The roll of pressure-sensitive adhesive (PSA) tape in the design and production of home appliances is a sticky one.
Patrick G. Mahoney
With consumers shopping for the sleekest, quietest, most-durable appliances, manufacturers and design engineers are looking at every possible means of improving product appearance and performance. One such method involves the use of pressure-sensitive solutions. By replacing mechanical fasteners and liquid adhesives with pressure-sensitive adhesive (PSA) tapes, appliance manufacturers can reduce sound and vibrations, bond to new low surface energy (LSE) substrates, protect electronics, and improve production efficiency.
Whether it's washer, range, refrigerator or microwave manufacturing, there's a full line of PSA tapes to help design engineers.
MACtac, Stow, Ohio, for example, makes MACbond double-coated tapes, MACfilm acrylic-free films, MACmount double-coated foam tapes, and GS Series highstrength free films. Each is designed to resolve issues that mechanical fasteners and liquid adhesives cannot, the company claims.
Other attachment systems, including hot melts, sprays, liquids, and epoxies, have greater processing demands. They might even need an adhesives expert to monitor such things as die cutting and "release" needs.
Release is the degree of ease with which the liner or backing may be removed from a tape. Typically, release is determined by a silicone coating that prevents the adhesive from forming a strong bond with the release liner. Some bond is necessary to keep the liner from falling off the tape until it needs to be removed. The composition and coat weight of the release determines the ease of removal. In some cases, an easy release, coupled with slitting (cutting) to a thin-roll size may make the liner delaminate prematurely. Applications requiring very narrow rolls, or high tension, may need a stronger release.
"Compressible," in the context of adhesives, means that pressure can be applied without drastically changing the form or function of the PSA. Some spray adhesives, sealants, and caulks remain somewhat conformable and compressible after drying. But they are limited in this aspect because they are difficult to apply uniformly and have no basic structure to keep them in place. The fluid structure of these alternative systems does not return to its original shape. PSA has structure which can be compressed, twisted, and pulled, and still retains memory of its shape better than liquid adhesives. Plus, because PSA forms an instant bond based on pressure, the more pressure the better. While it forms some bond instantly, different formulations build adhesion over time. In some cases, the application may require this.
PSA may be more expensive than metal or liquid fasteners. However, it may be more economical when processing costs are factored in. With little or no dwell time, lines can keep moving. PSA emits no hazardous VOCs and need no complicated application equipment. Even when hand applied, it guarantees a uniform coating. In short, PSA can solve design issues that produce noisy appliances, a growing problem with consumers.
PSA tapes let manufacturers replace metal components with low-surface-energy (LSE) materials such as plastics and apply powder-coated paints. Powder coating is a dry-finishing process using finely ground particles of pigment and resin that are electrostatically charged and sprayed onto a surface. This produces a highquality, durable finish while maintaining production and complying with environmental regulations. Powder coating is designed for metal substrates due to the high temperature required to cure the powder.
Surface energy is measured in dynes and represents the level of molecular activity in a substrate. Adhesive relies on molecular activity to form a bond. The higher the surface energy, the easier it is to adhere. Low surface energies present a challenge for adhesives. A good example is a car that has been waxed. The wax creates a low surface energy that whisks away water. Steel, for example, has a high surface energy, while plastics have a lower surface energy.
"LSE materials like plastics are very tricky," says B.J. Taylor, core products manager for MACtac Technical Products. "When securing plastics with a mechanical fastener they can split or crack. And until recently, their low surface energy did not tolerate the use of adhesives for bonding. Previous adhesives on the market did not have the right temperature and cohesive properties required for LSE applications."
Many powder-coated paints have low surface energy. Surface energy is necessary to create the bond with the adhesive. So adhesives stick better to high surface energy materials like stainless steel than to plastics like polypropylene, which is used extensively in appliances.
But the discovery of an adhesive that bonds well to LSE substrates changed all that. This advance in adhesive technology made possible the formulation of special tapes that can secure LSE surfaces such as polypropylene and high-density polyethylene. By incorporating plastics in their design, manufacturers can reduce the weight of an appliance while making it more durable. Acrylic-based adhesive tapes provide aggressive tack and peel for quick bonding and long-term adhesion. They also protect against damage caused by water, detergent, alcohol, and aliphatic and some aromatic hydrocarbons.
Besides trading metal for plastic, appliance manufacturers are also replacing dials with touchscreens. Custom tapes are compatible with electronics. They adhere to both polyester membrane and glass substrates, and can be die cut to fit around the edges of a touchscreen. These tapes help prevent common problems like corrosion, and protecting circuits against harmful chemicals and moisture.
Touchscreens use membrane-switch technology. But, instead of the spacer membranes used in flexible switches, conductive surfaces are kept apart by raised, printed dots.
Double-coated polyester films come in a range of thicknesses and flexibilities. "The polyester adhesive is die cut to provide enough gap between a membrane switch's top and bottom circuits and is flexible enough to compress both circuits together with the touch of a finger," says Taylor. PSA tapes have a proven track record of minimizing costs by improving efficiency and reducing production time, says Taylor. Mechanical fasteners may require additional steps such as drilling and riveting while liquid adhesives can delay production up to a day for curing or drying. PSA tapes reduce setup time by offering immediate tack.
Appliance manufacturers also use PSA tapes as assembly aids. Because of their ability to adhere to irregular surfaces, they help secure door handles and affix wires and cable clips, reducing slicingor entanglement. Design engineers use MACfilm tapes for nameplate, emblem, and backgraphic mounting because of their edge-to-edge bonding capabilities. The term "back graphics" refers to printing on the back of the material to protect or "bury" the print. When mounted, the adhesive touches the inks. The ability to backgraphic-mount means the adhesive won't negatively react with the ink. It will neither distort the ink nor deaden its own properties. Also, it does not compromise the quality of the print as seen through the surface. PSAs are also used to adhere decorative trim for improved aesthetics.
A number of assembly needs rely on PSAs. Among them are gasket and seal mounting, insulation bonding, microwave-door lining, sound dampening, nameplate and graphic-overlay mounting, and high-temperature applications.
The Four Basic PSAs
The oldest tapes masking, electrician's, box sealing, duct, and household cellophane come without a liner.
Next is the so-called one-side-coated or single-faced tape. Usually, it consists of a flexible material with pressure-sensitive adhesive to bond it to a substrate. The face may be decorative, printable, protective, and so forth.
Then, there's two-side-coated or double-faced tape. It is used to join or bond two substrates, often replacing mechanical fasteners or liquid glue. The two adhesive layers can be the same or different adhesives. The adhesives can be thick or thin. The carrier membrane can be paper, film, foil, cloth, or foam.
Double-faced tape can bond similar or dissimilar surfaces. It's often used in an intermediate step where it bonds to a single substrate that then gets processed (e.g., die-cut).
Double-faced tape can be the primary means of fastening or used to temporarily hold substrates together during assembly. Some examples are the bonding of gaskets or insulation materials, joining electronic components, and attaching molded and extruded-plastic parts.
The fourth is transfer tape or free film, which has the same basic function as the double faced. However, this formulation does not have a carrier membrane to reinforce the adhesive. As a result, the adhesive is extensible and more conformable. With only one layer of adhesive, the tape can be relatively thin.
This tape comes in various thicknesses to fill the gap between substrates. Applications include mounting nameplates and printing second-surface signs or labels. Double-faced tape can also be applied to substrates that are extensible and conformable without significantly affecting those properties.