Mechanical Properties, Morphologies and Thermal

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Mechanical Properties, Morphologies and Thermal Decomposition Kinetics of Poly(lactic acid) Toughened by Waste Rubber Powder ARTICLE in INTERNATIONAL POLYMER PROCESSING JOURNAL OF THE POLYMER PROCESSING SOCIETY · AUGUST 2015 Impact Factor: 0.51 · DOI: 10.3139/217.3049

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J.-N. Yang1 *, S.-B. Nie2 , G.-X. Ding1 , Z.-F. Wang1 , J.-S. Gao1 , J.-B. Zhu1 1 School 2

of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, PRC School of Mining and Safety Engineering, Anhui University of Science and Technology, Huainan, PRC

International Polymer Processing downloaded from www.hanser-elibrary.com by Carl Hanser Verlag on August 19, 2015 For personal use only.

Mechanical Properties, Morphologies and Thermal Decomposition Kinetics of Poly(lactic acid) Toughened by Waste Rubber Powder To improve the impact resistance and reduce the product cost, poly(lactic acid) (PLA) blends containing varying mass fraction of waste rubber powder (WRP) were fabricated via melt compounding. The effects of WRP contents on the mechanical properties, morphologies and thermal stabilities of PLA/WRP blends were investigated. Mechanical tests showed that WRP could increase the ductilities of PLA, leading to the significant improvements in the impact toughness and elongation at break. In contrast, the tensile strength was just heightened slightly, while elastic modulus declined gradually. Scanning electron microscopy observations indicated that well bonded interfacial morphologies were formed between PLA and WRP. From the results of thermo gravimetric analysis, WRP decreased the onset and peak decomposition temperatures of PLA phase and increased the char contents of samples significantly. Average activation energies of samples were increased first and then decreased with increasing WRP. Finally, theoretical lifetimes of PLA/WRP blends were also estimated.

1 Introduction Poly(lactic acid) (PLA) was an authentic biodegradable thermoplastic resin, which has obtained extensive attentions in recent years. It was the excellent biodegradability and biocompatilibity that usually made PLA very popular in the applications of internal sutures, implant devices, tissue scaffold and drug delivery devices (Nampoothiri et al., 2010). For the environmentally aware, PLA should be an especially attractive candidate instead of petroleum-based polymers, due to its sources from renewable plants (such as corn and starch). Although PLA possessed competitive strength and stiffness to general resins of polypropylene (PP) and polyethylene (PE), its applications were largely limited by the high brittleness, poor impact resistance and the relatively higher cost of products. * Mail address: Jinian Yang, School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, PRC E-mail: [email protected]

Intern. Polymer Processing XXX (2015) 4

Therefore, it was essential to improve the comprehensive properties of PLA and widen the scope of potential applications. In recent years, several research efforts have been proposed and demonstrated to modify the toughness of PLA (Anderson et al., 2008) including introducing the second phase to create PLA blends has been proved to be efficient. So far, many kinds of polymers were considered to improve the impact toughness of PLA. It was not only these biodegradable polymers, such as poly(hydroxyalkanoate)s (PHAs) (Noda et al., 2004), poly(caprolactone) (PCL) (Broz et al., 2003; Semba et al.; 2006), poly(butylene succinate) (PBS) (Wang et al., 2009; Yokohara and Yamaguchi, 2008) poly(butylene adipate-co-terephthalate) (PBAT) (Jiang et al., 2006; Zhang et al., 2009), poly(ether)urethane (PU) (Li and Shimizu, 2007), but those of conventional elastomers, including linear low density polypropylene (LLDPE) (Anderson et al., 2003; Anderson and Hillmyer, 2004), ethylene-a-octene copolymer (EOC) (Ho et al., 2008; Su et al., 2009), ethylene-co-vinyl acetate copolymer (EVA) (Ma et al., 2012), hydrogenated styrene-butadiene-styrene block copolymer (SBS) (Hashima et al., 2010) and methyl methacrylate-butadiene-styrene (MBS) (Zhang et al., 2012). By comparing the effects of polymers on the impact toughness of PLA, the addition of elastomers usually led to superior toughening effects, though there might be some negative influences on the biodegradability of matrix. However, due to the prices of the elastomers mentioned above, they were not low cost enough for PLA products and it was still a challenge of searching for more cost-effective impact modifiers. Waste rubber powder (WRP) was the organic particle and primarily produced from waste tires via mechanical cut and milling process. WRP was usually characterized by low cost, fine particle size (general in micro scales) and elastomeric feature. WRP has been utilized considerably as fillers in rubber, and notably impact resistance modifiers in plastic and asphalt. It could improve the toughness of rigid polymers indeed, and the optimal mass fraction located during the range of 10 %*25 %; however, it certainly depended on the matrix, WRP size and the interfacial reactions between WRP and polymers (Sonnier et al., 2007; 2008; Ouyang et al., 2012; El-Nemr and Khalil, 2011; Meszaros et al., 2012; Li et al., 2004; Shanmugharaj et al., 2005; Lee et al., 2009; Ghaisas et al., 2004; Tan et al., 2009; Hassan et al., 2010). Although, to our best

