Article pubs.acs.org/IECR
Fabricating Highly Reactive Bio-based Compatibilizers of Epoxidized Citric Acid To Improve the Flexural Properties of Polylactide/ Microcrystalline Cellulose Blends Xinyan Dai, Zhu Xiong, Songqi Ma, Chao Li, Jinggang Wang, Haining Na,* and Jin Zhu* Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, People’s Republic of China ABSTRACT: Epoxidized citric acid (ECA) is synthesized as a highly reactive bio-based compatibilizer to improve the flexural property of polylactide (PLA)/microcrystalline cellulose (MCC) blends. Confirmed by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) analysis, ECA includes three oxirane groups in its chemical structure with the rather high epoxy value at 0.76. After adding 1−5 wt % ECA in PLA/MCC blends, the interfacial adhesion between PLA and MCC is significantly improved. Accordingly, flexural strength, flexural modulus as well as impact strength of PLA/MCC/ECA blends are all improved and thus the increase of the flexural properties. This work suggests an effective way to create biobased compatibilizer with high reactivity and hereby displays a feasible way to fabricated fully bio-based PLA/MCC blends with high performance.
1. INTRODUCTION Polylactide (PLA) has received great attention as one of the most important bio-based plastics, because of the merits of superior mechanical strength, easy processability, and good biocompatibility.1 The development of PLA/cellulose blends to produce fully environmental friendly materials with a high ratio of performance to price has become a promising research direction of polymer processing.2 During the past few years, many attempts have been made to pursue high-performance PLA/cellulose blends. Generally, the improvement of interfacial compatibility between PLA and cellulose is realized as a critical issue.3 Cellulose with plenty of polar hydroxyl groups is fairly hydrophilic in nature, but PLA is relatively hydrophobic. This phenomenon causes an obvious interfacial difference and poor compatibility between PLA and cellulose.4,5 As reported in the literature, some physical or chemical pretreatments of cellulose, such as microwave treatment, mercerization, silane coupling, acetylation, and grafting co-polymerization, have been applied to improve the compatibility.3,6 Although researchers have given majority evidence to show the improvement in compatibility, because of the limitation of the complex processing procedures, excessive energy consumption, and the toxicity of pretreated agent,7−10 it has not found a feasible way to produce PLA/cellulose blends with higher properties for extensive application. To establish a facile and efficient method, the utilization of reactive compatibilizer to reduce the interfacial difference between PLA and cellulose is recognized as the most suitable methodology. The reactive compatibilizer is usually a hydrophobic active chemical with reactive functional groups. It can react with the hydroxyl group of cellulose to form a particular interface between PLA and cellulose, thus improving the interfacial adhesion and then the mechanical properties of PLA/cellulose blends.6,11−13 However, not all of the reactive compatibilizers are thoroughly environmental friendly. The non-bio-based compatibilizer11,13−16 is always impossible to completely avoid the © 2015 American Chemical Society
adverse impacts of nonbiodegradability and nonsustainable in fabricating green PLA/cellulose blends. In order to deposit highly effective bio-based compatibilizers, our previous work tentatively used epoxidized soybean oil (ESO) to increase the interfacial adhesion between PLA and MCC, thus suggesting a feasible way to prepare fully bio-based PLA/microcrystalline cellulose (MCC) blends with high performance.17 However, the relative low reactivity between cellulose and ESO are easy to result in inadequate reaction and unexpected coalescence of ESO. This phenomenon usually leads to an inevitable decrease of flexural property impacted by partially local softening of the PLA/MCC/ESO blend. Therefore, it is necessary to exploit a highly reactive bio-based compatibilizer to fabricate PLA/MCC blends. Considering the design of a bio-based compatibilizer with high reactivation, citric acid (CA) with multifunctions in its chemical structure is emphasized as the initial substance. The functional groups of CA then are modified to create epoxidized citric acid (ECA). Along with the high reactivity of the multiepoxy groups, ECA is expected to produce chemical bonding linkages as much as possible with MCC. It is expected to form a flexible layer as ESO-modified MCC17 to increase the interfacial adhesion between PLA and MCC. Accordingly, flexural property of the PLA/MCC blends can be also expected to be improved. In this paper, the high reactive ECA is first synthesized in the experiment and then applied to fabricate the PLA/MCC blends. The mechanism and the effect on the improvement of compatibility are examined in detail. In addition, the mechanical property impacted by the change of interfacial compatibility is also evaluated. Received: Revised: Accepted: Published: 3806
December 17, 2014 March 24, 2015 April 1, 2015 April 1, 2015 DOI: 10.1021/ie504904c Ind. Eng. Chem. Res. 2015, 54, 3806−3812
Article
Industrial & Engineering Chemistry Research Scheme 1. Reaction Route for the Synthesis of ECA
Figure 1. (A) FTIR spectrum of CA, TACA and ECA; (B) 1H NMR spectra of TACA; (C) 1H NMR spectra of ECA; and (D) 13C NMR of ECA.
