Fire Propagation

Fire propagation is the most critical stage in a fire in terms of hazard assessment, and thus most of the standard test methods use it for the flammability assessment criterion for polymers. Fire propagation represents growth of the combustion process including surface flame spread, nonflaming fire growth, and the fireball in premixed flame propagation [14, 15, 34, 35]. It can be considered as an advancing ignition front in which the leading edge of the flame acts as the source of heat and the source of ignition. It can occur on a horizontal surface, an inclined surface, and on a vertical surface parallel to or opposite to the airflow direction (upward/concurrent or downward or lateral fire propagation). One of the following fire propagation behaviors may be observed for the polymers:

• Nonpropagating There is no fire propagation beyond the ignition zone.

• Decelerating Fire propagation rate17 beyond the ignition zone decreases and propagation stops before covering the entire polymer surface.

• Propagating Fire propagates beyond the ignition zone until the entire polymer surface is involved on fire.

• Accelerating Fire propagation rate beyond the ignition zone increases rapidly covering the entire polymer surface and far beyond with flames in a relatively short time.

For propagating fires, the leading edge of the flame transfers heat ahead of the zone, raises the surface temperature to the ignition temperature of the polymer (satisfying the CHF value), and maintains the temperature until polymer vapors ignite (satisfying the TRP value) [14, 15, 21, 22, 34, 35]. The heat flux provided by the leading edge of the flame depends on the chemical heat release rate. Thus, fire propagates at different rates depending on the heat release rate.

Figures 11.12 and 11.13 illustrate nonpropagating and rapidly propagating fires between two vertical parallel panels of polymers [63]. The panels are about 0.61 m (2 ft) wide, 2.44 m (8 ft) high, and 25 mm (1 inch) thick separated by

17 Fire propagation rate is the velocity at which the ignition front travels over the surface.

Figure 11.12. Nonpropagating fire between two vertical parallel panels of a polymer for test duration of 15 min [63]. The panels are about 0.61 m (2 ft) wide, 2.44 m (8 ft) high, and 25 mm (1 inch) thick separated by 0.30 m (1 ft). Ignition source is a 60-kW, about 0.30-m wide, 0.61-m long, and 0.30-m high propane sand burner. The tip of the flame from the burner reaches a height of about 0.91 m (3 ft). Marks on the scale are in feet.

Figure 11.12. Nonpropagating fire between two vertical parallel panels of a polymer for test duration of 15 min [63]. The panels are about 0.61 m (2 ft) wide, 2.44 m (8 ft) high, and 25 mm (1 inch) thick separated by 0.30 m (1 ft). Ignition source is a 60-kW, about 0.30-m wide, 0.61-m long, and 0.30-m high propane sand burner. The tip of the flame from the burner reaches a height of about 0.91 m (3 ft). Marks on the scale are in feet.

0.30 m (1 ft). The ignition source is a 60-kW, about 0.30-m wide, 0.61-m long, and 0.30-m high propane sand burner. The tip of the flame from the burner reaches a height of about 0.91 m (3 ft). Marks on the scale are in feet.

The common measurements that are made in the standard flammability tests for fire propagation are:

Figure 11.13. Rapidly propagating fire between two vertical parallel panels of a polymer [63]. Panels are about 0.61 m (2 ft) wide, 2.44 m (8 ft) high, and 25 mm (1 inch) thick separated by 0.30 m (1 ft). Ignition source is a 60-kW, about 0.30-m wide, 0.61-m long, and 0.30-m high propane sand burner. Tip of the flame from the burner reaches a height of about 0.91 m (3 ft). Marks on the scale are in feet. The photograph was taken a few seconds before flames extended far beyond top of the panels.

Figure 11.13. Rapidly propagating fire between two vertical parallel panels of a polymer [63]. Panels are about 0.61 m (2 ft) wide, 2.44 m (8 ft) high, and 25 mm (1 inch) thick separated by 0.30 m (1 ft). Ignition source is a 60-kW, about 0.30-m wide, 0.61-m long, and 0.30-m high propane sand burner. Tip of the flame from the burner reaches a height of about 0.91 m (3 ft). Marks on the scale are in feet. The photograph was taken a few seconds before flames extended far beyond top of the panels.

• Extent of fire propagation on a horizontal or vertical surface

• Melting, dripping, and ignition of materials in close proximity to the sample by the hot burning molten droplets

• Minimum heat flux or surface temperature for flame spread

• Minimum oxygen concentration for flame spread

• Flame spread or fire propagation rate

• Heat release rate

• Smoke release rate

The above measurements made in the fire propagation tests are used in different ways to set up the standard test criteria, such as discussed in the following sections for the selected standard tests as illustrations.

