Biodegradation and Mechanical Property of Polylactic Acid-2013(贾明印)

Biodegradation and Mechanical Property of Polylactic Acid/Thermoplastic Starch Blends with Poly (ethylene glycol)XUE Ping, WANG Kejian, JIA Mingyin *, YANG Meijuan(Institute of Plastics Machinery and Engineering, Beijing University of Chemical Technology, Beijing 100029, China )Abstract: The effects of adding poly (ethylene glycol) (PEG) into polylactic acid/thermoplastic starch blends (PLA/TPS) on the properties were investigated by DSC, SEM and mechanical property-testing. The blends of PLA/TPS blended with increasing content PEG exhibited lower temperature of glass transition (T g ) and lower temperature of melting (T m ) as well as higher melt flow index (MFI), which indicates the plasticization and processability of the composites were dramatically improved. The tensile strength, fl exural strength and izod impact strength of PLA/TPS (80/20) increased at first and then decreased with increasing content of PEG due to stronger interfacial adhesion. The optimized mechanical property can be obtained for the blend with 3 wt % PEG. The samples containing PEG after soil burial for 5 months showed quicker degradation being accompanied with large weight loss and mechanical properties loss.Key words:biodegradation;mechanical property;polylactic acid; starchDOI 10.1007/s11595-013-0658-91 IntroductionWith the increased applications of plastics, a remarkable amount run into ecosystem as wastes [1]. The resulting waste plastic products caused serious environmental pollution being threat to ecology and human health. One of the important ways to reduce the waste plastic pollution is to develop biodegradable materials from renewable resources being capable of natural degradation either inside or outside of organisms.Polylactic acid (PLA) is widely studied and utilized in the areas of tissue engineering, bone healing materials and drug delivery because of its good biodegradability, biocompatibility and reasonable mechanical properties. Pure PLA degrades slowly to carbon dioxide, methane and water in the natural environment over a period of several months to 2 years [2]. However, the relatively higher price of PLA than conventional petroleum polymers prevents its widecommercialization. In addition, amorphous polylactide is rigid and brittle under ambient conditions with an elastic modulus of about 3 GPa and a low ability for plastic deformation as well as T g of 50-60 ℃[3,4].In order to modify the disadvantageous properties, PLA blends have been prepared with other polymers such as polyethylene oxide [5], polyvinyl acetate [6], polyethylene glycol [7] and polyhydroxy butyrate [8]. Starch was often used to blend with PLA to enhance the biodegradable characteristics and to reduce the cost because starch was commercially available and was readily derived from renewable resources [9-12].However, starch is a hydrophilic polymer absorbing water while PLA is hydrophobic and water- resistant. Previous studies have shown that the blend of PLA and thermoplastic starch (TPS) were of rather poor mechanical properties due to poor adhesion between components especially at high starch concentration [13,14].Blend of PLA/TPS showed lower tensile strength and elongation in comparison with neat polylactide [15].In addition, water absorption increases with starch concentration resulting in the brittleness of PLA/TPS, which is a major drawback in many applications. To remedy this limitation, some plasticisers were used. Poly (ethylene glycol) (PEG) was found to be effective in modifying PLA [16,17].With the addition of the modifi ers, the mechanical property and the degradation property of PLA[18] can be weakened regardless of pure PLA was completely degraded to CO2, water and a small amount of biomass, and even the blends were buried in soil[19,20]. However, relatively few systematic studies on the effects of the degradation of PLA/TPS with the addition of compatibilizer were made.In this work, blends of PLA/TPS were prepared for the purpose of combining the good mechanical properties of PLA with the low cost of thermoplastic starch. PEG as a compatibilizer was used in order to achieve