Tannin-based bio-adhesives for the wood panel industry as sustainable alternatives to petrochemical resins

DOI: 10.1111/jiec.13210 RESEARCH AND ANALYSIS Tannin-based bio-adhesives for the wood panel industry as sustainable alternatives to petrochemical resins Ana Arias Sara González-García Gumersindo Feijoo Maria Teresa Moreira CRETUS Institute, Department of Chemical Engineering, School of Engineering, Universidade de Santiago de Compostela, Santiago de Compostela, Spain Correspondence Ana Arias, CRETUS Institute, Department of Chemical Engineering, School of Engineering, Universidade de Santiago de Compostela, Santiago de Compostela, Spain. Email: [email protected] Editor Managing Review: Michael Zwicky Hauschild Funding information ERA-CoBIOETCH project (PCI2018-092866) Programación Conjunta Internacional 2018 - WooBAdh project, Grant/Award Number: PCI2018-092866; Spanish Ministry of Economy and Competitiveness, Grant/Award Number: RYC-2014-14984; Galician Competitive Research Group, Grant/Award Number: ED431C 2017/29; CRETUS Strategic Partnership, Grant/Award Number: ED431E 2018/01. Abstract The development of natural-based adhesives is a key aspect in reducing the consumption of fossil resources, in adapting to legislation on the use of formaldehyde and in ensuring the sustainability of the wood panel industry. This study focuses on five alternatives for tannin-based adhesives (tannin-paraformaldehyde, tannin-glyoxal, tanninhexamine, tannin non-isocyanate polyurethane (NIPU) with ammonium hydroxide and tannin NIPU with HDMA bio-adhesives) as potential proposals for the substitution of synthetic adhesives currently used: Urea-formaldehyde (UF), Phenol-formaldehyde (PF) and melamine urea formaldehyde (MUF). Modeling and simulation tools were used for the conceptual design of the reaction stage of the bio-adhesives due to the lack of real data from industrial scale production systems. The life cycle assessment methodology was used to identify the environmental impacts associated with the production processes of these bio-adhesives. The results obtained showed that tannin NIPU bio-adhesive with ammonium hydroxide has the best environmental profile, although the other alternative bio-adhesives also show good environmental profiles compared to synthetic adhesives. Sensitivity analyses have been carried out after evaluating the main hot spots in the production processes. In all of them, reductions of the environmental impacts have been observed, with the replacement of DMC by EC and the reduction of energy consumption being the options that showed the greatest improvements. KEYWORDS bio-adhesives, environmental impacts, industrial ecology, life cycle assessment, non-isocyanate polyurethanes, tannins 1INTRODUCTION Wood-based panels cover a wide range of wood-based products whose predominant application is in the construction and furniture sectors (Thoemen et al., 2010). Technological developments driven by market requirements, safety and environmental standards have included changes in the process, demonstrated by the considerable variety of adhesive types used for wood panel manufacture. In particular, with the aim of drastically reducing formaldehyde emissions from wood panels (Zhang et al., 2018), not only the reduction in the use of certain chemicals has been This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2021 The Authors. Journal of Industrial Ecology published by Wiley Periodicals LLC on behalf of Yale University Journal of Industrial Ecology 2022;26:627–642. wileyonlinelibrary.com/journal/jiec 627 628 ARIAS ET AL. considered, but also the development of bio-based options to replace adhesives of a petrochemical nature (Jarre et al., 2020). The use of renewable materials or even biowaste streams for the formulation of bio-based adhesives is conveyed as a strategy aiming a low carbon footprint process (Heinrich, 2019). However, to be a viable alternative, bio-adhesives must have similar bonding performance to synthetic ones, a competitive cost and a low environmental impact. Of particular importance is the fact that the biodegradability of bio-adhesives is higher than that of petrochemical ones (Heinrich, 2019). From an economic perspective, it is important to assess the position of bio-adhesives in the market and future trends, whereas a large increase in demand is expected at a compound annual growth rate (CAGR) of 10% over a period of 5 years (Markets & Markets, 2020). The raw materials used for the formulation