Thermochimica Acta431(2005)中煤第七十一工程处
se.20sqw
76–80
Glass transition and thermal decomposition of
epoxy resins from the carboxylic acid system
consisting of ester-carboxylic acid derivatives of
alcoholysis lignin and ethylene glycol with various
dicarboxylic acids
Shigeo Hirose a,∗,Tatsuko Hatakeyama b,Hyoe Hatakeyama c
a National Institute of Advanced Industrial Science and Technology,Central5,Tsukuba,Ibaraki305-8565,Japan
b Otsuma Women’s University,12Sanbancho,Chiyoda-ku,Tokyo102-8537,Japan
c Fukui University of Technology,3-6-1Gakuen,Fukui910-8501,Japan
Received14October2004;received in revised form10December2004;accepted21January2005
Available online23May2005
Abstract
Alcoholysis lignin(AL)was dissolved in ethylene glycol and the obtained mixture was reacted with succinic anhydride to form a mixture of ester-carboxylic acid derivatives of AL and ethylene glycol(AL-poly(ester-carboxylic acid),ALEGPA).The obtained ALEGPA was mixed with dicarboxylic acids with different alkylene chain length such as succinic acid(alkylene chain,C2),adipic(C4)acid and sebacic acid(C8). The obtained mixture of ALEGPA and dicarboxylic acid was reacted with ethylene glycol diglycidyl ether in the presence of a catalytic amount of dimethylbenzylamine to form ester-epoxy resins.The curing reaction was carried out at130◦C for5h.The molar ratio of epoxy groups to carboxylic acid groups([EPOXY]/[AA]ratios,mol/mol)was1.0.The ALEGPA content in the above mixture was varied from0to100%. Thermal properties of epoxy resins were studied by differential scanning calorimetry(DSC)and thermogravimetry(TG).Glass transition temperatures(T g’s)increased with increasing ALEGPA contents,suggesting that lignin acts as a hard segment in epoxy resin networks.The values of T g’s of epoxy resins with dicarboxylic acids increased in the following order;epoxy resins with succinic acid(alkylene chain,C2), adipic acid(C4)and sebacic acid(C8).Thermal degradation temperatures(T d’s)of epoxy resins slightly decreased with increasing
ALEGPA contents.The values of mass residue at500◦C(MR500)increased with increasing AL contents in epoxy resins and also with decreasing chain lengths of dicarboxylic acids.
©2005Elsevier B.V.All rights reserved.
Keywords:Glass transition;Thermal decomposition;Epoxy resins;Alcoholysis lignin;Aliphatic dicarboxylic acids
1.Introduction
Lignin is recognized as one of the most important renew-able resources,since the amount of production is very large [1].Lignin has a highly branched chemical structure con-sisting of phenyl propane units which are connected mainly by ether linkage.It is known that lignin shows insufficient ∗Corresponding author.Tel.:+81298616250;fax:+81298616250.
E-mail address:jp(S.Hirose).mechanical properties in solid state as a polymeric material [2].Many attempts in chemical and physical modifications of lignin have been made in order to solve the above problems in its utilization as a polymeric material.In the last10years, we have extensively studied synthetic polymers from lignin [3,4].In the above studies,synthetic polymers were
derived from lignin on the basis of molecular design concerning the basic structures such as phenyl propane units,and also the functional groups in lignin molecules such as hydroxyl and methoxyl groups.Recently,it was found that polyurethanes
0040-6031/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.a.2005.01.043
S.Hirose et al./Thermochimica Acta431(2005)76–8077
阻垢剂评价derived from lignin and also from lignin-based caprolactones show excellent thermal and mechanical properties and also biodegradability[5–9]. Epoxy resins are recognized as one of the important ther-moset polymers,since they are used in various materials such as adhesives,matrix of composites and elastomers. In the past,many researchers studied ether type of lignin-based epoxy resins prepared from lignin[10–12].Recently, aliphatic polyesters,such as polycaprolactones,poly ethy-lene succinate,polylactic acid,have received considerable attention due to the fact that they are biodegradable.In our previous study,we investigated synthesis and thermal prop-erties of the ester type of epoxy resins,which can be derived from lignin,polyethylene glycol diglycidyl ether and azelaic anhydride[13].We also studied epoxy resins which can be prepared from an ester-carboxylic acid derivative of lignin synthesized from alco
holysis lignin(AL)and succinic acid anhydride.The obtained ester-carboxylic acid derivative of AL(ALEGPA)was reacted with ethylene glycol diglycidyl ether(EGDGE)to form epoxy resins under various condi-tions.The behavior in curing reactions was studied by differ-ential scanning calorimetry(DSC).Furthermore,the thermal properties of the obtained epoxy resins were studied by DSC and thermogravimetry(TG)[14].In the present study,epoxy resins were prepared from a carboxylic acid system consist-ing of ALEGPA with aliphatic dicarboxylic acids with dif-ferent alkylene chain lengths such as succinic acid(alkylene chain,C2),adipic acid(C4)and sebacic acid(C8).The ther-mal properties such as glass transition and thermal decompo-sition of the obtained epoxy resins were studied by DSC and TG.The influence of the difference in chemical structure of epoxy resins on thermal properties is investigated.
