UNIVERSITÉ DU QUÉBEC

THÈSE PRÉSENTÉE À
L'UNIVERSITÉ DU QUÉBEC À TROIS-RIVIÈRES

COMME EXIGENCE PARTIELLE
DU DOCTORAT EN GÉNIE PAPETIER

PAR
DANIEL MATTE

PRECOAT'S INFLUENCE ON TOPCOAT'S PHYSICAL
PROPERTIES AND PRINTABILITY

Février 2012
Université du Québec à Trois-Rivières

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UNIVERSITÉ DU QUÉBEC À TROIS-RIVIÈRES

DOCTORAT EN GÉNIE PAPETIER (PH.D.)

Programme offert par l'Université du QUÉBEC À TROIS-RIVIÈRES

PRECOAT'S INFLUENCE ON TOPCOAT'S PHYSICAL PROPERTIES AND
PRINTABILITY

INFLUENCE DE LA PRÉ-COUCHE SUR LES PROPRIÉTÉS PHYSIQUES ET
D'IMPRESSION DE LA COUCHE SUPÉRIEURE

PAR

Daniel Matte

<table><tr><td>Claude DANEAULT<br>Directeur de recherche</td><td>Université du Québec à Trois-Rivières</td></tr><tr><td>Bruno CHABOT<br>Président du jury</td><td>Université du Québec à Trois-Rivières</td></tr><tr><td>Patrice MANGIN<br>Co-directeur de recherche</td><td>Université du Québec à Trois-Rivières</td></tr><tr><td>Jean-Philippe BOISVERT<br>Évaluateur externe</td><td>Centre de recherche Lafarge,<br>St-Quentin Fallavier, France</td></tr><tr><td>François BERTRAND<br>Évaluateur externe</td><td>École Polytechnique de Montréal,<br>Département de génie chimique</td></tr></table>

Thèse soutenue le 20 décembre 2011
Preface

The objective of present thesis is to verify the possibility to establish interactions be-
tween different properties of the precoat and/or topcoat layers in order to relate these
properties to printability. To reach the thesis objective we designed an experiment based
on the analysis of various groups of coated samples. The study is divided in 4 different
groups representing for each one a different precoat, coating layer, pigment-latex, and
calendering combinations.

The first group (Group 1) is used to study the replacement of 20 parts of GCC (Ground Calcium Carbonate; 1.4 µm average particle size) by 4 clays of different average particle size. The second group (Group 2) serves to study the calendering effect on the base paper or on the precoated layer. Two temperature conditions combined with 2 different pressures for a total of 8 conditions are included in Group 2. The third group (Group 3) has only 2 conditions: for the first condition, the latex S/B (Styrene/Butadiene) is replaced by an acrylic in the precoat; for the second condition, the CMC (Carboxy Methyl Cellulose) is replaced by an amphoteric protein. Finally, the last group (Group 4) is used to study what occurs when the GCC pigment is totally replaced in the precoat formulation by other pigments. Group 4 has five different formulations: one with PCC (Precipitated Calcium Carbonate), one with Brazilian Clay and 3 with GCC different particle size and different particle size distribution. The topcoat applied over each of these 20 precoat conditions is exactly the same.

The latex is changed for the topcoat using 3 different latexes for a total of 60 conditions.
The latexes used are Styrene/Butadiene/Acrylonitrile (S/B/ACN), Styrene/Acrylic (S/A),
Styrene/Acrylic/ Acrylonitrile (S/A/ACN). All these conditions are analyzed for pore
structure, contact angle, SEM (Scanning electron microscopy), and offset printability
test (DELTACK).
**Acknowledgements**

My first thoughts go to my family who believed in me from the very beginning of this
project. Their support and help were a huge part in supporting me to successfully com-
plete such a huge undertaking. I want to thank all my family members, my wife Jo-
hanne, and my children Kathleen, Marilyn, Pier-Luc, and Jean-Christophe, for their sup-
port and their good comments during all the time and the long hours I worked on my
thesis.

Big thanks are due to Professors Claude Daneault and Patrice Mangin for their com-
ments, support, and help. Without them, I would certainly have not been able to com-
plete the work in its present form. They showed me various ways to improve myself in
this “adventure”. Very special thanks are due to Patrice Mangin for the time he spent
with me to re-write and re-organise the document, and to teach me how to analyse data
in further depth, with a scientific mind, before reaching the final defence of the thesis.
All his comments and suggestions surely improved this document to a high scientific
level.

I also want to thank Centre International de Couchage (CIC) and the people at CIC for
their support and their help. Thanks are due to Omya Development AG for the great
support offered, especially for all the testing performed on my behalf, and finally thanks
are due to Kamin LLC for allowing me to utilise some results from their trials.

Very special thanks are due to Nancy Jacob, CIC for all the data compilation and her
patience with the changes in the direction of my research. Special thoughts are con-
veyed to Charlie Mazza, CIC, and Patrice Mangin, CIPP/UQTR for their help in improv-
ing the English structure of my document.

The author also wishes to thank Dr Patrick Gane and Dr Cathy Ridgway, Omya Devel-
opment AG for their help and support in the mercury testing and their useful comments
on the different methods. Discussions were particularly valuable in conducting and pre-
paring the research work and strategy. Their input is very much appreciated.
The last words and thanks go for all the persons listed below who helped me in different ways and at different times during the project, namely:

<table><tr><td>Joseph Ishley</td><td>for comments on the English document,</td></tr><tr><td>Sylvie St Amour</td><td>for mercury intrusion and CLC testing,</td></tr><tr><td>Sylvie Sauriol</td><td>for CLC testing and printability measurements,</td></tr><tr><td>Martin Dubé</td><td>for profilometry, mathematical and pore analysis,</td></tr><tr><td>Agnès Lejeune</td><td>for Scanning Electron Microscopy (SEM) wonderful pictures,</td></tr><tr><td>Josée Doucet</td><td>for contact angle measurement,</td></tr><tr><td>Amy Dimmick</td><td>for comments and corrections on the English document, and</td></tr><tr><td>Robert Lanouette</td><td>for PLS analysis, comments, and useful discussions.</td></tr></table>

February 2012
Résumé en français

La fabrication du papier enduit ou papier couché connaît des développements importants depuis une vingtaine d'années. Une partie de ces développements concerne les papiers enduits de plus d'une couche, communément appelés «papier multicouches.» Durant toutes ces années, la caractérisation de la structure des couches, les unes par rapport aux autres, n'a fait l'objet que de quelques études qui traitaient principalement de la couche finale appliquée sur le papier. L'influence de la pré-couche sur les caractéristiques finales d'impression n'a quant à elle pratiquement jamais été abordée de façon fondamentale. Une étude sur les interactions entre les couches pour un carton de pâte kraft blanchie mentionne l'effet de la pré-couche mais aucune étude systématique ne traite le papier sans bois (wood free).

L'objectif premier de ce travail a donc été de déterminer et de comprendre l'effet de la pré-couche sur les caractéristiques finales d'impression de la feuille. Pour ce faire, vingt (20) différentes conditions de préparation de pré-couche ont été analysées avant d'y appliquer trois types d'enduits de surface différents. Les différentes conditions d'application des pré-couches ont été réalisés sur une coucheuse pilote selon des conditions d'opération industrielles normales pour ce type de papier. L'application de la couche supérieure a été réalisée sur un équipement de laboratoire qui reproduits des conditions industrielles d'opérations : le CLC pour « Cylindrical Laboratory Coater ». L'impression de type offset des papiers ainsi préparés a été faite à l'aide d'un appareil Prüfbau adapté pour la mesure du maximum de tirant lors de la prise d'encre (DELTACK). Les structures ont été déterminées par intrusion au mercure et par microscopie électronique à balayage complétée par des mesures d'angle de contact. Outre les mesures normalisées sur les propriétés des papiers, ces mesures ont servi à évaluer les propriétés structurelles et de surface des papiers en vue de l'analyse et de la compréhension des comportements des différentes pré-couches d'un point de vue structurel.

Le support de couche utilisé est un papier sans bois avec un grammage ou poids de base de 61 g/m², soit le papier commercial « Thermal Transfert » (Fraser Paper Company). Quatre différents groupes de papiers pré-couchés ont été produits en usine pilote afin
d'obtenir une qualité et une stabilité jugées primordiales avant l'application en laboratoire à l'aide du CLC d'une couche finale ou couche supérieure, au-dessus de la pré-couche. Toutes les formulations de pré-couche ont été ajustées à une concentration de départ de 66 % avec cependant parfois une dilution additionnelle afin de permettre une application de qualité lors de la production en usine pilote. La pré-couche de 12 g/m² par côté, le coté rugueux étant couché en premier, a été appliquée avec la station de couche jet/lame de la coucheuse pilote. La vitesse de la coucheuse était de 1400 m/min et la lame choisie pour le contrôle de poids de couche correspondait à une nomenclature industrielle « 0.457 mm / 84 mm / 45° » (dimension et angle de la lame d'application).

Pour chacune des conditions expérimentales de l'étude, les tests d'intrusion au mercure ont été effectués sur le support de couche, sur le papier pré-couché, sur le papier double couche pré-couché et couché et sur le papier double couche calandré. L'objectif final de ces analyses était d'obtenir les renseignements sur la partie couchée, pré-couche et couche supérieure, uniquement. En effet, lorsque le test est effectué, la structure complète de la feuille est analysée, soit le support de couche et les deux couches. Une caractéristique spécifique de cette étude est d'avoir utilisé une particularité du programme Autopore® de la mesure d'intrusion au mercure pour optimiser l'analyse des données de la double-couche. En effet, le programme Autopore® permet de soustraire à l'analyse de porosité les données relatives à une mesure à vide du bulbe contenant l'échantillon afin de tenir compte de l'expansion du bulbe sous l'effet de la pression d'intrusion ainsi que du tube indiquant le volume d'intrusion lors du test à haute pression. Nous avons utiliser cette particularité afin de « retrancher » les données de porosité du papier support de couche en le considérant alors comme une valeur de porosité de base ou une porosité hors étude « à enlever » pour faciliter l'analyse de la pré-couche. Ainsi, il devenait possible de conserver uniquement les valeurs reliées directement à la pré-couche sans une distorsion qui serait causée par le support de couche. La même procédure a été appliquée pour la couche supérieure et la couche calandrée afin de pouvoir isoler les résultats perlinents pour chacune des composantes expérimentales au niveau de la structure de la feuille. Cette nouvelle méthode, avec ses limites (la mesure étant destructive, ce n'est jamais « exactement le même échantillon de papier » qui peut être évalué avant et après couchage ou calandrage), permet cependant d'isoler chacune des couches les unes des
autres en plus d'avoir une vue d'ensemble complète de la structure étudiée. Il devient
alors plus facile d'analyser les résultats des traitements mécaniques ou modifications
appliqués.

Le tableau suivant présente la liste de toutes les conditions qui composent les 4 groupes
de l'étude, conçus avec pour objectif principal l'étude des différentes modifications de la
pré-couche, soit : (1) 20 parties de substitution de kaolin de la pré-couche, (2) pré-
calandrage du support de couche et de la pré-couche, (3) substitution du latex et de
l'épaississant de la pré-couche et (4) 100 parties de substitution du/des pigments de la
pré-couche. Nous avons choisi de présenter dans ce sommaire étendu en plus en détail
les résultats et l'analyse effectués lors de la substitution de 20 parties de kaolin dans la
pré-couche (groupe 1) et seulement une brève conclusion pour chacun des trois autres
groupes. Ce choix a été fait afin de démontrer comment sont présentés et analysés les
résultats tout en préservant le modèle du résumé en français.

Dans le groupe 1 qui comporte 4 conditions de substitution de 20 parties de kaolin par
des pigments de différentes grosseurs de particules et de forme, la recette « standard »
ou de base pour la pré-couche, aussi valable pour l'ensemble de l'étude, comprend 100
parties de GCC (1,4 µm). Pour ce groupe 1, les kaolins de substitution sont : un kaolin
No.1 (brillance élevée, particule fine de dimension moyenne 0.469 µm), un kaolin No.2
(brillance régulière, dimension 2.13 µm), un kaolin grossier délaminé (brillance réguliè-
re, dimension 5.312 µm) et un kaolin d'ingénierie grossier (rapport longueur/épaisseur
élevé, dimension 6.329 µm). L'analyse des résultats du groupe 1 démontre que substi-
tuer 20 parties de kaolin dans une formulation contenant 100 parties de GCC dans la
formulation de la pré-couche permet de moduler la structure de la pré-couche et
d'obtenir les propriétés d'impression désirées. De la même façon, nous avons montré
que l'absorption capillaire, le mouvement préférentiel de divers éléments de la pré-
couche et de la couche ou la pénétration forcée des liquides de la couche supérieure dans
la pré-couche sont des mécanismes potentiels pour expliquer comment la structure de la
couche supérieure est modifiée par la présence de la pré-couche. Nous avançons de plus
l'hypothèse que la pénétration forcée ou une partie de celle-ci est probablement causée
par le remouillage de la pré-couche lorsqu'une couche y est appliquée par-dessus.
<table><caption>Tableau Formulations de pré-couches reliées à chacun des groupes de l'étude.</caption><thead><tr><th>Formulations de pré-couche</th><th>Groupe</th><th>Nombre de formulations</th></tr></thead><tbody><tr><td>100 parties GCC (1.4 µm)</td><td>Contrôle</td><td>3 couches = 3 échantillons</td></tr><tr><td>20 parties de kaolin No. 1 - Brillance élevée et particule</td><td rowspan="3">Groupe 1<br>20 parties<br>de substitution<br>de kaolin</td><td rowspan="3">3 couches x 4 pré-couches<br>= 12 conditions</td></tr><tr><td>20 parties de kaolin No. 2 - Brillance régulière (2.13 µm)</td></tr><tr><td>20 parties de kaolin grossier délaminé (5.312 µm)</td></tr><tr><td>20 parties de kaolin d'ing. grossier -facteur de forme élevé (6.329 µm)</td><td rowspan="7">Group 2<br>Pré-calandrage<br>du support de<br>couche et de la<br>pré-couche</td><td rowspan="7">3 couches x<br>4 calandrage du support de<br>couche<br>et<br>4 calandrage de la<br>pré-couche<br>= 24 conditions</td></tr><tr><td>Support de couche calandré 100°C - 50 kN/m</td></tr><tr><td>Support de couche calandré 100°C - 200 kN/m</td></tr><tr><td>Pré-couche calandrée 100°C - 50 kN/m</td></tr><tr><td>Pré-couche calandrée 100°C - 200 kN/m</td></tr><tr><td>Support de couche calandré 200°C - 50 kN/m</td></tr><tr><td>Support de couche calandré 200°C - 200 kN/m</td></tr><tr><td>Pré-couche calandrée 200°C - 50 kN/m</td><td rowspan="2">Groupe 3<br>Substitution<br>S/A + Protéine</td><td rowspan="2">3 couches x 2 pré-couches<br>6 = conditions</td></tr><tr><td>Pré-couche calandrée 200°C - 200 kN/m</td></tr><tr><td>Pré-couche 100 parties GCC (1.4 µm) + Latex acrylique</td><td rowspan="2">Group 4<br>Substitution<br>de 100 parties<br>de pigment</td><td rowspan="2">3 couches x 5 pré-couches<br>= 15 conditions</td></tr><tr><td>Pré-couche 100 parties GCC (1.4 µm) + Protéine</td></tr><tr><td>100 parties BPSD GCC (0.659 µm)</td><td rowspan="5">Group 4<br>Substitution<br>de 100 parties<br>de pigment</td><td rowspan="5">3 couches x 5 pré-couches<br>= 15 conditions</td></tr><tr><td>100 parties BPSD GCC (0.3598 µm)</td></tr><tr><td>100 parties PCC (0.499 µm)</td></tr><tr><td>100 parties de kaolin brésilien (0.548 µm)</td></tr><tr><td>100 parties NPSD GCC (0.6593 µm)</td></tr></tbody></table>

BPSD= pigment avec une distribution de particules larges

NPSD= pigment avec une distribution de particules étroites

Une autre conclusion clé de notre étude est d'avoir mis en évidence l'importance des différents types de latex utilisés pour la fabrication de la couche supérieure dans la prise d'encre (cf. l'analyse des propriétés d'impression DELTACK). Le latex avec une base de butadiène résulte toujours en une force maximale de triant d'encre plus élevé que pour le latex à base d'acrylique. Les deux latex à base d'acrylique utilisés dans la fabrication de la couche supérieure ont un comportement similaire sur la prise d'encre comme il est possible de le déduire sur l'analyse de courbes DELTACK d'aspect similaire. Finalement, nous avons montré qu'il est possible d'obtenir la même prise d'encre avec
des structures poreuses très différentes. Les deux situations observées sont (a) le cas de structures avec des volumes de pores identiques mais des diamètres de pores moyens très différents ou (b) le cas de volumes de pores très différents avec des diamètres moyens de pores identiques.

Nous avons proposé un mécanisme fondamental qui permet d'expliquer simplement ce phénomène particulier sur la base des changements dans la structure de la couche ayant pour origine les différentes pré-couches utilisées. De façon globale, les changements structuraux que l'on retrouve dans un complexe de structure pré-couche/couche sont en mesure de modifier comment l'encre et/ou les solvants de l'encre pénètrent préférentiellement à l'intérieur des deux différents types de structure, le tout résultant en des compositions finales du film d'encre très différentes. Le film d'encre peut être épais ou mince mais cependant posséder la même force de tirant reliée à la prise d'encre dans des structures différentes.

L'illustration suivante permet de visualiser le mécanisme proposé pour la fixation de l'encre dans les 2 cas. Elle représente l'épaisseur du film d'encre en relation avec une force de tirant maximum égale et la structure de la pré-couche : structure de pré-couche fermée à gauche et ouverte à droite démontrant la possibilité de filtration/pénétration préférentielle des composantes de l'encre.

<figure><img src="image_1.png" /></figure>

Bas = pré-couche / milieu = couche/ dessus = film d'encre (vert)
Le groupe deux (2) de notre étude a été conçu afin de vérifier l'effet du pré-calandrage du support de couche et de la pré-couche sur un papier double-couche basé sur l'hypothèse que ce pré-calandrage pouvait affecter la structure supérieure sur la base d'effets physiques de pénétration des fluides vus pour la prise d'encre mais ici appliqués à la pré-couche ou à la couche supérieure. Pour toutes les conditions, le pré-calandrage résulte en une diminution de la dimension des pores et conséquemment, d'une réduction du volume total de pores lorsque comparé à la condition de contrôle qui n'a subit aucun pré-calandrage. L'analyse des courbes de distribution des pores n'a cependant pas permis d'identifier de tendance généralisable pouvant expliquer comment une structure double-couche pouvait être affectée par le pré-calandrage. Par contre, l'analyse du volume total de pores a montré des tendances généralisables en ce qui concerne les effets de température ou de pression; ce qui s'avère un outil puissant pour comprendre comment le pré-calandrage peut modifier les structures de papier double-couche.

En effet, nous avons trouvé que le pré-calandrage à 100°C résulte en un volume de pores de la structure de la pré-couche plus élevé que lors d'un pré-calandrage à 200°C. Comme anticipé, le pré-calandrage à 200 kN/m produit quant à lui un volume de pores total plus faible que le pré-calandrage à 50 kN/m. Les résultats obtenus pour la structure de la couche sont plus surprenants et ce, pour le pré-calandrage du support de pré-couche et de la pré-couche elle-même. Nous avons observé deux effets qui semblent *a priori* en opposition mais que nous avons pu expliciter simplement par transposition d'effet de filtration d'une structure à l'autre. En effet, (a) lorsque le support de couche a été pré-calandré, le volume total de la couche est plus élevé à 100°C qu'à 200°C, et (b) à l'opposé, lorsque la pré-couche a été pré-calandrée, le volume total de la couche est plus élevé à 200°C qu'à 100°C.

Le mécanisme que nous proposons afin d'expliquer de tels résultats en apparence contradictoire est basé sur la filtration et la compaction d'une structure qui survient lors des opérations de couchage, les effets étant dus au mouvement de l'eau et des matériaux constituants de la couche lorsqu'une couche humide est appliquée par-dessus une autre surface. Assurément, la surface d'une structure poreuse devient plus ou moins ouverte selon le calandrage appliqué et peut ainsi contrôler la filtration et la compaction. Le pré-
calandrage à haute température de la précouche a un impact direct sur la structure de la
couche (fermeture des pores). Le pré-calandrage du support de couche a le même effet
lorsque la pré-couche est appliquée mais la situation est nécessairement inversée lorsque
la couche est appliquée; il y a donc transposition d'effet de filtration d'une couche à
l'autre ou « effet domino ». En effet, considérant que le papier couché double-couche
comporte trois couches, soit papier de base, pré-couche et couche supérieure : une struc-
ture de base (papier de base ou pré-couche) plus fermée rend par filtration préférentielle
la couche supérieure plus ou moins ouverte avec un effet subséquent (« domino ») sur la
couche supérieure finale. Finalement, nous expliquons qu'il est possible d'obtenir les
mêmes caractéristiques d'impressions avec les différentes structures créées par pré-
calandrage. Une fois encore, l'effet de la température du pré-calandrage est ici le facteur
prédominant. En conclusion, nous avons donc pu montrer qu'il est possible de contrôler
la microstructure et même la nanostructure (jusqu'à 30 nm) d'un papier double-couche
par le pré-calandrage, et ainsi, qu'il est possible de contrôler les caractéristiques
d'impressions des papiers double-couches.

Concernant l'utilisation de protéine ou de latex S/A dans la pré-couche en substitution
d'une combinaison initiale S/B+CMC (groupe 3), la conclusion générale est que cette
pratique n'apporte pas d'avantages significatifs, spécialement pour la substitution du
latex S/A. Par contre, dans le cas de la substitution du CMC par la protéine, nous avons
pu montrer que la protéine change la structure en élargissant la distribution des pores de
la pré-couche. Il serait par contre hasardeux de généraliser les avantages de l'utilisation
de ce produit car une seule condition a été réalisée. Une étude plus approfondie impli-
quant d'autres conditions expérimentales serait nécessaire pour de futurs travaux. Les
résultats d'impression n'ont quant à eux apporté aucune conclusion qui soit différente
des autres conditions des autres groupes d'étude.

