Pigment Coating and Surface Sizing of Paper

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1 Papermaking Science and Technology a book series covering the latest technology and future trends Book 11 Pigment Coating and Surface Sizing of Paper Second Edition Totally updated version Book editor Jouni Paltakari, D.Sc. (Tech.), Professor, Department of Forest Products Technology, Helsinki University of Technology Publisher Paper Engineers Association/Paperi ja Puu Oy Paperi ja Puu Oy

2 Table of Contents 1 Introduction to pigment coating and surface sizing of paper and board Pigment-coated products Base paper Coating materials general Pigments Binders Co-binders and thickeners Additives Surface sizing chemicals Rheology of pigment slurries and coating formulations Coating layer consolidation and formation of the coating layer structure Coating layer measurement and analysis techniques Coating colour preparation and handling Coating and surface sizing technologies Drying of paper coatings and drying equipment Process control and automation in coating machine Development trends and future outlook Conversion factors Index

3 CHAPTER 1 Introduction to pigment coating and surface sizing of paper and board 1 Objectives of coating and surface sizing How coating affects paper properties Coating colour composition Coating materials Coating colour preparation coating formulations Surface sizing general Coating process general Coating process Coating quality and process runnability Economics of coating Coating material viewpoint Technological viewpoint...25 Appendix Principle of a form for coating colour make-down

4 CHAPTER 2 Pigment-coated products 1 Introduction Coated paper and board grades Coated papers Coated boards End-use demands for coatings Printing processes General printing paper requirements Requirements for offset printing paper Requirements for rotogravure printing paper Requirements for flexo printing paper Requirements for inkjet printing paper Requirements for electrophotography paper Summary of requirements for printing Converting processes...39 References

5 CHAPTER 310 Base paper 1 Introduction Transport mechanisms Coating colour penetration Coating colour liquid phase penetration General requirements for base papers and how these are influenced Strength and its uniformity Sheet structure in z-direction Surface properties and pore structure Interactions between base paper and coating colour influence on coated paper properties Coating holdout Coating consolidation / coating layer structure Roughening of the base sheet in contact with water Runnability at the coater...54 References

6 CHAPTER 4 Coating materials general 1 Coating pigments general Functions of pigment in the coating layer Ideal pigment Classification of pigments Key properties of pigments Criteria for selecting pigments Coating binders general Functions of binders Ideal binder Types of binder Criteria for selecting binders Binding power of binders Binder demand of pigment Co-binders and thickeners Coating additives...70 References

7 CHAPTER 5 CHAPTER 5 Pigments 1 Kaolin Occurrence and mineralogy Manufacturing coating kaolin grades Properties of coating kaolins Particle shape Particle size Typical physical properties of North American, and Brazilian kaolins Calcined, structured and engineered kaolins Dispersion of kaolins Flow properties and runnability of kaolin-containing coating colours Kaolin applications in paper and board coating Brazilian kaolins North American kaolins Structured and engineered kaolins Compositions and makedown of coating colours containing kaolin Properties of coated paper containing kaolin Woodfree topcoating LWC/MWC offset LWC rotogravure Ground calcium carbonate Material description Classification Methods of manufacture Calcium carbonate solubility Suspension stability slurries and their response in coating colours Product characteristics, availability and handling Reasons for use in paper coating Analytical procedures

8 Pigments 2.2 Economic and market factors Future outlook Acknowledgements Talc History of talc in paper Occurrence and mineralogy Magnesium carbonate derivative ore bodies Serpentine derivative ore bodies Silica aluminous rock derivative ore bodies Magnesium sedimentary deposit derivative ore bodies Mineralogy and morphology of refined coating talc Structural definition of talc Coating talc morphology Surface free energy of talc and talc/chlorite mixtures Processes for refining paper coating talc Raw material refining processes Milling Production of talc slurries Delivery forms, costs and logistics Talc applications in paper coating Talc in the production of LWC rotogravure publication papers Talc in the production of offset printing grades Talc in special coating applications Acknowledgements Calcium Sulphate (Gypsum) Properties of Calcium Sulphate (Gypsum) Physical properties Solubility Manufacturing Crystallisation Grinding Environmental aspects Composition and make-down of gypsum-containing coating colours Flow properties and runnability of gypsum-containing coating colours Coating applications of various grades of gypsum Properties of gypsum-containing coatings and coated papers Coated broke, gypsum solubility and wet end control

