Article pubs.acs.org/IECR
Compatibilizing Effects of Maleated Poly(lactic acid) (PLA) on Properties of PLA/Soy Protein Composites Rui Zhu, Hongzhi Liu, and Jinwen Zhang* Composite Materials and Engineering Center, Washington State University, Pullman, Washington 99164-1806, United States ABSTRACT: Poly(lactic acid) (PLA)/soy protein concentrate (SPC) composites were prepared using a twin-screw extruder. Extra amounts of water and glycerol were added to SPC prior to compounding with PLA to ensure that SPC behaved like a thermoplastic during subsequent mixing. Free radical grafting of maleic anhydride (MA) onto PLA was performed by reactive extrusion, and PLA-g-MA was used as a compatibilizer in the composites. The effects of compatibilizer concentration and degree of functionality on the tensile, morphological, and thermal properties of PLA/SPC composites were studied. The tensile strength of the composites containing 4 phr compatibilizer increased by 19% compared to that of the uncompatibilized one. Also, the presence of compatibilizer resulted in finer domain sizes of SPC and a lower damping peak, both of which suggested that good interfacial adhesion between the two phases was achieved. Differential scanning calorimetry analysis indicated that SPC induced and accelerated cold crystallization of PLA.
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INTRODUCTION In recent years, biodegradable and biobased polymers have received extensive interest for plastic applications as an alternative to conventional petroleum-based plastics. Poly(lactic acid) (PLA) is the most promising biobased polymer commercially available in the market. Commercial PLA is mainly synthesized by ring-opening polymerization of lactides which are the cyclic dimers of lactic acids and are typically derived from corn starch fermentation. PLA is biodegradable and compostable and demonstrates good tensile strength and modulus, making it attractive for disposable and biodegradable plastic products. However, PLA is still relatively more expensive than many petroleum-based commodity plastics. In addition, the biodegradation rate of PLA is relatively low. Polymer blending is a cost-effective method for improving the properties of one or both of the components. Blending PLA with some natural polymers such as starch and soy protein (SP) could also substantially reduce the total cost while increasing the degradation rate of the composites. As the leftover from soybean oil crushing, soy meal is an agricultural residue. Soy meal can be grinded into soy flour (54% protein), or further purified into soy protein concentrate (SPC) (65−72% protein) and soy protein isolate (SPI) (90% protein). Zhang et al.1 melt compounded SPI with sufficient water and glycerol by extrusion and subsequently extruded the compounded SP pellets into smooth sheets. Recently, we have conducted an extensive investigation of water-assisted SPC blending with PLA2,3 or poly(butylene adipate-co-terephthtalate) (PBAT)4,5 and found that in the presence of water SPC behaved like a plastic during mixing as it could be deformed greatly. The results also demonstrated the resulting blends became in situ formed composites after water was evaporated.6 However, PLA and SP are thermodynamically immiscible due to the large difference in their hydrophilicities. The interfacial bonding of simple PLA/SP blends is fairly weak. Interfacial modifiers containing reactive functional groups are able to generate in situ formed blocks or grafted copolymers at © 2012 American Chemical Society
the interface to improve the compatibilization. Poly(2-ethyl-2oxazoline) (PEOX) is regarded as a broadly compatible polymeric solvent or compatibilizing agent for many polymer pairs.7 PEOX was found to have good affinity with SP, resulting in a fine phase morphology, and thus substantially improved mechanical properties of the PLA/SPI or PLA/SPC composites.8 Methylene diphenyl diisocyanate (MDI) is highly reactive, and it can form urethane linkages with hydroxyl and carboxylic groups. It has been widely used as a compatibilizer in natural polymer and PLA blends.9,10 The incorporation of 0.5 wt % MDI in a PLA/starch composite significantly improved the tensile strength from 36.0 to 66.7 MPa.9 However, MDI is considered an environmentally hazardous material and is not suitable for food packaging or related applications.11 Recently, a more promising interfacial modification route was adopted in binary immiscible PLA blends and composites. This route includes grafting a reactive moiety, for instance maleic anhydride (MA), onto the polymer matrix, and then having this moiety to react with the natural polymers in some ways.12 MA-grafted PLA (PLA-g-MA) has been widely used as a compatibilizer in PLA blends with starch,11,13 talc,14 and silicate.15 The polysaccharide components of SPC have a lot of hydroxyl groups that may react with MA similarly. On the other hand, the protein component of SPC has amide and free amino groups which are probably more active in reacting with MA. So far, PLA-g-MA has not been used as a compatibilizer in PLA/ SP composites. Furthermore, studies regarding the effects of functionality of the compatibilizer are also lacking in the literature. In this study, the compounding of PLA/SPC composites was performed using extrusion and the test specimens of the composites were prepared by injection molding. The water Received: Revised: Accepted: Published: 7786
