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Study on preparation and fire retardant mechanism analysis of


Surface & Coatings Technology 201 (2007) 7835 – 7841 www.elsevier.com/locate/surfcoat

Study on preparation and fire-retardant mechanism analysis of intumescent flame-retardant

coatings
Jun-wei Gu ?, Guang-cheng Zhang, Shan-lai Dong, Qiu-yu Zhang, Jie Kong
Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an, Shannxi Province, 710072, PR China Received 2 May 2006; accepted in revised form 12 March 2007 Available online 21 March 2007

Abstract An intumescent flame-retardant coating was prepared by unsaturated polyester resin and epoxy resin as two-component matrix resins, ammonium polyphosphate (APP) as acid source, melamine (Mel) as the blowing agent and pentaerythritol (PER) as carbon agent, expandable graphite as synergistic agent, adding titanium dioxide (TiO2), solvent and other assistants. Results showed that such a coating had excellent physical–chemical properties. When the thickness of the coating on the wood matrix reached 2.0 mm, the limit of fire-endurance could get to 210 min. And the various component thermal characteristics, decompose processes and interactions of the flame-retardant coating system were investigated by DSC and TGA. The contribution of phosphorus to the formation of the final charring layer and their morphological structures was studied by SEM, XRD and FTIR. On the basis, the flame-retardant mechanism of the intumescent flame-retardant coating was systematically investigated. ? 2007 Elsevier B.V. All rights reserved.
Keywords: Intumescent flame-retardant coating; Thermal characteristics; Charring layer; Flame-retardant mechanisms

1. Introduction The protection of materials against fire has become an important issue in the construction industry. The use of intumescent flame-retardant coating is one of the easiest, economical and the most efficient ways to protect materials against fire [1,2], and it is easily processed and may be used onto wood [3]. It presents two main advantages: it can prevent heat from penetrating and flames from spreading. Moreover it does not modify the intrinsic properties of the materials (e.g. the mechanical properties) [4,5]. The action of the flame retardant can occur across both or either of the vapor phase and the condensed phase. The combustion is a complex process: there may be different mechanisms with different matrix resins and different flame retardants. However, in our knowledge, the mechanism analysis of the flame-retardant coating is seldom reported at present.

In this paper, an intumescent flame-retardant coating was prepared by unsaturated polyester resin and epoxy resin as twocomponent matrix resins, APP as acid source, Mel as the blowing agent and PER as carbon agent, expandable graphite as synergistic agent, adding titanium dioxide (TiO2), solvent and other assistants. The thermal characteristics, decompose processes and interactions of flame-retardant coating system were investigated via DSC and TGA. The contribution of phosphorus to the formation of the final charring layer and their morphological structures was studied by SEM, XRD and FTIR. On the basis, the flame-retardant mechanism of intumescent flame-retardant coating was systematically investigated. 2. Experimental 2.1. Materials Unsaturated polyester resin (commercial grade) supplied by Wujin TianLong Chemical Co., Ltd, Wuhan, China; epoxy resin E-51 (commercial grade) purchased from Xi'an Epoxy Resin Factory, Xi'an, China; ammonium polyphosphate (commercial

? Corresponding author. Tel./fax: +86 29 82375990. E-mail address: nwpugjw@163.com (J. Gu). 0257-8972/$ - see front matter ? 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.03.020

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Fig. 1. The process of preparation of the coating.

