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.

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.

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.

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

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 ...... 14 2.2 Limitations...... 14 2.3 Key Questions ...... 14 3 Theory ...... 15 3.1 Wood morphology ...... 15 3.1.1 Cellulose ...... 16 3.1.2 Hemicellulose ...... 16 3.1.3 Lignin ...... 16 3.2 The pulping process ...... 16 3.3 Brief overview of Kraft pulping ...... 17 3.3.1 Impregnation ...... 18 3.3.2 Cooking ...... 18 3.3.3 Washing ...... 19 3.3.4 Recovery process ...... 19 3.3.5 Bleaching ...... 20 3.3.6 Papermaking process ...... 21 3.4 Factors that can affect brightness ...... 22 3.4.1 Chemical factors ...... 23 3.4.1.7.1 Fillers ...... 27 3.4.2 Physical factors ...... 31 3.5 Optical properties ...... 35 3.5.1 Brightness ...... 36 3.5.2 Y-value ...... 36 3.5.3 Color ...... 37 3.5.4 Measurements of optical properties ...... 39 4 Method...... 43

VII

4.1 Accelerated ageing ...... 43 4.2 Brightness & color measurements ...... 43 4.3 Preparation of laboratory sheets for brightness measurements ...... 43 4.4 Factorial design ...... 44 4.5 pH variation ...... 46 4.6 Temperature variation ...... 46 4.7 Storage of pulp ...... 46 4.8 Variation of relative humidity ...... 47 4.9 Applying cooling ...... 47 4.10 Preparation of laboratory sheets for physical testing ...... 47 4.11 Measurement of physical properties ...... 47 4.11.1 Grammage ...... 47 4.11.2 Thickness ...... 47 4.11.3 Bursting strength ...... 48 4.11.4 Porosity ...... 48 4.11.5 Roughness ...... 48 4.11.6 Short Span Compression Test (SCT) ...... 48 4.12 Mill trial ...... 48 5 Results ...... 49 5.1 Factorial design ...... 49 5.1 pH-variation ...... 50 5.2 Temperature variation ...... 53 5.3 Pulp storage ...... 54 5.4 Relative humidity ...... 54 5.5 Cooling of the laboratory sheets ...... 55 5.6 Cooling of archived samples ...... 57 5.7 Paper properties ...... 57 6 Discussion ...... 59 6.1 General discussion ...... 59 6.1.1 Chemical factors ...... 59 6.1.2 Physical factors ...... 60 6.2 Economical & environmental evaluation ...... 61 6.3 Possible sources of errors ...... 62 7 Conclusion ...... 63 7.1 Future work ...... 63 8 References ...... 64

VIII

List of Figures Figure 1 Worldwide production of pulp and paper & usage of waste paper (Skogsindustrierna, n.d.) 11 Figure 2 Production of pulp & paper by region during 2017 (Skogsindustrierna, n.d.) ...... 11 Figure 3 Kraftliner is the outer layer of corrugated board (A40 Packaging, 2018) ...... 13 Figure 4 Schematic illustration of the cell wall layers showing microfibril orientation (Solala, 2015) 15 Figure 5 Overview of the Kraft process (Andersson , 2014) ...... 17 Figure 6 Stages during the recovery of chemicals within the Kraft pulping process (Brännvall, 2009a) ...... 20 Figure 7 A simplified illustration of the paper machine (Brännvall, 2009a) ...... 22 Figure 8 Factors affecting yellowing within the papermaking process (Forsskåhl, 2000) ...... 22 Figure 9 Catalytic decomposition of hydrogen peroxide by the Fenton cycle (Backlund , 2009) ...... 24 Figure 10 Elimination of methanol from xylan during pulping (Gellerstedt , 2009)...... 25 Figure 11 Increased brightness stability trough degradation of the hexenuronic acid group’s (Sandin, 2008) ...... 26 Figure 12 Factors affecting paper permanence (VERSO, 2016) ...... 26 Figure 13 Sizing agents used worldwide (Auhorn, 2006) ...... 28 Figure 14 Anchoring of a rosin molecule onto a fiber surface (Hubbe M. A., 2004) ...... 29 Figure 15 The fortification of abietic acid with maleic anhydride to produce fortified rosin (Hubbe M. A., 2004) ...... 29 Figure 16 Aqueous solubility of Aluminum compounds (Lindström T. , 2009) ...... 30 Figure 17 Decrease of the temperature in paper-reels (Lindström M. , 1990) ...... 32 Figure 18 Applied cooling after drying in able to lower the temperature, hence reducing the brightness reversion (Larsson & Karlsson, 2000) ...... 33 Figure 19 Amount of water in air at different RH across a temperature range (Lenntech, 2020) ...... 33 Figure 20 The amount of water that pure cellulose can bind increases with the relative humidity (Hubbe, o.a., 2017) ...... 34 Figure 21 Illustration of how light interacts with a paper (Vaarasalo, 1999) ...... 35 Figure 22 Difference between Y-value and ISO brightness (Vaarasalo, 1999) ...... 36 Figure 23 The CIE 1931 color space chromaticity diagram (Wikipedia, the free encyclopedia, 2020) 38 Figure 24 The CIELAB color space (Konica Minolta Sensing Americas, Inc, 2020a) ...... 39 Figure 25 Standard illuminants A, C & D65 (Konica Minolta Sensing Americas, Inc, 2020b) ...... 39 Figure 26 Folding the sheet for brightness evaluation ...... 43 Figure 27 Preparation of laboratory sheets from bleached pulp (Nygren, 2019) ...... 44 Figure 28 Pulp in a stainless-steel container ...... 46 Figure 29 Interaction plot where pH4 is denoted with (-), pH5 is (0) and pH7 is (+) ...... 49 Figure 30 b*-values obtained by subjecting paper sheets of an thermal accelerated ageing according to ISO 5630-1 at different pH ...... 50 Figure 31 Visual differences between acidic conditions (pH 5) to the right (yellowish) and neutral/alkaline conditions (pH 7) to the left (whitish) (Nygren, 2019) ...... 51 Figure 32 Changes in the b*-value for paper-sheets with different pH during an thermal accelerated ageing according to ISO 5630-1 ...... 51 Figure 33 ∆b*-values and pH during points of sample for different tambours ...... 52 Figure 34 The effect on b*-value at different temperatures ...... 53 Figure 35 Decreased b*-value and pH due to storage time ...... 54 Figure 36 Relative humidity versus b*-value ...... 55 Figure 37 Heating and cooling the laboratory sheets ...... 56 Figure 38 Cooling and heating the laboratory sheets ...... 56 Figure 39 Cooling and heating the archived papers ...... 57

List of Tables Table 1 Components in softwood and hardwood (Sixta, 2006) ...... 15 Table 2 Pulp characteristics depending on which process being used (Brännvall & Annergren, 2009) 17 Table 3 Bleaching stage with corresponding symbol (Germgård, 2009) ...... 21 Table 4 Optical properties (Bristow , 2009) ...... 35 Table 5 Brief description of optical properties (Hubbe, Pawlak, & Koukoulas, 2008) (Dürholz & Tollander , 2004) ...... 41 Table 6 Frequently used standardized ageing methods for paper (Forsskåhl, 2000; Zervos, 2010) ..... 42 Table 7 Factors with corresponding levels ...... 44 Table 8 Design matrix for a 32 factorial experiment ...... 45 Table 9 Design matrix with obtained results ...... 49 Table 10 ANOVA-results ...... 50 Table 11 Testing of paper properties ...... 58

1 Introduction 1.1 Background The paper industry has been an important factor for the improvement of human development throughout the history and has since being invented in China around 100 BC, been used mainly for literature and educational reasoning (S.AbuBakr, 2004). The paper production has increased significantly over the last decades as seen in Figure 1, mainly due to the economic growth of our developing nations hence resulting in established uses around the world. Figure 2 shows how the production of pulp and paper is distributed worldwide where Asia has become the largest producer of paper as of 2017 (Skogsindustrierna, n.d.).

Figure 1 Worldwide production of pulp and paper & usage of waste paper (Skogsindustrierna, n.d.)

Figure 2 Production of pulp & paper by region during 2017 (Skogsindustrierna, n.d.)

The increased usage of paper products is also a result of today’s environmental focus, which will probably increase even more since the European Parliament approved a new directive during 2019 which to ban single-use plastics, such as plastic bags by 2021 (European Parliament, 2019). Producers of e.g. packaging has therefore began to lean more towards paper over the past decade, especially since paper has many properties and advantages compared to other materials. Paper has thereby become a key factor for the reduction fossil emissions since wood is, from a sustainability point of view, a renewable, biodegradable, easy to recycle and a relatively low-cost raw material (S.AbuBakr, 2004). This does although, put a high demand on the manufactures since the requirements and properties can vary for different kinds of paper products. An example of a property that is important for most of the paper product being available is the brightness, which is achieved by bleaching the pulp. Bleached paper products are common among printing paper and tissue- paper etc. However, post discoloration, also known as the tendency to yellow (or yellowing), of paper is a well-known technical problem that is faced by manufacturers within the paper industry. This phenomenon is believed by researchers to be caused by a combination of chemically and physically factors (Brännvall, 2009a). Smurfit Kappa Piteå, located in northern Sweden, has during periods noticed instability of their bleached products due to yellowing. It has therefore been decided to examine if it is possible to reduce the yellowing since it is undesired from a customer point of view. This thesis/project will focus on finding causes within the paper-making process at Smurfit Kappa Piteå that results in an increased yellowing and thereby to propose measures to reduce the variation.

1.2 Smurfit Kappa The company was founded during the early 1930th in Ireland and were in 1938 acquired by Jefferson Smurfit where the business quickly grew into one of Ireland’s leading manufacturers of cardboard boxes and packaging boxes. In 2005 Jefferson Smurfit merged with Kappa Packaging to form Smurfit Kappa. As of today, Smurfit Kappa is one of the world-leading producers of paper-based packaging and has over 46,000 employees in 350 production sites across 35 countries (Smurfit Kappa , 2019a). Smurfit Kappa Piteå is a part of the Smurfit Kappa group whereas the unit in Piteå is the largest producer of Kraftliner in northern Europe, with an annual production of 700 000 tonnes. The number of employees is around 500 as of today, which makes Smurfit Kappa Piteå the largest private employer in Piteå (Smurfit Kappa, 2019b).

1.2.1 Kraftliner Kraftliner, as seen in Figure 3, is a paper grade that is primarily used as the outer surface layer of corrugated board which is usually utilized for packaging applications. The production line at Smurfit Kappa in Piteå consists of two paper machines that produces different qualities of Kraftliner. Paper machine 1 (PM1) produces brown Kraftliner with basis weights ranging from 100 to 300 g/m2. Paper machine 2 (PM2) produces paper with a white top layer that have a basis weight between 115 and 200 g/m2.

Figure 3 Kraftliner is the outer layer of corrugated board (A40 Packaging, 2018) 1.3 Contents description The theory regarding this thesis, based on the literature research is presented in chapter 3 where sections 3.1-3.3.6 provides a short description of wood as raw material and the overall pulping & papermaking process. The final sections of chapter 3 describe the causes of yellowing and some theory of the optical measurements. The materials and methods used are presented in chapter 4. The results of the experimental trials are presented in chapter 5. Chapter 6 contains general discussions and chapter 7 presents the conclusion of the research along with some ideas for future work.

2 About the project This master’s thesis project is the final assignment within the engineering program “Sustainable Process Engineering” at Luleå University of Technology. The project has been carried out at Smurfit Kappa Piteå from October 2019 through April 2020. 2.1 Objective The scope of this project is about finding causes within the pulp and papermaking process at Smurfit Kappa that results in an increased yellowing, and hence to propose measures in order to reduce the variation.

From previous studies performed at Smurfit Kappa it is known that paper made from totally chlorine-free (TCF) bleached pulps tend to lose brightness during storage when exposed to heat (Sandin, 2008). However, it is unknown which step in the process that causes the instability towards a thermal induced yellowing.

