Fluorescence study of the hydrolytic degradation process of the polysiloxane coatings of basalt fibers

Basalt fiber surfaces were modified using different silane aqueous solutions to generate a variety of polyorganosiloxane coatings. After removing the commercial coating of the fibers by calcination and subsequent activation processes, polysiloxanes were grafted on the fiber surfaces. Three aqueous solutions were used for the silanization: (i) γ-aminopropyltriethoxysilane, APTES; (ii) γ-aminopropylmethyldiethoxysilane, APDES, and (iii) a mixture of 50% by weight of both APTES + APDES. The silanized fibers were chemically labeled with fluorescein isothiocyanate to be immersed afterwards in different aqueous solutions (pH = 7) to study the hydrolytic degradation of the polysiloxane coatings. The hydrolysis phenomena were monitored by steady state fluorescence at different temperatures to subsequently study the kinetics of the process. The hydrolysis process was also studied by monitoring the pH of the solution in which the silanized fibers were immersed as a function of time. The data obtained from fluorimetry were fitted to an integrated expression arising from a first order kinetic process, which allowed estimation of the activation energies of the hydrolytic degradations. The results indicated that although the hydrolytic rate of the polysiloxane coatings increased in the order APDES < APDES + APTES < APTES, differences in the mechanism were not the cause of that order; the initial concentration of siloxane bonds able to be hydrolyzed was responsible.