Concernant la substitution de 100 parties de pigment (groupe 4) de la pré-couche,
l'analyse effectuée nous permet de confirmer que la structure de la pré-couche influence
grandement la structure finale d'un papier double couche. Nous avons observé ente au-
tres l'importance clé de la distribution et le diamètre moyen de particules afin de déter-
miner comment ces facteurs influencent la structure globale constitué par l'ensemble
pré-couche/couche supérieure. Plus spécifiquement, nous avons trouvé et expliqué comment le volume de pores total est inversement proportionnel à la distribution des particules de pigment. Nous avons aussi montré que la surface totale des pores décroit avec la dimension de particules et qu'elle est inversement proportionnelle à la dimension de particules dans le cas de distribution de particules élargie. Finalement, nous avons confirmé notre hypothèse, à savoir que le diamètre moyen des particules est directement relié à la dimension du pigment utilisé dans la pré-couche.

En conclusion générale, nous avons proposé un mécanisme en deux phases, qui a pu être validé dans d'autres conditions au niveau de la formation de la pré-couche et de la couche supérieure pour expliquer comment la même force de tirant, résultant de prises d'encre donc de mécanismes d'absorption, pouvait être obtenue avec structures de couche très différentes. En effet, la filtration d'une encre ou d'une sauce de couchage est basée sur les mêmes mécanismes physiques. Concernant la prise d'encre, nous avons montré que, dans un cas, des volumes de pores similaires combinés à des diamètres de pores moyens très différents et, dans un autre cas, des volumes de pores passablement différents combinés à des diamètres moyens de pores similaires pouvait résulter en une même force de tirant maximum reliée au film d'encre transférée au papier double-couche. Le mécanisme proposé pour expliquer cet effet est basé sur la pénétration préférentielle des solvants de l'encre qui contrôle la prise d'encre ainsi que la variation du transfert de l'encre relié aux différentes structures de couche qui contrôle l'épaisseur du film d'encre : les deux ayant des effet opposés. La filtration de la sauce de couchage obéit quant à elle au même mécanisme.
**Abstract**

The main objective of present study was to establish how modification in the design and structure of the precoat in double-coated papers would control the final properties, mainly printing characteristics, of the double-coated paper. To achieve such an objective we needed first to develop different precoat structures based on commercially feasible and applicable practices and second to fine tune the mercury intrusion method to be able to differentiate sometimes minute but important structural differences brought about by modifications in the precoat formulation, application, and treatment.

Mercury intrusion is a well established method which is used extensively within the pulp and paper industry. The test provides a great deal of information to the papermaker about the paper and coating structure which may vary widely depending on the choice of materials used in the coated paper production. The mercury intrusion test has been shown to be invaluable in predicting or troubleshooting many areas such as blister resistance and printing issues. As mercury intrusion is time consuming and expensive, every effort must be made to ensure that the data collected is fully investigated and interpreted to take complete advantage of the information on pores structure and pores distribution.

The specifically designed method of mercury intrusion curves analysis was developed to examine and analyse a wide range of commercial operating conditions, including base paper calendering, different pigment and binder combinations. In essence, we determine cut-off values for the different sections of the mercury intrusion curves to be able to compare these curves on a same base-line. The mercury intrusion curves thus modified are designed as "normalized".

Present study of double-coated layer structures focuses on the effect of the precoat portion of a double layer wood free coated paper. Many different precoat and surface treatment conditions are analyzed to verify, and understand the relationships between the various precoat combinations and printing characteristics in order to provide the best printability.
In the experimental design, 12 different precoat formulations and 8 calendering conditions are analysed. The pigment of the precoats varies for each of the 12 precoats, and consists of different pigments, including broad ground calcium carbonate, fine number one (#1) clay, delaminated clay, number 2 (#2) clay, structured clays, and fine precipitated calcium carbonate. The binder and additive system for all of the coatings are kept constant, with one exception where the latex is modified. The latex used for the majority of the coatings is standard styrene butadiene (S/B) latex.

Additionally, one selected precoat serves as a standard or reference. Eight different calendering iterations are used, i.e. calendering the base paper at four different temperature and pressure combinations, and calendering the precoated paper at 4 different temperature and pressure combinations. All of the precoat application and calendering is performed at a pilot scale at the Centre International de Couchage (CIC) in Trois-Rivières, Quebec, Canada. Each precoat is applied to both sides of the base paper by blade coating at 12 g/m².

Including the calendering iterations, there are then 20 total precoat conditions. Three different topcoats are applied to each precoat, bringing the total number of conditions examined to 60. The pigment slip and additive portion of the topcoats are kept constant, and only the latex varies. The three different latexes used are styrene-butadiene acrylonitrile (S/B/ACN), styrene acrylate (S/A), and styrene-acrylate acrylonitrile (S/A/ACN). The topcoats are applied to one side of the precoated paper at 12 g/m² via the laboratory CLC equipment. All these topcoats conditions are tested with the Prüfbau DELTACK and a curve of each condition is used to examine which conditions would have the best surfaces for offset printing.

Finally, we have proposed and validated a physical mechanism based on preferential filtration, structural rearrangement, and filtration domino effect of one structure to the next to explain how the precoat structure influences both the structure and the printing characteristics of the double-coated paper.
**Keywords**

English:

Coated paper, double-coated paper, precalendering, calendering, soft nip calendering, pilot coater, pigment, latex, filtration mechanisms, coating structure, layered structure, pigment packing, pore size, pore distribution, pore volume, micro-structure, nano-structure, printing characteristics, print quality, mercury intrusion, contact angle, Prüfbau DELTACK, microscopic evaluation, projection on latent structure (PLS), normalization method.

Français :

Papier couché, papier couché double-couche, pré-calandrage, calandrage, calandrage à pince molle, coucheuse pilote, pigment, latex, mécanismes de filtration, structure de la couche, structure stratifiée, empilement des pigments, tailles de pores, distribution des pores, volume des pores, microstructure, nanostructure, caractéristiques d'impression, qualité d'impression, intrusion au mercure, angle de contact, Prüfbau DELTACK, évaluation microscopique, projection sur structure latente (PLS), méthode de normalisation.
Table of Contents

<table><tr><td>Preface</td><td>iii</td></tr><tr><td>Acknowledgements</td><td>iv</td></tr><tr><td>Résumé en français</td><td>ii</td></tr><tr><td>Abstract</td><td>x</td></tr><tr><td>Keywords</td><td>xii</td></tr><tr><td>Table of Contents</td><td>xiii</td></tr><tr><td>List of Figures</td><td>xvii</td></tr><tr><td>List of Tables</td><td>xxiv</td></tr><tr><td>List of Equations</td><td>xxvi</td></tr><tr><td>List of Appendix</td><td>xxviii</td></tr><tr><td>Abbreviations and Symbols</td><td>xxix</td></tr><tr><td>Abbreviations</td><td>xxix</td></tr><tr><td>Symbols</td><td>xxxii</td></tr><tr><td>Chapter 1 - Introduction</td><td>35</td></tr><tr><td>Chapter 2 - Literature review</td><td>36</td></tr><tr><td>2.1 Paper (Base stock)</td><td>37</td></tr><tr><td>2.2 Pigments and latex</td><td>39</td></tr><tr><td>2.2.1 Clay</td><td>43</td></tr><tr><td>2.2.2 Calcium Carbonate</td><td>45</td></tr><tr><td>2.2.3 Latex</td><td>47</td></tr><tr><td>2.3 Calendering</td><td>54</td></tr><tr><td>2.4 Multilayer coated paper</td><td>59</td></tr><tr><td>2.5 Conclusions and implications</td><td>61</td></tr></table>
Chapter 3 - Discussion and analysis of the characterization methods 64

3.1 Introduction 64

3.2 Contact angle method 67

3.2.1 Contact angle theory 69

3.2.2 Contact angle: test description 70

3.3 Mercury intrusion 71

3.3.1 Darcy's Law 71

3.3.2 Washburn Equation 72

3.3.3 Mercury porosimetry 74

3.3.4 Further considerations on liquid penetration of complex structures 75

3.4 Equipment and methods – Mercury intrusion 77

3.4.1 Repeatability of the mercury intrusion test 81

3.5 Normalisation and optimization of the mercury intrusion data analysis 83

3.5.1 The hexadecane imbibition method and analysis 83

3.5.2 Coating layer cut off point 86

3.5.3 Separation of the precoat and the topcoat layers 90

3.6 Mercury Intrusion normalisation method 93

3.7 Scanning Electron Microscopy (SEM) 106

3.8 Prüfbau DELTACK method 108

3.8.1 Description of the method 108

3.8.2 DELTACK curve analysis 111

3.8.3 Maximum in ink tack force 112

Chapter 4 - Experimental design 114

4.1 Experimental levels selection 114

4.2 Precoated paper roll preparation 116

4.3 Top Coated Samples 118

4.3.1 Control of the top coat weight for the CLC samples 119

4.4 Calendering the top coated samples 120

Chapter 5 - Results and Analysis 121

5.1 Introduction 121
5.2 Effects of 20 parts clay substitution in the precoat layer (Group 1) ........ 123

5.2.1 Analysis of the precoat and topcoat structures: 20 parts clay substitution (Group 1) .................................................................................. 123

5.2.1.1 Analysis of the precoat and topcoat structure through pore size distribution.................................................. 123

5.2.1.2 Influence of the precoat (20 parts substitution) on the topcoat/precoat combined structure ........................ 126

5.2.1.3 Influence of the precoat structure (20 parts substitution) on the topcoat structure ........................................ 130

5.2.2 Printability variations related to 20 parts clay substitution in the precoat (Group 1) .................................................................................. 134

5.2.3 Contact Angle results comparison from Group 1 (20 parts clay substitution) ................................................................................ 141

5.2.4 Summary: 20 parts substitution in the precoat layer (Group 1) .................................................................................................................. 143

5.3 Effect of calendering the base stock or the precoat layer (Group 2) ........ 144

5.3.1 Analysis of the precoat and topcoat structures: calendering base paper and precoated layer (Group 2) ........................................ 145

5.3.1.1 Analysis of the precoat and topcoat structure through pore size distribution.................................................. 146

5.3.2 Printability variations related to precalendering the base stock or the precoat layer (group 2) .................................................. 154

5.3.3 Summary: base paper and precoated layer calendering group (Group 2) .................................................................................. 155

5.4 Effect on Styrene/Acrylic and Protein substitution in precoat layer (Group 3) .................................................................................................................. 157

5.4.1 Analysis of the precoat and topcoat structures through pore size distribution: Styrene/Acrylic and Protein substitution (Group 3) .................................................................................................................. 157

5.4.2 DELTACK results comparison from Group 3 (SA latex and protein substitution) .................................................................................. 164

5.4.3 Summary: Latex and thickener substitution in precoat layer (Group 3) .................................................................................................................. 168

5.5 Effect on 100 parts substitution in the precoat layer (group 4) ................. 168

5.5.1 Analysis of the uncalendered precoat and topcoat structures, pore size distribution: 100 parts precoat pigment substitution (group 4) .................................................................................. 169
<table>
  <tbody>
    <tr>
      <td>5.5.2</td>
      <td>Analysis of the calendered precoat and topcoat structures total pore volume, total pore area, and average pore diameter: 100 parts precoat pigment substitution (group 4)</td>
      <td>173</td>
    </tr>
    <tr>
      <td>5.5.3</td>
      <td>DELTACK results comparison from Group 4 (100 parts pigments substitution)</td>
      <td>175</td>
    </tr>
    <tr>
      <td>5.5.4</td>
      <td>Summary: 100 parts substitution in the precoat layer (Group 4)</td>
      <td>179</td>
    </tr>
    <tr>
      <td>5.6</td>
      <td>DELTACK and ink tack comparison of Groups and latex – Average Curves</td>
      <td>179</td>
    </tr>
    <tr>
      <td>5.7</td>
      <td>Projection on a Latent Structure (Partial least Square) Study</td>
      <td>185</td>
    </tr>
    <tr>
      <td>5.7.1</td>
      <td>General presentation of the PLS method</td>
      <td>185</td>
    </tr>
    <tr>
      <td>5.7.2</td>
      <td>PLS Results</td>
      <td>188</td>
    </tr>
    <tr>
      <td>5.7.3</td>
      <td>PLS Study for 2 components - Effect of precoat pore volume on the final results</td>
      <td>189</td>
    </tr>
    <tr>
      <td>5.7.4</td>
      <td>Effect of the latex</td>
      <td>190</td>
    </tr>
    <tr>
      <td>5.7.5</td>
      <td>PLS study for 3 components</td>
      <td>193</td>
    </tr>
    <tr>
      <td>5.7.6</td>
      <td>PLS study for 4 components</td>
      <td>194</td>
    </tr>
    <tr>
      <td colspan="2">Chapter 6 - Conclusion</td>
      <td>201</td>
    </tr>
    <tr>
      <td colspan="2">Bibliography</td>
      <td>207</td>
    </tr>
    <tr>
      <td colspan="2">Appendix</td>
      <td>216</td>
    </tr>
    <tr>
      <td colspan="2">Appendix 1</td>
      <td>216</td>
    </tr>
    <tr>
      <td colspan="2">Appendix 2</td>
      <td>219</td>
    </tr>
    <tr>
      <td colspan="2">Appendix 3</td>
      <td>226</td>
    </tr>
  </tbody>
</table>
List of Figures

