Injection Molded Sustainable Biocomposites From Poly(butylene

Research Article pubs.acs.org/journal/ascecg

Injection Molded Sustainable Biocomposites From Poly(butylene succinate) Bioplastic and Perennial Grass Rajendran Muthuraj,†,‡ Manjusri Misra,†,‡ and Amar Kumar Mohanty*,†,‡ †

School of Engineering, Thornbrough Building, University of Guelph, 50 Stone Road E, Guelph, Ontario N1G2W1, Canada Bioproducts Discovery and Development Centre (BDDC), Crop Science Building, Department of Plant Agriculture, University of Guelph, 50 Stone Road E, Guelph, Ontario N1G2W1, Canada

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ABSTRACT: Biocomposites from poly(butylene succinate) (PBS) and perennial grass (miscanthus fibers) were successfully prepared by extrusion and injection molding methods with different fiber loadings. The tensile strength of uncompatibilized PBS/miscanthus composites was much lower compared to that of neat PBS. Unlike tensile strength, the flexural and impact strengths were significantly enhanced after incorporation of miscanthus fibers into the PBS matrix. The enhanced flexural strength was attributed to the reinforcing effect of miscathus fibers. The fiber pull-out mechanism is likely responsible for the observed impact strength improvement. Addition of 5 wt % maleic anhydride (MAH) grafted PBS (MAH-g-PBS) into PBS composites showed a significant improvement in tensile and flexural strength compared to the corresponding uncompatibilized composites and neat matrix. For example, the PBS composites with 50 wt % miscanthus fiber and 5 wt % MAH-g-PBS resulted in 22, 139, and 47% improvements in tensile, flexural, and impact strength compared to neat PBS. These improvements were attributed to the enhanced interfacial interaction between the components, as confirmed by adhesion parameter values and by surface morphological analysis. The load-bearing capacity of the compatibilized and uncompatibilized PBS/miscanthus composites was analyzed using a mathematical model. Overall, this study provides an option for preparing a sustainable biocomposite with superior mechanical and thermo-mechanical properties. KEYWORDS: Miscanthus fibers, Biodegradability, Biocomposites, Compatibilizer, Mechanical properties, Interfacial adhesion

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based monomer with biobased content of ∼54%.3 These attractive properties may be responsible for the increase in the use of PBS based green composites for various applications.2 The cost of PBS is more expensive than conventional nonbiodegradable polymers.6 It is well-known that this shortcoming can be overcome by blending with inexpensive natural fibers/fillers while maintaining or enhancing the matrix performance. Therefore, a variety of natural fibers are used to fabricate composites with the PBS matrix.3 In addition to being inexpensive, natural fibers are renewable, sustainable, biodegradable, abundant, and have good specific properties and low density compared to synthetic fibers such as glass fiber.7 Among the natural fibers, miscanthus fiber (perennial grass) is mainly

INTRODUCTION Increasing environmental pollution, global warming, and waste accumulation issues are impetus for developing sustainable and environmentally friendly biodegradable materials to replace nonbiodegradable materials. One category of biodegradable materials is green composites that can be produced from biodegradable polymer matrices with natural fibers as reinforcement. These green composite materials have been finding increased favor across packaging, horticultural, automotive, and biomedical applications.1−4 There are several biodegradable polymers commercially available in the market that are being used for green composite fabrication. Among them, poly(butylene succinate), PBS, is one of the promising candidate for green composite fabrication because it has good melt processability and biodegradability under composting environments.5 PBS is typically produced from petroleum based monomers but also can be produced from renewable resource © XXXX American Chemical Society

Received: July 7, 2015 Revised: August 20, 2015

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DOI: 10.1021/acssuschemeng.5b00646 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

