A renewable agricultural waste material for the synthesis of the novel thermal stability epoxy resins.
Lignin is the second most abundant group of biopolymers . So lignin is considered to be one of the most promising future biomass resources. However, the utilization of lignin-based materials is limited in practical human life, and the widespread application of lignin has not been achieved.
Lignin is generally viewed as the waste material in the majority of factories. It was treated as sewage effluent frequently by most paper factories and burned directly in many farms. We have known that only about 1% of all the lignin from the paper production was recycled , and the rest was either burned as fuel in production step or discharged into the stream. In recent decades, more and more people pay attention to the synthesis of polymer compounds based on lignin [3-6]. Lignin is one of renewable aromatic compounds among non-oil resources. Due to the lignin molecules containing a large number of active groups, it can replace bisphenol A to synthesize epoxy resins [7-9]. Epoxy resins have been commercialized for more than 50 years. There are three main corporations focusing on the production of epoxy resins: Hexion (formerly Shell's Epoxy Resins and Resolution Performance Products), Dow, and Huntsman. The three companies hold about 75% of the world's epoxy resin production. The demand of epoxy resins in practical human life is growing quickly with the development of the global economy. Epoxy resins are widely used in industrial fields such as coating, potting, adhesives, laminates, and composites [10-12], due to their many attractive characteristics of excellent chemical and corrosion resistance, good adhesion to many substrates, high strength and modulus, and superior electrical properties. However, one of the main drawbacks of epoxy resins is their inherent flammability, which restricts their application in many fields for safety consideration. Therefore, it is an important issue to improve the flame retardancy of epoxy resins. For example, epoxy resins with superior thermal stability are expected for using as encapsulation materials and molding compounds in advanced electronic components . Epoxy resins can be rendered fire retardant either by incorporating fire-retardant additives [14, 15] or by copolymerization with reactive flame retardants [6, 16, 17].
The addition of fire-retardant additives is a simple and common way to promote the flame retardancy of epoxy resins. But this method faces the problem of poor dispersion of flame retardants, which deteriorates the physical properties of epoxy resin matrix and decreases the efficiency of flame retardants .
Lightning has a highly branched structure (Guayaquil, syringyl, and hydroxyl phenyl) [18, 19], which can make lignin-based epoxy resins that have a higher bonding strength and good thermal performance without fire-retardant additives. Until now, the methods of synthesizing lignin-based epoxy resins can be summarized into three categories as follows.
a. Blend the derivatives of lignin with general epoxy resins directly [20, 21].
b. Modify lignin derivatives such as kraft lignin by epoxides directly [22, 23].
c. Modify lignin derivatives to improve their chemical activity, followed by epoxidization [24, 25].
Because the alkali lignin macromolecules are curled and the reactive groups can hardly be effectively used, they need to be modified before reacting. In previous studies, the methods of lignin modification are many and varied. According to the requirements of synthetic products in our research, the lignin needs to be modified by Mannich reaction [26, 27].
After Mannich reaction, the alkali lignin has more active groups that are conducive to the followed reactions. Lignin can replace or partially replace bisphenol A to react with epichlorohydrin to synthesize lignin-based epoxy resins [9, 25, 28]. In this article, our method has three advantages as below.
Firstly, most people added formaldehyde into mixed solutions of the amine and lignin for modifying lignin by Mannich reaction in recent years. Then lignin achieved the purpose of modification. But in our article, we developed a new way of feeding to modify the lignin and then we compared our method with others.
Secondly, the lignin modified by diethanolamine (Diethanolamine can be used as not only modifying reagent to lignin, but also internal curing agent during the application of epoxy resins.) can directly be used to prepare epoxy resins. In this way, we can avoid lignin extraction and have no sewage discharge. Up to now, this is the first time to use this method to synthesize lignin-based epoxy resins. By this way, pollution would be reduced and the environment could be protected.
Thirdly, bisphenol A is expensive and can cause worse environmental pollution . Meanwhile, lignin is a kind of inexpensive and environmentally friendly resource. Pollutant emissions and production cost will be reduced if we use lignin to replace bisphenol A.
Alkali lignin was obtained from the straw . The content of lignin in the straw was about 10%-20% . The straw was obtained from Liu'an city, Anhui province.
