Adhesives + Composites A Natural Combination

A structural adhesives tutorial

—by Kathrine Lewis, Technical Development Manager, Huntsman Advanced Materials

As design engineers continually push the limits of materials performance, they are increasingly using advanced composites for structural applications. From racecar bodies and aircraft components to wind turbine blades and building walls, composites are replacing traditional all-metal constructions to reduce weight, increase corrosion resistance and support greater design flexibility. Moreover, with the right adhesive, composite assemblies can withstand exposure to temperature and environmental extremes as well as vibration even when the substrates have dissimilar coefficients of thermal expansion. 

The three primary types of structural adhesives used in bonding composite parts are epoxies, polyurethanes and methacrylates. Each has its own distinct chemical composition and physical properties; all bond substrates with sufficient strength to transfer high loads without failure. To determine which structural adhesive is appropriate for a specific project, its characteristics and capabilities must be evaluated along with substrates, processing requirements, anticipated operating conditions and desired end-part performance.

Epoxies

The majority of epoxies are two-component systems comprising of resin and hardener. The easy-to-handle adhesives are formulated with viscosities ranging from thin liquids to thick pastes to accommodate numerous application methods. Epoxies feature working gel times from a rapid 90 seconds to two hours, to meet the processing and lead-time requirements of specific projects. The adhesives attain handling strength in as little as five minutes at room temperature. At this point, a bonded assembly can be removed from clamps or fixtures, and the epoxy allowed to complete its cure cycle at either room or elevated temperatures. 

Once cured, epoxy adhesives produce strong, rigid bonds on the thermoset plastics typically used in composite structures as well as on metals, which may be part of the assembly. Epoxies feature good electrical insulating characteristics as well as excellent solvent and chemical resistance. Hardnesses are in the Shore 66 to 90D range and lap shear strengths are from 1990 to 4100 psi. Maximum service temperatures can be as high as 350°F (depending on formulation and cure temperatures) and elongations are typically from 1 to 55%. 

With this combination of characteristics, epoxies are often the materials of choice for applications requiring outstanding durability and environmental/heat resistance. In building its LeMans-winning racecars, for example, Audi bonds aluminum water coolers to the carbon fiber reinforced plastic (CFRP) monocoque frames of the vehicles . Audi engineers selected epoxy for the bonding application because it produces tough, durable joints that retain their strength even at elevated temperatures and when exposed to mechanical stresses and vibration. The two-component paste epoxy used by Audi features a glass transition temperature of 284° to 302°F, and maintains bondline integrity in the presence of moisture, fuel and other fluids.

Structural epoxies have also proved tough enough to assemble glass reinforced plastic (GRP) sandwich construction wall systems used by Norway’s Marine Composites AS to build hospitals, food processing and pharmaceutical plants. The high-performance epoxy specified for joining the “LEGO”-like blocks for the buildings exhibits good gap filling properties to produce 0.002- to 0.004-in. thick bondlines, while resisting sagging up to a 0.4-in. thickness. The adhesive also has good environmental characteristics to maintain resilient joints in Norway’s corrosive marine atmosphere at temperatures varying from -40° to 95°F. 

The load-carrying capability of epoxies also makes the adhesives suitable for bonding metal hubs to GRP rotor blades in wind generators. The application demands an adhesive that can withstand the centrifugal forces applied to each blade. With lap shear strength in the 4350 psi range, epoxy provides the needed high resistance to stress, fatigue and aging to ensure long-lasting joints.

Polyurethanes

For applications such as thermoplastic bonding, joining dissimilar substrates and projects requiring good joint flexibility, polyurethanes are typically used. Chemically, the adhesives are comprised of an isocyanate resin that is mixed with a polyol hardener to produce the bonding system. (Care must be taken, however, to avoid contamination of the resins and hardeners with moisture that can render the polyurethane unusable.) Polyurethanes are formulated with viscosities from liquid to paste and have work lives from one to 20 minutes. Precise ratio control is critical with polyurethanes to achieve the desired hardening of the adhesives. To facilitate mixing, many polyurethanes are formulated with 1:1 ratios and are packaged in 50 and 200 ml dual-barrel cartridges that provide for accurate blending and neat, waste-free dispensing. Polyurethanes generally can be applied to surfaces after minimal pretreatment. They exhibit good gap-filling and sag-resistant characteristics, and attain handling strength in as little as an hour. The adhesives cure at room temperature or elevated temperatures, offering users the flexibility to vary adhesives choice according to processing capabilities and project lead-time. 