strong interfacial adhesion improving the thermal and mechanical properties. Besides, the effect of PEG on the weight loss rate and mechanical property of the injection molded PLA/TPS bars was also examined after being buried in soil over 5 months.2 Experimental2.1 MaterialsPLA (3501D) was purchased from Cargill (USA), and it had a weight-average molecular weight of about 100 000 Da and was polymerized mainly from L-lactic acid. Thermoplastic wheat starch (12% moisture) was obtained from Chengdu New Keli Chemical Science Co. Ltd, China. The contents of PLA and thermoplastic wheat starch used in the study were 80 wt% and 20 wt%, respectively. Polymeric PEG (PEG-400) with weight-average molecular weight of 400 g/mol was obtained from Tianjin Letai Chemical Reagent Factory. The content of PEG is the percent of total weight of PLA and TPS and the contents ranged from 0 to 5 wt%.2.2 Blending and processingPrior to blending, PLA and TPS were dried for 24 h at 80 ℃ and 50 ℃ respectively. Blends of PLA /TPS with different contents of PEG were then dry-blended in a 5 L high intensity kinetic mixer at 3 200 r/min. After compounding, materials were pelletized using a twin-screw extruder of diameter 20 mm by Kunshan Kesun Rubber & Plastic Machinery Co., Ltd. The temperatures of the extruder barrel from feed zone to die were 160, 165, 165, 170 and 170 ℃. The screw rotation speed was 330 r/min. The pellets were dried at 50 ℃ for at least 24 h before injection molding into bar samples. The temperatures of the injection-molding machine were 165 ℃(near the inlet), 170 ℃ and 175 ℃ with injection pressure of 8 MPa.The injection time was 8 s and the cooling time was 60 s.The injection molding bars were sewn into nylon mesh bags and buried in soil approximately 150 mm deep and over 5 months for degradation analysis.2.3 Testing and analyses2.3.1 Mechanical propertiesSamples were oven-dried at 80 ℃ for 24 h before testing. Tensile strength and elongation at break were measured with a crosshead speed of 10 mm/min and a gauge length of 25 mm on a Universal Testing Machine (INSTRON 1185, UK) in terms of ASTM D 638. Flexural tests were carried out according to ASTM D 790. A three-point loading system was utilized with a crosshead speed of 1.3 mm/min. The MOE (Flexural modulus) and MOR (fl exural strength) were calculated in the standard. Unnotched izod impact strength was tested on an impact tester UJ-40 (Hebei Chengde Materials Laboratory Machines Plant ) according to ASTMD256-97. Each reported values were the average for fi ve samples.2.3.2 Melt fl ow index (MFI)Melt flow index (MFI) of the PLA/TPS blends was tested using a capillary rheometer (PXRZ-400A, Jilin University Instrument Factory, Jilin, China).The temperature of the die was 190 ℃ and the load was 2.16 kg.2.3.3 Differential scanning calorimetry (DSC)The thermal properties were determined via DSC (PerkinElmer TGS-2, USA) according to ASTM Method D 3417-83. About 5-10 mg of each sample was sealed in an aluminum pan. Thermal history of a sample was relaxed by heating it from 20 to 200 ℃ at a rate of 10 ℃/min, holding it at 200 ℃ for 10 min and then cooling it to 20 ℃ at the same rate. The thermal behavior was recorded by the reheating of the sample from 20 to 200 ℃ at the same rate. The heats of fusion and crystallization were determined.2.3.4 Scanning electron microscopy (SEM)The microstructure of fracture surfaces of samples was observed with a scanning electron microscope (S-4700, Japan Hitachi Inc.) at a working distance of approximately 25 mm, a voltage of 15 kV, and a probe current of 6 × 10-15A. The samples were cooled in liquid nitrogen to be broken. The fracture surface was vacuum-coated with gold for SEM analysis.3 Results and discussion3.1 Mechanical property of PLA/TPS/PEGEffect of the content of PEG-400 on the melt fl ow index (MFI) of PLA/TPS blend is shown in Fig.1. It can be seen that the MFI of the composites is increasedgreatly even though a small amount of PEG-400 is contained, which indicates that adding PEG-400 effectively improves the plasticization of the blends and decreases the melt viscosity. It can also be found from the figure that the improvement of MFI of the