In response to the cargo trailer market’s demand for more smooth-sided trailers for both durability and aesthetics, manufacturers have been exploring various methods of side-panel bonding in order to create trailers in the most efficient and cost effective way: tape, adhesives, and a combination of tape and adhesives.
“We’ve seen a large infiltration of those types of solutions in the marketplace in assembling a trailer,” said Jeff Sprout, transportation segment leader for 3M Company, “so we’ve been working with NATM and technical organizations to put together published solutions out there for the most effective ways to build.
“The design and assembly process associated with these three methods differ greatly.”
In a presentation, “A Comparison of Cargo Trailer Assembly Methods,” Sprout joined Dave Herington, US bonding business manager, and Brent Bystrom, technical service specialist.
Bystrom said there are four methods of assembling trailers:
• Mechanical fasteners (screw and rivets).
“This is attaching side panels to the frame with screws or rivets,” he said. “It’s the most common approach seen in the market today and is typically chosen due to familiarity of method, ease of training crews, and low material cost.”
• Liquid adhesive bonding.
“One or two-part liquid adhesives are used to bond the side panels to the frame and bond panel seams. Spacer tape is used to prevent ‘squeeze-out’ on the overlap seam. Panels are typically fixtured in place with a few screws or clamps while the adhesive cures. There is an increasing adoption of this method over screwed trailers due to the speed of application, durable performance, and improved aesthetics.”
• Acrylic foam tape.
“The panel is bonded to the frame and overlap seams are bonded with a fully cured ‘double-sided’ acrylic foam tape. Durability and performance are proven with extensive third-party testing and a 25-plus-year track record. It’s chosen due to performance, durability, smooth look, and immediate bonding or handling strength.”
• Liquid and tape combo/hybrid.
“This is attaching panels and overlap seams with a combination of tape and liquid adhesives. It combines the immediate handling strength benefits of tape with the easy application of adhesives. Tape typically is used for overlap seams and some of the posts while adhesive is used on intermediate posts.”
Bystrom said the advantages of the mechanical fastener method are: immediate holding strength; easy-to-train operators; no surface preparation is required; and the application is not affected by temperature.
But there are limitations and challenges: the screw location must be identified to make sure the fastener doesn’t miss the frame (X on posts and 2X at seams); significant tool noise, ergonomic issues, and repetitive motion injuries; workers must use ladders or scaffolding; there is damage caused by driver slippage; and screws are not recommended on aluminum framed trailers due to the softness of the metal and the risk of them coming loose.
The performance considerations: panel fatigue (point stresses); metal-to-metal contact creates galvanic corrosion; reduced durability as screws loosen; and no sealing, allowing water intrusion.
The aesthetic considerations: visible fasteners; fastener and post corrosion; panel distortion and dimpling; and it’s harder to wrap or apply graphics, leading to reduced durability of graphics.
Herington said the advantages of the liquid adhesives method are: they’re easy to apply onto the frame; minimal training is required; and the application process is quicker than screwed trailers.
The limitations and challenges: some adhesives require surface preparation for adequate performance; adhesives don’t have immediate handling strength; clamping or fixturing is required; prevention of adhesives squeeze-out at the seams is necessary, requiring a separate step; the panel needs to be applied to the frame within a certain period of time; and cure rates are affected by temperature.
The performance considerations: adhesives provide bonding and sealing; adhesives distribute stress over a larger area; some adhesives have flexibility and elongation to withstand vibration and impact; and adhesive bead dimension is irregular, which could lead to false bonds.
The aesthetics considerations: smooth-sided trailers, with no visible fasteners; some adhesives can cause dimpling or read-through due to shrinkage; there’s a “pillowing” appearance at high temperatures; adhesive bead dimension is irregular, which could lead to “wavy” appearance; and uncured adhesive can get on panels, requiring extra cleanup.
