J. Coat. Technol. Res. DOI 10.1007/s11998-016-9815-3

REVIEW PAPER

Intumescent coatings: A review on recent progress Ravindra G. Puri, A. S. Khanna

 American Coatings Association 2016 Abstract Fire is a serious threat to people and the structures they build. There is a continuous development of newer methods and materials to prevent its effects on them. Nowadays, a lot of attention is being paid in the design of public and commercial buildings by incorporating fire safety. Passive fireproofing of highrise structures has become very important due to the use of steel in load bearing mode and has attracted increased attention after the collapse of the WTC towers. Conventional passive fireproofing materials include concrete covering, gypsum board and cementitious coatings which have a poor aesthetic. Intumescent fire-resistive coatings are a newer type of passive fireproofing coatings usually applied as thin film and they swell many times their original thickness forming an insulating char which acts as a barrier between the fire and the structural steel. It prevents the temperatures of the steel members from reaching a critical value and helps in maintaining the integrity of the structure in fire event. They are the preferred choice for passive fire protection of load bearing steel frame structures of architects and designers as they offer aesthetic appearance, flexibility, speed of application, and ease of inspection and maintenance. The present review covers recent developments in the field of intumescent coatings with a major emphasis on organic intumescent coatings. The role of various ingredients, their interactions in intumescent coatings, effects of various pigments, binders and additives are discussed briefly. Keywords Intumescent coatings, Passive fireproofing, Fire safety

R. G. Puri (&), A. S. Khanna Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India e-mail: [email protected]

Abbreviations IFR Intumescent fire retardant APP Ammonium polyphosphate PER Pentaerythritol MEL Melamine AFP Active fire protection PFP Passive fire protection GRP Glass fiber-reinforced polyester MF Melamine-formaldehyde PVA Poly vinyl acetate EVA Ethylene vinyl acetate DP Degree of polymerization DAHP Di-ammonium hydrogen phosphate MP Melamine phosphate EDAP Ethylene diamine phosphate DPER Dipentaerythritol TPER Tri pentaerythritol PEPA Pentaerythritol phosphate PEPAMP Pentaerythritol phosphate blend with melamine phosphate THEIC Tris (hydroxyethyl) isocyanurate PETA Poly-ethylene terephthalamide MC Maleated cyclodexdrin ZHS Zinc hydroxystannate CES Chicken eggshell ATH Aluminum tri hydrate PP Polypropylene RPU Rigid polyurethane CNT Carbon nanotubes OMMT Organically modified montmorillonite LDH Layered double hydroxides EG Expandable graphite IPTS 3-Isocyanatopropyltriethoxysilane

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Introduction Fire is destructive in nature and there is always a great risk associated with it. Catastrophic fire can result in the loss of wealth, possessions and even life. In recent years, increasing attention is paid to fire safety regulations and there is high demand to reduce the fire hazard associated with combustibles such as wood, plastics, textiles, etc.1 Newer methods and materials are in continuous development for preventing and reducing the effects of fire. Inherently flame-retardant materials and flameretardant-incorporated materials such as coatings, composites, firestops and intumescent coatings are currently used to provide protection against fire.2 Intumescent coatings are the recent developments in the passive fireproofing of load bearing structures. In the past, Vandersall and Hao et al. reviewed intumescent coatings.3,4 Weil reviewed fire-protective and fireretardant coatings with major emphasis on intumescent coatings.5 However in recent years, the scientific community has focused extensively on research of intumescent coatings, and due to the large number of publications in recent years, there is a need to review the current progress in the field. The present review focuses on the recent developments mainly in the field of intumescent coatings. Some attention is also given to polymers and composite systems which may be useful in developing coating formulations. Fire protection systems Although the regulations and standard operating procedures in the event of a fire are laid down by many government and private agencies, there still is a need to educate society about prevention and rapid response in the event of a fire. The following few methods are usually applied for fire prevention: • •

•

Fire prevention by awareness It includes fire safety education, fire drills, fire service response and emergency evacuation etc. Active fire protection (AFP) It is the action to control and extinguish the fire by fire fighting systems, extinguishers, sprinklers, etc. They can be further modified by installing detection systems and acoustic alarms. Passive fire protection (PFP) This limits and controls the fire once it has occurred, for example, using (i) Fire retarding chemicals (ii) Modified fire rated fabricated structure (iii) Intumescent coatings.

Why structural steel needs fire protection? Currently, structural steel is used extensively in the construction of high-rise commercial and industrial

buildings. There is wider acceptance of the use of steel framed structures in the design of new multistoried buildings by architects because of rapid design, quick fabrication, and erection which speeds up construction. Although the steel is not susceptible to fire, it has a serious limitation and loses approximately 50% of its load bearing capability at around 500C. In an extreme fire event, an unprotected steel structure may collapse due to a loss in mechanical properties at extremely high temperature during the fire. Therefore, a major design consideration in high-rise buildings during a fire event is to prevent fire-induced structural collapse by providing a suitable passive fireproofing system which provides sufficient time to evacuate humans and valuables.6

Traditional passive fireproofing coatings Traditionally, passive fireproofing coatings are mainly of a cementitious type. They are usually categorized as dense concrete and lightweight cementitious coatings. They are based on portland cement, vermiculite, gypsum and other materials. On construction sites during application, they are mixed in water with fillers, binders and are applied by spraying at a thickness of few inches. They provide fire protection by thermal insulation effects and water release from a few minutes to several hours. They have low cost and are easier in application but have some serious limitations in terms of their weight, thickness and poor aesthetics. Since they have poor aesthetics, building architects are reluctant to use them on visually exposed steel sections.7 They have a tendency to crack and dislodge in a fierce fire. Their poor surface finish allows moisture to penetrate and accumulate, promoting corrosion which limits their performance.8

Intumescent coatings Intumescent coatings have been used as passive fire protection for over 20 years to protect structural steel in high-rise buildings to avoid fire-induced structural collapse. An intumescent coating swells on exposure to fire in a controlled manner several times their original thickness, producing a carbonaceous protective char. The char acts as heat transfer barrier to physically and thermally protect the coated steel structure (Fig. 1). Intumescent coatings can swell upto 100 times on heating (from 1-mm to 10-cm-thick foam). In the event of fire, they can act as passive fire protection for steel which loses half of its load bearing capacity above 500C limiting structural damage and prolonging evacuation time during fire breakout, preventing the loss of lives and property.9 Intumescent coatings are increasingly used because they provide a high-quality finish and also have good

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Fire

Char

dioxide and water is a major limitation. On environmental exposure, they gradually lose their intumescence, adhesion and become brittle. K2 SiO3  nH2 O ! K2 SiO3 þ nH2 O Na2 SiO3  nH2 O ! Na2 SiO3 þ nH2 O

Intumescent coating

Fig. 1: Structural protection using intumescent coatings

fire ratings on buildings requiring high fire protection, like stadiums, skyscrapers, multistoried buildings. Most high-rise and steel-framed constructions use intumescent coatings to protect them from the fire. A char with enhanced strength can help to protect the steel structure exposed to heat or fire and remain stable in high wind velocities. Inorganic intumescent coatings based on soluble alkali silicates were widely used in earlier days because of their low cost but they have serious drawbacks in durability and water resistance. Due to their aesthetics and performance, nowadays organic thin film intumescent coatings are becoming more popular. An organic intumescent coating consists of the following ingredients: • • • •

Carbon donor or char former, e.g., Pentaerythritol; Acid donor or catalyst, e.g., Ammonium polyphosphate (APP); Blowing agent, e.g., Melamine; Binder, e.g., Polyvinyl acetate, Epoxy.