purchased from Acros Organics, and maleic anhydride (MAH) was obtained from Sigma-Aldrich, USA. Synthesis of Maleic Anhydride Grafted PBS. To reduce hydrolytic degradation of PBS, the PBS pellets were dried at 80 °C for 12 h before processing. Maleic anhydride grafted PBS (MAH-g-PBS) was prepared with the initiator and MAH concentration of 1 and 5 phr, respectively. The MAH-g-PBS sample was prepared in a laboratory-scale internal batch mixer (Torque Rheometer, Haake PolyLab QC, Thermo Scientific). The batch mixer barrel was divided into three heating zones. The temperatures of these three heating zones were kept constant at 160 °C with a screw speed of 60 rpm and reaction time of 6 min for synthesizing MAH-g-PBS sample. A reaction time of 6 min was divided into three steps. In the first step, polymers were melted at 160 °C for 2 min and in the second step a free radical initiator was introduced into the molten polymers to make reactive sites for 2 min. Finally, the maleic anhydride was added into the reaction medium and the reaction was carried out for another 2 min to obtain a desired amount of grafting content onto PBS. It is known that homopolymerization of MAH is difficult with molten polymers because of the 1,2-disubstituted double bond present in the MAH. To diminish the homopolymerization of the MAH in the reaction medium, the reaction was carried out above the ceiling temperature (150 °C). During the reaction, the reaction mixture torque was monitored over time showing a significant increment. This increased torque was likely due to grafting and gel formation that occurred after addition of MAH and DCP into polymers. The MAH grafted PBS was taken out from the batch mixer and ground into small pieces for further experiments. Purification of Maleic Anhydride Grafted PBS and Determination of Maleic Anhydride Grafting Percentage. The purification of the MAH grafted PBS was performed according to a modified procedure.22 The unreacted maleic anhydride was removed by vacuum drying at 80 °C for 1 day. Vacuum dried maleic anhydride grafted PBS was dissolved in chloroform at room temperature overnight. Then, the grafted maleic anhydride groups were hydrolyzed into carboxylic acid by adding a few drops of 1 N hydrochloric acid (HCl) at room temperature. After dissolution of MAH-g-PBS samples, they were selectively precipitated in methanol and filtered. The filtered samples were repeatedly washed with excess methanol to remove residual MAH, DCP, and HCl, followed by drying at 80 °C under vacuum for 24 h. The dried samples were used for further analysis. The maleic anhydride grafting percentage onto the PBS backbone was determined by back-titration, which was modified from Nabar et al.22 About 1 g of purified MAH grafted PBS was dissolved in chloroform (100 mL) at room temperature for 2 h. Immediately, the solutions were titrated against 0.025 N alcoholic KOH solution and phenolphthalein indicator. Under this condition, MAH grafted samples were completely soluble in chloroform and did not precipitate during titration against alcoholic KOH solution. The MAH grafting percentage was calculated as follows

used for green energy production, soil preservation, and composite application.8 It is an attractive fiber for composite fabrication because of suitable fiber properties,7,8 higher yield per hectare, lower production cost,8 realistic price,9 and good thermal stability up to 210 °C.10 According to Bourmaud and Pimbert,10 the miscanthus fibers have modulus of 9.49 GPa, which is between hemp (12.14 GPa) and sisal (8.52 GPa) fibers modulus. Kirwan et al.9 reported that miscanthus fibers have mechanical properties similar to commodity thermoplastics. Because of these inherent properties, recently, miscanthus fiber has been used as a reinforcement agent in biodegradable polymer matrix such as polylactide (PLA),7 Mater-Bi,11 poly(vinyl alcohol) (PVA),9 PLA/poly(hydroxybutyrate-covalerate) (PHBV) blend,8 and preblend of poly(butylene adipate-co-terephthalate) (PBAT)/PHBV12 matrices. There are four major issues that have been identified in the development of natural fiber reinforced polymer composites such as poor thermal stability of fibers (CO) groups was observed at 1718 cm −1 . 25,26 The band at 1041 cm −1 corresponds to the −OCC− stretching vibration PBS.24 Most of the saturated hydrocarbons contain methyl groups. These methyl groups show a symmetric stretching band at 2962 cm−1 and an asymmetric stretching band 2872 at cm−1.27 In PBS, methyl and methylene CH stretching bands occur at 2945 and 2854 cm−1, respectively. Two new small peaks (1859 and 1788 cm−1) were formed in maleic anhydride grafted PBS, which correspond to the saturated cyclic anhydride carbonyl ring (succinic anhydride group).28,29 These characteristic peaks confirm that the MAH moieties were successfully grafted onto the PBS backbone. The characteristic functional group of ester linkages is −COC−. A strong −COC− stretching peak (1151 cm−1) was observed in PBS, which confirms that the PBS contains ester linkages. In Figure 1, observed peaks at 955, 806, and 644 cm−1 were attributed to CO stretching, CH2 in OC(CH2)2CO in-plane bending, and −COO bending bands of the PBS and MAH grafted PBS.27 These peaks are clear evidence to differentiate PBS from other polymers. Also, elimination of unreacted MAH in the grafted samples can be confirmed by disappearance of the maleic anhydride ring at 684 cm−1 in the FTIR spectra of MAH-g-PBS.30 In addition, the peak at 1590 cm−1 belongs to the CC stretching of the maleic anhydride.30 This CC peak was not found in the MAH grafted PBS sample, which suggests that the MAH was grafted on the PBS backbone. Mechanical Properties of PBS Composites. The mechanical properties of the natural fiber composites are dependent on both the matrix and fiber characters. The tensile strength of a natural fiber reinforced composite is mainly influenced by three factors such as stress concentration, fiber orientation in the matrix, and interfacial interaction between the components.13 On the other hand, wettability of fibers by the matrix, volume fraction of fibers, and aspect ratio of fibers