January 13, 2012 April 20, 2012 May 18, 2012 May 18, 2012 dx.doi.org/10.1021/ie300118x | Ind. Eng. Chem. Res. 2012, 51, 7786−7792
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Table 1. Formulations of PLA-g-MA sample neat PLA 1 2 3 4
L101 (wt %) 0.25 0.25 1 0.5
MA/St ratio 1:0 1:0.5 1:0.5 1:0.5
MA (wt %) 2 2 2 2
T (°C) 180 180 180 200
Mw
Mn
Mw/Mn
0.25 0.54 0.68 0.90
128 000 108 300 111 500 103 600 85 000
88 900 74 700 77 500 72 300 54 400
1.44 1.45 1.44 1.43 1.56
phenolphthalein end point using a 0.04 M KOH solution in methanol. Each sample was tested in triplicate, and the average values are listed in Table 1. In some places of the discussion, the grafted copolymer is also denoted as PLA-g-X% MA, where X indicates the grafting degree. The molecular weight of PLA-g-MA was analyzed by a Viscotek gel permeation chromatography (GPC) system (TDA305). The triple detector platform was composed of light scattering (LS), refractive index (RI), and viscometer (VIS). Chloroform was used as the eluent at a flow rate of 1.0 mL/min. The detector temperature was 35 °C. The sample concentration was 2.0 mg/mL in chloroform, and the injection size was 120 μL. Preparation of PLA/SPC Composites. SP was first formulated with the following ingredients by weight: SPC (100 parts, dry weight), glycerol (10 parts), water (15 parts), and sodium sulfite (0.5 parts). The ingredients were mixed in a kitchen mixer and then stored in sealed plastic bags to equilibrate at room temperature overnight. The formulated SPC, PLA, and/or PLA-g-MA were then compounded using the same extruder used for the preparation of PLA-g-MA. For the compatibilized composites, PLA-g-MA was incorporated based on the total weight of PLA and SPC (dry weight). The screw speed was maintained at 100 rpm for all runs, and the eight controlled temperature zones from the first heating zone to the die were set at 90, 100, 130, 145, 160, 160, 160, and 155 °C, respectively. The extrudate was cooled in a water bath and subsequently granulated by a strand pelletizer. Pellets were dried in a convection oven at 80 °C for 12 h prior to injection molding. Standard tensile bars (ASTM D638, Type I) were prepared using a Sumitomo injection molding machine (SE 50D). The three zone temperatures of the barrel were set at 150, 160, and 170 °C, respectively, and the nozzle was set at 165 °C. The mold temperature was set at 40 °C, and the cooling time was ca. 40 s. All samples were conditioned for 1 week at 23 ± 2 °C and 50 ± 5% relative humidity prior to mechanical testing and other characterizations. Tensile Tests. Tensile tests were performed on a screwdriven universal testing machine (Instron 4466) equipped with a 10-KN electronic load cell and mechanical grips. The crosshead speed was 5 mm/min, and the strain was measured using a 50-mm extensometer (MTS 634.12E-24). All tests were carried out according to ASTM D638. Five replicates were tested for each sample to obtain an average value. Microscopy. Field emission scanning electron microscopy (FE SEM, Quanta 200F) was applied to investigate the microstructure of the PLA/SPC composites. Tensile fracture surfaces and microtomed smooth surfaces of the composites were dried and sputter-coated with gold prior to examination. Fourier Transform Infrared Spectroscopy (FT-IR). The FT-IR analysis was performed using a Thermo Nicolet Nexus 670 spectrometer. The spectrum was recorded between 400 and 4000 cm−1 with 32 scans at a resolution of 2 cm−1. To analyze the reactive interfacial compatibilization, the injection
content and SPC loading level in the preformulated SPC were fixed at 15% and 30%, respectively. PLA-g-MAs of different grafting degrees were prepared via free radical grafting reaction in the molten state and used as compatibilizers in PLA/SPC composites. The maleated PLA was characterized by titration and Fourier transform infrared spectroscopy (FT-IR). The amount of PLA-g-MA in the composites was varied between 1 and 4 phr (per hundred resin) based on the total weight of PLA and SPC. The main objective of this study was to investigate the effects of concentration and grafting degree of PLA-g-MA on the mechanical, morphological, and thermal properties of PLA/SPC composites.
