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polylactic acid (PLA) green composites
Accepted Manuscript Preparation, Characterization and Crystallization Kinetics of Kenaf Fiber/Multiwalled Carbon Nanotube/Polylactic Acid (PLA) Green Composites
Po-Yuan Chen, Hong-Yuan Lian, Yeng-Fong Shih, Su-Mei Chen-Wei, Ru-Jong Jeng PII:
S0254-0584(17)30367-X
DOI:
10.1016/j.matchemphys.2017.05.006
Reference:
MAC 19679
To appear in:
Materials Chemistry and Physics
Received Date:
15 October 2016
Revised Date:
30 April 2017
Accepted Date:
06 May 2017
Please cite this article as: Po-Yuan Chen, Hong-Yuan Lian, Yeng-Fong Shih, Su-Mei Chen-Wei, Ru-Jong Jeng, Preparation, Characterization and Crystallization Kinetics of Kenaf Fiber/Multiwalled Carbon Nanotube/Polylactic Acid (PLA) Green Composites, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.05.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Kenaf fibers (KF) and MWCNT are used to reinforce PLA.
Functionalized KF and MWCNT significantly improve the physical properties of PLA.
KF and MWCNT speed up crystalline growth and enrich the crystallinity of PLA.
The reinforcement is helpful to turn PLA crystallinity from 3D to 2D structure.
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Preparation, Characterization and Crystallization Kinetics of Kenaf Fiber/Multiwalled Carbon Nanotube/Polylactic Acid (PLA) Green Composites Po-Yuan Chena, Hong-Yuan Lianb, Yeng-Fong Shih*a, Su-Mei Chen-Weib, Ru-Jong Jengc aDepartment
of Applied Chemistry, Chaoyang University of Technology, No. 168,
Jifeng E. Rd., Wufeng District, Taichung 41349, Taiwan, R.O.C. bDepartment
of Advanced Coating, Division of Applied Chemistry, Material and
Chemical Research Laboratories, Industrial Technology Research Institute, No.321, Sec.2, Kuang Fu Rd., Hsinchu, 30011, Taiwan, R.O.C. cInstitute
of Polymer Science and Engineering, National Taiwan University, No.1, Sec.
4, Roosevelt Rd., Taipei, 10617, Taiwan, R.O.C.
* Correspondence to: Yeng-Fong Shih Email: [email protected]
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Abstract In this study, chemically functionalized reinforcements, such as Kenaf fiber (KF) and multi-walled carbon nanotubes (MWCNTs), are used to enhance the crystallinity, mechanical properties and heat resistance of polylactic acid (PLA) composites. The functionalized KF and MWCNTs evidence excellent compatibility in the PLA matrix through the generation of chemical bonding, and a transcrystalline structure is generated around the reinforcements, thereby significantly improving the physical properties. The DSC analysis reveals that functionalized KF can speed up crystalline growth and increase the crystalline content. The annealing treatment further drives the PLA recrystalization and enhances crystallinity, according to the DSC results. Moreover, the Avrami equation shows that the functionalized KF and MWCNTs are helpful in turning the PLA crystallinity from a 3-dimensional bulk (or sphere) structure into a 2dimensional sheet structure.
Keywords: Kenaf fiber, polylactic acid, multi-walled carbon nanotubes, annealing
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1. Introduction Traditional petroleum-based plastics are used widely due to their high strength, durability,
stability
and
well-developed
processes.
However,
the
inherent
hydrophobicity, high molecular weight and additives, such as antioxidants and stabilizers, of petroleum-based plastics make decomposition difficult. Plastic waste now accounts for some 150 million tons per year, but landfills and incineration treatments present serious pollution issues, such as desertification, toxic substances, greenhouse gases, etc. As the awareness of these hazards as well as the necessity of environment protection has increased, polymer materials which naturally decompose, such as polylactic acid (PLA), polyhydroxyalkanoates (PHA) and polybutylene succinate (PBS), have been investigated [1-4]. Of these green materials, PLA has been shown to have superior properties, such as a high melting point (~170℃), low glass transition temperature (~60℃) and high mechanical strength. As a thermoplastic, PLA usually undergoes preheating before the forming process. Due to its semi-crystalline nature, PLA re-crystallizes during heating and results in distinct physical properties.[5-7] Therefore, crystallization and crystallization kinetics play important roles in the chosen manufacturing process and product performance. Some studies have reported that blending with other polymers, fillers or nucleation agents can improve PLA crystallinity [8-13]. However, these additives may prevent the PLA composite from biodegrading or result in a poor physical performance due to the poor compatibility between the heterogeneous phases. Taking into account biodegradation and physical performance, in this study we used Kenaf fiber (KF), an efficient lignocellulosic crop that can reach a height of 4-6 m and can yield 24 Mg/ha in 5-7 months with low inputs [12], to serve as both
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reinforcement and nucleation agent. To overcome the poor compatibility between the hydrophilic KF and the slightly hydrophobic PLA matrix, the KF was modified with a functional group that could chemically bond to the PLA matrix. Moreover, to increase the conductivity of PLA for antistatic applications, the KF was blended with small amounts of multi-walled carbon nanotubes (MWCNTs) [13]. The purpose of this research is to develop a green composite which exhibits not only desirable biodegradability, low density and good mechanical properties, but anti-static characteristic as well. Thus, this green composite can be used for high value-added fields. In addition to the surface modification of KF, this study also investigated the influence of KF content and annealing treatment on the mechanical properties, heat resistance and crystallinity kinetics of PLA.
2. Materials and methods 2.1 Materials PLA pellets (MW > 100000) were purchased in 2003D grade from NatureWorks. The KF and MWCNTs were provided by JPSeco and Scientech Corporation, respectively.
Sodium
hydroxide
(NaOH),
acetic
acid,
acetone,
3‐
glycidoxypropyltrimethoxysilane (OX-silane) and nitric acid were purchased from Echo Chemical.
2.2 Surface modification of reinforcement The raw KF was pretreated with a 4% NaOH solution for 5 min to remove the lignin, hemicellulose and impurities. Then, it was neutralized using 1% acetic acid before being dried at 60℃. The purified KF was coupled to OX-silane by immersion in
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a 3% silane/acetone solution for 48 h at room temperature; the solvent was then removed, and after drying at 60℃ for 24 h, the modified KF (KF-OX) was ready for use. The MWCNTs were modified using a carboxyl group via acid treatment under reflux at 120℃ for 1 h; during the acidization, each 1 g of MWCNTs was treated with 100 ml of HNO3. After cooling, the modified MWCNTs were filtered and washed in water several times to remove residual HNO3 before being dried.
2.3 Preparation of PLA composite The PLA, KF, and MWCNTs had been dried in an oven at 100℃ for 4 h under reduced pressure until the moisture content was below 1.0 wt%. Immediately after drying, the PLA pellets were pre-melted at 175℃ in a counter-rotating internal mixer (Brabender PL2000, Duisburg, Germany) with a rotation speed of 50 rpm. Subsequently, the MWCNTs were loaded into the mixer and blended with PLA for 5 min. KF was then further loaded into the mixer and blended with the mixture for another 10 min. Afterwards, the PLA composite was granulated. The sample was then processed by compression molding at 175℃ for 5 min (under 20 kgf/cm2 or 50 kgf/cm2), 3 min (under 75 kgf/cm2) and 1 min (under 100 kgf/cm2) in sequence. In the annealing treatment, the PLA composite was further heated at 80℃ for 8 h. The formulations for the samples are shown in Table 1.
2.4 Characterizations The infrared spectra were obtained with an FTIR spectrometer (Paragon 500, PerkinElmer) with a resolution of 2 cm−1 that scanned 50 times from 300 to 4000 cm−1 at room temperature. All film samples were taken using the conventional KBr disk
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method. X-ray diffraction (XRD) was performed using a PANalytical X'Pert PRO using Cu-Kαradiation (λ = 0.154nm) at a scanning rate of 1∘/min. Differential scanning calorimetry (DSC) was performed under nitrogen flow by a TA Instruments Q20 apparatus; the scanning included the following three cycles: (1) keep at 40℃ for 3 min followed by heating to 180℃ at a heating rate of 10℃/min, and terminate at 180℃ for 5 min; (2) cool from 180℃ to 40℃ at a cooling rate of 5℃/min, and keep at 40℃ for 5 min; and (3) heat from 40℃ to 180℃ at a heating rate of 10℃/min. For the analysis of isothermal crystallization kinetics, samples of 5-10 mg were weighed and placed in the crucibles. The temperature was rapidly raised to 190℃, and then the thermal history was erased by holding the sample at this temperature for 3 min. The isothermal crystallization kinetics of neat PLA and KF/CNTs/PLA composites was investigated at their peak crystallization temperatures (Tpeak) (that is, the temperature where each sample’s crystallization rate was at its highest). After the removal of the thermal histories, the samples were rapidly cooled down, at 20 ℃/min, to the test temperature and the temperature was maintained until full crystallization occurred. Moreover, the morphology was observed with a Carl Zeiss polarizing microscope (POM) equipped with a Mettler-Toledo hot stage. The samples were melted at 190℃ for 10 min and then cooled down to 130℃ at 20℃/min for isothermal crystallization to occur. The tensile test was carried out according to the ASTM D638 test method at a strain rate of 50 mm/min using an Instron universal tester, model HT-9102 (Hung Ta Instrument Co., Taiwan). The impact strength was analyzed according to the ASTM D256 test method using an impact resistance testing machine, model GT-70045-MDL (Gotech Testing Machines Co., Taiwan). A GT-HV 2000 analyzer was used to measure the heat
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deflection temperature (HDT) of the composites. The specimen was loaded for the three-point bending test in the edgewise direction with a load of 66 psi, and the temperature was increased at a rate of 2°C/min until the specimen deflected 0.25 mm, as specified by ASTM D648. All of the results were taken as the average value of five samples.