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J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder knowledge, there was little literature on WRP modified PLA, the possible excellent toughening effect of WRP was expected, and also for the purpose of reducing the product cost of PLA. In present paper, WRP was chosen as impact modifier for toughening PLA. PLA blends with varied mass fraction of WRP were prepared, and their interfacial morphologies and mechanical properties were investigated. Also, this study focused on the thermal properties of PLA/WRP blends. The activation energies of PLA phase versus increasing WRP content were calculated and evaluated according to different equations. Finally, the theoretical lifetime of investigated samples was estimated through thermo gravimetric analysis (TGA).


2 Experimental 2.1 Raw Materials Poly(lactic acid) (PLA, REVODE101) was kindly supplied by Zhejiang Hisun Biomaterials Co., Ltd. (Zhejiang, PRC). Its melt flow rate was 2*10 g/10 min and the specific gravity was 1.25 g/cm3. Waste rubber powder (WRP) was purchased from Hangzhou Baoli Recycling Co., Ltd. (Hangzhou, PRC). It was a blend of car and truck waste tires containing a small amount of fibers. The specific surface area of WRP was less than 0.1 m2/g. The silane coupling agent used in this study was c-aminopropyltriethoxysilane, which was usually named as KH-550. It was the product of Nanjing Shuguang Chemical Group Co., Ltd. (Nanjing, PRC) A. R. grade. Absolute alcohol and silicone oil were both commercial available with A. R. grade.

2.2 Fabrication of PLA/WRP Blends WRP was passed through a sieve of 120 mesh, and then immersed into 1.5 vol.% A1100/alcohol solution with vigorous stirring for about 4 h at 40 8C. The treated WRP was filtered, washed and dried to constant weight in a vacuum oven. Before use, PLA and WRP were dried in a vacuum oven at 60 8C for 24 h. All the samples were melted and compounded in a double-roll open mill at 135 8C for 30 min. The blends were then compression molded (145 8C and 15 MPa) into sheets (200 mm · 200 mm · 4 mm) for subsequent measurements. The compositions of PLA/WRP blends were 95/5, 90/10, 85/

15, 80/20 and 75/25 (mass fraction ratio), and the samples were marked as PLA/WRPx% (x = 5, 10, 15, 20 and 25). The pristine PLA was chosen as control sample and underwent the same procedure. Standard dumbbell and bar specimens were cut from the as-molded sheets for tensile and impact tests, respectively.

2.3 Characterization Uniaxial tensile tests were carried out at room temperature by using a computer aided WDW-50 universal testing machine. The crosshead speed of testing was 2 mm/min. Chinese Standards GB/T 1040.1-2006 was followed to measure the elastic modulus, tensile strength and elongation at break of samples. The testing sample was 1A type with the gauge length of 50 mm. Charpy impact tests were performed in a TCJ-25 J impact test. The impact toughness was calculated via dividing the impact work by cross-sectional area of the sample according to GB/T 1043.1-2008. The sample was un-notched and its dimensions were 80 mm for length, 10 mm for width and 4 mm for thickness, with the span length of 62 mm. All results were the average of at least five replications. Micrographs of impact fractured surfaces of samples were taken in a Hitachi S-4800 field emission scanning electron microscopy (SEM) with an accelerating voltage of 3.0 kV. Before examination, non-conducting samples were coated with a thin layer of gold (about 10 nm) by sputtering technique. Thermo gravimetric analysis (TGA) was done using a SDT2960 thermo-gravimetric analyzer. Tiny samples with about 10 mg were picked up from the core part of bar specimens. Non-isothermal TGA data were obtained with samples heated at different heating rates (10, 20, 30 and 40 K/min). The temperature range was from 50 8C (323.15 K) to 550 8C (823.15 K). All TGA tests were carried out under nitrogen (N2) atmosphere, with the flow rate of 60 ml/min. 3 Results and Discussion 3.1 Mechanical Properties The mechanical properties of PLA/WRP blends investigated were shown in Table 1. As could be seen clearly, the impact toughness was increased first and then decreased with increas-