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grafting deg (%)
EXPERIMENTAL SECTION
Materials. NatureWorks PLA (2002D) in pellet form was used in this work. Commercial grade SPC (Arcon S) in fine powder form was provided by Archer Daniels Midland Co. (Decatur, IL, USA). It contained ca. 72% protein (on dry weight basis), 20% carbohydrate, 3% fat, 6 wt % moisture, and a small amount of minerals as received. Maleic anhydride (MA) (95%), Styrene (St), and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (L101) were purchased from Sigma-Aldrich Chemical Co. All chemicals in this study were used as received without further purification. Preparation and Characterization of PLA-g-MA. MA, St, and L101 were first dissolved in 20 mL of acetone, and then manually mixed thoroughly with PLA in a zip-lock plastic bag. After the acetone was evaporated completely, the mixture was compounded using a corotating twin-screw extruder (Leistriz ZSE-18HP) equipped with a volumetric feeder and a strand die. The diameter of the screw was 18 mm with a length-todiameter ratio (L/D) of 40. The screw speed was maintained at 60 rpm. From the feed first heating zone to the die, the temperatures were set at 140, 160, 180, 180, 180, 180, 165, and 145 °C, respectively. A vacuum pump was connected to the vent at the sixth heating zone on the barrel to remove vapors generated during extrusion. The extruded strands went through a water bath and were subsequently pelletized. The pellets were dried at 80 °C under vacuum for 12 h. The degree of grafting was determined by the following purification and titration procedure. The sample was first purified in the following steps. Approximately 2.5 g of PLA-gMA was dissolved in 40 mL of chloroform, and 0.75 mL of 1 M HCl solution in water was added to hydrolyze the anhydride groups to carboxylic acid at room temperature. The solution was stirred vigorously for 30 min. This solution was added dropwise to excess acetone (400 mL). The grafted polymer was precipitated while any possible MA, St, and MA-co-St oligomers remained in the solution. The filtered precipitates were then washed by acetone and then distilled water several times to remove the possible residual unreacted monomers and MA-coSt oligomers, and dried at 85 °C in a vacuum for 24 h. The above purified PLA-g-MA sample was first dissolved in chloroform, and then the solution was titrated to a 7787
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molded composite samples were sliced to flakes of ∼40 μm in thickness and then extracted with chloroform under stirring at ambient temperature for 72 h to remove the free PLA. The collected residues were ground with KBr powder at the same sample/KBr weight ratio and then molded into disks using a press. The disks were oven-dried under vacuum to further remove residual solvent and moisture. Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties were measured using a dynamic mechanical analyzer (Rheometrics Solids Analyzer, RSAII). DMA specimens (12.8 × 3.2 × 35 mm3) were cut from the injection molded samples. A single-cantilever fixture at a vibration frequency of 1 Hz was applied in the DMA test. All tests were conducted at an amplitude of 15 μm using a 2 °C·min−1 temperature ramp from −10 to 150 °C. Differential Scanning Calorimetry (DSC). The melting and crystallization behaviors of the PLA/SPC composites were studied by DSC (Mettler Toledo 822e) using the samples sliced from injection molded specimens. Approximate 4 mg of sample was sealed in a 40 μL aluminum pan. The sample was first heated from 25 to 180 °C at the heating rate of 20 °C·min−1 to erase its thermal history, isothermally conditioned at 180 °C for 2 min, and then cooled to 0 °C at 20 °C min−1. Finally, the sample was heated again from 0 to 180 °C at 5 °C·min−1 to examine the glass transition temperature and crystallinity of PLA in the PLA/SPC composite.
