Comparison of the properties of flax shives based particleboards prepared using binders of bio-based lignin and partially bio-based epoxy resin

Abstract

EIn order to reduce formaldehyde emissions, an experimental investigation of lightweight flax shives based particleboards, prepared using two different binders: bio-based lignin (lignosulfonate) and partially bio-based epoxy resin (greenpoxy 56), was undertaken. The flax based particleboards (with a target density of 500 kg/m³), were elaborated by a thermo-compression process using 20 wt% of binder content. The flax shives were directly valorised as they had no pre-treatment, in order to qualify their potential as a raw material. Bending, compression and fire resistance tests showed that the mechanical and flame performances of lignosulfonate-based particleboards were high compared to those of greenpoxy56 based panels. Nevertheless, the latter were found to have an interesting dimensional stability after water immersion and good insulating properties. Moreover, both lignosulfonate and greenpoxy 56 based particleboards met the minimum EN 15197 requirements for non-load bearing flaxboards for use in dry conditions (Type FB2). Such results illustrated the competitiveness of the studied particleboards, as bio-sourced structures, compared to urea-formaldehyde based panels.

Introduction

Binding agents are widely used in particleboard applications and they create most of the bonds in a particleboard [1]. Phenol-formaldehyde (PF) resin is a widely used binding agent, owing to its strength and moisture resistant properties [2,3]. Other resins, such as urea-formaldehyde (UF), resorcinol-formaldehyde (RF), melamine-formaldehyde (MF) and diisocyanates are used in particleboards [4,5]. However, these are toxic and expensive [2,6]. Thus, the demand for the replacement of such adhesives is not only due to environmental concerns but also out of economic interest [2].

Lignin has a role as a binding agent in the biomass itself. Several studies have reported on the partial replacement of phenol in PF resin by several types of lignin, varying the proportions and incorporation of lignin with PF or other commercial resins in order to fabricate particleboards. The partial replacement of phenol in PF resin by organosolv lignin and modified organosolv lignin (phenolated-lignin) was tested [2]. The chemical modification of lignin is usually performed to increase the reactive sites and thus, their reactivity while undergoing condensation reactions. Many studies have been undertaken on the modification of lignin. Particleboards were produced using resins where chemically modified lignin (such as methylolated lignin, demethylated lignin and methylolated black liquor) partially replaced phenol in PF resins [3]. In certain studies, lignin replaced phenol both partially and completely in phenol-formaldehyde resins in the fabrication of fibreboards [7].

With respect to the incorporation of lignin with conventional resins, modified lignin (such as hydroxymethylated lignin and glyoxalated lignin) were incorporated as a resin component (diisocyanates and with/without PF resins) in various proportions and used for the fabrication of particleboards [8]. Lei H. et al. [9] prepared particleboards using different resin components, such as glyoxalated lignin, glyoxal, tannin, diisocyanates and PF resins. Stephanou and Pizzi [10] used various proportions of methylolated kraft lignin, diisocyanates and PF resins as binders for the preparation of particleboards.

Anglès et al. [11] used pre-treated sawdust material for the fabrication of panels with different types of lignin as the only binder, which varied from 5% to 20% on a dry solids basis. They also prepared binderless panels and compared the effect of adding lignin to the panels. Velasquez et al. [12] used kraft lignin at different proportions, mixed with steam-exploded pulp for the fabrication of particleboards. Privas and Navardused [13] pre-treated flax fibres for fabrication of panels using lignosulfonate as the only binder.

On the other hand, the use of thermosetting matrices as a binder in fibre-reinforced composites is increasingly common because of its mechanical performance and compatibility with different fabricating processes [14,15]. The thermosets have low viscosity compared to thermoplastics, which permits relatively better impregnation among the fibres. Epoxy resin is one of the most widely used thermosetting matrices. It is even used with plant fibres such as flax, hemp, bagasse and sisal [[16], [17], [18], [19], [20]].

In recent decades, the development of an alternative matrix to the conventional oil-based ones has gained interest. The context extends to reducing the CO₂ footprint and dependence on fossil fuels. Formaldehyde-free bio-sourced phenolic resins and bisphenol A-free epoxy resins are the outcomes of such an approach [21].

Several studies have focused on the use of long flax fibres as a reinforcement in bio-sourced composites but few have been interested in the valorisation of flax shives, which comprise the woody core generated as a by-product in the flax scutching process. These shives are not valorised as primary materials but for bio-fuel, animal bedding and mulch. These plant particles which are widely available in the north-west of France, have a porous structure and mechanical strength that promises to become a potential replacement for wood particles in light agglomerated panels.

In general, a wide variety of agro-resources such as bagasse [22], hazelnut and almond husk residues [5,23] and flax shives [24] have been combined with thermosetting based formaldehyde. However, there has been little mention of the use of lignosulfonate or epoxy as being the only binders for the fabrication of plant particleboards [25,26].

In this study, GreenPoxy 56 (a partially bio-sourced thermosetting resin, which has up to 56% of its molecular structure from plant origins) and calcium lignosulfonate are used as a matrix for particleboards. Furthermore, flax shives, an agricultural residue, are directly valorised as they were and with no pre-treatment. Hence, the fabricated particleboard is novel owing to such a combination of the material and the binder.

In the present study, because of environmental concerns and local resource development, a straightforward method for the fabrication of innovative flax shives based particleboards using the bio-based binder lignosulfonate (PLS) and a conventional, partially bio-based binder epoxy resin (PER) has been established and presented. The mechanical and thermal properties of fabricated PLS and PER particleboards are studied, as well as water absorption, thickness swelling and flame tests. An attempt to understand the relationship between fabrication, structure and properties of bio-based particleboards and interactions within them has been undertaken.