Phenol formaldehyde impregnated wood

With the advent of these compounds in the 1960s, the hitherto more conventional insulating materials, such as phenol formaldehyde (popularly known as Bakelite) and wood (veneered impregnated) have been almost replaced by them. These compounds offer better electromechanical properties than conventional materials. Below we describe the basic mix and properties of these two basic compounds, for a brief reference. 

Phenolic phenol formaldehydes (PFs) are the low-cost workhorse of the electrical industry (particularly in the past) low creep, excellent dimensional stability, good chemical resistance, good weatherability. Molded black or brown opaque handles for cookware are familiar applications. Also used as a caramel colored impregnating plastics for wood or cloth laminates, and (with reinforcement) for brake linings and many under-the-hood automotive electricals. There are different grades of phenolics that range from very low cost (with low performances) to high cost (with superior performances). The first of the thermosets to be injection-molded (1909). 

Burr, H. K., and Stamm, A. J., "Comparison of Commercial Water-Soluble Phenol-Formaldehyde Resinoids for Wood Impregnation", U. S. For. Prod. Lab.Mimeo T384 (1943). 

Figure 2. SEM view of wood fibers impregnated with brominated phenol-formaldehyde resin. The line trace indicates bromine concentration in the wood...

The natural product comprises veneers of real wood which have been sorted and joined edge-to-edge (for example, by stitching), and bonded under heat and pressure to layers of kraft paper impregnated with phenol-formaldehyde resin. A barrier layer immediately below the veneer is impregnated with melamine-formaldehyde resin and prevents upward migration of the darker phenolic resin. 

Figure 2 Creep-recovery tests of chemically treated woods. U, untreated wood Fs, vapor phase formalization F, liquid phase formalization A, acetylation PO, etherification with propylene oxide MG, treatment with maleic acid and glycerol PFl, impregnation with low molecular weight phenol-formaldehyde resin PEG-ICP, impregnation with polyethylene glycol (PEG-IOOO) WPC, formation of a wood- polymer composite (PMMA) WIC, formation of a wood-inorganic material composite.

The impregnation of PF (phenol-formaldehyde) resin into wood was studied in the early part of the twentieth century. PF resin wood composites have been commercialized as Impreg and Compreg for their high mechanical strength and dimensional stability. Although the enhancement of decay resistance by PF resin treatment was also found earlier [1], it has not been applied to wood preservation techniques for the prevention of biodeterioration. 

Figure 7. Formaldehyde liberation from particleboards and CH20-sorbed wood at 27°C and 33 percent relative humidity (RH) weighing bottle test with -80 mesh materials (o Southern pine impregnated with pH 2 tartaric acid and vapor-equilibrated with CH20/salt solution at pet RH O as before except heated 4 min. 16O°C after CH2O sorption 0 urea-formaldehyde particle board (b) phenol-formaldehyde particleboard, values approximate P = Perforator value at indicated moisture content (MC)).

Reduction of furfural leads to furfuryl alcohol 63 which can be easily transformed into resins, thus replacing phenol formaldehyde resins (Scheme 21) [115]. A recent apphcation concerns impregnation of wood (furfurylated wood). For instance, high retention of grafted or polymerized furfuryl alcohol improves a variety of properties such as an increased hardness, resistance to microbial decay and insect attack, as well as resistance to chemical degradaticai. Modulus of rupture (MOR), elasticity (MOE), and the dimensional stabihty are also significantly increased. 

Water-soluble phenol-formaldehyde solutions can also be impregnated into degraded wood and polymerized (14), Although this resin system greatly improves strength and integrity, the color of the treated wood is always reddish-brown. This color may not be a problem for darker wood, but it is a serious drawback for many potential applications. 

A simpler approach for depositing water insoluble chemicals within the cell walls of wood is to impregnate the wood with solvent soluble resin forming chemicals containing a catalyst that penetrate the cell walls followed by evaporation of the solvent and then heating to polymerize the resin. This has been accomplished with the following water soluble resin forming systems phenol, resorcinol, melamine and urea-formaldehydes, phenol-furfural, furfuryl-aniline and furfuryl alcohol (44K The most successful of these has been phenol-formaldehyde Us),... 

The first applications of adhesives for bonding aircraft structure probably date back to about 1920, when light-weight airframes were fabricated with plasticized nitrocellulose-impregnated fabric stretched over a framework of wooden stringers. The wood adhesives used for these applications were first based on casein, then urea formaldehyde resins, and finally phenol formaldehyde resins. 

Some reactive adhesives are applied with the aid of a volatile diluent. When a phenol-formaldehyde resin is used to impregnate and glue together the layers of wood that make up plywood, water is evaporated from the system. Alkyd resins and oleoresinons varnishes with a volatile diluent can be applied to a surface that is glued to another snrface after the diluent is evaporated. 

Phenolic and other formaldehyde condensation polymers are also important reactive adhesives. Powdered phenolic resin is mixed with abrasive grains and the mixture is compression molded to form grinding wheels. A B-stage phenolic (Chapter XX) in a solvent is used to impregnate tissue paper. The solvent is evaporated, and the dry sheets are placed between layers of wood in a heated press, where the resin first melts and then cures, bonding the wood to form plywood. Similarly, sheets of paper impregnated with a B-stage melamine-formaldehyde resin are laminated and cured to form the familiar Formica counter tops. 

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