Thermal Properties of Epoxy Resins from Ester-Carboxylic Acid Derivatives of Mono- and Disaccharides

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1 Netsu Sokutei Thermal Properties of Epoxy Resins from Ester-Carboxylic Acid Derivatives of Mono- and Disaccharides Shigeo Hirose, Tatsuko Hatakeyama, and Hyoe Hatakeyama (Received May 20, 2003; Accepted June 17, 2003) Mono- and disaccharides (SAC) such as glucose (Glc), fructose (Frc) and sucrose (Suc) were dissolved in ethylene glycol and each of the obtained mixture was reacted with succinic anhydride to form a mixture of ester-carboxylic acid derivatives (SAC-polyacid, SACPA). Ethylene glycol-polyacid (EGPA) was also prepared from ethylene glycol. Each of the obtained mixtures of ester carboxylic acid derivatives was reacted with ethylene glycol diglycidyl ether in the presence of a catalytic amount of dimethylbenzylamine to form ester-epoxy resins. The molar ratios of epoxy groups to carboxylic acid groups ([EPOXY]/[ACID] ratios, mol mol 1 ) were varied from 0.5 to 1.3. The contents of SACPA in the mixture of SACPA and EGPA were also varied from 0 to 100 %. Thermal properties of epoxy resins were studied by differential scanning calorimetry (DSC) and thermogravimetry (TG). Glass transition temperatures (T g's) of epoxy resins showed a maximum value of 8.1 when [EPOXY]/[ACID] ratio was 1.0. T g's increased with increasing SACPA contents suggesting that saccharides act as hard segments in epoxy resin networks. Thermal decomposition temperatures (T d's) of epoxy resins slightly decreased with increasing SACPA contents, while mass residue at 500 increased with increasing SACPA contents. 1. Introduction The main plant components such as cellulose, hemicellulose and lignin are recognized as the most important renewable resources, since the amount of their production is large. 1) Mono- and disaccharides such as glucose, fructose, sucrose, etc. are also abundantly produced from sugar cane and other plants. 2) Therefore, the utilization of the above plant components and saccharides has been extensively studied in the past. Many attempts in chemical and physical modifications of the above materials have been made in order to obtain various kinds of polymers. In the last ten years, synthetic polymers, which were derived from saccharides, and lignin, which are the main components of plant materials, have been extensively studied in our laboratory. 3,4) The above studies have been carried out on the basis of molecular design concerning the chemical structures in their molecules such as pyranose and furanose rings, phenylene, hydroxyl and methoxyl groups. Recently, it was found that polyurethanes derived from molasses, mono- and disaccharides and also from saccharide-based caprolactones show excellent thermal and mechanical properties and also biodegradability. 5-9) Epoxy resins are known to be one of the important thermoset polymers, since they are used as various materials such as adhesives, composite matrices and elastomers. 10) Concerning saccharide-based epoxy resins, studies on oxyalkylated sucrose epoxy resins, 11) sucrosebased polyether-polyol interpenetrating networks with epoxy resins 12) and epoxy resins from glucose maleic acid ester vinyl copolymers 13) have been reported. Recently, aliphatic polyesters, such as polycaprolactones, poly(ethylene succinate), and poly(lactic acid), have received considerable attention due to the fact that they are biodegradable. 14) In our previous study, we investigated 2003 The Japan Society of Calorimetry and Thermal Analysis. 154

