After the bonded veneers had been treated with conditioning method B, where they were immersed in water for 3 h and re-dried for one week, several veneers had delaminated, i.e. the water resistance was poor. However, the wood substrates bonded with Cascol and locust bean gum dispersed in water did not delaminate, showing good water resistance. The tensile shear strengths decreased for both the veneers bonded with Cascol and locust bean gum dispersion but the tensile shear strength was higher for Cascol.
Even though locust bean gum dispersed in brown liquor resulted in higher tensile shear strength after conditioning method A, locust bean gum dispersed in water show the best bonding performance in humid conditions since it also endures conditioning method B, whereas all veneers bonded with locust bean gum dispersed in brown liquor delaminated in water. The brown liquor contains salts which may be the reason for the low water resistance. The low water resistance was also indicated in the contact-angle measurements. Locust bean gum has the best water resistance of all tested gums. In the comparison with the reference Cascol, locust bean gum has lower tensile shear strength. However, the dry solids content of the locust bean gum dispersion is 6 wt% whereas Cascol has a dry solids content of ∼50 wt%. The amount of binder in the glue line is thereby larger in the samples bonded with Cascol than in the samples bonded with locust bean gum dispersion.
To overcome this, a multiplied gluing of veneers, Application Method 3 (AM3), was performed with the locust bean gum dispersion. Four layers of locust bean gum dispersions were applied on the veneer resulting in a bond line corresponding to a dispersion with 24 wt% dry solids content. The result of the tensile shear strength test of the multiplied gluing is also presented in Fig. 4. The tensile shear strength of the bonded veneers was significantly improved and higher than the veneers bonded with Cascol for both the dried veneers, conditioning method A and the re-dried veneers, conditioning method B. The adhesive was very strong; the veneer failed before the adhesive, resulting in fiber tear or wood failure. Unfortunately, all veneers delaminated in water when treated with conditioning method C. Even though the thickness of the glue line only corresponds to a 24 wt% dispersion it has superior tensile shear strength compared to Cascol with ∼50 wt% dry solids content. The tensile shear strength is only slightly reduced after conditioning method B, demonstrating a very good water resistance of the adhesive.
The tensile shear strengths of the wood substrates that have been bonded with Application Method 4 are presented in Fig. 5. All dried wood substrates bonded with gum dispersions have higher tensile shear strength than 10 MPa and thereby fulfill the D1 criterion according to EN 204 except those that were bonded with xanthan gum dispersion and 11 wt% tamarind gum dispersion. The wood substrates bonded with locust bean gum, guar gum and 6 wt% tamarind gum dispersions as well as locust bean gum dispersed in brown liquor show tensile shear strengths comparable to Cascol and all result in fiber tear.