Polycaprolactone-Based Green Renewable ... - ACS Publications

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Polycaprolactone-Based Green Renewable Ecocomposites Made from Rice Straw Fiber: Characterization and Assessment of Mechanical and Thermal Properties Chin-San Wu* and Hsin-Tzu Liao Department of Chemical and Biochemical Engineering, Kao Yuan University, Kaohsiung County, Taiwan 82101, Republic of China ABSTRACT: The biodegradability and mechanical and thermal properties of composite materials made from maleic anhydridegrafted polycaprolactone (PCL-g-MA) and agricultural residues (rice straw fiber, RSF) were evaluated. Composites of PCL-g-MA and RSF (PCL-g-MA/RSF) exhibited noticeably superior mechanical properties compared to those of PCL/RSF due to greater compatibility of PCL-g-MA with RSF. The dispersion of RSF in the PCL-g-MA matrix was highly homogeneous due to ester formation between the anhydride groups of PCL-g-MA and the hydroxyl groups of RSF that resulted in the formation of branched and cross-linked macromolecules. In addition, the PLA-g-MA/RSF composites were more easily processed due to their lower melt viscosity. Water resistance of PCL-g-MA/RSF was higher than that of PCL/RSF, although weight loss of composites buried in soil compost indicated that both were biodegradable, especially at high levels of RSF substitution. The PCL/RSF and PCL-g-MA/RSF composites were more biodegradable than pure PCL, which implies a strong connection between RSF content and biodegradability. converted into industrially useful products.15,16 Composites of PCL and agricultural residues are not only expected to have markedly lower costs than pure PCL, but their biodegradability and mechanical properties can also be adjusted by varying their compositions.17 A significant hurdle in the production of PCL composites is that PCL is relatively hydrophobic while most agricultural residues are hydrophilica situation that leads to poor compatibility between the two phases of the composites. To alleviate this problem, “compatibilizers” in the form of reactive functional groups can be incorporated into the PCL polymer backbone to enhance the miscibility of the system and improve the composite’s overall mechanical properties.18 Maleated polymer is an effective compatibilizer in polymer/agricultural residue composites,19 and various coupling agents have similarly been used to reinforce other polymer/natural fiber composites.20 Numerous past studies have attempted to develop new composite synthetic plastics from biodegradable biopolymers.21,22 There is also growing interest in exploiting renewable resources as raw materials for the production of commercially useful biodegradable plastics.23 Several researchers have successfully developed composites of thermoplastic polymers and cellulose-lignin from rice straw fiber (RSF).24 RSF is an abundant, natural agricultural resource. Although cellulose fibers in plastic composites can yield many desirable properties, fiber dispersion and fiber-matrix compatibility remain problematic.25 In addition, although composites with high fiber content are inexpensive, high viscosities and other undesirable

1. INTRODUCTION Despite plastics recycling efforts, a large amount of plastic is not recovered and continues to be discarded in municipal landfills. There are a host of environmental problems associated with the disposal of plastics, and it has become increasingly difficult to find available landfill sites capable of handling the material. As a result, efforts to develop renewable, degradable, and recyclable materials, that is, green materials, have expanded in recent years.1−5 In particular, biodegradable polymer composites have attracted attention for their potential as a renewable raw material, especially in the context of reducing dependence on landfills for disposal of plastics. Several biodegradable aliphatic polymers, including poly(butylene succinate) (PBS), polycaprolactone (PCL), and poly(lactic acid) (PLA), have been identified for their commercial potential. In contrast to conventional plastics like polypropylene (PP) and polyethylene (PE), both of which require hundreds or even thousands of years to fully degrade, PCL biodegrades into naturally occurring products within only a few years.6,7 Because of its excellent biocompatibility, flexibility, and thermoplasticity, PCL and its copolymers have been proposed for use in various biomedical and biomaterial applications, and several commercially successful applications have emerged.8−11 Unfortunately, widespread commercialization of PCL has been limited because its production is both complex and expensive. However, these limitations can potentially be overcome by the use of PCL composites that combine PCL with polymers as well as natural fibers found in cost-effective biodegradable agricultural residues such as rice husks, rice straw, and corn stover, all of which are abundant, inexpensive, renewable, and fully biodegradable.12,13 Islam et al. explored the use of underutilized byproducts from both agricultural processing and industrial production to reduce environmental pollution.14 Materials including rice husks, rice straw, wheat straw, corn stover, and their composites were © 2012 American Chemical Society

