To fulfill the D2 criterion the bonded wood substrates must have higher tensile shear strength than 8 MPa after been immersed in water for 3 h and dried for a week according to conditioning method B. All wood substrates bonded with the xanthan gum dispersion and all except one wood substrate bonded with the 6 wt% tamarind gum from Premcem dispersion delaminated in water. All wood substrates bonded with locust bean gum dispersion fulfill the D2 criterion. The specimens bonded with locust bean gum dispersed in brown liquor do also fulfill the D2 criterion, but the tensile shear strength is lower than the water dispersion of locust bean gum. The wood substrates bonded with the other gum dispersions have lower tensile shear strength and does not fulfill the D2 limit. Cascol barely fulfills the D2 criterion and the standard deviation is large.
All wood substrates delaminate when soaked in water for 4 days according to conditioning method C, except the specimens bonded with locust bean gum. All wood substrates bonded with locust bean gum held together after the treatment. However, none of the bonded wood substrates fulfill the D3 criterion of 2 MPa. The fact that all wood substrates held together after the treatment show the potential of locust bean gum with good bonding performance as well as water resistance.
The tensile shear strength of the wood substrates that were treated at elevated temperature with conditioning method D is also presented in Fig. 5. None of the wood substrates bonded with gum dispersions are significantly affected by the heat treatment. They all show a very good heat resistance. The wood substrates bonded with the xanthan gum dispersion do not fulfill the WATT 91 limit of 7 MPa but an improvement in the tensile shear strength is actually seen after the heat treatment. The wood substrates bonded with the other gum dispersions fulfill the limit of 7 MPa. The cyclic structure, the possibility of hydrogen bonding and the high molar mass of the polysaccharides probably make the polysaccharide heat resistant. Cascol, which is a thermoplastic poly(vinyl acetate)-based adhesive, is not classified as a WATT 91 adhesive, the tensile shear strength is considerably reduced with the heat treatment.
The bonding performance of tamarind gum from Innovassynth is significantly better than tamarind gum from Premcem. Furthermore, wood substrates bonded with the 11 wt% dispersion show lower tensile shear strengths than the wood substrates bonded with the 6 wt% dispersions. These results are contradictory to the hypothesis and the results where veneers were bonded with Application Method 2, Fig. 4. The hypothesis that tamarind gum from Premcem should result in a better bonding performance due to the higher amount of residual protein and that the 11 wt% dispersion should be stronger than the 6 wt% dispersion, were not corroborated. However, Application Method 2 and 4 cannot be completely correlated to each other; the wood substrates, the bond line, the method etc. are different with the two methods.
The locust bean gum dispersion show by far the best bonding performance, water resistance and heat resistance, fulfilling the requirement for a D2 and WATT 91 wood adhesive, respectively. This result is superior compared to the commercial wood adhesive Cascol. However, Cascol is a thermoplastic polymer that is sensitive toward elevated temperatures. It is normally pressed at room temperature and not at 120 °C as in this case.
Therefore, Cascol and the locust bean gum dispersion were further used to bond wood panels at room temperature using Application Method 5. The tensile shear strengths of the bonded wood substrates are presented in Fig. 6. It was possible to bond the wood panels with the locust bean gum dispersion at room temperature even though the water content is 94 wt%. The wood substrates bonded with locust bean gum dispersion fulfill the D1 criterion (10 MPa) with much fiber tear and reveal an even higher tensile shear strength than Cascol. All wood substrates bonded with locust bean gum dispersion held together after 3 h in water, conditioning method B, whereas three wood substrates bonded with Cascol delaminated in the water. The wood substrates bonded with locust bean gum dispersion also fulfill the D2 criterion with similar results as Cascol, showing a good water resistance. However, the standard deviation is very large for both Cascol and locust bean gum dispersion. The locust bean gum dispersion also shows a good heat resistance since it also fulfills the WATT 91 criterion, whereas Cascol does not.
