Pressure-Sensitive Adhesives 101 | 2009-10-01 | ASI Magazine | Adhesives & Sealants In...
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`Pressure-Sensitive Adhesives 101
`
`This article explains what a pressure-sensitive adhesive is, how
`it is constructed and what adhesive options are available.
`
`By Kurt Schramer
`October 1, 2009
`
`Pressure-sensitive adhesives (PSAs) are used in applications ranging from labeling
`food packaging to assembling electronic devices to mounting graphics displays.
`Although they have been around for decades and their use is widespread,
`manufacturers, converters, and printers may not understand the key differences
`between the adhesive choices available, how they’re constructed, or which are best
`suited for a particular job.
`
`This article explains what a pressure-sensitive adhesive is, how it’s constructed and
`what adhesive options are available. The advantages and disadvantages of the
`primary adhesive types will be addressed, as well as how they are formulated and
`their common applications.
`
`What is a Pressure-Sensitive Adhesive?
`
`A pressure-sensitive adhesive is a material that holds two surfaces together solely
`by surface contact, which is achieved by slight initial external pressure. These dry
`adhesives require no activation with water, solvent or heat, and firmly adhere to
`many dissimilar surfaces with minimal pressure. Aggressive and permanently tacky
`at room temperature, PSAs can be used for bonding materials such as plastic,
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`paper, metal, glass, and wood.
`
`Pressure-sensitive adhesives offer numerous advantages. Application can be easily
`automated and usually requires no elaborate equipment. When used in assembly,
`PSAs save time compared to liquid adhesives because they don’t require any setup
`or long curing times. In addition, adhesion is immediate, allowing manufacturing
`procedures to continue uninterrupted, which results in significant time and labor
`savings. Pressure-sensitive adhesives eliminate the need for screws, rivets, clips or
`drilling holes, thus preventing cracks and corrosion and improving the integrity and
`appearance of the final product. PSAs also provide an insulating seal, as well as
`sound and vibration control.
`
`Pressure-Sensitive Adhesive Constructions
`
`Pressure-sensitive adhesives are available in four different product constructions;
`the most appropriate type depends on the application in question and the bonding
`requirements. The oldest and most commonly known type of PSA is an adhesive
`tape that winds onto itself without a liner; examples include masking tape, duct
`tape, cellophane tape and electrician’s tape.
`
`A second type of pressure-sensitive adhesive construction is a one-side coated or
`single-faced product. This involves a flexible facestock coated on one side with
`adhesive. The adhesive side is protected by a release liner, which is removed when
`ready for application. Examples of this construction include protective
`overlaminates, labels, graphics or any application in which the face serves a unique
`function—whether it needs to be printable, decorative, or used for protection.
`
`Another pressure-sensitive adhesive is a two-side coated or double-faced
`construction. With this type, the adhesive is applied to both sides of a carrier
`membrane (film, paper, foam, foil, scrim or cloth), then protected by a release liner.
`The two-sided constructions can be customized for an application by using a
`different type of adhesive on each side of the carrier membrane. Applications for
`these PSAs include situations where two substrates need to be bonded
`together—often replacing the use of mechanical fasteners or liquid adhesives.
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`The final example of a pressure-sensitive adhesive construction is a transfer tape or
`free film. This is simply a strip of adhesive that is wound around a release liner. It
`functions like a two-side-coated PSA in that it enables two substrates to be bonded
`together, but the transfer tape construction does not have a carrier membrane in
`the middle to reinforce the adhesive. The result is an adhesive strip that is more
`conformable and extensible, and can be made to almost any thickness to meet the
`specific application requirements. Examples of applications include the mounting
`of nameplates, construction of electronics or products with slim profiles, and
`decorative substrates.
`
`Manufacturing Methods for Pressure-Sensitive
`Adhesives
`
`There are four popular chemistries resulting in a pressure-sensitive adhesive
`laminate. One chemistry is achieved through the use of a solvent. With this method,
`the adhesive components (usually rubber or acrylic) are dissolved in a solvent and
`then coated onto a web. The solvent is then evaporated using heat and air flow,
`leaving a dry yet sticky adhesive (pressure-sensitive) that is wound in a roll.