3. Results and discussion 3.1 Surface modification of MWCNTs and functionalization of KF In the FTIR spectrum for surface modified MWCNTs (Figure 1(1)), the absorption peaks at 2919 cm−1 and 2850 cm−1 are assigned to the C-H asymmetric and symmetric stretching vibration, whereas the peaks at 1634 cm−1 and 1249 cm−1 are assigned to the conjugated C=C and C-O stretching. The peak around 3436 cm−1 is the characteristic absorption peak for hydroxyl group, resulting from the acidic modification of MWCNTs [14]. KF is an important natural reinforcement material which has excellent mechanical properties because of its cellulose content. However, in raw KF, impurities such as lignin and hemicellulose can reduce its strengthening effect. In order to remove these impurities and thereby ensure the best reinforcement, the KF was purified using a general alkali treatment [15]. In the FTIR spectrum of the raw Kenaf (KF-UN), shown in Figure 1(2), the existence of lignin and hemicellulose was indicated from the C=O (1737 cm-1) and C-O (1261 cm-1) peaks [16]. The C=O and C-O peaks disappeared after the alkali treatment (KF-Alkali), thus demonstrating the successful removal of the impurities. During the surface modification, as shown in Scheme 1(1), the silane underwent hydrolysis-condensation or hydrogen bonding to the KF, and grafted the KF
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with the epoxy groups (KF-OX). The KF-OX appeared as Si-O-Si (897 cm-1) and Si-Ocellulose (1203 cm-1) peaks, indicating that the silane underwent condensation with another silane (Si-O-Si) and KF (Si-O-cellulose); these peaks demonstrated the successful modification using silane coupling agents [17].
3.2 X-ray diffraction (XRD) Figure 2 shows the XRD patterns of bare PLA, PLA composite and the specimens after annealing. The bare PLA exhibited a trace of crystallinity before annealing, as identified by the broad peak around 2θ =16~20o, which indicated that the PLA chains were poorly ordered (i.e., low degree of crystallinity). This could have been due to the rapid cooling of the PLA during molding [18]. After annealing, the PLA-A showed sharp peaks at 16.7° and 19.1°, attributed to the lattice planes (110/200) and (203) of α crystal form of PLA, respectively.[19] After adding unmodified KF, PF30-UN, a broad peak appeared at 23.0° corresponding to the lattice plane (002) for cellulose I.[20] The PC1F30-OX was similar to bare PLA in that a broad peak was apparent around 2θ =16~20o, indicating the composite’s low degree of crystallinity before annealing. After annealing, PC1F30-OX-A showed sharp peaks at 16.7° and 19.1° corresponding to the PLA crystallinity of the (110/200) and (203) phases. This was due to molecules migrating in the crystal lattice and the number of dislocations decreasing in the annealing procedure, leading to the higher degree of crystallinity.
3.3 Differential scanning calorimetry (DSC) The DSC curves shown in Figure 3 illustrate the phase transition of the PLA composite during heating; the derived transition temperatures are summarized in Table
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2. Before annealing, the bare PLA showed a sharp exothermic peak at 106.2℃ (Tc) for re-crystallization. The re-crystallized PLA started to melt and contributed two endothermic peaks above 140℃, with the small peak at 146.8℃ (Tm1) and the large peak at 156.6℃ (Tm2) attributed to imperfect and perfect crystallinity, respectively.[21] After introducing KF-OX and MWCNTs, all transition temperatures (Tg, Tc, Tm1 and Tm2) decreased, and the decrements below 30 wt% of KF-OX appeared to be positive to the KF-OX content. Generally speaking, the Tc decrement was contributed by the nucleating effect of KF and led to the faster recrystallization. In the PLA composite reinforced only by unmodified KF (PF30-UN), the DSC curve showed characteristic peaks similar to that of bare PLA, except for a slight shift to a lower temperature. By using KF-OX, PC1PF30-OX obviously decreased the Tc to 84.5℃, demonstrating that the chemical bonding between KF-OX and PLA acted effectively as a nucleating agent due to the better dispersion. Furthermore, the additions of KF-OX and MWCNTs were likely to help the crystal growth reach uniform lamellar thickness. Therefore, all the crystals melted at the same temperature. However, the crystallization peak of bare PLA is still large as compared with the composites. This indicates that the addition of MWCNTs and KF may play an important role in accelerating the crystallization process, but not increasing the crystallinity of PLA [22,23]. Figure 3(2) shows the DSC curves of PC1F10-OX before and after annealing, with the peak variations reflecting the annealing effect on the crystalline phase. Before annealing, PC1F10-OX had a curve similar to that of the bare PLA, except for a smaller Tm1 peak. On the other hand, for the PC1PF10-OX-A, the Tg and Tc peaks vanished after the annealing treatment and the Tm increased. This demonstrated that the recrystallization was totally finished during the annealing and resulted in more perfect
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crystallinity.
3.4 Isothermal crystallization kinetics The relative crystallinity (Xt) can be derived by the division calculation between the exothermal area at specific periods and the whole peak area; the obtained isothermal crystallization kinetics. To deduce the nucleation mechanism and crystal growth, the Avrami equation was used to fit the isothermal crystallization kinetics. The Avrami equation is defined as follows: [24, 25]
1 X t exp K T t n
where Xt: relative degree of crystallinity at different periods K(T): rate constant of isothermal crystallization n: Avrami index for describing nucleation mechanism and crystal growth t: time By plotting log(-ln(1-Xt)) versus log(t), n and K can be obtained; the related parameters are summarized in Table 3. In the PC1F-OX series containing 10-30 wt% KF-OX, the K value increased dramatically with the higher KF content. For example, PC1F30-OX showed an extremely high K value of 0.0513 compared to the bare PLA (1.4855×10-7). The results of the K value demonstrated that KF-OX accelerated the crystallization rate. However, the K values of PC1F40-OX and PF30-UN (3.510×10-4 and 4.169×10-3) were smaller than that of PC1F30-OX. This indicates that the well dispersed plant fibers in PC1F30-OX can more effectively accelerate the crystallization rate. On the other hand, the aggregates of excessive plant fibers in PC1F40-OX and poor compatibility of the un-treated plant fibers with PLA in PF30-UN both reduced the
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accelerating effect. The rebounding K values of PC1F40-OX and PF30-UN due to poor dispersion were evidenced by the slowing crystallization rate, which prolonged the recrystallization time. The n value in the Avrami equation can be used to estimate crystallinity growth. In the bare PLA, re-crystallization started from homogeneous nucleation, and the n value near to 4 indicates that the crystallinity grew as a 3-dimensional bulk or sphere structure. After introducing KF-OX and MWCNTs, the crystallinity nucleation became heterogeneous, with the decreasing n values coming from the heterogeneous growth independent of time. The n values of PC1F10-OX, PC1F20-OX and PF30-UN were close to 3, which meant their crystals grew as a 3-dimensional bulk or sphere structure. The n value of PC1F30-OX decreased to nearly 2, which explained the crystals now growing as a 2-dimensional sheet. In PC1F40-OX, the n value rebounded to nearly 3, indicating serious fiber entanglement and confined growth space, which resulted in crystals growth in 3-dimensions.