(2)

The percentage of crystallinity of biocomposite samples was calculated as follows24 χ=

ΔHc × 100% ΔHm° (1 − Wf )

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(3)

The parameter ΔHc is crystallization enthalpy, and ΔHm ° is the theoretical melting enthalpy of one-hundred percentage crystalline PBS taken to be 110.3 J/g.24 The term Wf is the weight fraction of the fibers in the composite samples. Morphological Analysis. To examine the fracture, surface morphology of the fractured sample was analyzed using scanning electron microscopy (Inspect S50-FEI Company SEM). Before sample morphology was observed, all the samples were gold coated with a final thickness of 20 nm with 20 mA in order to make them electrically conductive. C


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Figure 2. (a) Tensile properties of the PBS and PBS/miscanthus composites: (A) neat PBS, (B) MAH-g-PBS (compatibilizer), (C) PBS/ compatibilizer (95/5 wt %), (D) PBS/miscanthus (70/30 wt %), (E) PBS/miscanthus/compatibilizer (65/30/5 wt %), (F) PBS/miscanthus (60/40 wt %), (G) PBS/miscanthus/compatibilizer (55/40/5 wt %), (H) PBS/miscanthus (50/50 wt %), and (I) PBS/miscanthus/compatibilizer (45/50/5 wt %). (b) Reduced tensile strength of the uncompatibilized and compatibilized PBS/miscanthus composites plotted against volume fraction of fibers according to eq 5.

determine tensile modulus of the natural fiber composite.13 Figure 2a shows the variation of tensile modulus and tensile strength of PBS composites with different fiber loadings with and without compatibilizer. The neat PBS had a tensile strength of 39 MPa and tensile modulus of 0.66 GPa. The tensile strength and modulus of MAH-g-PBS and PBS/5 wt % MAH-gPBS did not show any significant difference compared to neat PBS. Because of the lack of interfacial interaction between the phases and incompatibility between the miscanthus fiber and the PBS, the tensile strength decreased significantly after inclusion of miscanthus fibers into the PBS. Particularly, the addition of the 50 wt % miscanthus fiber into the PBS matrix yielded a 22% reduction in tensile strength. As expected, the tensile modulus of the natural fiber composites depends on the fiber content. Consequently, the tensile modulus of the PBS composites was steeply increased with increasing miscanthus fiber loading up to 50 wt %. For instance, the PBS composite with 50 wt % miscanthus fiber showed a maximum tensile modulus value of 3.88 GPa, which is 488% higher than that of neat PBS. Among the tensile modulus and tensile strength of the composites, tensile strength was influenced by the addition of compatibilizer. The tensile strength of all the compatibilized composites was significantly higher than that of the uncompatibilized composites as well as of PBS matrix. The observed improvement in the tensile strength of the compatibilized samples is attributed to good interfacial interaction between the components. The tensile modulus of the compatibilized composites did not show any substantial difference compared to that of the corresponding uncompatibilized composites. This observation is consistent with PLA/ wood composites.31 In the presence of compatibilizer, the tensile strength improvement of the PBS/miscanthus composite leveled off beyond 30 wt % fiber content. The percentage of elongation at break of neat PBS was around 250%. After incorporation of miscanthus fibers into the PBS matrix, all the composites showed 2−4% of elongation at break. This

elongation reduction is a common observation in natural fiber reinforced composites because of phase separation phenomena and the reduced polymer chain entanglement in the presence of rigid fibers.12 The load-bearing capacity/reinforcing effect of the particulate or short-fiber-filled composites can be expressed quantitatively with the help of a simple model developed earlier eq 3.32−36 This equation can be used to describe the compositiondependent tensile properties of the biocomposites.31,33 σT = σT0λ n

