Adhesives and Coating Decision Making

Published: September 22, 2025

By Tom Kerchiss, Chairman, RK Print Coat Instruments Ltd.

Consumer expectations coupled with industry demands continue to exert pressure on adhesive and sealant manufacturers to provide new adhesives or refine existing formulations that are environmentally acceptable and yet will enable users to achieve the desired degree of bonding or sealing of often dissimilar surfaces. A tall order, perhaps.

Developments in adhesives require the various components involved to be tested under realistic processing environments. Naturally enough, as most adhesives used in converting are coated there is considerable interest in pilot coating systems that enable various industry technologists, formulators and packaging professionals for example: to trial different materials using appropriate coating applicator and support technologies.

Every segment of industry has different adhesive requirements and associated areas of concern in terms of processing. In flexible packaging adhesives must provide good clarity and bond strength. Adhesives often need to be resistant to heat; others must be able to withstand lengthy periods of low temperature and rapid thaw conditions. Adhesives must seal seams and welds sufficiently; bond and remain fail free for the lifetime of the product.

Manufacturers, environmentalist and recycling authorities also want adhesives that can be recycled easily in with current recycling strategies. Laminated pouches and laminating adhesives are difficult to dispose of in normal waste streams. Mono based materials and those that engineered to comply with recycling or sustainability strategies hold out great promise.

Heat seal coating may be appropriate in certain circumstances and are an alternative to laminating adhesives. Heat seal coatings are applied to a substrate via a coater and then wound up into a roll. This roll of coated material can then be unwound at a later date and the heat seal can be adhered to another substrate surface using a combination of heat and pressure.

An advantage of this type of adhesive is that it does not have to bond immediately. It also means that bonding is a distinctly separate and controllable operation. Heat seal coating is used in electronic and medical applications or when precision is required. For example: laying down print and coated lacquers in discrete patterns.

No process is trouble free. A problem that can occur with heat seal coating is that there is a risk of blocking when wound up into a roll. Blocking transfers the roll of heat sealed material into a solid log. Blocking may be due to depositing too thicker layer of coated adhesive. However, there is some evidence to suggest that drying, either too much or too little may cause blocking.

Products such as pressure sensitive adhesives for labels and tapes are primarily a coating and perhaps process. The objective may sound simple; deposit a uniform amount of adhesive onto a web. Adhesives are available in a variety of adhesive formulations, each subject to specific viscosities and levels of solids. The converter or coating operative may be required to coat labels or other stock in various thicknesses and coating weights.

Pressure sensitive adhesive items may be coated via the pre-metered method; that is the thickness of the adhesive is predetermined prior to applying it to the web. Post metering is another option. In post metering the adhesive or amount of adhesive is determined after the adhesive has been deposited onto the web. So what factors govern the chosen method of coating?

The actual choice of coating method depends upon the viscosity of the adhesive, the solids content, the properties and characteristics of the substrate to be coated, the coat weight parameters and the specifics of the application.

Pre-metered methods of coating include reverse roll and gravure. The latter can provide for a smooth coating profile and as such may be regarded as being very good for coating pressure sensitive materials.

For post metered coating of adhesives meter bar is a very serviceable and easy coating method. However, there is a risk of ridging and therefore this wire wound coating method is limited to low viscosity formulations with good flow characteristics.

When round table discussions and briefing takes place thought must be given to the thickness of the PSA, which may vary somewhere between 0.8 ml on up to and sometimes more than 5ml. Also up for consideration is the adhesive formulation itself. Is it emulsion, hot melt or more often than not these days, a custom configured formulation, involving water based or a bio-based, bio-compostable or a synthetic.

Converters and suppliers are not only concerned with the performance of the bond at point of bonding or sealing but the overall performance of the converted and adhesive coated product throughout its practical life. Many of the changes that take place are dynamic and are often the ones the coating operative, the converter and others may have little control over. For example, gauge band variations, even changes in substrate.

The environment in the workplace can pose problems. Heat or lack of it, moisture, humidity and static can disrupt production. Many of the processing transients are resolvable. Pilot coating systems, especially, those offering short run capability and configured appropriately enable users to trial materials and formulators and undertake processes that would generally not be feasible using a production line coater.

We live in interesting times with new formulations for adhesives and with materials in general changing all the time. Packaging, labeling and other segments of industry are endeavoring to become more environmentally acceptable, reducing carbon footprint and conserving scarce earth resources.

Coating systems, particularly custom configured machines, enable the manufacturer and converter to work towards sustainability and efficiency. It remains to be seen what A1 and other forms of machine learning have to offer, as these are early days and confidentiality clauses are often in place.

About the Author

Tom Kerchiss is the chairman of sample preparation system and print/coat/laminating technology specialist at RK PrintCoat Instruments Ltd. The company, which won an Innovator in Pre-Press Award for the FlexiProof 100, supplies printing ink manufacturers, both large and small, as well as printers, converters and other businesses with color communication devices for all of the major print disciplines.