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Thermal Induced Yellowing of Peroxide Bleached Birch Pulp

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Thermal Induced Yellowing of Peroxide Bleached Birch Pulp Thermal induced yellowing of peroxide bleached birch pulp Alexander Nygren Sustainable Process Engineering, master's level 2020 Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Student: Alexander Nygren [email protected] Examiner: Prof. Ulrika Rova Supervisors: Dr. Io Antonopoulou Marianne Tollander Program: TCMPA Course: X7003K Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Division of Sustainable Process Engineering Chemical Technology Preface The work during this master thesis has been performed at Smurfit Kappa in Piteå, a producer of Kraftliner. The thesis was carried out from October 2019 through April 2020 and is the final assignment within the engineering program “Sustainable Process Engineering” at Luleå University of Technology. II Acknowledgement I would like to start this report by expressing my gratitude to Professor Ulrika Rova, Dr. Io Antonopoulou, quality manager Marianne Tollander and quality engineer Johan Lundberg; my supervisors, for their great guidance and useful critique during this master thesis. I am also grateful to Smurfit Kappa and Luleå University of Technology (LTU) for arranging this thesis which is the final assignment of the master’s degree studies in sustainable chemical engineering at LTU. Further, I want to thank all employees at Smurfit Kappa in Piteå for being friendly, welcoming and helping me when needed. Finally, I would like to thank my loving family and partner Malin Sanderyd for always supporting and encouraging me when needed. And the most important part of these years at the university will always be my wonderful friends that I’ve had the benefit of socializing with throughout my studies; thank you all for the patience, support and for making my time as a student the best five years of my life that will always live on through my memories. Luleå, 2020 __________________________ Alexander Nygren III Abstract Brightness reversion, also known as yellowing, is a well-known phenomenon which means that the brightness of paper products decreases during ageing. The name of this phenomenon is based on that paper products usually change in color towards yellow during ageing. Yellowing is considered to occur due to a mixture of chemical and physical factors, which makes it a complex problem for the pulp & paper industry. The majority of the literature and research conducted with respect towards yellowing claims that light and heat is the two main factors that contributes the most to a brightness reversion, depending on the type of pulp and process that is utilized. Smurfit Kappa in Piteå is a manufacturer of the paper grade Kraftliner and has during some occasions noted unstable brightness. Based on previous work at Smurfit Kappa, it is known that the finished liners produced from bleached pulp in a completely chlorine-free process is very sensitive to heat, especially for longer periods of time during storage. It has also been documented that the storage temperature for paper products is of great importance, especially the cooling rate of the paper-rolls from production, which could take around two weeks to reach the ambient temperature. This thesis work, alongside with a literature study as a basis will examining the effect of pH towards yellowing during thermal exposure. Through a factorial experiment it was initially found that the yellowing is favored by higher temperatures in conjunction with lower pH values. In order to obtain a brightness reversion of a paper product within a reasonable timeframe an accelerated aging method was used according to the ISO standard 5630-1. Throughout this thesis is the brightness reversion expressed in the so-called b* value, which indicates the color change from blue to yellow. Further experiments, including ageing methods with moisture, also concluded that an acidic pH results in a more severe yellowing. It was also observed that the pH was decreasing during experiments of pulp storage, this most likely to the chemical phenomena known as acidic hydrolysis. The b*-value seemed to be favorable of the decreased pH, thus could the pulp be stored at pH around 8 instead of 9-10 in order to suppress potential yellowing reactions. Furthermore, it was found that cooling of the paper resulted in a decrease of the b* value, it is however unclear what causes this phenomenon but a theory could be that chromophoric groups are being deactivated/activated due to the temperature changes and hence making the phenomena reversible. IV Sammanfattning Eftergulning är ett välkänt fenomen som innebär att ljusheten hos pappersprodukter minskar när dessa åldras. Namnet på detta fenomen grundar sig i att pappersprodukter vanligtvis skiftar i färg mot det gula hållet. Detta