Received: Revised: Accepted: Published: 3329

September 2, 2011 January 20, 2012 February 7, 2012 February 7, 2012 dx.doi.org/10.1021/ie202002p | Ind. Eng. Chem. Res. 2012, 51, 3329−3337

Industrial & Engineering Chemistry Research

Article

2.3. Determination of Grafting Percentage. The MA loading of the tetrahydrofuran-soluble polymer (expressed as grafting percentage) was calculated from the acid number and was determined as follows. First, about 2 g of copolymer was heated in 200 mL of refluxing tetrahydrofuran for 2 h. The hot solution was then titrated immediately with 0.03 N ethanolic potassium hydroxide (KOH) solution, which was standardized against a solution of potassium hydrogen phthalate, with phenolphthalein used as an indicator. The acid number was calculated using eq 1 below, and the grafting percentage was calculated using eq 2.26

rheological properties during processing have limited the application of these materials. To our knowledge, this is the first study using agricultural residues (RSF) as the filler for the preparation of PCL-based green renewable eco-composites. In this work, we studied the structural and thermal effects of replacing pure PCL with a more compatible maleic anhydride-grafted PCL (PCL-g-MA) in RSF-containing composites. The composites were characterized to identify bulk structural changes induced by the MA moiety using Fourier transform infrared (FTIR) spectroscopy, 13 C nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). In addition, the water resistance and biodegradability of the composites were assessed by measuring the water absorption and weight loss of samples buried in soil.

acid number (mg KOH/g) =

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial grade PCL (CAPA 6800) with a molar mass of 80 000 g/mol was obtained from Solvay Chemicals (Green River, WY). MA and benzoyl peroxide (BPO) were supplied by Aldrich Chemical Corporation (Milwaukee, WI). Before use, the MA was purified by recrystallization from chloroform. BPO, used as the polymerization initiator, was purified by dissolution in chloroform and reprecipitation from methanol. Other reagents were purified using conventional methods. RSF was obtained from Lu-chu (Taiwan, R.O.C.). 2.2. Grafting Reaction and Sample Preparation. The grafting reaction of MA onto PCL is illustrated in Scheme 1.

VKOH (mL) × CKOH(N) × 56.1 polymer (g)

grafting percentage (%) =

acid number × 98.1 100 2 × 561

(1)

(2)

The grafting percentage was found to be 1.02 wt % when BPO and MA loadings were maintained at 0.3 wt % and 10 wt %, respectively. 2.4. RSF Processing. RSF was extracted as a byproduct of rice husk processing from 4- to 5-month Penglai rice supplied by Lu-chu of Taiwan. As shown in Scheme 2, purification consisted of immersing 60 g of ground and dried RSF in 1000 mL distilled water for 2 days to remove any water-soluble components. The product was then dried at 50−60 °C for 2 days under vacuum. The resulting brown fibers were 1−2 cm long. The fibers were dried, ground, and sorted. After grinding, the fiber mixture consisted of a fine brown powder with single, dispersed pale-yellow fibers about 100−350 μm long. The samples were passed through 80-mesh (0.178 mm) and 100mesh (0.152 mm) sieves, air-dried for 2 days at 50−60 °C, and vacuum-dried for at least 6 h at 100−110 °C until the moisture content fell to 5 ± 2%. 2.5. Composite Preparation. Prior to composite fabrication, RSF samples were cleaned with acetone and dried in an oven at 105 °C for 24 h. Composites were prepared in a Plastograph 200-Nm mixer W50EHT with a blade rotor (Brabender, Dayton, OH). The blends were mixed between 70 and 80 °C for 20 min at a rotor speed of 50 rpm. Composite samples were prepared with RSF:PCL or RSF:PCL-g-MA mass ratios of 10/90, 20/80, 30/70, and 40/60. Residual MA in the PCL-g-MA reaction mixtures was removed via acetone extraction prior to the preparation of PCL-g-MA/RSF composites. After mixing, the composites were pressed into thin plates with a hot press and placed in a dryer for cooling. These thin plates were cut to standard sample dimensions for further characterization. 2.6. NMR, FTIR, and XRD Analyses. Solid-state 13C NMR spectra were acquired with an AMX-400 NMR spectrometer at 100 MHz under cross-polarization while spinning at the magic angle. Power decoupling conditions were set with a 90° pulse and a 4-s cycle time. Infrared spectra of the samples were obtained using an FTS-7PC FTIR spectrophotometer (BioRad, Hercules, CA). XRD diffractograms were recorded using a D/max 3-V X-ray diffractometer (Rigaku, Tokyo, Japan) with a Cu target and Kα radiation at a scanning rate of 2° per min. 2.7. DMA, DSC, and TGA Analyses. A TA Instruments model 2080 DMA (New Castle, DE) operating in film tension mode was used to evaluate phase compatibility in the composites. The test specimen was a rectangular thin film

