Interest in natural “bio” adhesives has been growing to replace synthetic adhesives due to environmental and health concerns over the latter, particularly those based on formaldehyde.
There is a sense that adhesives trends may effectively turn full circle in the future as the first adhesives to be used were of a natural origin using the likes of soy protein, animal blood and casein.
Of course the onset of synthetic products brought many advantages such as ease of use, improved water resistance and efficiency and their continued use for many years ahead is not in doubt. But continual lobbying and regulation on formaldehyde levels and interest in environmental matters has created renewed interest in so-called bio-adhesives.
Wood-based panels giant Arauco is one of the companies giving close attention to this area as it seeks to fulfil the highest environmental and sustainable standards and maximise the value of its wood raw material streams.
The company has identified nanocellulose and tannins as having potential for the manufacture of adhesives in the wood-based panels industry.
Arauco’s research and development division Bioforest SA conducts research and develops and applies the latest technologies at the centre’s wood pulp and biotechnology labs and greenhouses, as well as the company’s forestland and industrial facilities.
Its research focuses on improving the panel production process, studying the physical-mechanical properties of products, searching for new adhesives and developing new products.
Bruno Gorrini, the panels R&D manager at Bioforest, attended the International Panel Products Symposium (IPPS) in Wales last year to share lab results demonstrating the potential of radiata pine tannin and nanocellulose to reinforce amino resin for wood-based panels. Research has been done with particleboard (PB), MDF and plywood.
Mr Gorrini said the challenge for green adhesives was achieving the same performance at similar or lower cost as synthetic adhesives.
Challenges to be overcome in using the alternatives include high viscosity of adhesives and its high-water content which can be limiting in industrial applications. Also, tannins are not yet available in sufficient quantities, although there is potential for production in South America.
Nanocellulose has enhanced properties and has several benefits in reinforcement in adhesives, including improvements in mechanical and physical properties of panels and a reduction in formaldehyde emissions compared to standard panels using synthetic adhesives.
Tannin-based Adhesives
In its trials, radiata pine barks from Arauco’s Horcones sawmill in Chile were used for tannin extraction, while cross-linking agents glyoxal and tris hydroxymethyl nitromethane were also added with wood particles (0.9- 2.7mm) from Arauco’s Teno mill to make particleboard and MDF samples.
Polyphenols from the bark were extracted using hot water, concentrated by evaporation and then dried to a brown powder with a moisture content of 10%.
Adhesives were prepared with either a 45% or 36% aqueous solution. The selected cross-linker was added and mixed at room temperature. Adhesive pH was modified using a 40% sodium hydroxide aqueous solution. In some formulations, citric acid was used as a stabilising agent.
Parameters to be evaluated, compared with UF resin were: gel time, viscosity, pH, mechanical cure and chemical cure.
The adhesive coated wood particles or fibres were cold pre-pressed, then compressed particles and fibre mats were hot-pressed in a Bürkle press. The board dimensions were 300x300x12mm.
For the PB testing, three resin contents (8%, 9% and 10%) of the tannin-based adhesive were tested for the core layer, while the surface layer was maintained at 12%. Target density was 640kg/m3.
For MDF, resin content of the tannin adhesive was maintained at 15%. Target density was 620kg/m3.
Gel time was found to be between 250sec to over 600sec compared to 92sec for commercial UF adhesive. In general, when solids content of tannin-based adhesives is over 40% high viscosity becomes a problem, so citric acid is added to allow a solids content increase from 36% to 45% but also lowering viscosity.
Testing showed all adhesives prepared had similar or higher maximum shear strength values than commercial UF resin. One formulation (highest viscosity) has a shear strength performance almost twice that of UF – this one had a higher enthalpy value and required a lower temperature for reaction.
Results showed that the bio-adhesives reached similar performance to commercial wood adhesives based on phenol and formaldehyde. Addition of citric acid promoted the reaction between tannins and cross-linkers, increasing solids content by lowering viscosity.
The internal bond (IB) strength of PB test boards satisfied European norm standards and were competitive compared to UF resin. Slight differences were found when comparing the IB values between the different resin contents of the core layer and surface layer, with best results found for boards with a core layer of 9% and surface layer of 12%.