Organic intumescent coatings are thin film intumescent coatings that swell considerably when exposed to heat. They have a good quality finish and can also be topcoated if required for outdoor exposure. They have many advantages like ease of application, aesthetic, flexibility but have a limitation of sometimes forming a fluffy char which may dislodge at high wind velocities.10 Intumescent silicate coatings Inorganic intumescents are based on alkali silicates. They find their main application in fire protection of wood. They have limited application as coatings and are mainly used as firestops. They swell on exposure to fire mainly due to the endothermic loss of water of hydration and their ability to melt forming solid rigid foam mainly consisting of hydrated silica.11,12 Environmental and health concerns with their use are decreased as only water vapor is released during intumescence, but their sensitivity to both carbon

An alkali silicate fire-protective coating with an acidic curing agent was investigated for the effect of relative humidity on intumescence, water sensitivity and water absorption. Atmospheric CO2 reacts with the coating resulting in cracking and aging. It is suggested that over-coating prolongs the aging.13 An inorganic coating composition has been reported containing sodium silicate, potassium silicate and silicon carbide powder applied to the substrate, typically aluminum intumesces at high temperatures and forms a heat insulating char and retains this structure for prolonged periods at temperatures up to 1000C.14 Mostly silicates are preferred in interior applications and in fire stops because of their poor water resistance. Making an intumescent powder and mixing it with a binder is also one method of using them. A product using lithium– sodium-potassium silicate composition having an intumescent range above 195C, suitable as a coating or firestop has already been reported.15 Phosphosilicate coatings formulated by incorporation of up to 5% sodium phosphate in the alkali silicate coating shows high temperature resistance and is useful in fuel fires. Exposure of these coatings to high temperatures results in polymerization of silicate to crystalline silica.16 Fire and heat protection of glass fiber-reinforced polyester (GRP) composites using intumescent silicate coating is also reported. The inclusion of intumescent surface barriers results in less temperature rise within the core GRP composites.17 An intumescent inorganic silicate fibrous material containing CaO, MgO, Al2O3, and calcium magnesium silicate along with an acrylic or polyvinyl acetate is used in a coating or mastic.18 An aqueous fire barrier composition can be prepared by a polymer latex and an intumescent agent that includes granular alkali metal silicate.19 A fire-retardant, intumescent, ceramic coating for structural steel and wood is prepared by mixing a finely ground borate compound to sodium silicate solution then drying, pulverizing and adding it to a second sodium silicate solution.20 A lowcost intumescent composition using alkali silicate, microspheres and frit materials is said to have been stable for several years.21 A self-curing alkali metalbased intumescent composition having alkali metal silicate has been reported.22 An intumescent aqueous composition of alkali silicate, clay, and inorganic materials like hydrated aluminum oxide, hydrated borate, carbonates, bicarbonates can be applied to metal, wooden and polymeric materials and shown to provide a thermal barrier when exposed to high temperatures.23 The intumescent composition containing sodium silicate, cenospheres, water, and sodium

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tetradecyl sulfate is also reported for wood structural panels for protection against high temperatures and heat exposure for longer duration.24 Organic intumescent coatings Organic intumescent coatings are thin film intumescent coatings and are based on an acid catalyst, e.g., ammonium polyphosphate (APP), a char former e.g., pentaerythritol (PER) and a blowing agent, e.g., melamine (MEL) in solventborne or waterborne binders. These kinds of coatings are more weather stable and water resistant than alkali silicate-based coatings. They are favored by architects and designers for passive fire protection of steel-framed structures as they provide a finish that does not detract from the appearance of the exposed steelwork as in the case of cementitious coatings. They are increasingly used today in modern day airports, skyscrapers, sports complexes, shopping malls, hotels and other places, and enabled the architect to create and design elements using the steel itself.

strained in the melted matrix making it swell and forming an insulating multi-cellular protective char layer. This carbonaceous char shield restricts the transfer of heat from the source to the substrate and prevents further degradation of the underlying material (Fig. 2).4,25 The sequence of the intumescence process and reactions are given as follows: 1. 2.

Softening and melting of the polymeric binder. Releasing of inorganic acids. [250 C

ðNH4 PO3 Þn ƒƒƒƒƒ!ðHPO3 Þn nNH3

3.

Carbonization of char formers (e.g., of polyalcohols).

ðHPO3 Þn þCx ðH2 OÞm ! ½Cx þðHPO3 Þn mH2 O 4.

Release of gaseous products by blowing agents (e.g., melamine). N

NH2

NH2

Intumescence mechanism The intumescence process usually takes place in following steps: a mineral acid is released by breaking the acid source which results in dehydration and carbonization of char formers. Further, the blowing agent decomposes to release gases which get con-

Post heating

During heating Fully expanded char Blowing Pre heating

Melting Virgin zone Protected substrate

Fig. 2: Schematic representation of intumescence process

Fig. 3: Intumescent coatings char

N

N

NH3

O2

N2 +

H2O

NH2

5. 6.

Foaming and expansion of the mixture. Crosslinking and solidification of char.

There are three intumescent ingredients: an acid source, a carbon source and a blowing agent. The intumescent coating has to be formulated and optimized so as to form an efficient protective char. The protective and heat barrier properties of the char are largely influenced by its structure. A coating should form a large char volume, thick and continuous inner char structure. The char should be compact, and should have smaller cell size and narrower cell size distribution. The cells should be closed like solid foam. The coating should have other performance properties to withstand the service environment in the long term26 (Figs. 3, 4).