2 Thermal Properties of Epoxy Resins from Ester-Carboxylic Acid Derivatives of Mono- and Disaccharides the synthesis and thermal properties of the ester type of epoxy resins, which can be derived from lignin, polyethylene glycol diglycidyl ether and azelaic anhydride. 15) It has been reported that the ester type of epoxy resins with hydroxyl groups attached to the β -carbons of ester groups can be prepared by the reaction of epoxy compounds with carboxylic acids in the presence of amine catalysts. 16) Accordingly, in our previous study, epoxy resins were prepared from an ester-carboxylic acid derivative of alcoholysis lignin. 17) In the present study, ester-carboxylic acid derivatives were synthesized from mono- and disaccharides (SAC) such as glucose (Glc), fructose (Frc) and sucrose (Suc). The obtained ester-carboxylic acid derivative of SAC was reacted with ethylene glycol diglycidyl ether (EGDGE) to form epoxy resins under various conditions. The thermal properties of the obtained epoxy resins were studied by differential scanning calorimetry (DSC) and thermogravimetry (TG). 2. Experimental 2.1 Materials Mono- and disaccharides (SAC) such as Glc, Frc and Suc were commercially obtained from Wako Pure Chemical Industries Ltd., Japan. They were dried in vacuum at 70 in the presence of phosphorous pentoxide. Other reagents such as ethylene glycol (EG), glycerin (Gly), succinic anhydride (SAH), ethylene glycol diglycidyl ether (EGDGE), and dimethylbenzylamine (DMBA) were also commercially obtained from Wako Pure Chemical Industries Ltd., Japan. The above reagents were used without further purification. 2.2 Synthesis of ester-carboxylic acid derivatives of SAC and EG SAC polyacid (SACPA) was obtained as follows. Glc 50 g was dissolved in EG 50 g at 80. Succinic anhydride 60 g (mol) and DMBA 3.75 g were added to the above solution, and the mixture was stirred at 80 for 6 h. Polyacids were also synthesized from Frc, Suc and Gly. EG polyacid (EGPA ) were also prepared from EG in the same manner as SACPA. The synthetic scheme of the above process is shown in Scheme Synthesis of epoxy resins Each SACPA was mixed well with EGDGE at 80, Scheme 1 Scheme for synthetic procedure of epoxy resins. and the mixture was allowed to stand at 130 for 5 hr in an oven. The synthetic scheme of the epoxy resins is also shown in Scheme 1. Epoxy resins were also synthesized from GlyPA in the same manner, as a reference. The molar ratios of carboxylic acid groups to epoxy groups [EPOXY]/[ACID] ratio (mol mol 1 ) were varied at 0.5, 0.6, , 0.9, 1.0, 1.1, 1.2 and 1.3. SACPA contents were also varied at 0, 20, 40, 60, 80 and 100 %. The [EPOXY]/[ACID] ratios and the SACPA contents were calculated by the following equations. [EPOXY]/[ACID] ratio (mol mol 1 ) (M EGDGE W EGDGE) / (M SACPA W SACPA M EGPA W EGPA) SACPA content (%) [W SACPA /(W SACPA W EGPA)] 100 where M EGDGE is the amount of epoxy groups per gram of EGDGE (7.7 mmol g 1 ), W EGDGE the weight of EGDGE, M SACPA the amount of carboxylic acid groups per gram of SACPA (Glc 7.50, Frc 7.50 and Suc 7.35 mmol g 1 ), W SACPA the weight of SACPA, M EGPA the amount of carboxylic acid groups per gram of EGPA (7.63 mmol g 1 ), W EGPA the weight of EGPA. 2.4 Measurements A Perkin-Elmer Spectrum One Fourier transform infrared spectrometer was used for infrared spectrometry. The measurements were carried out using a universal ATR unit. A Seiko DSC 220 was used for differential scanning calorimetry (DSC). The measurements of phase transition of epoxy resins were carried out ranging from 60 to 80 at a heating rate of 10 min 1 using ca. 5 mg of samples. The samples were heated to

3 Absorbance / Arbitrary Unit Wavenumber / cm 1 Fig.1 Chemical structure of glucose, fructose and sucrose. Fig.2 FTIR spectra of GlcPA and epoxy resin from GlcPA at [EPOXY]/[ACID] ratio 1.0. Absorbance / Arbitrary Unit Wavenumber / cm 1 Fig.3 Enlarged FTIR spectra of GlcPA and epoxy resin from GlcPA in the range of 1500 and 4000 cm 1. Scheme 2 Reaction scheme for the synthesis of APLA and epoxy resins. and maintained for 10 min, and then they were quenched to 60 in DSC aluminum vessels before measurements. The glass transition temperatures (T g's) were determined according to a method reported by Nakamura et al. 18) The heat capacity shift at T g ( C p) was also determined using DSC curves. A Seiko TG/DTA 220 was used for thermogravimetry (TG). The measurements were carried out using ca. 5 mg of samples at a heating rate of 10 min 1 in nitrogen flow of 300 ml min 1. Thermal decomposition temperatures (T d's) were determined according to a method reported by Hatakeyama. 19) Mass residues at 500 (MR's) were also determined for the samples. 3. Results and Discussion In the present study, epoxy resins were synthesized as follows. Mono- or disaccharides (SAC) (the chemical structure of the above saccharides are shown in Fig.1) were dissolved in the same amount by weight of EG, and the obtained mixture was reacted with succinic anhydride to form the mixture of ester-carboxylic acid derivatives of SAC and EG (SAC polyacid, SACPA). Epoxy resins were obtained by the reaction of SACPA with EGDGE. The reaction scheme is shown in Scheme 2. The chemical structure of the obtained SACPA was analyzed by FTIR. FTIR spectrum of GlcPA is shown in Fig.2. The characteristic absorption peaks of carboxylic acid groups at ca. 1690, around 2650 and 3000 cm 1, and also of ester groups at 1730, 1200 cm 1 are observed in the spectrum. The FTIR spectrum of an epoxy resin 156

