Article pubs.acs.org/IECR
Poly(butylene succinate-co-adipate) Green Composites with Enhanced Rigidity: Influences of Dimension and Surface Modification of Kenaf Fiber Reinforcement Fang-Chyou Chiu,*,† Yu-Chi Hsieh,† Yi-Ching Sung,‡ and Nai-Yun Liang‡ †
Department of Chemical and Materials Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC Polymer Materials Section, Taiwan Textile Research Institute, New Taipei City 236, Taiwan, ROC
‡
ABSTRACT: Kenaf fibers (KFs) were utilized as reinforcements to prepare poly(butylene succinate-co-adipate) (PBSA) green composites. Untreated KFs of KF35 (large dimension) and KF120 (small dimension) imparted the nucleation effect for PBSA crystallization. KF120 exhibited superior nucleation efficiency compared with KF35. The tensile/flexural moduli of PBSA drastically increased after the addition of KF35 or KF120. Successful modifications of KF35 through NaOH(aq) and 3aminopropyltriethoxysilane (APS) were confirmed. The APS-treated KF (KF35AS) exhibited enhanced interaction with the PBSA matrix compared with NaOH-treated (KF35A) and untreated KF35. KF35A and KF35AS also facilitated the nucleation of PBSA crystallization. The KF35AS-incorporated composites exhibited the highest tensile/flexural moduli among the different KF-added systems at identical loadings. The tensile and flexural moduli increased to 629% and 360% (40 wt % loading), respectively, compared with PBSA. The enhanced interfacial interaction between KF35AS and PBSA lessened the negative influence of KFs on the thermal stability and water absorption of PBSA.
1. INTRODUCTION Given the growth in environmental consciousness, biopolymers possessing biobased and/or biodegradable characteristics have increasingly attracted attention during the past two decades.1−3 Biobased polymers, which are made of renewable resources, reduce carbon dioxide emission during the manufacturing process compared with petroleum-based polymers. Biodegradable polymers that produce nontoxic materials (e.g., water and carbon dioxide) during biodegradation are the main solution to the waste disposal problem caused by conventional polymers. Biopolymers, such as poly(lactic acid) (PLA), poly(hydroxyalkanoate), thermoplastic starch, and poly(butylene succinate) (PBS), have been manufactured into commercial products to replace commodity plastics in certain applications. However, disadvantages such as inferior mechanical properties, low heat resistance, and high cost have limited the end-use applications of biopolymers. Efforts to improve biopolymer properties have been made by blending them with other polymers and/or by incorporating various fillers. Among the fillers investigated, plant fibers are receiving great interest because of their renewability, low density, nonabrasiveness, and high specific properties.4−6 The plant fiber-reinforced biopolymers, called green composites, are continuously being developed. The biodegradable green composites are considered the most environmentally benign materials because they can be fully decomposed at the end of their life cycle. PLA is a widely utilized biodegradable polymer and has been extensively investigated.7,8 The plant fiber-reinforced PLA green composites prepared through various techniques have been studied. Kenaf, flax, bamboo, coir, and jute were chosen as reinforcements.9−13 Good combinations of rigidity, processability, and resistance in thermal deformation at moderate fiber loading were achieved. For example, Nishino et al.9 prepared and characterized the kenaf sheet-reinforced PLA composites. © 2015 American Chemical Society
The prepared green composites (70 vol % of kenaf) were comparable with traditional composites in terms of high tensile modulus (6.3 GPa) and tensile strength (62 MPa). Le Moigne et al.10 investigated the PLA/flax fiber green composites with and without flax fiber organosilane treatment. Optimized treatment conditions increased the hydrophobicity of the fibers and significantly improved the mechanical properties of PLA composites. PBS is among the valuable biodegradable polymers. PBS-based composites and blends with different formulations have also been investigated.14−17 Nam et al.14 studied the PBS/ coir fiber composites with coir fiber alkali treatment. The best mechanical properties