Fire Hazards of Materials and Products

The presence of combustible material in combustible systems represents an obvious condition of burning. Burning phenomena and the phases of the burning process fundamentally depend on the physical and chemical properties of the material involved. Therefore, it seems reasonable to make a survey of the flammability of the various materials and products with respect to their character and properties. For this section, the ordering principle for the grouping of materials is governed by technical aspects rather than by theoretical conceptions (NFPA 1991).

Wood and wood-based products

Wood is one of the most common materials in the human milieu. Houses, building structures, furniture and consumer goods are made of wood, and it is also widely used for products such as paper as well as in the chemical industry. Wood and wood products are combustible, and when in contact with high-temperature surfaces and exposed to heat radiation, open flames or any other ignition source, will carbonize, glow, ignite or burn, depending upon the condition of combustion. To widen the field of their application, the improvement of their combustion properties is required. In order to make structural units produced from wood less combustible, they are typically treated with fire-retardant agents (e.g., saturated, impregnated, provided with surface coating). The most essential characteristic of combustibility of the various kinds of wood is the ignition temperature. Its value strongly depends on some of the properties of wood and the test conditions of determination, namely, the wood sample’s density, humidity, size and shape, as well as the ignition source, time of exposure, intensity of exposure and the atmosphere during testing. It is interesting to note that the ignition temperature as determined by various test methods differs. Experience has shown that the inclination of clean and dry wood products to ignition is extremely low, but several fire cases caused by spontaneous ignition have been known to occur from storing dusty and oily waste wood in rooms with imperfect ventilation. It has been proven empirically that higher moisture content increases the ignition temperature and reduces the burning speed of wood. The thermal decomposition of wood is a complicated process, but its phases may clearly be observed as follows:

· The thermal decomposition with mass loss starts already in the range 120-200 °C; moisture content releases and the non-combustible degradates occur in the combustion space.

· At 200-280 °C, mainly endothermic reactions occur while the heat energy of ignition source is taken up.

· At 280-500 °C, the exothermic reactions of decomposition products are steadily accelerating as the primary process, while carbonization phenomena may be observed. In this temperature range, sustaining combustion has already developed. After ignition, burning is not steady in time because of the good heat-insulating ability of its carbonized layers. Consequently, the warming up of the deeper layers is limited and time consuming. When the surfacing of the combustible decomposition products is accelerated, burning will be complete.

· At temperatures exceeding 500 °C, the wood char forms residues. During its additional glowing, ash containing solid, inorganic materials is produced, and the process has come to an end.

Fibres and textiles

The majority of the textiles produced from fibrous materials that are found in the close surrounding of people is combustible. Clothing, furniture and the built environment partly or totally consists of textiles. The hazard which they present exists during their production, processing and storing as well as during their wearing. The basic materials of textiles are both natural and artificial; synthetic fibres are used either alone or mixed with natural fibres. The chemical composition of the natural fibres of plant origin (cotton, hemp, jute, flax) is cellulose, which is combustible, and these fibres have a relatively high ignition temperature (approx. 400 °C). It is an advantageous feature of their burning that when brought to high temperature they carbonize but do not melt. This is especially advantageous for the medical treatments of burn casualties. The fire hazardous properties of fibres of protein base of animal origin (wool, silk, hair) are even more favourable than those of fibres of plant origin, because a higher temperature is required for their ignition (500-600 °C), and under the same conditions, their burning is less intensive.

The plastics industry, utilizing several extremely good mechanical properties of polymer products, has also gained prominence in the textile industry. Among the properties of acrylic, polyester and the thermoplastic synthetic fibres (nylon, polypropylene, polyethylene), those associated with burning are the least advantageous. Most of them, in spite of their high ignition temperature (approx. 400-600 °C), melt when exposed to heat, easily ignite, burn intensively, drop or melt when burning and release considerably high quantities of smoke and toxic gases. These burning properties may be improved by addition of natural fibres, producing so-called textiles with mixed fibres. Further treatment is accomplished with flame-retardant agents. For the manufacture of textiles for industrial purposes and heat-protective clothing, inorganic, non-combustible fibre products (including glass and metal fibres) are already used in large quantities.

The most important fire hazard characteristics of textiles are the properties connected with ignitability, flame spread, heat generation and the toxic combustion products. Special testing methods have been developed for their determination. The test results obtained influence the fields of application for these products (tents and flats, furniture, vehicle upholstery, clothes, carpets, curtains, special protective clothing against heat and weather), as well as the stipulations to restrict the risks in their use. An essential task of industrial researchers is to develop textiles that sustain high temperature, treated with fire-retardant agents, (heavily combustible, with long ignition time, low flame spread rate, low speed of heat release) and produce small amounts of toxic combustion products, as well as to improve the unfavourable effect of fire accidents due to the burning of such materials.