9 CHAPTER 5 5 Precipitated calcium carbonate Manufacturing of PCC and PCC coating grades Physical properties of PCC coating grades Compositions and makedown of coating colours containing PCC Flow properties and runnability of PCC-containing coating colours Coating applications of various grades of PCC Properties of coated papers containing PCC Future outlook Summary Synthetic plastic pigments Principal classes Chemical and physical properties Method of manufacture Shipping, handling, and storage Application and properties Typical formulations Rheology Optical performance Specific uses Specialty uses Advancements in hollow-spheres Mechanism of performance Gloss Opacity Brightness Bulk Thermal insulation Analytical procedures Critical properties and specifications Measurement methods and techniques Economic and market factors Future outlook Market trends and forecast Impact of developing technologies Alumina trihydrate Origin Manufacturing process

10 Pigments 7.3 Crystal structure Chemical properties Physical properties Rheological behaviour Formulation Coating applications ATH as an extender Analytical procedures Handling and packaging Future outlook Titanium dioxide Introduction Manufacturing titanium dioxide pigments Titanium raw materials Sulphate process Chloride process Surface treatment and finishing Properties of titanium dioxide pigments Chemical and physical properties Optical properties Other properties of TiO 2 pigments Titanium dioxide pigments in paper applications Dispersing of titanium dioxide pigments Properties of titanium dioxide pigment slurries Properties of coating colours containing titanium dioxide pigment Dry hiding of coating colours Properties of coatings and coated papers containing titanium dioxide pigment Optical properties Interaction of TiO 2 with fluorescent whitening agents Product safety Future prospects for titanium dioxide pigment References

11 CHAPTER 6 Binders 1 Starch Origin and manufacture of starch Handling of starch Modification of starch Degradation Substitution General characteristics of starch as a binder in surface sizing and coating Surface sizing Coating Effects of starch on the properties of paper and board in sizing and coating Surface sizing Coating Latex Introduction What is a latex? Physicochemical characterisation of polymer dispersions used as paper coating binders Emulsion polymerisation Storage and handling of latexes Types of paper coating latexes and their characteristics PVAc latexes Acrylic based latexes Styrene-butadiene latexes Other latexes Properties of styrene-acrylic and styrene butadiene-latexes Impact of latex on the paper coating process and paper quality Influence of latex on the coating colour Coated Paper Printing Trends and outlook References

12 CHAPTER 7 Co-binders and thickeners 1 Carboxy methyl cellulose Introduction Manufacturing and chemistry of CMC CMC grades for sizing and pigment coating of paper Addition of CMC to the coating colour mixture Effect of CMC on coating colour water retention properties Effect of CMC on coating colour flow properties Effect of CMC on coating properties Soy protein Introduction Origin, manufacturing, and chemistry Soy protein product range Key mechanism in coatings Coating rheology Runnability and water-holding capacity Anti-blocking and calender hazing Soy protein coating: formulation strategy Preparation of soy protein for coating formulations Conclusion Polyvinyl alcohol Manufacture Composition, structure, and properties Dissolution procedure, speed of dissolution and aging of PVOH solutions Polyvinyl alcohol grades for coating Effect on coating colour/flow properties Polyvinyl alcohol as coating binder

13 Co-binders and thickeners 3.7 Polyvinyl alcohol for surface sizing Effect on optical brightening agents/carrier properties Mechanism of PVOH as a carrier Comparison of different co-binders Which PVOH grade to select for optimum whiteness Amount of PVOH to be used Which OBA works best with PVOH Performance/economy balance when using OBA/PVOH combinations Synthetic co-binders and thickeners by Introduction Synthetic thickeners Chemical structure Main functions of thickeners Thickening mechanisms in the aqueous phase Thickening mechanisms in the coating colour Influence of synthetic thickeners on water retention Rheology of synthetic thickeners Associative thickeners Chemical structure and classification Associative thickeners in comparison with conventional synthetic thickeners Synthetic co-binders Chemical structure Main functions of co-binders Influence of co-binders and thickeners on coating properties Binding power and water resistance Brightness and opacity Smoothness and gloss Porosity, print gloss and glueability Printability Application Preparing coating colours Applications in coatings applied to different grades of paper Summary References For further reading about soy protein