Acetone, anhydrous ethanol, formaldehyde, hydrochloric acid, and sodium hydroxide used in the synthetic process were purchased from the Beijing Chemicals Co. Epichlorohydrin was purchased from the BASF Co. Bisphenol A and diethanolamine were purchased from SINOPHARM. These reagents are all of analytical grade. Distilled water was applied for all synthesis and treatment processes.
Modify the Alkali Lignin
Mannich reaction is also known as aminomethylation reaction , because the essence of this reaction is that formaldehyde and amines are reacted with the compound containing active hydrogen, then the active hydrogen is replaced by amine methyl.
We took two different orders to add the reagents into the reacting system in Mannich reaction processes as follows.
a. Put the alkali lignin (We call this kind of unmodified lignin OL.) and diethanolamine into a 250 mL four-neck round-bottom flask equipped with a thermometer and mechanical stirrer. Formaldehyde was added into mixed solutions in nitrogen atmosphere while the mixture was heated at the specified temperature (50[degrees]C, 60[degrees]C, 70[degrees]C, and 80[degrees]C, respectively). The mixture was needed to react at different temperatures for 1.5-2 hr for obtaining the modified lignin, respectively. We call this kind of modified lignin DLF.
b. Diethanolamine and formaldehyde were put into a 250 mL four-neck round-bottom flask equipped with a thermometer and mechanical stirrer. After the mixture was heated at 60[degrees]C for 2 hr, we moved it into the separating funnel. Then the mixture was added dropwise into the lignin solution at the specified temperature (50[degrees]C, 60[degrees]C, 70[degrees]C, and 80[degrees]C, respectively) in nitrogen atmosphere. The reaction need to be continued for 2 hr after the completion of dropping. We call this kind of modified lignin DFL.
Synthesis of Lignin-Based Epoxy Resins
Bisphenol A epoxy resin is a macromolecular compound which contains many epoxy groups, ether linkages, and hydrogen bonds . Ether linkages make intramolecular rotation easily. Meanwhile, hydroxyl bonds enhance the chemical activity of epoxy resins and improve their bonding capacity.
In the experiment of synthesizing lignin-based epoxy resins, we blended bisphenol A with epoxy chloropropane in a 250 mL three-neck round-bottom flask equipped with a thermometer and mechanical stirrer, and the mixture was heated until bisphenol A dissolved completely into solution at 50[degrees]C. Then 1 mL lignin solution was added dropwise as catalyst into the mixture to keep reaction for 30 min. The water bath was set to the specified temperature (60[degrees]C, 70[degrees]C, 80[degrees]C, and 90[degrees]C, respectively) and then we dripped the residual lignin into the reaction system. We would obtain lignin-based epoxy resin after 1 hr when the dropping process was accomplished.
Synthesis of the neat epoxy resin, we took the same method as lignin-based epoxy resin. Here is only a small difference. In a typical procedure for the neat epoxy resin, 1 mL 30% sodium hydroxide solution (NaOH) instead of lignin solution was drop-wise added to the three-neck round-bottom flask and kept reaction for 1 hr at 90[degrees]C. We would obtain the colorless and transparent neat epoxy resin.
Figure 1 shows the simple processes of our experiment. OL, DLF, and DFL reacted with epoxy chloropropane to form epoxy resins were called OL-E, DLF-E, and DFL-E, respectively.
The microstructure of the modified lignin was characterized by using Hitachi H-800 transmission electron microscope (TEM) at the accelerator voltage value of 200 kV. The powder was dispersed completely in ethanol by ultrasonication before TEM characterization.
We utilized JEOL JSM 6700F scanning electron microscopy (SEM) to examine the microstructure of the lignin-based epoxy resins at 5.0 kV. The lignin-based epoxy resins were dispersed completely into acetone before SEM characterization.
The content of phenolic hydroxyl was examined by using the Shimadzu UV-2450 ultraviolet spectra (UV) and other functional groups were examined by the Nicolet Impact 410 infrared spectra (FT-1R).
