Starch Composites

Blending poly(lactic acid) (PLA) with wheat starch compatibized by maleic anhydride (MA) was performed with a lab-scale co-extruder. An initiator, 2 ...
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Biomacromolecules 2004, 5, 1446-1451

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Mechanical Properties of Poly(lactic acid)/Starch Composites Compatibilized by Maleic Anhydride Jian-Feng Zhang and Xiuzhi Sun* Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas 66506 Received January 8, 2004; Revised Manuscript Received April 19, 2004

Blending poly(lactic acid) (PLA) with wheat starch compatibized by maleic anhydride (MA) was performed with a lab-scale co-extruder. An initiator, 2,5-bis(tert-butylperoxy)-2,5 dimethylhexane (L101), was used to improve compatability among PLA, starch and MA. Interfacial adhesion between PLA and starch was significantly improved. Mechanical properties increased markedly compared to the virgin composites of PLA/starch. The PLA/starch composites at a constant ratio of 55/45 compatibilized by 1% MA and initiated by 10% L101 (MA basis) resulted in the highest tensile strength and elongation. Introduction Environmental concerns and a shortage of petroleum resources have driven efforts aimed at bulk production of biodegradable materials. Poly (lactic acid) (PLA) is a promising polymer with high biodegradability and good mechanical properties for industrial plastic applications.1-5 However, PLA is an expensive material, limiting its uses for disposable items. Starch, an inexpensive renewable natural biopolymer, is a strong candidate for composites with PLA. However, PLA and starch are thermodynamically immiscible.6 Interfacial fracture strength is low due to low interdiffusion of molecules necessary for creating entanglements at the interphase between PLA and starch.6 Interfacial adhesion plays a vital role in mechanical properties of polymeric composites. Reactive interfacial coupling agents are often used to improve interfacial properties and control morphologies of polymeric composites. Coupling agents containing reactive functional groups are able to generate in situ formation of blocks or grafted copolymers at the interface by hot-melting blending. Reactive compatibilisation has been proven an effective method for morphology control in a variety of composites systems, including poly(ethylene oxide)/poly(methyl vinyl ethermaleic acid),7 poly(butylene terephthalate)/ethylene-vinyl acetate,8 styrene-acrylonitrile/ethylene-propylene-diene-monomer,9 polyphenylene oxide/polyamide-6,10 polystyrene/polyamide-6,11 and polypropylene/polyamide.12 Based on previous research,6,13 methylenediphenyl diisocyanate (MDI) improved mechanical properties of a PLA/starch blend, significantly improving tensile strength from 36.0 MPa of virgin PLA/starch composite to 66.7 MPa at 0.5 wt % of MDI.6 However, MDI is considered an environmentally hazardous material, which may make it unsuitable for food packaging or related applications, although the amount used was only 0.5% and it was safe after reacting. Maleic anhydride (MA) was considered for the present investigation based on the wide use of grafting MA or maleic * To whom correspondence should be addressed. Tel.: 785-532-4077. Fax: 1-785-532-7010. E-mail: [email protected].

Figure 1. Proposed chemical reactions among PLA, starch, MA, and initiator L101.

copolymer as a compatabilizer in binary immiscible polymer blends.7-12 MA is highly reactive with PLA free radicals induced by an initiator,13,14 and the anhydride group could react with hydroxyls from starch to form ester linkages,15 as schematically shown in Figure 1. The carboxylic groups, arising from the hydrolyzed anhydride, could also form hydrogen bonding with the hydroxyl groups.15 Besides these reactions, hydrogen bonding would occur between carbonyl groups from PLA and hydroxyl groups from starch.16 The objective of this study was to prepare a PLA and starch blend compatibilized by MA in the presence of an initiator. Experimental Section Sample Preparation. PLA was obtained from Shimadzu, Inc. (Tokyo, Japan). It had a weight-average molecular weight of 120 kDa and was polymerized mainly from L-lactic acid with a glass transition temperature of 60.0 °C, crystallization temperature of 125.2 °C, and melting temperature