2.2 Limitations The project will be limited to Kraftliner produced at PM2 since the white top layer that is composed of TCF bleached birch pulp has been shown to be more unstable towards yellowing, compared to pulps bleached with an elementary chlorine free (ECF) process. Smurfit Kappa Piteå utilizes a TCF bleaching process, mainly due to environmental reasoning, so a change of the bleaching process is not a relevant solution as of today, especially since it would be rather expensive and would involve a lot of new authorizations. The project will therefore be limited to the yellowing of peroxide bleached birch pulp. 2.3 Key Questions

• Can the brightness reversion be reduced by decreasing the thermal exposure? • Are there other factors (chemical or physical) present during pulping/papermaking that contributes to a thermal induced yellowing?

3 Theory This section provides theoretical information in order to facilitate the interpretation of the results which is later presented in section 5. This section starts with a short description of the wood morphology followed by a brief overview of the pulping and papermaking process. Next, is the causes of yellowing explained, and the last subsection explicates the optical properties and evaluation of paper. 3.1 Wood morphology Paper is made from pulped fibres (cells) which can be acquired from either wood, plants or other renewable sources. Wood as a raw material, which is normally used to produce paper within the papermaking industry, is composed of a network of fibres. Wood is normally divided into two groups; hardwood and softwood. Hardwood can be found in deciduous trees such as birch and eucalyptus, and softwoods is from coniferous trees such as spruce and pine. The fibres are composed by cellulose, hemicellulose, lignin and some extractives whereas the distribution looks different depending on the wooden species. A general distribution can be seen in Table 1 (Raimo, 2000).

Table 1 Components in softwood and hardwood (Sixta, 2006)

Component Softwoods [%] Hardwoods [%] Cellulose 40-44 40-44 Hemicelluloses 30-32 15-35 Lignin 25-32 18-25

The wooden fibre consists out of several cell wall layers of different composition and thickness as illustrated in Figure 4. Almost all wooden fibres have a tubular structure whereas the hollow void of the fibre is called lumen, which main purpose is to transport liquids and nutrients throughout the cell. The outer layer, called middle lamella, is mainly containing lignin which bonds the fibres together. The primary wall is the first layer that is developed during the growth of a cell and the secondary wall consists of three different layers S1, S2 and S3 of different thicknesses which contains the largest amount of cellulose (Daniel , 2009).

Figure 4 Schematic illustration of the cell wall layers showing microfibril orientation (Solala, 2015)

3.1.1 Cellulose Cellulose is the most common polysaccharide found in the cell wall of plants. These polysaccharides are composed out of glucose units mainly which are organized into chains. The chains are further bond together by hydrogen bonds thus making the structure very strong. Cellulose is utilized industrially during papermaking and for making clothes (Henriksson & Lennholm , 2009).

3.1.2 Hemicellulose Hemicellulose can be found within most plant cell walls and is composed out of several different polysaccharides, which differs compared to cellulose that mainly contains glucose units. The composition of the polysaccharides in hemicelluloses varies depending on the wooden species (hardwood or softwood). Xylose, glucose, mannose, arabinose and galactose units are the most common sugar monomers that can be found in hemicelluloses whereas the xylose-structures has been found to degrade into specific acid groups that can promote optical defects post bleaching (Teleman , 2009).

3.1.3 Lignin Lignin is a hydrophobic polymer that give stiffness to the fibres, act as a glue to keep the cell wall layers together, assists in the transport of nutrients and protects against microorganisms. Despite the many important advantages of lignin, it is undesirable during papermaking since the goal of the pulping process is to liberate the fibres (Henriksson , 2009).

3.2 The pulping process The main intention of the pulping process is to liberate cellulose from the wooden framework, this is achieved by removing the lignin polymer. The removal of lignin, also known as delignification, is essential in order to liberate the cellulose since lignin behaves like a glue that bonds the fibres together. The delignification can be performed either by a mechanical or chemical unit process. Table 2 displays how the pulp characteristics can vary depending on which process being used (Brännvall, 2009a). The mechanical pulping process does often rely on a lot of electric power in order to operate the process but results in a high product yield. Chemical pulping could be described as the opposite since it generally has a lower product yield where approximately half of the material that is used becomes pulp and the rest is dissolved within the chemicals being used. However, the chemicals and energy within the dissolved organic material can be recovered, hence making the process economically feasible where a small amount of external energy must be used (Brännvall, 2009a). The pulping methods also differs in terms of delignification whereas the mechanical pulping process has most of the lignin remaining post processing and the chemical pulping process has almost no remaining lignin. The chemical pulping process is, however, unable to remove all lignin during processing, otherwise would damage be caused to the carbohydrates which is why the delignification is terminated with some residual-lignin remaining in the pulp. The amount of residual-lignin in the pulp can be estimated by determining the so called Kappa number, which is the measure of consumed permanganate that is added the pulp, hence giving an estimation of the amount of chemicals required during processing (Brännvall, 2009a).

Table 2 Pulp characteristics depending on which process being used (Brännvall & Annergren, 2009)

Pulp characteristics Chemical pulp Mechanical pulp Fibre characteristics Long, strong, collapsed, flexible Short, weak, uncollapsed, stiff Fines content 1-5% 20-30% Lignin content 0-3% 20-28% Porosity Low High Pulp strength High Low Pulp yield 45-55% 90-95% Sheet density High Low Yellowing sensitivity Medium High

3.3 Brief overview of Kraft pulping Smurfit Kappa Piteå utilizes the Kraft pulping process which is a chemical pulping method with the means of removing lignin from wood by chemically altering it with sodium hydroxide (NaOH), and sodium sulfide (Na2S) to produce soluble fragments of the lignin polymer. The Kraft process was invented around the late 19th century in Germany and was named Kraft (German and Swedish for strength) due to the strength of the paper being produced which is a major reason for its predominance today (Brännvall, 2009a). The Kraft process as displayed in Figure 5 includes the process steps Impregnation, Cooking, Washing, Bleaching, Papermaking and Recovery of chemicals.

Figure 5 Overview of the Kraft process (Andersson , 2014)

The Kraft process has some additional advantages compared to other chemical pulping processes such as an efficient recovery of chemicals along with the production of heat and valuable materials from by-products such as tall oil and turpentine. There are however some disadvantages to Kraft pulping as well such as the odor being generated and the dark color of the finalized pulp which often requires a bleaching step depending on the final product (Biermann, 1996).

3.3.1 Impregnation The first unit process of Kraft pulping is to debark and cut the wooden raw material into smaller fragments. This operation is performed because bark and twigs usually contain non-fibrous substances such as dirt and thus cannot be used for pulping since they would contaminate the pulp. However, those can instead be used for fuel in the mill's bio-boilers. The smaller fragments of the raw material are now referred as wooden chips. The chips are then preheated with steam, this operation causes air in the lumen to expand and is thereby removed. If air were to remain within the lumen would liquor be prevented from penetrating into the chips. The final step is to saturate the chips with the active chemicals NaOH and Na2S, those are also known industrially as white liquor due to its transparent appearance. The white liquor penetrates into the chips and chemical reactions with the wooden components can thus begin. The impregnation is a requirement in order to achieve a homogeneous pulp and low amount of rejects (Brännvall, 2009b).

3.3.2 Cooking The white liquor and the wooden chips are charged into a pressurized vessel called digester. The digester normally operates at around 150 °C, this unit operation can be performed either in batch or by a continuous process. However, the delignification procedure is highly temperature dependent which directly affects the reaction rate, thus making it difficult to distinguish the delignification by time if the temperature varies a lot. Hence can the amount of time required for the cook vary depending on the degree of delignification that is being desired (Brännvall, 2009b). During the so-called cooking procedure are the hydroxide and hydrosulfide anions reacting with lignin which causes the polymer to degrade into soluble fragments, thereby separating cellulose and hemicellulose from the lignin structures. The main delignifying agent of the white liquor is Na2S whereas NaOH keeps the lignin fragments in solution (Brännvall, 2009a). The active chemicals dissolves during the delignification process according to reactions 1-4 (Gullichsen, 2000):

+ + + (1) + − 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 + 𝐻𝐻2𝑂𝑂 ↔2𝑁𝑁𝑁𝑁 + 𝑂𝑂𝑂𝑂 + 𝐻𝐻2𝑂𝑂 (2) + 2− 𝑁𝑁𝑁𝑁2𝑆𝑆 +𝐻𝐻2𝑂𝑂 ↔ 𝑁𝑁𝑁𝑁 +𝑆𝑆 𝐻𝐻2𝑂𝑂 (3) 2− − − 𝑆𝑆 +𝐻𝐻2𝑂𝑂 ↔ 𝐻𝐻𝐻𝐻 + 𝑂𝑂𝑂𝑂 (4) − − The solid pulp is at this point known𝐻𝐻𝐻𝐻 as brown𝐻𝐻2𝑂𝑂 ↔ stock𝐻𝐻2𝑆𝑆 because𝑂𝑂𝑂𝑂 of its color and consists of about 50% by weight of the dry wood.

3.3.3 Washing The pulp from the cooking process is transferred to a collection tank, this procedure is known as blowing and releases steam and volatiles from the pulp. However, there is still a lot of used chemicals absorbed by the pulp and therefore a washing stage is introduced with the main purpose to get a clean pulp whereas the used chemicals are separated from the fibres. A pulping mill usually has several washing stages in series where the processes displacement, thickening/dilution and diffusion are involved (Miliander, 2009). Listed below are some types of washing equipment that is used:

• Atmospheric diffusers • Drum displacers • Pressure diffusers • Vacuum drum washers • Wash presses 3.3.4 Recovery process The purpose of the recovery process is both to recycle the used chemicals and also to extract the energy from the organic compounds within the excess chemicals that remains from the washing stages. These excess chemicals are industrially being referred as black liquor, mainly due to its color, and contains organic components, inorganic compounds and water (Theliander, 2009a). The chemical recovery process is illustrated according to Figure 6 down below and displays theses five steps:

• Black liquor burning • Black liquor evaporation • Condensate treatment • Lime mud reburning • White liquor preparation The black liquor is first being evaporated where mainly water is separated, this is done in order to increase the dry organic material which is later separated from the water and thereafter burnt in a furnace. The black liquor is by this state concentrated and is combusted in a so-called recovery boiler where heat is extracted from the gases being formed. A molten mixture now being referred as green liquor exits the recovery boiler, this mixture is composed of sodium carbonate (Na2CO3) and sodium sulfide dissolved in water (Theliander, 2009a). The green liquor is later processed in the so-called “white liquor preparation plant” where calcium oxide (CaO) is added to the green liquor, thus forming so-called slaked lime (Ca(OH)2). Sodium carbonate is then causticized with calcium hydroxide in order to form sodium hydroxide and solid lime mud (CaCO3) according to reaction (5), thereby is white liquor recycled and obtained. The lime mud is then separated from the white liquor by either a filtration or thickening step and is now ready to be used for the cooking of wood chips again (Theliander, 2009b).

+ ( ) 2 + (5)

𝑁𝑁𝑁𝑁2𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶 𝑂𝑂𝑂𝑂 2 ←→ 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶3

Calcium carbonate which remains in the lime mud is calcined in the “lime mud reburning plant” as seen in reaction (6) and thus is so called reburned lime (CaO) formed (Theliander, 2009b).

+ (6)

The reburned lime reacts with water𝐶𝐶𝐶𝐶𝐶𝐶 𝐶𝐶according3 → 𝐶𝐶𝐶𝐶𝐶𝐶 to reaction𝐶𝐶𝐶𝐶2 (7) and calcium hydroxide is regenerated which is recycled to the white liquor preparation plant (Theliander, 2009b).

+ ( ) (7)

𝐶𝐶𝐶𝐶𝐶𝐶 𝐻𝐻2𝑂𝑂 → 𝐶𝐶𝐶𝐶 𝑂𝑂𝑂𝑂 2

Figure 6 Stages during the recovery of chemicals within the Kraft pulping process (Brännvall, 2009a)

3.3.5 Bleaching Bleaching within the chemical pulping process is used to remove the residual lignin which is still present in the pulp post cooking since the Kraft process doesn’t guarantee a complete delignification without damaging the fibres, due to reasons of pulp yield and the viscosity. Mechanical pulps, however, still contains a high grade of lignin structures and is therefore bleached with means to remain lignin within the pulp and only to brighten the fibres. As mentioned in previous sections, pulps post cooking has a brownish color, which is a result of various substances in the pulp, mainly found in the residual lignin. The brown color of the pulp is another reason for the usage of bleaching as some paper products requires a bright pulp. Bleaching also results in a higher cleanliness, since impurities such as shives and dirt are removed from the pulp during the bleaching step, otherwise could these impurities end up on the finished paper. The bleaching process is usually performed in several stages by using different chemicals during each stage, the most common bleaching agents as of today is listed in Table 3 (Germgård, 2009).