Thermal gravimetric analysis (TGA) is valuable techniques for studying the thermal properties of various compounds, which were carried out by means of a DTG-60H analyzer (SHIMADZU). Tests were performed with about 6 mg of samples on alumina crucibles with the heating rates of 20[degrees]C/min in air atmosphere and weight losses were calculated from the TGA using tabular [alpha]-[Al.sub.2][O.sub.3] as reference.
The hydroxymethyl content of lignin and the epoxide equivalent (EEW) of lignin-based epoxy resins were measured by ASTM D 1763-00 (2005).
RESULTS AND DISCUSSION
Effects of Lignin Modification
Many factors, such as the adding order of reactants, reaction temperature and the value of pH can influence the performance of the modified lignin. Therefore, we investigated those factors, respectively.
The Change of Phenolic Hydroxyl and Hydroxymethyl Contents. Formaldehyde was Dripped into Mixed Solutions of Lignin and Diethanolamine. We examined the influences of the pH value and the reaction temperature on the reaction products in the same reaction time when formaldehyde was dripped into mixed solutions of lignin and diethanolamine.
In order to test the change of phenolic hydroxyl content, we measured the UV absorption of the neutral solution against the one of alkaline solutions for getting the UV difference spectra, and we defined [DELTA][[DELTA].sub.max] was the difference value of the maximum absorbance of the two solutions above, and we took the vanillin as the model compound [31, 32] with the detection wavelength at 279 nm.
The phenolic hydroxyl group content of the lignin sample was calculated using the [DELTA][[DELTA].sub.max]. The computational formula of phenolic hydroxyl was shown as follows.
Phenolic hydroxyl (%) = [DELTA][[DELTA].sub.max] X 0.41078 x 100. (1)
The changes of phenolic hydroxyl content and hydroxymethyl content are shown in Table 1.
Table 1 shows that pH (from 5 to 11) affected the contents of phenolic hydroxyl and hydroxymethyl in lignin during the modifying process when the temperature was 80[degrees]C for 2 hr. From Table 1 we could see the content of phenolic hydroxyl having no regularity and the content of hydroxymethyl in lignin increased faster than the increasing of pH value. But the hydroxymethyl content decreased rapidly when the pH value was >10. While the value of pH was 10, the contents of phenolic hydroxyl and hydroxymethyl in lignin reached the maxima of 3.63% and 23.03%, respectively.
Table 2 shows the effect of temperature (from 50[degrees]C to 80[degrees]C) on the phenolic hydroxyl content and the hydroxymethyl content in lignin during modifying processes when the pH was 10 and the reaction time was 2 hr. From Table 2 we could see that the contents of phenolic hydroxyl and hydroxymethyl were curve with an upward trend until the temperature reaching 60[degrees]C, then they exhibited a downtrend. The content of phenolic hydroxyl in lignin got a maximum of 4.04% in this process. The content of hydroxymethyl could get up to 24.38% at the same temperature.
Therefore, we could draw the conclusion obviously that if we took this way of adding lignin in the modifying process, the contents of phenolic hydroxyl and hydroxymethyl in lignin could be increased by 39.3% and 18.4%, respectively (The content of phenolic hydroxyl was 2.90% and hydroxymethyl content was 20.6% in unmodified lignin.), when the pH was 10 and the reaction temperature was 60[degrees]C.
Mixed Solutions of Formaldehyde and Diethanolamine were Dripped into Lignin. We examined the influences of pH value on the reaction products in the same reaction temperature and time, which are shown in Table 3.
Table 3 shows the effects of pH (from 5 to 11) on the phenolic hydroxyl and hydroxymethyl content of lignin in the modification process when the temperature was 80[degrees]C and the reaction time lasted for 2 hr. From Table 3 we could see that when the pH value getting 10 the phenolic hydroxyl and hydroxymethyl content of lignin reached a maximum of 4.00% and 26.63%, respectively. They increased about 37.9% and 29.1% compared with the content of unmodified lignin.
The process of modifying lignin by Mannich reaction is a substitution reaction, that is, 2 moles of hydroxyl in diethanolamine was added into 1 mole lignin molecule. Therefore, the modified lignin has more active groups than the original lignin.
