POLYAMINE-MODIFIED UREA-FORMALDEHYDE RESINS .2. RESISTANCE TO STRESS-INDUCED BY MOISTURE CYCLING OF SOLID WOOD JOINTS AND PARTICLEBOARD

The objective of this study was to improve the durability and stability of urea-formaldehyde-bonded wood products by decreasing the internal stress developed during resin cure and by improving the ability of the cured system to withstand cyclic stresses. Urea-formaldehyde resins were modified by (i) incorporating urea-terminated di- and trifunctional flexible amines into the resin structure, (ii) curing the resins with the hydrochloride derivatives of some of these amines (in place of ammonium chloride), or (ii) using both processes. The cyclic stress resistance of solid wood joints and the internal bond strength and thickness swell of particleboards made with these resins were compared with these properties in joints and particleboards made with an unmodified resin and a commercial urea-formaldehyde resin. The strength of joints and the internal bond strength of particleboards made with resins modified with urea-terminated hexamethylenediamine, bishexamethylenetriamine, and poly(propylene oxide) triamine at modifier contents of 13, 16, and 28 wt %, respectively, showed excellent stability even after repeated wet-dry cycles. Similar stress resistance was shown by wood joints and particleboards bonded with the unmodified resin cured with the hydrochloride salts of hexamethylenediamine and triethyleneaminetriamine. In contrast, solid wood joints and particleboards bonded with the unmodified resin cured with ammonium chloride showed lower resistance to cyclic stress. Particleboards that possessed good cyclic stress resistance also showed less thickness swell.