Wood composites -adhesives

There have been many attempts to replace these resins with lignin derivatives for wood composite adhesives suitable for plywood, particleboard and waferboard. Most of these studies have been empirical in nature, and few have achieved further consideration for industrial application. As wood binders, technical lignins are variable in quality and poorly reactive in comparison to conventional resin systems such as phenol-formaldehyde (PF) resins. Consequently, they are not utilized on their own. Indeed, if they were, this would adversely affect production quality and times, and necessitate equipment changes. In the wood composite industry, resins having such deleterious effects are not likely to be used even if savings could be made in terms of material costs. 

Steam-explosion lignins have received some attention as adhesives mainly because this process offers some potential for utilizing low-quality hardwoods. Like kraft lignin, these preparations also require activation [e.g., by hydroxymethy-lation (70), isocyanation (75,80)] to achieve any type of acceptable wood composite adhesive properties. 

Considerable research activity has been directed toward producing wood composite adhesives from lignin, and this has been accompanied by very little practical success in terms of commercial implementation. By themselves, and regardless of source, lignins offer no advantages in terms of chemical reactivity, product quality, or color when compared to conventional wood composite adhesives. At low replacement levels (10 to 30%), lignins can and will continue to be employed as extenders for UF and PF resins. When they are used as extenders, best results are obtained when chemically activated (e.g., by methylolation). 

However, lignin may find application as a suitable wood-composite adhesive in adhesive formulations if chemically modified in some manner. In this respect, the recent developments with soda bagasse lignin-resorcinol-formaldehyde and lignin-isocyanate adhesives allow for some cautious optimism for the future. 

Roy, C., Calve, L., Lu, X., Pakdel, H., and Amen-Chen, C. (1999) Wood Composite Adhesives from Softwood Bark-Derived Vacuum Pyrolysis Oils.In Biomass A Growth Opportunity in Green Energy and blue Added Products. 

Himmelblau D.A., Grozdits G. A. and Gibson M. A. (2000) Performance of Wood Composite Adhesives Made With Biomass Pyrolysis Oil To be published as part of fVood Adhesives 2000, Forest Products Society, Madison, WI. 

Himmelblau D.A. and Grozdits G.A. (1999) Production and Performance of Wood Composite Adhesives with Air-Blown, Fluidized-Bed Pyrolysis Oil. hiPrvceedings. Fourth Biomass Conference of the A mericas, Elsevier, New York. 

Properties Liq. insol. in water m.w. 226.28 dens. 1.010 b.p. 290 C flash pt. (COC) 124 C ref. index 1.4520 Toxicology TSCA listed Storage Light-sensitive Uses Peroxide crosslinker/coactivator for elastomers, PVC plastisols, syn. resins, cast acrylic sheet/rods coagent for rubber compding. impregnant for metal and wood composites, adhesives, and glass-reinforced... 

Related articles include Wood adhesives - hot melts, Wood adhesives - isocyanates/ urethanes, Wood adhesives - phenolics and Wood composites - adhesives. 

Wood composites - adhesives B H PAXTON Scope for different adhesive types... 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

---