PLA. According to John et al.,16 cyclic anhydrides should exhibit an intensive absorption band near 1780 cm−1 and a weak band near 1850 cm−1 which were attributed to the symmetric stretching and asymmetric stretching of CO. Because the MA degree of grafting is only ∼0.9% on PLA in this sample, the absorption at 1780 cm−1 appeared only like a shoulder on the large CO of PLA, while the absorption at 1850 cm−1 was hardly noted. Deconvolution of the spectrum of the PLA-g-MA in Figure 1 was performed (deconvolution curves not shown), and the result further proved the shoulder was attributed to the absorption of the grafted MA. Overall, the FT-IR results confirmed that MA was grafted onto PLA successfully. Mechanical Properties. Representative stress−strain curves of neat PLA and PLA/SPC composites are shown in Figure 2. Both PLA/SPC composites with and without
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RESULTS AND DISCUSSION Characterization of PLA-g-MA. In general, all the samples showed grafting degrees ranging from 0.25 to 0.90%. GPC analysis revealed that the molecular weight of the grafted copolymer was comparable to that of the neat PLA, except for the sample whose grafting was conducted at 200 °C. Thermal degradation became severe at the higher temperature, so the Mw decreased sharply from 128 000 to 85 000 g/mol. FT-IR analysis is mainly used to characterize grafted PLA in this study. Figure 1 shows the absorbance spectra of neat PLA and a PLA-
Figure 2. Stress−strain curves of (A) neat PLA, (B) PLA/SPC (70/30 w/w) composite without PLA-g-MA, and (C) PLA/SPC (70/30 w/w) composite with 4 phr PLA-g-0.54% MA.
compatibilizer showed brittle fracture and did not exhibit any yielding. The modulus of the composites was higher than that of neat PLA due to the incorporation of rigid SPC. The tensile properties of neat PLA and PLA/SPC composites with different PLA-g-MA concentrations are summarized in Table 2. One-way ANOVA statistical analysis was applied to evaluate the data at the 95% confidence level. Examination of the p-value of each property showed that the addition of SPC and the PLA-g-MA concentration significantly affected the strength, modulus, and elongation of the composites. Tukey multiple comparison further identified the significant differences among samples at α = 0.05. Compared to the neat PLA, the composite with 30% SPC exhibited a ∼21% increase in modulus which was not statistically varied with PLAg-MA concentration. However, both tensile strength and elongation of the composites increased with concentration of PLA-g-MA. For the composite with 4 phr PLA-g-MA, the tensile strength and elongation increased ∼19 and ∼21%, resepctively, as compared to the one without PLA-g-MA. The anhydride groups were likely to react with the amino groups of proteins and hydroxyl groups of carbohydrates in SPC, therefore greatly enhancing the interfacial adhesion which played a key role in reducing the size of the dispersed phase.17 Nevertheless, the increasing trend of tensile strength and elongation leveled off when the PLA-g-MA concentration was
Figure 1. FT-IR absorption spectra of neat PLA and PLA-g-MA.
