Abstract
The overall aim of the project was to systematically evaluate the suitability of softwood tannins for wood resin and hot melt applications including the development of adhesive formulations for both applications. Furthermore the aim is to improve the process economy. Therefore, optimizing the extraction step and improving the extraction yield of tannins and the stability of tannins in liquids were project objectives.
In the project, VTT Technical Research Centre of Finland Ltd. (coordinator), the Natural Resources Institute Finland (Luke) and the South-Eastern Finland University of Applied Sciences (Xamk) have evaluated technologies for an industrial production method of tannin from softwood bark and performed application tests.
The project delivered a realistic estimate of the yield for crude tannin in an industrial set-up. The yield of solubilized material in pilot extractions were up to 14 w% from finely ground spruce bark in a batch process in VTT’s pilot extraction, 10 w% in a batch process with flow through in Luke’s pilot extraction and up to 7 w% in the newly developed continuous extraction using 8% consistency at Xamk. The lower yield compared to up to 20% in the literature, can be partly explained by the fact that bark stream in pulp and saw mills contain substantial amounts of wood. In project samples from a saw mill, 20 % and 30% wood content were estimated for bark from spruce and bine bark, respectively. The yield of crude tannin from pine bark remained lower in pilot extraction trials with 7.8 w% yield. In agreement with earlier findings, the most potential crude tannin extracts from spruce bark contained ~45% phenolic compounds, ~40% carbohydrates and ~5% ash for batch extractions at 90 or 110 °C.
An approach to produce a purified tannin product enriched in phenolic compounds was developed. The performed steps were liberation of monosaccharides from the crude tannin by enzymatic hydrolysis of carbohydrates followed by nanofiltration in order to remove the liberated monosaccharides. In total, 50% of carbohydrates were liberated with an enzyme dosage of 10 mg/g DM. The fractionation with a nanofiltration membrane enabled 9-fold concentration of the enzymatically treated crude spruce bark tannin in pilot scale with successful removal of the liberated carbohydrates, resulting in enriched phenolic fraction. However, also the permeate contained some phenols. Thus, both monosaccharides and low molecular weight phenols can pass through NFG membrane, enlarging the average molecular weight of the fraction.
In a shelf life study, it was observed that aqueous crude tannin products can be stored at +4°C for a period of 7 months preserving the reactivity against formaldehyde and without notable microbiological growth.
The tannins from softwood bark were tested as phenol replacement in phenol-formaldehyde resins (thermosets) for plywood gluing and at a replacement level of 30%, crude pine tannin, the resins showed comparable adhesive bond strength as commercial wood resins in laboratory testing. Even 50% replacement level gave reasonable bond strength. Future work should be directed towards formulation to ensure water resistance. It could be demonstrated that a mixture of tannin and poly(DL-lactide-co-ε-caprolactone) is a potential hot melt adhesive. The copolymer can be used as a hot melt adhesive by itself, but the processability and adhesion towards uncoated boxboard increased with tannins.
In the project, VTT Technical Research Centre of Finland Ltd. (coordinator), the Natural Resources Institute Finland (Luke) and the South-Eastern Finland University of Applied Sciences (Xamk) have evaluated technologies for an industrial production method of tannin from softwood bark and performed application tests.
The project delivered a realistic estimate of the yield for crude tannin in an industrial set-up. The yield of solubilized material in pilot extractions were up to 14 w% from finely ground spruce bark in a batch process in VTT’s pilot extraction, 10 w% in a batch process with flow through in Luke’s pilot extraction and up to 7 w% in the newly developed continuous extraction using 8% consistency at Xamk. The lower yield compared to up to 20% in the literature, can be partly explained by the fact that bark stream in pulp and saw mills contain substantial amounts of wood. In project samples from a saw mill, 20 % and 30% wood content were estimated for bark from spruce and bine bark, respectively. The yield of crude tannin from pine bark remained lower in pilot extraction trials with 7.8 w% yield. In agreement with earlier findings, the most potential crude tannin extracts from spruce bark contained ~45% phenolic compounds, ~40% carbohydrates and ~5% ash for batch extractions at 90 or 110 °C.
An approach to produce a purified tannin product enriched in phenolic compounds was developed. The performed steps were liberation of monosaccharides from the crude tannin by enzymatic hydrolysis of carbohydrates followed by nanofiltration in order to remove the liberated monosaccharides. In total, 50% of carbohydrates were liberated with an enzyme dosage of 10 mg/g DM. The fractionation with a nanofiltration membrane enabled 9-fold concentration of the enzymatically treated crude spruce bark tannin in pilot scale with successful removal of the liberated carbohydrates, resulting in enriched phenolic fraction. However, also the permeate contained some phenols. Thus, both monosaccharides and low molecular weight phenols can pass through NFG membrane, enlarging the average molecular weight of the fraction.
In a shelf life study, it was observed that aqueous crude tannin products can be stored at +4°C for a period of 7 months preserving the reactivity against formaldehyde and without notable microbiological growth.
The tannins from softwood bark were tested as phenol replacement in phenol-formaldehyde resins (thermosets) for plywood gluing and at a replacement level of 30%, crude pine tannin, the resins showed comparable adhesive bond strength as commercial wood resins in laboratory testing. Even 50% replacement level gave reasonable bond strength. Future work should be directed towards formulation to ensure water resistance. It could be demonstrated that a mixture of tannin and poly(DL-lactide-co-ε-caprolactone) is a potential hot melt adhesive. The copolymer can be used as a hot melt adhesive by itself, but the processability and adhesion towards uncoated boxboard increased with tannins.
| Original language | English |
|---|---|
| Publisher | SusBinders project |
| Number of pages | 78 |
| Publication status | Published - 2020 |
| MoE publication type | D4 Published development or research report or study |