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1.Introduction
Water uptake in epoxy systems is a challenging issue due to the irreversible changes that water operates on the polymer properties.It is believed and there are enough experimental evidences that entrance of water induces:(a) swelling of the system and build-up of residual stresses at and near the interfaces [1],(b) rupture of adhesive bonding between the system and a given substrate [2],(c) aperture of oxirane remaining groups [3],(d) modification of the local stress state and creation of microcrazes through environmental stress cracking [4].
There is no universal model to cover all types of water diffusion [1].Several mechanisms for the water entrance have been proposed:(a) diffusion governed by Fick’s law through the free volume of the polymer [5],(b) case II diffusion mechanism by which osmotic swelling is limited by polymer creep [6],(c) swelling induced by a favourable polymer–solvent parameter,(d) osmotic phenomena due to the presence of micropores,channels and other defects in the polymer [7].
It is also frequently found in the literature that the water diffusion coefficient in different epoxy systems is around 10−8 to 10−10 cm2 s−1 [1,2,8],even for rubber modified compositions [8].Similar values are obtained for other glassy polymer systems [2].Diffusion coefficient measures the initial rate of water absorption and,in principle,it should be dependent on the chemical nature of the polymer system and on the cross-linking degree for cross-linked systems such as epoxy’s.But,as it will be shown in this paper,similar diffusion coefficients are found even for not fully cured systems.
In an effort for designing water resistant epoxy systems,it is necessary to know which material parameters are really involved and control the process of water absorption.In this paper,we study the water absorption properties of a new epoxy formulation based in the use as hardener of a reactive derivative of a hydrophobic polymer,such as polysiloxane.It will be shown that when common bisphenol-A diglycidyl ether (DGEBA) resins are cured in the presence of poly(3-aminopropylmethylsiloxane) (PAMS),equilibrium properties are greatly enhanced.In addition,due to the special characteristics and morphology of this epoxy system [9],a different behaviour against postcuring temperature is found.
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1.Introduction
Water uptake in epoxy systems is a challenging issue due to the irreversible changes that water operates on the polymer properties.It is believed and there are enough experimental evidences that entrance of water induces:(a) swelling of the system and build-up of residual stresses at and near the interfaces [1],(b) rupture of adhesive bonding between the system and a given substrate [2],(c) aperture of oxirane remaining groups [3],(d) modification of the local stress state and creation of microcrazes through environmental stress cracking [4].
There is no universal model to cover all types of water diffusion [1].Several mechanisms for the water entrance have been proposed:(a) diffusion governed by Fick’s law through the free volume of the polymer [5],(b) case II diffusion mechanism by which osmotic swelling is limited by polymer creep [6],(c) swelling induced by a favourable polymer–solvent parameter,(d) osmotic phenomena due to the presence of micropores,channels and other defects in the polymer [7].
It is also frequently found in the literature that the water diffusion coefficient in different epoxy systems is around 10−8 to 10−10 cm2 s−1 [1,2,8],even for rubber modified compositions [8].Similar values are obtained for other glassy polymer systems [2].Diffusion coefficient measures the initial rate of water absorption and,in principle,it should be dependent on the chemical nature of the polymer system and on the cross-linking degree for cross-linked systems such as epoxy’s.But,as it will be shown in this paper,similar diffusion coefficients are found even for not fully cured systems.
In an effort for designing water resistant epoxy systems,it is necessary to know which material parameters are really involved and control the process of water absorption.In this paper,we study the water absorption properties of a new epoxy formulation based in the use as hardener of a reactive derivative of a hydrophobic polymer,such as polysiloxane.It will be shown that when common bisphenol-A diglycidyl ether (DGEBA) resins are cured in the presence of poly(3-aminopropylmethylsiloxane) (PAMS),equilibrium properties are greatly enhanced.In addition,due to the special characteristics and morphology of this epoxy system [9],a different behaviour against postcuring temperature is found.
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