Intumescent paints - efficient way of protection of steel materials against fire

Intumescent coating, often referred to as intumescent paint, is one of the easiest and most efficient ways to protect load-bearing elements of buildings against fire. Intumescent coating delays the collapse of the structure through insulating the structural elements (columns, beams, floors and roofs) that support the building, thus helping achieve fire resistance levels specified in terms of time. Therefore, it fulfills the highest priority of passive fire protection: preventing the collapse of the building, allowing the time for safe evacuation of people from it and making it safer for the emergency services and rescue team.

Intumescent coating is an increasingly used way of providing passive fire protection to the load-bearing structures, especially structural steel, which is becoming more and more popular in modern architectural design of both industrial and commercial buildings. As a means of fire protection, intumescent coating presents several advantages:

• It does not modify the intrinsic properties of materials, for example, the mechanical properties;
• It is easily processed, and
• Different kinds of intumescent paint can be used on a variety of materials, such as steel, timbers, composite elements and concrete.

How do intumescent paints work?

Intumescent paint is a reactive coating which swells as a result of heat exposure, thus increasing in volume and decreasing in density. Specifically, an intumescent paint is a coating that reacts to heat by swelling in a controlled manner to many times its original thickness, producing a carbonaceous char formed by a large number of small bubbles that act as an insulating layer to protect the substrate.

The purpose of intumescent products is the prevention of the structural collapse of the building, which can occur if load- bearing steel elements reach a critical state.

For steel, this is linked to the critical temperature, defined as the temperature at which the load-bearing capacity becomes equal to the effect of the applied loads (so the steel element is very close to collapse). Critical temperature of steel can vary from 350 °C to 750 °C, depending mainly on the loading scheme, but in most of the cases between 500 °C and 620 °C.

For concrete, critical state is linked to the critical temperature of the reinforcing bars (normally from 350 °C to 500 °C) and to reaching a temperature of 500 °C inside the concrete element.

For wood, critical state is linked to the residual section of the timber load-bearing element after burning.

How to correctly apply intumescent paints to steel materials - preparation tips

Intumescent paints are always part of a system. For steelworks, the system includes an anticorrosive primer and (eventually) a topcoat. For the former, the purpose is assuring adhesion to the substrate in the cold state, anticorrosion protection and stickability of intumescent char formed during fire exposure, while for the latter, the purpose is serving an aesthetic function and, in case of specific atmospheric aggression, a sealer function to prevent early degradation and inactivation of the intumescent layer and to promote weathering resistance in end-use conditions.

Before being coated with a compatible primer, steelwork must be prepared according to the SA 2.5 Swedish Standard. If that is already the case, it must be cleaned and free from grease, oil, rust, dirt or any other contaminant that may inhibit the bonding.

Compatible primers, tested with the main Promat products are:

• Acrylic
• Short or medium oil alkyd
• Two-component epoxy
• Zinc-rich epoxy (containing about 80% by mass of metallic zinc powder)
• Zinc-rich epoxy (containing about 96% by mass of metallic zinc powder)
• Zinc silicate
• Polybutadien (Promat® TY ROX).

The preparation for concrete and timber depends on the physical state of the support. For more information, contact our technical department.

What is dry film thickness (DFT) of intumescent paints?

The dry film thickness and quantity of material required for a certain fire resistance time (R 30, 60, 90, 120 minutes or more) depends upon various factors.

Dry film thickness of intumescent products for structural steel is determined by the following factors:

• Mass factor (called also massivity, section factor or H_p /A or A_p /V), a ratio between the area of the steel exposed to the fire and the volume of the steel section. The higher the mass factor, the faster the steel section heats up, and the greater the thickness of fire protection material required.

• Exposure or number of faces exposed to fire. Is the structural steel a column or a beam, a composite element, a hollow section or something else?

• Critical temperature, the limiting temperature as a function of the degree of utilization. The lower the critical temperature, the faster the steel section will reach it, and the greater the thickness of fire protection material required.

• Duration or fire rating or the level of protection required (R 60, R 120, etc.).

• Test Standards and Approvals. There are various such Test Standards and Approvals giving different thicknesses for the same protection.

When should wet film thickness (WFT) and dry film thickness (DFT) be measured?

During the application, it is necessary to frequently measure the wet film thickness (WFT) with a wet film thickness gauge.

To determine dry film thickness (DFT) based on the wet film thickness (WTF), it is necessary to multiply the WFT by a specific number that is different from product to product. For Promat’s product range, the value is between 0.68 and 0.7 (see the specific product data sheet for more information).

The maximum thickness that can be applied for each coating varies by product as well as by related consumption (the TSR, theoretical spread rate).

Following the sufficient period of drying, a survey of dry film thickness should be carried out using a suitable calibrated gauge. An electromagnetic induction instrument with a statistical function to store readings and calculate their averages is most useful. Where dry film readings include a primer or topcoat, allowance must be made for these coatings and subtracted from the total reading.

Do I need to apply topcoating on intumescent paint for steel protection?

Intumescent paint for normal interior application can be used without any additional decorative top layer. Adding a topcoat is necessary for exterior semi-exposed or high ambient- humidity applications.

The nature of the environment to which the coatings will be exposed may affect their durability or their performance in a fire situation. If necessary, a topcoat must be applied to the surface of the intumescent coating, either as a protection against environmental degradation or for decorative purposes.

How do I choose the best type of intumescent paint for my project?

The topcoat should be specified based upon the intended use of the system and the environmental conditions.

The following use categories are defined for fire protective products according to ETAG 018:

• Type X: Reactive coating system intended for all conditions (internal, semi-exposed and exposed).
• Type Y: Reactive coating system intended for internal and semi-exposed conditions. The latter includes temperatures below zero, but no exposure to rain and limited exposure to UV (ultraviolet) (which is not assessed).
• Type Z₁: Reactive coating system intended for internal conditions (excluding temperatures below zero) with high humidity.
• Type Z₂: Reactive coating system intended for internal conditions (excluding temperatures below zero) with humidity classes other than Z₁

If protected with a specific topcoat (depending on the weather condition), intumescent paints can also be applied in high-humidity, semi-exposed or exposed conditions.

The main chemical families of topcoats used for environmental protection are:

• two-component polyurethane top coat
• two-component acrylic polyurethane
• copolymer acrylic
• polyurethane
• acrylic PU
• epoxy
• urethane alkyd

Can I use intumescent paints to protect concrete or timber constructions?

Intumescent paints can also be used for protecting concrete, in which case the thickness necessary for the given fire resistance time is calculated with regard to the critical temperature of both the steel reinforced bars (between 350 °C and 500 °C) and the concrete cover (the least distance between the surface of embedded reinforcement and the outer surface of the concrete).

Intumescent coating, especially if transparent, can also be used for protection of timber, reducing the fire reaction and improving the fire rating.