Venla Hemmilä Linnaeus University Dissertations No 363/2019

Venla Hemmilä − Formaldehyde emission test methods and adhesives from biorefinery − Formaldehyde biorefinery emission test methods and adhesives from Towards and sustainable low-emitting particle- and fibreboards Towards low-emitting and sustainable particle- and fibreboards − Formaldehyde emission test methods and adhesives from biorefinery

Lnu.se isbn: 978-91-88898-97-5 (print), 978-91-88898-98-2 (pdf )

linnaeus university press Towards low-emitting and sustainable particle- and fibreboards

− Formaldehyde emission test methods and adhesives from biorefinery lignins

Linnaeus University Dissertations

No 363/2019

TOWARDS LOW-EMITTING AND

SUSTAINABLE PARTICLE-

AND FIBREBOARDS − Formaldehyde emission test methods and adhesives from biorefinery lignins

VENLA HEMMILÄ

LINNAEUS UNIVERSITY PRESS

Towards low-emitting and sustainable particle- and fibreboards − Formaldehyde emission test methods and adhesives from biorefinery lignins Doctoral Dissertation, Forestry and Wood Technology, Linnaeus University, Växjö, 2019

ISBN: 978-91-88898-97-5 (print), 978-91-88898-98-2 (pdf) Published by: Linnaeus University Press, 351 95 Växjö Printed by: Holmbergs, 2019

For Leo and Linus

Abstract

High volumes, fast production speed, and low material costs have been historically the driving factors of the particle- and fibreboard industries. However, in recent years the fossil-fuel dependency and health issues of the formaldehyde-containing adhesives used in the production have gained attention from both legislators and consumers. The latest example of legislation development is the change that the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany (Bundesministerium für Umwelt, Naturschutz und Nukleare Sicherheit) made to their testing method, effectively lowering the formaldehyde emission levels of wood-based panels in Germany from the European emission level of 0.1 ppm (E1, EN 717-1) to 0.05 ppm. As the emission levels of requirements decrease, market opportunities arise for formaldehyde-free bio-based adhesive systems. The aim of this thesis was thus to evaluate the different formaldehyde test methods at low emission levels (<0.05 ppm), and to explore new adhesive alternatives to the formaldehyde and petroleum-based systems used today. As formaldehyde emissions decrease, choosing the right measurement method becomes increasingly important. Repeatability and correlation between the main European and American formaldehyde measurement chambers, described in EN 717-1 and ASTM D 6007 standards respectively, were determined. In addition, an alternative fast factory method based on emissions was evaluated, and the effect of reducing the conditioning time before emission measurements was investigated. A literature research was conducted on different bio-based raw materials in order to review their potential, from both scientific and industrial viewpoints, as alternatives to the current petroleum- derived and formaldehyde-based adhesives. residues from biorefinery processes were chosen for further testing due to their increasing volumes and potential to suit various pathways for adhesive making. Three different biorefinery lignins were compared, and ammonium lignosulfonate was chosen for making adhesives for particleboards by using one petroleum-based and one bio-based crosslinker. The main conclusion of the formaldehyde emission part of the thesis was that formaldehyde emissions can be measured both accurately and quickly at low levels using chamber methods, even at factory environment. There was a good correlation between the American D 6007 and European EN 717-1 chamber methods at emission levels <0.05 ppm for both particleboards (r2 = 0.9167) and fibreboards (r2 = 0.9443). Further understanding on the effect of edge-sealing of boards and analytical methods described in the standards was obtained. It was confirmed that a fast chamber method with 1 day conditioning and 15 minutes measuring time could be used for factory formaldehyde control for most board types.

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The bio-based adhesives’ literature review revealed a large amount of studies on different sustainable adhesive systems, some of which seem promising. Both soy protein and tannin were found to be partially commercialized, with certain pre-requisites. Kraft-lignin was especially well researched, but was found to be difficult to use for other applications than partial replacement of phenol in phenol-formaldehyde (PF) adhesives due to poor water solubility and purity. Lignin residues from biorefinery processes were found to be a less studied, growing raw-material source with a lot of potential. Thus, supercritical water hydrolysis lignin (SCWH) and two biorefinery were chemically and thermally characterized, and evaluated as raw materials for value-added applications, including adhesives. SCWH lignin was found to have more β-R linkages and lower amount of impurities than the lignosulfonates. High amount of phenolic hydroxyl groups indicated that SCWH would be well suited for phenol replacement in PF adhesives. The two lignosulfonates had more aliphatic hydroxyl groups, which can be interesting for other crosslinking reactions than PF. Ammonium lignosulfonate (ALS) was chosen for further evaluation as having slightly better properties than sodium lignosulfonate (SLS). ALS was combined with one bio-based crosslinker, furfuryl alcohol (FOH), and one synthetic crosslinker, 4,4’-diphenylmethane diisocyanate (pMDI), and tested as particleboard adhesive. Although in veneer tensile shear strength testing the crosslinkers worked equally well, pMDI provided significantly better results in particleboards. In addition, higher emissions than what can be expected from wood particles alone were detected from the particleboard samples crosslinked with FOH, even though FOH can be classified as non-formaldehyde added adhesive system. Further research is needed to elucidate how much the lignin contributes to the final adhesion strength when it is used together with pMDI. This thesis has provided new insights on formaldehyde emissions and bio- based adhesives towards healthier and more sustainable particle- and fibreboards. It has been proven that formaldehyde emissions can be measured accurately at emission levels of wood, enabling comparisons of formaldehyde- free systems. Formaldehyde-free adhesives based on a biorefinery lignin type and pMDI showed promising results for particleboards. However, these results need to be improved by different modifications of the lignin in order to bring the adhesive system to the economical and performance level required by the particleboard industry.

Keywords: Wood board industry, chamber methods, dynamic microchamber (DMC), bio-based adhesives, lignosulfonates, supercritical water hydrolyzed lignin, crosslinkers, fibreboards, particleboards

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Sammanfattning (in Swedish)

Stora volymer, hög produktionshastighet och låga materialkostnader har historiskt varit drivande faktorer för spån- och fiberskivindustrin. På senare tid har dock beroendet av fossila bränslen samt de hälsoproblem associerad med formaldehydinnehållande lim fått alltmer uppmärksamhet från både lagstiftare och konsumenter. Det senaste exemplet på lagstiftningsutveckling är den förändring som det Federala Ministeriet för Miljö, Naturskydd och Kärnsäkerhet i Tyskland (Bundesministerium für Umwelt, Naturschutz und Nukleare Sicherheit) har gjort i sin testmetod, vilket effektivt sänker emissioner av formaldehyd från träbaserade paneler i Tyskland från den europeiska emissionsnivån på 0.1 ppm (E1, EN 717-1) till 0.05 ppm. I takt med att de godkända emissionsnivåerna sänks, uppstår marknadsmöjligheter för formaldehydfria biobaserade limsystem. Syftet med denna avhandling var att utvärdera noggrannheten hos mätmetoder för formaldehyd vid låga emissionsnivåer (<0,05 ppm) samt att utforska nya limalternativ till de formaldehyd- och petroleumbaserade system som används idag. När emissionerna av formaldehyd minskar, blir det allt viktigare att välja rätt mätmetod. Repeterbarhet och korrelation mellan de viktigaste europeiska och amerikanska formaldehyd-mätningskamrarna, beskrivna i EN 717-1 respektive ASTM D 6007 standarder, bestämdes. Dessutom utvärderades en alternativ snabb emission mätmetod för produktion samt effekten av att reducera konditioneringstiden innan mätning. En litteraturundersökning genomfördes på olika biobaserade råvaror för att granska deras potential, både ur akademiska och industriella synvinklar, som alternativ till nuvarande petroleum- och formaldehydbaserade limmen. Ligninrester från bioraffinaderiprocesser valdes för vidare testning på grund av deras ökande volymer och potential att passa olika typer av limframställning. Tre olika bioraffinaderi ligniner jämfördes och ammonium lignosulfonat valdes för framställning av lim för spånskivor med användning av en petroleumbaserad och en biobaserad tvärbindare. Den huvudsakliga slutsatsen var att formaldehydemissioner kan mätas både noggrant och snabbt på låga nivåer med hjälp av kammarmetoder, även i fabriksmiljö. Det fanns en god korrelation mellan de amerikanska ASTM D 6007 och europeiska EN 717-1, kammarmetoderna vid emissionsnivåer <0,05 ppm för både spånskivor (r2 = 0,9167) och fiberskivor (r2 = 0,9443). Dessutom erhölls ytterligare förståelse för effekten av kanttätning av träpaneler och analysmetoder beskrivna i standarderna. Vidare bekräftades att en snabbkammarmetod med 1 dags konditionering och 15 minuters mättid kunde användas som produktionskontroll av formaldehyd för de flesta träpaneltyper. Det biobaserade limmens litteraturöversikt avslöjade en stor mängd studier om olika hållbara limsystem, av vilka några verkar lovande. Både sojaprotein

iii och tannin visade sig vara delvis kommersialiserade, med vissa förutsättningar. Kraft lignin undersöktes särskilt noggrant, men visade sig vara svårt att använda för andra tillämpningar än partiell ersättning av fenol i fenol-formaldehyd (PF) lim på grund av dålig vattenlöslighet och renhet. Ligninrester från bioraffinaderiprocesser visade sig vara en mindre studerad, växande råmaterialkälla med mycket potential. Således karakteriserades superkritiskt vattenhydrolys lignin (SCWH) och två bioraffinaderi lignosulfonater kemiskt och termiskt, och utvärderades som råmaterial för mervärde applikationer, inklusive lim. SCWH-lignin visade sig ha fler β-R-bindningar och lägre mängd föroreningar än lignosulfonaterna. Hög mängd fenoliska hydroxylgrupper indikerade att SCWH skulle vara väl lämpat för ersättning av fenol i PF-lim. De två lignosulfonaterna hade mer alifatiska hydroxylgrupper, vilket kan vara intressant för andra tvärbindningsreaktioner än PF. Ammonium lignosulfonat (ALS) valdes för vidare utvärdering eftersom den hade något bättre egenskaper än natrium lignosulfonat (SLS). ALS kombinerades med en biobaserad tvärbindare, furfurylalkohol (FOH) och en syntetisk tvärbindare, 4,4'- difenylmetandiisocyanat (pMDI), och testades som spånskivlim. De båda tvärbindarna fungerade lika bra vid draghållfasthetstest av fanér, men pMDI gav betydligt bättre resultat i spånskivor. Dessutom detekterades högre emissioner än vad som kan förväntas från enbart träpartiklar från spånskivsproverna tvärbundna med FOH, även om FOH kan klassificeras som ett limsystem med icke tillsatt formaldehyd. Ytterligare forskning behövs för att klargöra hur mycket ligninet bidrar till den slutliga vidhäftningsstyrkan när det används tillsammans med pMDI. Denna avhandling har gett ny insikt om formaldehyd emissioner och biobaserade lim för mer hälsosamma och miljömässigt hållbara spån- och fiberskivor. Det har också visats att emissioner av formaldehyd kan mätas noggrant vid de nivåer som trä har, vilket möjliggör jämförelser av formaldehydfria system. Formaldehydfria lim baserade på en bioraffinerad lignintyp och pMDI visade lovande resultat för spånskivor. Dessa resultat måste emellertid förbättras genom olika modifieringar av ligninet för att ta limsystemet till den nivå av ekonomi och prestanda som krävs av spånskivebranschen.

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Acknowledgements

The present thesis consists of results and work done at Linnaeus University 2013-2019 within the project “New environment-friendly board materials”, co- financed by the Knowledge Foundation, Linnaeus University, IKEA Industry AB and IKEA of Sweden. Since 2014, I have been employed by IKEA Industry AB, with partial time for studies. There are so many people at IKEA, LnU, and around me that deserve an acknowledgement for helping me these last years, but I can only mention a few. I wish to express my gratitude to my supervisor Stergios Adamopoulos. He became the “supervisor-who-stayed”, formed a research group, and built a lab for my studies. I thank Åsa Blom, IoS GTF, the LnU GoFP, Hultsfred colleagues, and my co-authors for the invaluable help during my studies. And Jonaz Nilsson and Charlotta Parsland for all the practical help you have provided. Not forgetting Josefin, Tinh, Elaheh and the other “M-hus girls”! And Diana, for being so much more than “statistically significant” to my studies. This work could not have been accomplished without the support of my boss Martin Strand. He and my colleagues at PDC Älmhult never stopped joking about never seeing me around, while always making me feel that it was fine. I could not wish for better colleagues or work place. The two people who have always believed in me are my parents Soili and Ari. Who I am today, I can credit to you. Huge thanks to you, and to my dear brother Ville, who has been my greatest inspiration. I hope you are not too disappointed for the lack of a sword. I want to thank my extended family in Sweden, Anneli and Kent, for being there for me, and for keeping me fed during the intense study periods. The greatest thanks of all belong to my family, Leo and Linus, for giving me energy and for keeping me grounded. For the countless of times you have helped me to prioritize my activities and to keep things in perspective. For giving me a hug or a cola just when it was needed. For always being there for me. This thesis would not exist without you.

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Appended

This doctoral thesis is based on the following appended papers, which in the text are referred to by their Roman numbers.

I. HEMMILÄ, V.; ADAMOPOULOS, S.; KARLSSON, O.; KUMAR, A. 2017. Development of sustainable bio-adhesives for engineered wood panels: a review. RSC Advances, vol. 7, no. 61, pp. 38604–38630. http://pubs.rsc.org/en/content/articlepdf/2017/ra/c7ra06598a

II. HEMMILÄ, V.; MEYER, B.; LARSEN, A., SCHWAB, H.; ADAMOPOULOS, A. 2019. Influencing factors, repeatability and correlation of chamber methods in measuring formaldehyde emissions from fiber- and particleboards. International Journal of Adhesion and Adhesives, vol. 94, 102420. https://doi.org/10.1016/j.ijadhadh.2019. 102420

III. HEMMILÄ, V.; ZABKA, M.; ADAMOPOULOS, S. 2018. Evaluation of dynamic microchamber as a quick factory formaldehyde emission control method for industrial particleboards. Advances in Materials Science and Engineering, vol. 2018, Article ID 4582383. https://doi.org/10.1155/2018/4582383

IV. HEMMILÄ, V.; HOSSEINPOURPIA, R.; ADAMOPOULOS, S.; ECEIZA, A. 2019. Characterization of wood-based industrial biorefinery lignosulfonates and supercritical water hydrolysis lignin for value-added applications. Under review in Waste and Biomass Valorization

V. HEMMILÄ, V.; ADAMOPOULOS, S.; HOSSEINPOURPIA, R. AHMED, S.A. 2019. Ammonium lignosulfonate adhesives for particleboards with pMDI and furfuryl alcohol as crosslinkers. Accepted for publication in Polymers

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Contribution to the appended papers I. Hemmilä and Adamopoulos conceptualized and designed the structure of the review. Hemmilä conducted the literature review and wrote the first version of the manuscript. Adamopoulos wrote and added parts in the first version and Karlsson revised the manuscript. Kumar, Adamopoulos and Hemmilä did the final revisions and corrections, and Kumar submitted the paper.

Paper II. Hemmilä, Meyer and Larsen conceptualized and designed the study. Meyer and Schwab acquired the data. Hemmilä and Meyer analyzed and interpreted the data. Hemmilä wrote the first draft of the manuscript under supervision of Meyer and Larsen. Adamopoulos revised the manuscript and Hemmilä submitted it.

Paper III. Hemmilä and Zabka conceptualized and designed the study. Zabka acquired the data. Hemmilä analyzed and interpreted the data and wrote the manuscript under supervision of Adamopoulos.

Paper IV. Hemmilä and Adamopoulos conceptualized and designed the study. Hosseinpourpia, Hemmilä and Eceiza performed the characterizations. Hemmilä analyzed all measurements, and wrote the first draft of the manuscript. Hosseinpourpia, Adamopoulos and Eceiza revised the manuscript and Hemmilä submitted it.

Paper V. Hemmilä and Adamopoulos conceptualized and designed the study. Hosseinpourpia and Hemmilä performed the chemical analysis and made the particleboards. Ahmed assisted in the microscopy measurements. Hemmilä measured the particleboard properties, analysed all results and wrote the first draft of the manuscript. Adamopoulos revised the manuscript and Hemmilä submitted it.

