Decay resistance

Extractives and Ash. The amount of extractives in wood varies from 5 to 20% by weight and includes a wide variety of organic chemicals (11). Many of these function as intermediates in tree metaboUsm as energy reserves or participate in the tree s defense mechanism against microbiological attack. The extractives contribute to wood properties such as color, odor, and decay resistance. 

Eor practical purposes, the sapwood of all species may be considered to be susceptible to biodeterioration. The heartwood of some species, however, contains toxic extractives that protect it against biological attack. Among the native species that have decay-resistant or highly decay-resistant heartwood are bald cypress, redwood, cedars, white oak, black locust, and black walnut (60). Douglas-fir, several of the pines, the larches, and honey locust... 

Comparative Decay Resistance of Heartwood of Native Species, Research Note PPL 0153, U.S. Department of Agriculture, Eorest Service, Eorest Products Laboratory, Madison, Wise., 1967. 

Sodium borate solutions near the Na20 B202 ratio of maximum solubihty can be spray-dried to form an amorphous product with the approximate composition Na20 4B202 4H20 commonly referred to as sodium octaborate (64). This material dissolves rapidly in water without any decrease in temperature to form supersaturated solutions. Such solutions have found apphcation in treating ceUulosic materials to impart fire-retardant and decay-resistant properties (see Cellulose). 

Wood modification is a means of altering the material to overcome or ameliorate one or more of its disadvantages. The aim of a modification may be to bring about an improvement in decay resistance or dimensional stability, to reduce water sorption, to improve weathering performance and so on. The term wood modification applies to the application of a process that alters the properties of the material such that during the lifetime of a product no loss of the enhanced performance of the wood should occur. 

Blocking of the cell wall micropores may be a mechanism in explaining some of the properties of modified wood (e.g. decay resistance). 

It is worth considering the measurement of the ESP in some detail, since many wood modification techniques result in a reduction of EMC at a given RH and there have been smdies in which attempts have been made to correlate the ESP with other properties, such as decay resistance. 

Testing of decay resistance can be performed in a laboratory environment or in outdoor field trials, and there are many standards defined for these tests. The first objective of a laboratory-based test is to provide a methodology for the rapid screening of a candidate wood preservative, treatment or modification in order to assess which ones exhibit decay resistance. Broadly speaking, laboratory-based tests can be divided into sterile (pure culture) tests and unsterile tests (such as fungal cellar, soil burial etc.). 

Goldstein etal. (1961) exposed acetylated ponderosa pine for 12 weeks to six basidio-mycete fungi, according to ASTM D1413-56T, with a WPG of 18 % reported to be sufficient to provide decay resistance. Peterson and Thomas (1978) acetylated loblolly pine (Pinus taeda), green ash (Fraxinus americana) and yellow poplar Liriodendron tulipfera), also... 

Table 3.1 Table summarizing experiments on decay resistance of acetylated wood exposed to brown rot fungi... 

Beckers etal. (1994) examined the decay resistance of acetylated beech and poplar against C. versicolor, finding that the decay protection threshold was around 12%, whereas acetylated pine required a WPG of 20 % to prevent attack by P. placenta. Attack by C. puteana and G. trabeum was prevented at about 17 % WPG, for acetylated beech and poplar, with pine requiring a slightly higher WPG of 20 %. 

In a round robin test conducted as a work package within the European Thematic Network for Wood Modification, considerable differences in decay resistance were found... 

Loblolly pine modified by 1,6-diisocyanatohexane (HDI) was found to be resistant to attack by G. trabeum at a WPG of 26 % (Chen, 1992c). At 26 % WPG, 6 % of bonded chemical was lost during a 12-week soil decay test. When moist wood was used for reaction, the HDI reacted mainly to form ureas and biurets. It was stated that the decay resistance of HDI modified wood was probably due to the inability of the modified cell wall to absorb sufficient amounts of water to support decay. Although wood reacted with chloro-sulphonyl isocyanate lost only 1.3 % mass when exposed to G. trabeum in a decay test, it was reported that 50 % of the bonded chemical was lost in this test. 

Forster etal. (1997) found that butyl isocyanate modification was more effective in enhancing decay resistance compared to the anhydrides studied in the same experiment. Decay protection thresholds were of the order of 15-20 % WPG, with the fungi studied. 

Cardias Williams and Hale (1999, 2003) studied the effect of the carbon chain length of the isocyanate upon decay resistance of the modified wood. Wood was reacted with butyl or hexyl isocyanate, or HDI in a pyridine/acetone solvent mix. Modified wood was... 

The resistance of propylene and butylene epoxide modified wood to subterranean termites was studied (Rowell etal., 1979). A threshold of 34 % WPG was found to give good protection. Giant ipil ipil (Leucaena leucocephala) was modified with acetic anhydride, maleic anhydride - glycerol or propylene oxide (Mallari etal., 1988, 1990). All modified samples showed good decay resistance to C. versicolor and Tyromyces palustris. 

Ibach and Rowell (2000) reacted wood with propylene oxide and butylene oxide, and determined the decay resistance of the modified wood. Propylene oxide modification was found to be ineffective in preventing decay by G. trabeum, whereas butylene oxide modification proved to be effective at 23 % WPG. Reaction of epicholorohydrin with wood was found to provide decay protection at 31 % WPG against G. trabeum, although such modification did not lower the EMC significantly (Ibach and Lee, 2002). It was concluded that decay protection was due to substrate modification. 

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