<table>
  <tbody>
    <tr>
      <td>Figure 2-1</td>
      <td>A MSP (left) and blade (right) coated ULWC paper, where a 28 g/m² wood containing base paper is coated with 5 g/m².</td>
      <td>39</td>
    </tr>
    <tr>
      <td>Figure 2-2</td>
      <td>Scanning electron microscopy of talc.</td>
      <td>41</td>
    </tr>
    <tr>
      <td>Figure 2-3</td>
      <td>Pore size comparison for large and small particle size pigments</td>
      <td>42</td>
    </tr>
    <tr>
      <td>Figure 2-4</td>
      <td>Packing efficiency of pigments with two different particle size diameter.</td>
      <td>43</td>
    </tr>
    <tr>
      <td>Figure 2-5</td>
      <td>Packing efficiency for a platy and blocky pigments.</td>
      <td>44</td>
    </tr>
    <tr>
      <td>Figure 2-6</td>
      <td>Coating layer structure with high aspect ratio clay.</td>
      <td>44</td>
    </tr>
    <tr>
      <td>Figure 2-7</td>
      <td>Packing efficiency of precipitated calcium carbonate compared with a blocky pigment.</td>
      <td>45</td>
    </tr>
    <tr>
      <td>Figure 2-8</td>
      <td>Pore structure for a GCC with a broad particle size distribution.</td>
      <td>47</td>
    </tr>
    <tr>
      <td>Figure 2-9</td>
      <td>Pore structure for a PCC with a narrow particle size distribution</td>
      <td>47</td>
    </tr>
    <tr>
      <td>Figure 2-10</td>
      <td>Glass transition temperature (Tg) related to the level of styrene in the latex.</td>
      <td>50</td>
    </tr>
    <tr>
      <td>Figure 2-11</td>
      <td>Film forming during drying process for two different Tg</td>
      <td>51</td>
    </tr>
    <tr>
      <td>Figure 2-12</td>
      <td>Effect of the calendering on the pore structure.</td>
      <td>55</td>
    </tr>
    <tr>
      <td>Figure 2-13</td>
      <td>Water movement for a platy pigment for an uncalendered (left) and calendered (right) coating layer.</td>
      <td>57</td>
    </tr>
    <tr>
      <td>Figure 2-14</td>
      <td>Double coated paper and base stock.</td>
      <td>59</td>
    </tr>
    <tr>
      <td>Figure 3-1</td>
      <td>Water drop with an angle lower than 90° (left) and greater than 90° (right).</td>
      <td>67</td>
    </tr>
    <tr>
      <td>Figure 3-2</td>
      <td>Contact angle θ between a substrate and a liquid</td>
      <td>68</td>
    </tr>
    <tr>
      <td>Figure 3-3</td>
      <td>Droplet of non-wetting liquid on solid surface.</td>
      <td>69</td>
    </tr>
    <tr>
      <td>Figure 3-4</td>
      <td>Droplet on a solid surface at various absorption stages: (a) initial deposition, (b) during absorption, and (c) at equilibrium, end of the absorption.</td>
      <td>70</td>
    </tr>
    <tr>
      <td>Figure 3-5</td>
      <td>Droplet deposit, measurement and final results for angle lower (left) and greater (right) than 90°</td>
      <td>71</td>
    </tr>
    <tr>
      <td>Figure 3-6</td>
      <td>Wetting and non-wetting fluid in a capillary</td>
      <td>73</td>
    </tr>
    <tr>
      <td>Figure 3-7</td>
      <td>Mercury Intrusion equipment</td>
      <td>78</td>
    </tr>
    <tr>
      <td>Figure 3-8</td>
      <td>Sample holder with his stem mounted for the mercury intrusion test</td>
      <td>79</td>
    </tr>
    <tr>
      <td>Figure 3-9</td>
      <td>Paper sample set up for the mercury intrusion test.</td>
      <td>80</td>
    </tr>
    <tr>
      <td>Figure 3-10</td>
      <td>Paper sample in his penetrometer</td>
      <td>81</td>
    </tr>
  </tbody>
</table>
<table><tr><td>Figure 3-11</td><td>Mercury intrusion curves with samples from the same piece of paper</td><td>82</td></tr><tr><td>Figure 3-12</td><td>Example of surface defect affecting the mercury intrusion volume</td><td>84</td></tr><tr><td>Figure 3-13</td><td>Hexadecane absorption by a base paper sample</td><td>85</td></tr><tr><td>Figure 3-14</td><td>Mercury intrusion curves without normalization</td><td>88</td></tr><tr><td>Figure 3-15</td><td>Base paper pore structure with coating structure</td><td>88</td></tr><tr><td>Figure 3-16</td><td>Mercury intrusion curves with base paper blank - cut off point for coating</td><td>89</td></tr><tr><td>Figure 3-17</td><td>Base paper pore structure with coating structure</td><td>89</td></tr><tr><td>Figure 3-18</td><td>Precoated subtraction for the coating structure</td><td>91</td></tr><tr><td>Figure 3-19</td><td>Example of curves with the precoat used as blank in the Autopore program</td><td>92</td></tr><tr><td>Figure 3-20</td><td>Top coating layers for the 3 different latexes with condition 1</td><td>92</td></tr><tr><td>Figure 3-21</td><td>Top coating layers for Standard precoat-100 parts GCC (1.4 µm) and 20 parts clay substitution: High Glossing clay (0.469 µm), No. 2 clay (2.13 µm), delaminated clay (5.312 µm), and High Aspect Ratio clay (6.329 µm)</td><td>93</td></tr><tr><td>Figure 3-22</td><td>Reference file to store the results related to a condition</td><td>94</td></tr><tr><td>Figure 3-23</td><td>Screen to open the mercury intrusion file with the Autopore® program</td><td>95</td></tr><tr><td>Figure 3-24</td><td>Dialog box to choose the desired file to analyse</td><td>95</td></tr><tr><td>Figure 3-25</td><td>Mercury intrusion file while open with Autopore®</td><td>96</td></tr><tr><td>Figure 3-26</td><td>Mercury intrusion file after clicking on advanced button</td><td>97</td></tr><tr><td>Figure 3-27</td><td>Dialog boxes to obtain the results normalised for the base paper</td><td>99</td></tr><tr><td>Figure 3-28</td><td>Dialog boxes to obtain the results normalised for the precoated layer</td><td>100</td></tr><tr><td>Figure 3-29</td><td>Dialog boxes to obtain the results normalised for the uncalendered double coated paper</td><td>101</td></tr><tr><td>Figure 3-30</td><td>Dialog boxes to obtain the results normalised for the calendered double coated paper</td><td>102</td></tr><tr><td>Figure 3-32</td><td>Dialog box to start to generate excel spreadsheet with Autopore® program</td><td>103</td></tr><tr><td>Figure 3-33</td><td>Dialog box used to choose the .smp file to convert in excel spreadsheet</td><td>104</td></tr><tr><td>Figure 3-34</td><td>Dialog box to choose the reports to convert in excel file</td><td>105</td></tr><tr><td>Figure 3-35</td><td>Converted file</td><td>105</td></tr></table>
<table>
  <tbody>
    <tr>
      <td>Figure 3-36</td>
      <td>Save the document on excel spreadsheet form</td>
      <td>106</td>
    </tr>
    <tr>
      <td>Figure 3-37</td>
      <td>Defect in the calendered coating layer</td>
      <td>107</td>
    </tr>
    <tr>
      <td>Figure 3-38</td>
      <td>Defect generated by blade coating</td>
      <td>107</td>
    </tr>
    <tr>
      <td>Figure 3-39</td>
      <td>SEM measurement equipment</td>
      <td>108</td>
    </tr>
    <tr>
      <td>Figure 3-40</td>
      <td>DELTACK measurement instrument</td>
      <td>110</td>
    </tr>
    <tr>
      <td>Figure 3-41</td>
      <td>Example of tack development from a DELTACK test curve</td>
      <td>111</td>
    </tr>
    <tr>
      <td>Figure 3-42</td>
      <td>Example of a complete DELTACK curve</td>
      <td>112</td>
    </tr>
    <tr>
      <td>Figure 3-43</td>
      <td>Truncated DELTACK Curve to fit the Polynomial Curve</td>
      <td>113</td>
    </tr>
    <tr>
      <td>Figure 3-44</td>
      <td>DELTACK Curves means and polynomial curves fitting</td>
      <td>113</td>
    </tr>
    <tr>
      <td>Figure 4-1</td>
      <td>Schematic of the experimental design</td>
      <td>115</td>
    </tr>
    <tr>
      <td>Figure 4-2</td>
      <td>Styrene/Acrylic latex molecule</td>
      <td>116</td>
    </tr>
    <tr>
      <td>Figure 4-3</td>
      <td>Styrene/Butadiene latex molecule</td>
      <td>116</td>
    </tr>
    <tr>
      <td>Figure 4-4</td>
      <td>Schematic representation of the cross section of the final paper samples produced for the study</td>
      <td>119</td>
    </tr>
    <tr>
      <td>Figure 5-1</td>
      <td>Mercury Intrusion in Precoated layer - Precoat 20 parts clay substitution Group</td>
      <td>124</td>
    </tr>
    <tr>
      <td>Figure 5-2</td>
      <td>Mercury Intrusion in Coating structure for condition with Standard Precoat - 100 parts GCC (1.4µm) - uncalendered</td>
      <td>125</td>
    </tr>
    <tr>
      <td>Figure 5-3</td>
      <td>Mercury Intrusion in Topcoating Layer - 20 parts clay substitution group - S/B/ACN latex - uncalendered</td>
      <td>127</td>
    </tr>
    <tr>
      <td>Figure 5-4</td>
      <td>Mercury Intrusion in Topcoating Layer - 20 parts clay substitution group - S/A latex - uncalendered</td>
      <td>127</td>
    </tr>
    <tr>
      <td>Figure 5-5</td>
      <td>Mercury Intrusion in Topcoating Layer - 20 parts clay substitution group - Latex S/A/ACN - uncalendered</td>
      <td>128</td>
    </tr>
    <tr>
      <td>Figure 5-6</td>
      <td>Total Pore Volume (TPV) Uncalendered Paper for Group 1 - Comparing the 3 latexes</td>
      <td>129</td>
    </tr>
    <tr>
      <td>Figure 5-7</td>
      <td>Total Pore Volume (TPV) Calendered Paper for Group 1 - Comparing the 3 latexes</td>
      <td>129</td>
    </tr>
    <tr>
      <td>Figure 5-8</td>
      <td>Average Pore Diameter (APD) Calendered Paper for Group 1 - Comparing the 3 latexes</td>
      <td>130</td>
    </tr>
    <tr>
      <td>Figure 5-9</td>
      <td>Total Pore Volume (TPV) Topcoated layer for Group 1 as a function of the particle size of the precoat layer - Comparing the 3 latexes</td>
      <td>131</td>
    </tr>
    <tr>
      <td>Figure 5-10</td>
      <td>Average Pore Diameter (APD) Topcoated layer for Group 1 as a function of the particle size of the precoat layer - Comparing the 3 latexes</td>
      <td>133</td>
    </tr>
  </tbody>
</table>
<table>
  <tbody>
    <tr>
      <td>Figure 5-11</td>
      <td>Capillary theory related to smaller pores in the precoat. Topcoat: GCC is in purple-blue, clay in light red. Precoat: GCC is in orange and N°1 fine clay is in yellow.</td>
      <td>133</td>
    </tr>
    <tr>
      <td>Figure 5-12</td>
      <td>Preferential movement theory related to bigger pores in the precoat. Topcoat: GCC is in purple-blue, clay in light red. Precoat: GCC is in orange and high aspect ratio clay is in grey.</td>
      <td>134</td>
    </tr>
    <tr>
      <td>Figure 5-13</td>
      <td>DELTACK – 20% clay substitution Group, S/B/ACN latex.</td>
      <td>135</td>
    </tr>
    <tr>
      <td>Figure 5-14</td>
      <td>Time to reach the maximum force function of the maximum force</td>
      <td>136</td>
    </tr>
    <tr>
      <td>Figure 5-15</td>
      <td>Total Pore Volume function Time to reach the maximum force</td>
      <td>136</td>
    </tr>
    <tr>
      <td>Figure 5-16</td>
      <td>Ink film thickness related to an equal ink tack force and the precoat structure: close precoat structure (left) and open precoat structure (right).</td>
      <td>138</td>
    </tr>
    <tr>
      <td>Figure 5-17</td>
      <td>DELTACK maximum force versus total pore volume for 20 parts clay substitution group</td>
      <td>139</td>
    </tr>
    <tr>
      <td>Figure 5-18</td>
      <td>DELTACK maximum force versus average pore diameter for 20 parts clay substitution group</td>
      <td>140</td>
    </tr>
    <tr>
      <td>Figure 5-19</td>
      <td>Contact angle for Coated Paper– 20% clay substitution Group</td>
      <td>142</td>
    </tr>
    <tr>
      <td>Figure 5-20</td>
      <td>Contact angle for calendered Paper– 20% clay substitution Group</td>
      <td>143</td>
    </tr>
    <tr>
      <td>Figure 5-21</td>
      <td>Uncalendered (left) and calendered (right) base stock use in this study</td>
      <td>145</td>
    </tr>
    <tr>
      <td>Figure 5-22</td>
      <td>Mercury Intrusion in Precoated layer – Base Paper and Precoat Calendered Group</td>
      <td>146</td>
    </tr>
    <tr>
      <td>Figure 5-23</td>
      <td>Mercury Intrusion in Topcoated layer – Base Paper and Precoat Calendered Group Separated – S/B/ACN latex</td>
      <td>148</td>
    </tr>
    <tr>
      <td>Figure 5-24</td>
      <td>Mercury Intrusion in Coating Structure – Base Paper and Precoat Calendered Group</td>
      <td>151</td>
    </tr>
    <tr>
      <td>Figure 5-25</td>
      <td>Total pore volume (mL/g) for the standard reference (yellow), calendered base paper (blue) and precoat layer (purple) for the different pressures and 100°C used in calendering.</td>
      <td>152</td>
    </tr>
    <tr>
      <td>Figure 5-26</td>
      <td>Total pore volume (mL/g) for the standard reference (yellow), calendered base paper (blue) and precoat layer (purple) for the different pressures and 200°C used in calendering.</td>
      <td>153</td>
    </tr>
    <tr>
      <td>Figure 5-27</td>
      <td>DELTACK mean for each latex (8 calendering conditions)</td>
      <td>154</td>
    </tr>
    <tr>
      <td>Figure 5-28</td>
      <td>Localisation of the dots for the precalendering, both base paper and precoat, at 100°C and 200°C for the S/B/ACN, S/A, and S/A/ACN latex</td>
      <td>155</td>
    </tr>
    <tr>
      <td>Figure 5-29</td>
      <td>Mercury Intrusion in Precoating layer with 100 parts GCC (1.4 μm) –SA Latex and Amphoteric Thickener (protein).</td>
      <td>158</td>
    </tr>
  </tbody>
</table>
<table>
  <tbody>
    <tr>
      <td>Figure 5-30</td>
      <td>Mercury Intrusion in Coating structure for condition with 100 parts GCC (1.4 µm) and Acrylic Latex - uncalendered</td>
      <td>160</td>
    </tr>
    <tr>
      <td>Figure 5-31</td>
      <td>Mercury Intrusion in Coating structure for condition with 100 parts GCC (1.4 µm) – Amphoteric Thickener (protein) – Uncalendered</td>
      <td>160</td>
    </tr>
    <tr>
      <td>Figure 5-32</td>
      <td>Mercury Intrusion in Calendered Structure with 100 parts GCC (1.4 µm) –S/B/ACN Latex and Amphoteric Thickener (protein)</td>
      <td>161</td>
    </tr>
    <tr>
      <td>Figure 5-33</td>
      <td>Mercury Intrusion in Calendered Structure with 100 parts GCC (1.4 µm) –SA Latex and Amphoteric Thickener (protein)</td>
      <td>162</td>
    </tr>
    <tr>
      <td>Figure 5-34</td>
      <td>Mercury Intrusion in Calendered Structure with 100 parts GCC (1.4 µm) –S/A/ACN Latex and Amphoteric Thickener (protein)</td>
      <td>162</td>
    </tr>
    <tr>
      <td>Figure 5-35</td>
      <td>Total Pore Volume (TPV) Coated Paper for Group 3 - Comparing the 3 latexes. Precoat: all 3 groups 100 parts GCC (1.4 µm and (A): SBR+CMC, (B): S/A+CMC, and (C) SBR+Protein.</td>
      <td>163</td>
    </tr>
    <tr>
      <td>Figure 5-36</td>
      <td>DELTACK – S/A Precoating layer and protein Group, latex, S/B/ACN</td>
      <td>165</td>
    </tr>
    <tr>
      <td>Figure 5-37</td>
      <td>DELTACK – S/A Precoating layer and protein Group, S/A latex</td>
      <td>166</td>
    </tr>
    <tr>
      <td>Figure 5-38</td>
      <td>DELTACK – S/A Precoating layer and protein Group, latex, S/A/ACN latex</td>
      <td>166</td>
    </tr>
    <tr>
      <td>Figure 5-41</td>
      <td>Mercury Intrusion in precoat layer – 100 parts substitution Group. The reference curve GCC is in red.</td>
      <td>170</td>
    </tr>
    <tr>
      <td>Figure 5-42</td>
      <td>Mercury Intrusion in Coating Structure layer – 100 parts substitution Group. Uncalendered samples.</td>
      <td>171</td>
    </tr>
    <tr>
      <td>Figure 5-43</td>
      <td>Mercury Intrusion in Topcoat Structure – 100 parts substitution Group – S/B/ACN latex. Uncalendered samples.</td>
      <td>171</td>
    </tr>
    <tr>
      <td>Figure 5-44</td>
      <td>Mercury Intrusion in Topcoat Structure – 100 parts substitution Group – SA latex. Uncalendered samples.</td>
      <td>172</td>
    </tr>
    <tr>
      <td>Figure 5-45</td>
      <td>Mercury Intrusion in Topcoat Structure – 100 parts substitution Group – S/A/ACN latex. Uncalendered samples.</td>
      <td>172</td>
    </tr>
    <tr>
      <td>Figure 5-46</td>
      <td>Total Pore Volume (TPV) of the calendered precoat/topcoat structure for Group 4 - Comparing the 3 latexes. The samples are ordered in decreasing particle size distribution.</td>
      <td>174</td>
    </tr>
    <tr>
      <td>Figure 5-47</td>
      <td>Total Pore Area (TPA) Coated Paper for Group 4 - Comparing the 3 latexes. The samples are ordered in decreasing particle size distribution.</td>
      <td>174</td>
    </tr>
    <tr>
      <td>Figure 5-48</td>
      <td>Average Pore Diameter (APD) Coated Paper for Group 4 - Comparing the 3 latexes. Samples are ordered as a function of the particle size.</td>
      <td>175</td>
    </tr>
  </tbody>
</table>
<table>
  <tr>
    <td>Figure 5-49</td>
    <td>DELTACK mean for each latex (5 conditions – 100 parts substitution)</td>
    <td>176</td>
  </tr>
  <tr>
    <td>Figure 5-50</td>
    <td>DELTACK maximum force versus total pore volume for 100 parts pigments substitution group- all conditions</td>
    <td>178</td>
  </tr>
  <tr>
    <td>Figure 5-51</td>
    <td>DELTACK maximum force versus average pore diameter for 100 parts pigments substitution group – all conditions</td>
    <td>178</td>
  </tr>
  <tr>
    <td>Figure 5-52</td>
    <td>General DELTACK Comparison curves for the 3 latex</td>
    <td>181</td>
  </tr>
  <tr>
    <td>Figure 5-53</td>
    <td>DELTACK Curve for the control precoating layer – 100 parts GCC (1.4 µm)</td>
    <td>182</td>
  </tr>
  <tr>
    <td>Figure 5-54</td>
    <td>DELTACK Average Curves for the 3 latexes for the 20 parts substitution Group</td>
    <td>182</td>
  </tr>
  <tr>
    <td>Figure 5-55</td>
    <td>DELTACK Average Curves for the 3 latexes for the base paper and precoating layer calendering Group</td>
    <td>183</td>
  </tr>
  <tr>
    <td>Figure 5-56</td>
    <td>DELTACK Average Curves for the 3 latexes for the Acrylic latex and Protein substitution Group</td>
    <td>183</td>
  </tr>
  <tr>
    <td>Figure 5-57</td>
    <td>DELTACK Average Curves for the 3 latexes for the 100 parts substitution Groups - Carbonates</td>
    <td>184</td>
  </tr>
  <tr>
    <td>Figure 5-58</td>
    <td>DELTACK average curves for the 3 latex – Brazilian clay</td>
    <td>184</td>
  </tr>
  <tr>
    <td>Figure 5-59</td>
    <td>2 principal axis on a fish picture</td>
    <td>186</td>
  </tr>
  <tr>
    <td>Figure 5-60</td>
    <td>Graphic showing 2 principal components, the best correlation with Y and the projection on a surface</td>
    <td>187</td>
  </tr>
  <tr>
    <td>Figure 5-61</td>
    <td>PLS Regression – Projection graphic with the +/- 0.2 square low correlation area</td>
    <td>188</td>
  </tr>
  <tr>
    <td>Figure 5-62</td>
    <td>Graphic projection for the Component 1 and 2 without precoated pore volume variable</td>
    <td>191</td>
  </tr>
  <tr>
    <td>Figure 5-63</td>
    <td>Graphic projection for the Component 1 and 2 with precoated pore volume variable</td>
    <td>191</td>
  </tr>
  <tr>
    <td>Figure 5-64</td>
    <td>PLS Result (2 Components) – Graphic for the 3 latex</td>
    <td>192</td>
  </tr>
  <tr>
    <td>Figure 5-65</td>
    <td>PLS Result (2 Components) – Correlation factor R² projection graphic</td>
    <td>192</td>
  </tr>
  <tr>
    <td>Figure 5-66</td>
    <td>PLS Result (3 Components) – Most important variables in order of their importance</td>
    <td>194</td>
  </tr>
  <tr>
    <td>Figure 5-67</td>
    <td>PLS Result (3 Components) – Correlation factor R² projection graphic</td>
    <td>194</td>
  </tr>
  <tr>
    <td>Figure 5-69</td>
    <td>PLS Result (4 Components) – Most important variables in order of their importance</td>
    <td>196</td>
  </tr>
  <tr>
    <td>Figure 5-70</td>
    <td>PLS Result (4 Components) – Graphic projection for the Component 1 and 2 with precoated variable</td>
    <td>196</td>
  </tr>
</table>
<table>
  <tbody>
    <tr>
      <td>Figure 5-71</td>
      <td>PLS Result (4 Components) – Graphic projection for the Component 3 and 4 with precoated variable</td>
      <td>198</td>
    </tr>
    <tr>
      <td>Figure 5-72</td>
      <td>PLS Result (4 Components) – Ink Tack Force</td>
      <td>198</td>
    </tr>
    <tr>
      <td>Figure 5-73</td>
      <td>PLS Result (4 Components) – Time to get the maximum force with DELTACK</td>
      <td>199</td>
    </tr>
    <tr>
      <td>Figure 5-74</td>
      <td>PLS Result (4 Components) – Pore Volume for uncalendered paper</td>
      <td>199</td>
    </tr>
    <tr>
      <td>Figure 5-75</td>
      <td>PLS Result (4 Components) – Pore Volume for calendered paper</td>
      <td>200</td>
    </tr>
    <tr>
      <td>Figure 5-76</td>
      <td>PLS Result (4 Components) – Contact Angle</td>
      <td>200</td>
    </tr>
    <tr>
      <td>Figure 6-3</td>
      <td>DELTACK Average Curves for S/B/ACN latex, S/A latex and S/A/ACN, 100 parts GCC (1.4 µm)</td>
      <td>219</td>
    </tr>
    <tr>
      <td>Figure 6-4</td>
      <td>DELTACK – 20% clay substitution Group, S/B/ACN latex</td>
      <td>219</td>
    </tr>
    <tr>
      <td>Figure 6-5</td>
      <td>DELTACK – 20% clay substitution Group, S/A latex</td>
      <td>220</td>
    </tr>
    <tr>
      <td>Figure 6-6</td>
      <td>DELTACK – 20% clay substitution Group, S/A/ACN latex</td>
      <td>220</td>
    </tr>
    <tr>
      <td>Figure 6-7</td>
      <td>DELTACK – Base Paper and Precoated layer Calendering Group, S/B/ACN latex</td>
      <td>221</td>
    </tr>
    <tr>
      <td>Figure 6-8</td>
      <td>DELTACK – Base Paper and Precoated layer Calendering Group, S/A latex</td>
      <td>221</td>
    </tr>
    <tr>
      <td>Figure 6-9</td>
      <td>DELTACK – Base Paper and Precoated layer Calendering Group, S/A/ACN latex</td>
      <td>222</td>
    </tr>
    <tr>
      <td>Figure 6-10</td>
      <td>DELTACK – S/A Precoating layer and protein Group, latex, S/B/ACN</td>
      <td>222</td>
    </tr>
    <tr>
      <td>Figure 6-11</td>
      <td>DELTACK – S/A Precoating layer and protein Group, S/A latex</td>
      <td>223</td>
    </tr>
    <tr>
      <td>Figure 6-12</td>
      <td>DELTACK – S/A Precoating layer and protein Group, latex, S/A/ACN latex</td>
      <td>223</td>
    </tr>
    <tr>
      <td>Figure 6-13</td>
      <td>DELTACK – 100% Pigment substitution Group, latex, S/B/ACN</td>
      <td>224</td>
    </tr>
    <tr>
      <td>Figure 6-14</td>
      <td>DELTACK – 100% Pigment substitution Group, S/A latex</td>
      <td>224</td>
    </tr>
    <tr>
      <td>Figure 6-15</td>
      <td>DELTACK – 100% Pigment substitution Group, S/A/ACN latex</td>
      <td>225</td>
    </tr>
    <tr>
      <td>Figure 6-16</td>
      <td>Correlation using equation 6-2 and 6-3 and showing correlating latent factor</td>
      <td>228</td>
    </tr>
  </tbody>
</table>
List of Tables

<table><tr><td>Table 2-1</td><td>Monomers used to make S/B latex</td><td>48</td></tr><tr><td>Table 2-2</td><td>Monomers used to make S/A latex</td><td>49</td></tr><tr><td>Table 2-3</td><td>Influence of different monomer latex composition on printing properties</td><td>49</td></tr><tr><td>Table 3-1</td><td>Repeatability of the mercury intrusion test</td><td>82</td></tr><tr><td>Table 3-2</td><td>Surface Tension (mN/m) and Contact Angle (°) of the chemicals used in this study</td><td>84</td></tr><tr><td>Table 3-3</td><td>Hexadecane test results BS 2007-1-1</td><td>85</td></tr><tr><td>Table 3-4</td><td>Base paper hexadecane and pore diameter cut off points</td><td>86</td></tr><tr><td>Table 3-5</td><td>Coating layers cut off points (larger pores)</td><td>90</td></tr><tr><td>Table 3-6</td><td>Autopore® files use to explain the mercury intrusion method. Related to BS1Latex1</td><td>93</td></tr><tr><td>Table 3-7</td><td>Prüfbau Instrument Settings for Sheet Offset and Web Offset</td><td>109</td></tr><tr><td>Table 3-8</td><td>Ink Test Settings for DELTACK Prüfbau test</td><td>109</td></tr><tr><td>Table 3-9</td><td>Prüfbau test settings</td><td>110</td></tr><tr><td>Table 4-1</td><td>Latex characteristics used in the study</td><td>115</td></tr><tr><td>Table 4-2</td><td>Precoated standard formulations and 20 parts substitution</td><td>117</td></tr><tr><td>Table 4-3</td><td>Calendering and Soft Nips Settings</td><td>117</td></tr><tr><td>Table 4-4</td><td>Precoat formulations – Latex and Amphoteric thickener</td><td>117</td></tr><tr><td>Table 4-5</td><td>Precoat formulations – 100 parts substitution</td><td>118</td></tr><tr><td>Table 4-6</td><td>Precoat Operation Parameters - Coater</td><td>118</td></tr><tr><td>Table 4-7</td><td>Topcoat Formulations used with the CLC</td><td>120</td></tr><tr><td>Table 4-8</td><td>Laboratory Calendering Conditions</td><td>120</td></tr><tr><td>Table 5-1</td><td>Precoated formulations related with the group and the condition number</td><td>122</td></tr><tr><td>Table 5-2</td><td>Conditions for the 20 parts clay substitution - Group 1</td><td>123</td></tr><tr><td>Table 5-3</td><td>Mean Contact Angle (°) values for Group 1 – Coated and Calendered Paper</td><td>143</td></tr><tr><td>Table 5-4</td><td>Conditions for the calendering group - Group 2</td><td>145</td></tr><tr><td>Table 5-5</td><td>Average volume structure (mL/g) for the precoat layer (Group 2)</td><td>147</td></tr><tr><td>Table 5-6</td><td>Average pore volume (mL/g) for the topcoat layer for each latex (Group 2)</td><td>150</td></tr></table>
<table><tr><td>Table 5-7</td><td>Conditions for the S/A precoat latex and amphoteric thickener - Group 3</td><td>157</td></tr><tr><td>Table 5-8</td><td>Conditions for 100 parts substitution - Group 4</td><td>168</td></tr><tr><td>Table 5-9</td><td>Maximum ink tack force and time for all conditions for the 3 latexes used in this study</td><td>181</td></tr><tr><td>Table 5-10</td><td>Modeling structure related to the components used</td><td>189</td></tr><tr><td>Table 5-11</td><td>Correlation factor R<sup>2</sup> for 2 components - Results</td><td>191</td></tr><tr><td>Table 5-12</td><td>Correlation factor R<sup>2</sup> for 3 components - Results</td><td>193</td></tr><tr><td>Table 5-13</td><td>Correlation factor R<sup>2</sup> for 4 components - Results</td><td>195</td></tr></table>
List of Equations

$$ \frac{1}{Tg} = \frac{W_1}{Tg_1} + \frac{W_2}{Tg_2} \qquad \text{Equation 2-1} \qquad 51 $$

$$ S = \gamma_s - \gamma_l - \gamma_{sl} \qquad \text{Equation 3-1} \qquad 67 $$

$$ \cos \theta = \frac{\gamma_s - \gamma_{SL}}{\gamma_L} \qquad \text{Equation 3-2} \qquad 68 $$

$$ \gamma_{S-V} + \gamma_{L-V} \cos(\theta) = \gamma_{S-L} \qquad \text{Equation 3-3} \qquad 69 $$

$$ Q = \frac{k \Delta P}{\eta L} A \qquad \text{Equation 3-4} \qquad 71 $$

$$ k = \frac{Q}{A} \frac{L}{\Delta P} \eta \quad (\text{with unit m}^2) \qquad \text{Equation 3-5} \qquad 72 $$

$$ K = \frac{k}{\eta} \qquad \text{Equation 3-6} \qquad 72 $$

$$ F_{out} = 2 \pi r \gamma \cos \theta \qquad \text{Equation 3-1} \qquad 73 $$

$$ P = \frac{F_{out}}{\pi r^2} \qquad \text{Equation 3-2} \qquad 73 $$

$$ P = \frac{-2 \pi r \gamma \cos \theta}{\pi r^2} \qquad \text{Equation 3-3} \qquad 74 $$

$$ P = \frac{-4 \gamma \cos \theta}{D} \qquad \text{Equation 3-4} \qquad 74 $$

$$ D = \frac{712,54}{P} \qquad \text{Equation 3-5} \qquad 77 $$

$$ V_{\text{int}} = V_{\text{obs}} - \delta V_{\text{blank}} + \left[ 0,175(V_{\text{bulk}}^1) \log_{10} \left( 1 + \frac{P}{1820} \right) \right] - V_{\text{bulk}}^1 (1 - \Phi^1) \left( 1 - \exp\left[ \frac{P^1 - P}{M_{ss}} \right] \right) \qquad \text{Equation 3-6} \qquad 78 $$

$$ (\text{MaximumValue} - \text{MinimumValue}) \div \text{Mean} \times 100 \qquad \text{Equation 3-7} \qquad 81 $$
$$W_{\text{absorbed hexa}} = W_{\text{hexa saturated base paper}} - W_{\text{dry base paper}} \qquad \text{Equation 3-8} \qquad 84$$

$$V_{\text{absorbed hexa}} = W_{\text{absorbed hexa}} / 0.77 \qquad \text{Equation 3-9} \qquad 84$$

$$\text{Total Pore Volume} = V_{\text{absorbed hexa}} / W_{\text{dry base paper}} \qquad \text{Equation 3-10} \qquad 84$$

$$y = ax^2 + bx + c \qquad \text{Equation 3-11} \qquad 112$$

$$Y = b_0 + b_1 X_1 + b_2 X_2 + \dots + b_p X_p \qquad \text{Equation 6-1} \qquad 227$$

$$[X]_N^K = [T]_N^A * [P]_A^K + [E]_N^K \qquad \text{Equation 6-2} \qquad 227$$

$$[Y]_N^M = [U]_N^A * [C]_A^M + [F]_N^M \qquad \text{Equation 6-3} \qquad 227$$

$$u_{n,a} = b_a t_{n,a} + h_{n,a} \qquad \text{Equation 6-4} \qquad 227$$
List of Appendix

