Polymers, burning limiting oxygen index

Of the several test methods for evaluating the burning behavior of different polymers, the limiting oxygen index (LOI) will be used here to illustrate the relative flammability of materials. This test measures the minimum concentration of oxygen in an oxygen-nitrogen atmosphere that is necessary to initiate and support a flame. 

The maleimides can be cured by an addition reaction in which no volatiles are evolved. One of the remarkable features of the polymers is their high LOI s (limiting oxygen index) of 100. In other words, these materials do not burn in the presence of oxygen, even when the samples are preheated to 300 0. 

Figure 24.1. Relationship between the limiting oxygen index LOI and the fraction f of charcoal-like residues by the burning of various polymers (after D. W. van Krevelen).

Figure 11.14. Relationship between limiting oxygen index (LOI) and UL 94 classification of polymers. HB, horizontal burning, V, vertical burning.

Flammability of polymers can be determined by their reactivity with oxygen. The limiting oxygen index (LOI), defined as the minimum amount of oxygen required to sustain combustion under specified conditions, is a quantitative measure of the tendency of materials to burn (Table 17.1). In a flame-retarded system, the study ofthe LOI can provide information on the effectiveness of flame-retardant materials [29]. 

The limiting oxygen index (LOI) is the minimum concentration of oxygen, expressed as a percentage, that will support combustion of the polymer. It is measured by passing a mixture of O2 and N2 over a burning specimen, and reducing the O2 level until a critical level is reached. 

A simplified model of ignition and burning of polymers which docs not require one to work with partial differential equations has been proposed by Rychly [49]. It was applied to the combustion carried out in a mass loss cone calorimeter system [50]. A series of simulations of heat release rate curves was performed for polymers with intumescent additives [51]. The model was also used for the prediction of limiting oxygen index (LOI) [52],... 

Polyphosphazenes have low inflammability, very high values of limiting oxygen index (LOI) equal to 24-65, and evolve no toxic gases during burning. Most of them do not burn and some are used as additives to other polymers as flame... 

For the past several years there was hope that the formation of a nanocomposite alone would permit one to impart fire retardancy to a polymer in all combustion conditions. Flame retardancy is achieved with nanocomposites alone, but not enough for an ignition resistance test such as the limiting oxygen index (LOI) and the UL-94 vertical burning test (ANSI//ASTM D-635/77). 

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