Archivum Combustionis Vol. 30 (2010) no. 4
Montmorillonite as a Polyurethane Foams Flame Retardant A. Ubowska Department of Ship Safety Engineering, West Pomeranian University of Technology, Szczecin al. Piastów 41, 71-065 Szczecin, Poland email:
[email protected]
Polyurethane foams are used in many applications for example in automotive industry. They are attractive especially because of cushioning and physical properties and easy handling. The use of polyurethanes for car, train or railway seats is determined by their behaviour during fire conditions. To improve the safety of polyurethane foams usage, flame retardants are applied for their production. The most popular are polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE), reputed as possibly environmentally hazardous (possibility of carcinogenesis by accumulation in the human body). Car producers like Volvo or Scania have published “Grey and black chemical list” to limit and eventually phase out hazardous substances from products and production process. Among these substances are PBB and PBDE [1, 2]. To solve this problem research for new fire retardants for polyurethane is intensified. In recent years some research has focused on montmorillonite (MMT) based fire retardants. MMT is one of the 2:1 type phyllosilicate clays with exchangeable cations in the interlamellar galleries. Its use in polymeric matrix improve mechanical properties, gas barrier performance and thermal properties. Polymer clay nanocomposites have been used by Toyota for barrier and under-hood engine parts since the 1990s. Recent usage in the 2004 Hummer H2 and the Chevrolet Impala are noted [3]. The improvement of fire retardant and thermal stability can be achieved when modified clay is applied. The exchangeable cations can be replaced with phosphonium [4], onium ions [5] or other.
Flexible polyurethane foams Flexible polyurethane foams (PUR) are widely used in upholstered chairs and mattresses. A combination of low thermal conductivity and high effective heat of combustion renders pure flexible PUR foam easily ignitable and results in a fast flame spread. The combustion of flexible PUR foam is strongly affected by its physical behavior [6]. The combustion of polyurethane foams is a two step process: (i) the first step is attributed to the degradation of the foam to produce the tar, (ii) the second step is attributed to the combustion of the tar produced. The fire behaviour of a foam is weakly related to the isocyanate index, the surfactant as well as catalyst content. The blowing agent mass fraction is the major factor modifying the combustion of a foam [7].
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Flexible polyurethane foams flame retardants in actual commercial use are mostly phosphate such as: tris(chloroisopropyl) phosphate, tris(dichloroisopropyl) phosphate, oligomeric chloroethyl ethylene phosphate, isopropylphenyl diphenyl phosphate, highmolecular-weight ammonium polyphosphate and others [8]. Montmorillonite Montmorillonite belongs to smectite group of clay minerals which has 2 : 1 type of layer structure (Fig. 1). It consists of negatively charged silica sheets which are held together by charge-balancing counterions such as Mg2+, Na+ and Ca2+. The general formula of MMT is (M+y ×nH2O)(Al4-yMgy)Si8O20(OH)4, where M (M = Na+, Ca2+, Mg2+ etc) is interlayer cation (Fig. 1). These interlayer cations balance the negative charges which are generated by the isomorphous substitution of Mg2+ and Fe2+ for Al3+ in the octahedral sheet and Al3+ for Si4+ in tetrahedral sheet. The interlayer cations can be replaced easily by either organic or inorganic molecules through a process called intercalation [9]. Organo-modified MMT (OMMT), is produced via ion-exchange reactions. The aim of this modification is to encourage monomers or polymer chains to diffuse (intercalate) into the interlayer spacings, swelling the layered structure to form intercalated lamellae stacks [10].
