International Paint: Intumescent coatings for fire protection
By Dr. Allan Jowsey MEng AIFireE MSFPE, Fire Engineering Manager for International Paint The intumescent coatings industry has moved at a fast pace over recent years. Understanding passive fire protection technology and the impact of legislation is important to ensure compliance with international standards for fire and life safety. Introduction 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 fire-fighters. 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. Traditional methods of fireproofing, such as cementitious coatings and gypsum boards, can only be applied on-site and are often considered aesthetically unappealing for visually exposed steelwork and labour-intensive to apply. Intumescent coatings allow for easy off-site and on-site application during construction and provide an attractive finish that does not compromise intricate designs and shapes created from the steel. This allows maximum architectural expression for structures such as airports, stadia, leisure facilities, hospitals and office buildings. As with any fire protection product, it is important to understand its basis to ensure correct specification and one that is fit for purpose. This article outlines some of the keys issues that architects, engineers, fabricators, applicators and Approving Authorities should be aware of when dealing with intumescent coatings. To fully appreciate the role of passive fire protection, it is important to understand three main things: How a structure will perform in the event of a fire? What fire scenario may it be exposed to? What benefits can passive fire protection offer? 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 3 hours 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 and can permit engineers to specify temperatures of their structural elements as part of an optimised design. Fire types The fire protection industry has adopted standard “fire curves” for different types of fires: Cellulosic fires are fuelled by combustibles such as wood, paper and textiles. They are typically associated with commercial infrastructure. Hydrocarbon fires, or pool fires, are fuelled by oil and gas and have a very rapid heat rise. They can be extremely turbulent as they entrain oxygen to maintain combustion. Jet fires are a particular group of hydrocarbon-fuelled fires expelled from an orifice under high pressure. They can have high erosive forces in addition to high heat fluxes above those experienced in open pool fires. Epoxy intumescents are used where there is a risk of hydrocarbon and jet fires, Thin-film acrylics are typically used where there is risk of a cellulosic fire, although an epoxy coating may be required for durability. 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 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. The choice of which to use on a specific project is dependent on factors that include : The fire resistance rating and fire exposure type: Often set by a design standard and influenced by the occupancy use of the structure Legislative requirements including the fire test standard: Strongly linked to global geographic location with fire test standards being referenced in design documents Durability and anti-corrosion requirements: The degree of environmental exposure of the steel is important in selecting an intumescent coating. Environmental Classifications are set out in ISO 12944 Part
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