2. EXPERIMENTAL SECTION
condenser and mechanical agitator. The reaction was processed at 60 °C for 50 h with consistent stirring and then cooled to room temperature. The product was first filtrated to remove the solid precipitation, and the residual solvents and unreacted components in the filtrate were also removed with a rotatory evaporation under vacuum. After dried with anhydrous sodium sulfate, TACA (78% yield) was obtained and then dried at 60 °C with vacuum overnight for further use. During the second step, TACA (7.07 g, 0.023 mol), together with m-CPBA (17.60 g, 0.0102 mol) and 106 g of dichloromethane, were incorporated in the reaction flask. After dissolving with agitation, the reaction was initiated at 35 °C and lasted 48 h, and then was cooled to −5 °C. The product was filtrated and then dried through rotary evaporation. Subsequently, the product continued to be washed with 10% Na2SO3 and saturated salt water for several times. ECA then was extracted by dichloromethane and purified by dissolved in dichloromethane and poured into 5-fold cold methanol. Finally, pure ECA (yield 58%) was obtained via rotary evaporation and dried under vacuum at 40 °C for 6 h. The epoxy value of ECA
2.1. Materials. CA, allyl bromide (C3H5Br), and 3chloroperoxybenzoic (m-CPBA, 85%) were supplied by Aladdin Industrial, Inc. Acetone, potassium carbonate (K2CO3), sodium chloride (NaCl), methanol, anhydrous sodium sulfate (Na2SO4), dichloromethane, N,N-dimethylformamide (DMF) and sodium sulfite (Na2SO3) were obtained from Sinopharm Chemical Reagent and used without further purification. PLA (Lot No. 4032D) was obtained from NatureWorks LLC (Minnetonka, MN, USA). MCC fillers were purchased from Yuzhong Biotechnology Company (Henan, China) with the average length and diameter of ∼80 μm and ∼20 μm, respectively. 2.2. Synthesis of ECA. The synthesis of ECA includes two steps: the synthesis of triallylation of citric acid (TACA), and the epoxidation of TACA, as shown in Scheme 1. In the first step, CA (5.00 g, 0.026 mol), DMF (10 g), acetone (10 g), K2CO3 (11.87 g, 0.086 mol), and C3H5Br (18.89 g, 0.156 mol) were placed into a three-necked flask equipped with a reflux 3807
DOI: 10.1021/ie504904c Ind. Eng. Chem. Res. 2015, 54, 3806−3812
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Figure 2. SEM images of (A, B) PLA/MCC (80/20), (C, D) PLA/MCC/ECA (80/20/1), and (E, F) PLA/MCC/ECA (80/20/5).