10.1021/bm0400022 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/29/2004

Properties of PLA/Starch Composites

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Table 1. Effect of Initiator L101 and MA on Mechanical Properties of PLA and Its Composite with Starch at a Ratio of 55/45 sample

tensile strength (MPa)

elongation of blend (%)

neat PLA PLA+0.01%L101 (PLA basis) PLA/starch (55/45) (PLA+0.01%L101)/starch (55/45) PLA/starch/MA/(55/45/0.25)+10% L101 (MA basis) PLA/starch/MA/(55/45/0.50)+10% L101 (MA basis) PLA/starch/MA/(55/45/1.00)+10% L101 (MA basis) PLA/starch/MA/(55/45/2.00)+10% L101 (MA basis)

61.6 ( 3.8 54.3 ( 3.5 30.0 ( 2.6 35.4 ( 3.6 44.6 ( 1.1 50.0 ( 1.9 52.4 ( 1.9 45.4 ( 0.3

5.2 ( 0.5 4.9 ( 0.3 2.7 ( 0.1 4.0 ( 0.2 3.7 ( 0.1 3.7 ( 0.4 4.1 ( 0.1 3.6 ( 0.2

of 172.0 °C. Crystallinity of PLA was about 36.8%.17 Wheat starch was purchased from Midwest Grain Products, Inc (Atchison, KS). It had an amylose content of 23-28% and a particle size distribution of 17.95-18.09 µm. The MA was purchased from Aldrich Chemical Co., Inc (Milaukee, MI) and had a boiling point of 200 °C. The initiator used in this work was 2,5-bis(tert-butylperoxy)-2,5 dimethylhexane (L101), Atofina Chemicals Inc (Milaukee, MI), trade name Luperox 101. PLA chips were ground by a laboratory mill (model 4 Laboratory Mill, Thomas-Wiley Co., Philadelphia, PA) into about 2 mm powder prior to blending in order to uniformly wet by L101 liquid. Wheat starch was dried in an oven at 135 °C for 2 h according to AACC method 44-15A. PLA and starch were premixed using a stand mixer (Ultra Power Kitchen Aid, St. Joseph, MI) at room temperature for 10 min. The ratio of PLA/starch was 55/45, which was chosen based on the works by Ke and Sun.17 MA and L101 were added into the PLA/starch system at various concentrations and the blend was mixed for another 10 min. The mixture was then extruded by twin-screw extruder (TW-100, Haake, Paramus, NJ) with a screw diameter of 19.1 mm and a length-to-diameter ratio of 25/1. The temperature profile of the extruder was set at 125, 185, and 185 °C from feed inlet to die, respectively. The extruded strand was cut into pieces and then ground into about 2 mm powder for tensile bar preparation. Tensile bars (type IV) according to ASTM D638-91 were hot compression-molded at 180 °C, 4.2 MPa for 7 min by a Carver hot press (model 3889, Auto “M”, Carver Inc., Wabash, IN) then cooled to room temperature in air. All tensile bars prepared were preconditioned at 25 °C with a relative humidity of 50% for 48 h to relax internal stress prior to mechanical testing. To compare with simultaneous blending, the so-called “one-step” procedure described above, 1% MA and 10% L101 (MA basis) were first blended with PLA to produce grafted copolymer MA-g-PLA. The MA-g-PLA was then added into the PLA/starch blending system at various ratios. Preparation procedures for mixing composites and tensile bars were the same as described above for the one-step approach. Properties Measurement. Tensile strength and elongation at break of each sample were determined using an Instron testing machine (model 4465, Canton, MA) according to ASTM D 638-91 at room temperature with a crosshead rate of 5 mm/min and a 30 mm gauge length. The Young’s modulus was measured between 0.05 and 0.25% linear elongation. At least 6 replicates were tested, and average values were reported.

Microstructure of a fracture surface of samples from tensile tests was performed by scanning electron microscopy (SEM; Hitachi S-3500N, Hitachi Science Systems, Ltd., Japan). Dimethyl sulfoxide (DMSO, Fisher Chemicals) is a good solvent for starch, and PLA becomes only a little swollen in DMSO. Therefore, the amount of starch reacted with MA in the PLA/starch composites system can be determined by the following equation: ms ) Wb45% - (Wb - Wr)

