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Pointers for Pressure-Sensitive Adhesive Selection

(December 2002) posted on Mon Dec 16, 2002

Learn about the different types of adhesives you can choose from, the kinds of applications they support, and the questions to ask yourself and your film supplier before selecting an adhesive.


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Choosing the best pressure-sensitive adhesive system for a screen-printed graphic can be challenging. While some performance criteria, such as whether the adhesive is permanent or removable, may seem obvious, other requirements may be less clear cut. Thinking of the adhesive as an integral component of the overall graphic or label helps set the stage for success. Consider the production requirements (how the graphic will be printed and finished) as well as the end-use needs (including where the graphic will be placed and how long it needs to last), and you'll be in a good position to select the best adhesive for any job.

Choosing the best pressure-sensitive adhesive system for a screen-printed graphic can be challenging. While some performance criteria, such as whether the adhesive is permanent or removable, may seem obvious, other requirements may be less clear cut. Thinking of the adhesive as an integral component of the overall graphic or label helps set the stage for success. Consider the production requirements (how the graphic will be printed and finished) as well as the end-use needs (including where the graphic will be placed and how long it needs to last), and you'll be in a good position to select the best adhesive for any job.

Understanding adhesive families

Pressure-sensitive adhesives (PSAs) are characterized by their ability to bond to a surface without the need for heat or mechanical force. Within the realm of PSAs are three main polymer families: silicone, acrylic, and rubber. Each of these adhesive types offers specific characteristics that are particularly suited for certain applications.

Rubber and silicone adhesives are often ideal for niche applications. For example, if you know your graphic or label will be bonded to something that will be exposed to high temperatures, a silicone-based adhesive may be the best choice. A rubber-based adhesive is a good option for an application that requires a strong, permanent bond at normal room temperatures of 60-80°F (16-27°C).

For most screen-printed applications, however, acrylic adhesives provide the best range of performance characteristics, enabling the label, graphic, or decal to remain adhered through a wide variety of environmental conditions. But variations abound even within the acrylic-adhesive family.

Application considerations

You should examine three fundamental areas when deciding which adhesive to specify for a job: the production process, the end use of the product, and the desired physical properties of the adhesive. This examination will lead you to several critical questions that must be addressed before you look at particular adhesive options.

To what type of surface will the label or graphic be bonded? In some cases, the surface may be rough or textured, requiring a thicker adhesive coating. Certain surfaces, such as powder-coated metals and plastics with low surface energies, have unique requirements that may also require a heavier coating. A more aggressive adhesive may be required if the surface is curved. All of these factors can affect which adhesive you select.

What type of film will be used? The characteristics of the substrate also must be considered. For example, polyester is a rigid and dimensionally stable film, while other stocks, such as polyethylene and vinyl, are more flexible. The same adhesive coated on these different materials may not deliver the same performance, so you need to consider the impact the substrate will have.

How will the label or graphic be produced? Rotary screen printing, sheet-fed screen printing, and digital printing are very different processes. Among other things, each delivers a completely different ink-deposit thickness. The ink deposit is important because the amount of ink on the substrate may affect its tendency to curl. So processes that lay down more ink may require a more aggressive adhesive.

Different finishing processes can also influence adhesive selection. Adhesives that are compatible with diecutting, for example, may not work well in applications that will be cut with a knife (i.e., cutting plotter, guillotine cutter).

What conditions will the label or graphic ultimately face in the field? Some graphics will be used exclusively indoors. Others will be exposed to the outdoors and subjected to a host of different environmental conditions. You must account for these conditions during adhesive selection and determine which adhesive will survive the application. The intended life span of the graphic is another consideration. For example, a floor graphic that needs to last for a six-month indoor promotion may not require the same adhesive as a product-identification label on a leaf blower that is expected to last ten years.

The previous questions will often lead to a host of further queries: Will the graphic be applied by hand or by machine? Will it be applied using a wet or dry method? Is there a window of time during which the graphic or label may need to be repositioned? All of these questions are valuable in that they help narrow the field of adhesive choices early in the process. Once you have the answers, you will be able to determine which adhesives will work best for your particular application.

Some graphics must last through the life of a consumer product. Some may need to form a permanent bond with the application surface eventually, but must be repositionable during installation. Others only need to last for a relatively brief period, but must be easy to remove. You have an assortment of PSAs to choose from for each set of job requirements.

Analyzing adhesive properties

Once you understand the production needs and end-use requirements of the application, you can focus on the physical properties of the adhesive. The characteristics of the adhesive--including the sometimes misunderstood trio of shear, tack, and peel--can weigh heavily on your decision making. Although these properties can be challenging to relate to the needs of the application at hand, they can help you anticipate how an adhesive will perform.