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List of abbreviations

Symbol Description ACAC Acetyl acetone ALS Ammonium lignosulfonate ASTM ASTM International (originally: American Society for Testing and Materials) CARB Californian Air Resource Board 13C NMR Carbon-13 nuclear magnetic resonance DNPH 2,4-dinitrophenylhydrazine DMC Dynamic microchamber DSC Differential scanning calorimetry EN European Standard EPA Environmental Protection Agency FOH Furfuryl alcohol FTIR Fourier Transform Infrared Spectroscopy GPC Gel permeation chromatography HSD Tukey’s honestly significant difference IB Internal bond ISO International Organization for Standardization MBTH 3-methyl-2-benzothiazolonehydrazone MDF Medium density fibreboard MOE Modulus of elasticity MOR Modulus of rupture OSB Oriented strand board PAE Polyamidoamine-epichlorohydrin PB Particleboard PEI Polyethyleneimine PF Phenol-formaldehyde pMDI Polymeric 4,4’-diphenylmethane diisocyanate ppm Parts per million PVAc Polyvinyl acetate SCWH Supercritical water hydrolysis lignin SLS Sodium lignosulfonate TGA Thermogravimetric analysis TRIS Tris(hydroxylmethyl)nitromethane UF Urea-formaldehyde () UmF Melamine reinforced urea-formaldehyde (resin)

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Content

Abstract ...... i Sammanfattning (in Swedish) ...... iii Acknowledgements ...... v Appended papers ...... vi List of abbreviations ...... viii Content ...... ix 1. Introduction ...... 1 1.1 Context ...... 1 1.2 Background ...... 2 1.2.1 Particle- and fibreboards ...... 2 1.2.2 Wood-based panel adhesives ...... 4 1.2.3 Formaldehyde ...... 5 1.2.4 Formaldehyde in wood-based panels ...... 5 1.2.5 Formaldehyde limits ...... 7 1.2.6 Measurement of formaldehyde from wood-based panels...... 8 1.2.7 Bio-based raw materials for adhesives ...... 10 1.2.8 Crosslinkers for bio-based raw materials ...... 12 1.2.9 Lignin extraction methods and structure ...... 15 1.2.10 Lignin applications ...... 18 1.2.11 Research questions ...... 19 1.3 Aim and objectives...... 20 1.4 Overview of the appended papers ...... 21 1.5 Limitations ...... 24 2. Materials and methods ...... 25 2.1 Review on bio-based adhesives (Paper I) ...... 25 2.2 Industrial boards (Papers II and III) ...... 25 2.3 Formaldehyde measurements (Papers II, III, and V) ...... 26 2.4 WKI-formula (Paper II) ...... 28 2.5 Lignin types and their characterisation (Papers IV and V) ...... 28 2.6 Adhesive formulations and evaluation of bonding (Paper V) ...... 28 2.7 Particleboard manufacturing and testing (Paper V) ...... 30 2.8 Statistical analysis (Papers II, III and V) ...... 30 3. Results and discussion ...... 31 3.1 Formaldehyde emissions ...... 31 3.1.1 Evaluation of chamber methods at low emission levels ...... 31

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3.1.2 Correlations between test methods at low emission levels ..... 32 3.1.3 Factors influencing emissions: edge-sealing, analytical method and conditioning time ...... 34 3.1.4 Reduction of conditioning time before emission measurements ...... 38 3.2 Bio-based raw-materials ...... 39 3.2.1 Chemical structure and thermal properties of the biorefinery lignins ...... 41 3.2.2 Biorefinery lignins for adhesive applications...... 43 3.2.3 Crosslinkers for lignins ...... 43 3.2.4 Curing of ALS with pMDI and FOH crosslinkers ...... 44 3.2.5 Bonding strength of ALS adhesives and particleboard properties ...... 45 3.2.6 Formaldehyde emissions of the ALS-based adhesives compared to wood emissions ...... 47 4. Conclusions ...... 50 5. Future work ...... 52 6. References ...... 53 Appended papers I-V

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Introduction

1. Introduction

1.1 Context Development in the wood-based panel industry has been driven by the need for cheaper, faster-curing and more specialized products. As the properties of boards are closely related to the adhesive used in bonding them, these requirements have led to formaldehyde-based systems dominating the market. Formaldehyde, with urea, melamine, phenol, resorcinol, or combination thereof, can create fast curing and cheap systems that are highly flexible and adaptable to different needs depending on resin formulation or “cooking” procedures (Dunky, 2003). In recent years, two other needs have arisen for wood-based panels: low formaldehyde emissions and sustainability. Formaldehyde has been known to be one of the primary indoor air pollutants since the mid-1980s. Particle- and fibreboards used in furniture and in construction elements are one of the main permanent sources of formaldehyde to indoor air (Salthammer et al., 2010). The lower air exchange rates of modern housing enable even the low emissions of wood-based panels to build up over time. Formaldehyde testing methods are a current topic with the U.S. Environmental Protection Agency (EPA) adapting the California Air Resource Board (CARB) emission limits in 2017, and the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany changing their legislation in 2018 and introducing the EN 16516 chamber method for both wood-based furniture and construction boards. The German legislation is also the latest example of lowering legal limits for formaldehyde emissions, effectively lowering the European emission level (E1) measured with EN 717-1 from 0.1 ppm to 0.05 ppm in Germany as from 1st of January 2020. This has generated the need for more research on the accuracy of different test methods at such very low emission levels. An additional issue is the quality control methods used at factories in Europe and Asia, which are based on formaldehyde content. Content methods, such as perforator according to ISO 12460-5, are not accurate at low emission levels (Mayer, 1978; Yu & Crump, 1999), and thus have poor correlation to the

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Introduction chamber methods, which are used to define the limits in most legal requirements. This is a major risk to the wood-based panel industry today. The content methods are used due to the low cost of the required equipment and relatively fast testing speed. Using the perforator method, results can be obtained in eight hours, while the relatively fast chamber test method, American small chamber ASTM D 6007, takes seven days to obtain the final result. Lower formaldehyde emissions of boards can be achieved by modifying the urea-formaldehyde with phenol or melamine. Using formaldehyde capturing scavengers, such are urea, organic amines, sulphites, and bio- polymers can also lead to lower emissions, however, they also often have negative effect on the board properties (Carvalho et al., 2012). Another alternative is to use bio-based formaldehyde-free adhesives. Bio-based raw materials provide low emitting and more sustainable alternatives to the fossil- based adhesives used today by the industry. The most researched bio-based raw materials for adhesives are natural phenolics (tannins and lignins), carbohydrates (starches, gums), proteins (soy, wheat, clam) and vegetable oils. The bio-based adhesives struggle to compete with the formaldehyde-based synthetic resins when it comes to reaction speed, price and water tolerance (Solt et al., 2019). Most bio-adhesives have high amount of hydroxyl groups (-OH) that are subjectable to hydrolysis. Specific, and often expensive, crosslinkers such as isocyanates are required for the bio-adhesives to create strong and water-resistant bonds (Solt et al., 2019). Because of this, the low price levels of synthetic resins cannot, as of yet, be reached. However, the gap is expected to close since more expensive synthetic resins will be required to meet the new lower formaldehyde emission levels. This gives bio-based adhesives an opportunity to enter the market and to increase their volumes. Volume increase can lead to competitive price levels compared to formaldehyde-based adhesives but improved performance is still required.

1.2 Background 1.2.1 Particle- and fibreboards One of the largest wood markets is manufacturing of wood-based panels. In wood-based panel production, adhesive is combined with particles or fibres to create a wood-adhesive matrix. When compared with timber products, the wood-based panels have increased dimensional stability and have the possibility of using even the smallest residues from sawmills giving them an economical advantage. Boards are also more resistant towards insect attacks, and have potential for designing engineered products with properties that suit special end- uses, such as low thermal conductivity, fire-resistance or better bio-resistance (Irle et al., 2010).

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Introduction

The focus of this doctoral thesis is particle- and fibreboard industries while the term “emissions” is specific to formaldehyde they emit. However, the studied bio-adhesives are also applicable for other wood-based panel products such as plywood and oriented strandboard (OSB). Particleboard is a generic term for any board produced from wood particles using adhesives, and then pressing them at high pressure and temperature. This thermomechanical processing influences the final properties of particleboards. Almost all physical and mechanical properties of particleboards are related to their density (Rowell, 2005). A typical particleboard has a density of 600 to 650 kg/m3. There is an increasing interest in lower density particleboards in order to save material and to produce lightweight furniture. For this purpose, the boards need to be engineered to reach the desired strength properties, as an example by having non-homogeneous density distribution. The strength requirements of any particleboard depend on its application. Particles can be obtained from logs, residues from sawmills, or recycled material. Particles are sorted by size, dried, and resinated before forming and pressing. Typical particleboard consists of three layers, two fine surface layers and a core layer with longer thin chips in the middle. Fine particles increase the surface quality and internal bond, while the larger slender particles in the middle increase the other board properties. Having different layers also enables the usage of different adhesive systems in the core and surface of the board to create specific properties of the particleboard. Fibreboards, such as medium- (MDF) and high-density fibreboard (HDF), are homogeneous, one-layer boards. They are composed of fine lignocellulosic fibres, and combined with an adhesive under heat and pressure, similar to particleboards. The density of MDF varies between 500-900 kg/m3 (Rowell, 2005). Fibreboard production consists of four steps: chip production, fibre production by steaming and refining, blowline resination, drying, and sheet forming. Unlike in particleboard production, fibreboards cannot use recycled materials or sawmill residues. Instead, logs are debarked and chipped into suitable sizes for refining. In this process, the chips are heated and fed through the that uses centrifugal forces to separate the fibres. The resulting enter a circular pipeline, the blowline, where the adhesive and additives are added to the fibres. The pressure in the blowline causes the fibres mix to turbulent, allowing good blending of the adhesive. After this, the material enters the dryer. Fibreboards typically have better adhesive distribution than particleboards, but the adhesive undergoes some pre-curing before mat formation as the resination typically happens before the dryer unlike in the particleboard production. The particle- and fibreboard industry is driven by fast production speed and high volumes. Because of this, the final cost of a particle- or fibreboard is not just the cost of raw materials, but also the total cost of production. Any increase in pressing speed will directly reflect on the product cost. In membership

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Introduction countries of the European Panel Federation, a total of 30.25 million m3 of particleboard were produced in 2016. The majority of this (66%) went into furniture production, the building sector being the second largest buyer at 22%. Similarly, the majority of the 12.0 million m3 fibreboard produced went into furniture manufacturing (45%), while the laminate flooring application hold the second place, consuming 32% of the total MDF produced (European Panel Federation, 2017; Mantanis et al., 2017).

1.2.2 Wood-based panel adhesives Final properties of wood-based panels are determined by the density, production technology and foremost the adhesive used. Adhesive is key to the bond strength and long term performance of wood-based panels (Irle et al., 2010). However, in order to work optimally, correct pressing parameters, cooling procedure, additives, and type and amount of hardeners needs to be used (Irle et al., 2010). Urea formaldehyde (UF) is the most common adhesive used in the particle- and fibreboard industry. UF is a polymeric condensation product of the chemical reaction of urea and formaldehyde. UF is low cost, non-flammable, fast curing, has long shelf life and can easily be adjusted to suit different processes. The major drawbacks are the poor water resistance and formaldehyde emissions. UFs are thermosetting polymers that contain amino groups. The aminomethylenic bonds are susceptible to hydrolytic degradation under acidic and moisture conditions, which leads to bond failure and release of formaldehyde (Salthammer et al., 2010). Addition of melamine to UF increases the bond strength, water resistance and reduces the emissions, but also increases the cost of the resin (Paiva et al., 2012). Phenol-formaldehyde (PF) resins provide permanent resistance under humid climate, but are also significantly more expensive and mainly suited for other board types meant for outdoor use, such as oriented strandboard (OSB) and plywood. Polymeric 4, 4’-diphenylmethane diisocyanate (pMDI) is used especially in the production of OSB and MDF (Mantanis et al., 2017), but can also be used to produce particleboards. While the application amount of UF for particle- and fibreboards is typically around 10% (w/w dry on dry wood), only around 3% of pMDI is required to reach similar board properties. In North America, this can be achieved at similar costs, while in Europe, where the UF resin market is more developed, UF-based resins remain the most economical options. pMDI cures very fast in presence of moisture and materials containing hydroxyl groups, and creates strong covalent urethane bonds (Bao et al., 2003). pMDI can also be used in small amounts (0.3-0.5% on dry wood) as an accelerator to UF-based resins to increase production speed and improve the final board properties (Dunky, 2003; Mantanis et al., 2017). pMDI release no formaldehyde, but it is

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Introduction hazardous to human health during production, and needs extra precaution during board production.

1.2.3 Formaldehyde Formaldehyde (systematic name: methanal) is a naturally occurring chemical compound found in most living things. It is the smallest aldehyde with a chemical formula CH2O. Saturated water solution consists around 37 w% formaldehyde and is called “formalin”. Formaldehyde is often sold and used in its linear and more stable polymer form, as paraformaldehyde. Formaldehyde is produced by the catalytical oxidation of methane or methanol. Versatility of formaldehyde makes it suitable for various industrial applications. It is used as intermediate in manufacturing of industrial chemicals, and as a preservative for consumer products, such as shampoo and cosmetics. Since 1930s, formaldehyde has been an important precursor for wood adhesives and wood composites (Salthammer et al., 2010). Formaldehyde is also the most known indoor air pollutant (Salthammer et al., 2010). Sources of formaldehyde can be divided into temporary (e.g. combustion), intermitting (e.g. air cleaners) and permanent sources (e.g. building products and furniture) (Salthammer, 2019). In mid-1960s, urea- formaldehyde (UF) glued particleboards in prefabricated houses were identified as the source of negative health effects, in particular irritation of the eyes and upper airways. Because of this, in 1977 the former German Federal Agency of Health proposed a guideline value of 0.1 ppm for human exposure in dwellings (Salthammer et al., 2010). In 1980, the carcinogenicity of formaldehyde in rats and mice was reported (Swenberg et al., 1980), but it took until 2004 before formaldehyde was classified as a human cancer causing carcinogen (Group 1) by the International Agency for Research on Cancer (IARC) (2004). U.S. Environmental Protection Agency (EPA) has considered formaldehyde as probable human carcinogen (category 1B) since 1991, and the European Commission classified it as a carcinogen Category 1B and mutagen Category 2 in June 2014 (Commission Regulation (EU), 2014), which was confirmed by the World Health Organization (WHO) in 2010. At the same time, WHO also set an indoor guideline value of 0.1 mg/m3 for formaldehyde (Salthammer, 2019; World Health Organization, 2010).

1.2.4 Formaldehyde in wood-based panels Formaldehyde-based resins, mainly urea-formaldehyde (UF), is the main source of formaldehyde emissions from wood-based panels. Formaldehyde is sourced from the free formaldehyde left in the boards, and from the hydrolysis of the cured UF resin. The hydrolysis of the cured UF resin is a long term degradation process, and increases at high temperatures, humidity, and at acidic conditions

5

Introduction

(Frihart et al., 2012). The hydrolytic stability also depends on the resins’ chemical structure and degree of crosslinking. Due to legal requirements, the formaldehyde content of particleboards have reduced enormously since 1960s (Figure 1). This change has been done by lowering the formaldehyde to urea (F:U) molar ratio, and by copolymerization with more expensive compounds such as melamine (Solt et al., 2019). Lower F:U molar ratio decreases the emissions, but at the same time it can lower the mechanical properties and degree of crosslinking and reduce the curing speed. It has also been suggested to increase the crystallinity of the UF resin, leading to increased stability (Park & Jeong, 2011),

Figure 1. Development of formaldehyde content of industrially produced particleboards in Europe. Redrawn by (Solt et al., 2019) from (Durkic, 2009; Fliedner et al., 2010; Marutzky, 2008).

However, fully formaldehyde-free and non-emitting adhesive systems will not lead to formaldehyde-free boards (Birkeland et al., 2010; Frihart et al., 2012; Navarrete et al., 2013; Salem et al., 2017; Salem et al., 2018). Natural wood particles and fibres will always emit formaldehyde due to the thermohydrolytic cleavage of wood constituents. The amount of emissions is dependent on the wood species, wood type, and most importantly the size, thermal treatment and drying of the particles (Böhm et al., 2012; He et al., 2012; Martínez & Belanche, 2000; Roffael et al., 2012; Salem & Böhm, 2013; Weigl et al., 2009). In

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Introduction addition, increasing amounts of recycled wood is used in the particleboard production, leading to higher formaldehyde emissions if there is a lot of high emission particleboards in the recycling mix (Himmel et al., 2014). Wood-based emissions are low compared to those from UF adhesives, but become significant as the share of low-emitting adhesives increase in board industry (Costa et al., 2011). These levels are not a concern to human health, but because modern energy saving houses are built with lower air exchange rates (Pierce et al., 2016), the emissions can build up over time and worsen the indoor air quality.

1.2.5 Formaldehyde limits There are multiple governmental and voluntary guidelines and regulations for formaldehyde emissions from wood-based materials. As most of the regulations use different measuring methods to determine the limits, understanding the exact relation between the methods is difficult. Table 1 shows some of the most known legal limits stated in the regulations. In addition to these, New Zealand has their own standards for classifying board emission classes. China and Russia have their own levels for furniture, the one in Russian being the most severe formaldehyde limit in the world for at 0.01 ppm (GOST 30255, 2014).

Table 1. The most common formaldehyde limits based on emission methods. PB: particleboards, MDF: medium-density fibreboards.

Region Emission test method Limit Europe, E1 board  PB, MDF EN 717-1 ≤ 0.1 ppm (≤ 0.12 mg/m3) Germany  PB, MDF EN 16516 ≤ 0.1 ppm  PB, MDF EN 717-1 ≤ 0.05 ppm U.S., CARB II/EPA  PB ASTM E 1333 ≤ 0.09 ppm  MDF ASTM E 1333 ≤ 0.11 ppm Japan, F****  PB, MDF JIS A 1460 ≤ 0.3 mg/L

During the last two years, there has been a continued interest in formaldehyde emissions from wood-based panels. In 2017, the U.S. Environmental Protection Agency (EPA) (2017) adapted the Californian Air Resource Board (CARB) limit values. In 2018, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany (Bundesministerium für Umwelt Naturschutz und nukleare Sicherheit (2018)) made a change in the legislation concerning the testing method, effectively causing the allowed emission level to be halved from the current European level (from 0.1 ppm to 0.05 ppm

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Introduction according to EN 717-1). This is challenging for the European particle- and fibreboard industries. Formaldehyde emissions are subjectable to variations in raw material source and to the minor changes in the pressing process. Due to the slow quality control methods and lack of knowledge on the accuracy at low emissions, a safety tolerance needs to be in place, leading to average emissions far below 0.05 ppm. To reach this level, modifications to the process or glue need to be implemented by most of the European board producers for products meant for the German market. Emissions of UF bound wood-based panels can be reduced by decreasing the formaldehyde/urea ratio, increasing the pressing time, adding other co- monomers such as melamine, and by using more scavengers (Myers & Koutsky, 1990; Paiva et al., 2012). Another approach is to use alternative, formaldehyde- free adhesives, such as pMDI or bio-based adhesives.

1.2.6 Measurement of formaldehyde from wood-based panels Perforator method (ISO 12460-5 (2015)) is a content method that measures the free formaldehyde found in the board. Perforator method is a common factory quality control method in Europe and Asia due to its relatively fast measuring time (3h) and low cost of the equipment (Carvalho et al., 2012). According to the perforator method ISO 12460-5, formaldehyde is extracted from test pieces by boiling toluene, transferred to distilled water, and analysed photometrically by the acetylacetone method (ISO 12460-5 (2015). This extraction of formaldehyde with solvent increases the speed of the measurement, but cannot be expected to always correlate with formaldehyde emissions. The fact that the sensitivity of perforator method decreases as emissions get lower is known since the 1970s (Mayer, 1978; Roffael & Johnsson, 2012; Yu & Crump, 1999). As can be seen in Figure 2, the perforator value changes little below a formaldehyde to urea molar ratio of 1:1. Most adhesives meant to fulfil CARB II emission level (0.09 ppm) are below this molar ratio. The poor correlation of perforator method to chamber methods at low emission levels is a major drawback, as most legislations are focusing on emission levels (Roffael et al., 2010; Yu & Crump, 1999). Still, the perforator method is widely used in the particle- and fibreboard industry.

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Introduction

Figure 2. Changes in formaldehyde release (perforator value) with molar ratio of UF resins (Mayer, 1978; Roffael & Johnsson, 2012).

