Petri Widsten a & Andreas Kandelbauer bc a Scion, Rotorua, New Zealand. Reutlingen, Germany Published online: 14 Mar 2014.

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1 This article was downloaded by: [Scion (New Zealand Forest Research Institute)], [Petri Widsten] On: 18 March 2014, At: 18:36 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Adhesion Science and Technology Publication details, including instructions for authors and subscription information: Industrial scale evaluation of cationic tannin as a binder for hardboard Petri Widsten a & Andreas Kandelbauer bc a Scion, Rotorua, New Zealand b Wood Kplus, St. Veit an der Glan, Austria c Department of Applied Chemistry, Reutlingen University, Reutlingen, Germany Published online: 14 Mar To cite this article: Petri Widsten & Andreas Kandelbauer (2014): Industrial scale evaluation of cationic tannin as a binder for hardboard, Journal of Adhesion Science and Technology, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

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3 Journal of Adhesion Science and Technology, Industrial scale evaluation of cationic tannin as a binder for hardboard Petri Widsten a * and Andreas Kandelbauer b,c a Scion, Rotorua, New Zealand; b Wood Kplus, St. Veit an der Glan, Austria; c Department of Applied Chemistry, Reutlingen University, Reutlingen, Germany (Received 29 September 2013; final version received 9 February 2014; accepted 15 February 2014) Hardboards (HBs) (wet-process high-density fibreboards) were made in an industrial trial using a binder system consisting of cationic mimosa tannin and laccase or just cationic tannin without any thermosetting adhesive. The boards displayed superior mechanical strength compared to reference boards made with phenol formaldehyde, easily exceeding the European standards for general-purpose HBs. The thickness swell of most of the boards was slightly greater than the standards would allow, so some optimisation is required in this area. The improved board properties appear to be mainly associated with ionic interactions involving quaternary amino groups in cationic tannin and negatively charged wood fibres rather than to cross-linking of fibres via laccase-assisted formation and coupling of radicals in tannin and fibre lignin. Keywords: adhesion; binder; cationic; fibre; fibreboard; hardboard; laccase; tannin 1. Introduction Hardboards (HBs) are high-density ( 900 kg m 3 ) fibreboards made from wood fibres by a wet process, i.e. using water as the fibre distributing medium during mat forming. Although dry-process fibreboards, medium-density fibreboard (MDF, kg m 3 ) and high-density fibreboard (HDF, 880 kg m 3 ) are nowadays more commonly used than HBs, the latter have distinct advantages. First, dry-process fibreboards require large amounts of resin binder (8 12% of urea formaldehyde (UF) or other formaldehyde-based adhesive), whereas HBs are made with little (~1%) or sometimes with no added phenolic binder. Health concerns over the emissions of formaldehyde, a human carcinogen, from fibreboard-based products such as furniture and kitchen cabinets and during board production and machining have resulted in tightening limits for maximum allowable airborne formaldehyde levels for adhesively bonded wood products. Formaldehyde-free or ultra-low-emitting adhesives can be used instead of the high-emitting UF resins,[1,2] but at a significantly higher cost. Moreover, fibreboards, containing formaldehyde resins cannot be disposed of as normal household waste by burning or other means and must be treated as hazardous waste. Part of the phenol in synthetic adhesives such as phenol formaldehyde (PF) can be substituted for lignosulphonate, which is a lignin by-product of sulphite pulping, or flavonoid-based tannins extracted mainly from tree bark.[3] Enzymatic cross-linking of lignin and/or phenolic binders is another strategy; a recent review [4] covers the various *Corresponding author. petri.widsten@scionresearch.com 2014 Taylor & Francis

