Ind. Eng. Chem. Res. 2006, 45, 7475-7481
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High-Throughput Fire Testing for Intumescent Coatings M. Jimenez, S. Duquesne, and S. Bourbigot* Laboratoire des Proce´ de´ s d’Elaboration de ReVeˆ tements Fonctionnels (PERF), LSPES UMR 8008, Ecole Nationale Supe´ rieure de Chimie de Lille, BP90108, F-59652 VilleneuVe d’Ascq, France
A new small-scale fire test is described in this paper. The goal was to design a high-throughput test permitting the optimization of intumescent mastic coatings formulated to protect steel in the case of hydrocarbon fire. This type of coating, applied on steel plates, beams, or columns, is usually evaluated in large industrial furnaces. Such experiments are expensive and time-consuming. The development of a small-scale laboratory test using an external heat flux is investigated. Temperature as a function of time is recorded on the back of the coated steel panel. It is shown that temperature profiles are very well correlated with those measured in industrial furnaces. This low-cost and fast test is very repeatable, and it allows rapid screening of a large number of different coating formulations. The test also shows good correlation with the results obtained in cellulosic fire testing. 1. Introduction The protection of metallic materials against fire has become an important issue in the construction industry. Indeed, prevention of the structural collapse of the building is paramount to ensure the safe evacuation of people from the building, and is a prime requirement of building regulations1 in many countries. Steel usually begins to lose its structural properties above 500 °C. Intumescent coatings are designed to perform under severe conditions and to maintain the steel integrity for between 1 and 3 h in some cases when the temperature of the surroundings is in excess of 1100 °C.2-4 Upon heating, foamed cellular charred layers are formed on the surface, which protect the underlying material from the action of the heat and/or flame. Different types of fire curves, corresponding to different standards, can be used during industrial tests, the main ones being cellulosic and hydrocarbon fire test curves,5,6 as shown in Figure 1. The cellulosic fire test curve (ASTM E-119) intends to simulate the rate of temperature increase that can be observed in a residential or commercial building fire where the main sources of combustion fuel are cellulosic in nature, such as wood, paper, furniture, and common building materials. The fire curve is characterized by a relatively slow temperature rise to around 927 °C after 60 min. But although this fire curve is still used, it is noteworthy that the burning rates for certain materials, e.g., petrol gas and fuels, etc., are much larger than the rate at which, for instance, timber would burn. As such, there was a need for an alternative exposure for the purpose of carrying out tests on structures and materials used within the petrochemical industry, and thus the hydrocarbon curve was developed. The hydrocarbon test curve (UL1709) intends to simulate or to be indicative of the rapid temperature rise measured when a hydrocarbon fuel such as oil or natural gas burns: the temperature rises rapidly to 1000 °C within 4 min until reaching temperature between 1100 or 1200 °C. This hydrocarbon fire test curve, developed by the Mobil Oil Co. in the early 1970s and adopted by a number of organizations, is now a well-accepted reference in high-risk environments such * To whom correspondence should be addressed. Tel.: + 33 (0)3 20 43 48 88. Fax: +33 (0)3 20 43 65 84. E-mail: serge.bourbigot@ ensc-lille.fr.