Other ways to increase the measurement speed is to increase the temperature, as in the gas analysis method, ISO 12460-3 (2015). ISO 12460-3 can be classified as a chamber method, but the accelerated testing conditions (temperature of 60 °C, low humidity (≤ 3%) and high air exchange rate (15 h-1) can lead to unsatisfactory correlations to chamber methods that use more moderate conditions (Risholm-Sundman et al., 2007). Similarly to gas analysis method, the flask method (e.g. EN 717-3) is an emission test method with accelerated conditions. It uses small test pieces, relatively high temperature (40 °C) and high humidity, as the test pieces are suspended over water in a closed container. Dynamic microchamber (DMC) is a small (0.044 m3) chamber which uses an electrochemical sensor for the formaldehyde detection (Carvalho et al., 2012). In this method, high loading factor (10.39 m2/m3) and air exchange rate (12.19 h-1) are required to keep the ratio of the air flow through the chamber (Q) to sample surface area (A) at 1.173 m/h, as required by ASTM D 6007 standard. Non-accelerated chamber methods such as EN 717-1, ASTM E 1333, ASTM D 6007 and JIS A 1901, are considered the most relevant, as the boards are tested in the state of which they would typically emit in a residence. They are also the base for many legal requirements. The boards are placed after defined conditioning into chambers, and kept there for a defined period, or until steady state is reached. The testing parameters such as edge sealing (Kim et al., 2006), testing temperature (Liang et al., 2015), humidity (Frihart et al., 2012) and

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Introduction conditioning of the samples have all been found to have a major impact on the final emission values (Risholm-Sundman et al., 2007; Zhang et al., 2018). Humidity affects especially UF-bonded boards due to hydrolysis, which increases the formaldehyde released (Salthammer et al., 2010). Particleboards and MDF are also affected differently by these variables. Another difference among the chamber methods is the analytical method used. There are four main analytical methods mentioned in different standards. ASTM E 1333 and some Chinese standards allow the use of 3-methyl-2- benzothiazolonehydrazone (MBTH) for detection of formaldehyde from wood- based materials. However, even though MBTH is reported to be selective to formaldehyde, in reality it delivers a parameter that is the sum of all aliphatic aldehydes (Giesen et al., 2016). This makes MBTH unsuitable analytical method for wood-based materials, which emit other aldehydes besides formaldehyde. The second methods is the chromotropic acid test procedure, which is the official method for ASTM E 1333 and ASTM D 6007. However, this method it is not recommended by CARB, due to poor detection limit (0.02 ppm) (California Air Resources Board (CARB), 2010). This leaves the 2.4- dinitrophenylhydrazine (DNPH) method used in ASTM and ISO methods, and the acetyl acetone (ACAC) method based on Hantzsch reaction used particularly in Europe. Both analytical procedures are also used in Japanese standards (ACAC in JIS A 1460 and DNPH in JIS A 1901) (JIS A 1460, 2001; JIS A 1901, 2009). Previous studies have shown good correlations between these analytical methods (Giesen et al., 2016; Salthammer et al., 2010), but behaviour at very low emission ranges (< 0.02 ppm ref. ASTM) is not well investigated. The EN 717-1 method that is carried out until steady state is the most commonly used for measuring emissions at low emission levels, close to that of natural wood. However, this is not industrially viable, as receiving the result can take up to 28 days. Even the relatively fast chamber method, ASTM D 6007, requires 7 days for final emission result. In addition, there is no published information on how accurate the chamber methods are at low emission levels. In order to justify the use of bio-based adhesive systems due to their lower emissions, it is important to be able to measure those emissions accurately both in laboratory and in factory settings with faster emission test methods.

1.2.7 Bio-based raw materials for adhesives “Bio-based product” is defined in U.S. as “a commercial or industrial product (other than food or feed) that is composed, in whole or in significant part, of biological product or renewable domestic agricultural materials (including plant, animal, and marine material) or forestry materials” (Congress Bill, 2002). Bio-based materials that can be used as adhesives include, as an example, natural phenolics (tannins and lignins), carbohydrates (e.g. starches and gums),

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Introduction proteins (e.g. soy, wheat, clam) and vegetable oils. The large amount of reviews written on bio-based adhesives in the last 15 years shows the amount of interest in the topic. Each of these reviews has a unique touch, focusing on different topics, such as adhesive formulations (Pizzi, 2006), and evaluation and characterization techniques of bio-based raw materials (Ferdosian et al., 2017). In addition, several reviews exist for individual raw materials, such as protein (Frihart & J. Birkeland, 2014) and lignins (Thakur et al., 2014). Although our knowledge on the raw materials themselves, as well as the various modifications and formulations is high, the feasibility of bio-based systems for industrial applications are rarely evaluated. Often, pressing times and adhesive amounts that are not even close to being industrially viable are used. Only recently, some authors have brought up these questions from the industrial point of view (Solt et al., 2019). Especially, the requirement for formaldehyde-free adhesives has given more drive to the development of sustainable adhesive systems based on bio-based raw materials. However, most of the bio-based adhesive systems for particle- and fibreboards are still in research stage. They struggle to match synthetic UF- and pMDI-based resins when it comes to curing speed, adhesion strength and moisture resistance while maintaining the low costs. Adhesives based on biopolymers can also have specific issues, when it comes to storing, distributing, around the year availability or application. In addition, the long-term properties of wood boards manufactured with bio-based adhesives, such as creep and mould resistance, are not fully known. Most importantly, the potential bio-based adhesives need to fulfil the basic needs of the wood-based industry: availability, low price, reactivity, mechanical strength and humidity resistance. Thus far, there are only a few commercialized solutions that can be used to produce boards that can reach the performance requirements of wood-based panel industry. These boards are sold at a premium price (Solt et al., 2019), and they contain a synthetic crosslinkers. Due to the huge production volumes of particle- and fibreboard industry, availability and cost are the first limitations to raw materials for bio-based adhesives. From all lignocellulosic materials (trees, plants, and biomass), lignin, and hemicelluloses can be extracted in large amounts. However, as cellulose and hemicelluloses are important raw materials for bio-energy, pulp and paper, and chemical industry, lignin is the only raw material that is available in large amounts for the adhesive industry at reasonable price. Lignin comes in various different forms depending on the extraction process, as discussed in the following part 1.2.10. Tannins are polyphenolic plant compounds that can be classified into two major categories: the oligomeric condensed tannins and the non-polymeric hydolysable tannins (Engström et al., 2016). Tannins can be extracted from barks of trees, wood, leaves and fruits. However, the concentration of tannin is high enough only in few wood species making the extraction worthwhile, and

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Introduction thus tannin availability is dependent on the local tree species. As adhesive raw material, tannin is considered to have good moisture and fungi resistance (Pizzi, 2006). Most proteins are available as by-products of soy, palm, canola, cotton seed and sunflower oil extractions. In Europe, protein from wheat gluten is also available as by-product of bioethanol production (Nordqvist et al., 2013). The value of proteins varies based on the isolation stage. As an example, soy protein meal (44-50% protein content) can be considered a by-product of soybean oil process, but can further be purified to soy flour and protein isolates (80-90% protein content). These can be used as animal feed and raw materials for other chemicals, increasing the price of protein isolates. In 2015, the world production of soy beans was estimated to be over 317×106 t (Zhang et al., 2017). Starch consists of two types of glucose polymers, the helical and linear amylose and the branched amylopectin (Ferdosian et al., 2017). It is widely used as coating binder, agent, adhesive and wet-end additive (Gadhave et al., 2018). Starch can be derived from seeds, roots and leaves of plants using it as the energy storage, such as corn, wheat, potato, rice and tapioca. The appearance and properties of starch from these different sources varies greatly. Many of the plants used to produce starch are modified to have high amounts of starch, making them unsuitable as food sources. There is no direct competition with food, however, the land to grow the starch can be used to grow edible corps. Modification of starch is required to reach the bonding strength and water- resistance required by most adhesive applications (Ferdosian et al., 2017). Another carbohydrate of interest is chitin extracted from shell fish, fin fish and fish waste. Vegetable oils and pectin can also be used as adhesive raw materials (Solt et al., 2019). It should be noted that there is also another way to increase the bio-content of adhesives. This is by making the adhesive building blocks from bio-based material sources (Cavani, 2016). This requires no disruption of the market, as practically the same adhesives are still used, but only the adhesive building block sourcing is changed from petroleum-based to bio-based. This way, a stepwise approach can be taken into moving towards bio-based, as the bio-based building blocks are still more expensive due to the low availability. However, this route will not lower the formaldehyde emissions.

1.2.8 Crosslinkers for bio-based raw materials As can be seen from Figure 3, most bio-based raw materials have high amounts of hydroxyl groups. Because of this, a large amount of hydrogen bonds can be created. This is enough to secure adhesion strength in dry conditions; however, these bonds are reversible and can be disrupted by water molecules. Covalent bonds have superior strength (bond energy 60-700 kJ/mol) compared to hydrogen bonds (1-25 kJ/mol) (Pizzi, 1994). Although they require a lot of

12

Introduction energy for formation, covalent bonds are non-reversible and will hold even in moist environment. Covalent bonds of adhesives are also responsible for the creep resistance of wood-based panels. In order to form networks of covalent bonds, most bio-based adhesives require a so called “crosslinker” (Figure 4).

Figure 3. Examples of repeating structures of some biopolymers: a) syringyl lignin (Tejado et al., 2007), b) condensed tannin (Engström et al., 2016), c) two amino acids bound with a peptide bond as a part of the complex protein structure, and d) starch (Masina et al., 2017).

Figure 4. Example of a covalent bond: Isocyanate reaction with hydroxyl to form urethane.

Specific crosslinkers are often required but some are suitable for most of the bio-based raw materials. One of these is polymeric diphenylmethane diisocyanate (pMDI). pMDI has been used in combination with most bio-based adhesives, such as lignins (El Mansouri et al., 2006), proteins (Lei et al., 2010), and starches (Tan et al., 2011). pMDI reacts with hydroxyls, which bio-based materials have plenty of, to form covalent urethane linkages (Figure 4). However, pMDI can work as adhesive on its own. Thus, when bio-based systems with pMDI as crosslinker are studied, the results should be evaluated

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Introduction in relation to the total pMDI amount. Figure 5 shows the internal bond (IB) strength in relation to pMDI amount for various bio-based and pMDI systems, fully ignoring how much bio-based component is in the systems. In many cases, it seems that the bio-based systems with certain amount of pMDI are performing very similarly to those containing the same amount of pMDI as sole adhesive, and thus addition of the bio-based component does not seem to have significant beneficial effect on the IB strength (Solt et al., 2019). The high press factors and adhesive addition rates show that the majority of research ignores what can be considered even remotely industrially viable. For industrial use of pMDI- based particleboards, press factors below 5 s/mm have been reported (Pizzi, 2014).

Figure 5. Dependence of internal bond strength on isocyanate (pMDI only) content used for particleboards. Connected data points indicate values from same studies using similar parameters. Redrawn from (Solt et al., 2019), see original publication for references for each point.

An interesting alternative to the fossil-based isocyanates such as pMDI is pentamethylene diisocyanate (PDI), which can be sourced up to 70% from biomass. This bio-based PDI is converted from industrial sugar (glucose) and exists on the market on a small scale at a premium price. Lignin has primarily been used as partial replacement of phenol in phenol- formaldehyde (PF) resins due to its phenolic nature. Thus, most crosslinkers studied are aldehydes, such as formaldehyde, glutaraldehyde (da Silva et al., 2013) and glyoxal (Navarrete et al., 2012). Lignin-tannin adhesives are also well researched due to the similar phenolic nature of lignin and tannin (Mansouri et

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Introduction al., 2011; Navarrete et al., 2012). Furfuryl alcohol (FOH) has also been used as a crosslinker for lignin. Best results have been obtained with FOH amount of 40% and lignin:tannin ratios of 50:10 and 40:20 (respective IB 0.55 N/mm2 and 0.53 N/mm2, with adhesive addition amount of 15%) (Luckeneder et al., 2016). Although high IB strength values (e.g. 0.67 N/mm2 at press factor of 15 s/mm (El Mansouri et al., 2006)) have been reported for lignin-pMDI systems, the total pMDI amount in many studies is so high, that the contribution of lignin into the final strength can be questioned (Solt et al., 2019). Aldehydes are popular crosslinkers for tannins. However, due to short pot life when using tannin with formaldehyde, less reactive aldehydes are preferred (e.g. glyoxal (Vázquez et al., 2012), and tris(hydroxylmethyl) nitromethane (TRIS) (Trosa & Pizzi, 2001)). In this study by Trosa and Pizzi, good IB strength values (up to 0.8 N/mm2 dry and 0.20 N/mm2 after 2h boiling) could be achieved at press time of 14 s/mm using mimosa tannin with 12% of TRIS as hardener. Hexamethylenetetramine (hexamine) is a another well known crosslinker for tannins (Pichelin et al., 1999; Valenzuela et al., 2012). Proteins are combination of polar and non-polar groups, making them good adhesives for a variety of surfaces. However, many of the reactive groups are bound with internal hydrogen bonds inside the globular structure of protein. Thus, denaturation, e.g. with sodium hydroxide or urea, has been found to be necessary to increase the bonding strength of soy adhesives (Frihart & J. Birkeland, 2014; Jang et al., 2011; Nordqvist et al., 2013). The most commercialized protein technology is using polyamidoamine-epichlorohydrin (PAE) as co-reagent for soy proteins (Li et al., 2004). This technology developed by Professor Kaichang Li at Oregon State University is widely used in interior plywood and engineered wood flooring and is covered by multiple patent applications (Allen et al., 2011; Li, 2007). Other protein crosslinkers include polyamides, polyethyleneimines (PEI) (Gu & Li, 2011) and ketones (Hamarneh et al., 2010). Starch adhesives rely heavily on hydrogen bonding, thus leading to poor water resistance. To overcome this drawback, starch needs to be combined either with a strong crosslinker, such as pMDI (Qiao et al., 2015), or a suitable co-polymer such as polyvinyl alcohol (Imam et al., 1999), tannins (Moubarik et al., 2011) and epoxies (Nie et al., 2013). However, many studies report extremely high pressing time for starch-based adhesives, and best results have been reported with veneer-lap joint tests (Solt et al., 2019).

1.2.9 Lignin extraction methods and structure Lignin is the second most abundant biopolymer found in nature that consists of randomly crosslinked phenylpropanoid units (Figure 6) (Tejado et al., 2007). The word “lignin” refers both to natural lignin found in plants, as well as to the by-product from extraction processes (Calvo-Flores, 2015). Traditionally, most

15

Introduction lignin available in market comes as a by-product from pulping processes. However, in the recent years both energy and material driven biorefineries have increased in number, giving raise to the amount of residual lignin available for other industries. The main objective of paper and pulp industry is to preserve cellulose at maximum yield and quality by fibre liberation, often leading to highly damaged lignin residues. is a good example of this, being the most common pulp extraction process for forest biomass. This hydrolytic treatment is performed with high pH with sodium hydroxide (NaOH) and sodium sulphide (Na2S) at temperatures above 170 °C (Chakar & Ragauskas, 2004). Kraft lignin is often burned for the high-energy consuming pulping process. Biorefineries focus on structurally opening the fibres to enable the breakdown of the cellulose and hemicelluloses into monomeric sugars (Santos et al., 2013). Due to the focus on value-added chemicals, the lignin is often treated as co-product, instead of waste product. This leads to potentially higher quality and purer lignins. Some processes, like the , can be utilized in both pulping and biorefinery processes. The sulfite process uses sulphur aqueous solution (SO2) and a base consisting of ammonium, sodium, magnesium or calcium, leading to different types of lignosulfonates. Lignins from both Kraft and sulphite processes can be classified as sulphur containing lignins (Carvajal et al., 2016). Because sulfur is an environmental concern, many sulphur-free extraction processes have been developed, especially for biorefinery processes (Holladay et al., 2007). Out of the sulphur- free processes, process uses solvents such as formic acid, methanol, ethanol and acetic acid, to isolate the lignin from the lignocellulosic matrix. The solvent can then be recovered for reduced environmental impact. However, although leading to better quality lignin, organosolv process is significantly more expensive than the Kraft and sulphite processes (Carvajal et al., 2016). Water above its critical point (supercritical water, 374.2 °C and 22.1 MPa) can also be used as the reactant and catalyst for breaking down biomass in so called supercritical water hydrolysis (SCWH) process (Cantero et al., 2015; Kruse & Dahmen, 2015).

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Introduction

Figure 6. The structural units of lignin (modified from (Tejado et al., 2007). Name on top is the precursor (cinnamyl alcohol) name, and name on the bottom is the matching aromatic constituent name.

The structure of lignin depends both on its origin, as well as on the extraction process (Azadi et al., 2013; Calvo-Flores, 2015; Holladay et al., 2007), As an example of the differences caused by origin, more than 95% of softwood lignin consists of coniferyl alcohol, while hardwoods can have different ratios of conferyl/sinapyl alcohols (Figure 6). Grasses and cereal straws on the other hand have coumaryl alcohol present (Gosselink et al., 2004). The lignin structure also differs depending on which part of the plant it is taken from. The extraction process has even bigger effect on the final structure. Typically the extraction processes cleave the β-O-4 linkages between the phenylpropane units. In addition, some cleavage of propane side chains and demethoxylation can occur (Cocero et al., 2018; Moon et al., 2011). Kraft and sulfate processes strongly degrade the lignin leading to low amount of β-O-4 linkages (less than 10%), low molecular weight and low reactivity (Azadi et al., 2013; Chakar & Ragauskas, 2004) (Gan et al., 2012; Inwood et al., 2017). The difference is that, unlike Kraft lignin, lignosulfonates from sulfite process are water-soluble over almost the entire pH range, making them easier to handle and enabling them to be used as dispersants for various applications (Azadi et al., 2013) (Aro & Fatehi, 2017). For SCWH process, the depolymerisation pathway and degree is highly dependent on the reaction temperature and time (Wahyudiono et al., 2007). SCWH is also not water soluble, except at high pH. As discussed earlier, Kraft lignin and lignosulfonates have residual sulfonate groups, while SCWH lignin is free of chemical contaminants.

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Introduction

The complex and varying aromatic structure of the different lignin types makes their utilization difficult. Annually, more than 50 million tons of lignin worldwide are used as a combustion fuel in pulp mills while a very small portion of lignin (around 2-5 wt%) is used commercially, mainly in low-value applications (Costello, 2017; Farag et al., 2014).

