cm1 lower than that for pure PPC. Because starch is a multi-hydroxyl polymer with three hydroxyl groups per monomer, the shift of carbonyl stretching absorption to lower wave number is ascribed to the interaction between carbonyl groups of PPC and hydroxyl groups of starch by hydrogen bonding. [14,15] At the existence of SA, with the increasing of starch content in PPC/ starch/SA composites the absorption peak shifts more towards lower wave number. For 70/30 PPC/starch/SA
e d Transmittance
Df ¼ fb fnb ¼
1739.5
c 1741.43
b a
1745.58
a: PPC b: PPC/starch 70/30 c: PPC/starch 50/50 1747.5 d: PPC/starch/SA 70/30/2 e: PPC/starch/SA 50/50/2
1749.43
2000
1900
1800
1700
1600
1500
-1
Wavenumber (cm )
Fig. 2. The FT-IR spectra of PPC and PPC/starch composites.
composite, the wave number of absorption peak is 1741.43 cm1, 8 cm1 lower than that for pure PPC. For 50/50 PPC/starch/SA composite, the wave number of absorption peak is 1739.5 cm1, about 10 cm1 lower than that for pure PPC. Obviously, SA can improve the interaction between PPC and starch. 3.3. Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) is performed for the composites, where the weight loss due to the volatilization of the degradation products is monitored as a function of
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Mass Loss (%)
temperature. The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of pure PPC, SA endcapped PPC, PPC/starch and PPC/starch/SA in air at a heating rate of 15 C/min are shown in Figs. 3 and 4, respectively. The decomposed temperature, Tmax is the temperature at maximum rate of weight loss, i.e., the peak temperature shown in Fig. 4. It can be seen that the degradation of end-capped PPC takes place at higher tempera-
100 80 60 40 20 0
PPC/starch/SA
100 80 60 40 20 0
PPC/starch
100 80 60 40 20 0
PPC capped with SA
100 80 60 40 20 0
PPC 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 3. Thermogravimetric curves for PPC and PPC/starch composites at a heating rate of 15 C/min in air.
-0.5 -1.0 -1.5
PPC/starch/SA
-0.5 -1.0
DTG
-1.5
PPC/starch
-2.0 -0.5 -1.0 -1.5 -2.0 -2.5
PPC capped with SA
-0.5 -1.0 -1.5 -2.0 -2.5
PPC 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 4. Derivative thermogravimetric curves for PPC and PPC/starch composites at a heating rate of 15 C/min in air.
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ture than that of uncapped PPC. Peng et al. [12] report that PPC is easily decomposed to cyclic carbonate. Li et al. [20] have found that PPC pyrolysis obeys two-step pyrolysis mechanism: main chain random scission and unzipping. During end capping, nucleophilic terminal hydroxyl groups are replaced with less reactive groupings. Although the potential for depolymerisation via chain scission is not affected by the transformation, due to inhibition of the formation of five-member rings in the chain end after end capping, end-initiated depolymerisation via chain unzipping is prevented. For end-capped PPC, random chain scission dominates the whole degradation process. The addition of SA improves the thermal stability of PPC. Starch can accelerate the thermal decomposition of PPC. As shown in Fig. 4 and Table 1, the introduction of starch decreases Tmax from 528.85 K of pure PPC to 504.79 K of PPC in PPC/starch composites. However, SA increases the decomposed temperatures of both PPC and starch in PPC/starch/SA composites, compared to PPC/ starch composites. And the decomposed temperature of PPC in PPC/SA (561.24 K) is just equal to the decomposed temperature of PPC in PPC/starch/SA (561.24 K). Activation energy of decomposition, Et, of the polymer composites can be calculated from the TGA curves by the integral method proposed by Horowitz and Metzger using [21] 1
ln½lnð1 aÞ ¼ Et h=RT 2max
ð2Þ
where a is the decomposed fraction, Et is the activation energy of decomposition, Tmax is the temperature at maximum rate of weight loss, h is (T Tmax), and R is the gas constant. From the plots of ln[ln(1 a)1] versus h, which are shown in Fig. 5, the activation energy (Et) for decomposition can be determined from the slope of the straight line of the plots. Et of both PPC and starch in composites are listed in Table 1. As demonstrated in Table 1, Et of both PPC and starch in composites are raised when SA is added. Et of PPC in PPC/SA is 116.3 kJ/mol, while Et of pure PPC is 56 kJ/ mol. In view of Et, PPC (120.5 kJ/mol) in PPC/starch/SA is more thermally stable than pure PPC (56 kJ/mol) and PPC (45.3 kJ/mol) in PPC/starch. SA also improves Et of starch from 34.3 kJ/mol in PPC/starch to 57.4 kJ/mol in PPC/starch/SA. At the existence of SA, the improvement of thermal stability for both PPC and starch is attributed to the specific interactions between the chains of starch and SA end-capped PPC.
