2938 . Chapter 24 



exposure ratings, neither of which qualify products for unprotected exterior 

 appHcation. 



With few exceptions, the hardwood structural flakeboard industry uses phe- 

 nol-formaldehyde resins as binders. One company has manufactured wafer- 

 board from a spray-dried mixture of co-reacted kraft black liquor and a phenolic 

 resol (Dolenko and Clarke 1978), and much research has been aimed at binding 

 aspen waferboards with spent sulfite liquor powder (Shen 1974, 1977a; Shen 

 and Fung 1979). Polyisocyanates, in a variety of formulations, also show con- 

 siderable promise for hardwoods (Hse 1978; 1981; Steiner et al. 1980). To 

 reduce dependence on petroleum-based adhesives, researchers throughout the 

 world are vigorously studying bark tannins as partial replacements for phenol, 

 e.g., see Madle and Molnar (1980). 



This book does not delve deeply into resin technology. Readers interested in 

 the technology will find useful the treatise being prepared on the subject by C- 

 Y. Hse of the Southern Forest Experiment Station, Pineville, La. Texts on the 

 subject already in print include Martin (1956), Megson (1957), and Lloyd 

 (1981). Some knowledge is needed, however for appreciation of manufacturing 

 procedures and panel performances. 



PHENOL-FORMALDEHYDE RESOL AND NOVOLAC RESINS 



Resol resins. — Phenol formaldehyde resins can be prepared by two methods. 

 For resol resin, phenol is reacted with excess formaldehyde in the presence of an 

 alkali catalyst. Mole ratios of formaldehyde to phenol of 1.8:1 to 2.2:1 are 

 employed. Curing to produce a rigid thermoset resin is activated by increased 

 temperature. Resol-type resins may be purchased either as a liquid or as a spray- 

 dried powder which has to be mixed with water. 



Novolac resins. — Reacting excess phenol with formaldehyde in the presence 

 of an acid catalyst produces a hard resin known as novolac. Mole ratios of 0.8: 1 

 to 1 : 1 are used. The novolac is ground into a fine powder to which is added about 

 15 percent of hexamethylene tetramine. When heated in a hot press, the powder 

 forms ammonia, which acts as a catalyst, and formaldehyde — which reacts with 

 the heat-liquified novolac — to produce a thermosetting resin. Novolac resins are 

 more stable than the resols, but must be kept dry. 



Powdered resins. — In the production of aspen waferboard, the primary adhe- 

 sive type used is spray-dried phenolic resol powder which requires only about 

 150°C to thermoset, some 10° to 30°C lower than novolacs. Spray-dried resols 

 also yield board properties superior to novolacs (Hickson 1980). 



Powdered resin is easily applied to flakes of certain species (e.g. , aspen) with 

 simple equipment. Distribution is good because the resin tends to cling in a 

 uniform monolayer over all surfaces, thus minimizing over- application, and 

 affording high resin efficiency. Because of the added manufacturing operations 

 of spray drying and grinding, however, powdered resins are usually more 

 expensive than liquid. For optimum flow, they also require 10 to 20°C higher 

 press temperatures than those needed for liquid resins, and they sometimes 

 create an annoying dust problem (Hse 1975a). 



