Fluorescence monitoring of curing process and water accessibility at glass fiber/epoxy interphase on composite materials

Modified glass fibers with three different amino silanes-(1) 3-aminopropyltriethoxysilane (APTES), (2) 3-aminopropylmethyldiethoxysilane (APDES), and (3) 3-aminopropyldimethylethoxysilane (APMES)-were labeled with 5-dimethylaminonaphthalene-1-sulfonylchloride (DNS), and its fluorescence was followed as a function of curing time for two epoxide formulations (TRIEPOX-LM, Gairesa, Spain, and diglycidyl ester bisphenol A-ethylenediamine), as well as a function of water uptake. It was demonstrated that the fluorescence response from fluorophores directly attached to the coupling region in glass fibers/epoxy composites is a very sensible method for nondestructive and "in situ" monitoring of any changes that appear at the interphase (rigidity and solvent accessibility). In addition, fluorescence from a dansyl label shows more rigidity when APMES is used as the coupling agent. This suggests that the greater the cross-linking degree at the coupling region, the smaller the epoxy chain interdiffusion. Finally, the water uptake process in glass fiber/epoxy composites can be divided into two steps; the first step corresponds to the diffusion of water to the coupling region, and the second step corresponds to the hydrolysis of siloxane bonds in the interphase. Both steps are dependent on the surface treatment of the glass fibers.