1−ϕ exp(Bϕ) 1 + 2.5ϕ

(4)

The term σT0 is true tensile strength of matrix; σT is true tensile strength of composites; λ is ratio of gauge length measured after (L) and before (L0) the tensile test; n is the strain hardening parameter; B is the load-bearing capacity of the dispersed component, which depends on interfacial adhesion/ interaction; φ is the volume fraction of fibers in the composites, calculated based on the density values. The φ values obtained were also confirmed by extracting the fibers from the composite samples by dissolving in chloroform. From eq 4, n is strain hardening tendency of matrix. The tensile strength (σT) divided by the terms λn and 1 − φ/1 + 2.5φ in eq 4 gives a reduced tensile strength (σTred), which is the yield stress of the composites. Equation 5 is obtained from simplified linear form of eq 4.32,36 when the deformation of composites is small, n can be neglected from eq 4.32 ln σTred = ln σT0 + Bϕ

(5)

If we plot the natural logarithm of reduced tensile strength as a function of fiber volume fraction, this reduced tensile strength must show straight line with a slope of B, which corresponds to load-bearing capacity or load carried by fibers in the composites.34,36 This load bearing capacity is also depends on the interfacial adhesion between the fiber−matrix. In Figure 2b, the reduced tensile strength of both compatibilized and D


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Figure 3. (a) Flexural properties of the PBS and its composites. (b) Nothched Izod impact strength of the PBS and its composites; (A) neat PBS, (B) MAH-g-PBS (compatibilizer), (C) PBS/compatibilizer (95/5 wt %), (D) PBS/miscanthus (70/30 wt %), (E) PBS/miscanthus/compatibilizer (65/30/5 wt %), (F) PBS/miscanthus (60/40 wt %), (G) PBS/miscanthus/compatibilizer (55/40/5 wt %), (H) PBS/miscanthus (50/50 wt %), and (I) PBS/miscanthus/compatibilizer (45/50/5 wt %).

uncompatibilized composites is plotted against fiber content in eq 5. It was observed that a linear correlation occurred for compatibilized and uncompatibilized composites with dissimilar slope (B) value. The B value was proportional to the loadbearing capacity/reinforcing effect of the resulting composites. The compatibilized composites showed superior load-bearing capacity with a B value of 4.85 whereas uncompatibilized composites yielded least reinforcing effect with a B value of 3.65. The enhanced load-bearing capacity of the compatibilized composites was consistent with observed tensile modulus and strength of the PBS/miscanthus composite. The B value (4.85) of the compatibilized PBS/composites found in this study was much higher than that of compatibilized PLA/wood composites,31,32,35 suggesting that the load-bearing capacity of compatibilized PBS/miscanthus composites is greater than that of compatibilized PLA/wood composites. Figure 3a shows the flexural properties of PBS and its composites with and without compatibilizer. The flexural strength and modulus of MAH-g-PBS and PBS/5 wt % MAH-g-PBS are not significantly different compared to neat PBS. Unlike tensile strength, the flexural strength of all the uncompatibilized composites is remarkably increased compared to neat PBS. This increase can be attributed to the enhanced stiffness of the PBS matrix after incorporation of the miscanthus fibers. However, there appears to be a slight reduction in flexural strength of the uncompatibilized composites with increasing fiber loadings from 30 to 50 wt % but they are not statistically significant. In contrast, flexural modulus of the composites gradually increased with increasing fiber content up to 50 wt %. This trend has good agreement with tensile modulus observation in this current study. Regarding flexural modulus, there was an insignificant difference observed between the compatibilized and uncompatibilized composites. In contrast, the compatibilized composites exhibit superior flexural strength compared to the uncompatibilized composites. The flexural strength of compatibilized PBS composites levels off when fiber loading is increased from 30 to 50 wt %. This observation has good agreement with observed tensile strength of compatibilized PBS composites. Compared to neat PBS, the flexural modulus and flexural strength of compatibilized PBS composites with 50 wt % miscanthus fibers were found to increase 515 and 139%, respectively. The observed improve-

