CHAPTER 11 |
 |
Fire-Retarded Environmentally Friendly Flexible Foam Materials Using Nanotechnology
Richard A. Pethrick
WestCHEM, Department of Pure and Applied Chemistry,
University of Strathclyde, Glasgow, Scotland
Contents
2. Analysis of the Various Stages in Which a Fire is Created
3. How can Nanotechnology be Used to Help Control Fires?
4. Can Such a Structure be Created in Practice?
5. Do We Need Anything Else to Make the Foam Fire Resistant?
1. INTRODUCTION
Fire kills many thousands of people a year. Death is usually a consequence of asphyxiation rather than a consequence of burning. Dense smoke and fumes emitted during the early stages of a fire can cause disorientation of person caught in the fire. High levels of carbon monoxide, carbon dioxide, and to a lesser extent sulphur dioxide, hydrogen chloride, hydrogen cyanide, and various gases, which reflect the chemical nature of the combustible fuels, are the principle cause of asphyxiation. Forensic analysis of a fire situation will usually indicate that some source of heat initiated the event. The source may be electrical wires or heaters, which have overheated, a lighted match, which is left to smolder, a cigarette butt, which was not fully extinguished, etc. In most cases, the fire is created by a flammable object fueling the fire and causing it to spread. It is therefore critical that the susceptibility of materials to undergo combustion is reduced when they are used in a domestic environment.
2. ANALYSIS OF THE VARIOUS STAGES IN WHICH
A FIRE IS CREATED
To design a material that will be effective in suppressing combustion, it is useful to understand the various stages involved in the growth of a fire. We can separate the growth of the fire into three stages.
2.1. Precombustion
In this stage, the object that will form the fuel for the fire is heated by the source. Usually, the effect of heating will be to cause the material to soften and even melt. In the melt phase, the material may experience temperatures, which are above the ceiling temperature, and degradation of the material may occur. In the case of a polymer, the degradation may yield the monomer from which the polymer was formed or some other organic material of molar mass of the order of 60–200 daltons. These volatile fragments may now be at a temperature which is above the required for spontaneous combustion and a fire can start. In the initial stages, there will be sufficient oxygen present to sustain combustion and a cold flame will be generated (Fig. 11.1).
Figure 11.1 Schematic of the stages of combustion.
2.2. Initiation of Combustion
The cold flame that is created depends on a constant supply of the low molar mass fragments and plenty of oxygen. This flame can be extinguished by trapping the various radicals that are formed and by suppressing the combustion chemistry. Many fire retardants work by interrupting the combustion chemistry and damping the flame. If the combustion chemistry is allowed to establish itself, the nature of the reactions may change and become less reliant in the presence of oxygen. In a major fire, the process is able to draw air into the fire, but the process is less dominated by the oxygen chemistry. A very important factor in this stage of the growth of the fire is the amount of heat, which is radiated back onto the sample. The heat from the combustion can further melt the material and hence add fuel to the fire (Fig. 11.1). One way of suppressing the growth of a fire is to create an insulating layer on the top of the material, which is fueling the fire. Char, which is naturally formed during the degradation process, is in fact a good insulator and can effectively produce a thermal barrier and also suppress the evolution of volatiles from the material. The combination of both these effects can lead to a suppression of the growth of the fire.
2.3. Self-sustained Combustion
Once the sustained combustion process has been achieved, the growth of the fire will depend on a supply of fuel to the flame. If the volume of combustible material is small, the fire may burn to a point where it ceases to be sustainable and will go out. In practice, the flames are usually propagated by either traveling across the surface of a material or, alternatively, dropping material into the flames and increasing the volume of the material available for the fire. Some chemical additives, because they suppress the initiation of combustion, are good at controlling flame spread. Forensic analysis of a fire situation often indicates that a molten material is dripped into the source of the flame. It is therefore desirable to stop dripping, and this, in principle, can be achieved by making the polymer melt viscoelastic.