Scheme 1. The Grafting Reaction of MA onto PCL

During the preliminary test, using tetrahydrofuran as the solvent, different amounts of BPO and MA were used in the grafting reaction under the conditions of 40 ± 2 °C and 60 rpm for 10 h. The best grafting percentage was 1.02 wt % when BPO and MA loadings were maintained at 0.3 and 10 wt %, respectively. The grafted product (4 g) was then dissolved in 200 mL of tetrahydrofuran at 40 ± 2 °C, and the hot solution was filtered through several layers of cheesecloth. The cheesecloth was washed with 600 mL of acetone to remove the tetrahydrofuran-insoluble, unreacted MA, and the remaining product was dried in a vacuum oven at 50 °C for 24 h. The tetrahydrofuran-soluble component in the filtrate was extracted five times using 600 mL of cold acetone for each extraction. 3330

dx.doi.org/10.1021/ie202002p | Ind. Eng. Chem. Res. 2012, 51, 3329−3337


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Scheme 2. Modification of PCL with Agricultural Residues (Rice Straw Fiber, RSF) and the Preparation of Composite Materials

25 × 5 × 1 mm3. The tests were performed at a frequency of 1 Hz, a strain level of 0.075%, and a temperature rising from −100 to +60 °C at a heating rate of 3 °C/min. Static forces were selected from the experimental results in the linear elastic region in the absence of drawing effects. To specify this force, several stress−strain experiments were conducted beforehand, so that the ratio of static force to dynamic force could be kept constant during the measurements. The glass-transition temperatures (Tg), melting temperatures (Tm), and heats of fusion (ΔHm) were determined with a differential scanning calorimeter (TA Instruments model 2010 DSC, New Castle, DE). Sample quantities ranged from 4 to 6 mg; melting curves were recorded between −100 and 100 °C at a heating rate of 10 °C per min. Values of Tg, Tm, and ΔHm were extracted from the temperatures and areas of melting peaks in the DSC heating thermograms. Thermogravimetric analysis (TA Instruments model 2010 TGA, New Castle, DE) was used to assess whether interactions between the two organic phases influenced the thermal degradation of the composite. Samples were placed in alumina crucibles and tested with a thermal ramp over a temperature range of 30−600 °C at a heating rate of 10 °C per min, and then the initial decomposition temperatures (IDTs) of the composites were obtained. 2.8. Mechanical Testing. A mechanical tester (Lloyd Instruments, model LR5K) was used to determine each specimen’s tensile strength and elongation at break in accordance with ASTM D638. Test specimens were prepared in a hydraulic press at 80 °C and conditioned at 50 ± 5% relative humidity for 24 h before making measurements. Measurements were made using a crosshead speed of 20 mm/min. Five specimens were tested, and a mean value was determined. 2.9. Composite Morphology. A thin film (150 × 150 × 1 mm3) of each composite was prepared with a hydraulic press and treated with hot water at 60 °C for 24 h. Specimens were cut according to ASTM D638. After rupture, a thin section of the fracture plane was removed. The thin sections were then coated with gold, and the fracture surface morphologies were observed using a scanning electron microscope (SEM, Hitachi Microscopy model S-1400, Tokyo, Japan).

