The coatings application process is a continually developing technology. Over the past 100 years coating methods have gradually developed from the first basic roll coater to the wide variety of techniques currently used in the converting industry. The development of new technology is continuing with the following current basic trends:

• Improved applicator performance and flexibility
• Increased use of specialized coating devices
• Increase in use of pre-metered applicators
• Availability of versatile laboratory coating machines
• Improved process control

Improved applicator performance and flexibility: A basic trend has been an increase in the performance and quality of all applicators. The range of coating weights, line speeds, operating ranges and quality have all been extended. This is a result of an increase in coating process research by both the academic and industrial research communities.

There are 30 basic coating applications techniques and about 1000 specific configurations of these methods. In this discussion, the coating applicators are classified by the mode used to control the final coverage, or thickness of the layers. This classification will help to gain an understanding of the mechanisms involved and the current trends in applicator choice. The four basic modes are:

• Self-metered, in which the coverage is determined by interaction of fluid with device and by conditions in the coating meniscus, viscosity, gap, line speed, roll speed ratios, etc. Examples are dip coating, reverse roll and forward roll.
• Doctored, in which a post-coating device is used to control the final coverage. Examples are Mayer rod, blade coating and air-knife coating.
• Pre-metered, in which the coverage is determined by controlling the supply of the coating solution to the applicator. Examples are slot die and slide coating.
• Hybrid methods, which use mixtures of the above to control coverage. Gravure coating is an example.

Increased of use of specialized coating devices: As a result of the increased performance of all of the coating methods, it is now apparent each method has its own advantages and disadvantages and that no one method can meet all of the coated product requirements. Therefore, there is a trend to use multiple methods to closely match the needs of the product with the capability of a specific coating method.

A successful coating line must be versatile and have the capability to use multiple coating methods. Cartridge coaters are increasingly used to meet this need. They are coating stands which permit the rapid change of a variety of coating heads in a plant environment. Direct and reverse gravure, offset gravure, reverse roll slot die and metering rods are among the techniques which can be used with this approach.

Another factor in the trend towards specialized coating devices is a growth of high-value specialty products, which have unique coating needs. Specialized techniques are being developed to coat these products. An example is in the coating of fuel cell membranes and coating of thin film battery anodes and cathodes. These require discrete coatings on the web and not the typical continuous coatings; therefore, there is a trend toward coaters which apply patches of coating on a web (see diagram on previous page).

Slot die coaters are used for this application. The flow to the die is started and stopped as needed to give the machine direction spacing and inserts in the die are used to control transverse direction spacing. Gravure coating can be also be used for patch coating. However, the slot die method is preferred because of wide range of coat weights, coating weight uniformity, and no recirculating solutions.

Gravure coating is an old technique whose versatility has been increased by recent technology developments. A very useful new gravure-coating technique is the Micro-gravure™ technique, which was introduced in the early 1990's by Yasui Seiki Co., Ltd. It is intended for low coating weight products on light-gauge films for imaging, electronics, packaging, batteries and other specialty applications.

The unique features are the use of small diameter rolls, 20-50 mm, versus 150-300 mm for conventional gravure. This results in a small stable bead, which when combined with reverse application gives very good quality and a low coating weight. A typical configuration is shown in the diagram above.

Another example of the increase in specialized coating techniques is the trend towards 100% solids coating using hot melt and extrusion applicators. Concerns over solvent emission and waste disposal are resulting in eliminating of solvents and more products being coated at 100% solids. The development of die coaters with lower coating weight capability is contributing to this increased usage.

Increased use of pre-metered applicators: There is a trend towards the pre-metered coating methods replacing the self-metered and doctor mode methods in a variety of applications. This trend is expected to accelerate in coming years. Technology improvements in the last 10 years have resulted in an increased range of coat weight, particularly on the low end. Quality and uniformity of both the final coated product and the applicator has been significantly improved. A better understanding of fluid flow mechanisms has led to improved die design via computer modeling. This permits dies to be designed and fabricated for specific applications at reasonable costs. In addition, support equipment such as pumps is more available, at less cost.