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(NH4PO3)n

>250°C

(HPO3)n

+n NH3

Polyphosporic acid + Binder (HPO3)n

Dehydration

+

+ Char former

H3PO4

x Char

Fig. 5: APP response to fire and its interactions with other ingredients Fig. 4: SEM image of (a) char surface (b) internal of char showing cell size distribution

As discussed earlier, the intumescent coating consists of three major ingredients, i.e., acid source, char former and blowing agent. The examples of various intumescent ingredients are given in Table 1.2,27

Interaction of acid source-char former-blowing agent The intumescent process is triggered by an acidliberating catalyst, e.g., ammonium polyphosphate (APP) releasing acid above 250C in the event of a fire. The acid reacts with the char former, e.g., polyol resulting in its dehydration and forming carbonaceous char (Fig. 5). The blowing agent, e.g., melamine, decomposes and releases gases in the molten mass resulting in foaming which then solidifies making a heat transfer barrier. Camino et al. studied in detail the degradation of APP. APP loses ammonia and water when heated. However at higher temperatures, crosslinking occurs through P–O–P and P–N–P structures.28 The reaction takes place with the disruption of

chain structure of polyphosphate with the elimination of ammonia, water, and releasing polyphosphoric acid. Polyphosphoric acid then reacts with pentaerythritol forming cyclic phosphoric acid esters which also act as blowing agents.25,29 During this process, melamine and polyphosphoric acid also combine to form melamine polyphosphate and dipolyphosphate and further, polyphosphoric acid dehydrates to phosphorus pentoxide. Cyclic phosphoric acid esters of char formers also act as the blowing agents. Glycerol as a char former results in an ester with a low temperature of decomposition in the coating and shows the best performance in the fire test.30 Ammonium polyphosphate (APP) and its modifications APP is most widely used acid source in intumescent formulations. However, water sensitivity and poor compatibility with the binder are some of its limitations. The properties of ammonium polyphosphate depend on the number of monomer units and degree of branching. APP with shorter chains (n < 100) has more water sensitivity and less thermal stability than with

Table 1: Examples of various components of intumescent systems Acid source Inorganic acids Phosphoric Sulfuric Boric Ammonium salts Phosphates, polyphosphates Borates, polyborates Sulfates Halides Phosphates of amine Melamine phosphate Melamine pyrophosphate Urea with phosphoric acids Other phosphates Tricresyl phosphate Alkyl phosphates Haloalkyl phosphates

Char formers

Blowing agent

Pentaerythritol, Dipentaerythritol, Tripentaerythritol, Starch Dextrins Sorbitol Mannitol Phenol–formaldehyde resins Methylol melamine Char-forming polymers

Melamine Urea Urea–formaldehyde Melamine formaldehyde Dicyandiamide

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longer chains (n > 1000). There are two main classes of APP: Crystal phase I (APP I) and Crystal phase II (APP II). APP I having phosphate units generally lower than 100 is a linear chain with the lower decomposition temperature of approximately 150C and has higher water solubility. APP II with number of phosphate units higher than 1000 has a branched structure and much higher molecular weight than APP I. The thermal stability of APP II is higher with decomposition temperature of approximately 300C and has less water solubility than APP I. Surface modification of APP with melamine can be helpful in overcoming the poor water resistance and low compatibility with polymeric binders. Some of the ammonium cations in APP get replaced by melamine increasing its water resistance.31 Microencapsulated ammonium polyphosphate (MCAPP) with a melamine–formaldehyde (MF) resin coating layer leads to reduction in water absorption and gives a still more water-resistant APP. It also increases its compatibility in the matrix and gives higher thermal stability.32,33 Most of the reported studies on the surface modification of APP are in the composite system but can also be useful in formulating coating. In microencapsulated APP, the shell can have additional effect as a char former or blowing agent and it may also prevent water from coming in contact with the acid source. Tang et al. synthesized glycidyl methacrylate microencapsulated ammonium polyphosphate which shows better flame retardancy, thermal stability, and better dispersion in the epoxy matrix.34 Microencapsulated ammonium polyphosphate with a urea-melamine–formaldehyde resin prepared by in situ polymerization shown to increase water resistance of composites.35 Many other materials like PVA-melamine–formaldehyde resin, hydroxyl silicone oil, polyurethane, and epoxy can be used for the microencapsulation of APP.36–38 Composites containing them are reported to have better thermal stability and water resistance.39–41 Microencapsulation with cellulose acetate butyrate enhances the water resistance of APP and its compatibility with ethylene vinyl acetate. The results also conclude that it increases interfacial adhesion, mechanical, electrical and thermal stability of the system.42 Sol–gel encapsulated ammonium polyphosphate (APP) has better water resistance and also its epoxy resin composites have better flame retardancy because of a synergistic effect between polysiloxane and APP. The results demonstrate that the silanes hydrolyze and condense forming a dense layer on the surface of ammonium polyphosphate. They react with the hydroxyl groups on the surface forming a new Si–O–P bond.43,44 APP modification with ethanolamine (ETA) improves its flame-retardancy via ion exchange reaction. Ethanolamine-modified ammonium polyphosphate (ETA-APP) thus formed acts as the acid source, blowing agent and also has excellent charring property.45 Ethylene diamine-modified ammonium polyphosphate forms some stable structures such as P–N–C and C–N which dramatically increases its charring ability resulting in

much better flame retardancy.46 Zheng et al. synthesized microencapsulated ammonium polyphosphate via a two-step surface polymerization process using 4, 4¢diphenylmethane diisocyanate (MDI), melamine (MEL) and pentaerythritol (PER), which introduces the carbon source and blowing agent into the microcapsules simultaneously. The microencapsulation combines the flame-retardant system (APP/PER/MEL) in one compound and also improves its compatibility, water resistance and flame retardancy.47 The degree of polymerization (DP) of APP also influences fire protection of intumescent coating. The fire protection of the coating significantly improved with the increase of DP. The improvement was significant when DP was less than 100 but slows when DP was more than 100.48 Other phosphates and phosphorus compounds Many other phosphorus compounds are also useful in intumescent coatings and polymeric systems. Other phosphates such as mono-, di-ammonium phosphates may have higher water solubility and are not generally preferred in coatings but may found some use in other polymeric systems. However, ammonium dihydrogen phosphate and partially substituted ammonium polyphosphates in fireproofing formulations are shown to enhance adhesion of foamed coke to the metal surface.49 Synthesis of hybrid salt composition with substantially reduced solubility and low thermal activation temperature is reported by the interaction of mono ammonium phosphate, phosphoric acid and melamine.50 Commercial polyurea coating containing microcapsules of di-ammonium hydrogen phosphate (DAHP) with polyester-polyurethane shells has been shown to involve the smallest quantity of smoke and carbon monoxide.51,52 Ammonium polyphosphate is the most widely used acid source in intumescent formulation but other salts, e.g., melamine phosphate (MP), melamine pyrophosphate also find use. MP has slightly less solubility in water and is a suitable choice in combination with APP. Various melamine phosphates differ in their stabilities and water solubility. Melamine phosphate and pyrophosphates have higher residues than APP at higher temperatures. Melamine pyrophosphate is thermally more stable and also has less water solubility. Melamine pyrophosphate has better water resistance and gives char with better thermal barrier properties than melamine phosphate. They form an infusible residue of phosphorus oxynitride having extremely high thermal stability when heated.53,54 Different crystal types of melamine orthophosphates have different behaviors when incorporated in epoxy intumescent coatings.55 Melamine phosphate starts to decompose at about 250C and converts to melamine pyrophosphate, releasing small amount of melamine.56 MP contains both acid source and blowing agent and is useful in combination with char-forming polymers. Fireproof coating composing of melamine salts or

