Steel Coatings Articles

Science Behind It: Intumescent Coatings on Steel

Photo courtesy of PK Industrial

The ever-increasing use of steel as a construction material has led to enhanced flexibility in design as well as significant time savings in the construction industry. However, its use in social infrastructure and buildings has brought an additional challenge — that of fire safety. In the event of a fire, steel can lose its strength and collapse, resulting in damage to property and, in the worst case, loss of life.

Almost all buildings incorporate some fire safety measures. In the event of a fire, structures are required to maintain their stability for a reasonable period of time to enable occupants to evacuate and to provide safety to firefighters. There are numerous products available to designers to satisfy the fire resistance requirements of their projects. Intumescent coatings provide one such method. They react in the presence of intense heat to form an insulating layer, protecting the underlying steel and extending the duration of its structural integrity. Intumescent coatings allow for easy off- and on-site application during construction and provide an attractive finish that does not compromise intricate designs and shapes created from the steel.

Structural Response

Structural steel reduces in strength and stiffness with increased temperature. This can have a detrimental effect on the stability of a structure. Unprotected steel will heat very rapidly in a fire. The aim of an intumescent coating is to insulate the steel and keep it relatively cool for the required fire resistance rating.

The response of a steel structure in a fire can be further influenced by the maximum temperature attained, the degree to which it is loaded, its restraint, and the mechanical properties of the steel itself.

The term ‘Fire Resistance Rating’ is associated with the ability of a building element to perform its function as a barrier or structural component for a specified time during the course of a fire. It is often specified in combination with a critical steel temperature as set by a qualified engineer. Durations vary with legislation around the world, but a typical period may be 60, 90, or 120 minutes.

The basis for the rating is typically specified in accordance with design standards and guidance documents. These documents vary in nature around the world, but fire resistance requirements are strongly related to the risk of fire (occupancy use), the height of the structure, and may be associated with provision of a suppression system. It is critical to understand the correct legislative requirements for a project.

Intumescent coatings can cover a wide range of structural sections, including universal beams and columns, circular and rectangular hollow sections, and concrete-filled tubes. Depending on the type of intumescent coating, it is possible to protect members for up to three hours of fire resistance. Manufacturers also assess their products over a wide range of critical temperatures — known as Multi-Temperature Assessments (MTAs). These may typically be 350° C to 750° C (662° F to 1,382° F) and can permit engineers to specify temperatures of their structural elements as part of an optimized design.

Intumescent Coatings

Intumescent coatings work by undergoing a chemical reaction when heated to form an expanded, thermally insulating layer. The coatings include an acid source — typically phosphorous-based, a carbon source, and one or more blowing agents dispersed in a suitable resin system. At temperatures of around 200° C (392° F), the acid and carbon source react to form a carbonaceous melt, which is then expanded by gases generated during the thermal decomposition of the blowing agents resulting in a sponge-like char.

Intumescents are available in categories that include thin-film water-borne or solvent-borne acrylics and high-build epoxies. Intumescent coating manufacturers often work closely with their clients throughout their projects to ensure a product is chosen that fits with the fire resistance rating requirements, meets the durability specifications, and aligns with construction timeframes.

Intumescent coatings provide a proven method to achieve a required fire resistance rating for a structure. However, before a specific coating can be chosen, it is important to ensure that the coating meets the specifications and is fit for purpose.

Understanding global and local fire safety legislation, fire testing requirements, and product performance is key. Intumescent products are subject to extensive testing to a wide range of global test standards, and these tests and their associated assessments, together with independent third-party certification, provide confidence that a product will be fit for purpose and meet expectations.

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