Surface & Coatings Technology Influences of binder on fire protection and anticorrosion properties of intumescent fire resistivecoating for steel structureArticleinfo：Article history:Received 31 May 2009Accepted in revised form 23 October 2009Available online 30 October 2009Keywords:IntumescentFire resistive coatingBinderFire protectionAnticorrosion propertyAbstract：The combination of epoxy emulsion and self-crosslinked silicone acrylate (SSA) emulsion was used as mixed binder for preparing water-borne intumescent fire resistive coating. The influences of binders on the properties and char formation of the coatings were investigated in detail by using thermo gravimetry (TG), scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), rheological measurement, fire protection test and anticorrosion property test. It was found that the fire protection and foam structure of the coating was significantly improved by adding 14% SSA (by mass). The TG results showed that adding SSA increased the residue weights of the coatings. The XRF results demonstrated that anti-oxidation of the coatings was improved by adding SSA into the mixed binders. The results of anticorrosion property test indicated that the anticorrosion property of the coatings was enhanced with increase of content of SSA.Crown Copy right . 2009 Published by Elsevier B.V. All rights reserved.1. IntroductionIntumescent fire resistive coatings have been found widespread used as passive fire protection for steel structure which applied in civil buildings, chemical plants and other facilities [1,2]. Intumescent coatings were composed of three fire retardant additives: an acid source(such as ammonium polyphosphate, APP), a carbon source (such as pentaerythritol, PER) and a blowing agent (such as melamine, MEL) bound together by a binder. During the intumescent process, the binder became important due to two effects: it contributed to the char layer expansion and ensured the formation of uniform foam structure [3–5]. However, hydrophilic fire retardant additives (APP and PER) in the coatings were very sensitive to corrosive substances, such as water, acid and alkali [6]. They could easily migrate to the surface of the coatings in corrosive environment [7]. This would significantly depress the expected effect of intumescent coatings. The binder as a film-forming component could prevent or remarkably reduce migration of fire retardant additives and access of the corrosive substances [8,9]. However, somepolymer binders, such as acrylic resin,were not efficient enough to provide good corrosion resistance [10].The objective of this work was to develop a highly protective intumescent coating, which not only had advantages of good fireproof performance, but also showed great anticorrosion property. To achieve this objective, epoxy emulsion and self-crosslinked silicone acrylate (SSA) were selected as mixed binders. Epoxy was used to bind fire retardant additives and provided a carbon source of the intumescent system. Moreover, the chemical structure of epoxy imparted them high chemical resistance against severe corrosive conditions [11]. The crosslinking structure of SSA could increase intumescent rate of coating and improve the foam structure of char layer. Furthermore, when SSA was mixed with epoxy emulsion, the reaction might occur between the two resins, which could enhance crosslinking degree of the mixed resin and lead to an increase of corrosion resistance of the coatings [12,13]. The influences of the mixed binder on the fire protection and anticorrosion properties of intumescent coating were analyzed. Based on the obtained results, the effects of mixed binder on the coatings were evaluated.2. Experimental2.1. MaterialsEpoxy emulsion was supplied by Anbang New Material Development Co., Zhejiang, China.SSA was supplied by Duokete Chemical Reagent Co., Jiangsu, China.APP (nN1000) was supplied by Weidong Chemistry Co., Shandong, China.MEL was supplied by Luming Chemistry Co., Shandong, China.PER and titanium dioxide (TiO2, rutile) were supplied byGuoyao Chemical Reagent Co., Shanghai, China.Kaolin was supplied by Sanbao Kaolin