3.5 Tensile strength The composition and physical properties of the PLA composite are summarized in Table 4. As shown in Figure 4, before annealing, the KF-OX was effective in reinforcing the PLA composite. For example, PC1F30-OX increased the tensile strength to 78.5 MPa, 57% higher than that of the bare PLA. The effective promotion of tensile strength was inferred to the chemical reaction between the KF-OX and PLA, as shown in Scheme 1(2), and the accompanying compatibility served to optimize the stress delivery, resulting in high tensile strength. In PC1F40-OX, the high content of fibers resulted in their being seriously entangled, which impeded the stress delivery and the
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tensile strength dropped to 47.7 MPa. As a semi-crystalline polymer, PLA can re-crystallize into spherical microdomains via annealing.[26, 27] In our study, the annealed composite only enhanced the tensile strength when the KF content was higher than 20 wt%. Therefore, it was deduced that a low KF-OX content was not enough to drive the generation of the transcrystalline structure required for high strength [28, 29]. On the other hand, a high KF-OX content helped develop a transcrystalline structure around the fibers (as shown in Figure 5) , with the PC1F30-OX-A boosting the tensile strength to 91.5 MPa, 84% higher than that of the bare PLA before annealing. This was because the crystalline morphology which formed at the interfaces between the polymer matrix and fibers had a significant influence on the mechanical properties of the composites. The high heterogeneous nucleation activity on the surface of the fibers forced the polymer to grow in one direction, resulting in the formation of a columnar crystalline layer perpendicular to the axis of the fiber surface. This is known as transcrystallinity.[28, 30] The presence of an anisotropic layer, such as the transcrystallinity in the composite material, effectively provides a high resistance to external loading due to the enhanced fiber/matrix adhesion. The enhanced interfacial properties also allowed for a more effective transfer of applied stress, resulting in the superior strength and stiffness of the composite material. Although the seriously entangled fibers of the PC1F40-OX-A reduced the tensile strength to 53.6 MPa, the re-crystallization still promised a higher strength than that of the PC1F40-OX.
3.6 Impact strength As shown in Figure 6, the impact reinforcement before annealing appeared to be
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positive to the KF content, with the PC1F40-OX impact strength approaching a high of 43.8 J/m, 149% higher than that of the bare PLA. The impact enhancement of the KFOX was because of its hollow cellulose structure, which tolerates higher deformation under impact than a solid structure.[31] After annealing, PLA-A re-crystallized, which elevated the impact strength to 30.9 J/m, 76% higher than that of the PLA before annealing. However, the impact strength was similar before and after annealing for the composites with different KF contents. In composites, the interface plays a key role in transferring the stress from the matrix to the fiber. A weak interface generally results in low strength and stiffness, but high resistance to fracture, whereas a strong interface produces high strength and stiffness, but often low resistance to fracture [32]. Ye et al. [33] found that the toughness of GF/PP thermoplastic composites was decreased as the degree of crystallinity was increased. The high crystallinity matrix with high modulus cannot transfer external stress effectively to the reinforcing fiber. Lin et al. [34] also discovered that the increased degree of crystallinity enhanced the stiffness but reduced the ductility of the PLA. The tested specimen broke early because of its brittleness and easy crack propagation through a grain boundary of large spherulites. Moreover, the crystallinity was not the single reason, but more due to the various mesostructures, i.e. the existence of different spherulitic morphologies near or far from the fibers resulted from the annealing procedure.
3.7 Heat distortion temperature The heat distortion temperature (HDT) reflects the heat resistance of the composite; the HDT results are summarized in Figure 7 and Table 4. Before annealing, the bare
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PLA exhibited an HDT of 60.6℃, while the HDT of the PC1F30-OX composite decreased slightly to 58.1℃. Moreover, the HDT of the composite without MWCNTs (PC30-OX) was 60.7℃, which was higher than that of PC1F30-OX. This might be the relatively high KF to CNT ratios in the composites, meaning that the former effectively builds up the ‘‘backbone” of samples and is expected to play a dominating role in the HDT properties [35]. Tang et al. [36] found a percolation threshold of crystallinity corresponding to HDT. HDT would start to increase when the crystallinity exceeds 20%. Moreover, other researchers [37] found that the heat of fusion (related to crystallinity) of the semicrystalline biopolymers would decrease, particularly in the low fiber content biocomposites. Therefore, lower crystallinity might possibly lead to lower HDTs for the unannealed samples. On the other hand, annealing significantly enhanced the HDT. For example, the HDT of PLA-A and PC1F40-OX-A increased to 102.5℃ and 134.6℃, 69% and 122% higher than that of the PLA before annealing, respectively. Moreover, the HDT of the composite without MWCNTs after annealing (PC30-OX-A) was 129.5 ℃, which was higher than that of PC1F30-OX-A (125.5℃). This is analogous to those mentioned above. To sum up, the re-crystallization induced by annealing made the PLA composite more resistant to high temperature, and the transcrystalline structure growth around the KF-OX further increased its heat resistance.
4. Conclusions The KF-OX improved the compatibility in the PLA matrix and resulted in better mechanical and heat properties. With the annealing treatment, more transcrystallinity and perfect crystallinity developed around KF-OX during re-crystallization and further enhanced the physical performance. The DSC analysis revealed that the KF-OX
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accelerated the rate of crystalline growth and accounted for the more perfect crystallinity. In the analysis using the Avrami equation, the KF-OX addition obtained a higher rate constant (K) evidencing the fast crystallization, and the Avrami index (n) indicated that PLA crystallinity growth turned into a 2-dimensional sheet structure. Moreover, it is found that the existence of crystallographic relationships between the structures of the fiber and the polymer crystals is a key parameter to determine the properties of the composites. Finally, this study shows that the annealing procedure may be a route to tune the microstructure and physical properties of a polymer, which might open a promising door to produce the natural fiber/PLA green composites without using any environmentally harmful reagents. Our study indicates that the annealing treatment plays an important role in controlling the microstructure as well as influencing the final mechanical performances of the PLA composites.
References [1] Chen G-Q, Wu Q. The application of polyhydroxyalkanoates as tissue engineering materials. Biomaterials. 2005;26(33):6565-6578. [2] Garlotta D. A literature review of poly(lactic acid). J Polym Environ. 2002;9(2):6384. [3] Lasprilla AJR, Martinez GAR, Lunelli BH, Jardini AL, Maciel Filho R. Poly-lactic acid synthesis for application in biomedical devices - A review. Biotechnol Adv. 2012;30(1):321-328. [4] Chen D, Li J, Ren J. Influence of fiber surface-treatment on interfacial property of poly(L-lactic acid)/ramie fabric biocomposites under UV-irradiation hydrothermal aging. Mater Chem Phys. 2011;126(3):524-531.
15
ACCEPTED MANUSCRIPT
[5] Solarski S, Ferreira M, Devaux E. Characterization of the thermal properties of PLA fibers by modulated differential scanning calorimetry. Polymer. 2005;46(25):1118711192. [6] As'habi L, Jafari SH, Khonakdar HA, Haussler L, Wagenknecht U, Heinrich G. Non-isothermal crystallization behavior of PLA/LLDPE/nanoclay hybrid: synergistic role of LLDPE and clay. Thermochim Acta. 2013;565:102-113. [7] Chiu H-T, Huang S-Y, Chen Y-F, Kuo M-T, Chiang T-Y, Chang C-Y, et al. Heat treatment effects on the mechanical properties and morphologies of poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends. Int J Polym Sci. 2013:951696, 951612 pp, doi:10.1155/2013/951696. [8] Ren Z, Dong L, Yang Y. Dynamic mechanical and thermal properties of plasticized poly(lactic acid). J Appl Polym Sci. 2006;101(3):1583-1590. [9] Picard E, Espuche E, Fulchiron R. Effect of an organo-modified montmorillonite on PLA crystallization and gas barrier properties. Appl Clay Sci. 2011;53(1):58-65. [10] Kawamoto N, Sakai A, Horikoshi T, Urushihara T, Tobita E. Nucleating agent for poly(L-lactic acid) - an optimization of chemical structure of hydrazide compound for advanced nucleation ability. J Appl Polym Sci. 2007;103(1):198-203. [11] Suryanegara L, Nakagaito AN, Yano H. Thermo-mechanical properties of microfibrillated cellulose-reinforced partially crystallized PLA composites. Cellulose. 2010;17(4):771-778. [12] Brown RC, Brown TR, Editors. Biorenewable Resources: Engineering New Products from Agriculture, 2nd Edition: Wiley-Blackwell; 2014. [13] Lin W-Y, Shih Y-F, Lin C-H, Lee C-C, Yu Y-H. The preparation of multi-walled carbon nanotube/poly(lactic acid) composites with excellent conductivity. J Taiwan Inst
16
ACCEPTED MANUSCRIPT
Chem Eng. 2013;44(3):489-496. [14] Aviles F, Cauich-Rodriguez JV, Moo-Tah L, May-Pat A, Vargas-Coronado R. Evaluation of mild acid oxidation treatments for MWCNT functionalization. Carbon. 2009;47(13):2970-2975. [15] Mohsenzadeh A, Jeihanipour A, Karimi K, Taherzadeh MJ. Alkali pretreatment of softwood
spruce
and
hardwood
birch
by
NaOH/thiourea,
NaOH/urea,
NaOH/urea/thiourea, and NaOH/PEG to improve ethanol and biogas production. J Chem Technol Biotechnol. 2012;87(8):1209-1214. [16] Magniez K, Voda AS, Kafi AA, Fichini A, Guo Q, Fox BL. Overcoming Interfacial Affinity Issues in Natural Fiber Reinforced Polylactide Biocomposites by Surface Adsorption of Amphiphilic Block Copolymers. ACS Appl Mater Interfaces. 2013;5(2):276-283. [17] Le Moigne N, Longerey M, Taulemesse J-M, Benezet J-C, Bergeret A. Study of the interface in natural fibres reinforced poly(lactic acid) biocomposites modified by optimized organosilane treatments. Ind Crops Prod. 2014;52:481-494. [18] Oksman K, Skrifvars M, Selin JF. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos Sci Technol. 2003;63(9):1317-1324. [19] Sullivan ME, Moon JR, Kalaitzidou K. Processing and Characterization of Cellulose Nanocrystals/Polylactic Acid Nanocomposite Films. Materials. 2015;8(12). [20] Chen Q, Wang Q, Mitsumura N, Niida H. Improved cellulose by ionic liquid mixture with solid acid catalysis and its application in polyethylene glycol liquefaction. Mater Sci Appl. 2013;4(12):839-845. [21] Tabi T, Sajo IE, Szabo F, Luyt AS, Kovacs JG. Crystalline structure of annealed polylactic acid and its relation to processing. eXPRESS Polym Lett. 2010;4(10):659-
17
ACCEPTED MANUSCRIPT
668. [22] Robles E, Urruzola I, Labidi J, Serrano L. Surface-modified nano-cellulose as reinforcement in poly(lactic acid) to conform new composites. Ind Crop Prod. 2015;71:44-53. [23] Gil-Castell O, Badia JD, Kittikorn T, Str€omberg E, Ek M, Karlsson S, RibesGreus A. Impact of hydrothermal ageing on the thermal stability, morphology and viscoelastic
performance
of
PLA/sisal
biocomposites.
