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starch blend-based biocomposites
Accepted Manuscript Preparation and Properties of luffa fiber- and kenaf fiber-filled poly(butylene succinate-co-lactate)/starch blend-based biocomposites Sun-Mou Lai, Yu-Hsiang Kao, Yu-Kuo Liu, Fang-Chyou Chiu PII:
S0142-9418(15)30154-9
DOI:
10.1016/j.polymertesting.2016.01.015
Reference:
POTE 4575
To appear in:
Polymer Testing
Received Date: 2 November 2015 Accepted Date: 20 January 2016
Please cite this article as: S.-M. Lai, Y.-H. Kao, Y.-K. Liu, F.-C. Chiu, Preparation and Properties of luffa fiber- and kenaf fiber-filled poly(butylene succinate-co-lactate)/starch blend-based biocomposites, Polymer Testing (2016), doi: 10.1016/j.polymertesting.2016.01.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Material Properties Preparation and Properties of luffa fiber- and kenaf fiber-filled poly(butylene succinate-co-lactate)/starch blend-based biocomposites
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Sun-Mou Lai1, Yu-Hsiang Kao2, Yu-Kuo Liu3, Fang-Chyou Chiu2* 1. Department of Chemical and Materials Engineering, National I-Lan University, I-Lan 260, Taiwan, ROC
2. Department of Chemical and Materials Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC
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3. Graduate Institute of Biochemical and Biomedical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC
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Abstract
Biodegradable poly(butylene succinate-co-lactate) (PBSL)/starch blends that contain various amounts of starch were prepared. In addition, luffa fiber (LF) and kenaf fiber (KF) were incorporated, individually, into PBSL/starch (70/30) blend to achieve biocomposites. The LF and
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KF were treated with NaOH(aq) prior to their addition to the blend. The Young’s modulus and flexural modulus of PBSL increased with the addition of starch and increased further after the formation of the biocomposites. The highest Young’s modulus increment, which was found in the
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KF-added system, was up to a 2.2-fold increase compared with neat PBSL. The tensile/flexural/impact strength of PBSL declined after the formation of the blends. With the
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further addition of LF/KF, the said properties leveled off. The blends exhibited higher complex viscosity and dynamic storage modulus in the melt state than the neat PBSL, and the values further increased in the biocomposites. The crystallization temperature of PBSL slightly decreased in the blends. By contrast, the biocomposites showed an increment in PBSL crystallization temperature, from 73.0 °C (PBSL) to 75.3 °C (KF-added composite), thereby confirming the surface nucleation effect of LF/KF. The blends showed a higher degree of water absorption than PBSL. The formation of biocomposites led to an even higher degree of water absorption. The current approach of including LF/KF in the PBSL/starch blend to enhance the 1
ACCEPTED MANUSCRIPT rigidity and biodegradability was advantageous in expanding the applications of PBSL.
Keywords: poly(butylene succinate-co-lactate); starch; luffa fiber; kenaf fiber; biocomposites;
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physical properties *Corresponding author. Tel: +886-3-2118800 ext.5297; fax: +886-3-2118668.
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E-mail address: [email protected] (F. C. Chiu)
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ACCEPTED MANUSCRIPT 1. Introduction Given growing environmental concerns, biodegradable polymers with ecological advantages over petroleum-based polymers in terms of sustainability have received much attention in
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academia and industry in the past two decades. Several renowned companies, such as Natureworks, Dow, Procter & Gamble, and others, have begun developing several types of biodegradable polymers in the commercial market, including poly(lactic acid) (PLA), polycaprolactone (PCL) and poly(butylene succinate) (PBSU). In addition, biomass-based
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polymers made from natural resources have also been the development target to reduce carbon dioxide emissions from the manufacturing process. In particular, these biopolymers, such as
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starch, poly(hydroxy alkanoate) (PHA) and cellulose materials, are considered cost-competitive with commodity plastics. Thus, their potential applications are anticipated to continuously expand for years to come.
The abundant natural food source from most plants, starch is primarily composed of amylose and amylopectin. Amylose is a linear polymer with α-1,4-linked glucose units, whereas
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amylopectin is a branched polymer with α-1,4-linked chains connected by 1,6-linkages [1]. Griffin [2] pioneered the blending of granular starch with plastics, which incorporated the eco-feature in conventional polymers. However, starch failed to be well-dispersed in the plastic
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matrices because of the inherent hydrogen bonding between the adjacent molecules in starch and the non-efficient mixing processes. Therefore, the gelatinization technology to prepare
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thermoplastic starch was developed, with addition of water, plasticizers, or both [3]. In addition to starch, plant fibers have been utilized as eco-reinforcement to develop polymer composites. Among the examined plant fibers, luffa fiber (LF) and kenaf fiber (KF) are two of the suitable choices as reinforcing fillers for preparing polymer composites. LF sourced from luffa cylindrica, a subtropical plant, is readily available in a vascular form [4]. KF, obtained from a natural tropical plant, is also globally available in a cylindrical structure [5]. Given their low density, nonabrasive nature, biodegradability and low cost, starch and plant fibers are considered to be attractive counterparts for manufacturing biopolymer blends and biocomposites. Several recent studies 3
ACCEPTED MANUSCRIPT have investigated biodegradable polymers filled with starch or different plant fibers for the formation of blends and biocomposites, such as the blends of PLA/starch [6–9], PCL/starch [10] and PBSU/starch [11–13], and the composites of PLA/corn fiber [14], PLA/bamboo fiber [15],
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PBSU/curaua fiber [16], starch/LF [17] and starch/KF [5]. Poly(butylene succinate-co-lactate) (PBSL) is a newly developed aliphatic polyester that possesses biodegradable characteristics. It is synthesized by the polycondensation of 1,4 butanediol, succinic acid, and L-lactic acid [18, 19]. Hitherto, only a limited number of
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biodegradable PBSL blends and composites have been investigated [18–26]. Among the studies, none has been conducted on PBSL/starch blends and plant fiber-filled PBSL composites. From
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academic and practical viewpoints, blending starch with PBSL and then adding plant fibers to the blends should merit a detailed study and is an attractive approach to enhance the performance of PBSL. For instance, LF has been reported to increase the tensile strength of thermoplastic starch (TPS) by up to twofold [17], and KF has been added to cassava starch as reinforcing filler [5]. In the present work, both LF and KF were chosen to compare their reinforcing capability for
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the PBSL/starch blend. The melt-mixed samples were characterized by differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA). The rheological/water absorption properties and fractured multi-phasic morphology of the samples were also evaluated. In addition,
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the influences of incorporating starch and LF/KF on the tensile and flexural properties of PBSL were measured. This study should facilitate further understanding in developing novel
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biopolymer composites for our environment.
2. Experimental
2.1 Materials The PBSL utilized in this study was a product of Mitsubishi Chemical Corporation, Japan, under the trade name of AZ71T. Its density was 1.26 g/cm3, and its lactate content was ca. 3 mol.%. Cassava starch of edible grade grown in Taiwan was used as the counterpart for preparing 4
ACCEPTED MANUSCRIPT the PBSA blends. LF and KF grown in China and India, respectively, were utilized as the reinforcements for preparing the composites.
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2.2 Preparation of the Samples Both LF and KF fibers were surface pretreated to improve their compatibility with the polymer matrix. The chopped LF and KF were first washed with distilled water to remove surface impurities and then allowed to dry in an oven at 70 °C for 1 day. The washed/dried fibers were
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alkali-treated with NaOH by immersion in 1% aqueous NaOH solution and then stirred for 1 h at room temperature. The alkali-treated LF and KF were thoroughly washed with distilled water
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until pH 7 was reached and allowed to dry in an oven. The fibers were then pulverized and screened with a sieve size of 18 mesh into a powder form to blend them with the polymer matrix. Prior to the mixing procedure, PBSL, starch and treated LF/KF were dried in a vacuum oven to remove the absorbed moisture. The mixing was conducted in an intermeshing twin-screw extruder (SHJ-20B, L/D = 40) in co-rotating mode. The extruder was operated at a screw speed of
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360 rpm and barrel temperatures ranging from 90 °C to 120 °C. The formulation and sample designations are listed in Table 1. The pelletized samples of different formulations were fed into an injection molding machine to prepare specimens for further characterization of their
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mechanical properties.
