Got a glue? Replacing phenol with bark
Can bark be used as an eco-friendly replacement for phenol in wood composite adhesives? Ning Yan explains the results of a recent study.
Phenol-formaldehyde (PF) adhesives are the dominant resin for bonding in wood composites such as oriented strand board (OSB), medium density fibreboard (MDF) and exterior plywood. These thermosetting polymers have the advantages of good mechanical properties, high moisture resistance and stability. But with increasing global concerns about fossil fuel depletion and environmental footprint, there is more interest in exploring renewable resources as alternative feedstocks to replace phenol for the production of PF-based adhesives.
Bark is a renewable biomass material available in large quantities as waste residue from the conversion of wood logs to various forest products. It has a similar chemical composition to wood, although it contains more extractives and phenolic compounds. In forest product mills, bark is usually mixed with other woody residues and used as hog fuel for heat recovery. However, the heating value of bark is quite low and drops sharply when wet, so exploring other value-added uses for bark would be highly advantageous.
Previous studies have shown the potential of applying bark, bark extractives and bark components for phenolic resin synthesis. The adhesive application of bark from the mountain pine beetle-infested lodgepole pine was found to be highly promising. Both the liquefied bark and the bark extractives were found to be able to partly replace petroleum-based phenol for resin synthesis. The bondability of the resulting bark-derived PF resins was comparable to the commercial PF resins when 30wt% petroleum-based phenol was replaced by either the liquefied bark or the bark extractives.
In this study, bark from the sugar maple, Acer saccharum, was extracted with alkaline solution by using an autoclave. The acquired alkaline extractives were used as an alternative to partially replace phenol for reacting with formaldehyde under alkaline conditions to give bark extractive-based phenol formaldehyde resins. The resulting bark extractive PF resins were tested and compared with a lab PF resin and a commercial PF resin in terms of their curing characteristics and bonding performance.
Experimental bark extraction
The bark flakes from sugar maple were extracted by 1% NaOH solution in autoclave at 120°C for 30 minutes. Extraction was repeated twice and each extract was then passed through a Whatman paper filter. The resulting alkaline filtrates from the three-stage extractions were combined and the alkalinesoluble fraction was oven-dried at 60°C to constant weights. The resulting solid extractives were ground by mortar and pestle into powders prior to resin synthesis.
A calculated amount of bark extractives in powder form (neutralised and alkaline), phenol (crystal form), 37% formaldehyde, and 40% sodium hydroxide (one third of total NaOH weight) were mixed in a three-neck flask. The phenol replacement levels by the bark extractives were set as 30%, 50% and 70% by weight. The reaction temperature increased from room temperature to 65°C within 30 minutes and was kept at 65°C for 10 minutes in an oil bath, followed by the addition of the remaining two thirds of 40% NaOH. The reaction mixture was then heated to 85°C and kept there for 60 minutes. After the reaction, the reactor mass was cooled down to room temperature. The resulting bark extractive PF resins were viscous liquids with black colour. These bark extractive PF resins were subjected to various tests without further drying.
The laboratory-made PF resin (lab PF) without bark extractives was prepared by following exactly the same reaction steps used for the synthesis of bark extractive PF resins. A commercial PF resin for the face layers of oriented strandboard production was used for comparison.
Curing behaviour and kinetics
The curing curves of the bark extractive PF resins made with 30wt% replacement of phenol as measured by differential scanning calorimetric (DSC) are shown in Figure 1. Two exothermic peaks were observed for the bark extractive PF resins made at all phenol substitution levels. However, it was different from what had been reported for liquefied bark PF resins and the bark extractive PF resins from the mountain pine beetleinfested lodgepole pine. The first peak represented the addition reaction, the second peak was associated with the condensation reaction.
The specimens bonded with sugar maple bark extractive PF resins made with 30wt% and 50wt% replacement of phenol had dry shear strength values similar to those of specimens bonded with the commercial PF resin and higher than the lab PF resin. After the water soaking and drying (WSAD) treatment and the boiling water treatment wet test (BWT/W), no delamination occurred in any of the specimens. The bark extractive PF resins made with 30wt% replacement of phenol gave better lap shear strengths than the commercial and lab PF resins in the dry and WSAD tests and a similar lap shear strength in BWT/W test. At the 50wt% substitution level, the bark extractive-PF resins had similar bond strengths as the commercial PF resins in the dry and WSAD tests, but a lower BWT/W bond strength. When the weight replacement of phenol increased to 70%, the dry, WSAD and BWT/W shear strength values of the specimens decreased significantly. At 70wt% replacement of phenol, the bark extractive-PF resins gave lower dry and wet bond strength than the commercial PF resins. Even though the bond strength of the resins made with 70wt% replacement of phenol was still similar to that of the lab PF resin in dry and WSAD tests, it was much lower than that of the lab PF resin in BWT/W tests. Attempts were made to synthesise the BEPF resin with 100wt% replacement of phenol, but most of the bonded specimens were delaminated.
(Lab PF: Laboratory made PF resin, Com PF: Commercial PF resin, MEPF 30, MEPF 50 and MEPF 70: Bark extractive-PF resins made from sugar maple bark extractives with 30wt%, 50wt%, and 70wt% phenol substitution level, respectively)
The above results suggest that bark extractives from sugar maple could replace up to 50wt% of phenol to make bark extractive PF resins without significantly reducing the bond strength. Good wet bond strength of the bark extractive PF resins at 30wt% and 50wt% replacements of phenol could be attributed to the catechol moiety of the tannins in the bark extractives. The bond strength of the resins made from sugar maple bark extractives was different from what was observed for the bond strength of the resins made from mountain pine beetle-infested lodgepole pine bark extractives at the same phenol substitution level. The difference could be caused by the difference in structure and composition of the extractives. Bark extractives from sugar maple were found to be suitable for partly replacing phenol in synthesising biomass-based phenolic resins.
The author wishes to acknowledge the financial support from the Ontario Research Fund (ORF), and thank FP-Innovations for providing bark and commercial PF resin samples. Mr Nicolas Tanguy is also acknowledged for helping with the resin synthesis and characterisation.