Thermo-Mechanical and Morphological Properties of ... - Springer Link

Fibers and Polymers 2014, Vol.15, No.6, 1303-1309

DOI 10.1007/s12221-014-1303-8

Thermo-Mechanical and Morphological Properties of Short Natural Fiber Reinforced Poly (Lactic Acid) Biocomposite: Effect of Fiber Treatment A. K. M. Moshiul Alam, M. F. Mina, M. D. H. Beg*, A. A. Mamun1, A. K. Bledzki1, and Q. T. H. Shubhra2 Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Gambang 26300, Kuantan, Malaysia 1 Institute of Material Engineering, University of Kassel, Germany 2 Faculty of Information Technology, University of Pannonia, Egyetem u.10, H-8200 Veszprém, Hungary (Received October 4, 2013; Revised December 23, 2013; Accepted January 4, 2014) Abstract: Untreated oil palm empty fruit bunch (REFB), alkali treated EFB (AEFB), ultrasound treated EFB (UEFB) and simultaneous ultrasound-alkali treated EFB (UAEFB) short fibers were incorporated in poly(lactic acid) (PLA) for fabricating bio-composites. The REFB fiber-PLA (REPC) and treated EFB (TEFB) fiber-PLA (TEPC) composites were prepared and characterized. Glass transition temperature, crystal melting temperature, decomposition temperature, melt flow index, density and mechanical properties (tensile strength, tensile modulus and impact strength) of TEPC are found to be higher than those of REPC. The observed crystallization temperature of TEPC is lower than that of REPC. Among all samples, TEPC prepared from UAEFB fiber shows better performances than other samples fabricated by REFB and AEFB fibers. Scanning electron microscopy, Fourier transform infrared spectroscopy and XRD analyses well support all the observed results. Keywords: Biopolymers and renewable polymers, Extrusion, Morphology, Differential scanning calorimetry (DSC), X-ray

respect, EFB fiber can play an important role, because both PLA and EFB fibers have the affinity to make hydrogen/ covalent bonds after melt processing [7,8] and this tendency can induce the crystallinity in PLA more than other nucleating agents. However, the inherent drawbacks of EFB fibers, such as high moisture absorption, low thermal stability and poor incompatibility with polymer matrix, can adversely affect the interaction between the fiber and PLA [7]. To overcome these drawbacks, alkaline treatment, heat treatment, acetylation, coupling agent treatment, etc. of fibers were proposed [9]. Of these techniques, alkali treatment is a potentially useful approach, leading to the increase in the number of reactive hydroxyl groups (-OH) on the fiber surface for chemical and hydrogen bonding [10]. A notable other method, not frequently employed for modifying EFB fiber surface, is the ultrasound treatment [8], which can also help the free -OH groups increase in fiber surface. Using the ultrasonic method, a few natural fibers other than EFB fiber have been treated to reinforce PLA [8,11,12], while published works on alkali treated natural fibers for composites are plenty [13-16]. Moreover, no published article is available in the literature on EFB fibers loaded PLA composites, where these fibers were processed simultaneously by ultrasound and alkali. Therefore, we have been enthusiastic to develop methods by using the ultrasound treatment under both water and alkaline medium for modifying EFB fibers in order to compare the effectiveness of ultrasonic energy in exposing -OH groups on the fiber surface. By this way, we expect that EFB fibers could make more compatible with PLA. Under these circumstances, the present study has been emphasized in order to explore the underlying phenomena affecting the crystalline structures, mechanical and thermal properties of

