Modification of novel bio-based adhesive made from citric acid and sucrose by ZnCl₂

Abstract

Due to environmental and health concerns, a substitute for conventional wood adhesives has been sought, with bio-based adhesives being one of the most-researched alternatives. Recently, a bio-based adhesive made from citric acid and sucrose was used to manufacture sweet sorghum bagasse particleboard. However, the pressing conditions (200 °C, 10 min) consume a high amount of energy, and so optimization of the pressing condition is required. Therefore, the effects of ZnCl₂ as a catalyst for the curing reaction of citric acid-sucrose adhesive were investigated. Thermal analyses such as differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) as well as Fourier transform-infrared (FT-IR) spectroscopy and insoluble matter analysis were used to investigate the effect of the catalyst on the curing behaviour of a citric acid-sucrose adhesive. DSC and TGA showed that ZnCl₂ decreased the curing temperature, enthalpy reaction, and apparent activation energy. The apparent activation energy obtained using the Kissinger method decreased from 123.7 kJ/mol to 108.7 kJ/mol after 1% ZnCl₂ catalyst was added. The level of insoluble matter in the cured adhesive after 4 h of boiling treatment at 150 °C, 180 °C, and 200 °C with various heating times (6 and 10 min) was higher after the addition of 1% ZnCl₂. FT-IR analysis of the cured adhesive showed that ZnCl₂ could accelerate the thermal degradation of sucrose, the formation of 5-hydroxymethylfurfural, and the formation of polymeric humins.

Introduction

Commonly, wood-based composites like particleboard use synthetic resin adhesives such as formaldehyde-based adhesives. However, synthetic resin adhesives are derived from fossil resources and can lead to environmental and human health problems [1,2]. To address this issue, research on bio-based adhesives has been performed. Various bioresources such as protein, tannins, lignin, and carbohydrates were used as raw materials for bio-based adhesives [[3], [4], [5]]. However, the conventional bio-based adhesives showed poor bonding performance and complicated preparation methods. In addition, synthetic compounds such as polymeric-methylene diphenyl diisocyanate (PMDI) were sometimes needed to obtain satisfying performance. Therefore, bio-based adhesives that are easy to prepare, safe for the environment and for health, and perform well are in need of development.

Recently, it was reported that citric acid-sucrose adhesive, a novel bio-based adhesive, performed well on particleboard made from wood and sorghum bagasse [[6], [7], [8]]. The advantage of this adhesive is the simplicity of its preparation, which involves dissolving citric acid and sucrose in water at room temperature. However, novel bio-based adhesives still have disadvantages, namely a high press temperature and long press time (200 °C and 10 min) [8]. To overcome these problems, a way to accelerate the reaction of the citric acid-sucrose adhesive is needed. The reaction of citric acid-sucrose adhesive is predicted to have several steps. The first step is the thermal degradation of sucrose to produce glucose and fructose [9,10]. The fructose and glucose then undergo a further reaction to form 5-hydroxymethylfurfural (5-HMF); the 5-HMF itself then polymerizes into a polymer that is insoluble in water, known as a humins polymer [11,12]. Some possible structures of humins polymers have been widely researched and proposed [[13], [14], [15]]. Several studies have suggested that ZnCl₂ can be used as a catalyst for 5-HMF formation directly from sucrose or cellulose [12,[16], [17], [18]]. In addition, it was reported that citric acid acts as a cross-linking agent between various polymers [[19], [20], [21], [22]]. Therefore, in this research, citric acid is proposed to be a cross-linking agent between humins polymers, and ZnCl₂ would act as a catalyst to accelerate the reaction of citric acid-sucrose adhesive. The effect of the addition of ZnCl₂ on the thermal properties of citric acid-sucrose adhesive was investigated using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Fourier transform-infrared (FT-IR) spectroscopy analysis was used to clarify the chemical reactions of the adhesives during heating. Insoluble matter against boiling water was measured to determine the polymerization of the adhesive. And in addition, the citric acid-sucrose adhesive as a control and the modified adhesive with ZnCl₂ were used to make particleboards from sorghum bagasse and then compared their mechanical and physical properties.