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Why Sticky Stuff Sticks, Part Two

Jim continues his discussion about adhesives.

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Editor’s Note: Last month, Jim discussed the various components that comprise pressure sensitives, the difference between tack and adhesion, and how the energy levels of different substrates affect adhesion. The pressure-sensitive sandwich The pressure required to make a pressure-sensitive material work isn’t its sole trait. Pressure sensitives are also distinct from other adhesive families because they comprise multi-layered constructions called pressure-sensitive sandwiches."

Like a local deli that offers various sandwiches, the pressure-sensitive sandwich utilizes many different formulations. These constructions can be divided into two broad categories: self-wound and linered. The simplest self-wound construction merely comprises an adhesive applied to a facestock, such as paper or a plastic film.

This product category includes masking tape, paper and film application tapes, and self-wound overlaminates. There are two construction variations: priming the facestock’s second surface — the adhesive side — and applying a release coat to the first surface. Just as priming ensures a better final coat of paint, it seals the surface if you’re coating a paper facestock.

By sealing the paper, less adhesive soaks into the facestock; thus, more adhesive stays on the surface. This allows more consistent performance throughout the roll. Because the adhesive lays atop the paper, there’s more adhesive, which improves the adhesive’s cold flow. Priming also enhances the adhesive’s bond to the facestock. A better adhesive anchor means that, if adhesive touches adhesive, less adhesive delaminates when you pull them apart.

Also, with better anchorage, less adhesive transfers to the substrate after the tape is removed. Some self-wound constructions are release-coated to provide easier unwinding; also, the process prevents tape from "blocking" on the roll. Blocking occurs when a self-wound product sticks together and doesn’t unwind. Self-wound products include masking tapes, application tapes and surface-protection films.

Linered materials include a siliconized release liner, such as cast or calendered vinyl, double-sided foam tapes, supported and unsupported transfer adhesives, and linered overlaminates. The release liner protects the adhesive from the pressure-sensitive material, stabilizes the construction during conversion operations and smoothes the adhesive to provide the clarity needed for overlaminating films.

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Release liners are constructed from various base materials, such as polyester and polypropylene films, and densified or polycoated kraft papers. Like anything else, each material exhibits strengths and weaknesses. Liners are manufactured in various thicknesses. Commonly used in the sign industry, 78-lb. liners have the flexibility necessary for plotter cutting, while 96-lb., polycoated liners are ideal for screenprinters.

Heavier paper lays more flatly on the press, and the polycoating prevents moisture absorption. Thus, the liner doesn’t grow and tamper with registration. Among the linered, pressure-sensitive products on the market, the most basic include single-liner, unsupported transfer adhesives, or transfer tapes. A transfer tape is not the same as an application tape.

Transfer tapes merely entail an adhesive applied to a release liner, which some refer to as "glue on a roll." A fancier version is transfer tape with two liners, which requires users to remove one of the liners prior to application to a print, nameplate or graphic panel. Transfer tapes are often used to manufacture polycarbonate control panels. These panels are printed sub-surface, which means the polycarbonate film is printed in reverse, on its second surface, or underside.

Then, transfer tape is laminated to the printed side. The release liner protects the panel’s adhesive until application. In this application, the two most commonly used adhesive thicknesses are 2-mil for smooth surfaces and 5-mil for rough applications. A supported transfer adhesive takes its name from the film carrier sandwiched between two adhesive layers. In the digital-graphics industry, this product is usually called a mounting tape. The carrier serves numerous functions.

It stabilizes the tape during the lamination process, provides additional rigidity for the print and allows the use of two different adhesives. Commonly, a mounting tape comprises the following layers: release liner, a removable or non-removable adhesive, film carrier, permanent adhesive and the second release liner. The permanent adhesive applies to the graphic, while the other side adheres to the substrate. Dozens of double-sided, foam-tape constructions are designed for numerous applications.