ment in the compatibilized composites can be due to the strong reinforcing effect of miscanthus fibers and reduced fiber agglomeration in the matrix. The notched Izod impact strength of PBS and its composites with and without compatibilizer is shown in Figure 3b. The impact strength of neat PBS is around 28 J/m, which can be comparable to homopolypropylene.26 The impact strength of MAH-g-PBS (64 J/m) is significantly higher compared to that of neat PBS. The observed impact strength improvement in the MAH-g-PBS sample is attributed to partial cross-linking, which occurred during MAH grafting on the PBS backbone in the presence of DCP free radical initiator. A similar type of observation has been reported in the partially cross-linked PBS in the presence of DCP initiator.37 However, there was no change observed in the impact strength of PBS after incorporation of 5 wt % MAH-g-PBS compatibilizer. Unlike impact strength of compression molded PBS/30 wt % kenaf composites38 and injection molded PLA/30 wt % wheat straw composites,39 there is a trend for impact strength of injection molded PBS composite to increase with miscanthus fibers loading up to 40 wt %. In contrast, the PBS composites with 50 wt % miscanthus fibers did not show any significant impact strength improvement when compared to neat PBS. This could be due to the uneven fiber dispersion in the matrix when composites contain more fibers, i.e., 50 wt %. The impact strength improvement was more pronounced in the compatibilized composites in comparison to their corresponding uncompatibilized composites. When the impact strengths of compatibilized and uncompatibilized PBS/miscanthus composites were compared, a maximum improvement (47%) was noticed in compatibilized composites with 50 wt % fiber loading. Avella et al.40,41 have observed a similar trend in compatibilized PHBV/kenaf composites and compatibilized PLA/kenaf fiber composites. According to these studies, the observed improvement was due to the better adhesion between the components, less fiber pull-out under impact load, and enhanced uniform fiber dispersion in the matrix. However, the impact strength of compatibilized composites gradually declined with increasing fiber loading from 30 to 50 wt %, which follows a similar trend to uncompatibilized composites. This could be due to the fiber−fiber contact that was increased with increasing fiber loadings. Overall, all the compatibilized E


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Figure 4. (a) Dynamic mechanical analysis of the PBS and its composites. (b) Tan δ curves of the PBS and its composites (A) neat PBS, (B) PBS/ miscanthus (70/30 wt %), (C) PBS/miscanthus/compatibilizer (65/30/5 wt %), (D) PBS/miscanthus (60/40 wt %), (E) PBS/miscanthus/ compatibilizer (55/40/5 wt %), (F) PBS/miscanthus (50/50 wt %), and (G) PBS/miscanthus/compatibilizer (45/50/5 wt %).

their corresponding uncompatibilized counterparts. These reductions were due to the improved interaction between the phases with addition of compatibilizer. This can be further confirmed by evaluating the interface adhesion factor (A) between the filler and matrix. Adhesion Factor Calculation. From the height of tan δ peak values, Kubát et al.43 described a methodology to evaluate degree of interaction between the composite components. When there is strong interfacial interaction/adhesion between the phases and reduction of macromolecular mobility around the reinforcement surface, the value of adhesion factor (A) decreases. Thus, lower values of adhesion factor (A) are evidence of a high degree of interactions between the fibers and the matrix. According to this methodology, the adhesion/ interaction between the fiber and the matrix can be estimated by adhesion factor (A), which was calculated in the following equation.43