Polym
Degrad
Stab.
2016;132:87-96. [24] Augis JA, Bennett JE. Calculation of the Avrami parameters for heterogeneous solid state reactions using a modification of the Kissinger method. J Therm Anal. 1978;13(2):283-292. [25] Lorenzo AT, Arnal ML, Albuerne J, Mueller AJ. DSC isothermal polymer crystallization kinetics measurements and the use of the Avrami equation to fit the data: Guidelines to avoid common problems. Polym Test. 2007;26(2):222-231. [26] Rathi S, Chen X, Coughlin EB, Hsu SL, Golub CS, Tzivanis MJ. Toughening semicrystalline poly(lactic acid) by morphology alteration. Polymer. 2011;52(19):41844188. [27] Fang Q, Hanna MA. Rheological properties of amorphous and semicrystalline polylactic acid polymers. Ind Crops Prod. 1999;10(1):47-53. [28] Xu H, Xie L, Chen Y-H, Huang H-D, Xu J-Z, Zhong G-J, et al. Strong Shear FlowDriven Simultaneous Formation of Classic Shish-Kebab, Hybrid Shish-Kebab, and Transcrystallinity in Poly(lactic acid)/Natural Fiber Biocomposites. ACS Sustainable Chem Eng. 2013;1(12):1619-1629. [29] Xu H, Xie L, Jiang X, Hakkarainen M, Chen J-B, Zhong G-J, et al. Structural Basis
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for Unique Hierarchical Cylindrites Induced by Ultrahigh Shear Gradient in Single Natural Fiber Reinforced Poly(lactic acid) Green Composites. Biomacromolecules. 2014;15(5):1676-1686. [30] Quan H, Li Z-M, Yang M-B, Huang R. On transcrystallinity in semi-crystalline polymer composites. Compos Sci Technol. 2005;65(7-8):999-1021. [31] Kalia S, Dufresne A, Cherian BM, Kaith BS, Avérous L. Cellulose-Based Bio- and Nanocomposites: A Review2011. [32] Liu FP, Wolcott MP, Gardner DJ, Rials TG. Characterization of the interface between cellulosic fibers and a thermoplastic matrix, Compos Interface. 1994; 2(6):419432. [33] Ye L, Beehag A, Friedrich K. Mesostructural aspects of interlaminar fracture in glass fiber-reinforced thermoplastics: Is crystallinity a key? Compos Sci Technol. 1995;53(2):167-173. [34] Li L, Hashaikeh R, Arafat HA. Development of eco-efficient micro-porous membranes via electrospinning and annealing of poly (lactic acid). J Membr Sci. 2013; 436: 57-67. [35] Shen Z, Bateman S, Wu DY, McMahon P,
Dell’Olio M, Gotama J. The effects of
carbon nanotubes on mechanical and thermal properties of woven glass fibre reinforced polyamide-6 nanocomposites. Compos Sci Technol. 2009;69(2):239-244. [36] Tang Z, Zhang C, Liu X, Zhu J. The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent. J Appl Polym Sci. 2012;125(2):1108-15. [37] Sanchez-Garcia MD,
Gimenez E, Lagaron JM. Morphology and barrier
properties of solvent cast composites of thermoplastic biopolymers and purified
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cellulose fibers. Carbohydr Polym. 2008;71 (2):235-244.
(1)
(2)
Figure 1. FTIR spectrum of (1) surface modified MWCNTs and (2) raw KF (KF-UN), purified KF (KFAlkali) and KF modified with OX-silane (KF-OX)
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(1)
(2)
Scheme 1. Reaction mechanism of (1) modification on KF surface by OX-silane and (2) reaction between PLA and OX-grafted KF
Figure 2. XRD patterns of PLA and composites before and after annealing
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(1)
(2)
Figure 3. DSC curves at (1) reheating and (2) first heating
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Figure 4. Tensile strength of PLA composite before and after annealing treatment
Figure 5. POM micrograph of PC1F30 OX
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Figure 6. Impact strength of PLA composite before and after annealing treatment
Figure 7. HDT of PLA and composite before and after annealing treatment
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Table 1. Formulations of samples
PLA (wt%) PLA 100 PC1F10-OX 89 PC1F20-OX 79 PC1F30-OX 69 PC1F40-OX 59 PLA-A 100 PC1F10-OX-A 89 PC1F20-OX-A 79 PC1F30-OX-A 69 PC1F40-OX-A 59 PF30-UN 70 *The KF is un-modified.
Modified MWCNTs content (wt%) 0 1 1 1 1 0 1 1 1 1 0
Modified KF content (wt%)
Annealing treatment
0 10 20 30 40 0 10 20 30 40 30*
No No No No No Yes Yes Yes Yes Yes No
Table 2. Thermal properties derived from DSC measurement at reheating
PLA PC1F10-OX PC1F20-OX PC1F30-OX PC1F40-OX PF30-UN
Tc (℃) 106.2 96.8 90.7 84.5 91.8 98.3
Tm1 (℃) 146.8 143.3 141.1
Tm2 (℃) 156.6 154.9 152.6 151.0 153.2 152.1
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Table 3. Parameters of isothermal crystallization kinetics derived from Avrami equation
PLA PC1F10-OX PC1F20-OX PC1F30-OX PC1F40-OX PF30-UN
Crystallization time (min) 120 49 47 16 22 37
n 3.69 2.82 2.73 1.83 3.24 2.80
K (min-n) 1.486×10-7 8.691×10-5 2.181×10-4 0.0513 3.510×10-4 4.169×10-3
R2 0.9886 0.9940 0.9815 0.9969 0.9906 0.9903
Table 4. Mechanical properties of PLA and its composites TS IS TS* IS** Increment Increment (MPa) (J/m) (%) (%)
PLA 49.7±4.9 PC1F10-OX 61.6±8.0 PC1F20-OX 62.8±5.2 PC1F30-OX 78.5±3.4 PC1F40-OX 47.7±8.5 PLA-A 46.8±5.1 PC1F10-OX-A 61.6±3.6 PC1F20-OX-A 70.4±3.5 PC1F30-OX-A 91.5±8.6 PC1F40-OX-A 53.6±8.0 *TS: tensile strength **IS: impact strength
23 26 57 -4 -5 23 41 84 7
17.5±2.6 30.8±4.1 36.8±0.7 37.4±1.9 43.8±4.5 30.9±7.5 30.3±1.3 35.5±1.5 35.4±1.7 44.9±3.1
76 110 113 149 76 73 102 102 156
t1/2 (min) 50.00 22.45 22.60 3.98 9.60 8.37
HDT (℃)
HDT Increment (%)
60.6 59.3 57.7 58.1 58.1 102.5 113.7 120.9 125.5 134.6
-2 -5 -4 -4 69 87 99 107 122
26
Po-Yuan Chen, Hong-Yuan Lian, Yeng-Fong Shih, Su-Mei Chen-Wei, Ru-Jong Jeng PII:
S0254-0584(17)30367-X
DOI:
10.1016/j.matchemphys.2017.05.006
Reference:
MAC 19679
To appear in:
Materials Chemistry and Physics
Received Date:
15 October 2016
Revised Date:
30 April 2017
Accepted Date:
06 May 2017
Please cite this article as: Po-Yuan Chen, Hong-Yuan Lian, Yeng-Fong Shih, Su-Mei Chen-Wei, Ru-Jong Jeng, Preparation, Characterization and Crystallization Kinetics of Kenaf Fiber/Multiwalled Carbon Nanotube/Polylactic Acid (PLA) Green Composites, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.05.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Kenaf fibers (KF) and MWCNT are used to reinforce PLA.