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2.3 Measurements
A Fourier transform infrared (FTIR) spectrometer was used to confirm the surface treatment
on LF and KF. The spectra of treated fibers were recorded on a Bruker Optics Tensor 27 system at a resolution of 4 cm-1 for 32 scans. The appearance of 18-mesh sieved LF and KF was observed using a scanning electron microscope (SEM, Hitachi S-3000N). The phase morphology of the liquid nitrogen cryo-fractured samples was observed through the same SEM. All the specimens were sputtered with gold before observation. The crystallization temperatures (Tc) of the samples were measured using a differential scanning calorimeter (DSC, TA Q10) at a cooling 5
ACCEPTED MANUSCRIPT rate of 10 °C/min from 140 °C. The melting temperatures (Tm) of the pre-cooled samples were then determined at a heating rate of 10 °C/min from 25 °C to 140 °C. A thermogravimetric analyzer (TGA, TA Q50) was used to evaluate the thermal stability of the samples. The samples
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were heated from room temperature to 700 °C at 10 °C/min under nitrogen environment. To avoid the water absorption effect on the scanned results, the peak temperature of the derivative TGA curve was used as an indication for the thermal stability comparison.
The tensile properties, including the Young’s modulus, tensile strength, and elongation at
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break, of the dumbbell-shaped samples (according to ASTM-D638) were measured using a Gotech AI-3000 system at a crosshead speed of 5 mm/min. Flexural property tests were
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conducted per ASTM D790 at a crosshead speed of 10 mm/min using the same Gotech AI-3000 system. Izod impact strength tests (Ceast Resil 5,5) were conducted on notched specimens in accordance with ASTM D256. The acquired mechanical properties were average values of five specimens of the same formulation. The rheological properties of the samples were determined at 140 °C using an Anton Paar rheometer (MCR 101) at a strain of 1% as a function of angular
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frequencies. The water absorption was evaluated by determining the weight gained by the dried samples after their immersion in distilled water for different periods. The equation used is as follows:
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Water absorption (%) = (Wt − Wo)/Wo × 100
where Wt is the weight of the sample treated for t day(s) after drying excess distilled water, and
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Wo is the sample weight prior to the test.
3. Results and discussion
3.1 Alkaline Treatment of LF and KF Figure 1 shows the variations in the FTIR spectra of the untreated and treated LF and KF. As commonly known, plant fibers are mainly composed of cellulose, hemi-cellulose and lignin. The absorption peaks between 1500 and 1000 cm-1, including CH2 bending, O-H bending, C-OR 6
ACCEPTED MANUSCRIPT stretching and anhydroglucose ring, are associated with the internal structure of cellulose fibers [27]. In addition, the absorption of C=O vibration around 1735 cm-1, which is attributed to the carbonyl and acetyl groups in the hemi-cellulose and lignin [28], was observed in the untreated
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LF and KF. The C=O absorption disappeared accordingly in the treated LF/KF, which indicated the successful removal of hemi-cellulose and lignin by NaOH(aq) treatment. The difference in surface topography between the untreated and treated LF/KF are depicted in the SEM images of Figure 2. The waxy substance and impurity particles on the untreated fiber surfaces were
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evidently removed after the alkaline treatment, as shown in the comparisons in Figures 2(a) and (c) and in Figures 2(b) and (d). The treated fibers also exhibited a bundle-like structure caused by
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the removal of semi-cellulose and lignin, as reported by Ghali et al. [29]. Overall, with the successful NaOH(aq) treatment, the adhesion between LF/KF and polymer matrix was anticipated to be improved. Furthermore, the high aspect ratios of LF and KF were also noted. The aspect ratio of KF is about 31, which is more than twofold of that of LF (ca. 14), as depicted in Figures 2(e) and (f). The surface treatment and the high aspect ratio of LF and KF are beneficial to the
mechanical properties.
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3.2 Phase Morphology
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reinforcing efficiency of the PBSL/starch blends, as discussed in the following section on
The fractured surfaces of the PBSL/starch blends containing various amounts of starch are
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shown in the SEM images of Figure 3. The starch granules (circled) were finely dispersed, and their average dimension was about 14 µm. Specifically, some interfacial cavities were observed in between the PBSL and dispersed starch, indicating a lack of strong interaction between the two components. This morphological observation justified the mechanical properties discussed below. For the fractured morphology of the LF/KF-added composites, the SEM image of the PB7ST3 (70/30) blend containing 10 phr of LF is shown in Figure 4(a). A clear fracturing phenomenon of broken fibers (circled) without pulled-out characteristics was observed. This morphology indicated good bonding between the LF and PBSL/starch matrix, which was attributed to the 7
ACCEPTED MANUSCRIPT good anchor provided by the alkaline treatment and the coarse fiber surfaces to the matrix molecules. However, interfacial de-bonding phenomenon (gap) was visible in some regions (arrowed) around the interfaces as well. Kaewtatip et al. [17] observed a similar morphology of
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LF dispersed within the thermoplastic starch matrix. Figure 4(b) shows the SEM morphology of the corresponding KF-added composite. Slightly different from that in the LF-added composite, the gap between the KF and blend matrix is barely observable (arrowed), suggesting stronger interaction between the KF and matrix. Based on the dispersion status of the LF/KF within the
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blend matrix, improvement in the stiffness of PBSL/starch blend is anticipated.
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3.3 Mechanical Properties
Figure 5 illustrates the effect of starch content and fiber type on the tensile properties of the prepared samples. Figure 5(a) shows that, first, the Young’s modulus increased significantly, from 210 MPa (neat PBSL) to 354 MPa (30 wt.% starch loaded), as the starch content increased. This increment was attributed to the rigid nature of the starch component. Second, after the individual
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addition of LF and KF into the PB7ST3 blend, the Young’s modulus increased further. The largest increment was found in the KF-added system, that is, an increase of up to 2.2-fold compared with neat PBSL. The higher aspect ratio of KF than that of LF was mainly responsible for this result.
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Salleh et al. [30] also reported a similar reinforcing capability of KF on the modulus of polyethylene. The good adhesion between the dispersed KF and matrix also had a key role in the
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observation. The Young’s modulus of the individual samples is summarized in Table 2. Figure 5(b) compares the tensile strength of the samples. The values progressively decreased with an increase in the starch content from 33.5 MPa (neat PBSL) to 17.4 MPa (30 wt.% starch loaded). This finding agreed with the result reported for the PBSU/starch system [11]. In general, the tensile strength obtained under a large deformation is more sensitive to crack defects than the Young’s modulus acquired at the initial deformation. The visible interfacial cavities between the two phases of PBSL and starch (discussed above) were considered defects. Therefore, the cracks eventually propagated under load, which caused the PBSL/starch blends to have lower strength at 8
ACCEPTED MANUSCRIPT higher amounts of starch. After the individual addition of LF/KF into the PB7ST3 blend, the tensile strength tended to level off without a continuous decrease. A measurable increase or decrease in the tensile strength of the thermoplastic starch/LF and polyethylene/KF composites
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was reported previously [17, 30], which demonstrated the importance of interfacial gaps to the tensile strength of the composites. Based on the SEM observation, the interfacial adhesion between the individual LF/KF and matrix was fine, and stress transfer should have partially occurred during the deformation process. No further deterioration in tensile strength was
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consequently observed. The elongation at break of the individual samples is compared in Figure 5(c). The elongation of PBSL decreased significantly after the addition of starch, mainly as a
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result of the rigid nature of starch. This decline was exacerbated with the further addition of LF and KF into the blend, which was attributed to the more rigid feature of both fibers than that of the PBSL. A similar finding was reported for thermoplastic starch/LF and polyethylene/KF composite systems [17, 30].