of bio-based adhesives are diverse, from lignocellulosic biomass such as lignin and tannins, plant proteins such as soy and starch-based polysaccharides, among others (Hemmilä et al., 2017). It is important to mention that even if a renewable source is used in the formulation of the adhesives, a synthetic crosslinker is usually required to provide the necessary bonding properties for manufacture of the wood panels (Hemmillä et al., 2017). In the case of lignin-based adhesives, a previous stage of lignin activation is required for resin production, in which lignin powder is added to sodium hydroxide and glyoxal under mechanical stirring (Mansouri et al., 2007). It should be noted that formulations based on low-molecular mass lignin have a higher relative proportion of reactive sites, which ensures better results than those achieved with higher-molecular mass lignin. These lignin-based adhesives also give acceptable results in particleboard pressing times, as well as good results in terms of internal panel bond strength. The tannin-based option has experienced significant interest (Thébault et al., 2014) not only because of its renewable origin and the wide variability of manufacturing process conditions (Pizzi, 2006), but also because of the superior performance of the final product, which is highly competitive with petrochemical resins. Two alternatives can be identified: condensed tannins (Pizzi, 1982), whose structure is based on polymers of flavanol units (Schofield et al., 2001) and hydrolysable tannins (Spina et al., 2013), which are esters of carboxylic acids and sugars. Although both could be used as substitutes for phenol-formaldehyde resins, hydrolysable tannins represent a worse option, as they have a low level of phenol substitution and lack polymeric structure (Pizzi, 1982). As in the case of lignin, tannins also need to be extracted from different parts of the plant (Hemillä et al., 2017) and hot water extraction is the most developed and widely used method among the different options available (Bianchi et al., 2015;Ding et al., 2017; González et al., 2016; Krogell et al., 2012; Kilpelainen et al., 2019). Despite the numerous papers reporting different formulation protocols for bio-adhesives production (Amari et al., 2021; Li et al., 2016; Moubarik et al., 2010; Navarrete et al., 2010; Ndiwe et al., 2019; Ramires & Frollini, 2012), there is a need for environmental information on the impacts associated, especially important at an early stage of development. We cannot ignore the fact that if the production process has higher environmental impacts than those of its petrochemical counterparts, there are few options for new bio-adhesives to be considered as feasible, viable and environmentally sustainable alternatives (Arias et al., 2020, 2021). Different research reports have demonstrated the benefits on considering bio-based adhesives as sustainable alternatives to synthetic resins: being biodegradable, having lower carbon footprint and reduced toxicity, and positive impact on climate change—the main advantages that stands out (Heinrich, 2019). To this end, the life cycle assessment (LCA) methodology (Brusseau, 2019; Tillman, 2010) has been considered to provide extensive information on the environmental indicators in the different impact categories based on a detailed analysis of the inventory of raw materials, products, energy and emissions associated with a product or process throughout its life cycle. The different bio-adhesives alternatives considered within this manuscript are included on Table SM1, Supporting Information S1. Inventory data for developing LCA studies has been taken from Ecoinvent® version 3.5 database, a summary of the processes selected for the different scenarios is reported on Table SM2, Supporting Information S1. This methodology has just been considered to demonstrate the environmental benefits associated with the production of alternative bioadhesives to be used in wooden sector (González-García et al., 2011; McDeviit & Grigsby, 2014; Yang and Rosentrater, 2021; Arias et al., 2020). Nevertheless, to the best of our knowledge, tannin-based bio-adhesives have not been assessed in such level of detail. In this sense, the main objective of the study will be to compare the environmental impacts of five alternatives of tannin-based bio-adhesives with those of the most common synthetic-based resins: Urea-formaldehyde (UF), phenol-formaldehyde (PF) and melamine urea-formaldehyde (MUF). The lack of real data on the manufacturing process of bio-adhesives at full scale makes it necessary to use modeling and simulation tools for the conceptual design of the reaction stage. 