Enhancement of fluorescent stability of 2,5-bis-2-(5-tert-butyl)-benzoxazolyl-thiophene (BBOT) containing LDPE films

The photostability of an optical brightener 2,5-bis-2-(5-tert-butyl)-benzoxazolyl-thiophene (BBOT) was investigated in solution and in LDPE films, respectively. The films were either laminated with light-screening transparent films containing ultrafine titanium dioxide or taken into multilayer systems containing a cross-linked component. It could be demonstrated that in the 250 through 400 nm irradiation spectra of BBOT the range between 250 and 300 nm was responsible for the light sensitivity of the system, whereas that between 300 and 400 nm was responsible for the fluorescence of the system. The influence of systems prepared with lamination and the cross-linked layer on the intensity of fluorescence of BBOT was determined. The simple lamination with titanium-dioxide-containing films did not prove to be an effective way of protection as both the photostability and the fluorescence intensity decreased simultaneously in systems of this type. The drop in light stability could be attributed to the migration of BBOT into the TiO2-containing film. A five-component system had to be produced for improving the light stability of the wanted product. LDPE film was linked to one side of a BBOT-containing film by means of a cross-linked adhesive. The other side of this film was linked to either another LDPE film or to TiO2-particle-filled LDPE film by the same type of cross linked adhesive. The initial intensity of fluorescence dropped simultaneously by 15 to 40%. The light stability could be increased up to 400% of the original value. (C) 1998 Elsevier Science S.A. All rights reserved.