of the coir fiber-incorporated composite were achieved at a fiber content of 25 wt %, which exhibited a 141.9% increase in tensile modulus and a 97.4% increase in flexural modulus compared with those of neat PBS. To modify the properties of PBS, poly(butylene succinate-co-adipate) (PBSA) was synthesized through the polycondensation of 1,4butanediol with succinic and adipic acids. As an emerging biodegradable copolymer, PBSA possesses good processability, toughness, and chemical resistance. It can be used in service ware, mulch films, and waste-composting bags. Several studies on the PBSA and its blends/composites were reported.18−25 Wang et al.18 investigated multiple melting peaks of PBSA using differential scanning calorimetry (DSC). The meltings of primary/secondary lamellae and recrystallized crystals were responsible for the complex melting behavior of PBSA. Siracusa et al.19 reported that the chemical compositions of PBS and PBSA strongly affected their permeability to gases. Ojijo et al.20,21 examined the properties of immiscible PBSA/PLA Received: Revised: Accepted: Published: 12826
September 10, 2015 November 21, 2015 December 9, 2015 December 9, 2015 DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
Article
Industrial & Engineering Chemistry Research
It was first alkali-treated with NaOH by immersion in 6% aqueous NaOH solution for 24 h at room temperature. The alkali-treated KF (denoted KF35A) was then thoroughly washed with distilled water until pH 7 was reached and allowed to dry in an oven. Some of KF35A was additionally treated with APS following the method of Le Moigne et al.,10with slight modifications. First, an acid solution (pH 4.5) of 5 wt % APS diluted in an aqueous solution of methanol (50/ 50) was prepared with acetic acid and stirred for 1 h at 20 °C for the activation of APS. KF35A, at weight ratio 9:1 with respective to the APS, was then immersed in the APS solution for 4 h at 20 °C. Afterward, the APS-treated KF (denoted KF35AS) was washed with distilled water and oven-dried before being mixed with PBSA. A Haake PolyDrive mixer (R600) with a pair of Banburytype rotors was used to prepare the composites. Different KFs at loadings of 10, 30, 40 wt % were dry mixed with PBSA first before being introduced into the mixer. Each composite was mixed for 8 min at 140 °C under a rotor speed of 60 rpm. The composite sample was designated as KF35-X, KF120-X, KF35A-X, and KF35AS-X to indicate the KF35-, KF120-, KF35A-, and KF35AS-added composites, respectively; here X represents the X wt % of KFs included in the composites. The composites were subsequently hot pressed at 140 °C to prepare the specimens for further analyses. Neat PBSA was also treated under the same conditions for comparison. 2.3. Characterization. Scanning electron microscope combined with energy dispersive spectrometer (SEM-EDS, Hitachi S-3000N SEM/JEOL 6587 EDS) was used to examine the surface topography of untreated/treated KFs. The average length and diameter of KF35 and KF120 were determined based on 30 individual sieved KFs with the same SEM. To characterize the dispersion status of different KFs within the PBSA matrix, SEM was also employed to observe the cryofractured (in liquid nitrogen) surfaces of prepared composites. Fourier transform infrared spectroscopy (FTIR) was used to confirm the modification of KFs after surface treatments. A Bruker Optics Tensor 27 system was used to obtain FTIR spectra at a resolution of 4 cm−1. Crystallization and melting behavior of the samples were studied using a DSC (TA DSC Q10) equipped with an intercooler. Samples ca. 5 mg were first melted at 140 °C for 3 min and then cooled to 20 °C at a rate of 10 °C/min or 40 °C/min for nonisothermal crystallization experiments or fast-cooling (80 °C/min) to predetermined temperature (Tc) for isothermal crystallization. The crystallized samples were followed by heating to 140 °C at 20 °C/min to evaluate their melting behavior. A thermogravimetric analysis (TGA) system (TA TGA Q50) was used to evaluate the thermal stability of samples under N2 environment. The samples were heated from room temperature to 700 °C at 10 °C/min. The tensile modulus of the dog-bone-shaped specimens (according to ASTM D638) was measured at a crosshead speed of 2 mm/min using a Gotech Al-3000 system. The flexural modulus of rectangular specimens in accordance with ASTM D790 was measured by the same Gotech Al-3000 system at a speed of 1 mm/min. The obtained tensile/flexural properties were average values of at least five specimens of the same formulation. The water absorption characteristics of the samples were evaluated by determining the weight gained by the dried samples after immersion in distilled water for 2 weeks. The equation used is as follows
