CHAPTER TWO
LITERATURE REVIEW
2.1 Fire, Pyrolyses and Combustion
Fire is the reaction involving fuel and oxygen that produces heat and light [9]. It results from rapi d chemical reaction between a fuel (wood, gasoline) or polymeric materials (plastic, cellulose) and oxygen. In order to produce fire a combustible materials and oxygen must be present and in contact at sufficient high temperature to initiate combustion. The two substances (oxygen and fuel) must continue to be in contact for combustion to be sustained.
A flame is a gas phase combustion reaction which is able to propagate through space [10]. Fire is a common term for combustion especially when out of control combustion refers to exothermic reaction in any phase. It usually implies propagation and oxidation. In most combustion processes the exothermic stages occur in the gas phase regardless of the initial phases of the reactants. Therefore , flames are associated with most combustion processes. The combustibility of a substance depends on its chemical composition and physical state [3,10]. For instance, if the source of oxygen is air, then the molecules of any flammable gas escaping into the air, will mix with oxygen molecules and at ignition temperature, will burn. In case of a liquid, the flammable liquid must first be vapourized and its vapour mixed with oxygen, and it will burn. Similarly, solids must usually be liquefied and vapourized, or at lease reduced to small particles, with large surface area before it will burn. Every material must be raised to its specific ignition temperature before a fire will occur, though oxidation of the material may take place below this temperature. Oxidation normally involves atmospheric oxygen, but many other oxidizers produce flames and some flames do not involves oxidation reactions. Above the ignition temperature, the heat of oxidation does not dissipate fast enough and raises the next area of unburned fuel to ignition te mperature. Normally the ignition temperature of solids are higher than those of the liquids.
When a part of any material is exposed to external source of heat, its temperature will rise as a result of heat transfer. As temperature progressively increases, a point is reached when enough thermal energy has been imbibed as to break bonds. This result in degradation often called pyrol ysis. Pyrolysis may or may not be influenced by oxygen; all that is required is heat or high temperature. The pyrolysate or pyrolysis product, whose composition depends on the material, includes combustible and non -combustible gases as well as carbonaceous char [3]. At optimum oxygen combustible gas ratio and at the right temperature ignition occurs. Flame is produced and heat is ev olved. Burning is then sustained by part of the heat of the combustion produced within the flame, some of which is channeled back to the material. The remainder is “lost†to the surrounding.
2.1.2 Pyrolysis of Plastics
Pyrolysis of plasties or elastomer starts with liberation of volatiles when the plastics are exposed to a source of sufficient energy. These volatiles then mix with air and undergo ignition when the temperature is sufficiently high. Three steps are involved for the whole combustion process : heating, thermal decomposition or pyrolysis and ignition into flame[11]. If the heat of combustion is enough to sustain the pyrolysis and the subsequent ignition, plastics or elastomer burn spontaneously even after the removal of the external heat source. Thus, a self sustaining combustion cycle is established as illustrated in Fig I.
2.1.3 Pyrolysis of Polyurethane foams
Polyurethane foams as an elastomer have highly cellular structures, which are easily ignitable and highly flammable. Flame spread is very fast on the surface of these materials and results in engulfing the entire area in a few minutes.
The burning process of polyurethane foam may be considered as occurring in five stages [ 12] viz.
1. Heat from the external source is applied to the foam, and it progressively raises the temperature. The thermal insulation properties of the foam promote a rapid temperature rise at the exposed surface because heat cannot be transmitted to the lower layer of the foam.
2. The polyurethane foam re aches its temperature of initial decomposition and begins to form combustible gases, non-combustible gases, entrained solid particles and carbonaceous char. The evolution of gases expands the foam structure and thus combustible and non-combustible gases will both cause difficulty by disrupting the chemical and physical structure of the foam, exposing new surfaces to destructive temperatures.
3. The resulting combustible gases ignite in the presence of sufficient oxygen and further combustion begins. The condition of ignition depends on the presence of an external source of ignition, the temperature and composition of the gas phase.
4. The heat of combustion raises the temperature of the gaseous products of combustion and of the non combustible gases result ing in an increased heat transfer by conduction.
5. The heat will transfer from the combustible zone to adjacent foam surface producing further decomposition and ignition thus flame propagatio n [13].
The above pattern of pyrolysis seem to indicate that polyurethane flame retardant requires cheap, reactive and miscible chemicals that can bind easily with the foam rate without adversely affecting the foam density.
