Environmental benefits of soy-based bio-adhesives as an alternative to formaldehyde-based options

Environmental Science and Pollution Research Environmental benefits of soy-based bio-adhesives as an alternative to formaldehydebased options --Manuscript Draft-- Manuscript Number: ESPR-D-20-12999 Full Title: Environmental benefits of soy-based bio-adhesives as an alternative to formaldehydebased options Article Type: Research Article Keywords: Soybean; Soy protein; Wood panel; Sensitivity analysis; Life Cycle assessment; Environmental profile Corresponding Author: Ana Arias Universidade de Santiago de Compostela - Campus Vida Santiago de Compostela, Galicia SPAIN Corresponding Author Secondary Information: Corresponding Author's Institution: Universidade de Santiago de Compostela - Campus Vida Corresponding Author's Secondary Institution: First Author: Ana Arias First Author Secondary Information: Order of Authors: Ana Arias Sara González-García Gumersindo Feijoo Maria Teresa Moreira Order of Authors Secondary Information: Funding Information: Ministerio de Economía y Competitividad (RYC-2014-14984) Mrs. Sara González-García ERA-CoBIOTech WooBAdh project (PCI2018-092866) Not applicable Galician Competitive Research Group (ED431C 2017/29) Not applicable CRETUS Strategic Partnership (ED431E 2018/01) Not applicable Abstract: The restrictions imposed in relation to the use of formaldehyde in the formulation of the synthetic resins used in the manufacture of wood panels have been the driving force in the development of clean production strategies. The use of non-renewable raw materials is desirable to be changed by bio-based options. In this context, the environmental profile of different alternatives of soy-based adhesives is investigated, as possible options for replacing the commonly used synthetic resins. This report includes the modeling of the reaction stage on a large scale as well as the evaluation of life cycle impacts associated with each soy-based alternative. Among the six proposals presented, the one that stands out for its potentiality to replace synthetic resins is the soy protein bio-adhesive with tannin-based resin. Suggested Reviewers: Cristiano Alves Federal University of Santa Catarina: Universidade Federal de Santa Catarina [email protected] For his extensive work and experience in the fields of Natural Composite Materials, Eco-Design, Sustainability, Sustainable Product Development. Ioan-Robert Istrate IMDEA Energy Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation [email protected] One of the topics on which his research focuses is on the Life Cycle Sustainability Analysis, which is the methodology used in the development of this study. Paula Sofia Quinteiro Department of Environment and Planning [email protected] PhD in Science and Enviromental Engineering, forming part of a research group based on resources circularity assessment and technology (RCAT). Topics related with the submited article. Dieter Boer Universitat Rovira I Virgili Facultad de Ciencias Economicas y Empresariales [email protected] Due to his skills and expertise on chemical and environmental engineering, and sustainability. Topics on which this article is based on. Opposed Reviewers: Additional Information: Question Response §Are you submitting to a Special Issue? No Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Philippe Garriges Editor-in-Chief, Biomass & Bioenergy. Dear Philippe Garriges: We are pleased to enclose an original manuscript of our paper entitled “Environmental benefits of soy-based bio-adhesives as an alternative to formaldehyde-based options” by Ana Arias, Sara González-García, Gumersindo Feijoo and Maria Teresa Moreira which can hopefully be published in the Environmental Science and Pollution Research. This paper has not been previously published, in whole or in part, and is not under consideration by any other journal. The restrictions imposed in relation to the use of formaldehyde in the formulation of the synthetic resins used in the manufacture of wood panels have been the driving force in the development of bio-based options. In this context, the environmental profile of different alternatives of soy-based adhesives is investigated, as possible options for replacing the commonly used synthetic resins. This report includes the modeling of the reaction stage on a large scale as well as the evaluation of life cycle impacts associated with each soy-based alternative. Among six different options, soy protein bio-adhesive with tannin-based resin has an overall better profile. Sensitivity analysis on the formulation and process conditions lead to identify potential improvements in the environmental profile. This study provides useful information for researchers on where to focus on the development of bio-adhesives. We hope that this work is appropriate for publication in Environmental Science and Pollution Research. Yours sincerely, Ana Arias Calvo Cover letter 1 Environmental benefits of soy-based bio-adhesives as an alternative to 1 formaldehyde-based options 2 Ana Arias, Sara González-García, Gumersindo Feijoo and Maria Teresa Moreira 3 CRETUS Institute, Department of Chemical Engineering, School of Engineering, 4 Universidade de Santiago de Compostela, Spain. 