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In situ compatibilization of maleated thermoplastic starch/polyester melt-blends by reactive extrusion.

INTRODUCTION

Plastic packaging is now subjected to strong pressure from environmental and disposal regulations in the design of single-use disposable packaging (1), (2). Obviously, there is a need for the development of environmentally friendly and biodegradable products derived from renewable resources, which retain all of the current plastics performances (3). Because of the biodegradable character, natural abundance and low-cost, many researchers have attempted to incorporate starch into a great variety of plastic materials. Native starch may be added as unmodified filler (4), (5) in melt-blends with, e.g. poly(hydroxybutyrate-co-hydroxyvalerate). However, the use of unmodified granular starch as a particulate filler cannot provide any appreciable reinforcement due to the poor adhesion of the polymer-granule interface.(6).

The main uses of starch have been as a binder, as a thermoplastically processable constituent within thermoplastic polymer blends, and as a thermoplastic material by itself. In general, thermoplastic starch (TPS) is obtained from native starch in the presence of a plasticizer when heated and sheared (3), (6-9). Glycerol and water are the most used plasticizers. Unfortunately, the main limitation to use TPS as commodity plastics is due to its moisture sensitivity and thermal instability. It is well understood that TPS has to be combined in melt-blend with hydrophobic polymeric materials to produce high-performance TPS-based products (10). At the earliest stage, polystyrene, polyethylene, poly(methy1 methacrylate) and others have been largely studied (11-16). However, from an environmental viewpoint, the residues from nondegradable polymers are not environmentally friendly after biodegradation of starchy fraction. In this respect, some authors have preferred biodegradable hydrophobic polymers such as poly (caprolactone) and cellulose acetate to manufacture biodegradable products. Recently, poly (butylene adipate-co-terephthalate) (PBAT), a biodegradable aliphatic-aromatic copolyester, has shown to be an interesting polymeric partner in the starch-based melt-blends. This is due to its interesting thermomechanical properties (17). However, because of the large difference in the hydrophilic balance, it is necessary to carry out a chemical functionalization on the biodegradable hydrophobic polymers in the preparation of starch-based polymer melt-blends (18-38). Grafting of maleic anhydride (MA) onto the polymer backbone is currently used for functionalization. These grafted reactive functions can then react with the hydroxyl groups from starch to form covalent bonds; and thus, to provide better size control of the dispersed phase and stronger interfacial adhesion. In a similar way, we have reported a novel in situ chemically modified TPS, called maleated TPS (MTPS). MTPS was prepared, through reactive extrusion processing of starch in the presence of glycerol (plasticizer) and MA (esterification agent) (39). It has been shown that MA could be efficiently grafted onto the starch backbone, preferentially at the C6 positions of glucose units. This leads to a starch ester bearing a free carboxylic acid group (Scheme 1). Interestingly, the MA moieties grafted onto the starch backbone could promote some hydrolysis and glucosidation reactions that reduce the relative molecular weight for MTPS (Scheme 2). This reduction for TPS represents a major issue in the melt blends based on TPS that most of the authors have not addressed. Indeed, TPS most often exhibits (too) high melt-viscosity making its fine dispersion very difficult throughout the polymer matrix. This significantly limited the mechanical properties of the resulting melt-blends (32). In addition to its reduced melt-viscosity, the interest of MTPS is also the presence of free carboxylic acid groups, i.e. open MA moieties grafted onto the starch backbone. Such functions are able to promote acid-transesterification reactions with polyester chains such as PBAT. From these interfacial transesterification reactions, one can expect the formation of graft copolymers with good properties (32), (33).

[ILLUSTRATION OMITTED]

This article aims at reporting the utilization of MTPS in the reactive extrusion melt blending with PBAT. The ultimate objective of this work is to use the resulting (reactive) melt-blends in blown film applications. The effects of both MA and polyester contents were studied, whereas selective Soxhlet extraction experiments together with FTIR analyses were used to attest for the formation of a graft copolymer generated by acid-promoted transesterification reactions.