3. HOW CAN NANOTECHNOLOGY BE USED TO HELP CONTROL FIRES?
The critical stages in the development of a fire are precombustion and initiation of combustion. Although the self-sustained combustion stage is very important, at this stage the fire is usually growing at a rate that makes simple chemical control very difficult. So, what can we do with nanotechnology that might aid to control the growth of the precombustion and early stage combustion process? In the case of foam, we are dealing with large surface areas that are ideal for the fast release of volatile components. If we wish to control the rate of release of these low molar mass fragments, it would be desirable to introduce into the material a barrier layer. Nanotechnology provides us with that possibility. A further aid to suppression of fire is the promotion of char formation. Certain nanomaterials have a potential of aiding char formation through their ability to assist recombination reactions of low molar mass radical fragments in the solid phase. A further advantage of nanomaterials is their ability to enhance the viscosity of the melt, and this adds the desirable viscoelastic characteristics that will suppress dripping. The desirable nanomaterial would have a platelet structure and form a barrier structure in the solid, which would suppress low–molar mass emissions from escaping from the solid as it melts (Fig. 11.2).
Figure 11.2 Schematic showing the nanoplatelets organized in the walls of the foam structure.
4. CAN SUCH A STRUCTURE BE CREATED IN PRACTICE?
Over the last 10 years, a lot of interest has been directed toward the study of nanoorganically modified clay composites [1]. Clay is a naturally occurring nanomaterial that has been used for many years as a cheap additive to reduce the cost of articles made from plastics. Clay is constituted of stacks of aluminosilicate layered structures, the thickness of the layer being ~1 nm (Fig. 11.3). The chemistry of clay reflects the geological conditions that existed during its creation. One particular form montmorillonite contains low levels of impurity ions, e.g., Fe, Mg, and Li. Incorporation of these ions into the layer leaves a charge deficiency that creates the binding site for the sodium ions. The characteristics slip of clays reflects the ability for platelets in a stack to be moved relative to one another when a shear force is applied. However, the relatively strong electrostatic forces keep the stacks together and clay is usually observed as nodules of approximately 1–5µ. The platelets are of length 400–100 nm and the balance of forces favors a stacked structure.
Figure 11.3 Schematic of a clay stack, showing expanded view of a typical layer formed from aluminosilicates held together by a layer of sodium ions.
The major breakthrough with these materials came with the recognition that it was possible to exfoliate and disperse these individual platelets if the sodium ions were replaced by quaternary ammonium ions [2].
There is a very large literature on the physical property enhancement that can be achieved when the organically modified clay platelets are dispersed in the matrix [1, 2]. Flexible foams are usually obtained from polyurethanes (PUs), which is produced by the reaction of a polyether with a diisocyanate. Depending on the nature and proportions of the polyether and the diisocyanate, flexible or rigid foams will be created. Methylene diphenyl isocyanate is typically used for more rigid materials, whereas toluene diisocyanate is used predominantly for flexible foams. The nanoclay can be readily incorporated into the PU by predispersion in the polyether prior to reaction with the isocyanate [3]. To aid dispersion, it is often useful to use ultrasound. The ultrasound can interact with the clay stack, and effective exfoliation of the platelets can be achieved. A problem with the efficient exfoliation of the clay is that the platelets can form a transient structure in the liquid and significantly enhance the viscosity (Fig. 11.4).
The platelets have, as a consequence of the charge deficiency associated with the impurity ions, an “effective” negative charge on the surface. It is this negative charged region on the platelets surface which in nature clay attracts the sodium ions and in the organically modified clay is the location for the quaternary ammonium ions. What is not always recognized is that the edges of the clay platelets will also have a charge associated with the growth sites for the inorganic matrix. These edge sites will tend to have a preponderance of positive charges. The favorable interaction of the positively charged edges with the negatively charged sites on the surface of the clay platelets allows the creation of a network structure. This phenomenon has been recognized for many years and is the basis of the thickening action of clay and related system to produce pastes. Although this effect is desirable in a tooth paste, it is not at all desirable in the polyether that has to be effectively mixed with the isocyanate to form the flexible foam. To recover the desirable low viscosity, the electrostatic interactions need to be blocked. Addition of organically modified titanium compounds have been found to be effective in achieving the reduction of the viscosity (Fig. 11.4).
