Pressure-sensitive adhesives build stronger bonds.
Specialty Tape Div.
Edited by Vicky Waltz
In light of tough economic times and competition from around the globe, manufacturers are under pressure to improve the quality and performance of their products while cutting costs and speeding time to market.
One way they are addressing these issues is in how they assemble their products. Tried-and-true mechanical fasteners such as nuts and bolts, screws, and rivets are giving way to adhesive technology, with the use of pressure-sensitive adhesive tapes on the rise.
Permanently sticky at room temperature, pressure-sensitive adhesives (PSAs) bond to a variety of materials with the slightest pressure. Provided in dry form, PSA tape requires no heat, water, or solvent to activate. It has good cohesive strength yet can be manipulated by hand, which provides flexibility during assembly and simplifies design and production. Finally, wetting and bond strength increase with time, making for rugged and longlasting assemblies.
With a projected growth of 4 to 6% annually for the next five to 10 years, PSAs are rapidly replacing mechanical fasteners and liquid and spray glues. While raw material costs of these latter methods may be lower, they require an investment in capital equipment and are more labor intensive. Ease of application, low capital expenses, and faster assembly make PSAs cost-effective bonding alternatives.
For these reasons, PSAs are making major inroads in assembly technology. They are used in a number of areas, including the automotive, medical, packaging, distribution, retail, and assembly industries. The automotive industry alone uses an average of 2,000 to 3,000 in.2 in each vehicle. For instance, PSAs hold carpet to interior trims and bond antisqueak materials and seals in instrument panels.
They also economically improve performance. PSAs provide 100% coverage for adhesion to bonding areas, which eliminates high stress zones and improves aesthetics compared with mechanical fasteners. In addition, they provide built-in damping that absorbs sound waves and vibrations.
PSA tape also addresses ergonomic issues that have become increasingly important, and costly, in recent years. Many applications are "peel and stick" and only require hand pressure. Specialized tools, such as vacuum extension bars, let assemblers reach crevices and other compact spaces. Other fasteners can cause ergonomic risks due to awkward positions required for assembly.
During production, the adhesive is coated in rolls up to 60 in. wide. Afterward, it is converted into various widths and shapes.
Because virtually no assembly equipment is required for use of PSAs, production changes generally require only changing the die-cut shapes of PSA-backed parts. Retooling and expensive capitalequipment changes are unnecessary because the same process equipment may be used for various assembly lines. Fabricating specific parts and shapes allows for this flexibility.
PSA tapes consist of up to four basic components: the carrier (or facestock), an adhesive, a release coating, and liner paper.
The carrier, or face-stock, is the part of PSA tape that has an adhesive coating. The term facestock generally refers to double-coated tapes. Materials that use facestocks include films, foams, nonwoven and woven fabrics, foils, and paper. Choice of carrier or facestock determines the final PSA tape's conformity, tensile strength, elongation rate, printability, adhesive anchorage, tear, porosity, stiffness, temperature resistance, dimensional stability, cost, and moisture vapor transmission rate.
The chemical composition of adhesives and the methods by which the adhesive is delivered to the web lets manufacturers produce tapes with a wide range of properties. For instance, PSAs are available in rubber, acrylic, silicone, solvent-based, hot-melt, and emulsion compositions.
Rubber-based adhesives consist of natural or synthetic rubber, various resins, oils, and antioxidants. Because natural rubber is expensive, a tackifier or blend of tackifiers is added to make the rubber an adhesive. Other components, such as flame retardants, pigments, and fillers are also added. Rubber-based adhesives are said to be the most cost-effective PSA system. However, longterm aging stability is low.
Acrylic-based adhesives are made with acrylic monomers that have been polymerized to high molecular weight. Monomer composition and molecular weight of the polymer determine most of the adhesive's properties. Modified acrylic adhesives are formulated with other components such as tackifiers, flame retardants, pigments, and fillers. While acrylicbased adhesives are more expensive than rubber-based adhesives, they ensure long-term aging stability and adhesion to high surface energy materials.
Modified acrylic adhesives contain tackifiers that improve initial tack and adhesion levels while decreasing resistance to solvents, plasticizers, and high temperatures.
Silicone-based adhesives, which are compounded from silicone polymers, are the only PSA that consistently bond to silicone substrates. Silicone-based adhesives are expensive in relation to other types of systems, and they have low initial tack and adhesion.
Solvent-based adhesives are polymerized in solvent and cast onto a web. Following coating, the solvent evaporates, leaving a functional adhesive.
Hot melt and emulsion are two additional types of adhesives. Hot-melt adhesives are made from thermoplastic rubbers that formulate with tackifying resins, oils, and antioxidants to achieve coating on the web at high temperatures. Emulsion adhesive ingredients are polymerized in water, applied to the web, and dried to create a functional adhesive.
Release liners are necessary to produce, process, and apply PSAs. Densified kraft paper or plastic film is used as the liner and coated on one or both sides with silicone release agents. Paper liners can be coated with polyethylene to prevent moisture absorption and to allow the liner to lay flat during processing. The liner protects the adhesive as the tape web is transported and handled. It also assists in the converting process as tapes are laminated, die-cut, or slit.