composites is not large when the content of PEG-400 is more than 3 wt%, considering that the slope increases slowly.Table 1 shows the effect of content of PEG-400 on the mechanical properties of the PLA/TPS blend. It can be seen from Table 1 that tensile strength, MOR and MOE of the blends all increase when adding PEG-400 due to the improvements of compatibility and interfacial adhesion between PLA and TPS. However, these values decrease when the content of PEG is more than 3 wt%. One hand, with the large amount of PEG-400, the redundant part penetrates the inside of the TPS particles and breaks the internal hydrogen bond resulting in the decreasing of the force between TPS molecules and declining of the strength. On the other hand, more content of PEG-400 easily induces the aggregation and decreases the dispersibility. In this way, the best mechanical properties would be obtained with 3 wt% content of PEG-400.Table 1 also shows that elongation of the blend decreases greatly at fi rst due to poor adhesion between matrix and TPS and then increases with enhancing content of PEG-400, which indicates PEG-400 as one kind of compatibilizers improves the motion ability of macromolecular chain resulting in the enhancement of toughness of the PLA/TPS blend. It also can be found from Table 1 that unnotched izod impact strength of the PLA/TPS blend decreases when the content ofPEG-400 is less than 1 wt%.Then, unnotched izod impact strength increases until the content of PEG-400 gets to 3 wt%. However, when continuing to increase the content of PEG-400, unnotched izod impact strength decreases.3.2 Thermal property of PLA/TPS/PEGThe DSC curves of the composites are shown in Fig.2. It can be seen from Fig.2 that blends of PLA and TPS exhibit cold-crystallized peak temperature around 110 ℃ and melting peak temperature around 150 ℃. The phase segregation of the blends of PLA/TPS is also observed based on the presence of double peaks, especially for the blends without PEG-400(curve a), which indicates the poor compatibility of PLA/TPS and the lack of interaction between the two polymers. However, the distance of the double peaks becomes narrow and small with increasing content of PEG whose content is less than 3 wt%, which proves that PEG-400 is effective to enhance the interfacial interaction of PLA/TPS composites. However, the distance of the double peaks becomes large when the content of PEG is more than 3 wt% (curve d), which indicates the poor compatibility of PLA/TPS because of the aggregation of redundant PEG-400.To take a closer look at the effect of content of PEG-400 on thermal properties of composites, the values of glass transition temperature (T g ), melting enthalpy (H m ) ,as well as cold crystallization and melting temperature(T c and T m ) are shown in Table 2. As shown in Table 2, blends of PLA/TPS exhibit lower T g ,T c and T m than those of pure PLA. It alsocan be found from Table 2 that the effect of PEG-400on reducing T c is more than that on T m . Besides, T c is seen to decrease with increasing PEG content, which is consistent with the fact that PLA/TPS composites crystallizes more easily at lower temperature due to enhanced chain mobility as the PEG level increases [21]. In the meantime, melting enthalpy of the blends is also seen to decrease with enhanced content of PEG-400.The melting enthalpy always corresponded to the crystallization enthalpy, indicating that no crystallinity developed in these materials during quenching to room temperature. Thus, it can be concluded that containing PEG-400 is effective to enhance the plasticization of PLA/TPS.Fig.3 shows the SEM of composites with different content of PEG. From Fig.3 (a), it can be seen that the composites without PEG-400 shows large grooves and gaps induced by TPS granules and the composites exhibit obvious “sea-island” structure. Besides, phase segregation between TPS granules and PLA matrix is very clear, which indicates the poor compatibility of the blends. When containing 1 wt% PEG-400, the composites show lower interfacial gaps between TPS granules and PLA matrix, whereas the interface line between the two phases is still obvious. However, this phenomenon