Intumescent coating is a type of passive fire protection system used in steel structures. When it is exposed to heat these coatings will swell to form a low density and low thermal conductive foam which prevents steel structure from losing its stability. Intumescent paint is a coating that reacts to heat in a controllable manner to many times of its original thickness to form a carbonaceous char, formed by a large number of bubbles, which acts as an insulating layer to protect the substrate. Passive fire protections insulate the steel structures from high temperatures generated from fire. Fireproofing of steel is most common in refinery and chemical plants. When exposed to heat, steel will lose its strength. Fireproofing helps the structure to maintain its strength. Intumescent paint reacts once the temperature crosses 120℃ and it results in a charring effect at the surface and results in the release of water vapor. pigments are added with the final layer of intumescent paints to get the desired finishing color of the steel.
Properties Of Intumescent Paint
During a fire, the load-bearing capability of the structure gets weaker and it can result in damage of structure which results in life and property loss. The duration of protection in case of fire is between 60 -120 minutes. Timber substrates can be coated with intumescent paint to comply with standards of fire regulation. The intumescent coating has several advantages.
- a) It never changes the mechanical properties of structure
- b) It can be processed easily
- c) It is applied on timbers, steel, composite columns, and concrete.
- d) It dries approximately 1hr under normal conditions.
- e) It can be applied to untreated and previously coated surfaces with the use of internal and external primers.
Intumescent paints are inert at low temperature and it provides insulation due to chemical reaction that occurs at temperatures about 200- 250℃. This temperature will not affect the properties of steel. As a result of this reaction, the paint will swell and form a low conductivity char. The application of the intumescent coating is a complicated process that requires trained technicians and specialized equipment.
Types Of Intumescent Paint
There are two types of intumescent coatings
- Thin – Film intumescent coating
- Thick Film intumescent coating
- Thin- Film Intumescent Coating
The thin-film intumescent coating contains 3 components primer, base coat, and sealer coat.
The main ingredients of the base coat are
- Decomposing catalyst producing phosphoric acid
- A carbonic such as starch combines with a mineral acid to form carbonaceous char
- A binder or resin, heated at a predetermined temperature, which aids to softens.
- Formation of the decomposing agent with the melting of binder to form a large volume of non-flammable gases. carbon dioxide, ammonia, and water vapor causes the carbonaceous char to swell and expands to insulate the structure
Expansion ratios of Intumescent coating are about 50:1 when the coating is subjected to fire, 1mm thick coating will expand about 50mm. Intumescent coatings can be used in offline applications, this results in faster construction, improved quality, reduced site disruption, improved site safety, and easy servicing installations.
- Thick Film Intumescent Coating
These coatings are having higher dry film thickness than thin-film intumescent coatings. They are developed originally to use in hydrocarbon fires. They have excellent strength and durability which makes them best for usage in higher temperatures. The modified products can be used against cellulosic fires. The thin-intumescent coatings cannot be used in areas where the conditions are more severe and maintenance problems exist. Thick Intumescent coatings can be used in areas like high rise buildings and marine applications. The expansion ratio of thick film intumescent coating is much less than thin-film coating.
The dry film thickness of intumescent paints depends on various factors.
- Mass factor
It is the ratio of the area of steel exposed to the fire and volume of the steel section. The higher the mass factor, the faster the steel heats up and the greater thickness of the coating required.
It is the number of faces exposed to fire
- Duration of fire rating
The level of fire protection required according to usage.
Critical temperature: It is the limiting temperature as a function of utilization and lowers the temperature faster the steel section will reach it.
Procedure For Application Of Paint On Steel Structure
The coating of intumescent paint is done by 3 process
- Primer Coat
Before the application of this paint to steel structure, the steel should be sandblasted for making it free of dirt, grease, oil, and dust, or any other things which can affect its binding properties. The material inspection before painting must be done by a Qc engineer and after the inspection gets approved the raw materials can be subjected to primer coating.
Abrasive blasting is done by propelling a stream of abrasive material against a surface under high pressure to remove mill scale and contaminants. The most common abrasives used are steel grit, garnet, and coal slag.
Once the surface contaminants are removed, an anchor profile has been created. The next step is to apply the primer paint to a steel structure. There is a proper method for the application of this coat on the structures. In structures, anti-corrosive primer paints are applied initially and then a topcoat. The main purpose of these coats is to provide adhesion to substrate in a cold state, anti-corrosion properties, and sticking capacity of char formed during a fire.
The most commonly used primers are acrylic, short-medium oil alkyd, two-component epoxy, and zinc-rich epoxy. The intumescent paints are applied as a spray with airless paint equipment. Brush and rollers also can be used. The types are
- Fire Proofing
The next step after the primer coating is fireproofing which is having a fire rating of 1.5 to 3hrs. The fire rating is determined by the number of mils of fireproofing applied. The mils are thickness or quantity of fireproofing added to the steel. Multiple coats are required for getting maximum thickness and after 25 percent of fireproofing is completed mesh material is added. After the mesh is applied the remaining fireproofing is done.
- Final Coat
A final topcoat is applied over the fireproofing coat. If it is used for the external purpose, polyurethane paint is applied. Polyurethane paints provide ultraviolet protection to steels. After the final coating is finished the materials are ready to be delivered. During transportation cracking and disbondment are the common damages that occur in steel structures. If the steel is placed one over the other cracks can occur due to the weight. Disbondment occurs if the steel structure gets in contact with water. If the fireproofing coating will come in contact with water, the fireproofing will begin to pull away from the steel. The fireproofed steel should be compulsorily placed in dry areas. The material engineer should oversee the stacking of these materials. This requires well-qualified staff to carry out the activity.
Intumescent Paints For Concrete, Steel And Cast Iron
Intumescent paint can extend the fire resistance of concrete structures. When exposed to rapid-fire, concrete loses its fire resistance capability due to spalling. Without passive fire, protection structures will get damaged. It only takes a few minutes of unprotected steelwork to reach temperatures of around 550℃. At this temperature, steel loses its load-bearing capacity, and structure will collapse. It comes as a waterborne product that can be used in onsite applications. Waterborne and solvent-based top sealers are available. If intumescent paints are protected with a top coat, then it can be used in high humid and exposed conditions. The main chemicals used for topcoats are two-component acrylic polyurethane, two-component polyurethane topcoat, epoxy, copolymer acrylic, and acrylic PU.
Evaluation Of Intumescent Paints
The most important and commonly used measurements of intumescent coatings are adhesion, cohesion, and how sticky it is with the substrate. pull-off adhesion test and tape test are two ways to evaluate the adhesion and cohesion of the paint.
For the tape test, cross or ribbon cuts are made into the surface and special tape is used to pull off the paint.
The pull-off test can be done in the following ways
- A steel plug is installed on the coating surface
- High pressure is applied to pull off the plug
- It can determine the failure is adhesive or cohesive.
- The pressure of pull is increased further until the bond fails.
- Higher the pressure required to cause the failure in bond, better is the quality of the bond
The adhesive failure commonly occurs at the substrates and not within the coating.
The intumescent coating plays a vital role in passive fire protection of the building. The most common cause of emergency situations is the occurrence of fire. The occurrence of fire can cause severe loss to property and lives. The fireproofing improves the longevity and safety of the buildings. Fireproofing design and execution is one of the important aspects of building construction. The execution of this job needs qualified engineers, technicians, and supervisors.
In a nutshell, Intumescent Paint is used as a fireproof Coating and there are many properties associated with it. Hope you understand the procedure to use intumescent Paint on a steel structure.