Table 3 Bleaching stages (Germgård, 2009)

Bleaching chemicals Acid treatment Chlorine Chlorine dioxide Alkaline extraction Hypochlorite Oxygen Hydrogen peroxide Chelating stage Peracetic acid Water treatment Enzyme stage Dithionite Ozone

That pulp fibres could be bleached has been known since the 18th century where chlorine (Cl2) and hypochlorite (OCl) was originally used as bleaching agents. Chlorine dioxide (ClO2) was however, during the 1920s, found to be a more effective bleaching agent where less damage on the cellulose was noticed which is therefore ClO2 as of today is one of the dominating bleaching agents used in pulp mills globally. Bleaching that includes chlorine dioxide is usually denoted as ECF and the bleaching where chlorine compounds is absent is instead called TCF. The TCF term was established around the 1990s due to the increased environmental awareness, whereas the usage of chlorinated organic compounds during paper production was a major issue. Modern TCF bleaching stages utilizes either hydrogen peroxide (H2O2), oxygen (O2) delignification or ozone (O3) as bleaching agents (Germgård, 2009).

3.3.6 Papermaking process The fibres from the pulping process is at this point liberated from the lignin polymer and is sent to the paper machine where a slurry of fibres is sprayed onto a wire that is operating at constant motion. In order to obtain certain properties of the paper some additives are added to the slurry, these additives can be acids and bases to control the pH, dyes/pigments to get a desired color, starch and resins to improve dry/wet strength etc. The paper enters a so-called pressing section where water is removed by squeezing/pressing the paper-web in-between steel rolls. However, according to Figure 7, the paper still contains water and in order to increase the dryness of the paper is it further dried at the so-called drying section. Finally, at the end of the paper machine, the web is reeled into large paper-rolls and the papermaking is completed (Brännvall, 2009a).

Figure 7 A simplified illustration of the paper machine (Brännvall, 2009a)

3.4 Factors that can affect brightness Paper produced from both bleached and unbleached pulps undergoes a brightness reversion during ageing whereas they change in color. This post-discoloration where bleached paper turns yellowish, also known as yellowing, has been a matter of research for decades but the chemical pathways are not yet fully understood, thus making it a complex problem for manufacturers of pulp and paper products. Yellowing is in other words the decrease of brightness, which is normally occurring post bleaching, for example upon storage of the pulp or paper. According to the literature yellowing is a result of a combination of several factors, thus making it difficult to pinpoint which combination of factors that contributes the most to the decrease of brightness within the process. It has however been demonstrated that yellowing is, depending on the pulping method, vastly accelerated by the presence of heat and ultraviolet light (UV). Yellowing does, as seen in Figure 8, depend upon a complex combination of chemically and physically factors, some of these will be described briefly in the following subsections (Forsskåhl, 2000).

Manufacuting Raw material & pulp Storage of product

•Age •Additives •Light •Composition •Bleaching method •Heat •Chromophores •pH •Humidity •Extractives •Pulping method •Oxygen •Fibre species •Pollutants •Growth •Time •Hexenuronic acids •Lignin •Transition metals

Figure 8 Factors affecting yellowing within the papermaking process (Forsskåhl, 2000)

The chemical and physical factors each have different ageing mechanisms which causes the degree of polymerization of cellulose to be reduced, those can be divided into the following categories (Fellers et al., 1989):

1) Chemical degradation mechanisms: • Cross-linking - links are formed between the cellulose chains which has an embrittling effect • Hydrolysis - breakdown of a compound due to reaction with water • Mechano chemical breakdown - degradation trough chain-cleavage and oxidation • Microbiological breakdown • Oxidation – formation of carboxylic groups

2) Physical ageing mechanisms: • Crystallization - changes in the state of order of the cellulose polymer

3.4.1 Chemical factors 3.4.1.1 Lignin Despite the many important advantages of lignin, such as keeping the fibres together, assisting in the transport of nutrients, and protecting the wood from microbial degradation, lignin is also the cause of why pulp fibres have a brownish color (Henriksson , 2009). Most wooden species have a natural brownish color whereas the major contribution for the color of the fibres comes from the specific lignin structures called chromophores, which is the main contributor to yellowing regarding mechanical pulps (Gellerstedt, 2009). The color formation and the binding of fibres is some of the reasons why lignin is undesirable during papermaking and is therefore removed either by pulping or bleaching processes. The residual lignin is determined by the Kappa numbers which measures the consumption of added permanganate, however lignin is not the only compound that can consume permanganate.

3.4.1.2 Chromophores A chromophore is defined according to the International Union of Pure and Applied Chemistry (IUPAC) as “the part of a molecule where the electronic transition responsible for a given spectral band is localized”. Thus meaning, in other words, that a chromophore is the region within a molecule where the energy difference between two molecular orbitals can be absorbed by exciting an electron within certain wavelengths of the visible spectrum ( International Union of Pure and Applied Chemistry, 1997). Chromophores can thus, through absorption of light cause an increase of energy which can result in a so-called photolytic reaction. During a photolytic reaction, molecules are cleaved which can lead to the formation of new chromophoric groups, hence resulting in a chain reaction. If more chromophores are being formed, it would result in the ability to absorb more energy through light and the affected material is thereby perceived as either discolored or bleached. This increase of energy required for the reactions to occur could theoretically also be introduced by heat, thus can yellowing by chromophores either be light-induced or thermal- induced (Forsskåhl, 2000).

3.4.1.3 Extractives Wood extractives is defined as compounds that are soluble either in organic solvents or water and is therefore “extractable”. Some extractives have an important role regarding the metabolism of the wooden cells while other extractives protect the wooden material against microbial degradation. Extractives can be of either lipophilic or hydrophilic type, where the lipophilic extractives is referred as wood resins. Wood resins usually consists of fatty acids, esters, terpenoids or waxes, which can give rise to certain defects that could affect the pulp and paper properties negatively. Some of these defects due to extractives are:

• Visible defects - spots or holes in the produced paper • Surface energy defects – decreases the bonding between the fibres thereby affects the paper strength • Odor of the pulp • Poor optical properties However, during pulping the main part of the wood extractives are dissolved in the black liquor and either burnt for energy or separated to be further used as a source for production of bi- products such as tall oil (Nilvebrant & Björklund Jansson, 2009).

3.4.1.4 Transition metals The presence of transition metal ions, in particular the cations Mn2+ and Fe2+ (also Fe3+ and Cu2+) are considered to promote yellowing since they are involved in catalytic degradation reactions in the presence of hydrogen peroxide as seen in Figure 9, hence reducing the major bleaching agent used during Kraft pulping and thereby decreasing the bleachability efficiency (Backlund , 2009).

It is assumed that during the pulping process the transition metals can be absorbed from either process water or the process equipment. Although the transition metals can be removed trough addition of chelating agents like DTPA or EDTA to the pulp, thus being able to avoid the breakdown of hydrogen peroxide due to chelation of the transition metals (Forsskåhl, 2000). However, there are also some transition metal ions which are beneficial for the pulping process as these tend to reduce the influence of the harmful ions; magnesium (Mg) and calcium (Ca) ions have been shown to have this influence (Germgård, 2009).

Figure 9 Catalytic decomposition of hydrogen peroxide by the Fenton cycle (Backlund , 2009)

3.4.1.5 Hexenuronic acids (HexA) Softwood and hardwood both contain xylan which is a group of hemicelluloses usually present in wood and plants. During strong alkali conditions, such as at the pulping process, methanol can be eliminated from xylan substituted with a 4-O-methyl-glucuronic acid group which results in 4-deoxy-hex-4-enuronic acid group formation (Gellerstedt , 2009). The reaction is presented in Figure 10. The acid being formed is commonly known as hexenuronic acid (HexA) which is rather stable in alkali environments, and has also been shown to consume permanganate which makes the Kappa number correlation to lignin dubious, thus being considered to be a major contributing factor for the brightness reversion (Sixta et al., 2006).

Figure 10 Elimination of methanol from xylan during pulping (Gellerstedt , 2009).

However, hexenuronic acid has been shown to be rapidly hydrolyzed at high temperatures and extremely low pH-values. The inclusion of a hot acid hydrolytic stage could thereby through a selective hydrolytic degradation of the hexenuronic acid group’s increase the brightness stability. The products which are formed during the degradation of hexenuronic acid, mainly formic acid, furancarboxylic acid and 5-formyl furancarboxylic acid have been shown to be soluble in water and can therefore easily be removed (Suess, 2010). This is why pulps that contain less residual-lignin, such as TCF pulps, have been shown to contain a larger amount of hexenuronic acids due to the lack of acidic steps during the chemical pulping process, hence leaving the hexenuronic acids unaffected could result in a possible reversion of brightness (Granström et al.,2001; Sevastyanova et al., 2006).

It has been illustrated that the b*-value can be decreased by the elimination of the hexenuronic acid groups through a hot acid hydrolytic stage. The degradation of hexenuronic acid seems to be largely dependent upon the time of treatment accordingly to Figure 11 (Sandin, 2008).

Hot acid hydrolysis 16 15 14 13 12 11 10 9

b* 8 7 6 5 4 3 2 1 0 0 24 48 72 Time [h] 80°C 65 % RH

Figure 11 Increased brightness stability trough degradation of the hexenuronic acid group’s (Sandin, 2008)

3.4.1.6 pH The pH scale is logarithmic and ranges in-between 0-14 and is used to indicate the concentration of hydrogen ions in a solution, namely, to specify the acidity or alkalinity of the solution. A pH value of 7 has been defined as neutral, thus meaning that a pH lower than 7 indicates acidic conditions and a pH higher than 7 is therefore alkaline (Lawn & Prichard, 2003). The pH value is crucial both during the pulping and papermaking process since a change in pH will greatly inﬂuence the behavior of the paper properties as illustrated by Figure 12 (Forsskåhl, 2000). Several studies have shown that acidity tends to accelerate the degradation of cellulose through acid-catalyzed hydrolysis (Lundeen, 1983).

Figure 12 Factors affecting paper permanence (VERSO, 2016)

There are several factors within the papermaking process which can influence the pH of the paper but the primary source of acidification is the salt aluminum sulfate (Al2(SO4)3) or industrially known as alum, which is commonly used as an sizing agent (Biermann, 1996).

Furthermore, the pH value will gradually decrease during aging of the paper due to carboxyl groups (COOH) being formed through oxidation processes, hence enhancing the acidic hydrolysis effects (Enberg et al., 2013). Depending on both the hydrolytic and oxidative processes, the acidity of a paper product can increase even during natural ageing, e.g. during storage (Fellers et al., 1989).

Defects such as brittle and discolored paper products have been shown to correlate with lower pH-values upon ageing, while neutral or alkaline paper products generally have a much better permanence. It has also been shown that paper products which been deacidified tends to degrade slower compared to before the deacidification process (Zervos, 2010). It is therefore recommended that the paper product has some sort of buffer capacity in order to resist the ageing due to acidic hydrolysis (Fellers et al., 1989).

3.4.1.7 Additives Additives is a group of chemicals used to attain certain desirable properties of the finished paper. Some additives such as sizing agents, dyes and fillers are referred as functional additives and is commonly used to improve the qualities of the paper product and must therefore be retained on the sheet in order to achieve the desired effect. Another group of additives is referred to as control additives, which includes biocides, retention aids, pH control agents and defoamers. Control additives is added during the process of manufacturing to improve the overall process without affecting the physical properties of the paper (Biermann, 1996).

Previous studies at Smurfit Kappa Piteå has shown that addition of certain additives has a positive effect regarding the brightness reversion, hence showing the importance of adding them during processing (Sandin, 2008).

