Factors Influencing the Long-Term Performance of Prepainted Metal Buildings

Most metal buildings are made with prepainted metal building panels. The coil coating process used to make prepainted metal provides many benefits, including superior bonding of paint to metal, durability, environmental friendliness, etc. As a result of the use of prepainted metal, metal buildings are aesthetically pleasing, durable, long-lasting, and economically-efficient structures.

The long-term performance of metal building panels and metal buildings is influenced by three principal factors:

• Choice of materials

• Environment in which the products are placed

• Variability of processes

Choice of Materials

Of the three factors mentioned above, selection of materials has the greatest influence on long-term performance. These materials include the aluminum or steel substrate, metallic coating (for steel), the type of pretreatment used to prepare the substrate for coil coating, and the chemistry of the primer and topcoat.

In addition to the mechanical properties, the metal substrate also plays a principal role in the corrosion resistance of building panels. Aluminum is inherently resistant to corrosion, except where exposed to aggressive alkaline conditions. For a steel substrate, the application of the metallic coating is a well-developed method of protecting the base, steel substrate, which is susceptible to corrosion. Metallic coatings protect the base steel and any bare, cut edges from corrosion through galvanic (cathodic) protection, where the steel is protected by the sacrificial corrosion of the zinc coating (i.e. the cold-rolled steel base metal would corrode on its own, but the zinc-containing metallic coating will preferentially corrode, protecting the steel substrate).

Cleaning and pretreating the substrate prior to painting are important process steps that provide adequate adhesion of the paint system to the substrate, thereby influencing the long-term performance of metal building panels. Since the coatings are expected to remain intact and adhered to the substrate for decades, these steps in the process are critical.

Environment

 As a metal building product is placed in use, it is subjected to the rigors of the environment for the remainder of its lifetime. There is not much that one can do with the environment into which a product is placed, but the choice of building material can be—and must be—carefully considered so it can deliver satisfactory performance over the lifetime of the building. There are many aggressive conditions to consider, such as:

• Aggressive UV Conditions.

• High Temperature Exposure

• Aggressive Chemical Environments

All of the above conditions require the specifier to carefully consider the type of metal substrate to be used, as well as pretreatment and coatings selection. Unfortunately, there is no “one size fits all” solution.

Process Variability

Variability exists in all industrial processes, and to a degree this variability may affect long-term performance. In general, process variability is a minor factor along the coil coating supply chain. Far more important are the choices of substrate, pretreatment, and coatings (discussed above). A well-controlled coating process will produce inferior product if a poor choice is made of substrate (e.g., choosing a metallic-layer thickness that is too thin, which leads to early corrosion), or if a topcoat with poor durability pigmentation is used, producing excessive chalking and color change. Process variability may affect long-term performance resulting in corrosion, delamination or chalk and fade in the product.

Conclusion

Coil coated metal used for building products provides unrivaled long-term performance. The critical factors necessary to provide superior performance are to make an informed choice of the metal substrate, pretreatment, primer, and topcoat. Once these choices are made, the processes used to manufacture the metal substrate, pretreatment, and coatings, as well as the coil coating process used to produce the prepainted metal (if properly run and controlled), provide a level of consistency that sustains the performance of a metal building panel, and a metal building, for decades.

To learn more about the principal factors for long-term performance of metal building panels click here to access a copy of NCCA’s recently published Tool Kit #26 covering this topic.

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