determined by the chlorhydric acid-acetone method18 was ∼0.76, which is slightly lower than the theoretical value of 0.83. 2.3. Fabrication of PLA/MCC Blends. PLA and MCC were first vacuum-dried at 80 °C for 12 h. Then, pure PLA, PLA/MCC (weight ratio of 80/20), and PLA/MCC/ECA (weight ratios of 80/20/1, 80/20/3, 80/20/5) were meltblended by a laboratory-scale conical twin screw extruder (Ruiming Plastics Machinery, Wuhan, China) with rotational speed of 40 rpm at 175 °C for 5 min. All of the materials were sequentially injected into flexural and impact bars (80 mm × 10 mm × 4 mm), according to the ISO 178:2010 standard via a miniature injection machine (Model SZ-15, Wuhan, China). The injection pressure, temperature, and time were chosen as 3 MPa, 200 °C, and 30 s, respectively. The mold temperature was set at 40 °C. 2.4. Extraction of MCC Fillers from the Blends. The MCC fillers were extracted from the blends via Soxhlet extraction with chloroform for at least 7 days to completely remove unbonded ECA. The extracted fillers were marked as EMCC0, E-MAMCC1, E-MAMCC3, and E-MAMCC5, based on the ratio of ECA used in the blends. Thereafter, the extracted fillers were vacuum-dried for 5 h at 50 °C. 2.5. Characterization. Fourier transform infrared (FT-IR) spectra of the samples prepared with KBr were recorded by a spectrophotometer (namely, a Nicolet Model FTIR6700 infrared spectrophotometer) over the range of wavenumbers at 4000−400 cm−1. The 1H NMR and 13C NMR spectra were recorded on Bruker AVANCE III 400 M nuclear magnetic resonance (NMR) spectrometer (deuterated chloroform and deuterated DMSO were respectively used as the solvents). The
cryogenically fracture surface of blends with liquid nitrogen was examined via scanning electron microscopy (SEM) (Hitachi, Model TM-1000) after being sputter-coated with gold. The flexural properties were measured on an Instron Model 5567 (Boston, MA) instrument at a crosshead speed of 2 mm/min. Results of at least five samples were averaged to calculate the flexural modulus, strength and elongation. The notched impact test was conducted on an impact tester (Model XJ-50Z, Chengde Dahua Testing Machine Co. Ltd., Chengde, China) with the use of Model 1J pendulum. X-ray photoelectron characterization for extracted MCC fillers was performed on a Kratos AXIS Ultra DLD photoelectron spectrometer (Shimadzu, Japan) with a hemispherical sector energy analyzer. Al Kα X-ray radiation was used as the excitation resource. All spectra were calibrated with graphitic carbon at a binding energy of 284.8 eV. The static contact angles with water at equilibrium were measured on a Data Physics instrument (Model OCA20, Germany), which was equipped with a camera, with a precision of ±0.1°. In order to run the test of static contact angle, virgin MCC and the extracted MCC (E-MCC5) are respectively prepared in the form of flakelets, using a tablet machine. The sample of PLA is prepared by injection molding.
3. RESULTS AND DISCUSSION 3.1. The Chemical Structure of ECA. As shown in Scheme 1, a two-step process is employed for preparing ECA. FT-IR and NMR analyses are respectively conducted to determine the chemical structures of the products. With regard to the intermediate product obtained by reacting CA with C3H5Br, some absorption bands at 3087, 1650, 930, and 989 3808
DOI: 10.1021/ie504904c Ind. Eng. Chem. Res. 2015, 54, 3806−3812
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Figure 3. Surface analysis of MCC and extracted MCC: (A) FTIR spectra and (B) C 1s XPS spectra with peak deconvolution.
cm−1 are shown in the FT-IR spectrum (see Figure 1A). They are assigned to the stretching vibration of CH and CC, wagging vibration of CH2 and CH, respectively. Further to examine by 1H NMR, the peaks at 2.7−3.0 ppm shows the protons on the methylene. Moreover, the peaks at 4.4−5.9 ppm are well-attributed to typical absorption of the protons from the unsaturated double bonds and the methylene linking to ester groups (see Figure 1B). Considering with the ratio of peak area, the hydroxyls on CA are all successfully transferred to carbon− carbon double bonds. After TACA continues to react with mCPBA, there occurs the absorption bands at 908 and 762 cm−1 on FT-IR spectrum (see Figure 1A). It indicates the oxirane groups on ECA. As shown on the spectrum of 1H NMR spectrum (see Figure 1C), some peaks at 3.1−3.3 ppm and 2.6−3.0 ppm are clearly exhibited. This result is accord with the typical peaks of protons on oxirane groups. The 13C NMR spectrum is also shown in Figure 1D. The peaks of carbons on the oxirane groups can be easily found. After examining the peak ratio shown in Figures 1C and 1D, it confirms that ECA includes three oxirane groups in its chemical structure. In addition, the epoxy value of ECA is up to 0.76, which is determined by the chlorhydric acid−acetone method. 