3.4.1.7.1 Fillers Fillers are pigments which is used for brightness improvements. Some common fillers are calcium carbonate (CaCO3), titanium dioxide (TiO2) and clay (Biermann, 1996).

3.4.1.7.2 Dyes & brighteners Dyes are colors that are soluble in water. They are absorbed onto the fibre surfaces and thereby conveying their color to the fibres. Blue dye is commonly added to the fibres in order to offset the tendency for the paper to undergo yellowing. However, the blue dye doesn’t make the fibres brighter as it only masks the yellowness of the paper, hence giving the perception of a brighter paper (Biermann, 1996). There are four types of dyes used within the papermaking industry, namely:

• Basic dyes - cationic organic dyes which contain amine groups, and has to be utilized alongside with inorganic anions in order to fixate them onto the fibre surface • Acid dyes - anionic organic dyes that contain sulfonate groups and are used together with organic cations to attach them on the surface of the fibres • Direct dyes - have a direct affinity for cellulose • Fluorescent brightening agents - also known as optical brightener agents (OBA), are used to further brighten paper via conversion of UV light to visible light

3.4.1.7.3 Internal & surface sizing Sizing, also known as “gluing” of paper is the procedure of adding chemical additives to the paper with the goal of achieving hydrophobic properties. Internal sizing and surface sizing are the two sizing methods being used during the papermaking process whereas internal sizing is referred to as the addition of chemicals that provides the paper with hydrophobic properties, thus enhancing the resistance of penetration of liquids throughout the sheet. Surface sizing utilizes an addition of starches and polymer dispersions where the capillaries of the paper is clogged, hence, to improve the surface strength of the paper. The finished paper product can be divided into three groups referring to its sizing:

• Hard-sized - high resistance to liquid penetration, such as packaging papers • Slack-sized - low resistance to liquid penetration, such as newsprint • No-sized – very low resistance to liquid penetration, such as toilet paper

There are two common methods used during internal sizing; rosin sizing and alkaline sizing (Biermann, 1996; Lindström T. , 2009).

3.4.1.7.3.1 Rosin sizing Rosin is a material composed of various resin acids which is usually found naturally in softwoods. The sizing method with rosin was developed during the early 1800s and is as of today the most common sizing method being used as seen in Figure 13, mainly because of the relatively inexpensive price of the raw material due to its natural occurrence (Biermann, 1996). However, there are also certain drawbacks with rosin sizing due to the acidic conditions during the process which can cause the infamous yellowing, corrosion to the paper machine and embrittlement of the paper (Lindström T. , 2009).

Figure 13 Sizing agents used worldwide (Auhorn, 2006)

Rosin is anionic by nature and has therefore no affinity for the cellulose fibres, hence requiring a second component with a strong positive charge in order to anchor the rosin molecules onto the fibre surface. The most common choice of such an ionic compound is the salt aluminum sulfate, known industrially as alum. It has been shown that fibres with a low amount of carboxylic groups are more difficult to size with alum and rosin, mainly because the carboxylic acid groups facilitates the anchoring of rosin onto the fibre surfaces through the aluminum salts as illustrated in Figure 14 (Lindström T. , 2009).

Figure 14 Anchoring of a rosin molecule onto a fiber surface (Hubbe M. A., 2004)

Therefore, most of the sizing agents being available as of today, so called fortified rosins, have additional carboxylic groups added through reactions with either maleic anhydride or fumaric acid, hence in order to increase the sizing efficiency. Figure 15 illustrates the reaction of abietic acid with maleic anhydride where two additional carboxylic groups have been added to the molecule. The addition of carboxylic groups and the acidic nature of alum gives this sizing method acidic properties, hence being optimum within the pH range between 4–5 (Lindström T. , 2009).

Figure 15 The fortification of abietic acid with maleic anhydride to produce fortified rosin (Hubbe M. A., 2004)

However, rosin is not soluble in water and can therefore not be applied directly to the fibre surface. There are, however, different ways to implement the sizing with rosin, for those are the following terms used (Auhorn, 2006):

• Dispersed size: particles which are stabilized by either starch, polymers, or proteins • Dry size: 100% neutralized rosin that dissolves easily in water • Extended size: a 50:50 mixture of neutralized resin acids and urea • Fortified size: contains additional carboxylic groups • Free rosin: resin acids whose carboxyl groups are in protonated form. • Paste size: 80% neutralized rosin • Rosin soap size: formed by saponifying resin acids with a sodium base

3.4.1.7.3.2 Aluminum sulfate (Alum) Aluminum sulfate (Al2(SO4)3), also known industrially as alum, is a chemical compound which is used during sizing with rosin. Due to the positive charge of the aluminum ion, alongside with the small radius, provides alum with properties that are suitable for the anchoring of rosin onto the fibre surface. Aluminum tends to act as a weak acid in aqueous solutions and can thereby undergo hydrolysis according to the reaction formula below:

( ) ( ) ( ) + (8)

Thus, the pH is lowered due𝐴𝐴𝐴𝐴 to𝐻𝐻 the2𝑂𝑂 formation6 ↔ 𝐴𝐴𝐴𝐴 𝐻𝐻2 𝑂𝑂of 6hydrogen−𝑥𝑥 𝑂𝑂𝑂𝑂 𝑥𝑥 ions,𝑥𝑥𝑥𝑥 which restricts the usage of alum within the alkaline region due to acid conditions. However, as mentioned earlier does rosin sizing have a few drawbacks due to the acidic conditions needed in order to utilize the method. It would therefore, theoretically, be better to avoid these acidic conditions by sizing at neutral or alkaline pH-values to achieve an alkali excess able to suppress the yellowing reactions during ageing (Lindström T. , 2009). But sizing with alum and rosin at higher pH-values is unfortunately problematic since it could form Al(OH)4 instead of anchoring the aluminum resinate onto the fibres according to Figure 16. Furthermore, the rosin molecule could be deprotonated if utilized at higher pH-values, and would thereby resulting in the polar groups turning towards the water molecules (also known as overturning) hence causing a loss of the sizing efficiency. Due to these disadvantages, neutral or alkaline sizing can instead be utilized by using either of the sizing agent’s alkenyl succinic anhydride (ASA) or alkyl ketene dimer (AKD). These sizing agents are at the opposites against each other, whereas AKD is the least reactive out of the two, but yet rather stable against hydrolysis and ASA is, vice versa, very reactive towards cellulose but instead sensitive to hydrolysis (Lindström T. , 2009).

Figure 16 Aqueous solubility of Aluminum compounds (Lindström T. , 2009)

Utilizing a neutral-alkaline sizing method during the papermaking process provides certain advantages such as:

• The produced paper tends to age better since there are less residual acid present to cause degradation of the carbohydrates • Calcium carbonate can be used as filler • The produced paper will be stronger and less brittle • Less corrosion on the equipment

Using calcium carbonate (CaCO3) as a filler is one of the major advantages when utilizing neutral or alkaline papermaking conditions, especially since it is an inexpensive raw material, tends to buffer pH towards alkaline levels and has good optical properties that would result in higher brightness of the paper (Lundeen, 1983). Calcium carbonate is however soluble during acidic conditions which can cause CaSO4 (gypsum) precipitates and carbon dioxide (CO2) to be formed, and hence is it therefore not appropriate to use during sizing with rosin and alum at acidic conditions (Auhorn, 2006; Neimo , 1999; U.S. Congress, Office of Technology Assessment, 1988).

3.4.2 Physical factors Physical ageing of a material is defined as a process whereas molecules adjust their positions so a closer molecular structure is formed, thereby being characterized as a decrease in volume with time. It should therefore signify that physical ageing is reversible due to the fact that the molecular mobility can be both increased and decreased, hence being the opposite of chemical ageing, which causes a chemical change in the molecular structure thus being an irreversible process (Fellers et al., 1989). Physical factors that is known to influence the brightness reversion of pulps and papers are for example the exposure to UV-light (daylight or indoor illumination) and high temperatures, where generally speaking, paper from mechanical processes are more sensitive to light-induced yellowing and paper from the chemical process is more affected from thermal-induced yellowing (Ragnar, 2007). The decrease in brightness which occurs due to light-induced yellowing is limited to the surface of the material while heat-induced yellowing is causing chemical reactions to occur throughout the whole material (Forsskåhl, 2000). There are other factors as well, that have been shown to affect the brightness in combination with high temperatures such as the moisture content and some oxygen-compounds (Anders, 2006; Zou, 2002; Iversen & Kolar , 1991).

3.4.2.1 Temperature Temperature is the measure of kinetic energy which causes molecular motion. In other words, atoms move faster alongside with increasing temperature which allows collisions in-between reactants to occur more frequently, thus improving the chance of reactants forming products and hence resulting in the rate of a reaction increasing. It is known that a rise in about 10 degrees of Celsius normally doubles the reaction rate, this rule of thumb also seems somewhat valid for the thermal induced degradation of cellulose (Zervos, 2010). The rate of a reaction is therefore highly dependent upon temperature, as shown by Arrhenius equation below:

= (9) 𝐸𝐸𝑎𝑎 − 𝑅𝑅𝑅𝑅 𝑘𝑘 𝐴𝐴𝑒𝑒

Whereas R is the gas constant, Ea is the activation energy, T is temperature and A is the frequency factor. The temperature is therefore a critical factor regarding the degradation of cellulose, yet also a useful tool while conducting accelerating ageing experiments specified by various standardized methods (Zervos, 2010).

3.4.2.1.1 Temperature during storage time Paper (and pulp to some degree) seems to suffer from yellowing upon storage time which has been demonstrated by several studies. One of these reports was conducted during an internal study at Smurfit Kappa Piteå by Maria Lindström whom investigated the cooling process of the paper-reels. The study was conducted inside a storage magazine where the temperature ranged between 5 to 10 °C, which during these conditions showed that it would take approximately 20 days for the temperature of the roll to be equalized at around 10 °C as seen in Figure 17. The slow cooling process was explained by the initial temperature of the paper-reels from production which could reach temperatures close to 50 °C at the center of the roll, and due to the packaging of the paper-roll which isolated the heat at the center of it (Lindström M. , 1990). Frida Sandin repeated this experiment during a thesis-project in 2008 at Smurfit Kappa where similar results was obtained. However, Sandin showed that the slow cooling process had an impact on the brightness reversion and thus concluded that the paper-reels should be stored as cold as possible (Sandin, 2008).

Figure 17 Decrease of the temperature in paper-reels (Lindström M. , 1990)

Due to the slow cooling process caused by the high temperature during bailing, cooling of the paper sheet could be an alternative solution in order to reduce the temperature and hence reducing the brightness reversion as well. It is mentioned in a few sources that this kind of technology, explained by Figure 18, is applied within the papermaking industry in able to suppress the brightness reversion (Larsson & Karlsson, 2000).

Figure 18 Applied cooling after drying in able to lower the temperature, hence reducing the brightness reversion (Larsson & Karlsson, 2000)

It is therefore, also suggested throughout various reports that in order to slow down the degradation processes of cellulose, the temperature at the storage areas such as in archives and libraries (where paper usually is kept) should be maintained at temperatures under 20 °C (Zervos, 2010).

3.4.2.2 Relative humidity & moisture content The relative humidity (RH) is the amount of water molecules that the air can hold at a certain temperature. Figure 19 illustrates how the amount of water in the air changes depending on the relative humidity whereas the ability to retain moisture also increases/decreases alongside with the temperature (Stolow, 1987).

Figure 19 Amount of water in air at different RH across a temperature range (Lenntech, 2020) The moisture content of cellulose and thereby also of paper is, as shown in Figure 20, defined by the relative humidity whereas the water content that the paper can hold increases with an increased RH. Hence, the temperature of the environment is also an important parameter since it affects the RH (Zervos, 2010). The geographical location of a paper production facility and season of the year should thereby be the primary factors that could affect the RH (Stolow, 1987).

Figure 20 The amount of water that pure cellulose can bind increases with the relative humidity (Hubbe, o.a., 2017)

The effect of moisture regarding ageing of paper has been shown to increase the speed of the degradation rate of cellulose which is not surprising, especially since water is an essential reactant during acid hydrolysis (Zervos, 2010).