`
`Adhesives made with solvents often provide high performance, but can be
`expensive and require extra care because of processing restrictions. Special
`equipment is needed to properly dispose of the solvent, and there must be vigilant
`awareness of safety issues when dealing with solvents.
`
`Another way to manufacture a pressure-sensitive adhesive is to create an emulsion
`(water-based). Here, the acrylic polymer and other additives are dispersed, not
`dissolved, in water and coated onto a web. Again, heat and air flow are used to
`evaporate the water before winding into a roll. Today’s water-based emulsions can
`often provide the same high level of performance as solvent-based acrylics and are
`generally a good choice when lower cost and safety are important, since water-
`based emulsions don’t require the extra steps that are needed with solvents.
`
`The third method for creating a pressure-sensitive adhesive is developing a hot
`melt. Hot melts are typically a mixture of a polymer (such as thermoplastic rubber),
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`tackifying resins and a hydrocarbon diluent (wax or oil plasticizer), which is heated
`until flowable. The hot-melt adhesive is then coated on a web and cooled before
`winding into a roll.
`
`Hot-melt adhesives typically exhibit excellent adhesion to substrates and
`facestocks, have very good coatability, and are less expensive than most solvent-
`based adhesives. Drawbacks generally include processing and safety challenges, as
`well as difficulty performing under high temperatures. The major advantage of a
`hot-melt system is its lack of solvents and water - meaning it doesn't have to be
`dried in an oven. Hot melts are frequently referred to as 100% solids (no water or
`solvent) technology.
`
`Finally, pressure-sensitive adhesives can be created through the use of a UV-curable
`adhesive. These adhesives usually require no mixing and cure in seconds when
`exposed to a UV light source. These environmentally friendly adhesives are reactive
`compounds that contain no solvents or other volatile substances. Because UV
`adhesives cure so quickly and at temperatures much lower than thermal heating,
`they can be used successfully on several heat-sensitive substrates such as plastic
`films, paper, and synthetic fibers. The quick curing minimizes shrinkage, providing
`the dimensional accuracy that is often called for in applications such as electronics
`manufacturing. UV-curable adhesives also offer durability, strength, insulation
`characteristics, and resistance to chemicals, moisture, and temperature. Like hot
`melts, UV adhesives do not require an oven.
`
`A decision on which method of adhesive manufacturing technology should be used
`for a particular application is determined by the performance characteristics
`needed.
`
`Common Adhesive Types and Their Properties
`
`In addition to considering the method in which the adhesive is created (solvent-
`based, water-based, hot melt, or UV-cured), it is important to understand the key
`adhesive component choices and how they will affect the pressure-sensitive
`adhesive’s performance in the application. The three main adhesive chemistry
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`options are rubber/resin, acrylic and silicone.
`
`For many years, rubber/resin formulations have been available that combine
`natural or synthetic rubber with tackifying resins, oils, antioxidants, or other
`ingredients as needed. They are typically either solvent-based or used in hot melts,
`and the aggressive nature of rubber adhesives makes excellent bonding easy to
`achieve.
`
`These adhesives are known for their high level of adhesion to a variety of substrates
`and facestocks, as well as their high coatability. Rubber-based adhesives provide
`high tack and peel, and make excellent general-purpose adhesives, because of their
`ability to adhere well to several non-polar, low-energy surfaces, including
`polyethylene and polypropylene. In general, rubber-based adhesives are often a
`more economical choice compared to most acrylics.
`
`Disadvantages of rubber-based adhesives include limited effectiveness when
`exposed to certain chemicals, UV rays, or high temperatures (over 150°F/66°C). In
`addition, they are more susceptible to oxidation and may darken, lose their tack,
`and become brittle if overexposed. Also, rubber/resin adhesives may turn soft and
`gummy if plasticizers, used in most polyvinyl chloride films (PVC), migrate into the
`adhesive.
`
`The other type of adhesive chemistry commonly used in pressure-sensitive
`adhesives is acrylic. Acrylic adhesives can either be solvent- or water-based, and are
`formulated by reacting monomers with the desired properties, which are then
`typically crosslinked to form the type of polymer needed. Monomers are the
`building blocks of polymers and are considered to be either “soft” or “hard.” The
`combination of hard and soft monomers can be adjusted based on the level of
`adhesive (polymer) performance needed.