Samples

Impact toughness kJ/m2

Tensile strength MPa

Elastic modulus GPa

Elongation at break %

PLA PLA/WRP5 % PLA/WRP10 % PLA/WRP15 % PLA/WRP20 % PLA/WRP25 %

14.47 20.36 29.45 37.39 16.86 16.48

47.14 47.61 48.57 41.94 31.38 28.97

1.98 1.91 1.83 1.72 1.55 1.49

0.83 0.94 1.37 2.03 1.89 1.86

Table 1. Mechanical performance of PLA/WRP blends

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J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder ing WRP. With the WRP added up to 15 %, the peak value of 37.39 kJ/m2 was achieved, which showed the improvement of 158.4 % on the impact toughness than that of pure PLA (14.47 kJ/m2). This tremendous toughening effect derived from the WRP was mainly due to its rubbery nature. When the samples were subjected to high speed impact loadings, the granular WRP dispersed well in matrix should have the functions of releasing the plastic constraint, and inducing shear deformation in the matrix. On the other hand, WRP was acted as the second phase particles in the matrix and had the ability of hindering or deflecting crack propagation in the process of crack generation and growth. Thus, large amounts of energy should be consumed before samples crushed. As a result, the impact toughness increased greatly. However, the excessive WRP caused the obvious negative influence on the impact toughness, although it was still higher than the control sample. This should be explained by the excessive WRP tended to aggregate, and thus acted as points of stress concentration to facilitate the failure of samples. From Table 1, WRP seemed to have little positive effect on the tensile properties of samples. With WRP increased to 10 %, the tensile strength just increased slightly from 47.14 MPa to 48.57 MPa, but then decline sharply to 28.97 MPa with 25 % WRP. The elastic modulus exhibited a nearly linear decrease from 1.98 GPa to 1.49 GPa with growing content of WRP. The following factors that should affect the tensile properties of PLA/WRP blends could be considered. WRP was possessed of much lower strength and stiffness than pristine PLA. Of course, the WRP addition resulted in the decrease in strength and stiffness according to rules of mixture.

However, the WRP particles, as well known, were generated from those waste tires. During the production process, the mechanical forces just could break the surface cross-linked structures of WRP. However, it was hardly to affect the inner morphologies. Thus, the WRP was generally characterized by inherent cross-linked structure (Adhikari et al., 2000). So, the strength of WRP was higher than that of conventional elastomer, such as EOC. The negative effect of WRP on the tensile strength should not as obvious as the EOC on the polymers. On the other hand, when the content of WRP was low (not more than 10 %), the WRP could disperse as single particles in the matrix, and their interfaces were well bonded (shown in Fig. 1). The enhanced interfacial adhesion might certainly conquer the negative influence of WRP on the tensile strength. As a result, the tensile strength was not declined and even had a very slight improvement. However, the negative effect behaved increasingly dominant with the continuous rise of WRP, and subsequently led to the drastic decrease in the tensile strength and elastic modulus of PLA/WRP blends. The elongation at break, another indication of material ductility, displayed the similar variation trend to that of impact toughness. Adequate addition of WRP increased the elongation at break, while excessive WRP resulted in a moderate decline. However, all of the values were much higher than that of virginal PLA. This could be attributed to the softer WRP resulting in the improvements on the ductile of PLA/WRP blends under the quasi-static testing. However, it was also noted that, despite that the highest value was 144.6 % higher than that of virginal PLA; the absolute values of elongation at break for all the samples were still too low. Therefore, in the further experiments,

A)

B)

C)

D)

E)

F)

Fig. 1. Morphologies of impact fracture surfaces of (A) PLA/WRP5 %, (B) PLA/WRP5 % with high magnification, (C) PLA/WRP15 %, (D) PLA/ WRP15 % with high magnification, (E) PLA/WRP25 % and (F) PLA/WRP25 % with high magnification


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J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder the more effective technologies should be proposed and carried out to enhance the ductility of PLA.