14 CHAPTER 8 Additives 1 Introduction Coating dispersants Dispersion General aspects concerning dispersions Wetting Disruption of particle clusters Stabilisation General aspects concerning dispersants Amount of dispersant required for stabilisation Dispersant types Polyacrylate salts Other dispersants Foam control agents in surface sizing and paper coating Physicochemical basis of foam formation and stabilisation Physicochemical basis of foam control Foam and foam control in surface sizing and paper coating Process design and mechanical foam control Chemical foam control Methods of foam measurement Dosage of foam control agents Adverse effects of foam control agents Conclusion Water retention, rheology modifiers and ph control Optical brightening agents Chemical structure and fluorescence Types of optical brightener Role of carrier

15 Additives 6 Lubricants Calcium stearate Wax emulsions Other lubricants Insolubilisers Types of insolubiliser Glyoxal and its resin derivatives Melamine- and urea-formaldehyde resins Zirconium-based crosslinkers Use of crosslinkers Preservatives Types of preservative Conclusions References

16 CHAPTER 9 Surface sizing chemicals 1 Background and target of surface sizing Surface sizing materials Starch Carboxy methyl cellulose Polyvinyl alcohol Surface sizing additives Effect of size properties in sizing process Surface size formulations Pigmented surface sizing References

17 CHAPTER 10 Rheology of pigment slurries and coating formulations 1 Introduction Definitions of key rheological terms Types of flow behaviour Shear-rate-dependent rheology Time-dependent rheology Viscosity of suspensions Viscoelasticity Measurement of viscoelasticity Stress-relaxation experiments Creep experiments Normal forces Viscoelasticity of paper coatings Extensional viscosity Measurement of viscosity Controlled-strain versus controlled-stress measurement Measuring devices for rotational rheometers Capillary rheometers Correction for inertial effects Correction for non-newtonian fluids Correction for end effects Correction for wall-slip effects Using a capillary rheometer to characterise paper coatings Rheology of paper coatings Shear-rate ranges important to paper coatings Examples of paper coating rheograms

18 Rheology of pigment slurries and coating formulations 8 Effect of coating-formulation components on rheology Pigment-slurry rheology Effect of latex on paper coating rheology Effect of rheology modifiers on paper coating rheology Natural thickeners Synthetic thickeners Using rheological measurements to predict paper coating behaviour References

19 CHAPTER 11 Coating layer consolidation and formation of the coating layer structure 1 Introduction Wet coating structure Association between pigments and latex Association between pigment, latex and starch Association between pigment, latex and CMC Coating-base paper interaction Coating consolidation Introduction Water flow from coating to base paper, water retention and immobilisation Coating coverage and immobilisation Coating consolidation mechanisms Coating consolidation in blade coating Dry coating structure Packing and volume fraction of pigment Openness of the coating layer Surface properties Structure of coating layer and its thermal properties Modelling of coating consolidation and coating structure Consolidation, dewatering and immobilisation Pore structure simulation References

20 CHAPTER 12 Coating layer measurement and analysis techniques 1 Introduction Surface structure Topographical mapping Stylus profilometry Optical profilometry Confocal microscopy Scanning probe microscopy Optical interferometry Stereoscopy Air flow roughness Coefficient of friction Optical imagery Contact angle analysis Fluid droplets Immersion and extraction Optical properties Gloss Dynamic print gloss Goniophotometry Specular reflectance uniformity Chemical distribution and chemical composition of coating Bulk structure Mercury porosimetry Fluid imbibition Air permeability Staining

