Winning time. Seite/Page: 30

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1 Winning time New developments in intumescent paints for structural-steel protection The need for fire resistant coatings for structural steel in modern architecture is increasing Ammonium polyphosphate is an environmentally friendly and versatile flame retardant which is used in modern steel protective coatings Improved, polymer coated ammonium polyphosphate grades together with new technologies such as ceramifying agents will widen the existing range of applications in architecture and offer a considerably higher safety standard Thomas Futterer, David Garcia, Vicens Mans, Hans-Dieter Nägerl, Eduardo Tortosa The ammonium salts of orthophosphoric acid, MAP and DAP, have been used as additives for flame retardants (FR) for decades They have also been used in coatings for construction elements, mainly made from steel, wood, and other building materials Because of their high solubility and low thermal stability, however, the application of these products in modern architectural applications, where higher standards apply, is very limited Halogen-free polyphosphates find wider use Ammonium polyphosphate (APP, FR CROS 484) is the ammonium salt of polyphosphoric acid In contrast to MAP and DAP, it has a polymeric structure with the general formula [NH 3 HPO 3 ] n (Figure 1) APP is an efficient and multi-purpose FR, which is widely used Due to the environmental and toxicological concerns surrounding many halogenated products, FRs based on APP, which are halogen-free, are finding a broader range of uses Especially in construction and building, APP is playing a mayor role as flame-retardant additive in intumescent paints for the fire protection of structural steel Two crystal forms of APP (APP I and APP II) are commercially available and used as major ingredients for fire-retardant coatings APP I has a chain length of about 100, and that of APP II is more than 1,000 APP II is finding more applications than APP I, because of its better water resistance and higher thermal stability in paints and coatings, especially for exterior applications APP II decomposes at temperatures above 280 C Ammonia is released and the remaining polyphosphoric acid forms a melted, glassy surface In water, APP is slowly hydrolysed to MAP Large amounts of standard APP II are used worldwide in industrial coatings for fire protection of structural steel New building regulations, such as ENV , with stronger requirements and more severe fire-testing conditions require APP grades that have even better fire performance, lower water solubility and better compatibility to new resin and coating systems Preventing or retarding structural steel collapse Intumescence is the result of a sequential chemical reaction between several chemical products activated by the heat of flames during the fire Intumescent products are those which expand (or intumesce) to several times their original size when activated by high temperatures The final result of the chemical reaction is a foam that isolates and protects the underlying material from fire and oygen and reduces the smoke and the heat transmission As the name implies, intumescent coatings swell up when exposed to higher temperatures and for example build up a thick insulation layer around a steel beam APP plays an important role in intumescent coatings Its reaction with the other components takes place at around 250 C to form a thermally insulating carbonaceous char The char can be expanded up to 50 times the original thickness of the coating This layer reduces the rate of heating of the steel and prolongs its load bearing capacity The combined use of steel construction methods and intumescent fire protection has proved the most cost effective and reliable solution in many different situations Although steel does not burn, it looses stability at temperatures above 550 C The consequences of this effect were catastrophically seen in the collapse of the World Trade Center due to the high temperatures of the fires The rate of heating of steel can be greatly reduced by the use of insulating materials such as an intumescent coatings, which increases the time until the steel reaches its critical temperature The primary objective is the prevention of structural steel collapse, which in turn will win valuable time and possibilities for the escape of the occupants and ensure the safety of rescue services How do intumescent coatings operate? Intumescent coatings contain four basic "synergistic" ingredients: 1 a catalyst, ie a dehydrating or carbonising agent, such as APP, which at temperatures above 200 C liberates polyphosphoric acid 2 a source of carbon, ie organic substances which can be charred and turned into coal-like products by polyphosphoric acid, for example polyols such as pentaerythritol or dipentaerythritol 3 a blowing agent such as melamine, which under decomposition releases gases (N 2, NH 3 ) which expand the char 4 a binder/resin system, for instance acrylic resins, epoxy resins etc Typical formulations of solvent or water based intumescent coatings are given in Table 1 Coated APP grades yield higher weathering resistance New coated grades have been developed in which the APP II particle is surface treated with different resins, plastic materials or agents Various coating methods can be used, such as adhesion, ionic bonding or covalent bonding For instance, APP II can be coated with amines, amino resins (shell-coated or reaction-coated), silanes (reaction-coated or coating by adhesion), but also with silicones, thermosets or thermoplastic resins Materials which have a low but significant water solubility, such as APP, can be washed out over time Coating of APP was found to be a very efficient method of getting around this problem This solution is especially advantageous for paints and coatings that are applied on steel for external use, where humidity can be high and persistent Coated APP grades will remain much longer in the protection layer (Figure 2) and thus the flame retardant properties can be guaranteed for a longer period Weathering tests of three basic paints, where the only difference was the type of APP used (A, B, C: water based, and D, E, F: solvent based), were performed The behaviour of the standard APP ("FR CROS 484") versus the coated grades ("FR CROS 487" and "FR CROS 489") was observed (Figure 3) Painted steel laminates of 15 x 15 cm with a dry film thickness of 1000 microns without top coatings were tested in a weathering chamber at a relative moisture of 100% and at 70 C for two days The blistering