Functionalized KF and MWCNT significantly improve the physical properties of PLA.
KF and MWCNT speed up crystalline growth and enrich the crystallinity of PLA.
The reinforcement is helpful to turn PLA crystallinity from 3D to 2D structure.
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Preparation, Characterization and Crystallization Kinetics of Kenaf Fiber/Multiwalled Carbon Nanotube/Polylactic Acid (PLA) Green Composites Po-Yuan Chena, Hong-Yuan Lianb, Yeng-Fong Shih*a, Su-Mei Chen-Weib, Ru-Jong Jengc aDepartment
of Applied Chemistry, Chaoyang University of Technology, No. 168,
Jifeng E. Rd., Wufeng District, Taichung 41349, Taiwan, R.O.C. bDepartment
of Advanced Coating, Division of Applied Chemistry, Material and
Chemical Research Laboratories, Industrial Technology Research Institute, No.321, Sec.2, Kuang Fu Rd., Hsinchu, 30011, Taiwan, R.O.C. cInstitute
of Polymer Science and Engineering, National Taiwan University, No.1, Sec.
4, Roosevelt Rd., Taipei, 10617, Taiwan, R.O.C.
* Correspondence to: Yeng-Fong Shih Email: [email protected]
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Abstract In this study, chemically functionalized reinforcements, such as Kenaf fiber (KF) and multi-walled carbon nanotubes (MWCNTs), are used to enhance the crystallinity, mechanical properties and heat resistance of polylactic acid (PLA) composites. The functionalized KF and MWCNTs evidence excellent compatibility in the PLA matrix through the generation of chemical bonding, and a transcrystalline structure is generated around the reinforcements, thereby significantly improving the physical properties. The DSC analysis reveals that functionalized KF can speed up crystalline growth and increase the crystalline content. The annealing treatment further drives the PLA recrystalization and enhances crystallinity, according to the DSC results. Moreover, the Avrami equation shows that the functionalized KF and MWCNTs are helpful in turning the PLA crystallinity from a 3-dimensional bulk (or sphere) structure into a 2dimensional sheet structure.
Keywords: Kenaf fiber, polylactic acid, multi-walled carbon nanotubes, annealing
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1. Introduction Traditional petroleum-based plastics are used widely due to their high strength, durability,
stability
and
well-developed
processes.
However,
the
inherent
hydrophobicity, high molecular weight and additives, such as antioxidants and stabilizers, of petroleum-based plastics make decomposition difficult. Plastic waste now accounts for some 150 million tons per year, but landfills and incineration treatments present serious pollution issues, such as desertification, toxic substances, greenhouse gases, etc. As the awareness of these hazards as well as the necessity of environment protection has increased, polymer materials which naturally decompose, such as polylactic acid (PLA), polyhydroxyalkanoates (PHA) and polybutylene succinate (PBS), have been investigated [1-4]. Of these green materials, PLA has been shown to have superior properties, such as a high melting point (~170℃), low glass transition temperature (~60℃) and high mechanical strength. As a thermoplastic, PLA usually undergoes preheating before the forming process. Due to its semi-crystalline nature, PLA re-crystallizes during heating and results in distinct physical properties.[5-7] Therefore, crystallization and crystallization kinetics play important roles in the chosen manufacturing process and product performance. Some studies have reported that blending with other polymers, fillers or nucleation agents can improve PLA crystallinity [8-13]. However, these additives may prevent the PLA composite from biodegrading or result in a poor physical performance due to the poor compatibility between the heterogeneous phases. Taking into account biodegradation and physical performance, in this study we used Kenaf fiber (KF), an efficient lignocellulosic crop that can reach a height of 4-6 m and can yield 24 Mg/ha in 5-7 months with low inputs [12], to serve as both
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reinforcement and nucleation agent. To overcome the poor compatibility between the hydrophilic KF and the slightly hydrophobic PLA matrix, the KF was modified with a functional group that could chemically bond to the PLA matrix. Moreover, to increase the conductivity of PLA for antistatic applications, the KF was blended with small amounts of multi-walled carbon nanotubes (MWCNTs) [13]. The purpose of this research is to develop a green composite which exhibits not only desirable biodegradability, low density and good mechanical properties, but anti-static characteristic as well. Thus, this green composite can be used for high value-added fields. In addition to the surface modification of KF, this study also investigated the influence of KF content and annealing treatment on the mechanical properties, heat resistance and crystallinity kinetics of PLA.
2. Materials and methods 2.1 Materials PLA pellets (MW > 100000) were purchased in 2003D grade from NatureWorks. The KF and MWCNTs were provided by JPSeco and Scientech Corporation, respectively.
Sodium
hydroxide
(NaOH),
acetic
acid,
acetone,
3‐
glycidoxypropyltrimethoxysilane (OX-silane) and nitric acid were purchased from Echo Chemical.
2.2 Surface modification of reinforcement The raw KF was pretreated with a 4% NaOH solution for 5 min to remove the lignin, hemicellulose and impurities. Then, it was neutralized using 1% acetic acid before being dried at 60℃. The purified KF was coupled to OX-silane by immersion in
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a 3% silane/acetone solution for 48 h at room temperature; the solvent was then removed, and after drying at 60℃ for 24 h, the modified KF (KF-OX) was ready for use. The MWCNTs were modified using a carboxyl group via acid treatment under reflux at 120℃ for 1 h; during the acidization, each 1 g of MWCNTs was treated with 100 ml of HNO3. After cooling, the modified MWCNTs were filtered and washed in water several times to remove residual HNO3 before being dried.
2.3 Preparation of PLA composite The PLA, KF, and MWCNTs had been dried in an oven at 100℃ for 4 h under reduced pressure until the moisture content was below 1.0 wt%. Immediately after drying, the PLA pellets were pre-melted at 175℃ in a counter-rotating internal mixer (Brabender PL2000, Duisburg, Germany) with a rotation speed of 50 rpm. Subsequently, the MWCNTs were loaded into the mixer and blended with PLA for 5 min. KF was then further loaded into the mixer and blended with the mixture for another 10 min. Afterwards, the PLA composite was granulated. The sample was then processed by compression molding at 175℃ for 5 min (under 20 kgf/cm2 or 50 kgf/cm2), 3 min (under 75 kgf/cm2) and 1 min (under 100 kgf/cm2) in sequence. In the annealing treatment, the PLA composite was further heated at 80℃ for 8 h. The formulations for the samples are shown in Table 1.
2.4 Characterizations The infrared spectra were obtained with an FTIR spectrometer (Paragon 500, PerkinElmer) with a resolution of 2 cm−1 that scanned 50 times from 300 to 4000 cm−1 at room temperature. All film samples were taken using the conventional KBr disk
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method. X-ray diffraction (XRD) was performed using a PANalytical X'Pert PRO using Cu-Kαradiation (λ = 0.154nm) at a scanning rate of 1∘/min. Differential scanning calorimetry (DSC) was performed under nitrogen flow by a TA Instruments Q20 apparatus; the scanning included the following three cycles: (1) keep at 40℃ for 3 min followed by heating to 180℃ at a heating rate of 10℃/min, and terminate at 180℃ for 5 min; (2) cool from 180℃ to 40℃ at a cooling rate of 5℃/min, and keep at 40℃ for 5 min; and (3) heat from 40℃ to 180℃ at a heating rate of 10℃/min. For the analysis of isothermal crystallization kinetics, samples of 5-10 mg were weighed and placed in the crucibles. The temperature was rapidly raised to 190℃, and then the thermal history was erased by holding the sample at this temperature for 3 min. The isothermal crystallization kinetics of neat PLA and KF/CNTs/PLA composites was investigated at their peak crystallization temperatures (Tpeak) (that is, the temperature where each sample’s crystallization rate was at its highest). After the removal of the thermal histories, the samples were rapidly cooled down, at 20 ℃/min, to the test temperature and the temperature was maintained until full crystallization occurred. Moreover, the morphology was observed with a Carl Zeiss polarizing microscope (POM) equipped with a Mettler-Toledo hot stage. The samples were melted at 190℃ for 10 min and then cooled down to 130℃ at 20℃/min for isothermal crystallization to occur. The tensile test was carried out according to the ASTM D638 test method at a strain rate of 50 mm/min using an Instron universal tester, model HT-9102 (Hung Ta Instrument Co., Taiwan). The impact strength was analyzed according to the ASTM D256 test method using an impact resistance testing machine, model GT-70045-MDL (Gotech Testing Machines Co., Taiwan). A GT-HV 2000 analyzer was used to measure the heat
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deflection temperature (HDT) of the composites. The specimen was loaded for the three-point bending test in the edgewise direction with a load of 66 psi, and the temperature was increased at a rate of 2°C/min until the specimen deflected 0.25 mm, as specified by ASTM D648. All of the results were taken as the average value of five samples.