2METHODOLOGY In order to properly compare the environmental profile of bio-adhesives with their conventional counterparts, it is necessary to quantify the main inputs and outputs of each unit of the process. From a detailed flow sheet of the different manufacturing schemes, mass and energy balances will be developed using the Aspen Hysys® software. It is from the conceptual design of the process when it is possible to elaborate the inventories of the life cycle as a previous stage to the quantification of the environmental impacts by means of the LCA methodology (Tillman, 2010). ARIAS ET AL.629 2.1 Definition of the goal and scope of the study The main objective of this assessment is to determine whether the production processes for tannin-based adhesives can be carried out with a lower environmental impact than those equivalent to synthetic resins. For this purpose, the life cycle environmental impacts associated with the resources consumed and the manufacturing processes of the bio-adhesives will be addressed. A decision in the evaluation process will focus on the functional unit, as this is the basis of the calculation for all input and output flows. The system boundaries are considered from a cradle-to-gate approach, that is, the extraction raw materials, the production of tannin-based adhesives and the management of waste streams within the factory. As functional unit, 1 kg of adhesive was considered as an straightforward reference for the comparison of the different bio-adhesives proposed, which is in line with the one chosen by other authors (Yang and Rosentrater, 2021; Hellweg and Messmer, 2015; Silva et al., 2015). Transport and infrastructure activities associated with the distribution of raw materials needed for the formulation of adhesives are deemed identical for the different scenarios; accordingly, both stages were left out of the study. 2.2 Description of the systems and collection of inventory data Five scenarios of tannin bio-adhesive processes have been considered, according to the use of different chemicals for the formulation of resins: paraformaldehyde, glyoxal, hexamine, ammonium hydroxide and hexamethylenediamine (HDMA). These chemicals are used as crosslinking agents and hardeners, which can promote the formation of covalent and ionic bonds between polymers, thus creating high molecular weight polymer chains (Solt et al., 2019). This conversion process, from a short polymer chain to a dimensional network, is called adhesive curing. In all the scenarios of the bio-adhesives proposed, the first stage of the production process consists of the extraction of tannin from the bark based on hot water extraction method the most widespread and therefore, the one considered in our study. Although the addition of chemicals is not necessary, the process involves a high consumption of energy (Ding et al., 2017). Information on the manufacturing process to obtain the condensed tannin has been taken from González-García et al. (2016). This process includes not only the extraction of the condensed tannin, but also the background activities associated with forestry activities and the management of waste streams. Although the addition of chemicals is not necessary, the process involves a high consumption of energy (Ding et al., 2017). The inventory data for the production process of condensed tannins is shown on Table SM3 in Supporting Information S1. Mass and energy flows of the different tannin bio-adhesive processes are estimated on the basis of laboratory-scale data and implemented through Aspen Hysys® as a process simulation software. The modeling of the reaction unit is the main stage of the process in a facility producing 24 metric tons per day (t/d), which is in the range of the production capacity of formaldehyde-based adhesives in other studies: 15,000 t/year (Yang and Rosentrater, 2021). 2.2.1 Tannin-paraformaldehyde bio-adhesive The manufacturing process of the bio-adhesive is formulated from the following mixture: extracted tannin, water, paraformaldehyde and NaOH (Ballerini et al., 2005). In substitution of formaldehyde, paraformaldehyde is considered in this alternative as it could be rapidly depolymerized under alkaline conditions (Zhou & Du, 2020). One of the advantages of this bio-adhesive is based on the fact that no pre-reaction steps are needed, they are simply mixed in a glue blender just before the mixture is used as an adhesive. The hardening reaction occurs exclusively during the hotpressing step of the wood panels. The inventory data for the impact assessment are shown in Table SM4 in Supporting Information S1. 