As tannins do not melt they need water to be mobile so a good IB result depends on moisture content of the board at the time of hot pressing. Optimal moisture content of the glued particles was around 15% (compared to 8-10% of UF resins).
Best results for the MDF boards were with adhesives formulated with citric acid, with evidence that adding the latter might promote reaction between tannins and cross-linker, enhancing the IB value at the same temperature and pressing time
CNF Reinforced UF Adhesives
This involved eucalyptus bleached cellulose from Arauco in Chile, UF resins, aqueous ammonium sulphate (hardener) and paraffin wax emulsion. Cellulose nanofibres (CNF) were isolated from the pulp, swollen in distilled water and then dispersed via a high shear lab mixer. Further processes were used to produce homogenised CNF.
It is known that CNF addition increases water content and the viscosity. Cellulose fibres are built up by strong long filaments and after multiple shearing treatments it is possible to release individual nanofibres.
Reinforcement with homogenised cellulose fibres (H-CNF) has shown a positive effect at increasing PB board mechanical properties but the production of CNF through homogenisation requires high energy costs, which limits its application at industrial level, whereas grinding technology (G-CNF) allows for lower costs.
The addition of CNF in resins decreases over-penetration and agglomeration of adhesive in the particles, allowing use of smaller volumes of resin while obtaining similar mechanical properties as commercial UF resin formulations. Results of testing on boards reinforced with G-CNF showed test PB boards of 620kg/m3 could reduce resin consumption by up to 10%, while the reduction for 550kg/m3 boards was 10-15%.
Reinforcement of MDF with G-CNF evaluated under a 120sec pressing time and 16% resin content showed IB values were on average 9.5% higher for test boards compared to normal UF based boards, while test boards subject to a 180sec pressing time obtained a 7.7% higher value when using a 16% resin content. Compared with earlier research by others, the results may indicate that larger wood particles are best suited for UF reinforced with G-CNF as adhesive distribution may play an important role: when UF resin is mixed with G-CNF, the bond line increases by 10%, improving interactions contributing towards adhesive strength.
Thickness swelling was also lower for the test boards in both pressing times. This also confirms earlier research and may be due to the final moisture content of the boards increasing when adding G-CNF, therefore not allowing more water to be absorbed into the compressed fibres.
Conclusions
The conclusions show adhesives formulated based on tannin and GLY or TRIS crosslinkers reached similar performance to the commercial wood adhesives based on phenol and formaldehyde.
The addition of citric acid promoted reaction between tannins and cross-linkers, allowing an increase in the solids content by lowering viscosity values. IB strength of boards bonded with tannin-based adhesives satisfied the requirements of the European norm EN-319: 1993.
Using homogenised cellulose fibres (H-CNF) and ground cellulose nanofibres (G-CNF) to reinforce UF resin in particleboard allowed a 10-20% reduction in resin consumption, with the same adhesive strength improvement and also suggesting a reactivity increase.
In MDF, internal bond strength was increased when adding 1 wt% G-CNF to UF resin, with improvements in both pressing times of 120sec and 180sec. Final moisture content of the MDF boards was higher with the G-CNF addition and also helped to reduce thickness swelling.
At the IPPS conference Mr Gorrini was positive about the future potential of tannins and cellulose nanofibres.
“I think in the future that many companies will be using environmentally-friendly adhesives and it will make for a better environment,” he said.
Updating WBPI in February, 2020, Mr Gorrini said, to date, Bioforest’s tests on MDF and PB using formulations with radiata pine tannin without formaldehyde had been at laboratory scale.
“In the case of plywood, we have replaced 60% of phenol for tannin in the resin and we have run several industrial trials to improve the resin formulation,” he added.
“Now, we need to have enough tannin for the production of plywood without phenol.
“For that, we have developed a patent pending process to extract the tannin for the pine bark and recently we have built a pilot plant using this concept. We are now in the validation process of this concept in order to have well defined the process parameters for an industrial scale tannin extraction plant.”