(1)

where ms is the amount of starch reacted with MA, Wb is PLA/starch sample weight, Wr is the residue weight of the composites containing PLA and starch reacted with MA, 45% is the ratio of starch in the composites, which is based on uniform dispersion of starch and PLA in the composites. Extraction with DMSO continued for 3 days. The composites residues were then dried at 70 °C to constant mass. Since starch is insoluble in chloroform, the amount of PLA reacted with MA can be determined by extracting PLA with chloroform (Fisher Chemicals) using the equation described below: mPLA ) Wb55% - (Wr - Wrr)

(2)

where mPLA is the amount of PLA reacted with MA, Wb and Wr are the same as above, Wrr is the residual weight of the composites containing residual PLA and starch reacted with MA extracted by DMSO and chloroform, and 55% is the ratio of PLA in the composites. Results and Discussion Blending of PLA/Starch/MA. PLA and starch composites have shown poor mechanical properties in previous research.6,16,17 The results of those studies are indicative of poor interfacial adhesion between PLA matrix and starch granules for their immiscible characteristics. Mechanical properties of PLA/starch composites were improved by incorporating MA with 10% L101 (MA basis; Table 1). The tensile strength of composites in the presence of MA, even at a 0.25% level, was increased by 14.6 MPa compared to the composites without MA. For the composites with 1.0% MA, the average tensile strength was enhanced up to 52.4 MPa. At 2.0% MA, the tensile strength of the composites was about 45.4 MPa, which was lower than that with 0.5% MA. Recently, O’Shaughnessy et al.18 proposed that there existed a critical density (C) for in situ formed molecules by compatibilizer at the interface, which was dependent on the molecular weight of the compatibilizer C ∼ N-0.5, where N

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Table 2. Effect of L101 Content (MA Basis) on Mechanical Properties of PLA/Starch (55/45) Compounded with 1% MA content of L101 (MA basis wt %)

tensile strength (MPa)

elongation of blend (%)

0 5 10 15

28.6 ( 0.9 48.0 ( 2.9 52.4 ( 1.9 52.9 ( 1.6

2.6 ( 0.1 3.9 ( 0.1 4.1 ( 0.1 3.9 ( 0.1

is the molecular weight of the compatibilizer. At the critical density, enough copolymers were formed at the interphase to prevent bulk polymers from penetrating the copolymer barrier for further reaction. In the current study, 1% MA with 10 wt % L101 (MA basis) was enough to reach the critical copolymer density. Thus, mechanical properties of the composites were not further enhanced as MA and L101 concentration increased beyond the critical level. Elongation at break of these composites had similar trends as tensile strength. The composites with 1.0% MA and 10% Ll01 (MA basis) showed the greatest elongation among the PLA/starch composites, though still slightly lower than pure PLA. The composite without MA and L101 had the lowest elongation because of its poor interfacial adhesion, as discussed by Wang et al.6 and Ke and Sun.16,17 At a fixed MA content of 1%, tensile strength and elongation at break increased significantly as L101 content increased from 0% to 10% (MA basis) and then leveled off (Table 2). Adding 10 wt % L101 (MA basis) in the composite could be an optimum amount to initiate enough PLA radicals that could react with MA, which further reacted with starch strengthening the interfacial adhesion. Adding too much initiator, such as 15%, would generate more radicals than required. The composite without L101 had the lowest tensile strength and elongation at break. PLA without initiator was relative stable, indicating there were not enough free radicals reacting with MA, resulting in poor interfacial adhesion between the PLA matrix and starch granule. As expected, the function of initiator was to induce free radicals on PLA that could react with MA. Pure PLA initiated by 0.01% L101 (PLA basis) formed l-PLA, which had lower tensile strength and elongation than pure PLA (Table 1). PLA could be degraded by the formation of PLA free radicals by L101,19 weakening mechanical strength. However, the l-PLA slightly enhanced mechanical properties of its composite with starch. The degraded PLA had smaller molecular weight than the regular PLA, which would improve starch dispersion. MA Grafted PLA and PLA/Starch Blends. MA grafted PLA (MA-g-PLA) had higher tensile strength and elastic modulus than pure PLA (Table 3), though there was a slight

