Hot Melts Hold Fast
Understanding the variables involved in pressure-sensitive label construction can help optimize labeling operations and reduce costs.January 2012 By Hardi Doehler, Evonik Industries AG
The quality of pressure-sensitive adhesive (PSA) labels and tape is often judged long after products are wound and packaged. These products can be subject to wide swings in temperature and other environmental conditions well before they are called into action months later. Yet when the time comes, the adhesive must release cleanly from the liner, without tears or undue tack. Adhering too strongly (or releasing too early) can frustrate customers, and in high-speed food and beverage labeling operations, delays can be costly.
PSA label construction
A PSA "laminate" is composed of two main layers, the label (or tape) facestock and the liner, with a PSA and silicone release-coating laminated between (illustration below). The silicone coating and liner material make up the release liner. Understanding the variables involved in PSA label construction and performance can help optimize labeling operations and cut costs.
Many printers and converters simply buy ready-to-print label rollstock (with all four components). Some want to create their own rollstock for greater control and higher speeds. They may buy a pre-coated siliconized liner, then apply a PSA, and laminate the liner and label together. They may even formulate their own adhesive, which can be applied to either the label or release liner side. With clear facestock, the label can be reverse printed so there's no need for a protective coating. Or they may use in-line equipment that can siliconize the liner, apply adhesive, and print the label—all in one high-speed process.
With radiation-curable (RC) silicones as the liner release coating, energy savings can be significant compared to thermal-cure coating systems. And with in-line coating of silicone and adhesive, cost savings can range 30 percent or more. Equipment for coating and UV curing these silicones can be extremely compact, making retrofitting into existing adhesive coating lines easy. In-line coating equipment is available both for truly high-speed wide-web and narrow-web printing lines.
Hot melt adhesives are widely used to produce PSA labels and tapes on a variety of paper and plastic liner materials substrates. Because hot melts are designed to take the heat, shipping and storage temperatures are unlikely to present a problem for the adhesive (when properly stabilized with antioxidants). What can present an issue, however, occurs between adhesive and liner material. The cold-cure UV-silicone liner coating must provide a barrier between the liner material and adhesive without undergoing undue interactions that can affect proper release. One key measure of that performance is the opening force, and it should remain constant for a long period.