Table 1 Thermal gravimetric parameters of PPC and PPC/starch composites PPC, Tmax (K) PPC PPC/SA PPC/starch PPC/starch/SA
528.85 561.24 504.79 561.24
Starch, Tmax (K) – – 606.88 612.75
PPC
Starch
The slope
Et (kJ/mol)
The slope
Et (kJ/mol)
0.0250 0.0444 0.0214 0.0460
56.0 116.3 45.3 120.5
– – 0.0112 0.0184
– – 34.3 57.4
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366 -1.5
-1.0
-2.0
-1.5 -1
ln[ln(1-a) ]
-1
ln[ln(1-a) ]
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-2.5 -3.0 -3.5
PPC: y=-1.7166+0.0250x
-4.0 -60
-50
-40
-30
-2.0
-2.5
PPC in PPC/SA: y=-0.7095+0.0444x
-3.0 -3.5 -60
-20
-50
-40
1.5
0.5
1.0
0.0 -0.5
PPC in PPC/starch: y=-0.6327+0.02138x
0.5 0.0
Starch in PPC+starch: y=-0.6887+0.0112x
-0.5 -1.0
-1.5 0
5
10
15
20
25
30
35
80
40
85
90
95
theta (K)
105
110
115
120
1.5
-0.5 -1
ln[ln(1-a) ]
1.0
-1.0
-1.5
PPC in PPS/starch/SA: y=-0.4912+0.0460x
-2.0 -2.5 -35
100
theta (K)
0.0
-1
-20
-1
ln[ln(1-a) ]
1.0
-1.0
ln[ln(1-a) ]
-30
theta (K)
-1
ln[ln(1-a) ]
theta (K)
-30
-25
-20
-15
-10
-5
0
0.5
0.0
Starch in PPC/starch/SA: y=0.5970+0.0184x
-0.5 -1.0 -35
5
-30
-25
-20
theta (K)
-15
-10
-5
0
5
theta (K)
Fig. 5. Plots of ln[ln(1 a)1] versus h for determination of the decomposition activation energy Et. Straight line is the linear fit of the data points.
3.4. Mechanical properties 18 16 14 12
Stress(%)
Fig. 6 shows the effect of SA contents on mechanical properties. With the increasing of SA content, the break strain of PPC decreases. And the break stress is increased form 9.78 to 10.26 MPa at 1 wt% SA content, and then decreases to 6.53 MPa at 2 wt% SA content. The superfluous SA will result the degradation of PPC during the processing. Therefore, the content of SA is fixed at 1 wt% SA based on the gross of PPC and starch. Interfacial interaction between the fillers and matrix is an important factor affecting the mechanical properties of the composites. Thus, theoretical tensile yield strength and ultimate tensile strength of the composites are modelled for the cases of adhesion and no adhesion between the filler particles and matrix. In the case of no adhesion, the interfacial layer cannot transfer stress. The tensile strengths of the composites can be predicted using Nicholais–Narkis models [22]
b
10
a
8 6
c
4
a: PPC b: PPC/SA (100/1) c: PPC/SA (100/2)
2 0 0
50
100
150
200
250
300
350
Strain(%)
Fig. 6. The effect of SA contents on the stress–strain curves of PPC/SA composites.
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366
rc ¼ rm ð1 aUbf Þ
ð3Þ
18
where Uf, rc and rm are volume fraction of filler, and tensile strengths of the composite and matrix, respectively. In the Nicholais and Narkis model, parameters a and b are the constants related to filler–matrix interaction and geometry of the filler, respectively. The value of a less than 1.21 represents good adhesion for composites containing spherical fillers. In the absence of adhesion for the composites, Eq. (4) becomes
16
1.4
Yield stress ratio of composite/matrix
300
Break Stress (MPa)
14
1.2
240
12 10
180 8 120
6 4
60
ð4Þ
2 0
0 0
10
20
30
40
50
40
50
starch weight content (%)
2400
Youngs Modulus (MPa)
This model is based on the assumption that the decrease of tensile strength is due to the reduction in effective cross-section area caused by the spherical filler particles. If perfect adhesion were present between PPC and starch granules, the loading stresses would be transferred to the starch, and no reduction in effective surface area would result. The densities of cornstarch and PPC used in the work are 1.40 and 1.30 g/cm3, respectively. The weight fraction of filler is transferred to volume fraction. The experimental and theoretical curves are plotted in Fig. 7. From the figure it can be seen that the experimental value of PPC/starch is higher than that calculated by Eq. (4). This indicates that there is the adhesion with some degree between PPC and starch granules. And experimental curve of PPC/starch/SA is much higher. SA improves the degree of adhesion between PPC and starch granules. This result also proves that SA can enhance the interaction between the PPC chains and the surface of starch granule. When starch content is 28.4% (volume fraction), i.e., 30 wt%, the yield stress ratio of composite/matrix reaches the peak at 1.18 for PPC/starch composites without SA and 1.41 PPC/starch composites with SA. The yield stresses of PPC/starch composites without SA and with SA are, respectively, 19.20 and 22.94 MPa. The effect of starch weight contents on break strain, break stress and Young’s Modulus of PPC/starch compos-
360
Break Strain (%)
2=3
rc =rm ¼ ð1 1:21Uf Þ
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2000
1600
1200
800
0
10
20
30
starch weight content (%)
Fig. 8. The effect of starch weight contents on mechanical properties of PPC/starch composites.
