The effects of pigments on the photostability of polyethylene

Abstract

The affect of carbon black and various colourizing pigments on the ultraviolet (UV) stability of high and low density polyethylene (HDPE and LDPE) was determined using a novel method for the analysis of oxygen uptake profiles. Samples were exposed to 0.27 Wm−2 (measured at 340 nm) UV irrdiation at 25.0±0.1° C in air at 1.0 atm. The usefulness of this method of assessment of UV stability is demonstrated. The method also enables the rapid collection of data that enable the comparison of the relative photostabilities of experimental and commercial formulations containing pigments and stabilizing additives. The results show that carbon black is an effective UV screening agent for HDPE when added at levels as low as 0.05% (wt/wt) and that increased photoprotection is achieved with increasing concentration of carbon black, up to 5% (wt/wt), above which there is no further significant increase in photostability. LDPE containing ultramarine blue pigment (Na7Al6Si6O24S3) exhibits relatively poor photostability, whereas ferric oxide (Fe2O3) and chrome orange (PbCrO4.PbO) pigments are better photostabilizers for this material. Cadmium sulphide (CdS) was found to photosensitize LDPE. A compound containing 0.10% (wt/wt) carbon black, 0.12% (wt/wt) titanium dioxide (TiO2) and 1.78% (wt/wt) phthalocyanine green (C33H2N8Cl14Cu) is an effective formulation for the stabilization of LDPE. Formulations of LDPE containing ultramarine blue-TiO2 or ferric oxidecarbon black combinations absorb heat on exposure and this may affect their photostability.