In addition to a variety of carriers and adhesives, PSA manufacturers offer a range of typical tape constructions.
Single-coated, self-wound tape has adhesive on one side of a facestock and a silicone coating on the other. This ensures easy unwinding. Typical applications include diapers and electrical, masking, and packaging tape.
Single-coated, linered tape has adhesive on one side of the facestock protected by a release liner coated on one side with silicone. Uses include label stock, wound care products, electromechanical devices, and foil insulation tapes.
Unsupported transfer tape has a film of unsupported adhesive directly coated onto a siliconized release liner. The release liner is coated with silicone on both sides to ensure easy unwinding and easy lamination. Unsupported construction allows for conformity to irregular surfaces. Common uses include nameplate mounting and foam bonding.
Double-linered transfer tape is similar to transfer tape, but release liners appear on both sides of the adhesive. Only sides that come into contact with the adhesive are coated in silicone. Double-linered transfer tape is used when the adhesive needs to be die-cut before hand installation. Uses include membrane switch plates and decorative overlays.
Transfer tape conforms to irregular surfaces, including grained and textured panels, and it lets laminated material stretch without causing stress on bonded material. Transfer tape doesn't cause curl on laminated material, and it has no memory for contoured or edge-wrapped parts.
Heat-activated tape has a film of heat-activated adhesive that is applied to one side of a carrier film while the other side is coated with the PSA. A release liner that is coated with silicone on both sides protects the PSA side and allows for difficult-to-bond substrates such as low permeability foams and high density polyethylene foams. Uses include non-slip steel pads and sound damping materials.
Double-coated PSAs ensure dimensional stability to laminated material and prevent stretching of laminated thin foams and extensible fabrics. Adhesive is coated on both sides of the carrier material, and both sides are coated with silicone. Double-coated tapes offer more stability when narrow slitting, add support to otherwise stretchy gasketing materials (such as open cell foams), and reduce chances of overlamination on porous materials. Use of film carriers offers the ability to coat two dissimilar adhesives in a single product. Typical uses are self-adhesive gasketing and abrasive sanding/polishing pads.
Double-linered, doublecoated PSAs are similar to double-coated PSAs except they have release liners on both sides. The liners are silicone coated only on the side that comes in contact with the adhesive. The second liner requires careful handling to prevent delamination issues. Uses include die-cut parts and assembly aids or gaskets.
Performance properties and considerations
When selecting PSAs, manufacturing engineers need to consider four performance criteria:
Adhesion is the molecular force of attraction or bond between the adhesive and the surface to which it is joined. The material's surface energy determines bond strength.
Shear is the ability of the tape to resist slippage under shear stress. It is measured by applying a standard area of tape to a vertical test panel and suspending a standard weight on the bottom portion of the tape. Measurement is given in minutes until failure and distance slipped.
Tack or quick stick allows adhesives to adhere to surfaces under light pressure with minimum contact time.
Cohesion refers to a PSA's internal strength and its ability to resist external, shear forces that cause splitting and slippage. Cohesion is measured by resistance to forces parallel to the surface.
When selecting PSAs, several important points should be considered. While adhesive type and tape construction affect performance, so do:
Flammability: Does the adhesive have to meet any flammability requirements?
Foam bonding: Is the application bonding foam, and if so, what type?
Repositionability: Parts that are hand-applied and difficult to position may require repositioning during installation.
Plasticizer resistance: Is the adhesive bonding to PVC or rubber that may affect the adhesive?
Durability: Is the adhesive for assembly only and held mechanically into place, or does the adhesive bond have to last the life of the product?
Fogging: Is the tape applied near glass?
Weatherability: Will the part be exposed to any exterior environments, such as sun or moisture?
Traditional fastening techniques
While PSAs are becoming more common in assembly industries, tapes and foams have not completely replaced conventional fastening methods.
Engineers involved in the design, applications or processing of products, components, and subassemblies choose from a number of fastening and attaching methods. Greater knowledge in the design phase prevents costly mistakes and improves profitability.
Traditional fastening systems include mechanical fasteners such as nuts, bolts, rivets, staples, screws, tabs, and posts, hot-melt adhesives, and spray and liquid adhesives.
Mechanical fasteners attach components to substrates. Relatively easy assembly, low capital equipment costs, nominal labor costs, and good product aesthetics are all advantages of mechanical fastening methods. However, mechanical fasteners also have higher component costs, and small bonding areas may limit holding capacity. In addition, the fasteners are prone to loosen, weaken, and rust.
When applied as a molten substance in beads or swirls on substrates, adhesives form bonds after cooling. However, a quality bond requires constant inspection by the users, and performance is affected by the time between the adhesive's application on one substrate until contact with the second substrate. Equipment necessary to melt and deliver the adhesive can be costly, and application to the very edge of the substrate can be difficult.
Liquid and spray adhesives are generally solvent or water-based systems that are applied to substrates through air pressure mist. They may or may not be tacky to the touch after drying. Liquid and spray adhesives are inexpensive, and material costs are low. While they can achieve 100% coverage, it is difficult to limit to specific areas, and overspraying is common. Application equipment, maintenance, and labor can be expensive and carry environmental and regulatory implications.
Alternative fastening systems may be a more practical choice over PSAs when assembling small volumes or custom products.