2.3. Fire Testing

To study fire retardancy properties, limiting oxygen index (LOI), UL-94, and cone calorimeter (CC) tests were conducted. All testing devices were from Netzsch Taurus Instruments GmbH (Weimar, Germany).

The LOI fire test is particularly useful to track flammability properties occurring from slight formula changes within multi-material flame retardant systems. The testing setup includes a vertically clamped sample (125 × 10 × <10 mm³), which is surrounded by a controlled N₂/O₂ atmosphere. During the testing routine, a 50 watt propane flame is systematically applied to the sample in a candle-like setup from above. If the sample is ignited, flame propagation is observed, the propagation time and distance are measured, and the N₂/O₂ atmosphere contents are systematically adjusted. The resulting key figure “limiting oxygen index” (LOI) represents the lowest atmospheric oxygen content needed to provide either 50 mm downward fire propagation or 3 min of burning time. Tests were conducted following the DIN EN ISO 4589-2 [40] standards.

UL-94 (vertical) fire testing setups are used to study the self-extinguishing properties and fire-dripping behavior of polymeric materials under normal atmospheric conditions. During the test, a 50 watt methane flame is applied from underneath the sample (125 × 13 × <13 mm³) following a standardized routine. After the testing flame is removed, burning times and dripping behavior are observed. Testing results are then clustered into three classifications from best to worst: V0, V1, and V2. V0 represents instant self-extinguishing behavior with no burn dripping. V2 classifications allow burn dripping and, compared to V0 classifications, longer burning times. Tests were conducted in accordance with the DIN EN 60695-11-10 [41] standards.

The cone calorimeter testing method is a valuable tool to gain insights into the burning behavior under enforced flaming conditions. During the testing routine, 100 × 100 × < 50 mm³ sample geometry is placed underneath the cone heater. Pyrolysis gases are ignited via piloted ignition and the burning process is monitored by tracking the heat development over time until complete combustion. The amount of heat released as well as the heat development characteristics provide important information about the functionality of the flame retardant formulation. Within this study, tests have been conducted using three heater capacities 35, 50, and 65 kW/m² in order to calculate the heat release parameter (HRP). The appropriate literature targeting the correct use and interpretation of important key figures can be found in [42,43,44,45]. The sample geometries used were 100 × 100 × 4 mm³. All tests were repeated at least three times, and averaged curves are presented in the results section. Tests were conducted in accordance with the DIN ISO 5660-1 [46] standards.