Herington said polyurethane adhesive sealants provide the lowest-cost solution; require surface prep on aluminum and steel; and have great overall adhesion and durability. Silane Modified Polymers (SMP) provide similar performance to polyurethanes and typically no primer is needed for aluminum and steel. There is a little higher cost, but the priming step is eliminated.
“Adhesive sealants take a long time to build strength and ultimately cure,” he said. “Adhesives sealants are a moisture-curing technology. When sandwiched between two pieces of metal, they have to cure from the outside in. This can take 14 to 28 days.
“There’s a risk of delamination due to thermal expansion/contraction of the panel if not adequately cured. The recommendations are to allow 14 days to build strength before exposing to environment, and to use acrylic foam tape in combination with sealant to keep the panel secure while curing.”
He said acrylic adhesives have a fast rate of strength build (10 to 20 minutes to handling strength), require minimal surface prep, and are plenty strong for panel bonding.
Epoxy adhesives have a slower rate of strength build and typically require more surface prep than acrylics. They have the strongest overall performance and are more common in bus/truck applications.
There is a caution with structural adhesives.
“False bonds are a common issue when using liquid adhesives with an open-cell foam barrier tape,” he said. “The pressure on the bond ‘flattens’ the adhesive, and the adhesive separates upon recovery of the foam.”
Bystrom said the advantages of the acrylic foam tape method are: ease and convenience of tape; flexibility in the assembly process (pre-tape panels, pre-tape posts prior to frame set, and automation possibilities); immediate handling strength because they are fully cured, with no fixturing or cure time, and they can be shipped immediately after assembly; bonds dissimilar materials; and there is opportunity for lightweighting.
The limitations and challenges: some surface preparation is required; there is planarity of bonded substrates; some design change is required; operator training is required (the process is important, but easy to learn); and the application temperature should be above 50 degrees F.
The performance considerations: proven performance and durability (a Bosch study showed 100,000 miles with no delamination); the closed cell acrylic adhesive core allows bonding and sealing; stresses are distributed across the entire bond line; viscoelastic material provides energy absorption and stress relaxation; there is reduced noise (41%) and vibration (30%); it allows unconstrained panel movement caused by thermal expansion and contraction; it bonds dissimilar materials, eliminating galvanic corrosion; surface preparation and contact pressure are critical to making good bond; and the unconstrained design is optimal to allow for expansion and contraction of the panel.
The aesthetics considerations: it is the ultimate smooth-sided appearance, with an unconstrained design; improved appearance and durability of graphic overlays; and the protective film stays on the sidewall panel throughout production.
The technical and design considerations; there is a need to design for thermal expansion and contraction; stress can cause popping of the panels; an unconstrained design is optimal; not all foam tapes are created equally (there are various acrylic adhesives and various core densities, including firm, conformable, and soft); poor tape selection can lead to failure; and the Cyclic Fatigue Test is a potential predictor of tape failure (core strength).
The advantages of the liquid and tape combo method: it leverages the benefits of tape and adhesive; it holds the panel in place while the adhesive cures, so the trailer can be shipped immediately after assembly; adhesive can be applied quickly; and there is assembly flexibility, with the pre-taping of panel seams and staging of panels.
The limitations and challenges: some surface preparation required; operator training is required because two different methods requires additional training; uncured adhesive can get onto the panel surface, causing rework; and the proper bead thickness is critical.
Performance considerations; tape and adhesive sealants both have viscoelastic properties; tape and adhesive sealants both provide bonding and sealing; tape is holding the panel in place while the adhesive is curing; it is a newer assembly method and doesn’t have as many years with a proven track record; and there are potential false bonds if not enough adhesive is applied.
Aesthetics considerations: it’s still a smooth-sided trailer with good aesthetics; adhesive bead dimension is irregular and could lead to a “wavy” appearance; and there is improved appearance and durability of graphic overlays.
“There are pros and cons to each application method,” Bystrom said. “You need to determine what makes the most sense for your operation and your customer base.” ♦