To further demonstrate the effects of adding starch and LF/KF on the mechanical properties
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of PBSL, a flexural experiment was performed, and the results are shown in Figure 6. Similar to the tensile modulus, the flexural modulus increased with the inclusion of starch into the PBSL matrix, from 540 MPa (neat PBSL) to 818 MPa (30 wt.% starch loaded), as depicted in Figure
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6(a). A significant increment was noticed with the further addition of individual LF and KF. The flexural modulus values of the samples are listed in Table 2. The largest improvement in the
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flexural modulus, which was attained in the KF-added composite, consisted of up to a twofold increase. This finding was again attributed to the high aspect ratio of KF, which had a better reinforcing effect than LF. Interestingly, the values for the flexural strength (Figure 6(b)) changed slightly with the addition of starch and LF/KF, in contrast to the evident decrease in the tensile strength. The different results resulted from the different characteristics of the two tests. The loading force tended to seal the crack defects to some degree under the bending deformation; thus, the flexural strength was not so sensitive to the crack defects, in contrast to the strength under tensile deformation. Apparently, the practical applications under different types of deformation 9
ACCEPTED MANUSCRIPT should be carefully considered to better exploit the merit of adding starch/plant fibers to polymer matrices. The effects of adding starch and LF/KF on the toughness of the samples were revealed by the
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Izod impact tests, and the results are shown in Figure 6(c). The impact strength of PBSL was 22.6 J/m. However, given the rigid nature of starch, the impact strength decreased by a large degree with the presence of starch. Enhancement of impact strength was not observed in the two composites because of the rigid characteristic of LF and KF. However, the impact strength did not
3.4 Rheological Properties
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absorb impact energy by deflecting the crack path.
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deteriorate further after the addition of individual LF and KF, mainly because of their ability to
The rheological properties, including the storage modulus and complex viscosity, of the representative samples were measured. Figure 7(a) shows the storage modulus (G’) as a function of angular frequency (ω). As anticipated, the modulus increased with the frequency for all the
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samples as a result of restricted molecular motion. Also observable is that the storage modulus of PBSL increased after the addition of starch and increased further with the presence of LF and KF. Furthermore, in the low frequency region, PBSL exhibited an evidently larger slope than those of
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the PBSL/starch blends and LF/KF-added composites. The flattened slope of the blends and composites indicates a pseudo-network (solid-like) structure development caused by the
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dispersed starch and LF/KF. The LF was noted to increase the modulus by a higher extent than KF. The reduced “effective” aspect ratio resulting from the aggregate formation from the coil-up of higher flexible KF under flow stress was considered for the current finding. For the “rigid rods” of LF (larger diameter), the coil-up formation was not easy to attain, therefore the solid LF essentially contributed to the higher storage modulus. A related observation was reported on the comparison of the orientation degree of rigid ZnO and flexible carbon nanofiber during deformation [31], in which rigid ZnO with a low aspect ratio surprisingly conferred higher orientation than flexible carbon nanofiber with a high aspect ratio. 10
ACCEPTED MANUSCRIPT The complex viscosity (η*) versus angular frequency (ω) of the samples is shown in Figure 7(b). The viscosity of PBSL decreased slightly with an increasing angular frequency, as commonly seen for polymer melts. A similar behavior was observed for the starch-incorporated
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blends, and the blends showed higher viscosity than PBSL over the tested frequencies. The viscosity of the LF/KF-added composites increased further. The Newtonian behavior of PBSL and its blends in the low frequency region became non-Newtonian after the LF and KF additions. The addition of individual LF/KF resulted in a more solid-like flow behavior of the sample. The
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LF induced a higher increase in the viscosity than KF. The larger diameter of LF had a significant
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role in the observation, as discussed above for the result on the storage modulus.
3.5 Thermal Properties
The crystallization (peak) temperature (Tc) and melting temperature (Tm) of PBSL in neat state and in blends/composites were determined by DSC. Figure 8(a) shows the cooling curves of the samples at 10 °C/min rate. The Tc of PBSL shifted from 73.0 °C to slightly lower
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temperatures after the addition of starch, and that of the PB7ST3 blend reached 71.9 °C. The further incorporation of LF and KF into the blend, by contrast, increased the Tc and the KF-added composite showed a higher Tc up to 75.3 °C. This increase in Tc indicated that LF and KF both
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assumed the role of a nucleation agent in the crystallization of PBSL. The Tm of PBSL remained almost unchanged in all the samples, as shown in the 10 °C/min heated curves of the pre-cooled
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samples in Figure 8(b). This observation indicated a limited change in the crystalline structure of PBSL with the addition of starch and LF/KF. The determined Tc and Tm of each sample are listed in Table 2.
Figure 9 illustrates the derivative TGA curves of PBSL and its blends/composites. PBSL
showed one degradation peak, whereas the blends/composites exhibited one minor low temperature peak and one major high temperature peak (indicating a two-staged degradation). The inset in the figure compares the derivative degradation curves of the neat components. Based on the peak temperature (Tmax, temperature of the fastest degradation rate), the thermal stability 11
ACCEPTED MANUSCRIPT of the components follows the sequence PBSL > KF > LF > starch. The degradation of starch started at around 250 °C, and that of PBSL started at above 300 °C. The low thermal stability of the carbohydrate compound (starch) was confirmed. The one Tmax of PBSL and the two Tmaxs of
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the blends/composites were determined and are listed in Table 2. The low Tmax of the blends/composites mainly represented the degradation of starch, whereas the high Tmax was associated with the degradation of PBSL. Both the Tmax values were slightly lower than those of
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neat starch (306 °C) and PBSL (386 °C).
3.6 Water Absorption
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A moist environment is essential for the decomposition of biodegradable polymers at the end of their life. The influence of adding starch and LF/KF on the water absorption of PBSL was evaluated, as shown by the data in Figure 10. Neat PBSL showed the lowest water absorption among the samples. The water absorption of neat PBSL increased with the immersion period and then leveled off (ca. 1 wt.%) within the test duration of 7 days. For the blends, a longer period is
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needed to attain the equilibrium water absorption. In general, water absorption increased with the starch loading in any period of immersion because of the natural high water absorption feature of starch. With the inclusion of individual LF and KF, the water absorption increased further and
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reached up to 5 wt.% for the LF-filled composite. This increase was mainly attributed to the porous structure and hydrophilic characteristics of plant fibers. This high water absorption
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property was expected to be beneficial in reducing the degradation time under compost soil. Thus, the current approach of including starch and LF/KF in the PBSL matrix to enhance their biodegradability was advantageous in expanding their applications.
4. Conclusions PBSL/starch blends were prepared through a melt-mixing process. Alkali-treated LF and KF with a high aspect ratio were added, individually, into the PB7ST3 blend to produce biocomposites. The blends showed higher rigidity than neat PBSL. Given the good bonding 12
ACCEPTED MANUSCRIPT between individual LF/KF and the blend matrix, the Young’s modulus of the biocomposites increased significantly by up to 2.2-fold compared with neat PBSL for the KF-reinforced system. The LF/KF tended to compensate for the deteriorated flexural strength from the rigid starch
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component in the blend, which was mainly attributed to their ability to absorb loading energy by deflecting the crack path. The PBSL/starch blends exhibited higher complex viscosity and dynamic storage modulus in the melt state than neat PBSL, and the properties further increased in the biocomposites. The composites showed an increment in the crystallization temperature of
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PBSL, confirming the surface nucleation effect of both fibers. The blends and biocomposites exhibited slightly lower thermal stability than neat PBSL. In addition, the water absorption of
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PBSL increased after the formation of the blends and further increased in the biocomposites. The current approach of including plant fibers in PBSL/starch blends to enhance their mechanical properties and biodegradability was advantageous in expanding the applications of PBSL.
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Acknowledgements
Grant-in-aid from the Ministry of Science and Technology (Taiwan, ROC) under the contract
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R.L. Whistler, J.N. Bemiller, E.F. Paschall, Starch: Chemistry and Technology, 2nd Edition,
2.
