Effect of Carbon Black on UV stability of LLDPE films under artificial weathering conditions M. Liu, A.R. Horrocks* Faculty of Technology, Bolton Institute, Bolton BL3 5AB, UK Received 1 October 2001; accepted 12 October 2001 Abstract Carbon Black is recognised for its ability to stabilise polyolefins against UV degradation. Linear low density polyethylene (LLDPE) 75 mm extruded films containing a variety of carbon black with different particle size, structure and concentrations were exposed to two accelerated artificial weathering devices, a xenon arc source, e.g. Xenotest 150S, and fluorescent tube sources with UVA and UVB lamps, under controlled temperature and humidity. The changes in physicochemical properties during exposures were studied using tensile testing, Fourier transform infrared (FTIR) spectroscopic and differential scanning calorimetric (DSC) methods. Presence of each carbon black shows significant improvement in UV stabilisation compared to clear films, especially for those with small particle sizes as expected. There is no consistent effect of carbon black structure on UV stabilisation for various particle-sized carbon blacks. An increase in carbon black concentration from 1.5 to 3.5% w/w also improved UV stabilisation. For UVA and UVB sources, presence of carbon black, while increasing carbonyl group generation with respect to unit loss in tensile property with respect to unfilled LLDPE, also appears to suppress Norrish Type II scissions at photochemically generated carbonyl centers in polymer chains. This is especially the case for the smallest (20 nm) particle sizes. Thus the photostabilising efficiency of carbon black is based on both physical surface-area-dependent UV absorption and photochemical activity. Under xenon arc exposure, however, this latter is minimal. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyethylene; LLDPE; Carbon Black; UV; Carbonyl; Vinyl; Photodegradation 1. Introduction Carbon black has been an established light stabilising additive in polyolefins (and other polymers) for many years [1]. It is believed to function as a simple physical screen, a UV absorber, a radical trap and a terminator of the free radical chains through which the photo-oxi- dative reactions are propagated [1,2]. The resistance to UV degradation is usually related to the type and par- ticle size of the carbon black used as well as to the con- centration and dispersion of the carbon black in the matrix [3]. Small particle sized-carbon blacks are known to have the greatest UV stabilising efficiency but tend to agglomerate into aggregates or clusters which are not easily dispersed [1,4]. The complex mixture of chemisorbed, oxygenated groups on carbon black particle surfaces may also account for their UV protective capacity [5], and the quinonic and phenolic groups present may especially function as antioxidants [6]. The understanding of the role of carbon black towards oxidation may be sum- marised in terms of an antioxidant effect, probably via catalytic decomposition of peroxides and free radical scavenging with greater effectiveness at low tempera- ture, balanced by a pro-oxidant property under some conditions [7]. The observed effect of a given carbon black–polyolefin combination is the resultant of anti- and pro-oxidant effects influenced by both content and inherent properties of carbon black [8]. Recently pub- lished results [9,10] regarding the behaviour of carbon black-filled polypropylene tapes showed that the con- centration, particle diameter and structural character- istics of carbon black have significant effects on the tensile and morphological properties as well as thermal and photo stabilities. This paper discusses the first part of an investigation of the effect of a series of carbon blacks having a range of particle properties on the UV stability of linear low 0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00252-X Polymer Degradation and Stability 75 (2002) 485–499 www.elsevier.com/locate/polydegstab * Corresponding author. Tel.: +44-1204-528851; fax: +44-1204- 399074. E-mail address: arch1@bolton.ac.uk (A.R. Horrocks).