Introduction Owing to their advantages like light-weight, low-cost, availability, reproducibility, biodegradability and environmentally friendly characteristics over glass/carbon fibers, natural fibers have attracted increased attentions as suitable reinforcements in petroleum-based thermoplastics [1]. The reinforced thermoplastics are used as potential composite materials for applications in automotive, construction and building industries [2]. Despite huge markets of petroleum based thermoplastic composites, researchers are trying to reduce the consumption of feedstock energy and seek the alternative renewable resources for fabricating composite materials considering the clean environment. To alleviate the use of fossil-fuel derived thermoplastics, bio-based plastic like poly (lactic acid) (PLA), due to its ease of processes ability and eco-friendly behavior, has emerged as a promising candidate in composite manufacturing [3]. On the other hand, oil palm empty fruit bunch (EFB) fiber, among other natural fibers, is a readily obtainable fiber in some countries in the world, wasted in the premise of palm oil industries despite its huge potentials in many applications [4]. This trash of EFB fiber is polluting the environment and makes a threat to the human life. If these EFB fibers can be used in reinforcing bioplastics thus their use can be extended as well as their disposable problem can be resolved accordingly. It is notable that the use of PLA is restricted due to its relatively low toughness [5], which needs to be improved for its wider applications. Toughness of a material is generally related to its crystalline level that can be changed by incorporating various nucleating agents into it [6]. In this *Corresponding author: [email protected] 1303

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Fibers and Polymers 2014, Vol.15, No.6

the resulting composites prepared from these processed fibers.

Experimental Materials PLA thermoplastic was collected from Nature Works® PLA, 2002D, USA, it has derived from renewable resources and is specifically designed for extrusion/thermoforming applications. It is a clear extrusion sheet grade, having the melt-flow index of 5-7 g/10 min at 210 oC and specific gravity of 1.24. Oil palm EFB fibers were obtained from FELDA Palm Oil Refinery, Pahang, Malaysia. Sodium hydroxide (NaOH) (solubility 1090 g/l at 20 oC, molar mass 40 g/l, density 2.13 g/cm3 at 20 oC) and acetic acid (Density 1.06 g/cm3, water soluble at 20 oC) were purchased from Merck, Germany. Fibers Treatment and Composites Fabrication The EFB fibers were separately subjected to treatments in alkali medium and by ultrasound in both water and alkali medium using a Daihan Ultrasonic bath. The fiber treatments were conducted at 3 wt% NaOH solution, 60 minutes exposure time and 80 oC treatment temperatures with and without ultrasound, maintaining a weight ratio for EFB fiber to solution of 1:10. These were the optimal treatment parameters, as disclosed elsewhere [17]. After completing treatments, fibers were washed thoroughly with water-flow to remove alkali solution from the fiber surface and subsequently neutralized to pH 7 with a few drops of acetic acid solution and then washed and dried in an oven. Thereafter, they were chopped into a length of 2-3 mm by a chopper machine to produce short fibers. Four types of short EFB fibers used in this study were raw EFB (REFB), alkali-treated EFB (AEFB), ultrasound-treated EFB (UEFB) and simultaneous ultrasoundalkali treated EFB (UAEFB) fibers. Then, REFB, AEFB, UEFB and UAEFB fibers of 30 wt% were first extrusion molded and then injection molded with PLA at 180 oC to prepare the final composites, such as REPC, AEPC, UEPC and UAEPC, respectively. Tensile Testing of Composites Tensile testing of composites was conducted according to ASTM 638-08, using a Shimadzu (Model: AG-1) universal tensile testing machine fitted with a 5 kN load cell operated at a cross-head speed of 10 mm/min. Five replicates were evaluated for each type of samples for tensile strength (TS) and tensile modulus (TM) measurements, keeping 65 mm as the gauge length. Impact Testing Impact testing was carried out according to the EN ISO 179 standard by a Ray-Ran Pendulum Charpy Impact System. The impact velocity was 2.9 m/s and the hammer weight

A. K. M. Moshiul Alam et al.

was 0.475 kg for this test. Dimensions of the samples were 80×8×3.5 mm with a single notch of 0.25 mm. Five replicates were evaluated for each type of samples to obtain the impact strength (IS). Melt Flow Index The melt flow index (MFI) measurement was conducted by using a Dynisco MFI2 melt flow indexer, following the ASTM D1238 Standard test. Melt flow rates of the samples were measured by an extrusion plastometer, using an applied load of 2.16 kg at 210 oC. Three independent tests were carried out for each type of sample to obtain the MFI. Density Measurement The density of untreated and treated fibers and composites were measured by a Micromeritics AccuPyc II 1340 Gas Pycnometer through helium gas flow into the sample cell. The theoretical density of composites was calculated by the following equation given by Agarwal and Broutman [18]: 1 ρth = -----------------------------WPLA WEFB ------------ + -----------ρPLA ρEFB