2.10. Water Absorption. Samples were prepared for water absorption measurements by cutting the composites into 50 × 25 mm strips (150 ± 5 μm thickness) in accordance with ASTM D570. The samples were dried in a vacuum oven at 50 ± 2 °C for 8 h, cooled in a desiccator, and then immediately weighed to the nearest 0.001 g. This weight was designated Wc. Thereafter, the samples were immersed in distilled water and maintained at 25 ± 2 °C for a 14-week period. During this time, they were removed from the water at 2-week intervals, gently blotted with tissue paper to remove excess water from their surfaces, immediately weighed to the nearest 0.001 g three times, and then returned to the water. An average value of the weight measured at each 2-week interval was calculated, and these average weights were designated Ww. The percentage of weight increase due to water absorption (Wf) was calculated to the nearest 0.01% according to %Wf =

Ww − Wc 100 Wc

(3)

2.11. Biodegradation Studies. Biodegradability of the samples was assessed by measuring the weight loss of the composites over time in soil. Samples measuring 50 × 30 × 1 mm3 were weighed and buried in boxes containing alluvial-type soil obtained from farmland topsoil before planting. The soil was sifted to remove large clumps and plant debris. Procedures for soil burial were as described by Alvarez et al.27 Soil was maintained at approximately 35% moisture by weight, and the samples were buried at a depth of 12−15 cm. A control box consisted of samples and no soil. The samples were unburied after 2 weeks, washed in distilled water, dried in a vacuum oven at 50 ± 2 °C for 3 days, and equilibrated in a desiccator for at least 1 day. The samples were then weighed before being returned to the soil.

3. RESULTS AND DISCUSSION 3.1. Characterization of PCL and its composites. The FTIR spectra for pure PCL and PCL-g-MA are shown in Figure 1 bands A and B, respectively. The characteristic absorption bands of PCL at 3300−3700, 1700−1750, and 500−1500 cm−1 3331



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Figure 1. FTIR spectra of (A) PCL, (B) PCL-g-MA, (C) PCL/RSF (20 wt %), and (D) PCL-g-MA/RSF.

appeared in the spectra of both polymers.28 Two extra shoulders characteristic of anhydride carboxyl groups were observed at 1786 and 1857 cm−1 in the modified PCL-g-MA spectrum. Similar results have been reported previously.29,30 The shoulders represent free acid in the modified polymer PCL-g-MA and thus indicate successful grafting of MA onto PCL. The FTIR spectrum of RSF (Figure 1E) exhibited peaks at 3200−3500, 1600−1700, and 1100−1500 cm−1 attributable to hydroxyl groups and −CO stretching.31 The −OH stretching peak at 3200−3700 cm−1 intensified in the PCL/RSF (20 wt %) composite (Figure 2C) due to additional vibrational contributions from the −OH groups of RSF. As shown in Figure 2D, the FTIR spectrum of the PCL-gMA/RSF (20 wt %) composite exhibited a peak at 1738 cm−1 that was not present in the FTIR spectrum of the PCL/RSF (20 wt %) composite. The appearance of this peak was attributed to the ester carbonyl stretching vibration of the copolymer and is consistent with results obtained by Kim et al.32 Collectively, these FTIR spectral data suggest the formation of branched and cross-linked macromolecules in the PCL-g-MA/RSF composite through covalent linkage of the anhydride carboxyl groups in PCL-g-MA and the hydroxyl groups of RSF. To further confirm this finding, solid-state 13C NMR spectra of PCL and PCL-g-MA were obtained and compared; the results are shown in Figure 2 bands A and B, respectively. Three peaks were observed, corresponding to carbon atoms in the unmodified PCL (1, δ = 64.3 ppm; 2, δ = 28.9 ppm; 3, δ = 25.8 ppm; 4, δ = 25.1 ppm; 5, δ = 34.4 ppm; 6, δ = 172.9 ppm).33 The 13C NMR spectrum of PCL-g-MA showed additional peaks (7, δ = 42.3 ppm; 8, δ = 36.2 ppm; 9, −CO δ = 174.3 ppm), thereby confirming that MA was covalently grafted onto PCL. The solid-state 13C NMR spectra of PCL/RSF (20 wt %), PCL-g-MA/RSF (20 wt %), and RSF are shown in Figure 3 (C−E, respectively). The spectra are similar to those reported by Xiao et al.34 Relative to unmodified PCL/RSF (Figure 3C), additional peaks at δ = 42.3 ppm (7) and δ = 36.2 ppm (8) were observed in the spectra of PCL-g-MA/RSF composites

Figure 2. Solid-state 13C NMR spectra of (A) PCL, (B) PCL-g-MA, (C) PCL/RSF (20 wt %), (D) PCL-g-MA/RSF (20 wt %), and (E) RSF.