2.2.2 Tannin-glyoxal bio-adhesive Tannin-glyoxal bio-adhesives have potential for the formulation of adhesive polymers used for wood bonding (Baaka et al., 2017), where glyoxal is a substitute of formaldehyde as a hardening agent. The glyoxylation reaction consists of the formation of cross-linked bridges between glyoxal and the aromatic sites of the tannin molecular structure. The inventory data of tannin-glyoxal bio-adhesive includes extracted tannin, water, glyoxal and NaOH as main inputs (Table SM5 in Supporting Information S1). As with the tannin-paraformaldehyde adhesive, the hardening reaction occurs exclusively during the hot-pressing stage of the wood-based panels, so no pre-reaction stage is required. 2.2.3 Tannin-hexamine bio-adhesive Hexamine can be also an alternative hardener that does not lead to formaldehyde emissions (Pena et al., 2009). The cross-linking reaction process between hexamine and tannins takes place through the formation of reactive or intermediate fragments of hexamine and their reaction with the 630 ARIAS ET AL. phenolic nuclei of polyflavonoid tannins (Mosiewicki et al., 2004). In this scenario the bio-adhesive is formulated based on extracted tannin mixed with water, hexamine and NaOH. Inventory data for this bio-adhesive is shown in Table SM6 in Supporting Information S1. 2.2.4 Tannin-NIPU based bio-adhesives Polyurethanes are classified as thermosetting polymers, which are composed of organic units linked by urethane (or carbamate) bonds. They are versatile polymers of strong chemical structure (Lanaro et al., 2018), with an outstanding role as binders in medium-density fiberboards, laminated wood, particleboards, among others (Madhav et al., 2019). Although its use is widespread, currently the formulation of polyurethanes that do not contain isocyanates in its structure is pursued, due to the need to eradicate phosgene in its production process (Figovsky et al, 2013; Kathalewar et al., 2013). Thus, the production of so-called non-isocyanate polyurethanes (NIPU) is based on the chemical reaction between carbonates and primary amino groups, forming a cross-linked polymer with a lower environmental impact compared to conventional polyurethanes (Figovsky et al., 2013). In addition, other positive characteristics of NIPU polymers are structural and thermal stability, water tolerance and chemical resistance. Based on these assumptions, two tannin-based scenarios based on NIPU polymers have been proposed. Tannin-NIPU with ammonium hydroxide bio-adhesive There are different methods for preparing NIPU polymers, but one of the most widespread is the polycondensation reaction of cyclic carbonates and primary amines, which results in the production of a polymer chain with hydrogen bonds (Datta & Wloch, 2016). The formulation of the bioadhesive starts with the condensation of the tannins in an ammonium hydroxide medium (Thébault et al., 2017), becoming a viscous mixture with the formation of nitrogen bonds between the flavonoid molecules of the tannins. The tannin extract is then mixed with dimethylcarbonate (DMC), which requires a reaction time of 2 h under mechanical agitation and 90◦C. At this stage, the carboxymethylation process of the hydroxyl phenolic groups of DMC takes place. This mixture is added to the condensed amino tannins for the final formulation of the bio-adhesive. Inventory data are presented in Table SM7 in Supporting Information S1. Tannin-NIPU with HDMA bio-adhesive The formulation of this type of tannin-NIPU bio-adhesive is similar to the previous one, with the difference that hexamethylenediamine (HDMA) is used for the formation of the urethane bonds. In this case, tannins are mixed with deionized water and DMC and the mixture is heated to 65◦C for 60 min (Chen et al., 2020). Finally, for the formation of the urethane bonds, HDMA is added to the mixture at 90◦C for 2 h. The last step is to reduce the temperature of the adhesive to room temperature (Chen et al., 2020). For the case of HDMA, the modeling of its production process was also performed from the available data reported in the literature (Dros et al., 2015). Tables SM8 and SM9 in Supporting Information S1 show the main inputs and outputs of the HDMA and tannin-NIPU bio-adhesive manufacturing process. 