1.2.10 Lignin applications Currently, there are several commercialized applications for lignin, all of which are dependent on the initial lignin structure. Vanillin is one of the biggest markets for lignin, but other aromatic compounds can be extracted from lignin by combining solvotic extraction with reductive depolymerisation. Some examples of such chemicals are benzene, toluene, xylenes and phenols (Huang et al., 2017). For depolymerisation, low molecular weight and high amount of reactive groups is required from the lignin (Shu et al., 2016). Some applications utilize lignin in its macromolecular state. High molecular weight lignins, such as some water soluble lignosulfonates, are well suited to be used as dispersants (Aro & Fatehi, 2017). Out of the adhesive applications, the most industrially viable and developed is the phenol replacement in phenol-formaldehyde adhesives. The amount of available reactive sites is important for this application. In addition, the amount of phenolic and aliphatic hydroxyl groups is critical for all adhesive applications that require reaction of lignin with different aldehydes, tannins, phenols or isocyanates (El Mansouri & Salvado, 2006; Wang et al., 2018).

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Introduction

1.2.11 Research questions As seen from the amount of reviews and research articles available, there is a large amount of information available on the different bio-based raw materials, their reactions, and evaluation methods for wood board adhesives. However, the bio-based systems struggle to enter the market due to higher costs, poor availability and lack of interest from the industry. The bio-based alternatives also often have special requirements on the particle- and fibreboard production lines in order to be used, making the transition from formaldehyde-based systems problematic. Lower formaldehyde emission can be an enabler for bio-based adhesive systems. The new stricter emission limits and changing methods, like the German UBA adopting EN 16516 for wood-based panels, create uncertainty in the industry. There is no clear picture on what these levels are compared to the industrially familiar emission methods, and there is lacking information on what methods are accurate at these new low levels. Until these questions are answered, it is difficult to judge whether using modified low emitting MUF adhesives is enough, or if more radical transformation to emission-free adhesive systems is necessary. If industrial acceptance can be reached for bio-based adhesives, one raw material with a lot of potential is lignin. This is especially true due to the increasing amount of biorefineries that bring more lignins to market, some of which is of higher quality than the traditional lignins from the pulping industry.

In this context, the following research questions have been defined: RQ1: How the different formaldehyde measuring methods perform at the lower emission levels required by the new tighter legal requirements? RQ2: Which are the key parameters affecting the final formaldehyde emission value provided by the different methods, and can their effect be predicted at lower emissions? RQ3: What is the current state of bio-based adhesives’ research from the viewpoint of the industrial needs for performance, emissions and availability, and what are the requirements to succeed in replacing the formaldehyde-based adhesives used today? RQ4: Which are the determining properties of biorefinery lignins, and how these can be used with appropriate crosslinking for developing bio- based adhesive systems for particleboards?

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Introduction

1.3 Aim and objectives The aim of this thesis was to look into research questions related with the amino- based adhesives used today in the particle- and fibreboard industries. More specifically, it was aimed to address the uncertainty around the new lowered formaldehyde emission levels and changes in the measuring methods, and also the need for more sustainable wood board products with less fossil-fuel dependency. One part of the thesis was thus focused on studying the appropriateness of formaldehyde measuring methods for low emission levels, their accuracy at the new low emission levels, correlation to the main global emission requirements, and possibility to provide reliable results in shortest time possible. Another part of the thesis explored the potential of different bio- based raw materials and adhesive systems produced thereof to fulfil the new low emission requirements without compromising the quality of boards. The work was further progressed on evaluating less researched biorefinery lignins and proposing crosslinking paths for their eventual use as particleboard adhesives. The following specific objectives were then defined, while the corresponding papers in the thesis are mentioned in parentheses:

OBJ1: To review the different formaldehyde test methods for wood-based panels and the recent developments regarding bio-based adhesives from both scientific and industrial points of view (Paper I). OBJ2: To determine the emission levels of particle- and fibreboards originating only from the wooden materials used in their production, and when glued with formaldehyde-free adhesives (Papers II and V). OBJ3: To evaluate the correlations between the main formaldehyde measuring methods according to perforator ISO 12460-5, European chamber EN 717-1, and American chamber ASTM D 6007 (Papers II and III) OBJ4: To evaluate the effect of edge-sealing, analytical methods and chamber types on formaldehyde emissions at low levels, and the possibility of shorter conditioning time for faster measurement at the industry (Papers II and III). OBJ5: To characterise emerging biorefinery lignins and evaluate their potential and best pathways for their utilization for value-added products, including adhesives for wood-based panels (Paper IV). OBJ6: To evaluate different crosslinkers for the common bio-based raw materials used as wood board adhesives, and test a biorefinery lignin type with most potential for production of particleboards by employing appropriate crosslinkers (Papers I and V)

20

Introduction

1.4 Overview of the appended papers This thesis is based on experimental work of the author and research colleagues in combination with a scientific review of the literature in the field. In order to evaluate the current status of formaldehyde test methods and bio-based adhesives and their crosslinkers, a literature review on academic publications and some patents was conducted (Paper I). Based on the review, the most critical formaldehyde testing methods to Europe were identified and tested (Papers II and III). A raw material group with quite unexplored potential for wood board adhesives, biorefinery lignins (Papers IV and V), and suitable crosslinkers for them (Paper V) were studied. The relations of the appended papers for collectively contributing to the overall results of the thesis can be seen in Figure 7.

Figure 7: Schematic representation showing how the objectives (OBJ) in the appended papers relate to each other, and collectively contribute to the overall results of the thesis

21

Introduction

PAPER I: HEMMILÄ, V.; ADAMOPOULOS, S.; KARLSSON, O.; KUMAR, A. 2017. Development of sustainable bio-adhesives for engineered wood boards: a review. RSC Advances, vol. 7, no. 61, pp. 38604–38630. http://pubs.rsc.org/en/content/articlepdf/2017/ra/c7ra06598a

This article presents a review of the research undertaken on various bio-based adhesive systems usable for wood boards as alternatives to the existing amino- based thermosetting adhesives. The sources of formaldehyde emissions in wood-based panels as well as the different emission test methods were discussed, while the focus of this review was on the research conducted on sustainable bio-based adhesive systems for wood boards. The first part focuses on the structure and availability of different sustainable raw materials, such as lignin, starch, protein and tannin. The second part focuses on the suitability of these raw materials as adhesive alternatives to the existing amino-based thermosetting adhesives and their crosslinkers.

PAPER II: HEMMILÄ, V.; MEYER, B.; LARSEN, A., SCHWAB, H.; ADAMOPOULOS, A. 2019. Influencing factors, repeatability and correlation of chamber methods in measuring formaldehyde emissions from fiber- and particleboards. International Journal of Adhesion and Adhesives, vol. 94, 102420. https://doi.org/10.1016/j.ijadhadh.2019.102420

This article focuses on the accuracy and correlation between EN 717-1 and ASTM D 6007 chamber methods at emission levels below 0.05 ppm. The effect of analytical methods and edge-sealing on chamber emissions is determined, and accuracies and correlations of the EN 717-1 and ASTM D 6007 chambers defined at low emission levels (< 0.05 ppm). In addition, some emission values are compared to those obtained with EN 16516.

PAPER III: HEMMILÄ, V.; ZABKA, M.; ADAMOPOULOS, S. 2018. Evaluation of dynamic microchamber as a quick factory formaldehyde emission control method for industrial particleboards. Advances in Materials Science and Engineering, volume 2018, Article ID 4582383. https://doi.org/10.1155/2018/4582383

This study investigates alternative to the currently common European perforator methods, such as ISO 12460-5, that measure formaldehyde content, instead of emissions. As the conditioning and measuring times of chamber methods are

22

Introduction typically long (minimum seven days), it was of interest whether emission values of particleboards measured one day after production would be usable for quality control purposes. The correlation between 1-day and 7-day emission values was determined using a dynamic microchamber (DMC). Three industrial particleboard types that differed in density and emission levels were used for the evaluation. The online emission measuring equipment Aero-laser AL4021 connected to the 1 m3 chamber was used to gain further information on the emission reduction behaviour of the different board types.

PAPER IV: HEMMILÄ, V.; HOSSEINPOURPIA, R.; ADAMOPOULOS, S.; ECEIZA, A. 2019. Characterization of wood-based industrial biorefinery lignosulfonates and supercritical water hydrolysis lignin. Under review in Waste and Biomass Valorization

Understanding the properties of any particular lignin type is crucial when choosing the right lignin for the right end use. In this paper, three different lignin types (supercritical water hydrolysis lignin SCWH, ammonium lignosulfonate ALS, and sodium lignosulfonate SLS) were evaluated for their chemical structure, thermal properties and water vapor adsorption behavior.

PAPER V: HEMMILÄ, V.; ADAMOPOULOS, S.; HOSSEINPOURPIA, R. AHMED, S.A. 2019. Ammonium lignosulfonate adhesives for particleboards with pMDI and furfuryl alcohol as crosslinkers. Accepted for publication in Polymers

In this work, two crosslinkers were tested for ammonium lignosulfonate (ALS); bio-based furfuryl alcohol (FOH) and synthetic polymeric 4,4’- diphenylmethane diisocyanate (pMDI). In addition, the effect of adding a small amount of mimosa tannin (10% of lignin dry weight) to the lignin before crosslinking was evaluated. The reactivity of the crosslinked mixtures were determined by measuring the activation energy using Differential Scanning Calorimetry (DSC). Properties of the crosslinked ALS adhesives were tested on 2-layered beech veneers and 3-layer particleboards. Besides mechanical and moisture properties, formaldehyde emissions were measured of the particleboards glued with ALS in combination with the two different crosslinekrs. In addition, light microscopy was employed to evaluate the interaction of the adhesives with wood tissues on veneer samples.

23

Introduction

1.5 Limitations This thesis focuses on particle- and fibreboards. Results from veneer, plywood and solid wood testing were included in the literature review Paper I, as they can be used at initial stages of particle- and fibreboard adhesive development. As the thesis is industrially driven, the literature study was limited to adhesive systems that have potential of being available in large volumes. In evaluation of influencing factors of formaldehyde test methods, errors caused by handling were only included in the discussion. Only the EN 717-1 and ASTM D 6007 chamber methods were evaluated in depth, as they are the most relevant test methods for particle- and fibreboard industries in Europe. Methods and standards from other parts of the world were not considered in this thesis. In evaluation of the dynamic microchamber (DMC), particleboards from only one production line were evaluated, and should be considered as examples of different emission behaviours of different materials. Implementing this equipment requires additional testing for each product and production line. The ammonium lignosulfonate (ALS) results were based on crosslinking of unmodified ALS and as such are indicative. As the crosslinkers can be used as adhesives themselves, it is important to note that the contribution of ALS to the final adhesion strength was not evaluated. Lignin from other extraction methods, lignosulfonates with other counter ions, and even other ammonium lignosulfonates can behave differently, and final results depend on the structure of the lignin in question. All wood boards, wooden material, and chemicals used in this thesis were sourced from European suppliers. Especially for board materials, differences can be expected to those from other parts of the world due to different pressing parameters, glues and raw materials origin.

24

Material and methods

2. Materials and methods

2.1 Review on bio-based adhesives (Paper I) A literature study (Paper I) was conducted in order to evaluate the research done on bio-based adhesive systems for wood-based panels. Scientific publications and some patents, mainly from Linnaeus University OneSearch and Google Scholar search engines, up to year 2017, were included in the study. The publications were evaluated by taking into account industrially important parameters, such as performance, speed and volumes. The review was divided into two main parts: the first part focused on the structure and availability of different sustainable raw materials and the second part focused on the suitability of these raw materials for wood adhesives. Both parts were divided by raw material. Crosslinkers for different bio-based materials were included into the second part. In addition, sources of formaldehyde and different formaldehyde measuring standard methods were discussed and compared.

2.2 Industrial boards (Papers II and III) Different industrially produced three-layered particleboards (PB) and standard medium-density fibreboards (MDF) with formaldehyde emission values ranging from 0.004 to 0.15 ppm (ref. ASTM D 6007) were used in the Papers II and III, as can be seen in Table 2. More details of the industrial boards used in the thesis can be found in the appended Papers II and III.

25

Material and methods

Table 2. Information on the industrial fibre- (MDF) and particleboard (PB) types used in Papers II and III.

Identifier Board type Target Thickness [mm] [kg/m3] PB (for Standard, low - 3-25 II correlation) emission PBs MDF (for Standard, low - 3-25 II correlation) emission MDFs A Standard PB 660 18, 20 III B Standard, 630 12, 15, 18, 22 III low emission PB C Engineered, 530 18 III low density PB

2.3 Formaldehyde measurements (Papers II, III, and V) In total, four different chamber methods were used in this thesis: European EN 717-1 and EN 16516 chambers (Paper II), and American 1 m3 ASTM D 6007 (Papers II and III) and a smaller version of it, the 0.044 m3 dynamic microchamber (DMC) (Papers III and V). The chamber methods are listed in Table 3.

Table 3. Differences between the experimental settings used for different chamber methods in Papers II, III and V.

Chamber method EN 717-1 EN 16516 ASTM D 6007 1 m3 DMC Volume (V) [m3] 1.00 1.00 1.00 0.044 Temperature [°C] 23 ± 0.5 23 ± 1 25 ± 1 25 ± 1 Humidity [%] 45 ± 3 50 ± 5 50 ± 5 50 ± 5 Loading factor (L) [m2/m3] 1.00 1.801 0.43 10.39 Edge sealing (boards) Partly: Partly1: Fully sealed2 Fully sealed 1.5 m/m2 1.5 m/m2 Air exchange rate (N) [1/h] 1.00 0.50 0.50 12.19 Analytical method ACAC ACAC ACAC3 Sensor4 Used in papers II II II, III III, V 1 Measurement procedure for determination of formaldehyde emissions from wood-based panels according to the German Chemical Prohibition Ordinance (Chemikalien Verbotsverordnung) (Bundesministerium für Umwelt Naturschutz und nukleare Sicherheit, 2018). 2 According to ASTM D 6007, the edges need to be completely sealed if edge area is more than 5% related to surface area. 3 For analytical method comparison DNPH and modified ACAC (reagent) were also used in Paper II. 4 Electrochemical sensor with calibration to ACAC.

26

Material and methods

In addition, measurements were performed using the perforator method (ISO 12460-5:2015) in Paper III. For the emission reduction behaviour analysis in Paper III, Aero-laser AL4021 with inbuilt ACAC was used, as seen in Figure 11. According to ASTM D 6007, three full air changes or 15 min, whichever is greater, are required before measurements (ASTM D 6007, 2014). Thus, the samples were kept in the DMC for 15 min before taking the emission value. For the comparison of 1-day to 7-day emission values, samples were condition 24 h before, and 6 days in between measurements at 24 °C and 50% RH.

Figure 8. Air flow inside the 1 m3 chamber and the dynamic microchamber (DMC) (William H. Anderson, 1995), the different measurement pathways and how they are referred to in this thesis (Paper III).

To evaluate the accuracy of ASTM D 6007 chamber method at low formaldehyde emission levels in Paper II, 12 boards with different emissions were each cut into 15 samples that were tested in 15 different ASTM D 6007 chambers. As a comparison, one board with average emission of 0.064 ppm was cut into 12 samples and tested in 12 EN 717-1 chambers. The accuracy of EN 717-1 and ASTM D 6007 chamber methods was determined using previously collected data (Fraunhofer WKI Quality Assessment, internal investigations 2009-2017) as explained in Paper II. Details on the special edge-sealing for edge emission analysis, and effect of 5% open edge and the testing procedure for particle, fibre and gluesless board emission testing can be found at Materials and Methods part of the appended Paper II.

27

Material and methods

2.4 WKI-formula (Paper II) The correlations between ASTM D 6007 and EN 717-1 determined in this thesis were compared to values calculated using the WKI formula (Meyer et al., 2014) at currently critical emission levels (e.g. E1, UBA and EPA/CARB II levels). The WKI formula for the emission concentration in ppm is: 1 퐶 = 0.00555 × (퐶퐵푒푧푢푔 + 0.008) × (푇 − 12.7) × (푈 − 1.2) × 푁 1+1.75× 퐿 Where 퐶퐵푒푧푢푔 is the EN 717-1 chamber result at steady state concentration, T is the temperature in °C, U is the relative humidity in %, N is the air exchange rate in l/h and L the loading rate in m2/m3. The formula is only valid for relative humidity 30-50%.

2.5 Lignin types and their characterisation (Papers IV and V) Ammonium lignosulfonate (ALS, Papers IV and V) and sodium lignosulfonate (SLS, Paper IV) samples were obtained from commercial biorefinery processes and received from the industrial supplier in dry powder form. The wood mixtures used in both lignosulfonate pulping processes were mainly softwood. The supercritical water hydrolysis lignin (SCWH, Paper IV) was sourced from another industrial supplier as a side product of a smaller non-commercial biorefinery batch, and delivered in dry powder form. The composition of the SCWH lignin was 65-85% lignin, 10-25% carbohydrates and <10% water. The wood mix used for the biorefinery process was mainly Scots pine (Pinus sylvestris L.) with minor traces of other wood species. The lignins were analysed using Gel Permeation Chromatography (GPC), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), elemental analysis, Fourier transform infrared (FTIR), and solid-state 13C Nuclear Magnetic Resonance (13C NMR) methods. Details of the methods can be found in Paper IV. The adhesive formulations of ALS with crosslinkers was analysed using DSC as described in Paper V.

2.6 Adhesive formulations and evaluation of bonding (Paper V) Ammonium lignosulfonate (ALS) glue recipes were prepared as shown in Table 4. As reference, industrial melamine reinforced urea formaldehyde (UmF) resin with melamine content of 5% was used (AkzoNobel, Sweden). For UmF, 3% (w/w of dry resin) of ammonium sulphite with solid content of 45% was added

28

Material and methods as hardener (Yara, Sweden). Two different crosslinkers were used, polymeric 4,4’-diphenylmethane diisocyanate (pMDI) (Huntsman, Belgium) and furfuryl alcohol (FOH) (Sigma Aldrich USA). As a reinforcement, commercial mimosa tannin powder was added to the lignin powder before dilution to water to form the base before crosslinking. The tannin was received from Silvachimica S.r.l. (San Michele Mondovì, Italy). Lignin (or lignin and tannin) was then diluted to water in ratio of 1:2. For the formulations with the bio-based crosslinker FOH (ALS-FOH and ALS-FOH + m), the base solution consisting of ALS or ALS + m was first heated to 70±3 °C under constant magnetic stirring, after which the FOH was added. The pH was adjusted to 2.0 with 30% H2SO4 (2% w/w of solids) and the reaction proceeded for 45 min for adhesive evaluations on veneer samples. An identical procedure was applied for preparation of the same adhesives for particleboard manufacturing. A shorter reaction time (30 min) was required to keep the viscosity of the adhesive at a spray-able range (< 300 mPa.s) for the lab-scale semi-manual blender used in this study.

