Performance of Natural-Fiber–Plastic Composites under Stress for Outdoor Applications: Effect of Moisture, Temperature, and Ultraviolet Light Exposure J. L. Lopez, M. Sain, P. Cooper Faculty of Forestry, Centre for Biocomposites and Biomaterials Processing, University of Toronto, Toronto M5S 3B3, Canada Received 20 August 2004; accepted 15 April 2005 DOI 10.1002/app.22884 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The effects of moisture, temperature, and ul- traviolet (UV) light on performance of natural-fiber–plastic composites (NFPC) were assessed. We conducted short-term tests in the laboratory and long-term tests under natural exposure and measured changes in mechanical properties and color in samples of the composite. Chemical changes of the composite’s materials were measured by X-ray photo- electron spectroscopy to elucidate the mechanisms of chem- ical transformations on the material surface. Relative humid- ity highly affected the modulus of rupture (MOR) and the modulus of elasticity (MOE), and had a greater effect than temperature and UV exposure on performance of the com- posite. The lightness of the composite was increased by the UV effect in the short- and the long-term tests. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the composite was protected by the UV absorber. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2570 –2577, 2006 Key words: mechanical properties; natural-fiber–plastic composite; XPS; UV irradiation; photo- and thermo-oxida- tion INTRODUCTION Durability of natural fiber composites is uncertain and performance is still under investigation. Outdoor du- rability of wood and nonwood–plastic composites might be influenced by thermal stability, moisture uptake, fungal resistance, and ultraviolet (UV) stabil- ity. 1 To overcome the detrimental effects of these agents, the industry enhances properties to prolong the lifespan of the natural fiber composites, using coupling agents, fungicides, and UV absorbers during the manufacturing process. Durability is measured through performance and appearance. When physical and mechanical proper- ties, as well as esthetic properties, deteriorate, the service life of the product shortens. Some of the most common damaging effects in the composites are the following: color change and fading, surface erosion, loss of mechanical properties, and weight loss. Results of recent studies 1–5 show that solar irradiance (UV component of the sunlight), relative humidity, tem- perature, and/or other environmental variables are the causal agents of this deterioration in weathered natural–fiber–plastic composites. Each component of a natural-fiber–plastic composite absorbs UV energy. Free radical photo-oxidation and thermo-oxidation re- actions occur as a result of sunlight and high temper- atures, respectively. This is relevant because the poly- mer chain scissions originating from these reactions lead to deterioration of the mechanical properties in the composites, 3 reducing their durability. Degradation of polymers through photo-oxidation has been studied in different materials. In the pulp and paper industry, yellowing and bleaching of news- print and mechanical pulps has been extensively stud- ied using light at different wavelengths. Lignocellu- losic materials such as wood and paper readily un- dergo light-induced yellowing. 6 Lignin contains numerous chromophores that efficiently absorb UV radiation. 7 As much as 80 –95% of the absorption co- efficient of wood can be ascribed to the lignin frac- tion. 8 Forsskahl and Tylli 9 summarize the results of several studies, showing that the maximum yellowing effect on wood fibers was found at 310 –328 nm 10 and at 310 –320 nm. 11 Results of their study on chemime- chanical pulps confirmed these values. In the case of polyolefins, Xingzhou 12 and Zhenfeng et al. 13 showed that the most effective spectral regions for causing PE and PP photo-oxidation are in the ranges 330 –360 nm and 335–360 nm, respectively. Stark and Matuana 1 determined the effectiveness of various photo-stabilizers and their interactions on the weathering of wood flour filled HDPE composites. Under the conditions of their experiment they found Correspondence to: M. Sain (m.sain@utoronto.ca). Journal of Applied Polymer Science, Vol. 99, 2570 –2577 (2006) © 2005 Wiley Periodicals, Inc.