Thermal Properties and Morphology of Biodegradable PLA/Starch Compatibilized Blends

Transcription

1 J. Ind. Eng. Chem., Vol. 13, No. 3, (2007) Thermal Properties and Morphology of Biodegradable PLA/Starch Compatibilized Blends Woo Yeul Jang, Boo Young Shin, Tae Jin Lee, and Ramani Narayan* School of Display and Chemical Engineering, Yeungnam University, Gyoungsan , Korea * Department of Chemical Engineering & Material Science, Michigan State University, East Lansing, MI Received October 23, 2006; Accepted February 7, 2007 Abstract: Maleic anhydride (MA) and maleated thermoplastic starch (MATPS) are used as reactive compatibilizers to improve interfacial adhesion in preparing PLA/starch blends. The morphological and thermal properties were examined by using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). SEM study revealed that MA is a good compatibilizer, while MATPS is not as effective for PLA/starch blend systems. DSC showed that the PLA/starch blends had increased crystallinity with MA as the reactive compatibilizer. The structural changes of constituents and molecular weight change of PLA were characterized by using fourier transform infrared (FT-IR) spectroscopy and gel permeation chromatography (GPC). MA compatibilized blends showed higher biodegradability than simple PLA/starch blends at the same PLA starch ratio. Keywords: PLA, starch, morphology, thermal property, biodegradability Introduction 1) Interest in biobased and biodegradable polymers is increasing due to concerns on managing our carbon emissions in a sustainable manner, and the environmental requirements on safe and effective disposal of polymers (like plastics) after use when it enters the waste stream [1]. Poly(lactic acid) (PLA), produced from annually renewable biofeedstock like corn, is one of the most important biobased, biodegradable polymer and is now finding a lot of commercial applications. Starch and cellulose polymers derived from biofeedstocks are also finding use directly or as blends with other biobased and biodegradable polymer materials [2,3]. Blends of PLA and starch offer cost-performance benefits with increased biodegradability, without compromising environmental and carbon management benefits: Starch can enhance biodegradability and reduce cost while PLA offers superior mechanical properties. Starch has been added in granular [4] and gelatinized form [5] to form blends. However, these blends do not show good interfacial morphology because of the inherent in- To whom all correspondence should be addressed. ( byshin@ynu.ac.kr) compatibility between the starch-pla polymer systems. To improve interfacial adhesion, reactive compatibilization with MA, dioctyl maleate, and methylene diphenyl diisocyanate was extensively studied by Sun and cowerkers [5-13]. Narayan and Dubois [14] achieved very good interfacial adhesion in PLA/starch blend by using maleic anhydride grafted PLA. This was prepared by reactive extrusion of maleic anhydride and PLA with a peroxide initiator, prior to blending with starch. In this paper we report on our work to improved interfacial adhesion between PLA and granular starch by using MA and MATPS [15] as reactive compatibilizers. For comparison, and as control we also prepared pure PLA/starch blends with no compatibilization. Experimental Materials Poly(lactic acid) (PLA Polymer 2100D) was obtained from Cargill Dow LLC. It was dried in vacuum oven for 24 h at 50 C. 2,5-bis((tert-butylperoxy)-2,5-dimethyl hexane (Luperox) and maleic anhydride were provided by Aldrich. Corn starch (11 % inherent moisture) was obtained from Shindongbang Inc. Korea. Starch was

2 458 Woo Yeul Jang, Boo Young Shin, Tae Jin Lee, and Ramani Narayan Table 1. DSC Characteristics of the Blends of PLA and Starch Ratios of PLA/starch by wt% Compatibilizer T m ( C) Crystallinity (%) Sample codes MA* phr MATPS* phr T g ( C) T c ( C) T m1 T m2 PLA PS10 90/ PS20 80/ PS30 70/ PS40 60/ PS50 50/ PSMA10 90/ PSMA20 80/ PSMA30 70/ PSMA40 60/ PAMA50 50/ PSMATPS5 70/ PSMATPS10 70/ PSMATPS15 70/ * MA and MATPS contents on PLA weight basis dried in convection oven at 100 C for 24 h prior to use. The moisture content of starch was reduced to about 3 %. MATPS was prepared as described in previous study [15]. Compositions and Mixing Conditions The compositions of PLA/starch blends and sample codes are listed in Table 1. The amount of initiator and maleic anhydride were fixed at 0.3 and 3 phr respectively on PLA weight basis. The amount of initiator and maleic