Table 4. Adhesive compositions for testing of 2-layered veneer samples and particleboards based on ammonium lignosulfonate (ALS), furfuryl alcohol (FOH) and pMDI as crosslinkers, and with or without mimosa tannin (m) (Paper V).

Base: Crosslinker Application Application Final viscosity for Lignin to amount in amount for amount for veneer/particleboard Formulations tannin relation to 2-layer particleboard (mPa.s) ratio base1 (%) veneer (g (w% to dry solids /m2) particles) UmF2 100 12 370/370 ALS 10:0 - 100 12 80/90 ALS FOH 10:0 25 100 8 2100/210 ALS FOH + m 9:1 25 100 8 2350/190 ALS pMDI 10:0 25 100 8 120/110 ALS pMDI + m 9:1 25 100 8 110/100 1Base refers to lignin or to the lignin-tannin mixture 2Melamine-urea formaldehyde with 3% (w/w of dry resin) of ammonium sulfite as hardener

Beech veneers samples with the adhesive formulations were pressed in a single daylight press at temperature of 150 °C for 90 s. The shear bond strength of all the adhesive formulations was evaluated by tensile shear strength using a lap joint test according to EN 205 (2016) using an Instron universal testing machine (Buckinghamshire, England). Total of 10 repetitions of each adhesive formulation were tested. To observe the bondline thickness and adhesive penetration into the wood tissue, a motorized BX63F light microscope equipped with the DP73 colour CCD cooled camera (max. 17.28 megapixel) and the software program cellSens DIMENSION 1.18 (all Olympus, Lund, Sweden) were used as described in Paper V.

29

Material and methods

2.7 Particleboard manufacturing and testing (Paper V) For 3-layered particleboard production (Paper V), the adhesives were applied using a semi-manual rotation blender constructed for this purpose. Forming was done by hand. The target density of the boards was 620 kg/m3, core percentage 64%, and dimensions 500 × 450 × 12 mm. All boards were pressed for 4.5 min with 0.5 min opening time at temperature of 190 °C as described in Paper V. Internal bond (IB) was measured according to EN 319 (1993) standard, modulus of rupture (MOR) and modulus of elasticity (MOE) according to EN 310 (1993) standard and thickness swelling (TS) according to EN 317 (1993) standard.

2.8 Statistical analysis (Papers II, III and V) For regression analysis of chamber correlations in Paper II and for 1-day and 7-day emission correlation in Paper III, the Design-Expert® software version 11 was used. The significance of regressions were based on the t-test (p < 0.001). Mean values, standard deviations and coefficients of variations were calculated with Microsoft Excel. Standard deviations (sd) were calculated using the formula:

(푥 − 푥̅)2 푠푑 = √∑ 푖 (푛 − 1)

Where 푥푖 is the observed values, 푥̅ the mean of the values, and n is the sample size. Coefficient of variation (CoV) was calculated from the standard deviation (sd) using the formula:

푠푑 퐶표푉 = 푥̅

The statistical differences between the mean values in Paper V were assessed by ANOVA and Tukey’s honestly significant difference (HSD) using a p-value of under 0.05 as threshold of statistical significance.

30

Results and discussion

3. Results and discussion

3.1 Formaldehyde emissions 3.1.1 Evaluation of chamber methods at low emission levels Although chamber methods are considered accurate, there is little data available on chamber repeatability at low emission levels. Most emissions near the levels of natural wood are measured using steady state methods, such as EN 717-1. In this thesis, the possibility of using ASTM D 6007 to detect very low emission levels was also evaluated. The standard deviation was found to be very low for ASTM D 6007 chamber at all emission ranges (Table 5).

Table 5. Results of chamber accuracy evaluations (Paper II).

Chamber Emission Number of Average Standard Variation Average of Average of method range chambers emission deviation coefficient standard variation (ppm) tested (ppm) (ppm) (%) deviations coefficients (ppm) (%) EN 717-1 12 0.064 0.003 4.4 15 0.013 0.002 11.5 15 0.016 0.001 8.1 < 0.05 0.002 9.6 15 0.022 0.002 10.5 15 0.023 0.002 8.3 15 0.053 0.005 8.9 ASTM D 15 0.077 0.008 10.0 0.05 - 6007 15 0.081 0.008 10.4 0.008 9.7 0.10 15 0.084 0.010 11.6 15 0.094 0.007 7.8 15 0.124 0.018 7.3 0.10 - 15 0.146 0.028 9.5 0.010 7.5 0.15 15 0.146 0.016 5.6

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Results and discussion

For the low emitting boards (< 0.05 ppm) the standard deviation was slightly lower (sd = 0.002) than for the higher emission ranges (0.02 - 0.09 ppm; sd = 0.008 and 0.09 - 0.15 ppm; sd = 0.010). However, the variability in formaldehyde emissions is higher in ASTM D 6007 chamber than in the EN 717-1 chamber as revealed by the variation coefficient results. This difference was small, and it could be concluded that the accuracy of both EN 717-1 and ASTM D 6007 chambers is good, even at very low emission levels, as long as heterogeneity in samples and chamber conditions is eliminated.

3.1.2 Correlations between test methods at low emission levels The correlations between formaldehyde measuring methods at emission levels of wood and boards glued with bio-based binders are not well investigated. Because of the high amount of variables in board types and measuring procedures, there are no universal correlations between emission measuring methods. However, for methods measuring the same parameter, such as emission, accurate correlations can be created for boards with relatively similar emission profiles. In this work, correlation between the 1 m3 ASTM D 6007 chamber method and the EN 717-1, DMC, and ISO 12460-5 methods were investigated at low emission values (Table 6).

Table 6. Correlations between the different formaldehyde emission and content measuring methods used in this thesis.

Emission range (ppm) Correlation to Method and board type Paper (acc. ASTM D 6007) 1 m3 ASTM D 6007 ISO 12460-5 PB < 0.10 0.1030 (ns) III DMC PB all < 0.10 0.7218 (*) III DMC PB standard boards < 0.10 0.8387 (*) III EN 717-1 PB < 0.05 0.9167 (*) II EN 717-1 MDF < 0.05 0.9443 (*) II ns = not-significant, p > 0.05 * = significant, p < 0.0001

Perforator method (ISO 12460-5) is the most common factory quality control method used in Europe due to its fast measuring time and low cost of equipment. It gives satisfactory correlation to chamber methods at high emission levels (> 0.09 ppm), but at lower emission levels the correlation between perforator to chamber values have been found to be poor (Roffael & Johnsson, 2012; Yu & Crump, 1999). In this thesis no correlation between the perforator ISO 12460-5

32

Results and discussion and a chamber emission method, ASTM D 6007, was found (r2 = 0.1030, p > 0.05), highlighting the need for better factory quality control methods in Europe. Because of this, an emission based factory quality control method, dynamic microchamber (DMC), was tested for three different board types. Smaller versions of ASTM D 6007 chamber, such as DMC, have the benefit of shorter measurement times. According to ASTM D 6007, three full air changes or 15 minutes, whichever is greater, is required before measurements (ASTM D 6007, 2014). Measurements with 0.044 m3 DMC are thus limited by the 15-minute limit, while the 1 m3 chamber requires a minimum of six hours to reach three air changes before measurements. The small size and high air exchange rate of DMC can lead to larger errors compared to 1 m3 version of the ASTM D 6007 chamber. The errors can be sourced from the un-homogeneous nature of the boards themselves, which was supported by the results of this study. Removing results of the non-standard type board increased the correlation from r2 = 0.7218 to correlation of r2 = 0.8387 (p < 0.0001) (Table 6). The best correlation was obtained between the EN 717-1 and ASTM D 6007 chamber methods (emission range < 0.05 ppm). A strong correlation was found for 3-25 mm thick PBs (r2 = 0.9167) and for MDFs (r2 = 0.9443) (Table 6). The statistically significant (p < 0.0001) regression formulae were:

Formula 1 (PBs): 푦 = 0.7609푥 + 0.002, r2 = 0.9167 Formula 2 (MDF): 푦 = 0.9793푥, r2 = 0.9443

Where y is the EN 717-1 emission value in ppm, and x is the ASTM D 6007 emission value in ppm. These formulae are not meant as universal correlations, but show that good correlations can be achieved between these methods at emission levels below 0.05 ppm. Boards of thicknesses between 3 and 25 mm were used, and the correlations could be further improved by dividing the results into different thickness groups, as is the industrial practice. The industrially interesting emission levels, E1, UBA, and EPA/CARB II levels, are presented in Table 7 with conversions using formulae 1 and 2, and the WKI formula. WKI formula is based on experimental PB data, and calculates emissions based on changes in chamber settings (Meyer et al., 2014). For PB the Formula 1 and the WKI formula gave very similar results. However, variations were detected for MDF, and the WKI formula gave lower ASTM D 6007 results than Formula 2. There are two explanations for this. In this work, only 10 points were used to form the correlation, and further improvement is required. In addition, the WKI formula is originally based on PB data, and only few MDF measurements were used as verification tests (Meyer et al., 2014).

33

Results and discussion

Table 7: The currently critical emission levels calculated to the formaldehyde emission values corresponding to EN 717-1 and ASTM D 6007 methods using Fomula 1 for PB and Formula 2 for MDF, as well as the WKI formula (Meyer et al., 2014) (Paper II).

Calculated formaldehyde emission values (ppm) PB MDF Formaldehyde emission level Formula WKI Formula 2 WKI 1 formula formula ASTM D 6007 European E1 (0.10 ppm ref. EN 717- 0.129 0.119 0.102 0.082 1) UBA (0.05 ppm ref. EN 717-1) 0.063 0.064 0.051 0.044 EN 717-1 (ppm) EPA/CARB II (ref. ASTM D 6007) - 0.09 ppm for PB 0.070 0.074 - 0.11 ppm for MDF 0.108 0.136

3.1.3 Factors influencing emissions: edge-sealing, analytical method and conditioning time It is clear that there are differences between measurement methods that make creating correlations among them difficult. Figure 9 explains some of the parameters that can cause different emissions to be detected from wood-based materials.

Figure 9. Some factors affecting final formaldehyde emission values of chamber methods.

The first two parts to the left, board type and material handling, are independent of the standard used for the measurements and relate to the production parameters and other practices applied by the board manufacturers.

34

Results and discussion

Thickness is the most well known cause of differences in emission values, and because of this, manufacturers often divide boards into thickness groups for certifications, such as CARB II. However, it is important to understand that thickness is not the only variable affecting the final board emission. Amount of recycled wood, the density of the board, glue type used and pressing parameters are also important in defining the behaviour of a board in formaldehyde emission measurements. The part causing most differences in measurements between laboratories and/or board mills is the handling of the boards. The time after the production the samples are taken, and the way they are stored (sealed, stacked, non-stacked, and potential placement in the stack) affect the emissions. In addition, it is important to be aware of the exact board position where the samples have been taken from, as emissions can vary from edges to the centre of the board. The parameters defined by different measurement standards, such as EN 717- 1 and ASTM D 6007, are the board preparation before measurement, and the settings during emission measurement. In this work, the effect of some of these parameters (conditioning, edge-sealing, and analytical method) have been evaluated. Emissions from edges account to approximately one third of the total emissions on a standard board (see Paper II). This can be expected, even though the area of edges is much smaller, as emissions from core are higher than from the surface layer due to core layer having typically lower density, higher porosity and different particle geometry (Roffael et al., 2012; Salem et al., 2011; Zhang et al., 2018). With increasing amount of formaldehyde free resins on the market, it is interesting to discuss how exactly the different edge-sealing of different chamber methods affect the final emissions. If, as an example, a formaldehyde-free bio-based system is used in the surface layer of a particleboard, and a E1-emission level amino-based resin in the core, how much will the edge contribute to the final emissions? Chambers can have different edge-sealing requirements. According to ASTM D 6007 standard, the edges need to be fully sealed, except for the thinnest boards (< 4 mm for PB and < 6 mm for MDF) according to the standard. In oppose to this, both European chambers, EN 717-1 and EN 16516, and the large American chamber ASTM E1333 have some open edges. To evaluate the effect of these small changes in edge sealing, ASTM D 6007 measurements were done with normal full sealing, and with 5% of open edges. The boards with 5% of open edges had in average 10.7% higher emissions, which varied greatly depending on board type from -1% to 28% (see Paper II). The effect was greater on the thickest board, which is in line with previous reports (Kim et al., 2006; Salem et al., 2011). If surface is fully sealed, no emissions of the core layer can affect the final board emissions for methods that seal edges fully. Edge sealing has smaller effect for thinner boards, boards with open surface, and for boards that do not vary greatly between core and surface emissions.

35

Results and discussion

As discussed in the Introduction, MBTH is unsuitable analytical method for wood based products, as it measures all aliphatic aldehydes and not just formaldehyde (Giesen et al., 2016), and chromotropic acid has poor detection limit (0.02 ppm) (California Air Resources Board (CARB), 2010). Because of this, in this thesis only ACAC and DNPH were looked into at low emission level. With standard ACAC method according to EN 717-1 (ACAC water) the difference to DNPH was small at ranges above 0.02 ppm (Figure 10). However, below 0.02 ppm the standard deviation of the absolute percentage difference was 25.6%. This difference could be reduced to 5.6 % by using direct absorber solution (ACAC reagent), where the formaldehyde was directly trapped into the flask containing ACAC solution, instead of the standard handling way. In the standard (ACAC water) method, the formaldehyde is first trapped into two connecting wash bottles with water, and then 10% of this absorption solution is transferred into the analysis flask, and combined with the ACAC solution (EN 717-1, 2004). This leads to potential errors caused by the handling of the absorber solution and using multiple containers. It should be noted that even at the < 0.02 ppm emission level the 25.6% error equals to only 0.005 ppm in emission value, which is very small.

30 25.6

20

Standarddeviation 10 8.1 8.3 5.6 4.1

ofabsolutethe % difference [%] 3.5

0 ≤ 0.02 0.02 - 0.05 0.05 - 0.10 Formaldehyde release range [ppm]

Standard deviation of ACAC (water) to DNPH Standard deviation of ACAC (reagent) to DNPH

Figure 10: Effect of absorber solution (ACAC water: 25 ml water and ACAC reagent: 30 ml acetyl acetone reagent) of ACAC analytical evaluation method on ACAC to DNPH standard deviation of the absolute percent difference at different formaldehyde emission ranges (< 0.02 n=5, 0.02 - 0.05 n=7, 0.05 - 0.10 n=6) (Paper II).

36

Results and discussion

The formaldehyde reduction behavior between different board types was found to vary greatly, as can be seen from Figure 11. As could be expected from general knowledge (Navarrete et al., 2013), all boards had highest emissions in the first day. The low density board continued to reduce emissions even after seven days, while the standard boards stabilized in emissions after 4-6 days. Differences can be expected between board types measured using different methods, as gas analysis is measured after one day of production, ASTM D 6007 after seven days of conditioning, and EN 717-1 and EN 16516 after steady state has been reached (up to 28 days). Thus correlations between methods using different conditioning times need to always be board type specific, and boards with different emission reduction behaviors can not be combined into the same correlations. These results can be considered a rough example on how the emission reduction behavior can differ between board types. More continuous emission measurements of different board types are required for understanding what board properties cause the differences. 0,12

0,1 0.09 ppm 0,08

0,06

0,04

0,02

chamberwith 4021 AL (ppm) 3

1m 0 0 1 2 3 4 5 6 7 8 Time (Days)

Figure 11. Formaldehyde emission (measured with the 1 m3 chamber using the Aero-laser AL4021) reduction with time (days) after the production for three different particleboard types: low density, standard, and standard low-emission. Day 0 is after 2h conditioning from the production. The EPA TSCA Title VI level of 0.09 ppm for particleboards is marked in the graph (Paper III).

In summary, the ACAC and DNPH methods are well comparable, and the differences could be almost fully removed by using direct reagent solution. Opening of 5% of the edges increased the emissions in average 0.006 ppm at emission range 0.03-0.16 ppm. Shortening the conditioning time from seven days to one day had major effect on the boards with emission range 0.03-0.09

37

Results and discussion ppm. This was also highly dependent on the board type, with low density board having significantly higher initial emissions (p < 0.0001, see Paper III). Although the one day conditioning time is not stated in any emission standards, it is the maximum time for production control methods, and will be evaluated in the next part.

3.1.4 Reduction of conditioning time before emission measurements Conditioning time required for DMC according to the ASTM D 6007 standard is seven days. In this thesis, emission values after just 1-day of conditioning were compared to those after 7-days of conditioning for three different board types. No correlation was detected (p > 0.05, Figure 12a) for un-homogeneous low density particleboards (530 kg/m3). However, the emission reduction from day one to day seven was significant (p < 0.0001) for this particleboard type. The particleboard types with higher density, above 600 kg/m3, had significant positive correlations between the 1-day and 7-day DMC emissions (standard board A: r = 0.8721, p = 0.0022 and standard low emission board B: r = 0.8857, p < 0.0001). The A and B particleboard types had similar densities and formaldehyde emission profiles, and the combined correlation between the 1- day and 7-day emissions was better than the individual ones (AB: r = 0.9099, p < 0.0001, Figure 12b). Based on these results, grouping of particleboard types is acceptable only in some cases. This needs to be evaluated separately for each particleboard type.

Figure 12. Correlation between the 1-day and 7-day DMC values for a) particleboard type C (low-density), b) particleboard types A (standard) and B (standard, low-emission) combined. Circles represent values measured from different production batches, dotted lines confidence bands, and numbers before circles indicate more than one measurement at the same location. No outliers exist for a), and two values have been marked in the figure as outliers in b) (Paper III).

38

Results and discussion

These results show that it is possible to get fast and reliable test results with just one day of conditioning using the DMC method. The 1-day to 7-day correlation is however board dependent, and needs to be evaluated separately for each board type if the initial 1-day values are above permitted level. Special boards can behave differently, leading to poor, or no, correlation between the 1- day and 7-day emissions. As an example, less pressure is used in production of the low density particleboards, which can lead to less free formaldehyde escaping during the pressing. Thus, after one day of conditioning the emissions for low emission boards can still be high, and small changes in the particleboard production can affect the initial level greatly. In such case, larger amount of data point needs to be collected to confirm that the final emissions are below the wished control limit. Conditioning with higher air-exchange might have a positive impact on the 1-day to 7-day correlation.