as petrochemical complexes and offshore platforms, with a typical rating of 2 h. This hydrocarbon test curve is also used to simulate jet fire scenarios in which leaking high-pressure hydrocarbon gases ignite to produce intense, erosive jet flames that can reach speeds of 150 m/s. A standard jet fire test, denominated OTI 95 634, has been developed jointly by the U.K. Health and Safety Executive and the Norwegian Petroleum Directorate for use predominantly on offshore installations.7 The test impinges the high-speed stream of ignited propane fuel onto a substrate coated with the product.8 The propane is delivered at a rate of 0.3-20 kg/s, depending on the test site setup, and can consume 1 ton of fuel/min. The industrial test taken as reference in this paper is test UL1709, and the intumescent formulations used in this study are thick-film intumescent systems, designed to provide protection of steel in both hydrocarbon and jet fires. Intumescent thick films are usually based on epoxy, vinyl, or other elastomeric resins and contain ingredients providing intumescence upon heating. They are available as solvent-free systems, permitting the application of up to 8-10 mm thick coating.9-12 They are hard and durable, and some of them can provide excellent protection from corrosion.13,14 They exhibit very high adhesion to the substrate and resistance to impact, abrasion, and vibration damage. High tensile and compressive strengths can be obtained, and weather resistance is excellent. These intumescent coatings are high-value products for industries, and the competition in developing new products is high. But the development of new intumescent coatings depends on many parameters: the intumescence concept15-18 requires a balance between the fire properties and the level of additives in the material. The formulation of the coating has first to be optimized in terms of physical and chemical properties (char strength, expansion, and viscosity, etc.) in order to form an effective protective char.19 This latter should limit both the heat transfer from the heat source to the substrate and the mass transfer from the substrate to the heat source, resulting in conservation of the underlying material. The formulations are then evaluated in an industrial furnace using UL1709,20 in order to compare their ability to protect the steel substrate from fire. In a typical fire test, steel substrates coated with intumescent coating are tested. Thermocouples are attached to the isolated back of the plate, and the evolution of temperature is plotted against time (Figure 2).
10.1021/ie0608410 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/28/2006
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Figure 1. Main standard fire test curves.
Figure 2. Example of time/temperature curve registered.
The key parameter of this test is when the nonheated side of the steel plate reaches 400 °C: the critical temperature of the steel is an essential parameter; this parameter is defined as the temperature at which only 60% of the original strength remains, the point at which failure is imminent under full design loads. It is important to take into account the load of the structure: if the steel member is part of the main structure, it follows that the load also needs to be considered when determining the protection requirements. For regular reinforcing steel, the critical temperature is 538 °C, while for prestressing steel bars, which are made of high-carbon, cold-drawn steel instead of low-carbon, hot-rolled steel, the critical temperature is significantly lower at 427 °C. It is the reason 500 °C has been selected as a standard for normally loaded structural components, while 400 °C has been selected as a standard for heavily loaded structural components such as, for example, offshore platforms.
Figure 3. Industrial furnace.
This temperature is known as the “failure temperature”. The corresponding time is called the “time of failure”. The most efficient coatings are the ones having the longest time of failure. These furnace tests are however very expensive, and panel preparation is time-consuming; moreover only a few panels can be tested in 1 day. This is why small-scale and high-throughput tests should be developed to allow rapid screening of formulations21 and accelerate the research on new and more efficient intumescent formulations. The test we have developed in our laboratory tries to evaluate very rapidly the heat barrier effect of intumescent coating when exposed to a heat flux. Tests have been carried out on different intumescent epoxybased thick-film coatings: results and correlations are presented and commented on in the first parts of this paper. The last part of the paper discusses results also measured on cellulosic intumescent coatings as test materials. 2. Experimental Apparatus and Methods 2.1. Industrial Fire Test. Industrial furnace tests have been carried out in a 1.5 m3 furnace (Figure 3) according to UL1709. The coatings are applied on a steel plate (thickness, 3.5 mm) and cured for 1 week at room temperature. Thermocouples are attached to the back of the coated plates. Five thermocouples are used on each plate, so that an average temperature can be obtained. The burning conditions try to fit as much as possible the ramp of temperature of a hydrocarbon fire (about 200 °C/ min). Figure 4 shows the difference between the experimental
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Figure 4. Comparison between standard and experimental hydrocarbon curves.
