Objective - To develop the measurement science that enables the design of engineered fire safe materials and products for residential applications.
What is the new technical idea? In the past, efforts at flammability reduction have primarily utilized flame retardant (FR) additives. The development of FRs has largely been accomplished through trial-and-error with the help of several bench-scale standardized flammability tests. Two problems with the existing approach are: (1) relatively large samples must be prepared to perform most standard flammability tests, and (2) since fire problems are notoriously difficult to scale, correlation of the results of bench-scale tests to realistic full-scale fire scenarios is tenuous. Additionally, a number of concerns have been raised in recent years concerning the toxicity of halogenated FRs. Consequently, there is significant pressure to develop flammability reduction technologies that do not rely on halogenated FRs. For example, barrier fabrics have proven to be a successful flammability reduction technology in several industries. Unfortunately, many standard flammability tests are not appropriate for testing the effectiveness of novel flammability reduction technologies such as barrier fabrics.
Progress in flammability reduction requires an approach for predicting the full-scale flammability of increasingly sophisticated material systems using increasingly smaller sample sizes. In this project, such an approach will be developed based on an integrated suite of microgram scale tests to parameterize applied pyrolysis models for predicting the burning behavior of novel technologies for flammability reduction.
What is the research plan? In this phase of the project, the focus is on creating a material property database for characterizing flammability (Task 1), generating small-scale fire model validation data (Task 2), and advancing pyrolysis modeling in order to simulate standard tests for flammability (Task 3). Accomplishing these tasks provides the ability to predict the flammability of any arbitrary material or product, but the focus for FY18 will be on the scenario of a modified cone calorimetry setup for assessing the flammability reduction gained by the use of barriers in residential upholstered furniture.
Task 1: Material Flammability Database
Simulation of burning materials requires quantification of material properties. Most existing pyrolysis models require quantification of the following properties:
- Thermal conductivity,
- Emissivity and attenuation coefficient,
- Specific heat capacity,
- Heat of gasification,
- Kinetic rate parameters, and
- Heat of combustion.
These properties are often difficult to measure as they are needed for the large range of temperatures and compositions experienced by a burning material. Although these properties have been characterized for several materials, the values are often difficult to find as there is currently no widely-accepted flammability repository comparable the NIST-JANAF tables for thermochemical properties. The planned database would serve as both a repository for storing data routinely generated in the Flammability Reduction Group’s Analytical Lab as well as a source of property values for fire protection engineers seeking to simulate the burning behavior of a known material.
The Material Flammability Database (MFD) will include four major components: data, tools for parameter estimation, a standardized data storage protocol, and a web-based interface for accessing and analyzing the data Of course, the foundation of the MFD is data. Data will be obtained using several available small-scale tests, specifically:
- Thermogravimetric analysis (TGA) for pyrolysis kinetics,
- Simultaneous thermal analysis (STA) for specific heat capacity and heat of gasification,
- Microcombustion calorimetry (MCC) for heat of combustion,
- Gas pycnometry for density
- Transient plane and line source devices for thermal conductivity,
- Gasification apparatus for thermal conductivity and attenuation coefficient, and
- Reflectometer for emissivity.
With the exception of the transient plane/line source techniques for measuring thermal conductivity, all of the above experiments require only microgram scale samples. At this point, the principal challenge to developing a complete MFD is finding a robust methodology for estimating thermal conductivity at a range of temperatures using only microgram scale samples.
Although the experiments listed above are small, they are far from physically simple. Rational model reduction is needed to extract useful material property estimates from the data. Current approaches rely on ad hoc manual estimates of the properties. A critical element of the MFD will be a suite of automated data reduction tools for extracting material properties from the raw data.
Once the data has been obtained, and the material properties have been extracted, the data must be organized and stored in an easily searchable format. This storage requires careful specification of the metadata describing both the material and the experimental scenario. Furthermore, once a protocol for metadata is determined, an appropriate format for storing the metadata must be chosen. The stored data will then need to be accessible in a user-friendly manner. An internal or external web-based interface is most appropriate. Data storage and accessibility of the MFD require not only detailed knowledge of material flammability, but also experience with the state-of-the-art in database management. Therefore, we will need to work closely with ELDST staff on this component of the MFD.
In addition to providing a resource for fire modeling, additional applications of the MFD will include uncertainty quantification of the properties of a given material, analysis of the effects of ageing, and material identification for forensics.
Task 2: Small-scale Fire Model Validation Data
As discussed above, this project takes the approach that modeling and microgram scale can be used in conjunction to predict the flammability reduction effect of a novel technology. Model development requires model validation. Therefore, the second major task of this project is to perform a range of bench-scale flame spread experiments for model validation.
Previous work has focused on upward vertical flame spread of thermoplastics. This work is to be extended to more complex material systems burning in a range of configurations.
Task 3: Applied Pyrolysis Modeling
Existing pyrolysis models are capable of predicting gasification and cone calorimetry for a limited number of relatively simple materials. Although the burning behavior of combustible materials is complex, it is possible to extend existing models in order to obtain broader applicability to realistic fire scenarios. At this phase of the FRT project, focus will be on the modified cone setup being developed to test the flammability reduction of novel barrier fabrics for polyurethane furniture.
Several aspects of the modified cone presents obstacles to the current pyrolysis model used in FDS. Specifically, the FDS solid model does not account for finite rate mass transfer or the allowance of zero-dimensional thermally-lumped layers needed to model the growing air gap in the modified cone setup. Furthermore, gas-solid phase coupling is still very much a work in progress for FDS, and so our efforts at modeling the modified cone will support validation and development of advanced flame spread modeling.