Modifying the Properties of Wood

Although the use of simple diluents and adulterants almost certainly predates recorded history, the use of fillers to modify the properties of a composition can be traced as far back as eady Roman times, when artisans used ground marble in lime plaster, frescoes, and po22olanic mortar. The use of fillers in paper and paper coatings made its appearance in the mid-nineteenth century. Functional fillers, which introduce new properties into a composition rather than modify pre-existing properties, were commercially developed eady in the twentieth century when Goodrich added carbon black to mbber and Baekeland formulated phenol— formaldehyde plastics with wood dour. 

Considerable efforts have been directed, during the last few years, to grafting various vinyl and acrylic monomers on cellulosic fibers or films in order to modify their properties in a predetermined way. It was shown that one can graft large amounts of vinyl-type monomers, such as acrylonitrile or acrylic esters on rayon, cotton, cellophane, paper, and wood, thus modifying the properties of the base material in many respects, and improving, particularly, dimensional stability, water repellency, and resistance to thermal and chemical degradation. 

Indonesia is a tropical country with about 144 million Ha of tropical forest, producing a large amount of natural polymers, mainly natural rubber and wood products. In 1991 the total production of natural rubber was about 1.36 million tonnes while the production of log was about 40 million. Such large amount and various kinds of natural polymers were subject to investigation by a number of research centers since before the second WW. Center for the Application of Isotopes and Radiation, using radiation technique as a tool to modify the property of various natural polymers in order to meet the industries. Several Cobalt-60 sources and electron beam machine (EBM) from a small scale to a pilot scale are available. Other polymers such as synthetic fibers also becoming a subject of investigation. 

In a study conducted by Yuan et al, argon and air-plasma treatments were used to modify the surface of wood fibers in order to improve the compatibility between the wood fibers and a polypropylene matrix [55]. The improvement in the mechanical properties of the resulting composites, as depicted by SEM, was attributed to an increase in the surface roughness of the wood fibers following plasma treatment. The increase in surface roughness can facilitate better mechanical interlocking, but the increase in... 

Hazarika A, Maji TK (2013a) Study on the properties of wood polymer nanocomposites based on melamine formaldehyde-furfuryl alcohol copolymer and modified clay. J Wood Chem Technol 33 103-124... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

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

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

For work where it is critical to examine the properties of the chemically modified wood with the certainty that there are no by-products or solvents present, it is advisable not to rely solely on an oven-drying stage, or on vacuum-heating. More effective extraction can be achieved with the use of Soxhlet apparatus followed by oven-drying. However, the efficiency of this process is very dependent upon sample size and the scale at which the work is being performed. To ensure total removal of residual chemicals, it is usually a good idea... 

Popper and Bariska (1972) studied the moisture sorption properties of wood chemically modified with acetic (or phthalic) anhydride and analysed the results using Brunauer-Emmett-Teller (BET) theory and the H-H model. Acetylation was found to reduce the number of sorption sites, whereas little effect was noted with phthaloylation. By dividing the sorption isotherm into a monolayer component and a multilayer component using the H-H model, it was shown that there was a large reduction in the... 

It has been reported that thermoplastic properties can be imparted to wood by modification of wood particles with fatty acid chlorides in a dinitrogen tetroxide -dimethylformamide - pyridine mixture (Funakoshi etal., 1979 Shiraishi etal, 1979, 1983). A method has also been developed for the modification of wood sawdust without the addition of organic solvents (Thiebaud and Borredon, 1995), and the thermal properties of such modified wood determined (Thiebaud etal, 1997). 

Wood wafers were treated with mixtures of propylene oxide and oligomeric isocyanate (Guevera and Moslemi, 1983). The best treatment was found to be a mixture of 9 1 propylene oxide to isocyanate. In another study, Guevera and Moslemi (1984) studied the swelling properties of wood modified with propylene or butylene oxide and compared the data with modifications using a furan resin, or vinylpyrrolidinone. The best results were obtained by the use of alkylene oxides in combination with a cross-linking agent (trimethylol propane trimethacrylate). 

Benzylation has been performed on wood in order to impart thermoplastic properties to the substrate (Hon and Ou, 1989). Wood is pre-treated with aqueous NaOH solution, then with benzyl chloride. Benzylation of the surfaces of wood blocks and chips for selfbonding of wood surfaces has also been reported (Kiguchi, 1990a,b Kiguchi and Yamamoto, 1992). A vapour-phase benzylation method has also been developed (Kiguchi, 1993). Carboxymethylation of NaOH-treated wood using various solvent systems has been studied (Shiraishi and Kishi, 1986 Honma and Nakano, 1991). Wood modified in this way has been used to make wood-phenolic adhesives (Kishi and Shiraishi, 1986). 

The properties of thermally modified wood are highly dependent upon the thermal treatment employed, and it is very important to take these into account when comparing the various treatment methods employed. This chapter examines the effect of the process variables upon the properties of thermally modified wood, and then considers the chemistry of thermal modification. Studies of physical changes are discussed, followed by an overview of the biological properties of thermally modified wood. A short description of some recent literature on the use of thermal treatment combined with compression and on hot oil treatments is also included. 

There are a variety of thermal modification methods that can be applied to wood, and the exact method of treatment can have a significant effect upon the properties of the thermally modified wood. Important process variables include the following ... 

The standards that have been developed to date are concerned with determining the properties of conventional wood products. There is now an urgent need to develop appropriate standards and agree them at an international level. At present, claims made with respect to modified wood are based upon the current set of standards, which may not always be appropriate to the performance of these new materials in real-life situations. This is an area that has to be addressed in the very near future. 

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