Web Coating: Process Characterization

Published: November 01, 2008, By By Edward D. Cohen, Edward D. Cohen Consulting Inc.

The web coating process has about 15 major components, and each of these components, in turn, has many hardware subsystems. All these hardware subsystems contain process variables that must be measured and controlled to ensure they are stable and within control limits.

The result is several hundred process variables that need to be characterized to guarantee a reproducible, cost-effective process. All process components are critical, and none should be ignored.

Process Variables

There are three basic data classes to be characterized.

• Process control variables include line speed, web tension, drying air, dry-bulb temperature, coating solution temperature, and flow rate to the applicator, pump speed, and many others. These vary depending on the product type and must be measured and maintained with established control limits.
• Process measurements result from process control properties, e.g., the dry bulb and solvent concentration of the returning dryer air. This can be used to calculate the actual drying performance in each zone. The actual coating weight on the substrate is another variable.
• Fundamental coater properties include drying air slot velocity and roll conditions as well as coating applicator conditions.

These measurements should be stored in a historical database. Then they can be used to troubleshoot defects, maintain quality, and identify process components that need to be improved.

Technology Has Advanced

Previously, the ability to obtain and analyze all the needed characterization data was limited because of several factors. The measurement, control, and data acquisition technology was expensive and limited in the variables that could be measured. The needed data initially was obtained on a strip chart or manually recorded on a paper log sheet. This data could be entered manually into a computer system. However, the process was time consuming, expensive, and could have significant errors because of incorrect data entry into the computer or missing data.

As the technology developed and hardware costs decreased, the utilization of computer data acquisition increased. In addition, the low hardware costs also resulted in additional data points being collected and more accurate process characterization. There was an improved understanding of the fundamentals of the process.

Recent technological advances in process measurement, data collection and information systems, computer performance, and data storage have resulted in the availability of new, low-cost, precise measurement and digital data acquisition systems for the web coating process. These systems give the converter the ability to do a thorough process characterization at a very low cost.

Also, since the data is available in digital format, it can be analyzed easily to determine defect causes, areas of poor reproducibility, and performance of individual products. In addition, the fundamental mechanisms of the key coating process subsystems continually have been developed and have resulted in the identification of additional process variables to be measured. Use of this new technology will improve process performance and economics significantly.

The majority of the necessary process measurements can be made with conventional, commercially available instrumentation, such as thermometers and infrared (IR) thermometers for temperature, velometers for air velocity, speed sensors for line speed, footage counters, viscometers, coating weight measurement devices, etc.

However, some characterizations still are subjective, such as substrate roll condition, substrate planarity, and coater roll surface quality. As a result, visual analysis is still a very important characterization method. The figure to the right shows the major subsystems that need to be characterized.

Raw Materials

Raw material measurements are needed to ensure all compounds have the desired purity and they are the correct compounds. Product performance can be very sensitive to the quality of the coating solution and the substrate.

Several instruments can be used to characterize raw materials' chemical and thermal properties. The most versatile methods for purity and composition are IR and ultraviolet (UV) spectroscopy, atomic absorption, gas chromatography, and scanning electron microscopy (SEM) with energy-dispersive X-ray analysis. Thermal analysis techniques can be used to determine transition temperatures and purity levels. The exact tests and specification will depend on your product needs.

If the vendors are ISO certified, then their test results can be used. However, some key properties occasionally should be taken to verify vendor results.

The properties of the substrate are critical to ensure good coating quality and product performance. The substrate needs to be uniformly wetted and free of contamination to give a uniform wet coating. Also, the coating solution must adhere to the substrate.

A defective substrate will cause yield loss and cannot be corrected by any of the process in the web coating line. Following are the key substrate properties that need to be measured:

• Uniform thickness in machine direction (MD) and transverse direction (TD), which is particularly important in a doctoring process in which the distance from blade to substrate determines final coating weight;
• Planarity of the substrate — it should have good planarity and not curl, otherwise it may get damaged in the coating applicator or in the dryer;
• Quality of uncoated substrate roll — it should have straight sides and no telescoping;
• Contamination level in the substrate;
• Transition temperatures — Tg (glass transition temperature), Tm (melting temperature), and Td (temperature difference);
• Location in master roll if substrate is slit before coating;
• Surface wettability and adhesion;
• Identification of the side to be coated;
• Surface roughness.

Solution Preparation and Characterization

The coating solution properties need to be measured to ensure there will be excellent coating quality and no coating defects. These are the important properties to characterize in the mixing process:

• The correct raw materials added in the proper level and at the right addition points;
• Batch size;
• Mixing time and temperature profile;
• Agitator speed;
• Solution/dispersion properties — percent solids, pH, particle size, purity;
• Viscosity, surface tension, and contact angle;
• Special performance properties;
• Correct filter type and size — monitoring filter pressure to ensure filter is functional;
• Storage conditions of solution if not coated immediately after preparation;
• Cleanliness of kettles and transfer lines.

