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Currently, 90% of the total market of this compound corresponds to lignosulphonates (LS). The most successful industrial attempts to use lignin for wood adhesives rely on using it as a partial substitute in phenol\u2013formaldehyde or urea\u2013formaldehyde resins. However, lignin\u2019s aromatic ring presents a low number of reactive sites. Several methods have been proposed to improve its reactivity, such as prior methylolation with formaldehyde. Off-line methods are commonly applied to monitor this reaction\u2019s progress, but this introduces a significant delay in the analysis. This study proposes a new method for in-line monitoring of the methylolation reaction using visible spectroscopy. In order to monitor the reaction progress, principal component analysis was applied to the spectra, and the obtained scores were analyzed. When these results were plotted against those obtained by the off-line methods, a satisfactory regression was obtained at 50 \u00b0C (R2 = 0.97) and 60 \u00b0C (R2 = 0.98) for two different LS samples. Therefore, it was concluded that visible spectroscopy is a promising technique for studying lignin methylolation.<\/jats:p>","DOI":"10.3390\/polym15010178","type":"journal-article","created":{"date-parts":[[2022,12,30]],"date-time":"2022-12-30T03:18:18Z","timestamp":1672370298000},"page":"178","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":8,"title":["The Potential of Visible Spectroscopy as a Tool for the In-Line Monitoring of Lignin Methylolation"],"prefix":"10.3390","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9216-3867","authenticated-orcid":false,"given":"Sofia","family":"Gon\u00e7alves","sequence":"first","affiliation":[{"name":"LEPABE\u2014Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal"},{"name":"Associate Laboratory in Chemical Engineering (ALiCE), FEUP, 4200-465 Porto, Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jorge","family":"Martins","sequence":"additional","affiliation":[{"name":"LEPABE\u2014Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal"},{"name":"Associate Laboratory in Chemical Engineering (ALiCE), FEUP, 4200-465 Porto, Portugal"},{"name":"DEMad\u2013Departamento de Engenharia de Madeiras, Instituto Polit\u00e9cnico de Viseu, Campus Polit\u00e9cnico de Repeses, 3504-510 Viseu, Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"N\u00e1dia T.","family":"Paiva","sequence":"additional","affiliation":[{"name":"Sonae Arauco Portugal S.A., Lugar do Espido\u2014Via Norte, 4470-177 Porto, Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9878-8411","authenticated-orcid":false,"given":"Diana","family":"Paiva","sequence":"additional","affiliation":[{"name":"LEPABE\u2014Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal"},{"name":"Associate Laboratory in Chemical Engineering (ALiCE), FEUP, 4200-465 Porto, Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4557-7113","authenticated-orcid":false,"given":"Lu\u00edsa H.","family":"Carvalho","sequence":"additional","affiliation":[{"name":"LEPABE\u2014Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal"},{"name":"Associate Laboratory in Chemical Engineering (ALiCE), FEUP, 4200-465 Porto, Portugal"},{"name":"DEMad\u2013Departamento de Engenharia de Madeiras, Instituto Polit\u00e9cnico de Viseu, Campus Polit\u00e9cnico de Repeses, 3504-510 Viseu, Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4447-4473","authenticated-orcid":false,"given":"Fern\u00e3o D.","family":"Magalh\u00e3es","sequence":"additional","affiliation":[{"name":"LEPABE\u2014Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal"},{"name":"Associate Laboratory in Chemical Engineering (ALiCE), FEUP, 4200-465 Porto, Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2022,12,30]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"11559","DOI":"10.1021\/acs.chemrev.5b00155","article-title":"Catalytic Transformation of Lignin for the Production of Chemicals and Fuels","volume":"115","author":"Li","year":"2015","journal-title":"Chem. Rev."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"38604","DOI":"10.1039\/C7RA06598A","article-title":"Development of sustainable bio-adhesives for engineered wood panels\u2014A Review","volume":"7","author":"Adamopoulos","year":"2017","journal-title":"RSC Adv."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"65","DOI":"10.1007\/s00107-006-0130-z","article-title":"Lignin-based wood panel adhesives without formaldehyde","volume":"65","author":"Pizzi","year":"2007","journal-title":"Holz Als Roh Und Werkst."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Gon\u00e7alves, S., Ferra, J., Paiva, N., Martins, J., Carvalho, L.H., and Magalh\u00e3es, F.D. (2021). Lignosulphonates as an Alternative to Non-Renewable Binders in Wood-Based Materials. Polymers, 13.","DOI":"10.3390\/polym13234196"},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Lubis, M.A.R., Labib, A., Akbar, F., Nuryawan, A., Antov, P., Kristak, L., Papadopoulos, A.N., and Pizzi, A. (2022). Influence of Lignin Content and Pressing Time on Plywood Properties Bonded with Cold-Setting Adhesive Based on Poly (Vinyl Alcohol), Lignin, and Hexamine. Polymers, 14.","DOI":"10.3390\/polym14102111"},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Hemmil\u00e4, V., Adamopoulos, S., Hosseinpourpia, R., and Ahmed, S.A. (2019). Ammonium Lignosulfonate Adhesives for Particleboards with pMDI and Furfuryl Alcohol as Crosslinkers. Polymers, 11.","DOI":"10.3390\/polym11101633"},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Ferreira, A.M., Pereira, J., Almeida, M., Ferra, J., Paiva, N., Martins, J., Magalh\u00e3es, F.D., and Carvalho, L.H. (2018). Biosourced Binder for Wood Particleboards Based on Spent Sulfite Liquor and Wheat Flour. 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