3.2. Compatibility of PLA/MCC Blends Impacted by ECA. The compatibility between PLA and MCC before and after adding ECA is distinguished from the morphology of fracture surfaces of the PLA/MCC blends. SEM images of the cross sections of PLA/MCC (80/20), PLA/MCC/ECA (80/ 20/1), and PLA/MCC/ECA (80/20/5) are presented in Figure 2. There exists a large amount of aggregation of MCC fillers and many clear gaps (see Figures 2A and 2B). This phenomenon can be attributed to the poor interfacial compatibility between MCC and PLA. After the addition of 1 wt % ECA in the PLA/MCC blends, it shows a very smooth morphology at the cross section (see Figures 2C and 2D. MCC are well-embedded in the PLA matrix. Almost no clear gaps are observed between MCC and PLA. With the addition of 5 wt % ECA, SEM images show rather smooth cross sections (see Figures 2E and 2F). There are no gaps existed between PLA and MCC. Therefore, all the results reveal an effective improvement in the interfacial compatibilization of the PLA/
MCC blends. The mechanical propertyparticularly, the flexural propertyis expected to be improved accordingly. To understand the effect produce by adding the high active ECA in PLA/MCC blends, the MCC is extracted from the PLA/MCC and PLA/MCC/ECA blends for analysis in detail. From the spectrum of FTIR (see Figure 3A), the MCC extracted from the PLA/MCC (80/20) blend shows the absorption bands of hydroxyl stretching vibrations (3100−3600 cm−1), deformation of glucopyranose ring (1110 cm−1), stretching of C−O on skeleton (1058 and 1027 cm−1), and linkages (897 cm−1) related to the β-(1,4) glycosidic bond. It is almost the same as the FTIR spectrum of the virgin MCC. However, the MCC extracted from PLA/MCC/ECA blends exhibits another absorption band at 1754 cm−1. The result confirms the carbonyl group on the surface of MCC. It clearly indicates that ECA is chemically bonded on MCC. In fact, the reaction between MCC and ECA happens at high temperature during processing. XPS analysis is also used to show the accurate chemical composition on the surface of extracted MCC. From Figure 3B and Table 1, it presents the deconvoluted C 1s XPS spectra of MCC, E-MCC0, E-MMC1, E-MCC3, and E-MCC5, respectively. Four types of peaks are obtained: C1 (C−C, 284.8 eV), C2 (C−O, 286.4 eV), C3 (O−C−O, 287.7 eV), and C4 (O− CO, 289.0 eV). In comparison with the virgin MCC, the peak intensity of C4 from E-MCC1, E-MCC3, and E-MCC5 significantly increases (see Figure 3B and Table 1). As we Table 1. XPS Results for MCC and Extracted MCC Peak Intensity (%)
MCC EMCC1 EMCC3 EMCC5 3809
[C1], 284.8 eV
[C2], 286.4 eV
[C3], 287.7 eV
[C4], 289.0 eV
C1/ C3
C4/ C3
17.3 18.2
63.4 51.0
17.8 16.1
1.5 14.7
0.97 1.13
0.01 0.91
21.7
44.8
15.1
18.4
1.43
1.21
22.4
42.5
11.8
23.3
1.90
1.97
DOI: 10.1021/ie504904c Ind. Eng. Chem. Res. 2015, 54, 3806−3812
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Industrial & Engineering Chemistry Research
Figure 4. Tests of static contact angle with water.
Figure 5. Flexural properties of PLA and PLA blends. (A) stress−strain curve, (B) flexural strength, (C) flexural modulus, (D) flexural elongation. In panels (A)−(D), labels “a”, “b”, “c”, “d”, and “e” represents the samples of PLA, PLA/MCC (80/20), PLA/MCC/ECA (80/20/1), PLA/MCC/ ECA (80/20/3), and PLA/MCC/ECA (80/20/5), respectively.
MCC and PLA is decreased and, accordingly, the interfacial adhesion increased. 3.3. The Improvement of Flexural Property. The flexural properties of PLA, PLA/MCC (80/20), and PLA/ MCC/ECA are shown in Figure 5. PLA has good flexural properties of high flexural modulus (3.3 GPa), flexural strength (95.4 MPa), and elongation (4.1%). However, after adding 20 wt % MCC, only the flexural modulus keeps increasing to 4.2 GPa. The flexural strength and elongation respectively decreases to 71.7 MPa and 2.2%. These results clearly reflect the increase of stiffness produced by the usage of MCC. However, because of the poor compatibility, the toughness of PLA/MCC is low. After adding 1 wt % ECA (based on the total weight of PLA and MCC), the flexural strength reaches to 79.5 MPa. It is ∼11% higher than the flexural strength of unmodified PLA/MCC blend. Furthermore, the flexural strength can be increased to 80.7 and 84.6 MPa by using 3 and 5 wt % ECA. As similar as the flexural strength, the elongation of PLA/MCC blends with ECA also increases. The elongation is up to 2.9% with the addition of 5 wt % ECA. Figure 6 gives the notched impact strength of PLA/MCC blends before and after the addition of ECA. After using 3−5 wt % ECA in the blends, the notched impact strength increases from 2.44 kJ/m2 to ∼3.5 kJ/ m2. This value is almost equal to the notched impact strength of PLA (∼3.54 kJ/m2, as shown in Figure 6). These results show the toughness of PLA/MCC blends can be effectively improved