2.Experimental
2.1.Materials
Alcoholysis lignin(AL)was kindly supplied by Repap Co.,USA,and was dried in vacuum at70◦C.Other reagents such as ethylene glycol(EG),ethylene glycol diglycidyl ether(EGDGE),dimethylbenzylamine(DMBA),succinic acid,adipic acid and sebacic acid were comme
rcially ob-tained from Wako Pure Chemical Industries Ltd.,Japan.The above reagents were used without further purification.
2.2.Preparation of epoxy resins
AL polyacid(ALEGPA)was prepared from AL,EG and succinic anhydride,according to the method previously re-ported[14].ALEGPA and a dicarboxylic acid(DCA)was mixed well with EGDGE at80◦C,and the mixture was al-lowed to stand at130◦C for5h in an oven.Each of succinic acid,adipic acid and sebacic acid was used as an aliphatic di-carboxylic acid(DCA).The molar ratios of carboxylic acid groups to epoxy groups[EPOXY]/[ACID]ratio(mol mol−1)was maintained at1.0.ALEGPA contents were varied at0, 20,40,60,80and100%.The[EPOXY]/[ACID]ratios and the ALEGPA contents were calculated by the following equa-tions:
[EPOXY]/[ACID]ratio(mol/mol)=(M EGDGE W EGDGE)/ (M ALEGPA W ALEGPA+M DCA W DCA),ALEGPA content(%) =[W ALEGPA/(W ALEGPA+W DCA)]×100
where M EGDGE is the mole number of epoxy groups per gram of EGDGE(7.7mmol g−1),W EGDGE the weight of EGDGE, M ALEGPA the mole number of carboxylic acid groups per gram of ALEGPA(6.62mmol g−1),W ALEGPA the weight of ALEGPA,M DCA the mole number of carboxylic a
cid groups per gram of DCA,W DCA the weight of DCA.
2.3.Measurements
A Perkin-Elmer Spectrum One Fourier transform infrared spectrometer equipped with a universal ATR unit was used for infrared spectrometry.A Seiko DSC220was used for dif-ferential scanning calorimetry(DSC).The measurements of glass transition of epoxy resins were carried out ranging from −60to80◦C at a heating rate of10◦C min−1using ca.5mg of samples.The samples were heated to130◦C and main-
tained for10min,and then they were quenched to−60◦C in DSC aluminum vessels before measurements.The glass transition temperatures(T g’s)were determined according to a method reported by Nakamura et al.[15].A Seiko TG/DTA 220was used for thermogravimetry(TG).The measurements were carried out using ca.5mg of samples at a heating rate of10◦C min−1in nitrogenflow of300mL min−1.Thermal decomposition temperatures(T d’s)were determined accord-ing to a method reported by Hatakeyama and coworkers [16].
3.Results and discussion
In the present study,epoxy resins were obtained by the reaction of a mixture ALEGPA/DCA with EGDGE. The reaction scheme is shown in Scheme1.The chemical structure of the obtained ALEGPA is confirmed by FT-IR. FT-IR spectrum of ALEGPA is shown in Fig.1.The characteristic absorption peaks of carboxylic acid groups at1780,around2700and3200cm−1,and also of ester groups at1720,1200cm−1are observed in the spectrum. The FT-IR spectrum of an epoxy resin with succinic acid with ALEGPA content60%after curing at130◦C for5h is also shown in Fig.1.The characteristic absorption peaks for ester groups1720and1200cm−1and also hydroxyl groups at3300cm−1are observed.
Glass transition of epoxy resins was studied by DSC.Fig.2 shows DSC curves of epoxy resins with various ALEGPA contents.A heat capacity gap in baseline due to glass transi-
78S.Hirose et al./Thermochimica Acta 431(2005)
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Scheme 1.Reaction scheme for the preparation of epoxy
resins.
Fig.1.FT-IR spectra of ALEGPA,EGDGE and an epoxy resin with succinic
acid.