is not clear when the content ofPEG gets to 3 wt%, as shown in Fig.3(c). The reason is that hydroxyl group of PEG possibly reacts with the carboxyl group of PLA and forms ester bond resulting in enhanced interfacial adhesion of PLA/TPS. However, the interface line between the two phases becomes clear again when the content of PEG increases to 5 wt% because that more content of PEG easily induces the aggregation and decreases the dispersibility of PEG. This trend is similarly to that of mechanical property discussed in Table 1.3.3 Degradation property of PLA/TPS/PEGIn order to verify the effects of TPS on the biodegradation of PLA, the biodegradation rates of unmodi fi ed blends of PLA/TPS bars after buried in soil for 5 months were researched. The content of TPS in PLA blends varied from zero to 10 wt%, 20 wt%, 30 wt%, 40 wt% respectively. The studies show that the degradation rate is not obvious for pure PLA which just shows rough surface. It is because that main chain of PLA is rigidity and side chain containing methyl, which results that pure PLA is not easy degraded by natural microbe. With increasing content of TPS, the blends show small void and fi ne cracks on the surface and the phenomenon is very obvious when the content of TPS gets to 30 wt% corresponding to increased void and crack. Importantly, it was also found that the blend with 40 wt% content of TPS has been cracked when taken out from the soil. Thus, it is believed that the degradation of the blends of PLA/TPS can be improved with the increased content of TPS.Corronspondly, biodegradation rates of the blends of PLA/TPS bars with different content of PEG-400 were also studied. All the samples show rough surfaces after being soil-buried for the microbial growth. Besides, the samples containing PEG-400 show more obvious color fading and fine cracks on the surface than those of the samples without PEG-400 at the same conditions. However, there is no great difference in the effect of the amount of PEG on the surface degradationsituation.Fig.4 shows the weigh loss of tensile bars and izod impact bars of the composites after being buried in soil for 5 months. From the fi gure, it can be observed that the bars containing PEG shows greater weight loss than those of bars without PEG after buried in soil for 5 months. The largest weight loss of izod impact bars is 18% for the samples with 5 wt% content of PEG-400. It also can be seen that the weight loss of the bars increases in essence with enhanced content of PEG-400, which indicates the addition of PEG-400 promotes the degradation rate of PLA/TPS blends.Effects of the content of PEG-400 on tensile strength, elongation and unnotched izon impact strength of the blends before and after degradation are shown in Fig.5. The fi gure shows that the mechanical properties of the samples after degradation are much lower than that of the original samples. The more PEG-400 is, the greater the decrement is. The drops may be both of absorbing more water into the material inducing hydrolysis of ester bonds and lowering of molecular weight followed by intracellular uptake of lactic acid oligomers and catabolism. The small molecular chains of PLA then are degraded into CO2 and water by the fungus. On the other hand, blends with hydrophilic PEG-400 improves the hydrophilicity of the composites, resulting that the composites absorb more easily the moisture in the soil .Rates of hydrolysis increase with water content and are catalyzed by free carboxyl groups. It can also be found from Fig.5 that the largest degradation rate, corresponding to the lowest mechanical properties, can be got with 5 wt% content of PEG-400.Table 3 shows the mechanical properties losses of the composites after soil burial for 5 months. It can be found that losses in tensile strength, elongation and unnotched izod impact strength are 84.43%, 95.80% and 90.89 %, respectively, when the content of PEG is 5 wt%. The trend that the losses of mechanical properties increase with increment of PEG is also seen, which indicates that adding PEG-400 promotes the degradation of PLA/TPS blends.4 ConclusionsIt showed that poly (ethylene glycol), as a modifi er, enhanced