Study On Preparation And Properties Of Hot-wet Resistant Epoxy Resins

Fluorene-containing epoxy resins are widely used as resin matrices of fiber reinforced composites and electronic encapsulation materials as a kind of two-function solid epoxides, due to their advantages of high strength, good thermal stability and desirable moisture resistance.Two novel fluorene-containing epoxy resins, diglycidyl ether of 9,9-bis(4-hydroxyphenyl)-fluorene (DGEBF) and diglycidyl ether of 9,9-bis(3-methyl-4-hydroxyphenyl)-fluorene (DGEMBF), and two fluorene-based curing agents, 9,9-bis-(4-aminophenyl)-fluorene (BPF) and 9,9-bis-(3-methyl-4-aminophenyl)-fluorene (BMAPF), were prepared based on the recent literatures and the synthetic conditions such as reagent formula, catalyst dosage, reaction temperature and reaction time were optimized. The chemical structures of the obtained products were characterized by FTIR, NMR and MS. Fluorene-containing epoxy resins could be synthesized effectively by phase transfer catalysis, and the epoxy values of obtained epoxides reached up to 95% approximately of the theoretic calculated values. Fluorenyl diamines were prepared by the nucleophilic substitution of fluorenone and aromatic amines. Under optimal process, the average weight yield of BPF and BMAPF was 59.4% and 69.2% respectively, and the mass percentages of these two curatives were both up to 99.4%.The epoxy blends were prepared with fluorene-containing epoxy resins (DGEBF or DGEMBF), bisphenol A epoxide (E-44) and cycloaliphatic epoxy resin (TDE-85), and then cured with aromatic diamines such as BPF, BMAPF, diaminodiphenylsulfone (DDS) and diaminodiphenylmethane (DDM). The curing processes of the resulting epoxy systems were firstly investigated by gel curve tests to evaluate apparent curing activities of those epoxy systems, and then studied by differential scanning calorimetry (DSC) to obtain initial, peak and final extrapolated curing temperatures, and the optimal curing processes were finally concluded based on the results of the abovementioned testment. DSC results indicate that the cure activities of epoxy systems are restrained and all the extrapolated exothermic temperatures shifted to higher temperatures, due to the introduction of fluorene moieties to the chain backbone and the substitution of methyl for hydrogen atoms.Shear strength tests show that the cured fluorene-containing epoxy resins exhibit good high-temperature mechamcal performance and relatively poor ductility at room temperature, however, the shear strength could be improved by the substitution of methyl for hydrogen atoms in the side segments. Furthermore, the polymers cured with fluorenyl diamines possess better mechanical properties at elevated temperature than that of the polymers cured DDS or DDM. Moreover, the molar ratio of epoxide to curing agent also affects the mechanical performance of the epoxy thermosets, and a molar ratio of epoxide to curing agent ranged from 1:1 to 1:1.1 is acceptable.The thermal properties of epoxy blends were further evaluated by dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). DMTA results indicate that cured polymers bearing fluorene moieties display remarkably higher elastic modulus (E) and glass transition temperature (T_g) than those of neat cured E-44 and TDE-85 resins, and an elevated T_g would be obtained with the increased content of DGEBF in the epoxy blends. TGA results are in agreement with that of DMTA. T_g of blend reached up to 260℃when cured with BPF. Furthermore, for the fluorenyl diamines, a decreased T_g and E would be induced by the incorporation of methyl to the side groups.Moisture absorption tests indicate that the moisture resistance of fluorenyl diamines are better than that of DDS and DDM. The balance water uptake of epoxy/fluorenyl diamine system is less than 2.3% except that of TDE-85/fluorenyl diamine systems. The moisture absorption of cured resins could be reduced via blending DGEBF with general epoxides. It can be concluded that the water uptake of cured polymer could be restricted by the incorporation of hydrophobic fluorene rings and methyl groups.