ites are shown in Fig. 8. At the starch weight content range from 0% to 50%, both break stress and Youngs Modulus curves of PPC/starch/SA composites are higher than ones of PPC/starch composites. And at the starch weight content range from 10 to 50%, break strain curve of PPC/ starch/SA composites is also higher than one of PPC/ starch composites. These all illustrates that SA can increase mechanical properties of PPC/starch composites because SA enhances the interaction between PPC and starch granule.
1.0
4. Summary 0.8
0.6
theoretical curve by Eq.2 experimental curve of PPC/starch experimental curve of PPC/starch/SA
0.4
0.2 0.0
0.1
0.2
0.3
0.4
0.5
starch volume fraction (%)
Fig. 7. The effect of starch volume fraction on yield stress ratio of PPC/ starch composites and PPC.
SEM has proved that SA reduces interfacial tension and improves the interfacial compatibility between PPC matrix and dispersed starch granules. As revealed by FT-IR, the wave number of absorption peak for 50/50 PPC/starch/ SA composite is about 6 cm1 lower than that for 50/50 PPC/starch composite. SA obviously improves the interaction between PPC and starch, which results in the improvement for thermal stability and mechanical properties of PPC/starch composites. In view of Et, PPC (120.5 kJ/ mol) in PPC/starch/SA is more thermally stable than
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PPC (45.3 kJ/mol) in PPC/starch. SA also improves Et of starch from 34.3 kJ/mol in PPC/starch to 57.4 kJ/mol in PPC/starch/SA. At starch weight content range from 10 to 50%, break strain, break stress and Youngs Modulus of PPC/starch/SA composites are all higher than ones of PPC/starch composites. At 28.4% volume fraction starch, the yield stresses of PPC/starch composites without SA and with SA, respectively, reach the vertex at 19.20 and 22.94 MPa. References [1] Fang Q, Hanna MA. Characteristics of biodegradable Mater-Bistarch based foams as affected by ingredient formulations. Ind Crops Prod 2001;13(3):219–27. [2] Martin O, Averous L. Poly (lactic acid): plasticization and properties of biodegradable multiphase systems. Polymer 2001;42(14):6209–19. [3] Averous L, Fauconnier N, Moro L, Fringant C. Blends of thermoplastic starch and polyesteramide: processing and properties. J Appl Polym Sci 2000;76(7):1117–28. [4] Yoshioka M, Shiraishi N. Biodegradable plastics from cellulose. Mole Cryst Liq Cryst 2000;353:59–73. [5] Pantazaki AA, Tambaka MG, Langlois V, Guerin P, Kyriakidis DA. Polyhydro- xyalkanoate (PHA) biosynthesis in thermus thermophilus: purification and biochemical properties of PHA synthase. Mol Cell Biochem 2003;254(1–2):173–83. [6] Swain SN, Biswal SM, Nanda PK, Nayak PL. Biodegradable soybased plastics: opportunities and challenges. J Polym Environ 2004;12(1):35–42. [7] Wool RP. Development of affordable soy-based plastics, resins, and adhesives. Chemtech 1999;29(6):44–8. [8] Mohanty AK, Misra M, Drzal LT. Sustainable bio-composites from renewable resources: opportunities and challenges in the green materials world. J Polym Environ 2002;10(1–2):19–26. [9] Chisholm MH, Navarro-Llobet D, Zhou ZP. Poly(propylene carbonate). 1. More about poly (propylene carbonate) formed from the copolymerization of propylene oxide and carbon dioxide employing a zinc glutarate catalyst. Macromolecules 2002;35:6494–504.
