John McGann
Scapa North America
Windsor, Conn.
www.scapa.com

PSAs with film carriers and liners hold circuit boards in cell phones.

Look at how most modern automobiles are put together and you'll notice plenty of pressure-sensitive adhesives (PSAs). Ditto for personal electronics, consumer and industrial products, and medical devices. PSA films and tapes seal and bond joints, dampen vibration and shock, reduce noise, and fill gaps. Compared with most mechanical fasteners, PSAs weigh less and better distribute stress over a wider area to lessen component distortion. PSAs can be cut to the desired shape with minimal waste and apply easily which helps speed assembly and cut associated costs.

But PSAs -- like all fastening methods -- have limitations. For example, bond strength depends on load direction and is higher in shear, compression, and tension, and lower in peel and cleavage directions. Further, long-lasting bonds require clean, smooth surfaces for PSAs to stick to. Such adhesives work only over a limited temperature range or about -60 to 500°F. Not to mention, PSAs typically cost more than mechanical fasteners and no one formulation works for everything.

Consider PSAs during product design or redesign to avoid production delays and potential quality issues down the road. A quick overview of available PSAs will help determine which one is right for the job.

Chemistry and construction

PSAs come in three basic chemistries: rubber based, acrylic, and silicone. Rubber-based PSAs adhere to a wider variety of surfaces than acrylics, though the former has a narrower service temperature range and doesn't resist UV as well, important considerations for outdoor applications. Silicone formulations work best for assemblies subjected to temperature extremes. Acrylics fill the gap between silicones and rubber-based products in terms of cost and performance.

There are also three basic PSA constructions: transfer films, single-coated tapes, and double-coated tapes. Transfer films join two surfaces by virtue of an unsupported adhesive layer packaged on one or more release liners. Roll-form transfer films typically employ one release liner while sheets use two. Adhesive transfer films have no carrier or support to minimize thickness though the film itself may contain glass fibers for added strength and easier die cutting.

Single-coated adhesive tapes have adhesive on only one carrier side or backing and come with or without release liners. Backing materials include soft PVC, polyethylene, aluminum, cellulose, cloth, foam, and others. Single-coated tapes stick to another substrate as with a bandage to skin. They can also wrap around an object (aluminum tape around HVAC duct work or PVC tape around a bundle of wires, for example), act as a protective film, closure, or splicing tape.

Double-coated tapes also join two surfaces but adhesive coats both sides of a support or carrier, sometimes with different adhesive types. For example, one carrier side may contain a permanent adhesive while the other side uses a removable formulation. Like roll-form transfer films, roll-form, double-coated tapes typically employ one release liner while sheets use two. Double-coated tapes can come in different colors, are imprintable, and can be made to resist tampering or provide authentication. Such tapes are noted for dimensional stability, good gap filling and sealing properties, ease of application, an ability to be die cut, and for electrical and thermal conductivity or resistance.

Automakers use PSAs for several applications including wrapping underhood wiring, attaching running-board treads, and mounting luggage racks to roofs.

Carriers and liners

Next consider PSA carriers. The four most common carrier types are papers, films, foils, or foams. Papers tend to be the least expensive of these. Papers can be scored, folded, easily printed on and die cut, and they're recyclable. Foams fill gaps and cushion joints better than the other carriers, while foils resist heat, reflect light, insulate, and retard flames. Films have high tensile and tear strength, are highly flexible and dimensionally stable, and resist moisture. Films are also thin, clean, fiber-free, transparent, and work well at high temperatures.

Most tapes require one or more release liners. But release liners are eventually thrown away so they are typically neglected during the design phase. This can be a mistake, especially when using transfer films, because it's the release liner that lets the unsupported adhesive film stick to the correct side (of the liner) as it unwinds.

There are four basic liners. Kraft-paper release liners are generally the least expensive but tend to wrinkle when exposed to humidity. Claycoated paper liners, in contrast, hold their shape in humid conditions and withstand high temperatures but tear easier than Kraft papers. Polycoated-paper liners cost more than Kraft or claycoated paper but better resist humidity, won't wrinkle, and also lay flatter. However, polycoated paper can't withstand elevated temperatures. LDPE, HDPE, polyester, or polypropylene film release liners are generally the most expensive of all but give the highest possible resistance to humidity and the smoothest surface for adhesives to coat.