Self-metered roll coaters are an old technology. They have been the backbone of the industry for over 100 years and are a known reliable process. They have the advantage of a strong practical and theoretical knowledge base. They can be used for a wide variety of products and can coat over a wide range of coat weight, line speeds and viscosities. However, they have several disadvantages which has lead converters to seek other methods:

• they are open to the atmosphere, creating solvent evaporation and environmental problems
• it is difficult to control coverage because of sensitivity to viscosity change
• the excess coating solution is constantly recirculating and aging, and can pick up contaminants.

Self-metered processes are increasing in usage because the coverage uniformity range and control are excellent and are not dependent on fluid properties. Other advantages are that coating uniformity and quality are excellent; it is a closed system, which minimizes solvent evaporation; it can coat multiple layers; there are no recirculating coating solutions; and the basic principles are well understood, so users have the knowledge to effectively run the process.

Slot die, for solution coatings, and, extrusion dies, for 100% solids coatings, are the major methods in this category. They are used for a wide range of coatings, and use will continue to grow. A typical multilayer slot die is pictured on page 50.

An emerging method is the slide coater, pictured on page 52. This method has been used extensively to coat multilayer photographic films. Until recently, these coaters were not made commercially, because film manufacturers tended to make their own applicators. A commercially available slide coater was shown at the April 2001 CMM show in Chicago. There are now commercial sources of these slide coaters and they will find increased application as converters learn to apply multiple layers simultaneously instead of sequentially.

Availability of versatile laboratory coating machines: The typical development of a coated product starts with a laboratory bench process to coat sheets, then moves to a continuous web pilot coater and then to the final production coater. A recent trend has been the development of improved hardware for the first two steps in the development. The bench top sheet coaters are moving from hand coater with a die frame or Mayer rod followed by poorly controlled air drying, to automated bench top sheet coater/dryers which have automated coating followed by controlled dry drying. The automation improves the quality, reproducibility and productivity. Another advantage is that drying can be controlled so that experiments to determine drying/curing effects on product performance can be run.

Continuous-web benchtop coaters, which use a narrow web (typically 6 in.) and have all of the needed elements for continuous coating except for automated roll changes, are also being introduced and are finding many applications. Typical applicators available include reverse roll, knife over roll, or forward roll, which more closely simulates a commercial process. These coaters give a more uniform coated product, improved reproducibility and better productivity then with hand methods.

While the purchase price of both classes of laboratory coaters are higher then the hand tools, the increases in productivity and uniformity gives them an overall advantage. Both of these can increase the output of coated samples by at least an order of magnitude.

Another recent trend in laboratory coaters has been the development and introduction of compact modular machines. These machines contain all of the elements needed to continuously coat a precision saleable product: unwind, coater, dryer, rewind, and control systems. The modular feature permits customization of the dryer type, length and coating methods. Since they are available in widths up to 18" and at line speeds up to 130 f.p.m, they can also be used to produce commercial quantities of products, if a wider width is not needed. For specialty low volume products the economics are favorable. The purchase cost is typically under $1,000,000, productivity can be as much as several million square feet a year, and a small crew can be used to run the unit. Often the route to commercialization may be more effective using several of these modular coaters, with each dedicated to a specific product version, instead of one large machine.

Improved process control: The control systems for the coating line are becoming increasingly sophisticated with the addition of computer control and information systems. This sophistication is leading to higher line speeds, more up-time, better quality and improved problem solving. Advances in on-line measurements of coating weight, viscosity, defects and dry point determination provide the input signals needed for effective closed-loop process-control systems.

The computers that implement these control system also are being used to store process data. This permits rapid data retrieval to help diagnose a problem as it is found. These computers are also connected to company networks, so experts at remote locations can view data.

Editor's note: a handy coating methods table will appear with the online version of this article (www.convertingmagazine.com). It summarizes the range of key variables, wet thickness, line speed and coverage uniformity of the major methods. Data are based on published values from several sources and the experience of the author. An estimate of the cost of each applicator is also included.