ABSTRACT:Three bisphenol-A based novolac epoxy resins with different bridging groups: methylene, methinephenyl and methine-naphthyl, respectively, between bisphenol-A phenyl rings were prepared to study the effect of structure of bridging group on curing and properties of the epoxy resins. The structures of the obtained epoxy resins were characterized using FT-IR and 1 H NMR spectra, the molecular weight and polydispersity index were determined using GPC. The effect of bridging groups on the curing kinetics, thermal mechanical properties, thermal stability, and moisture resistance of the synthesized epoxy resins cured with 4,4 0 -diaminodiphenyl sulphone (DDS) were investigated by dynamic differential scanning calorimetry, dynamic mechanical analysis, thermogravimetric analysis, X-ray diffraction and moisture absorption measurement. It was concluded that the methylene-bridged epoxy resin possessed the highest curing reaction reactivity toward DDS and the methine-naphthyl-bridged epoxy network possessed the highest storage modulus, glass transition temperatures, thermal stability, and moisture resistance. Epoxy resin has been utilized in many applications such as surface coatings, structural adhesives, printed circuit board, insulation materials for electronic devices, and advanced composites matrices due to its good thermal and dimensional stability, excellent chemical and corrosion resistance, and superior mechanical and electrical properties, in addition to ease of handling and processabilities. However, in hightech applications lower thermal expansion, higher toughness, better heat and moisture resistance were required.1 For this reason, much work has been continuously directed toward improving their thermal and physical properties by modifications of epoxy resins, in both backbone and pendant groups.
2-5Several approaches were adopted to enhance the heat resistance of epoxy resins with increasing crosslink density or introducing bulky structures such as biphenyl or naphthalene.6-8 The introduction of naphthalene moiety is expected to greatly improve the thermal property and moisture resistance owing to its excellent rigidity, facile packing of molecules and significant degree of hydrophobicity.9-14 Many studies reported that epoxy resin showed remarkably higher glass transition temperature (T g ), higher thermal stability, higher flexural modulus, better moisture resistance, lower coefficient of thermal expansion when stiff naphthalene was inserted or appended by chemical modification. [15][16][17][18] In order to design and develop more novel epoxy resins with higher thermal and mechanical properties to meet the application need, the deeply understanding the effect of the molecular structure of materials on curing behavior, chemical and physical properties of epoxy resin was increasingly more important, because it can predict the end product properties of the new materials and provide a guide for the development process and speed up the development cycle. Varley et al. utilized the relationship between the ...
