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

Modified glass fibers with three different amino silanes-(1) 3-aminopropylt riethoxysilane (APTES), (2) 3-aminopropylmethyldiethoxysilane (APDES), and (3) 3-aminopropyldimethylethoxysilane (APMES)-were labeled with 5-dimethyla minonaphthalene-1-sulfonylchloride (DNS), and its fluorescence was followed as a function of curing time for two epoxide formulations (TRIEPOX-LM, Gai resa, Spain, and diglycidyl ester bisphenol A-ethylenediamine), as well as a function of water uptake. It was demonstrated that the fluorescence respo nse from fluorophores directly attached to the coupling region in glass fib ers/epoxy composites is a very sensible method for nondestructive and "in s itu" 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 t he epoxy chain interdiffusion. Finally, the water uptake process in glass f iber/epoxy composites can be divided into two steps; the first step corresp onds to the diffusion of water to the coupling region, and the second step corresponds to the hydrolysis of siloxane bonds in the interphase. Both ste ps are dependent on the surface treatment of the glass fibers.