Solution Delivery

Typically, a separate feed system is used to supply a uniform coating solution to the coating applicator. Key variables for this process are as follows:

• Pump speed and flow rates to applicator;
• Recycle flow rate and line location;
• Consumption rates;
• Filter pressures versus time;
• Temperature and viscosity values during coating;
• In-line injection addition rate;
• Cleanliness of kettles and transfer lines.

Coating Applicator

There are several variables that influence the coating quality and coverage level. The specific variables to be characterized for a roll coating process include the following:

• Gap between applicator and coating rolls, measured with substrate in place;
• Gap TD uniformity;
• Roll speeds, gaps, and direction for each roll in multiroll coaters;
• Roll surface quality;
• Flow rates to roll coating pan;
• Solution and applicator temperatures.

For a slot die coater, these are the specific variables to be characterized:

• Gap between slide lip and coating roll;
• Vacuum box vacuum level and clearance;
• Flow rate to slot die;
• Solution and applicator temperatures;
• Shims in distribution channel;
• Die lip configuration;
• Die plate and support identifying numbers;
• Die lip sharpness.

There are some general variables that should be characterized for all coating methods:

• Vibration levels of the coating station and coating rolls;
• Cleanliness of all coating applicator hardware and transfer lines;
• Temperature and relative humidity (RH) of coating applicator ambient air;
• Coater room temperature and RH;
• Level of airborne particulates, contaminants, and pollutants in coater room ambient air.

The Drying Process

The function of the dryer is to remove solvent from wet coating without introducing any physical or chemical defects in the dried coating. The drying rate is determined by the air velocity level and uniformity, line speed, solvent concentration in drying air, and the difference in temperature between coating and dry bulb of air.

Quality can be affected by high air velocity disturbing the coating, contamination from drying air, and non-uniform air delivery to coating surface. To get good quality and high productivity, the following variables must be controlled:

• Web temperature all zones;
• Dry bulb, both supply and return, in all zones;
• Solvent concentration, both supply and return, in all zones and wet bulb or dew point if aqueous;
• Air flow volume in supply and return ducts;
• Slot velocity in selected nozzles both average and profile;
• Filter pressure on dryer air in all zones and motor amperage;
• Dryer air balance;
• Actual solvent removal in each zone, which can be calculated from drying air volume and solvent leveling return air;
• The dry point of a coating, which is the location in the dryer where the coating is essentially free of liquid. It can be detected by measuring solvent level, film web temperatures, or by visual observation. The exact location in the dryer and the uniformity in the TD should be measured and recorded. Under no circumstances should the moving web ever be touched.

On-Line Measurements

There are two new technology systems — on-line coating measurement and surface defect inspection — that are essential characterization techniques. Each of these has its own data storage and analysis system, which also can be included in an overall plant information system.

• On-line coating weight gauge to measure coating weight, MD and TD profiles, and roll-to-roll averages;
• Surface inspection systems to identify defects, frequency, type, and location.

Web Handling

The web handling system must transport the web through the coater at a uniform line speed without distorting the substrate or creating defects. Uniform line speed is essential to uniform coating weight. These are the key variables to be measured in the transport system:

• Line speed, average, and variability;
• Tension levels in each zone — average and variability;
• Drive motors' performance;
• Edge alignment;
• Substrate roll physical conditions;
• Coated roll length;
• Performance and conditions of transport rolls;
• Vibration levels;
• Roll speeds.

Storage

The coating process raw materials, coating solutions, and coated rolls are stored at various times. The ambient temperature and RH of the storage facilities should be controlled and measured.

Process Control & Information Systems

Process control and information systems are computer systems that automate and regulate the operations in a continuous coating line. They control, monitor, record, and store all the independent process variables.

They can use several types of control logic based on the needs of the process. In addition, they perform several other functions, such as measure and record the dependent variables in the process. Also, they record all of the roll bookkeeping functions, uncoated and coated roll information, and raw material lots.

The system also can interact with surface inspection systems so defect levels can be correlated with process variables. The obtained data can be analyzed by several statistical routines, such as JMP and Minitab statistical software.

Edward D. Cohen, Ph.D., is a technical consultant for the Assn. of Industrial Metallizers, Coaters & Laminators (AIMCAL). He has 40+ years of experience in research and manufacturing technology. Contact him at 480-836-9452; This email address is being protected from spambots. You need JavaScript enabled to view it..

This article, along with future articles by other authors, is provided as a cooperative effort between PFFC and AIMCAL. Authors contribute to AIMCAL's technical and education offerings, which include the association's Fall Technical Conference, Converting School, and Ask AIMCAL.

Want To Learn More?

For more information on optimization of the coating process, see PFFC's Special Report on coating and laminating in February 2008, or visit our website at www.pffc-online.com and click on the “One Stop” category: Coat/Laminate.

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s).