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20. P.M. Chou, M. Mariatti, A. Zulkifli, M. Todo, Changes in the crystallinity and mechanical properties of poly(L-lactic acid)/poly(butylene succinate-co-L-lactate) blend with annealing process, Polym. Bull. 67 (2011) 815-830.
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21. P.M. Chou, M. Mariatti, A. Zulkifli, M. Todo, Effect of secondary forces in the compatibility of two incompatible biodegradable polymers, Polym. Bull. 69 (2012) 455-469.
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22. V. Vannaladsaysy, M. Todo, T. Takayama, M. Jaafar, Z. Ahmad, K. Pasomsouk, Effects of lysine triisocyanate on the mode I fracture behavior of polymer blend of poly (L-lactic acid) and poly (butylene succinate-co-L-lactate), J. Mater. Sci. 44 (2009) 3006-3009.
23. V. Vilay, M. Mariatti, Z. Ahmad, K. Pasomsouk, M. Todo, Characterization of the mechanical and thermal properties and morphological behavior of biodegradable poly(L-lactide)/poly(ε-caprolactone) and poly(l-lactide)/poly(butylene succinate-co-L-lactate) polymeric blends, J. Appl. Polym. Sci. 114 (2009) 1784-1792. 24. V. Vilay, M. Mariatti, Z. Ahmad, K. Pasomsouk, M. Todo, Effect of PEO-PPO-PEO 15
ACCEPTED MANUSCRIPT copolymer on the mechanical and thermal properties and morphological behavior of biodegradable poly (L-lactic acid) (PLLA) and poly (butylene succinate-co-L-lactate) (PBSL) blends, Polym. Advan. Technol. 22 (2011) 1786-1793.
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25. Y. Wang, S.M. Chiao, T.-F. Hung, S.-Y. Yang, Improvement in toughness and heat resistance of poly(lactic acid)/polycarbonate blend through twin-screw blending: Influence of compatibilizer type, J. Appl. Polym. Sci. 125 (2012) E402-E412.
26. Y. Wang, S.M. Chiao, M.-T. Lai, S.-Y. Yang, The role of polycarbonate molecular weight in
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the poly(L-lactide) blends compatibilized with poly(butylene succinate-co-L-lactate), Polym. Eng. Sci. 53 (2013) 1171-1180.
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27. V.O.A. Tanobe, T.H.D. Sydenstricker, M. Munaro, S.C. Amico, A comprehensive characterization of chemically treated Brazilian sponge-gourds (luffa cylindrica), Polym. Test. 24 (2005) 474-482.
28. G. Siqueira, J. Bras, A. Dufresne, Luffa cylindrica as a lignocellulosic source of fiber, microfibrillated cellulose, and celllulose nanocrystals, BioResources 5 (2010) 727-740.
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29. L. Ghali, S. Msahli, M. Zidi, F. Sakli, Effect of pre-treatment of luffa fibres on the structural properties, Mater. Lett. 63 (2009) 61-63.
30. F.M. Salleh, A. Hassan, R. Yahya, A.D. Azzahari, Effects of extrusion temperature on the
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rheological, dynamic mechanical and tensile properties of kenaf fiber/HDPE composites, Compos. Part B: Engineering 58 (2014) 259-266.
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31. H. Koerner, J. Kelley, J. George, L. Drummy, P. Mirau, N.S. Bell, J.W.P. Hsu, R.A. Vaia, ZnO nanorod−thermoplastic polyurethane nanocomposites: Morphology and shape memory
performance, Macromolecules 42 (2009) 8933–8942.
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ACCEPTED MANUSCRIPT FIGURE CAPTIONS Fig. 1 FTIR spectra of untreated and treated LF/KF fibers. Fig. 2 SEM images of (a) untreated LF; (b) untreated KF; (c) treated LF; (d) treated KF; (e) LF
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appearance; (f) KF appearance. Fig. 3 SEM images of fractured surfaces of the blends with two magnifications: (a)(b) PB9ST1; (c)(d) PB8ST2; (e)(f) PB7ST3.
Fig. 4 SEM images of fractured surfaces of the biocomposites with two magnifications: (a)(b)
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PB7ST3-LF; (c)(d) PB7ST3-KF.
Fig. 5 Tensile properties of the samples: (a) Young’s modulus; (b) tensile strength; (c) elongation
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at break.
Fig. 6 Flexural/impact properties of the samples: (a) Flexural modulus; (b) flexural strength; (c) impact strength.
Fig. 7 Rheological properties of selected samples: (a) G’ vs. ω; (b) η∗ vs. ω. Fig. 8 (a) DSC cooling thermograms of the samples; (b) DSC heating thermograms of the
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pre-cooled samples.
Fig. 9 Derivative TGA-scanned curves of the blends and biocomposites. (Insets: scanned curves of neat components).
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Fig.10 Water absorption of the samples as a function of immersion duration.
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ACCEPTED MANUSCRIPT Table 1 Sample designation and formulation Composition
PBSL
Parts (wt.%)
PBSL
100
PB9ST1
PBSL/Starch
90/10
PB8ST2
PBSL/Starch
80/20
PB7ST3
PBSL/Starch
70/30
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Designation
PBSL/Starch/LF
70/30/10phr
PB7ST3-KF
PBSL/Starch/KF
70/30/10phr
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PB7ST3-LF
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ACCEPTED MANUSCRIPT Table 2 Representative physical properties of the samples Properties YM (MPa)
FM (MPa)
Tc (°C)
Tm (°C)
210
540
73.0
110.1
PB9ST1
247
603
72.8
109.7
PB8ST2
296
686
72.6
110.1
PB7ST3
354
818
71.9
110.0
PB7ST3-LF
404
1006
75.0
110.3
PB7ST3-KF
468
1062
75.3
110.0
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*Starch: Tmax = 306 °C.
386
19
297/385
294/382 299/384
305/385
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PBSL
Tmax (°C)*
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Samples
304/384
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untreated LF
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-1
1735 cm
Transmittance (a.u.)