2.3 Methodology used for the LCA study Three assessment methods were used for performing LCA methodology. Firstly, ReCiPe 2016 hierarchist Midpoint method V1.03 World (2010) to evaluate the environmental profiles of the bio-adhesives, taking into account the following impact categories: global warming potential (GWP), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), and fossil resource scarcity (FRS). Subsequently, ReCiPe 2016 hierarchist Endpoint method V1.03 World (2010) H/H and USEtox ® V1.01 (Acero et al., 2014)were considered to benchmark the bio-adhesives alternatives with the synthetic ones. Regarding the former, it was used in terms of single score values related to the categories of human health, ecosystem quality and fossil resource scarcity. Concerning the latter, it was used to evaluate the impacts of human toxicity, both cancer and non-cancer categories. The rationale behind the selection of this method is based on the fact that it provides a specialized approach of human toxicity-related impacts and is recommended by numerous research organizations (Acero et al., 2014). 3RESULTS AND DISCUSSION The results obtained after applying the LCA methodology to the different alternatives of bio-adhesives proposed are shown in Table 1, using Recipe MidPoint method for the calculations. Despite the fact that a detailed analysis of the environmental impact values obtained is reported in the following sections, tannin-NIPU ammonium hydroxide bio-adhesives have a greater potential for the substitution of synthetic resins, from an environmental point of view, since lower impact results are obtained in the MidPoint categories considered. ARIAS ET AL.631 TABLE 1 Life cycle assessment results obtained for the bio-based adhesives by applying Recipe MidPoint calculation method Tannin paraformaldehyde Tannin glyoxal Tannin hexamine Tannin NIPU with ammonium hydroxide Tannin NIPU with HDMA GWP kg CO2eq 2.95 3.10 3.15 3.52 1.57 SOD mg CFC11 eq 16 16 17 19 4.36 TA gSO 2eq 152 156 159 163 50 FE gPeq 27 28 28 27 5.05 ME g N eq 2.16 2.19 4.24 2.07 0.61 TET kg 1,4-DCB 2.01 2.04 2.39 2.75 2.85 FET g 1,4-DCB 72 73 73 77 15 MET g1,4-DCB 100 101 102 107 21 FRS kg oil eq 0.80 0.88 0.89 1.45 0.69 Abbreviations: FE, freshwater eutrophication; FET, freshwater ecotoxicity; FRS, fossil resource scarcity; GWP, global warming potential; ME, marine eutrophication; MET, marine ecotoxicity; SOD, stratospheric ozone depletion; TA, terrestrial acidification; TET, terrestrial ecotoxicity. 3.1 Environmental assessment of tannin-paraformaldehyde bio-adhesive The characterization results obtained for tannin-paraformaldehyde bio-adhesive showed that energy requirements are the main hotspots of the production process (Figure 1a), reaching contribution values higher than 50% in the overall impact categories. Although electricity requirements are the main contributor to the environmental impacts associated with the production of this type of adhesive, it was observed that the use of tannin in the formulation of bio-adhesives also plays a key role on the environmental profile. This contribution is derived from the background activities required for the manufacture of condensed tannins, specifically on the energy requirements of the process (Figure 1b). In this way, the environmental profile of this first bio-adhesive alternative shows that the use of paraformaldehyde is not the main cause of the impact, being energy demand and the background activities derived from the use of condensed tannins the main responsible stages contributing to the environmental profile of the tannin paraformaldehyde bio-adhesive. 3.2 Environmental assessment of the tannin-glyoxal bio-adhesive Similarly to the previous formulation, the production of electricity requirements is the main hotspot all over the environmental profile associated with this type of adhesive, with a significant contribution from condensed tannins (Figure 2). In this case, the contribution from the chemical used as a cross-linker for the bio-adhesive production, that is, glyoxal, is slightly higher than the one achieved by the paraformaldehyde for the categories of global warming and fossil resource scarcity: 8% and 14% respectively. On the other hand, it is important to specify that the formulation of the tannin paraformaldehyde bio-adhesive is different from that of glyoxal. The amount of cross-linker used for tannin paraformaldehyde bio-adhesive is half to that of the tannin glyoxal bio-adhesive. This fact implies a higher contribution of chemicals in the environmental profile. From the point of view of formulating a bio-adhesive with a higher natural base and a lower chemical content, the paraformaldehyde-based adhesive would be better. 