3.2 Bio-based raw-materials Understanding the properties of adhesives used by the particle- and fibreboard industries is important in order to evaluate the potential of bio-based alternatives for their replacement. In Table 8, the advantages and disadvantages of synthetic adhesives used for particle- and fibreboard production are compared.

Table 8. Advantages and disadvantages of synthetic adhesives used in the manufacturing of wood-based panels (Adjusted from Paper I).

Adhesives Parameters UF MUF PF pMDI Price Low Medium to high Medium High

Cure temperature Low Medium High Medium

Press time Very low Low Medium to long Low

Susceptibility against wood High Medium Low High species Compatibility with bio-based Medium Medium Medium to high High raw-materials Manipulations Easy Easy Easy Difficult

Resistant against hydrolysis No Medium to high High High

Use in wet conditions No Partially yes Yes Yes

Formaldehyde emissions E1. CARB E1. CARB II Very low No I (CARB, (CARB, 2007) emission 2007) possible possible

39

Results and discussion

The UF and MUF resins for particle- and fibreboards are low cost, react fast, and are easy to modify for multiple purposes. Comparison to synthetic adhesives makes it obvious that 100% bio-based systems struggle to match their performance and cost. Thus, in order to enter the market, a step-wise approach is needed, using bio-based materials with some amounts of synthetic crosslinkers. Table 9 summarizes the potential of the bio-based raw materials and their most used crosslinkers (Paper I). Tannin was found to differ from the other bio- based raw materials due to its high adhesion and moisture tolerance. It could be crosslinked using formaldehyde or slowly formaldehyde releasing hardeners, such as tris(hydroxylmethyl)nitromethane (TRIS). However, the most researched tannins are not available in large volumes, except in South Africa and South America, where they have been partially commercialized.

Table 9. Potential of biopolymers for wood-based panel adhesives and their main crosslinkers (Adjusted from Paper I).

Biopolymer Potential for adhesives Crosslinker examples Lignin Available, low cost, low reactivity, requires Aldehydes (e.g. glutaraldehyde modification, low moisture resistance and glyoxal), pMDI, tannin Tannin Good adhesion, fast curing, high viscosity, good Hexamine, glyoxal, TRIS water resistance, poor geographical availability Protein Available, low pressing temperature, high Poyamines, PAE, PEI, pMDI, viscosity, low water resistance (most), (ketones) denaturation required Starch Medium cost, low reactivity, low water resistance, Epoxies, pMDI, tannin, chitosan modification/ grafting required

The most developed crosslinker for soy protein is polyamidoamine- epicholohydrin (PAE) (Li, 2007). Soy-PAE systems are already industrially available for plywood and veneer applications with some testing done also on particleboards. Starch based adhesives were found to be most suited for solid wood and plywood applications due to the long pressing time required for curing. In addition, the water tolerance of these systems was found to be lacking. In order for the starch-based adhesives to compete with synthetic resins, extensive modification of the starch, with reactive crosslinker such as pMDI or epoxy, is required. Lignin was found to have high potential due to the low cost and large volumes available. However, press factors in many studies were much higher (up to 30 s/mm) than what can even be considered for lab scale particleboard testing (Lei et al., 2008; Solt et al., 2019). Most testing was done on replacement of phenol in PF resins, with 30% replacement being the maximum without decreasing performance below accepted limits for the industry. The modifications best

40

Results and discussion suited to increase adhesion performance of lignin are methylolation, phenolation and demthylation (Hu et al., 2011). However, most of the studies evaluated Kraft lignin and other lignins from pulping processes. As discussed earlier, the lignins from different extraction processes differ greatly, and some biorefinery lignins might have better reactivity. Less attention has been paid so far on the potential of these lignin residues for wood-based panel adhesives. For the majority of the studies, a lack of industrial viability was evident. As an example, extreme pressing times and high adhesive addition amounts we common. The need for using highly reactive crosslinkers, such as pMDI, has been evident in the research conducted so far. Although bio-based adhesives have the potential to have considerably lower impact on environment than petrochemical adhesives (McDevitt et al., 2014), being more sustainable is not yet enough to replace the urea-formaldehyde based adhesives used today by the particle- and fibreboard industry. Additional drivers towards bio-based adhesives are required, and the increasing interest and control of formaldehyde emissions might be that changing force.

3.2.1 Chemical structure and thermal properties of the biorefinery lignins The word “lignin” is often used to describe everything from natural lignin found in plants, to highly degraded by-products from pulping, and biorefinery processes. In order to utilize lignin residues for higher-value applications, basic understanding of the structure and behaviour in necessary. In this work, two different biorefinery lignosulfonate residues, ammonium lignosulfonate (ALS) and sodium lignosulfonate (SLS) and one supercritical water hydrolysis lignin (SCWH) were compared to gain initial understanding of the properties and variations between the lignins. No purification of the lignins was performed before the analysis, as “use-all” approach was aimed for in this thesis. Elemental analysis revealed low carbon content for the lignosulfonates compared to SCWH lignin, which has previously been reported (Sahoo et al., 2011). Lignosulfonates had also some sulphur (5.5% for ALS and 6.0% for SLS), which improves their performance as dispersants (Ouyang et al., 2009). This can also have an added benefit for adhesive applications. Of interest is also the low amount of inorganics of the SCWH lignin compared to the lignosulfonates. Both sulphite process and the supercritical water hydrolysis process cleave β-O-4 linkages between the phenylpropanoid units in order to extract the cellulose and hemicellulose. The solid state 13C NMR revealed low amounts of β-O-4 likages in lignosulfonates, which also is in line with previous findings, where less than 10% of these linkages was found to remain after the relatively rough sulphite pulping process (Azadi et al., 2013; Chakar & Ragauskas, 2004).

41

Results and discussion

Although low amount of β-O-4 can also be expected from the super critical water hydrolysis lignin (Cocero et al., 2018; Moon et al., 2011), the results of this study indicate that more of the β-R linkages remained in SCWH lignin compared to the lignosulfonates (see Paper IV). The Fourier transform infrared (FTIR) spectra of the three lignins indicated more phenolic -OH groups in the SCWH lignin, and more aliphatic -OH groups in the ALS and SLS lignin samples. Both aromatic and aliphatic hydroxyl groups increase the lignins’ reactivity towards adhesive precursors such as aldehydes, tannins, phenols and isocyanates (El Mansouri & Salvado, 2006; Wang et al., 2018). They also determine the reinforcement effect of lignin in different polymer blends, as grafting of lignin is typically required to make it compatible with polymers such as polylactic acid, polypropylene and polycinyl alcohol (Dias et al., 2017). Thermograms (TG) and derivates of thermograms (DTG) of SCWH, ALS and SLS are shown in Figure 13. The main differences between the lignins was that, unlike the other lignin samples, SLS showed a third maximum degradation temperature (Tmax3) at 760 °C. Loss of mass above 500 °C could be caused by the degradation of aromatic rings (Tejado et al., 2007) or by the release of CO from degenerating C-O-C and C=O bonds (Sahoo et al., 2011). SLS lignin showed also higher residual mass than ALS and SCWH, which can be attributed to the higher decomposition degree of SLS caused by the more severe pulping process (Ghorbani et al., 2018; Wörmeyer et al., 2011), and partially to the existence of inorganic impurities (Sahoo et al., 2011).

100 2

90 C) ° 0 80 70 -2 60 -4 50

Weight Weight (%) -6 40

30 -8 Derivative Derivative weight (%/ 20 -10 0 500 0 200 400 600 800 Temperature (°C) Temperature (°C)

SCWH A-LS S-LS SCWH A-LS S-LS

Figure 13. a) Thermograms (TG) and b) derivate of thermograms (DTG) of supercritical water hydrolysis lignin (SCWH), ammonium lignosulfonate (ALS) and sodium lignosulfonate (SLS) (Paper IV).

42

Results and discussion

3.2.2 Biorefinery lignins for adhesive applications The low amount of inorganics in SCWH lignin, in addition to its low molecular weight and high amount of β-O-4 linkages, makes it suitable for other, more value-added applications, such as conversion to aromatic compounds (Rahimi et al., 2014; Shu et al., 2016). The SCWH lignin is also water soluble only at high pH, limiting its usability without modification. The best adhesive application for SCWH would be phenol replacement in PF resins due to the high amount of phenolic hydroxyl groups. The lignosulfonates were not suitable in their current form for conversion to chemicals due to their high molecular weights and, especially in the case of SLS, the high amount of inorganics. However, the water solubility makes lignosulfonates easy to work with in applications where the lignin is used in its macromolecular state. They are well suited for dispersants, and as the lignosulfonates had more aliphatic hydroxyl groups, there is possibility for interesting crosslinking opportunities. Especially ALS seemed to be a promising material for wood-based panel adhesives and had lower decomposition degree compared to SLS.

3.2.3 Crosslinkers for lignins As discussed in Introduction, there are multiple available crosslinkers for bio- based adhesives. Some of these are universal and work for most bio-materials, such as pMDI (Solt et al., 2019), and some are highly specific for certain bio- materials, such as polyamidoamine-epichlorohydrin (PAE) for soy-protein (Li et al., 2004). In the literature review (Paper I), several crosslinkers for lignins were identified, some of which are shown in Table 10.

Table 10. Examples of major crosslinkers for lignins. Modified from Paper I. The letters L stand for lignin and GL for glyoxalated lignin. In the strength column, shear refers to the tensile shear strength measured for 2-layered veneers and IB to the internal bond measured for particleboards.

Composition Pressing Strength Reference Crosslinker time (MPa) Formaldehyde (F) 1:2 L:F 4 min 3.6 (shear) (Kalami et al., 2017) pMDI 60/40 GL/pMDI 32 s/mm 0.46 (IB) (Lei et al., 2008) 60/40 GL/pMDI 13.1 s/mm 0.67 (IB) (El Mansouri et al., 2006) Tannin (T) 40/60 GL/T 32 s/mm 0.53 (IB) (Navarrete et al., 2012) FOH 50/10/40 L/T/FOH 19 s/mm 0.55 (IB) (Luckeneder et al., 2016)

43

Results and discussion

As discussed earlier, lignins require a strong crosslinker in order to form water resistant networks of covalent bonds. In addition, for wood-based panel applications the reactivity of lignin needs to be increased. The most researched crosslinkers for lignins are aldehydes, pMDI, tannin and furfuryl alcohol (FOH). Out of the aldehydes, the most reactive is the smallest one, formaldehyde (Trosa & Pizzi, 2001). However, due to the formaldehyde emissions, alternative aldehydes, such as glyoxal, have been evaluated. Glyoxalated lignins are often used in combination with more reactive crosslinkers to reach the properties required by particle- and fibreboard industries (Solt et al., 2019). The universal crosslinker, pMDI, has been used with good results to crosslink lignin. However, the pressing times are often very long, and while pMDI is formaldehyde free, it is still fossil-sourced. FOH, on the other hand can be sourced from fully bio-based sources, and there has been some promising results with lignin-FOH systems (Luckeneder et al., 2016). In this work, these two were evaluated as crosslinkers for ammonium lignosulfonate (ALS), pMDI as a synthetic universal crosslinker, and furfuryl alcohol (FOH) as a bio-based more experimental crosslinker. FOH was tested in order to get an indication if the derived 100% bio-based adhesive system could have any potential as particleboard adhesive.

3.2.4 Curing of ALS with pMDI and FOH crosslinkers Reactivity of the adhesive systems can be evaluated using Differential Scanning Calorimetry (DSC). As shown in Figure 14, addition of FOH as a crosslinker to ALS lowered the curing peak from 129 to 119 °C for the ALS FOH sample, and to 120 °C for the mimosa tannin reinforced ALS FOH sample (ALS FOH + m). In addition, it lowered the required curing heat from 172 J/g to 117 J/g for the ALS FOH sample and to 126 J/g for the ALS FOH + m sample. These results are promising for the ALS FOH systems, as the energy required for the curing them is lower than that of the pure ALS.

44

Results and discussion

5

) 1 - 0

-5 ALS -10 ALS FOH

ALS FOH + m Heat flow Heat flow (mW.mg -15 ALS pMDI ALS pMDI + m -20 25 50 75 100 125 150 175 200 Temperature (°C)

Figure 14. DSC heat of reaction thermograms of ammonium lignosulfonate (ALS) with two different crosslinkers: pMDI and furfuryl alcohol (FOH), and without and with mimosa tannin (+ m) (Paper V).

Measuring DSC of samples containing pMDI is challenging, as pMDI starts the curing process as soon as it comes in contact with the water of the ALS, even in room temperature. Despite this, a clear lowering of the curing temperature peak and the curing heat could be seen for the ALS pMDI sample (107 °C, 123 J/g). Strangely enough, a similar effect could not be seen for the ALS pMDI + m sample. This could be due to pre-curing of pMDI, as tannin is more reactive than lignin. Based on this the ALS pMDI sample required least amount of energy for curing at lowest temperature.

3.2.5 Bonding strength of ALS adhesives and particleboard properties

Adhesives for particleboards are often tested on veneer or solid wood to determine bonding strength. However, the requirements for particleboard adhesives differ greatly compared to those of solid wood and veneer/plywood applications. The pressing times are shorter, the adhesives need to have better sprayability, and adhesion penetration requirements are different. In this work, the properties of the ALS samples with pMDI and FOH crosslinkers were tested first with 2-layered veneer samples and then for making particleboards. Tensile strengths of all crosslinked samples were at the level of the UmF reference and significantly higher than the ALS sample without crosslinker (Table 11, and for a more detailed analysis see paper V). There was no

45

Results and discussion significant difference (p > 0.05) between the FOH and pMDI crosslinked samples, or between the samples with or without tannin (p > 0.05).

Table 11. Bonding properties of tested adhesive combinations of ammonium lignosulfonate (ALS), furfuyl alcohol (FOH), pMDI and mimosa tannin (m) with 2-layered veneer samples. For each property statistically different values are marked with different letter in parenthesis at an error probability of α = 0.05 (ANOVA and Tukey’s HSD tests)

ALS ALS ALS ALS Property ALS UmF FOH FOH + m pMDI pMDI + m

2 Tensile strength (N/mm ) 0.56 (a) 1.30(b) 1.72 (b) 1.69 (b) 1.47 (b) 1.54 (b)

Bondline thickness (µm) - - 52 (a) 67 (a) 12 (b) 26 (b) Hyphens (-) denote not measured samples

Although the tensile shear strength values were not different between the pMDI and FOH crosslinked samples, there was a significant difference (p < 0.0001) in the bondline thickness. The bondline thickness of ALS FOH and ALS FOH + m samples were 52 µm and 67 µm, respectively, while the ALS pMDI and ALS pMDI + m samples reached only bondline thickness of 12 µm and 26 µm, respectively. In addition, thick and dark color bondlines could be detected for ALS FOH samples, while the ALS pMDI samples had light, imperfect and starved bondlines (see paper V). Reaction of isocyanate groups with moisture can result in the production of CO2 gas, which causes an inner vapour pressure that drives more adhesive from the bondline towards the wooden structure (Bastani et al., 2016; Bastani et al., 2017). The good tensile shear strength results of the veneer testing did not carry on to the particleboard tests. Table 12 shows the internal bond (IB) strength of the tested particleboards. Addition of FOH increased the strength to a level where the board did not laminate after pressing and could be measured, unlike the ALS sample. However, the strength values were significantly lower (0.17 N/mm2 for ALS FOH and 0.18 N/mm2 for ALS FOH + m) than that of the UmF reference (0.33 N/mm2). This might be due to the fact that less reaction time was used for the FOH crosslinked adhesives meant for the particleboard production than for the veneers to keep the viscosity at a level suitable for spraying. The pMDI crosslinked ALS samples had significantly higher IB values (0.62 N/mm2 for ALS pMDI, and to 0.49 N/mm2 for ALS pMDI + m) than the UmF reference (0.33 N/mm2). Interestingly enough, the addition of tannin gave notably lower (p > 0.05) IB strength, indicating that the higher curing heat of ALS pMDI + m sample noticed in the DSC measurements might not have only been caused by precuring of the pMDI.

46

Results and discussion

Table 12. Properties of tested adhesive combinations of ammonium lignosulfonate (ALS), furfuyl alcohol (FOH), pMDI and mimosa tannin (m) with particleboard. For each property statistically different values are marked with different letter in parenthesis at an error probability of α = 0.05 (ANOVA and Tukey’s HSD tests)

ALS ALS ALS ALS Property ALS UmF FOH FOH + m pMDI pMDI + m 2 IB (N/mm ) x 0.33 (a) 0.17 (b) 0.18 (b) 0.62 (c) 0.49 (d) 2 MOR (N/mm ) x 7.28 (a) 3.25 (b) 4.13 (ab) 11.08 (c) 10.85 (c) 1632 MOE (N/mm2) x (a) 691 (b) 936 (b) 2057 (a) 1856 (a) 2h TS (%) x 40 (a) 122 (b) 108 (b) 14 (c) 9 (c) 24h TS (%) x 52 (a) x x 26 (b) 19 (b) Emission (ppm) x 0.073 0.059 0.050 0.027 0.028 TS: thickness swelling x: failure before measurement

Swelling test, where the boards are immersed into water for 2h and 24h, does not fully reflect the environment where the particleboards will be used eventually. In most applications, moisture enters particleboards as vapour from the air and not as liquid water. However, the swelling test gives an indication on the moisture resistance properties and the curing of the adhesive. As seen in Table 12, both of the FOH samples delaminated in water already after 2h. The pMDI bonded samples performed better than the UmF reference: ALS pMDI sample having swelling of 14% after 2h, and 26% after 24h, and ALS pMDI + m swelling of 9% after 2h and 19% after 24h. In this case, the mimosa tannin reinforced board performed slightly better.

3.2.6 Formaldehyde emissions of the ALS-based adhesives compared to wood emissions The fact that formaldehyde-free adhesives do not lead to formaldehyde-free boards has already been discussed in the Introduction. The final emission from wood-based panel is the sum of emissions from the wooden part and the glue, both affected by the processing. The natural formaldehyde emission level from the wood part of boards is affected by the raw material source, size, drying and heat treatment of the particles and fibres, as seen in Figure 15 and supported by previous findings (Böhm et al., 2012; He et al., 2012; Martínez & Belanche, 2000; Roffael et al., 2012; Salem & Böhm, 2013; Weigl et al., 2009).