(1)

where ρth, ρPLA and ρEFB, represent the theoretical densities of composite, PLA and EFB, respectively, and WPLA and WEFB represent the weights of PLA and EFB fiber, respectively. The ρth and experimental density (ρexp) values of composites were used to calculate the void fraction (Vf) in composites by using the following equation. ρth – ρexp - × 100% Vf % = ------------------ρth

(2)

Fourier Transformed Infrared Spectroscopy Chemical textures of composites with untreated and treated fibers were detected by a Nicolet 6700 FT-IR spectrometer, Thermo Scientific, Germany. Fourier transformation infrared (FTIR) spectroscopy was performed using the standard KBr pellet technique in the wave number range of 4000-500 cm-1. Scanning Electron Microscopy (SEM) Fractured surface morphologies of PLA composites with treated and untreated EFB fibers were investigated by using a scanning electron microscope (ZEISS, EVO50, Germany). Samples were mounted on aluminium stubs with carbon tape and then sputter coated with platinum to make them conductive prior to scanning electron microscopy (SEM). Differential Scanning Calorimetry Glass transition temperature (Tg), melting temperature (Tm) and crystallization temperature (Tc) were monitored by the differential scanning calorimetry (DSC) using a TA/ Q1000 apparatus under nitrogen atmosphere. In order to avoid moisture effect, all the test specimens were dried at 90 oC for 7 h prior to the measurements. For DSC measurements,

Short Natural Fiber Reinforced Biocomposite


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the samples were initially heated at temperatures in the range of 30-250 oC with a heating rate of 10 oC min-1, then cooled down to 30 oC with a cooling rate of 5 oC min-1 and re-heated from 30 to 250 oC to monitor calorimetric properties. The degree of DSC crystallinity (Xdsc) of the samples was calculated by the heat of fusion for the tested sample and a reference sample with 100 % crystallinity using the following relation [19]: ΔH Xdsc = ⎛ ----------------⎞ × 100 ⎝ ΔHm W⎠

(3)

where ∆H is the heat of fusion of the sample under study, ∆Hm of value 93.6 J g-1 represents the heat of fusion for 100 % crystalline PLA and W is the mass fraction of the matrix [12]. X-ray Diffraction Measurements X-ray diffraction (XRD) measurements were conducted on disc specimen for EFB fibers and tensile specimen for composites using a Rigaku MIniflex II, Japan, equipped with computer controlled software. The operating voltage and the tube current of the X-ray generator were 30 kV, 15 mA, respectively. For XRD studies of composites, tensile specimens were cut into a size of 2×2 cm. The specimens were step-wise scanned over the scattering angle (2θ) from 5 to 40 o, with a step of 0.02 o, using the CuKα radiation of wavelength λ=1.541 Å. The data were collected in terms of the diffracted X-ray intensity (I) versus 2θ. Thermogravimetric Analysis Thermogravimetric analysis (TGA) was carried out by a TGA Q500 V6.4, Germany in a platinum sample pan under nitrogen atmosphere (flow rate 60 ml/min) with a heating rate of 20 oC/min. The temperature range was scanned from 25 to 600 oC.

