Spruce Bark-Extracted Lignin and Tannin-Based Bioresin-Adhesives: Effect of Curing Temperatures on the Thermal Properties of the Resins.

Sunanda Sain,Paul Christakopoulos,Ulrika Rova,Leonidas Matsakas,Tommy Öman,Mikael Skrifvars

doi:10.3390/molecules26123523

Abstract

In this study, formaldehyde-free bioresin adhesives were synthesised from lignin and tannin, which were obtained from softwood bark. The extraction was done via organosolv treatment and hot water extraction, respectively. A non-volatile, non-toxic aldehyde, glyoxal, was used as a substitute for formaldehyde in order to modify the chemical structure of both the lignin and tannin. The glyoxal modification reaction was confirmed by ATR–FTIR spectroscopy. Three different resin formulations were prepared using modified lignin along with the modified tannin. The thermal properties of the modified lignin, tannin, and the bioresins were assessed by DSC and TGA. When the bioresins were cured at a high temperature (200 °C) by compression moulding, they exhibited higher thermal stability as well as an enhanced degree of cross-linking compared to the low temperature-cured bioresins. The thermal properties of the resins were strongly affected by the compositions of the resins as well as the curing temperatures.

Highlights

Recent environmental concerns have made researchers focus on the development of biobased materials in order to develop new valuable products and chemicals of natural origin
Different bioresin adhesives were formulated with chemically modified L, T, and lignin–tannin mixtures (LT) (i.e., glyoxalated lignin (GOL), glyoxalated tannin (GOT), and glyoxal modification of the lignin–tannin mixture (GOLT)) and hexamine (H), which served as a curing agent
The presence of a broad signal at 3320 cm−1 attributed to the –OH stretching due to phenolic structure in L, T, and modified components (GOL, GOT, and GOLT) [11]

Summary

Introduction

Recent environmental concerns have made researchers focus on the development of biobased materials in order to develop new valuable products and chemicals of natural origin. In the wood adhesive industry, synthetic resin adhesives such as phenol–formaldehyde (PF), urea–formaldehyde (UF), and melamine urea–formaldehyde (MUF) have dominated the market due to their easy processing, good mechanical performances, and low price [1]. These resin adhesives have major uses in wood panel production with plywood, particle boards, and medium-density fibreboards (MDF), which are very promising materials for the furniture, packaging, and construction sectors. These resins have formaldehyde as a major component, which is toxic and a potentially carcinogenic compound [2]. Many research works have been carried out in order to synthesise formaldehyde-free bioresin adhesives from biobased resources such as cellulose, lignin, and tannin [3]

Objectives

Results

Conclusion