4 2 P. Widsten and A. Kandelbauer laboratory and a pilot scale methods based on the treatment of lignins with phenoloxidising enzymes such as laccase.[5] The disadvantages of HBs compared to MDF/ HDF are the large water consumption during production, the need for effluent water treatment and the higher quantities of fibre needed to make the boards (unlike MDF/ HDF, HBs do not have a density profile with the density decreasing towards the panel core but are of uniform density). To make HBs a more attractive option, the phenolic resin, which is still often required for the panels to meet the standard requirements [6] (EN 622-2) for mechanical strength and thickness swell (TS), should be replaced with a bio-based binder. This also reduces the overall human toxicity and fresh water aquatic ecotoxicity of the HB production process, to which PF makes a larger contribution according to a recent HB life-cycle assessment.[7] A recent pilot-scale trial at the HB plant where the present industrial trial was carried out showed that HBs meeting the EN requirements [6] for general-purpose HB with excellent mechanical properties could be made with tannic acid (a hydrolysable tannin based on polygalloyl esters of glucose) and laccase as the fibre cross-linking system without added synthetic binder.[8] Flavonoid-based condensed tannins such as mimosa tannin did not produce industrial standard HB, but it was of interest to find out whether cationic mimosa tannin (CT) would perform better. CT is used mainly as a flocculant in wastewater treatment.[9,10] Because of its cationic nature, imparted by quaternary amino groups, it adheres well to the negatively charged [11] wood pulp fibres. Its bulk price is not prohibitive (ca. 1.5/kg) compared to mimosa tannin which is produced in vast amounts at low cost for the leather tanning industry. The present investigation is concerned with an industrial evaluation of HB production using CT, with or without laccase, as the binder. 2. Materials and methods 2.1. Chemicals Cationic mimosa tannin (Tanfloc SG) and mimosa tannin (Weibull) powders were purchased from Christian D. Markmann GmbH, Hamburg, Germany. The tannin doses are on oven-dry fibre basis. The laccase was a liquid Novozym (Myceliophtora thermophila) preparation supplied by Novozymes, Nieder-Olm, Germany. Its activity was expressed in nanokatals (nkat) and determined spectrophotometrically against ABTS (2,2-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid)) at ph 4.5 and 30 C according to Niku-Paavola et al. [12]. The laccase dose was 60 nkat/g based on combined dry mass of fibre, and CT. Folin Ciocalteau reagent, anhydrous (+)-catechin and gallic acid were from Sigma Aldrich Total phenol content Total phenol content (TPC) of tannins was determined by the Folin Ciocalteu assay according to Kähkönen et al. [13]. Absorption was measured at 765 nm using a Cary 300 Bio UV visible spectrophotometer. A calibration curve (linear fit R 2 = ; mg/ml gallic acid; A765 = ) was constructed with gallic acid and the sample TPC (mean of three or more replicates) expressed as gallic acid equivalents (mg GAE/g) on dry extract weight basis. Anhydrous (+)-catechin was used to check the accuracy of the method (yielding 977 ± 40 mg/g GAE).

5 Journal of Adhesion Science and Technology HB raw material and preparation The HBs produced at this trial were made at an Austrian HB plant according to its normal production practice using a fibre furnish consisting of 22.2% European beech (Fagus sylvatica), 55.6% Norway spruce (Picea abies) and 22.2% debarked Norway spruce. However, when paraffin wax was used it was applied by spraying onto the fibre mats before pressing. Sieve water ph and temperature were 4.5 and 64 C, respectively. The boards (12 per experimental point) measured mm and had a density of ~1000 kg/m 3. The press conditions were: 1st step: 203 C, 220 bar, 65 s; 2nd step: 203 C, 50 bar, 210 s; 3rd step: 203 C, 200 bar, 40 s. Reference boards with the same method were produced with 1% PF and no wax HB testing The European Norm (EN) HB test methods used were: internal bond (IB) test: EN 319; modulus of rupture (MOR) and modulus of elasticity (MOE): EN 310; TS: EN Results and discussion Standard, general-purpose 3.5 mm HBs produced during regular industrial production at the plant where the trial was carried out have an IB strength of roughly 1.5 MPa. These boards, however, are made with 1% PF and 1% wax. As shown in (Figure 1), the HBs produced in the present trial with CT and without wax compared favourably with the standard boards: those made without and with laccase had 17 or 7% higher IB strength, respectively. Even the HBs made without binder easily meet the European norm EN (2006) for general-purpose 3.5-mm-thick HBs (0.5 MPa).[6] Cationic tannin can be produced by aminomethylation (Mannich reaction) using an aldehyde and an amine or with epoxy reagents. (Figure 2) shows the production method of cationic tannin from mimosa tannin and an epoxy reagent, and the likely main bonding mechanisms between wood fibres and cationic tannin in the presence of laccase. Wood pulp fibres carry a net negative charge due to the carboxyl groups found in hemicelluloses and lignin. The quaternary amino groups of CT are attracted to the negatively charged fibres, binding the fibres together via ionic interactions. In addition, IB strength, MPa No wax 1% wax 0.0 No binder 3.6% CT 2.9% CT + laccase Figure 1. IB strength of 3.5 mm HBs. Error bars indicate standard deviation.