To fulfill the D2 criterion the bonded wood substrates must have higher tensile shear strength than 8 MPa after been immersed in water for 3 h and dried for a week according to conditioning method B. All wood substrates bonded with the xanthan gum dispersion and all except one wood substrate bonded with the 6 wt% tamarind gum from Premcem dispersion delaminated in water. All wood substrates bonded with locust bean gum dispersion fulfill the D2 criterion. The specimens bonded with locust bean gum dispersed in brown liquor do also fulfill the D2 criterion, but the tensile shear strength is lower than the water dispersion of locust bean gum. The wood substrates bonded with the other gum dispersions have lower tensile shear strength and does not fulfill the D2 limit. Cascol barely fulfills the D2 criterion and the standard deviation is large.
All wood substrates delaminate when soaked in water for 4 days according to conditioning method C, except the specimens bonded with locust bean gum. All wood substrates bonded with locust bean gum held together after the treatment. However, none of the bonded wood substrates fulfill the D3 criterion of 2 MPa. The fact that all wood substrates held together after the treatment show the potential of locust bean gum with good bonding performance as well as water resistance.
The tensile shear strength of the wood substrates that were treated at elevated temperature with conditioning method D is also presented in Fig. 5. None of the wood substrates bonded with gum dispersions are significantly affected by the heat treatment. They all show a very good heat resistance. The wood substrates bonded with the xanthan gum dispersion do not fulfill the WATT 91 limit of 7 MPa but an improvement in the tensile shear strength is actually seen after the heat treatment. The wood substrates bonded with the other gum dispersions fulfill the limit of 7 MPa. The cyclic structure, the possibility of hydrogen bonding and the high molar mass of the polysaccharides probably make the polysaccharide heat resistant. Cascol, which is a thermoplastic poly(vinyl acetate)-based adhesive, is not classified as a WATT 91 adhesive, the tensile shear strength is considerably reduced with the heat treatment.
The bonding performance of tamarind gum from Innovassynth is significantly better than tamarind gum from Premcem. Furthermore, wood substrates bonded with the 11 wt% dispersion show lower tensile shear strengths than the wood substrates bonded with the 6 wt% dispersions. These results are contradictory to the hypothesis and the results where veneers were bonded with Application Method 2, Fig. 4. The hypothesis that tamarind gum from Premcem should result in a better bonding performance due to the higher amount of residual protein and that the 11 wt% dispersion should be stronger than the 6 wt% dispersion, were not corroborated. However, Application Method 2 and 4 cannot be completely correlated to each other; the wood substrates, the bond line, the method etc. are different with the two methods.
The locust bean gum dispersion show by far the best bonding performance, water resistance and heat resistance, fulfilling the requirement for a D2 and WATT 91 wood adhesive, respectively. This result is superior compared to the commercial wood adhesive Cascol. However, Cascol is a thermoplastic polymer that is sensitive toward elevated temperatures. It is normally pressed at room temperature and not at 120 °C as in this case.
Therefore, Cascol and the locust bean gum dispersion were further used to bond wood panels at room temperature using Application Method 5. The tensile shear strengths of the bonded wood substrates are presented in Fig. 6. It was possible to bond the wood panels with the locust bean gum dispersion at room temperature even though the water content is 94 wt%. The wood substrates bonded with locust bean gum dispersion fulfill the D1 criterion (10 MPa) with much fiber tear and reveal an even higher tensile shear strength than Cascol. All wood substrates bonded with locust bean gum dispersion held together after 3 h in water, conditioning method B, whereas three wood substrates bonded with Cascol delaminated in the water. The wood substrates bonded with locust bean gum dispersion also fulfill the D2 criterion with similar results as Cascol, showing a good water resistance. However, the standard deviation is very large for both Cascol and locust bean gum dispersion. The locust bean gum dispersion also shows a good heat resistance since it also fulfills the WATT 91 criterion, whereas Cascol does not.
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