The moisture content also serves as a swelling/shrinking agent, thus changing the surface area of the fibres, allowing for either more or less ageing reactions taking place (Zervos, 2010). A low RH level (and thereby low moisture content) usually shrinks the fibres whereas higher RH levels tends to increase the fibre size due to water absorption (Stolow, 1987). Since the moisture content contributes to the rate of degradation of paper and allows changes in the molecular mobility, the evaluation of paper properties must be measured under standard conditions for both temperature and relative humidity. The standard testing conditions according to ISO 187:1990 is 23 °C and 50% RH, whereas if not conditioned properly it can result in misleading data (International Organization for Standardization, 2020a).

3.4.2.3 Light Light has an important role during the ageing process of paper products, especially for papers containing a larger amount of lignin and thus also more chromophores. The exposure of light initiates the photooxidation of cellulose, which reduces the degree of polymerization and produces free radicals on the cellulose backbone such as carbonyls and carboxyl’s. However, both cellulose and hemicelluloses are only able to absorb UV light, which restricts these photochemical reactions. Paper containing more residual-lignin does on the other hand strongly absorb light from the UV and the visible spectra, hence being more affected towards photochemical reactions (Zervos, 2010). The light induced degradation of cellulose can be divided into three mechanisms:

• Photolysis from UV light • Photochemical production of free radicals • Photosensitized degradation from visible light

3.5 Optical properties Optical properties are properties which are associated with the appearance of the paper product. Among those are brightness and color important definitions that has to be understood in order to measure and review the optical properties. Additional properties commonly used for optical measurements is listed in Table 4 below (Bristow , 2009). Some of these properties, namely the ability to absorb light, reflectance of light, scattering of light in different directions and to transmit light through a material is illustrated by Figure 21. The Kubelka-Munk theory links together the properties previously mentioned above and is used for describing the optical properties of paper (Brännvall & Annergren, 2009).

Figure 21 Illustration of how light interacts with a paper (Vaarasalo, 1999)

Table 4 Optical properties (Bristow , 2009)

Property Symbol Description Chromaticity x, y Coordinates derived from the X,Y,Z values which indicate the coordinates color content of a material CIELAB values L*, a*, b* Transformation of the X,Y,Z values to define a three- dimensional color space

Dominant wavelength λD, pE Values derived from the chromaticity coordinates which and spectral purity corresponds to the color strengths of the material

ISO brightness R457 Reflectance factor at 457 nm

Opacity W Value calculated as 100 times the ratio of R0 divided by R∞

Reflectivity R∞ Reflectance factor of a pad of material, so thick that an increase of the thickness wouldn’t affect the value

Reflectivity factor over R0 Reflectance factor of a single sheet over a non-reflecting area black

Tint Tw Value to determine if any color deviation is towards green or red

Tristimulus values X,Y,Z Reflectance factors in the red, blue and green regions which in combination with each other defines the color of a material

Whiteness W Transformation of the X,Y,Z values which indicate the whiteness of a material

Y-value, luminance Ry A measure of the lightness of a material at 557 nm

3.5.1 Brightness Brightness is defined according to ISO as a measure of the relative amount of light reﬂected within the visible spectrum at an effective wavelength of 457 nm and the term is commonly used within the papermaking industry in order to measure the effect of the bleaching process (Vaarasalo, 1999). Brightness is mathematically expressed as a function of the k/s quota which Kubelka and Munk defined during the early 1930th, where k is the absorption coefficient, s is the scattering coefficient, and R∞ the reflectance of the material. However this correlation has some flaws as it assumes that the material is homogeneous which doesn’t apply to the surface area of all paper products, and brightness is also unproportioned to the quantity of colored species in pulp, but the results are still considered to be accurate and therefore is the Kubelka- Munk theory often used in the field of paper optics (Brännvall & Annergren, 2009).

(1 ) = (10) 2 2 𝑘𝑘 − 𝑅𝑅∞ ∞ R∞ is proportional to k/s whereas a decreased𝑠𝑠 k𝑅𝑅 would result in an increased brightness thus signifying a decrease of the colored substances within the pulp. Brightness also decreases with a decreased s, as the light scattering surfaces are being reduced (Brännvall & Annergren, 2009). Pulps are in general, often bleached to a brightness of around 85% ISO units which is normally referred to as semibleached and pulps which brightness has exceeded 85% ISO units are referred as fully bleached (Germgård, 2009). To put this into aspect, a non-reflective surface has a brightness of 0% ISO units and a perfect diffuser has a brightness of 100% ISO units (Vaarasalo, 1999).

3.5.2 Y-value The luminous reflectance, also known as the Y-value, is the intrinsic reflectance factor measured at an effective wavelength of 557 nm, hence being an indicator for perceived lightness and must not be confused with whiteness or ISO Brightness, especially since the major difference from brightness is that the Y-value covers the entire visible spectra as seen in Figure 22 (Vaarasalo, 1999).

Figure 22 Difference between Y-value and ISO brightness (Vaarasalo, 1999)

3.5.3 Color Color depends on the reﬂectance and absorption properties of the object, the light illuminating the object and the sensitivity of the eye to different wavelengths of light (Biermann , 1996). Our ability to perceive colors reckon on the retina of the human eye which has three types of receptor cells that are speciﬁcally sensitive to red, green, and blue light, therefore is color measurements largely based upon the so-called additive mixing rule. According to the additive mixing rule can any color within the visible spectra be produced by combining the wavelengths corresponding to the color red (X), green (Y), and blue (Z) (Vaarasalo, 1999).

The sensitivity of the retina is however varied individually, thus can color appear different for each individual which can lead to inconsistencies when evaluating color measurements (Bristow , 2009). A numerical description that reflects the human perception of color based of the sensitivity for the receptors of the retina was therefore created in the early 1930th by scientists from the Commission International de l'Eclairage (CIE) in order to measure color differences. This numerical description, also known as the standard observer, was determined by subjecting individuals to different shades of color while looking through a hole. The hole only allowed a 2° field of view due to the hypothesis of where the color-sensing cones was located in the retina. The subjects were then assigned to match each color by combining various intensities of red, green, and blue lights. The responses from the experiment were used to calculate a mean value which was further used to develop different mathematical models for color description. The standard observer system was used to plot all the color nuances in a diagram that the human eye is expected to perceive, thus were a so-called color space obtained. CIE has developed several different types of color spaces which are commonly used within the papermaking industry, where two among these color spaces are the CIEXYZ and CIELAB (Vaarasalo, 1999).

3.5.3.1 CIE XYZ color space The CIE XYZ color space utilizes the Y-value, which is a measure of the luminance, and the chromaticity which is the normalized tristimulus values X, Y, and Z, denoted by the parameters x, y, and z according to the equations below:

= (11) + + 𝑋𝑋 𝑥𝑥 = 𝑋𝑋 𝑌𝑌 𝑍𝑍 (12) + + 𝑌𝑌 𝑦𝑦 = 𝑋𝑋 𝑌𝑌 𝑍𝑍 (13) + + 𝑍𝑍 𝑧𝑧 𝑋𝑋 𝑌𝑌 𝑍𝑍 According to multivariable calculus, the coordinates x, y, and z are equal to 1 which thereby makes only two coordinates necessary to describe the third, hence usually a xy coordinate system is used accordingly to CIE's recommendations (Biermann , 1996).

Figure 23 illustrates how the chromaticity coordinates is plotted in a xy coordinate system according to each visible wavelength listed in nanometers. The diagram specifies how the human eye will comprehend light within a given spectrum, however it cannot specify colors of objects since the chromaticity observed while looking at an object also depends on the light source (Vaarasalo, 1999).

Figure 23 The CIE 1931 color space chromaticity diagram (Wikipedia, the free encyclopedia, 2020)

3.5.3.2 CIELAB color space The CIELAB color space was created in order to obtain a relatively uniform, more manageable and easier to understand color space. The color space was derived from the CIE XYZ color space (Bristow , 2009). The L*, a* and b* values were calculated from a transformation of the XYZ values and is defined according to the formulas below:

= 116 1 16 (14) 3 ∗ 𝑌𝑌 𝐿𝐿 � � − 𝑌𝑌𝑛𝑛

= 500 1 1 (15) 3 3 ∗ 𝑋𝑋 𝑌𝑌 𝑎𝑎 �� − � �� 𝑋𝑋𝑛𝑛 𝑌𝑌𝑛𝑛

= 200 1 1 (16) 3 3 ∗ 𝑌𝑌 𝑍𝑍 𝑏𝑏 �� − � �� 𝑌𝑌𝑛𝑛 𝑍𝑍𝑛𝑛

Xn, Yn and Zn are the tristimulus values of a reflecting diffuser, whereas the illuminant and observer also are known (Vaarasalo, 1999)

Due to the usage of three parameters within the CIELAB color space is the space itself a three- dimensional space which can be seen in Figure 24 (Bristow , 2009). The L-axis corresponds from black at 0 and to perfect whiteness at a value of 100. The a-axis indicates a color towards green at a negative (−) value and red at a positive (+) value whereas the b-axis ranges from blue at a negative (−) value to yellow at a positive (+) value (Biermann , 1996).

Figure 24 The CIELAB color space (Konica Minolta Sensing Americas, Inc, 2020a)

3.5.4 Measurements of optical properties 3.5.4.1 Standard Illuminants Color and brightness measurements are heavily dependent on the light source as the color of an object may appear different since light emits different amounts of energy at each wavelength of the visible spectrum. An illuminant is in other words a representation of a light's spectral power distribution at a given wavelength as illustrated in Figure 25 (Vaarasalo, 1999). CIE has defined several illuminants to represent certain light sources, as listed below (Bristow , 2009).

• Standard Illuminant A - Incandescent light • Standard Illuminant C - Average daylight (excluding the region of UV wavelengths) • Standard Illuminant D65 - Average daylight (including the region of UV wavelengths)

Figure 25 Standard illuminants A, C & D65 (Konica Minolta Sensing Americas, Inc, 2020b)

3.5.4.2 Calculation of brightness reversion There are various ways how to calculate the brightness reversion but the two most common ways how yellowing is reported is either as loss of ISO-brightness units or as a recalculation as the post color number (PC). The simplest method to use is the decrease in brightness, ∆R∞. However, the decrease of brightness for an object with a high initial brightness level is unfortunately not comparable with the same decrease at a lower level of brightness, hence the amount of chromophoric groups must be considered. A larger number of chromophores have thus to be formed in order to cause the same decrease in brightness for a brighter pulp (Forsskåhl, 2000). Post color numbers is calculated according to the equation below:

= 100 (17) 𝑘𝑘 𝑘𝑘 𝑃𝑃𝑃𝑃 ∗ �� − � � � Where index 1 is referring to the ratio of the absorption𝑠𝑠 2 and𝑠𝑠 1 scattering coeﬃcients before being subjected to ageing, and index 2 is the same ratio after an accelerated aging test has been performed. Hence, the PC number makes it possible to compare objects with different initial brightness (Forsskåhl, 2000). One disadvantage of the PC number is that it is kind of difficult to understand what a certain PC number value actually means. Yellowing can therefore be expressed in terms of Δb* instead, hence giving a better understanding of the measure of the change in color and not just the brightness (Ragnar, 2007).

3.5.4.3 Measurement standards There are several standardized methods developed for the measurements of optical properties as seen in Table 5 below, but the results are however incomparable in-between the methods since the results are influenced for instance of the lamp being used and the wavelength used during the measurement. Furthermore, color perception is another factor that could influence the results of an optical measurement since the perception of color nuances are different in- between individuals. Hence, could a measured value be doubtful if it does not appear the same as the perceived value. It is therefore of great importance to indicate which method that has been used to perform the optical measurements (Forsskåhl, 2000).

