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Huzyan, H. I., Abdul Aziz, A., and Hussin, M. H. (2021). "Ecofriendly wood adhesives from date palm fronds lignin for plywood," BioResources 16(2), 4106-4125.

Abstract

Utilization of lignin phenol glyoxal (LPG) resins was studied as a potential alternative for phenol formaldehyde (PF) resins. Lignin was extracted by alkaline pulping processes (kraft and soda) from date palm fronds (DPF) and was used as an alternative for phenol in LPG resins. The isolated lignin samples were characterized using complementary analyses that included Fourier transform infrared (FTIR) spectroscopy, 13C nuclear magnetic resonance (NMR) spectroscopy, thermal stability, thermogravi-metric analysis (TGA), and differential scanning calorimetry (DSC). Kraft lignin phenol glyoxal (KLPG) and soda lignin phenol glyoxal (SLPG) resins also were characterized in terms of solid content, viscosity, and gel time. Finally, physico-mechanical tests were performed on plywood panels that were treated with different molar ratios of LPG resins. The results revealed that 50% (w/w) KLPG resin resulted in higher tensile strength (65.3 MPa) than PF resin (58.57 MPa), which was potentially attributed to the higher amount of phenolic groups compared to soda lignin. Therefore, the substitution of DPF lignin in LPG resins enhanced the adhesive in terms of its chemical and mechanical properties, enabling it to produce a more environmentally friendly wood adhesive.


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Ecofriendly Wood Adhesives from Date Palm Fronds Lignin for Plywood

Hiba Ibrahim Huzyan, Alia Abdul Aziz, and M. Hazwan Hussin *

Utilization of lignin phenol glyoxal (LPG) resins was studied as a potential alternative for phenol formaldehyde (PF) resins. Lignin was extracted by alkaline pulping processes (kraft and soda) from date palm fronds (DPF) and was used as an alternative for phenol in LPG resins. The isolated lignin samples were characterized using complementary analyses that included Fourier transform infrared (FTIR) spectroscopy, 13C nuclear magnetic resonance (NMR) spectroscopy, thermal stability, thermogravi-metric analysis (TGA), and differential scanning calorimetry (DSC). Kraft lignin phenol glyoxal (KLPG) and soda lignin phenol glyoxal (SLPG) resins also were characterized in terms of solid content, viscosity, and gel time. Finally, physico-mechanical tests were performed on plywood panels that were treated with different molar ratios of LPG resins. The results revealed that 50% (w/w) KLPG resin resulted in higher tensile strength (65.3 MPa) than PF resin (58.57 MPa), which was potentially attributed to the higher amount of phenolic groups compared to soda lignin. Therefore, the substitution of DPF lignin in LPG resins enhanced the adhesive in terms of its chemical and mechanical properties, enabling it to produce a more environmentally friendly wood adhesive.

Keywords: Lignin; Wood adhesive; Date palm fronds

Contact information: Materials Technology Research Group (MaTReC), School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia;

* Corresponding author: mhh@usm.my; mhh.usm@gmail.com

GRAPHICAL ABSTRACT

INTRODUCTION

Phenolic resins are credited as the oldest synthetic thermosetting polymers. Due to their outstanding chemical and mechanical properties and characteristics, i.e., high water resistance, dimensional and thermal stability, chemical resistance, and electrical insulation, they are widely applied as acid-resistant coatings, fiber-reinforced composites, electric laminates, and in wood industrial production of exterior-grade plywood panels, oriented strand board panels, and particleboards (Danielson and Simonson 1998; Ramires et al. 2010; Foyer et al. 2016b). However, the toxicity of formaldehyde (McGregor et al. 2006) and the potential shortage of phenol production from the non-renewable petrochemical resources have triggered researchers to conduct numerous studies to investigate possible alternative ecofriendly substances as a replacement to petroleum-based materials in phenol formaldehyde resins (Wu 1997; Ibrahim et al. 2011). Some of the bio-based polymers that have been used are proteins, tannins, lignin, and polysaccharides (Ballerini et al. 2005; Moubarik et al. 2015; Hussin et al. 2017).