treated LF
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untreated KF
2000
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treated KF
1800
1600
1400
1200
1000
Wavenumber (cm-1)
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Fig. 1
800
600
(b)
(c)
(d)
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(a)
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(f)
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2 mm
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(e)
2 mm
Fig. 2
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(a)
(b)
30 m
(d)
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(c)
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100 m
30 m
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100 m
(f)
(e)
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EP
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100 m
Fig. 3
30 m
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(a)
(b)
30 μm
(d)
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(c)
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100 μm
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100 μm
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Fig. 4
30 μm
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500
(a)
300
200
100
0 L PBS
S T1 PB9
S PB8
T2
- LF - KF ST3 S T3 S T3 PB7 PB7 PB7
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(b)
20
10
0 L PBS
PB9
ST1
120
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Tensile strength (MPa)
30
S T2 PB8
- LF -KF S T3 ST3 ST3 PB7 PB7 PB7
(c)
100 40
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Elongation at break (%)
110
30 20 10
0
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L PBS
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Young's modulus (MPa)
400
S T1 PB9
S PB8
T2
PB7
- LF - KF ST3 S T3 S T3 PB7 PB7
Fig. 5
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1200
(a) Flexural modulus (MPa)
1000
800
400
200
0 L PBS
S PB9
T1
S PB8
T2
S PB7
T3 -LF -KF ST3 S T3 PB7 PB7
40
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30
20
10
0 PBS
L
S PB9
25
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Flexural strength (MPa)
(b)
T1
S PB8
T2
S PB7
T3 -LF -KF ST3 S T3 PB7 PB7
(c)
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Impact strength (J/m)
20
15
10
5
0
L
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PBS
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600
S PB9
T1
S PB8
T2
S PB7
Fig. 6
T3 -LF -KF ST3 S T3 PB7 PB7
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1e+5
1e+3
1e+2
PBSL PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
1e+1
1e+0
1
10
100
PBSL PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
(b)
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1e+4
1e+3
1e+2
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Complex viscosity, η* (Pa s)
Angular frequency, ω (rad/sec)
1
10
Angular frequency, ω (rad/sec)
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Fig. 7
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Storage modulus, G' (Pa)
(a) 1e+4
100
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(a)
Tc
PBSL PB9ST1 Endo up
PB8ST2
PB7ST3-LF PB7ST3-KF
50
60
70
Temperature (oC)
80
90
(b)
Tm
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PB9ST1 PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
80
90
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Endo up
PBSL
100
110
Temperature (oC)
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Fig. 8
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PB7ST3
120
130
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6 PBSL PB9ST1 PB8ST2 PB7ST3 PB7ST3-LF
4
PB7ST3-KF
3
2
1
0 1
2
3
4
5
6
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Immersion period (day)
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Fig. 10
7
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0
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Water absorption (wt.%)
5
S0142-9418(15)30154-9
DOI:
10.1016/j.polymertesting.2016.01.015
Reference:
POTE 4575
To appear in:
Polymer Testing
Received Date: 2 November 2015 Accepted Date: 20 January 2016
Please cite this article as: S.-M. Lai, Y.-H. Kao, Y.-K. Liu, F.-C. Chiu, Preparation and Properties of luffa fiber- and kenaf fiber-filled poly(butylene succinate-co-lactate)/starch blend-based biocomposites, Polymer Testing (2016), doi: 10.1016/j.polymertesting.2016.01.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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2.5 2.5
1.5
PBSL ST LF KF
2.0
1.5
1.0
0.5
0.0
1.0
200
300
400
Temperature (oC)
0.0
250
300
350
400
500
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Temperature (oC)
450
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Fig. 9
500
600
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0.5
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Derivative weight (%/oC)
2.0
Derivative weight ( %/oC)
PBSL PB9ST1 PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
550
ACCEPTED MANUSCRIPT Material Properties Preparation and Properties of luffa fiber- and kenaf fiber-filled poly(butylene succinate-co-lactate)/starch blend-based biocomposites
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Sun-Mou Lai1, Yu-Hsiang Kao2, Yu-Kuo Liu3, Fang-Chyou Chiu2* 1. Department of Chemical and Materials Engineering, National I-Lan University, I-Lan 260, Taiwan, ROC
2. Department of Chemical and Materials Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC
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3. Graduate Institute of Biochemical and Biomedical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC
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Abstract
Biodegradable poly(butylene succinate-co-lactate) (PBSL)/starch blends that contain various amounts of starch were prepared. In addition, luffa fiber (LF) and kenaf fiber (KF) were incorporated, individually, into PBSL/starch (70/30) blend to achieve biocomposites. The LF and
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KF were treated with NaOH(aq) prior to their addition to the blend. The Young’s modulus and flexural modulus of PBSL increased with the addition of starch and increased further after the formation of the biocomposites. The highest Young’s modulus increment, which was found in the
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KF-added system, was up to a 2.2-fold increase compared with neat PBSL. The tensile/flexural/impact strength of PBSL declined after the formation of the blends. With the
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further addition of LF/KF, the said properties leveled off. The blends exhibited higher complex viscosity and dynamic storage modulus in the melt state than the neat PBSL, and the values further increased in the biocomposites. The crystallization temperature of PBSL slightly decreased in the blends. By contrast, the biocomposites showed an increment in PBSL crystallization temperature, from 73.0 °C (PBSL) to 75.3 °C (KF-added composite), thereby confirming the surface nucleation effect of LF/KF. The blends showed a higher degree of water absorption than PBSL. The formation of biocomposites led to an even higher degree of water absorption. The current approach of including LF/KF in the PBSL/starch blend to enhance the 1
ACCEPTED MANUSCRIPT rigidity and biodegradability was advantageous in expanding the applications of PBSL.
Keywords: poly(butylene succinate-co-lactate); starch; luffa fiber; kenaf fiber; biocomposites;
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physical properties *Corresponding author. Tel: +886-3-2118800 ext.5297; fax: +886-3-2118668.
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E-mail address: [email protected] (F. C. Chiu)
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ACCEPTED MANUSCRIPT 1. Introduction Given growing environmental concerns, biodegradable polymers with ecological advantages over petroleum-based polymers in terms of sustainability have received much attention in
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academia and industry in the past two decades. Several renowned companies, such as Natureworks, Dow, Procter & Gamble, and others, have begun developing several types of biodegradable polymers in the commercial market, including poly(lactic acid) (PLA), polycaprolactone (PCL) and poly(butylene succinate) (PBSU). In addition, biomass-based
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polymers made from natural resources have also been the development target to reduce carbon dioxide emissions from the manufacturing process. In particular, these biopolymers, such as
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starch, poly(hydroxy alkanoate) (PHA) and cellulose materials, are considered cost-competitive with commodity plastics. Thus, their potential applications are anticipated to continuously expand for years to come.
The abundant natural food source from most plants, starch is primarily composed of amylose and amylopectin. Amylose is a linear polymer with α-1,4-linked glucose units, whereas
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amylopectin is a branched polymer with α-1,4-linked chains connected by 1,6-linkages [1]. Griffin [2] pioneered the blending of granular starch with plastics, which incorporated the eco-feature in conventional polymers. However, starch failed to be well-dispersed in the plastic
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matrices because of the inherent hydrogen bonding between the adjacent molecules in starch and the non-efficient mixing processes. Therefore, the gelatinization technology to prepare
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thermoplastic starch was developed, with addition of water, plasticizers, or both [3]. In addition to starch, plant fibers have been utilized as eco-reinforcement to develop polymer composites. Among the examined plant fibers, luffa fiber (LF) and kenaf fiber (KF) are two of the suitable choices as reinforcing fillers for preparing polymer composites. LF sourced from luffa cylindrica, a subtropical plant, is readily available in a vascular form [4]. KF, obtained from a natural tropical plant, is also globally available in a cylindrical structure [5]. Given their low density, nonabrasive nature, biodegradability and low cost, starch and plant fibers are considered to be attractive counterparts for manufacturing biopolymer blends and biocomposites. Several recent studies 3
ACCEPTED MANUSCRIPT have investigated biodegradable polymers filled with starch or different plant fibers for the formation of blends and biocomposites, such as the blends of PLA/starch [6–9], PCL/starch [10] and PBSU/starch [11–13], and the composites of PLA/corn fiber [14], PLA/bamboo fiber [15],
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PBSU/curaua fiber [16], starch/LF [17] and starch/KF [5]. Poly(butylene succinate-co-lactate) (PBSL) is a newly developed aliphatic polyester that possesses biodegradable characteristics. It is synthesized by the polycondensation of 1,4 butanediol, succinic acid, and L-lactic acid [18, 19]. Hitherto, only a limited number of
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biodegradable PBSL blends and composites have been investigated [18–26]. Among the studies, none has been conducted on PBSL/starch blends and plant fiber-filled PBSL composites. From
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academic and practical viewpoints, blending starch with PBSL and then adding plant fibers to the blends should merit a detailed study and is an attractive approach to enhance the performance of PBSL. For instance, LF has been reported to increase the tensile strength of thermoplastic starch (TPS) by up to twofold [17], and KF has been added to cassava starch as reinforcing filler [5]. In the present work, both LF and KF were chosen to compare their reinforcing capability for
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the PBSL/starch blend. The melt-mixed samples were characterized by differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA). The rheological/water absorption properties and fractured multi-phasic morphology of the samples were also evaluated. In addition,
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the influences of incorporating starch and LF/KF on the tensile and flexural properties of PBSL were measured. This study should facilitate further understanding in developing novel
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biopolymer composites for our environment.
2. Experimental
2.1 Materials The PBSL utilized in this study was a product of Mitsubishi Chemical Corporation, Japan, under the trade name of AZ71T. Its density was 1.26 g/cm3, and its lactate content was ca. 3 mol.%. Cassava starch of edible grade grown in Taiwan was used as the counterpart for preparing 4
ACCEPTED MANUSCRIPT the PBSA blends. LF and KF grown in China and India, respectively, were utilized as the reinforcements for preparing the composites.