[10] Chisholm MH, Zhou ZP. Concerning the mechanism of the ring opening of propylene oxide in the copolymerization of propylene oxide and carbon dioxide to give poly (propylene carbonate). J Am Chem Soc 2004;126:11030–9. [11] Chen SM, Tan L, Qiu FR, Jiang XL, Wang M, Zhang HD. The study of poly (styrene-co-p-(hexafluoro-2-hydroxylisopropyl)-ame(propylene (propylene carbonate))) blends by ESR spin probe and Raman. Polymer 2004;45:3045–53. [12] Peng SW, An YX, Chen C, Fei B, Zhuang YG, Dong LS. Thermal degradation kinetics of uncapped and end-capped poly (propylene carbonate). Poly Degrad Stabil 2003;80:141–7. [13] Petersen K, Væggemose NP, Bertelsen G. Potential of biobased materials for food packaging. Trends Food Sci Technol 1999; 10(2):52–68. [14] Peng SW, Wang XY, Dong LS. Special interaction between poly (propylene carbonate) and corn starch. Poly Compos 2005;26:37–41. [15] Ge XC, Li XH, Zhu Q, Li L, Meng YZ. Preparation and properties of biodegradable poly (propylene carbonate)/starch composites. Poly Eng Sci 2004;44:2134–40. [16] Pawlak A, Mucha M. Thermogravimetric and FT-IR studies of chitosan blends. Thermochimi Acta 2003;396(1–2):153–66. [17] Liu W, Wang YJ, Sun Z. Effects of polyethylene-grafted maleic anhydride (PE-g-MA) on thermal properties, morphology, and tensile properties of low-density polyethylene (LDPE) and corn starch blends. J Appl Poly Sci 2003;88(13):2904–11. [18] Sailaja RRN, Chanda M. Use of maleic anhydride-grafted polyethylene as compatibilizer for HDPE-tapioca starch blends: effects on mechanical properties. J Appl Poly Sci 2001;80:863–72. [19] Bhandari PN, Singhal RS. Studies on the optimisation of preparation of succinate derivatives from corn and amaranth starches. Carbohyd Polym 2002;47(3):277–83. [20] Li XH, Meng YZ, Zhu Q, Tjong SC. Thermal decomposition characteristics of poly (propylene carbonate) using TG/IR and PyGC/MS techniques. Polym Degrad Stabil 2003;81:157–65. [21] Chen GX, Yoon JS. Morphology and thermal properties of poly(Llactide)/poly (butylene succinate-co-butylene adipate) compounded with twice functionalized clay. J Polym Sci Part B 2005;43:478–87. [22] Metin D, Tihminlioglu F, Balkose D, Ulku S. The effect of interfacial interactions on the mechanical properties of polypropylene/natural zeolite composites. Compos: Part A 2004;35:23–32.
Properties of biodegradable poly(propylene carbonate)/starch composites with succinic anhydride Xiaofei Ma, Jiugao Yu *, Ang Zhao School of Science, Tianjin University, Tianjin 300072, China Received 12 July 2005; received in revised form 26 October 2005; accepted 25 November 2005 Available online 10 January 2006
Abstract Biodegradable composites of poly(propylene carbonate) (PPC) reinforced with granular cornstarch are prepared in a single screw extruder. The effects of succinic anhydride (SA) on the morphology, thermal properties, as well as mechanical properties of PPC/starch composites, are investigated. Scanning electron microscope (SEM) shows that starch surface becomes coarse, the interface is not clear and compatibility is increased when SA is added. Fourier transform infrared (FT-IR) Spectroscopy reveals that SA can improve the interaction between PPC and starch. Thermogravimetric analysis (TGA) results show that SA leads a significant improvement of thermal stability for PPC/starch composites. Mechanical testing illustrates that SA can increase mechanical properties of PPC/starch composites. The yield stresses of PPC/starch composites without SA and with SA are, respectively, 19.20 and 22.94 MPa. SA enhances the properties of PPC/starch composites, which are all ascribed to the improvement of the interaction between PPC and granular starch at the existence of SA. 2005 Elsevier Ltd. All rights reserved.
1. Introduction Much effort [1–3] has recently been made to develop biodegradable materials because of the worldwide environment problems resulted from petroleum-derived plastics. Many renewable resource-based biopolymers such as starch plastics, cellulose plastics [4], polylactides, polyhydroxyalkanoates (bacterial polyesters) [5], and soy-based plastics [6,7] have been investigated to alternate conventional non-degradable or incompletely degrading synthetic polymers (e.g. polyolefin) in the application scopes of oneoff materials [8]. Poly(propylene carbonate) (PPC) is a biodegradable aliphatic polycarbonate. At the existence of heterogeneous catalyst system, propylene oxide and carbon dioxide produce a polymer involving the regular alternating copolymer PPC [9,10], which can be used as adhesives, solid electrolytes, polyols, photoresists, barrier materials, flexibilizers and plasticizers [11]. However, such materials *
Corresponding author. Tel.: +86 22 27406144; fax: +86 22 27403475 E-mail address: [email protected] (J. Yu).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.11.028
are generally easy to decompose under the influence of heat and catalysts, which is resolved by end-capping using different active agents. [12] And their mechanical properties need to be improved by blending with other polymers to form polymer composites [11]. Starch is one of the promising raw materials for the production of biodegradable plastics [13]. Starch is renewable and biodegradable from a great variety of crops. Since 1970s starch has been incorporated into polyethylene in order to increase the biodegradability. Biodegradable composites of poly(propylene carbonate) (PPC) reinforced with unmodified cornstarch have been studied by Peng et al. [14] and Ge et al. [15]. In this paper, succinic anhydride (SA) as an active agent is introduced to PPC/starch composites. On the one hand, it is expected that SA can end-cap PPC to improve thermal stability. On the other hand, succinic anhydride is prone to react with hydroxyl groups in starch, introduce ester groups into starch and improve the compatibility between PPC matrix and starch. In this paper, the morphology, FT-IR, thermal properties and mechanical properties of PPC/starch composites are investigated. SA increases
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366
the interaction between PPC and starch, which results in the improvement of starch dispersion, thermal stability and mechanical properties in PPC/starch composites. 2. Experimental section
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2.6. Mechanical testing Samples 8 cm · B3 mm in size are cut from the extruded strips. The Testometric AX M350-10KN Materials Testing Machines operates and a crosshead speed of 100 mm/min is used for tensile testing (ISO 1184-1983 standard).
