Edited by Jessica Shapiro
• Surface energy is a relative quantity that affects an adhesive’s ability to wet the surface.
• Raising the polymer surface energy through pretreatment or lowering the adhesive’s surface energy promotes bonding.
Basics of Design Engineering: Adhesives
Basics of Design Engineering: Joining plastic
Polypropylene (PP), polyethylene (PE), and other thermoplastic olefins attract designers because of their low cost and excellent physical properties. The polymers are durable, flexible, and resist moisture, heat, and solvents. Unfortunately, some of these properties also make the plastics difficult to bond with conventional adhesives.
PP, PE, Teflon, and other hard-to-bond polymers are called low-surface-energy (LSE) plastics, and engineers typically turn to mechanical fasteners and ultrasonic welding to assemble LSE plastic structural components. But these approaches can affect cost, service life, and aesthetics. As designers specify more LSE plastics, they need more-efficient, reliable, and economical ways to join LSE plastics.
Mechanical fasteners constrain components, increasing the risk of cracking and failure at attachment points from material expansion and contraction, flexing, and vibration. Ultrasonic welding is time consuming and often dimples the surface, harming product appearance.
Adhesives, on the other hand, distribute loads evenly, reducing joint stress. They resist flexing, vibration, and impact. Adhesives seal joints to minimize corrosion and fill in surface gaps that mechanical fasteners or ultrasonic methods can’t fix.
Practical and economical adhesives for joining LSE plastics were not available until recently, however. New liquid and viscoelastic adhesives are changing the outlook for adhesive-based assembly of LSE plastics.
Polymer surface energy is important because it determines if adhesives can spread out over the bonding surface, a process called wetting out that is necessary to create strong bonds. The surface energy, or wettability, of a material is measured in dynes per centimeter. Plastics with relatively high surface energy — acrylonitrile butadiene styrene (ABS) and polycarbonate, for example — bond readily because they are easily wet by conventional adhesives.
Properly prepared aluminum with a surface energy of 840 dynes/cm strongly bonds with adhesives, which explains why modern aluminum-skinned aircraft are relying more on adhesives and less on rivets for structural assembly.
At the lower end of the spectrum, PP and PE have surface energies around 30 dynes/cm. Polytetrafluoroethylene (PTFE), the original coating for nonstick cookware, is the LSE champ at 19 dynes/cm.
Measuring the contact angle of a water droplet on the surface of a material determines surface energy. Contact angles greater than 90° indicate lower surface energy and a surface that is more difficult to wet. When the contact angle approaches 180°, the surface is very difficult to wet and water “beads up” as on a waxed car.
Conversely, contact angles less than 90° indicate higher surface energy and a surface that is easier to wet. On a surface producing a contact angle approaching 0° water forms a sheet.
The better a liquid adhesive wets a material surface, the more area it can cover. This has two beneficial results: a stronger chemical bond and stronger mechanical bond.
The chemical bonds get a boost from more attraction and interaction of reactive groups in the adhesive and the substrate. Mechanical bonds form when adhesive penetrates the surface, filling in microscopic irregularities and producing adhesive interlocks.
Absolute surface energy numbers are of less interest to engineers than the relative magnitude of the plastic’s surface energy and that of the adhesive. Ideally, the surface energy of a plastic should be 7 to 10 dynes/cm higher than the surface energy of an adhesive. A liquid or pressure-sensitive adhesive with a surface energy of 20 dynes/cm will spontaneously wet out LSE plastics with surface energies of 30 dynes/cm or more.
Therefore, engineers can make LSE plastics easier to bond by either raising the surface energy of the plastic or lowering the surface energy of the adhesive.
To boost a plastic’s surface energy, engineers usually pretreat the plastic with primers, flame, plasma, or corona-discharge processes. These change the surface chemistry and render the plastic wettable by conventional adhesives.
Plastic pretreatments add cost and time to production processes. In addition, the surface energy effects of flame, plasma, and corona-discharge treatments may only last minutes, days, or weeks depending on the plastic. Primers may pose environmental issues that engineers must weigh.
The other approach, lowering the surface energy of the adhesive to attain an aggressive bond, involves adding tackifiers to the adhesive formulation. In some cases, engineers use double-coated or transfer tapes to improve tackiness.
Newly formulated acrylic-liquid adhesives and pressure-sensitive-adhesive tapes strongly bond with many LSE plastics without priming or other pretreatments. One approach uses a two-part, solvent-free, room-temperature-curing acrylic adhesive that skips curing ovens, UV lamps, and heaters. The resulting structural bonds resist chemical attack, water, humidity, and corrosion and have overlap shear strength over 1,000 psi.
When substrates are thin, lightweight, or flexible, a thin bonding product adheres as well or better than liquid adhesives. For such parts, a pressure-sensitive, double-coated or transfer tape gives LSE materials bonds that resist temperature extremes and solvents and have peel strength comparable to those of liquid structural adhesives.
Other adhesives for LSE plastics use cyanoacrylate chemistry. The high-strength, one-part adhesives cure at room temperature and produce strong bonds on LSE plastics without olefin primers. Accelerators can be added to the formulation to speed cure in low-humidity environments.
Some cyanoacrylate adhesives permit light-accelerated curing to increase bond depth and strength. The short cure times let manufacturers process relatively small plastic parts in seconds rather than minutes in high-volume applications. Cure strength can be boosted further with plasma, primer, or corona discharge pretreatment of the LSE plastic.
This new generation of liquid adhesives, adhesive tapes, and thin-film/foam bonding systems create strong bonds with LSE plastics that resist impact, shock, and fatigue better than conventional mechanical and ultrasonic fastening.
The availability of reliable adhesive-bonding alternatives for LSE plastics promises faster production and assembly using less-skilled labor. Adhesives add little additional weight to assemblies, cause no change in part dimensions or geometry, and readily bond dissimilar substrates and heat-sensitive materials. And because the limitations of conventional fastening techniques are not an issue, engineers have more latitude in specifying component thickness and shape.
Designers are using new liquid-adhesive, thin-film/foam bonding systems instead of mechanical fasteners and ultrasonic welding to attach polypropylene fenders, bumpers, body trim, body panels, and other items on vehicles and recreational trailers. They attach name plates, protective windows, and warning labels to industrial equipment and are finding specialized niches in diagnostic medical products.
Help for hard-to-bond surfaces
Most automobile interior trim pieces are now LSE plastics. Thin bonding systems can handle the extreme temperatures in auto interiors and hold these parts in place for the life of the vehicle without unsightly mechanical fasteners.
Flexible LSE-rubber gaskets laminated with die-cut, thin bonding films, let assemblers peel and stick the gaskets where they are needed. These gaskets bond to advanced LSE paint systems that are impervious to conventional adhesives.
Die-cut, thin bonding films also permit peel-and-stick attachment of heat-shield materials and sound-dampening foams in all-terrain recreational and work vehicles with hoods and other body panels made from LSE plastics or blends. This approach eliminates time-consuming, labor-intensive, and environmentally hazardous liquid-contact-adhesive processes.
Electronic and Medical
In the competitive medical disposables market, thin bondings film laminate low-cost LSE plastic into diagnostic strips.