Lignin is known as a complex phenolic polymer that is built up by oxidative coupling of three major phenylpropanoid units, namely trans-para-coumaryl alcohol, trans-coniferyl alcohol, and trans-sinapyl alcohol. Thus, it is characterized by having a high aromatic density and high cross-linked structure that is comparable to phenol network structure (Ibrahim et al. 2007). The functional groups in unmodified lignin are mainly aliphatic and aromatic hydroxyl groups, and methoxyl groups, of which the phenolic –OH groups play a main role in substitution reactions. The structure similarity between phenol and lignin renders the condensation reaction between phenol and formaldehyde in phenolic resins the same as the condensation reaction between lignin and glyoxal in lignin phenol glyoxal resins (Younesi-Kordkheili and Pizzi 2019). Currently, lignin has been isolated from different lignocellulosic waste and exploited in phenol-formaldehyde adhesives as a phenol substituent, due to the structural similarity of both polymers (Zhang et al. 2013).

One of the types of lignocellulosic biomass that lignin could be isolated from is date palm fronds. Date palm trees (Phoenix dactylifera) are widely cultivated across Northern Africa and the Middle East and are naturalized in many tropical and subtropical regions worldwide. Analysis of date palm trees showed a low to medium hemicelluloses content (13% to 31%), cellulose (33% to 48%), a high ash content (1% to 15%), and extractives (8% to 33%) comparing to other lignocellulosic materials, and a moderate lignin amount (26% to 40%). More specifically, date palm fronds presented 26% of lignin, which is similar to those found in other wood species and non-wood plants (Nasser et al. 2016). The reasonable lignin content in the fronds, the increasing amount of waste from the date palm trees that can reach 40 kg (most of them are leaves and surface fibers around the trunk) in a country like UAE (Mallaki and Fatehi 2014), were the reason behind choosing the date palm fronds as a lignin source.

Furthermore, formaldehyde is regarded as carcinogenic by the International Agency for Research on Cancer (IARC) and the Scientific Committee on Occupational Exposure Limits (SCOEL) (IARC 2006). Thus, to produce an environmentally friendly wood adhesive, formaldehyde can be fully replaced by hexamine, glyoxal, and furanic materials (i.e., furfural and furfuryl alcohol) (Norström et al. 2014; Foyer et al. 2016a; Santiago-Medina et al. 2016; Hussin et al. 2019). Glyoxal is considered as a non-toxic aldehyde, which is non-volatile, biodegradable, and has a low cost. In addition, it could react with lignin in a similar way to the condensation of formaldehyde with lignin (Faris et al. 2017). Many studies have investigated bio-based wood adhesives by producing lignin-phenol-glyoxal resins (Hussin et al. 2019; Younesi-Kordkheili 2019). Younesi-Kordkheili (2019) used ionic liquid treated lignin in the preparation of lignin phenol glyoxal adhesive to bond particleboard panels, which demonstrated mechanical properties in par with panels treated with phenol formaldehyde resins. As well, Hussin et al. (2019) obtained lignin from kenaf core (Hibiscus cannabinus), and partially substituted phenol in lignin-phenol-glyoxal adhesives. It was a 3:7 ratio of lignin to phenol in lignin-phenol-glyoxal resins that gave the highest internal bonding (53.83 MPa) and tensile strength (72.08 MPa) compared to that of phenol formaldehyde resin.

There have been a great number of studies regarding lignin and glyoxal utilization in wood adhesives, including the study of the preparation kraft lignin phenol glyoxal (KLPG) and soda lignin phenol glyoxal (SLPG) adhesives. However, to the best of our knowledge, the adhesives were not modified by date palm fronds lignin. In this paper, lignin was extracted from date palm fronds by the use of alkaline pulping processes (kraft and soda). Subsequently, kraft lignin (KL) and soda lignin (SL) were used to prepare both adhesives, kraft lig