anhydride selected were based on values used for preparing maleated PLA [14,16]. All components of the blend were pre-mixed in plastic bag before extrusion in a twin-screw co-rotating extruder (SM PLATEK Co. Ltd., TEK 30, Korea). The screw diameter was 30 mm with an L/D ratio of 36. The extruder was operated at 150 rpm with a constant feed rate of 10 kg/hr and barrel temperature range was C. Die temperature was 180 C. Characterization Thermal properties were determined using differential scanning calorimetry (DSC; Perkin-Elmer Pyris 6). DSC analysis was done at 10 C/min up to 200 C under nitrogen atmosphere. The crystallinity of PLA was calculated according to the following equation [12]: ø % where H m and H m o are enthalpies (J/g) of fusion of blend and PLA crystal of infinite size with a value of 93.6 J/g, respectively; ø PLA is the PLA weight fraction in the blend. Morphology of the blends was studied using fractured surfaces under cryogenical conditions using Scanning Electron Microscope (SEM; Hitachi model s-4100; Japan). Pure PLA and starch were extracted from the reactive blend using soxhlet extraction to investigate the reaction of MA with PLA and starch. The chloroform soluble part from a 24 h extraction is PLA. The residual starch was rinsed in chloroform and then dried in oven at 100 C. After weighing the starch, it was dissolved again in DMSO at room temperature and then filtered to obtain any resultant copolymer (PLA-co-starch) connected by the reaction with MA. Molecular weight and molecular weight distribution of extracted PLA and pure PLA were determined using gel permeation chromatography (GPC; Waters Alliance GPC 2000) operating in THF at 40 C. The average molecular weights were calibrated using polystyrene standards. Fourier transform infrared (FT-IR, Excalibure Spectrometer FTS 3000 MX, BIO RAD, USA) was used to obtain IR spectra of extracted PLA, starch and blends. Biodegradability and rate of biodegradation of the pure PLA and blends were investigated in a controlled and reproducible test environment [17]. Results and Discussion Thermal Properties The thermal properties of pure PLA and PLA/starch blends are listed in Table 1. Starch content does not affect the glass transition temperature (T g ) and the main melting temperature (T m ) of PLA in blends, while the heat of fusion was affected by the addition of starch.

3 Thermal Properties and Morphology of Biodegradable PLA/Starch Compatibilized Blends 459 Table 2. Molecular Weight of Pure PLA and Extracted PLA Sample codes M n (g/mole) M w (g/mole) M w/m n Pure PLA M1.6 PS PS PS PS PS PSMA PSMA PSMA PSMA PSMA PSMATPS PSMATPS PSMATPS These results are consistent with other literature reports [4-13] for PLA/starch blends without plasticizer. Cold crystallization temperature (T c ) was ca. 129 C and was not changed by the starch content. The very weak T m peak of pure PLA was shown at about 154 C and distinct T g curve was observed at 63 C. The melting peaks of blends become separate and distinct and heat of fusion increased slightly with increasing starch content. Calculated crystallinity increased from about 0 % in pure PLA to 11 at 50 % starch content in the blend. The increased crystallinity might be caused by starch nucleation and degradation of PLA polymer chains. It has been reported that starch acts as nucleating agent for crystallization and molecular weight affects the crystallization of polymers [4-13]. Generally, low molecular weight polymers have high crystallinity and high crystallization rate [18]. In this pure blend case, both factors, that is, nucleation and decreased molecular weight might be the cause of increased crystallinity. It is unclear from previous reports whether moisture degrades the molecular weight of PLA during melt mixing or not [4,6]. We observed that the number and weight average molecular weight of PLA in the starch/pla blends reduced to about half compared to that of pure PLA, while polydispersity increased (see Table 2 for the molecular weight data of the extracted PLA from the blends.). This result suggests that the moisture present in the starch decreased the molecular weight of PLA due to hydrolysis resulting in lower mechanical properties [6,16]. For the MA compatibilized PLA/starch blends, T g decreased a little compared to that of the pure blend. It is known that plasticizer decreases the glass transition temperature of polymer. Though MA was introduced as a reactive compatibilizer, MA might act as a plasticizer