6 4 P. Widsten and A. Kandelbauer Figure 2. Preparation of CT [9,10] and bonding mechanisms (red dotted lines) between wood fibres and tannin (ionic interaction and radical coupling after laccase-assisted oxidation). laccase catalyses the oxidation of phenolic hydroxyl groups of tannin and wood lignin to phenoxy radicals which may cross-link to give covalently linked fibres. However, the presence of laccase made no difference to IB, indicating that the main bonding mechanism is that between quaternary amino groups and negative charges on the fibre surface. It has been shown earlier that laccase with regular (non-cationised) condensed

7 Journal of Adhesion Science and Technology 5 tannin [8] or laccase alone [14 17] can significantly improve the mechanical properties of HB or MDF, respectively. Due to the introduction of ether groups to mimosa tannin during its manufacture, the phenol content of cationic tannin (TPC = 314 ± 10 GAE mg/ g GAE) was also significantly lower than that of mimosa tannin (TPC = 498 ± 13 mg/g GAE), which should reduce its reactivity toward laccase compared to mimosa tannin. This would be the case especially if the modification takes place on the B-ring; phenolic rings bearing only one phenolic hydroxyl and no methoxyl or other activating group and/or a large substituent ortho to the phenolic hydroxyl causing steric hindrance are not easily oxidised.[18] Coupling between tannin-based radicals could also be detrimental to fibre-to-fibre bonding by removing radicals that would otherwise couple with fibre lignin-based radicals. In the dry boards, interfibre attractive forces such as hydrogen bonds and hemicellulose condensation will also contribute to bonding.[19] Wax was clearly detrimental to fibre bonding, probably acting as a barrier between the opposite charges and phenoxy radicals which are able to form interfibre bonds. In the case of laccase, wax can also prevent the enzyme from oxidising phenolic groups in lignin and tannin. The mean IB value of the laccase-ct boards was lower than that of the CT-only boards, although the difference was not statistically significant. Based on the IB values, it therefore appears that there is no benefit in combining laccase with CT. This is in a marked contrast to prior pilot-scale tannic acid-bonded boards [8] where laccase was crucial to obtaining high IB strength and the bonding was assigned mainly to laccase-catalysed formation of phenoxy radicals in fibre lignin and tannic acid followed by their coupling to give interfibre bonds. Obviously, CT is able to bind fibres in a completely different way, although other bonding mechanisms contribute to adhesion as well.[19] Because of its cationic nature, imparted by quaternary amino groups, CT has a strong affinity to the negatively charged fibres, so that it is not washed away during the wet forming process of the fibre mat. This was a problem with the tannic acid during the pilot scale trial where the normal wet-process method was unable to be used. In terms of MOR (Figure 3), the HBs manufactured with CT and without wax were superior to the boards made without CT (19 21% improvement). These values are also higher than those of standard boards made with 1% PF (~51 MPa). The use of laccase had no significant effect on MOR. When wax was present, all three sets of boards had similar MOR values. Similar to IB, MOR was negatively impacted by wax. Despite MOR, MPa No wax 1% wax 10 0 No binder CT CT + laccase Figure 3. MOR of 3.5 mm HBs. Error bars indicate standard deviation.

8 6 P. Widsten and A. Kandelbauer Figure 4. MOE, MPa No binder CT CT + laccase MOE of 3.5 mm HBs. Error bars indicate standard deviation. No wax 1% wax this, all the boards clearly exceeded the EN [6] minimum MOR requirement for general-purpose 3.5 mm HBs (30 MPa). As for MOE, there is no official European standard for general-purpose HBs but all the boards had OE values well above the requirements for boards used in exterior applications (3600 MPa); (Figure 4). Based on the mean values, the best treatment in terms of MOE was CT/laccase without wax which improved MOE by 19% compared to the boards made without binder while CT alone gave a 10% improvement. However, this difference is not statistically significant. Wax reduced the MOE for all boards, having the greatest impact on the CT/laccase HBs. Wax did not reduce the TS of the boards (Figure 5) so there seems to be little point using wax as a surface treatment with CT because any hydrophobisation of fibres reducing water absorbance is probably offset by a decline in interfibre and intrafibre bonding that adds to it. Because of time constraints on this trial, no attempt was made to optimise the pressing conditions or to add the wax to the stock together with CT to reduce TS down to its industrial (EN 622-2) standard [6] (37%). This seems quite achievable as 25% of the boards made with CT or CT/laccase already met this target without any optimisation. In the pilot-scale study, [8] 2% wax added together with tannin reduced TS from 47 to 27% but also reduced the mechanical properties. Thickness swell, % No binder CT CT+laccase No wax 1% wax Figure 5. deviation. TS of 3.5 mm HBs after a 24-h cold water soak. Error bars indicate standard

