The ignition resistance of polymers has been investigated in detail both experimentally and theoretically [14, 15, 21, 22, 34, 35]. It is well recognized that the ignition behavior of a polymer is governed by its actual thickness (d) relative to the thermal penetration depth (5) [14, 15, 34, 35]. The thermal penetration depth is expressed as: _

where a is the thermal diffusivity of the polymer (mm/s), t is the heat exposure time of the polymer surface (s), k is the thermal conductivity of the polymer (W/m K), p is the density of the polymer (g/cm3), and c is the heat capacity of the polymer (J/g K). For thermally thick conditions, 5 < d and for thermally thin conditions, 5 > d. Polymers are generally physically thick in their end-use applications, and thus the ignition under thermally thick condition is more common than under thermally thin condition.

In some of the standard flammability tests, ignition time or time to flame extinguishment is specified as the ignition resistance criterion. In some of the tests a single measurement is made, while in others multiple measurements are made for the ignition time or time to flame extinguishment, using either a burner flame or an external heat source with a pilot. The data from the standard tests where ignition time is measured at several external heat flux values [31, 36-38] can be used to derive the ignition properties of the polymers under thermally thick condition.

The following is the most commonly used expression for the relationship between the ignition time and external heat flux based purely on the thermal arguments [14, 15, 21, 22, 34, 35]:

where tig is the ignition time (s), a is the polymer surface absorptivity,9 tj'^ is the critical heat flux (CHF)10 per unit polymer surface area (kW/m2), Tig is the ignition temperature (°C), and 7'u is the ambient temperature (°C). The term (Tig — Ta)^/kpc is defined as the thermal response parameter (TRP) of the polymer (kW-s1/2/m2) [21, 22]. CHF and TRP are defined as the ignition properties of the polymers.

The ignition properties of polymers can be obtained by using the standard test procedure of exposing the polymer surface in a horizontal or vertical orientation to external heat flux, in the presence of a pilot flame and measuring the ignition time [31, 36-38]. Figure 11.3 shows an example of the ignition test for a circular sample, about 100 mm in diameter and 25 mm in thickness in the ASTM E2058 Apparatus. Figure 11.4 show an example of the ignition data.

9 In Eq. (11.6), the polymer surface absorptivity, a, is taken as unity when the ignition experiments are performed with sample surfaces coated black, such as in the ASTM E2058 Apparatus [31].

10 CHF value is taken as the external heat flux value at which there is no ignition under quiescent airflow conditions.

Figure 11.3. About 100-mm diameter and 25-mm thick polymer sample with black-coated surface exposed to external heat flux in the presence of a pilot flame in the ASTM E2058 Apparatus [31]. Release of polymer vapors prior to ignition can be noted.

Figure 11.4. Relationship between the time to ignition measured in the ASTM E2058 Apparatus and external heat flux for 100-mm square and 25-mm thick slab of PMMA. The surface was coated black in the experiments. Data are taken from Ref. [39].

Figure 11.4. Relationship between the time to ignition measured in the ASTM E2058 Apparatus and external heat flux for 100-mm square and 25-mm thick slab of PMMA. The surface was coated black in the experiments. Data are taken from Ref. [39].

These data were measured in the ASTM E2058 Fire Propagation Apparatus under quiescent airflow conditions for about 100 mm2 and 25-mm thick slab of polymethylmethacrylate (PMMA), with surface coated black [39]. The experimental TRP value, TRPExp, is taken as the inverse of the slope of the linear relationship between (1/iig)1/2 and for black coated surface (a = 1.0).

The ignition properties from ignition time versus external heat flux data for a large number of polymers have been determined [19, 21, 22, 39-44]. Tables 11.5 and 11.6 list the values of the ignition properties obtained from the ignition data measured in ASTM E2058 fire propagation apparatus for selected polymers from parts of minivan and ordinary and advanced engineered polymers [21, 22, 39-41]. Table 11.7 lists the ignition property values obtained from the ignition time with sustained combustion for at least 10 s at various heat flux data measured in ASTM E1354 cone calorimeter and reported in Refs. [19, 42-44]. The data in Tables 11.5 to 11.7 show that the CHF values vary in the range of 10-50 kW/m2 and the TRPExp values in the range of 73-1111 kW s1/2/m2, showing very wide variations in the ignition resistance of polymers.

In addition to determining the experimental TRPExp values, they can also be calculated for polymers with known k, p, c, Tv(Td) and Tig values. These values are available in the literature for many polymers [25-27, 29, 30, 39, 45-48], some of which are listed in Tables 11.5 and 11.6 along with the TRPExp and its calculated value, TRPCal. In these tables, TRPCal values calculated from Tv(Td) as well as Tig values with known k, p, and c have been listed.

Table 11.6 also includes the +Jkpc values calculated from k, p, and c values from the literature. The average +Jkpc values for each class of solid polymers in the table vary within a narrow range, suggesting that they are independent of the generic natures of the solid polymers.11 The average value of «Jkpc for ordinary solid polymers is 0.778 ± 18%, for high-temperature polymers; it is 0.640 ± 15%, and for highly halogenated solid polymers it is 0.624 ± 18%. The overall average value of sfkpc for solid polymers is 0.781 ± 18%. Thus, the sfkpc component of TRP can be considered as approximately constant and independent of the generic nature of the polymer. The variations in the TRP values thus would mainly be due to the variations in the Tig values.

The Tig values of polymers can be measured directly using standardized tests or estimated from the CHF values.12 The Tig values estimated in this fashion from the CHF values have been reported in the literature [21, 22, 39-41], some of which are included in Tables 11.5 and 11.6 along with the Tv/Td values taken from previous tables. It can be noted that for ordinary and high-temperature polymers, Tv(Td) « Tig and the TRPCal values calculated from Tv(Td) values and from Tig values are similar. For highly halogenated polymers, Tig » Tv(Td) and

11 The ^Jkpc values of polymers modified for use as foams, expanded elastomers, and fabrics are significantly lower and variable than the values for the solid polymers.

12 Tig(°C) ^ [(qc'r)°.25 x 364] — 273, assuming heat losses to be mainly due to re-radiation and polymer surface acting as a black body and an ambient temperature of 20° C.

Table 11.5 Thermo-Physical and Ignition Properties of Polymers from Parts of a Minivan3

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