The Effect of Masked Isocyanates on the Mechanical Properties of My720/dds Epoxy Resin
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THE EFFECT OF MASKED ISOCYANATES ON THE MECHANICAL PROPERTIES OF MY720/DDS EPOXY RESIN N. RUNGSIMUNTAKUL, S.V. LONIKAR, R. E. FORNES*, AND R.D. GILBERT
North Carolina State University, Fiber and Polymer Science Program, and *Physics Department, P. 0. Box 8202, Raleigh, NC 27695-8202 ABSTRACT
The mechanical properties of epoxy resins and epoxy resin/graphite fiber composites are adversely affected by moisture absorption. Incorporation of masked isocyanates that unmask to release isocyanates in situ at the cure temperatures (150-177°C) reduce the equilibrium absorption up to -70%. Dynamic mechanical analyses and stress-strain properties of epoxy resins containing masked isocyanates were examined to determine their effect on mechanical properties. The ultimate Tg of the epoxy is reduced by incorporation of masked isocyanate, but the actual Tg is comparable to the "as cured" Tg of the epoxy. The dynamic moduli up to the Tg are relatively unaffected. In a number of cases, the initial modulus, elongation at break and peak stress are equal or better than the unmodified resins. INTRODUCTION
In structural and aerospace applications, epoxy resins are used as the matrices in fiber reinforced composites that have high strengths and moduli and are light weight. The adverse effect of moisture on mechanical properties is one major drawback of epoxy resins. Upon curing, the epoxy-amine systems generate hydrophilic groups such as hydroxyl, secondary and tertiary amine groups and residual oxirane and primary amine groups (and in some cases sulfone) that can interact with water. Water is also absorbed into the unoccupied volume of the cured epoxy resins. The sorbed water plasticizes the resins, lowers the glass transition temperature (Tg), causes swelling, induces stresses, chemical bond cleavage and debonding of the fiber-matrix interface. Enhanced craze initiation and crack propagation reduce the serviceability of composites. Attempts to reduce moisture absorption of epoxy have been reported by several workers. (See Fisher et al.[1], Hu et al. [2] and references therein.) Fisher et al. [1] showed that the polar functional groups in cured tetraglycidyl-4,4'-diaminodiphenyl methane (TGDDM) - diaminodiphenyl sulfone (DDS) (73/27 wt%) thin films reacted with cc, ax, ca-trifluoro-m-tolyl isocyanate (MTFPI) in dimethylsulfoxide (DMSO) at 70'C, and reported a 54 % reduction in moisture absorption. Hu et al. [2] obtained 75 and 69 % reductions in equilibrium water absorption by reacting the hydroxyl, amine, and epoxide functional groups of cured TGDDM-DDS epoxy thin film (20-50 ptm thick) with pentafluorobenzoyl chloride and 2, 4-difluorophenyl isocyanate in N, N'-dimethylacetamide, respectively. The reactants were allowed to diffuse into
Mat. Res. Soc. Symp. Proc. Vol. 171. '1990 Materials Research Society
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the resin films which had been previously swollen with solvent. This technique is not feasible with the thicker films >150 jim) because the films crack due to successive swelling. The objectives of this study were to study th
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