decrease in elongation at break. However, mechanical properties of MA-g-PLA blended with PLA/starch or starch alone were similar to those PLA/starch composites with 1% MA and 10% L101 (MA basis) presented in Table 1, regardless of pure PLA content in the system. As the maleated PLA was further compounded with starch, the other free end of MA from the maleated PLA would react with starch, and some free MA and/or reactive groups from grafted MA would further react with both starch and PLA during extrusion. Microstructure and Interphase Thickness Analysis. Mechanical properties of a polymer blend are closely related to its morphology. Micrograph of the fracture surface of PLA/starch specimen without MA and L101 showed typical characteristics of an immiscible composite (Figure 2A). As suggested by Ke and Sun,17 starch is a disperse phase and PLA is continuous phase at a ratio of 55/45 (PLA/starch). A clear edge and cavity between starch granules and PLA matrix were observed and starch granules had a broad size distribution. Poor adhesion between PLA and starch was confirmed by weak tensile strength at 30.0 MPa (Table 1). For the PLA/starch composite with MA but without L101 initiator, similar dispersion of starch was observed (Figure 2B), where some starch granules detached from the PLA matrix and then hollow cavities. At this condition, a tensile strength of 28.6 MPa was obtained. For the PLA/starch/MA composite with L101 initiator (Figure 2C), a uniform dispersion of starch granules in PLA matrix and some pullouts were observed. The surface to surface intergranular distance (matrix ligament thickness) is even, and local deformation in PLA matrix near starch granule increases. PLA matrix exhibits slightly plastic deformation having some extended strands. It is noteworthy that both dispersed starch granules and PLA matrix were fractured upon tensile test (Figure 2C), indicating a strong adhesion between PLA and starch, resulting in enhanced tensile strength at 52.4 MPa. This is consistent with the mechanism of debonding starch granules from PLA matrix. In the composite filling with wheat starch, where the starch granules acted as a filler.18,20,21 Starch is a typical brittle material with high modulus (>1 GPa).22 The thickness of the interfacial zone between PLA and starch is closely related to mechanical properties of a compounded system. Normally, improved mechanical properties are attributed to the thickness of the interphase which results in a good interfacial adhesion. The relative interfacial thickness between PLA and starch could be estimated using a method developed by Dedecker et al.23 with the following assumptions: (i) the dispersed starch granules are perfectly spherical in shape with

Table 3. Effect of Using a Two-Step Method to Prepare Maleated PLA by 1% MA and 10 wt % L101 (MA Basis) on Mechanical Properties of Maleated PLA and Its Blend with PLA and Starch sample

tensile strength (MPa)

elongation of blend (%)

modulus (GPa)

neat PLA MA-g-PLA MA-g-PLA/starch (55/45) PLA/MA-g-PLA/starch (44/11/45) PLA/MA-g-PLA/starch (22.5/22.5/45) PLA/MA-g-PLA/starch (11/44/45)

61.6 ( 3.8 63.4 ( 1.6 53.7 ( 1.1 49.0 ( 0.7 53.9 ( 2.1 53.7 ( 1.6

5.2 ( 0.5 5.2 ( 0.2 4.0 ( 0.1 4.3 ( 0.1 3.9 ( 0.2 3.6 ( 0.2

1.61 ( 0.08 1.75 ( 0.03 2.10 ( 0.09 1.99 ( 0.02 2.10 ( 0.05 1.98 ( 0.10

Properties of PLA/Starch Composites

Biomacromolecules, Vol. 5, No. 4, 2004 1449

Figure 3. (a) Schematic structure of PLA/starch compounded with MA. The insert text A stands for starch core; B stands for interphase zone; C stands for PLA phase; t stands for the thickness of interphase layer. (b) Weight fraction of starch and PLA in PLA/MA/starch composites as a function of the distance from the particle center. A linear concentration gradient is assumed. The insert text in the drawing r1 represents the radius of starch core; r2 represents the radius of PLA particle; t is thickness of interphase between starch and PLA; t1 is the thickness of interphase containing starch; t2 is the thickness of interphase containing PLA.