Fig.2.DSC curves of epoxy resins with various ALEGPA contents in a ALEGPA/succinic acid system.Numbers indicate ALEGPA contents.tion is observed in each DSC curve.T g ’s change according to the change in ALEGPA contents of epoxy resins in the ALEGPA/succinic acid system.Fig.3shows the relationship between T g and ALEGPA content of epoxy resins with dicar-boxylic acids (DCA)with various alkylene chain lengths.T g increases with increasing ALEGPA content of epoxy resin.The above results indicate that lignin acts as a hard segment in epoxy resin molecules.It is known that lignin is a highly branched polymer consisting of phenylpropane units mainly connected by ether linkage.It has also a number of hydroxyl groups in a molecule [1].Therefore,it is considered that lignin exists as cross-linking points.The chain lengths of epoxy resins between cross-linking points decrease with in-creasing ALEGPA content in epoxy resins.The increase in the chain lengths between cross-linking points enhances the main chain molecular motion.The T g values are high in
the
Fig.3.Relationship between T g and ALEGPA content of epoxy resins with DCA with various alkylene chain lengths.C 2(᭹),C 4( )and C 8( ).
S.Hirose et al./Thermochimica Acta 431(2005)76–80
79
Fig.4.TG and differential TG (DTG)curves of the starting materials such as ALPA,EGDGE and AL.
order of epoxy resins with succinic acid (C 2),adipic acid (C 4)and sebacic acid (C 8).The above results are reasonable when we consider that the flexibility of main chains in epoxy resin molecules increases with increasing chain lengths of dicarboxylic acids.
Thermal decomposition behavior of starting materials and epoxy resins was studied by TG.Fig.4shows TG and dif-ferential TG (DTG)curves of the starting materials such as ALEGPA,EGDGE and AL.TG and DTG curves of DCA are not shown in Fig.4,since only the evaporation of the above compounds was observed in TG measurements.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 start-ing materials are 189.6and 133.3◦C while T d2’s 335.3and 233.2◦C,respectively.It is known that epoxy groups are rel-atively unstable [17].Accordingly,it is considered that the above group starts to decompose at T d1region.
Fig.5shows TG and differential TG (DTG)curves of epoxy resins with various ALEGPA contents in the ALEGPA/DCA systems with succinic acid,adipic acid and sebacic acid.It is observed that the dec
omposition apparently proceeds in a smooth step.The thermal degradation at T d1re-gion,that is observed in TG curves of the starting
materials
Fig.5.TG and differential TG (DTG)curves of epoxy
resins.
Fig.6.Relationship between T d ALEGPA content of epoxy resins.C 2(᭹),C 4( )and C 8( ).
(Fig.4),is not observed.This indicates that thermally un-stable carboxylic acid and epoxy groups were converted into thermally stable ester groups.Thermal decomposition tem-peratures (T d ’s)and mass residue at 500◦C (MR 500)were determined from TG curves.Fig.6shows the relationship be-tween T d and ALEGPA content of epoxy resins.T d slightly decreases with increasing ALEGPA content.However,the degree of the decrease in T d values is very small.It is known that lignin is relatively thermally unstable [6].As shown in Fig.4,T d of AL was determined as 284.3◦C.The T d values of epoxy resins from AL are much higher than that of AL.There-fore,it can be considered that lignin becomes thermally stable after introduction into the epoxy resin molecules.As shown in Fig.6,T d values are almost the same regardless of the dif-ference in alkylene chain lengths of DCA.In order to clarify the influence of lignin in epoxy resins on MR 500values,AL contents in epoxy resins were calculated.The relationship be-tween AL contents and MR 500is shown in Fig.7.As clearly seen in Fig.7,MR 500values increase with increasing AL con-tents in epoxy resins.It is known that lignin mol
ecules react with each other to form a condensed char-like material,when they are heated in nitrogen.Therefore,it is considered that the materials in the residue at 500◦C are mainly formed by the reaction with lignin in epoxy resins during the decompo-sition process.The above consideration can be supported by the fact that the MR 500value of ALEGPA is higher than that of SA,as shown in Fig.4.As shown in Fig.7,MR 500vales are always high in the order of epoxy resins with succinic acid (C 2),adipic acid (C 4)and sebacinic acid (C 8),suggesting that dicarboxylic acids with longer alkylene chains give smaller amounts of residual materials after thermal decomposition up to 500◦C.However,the above difference in MR 500values be-comes smaller when AL contents are increased.Accordingly,it can be said that the AL contents in epoxy resins strongly affect MR 500values in the higher AL contents region.
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Fig.7.Relationship between MR500’s and AL contents in epoxy resins.C2 (᭹),C4( )and C8( ).
4.Conclusion
Epoxy resins were obtained by the reaction of the ester-carboxylic acid derivatives of alcoholysis lignin (ALEGPA)/aliphatic dicarboxylic acids(DCA)system with EGDGE.T g increased with increasing ALEGPA contents in epoxy resins,suggesting that lignin acts as a hard segment in epoxy resins.It was found that T g’s of epoxy resins decreased with increasing alkylene chain lengths of dicarboxylic acids from C2to C8.T d slightly decreased with increasing ALEGPA content in epoxy resin.The difference in alkylene chain length of DCA does not affect T d values of epoxy resins,while it affects MR500values.It was also found that AL contents in epoxy resins strongly affect MR500values particularly in the higher AL contents region.References
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