as well in this blend. Over 20 % starch content, two melting endothermic peaks were observed at 146 and 154 C approximately. Cold crystallization exothermic peak was seen at C, which was lowered by about 10 C compared to that of the pure blend. However, PSMA10 does not show the crystallization double peak and the change of cold crystallization temperature at 127 C. The percent crystallinity of MA compatibilized blends are much higher than those of pure blends with the same starch content. The crystallinity of MA compatibilized blend increases with increasing starch content. PSMA40 shows the highest crystallinity of 48 % among all the blends. This value is about 5 times higher than that of the pure blend. This may be attributed to the increased molecular motion of PLA chains. There are two reasons for this increased mobility of the polymer chains. The first one is the lower molecular weight due to degradation during melt mixing process and the other is plasticizer effect of MA. As shown in Table 2, the PLA component in MA compatibilized blends and pure blends have very similar molecular weight and molecular distribution. Thus the reason for increased crystallinity in MA compatibilized blends can be attributed to the plasticizer effect of MA. Moreover, the T c value indicates that crystallization occurs at lower temperature during the heating process due to the easy motion of molecules. At highest starch content (PSMA50) the crystallinity of PLA decreased a little because starch particles may interfere with the motion of PLA molecules. Thus, the optimum content of starch as a nucleating agent may be around 40 % starch content for MA compatibilized PLA/starch blend system. Furthermore, there are double melting peaks in MA compatibilized blends with high starch content. The double melting peaks of PLA has been reported in plasticized PLA/starch blend [11], reactive compatibilized blend [12], and star-shaped PLA/starch blend [5]. Thus these double melting peaks are caused by the plasticizer effect as well as nucleation effect. The higher melting temperature might be related to nucleating effects, while lower one related to plasticization. The thermal characteristics of MATPS compatibilized PLA/starch blends are slightly different from those of pure blends and MA compatibilized blends. As reported in literature [19-21], thermoplastic starch (TPS) is obtained after disruption and plasticization of native granular starch with plasticizer under high pressure and high temperature. In previous work using glycerol (GL) and maleic anhydride, a maleated thermoplastic starch (MATPS) was obtained. The detail characterization and synthesis of MATPS is described in patent [15]. The MATPS can function as a reactive compatibilizer because of the maleic anhydride functionality attached to it. In addition, it is reasonable to expect the MATPS can wet the starch because MATPS has the same chemical structure of starch. Thus, we can expect that MATPS to

4 460 Woo Yeul Jang, Boo Young Shin, Tae Jin Lee, and Ramani Narayan (a) PS30 (b) PS40 (c) PS50 Figure 1. SEM micrographs of PLA/starch blends without compatibilizer. be a good compatibilizer for PLA/starch blend system - chemical bond with PLA through the anhydride functionality and good compatibility/wetability with the starch through the starch backbone of the reactive compatibilizer. Thermal properties of MATPS compatibilized blends are shown in Table 1. T g and T c were slightly decreased by addition of MATPS. This decrease may be caused by the migration of plasticizer into PLA during blending. The lowering of T g and T c is well recognized in plasticized polymers [22]. Im [5] reported that glycerol is very good plasticizer of PLA and T g and T m decreased, but crystallinity increased in GL plasticized PLA/starch blend. Though the amount of GL that migrated into PLA was very low, it impacted the thermal properties and crystallization behavior of PLA in blends because GL is a good plasticizer for PLA [5,12]. The crystallinity of MATPS compatibilized blend is higher than those of pure blend (PS30), while lower than those of MA compatibilized blend (PSMA30). MATPS compatibilized blends show single melting peaks at lower MATPS content (PSMATPS5, PSMATPS10) and weak double peaks at higher MATPS content (PSMATPS15). The double peaks at high MATPS content may be explained by plasticization effect like in the case of MA compatibilized blends. PSMATPS15 has higher crystallinity than that of other MATPS