5 E-mail address: [email protected] 6 Abstract 7 The restrictions imposed in relation to the use of formaldehyde in the formulation of 8 the synthetic resins used in the manufacture of wood panels have been the driving force 9 in the development of clean production strategies. The use of non-renewable raw 10 materials is desirable to be changed by bio-based options. In this context, the 11 environmental profile of different alternatives of soy-based adhesives is investigated, as 12 possible options for replacing the commonly used synthetic resins. This report includes 13 the modeling of the reaction stage on a large scale as well as the evaluation of life cycle 14 impacts associated with each soy-based alternative. Among the six proposals presented, 15 the one that stands out for its potentiality to replace synthetic resins is the soy protein 16 bio-adhesive with tannin-based resin. 17 Keywords: Soybean; Soy protein; Wood panel; Sensitivity analysis; Life Cycle 18 assessment; Environmental profile 19 20 Manuscript Click here to access/download;Manuscript;Manuscript.docx Click here to view linked References 2 1. Introduction 21 The wood-based panel sector has achieved significant positions in the global market, 22 with a size of 144.67 USD billion by 2019, and grow significantly in terms of CAGR of up 23 to 6.9% from 2020 to 2027 (Grand View Research, 2020). Regarding the different types 24 of wood panels, the category that stands out is plywood: a wood product manufactured 25 from the gluing of multiple thin layers of wood with excellent properties in terms of 26 strength, durability, water resistance, among others (Jia et al., 2019). Although this type 27 of product is made from wood and is supposed to be environmental-friendly, the 28 manufacture of boards requires adhesives in the gluing stage (González-García et al., 29 2009), which are traditionally fossil-based (phenol-formaldehyde, urea-formaldehyde, 30 etc.). Their use has posed some environmental problems related to formaldehyde 31 emissions, not only in the manufacturing process but also diffuse emissions that can 32 affect indoor air quality (Hemmilä et al., 2017). 33 One of the options being considered to develop adhesives with lower environmental 34 impact is based on the partial or total replacement of formaldehyde by the formulation 35 of vegetable protein-based bio-adhesives (Kajaks et al., 2012). The techno-economic 36 viability of this type of bio-adhesives should ensure that a number of requirements are 37 met; similar performance to synthetic alternatives, adequate strength and stiffness 38 properties according to standards and competitive cost (Frihart et al., 2014). In addition, 39 it is anticipated that the use of bio-adhesives could lead to a number of environmental 40 benefits, especially regarding the depletion of fossil resources and associated CO2 41 emissions. Despite the promising benefits of bio-based adhesives, an unbiased holistic 42 perspective must be conducted, so that all environmental aspects related to the process 43 are addressed; especially if the impact categories linked to the energy consumption of a 44 3 process that has not been optimized or also the land use for biomass cultivation are 45 taken into account. 46 Soy is a renewable resource with favorable characteristics for use as a raw material in 47 the manufacture of bio-adhesives, namely, abundance, ease of processing and low cost 48 (Vnuĉec et al., 2017). Its derivatives, including soy protein (SPI), defatted soy flour (DSF) 49 and soy flour (SF) are ideal materials for the production of bio-adhesives. Although soy 50 protein has high protein purity, with levels exceeding 90% raw protein (Hojilla51 Evangelista 2010), the transition to an industrial-scale process has been significantly 52 limited due to its high cost. For this reason, recent research related to soy-based 53 adhesives has been directed primarily at modifying soy flour fractions. Analysis of the 54 performance of soy-based bio-adhesives shows acceptable but not excellent water 55 resistance, which may be a disadvantage compared to well-established alternatives of 56 petrochemical origin (Lei et al., 2014). In order to solve these limitations, it is considered 57 that the addition of cross-linking agents can improve the performance of the adhesive 58 (Ferdosian et al., 2017). For this reason, the optimization of the use of crosslinking 59 agents and the adaptation of the process operating conditions are key aspects to 60 improve the environmental, technical and economic profiles of the bio-adhesives. 