<table><tr><td>Appendix 1 Mercury Intrusion Test Procedure by Micromeritics and calculating values description</td><td>216</td></tr><tr><td>Appendix 2 DELTACK Curves</td><td>219</td></tr><tr><td>Appendix 3 Projection on Latent structure (PLS)</td><td>226</td></tr></table>
# Abbreviations and Symbols

## Abbreviations

<table><tr><td>3D</td><td>Three Dimensional Space</td></tr><tr><td>AVP</td><td>Average Pore Diameter</td></tr><tr><td>BPSD</td><td>Broad particle size distribution</td></tr><tr><td>CA</td><td>Contact Angle</td></tr><tr><td>CLC</td><td>Cylindrical Laboratory Coater</td></tr><tr><td>CMC</td><td>Carboxy-methyl cellulose</td></tr><tr><td>COV</td><td>Coefficient of variation</td></tr><tr><td>Force</td><td>Maximum force (DELTACK Test)</td></tr><tr><td>GCC</td><td>Ground Calcium Carbonate</td></tr><tr><td>IA</td><td>Image Area</td></tr><tr><td>IGT</td><td>Surface strength printability test</td></tr><tr><td>kN/m</td><td>Kilo-Newton per meter (5.714 PLI per kN/m)</td></tr><tr><td>MPa</td><td>MegaPascal (1MPa=145.03 PSI)</td></tr><tr><td>MPDV</td><td>Median Pore Diameter Volume</td></tr><tr><td>MPDA</td><td>Median Pore Diameter Area</td></tr><tr><td>MSP</td><td>Metering size press</td></tr><tr><td>NPSD</td><td>Narrow particle size distribution</td></tr></table>
<table><tr><td>NIA</td><td>Non-Image Area</td></tr><tr><td>P</td><td>Porosity</td></tr><tr><td>PCA</td><td>Principal Component Analysis</td></tr><tr><td>P_Cal_PC</td><td>Precoating layer calendering pressure</td></tr><tr><td>PCC</td><td>Precipitated Calcium Carbonate</td></tr><tr><td>PLI</td><td>Pounds per linear inches</td></tr><tr><td>PLS</td><td>Projection on a latent Structure or Partial Least Squares</td></tr><tr><td>PPS</td><td>Parker Print Surf – roughness of the sheet</td></tr><tr><td>PrVP</td><td>Precoated layer Pore Volume</td></tr><tr><td>PSI</td><td>Pounds per square inches</td></tr><tr><td>RDP</td><td>Research, Development and Production</td></tr><tr><td>RMS</td><td>Root mean square</td></tr><tr><td>S/A</td><td>Styrene/Acrylic Latex</td></tr><tr><td>S/A/ACN</td><td>Styrene/Acrylic/Acrylonitrile Latex</td></tr><tr><td>S/B</td><td>Styrene/Butadiene Latex</td></tr><tr><td>S/B/ACN</td><td>Styrene/Butadiene/Acrylonitrile Latex</td></tr><tr><td>SBR</td><td>Styrene/Butadiene Latex</td></tr><tr><td>SEM</td><td>Surface electronic microscopy</td></tr><tr><td>Tg</td><td>Glass transition temperature</td></tr><tr><td>Time</td><td>Time need to reach the maximum force (DELTACK Test)</td></tr></table>
TPA Total Pore Area

T Cal PC Precoating layer calendering temperature

TPV Total Pore Volume

TPA Total Pore Area

VP Pore Volume for uncalendered paper

VPC Pore Volume for calendered paper

W Weight
## Symbols

<table><tr><td>A</td><td>filtration area</td></tr><tr><td>b</td><td>flow coefficient</td></tr><tr><td>b</td><td>correlation factor</td></tr><tr><td>C</td><td>concentration</td></tr><tr><td>D</td><td>coefficient of diffusion</td></tr><tr><td>d<sub>eff</sub></td><td>effective particle diameter</td></tr><tr><td>ΔP</td><td>pressure drop, pressure differential (Pa)</td></tr><tr><td>F</td><td>filtration elements</td></tr><tr><td>h</td><td>distance</td></tr><tr><td>H</td><td>residual matrix of error</td></tr><tr><td>k</td><td>permeability</td></tr><tr><td>K</td><td>coefficient of permeability or hydraulic conductivity.</td></tr><tr><td>L</td><td>length or layer thickness</td></tr><tr><td>L<sub>filter cake</sub></td><td>thickness of filter cake layer</td></tr><tr><td>[Matrix]</td><td>symbol for a matrix calculation; the name of the matrix is symbolized by a capital letter.</td></tr><tr><td>P</td><td>applied pressure</td></tr><tr><td>p<sub>e</sub></td><td>additional pressure applied with the capillary pressure</td></tr><tr><td>ΔP/L</td><td>pressure drop across the sample length (L)</td></tr><tr><td>Q</td><td>flow</td></tr><tr><td>Q<sub>liquid</sub></td><td>amount of liquid phase removed through dewatering</td></tr></table>
r capillary radius

W circumference of the ring (µm, around 10 cm)

Q air flow (µm³ *s⁻¹)

Ra roughness average

r_h hydraulic radius

Rq root mean square (RMS) roughness

Rt maximum peak to valley height

R_pps Parker print surf roughness of paper (µm)

S spreading coefficient

S area of the porous medium cylinder defined by πR² (R is the radius)

t time

u and t vector reducing number of planes between x and y

V IMB volume of ink imbibed by the layer

V_b corresponding bulk volume

X flow direction

θ contact angle of the mercury on the pores (usually 130°-140° for paper)

μ dynamic viscosity (in this case, the coating color viscosity)

μ_f pore coefficient

μ_l path coefficient

φ average mass flow

Φ fractional void volume

ξ porosity

Ψ Kozeny constant
ρ fluid density

μ air viscosity at the measurement temperature (PA*s)

γ surface tension

γs superficial tension of the solid (free energy of the solid)

γL superficial tension of the fluid (free energy of the fluid)

γSL interfacial tension between the liquid and the solid.

γHg surface tension of mercury (usually 480-485 mPa·s)
# Chapter 1 - Introduction

The need to improve paper properties is always what consumers request from the print-
ers and the paper producers. As coated paper is a big part of the improvement, there is a
need for high performance coatings. Coatings are applied to board and paper to provide
a smooth surface and to improve the printing snap, often defined as the overall impres-
sion of the print quality as perceived through human eyes in the pressroom. Most stud-
ies are performed to identify the best pigment-binder combinations and thus obtain the
right properties required by the papermaker for operations. The type and the amount of
the various coating color components affect the mechanical, optical and liquid absorp-
tion characteristics of the coated paper in a very complex manner. Considering printing
characteristics, it is why the biggest challenge for the researchers still remains to find an
optimum balance of paper properties for an ink droplet to provide a sharply defined
printed dot on the coated paper surface. The droplet needs to have an optimal
spread/penetration ratio [1] and to set fast, i.e. within the runnability limits of the print-
ing process in the coating structure. To achieve such ink setting properties, the coating
color particles need to be located close to the paper surface [2]. This is particularly true
for the offset printing process where a large amount of ink and water, up to 300% ink
coverage through 4, and more, ink layer applications, are added to the paper during
printing. A properly formulated coating offers several options and the papermaker is
continuously looking for new ways to take advantage of formulation options to improve
properties such as brightness, opacity, smoothness, surface strength, ink setting, ink dry-
ing, print gloss, etc [3]. The type of coating technology used also increases the chal-
lenge to meet new operation and quality objectives. For example, the runnability of high
solids pigmented coating colors on the metering size press depends on a complex bal-
ance between rheology and water retention [4-7]. The pulse dynamics generated by the
nip in a metering size press or a roll applicator acting on coating colors play an impor-
tant role in film splitting phenomena. Therefore, the development strategy for the meter-
ing size press requires a new approach to create a necessary balance between dynamic
water retention and rheology when using the new high viscosity (1000-1200 poise or
Pa.s⁻¹) formulations [8].
Chapter 2 - Literature review

One common way to increase the print quality of a paper sheet is through surface treat-
ment, either coating and/or calendering. Papers designed to improve image sharpness
and brightness need to be coated, i.e. one needs to apply a thin layer of aqueous suspen-
sion containing mineral pigments and chemicals over a base sheet. This suspension is
dried after application, the drying method contributes to the characteristics, either good
or bad, of the dry thin, actually micrometers thick, layer.

In the last 2 decades, several studies tried to explain the factors that control the printabil-
ity of the coated paper. The pore structure of paper coatings that could be modified us-
ing a range of different mineral pigments and binder contents, has been found by many
researchers to have a significant effect on the coated paper printing characteristics [9].
The first identified paper structure parameter was the architecture of the pore structure
for the base paper and the coating layer which together drive the final print properties.
Most physical and functional properties of pigment coated papers are influenced to a
greater or lesser extent, by the porous structure of the coating layer. For example, the
number and the size of microvoids contribute to macroscopic properties such as bright-
ness, light scattering efficiency, and opacity. The void structure also affects the flow of
liquids and gases through the coating layer. Absorption of inks and fountain solutions
during printing is controlled by the structure of the pore space close to the coating layer
surface. The porous structure of a coating layer depends on the coating color formula-
tion, pigments and their combinations, synthetic and natural binders, chemical additives
(e.g. associative thickeners), coating color application, and the drying processes [10].
Therefore, it is obvious that the coated paper structure can be considered as a combina-
tion of fibres, pigment(s), binder, and air volume. The air volume is directly influenced
by the others parameters. A wide range of control of the coating pore size is possible
using narrow particle size distribution kaolins and carbonates. Tuning the pore diameter
to a desire target enables to control the print properties while still maintaining the de-
sired gloss and optical properties. Once pore diameter and volume are controlled to the
targeted, desired range, optimizing the binder package will further enhance the proper-
ties. While the pigments can create exceptional value to the final coated paper, mis-
matched pigments may negatively affect optical performance and result in a poor print
performance [11]. We may conclude that the pore structure of the surface layer will
determine both the optical and print performance of the coated paper [12]. It is however
important to understand how the structure may be affected by the size and shape of the
components in the coating layer and how they are packed together [12].

The literature analysis is then closely related to the pore structure or network: a concept
with its own terminology. To discuss pore structure, two terms needs to be defined;
skeleton and skeletal element, as was done by Gane [13] : ‘’If you think about a porous
structure, what do you see? Imagine you are walking through the structure as you might
walk through a network of caves underground. You perhaps see big rocks, sands and
soil made walls of the cave. Perhaps you see other caves and channels on either side or
even above and below. You are walking around in the pores; the solid material between
the pores is the skeleton of the structure. Each part of the skeleton (big rocks, sands)
between the pores/caves is a skeletal element. In other words, if you know the pore
structure (the pore elements) you then need to know the solid structure to determine how
the pores are arranged in space. The arrangement of the solid structure is known as the
skeleton, and each part between the pore elements is a skeletal element’’.

## 2.1 Paper (Base stock)

The application of a coating layer is a complex process dependent of many interactions
between the components and the process. Notwithstanding all factors, the application
must still be achieved on a paper surface designed to adequately receive a layer of coat-
ing. The coating layer does not and will not hide a defect (Figure 2-1) or enhance the
poor quality of the base paper. Base paper characteristics are very important to obtain a
high quality final product and insure good printability [14, 15]. As an example, Al-
Turaif [16] shows that gloss increases and roughness decreases when the substrate is
changed from absorbent to non-absorbent. Another important aspect is the filler content
in the base paper. Increasing the base paper filler content improves surface roughness,
print gloss, paper gloss, and has a positive effect on print mottle [17].
The tendency to increase the filler level was initiated many years ago. Nowadays, it becomes more and more important when considering the price of the fibres. The two sidedness of the base paper may become critical, especially when reducing the coatweight and consequently, the possibilities for coat weight adjustment between both sides of the sheet [6]. Another aspect, especially for wood free papers, is the sizing of the base stock. Huang and Lepoutre [18] report that sizing is added to control the penetration of the fluids in the base sheet but, if absorbency is changed by hydrophobic sizing, just a small effect may be observed. However, if pore size and void fraction (pore volume) are modified, the effect on absorbency is large. Overall, hydrophobic sizing improves coated gloss slightly on dense base sheet, but not on porous sheets. It does not have an effect on coated roughness, irrespective of base sheet density. The fluid penetration causing the base paper roughening under the influence of moisture and the hydraulic of fluid penetration are a function of the dwell time and dewatering characteristics. They have been shown to be dominant factors with respect to modification and coverage in the final coated paper properties [19].

Physical coverage of a porous base paper is a complex function of the application method, solids concentration, coating formulation, pigment particle size, and shape distribution, including interactions with the void dimensions, the anisotropic rheology and the drainage during dewatering [20]. Another article by Allem and Uesaka [21] finds that for LWC papers, the coating thickness is primarily affected by the base sheet roughness. They show that other coating parameters are also related, albeit to a lesser extent, to the coating thickness.

It is important to recognize the distinction between micro and macro roughness when considering a base paper problem. Gane and Hooper [22] show that the macro roughness is primarily associated with initial base sheet roughness, surface void structure, and fibre compaction. The macro profile is in the important region related to coating coverage. Micro profile is related to the structural packing of the pigmented coating layer and is determined by the pigment size and shape distributions, and by the interactions between coating color and base sheet.
<figure><img src="image_2.png" /><figcaption>**Figure 2-1** A MSP (left) and blade (right) coated ULWC paper, where a 28 g/m² wood containing base paper is coated with 5 g/m².</figcaption></figure>

The structure of the coating layer influences the macro roughness through the effective coating holdout and coverage associated with coating bulk and void bridging [22]. Base paper fibres, however, do not constitute a static interlocking network but introduce dynamic and chemicals interactions, particularly with the fluid component of the coating color. Acting together, such phenomena collectively control the overall absorbency of the base sheet, which in conjunction with the dewatering rate of the coating color, determines surface coverage, coating holdout, and roughness of the coated sheet for a given pigment system [22].

Therefore, control of the absorbency of the base paper is an important parameter because the relaxation of fibre stress occurs during the wetting generated by the coating color application. The relaxation phenomenon originates from the base sheet formation and/or any surface treatment performed on the base stock prior to the application of the coating layer. A good water holding, i.e. slow dewatering, combined with a fast immobilization limits the extent of the surface relaxation of the base sheet.

## 2.2 Pigments and latex

The coating layer applied on the base stock or another coating layer is mainly produced with minerals pigments. The pigments are the products of mining exploitation or are manufactured under controlled industrial process conditions. Today some pigments combine the two processes: they are called “engineered pigments”. Many pigments are
commercially available. Choosing the right one is a complex process where several factors must be considered to achieve the quality needed for a specific coated paper.

Poor dispersion of pigments in a coating color recipe is known to lead to problems such as blade scratches, excess rejects at screen, rheological fluctuations, and runnability disturbances at the coater [23]. It is however related to the process itself and fundamentals drive the final behaviour in a pressroom. Lee [24] shows that pigment particle size distribution, particle shape, binder type (film shrinkage), drying temperature and time, pigment wetting, coating holdout (especially at low coating weights), and varying coating weights contribute to micro scale roughness. The micro scale variations are partly responsible for instabilities, variations during the printing process. The characterization of the coating layer and understanding the mechanisms and interactions of the layer various parameters is a key factor to improving the printing characteristics and the printing quality.

Several studies designate the pore structure as the key factor, or at least, a very important aspect of the coating layer structure. On the one hand, Di Risio and Yan [25] find that pigment particle packing, one of the most important pigment properties, governs the coating pore structure, which in turn affects the optical, mechanical, and printing qualities of the coated paper.

On the other hand, David and Bertrand [26] report that the particle size and shape distributions control the packing ability of pigments; these parameters thus impacting the resulting coating structure. In addition, a relationship between the shape factor of the different pigments and the flow rate, as related to coating movement, in conjunction with the pore structure is rather straightforward. For high shape factor pigments, example talc (Figure 2-2 [27]), the coating penetration is small and does not depend on rheology.

It suggests that high shape factor pigments can block pores to reduce coating penetration. For moderate shape factor pigment, the results indicate that slowing the absorption of the water phase into the paper helps reduces coating penetration [28].
<figure><img src="image_3.png" /><figcaption>**Figure 2-2** Scanning electron microscopy of talc</figcaption></figure>

All of the concepts previously discussed indicate that it is feasible and possible to tailor-
make coating structures [29]. Coating structure design may be achieved by modeling
the pigment particle size and particle size distribution. The particle size determines the
paper gloss, while the pore size is determined by both the particle size and the distribu-
tion (Figure 2-3). More specifically, decreasing the particle size increases gloss and
decreases pores size, but it has only a minor influence on porosity. The last assertion is
only true if the particle size distribution is kept constant, which is unlikely to happen if
the particle size is changed by a modification of the pigments in the formulation. Gen-
erally, as the particle size decreases, the distribution narrows.

However, decreasing the particle size distributions increases pore size and porosity. The influence on paper gloss is minor or may be considered negligible [3]. The pore structure, as previously described, controls the porosity of the coating layer. The porosity of the coating layer itself can be controlled through modification of the pigment size and morphology, the binder type and level, and inter-particle interaction. The morphology of the particles is here related to particles with a regular shape as such type of particle packs tighter, closer than irregularly shaped particles [30].
<figure><img src="image_4.png" /><figcaption>**Figure 2-3** Pore size comparison for large and small particle size pigments</figcaption></figure>

Previous considerations indicate the importance of understanding the coating structure and how the coating layer interacts with the base sheet and the coating process itself. It is well established that gloss is dependent on surface roughness and pore structure [31]. Therefore, these parameters must be considered upon designing a new coating or improving an existing one. To build a coating structure in accordance with our goals, the coating layer has to be designed with a thorough understanding of all parameters which affect its structure. Larsson and Engstrom [32] find that the thickness of the coating layer is governed by the pigment packing in the final dry coating layer, which in turn is governed by the particle shape and particle size distribution of the pigment and by the interactions between the constituents of the coating color. Pigments with narrow particle size distribution are known to give superior coverage. Okomori and Lepoutre [33] also show that the pigment size distribution and the pigment shape influence pigment packing. The only parameter with no effect is pigment surface area which does not affect the coating pore volume to any great extent [34]. Of course, the last statement is heavily dependent on what type of pigment, and what will be the change in coating morphology.

The shape, size and size distributions also affect the optical properties of the coated paper. Devisetti and Malla [35] investigating coarse pigments find that a high coated sheet brightness can be obtained despite the fact that the lower pigment brightness than that of other kaolins. This is probably due to the improved coverage of groundwood base stock with the use of coarse pigments.

The role and the way each kind of pigment behaves are pigment specific. Each pigment has unique morphologies, unique crystal structures. The ways a pigment and its morphology interact with the other components has a strong influence on the coated layer structure (Figure 2-4). For example, the benefits of a plate like particle morphology has
been frequently described in terms of contour-like coverage in lightweight coating, whereas blocky particles tend to fill surface voids and leads to uneven coverage [36]. Finally, all the differences among the pigments and certain orders of mixing components can result in a significant change in the shear viscosity and/or viscoelastic parameters characterizing the colors. Such interactions will influence the rheological properties of the coating color, and changes in the rheological behaviour may have consequences for the runnability of the color [37] in the costing operation.

The pigments can be separated into four groups; clay, carbonate, titanium, and special pigments. The 2 first groups are covered by many different studies because these pigments are, and have been, widely used for coating paper.

<figure><img src="image_5.png" /><figcaption>Figure 2-4 Packing efficiency of pigments with two different particle size diameter</figcaption></figure>

### 2.2.1 Clay

Clay is a pigment that has been used in the coating for more than 100 years in the paper industry. It is mined around the world and the main producing areas are mainly England, Georgia in the United States of America, and Jari River district in Brazil. All these kaolins have their specific characteristics. They are used because they are locally available or because they have specific characteristics permitting to meet specific coating layer structures. Clay coating colors usually exhibit higher viscosities than ground calcium carbonate (GCC), for a given pigment concentration. The difference is mainly due to different particle morphologies. The platelet morphology of kaolinite confers poor
packing ability but, at the same time, good coverage properties of the base paper (Figure 2-5) [38].

<figure><img src="image_6.png" /><figcaption>**Figure 2-5** Packing efficiency for a platy and blocky pigments</figcaption></figure>

The pore volume of the clay coated layer is reduced almost twice as much for the clay coating as for the GCC coating. The phenomenon is attributed to the greater capability of platy clay (Figure 2-6) to reorient and rearrange under compression than the isometric GCC particles [39]. Furthermore, the coarser kaolin particles seem to produce bigger pores than produced by the finer kaolin particles. For particles of a similar median size, narrow kaolin distribution generates pore distributions with more uniform pore sizes than generated by broader clay particle distributions [25].

<figure><img src="image_7.png" /><figcaption>**Figure 2-6** Coating layer structure with high aspect ratio clay</figcaption></figure>

Di Risio and Yan [25] report that coarse clays particles produce coating layers with a similar number of pores but with a larger pore size than finer clay. For the same particle size, clay pigments with a broad particle size distribution produce a low pore volume. Notwithstanding the clay source, the effect of varying the aspect ratio from low to high is studied by Gane [40, 41] who finds that English platy clay runs clean at high speed,
without interactive binders and water retention aids, whereas the less water-retaining US clays and coarse ground carbonates requires some controlled level of water retention aid. The problem, thus, arises that many water retention aids and natural binders used today create an increased viscoelasticity in the coating color by interacting with pigments and creating depletion and flocculation together with partially or strongly associated water. These interactions will result in a predicted effect of relaxation induced dilatancy – dry stalagmites, dusting, scratching etc.