2 and weight loss of the samples were assessed In both systems, water- and solvent-borne paints, the coated APP grades reduce the weight loss and improve the surface defects The best result was achieved with the reacted coated APP grades, where the weight loss was lowest A more uniform intumescence with coated APP grades The intumescent behaviour of all paints, A to F, was tested using a small scale test furnace consisting of a propane burner This method has proved to be an excellent way of studying char evolution and its cellular structure The following parameters were measured (Table 2): - Intumescence thickness - Cellular char structure - Temperature curves versus time Water based paints (A-C) generally show a higher intumescence than solvent based (D-F) paints Interestingly, it was found that the use of coated APP grades lowered the thickness of the intumescence, but the cell structure is more uniform and closed, with no cracking Accordingly, the temperature rise of the steel during the fire test is lower when paints with coated grades are tested This leads to a better insulation and structural stability of the foamed layer and finally to a better steel protection under high temperature conditions The fire resistance test was also performed with the weathered samples (Table 2) The results show that the intumescence and heat insulation were always better when coated APP grades were used in the paint formulation, because of their better humidity resistance Coated APP grades therefore can contribute to more durable intumescent coatings which give longer and better performance, especially under high humidity and outdoor conditions ph-adjustment for better compatibility Coated APP grades also offer a higher thermal stability with a 2 % weight loss at temperatures over 300 C, measured by TGA Such materials are less sensitive to shear forces during mixing and allow users a greater flexibility and faster processes The ph value (typically 5) of the APP can be adjusted to a neutral ph by applying different coatings The ph-adjusted APPs offer a greater compatibility to certain resins Also the viscosity stability of intumescent paints during storage should be more constant and not change dramatically, as is the case with some lower quality APP types The hydrophilic or hydrophobic properties of the APP can be adjusted by changing the surface polarity of the particles Such products can be used in either solvent- or water-based paint systems In general, coated APP particles are more compatible with the matrix in which they are used Ceramifying agents improve the foam consistency Ceramifying agents are an interesting and promising technology to improve the consistency of intumescent foam The use of selected ceramifying agents will increase the strength and the resistance of the intumescent foam and at the same time the adherence to the steel profile Various additives have been tested in a traditional intumescent system used in reactive systems for steel profile protection The basic formulation of the intumescent coating contains three parts of APP II ("FR CROS 484"), one part of melamine and one part of pentaerythritol (compare formulations above) The basic formulation used was: APP II 357% melamine 119% pentaerythritol 119% titanium dioxide 167% special ceramifying additive 238% The ceramifying additives tested were salts of phosphoric acid and others Mixtures of one gram of the above formulations were put on a steel plate, placed in an electric furnace and heated according to the ISO curve up to 850 C (approx 30 minutes) Only certain materials showed a "ceramifying effect", which can be characterised by a harder and more compact intumescent char A visualisation of this effect is shown in Figure 4 Three "ceramification" categories were defined: "Good" (hard intumescence), "Regular" (semi-hard intumescence) and "Bad" (soft intumescence) Not all salts and combinations gave good results and improved the hardness of the intumescence (Table 3) Ceramified intumescent layers are mechanically stable The efficiency of all intumescent systems with positive ceramifying effects was tested in a solvent-based formulation by using a small scale test consisting of direct heating by a propane burner and determination of the time vs temperature curves (Figure 5, salts 1, 3 and 5) A steel plate 140 x 140 x 10 mm, cleaned by sand blasting, was coated with one layer of a primer The primer was applied by brushing and had a film thickness of microns Afterwards, several layers of the intumescent paint with an total thickness (dry) of microns were applied The coated specimens were dried for 30 days at room temperature before the burning test The fire resistance test was performed in an electrical furnace according to ISO 834 The starting temperature was 25 C The temperature was increased slowly and the test was stopped when the temperature reached 550 C The temperature was measured by a thermocouple in the steel The paint without any ceramifying agent produced a very soft intumescent foam Even when small weights (100 g) were applied, the cellular structure of the foam collapsed under the weight In contrast, the coating formulation with ceramifying agents (salts 1 and 5) produced a thinner, but much harder intumescence foam A weight of one kilo neither penetrated the foam nor produced any significant foam deformation, thereby maintaining the original structure In addition, intumescent foams with ceramifying agents show better thermal insulation than those without agents or less effective salts As can be seen in Figure 5, the intumescent coatings with the ceramifying agents 1 and 5 reach the critical temperature (500 C) eight minutes later than the intumescent system without any additive A fire-resistance test carried out by an official testing laboratory was applied to this system, and resulted in a glassy and hard structure without any cracking and flaking of the protective coating Thus, further optimisation of intumescent coatings using ceramifying agents will lead to new and improved steel protection systems with better fire performance Results at a glance - Ammonium polyphosphates (APP) are playing an important role in as flame-retardant additive in intumescent paints for the fire protection of structural steel - Coated grades of APP significantly improve the coatings' performance: Formulations based on these grades show better moisture resistance, higher thermal stability and more uniform intumescence layers, leading to better thermal insulation - The use of specific ceramifying agents in the formulations greatly improves the consistency, ie the mechanical stability, of the intumescent foams, adding to their fire protection performance