3.2 Morphologies Figure 1 showed typical morphologies of fractured surfaces of samples, which were selected from impact tests at room temperature. These micrographs demonstrated the distribution of WRP, as well as the interfacial morphologies between WRP and PLA matrix. Clearly to see, WRP behaved as the second phases (white areas in the Fig. 1) and distributed randomly in the matrix, with irregular shape and sizes. That was to say, the PLA/WRP blends exhibited the typical droplet-matrix structures. From SEM pictures of high magnification, the boundaries between WRP and matrix were blurry. There were no obvious interfacial de-bonding observed. It was indicated that well adhered interfaces between WRP and PLA were formed. When the mass fraction was not more than 15 %, WRP dispersed very well and almost existed with a single particle. However, with further increment of WRP, the large aggregates came to appear due to the incompatibility between WRP and PLA. As a result, the obvious negative effects on the mechanical properties should be presented, as shown in Table 1.

However, it seemed to have no obvious effect on rate of thermal decomposition of PLA phase, because of DT for PLA and PLA/WRP blends depicting similar values. In the third region (658.15 K*823.15 K), the WRP apparently had the highest WR, which was up to 65.5 % shown in Table 2. However, PLA possessed the lowest WR (only about 1.0 % char contents left). This was mainly ascribed to organic nature of PLA,

3.3 Thermal Stability

A)

Figure 2 depicted the TGA and DTG thermograms of samples at 10 K/min. the characteristic temperatures, including onset temperature of decomposition (defined as mass conversion ( ) up to 5 %, To), peak temperature of maximum mass conversion rate (da/dt) (TP) and DT (defined as temperature region between To and Tp), as well as remaining percentage (WR), were all listed in Table 2. The decomposition of PLA proceeded according to a unique reaction within a narrow temperature range. Although WRP decomposed within a relative wider temperature range, it seemed to have little evident influence on the thermal degradation model of PLA/WRP blends, which still presented the similar sigmoid shape curves as that of pure PLA. With the temperature increased from 323.15 K to 823.15 K, there were three stages displayed for thermal degradation behavior. Within the first stage up to 543.15 K, little difference in thermal stability could be observed between PLA and PLA/ WRP blends. However, during the major degradation region from 543.15 K to 658.15 K, To and TP of PLA/WRP samples shifted to lower temperature obviously with increased WRP, suggesting that WRP accelerated the thermal degradation of PLA in this temperature region. Generally, the low thermal stability of WRP might lead to the decline of To and TP of blends, because thermal properties of blends were largely determined by the relative lower component according to the rule of mixture. On the other hand, PLA was characterized by the repeated aliphatic ester structure, which was easy to hydrolyze and breakdown. The governable mechanism of thermal degradation of PLA was usually known as random chain scission or specific chain end scission (Fan et al., 2004). The plasticizer, antioxidant and other auxiliary agents remained in WRP might have the promotional influences on the ester bonds scission.

B)

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Fig. 2. TGA (A) and DTG (B) thermograms of samples at 10 K/min

Samples

To K

TP K

DT K

WR %

WRP PLA PLA/WRP5 % PLA/WRP10 % PLA/WRP15 % PLA/WRP20 %

536.9 603.3 601.8 595.4 586.6 573.8

610.2 634.2 632.6 625.2 619.4 606.4

73.3 30.9 30.8 29.8 32.8 32.6

65.5 1.0 3.4 5.5 9.6 11.3

Table 2. Characteristic data for thermal decomposition of samples


J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder which was quasi-totally decomposed in an inert atmosphere of nitrogen. The values of WR for PLA/WRP blends increased steadily with the content of WRP. As well known, the inorganic fillers including carbon black and other inorganic fillers in WRP were stable at high temperatures, and could not be removed by ignition under nitrogen. Therefore, the values of WR increased with the growing proportion of WRP in the blends.