2.1. Materials 3. Results and discussion PPC is obtained from State Key Polymer Physics and Chemistry Laboratory, Changchun Institute of Applied Chemistry, China. Its molecular weight measured by GPC is 130,000 and Mw/Mn = 4.3. Cornstarch (11% moisture) was obtained from Langfang Starch Company (Langfang, Heibei, China). Succinic anhydride (SA) is purchased from Tianjin Chemical Reagent Factory (Tianjin, China). SA is analytical reagent.
3.1. Morphology
The cryo-fractured surfaces of extruded PLA/PPC composites are performed with Scanning Electron Microscope Philips XL-3 (FEI Company, Hillsboro, Oregon, USA), operating at an acceleration voltage of 20 kV. The composites are cooled in liquid nitrogen, and then broken. The fracture surfaces are vacuum coated with gold for SEM.
The morphology structure of polymer composites is a very important characteristic because it ultimately determines many properties of the polymer composites, such as thermal stability and mechanical properties. In the present study, starch functioned as the filler because the granular structure of starch is retained after extrusion and is homogeneously dispersed in the PPC matrix (shown in Fig. 1). There are the smooth surface of corn starch and the distinct interfacial appearance between corn starch and PPC. Therefore, their interfacial tension is large and their interfacial adhesion is low, which agrees with recent results of Liu et al. [17] and Sailaja et al. [18] for polyethylene and granular starch composites. The morphology of cryo-fractured surfaces is shown in Fig. 1. At the existence of SA (Fig. 1(c) and (d)), the distinction between corn starch and PPC is not as clear as that of the composites without SA (Fig. 1(a) and (b), and the surface of corn starch becomes coarse. These characteristics are typical of compatibility, suggesting the occurrence of good interaction between starch and PPC. PPC is an aliphatic polycarbonate, which is hydrophobic. Starch is a multi-hydroxyl polymer with three hydroxyl groups per monomer, which is hydrophilic. SA improves the interfacial adhesion between PPC and corn starch, and the improved interfacial adhesion results in increased compatibility. The improved interfacial adhesion attributes to the strong chemical and physical interaction. Besides the end-capped reaction between PPC and SA, the chemical interaction presumably results from reaction of hydroxyl groups in starch with anhydride groups in SA [19] under the extrusion conditions of high temperature and high shear. This reaction makes starch surface become coarse, as shown by arrows in Fig. 1(d). The strong physical interactions occur between PPC and SA modified starch, between SA end-capped PPC and starch, between SA end-capped PPC and SA modified starch. When SA is situated at the interface between starch and PPC and interacted with both, the interfacial tension is reduced and compatibility is increased.
2.5. Thermogravimetric analysis (TGA)
3.2. FT-IR
The thermal properties of the composites are measured with a ZTY-ZP type thermal analyzer. The sample weight varies from 10 to 15 mg. Samples are heated from the room temperature to 500 C at a heating rate of 15 C/min.
The interaction of polymer composites can be identified by the FT-IR spectra. On the basis of the harmonic oscillator model the reduction in force constant f can be represented by [16]
2.2. Composite preparation Starch samples were dried at 115 C for 4 h to eliminate water. PPC is blended (3000 rpm, 2 min) with starch or/and SA by use of High Speed Mixer GH-100Y (made in China). The composites are mixed in the single screw Plastic extruder SJ-25(s) (Axon Australia Pty. Ltd., Australia) with a screw diameter of 30 mm and a length to diameter ratio of 25:1. The temperature profile along the extruder barrel was 110/115/120 C, and the temperature at the die is 90 C. The screw speed is 20 rpm. The die is a round sheet with the diameter 3 mm holes. The optimum content of SA is 1 wt/gross of PPC and starch. The superfluous SA can result the degradation of PPC during the processing. 2.3. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra are obtained at 2 cm1 resolution with BIO-RAD FTS3000 IR Spectrum Scanner. Typically, 64 scans are signal-averaged to reduce spectral noise. The extruded TPS strips are compressed to the transparent slices with the thickness of around 0.2 mm in the Flat Sulfuration Machine, tested by the transmission method [16]. 2.4. Scanning electron microscope (SEM)
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Fig. 1. SEM micrographs of cryo-fractured PPC/starch and PPC/starch/SA composites: (a) PPC/starch (70/30), 500·; (b) PPC/starch (70/30), 2000·; (c) PPC/starch/SA (70/30/1), 500· and (d) PPC/starch/SA (70/30/1), 2000·.