### 2.2.2 Calcium Carbonate

Calcium carbonate is a very useful common coating pigment. The calcium carbonate pigments are separated in 2 major groups; either ground (GCC) or precipitated (PCC). Ground carbonate or GCC comes from chalk, limestone or marble which is the primary GCC source around the world. PCC is manufactured under controlled conditions to produce specific particle size, shape, and particle size distribution. Both forms of calcium carbonate are used to “build structure” in the coating layer. PCC enhances pore volume, but the average pore diameter is somehow similar when PCC and GCC have a similar size distribution (Figure 2-7). Coarse GCC increases the average pore diameter significantly, but pore volume and porosity decrease. Coating pore structure also impacts optical properties and the printability of low gloss coated paper[42].

<figure><img src="image_8.png" /><figcaption>**Figure 2-7** Packing efficiency of precipitated calcium carbonate compared with a blocky pigment</figcaption></figure>

Yulong and Zhenlei [42] show that brightness depends strongly on the pore diameter of coating structure and that coatings with relatively large pore size enhance light scattering
and opacity. Sheet gloss is reduced significantly with a roughness increase, as measured with the Parker Print-Surf (PPS). Print gloss and delta gloss are improved with an appropriate balance between pore sizes and pores volume. Increasing average pore diameter and decreasing pore volume enhance print gloss and the gloss variation (delta gloss). Larsson and Engstrom [32] find that porosity of the final coating layer (GCC) is not controlled by the network structure, but by the packing of the aggregates and shrinkage during consolidation of the coating layer. The pigment packing is in turn controlled by the relative plastic viscosity of the suspension, by the particle size distribution of the pigments and its stabilization upon drying. Knappich et al [43] find a similar behaviour for GCC with narrow particle size distribution resulting in a coating structure with a large void diameter and a high total void volume. Therefore, the optical and physical properties of the coated paper are significantly improved. Fukui [44, 45] investigating blends of coarse and fine ground carbonate (GCC) in several ratios finds results in accordance with Osterhuber [44, 45]. Osterhuber, McFadden and Roman [29, 45] show that the pore volume, measured with mercury intrusion, remains relatively constant when particle size is varied within 0.45 - 0.85µm. With a particle size above 0.85µm, the pore volume decreases slightly, which could indicate an improved, almost ideal packing. The lowest pore volume is obtained using 100% coarse carbonate, while addition of fine carbonate increases the pore volume.

The type of calcium carbonate particles and the mixing ratio with other pigments directly determine how the structure of the coated paper will be developed. A carbonate with a broad distribution results in dense particle packing (Figure 2-8) which closes the coating. Conversely, the narrow particle size distribution of PCC generates a coating structure that contains substantially larger pores and higher pore volumes (Figure 2-9) than a broad particle size distribution GCC [45]. In addition, reducing [11, 46, 47] the particle diameter reduces the pore diameter. Works from Hagemeyer suggest that organic dispersant plus the chemical nature of the pigment have an even larger effect than the shape itself. Another aspect of the PCC is how it affects the final paper properties. Al-Turaif shows that introducing a prismatic shaped PCC pigment into the plate like kaolin pigment coating causes a gloss decrease and a roughness increase. Furthermore, adding PCC particles upsets the packing thus increasing the pathways within the coating
and subsequently increasing the movements of latex particles toward the surface [16]. The calcium carbonate is also a good pigment to control the blistering resistance. Okomori and Lepoutre [33, 48] find that using 100 parts CaCO₃ is considerably better for blistering resistance than using 100 parts clay. The blistering of coated paper depends greatly on the properties of the coating layer near the base stock surface.

<figure><img src="image_9.png" /><figcaption>Figure 2-8 Pore structure for a GCC with a broad particle size distribution</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 2-9 Pore structure for a PCC with a narrow particle size distribution</figcaption></figure>

### 2.2.3 Latex

The latex is an important constituent of a coating structure. The pigments create the structure and the latex keeps the structure holding together after drying in the coating process. Latex has a complex chemical structure. It is composed of particles and liquid forming an emulsion. The type and particle size of the latex give the final properties of
the film formed when the sheet is dried. The latex is one of main components used to increase the strength of the coating structure. The type and particle size of the latex also affect the coating structure. It is the reason why coating strength increases as latex content increases and, at a given latex content level, it is why coating strength increases when pigment particle size distribution increases.

Latex emulsion is a dispersion of spherical particles (100-300 µm) in water. A latex solution is always produced by reaction with 2 or more different monomers. The lists of possible monomers for the two main groups of latex used in paper industry are listed in table 2-1 and 2-2 [49]. Latex always contains styrene monomer combined with one or more other monomers. Each latex is generally combined with other chemical components e.g. carboxylic acid (-COOH), amide (CONH₂), hydroxyl groups (-OH).

<table><caption>Table 2-1 Monomers used to make S/B latex</caption><thead><tr><th>Styrene / Butadiene Latex</th><th>Basic chemical group</th></tr></thead><tbody><tr><td>Styrene</td><td>CH<sub>2</sub> = CH = CH<sub>2</sub></td></tr><tr><td>Butadiene</td><td>CH<sub>2</sub> = CH - CH = CH<sub>2</sub></td></tr><tr><td>Acrylamide</td><td>CH<sub>2</sub> = CH - C - NH<sub>2</sub></td></tr><tr><td>Acrylic acid</td><td>CH<sub>2</sub> = CH - C - OH</td></tr><tr><td>Acrylonitrile</td><td>CH<sub>2</sub> = CH - C = N</td></tr><tr><td>Itaconic acid</td><td>CH<sub>2</sub> = C - C - OH<br>CH<sub>2</sub> = C - OH</td></tr></tbody></table>
The choice of the other monomers is related to the end use of the product and the desired end-properties. The general rule of thumb in the pulp and paper industry is that the S/B binders are mainly comprised of styrene and butadiene. In addition, acrylic binders have a wider variety of composition than S/B binders. The key parameters for binder performance are monomer composition, glass transition temperature Tg, gel content, and particle size.

<table><caption>**Table 2-2 Monomers used to make S/A latex**</caption><thead><tr><th>Acrylic Latex</th><th>Basic chemical group</th></tr></thead><tbody><tr><td>Styrene</td><td>CH<sub>2</sub> = CH<sub>2</sub></td></tr><tr><td>N-Butyl - acrylate</td><td>CH<sub>2</sub> = CH - C - O - (CH<sub>2</sub>)<sub>3</sub> - CH<sub>3</sub></td></tr><tr><td>Methyl acrylate</td><td>CH<sub>2</sub> = CH - C - O - CH<sub>3</sub></td></tr><tr><td>Ethyl acrylate</td><td>CH<sub>2</sub> = CH - C - O - CH<sub>2</sub> - CH<sub>3</sub></td></tr><tr><td>Acrylonitrile</td><td>CH<sub>2</sub> = CH - C = N</td></tr></tbody></table>

The choice of the monomers has a great impact in the final performance of the latex. The influence of the monomer composition for latex is listed in table 2-3. The table shows the printing properties for the most important monomers used in industry coating processes. Styrene is the monomer present in all latex used in coating industry. The glass transition temperature of the styrene is therefore an important factor. The relation between the percentage of styrene and the Tg is defined in Figure 2-10 [49].
<table><caption>**Table 2-3** Influence of different monomer latex composition on printing properties</caption><thead><tr><th>Butadiene</th><th>n-Butyl-acrylate</th><th>Styrene</th><th>Acrylonitrile</th></tr></thead><tbody><tr><td>Elasticity, flexibility</td><td>Elasticity, flexibility</td><td>Stiffness</td><td>Stiffness</td></tr><tr><td>Fold endurance</td><td>Fold endurance</td><td>Gloss</td><td>Solvent resistance</td></tr><tr><td>Bond Strength</td><td>Bond Strength</td><td>Porosity</td><td>Bond Strength</td></tr><tr><td colspan="4">Resistance to UV</td></tr><tr><td colspan="4">Print evenness in offset</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>**Figure 2-10** Glass transition temperature (Tg) related to the level of styrene in the latex</figcaption></figure>
Therefore, it can be concluded that the lower the Glass transition temperature (Tg), the softer the latex. The Tg of a latex is calculated by the Gordon-Taylor equation (equation 2-1) [49] as follow:

$$ \frac{1}{Tg} = \frac{W_1}{Tg_1} + \frac{W_2}{Tg_2} \qquad \text{Equation 2-1} $$

Where

Tg = glass transition temperature of the monomer

W = weight fraction of the monomer

The particle size influences bond strength, ink gloss, blister resistance, and gloss. Increasing the particles size of a latex negatively affect (decrease) the two first parameters (bond strength and ink gloss) while the two last one (blister resistance and gloss) are positively affected (increase). The glass transition temperature Tg is important to evaluate the power needed in the drying process to make the film upon surface application. Figure 2-11 [49] shows how the film is formed with different drying temperatures for two different latexes with different glass transition temperatures Tg.

<table><caption>Figure 2-11 Film forming during drying process for two different Tg</caption><thead><tr><th rowspan="2">Tg temperature</th><th colspan="2">Drying temperature</th></tr><tr><th>30 °C</th><th>80 °C</th></tr></thead><tbody><tr><td>28 °C</td><td></td><td></td></tr><tr><td>6 °C</td><td></td><td></td></tr></tbody></table>
A low specific area of the broad distribution pigments in a coating indicates a low pro-
portion of unbounded pigment surface [33]. The mechanical properties of the coating
layer are greatly affected by the mechanical properties of the binder [50]. The mechani-
cal properties of the coating layer will thus depend on the cohesion of the latex bridges
formed between the pigments particles and the void structure of the coating layer. For
example, stiff latex should give strong bridges and prevent or reduce shrinking during
drying and consolidation phase; therefore resulting in an improved porous structure [50].
The stiffness, as a latex property, is neither good nor bad. It must be aligned with the
final sheet property required by the customer. An interesting characteristic used to
evaluate the mechanical properties of a latex is still the glass transition temperature,
"Tg", Kugge et al [51] demonstrated that above Tg the polymer chains are considerably
more mobile, enabling the latex particles to easily deform and readily film-form during
drying, than below Tg. A low Tg latex greatly retards the film formation during the dry-
ing process. The differences in latex mechanical properties influence the mobility of the
latex particles during the drying process and therefore strongly influence the distribution
of latex within the coating [24, 52]. The latex binder Tg has therefore a considerable
influence on the latex behaviour inside the coating layer. For example, high Tg latex is
known to migrate more easily to the top surface during drying [53] than low Tg latex
Binder migration to the top layer during drying is effective resulting in a closed, smooth,
high gloss top layer [51]. The amount of latex strongly influences the properties of pig-
ment coating. Sufficient latex is required to confer mechanical strength to the coating
layer. Excess addition of latex has adverse effects on a range of optical and physical
properties.

Theoretical models often propose that latex particles are well dispersed through the wet
coating layer and form a film including all pigments particles during the drying phase.
However, Hiorns et al [12] show that the latexes do not necessarily form a film at the
surface of the pigments. The latex plays an important role in the structure of the coated
sheet. Carte et al [11] show that despite the large change in coating sheet properties,
little change occurs in the pore structure of the coating, relatively to a change in the latex
used in the formulation. This would indicate that chemical effects of the latex might
have a dominant effect on the final properties of the coating.
Another important latex concept to consider is how the latex particles behave in a solu-
tion with other pigments particles. The size of the latex particles is very small compared
to the majority of the pigment particles. The small latex particles increase the packing
volume of the coating color. Therefore, the amount of binder impacts the structure of
the coating and affects the porosity of paper coatings. This is confirmed by Knappich et
al [43] where latex particles replace pigment fines in the wet coating. Both coating pore
volume and average pore diameter are reduced when fines content and binder level are
increased. Malla et al [54] confirm that pore volume is mostly affected by binder level
while pore size is primarily affected by the amount of fines in the pigment. Finally Au-
dette and Hiorns et al [12, 55] show that the use of hard latex, with increased Styrene in
S/B latex, in the coating process usually provides benefits in terms of coating smooth-
ness and hence sheet gloss. It is here important to consider the latex melting in the dry-
ing process. The impact is no longer comparable to the one of a hard sphere-like particle
but to a liquid forming a film and filling the pore network.

The latex alone, or added to other chemicals, is known to influence the coating structure.
Several papers mention the different ways how latex behaves in the coating structure.
Lepoutre [56] reports the now well known effect that binders and thickeners affect the
coating structure during coating layer consolidation. Larsson and Engstrom [32] agree.
However, when using carboxy-methyl cellulose (CMC), they find that the CMC inter-
acted with both, GCC-pigment and the latex, to build the network structure. Wallström
and Järnström [57] find that at low binder levels, coating structure is mainly controlled
by the packing of pigment particles. In addition, the binder redistribution may impact
the pore structure. The binder redistribution is defined by two different concepts; the
binder migration to the surface and the binder penetration into the paper. This is con-
firmed by Matsubayashi et al [48] who find that high concentration of latex occurs at the
coating-base stock interface. It also indicates that latex tends to migrate into the base
stock as well as into the coating structure. On this basis, pigments which increase the
porosity of the coating should require more binder. Dimmick [58] believes such a
statement is not supported by all researchers and might not be necessarily true as the
binder does not completely cover the surface of the pigments. On the one hand, it is
more like a spot welded theory where the binder welds the point of contacts between the
pigment particles. As a consequence, increasing the porosity does not necessarily increase the binder demand to keep the strength of the coating layer. Indeed, the spot welding effect does not automatically decrease. On other hand, and over a level of porosities, the statement is validated. However, Husband *et al* [59] indicate that such pigment types may lose binder readily into the base paper, which in turn might weaken the coating strength while, paradoxically, strengthening the coating – base sheet interface.

Identifying the right type of latex for a given application is important. For example, in a study of the effect of binder on rotogravure printability Fernandez *et al* [60] and Devisetti [35] show that soft, vinyl-acrylic latex provides optimum printability. Laudome *et al* [61] complete the information in an analysis of the different behaviour of latex and starch. They show that shrinkage during drying with starch-based formulations causes stresses far higher than those occurring in latex-based formulations.

## 2.3 Calendering

Paper has been calendered since the paper machine invention by Nicolas Robert over 200 years ago. The calendering has evolved to become a very important part of the final quality of uncoated and coated paper. Today, calendering is a main factor for the improvement of the surface of both paper and printing quality. As the factors controlling the surface properties are numerous and complex, a thorough understanding of the relationship between base stock, coating color and type of calendering is essential. It is well accepted that producing a good quality for printing requires a smooth and uniform surface. Coating application and subsequent calendering greatly improve the paper surface characteristics but, in counterpart, both operations must be performed according to the specifications and characteristics required for the end product and the end user. When coating layers are superposed, added one over the other as it is done for double coated and triple coated papers, the final product quality is significantly enhanced. During coating, the liquid phase in the coating color comes into contact with the base sheet. The fluid absorption by the fibre network and the fibres themselves increases both the thickness and roughens the surface of the base sheet. The increase in surface roughness of the base sheet may affect the coating mass distribution and so requires more calender-
ing energy to attain the desired smoothness [14]. When the base stock is precalendered, the entire effect of precalendering is lost due to the release of intra-fibre stresses. As the fibres come into contact with the water contained in the coating color, they have a tendency to recover their original shape: therefore the roughness of both the fibre surface and by consequence of the paper surface tends to increase [22, 62]. The effect is well documented and observed for light weight coated (LWC) paper grades [63].

When a calendering load is applied to a paper surface, most of the desired smoothening effect of the surface is already achieved at 110 kN/m. A further increase of the loading pressure only maintains the surface properties up to a maximum but also negative affects some paper properties. The pressure applied by calendering has an important effect on modifying the pore structure of the coating and globally the paper structure (Figure 2-12). Calendering has a greater influence, like reducing the pore size, on large pores than on small pores; the effect decreasing with the size of the pores. The reduction in pore volume and pore size results in a significant reduction of the light-scattering coefficient, up to 35%, of the coated sheet. The particle size distribution has a significant effect on the coating structure; for instance upon calendaring, a GCC with a narrow particle size distribution results in a larger pore volume and larger pore size than a GCC with a broad particle size distribution. Although the coating with narrow particle size distribution pigments suffers a somewhat larger reduction in pore volume and pore size during calendering, it still presents a larger pore volume and pore size than a coating with broad particle size distribution pigments. The pore volume of a clay coating is reduced almost twice as much as a GCC coating [39].

<figure><img src="image_12.png" /><figcaption>**Figure 2-12 Effect of the calendering on the pore structure**</figcaption></figure>

As far as calendaring temperature is concerned, paper properties are more sensitive to the effect of temperature than of calendering pressure. The main effect of high calender-
ing temperatures and low pressures is on the coating layer itself which will deform and
smoothen under such conditions. Oppositely, the combination of a low temperature and
high pressure implies that the whole sheet will compress evenly [12]. The smooth sur-
face then results in an even thickness profile of the paper. Concomitantly, the bending
stiffness and opacity of the paper are reduced due to combined effect of paper thickness
reduction [50], fibres being crushed in the process, and the reduction in the void volume
in the fibres, fibre network, and coating layer. The finest pore structure of the coating
layer, i.e. a precoat or a topcoat layer, is the controlling factor for ink setting behaviour
as it controls liquid penetration into the layer (Figure 2-13). Kugge [52] works confirm
previous assertion as he surmises that both the topcoat, which is directly into contact
with the ink, or the underlying precoat and even the precalendering and/or calendering
may modify the latex distribution inside the coating layer. The rearrangement of the
latex particles with water movement upon drying of the sheet may be the reason why the
latex content on the surface of the coated sheets is affected by the precalendering condi-
tions. The type of calendering (hard nip, soft nip, supercalendering, etc.) used is an im-
portant factor as it will further influence the quantity of latex ending-up on the coated
sheet surface: the highest latex content on the surface being reached with soft nip precal-
endered sheets. The intensity of precalendering also affects the surface latex content. In
addition, Venkata *et al* [62] find that different precalendering methods such as machine
calendering, supercalendering, and soft nip calendering do not affect the distribution of
latex on the surface of the coated sheet. The latex influences the whole coating process
due to film forming behaviour and compressive response to calendering. The viscoelas-
tic properties of the coatings correlate with the compressive behaviour of the calendering
of coated paper and the bending behaviour (Gurley stiffness testing) [64].

Precalendering is then an interesting strategy to control the total calendering operation
load applied onto a coated sheet. However, as we have just seen, different aspects of the
process need to be taken into account [5]. In essence, following points may be outlined:

* precalendering is one of the effective methods to achieve a uniform coating layer and to reduce the roughness and porosity of the surface base paper,
* the intensity of calendering is the critical factor in achieving the final surface properties of the coated sheet,

* precalendering the base sheets as a pre-treatment is effective only if the coated sheet is not post calendered [62],

* precalendering at high temperature has the same effect as increasing the coat weight around 1 g/m² [65],

* the precalendering effect on the base paper with calendering at high temperature increases the number of small pores on the surface, and

* a base sheet without precalendering and with precalendering at low temperature gives the same coated paper properties [65].

<figure><img src="image_13.png" /><figcaption>**Figure 2-13** Water movement for a platy pigment for an uncalendered (left) and calendered (right) coating layer</figcaption></figure>

The effect of precalendering has a different impact depending on the nature of the base paper, either wood free or mainly mechanical pulp (as for LWC) sheet. In the case of the wood free paper, precalendering before coating has generally positive effects on the properties of the coated end product. The gloss and the smoothness are somewhat enhanced. As expected, the coating mass distribution in coating layer is more homogenous when a precalendered paper is used [66]. For LWC paper precalendering does not always improve the coated sheet final properties. For instance, Ahlroos, Alexandersson, and Grön [17, 22], and Gane and Hooper [17, 22] show that during precalendering, ex-
pansion of fibre flocks may affect the coating mass distribution and deteriorate print quality. Here, precalendering the base sheet (for LWC) does not significantly improve surface roughness or gloss. Another example is provided by Engström and Lafaye [67]. They find that precalendering a wood containing base paper (LWC) results in a reduction in the covering ability of the coating with a subsequent increase of mottling in offset printing. In a study of the effect of precalendering and low coat weight application, as low as 2-3 g/m², Gane and Hooper [22] explain that surface of the base sheet must have a macro-roughness adapted to the application of the coating layer; the base stock being the same apart from the reduced macro roughness of the precalendered sample. They show that an improved coating coverage is achieved when coating a smooth precalendered base stock. It is clear that the rough base stock has a great proportion of uncovered surface fibres in the finished sheet. However, the gloss of the two samples is similar.

The same authors show that the surface profile generated after coating reproduced many of the surface features present in the uncalendered base stock. More critical is that the smoothing effect due to precalendering coupled to the regeneration of the original base profile via relaxation of the fibre network results in a coated sheet which does not achieve the gloss advantage that might have been expected from the improved coverage [22].

Today, soft nip calendering is a useful technology mainly used for LWC and board grades with some applications in wood free papers. The impact of soft nip calendering on density is small: the paper surface developed is a closed surface with small pores [65]. Furthermore, Nissinen [68] finds that the effects of soft calendering on quality characteristics are usually small, except for roughness and gloss. It is the reason why precoat soft calendering remains an interesting process variable. In their work, although they expected a negative effect on the coating process, topcoating, at 10 g/m², went surprisingly well with no streaking problems. However, the coatweight varies upon blade load modifications. Finally, they show that the uniformity of coating layer and post-calendering at low load are important for print quality characteristics, and more specifically mottling [65].
## 2.4 Multilayer coated paper

Double layer coating is used to obtain a paper with maximum gloss and printability characteristics (Figure 2-14) [69]. Usually, double-coating is used to apply high coat weight applications as it mainly provides a large flexibility for the coating colors and coat weights, in both the precoat and topcoat layer, applied on the paper. It should be emphasized that with the same total coatweight, double-coating provides a paper with a smoother surface after printing than single-coating where fibre swelling will increase the paper surface roughness [6, 70]. The interactions between the coating layers and the base paper are critical parameters to consider when producing a new or a modified coated paper grade. Multilayer coating is generally done to apply high coat weights in a strategy to minimize total production cost. Indeed, cost savings can be realized by the use of less expensive pigments and additives in the precoat, thus keeping the high glossing pigments and binder in the topcoat. Such a cost reduction strategy may be optimized by varying the coatweight ratio between precoat and topcoat with the same total coatweight applied to the base paper [71].

