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 xv

Preface to the First Edition xvii

About the Authors xxi

1 Introduction 1

2 Troubleshooting or Problem-Solving Procedure 5

Basic Troubleshooting Principles 6

Detect the Defect 8

Define the Defect or Problem 9

Name the Defect 11

Problem Statement 15

Collect and Analyze Additional Data 16

Process Information 16

General Process Information 17

Analytical Data 19

Standard Operating Procedures 19

Analyze the Data and Identify Potential Causes of Problem 19

Eliminate the Problem 25

Document the Results 25

Use Results to Prevent Reoccurrence 27

References 27

3 Coater and Defect Analytical Tools 29

Overview of Analytical Tools 30

Analytical Methods to Characterize Defects 30

Defect Test Procedures 31

Video Enhanced Microscopy 31

Surface Characterization Techniques 33

Coater Characterization Instruments 38

Data Loggers 42

Rheological Measurements 44

Surface Properties 49

On-Line Characterization Systems 50

On-Line Inspection Systems 50

On-Line Coating Weight Measurement 52

On-Line Viscosity 54

Data Analysis Techniques 55

Keeping Current 56

References 61

4 Problems Associated with Feed Preparation 63

Dirt and Other Particulates 63

Filtration 64

Agglomerates 66

Bubbles 68

Poor Cross-Web Uniformity 69

In-Line Mixers 70

Temperature Control 70

Flow Control for Down-Web Uniformity 70

Feeding the Coating Liquid 70

Flow Control 72

Magnetic Flow Meters 72

Ultrasonic Meters 73

Coriolis Meters 73

References 74

5 Problems Associated with Roll Coating and Related Processes 75

Coating Methods 75

Control of Coating Weight 82

Ribbing 86

Ribbing in Forward Roll Coating 87

Ribbing and Cascade (or Herringbone or Seashore) in Reverse Roll Coating 89

Gravure Coating 92

Streaks and Bands in Roll Coating 92

Chatter in Roll Coating 94

Sagging 94

References 95

6 Problems in Slot Extrusion Slide and Curtain Coating 97

Description of Coating Methods 97

Limits of Coatability 107

The Window of Coatability 107

Slide Coating 109

Vortices and Eddies 110

Slot Coating 117

Extrusion Coating 124

Curtain Coating 126

Effect of Surface Roughness on the Limits of Coatability 128

Instabilities in Premetered Coating 131

Chatter 131

Ribbing 131

Neck-in 133

Scalloped Edges 133

Edge Beads and Heavy Edges 133

Waves in Slide and in Curtain Coating 135

Streaks and Bands in Premetered Coating 137

References 141

7 Coating Problems Associated with Coating Die Design 143

Cross-Web Uniformity 143

Temperature Control 148

Corrections to the Coating Die Internals 149

Die Internal Pressures and Spreading 150

Extrusion Dies 151

Wall Shear Stress Control and Residence Time Control 153

References 155

8 Surface Tension Driven Defects 157

Surfactants 158

Surface Tension Effects in Coating 159

Surface Tension Driven Defects 162

Convection or Bénard Cells 162

Craters 163

Fat Edges or Picture Framing 164

Nonuniform Edges in Multilayer Coatings 165

Dewetting and Crawling 166

Adhesive Failure and Delamination 167

Nonuniform Base Surface Energies 168

References 168

9 Problems Associated with Static Electricity 169

Formation of Charges 171

Electrostatic Fields 173

Surface Resistivity 174

Measurement of Static Charges 177

Removal of Charges 180

Free Charges 180

Bound Charges 182

References 183

10 Problems Associated with Drying 185

Dryer Control and Solvent Removal 189

Drying Condition Case History 195

Drying Defects 198

Air Bar Rubs 199

Blushing 199

Bubbles and Blisters 200

Cockle or Wrinkling 201

Curl 201

Mud Cracking 204

Delamination 204

Dryer Bands 205

Dryer Remelt 206

Dryer Contamination Spots 207

Dryer Condensation Spots 208

Haze 208

Mottle 208

Orange Peel and Crinkling 209

Overspray 209

Reticulation 209

Surface Blow-Around 211

Microscopic Defects 211

Starry Night 211

Pinholes 212

The Role of Dryer Simulation in Troubleshooting 212

The Role of Radiative Heating in Defect Reduction 214

References 215

11 Problems Associated with Web Handling 217
by Gerald I. Kheboian

Web 218

Web Characteristics and Problems Associated with the Web 219

Base Web Defects that Cause Wrinkling 219

Causes of Web Defects 221

Web Attributes 221

Web Camber 221

Web Cross-Machine Thickness Profile Variation 223

Basis Weight Profile Variation 225

Surface of the Web 225

Plastic Web 227

Web Transport Systems 228

Tension 230

Reasons Why Operators Must Change Tensions in Order to Successfully Transport Web 231