Figure 2. Microstructure of PLA/starch composites prepared using the one-step method: (a) virgin PLA/starch (55/45); (b) PLA/starch/ MA (55/45)99/1; (c) PLA/starch/MA (55/45)99/1 initiated by 10 wt % L101 (MA basis).

a uniform size, (ii) the dispersed granules have a core that consists of starch and an interphase zone in which the composition gradually changes from pure starch to grafting copolymer then to pure PLA, (iii) the concentration gradient in the interphase is linear, (iv) all MA are completely reacted with starch and PLA, (v) the interphase has a uniform thickness, and (vi) the PLA matrix surrounding the starch granule and interphase formed between PLA and starch are perfectly circular in shape. A schematic model is shown in

Figure 3. The relative interphase thickness (t) represents the reaction degree between starch (A zone) and PLA (C zone) induced by MA, which is composed of two parts, t1 and t2, representing the relative interphase thickness from starch to MA grafting starch and from MA grafting starch to MA grafting PLA then to PLA, respectively. For a real system, after starch and PLA reacted with MA, the real thickness at the interface was illustrated in the Figure 3 with dash line, where the boundary varies with reaction condition. Also, the critical point where the component varies from pure starch to MA grafting copolymer to pure PLA shifted left or right depending on the reaction between starch and MA, and between PLA and MA. The weight ratio of starch is described by a mathematical function. The starch ratio at the critical point may range from an extreme value of 0 to 1. This function is 1 for the case r < r1 because it is within starch granule and can be described by the function (r1 + t1 - r)/t1 for the case where r1 < r < r1 + t1. For r > r1 + t1, the function of starch description is invalid for the component in this area changed into PLA. The description of PLA in the interfacial zone is similar with that of starch. The amount of starch (Ms) in the granule (zone A) and the amount of starch (Mi) in the interfacial region (zone B) can be calculated

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Table 4. Residual Starch and PLA in PLA/Starch Composites Extracted by Methyl Sulfoxide and Chloroform, Respectively, and Estimated Interphase Thickness between PLA and Starch Granules blends

residual starch (%)

residual PLA (%)

estimated interphase thickness (×10-7µm)

PLA/starch (55/45) PLA/starch/MA/L101 55/45/(0.25/10%) PLA/starch/MA/L101 55/45/(0.5/10%) PLA/starch/MA/L101 55/45/(1/10%) PLA/starch/MA/L101 55/45/(2/10%)

8.42 20.7 26.3 35.4 42.0

6.34 18.2 21.6 38.6 44.1

0.17 0.47 0.65 1.01 1.37

Figure 4. Theoretical calculation of relative interphase thickness t1 containing starch.

by the following equations:23

∫0r r2 dr

(3)

r1 + t1 - r 2 r dr t1

(4)

Ms ) 4π Mi ) 4π

∫rr + t 1

1

1

1

The percentage of starch that is located in the interfacial region can be expressed as R ) Mi/(Ms + Mi)

(5)

After integrating, the thickness t1 can be theoretically calculated with regard to the percentage of starch according to eq 5, as shown in Figure 4, assuming that the starch granule diameter is 18 µm, which was indicated from SEM observation (Figure 2). The relative interphase thickness increased as the percentage of starch in the interphase region B increased, but the relative interphase thickness was fairly thin, ranging from 0 to 72 µm corresponding to 0-80% starch in the region B. In the case where no reaction occurred in the PLA/starch composites, there is no interphase between these two components; therefore, the thickness of the interphase is zero. Another extreme case would be 100% starch located in the interphase area, meaning an unlimited thickness of the interphase. However, this is unachievable. Between these two extremes is that some reaction among MA, starch and PLA, and the thickness of interphase varies with the reaction degree. Estimation of the Extent of Reaction. Solvent extraction tests provided evidence that some PLA and starch were compatibilized by MA in the presence of initiator L101, where the MA grafting copolymer in the interface is