Figure 3. X-ray diffraction patterns for (A) PCL, (B) PCL/RSF, (C) PCL-g-MA/RSF, and (D) RSF.

(Figure 3D); these results are consistent with previous studies and indicate grafting of MA onto PCL.35 However, the carbonyl (CO) peak at δ = 174.3 ppm (Figure 2B, peak 9), which is also typical for MA grafted onto PCL, was absent in the solid-state spectrum of PCL-g-MA/RSF (20 wt %). The absence of this peak was most likely a result of an additional 3332



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To evaluate compatibility, the dynamic mechanical properties of the PCL/RSF and PCL-g-MA/RSF composites were measured (Figure 5). Figure 5 shows the variation of G′ and

condensation reaction between the anhydride group of MA and the −OH group of RSF that split the carbonyl peak into two bands (δ = 177.3 and 178.1 ppm). This additional condensation reaction converted the fully acylated groups in the original RSF to esters (represented by peaks 10 and 11 in Figure 2C) and did not occur between PCL and RSF, as indicated by the absence of corresponding peaks in the FTIR spectrum of PCL/RSF (20 wt %) in Figure 2D. The formation of ester groups significantly affects the mechanical, thermal, and biodegradation properties of PCL-g-MA/RSF and is discussed in greater detail in the following sections. 3.2. X-ray Diffraction. X-ray diffraction patterns of pure PCL, PCL/RSF (20 wt %), PCL-g-MA/RSF (20 wt %), and RSF are shown in Figure 3A−D. Similar to the results of Chen and Chang,36 pure PCL (Figure 3A) exhibited two diffraction peaks at about 23.7° and 21.2°, designated in the Figure as “1” and “2”, respectively. An additional peak was observed for the PCL/RSF composites (Figure 3B) at about 15.2°, designated as peak 3. Peak 3 was likely caused by a change in the organization of PCL molecules upon blending with RSF, and it indicates that the RSF was physically dispersed throughout the PCL matrix.37 Figure 3C shows an additional peak at 18.1° (designated 4) in the diffraction pattern of the PCL-g-MA/RSF composites. This peak, also identified by Danyadi et al.,38 was likely caused by the formation of ester carbonyl groups, indicating that the crystalline structure of the RSF composites was altered when PCL-g-MA was used in place of PCL. Figure 3D shows that the XRD pattern of RSF contained two peaks at 22.6° and 15.1°. Chen et al. reported a similar result.39 3.3. Torque Measurements during Mixing and DMA Analysis. The effects of RSF content and mixing time on the melt torque of PCL/RSF and PCL-g-MA/RSF composites are shown in Figure 4. When preparing PCL/RSF or PCL-g-MA/

Figure 5. Dynamic viscoelastic behaviors of PCL/RSF and PCL-gMA/RSF composites at various RSF contents.