Shear measures the internal strength of an adhesive in the same plane as the bond. A graphic backed with a low-shear adhesive may slide and fall off over time. In contrast, a high-shear adhesive will keep a graphic in place. Shear is a particularly important factor to consider with curved application surfaces. Curved surfaces can induce gradual peeling or "flagging," often a slow process that can have a negative effect on the graphic's appearance.

Tack measures an adhesive's ability to form an instantaneous bond. A high-tack adhesive is called for in applications where the graphic must adhere to the application surface as quickly as possible. Conversely, a low-tack adhesive allows for the ability to reapply or reposition the graphic.

Peel is a measure of debonding, the force required to remove an adhesive from a surface. This measurement allows film manufacturers and converters to determine whether a product will be able to resist an anticipated force that may work against the adhesive bond. For example, an outdoor graphic may be subjected to high winds. The adhesive bond must be stronger than the anticipated force of the wind in order for the graphic to remain intact. Obviously, the substrate on which the adhesive has been coated also plays an important role in determining whether a graphic will hold up in such conditions.

Shear, tack, and peel must be considered in relation to one another when evaluating which adhesive will do the best job. For example, a higher-shear adhesive will likely have a lower level of peel. An application requiring a high-shear adhesive may also benefit from low-tack characteristics.

Once you have examined the production and end-use requirements, as well as the necessary adhesive properties, you are ready to apply these principles to actual applications. Below, we discuss adhesive selection for some common applications to help demonstrate how you can use these guidelines in your own business.

Window graphics

A solid knowledge of PSAs is indispensable for screen printers looking at the lucrative market for window graphics. The clear and white vinyls favored for both indoor and outdoor window graphics can be backed with a low-tack, removable PSA that allows for easy installation, repositioning, and removal without leaving residue.

Adhesives have a natural affinity for glass. Sometimes, this tendency to adhere quickly needs to be countered, especially when window graphics are large and require extra time and effort to position correctly. If the adhesive were to stick to the window too quickly, the installer might accidentally tear or stretch the graphic during an attemp to reposition it. A low-tack adhesive gives the installer a greater ability to reposition and apply the graphic without risking such damage.

Some window applications, however, call for permanent PSAs. Two-way window decals, for example, are often used for longer-term promotional activities. The adhesive needs to last for the decal's anticipated life span, and it usually includes a non-yellowing agent to help maintain graphic clarity. Although not as common, custom removable adhesives may be used for short-term, two-way window-decal promotions.

The window-graphics market also includes perforated one-way signage. This technology allows graphics to be printed on one side of a material while permitting viewers on the other side to look through the window. Perforated adhesive film is ideal for point-of-purchase signage and event promotion, and it is also used widely for transit advertising. A removable/repositionable adhesive is the preferred PSA here so that installers can quickly remove and change messages as needed.

Floor graphics

Screen- and digitally printed floor graphics provide an eye-catching way to direct consumer attention to specific products or promotions. These advertisements are placed on floors, carpets, and even sidewalks. Although you can enhance the visibility of your customer's advertising message through the inks, substrates, and finishing processes you use, the adhesive you select will ultimately determine the success of the graphic.

For interior applications, permanent/removable PSAs bond firmly to a wide variety of flooring surfaces such as waxed commercial PVC tile, certain ceramic tile, and sealed concrete. The strength of the bond is particularly important in high-traffic areas where a loose graphic could become a tripping hazard. At the same time, the graphic must be easy to remove completely when the promotion is over. Often used in conjunction with white, clear, and holographic vinyls, these adhesive systems must meet ASTM non-slip standards.

Many of these same requirements come into play when looking at outdoor floor graphics. But here, the PSA must adhere to more challenging surfaces such as asphalt, concrete, and other outdoor walking surfaces. For outdoor work, an aggressive permanent/removable adhesive is often used with an opaque vinyl.

Recent PSA developments have opened the way for new carpet-graphic opportunities. Designed with low-pile, commercial carpets in mind, these adhesives minimize the amount of fiber pull so they remove cleanly when the promotional period ends.

Transit advertising

According to industry trade publications and associations, the transit-advertising market has been growing exponentially in the US over the past decade. Buses, cars, bus shelters, kiosks, and subways now sport a variety of advertisements on pressure-sensitive film and other substrates. The bus-graphics market, in particular, has exploded and driven PSA and film suppliers to create new systems that meet the unique challenges of this rapidly expanding segment.

Success in the transit-advertising field hinges on thoroughly understanding the requirements of the finished graphic. The obvious challenge with a bus graphic is to adhere dozens of square feet of film to the painted, contoured metal of the bus, but this isn't the only consideration. Will the graphic be screen printed or produced on a wide-format digital printer? Will the substrate be shipped in roll or sheet form? These factors also play a role in determining the best adhesive for the job.