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guanidine salt and microencapsulated melamine with stability in a tropical environment is reported.57 A fireretardant additive containing a mixture of melamine polyphosphate (MP)/dipentaerythritol (DPER) has been used for preparing waterborne intumescent fireresistive coating. The results confirmed that the MP/ DPER mixture and the coating have similar thermal degradation behavior. They degrade, releasing nonflammable gases and form phosphorus oxide containing intumescent char layer at high temperature.58 An intumescent coating is also formulated using dicyandiamide phosphate along with pentaerythritol, especially as a clearcoat for wood and wood-related surfaces.59 An intumescent polymer composition with pentaerythritol phosphate is reported where pentaerythritol phosphate (PEPA) functions as both an acid source and a carbonific.60 Pentaerythritol phosphate (PETA) and its blend with melamine phosphate (PEPAMP) act as an intumescent additive and are shown to produce equal amount of char; however, PEPAMP produces less stable char.61,62 Ethyl diamine phosphate has an advantage of its relatively high phosphorus content. It is also inexpensive and does not require the use of synergists in order to provide sufficient effectiveness.63,64 Chloroalkyl phosphates and phosphonates find major use as flame retardants.65,66 It is obvious that both chlorine and phosphorus contribute to the flame retardancy. They may also be useful in intumescent coating formulations. The organic phosphate or phosphonate acts as blowing agent and charring catalyst by releasing phosphorus acids and can also be useful as plasticizer improving coating properties.67 Intumescent formulations using monophosphates such as tris (2-chloroethyl) phosphate are shown to have better flexibility due to its plasticizing effect.68 Clear intumescent lacquers for metal, wood and textiles can also be synthesized by reaction of the polyphosphoric acid with polyols forming partial esters and further curing with epoxy and amino resins.69 Pentaerythritol phosphate alcohol aryl phosphonate and phosphate compounds contain both char former and acid source. They may contain from one to three pentaerythritol phosphate and are useful as flame retardants.70 Intumescent coatings for fiberglass mats can be prepared by mixed glycerol/pentaerythritol acid phosphate as a base, cured with amino resin.71 Ma et al. synthesized poly (diamino diphenyl methane spiro cyclic pentaerythritol bisphosphonate) (PDSPB), having a high thermal stability. The phosphate ester releases phosphoric acid and forms a complex P–O–Ph and aromatic/graphitic structure having improved fire performance.72

Various char-forming ingredients The major role of char former is to form a carbonaceous char by dehydration in the presence of acid source. In literature, pentaerythritol has been the

mostly reported charring agent in combination with APP as an acid source in intumescent formulations. Its lower melting point is advantageous in early intumescence; however, its water solubility restricts its use sometimes for weather stable applications. Dipentaerythritol and tripentaerythritol although more expensive are somewhat less soluble. The comparison of pentaerythritol with its dimer and trimer shows that pentaerythritol intumesces earlier. The embrittlement temperature is, however, higher for di- and tripentaerythritol and has resistance to the wind and cracking of char at high temperature, especially on curved surfaces.73 Other char former as replacements for the pentaerythritol such as tris (hydroxyethyl) isocyanurate (THEIC) is also reported. THEIC, a derivative of triazine, and its derivatives serve as the charring agent. There is a synergistic interaction between APP and THEIC but water solubility limits its use.74 THEIC, when used along with long chain amine in intumescent flame-retardant compositions, reduces its blooming tendency.75 Formals of pentaerythritol and dipentaerythritol are the by-products in their frequently used production processes and cheap carbon sources as a partial replacement.76 Char-forming ingredients such as starches, sugars and cellulose having pendant hydroxyl groups are cheaper char formers and find use in intumescent formulations.77,78 Alongi et al. prepared a cyclodextrin nanosponge compound which acts as a carbon source and also helps in foam formation.79 Poly-hexa methylene terephthalamide (PA6T) is a novel charring agent in combination with ammonium polyphosphate (APP). A synergistic interaction takes place between PA6T and APP, which aids in char formation. The results revealed the formation of uniform and compact intumescent char after combustion.80 Maleated cyclodexdrin (MC) and its metal salts (Metal MC) in poly (vinyl alcohol) show improved thermal stability of the composites. It is concluded that during degradation the metal ions enhances formation of organized and compact char.81 Bi(4-methoxy-1-phospha-2, 6, 7-trioxabicyclo [2.2.2]octane-1-sulfide) phenylphosphate (BSPPO) is reported as a novel halogen-free charring agent having excellent charring ability.82 Alkali lignin and ureamodified lignin as a carbonization agent in intumescent formulations are also reported. The results showed that the urea-modified lignin with APP has better flame retardancy and thermal stability as compared with virgin lignin.83

Melamine and other blowing agents Melamine is the most widely used blowing agent in intumescent formulations. Its action mainly takes place by its degradation at high temperature, releasing gases and volatile products. Various melamine compounds like melamine phosphate and polyphosphate also find use as blowing agents. The TGA data of melamine

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suggest that it sublimes completely at about 250– 350C. Melamine, apart from its role as a blowing agent, also interacts with ammonium polyphosphate.84 Costa et al. demonstrated that on heating, melamine undergoes condensation with liberation of ammonia and forms insoluble products. Upon heating in a close system ammonia is eliminated and fused ring structures, e.g., melam, melem and melon are formed.85,86 The sublimed melamine and ammonia are poor fuels and flame inhibitors and act as flame retardants in the gaseous phase. Melamine phosphate formed by the interaction of melamine with ammonium polyphosphate also acts as an acid source. Various phosphorus compounds of melamine such as phosphate, pyrophosphate and polyphosphate are useful flame retardants. In addition to an acid source, they are also useful as blowing agents in intumescent formulations.87 Urea can also act as a blowing agent but is shown to reduce the intumescence in APP-PER mixture because of less amount of gases evolved during decomposition.88 Dicyandiamide and guanidine can also be a useful blowing agent along with melamine in intumescent formulations.