blends and the blend-based nanocomposites. The 70PLA/ 30PBSA blend exhibited optimal properties because of the maximum specific interfacial area of the PBSA droplets. The significance of nanoclay content and localization on the properties of blend-based composites was also disclosed. Zeng et al.22 synthesized and characterized PBSA-block-PLA copolymers. The mechanical properties of the copolymers can be tuned by the content of PBSA. Yang et al.23,24 reported the miscible feature of PBSA/poly(hydroxyl ether biphenyl A) and PBSA/poly(vinyl phenol) blends. The miscible counterparts reduced the crystallization rate of PBSA. Kim et al.25 studied the PBSA/carbon nanotube (CNT)-coated silk fiber composites. The mechanical properties of PBSA were improved after the formation of composites with a small amount (3 wt %) of CNT-coated silk fibers incorporation. To extend the versatility of PBSA, plant fiber-incorporated green composites should be developed because they show potential in considerably improving PBSA mechanical properties. However, studies on PBSA green composites are relatively few. Su and Wu26,27 utilized bamboo fiber and rice husk as reinforcements in preparing PBSA green composites. Composites made of acrylic acid-grafted PBSA showed superior mechanical properties compared with those of neat PBSAbased composites. Of the plant fibers examined for green composites preparation, kenaf fiber (KF) is well-known for its high modulus and economical/ecological advantages.9 However, KF is not so widely used as compared with flax, hemp, ramie, and others. For preparing the green composites, the dimension of reinforcing fibers should play an important role in enhancing the physical properties of polymer matrices. To our knowledge, the influence of KF dimension on the physical properties of KF-reinforced green composites was not studied before. In the present study, KF was thus chosen in the preparation of PBSA green composites. This study aimed to examine the effects of dimension and different surface treatments of KF on the resultant properties of melt-mixed PBSA/KF green composites. Untreated KFs of two dimensions, alkaline (NaOH)-treated and silane-treated KFs, were respectively incorporated into PBSA at loadings varying from 10−40 wt % to form the composites. The dispersion status of different KFs within the matrix, as well as the crystallization/ melting behavior, thermal stability, and mechanical/water absorption properties of the prepared composites, was reported and compared.
2. EXPERIMENTAL SECTION 2.1. Materials. PBSA used in this study was a commercial product (GS Pla AD-type) from Mitsubishi Chemical Corporation, Japan, with a density of 1.24 g/cm3. The average Mw and Mw/Mn values were 1.54 × 105 g/mol and 1.87, respectively.27 Chopped KF grown in India and of approximately 10 mm in length was utilized as the reinforcement for preparing composites. High purity sodium hydroxide (NaOH) obtained from J.T. Baker and 3-aminopropyltriethoxysilane (APS) purchased from Sigma-Aldrich were used for KF surface treatments. 2.2. KF Surface Treatments and Composites Preparation. The as-received KF was 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 KF was then pulverized and screened with sieve size of 35 and 120 mesh, respectively, into different dimensions (denoted as KF35 and KF120, respectively). KF35 was selected for further surface treatments. 12827
DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
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Figure 1. SEM surface topography of (a) KF35, (b) KF35A, and (c) KF35AS (insets: EDS data of the corresponding image).
water absorption (%) = (Wf − Wo)/Wo × 100
(1)
where Wo is the dried sample weight prior to the test, and Wf is the weight of the treated sample after drying excess distilled water.