Cured polyurethanes exhibit good flexibility even at temperatures as low as -40°F, along with high impact resistance and the ability to maintain bond strength when exposed to high humidity and moisture. Hardness of the systems is significantly broader than with epoxies, from a rubbery Shore 10A to a rigid Shore 70D. Service temperatures (typically from 120°F to 140°F) are lower than for epoxies as well as lap shear strengths, which are in the 1700 to 2800 psi range. Polyurethanes have slightly higher peel strengths than epoxies (up to 55 pli) and outstanding elongation (up to 250%), making them suitable for use on hard-to-join and dissimilar substrates that will be exposed to vibration and other mechanical stresses. Polyurethanes used in manufacturing and repairing aircraft are also self-extinguishing to meet FAA flammability standards.

With this combination of features, polyurethanes are frequently used in assembly and repair of thermoplastic components used in aircraft interiors. ABS aircraft tray tables are, for example, quickly bonded with a rapid-setting polyurethane that can hold a 150-lb. load without disbonding. The same polyurethane was the material of choice for repairing thermoplastic cockpit side-pocket panels. For the panel project, new aluminum bottoms were added to side-pockets using polyurethane to attain the required durability between the dissimilar substrates. The adhesive also exhibited the strength needed for long service life with a lap shear of 2500 psi and T-peel of 40 pli. 

Durability under a broad range of temperature and weather extremes is also a key factor in joining ABS and resilient Telene thermoplastic panels to produce tonneau covers for pick-up trucks. The polyurethane adhesive selected for the application features an easy-to-handle flowable viscosity, resists sagging and run-out as panels are inverted and joined, and bonds cover halves without read-through for an aesthetically attractive surface. 

Aircraft windshields, like tonneau covers, must be able to withstand ongoing environmental and temperature changes. A leading windshield manufacturer uses a versatile polyurethane to bond nylon-reinforced acrylic sheet and epoxy edging to the acrylic windshield The adhesive has proved ideal for the application because, unlike other products the engineers tried, it doesn’t craze or crack windshields as the aircraft takes off and climbs to the much colder temperatures at the cruising altitude.

Methacrylates

Structural adhesives in the methacrylate family combine epoxy-like strength with the ability to set and cure quickly. Versatile, two-component methacrylates are designed to securely join substrates including thermoplastics, thermoset composites, metals, ceramics and glass. Like polyurethanes, methacrylates require little surface preparation. The acrylic-based adhesives have work lives of three to 10 minutes and attain handling strength as quickly as four minutes to support high-productivity assembly operations. Typical cure cycles are 18 to 30 minutes at room temperature; increasing temperatures to 104°F can accelerate curing. Methacrylates fill gaps up to 0.16 in. wide.

Once cured, methacrylates exhibit lap shear strengths approaching that of epoxies (as high as 3600 psi), peel strengths up to 63 pli, and elongations that fall between epoxies and polyurethanes in the 35 to 75% range. The structural adhesives exhibit good chemical and water resistance.

With these characteristics, methacrylates were recently used by Volvo to bond thermoplastic components to ABS steering columns and cab hinges on its trucks. In service, the adhesive-bonded parts featured the required combination of strength and flexibility even when exposed to oil, gas and temperature extremes. 

In another transportation application, methacrylates are producing durable joints on recreational vehicles manufactured in Italy for sale throughout the world. The adhesives are used to bond fiberglass components onto external walls and to join the RV body to the chassis. The manufacturer has found that the bonded joints produce a high-quality appearance that could not have been attained with welding or riveting. In service, after use on nearly 5000 RV’s, the joints have demonstrated their ability to withstand vibration and heavy loads for excellent long-term durability.

The use of adhesive-bonded advanced composites can be expected to continue to grow in the production of the cars, trucks, planes and structures. As industry switches from mechanically fastened metal components to bonded composites for the desired weight reduction and other benefits, it is more important than ever for design engineers to have a thorough knowledge and understanding of the latest advances in structural adhesives and their capabilities. 

Comparison of Typical Properties of Epoxies, Polyurethanes and Methacrylates

Typical Mixed Properties

Typical Cured Properties