Influence of Resin Formulation Variables on Bond Quality of Southern Pine Plywood

Thirty-six resins, each with 2 replicates, were factorially prepared with 3 formulation variables: molar ratio of sodium hydroxide to phenol 0.4, 0.7, and 1.0; level of resin solids content 37, 40, and 43 percent; and molar ratio of formaldehyde to phenol 1.6, 1.9, 2.2, and 2.5. Glue bond quality decreased substantially with a change of NaOH/phenol ratio from 0.4 to 0.7. A further change in ratio from 0.7 to 1.0 failed to affect it significantly, apparently because conversion of formaldehyde by the Cannizzaro reaction became important where more than half a mole of caustic was present in the reaction mixture. On the average, bond quality increased as solids content increased. Changes in the CH2O/phenol ratio affected delamination but not wet shear strength or wood-failure percentages; the lower molar ratios yielded gluelines that delaminated least. Latewood-to-latewood delaminated substantially more than earlywood-to-earlywood gluelines. Of 72 batches of resins, two met the wood-failure standard for exterior gluelines. Three yielded latewood-to-latewood gluelines that averaged less than 20 percent delamination in 3 months of outdoor exposure in central Louisiana.

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