g-MA with a grafting degree of 0.90% in the range 1600−1900 cm−1. The test specimens for FT-IR analysis in Figure 1 were the film samples prepared by pressing the melt between Teflon sheets using a small press. Neat PLA had a strong absorption band at 1760 cm−1 (CO stretching). In comparison, the PLA-g-MA exhibited a spectrum very similar to that of the neat 7788
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Table 2. Mechanical Properties of PLA/SPC Composites with Different PLA-g-MA Concentrations tensile properties strength (MPa)
modulus (GPa)
elongation (%)
a
PLA/SPC
PLA-g-MA (phr)
100/0 70/30 70/30 70/30 70/30
0 0 1 2 4
impact strength (J/m) 65.1 44.7 48.6 52.2 53.1
± ± ± ± ±
0.6 0.5 0.7 1.1 1.5
Ab B C D D
3.47 4.17 4.30 4.21 4.19
± ± ± ± ±
0.04 0.07 0.11 0.03 0.04
Ab B B B B
3.59 1.54 1.68 1.85 1.86
± ± ± ± ±
0.14 0.07 0.03 0.05 0.11
Ab B B C C
11.3 ± 0.8 4.2 ± 0.1 5.6 ± 1.1 5.8 ± 0.9 5.0 ± 1.1
a
PLA-g-MA is added on the basis of per hundred resin (total weight of PLA and SPC), and the grafting degree was 0.54%. bOne-way ANOVA Analysis (SAS) and Tukey multiple comparison of the effects of SPC and PLA-g-MA concentration. Values with different letters mean they are significantly different from each other. The confidence level was set at 95%.
relatively large and unevenly distributed. The interstices between the SPC particles and the PLA matrix were clearly observed for the uncompatibilized composite, indicating poor interfacal adhesion. With the presence of 4 phr PLA-g-0.25% MA, (Figure 4b), however, the SPC granules became much finer and evenly dispersed within the matrix. Therefore, the mechanical properties were improved as already discussed in the section Mechanical Properties. Moreover, the grafting degree of MA also exhibited influences on SPC domain size and phase structure of the composites. With grafting degree increasing from 0.25 to 0.90% (Figure 4c), SPC changed from large granules to small particles in micrometer sizes and the phase structure of the composites changed from coarse to fine. The reaction between the anhydride groups of the MAgrafted PLA and amino groups of SPC would decrease the interfacial tension. Therefore, the breakup of SPC suppressed the coalescence, and the SPC granules were smaller than those in the simple PLA/SPC composites. Some interstices were still visible in the micrographs of the compatibilized samples but were much smaller and fewer compared to that of uncompatibilized sample. Cryo-fractured surfaces of the PLA/ SPC composites were also prepared and examined using SEM. The micrgraphs (not shown) exhibited very similar morphological characteristics compared to those from the microtomed surfaces. Figure 5 shows the tensile fracture surface of the neat PLA and PLA/SPC composites with different contents of compatibilizer. Neat PLA and PLA/SPC composites all exhibited brittle failure behaviors as the fractures showed little sign of plastic deformation. Without compatibilizer (Figure 5b), SPC appeared in fairly large particles, corresponding to the observation in Figure 4. The flat fracture surface suggests the fracture propagated through the weak interfaces of SPC particles with the PLA matrix. As a result, the large SPC particles in this sample were simply broken apart (as indicated by the arrows in Figure 5b), contrasting the clear pullout as they were in the compatibilized samples. This result was probably attributed to the severe stress concentration at the large SPC particles which broke the particles instead of pulling out. For the compatibilized PLA/SPC composites (Figure 5c,d), pullout of the SPC particles which were much finer than those in the uncompatibilized samples was obvious. As the grafting degree increased from 0.25 (Figure 5c) to 0.90% (Figure 5d), the SPC particles became even finer. This result corresponded to the trend of tensile strength as shown in Figure 3. The inferior strength of the blends to neat PLA was probably due to the weak interfacial adhesion which was probably true even in those blends with PLA-g-MA added. Because of the relatively weak interfacial adhesion, the PLA/
above 2%. It is well-known that, in reactive blending, only a certain concentration of block or graft copolymer is required to saturate the interface and produce optimum compatibilization. The emulsification curves in several polymer blends systems display similar characteristics, that is, an initial significant drop in the size of the dispersed phase with the addition of copolymer followed by the attainment of an equilibrium value at high concentrations of the copolymer.18 Figure 3 shows the effects of grafting degree of PLA-g-MA on tensile strengths of the composites. It can be noted that the
Figure 3. Tensile strengths of PLA/SPC composites comprising PLAg-MA with different grafting degrees.
