Choosing the right intumescent paint for your project is not a simple criteria decision. The specifier / owner / architect should ensure that the choice would satisfy multiple factors with relation to the architectural finish, service environment, environmental safety factors and most important the required fire resistance rating. In general intumescent paint is a functional coating that would provide the structural steel additional resistance to ensure the building does not collapse during building evacuation and fire suppression services.
In this post we will be looking at the most important aspects of choosing the right product, such as the required fire resistance rating, and ensuring that the product achieves the required fire resistance rating. We will also look at if the coating system is within the design life cycle and can sustain the corrosion and erosion rating of the serving environment. Additionally, we would be discussing the cosmetic finish of intumescent paint and how would the preceding factors affect the cosmetic finish that can be achieved. Usually architects get in the pitfall of assuming intumescent paint no matter what the service environment or the fire resistance rating, would be similar to anti-corrosion paint, unfortunately this is not true and we will be detailing that in our discussion on cosmetic finishes. Finally, the environmental impact and VOC contents will be discussed.
Fire Resistance Rating: The first part of choosing an intumescent paint is knowing the fire resistance rating. Often in the middle east we come across buildings that are over specified by the designers. Typically, in prescriptive design there is not a valid reason to over specify fire protection requirements unless it was shown necessary by the fire protection engineer to comply with one of the goals and objectives of the design against a specific hazard. In this case it would be a performance based design to comply with an additional requirement that is required by stakeholders or local AHJ (e.g. additional insurance requirement). A careful code compliance should be done by a competent experienced engineer. The fire resistance rating required would affect a lot of aspects of the choice of the right intumescent paint. When the fire resistance rating is specified the range of products capable of protecting the structure is then determined.
Product Fire Rating Limitations: each product is tested by a third party laboratory. The fire test sets important limitations to the tested product to ensure that the product is used in accordance to its capabilities. Not all intumescent paints are capable of covering all sections on the structure and it would highly depend on the third party lab testing. In our previous article “How does Structural Steel Fail Due to Fire Exposure? How to Protect it?” the section factors and fire testing were discussed and we recommend the reader to visit the article to thoroughly understand section factors and basics about the fire testing. The most widely used fire testing for intumescent paint in the Middle East is BS476 part 21. Typically, in labs such as Warrington / Exova, Redbook Live, LPBC, BRE ..etc. The test results is provided in tables that provide the following:
1) The Fire Resistance Rating: each table in the testing certificate is for a specific fire rating.
2) The Type of Element Protected: I or H section 4-sided columns and beams are protected under I/H columns, I or H sections beams that are supporting a concrete floor are protected under I/H beams and are exposed from three sides only. Rectangular and Circular Hollow sections are protected under hollow rectangular columns and hollow circular columns respectively. Not all products are tested for all types of elements and care should be taken when choosing a product for your project.
3) The Section Factor: The tests show a list of section factors (Hp/A aka A/V factor) all parties should ensure that the Hp/A factor for each element of the structure was calculated.
4) Dry Film Thickness (DFT): the DFT for each section factor is shown in the test for a range of applicable failure temperature. In accordance to our previous article “How does Structural Steel Fail Due to Fire Exposure? How to Protect it?”, only a single standard failure temperature shall be used unless a fire engineering analysis is determined.
5) Critical Temperature (failure temperature): a range of critical temperatures is tested such that they are assumed to be the temperature at which the steel would fail. Only standard failure temperatures of 550 deg C for 4-side exposed beams and columns, 620 deg C for 3-sided beams with concrete on top and 520 deg C for hollow sections.
Below is a sample extracted from a sample test certificate:
If the AV Factor of the element is calculated to be 210, and the required fire resistance rating was 120 mins Fire Resistance and the element was a four sided column. The above product testing cannot cover this section and a choice of another material has to be considered. As can be seen in the above table, at the standard failure temperature for 4-sided columns which is 550 deg C the maximum section that can be covered is 145. It is similar to ASTM E119 or UL263 tests, the test usually provides the minimum section size or the W/D section factor limitation, the DFT of the product that is required and the fire resistance rating. ASTM E119 has a critical temperature of 1000 deg F.
