Effect of water on the properties of epoxy matrix and composite
The complex sorption behavior of the water in amine-epoxy thermosets is discussed and related to depression of the mechanical properties. The hypothesized sorption modes and the corresponding mechanisms of plasticization are discussed on the basis of experimental vapor and liquid sorption tests, differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis. In particular, two different types of epoxy materials have been chosen: low-performance systems of diglycidyl ether of bisphenol-A (DGEBA) cured with linear amines, and high-performance formulations based on aromatic amine-cured tetraglycidyldiamino diphenylmethane (TGDDM) which are commonly used as matrices for carbon fiber composites.
Three modes of moisture sorption are assumed: dilution of the free volume in the network, hydrogen bonding involving hydrophilic groups of the polymer and adsorption onto the surfaces of “holes” which define the excess free volume of the glassy structure and are induced hygrothermally. The DGEBA systems are described to have no appreciable sorption via hydrogen bonding, but rather most of the moisture absorbed is through dilution of the free volume and adsorption in the “holes”. On the other hand, the TGDDM systems are described to absorb more moisture through significant hydrogen bonding and dilution of the free volume. High plasticization, evident as Tg depressions of 30 to 80 °C, are possible and experimentally observed especially for the stiffer TGDDM resins. Relationships derived from the free volume theory or the classical thermodynamic treatment may be used to describe the compositional dependence of the Tg in miscible blends.
KeywordsFree Volume Epoxy Matrix Moisture Sorption Epoxy System Sorbed Water
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