Co., Neimenggu, China.Expandable graphite (EG) was supplied by Baoding Action Carbon Co., Hebei, China.The particle size and expansion volume of EG were 0.18 mm and 180 cm3/g, respectively (specified by the suppliers).2.2. Samples preparationThe composition of intumescent coatings was listed in Table 1. APP, MEL, PER, TiO2, kaolin, EG and distilled water were mixed by high-speed disperse mixer. The mixture was added into the mixed binder composed with epoxy emulsion and SSA, then the coatings were dispersed by high speed disperse mixer. Prepared coating was coated on steel board (35# carbon steel, 15 cm×7 cm×1 mm) and then the sample boards were dried. This process was repeated 10–15 times until dry film thickness of 2±0.1 mm was reached.2.3. Measurement and characterizationPyris 1 TGA analyzer (Perkin Elmer Co., America) was used for TG analysis, which was performed from 50 to 800°C at a heating rate of 20°C/min under nitrogen gas (40 mL/min). XRD measurements were performed on D/max2550VB3+/PC X-ray diffract to meter (Rigaku International Co., Japan) in the range (10°b2θb70°). XRF measurements were performed on SRS3400 X-ray fluorescence spectrometer (Bruker Co., Germany). S-2360 N SEM (Hitachi Co., Japan) operated at 15 kV was used to observe the morphologies of char layers and the distribution of the cell size was characterized with SEM pictures of char layers.Rheological measurement was recorded on EQUINOXSS/HYPERION2000 ARES rheometer (TA Co., American) from 1 to 100 rad/s at 270 °C. The testing temperature (270 °C) was selected based on the DTG results. According to the DTG curves (derivatives of the TG curves) of the coatings, the decomposition of APP/PER/MEL system began at 230 °C. The inert gases (NH3, H2O) sent out and the coatings began to expand. The mixed emulsion was dried and formed film at room temperature for 240 h. The swelling ratio (SR)was determined by immersing the film in toluene for 48 h. After immersing, the swollen film was patted dried and weighed. The ratio of the mass of swollen film to that of the unswollen film was SR. Moreover,the initial film was extracted in Soxhlet extractor with acetone for 24 h. The film after extracting was dried at 100 °C for 5 h in oven and then weighed. The ratio of the mass of dried film to that of the initial film was the gel content (GC),which was a measurement of crosslinking degree of the film.2.4. Fire protection testThe fire protection test was carried out using the equipment in Fig.1 and the gas consumption of the gas lamp was 130 g/h. The side of steel board coated by intumescent coating was exposed to the gas lamp and burned for 1 h. During the test, the temperature of backside of steel board was recorded by digital thermometer (Center305, Center Co., Taiwan) and drawn as a function of time and the time–temperature curve of uncoated steel board was drawn as Fig. 2. Moreover, the intumescent process of coating was recorded by video camera (FS10, Canon Co., Japan) and the time–intumescent rate curve of coating was drawn to characterize the speed of intumescent process of coating.Moreover, the intumescent rate (I) was calculated by Eq. (1).I = （d2-d0）/（d1-d0）（1）In Eq. (1), d0 was the thickness of the steel board, d1 was the thickness of the sample board coated intumescent coating, d2 was the thickness of the sample board after fire protection test.2.5. Anticorrosion property testThe sample broads of coatings were weighed and then immersed in alkali liquor (5% sodium hydroxide solution by mass) at room temperature for 24 h and where after dried at room temperature for 12 h. The weight change rate x (%) was calculated by Eq. (2) and the anticorrosion property of coating was evaluated.X（%）= （m1-m2）/m1 × 100 （2）In Eq. (2), m1 was the weight of coating before test, m2 was the coating weight after a specific time of test.The sample broads of coatings were immersed in 5% hydrochloric acid solution (by mass) until the coatings were blistered, cracked or split, and the immersed time was defined as acidproof time.3. Results and discussion3.1. Influences of content of SSA on properties of bindersBoth epoxy emulsion and