G″ with temperature for these composites. Regardless of the RSF content, G′ and G″ increase with increasing temperature, and a transition can be seen in the vicinity of −40 °C. Roughly speaking, G′ and G″ at a given temperature increase with increasing RSF content. Furthermore, the temperature dependence of G′ and G″ of PCL-g-MA/RSF composites is weaker than that of PCL/RSF composites, that is, G′ and G″ remain at a high level over a wide temperature range. This is because of the formation of ester carbonyl groups. The results indicate that there is only a small enhancement in the rigidity of the amorphous phase and a decrease in the chain mobility for the PCL-g-MA/RSF composites, because the compatibility between the PCL and RSF is poor. The increase in the peak widths of the composites could also explain the poorer elongational properties. 3.4. Morphology and Mechanical Properties of PCL and Its Composites. In most composite materials, effective wetting and uniform dispersion of all components in a given matrix and strong interfacial adhesion between the phases are required to obtain a composite with satisfactory mechanical properties. In the current study, RSF may be considered as a dispersed phase within a PCL or PCL-g-MA matrix. To evaluate the composite morphology, SEM was used to examine tensile fracture surfaces of PCL/RSF (20 wt %) and PCL-gMA/RSF (20 wt %) samples. The SEM microphotograph of PCL/RSF (20 wt %) in Figure 6A shows that the RSF in this composite agglomerated into bundles unevenly distributed in the matrix. This poor adhesion was due to the formation of hydrogen bonds between RSF and the disparate hydrophilicities of PCL and RSF. Poor wetting in these composites was also observed (marked in Figure 6A) and was attributed to large differences in surface energy between the RSF and the PCL matrix.41 The PCL-g-MA/RSF (20 wt %) microphotograph in Figure 6B shows more homogeneous adhesion and better wetting of RSF in the PCL-g-MA matrix, as indicated by the complete coverage of PCL-g-MA on the fiber and the removal of both materials when a fiber was pulled from the bulk. This improved interfacial adhesion was due to the similar hydrophilicity of the two components, which allowed for the

Figure 4. Torque values as a function of mixing time for PCL/RSF and PCL-g-MA/RSF composites at various RSF contents.

RSF, the polymer was first melted, and then the RSF (in a fibrous state) was added. Hence, the resulting polymer composite contained fibrous filler. Torque values decreased with increasing RSF content and mixing time, and they approached a stable value when the mixture was mixed after 8 min. The final torque values decreased with increasing RSF content in the composites because the melt viscosity of RSF was lower than that of either PCL or PCL-g-MA. In addition, the melt viscosity values of the PCL-g-MA/RSF composites were significantly lower than those of PCL/RSF composites at the same RSF content. According to Tronc et al.,40 improved rheological behavior in MA-grafted PCL is due to the formation of ester carbonyl groups (as discussed earlier), which leads to conformational changes in the polymer. 3333



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PCL/RSF composites (Figure 7A), the tensile strength decreased markedly and continuously with increasing RSF content (from 37.5 to 15.9 MPa). This was attributed to poor dispersion of RSF in the PCL matrix, as previously discussed and as shown in Figure 5A. The effect of this incompatibility on the mechanical properties of the composites was substantial. The PCL-g-MA/RSF composites shown in Figure 7A exhibited unique behavior; tensile strength at break increased with increasing RSF content despite the fact that PCL-g-MA had a lower tensile strength than pure PCL. It was also found that the higher grafting extent of MA could lead the larger tensile strength at break for the PCL-g-MA/RSF composites. Furthermore, the tensile strength of the PCL-g-MA/RSF composites was constant or slowly declined with RSF content greater than 20 wt %. This behavior was likely due to enhanced dispersion of RSF in the PCL-g-MA matrix resulting from the formation of branched or cross-linked macromolecules.42 As shown in Figure 7B, elongation at break was lower for the PCL/RSF composites than for the PCL-g-MA/RSF composites. In PCL/RSF, the RSF tended to agglomerate into bundles, which is indicative of poor compatibility between the two phases. As shown by the solid line in Figure 7B, the elongation at break in the PCL-g-MA/RSF composites also decreased with increasing amounts of RSF. However, these values were still lower than those of pure PCL. Comparing PCL/RSF and PCL-g-MA/RSF, it can be seen that tensile strength and elongation of the latter were approximately 8−25 MPa and 50−200% higher than those of the former. For the PCL-g-MA/RSF composites, it was also found that the effect of grafting extent of MA on the elongation was the same as the tensile strength at break. 3.5. Thermal Properties of PCL and Its Composites. The heats of fusion (ΔHf), melt temperatures (Tm), and glass transition temperatures (Tg) of PCL/RSF and PCL-g-MA/RSF composites with different RSF content were determined using DSC. The results are tabulated in Table 1. For both

Figure 6. SEM microphotographs showing the distribution and wetting of RSF in PCL/RSF (20 wt %) and PCL-g-MA/ RSF (20 wt %) composites.