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2.2 Preparation of the Samples Both LF and KF fibers were surface pretreated to improve their compatibility with the polymer matrix. The chopped LF and KF were first washed with distilled water to remove surface impurities and then allowed to dry in an oven at 70 °C for 1 day. The washed/dried fibers were
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alkali-treated with NaOH by immersion in 1% aqueous NaOH solution and then stirred for 1 h at room temperature. The alkali-treated LF and KF were thoroughly washed with distilled water
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until pH 7 was reached and allowed to dry in an oven. The fibers were then pulverized and screened with a sieve size of 18 mesh into a powder form to blend them with the polymer matrix. Prior to the mixing procedure, PBSL, starch and treated LF/KF were dried in a vacuum oven to remove the absorbed moisture. The mixing was conducted in an intermeshing twin-screw extruder (SHJ-20B, L/D = 40) in co-rotating mode. The extruder was operated at a screw speed of
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360 rpm and barrel temperatures ranging from 90 °C to 120 °C. The formulation and sample designations are listed in Table 1. The pelletized samples of different formulations were fed into an injection molding machine to prepare specimens for further characterization of their
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mechanical properties.
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2.3 Measurements
A Fourier transform infrared (FTIR) spectrometer was used to confirm the surface treatment
on LF and KF. The spectra of treated fibers were recorded on a Bruker Optics Tensor 27 system at a resolution of 4 cm-1 for 32 scans. The appearance of 18-mesh sieved LF and KF was observed using a scanning electron microscope (SEM, Hitachi S-3000N). The phase morphology of the liquid nitrogen cryo-fractured samples was observed through the same SEM. All the specimens were sputtered with gold before observation. The crystallization temperatures (Tc) of the samples were measured using a differential scanning calorimeter (DSC, TA Q10) at a cooling 5
ACCEPTED MANUSCRIPT rate of 10 °C/min from 140 °C. The melting temperatures (Tm) of the pre-cooled samples were then determined at a heating rate of 10 °C/min from 25 °C to 140 °C. A thermogravimetric analyzer (TGA, TA Q50) was used to evaluate the thermal stability of the samples. The samples
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were heated from room temperature to 700 °C at 10 °C/min under nitrogen environment. To avoid the water absorption effect on the scanned results, the peak temperature of the derivative TGA curve was used as an indication for the thermal stability comparison.
The tensile properties, including the Young’s modulus, tensile strength, and elongation at
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break, of the dumbbell-shaped samples (according to ASTM-D638) were measured using a Gotech AI-3000 system at a crosshead speed of 5 mm/min. Flexural property tests were
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conducted per ASTM D790 at a crosshead speed of 10 mm/min using the same Gotech AI-3000 system. Izod impact strength tests (Ceast Resil 5,5) were conducted on notched specimens in accordance with ASTM D256. The acquired mechanical properties were average values of five specimens of the same formulation. The rheological properties of the samples were determined at 140 °C using an Anton Paar rheometer (MCR 101) at a strain of 1% as a function of angular
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frequencies. The water absorption was evaluated by determining the weight gained by the dried samples after their immersion in distilled water for different periods. The equation used is as follows:
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Water absorption (%) = (Wt − Wo)/Wo × 100
where Wt is the weight of the sample treated for t day(s) after drying excess distilled water, and
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Wo is the sample weight prior to the test.
3. Results and discussion
3.1 Alkaline Treatment of LF and KF Figure 1 shows the variations in the FTIR spectra of the untreated and treated LF and KF. As commonly known, plant fibers are mainly composed of cellulose, hemi-cellulose and lignin. The absorption peaks between 1500 and 1000 cm-1, including CH2 bending, O-H bending, C-OR 6
ACCEPTED MANUSCRIPT stretching and anhydroglucose ring, are associated with the internal structure of cellulose fibers [27]. In addition, the absorption of C=O vibration around 1735 cm-1, which is attributed to the carbonyl and acetyl groups in the hemi-cellulose and lignin [28], was observed in the untreated
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LF and KF. The C=O absorption disappeared accordingly in the treated LF/KF, which indicated the successful removal of hemi-cellulose and lignin by NaOH(aq) treatment. The difference in surface topography between the untreated and treated LF/KF are depicted in the SEM images of Figure 2. The waxy substance and impurity particles on the untreated fiber surfaces were
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evidently removed after the alkaline treatment, as shown in the comparisons in Figures 2(a) and (c) and in Figures 2(b) and (d). The treated fibers also exhibited a bundle-like structure caused by
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the removal of semi-cellulose and lignin, as reported by Ghali et al. [29]. Overall, with the successful NaOH(aq) treatment, the adhesion between LF/KF and polymer matrix was anticipated to be improved. Furthermore, the high aspect ratios of LF and KF were also noted. The aspect ratio of KF is about 31, which is more than twofold of that of LF (ca. 14), as depicted in Figures 2(e) and (f). The surface treatment and the high aspect ratio of LF and KF are beneficial to the
mechanical properties.
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3.2 Phase Morphology
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reinforcing efficiency of the PBSL/starch blends, as discussed in the following section on
The fractured surfaces of the PBSL/starch blends containing various amounts of starch are
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shown in the SEM images of Figure 3. The starch granules (circled) were finely dispersed, and their average dimension was about 14 µm. Specifically, some interfacial cavities were observed in between the PBSL and dispersed starch, indicating a lack of strong interaction between the two components. This morphological observation justified the mechanical properties discussed below. For the fractured morphology of the LF/KF-added composites, the SEM image of the PB7ST3 (70/30) blend containing 10 phr of LF is shown in Figure 4(a). A clear fracturing phenomenon of broken fibers (circled) without pulled-out characteristics was observed. This morphology indicated good bonding between the LF and PBSL/starch matrix, which was attributed to the 7
ACCEPTED MANUSCRIPT good anchor provided by the alkaline treatment and the coarse fiber surfaces to the matrix molecules. However, interfacial de-bonding phenomenon (gap) was visible in some regions (arrowed) around the interfaces as well. Kaewtatip et al. [17] observed a similar morphology of
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LF dispersed within the thermoplastic starch matrix. Figure 4(b) shows the SEM morphology of the corresponding KF-added composite. Slightly different from that in the LF-added composite, the gap between the KF and blend matrix is barely observable (arrowed), suggesting stronger interaction between the KF and matrix. Based on the dispersion status of the LF/KF within the
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blend matrix, improvement in the stiffness of PBSL/starch blend is anticipated.
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3.3 Mechanical Properties
Figure 5 illustrates the effect of starch content and fiber type on the tensile properties of the prepared samples. Figure 5(a) shows that, first, the Young’s modulus increased significantly, from 210 MPa (neat PBSL) to 354 MPa (30 wt.% starch loaded), as the starch content increased. This increment was attributed to the rigid nature of the starch component. Second, after the individual
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addition of LF and KF into the PB7ST3 blend, the Young’s modulus increased further. The largest increment was found in the KF-added system, that is, an increase of up to 2.2-fold compared with neat PBSL. The higher aspect ratio of KF than that of LF was mainly responsible for this result.
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Salleh et al. [30] also reported a similar reinforcing capability of KF on the modulus of polyethylene. The good adhesion between the dispersed KF and matrix also had a key role in the
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observation. The Young’s modulus of the individual samples is summarized in Table 2. Figure 5(b) compares the tensile strength of the samples. The values progressively decreased with an increase in the starch content from 33.5 MPa (neat PBSL) to 17.4 MPa (30 wt.% starch loaded). This finding agreed with the result reported for the PBSU/starch system [11]. In general, the tensile strength obtained under a large deformation is more sensitive to crack defects than the Young’s modulus acquired at the initial deformation. The visible interfacial cavities between the two phases of PBSL and starch (discussed above) were considered defects. Therefore, the cracks eventually propagated under load, which caused the PBSL/starch blends to have lower strength at 8
ACCEPTED MANUSCRIPT higher amounts of starch. After the individual addition of LF/KF into the PB7ST3 blend, the tensile strength tended to level off without a continuous decrease. A measurable increase or decrease in the tensile strength of the thermoplastic starch/LF and polyethylene/KF composites
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was reported previously [17, 30], which demonstrated the importance of interfacial gaps to the tensile strength of the composites. Based on the SEM observation, the interfacial adhesion between the individual LF/KF and matrix was fine, and stress transfer should have partially occurred during the deformation process. No further deterioration in tensile strength was
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consequently observed. The elongation at break of the individual samples is compared in Figure 5(c). The elongation of PBSL decreased significantly after the addition of starch, mainly as a
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result of the rigid nature of starch. This decline was exacerbated with the further addition of LF and KF into the blend, which was attributed to the more rigid feature of both fibers than that of the PBSL. A similar finding was reported for thermoplastic starch/LF and polyethylene/KF composite systems [17, 30].
