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Structural advances result in safer aluminum composite panels

Changes to core composition and panel attachment methods are improving the safety, strength and design flexibility of aluminum composite material panels.

Designers are using aluminum composite material (ACM) panels for architectural applications with increasing frequency. Constructed of two thin sheets of aluminum with a low-density material sandwiched between them, the panels lend a modern aesthetic to buildings and have practical benefits such as being lightweight, offering sound and thermal insulation, requiring minimal maintenance and using recyclable materials.

The majority of ACM cladding is installed on office buildings, warehouses and retail stores. These structures typically use flat ACM panels. In some high end applications, designers work with manufacturers and fabricators to customize the size, shape and curvature of panels. Their flexibility makes ACM panels a popular material choice for projects ranging from standard low‐rise commercial structures to high‐end designs. In any application, the components of the engineered system and the assembly as a whole—including the method of attaching the panel to the building—should be carefully evaluated for safety, effectiveness and efficiency.

Panel attachment systems play a large role in the final appearance and performance of a building's façade. Relying on mechanical fasteners has been a common strategy. However, hybrid systems consisting of both mechanical fasteners and structural adhesives are coming into greater use, in part because of their superior damage resistance and safety advantages.

Wind and Fire Resistance in Panel Systems

The most recent panel to come to market is the honeycomb panel, a technology adapted from the aircraft industry that offers superior weight, rigidity and fire‐resistance. To create the honeycomb grid, aluminum is formed into a hexagonal structure; z‐axis faces of the hexagons are oriented at 90 degrees to the aluminum sheets that sandwich the core.

Panels with a honeycomb interior present a challenge when affixing a panel using mechanical fasteners, as fasteners that are positioned to penetrate the full depth of the panel have no solid surface into which they can "bite." A common attachment method is the potted insert. Like honeycomb panels, potted inserts were first used in the aircraft industry. A hole is bored through the back surface of the panel and the core, leaving the front surface untouched. An insert is placed in the hole and potted with adhesive, often an epoxy. Another method is to directly attach brackets to the back of the panel with a structural adhesive without having to bore out the inserts (which can potentially damage the panel’s surface).

Based upon the performance of adhesive in the transportation industry, an acrylic adhesive/rivet hybrid assembly that used non-organic core composite panels with fixation bracket bonding was approved for an architectural application as part of the scope of an "Appreciation Technique D'Experimentation" (ATEX) awarded by the French governmental entity Centre Scientifique et Technique du Batiment (CSTB).

In addition to renewed emphasis on fire safety, severe weather episodes are affecting the ACM industry. An increased incidence of hurricanes in Florida, beginning with hurricane Andrew in 1992, has spurred more stringent wind force codes and standards for cladding in the United States. With ongoing climate evolution, European countries are also preparing for stronger wind events in the future. Panel assemblies are now regularly tested to withstand high wind loading, with particular attention placed on the negative pressure conditions that pull cladding from a structure with suction. Testing procedures include impact resistance (using standardized projectile objects aimed at various locations on a panel) and cyclic air pressure differential testing that represents inward and outward acting pressures.

Façade performance in high wind is heavily dependent upon the panel fixation method, and hybrid methods are delivering excellent results in testing situations. In a test performed by CSTB for a hybrid system of rivets and acrylic adhesives, the assembly exhibited resistance to high winds. The panel‐to‐ building bond did not fail even though the panel coating did. In the same test, a mechanical fastener system alone did not pass; rivets tore the panel skin. This illustrated the benefits of surface bonding over spot bonding, which concentrates stresses in single locations.

Designers who are contemplating using an adhesive or hybrid method should not only work with product manufacturers to determine the fire safety of the assembly, but should conduct fire testing of the entire rainscreen unit.

Applications

In hybrid bonding systems, the main role of the adhesive is to provide additional support and rigidity to the bonded panel. There are several common structural arrangements for panel assemblies:

  • Extrusion‐to‐Panel Bonding

Aluminum extrusions are often applied to the back side of panels to give extra support and rigidity. Manufacturers generally recommend that these stiffeners be used on large panels (20 square feet or larger), where they also reduce deflection. Traditionally, extrusions were welded in place; today, structural adhesives are delivering better efficiency and performance.

  • Frame‐to‐Panel Bonding

Some panels are factory‐applied to a rigid metal frame. A continuous bead of adhesive is applied directly to the frame, which is then positioned on the perimeter of the metal (or composite) panel. Cure time can be as little as 15 minutes, after which time weights can be removed from the panel and it can be packed for shipping.

  • Hook/Fixing Plate‐to‐Panel Bonding

Often, panels are attached to a building using hooks, brackets or plates. Adhesives can be used in these situations as a primary or secondary (to mechanical fasteners) fixation method.

  • Butt Stripe Bonding

When two panels are attached at a butt joint, a strip of material (usually constructed of the same material as the panel) is overlapped behind the panels to provide support. Affixing the strip with structural adhesive can provide strength and reinforcement at the joint.

Fabrication drawings specify the size, placement, material and bonding method of extrusions, frames, plates and other attachment hardware. Manufacturers also work with the design team to develop the adhesive bonding process.