effectively the interfacial adhesion between TPS and PLA. All of the tensile strength, flexural strength and izod impact strength increase at first and then decrease with increasing PEG at the critical content of 3 wt %. Mixing a small amount of PEG into PLA/TPS blend improved its fl owability. The blends of PLA/TPS with increasing PEG exhibits lowerTgand Tm. The blends containing PEG after being soil-buried for 5 months exhibited greater degradation rate corresponding to large weight loss and mechanical properties loss.References:[1] Shah A A, Hasan F, Hameed A, et al. Biological Degradation ofPlastics: a Comprehensive Review[J]. Biotechnol. Adv., 2008, 26(3):246-265[2] Wang N, Yu J G, Ma X F. Preparation and Characterization ofThermoplastic Starch/PLA Blends by One-step Reactive Extrusion[J].Polym. Int., 2007, 56(5): 1 440-1 447[3] Kozlowski M, Masirek R, Piorkowska E, et al. Biodegradable Blendsof Poly(L-lactide) and Starch[J]. J. Appl. Polym. Sci., 2007, 105(1): 269-277[4] Kulinski Z, Piorkowska E. Crystallization, Structure and Properties ofPlasticized Poly(L-lactide) [J]. Polymer, 2005, 46(3): 10 290–10 300 [5] Gajria A M, Dave V, Gross R A, et al. Miscibility and Biodegradabilityof Blends of Poly (lactic acid) and Poly (vinyl acetate)[J]. Polymer, 1996, 37(3): 437-444[6] Nijenhuis A J, Colstee E, Grijpma D W, et al. High MolecularWeight Poly (L-lactide) and Poly (ethylene oxide) Blends: Thermal Characterization and Physical Properties[J].Polymer,1996, 37(26):5 849-5 857[7] Sheth M, Kumar R A , Dave V, et al. Biodegradable Polymer Blends ofPoly (lactic acid) and Poly (ethylene glycol) [J]. J. Appl. Polym. Sci., 1997, 66(8) : 1 495-1 505[8] Focarete M L, Ceccourulli G, Scandola M, et al. Further Evidence ofCrystallinity-Induced Biodegradation of Synthetic Atactic Poly(3-hydroxybutyrate) by PHB-Depolymerase a from Pseudomonas lemoignei. Blends of Atactic Poly(3-hydroxybutyrate) with Crystalline Polyesters[J]. Macromolecules, 1998, 31(24): 8 485-8 492[9] Wang H, Sun X Z, Seib P. Effects of Starch Moisture on Properties ofWheat Starch/Poly(lactic acid) Blends Containing Methylene Diphenyl Diisocyanate[J]. J. Polym. Environ., 2002, 10(4):133 -138[10] Zhang J F, Sun X Z. Mechanical Properties of Poly(lactic acid)/starch Composites Compatibilized by Maleic Anhydride[J].Biomacromolecules, 2004, 5(10) : 1 446-1 451[11] Wu C S. Characterizing Biodegradability of Polylactide (PLA) orPLA-g-AA/starch Encapsulating Phosphate-solubilizing BacteriumBacillus[J]. Macromoecularl Bioscencei, 2005, 5(5): 352-361[12] Chen L, Qiu X Y, Xie Z G, et al. Poly(L-lactide)/starch BlendsCompatibilized with Poly(L-lactide)-g-starch Copolymer[J].Carbohydr. Polym., 2006, 65(1):75-80[13] Kim S H, Chin I, Yoon J, et al. Mechanical Properties of BiodegradableBlends of Poly(l-lactic acid) and Starch[J]. Korea Polym. J., 1998,6(5):422-427[14] Ke T, Sun X. Physical Properties of Poly(lactic acid) and StarchComposites with Various Blending Ratios[J]. Cereal. Chem., 2000, 77(6): 761-768[15] Martin O, Averous L. Poly(lactic acid): Plasticization and Propertiesof Biodegradable Multiphase Systems[J]. Polymer, 2001, 42(14):6 209-6 219[16] Baiardo M, Frisoni G, Scandola M, et al. Thermal and MechanicalProperties of Plasticized Poly(L-lactic acid) [J]. J. Appl. Polym. Sci., 2003, 90(7): 1 731-1 738[17] Hu Y, Hu Y S, Topolkaraev V, et al. Aging of Poly(lactide)/poly(ethylene glycol) Blends. Part 1. Poly(lactide) with Low Stereoregularity[J].Polymer, 2003, 44(14): 5 701-5 710[18] Itävaara M, Karjomaa S, Selin J. Biodegradation of Polylactide inAerobic and Anaerobic Thermophilic Conditions[J]. Chemosphere, 2002,46(6):879-885[19] Calmon A, Guillaume S, Bellon-Maurel V, et al. Evaluation of MaterialBiodegradability in Real Conditions-development of a Burial Test and an Analysis Methodology Based on Numerical Vision[J]. J. Environ.Polym. Degrad., 1999, 7(3): 157-166[20] Osawa S, Tsukegi T, Ogawa T, et al. Biodegradation Behavior ofMolded Poly(l-lactic acid)/starch Blend in a Land-fi ll Test[J]. Materials Life, 2000, 12(4): 199-205[21] Kozlowski M, Masirek R, Piorkowska E, et al. BiodegradableBlends of Poly(L-lactide) and Starch[J]. J. Appl. Polym. Sci., 2007,105(1):269-277。