[ILLUSTRATION OMITTED]

EXPERIMENTAL SECTION

Materials

Regular silver medal pearl cornstarch (unmodified) was obtained from Cargill-grade SMP 1100 (MN, USA), with equilibrium moisture content of about 12% (w/w). Anhydrous glycerol (99.9%) and MA were obtained from J.T. Baker (NJ, USA) and Sigma-Aldrich (WI, USA), respectively, and used as received. PBAT was purchased from BASF Corporation (Germany), under the trade name Ecoflex (F). PBAT is made by condensing 1,4 butanediol with 1,4-benzenedicarboxylic acid (terephthalic acid) and hexanedioic acid (adipic acid).

Synthesis of PBAT-g-TPS Graft Copolymers. PBAT-g-TPS graft copolymers were prepared through reactive extrusion using MA as a transesterification promoter. TPS was produced by plasticization of regular silver medal pearl cornstarch with glycerol (20 wt%) as a plasticizer in a twin-screw corotating CENTURY ZSK-30 extruder with a screw diameter of 30 mm and a L/D ratio of 42 at a melt temperature of about 153[degrees]C. MA was ground to a fine powder using a mortar and pestle and preblended with PBAT before being fed to the feed port of the extruder. The concentration of MA used was 2.5-wt% by starch. Meanwhile, TPS (previously oven dried overnight at 75[degrees]C) was ground to a fine powder and fed using an external feeder to the extruder. The feeder rates were adjusted accordingly to obtain a ratio of 70:30 (PBAT:TPS). The temperature profile was 15/95/125/145/160/165/165/165/150/145 from the feed throat to the die, and the melt temperature was 153[degrees]C The screw speed was 150 rpm The vent port was kept open for all studies to remove unreacted MA and moisture. The extruded strand was cooled using a water bath and pelletized in line. The pellets were dried in an oven overnight at 75[degrees]C before being film-blown. The grafting reaction was determined by Soxhlet extraction in dichloromethane (Sigma-Aldrich, WI, 99+%) overnight.

Synthesis of PBAT-g-MTPS Graft Copolymers. In the first step, MTPS was directly prepared from regular silver medal pearl cornstarch and 2.5 wt% MA (by starch) in the same extruder, as reported elsewhere (39). During the preparation of MTPS, glycerol as a plasticizer was pumped to the extruder through a peristaltic pump for a mass composition of starch/glycerol close to 80:20. Vacuum (using water jets) was applied at the vent port downstream to remove out the unreacted MA and excess moisture. Subsequently, PBAT-g-MTPS graft copolymers were prepared from MTPS (previously oven-dried overnight at 50[degrees]C), again in a twin-screw corotating CENTURY ZSK-30 extruder following the same experimental conditions like TPS. The grafting reaction was determined by Soxhlet extraction in dichloromethane (Sigma-Aldrich, WI, 99+%) overnight.

Blown Film Studies. Films made of the PBAT-g-TPS, PBAT-g-MTPS graft copolymers as well as the pure PBAT resin were blown-molded using a Killion single-screw blown film unit at a melt-temperature of 167[degrees]C under a screw speed of about 17 rpm (draw-up speed = 5-6 foot/min.). The screw diameter was 25.4 mm with an L:D ratio of 25:1. The die inner diameter was 50.8 mm with a die gap size of 1.5 mm.

