Experimental Methodology

An STA test increases the temperature of a milligram-scale sample following a predefined temperature program while maintaining a well-defined gas atmosphere. The STA collects data over time on the sample mass, sample temperature, and the heat flow rate into the sample. Decomposition kinetics, heat capacity, and energetics (heats of melting and heats of reaction) during thermal decomposition are obtained through analysis of the collected data. These derived quantities are important when using computational models to predict the burning rate of materials.

STA experiments were conducted on samples prepared as powders with masses in the range of 4.0 ± 0.1 mg. Sample specimens were placed in platinum-rhodium crucibles with lids. Experiments were conducted with an initial temperature of 323 K (120°F), a constant heating rate of 3 K/min (5.4°F/min), 10 K/min (18°F/min), or 30 K/min (54°F/min), and a minimum final temperature of 923 K (1200°F). Tests were conducted in a nitrogen atmosphere with a total gas flow rate of 70 mL/min.

Because flames with different temperatures emit radiation in different spectral ranges, to predict ignition and fire growth most accurately, spectral optical properties for target materials and products need to be characterized. An integrating sphere (IS) accessory for the Bruker Fourier-Transform Infrared (FTIR) spectrometer that allows for the measurement of the spectrally resolved absorption and transmission of radiation incident to materials.

The IS was utilized to measure spectral reflectivity and transmissivity according to ASTM E903 "Standard Test Method for Solar Absorptance, Reflectance, and Transmittance or Materials Using Integrating Spheres." The reference was a gold mirror which reflected almost all incident infrared radiation. The baseline measurement involved a light trap which reflected close to zero infrared radiation. The data collected through these techniques yielded the spectral emissivity and absorption coefficient.

The microscale combustion calorimeter (MCC) measures the specific heat release rate of milligram-scale material samples as the temperature of the sample is increased following a predefined temperature program while maintaining the sample in a well-defined atmosphere. MCC experiments can validate kinetic mechanisms, identify chemical changes in gases and vapors evolved during decomposition, and determine the heat of combustion of the gases evolved during decomposition.

MCC experiments were conducted on samples prepared as powders with masses in the range of 4.0 ± 0.1 mg. Sample specimens were placed in open aluminum oxide (ceramic) crucibles for the experiments. Experiments were conducted with an initial temperature of 423 K (300°F), a constant heating rate of 30 K/min (54°F/min), and a minimum final temperature of 923 K (1200°F). Tests were conducted in triplicate according to ASTM D7309-21a "Standard Test Method for Determining Flammability Characteristics of Plastics and Other Solid Materials Using Microscale Combustion Calorimetry."

A TA instruments FOX 200 heat flow meter (HFM) was used to measure the thermal conductivity and heat capacity of planar material samples. The HFM has an effective temperature range of 0 °C K (32°F). to 50 °C K (120°F).

Thermal conductivity tests were generally conducted according to ASTM C518 "Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus." Tests were typically conducted with a 20 K (36°F) or 25 K (45°F) temperature difference between the hot and cold plates with an upward heat flow direction. A minimum of 2 temperature set points were defined for each test in the range of 283 K (50°F) to 328 K (130°F), and tests were conducted in triplicate.

Specific heat capacity tests were conducted with the HFM in general accordance with ASTM C1784-20. The experimental procedure consisted of setting the hot and cold plate temperatures equal and measuring the heat flow to the sample and the time required to reach an equilibrium condition. After the equilibrium condition was achieved, the hot and cold plate temperatures were increased, and the amount of time required to achieve a new equilibrium was recorded. Tests were conducted with temperature set points in the range of 283 K (50°F) to 328 K (130°F).

The cone calorimeter measures heat release properties of materials. Measurements include heat release rate per unit area, mass loss rate, smoke release rate, effective heat of combustion, and rate of release of combustion gases.

Cone calorimeter tests were conducted in general accordance with ASTM E1354 "Standard Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter." Tests were conducted with incident heat fluxes of 25, 50, and 75 kW/m². The default sample preparation did not include any restraints such as an edge frame or wire grid due to the difficulty of characterizing boundary conditions created by these optional accessories. However, some accessories were used when necessary, according to the guidance in ASTM E1354, and these instances are described in the test notes. Prior to testing, samples were conditioned at a temperature of 20 °C ± 0.1 °C (68°F), and a relative humidity of 50% ± 0.5%.

A C-Therm Trident which allows for conduction of transient plane source and modified transient plane source techniques to determine the thermal conductivity and heat capacity of materials. The modified transient plane source has an effective temperature range of 0 °C to 500 °C and the transient plane source has an effective temperature range of 0 °C to 300 °C.

Samples were prepared for STA and MCC experiments through a cryogenic grinding process. Sample materials were cut by hand with various cleaned and sanitized hand tools to produce a collection of prismatic pieces with lengths of 2 mm to 10 mm. The pieces were placed in a 50 mL Retsch GmbH zirconium oxide grinding jar with a 1.5 mm diameter zirconium dioxide grinding ball. The sealed jar was submerged in liquid nitrogen until the jar was in thermal equilibrium with the nitrogen (at least 10 minutes). After cooling, the jar was secured in the grinding station of a Retsch Model MM301 mixer mill, and the sample was ground for a minimum of 10 minutes. This process creates a fine, homogeneous powder while avoiding thermal decomposition during grinding. This sample preparation process allowed for consistent sample masses and composition between replicates and minimized thermal decomposition during the sample preparation process.

Samples were prepared for the HFM by cutting the material with a band saw or table saw to produce planar sample specimens with dimensions of approximately 200 mm x 200 mm. For materials not large enough for a 200 mm x 200 mm sample, the material was instead cut to at least 100 mm x 100 mm. This smaller piece was used to fully cover the HFM sensor (central 75 mm x 75 mm (3 in x 3 in)). Additional sample pieces were placed around the central test sample to create at least a 200 mm x 200 mm area. This was done to minimize heat loss from the edges of the sample in the sensor region. When received sample materials were non-planar or had a rough surface finish that did not facilitate measurement with the HFM, the sample materials were pre-processed with a planer to ensure the faces of the material were parallel.

Experimental data is presented for conditioned ('Dry') and unconditioned ('Wet') material. Sample materials were stored indoors at a temperature of 22 °C ± 3 °C (72 °F ± 6 °F) and a relative humidity of 40% ± 20%. 'Dry' conditioned material samples were held in an oven at a temperature of 105 °C for a minimum of 48 hours. Sample mass was monitored to determine when all moisture had evaporated from the sample. After drying, samples were stored in a desiccator in the presence of Drierite until they were tested.