3. RESULTS AND DISCUSSION 3.1. Characterization of Untreated and Treated KFs. Surface topography of KFs with and without chemical treatments is shown in the SEM images of Figure 1. Figure 1(a) displays the surface roughness and distributed impurity of water-washed KF35. After treatment with NaOH(aq), the surface of KF35A became evidently smooth because of the removal of wax, hemicellulose, and impurities, as shown in Figure 1(b). The bundle-like structure composed of individual microcelluloses was discernible. For KF35AS (further treated with APS), a smooth feature similar to that of KF35A became observable, as shown in Figure 1(c). Some jagged points (arrowed) that appeared on the surface may be attributed to the deposition of the APS coupling agent. The insets in Figures 1(b) and 1(c) show the EDS results of KF35A and KF35AS, respectively. The silicone element became observable on the KF35AS surface but not on the KF35A surface, which confirmed the successful attachment of APS on the KF35AS surface. The changes in chemical structure of KFs after the treatments were revealed by FTIR spectroscopy. Figure 2 compares the FTIR spectra of untreated and treated KFs. The original absorption peaks at 1745 and 1260 cm−1 in KF35 disappeared in the treated KF35A and KF35AS, which indicated the removal of hemicellulose and lignin components in KF35, respectively, by alkaline treatment.28 The difference between the spectra of KF35A and KF35AS was also discernible. More evident peaks located at 1167, 1508, and
Figure 2. FTIR spectra of KF35, KF35A, and KF35AS.
1594 cm−1 were exhibited in KF35AS as a result of specific interactions between APS and KFs. The absorptions were mainly caused by the vibration of NH2 groups bonded to the hydroxyl groups and the Si−O-cellulose-related vibration, which complied with the results of Zhou et al.29 The FTIR data demonstrated the successful modifications of waterwashed KF35 using NaOH(aq) and APS. 3.2. Influence of KF Dimension on Composite Properties. 3.2.1. Dimension and Dispersion Status of KFs. Figure 3 shows the SEM images of KF35 and KF120. The average length and diameter of KF35 were 486 and 53 μm, respectively. Smaller corresponding values of 104 and 14 μm were noted for 12828
DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
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Industrial & Engineering Chemistry Research
Figure 3. SEM images of (a) KF35 and (b) KF120 (scale bar: 1 mm).
Figure 4. SEM fractured surface of composites: (a) KF35-10, (b) KF35-40, (c) KF120-10, and (d) KF120-40.
KF120. The aspect ratios were determined to be 9.2 for KF35 and 7.4 for KF120. To disclose the dispersion status of KF35 and KF120 within the PBSA matrix, Figures 4(a)−4(d) show the fractured surfaces of composites with 10 and 40 wt % KF
loadings. From the SEM images, KF35 and KF120 were both randomly dispersed within the individual matrix, which indicated satisfactory preparation of PBSA composites through the melt-mixing process. In addition, some pulled-out KFs with 12829
DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
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Industrial & Engineering Chemistry Research
Figure 5. (a) DSC curves of PBSA and KF35/KF120-added composites at 10 °C/min cooling, (b) DSC isothermal crystallization traces of representative samples, (c) plots of tp−1 vs Tc for representative samples, and (d) DSC heating curves of the 10 °C/min-cooled samples.