To further demonstrate the effects of adding starch and LF/KF on the mechanical properties
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of PBSL, a flexural experiment was performed, and the results are shown in Figure 6. Similar to the tensile modulus, the flexural modulus increased with the inclusion of starch into the PBSL matrix, from 540 MPa (neat PBSL) to 818 MPa (30 wt.% starch loaded), as depicted in Figure
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6(a). A significant increment was noticed with the further addition of individual LF and KF. The flexural modulus values of the samples are listed in Table 2. The largest improvement in the
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flexural modulus, which was attained in the KF-added composite, consisted of up to a twofold increase. This finding was again attributed to the high aspect ratio of KF, which had a better reinforcing effect than LF. Interestingly, the values for the flexural strength (Figure 6(b)) changed slightly with the addition of starch and LF/KF, in contrast to the evident decrease in the tensile strength. The different results resulted from the different characteristics of the two tests. The loading force tended to seal the crack defects to some degree under the bending deformation; thus, the flexural strength was not so sensitive to the crack defects, in contrast to the strength under tensile deformation. Apparently, the practical applications under different types of deformation 9
ACCEPTED MANUSCRIPT should be carefully considered to better exploit the merit of adding starch/plant fibers to polymer matrices. The effects of adding starch and LF/KF on the toughness of the samples were revealed by the
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Izod impact tests, and the results are shown in Figure 6(c). The impact strength of PBSL was 22.6 J/m. However, given the rigid nature of starch, the impact strength decreased by a large degree with the presence of starch. Enhancement of impact strength was not observed in the two composites because of the rigid characteristic of LF and KF. However, the impact strength did not
3.4 Rheological Properties
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absorb impact energy by deflecting the crack path.
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deteriorate further after the addition of individual LF and KF, mainly because of their ability to
The rheological properties, including the storage modulus and complex viscosity, of the representative samples were measured. Figure 7(a) shows the storage modulus (G’) as a function of angular frequency (ω). As anticipated, the modulus increased with the frequency for all the
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samples as a result of restricted molecular motion. Also observable is that the storage modulus of PBSL increased after the addition of starch and increased further with the presence of LF and KF. Furthermore, in the low frequency region, PBSL exhibited an evidently larger slope than those of
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the PBSL/starch blends and LF/KF-added composites. The flattened slope of the blends and composites indicates a pseudo-network (solid-like) structure development caused by the
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dispersed starch and LF/KF. The LF was noted to increase the modulus by a higher extent than KF. The reduced “effective” aspect ratio resulting from the aggregate formation from the coil-up of higher flexible KF under flow stress was considered for the current finding. For the “rigid rods” of LF (larger diameter), the coil-up formation was not easy to attain, therefore the solid LF essentially contributed to the higher storage modulus. A related observation was reported on the comparison of the orientation degree of rigid ZnO and flexible carbon nanofiber during deformation [31], in which rigid ZnO with a low aspect ratio surprisingly conferred higher orientation than flexible carbon nanofiber with a high aspect ratio. 10
ACCEPTED MANUSCRIPT The complex viscosity (η*) versus angular frequency (ω) of the samples is shown in Figure 7(b). The viscosity of PBSL decreased slightly with an increasing angular frequency, as commonly seen for polymer melts. A similar behavior was observed for the starch-incorporated
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blends, and the blends showed higher viscosity than PBSL over the tested frequencies. The viscosity of the LF/KF-added composites increased further. The Newtonian behavior of PBSL and its blends in the low frequency region became non-Newtonian after the LF and KF additions. The addition of individual LF/KF resulted in a more solid-like flow behavior of the sample. The
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LF induced a higher increase in the viscosity than KF. The larger diameter of LF had a significant
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role in the observation, as discussed above for the result on the storage modulus.
3.5 Thermal Properties
The crystallization (peak) temperature (Tc) and melting temperature (Tm) of PBSL in neat state and in blends/composites were determined by DSC. Figure 8(a) shows the cooling curves of the samples at 10 °C/min rate. The Tc of PBSL shifted from 73.0 °C to slightly lower
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temperatures after the addition of starch, and that of the PB7ST3 blend reached 71.9 °C. The further incorporation of LF and KF into the blend, by contrast, increased the Tc and the KF-added composite showed a higher Tc up to 75.3 °C. This increase in Tc indicated that LF and KF both
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assumed the role of a nucleation agent in the crystallization of PBSL. The Tm of PBSL remained almost unchanged in all the samples, as shown in the 10 °C/min heated curves of the pre-cooled
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samples in Figure 8(b). This observation indicated a limited change in the crystalline structure of PBSL with the addition of starch and LF/KF. The determined Tc and Tm of each sample are listed in Table 2.
Figure 9 illustrates the derivative TGA curves of PBSL and its blends/composites. PBSL
showed one degradation peak, whereas the blends/composites exhibited one minor low temperature peak and one major high temperature peak (indicating a two-staged degradation). The inset in the figure compares the derivative degradation curves of the neat components. Based on the peak temperature (Tmax, temperature of the fastest degradation rate), the thermal stability 11
ACCEPTED MANUSCRIPT of the components follows the sequence PBSL > KF > LF > starch. The degradation of starch started at around 250 °C, and that of PBSL started at above 300 °C. The low thermal stability of the carbohydrate compound (starch) was confirmed. The one Tmax of PBSL and the two Tmaxs of
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the blends/composites were determined and are listed in Table 2. The low Tmax of the blends/composites mainly represented the degradation of starch, whereas the high Tmax was associated with the degradation of PBSL. Both the Tmax values were slightly lower than those of
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neat starch (306 °C) and PBSL (386 °C).
3.6 Water Absorption
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A moist environment is essential for the decomposition of biodegradable polymers at the end of their life. The influence of adding starch and LF/KF on the water absorption of PBSL was evaluated, as shown by the data in Figure 10. Neat PBSL showed the lowest water absorption among the samples. The water absorption of neat PBSL increased with the immersion period and then leveled off (ca. 1 wt.%) within the test duration of 7 days. For the blends, a longer period is
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needed to attain the equilibrium water absorption. In general, water absorption increased with the starch loading in any period of immersion because of the natural high water absorption feature of starch. With the inclusion of individual LF and KF, the water absorption increased further and
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reached up to 5 wt.% for the LF-filled composite. This increase was mainly attributed to the porous structure and hydrophilic characteristics of plant fibers. This high water absorption
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property was expected to be beneficial in reducing the degradation time under compost soil. Thus, the current approach of including starch and LF/KF in the PBSL matrix to enhance their biodegradability was advantageous in expanding their applications.
4. Conclusions PBSL/starch blends were prepared through a melt-mixing process. Alkali-treated LF and KF with a high aspect ratio were added, individually, into the PB7ST3 blend to produce biocomposites. The blends showed higher rigidity than neat PBSL. Given the good bonding 12
ACCEPTED MANUSCRIPT between individual LF/KF and the blend matrix, the Young’s modulus of the biocomposites increased significantly by up to 2.2-fold compared with neat PBSL for the KF-reinforced system. The LF/KF tended to compensate for the deteriorated flexural strength from the rigid starch
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component in the blend, which was mainly attributed to their ability to absorb loading energy by deflecting the crack path. The PBSL/starch blends exhibited higher complex viscosity and dynamic storage modulus in the melt state than neat PBSL, and the properties further increased in the biocomposites. The composites showed an increment in the crystallization temperature of
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PBSL, confirming the surface nucleation effect of both fibers. The blends and biocomposites exhibited slightly lower thermal stability than neat PBSL. In addition, the water absorption of
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PBSL increased after the formation of the blends and further increased in the biocomposites. The current approach of including plant fibers in PBSL/starch blends to enhance their mechanical properties and biodegradability was advantageous in expanding the applications of PBSL.