Figure 5. Four steel plates coated, before and after the test.
and the standard hydrocarbon curve. The main difference occurs during the first minutes of heating: the increase of temperature is much more rapid during the test compared to the standard. This is not a problem, as the aim is to test the coatings under the most severe conditions possible. As four plates are tested at the same time, the plates are isolated using glass wool. The plates are mounted vertically in the furnace and burnt (Figure 5) until the thermocouples attached to the back of the plates reach a temperature higher than 400 °C (the failure temperature). Time/temperature curves are recorded, which characterize the heat protective effect of the different coatings in a hydrocarbon fire. 2.2. Small-Scale Test. Some small-scale tests such as the cone calorimeter test,22 limiting oxygen index (LOI),23 and flammability tests (UL94)24 already exist and are widely used to evaluate different parameters of intumescent formulations, for example, the heat release rate, the ignition time, and the gases released, etc. By developing the current small-scale test, the aim was to evaluate the efficiency of the intumescent coating in terms of heat transfer. As in the large furnace test, the aim is to obtain time/ temperature profiles measured on the back of the coated steel plate. The heat source is a heat radiator provided by Saint Gobain (France). The steel plates used are squares of 5 × 5 cm2 and 5 mm thick; the steel is exactly the same as the one used in the industrial test. About 1 mm of the intumescent coating is applied on the surface of the steel plate. A black coating, provided by
Medtherm Corp. (Huntsville, Alabama), resistant to 800 °C and having a constant emissivity of 0.92, is applied on the nonheated side of the steel plates. The constant emissivity of the backside of the plate allows accurate measurement of the surface temperature of the plate using an infrared pyrometer. The infrared pyrometer is positioned at a constant distance from the steel plate, and the beam is pointed on the center of the plate. It detects the temperature on the nonheated face of the steel plate and registers the time/temperature curve on a computer. This time however no failure temperature is considered, as the heating temperature does not reach 1200 °C as in the large furnace. This test is used mostly by comparing temperature obtained after 20 min for one formulation to the test is shown in Figure 6. This small-scale test is very stable and repeatable. The reference curve is the time/temperature curve obtained for the virgin steel plate black-coated on its nonheated face. The whole system is placed into a box in order to avoid the effect of the fume cupboard: the aim is to reduce the convective and chimney effects. 2.3. Materials. Different intumescent formulations have been studied using both tests, to compare the results obtained. Because all the formulations are provided by an industrial partner, the identity of the components will not be given in this paper. Three different types of formulations have been tested: the first type comprised three basic intumescent epoxy-based formulations (IF1, IF2, and IF3), which are compared to a reference commercial intumescent mastic (IF4). The second series of tests
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tested in a large furnace using the cellulosic fire curve. The purpose was to validate our test in different conditions. 3. Results and Discussion
Figure 6. Presentation of the small-scale test.
Figure 7. Repeatability test on a virgin plate and on a virgin plate coated with IF4.
were carried out on three intumescent epoxy-based mastics (IF5, IF6, and IF7), also compared to the commercial formulation (IF4). The final tests were carried out on three unknown cellulosic coatings (IFA, IFB, and IFC), which have also been
3.1. Reproducibility Tests. Reproducibility tests were carried out on the uncoated steel plate and on the steel plate coated with 1 mm of the commercial reference IF4. The experiments were performed on different days, and as it can be seen in Figure 7, the experimental reproducibility is very good in all cases throughout the entire test. 3.2. First Series of Tests on “Basic” Formulations. Heat radiator tests were first carried out on different formulations (IF1, IF2, IF3, and IF4), and the results are compared with those obtained in furnace tests. The formulations IF1, IF2, and IF3 are constituted by the main intumescent compounds of the formulation IF4. The time/temperature curves obtained both at the industrial furnace tests and with the heat radiator test are shown in Figure 8a,b, respectively. The time/temperature curves carried out in the industrial furnace are shown up to 400 °C (failure temperature). The parameter taken into account is the time of failure, namely, the time when temperature reaches 400 °C. The materials exhibit very different behavior: the most efficient coating is IF4 (time of failure of about 40 min), while the worst one is IF1 (time of failure of 10 min). During the heat radiator test, there is an important increase of temperature at the beginning of the test (from 0 to 350 s) in all cases, but after 450 s the temperature remains quite steady. If the temperatures reached at the end of the experiment are compared, the best performance is obtained when the temperature increases slowly during initial heating (t < 350 s) and when the lowest temperature is reached at the end of the test (t > 450 s). According to those observations, the curves can be well-distinguished. The best formulation is still IF4 with a temperature of about 320 °C reached at 1200 s, and the worst one is still IF1, which reaches about 400 °C at 1200 s. The ranking of results obtained after 1200 s in the heat radiator test and at 400 °C in the furnace tests are then summarized in Table 1. The rank orders between the heat radiator test and the industrial furnace agree well each other. So the heat radiator test appears to be a very interesting tool to carry out an initial
Figure 8. (a) Industrial furnace and (b) heat radiator tests on five intumescent formulations.
Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7479 Table 1. Ranking of Results Obtained for Tests on Intumescent Formulations (1 w Best Result) IF1 IF2 IF3 IF4
heat radiator test
industrial furnace test
4 3 2 1
4 3 2 1
assessment of whether a coating might perform well in a hydrocarbon fire. Another interesting facet of the heat radiator test is that it is possible to look at the texture of the resulting formed char to estimate its mechanical resistance. It is a major advantage since an intumescent coating should also resist external stresses and some prediction of the behavior on the large scale might be done. Another observation we made during the industrial furnace test is that the formulations IF1 and IF2 fell off the steel plate during the test. For this reason it was decided to set up the same test with the coated plate mounted vertically (Figure 9).
Figure 9. Small-scale test with the steel plate put vertically.
The same coating formulations were tested with this experimental setup (Figure 10).
Figure 10. Tests carried out with plates mounted vertically.
As above, the temperature/time curves show that IF1 is the least effective formulation. Nevertheless it is difficult to determine which one of the three other formulations is the best one, as they reach the same temperature after 1200 s. First experimental observations show that there are more heat losses on sides due to convective phenomena than when the plates are mounted horizontally: the heat radiator is still at the same distance from the plate, but the uncoated steel plate reaches
only 380 °C, whereas it reached 450 °C when the plate was mounted horizontally. The second observation is that the coatings swell less than in the first test and do not ignite. Those comments explain then why this particular experimental setup does not permit discrimination of the formulations. It is not our goal here to compare further the two setups. Our initial aim was to test the mechanical resistance of the char on plates and see if formulations IF1 and IF2 fell off from the plate, as it could be observed during furnace testing. However, the temperatures should be too low here: the char, which does not swell a lot, does not fall off the plate. Considering that the curves are not very distinguishable and that the char remains on the steel plate, the horizontal plate system will be considered in the following part. 3.3. Second Series of Tests Carried out on Intumescent Mastics. A second series of tests was carried out on four different intumescent mastics: IF4 (reference formulation), IF5, IF6, and IF7. Curves obtained in the industrial furnace and heat radiator test are presented in Figure 11a,b, respectively. The best formulation in the furnace evaluated up to 30 min is IF5, and the worst formulation is IF7. After 30 min, the time/ temperature curve of the formulation IF5 crosses over the curve of the formulation IF4, and at the end of the test, IF4 is still the most efficient formulation. Considering the results of the small-scale test, the best performing formulation up to 13 min is IF5 and the worst is IF7. Interestingly the same crossover of curves between IF5 and IF4 is subsequently observed: after 13 min the formulation IF4 shows the best performance. This crossover can be the consequence of different phenomena: this can be due to changes in the structure of the char (cracks, change of viscosity, bubbles, and mechanical strength of the char) which modify the heat transfer between the intumesced coating and the plate. Or some components inside the coating, such as fibers, might provide an additional insulating effect after a certain time of heating accumulating at the surface. The ranking of results obtained after 1200 s with heat radiator and at 400 °C in the furnace test are summarized in Table 2. As above, the heat radiator and the industrial furnace tests agree very well each other. It is noteworthy that the temperature/ time curves exhibit a similar behavior including the crossover. According to those results, we may conclude that the heat radiator test can be used to effectively rank the efficiency of intumescent mastics. However, there are not only intumescent epoxy mastics in the fire protection coating market: two main kinds of coatings exist, the thick-film intumescent coatings (mastic) and thin-film intumescents (cellulosic coatings). The next part of the paper will examine whether the heat radiator test can also be used to evaluate cellulosic intumescent coatings. 3.4. Third Series of Tests Carried out on Cellulosic Coatings. Thin intumescent films are used for protection from cellulosic type fires. They are generally available as solventor water-based systems and applied by spray or brush-roller in thin-film coats up to 3 mm thick. They typically use thermoplastic acrylic based resin systems, and they intumesce quickly when exposed to a cellulosic type fire environment. Protection for 1 h can be achieved with 1-3 mm thick coatings. Thin intumescent films are often referred to as “fire retardant paints” rather than “fireproofing” materials due to their lower fire resistance compared to thick intumescent films. Many of them are unsuitable for exterior use unless a topcoat is applied over them, and the test ratings are limited to cellulosic fires only. Advantages of these products include that they are available in
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Figure 11. (a) Industrial furnace and (b) heat radiator tests on four intumescent mastics.