Anti-Corrosion and Environmental Rating: Typically, intumescent paint can either be waterbased, solvent based or epoxy intumescent. The later is typically used for oil and gas applications. Waterbased products are typically used for internal applications where the paint is not exposed to weather or direct sunlight. Solvent based products for anti corrosion point of view can be either used in internal or external exposed to weather conditions. However, solvent based have some limitation in internal applications in terms of VOC content that we would discuss later in our article. Typically, intumescent paints (solvent and waterbased) cannot exceed a C4 Environmental classification in accordance with ISO-12944:2018, for C5 or CX ratings an epoxy intumescent paint shall be used.
Cosmetic Finish: Intumescent paint can achieve high level of cosmetic finish when applied at DFTs lower than 1mm. However, when the fire rating exceeds 60mins, DFTs are required in the range of 2-5 mm, depending on how thin the structure is. In structural steel Hp/A is smaller than Pre-Engineering Buildings (PEB), and the DFTs are significantly lower. When applying intumescent paint at high DFTs a lot of unsmooth effects are expected such as orange peel. Orange Peel Effect results from having the top skin of intumescent paint losing the liquid content faster than the lower portions. The higher the temperatures or higher the wind the less the humidity it is more expected to happen. Access is always an issue for finish as the steel is not evenly accessible to the applicator to apply paint in consistency. In buildings exposed to wind and in high temperatures these effects are expected to be more dominant as well as when applied at high DFTs in one coat.
These orange peel effects are typically not seen when viewed 4-5 meters away from the steel surface as shown in the sample above. However, when are not homogeneous and a topcoat is applied the orange peel will be obvious due to the difference in gloss and reflections. These effects are more dominant as well for water base coatings than solvent base as water base cure much faster. At the same time, water based intumescent coatings are typically thinner for the same section and fire rating.
If cosmetic finish is an important aspect of the architectural finish of the structure and will be on view. It is important to ensure that the fire resistance rating of the structure is not over specified. The type of section (e.g. small hollow sections or small sections are harder to spray evenly) and the required DFT should be taken into consideration. Application typically should be site applied to avoid damages as intumescent paint is almost impossible to restore to the spray finish if damaged. Enough time in the construction program should be provided to ensure that the paint is applied at thin layers and enough time is given for curing between layers. Finally, it is always important to consult an experienced contractor to understand what can be achieved. ASFP provides a set of cosmetic finishes, which are basic, decorative and be spoke. It is important that the finish is specified in the project specification by the designer to be considered during the procurement and execution. Applying solvent base products can reduce the orange peel however, due to the high DFTs required and long drying times, texture due to high DFT build are expected.
Off-site application can provide more challenges, while having the steel at perfect conditions and allowing for shorter on-site program. Intumescent paint is very thin and low in density and is easily damaged during transportation and erection. These damages are very hard to restore to the same level of finish as the sprayed finish. When high level of finish is required it is important to apply the intumescent paint at site as it is very unlikely to be able to avoid damages.
Volatile Organic Compounds (VOCs): are chemical compounds that are emitted as gases from either liquids or solids. VOCs have versatile of health effects that can be short or long term. VOCs are used as ingredients in various products that are used in construction from furniture, paints, varnishes, plastics etc. VOCs can cause illness, irritation, allergies, damage to liver and kidneys, damage to central nervous systems, cancer, nausea and a lot more. VOCs are also linked to lack of focus, and reduction in productivity . It is important to reduce those especially when indoors due to the limited ventilation in comparison to indoors. There are a lot of regulations that prohibit an upper limit of VOC (g/l). The specifier engineer should ensure to choose the lowest VOC content for intumescent paint which is typically water based.
Author: Moath Quraini, Technical and Commercial Manager at Passifire.
Author Background: Bachelor of Science in Mechanical Engineering, Masters of Engineering in Mechanical Engineering, Graduate Certificate in Fire Protection Engineering, Master of Engineering in Fire Protection Engineering.