Catalysts and synergists in intumescent formulations In addition to the acid catalyst used in typical intumescent formulations, many other compounds like amines and metal oxides show catalytic and synergistic effects. Most of the reported catalysts are in noncoating systems but are useful in developing coatings. Epoxy resin showed improved flame retardancy through the addition of APP with metal compounds. It is reported that the flame retardancy is affected by the composition of metal compounds (metal ions and ligands/anions) and the mass ratios of APP to metal compounds.89 Metals like iron, magnesium, aluminum, and zinc also influence the thermal degradation of paraffin/intumescent flame retardants (IFR) system and could also increase the char yield.90 Calcium gluconate monohydrate is a new base-catalyzed intumescent compound. It forms low-density closed-cell carbonaceous foam when exposed to heat. The volume expansion can be as high as two hundred times the original volume.91 Cerium (IV) phosphate with intumescent flame retardant in styrene butadiene rubber acts synergistically and results in the formation of compact charring layers.92 La2O3 promotes formation of a homogenous and compact intumescent char layer and shows a synergistic effect in the flame retardancy and smoke suppression of IFR system.93 Sheng et al. reported excellent improvement in flammability due to the catalytic carbonization effect of LaMnO3 in the intumescent flame-retardant system. The incorporation of LaMnO3 improves the thermal stability and it also acts as the nuclei to induce the formation of the continuous, compact, and smooth condensed phase

intumescent charred layer.94 N-alkoxy-hindered amine-containing silane (Si-NORs), prepared by sol gel reaction combining N-alkoxy-hindered amine and silane coupling, are shown to improve significantly the thermal stability, UV stability, mechanical properties, compatibility and residual char structure.95 Zinc hydroxyl stannate (ZHS) and Ferrous disulfide (FeS2) can also improve the char yields of intumescent systems. They could promote formation of stable, homogeneous, and compact intumescent char and thus protect the underlying polymer from degradation.96,97 Zinc and nickel salts and oxides can enhance the performance of the intumescent system. ZnSO4Æ7H2O can optimize the intumescent process and strengthens the char layer under combustion conditions.98 Supported nickel catalyst (Ni-Cat) improves the flame retardancy of IFR system based on APP and PER in polypropylene (PP) matrix. The catalyst was shown to act synergistically and changes the char microstructure and improves the thermal stability.99 Zeolite in intumescent formulations results in the formation of stable structures with the polymer and helps in stabilizing intumescent protective shield. It enhances the stability of the phosphocarbonaceous species and hinders the formation of small molecules in the polyaromatic network.100,101 Spent refinery catalyst is also reported as synergistic agent in intumescent formulations. The use of finest catalyst exhibits a higher amount of residue at high temperature and low heat release rates. Silica–aluminates are found to be more efficient than silica or alumina alone.102

Influence of binder in intumescent coatings Various studies have reported on the role of the binder in intumescent formulations. The binders, besides providing typical protective performance, also have to degrade over the same temperature range as other intumescent fire retardant additives in the formulation so as to produce intumescence, leading to a carbonaceous char. Additionally, it must have a melt viscosity which would not be too low to cause the molten coating to slump leading to dislodging of char during intumescence, nor too high to prevent the expansion of the foam. The polymer melt should be so viscoelastic that the gases released during the intumescence process should remain contained within it, forming a foamed char. Some binder also aids in char formation process. They have synergistic interaction with ammonium polyphosphate and produce thermally more stable chars. A range of polymeric resins can be used as a binder in intumescent coatings. Selection of binder also depends upon the service environment to which the coating is exposed and its expected durability. Waterborne acrylics are used mostly in dry, internal locations, solventborne acrylics in internal or sheltered external locations while solvent-less or solventborne

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epoxy coatings find use in any location where high weatherability and protective performance are anticipated. Thermoplastic binders have good char-forming behavior compared to thermosets. Polyvinyl acetate emulsion is mostly preferred in waterborne while epoxies are in solventborne intumescent formulations. The combination of binders also gives good performance. Chlorinated rubber with chlorinated paraffin gives good performance and withstands aging in a conventional APP-PER-MEL formulation. In addition to their fire retardancy, the halogen-containing materials also act as a plasticizer and flexibilizer for the dried coating.103 Phosphorus-containing styrene-acrylic copolymers are shown to form a dense swollen char. The results demonstrated that along with thermal stability, the crosslinking degree of the binder has an important role in the performance of intumescent coatings.104 A copolymer with substituted styrene has good reactivity with APP and shows enhanced thermal stability. Copolymers of 2-ethylhexyl acrylate p-methyl styrene are relatively efficient in thin film intumescent coating. Linear and crosslinked copolymers, more particularly those synthesized from monomers having good reactivity with ammonium polyphosphate, greatly improve the thermal insulation of char.105–109 The addition of a chloroparaffin in intumescent paints improves flame-retardant properties of polyvinyl acetate. A condensed heterocyclic system is formed by C=C bonds. When combining it with melamine, it reacts with the polyene on a large temperature range, forming a condensed heteroaromatic structure having high thermal stability.110 Polymers like polyamide-6 act as a carbonization agent in an intumescent formulation. Insertion of this additive system in EVA leads to a significant improvement of the fire performance of the material.111 Molecular weight of binder can also affect fire protection of the intumescent coating. High molecular weight can improve the thermal stability of the coating and improve the antioxidation property of the char at high temperature. It is observed that the higher melt viscosity with increasing molecular weight may reduce the expansion speed of the coating leading to an increase in the cell size of the char layer. However, loose and uneven foam structure may be formed with excessive high molecular weight significantly reducing the fire performance.112 A combination of thermosetting and thermoplastic binders can be a suitable choice. Mostly, thermosetting binder is an epoxy resin. Various thermoplastic binders i.e., ketonic resin, hydrogenated castor oil are used to modify melt viscosity.113,114 A cost-effective and environmentally friendly polymer emulsion using phosphonate or phosphate monomer along with acrylic and methacrylic monomers is also reported to be useful in intumescent paints.115 Epoxidized polysulphide-based intumescent composition forms a substantially stable and protective carbonaceous char.116 Due to high thickness, slow drying remains a major problem with intumescent coatings particularly in water-based formulations when the relative humidity is high. An

intumescent composition curable by free radical polymerization is reported to be useful in these conditions.117–119 Waterborne intumescent fire-retardant varnish prepared from phosphate resin acid (PRA) and cold cured amino resin is reported. The fire retardancy test demonstrated that a high phosphorus content is useful to fire retardancy of coatings, but the quality of the char formed has a major role.120 Intumescent coatings to the metal of acrylic binder with copolymerizable acid monomers such as a methacrylic, maleic, fumaric or itaconic acid is claimed to have improved metal adhesion.121 The combination of self-crosslinking polyacrylate emulsion and silicone emulsion improves the fire protection of the coating significantly. The silicone emulsion improves the thermal stability of the coating and increases the char residue at high temperature.122 A self-crosslinking silicone acrylate when added to epoxy emulsion leads to increase in the fire protection. It increases the crosslinking degree of the binder and reduces permeation of water and migration of intumescent ingredients increasing its corrosion resistance.123 Fireresistance behavior of the intumescent coating based on silicone-epoxy resin containing intumescent additives is also evaluated. Intumescent coatings based on this resin forms a thermally stable thin ceramic-like layer, which improves the thermal insulation properties of the char.124 Gardelle et al. evaluated silicone-based binder in the intumescent coating. It is shown that the incorporation of a mixture of poly dimethyl siloxane and silica-coated by silane as a modifier in the silicone results in the formation of char having low thermal conductivity after swelling. Incorporation of expandable graphite, organoclay, and calcium carbonate in silicone-based coatings results in improved performance. This is due to interactions of calcium carbonate and silicone matrix which increases the thermal stability of the resin and forms a protective ceramic layer of calcium silicate.125,126 The concentration of binder used in the formulation also influences char expansion and its antioxidation behavior.127

