2940 



Chapter 24 



G - 



D - 



100 



60 



20 



30 



60 90 



REACTION TIME (MIN.) 



120 



Figure 24-18. — Viscosity, pH, and reaction temperature as related to reaction time 

 during preparation of phenol-formaldehyde resin. (Drawing after Hse 1975b.) 



IMPROVED PHENOLIC LIQUID RESINS 



Resorcinol-modified liquid phenolic resin. — Although resorcinol adhe- 

 sives have outstanding durability under severe test conditions, Hse ( 1 98 1 ) found 

 that a resorcinol-modified phenolic system gave little improvement in dimen- 

 sional stability of hardwood flakeboard. 



Alloy of liquid phenol-formaldehyde resin and polyisocyanate. — Hse 

 ( 198 1 ) found that an alloy of two types of liquid resin could bond flakeboards of 

 high-density hardwoods (white and southern red oak) so that they had acceptable 

 stability (table 24-10). In this alloying process, minor amounts of polyisocya- 

 nate are applied to the flakes before application of major amounts of phenol- 

 formaldehyde resin. The combined adhesive reacts in situ to obtain an improved 

 thermosetting adhesive resin. Performance of the new phenolic alloy is superior 

 to that of phenolic resin under conditions of high flake moisture content, low 

 resin content, and low panel density. The process produced internal bond 

 strength in all the white oak and southern red oak panels about 500 percent 

 greater than in those with phenol-formaldehyde resin alone. This increased 

 internal bond strength should permit reduction of panel density to improve 

 dimensional stability. The alloyed resin, is, however, more expensive and 

 presents more hazard to health during panel fabrication than conventional liquid 

 phenol-formaldehyde resin. 