21 Coating layer measurement and analysis techniques 4.5 Brightness, colour and opacity Transmitted light imaging Coating burnout imaging Transmitted electron imaging Applicability of the Kubelka-Munk theory Physical cross sectioning and imaging Neutron scattering X-ray diffraction Effects of coating structure on measurements and analysis Brightness, opacity, and colour Sheet gloss Print gloss Print mottle Offset halftone dot fidelity Blister resistance Glueability and surface strength Rotogravure printability Inkjet printability Conclusions References

22 CHAPTER 13 Coating colour preparation and handling 1 Introduction Coating colour chemicals their share of material flows and logistics Basic design data of coating colour plants Delivery and storage of coating chemicals Handling of pigments Pigment make-down Industrial batch-type dispersers Commercial pigment slurries Processing of binders Latexes Starch processing in coating colour plants Batch cooking of starch Continuous cooking and conversion of starch Polyvinyl alcohol Protein Preparation of additives Coating colour preparation process Batch preparation of coating colours Continuous coating colour preparation Coating colour supply systems for coaters Ultrafiltration process References

23 CHAPTER 14 Coating and surface sizing technologies 1 Introduction Coating machine General Future of coating machines Coating machine components Flying splice unwind Off-machine tail threading On-machine tail threading Coating section web run Multiple coating of paper and board Blade coating techniques Coating station types Applicator roll coating station Short dwell coating station Jet applicator coating station Metering systems Blade metering Forces acting on the blade Blade loading systems Tube loading Constant tip angle loading Bent blade loading CD control of coat weight Surface sizing and film coating Introduction Surface sizing and coating techniques Pond size press Film size press Sizing and coating process layouts Size amount and coat weight development Film metering process Film transfer process

24 Coating and surface sizing technologies 4.4 Effect of running parameters on starch penetration and consequent paper and board quality Starch penetration analysis method Effect of linear load and nip dwell time on starch penetration Size press technology for different paper and board grades Printing and writing paper grades Paperboard grades Specialty paper grades Spray coating Spray coating process Integrated coating colour management for spray coating Industrial applications/installations Curtain coating Introduction Application and metering in curtain coating Principle Types of curtain coater Curtain generation Curtain impingement and elongation Coating colour considerations in curtain coating Rheology Curtain stability Deaeration Coating layer development and structure Curtain coating process layouts Specialty papers Commodity grades New technologies Introduction Modified coating techniques Spot/pattern coating coating specific areas Transfer and cast coating forming the coating layer separately Methods with reduced water and energy consumption Foam coatings from special additives to pigment coatings Wet end coatings making use of paper drying Waterless coating methods Powder coating Thin surface treatments also used to improve printability Radiation curable coatings Hot melt and lamination methods Summary References

25 CHAPTER 15 Drying of paper coatings and drying equipment 1 Introduction Heat and mass transfer in the drying of a coated web Infrared drying Introduction Principles of radiation drying Electrical IR dryers Gas IR dryers Air drying Air dryer types Operating parameters Construction of an air flotation dryer Air flotation dryer retraction system Air circulation system Air dryer nozzles Heat transfer and drying rate Energy consumption Cylinder drying Felting geometry Drying rate Hood enclosure Correct drying strategy Mottling Heating period and initial drying Consolidation, constant-rate drying Final drying, falling-rate drying Summary References

26 CHAPTER 16 Process control and automation in coating machine 1 Introduction On-line coat weight measurement methods Oven-dry weight difference measurement Ash difference measurement method Direct coat weight measurement method Machine-direction coat weight control Start-up control Cross-direction coat weight profile control Machine-direction moisture control Drying optimisation Cross-direction moisture control

27 CHAPTER 17 Development trends and future outlook 1 Pigments Binders Co-binders and thickeners Coating and sizing methods and technology Drying technology Measurement and automation technology Analysis techniques, modelling and simulation methods

Index. Page numbers in bold refer to figures and page numbers in italic refer to tables.

Index. Page numbers in bold refer to figures and page numbers in italic refer to tables. Index Page numbers in bold refer to figures and page numbers in italic refer to tables. Air permeance 66, 135-6 AFM 60,60-1 'Apron size press' 84, 85 Atomic force microscopy (AFM) 60,60- Barrier dispersion