SSA had crosslinking structure and formed three-dimensional network structure which could significantly improve anticorrosion property of themselves [14,15]. Moreover, it has been reported that the reaction might occur between the two resins [12]. Six kinds of films (marked as A1–A6) were prepared according to the composition in Table 2 and the GC and SR of films were shown in Table 2.From Table 2, there was a continuous increase in GC value of films with increase of content of SSA. The GC values of A1 film and A6 filmwere 28.55% and 32.24% respectively, and both of them were less than 33%. But the GC of A5 film was 51.97% and almost two times higher than that of A1 film. This suggested that the crosslinking reaction (as shown in Fig. 3) has occurred between epoxy emulsion and SSA [16,17] and led to an increase of crosslinking degree of binders. This would contribute to improvement of anticorrosion property of binders. Moreover, it was shown that there was a gradual decrease in SR values of films with increase of content of SSA. This indicated that the solvent resistance of films was gradually improved because of increase of crosslinking degree of binders.3.2. Fire protection of coatingsAccording to the composition in Table 1, five kinds of coatings (marked as F1–F5) were studied by the fire protection test. During the test, the temperature of backside of steel broad was plotted as a function of time (as shown in Fig. 4) and the influences of content of SSA on fire protection of coatings were showed in Table 3.The shape of the temperature profiles was similar for all the coatings. During the first 300 s, there was no difference in the temperature of each coating, and the temperature increased rapidly and attained 180 °C. Then after 20 min of the test, the temperature reached an equilibrium value and almost remained unchanged at a long time. The experimental results showed that the equilibrium temperatures of F1, F2, F3, F4 and F5 were 270 °C, 255 °C, 243 °C, 272 °C and 290 °C respectively. It indicated that the equilibrium temperature of coatings was firstly decreased and then increased with increase of content of SSA, and the equilibrium temperature of F3 was obviously lower than that of other coatings.3.3. Thermal analysis of coatingsThermal degradation of F1 and F5 was analyzed by DTG curves in Fig. 5. In the case of F1, two weight loss peaks at 248.7 °C and 285.1 °C were attributed to the decomposition of APP and MEL, respectively. APP began to decompose at about 230 °C and liberated phosphoric acid, NH3 and H2O at the same time. The phosphoric acid took part in the dehydration of PER by esterification and induced char formation of the coating.MEL decomposed to yield gaseous products such asNH3 andH2O at about 290 °C. The inert gases caused fused char to swell and formed an expanded char layer [18]. The thirdweight loss peak at 394.5 °C in Fig. 5a was due to thermal degradation of the binder. Epoxy was a kind of high carbon-yielding polymer, which was helpful to char formation.SSA could react with epoxy and increased crosslinking degree of the mixed binder [12]. The binder with high crosslinking degree could enhance anti-oxidation and residue weight of the coatings at high temperature. Compared with decomposition temperatures of F1, the decomposition temperatures of F5 were much higher than those of F1. The results indicated that increasing SSA content could improve thermal destabilization of fire resistive coating and was favorable to fire protection of the coatings [19,20].TG curves of the coatings were presented in Fig. 6. The curves of the coatings were similar at 50–250 °C and weight loss of each coating was less than 20% at 250 °C.When the temperature was higher than 250 °C, the TG curves of the coatings were obvious different. The TG curves demonstrated that the residue weight of coating was increased with increase of content of SSA at high temperature. The residue weight of F1, F3 and F5 at 750 °C was 26.3%, 32.1% and 40%, respectively. The higher residue weights of F3 and F5 indicated that increasing SSA content in mixed binder could enhance anti-oxidation of