Ignition of Polymers

Polymeric materials are found all around us. They occur in the objects we use, in plants and animals, and even in our own bodies. Synthetic polymers, such as polypropylene, have a significant impact on the industrial use. Of the natural ones, lignocellulosic materials forming plant bodies are often used. The properties of polymeric materials are influenced both by the composition and shape of their macromolecules and by the additives that are physically bound in them. Many factors play an important role in the process of their thermal decomposition, such as the elements contained in their molecules, the bonds between them and the spatial structure that results from them, the form in which they are found, the additives used, and so on. This chapter focuses on the composition of the most commonly used polymers. It emphasises information about their thermal decomposition, moving from generally accepted principles to describing the behaviour of specific macromolecules. Of the synthetic polymers, the materials most commonly used in industry are described, supplemented by a selection of polymers used for the production of filaments for 3D printers. In the case of natural polymers, the chapter contains information concerning not only cellulose and lignin, but wood in general, which consists of about 70% of them.

The action of an external source of thermal radiation on the surface of a combustible material causes a number of physical and chemical processes. Depending on the properties of the material and the ambient conditions, its temperature is increased until a point is reached where the amount of gaseous combustion products released is sufficient to initiate the flame. This temperature is called the ignition temperature. With a uniform heat flux, the time required to achieve it is also a significant characteristic. It is called time to ignition and is important information pertaining to the initiation process. Based on its dependence on the external heat flux density, mathematical relationships can be compiled that allow the ignition to be characterised. A relatively large number of such relationships are proposed, each of which is suitable for certain materials or measurement conditions on the basis of specific simplifications. In the first part of this chapter, the physical processes associated with the impact of thermal radiation on the surface of the material are presented. The basic mathematical relationships that describe these physical processes are explained. For a clearer understanding of the text, the characteristics of selected plastics, which are necessary in the calculations, are also given. The second part focuses on suggested correlations. It provides an overview of the various correlations proposed in the scientific literature. Their order is chronological and thus, to a certain extent, also provides some insight into the development of ideas about the initiation of flame combustion.

Ignition may be described by various characteristics. The most commonly used characteristics are pilot ignition temperature, self-ignition temperature, and autoignition temperature. In every case, it is the temperature to which the sample must be heated in order to cause ignition under the test conditions. From the perspective of fire protection, the state of the system leading to the ignition of a flammable substance may be characterised in various ways. The following text will especially focus on those characteristics that may be obtained from data regarding the external heat flux and the time to ignition of the sample. Since the environmental conditions interact with the sample during the process of ignition, it largely depends on the test method used.

As should be clear from the previous sections, the correlation between the time to ignition of flaming combustion and the corresponding external heat flux, providing that appropriate simplifications are made, allows the specification of various characteristics. Some of them change during thermal loading, and the values obtained are therefore sometimes referred to as apparent. Their significance lies in the fact that they are related to the conditions at the moment of ignition which can be deduced based on these values even under other ambient conditions. Measurements were performed on samples of both synthetic and natural polymers. Synthetic polymers were made by 3D printing. Polylactic acid, acrylonitrile–butadiene–styrene copolymer with methylmethacrylate, and poly (ethylene terephthalate)-glycol were used as basic materials. Of the natural polymers, wood-based materials were used: pine wood, ash wood, thermowood, plywood, and blockboard. The samples were thermally loaded by a cone radiator with seven external heat fluxes in the range from 25 to 55 kW m⁻². Based on the achieved dependences of time to ignition on the density of the incident heat flux, ignition parameters were calculated and compared with the results from other authors.