3. Results and discussion 3.1 Surface modification of MWCNTs and functionalization of KF In the FTIR spectrum for surface modified MWCNTs (Figure 1(1)), the absorption peaks at 2919 cm−1 and 2850 cm−1 are assigned to the C-H asymmetric and symmetric stretching vibration, whereas the peaks at 1634 cm−1 and 1249 cm−1 are assigned to the conjugated C=C and C-O stretching. The peak around 3436 cm−1 is the characteristic absorption peak for hydroxyl group, resulting from the acidic modification of MWCNTs [14]. KF is an important natural reinforcement material which has excellent mechanical properties because of its cellulose content. However, in raw KF, impurities such as lignin and hemicellulose can reduce its strengthening effect. In order to remove these impurities and thereby ensure the best reinforcement, the KF was purified using a general alkali treatment [15]. In the FTIR spectrum of the raw Kenaf (KF-UN), shown in Figure 1(2), the existence of lignin and hemicellulose was indicated from the C=O (1737 cm-1) and C-O (1261 cm-1) peaks [16]. The C=O and C-O peaks disappeared after the alkali treatment (KF-Alkali), thus demonstrating the successful removal of the impurities. During the surface modification, as shown in Scheme 1(1), the silane underwent hydrolysis-condensation or hydrogen bonding to the KF, and grafted the KF
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with the epoxy groups (KF-OX). The KF-OX appeared as Si-O-Si (897 cm-1) and Si-Ocellulose (1203 cm-1) peaks, indicating that the silane underwent condensation with another silane (Si-O-Si) and KF (Si-O-cellulose); these peaks demonstrated the successful modification using silane coupling agents [17].
3.2 X-ray diffraction (XRD) Figure 2 shows the XRD patterns of bare PLA, PLA composite and the specimens after annealing. The bare PLA exhibited a trace of crystallinity before annealing, as identified by the broad peak around 2θ =16~20o, which indicated that the PLA chains were poorly ordered (i.e., low degree of crystallinity). This could have been due to the rapid cooling of the PLA during molding [18]. After annealing, the PLA-A showed sharp peaks at 16.7° and 19.1°, attributed to the lattice planes (110/200) and (203) of α crystal form of PLA, respectively.[19] After adding unmodified KF, PF30-UN, a broad peak appeared at 23.0° corresponding to the lattice plane (002) for cellulose I.[20] The PC1F30-OX was similar to bare PLA in that a broad peak was apparent around 2θ =16~20o, indicating the composite’s low degree of crystallinity before annealing. After annealing, PC1F30-OX-A showed sharp peaks at 16.7° and 19.1° corresponding to the PLA crystallinity of the (110/200) and (203) phases. This was due to molecules migrating in the crystal lattice and the number of dislocations decreasing in the annealing procedure, leading to the higher degree of crystallinity.
3.3 Differential scanning calorimetry (DSC) The DSC curves shown in Figure 3 illustrate the phase transition of the PLA composite during heating; the derived transition temperatures are summarized in Table
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2. Before annealing, the bare PLA showed a sharp exothermic peak at 106.2℃ (Tc) for re-crystallization. The re-crystallized PLA started to melt and contributed two endothermic peaks above 140℃, with the small peak at 146.8℃ (Tm1) and the large peak at 156.6℃ (Tm2) attributed to imperfect and perfect crystallinity, respectively.[21] After introducing KF-OX and MWCNTs, all transition temperatures (Tg, Tc, Tm1 and Tm2) decreased, and the decrements below 30 wt% of KF-OX appeared to be positive to the KF-OX content. Generally speaking, the Tc decrement was contributed by the nucleating effect of KF and led to the faster recrystallization. In the PLA composite reinforced only by unmodified KF (PF30-UN), the DSC curve showed characteristic peaks similar to that of bare PLA, except for a slight shift to a lower temperature. By using KF-OX, PC1PF30-OX obviously decreased the Tc to 84.5℃, demonstrating that the chemical bonding between KF-OX and PLA acted effectively as a nucleating agent due to the better dispersion. Furthermore, the additions of KF-OX and MWCNTs were likely to help the crystal growth reach uniform lamellar thickness. Therefore, all the crystals melted at the same temperature. However, the crystallization peak of bare PLA is still large as compared with the composites. This indicates that the addition of MWCNTs and KF may play an important role in accelerating the crystallization process, but not increasing the crystallinity of PLA [22,23]. Figure 3(2) shows the DSC curves of PC1F10-OX before and after annealing, with the peak variations reflecting the annealing effect on the crystalline phase. Before annealing, PC1F10-OX had a curve similar to that of the bare PLA, except for a smaller Tm1 peak. On the other hand, for the PC1PF10-OX-A, the Tg and Tc peaks vanished after the annealing treatment and the Tm increased. This demonstrated that the recrystallization was totally finished during the annealing and resulted in more perfect
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crystallinity.
3.4 Isothermal crystallization kinetics The relative crystallinity (Xt) can be derived by the division calculation between the exothermal area at specific periods and the whole peak area; the obtained isothermal crystallization kinetics. To deduce the nucleation mechanism and crystal growth, the Avrami equation was used to fit the isothermal crystallization kinetics. The Avrami equation is defined as follows: [24, 25]
1 X t exp K T t n
where Xt: relative degree of crystallinity at different periods K(T): rate constant of isothermal crystallization n: Avrami index for describing nucleation mechanism and crystal growth t: time By plotting log(-ln(1-Xt)) versus log(t), n and K can be obtained; the related parameters are summarized in Table 3. In the PC1F-OX series containing 10-30 wt% KF-OX, the K value increased dramatically with the higher KF content. For example, PC1F30-OX showed an extremely high K value of 0.0513 compared to the bare PLA (1.4855×10-7). The results of the K value demonstrated that KF-OX accelerated the crystallization rate. However, the K values of PC1F40-OX and PF30-UN (3.510×10-4 and 4.169×10-3) were smaller than that of PC1F30-OX. This indicates that the well dispersed plant fibers in PC1F30-OX can more effectively accelerate the crystallization rate. On the other hand, the aggregates of excessive plant fibers in PC1F40-OX and poor compatibility of the un-treated plant fibers with PLA in PF30-UN both reduced the
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accelerating effect. The rebounding K values of PC1F40-OX and PF30-UN due to poor dispersion were evidenced by the slowing crystallization rate, which prolonged the recrystallization time. The n value in the Avrami equation can be used to estimate crystallinity growth. In the bare PLA, re-crystallization started from homogeneous nucleation, and the n value near to 4 indicates that the crystallinity grew as a 3-dimensional bulk or sphere structure. After introducing KF-OX and MWCNTs, the crystallinity nucleation became heterogeneous, with the decreasing n values coming from the heterogeneous growth independent of time. The n values of PC1F10-OX, PC1F20-OX and PF30-UN were close to 3, which meant their crystals grew as a 3-dimensional bulk or sphere structure. The n value of PC1F30-OX decreased to nearly 2, which explained the crystals now growing as a 2-dimensional sheet. In PC1F40-OX, the n value rebounded to nearly 3, indicating serious fiber entanglement and confined growth space, which resulted in crystals growth in 3-dimensions.