Figure 11.4 The initial viscosity and reduction of the viscosity on the addition of titanium-based coupling agents. Key: no coupling agent (−), 0.8% coupling agent (---), 1.5% coupling agent (– – –), 2.0 % coupling agent (—).
Using these nanomodified polyether, it is possible to create structures that mimic (Fig. 11.2). Electron microscopic examination of the walls of the flexible foams indicates that the blowing process has aligned the platelets to form the pseudo nematic order, which is desirable to create a barrier for the emission of the low–molecular mass fragments created when the polymer is heated (Fig. 11.5).
5. DO WE NEED ANYTHING ELSE TO MAKE THE FOAM FIRE RESISTANT?
The nanoclay by itself is able to reduce the emission rate of low–molecular mass fragments, but this is usually not sufficient to make the foam adequately fire resistant to pass a Crib 5 test [4]. The test involves placing two pieces of foam in the form of a chair and covering these with a loose-textured cloth. Onto the base of the chair is placed a small, square crib formed from 20 pieces of wood so as to form a structure that is five rungs high. In the center of the crib is placed a piece of cotton wool soaked in isopentane. The cotton wool is ignited, and for the foam to pass the test, the fire must be constrained to the locality of the crib and there must not be more than 15% of the foam destroyed in the test. This is a very stringent test of a material as the crib will sustain the source of heat for several minutes and mimics the sort of fire that would be created by a cigarette butt.
Figure 11.5 Electron micrograph of a nanomodified flexible foam (A) and a close up of the wall structure (B). The dark lines are the clay platelets aligned in the wall structure to form a barrier for low–molar mass emission.
To get a nano-modified flexible material to pass the Crib 5 test, it is usually necessary to add components that will aid char formation and also suppress flame spread. Char formation can be promoted by a range of inorganic nonvolatile agents that are able to interact with radicals formed during the degradation. The large surface area of the clay can be synergistic in this action and increase the efficiency of the char formation process. Once a char is formed, it will protect the polymer beneath and reduce the rate of low–molar mass emissions. This process is more efficient in rigid foams and less char promoter is required to achieve the required fire resistance. Control of the spread of the fire requires influencing the vapor-phase chemistry. Once more, this can be achieved using solid species that are only volatilized once the foam is burning. Combining a number of elements in a synergistic manner, it is possible to achieve fire-retardant characteristics without the use of the usual volatile organic compounds. This technology is disclosed by patents held by the author [5, 6].
6. SUMMARY
By the careful design of formulations and the use of an understanding of nanotechnology, it is possible to create flexible foams that have the desired fire-retardant characteristics for them to pass the Crib 5 test. The nanotechnology reduces the rate of emission of the degradation products and is synergistic in the formation of the char and reduces dripping of the molten polymer, hence reducing the spread of the fire. By itself, it is unable to achieve fire retardancy, but when combined with other agents, it achieves the desired effect. Nanotechnology has a significant role to play in the development of fire-retardant technology for the future.
ACKNOWLEDGMENTS
The author thanks EPSRC and Scottish Enterprise through its provision of a Proof of Concept grant for their support. This study was carried out in collaboration with Dr Liggat JJ, Dr Daly JH and Dr Rhoney I, and support by Mr McCulloch L.
References
[1] Krishnamoorti R, Silva AS. Rheology of polymer layered silicate nanocomposites. In: Pinnavaia TS, Beall GW, editors. Chapter 15. Chichester: Wiley Polymer Science; 2001. p. 316–43.
[2] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8:29–356.
[3] Rhoney I, Brown S, Hudson NE. Pethrick RA. Influence of processing method on the exfoliation process for organically modified clay systems. I. Polyurethanes. J Appl Polymer Sci 2004;91(2):1335–43.
[4] British Standard 5852 (Section 4) Part II, Methods of test for assessment of the ignitability of upholstered seating by smouldering and flaming ignition sources. 1990.
[5] Liggat JJ, Daly JH, McCulloch L, Pethrick RA. British Patent 0713394.5, 2007.
[6] Liggat JJ, Rhoney I, Pethrick RA. World Patent 2006/003421, 2005.