61 Looking at the most recent reports in literature, the environmental profile of bio62 adhesives has been scarcely addressed (McDevitt & Grigsby, 2014; Yang et al., 2020; 63 Arias et al., 2020). Moreover, the comparison between different bio-adhesive options is 64 not simple because it will depend on whether the selection of the functional unit, the 65 system boundaries, and the impact assessment methodology are similar. The available 66 reports on soy-based adhesives focus on the formulation of the adhesive and its 67 4 validation for use in the wood processing industry (He, Z., 2017; Ferdosian et al. 2017). 68 This study aims to provide essential information on the different alternatives for soy69 based adhesives for which no environmental studies have been carried out. For this 70 purpose, due to the lack of production data on an industrial scale, it is necessary to pose 71 mass and energy balances required in the process simulation using the Aspen Hysys® 72 tool. The conceptual design of the process is essential to provide key inventory data on 73 the production process, not only raw materials, products and energy but also emissions 74 from each stage of the manufacturing process. It will be based on the quantitative 75 analysis of global environmental impacts according to the life cycle methodology, when 76 it will be possible to identify real competition opportunities between a widely tested 77 fossil-based adhesive with a biotechnological alternative that aims to stand out as a 78 more sustainable product. 79 2. Methodology 80 2.1. Definition of the goal and scope of the study 81 The aim of this study is to determine the environmental burdens associated with the 82 production of soy-based adhesives on a large scale, in order to evaluate their potential 83 for use in the wood panel industry as substitutes for the fossil-based resins currently in 84 use. Given that the production processes of bio-adhesive are not developed on an 85 industrial scale, a simulation tool has been used with the aim of extrapolating the 86 available data reported at laboratory and pilot scale. 87 As for soy-based adhesives, the most commonly used crosslinkers for their production, 88 and those that have proven to be effective, are those based on epoxy chemicals and 89 aldehydes, as well as their derivatives (Lei et al., 2014), which has permitted to identify 90 5 six scenarios of soy-based bio-adhesives, which result of the combination of soy flour 91 and soy protein with the following crosslinkers and additives: dicyandiamide, 92 waterborne polyurethane, Sodium dodecylbenzene sulfonate (SDS) and epoxy resin, 93 polyacrylamide and epoxy resin, maleic anhydride and tannin-based resin. 94 Thus, although the raw material used to formulate this type of bio-adhesive is based on 95 soy protein, other chemicals used in its production can also have a significant 96 environmental impact. For this reason, this study aims to determine the environmental 97 profiles associated with the production of the different alternatives of soy-based bio98 adhesives and to demonstrate (or not) their environmental benefits when compared 99 with their fossil-based counterparts. Thus, a comparison will be made with the most 100 used fossil adhesives in the manufacture of wood panels: urea-formaldehyde (UF), 101 phenol-formaldehyde (PF) and melamine-urea-formaldehyde (MUF). Therefore, the life 102 cycle assessment (LCA) methodology (ISO 14040, 2016) is proposed for the assessment 103 of environmental impacts from a global point of view. 104 This methodology allows the use of different evaluation methods: ReCiPe 2016 105 hierarchist Midpoint method V1.03 World (2010), which was the tool for reporting the 106 environmental profile and identifying the environmental hotspots in terms of global 107 warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), 108 freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity 109 (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET) and fossil resource scarcity 110 (FRS). 111 Once the environmental profiles of soy-based bio-adhesive alternatives have been 112 determined, their comparison with fossil-based alternatives will be conducted in terms 113 6 of a single score that integrates three impact categories (human health, ecosystem 114 quality, and resource scarcity). Finally, it is important to mention that, in addition to the 115 need to reduce dependence on fossil resources and improve the quality of the 116 environment, another reason that drives the development of studies and research for 117 alternatives to synthetic adhesives is the interest of consumers to acquire more 118 sustainable products with less potential risk to human health. Therefore, it is important 119 to study the impact of adhesives on human toxicity, for which the USEtox ® V1.01 120 assessment method has been selected. 