grade) supplied by Hang Zhou JLS Flame Retardants Chemical Co., Ltd, Hangzhou, China, and its polymerization degree exceeds 1000; pentaerythritol (chemically grade) purchased from Shanghai Chemical Reagent Co., Ltd, Shanghai, China; melamine (analytically grade) supplied by Tianjin Bodi Chemical Co., Ltd, Tianjin, China; titanium dioxide (commercial grade) purchased from Chu Zhou GeRui Mining Co. Ltd, Chuzhou, China. 2.2. Preparation of intumescent flame-retardant coating Process of preparation of the intumescent flame-retardant coating is presented in Fig. 1. 2.3. Analysis and characterization 2.3.1. Thermogravimetric analysis (TGA) The thermogravimetric analyses of samples (approx. 10 mg) were carried out at 10 °C/min under N2, over the whole range of

temperatures (50–800 °C) by TGA Q50. And the curves of TGA were disposed by TA Universal Analysis software. 2.3.2. Differential scanning calorimentry (DSC) The DSC analyses of the samples (approx. 5 mg) were carried out at 10 °C/min under N2 by TA Instrument-2910 DSC, and the curves of DSC were disposed by TA Universal Analysis software. 2.3.3. Scanning electric microscope (SEM) Charring layer and their morphological structures were observed and analyzed by AMARY1000 SEM. 2.3.4. X-ray diffraction (XRD) The white materials of charring layer were carried out by X'PRO XRD equipment produced by PANalytical Corp. in Holand to know about the microcosmic structure and analyze the chemical components, with copper as the target, pressure/ current 40 kV/35mA. 2.3.5. Fourier Transform Infrared Ray (FTIR) The burned charring layer of coatings in the form of powder was planished to be samples with KBr. The changes of groups in the system were analyzed by FTIR, the rate of differentiate is 4 cm? 1 and the number of scanning is eight.

Table 1 The prescription of flame-retardant coatings (mass ratio, %) Components Unsaturated polyester resin Styrene Cobalt naphthenate Methyl ethyl ketone peroxide Epoxy resin Triethylenete tramine Mass/g 25.0 0.50 0.04 0.50 6.25 0.80 Components Olefin solution Ammonium polyphosphate Pentaerythritol Melamine Fillers and assistants Solvent Mass/g 0.25 28.00 10.50 17.50 8.00 2.66

Fig. 2. Experimental set-up of measurements.

J. Gu et al. / Surface & Coatings Technology 201 (2007) 7835–7841 Table 2 The results measured by the simulative board-burning method Coating number 1 2 3 4 5 Thickness/mm 2.02 2.00 1.98 2.01 1.99 Time of fire-endurance/min 225 230 220 228 216

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2.4. Fire resistance The experimental set-up which can measure the performance of the flame-retardant coating is presented in Fig. 2. 3. Results and discussion 3.1. Flame-retardant performance of the coating The prescription of flame-retardant coating is preferably finished by frequent orthogonal experiments, and the results are listed in Table 1. The flame-retardant coatings were determined by the current technical condition of veneer flame-retardant coatings. The results presented in Table 2 showed that the time of fireendurance could exceed 210 min. 3.2. TG analysis The TG curves of APP, PER, Mel and coating are presented in Fig. 3. From the TG curves in Fig. 3, it can be seen that APP has nearly no any weight loss before 290 °C. The weight loss gets to 16% over the range of 290–500 °C due to APP decomposing to release the gas of NH3 and H2O. The weight loss gets to 81% over the range of 500–700 °C, which can be contributed to the releasing of phosphoric acid, poly-phosphoric acid and polymetaphosphoric acid, meantime, poly-metaphosphoric acid is vaporized with APP decomposing. The weight loss gets to 88% around 790 °C. It can be also seen that PER begins to decompose and loses weight over 277 °C. The weight loss is mostly due to PER dehydrating intra-molecularly or intermolecular, then dehydrogenizing, charring and rupture of chemical bonds. The weight loss of Mel gets to 100% under 370 °C which relates to the decomposing of Mel per se and NH3 releasing. The ammonia released by Mel can dilute the oxygenous concentration, blow the charring layer and form intumescent, microporosite foam charring layer. From Fig. 3, it is also shown that the weight loss of the coating is 1% at the beginning of the experiment (20–230 °C), the moment is mostly due to fraction substance and resin decomposing, rudimental solvent and other volatilization vaporizing. The medium-term of the experiment (230– 510 °C) is the key weight loss region that the coating begins to decompose largely, the weight loss gets to about 54%. The coating melts, APP decomposes to release NH3, H2O and
Fig. 4. DSC curves of APP–PER–Mel flame-retardant system.