Change of the Lignin Morphology. The microstructure of the modified lignin was characterized by using TEM. Figure 2 shows the effect of Mannich reaction on the morphology of lignin. Figure 2a represents the original lignin, and Fig. 2b and c show the lignin which was modified by Mannich reaction.
The TEM image of Fig. 2a is the original lignin. We can see that the morphology of the original lignin was irregular, heterogeneous, cross-linked, and non-uniform cluster (shown in Fig. 2a). Meanwhile the modified lignin in Fig. 2b and c was spherical and uniform. So we could realize that Mannich reaction could make the macromolecule lignin more dispersed than the original lignin. Mannich reaction degraded the curling structure in lignin and increased the number of lignin active groups exposing to the external, which would improve the chemical activity of lignin.
FT-IR Spectra of Modified Lignin. The FT-1R spectra were used to determine which functional groups would be present in the samples. The FT-IR spectroscopy data indicated that the lignin after being modified (shown in Fig. 3, DFL and DLF) had different frameworks comparing with the original one (Fig. 3, OL). From Fig. 3, we found that the peak of 3443 [cm.sup.-1] showed the evidence of hydroxyl vibrating; 2943 [cm.sup.-1] was the characteristic peak of C-H deformations in methyl molecule; 1596 [cm.sup.-1] peak indicated the existence of vibration in the aromatic rings skeleton.
Comparing the three sets of FT-IR spectroscopy data, we could find that there were some other different peaks. In the FT-IR spectra of DFL and DLF, 1701 [cm.sup.-1] was the characteristic peak of C=O vibration in aromatic rings; 1334 [cm.sup.-1] showed the C-N stretching vibration in the tertiary amine. The absorption peaks of C=O and C-N could not be found in the original one (shown in Fig. 3). This illustrates that the C-N groups in diethanolamine were added to the modified lignin molecules, which could also be proved from the reaction Eqs. 2-5. These results demonstrated Mannich reaction can modify lignin. The reaction Eq. 2 is the total process of modified lignin by Mannich reaction. The reaction Eqs. 3-5 are the processes of modified lignin by two different orders to add the reagents into the reacting system in Mannich reaction as mentioned.
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Effects of Lignin-Based Epoxy Resins
There are many factors such as the amount of lignin added and the reaction temperature at the same reaction time can influence the performance of lignin-based epoxy resins. Therefore these factors were investigated, respectively.
FT-IR Spectra of Lignin-Based Epoxy Resins. We could see some characteristic peaks of functional groups as shown in Fig. 4. The peak near 3416 [cm.sup.-1] was hydroxyl vibration. The peak at 2963 [cm.sup.-1] showed C-H stretching vibration in ethyl. Peaks near the 1602 [cm.sup.-1] indicated the existence of the vibration in the aromatic rings skeleton. About 1508 [cm.sup.-1] peak was the evidence of asymmetric deformation of epoxy groups. About 1243 [cm.sup.-1] was C-O-C asymmetric stretching vibration. About 1043 and 829 [cm.sup.-1] were the vibration peak of substituent groups on the benzene ring. We could see from the FT-IR spectra that lignin-based epoxy resins and the neat epoxy resin had the same structure. Therefore, we can draw the conclusion that the modified lignin by Mannich reaction has higher reaction activity than the original one and they both could replace bisphenol A to synthesize epoxy resins.
SEM of Lignin-Based Epoxy Resins. We used the SEM micrograph to observe the morphology of the lignin-based epoxy resins. Figure 5a shows the neat epoxy resin, and Fig. 5b-d is OL-E, DLF-E, and DFL-E, respectively. The quantity of lignin in all kinds of lignin-based epoxy resins was 25 wt%. From Fig. 5b-d, we could see that the lignin-based epoxy resins existed obvious particles, which might be the macromolecules of lignin-based epoxy resins. Because the molecular weight of lignin is much bigger than that of bisphenol A, the molecular weight of lignin-based epoxy resins are bigger than that of the neat epoxy resin, when we use the same synthesis method. And the particles of lignin-based epoxy resins are more branched than those of the neat epoxy resin.