anses bero på en blandning av kemiska och fysikaliska faktorer, vilket därmed gör det till ett komplext problem som berör massa & pappersindustrin. Majoriteten av litteraturen och den forskning som utförts inom området gällande eftergulning visar att de två faktorerna ljus och värme påverkar förändringen av ljusheten i högst omfattning, beroende på vilken typ av massa och process som pappret är producerat av. Smurfit Kappa i Piteå är en tillverkare av papperstypen Kraftliner och har under olika perioder noterat ostabila ljushetsvärden på grund av eftergulningen. Frida Sandin konstaterade genom sitt examensarbete hos Smurfit Kappa under 2008 att massan som bleks i en helt klorfri process är extra känslig mot värme, speciellt under längre tidsperioder. Sandin konstaterade även att lagringstemperaturen för pappersprodukter har stor betydelse, speciellt gällande avsvalningsförloppet i de nytillverkade pappersrullarna under papperstillverkning vilka kunde ta ungefär två veckor på sig att nå omgivningens temperatur. Detta arbete fortsätter på Sandins tidigare studier, samt med en litteraturstudie som grund, genom att undersöka pH:s påverkan på papper i samband med varierande temperaturer mer noggrant. Genom ett inledande faktorförsök konstaterades det att eftergulningen gynnas av högre temperaturer i samband med lägre pH värden. För att kunna få en eftergulning på en pappersprodukt inom en rimlig tid så användes en accelererad åldringsmetod, i detta arbete användes främst en metod med en temperatur på 105 °C enligt ISO standarden 5630–1. Eftergulningen inom detta arbete uttrycks i det så kallade b*- värdet vilket indikerar en färgskiftning mellan blått och gult. Ytterligare försök, bland annat i kombination med fukt, konstaterade vidare att ett surt pH starkt missgynnar stabiliteten på eftergulningen för pappersprodukter. Det observerades också att pH-värdet sjönk under experiment där pappersmassa lagrades, detta skedde mest troligt på grund av det kemiska fenomenet som kallas sur hydrolys. b*-värdet tycktes däremot vara gynnsamt av det sänkta pH-värdet. Förslagsvis så borde massan lagras vid pH runt 8 istället för 9–10 för att reducera potentiella eftergulnings-reaktioner. Det konstaterades också att kylning av papperet resulterar i en sänkning av b*-värdet, det är dock oklart vad som får detta att ske men en teori är att det är kromofora grupper som aktiveras/inaktiveras. V Abbreviations A Acidic stage D Chlorine dioxide E Alkaline extraction ECF Elemental chlorine free EDTA Ethylenediaminetetraacetic acid ISO International Organization for Standardization IUPAC International Union of Pure and Applied Chemistry k Kubelka-Munk absorption coefficient O Oxygen stage P Peroxide stage PM1 Paper machine 1 PM2 Paper machine 2 Q Chelation stage R∞ Reflectance of an optically thick material R0 Reflectance of a single transmitting sheet s Kubelka-Munk scattering coefficient TAPPI Technical Association of the Pulp and Paper TCF Total chlorine free UV Ultraviolet Z Ozone stage Keywords: Accelerated ageing, Bleaching, Brightness reversion, Kraft pulp, pH, Yellowing VI Contents Preface ..................................................................................................................................................... II Acknowledgement ................................................................................................................................. III Abstract ................................................................................................................................................. IV Sammanfattning...................................................................................................................................... V Abbreviations ........................................................................................................................................ VI 1 Introduction ................................................................................................................................... 11 1.1 Background ........................................................................................................................... 11 1.2 Smurfit Kappa ....................................................................................................................... 12 1.2.1 Kraftliner ....................................................................................................................... 13 1.3 Contents description .............................................................................................................. 13 2 About the project ........................................................................................................................... 14 2.1 Objective ..............................................................................................................................
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