tensile strength gradually increased with increasing grafting degree of PLA-g-MA. With more MA moieties in PLA-g-MA, the compatibilizing reaction between PLA-g-MA and SPC became more intense at interfaces, thus achieving higher interfacial adhesion. In addition, the composites with 4 phr PLA-g-MA universally showed higher tensile strengths than the ones with 2 phr PLA-g-MA. A notched Izod impact test was also performed according to ASTM D256. The result indicated that the neat PLA displayed very brittle behavior. With 30% SPC, the PLA/SPC composites became more brittle (Table 2). A similar deterioration in impact strength was also noted for other PLA/soy protein blends3 and is very common for a polymer composite with this level of filler loading. The addition of compatibilizer only had a slight effect on improving the impact strength of PLA/SPC composites. Morphology. Figure 4 shows SEM images of the microtomed surfaces of the PLA/SPC composites. In the sample without PLA-g-MA (Figure 4a), the SPC granules were 7789
dx.doi.org/10.1021/ie300118x | Ind. Eng. Chem. Res. 2012, 51, 7786−7792
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Figure 4. SEM micrographs of microtomed surfaces of PLA/SPC (70/30 w/w) composites: (a) without PLA-g-MA; (b) with 4 phr PLA-g-0.25% MA; (c) with 4 phr PLA-g-0.90% MA.
mode, molecular segment mobility, and maximum damping temperature (Tg).21 Tan δ and storage modulus (E′) of neat PLA and PLA/SPC composites with different concentrations of PLA-g-MA as a function of temperature are shown in Figure 7. Neat PLA exhibited a sharp and high damping peak, whereas the PLA/SPC composites showed broad and low damping peaks. Because of its low crystallinity, neat PLA became very soft when the temperatures were above its α-transition (peak at ∼70 °C),8 and hence a high damping peak was observed in the transition zone. In a previous study, neat soy protein displayed a Tg of ∼145 °C, and adding 10% glycerol only led to a small reduction in Tg.22 Therefore, the SPC phase was still in its glassy state in the neighborhood of the PLA glass transition and its damping was quite low with respect to that of PLA.8 In other words, SPC particles behaved like rigid fillers in the molded composites. Consequently, the damping peak of PLA in the composite was greatly reduced with respect to that of neat PLA. Filler would restrict the motion of the matrix chain, thereby affecting the relaxation of the matrix chain and resulting in broadening. Compared to PLA/SPC composites without PLAg-MA, composites with PLA-g-MA demonstrated broader and lower damping peaks, which means that the blend had improved interactions between PLA and SPC. The height of the damping peak decreased with increasing compatibilizer concentration, which was probably attributed to the more effective contribution of the SPC phase to the E′ in the rubbery
SPC composites underwent debonding at the PLA/SPC interface under the stress level which was smaller than that of matrix yielding. Support for this argument can be found in the evidence that using more reactive isocyanate group containing coupling agents, such as polymeric diphenylmethane diisocyanate (pMDI), at relatively low concentrations, could result in PLA composites with significantly enhanced strength or even higher strength than that of the neat PLA.19 Figure 6 shows the FT-IR spectra of neat PLA, neat SPC, and the residues of composites with and without PLA-g-MA after solvent extraction to remove free PLA and PLA-g-MA. Neat PLA (Figure 6a) had a strong absorption band at 1760 cm−1 (CO stretching). Neat SPC (Figure 6b) showed strong absorption bands at 1514 (NH stretch, amide II), 1658 (CO stretch, amide I), and 3338 cm−1 (NH stretch).20 A small shoulder at 1760 cm−1 (CO stretching of PLA) was noted in the extracted residue from the composite with 4 phr PLA-g-MA (Figure 6d), which was not observed in the residue of the composite without PLA-g-MA (Figure 6c). This result substantiated the postulation that the grafted MA groups on the PLA-g-MA can react with the functional groups of SPC. The enhanced interaction between the PLA and SPC phases improved the compatibility between the two components, which were responsible for enhanced mechanical properties. Dynamic Mechanical Properties. DMA measurements are often used to evaluate polymer stiffness under dynamic 7790
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Figure 5. SEM micrographs of tensile fracture surfaces: (a) neat PLA; (b) PLA/SPC without PLA-g-MA; (c) PLA/SPC with 4 phr PLA-g-0.25% MA; (d) PLA/SPC with 4 phr PLA-g-0.90% MA.