4 Thermal Properties of Epoxy Resins from Ester-Carboxylic Acid Derivatives of Mono- and Disaccharides Endo. Fig.4 DSC curves of epoxy resins with various [EPOXY]/[ACID] ratios. Numbers indicate [EPOXY]/[ACID] ratios. Tg / Endo. Fig.6 DSC curves of epoxy resins with various SACPA contents. Numbers indicate SACPA contents. Tg / Cp / J deg 1 g 1 SACPA content / % [EPOXY]/[ACID] Ratio / mol mol 1 Fig.5 Relationship between T g and [EPOXY]/[ACID] ratio of epoxy resins. from GlcPA after curing at 130 for 5 h is also shown in Fig.2. The enlarged spectra are also shown in Fig.3. The characteristic absorption peaks for ester groups at 1730 and 1200 cm 1 and also hydroxyl groups at 3450 cm 1 are observed, while characteristic peaks of carboxylic acid groups at 1690, 2650 and 3000 cm 1 are not observed. The above results suggest that carboxylic acid groups react with epoxy groups to form ester groups and hydroxyl groups after the curing reaction. Phase transition of epoxy resins was studied by DSC. Fig.4 shows DSC curves of epoxy resins with various [EPOXY]/[ACID] ratios. In each DSC curve, the shift in the baseline due to glass transition is observed. The glass transition temperatures (T g's) were determined using DSC curves. Fig.5 shows the relationship between T g and [EPOXY]/[ACID] ratio of epoxy resins. T g initially Fig.7 Changes of T g and C p against SACPA content of epoxy resins. Glc ( ), Frc ( ) and Suc ( ). increases slightly with increasing [EPOXY]/[ACID] ratios and reaches a maximum at [EPOXY]/[ACID] ratio 1.0. It is considered that the very slight increase in T g values when [EPOXY]/[ACID] ratios are less than 1.0 is due to the existence of a number of carboxylic acid groups in the reaction mixtures which cause the restriction of molecular motion of polymer chains due to hydrogen bondings. The above results indicate that the cross-linking densities become highest at [EPOXY]/[ACID] ratio 1.0. Fig.6 shows DSC curves of epoxy resins with various GlcPA contents. T g's change according to the change in GlcPA contents of epoxy resins. Fig.7 shows the changes of T g and C p against SACPA content of epoxy resins. T g increases with increasing SACP content, while C p decreases with increasing SACPA content of epoxy resin, although the C p data are somewhat scattered. The above results indicate that saccharides act as hard segments in epoxy resin molecules. Accordingly, it is 157

