Performance Evaluation of Fire-Retardant Fabrics & Clothing

WHEN CONSIDERING A HIGH-PERFORMING FIRE PROTECTIVE ENSEMBLE, WHAT CHARACTERISTICS COME TO MIND? FIRST AND FOREMOST IS PROTECTION FROM FLAME AND HEAT HAZARDS, BUT MINIMIZING DISCOMFORT AND HEATSTRESS MUST FOLLOW CLOSELY.

The following discussion will provide technical insight into the methods and use of the TPP (thermal protective performance) and flame manikin systems to assess protection levels, and the sweating guarded hotplate and sweating manikin to evaluate the physiological impact of an ensemble.

Functional garment performance is jointly affected by the design of the garment and by the components the garment is comprised of. It follows naturally that testing methods must also consider both the componentlevel and system-level performance.

Furthermore, tests can be categorized as either properties tests, intended to quantify intrinsic material properties, or performance tests, designed to demonstrate performance in end use. Figure 1 illustrates this in a matrix showing increasing relevance and utility of the experimental results moving from upper left to lower right.

UNRAVELING THE TPP MYSTERY: ASTM F2700, F2703, ISO 17492, NFPA 1971, NFPA 1977, NFPA 2112

The TPP test device is designed to measure component performance, residing in the lower left quadrant of Figure 1. Navigating the history, terminology, and multiple test methods associated with TPP can be intimidating. The TPP testing device exposes a fabric sample to a combined radiant and convective load via quartz tubes and direct flame. Energy transmitted through the sample during exposure is measured using a copper slug calorimeter. However, depending on which standards you follow, your TPP device might be measuring the HTP, TTI or TPE, and these may or may not actually be a measure of protection at all. Simple, right?

The most important distinction between TPP methodologies is the criterion used to determine when exposure will end. The duration of exposure is typically based on comparing the transmitted energy to an empirical performance curve, commonly known as the Stoll curve, which estimates the energy required to generate seconddegree burn injury. Both ASTM F2700 and ISO 17492 specify that the heat exposure be terminated when the transmitted energy crosses the Stoll curve. This simulates exposure to heat until second-degree burn occurs.

In contrast, ASTM F2703 uses the same test apparatus, but modifies the exposure termination to consider the energy stored in the test sample. It has been widely reported that many burn injuries occur as energy absorbed by the protective garment continues to heat the skin. Under F2703, the sample exposure time is iteratively adjusted until the total energy during and after exposure just intersects the Stoll curve. The exposure time as determined by the ASTM F2703 test method will always be shorter than ASTM F2700/ISO 17492. This fundamental difference between ASTM F2700 and F2703 is why the term “thermal protective performance” has been redefined as heat transfer performance (HTP) by ASTM F2700, and thermal performance estimate (TPE) by ASTM F2703. The HTP is most representative of historical TPP measurements, but the TPE is more representative of actual performance against second-degree burn injury.

Choosing the best test methodology is primarily driven by regulatory compliance and the specific standards or protocol required. To illustrate, NFPA 1971 (Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting) and NFPA 1977 (Standard on Protective Clothing and Equipment for Wildland Fire Fighting) both incorporate minimum TPP ratings for garments or ensemble components. Both of these standards reference the experimental method from ISO 17492, modified for heat flux units (cal/cm2/ sec) and exposure heat flux (2 cal/cm2/sec) from ASTM F2700. NFPA 2112 (Standard on Flame-Resistant Garments for Protection of Industrial Personnel against Flash Fire) has been rewritten to incorporate the HTP measurement and method guidance from ASTM F2700 directly.

This is a subtle difference, so it is imperative that anyone specifying and performing the tests have a detailed understanding of the experimental parameters and calculation of results beyond simply referencing a test method.

FLAME MANIKIN – FROM COMPONENT TO SYSTEM TESTING

While it’s an oversimplification to describe the flame manikin method described by ASTM F1930 as a TPP test for a full garment, the fundamental measurement concept is similar. The TPP apparatus exposes a component sample to a controlled amount of energy for a finite time and measures performance relative to burn injury. The flame manikin system exposes a complete garment ensemble to a controlled flash fire simulation for a fixed duration and measures locations and severity of burn injury over the entire body. Flame manikin systems use a more sophisticated analysis than the TPP, since the results must include both predicted second- and third-degree burn injury, Optionally, they should also include pain threshold and first-degree burn for each sensor on the manikin. Manikins typically include 100 to 150 or more individual sensors distributed over the body surface area.