Table 1. Representative Data of PBSA and Its (Un)treated KFs-Added Composites properties
a
samples
To (°C)
Tp (°C)
ΔT (°C)
Tm2 (°C)
T10 (°C)
TM(MPa)a
FM(MPa)a
PBSA KF35-10 KF35-30 KF35-40 KF120-10 KF120-30 KF120-40 KF35A-10 KF35A-30 KF35A-40 KF35AS-10 KF35AS-30 KF35AS-40
50.9 53.4 56.4 57.0 54.2 57.2 57.8 53.9 56.4 57.3 53.8 56.1 57.7
38.6 41.5 46.1 48.0 43.1 47.8 48.4 41.8 45.8 48.7 41.4 46.1 49.8
21.3 30.9 20.6 18.4 33.7 23.3 21.6 25.0 22.3 18.9 20.9 18.2 17.6
87.8 88.5 88.4 88.8 88.6 88.5 88.9 88.4 88.3 88.6 88.4 88.6 88.7
353 342 325 322 343 327 318 338 323 314 346 336 331
250 (29) 533 (18) 1023 (69) 1653 (98) 541 (18) 1117 (39) 1592 (56) 683 (42) 1156 (152) 1554 (150) 794 (19) 1538 (98) 1822 (176)
179 (16) 253 (26) 561 (32) 720 (59) 267 (18) 574 (23) 712 (38) 264 (4) 577 (16) 730 (25) 284 (22) 670 (37) 823 (18)
Standard deviations are listed in parentheses.
(To and Tp) of neat PBSA were 50.9 and 38.6 °C, respectively. Both temperatures shifted to higher temperatures in the composites, and a higher KF loading caused a further increase in temperature. Table 1 summarizes the To and Tp values of the samples. The nucleation effect of KF35 and KF120 on PBSA crystallization in the composites was confirmed. Furthermore, the KF120-added composites exhibited greater increments in terms of To and Tp compared with the KF35-added composites at identical KF loadings. This result suggested that the
a negligible amount of PBSA attached to the surfaces were observed. Thus, the SEM observations suggested minimal adhesion (interaction) between the untreated KF35/KF120 and PBSA matrix. 3.2.2. Thermal Properties. The crystallization and melting behavior of PBSA in neat state and in the composites were examined and compared using DSC. Figure 5(a) shows the cooling curves of representative samples at a rate of 10 °C/min. The crystallization onset and crystallization peak temperatures 12830
DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
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Figure 6. (a) Tensile modulus and (b) flexural modulus of PBSA and its KF35/KF120-added composites.
Figure 5(d) shows the heating curves of 10 °C/min-cooled PBSA and the composites. Neat PBSA exhibited a shallow exotherm around 75 °C (arrowed) prior to the main melting peak. The melting−recrystallization−remelting of PBSA crystals during the heating scan was responsible for this observation.18 Rather than the shallow exotherm, a (minor) melting endotherm (Tm1) of the original crystals appeared prior to the main melting peak in the composites, and the minor endotherm became gradually evident as KF35 and KF120 loading increased. The temperature of the main melting peak (Tm2) of each sample is listed in Table 1. The Tm2 value as well as the Tm1 value (not listed for brevity) shifted to higher temperatures after forming the composites, which implied that the presence of KF35/KF120 enhanced the stability of original and regrown PBSA crystals. A similar melting temperature increase was reported in polyamide and poly(vinylidene fluoride) composite systems.31,32 The decline in entropy change (ΔS) upon crystals melting by the dispersed KFs should be considered for the observation as well. The additions of KF35 and KF120 demonstrated a slight difference in increasing Tms of PBSA. The thermal stability of neat PBSA and its composites was compared using TGA. The temperatures at 10 wt % loss (T10) of individual samples under N2 atmosphere are listed in Table 1. The water-washed KF35/KF120 exhibited T10 of 268 °C, which was evidently lower than that of PBSA (353 °C). The addition of KFs shifted the degradation of PBSA to lower temperatures. Higher KF loading led to lower degradation temperatures. Different KF dimensions exhibited a comparable effect on the thermal stability decline for PBSA. 3.2.3. Mechanical Properties. Figure 6(a) compares the tensile modulus of neat PBSA and its composites. The tensile modulus increased after forming the composites, and higher KF loadings caused a further increase in the tensile modulus. Table 1 lists the average tensile modulus (TM) of individual samples. The incorporation of 10 wt % KF35 or KF120 into PBSA resulted in an evident increase (>100%) in the tensile modulus. The value (>1500 MPa) became six times higher than the original value (250 MPa) of PBSA after the addition of 40 wt % KFs. However, the elongation at break of PBSA drastically dropped from 215% to less than 10% after formation of the composites (not shown for brevity). The flexural modulus is another important property for engineering applications. Similar to the tensile modulus, the flexural modulus increased