Characterization and Analyses

FTIR analysis was conducted on blown films derived from PBAT, PBAT-F-TPS, and PBAT-g-MTPS from 400 to 4000 [cm.sup.-1] using a Perkin Elmer Model 2000 FTIR. Differential scanning calorimetry was used to determine the thermal properties of the PBAT-g-MTPS graft copolymers and PBAT. A Q100 TA Modulated differential scanning calorimeter (DSC) was used under nitrogen flow (50 ml/min). The sample was heated to 150[degrees]C at the rate of 10[degrees]C/min. to remove any prior history, and then cooled back to -60[degrees]C at the rate of 10[degrees]C/min. An environmental scanning electron microscope (Phillips Electroscan 2020; ESEM) was used to observe the morphology of the samples. The samples were submerged in liquid nitrogen to fracture them with a pestle in a mortar. They were then mounted on aluminum stubs. Wide angle X-ray (WAXS) diffraction analysis was performed on blown film compositions with a Rigaku Rotaflex Ru-200BH X-ray diffractometer operated at 40 KV, 100 mA with a nickel filtered Cu [K.sub.[alpha]] radiation ([lambda](Cu [K[alpha].sub.1]) = 0.15406 nm), and a [theta] compensating slit. Data were acquired in 0.1[degrees] 2[theta], 10 s steps. Specimens were studied using films obtained by blown-film extrusion. Tensile properties of the films were determined using UTS Mechanical Testing Equipment Model SFM-20 fitted with a 45.5 kg load cell. The cross-head speed was 50 cm/min. Rectangular (10 cm X 2.5 cm) film samples were conditioned at 23[degrees]C [+ or -] 1[degrees]C and 50% [+ or -] 2% relative humidity for 48 h before being tested according to ASTM D-882 testing.

RESULTS AND DISCUSSION

Table 1 reports the effect of initial polyester content on the extracted fraction for MTPS-based melt-blends. In this study, MTPS was chemically modified with 2.5 wt% MA as described by us (39). The extracted fraction was determined by selective Soxhlet extraction carried out in dichloromethane overnight to strip off the unreacted PBAT selectively. For the sake of comparison, two direct PBAT/TPS melt-blends (70/30 wt/wt) prepared without and with 2.5 wt% MA (by starch) were also reported (entries 1-2, Table 1). TPS was prepared in the presence of 20 wt% glycerol (by starch) through the same processing conditions like MTPS.
TABLE 1. Effect of initial polyester content on the extracted fraction
(a) of reactive melt-blends prepared from MTPS and PBAT, and of PBAT/
TPS melt-blends prepared with and without 2.5 wt% MA.

Entry Nature Initial Extracted
 polyester fraction (a)
 content (wt%) (wt%)

1 PBAT/TPS 70 73
2 TPS-g-PBAT (b) 70 97
3 MTPS-g-PBAT (c) 70 98
4 MTPS-g-PBAT (c) 60 96
5 MTPS-g-PBAT (c) 50 47
6 MTPS-g-PBAT (c) 40 38

(a) As determined by gravimetry after Sohxlet extraction carried out in
dichloromethane overnight.

(b) Direct PBAT/TPS melt-blend prepared in the presence of 2.5 wt% MA
as transesterification agent.

(c) MTPS chemically modified with 2.5 wt% MA (see experimental).


In the simple PBAT/TPS melt-blend (entry 1, Table 1), no reaction between TPS and PBAT occurred. This was confirmed by the amount of extracted polyester close to the initial value (70 wt% polyester). When MTPS was used, the resulting PBAT/TPS melt-blend (70/30 wt%) was completely extracted out (entry 2, Table 1). Interestingly, it was observed that after Soxhlet extraction, the solvent turned to a colloidal solution without any precipitate. In other words, such observations attested for the formation of a PBAT-TPS graft copolymer derived from PBAT and TPS, covalently linked to each other through acid-promoted transesterification reactions between the ester functionalities from the PBAT backbone and the hydroxyl groups from TPS (Scheme 3). Almost complete extraction was also achieved with the PBAT-g-MTPS graft copolymer sample containing 60-wt% polyester content, leading again to the formation of a colloidal solution. It is worth noting that these transesterification reactions were promoted by acidic MA-moieties grafted onto the starch backbone, but to some extent with acidic MA-moieties grafted onto glycerol. Indeed, beside esterification reactions between starch and MA, MA could partially react with glycerol during the preparation of MTPS (39). Whichever the species, open/acidic MA moieties are obtained, and can efficiently promote in a second downstream extrusion operation acid-promoted transesterification reactions with polyester chains such as PBAT. However, for the 50/50 and 40/60 PBAT-MTPS blends, only 47 and 38%, respectively (close to the respective PBAT amount) were extracted out. This was likely due to the inversion in phase morphology between PBAT and MTPS. Therefore, the reaction was dependent on the relative amount in polyester and MTPS phases.