responsible for the compatibilization. In a PLA/starch composite without MA and initiator L101, 8.42% starch and 6.34% PLA were not extractable (Table 4). This is attributed to the physical entanglement between PLA macromolecules chains and starch granules. With increased MA and L101 concentrations in the composite, the amount of residual starch and PLA increased (Table 4). Taking the measured residual amount of starch as the percentage of starch in the interphase region B, the total relative interphase thickness of the composite was estimated using the curve given in Figure 4. For example, for the composite with 1.0% MA and 10% L101 (MA basis), the residual starch amount was 35.4%, and the estimated relative interphase thickness, including physical entanglement, was about 10.2 µm (Figure 4). Referring to Figure 2C, the average relative thickness at the interphase of each starch granule should be divided by the number of starch granules in the composite (e.g., 0.138 g sample). Therefore, the average relative thickness was approximately 1.01 × 10-7 µm, with the assumption of a circular starch granule and 1.4 g/cm3 density. Though a thin interfacial layer, it is strong enough to withstand and transfer a certain load between the PLA matrix and starch granules on account of chemical reactions, which was reflected by the improved mechanical properties as the relative interphase thickness increased (Table 4). It is noteworthy that the relative interphase thickness of the PLA/starch composite with 2% MA and 10% L101 (MA basis, would be 0.2% on blend basis) was thicker than that with 1% MA and 10% L101 (MA basis, would be 0.1% on blend basis); however, its tensile strength and elongation at break were lower (Table 1). This could be the reason for PLA degradation due to the extra L101, for the blend became slight yellowish after extrusion. Conclusion MA was a good compatibilizer for the PLA and starch system in the presence of initiator L101, regardless of whether the one-step or two-step approach was used. Mechanical properties of PLA/starch composites were significantly improved. A PLA/starch (55/45) composite with 1.0% MA and 10 wt % L101 (MA basis) had tensile strength of 52.4 MPa and 4.1% elongation, which was close to neat PLA. Acknowledgment. Authors appreciated the financial support of this research from the Kansas Wheat Commission and CPBR (Grant EPA82947901-148). References and Notes (1) Grijpma, D. W.; Pennings, A. J. Macromol. Chem. Phys. 1994, 195, 1633.

Properties of PLA/Starch Composites (2) Storey, R. F.; Wiggins, J. S.; Mauritz, K. A. Polym. Compos. 1993, 14, 7. (3) Huang, S. J. Polymers-Biomaterials and Medical Applications; Kroschwitzed, J. J., Ed.; Wiley: New York, 1989. (4) Jamshidi, K.; Hyon, S. H.; Ikada, Y. Polymer 1988, 29, 2229. (5) Mayer, J. M.; Kaplan, D. L. Trends Polym. Sci. 1994, 2 (7), 227. (6) Wang, H.; Sun, X. Z.; Seib, P. J. Appl. Polym. Sci. 2001, 82, 1761. (7) Rocco, A. M.; Pereira, R. P.; Felisberti, M. I. Polymer 2001, 42 (12), 5199. (8) Kim, S. J.; Shin, B. S.; Hong, J. L.; Cho, W. J.; Ha, C. S. Polymer 2001, 42 (9), 4073. (9) Pagnoulle, C.; Jerome, R. Polymer 2001, 42(5), 1893. (10) Son, Y.; Ahn, K. H.; Char, K. 2000 Polym. Eng. Sci. 40 (6), 1385. (11) Dedecker, K.; Groeninckx, G. Pure Appl. Chem. 1998, 70 (6), 1289. (12) Cho, K.; Li, F. Macromolecules 1998, 31, 7495. (13) Carlson, D.; Nie, L.; Narayan, R.; Dubois, P. J. Appl. Polym. Sci. 1999, 72, 477.

Biomacromolecules, Vol. 5, No. 4, 2004 1451 (14) Mani, R.; Bhattacharya, M.; Tang, J. J. Polym. Sci. A: Polym. Chem. 1999, 37 (11), 1693. (15) Vaidya, U. R.; Bhattacharya, M. J. Appl. Polym. Sci. 1994, 52, 617. (16) Ke, T. Y.; Sun, X. Z. J. Appl. Polym. Sci. 2001, 81, 3069. (17) Ke, T. Y.; Sun, X. Z. Cereal Chem. 2000, 77 (6), 761. (18) O’Shaughnessy, B.; Sawhney, U. Phys. ReV. Lett. 1996, 76, 3444. (19) Babanalbandi, A.; Hill, D. J. t.; Hunter, D. S.; Kettle, L. Polym. Int. 1999, 48, 980. (20) Willett, J. L. J. Appl. Polym. Sci. 1994, 54, 1685. (21) Arvanitoyannis, I.; Psomiadou, E.; Biliaderis, C. G. Starch 1997, 49, 306. (22) Mani, R.; Bhattacharya, M. Euro. Polym. J. 2001, 37, 515. (23) Dedecker, K.; Groeninckx, G. Macromolecules 1999, 32, 2472.

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