PBS/miscanthus composites performance can be comparable to polypropylene/10−20 wt % short glass fiber reinforced composites.42 Dynamic Mechanical Analysis. DMA was used to investigate the viscoelastic properties of PBS and its composites with respect to temperatures. The interfacial interaction between the fibers and the matrix can be estimated by storage modulus (E′) and loss factor (tan δ) properties. For example, the E′ values of the natural fiber composites are sensitive to the dispersion of fibers in the matrix as well as interfacial bonding between the phases. Figure 4a illustrates the storage modulus of PBS and its composites over the wide temperature range. It can be remarked that the E′ value of composites monotonically increased with the addition of miscanthus fibers up to 50 wt %. This improvement was due to the reinforcing effect of miscanthus fiber in the PBS matrix. In addition, the E′ value of compatibilized composites is slightly higher with respect to corresponding uncompatibilized composites. For instance, the storage modulus of compatibilized composites with 50 wt % miscanthus fibers showed 6.8 GPa at −60 °C, which is higher than corresponding uncompatibilized composites (6.52 GPa) as well as neat PBS (2.98 GPa) at the glassy region (−60 °C). The same trend has been observed across the entire investigated range of temperature. This suggests that compatibilized composites had uniform fiber dispersion and the high degree of interaction between the phases. However, all the samples showed a drastic reduction of E′ at −16 °C, which is attributed to the glass transition temperature (tan δ) of PBS. Moreover, the E′ values of all the samples were gradually decreased with increasing temperature up to 100 °C. This decrease in E′ is an expected consequence of the molecular motion/relaxation while increasing temperatures. Figure 4b shows the tan δ curves of PBS and its composites with different weight percentage fiber loadings. The glass transition temperature (position of tan δ peak maximum) of the PBS is not affected considerably as content of miscanthus fiber increases up to 50 wt %. In contrast, the height of the loss factor (tan δ) of the PBS was reduced by incorporation of miscanthus fibers. This reduction corresponds to the stiffness improvement of the PBS in the presence of fibers and is evidenced by increase of the E′. The height of the tan δ value was reduced in the compatibilized composites compared to

A=

tan δc(T ) 1 −1 1 − Vf tan δm(T )

(7)

where tan δm (T) and tan δc (T) represent the relative damping values of the neat matrix and the composite at a given temperature, respectively, and Vf is the volume fraction of fiber. Figure 5 shows the adhesion factor values of the composites with respect to temperatures. Unlike composites with higher fiber content (40 and 50 wt %), the composites with lower fiber content (30 wt %) showed lower adhesion factor values. Better interfacial interaction between components in the composites with 30 wt % fibers in contrast to composites with 50 wt % fibers content indicates the lower adhesion factor value. Moreover, the adhesion factor value of all the compatibilized composites was found to be lower in comparison to that of corresponding uncompatibilized composites. For instance, the lower adhesion factor values of PBS/30 wt % miscanthus composites is due to an optimum fiber content which helps to form better fiber dispersion in the PBS matrix and good interfacial adhesion between fiber and matrix. A strong adhesion between the fiber−matrix resulted less polymer chain mobility and thus showed lower adhesion factor values.44,45 Heat Deflection Temperature. The HDT value is used to determine physical deformation of a polymeric material at F


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was analyzed by DSC. It was not possible to detect the Tg values of neat PBS and its composites from DSC thermograms. Most of the literature on PBS composites also have reported Tg values from DMA analysis, probably due to the same reason.47 Table 1 shows the melting enthalpy, melting temperature, crystallization enthalpy, crystallization temperature, and percentage crystallinity of PBS before and after incorporation of miscanthus fibers. The crystallization temperature of neat PBS was found to be 93 °C. An obvious crystallization temperature reduction was observed in MAH-g-PBS compared to neat PBS. The observed crystallization temperature reduction is attributed to the decrease in polymer chain regularity, which hinders crystal growth, leading to lower crystallization temperature. The crystallinity of the PBS matrix was not heavily affected after incorporation of miscanthus fibers from 30 to 50 wt %; consequently, it can be expected that the biodegradability of the PBS composites will be similar to that of neat PBS.48 Interestingly, the PBS and it composites showed two distinct endothermic peaks, which may be attributed to two different lamellar thicknesses presented in the PBS and its composites. Usually, double melting peaks are observed for semicrystalline polymers and they can be explained by a melt recrystallization mechanism.49,50 These two melting points had a significantly lower temperature shift after MAH grafting on PBS. This is possibly due to the fact that the MAH group may prevent the lamella growth and nucleation of MAH grafted samples, thus leading to imperfect crystal structure compared to their parent polymer.51 However, the MAH grafted PBS sample had a sharp melting point and a weak shoulder melting point. The weak shoulder melting peak and sharp melting peak are attributed to lamella with more imperfect crystals and lamella with perfect crystals, respectively. In addition to that, after MAH grafting, the PBS showed one additional exothermic peak prior to melting point. This additional exothermic peak resulted from the fusion and recrystallization of PBS crystals during heating.52 The melting temperature of neat PBS was found to be 113 °C. The melting temperature of PBS and its composites is very similar, which suggests that the PBS melting temperature is not affected in the presence of miscanthus fibers. A similar trend was observed in crystallization temperature and percentage of crystallinity of PBS after addition of miscanthus fibers. During the DSC heating cycles (Figure 6), a broad bimodal melting peak for neat PBS was observed and can be attributed to the meltrecrystallization phenomena. This bimodal melting peak of PBS became more distinct after addition of miscanthus fibers. Morphological Analysis. Scanning electron microscopy (SEM) was employed to investigate the interfacial bonding between the component in the composites and the degree of fiber dispersion in the matrix. The surface morphology of