[ILLUSTRATION OMITTED]

To attest for the formation of PBAT-g-MTPS graft copolymers, the graft copolymers with 70 wt% polyester and their precursors (MTPS and PBAT) have been characterized by FTIR spectroscopy. Interestingly, the PBAT-g-MTPS graft copolymer fraction displayed a carbonyl stretch peak at 1720 [cm.sup.-1], assigned to PBAT (Fig. 1, compared expanded spectra a and c). Furthermore, the ester C--O stretch at 1270 [cm.sup.-1] was also observed in the graft copolymer but not in the MTPS precursor. The graft copolymer also exhibited a peak between 3200 and 3400 [cm.sup.-1] corresponding to --OH stretch as well as a peak at 1025-1060 [cm.sup.-1], which can be attributed to primary alcohols. This peak was also observed in MTPS, but not in PBAT. As compared to MTPS, the most striking feature was the complete disappearance of --OH bending peak absorption at 1640 [cm.sup.-1] in the graft copolymer fraction, which is characteristic to the chemical modification of starch (40). This attests that hydroxyl functions from starch participated in transesterification reactions with MTPS, yielding the formation of a graft copolymer structure.

[FIGURE 1 OMITTED]

The tensile properties were studied from the PBAT-g-MTPS graft copolymers containing 70 wt% polyester, which had been prepared from MTPS modified with different MA contents, i.e., 2.5, 5, 8 wt% (Table 2). These data were compared to the tensile properties of both PBAT and the PBAT/TPS melt-blend modified in the presence of 2.5 wt% MA (entries 1 and 2, Table 1). It is worth noting that the unmodified TPS/PBAT melt-blend (entry 1, Table 1) could not be blown since the resulting film was too much brittle (lack of compatibility between both partners). The tensile properties of films were only determined in the machine direction.
TABLE 2. Tensile strength, modulus of elasticity (MoE), elongation at
break (recorded in the machine direction) of blown films from
MTPs-g-PBAT graft copolymers (containing 70 wt% polyester) wherein MTPS
was modified with different MA contents, together with those of
pristine PBAT and the direct PBTA/TPS melt blend (with 70 wt%
polyester) added with 2.5 wt% MA (see entry 2. Table 1).

 Tensile MoE (MPa) Elongationa
Entry Nature strength at break (%)
 (MPa)

1 PBAT 38.6 [+ or -] 13.8 [+ or -] 600 [+ or -]
 4.0 1.3 35

2 TPS-g-PBAT 6.7 [+ or -] 41.4 [+ or -] 500 [+ or -]
 (205% MA by starch) 0.5 5.0 40

3 MTPS-g-PBAT 1 6.2 [+ or -] 77.6 [+ or -] 700 [+ or -]
 (2.5% MA by starch) 1.5 9.4 50

4 MTPS-g-PBAT 16.5 [+ or -] 77.7 [+ or -] 700 [+ or -]
 (5% MA by starch) 1.2 904 50

5 MTPS-g-PBAT 16.2 [+ or -] 78.1 [+ or -] 730 [+ or -]
 (85% MA by starch) 1.7 7.2 50


From Table 2, it comes out that the lowest tensile properties (tensile strength, modulus of elasticity, and break elongation) were obtained in the direct PBAT-g-TPS melt-blend as modified in the presence of MA (entry 2). Indeed, TPS has found to display a gel-like viscoelastic behavior. This is related with the formation of a crystalline elastic network produced by the complexation of amylose molecules with lipids/plasticizers and the physical entanglement of starch chains caused by its high molecular weight. Such a physical entanglement is responsible for the incomplete homogenization in the melt phase, and therefore the tensile properties (41). As a result, the grafting reaction could only occur at the (limited) interface between PBAT and TPS. It resulted in a reactive TPS-g-PBAT melt-blend with a coarse phase morphology, and hence, poor tensile properties. In contrast to TPS, the lower molecular weight for MTPS (39) together with its increased reactivity led, by melt blending with PBAT, to the formation of stronger covalent linkages between partners, a more complete homogenization, and therefore finer phase morphology for the resulting MTPS-PBAT melt-blends. As a result, the tensile strength and modulus of elasticity values for all MTPS-PBAT meltblends were much higher as compared to the TPS-g-PBAT melt-blend prepared in presence of MA. These values for MTPS-g-PBAT melt-blends in terms of tensile strength and modulus of elasticity did not change by increasing the MA content. This suggests that either the entanglement of PBAT and MTPS chains did not change after reactive extrusion or the MA content reached to a certain level, which did not allow promoting further reactions (17).