the coatings [21].3.4. Rheological properties of bindersIt has been reported that the viscoelastic behavior of the binder significantly affects the fire protection of intumescent coating [22,23]. Thus, high temperature rheological measurements of the binders were performed in order to identify changes in the rheological properties of the binders under burning condition. Loss tangent (tanδ) was the ratio of loss modulus and storage modulus, which represented the deformation capacity of materials [22]. The tanδ values of different binders were shown in Fig. 7 and the intumescent rates of coatings were presented in Table 3.It was shown that the tanδ values of binders increased at first, and then turned to decrease with increase of content of SSA, and the trend of intumescent rate of coatings was similar. The tanδ value was increased from 0.24 to 0.26 with increase of content of SSA from 0 to 14% when shear frequency was 10 rad/s. Meanwhile, the intumescent rate was increased from 10.7 to 14, which was improved by 30.8%. However, when the content of SSA continued to increase from 14% to 28%, the tanδ value was reduced to 0.16 and the intumescent rate was also decreased to 10.7. In fact, the binder with appropriate crosslinking degree could improve its melt rheological property and increased the intumescent rate of coatings [24,25].The melting viscosity of binder had a close relation with the rate and speed of intumescent process of coating [26,27]. The relationship between intumescent rate and time under fire protection test for different coatings was presented in Fig. 8 and the melting viscosity of different binders was showed in Fig. 9. When the content of SSA was increased from 0 to 14%, the melting viscosity of binders was obviouslyenlarged, and the required time of intumescent process of coating was increased from 263 s to 390 s. This suggested that the speed of intumescent process of coating became slowly. Moreover, the melting viscosity of binder was continued to significantly enlarge owing to increasing of content SSA to 28%, and the required time of intumescent process of coating was also increased to 502 s. The results showed that the required time of intumescent process of coatings was significantly increased with increase of content of SSA. These indicated that increasing viscosity of binder could slow the speed of intumescent process of coating.3.5. Morphology of intumescent char layersThe efficiency of char layer depended strongly on its physical structure [28]. The SEM micrographs of foam structure (a, b, c) of char layers of F1, F3 and F5 were shown in Fig. 10 and the cell size distribution of foams was presented in Fig. 11. It was shown that the cell size of char layers was gradually bigger due to the significantly enlargement of the melting viscosity of binders with increase of content of SSA [29,30]. The char layer of F1 had a uniform distribution of the cell size, but a tiny foam structure with some cracks was observed in Fig. 10a. The tiny foam structure could insulate steel substrate from heat and fire. However, heat and fire might transfer to steel substrate through the cracks in foam structure, which could lead to a decline of fire protection of F1. A dense foam structure of char layer was found in Fig. 10b and the cell size distribution of F3 was uniform too. This foam structure could isolate steel substrate from fire and provide a better fire protection of F3. Moreover, the char layer of F5 showed a broad distribution of the cell size and had a lot of large cells (Fig. 10c). This foam structure demonstrated that some cells burst and coalescedtogether [31],which could increase efficiency of heat transfer and damaged fire protection of F5.3.6. Elemental analysis of intumescent char layersXRF analysis could provide the detail information about element composition of char layer. The element composition of surface material of char layers obtained from F1, F3 and F5 was shown in Table 4. It was shown that the silicon content was increased from 0.35% to 2.64% with increase of content of SSA. However, there were no obviously differences among the main elements