formation of both branched and cross-linked macromolecules and prevented unfavorable hydrogen bonding in the RSF. Figure 7 shows the variation in tensile strength and elongation at break with RSF content for PCL/RSF, PCL-g-

Table 1. Effects of RSF Content on the Thermal Properties of PCL/RSF and PCL-g-MA/RSF Composites PCL/RSF

PCL-g-MA/RSF

RSF (wt %)

Tg (°C)

Tm (°C)

ΔHm (J/g)

Tg (°C)

Tm (°C)

ΔHm (J/g)

0 10 20 30 40

−59.5 −56.8 −53.6 −52.7 −51.9

62.5 61.1 60.2 59.5 58.9

72.6 53.7 42.6 36.1 31.5

−58.5 −54.6 −50.1 −48.7 −47.7

61.5 59.9 58.5 58.1 57.8

67.5 58.2 51.6 46.2 44.5

composites, Tm decreased with increasing RSF content, presumably due to the aforementioned decreasing melt viscosity. At the same RSF content, the PCL/RSF composite had a higher Tm than the PCL-g-MA/RSF composite. This result is consistent with the corresponding torque measurements shown in Figure 4. The lower melt viscosity indicated that PCL-g-MA/RSF was more easily processed than PCL/ RSF. As shown in Table 1, the glass transition temperature (Tg) increased with increasing RSF content for both PCL/RSF and PCL-g-MA/RSF composites. This increase was likely a result of reduced space available for molecular motion as RSF content in the composites increased. Tg values were higher for the PCL-gMA composite by about 1−5 °C, suggesting that grafting of MA onto the PCL further restricted molecular motion.

Figure 7. Effect of RSF content on the (A) tensile strength and (B) elongation at break for PCL/RSF, PCL-g-MA/RSF-1 (grafting percentage = 0.51 wt %) and PCL-g-MA/RSF-2 (grafting percentage = 1.02 wt %) composites.

MA/RSF-1, and PCL-g-MA/RSF-2 composites. The tensile strength and elongation of pure PCL (37.5 MPa and 675%) decreased after grafting with MA (36.1 MPa and 655%). For 3334



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The ΔHf of pure PCL was 72.6 J/g, whereas that of PCL-g-MA was 67.5 J/g. The lower ΔHf of PCL-g-MA was likely due to the grafted branches, which disrupted the regularity of the chain structures in PCL and increased spacing between the chains.43 The values of ΔHf for PCL-g-MA/RSF were approximately 5−15 J/g higher than those for PCL/RSF, likely due to the formation of ester carbonyl groups as discussed above. The ΔHf may be used to indicate composite crystallinity. Whereas the ΔHf of both PCL/RSF and PCL-g-MA/RSF composites decreased with increasing RSF content, the extent of the decrease was significantly greater in PCL/RSF, indicating a lower degree of crystallinity in PCL/RSF compared to PCL-gMA/RSF. These results are analogous to those obtained by Wu et al. for a maleated polyester bioplastic/natural fiber composite system.44 The marked decrease in crystallinity of PCL/RSF compared to PCL-g-MA/RSF was most likely a result of the sterically hindered motion of the PCL polymer segments due to the presence of RSF in the composite matrix; the hydrophilic nature of RSF led to poor dispersibility with the more hydrophobic PCL-g-MA polyester.45 The thermal stabilities of the PCL, PCL-g-MA, RSF, and the composites were measured by TGA under a nitrogen atmosphere. Because thermal decomposition can cause defunctionalization of RSF, we used TGA to determine the effect of RSF content on the weight loss of the composites. The results of these analyses are presented in Figure 7 and Table 2.

chains by RSF, and/or the condensation reaction, which leads to increased adhesion of RSF with PCL-g-MA compared to ungrafted PCL. These results are similar to those obtained for other polymer composites with natural fibers46 and are a consequence of the difference in interfacial forces in the two composites: PCL/RSF has relatively weak hydrogen bonds, while PCL-g-MA/RSF has strong coordination sites associated with the presence of anhydride groups. The thermal stability (IDT) values for PCL-g-MA/RSF composites were approximately 7−15 °C higher than those of analogous PCL/RSF composites. As discussed above, the higher IDT values were likely due to the formation of ester carbonyl groups in the PCLg-MA/RSF composites. 3.6. Water Absorption of PCL and Its Composites. At the same RSF content, the PCL-g-MA/RSF composites exhibited a higher resistance to water absorption than did the PCL/RSF composites (Figure 9). The water resistance of the

