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Flexography is the preferred method for producing pressure-sensitive labels. The process consists of a series of operations designed to apply a variety of coatings to a continuous moving web or roll of material. The final construction will be made up of a minimum of two or as many as four separate layers that are laminated together.
Pressure-Sensitive Sandwich
The pressure-sensitive product derives its name from the way the adhesive works. Unlike other types of adhesives that form a body with the surface they are applied to through evaporating or absorbing a solvent to form a solid material, pressure-sensitive adhesives stick to the substrate through the use of slight pressure.
The total construction consists of a variety of layers, each with a specific function and purpose. The typical layers include top coating, face stock, primer, adhesive, release coating, release liner, and anti-block coating. For most applications, the material will contain one face stock, adhesive, and release liner. However, in some applications, multiple layers can be added to produce a label on top of a label, or what is called a piggyback product.
Face stock is the term used to describe the label material. It is the portion of the pressure-sensitive construction that will contain the graphics and be applied to the final object. The face stock will always be the layer with the adhesive permanently bonded to it. Face stocks are available in a variety of materials. They can be made from paper, plastic, metal, and specialty materials such as cloth or embossed, metallized foils that give the effect of a prism. The physical requirements of a face stock are dependent on both the end use and converting requirements of the finished label, as well as the need to pull the web through the coater during manufacturing.
Adhesive is the portion that allows the label to adhere to the final object. Adhesives are available in a variety of formulations to provide the desired performance. Pressure-sensitive adhesives are classified in three ways: Coating technology (hot melt, solvent, emulsion), chemistry (rubber-based, acrylic), and the type of adhesive (permanent or removable).
A pressure-sensitive adhesive functions as the name implies. It is generally a soft, viscous material that is sticky to the touch. When applied to a substrate with slight pressure, it will begin to flow into the pores of the surface and develop a specific level of adhesion. Differences in the surfaces of the substrate, the conditions the label is applied under, and the properties of the adhesive will affect the ultimate performance of the label.
Permanent vs. Removable
Pressure-sensitive adhesives may be permanent or removable. A permanent adhesive is designed to adhere to a substrate without edge lifting, which cannot be removed without damaging either the label or the substrate. A removable adhesive is designed to adhere to a substrate without edge lifting, and can be removed without damage to either the label or the substrate.
The definition of a permanent or removable adhesive is clear and understandable; however, classifying an adhesive is not so easy. A variety of factors—including the composition of the substrate, the length of time the label is applied, and the conditions the label is exposed to during its life—affect the performance of the product. In addition, permanence or removablility is a subjective evaluation that can vary from person to person.
In many cases, customers require an adhesive that maintains some short-term ability to be removed before becoming permanent. This is sometimes incorrectly referred to as repositionability, as the label is rarely reapplied. This feature is desirable when labels are misapplied and set aside until, sometimes hours later, it can be removed, and the bottle replaced on the line for processing. The length of time the label remains removable will vary from adhesive to adhesive and from substrate to substrate.
Some adhesives may be further categorized according to special end-use requirements. For example, dairy label adhesives are designed specifically to adhere to polyethylene milk jugs. Freezer-grade adhesives are for substrates that are used on products as cold as -20° F.
Release coating is a material that is applied to the top side of the release liner, which prevents the adhesive from adhering to it. The release coating is a silicone-based material, which is applied either prior to the manufacturing process or in-line with the adhesive coating operation. Release coatings are available in both solvent and solventless versions.
Label application is the process of removing the label from the liner and applying it to the final object. Self-adhesive labels can be applied in a variety of ways. The simplest and sometimes the most practical method is by hand, whereas for high-speed, high-volume applications an automatic applicator may be employed.
Finishing Process
Labels can be finished in rolls, sheets, or fanfolded. As with any printed material, printed labels could be processed in any bindery postpress operations.
Roll labels are often made into master press rolls, which are then run through inspection equipment before packaging into final form. Inspection equipment will wind roll-to-roll, often employing a strobe light and an optical counter to aid the operator. Rewinding can be done either on cores or coreless. Finished rolls of labels can be used manually, or for automatic dispensing. Roll labels can also be placed in dispenser boxes where a label is dispensed by pulling on the liner.
Sheeted labels can be used as inserts to a product, packaged for resale, or for sheet-fed printers. Sheeted labels may be packaged on-press or palletized for later packaging.
Waste around a sheeted label can be removed immediately on press. The critical aspect of sheeted label production is keeping trim and sheeter dust to a minimum. Edge standards need to be established and monitored so that tooling can be replaced when needed.
Fanfolded labels are typically used in a computer printer, where this is the traditional form of print media. Fanfolded labels may also be used in automatic labeling equipment to provide a "non-stop" supply of labels—the lead edge of a fanfolded stack can be taped to the tail edge of the previous stack to extend running.
Chris Freddo is the vice president of New York Label & Box Works. For information, contact him at chris@nylabel.com. Elizabeth Anselmo also contributed to this article.
author: By Chris Freddo