Effect of polymer matrix on photo-stability of photo-luminescent dyes in multi-layer polymeric structures I. Baumberg, O. Berezin, A. Drabkin, B. Gorelik*, L. Kogan, M. Voskobojnik, M. Zaidman ELAM Ltd, Har Hotzvim, PO Box 45071, Jerusalem 91450, Israel Received 4 September 2000; received in revised form 30 January 2001; accepted 6 February 2001 Abstract The present study considers the photo-stability of luminescent dyes in multi-layer coatings made of various polymers [low-density polyethylene (LDPE), poly(vinyl chloride) (PVC) and poly(vinylidene fluoride) (PVDF)] . Dye stability is shown to depend on both the chemical structure of the colourant and the photo-stability of the polymer matrix. The influence of colourant self absorption on its photo-stability is shown. This effect was used to increase dye lightfastness in multi-layer coatings. In the case of a coloured PVC intermediate layer covered by an upper PVDF layer it is possible either to decrease or to increase lightfastness depending on the presence of UV-absorbers in the upper layer and its thickness and flexibility. Use of blends of PVDF based polymers with coloured PMMA substantially improves photo-stability of dyes compared to colourants introduced into pure PVDF. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photo-stability; Luminescent dyes; Coatings; PVC; PVDF 1. Introduction Multi-layer coloured coatings have found many appli- cations in the automotive industry, building decoration etc. The problem of photo-fading of coloured polymers is well known, in particular in the case of outdoor usage [1]. This paper gives experimental data on the photo-sta- bility of luminescent dyes and pigments in multi-layer polymer coatings used in manufacturing coloured elec- troluminescent fibres (ELF). The structure and properties of ELF and their con- stituents are described elsewhere [2–4]. Coloured ELF are used in various types of signs, lifejackets, car light accessories, etc. The general appearance of an ELF is shown in Fig. 1. As can be seen from Fig. 1 an ELF consists of a few constituents each having a specific function. The dielectric layer protects the article from short circuiting. The active electroluminescent layer consists of electroluminescent powder (ELP) dispersed in a polar binder. The transparent electrode is made of a thin (300 A ˚ ) tin oxide film. Polymer insulating layers (two or three) serve for protection of ELF from the environment and for colouration of the product. The latter is achieved by introduction of colourants into the polymer insulation. In some cases dyes can be intro- duced into the electroluminescent layer for tinting effect. Industrial ELP is able to give light only in the blue– green region of the spectrum. Photo-luminescent additives allow the creation of various colours from the initial blue–green light. An application of coloured ELF in building decoration and road signs can be accompanied with fast photo-dis- colouration as seen from Fig. 2, which shows transfor- mation of chromaticity coordinates of ELF of initially yellowcolourcausedbytheinfluenceofdirectsunlightin Israel. Development of coloured ELF with high light- fastness (durability more than 1 year in direct sunlight) included selection of appropriate colorant, polymer matrix, application of stabilization systems for dyes and polymers and usage of polymer multi-layer structure providing protection of colourants from photo-fading. 2. Experimental Various types of coloured ELF were produced on ELAM LTD production line. The composition of the 0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00119-7 Polymer Degradation and Stability 73 (2001) 403–410 www.elsevier.com/locate/polydegstab * Corresponding author. Fax: +972-2532-8889. E-mail address: borisg@elam.co.il (B. Gorelik).