Review of the weathering and photostability of modified wood

Abstract

Chemical modification of the molecular structure of polymers is a fundamental approach to improving the resistance of materials to photodegradation, but it is seldom employed because other techniques, such as the use of additives (ultraviolet absorbers and antioxidants) can provide adequate levels of protection. Wood is an exception to this rule because the additives that can protect polymers from photodegradation are less effective at photostabilizing wood. Conversely, chemical modification of wood surfaces with chromic acid is remarkably effective at photostabilizing wood, and this led to significant interest in chemical modification as a way of protecting wood from photodegradation. Early research focused on understanding why chemical modification with chromic acid was so effective at photostabilizing wood, and significant effort was directed towards finding effective alternatives to chromic acid. This search was largely unsuccessful and hence attention then shifted to the development of alternative methods of chemically modifying wood to improve its photostability. Research was also performed to examine the extent to which chemical and other modification systems designed to improve dimensional stability and decay resistance could protect wood from weathering. This paper reviews this research and describes the progress made to date to develop modification systems that can block the photodegradation of wood.

Keywords:

• Chromic acid
• esterification
• grafting
• photostability
• resin treatments
• thermal modification
• weathering

Acknowledgements

I would like to thank the following scientists who have worked or corresponded with me over the last three decades and shared their valuable insights into the weathering and photostability of wood (modified and unmodified): Bart Banks (UCNW, Bangor); Roy Miller, Hilary Derbyshire, Janice Carey, Ed Suttie and John Boxall (BRE, Watford); Karl Schmalzl, Tony Michell, Craig Forsyth and Adrian Wallis (CSIRO Forestry & Forest Products, Melbourne); Noel Owen (Brigham Young University, Provo); Peter Beutel, Phan Thay, Zhilin Qi, Roger Heady, John Broomhead, Fabiano Ximenes, Shakri Seman, Richard Webster and Siggi Schmid (ANU, Canberra); Peter Klemchuk, John Bolton and Daniel Rogez (CIBA-GEIGY); Makoto Kiguchi, Yutaka Kataoka and Hiroshi Matsunaga (FFPRI, Tsukuba); Bill Feist, Sam Williams and Roger Rowell (US FPL, Madison); Sigurd Heimdal and Beate Bendixen (Jotun, Sandefjord); Alan Preston (Viance, Charlotte); Carsten Mai, Yanjun Xie, Steffen Donath, Andreas Krause and Holger Militz (University of Göttingen); Gilles Sèbe, Mohamed Jebrane, Stephane Grelier and Alain Castellan (University of Bordeaux); Martina Meincken and Tim Rypstra (University of Stellenbosch); Krishna Pandey (Institute of Wood Science and Technology, Bangalore); Emma Ostmark, Ylva Karrfelt, Mats Westin and Magnus Willander (SP Stockholm); Paul Morris (FP-Innovations, Vancouver); and Mohammed Chowdhury, Kathrin Urban, Ricky Ratu, Johannes Weizenegger and Ian Cullis (UBC, Vancouver).