3.3 Environmental assessment of tannin-hexamine bio-adhesive In the case of tannin-hexamine bioadhesives, the production of energy requirements is once again the main hotspot, together with the contribution derived from the production of condensed tannins, which, as previously mentioned, is a consequence of the energy demand of the process. As it can be observed in Figure 3, the contribution of hexamine on the category of marine eutrophication is significant as it is a nitrogenous organic compound. This contribution is derived due to the fact that, when hexamine is introduced into the aquatic environment, it hydrolytically degrades to ammonium and formaldehyde, which leads to eutrophication associated with algae bloom. On the other hand, certain hexamine contribution is also observed on TET category. It can be identified as a cause of this contribution that, from the water-phase, ammonium can volatilize into air. Its presence in the air medium can cause the deposition of this compound in the ground, which can cause soil nitrification, since, under aerobic conditions, it is transformed into nitrite and nitrate by the action of nitrifying bacteria present in the floor. 632 ARIAS ET AL. (a) (b) 0% 20% 40% 60% 80% 100% GWP SOD TA FE ME TET FET MET FRS Electricity Tannin Sodium Hydroxide Paraformaldehyde Tap water 0% 20% 40% 60% 80% 100% GWP SOD TA FE ME TET FET MET FRS Bark chips Sodium Bisulfate Sodium Carbonate Tap water Electricity FIGURE 1 Distribution of burdens between contributing parameters/processes involved in tannin paraformaldehyde bio-adhesive production. (a) Distribution of burdens between contributing parameters/processes involved in tannin paraformaldehyde bio-adhesive. (b) Distribution of burdens between contributing parameters/processes involved in condensed tannin production. Abbreviations: FE, freshwater eutrophication; FET, freshwater ecotoxicity; FRS, fossil resource scarcity; GWP, global warming potential; ME, marine eutrophication; MET, marine ecotoxicity; SOD, stratospheric ozone depletion; TA, terrestrial acidification; TET, terrestrial ecotoxicity. Underlying data used to create this figure can be found in Supporting Information S2 3.4 Environmental assessment of tannin-NIPU with ammonium hydroxide bio-adhesive The environmental impacts of the tannin-NIPU-ammonium hydroxide bio-adhesive are shown in Figure 4. It can be observed that the environmental contribution from the production of dimethylcarbonate (DMC) is the main hot spot in the profile of the global system, with a share of more than 50% in all impact categories. Its lowest contribution is in TET, where the production process of ammonium hydroxide required for the formulation of the adhesive represents a significant impact. ARIAS ET AL.633 0% 20% 40% 60% 80% 100% GWP SOD TA FE ME TET FET MET FRS Tap water Glyoxal Sodium Hydroxide Tannin Electricity FIGURE 2 Distribution of burdens between contributing parameters/processes involved in tannin glyoxal bio-adhesive production. Abbreviations: FE, freshwater eutrophication; FET, freshwater ecotoxicity; FRS, fossil resource scarcity; GWP, global warming potential; ME, marine eutrophication; MET, marine ecotoxicity; SOD, stratospheric ozone depletion; TA, terrestrial acidification; TET, terrestrial ecotoxicity. Underlying data used to create this figure can be found in Supporting Information S2 FIGURE 3 Distribution of burdens between contributing parameters/processes involved in tannin hexamine bio-adhesive production. Abbreviations: FE, freshwater eutrophication; FET, freshwater ecotoxicity; FRS, fossil resource scarcity; GWP, global warming potential; ME, marine eutrophication; MET, marine ecotoxicity; SOD, stratospheric ozone depletion; TA, terrestrial acidification; TET, terrestrial ecotoxicity. Underlying data used to create this figure can be found in Supporting Information S2 With regard to the impact of DMC, its production process based on the oxidative carbonation of methanol has as its main limitation the high energy demand of the process (García-Herrero et al., 2016), with electricity being the most determining factor in all impact categories, except for the FRS