Result and Discussion FTIR Structural Analyses Figure 1 illustrates the FTIR spectra of various composites. Noticeable differences among the observed spectra of different composites are marked with arrows and dotted enclosures. The differences in the spectra inside the rectangular enclosure in the range 3300-3600 cm-1 gives an indication of the degree of hydrogen bond formation between the OH groups of fibers and the C=O or COOH groups of PLA. Evidence on esterification among OH groups of EFB fibers and terminal COOH groups of PLA is the intensity changes at 1756 cm-1 peak (arrows) [20]. Other significant differences regarding adhesion between fiber and PLA are the spectral changes in the range 1000-1300 cm-1. These may indicate that a fairly large number of available OH groups are exposed on the fiber surface after the alkali, ultrasound and ultrasound-alkali treatments. Conversely, a limited number

Figure 1. FTIR spectra of (a) REPC, (b) AEPC, (c) UEPC, and (d) UAEPC.

of the surface OH groups of untreated fibers takes part in bonding with the C=O and -COOH groups of PLA, as they are mostly engaged with impurities like lignin, pectin etc. Thus, the increased exposure of the OH groups of fibers provides improved potential for hydrogen and covalent bonds formation with C=O and COOH groups of PLA, respectively. We can, therefore, suggest an improved compatibility between treated EFB fibers and PLA matrix in AEPC, UEPC and UAEPC, where the effect of ultrasound for fiber-treatment is conspicuous. Surface Morphology SEM micrographs of the fractured surface of composites are presented in Figure 2, wherein holes, fiber pull-out, fiberfracture and PLA crystallites are marked. REPC surface seems to show fiber pulled out from the PLA matrix with the creation of noticeable large holes. These could be due to either free volumes developed in REPC or poor adhesion between REFB fibers and PLA. The poor adhesion is a result of the presence of impurities in the surface of untreated fibers. Contrary to these, the SEM micrographs of AEPC, UEPC and UAEPC show fiber-fracture and less holes, rather than fiber pull-out. Fiber fracture and fiber pull-out are indicators of the degree of stress transfer from the matrix to the fiber. The presence of fiber fracture and few holes in SEM micrographs of AEPC and UAEPC denote efficient stress transfer from the matrix when the load is applied. Thus, the SEM observations reveal a relatively poor interfacial adhesion in the REPC as compared to composites of the treated fibers. This improved adhesion of treated fibers with PLA may indicate the formation of hydrogen and covalent bonding in composites as discussed earlier. Improvement in physical and chemical bonding by treated fiber reinforced polymer composites has been reported earlier [21].

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Figure 2. SEM micrographs of (a) REPC, (b) AEPC, (c) UEPC, and (d) UAEPC.

Figure 3. XRD profiles of (a) REPC, (b) AEPC, (c) UEPC, and (d) UAEPC.

Structural Analyses by XRD Figure 3 represents the XRD profiles of different composites. The diffraction patterns from REPC contain a largely diffused peak with the peak-position at 2≈21 o, which is probably the combination of scattering intensities from the (200)p plane (at the vertical line) of the crystalline parts of PLA and the (200)f plane of REFB fibers [22]. The

diffraction pattern suggests a poor ordering of PLA molecules in REPC. In contrast, two distinct peaks appear in case of AEPC, UEPC and UAEPC. A shoulder is visible at 2θ ≈16 o, which has been indexed as (110)f plane of crystalline cellulose or (200)p plane of the α-form of PLA crystallites [23]. The (200)f peak intensity gradually increases and is shifted to the higher angle more for UAEPC than for UEPC or AEPC. The intensity of (200)p peak also increases. This increase in intensity and sharpness of the peak for the treated fiber based composites indicate that the degree of PLA crystallization increases more by treated fibers than by untreated fiber. Since fiber surfaces are modified by alkali treatment, hydrogen and covalent bonded interactions and mechanical interlocking between the fibre and PLA matrix can be improved. These increased interactions can help the fiber surface act as favorable nucleation sites for better crystallization in PLA and fibers. Glass Transition, Crystallization and Melting Temperatures DSC thermograms obtained from the 2nd heating course are shown in Figure 4. The pure PLA has been reported to display glass transition, cold crystallization and melting, which are characterized by temperatures Tg, Tc and Tm, respectively [24]. In this study, the composites show three distinct peaks, including the Tg and the Tc as well as the double-melting at two temperatures (Tm1 and Tm2), where Tm1



Figure 4. DSC thermograms of (a) REPC, (b) AEPC, (c) UEPC, and (d) UAEPC from the 2nd heating run.