121 Considering that the manufacturing process of bio-adhesives is not developed on a real 122 scale, mass and energy balances will be developed within the framework of a conceptual 123 design based on the data available at laboratory scale. Simulations of the bio-adhesive 124 production systems are then carried out using Aspen Hysys® software. The functional 125 unit used to report the environmental results is 1 kg of adhesive. 126 2.2. System boundaries and assumptions for the study. 127 A cradle-to-gate approach has been considered in this study (Figure 1), which implies 128 that the stages from the extraction of raw materials, bio-adhesive production and on129 site emissions are included within the system boundaries, excluding from the analysis 130 the production and maintenance of infrastructure, as well as all transport activities of 131 inputs (chemicals and raw materials) to the factory gate. The rationale behind this 132 hypothesis is based on the consideration that both activities entail minor environmental 133 loads (Jia et al., 2018; Yang & Rosentrater, 2020). 134 < Figure 1 over here> 135 13 used to determine the environmental profile of each bio-adhesive based on the impact 273 categories selected for analysis. Although the burdens shown in Table 1 allow for an 274 overall environmental assessment of the proposed bio-adhesive alternatives, it is 275 important to conduct a separate study for each in order to determine the contribution 276 of the components that conform the production system to the overall environmental 277 profile.<Table 1 around here> 278 3.2. Environmental assessment of the SF + D bio-adhesive 279 The use of soybean meal is one of the main contributors to the environmental impact 280 associated with the bio-adhesive (Figure 2). This contribution is the result of the 281 background processes associated with the soybean cultivation processes, specifically on 282 in situ emissions from agricultural activities and the use of fertilizers in the cultivation 283 stages. The second main contributor to the environmental profile of this bio-adhesive is 284 the dicyandiamide production process, with a contribution share of more than 50% in 285 half of the impact categories studied. On the other hand, this chemical compound has 286 some potential to cause toxicity to aquatic species. Although it is a stable compound 287 under normal conditions, its decomposition results in the release of carbon monoxide, 288 carbon dioxide and nitrogen oxides, recognized as GHG emissions. Due to these possible 289 effects on the environment, the possibility of replacing it with another crosslinking agent 290 should be studied, or considering that this process is not optimized, a reduction of the 291 dose used for the formulation of the bio-adhesive could be proposed. 292 We should be aware of another relevant aspect in the selection of a cross-linking agent. 293 Beyond its contribution to the environmental profile, its potential adverse effect on 294 human health must also be considered. When developing a new product, it is important 295 14 not only to study the environmental impact of the process, but also whether the use of 296 the product could pose a risk to human health and natural ecosystems in terms of 297 toxicity. According to the EU classification, dicyandiamide is a compound that, despite 298 not having carcinogenic or mutagenic effects, as was the case with formaldehyde, 299 exposure could entail certain potential health effects such as skin irritation, formation 300 of methemoglobin if absorbed by the body and gastrointestinal discomfort if inhaled. 301 Although the concentration of dicyandiamide used for the formulation of the proposed 302 bio-adhesive is low, which considerably reduces its possible adverse effects, the toxicity 303 potential should be assessed in the weighting for the selection of the different bio-based 304 alternatives. 305 < Figure 2 over here> 306 3.3. Environmental assessment of the SF + WP bio-adhesive 307 Two main hotspots could be identified within this environmental profile (Figure 3): the 308 soy flour and waterborne polyurethane as the cross-linking agent. This compound is 309 obtained from non-renewable sources, specifically, produced by a petrochemical route 310 (Yu et al., 2014), which leads to the emission of pollutants into the air, water and soil. 311 As an alternative to improve the environmental profile of this bio-adhesive, different 312 recent works have been developed on the production of adipic acid, one of the 313 compounds used for the formulation of WPU, through a biological synthesis (Raj et al., 314 2018; Sun et al., 2018; Kruyer et al., 2020), thus avoiding the use of the petrochemical 315 route. 