The objective is to develop an economically, technically and environmentally viable process which will provide long-term protection against wood weathering without masking the natural colour and texture of wood. Progress is described.
Wood is very susceptible to weathering when used outdoors because of its inherent nature of UV degradability and dimensional instability. Without effective protection, wood rapidly loses its attractive natural appearance and becomes unserviceable within a few years. Protection has traditionally taken the form of system design to avoid trapping water, and surface finishing to shed water and prevent UV degradation. Unfortunately, surface finishes such as paint and stain last only a few years and the need to repeatedly refinish wood has led to the perception of wood products as high maintenance building materials. The objective of this project is to develop fundamental information leading to processes which can be used by our industry to produce high- performance wood products with extended service lives and low maintenance requirements. The project has been focusing on UV resistance to the present stage. Wood samples were treated with chemicals that have UV stabilizing properties and their weathering performance evaluated by both accelerated and natural weathering tests. UV-visible spectroscopy was used to monitor the color change, and FTIR (Fourier Transform Infrared spectroscopy) was used to monitor lignin degradation on the wood surface. Rate of erosion, water absorption, and dimensional change during accelerated weathering was also studied. Preliminary weathering results are discussed in this report and recommendations made for further research.
In the built environment wood surfaces often experience different exposure conditions, particularly exposure to sunlight. This can result in part of a surface receiving full sun, while other parts may be completely shaded. Methods that minimize the resulting colour differences between exposed and shaded areas would enhance the aesthetic appeal of wood. In the present work a “pre-weathering” approach to wood colour stabilization was investigated. After two months of natural exposure, colour change was similar regardless of pre-exposure. The heartwood samples in a shade exposure were much more discoloured than the equivalent sapwood samples. This suggests that heartwood extractive oxidation is a major cause of initial heartwood discolouration. The samples exposed to the sun did discolour more, indicating a UV/Visible light effect as well.
This second status report describes the effects of 1 year exterior weathering at the Ottawa site and three accelerated aging tests on thickness, bending properties, internal bond strength, surface hardness, and nail withdrawal resistance of small specimens of two commercial waferboards. Following 1 year exterior weathering, all but two properties of random short wafer waferboards were adversely affected to a greater extent than aligned long wafer specimens. Thickness change and internal bond strength retention were more or less equal for the two waferboards. Relative to 1 year exterior weathering, accelerated aging was more severe in all cases except modulus of rupture following the AFNOR V 313 test and modulus of elasticity following both the AFNOR V 313 test and the ASTM 6-cycle test. Of the three accelerated aging tests studied, the AFNOR V 313 test was the least severe followed by the ASTM 6-cycle test and the CSA 2-hour boil, the most severe test. A second outdoor test site was set up in Saskatoon, Saskatchewan and similar waferboards were mounted for exposure.
Weathering stains turn wood a grey colour to mimic weathered wood, and thereby minimize colour change after installation. A field test was initiated to evaluate the performance of several commercial weathering stains for their ability to reduce colour change after field exposure, relative to uncoated wood. Samples should be inspected after one year of exposure.