Figure 12. (a) Heat radiator and (b) furnace tests on three intumescent cellulosic coatings. Table 2. Ranking of Results Obtained for Intumescent Mastics (1 w Best Result)
IF4 IF5 IF6 IF7
heat radiator test
industrial furnace test
1 2 3 4
1 2 3 4
wide range of colors and they are inexpensive and relatively easy to apply. They are mostly used inside buildings. Testing of cellulosic coatings is different from testing intumescent mastics: the hydrocarbon fire curve is replaced by the standard cellulosic fire curve (Figure 1). During this test, coatings can be applied on steel plates, or also on columns and beams, which are put inside the furnace, and the standard cellulosic fire is applied to them. As these coating are not designed to be applied on heavily loaded components, 500 °C is taken as the failure temperature, with the corresponding time of failure. Three commercial cellulosic coatings (IFA, IFB, and IFC) are evaluated using the heat radiator and furnace tests. The curves obtained for both tests are presented in Figure 12a,b). This was a “blind test”, as nothing was known about the composition of the coatings tested. The heat radiator test gives three distinct curves: the coating IFA shows much better efficiency than the other coatings. The
Table 3. Ranking of Results Obtained for Intumescent Cellulosic Coatings (1 w Best Result) IFA IFB IFC
heat radiator test
industrial furnace test
1 3 2
1 3 2
difference between IFB and IFC is less clear; at the end of the test IFC reaches a slightly lower temperature than IFB, but at the start of the test (up to 3 min) IFC provides the better performance The furnace test shows distinct curves, the most efficient formulation being IFA and the least efficient being IFB. The rankings of results obtained at 1200 s with the heat radiator and at 500 °C in the furnace tests are presented in Table 3. As above, correlation between the two tests is excellent, with the same ranking of coating performance obtained. 4. Conclusion A reliable high-throughput test has been developed in the laboratory in order to evaluate the heat barrier effect of different intumescent formulations. This is a small-size, rapid, and lowcost test, easy to use, showing great repeatability and allowing rapid screening of thick- and thin-film intumescent formulations.