lðm2b m2nb Þ 4p2
ð1Þ
where l = m1m2/(m1 + m2) corresponded to the reduced mass of the oscillator, m the oscillating frequency and f the force constant. The subscripts b and nb denote bonded and non-bonded oscillators, respectively. The reduction of force constant brought about by some interaction is directly related to the frequency (or wave number) shift of stretching vibrations. Thus, the lower the wave number corresponding to absorption peak, the stronger is the hydrogen bond interaction between polymer composites. Fig. 2 shows the spectra of PPC and PPC/starch composites without or with SA at room temperature in the carbonyl stretching region. PPC had a strong carbonyl stretching absorption at about 1750 cm1. With the increase of starch content in PPC/starch composites without SA, the absorption peak shifts towards lower wave number. For 70/30 PPC/starch composite, the wave number corresponding to absorption peak is 1747.5 cm1, about 2 cm1 lower than that for pure PPC. For 50/50 PPC/starch composite, the wave number corresponding to absorption peak is 1745.58 cm1, about 4 cm1 lower than that for pure PPC. Because starch is a multi-hydroxyl polymer with three hydroxyl groups per monomer, the shift of carbonyl stretching absorption to lower wave number is ascribed to the interaction between carbonyl groups of PPC and hydroxyl groups of starch by hydrogen bonding. [14,15] At the existence of SA, with the increasing of starch content in PPC/ starch/SA composites the absorption peak shifts more towards lower wave number. For 70/30 PPC/starch/SA
e d Transmittance
Df ¼ fb fnb ¼
1739.5
c 1741.43
b a
1745.58
a: PPC b: PPC/starch 70/30 c: PPC/starch 50/50 1747.5 d: PPC/starch/SA 70/30/2 e: PPC/starch/SA 50/50/2
1749.43
2000
1900
1800
1700
1600
1500
-1
Wavenumber (cm )
Fig. 2. The FT-IR spectra of PPC and PPC/starch composites.
composite, the wave number of absorption peak is 1741.43 cm1, 8 cm1 lower than that for pure PPC. For 50/50 PPC/starch/SA composite, the wave number of absorption peak is 1739.5 cm1, about 10 cm1 lower than that for pure PPC. Obviously, SA can improve the interaction between PPC and starch. 3.3. Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) is performed for the composites, where the weight loss due to the volatilization of the degradation products is monitored as a function of
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366
Mass Loss (%)
temperature. The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of pure PPC, SA endcapped PPC, PPC/starch and PPC/starch/SA in air at a heating rate of 15 C/min are shown in Figs. 3 and 4, respectively. The decomposed temperature, Tmax is the temperature at maximum rate of weight loss, i.e., the peak temperature shown in Fig. 4. It can be seen that the degradation of end-capped PPC takes place at higher tempera-
100 80 60 40 20 0
PPC/starch/SA
100 80 60 40 20 0
PPC/starch
100 80 60 40 20 0
PPC capped with SA
100 80 60 40 20 0
PPC 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 3. Thermogravimetric curves for PPC and PPC/starch composites at a heating rate of 15 C/min in air.
-0.5 -1.0 -1.5
PPC/starch/SA
-0.5 -1.0
DTG
-1.5
PPC/starch
-2.0 -0.5 -1.0 -1.5 -2.0 -2.5
PPC capped with SA
-0.5 -1.0 -1.5 -2.0 -2.5
PPC 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 4. Derivative thermogravimetric curves for PPC and PPC/starch composites at a heating rate of 15 C/min in air.
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ture than that of uncapped PPC. Peng et al. [12] report that PPC is easily decomposed to cyclic carbonate. Li et al. [20] have found that PPC pyrolysis obeys two-step pyrolysis mechanism: main chain random scission and unzipping. During end capping, nucleophilic terminal hydroxyl groups are replaced with less reactive groupings. Although the potential for depolymerisation via chain scission is not affected by the transformation, due to inhibition of the formation of five-member rings in the chain end after end capping, end-initiated depolymerisation via chain unzipping is prevented. For end-capped PPC, random chain scission dominates the whole degradation process. The addition of SA improves the thermal stability of PPC. Starch can accelerate the thermal decomposition of PPC. As shown in Fig. 4 and Table 1, the introduction of starch decreases Tmax from 528.85 K of pure PPC to 504.79 K of PPC in PPC/starch composites. However, SA increases the decomposed temperatures of both PPC and starch in PPC/starch/SA composites, compared to PPC/ starch composites. And the decomposed temperature of PPC in PPC/SA (561.24 K) is just equal to the decomposed temperature of PPC in PPC/starch/SA (561.24 K). Activation energy of decomposition, Et, of the polymer composites can be calculated from the TGA curves by the integral method proposed by Horowitz and Metzger using [21] 1
ln½lnð1 aÞ ¼ Et h=RT 2max
ð2Þ
where a is the decomposed fraction, Et is the activation energy of decomposition, Tmax is the temperature at maximum rate of weight loss, h is (T Tmax), and R is the gas constant. From the plots of ln[ln(1 a)1] versus h, which are shown in Fig. 5, the activation energy (Et) for decomposition can be determined from the slope of the straight line of the plots. Et of both PPC and starch in composites are listed in Table 1. As demonstrated in Table 1, Et of both PPC and starch in composites are raised when SA is added. Et of PPC in PPC/SA is 116.3 kJ/mol, while Et of pure PPC is 56 kJ/ mol. In view of Et, PPC (120.5 kJ/mol) in PPC/starch/SA is more thermally stable than pure PPC (56 kJ/mol) and PPC (45.3 kJ/mol) in PPC/starch. SA also improves Et of starch from 34.3 kJ/mol in PPC/starch to 57.4 kJ/mol in PPC/starch/SA. At the existence of SA, the improvement of thermal stability for both PPC and starch is attributed to the specific interactions between the chains of starch and SA end-capped PPC.