category, where CO and methanol also present a significant contribution. Methanol is included in the EPA list of 188 Air Toxics, which could contribute to the formation of smog and subsequently, to the impact categories related to ecotoxicity (Figure 4). On the other hand, although it is not considered as a toxic compound for the aquatic medium, its biodegradation can cause oxygen depletion in water, which could be harmful to aquatic species. In the case of carbon monoxide (CO), its contribution is significant compared to methanol. The environmental contribution of this compound to the impact categories of ecotoxicity and acidification is the result of 634 ARIAS ET AL. (a) (b) 0% 20% 40% 60% 80% 100% GWP SOD TA FE ME TET FET MET FRS Ammonium Hydroxide Tap water Condensed tannin DMC Heat Electricity On-site emissions 0% 20% 40% 60% 80% 100% GWP SOD TA FE ME TET FET FRS DMC Methanol Carbon monoxide Oxygen Hydrochloric acid Electricity Wastewater FIGURE 4 Distribution of burdens between contributing parameters/processes involved in tannin-NIPU with ammonia hydroxide bio-adhesive production. (a) Distribution of burdens between contributing parameters/processes involved in tannin NIPU with ammonia hydroxide bio-adhesive. (b) Distribution of burdens between contributing parameters/processes involved in DMC production process. Abbreviations: FE, freshwater eutrophication; FET, freshwater ecotoxicity; FRS, fossil resource scarcity; GWP, global warming potential; ME, marine eutrophication; MET, marine ecotoxicity; SOD, stratospheric ozone depletion; TA, terrestrial acidification; TET, terrestrial ecotoxicity. Underlying data used to create this figure can be found in Supporting Information S2 the different chemical reactions that take place at the atmospheric level, especially its oxidation to carbon dioxide and ozone, both considered as greenhouse gases. 3.5 Environmental assessment of tannin-NIPU with HDMA bio-adhesive The environmental impact results (Figure 5a) showed that the use of chemicals in the formulation of the adhesive is the main cause of impacts in all the Midpoint categories. The reason for the environmental contribution of the DMC is analogous to the one just mentioned in the previous section: ARIAS ET AL.635 (a) (b) 0% 20% 40% 60% 80% 100% GWP SOD TA FA ME TET FET MET FRS On-site emissions Tannin Water, deionised Tap water HDMA DMC Electricity Heat 0% 20% 40% 60% 80% 100% GWP SOD TA FA ME TET FET MET FRS On-site emission Butadiene Phosphoric acid Ammonia Dihydrogen Sodium (bi)sulfate Methane Water, cooling Electricity Steam FIGURE 5 Distribution of burdens between contributing parameters/processes involved in tannin-NIPU with HDMA bio-adhesive production. (a) Environmental profile distribution between inputs/outputs involved on tannin NIPU with HDMA bio-adhesive (b) Environmental profile distribution between inputs/outputs involved on HDMA production process. Abbreviations: FE, freshwater eutrophication; FET, freshwater ecotoxicity; FRS, fossil resource scarcity; GWP, global warming potential; ME, marine eutrophication; MET, marine ecotoxicity; SOD, stratospheric ozone depletion; TA, terrestrial acidification; TET, terrestrial ecotoxicity. Underlying data used to create this figure can be found in Supporting Information S2 the high energy demand of the process. This chemical contributes more to the categories of FA, ME, FET and MET where, for the other categories, HDMA is the fossil-based raw material with the large environmental contribution. The reaction of butadiene with hydrogen cyanide is the most extended manufacturing process to produce HDMA. This reaction gives rise to an intermediate product, adiponitrile (ADN), which is converted to hexamethylenediamine by hydrogenation. The by-products and waste streams are amines, CO2and imines (mainly hexamethylenimine). 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International Journal of Polymer Science,2018. Zhou, X., & Du, G. (2020). Applications of tannin resin adhesives in the wood industry. In Tannins-structural properties, biological properties and current knowledge. IntechOpen. SUPPORTING INFORMATION Additional supporting information may be found in the online version of the article at the publisher’s website. How to cite this article: Arias A, González-García S, Feijoo G, Moreira MT. Tannin-based bio-adhesives for the wood panel industry as sustainable alternatives to petrochemical resins. J Ind Ecol. 2022;26:627–642. https://doi.org/10.1111/jiec.13210