双官能度环氧树脂的双固化研究

编辑:wangyun 作者:张军营 程珏 北京化工大学 时间:2016年08月12日 访问次数:3630

 双官能度环氧树脂的双固化研究

张军营程珏

Dual Curing of Bifunctional Epoxy resin

Junying Zhang, Jue Cheng

Lab of adhesive and in-situ polymeriazation,Beijing university of chemical technology

Abstract: Based on long-standing fundamental research in our laboratory , the related some scientific problems hidden in industrial application of epoxy resin could be abstracted and solved. The important matter for epoxy resin is difficult to balance the long-term stability during shelf life and fast curing speed during application because the same curing reaction always take place in the system even in room temperature during storage. A series of bifunctional epoxy resin was designed and synthesized, and the related dual curing mechanism was revealed including “Synergistic” curing reaction for Diallyl-bearing epoxy resin(Diglycidal Ether of 4,4-diallyl bisphenol A, DGEDABA) and sulfur donors system, Ring-open reaction and Diels-Alder (D-A) reaction for furan-group containing bisphenol epoxy resin (Diglycidal Ether of bis(4-hydroxy-3,5-di-methylphenyl)-furylmethane,DGEDMF) and maleic anhydride system, and ring-open reaction and humidity-curing reaction for siliconized epoxy resin (SEP).
KeyWords: Epoxy, Dual curing, SEP ,D-A reaction

1. Introduction

Epoxy resins are widely used in diverse applications including adhesives, coatings, electronics and high-performance thermosetting matrix composites because of their outstanding mechanical and chemical properties, such as high bonding strength, low shrinkage, good chemical resistance and good processability. In all of these above applications a curing process is involved where a low molecular weight resin is transformed into an infinite molecular weight polymer with a three-dimensional network structure. The performance of epoxy resin is influenced by the curing mechanism, the structure of epoxy resin and the types of curing agent. The curing way of an epoxy resin is the most essential factor determining the performance of epoxy resin.

Bifunctional Epoxy resin is different from normal epoxy resin, and it contains two reactive functional groups--epoxy ring and the other reactive group. Dual-curing epoxy system can react some other curing behavior as well as the epoxy ring opening reaction which the conventional epoxy resin can do.There is a new method for epoxy resin modification that is introduction of other reactive groups to epoxy resin through the resin design so that curing agents can react with resin in two different curing mechanisms to adjust the performance and characteristics of epoxy resin. Dual-curing technology is an important means of curing , which can change the cross-linking pointsthe crosslink densitythe heat resistance of the epoxy resin.

2. Design of Double-bond contained epoxy resin and the related “Synergistic” curing mechanism

Based on long-standing fundamental research in our laboratory, the related some scientific problems hidden in industrial application of epoxy resin could be abstracted and solved. The important matter for epoxy resin is difficult to balance the long-term stability during shelf life and fast curing speed during application because the same curing reaction always take place in the system even in room temperature during storage. A series of bifunctional epoxy resin was designed and synthesized, and the related dual curing mechanism was revealed including “Synergistic” curing reaction for Diallyl-bearing epoxy resin (Diglycidal Ether of 4,4-diallyl bisphenol A, DADGEBA) and sulfur donors system, Ring-open reaction and Diels-Alder (D-A) reaction for furan-group containing bisphenol epoxy resin (Diglycidal Ether of bis(4-hydroxy-3,5-di-methylphenyl)-furylmethane, DGEDMF) and maleic anhydride system, and ring-open reaction and humidity-curing reaction for siliconized epoxy resin (SEP).

DADGEBA is a novel bifunctional epoxy resin which has both double bond and epoxide, and the synthetic process of DADGEBA is shown in figure 1. DADGEBA could not only be cured with general hardeners (amine, anhydride, imidazole, et al) but also be cured with sulfur donors which could not make the normal bisphenol A type epoxy resins (Diglycidal Ether of Bisphenal-A, DGEBA) cured. The curing behavior of DGEDABA with sulfur donors were carried out and the reaction mechanisms were investigated by using the structure characterization methods such as fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance(NMR) to monitor the changes of epoxy and vinyl groups in DADGEBA /S and DADGEBA/TMTD curing systems, and the related curing exotherms were also measured by differential scanning calorimetry (DSC). The vinyl and epoxy groups content were reduced with the increase of time when the systems heated. 1H-NMR and mass spectrum were adopted to monitor the structure changes before and after the reaction in 4,4’-diallyl bisphenol A (DAPBA)/S and (DAPBA)/TMTD systems, and the mercapto groups was formed and measured. The concluded“Synergistic” curing-reaction theory was as follows: First, sulfur donor break to form sulfur radicals at high temperature; Next, sulfur radicals capture α-H in allyl group to form mercapto groups and transfer the radicals to allyl structure. The sulfur radicals take a addition reaction with vinyl groups, while sulfhydryl groups react with epoxy ring, as in Fig2. The epoxy ring open reaction and the radicals-coupling reaction is the dual curing reactions for allyl contained epoxy resin and sulphur doner system. Curing kinetics of DADGEBA with sulfur was investigated by non-isothermal and isothermal DSC. The double exothermic peaks in DSC curve showed that the curing process of DADGEBA/S system contains two steps. Isothermal DSC analysis confirmed that the curing reaction followed Kamal’s autocatalytic model when the reaction temperature was between 170℃and 210℃, and when the temperature was above 210℃ the nth-order moder can be used to describe the curing reaction process of the DADGEBA/S system. The two kinetic equations in the entire reaction process of DADGEBA/S system are consistent with the two-step reaction mechanisms of the same system. The proposed theory would boost to come up with some new ideas and methods in rubber-metal bonding.