3.4 Analysis of Thermal Decomposition Kinetics The TGA and DTG curves of samples obtained at different heating rate of 10, 20, 30 and 40 K/min were depicted in Fig. 3. As could be seen, both of To and Tp shifted to higher temperature with increased heating rate. The decomposition temperatures of samples were rate dependence (actually thermal hysteresis). It was mainly caused by the fact that molecular chain scission was a thermal activated process. Elevated heating rate generated more degree of superheat and then To and Tp became higher. However, there were little variations of a for samples with identical WRP content, suggesting degree of decomposition was rate independence. The effective activation energy (E) of decomposition processes was calculated by Kissinger equation (Kissinger, 1957): b E A R ln 2 ¼ þ ln ; R TP E TP

where b was the heating rate, R was the gas constant, A represented the pre-exponential factor. By plotting ln(b/Tp2) vs. 1/ Tp, an approximately straight line with a slope of E/R could be obtained, as was shown in Fig. 4, in which enclosed with the E value, relativity (r) and the fitted linear equation. From the slopes and intercepts of the straight lines, we calculated the values of activation energy and obtained E = 166.4 kJ/mol for PLA, E = 187.7 kJ/mol for PLA/WRP5 %, E = 176.4 kJ/ mol for PLA/WRP10 %, E = 152.1 kJ/mol for PLA/WRP15 % and E = 127.6 kJ/mol for PLA/WRP20 %. The value of activation energy of blends decreased gradually with increasing WRP content, while the case of PLA/WRP5 % and PLA/ WRP10 % exhibited E values about 12.8 % and 6.0 % higher than that of pure PLA. This suggested that lower content of WRP (£ 10 %) could enhance energy barriers of peak decomposition for PLA/WRP5 % and PLA/WRP10 %. Nevertheless, further increased WRP decreased the energy barriers of the samples. As well known, the degradation for polymer was a process consisted of permeating and volatilization of low molecular matter (gas), breaking down of polymer chain and carbonization of remainder. The introduction of WRP into PLA matrix affected the thermal decomposition process mainly from two aspects. First, rubber ingredients left in WRP accelerated the break down of polymer chains and reduced the energy barrier. On the other hand, the fillers of carbon black and other inorganic particles contained in WRP could provide the tortuous path for the diffusion of low molecular matter and significantly reduced the permeation rate (Kim et al., 2010). Herein, the fillers could have acted as physical barriers and delayed the escape and volatilization of degradation products Intern. Polymer Processing XXX (2015) 4

(Bao et al., 2011). Then the energy barrier of decomposition increased. When WRP content was lower, the latter was more competition than the former, and then activation energy increased. However, if the WRP content was higher than 15 %, the former showed a greater influence than the latter, subsequently an opposite result was presented, i. e. the E values decreased. However, the activation energy value derived from Kissinger equation just represented the average value of E (i. e. apparent of activation energy) belonged to the whole thermal degradation processes. It did not indicate the E values varied with a. Thus, Flynn-Wall-Ozawa equation (Flynn and Wall, 1966a; Ozawa, 1970) was also employed. 0:4567Ea A E lg b ¼ 2:315; þ lg R T gðaÞ R where g(a) was the integral conversion function, T represented the corresponding temperature to a given constant value of a, Ea was the activation energy related to a. By plotting lg b vs. 1/T, we obtained an approximately straight line and then Ea could be calculated, as was very similar to Kissinger equation. Figure 5 showed the variation of Ea as a function of a. It was found that Ea did not maintaining the constant value absolutely. The Ea for PLA in the initial stage was higher than all the blends and then declined sharply with increasing a, while the values of Ea for PLA/WRP blends increased steadily. This fact suggested that WRP weakened the energy barrier at low temperature, but strengthened the energy barrier at high temperature during the whole decomposition process. That was why the PLA/WRP5 % and PLA/WRP10 % were characterized by lower To and higher E values than that of PLA. The average values of E’ for all samples were also attached in Fig. 5. Although the E’ values were a little lower than that derived from Kissinger equation (E), both of the two group data showed exactly the same trends, indicating well consistent in calculating values of activation energy by Kissinger and Flynn-Wall-Ozawa equations.