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It can be used in two experimental configurations (plates mounted vertically or horizontally), leading to complementary results. It is a very interesting preliminary tool for the development of new intumescent coatings as it can avoid the requirement to carry out large furnace tests on many coating formulations. Only the promising formulations will be progressed to full-scale furnace testing, saving both time and money. This test can be used to study coatings on different substrates (steel, aluminum). Literature Cited (1) Kruppa, J.; Twilt, L.; Wesche, J.; Cooke, G. Fire Protection of Structural Steel Work. European Commission, [Report] EUR 17987; Commission of the European Communities: Luxembourg, 1998; pp 1-137. CODEN: CECED9 ISSN: 1018-5593. CAN 129:43718 AN 1998:341614. (2) Seiner, J. A.; Ward, T. A. Polym. Paint Colour J. 1988, 178 (4207), 75. (3) Castle, G. K. Fire Protection of Structural Steel. Loss PreV. 1974, 8, 57. (4) Upadhya, S. C. Asian Paints (I) Ltd. Paintindia 2000, 50, 45. (5) Promat Tunnel Fire Protection, http://www.promat-tunnel.com/ idprt004.htm. (6) Rockwool, Fire Safe Insulation. A Guide to Risk and Changes in the Legislation; http://www.rockwool.com/graphics/RW-GB-implementation/brochures/Fire-Protection.pdf. (7) Jet Fire Resistance Test of Passive Fire Protection Materials; HSE Offshore Technology Report OTI 95634; 1996. (8) Norsok Standard. Piping and Equipment Insulation; 2005. http:// www.standard.no/pronorm-3/data/f/0/10/28/4_10704_0/R-004d1r3.pdf. (9) Ward, T.; Greer, S.; Boberski, W.; Seiner, J. (PPG industries) U.S. Pat. 4 529 467, 1985. (10) Nugent, R.; Ward, T.; Greigger, P.; Seiner, J. (PPG Industries) U.S. Pat. 5 070 119, 1991; 5 108 832. (11) Sinclair, M.; Watts, J. (Chance & Hunt Ltd. and Ferro (GB) Ltd.) PCT Pat. Appl. WO 02 077110, 2002. (12) Hanafin, J.; Bertrand, D. (Textron Systems Corp.) U.S. Pat. 6 096 812, 2000. (13) Phillips, L. N., Ed. Design with AdVanced Composite Materials; Springer-Verlag: London, 1989.
(14) Malmgren, N. Epoxy Plastics’ General Chemical and Physical Properties. NM Epoxy Handbook; http://www.nilsmalmgren.se/en/kemi/ allman.html. (15) Buckland, I. Characterisation of Passive fFire Protection Materials against Jet Fire Impingement, DIN TD5/005, 2003. http://www.hse.gov.uk/ foi/internalops/hid/din/505.pdf. (16) Camino, G.; Costa, L.; Martinasso, G. Intumescent Fire Retardant Systems. Polym. Degrad. Stab. 1989, 23, 359. (17) Delobel, R.; Le Bras, M.; Ouassou, N.; Alistiqsa, F. Thermal Behaviour of Ammonium Polyphosphate-Pentaerythritol and Ammonium pyrophosphate-pentaerythritol intumescent additives in polypropylene formulations. J. Fire Sci. 1990, 8 (2), 85. (18) Camino, G.; Martinasso, G.; Costa, L. Thermal degradation of Pentaerythritol diphosphate, Model Compound for Fire Retardant Intumescent Systems. Part I. Overall Thermal Degradation. Polym. Degrad. Stab. 1990, 27 (3), 285. (19) Bourbigot, S.; Le Bras, M.; Duquesne, S.; Rochery, M. Recent Advances for Intumescent Polymers. Macromol. Mater. Eng. 2004, 289 (6), 499. (20) UL. Rapid Rise Fire Tests of Protection Materials for Structural Steel, UL 1709; Underwriter Laboratories: Northbrook, IL, 1994. (21) Gilman, J. W.; Bourbigot, S.; Shields, J. R.; Nyden, M.; Kashiwagi, T.; Davis, R. D.; Vanderhart, D. L.; Demory, W.; Wilkie, C. A.; Morgan, A. B.; Harris, J.; Lyon, R. E. High Throughput Methods for Polymer Nanocomposites Research: Extrusion, NMR Characterization and Flammability Property Screening. J. Mater. Sci. 2003, 38, 4451. (22) Babrauskas, V. DeVelopment of the Cone Calorimeter: A BenchScale Heat Release Rate Apparatus Based on Oxygen Consumption; National Bureau of Standards, [Technical Report] NBSIR 82-2611; U.S. National Bureau of Standards: Washington, DC, 1982. (23) Standard Method of Test for Flammability of Plastics Using the Oxygen Index Method (ASTM D 2863); American Society for Testing and Materials: Philadelphia, PA. (24) Tests for Flammability of Plastic Materials for Parts in DeVices and Appliances (UL 94); Underwriters Laboratories: Northbrook, IL.
ReceiVed for reView June 30, 2006 ReVised manuscript receiVed August 21, 2006 Accepted August 23, 2006 IE0608410