The Content of Lignin in the Lignin-Based Epoxy Resins. The lignin content in epoxy resins was measured by ultraviolet spectra. According to the law of Beer-Lambert, the absorbance of material is proportional to its mass absorption coefficient, mass concentration, and the thickness of sample pool, so we take acetone and water as the solvent to measure the content of lignin in the lignin-based epoxy resins. From the experiments we found that the lignin in lignin-based epoxy resins had strong absorption peak at 327 nm, which is shown in Fig. 6.
Figure 7 illustrates the contents of unreacted lignin in lignin-based epoxy resins. We found that the unreacted lignin content of lignin-based epoxy resins was growing with the increasing of the lignin addition amount (shown in Fig. 7). The unreacted lignin content in DFL-E was the least among the three samples when the adding amount was same, that is, 0.114% of the lignin did not react when 25 wt% of lignin was added. So we could draw the conclusion that the modified lignin had more active groups, which were conducive to the modification by Mannich reaction. Dripping mixed solutions of formaldehyde and diethanolamine into lignin was more effective than the formaldehyde adding dropwise into mixed solutions of lignin and diethanolamine.
TGA of Lignin-Based Epoxy Resins. TGA is also widely used to investigate the thermal decomposition of polymers. This parameter can be used to give a better understanding of the thermal stability of lignin-based epoxy resins.
Figure 8 shows us TGA results of the neat epoxy resin, OL, OL-E, and DFL-E. (The quantity of lignin was 25 wt% in the OL-E and DFL-E.) The thermal stability and the decomposition temperature of OL-E and DFL-E were clearly improved (Fig. 8). The decomposition temperature of the OL-E, DFL-E, and the neat epoxy resin were divided into two processes. Before 400[degrees]C, the decomposition temperature of OL-E was 226[degrees]C, DFL-E was 241[degrees]C, and the neat epoxy resin was 210[degrees]C. We can clearly draw a conclusion that the decomposition temperature of OL-E was increased by 16[degrees]C and DFL-E increased by 31[degrees]C compared with the neat epoxy resin. In addition, the decomposition temperatures of OL-E and DFL-E were increased from 446[degrees]C to 452[degrees]C after 400[degrees]C. It showed that the lignin obviously improved the thermal resistance of the neat epoxy resin especially using the modified lignin. The reason is that the macromolecular structure of lignin increased the decomposition temperature of the neat epoxy resin. The modified lignin has more active groups is conducive to synthesize of the epoxy resin. From Fig. 8 we also know that the lignin is a reactant for the synthesis of the epoxy resin but not an additive because there is no lignin in OL-E and DFL-E systems beyond 650[degrees]C.
EEW of Lignin-Based Epoxy Resins. We examined EEW at different reaction temperatures and in different amounts of lignin added, which are shown in Table 4 and Fig. 9.
From Table 4 we could see that the EEW of lignin-based epoxy resins tended to decrease with the temperature keeping upward while the additive content of original lignin stay the same. The EEW would reach the maximum of 347 when the temperature was 90[degrees]C, so we set the reaction temperature at 90[degrees]C in the later experiments.
From Fig. 9, the EEW of lignin-based epoxy resins was constantly increasing with the addition in three kinds of lignin. (The quality ratio of lignin and bisphenol A was increased.)
The main reasons were that the proportion of phenol hydroxyl was low because the relative molecular mass of lignin was large. This led to the reactive activity of lignin less than that of bisphenol A, especially when their masses were same. Increasing lignin content would result in the number of epoxy groups in epoxy compound reducing and that lead the EEW of lignin-based epoxy resins to increase.
Figure 9a (OL-E) shows that the epoxy resin was synthesized by original lignin partly replacing of bisphenol A. We could sec that the curve of EEW had a little increase tend with the lignin added amount increasing, at the same time the maximum addition of lignin was 25 wt%. Figure 9b shows the EEW of DLF-E, which means we used modified lignin (Original lignin firstly reacted with diethanolamine and then formaldehyde was added into the mixture.) to synthesize the epoxy resin. We could find the EEW tended to an increase until the additive amount getting to 30 wt%. Figure 9c shows the EEW of DFL-E. (Diethanolamine and formaldehyde firstly reacted and then we used mixed solutions to modify lignin. Then we used the modified lignin to synthesize the epoxy resin.) We could find that the EEW exhibited a slow increase, meanwhile the maximum addition was 30 wt%.