nucleation efficiency of KF120 was superior to that of KF35, which may be ascribed to the greater surface contacts of KF120 with PBSA compared with KF35 with PBSA in the composites. Considering the equilibrium enthalpy of fusion of PBS for calculations, the crystallinity (Xc) of PBSA in various samples was determined based on the following equation Xc = ΔHc/[(1 − ϕ) × ΔHf °] × 100%
(2)
where ΔHc is the crystallization enthalpy of individual samples, ΔHf° is the equilibrium enthalpy of fusion (110.3 J/g) of the PBS crystal,30 and ϕ is the wt % of the KF35/KF120 in the composites. The values were 42% ± 2% for neat PBSA and the composites, indicating a slight change in PBSA crystallinity after formation of the composites. The crystallization peak width (ΔT) of each sample is also provided in Table 1. The composites with a low KF loading (10 wt %) possessed ΔT evidently broader than that of neat PBSA, which implied the retarded crystal growth of PBSA in these composites, especially with KF120 inclusion. The confinement effect induced by the low-loaded KFs could be considered for this behavior.31,32 The 40 °C/min-cooled samples showed a similar trend to the 10 °C/min-cooled samples (data not shown). Figure 5(b) shows the typical DSC isothermal crystallization traces of PBSA, KF120-10, and KF120-40 at different temperatures (Tcs). The time required to reach the exothermic peak (tp) was close to one another for the three curves. However, the time required to complete the crystallization (broadness of the peak) followed the sequence KF120-10 (3.76 min) > KF120-40 (3.45 min) > PBSA (3.21 min). This result suggests the retarded crystal growth of PBSA after KF addition, which corresponds to the nonisothermal crystallization result. The t p value was determined for the representative samples at different Tcs, and the reciprocal value (tp−1) is proportional to the overall crystallization (nucleation plus crystal growth) rate. Figure 5(c) shows the plots of tp−1 vs Tc for various samples. The overall crystallization rate of PBSA decreased (lower tp−1 value) with increasing Tc for all the samples. These plots also confirm that PBSA crystallized faster in the composites, and higher KF loading caused faster PBSA crystallization at identical Tcs. The KF120 showed a better enhancing effect compared with KF35 at 10 wt % loading. Due to some aggregation of KF at 40 wt % loading, KF120 and KF35 exhibited a similar effect in the increase of the overall crystallization rate of PBSA. 12831
DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
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Industrial & Engineering Chemistry Research with the loading of KF35 and KF120, as compared in Figure 6(b). The values (FM) are listed in Table 1 as well. After the addition of 40 wt % of either KF, the flexural modulus increased about four times the original value of PBSA. Upon comparing KF35 with KF120 in terms of rigidity reinforcement for PBSA, no significant difference was observed. The similar aspect ratios (9.2 vs 7.4) of KF35 and KF120 might have played a major role for this result. 3.3. Influence of KF Surface Treatments on Composite Properties. 3.3.1. Dispersion Status of KFs. Given that both untreated KF35 and KF120 exhibited similar reinforcing effects on the tensile/flexural properties of PBSA, KF35 was chosen for further surface treatments and then formed the composites. Figure 7 shows the SEM images of fractured composites
