Pressure-sensitive labels are seamlessly integrated into our physical environment. As product labels, UPC stickers, and price labels, they can be found in an enormous variety of tapes and labels in every room of a house, every building, every manufacturing facility, every vehicle, and even outdoor environments. From bathroom and kitchen fixtures, to cars, trucks, buses, planes, trains, motorcycles and bikes, to computer components, food packaging, clothing and fabric products, our world is literally covered with them. Masking tape, scotch tape, package sealing tape, product labels, bandaids, colorful kids’ stickers……it’s hard to imagine modern life without them.
The history of adhesive labels goes back to the development of surgical tape by an enterprising surgeon and a cotton buyer in the 19th and early 20th centuries. However, technological development of pressure sensitive stickers took a leap forward as a result of manufacturing advances during World War II. Today, continued improvement in these materials has made it possible to use them in an ever-widening range of circumstances and conditions, extending to extreme environments such as underwater, outer space, and very cold or hot applications.
Pressure-sensitive adhesive, or PSA, has the characteristic of being permanently tacky, which means that it can bond to a surface using low pressure. This trait distinguishes it from liquid adhesive, which requires a period of drying and solidifying before it adheres. A pressure sensitive adhesive combines the attributes of viscosity (a measure of its resistance to flow, also defined as thickness or stickiness) and elasticity.
Adhesive coating methods are what give pressure-sensitive labels their usefulness.
Adhesive labels are basically a sandwich made of a combination of materials, called a construction. It is composed of several layers. The first is the facestock, (so termed because it is the part of the label that faces the viewer or reader). It is the surface layer of the label and can be created from a number of materials including paper, film, foil, or fabric. Next is the adhesive itself, which will vary depending on the client’s needs and the surface that the facestock will be bound to. The liner, the final component, is a backing made of plastic film or paper and is meant to be discarded after use. The construction may also include a primer coat which is applied in the pre-treatment process and adds extra adhesive properties where these are desired.
The manufacturing process for these products consists principally of coating and laminating. There are a number of different options here, dependant on a variety of factors including intended use, cost, thickness and viscosity needed, the specific technology used (hot melt, emulsion, solvent or 100% solids), solids content, characteristics of the material to be coated, accuracy of coating weight, and, given all these factors, whether transfer or direct coating should be employed.
Transfer coating and direct coating are the two main coating methods for PSA’s. In transfer coating, the adhesive is applied to the liner and then transferred to the backing or facestock in the course of the lamination process. With direct coating, the PSA is applied directly to the facestock.
There is a distinction between pre-metered coating methods, in which the adhesive thickness is measured prior to application to the substrate or base, and post-metered, in which the thickness is determined after application. Gravure roll coating, reverse roll coating, slot die coating, extrusion slot die coating, and curtain coating are all pre-metered methods. Let’s consider each of these in turn.
Gravure Roll Coating
This method, which includes offset gravure, reverse gravure, and direct gravure, produces a very smooth coating surface. It is one of the most expensive coating methods, delivering highly accurate coat weights at high rates of speed, and is used for fluids in a wide range of viscosities. Consequently, it is highly versatile and can be used across a wide variety of market sectors. All gravure coating methods use a pre-metered or pre-measured method of delivering coating solution to the substrate or base layer.
Reverse Roll Coating
Reverse roll methods utilize a dual-roll system. A metering roll and an applicator roll run in counter-rotation. They are distinguished from other coating methods by the use of two reverse-running nips. The metering nip and the application nip work in tandem to deliver a precise amount of coating to the substrate while the rolls run in opposite directions to the substrate. Highly polished finishing coatings are possible with this method. Magnetic tapes and coated papers, as well as pressure-sensitive tapes, are manufactured using reverse roll coating.
Slot Die Coating
In slot die coating, the adhesive solution is applied to a surface via pump pressure through a narrow slot or channel that is positioned close to the substrate. An advantage of this method is that it has the ability to deliver very uniform coatings over large surfaces. It can be used for a variety of viscosities and solution types and can easily be scaled up to systems that use roll-to-roll coating and sheet-to-sheet deposition.
Extrusion Slot Die Coating
Extrusion coating differs from the above method in that it utilizes a coating head that is fed directly from an extruder. It features a gap and nip arrangement that delivers the polymer onto a substrate. This method has virtually replaced reverse roll coating on account of being more economical as well as being highly applicable to a wide range of coating needs. It offers coating uniformity, ability to coat on flexible or rigid surfaces, and scalability from small runs to large industrial scale operations. It is also highly efficient, uses a relatively small footprint thus requiring less floor space, and can utilize fluids at both high and low viscosity.
Curtain coating technology provides a steady flow of fluid from a slot die. A “curtain” or “waterfall” is formed as the fluid falls onto the substrate. This method is ideal for coating irregular surfaces. It requires high-speed application and a narrow application window due to the need to keep the fluid at a uniform temperature and viscosity. If the “curtain” is not positioned at the correct height above the substrate, gaps and incorrect coat weight may occur. Curtain coating has been most prevalent in European markets but is beginning to gain more favor in American manufacturing.
Next, we’ll consider a knife over roll coating and Mayer rod or wire-wound coating, both of which are post-metered coating methods.
Knife Over Roll Coating
Knife over roll or KOR coating is used when a large amount of adhesive needs to be applied to a surface that is uneven (such as textile, fiberglass, or cement). The coating is laid down on the substrate using a bolster roller and is spread with a “knife” or metering blade to the desired thickness. The excess coating is removed in the final step.
Mayer rod (Wire-Wound Rod Coating)
This is the third most popular method of manufacturing PSA’s as well as the least expensive. Wire-wound rods are typically used in low-viscosity applications such as solution or emulsion PSA’s. They are adaptable enough to be used with nearly any coating or substrate. The appearance of adhesive ridging, which results from the impression made by the wire on the substrate, is associated with Mayer rods.
With so many adhesive technology options to choose from and so many variables that affect the final product, it can be challenging to decide on a coating method that’s appropriate for your labels. Reach out to Arrow Systems for assistance with all of your PSA needs. We’re here to help.