Typically, these tapes contain a foam carrier, coated on both sides with adhesive, and a release liner that protects the adhesive on at least one side. A foam tape with a "closed-cell" construction (which resembles enclosed bubbles) forms an excellent moisture barrier. In contrast, if the foam carrier has a sponge-like structure, which resembles burst bubbles, the construction is called open-cell. Predictably, this construction sucks up water like a sponge.

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The following types of foam carriers make product selection confusing: high-density polyethylene, neoprene, acrylic, urethane and vinyl. The key difference among them is the internal, or shear, strength of the material. The last, but not least, type of linered products is paper and film products used in the sign, screenprinting and label industries.

In addition to cast and calendered vinyl, several products use various facestocks, such as polyester, polypro-pylene and paper. The typical construction includes a siliconized release liner, adhesive, occasionally an additional liner, facestock and, sometimes, a top coating or corona treatment to enhance surface energy and improve ink stability.

Rubber or acrylic?

Would you like rubber or acrylic adhesive on your pressure-sensitive sandwich? Adhesive systems fall into these two broad categories, and each has unique characteristics. Soft, rubber-based adhesives wet out well. Because soft adhesives flow readily, they coat the substrate and provide very high "quick stick" to a surface. Because rubber adhesives wet out better than acrylics, they require less pressure during application to form a bond. A rubber-based system’s tack allows adhesion to more surfaces.

The adhesive’s tackiness makes it an excellent choice for working with low-energy plastics. As an example, if a shop needs to make decals for a manufacturer of polypropylene outhouses (as I once did), a vinyl with a rubber-based adhesive will probably adhere well to this plastic substrate. By contrast, vinyl with an acrylic adhesive is likely to fall off unless the plastic has been either corona- or flame-treated.

Rubber-based adhesives also exhibit consistent bonding to the adherent, meaning, adhesion values don’t gradually grow as they do with acrylic adhesives. This is why rubber-based adhesives are used when making premasks. The adhesive bond doesn’t build on the vinyl graphics during long-term storage. Otherwise, you’d have difficulty removing the premask.

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However, rubber-based adhesives are susceptible to plasticizer migration. Thus, if you’ve applied a vinyl graphic with a rubber-based adhesive to a vinyl banner or flexible-face material, the banner’s plasticizers could soften the adhesive to the point of failure. Also, rubber-based adhesives are vulnerable to UV degradation and oxidation.

As oxidation and UV weathering progress, rubber polymers start to break down and, eventually, cause adhesive failure. Oxidation also causes rubber-based adhesives to yellow; that’s why your application tape yellows when exposed to light. The yellowing caused by slight adhesive degradation usually isn’t enough to be an issue.

In contrast to rubber, acrylic adhesives withstand higher temperatures, exhibit good shear and resist the degrading effects of UV light, plasticizers and chemicals. However, they don’t stick well to low-energy surfaces. Although acrylic adhesives cost more than their rubber counterparts, they offer a much wider range of performance characteristics. Because signshop owners and screenprinters have diverse needs and applications, vinyl manufacturers provide their customers with an extensive product menu, including vinyl films with permanent, removable and repositionable adhesives.

Vinyl makers blend their adhesives and provide end-users with products offering specific attributes. For example, highly aggressive, permanent adhesives can be coated on brittle, ultra-destructible films to produce safety labels. This construction is so fragile that it breaks down into tiny pieces when someone attempts to remove the label.

Repositionable vinyl films make graphics application significantly easier, because the installer can snap a material back onto the substrate and then reinstall the material without damaging the graphic. The latest improvement features an adhesive with a microstructure of tiny tunnels. These tunnels make graphics applications easier because they provide channels through which air can escape. The result is a bubble-free, vinyl application.

Coating technology Rubber and acrylic adhesives are available in several varieties. However, in the sign industry, the only widely used types are solvent and emulsion. When adhesive components combine with a solvent, the solvent dissolves the mixture and evenly dispenses elements to form a solution. Solvent-based systems have been the mainstay of the pressure-sensitive industry for decades.