9 Journal of Adhesion Science and Technology 7 An optimisation strategy for CT-bonded HB could include finding the exact minimum amount of wax that reduces TS to the desired level and avoids unnecessary deterioration of the mechanical properties. The CT dose should also be optimised bearing in mind that CT is roughly twice as expensive as PF resin, with the conversion of mimosa tannin to CT adding approximately 25% to the tannin price. Although HBs produced with CT are more expensive than standard HBs made with PF, the fact that they contain no thermosetting binder means that they have more recycling (repulping) and disposal options than the PF boards which must be treated as hazardous waste. Many eco-conscious consumers would be likely to prefer no-added formaldehyde and synthetic binder-free HB products even if they come with a moderate additional cost. However, although the CT-boards themselves are formaldehyde-free, the conversion of natural tannin into cationic tannin involves reagents that are not bio-based so CT cannot be considered as a fully bio-based product unless these chemicals are derived from renewable resources. The HBs made with CT/laccase had an extremely high gloss finish compared to those manufactured with CT only. However, since it had little effect on other properties, this may not be enough to justify the additional costs incurred on using laccase. 4. Conclusions Cationic tannin can be used as an adhesive to produce HBs of superior mechanical strength compared to boards made with 1% PF. There is no advantage in using laccase together with cationic tannin to increase interfibre bonding and most of the adhesive effect probably results from ionic interactions between tannin and fibres. The use of wax for improving dimensional stability on water soaking is counterproductive; it probably forms a barrier disrupting the ionic bonding and has no effect on TS. Some optimisation of treatment and pressing conditions is required for all the boards to comply with the TS standards. Acknowledgement The authors thank Carol Heathcote for her assistance in the experimental work. References [1] Harmon DM. In: Spear M, Loxton C, Ormondroyd G, Anthony R, Elias R, editors. Proc. 12th International Panel Products Symp. (IPPS); Espoo, 2008; p [2] Mullen JD. In: Spear M, Loxton C, Ormondroyd G, Anthony R, Elias R, editors. Proc. 12th International Panel Products Symp. (IPPS); Espoo, 2008; p [3] Pizzi A. Advanced wood adhesives technology. New York (NY): Marcel Dekker; [4] Widsten P, Kandelbauer A. Adhesion improvement of lignocellulosic products by enzymatic pre-treatment. Biotechnol. Adv. 2008;26:379. [5] Yaropolov AI, Skorobogatko OV, Vartanov SS, Varfolomeyev SD. Laccase: Properties, catalytic mechanism and applicability. Appl. Biochem. Biotechnol. 1994;49:257. [6] European norm EN 622-2, Fibreboards specifications part 2: requirements for hardboards [7] González-García S, Feijoo G, Widsten P, Kandelbauer A, Zikulnig-Rusch EM, Moreira MT. Environmental performance assessment of hardboard manufacture. Int. J. Life Cycle Assess. 2009;14:456. [8] Widsten P, Hummer A, Heathcote C, Kandelbauer A. A preliminary study of green production of fiberboard bonded with tannin and laccase in a wet process. Holzforschung. 2009;63:545.

10 8 P. Widsten and A. Kandelbauer [9] Bennison JJ, Clifford RP. United States patent US 4,964,952; [10] Pulkkinen E, Mikkonen H. In: Hemingway RW, Laks PE, editors. Plant polyphenols, basic life sciences. Vol. 59. Springer, 1992; p [11] Davison RW, Cates RE. Electrokinetic effects in papermaking systems: theory and practice. Paper Technol. Ind. 1975;16:107. [12] Niku-Paavola M-L, Karhunen E, Salola P, Raunio V. Ligninolytic enzymes of the white-rot fungus Phlebia radiata. Biochem. J. 1988;254:877. [13] Kähkönen MP, Hopia AI, Vuorela HJ, Rauha J-P, Pihlaja K, Kujala TS, Heinonen M. Antioxidant activity of plant extracts containing phenolic compounds. J. Agric. Food Chem. 1999;47:3954. [14] Kharazipour A, Huettermann A, Luedemann HD. Enzymatic activation of wood fibres as a means for the production of wood composites. J. Adhes. Sci. Technol. 1997;11:419. [15] Widsten P, Laine JE, Qvintus-Leino P, Tuominen S. Effect of high defibration temperature on the properties of medium-density fiberboard (MDF) made from laccase-treated hardwood fibers. J. Adhes. Sci. Technol. 2003;17:69. [16] Felby C, Thygesen LG, Sanadi A, Barsberg S. Native lignin for bonding of fiber boardsevaluation of bonding mechanisms in boards made from laccase-treated fibers of beech (Fagus sylvatica). Ind. Crops Prod. 2004;20:181. [17] Widsten P, Tuominen S, Qvintus-Leino P, Laine JE. The influence of high defibration temperature on the properties of fiberboard made from laccase-treated softwood fibers. Wood Sci. Technol. 2004;38:521. [18] Xu F. Oxidation of phenols, anilines, and benzenethiols by fungal laccases: correlation between activity and redox potentials as well as halide inhibition. Biochemistry. 1996;35:7608. [19] Back EL. The bonding mechanism in hardboard manufacture. Holzforschung. 1987;41:247.