Various pigments in intumescent formulations A variety of pigments are used in the intumescent formulation. Apart from their role as a typical coating ingredient, they are also useful in improving the fire performance of coatings. Use of titanium dioxide is reported in many intumescent formulations. Its use in the intumescent formulation is not only as a white pigment but is shown to interact with intumescent additives. The addition of pigments in intumescent formulation results in the formation of close cells, narrower cell size distribution and compact char structure which improves fire protection. Titanium dioxide-containing intumescent coatings often show the formation of white expanded foam on the exterior surface, which is mainly titanium pyrophosphate and is the result of the interaction of APP with TiO2.

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Horacek et al. proposed that the formation of titanium pyrophosphate is due to the reaction of P2O5 with TiO2 in the intumescent coating.30 Li et al. reported that fire resistance of coating containing rutile-TiO2 is much longer than that of the anatase-TiO2. It is suggested that the difference may be due to size distribution and crystalline forms of TiO2. Anatase-TiO2 is also converted to rutile-TiO2 at high temperature (above 900C).128 2TiO2 þ ðNH4 Þ4 P4 O12 ! 2TiP2 O7 þ 4NH3 þ 2H2 O Inorganic oxides form a highly reflective glassy layer on the surface of the char by interacting with phosphates. These compositions are effective at continuously maintained high temperatures for a long duration under severe environmental conditions, with reduced degradation of the mechanical properties.129 It is claimed that Molybdenum trioxide (MoO3) and ferric oxide (Fe2O3) improve the outer and inner surface structure of the residual char and enhance the thermal stability of the intumescent APP–PER–MEL coatings.130 Fumed silica (SF) as a binder and chicken eggshell (CES) as flame-retardant filler have been incorporated to synthesize water-based intumescent coatings. The coating showed improved adhesion to the steel substrate by adding fumed silica and provided longer-lasting protection.131 Liu et al. reported the use of sepiolite as a synergistic agent in the intumescent flame-retardant system. The addition of sepiolite in polypropylene results in low heat release rate, total smoke, and CO2 production.132 Intumescent flameretardant ethylene–vinyl acetate (EVA/IFR) composites containing lanthanide oxide, iron oxide, their mixture, and nanocrystalline lanthanum ferrite (LaFeO3) are also reported. It is confirmed that they act as effective additives functioning both as flame retardants and smoke suppressants. LaFeO3 could promote the formation of the homogeneous and compact intumescent char layer.133

Other additives, fillers, and fibers in intumescent formulations Many inorganic fillers and fibers find use in intumescent formulations. They increase char residue of the coatings at high temperatures. Selecting the right combination of flame-retardant fillers also strongly influence the fire protective as well as other properties of the coatings. Fillers such as magnesium hydroxide are reported to increase bonding strength to the metal surface due to its effective interface adhesion.134 Addition of inorganic fillers results in reinforcement of the char which improves the fire-protective performance, maintaining it for a longer time. Some inorganic fillers react with APP or with its degradation products and leads to the formation of thermally stable phosphate compounds which helps in maintain-

ing char integrity.135 Inert fillers like zirconium silicate help in the reinforcement of char. It is also reported that its addition results in an increase in residual weight of char and release of less gaseous products.136 Inorganic compounds e.g., borides, nitrides, or carbides of Titanium, Zirconium, or other metals are chemically inert and have a much higher thermal decomposition temperature. They serve as a filler and stabilize char in intumescent formulations.137 Glass flake as a modifier in intumescent coatings improves the fire protection and water resistance of the coatings. Coatings modified with glass flake restrict the migration of APP and PER in the presence of water, maintain their fixed ratio and have an excellent intumescent effect and fire protection.138 Refractory fibers like alumina and silica increase the strength of the residual char and also improve the fireprotective performance for a longer duration.129 Addition of nepheline syenite in the formulation has been reported to increase performance against high temperature fuel fires.139 Synthetic glasses also have been shown to increase the performance of intumescent coatings. The incorporation of vitreous fillers resulted in char stabilization, decreasing in optical density of smoke emitted and their toxicity index.119,140 Cenospheres as a filler in water-based intumescent formulation form a ceramifying layer of silicophosphates and aluminophosphates and protect the underlying char improving the fire performance.141 Koo et al. incorporated and compared high-temperature ceramic fibers and minerals in water-based epoxy intumescent systems.142 The high-temperature ceramic fibers are easier to incorporate than minerals and enhance the toughness of the char. Formulations with alumina, carbon, glass and aramid fibers are shown to increase the char strength.143 Use of wollastonite, kaolin clay, alumina, eggshell biofiller as an intumescent filler is also reported. The incorporation of filler results in even foaming and improved adhesion strength. The easy availability and good fire-protective performance of alumina make it a good choice. The coatings containing egg shell fillers are shown to have excellent fire protection, water resistance, thermal stability and adhesion strength.144–148 Different kinds of inorganic fillers incorporated into intumescent coatings are shown to have significant differences in the intumescent ratio, char morphology and thermal shielding performance. It is found that the high melt viscosity resulting from incorporation of inorganic fillers hinders the polymer chains from rotation and relaxation and influences the intumescent behavior which leads to lower intumescent ratio and poor fire performance.149

Use of fire-retardant additives in intumescent formulations Boron-containing additives are most common in the intumescent formulation and are well known to increase the fire-protective performance of the

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intumescent coatings. Interaction of boron compounds with intumescent additives results in formation of a vitreous layer over the surface of intumescent char. A low-density epoxy intumescent coating for steel against hydrocarbon fires containing boric acid is shown to have good performance.150 Jimenez et al. reported on the use of boric acid and coated ammonium polyphosphate (pure ammonium polyphosphate coated with THEIC) as flame retardants in an intumescent epoxybased formulation. Boric acid reacts with coated ammonium polyphosphate and forms borophosphate which provides superior mechanical resistance to the char and also enhances adhesion of char to the steel substrate.151 Colemanite (2CaOÆ3B2O3.5H2O), a commercial borate mineral, is shown to act as synergistic agent in an intumescent PP system. It is suggested that colemanite decomposes to CaO and B2O3 at high temperature which can react with flame retardants forming a thermally resistant layer.152 Boric acid along with ammonium polyphosphate gave best results from various intumescent formulations developed for steel in hydrocarbon fire based on thermoset epoxy-amine resin system. Boric acid reacts with APP to form borophosphate. It increases thermal protection as well as gives better adhesion and mechanical strength to the char.151,153 An intumescent epoxy coating is claimed to have improved fire protection by incorporation of zinc borate or zinc oxide along with it.68