Regardless of application, release liners must stay attached until the product is assembled. Then the liner must easily remove with a consistent force. The use of film liners in automated assembly minimizes tearing during removal. Film liners get the nod over paper for most electronics applications because paper fibers shed during slitting and conversion can contaminate delicate circuitry.

But there's more to selecting a liner than the material itself. For example, should a liner release easily or should it be more aggressive to stay put during processing? Should the liner be heavy to stiffen the assembly or lightweight to conform to other substrates? Will the liner be die cut or printed? Is product identification or color important? What is the price/performance limit and which properties can be sacrificed to get the right price? Answering these questions helps quickly determine the best liner for the job.

Applications abound

In many cases, PSAs have become the preferred method for assembling certain parts of computers and cell phones, medical devices, drug-delivery systems, automobiles, machines, and equipment.

In the electronics industry, for example, transfer films bond flexible circuits, EMI/RFI shielding, and other components. Double-coated tapes also work for these applications and for assembling flat keyboards and membrane switches, gap filling, and sealing. Single-coated tapes mask delicate circuitry during PCB manufacturing and protect surfaces and components as well as provide strain relief, electrical insulation, shielding, and thermal insulation.

PSAs have recently become an industry standard in automotive assembly. Prefabricated components typically arrive from suppliers with PSAs precut to shape and applied, simplifying assembly and lowering overall costs. Transfer films bond decorative trim, interior fabric, instrument panel assembly, and warning labels. Double-coated tapes attach exterior trim, emblems, nameplates, mirror assemblies, and carpet. Double-coated foams dampen vibration and attenuate sound. Single-coated tapes wrap wire harnesses, reduce wind noise, protect and mask surfaces, provide thermal or electrical isolation, and act as gaskets and seals.

Single-sided tapes hold IVs in place.

The medical market uses its share of PSAs as well. For example, special PSAs incorporating active ingredients such as nicotine, vitamins, and medications stick to the skin and allow medication to diffuse at a known rate without interacting with the drug. Adhesives for emerging wound dressings will bond to skin and allow moisture and air to pass through, accelerating healing of burns and other severe wounds. Transfer films and double-coated tapes assemble medical devices and strips for pregnancy and diabetes tests. Single-coated tapes become bandages, wrapping tapes and wound dressings, and skin attachments for electrodes, sensors, and IVs.

Lastly, makers of a variety of industrial and aerospace products use transfer films and double-coated tapes for a wide variety of applications including bonding foam, making specialty labels and insulation, and attaching carpeting to aircraft floors. Single-coated tapes protect surfaces, bundle wire, wrap and seal HVAC duct, mask powdercoated parts, and help build, repair, and winterize greenhouses.

When to push for pressure-sensitive adhesives

Consider PSAs when answering "yes" to any of the following questions:

Is weight reduction a goal?
Is a smooth surface important for aerodynamics or aesthetics?
Does the assembly require a seal?
Is reducing assembly time and complexity a goal?
Is corrosion a potential problem, especially when using different metals?
Is vibration and noise an issue?
Will end users or customers do the final assembly?

 

Common PSA terms

Tack: The condition of an adhesive when it feels sticky and begins adhering to a substrate immediately upon contact and with little pressure.
Peel adhesion: Force per unit width required to break a bond between a PSA and mating surface when the PSA is peeled back at a 180° angle (typically) at a set rate.
Static shear: A measure of an adhesive's internal or cohesive strength.
Plasticizer: Material added to a resin to boost flexibility.
Carrier or support: The material on which adhesive is coated to produce a double-coated tape.
Backing: A relatively thin, flexible material to which adhesive is applied to produce a single-coated tape.
Release liner: A sheet of material that acts as a protector for an adhesive film or mass that is easily removed by virtue of a silicone release coating.

COMPARING RUBBER, ACRYLIC AND SILICONE PSAS
Property Acrylic Rubber Silicone
Tack Low/moderate/high Typically high Typically low
180° peel adhesion Low/moderate/high Moderate to high Low to moderate
Static shear Low/moderate/high Low/moderate/high High
Humidity resistance Excellent Excellent Excellent
Aged peel adhesion Increases with time Constant with time May increase with time
Service-temperature range -40 to 250° F 30 to 150° F -60 to 500° F
Environment Indoor or outdoor Indoor only Indoor or outdoor
UV resistance Excellent Poor Excellent
Adhesive color Clear to straw Yellow (more with time) Clear
Solvent/chemical resistance Very Good Good Excellent
Plasticizer resistance Poor to fair Generally poor Good
Cost Low to medium Medium High