47

Results and discussion

Figure 15. Blue, orange and grey bars represent formaldehyde emissions from particles and fibres after the dryer from boards without glue, and from standard boards made out of the same particles and fibres for two particleboard suppliers and one MDF supplier measured using 1m3 ASTM D 6007 chamber. Green bars represent pMDI and FOH crosslinked ALS glued laboratory particleboards and their UMF reference measured using 0.044 m3 ASTM D 6007 chamber. Emission values in ppm (ASTM D 6007) are marked with numbers on top of the bars and lines. Modified from Papers II and V.

In Figure 15 these wood-based emissions are displayed with emissions of laboratory particleboards produced using the pMDI and FOH crosslinked ALS, with and without mimosa tannin (m). The samples glued with bio-based adhesives cannot be directly compared to the wood-based emissions due to different raw material source and smaller chamber size. However, it is indicated that the emissions of ALS pMDI bonded boards were lower (0.27 ppm for ALS pMDI and 0.28 ppm for ALS pMDI + m) than the emissions of boards bonded without adhesive (0.045 ppm for supplier 1, and 0.040 for supplier 2) but higher than particles before pressing. This is below any legal limit for wood-based panels, and much lower than the emission of the UmF bonded laboratory particleboard (0.073 ppm). The emissions of ALS FOH and ALS FOH + m samples were at the level of UmF bonded commercial particleboards (ALS FOH: 0.059 ppm and ALS FOH + m: 0.050 ppm). This is higher than what can be expected just from the wood itself. Although FOH can be classified as a non-added formaldehyde (NAF) adhesive, one of its polycondensation mechanism does lead to release of formaldehyde (Figure 16).

48

Results and discussion

Figure 16. One of furfuryl polycondensation pathways: reaction between the methylol groups of two furfuryl alcohol molecules, leading to dimethyl ether bride that can be transformed into a methylene bridge by release of formaldehyde (Bertarione et al., 2009; Szczurek et al., 2016).

The unexpected emission behavior of the boards bound with adhesives that can be classified as “non-added formaldehyde” highlight the need for accurate emission measuring methods at low levels. Especially, as the legal limits in some countries (e.g. Germany) are getting closer to the natural emission level of wood. In this thesis, the wood-based emissions were found to be below 0.045 ppm (ASTM D 6007).

49

Conclusions

4. Conclusions

This work was motivated by current needs of particle- and fibreboard industries regarding formaldehyde issues and new sustainable adhesives. The lack of correlations and information on the repeatability of chamber methods at the modern low levels (0.05 ppm EN 717-1) is problematic for the industry. This work has shown that measuring and controlling formaldehyde emissions of PBs and MDFs at low levels is possible at both laboratory and factory settings. The low levels are also opening the market for formaldehyde-free bio-based adhesives. These systems have been evaluated, and one group with growing potential, biorefinery lignins, has been further characterized and tested preliminary as board adhesives.

Main conclusions:  Formaldehyde emissions can be measured accurately at low emission levels (< 0.05 ppm) using EN 717-1 and ASTM D 6007 chambers.  There is a good correlation between EN 717-1 and ASTM D 6007 chambers below 0.05 ppm emission level (r2 = 0.9167 for PB and r2 = 0.9443 for MDF), as long as standardized handling of samples is secured.  Opening of 5% of the edges in ASTM D 6007 measurements was found to have an increasing effect on the emissions (in average 10.7%), which was highly dependent on the board type (variation between -1 and 28%).  Emission-based formaldehyde testing methods can, and should, be used at factory settings. Results obtained after 1-day of conditioning can be used to predict the final 7-day emissions for some board types. This work has highlighted the need for creating 1-day to 7- day correlation for each separate board type.  The literature study revealed large amount of different bio-based alternatives to the amino-based resins, some even commercialized at small scales. These systems, however, need a reactive crosslinker to reach acceptable moisture resistance and industrial parameters for their application.  New types of lignins are available with the increasing amount of bio- refineries. Supercritical water hydrolysis (SCWH) lignin varies greatly from sulphur containing lignin types, and should not be dismissed based on results from Kraft lignin research. SCWH lignin

50

Conclusions

is well suited for conversion to aromatic compounds, carbon fibres, and for phenol replacement in PF resins.  Lignosulfonates have best utilization possibilities in their macromolecular state, as dispersants, and potentially as adhesive precursors.  pMDI is better crosslinker than furfuryl alcohol (FOH) for ammonium lignosulfoante (ALS) for particleboard adhesive formulations. For gluing where higher viscosities and low pH are not an issue, polymerized furfuryl alcohol may also be a viable option.  Non-added-formaldehyde adhesives do not necessarily mean non- emitting adhesives. The ALS FOH combinations emitted more formaldehyde than the natural level of wood, while ALS pMDI samples were at the emission levels of natural wood after one day conditioning.

51

Future work

5. Future work

Correlations between chambers are affected by many factors, such as temperature, humidity and air-exchange rate. In this thesis, some additional factors have been discussed, including edge-sealing. Future investigations are required to evaluate how edge-sealing affects the new types of boards, which can have in the core and surface layers adhesive systems that differ considerably in their emission level and profile. The research done has shown that the conditioning time of ASTM D 6007 can be shortened to 1 day, and still obtain good correlation to the final 7-day emissions. There are some open questions remaining, such as how does the conditioning environment affect this correlation. For one of the board types tested, there was a poor correlation between the 1-day and 7-day emissions. It could also be seen that the emissions of this board type were higher after the first day of conditioning. Could the correlation between 1-day and 7-day emissions be improved by increasing the air-exchange rate in the conditioning chamber/room during the first day of conditioning? The combinations of ALS and crosslinkers were preliminary trials to see the behaviour of ALS as a particleboard adhesive. Further studies will reveal the interactions between ALS and the crosslinkers, as well as show how much the lignin does actually contribute to the final strength of the adhesive. Modifications of ALS, such as glyoxylation, could be studied for improved reaction speed and adhesion. Of interest is also to determine the lowest permitted amount of synthetic crosslinker while keeping the strength required by the industry. As it seems now, crosslinker development is the key to bring bio-based adhesives to the level of amino-based adhesives when it comes to the balance of price and performance.

52

References

6. References

Allen, A. J., Wescott, J. M., Varnell, D. F., & Evans, M. A. (2011). Protein adhesive formulations with amine-epichlorohydrin and isocyanate additives. US20110293934A1. Aro, T., & Fatehi, P. (2017). Production and application of lignosulfonates and sulfonated lignin. In ChemSusChem (Vol. 10, pp. 1861-1877). ASTM D 6007 (2014). Standard test method for determining formaldehyde concentrations in air from wood products using a small-scale chamber. In: ASTM International. Azadi, P., Inderwildi, O. R., Farnood, R., & King, D. A. (2013). Liquid fuels, hydrogen and chemicals from lignin: A critical review. Renewable and Sustainable Energy Reviews, 21, 506-523. Bao, S., Daunch, W. A., Sun, Y., Rinaldi, P. L., Marcinko, J. J., & Phanopoulos, C. (2003). Solid state two-dimensional NMR studies of polymeric diphenylmethane diisocyanate (PMDI) reaction in wood. Forest Products Journal, 53(6), 63. Bastani, A., Adamopoulos, S., Koddenberg, T., & Militz, H. (2016). Study of adhesive bondlines in modified wood with fluorescence microscopy and X-ray micro-computed tomography. International Journal of Adhesion and Adhesives, 68, 351-358. doi:10.1016/j.ijadhadh.2016.04.006 Bastani, A., Adamopoulos, S., & Militz, H. (2017). Effect of open assembly time and equilibrium moisture content on the penetration of polyurethane adhesive into thermally modified wood. The Journal of Adhesion, 93(7), 575-583. doi:10.1080/00218464.2015.1118621 Bertarione, S., Bonino, F., Cesano, F., Jain, S., Zanetti, M., Scarano, D., & Zecchina, A. (2009). Micro-FTIR and micro-Raman studies of a carbon film prepared from furfuryl alcohol polymerization.(fourier transform infrared spectroscopy). Journal of Physical Chemistry B, 113(31), 10571- 10574. doi:10.1021/jp9050534 Birkeland, M. J., Lorenz, L., Wescott, J. M., & Frihart, C. R. (2010). Determination of native (wood derived) formaldehyde by the desiccator method in particleboards generated during panel production. Holzforschung, 64(4), 429-433. doi:10.1515/HF.2010.070 Bundesministerium für Umwelt Naturschutz und nukleare Sicherheit. (2018). Bekanntmachung analytischer Verfahren für Probenahmen und Untersuching für die in Anlage 1 der Chemikalien-Verbotsverordnung genannten Stoffe und Stoffgruppen. Bundesanzeiger, BAnz AT 26.11.2018 B2. Böhm, M., Salem, M. Z. M., & Srba, J. (2012). Formaldehyde emission monitoring from a variety of solid wood, plywood, blockboard and flooring products manufactured for building and furnishing materials. Journal of Hazardous Materials, 221-222, 68-79. doi:10.1016/j.jhazmat.2012.04.013

53

References

California Air Resources Board (CARB). (2010). Third Party Certification Guideline: Limitations of Chromotropic Acid in Small-Scale Test Methods. Guidline CWP-10-002 for Title 17, California Code of Regulations, Sections 93120-93120.12. Calvo-Flores, F. G. (2015). Lignin and lignans as renewable raw materials: chemistry, technology and applications: John Wiley and Sons, Incorporated. Cantero, D. A., Dolores Bermejo, M., & José Cocero, M. (2015). Reaction engineering for process intensification of supercritical water biomass refining. The Journal of Supercritical Fluids, 96, 21-35. doi:10.1016/j.supflu.2014.07.003 CARB. (2007). California Air Resource Board, § 93120, Airborne toxic control measure to reduce formaldehyde emissions from composite wood products. California Code of Regulations, section 93120-93120.12, title 17. Carvajal, J. C., Gómez, Á., & Cardona, C. A. (2016). Comparison of lignin extraction processes: Economic and environmental assessment. Bioresource Technology, 214, 468-476. doi:10.1016/j.biortech.2016.04.103 Carvalho, L., Magalhães, F., & João, F. (2012). Formaldehyde Emissions from Wood-Based Panels -Testing Methods and Industrial Perspectives. In Formaldehyde: Chemistry, Applications and Role in Polymerization: Nova Science Publishers, Inc., Hauppauge, NY. Cavani, F. (2016). Chemicals and fuels from bio-based building blocks. Weinheim, Germany: Wiley-VCH Verlag GmBH & Company KGaA. Chakar, F. S., & Ragauskas, A. J. (2004). Review of current and future softwood kraft lignin process chemistry. Industrial crops and products, 20(2), 131- 141. doi:10.1016/j.indcrop.2004.04.016 Cocero, M. J., Cabeza, Á., Abad, N., Adamovic, T., Vaquerizo, L., Martínez, C. M., & Pazo-Cepeda, M. V. (2018). Understanding biomass fractionation in subcritical & supercritical water. The Journal of Supercritical Fluids, 133, 550-565. doi: 10.1016/j.supflu.2017.08.012 Commission Regulation (EU). (2014). No 605/2014. Official Journal of the European Union, L 167/36. Congress Bill. (2002). Farm Security and Rural Investment Act. H.R.2646 (107th) Congress of the United States of America. Costa, N., Pereira, J., Martins, J., Ferra, J., Cruz, P., Magalhães, F., . . . Carvalho, L. (2011). Alternative to latent catalysts for curing UF resins used in the production of low formaldehyde emission wood-based panels. International Journal of Adhesion and Adhesives. doi:10.1016/j.ijadhadh.2011.11.003 Costello, H. (2017). Lignin products global market size, sales data 2017–2022 & applications in animal feed industry Retrieved from (Market Analysis No. 188450), Orbis Research, Dallas, USA: da Silva, C. G., Grelier, S., Pichavant, F., Frollini, E., & Castellan, A. (2013). Adding value to lignins isolated from sugarcane and Miscanthus.

54

References

Industrial Crops & Products, 42, 87-95. doi:10.1016/j.indcrop.2012.04.040 Dias, O. A. T., Negrão, D. R., Gonçalves, D. F. C., Cesarino, I., & Leão, A. L. (2017). Recent approaches and future trends for lignin-based materials. Molecular Crystals and Liquid Crystals, 655(1), 204-223. doi:10.1080/15421406.2017.1360713 Dunky, M. (2003). Adhesives in the Wood Industry (Vol. 70). New York: Marcel Dekker. Durkic, K. (2009). Measures to reduce the formaldehyde emission of wood-based panels. Hannover, Germany: Formaldehyde Workshop. El Mansouri, N. E., Pizzi, A., & Salvadó, J. (2006). Lignin-based wood panel adhesives without formaldehyde. Holz als Roh- und Werkstoff, 65(1), 65. doi:10.1007/s00107-006-0130-z El Mansouri, N. E., & Salvado, J. (2006). Structural characterization of technical lignins for the production of adhesives: Application to lignosulfonate, kraft, soda-anthraquinone, organosolv and ethanol process lignins. Industrial crops and products, 24, 8-16. EN 205. (2016). Adhesives. Wood adhesives for non-structural applications. Determination of tensile shear strength of lap joints. In. Brussels, Belgium European Committee for Standardization. EN 310. (1993). Wood-based panels - Determination of modulus of elasticity in bending and of bending strength. In: European Standard. EN 317. (1993). Particleboards and fibreboards - Determination of swelling in thickness after immersion in water. In: European Standard. EN 319. (1993). Particleboards and fibreboards - Determination of tensile strength perpendicular to the plane of the board. In: European Standard. EN 717-1. (2004). Wood-based panels – determination of formaldehyde release – part 1: formaldehyde emission by the chamber method. In: European Standard. Engström, M. T., Karonen, M., Ahern, J. R., Baert, N., Payré, B., Hoste, H., & Salminen, J. P. (2016). Chemical structures of plant hydrolyzable tannins reveal their in vitro activity against egg hatching and motility of haemonchus contortus nematodes. Journal of agricultural and food chemistry, 64(4), 840. doi:10.1021/acs.jafc.5b05691 European Panel Federation. (2017). EPF Annual Report 2016–2017. Brussels. Farag, S., Kouisni, L., & Chaouki, J. (2014). Lumped approach in kinetic modeling of microwave pyrolysis of Kraft lignin. Energy & Fuels, 28(2), 1406-1417- 1406-1417. doi:10.1021/ef4023493 Ferdosian, F., Pan, Z., Gao, G., & Zhao, B. (2017). Bio-based adhesives and evaluation for wood composites application. Polymers, 9(2). doi:10.3390/polym9020070 Fliedner, E., Heep, W., & van Herwijnen, H. W. G. (2010). Verwendung nachwachsender Rohstoffe in Bindemitteln für Holzwerkstoffe. Chemie Ingenieur Technik, 82, 1161-1168. Frihart, C., & J. Birkeland, M. (2014). Soy properties and soy wood adhesives. In (Vol. 1178, pp. 167-192).

55

References

Frihart, C., Wescott, J., Chaffee, T., & Gonner, K. (2012). Formaldehyde Emissions from Urea- Formaldehyde- and No-Added- Formaldehyde-Bonded Particleboard as Influenced by Temperature and Relative Humidity. Forest Products Journal, 62(7/8), 551-558. Gadhave, R. V., Mahanwar, P. A., & Gadekar, P. T. (2018). Starch stabilized polyvinyl acetate emulsion: Review. Polymers from Renewable Resources, 9(2), 75-84. doi:10.1177/204124791800900203 Gan, L., Zhou, M., Yang, D., & Qiu, X. (2012). Preparation and evaluation of carboxymethylated lignin as dispersant for aqueous graphite suspension using Turbiscan Lab Analyzer. Journal of Dispersion Science and Technology, 34(5). doi:10.1080/01932691.2012.686248 Ghorbani, M., Konnerth, J., van Herwijnen, H. W. G., Zinovyev, G., Budjav, E., Requejo Silva, A., & Liebner, F. (2018). Commercial lignosulfonates from different sulfite processes as partial phenol replacement in PF resole resins. Journal of Applied Polymer Science, 135(8), 45893. doi:10.1002/app.45893 Giesen, R., Schripp, T., Markewitz, D., Meyer, B., Schwab, H., Uhde, E., & Salthammer, T. (2016). Comparison of Methods for the Determination of Formaldehyde in Air. Analytical Letters, 49(10), 1613-1621. doi:10.1080/00032719.2015.1107083 Gosselink, R. J. A., Abächerli, A., Semke, H., Malherbe, R., Käuper, P., Nadif, A., & van Dam, J. E. G. (2004). Analytical protocols for characterisation of sulphur-free lignin. Industrial Crops & Products, 19(3), 271-281. doi:10.1016/j.indcrop.2003.10.008 GOST 30255. (2014). Furniture, timber and polymers. Method for determination of formaldehyde and other volatile chemicals in the air of climatic chambers. In: Euro-Asian Council for Standardization Metrology and Certification (EASC). Gu, K., & Li, K. (2011). Preparation and evealuation of particleboard with a soy flour‐polyethylenimine‐maleic anhydride adhesive. Journal of the American Oil Chemists' Society, 88(5), 673-679. doi:10.1007/s11746-010- 1706-7 Hamarneh, A. I., Heeres, H. J., Broekhuis, A. A., Sjollema, K. A., Zhang, Y., & Picchioni, F. (2010). Use of soy proteins in polyketone-based wood adhesives. International Journal of Adhesion and Adhesives, 30(7), 626- 635. doi:DOI: 10.1016/j.ijadhadh.2010.06.002. He, Z., Zhang, Y., & Wei, W. (2012). Formaldehyde and VOC emissions at different manufacturing stages of wood-based panels. Building and Environment, 47(1), 197-204. doi:10.1016/j.buildenv.2011.07.023 Himmel, S., Irle, M., Legrand, G., Perez, R., & Mai, C. (2014). Effects of recovered wood on the formaldehyde release of particleboards. Holzforschung, 68(6), 669-678. doi:10.1515/hf-2013-0131 Holladay, J., Bozell, J., Johnson, D., & White, J. (2007). Top value added chemicals from bio-mass: Results of screening potential candidates from biorefinery lignin (Vol. 2). United States.

