The currently used thermal insulation tests for PPE fabrics are limited in the information they provide end users on the protective value of the fabrics and fabric systems (“composites”). These tests measure heat transfer through PPE fabrics and composites using different forms of heat exposure and specimen orientations while providing various types of information that can be related to protection. In the evolution of NFPA 1971, multiple tests have been established for setting the minimum requirements of turnout clothing, in terms of protection from flame and heat. And now, new testing technology has recently become available to further advance this evolution of thermal insulation measurements, which will be considered for inclusion in the standard.
TPP as the current metric
The thermal protective performance (TPP) test was first developed in the early 1980s and became the mainstay method for evaluating multi-layer garment composite materials for thermal insulation in turnout gear since the 1986 edition of NFPA 1971. Since then, the TPP test has become the most widely used bench-scale test for defining minimum insulative performance of different firefighting applications as well as for worker protection against industrial flash fires. In NFPA 1971, the application of the TPP test has also been extended to helmet ear cover composite, hood composite, garment wristlet, glove composite and glove wristlet materials, each with varying minimum performance requirements.
The TPP test uses a flat 6x6-inch fabric composite specimen and a single calorimeter, mounted in a weighted sensor assembly resting on top of the specimen to measure the amount of heat energy transfer, which in turn is used to determine the protection value for PPE fabrics. These specimens are subjected to an exposure of 84 kW/m2 (2 cal/cm2sec), representative of a flash fire or an emergency fireground condition, which is produced by the combination of two angled Meker burners and a bank of radiant lamps that are positioned below the horizontally mounted composite specimen.
The specimen is exposed to this combination of convective and radiant heat until the sensor registers that sufficient heat has transferred through the composite that would cause a second-degree burn injury. Because the prediction of a second-degree burn injury will vary with the insulation provided by the composite, the exposure time varies for each composite. Nevertheless, the time to a predicted second-degree burn injury is used to calculate the TPP rating as the principal output of the test when multiplied by the exposure energy of 2 cal/cm2sec. For structural firefighter protective clothing, garments and gloves are required to have a TPP rating of 35 cal/cm2 while interface components such as garment wristlets, helmet ear covers, hoods, and glove wristlets are required to have a TPP rating of 20 cal/cm2.
TPP testing limitations
While the TPP test has been used for structural firefighting protective clothing for nearly two decades, there are certain aspects of the test, like placing the full weight of the sensor directly on the test specimen, that can cause the test to overlook important fabric properties. In the course of providing the required protection, fabric shrinkage due to heat and/or flame exposure may occur that has a significant impact on the thermal protection offered by PPE fabrics after exposure.
For example, it is well known that gloves composites with a leather outer shell “wrinkle” due to localized shrinkage from the intense heat exposure and these wrinkles create air gaps between the glove shell and lining materials that result in artificially high TPP ratings. This type of predicted performance is contrary to what is observed in the field where firefighters facing emergency fireground conditions generally observe gloves to shrink, which may result in lower levels of thermal insulation.
There have also been other cases where material systems fool the TPP test by expanding in the high heat exposure and drooping toward the heat source from gravity that also causes an increase in the air gap between the moisture barrier and outer shell, also resulting in elevated TPP ratings that are not reflective of field experience.
The orientation and assumed shape of the composite specimens in the test can also be a concern for the TPP test. The horizontal position of flat fabric specimens in the TPP test with heat exposure from below the specimen is also not reflective of how garments and other protective clothing elements are worn. The human body more closely resembles a combination of cylinders (e.g., arms, legs, torso) rather than a group of flat planes. The flash fire manikin test, which is the most widely used full-scale test for PPE fabrics and as-sold industrial flame-resistant garments for thermal protection, makes use of a manikin form and over 120 thermal energy sensors distributed around that manikin.
However, the flash fire manikin test is very expensive to run and differences in garment construction and manikin form can lead to variability among test results. Its application to structural firefighting garments has been limited for these reasons as well as an inability to relate manikin results to composite ratings. This lack of correlation is significant because the manikin is presumed to be more realistic in comprehensively assessing the thermal insulation provided by garments under emergency fireground conditions.
Flash Fire Cylinder – the new thermal test
The Flash Fire Cylinder is a new bench-scale thermal test that aims to provide new and intermediate information between the TPP and manikin tests. The test exposes a PPE test specimen, in the form of a sleeve that fits around a vertically positioned cylinder, to a uniform flame front that produces an average heat flux of 84 kW/m2, the same intensity as both the TPP and manikin tests.
Fifteen thermal energy sensors on the cylindrical form measure the heat that is transferred from the flames through the test specimen. This transferred energy data is compared to the incident energy data collected during the system calibration and the resulting value is called the energy ratio value (ERV). So far, the ERV has been shown to provide a repeatable test metric and offers the potential for predicting field performance of garments.
The Flash Fire Cylinder is capable of providing percent burn injury prediction as well, but the digital burn/no-burn nature of these results lead to imprecision with only 15 thermal energy sensors. The ERV provides a continuous spectrum of heat transferred through the specimen relative to the incident energy and offers additional information about the actual protective properties of the specimen under test.
This proposed new test method allows for repeatable bench-scale evaluations of flash fire protective performance of materials used in construction of multilayer protective garment composites, wristlets, helmet ear covers, shrouds and hoods. The TPP test only assesses the response of flat samples of composites continuously exposed to flash fire conditions of a combined convective-radiant heat flux of 84 kW/m2 until a burn injury criterion is met; it does not account for heat transfer that occurs after the flame exposure ends.
In contrast, the Flash Fire Cylinder data acquisition software calculates the total accumulated energy that passes through the material specimens both during a 10-second exposure and a data acquisition period that extends at least 120 seconds following the exposure to evaluate additional heat transfer that occurs. It is believed that these results can provide a more reliable indicator of how fabrics and composites may perform as full clothing items.
In addition to the Flash Fire Cylinder, the device can be easily interchanged for an instrumented hand form (Flash Fire Hand). This allows for evaluation of whole glove products for thermal insulation protection from flash fire exposures, using the same principles as the FFC. The FFH includes 10 thermal energy sensors, 3 each on the palm and back of the hand and 2 each on the wrist and forearm.
FFC is not the only test that attempts to provide additional information beyond standard TPP measurements. There is further interest in a technique called cylindrical TPP, which is a modification of the standard TPP that similarly accounts for test sample shrinkage. This test, developed by North Carolina State University, uses the TPP platform but has the sample wrapped around a horizontal-positioned curved holder that allows sample movement due to shrinkage. This test has also been proposed as an alternative to TPP.
Future consideration for testing turnout clothing insulation
Initial testing with the FCC test method has measured the energy ratio value for 30 different turnout clothing composites. Comparison of these results with the TPP ratings for the same material systems shows that there is little correlation between the results of the two tests, indicating that the Flash Fire Cylinder test provides different information than the TPP test. Similar information is being developed for the cylindrical TPP test.
Both new tests are in the process of becoming standardized methods through ASTM International, and further work is expected to inform the NFPA technical committee for their possible inclusion within NFPA 1971 to supplement current TPP requirements. In the meantime, the standard TPP test remains valid for evaluating thermal protection.
Note: The views of the author do not necessarily reflect those of the sponsor.