The most striking feature was the high values for the elongation at break recorded for the MTPS-g-PBAT graft copolymers reaching values even higher than for the pristine PBAT. Such a behavior could be explained by the enhanced reactivity for MTPS, increasing the number of covalent (ester) bonds between the MTPS and PBAT phases. This improved their interfacial compatibility. It is worth noting that the whole quantity of MA initially added for producing MTPS (in the first step) was not present in the consecutive melt-blending step with PBAT. Indeed, the excess of unreacted MA was removed out in the vent port of the extruder along the reactive malcation of TPS, and did not get transferred to the next stage of the melt-blending process (17), (39). Such an approach reduced the risk of increasing the incidence of hydrolysis side-reactions within the polyester matrix that MA-derived acidic moieties could promote.

Environmental scanning electron microscopy (ESEM) was used to study the phase morphology on cryofractured samples for PBAT/TPS melt-blends (with and without MA), and the MTPS-g-PBAT graft copolymer initially modified with 2.5 wt% MA (see Fig. 2). All the samples contained 70-wt% polyester (entries 1-3, Table 1). For the direct melt-blend prepared in the absence of MA, Fig. 2a clearly evidences the lack of compatibility between plasticized starch and PBAT. In contrast, in presence of MA, some improvement in the compatibility between the two partners was observed because of the grafting reactions that occurred in the melt via acid-promoted transesterification reactions (see Fig. 2b). Interestingly, a uniform blend was obtained in the case of the PBAT-g-MTPS graft copolymer (see Fig. 2c). In addition to the grafting reactions taking place between the PBAT and MTPS phases, reduction of molecular weight for MTPS yielded a finer morphology of the dispersed phase (MTPS) in the continuous PBAT matrix, together with a concomitant increased interfacial area necessary for efficient grafting reactions.

[FIGURE 2 OMITTED]

WAXS diffractograms of regular cornstarch, TPS, the direct PBAT/TPS melt-blend prepared without MA, MTPS (modified with 2.5 wt% MA), and a PBAT-g-MTPS graft copolymer are shown in Figs. 3 and 4. By comparison to the X-ray pattern of cornstarch, new processing-induced characteristic crystalline starch peaks were observed at 13.2[degrees] and 20.5[degrees] for TPS assigned to the so-called V-structure (36), (37) (Fig. 3a and b). The appearance of these new diffraction peaks gave clear evidence for the formation of crystalline domains generated by retrogradation of the amorphous chains. Residual A-type crystallinity derived from cornstarch could also be observed in TPS (at ~ 17[degrees]) (38) (see Fig. 3), though diminished. Native A-type crystal lattice is due to amylopectin double helices consisting of double helical, sixfold structures, while the V-lattice corresponds to more contracted amylose helices with a low content in water and/or glycerol. It is worth noting that the presence of residual A-type crystallinity indicated that the energy input was not high enough for the complete plasticization of starch. Figure 3c and d show the WAXS patterns for PBAT and the PBAT-TPS melt-blend, respectively. It could be observed that the crystallinity from the TPS was not completely disrupted in the PBAT/TPS blend, attesting for the uncomplete plasticization for starch.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

For MTPS, it was observed that the residual crystalline peaks from the native starch crystals at ~17[degrees] were absent, suggesting that the inherent granular, crystalline structure of the native corn starch completely vanished (see Fig. 4). Since the MTPS had a lower molecular weight, the energy input in the extruder was sufficient to destructurize the native crystals completely. When MTPS was used in the PBAT-g-MTPS melt-blend, the resulting WAXS pattern showed that the processing-induced crystalline structure present in MTPS vanished in the PBAT-g-MTPS reactive blend, suggesting the homogenization of the MTPS in the polyester continuous phase. The peaks obtained in the WAXS diffraction pattern at 17.5[degrees], 20.5[degrees], and 23.2[degrees] were originated from the semicrystalline PBAT (34).