Fig. 1 Schema of sodium montmorillonite structure (www.bentonites.net)
Montmorillonite as a Polyurethane Foams Flame Retardant
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Polyurethane foam/montmorillonite PU foam/MMT nanocomposite can be prepared using a two step procedure. In the first step a fixed amount of montmorillonite (up to 10 wt.%) is dispersed in polyols mixture. After filler dispersion in polyol, the catalysts, surface-active agent, and blowing agents are added to the polyol–filler mixture and stirred. The isocyanate is added to the formulated polyol and the two components are mixed and poured into an open mould for free rise polymerization. The naturally occurring clays are of hydrophilic character and require modification by intercalating with amino acids, alkyl ammonium or phosphonium salts or with cationic surfactants to make them hydrophobic and organically compatible. Otherwise, phase segregation and filler agglomeration results, which decreases the overall performance of the PU composite. Thus, it is desirable to use stable and organically modified hydrophilic clay particles as fillers. The exchangeable cations can be replaced with phosphonium, onium ions or other. For example: -- Wang and Pinnavaia used montmorillonites exchanged with long-chain onium ions (carbon number > 12); -- Chang and An used three different organoclays such as hexadecylamine– montmorillonite (C16–MMT), Cloisite 25A and dodecyltrimethyl ammonium– montmorillonite (DTA–MMT). The effect of clay as a thermal insulator and mass transport barrier can be enhanced by improving the dispersibility of the organoclay [11]. Polyurethane foam/montmorillonite combustion process In order to stop, delay or retard the burning process of polyurethanes a flame retardant may be added. The flame retardance mechanism – the interference with the combustion process – can function either in the condensed phase or in the vapor phase through a physical mechanism, a chemical mechanism or a combination of these mechanisms. Combustion is a complex process and different mechanisms may be involved in the presence of different types of flame retardant. Figure 2. shows the processes occurring during polyurethane combustion without (Fig. 2 a) or with flame retardant (Fig. 2 b). Generally the incorporation of inorganic fillers dilute the polymer, produce a stable organic–inorganic interface, reduce the concentration of decomposition gases and increase the diffusion path barrier of the volatiles produced during the degradation process [11]. The use of nanoclays generally leads to an increase in thermal stability through a barrier effect, bringing a delay of thermal degradation products’ release [4]. It has been observed that in nanocomposites of polymers with MMT, which includes a high percentage of MMT, a surface barrier is formed upon pyrolysis and combustion [12]. Unmodified and
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organically modified MMT, act through a physical rather than a chemical mechanism. The thermal stability and fire resistance properties of polyurethane foam/clay nanocomposite are enhanced when a higher degree of dispersion of filler is achieved (i. e. using ultrasound treatment during montmorillonite modification with isocyanate) [4]. The presence of MMT in polymer-clay nanocomposite (1 to 5 wt%) can cause reduction in peak heat release rate (HRR) of 40 to 65% [3].
Fig. 2 The processes occurring during combustion of polyurethane a) without flame retardant-resulting large heat release and (b) with flame retardant-resulting low heat release [10]
References [1] [2] [3] [4]
SCANIA Standard STD 4158 (2007). Standard Volvo Group STD 100-0002 (2006). Morgan A. B., Polym. Adv. Technol. 2006, 17, 206. Modesti M., Lorenzetti A., Besco S., Hrelja D., Semenzato S., Bertani R., Michelin R. A., Polym. Degrad. Stab. 2008, 93, 2166. [5] Cao X., Lee L. J., Widya T., Macosko Ch., Polymer 2005, 46, 775. [6] Krämer R.H., Zammarano M., Linteris G.T., Gedde U.W., Gilman J.W., Polym. Degrad. Stab. 2010, 95, 1115. [7] Lefebvre J., Bastin B., Le Bras M., Duquesne S., Ritter Ch., Paleja R., Poutch F., Polymer Test. 2004, 23, 281. [8] Weil E. D., Levchik S. V., J. Fire Sci. 2004, 22, 183. [9] Pramanik M., Srivastava S. K., Samantaray B. K., Bhowmick A. K., J. Mater. Sci. Lett. 2001, 20, 1377. [10] Wilkinson A. N., Fithriyah N. H., Stanford J. L., Suckley D., Macromol. Symp. 2007, 256, 65. [11] Chattopadhyay D.K., Webster D. C., Prog. Polym. Sci. 2009, 34, 1068. [12] Lewin M., Fire Mater. 2003, 27, 1.