<figure><img src="image_14.png" /><figcaption>Figure 2-14 Double coated paper and base stock</figcaption></figure>

The coatweight/coating color interaction between each layer and the base paper directly controls the final quality of the paper. For instance, the gloss difference between a pre-
coat layer containing carbonate and a precoat layer containing clay is maintained after
application of the topcoat layer [71]. The gloss of the finished product appears to be
dependent on the roughness of the precoated sheet rather than the particle size of the
precoat pigments [71]. Another study by Renvall, Rautiainen and Rossitto concludes
that the type and the particle size of the pigments in a precoat formulation directly influ-
ence the finished properties of the double coated sheet [70]. Although the two preceding
studies have slightly different conclusions, we hypothesize that a combination of the
roughness of the base paper and the particle size are the key factors in the design of a
precoat layer. Some results show that, for a double-coated paper, the precoat application
has such a predominant importance that the topcoat weight has a relatively small influ-
ence on the IGT surface strength. Overall, the coatweight of the precoat and the method
of precoat application have also a predominant influence on the final product [70]. Us-
ing different binder system in the precoat layer, Burri *et al* [72] find no visible effect on
the ink setting rate. Renvall, Rautiainen and Rossitto [70] show that, for a given coat
weight level, when used for topcoat, the roll applicator provides a better gloss than the
short dwell coater. Finally, Hostetler [73] suggests that the precoat formulations have
significant effect on the topcoat dewatering.

The process runnability in topcoat layer application depends on the roughness of the
precoat. In a study using Solid Bleached Sulphate (SBS) paperboard, Gagnon and His-
cock [74] report that increasing pore diameter at a comparable total volume reduces the
“capillarity” of the precoat and tends to decrease the topcoat dewatering rates (i.e. dehy-
dration velocity). Furthermore, the inclusion of coarse carbonate in the precoat reduces
the incidence of streaking in double coated papers [71]. During coating color applica-
tion, the topcoat performance is solely controlled by the precoat layer. Nevertheless,
and it should be expected so, some final properties of the double coated paper are di-
rectly influenced by the topcoat layer. For instance, if cracking tendency is the key fac-
tor for sheet quality, the total coatweight applied becomes the most important controlling
factor [70]. Similarly, the drying of the topcoat layer is very important for the final
properties of double coated paper. Nissinen [68] points out that the quality of double
coated fine paper mostly depends on the moisture level, mainly after topcoat application.
He notices that the final moisture, and even the middle layer moisture in three layers, is quite related to mottling.

## 2.5 Conclusions and implications

We have reviewed and analysed the effects of different concepts and methods on the coating paper properties: among others calendering, pigment replacement, pigment and chemical substitution. From the literature review, it is apparent that the double coated papers properties in relationship with above concepts have not been largely and fundamentally analysed. Our remark is especially true when considering the architecture of the pore structure in the various layers in relationship to the double-coated paper properties: little understanding of the true fundamentals exits. Most of the studies evaluate the effect of the precoat on the final printability but are limited to the single precoated paper: understanding of the interaction between the two layers and on the final topcoat is still somehow lacking.

The characterization of the different layers and understanding the mechanisms and interactions of how the layer truly affects the final coated paper property, *i.e. how the layer truly works*, is a key factor to improving the printing characteristics and quality of double-coated papers. In present literature review, we explain how the pore structure may be an important factor to understanding such phenomena. The porous structure of coating layer depends on coating color formulation, pigments and their combinations, but also on pigment particle size distribution and particle shape. The porous structure is also impacted by the binders, either synthetic or natural, by the chemical additives, *e.g.* associative thickeners, by the coating color application, and by the drying processes. We emphasize here that the particle packing, considered as one of the most important pigment properties, is directly related to the coating pore structure, which in turn affects the optical, mechanical, and printing qualities of the coated paper. Such results suggest that high shape factor pigments may block pores to reduce the penetration of the coating color. Therefore, the type of pigments particles and their mixing ratio with other pigments directly determine how the structure of the coated paper is or can be developed. One study indicates that the type and the particle size of the pigments in a precoat for-
mulation may directly influence the finished properties of a double coated sheet. One author suggests that the precoat formulations have significant effect on topcoat dewatering while another concludes that “some final properties” of the double coated paper are directly influenced by the topcoat layer. Our review highlights the importance of the latex used in the topcoat formulation on the printing properties. Indeed, the latex alone, or in addition with other chemicals, is known to influence the coating structure.

Precalendering before coating is an interesting strategy to control the total calendering operation load applied onto a coated sheet. For wood free paper, it may result in positive effects on the properties of the coated end product. We have also shown that the interactions between the coating layers and the base paper are critical parameters to consider when producing a new or a modified coated paper grade.

As a consequence, on the basis of our own production and commercial experience comforted by present literature review, we have designed a study to verify how the precoat layer characteristics, mainly the structure parameters, could affect the final structure and printability of double coated papers. We thus designed experimental groups of paper to develop various coating structures in accordance with our research work objective and hypothesis. The objective is simple but quite complex: we surmise that it is possible to design the coating layers if a thorough understanding of the main parameters that affect the structure is achieved. With our industry perspective combined with present literature review, we focused the design parameters on partial or total pigment substitution in the precoat layer, precalendering of the precoat layer with a focus on temperature and calendering pressure, a point-study on the binder effect in the precoat. For the topcoat, due to the research work objective, we surmised that using the most commercially used latexes would bring the necessary additional topcoat parameters. Indeed, our hypothesis here is that the effect of the precoat may differ upon the various topcoat latexes.

Finally, as we focused the research on the structure of the various layers, we realized that we needed a method that would allow us to separate effects related to the finest pores in both the topcoat and precoat structures. Accordingly, we deemed it was feasible to improve the treatment of mercury intrusion data, and that is was indeed necessary, to be able to analyze in detail the pore size distribution and available pore volume of
each structure, i.e. the precoat and topcoat layers, both as a single combined layer and
separately.
# Chapter 3 - Discussion and analysis of the characterization methods

## 3.1 Introduction

We have seen that the pore structure of the paper and the coating layer are important concepts to characterize and to understand the final physical, optical, and printing properties of the coated paper. As the porous network controls both the ink absorption during paper printing and coating applications, in order to fully understand the basic differences brought about by precoating and calendaring, we selected characterization methods that involve liquid penetration.

The behaviour of a fluid in contact with a porous structure has been extensively studied in different industry sectors: petroleum, geology, agricultural etc. As both the base paper and the coating layer are porous structures, we may apply the penetration concepts that were developed in these other sectors. Nevertheless, the application of these concepts to the paper industry has to be tailored to describe more accurately the fluid behaviour on and within a base paper or any coating layer. Understanding the base paper structure and/or a coating structure are key factors to explain how the ink penetrates in a given substrate. Similarly, applying a coating layer over a substrate is done when the coating colour is still in a liquid form.

Therefore, it is very important to understand how the liquid movements behave after an application of a liquid over a structured substrate composed of a network of pores and capillaries (channels) with many varying shapes. The paper and/or coating layer pores and capillaries also present a large number of sizes and diameters. The wet coating layer is composed of pigment(s), binder(s), chemical(s), and water associated with these components and/or added to the coating mixture. The mixture rheological behaviour is controlled by the nature of the components and the interactions among the components. When the wet coating is applied to a substrate, the components and the water begin to move, rearrange themselves one versus the other (for instance "binder migration"); the
movements and rearrangements are controlled by the absorption behaviour of the fluid
into the paper and/or the coating layer.

When a coating layer is applied over a substrate, the components may either deeply penetrate in the substrate or preferably stay on the surface. The quality of the final product changes greatly with either process, depending on whether the coating layer stays on the surface or penetrates into the media. Thickeners or rheology modifiers are widely used to modify (normally slowing down) the dewatering and/or to minimize the movements of the pigment(s) and binder(s). That is why, on one hand, the relationships between the properties of the coating suspension and the final (dry) coating layer [28] has been widely studied. On the other hand, the differences between a fluid applied over a base paper (fibres with filler) versus over another coating layer (most of the time a dry layer) are also critical.

These concepts have been treated by many researchers [75-80] as fluid movements are not straightforward. Indeed, each parameter controlling the flow behaviour must be analysed separately from the others in order to understand the basic mechanisms. Schoelkopf *et al* [81] propose equations describing the liquid flow through porous substrate containing a driving force. The driving force can either be a wetting force leading to capillary and/or an external pressure, which then competes against flow penetration resistance force or so-called drag term. Viscous friction is the classic drag component. Schoelkopf *et al* [81] also propose other contributions to account for flow dissipation mechanisms like turbulent flow and liquid flow discontinuity close to the meniscus in the case of capillarity.

The fundamentals of fluid movement are further explained by Gane *et al* [76] in terms of viscosity, surface tension, density, and of course vapour pressure. Viscosity acts to slow flow in small capillaries after liquid has penetrated the larger pores of the porous system. The effect is most influenced by the permeability of the substrate. Permeability is defined by the Darcy law and is an average measure of liquid flow through a saturated substrate under external pressure. Surface tension interacts with the surface energy of the material making up the substrate and/or the coating layer. In the coating layer, it is usually defined by mineral, surfactant/dispersant on the mineral surface and the binder
system. Both polar and non-polar liquids will wet most of the coatings. Wetting the surface provides the force which will then drive absorption of the coating in the substrate. Density is important when considering short timescale effects within the substrate. Indeed, after the coating colour impacts the substrate, the liquid is facing the choice between penetrating a fine pore and a large pore. The large pore will absorb more liquid mass and so inertia will attempt to delay absorption. The fine pore (~0.1 µm) absorbs liquid rapidly for about 10 ns before viscosity acts to slow down the flow, but during that time interval the coating pore is already full of liquid [82].

Therefore, the fluid movement is separated into:

1. Coating color penetration or permeability (imbibition)

2. Coating penetration by capillarity

3. Thickening mechanism

4. Dewatering by filtration

In conclusion, although many methods are reported in the literature for characterizing pore structure and flow behaviour, for the reasons explained above, I have chosen the following methods to determine and/or characterize the porous structure of coated papers.

* Liquid drop contact angle

* Mercury porosimetry (normalized with the stain imbibition technique and data treatment within Autopore®)

* DELTACK Prüfbau

In my study, I have selected to use mainly mercury intrusion, although a very complex and even a quite complicated test to perform and to analyse, as it provides more detailed information on the pore structure and the pore structure distribution. The stain imbibition technique is nevertheless used to characterise and to normalize the base paper with the hexadecane method. The normalization method is explained later (heading 3.5)
## 3.2 Contact angle method

In order to measure contact angle, a very small droplet of liquid, around 5 picoliters, generally water, is applied on the surface of the substrate to be measured and the angle between the droplet and the substrate surface is accurately measured (Figure 3-1). The angle between the droplet and the substrate surface provides information on the substrate wettability (Figure 3-2).

<figure><img src="image_15.png" /><figcaption>Figure 3-1 Water drop with an angle lower than 90° (left) and greater than 90° (right)</figcaption></figure>

The wettability between a liquid and a surface is defined by equation 3-1

$$S = \gamma_s - \gamma_l - \gamma_{sl} \qquad \text{Equation 3-1}$$

where

$S$ = spreading coefficient

$\gamma_s$ = superficial tension of the solid (free energy of the solid)

$\gamma_l$ = superficial tension of the fluid (free energy of the fluid)

$\gamma_{sl}$ = interfacial tension between the liquid and the solid.

When the spreading coefficient is positive, the liquid spreads on the surface. When the spreading coefficient is negative, the liquid does not spread on the surface. The phenomenon is illustrated in the Figure 3-1 and Figure 3-2. In other words, when $\cos \theta > 0$, the surface is hydrophilic: the liquid is absorbed by the substrate; when $\cos \theta < 0$, the surface is hydrophobic: the liquid does not spread on the surface of the substrate. The co-
sine of the angle between the droplet and the substrate (solid) is defined by the equation 3-2.

$$ \cos \theta = \frac{\gamma_s - \gamma_{SL}}{\gamma_L} \qquad \text{Equation 3-2} $$

where Cos θ = contact angle between the fluid and the solid

<figure><img src="image_16.png" /><figcaption>Figure 3-2 Contact angle θ between a substrate and a liquid</figcaption></figure>

The contact angle may also be measured at different time intervals to analyse liquid absorption as a function of time. The phenomenon is illustrated in Figure 3-1. The red lines in the Figure 3-1 outline the contact angle and form a line around the droplet at the liquid-solid-vapour interface. The analysis of the variations of the red lines when a droplet is deposited on a horizontal solid surface from the start of the droplet deposition until the final stabilization describes the droplet absorption. Immediately after droplet deposition, the contact angle is necessarily close to 180°.

The angle measured at the liquid-solid-vapour interface determines the liquid-vapour tension vector. After complete deposition, the droplet contact angle decreases and therefore the horizontal component of the liquid-vapour tension decreases as energy balance should reach the minimum level. The equilibrium occurs when the sum of all vectors and horizontal components equal zero. If the contact angle is lower than 90°, the sign of the vector becomes negative. The liquid absorption theory will be further explained for the mercury intrusion method.
### 3.2.1 Contact angle theory

A drop of liquid on a solid surface is in contact with the substrate and with another fluid surrounding the droplet which is either the droplet liquid vapour or air. The configuration is illustrated in Figure 3-3 which presents the various interfaces: liquid-solid, liquid-vapour, and solid-vapour. Another interface is represented by a continuous line around the droplet that describes the liquid-vapour-solid boundary.

<figure><img src="image_17.png" /><figcaption>Figure 3-3 Droplet of non-wetting liquid on solid surface.</figcaption></figure>

Each interface corresponds to an interfacial tension symbolised as $γ_{l-v}$ for the liquid-vapour interface, $γ_{l-s}$ for the liquid-solid interface, and $γ_{s-v}$ for the solid-vapour interface. The force of surface tension is usually provided per unit length (dynes/cm) and act tangentially to the interface. The Young equation shown in equation Equation 3-3 gives the relation of the contact angle in the triple point energy, i.e. interfacial tensions, of the 3 interfaces: solid-vapour, solid-liquid, and liquid-vapour:

$$γ_{S-V} + γ_{L-V} \cos(θ) = γ_{S-L} \qquad \text{Equation 3-3}$$

The behaviour of a droplet deposited on a surface may be described by 3 vectors that change with time until equilibrium, minimal energy, is obtained. The phenomenon is described in Figure 3-4 [83]. In Figure 3-4 (a) the droplet has just been deposited on the surface. In Figure 3-4 (b), the droplet begins to spread and the forces are not yet balanced because the surface tension vectors of the interface solid-vapour and liquid-vapour are greater than the surface tension vector solid-liquid. When the contact angle goes over 90°, the sign of the cosine becomes positive. At energy equilibrium illustrated in Figure 3-4 (c), the droplet does not penetrate anymore in the substrate.
<figure><img src="image_18.png" /><figcaption>**Figure 3-4** Droplet on a solid surface at various absorption stages: (a) initial deposition, (b) during absorption, and (c) at equilibrium, end of the absorption.</figcaption></figure>

### 3.2.2 Contact angle: test description

In order to measure contact angle, a droplet, usually 10 nanoliters (nL) is deposited on a surface. The contact angle measuring apparatus takes measures as soon as the droplet hits the surface by evaluating the contact angles as a function of time. Water is normally used with paper but various liquids may be utilized to create the droplet. As seen in Figure 3-4, the angle is measured immediately after droplet deposition (a), and then again over a specific period of time (b), to evaluate the time needed for the droplet to stabilize (c) on the surface. The test procedure is illustrated in Figure 3-5. In the apparatus we used, the volume of the droplet is 10 nanoliters. The droplet is placed on the surface of the sample, securely fastened with tape, and a picture is taken every 100 ms. Three measurements are done for each test. If a measurement is significantly different from the
other, a new one is done. The contact angle result is the average of the 3 values measured on the CLC coated side from each sample.

<figure><img src="image_19.png" /><figcaption>Figure 3-5 Droplet deposit, measurement and final results for angle lower (left) and greater (right) than 90°</figcaption></figure>

## 3.3 Mercury intrusion

The mercury intrusion method is based on the principle of the penetration of a porous substrate by a non-wetting liquid under pressure and the pressure needed to penetrate a pore of a given size. The phenomena are essentially described by two laws: the Darcy law that relates the penetration rate as a function of pressure and permeability, and the Washburn equation that links the pore size to the pressure.

### 3.3.1 Darcy's Law

Darcy law is widely used and accepted to measure the permeability of a medium. The permeability (k) is a physical characteristic of a substrate that describes how a fluid passes through a porous network [84]. The Darcy law as defined by the equation 3-4 [84, 85] relates the penetration flow rate to the differential pressure; the differential pressure is generated by the characteristics of the skeletal element crossed (or permeated) which resist the fluid penetration:

$$Q = \frac{k \Delta P}{\eta L} A \qquad \text{Equation 3-4}$$
Where Q is the flow rate, A is the element area of the porous medium being penetrated, k is the permeability, η is the fluid dynamic viscosity, and ΔP is the pressure drop across the sample thickness or length (L). From equation 3-4 the permeability is calculated as follows (equation 3-5):

$$k = \frac{Q L}{A \Delta P} \eta \quad (\text{with unit m}^2) \qquad \text{Equation 3-5}$$

It is important to understand the difference between the permeability (k) and the coefficient of permeability (K) or hydraulic conductivity. The permeability (k) is a characteristic of the porous medium and highly dependent on the porosity which it is itself directly related to the physical characteristic of the layer, such as density, pore size, and tortuosity in the medium [86].

The coefficient of permeability (K) is a characteristic of the flow through a material for a specific fluid. The coefficient of permeability is defined by equation 3-6 [87]:

$$K = \frac{k}{\eta} \qquad \text{Equation 3-6}$$

Due to the fact that Darcy law does not fully explain all the results obtained from many research studies, it has been modified to consider additional parameters. The most famous modification is the known as the Carman-Kozeny law [88], only valid for laminar flows, which takes into consideration specific surface area, hydraulic radius, and tortuosity.

### 3.3.2 Washburn Equation

The penetration of a capillary by a liquid can be described as follows: along the boundary defined by the solid-vapour-liquid interface, all the forces involved are at equilibrium. The forces contributing to the formation of the meniscus inside the capillary are the cohesion of the fluid and the fluid adhesion to the capillary wall. The meniscus is concave for a wetting fluid and convex for a non-wetting fluid (Figure 3-6).
<figure><img src="image_20.png" /><figcaption>Figure 3-6 Wetting and non-wetting fluid in a capillary</figcaption></figure>

In 1921 Washburn [89] derived an equation relating internal, tension surface, and external forces, applied pressure, acting on a non-wetting fluid penetrating a circular capillary. Washburn demonstrated that the pressure to force a non-wetting fluid to enter a pore is directly proportional to the surface tension of the fluid and the contact angle of the liquid with the solid surface. The internal force acting against the non-wetting fluid penetration is related to the cosine of the fluid contact angle θ (Figure 3-6) around the circumference of the capillary (2π r, r being the capillary radius) as described in equation 3-1:

$$F_{out} = 2 \pi r \gamma \cos\theta \qquad \text{Equation 3-1}$$

In essence $F_{out}$ is the external force needed to force the non-wetting fluid to penetrate the capillary, $\gamma$ is the surface tension of the non-wetting fluid. Therefore to make the non-wetting fluid to penetrate the capillary we need a pressure P as (equation 3-2):

$$P = \frac{F_{out}}{\pi r^2} \qquad \text{Equation 3-2}$$
From equations 3-1 and 3-2 we may derive the well-known Washburn relation as described in equation 3-3.

$$P = \frac{-2 \pi r \gamma \cos \theta}{\pi r^2} \qquad \text{Equation 3-3}$$

or, considering the pore diameter (equation 3-4):

$$P = \frac{-4 \gamma \cos \theta}{D} \qquad \text{Equation 3-4}$$

### 3.3.3 Mercury porosimetry

The measurement of the porous structure with mercury as a non-wetting fluid was introduced by Ritter and Drake in 1945 [90]. Mercury porosimetry is based on the characteristic of mercury being a high surface tension liquid, strongly non-wetting for most substrates [91]. Mercury intrusion provides a complete pore size distribution spectrum as a function of applied pressure. At low applied pressures, the largest capillaries are filled by mercury while the smallest pores require very high pressures, up to 60 000 psi or 415 mPa [92]. From the Lucas-Washburn equation seen before section (3.3.2), the authors calculate the pore size distribution, the total pore volume, and the pore diameter calculate from capillary pressure-intrusion curves. The pore size distribution provided by mercury porosimetry is a physical characteristic of a porous medium.

The volume of mercury penetrating the substrate pores is measured directly as a function of the applied pressure P which is then converted to a plot of cumulative intrusion volume versus the pore diameter D as calculated from (equation 3-4) [92]. The method of calculation may nevertheless result in some inaccuracy because of the capillary penetration lag with large pores. Further measurement error may arise when a pore with a narrow entry or capillary (also called a bottleneck) is not filled until the pressure applied is high enough to overcome the capillary pressure. The result is that a large pore may actually be identified as a small pore. Although some studies show that small pores
could prevent the filling-in of larger connected pores, Yamazaki and Munakata [93] find
results which are not in agreement with the results explained above.

Ridgway and Gane [94] explain how some lack of experimental reproducibility may be
associated with large void features. The lack of reproducibility is however confined to
the low pressure intrusion region. It is a combination of variations in the samples prepa-
ration, the introduction of the sample in the measuring chamber of the mercury intrusion
penetrometer, and even the filling-in by mercury of surface and edge features that are
not an integral part of the porous structure under analysis.

### 3.3.4 Further considerations on liquid penetration of complex structures

In applying mercury intrusion technique and results to paper and paper coating, one
should consider some limitations and specific aspects of the technique. Indeed, Gane
[82] showed that liquid penetration into pores is driven by the liquid viscosity, surface
tension, liquid density, and of course the liquid vapour pressure.

Gane [82] shows that viscosity acts to slow flow in small dimension capillaries, but only
after a sufficient quantity of liquid has partially saturated the porous system. Conse-
quently, the effect is mainly influenced by the substrate permeability as defined in the
Darcy law (i.e. an average measure of a liquid flowing through a saturated medium un-
der external pressure).

Mercury intrusion is related to surface tension which interacts with the surface energy of the material making up the substrate. In paper coating, materials may be minerals, surfactant/dispersant attached to the mineral surface, and the binder system. It should also be reminded that both polar and non-polar liquids wet most coatings; the wetting thus provides the force which drives absorption.