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far we know that these effects of differently treated EFB fibers in PLA crystallization have not been published in any other article before. Considering crystal melting, it was suggested that that Tm1 is originated from small and imperfect crystals, which changed successively into more stable crystals through the melting and recrystallization and an exotherm between the two endothermic peaks is associated to recrystallization [26]. However, more detailed definitions have been given by Pan et al. [27], who described that Tm1 is associated with both the phase transition and the melting of the original α-phase crystals, while Tm2 arises from the αphase formed during the phase transition and meltrecrystallization. Other explanations have been the existence of either dual crystal structures in a polymer or dual lamellae population in the same crystalline structure of a polymer [28]. To distinguish the effect of differently treated fibers on crystallization in PLA, we invoke the free energy concept for the formation of a nucleus of a critical size [29]: 0

Table 1. The Tg, Tc, Tm, Xdsc (%) and Td for different samples Samples REPC AEPC UEPC UAEPC

o

Tg ( C) 56.5 59.9 60.6 62.2

Thermal properties Tc ( C) Tm1 ( C) Tm2 ( C) Xdsc (%) Td ( C) 119 140.1 146.6 31 324 108 143.6 150.8 35 332 105 143.6 151.9 41 346 101 143.6 151.9 43 364 o

o

o

o

and Tm2 correspond to the low- and high-temperature endotherms, respectively. The Tg, Tc, Tm, and Xdsc(%) values evaluated from the DSC runs are summarized in Table 1, showing different values for different samples. The observed Tg value in this study gradually increases based on fibers treatments according to the order UAEPC>UEPC>AEPC> REPC. Most often, it is a common trend to show a low Tg by a sample with a low degree of crystallinity, where the molecules can move easily. The more orderly crystalline state has the higher density, and consequently the noncrystalline molecular chains become constrained as they are anchored to the immobile crystallites. On the other hand, the increase in crystallinity in PLA has been found by the incorporation of nucleating agent, because this approach may lower the surface free energy barrier for nucleation that enables its crystallization at lower temperature on heating [25]. If we compare the degree of recrystallization in composites, REPC shows the highest Tc value (119 oC) and UAEPC shows the lowest value (101 oC). The lower Tc in composites implies that treated EFB fibers are more favorable nucleating agents for PLA crystallization during heating. The early crystallization of PLA by differently treated fibers is probably facilitated by the improved hydrogen bonding and covalent between treated fibers and PLA. As

4bσσe Tm * (4) ΔG = --------------------o ΔHm ΔT Obviously, G* increases with the decrease of ∆T. Using our DSC data for ∆T=Tm−Tc, we can infer that G* is the lowest for UAEPC followed by UEPC and AEPC. These results confirm that the energy barrier for nucleation in PLA is lowered by treated fibers, leading to an increase in the overall crystallization in PLA. Thus, we suggest that the increasing crystallization in biocomposites is due to the increased hydrogen/covalent interactions between PLA and treated fibers. Mechanical Properties The TS and TM of the composites are plotted in Figure 5, The TS and TM values are found to gradually increase with incorporation of TEFB fibers. These results indicate that simultaneous ultrasound-alkali treatment of the EFB fibers facilitates improvement in their mechanical performances.

Figure 5. Tensile strength and tensile modulus of composites.

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Figure 6. Impact strength and melt flow index of composites. Figure 7. A comparison between the experimental and theoretical densities of the composites.