Unwind Stand 231

When to Use Drive Motors on an Unwind Stand 233

Web Control 234

Core Selection and Build-Down Ratio 235

Draw Rolls Pull Rolls Capstans 235

Web Slippage and What Slip Affects 235

Nip Rolls 236

Unnipped Rolls (Elastomer Rolls) 237

Suction Rolls 241

Suction Aprons or Tables 242

Increasing the Reliability of Draw Roll Sections 243

Tension Isolation 243

Differential Tension 244

Driving Web-Carrying Rolls Within a Dryer 245

The Rewinder 246

Use of Accumulators at the Rewind and Unwind 246

Surface/Center Rewinds 247

Center-Driven Winders 248

Drives for Laminators 248

When to Drive Both Laminator Rolls 249

Coating Roll Drives 249

Coating Defects—Sources and Some Solutions 249

Backlash Exists in Timing Belts Gearboxes and Geared Couplings 250

Chatter Caused by the Roll 251

Roll Unbalance 251

Nonconcentric Rolls 252

Coating Roll Drive Systems 252

Feedback Devices 254

Coating Supply System 254

Transmission of Vibration Through Structural Members to the Coating Stand 254

Effect of Dryer Design on Coating Quality 255

Troubleshooting ‘‘Sudden’’ Coating Problems 255

Monitoring Speeds and Tensions of Web 257

A Speed Monitoring System 257

Monitoring the Speed of Idler Rolls 257

Monitoring for Slip Conditions on High-Speed Coaters 258

Tension Monitoring Systems 258

Specifying the Tension Range 259

Why Is Tension Control Needed? 259

Effects of Wiring Practices on Tension Control 260

When Do You Use a Pure Speed Control? 261

Equipment Lists Avoid Problems and Help Plant Personnel 264

Frequency and Direction of a Speed Perturbation 265

Problems that Occur After the Machine Is in Production 266

Types of Tension Control Systems 266

Motor Current Regulation 268

Dancer Roll Control 268

Example of the Use of a Dancer in Close Proximity to an Air Roll 270

Using Air Roll Air Gap Control as a Dancer in the Dryer 270

Force Transducer Control 272

Sizing Strain Gauge Load Cells 274

Section Characteristics that Ensure Good Web Transport Control 274

Master or Lead Section 274

Using Load Cell Control in Close Proximity to an Air Roll 275

Defects that Occur When the Master Section Slips 275

Sections with Limited Ability to Control Web Transport or Tension 275

Dealing with Sections that Have Limited Ability to Control Tension 276

Sections that Always Slip or Almost Always Slip 277

Drive Features that Give the Operator Additional Tools 278

Tools that Are Used for Troubleshooting Drive Problems 278

Optical Alignment Tooling and Laser Technology 278

Machine Alignment: Avoiding Many Web Handling Problems 278

Experience: Dryers Installed with the Dryer Centerline Skewed from the Machine Centerline 279

Alignment Problems Due to Building Location in a Swamp or Tidal River 279

The Alignment Process 280

The Baseline 281

Checking the Alignment of a Size Press (Roll Coater) on a Paper Web 282

Knowing How Your Machine Operates Can Avoid Problems in the Future 283

Summary 284

References and Bibliography 284

12 The Role of Process Support Equipment 287

Introduction 287

Definitions 288

Defect Support Equipment Interactions 291

Support Equipment Effect on Defects 291

Mechanical Chatter 291

Serpentine Chatter 292

Poor Physical Uniformity 294

Ribbing 294

Bubbles 294

Contamination Spots 294

Low Overall Productivity 295

Defects Due to Static Electricity 295

Scratches 295

Streaks 296

Mottle 296

Coating Weight Nonuniformity in Premetered Applicators 297

Coating Weight Nonuniformity in Self-Metered and Doctor Coaters 297

Air Bar Rubs in a Floater Dryer 297

Dry Point Variability 298

Selection of Equipment 298

13 Coating Defects Database 299

Need for Coating Defects Database 299

Defects Database: Examples 302

Base Flake 303

Roll Repeats 303

Bubbles 303

Bacteria Bubbles 306

Aimcal Defects Lexicon: Guide to Web Coating Technology 306

Description of Lexicon 306

Need for Lexicon 306

Lexicon Overview 308

Lexicon Sections 308

Glossary 308

Troubleshooting Guide 309

Defect Image Section 310

Search Capability 310

Availability 312

References 314

14 Defect Prevention 315

Introduction 315

Define Product and Process Requirements 316

Ensure Availability of Valid Analytical Techniques 319

Ensure Web Coating Process Meets Specifications 319

Use Appropriate Coating Method 320

Develop and Implement Operational Procedures 321

Quantify and Understand Defect Losses 321

Use Structured Troubleshooting Protocol 325

Use Computer Database to Store and Share Information 325

Implement Improved Process Technology 327

Use Statistical Process Control 327

Develop Fundamental Process Understanding 328

Index 331