ALKALINE TREATMENT OF SULFITED TANNIN-BASED ADHESIVE FROM MANGROVE TO INCREASE BOND INTEGRITY OF BEECH SLIPS

Abstract

PARIDAH, M. T. & MUSGRAVE, O. C. 2006. Alkaline treatment of sulfited tannin-based adhesive from
mangrove to increase bond integrity of beech slips. Sulfited tannin solutions were prepared by extracting
Rhizophora mucronata and R. apiculata barks with 4% aqueous sodium sulfite and 0.4% aqueous sodium
carbonate at 100 °C for two hours. The sulfited tannin solutions were pre-treated with 50% sodium hydroxide
at various amounts, and kept at 1 min, 5 min, 60 min or 24 hours prior to mixing with phenol resorcinol
formaldehyde (PRF) resin for bonding beech slips. Two types of bonding techniques were used: (1) normal,
one-component and (2) honeymoon, two-component adhesive mixtures. The adhesive mixtures were
evaluated for pH, water resistance property and gel time. The bond integrity of the jointed beech slips was
assessed based on shear strength and wood failure percentage. The study showed that the amount of aqueous
sodium hydroxide added to the sulfited-tannin solution had significant effects on viscosities of the final
adhesive mix, consequently influencing the penetration of the adhesive into the wood surface. The amount
of sodium hydroxide used depended on the type of tannin; R. mucronata tannin adhesive required 2–4%
and R. apiculata 0–2%. However, the viscosity of adhesive made from mixed R. mucronata-R. apiculata tannin
was not affected by the amount of sodium hydroxide added. The pre-treatment with sodium hydroxide had
successfully increased the gel time of this adhesive to an acceptable working condition for honeymoon
bonding technique. This technique improved the gluebond quality of the PRF-sulfited tannin-bonded
joints by at least 20%.