Table 5 Brief description of optical properties (Hubbe, Pawlak, & Koukoulas, 2008) (Dürholz & Tollander , 2004)

Properties ISO Description TAPPI Description Measurement of diffuse Brightness of pulp, paper and reflectance factor 452 paperboard (directional reflectance at 457 nm) Brightness 2470 Diffuse brightness of paper and 571 paperboard (d/0°) 525 Diffuse brightness of pulp (d/0°) Determination of color Color of paper and paperboard (C/2°) – Diffuse 524 (45°/0°, C/2°) Color 5631 reflectance method Color of paper and paperboard 527 (d/0°, C/2°)

Measurement of specular Specular gloss of paper and 8254-1 gloss Part 1: 75° gloss 480 paperboard at 75° with a converging beam. Measurement of specular Specular gloss of paper and Gloss 8254-2 gloss Part 2: 75° gloss paperboard at 20° with a parallel beam 653 Measurement of specular 8254-3 gloss Part 3: 20° gloss with a converging beam

Kubelka- Paper-determination of Interrelation of reflectance; Munk 9416 light scattering and 1214 reflectivity; TAPPI opacity; Coefficients absorption coefficients scattering, s; and absorption, k Determination of opacity Opacity of paper (illuminant (paper backing) 425 A/2°, 89% reflectance backing and paper backing) Opacity 2471 Diffuse opacity of paper (d/0° 519 paper backing) Determination of CIE CIE Whiteness and tint of paper 11475 whiteness (d65/10°) 560 and paperboard (Using d/0°) Whiteness Determination of CIE CIE whiteness and tint of paper 11476 whiteness (C/2°) 562 and paperboard (Using 45°/0°)

3.5.4.4 Accelerated aging The processes in which paper degrade during natural ageing conditions, namely at room temperature, is rather slow (Lundeen, 1983). Hence, when performing measurements for brightness reversion, an accelerated ageing method of either heat treatment or by light exposure is usually used (Forsskåhl, 2000). It is suggested that an accelerated ageing procedure of paper at 100 °C for the duration of 72 hours corresponds to around 25 years of ageing at room temperature. A major advantage with an accelerated ageing process is therefore that the results are obtained during a reasonable period of time (Fellers et al., 1989). However, the problem with these ageing methods is that an accelerated treatment doesn’t simulate the natural cause of ageing in an exact way, especially since the extreme conditions used during these methods could cause reactions that would never occur at realistic conditions (Hubbe, o.a., 2017). There are several methods for accelerated aging of paper where ISO, TAPPI (Technical Association of the Pulp and Paper) and ASTM (American Society for Testing and Materials) each have their own method respectively with a wide variation of humidity level, storage time and temperature. Table 6 below list commonly used accelerated ageing methods.

Table 6 Frequently used standardized ageing methods for paper (Forsskåhl, 2000; Zervos, 2010)

Standard Temperature [°C] Relative humidity [%] Time [h] Non-thermal

TAPPI 260 100 100 1 453 100 0 (dry) >2 544 90 25 >2

ISO 5630-1 105 0 (dry) >2 5630-2 90 25 >2 5630-3 80 65 >2 5630-4 120 or 150 0 (dry) >2 ASTM D 6819-02 100 50 120 D 6789-02 <40 50 48 Light ageing D 6833-02 25 50 - Pollutant ageing

4 Method This section describes the experimental trials, the sheet forming procedure, optical measurements and the measurements of paper properties. 4.1 Accelerated ageing The accelerated ageing method which was used during most of the experiments was ISO 5630- 1, a heat treatment at 0% relative humidity (RH) and at 105 °C. The laboratory sheets were placed in a standard heat cabinet where the heat treatment was carried out for the duration of 1 hour before brightness and color evaluation. ISO 5630-3 was used during experiments with higher relative humidity whereas the laboratory sheets was placed in an enclosed climate chamber for the duration of 1 hour at 65% RH and 80 °C. 4.2 Brightness & color measurements The brightness and color measurements were performed post accelerated ageing treatment whereas a L&W Elrepho spectrophotometer was used. The Elrepho spectrophotometer measures both brightness and the color values L*, a* and b* according to the ISO-standards ISO 2470 & ISO 5631. The laboratory sheet made for brightness evaluation was folded twice according to Figure 26 and the optical properties was measured on the top and bottom of the folded sheet.

Figure 26 Folding the sheet for brightness evaluation

4.3 Preparation of laboratory sheets for brightness measurements Bleached birch pulp was collected from the sample-point 047A190 which is outgoing pulp from the bleaching process. The pulp has not been in contact with any process water at this point, thus should contain less amount of transition metals and hence being optimal for optical measurements of thermal behavior. Chemicals such as additives, that is normally present during sizing, were chosen not to be added since they mostly affects the final pH, and the pH is, according to the theory, said to be a major factor affecting the rate of yellowing. The brightness of the pulp at this point is approximately 85% ISO units alongside with a pH value of around 10 and the outgoing temperature was in the range of 60-65 °C.

The laboratory sheets were made accordingly to SCAN-CM 11:95, where the pulp was disintegrated at first in deionized water. 2.5 ml of ethylenediaminetetraacetic acid (EDTA) with a concentration of 5 g/l was added to the mixture to chelate potential transition metals. The mixture was pH adjusted with sulfuric acid that had a concentration of 0.5 mol/l and sodium hydroxide with a concentration of 1 mol/l to reach the desired pH; as observed by the usage of a pH meter. The sheets were formed in a vacuum-funnel and was later dewatered further by the usage of pressing plates which was subjected at pressure of 300 kPa for the duration of 1 minute. The procedure is shown in Figure 27.

Figure 27 Preparation of laboratory sheets from bleached pulp (Nygren, 2019)

4.4 Factorial design Based on the literature, a reduced factorial experiment was performed to find the most suitable conditions (pH and temperature) for the reduction of yellowing. According to statistics, a factorial experiment is defined as an experiment which involves a set of two or more factors, each with discrete values or "levels". These types of experiments allow the investigator to study the effect of each factor, as well as the effects of interactions between factors on the response variable. The number of experiments that is to be performed is given by the formula:

(18) 𝑘𝑘 where n equals to the number of levels and 𝑛𝑛k represents the number of factors. During this experiment the number of levels were set to 3 and the number of factors equal to 2, which resulted in 9 experiments total. The factors during a factorial experiment as seen in Table 7 usually vary between low setting, denoted (-1), medium setting with denotation (0) and high setting denoted with (+1).

Table 7 Factors with corresponding levels

Factor LOW (-1) MEDIUM (0) HIGH (+1) Unit Temperature (A) 20-25 80 105 [°C] pH (B) 4 5 7 -

The factor A (temperature), was chosen since TCF pulps is especially sensitive against thermal induced yellowing, the second factor pH (B) was chosen since temperature and pH in correlation with each other are said to intensify yellowing according to most publications and reports. Temperature and especially pH are technically also the factors which are simplest and most inexpensive to manipulate during the papermaking process. The temperatures were chosen accordingly:

• 20-25 °C – Temperature range during storage of paper products • 80 °C – Average temperature during the drying section and also used during accelerated ageing • 105 °C – Frequently used temperature during accelerated ageing according to ISO- 5630-1 The pH-values was chosen accordingly to the minimum and maximum ranges recorded at Smurfit Kappa, during sizing at the wire pit. Table 8 displays the design matrix for the 32 factorial experiment. The levels indicate how the experiments shall be performed at each run. For example, at the first experiment the factors A and B are both denoted with a low level, hence shall the experiment be performed with low temperature and low pH. During the actual experiment the order of performance was randomized to protect against the effect of the so-called lurking variables. A lurking variable is a variable that has important effect of the experiment but is not included in the experimental configuration and hence could affect the results. It is therefore also important to perform at least two replicates (repetition of the experiment) to obtain an estimation of the residual (error) which represents the common cause of variation within the experiments. The results from the factorial experiment were analyzed by the method analysis of variance (or ANOVA), which is commonly used to analyze the differences among group means in a sample. The obtained data from the factorial experiment was analyzed by using statistical tools in the computer software’s MINITAB and Excel. Pulps were pH-adjusted according to Table 7 and, the laboratory sheets were made as described in section 4.3 and treated to the accelerated ageing method ISO 5630-1 as seen in section 4.1.

Table 8 Design matrix for a 32 factorial experiment

Run order A B AB 1 -1 -1 1 2 -1 0 0 3 -1 1 -1 4 0 -1 0 5 0 0 0 6 0 1 0 7 1 -1 -1 8 1 0 0 9 1 1 1

4.5 pH variation Pulps were pH-adjusted in the range of 4-9 in order to investigate how the b*-value would be affected. Laboratory sheets were made as described in section 4.3 and treated to the accelerated ageing method ISO 5630-1 as seen in section 4.1. 4.6 Temperature variation An experiment regarding the temperature variation, based on an experiment conducted by M. Ragnar in the report “Challenges and opportunities in measuring and maintaining brightness of bleached eucalypt kraft pulp” (2007), was performed to demonstrate the impact which temperature could have on the brightness reversion (Ragnar, 2007). Paper-sheets were made as described in section 4.3 where the pulp was pH-adjusted to either acidic or neutral. The sheets were subjected to different temperatures for the duration of 1 hour at dry conditions, similar to the ISO 5630-1 method as seen in section 4.1. 4.7 Storage of pulp Storage of pulps, that withhold high temperatures, are common process stages within pulp and paper mills. Since high temperatures are present within these storage towers were a storage trial performed in order to investigate how the storage of pulp would affect the yellowing. Based on a previous internal report at Smurfit Kappa Piteå, the storage parameters such as temperature was adjusted to 65 °C while pH was left unchanged to best simulate the conditions at a storage tower. The pulp was placed in containers made out of stainless steel, as seen in Figure 28, and was diluted with deionized water to a concentration of approximately 5% of dry weight basis. The containers were placed in a water bath to maintain the desired temperature for a duration of 24 hours. The laboratory sheets were then made from the stored pulps as described in section 4.3 and were dried in room temperature (around 25 °C) before evaluation of brightness and color according to section 4.2.

Figure 28 Pulp in a stainless-steel container (Nygren, 2019)

4.8 Variation of relative humidity An attempt was made to investigate how the brightness reversion would be affected under accelerated ageing conditions accordingly to ISO 5630-3 compared to ISO 5630-1. The laboratory sheets were pH-adjusted to both 5 and 7 in order to also compare the difference between acidic and neutral conditions. The sheets were made as described in section 4.3 and treated according to the accelerated ageing methods as described in section 4.1.

4.9 Applying cooling High temperatures are present during the final stage at the papermaking process, mainly at the drying section whereas the temperature can range between 80-90 °C. The paper web is however not subjected to these type of temperatures for a long duration of time, but it is enough to raise and maintain the temperature of the paper-roll for several days; hence resulting in negative effects on the brightness. Larsson & Karlsson mentions that the brightness reversion can be suppressed by cooling the paper web before reeling, cooling of the laboratory sheets was therefore applied to investigate how the b*-value would behave (Larsson & Karlsson, 2000). The laboratory sheets were made accordingly to section 4.3 and was dried at 80 °C mainly to simulate the drying section as much as possible. The optical properties were measured with 30 minutes intervals whereas cooling was applied after the thermal exposure and vice versa. 4.10 Preparation of laboratory sheets for physical testing The laboratory sheets made for physical properties testing were made in a so-called standard sheet former, hence resulting in isotropic sheets, with a surface weight of 100 g/m2 according to ISO 5269. The wet sheet exiting from the sheet former was placed in-between pressing plates along with blotting paper to absorb the excess water thus dewatering the sheet. The plates were subjected to a pressure of 0.55 MPa for the duration of 5 minutes and the blotting paper was replaced with dry ones to repeat the pressing procedure. The plates were once again subjected to the same pressure at 0.55 MPa for the duration of 2 minutes. The sheets were dried in conditioned climate operating at 23 °C and 50% RH. 4.11 Measurement of physical properties 4.11.1 Grammage The grammage is the weight in grams per unit area of a paper product, expressed as g/m². Determination of the grammage according to ISO 536 is done by weighing specimens with a known area (International Organization for Standardization, 2020b). The grammage is then calculated by the formula below:

= × 10000 (19) 𝑚𝑚 𝑔𝑔 4.11.2 Thickness 𝐴𝐴 The thickness, expressed in μm, is measured accordingly to ISO 534 by pressing the samples between two plates with a pressure of 100 kPa (International Organization for Standardization, 2020c).

