16 августа, 2015 
Malyar A. Pizzi
Ecole Nationale Superieure des Technologies et Industries du Bois, Universite de Nancy I, Epinal, France
Resorcinol-formaldehyde (RF), and phenol-resorcinol-formaldehyde (PRF) coldsetting adhesives are used primarily in the manufacture of structural, exterior-grade glulam fingerjoints, and other exterior timber structures. They produce bonds not only of high strength, but also of outstanding water and weather resistance when exposed to many climatic conditions [1,2]. PRF resins are prepared mainly by grafting resorcinol onto the active methylol groups of the low-condensation resols obtained by the reaction of phenol with formaldehyde. Resorcinol is the chemical species that gives to these adhesives their characteristic cold-setting behavior. At ambient temperature and on addition of a hardener, it provides accelerated and improved cross-linking not only to RF resins but also to the phenol-formaldehyde (PF) resins onto which resorcinol has been grafted by chemical reaction during resin manufacture. Resorcinol is an expensive chemical, produced in very few locations around the world (to date only three commercial plants are known to be operative: in the United States, Germany, and Japan), and its high price is the determining factor in the cost of RF and PRF adhesives. It is for this reason that the history of RF and PRF resins is closely interwoven, by necessity, with the search for a decrease in their resorcinol content, without loss of adhesive performance.
In the past decades, significant reductions in resorcinol content have been achieved: from pure RF resins, to PRF resins in which phenol and resorcinol were used in equal or comparable amounts, to the modern-day commercial resins for glulam and fingerjointing in which the percentage, by mass, of resorcinol on liquid resins is on the order of 15 to 18%. A step forward has also been the development and commercialization of the ‘‘honeymoon’’ fast-set system [3], either composed of just synthetic PRF resins or of a PRF resin coupled with the use of tannin extracts, which in certain countries are used to obtain PRFs of 8 to 9% resorcinol content without loss of performance and with some other advantages (such as gluing of high moisture content timber). This was a system improvement, not an advance on the basic formulation of PRF resins.
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