3 The authors: -> Dr Thomas Futterer is research and development manager for flame retardants at Chemische Fabrik Budenheim He received his PhD in 1997 from the University of Regensburg and joined Budenheim in > Dr Hans Dieter Nägerl is vice president at Chemische Fabrik Budenheim He received his PhD in 1970 from the University of Würzburg, Germany and joined Budenheim in > Vicens Mans is general manager of Budenheim Iberica He received his diploma of Industrial Chemistry from the Engineering School of Barcelona in 1976 He is member of the CEN-TC 127 and EAPFP, representing Spain -> Eduardo Tortosa is technical manager for flame retardants at Budenheim Iberica He received his Diploma Degree in Chemistry in 1963 at the Chemical Institute of Sarria (IQS) He joined Budenheim Iberica in > David Garcia is technical manager and qualitiy manager for flame retardants at Budenheim Iberica He received his Diploma Degree in Chemistry in 1996 from the University of Zaragoza, Spain He joined Budenheim Iberica in 1999 This paper was presented at the European Coatings Conference "Fire Retardant Coatings", Berlin, 14/15 September 2007

4 Figure 1: General formula of APP with chain length n

5 Figure 2: Improvement of the solubility of APP grades by different coating technologies

6 Figure 3 a-b: Moisture resistance of intumescent paints with different APP grades after artificial weathering a) water based paints A-C, b) solvent based paints D-F

7 Figure 4: Visual observation of ceramifying effect Left: sample with ceramifying additive (salt 1: hard intumescence) Right: sample without additive (soft intumescence)

8 Figure 5: Temperature vs time curves of standard paints without ceramifying additives and with ceramifying additives (salts 1, 3 and 5)

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