2710 



Chapter 22 



22-13 CHEMICALLY MODIFIED WOOD 



Treatments to modify properties of wood are summarized in Koch (1972, p. 

 1 128-1 143). Only two of these treatments will be discussed here — wood-plastic 

 composites and impregnation with polyethylene glycol. See also sub-section 

 STABILIZATION TREATMENTS in section 21-5 and figure 21-12. 



WOOD-PLASTIC COMPOSITES 



Vinyl monomers can be used to stabilize wood dimensionally and improve its 

 mechanical properties without impairing desirable aesthetic qualities. Unlike 

 deep-colored phenol-based, theremosetting polymers, the vinyl polymers are 

 clear, colorless, hard thermoplastic materials. Polymerizing the vinyl monomers 

 in the void spaces of the wood does not discolor the wood or alter in any way its 

 eye-appealing nature. The feel of the surface of a wood-vinyl plastic object is 

 like wood — in textile terminology, it has a good hand. Hardness is greatly 

 increased by the treatment. 



Vinyl monomers can be polymerized or cured in the wood by radiation or by 

 heating with free radical catalysts. Beall and Witt (1974) provided data on 

 bending properties of white ash and red oak sp. , impregnated when kiln-dry with 

 methyl methacrylate, and polymerized by radiation. Their specimens measured 

 1- by 1- by 16 inches and were broken in bending after equilibration to 6 percent 

 moisture content. Modulus of rupture, fiber stress at proportional limit, and 

 modulus of elasticity were all increased significantly by the treatment (table 22- 

 29). In spite of the difference in loading (61 percent for white ash, and 3 1 percent 

 for red oak), mechanical properties of treated specimens of the two species did 

 not differ significantly. 



Table 22-29 — Mechanical properties of small static-bending specimens of white ash and 



red oak impregnated with methyl methacrylate and cured, compared with untreated 



controls^ (Beall and Witt 1974) 



Fiber stress 



Modulus at Modulus 



of proportional of 



Species and treatment rupture limit elasticity 



- Psi - - 



White ash 



Untreated control 15,200 6,870 1,410,000 



Treated (61% loading)^ 23,900 11 , 100 2,200,000 



Red oak sp. 



Untreated control 16,000 7,600 1,560,000 



Treated (31% loading)^ 20,400 10,500 2,100,000 



'Specimens were tested at 6-percent moisture content; each value is the average for 16 specimens 

 of clear wood measuring 1 by 1 by 16 inches. 

 ^Ratio of polymer to ovendry wood mass. 