56

References

Hu, L., Pan, H., Zhou, Y., & Zhang, M. (2011). Methods to improve lignin's reactivity as a phenol substitute and as replacement for other phenolic compounds: A brief review BioResources, 6(3), 3515-3525. doi:10.15376/biores.6.3.3515-3525 Huang, X., Atay, C., Zhu, J., Palstra, S. W. L., Korányi, T. I., Boot, M. D., & Hensen, E. J. M. (2017). Catalytic depolymerization of lignin and woody biomass in supercritical ethanol: Influence of reaction temperature and feedstock. ASC Sustainable Chemistry & Engineering, 5(11), 10864- 10874. doi:10.1021/acssuschemeng.7b02790 Imam, S. H., Mao, L., Chen, L., & Greene, R. V. (1999). Wood adhesive from crosslinked poly(vinyl alcohol) and partially gelatinized starch: preparation and properties. Starch ‐ Stärke, 51(6), 225-229. International Agency for Research on Cancer (IARC). (2004). IARC classifies formaldehyde as carcinogenic to humans. Press Release June 15, No. 153. Inwood, J. P. W., Pakzad, L., & Fatehi, P. (2017). Production of sulfur containing Kraft lignin products. BioResources, 13(1). doi:10.15376/biores.13.1.53- 70 Irle, M., C. Barbu, M., Niemz, P., Carvalho, L., Martins, J., Costa, C., . . . Paul, W. (2010). Wood-Based Panels: An Introduction for Specialists. ISO 12460-3 (2015). Wood-based panels - Determination of formaldehyde release - Part 3: Gas analysis method. In. ISO 12460-5 (2015). Wood-based panels - Determination of formaldehyde release - Part 5: Extraction method (called the perforator method) In: International Organization for Standardization. Jang, Y., Huang, J., & Li, K. (2011). A new formaldehyde-free wood adhesive from renewable materials. International Journal of Adhesion and Adhesives, 31(7), 754-759. doi:DOI: 10.1016/j.ijadhadh.2011.07.003. JIS A 1460 (2001). Japanese Industrial Standard. Building boards. Determination of formaldehyde emission-desiccator method. In. JIS A 1901 (2009). Determination of the emissions of volatile organic compounds and aldehydes for building products - Small chamber method. Kalami, S., Arefmanesh, M., Master, E., & Nejad, M. (2017). Replacing 100% of phenol in phenolic adhesive formulations with lignin. Journal of Applied Polymer Science, 134(30), 45124. doi:10.1002/app.45124 Kim, S., Kim, J.-A., Kim, H.-J., Hyoung Lee, H., & Yoon, D.-W. (2006). The effects of edge sealing treatment applied to wood-based composites on formaldehyde emission by desiccator test method. Polymer Testing, 25(7), 904-911. doi:10.1016/j.polymertesting.2006.05.010 Kruse, A., & Dahmen, N. (2015). Water – A magic solvent for biomass conversion. The Journal of Supercritical Fluids, 96, 36-45. doi:10.1016/j.supflu.2014.09.038 Lei, H., Pizzi, A., & Du, G. (2008). Environmentally friendly mixed tannin/lignin wood resins. Journal of Applied Polymer Science, 107(1), 203-209. doi:10.1002/app.27011

57

References

Lei, H., Pizzi, A., Navarrete, P., Rigolet, S., Redl, A., & Wagner, A. (2010). Gluten Protein Adhesives for Wood Panels. Journal of Adhesion Science and Technology, 24(8-10), 1583-1596. doi:10.1163/016942410X500963 Li, K. (2007). Formaldehyde-free lignocellulosic adhesives and composites made from the adhesives. US7252735B2. Li, K., Peshkova, S., & Geng, X. (2004). Investigation of Soy Protein-Kymene Adhesive Systems for Wood Composites. Journal of the American Oil Chemists' Society (JAOCS), 81(5), 487-491. Liang, W., Yang, S., & Yang, X. (2015). Long-term formaldehyde emissions from medium-density fiberboard in a full-scale experimental room: Emission characteristics and the effects of temperature and humidity. Environmental Science & Technology, 49(17), 10349-10356. doi:10.1021/acs.est.5b02217 Luckeneder, P., Gavino, J., Kuchernig, R., Petutschnigg, A., & Tondi, G. (2016). Sustainable phenolic fractions as basis for furfuryl alcohol-based co- polymers and their use as wood adhesives. Polymers, 8(11), 396. doi:10.3390/polym8110396 Mansouri, H., Navarrete, P., Pizzi, A., Tapin-Lingua, S., Benjelloun-Mlayah, B., Pasch, H., & Rigolet, S. (2011). Synthetic-resin-free wood panel adhesives from mixed low molecular mass lignin and tannin. European Journal of Wood and Wood Products, 69(2), 221-229. Mantanis, G. I., Athanassiadou, E. T., Barbu, M. C., & Wijnendaele, K. (2017). Adhesive systems used in the European particleboard, MDF and OSB industries. Wood Material Science & Engineering, 13(2), 104-116. doi:10.1080/17480272.2017.1396622 Martínez, E., & Belanche, M. I. (2000). Influence of veneer wood species on plywood formaldehyde emission and content. European Journal of Wood and Wood Industries, 58(1), 31-34. doi:10.1007/s001070050381 Marutzky, R. (2008). Opening and thematic introduction. Proceedings of the technical formaldehyde conference, hannover, Germany. . Masina, N., Choonara, Y. E., Kumar, P., du Toit, L. C., Govender, M., Indermun, S., & Pillay, V. (2017). A review of the chemical modification techniques of starch. Carbohydrate Polymers, 157, 1226-1236. doi:10.1016/j.carbpol.2016.09.094 Mayer, J. (1978). Chemische Aspekte bei der Entwicklung formaldeydarmer Klebstoffe für die Holzwerkstoffindustrie. Spanplatten heute und morgen. Stuttgart: DRW-Verlag, 102–111. McDevitt, J., Grigsby, E., Grigsby, W., & Grigsby, J. (2014). Life cycle assessment of bio- and petro-chemical adhesives used in fiberboard production. Journal of Polymers and the Environment, 22(4), 537-544. doi:10.1007/s10924-014-0677-4 Meyer, B., Greubel, D., Schwab, H., & Marutzky, R. (2014). Formaldehydemissionen aus Spanplatten: Aktualisierung des WKI- Rechenmodells. Holztechnologie, 55(6), 20-26. Moon, S.-J., Eom, I.-Y., Kim, J.-Y., Kim, T.-S., Lee, S. M., Choi, I.-G., & Choi, J. W. (2011). Characterization of lignin-rich residues remaining after

58

References

continuous super-critical water hydrolysis of poplar wood (Populus albaglandulosa) for conversion to fermentable sugars. Bioresource Technology, 102(10), 5912-5916. doi:10.1016/j.biortech.2011.02.091 Moubarik, A., Causse, N., Poumadere, T., Allal, A., Pizzi, A., Charrier, F., & Charrier, B. (2011). Shear Refinement of Formaldehyde-Free Corn Starch and Mimosa Tannin (Acacia mearnsii) Wood Adhesives. Journal of Adhesion Science and Technology, 25(14), 1701-1713. doi:10.1163/016942411X576176 Myers, G. E., & Koutsky, J. A. (1990). Formaldehyde liberation and cure behavior of urea-formaldehyde resins. In Holzforschung - International Journal of the Biology, Chemistry, Physics and Technology of Wood (Vol. 44, pp. 117). Navarrete, P., Kebbi, Z., Michenot, F., Lemonon, J., Rogaume, C., Masson, E., . . . Pizzi, A. (2013). Formaldehyde and VOCs emissions from bio- particleboards. Journal of Adhesion Science and Technology, 27(7), 748- 762. doi:10.1080/01694243.2012.727149 Navarrete, P., Pizzi, A., Tapin-Lingua, S., Benjelloun-Mlayah, B., Pasch, H., Rode, K., . . . Rigolet, S. (2012). Low Formaldehyde Emitting Biobased Wood Adhesives Manufactured from Mixtures of Tannin and Glyoxylated Lignin. Journal of Adhesion Science and Technology, 26(10-11), 1667- 1684. doi:10.1163/156856111X618489 Nie, Y., Tian, X., Liu, Y., Wu, K., & Wang, J. (2013). Research on starch- g- polyvinyl acetate and epoxy resin-modified corn starch adhesive. Polymer Composites, 34(1), 77. Nordqvist, P., Nordgren, N., Khabbaz, F., & Malmström, E. (2013). Plant proteins as wood adhesives: Bonding performance at the macro- and nanoscale. Industrial Crops & Products, 44, 246-252. doi:10.1016/j.indcrop.2012.11.021 Ouyang, X., Ke, L., Qiu, X., Guo, Y., & Pang, Y. (2009). Sulfonation of alkali lignin and its potential use in dispersant for cement. Journal of Dispersion Science and Technology, 30(1), 1-6. doi:10.1080/01932690802473560 Paiva, N. T., Henriques, A., Cruz, P., Ferra, J. M., Carvalho, L. H., & Magalhães, F. D. (2012). Production of melamine fortified urea-formaldehyde resins with low formaldehyde emission. Journal of Applied Polymer Science, 124(3), 2311-2317. doi:10.1002/app.35282 Park, B.-D., & Jeong, H.-W. (2011). Hydrolytic stability and crystallinity of cured urea–formaldehyde resin adhesives with different formaldehyde/urea mole ratios. International Journal of Adhesion and Adhesives, 31(6), 524-529. doi:10.1016/j.ijadhadh.2011.05.001 Pichelin, F., Kamoun, C., & Pizzi, A. (1999). Hexamine hardener behaviour: effects on wood glueing, tannin and other wood adhesives. Holz als Roh- und Werkstoff - European Journal of Wood and Wood Industries, 57(5), 305. Pierce, J. S., Abelmann, A., Lotter, J. T., Ruestow, P. S., Unice, K. M., Beckett, E. M., . . . Finley, B. L. (2016). An assessment of formaldehyde emissions from laminate flooring manufactured in China. Regulatory Toxicology and Pharmacology, 81, 20-32. doi:10.1016/j.yrtph.2016.06.022

59

References

Pizzi, A. (1994). Advanced Wood Adhesives Technology. New York: Taylor & Francis. Pizzi, A. (2006). Recent developments in eco-efficient bio-based adhesives for wood bonding: opportunities and issues. Journal of Adhesion Science and Technology, 20(8), 829-846. doi:10.1163/156856106777638635 Pizzi, A. (2014). Reviews of adhesion and adhesives: Synthetic adhesives for wood panels: Chemistry and technology--a critical review. Cellular Polymers, 33(4), 218. Qiao, Z., Gu, J., Lv, S., Cao, J., Tan, H., & Zhang, Y. (2015). Preparation and properties of isocyanate prepolymer/corn starch adhesive. Journal of Adhesion Science and Technology, 29(13), 1368-1381. doi:DOI: 10.1080/01694243.2015.1030157. Rahimi, A., Ulbrich, A., Coon, J. J., & Stahl, S. S. (2014). Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature, 515, 249. doi:10.1038/nature13867 Risholm-Sundman, M., Larsen, A., Vestin, E., & Weibull, A. (2007). Formaldehyde emission—Comparison of different standard methods. Atmospheric Environment, 41(15), 3193-3202. doi:10.1016/j.atmosenv.2006.10.079 Roffael, E., Behn, C., & Dix, B. (2012). On the formaldehyde release of wood particles. Holz als Roh- und Werkstoff, 70(6), 911-912. doi:10.1007/s00107-012-0625-8 Roffael, E., & Johnsson, B. (2012). Perforator method in balance. International Wood Products Journal, 3(1), 43-50. doi:10.1179/2042645312Y.0000000012 Roffael, E., Johnsson, B., & Engström, B. (2010). On the measurement of formaldehyde release from low-emission wood-based panels using the perforator method. Journal of the International Academy of Wood Science, 44(3), 369-377. doi:10.1007/s00226-010-0355-1 Rowell, R. (2005). Handbook Of Wood Chemistry And Wood Composites: CRC Press. Sahoo, S., Seydibeyoğlu, M. Ö., Mohanty, A. K., & Misra, M. (2011). Characterization of industrial lignins for their utilization in future value added applications. Biomass and Bioenergy, 35(10), 4230-4237. doi:10.1016/j.biombioe.2011.07.009 Salem, M. Z. M., & Böhm, M. (2013). Understanding of Formaldehyde Emissions from Solid Wood: An Overview. BioResources, 8(3), 4790. doi:10.15376/biores.8.3.4775-4790 Salem, M. Z. M., Böhm, M., Beránková, J., & Srba, J. (2011). Effect of some manufacturing variables on formaldehyde release from particleboard: Relationship between different test methods. Building and Environment, 46(10), 1946-1953. doi:10.1016/j.buildenv.2011.04.004 Salem, M. Z. M., Böhm, M., & Nasser, R. (2017). Measuring the Formaldehyde Content from Different Types of Oriented Strand Board Manufactured with Different Thicknesses and Glued with Different Resins. Drvna Industrija, 68, 173-178. doi:10.5552/drind.2017.1640

60

References

Salem, M. Z. M., Böhm, M., Šedivka, P., Nasser, R. A., Ali, H. M., & Abo Elgat, W. (2018). Some physico-mechanical characteristics of uncoated OSB ECO-products made from Scots Pine (Pinus sylvestris L.) and bonded with pMDI resin. BioResources, 13(1), 1814-1828. doi:10.15376/biores.13.1.1814-1828 Salthammer, T. (2019). Formaldehyde sources, formaldehyde concentrations and air exchange rates in European housings. Building and Environment, 150, 219-232. doi:10.1016/j.buildenv.2018.12.042 Salthammer, T., Mentese, S., & Marutzky, R. (2010). Formaldehyde in the indoor environment. Chemical Reviews, 110(4), 2536-2572. doi:10.1021/cr800399g Santos, R., Hart, P., Jameel, H., & Chang, H.-m. (2013). Wood based lignin reactions important to the biorefinery and pulp and paper industries. BioResources, 8, 1456. doi:10.15376/biores.8.1.1456-1477 Shu, R., Long, J., Xu, Y., Ma, L., Zhang, Q., Wang, T., . . . Wu, Q. (2016). Investigation on the structural effect of lignin during the hydrogenolysis process. Bioresource Technology, 200, 14-22. doi:10.1016/j.biortech.2015.09.112 Solt, P., Konnerth, J., Gindl-Altmutter, W., Kantner, W., Moser, J., Mitter, R., & van Herwijnen, H. W. G. (2019). Technological performance of formaldehyde-free adhesive alternatives for particleboard industry. International Journal of Adhesion and Adhesives, 94, 99-131. doi:10.1016/j.ijadhadh.2019.04.007 Swenberg, J. A., Kerns, W. D., Mitchell, R. I., Gralla, E. J., & Pavkov, K. L. (1980). Induction of squamous cell carcinomas of the rat nasal cavity by inhalation exposure to formaldehyde vapor. Cancer Res, 40(9), 3398-3402. Szczurek, A., Fierro, V., Thébault, M., Pizzi, A., & Celzard, A. (2016). Structure and properties of poly(furfuryl alcohol)-tannin polyHIPEs. European Polymer Journal, 78, 195-212. doi:10.1016/j.eurpolymj.2016.03.037 Tan, H., Zhang, Y., & Weng, X. (2011). Preparation of the Plywood Using Starch- based Adhesives Modified with blocked isocyanates. Procedia Engineering, 15, 1171-1175. doi:10.1016/j.proeng.2011.08.216 Tejado, A., Peña, C., Labidi, J., Echeverria, J. M., & Mondragon, I. (2007). Physico- chemical characterization of lignins from different sources for use in phenol–formaldehyde resin synthesis. Bioresource Technology, 98(8), 1655-1663. doi:10.1016/j.biortech.2006.05.042 Thakur, V. K., Thakur, M. K., Raghavan, P., & Kessler, M. R. (2014). Progress in green polymer composites from lignin for multifunctional applications: A review. ACS Sustainable Chemistry & Engineering, 2(5), 1072-1092. doi:10.1021/sc500087z Trosa, A., & Pizzi, A. (2001). A no-aldehyde emission hardener for tannin-based wood adhesives for exterior panels. Holz als Roh- und Werkstoff, 59(4), 266-271. doi:10.1007/s001070100200 U.S. Environmental Protection Agency (EPA). (2017). Code of Federal Regulations (CFR) 40 CFR § 770 on Formaldehyde Standards for Composite Wood Products Act. In Title VI to the Toxic Substances Control Act, TSCA.

61

References

Wahyudiono, T., Kanetake, M., Sasaki, M., & Goto, M. (2007). Decomposition of a lignin model compound under hydrothermal conditions. Chemical Engineering & Technology, 30(8), 1113-1122. doi:10.1002/ceat.200700066 Valenzuela, J., von Leyser, E., Pizzi, A., Westermeyer, C., & Gorrini, B. (2012). Industrial production of pine tannin-bonded particleboard and MDF. European Journal of Wood and Wood Products, 70(5), 735-740. doi:10.1007/s00107-012-0610-2 Wang, S., Yu, Y., & Di, M. (2018). Green modification of corn stalk lignin and preparation of environmentally friendly lignin-based wood adhesive. Polymers, 10(6). doi:10.3390/polym10060631 Vázquez, G., Santos, J., Freire, M., Antorrena, G., & González-Álvarez, J. (2012). DSC and DMA study of chestnut shell tannins for their application as wood adhesives without formaldehyde emission. Journal of Thermal Analysis & Calorimetry, 108(2), 605. Weigl, M., Wimmer, R., Sykacek, E., & Steinwender, M. (2009). Wood-borne formaldehyde varying with species, wood grade, and cambial age. Forest Products Journal, 59(1 2), 88. William H. Anderson, C. W. H. (1995). Dynamic microchamber for measuring formaldehyde emissions 5,395,494. World Health Organization. (2010). WHO Guidelines for Indoor Air Quality: Selected Pollutants. Copenhagen. Wörmeyer, K., Ingram, T., Saake, B., Brunner, G., & Smirnova, I. (2011). Comparison of different pretreatment methods for lignocellulosic materials. Part II: Influence of pretreatment on the properties of rye straw lignin. Bioresource Technology, 102(5), 4157-4164. doi:10.1016/j.biortech.2010.11.063 Yu, C. W. F., & Crump, D. R. (1999). Testing for Formaldehyde Emission from Wood-Based Products – A Review. Indoor and Built Environment, 8(5), 280-286. doi:10.1159/000024655 Zhang, C., Garrison, T. F., Madbouly, S. A., & Kessler, M. R. (2017). Recent advances in vegetable oil-based polymers and their composites. Progress in Polymer Science, 71, 91-143. doi:10.1016/j.progpolymsci.2016.12.009 Zhang, J., Song, F., Tao, J., Zhang, Z., & Shi, S. Q. (2018). Research progress on formaldehyde emission of wood-based panel. International Journal of Polymer Science, 2018, 8. doi:10.1155/2018/9349721

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