`
`Crosslinking polymers improves the cohesive strength (internal strength of the
`adhesive, or the ability to stick to itself) of the acrylic formulation and can be
`accomplished in one of three ways: the use of thermal heat energy; by self-curing
`when a solvent is removed during the drying process; or through UV or EB non-
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`thermal radiation.
`
`In general, acrylics are relatively firm and durable, but are not as aggressive as
`rubber-based adhesives. An acrylic adhesive can be modified to alter its adhesion
`properties based on the selection of monomers used, the level of polymerization
`created, or by adding tackifying resins or other chemicals.
`
`The advantages of acrylic adhesives are typically the opposite of rubber adhesives.
`Acrylics provide excellent temperature, UV and oxidation resistance; therefore, they
`are often the preferred choice for outdoor applications. They resist the negative
`plasticizing action that can occur with rubber and hold up well to chemical
`exposure. Acrylic-based adhesives are very durable and have clarity and color
`stability. Acrylics bond well to polar surfaces like metal, glass, polyesters and
`polycarbonates, and have high peel, tack, and shear.
`
`Disadvantages of acrylics usually include poor adhesion to low-energy surfaces,
`such as polyethylene and polypropylene, as well as lower overall adhesion
`compared to rubber unless the adhesive is highly engineered. In general, acrylics
`are often more expensive than rubber-based adhesives.
`
`Silicone adhesives are another chemical option for pressure-sensitive adhesives but
`are less common than rubber or acrylic. These specialized adhesives consist of
`silicone polymers that provide adhesion to silicon and other hard-to-adhere-to
`materials. Silicone-based adhesives can maintain adhesion over a range of
`temperatures; however, beyond their ability to adhere to difficult surfaces, their
`overall adhesive strength is low. Silicone adhesives are also more expensive.
`
`Final Considerations
`
`In addition to choosing the most appropriate adhesive formulation, construction
`and manufacturing method, a few other factors must be considered when selecting
`a pressure-sensitive adhesive. It’s important to know exactly what kind of surfaces
`or substrates the adhesive will be bonded to or will be bonding together (smooth,
`textured, even or uneven, high-energy or low-energy, porous or non-porous, etc.).
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`The type of surface is a key factor in dictating which adhesive and construction will
`be most effective for the application. Knowing the surface properties will help
`address: adhesion requirements; thickness of adhesive needed; single-coat, double-
`coat, or transfer tape construction; and what kind of facestock or carrier membrane
`is needed.
`
`The adhesive’s end environment is also a consideration, including whether the
`pressure-sensitive adhesive will be exposed to extreme cold or heat, sun and UV
`rays, humidity, flexing, water or chemicals. All of these can greatly affect the ability
`of the adhesive to maintain its bond. The length of time the PSA will be in use also
`determines how strong the adhesion needs to be for the application.
`
`Choosing the most appropriate adhesive requires a lot of background work and
`careful consideration. The goal is to best match the needs of the application to the
`most suitable adhesive construction, while achieving the desired performance at the
`most reasonable cost. Always feel free to consult your adhesive supplier if questions
`arise or if consultation is needed to determine the best pressure-sensitive adhesive
`for the job.
`
`For more information, visit www.mactac.com.
`
`KEYWORDS: acrylics adhesives in assembly adhesives in electrical/electronic adhesives in
`packaging bonding silicones
`
`Share This Story
`
`Kurt Schramer is strategic business development director at MACtac.
`
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`Pressure-Sensitive Adhesives 101 | 2009-10-01 | ASI Magazine | Adhesives & Sealants In...
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`•
`
`•
`
`Transposable
`Adhesives: Acrylic
`Pressure-Sensitive
`Adhesives with
`Reactive Components
`
`See More
`
`Why use pressure-
`sensitive tape adhesives
`in the assembly of
`electronic devices?
`
`See More
`
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`Pressure-Sensitive Adhesives 101 | 2009-10-01 | ASI Magazine | Adhesives & Sealants In...
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`Page 9 of 9
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`•
`
`What Are Pressure-
`Sensitive Adhesives and
`Tapes?
`
`See More
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