316 <Figure 3 around here> 317 3.4. Environmental assessment of the SF + EP bio-adhesive 318 15 The environmental impact results (Figure 4) showed that the background activities 319 related with the production of the soybean flour and the epoxy resin play a key role in 320 the environmental profile in all the Midpoint categories considered for the assessment. 321 As for the contribution derived from the use of chemicals, the only one that stands out 322 in the environmental profile associated with the category of fossil resource scarcity 323 associated with the production of epoxy resin. There is also some implication in the 324 categories of ecotoxicity, eutrophication and acidification. Its release into the aquatic 325 environment causes reproductive effects in aquatic organisms, and in terms of toxicity 326 to soil and plants, the epoxy resin could be rapidly absorbed by the roots and 327 metabolized into glycosidic compounds (Carlisle et al., 2009), causing potential harmful 328 effects on seeds, leaves, stems, etc. (Plumlee, K.H., 2004). 329 < Figure 4 over here> 330 3.5. Environmental assessment of the SP + EP bio-adhesive 331 Bearing in mind the results depicted in Figure 5, the environmental profile is dominated 332 by the impacts derived from the background processes involved in the production of 333 soybean meal. Therefore, the main cause of the contribution of the soy protein is the 334 result of the combination of energy consumption associated to the extraction process 335 and the background processes required in the agricultural cultivation. The reason for the 336 environmental contribution of epoxy resin is analogous to the one mentioned in the 337 previous section. 338 Although the contribution of polyacrylamide is not significant in the environmental 339 profile of this bio-adhesive, its hazard classification should be taken into account. 340 According to the EU classification labels, exposure to this substance can have a number 341 16 of adverse effects due to its potential for carcinogenesis and mutagenesis, acute toxicity 342 if ingested, inhaled or in contact with the eyes, and it is also suspected to contribute to 343 impaired fertility. Due to the classification of this substance, although its content in the 344 bio-adhesive formulation is relatively low, which reduces its possible risk to human 345 health, it is an aspect that should be taken into account, as it can be a cause of exclusion 346 in the selection of the best bio-adhesive alternative. 347 <Figure 5 around here> 348 3.6. Environmental assessment of the SP + MA bio-adhesive 349 The distribution of burdens shown in Figure 6 showed that HDMA 350 (Hexamethylenediamine) production is the main hotspot in the global system profile, 351 with a share of more than 40% in categories such as GW, TA, ME, TET and FRS. The 352 reason for the environmental contribution of maleic anhydride is the result of the energy 353 needs of its production based on fossil resources. Once again, as in the previous 354 environmental profiles, the contribution of soy is also significant, mainly related to the 355 extraction of the soy protein. 356 < Figure 6 over here> 357 3.7. Environmental assessment of the SP + TR bio-adhesive 358 The results obtained for the environmental profile of the adhesive showed that the use 359 of tannin-based resin is a good choice of cross-linking agent, since its environmental 360 contribution on the entire system is not very significant (Figure 7). As for the previous 361 scenarios, the contribution of the soy protein extraction is remarkable. Therefore, 362 optimizing the protein extraction process, or even reducing the amount of soy protein 363 17 used for the formulation, becomes a key aspect of improvement to achieve a bio364 adhesive option with the potential to replace synthetic adhesives. 365 < Figure 7 over here> 366 3.8. Benchmarking the soy-based bio-adhesives and fossil-based 367 counterparts. 368 In order to develop a comparison with synthetic-based resins, the ReCipe 2016 369 hierarchist Endpoint V1.03 (2010) World H/H methodology has been applied (Table 2). 370 The damage categories considered were human health (HH), ecosystem quality (EQ) and 371 fossil resource scarcity (FRS). In this way, by applying the normalization and weighting 372 values of each of the categories, a single environmental score is estimated, which allows 373 for an overall value that includes three key elements to be considered for sustainable 374 development. 375 < Table 2 over here> 376 Considering soybean as a raw material, the soy-based bio-adhesives show better 377 environmental profiles than those for UF and PF. Even if compared with MUF, which is 378 the fossil resin with the best profile but the exception of the soy flour enhanced by the 379 WPU-based adhesive, where a greater contribution is observed in the HH category 380 compared to the value obtained by MUF (30 mPt versus 24 mPt, respectively), which 381 affects the overall environmental score (36 mPt versus 30 mPt, respectively). 