Table 2. Effect of RSF Content on the Thermal Properties of PCL/RSF and PCL-g-MA/RSF Composites RSF (wt %)

PCL/RSF IDT (°C)

PCL-g-MA/RSF IDT (°C)

0 10 20 30 40

337 321 305 293 283

333 328 316 307 298

Figure 9. The percent weight gain due to the absorption of water for PCL, PCL-g-MA, PCL/RSF, and PCL-g-MA/RSF composites.

For both composites, IDT decreased with increasing RSF content. This result may be attributed to a looser PCL or PCLg-MA polymer structure caused by RSF-induced expansion. For the same RSF content, the PCL/RSF composite had a lower IDT than the PCL-g-MA/RSF composite (Figure 8 and

PCL-g-MA/RSF composites was moderate, and it is likely that the hydrophobicity of RSF was enhanced by interactions with the MA-grafted PCL. As expected, for both PCL/RSF and PCL-g-MA/RSF, the percent water gain over the 14-week test period increased with RSF content. Because the arrangement of polymer chains in these systems is random, this result could be attributed to both decreased chain mobility in composites with greater amounts of RSF and to the hydrophilic character of RSF, which weakly adheres to the more hydrophobic PCL. 3.7. Biodegradation of PCL and Its Composites. Figure 10 shows the changes in weight ratio (degraded sample/initial sample) over time for the PCL/RSF and PCL-g-MA/RSF composites buried in soil compost. In the test environment, water diffused into the polymer, caused swelling, and enhanced biodegradation. As expected, the weight loss of PCL was somewhat smaller than that of PCL-g-MA because the latter had better water absorption. For both the PCL/RSF and PCLg-MA/RSF composites, the degree of weight loss increased with RSF content. Composites with 40% RSF degraded rapidly over the first 6 weeks; these composites lost a mass equivalent to their approximate RSF content and showed a gradual decrease in weight over the following 2 weeks. PCL-g-MA/RSF exhibited a weight loss of approximately 3−8 wt %.

Figure 8. Effect of RSF content on TGA for PCL/RSF and PCL-gMA/RSF composites.

Table 2). The reduced IDT in PCL/RSF can be attributed to two factors: increased inhibition of movement of polymer 3335



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Figure 10. Weight loss percentages of PCL, PCL-g-MA, PCL/RSF, and PCL-g-MA/RSF composites as a function of incubation time in soil compost.

4. CONCLUSIONS The component compatibility and thermal and mechanical properties of RSF composites with PCL and maleic anhydridegrafted PCL (PCL-g-MA) were examined. FTIR, NMR, and XRD analyses revealed the formation of ester groups formed by reactions between −OH groups in RSF and anhydride groups in PCL-g-MA. These ester groups significantly altered the crystal structures of the PCL-g-MA/RSF composites compared to the PCL/RSF composites. Although DSC tests indicated a decrease in the melting temperatures of both PCL/RSF and PCL-g-MA/RSF with increasing RSF content, the PCL-g-MA/ RSF composites were more easily processed due to lower melt temperatures and mixing torques. The morphology of PCL-gMA/RSF composites was consistent with good adhesion between the RSF phase and the PCL-g-MA matrix. In mechanical tests, PCL-g-MA enhanced the mechanical properties of the composites, especially the tensile strengths. The glass transition temperature of PCL-g-MA/RSF was higher than that of PCL/RSF, indicating more hindered molecular motion. The water resistance of PCL-g-MA/RSF was higher than that of PCL/RSF. When incubated in soil, the biodegradation rate of PCL-g-MA/RSF was lower than that of PCL/RSF, while still higher than that of pure PCL. The degree of biodegradation increased with increasing RSF content. Finally, there was a conflict between mechanical properties and biodegradability of the composites. For commercial applications of the proposed composites, the balance between mechanical properties and biodegradability of the composites should be considered.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +886-7-6077685. Fax: +886-7-6077788. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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