Liquid density is important when considering short timescale effects and interactions
within the substrate. The consideration does not necessarily apply at time zero (begin-
ning of the mercury intrusion) but whenever a liquid is facing the choice between pene-
trating a fine pore or a large pore. On the one hand, the large pore will absorb more mass
of liquid and so inertia will counter and delay the absorption. On the other hand, the fine pore will absorb liquid rapidly for about 10 ns (~0.1 µm) before viscosity will slow down the flow. However, due to the small pore size, by the time penetration lag should occur, the coating pore will most probably be already full of liquid.

Therefore, to understand liquid (here mercury) absorption into paper coatings, one needs to reason in terms of complex networks and not simple capillaries. In order to consider the effect of inertial wetting, two parameters should always be taken into account: first, how big are the pores in comparison with the substrate (for coating it will be the base paper structure), and second what is the substrate permeability that will be lost due to liquid absorption (liquid phase of the coating colour)?

Consequently and besides mercury intrusion, when analyzing a coated paper grade with a precoat layer, three types of structures should be considered: a very open pore structure formed by the base paper, a less open structure represented by the precoat, and a very fine pore structure constituted by the top coat layer.

In essence, it is only after a given time delay (sufficient to allow large pores to be satu- rated) that the network with the finest pores (in contact with the larger pore network) will be able to uptake liquid. For instance, a gloss topcoat will withhold liquid in its finest pores and allow liquid to penetrate the coarser precoat network by surface wetting the larger precoat pores thus the saturated top coated structure will act as a barrier to further liquid penetration. It implies that the precoat pores will not be filled by sole cap- illary forces but only when some external force will be applied.

Similarly, the same rule is applicable between the coating and the base paper structures. The base pores will not draw liquid out of the coating by capillarity as the coating pores are finer than the base paper pores. The reason for a base paper to draw liquid from the top coating layer does necessarily require another mechanism. It is pro- posed here, from our discussions with P. Gane [82], that a mechanism based on fibre surface wetting combined with fibre wall and lumen wall nanopore filling which may further be related to vapour pressure, and subsequent condensation of the vapour, may provide a better explanation than mere capillarity.
## 3.4 Equipment and methods – Mercury intrusion

To simplify the Lucas-Washburn equation [89] used as a basis for mercury intrusion, let us consider that the surface tension of mercury is 485 dynes/cm and the contact angle between mercury and most papers (including coated grades) is 130°, i.e. a cosine value of minus 0.36729133. Therefore the basic mercury intrusion equation that relates pore diameter to pressure becomes (equation 3-5) below:

$$D = \frac{712,54}{P} \qquad \text{Equation 1 3-5}$$

As the intrusion volume is measured with increasing pressures, each pressure applied corresponds to the measurement of pores of an exact pore size (given by above equation 3-5) which are filled with mercury. By measuring the volume of mercury penetrating a pore structure with increasing pressures, it is possible to calculate the pore volume and the pores size distribution.

The mercury intrusion is performed with a Micromeritics Autopore III that allows a maximum pressure to be applied of 414 MegaPascal (MPa) corresponding to about a 0.004 µm Laplace throat diameter (minimum pore dimension that can be measured). The equilibrium pressure time is set to 60 seconds. The mercury intrusion volume is obtained by measuring the variation inside a calibrated capillary tube (stem) that prolongs a bulb or sample holder.

All mercury intrusion data are normalised with a measure of the empty penetrometer, with no sample inside the penetrometer bulb (or blank), to allow for the penetrometer chamber deformation under pressure, mercury compression, and compressibility of the solid phase of the sample. The use of the blank penetrometer correction is explained below in the equation used for the Autopore (equation 3-6) to calculate the true volume intrusion in the sample. It is described in an article by Gane *et al* [95] as:

<footnote n="1">¹ 712.54 = -4 * cos 130° * 485 = -4 * -0.36729133 * 485</footnote>
$$V_{\text{int}} = V_{\text{obs}} - \delta V_{\text{blank}} + \left[ 0,175(V_{\text{bulk}}^{1})\log_{10}\left(1 + \frac{P}{1820}\right) \right] - V_{\text{bulk}}(1 - \Phi^{1})\left(1 - \exp\left[\frac{P^{1} - P}{M_{\text{ss}}}\right]\right) \quad \text{Equation 3-6}$$

where

$V_{\text{int}}$ volume of mercury intrusion into the sample

$V_{\text{obs}}$ intruded mercury volume reading

$\delta V_{\text{blank}}$ blank run volume reading

$V_{\text{bulk}}^{1}$ sample bulk volume at atmospheric pressure

$P$ applied pressure

$\Phi^{1}$ porosity at the atmospheric pressure

$P^{1}$ atmospheric pressure

$M_{\text{ss}}$ bulk modulus of the solid phase of the sample

The Micromeritics Autopore III equipment is built to run the mercury intrusion test in
two different sequences as shown in Figure 3-7.

<figure><img src="image_21.png" /><figcaption>Figure 3-7 Mercury Intrusion equipment</figcaption></figure>
The first sequence corresponds to the test performed at low pressure, followed by a second phase at high pressure. The equipment uses a transducer to detect the mercury volume variation in the stem mounted with the sample holder (Figure 3-8).

<figure><img src="image_22.png" /><figcaption>**Figure 3-8** Sample holder with his stem mounted for the mercury intrusion test</figcaption></figure>

An important experimental procedure point to provide good test accuracy is the method used to set-up the paper sample to be analysed. The method is described by Ridgway and Gane [94]. The sample has to be mounted to ensure an easy flow of the mercury on the entire surface. Furthermore, the paper sample needs to have enough total surface area to ensure that the mercury penetrating the pore structure will use up most of the mercury (around 80%) inside the stem.
To achieve that, a piece of metal wire (stainless steel as to avoid the formation of mercury amalgams) is wound into a spiral together with the paper sample. The outer diameter of the prepared sample should not allow any contact with the penetrometer wall.

For most papers, sample weight around 0.2-0.3 g will fit into a 15 cm³ penetrometer with a 0.392 cm³ stem volume (standard on the Autopore instrument). An example of the sample preparation with the wire and inside the penetrometer is shown in the Figures 3-9 and 3-10.

The Micromeritics Company further explains how the stem and bulb sample function (see Appendix 1). The Appendix 1 also describes the main parameters that can be calculated from the mercury intrusion curves.

<figure><img src="image_23.png" /><figcaption>**Figure 3-9** Paper sample set up for the mercury intrusion test</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Figure 3-10 Paper sample in his penetrometer</figcaption></figure>

### 3.4.1 Repeatability of the mercury intrusion test

Mercury intrusion testing is a useful tool to analyse any type of structure. However, this test is sample destructive and the repeatability is greatly related to the quality of the sample and the method used to treat the data. A study done by Ridgway [96] clearly shows the mercury intrusion test is highly repetitive. Table 3-1 and Figure 3-11 show the results of this study. The corrected data is acceptable when the hexadecane cut-off is done correctly and applied in the Autopore® program. It has been clearly demonstrated the occurrences and defects present on the surface of the paper, greatly affect mercury intrusion final results. Therefore the variations in the results found come from the paper sample itself instead of method of measurement.

The coefficient of variation (COV) is given by the equation 3-7:

$$ (Maximum\Value - Minimum\Value) \div Mean \times 100 \qquad Equation\ 3-7 $$
<table><caption>**Table 3-1** Repeatability of the mercury intrusion test</caption><thead><tr><th>Test</th><th>Total pore volume [96]</th></tr></thead><tbody><tr><td>Paper run #1</td><td>0.27759 mL/g</td></tr><tr><td>Paper run #2</td><td>0.27440 mL/g</td></tr><tr><td>Paper run #3</td><td>0.27674 mL/g</td></tr><tr><td>Paper run #4</td><td>0.27190 mL/g</td></tr><tr><td><strong>Mean</strong></td><td><strong>0.27516 mL/g</strong></td></tr><tr><td><strong>Standard Deviation</strong></td><td><strong>0.00256</strong></td></tr><tr><td><strong>Coefficient of variation (COV)</strong></td><td><strong>2.06%</strong></td></tr></tbody></table>

<figure><img src="image_25.png" /><figcaption>**Figure 3-11** Mercury intrusion curves with samples from the same piece of paper</figcaption></figure>
**3.5 Normalisation and optimization of the mercury intrusion data analysis**

The mercury intrusion test generates a data file with a series of results with pressure re-
lated to the volume of mercury intruded in the sample, from atmospheric pressure, about
0.1 mPa up to 414 mPa. The data file also contains the desorption data or decrease in
volume when the pressure is released. The difference between the upward and down-
ward curves is called hysteresis. As the file contains only raw untreated data, the data
may and must be normalised to enable proper analysis of the porous structure being ana-
lysed. We propose below a new method to extract pore distribution using successive
corrections from either a precoat or a top coat layer, calendered or not, from a correction
of the data based on the analysis of the base uncoated or precoated or uncalendered pa-
per and information from the analysis of the hexadecane imbibition technique.

**3.5.1 The hexadecane imbibition method and analysis**

In the normalization technique, and only for uncoated base stock papers, we use the
hexadecane analysis to determine the cut-off points for low mercury intrusion pressures.

In a paper sample, the very large pores, defects or mercury occlusions<fnref n="2" /> [94] generate a
high absorption volume of mercury (Figure 3-12) and affect the total pore volume re-
sults. The interstices and defects provide artificial information (artefacts) that must be
removed from the basic data file to ensure a good reliability and accurate analysis of the
test. Such data management or “normalisation” to remove the data artefacts can be
achieved by using information from the hexadecane imbibition method. The character-
istics of key fluids (mercury, hexadecane, and water) are shown in the table 3-2. Hexa-
decane is a non polar, aliphatic cyclic, non-wetting fluid that completely fills all void
volumes within a paper [76]. The different base *uncoated* papers used in our work were
analysed to find the total pore volume available with hexadecane. Practically, the base

<footnote n="2">² At very low pressure an “occlusion” first prevents mercury to intrude an artificial surface pore. Such a pore might be created by a raised fibre. When pressure increases, mercury will overcome the occlusion, penetrate the artificial surface pore and create an artefact in the pore distribution curve.</footnote>
paper samples are cut in small squares (2 cm X 2 cm) which are dried and then weighed. To ensure good repeatability, five tests are performed for each base paper sample. The paper samples are suspended by a thread with only a corner in contact with the hexadecane (Figure 3-13): the samples are not submerged in the fluid due to a risk of air entrapment which would affect the final results.

<figure><img src="image_26.png" /><figcaption>Figure 3-12 Example of surface defect affecting the mercury intrusion volume</figcaption></figure>

<table><caption>Table 3-2 Surface Tension (mN/m) and Contact Angle (°) of the chemicals used in this study</caption><thead><tr><td></td><td>Surface Tension</td><td>Contact Angle</td></tr></thead><tbody><tr><td>Mercury</td><td>485.5</td><td>130°</td></tr><tr><td>Hexadecane</td><td>28.1</td><td>1°</td></tr><tr><td>Water</td><td>71.99</td><td>15°</td></tr></tbody></table>

The hexadecane fills up the paper sample pores by capillarity action. Once the samples are saturated by hexadecane, they are weighed and the pore volume is calculated with the following equations 3-8, 3-9, and 3-10. An example of hexadecane calculations (base paper 2007-1-1) are shown in table 3-3.

$$W_{\text{absorbed hexa}} = W_{\text{hexa saturated base paper}} - W_{\text{dry base paper}} \qquad \text{Equation 3-8}$$

$$V_{\text{absorbed hexa}} = W_{\text{absorbed hexa}} / 0.77 \qquad \text{Equation 3-9}$$

$$\text{Total Pore Volume} = V_{\text{absorbed hexa}} / W_{\text{dry base paper}} \qquad \text{Equation 3-10}$$
<figure><img src="image_27.png" /><figcaption>Figure 3-13 Hexadecane absorption by a base paper sample</figcaption></figure>

<table><caption>Table 3-3 Hexadecane test results BS 2007-1-1</caption><thead><tr><th>Sample</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th></tr></thead><tbody><tr><td>sample dry weight (g)</td><td>0.0406</td><td>0.0394</td><td>0.0333</td><td>0.0357</td><td>0.0332</td></tr><tr><td>saturated sample weight (g)</td><td>0.056</td><td>0.0516</td><td>0.0468</td><td>0.049</td><td>0.0456</td></tr><tr><td>Hexadecane weight (g)</td><td>0.0154</td><td>0.0122</td><td>0.0135</td><td>0.0133</td><td>0.0124</td></tr><tr><td>Total pore volume (mL/g)</td><td>0.493</td><td>0.402</td><td>0.527</td><td>0.484</td><td>0.485</td></tr></tbody></table>

Once the total pore volume per gram of sample is determined from the hexadecane analysis, it is used to normalise the mercury intrusion curve. To obtain a normalised mercury intrusion curve, the hexadecane total pore volume is subtracted from the total pore volume results obtained with the base paper mercury intrusion analysis. Considering that the hexadecane total pore volume mainly represents very large pores, defects, and occlusions at low pressure, the volume is subtracted from the mercury total pore volume of the base paper. The difference between both values then represents the actual pore volume in the base paper, without the defects, large pores, and occlusions. The difference value is the subtracted from all the intrusion results in the data file. The point where the normalised curve crosses the x-axis becomes the cut off point for low pressure. The corresponding pressure (MPa) indicates the pore diameter corresponding to the cut off value. The cut off pore diameter represents the upper limit (large pores) used in the Micromeritics® Autopore III analysis for all samples (precoat, topcoat and calendered) derived from the uncoated base paper cut-off determination: i.e. data below the
cut-off limit are not considered for comparisons of the pore distributions. It also implies that data above the cut-off limit may be used to compare the actual effect of precoat, top coat, and calendering. Table 3-4 shows the hexadecane results and the pore diameter cut off point for each base paper used in the study.

<table><caption>Table 3-4 Base paper hexadecane and pore diameter cut off points</caption><thead><tr><th></th><th>HEXADECANE TOTAL PORE VOLUME</th><th>Cut off point Pore diameter</th></tr></thead><tbody><tr><td>BS 2007-1-1</td><td>0.478026900</td><td>53.8812</td></tr><tr><td>BS 2007-1-13</td><td>0.450303008</td><td>20.5708</td></tr><tr><td>BS 2007-1-15</td><td>0.453890373</td><td>39.2400</td></tr><tr><td>BS 2007-2-5</td><td>0.482528496</td><td>71.9300</td></tr><tr><td>BS 2007-10-5</td><td>0.500011513</td><td>72.0020</td></tr><tr><td>BS 2007-10-6</td><td>0.489464528</td><td>71.9236</td></tr><tr><td>BS 2007-10-7</td><td>0.511460924</td><td>71.9240</td></tr><tr><td>BS 2007-10-9</td><td>0.460735167</td><td>28.7859</td></tr><tr><td>BS 2007-10-12</td><td>0.466694138</td><td>71.9728</td></tr><tr><td>BS 2007-10-13</td><td>0.500236412</td><td>71.9728</td></tr></tbody></table>

### 3.5.2 Coating layer cut off point

The total mercury intrusion curves represent the total pore structure of the paper: fibre network and coatings included. However, for our study, only the portion of the intrusion curve representing the coating(s) is of interest. To do so, we first removed the portion of all mercury intrusion curves corresponding to the hexadecane pore volume as represented by the “hexadecane cut-off point pore diameter” (as listed in Table 3-4 for all 10 base papers used in the study).

An example of a series of pore distribution curves consisting of a base paper, the same base-paper with precoat, then with top coat, then calendered but with the pore section corresponding to the hexadecane cut-off removal is illustrated in Figure 3-14. To further the analysis and extract the sole information from the coating(s), the base paper was first
subtracted from the precoat, topcoat, and calendered intrusion results. To do so we used
the “blank” <fnref n="3" /> file capability available in the Micromeritics® Autopore III program [97,
98]. As illustrated in Figure 3-15 for a given base paper used for demonstration pur-
poses, the intrusion curves for the precoat, topcoat, and calendered samples all cross the
x axis in the same area, around 0.65 µm: i.e. the difference for the corresponding pore
volume between the base paper and the other 3 curves (precoat, top coat, calendered)
becomes negative. By analysing the graph and the intrusion curves before subtracting
the base paper, we conclude that the coating layer(s) begins where the curves cross the
X-axis. Therefore, all the pore size values higher than 0.65 µm have been removed as
they are not related to pores corresponding to the coating layers. Therefore, for this ex-
ample, the values above are nor significant for the analysis of he coating layers.

To further clean-up the data and truly isolate the pore distribution of the coating layers
of interest, we established another cut-off point for the pores smaller than 0.01 µm. The
0.01 µm value is chosen because the corresponding intrusion pressure is 148 MPa
(around 21 500 psi). Practically, such a pressure is much higher than any pressure en-
countered in a commercial press. We also know that very small pores affect the results
due to their impact on the final total pore volume. Last but not least, very high intrusion
pressures dilate the volume of the penetrometer bulb, thus creating a measure artefact.

According to the limits we just established for the high and low value in pore distribu-
tion, the pore interval used for the comparison and analysis of the coating layers (up to
calendering) lies between values established in table 3-5 and 0.01µm. Pores outside the
interval are considered to be irrelevant for our analysis.

In the interval analysis defined above, the mercury intrusion curves still include the
pores from the base paper. The base paper pores also affect the final total pore volume
of the coating layers: they also need to be removed to obtain an accurate, base-paper
independent, coating layer pore volume.

<footnote n="3"><sup>3</sup> The complete normalisation method is explained in section 3.6 of this document</footnote>
As shown in Figure 3-17 for the same sample as Figure 3-16, removing the corresponding base paper pores, we obtain normalized values corresponding exclusively to the coating structure itself.

<figure><img src="image_28.png" /><figcaption>**Figure 3-14** Mercury intrusion curves without normalization</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Figure 3-15** Base paper pore structure with coating structure</figcaption></figure>
Intrusion in Coated paper - Base paper intrusion
Precoat 100 parts GCC (1.4 µm)
Topcoat Latex S/B/ACN

<figure><img src="image_30.png" /><figcaption>Figure 3-16 Mercury intrusion curves with base paper blank – cut off point for coating</figcaption></figure>

Intrusion in Coating layer
Precoat 100 parts GCC (1.4 µm)
TOPCOAT LATEX S/B/ACN

<figure><img src="image_31.png" /><figcaption>Figure 3-17 Base paper pore structure with coating structure</figcaption></figure>
<table><caption>Table 3-5 Coating layers cut off points (larger pores)</caption><thead><tr><td>Precoat condition</td><td>Pore diameter (μm)</td></tr></thead><tbody><tr><td>1 to 10</td><td>0.65</td></tr><tr><td>11 to 14</td><td>0.80</td></tr><tr><td>15 & 16</td><td>0.65</td></tr><tr><td>17 to 20</td><td>0.51</td></tr></tbody></table>

The data obtained for the top coat and calendered top coat structures include both the precoat and the topcoat layers. To understand separately the effect of precoat and top-coat, it is then necessary to separate both precoat and topcoat layer structures.

### 3.5.3 Separation of the precoat and the topcoat layers

It was important to separate the base paper and the coating layers to be able to analyse correctly the effect of the coating layers themselves. Once this was achieved, the resulting curves of pore distribution for the coating layers present several interesting characteristics. As a case in point, using different latexes for the topcoat and different pigments in the precoat but with the same proportion (for example 20 parts/clay substitution) may affect the topcoat pore distribution. It then appears necessary to be able to separate the pore structures from the precoat and the topcoat layers. Therefore the same principle used to separate the coating structure from the base paper is applied to separate the pore structures from the precoat and the topcoat (Figure 3-18) layers. The procedure is similar to that of the base paper pore structure removal.

The procedure generates curves similar to the one shown in Figure 3-19. The cut-off point between the topcoat and precoat layer is readily identified and is applied to obtain the final topcoat pore distribution curves of interest. Indeed, the topcoat is the layer eventually printed. We are proposing here a method that allows a measure of the top-coat pore distribution *in situ*: i.e. the topcoat on top of a precoat applied on a base paper in realistic commercial conditions.
<figure><img src="image_32.png" /><figcaption>**Figure 3-18** Precoated subtraction for the coating structure</figcaption></figure>

Figures 3-20 and 3-21 show the final result obtained with the proposed method: *the pore distribution curves of the topcoat only*. Figure 3-20 presents the topcoat pore structure for double coated papers having the same base paper, the same precoat with 3 different latexes using the same topcoat pigment system: i.e. pigments are the same, only latexes vary. In this case, the low end for pore size is changed from 0.01 µm to 0.03 µm as pore sizes smaller than 0.03 µm result in negative pore values. The topcoat pores falls in the 0.15 to 0.03 µm interval. Figure 3-21 presents the topcoat pore structure for double coated papers having the same base paper, different precoats, and the same topcoat layer composition. As we surmised and as we will further detail in Chapter 5 - Results and Analysis, we see here that the topcoat layer latex and even precoat layer structure have a significant impact on the topcoat layer structure. It would not have been possible to see these differences without our method for mercury intrusion data treatment.

Finally, the values such as total pore volume (TPV), average pore diameter (APD), average pore area (APA), skeletal density, and total pore area (TPA) are also going to be more accurate and representative when using our proposed method of mercury intrusion data treatment. In conclusion, proper management and accurate analysis of mercury intrusion raw data have been shown here to be a key component for an in-depth interpretation of the pore distributions and hence the coating layer structures.
<figure><img src="image_33.png" /><figcaption>Figure 3-19 Example of curves with the precoat used as blank in the Autopore program</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>Figure 3-20 Top coating layers for the 3 different latexes with condition 1</figcaption></figure>
<figure><img src="image_35.png" /><figcaption>Figure 3-21 Top coating layers for Standard precoat-100 parts GCC (1.4 µm) and 20 parts clay substitution: High Glossing clay (0.469 µm), No. 2 clay (2.13 µm), delaminated clay (5.312 µm), and High Aspect Ratio clay (6.329 µm)</figcaption></figure>

## 3.6 Mercury Intrusion normalisation method

The method explains how the mercury intrusion results can be normalised to separate the different coating layers one from other and from the base paper. The method refers to the section 3.5 of the thesis. The table 3-6 shows the condition numbers of the study related to the Autopore® test number (highlight in yellow). They are the numbers used for the normalisation and to convert everything in an excel file.

