Intumescent Paint – An Efficient Way of Protecting Steel Against Fire

Promat 15/05/2018

intumescent coating
intumescent paint
fireproof paint
structural steel protection
Intumescent paints applied on steel structures
Intumescent paints applied on steel structures

An intumescent coating is one of the easiest and most efficient ways to protect load bearing elements against fire in a building. It is a technical fire protective product, which delays the collapse of the structure through insulating the structural elements (columns, beams, floors and roofs) that support the building, achieving specified fire resistance in terms of time. Therefore it fulfils with the highest priority of passive fire protection, which is to increase fire safety, to prevent building collapse allowing time for safe evacuation of people from the building and make the building safer for the emergency services and rescue team.

Intumescent coatings represent an increasingly used way to provide passive fire protection to the load-bearing structures, especially structural steel that is more and more used in modern architectural design in industrial and commercial buildings. It presents several advantages: it does not modify the intrinsic properties of the materials, like for example the mechanical properties; it is easily processed, and different version of intumescent paints can be used on a variety of materials such as steel, timbers, composite elements and concrete.

How intumescent paint works?

Intumescent 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 which reacts to heat by swelling in a controlled manner to many times its original thickness to produce a carbonaceous char, formed by a large number of small bubbles, which acts as an insulating layer to protect the substrate.

The scope 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 for which, the load bearing capacity becomes equal to the effect of the applied loads (so the steel element is very close to the collapse). Critical temperature of steel can vary from 350°C to 750°C, depending mainly for the loading scheme, but in most of the cases is between 550°C and 620°C. For concrete the critical state is linked to the critical temperature of the reinforcing bars (normally from 350°C to 550°C) and the reach of a temperature of 500°C inside the concrete element. For wood is linked to the residual section of the timber loadbearing element after burning.

How to correctly apply intumescent paints on steel materials? - Preparation Tips

Intumescent paints are always part of a system. For steelworks, the system include an anticorrosive primer and (eventually) a topcoat.

  • The scope of the primer is assuring adhesion to substrate in cold state, anticorrosion protection and stickability of intumescent char formed during fire exposure.
  • The scope of the topcoat is: aesthetic function and, in case of specific atmospheric aggression: sealer function to prevent early degradation and inactivation of intumescent layer, weathering resistance to end-use conditions.

Steelwork must be prepared to SA 2.5 Swedish Standard  before being coated with a compatible primer or, if already painted with a compatible primer, must be cleaned (free from grease, oil, rust, dirt or any other contaminant that may inhibit the bonding).

The compatible primers tested with the main Promat products are:

  • Acrylic
  • Short/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).

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

How to correctly apply intumescent paints
How to correctly apply intumescent paints

Intumescent coatings are preferably spray applied with airless paint equipment for speed and quality of finish. Brush and roller application may also be possible. 

  1. Spray, brush or roller applied intumescent paint
  2. Spray, brush or roller applied topcoat (if necessary).
  3. Primer approved by international certification or by Promat. Please contact our technical department.

Structural steel column and beam. Clean, dry and free from dust, oil, loose mill scale or rust and any other condition preventing good adhesion AND blast cleaned in accordance proper international standards.

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

The dry film thickness and quantity of material required for a certain fire resistance time (R30, 60, 90, 120 minutes or more),is dependent upon various factors.

Regarding structural steel, the mains are:

Dry film thickness of intumescent products is determined by:

  • Mass factor (called also massivity, section factor or HP/A or A/V). The mass factor is a ration 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 so the greater the thickness of fire protection material required.
  • Exposure: that is number of faces exposed to fire, is it a column or beam, composite elements, hollow section, etc.
  • Critical temperature: limiting temperature a s a function of the degree of utilization. The lower the critical temperature, the faster the steel section will reach it, and so the greater the thickness of fire protection material required.
  • Duration or fire rating: the level of protection required (R60, R120, etc.).

Test Standards and Approvals: different standard and assessment can give different thickness for the same protection.

When to measure wet film thickness (WFT) and dry film thickness (DFT)?

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

To determine dry film thickness (DFT) based on the wet film thickness (WTF), it’s necessary multiply  the WFT for a specific number, different products by products. For the Promat product range the value is between 0,68 and 0,7 (see specific product data sheet [link to product data sheets] for more information).

The maximum thickness that can be applied for each coat vary product by product (see specific product data sheet[link to product data sheets]) as well as the related consumption (TSR:  theoretical spread rate).

After sufficient drying time a survey of the dry film thickness should be carried out using a suitable calibrated gauge. An electromagnetic induction instrument with a statistical function to store readings and give an average is most useful. Where dry film readings include a primer and/or topcoat an allowance must be made for these coatings and subtracted from the total reading.

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

Intumescent paint for normal interior application can be used without any additional, decorative top coat.

For exterior, semi exposed or high ambient humidity applications, the addition of a topcoat is necessary.

 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 environment degradation or for decorative purposes.

How to 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 the 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. Semi exposed includes temperatures below zero, but no exposure to rain and limited exposure to UV (but UV 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

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

The main chemical families of topcoat 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 be used also for concrete protection. In this case the thickness necessary for a given fire resistance time is calculated keeping into consideration the critical temperature of the steel reinforced bars (between 350° 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 in the transparent version, can be used also for timber protection, reducing the fire reaction and improving the fire rating.

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