Coating and Drying Defects

EDGAR B. GUTOFF, SCD, is a consultant in coating and drying. He coedited Modern Coating and Drying Technology (Wiley) and coauthored The Application of Statistical Process Control to Roll Products as well as the first edition of this book. He was the first secretary of the International Society of Coating Science and Technology and served until 2002. He is a Fellow of the AIChE and of the Society for Imaging Science and Technology and an adjunct professor at Northeastern University.EDWARD D. COHEN, PhD, is a technical consultant in all aspects of the web coating process. He recently retired from DuPont Central Research and Development as a DuPont Fellow. Dr. Cohen is coeditor of Modern Coating and Drying Technology (Wiley) and coauthor of the first edition of this book.GERALD I. KHEBOIAN (Chapter 11 author) is a consultant on web transport systems. He has over thirty-five years' experience applying web handling systems, electrical drives, tension controls, and machine controls. He is past chairman of the national TAPPI Electrical Engineering Committee and of the TAPPI Drive Seminar Series.

• Preface xvPreface to the First Edition xviiAbout the Authors xxi1 Introduction 12 Troubleshooting or Problem-Solving Procedure 5Basic Troubleshooting Principles 6Detect the Defect 8Define the Defect or Problem 9Name the Defect 11Problem Statement 15Collect and Analyze Additional Data 16Process Information 16General Process Information 17Analytical Data 19Standard Operating Procedures 19Analyze the Data and Identify Potential Causes of Problem 19Eliminate the Problem 25Document the Results 25Use Results to Prevent Reoccurrence 27References 273 Coater and Defect Analytical Tools 29Overview of Analytical Tools 30Analytical Methods to Characterize Defects 30Defect Test Procedures 31Video Enhanced Microscopy 31Surface Characterization Techniques 33Coater Characterization Instruments 38Data Loggers 42Rheological Measurements 44Surface Properties 49On-Line Characterization Systems 50On-Line Inspection Systems 50On-Line Coating Weight Measurement 52On-Line Viscosity 54Data Analysis Techniques 55Keeping Current 56References 614 Problems Associated with Feed Preparation 63Dirt and Other Particulates 63Filtration 64Agglomerates 66Bubbles 68Poor Cross-Web Uniformity 69In-Line Mixers 70Temperature Control 70Flow Control for Down-Web Uniformity 70Feeding the Coating Liquid 70Flow Control 72Magnetic Flow Meters 72Ultrasonic Meters 73Coriolis Meters 73References 745 Problems Associated with Roll Coating and Related Processes 75Coating Methods 75Control of Coating Weight 82Ribbing 86Ribbing in Forward Roll Coating 87Ribbing and Cascade (or Herringbone or Seashore) in Reverse Roll Coating 89Gravure Coating 92Streaks and Bands in Roll Coating 92Chatter in Roll Coating 94Sagging 94References 956 Problems in Slot Extrusion Slide and Curtain Coating 97Description of Coating Methods 97Limits of Coatability 107The Window of Coatability 107Slide Coating 109Vortices and Eddies 110Slot Coating 117Extrusion Coating 124Curtain Coating 126Effect of Surface Roughness on the Limits of Coatability 128Instabilities in Premetered Coating 131Chatter 131Ribbing 131Neck-in 133Scalloped Edges 133Edge Beads and Heavy Edges 133Waves in Slide and in Curtain Coating 135Streaks and Bands in Premetered Coating 137References 1417 Coating Problems Associated with Coating Die Design 143Cross-Web Uniformity 143Temperature Control 148Corrections to the Coating Die Internals 149Die Internal Pressures and Spreading 150Extrusion Dies 151Wall Shear Stress Control and Residence Time Control 153References 1558 Surface Tension Driven Defects 157Surfactants 158Surface Tension Effects in Coating 159Surface Tension Driven Defects 162Convection or Bénard Cells 162Craters 163Fat Edges or Picture Framing 164Nonuniform Edges in Multilayer Coatings 165Dewetting and Crawling 166Adhesive Failure and Delamination 167Nonuniform Base Surface Energies 168References 1689 Problems Associated with Static Electricity 169Formation of Charges 171Electrostatic Fields 173Surface Resistivity 174Measurement of Static Charges 177Removal of Charges 180Free Charges 180Bound Charges 182References 18310 Problems Associated with Drying 185Dryer Control and Solvent Removal 189Drying Condition Case History 195Drying Defects 198Air Bar Rubs 199Blushing 199Bubbles and Blisters 200Cockle or Wrinkling 201Curl 201Mud Cracking 204Delamination 204Dryer Bands 205Dryer Remelt 206Dryer Contamination Spots 207Dryer Condensation Spots 208Haze 208Mottle 208Orange Peel and Crinkling 209Overspray 209Reticulation 209Surface Blow-Around 211Microscopic Defects 211Starry Night 211Pinholes 212The Role of Dryer Simulation in Troubleshooting 212The Role of Radiative Heating in Defect Reduction 214References 21511 Problems Associated with Web Handling 217by Gerald