Adhesives for Lightweighting of Structures

Lightweighting of structures is most often associated with automotive applications. However, lightweighting is a challenge across the transportation sector, including trucks, trailers, buses, passenger rail cars, and even heavy earth-moving equipment and cranes.

For cars, lightweighting is focused on improving fuel economy. Most of the load that the car moves involves the car itself. But in other areas, lightweighting is focused on load-carrying economy (see Table 1). The lighter the structure, the greater the load that can be conveyed at the same overall vehicle weight.

	Traditional Automotive	Electric Vehicles (EVs)	Buses and Light Rail	Trucks and Trains	Agriculture and Construction
Purpose	mostly moving the vehicle	mostly moving the vehicle	multiple people moving	freight moving	load moving
Fuel efficiency measure	miles per gallon	miles per charge	passengers per gallon	ton-miles per gallon	tons moved per gallon

Table 1. Load-carrying fuel economy.

Adhesives in Lightweighting

Adhesives serve an important role in lightweighting, with exploration focused on joining dissimilar materials for lightweight structures. A previous article (ASI October 2021) introduced the concepts needed for adhesive selection: suitability, compatibility, and capability:

Suitability – the appropriate use of an adhesive, as opposed to other joining methods (e.g., welding or mechanical fastening)
Compatibility – the ability of the adhesive to bond to both surfaces
Capability – the adhesive’s ability to perform structurally under all circumstances

Table 2 shows some of the many possible end uses for bonding in lightweight structures.

Market	Applications	Materials	Adhesives
Electric vehicles	facia, trim panels, interiors, battery boxes/floor pans, crash protection	TPUs, FG-polyester/VE, CF-polyamide, polyphenylene amide, dicyclopentadiene (with or without reinforcement), FG-olefins, olefins, aluminum, steels	acrylics, acrylated-ure, urethanes, epoxy, sealants in/out for batteries, WB, PSAs
Trailers	sidewalls, cored reefer walls, doors, flooring, glazing, pods	FG-polyester/VE, nat-fiber/resin, cored sandwich, VE-SMC, PC glazing, acrylic glazing, aluminum, steels	urethanes, acrylics, epoxies, tapes
Buses and light rail	interiors, fascia, flooring, side panels, roofs	FG-RIM, FG-polyester/VE, ABS, DCPD, TPOs, steel frames, aluminum	urethanes, acrylics, epoxy, WB, PSAs, tapes
Agriculture	cabs, engine covers, hoppers, fenders, boom arms, fascia, bushings and hard points, plastic tabs and gussets	FG-polyester/VE, ABS (comp-backed), cored sandwich, olefins, steel (structure and hard points)	acrylics, urethanes, epoxy
Construction	crane booms, cement pumping, bushings and hard points	CF-epoxy, steels	epoxy, direct joints
Infrastructure/civil engineering	bridges, walkways, spans, canal lock gates, bonded railings, structural repair, FRP wrap, reflectors, signage	FG-resin, CF-resin, polyester/VE, epoxy, steels, aluminum, concrete	epoxy, direct joints

Table 2. Potential end uses for bonded systems in lightweighting efforts.

Table 2 displays certain trends in adhesive selection for transportation. Epoxies offer the highest temperature resistance and have been used with steels for years. Acrylics and urethanes offer good versatility for bonding polymer surfaces and combining with metals. Interior components are bonded with pressure-sensitive adhesives (PSAs), sprayed/dried waterborne adhesives, films, and hot melts.

Many combinations are clearly possible, and selection depends on location and end use. Almost every adhesive type can conceivably be used on some structure, somewhere. Transportation is the largest adhesive end-use sector; automotive is the largest portion of that sector, by far.

The focus for automotive is on metal structures, most of which are spot welded, but many dissimilar metals combinations are non-weldable. The increased use of aluminum and special, high-strength steels is driving the use of bonded structures. Combinations of composites to metals and plastics to metals are non-weldable, as are combinations of dissimilar plastics and plastics to composites. In modern construction applications, most combinations can benefit from adhesive bonding.

Vehicles have been losing weight for 50 years. For electric vehicles (EVs), low weight is paramount for increasing range. However, another method for increasing range is making improvements in the battery itself. New chemistries and electrode materials are allowing increased energy density. This can expand the vehicle’s range by increasing the available power or by decreasing the battery size, thereby reducing weight. These new chemistries and constructions will require adhesive and sealant compatibility.

Challenges Facing Electric Vehicles

Electric vehicles have structural challenges, partly because the vehicle is built around the battery box. It becomes part of the floor pan structure and offers crash protection for the battery. EVs also encounter challenges for front, rear, side impact, and roll-over crash protection for occupants. Bumper systems and fascia are plastics and composites, which are now used for life-cage and roll-over protection.

Most EV battery boxes are aluminum, and electrical insulation is a concern. A thin, fiberglass composite facing can be applied to address this issue. Additional needs involve electrical and thermal insulation within the battery box and with the batteries themselves, offering more opportunities for adhesives and sealants.

Another challenge is that of structural design with adhesives. Structures are routinely modeled for both structural performance and crashworthiness. Because adhesives are viscoelastic, special testing and modeling techniques are required to include adhesives in the load path for structural design.

Test joints must also be evaluated under many environmental and mechanical stress conditions, often combined. Adhesive providers are increasingly involved with design teams to ensure the needed information is available. This goes beyond the common single lap shear test data to include fairly sophisticated testing methods to provide true engineering data (see ASI October 202 0).

For any vehicle, the incorporation of adhesives requires changes in manufacturing culture, which is another challenge. Purely bonded structures are rare. However, adhesives and sealants are combined with other joining methods to offer the advantages of better stress distribution, quieter rides, and corrosion control. The combination of self-piercing rivets (rivet bonding), spot welds, and friction-stir spot welds (weld bonding) is common for aluminum structures. Weld bonding steel structures has been commonplace for decades.

Lightweighting the Future

Lightweighting of structures for many end uses is becoming an obsession in the design community. Combinations of materials to take advantage of their best properties in concerted fashion is now part of the engineering culture. Adhesives will play an increasing role in future designs and applications.

To learn more, contact the author at gritter@chemquest.com.

Read the article in ASI.