12.68 In a study of wood-laminating adhesives by Kreibich and Hemingway (Forest Products Journal, 37(2), 1987, pp. 43–46), tannin is added to the usual phenol-resorcinolformaldehyde

(PRF) resin and tested for strength. For four mixtures of resin and tannin and two types of test (test A, dry shear test;

test B, two-hour boil test), the resulting shear strengths of the wood specimens (in psi) were as listed in table below. What can you conclude about the effects of the tannin mixtures on the strength of the laminate?

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PRF/Tannin Ratio Test A B 50/50 1,960 1,460 2,267 140 40/60 1,450 1,620 1,683 2.287 30/70 2,080 760 1,750 130 23/77 1,493 650 1,850 90