Wood fiberboard, formaldehyde

The bonding agents used in wood-based fiberboard are usually phenoplasts or aminoplasts. Formaldehyde emissions from wood fiberboard occur when formaldehyde (residual monomer) is still present after the crosslinking reaction, and in hydrolytic processes during use of the wood fiberboards. 

Formaldehyde-free wood fiberboard is bonded with isocyanate MDl. When hydrolytic processes ensue, wood fiberboard with UF is the worst, MF is better and PF is the best, since formaldehyde bonds most strongly with phenol (Novolak). The wood itself emits formaldehyde in small amounts, noticeably at higher temperatures [81]. 

Wood Bonding. This appHcation requires large volumes of phenoHc resins (5—25% by weight) for plywood, particle board, waferboard, and fiberboard. Initially, phenoHc resins were used mainly for exterior appHcations, whereas urea—formaldehyde (UF) was used for interiors. However, the concern over formaldehyde emission has caused the replacement of UF by phenol-formaldehyde adhesives. 

Phenolic Resins. Phenolic resins [9003-354] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-catalyzed one-stage or the acid-catalyzed two-stage process. The liquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, fiberboard, insulation, and cores for laminates. The physical properties for typical phenolic laminates made with wood are listed in Table 1. 

The first demonstration of the industrial importance of heme peroxidases in grafting applications has been the development of hybrid resins from renewable sources to replace phenol-formaldehyde based resins. Phenolic resins are widely used in surface coatings, adhesives, laminates, molding, friction materials, abrasives, flame retardants, carbon membranes, glass fiber laminates, fiberboards, and protein-based wood adhesives, [5]. Table 7.1 and Fig. 7.2 summarize some of the... 

Wood-based panel products are usually bonded with synthetic adhesives based on condensates of phenol, resorcinol, urea, or melamine with formaldehyde. Particleboards and fiberboards can also be bonded with mineral binders like cement or gypsum. Wood adhesives derived from natural products have more... 

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... 

The most widely used wood panel products are particleboard, softwood plywood, hardwood plywood, medium density fiberboard (MDF) and waferboard. The most common adhesive is urea-formaldehyde resin (UFR). Phenol-formaldehyde resins (PFR) are second in volume and melamine-formaldehyde resins (MFR) are a distant third. Recently,... 

If we consider as an example a relative air humidity of 50% and a temperature of 25 l, the wood moisture content would be 9.2 wt% (3. If we further consider that the product manufacturing process leaves about 1 wt% of the formaldehyde content of the UF resin as unreacted formaldehyde, we obtain for particleboard or medium density fiberboard (MDF), where UF-resin makes up 6-10 wt%, an approximate formaldehyde concentration of 0.2 M in the S-2 cell of the wood. 

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

Recently, adhesives that include lignin in their formulation have shown several properties similar to phenol-formaldehyde resins [11, 12] and are extensively utilized in the plywood, particleboard, fiberboard and laminated wood industries. The chemistry of the lignin-based adhesives has been studied from an empirical point of view [13] however, the molecular mechanism and the interactions of the structural cross-linked 4-hydroxyphenylpropanoids of lignin that drive the adhesion phenomenon remain unclear. 

These adhesives are suitable for exterior use and are unaffected by water (even boiling water), molds, grease, oil, and most solvents. Their appUca-tions primarily include wood, plywood, plastics, paper, and fiberboard. Resorcinol-formaldehydes are excellent marine-plywood adhesives. Curing at room temperature normally takes 8-12 hours, while phenoUc wood adhesives require a high-temperature cure. The adhesives are also used for indoor applications because of their high reliability. ... 

Its principal use, however, is as an adhesive for wood. In this application, resorcinol-formaldehyde has the unique property of setting at room temperature. The bond becomes completely waterproof in 24 h, and in another 2-4 days the bond is stronger than wood itself. Resorcinol resins also find application in bonding cellulose acetate, molded urea plastics, nylon, and various plastic laminates to wood core. They also bond to porous materials such as paper, textiles, leather, and fiberboard. 

Tricresyl phosphate, 2% pet (plasticizer in adhesives) Triethylenetetramine, 0.5% pet (epoxy catalyst) Turpentine, 10% olive oil (furniture polishes) Urea-formaldehyde resin, 10% pet (fiberboard glue) Various wood dusts, 10% pet... 

Even with only the three monomers melamine, formaldehyde, and urea, a variety of different types of resins exists which can fulfill nearly all requirements given in the wood-based panels industry. This field of application involves the production of wood-based panels like particleboards, medium density fiberboard (MDF), oriented strand board (OSB), plywood, blockboards, and others. In rare cases the resins and panels are also used in the fnrnitnre industry. According to the raw materials used, various types of melamine resins are possible ... 

Applications. UFs are the primary commodity binder in caul and caulless particleboard processes and medium density fiberboard (MDF). They can be formulated to develop tack and generally are used as a neat liquid. Acid catalysts are frequently used unless the wood species is acidic such as southern pine or oak. Products manufactured with UFs are rated for interior application. Use of MUFs or MFs increases bond durability and imparts water resistance to the bond, speeds cure rate, and reduces potential formaldehyde release. 

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