of surface material of char layers such as oxygen, phosphorus and titanium. The results indicated that increasing content of SSA in mixed binder had little influence on the element composition of surface material of char layers.The element composition of interior material of char layers was shown in Table 5.When the content of SSA was increased from0 to 28%, the carbon content in interior materials of char layers was increased from 39.14% to 50.1% and that of oxygen content was decreased from 32.77% to 22.86%. The values of C/O were increased from 1.19 to 2.19. Carbon content in char layer indicated residue degree, while oxygen content in char layer implied oxidation degree of char layer at high temperature, so higher carbon content and lower oxygen content in char layer contributed to improve anti-oxidation of char layers [32].After the char layers of coatings were burnt by fire, only some amorphous carbon and inorganic materials were remained [33] and the inorganic materials might be the main protecting layer at later stage of fire protection test. XRD results of char layer of F3 were shown in Fig.12. Several XRD peaks (marked a) and other peaks (marked b) were assigned to titanium pyrophosphate (TiP2O7) and TiO2, respectively. According to spectrum (3), TiO2 and TiP2O7 were mainly distributed on the surface of char layer. It was shown that TiO2 could react with APP or phosphoric acid and formed a ceramic material at high temperature which could enhance the strength of char layer.3.7. Anticorrosion properties of coatingsThe corrosion mediums, such as water, acid and alkali, could destroy some components of hydrophilic fire retardant additives and break some bonds of binders, so the corrosion resistance of intumescent coatings decreased significantly [7].The differences of weight change rate as a function of immersion time for the coatings were shown in Fig. 13. When F1 was immersed in alkali liquor, two main processes (permeation and migration) took place simultaneously. In the permeation process, water and corrosive ions could infiltrate into the pore structure of coating, which led to the increase ofweight of coating. Moreover, some hydrophilic fire retardant additives might migrate from coating and be solved in alkali liquor during the migration process, which resulted in weight loss of coating [34,35]. During the first 100 h of test, the permeation process of water and corrosive ions exceeded the migration process of fire retardant additives, the weight of coating was continuously increased and its weight gain rate was 4.7% at 100 h. After 100 h of test, the migration process became stronger, so the weight of coating was gradually decreased and its weight loss rate was 18% at 504 h. Furthermore, weight loss rate of coating maintained relatively constant between 504 and 576 h when the two processes of F1 reached equilibrium. After immersed in alkali liquor for 576 h, the cracking and blistering phenomena did not happen in any coating. However, the weight loss rate of coatings was gradually decreased with increase of content of SSA.It has been reported that the appropriate crosslinking degree of polymer binders could not only improve compactness of coatings, but also slow down permeation of water and ions and migration of fire retardant additives, which led to an improvement in corrosion resistance of coatings [36]. The influences of content of SSA on GC of binders and weight change rate of coatings were shown in Fig. 14. The GC of binders was increased linearly with increase of content of SSA, and the final weight loss rate of coatings was decreased gradually. The final weight loss rate of F2, F3, F4 and F5 was 16.7%, 12%, 10.5% and 9.1%, respectively. These results proved that increase of crosslinkingdegree of polymer binders was benefit to the alkali resistance of coatings. Furthermore, the acidproof time of intumescent coatings was listed in Table 6. It was shown that the acidproof time of coatings was gradually increased with increase of content of SSA. The acidproof time of F5 was remarkably achieved 219 h, which was improved by 52.1% compared with that of F1. Owing