382 Finally, the method of calculating USEtox has been used to carry out a more 383 comprehensive assessment of the human health category, in which two impact 384 categories are developed: human toxicity, cancer (HT, c) and human toxicity, non-cancer 385 18 (HT, nc) (Table 3). The values obtained showed that bio-adhesive alternatives have a 386 lower potential risk on human health, since their toxicity levels (both carcinogenic and 387 non-carcinogenic) are significantly lower than those obtained for UF and PF resins, which 388 are the most widely used in the field of wood panel production. 389 < Table 3 over here> 390 3.9. Sensitivity analysis and alternative formulations of the bio-adhesives 391 Although the results obtained for the soy bio-adhesives are promising, their 392 environmental profiles could be even better if the main critical points previously 393 identified for each bio-adhesive were improved and optimized. In the case of adhesives 394 that required the protein extraction stage, it was observed that the energy requirements 395 of the process contributed significantly to the environmental profile, and therefore, by 396 developing an energy optimization of the production process, they could be reduced by 397 up to 25%. On the other hand, it has also been found that the use of certain chemicals 398 as cross-linking agents has an important contribution to the environmental profile 399 obtained, as has been reported in the case of the use of dicyandiamide, WPU, epoxy 400 resin and, above all, hexamethylenediamine. Therefore, a sensitivity analysis has been 401 carried out focusing on the reduction of the amount of chemical dosage, assuming that 402 the reduction of the amount of cross-linking agent by 20% in the bio-adhesive 403 formulation. The first case considered for study (Figure 8a) refers to the reduction of the 404 amount of dicyandiamide used for the formulation of the SF+D bio-adhesive. The 405 greatest environmental improvement is observed in the categories of TA, FE and FRS, 406 where impact reductions of 13%, 14% and 12%, respectively, has been obtained. It 407 should be noted that in the categories of GW, SOD and ME no significant changes are 408 19 observed in comparison with the base case. The reason for this low level of 409 improvement stems from the fact that the main contributor in these impact categories 410 is not the use of dicyandiamide, but rather the background activities associated with the 411 production of soybean. 412 The second object of study is based on the SF + WP bio-adhesive, in which it has been 413 proposed to reduce the dose of WPU used for the formulation of the adhesive. The 414 results obtained (Figure 8b) showed a significant improvement in the environmental 415 profile, reaching reduction percentages of up to 20%. The only impact category where 416 the improvement obtained is negligible in the ME. As in the previous analysis, the 417 environmental contribution in this category of impact is the result of land management 418 and agricultural activities associated with soybean cultivation. 419 In the case of reducing the dose of HDMA used for the production of the SP + MA bio420 adhesive, improvement percentages of between 13% and 14% have been achieved for 421 the impact categories of ME, TET and FRS (Figure 8c). The formulation of this adhesive 422 requires the process of extracting the protein from the soybean, a stage that requires 423 significant energy consumption, resulting in a high contribution to the environmental 424 profile obtained for the adhesive. The fact that two hotspots of the bio-adhesive 425 production process are identified means that, although an improvement in the 426 environmental profile has been obtained by reducing the dosage of crosslinker used, the 427 impact reduction values are not as high as desired. A joint reduction of the two main 428 contributors to the system would lead to a more significant improvement in the profile. 429 The last alternative focused on reducing the amount of cross-linking agent used for the 430 formulation is that of SF+SE bio-adhesive (Figure 8d). The use of epoxy resin for the 431 20 formulation of the bio-adhesive implies the use of non-renewable fossil resources. 432 Therefore, a reduction in the dose of this chemical used leads to a significant 433 improvement in the impact category of SF+SE, reaching a percentage of reduction of 434 15%. However, in the SOD and ME categories, the percentages of impact reduction are 435 less than 1%. The reason for this low value is that in these categories the process of 436 extraction of the protein from the soybean, together with the background activities 437 associated with its cultivation, are the main contributors. 