Fig.1 The synthetic process of DADGEBA

 

Fig.2 “Synergistic” curing reaction for DADGEBA and sulfur doners

3. Design and curing mechanism of furan-group contained epoxy resin

Furan group containing the conjugated double bonds was introduced to bisphenol which was used in synthesis of epoxy resin with conjugated diene, the synthetic process of DGEBFM is shown in figure 3. There exist both the ring opening esterifying reaction of the epoxy ring and anhydride and Diels-Alder reaction of double bond in the resin and furan ring of anhydride, which are called dual-curing technology. The latter reaction creates a multi-ring structure, so that it not only improve the cross-linking density and heat resistance of the materials, but also add a new way of double-curing method to curing research of DGEBFM. A dual curing reaction for DGEBFM and maleic anhydride (MAAD) was designed and studied. The furan-ring in DGEBFM was employed to react with double-bond in MAAD followed by Diels-Alder reaction mechanism, while the epoxy-group in DGEBFM react with anhydride in MAAD followed by normal ring open reaction, as in figure 4. The dual curing reaction not only improve the cross-linking density and heat resistance of the materials, but also increase the curing speed of epoxy resins. The flow activation energy of viscosity for DGEBFM is 86kJ/mol and that for DGEBA is 73 kJ/mol, which contributed to the bigger furan group between the phenyl ring. The curing behavior of DGEBFM/MAAD was studied by FTIRDSC and NMR. The results of FTIR spectral analysis indicate that epoxy resin participates in both ring-opening and Diels-Alder (D-A) reaction. The dependence of both reaction on curing temperature was detected by DSC and NMR. The degree of D-A reaction in the curing system was about 35.8%. The heat resistance of epoxy resin with furan group was higher than DGEBA cured with MAAD, and the gelling-speed of DGEBFM was faster than that of DGEBA.

Fig.3 The synthetic process of DGEBFM

 

Fig.4 The ring-open and “D-A” reaction for DGEBFM and maleic anhydride

4. Design and curing mechanism of silane-group contained epoxy resin

It is of great significance to modify epoxy resins with organosilicon. Hydrosilylation is used to combine allyl-containing epoxy resin and hydrogen-containing organosilicon without reducing epoxy value and forming silicon -oxygen-carbon bond. The resultant epoxy-organosilicon hybrid has high crosslinking density and hydrolyzing stability. A siliconized epoxy resin (SEP) was synthesized by hydrosilylation of DADGEBA with trimethoxylsilane, and the synthetic process of SEP is shown in figure 5. A dual curing reaction for SEP and normal curing agent was designed and studied. The trimethoxylsilane in SEP was employed to react with H2O in atmosphere followed by the moisture curing mechanism, as in figure 6, while the epoxy-group in SEP react with curing agent followed by normal ring open reaction. FTIR and NMR were adopted to characterize the structure of SEP. Epoxy value and molecular weight were tested by hydrochloric acid-acetone and GPC method. Epoxy group reaction curing and moisture-curing were studied and mechanical properties and blending properties were tested as well. The ratio of 1-location addition product and 2-location addition product was 6.44 to 1. The molecular weight of SEP was similar to its theoretical value. Gel time and DSC results showed that SEP had slower curing reaction rate and less exothermic quantity than common bisphenol A epoxy resin such as E-51. SEP could be moisture-cured in the existence of catalyst. The glass transition temperature of DDM cured E-51 was 117.4 and the glass transition temperature of DDM cured SEP was 91.7 . The glass transition temperature of moisture-cured SEP was 55.0 . The mechanical strength of allyl-containing epoxy resin is 70~80% that of E-51, and SEP has a 50~65% that of E-51. The bonding property of SEP is similar to E-51 when silane coupling agent is used. Like E-51, SEP could be dissolved in most common solvents. Owing to containing silane structure, SEP has fine compatibility with organosilicon.

Fig.5 The synthetic process of SEP

Fig. 6 Moisture-curing reaction of siliconized epoxy resin (SEP)

Conclusion

Dual curing of bifunctional epoxy resin will improve the performance and characteristics of epoxy resin. Dual-curing technology is an important means of curing , which can change the cross-linking pointsthe crosslink densitythe heat resistance of the epoxy resin.

5. Acknowledgments:

This work was supported by Natural Science Foundation of China(NSFC), Grant No21176017 and 20376006.