Use of nanoparticles in intumescent formulations The addition of nano-scale additives has shown to enhance the fire protection performance of the intumescent coatings. Various nano-additives like nanoclays, e.g., montmorillonite, hectorite, saponite, double-layered Mg Al hydroxides improve thermal shielding performance of intumescent coatings. Carbon nanotubes (CNT), metals nanoparticles of silica, aluminum, and titanium, fullerenes, silsesquioxane are shown to perform synergistically in improving the performance of intumescent coatings. The introduction of CNT into intumescent formulation increases the thermal stability up to certain temperature range, but then causes a severe deterioration in performance due to the interaction of the network structure and the intumescent carbonaceous char. The increase in the concentration of MWCNT results in cracking of the char due to complex interactions between MWCNTs and IFR on heating in PP.154 MWCNT may be effective as a flame retardant in polymer nanocomposites, but are not suitable for intumescent coatings due to their degradation at high temperature.155 CNTs do not have enhanced reaction in APP system while organically modified polyhedral oligomeric silsesquioxane (OMPOSS) in combination with APP provides a large synergistic effect. Polyhedral oligomeric silsesquioxane also claimed to act as a flame retardant for thermoplastic polyurethane. There

is a formation of ceramified char of silicon network in a polyaromatic structure which acts as a thermal barrier at the surface and limits heat and mass transfer leading to a limited heat release rate.156 Use of nanoparticle in improving water resistance is also reported. Studies showed that about 4% of surface modified nano-size silica have improved the water resistance of APP-PERMEL coatings.157 Organically modified montmorillonite (OMMT) used as nano-filler is claimed to improve the fire protection, water and corrosion resistance of waterborne intumescent fire-retardant coating. It is due to the barrier effect of parallelarranged OMMT which significantly slows down the migration and solvation of fire retardant additives.158 Intumescent coatings prepared with organo-clays added with different types of Cloisite 30B, 10A and 15A in different concentrations (1%, 3%, 5%, and 10%) have shown significant enhancement in the fire retardancy. The formation of phosphocarbonous structure in the char enhances the fire performance.159 The addition of different nanoclays influences the char formation, its height, mass and structure along with its fracture behavior and hence influences its fire protection.160 Flame-retardant nano-coatings by the addition of nano-size layer double hydroxides (nano-LDHs) and titanium dioxide (nano-TiO2) to APP-PER-MEL systems have also been reported. Nano-LDHs lead to the formation of an intercalated nanostructure of the char and mixed metal oxides (Al2O3 and MgAl2O4) during thermal decomposition. The intercalated nanostructure enhances the antioxidation property of the char. APP and layered double hydroxide (LDH) show a synergistic effect on flame-retardant properties of poly vinyl alcohol.161 It is reported that only specific amounts (1.5%) of nano-LDHs could significantly improve its char layer structure and fire-resistant properties.162 The intumescent degree decreases with increasing LDH content, thus giving poor thermal shielding performances. However, it also depends on the functionalization of the LDH.163,164 Nano-aluminum hydroxide in intumescent PP leads to the formation of a more intact and homogeneous char during combustion. It also interacts with the degradation products of APP giving aluminum metaphosphate, which provides a synergistic effect in the condensed phase.165 Fumed silica in APP-based intumescent coatings is shown to have a beneficial effect. Silicon migrates on the surface in the heating process and three-dimensional interpenetrating networks were formed leading to the formation of a physically strong charred surface layer.163,166,167 Dong et al. reported the use of nano-sized boron nitride in the waterborne intumescent coating.168 It is demonstrated that nanosized boron nitride improves the cell size distribution, foam structure and the mechanical strength of the char layer improving its heat insulation. Polymer flameretardant composition which forms a homogeneous foamy char using nano-graphene is also reported.169 Cyclodextrin nanosponges are also claimed as novel green flame retardants. The nanosponge architectures

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keep phosphorus derivatives embedded and generate phosphoric acid directly in situ at high temperatures. As a result, they dehydrate, resulting in char formation and thus significantly increasing fire protection.170

Expandable graphite (EG)-based intumescent coatings EG is a new class of intumescent additives having an exfoliated structure intercalated with acid intercalating agents (H2SO4 or HNO3) into the graphite crystal structure (Fig. 6). Under fire at high temperatures, the intercalated agents release gases and expansion takes place in a perpendicular direction to the carbon layers in the crystal structure.171,172 Current research demonstrated that EG can be a useful additive in improving flame-retardant properties of the coatings. An intumescent coating for steel conduits using expandable graphite and glass or ceramic micro balloons is reported.173 Expandable graphite modified by polyethylene glycol improves the thermal shielding performance of the intumescent coating. It is reported that adding modified EG increases cell size of the char layer and also narrows the cell size distribution.174 Most of the flame-retardant additives in APP–PER– MEL coatings are water sensitive, and thus during water immersion, the compositional changes result in the reduction of the fire performance of coatings. The unique structure of graphite flakes with specific parallel-arrangement acts as a hindrance to the permeation

HSO4

–

H2SO4

H2SO4

HSO4

–

HSO4

–

–

H2SO4 HSO4

Fig. 6: Expandable graphite structure

of water and ions through the coating and, therefore, helps in reducing water sensitivity of the coating.175 Polyurethane/EG coating, which is obtained by poly condensation of an isocyanate with a polyol, eliminates drawbacks such as the loss of mechanical properties or processing difficulties improving fire retardancy of polymers.176 Boric acid modification ensures a uniform H3BO3 distribution over the surface, thereby considerably improving the fire protection of the material.177 Grafting 3-isocyanatopropyltriethoxysilane (IPTS) to EG as coupling agent reacts with a polymer matrix, resulting in increased thermal stability and flame retardancy.178 Gardelle et al. have reported on curable-silicone-based coatings containing EG and organoclay. It is observed that the presence of the nondegraded silicone matrix, coating of graphite pellets by silicone and also the organoclay increases the cohesion of the char.179,180 Higher addition of EG may result in poor bonding of coating.181 It is also reported that titanium-based catalyst gives higher fire protection and better mechanical properties to the char than the tin catalyst in intumescent silicone coatings. It is confirmed that during crosslinking, tin migrates to the surface leading to a low thermal stability and thus, low fire performances while titanium catalyst interacts with the silicone network forming Si–O–Ti linkage, increasing the thermal stability of the matrix and thus enhancing the fire performance of the coating.182