438 The most significant improvements in the environmental profiles have been achieved 439 by reducing the electrical requirements of the soy protein extraction process by 25% 440 (Figure 8e). The highest percentages of improvement were obtained for the FE (22%), 441 MET (21%) and FRS (20%) categories. The only impact categories in which the 442 improvement has not been as high as expected are SOD and ME, in which the 443 percentages of impact reduction were 3% and 2%, respectively. In these categories, the 444 influence of the background activities of soybean cultivation is highly significant, 445 reaching practically the entire impact generated. 446 < Figure 8 over here> 447 448 Conclusions 449 In this study LCA methodology was used for the environmental assessment of different 450 soy-based bio-adhesives as alternatives to fossil-based ones. Certain improvements in 451 production processes can be identified that would considerably reduce the 452 environmental impacts of the bio-adhesives production. The optimization of the protein 453 21 extraction process, in terms of the use of energy resources, is considered a key aspect 454 to be improved in those bio-adhesives that require this initial stage of the process. 455 In general, once the comparative studies have been carried out, it is considered that the 456 alternatives of SF + D, SP + EP and SF + SE are the best from the environmental point of 457 view, as they are the ones that have less impact on ecosystem quality, fossil resource 458 scarcity, human health categories. With regard to the health effects that may be caused 459 by dicyandiamide and polyacrylamide, toxicity data should be considered as a criterion 460 for selecting the best soy-based bio-adhesive to replace synthetic resins. With this in 461 mind, the SF+SE bio-adhesive is selected as the best alternative considering both 462 environmental and toxicity perspectives. 463 Ethics approval and consent to participate 464 Not applicable. 465 Consent for publication 466 Not applicable. 467 Availability of data and materials 468 All data generated or analysed during this study are included in this published article 469 [and its supplementary information files]. 470 Competing interest 471 The authors declare that they have no known competing financial interests or personal 472 relationships that could have appeared to influence the work in this paper. 473 Funding & Acknowledgements 474 22 This research has been financially supported by ERA-CoBIOTech (PCI2018-092866) 475 WooBAdh project. Dr. S. González García thanks to the Spanish Ministry of Economy and 476 Competitiveness for financial support (Grant reference RYC-2014-14984). The authors 477 belong to the Galician Competitive Research Group (GRC ED431C 2017/29) and to 478 CRETUS Strategic Partnership (ED431E 2018/01). 479 Authors’ contribution 480 Ana Arias: Methodology, Writing-original draft, Formal analysis, Writing-review & 481 editing. Sara González-García: Writing-review & editing, Validation. Gumersindo Feijoo: 482 Validation. María Teresa Moreira: Conceptualization, Writing-review & editing, 483 Validation. 484 485 References 486 Araujo, V. K. A., de Almeida, S., de Oliveira, S. B., Calixto, W. P., Furriel, G. P., & Barbosa, 487 D. P. (2017, May). Anaerobic digestion using residue of soybean processing: Biogas 488 production and it is potential to generate energy. In 2017 18th International Scientific 489 Conference on Electric Power Engineering (EPE) (pp. 1-4). IEEE. 490 Berardy, A., Costello, C., & Seager, T. (2015, May). Life cycle assessment of soy protein 491 isolate. In Proceedings of the International Symposium on Sustainable Systems and 492 Technologies, Dearborn, MI, USA (pp. 18-20). 493 Carlisle, J., Chan, D., Golub, M., Henkel, S., Painter, P., & Lily, W. K. (2009). Toxicological 494 Profile for Bisphenol A. September 2009. Office of Environmental Health Hazard 495 Assessment Ocean Protection Council under an Interagency Agreement, (07-055). 496 Chen, M., Luo, J., Shi, R., Zhang, J., Gao, Q., & Li, J. (2017). Improved adhesion 497 performance of soy protein-based adhesives with a larch tannin-based 498 resin. Polymers, 9(9), 408. 499 Figure 4. Impact assessment results per impact category for SF+SE bio-adhesive production. Acronyms: global warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET) and fossil resource scarcity (FRS). Figure 5. Impact assessment results per impact category for SP+EP bio-adhesive production. Acronyms: global warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET) and fossil resource scarcity (FRS). 