Reductive and Oxidative Bleaching of Aged Lignocellulosic Papers: A Comparative Study
Abstract
With natural ageing, wood pulp papers acidify, yellow and become very fragile, all of which pose a major preservation challenge. Bleaching is a remediation treatment primarily used to increase the brightness of paper, thereby enabling access to its historical or artistic content. Literature on paper bleaching is vast and the lack of standardised protocols makes it difficult to compare the results. Moreover, these studies often focus on papers containing no or little lignin. To tackle these issues, we compared the impact of reductive and oxidative bleaching on two naturally aged lignocellulosic papers' color, pH, water absorption and mechanical strength.
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Context & Objectives I References Reductive and Oxidative Bleaching of Aged Lignocellulosic Papers: A Comparative Study Camille Jallu1,2,4, Nathan Ferrandin-Schoffel1,3, Anne-Laurence Dupont4, Estelle Van Geyts1, Francisco Mederos-Henry1,2,5 1 École nationale supérieure des Arts Visuels de La Cambre (ENSAV La Cambre), Conservation and Restoration department, Brussels, Belgium 2 Royal Institute for Cultural Heritage (KIK-IRPA), Laboratories department, Brussels, Belgium 3 Institut Parisien de Chimie Moléculaire (UMR 8232), Equipe Chimie des Polymères, Sorbonne Université, CNRS, Paris, France 4 Centre de Recherche sur la Conservation, Muséum National d’Histoire Naturelle, CNRS, Paris, France 5 Research Centre in Archaeology and Heritage (CReA-Patrimoine), Université Libre de Bruxelles (ULB), Brussels, Belgium [email protected] With natural ageing, wood pulp papers acidify, yellow and become very fragile, all of which pose a major preservation challenge. Bleaching is a remediation treatment primarily used to increase the brightness of paper, thereby enabling access to its historical or artistic content. [1] Henniges, U., Potthast, A. (2009). Bleaching revisited: Impact of oxidative and reductive bleaching treatments on cellulose and paper. Restaurator, 30(4), 294–320 [2] Hey, M. (1979). The washing and aqueous deacidification of paper. The Paper Conservator, 4(1), 66–80 [3] Burgess, H. D. (1988). Practical considerations for conservation bleaching. Journal of the IIC-Canadian Group, 13 [4] Browning, B. L. (1977). Analysis of paper, M. Dekker, Ed., pp. 78–86 [5] Calvini, P., Gorassini, A. (2002). FTIR-deconvolution spectra of paper documents. Restaurator, 23(1), 48–66 Total Colour Change III ONNaBH4TBAB H2O2Sunlight The reduction protocols outperformed the oxidation ones in diminishing the yellowness of the degraded papers, especially NaBH4. Conclusions Reduction bleaching protocols outperformed oxidation ones in diminishing paper yellowing Reducing treatments improved the folding endurance of both papers but the results were more contrasted with the oxidising ones The tensile strength weakening of paper fibres due to bleaching with TBAB remains to be better understood Further research should include more lignocellulosic papers We compared reductive and oxidative bleaching on two naturally aged lignocellulosic papers UV-filtered sunlight bleaching Just Noticeable Difference, JND (∆E* < 2) 0 5 10 15 ∆E* BYK®spectro2guide CIE*Lab76 ISO 5631-2 Original Neutralised Sodium borohydride Tertbutylamineborane complex Hydrogen peroxide / No treatment Ca(OH) 232 mg.L1 0,2 M 0,2 M 1 % w/V UV-filtered** DI water Ethanol DI water + Ca(OH)2 pH 9 30 min 60 min 120 min Reducing agents A multi-step methodology [2—3] DI water 3 x 30 min DI water WASHING NEUTRALISATION Ca(OH)2* aqueous solution (32 mg.L-1) BLEACHING NEUTRALISATION RINSING DI water Reducing and oxidising agents Ca(OH)2 aqueous solution (32 mg.L-1) *Calcium hydroxide DI water NaBH4 NO TBAB H2O2Sunlight Images of the papers before and after bleaching TAPPI T-494 Mechanical Strength V Coaxial Tensile Strength Degraded paper Highly degraded paper Three different mechanical tests were performed to assess the impact of bleaching on the paper resistance. Overall NaBH4 showed the best potential as an optimal bleaching agent to stabilize both degraded lignocellulosic papers. TAPPI T-511TAPPI T-231 Zero-Span Tensile Strength Folding Endurance 6 7 8 9 Zero-Span TS (kg/15mm) 0 20 40 60 80 Number of double folds 10 20 30 Tensile strength (MPa) pH and Water Absorption IV ONNaBH4TBAB H2O2Sunlight 5 mm *Contemporary machine-made paper: GWP/BSP, kaolin filler, rosin sizing Hydrophilicity of the papers increased, especially with the reducing agents. Water droplet absorption of bleached paper* samples All the papers that underwent the multi-step methodology were alkaline. 4 5 6 7 8 9 pH TAPPI T-509 **Cutoff of UVA, UVB, UVC between 200 and 390 nm Papers Composition II FTIR Characterisation [5] 1000 1500 2000 2500 3000 3500 4000 Absorbance (a. u.) Wavenumbers (cm-1) Rosin Transmission Extracted dry residue Rosin reference 1000 1500 2000 2500 3000 3500 4000 Absorbance (a. u.) Wavenumbers (cm-1) Kaolin Lignin ATR Early 20th century newsprint paper (highly degraded) Whatman #1 pure cellulosic paper Pulp, fillers, and sizing of two early 20th century newsprint papers FTIR on the dry residues 2800 1710 1655 1450 1430 820 3690 3620 1510 1590 1725 Sizing Chemical test for rosin [4] A newsprint paper from the project X100 50 µm GWP USP GWP USP groundwood pulp unbleached sulfite pulp Fibres Identification Lofton-Meritt indicator staining BSP bleached sulfite pulp BSP Oxidising agents Literature on paper bleaching is vast and the lack of standardised protocols makes it difficult to compare the results [1]. Moreover, these studies often focus on papers containing no or little lignin.