3.5 Tensile strength The composition and physical properties of the PLA composite are summarized in Table 4. As shown in Figure 4, before annealing, the KF-OX was effective in reinforcing the PLA composite. For example, PC1F30-OX increased the tensile strength to 78.5 MPa, 57% higher than that of the bare PLA. The effective promotion of tensile strength was inferred to the chemical reaction between the KF-OX and PLA, as shown in Scheme 1(2), and the accompanying compatibility served to optimize the stress delivery, resulting in high tensile strength. In PC1F40-OX, the high content of fibers resulted in their being seriously entangled, which impeded the stress delivery and the
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tensile strength dropped to 47.7 MPa. As a semi-crystalline polymer, PLA can re-crystallize into spherical microdomains via annealing.[26, 27] In our study, the annealed composite only enhanced the tensile strength when the KF content was higher than 20 wt%. Therefore, it was deduced that a low KF-OX content was not enough to drive the generation of the transcrystalline structure required for high strength [28, 29]. On the other hand, a high KF-OX content helped develop a transcrystalline structure around the fibers (as shown in Figure 5) , with the PC1F30-OX-A boosting the tensile strength to 91.5 MPa, 84% higher than that of the bare PLA before annealing. This was because the crystalline morphology which formed at the interfaces between the polymer matrix and fibers had a significant influence on the mechanical properties of the composites. The high heterogeneous nucleation activity on the surface of the fibers forced the polymer to grow in one direction, resulting in the formation of a columnar crystalline layer perpendicular to the axis of the fiber surface. This is known as transcrystallinity.[28, 30] The presence of an anisotropic layer, such as the transcrystallinity in the composite material, effectively provides a high resistance to external loading due to the enhanced fiber/matrix adhesion. The enhanced interfacial properties also allowed for a more effective transfer of applied stress, resulting in the superior strength and stiffness of the composite material. Although the seriously entangled fibers of the PC1F40-OX-A reduced the tensile strength to 53.6 MPa, the re-crystallization still promised a higher strength than that of the PC1F40-OX.
3.6 Impact strength As shown in Figure 6, the impact reinforcement before annealing appeared to be
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positive to the KF content, with the PC1F40-OX impact strength approaching a high of 43.8 J/m, 149% higher than that of the bare PLA. The impact enhancement of the KFOX was because of its hollow cellulose structure, which tolerates higher deformation under impact than a solid structure.[31] After annealing, PLA-A re-crystallized, which elevated the impact strength to 30.9 J/m, 76% higher than that of the PLA before annealing. However, the impact strength was similar before and after annealing for the composites with different KF contents. In composites, the interface plays a key role in transferring the stress from the matrix to the fiber. A weak interface generally results in low strength and stiffness, but high resistance to fracture, whereas a strong interface produces high strength and stiffness, but often low resistance to fracture [32]. Ye et al. [33] found that the toughness of GF/PP thermoplastic composites was decreased as the degree of crystallinity was increased. The high crystallinity matrix with high modulus cannot transfer external stress effectively to the reinforcing fiber. Lin et al. [34] also discovered that the increased degree of crystallinity enhanced the stiffness but reduced the ductility of the PLA. The tested specimen broke early because of its brittleness and easy crack propagation through a grain boundary of large spherulites. Moreover, the crystallinity was not the single reason, but more due to the various mesostructures, i.e. the existence of different spherulitic morphologies near or far from the fibers resulted from the annealing procedure.
3.7 Heat distortion temperature The heat distortion temperature (HDT) reflects the heat resistance of the composite; the HDT results are summarized in Figure 7 and Table 4. Before annealing, the bare
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PLA exhibited an HDT of 60.6℃, while the HDT of the PC1F30-OX composite decreased slightly to 58.1℃. Moreover, the HDT of the composite without MWCNTs (PC30-OX) was 60.7℃, which was higher than that of PC1F30-OX. This might be the relatively high KF to CNT ratios in the composites, meaning that the former effectively builds up the ‘‘backbone” of samples and is expected to play a dominating role in the HDT properties [35]. Tang et al. [36] found a percolation threshold of crystallinity corresponding to HDT. HDT would start to increase when the crystallinity exceeds 20%. Moreover, other researchers [37] found that the heat of fusion (related to crystallinity) of the semicrystalline biopolymers would decrease, particularly in the low fiber content biocomposites. Therefore, lower crystallinity might possibly lead to lower HDTs for the unannealed samples. On the other hand, annealing significantly enhanced the HDT. For example, the HDT of PLA-A and PC1F40-OX-A increased to 102.5℃ and 134.6℃, 69% and 122% higher than that of the PLA before annealing, respectively. Moreover, the HDT of the composite without MWCNTs after annealing (PC30-OX-A) was 129.5 ℃, which was higher than that of PC1F30-OX-A (125.5℃). This is analogous to those mentioned above. To sum up, the re-crystallization induced by annealing made the PLA composite more resistant to high temperature, and the transcrystalline structure growth around the KF-OX further increased its heat resistance.
4. Conclusions The KF-OX improved the compatibility in the PLA matrix and resulted in better mechanical and heat properties. With the annealing treatment, more transcrystallinity and perfect crystallinity developed around KF-OX during re-crystallization and further enhanced the physical performance. The DSC analysis revealed that the KF-OX
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accelerated the rate of crystalline growth and accounted for the more perfect crystallinity. In the analysis using the Avrami equation, the KF-OX addition obtained a higher rate constant (K) evidencing the fast crystallization, and the Avrami index (n) indicated that PLA crystallinity growth turned into a 2-dimensional sheet structure. Moreover, it is found that the existence of crystallographic relationships between the structures of the fiber and the polymer crystals is a key parameter to determine the properties of the composites. Finally, this study shows that the annealing procedure may be a route to tune the microstructure and physical properties of a polymer, which might open a promising door to produce the natural fiber/PLA green composites without using any environmentally harmful reagents. Our study indicates that the annealing treatment plays an important role in controlling the microstructure as well as influencing the final mechanical performances of the PLA composites.
References [1] Chen G-Q, Wu Q. The application of polyhydroxyalkanoates as tissue engineering materials. Biomaterials. 2005;26(33):6565-6578. [2] Garlotta D. A literature review of poly(lactic acid). J Polym Environ. 2002;9(2):6384. [3] Lasprilla AJR, Martinez GAR, Lunelli BH, Jardini AL, Maciel Filho R. Poly-lactic acid synthesis for application in biomedical devices - A review. Biotechnol Adv. 2012;30(1):321-328. [4] Chen D, Li J, Ren J. Influence of fiber surface-treatment on interfacial property of poly(L-lactic acid)/ramie fabric biocomposites under UV-irradiation hydrothermal aging. Mater Chem Phys. 2011;126(3):524-531.
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[5] Solarski S, Ferreira M, Devaux E. Characterization of the thermal properties of PLA fibers by modulated differential scanning calorimetry. Polymer. 2005;46(25):1118711192. [6] As'habi L, Jafari SH, Khonakdar HA, Haussler L, Wagenknecht U, Heinrich G. Non-isothermal crystallization behavior of PLA/LLDPE/nanoclay hybrid: synergistic role of LLDPE and clay. Thermochim Acta. 2013;565:102-113. [7] Chiu H-T, Huang S-Y, Chen Y-F, Kuo M-T, Chiang T-Y, Chang C-Y, et al. Heat treatment effects on the mechanical properties and morphologies of poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends. Int J Polym Sci. 2013:951696, 951612 pp, doi:10.1155/2013/951696. [8] Ren Z, Dong L, Yang Y. Dynamic mechanical and thermal properties of plasticized poly(lactic acid). J Appl Polym Sci. 2006;101(3):1583-1590. [9] Picard E, Espuche E, Fulchiron R. Effect of an organo-modified montmorillonite on PLA crystallization and gas barrier properties. Appl Clay Sci. 2011;53(1):58-65. [10] Kawamoto N, Sakai A, Horikoshi T, Urushihara T, Tobita E. Nucleating agent for poly(L-lactic acid) - an optimization of chemical structure of hydrazide compound for advanced nucleation ability. J Appl Polym Sci. 2007;103(1):198-203. [11] Suryanegara L, Nakagaito AN, Yano H. Thermo-mechanical properties of microfibrillated cellulose-reinforced partially crystallized PLA composites. Cellulose. 2010;17(4):771-778. [12] Brown RC, Brown TR, Editors. Biorenewable Resources: Engineering New Products from Agriculture, 2nd Edition: Wiley-Blackwell; 2014. [13] Lin W-Y, Shih Y-F, Lin C-H, Lee C-C, Yu Y-H. The preparation of multi-walled carbon nanotube/poly(lactic acid) composites with excellent conductivity. J Taiwan Inst
16
ACCEPTED MANUSCRIPT
Chem Eng. 2013;44(3):489-496. [14] Aviles F, Cauich-Rodriguez JV, Moo-Tah L, May-Pat A, Vargas-Coronado R. Evaluation of mild acid oxidation treatments for MWCNT functionalization. Carbon. 2009;47(13):2970-2975. [15] Mohsenzadeh A, Jeihanipour A, Karimi K, Taherzadeh MJ. Alkali pretreatment of softwood
spruce
and
hardwood
birch
by
NaOH/thiourea,
NaOH/urea,
NaOH/urea/thiourea, and NaOH/PEG to improve ethanol and biogas production. J Chem Technol Biotechnol. 2012;87(8):1209-1214. [16] Magniez K, Voda AS, Kafi AA, Fichini A, Guo Q, Fox BL. Overcoming Interfacial Affinity Issues in Natural Fiber Reinforced Polylactide Biocomposites by Surface Adsorption of Amphiphilic Block Copolymers. ACS Appl Mater Interfaces. 2013;5(2):276-283. [17] Le Moigne N, Longerey M, Taulemesse J-M, Benezet J-C, Bergeret A. Study of the interface in natural fibres reinforced poly(lactic acid) biocomposites modified by optimized organosilane treatments. Ind Crops Prod. 2014;52:481-494. [18] Oksman K, Skrifvars M, Selin JF. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos Sci Technol. 2003;63(9):1317-1324. [19] Sullivan ME, Moon JR, Kalaitzidou K. Processing and Characterization of Cellulose Nanocrystals/Polylactic Acid Nanocomposite Films. Materials. 2015;8(12). [20] Chen Q, Wang Q, Mitsumura N, Niida H. Improved cellulose by ionic liquid mixture with solid acid catalysis and its application in polyethylene glycol liquefaction. Mater Sci Appl. 2013;4(12):839-845. [21] Tabi T, Sajo IE, Szabo F, Luyt AS, Kovacs JG. Crystalline structure of annealed polylactic acid and its relation to processing. eXPRESS Polym Lett. 2010;4(10):659-
17
ACCEPTED MANUSCRIPT
668. [22] Robles E, Urruzola I, Labidi J, Serrano L. Surface-modified nano-cellulose as reinforcement in poly(lactic acid) to conform new composites. Ind Crop Prod. 2015;71:44-53. [23] Gil-Castell O, Badia JD, Kittikorn T, Str€omberg E, Ek M, Karlsson S, RibesGreus A. Impact of hydrothermal ageing on the thermal stability, morphology and viscoelastic
performance
of
PLA/sisal
biocomposites.