Performance testing of fire-protective intumescent coatings No two fire conditions are similar. The extent of temperature rise mainly depends on the type and quantity of fuel, availability of oxygen, and ambient conditions. Thus, for testing, fire performance of this kind of coatings standard fire curves have been defined.183 In fire-resistance design of structures and tall buildings, performance-based structural fire engineering is mostly accepted method of structural design. This makes it important to understand the performance of these coatings under a range of potential designed fire conditions. Other methods for determining the heat transfer to structural elements are by using heat fluxtime curves and temperature–time curves. The fire resistance of building structures is mainly decided by the time within which the structural members achieves a critical temperature which reduces their load bearing capacity. In many countries, the critical temperature for steel structures is 500–550C, above which it starts losing its mechanical properties.184,185 The use of an intumescent coating on structural steel may prolong the fire rating time from few minutes to several hours. Apart from the formulation, the performance also depends on the thickness of the coatings, types of steel sections, e.g., I section, hallow sections and their orientation, e.g., beams, columns. The tests are carried out in the large test furnace nearly (3 m 9 4 m 9 6 m) where the conditions of tempera-

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T  T0 ¼ 345 Log ð8t þ 1Þ; where t is the time in minutes, T is the furnace temperature (C) within time t, T0 is the initial furnace temperature (C). An essential requirement for the intumescent coating to protect steel is that it should remain intact in the event of a fire. Adhesion and cohesion of char in fire scenario are also important issues. The durability of coatings is important for effective fire protection in a long run. Poor protection can lead to corrosion and possibly structural failure in fire and expensive maintenance cost. The major ingredients ammonium polyphosphate, pentaerythritol, and melamine are water sensitive and need careful formulation. For testing durability of the coatings, many standard coating tests are used. NORSOK M-501 and European Technical Approval Guidance ETAG 18-2 are some to mention here. The panels are exposed to the specified exposure environment according to these test procedures and are then fire tested against a non-exposed sample.188

Work done in our lab at IIT Bombay

ings provide good protection against fire. An intumescent coating containing sodium, potassium, and lithium silicate was formulated containing expandable graphite and silicon carbide. The expanded char and time– temperature profile of the coating are shown in Figs. 7, 8. The back panel temperature of these coatings on aluminum substrates after 60 min was 180C. The char produced was very tough and expansion was 15 times of the original thickness. Increasing the coating thickness resulted in better fire rating thus increasing the evacuation time. However, the mechanical properties of the coatings were not good and the coating had poor application properties. The inorganic-based intumescent coating was thus modified with organic binders to overcome the drawbacks. An epoxy-based silicate intumescent coating showed a good fire performance with a back panel temperature of 180C at the end of 60 min. The char was strong but the application of the coating was still not adequate (Figs. 9, 10). In order to further improve the application properties of the silicate-based intumescent coatings, waterbased emulsions were introduced into the formulation along with organoclay, aluminum hydroxide, silicon carbide, and zinc borate. The coating showed excellent 220 200 180 160

Temperature °C

ture and load are simulated. The temperature of the furnace, as well as the inside of the steel core, is measured by well-placed thermocouples and the test is carried out according to various fire curves. British standard BS 476 part 20, 21 and ISO 834 describes the test method for intumescent coatings in cellulosic fire exposure. Other standard test curves include UL 1709 for hydrocarbon fire and ISO 22899 for jet fire. The increase of temperature in the furnace follows the dependency as per the relation given below.186,187

140 120 100

6.5 mm 4.5 mm 2 mm

80 60

Various inorganic- and organic-based intumescent coatings were developed in our laboratory at IIT Bombay. Some of the findings of the work are described below: The coatings were developed using sodium, potassium, and lithium silicates along with other fireretardant additives. It was observed that only alkali silicate-based coatings cannot provide fire protection for more than 20 min and are not suitable for extreme conditions. Expandable graphite-based silicate coat-

Fig. 7: Char of silicate intumescent coatings

40 20 0

20

40

60

80

100

120

Time (min)

Fig. 8: Time–temperature profile of silicate intumescent coatings

Fig. 9: Epoxy silicate intumescent coatings (a) before and (b) after fire testing

J. Coat. Technol. Res. 240 2 mm Epoxy silicate coating

220 200

Temperature °C

180 160 140 120 100 80 60 40 20 0 0

10

20

30

40

50

60

Time (min)

The developed intumescent silicate coatings have excellent expansion and char strength but are poor in mechanical properties, over-coating ability and have a severe problem with moisture absorption in the humid atmosphere leading to severe corrosion. Currently, we are working on the development of both waterborne and solventborne organic thin film intumescent coatings. Various formulations are developed for improvement in char fragility and mechanical strength. The developed formulations also have a good fire rating and corrosion resistance. APP modifications in waterbased emulsions and epoxy systems with various incorporated additives like nano-clays, metal nanoparticles and vitreous fillers helped in improving the properties of the intumescent coatings. In addition, various test methods were designed for laboratory testing of the intumescent coatings, e.g., small-scale testing for adhesion and strength of char.

Fig. 10: Time–temperature profile of intumescent epoxy silicate coatings

Conclusion

Fig. 11: Silicate acrylic intumescent formulation

200 180 160

Temperature °C

140 120 100 80 60 40 20 0

10

20

30

40

50

60

Time (min)

Fig. 12: Time–temperature profile of silicate acrylic intumescent

intumescent property with higher expansion and strong char and good application properties (Figs. 11, 12).

Intumescent coatings provide passive fire protection with a rating of few minutes to several hours and are consistently used by architects and designers. In addition, they also offer aesthetic appearance and ease of application. They can be formulated for use in cellulosic fire and hydrocarbon fire including jet fire and are effective in prolonging fire-induced structural collapse. The present review focuses several aspects of intumescent coatings i.e., their classifications, composition variations and effect of several additives. The use of appropriate ingredients in formulation is of paramount importance as it largely influences the fire shielding performance. High-temperature stable fillers, nano-metal oxides, and clays modify the morphology of the intumescent char and increase thermal stability. The nano-additives are effective only at low concentrations. Modern day intumescent coatings provide effective passive fire protection in the event of a fire but still have some pitfalls. Lower fire ratings and application involving several layers with long drying times make the use of these coatings difficult on site. Due to use of water sensitive ingredients in the formulations, the outdoor exposed coatings have poor corrosion resistance. The ingredients leach out of the coating in moist environments and the coatings lose their intumescence with time. The carbonaceous char formed may have poor cohesive strength and fragility and may dislodge due to mechanical disturbances of falling objects during a fire event. Use of nanofillers and nano-clays, surface modification of ammonium polyphosphate, and formulating with vitreous fillers are some of the promising approaches in the formulation of high-performance intumescent coatings.

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