0% 20% 40% 60% 80% 100% GW SOD TA FE ME TET FET MET FRS Tap water CaO NaOH Soy flour Epoxy resin SBDS Electricity Heat 0% 20% 40% 60% 80% 100% GW SOD TA FE ME TET FET MET FRS Soy protein Tap water Polyacrylamide Epoxy resin Electricity Heat Figure 6. Impact assessment results per impact category for SP+MA bio-adhesive production. Acronyms: global warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET) and fossil resource scarcity (FRS). Figure 7. Impact assessment results per impact category for SP+TR bio-adhesive production. Acronyms: global warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET) and fossil resource scarcity (FRS). 0% 20% 40% 60% 80% 100% GW SOD TA FE ME TET FET MET FRS Soy protein Tap water Maleic anhydride HDMA Electricity Heat 0% 20% 40% 60% 80% 100% GW SOD TA FE ME TET FET MET FRS Tannin Tap water Methanol Resorcinol Glyoxal Sodium Hydroxide Soy protein Heat Figure 8. Sensitivity analysis of bio-adhesives production processes under study; a) Reduction of 20% on dicyandiamide dose on SF+D bio-adhesive b) Reduction of 20% on WPU dose on SF+WP bio-adhesive c) Reduction of 20% on HDMA dose on SP+MA bio-adhesive8d) Reduction of 20% on epoxy resin dose on SF+SE bio-adhesive e) Reduction of 25% on electricity needs on soy protein extraction process. Acronyms: global warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET) and fossil resource scarcity (FRS). 0 20 40 60 80 100 GW SOD TA FE ME TET FET MET FRS Base case Reduction of 25% on electricity needs 0 20 40 60 80 100 GW SOD TA FE ME TET FET MET FRS Base case Reduction of 20% on dicyandiamide dose 0 20 40 60 80 100 GW SOD TA FE ME TET FET MET FRS Base case Reduction of 20% on WPU dose 0 20 40 60 80 100 GW SOD TA FE ME TET FET MET FRS Base case Reduction of 20% on HDMA dose 0 20 40 60 80 100 GW SOD TA FE ME TET FET MET FRS Base case Reduction of 20% on epoxy resin dose (a) (b) (d) (c) (e) Table 1. Impact assessment values obtained for the bio-adhesives per impact category. Acronyms: global warming (GW), stratospheric ozone depletion (SOD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET), fossil resource scarcity (FRS), soy flour with dicyandiamide crosslinker bio-adhesive (SF+D), soy protein crosslinked with epoxy resin and polyacrylamide bio-adhesive (SP+EP), soy flour enhanced by waterborne polyurethane bioadhesive (SF+WP), soy protein with maleic anhydride bio-adhesive (SP+MA), soy flour crosslinked with SBDS and epoxy resin bio-adhesive (SF+SE) and soy protein with tannin-based resin bio-adhesive (SF+TR). Impact Category Unit SCENARIOS SF+ D SP + EP SF + WP SP + MA SF + SE SP + TR GW kg CO2 eq 0.61 0.71 2.56 2.14 1.20 0.67 SOD mg CFC11 eq 1.62 1.08 46.07 2.23 2.07 1.22 TA g SO2 eq 1.60 1.94 3.41 6.39 1.96 1.95 FE g P eq 0.19 0.30 0.22 0.76 0.19 0.33 ME g N eq 0.33 0.23 0.49 1.12 0.43 0.26 TET kg 1,4-DCB 0.44 0.43 1.00 2.19 0.62 0.40 FET g 1,4-DCB 6.14 10.42 12.24 24.15 10.35 10.77 MET g 1,4-DCB 5.30 11.85 12.33 31.06 8.46 12.61 FRS kg oil eq 0.06 0.15 0.30 0.73 0.28 0.11 Tables Table 2. Single score values obtained for the bio-adhesives per damage category, and its comparison with synthetic resins. Acronyms: Human Health (HH), Ecosystem Quality (EQ), Fossil Resource Scarcity (FRS), soy flour with dicyandiamide crosslinker bio-adhesive (SF+D), soy protein crosslinked with epoxy resin and polyacrylamide bio-adhesive (SP+EP), soy flour enhanced by waterborne polyurethane bio-adhesive (SF+WP), soy protein with maleic anhydride bio-adhesive (SP+MA), soy flour crosslinked with SBDS and epoxy resin bio-adhesive (SF+SE) and soy protein with tannin-based resin bio-adhesive (SF+TR). Damage Category Unit SCENARIOS SF+ D SP + EP SF + WP SP + MA SF + SE SP + TR HH mPt 7.16 8.34 30 25 14 7.89 EQ mPt 1.28 1.55 4.68 4.65 2.31 1.49 FRS mPt 0.23 0.45 1.00 2.65 1.04 0.25 TOTAL 8.67 10 36 32 17 9.64 Damage Category Unit SYNTHETIC RESINS UF MUF PF HH mPt 31 24 41 EQ mPt 6 5 7 FRS mPt 5 3 7 TOTAL 41 33 56 Table 3. Impact assessment values obtained for the bio-adhesives per impact category to evaluate the human impacts caused, referring to the environmental indicator of Human Toxicity (unit values: CTUh) and its comparison with synthetic resins. Acronyms: HT,c (Human Toxicity, cancer), HT,nc (Human Toxicity, non-cancer), soy flour with dicyandiamide crosslinker bioadhesive (SF+D), soy protein crosslinked with epoxy resin and polyacrylamide bio-adhesive (SP+EP), soy flour enhanced by waterborne polyurethane bio-adhesive (SF+WP), soy protein with maleic anhydride bio-adhesive (SP+MA), soy flour crosslinked with SBDS and epoxy resin bio-adhesive (SF+SE) and soy protein with tannin-based resin bio-adhesive (SF+TR). Impact Category Unit SOYBEAN AS RAW MATERIAL SF+ D SP + EP SF + WP SP + MA SF + SE SP + TR HT,c ·10-10 CTUh 3.55 2.46 5.79 36 5.49 2.30 HT,nc 4.41 2.76 5.60 3.87 6.29 2.22 Total 7.97 5.22 11 40 12 4.52 Impact Category Unit SYNTHETIC RESINS UF PF MUF HT,c ·10-10 CTUh 259 92 1.91 HT,nc 5.38 12 1.77 Total 264 104 3.68 Cover Letter Click here to access/download Supplementary Material Suplemental Material_Tables.docx