Figure 5. Adhesion factor of the PBS/miscanthus composites: (A) PBS/miscanthus (70/30 wt %), (B) PBS/miscanthus/compatibilizer (65/30/5 wt %), (C) PBS/miscanthus (60/40 wt %), (D) PBS/ miscanthus/compatibilizer (55/40/5 wt %), (E) PBS/miscanthus (50/ 50 wt %), and (F) PBS/miscanthus/compatibilizer (45/50/5 wt %).

elevated temperatures with a set of testing conditions. Table 1 shows the HDT values of neat PBS and its compatibilized and uncompatibilized composites. The HDT values of neat PBS and MAH-g-PBS were around 90 and 88 °C, respectively. The PBS shows a significant increase in HDT with increasing miscanthus fiber loading from 30 to 50 wt %. For instance, the PBS with 40 wt % miscanthus fiber composites showed a 29% improvement in comparison to neat PBS. The observed HDT improvements of PBS composites were attributed to the enhanced stiffness/reinforcement effect of resulting composites, as reports in the literature.8 The enhanced stiffness of all the PBS composites has good agreement with the observed tensile and flexural modulus. Similarly, the HDT values of PBS/basalt fiber (85/15 wt %) composites46 and PBS/switchgrass (50/50 wt %) composites20 were increased by 40 and 36%, respectively, in comparison to neat PBS. The HDT values of compatibilized and uncompatibilized composites were not significantly different in the present study. This can be ascribed to the HDT values of all the composites being very close to their melting temperature, i.e., ∼114 °C. Therefore, it can be concluded that an optimum HDT value of a PBS composite is 115 °C, which was able to be achieved in the PBS composites with 40 wt % miscanthus fibers loading. Differential Scanning Calorimetry. The effect of the incorporated miscanthus fibers on the melting temperature (Tm), melting enthalpy (ΔHm), crystalline temperature (Tc), crystalline enthalpy (ΔHc), and percentage crystallinity of PBS

Table 1. Summary of Heat Deflection Temperature (HDT) and Differential Scanning Calorimetry (DSC) Traces of neat PBS and Its Composites samples neat PBS MAH-g-PBS PBS/miscanthus (70/30 wt %) PBS/miscanthus/MAH-g-PBS (65/30/5 wt %) PBS/miscanthus (60/40 wt %) PBS/miscanthus/MAH-g-PBS (55/40/5 wt %) PBS/miscanthus (50/50 wt %) PBS/miscanthus/MAH-g-PBS (45/50/5 wt %)

HDT (°C) 89.59 87.77 108.63 112.25 114.52 115.89 115.62 116.94

± ± ± ± ± ± ± ±

4.15 5.20 3.97 0.31 0.98 0.84 0.39 0.32

Tc (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

crystallinity (%)