to the linear increase of GC of the mixed binders with increase of content of SSA, the results indicated that increase of crosslinking degree of the mixed binders was favorable to improve acid resistance of the coatings.4. ConclusionsThe combination of epoxy and SSA led to an increase of crosslinking degree of polymer binder. This could significantly improve compactness of coating and slow down permeation of water and migration of fire retardant additives, which resulted in an improvement in corrosion resistance of coating. The interaction of fire retardant additives and the mixed binder led to formation of foam structure of char layer. The TG results showed that adding SSA increased the residue weights of the coatings. The rheological property of the mixed binder remarkably improved by adding 14%SSA, and it was benefit to increase intumescent rate of coating and form a better foam structure of char layer. Moreover, the XRF results showed that carbon content of char layers was increased and oxygen content was decreased with increase of content of SSA. This could produce a significant enhancement in anti-oxidation of coatings. TiO2 and TiP2O7 were mainly distributed on the surface of char layer and became the main protecting layer at later stage of fire protection test. References[1] J.A. Rhys, Fire Mater. 4 (1980) 154.[2] J.W. Gu, G.C. Zhang, S.L. Dong, Q.Y. Zhang, J. Kong, Surf. Coat. Technol. 201 (2007) 7835.[3] M. Jimenez, S. Duquesne, S. Bourbigot, Thermochim. Acta 449 (2006) 16.[4] S. Duquesne, S. Magnet, C. Jama, R. Delobel, Polym. Degrad. Stab. 88 (2005) 63.[5] S. Duquesne, S. Magnet, C. Jama, R. Delobel, Surf. Coat. Technol. 180–181 (2004) 302.[6] B. Ostman, A. Voss, A. Hughes, P.J. Hovde, Q. Grexa, Fire Mater.25 (2001) 94.[7] Z.Y. Wang, E.H. Han, W. Ke, Corros. Sci. 49 (2007) 2237.[8] W. Funke, Prog. Org. Coat. 31 (1997) 5.[9] W. Funke, Prog. Org. Coat. 28 (1996) 3.[10] M.N. Sathyanarayana, M. Yaseen, Prog. Org. Coat. 26 (1995) 275.[11] J.J. Suay, M.T. Rodriguez, K.A. Razzaq, J.J. Carpio, J.J. Saura, Prog. Org. Coat. 46 (2003) 121.[12] T.Y. Guo, X. Chen, M.D. Song, B.H. Zhang, J. Appl. Polym. Sci. 100 (2006) 1824.[13] T.Y. Guo, X. Chen, G.J. Hao, M.D. Song, B.H. Zhang, Adv. Polym. Tech. 24 (2005) 288.[14] G.F. Levchik, S. Kun, S.V. Levchik, G. Camino, C.A. Wilkie, Polym. Degrad. Stab. 65 (1999) 395.[15] S.L. Case, E.P. O'Brien, T.C. Ward, Polymer 46 (2005) 10,831.[16] S.A. Kumar, T.S.N. Sankaranarayanan, Prog. Org. Coat. 45(2002) 323.[17] S.A. Kumar, Z. Denchev, M. Alagar, Eur. Polym. J. 42 (2006) 2419.[18] K. Wu, Z.Z. Wang, H.J. Liang, Polym. Compos. 29 (2008) 854.[19] M. Jimenez, S. Duquesne, S. Bourbigot, Polym. Degrad. Stab.94 (2009) 404.[20] Q. Wang, W. Shi, Polym. Degrad. Stab. 91 (2006) 1747.[21] S. Duquesne, R. Delobel, M. Le Bras, G. Camino, Polym. Degrad. Stab. 77 (2002) 333.[22] P. Anna, G. Marosi, I. Csontos, S. Bourbigot, M. Le Bras, R. Delobel, Polym. Degrad. Stab. 74 (2001) 423.[23] P. Anna, G. Marosi, S. Bourbigot, M. Le Bras, R. Delobel, Polym. Degrad. Stab. 77 (2002) 243.[24] K.M. Gibov, V.S. Mamleev, J. Appl. Polym. Sci. 66 (1997) 329.[25] V.S. Mamleev, E.A. Bekturov, K.M. Gibov, J. Appl. Polym. Sci.70 (1998) 1523.[26] T. Kashiwagi, R.H. Harris, X. Zhang, R.M. Briber, B.H. Cipriano, S.R. Raghavan, W.H.Awad, J.R. Shields, Polymer 45 (2004) 881.[27] L. Karlsson, A. Lundgren, J. Junqvist, T. Hjerberg, Polym. Degrad. Stab. 94 (2009) 527.[28] Z.Y. Wang, E.H. Han, W. Ke, Surf. Coat. Technol. 200 (2006) 5706.[29] A. Andersson, S. Lundmark, F.H.J. Maurer, J. Appl. Polym. Sci. 104 (2007) 748.[30] M. Jimenez, S. Duquesne, S. Bourbigot, Surf. Coat. Technol. 201 (2006) 979.[31] M. Yamaguchi, K.I. Suzuki, J. Polym, Sci: Pol. Phys. 39 (2001) 2159.[32] G.X. Li, G.Z. Liang, T.S. He, Q.L. Yang, X.F. Song, Polym. Degrad. Stab. 92 (2007) 569.[33] G.X. Li, J.F. Yang, T.S. He, Y.H. Wu, G.Z. Liang, Surf. Coat. Technol. 202 (2008) 3121.[34] V.H. Nguyen, F.X. Perrin, J.L. Vernet, Corro. Sci. 47 (2005) 397.[35] J.M. Yeh, H.Y. Huang, C.L. Chen, W.F. Su, Y.H. Yu, Surf. Coat. Technol. 200 (2006) 2753.[36] S. Zafar, U. Riaz, S. Ahmad, J. Appl. Polym. Sci. 107 (2008) 215.。