Today, manufacturers must deal with solvent recovery and disposal. By comparison, water-based adhesive systems are emulsions. Fine particles of the system’s solid components are suspended in water. Rubber-based adhesives that are coated on paper application tapes are also emulsions. Further, acrylic emulsions, which have been used for years, are becoming more popular because they’re more cost-effective and contain no VOCs.

Product testing

Product evaluation includes tests ranging from visual inspection to precise measurements. For example, vinyl and overlaminate manufacturers inspect adhesives for clarity and color. When performing a light-transmission analysis, the measured light transmitted through the test sample is compared with a reference sample. An adhesive’s clarity is important for such applications as window graphics. Technical managers evaluate an adhesive’s physical properties using "destructive" tests.

The two most common tests used in evaluating a cast or calendared vinyl check "tack" and "peel." The Pressure Sensitive Tape Council (PSTC) established the guidelines for these tests, which are called PSTC-1 and PSTC-5. PSTC-1 tests ultimate adhesion. In this exam, a strip of cast or calendered vinyl is applied to a stainless-steel substrate. An edge of the sample strip is pulled entirely against itself. The testing equipment measures the amount of force required to peel the tape from the panel.

To test the adhesion growth over time, manufacturers check samples at other time intervals, such as 24 hours, 72 hours, seven days or two weeks. PSTC-5 tests an adhesive’s loop tack. During this trial, the two ends of a sample strip are placed in the jaws of the test equipment to form a material loop with the sticky side exposed. The test machine touches the loop of material against a stainless-steel plate and measures the amount of force required to pull the two apart.

This tack test demonstrates an adhesive’s aggressiveness. For vinyl manufacturers, this test gives technical experts an idea of how repositionable vinyl markings will be during application. The rolling-ball test examines adhesive tack. A ball is rolled down the groove in a metal piece — shaped somewhat like a playground slide — onto a test sample at the end of the metal slide.

A technician then measures how far the ball rolls on the sample pieces. The ball rolls farther on a harder adhesive than on one that’s soft and tacky. Other specialized test equipment measures the force (the release value) required to peel a pressure-sensitive film from the release liner. In developing a product, a manufacturer tests values at different times to verify any changes.

High release values reveal difficulty when transferring a vinyl graphic from the liner, which can be problematic for the user. At the other end of the spectrum, low release values indicate poor stability of the film on the liner. In this case, the vinyl can slip on the liner during plotter cutting. In many cases, test samples will be tested in their "natural" state, compared to when they’ve been "aged."

The natural state means the tape or vinyl-film sample comes right off the roll. In contrast, an "aged" sample may have been cooked in an oven at, for example, 120°F for two weeks. By repeating the original tests after aging, examiners can predict how the product will perform after being on a distributor’s shelf for a couple years. Experts look for drops in adhesion values or loop tack, an increase in release values and any changes in the facestock’s physical properties.

With double-sided foam tapes, analysts conduct tests measuring shear, tensile and cleavage. Shear is the internal strength of an adhesive or foam carrier. A shear test measures parallel forces generated within an adhesive or a foam tape. For example, a shear test calculates the force acting upon a tape holding the weight of a heavy sign against a wall.

In testing the shear of a vinyl adhesive, a tape bearing a 500-1,000-gram weight is applied on top of the vinyl. If the adhesive doesn’t hold the facestock to the substrate for the specified time, the technician must determine whether it’s a cohesive or an adhesive failure. While shear measures parallel forces, tensile measures the perpendicular forces imposed on an adhesive bond.

Imagine grabbing two sides of a sign and pulling it away from the building. In this example, the stress is distributed equally over the entire area held by the adhesive or foam tape. Now, instead of pulling on two sides of the sign, suppose you pull only at one edge.

In this case, all your force is focused at one point. If you can break the bond at this point, you’ll likely tear the sign off the wall as easily as you would unzip a zipper. The strength of two materials bonded together at such an end point is called cleavage.

I’ve torn apart rigid demonstration panels held together with high-bonding foam tape several times, not because I’m exceptionally strong, but because I understood the concept of cleavage.

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