11.8.1. The FMVSS 302 Test for Flammability of Vehicle Interior Materials [64]

The test is a horizontal fire propagation test performed in a metal cabinet using 102-mm-wide and 356-mm-long polymer sample. Fire propagation rate and burning time are used as the selection criteria. A polymer that does not burn nor transmit a flame front across its surface at a rate of more than 102 mm/min (1.7 mm/s) is acceptable as a vehicle interior material. If a polymer stops burning after 60 s of heat exposure and has not burned more than 51 mm from the point where the timing was started, it is considered to meet the burn-rate requirement.

As intended, the test criteria are applicable to small polymer parts in the interior of vehicles exposed to small ignition sources. The test is not intended to be applied to large exposed polymer surface areas or polymers in the engine compartment exposed to high-intensity ignition sources. The test method has not been developed to predict the fire behavior of polymers expected in actual fires but rather to screen out polymers with higher resistance to fire propagation from those with low resistance.

11.8.2. UL 94 Standard Test Methodology for Flammability of Plastic Materials for Parts in Devices and Appliances [24]

In this test methodology, both horizontal burning (HB) and vertical burning (V) behaviors of polymers are examined. For horizontal burning test for classifying materials, 127-mm (5-inch) long and 12.7-mm (0.5-inch) wide samples with maximum thickness of 12.7 mm (0.5 inch) are placed on top of a wire gauge and ignited by a 30-s exposure to a Bunsen burner at one end.

For 94HB materials: (1) the burning rate does not exceed 38.1 mm/min (1.5 inches/min) over a 76-mm (3.0-inches) specimen having a thickness of 3-13 mm (0.12-0.50 inch), (2) the burning rate does not exceed 76 mm/min (3.0 in/min) over a 76-mm (3.0-inch) span for specimens having thickness less than 3 mm (0.12 inch), or (3) cease to burn before reaching 102 mm (4.0 inches).

For classifying materials as 94V-0, 94V-1, or 94V-2 in the vertical burning test, 127-mm (5-inches) long and 13-mm (0.5-inch) wide specimens with thickness limited to 13 mm are used. The bottom edge of the specimen is ignited by a 5-s exposure to a Bunsen burner with a 5-s delay and repeated five times until the sample ignites. The 94V-0, 94V-1, and 94V-2 polymer classification criteria are:

Criterion

(A) Flaming combustion time after removal of the test flame (s)

(B) Total flaming combustion time after 10 test flame applications for each set of 5 specimens (s)

(C) Burning with flaming or glowing combustion up to the holding clamp

(D) Dripping flaming particles that ignite the dry absorbent surgical cotton located 12 inches (305 mm) below the test specimen

(E) Glowing combustion persisting for more than 30 s after the second removal of the test flame (s)

The relative resistance of materials to burning according to UL 94 is HB < V-2 < V-1 < V-0. Examples of the UL 94 classification of polymers are listed in Table 11.3 along with their peak vaporization (decomposition) temperature, char yield, and limiting oxygen index (LOI) values, where data are taken from the Federal Aviation Administration (FAA) research study on polymers for aircraft [33]. All the ordinary polymers listed in the table, which generally have low fire resistance, are classified as HB. Most of the high-temperature and halo-genated polymers listed in the table that generally have high fire resistance are classified as V-0.

As intended, the test criteria are applicable to small polymer parts in metallic devices and appliances exposed to small ignition sources and not to devices and appliances made entirely of polymers and exposed to high-intensity ignition sources. The test method has not been developed to predict the fire behavior of polymers expected in actual fires but rather to screen out polymers with higher resistance to fire propagation from those with low resistance.

11.8.3. ASTM D 2863-70 Test Methodology for Limited Oxygen Index of Materials [65]

The limited oxygen index (LOI) is the minimum O2 concentration at or below which there is no downward fire propagation for a vertical polymer sheet inside a glass cylinder, with gas flowing in an upward direction. In the test, 70- to 150-mm (2.8- to 5.9-inch) long, 6.5-mm (0.26-inch) wide, and 3-mm (0.12-inch) thick vertical polymer samples are ignited at the top and fire propagates in a downward direction.

The test is performed at several O2 concentrations to determine the minimum O2 concentration at and below which there is no downward fire propagation. LOI values are listed in Table 11.19 taken from Table 11.3 and from Refs. [22] and [66]. The LOI values are arranged on the basis of the generic nature of polymers.

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