Fig. 3. TGA curves of APP, PER, Mel and coating.

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Fig. 5. DSC curves of flame-retardant system and the coating.

Fig. 7. SEM images of charring layer (APP:PER:Mel = 8:3:5).

phosphoric acid (the catalytic effect is considered as a benefit since to be efficient the intumescent protective layer has to be formed in the early stages of a fire. For both resin/APP mixtures, a thermally stable protective layer is formed later on in the higher temperature range) thermally degrades, dehydrates to release poly-metaphosphoric acid and pyrophosphoric acid, moreover reacts with pentaerythritol and other organic materials which contains hydroxy groups, dehydrates, to form the charring framework. At the same time, the vesicant ammonia begins to release NH3 gas over 296 °C. The NH3 gas blows the charring layer to form a compact, firm and black charring layer. The weight loss of the coating is about 18 percent in the later stages of the experiment, the moment, an opposite high temperature and high pressure air current occasion was formed on surface of the coating, the carbon in the charring layer is oxygenized to CO2 by O2. A part of charring layer is off by gas because of the weak strength of adhesion. Some inorganic framework maintains at last, the most important components of the rudimental is the matter of inorganic phosphate [6]. The matching of decompose temperature (Td) of Mel, APP and PER is the key factor to form an ideal charring layer. If the Td of Mel ? APP, the gas will escape before forming charring layer and can't play an important part in the intumescent state. If the Td of Mel ? APP, the gas will blow off the charring layer and can't form a good froth layer. At one time, if the Td of PER ? APP, PER has decomposed entirely before thermal decomposing of APP, and not to form good froth layer. From the analysis

of the thermal stability of components above, it is clear that the decompose temperature of Mel, APP and PER is contiguous. Bugajny [7] assumed that the simultaneous polymerization reaction decomposing was independent of intumescent actions on the basis of EGA and TG/DTA analysis. They found that the decompose temperatures of PER and APP were contiguous and intumescent phenomena appeared over the range of the whole decompose temperature. It was observed that three flameretardant assistants had an optimal matching in the intumescent reactions. 3.3. DSC analysis By DSC analysis of the APP–PER–Mel system and the coating, the thermal properties of the physical chemistry changing in the complex swelling phenomena are discussed (Fig. 4). The two endothermic peaks of the crystal type transition (cubic crystal type changing into cube crystal type) of PER are presented in Fig. 3. The former (8:3:5) heat enthalpy value (105.4 J/g) was larger than the latter (52.32 J/g) under the same condition. It indicates that both endothermic effect and flameretardant effect are better.

Fig. 6. Structures of charring layer: (a) uniformity and (b) asymmetry.

Fig. 8. XRD chart of inorganic layer.

J. Gu et al. / Surface & Coatings Technology 201 (2007) 7835–7841 Table 3 The corresponding data of XRD chart Visible ? Score 49 Compound name Titanium pyrophosphate Scale factor 0.957 Chemical formula TiP2O7

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In the curve of APP–PER–Mel (8:3:5), the maximal endothermic peaks of the crystal type transition of PER are 189 °C, and the corresponding heat of phase transition is 105.4 J/g. The several small endothermic peaks that followed are irregular. There are no apparent endothermic or heat releasing phenomena and we can testify that a series of complex changes has been occurred at the range of the temperature. Only a small quantity of reaction mixtures transforms into solid foam charring layer. However the majority loses up in the forming of gas production. The lack of matrix materials influences the flame-retardant performance and the matrix plays a very important role in the course of frothing. The two comparing DSC curves of the flame-retardant system (8:3:5) and the coating are presented in Fig. 5. There are two clear endothermic peaks in Fig. 5. The former is still corresponding to the endothermic peak of PER. The endothermic peak over 260 °C relates to the melting of PER, or to the adding matrix resins in the system. Different from the curve of the flame-retardant system, the latter part of the curve of the coating has changed a lot in shapes and trending. It demonstrates that the matrix resins influence the flame-retardant performance of the coating largely, and the flame-retardant system plays a definitive action. Because the matrix resins can well control the gas dispersing, it is helpful for the formation and the structure of the charring layer in the whole course. Therefore, the coating holds hard combustibility and excellent intumescent effect over the fire or high temperature. 3.4. SEM analysis The charring layer protects the matrix materials, and its protective property depends on the physical and chemical structure