While an aggressive permanent adhesive may seem like the best choice at first, other requirements must be taken into account. Because of their size, these graphics need to be lined up and applied carefully, much like enormous sheets of wallpaper. The adhesive must therefore be repositionable. Another important criterion is that the adhesive must be easily removed when the promotion has ended, ensuring that the vehicle is not damaged. These advertising messages change frequently, so the application and removal processes must take place in the field to minimize the amount of time that the vehicles are out of service.

The challenge of selecting the right adhesive is further complicated by the fact that the graphic will be exposed to the same grueling outdoor conditions that the vehicle encounters on a daily basis--dirt, debris, ice, high humidity, extreme temperatures, intense UV rays, and harsh chemicals. You must also know the environment in which the vehicle will be driven. An adhesive that works well in the arid Southwest may not be suitable for the muggy Southeast or the bitter cold of the Northeast.

P-O-P graphics

When it comes to creating point-of-purchase decals, displays, and promotional signage for both indoor and outdoor messages, screen printers can choose from a number of versatile general-purpose and high-performance permanent adhesives that bond excellently to a variety of surfaces. In an effort to attract and hold the attention of consumers, many screen printers and P-O-P designers have also begun to utilize specialty adhesives. Today, metallic flakes and fluorescent pigments are commonly added to adhesives so that, in conjunction with a clear substrate, the P-O-P graphic will truly "pop."

For larger P-O-P graphics, PSAs provide a powerful advantage over non-adhesive substrates, which require bulky mounting hardware for installation. Materials such as pressure-sensitive calendered vinyl, on the other hand, can be adhered easily with minimal labor. This has allowed designers to develop thinner and simpler backing panels on their displays, reducing the amount of time needed to apply new graphics and remove old ones.

Stickers and decals

Screen printers looking to break into the novelty and seasonal market for stickers and decals will find a wide range of PSAs ready to back them up. This segment includes college, alumni, and association decals, after-market product decoration, holiday, seasonal, and novelty stickers, and bumper stickers.

Permanent adhesives are often the choice for decals in the holiday/seasonal and novelty markets because they bond well to a variety of surfaces. A clear permanent adhesive is favored when it comes to college, alumni, and association decals, such as car-window decorations. Low-tack removable-adhesive systems are sometimes used for decals that may be applied to store windows.

Acid-free adhesives can be a valuable option for particularly delicate graphic applications. These products will not yellow over time and resist deterioration. This is particularly important when creating novelty stickers and decals that will be used in conjunction with scrapbooks, photo albums, and other personal memorabilia. Films designed for this market are available with permanent, repositionable, and removable adhesives. The removable and repositionable adhesives allow the decal to be removed cleanly, yet adhere to many different smooth, semi-smooth, flat, and curved surfaces.

Adhesives used in the novelty-decal market can also offer visual power. Adhesive manufacturers can mix in metallic flakes of varying sizes to give the decal a unique look. Similarly, fluorescent pigments can be added without diminishing the effectiveness of the adhesive. Color-matching capabilities are also available, allowing printers to address color-specific applications without incurring the large-volume commitment associated with purchasing custom colored vinyl. These adhesives can create brilliant results when used in conjunction with a clear substrate.

Product identification and safety/hazard labels

Durability and aesthetics are key concerns for this particular market segment. From outdoor power tools and lawn and garden equipment to appliances and consumer electronics, these durable-goods labels must remain attractive, legible, and permanently adhered for the product's life cycle. For product-identification labels, the branding and name recognition of the product hinges on the long-term durability of the label. Likewise, a safety/hazard/instructional label must remain intact to ensure consumer safety and protect OEMs from liability due to malfunction or misuse.

Adhesive selection depends heavily on the application surface and environment the label must endure. In many cases, these labels must withstand a wide range of conditions, such as exposure to moisture, UV light, and wide temperature ranges. In addition, these labels cannot lift or flag. If the label slips because the adhesive did not anchor it to the product's surface, it may leave behind a sticky residue that could collect dust and dirt. This can produce an unattractive "halo" effect around the label edges--not something that will enhance the OEM's image in the marketplace.

High-performance, permanent acrylic adhesives are often favored for these challenging applications. The adhesive thickness, or coat weight, can be tailored to meet the requirements of the intended surface. For example, a thicker coating may be necessary to counter the effects of a textured or powder-coated surface. Certain plastics with a low surface energy may also require a heavier adhesive coating.

Asking the right questions

In order to produce the best label, graphic, or decal, you must understand the production and end-use requirements of the job, as well as the physical properties of the adhesive system. That is why it is crucial to work with an experienced pressure-sensitive film supplier that can offer you a host of value-added choices. Selecting the right adhesive will do more than keep your graphic adhered--it will help you bond with your customers.


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