incorporated with 30 wt % of different KFs. Figure 7(a) shows the random dispersion and long pulled-out feature (arrowed) of KF35. KF35A and KF35AS were also finely dispersed within the matrix, whereas a lower amount of pulled-out features could be observed upon comparison with KF35, as shown in Figures 7(b) and 7(c). To reveal the interfacial morphology between KFs and the PBSA matrix, high magnification images are shown in Figures 7(d)−7(f). In Figure 7(d), gaps in the interfacial regions (circled) and smooth surfaces of pulled-out KF35 were observed, which confirmed a minimal interaction between the untreated KF35 and PBSA. Figure 7(e) displays the debundled texture of KF35A and slightly short pulled-out KF35A, suggesting a weak interaction between KF35A and the PBSA matrix. For the KF35AS-added composites (Figure 7(f)), imbedded KF35AS and close adhesion between the fiber and matrix were observed. The additional APS treatment on the NaOH-treated KFs led to a further KF-PBSA interaction enhancement. 3.3.2. Thermal Properties. Figure 8(a) shows the DSC cooling curves of PBSA and representative (un)treated KFadded composites at a rate of 10 °C/min. The addition of KF35A and KF35AS increased To and Tp of PBSA, similar to the KF35-added system. High KF loading shifted the crystallization of PBSA to higher temperatures. The determined To and Tp values are listed in Table 1 for comparison. The different surface treatments of KFs showed a slight influence on the enhanced crystallization of PBSA. Also noted the crystallinity of PBSA scarcely changed with the additions of different KFs. Figure 8(b) shows the DSC melting curves of the 10 °C/min precooled samples. Untreated and treated KFs imparted a similar effect on the melting of PBSA in the composites. Tm’s (Tm1/Tm2) of PBSA slightly increased after the addition of different KFs, and the minor endotherm of original crystals (arrowed) became more noticeable as the KF loading reached 40 wt %. The Tm2 values of the composites are summarized in Table 1. The stability of PBSA crystals slightly increased in all the (un)treated KF-added composite systems. Figure 9(a) shows the TGA curves of neat PBSA and KF35AS-added composites. KF35AS addition caused a decrease in PBSA degradation temperature. However, the residue (formed char) increased with KF35AS loading. The decline in thermal stability after KF addition was also observed in the KF35A-added system (curves not shown). Figure 9(b) depicts the degradation curves of neat PBSA and representative composites with 30 wt % (un)treated KF loading. The degradation temperatures (T10) of the samples, listed in Table 1, basically followed the sequence PBSA > KF35ASadded > KF35-added approximately equal to KF35A-added composites. These results indicated that the enhanced interfacial interaction between PBSA and KF35AS lessened the negative influence of KFs on the thermal stability of PBSA. 3.3.3. Mechanical and Water Absorption Properties. Figure 10(a) compares the tensile modulus of neat PBSA with different KF-added composites. The TM values are summarized in Table 1, too. The three (un)treated KF-added composite systems exhibited evidently higher tensile modulus than neat PBSA, and they showed a similar trend of increasing tensile modulus with increasing KF loading. Of the different composite systems, the KF35AS system possessed the highest tensile modulus at identical loadings. The modulus increased by 629% at 40 wt % KF35AS loading. For the other two systems, the KF35A system showed higher tensile modulus compared with the KF35 system at identical loadings, except for a slightly
Figure 7. SEM fractured surface of composites: (a),(d) KF35-30, (b), (e) KF35A-30, and (c),(f) KF35AS-30 at two magnifications. 12832
DOI: 10.1021/acs.iecr.5b03384 Ind. Eng. Chem. Res. 2015, 54, 12826−12835
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Industrial & Engineering Chemistry Research
Figure 8. (a) DSC curves of PBSA and representative (un)treated KFs-added composites at 10 °C/min cooling and (b) DSC heating curves of the 10 °C/min-cooled samples.
Figure 9. TGA-scanned curves of (a) PBSA and KF35AS-added composites and (b) PBSA and (un)treated 30 wt % KFs-added composites.
smaller value at 40 wt % loading. The elongation at break of PBSA drastically decreased (