Fig. 10. SEM image of JLS-APP.

of the charring layer. Researches indicate that there are other elements in the charring-layer structure. But the non-charring element is easy to be oxygenated and very unstable on chemistry aspects. There are two ideal typical charring structures listed as follows. There are a great deal of integrated closed honeycomb pores in Fig. 6(a); such structure can form adequate temperature grads in the charring layer and protect the molten mass and matrix below. But the other structure is nonideal; there are many channels and apertures, and the gas and molten mass of polymers can overflow to the entry of the flame-region. Thus the isolation effect of heat transfer is inferior [8]. The physical structure of the charring layer plays a very important role in the performance of the flame retardant [8]. The formation of the final charring layer and their morphological structures is studied by SEM and the charring-layer SEM of the charring layer (APP–PER–Mel = 8:3:5) is presented in Fig. 7. From Fig. 7, as the weight ratio of APP, PER, and Mel is 8:3:5, the structure of the charring layer inclines to be more compact and

Fig. 9. XRD chart of JLS-APP.

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The above study shows that JLS-APP is a mixture including many phosphates, its ideal II crystalline type and high polymerization degree (n N 1000) ensure low-water-solubility, preferable decentralization and excellent waterproof performance. In the meantime, JLS-APP possesses high heat stability and without transference performance. The flame-retardant coating with the addition of JLS-APP has a good adhesive attraction, forms steady intumescent charring layer under flame, prevents the coating from falling off and ensures the flameretardant performance. 3.6. FTIR analysis To further investigate the flame-retardant activity and mechanism of the coating, the burned residue of the coating is monitored by FTIR spectra in Fig. 12. Fig. 12 shows the residues contain a few group but inorganic simple substances. The weak characteristic vibration peak at 3400 cm? 1 (–N–H–), which implies the most organic materials have completely disappeared, only remains lots of inorganic charring layer and a few phosphate, amide typing compounds. The bands near 1105 cm? 1 can be assigned to the characteristic 3? vibration peaks of PO4 group. Phosphides make some organic materials drastic degrade irregularly, reduce flammable gas, and make the high-carbon compounds dehydrate, char to form compact charring-layer structure by bridging reactions in the heat degradation in the solid phase. The stretching vibration peaks of the P–O–C band from 1000 cm? 1 to 1100 cm? 1 and P=O band from 1200 cm? 1 to 1250 cm? 1 can keep shifting to lower frequency sharply, respectively. The trend manifests that phosphorous materials reduce sharply with the combustion. 4. Conclusions 3.5. XRD analysis The main conclusions can be described as follows: There is another white material formed on the surface of the charring layer besides the amorphous carbon in the burning. The white material reinforces the strength of the charring layer, makes the charring layer hard to subside and takes the effect of flame-retardant performance. The more the quantity and the more homogeneous the material, the longer the fire-endurance time, and the better performance of the flame retardant of the coating [9]. The XRD analysis of the white material is presented in Fig. 8 and the corresponding data are listed in Table 3. The conclusion displays that the main component of the white material is the keen-titanium typical TiO2, the gold-red typical TiO2 is fairly scarce. Based on some correlative literatures, observing the real white material and analyzing the XRD chart, we can conclude that the white material is made up mostly of TiO2 and TiP2O7. In fact, the corresponding strong peak in the Fig. 8 is the result of the diffraction peak piled up TiP2O7 and keen-titanium typical TiO2. The main reaction in the stage presents as follows: TiO2 Y TiP2 O7 ? NH3 ? H2 O And the dehydrate catalyst JLS-APP is investigated via XRD, SEM and NMR (Figs. 9–11).
Fig. 12. FTIR wave of burned residue of coatings.
?NH4 ?4 P4 O12