know, C4 does not exist in the molecular structure of MCC. ECA is proved to be successfully bonded onto the surface of MCC. The data from Table 1 also exhibits the ratio change of C1/ C3 and C4/C3. Because C3 only exists on pyran rings of MCC and its amount almost remains unchanged during the processing of the blends, the values of C1/C3 and C4/C3 ratios actually reflects the amount of ECA bonded on the surface of MCC. Table 1 show the ratio of C1/C3 gradually increases from 0.97 to 1.90. The ratio of C4/C3 also increases from 0.01 to 1.97. That is to say, the more ECA is added in PLA/MCC blends, the larger the amount of ECA that is finally bonded on the surface of MCC. The measurements of static contact angle of virgin and extracted MCC are also carried out in the experiment. As shown in Figure 4, PLA exhibits the contact angle at 80.1°. This value is equal to the value reported from ref 19. With regard to the celluloses, the contact angle of virgin MCC with water is 39.3°, but E-MCC5 exhibits a contact angle of 76.3°. The value of contact angles between PLA and EMCC5 is very close. Similar contact angle reflects similar hydrophobicity of ECA-modified MCC and PLA. With reference to our previous experience with ESO-modified MCC,17 after using ECA in the processing of the PLA/MCC blends, a flexible layer is also considered to form on the surface of MCC particles. As a result, the interfacial difference between 3810
DOI: 10.1021/ie504904c Ind. Eng. Chem. Res. 2015, 54, 3806−3812
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ACKNOWLEDGMENTS This work is supported by Ningbo Natural Science Foundation (Nos. 2013A610025 and 2013A610023), and Ningbo Key Lab of Polymer Materials (Grant No. 2010A22001). The authors gratefully appreciate the valuable help from Dr. Zhaobin Tang, Jinyue Dai, Renhao Hu, Junchao Guo, and Gongjun Zhang.
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Figure 6. Notched impact strength of PLA, PLA/MCC (80/20), PLA/MCC/ECA (80/20/3), and PLA/MCC/ECA (80/20/5).
by adding ECA. As to the flexural modulus of PLA/MCC blends, with 1 wt % ECA, it reaches the highest value at 4.71 GPa. After increasing the amount of ECA to 3−5 wt %, the flexural moduli of PLA/ECA/MCC blends gradually reduce to 4.30−4.49 GPa. But, it is still higher than the flexural modulus of unmodified PLA/MCC blend. Considering the improvement in the compatibility between PLA and MCC, it reflects the important effect on the increase of flexural property. As we know, ECA with a high epoxy value has a higher reactivity to react with MCC. In section 3.2, it is proven that ECA is helpful to increase the interfacial adhesion between PLA and MCC. This phenomenon promotes the compatibility and thus improves both the toughness and stiffness of the PLA/MCC blends. Besides, the crystallinity of PLA can be improved. As a result, the flexural property of PLA/ MCC/ECA blends is finally improved. In comparison with the results shown in the literature,11,14,20,21 the toughness and the stiffness of PLA/MCC/ECA blends are equal or even better than the properties of PLA/cellulose blends modified by the traditional compatibilizers.
4. CONCLUSIONS The work presents the synthesis of ECA with high epoxy value and its usage as a high reactive bio-based compatibilizer in PLA/cellulose blends. Results show that ECA has three oxirane groups in its chemical structure. The epoxy value is much high at 0.76. After adding a small amount of ECA in PLA/MCC blends, the interfacial adhesion between PLA and MCC is extremely promoted. Accordingly, the flexural property is obviously improved. This work not only suggests a feasible way to fabricate fully bio-based PLA/MCC blends with high performance, using the high reactivity of ECA with cellulose, it also provides an important theoretical basis to design and fabricate highly reactive compatibilizers in the preparation of PLA/cellulose blends.
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AUTHOR INFORMATION
Corresponding Authors
*Tel: +86-574-86685283. Fax: +86-574-86685186. E-mail:
[email protected] (H. N. Na). *Tel: +86-574-86685283. Fax: +86-574-86685186. E-mail:
[email protected] (J. Zhu). Notes
The authors declare no competing financial interest. 3811
DOI: 10.1021/ie504904c Ind. Eng. Chem. Res. 2015, 54, 3806−3812
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