We can see that the EEW curves of the two different kinds of modified lignin (Fig. 9b and c) were obviously different, although they were both modified by Mannich reaction. From the following reaction Eqs. 2-5, we suspect that the differences are possibly caused by the following two reasons.
The first reason is that for EEW of DLF-E being obviously more than that of DFL-E may be that when dropping formaldehyde to mixed solutions of diethanolamine and lignin (the preparation process of DLF), only a part of the formaldehyde could react with lignin to form epoxy resins. At the same time it caused the diethanolamine excess. That was the rest of diethanolamine to be modified by Mannich reaction after all lignin reacted. The rest would react with epoxy groups of epoxy resins which led EEW of DLF-E to rise.
The second one is that, if we use the mixture, the diethanolamine would avoid formaldehyde reacting with modified lignin to form epoxy resins, that also increased the number of the active groups (-OH) in lignin.
We took the way of feeding by dripping mixed solutions of formaldehyde and diethanolamine into the lignin solution by Mannich reaction, which effectively improves the chemical activity of the lignin. The EEW reached a minimum of 446 when the additive amount of modified lignin was 30 wt% by the above Mannich reaction. The decomposition temperature of DFL-E was 31[degrees]C higher than that of the neat epoxy resin. These results suggest that the thermal stability of the neat epoxy resin was improved by the addition of the modified lignin, that is, lignin-based epoxy resins will be used in many fields.
[1.] Y.N. Qu, Y.M. Tian, B. Zou, J. Zhang, Y.H. Zheng, L.L. Wang, Y. Li, C.G. Rong, and Z.C. Wang, Bioresour. Technol., 101, 8402 (2010).
[2.] J. Zhang, L.X. Yu, Z.C. Wang, Y.M. Tian, Y.N. Qu, Y. Wang, J.J. Li, and H.Q. Liu, J. Client. Technol. Biotechnol., 86, 1177 (2011).
[3.] M. R. Barzegari, A. Alemdar, Y.L. Zhang, and D. Rodrigue, Polym. Compos., 33, 353 (2012).
[4.] S. Hirose, T. Hatakeyama, and H. Hatakeyama, Macromol. Symp., 224, 343 (2005).
[5.] G. Sun, H.G. Sun, Y. Liu, B.Y. Zhao, N. Zhu, and K.A. Hu, Polymer, 48, 330 (2007).
[6.] R.M. Perez, J.K.W. Sandler, V. Altstadt, T. Hoffmann, D. Pospiech, M. Ciesielski, M. Doring, U. Braun, A.I. Balabanovich, and B. Schartel, Polymer, 48, 778 (2007).
[7.] B.Y. Zhao, G. Chen, and Y. Liu, J. Mater. Sci., 20. 859 (2001).
[8.] N. Mansour, Q.L. Yuan, and F.R. Huang, Bioresources, 6, 2492 (2011).
[9.] F. Chen, H.H. Dai, X.L. Dong, J.T. Yang, and M.Q. Zhong, Polym. Compos., 32. 1019 (2011).
[10.] H.O. Yu, J. Liu, X. Wen, Z.W. Jiang, Y.J. Wang, L. Wang, J. Zheng, S.Y. Fu, and T. Tang, Polymer, 52, 4891 (2011).
[11.] J. Kong, Y.S. Tang, X.J. Zhang, and J.W. Gu, Polym. Bull., 60. 229 (2008).
[12.] C. Kaynak, A. Ozturk, and T. Tincer, Polym. Int., 51, 749 (2002).
[13.] C.S. Wu, Y.L. Liu, and K.Y. Hsu, Polymer. 44. 565 (2003).
[14.] L.A. Mercado, G. Ribera, M. Galia, and V. Cadiz, J. Polym. Sci. Part A Polym. Chem., 44, 1676 (2006).
[15.] Y.L. Liu, Polymer, 42, 3445 (2001).
[16.] G.H. Chen, B. Yang, and Y.Z. Wang, J. Appl. Polym. Sci., 102, 4978 (2006).
[17.] A.S. Zerda, and A.J. Lesser, J. Appl. Polym. Sci., 84. 302 (2002).