92.97 62.98 92.48 92.63 92.08 92.33 91.71 92.30

69.57 64.25 48.81 52.06 41.46 42.53 32.30 32.74

113.39 109.42 112.95 113.03 113.72 114.34 114.47 113.61

67.60 66.31 47.86 51.00 41.37 42.47 31.45 32.93

61.28 58.25 61.98 66.03 62.51 64.17 57.26 59.70

G


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dispersion in the matrix, and fibers wholly embedded into the PBS matrix due to strong adhesion at the interfacial regions compared to that of uncompatibilized composites. In the compatibilized composite, it can be seen (Figure 7b) that many fibers are coated with PBS matrix without the interfacial gap formation. This result was attributed to the enhanced fiber/ matrix adhesion with the help of a reactive compatibilizer, i.e., MAH-g-PBS.23 Consequently, all the compatibilized composites enhanced mechanical performances as compared to their corresponding uncompatibilized composites, which is also consistent with the observed lower adhesion factor A values of compatibilized composites. Similar findings have been recently reported for the compatibilized PBS/kenaf fiber composites.38

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CONCLUSIONS Miscanthus fiber reinforced PBS composites were fabricated at various concentrations of fiber loading. The MAH-g-PBS was successfully synthesized with a grafting level of 2.56%. The effect of compatibilizer (MAH-g-PBS) on the mechanical performances of the resulting composites was examined by tensile, flexural, and impact properties. All the compatibilized composites had greater thermo-mechanical and mechanical properties as compared to their corresponding uncompatiblized composites as well as the neat PBS matrix. These mechanical and thermo-mechanical property improvements were attributed to an enhanced interfacial interaction between the components with the addition of maleic anhydride grafted compatibilizer. The improved interfacial adhesion between the components was confirmed by microscopic analysis and theoretical adhesion parameter values. Overall, this study provides an option for preparing a sustainable biocomposite with superior mechanical and thermo-mechanical properties.

Figure 6. DSC second heating cycles of the PBS and its composites: (A) neat PBS, (B) MAH-g-PBS (compatibilizer), (C) PBS/miscanthus (70/30 wt %), (D) PBS/miscanthus/compatibilizer (65/30/5 wt %), (E) PBS/miscanthus (60/40 wt %), (F) PBS/miscanthus/compatibilizer (55/40/5 wt %), (G) PBS/miscanthus (50/50 wt %), and (H) PBS/miscanthus/compatibilizer (45/50/5 wt %).

uncompatibililized and compatibilized PBS composites with 50 wt % fiber loading is presented in Figure 7. SEM micrographs

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AUTHOR INFORMATION

Corresponding Author

*A. K. Mohanty. E-mail: [email protected]. Phone: +1519-824-4120 x56664.

Figure 7. SEM micrograph of uncompatibilized and compatibilized PBS composites: (a) PBS/miscanthus (50/50 wt %) composite and (b) PBS/miscanthus/compatibilizer (45/50/5 wt %) composite.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), Canada/University of Guelph-Bioeconomy for Industrial Uses Research Program Theme (Project # 200004, 200005 and 200425); the Ontario Ministry of Economic Development and Innovation (MEDI), Canada, Ontario Research Fund, Research Excellence Round 4 program (ORF-RE04) (Project # 050231 and 050289); the Natural Sciences and Engineering Research Council (NSERC), Canada − Discovery Grants (Project # 400322); and the NSERC NCE AUTO21 (Project # 460372 and 460373) for the financial support to carry out this research work.

of uncompatibilized composites (Figure 7a) reveal interfacial gaps between the matrix and the fiber, poor fiber wetting, and fibers pull-out traces from the PBS matrix. In addition, increased fiber bundles/aggregates are evident with increasing fiber content up to 50 wt %. A similar occurrence has been reported in PLA/kenaf composites,41 PHBV/kenaf composites,40 PHBV/PLA/miscanthus composites,8 PBS/bamboo composites,53 PBS/kenaf fiber composites,38 and PP/bioflour composites.23 This is possibly due to an increase fiber−fiber interaction with increasing fiber contents in the matrix, which could hinder the interaction between the phases in the composites.54 This contributes to a reduction in the tensile stress of resulting PBS composite with increasing fiber content from 30 to 50 wt %. It is well documented that most of the natural fibers are not compatible with a hydrophobic polymer matrix; this lack of compatibility is responsible for fiber debonding from the matrix during tensile fracture.23,55 Figure 7b shows SEM micrograph of compatibilized PBS composites. The compatibilized composite clearly showed paucity of fiber pull-out traces from the matrix, good fiber

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Injection Molded Sustainable Biocomposites From Poly(butylene

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