5 Tg / DTG / % min 1 TG / mt m % n / mol Fig.8 Changes of T g against calculated number of repeating units between cross-linking points (n) of epoxy resins. Glc ( ), Frc ( ), Suc ( ) and Gly ( ). DTG / % min 1 TG / mt m % Fig.9 TG and differential TG (DTG) curves of the starting materials such as GlcPA, EGPA and EGDGE. considered that saccharides exist as cross-linking points and that the chain lengths of epoxy resins between crosslinking points decrease with increasing SACPA content in epoxy resins. The increase in the chain lengths between cross-linking points enhances the main chain molecular motion. Furthermore, T g values of epoxy resins from Suc are always highest and, in contrast, their C p values are lowest, particularly in the high SACPA content region. The above results suggest that the effect of the chemical structure of saccharides on the main chain motion of epoxy resins between cross-linking points, becomes prominent particularly in the higher SACPA content region. Fig.8 shows the relationship between T g and the calculated number of repeating units (one unit consists Fig.10 TG and DTG curves of epoxy resins with various [EPOXY]/[ACID] ratio. Numbers indicate [EPOXY]/[ACID] ratios. of EGPA 1 mol and EGDGE 1 mol) between cross-linking points. The results of epoxy resins from glycerin (Gly) are also shown in Fig.8, as a reference. The T g values of epoxy resins from Glc, Frc and Gly are almost the same, suggesting that the effect of ring structure on the main chain motion of epoxy resins are small in the present epoxy resin system. Thermal decomposition behavior of starting materials and epoxy resins was studied by TG. Fig.9 shows TG and differential TG (DTG) curves of the starting materials such as GlcPA, EGPA and EGDGE. It is observed that thermal decomposition apparently proceeds in two steps. T d's at lower temperature regions (T d1) and also T d's at higher temperature regions (T d2) were determined. T d1's of the above starting materials are 201.2, and 133.3, while T d2's are 354.7, and 203.2, respectively. It is known that carboxylic acid and epoxy groups are thermally unstable. Accordingly, it is considered that those groups decompose at the T d1 region. Fig.10 shows TG and differential TG (DTG) curves of epoxy resins with various [EPOXY]/[ACID] ratios. It is observed that thermal decomposition proceeds in two steps for the samples with [EPOXY]/[ACID] ratios less than 1.0. It is considered that the thermal decomposition at lower temperature region (T d1's) is related to the decomposition of carboxylic acid in the samples. The values of the T d's at higher temperature region (T d2's) are almost the same regardless of the [EPOXY]/[ACID] ratios, as shown in Fig.11. Fig.12 also shows TG and DTG curves of epoxy resins with SACPA contents. It is observed that the decomposition apparently 158

6 Thermal Properties of Epoxy Resins from Ester-Carboxylic Acid Derivatives of Mono- and Disaccharides Td / Td / MR / % [EPOXY]/[ACID] Ratio / mol mol 1 Fig.11 Relationship between T d and [EPOXY]/[ACID] ratio of epoxy resins. SACPA content / % Fig.13 Changes of T d and mass residue at 500 (MR) against SACPA content of epoxy resins. Glc ( ), Frc ( ) and Suc ( ). DTG / % min 1 TG / mt m % content. It is known that saccharide molecules react with each other to form a condensed char-like material, when it is heated in nitrogen. 20) Therefore, it is considered that the materials in the residue at 500 are mainly formed by the reaction with saccharides in epoxy resins during the decomposition process. The above results can be supported by the fact that the MR value of SACPA is higher than that of EGPA, as shown in Fig Conclusion Fig.12 TG and DTG curves of epoxy resins with various SACPA contents. Numbers indicate SACPA contents. proceeds in a smooth step, without thermal decomposition at the T d1 region, which are observed in TG curves of the starting materials (see Fig.9). This indicates that thermally unstable carboxylic acid and epoxy groups were converted into thermally stable ester groups. Thermal decomposition temperatures (T d's) and mass residue at 500 (MR) were determined from TG curves. Fig.13 shows the relationship between T d and SACPA content of epoxy resins. T d decreases with increasing SACPA content. It is known that saccharides are relatively thermally unstable. 20) However, the degree of the decrease in T d values is very small. Therefore, it can be said that saccharides become thermally stable after introduction into the epoxy resin molecules. Fig.13 also shows the relationship between MR and SACPA content of epoxy resins. MR increases with increasing SACPA Ester-carboxylic acid derivative of SAC was synthesized in a solution of SAC in EG. Epoxy resins were obtained by the reaction of the ester-carboxylic acid derivatives with EGDGE in the presence of a catalytic amount of BMBA at [EPOXY]/ [ACID] ratio 1.0. T g increased with increasing SACPA contents in epoxy resins, suggesting that saccharides act as hard segments in epoxy resins. T d slightly decreased with increasing SACPA content in epoxy resin. However, the decrease in T d was small, suggesting that the thermal stability of the obtained epoxy resins is not affected by the introduction of saccharide molecules into the epoxy resin molecular chains. References 1) K. Kringstad, in "Future Sources of Organic Raw Materials - CHEMRAWN I", (L. E. St. Pierre et. al. Eds.), Pergamon Press, p.627 (1980). 2) F. W. Lichtenthaler and S. Mondel, Pure & Appl. Chem. 69, 1853 (1997). 3) H. Hatakeyama, S. Hirose, T. Hatakeyama, K. Nakamura, K. Kobashigawa, and N. Morohoshi, J. 159

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