absorbed layer of polymer surrounding the filler surface, which restricted the molecular motion and resulted in lower damping.9 The slight decrease in Tg for PLA in PLA/SPC composites might be due to the migration of some small
Figure 6. FT-IR spectra of (a) neat PLA, (b) neat SPC, (c) residue of extracted PLA/SPC without PLA-g-MA, and (d) residue of extracted PLA/SPC with 4 phr PLA-g-0.90% MA. Figure 7. Tan δ and storage modulus (E′) of neat PLA and PLA/SPC (70/30 w/w) composites with different concentrations (phr) of PLAg-0.39% MA.
region of PLA.23 Moreover, the enhanced interfacial adhesion between PLA and SPC might be due to the appearance of an 7791
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(3) Liu, B.; Jiang, L.; Zhang, J. Study of Effects of Processing Aids on Properties of Poly (lactic acid)/Soy Protein Blends. J. Polym. Environ. 2011, 19, 239. (4) Chen, F.; Zhang, J. Effects of Plasticization and Shear Stress on Phase Structure Development and Properties of Soy Protein Blends. ACS Appl. Mater. Interfaces 2010, 2, 3324. (5) Chen, F.; Zhang, J. A new approach for morphology control of poly (butylene adipate-co-terephthalate) and soy protein blends. Polymer 2009, 50, 3770. (6) Chen, F.; Zhang, J. In-situ poly(butylene adipate-co-terephthalate)/soy protein concentrate composites: Effects of compatibilization and composition on properties. Polymer 2010, 51, 1812. (7) Aoi, K.; Okada, M. Polymerization of oxazolines. Prog. Polym. Sci. 1996, 21, 151. (8) Zhang, J.; Jiang, L.; Zhu, L.; Jane, J.; Mungara, P. Morphology and properties of soy protein and polylactide blends. Biomacromolecules 2006, 7, 1551. (9) Wang, H.; Sun, X.; Seib, P. Strengthening blends of poly (lactic acid) and starch with methylenediphenyl diisocyanate. J. Appl. Polym. Sci. 2001, 82, 1761. (10) Fang, K.; Wang, B.; Sheng, K.; Sun, X. Properties and morphology of poly (lactic acid)/soy protein isolate blends. J. Appl. Polym. Sci. 2009, 114, 754. (11) Zhang, J. F.; Sun, X. Mechanical properties of poly(lactic acid)/ starch composites compatibilized by maleic anhydride. Biomacromolecules 2004, 5, 1446. (12) Huneault, M. A.; Li, H. Morphology and properties of compatibilized polylactide/thermoplastic starch blends. Polymer 2007, 48, 270. (13) Orozco, V. H.; Brostow, W.; Chonkaew, W.; Lopez, B. L. Preparation and Characterization of Poly(Lactic Acid)-g-Maleic Anhydride + Starch Blends. Macromol. Symp. 2009, 277, 69. (14) Fowlks, A. C.; Narayan, R. The effect of maleated polylactic acid (PLA) as an interfacial modifier in PLA talc composites. J. Appl. Polym. Sci. 2010, 118, 2810. (15) Petersson, L.; Oksman, K.; Mathew, A. Using maleic anhydride grafted poly (lactic acid) as a compatibilizer in poly (lactic acid)/ layered silicate nanocomposites. J. Appl. Polym. Sci. 2006, 102, 1852. (16) John, J.; Tang, J.; Yang, Z.; Bhattacharya, M. Synthesis and characterization of anhydride-functional polycaprolactone. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 1139. (17) Tol, R.; Groeninckx, G.; Vinckier, I.; Moldenaers, P.; Mewis, J. Phase morphology and stability of co-continuous (PPE/PS)/PA6 and PS/PA6 blends: effect of rheology and reactive compatibilization. Polymer 2004, 45, 2587. (18) Li, J.; Favis, B. D. Strategies to measure and optimize the migration of the interfacial modifier to the interface in immiscible polymer blends. Polymer 2002, 43, 4935. (19) Liu, B.; Jiang, L.; Liu, H.; Zhang, J. Synergetic Effect of Dual Compatibilizers on in Situ Formed Poly(lactic acid)/Soy Protein Composites. Ind. Eng. Chem. Res. 2010, 49, 6399. (20) Zhong, Z.; Sun, S.; Hagan, S.; Gehrke, S. Soy protein isolate/ polycaprolactone blends: Compatibility reactions and water absorption mechanisms. Trans. ASAE 