Table 1 Thermal gravimetric parameters of PPC and PPC/starch composites PPC, Tmax (K) PPC PPC/SA PPC/starch PPC/starch/SA
528.85 561.24 504.79 561.24
Starch, Tmax (K) – – 606.88 612.75
PPC
Starch
The slope
Et (kJ/mol)
The slope
Et (kJ/mol)
0.0250 0.0444 0.0214 0.0460
56.0 116.3 45.3 120.5
– – 0.0112 0.0184
– – 34.3 57.4
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366 -1.5
-1.0
-2.0
-1.5 -1
ln[ln(1-a) ]
-1
ln[ln(1-a) ]
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-2.5 -3.0 -3.5
PPC: y=-1.7166+0.0250x
-4.0 -60
-50
-40
-30
-2.0
-2.5
PPC in PPC/SA: y=-0.7095+0.0444x
-3.0 -3.5 -60
-20
-50
-40
1.5
0.5
1.0
0.0 -0.5
PPC in PPC/starch: y=-0.6327+0.02138x
0.5 0.0
Starch in PPC+starch: y=-0.6887+0.0112x
-0.5 -1.0
-1.5 0
5
10
15
20
25
30
35
80
40
85
90
95
theta (K)
105
110
115
120
1.5
-0.5 -1
ln[ln(1-a) ]
1.0
-1.0
-1.5
PPC in PPS/starch/SA: y=-0.4912+0.0460x
-2.0 -2.5 -35
100
theta (K)
0.0
-1
-20
-1
ln[ln(1-a) ]
1.0
-1.0
ln[ln(1-a) ]
-30
theta (K)
-1
ln[ln(1-a) ]
theta (K)
-30
-25
-20
-15
-10
-5
0
0.5
0.0
Starch in PPC/starch/SA: y=0.5970+0.0184x
-0.5 -1.0 -35
5
-30
-25
-20
theta (K)
-15
-10
-5
0
5
theta (K)
Fig. 5. Plots of ln[ln(1 a)1] versus h for determination of the decomposition activation energy Et. Straight line is the linear fit of the data points.
3.4. Mechanical properties 18 16 14 12
Stress(%)
Fig. 6 shows the effect of SA contents on mechanical properties. With the increasing of SA content, the break strain of PPC decreases. And the break stress is increased form 9.78 to 10.26 MPa at 1 wt% SA content, and then decreases to 6.53 MPa at 2 wt% SA content. The superfluous SA will result the degradation of PPC during the processing. Therefore, the content of SA is fixed at 1 wt% SA based on the gross of PPC and starch. Interfacial interaction between the fillers and matrix is an important factor affecting the mechanical properties of the composites. Thus, theoretical tensile yield strength and ultimate tensile strength of the composites are modelled for the cases of adhesion and no adhesion between the filler particles and matrix. In the case of no adhesion, the interfacial layer cannot transfer stress. The tensile strengths of the composites can be predicted using Nicholais–Narkis models [22]
b
10
a
8 6
c
4
a: PPC b: PPC/SA (100/1) c: PPC/SA (100/2)
2 0 0
50
100
150
200
250
300
350
Strain(%)
Fig. 6. The effect of SA contents on the stress–strain curves of PPC/SA composites.
