Wood fiber surface properties

J.L. Pehanich, P.R. Blankenhor and M.R. Silsbee, Wood fiber surface treatment level effects on selected mechanical properties of wood fiber-cement composites , Cem. Conor. Res. 34, 2004, 59-65. 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

All of the chemical evidence that can be marshalled indicates that wood fiberboard manufacture exploits the thermoplastic properties of lignin. Defibering is effected by the thermal softening of lignin in the middle lamella at saturated steam pressures above 130C. Interfelted fiber mats are consolidated with or without densification pressure by the thermoplastic fusion of lignin-rich fiber surfaces at high board conversion temperatures. 

While paper can be made of wood fibers alone, little is actually made without some chemical addition or modification. These chemical additives are used to either assist in papermaking or to give the paper certain desirable end-use qualities. These chemicals can be added at virtually any step in papermaking. Some of the additives are used to influence the entire sheet properties. These chemicals are added to the pulp slurry prior to sheet formation (internal addition). When the surface properties of the sheet also need to be altered, additives are used on the sheet after some period of formation or drying (external addition). A number of these chemicals serve commonly as both internal or external additions. 

There have been several studies on the changes in mechanical properties of fiberboards made from chemically modified wood fiber. Hardboards made from control and acetylated hemlock fiber using 7% phenyl-formaldehyde adhesive were tested. In static bending, MOR was reduced by 23% and MOE reduced by 16% in acetylated boards as compared to control boards [38]. Tensile strength parallel to the surface was reduced by 5% but there was no... 

Liao B, Huang Y, Cong G (1997) Influence of modified wood fibers on the mechanical properties of wood fiber-reinforced polyethylene. J Appl Polym Sci 66 1561-1568 Mani P, Satyanarayan KG (1990) Effects of the surface treatments of lignocellulosic fibers on their debonding stress. J Adhes Sci Technol 4 17-24... 

Singh B, Gupta M, Verma A (1996) Influence of fiber surface treatment on the properties of sisal fiber reinforced polyester composites. Polym Compos 17 910-918 Schirp A, Wolcott M (2005) Influence of fungal decay and moisture absorption on mechanical properties of extruded wood-plastic composites. Wood Fiber Sci 37 643-652 Winfield AG (1979) Jute reinforced polyester project for UNIDO/Govt, of India Blast Rubber Int 4 23-28... 

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... 

Blends of ethylene copolymers (such as ethylene-vinyl acetate, ethylene-ethyl acrylate, ethylene-acrylic acid copolymers) have been typically added to LDPE, HDPE and LLDPE to improve filler and additive acceptance, balance film properties, improve environmental stress crack resistance, tear resistance, toughness and surface properties. These blends are particularly prevalent in film formulations and as such are rarely disclosed by the manufacturer. Equistar has introduced functionalized polyolefins (Integrate ) to improve dispersion and adhesion in wood fiber filled and mineral filled polyolefin composites. Arkema Group offers Lotryl ethylene-acrylate (methyl, butyl and 2-ethyl hexyl) copolymers and notes the wide range of compatibility with other polyolefins as well as polyamides and polyesters. 

In the case of fiber-reinforced composites, even where the filler consists of wood fibers and other ligno-cellulosic or proteic fibers, the main issue is the interfacial adhesion between matrix and fibers, as a sine qua non requirement for the load transfer, as the fibers carry the load, while the matrix distributes and transfers it from fiber to fiber. Hence, the relevance of the interphase that may be formed spontaneously, no matter the fibers are raw or modified by a surface treatment, and which strongly affects the properties of the composite through its thickness, structure and properties [3]. 

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