2005, 48, 1281. (21) Menczel, J.; Prime, R. Thermal Analysis of Polymers: Fundamentals and Applications; John Wiley & Sons: New York, 2008; p 387. (22) Zhang, J.; Mungara, P.; Jane, J. Mechanical and thermal properties of extruded soy protein sheets. Polymer 2001, 42, 2569. (23) Sumita, M.; Tsukihi, H.; Miyasaka, K.; Ishikawa, K. Dynamic mechanical properties of polypropylene composites filled with ultrafine particles. J. Appl. Polym. Sci. 1984, 29, 1523. (24) Jiang, L.; Wolcott, M. P.; Zhang, J. Study of biodegradable polylactide/poly(butylene adipate-co-terephthalate) blends. Biomacromolecules 2006, 7, 199.
molecules including glycerol and water from the SPC phase to the PLA matrix during compounding. However, with increasing compatibilizer concentration, the Tg of PLA in the PLA/SPC composites shifted to higher temperatures and became very close to that of the neat PLA. Such a shift of Tg could result from the better interaction between PLA and SPC with the addition of PLA-g-MA. DSC results (data not shown) also indicated that the Tg of PLA in the composites was slightly lower than that of neat PLA. On the other hand, neither the concentration nor the functionality of PLA-g-MA exhibited significant influences on the crystallinity and melting point of PLA in the PLA/SPC composites. The E′ values of all samples suddenly dropped when the Tg of PLA (∼60 °C) was reached, and then recovered to a significant degree between 85 and 100 °C due to cold crystallization of PLA.24 Neat PLA had the lowest E′ in the whole range of test temperatures. SPC acted as a rigid filler to improve the stiffness of the composites. The PLA/SPC composites with 4 phr PLA-g-MA had higher E′ values than composites without PLA-g-MA in the whole temperature range. The reinforcing effect of SPC appeared even more pronounced at higher temperatures. For instance, E′ of the composites with 4 phr PLA-g-MA was 182% that of the neat PLA at 30 °C, whereas the value was 361% at 90 °C. This could be due to the enhanced interfacial adhesion in the former.
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CONCLUSIONS This study demonstrated that MA-grafted PLA served as an effective compatibilizer for PLA/SPC composites. Free radical grafting of MA to PLA was achieved by reactive extrusion, and the use of styrene as a comonomer during the grafting exhibited a certain effect on promoting MA grafting. Both the grafting degree and concentration of maleated PLA showed significant effects on the morphology and properties of PLA/SPC composites. The tensile properties of the compatibilized PLA/SPC composites were significantly higher than that of the uncompatibilized one. The use of PLA-g-MA as a compatibilizer resulted in fine morphological structures of the blends. The damping peak height of the PLA/SPC composites was reduced by the addition of compatibilizer, while the storage modulus was increased, suggesting that better interfacial adhesion was achieved in the presence of PLA-g-MA.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (509) 335-8723. Fax: (509) 335-5077. E-mail: jwzhang@ wsu.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for financial support from the United Soybean Board (USB) and from the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, Grant 2007-35504-17818.
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REFERENCES
(1) Zhang, J.; Mungara, P.; Jane, J. Mechanical and thermal properties of extruded soy protein sheets. Polymer 2001, 42, 2569. (2) Liu, B.; Jiang, L.; Zhang, J. Extrusion Foaming of Poly (lactic acid)/Soy Protein Concentrate Blends. Macromol. Mater. Eng. 2011, 296, 835. 7792
dx.doi.org/10.1021/ie300118x | Ind. Eng. Chem. Res. 2012, 51, 7786−7792