I. KheboianWeb 218Web Characteristics and Problems Associated with the Web 219Base Web Defects that Cause Wrinkling 219Causes of Web Defects 221Web Attributes 221Web Camber 221Web Cross-Machine Thickness Profile Variation 223Basis Weight Profile Variation 225Surface of the Web 225Plastic Web 227Web Transport Systems 228Tension 230Reasons Why Operators Must Change Tensions in Order to Successfully Transport Web 231Unwind Stand 231When to Use Drive Motors on an Unwind Stand 233Web Control 234Core Selection and Build-Down Ratio 235Draw Rolls Pull Rolls Capstans 235Web Slippage and What Slip Affects 235Nip Rolls 236Unnipped Rolls (Elastomer Rolls) 237Suction Rolls 241Suction Aprons or Tables 242Increasing the Reliability of Draw Roll Sections 243Tension Isolation 243Differential Tension 244Driving Web-Carrying Rolls Within a Dryer 245The Rewinder 246Use of Accumulators at the Rewind and Unwind 246Surface/Center Rewinds 247Center-Driven Winders 248Drives for Laminators 248When to Drive Both Laminator Rolls 249Coating Roll Drives 249Coating Defects—Sources and Some Solutions 249Backlash Exists in Timing Belts Gearboxes and Geared Couplings 250Chatter Caused by the Roll 251Roll Unbalance 251Nonconcentric Rolls 252Coating Roll Drive Systems 252Feedback Devices 254Coating Supply System 254Transmission of Vibration Through Structural Members to the Coating Stand 254Effect of Dryer Design on Coating Quality 255Troubleshooting ‘‘Sudden’’ Coating Problems 255Monitoring Speeds and Tensions of Web 257A Speed Monitoring System 257Monitoring the Speed of Idler Rolls 257Monitoring for Slip Conditions on High-Speed Coaters 258Tension Monitoring Systems 258Specifying the Tension Range 259Why Is Tension Control Needed? 259Effects of Wiring Practices on Tension Control 260When Do You Use a Pure Speed Control? 261Equipment Lists Avoid Problems and Help Plant Personnel 264Frequency and Direction of a Speed Perturbation 265Problems that Occur After the Machine Is in Production 266Types of Tension Control Systems 266Motor Current Regulation 268Dancer Roll Control 268Example of the Use of a Dancer in Close Proximity to an Air Roll 270Using Air Roll Air Gap Control as a Dancer in the Dryer 270Force Transducer Control 272Sizing Strain Gauge Load Cells 274Section Characteristics that Ensure Good Web Transport Control 274Master or Lead Section 274Using Load Cell Control in Close Proximity to an Air Roll 275Defects that Occur When the Master Section Slips 275Sections with Limited Ability to Control Web Transport or Tension 275Dealing with Sections that Have Limited Ability to Control Tension 276Sections that Always Slip or Almost Always Slip 277Drive Features that Give the Operator Additional Tools 278Tools that Are Used for Troubleshooting Drive Problems 278Optical Alignment Tooling and Laser Technology 278Machine Alignment: Avoiding Many Web Handling Problems 278Experience: Dryers Installed with the Dryer Centerline Skewed from the Machine Centerline 279Alignment Problems Due to Building Location in a Swamp or Tidal River 279The Alignment Process 280The Baseline 281Checking the Alignment of a Size Press (Roll Coater) on a Paper Web 282Knowing How Your Machine Operates Can Avoid Problems in the Future 283Summary 284References and Bibliography 28412 The Role of Process Support Equipment 287Introduction 287Definitions 288Defect Support Equipment Interactions 291Support Equipment Effect on Defects 291Mechanical Chatter 291Serpentine Chatter 292Poor Physical Uniformity 294Ribbing 294Bubbles 294Contamination Spots 294Low Overall Productivity 295Defects Due to Static Electricity 295Scratches 295Streaks 296Mottle 296Coating Weight Nonuniformity in Premetered Applicators 297Coating Weight Nonuniformity in Self-Metered and Doctor Coaters 297Air Bar Rubs in a Floater Dryer 297Dry Point Variability 298Selection of Equipment 29813 Coating Defects Database 299Need for Coating Defects Database 299Defects Database: Examples 302Base Flake 303Roll Repeats 303Bubbles 303Bacteria Bubbles 306Aimcal Defects Lexicon: Guide to Web Coating Technology 306Description of Lexicon 306Need for Lexicon 306Lexicon Overview 308Lexicon Sections 308Glossary 308Troubleshooting Guide 309Defect Image Section 310Search Capability 310Availability 312References 31414 Defect Prevention 315Introduction 315Define Product and Process Requirements 316Ensure Availability of Valid Analytical Techniques 319Ensure Web Coating Process Meets Specifications 319Use Appropriate Coating Method 320Develop and Implement Operational Procedures 321Quantify and Understand Defect Losses 321Use Structured Troubleshooting Protocol 325Use Computer Database to Store and Share Information 325Implement Improved Process Technology 327Use Statistical Process Control 327Develop Fundamental Process Understanding 328Index 331

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