Failure, wood

Typical tensile strength and percentage wood failure results obtainable with synthetic PRF resins... 

Strength (N) Wood failure (%) Strength (N) Wood failure (%) Strength (N) Wood failure (%)... 

Tannins are polyhydroxyphenols. They are soluble in water, aleohols and aeetone and ean eoagulate proteins. They are yielded by extraetion from wood substanee, bark, leaves and fruits. Other components of the extraction solutions are sugars, pectins and other polymeric carbohydrates, amino acids and other substances. The content of non-tannins can reduce wood failure and water resistance of glued bonds. The polymeric carbohydrates especially increase the viscosity of the extracts. 

An early study by Rudkin (1950) showed that substitution of OH groups with acetyl resulted in a significant decrease in bond strength between the wood and a UF resin in lap-joint tests. Vick and Rowell (1990) studied the adhesive bonding of acetylated yellow poplar, with 18 different thermoplastic and thermosetting adhesives. The effectiveness of the adhesives was examined by determination of bond shear strength (and wood failure) of 6 mm thick, bonded wood strips after conditioning at 27 °C and... 

Figure 1. Comparison of wet-bond adhesion strengths of lignin-resol resin adhesives phenolated with and without acid catalyst. Legend phenola-tion with acid catalyst at 80°C for 60 min Q phenolation without catalyst at 200°C for 60 min. Note Numerical values in parentheses are percentages of wood failure hot-press temperature 120°C.

Figure 2. Relationship between hot-press time and wet-bond adhesion strength for lignin-resol resin adhesives with and without alkylresorcinol. Legend Q without alkylresorcinol 0 with 10 parts alkylresorcinol. Note Numerical values in parentheses are percentages of wood failure phenola-tion 200°C, 60 min, without catalyst hot-press 120°C, 6 min.

The various reduction systems can be classified into knife, hammer, and attrition units, each type producing a characteristic particle. A tramp metal detection system is included in all reduction steps to protect the equipment from serious damage. The shape and integrity of the component particles strongly influence the quality of the resultant particleboard therefore, the optimum in particle preparation is achieved when the desired particle is obtained with no damage to the structure of the wood. Wood failure within the particle will result in a particleboard of lower strength than one formed from intact particles. 

In contrast, compare with the picture of a joint made with the same glue and wood but after 60 minutes open time and 0 minutes closed, Figure 4. The very thick glue line and almost total lack of solvent absorption are quite apparent. The wood failure in testing this joint was very low, while that of the previous figure was very high. 

It is quite probable that the resin is insufficiently cured at the junction between latewood surfaces because the water has not been efficiently removed from the film. One observation which substantiates this is that when the specimens are heated, the sheer strength and wood failure values both increase, and the distinction between earlywood and latewood disappears. This definitely indicates that the problem is due to under cure. 

D 5266 Practice for Estimating the Percentage of Wood Failure in Adhesive Bonded... 

Figure 2. Strength and wood failures obtained on beech strips glued with lignin hydroxymethylated for different times at pH 13 or 12.

Entry Fortifier Percentage Fortifier On Resin Adhesive Strength (% Wood Failure) ... 

Figure 3. Strength and wood failures of beech strips glued with hydroxymethy-lated lignin and PF resin in different ratios cured at 90 °C for 4 hours.

Novolaks were prepared using a phenol-to-formaldehyde molar ratio of 4 1 with 5 mole percent of H2SO4 added as a catalyst. Typically, 47 g liquid phenol (91.7% assay), 3 g paraformaldehyde, and 30 mL water plus the required acid catalyst were added to a three-neck, 250-mL round bottom flask. The flask was fitted with a reflux condenser and stirrer. The mixture was refluxed for 2 to 4 hours with the oil bath at 115 °C then, the mixture was neutralized with 50% (w/w) NaOH and the excess phenol removed by steam distillation for 5 to 6 hours. The remaining viscous oily residue was washed repeatedly with boiling water. A novolak with the P/N fraction was prepared as described above with 1 1 by volume phenol and P/N fraction and half of the amount of formaldehyde. Initial wood-gluing testing with this novolak indicates wood failure rather than glueline failure. 

A few preliminary resols have also been made with a 50% replacement of phenol by the P/N fraction of the wood oil. Tests have shown these adhesives to have shear strengths and wood failure comparable to that obtained with Borden s Cascophen 313. This work is in progress. 

Although not normally measured and not a requirement of an industry standard, the high wood failure values of the end-joints obtained with these adhesives are further evidence of water-resistant bonds. It was difficult to discern differ-... 

Table III. Strength and Wood Failure of Plywood1 Bonded with Formulations of Diisocyanate (PMDI) and Tannin Extracts (50% Solutions)...

Figure 2. Effect of gum arabic on bond strength and wood failure of phenol-formaldehyde bonded wood.

In this study, up to about 50% of the phenol-formaldehyde was replaced with carbohydrates and the modified resins used to bond wood veneer panels. The carbohydrate modified resins were formulated and cured under neutral conditions. The resins bond wood with acceptable dry- and wet-shear strengths, and wood failures. Reducing as well as nonreducing carbohydrates can be used as modifiers. The carbohydrate modifiers are being incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and methylol groups in the phenol-formaldehyde resin. The resins formulated under neutral conditions are very light in color. 

The amount of wood failure in the two-ply specimens decreases as the amount of modifier in the resin is increased. The wood failure data also indicate that the performance of the modified resin is not severely affected until greater than about 0.6 moles of modifier per mole of phenol has been added to the neutral resin (Figure 4). 

Figure 4. Variation of the dry- and wet- (hatched) wood failures of modified phenol-formaldehyde resins. The moles of modifier were varied as indicated. The mole ratio of phenol to formaldehyde was 1 2.3. The wood failures of xylose- and prehydrolysate-modified resins were similar and were averaged.

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