R. Sam Williams

Violet light causes photodegradation of wood beyond the zone affected by ultraviolet radiation

Abstract The limited penetration of wood by light explains why the weathering of wood exposed outdoors is a surface phenomenon. Wood is rapidly degraded by short-wavelength UV radiation, but the penetration of light into wood is positively correlated with its wavelength. Hence, sub-surface degradation is likely to be caused by longer-wavelength light that still has sufficient energy to degrade wood. In this paper we test this hypothesis and determine the wavelengths of visible light that extend photodegradation into wood beyond the zone affected by UV radiation. Sugi (Cryptomeria japonica) earlywood was exposed to UV and visible light with narrow band gaps (20 nm) and the penetration of light into the wood was measured using a photodetector. Photodegradation was depth-profiled using FT-IR microscopy. There was a positive correlation between the penetration of light into sugi earlywood and the wavelength of the incident radiation within the range 246–496 nm. The depth of photodegradation also increased with wavelength up to and including the violet region (403 nm) of the visible spectrum. Blue light (434–496 nm) penetrated wood to a greater extent than violet light and was capable of bleaching the wood, but it did not significantly modify lignin, and hence it was not responsible for sub-surface photodegradation of wood. We conclude that violet light is the component of the visible spectrum that extends photodegradation into wood beyond the zone affected by UV radiation. Accordingly, surface treatments designed to protect wood used outdoors should shield wood from the effects of violet light.

Wood properties affecting finish service life

Wood is a biological material that has widely different properties depending on species, geographic area where the tree grew, the growth conditions, size of the tree at harvest, sawing, and other manufacturing processes. Some of the more important wood properties as they relate to wood finishing are discussed, e.g., growth rate, density, knots, extractives, juvenile wood, grain orientation, and weathering characteristics.

Duration of wood preweathering : Effect on the service life of subsequently applied paint

Previous studies of the effect of preweathering of wood (weathering of wood prior to painting) on subsequent paint performance have not linked short periods of preweathering (weeks) to paint service life. To examine the link between preweathering and paint service life, we analyzed paint performance (cracking and flaking) after 14 years outside on boards that were preweathered for various amounts of time. We then compared our results with previous results from paint adhesion tests of similar boards that were also preweathered for the same amount of time. There was a direct correlation between the amount of time the siding was preweathered and the long-term performance of paint. Paint on wood preweathered for 16 weeks began to fail after just three years. Paint on wood preweathered for shorter periods lasted longer, but even those boards that were preweathered for as little as one week showed paint failure earlier than boards that were not preweathered. There was also good correlation between paint adhesive strength results from the previous study and outdoor paint performance results from this study, showing that loss of paint adhesion may be linked to paint performance in outdoor field tests.

Evaluation of several finishes on severely weathered wood

Alkyd-, oil-modified-latex-, and latex-based finishes were applied to severely weathered western redcedar and redwood boards that did not have any surface treatment to ameliorate the weathered surface prior to painting. Six finishes were evaluated annually for 11 years for cracking, flaking, erosion, mildew growth, discoloration, and general appearance. Lowsolids-content latex finishes that contained about 10% raw linseed oil and 11% acrylic resin (i.e., the oil-modified latex finishes) performed better on badly weathered wood than did the alkyd and the other latex finish, even after 11 years. Latex finishes that contained raw linseed oil probably stabilized the weathered surface and plasticized the finish. The stabilization of the wood surface and the flexibility of the finish throughout its service life are the important factors in finish performance on these weathered substrates.

Durability of Building Joint Sealants

Predicting the service life of building joint sealants exposed to service environments in less than real time has been a need of the sealant community for many decades. Despite extensive research efforts to design laboratory accelerated tests to duplicate the failure modes occurring in field exposures, little success has been achieved using conventional durability methodologies. In response to this urgent need, we have designed a laboratory-based test methodology that used a systematic approach to study, both independently and in combination, the major environmental factors that cause aging in building joint sealants. Changes in modulus, stiffness, and stress relaxation behavior were assessed. Field exposure was conducted in Gaithersburg, MD, using a thermally-driven exposure device with capabilities for monitoring changes in the sealant load and displacement. The results of both field and laboratory exposures are presented and discussed.

Evaluating Cyclic Fatigue of Sealants During Outdoor Testing

A computer-controlled test apparatus (CCTA) and other instrumentation for subjecting sealant specimens to cyclic fatigue during outdoor exposure was developed. The CCTA enables us to use weather-induced conditions to cyclic fatigue specimens and to conduct controlled tests in-situ during the outdoor exposure. Thermally induced dimensional changes of an aluminum bar were fed to a computer that enhanced the movement, set limits on the movement, and supplied movement information to the test apparatus. As specimens moved, load/deformation and weather conditions (temperature, relative humidity, UV radiation, precipitation, and wind velocity) were measured every few minutes to give an extensive database containing these variables. In addition, controlled tests were done every four hours during the exposure. At these four hour intervals, the computer was programmed to interrupt weather-induced cyclic fatigue and conduct a standard-strain test. The data enabled us to calculate the elastic modulus of each specimen at any time during the exposure and to construct a model that fit the observed temperature and relative humidity effects on modulus.

Installation, care, and maintenance of wood shake and shingle siding

This article gives general guidelines for selection, installation, finishing, and maintenance of wood shake and shingle roofs. The authors have gathered information from a variety of sources: research publications on wood finishing, technical data sheets from paint manufacturers, installation instructions for shake and shingle roofs, and interviews with experts having decades of experience in constructing and inspecting shake and shingle roofs. Where possible, recommendations are based on research results; however, some information is determined from practical experience installing shake and shingle roofs. More detailed information is available from shake and shingle suppliers and the Cedar Shake and Shingle Bureau (CSSB). Note: Installation instructions contained herein are not intended to supercede local building codes.

Restoration of severely weathered wood

Severely weathered window units were used to test various restoration methods and pretreatments. Sanded and unsanded units were pretreated with a consolidant or water repellent preservative, finished with an oil- or latex-based paint system, and exposed outdoors near Madison, WI, for five years. Pretreatments were applied to both window sashes (stiles and rails) and sills. In most cases, pretreatment with consolidants was detrimental to the finish. These pretreatments generally caused more flaking and cracking of the paint compared with that of untreated controls or penetrating water-repellent preservatives. The best results were obtained by a combination of sanding and pretreatment with a water-repellent preservative containing copper naphthenate or with tung oil.