Fig. 11. NMR chart of JLS-APP.

more homogeneous, the intension of charring layer improves largely, and the effect of the flame retardant is better. There are many irregular mini-pore structures of spongy foams in the charring layer from Fig. 7. It explains the dehydration charring of PER and frothing of Mel proceeds in the range of rather appropriate temperature. Moreover, the intumescent layer is compact and spongy, and the heat insulation effect. The different aperture surface tensions in the course of gas cavities lead to the asymmetry of abscess, and the surface tension relies on the viscosity and symmetry of the coating. The intumescent charring layer with many mini-pores acts as the effect of the flame retardant, heat insulation and protecting inner matrix materials.

(1) JLS-APP is a mixture including many phosphates, its ideal II crystalline type and high polymerization degree (n N 1000) ensure low-water-solubility, preferable decentralization and excellent waterproof performance. In the meantime, JLS-APP possesses high heat stability and without transference performance. The flame-retardant

J. Gu et al. / Surface & Coatings Technology 201 (2007) 7835–7841

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coating with the addition of JLS-APP has a good adhesive attraction, forms steady intumescent charring layer under flame, prevents the coating from falling off and ensures the flame-retardant performance. (2) The flame-retardant assistant is the key factor to influence the performance of the flame-retardant coating. The thermal decompose temperature of the dehydration catalyzer reagent and vesicant should be very similar to the melting temperature of the charring reagent. During burning, the coating begins to intenerate and melt, APP decomposes to release poly-metaphosphoric acid and phosphoric acid which lead the hydroxyl of PER to dehydrate, char. Meantime, Mel decomposes to release NH3, plumps up the melting coating, and forms the homogeneous and compact charring layer to protect the matrix material well. The whole course takes its flameretardant effect by gas phase flame-retardant mechanism. (3) The SEM analysis indicates charring layer is compact and has excessive cavities. The heat insulation effect relates to the formation of swell extent and excessive cavity structures of the charring layer. The XRD analysis indicates the white material on the surface of charring layer is made

up mostly of TiO2 and TiP2O7. The white material benefits the performance of the flame retardant and the thermal insulation and protects inner matrix materials. The white material takes its flame-retardant effect by condensed phase flame-retardant mechanism. References
[1] H.L. Vandersall, J. Fire Flamm. 2 (1971) 97. [2] J.A. Rhys, Fire Mater. 4 (1980) 154. [3] J.H. Koo, W. Wootan, W.K. Chow, H.W. Au Yeung, S. Venumbaka, Fire and Polymers, ACS Symposium Series, vol. 797, 2001, p. 361. [4] Guang-cheng Zhang, Jun-wei Gu, J. Mater. Eng. 1 (2006) 47. [5] S. Duquesnea, S. Magnetb, C. Jamaa, R. Delobelc, Surf. Coat. Technol. 180–181 (2004) 302. [6] Jun-wei Gu, Guang-cheng Zhang, Shan-lai Dong, et al., Paint Coat. Ind. 35 (12) (2005) 26. [7] M. Bugajny, M. Le Bras, Fire Mater. 23 (1999) 49. [8] Jun Zhang, Kui-jian Ji, Yan-zhi Xia, Polymer Combustion and Fireretardant Technology[M], 21–22, Chemical Industry Press, Beijing, 2005, p. 94. [9] Xin-yan Xiao, Zhuo-ru Yang, Huan-qin Chen, Chem. Ind. Eng. 16 (2) (1999) 73.


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