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Acknowledgements
Grant-in-aid from the Ministry of Science and Technology (Taiwan, ROC) under the contract
1.
R.L. Whistler, J.N. Bemiller, E.F. Paschall, Starch: Chemistry and Technology, 2nd Edition,
2.
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16. Frollini, Elisabete (1); Bartolucci, Nadia (1, 2); Sisti, Laura (2); Celli, Annamaria (2), Biocomposites based on poly(butylene succinate) and curaua: Mechanical and morphological properties, Polym. Test. 45 (2015) 168-173.
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20. P.M. Chou, M. Mariatti, A. Zulkifli, M. Todo, Changes in the crystallinity and mechanical properties of poly(L-lactic acid)/poly(butylene succinate-co-L-lactate) blend with annealing process, Polym. Bull. 67 (2011) 815-830.
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21. P.M. Chou, M. Mariatti, A. Zulkifli, M. Todo, Effect of secondary forces in the compatibility of two incompatible biodegradable polymers, Polym. Bull. 69 (2012) 455-469.
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22. V. Vannaladsaysy, M. Todo, T. Takayama, M. Jaafar, Z. Ahmad, K. Pasomsouk, Effects of lysine triisocyanate on the mode I fracture behavior of polymer blend of poly (L-lactic acid) and poly (butylene succinate-co-L-lactate), J. Mater. Sci. 44 (2009) 3006-3009.
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ACCEPTED MANUSCRIPT copolymer on the mechanical and thermal properties and morphological behavior of biodegradable poly (L-lactic acid) (PLLA) and poly (butylene succinate-co-L-lactate) (PBSL) blends, Polym. Advan. Technol. 22 (2011) 1786-1793.
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25. Y. Wang, S.M. Chiao, T.-F. Hung, S.-Y. Yang, Improvement in toughness and heat resistance of poly(lactic acid)/polycarbonate blend through twin-screw blending: Influence of compatibilizer type, J. Appl. Polym. Sci. 125 (2012) E402-E412.
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28. G. Siqueira, J. Bras, A. Dufresne, Luffa cylindrica as a lignocellulosic source of fiber, microfibrillated cellulose, and celllulose nanocrystals, BioResources 5 (2010) 727-740.
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ACCEPTED MANUSCRIPT FIGURE CAPTIONS Fig. 1 FTIR spectra of untreated and treated LF/KF fibers. Fig. 2 SEM images of (a) untreated LF; (b) untreated KF; (c) treated LF; (d) treated KF; (e) LF
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appearance; (f) KF appearance. Fig. 3 SEM images of fractured surfaces of the blends with two magnifications: (a)(b) PB9ST1; (c)(d) PB8ST2; (e)(f) PB7ST3.
Fig. 4 SEM images of fractured surfaces of the biocomposites with two magnifications: (a)(b)
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PB7ST3-LF; (c)(d) PB7ST3-KF.
Fig. 5 Tensile properties of the samples: (a) Young’s modulus; (b) tensile strength; (c) elongation
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at break.
Fig. 6 Flexural/impact properties of the samples: (a) Flexural modulus; (b) flexural strength; (c) impact strength.
Fig. 7 Rheological properties of selected samples: (a) G’ vs. ω; (b) η∗ vs. ω. Fig. 8 (a) DSC cooling thermograms of the samples; (b) DSC heating thermograms of the
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pre-cooled samples.
Fig. 9 Derivative TGA-scanned curves of the blends and biocomposites. (Insets: scanned curves of neat components).
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Fig.10 Water absorption of the samples as a function of immersion duration.
17
ACCEPTED MANUSCRIPT Table 1 Sample designation and formulation Composition
PBSL
Parts (wt.%)
PBSL
100
PB9ST1
PBSL/Starch
90/10
PB8ST2
PBSL/Starch
80/20
PB7ST3
PBSL/Starch
70/30
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Designation
PBSL/Starch/LF
70/30/10phr
PB7ST3-KF
PBSL/Starch/KF
70/30/10phr
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PB7ST3-LF
18
ACCEPTED MANUSCRIPT Table 2 Representative physical properties of the samples Properties YM (MPa)
FM (MPa)
Tc (°C)
Tm (°C)
210
540
73.0
110.1
PB9ST1
247
603
72.8
109.7
PB8ST2
296
686
72.6
110.1
PB7ST3
354
818
71.9
110.0
PB7ST3-LF
404
1006
75.0
110.3
PB7ST3-KF
468
1062
75.3
110.0
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*Starch: Tmax = 306 °C.
386
19
297/385
294/382 299/384
305/385
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PBSL
Tmax (°C)*
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Samples
304/384
ACCEPTED MANUSCRIPT
untreated LF
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-1
1735 cm
Transmittance (a.u.)
treated LF
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untreated KF
2000
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treated KF
1800
1600
1400
1200
1000
Wavenumber (cm-1)
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Fig. 1
800
600
(b)
(c)
(d)
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(a)
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ACCEPTED MANUSCRIPT
(f)
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2 mm
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(e)
2 mm
Fig. 2
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(a)
(b)
30 m
(d)
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(c)
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100 m
30 m
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100 m
(f)
(e)
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EP
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100 m
Fig. 3
30 m
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(a)
(b)
30 μm
(d)
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(c)
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100 μm
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100 μm
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Fig. 4
30 μm
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500
(a)
300
200
100
0 L PBS
S T1 PB9
S PB8
T2
- LF - KF ST3 S T3 S T3 PB7 PB7 PB7
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(b)
20
10
0 L PBS
PB9
ST1
120
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Tensile strength (MPa)
30
S T2 PB8
- LF -KF S T3 ST3 ST3 PB7 PB7 PB7
(c)
100 40
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Elongation at break (%)
110
30 20 10
0
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L PBS
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Young's modulus (MPa)
400
S T1 PB9
S PB8
T2
PB7
- LF - KF ST3 S T3 S T3 PB7 PB7
Fig. 5
ACCEPTED MANUSCRIPT
1200
(a) Flexural modulus (MPa)
1000
800
400
200
0 L PBS
S PB9
T1
S PB8
T2
S PB7
T3 -LF -KF ST3 S T3 PB7 PB7
40
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30
20
10
0 PBS
L
S PB9
25
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Flexural strength (MPa)
(b)
T1
S PB8
T2
S PB7
T3 -LF -KF ST3 S T3 PB7 PB7
(c)
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Impact strength (J/m)
20
15
10
5
0
L
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PBS
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600
S PB9
T1
S PB8
T2
S PB7
Fig. 6
T3 -LF -KF ST3 S T3 PB7 PB7
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1e+5
1e+3
1e+2
PBSL PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
1e+1
1e+0
1
10
100
PBSL PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
(b)
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1e+4
1e+3
1e+2
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Complex viscosity, η* (Pa s)
Angular frequency, ω (rad/sec)
1
10
Angular frequency, ω (rad/sec)
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Fig. 7
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Storage modulus, G' (Pa)
(a) 1e+4
100
ACCEPTED MANUSCRIPT
(a)
Tc
PBSL PB9ST1 Endo up
PB8ST2
PB7ST3-LF PB7ST3-KF
50
60
70
Temperature (oC)
80
90
(b)
Tm
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PB9ST1 PB8ST2 PB7ST3 PB7ST3-LF PB7ST3-KF
80
90
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Endo up
PBSL
100
110
Temperature (oC)
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Fig. 8
AC C
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PB7ST3
120
130
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6 PBSL PB9ST1 PB8ST2 PB7ST3 PB7ST3-LF
4
PB7ST3-KF
3
2
1
0 1
2
3
4
5
6
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Immersion period (day)
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Fig. 10
7
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0
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Water absorption (wt.%)
5