The structures and properties of these samples are subsequently compared with those of REPC and UAEPC. The trend of IS and MFI changes are shown in Figure 6, apparently similar to the trend of TS and TM changes for all these samples investigated herein. The increasing trend in TS, TM, IS and MFI of composites occurs according to the following order: UAEPC>UEPC>AEPC>REPC. The increase in TS of UAEPC, UEPC and AEPC from REPC is 23.53, 1.96, 1.37 %, TM is 52.34, 0.43, 0.25 %, that IS is 47.24, 35.43, 2.36 %, and that MFI is 183, 145, 103 % respectively. These observations denote a significant enhancement in mechanical properties of composite materials prepared by simultaneous ultrasoundalkali treatment of EFB fibers. This enhancement in properties is due to the modification of fiber surface by different treatments and increased compatibility between EFB fibers and PLA, as demonstrated by FTIR, SEM, XRD and DSC studies. An increase in mechanical properties in chemically treated fiber reinforced polymer composites has been shown elsewhere [13,15]. Density and Void Fraction Analysis The ρth and ρexp values of different composites are plotted in Figure 7. The ρexp of composite increases depending on different treatments of fibers and the maximum value is observed in case of UAEFB incorporated composite. The ρth has been calculated by equation (1) using the measured density 1.15, 1.21, 1.24 and 1.27 g/cc for REFB, AEFB, UEFB and UAEFB fibers, respectively. It is found that the ρexp is lower than the ρth of the composites. Hence, the estimated void fraction Vf of the composites seems to reduce after incorporation of treated fiber, indicating the values of 10, 5, 4 and 2 % for REPC, AEPC, UEPC and UAEPC, respectively. These results demonstrate the increased compatibility between PLA and treated fibers, where simultaneous ultrasound and alkali treatment shows the best compatibility between them as compared to other treatments.

Figure 8. TGA thermograms of (a) REPC, (b) AEPC, (c) UEPC, and (d) UAEPC.

Thermal Degradation Figure 8 illustrates the thermal degradation of the composites investigated in a temperature range of 50-600 oC. TGA generally involves the release of absorbed water (if any) from a sample, the “onset” of degradation of molecules in the sample, the steps of degradation and the presence of residual char. The degradation in REPC composite begin at relatively lower temperature than that found in treated EFB incorporated composites such as AEPC, UEPC, and UAEPC. The TGA traces of composites seem to fall at 100 oC, indicating the emission of absorbed moisture. Besides, the TGA sharp-fall, which occurs at 275 oC for REPC look quite distinct from that for AEPC, UEPC, UAEPC, showing a two-step process in the temperature ranges of 275-380 oC and 380-519 oC. This may be connected to the decomposition behavior of the molecules of EFB fibers in the differently


treated composites. The degradation of natural fiber has been ascribed by the dissociation of C-C chain bonds along with H-abstraction at the site of dissociation [1]. Although, degradation temperature (Td) can be obtained from TGA traces, it is a common practice to consider the Td at 50 % weight loss of a sample as an indicator for the structural destabilization [29,30]. The Td values thus evaluated for REPC, AEPC, UEPC, and UAEPC from TGA curves are introduced in Table 1, revealing an increase in thermal stability in treated fiber loaded composites.

Conclusion Short EFB fiber reinforced PLA composites REPC, AEPC, UEPC and UAEPC have been fabricated with differently treated fibers. The corresponding glass transition temperatures of UAEPC, UEPC, AEPC and REPC are 62.2, 60.6, 59.9, 56.5 oC, in addition to, the crystallization temperatures are 101, 105, 108, 119 oC in that order, and the crystal melting temperatures are 151.9, 151.9, 150.8, 146.6 oC likewise. The treated fibers reinforced composites exhibit an increase in mechanical properties, such as tensile strength, tensile modulus, and impact strength along with melt flow index from REPC consequently. Furthermore, FTIR and SEM have provided clear evidences of more adhesion of PLA with treated fibers than with un-treated ones. Information on crystallization of the composites as observed by DSC and X-ray diffraction has been found to be consistent. Among various treatments, fibers treated by ultrasound in alkali medium have been found to be more effective than others. An increased thermal stability in UAEPC, UEPC and AEPC from REPC is observed. Therefore, these superior properties in treated fiber based composites are suggested to be due to the improved adhesion between PLA and fibers by hydrogen/covalent bond formations.

Acknowledgement Authors would like to acknowledge Ministry of Higher Education, Malaysia for providing financial support through the project FRGS (RDU 120106).

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