compatibilized blends at the same starch content. This increase in crystallinity for PSMATPS15 might be caused by the presence of an additional nucleating agent. The additional nucleating agent must be phase separated particles of MATPS in PLA matrix (see discussion in morphology section). From this result, we conclude that the higher T m peak of MATPS compatibilized blend was mainly caused by nucleating from granular starch and phase separated MATPS particles, while the lower melting peak was due the plasticizer migration of GL. The significant difference between the MATPS compatibilized blend, pure blend and MA compatibilized blend is in the molecular weight changes as listed in Table 2. The molecular weight change was very small compared to pure and MA compatibilized blends. From this result, we can hypothesize that the MATPS compatibilizer which is expected to locate at the interfacial region between PLA and granular starch, prevented the migration of water into PLA. If this is correct, it is certain that the major reason of decreasing molecular weight of PLA in blends is hydrolysis and minor cause is thermal degradation. It is important to reduce molecular degradation during mixing to minimize the loss of mechanical properties. Morphology It is well known that mechanical properties of polymer blends are strongly related to their morphology. The relation between mechanical properties and morphology for the blend of PLA and starch was studied by Sun and and coworkers [7-9,13]. Thus, control of morphology is very important in PLA/starch blends. So, we observed the morphologies of relatively high starch content blends and compared it with each other. As shown in Figure 1 (for pure blend), a clear edge and cavity can be seen between starch granule and PLA matrix. Starch granules had a broad size distribution and some of them detached from the PLA matrix. This morphology is typical of incompatible blends resulting in poor mechanical properties. For MA compatibilized PLA/starch blends (Figure 2), SEM micrographs show very good compatible morphologies without the edge, cavity, and holes resulting from poor interfacial adhesion. There are two kinds of fracture morphologies. One is a fracture occurring through the interface (see arrows in Figure 2(b)) and the other is broken starches (see arrows in Figure 2(c)) at its center. The fracture surface of PSMA30 and PSMA40 (Figure 2(a) and (b)) shows relatively many interfacial fractures. This result implies that any crazes formed that transformed to crack were near the poles of the particles. It is possible that the good adhesion of this blend is due to a reaction between the hydroxyl group of starch molecule and anhydride group, and this reacted molecule can further react with PLA. This kind of reaction scheme is how MA

5 Thermal Properties and Morphology of Biodegradable PLA/Starch Compatibilized Blends 461 (a) PSMA30 (b) PSMA40 (c) PSMA50 Figure 2. SEM micrographs of MA compatibilized PLA/starch blends. (a) PSMATPS5 (b) PSMATPS10 (c) PSMATPS15 Figure 3. SEM micrographs of MATPS compatibilized PLA/starch blends. functions as a reactive compatibilizer. However, other types of interaction between starch and PLA matrix such as wetting due to low surface tension and hydrogen bonding, cannot not be excluded. At 50 % of starch blend (PSMA50) there were many broken granules, which contained a central void. This void is typical of starch granules having equilibrium moisture content or less. As discussed by Sun [8], to form a good adhesion, good wetting at interface by lowering the surface tension prior to bonding reaction is needed. Good wetting is related to molecular diffusion and absorption. Thus, it is possible that the surface tension of starch could be reduced by the absorption of MA and that of PLA by the plasticizer role of MA. After wetting, the grafting reaction of MA to starch as well as PLA can be possible. The further reaction of MA grafted starch to plasticized PLA and MA grafted PLA to starch possibly occurred. As shown in Figure 2(c) (see arrows), there are distinct boundaries between broken starch and PLA matrix, whose thickness is about µm and filled with a certain material which differ from starch and PLA because the fractured boundry is much smoother than matrix surface. The material filling the cavities of boundary could be MA grafted starch (or MA wetted starch), MA grafted PLA (or MA plasticized PLA), or pure MA. Also, there might be PLA copolymerized with starch. The