Polym
Degrad
Stab.
2016;132:87-96. [24] Augis JA, Bennett JE. Calculation of the Avrami parameters for heterogeneous solid state reactions using a modification of the Kissinger method. J Therm Anal. 1978;13(2):283-292. [25] Lorenzo AT, Arnal ML, Albuerne J, Mueller AJ. DSC isothermal polymer crystallization kinetics measurements and the use of the Avrami equation to fit the data: Guidelines to avoid common problems. Polym Test. 2007;26(2):222-231. [26] Rathi S, Chen X, Coughlin EB, Hsu SL, Golub CS, Tzivanis MJ. Toughening semicrystalline poly(lactic acid) by morphology alteration. Polymer. 2011;52(19):41844188. [27] Fang Q, Hanna MA. Rheological properties of amorphous and semicrystalline polylactic acid polymers. Ind Crops Prod. 1999;10(1):47-53. [28] Xu H, Xie L, Chen Y-H, Huang H-D, Xu J-Z, Zhong G-J, et al. Strong Shear FlowDriven Simultaneous Formation of Classic Shish-Kebab, Hybrid Shish-Kebab, and Transcrystallinity in Poly(lactic acid)/Natural Fiber Biocomposites. ACS Sustainable Chem Eng. 2013;1(12):1619-1629. [29] Xu H, Xie L, Jiang X, Hakkarainen M, Chen J-B, Zhong G-J, et al. Structural Basis
18
ACCEPTED MANUSCRIPT
for Unique Hierarchical Cylindrites Induced by Ultrahigh Shear Gradient in Single Natural Fiber Reinforced Poly(lactic acid) Green Composites. Biomacromolecules. 2014;15(5):1676-1686. [30] Quan H, Li Z-M, Yang M-B, Huang R. On transcrystallinity in semi-crystalline polymer composites. Compos Sci Technol. 2005;65(7-8):999-1021. [31] Kalia S, Dufresne A, Cherian BM, Kaith BS, Avérous L. Cellulose-Based Bio- and Nanocomposites: A Review2011. [32] Liu FP, Wolcott MP, Gardner DJ, Rials TG. Characterization of the interface between cellulosic fibers and a thermoplastic matrix, Compos Interface. 1994; 2(6):419432. [33] Ye L, Beehag A, Friedrich K. Mesostructural aspects of interlaminar fracture in glass fiber-reinforced thermoplastics: Is crystallinity a key? Compos Sci Technol. 1995;53(2):167-173. [34] Li L, Hashaikeh R, Arafat HA. Development of eco-efficient micro-porous membranes via electrospinning and annealing of poly (lactic acid). J Membr Sci. 2013; 436: 57-67. [35] Shen Z, Bateman S, Wu DY, McMahon P,
Dell’Olio M, Gotama J. The effects of
carbon nanotubes on mechanical and thermal properties of woven glass fibre reinforced polyamide-6 nanocomposites. Compos Sci Technol. 2009;69(2):239-244. [36] Tang Z, Zhang C, Liu X, Zhu J. The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent. J Appl Polym Sci. 2012;125(2):1108-15. [37] Sanchez-Garcia MD,
Gimenez E, Lagaron JM. Morphology and barrier
properties of solvent cast composites of thermoplastic biopolymers and purified
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cellulose fibers. Carbohydr Polym. 2008;71 (2):235-244.
(1)
(2)
Figure 1. FTIR spectrum of (1) surface modified MWCNTs and (2) raw KF (KF-UN), purified KF (KFAlkali) and KF modified with OX-silane (KF-OX)
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(1)
(2)
Scheme 1. Reaction mechanism of (1) modification on KF surface by OX-silane and (2) reaction between PLA and OX-grafted KF
Figure 2. XRD patterns of PLA and composites before and after annealing
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(1)
(2)
Figure 3. DSC curves at (1) reheating and (2) first heating
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Figure 4. Tensile strength of PLA composite before and after annealing treatment
Figure 5. POM micrograph of PC1F30 OX
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Figure 6. Impact strength of PLA composite before and after annealing treatment
Figure 7. HDT of PLA and composite before and after annealing treatment
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Table 1. Formulations of samples
PLA (wt%) PLA 100 PC1F10-OX 89 PC1F20-OX 79 PC1F30-OX 69 PC1F40-OX 59 PLA-A 100 PC1F10-OX-A 89 PC1F20-OX-A 79 PC1F30-OX-A 69 PC1F40-OX-A 59 PF30-UN 70 *The KF is un-modified.
Modified MWCNTs content (wt%) 0 1 1 1 1 0 1 1 1 1 0
Modified KF content (wt%)
Annealing treatment
0 10 20 30 40 0 10 20 30 40 30*
No No No No No Yes Yes Yes Yes Yes No
Table 2. Thermal properties derived from DSC measurement at reheating
PLA PC1F10-OX PC1F20-OX PC1F30-OX PC1F40-OX PF30-UN
Tc (℃) 106.2 96.8 90.7 84.5 91.8 98.3
Tm1 (℃) 146.8 143.3 141.1
Tm2 (℃) 156.6 154.9 152.6 151.0 153.2 152.1
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Table 3. Parameters of isothermal crystallization kinetics derived from Avrami equation
PLA PC1F10-OX PC1F20-OX PC1F30-OX PC1F40-OX PF30-UN
Crystallization time (min) 120 49 47 16 22 37
n 3.69 2.82 2.73 1.83 3.24 2.80
K (min-n) 1.486×10-7 8.691×10-5 2.181×10-4 0.0513 3.510×10-4 4.169×10-3
R2 0.9886 0.9940 0.9815 0.9969 0.9906 0.9903
Table 4. Mechanical properties of PLA and its composites TS IS TS* IS** Increment Increment (MPa) (J/m) (%) (%)
PLA 49.7±4.9 PC1F10-OX 61.6±8.0 PC1F20-OX 62.8±5.2 PC1F30-OX 78.5±3.4 PC1F40-OX 47.7±8.5 PLA-A 46.8±5.1 PC1F10-OX-A 61.6±3.6 PC1F20-OX-A 70.4±3.5 PC1F30-OX-A 91.5±8.6 PC1F40-OX-A 53.6±8.0 *TS: tensile strength **IS: impact strength
23 26 57 -4 -5 23 41 84 7
17.5±2.6 30.8±4.1 36.8±0.7 37.4±1.9 43.8±4.5 30.9±7.5 30.3±1.3 35.5±1.5 35.4±1.7 44.9±3.1
76 110 113 149 76 73 102 102 156
t1/2 (min) 50.00 22.45 22.60 3.98 9.60 8.37
HDT (℃)
HDT Increment (%)
60.6 59.3 57.7 58.1 58.1 102.5 113.7 120.9 125.5 134.6
-2 -5 -4 -4 69 87 99 107 122
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