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This paper mainly investigated the hygrothermal fatigue of
three thermosetting resin systems. In part one, an anomalous
moisture absorption behavior of 4,4'-dicyanato-1,1'-diphenolentane
(DCDPE) cyanate ester resin was investigated. A two-stage
diffusion hypothesis was employed to modify the Fickian equation
and was found to better fit the data points. It was due to that
the swelling stress produced in the second stage of diffusion
decreased the diffusion rate. Long-term moisture absorption also
hydrolyzed the triazine into the dimer links as indicated by FTIR
analysis and the resulting hygrothermal fatigue further produced
some microvoids as observed by SEM. Finally, through the moisture
absorption investigation of various thickness samples, we found
that the thickness had a significant effect on the degree of
deviation from Fickian law.
In part two, the hygrothermal fatigue of the diglycidyl ether
of bisphenol A／ethylene diamine (DGEBA/EDA) resins blending with
various content of carboxy-terminated butadiene－acrylonitrile
(CTBN(AN=17%)) was investigation. The samples were exposured to
three different environments, i.e. 60℃/100%RH, 70℃/100%RH and
80℃/100%RH. The determination of saturated moisture gain and
apparent diffusivity was carried out by the measurement of weight
gain of the samples. Based on that, the activated energy of
diffusion and the ratio of Vf/V* (free volume/critical free volume)
were calculated. Besides, we employed the FTIR and DMA techniques
to analyze the change of chemical structure and thermal properties
during the moisture absorption process. The results showed that the
more of CTBN incorporated to DGEBA/EDA system, the lesser of the
moisture gain after saturation, the larger diffusion coefficient,
the more free volume and the lower activation energy of diffusion
were found. When the content of CTBN was less than 30phr, the Tβ
of wet samples were lower than that of the dry one. When the content
of CTBN was larger than 30phr, the Tg of wet samples were lower than
that of the dry one.
In part three, we found that the addition of chromium acetylacetonate
(Cr(acac)3) into tetra functional epoxy resin formulation can decrease
the moisture gain of laminates. It was attributed to the interaction of
Cr(acac)3 with the -OH group in the epoxy resins. If 2-undecyl-imidazole
(C11Z) accelerator was further incorporated into the epoxy resins, the
concentration of -OH group could be diluted and would not entirely
interact Cr(acac)3. Hence, the moisture gain was increased. On the
contrary, the drastic depression in moisture gain of laminate prepared
from the same formulation was found. This was attributed to the
processing effects, which introduce the inhomogeneous network. From the
analysis of RMS, EPMA and DMA results, we found that the lay-up laminate
heated at 120oC in the preliminary stage had rather high viscosity due
to the early reaction of C11Z with tetraglycidy-diaminodiphenyl methane
(TGDDM). The following compact stage caused the small molecules such as
uncured TGDDM, diaminodiphenyl sulfone (DDS) and Cr(acac)3 squeezed into
the surface of carbon fiber. The resulting inhomogeneity of polymer
network in the laminate decreased the moisture absorption.
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