Formaldehyde reaction with phenol extracts

If polymeric procyanidins extractable from conifer tree barks are to be used in adhesive formulations requiring condensation with phenol-formaldehyde prepolymers, these reactions must be performed at acidic pH conditions, and because of solubility limitations, this will probably require the use of sulfonate derivatives. 

Use of the phenolic components of bark extracts in preparing adhesive components used in plywood and particleboard manufacture has been proposed from time to time (8,15). Such preparations are based on the reaction of bark phenolic components with an aldehyde, usually formaldehyde. 

Attempts to make adhesive formulations by direct reaction of formaldehyde or its equivalent resulted in products that were excessively viscous, and the working time was too short for commercial application (57). It was concluded that formaldehyde, although readily reactive with the tannin molecule, provided much too short linkages to connect the bulky tannin molecules. This problem was circumvented by the preparation of a polymethylolphenol reagent that, when put in solution with the bark extract, formed a combination that was stable for several weeks at room temperature. When heated, the polymethylolphenol and bark extract reacted rapidly to form an infusible resin. Commercial trials were made to produce exterior-grade Douglas-fir plywood. Widespread use of the extracts for this purpose, however, was inhibited by a drop in the price of phenol below what the bark extracts could be manufactured for. (The best extract for adhesive purposes was an ammonia extract of hemlock bark converted to a sodium derivative prior to spray drying, a more costly extraction procedure than simple sodium hydroxide extraction of bark.)... 

Colorimetric Indirect Method based on the typical reaction of polyphenols with formaldehyde, Singleton " proposed an assay for the quantification of phenolic material condensable with formaldehyde. He described a colorimetric procedure, instead of the gravimetric method, combining the Folin-Ciocalteu assay wifii the method of Stiasny (Fig 2). First, the total phenol content of the extract was measured before the precipitation reaction by the Folin-Ciocalteu method following the... 

Methods aimed at the determination of phenolic materials present in the extract that can be reacted with formaldehyde These methods were devised particularly for tanning extract used in adhesives and are all based on the determination of some of the products of the reaction of the flavonoids with formaldehyde. 

Despite the similar chemical nature, there is no evidence for cross-reactivity between phenol-formaldehyde resin and PTBP-FR (Geldof et al. 1989). Some authors have attributed test reactions to PTBP-FR to free formaldehyde however, most patients are actually not allergic to formaldehyde. Schubert and Agatha (1979) performed patch-test studies with the chroma-tographically extracted ingredients of commercially available PTBP-FR. They found two linear condensates named 2-hydroxy-5-tert.-butylbenzylalcohol and 2,2 -dihydroxy-3,3-di-(2-hydroxy-5-tertiary butyl)-benzyl-5,5-ditertiary butyl-diphenylmethan to be the real allergens. 

The use of these intermediates to produce shikimates is shown in Figure 6.32. In principle, anethole (53) and estragole (methyl chavicol) (52) are available from phenol, but in practice, the demand is met by extraction from turpentine. Carboxylation of phenol gives salicylic acid (38) and hence serves as a source for the various salicylate esters. Formylation of phenol by formaldehyde, in the presence of a suitable catalyst, has now replaced the Reimer-Tiemann reaction as a route to hydroxybenzaldehydes. The initial products are saligenin (189) and p-hydroxybenzyl alcohol (190), which can be oxidized to salicylaldehyde (191) and p-hydroxybenzaldehyde (192), respectively. Condensation of salicylaldehyde with acetic acid/acetic anhydride gives coumarin (50) and 0-alkylation ofp-hydroxybenzaldehyde gives anisaldehyde (44). As mentioned earlier, oxidation of phenol provides a route to catechol (184) and guaiacol (188). The latter is a precursor for vanillin, and catechol also provides a route to heliotropin (61) via methylenedioxybenzene (193). 

Thirty-six copolymer resins were prepared with 20% by weight of phenol replaced by those extracts, which had been found to consume more than 50% of the available formaldehyde at the end of 120 minutes reaction time at the 85 C reaction temperature. 

---