boundary thickness of this blend was much larger than that of Sun s [13] calculated values. To classify the materials in the boundary is very difficult but we extracted the blend to recover the materials filling the cavities and then characterized the extracted materials by FT-IR. Figure 3 shows the morphology of MATPS compatibilized blends with varying amounts of compatibilizer. Unexpectedly, Figure 3(a) does not show improved interfacial adhesion compare to pure blend (Figure 1(a)). Some starches were attached at the surface and others well embedded in the PLA matrix, however the interface between starch and matrix still has some cavities. This morphology suggests that the amount of compatibilizer was not sufficient to promote compatible morphology. Figure 3(b) and (c) shows much improved interfacial morphology due to good interfacial adhesion compared to that of Figure 1. This morphology revealed that MATPS could be a compatibilizer for blend of PLA and starch blend. The morphology is approximately similar to the morphology of MA compatibilized blends (Figure 2),

6 462 Woo Yeul Jang, Boo Young Shin, Tae Jin Lee, and Ramani Narayan Figure 4. FT-IR spectroscopy of MA, pure PLA, extracted PLA from blends. but the morphology of interface between broken starch and matrix is somewhat different from that of MA compatibilized blend. There were some cavities at interface, which might be due to the lack of compatibilizer (see circles in Figure 3 (b)). The morphology of the highest MATPS content blend (Figure 3 (c)) still shows the same kind of cavities. These results indicate that MATPS could not easily diffuse into the interface due to low molecular motion of MATPS chains, which in turn is related to the viscosity of MATPS. On the other hand, we can see some very small particles (circles in Figure 3(c)) with a diameter of about 1 5 µm. These small particles seem to be MATPS particle phase imbedded in the matrix. These particles could not reach the interface during melt mixing. These MATPS particles might act as a nucleating agent as discussed above. From this result, though the morphology of MATPS compatibilized blends show improved interfacial adhesion, MATPS is not suitable as a compatibilizer because it could not effectively diffuse to the interface due to low molecular motion resulting from its high viscosity. To observe the possibility of reaction between MATPS and PLA, we also extracted the PSMATPS15 and characterized the extracts by FT-IR. Estimation of Reaction Unlike the results of Sun s experiment [13], we could not find a meaningful residual mass of starch-pla copolymer, maleated PLA and maleated starch from extraction experiment. Thus, it is impossible to classify the mass of maleated PLA and starch by weighing because the amount of reacted constituents is too small to measure. Therefore, to understand the reaction of MA with starch and PLA, the titration method should be applied [14]. To observe the reaction of MA with PLA and starch qualitatively, we obtained the FT-IR spectra of pure PLA, granule starch, MA, and extracted PLA from PS30, PSMA30, and PSMATPS30 as shown in Figure 4. In addition, FT-IR of pure starch and extracted starches are Figure 5. FT-IR spectroscopy of pure starch and extracted starch from blends. shown in Figure 5. There was no difference between the FT-IR spectra of blends with varying starch content (not shown in this paper). Thus, we selected fixed 70/30 by weight percent of PLA to starch blend ratio for comparison. Characteristic absorption ranges of starch include absorption bands of O-H ( cm -1 ), C-H (2927 cm -1 ), O-H bending of absorbed water (1641 cm -1 ), and C-H stretching ( cm -1 ) [5]. The spectrum of PLA shows the strong C=O absorption band at 1757 cm -1. Also the typical two C=O stretching mode of anhydride at around 1843 and 1778 cm -1 suggested the presence of MA. The FT-IR spectrum of extracted PLA from PSMA30 is shown in Figure 4. Unfortunately, we can not obtain information about the existence of maleated PLA because the anhydride band is not discernible due to overlap with C=O absorption band of PLA or there is an absence of maleated PLA. Thus, it is impossible to document with certainty that MA has reacted with PLA as we expected and as reported in previous studies [14,23]. The FT-IR spectrum of extracted starch from MA and MATPS compatibilized blends shows the very weak C=O absorption band at 1720 and 1760 cm -1 (Figures 5 (c) and (d)) owing to the reaction of MA with starch [15,24]. However, the spectrum of extracted starch from pure blend showed only the characteristic absorption band of pristine starch (Figures 5(a) and (b)). Biodegradability Figures 6 and 7 show the results of biodegradability study conducted on the pure and MA compatibilized blends. The test apparatus was calibrated using cellulose as reference material, which shows 91 % biodegradability after 42 days. The biodegradability value of pure PLA was very similar to that of the result of earlier study [25]. However, the initial time lag for hydrolysis of the polymer to diffusable oligomers before the onset of mass loss from the polymer and microbial utilization was about 13 days. This is much longer than the 5 days obtained in earlier research [25]. As shown in Figure 7, the

7 Thermal Properties and Morphology of Biodegradable PLA/Starch Compatibilized Blends 463 Conclusion Figure 6. Cumulative biodegradability of samples. Figure 7. Rate of biodegradation of samples. rate of biodegradation begins to steadily increase reaching a maximum between days 20, 25, and then maintained steady rate of 2.2 % per day. For pure blends, the biodegradability increases with increasing starch content and with reduced time lag. Unlike pure PLA and lower starch content blends, 50 % starch content blend (PS50) shows no time lag for biodegradation and two maximum peaks at days 2 and around days 20 are observed. Also this blend was asymptotically approaching complete biodegradation of 85 %. All the MA compatibilized blends show higher biodegradability and higher rate of degradation compared to those of pure blends. It is generally known that the higher crystallinity material has lower biodegradability. In spite of its higher crystallinity, MA compatibilized blends show much high biodegradability values when compared to those of pure blends. In fact, a MA compatibilized blend containing 10 % starch has almost same biodegradability as that of PS50. This result might be due to the existence of MA. Either reacted MA with PLA and starch or untreated MA might form an acid group due to plenty of water in the compost and then this acid accelerated the chain scission of PLA by hydrolysis and back biting resulting in high biodegradability [14]. DSC studies reveal that the crystallinity of blends increased with increasing starch content. In particular, MA compatibilized blends showed highest crystallinity amongst the three types of blends due to the dual effects of nucleation by starch and plasticization by MA. The MA compatibilized PLA/starch blends show good interfacial morphology. The cavities at the interface between PLA matrix and granular starch were well filled with a certain material, which could not be identified in this work. We could not find the evidence for the reaction between MA and PLA from extraction studies, while the spectrum of extracted starch from MA compatibilized blend reveals the presence of maleated starch. The morphology study for MA compatibilized blends confirmed that MA was a good reactive compatibilizer and might have reacted with PLA or starch. The GPC data indicated that the molecular weight of PLA in pure blends and MA compatibilized blends decreased by the chain scission of PLA molecules due to hydrolysis, backbiting and thermal degradation. The decrease of molecular weight of PLA in MATPS compatibilized blends was much less than that of pure blend. The biodegradability values of blends increased with increasing starch, while time lag in biodegradation decreased. Also MA compatibilized blends showed higher biodegradability than pure blends. References 1. R. Narayan, ACS Symp. Ser. 939, 282 (2006). 2. S. B. J. David, D. Geyer, A. Gustafson, J. Snook, and R. Narayan, in Biodegradable Plastics and Polymers, Y. Doi and K. Fukuda Eds., Elsvier, Osaka, pp601 (1993). 3. R. Narayan, ACS Symposium, 575, 1 (1994). 4. S. Jacobsen and H. G. Fritz, Polymer Eng. Sci., 36, 2799 (1996). 5. J. W. Park and S. S. Im, Polymer Eng. Sci., 40, 2539 (2000). 6. T. Ke and X. Sun, J. Appl. Polym. Sci., 81, 3069 (2001). 7. H. Wang, X. Sun, and P. Seib, J. Appl. Polym. Sci., 82, 1761 (2001). 8. H. Wang, X. Sun, and P. Seib, J. Appl. Polym. Sci., 84, 1257 (2002). 9. T. Ke and X. Sun, J. Appl. Polym. Sci., 88, 2947 (2003). 10. T. Ke and X. Sun, J. Appl. Polym. Sci., 89, 1203 (2003). 11. H. Wang, X. Sun, and P. Seib, J. Appl. Polym. Sci., 90, 3683 (2003). 12. J. F. Zhang and X. Sun, J. Appl. Polym. Sci., 94,

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