Mechanical properties woods

Caro s acid is effective ia delignifying wood pulp (qv) made by chlorine-free bleaching sequences. When conditions are carefully controlled, the mechanical properties of the final paper (qv) are not impaired. These processes were developed ia the 1980s and commercialized ia the 1990s (68). 

Many mechanical properties of wood, such as bending and cmshing strength and hardness, depend upon the density of wood denser woods are generally stronger (6). Wood density is determined largely by the relative thickness of the cell wall and by the proportions of thick-walled and thin-walled cells present. 

The mechanical properties of wood tend to increase when it is cooled and to decrease when it is heated (6,18). If untreated wood heated in air is not exposed to temperatures of more than - 70° C for more than about 1 year, the decrease in properties with increasing temperature is referred to as immediate or reversible ie, the property would be lower if tested at the higher temperature but would be unchanged if heated and then tested at room temperature. The immediate effect of temperature on strength and modulus of elasticity of clear wood, based on several different loading modes, is illustrated in Figures 4—6 (6). 

Higher temperatures result in permanent degradation. The amount of this irreversible loss in mechanical properties depends upon moisture content, heating medium, temperature, exposure period, and, to some extent, species. The effects of these factors on modulus of mpture, modulus of elasticity, and work to maximum load are illustrated in Figures 6—9 (6). The effects may be less severe for commercial lumber than for clear wood heated in air (Fig. 10). The permanent property losses shown are based on tests conducted after specimens were cooled to - 24° C and conditioned to a moisture content of... 

Wood preservatives ate appHed either from an oil system, such as creosote, petroleum solutions of pentachlorophenol, or copper naphthanate, or a water system. Oil treatments ate relatively inert with wood material, and thus, have Htde effect on mechanical properties. However, most oil treatments require simultaneous thermal treatments, which ate specifically limited in treating standards to preclude strength losses (24). 

J. E. Wiaandy, "Effects of Waterborne Preservative Treatment on Mechanical Properties A Review," ia Proceedings of the 91 st Annual Meeting of the American Wood-Preservers Association, New York, May 21—24, 1995, Vol. 91, AWPA, Woodstock, Md., 1995, pp. 17—33. 

E. C. O. Erikson, Mechanical Properties ofUaminated Modified Wood, Report 1639, U.S. Department of Agriculture, Eorest Service, Eorest Products Laboratory, Madison, Wise., 1965. 

Wood chipboard is free from grain and is thus essentially isotropic in its behaviour. The mechanical properties are approximately the same as the average of the properties of the original wood measured along and across the grain. The water resistance of chipboard is poor but, being isotropic, it does not warp as long as it is able to swell freely in all directions. 

Modifications of the wood surface can be performed by various physical, mechanical and chemical treatments. Chemical treatments especially are performed in order to enhance the dimensional stability, but also for amelioration of physical and mechanical properties or a higher resistance against physical, chemical and biological degradation. 

The improved physical and mechanical properties of the wood-plastic composites lead to a diversity of applications, e.g., automotive parts, furniture, construction (e.g., building panel, flooring veneers), toys, cutlery handles, industrial pattern, sports equipment, musical... 

The surface energy of fibers is closely related to the hydrophilicity of the fiber [38]. Some investigations are concerned with methods to decrease hydrophilicity. The modification, of wood cellulose fibers with stearic acid [43] hydrophobizes those fibers and improves their dispersion in polypropylene. As can be observed in jute-reinforced unsaturated polyester resin composites, treatment with polyvinylacetate increases the mechanical properties [24] and moisture repellency. 

The mechanical properties of composites reinforced with wood fibers and PVC or PS as resin can be improved by an isocyanate treatment of those cellulose fibers [41,50] or the polymer matrix [50]. Polymethylene-polyphenyl-isocianate (PMPPIC) in pure state or solution in plasticizer can be used. PMPPIC is chemically linked to the cellulose matrix through strong covalent bonds (Fig. 8). 

Analog-to-glass fibers silanes are used as coupling agents for natural fiber polymer composites. For example, the treatment of wood fibers with product A-175 improves wood dimensional stability [53]. In contrast, a decrease of mechanical properties was observed for coir-UP composites after a fiber modification with di-chloromethylvinyl silane [54]. The treatment of mercer-... 

The mechanical properties of plastics enable them to perform in a wide variety of end uses and environments, often at lower cost than other design materials such as metal or wood. This section reviews the static property tests. Chapter 5 provides more information on the meaning of these type data. 

Wood is anisotropic with distinct different properties in three directions. Its highest mechanical properties are in the growth (fiber) direction, with the perpendicular (or second plane) direction having lower properties and the other perpendicular (or third plane) direction having much lower properties. 

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