Study of Catalysts for Suberinic Acid-Based Adhesive Polymerization

Abstract

Birch outer bark is a byproduct accumulated from birch tree (Betula pendula Roth. and Betula pubescens Ehrh.) processing and contains up to 50 wt% suberin. Suberin is a biopolyester built mostly from α,ω-bifunctional fatty acids (suberinic acids) which after depolymerization together with lignocarbohydrate complex is a potential adhesive for obtaining particleboard composites. These composites can be made in hot-pressing process at around 225 °C temperature. The high hot-pressing temperature for suberinic acid-based adhesive is a disadvantage compared to hot-pressing temperatures for conventional resins such as urea-formaldehyde and phenol-formaldehyde (120–130 °C and 170–180 °C respectively). In this study we looked into possible ways to improve the properties (modulus of elasticity (MOE), bending strength (MOR) of suberinic acid-based particleboards by utilization of catalysts (additives) for SA-based adhesive polymerization, thus reducing hot-pressing temperature by potentially achieving the same mechanical properties as for the catalyst-untreated boards hot-pressed at higher temperature. The suitability of potential protonic acid, Lewis acid, as well as tin-based and basic catalyst was studied with differential scanning calorimetry (DSC) by quantifying specific enthalpy of the exothermic peak with an onset temperature of ≈185 °C. Based on DSC results, six catalysts (mixtures) were chosen (sulfanilic acid, p-toluenesulfonic acid, Al2(SO4)3, SnCl2, SnCl2/p-toluenesulfonic acid, dibutyltin oxide) for further studies on the effects of mechanical properties of particleboards. Treatment with 5 wt% sulfanilic acid, 5 wt% p-toluenesulfonic acid (free suberinic acid basis) significantly improved both MOE and MOR values. 6 wt% Al2(SO4)3 significantly improved MOE value.