DSC measurements confirmed the presence of a melting peak for the PBAT-g-MTPS copolymer very similar to the pristine PBAT sample (at ca. 122[degrees]C), while exhibiting identical glass transition temperatures for both samples (entries 1 and 5, Table 2; Fig. 5). The normalized melting enthalpy of the graft copolymer (11.6 J/g) slightly decreased as compared to the pristine PBAT (15.5 J/g), more likely due to some reduction in molecular weight for the polyester matrix. Such a reduction could be provoked by ester bond hydrolysis as promoted by MA-derived acidic moieties of MTPS. We can also assume that grafting of starch with polyester hindered the crystallization for the PBAT fraction. This decrease of normalized melting enthalpy was not observed in the case of simple melt blend (not reported). It is worth nothing that the melting temperature of MTPS was not detected in the graft copolymer (39), again supporting the good homogenization of MTPS within the polyester matrix.

[FIGURE 5 OMITTED]

CONCLUDING REMARKS

This article reports the use of MTPS in the melt blend with PBAT through reactive extrusion processing. As reported recently, MTPS with high reactivity and reduced molecular weight could be successfully prepared by plasticization of starch in the presence of glycerol as plasticizer and MA as transesterification promoter. This reduction of molecular weight has proved to be beneficial in the melt blending of MTPS with PBAT. In the first step, both effects of polyester and MA contents were studied on the physicochemical parameters of the resulting MTPS/PBAT melt-blends. For 60 and 70 wt% polyester fractions, PBAT-g-MTPS graft copolymers were selectively obtained through transesterification reactions promoted by MA-derived acidic moieties grafted onto the starch backbone. This was evidenced by selective Soxhlet extraction experiments and FTIR analyses. At lower polyester content, no reaction occurred more likely due to an inversion in phase morphology between PBAT and MTPS. Interestingly, tensile properties of the blown films resulting from the MTPS-g-PBAT graft copolymers containing 70 wt% polyester were much higher than those of TPS-g-PBAT melt-blends as produced in the presence of MA. As attested by ESEM, this yields a finer morphology of the dispersed phase in the continuous PBAT matrix, together with an increased interfacial area for the grafting reaction. Increased MA content in the preparation of MTPS did not affect the tensile values, suggesting that the entanglement of PBAT and MTPS chains, responsible for these values, did not change after reactive melt blending. Moreover, WAXS analyses evidenced that the native starch crystalline structure completely vanished in the "PBAT-g-MTPS" reactive blends, suggesting the grafting reaction/homogenization of the MTPS in the polyester continuous phase.

ACKNOWLEDGMENTS

The authors are very grateful to "Region Wallonne" and European Community (FEDER, FSE) for general support in the frame of "Objectif 1-Hainaut: Materia Nova". They really appreciate the kind advices provided by the reviewers to improve the quality of the manuscript.

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Jean-Marie Raquez, (1), (2) Yogaraj Nabar, (3) Ramani Narayan, (3) Philippe Dubois(1)

(1) Laboratory of Polymer and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons-Hainaut, B-7000 Mons, Belgium

(2) Polymers and Composites Technology and Mechanical Engineering Department, Ecole des Mines de Douai, BP 10838, F-59508 Douai, France

(3) Department of Chemical Engineering and Material Science, Michigan State University, East Lansing, Michigan 48824, USA

Correspondence to: Jean-Marie Raquez: e-mail: jean-marie.raquez@umh.ac.be

Contract grant sponsors: Corn Products International contract grant sponsor: Belgian Federal Government Office of Science Policy: contract grant number: SSTC-PAI 6/27
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Author:Raquez, Jean-Marie; Nabar, Yogaraj; Narayan, Ramani; Dubois, Philippe
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:1USA
Date:Sep 1, 2008
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