4.11.3 Bursting strength The bursting strength, expressed in MN/kg, is described as the maximum pressure which a paper product can withhold without breaking the fibres. The data received from a bursting strength measurement is often presented as the bursting index which is calculated by the formula below:

= (20)

𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠ℎ𝑡𝑡 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 Bursting strength measurements for paper products𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 is described𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡 according to ISO 2758 whereas the paper is placed over an elastic “balloon”. The pressure increases at a constant rate, thus making the “balloon” swell until the paper ruptures (International Organization for Standardization, 2020d).

4.11.4 Porosity The porosity of a paper product, expressed in ml/l, is the measure of the penetration of a gaseous or liquidous substance through its surface. The measure accordingly to ISO 5636-3 is performed whereas the paper is clamped between two plates and air is penetrating through the paper surface for a short period of time (International Organization for Standardization, 2020e).

4.11.5 Roughness The measurement of the roughness of a paper product, accordingly to ISO 8791-2 is performed by clamping the paper product between two plates and measuring the rate of airflow in ml/minute (International Organization for Standardization, 2020f).

4.11.6 Short Span Compression Test (SCT) The measurement is performed according to ISO 9895, whereas a paper strip is placed in between clamps that slowly moves closer to each other hence making the fibres collapse at some point (International Organization for Standardization, 2020g). The instrument being used measures both directions of the fibres in the paper as listed below:

MD = Machine Direction

CD = Cross Direction 4.12 Mill trial A mill trial was going to be performed during the 18-19th of March with the purpose to investigate how a pH neutral sizing method would affect the yellowing of the paper. Unfortunately, the trial was cancelled due to the restrictions regarding the COVID-19 virus. The sizing method of choice was going to be ASA due to previously trials and experience with the method at Smurfit Kappa. During the performance was samples from PM2 going to be treated to an accelerated ageing accordingly to ISO 5630-1. The testing for brightness reversion would have begun during acid sizing conditions and continued as the pH gradually increased towards neutral at the paper machine. Additional paper testing of physical properties was going to be performed to observe the paper qualities, thus comparing the effect of the different sizing methods.

5 Results This section displays and explains the obtained results from the laboratory experiments. 5.1 Factorial design The results from the factorial experiment measured the b*-value which indicates color changes towards yellow or blue according to the CIELAB color space, are displayed in Table 9. The experiment was replicated two times to achieve statistical variation.

Table 9 Design matrix with obtained results

A B Replicate 1 Replicate 2 AB [temperature] [pH] Y [b*] Y [b*] -1 -1 1 5.11 5.38 -1 0 0 5.26 5.46 -1 1 -1 5.33 4.43 0 -1 0 5.59 5.36 0 0 0 5.52 5.57 0 1 0 5.38 5.47 1 -1 -1 5.12 6.52 1 0 0 6.3 5.98 1 1 1 6.67 5.59

The results indicated that a change in pH have a significant effect of how sensitive the fibres are to yellowing when exposed to higher temperatures. According to Figure 29 a lower b*-value is achieved for the paper sheet with a pH value towards neutral or alkaline, thus being more sustainable at higher temperatures and hence suppressing the tendency to yellow during ageing.

Interaction Plot for b* 6,50 6,30 6,10 5,90 5,70 5,50 105°C 5,30 80°C 5,10 4,90 20-25°C Mean of Respnse [b*] 4,70 4,50 - 0 + pH

Figure 29 Interaction plot where pH4 is denoted with (-), pH5 is (0) and pH7 is (+)

The factorial experiment was further analyzed with ANOVA as seen below in Table 10, which indicated that the temperature has an large impact on the b*-value based on the obtained p- value. The p-value is accordingly to statistics the probability of obtaining results close to the observed results of a statistical hypothesis test, whereas a smaller p-value than 0.05 normally means that there is stronger evidence in favor of the result being significant. Hence indicating that temperature is significant for affecting the b*-value since the p-value is less than 0.05. A change in the pH of the pulp showed that it does not have an effect on the b*-value by itself since a p-value greater than 0.05 indicates a non-significant variable. However, the interaction (AB) between temperature and pH showed that it is indeed significant for affecting the b*- value.

Table 10 ANOVA-results

ANOVA Table Sums of Squares Degree of freedom Mean Square F-stat p-value Temperature (A) 2.288 2 1.14 23.86 0.000252633 pH (B) 0.128 2 0.06 1.34 0.309562913 AB 0.930 4 0.23 4.85 0.023128648 Residual 0.432 9 0.05

Total 3.779 17 0.22 5.1 pH-variation Based on the literature studies and the factorial experiment, the pulps pH-adjusted were to investigate how the b*-value would be affected. It is apparent according to Figure 30, that a low b*-value and thus a lower brightness reversion can be achieved by increasing the pH of the finalized paper. A pH-value towards neutral or alkaline seems to be most favorable for maintaining a resistance against yellowing.

b* versus pH 7,50

7,00

6,50 105 °C b* 6,00

5,50

5,00 3 4 5 6 7 8 9 10 pH

Figure 30 b*-values obtained by subjecting paper sheets of an thermal accelerated ageing according to ISO 5630-1 at different pH

Figure 31 illustrates the visual differences of acidic and neutral conditions when being affected by thermal accelerated ageing according to ISO 5630-1. The sheet on the right was pH adjusted to 5 thus being acidic, whereas it can be seen that the sheet has changed to a yellowish color.

The sheet at the left was pH adjusted to 7 and showed less signs of yellowing, this is further validated in Figure 32 where the sheet made with acidic conditions shows a very steep increment regarding the b*-value when exposed to a thermal ageing.

Figure 31 Visual differences between acidic conditions (pH 5) to the right (yellowish) and neutral/alkaline conditions (pH 7) to the left (whitish) (Nygren, 2019)

b* versus time 14 12 10 8 b* 6 pH5 4 pH7 2 0 0 20 40 60 80 Time [h]

Figure 32 Changes in the b*-value for paper-sheets with different pH during an thermal accelerated ageing according to ISO 5630-1

Figure 33, composed by the aid of quality engineer Johan Lundberg at Smurfit Kappa, further illustrates how the ∆b*-value correlates with the pH. The data was obtained from the software WinMOPS which Smurfit Kappa uses to observe the pulping and papermaking process. The ∆b*-values was measured as a part of the routine for standard quality control at Smurfit Kappa Piteå whereas the tambour samples had been treated to an accelerated ageing at 105 °C for the duration of 1 hour, hence being able to compare the difference in the b*-value before and after ageing. The pH is continuously measured at the headbox at PM2 to keep track of the sizing process, and the data is presented in the software WinMOPS. The data validates that acidic pH- values at the headbox of the paper machine, and thus within the top layer of the finished liners, tends to result in a greater difference regarding the ∆b*-value and hence indicating that the brightness reversion therefore is more severe.

∆b* versus pH

2,75 delta b* pH 7

2,5

2,25 6,5

2 6 1,75

1,5 5,5 ∆b* 1,25

1 5 0,75 pH measured in the headbox at PM2

0,5 4,5

0,25

0 4 2019-06-14 2019-06-24 2019-07-04 2019-07-14 2019-07-24 2019-08-03 2019-08-13 2019-08-23 2019-09-02 2019-09-12 2019-09-22 2019-10-02 2019-10-12 2019-10-22 2019-11-01 2019-11-11 2019-11-21 2019-12-01 2019-12-11 2019-12-21 2019-12-31 2020-01-10 2020-01-20 2020-01-30 2020-02-09 2020-02-19 2020-02-29 2020-03-10 2020-03-20 2020-03-30 2020-04-09 2020-04-19 2020-04-29

Figure 33 ∆b*-values and pH during points of sample for different tambours

5.2 Temperature variation Figure 34 illustrates that the pulp is very sensitive to higher temperatures as compared to when being dried at room temperature. The b*-value increases approximately 0,3 units when being subjected for a temperature of 70 °C, which might not be much, but if the timeframe of the ageing procedure had been longer it would definitely result in a greater increment. At temperatures exceeding 100 °C, it can be seen that the b*-value increases drastically, and it is also shown during this temperature range that acidic conditions is far more sensitive to higher temperatures. However, the papermaking process normally does not reach temperatures exceeding 100 °C, and especially not during longer periods of time, but the experimental data gives nevertheless a brief understanding of how sensitive a paper sheet can be towards temperature. The pulp is subjected to temperatures around 60-70 °C during the bleaching process which could hypothetically contribute to ageing reactions occurring if being stored for a longer period of time within the towers. The highest temperature that the paper is subjected to during the papermaking process at Smurfit Kappa is recorded at the drying section which normally ranges around 80-90 °C. The drying process is although a very rapid procedure, so the paper-web is not in contact with the drying cylinders for a long duration of time. Yet it seems to be enough time for the finished paper-rolls to maintain high enough temperatures, hence resulting in a slow decrease in temperature upon storage which both Lindström and Sandin have shown (Sandin, 2008; Lindström M. , 1990).

b* versus temperature 9,00 8,50 8,00 7,50 b* 7,00 pH 5 6,50 pH 7 6,00 5,50 0 20 40 60 80 100 120 140 Temperature [°C]

Figure 34 The effect on b*-value at different temperatures

5.3 Pulp storage The experimental trial of pulp storage indicated, after 24 hours of storage time, that the pH value had decreased from approximately 10 to around 8. The decreased pH- value could be an indicator that acid hydrolysis is taking place, probably due to the formation of HexA. However, as seen in Figure 35, the b*-value decreases due to the storage time. Figure 30, as mentioned in section 5.1, indicates that a pH of 8 is more favorable in order to achieve better optical properties than a pH around 9-10. Hence, it could indicate that the storage time is of great importance especially since the pH could decrease even more which would have the reverse effect (increasing) of the b*-value.

Storage of pulp 5,16 12 5,14 5,12 10 5,1 8 5,08

b* 5,06 6 pH b* 5,04 5,02 4 pH 5 2 4,98 4,96 0 0 5 10 15 20 25 Time [h]

Figure 35 Decreased b*-value and pH due to storage time

5.4 Relative humidity As mentioned previously, the drying in paper machine is usually performed at temperatures around 80 °C which could cause the paper-reels to maintain high temperatures upon storage. The isolated temperature within the paper-roll could in combination with a high relative humidity cause the yellowing reactions of the paper to accelerate. An attempt was made to investigate how the brightness reversion would be affected under accelerated ageing conditions accordingly to ISO 5630-3. The results, as seen in Figure 36, indicate that the yellowing is much more severe at a higher relative humidity, especially during acidic conditions.

However, this experiment was only performed with handmade sheets in an enclosed climate chamber and may not be relevant for the paper-reels during storage. Ronald Bredemo at Smurfit Kappa Piteå have shown during an internal investigation that the moisture content of a paper- reel appeared to be somewhat constant during storage and shipping, and thus may not affect the yellowing rate as much (Bredemo, 2006). Yet it should still be recommended, according to the literature and the experiment, to keep the relative humidity as low as possible.

Comparing different RH at 80°C 8,2 8 7,8 7,6 7,4 pH5 - 65% RH 7,2 pH7 - 65% RH b* 7 pH5 - 0% RH (Dry) 6,8 6,6 pH7 - 0% RH (Dry) 6,4 6,2 6 0 50 100 150 200 250 300 Time [min]

Figure 36 Relative humidity versus b*-value

5.5 Cooling of the laboratory sheets The laboratory sheets were dried at 80 °C according to Figure 37 mainly to simulate the drying section as much as possible. The optical properties were measured with 30 minutes intervals. The b*-value is, as seen in the graph, continuously increasing during the period of time from 0- 150 minutes. Here it can also be seen that pH neutral conditions are to be preferred since the brightness reversion is readily suppressed. After 150 minutes the sheets were placed in a cooling storage which operates at approximately 7 °C, and after 30 minutes of cooling within the storage the optical properties were measured again. Surprisingly the b*-value had decreased as seen in Figure 37. The explanation of this is however unknown, but it could be because the molecular mobility is being decreased due to the lower temperature. It could also be an effect of swelling/shrinkage of the fibres, thus increasing/decreasing the light scattering and absorbance of the sheets. Or it might be the chromophore activity being decreased due to less energy conversion due to the lower temperature. It seems very unlikely that the chemical reactions taking place during heating would suddenly revert due to cooling, especially since chemical ageing is irreversible as mentioned in section 3.4.2. It does, however, show that lowering the b*-value can be achieved by cooling the sheet.