<table><caption>Table 3-6 Autopore® files use to explain the mercury intrusion method. Related to BS1Latex1</caption><thead><tr><th colspan="8">Mercury intrusion results – Standard precoat</th></tr><tr><th>Base Paper CLC</th><th>Base Paper Intrusion no.</th><th>Precoat Intrusion no.</th><th>CLC Trial Point</th><th>Intrusion no.</th><th>Calendred Intrusion no.</th><th>Intrusion no.</th><th>Hg Intrusion Condition Excel file</th></tr></thead><tbody><tr><td>BS 2007-599-1</td><td>001-535</td><td>011-539</td><td>501</td><td>001-691</td><td>551</td><td>001-665</td><td>BS1 LATEX 1</td></tr></tbody></table>
The files numbers for the penetrometer blank runs are 000-140.SMP (blank run for
penetrometer number 500) and 000-171.SMP (blank run for penetrometer number 181).
For each coating condition, a file must be created to store the generated files and data. In
our example the file is named BS1-Latex1. The file refers to the condition 1 with the
latex 1 or named the standard or control made with 100 parts GCC (1,4 µm) and latex
S/B/ACN. The following steps refer to the figure 3-22.

Open Autopore® program

Click “File”

Click “Open”

Choose “sample information” (see figure 3-23)

Select the file you want to normalise (see figure 3-24)

Click “OK” to open the file

<figure><img src="image_36.png" /><figcaption>Figure 3-22 Reference file to store the results related to a condition</figcaption></figure>
<figure><img src="image_37.png" /><figcaption>Figure 3-23 Screen to open the mercury intrusion file with the Autopore® program</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>Figure 3-24 Dialog box to choose the desired file to analyse</figcaption></figure>
Once the file is open, the window presented in the figure 3-25 appears on the screen. The top of the dialog box shows a line called "sample". You will find the name of the file you open or, if you wish, you can modify the name to reflect more what you want to do with the file. After, you have to specify if a blank will be used to correct the data measured when the test is done. You have to click your chosen option on the line " correction method". If you use a blank file, you have to specify which one you want to use and write the name to the line "Blank correction sample". The correction file can also be found by using the browse button. In figure 3-25, the correction file used is the blank run of the penetrometer is the 181 5cc 0,366cc. Finally you click on the "advanced" button at the bottom of the screen to have access to the other information available.

<figure><img src="image_39.png" /><figcaption>Figure 3-25 Mercury intrusion file while open with Autopore®</figcaption></figure>

After clicking the "advanced" button, a dialog box opens as shown in figure 3-26. You have to choose the tab "report options" located on the top of the dialog box. After, you choose the range of pores you want to analyse. The choice is done in the section "calculation range" and you select pore size. Finally, you let know to the system which range
of pore you want. In our example, the pore range is between 10 nm à 650 nm (0,01 µm à 0,65 µm). This range is the one establish by the analysing done for the complete paper (base, precoat, and topcoat) with the negatives curves presented earlier in the text. The analysis shows the coating part begin with pores to 0,65 µm and finer. The finer limit has been establish arbitrary to 0,01 µm because, in printing, pores finer than this diameter are considered like non relevant. For information, the entire study does not consider the pores higher than 54 µm because, the bigger pores are eliminated by the hexadecane method.

<figure><img src="image_40.png" /><figcaption>Figure 3-26 Mercury intrusion file after clicking on advanced button</figcaption></figure>

Therefore, the mercury file from this study can be separated in 3 areas defined by:

<table><tr><td>Coating plus fiber</td><td>54 µm à 0,01 µm</td></tr><tr><td>Coating</td><td>0,65 µm à 0,01 µm</td></tr><tr><td>Fiber</td><td>54 µm à 0,65 µm</td></tr></table>

Please note that each study must establish its own range for the three areas. For the normalisation of the coating layers, the pores between 0,01 µm and 0,65 µm have been
used. This section refers to the coating and all pores related to the base stock have been eliminated because they have no effect on the desired results. It is important to remember that some pores from the base stock are still in the coating range analysis and this is the reason why we have to begin the normalisation method by removing those pores. Figure 3-27 shows the 2 dialog boxes we have to open in Autopore®.

On the right, we see the file for the base paper and the blank used for the correction. The blank use is the penetrometer run without paper sample. The left dialog box is to choose the pore size range for the analysis. In the example the range is between 10 nm à 650 nm (0,01 µm à 0,65 µm). Figure 3-28 shows the dialog boxes for the normalisation of the precoated layer. The base paper has been used as the blank to remove the base paper pores in the precoated layer. The same logic is used for the double coated paper i.e. precoat and topcoat. The figure 3-29 shows the dialog boxes used to do the normalisation for the uncalendered double coated paper. Same thing is done for the calendered paper but it is not shown in the method here.

Finally, the topcoat must be separated from the precoat. The same path is followed and the figure 3-30 shows the dialog boxes found by opening the file in the Autopore® program. The file used as a blank is the precoated layer and the range chosen in our example is 10 nm à 150 nm (0,01 µm à 0,15 µm). After the .smp files has been generated, the files need to be converted in the excel format to be able to do the mathematical treatment of the mercury intrusion results requested by the user. The conversion to an excel file also provided the option to easily draw graphics of these results. The figure 3-31 shows the .smp files and their corresponding excel file.

To generate the excel spreadsheet, you have to click on the “report” on the bar menu on the top of the dialog box. Figure 3-32 shows how to create the excel spreadsheet. First, you click on “report” in the bar menu and you select “start report”. The selection of “start report” opens another dialog box to let the user choose which file he wants convert in excel file. The figure 3-33 shows the screen with the new dialog box after clicking “start report”.
<figure><img src="image_41.png" /><figcaption>Figure 3-27 Dialog boxes to obtain the results normalised for the base paper</figcaption></figure>
<figure><img src="image_42.png" /><figcaption>Figure 3-28 Dialog boxes to obtain the results normalised for the precoated layer</figcaption></figure>
<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /><figcaption>Figure 3-29 Dialog boxes to obtain the results normalised for the uncalendered double coated paper</figcaption></figure>
<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /><figcaption>Figure 3-30 Dialog boxes to obtain the results normalised for the calendered double coated paper</figcaption></figure>
<figure><img src="image_47.png" /><figcaption>Figure 3-31 .SMP files and their corresponding excel files</figcaption></figure>

<figure><img src="image_48.png" /><figcaption>Figure 3-32 Dialog box to start to generate excel spreadsheet with Autopore® program</figcaption></figure>
<figure><img src="image_49.png" /><figcaption>Figure 3-33 Dialog box used to choose the .smp file to convert in excel spreadsheet</figcaption></figure>

It is very important to select "screen" in the destination line (see figure 3-33). The selection of "screen" in the destination line will generate the document on the computer screen and this is the only way to get the document and be able to convert in excel spreadsheet. After the option "screen" is selected, the user chooses the file (or files) which need to be converted in excel spreadsheet and then, click "OK". By clicking "OK", another dialog box opens and the user sees the initial selection of reports needed to be generated. The user can change something if necessary. Figure 3-34 shows the dialog box with the selection done for the study. If all the selected documents are okay, click "OK". A new document appears on the computer screen and the figure 3-35 shows what type of document it is. The document can be printed as shown in figure 3-35 or can be converted in an excel spreadsheet file. The tool menu is located in the right side of the document. The user selects "save as" and the dialog box showed in figure 3-36 appears on the computer screen. The user selects the destination file to "save as" (Enregistrer dans) and also defines the type of the document. The type chooses is "spreadsheet .xls". Then click save (Enregistrer) and the document is convert in excel spread-
sheet file in the destination the user has selected earlier in the procedure. From this point all the treatments of the results are done with the excel program.

<figure><img src="image_50.png" /><figcaption>Figure 3-34 Dialog box to choose the reports to convert in excel file</figcaption></figure>

<figure><img src="image_51.png" /><figcaption>Figure 3-35 Converted file</figcaption></figure>
<figure><img src="image_52.png" /><figcaption>Figure 3-36 Save the document on excel spreadsheet form</figcaption></figure>

## 3.7 Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy or SEM is used to visually analyse the paper surface. It is a very efficient technique to analyse the irregularities, defects, and the structure of the surface. We found the observations very useful to explain and/or understand any problem that may occur while the coated paper is printed.

Therefore, we used SEM pictures to perform a subjective analysis of paper and coating layer surfaces; to study, for instance, how the pigments have been affected after an external treatment like calendaring and potential effects on surface printability. As an example, Figures 3-37 and 3-38 illustrate how defects shown by SEM may become visually apparent after printing.

SEM is also a useful tool in supporting results and conclusions obtained with other measurement methods.
<figure><img src="image_53.png" /><figcaption>Figure 3-37 Defect in the calendered coating layer</figcaption></figure>

<figure><img src="image_54.png" /><figcaption>Figure 3-38 Defect generated by blade coating.</figcaption></figure>
Indeed, as previously said, SEM allows the identification of very small defects, poor coating application, pigment agglomeration etc. It may help in explaining some odd results related to any other measurement. The SEM used in this study is a scanning electronic microscope model Jeol JSM-5500 as seen in Figure 3-39.

<figure><img src="image_55.png" /><figcaption>Figure 3-39 SEM measurement equipment</figcaption></figure>

## 3.8 Prüfbau DELTACK method

### 3.8.1 Description of the method

The Prüfbau DELTACK method has been developed to simulate conditions occurring during commercial printing, mainly to appraise ink film splitting forces. It should be emphasized that the Prüfbau DELTACK results are dependent on the printing speed, the printing temperature, the printing tension, the substrates, and of course mainly the substrate surface properties. The instrument used in our study is a Multipurpose Ink Tack Measurement System DELTACK from the company Prüfbau, Munich, Germany. The instrument settings are described in table 3-7; the setting we used is “Web Offset”. The instrument used is shown in Figure 3-40. The paper strip used had a dimension of 5.4 cm width by 30 cm length. The ink used is a heat-set printing ink 40 8020 (Michael Huber, Munich, Germany). The ink set-up for the test is shown in table 3-8.
<table><caption>Table 3-7 Prüfbau Instrument Settings for Sheet Offset and Web Offset</caption><thead><tr><td>Instrument Setting</td><td>Web Offset</td></tr></thead><tbody><tr><td>Measuring Mode</td><td>Mode X</td></tr><tr><td>Measuring Unit</td><td>1</td></tr><tr><td>Printing Tension</td><td>1000 N</td></tr><tr><td>Printing Tension for Splitting – Print Unit A</td><td>1000 N</td></tr><tr><td>Printing Form – Print Unit B</td><td>Metal 5 cm Width</td></tr><tr><td>Printing Form for Splitting</td><td>Rubber Green 5 cm width</td></tr><tr><td>Print Sample Carrier</td><td>Rubber</td></tr><tr><td>Speed for printing and Splitting</td><td>1.0 m/sec</td></tr><tr><td>Minimum Waiting Time after Printing</td><td>3 sec</td></tr><tr><td>Measuring Interval</td><td>1 sec</td></tr><tr><td>Number of Measuring Cycles</td><td>30</td></tr><tr><td>Mean Value Range</td><td>80-170 mm</td></tr></tbody></table>

<table><caption>Table 3-8 Ink Test Settings for DELTACK Prüfbau test</caption><thead><tr><th colspan="3">Test Ink</th></tr><tr><th>Instrument Setting</th><th>Sheet Offset</th><th>Web Offset</th></tr></thead><tbody><tr><td>Ink Quantity</td><td>150 mm³</td><td>150 mm³</td></tr><tr><td>Ink Time for inking unit</td><td>30 sec.</td><td>30 sec.</td></tr><tr><td>Ink Time for Print Form</td><td>30 sec.</td><td>30 sec.</td></tr></tbody></table>
<figure><img src="image_56.png" /><figcaption>Figure 3-40 DELTACK measurement instrument</figcaption></figure>

In present study, we followed the DELTACK procedure used by Omya which can be obtained from reference [99]. However, only 2 tests per side were done (and not 3 as in the Omya procedure) but an extra care was taken to ensure that all measurements were done in exactly the same way. The settings used are listed in table 3-9.

<table><caption>Table 3-9 Prüfbau test settings</caption><tr><td>Printing Speed</td><td>1 m/sec</td></tr><tr><td>Printing Force, PU A</td><td>1000 N</td></tr><tr><td>Printing Force, PU B</td><td>810 N</td></tr><tr><td>Delay</td><td>1 second</td></tr><tr><td>Number of measurement</td><td>30 per test</td></tr><tr><td>Ink</td><td>150 mm³ of the Heat-Set ink<br/>#40 8020.</td></tr></table>
### 3.8.2 DELTACK curve analysis

The ink setting on the calendered samples is evaluated with the DELTACK method. An example of the curve obtained with the method is illustrated in Figure 3-41. The method was developed for testing ink setting on coated paper grades. It measures the tack force as a function of time which is determined by the interactions of the ink with the coating surface, the coating structure, and the ink on the tack disc.

<figure><img src="image_57.png" /><figcaption>Figure 3-41 Example of tack development from a DELTACK test curve.</figcaption></figure>

An evaluation of the initial ink setting, driven by the solvent loss from the ink to the coating pores, is provided both by the tack build-up slope to the maximum tack, and by the time necessary to reach the maximum in the ink tack force. The tack decay is the start of the final drying process where enough solvent is lost from the ink to provide the dry, non tacky, printed surface.

The porosity, pore size, and total pore volume are known to have a great impact on ink setting [24, 33] [52]. Latex chemistry also affects the ink setting rate. The two considerations mean that ink setting is controlled by capillary absorption in the coating pores as well as ink absorption by the latex [76, 100, 101]. The DELTACK approach is therefore a great tool to relate the printing characteristics of our samples to the structure modification brought about by modifications in the precoat and topcoat formulations.
### 3.8.3 Maximum in ink tack force

<figure><img src="image_58.png" /><figcaption>Figure 3-42 Example of a complete DELTACK curve</figcaption></figure>

A standard laboratory DELTACK printing curve as seen in Figure 3-42 shows that the tack force increases to a maximum then decreases. Both the maximum in ink tack force and the time it takes to reach the maximum (as evaluated by the measuring cycles) or “ink setting time” may be used to compare the curves. In our study, we propose to calculate the maximum ink tack and ink setting time for all curves using second degree polynomial equation 3-11:

$$y = ax^2 + bx + c \qquad \text{Equation 3-11}$$

with y the tack force value (in Newton) and x the time (in seconds).

It is understood that the equation we are proposing is merely empirical, in order to treat the data as the complete curves found with the DELTACK method do not have an exact second degree polynomial shape. To be able to use a second order polynomial, a part of the curve end could be, upon need, truncated to insure a good fit of the polynomial shape in the corresponding x-interval of the curve. The truncating process is illustrated in Figures 3-42 and 3-43. In these examples, values higher than 21 seconds (x-axis) have been
removed from the data set. The truncation results in a very good correlation between the DELTACK curve and the polynomial fit as demonstrated by the determination coefficient R² included in the graphs. It confirms that using a second-order polynomial provides accurate results, meaning the maximum ink tack force and time values calculated with the equation (Figure 3-44) are correct.

<figure><img src="image_59.png" /><figcaption>Figure 3-43 Truncated DELTACK Curve to fit the Polynomial Curve</figcaption></figure>

<figure><img src="image_60.png" /><figcaption>Figure 3-44 DELTACK Curves means and polynomial curves fitting</figcaption></figure>
# Chapter 4 - Experimental design

## 4.1 Experimental levels selection

We learned from the literature study that the precoat has a significant influence on the top coat properties. However, the effect of the different components and parameters of the precoat are yet to be established. Accordingly, we designed the experimental study to analyse the effect of key precoat parameters on the topcoat properties.

From standard precoat composition (initial conditions or Group 0), i.e. a precoat containing 100 parts ground calcium carbonate (GCC) which is the case of about 98% of commercial precoat, we modify the **precoat components/parameters** by:

- replacing 20% of the GCC by clay (later called 20% clay substitution): group samples 1 or Group 1;

- calendering either the base paper or the precoat before precoat and/or top coat application: Group 2.

- substituting the standard precoat latex by an acrylic latex and the carboxy methyl cellulose (CMC) by an amphoteric thickener *i.e.* protein: Group 3.

- replacing 100% of the standard GCC by different GCC or precipitated calcium carbonates (PCC) or Brazilian clay: Group 4.

In practice, above conditions corresponds to a total of 20 “precoat conditions”: 1 for the initial condition (Group 0), 4 for the 20% clay substitution (Group 1), 4 for the calendering of the base paper, the precoat being uncalendered (Group 2A) and 4 when the precoat is calendered but not the base paper (Group 2B), 2 for the latex/CMC substitution (Group 3), and 4 for the 100% pigment substitution (Group 4). On top of all these 20 precoat conditions we have applied 3 different topcoats for a total of 60 conditions (3 x 20). All 60 samples were calendered to a gloss target of 70 [21].
For the 3 topcoats we used 3 different latexes with the same pigments and additives systems. The 3 latexes used are styrene/butadiene/acrylonitrile latex (S/B/ACN), styrene/acrylic latex (S/A), and styrene/acrylic/acrylonitrile latex (S/A/ACN). The experimental design is schematically represented in Figure 4-1.

<figure><img src="image_61.png" /><figcaption>**Figure 4-1** Schematic of the experimental design</figcaption></figure>

The technical characteristics of the 3 latexes used for the topcoats are listed in table 4-1. The characteristics of the latex used for the precoat are also included in the same table. The 2 schematics presented in Figure 4-2 and 4-3 [102] showing the latex particle polymerization illustrate the differences between the two latex families (SB and SA).

<table><caption>**Table 4-1** Latex characteristics used in the study</caption><thead><tr><th>Latex</th><th>Tg temperature (°C)</th><th>Carboxyl°</th><th>Gel Content</th><th>Particle size (Å)</th></tr></thead><tbody><tr><td>SBR latex</td><td>+17</td><td>Low</td><td>Low</td><td>1800</td></tr><tr><td>Styrene / Butadiene / Acrylonitrile</td><td>+10</td><td>medium</td><td>Medium</td><td>1400</td></tr><tr><td>Styrene/Acrylic*</td><td>+23</td><td>medium</td><td>High</td><td>1400</td></tr><tr><td>Styrene/Acrylic*Acrylonitrile</td><td>+40</td><td>medium</td><td>high</td><td>1400</td></tr></tbody></table>

<footnote>*Acrylic latex uses the n-butyl acrylate like second monomer</footnote>
The precoat applications were performed on a pilot coater at CIC [103]. The calendar-
ing of the base paper and the precoat were done by soft nip calendering on the CIC pilot
coater. The topcoat applications were done on a CLC 7000 at CIPP. The final 60 top-
coated samples were calendered on a laboratory supercalender at CIC. All conditions
are detailed below.

<figure><img src="image_62.png" /><figcaption>**Figure 4-2** Styrene/Acrylic latex molecule</figcaption></figure>

<figure><img src="image_63.png" /><figcaption>Figure 4-3- Styrene/Butadiene latex molecule</figcaption></figure>

**4.2 Precoated paper roll preparation**

The precoated samples were coated in a roll form on CIC pilot coater facility [103] using
the jet applicator coating head. The complete formulations, calendaring conditions, and
coater settings are indicated in the table 4-2, 4-3, 4-4, and 4-5.

The coater operation parameters are listed in table 4-6. Although twelve different rolls
from the same base stock were used to make the 20 precoated conditions, the results
proved that the base stock paper is very consistent and stable. All the precoated rolls
were coated both sides on the pilot coater (as a control for curl). The rougher side was
coated first.
<table><caption>Table 4-2 Precoated standard formulations and 20 parts substitution</caption><thead><tr><th></th><th>Cond. 1</th><th>Cond. 2</th><th>Cond. 3</th><th>Cond. 4</th><th>Cond. 5</th></tr></thead><tbody><tr><td>GCC (1.4µm)</td><td>100</td><td>80</td><td>80</td><td>80</td><td>80</td></tr><tr><td>High glossing clay (0.469µm)</td><td></td><td>20</td><td></td><td></td><td></td></tr><tr><td>No. 2 clay (2.13µm)</td><td></td><td></td><td>20</td><td></td><td></td></tr><tr><td>Delaminated clay (5.312µm)</td><td></td><td></td><td></td><td>20</td><td></td></tr><tr><td>High Aspect Ratio clay (6.329µm)</td><td></td><td></td><td></td><td></td><td>20</td></tr><tr><td>Latex SBR</td><td>10</td><td>10</td><td>10</td><td>10</td><td>10</td></tr><tr><td>Optical Brightener Hexa.</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td>Calcium Stearate</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.4</td></tr><tr><td>Carboxy Methyl Cellulose(CMC)</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.7</td></tr><tr><td>Solids</td><td>66%</td><td>66%</td><td>66%</td><td>66%</td><td>66%</td></tr></tbody></table>

<table><caption>Table 4-3 Calendering and Soft Nips Settings</caption><thead><tr><th></th><th>Cond. 7</th><th>Cond. 8</th><th>Cond. 9</th><th>Cond. 10</th></tr></thead><tbody><tr><td></td><td>Base paper</td><td>Base paper</td><td>Precoated</td><td>Precoated</td></tr><tr><td>Temperature Soft Nips #1 and #2</td><td>100°C</td><td>100°C</td><td>100°C</td><td>100°C</td></tr><tr><td>Pressure Soft Nips #1 and #2</td><td>50 kN/m</td><td>200 kN/m</td><td>50 kN/m</td><td>200 kN/m</td></tr></tbody></table><table><thead><tr><th></th><th>Cond. 11</th><th>Cond. 12</th><th>Cond. 13</th><th>Cond.14</th></tr></thead><tbody><tr><td></td><td>Base paper</td><td>Base paper</td><td>Precoated</td><td>Precoated</td></tr><tr><td>Temperature Soft Nips #1 and #2</td><td>200°C</td><td>200°C</td><td>200°C</td><td>200°C</td></tr><tr><td>Pressure Soft Nips #1 and #2</td><td>50 kN/m</td><td>200 kN/m</td><td>50 kN/m</td><td>200 kN/m</td></tr></tbody></table>

<table><caption>Table 4-4 Precoat formulations – Latex and Amphoteric thickener</caption><thead><tr><th></th><th>Cond. 15</th><th>Cond. 16</th></tr></thead><tbody><tr><td>GCC (1.4µm)</td><td>100</td><td>100</td></tr><tr><td>Latex SBR</td><td></td><td>10</td></tr><tr><td>S/A latex</td><td>10</td><td></td></tr><tr><td>Optical Brightener Hexa.</td><td>0.5</td><td>0.5</td></tr><tr><td>Calcium Stearate</td><td>0.4</td><td>0.4</td></tr><tr><td>Carboxy Methyl Cellulose(CMC)</td><td>0.7</td><td></td></tr><tr><td>Protein</td><td></td><td>2</td></tr><tr><td>Solids</td><td>66%</td><td>66%</td></tr></tbody></table>