3.5 Estimation of Lifetime Once the value of activation energy was obtained from the kinetic analysis of TGA results, theoretical lifetime of materials could be estimated by Toop equation (Toop, 1971). Eab Eab Eab lg s ¼ ; þ lg p þ lg 2:303R T R b R Tc where s was the predicted lifetime and Tc was the temperature at a constant value of a and b, and p(Ea/RTc) could be gained directly from the given table in references Doyle (1961) and Flynn and Wall (1966b). As well known, thermal degradation behavior significantly weakened the mechanical properties of polymer, thus the lifetime related to a up to 5 % was usually defined as the effective lifetime. Relationships between the predicted lifetime and temperature for samples were illustrated in Fig. 6, under a constant values of a = 5 % and b = 10 K/min. As could be seen clearly, the predicted lifetime of PLA/ WRP blends was decreased exponentially with the increased 471

J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder


temperature. When the working temperature was raised from 300 K (at room temperature) to 630 K, the values of s decreased sharply from 2.75 · 1016 min to 0 in the case of PLA. Also, it could be noticed that the predicted lifetime values for PLA/WRP blends were all lower than that of pure PLA. The

more the contents of WRP were, the lower the values of s would be. This phenomenon was not in line with the variations of activation energy with WRP content. The higher activation energy did not always lead to the longer lifetime.

A)

B)

C)

D)

E)

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Fig. 3. Continuous heating TGA and DTG curves of (A) PLA, (B) PLA/ WRP5 %, (C) PLA/WRP10 %, (D) PLA/WRP15 % and (E) PLA/WRP20 % at different heating rates of 10, 20, 30 and 40 K/min


J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder


4 Conclusions

Fig. 4. Kissinger plots of samples

Samples of PLA blended with varying mass fraction of WRP were prepared and their properties were investigated. This paper was mainly focused on the mechanical performances, morphologies and thermal degradation kinetics of PLA/WRP blends, and came to the conclusions as follows: 1. In a way, WRP could toughen PLA. The impact toughness and elongation at break of PLA/WRP blends were both increased significantly. Improvements of 158.4 % and 144.6 % for impact toughness and elongation at break, respectively, were obtained with 15 % WRP. However, there was only a slight enhancement exhibited in the tensile strength, while elastic modulus of samples declined gradually with increasing WRP. 2. WRP distributed randomly in PLA matrix with irregular shape and size in typical droplet-matrix structures. Moreover, well adhered morphologies were formed in the interfaces between WRP and PLA. 3. The introduction of WRP showed the ability of accelerating the thermal decomposition of PLA phase. The onset (To) and peak (Tp) temperatures of decomposition were both shifted to low temperatures. However, the residual weight at infinite time (i. e. char content) increased considerably from 1.0 % for virgin PLA to 11.3 % for PLA/WRP20 %. 4. For all samples, the decomposition temperatures were rate dependence. Increased heating rate usually resulted in their elevated To and Tp. However, the degree of decomposition was nothing to do with the heating rate. 5. The average activation energies of samples were increased first and then decreased with increasing WRP. Furthermore, the data derived from Flynn-Wall-Ozawa equation revealed that activation energies were also varied with the mass conversion (a). 6. The predicted theoretical lifetimes of PLA/WRP blends were decreased exponentially with temperature, and also declined with increased content of WRP.

Fig. 5. Ea values of samples as a function of

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Fig. 6. The predicted lifetime of samples at different temperature


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J. Yang et al.: Poly(lactic acid) Toughened by Waste Rubber Powder Acknowledgements This work was supported by Anhui Province Post Doctoral Researchers in Scientific Research Projects (No: 2014B006), the National Natural fund (No: 51303004) and the Anhui Provincial Natural Science Research Projects in Colleges and Universities (No: KJ2013Z067).


Date received: November 28, 2014 Date accepted: April 12, 2015

Bibliography DOI 10.3139/217.3049 Intern. Polymer Processing XXX (2015) 4; page 467 – 475 ª Carl Hanser Verlag GmbH & Co. KG ISSN 0930-777X


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