X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366
rc ¼ rm ð1 aUbf Þ
ð3Þ
18
where Uf, rc and rm are volume fraction of filler, and tensile strengths of the composite and matrix, respectively. In the Nicholais and Narkis model, parameters a and b are the constants related to filler–matrix interaction and geometry of the filler, respectively. The value of a less than 1.21 represents good adhesion for composites containing spherical fillers. In the absence of adhesion for the composites, Eq. (4) becomes
16
1.4
Yield stress ratio of composite/matrix
300
Break Stress (MPa)
14
1.2
240
12 10
180 8 120
6 4
60
ð4Þ
2 0
0 0
10
20
30
40
50
40
50
starch weight content (%)
2400
Youngs Modulus (MPa)
This model is based on the assumption that the decrease of tensile strength is due to the reduction in effective cross-section area caused by the spherical filler particles. If perfect adhesion were present between PPC and starch granules, the loading stresses would be transferred to the starch, and no reduction in effective surface area would result. The densities of cornstarch and PPC used in the work are 1.40 and 1.30 g/cm3, respectively. The weight fraction of filler is transferred to volume fraction. The experimental and theoretical curves are plotted in Fig. 7. From the figure it can be seen that the experimental value of PPC/starch is higher than that calculated by Eq. (4). This indicates that there is the adhesion with some degree between PPC and starch granules. And experimental curve of PPC/starch/SA is much higher. SA improves the degree of adhesion between PPC and starch granules. This result also proves that SA can enhance the interaction between the PPC chains and the surface of starch granule. When starch content is 28.4% (volume fraction), i.e., 30 wt%, the yield stress ratio of composite/matrix reaches the peak at 1.18 for PPC/starch composites without SA and 1.41 PPC/starch composites with SA. The yield stresses of PPC/starch composites without SA and with SA are, respectively, 19.20 and 22.94 MPa. The effect of starch weight contents on break strain, break stress and Young’s Modulus of PPC/starch compos-
360
Break Strain (%)
2=3
rc =rm ¼ ð1 1:21Uf Þ
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2000
1600
1200
800
0
10
20
30
starch weight content (%)
Fig. 8. The effect of starch weight contents on mechanical properties of PPC/starch composites.
ites are shown in Fig. 8. At the starch weight content range from 0% to 50%, both break stress and Youngs Modulus curves of PPC/starch/SA composites are higher than ones of PPC/starch composites. And at the starch weight content range from 10 to 50%, break strain curve of PPC/ starch/SA composites is also higher than one of PPC/ starch composites. These all illustrates that SA can increase mechanical properties of PPC/starch composites because SA enhances the interaction between PPC and starch granule.
1.0
4. Summary 0.8
0.6
theoretical curve by Eq.2 experimental curve of PPC/starch experimental curve of PPC/starch/SA
0.4
0.2 0.0
0.1
0.2
0.3
0.4
0.5
starch volume fraction (%)
Fig. 7. The effect of starch volume fraction on yield stress ratio of PPC/ starch composites and PPC.
SEM has proved that SA reduces interfacial tension and improves the interfacial compatibility between PPC matrix and dispersed starch granules. As revealed by FT-IR, the wave number of absorption peak for 50/50 PPC/starch/ SA composite is about 6 cm1 lower than that for 50/50 PPC/starch composite. SA obviously improves the interaction between PPC and starch, which results in the improvement for thermal stability and mechanical properties of PPC/starch composites. In view of Et, PPC (120.5 kJ/ mol) in PPC/starch/SA is more thermally stable than
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PPC (45.3 kJ/mol) in PPC/starch. SA also improves Et of starch from 34.3 kJ/mol in PPC/starch to 57.4 kJ/mol in PPC/starch/SA. At starch weight content range from 10 to 50%, break strain, break stress and Youngs Modulus of PPC/starch/SA composites are all higher than ones of PPC/starch composites. At 28.4% volume fraction starch, the yield stresses of PPC/starch composites without SA and with SA, respectively, reach the vertex at 19.20 and 22.94 MPa. References [1] Fang Q, Hanna MA. Characteristics of biodegradable Mater-Bistarch based foams as affected by ingredient formulations. Ind Crops Prod 2001;13(3):219–27. [2] Martin O, Averous L. Poly (lactic acid): plasticization and properties of biodegradable multiphase systems. Polymer 2001;42(14):6209–19. [3] Averous L, Fauconnier N, Moro L, Fringant C. Blends of thermoplastic starch and polyesteramide: processing and properties. J Appl Polym Sci 2000;76(7):1117–28. [4] Yoshioka M, Shiraishi N. Biodegradable plastics from cellulose. Mole Cryst Liq Cryst 2000;353:59–73. [5] Pantazaki AA, Tambaka MG, Langlois V, Guerin P, Kyriakidis DA. Polyhydro- xyalkanoate (PHA) biosynthesis in thermus thermophilus: purification and biochemical properties of PHA synthase. Mol Cell Biochem 2003;254(1–2):173–83. [6] Swain SN, Biswal SM, Nanda PK, Nayak PL. Biodegradable soybased plastics: opportunities and challenges. J Polym Environ 2004;12(1):35–42. [7] Wool RP. Development of affordable soy-based plastics, resins, and adhesives. Chemtech 1999;29(6):44–8. [8] Mohanty AK, Misra M, Drzal LT. Sustainable bio-composites from renewable resources: opportunities and challenges in the green materials world. J Polym Environ 2002;10(1–2):19–26. [9] Chisholm MH, Navarro-Llobet D, Zhou ZP. Poly(propylene carbonate). 1. More about poly (propylene carbonate) formed from the copolymerization of propylene oxide and carbon dioxide employing a zinc glutarate catalyst. Macromolecules 2002;35:6494–504.
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