Wood Composites and Their Polymer Binders

Abstract

This review presents first, rather succinctly, what are the important points to look out for when preparing good wood composites, the main types of wood composites manufactured industrially, and the mainly oil-derived wood composite adhesives and binders that dominate and have been dominating this industry. Also briefly described are the most characteristic biosourced, renewable-derived adhesives that are actively researched as substitutes. For all these adhesives, synthetic and biosourced, the reviews expose the considerable progresses which have occurred relatively recently, with a host of new approaches and ideas having been proposed and tested, some even implemented, but with even many more already appearing on the horizon.

Keywords: biosourced wood adhesives; environment-friendly; new approaches; synthetic wood adhesives; wood composite binders; wood composites.

Figure 1

(a) Schematic, exaggerated representation of two different panel density profiles along the panel thickness leading to very different board properties; (b) an example of two real, different density profiles of two different wood particleboards [1].

Scheme 1

Basis reaction of urea and formaldehyde to prepare UF resin.

Scheme 2

Schematic representation of a MUF resin.

Scheme 3

Schematic representation of a PF resin.

Scheme 4

Schematic representation of a PRF resin.

Scheme 5

Schematic representation of pMDI.

Scheme 6

Schematic representation of monocomponent PUR.

Figure 2

Specific oxidation of carbohydrates by sodium periodate leading to nontoxic and nonvolatile biosourced aldehydes and to different crosslinked products according to the conditions used [54].

Figure 3

Schematic representation of the formation of non-isocyanate polyurethane (NIPU) oligomers from monosaccharides reacted with dimethyl carbonate and hexamethylene diamine. The two tetramers shown have been identified and are among the multitude of other NIPU oligomers identified [110].

Figure 4

Example of citric acid bridges linking glucoses or holocellulose chains, and actually identified reaction products of citric acid with lignin. The compounds shown have been identified among several others of the same type, showing that both holocellulose and lignin are involved in wood bonding and citric acid bridge-forming by citric acid alone [116].

Figure 5

Proposed bridges and possible structures formed by the reaction citric acid with sucrose and with the furfural and hydroxymethyl furfural generated from sucrose by the citric acid treatment in sugar + citric acid adhesives [118].

Figure 6

Example of a mixed higher molecular weight oligomer leading to adhesive crosslinking formed, among others, in the new adhesive system derived by the reaction of glucose or sucrose with triacetin and hexamethylene diamine [194].