Other Benefits of Adhesives

Using adhesives to augment (or instead of) mechanical fasteners reduces the weight of the building façade. Adhesives may also lower labor costs because they hasten the assembly process.

Since designers typically attempt to hide unsightly screws, rivets and other fasteners within folds of metal, using adhesives affords design flexibility because it allows for direct attachment of brackets and stiffeners without having to hide the attachment points. Structural adhesives are sandable and paintable, as well, which is important when using honeycomb panels (or other panels that are not prepainted) as it enables designers to achieve optimal aesthetics on the finished building.

Because mechanical fasteners create holes in the panel, water penetration can occur at points of attachment. In addition to the mechanical fasteners, fabricators must also use a sealant to prevent water ingress (in a dry rain screen system) or corrosion staining (on a wet screen system). Using adhesives to reduce or eliminate the number of mechanical fasteners can improve the water resistance of the finished structure. Adhesives contribute to better weathering of the façade by reducing the rust staining caused by metal fasteners.

Acrylic adhesives, used in metal‐to‐metal applications, can bond dissimilar metals and eliminate the galvanic corrosion that dissimilar metals cause. Additionally, acrylic adhesives can withstand the internal bond stresses caused by metals with differing coefficients of thermal expansion (CTE). Under conditions that cause panel vibrations, adhesives can dampen the vibrations which would normally be transmitted from the panel to the building via the attachment system.

Making Sense of Adhesive Choices

In addition to mechanical fasteners, panels can be affixed to buildings with a variety of adhesives. There are several basic types of adhesive chemistries used in panel fabrication and cladding systems. To name a few, there are acrylics (used for metal bonding and metal to ACM bonding), silicones (often used for sealing and to supplement mechanical fasteners), epoxies (insert potting) and polyurethanes (general bonding and sealing). Choosing the correct adhesive for an application is critical to the success of a project. Elastomeric adhesives such as polyurethanes (PU) or silicones are best suited for sealing and supplementing a mechanical fastener. Attention should be paid to the manufacturers recommendations for surface preparation to be successful. An example would be the need to use a primer with a PU when the substrate is bare metal.

Using structural adhesives with ACM panels has, in the past, been somewhat difficult due to the problem of adhesives “reading" or "printing through" the class A surface of the ACM panel. Also known as "read through," this distortion of the panel’s surface is caused by a number of factors, with shrinkage of the adhesive, exotherm of the adhesive and the soft core material used in ACM panels all contributing.

"MMA adhesives, a subset of acrylic adhesives, can pose a particular problem for ACM panels as some core materials may be softened by the heat," said Stephen Webb, senior application engineer with LORD Corporation. "MMAs typically exotherm more (get hotter) than other acrylic adhesives when they cure and the core of the ACM softens. This softening, combined with higher shrinkage in MMAs, will lead to the surface of the ACM panel pulling down, out‐of‐plane with the rest of the panel. This can have a significant effect on the appearance of the panel as it reflects light; it is very easy for the human eye to pick up distortions in the panel’s surface. LORD has spent years studying the issue of read‐though and has developed acrylic adhesives with low‐exotherm and low‐shrinkage specifically for the ACM and cladding industry.”

Acrylic adhesives offer several other advantages in architectural applications. They have superior environmental performance, being resistant to dilute acids, alkalis, solvents, greases, oils and moisture as well as being resistant to UV‐exposure and weathering. Compared to mechanical fasteners or other adhesive formulations, acrylic adhesives deliver superior peel strength—resistance to forces applied at a 180 degree angle—on a variety of metal substrates.

"Acrylic Adhesives are a great choice for designers who are working with multiple materials because they can bond dissimilar materials. They also bond well to pre‐painted surfaces and have excellent bake resistance on thin‐gauge aluminum up to typical powder‐coating temperatures," said Webb.

Other advantages include easy surface preparation and flexible working time. While polyurethane adhesives require time‐consuming substrate preparation (typically consisting of cleaning, application of a primer and possibly surface abrasion), acrylics require minimal or no preparation. A range of working times are available for acrylic adhesives, allowing for proper application and positioning.

"Fast cure times are another benefit of acrylic adhesive; it can be 90 percent cured in 30 minutes at room temperature, at which time the panel can be unclamped and moved," said Webb. By applying modest heat, (<150 degrees F/<66 degrees C), users can customize cure rates to their application.

Double‐sided tape is another familiar assembly method in the architectural market. However, acrylic adhesives offer a stronger bond, can be repositioned and can be applied to complex or irregular surface areas. Acrylic adhesives typically have a lower cost per lineal foot.

ACM panel use is expected to grow in coming years due to a global upsurge in construction as well as the adherence to green building practices, which favor aluminum as a material choice. Designing ACM cladding systems to include structural adhesives not only improves the strength of panel bonding, but the fast cure time results in reduced cycle time and increased throughput for manufacturers. By eliminating some welds and metal fasteners, adhesives improve design flexibility and the water resistance of finished facades. The result is overall cost savings, greater efficiency and enhanced safety.

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