Heating vs Cooling Heating Cooling 6,10 5,90 5,70 5,50

b* 5,30 pH 5 5,10 pH 7 4,90 4,70 4,50 0 50 100 150 200 Time [min]

Figure 37 Heating and cooling the laboratory sheets

The reverse procedure was performed as shown in Figure 38, where the laboratory sheets was initially dried (water removal) at 80 °C for 30 minutes and then continuously cooled. The b*- value was once again decreased due to the cooling, yet only after being heated in beforehand. The b*-value, as seen in the graph, is rather stable during the period of time from 30-150 minutes where cooling is continuously applied. Even during cooling, it seems that neutral/alkaline conditions is to be preferred since the brightness reversion is suppressed. After 150 minutes the sheets were placed back in the heating cabinet and after 30 minutes of heating was the optical properties measured again. The b*-value increased again due to the higher temperature as seen Figure 38.

Cooling vs Heating Cooling Heating 6,10 5,90 5,70 5,50 pH 5 b* 5,30 5,10 pH 7 4,90 4,70 4,50 0 50 100 150 200 Time [min]

Figure 38 Cooling and heating the laboratory sheets

5.6 Cooling of archived samples Smurfit Kappa archives tambours that is been evaluated in the laboratory, which is mainly done in case of retesting purposes. The tambours are stored absent from light and heat in order to prevent ageing and yellowing to occur. The effect of cooling was also performed on a sample of selected archived tambours that have been stored since the summer months of 2019. The tambour samples with corresponding numbers; 2937306 and 2926408 were more affected towards yellowing during the initial evaluation and a similar result was obtained during the measurement. The optical properties of each sample were measured as a reference point before being subjected with cooling. The effect of cooling, as seen in Figure 39, shows that the samples had slight to no change in the b*-value. However, the effect of cooling the tambour samples surprisingly resulted in a decrease of the b*-value only when the samples was subjected to a thermal accelerated ageing which at first resulted in an increase of the b*-value.

Figure 39 Cooling and heating the archived papers

5.7 Paper properties General physical paper properties were tested on the handmade laboratory sheets. These tests were performed in order to observe how the fibres would react to acidic or neutral pH conditions. Results from handmade laboratory sheets will only give a brief understanding of how the fibres are affected, especially since the finished liners from production have a lot more components such as dyes, sizing agents, fillers etc. which will affect the paper properties further. A neutral pH-value of the paper resulted, as seen in Table 11, in a slightly more robust paper compared to the paper made with acidic conditions; although the difference could be seen as negligible. The porosity and roughness are lower for the sheets with pH-neutral conditions, which could indicate in more swollen fibres due to the higher pH-value, hence allowing less fluids of both liquid and gaseous state to penetrate trough the surface of the paper.

Table 11 Testing of paper properties

Properties Standard Unit pH5 pH7 Grammage ISO 536 g/m² 113.65 114.69 Thickness ISO 534 μm 139.75 139.60 Density ISO 534 kg/m³ 813 822 Porosity ISO 5636-3 ml/min 3430 3215 Roughness ISO 8791-2 ml/min 248 230 Burst index ISO 2758 MN/kg 2.40 2.48 SCT index ISO 9895 kNm/kg 20.95 21.15

6 Discussion This section discusses the obtained results 6.1 General discussion The ageing mechanisms and hence the breakdown of cellulose, which is the main component in pulp and paper, depends on a complex mixture of physical and chemical factors. Therefore, the rate of the ageing procedure is nearly impossible to predict in-between samples, unless the ageing process would be dominated by a single degradation process which is highly unlikely.

Moreover, it is also difficult to pinpoint where in the process that the thermal induced yellowing is most severe. High temperatures, which obviously is the major factor regarding the thermal exposure during yellowing, is present both during the pulping process and throughout the papermaking process. The pulp can withhold temperatures around 60 °C during the pulping process and the papermaking process has also high temperatures recorded, especially during the drying section since the goal is to vaporize the water content from the pulp to achieve a dry paper product. The temperatures during drying of the paper web can be around 80 °C.

6.1.1 Chemical factors There are many chemical factors affecting the brightness of the paper during the process of pulping and papermaking. These chemical factors could presumably have an effect of the obtained results but were assumed to be negligible since most of them are removed during the pulping process.

Lignin, which contains the molecular structure called chromophores, contributes to the darker color of the pulp and can through light and heat absorption by the chromophore activity contribute to a brightness reversion. Lignin is however removed during chemical pulping and the pulp/paper should therefore, as mentioned in section 3.4.1, contain as little lignin as possible without damaging the carbohydrates. Smurfit Kappa Piteå measures the Kappa number, which is an indication of the remaining lignin in the pulp, and therefore the lignin content was not included during the study.

As for extractives and transition metals, the assumption was made that these also were somewhat negligible since they are removed during the pulping process by being dissolved and through chelation by using EDTA for example. It would had been possible to measure the content of these through inductively coupled plasma mass spectrometry (ICP-MS) for example, and thereby study the behavior of the brightness reversion.

Hexenuronic acid groups are one of the chemically factors that is discussed as being a major issue regarding yellowing as of today. Many publications have shown that chemical pulps contain a larger amount of these acid groups due to the lack of acidic environments during pulping. There are, however, methods by which the HexA groups could be removed, such as introducing a hot acidic stage after the bleaching sequence in order to terminate the hexA acid groups that has become a common technique as of today. Sandin showed during her thesis that HexA groups are present in the pulp at Smurfit Kappa Piteå and could be removed, and therefore was not HexA included in this project (Sandin, 2008). However, an additional treatment stage that probably would consist of another tower and a washer could be rather expensive due to all new equipment that has to be installed. And since that also would require a larger process change and investment, more research should be conducted regarding the subject. Since the chemicals (additives) used during sizing mostly affects the final pH of the finished liners, those were not added during the experimental trials. However, the tendency for a thermal induced yellowing to increase in TCF paper seems to be mostly affected by the final pH value of the paper. Most of the reports and publications that were reviewed during the literature studies imply that the pH value is crucial for yellowing to occur, even without an accelerated treatment. The produced white liners at Smurfit Kappa Piteå have a finalized pH-value of approximately 5 which is obtained during the sizing process. It would be, accordingly to the laboratory trials beneficial to change the sizing method to neutral or alkaline to avoid acidic conditions, hence giving the liners a buffer capacity towards acidic hydrolysis and thus making the liners more resistant towards undergoing yellowing. The pH was also observed to decrease during the experimental trial regarding the storage time. Since a pH of 8 is more favorable to achieve better optical properties than a pH around 9-10 would it possibly be better to store the pulp at lower pH values (around 7-8). However, the storage time must also be considered since the pH theoretically could continue to decrease to acidic levels due to acidic hydrolysis which thereby would have the reverse effect, thus making the b*-value increase.

6.1.2 Physical factors It is although apparent, according to both the literature and the obtained results that the degradation processes which results in a brightness reversion can be severely decreased by controlling the temperature. An option that could resolve the thermal-induced yellowing could be to reduce the heat exposure to the paper-web before reeling, especially since the slow cooling process of a paper-roll can cause the yellowing to escalate during storage. The results from the experiments where both the laboratory sheets and tambour samples were subjected to cooling implies that the b*-value, and thus the brightness reversion is somewhat reversible during a short period of time. This might be due to activation and deactivation of chromophore activity, since they are said being dependent upon both light and heat.

Hence, cooling of the paper-web at the paper machine might be a solution for how to suppress the surface reactions that is taking place. However, an increased b*-value which resulted in a noticeable change in color of the paper is obviously not reversible by cooling. Cold storage of the paper products would therefore also be preferred to limit the thermal effect on yellowing. However, there could arise problems with cold storage since condensate can form more easily, hence requiring the storage to be dry as well. The results further concluded that the relative humidity is also a critical factor, at least regarding the laboratory hand sheets. The papers sheets that was being subjected to higher grades of moisture content showed an enhanced tendency for yellowing to occur, and once again were the papers treated to a higher pH shown to be more stable regarding the b*-value. A paper-roll could presumably be more exposed to a brightness reversion depending on the RH conditions, hence would it be recommended if the produced liners was stored at low humidity levels, although that might be difficult and expensive to achieve. 6.2 Economical & environmental evaluation Utilizing a pH neutral or alkaline sizing method during papermaking could alongside with better a better resistance towards ageing and thereby yellowing also potentially offer various cost savings throughout the process. According to the theory could the following aspects have a positive affect for the economy at a mill (Lundeen, 1983) (U.S. Congress, Office of Technology Assessment, 1988):

• A more robust paper can permit savings through reduction of various chemicals and raw materials • Calcium carbonate is an inexpensive material that is used as a filler during coating. It also acts as a pH buffer and results in higher brightness of the paper since calcium carbonate has good optical properties, thus reducing chemical & material costs. • Improved strength of a neutral/alkaline paper can reduce the amount of refining required of the pulp. • Neutral and alkaline sizing is less corrosive, hence extending the life of the machinery and thus reducing maintenance costs. Although this advantage would obviously only have a major benefit if non-stainless-steel equipment is being used. • The equipment can remain the same at both acid, neutral or alkaline sizing during the papermaking process, hence is no capital expenditure required to convert from one method of sizing to another. However, the advantages mentioned above are unlikely to fit every individual mill, especially since each mill utilizes different processes during manufacturing and does not produce the same paper qualities. The advantages would also vary depending on local conditions such as temperature or humidity levels, among many other factors. There is although, also some disadvantages of utilizing pH neutral or alkaline sizing during papermaking, where one major flaw would be that the paper is more exposed towards microbial or fungal degradation.

6.3 Possible sources of errors Doubts could be raised about the conditions used for the accelerated aging tests especially since the conditions usually involve very high temperatures to speed up the reactions taking place. Although, a major advantage is that visible and measurable data is obtained within a reasonable time frame, however, these type of accelerated ageing methods could result in different chemical reactions and degradation that would never occur during the course of natural aging.

It is also plausible that other factors, so called lurking variables, that has not been investigated during this thesis, contribute to an enhanced yellowing. Such factors can be the transition metal and HexA content as an example. However, taking all of the contributing factors into consideration would be rather difficult to analyze within a reasonable timeframe.

Another possible source of error could be that the L&W Elrepho spectrophotometer, used for the measurements of optical properties, is sensitive to the temperature of the measured object and hence resulting in misleading data. However, the temperature of the laboratory sheets was measured by the aid of an infrared thermometer to ensure that the sheets indeed was close to room temperature.

7 Conclusion This study among several others indicates that limiting the temperature exposure is of great importance, at least after the drying section. The so-called b*value seems to be reversible when being stored at cooler temperatures, yet only after first being subjected to heating. This could be due to activation/deactivation of chromophoric groups that are present within the residual lignin. Hence, both cooling prior reeling at the paper machine and cold storage might be a suitable suggestion in order to reduce the yellowing. The chemical causes, however, are far more complex to make a conclusion about. Although it seems likely that the final pH of the produced liners is a major factor to consider. Smurfit Kappa Piteå is, although, already aware of the benefits/disadvantages of utilizing different sizing methods. Sizing with rosin (acidic) is as of today the most suitable method of sizing at Smurfit Kappa Piteå due to several reasons. The discussion and conclusion regarding the obtained results of this thesis is therefore mainly recommendations. 7.1 Future work It was very unfortunate that the mill trial was cancelled as a lot of interesting data could not be obtained. A suggestion for future investigations regarding the problem of yellowing would be to perform the mill trial since it would give an insight how the b*-value is affected when utilizing a different sizing method and hence taking real life parameters and factors into account.

It would also be rather interesting to evaluate the changes in pH and optical properties as a function of time in order to understand the pH dynamics within the papers. In other words; how does the pH of the paper change with time, both through an accelerated and normal ageing process.

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