[18.] C. Baptista, D. Robert, and A.P. Duarte, Chem. Eng. J., 121, 153 (2006).
[19.] S. Sarkar and B. Adhikari, Polym. Compos., 22, 518 (2001).
[20.] D. Feldman, D. Banu, and A. Natansohn, J. Appl. Polym. Sci., 42, 1537 (1991).
[21.] Y. Nonaka, B. Tomita, and Y. Hatano, Holzforschung, 51. 183 (1997).
[22.] Y. Nakamura, T. Sawada, and K. Kuno, J. Chem. Eng. Jpn., 34, 1309 (2001).
[23.] C. Asada, Y. Nakamura, and F. Kobayashi, Biochem. Eng. J., 23, 131 (2005).
[24.] S. Hirose, T. Hatakeyama, and H. Hatakeyama, Thermochim. Acta, 431, 76 (2005).
[25.] G. Sun, H.G. Sun, and Y. Liu, Polymer. 48, 330 (2007).
[26.] Y. Matsushita and S. Yasuda, J. Wood. Sci., 49, 166 (2003).
[27.] Y.T. Zhou, X.L. Zhang, Y. Gao, and L.P. Zhang, Mod. Chem. Indus., 31, 260 (2011).
[28.] H. Kishi, A. Fujita, and H. Miyazaki, J. Appl. Polym. Sci., 102, 2285 (2006).
[29.] T. Zhang, X.W. Zhang, and X.L. Yan, Colloid. Surf. A, 392, 198 (2011).
[30.] K. Minu, J.K. Kurian, and V.V.N. Kishore, Biomass Bioenerg., 39, 210 (2012).
[31.] J.M. Yang and A.D.A.I. Goring, Can. J. Chem., 58, 2411 (1980).
[32.] T. Dizhbite, T. Galina, and V. Jurkjane, Bioresour. Technol., 95, 309 (2004).
Na Ding, (1) Xiaofeng Wang, (2) Yumei Tian, (1) Liu Yang, (1) Hongzhuo Chen, (1) Zichen Wang (1)
(1) College of Chemistry, Jilin University, Changchun 130012, People's Republic of China
(2) State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, People's Republic of China
Correspondence to: Y. Tian; e-mail: firstname.lastname@example.org
Contract grant sponsor: Key Project of the National Twelfth Five-Year Research Program of China; contract grant number: 2011BAE06B06; contract grant sponsor: Scientific and Technological Planning Project of Jilin Province; contract grant number: 20130302019GX.
Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Effects of the value of pH on the phenolic hydroxyl groups and hydroxymethyl contents of modified lignin. PH Contents (wt%) 5 6 7 8 Phenolic hydroxyl 2.19 2.11 1.64 3.21 groups Hydroxymethyl 18.87 20.21 21.12 21.31 PH Contents (wt%) 9 10 II Phenolic hydroxyl 2.22 3.63 2.84 groups Hydroxymethyl 21.70 23.03 21.13 TABLE 2. The effects of reaction temperature on the phenolic hydroxyl groups and hydroxymethyl contents of modified lignin. Temperature ([degrees]C) Contents (wt%) 50 60 70 80 Phenolic 1.80 4.04 2.92 3.63 hydroxyl groups Hydroxymethyl 21.64 24.38 23.41 23.03 TABLE 3. Effects of the value of pH on the phenolic hydroxyl groups and hydroxymethyl contents of modified lignin. pH Contents 5 6 7 8 (wt%) Phenolic 2.48 2.56 3.10 2.69 hydroxyl groups Hydroxymethyl 18.11 19.57 20.28 25.58 pH Contents 9 10 11 (wt%) Phenolic 3.23 4.00 2.77 hydroxyl groups Hydroxymethyl 26.14 26.63 24.86 TABLE 4. The EEW of epoxy resins containing 10% of original lignin at different temperatures. Epoxy resins Temperature ([degrees]C) containing 10% of original lignin 60 70 80 90 EEW 469 384 358 347
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|Author:||Ding, Na; Wang, Xiaofeng; Tian, Yumei; Yang, Liu; Chen, Hongzhuo; Wang, Zichen|
|Publication:||Polymer Engineering and Science|
|Date:||Dec 1, 2014|
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