High-Strength Adhesives Offer Advantages Over Mechanical Fasteners

If you had a window seat on a recent flight, you may have noticed metallic strips along the top of the wing flaps as they extended during your approach to the airport. Did you know they were held on by adhesive tape?

Commercial aircraft have used thin, two-sided, acrylic adhesive tape to attach these stainless-steel scuff strips since 1984. The strips prevent abrasion and chafing between the aluminum flap and the underside of the wing, which can occur while flaps deploy or retract during takeoff and landing.

The tape withstands temperature swings, from 150°F in direct sunlight on the ground to –64°F at altitude, and also resists vibration, humidity, and deicing solvents. Its flexibility compensates for aluminum’s and stainless steel’s different coefficients of thermal expansion, and its film acts as a barrier against bimetallic corrosion.

These scuff strips illustrate key advantages of adhesives over mechanical fasteners in producing strong, durable joints for structural applications.

Adhesive alternative
Advances in epoxy, acrylic, and urethane liquid adhesives and flexible, thin adhesives let designers meet structural integrity and strength requirements without mechanical fasteners or welding. These adhesives work with metals, composites, plastics, rubbers, glasses, and other materials.

Beyond their strong bonds, structural adhesives can lower overall costs while increasing the durability of products. Adhesives distribute stress across the entire bonded joint, whereas fasteners and spot welding can create stress concentrations. So designers using adhesives can specify thinner, lighter materials yet meet strength requirements.

Adhesives also remain flexible throughout their lives, creating joints that are up to 20 times more resistant to fatigue than rivets or welds. The materials’ flexibility helps them absorb flexural, vibrational, and impact stresses.

Continuous contact in adhesive bonds seals joints against contaminants and the effects of harsh environments. And adhesives’ film barrier prevents galvanic corrosion between dissimilar metals.

Protruding fasteners and joint puckering caused by spot and fusion welding disrupt the clean look many designers strive for. But nearly invisible adhesive joints allow cleaner styling and cut down on the machining and finishing needed to get a desired effect.

Adhesives, particularly thin-bonding versions, simplify and speed assembly. Newer pressure-sensitive products — including thin foam tapes, adhesive transfer films, and custom die-cut adhesive shapes — eliminate fixture time and provide tough, flexible bonds that may be stronger than those of liquid adhesives. Both types of bonding require less training, which lowers overall production time and cost.

Assemblies put together with adhesives are also lighter than mechanically fastened ones. The average car now weighs about 130 lb less thanks to adhesives replacing many mechanical fasteners. In transportation, aerospace, and personal electronics, light weight is a crucial design concern and cost factor.

Designers with structural bonding applications can choose from three families of adhesives: epoxies, acrylics, and urethanes. The best choice depends on the substrates being bonded, the operating environment, and production considerations.

An allied decision is whether to use liquid or thin-bonding adhesives, such as pressure-sensitive-adhesive (PSA) tapes or adhesive-transfer tapes. In many cases, both methods meet design requirements, but one fits into the production process more easily.

For example, if immediate handling strength is needed, thin-bonding adhesives offer an advantage. But if the user needs time to position the parts, liquid adhesives work better.

Designers may benefit from working with adhesive-technology specialists early in the design process. They can suggest approaches that provide better value.

Epoxies

Epoxy resin combines strength with resistance to high temperatures. Epoxy pastes come in one-part formulations that require heat to cure and in two-part, room-temperature-curing versions.

Heat-curing epoxies have traditionally provided higher shear strength — about 5,000 psi at room temperature — and better high-temperature performance than two-part epoxies.

Advances in two-part epoxies have created adhesives with overlap shear strengths of 4,500 psi and peel strengths over 1,000 psi. The two-part versions don’t need hot-air ovens, UV lamps, or other heat sources. However, fixtures may need to hold parts during the minutes to hours-long cure.

Epoxy-film adhesives can be die cut, making it easy to bond complex parts. With this approach, adhesive bond thickness is uniform across the joint and produces a better combination of shear and peel strength and resistance to shock and fatigue. However, epoxy-film adhesives also require fixturing and heat curing.

Acrylics
Acrylic adhesives cure the fastest of the three types of adhesives. They bond a wide range of substrates, including oily metals, usually without surface preparation, and have maximum shear strength of about 4,200 psi and peel strengths over 1,000 psi. Acrylics’ maximum operating temperatures range from 160 to over 300°F.

Acrylics are available as one-part, anaerobic adhesives that cure in the absence of oxygen, one-part light-cure adhesives that set up in seconds, and two-part formulations. The two-part formulations have higher viscosities that help the adhesive stay put during cure.

Newer acrylic adhesives strongly bond low-surface-energy (LSE) plastics like polypropylene and polyethylene without cleaning, priming, or flame treating. As a result, adhesives are replacing mechanical fasteners and fusion welding in structural and trim components, especially for the auto industry.

Acrylics also bond to adhesion-challenged, powder-coated paints used on appliances and to thermoplastic polyolefins in personal electronics, power tools, lawn tractors, and medical devices.

Cyanoacrylates are one-part, acrylic-based adhesives that cure in a few seconds (although formulations are available with longer cures). They bond many plastics, rubbers, metals, and other materials with tensile strengths up to 5,000 psi. Some cyanoacrylates join materials that don’t bond by other methods — EPDM rubber gaskets to metal, for example. Others resist fuels and chemicals between –40 and 250°F.

Acrylic PSA-foam and adhesive-transfer tapes have high holding strengths, resist harsh environments, and prevent corrosion. They permit assembly without pre-treatment or cleanup.

Die-cut peel-and-stick acrylic adhesives fit specified shapes, sizes, or profiles for quick assembly. Acrylic PSAs bond parts instantly without fixtures or long cure times.

Acrylic tapes have overlap shear strengths of about 300 psi. Although this is lower than the 1,000-psi shear strengths available in other structural adhesives, the adhesive deeply wets substrates, creating a mechanical bond. The viscoelastic nature of the polymer gives it impact and shock resistance superior to many structural adhesives.

Urethanes
Polyurethane adhesives come in one and two-part versions. Both types offer greater toughness than epoxies, meaning they resist impact, shock, and dimensional changes. This ability to handle expansion and contraction makes them ideal for bonding dissimilar substrates and for applications that flex from wind or vibration, see temperature cycles, or require continuous flexibility.

While polyurethane adhesives have overlap shear of 2,500 psi and peel strengths over 1,000 psi, their strength falls more sharply as temperature rises compared to epoxies and acrylics. Polyurethanes withstand maximum temperatures from 160 to 300°F. Urethanes, especially the two-part formulations, set up faster than epoxies but have less resistance to solvents.

© 2010 Penton Media, Inc.