Bulk wood, strength

Figure 1. Chain-link diagram of well-formed (proper) wood-glue joint. The materials and conditions that were used have led to a joint with good bulk wood strength, a strong interphase with good adhesion, and a strong glue film. (Reproduced with permission from ref. 1. Copyright 1950 American Furniture Manufacturers Association.)...

JL HE POLYMERIZATION OF VINYL MONOMERS in the void spaces of bulk wood results in wood—polymer composites of increased strength properties and dimensional stability see Chapter 6). Because the different environmental conditions expose in-service timber to attack by numerous wood-deteriorating microorganisms, it is desirable to enhance the biodegradation resistance of wood, with simultaneous improvements in mechanical behavior. This chapter summarizes the formation of bioactive wood-polymer composites (1-4). The basic approach is still in situ polymerization of vinyl monomers in wood, with the appropriate choice of a bioactive, toxic, functional group incorporated in the monomer, and with other modifications based on wood-polymer reactions. 

Strength of the bulk wood will depend, first, on the wood s density, grain pattern, grain direction, and moisture content. Second, it will depend on any damage or deterioration the wood may have suffered. Deterioration will normally be quite significant with archaeological wood, and considerably more attention will be given to it later. 

The first reported study of a reaction of wood with an epoxide appears to be that of McMillan (1963). This involved the use of gaseous ethylene oxide (Figure 4.9, R=H) at a temperature of 93 °C and a pressure of 3 atmospheres (0.3 MPa). In some cases, the wood was diffusion pre-treated with trimethylamine vapour as the catalyst. A 65 % ASE at 20 % WPG was obtained, attributed to a bulking effect due to in situ polymerization of the epoxide. There was no effect on the static bending strength of samples, and the modified wood became distinctly brown at higher levels of treatment. 

The strength properties of polyethylene glycol treated wood are virtually those of the swollen wood. This is not surprising as the polymer tends to maintain green wood dimensions. Unlike heat stabilized and formaldehyde cross-linked wood and wood bulked by resin forming polymers within the cell walls (to be considered later), the toughness of the wood is not adversely affected by polyethylene glycol treatment (35). 

Keywords artificial woods, bulk specific gravity, carbon fibers, FRP powder, hardness, nail withdrawal, strengths... 

The flexural strength of artificial woods is greatly improved by the addidon of carbon fibers. The flexural deformation behavior of the artificial woods is markedly improved by using carbon fibers, HPMC and shirasu balloon. The bulk specific gravity, hardness and compressive strength of the artificial woods decrease with increasing carbon fiber content, HPMC content and shirasu balloon content. 

The mix proportions of artificial woods which can be nailed are recommended in 4. From an economical viewpoint, the carbon fiber content in the mix proportions can be reduced to 2 vol%. The optimum mix proportions with a carbon fiber content of 2 vol% is given in Table 7. The bulk specific gravity, flexural and compressive strengths of an artificial wood with the optimum mix proportions are 0.9, 12.0MPa and 19.0MPa respectively. The artificial wood also has good wood-processability like natural wood. 

The bulk specific gravity of an artificial wood with the above optimum mix proportions is larger than that of calcium silicate-based wood-like materials, and their flexural and compressive strength are higher than those of the calcium silicate-based materials. 

In the end, all that is left of the former wood is the frail system of middle lamellae containing small amounts of still-crystalline cellulose. Disintegrating lignin residues are enclosed in this cellular and brittle wood skeleton. As long as it stays waterlogged—that is, bulked with and supported by water—this structure has a certain physical strength. It still carries the anatomical characteristics of the wood it once was. 

Wood that has survived archaeological time and conditions will undoubtedly have gluing properties that are, in varying degrees, different from those of fresh unmodified wood. Bulk strength may have been reduced and fluid permeability and absorbency increased. Surfaces, in particular, may have strength and wettability that have been altered by aging. Treatment with consolidants and dimensional stabilizers may have been necessary, and this will often leave surface residues and reduce permeability and absorbency. 

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