The flame manikin test is designed to evaluate the complete garment system. Lower-percent burn area indicates higher garment performance, with a maximum permissible burn area of 50% (excluding hands and feet). The results of the test include quantitative results of burn severity and location, plus subjective and visual observations, including post-exposure burn duration of the garment, shrinkage or cracking of garment, and smoke generation.

The facilities and safety systems required to operate this test are expensive, but the results obtained and lessons that can be learned are critical for garment systems designed to protect wearers from burn injury.

UNDERSTANDING THE THL TEST – INSULATION AND VAPOR PERMEABILITY

If protective garments were only about external flame and heat hazards, ensemble design would be simple. Use reflective surfaces to reduce radiant loads, and add lots of thermal insulation to protect against high convective environments. Unfortunately, this design approach increases heat stress, putting users at higher risk during extended use. Enter total heat loss (THL), as measured on a sweating hotplate according to ASTM F1868 part C. THL is actually derived from two separate measurements: thermal resistance (Rcf) and apparent water vapor resistance (Refa ).

No experimental method can be fully representative of the wide range of possible exposure scenarios.

The calculated total heat loss is the amount of heat that a sweat-saturated person at 35 oC skin temperature could lose to a 25 oC, 65% RH (relative humidity) environment while wearing the test specimen. The left-side term in the equation represents the “dry” component of heat loss, based on a 10-degree temperature difference, the measured sample Rcf, and an estimated air layer resistance of 0.04 m2C/W. The right-side term represents the “evaporative” component of heat loss based on a 3.75 kPa vapor pressure differential, the measured Ref  and an estimated air layer evaporative  resistance of 0.0035 m2kPa/W.

To meet the minimum THL threshold of 205 W/m2 required by NFPA 1971, a fire protective ensemble must balance both dry and evaporative heat loss components to optimize protection and breathability. Figure 2 illustrates the sensitivity of this total heat loss calculation to thermal and apparent evaporative resistance. A typical sample is compared with imbalanced theoretical scenarios to show the sensitivity of THL to thermal and evaporative resistance.

FULL BODY  THL – FULL ENSEMBLE TESTING ON A SWEATING MANIKIN

Where the flame manikin test provides a quantitative measure of full-ensemble protection, a sweating thermal manikin offers broad capabilities to measure ensemble properties and characterize performance at the system level. Although thermal manikin testing is widely used in the design and competitive benchmarking of ensembles, it is not currently incorporated into certification for protective clothing.

For the protective clothing market, there are three relevant testing methods: 

  • ASTM F1291, Standard Test Method for Measuring the Thermal Insulation of Clothing Using a Heated Manikin 
  • ASTM F2370, Standard Test Method for Measuring the Evaporative Resistance of Clothing Using a Sweating Manikin 
  • ASTM F2371, Standard Test Method for Measuring the Heat Removal, Rate of Personal Cooling Systems Using a Sweating Heated Manikin. 

The dry manikin test of ASTM F1291 measures insulation, while the sweating manikin test of ASTM F2730 measures vapor permeability. These results are the ensemble-measured equivalent of sweating hotplate results (Rcf and Ref a ) when evaluated with the same environment conditions. Thus it is possible for a sweating thermal manikin to calculate a whole-body THL, which represents the area-weighted average total heat loss.

Thermal manikins are frequently used to supplement human simulation studies. For a given garment and environmental condition, a segmented thermal manikin will consistently measure the heat loss by body region. When used prior to human experiments, they can offer valuable insight into experimental methods and potential results, and increase the likelihood of a successful human subject experiment.  When manikins are used in combination with human trials, the data generated provides better quantitative measurements of environmental conditions. Following the completion of human experiments, further analysis of the thermal manikin measurements can help to clarify the varying (and sometimes confounding) data results gleaned from human subjects.

Sweating thermal manikins have traditionally been used only to measure dry and evaporative resistance, but are increasingly being used to simulate human wear trials. Advanced manikin technologies and regulation methods are continually improving thermal manikin physiological realism, providing another tool to evaluate product performance.

We must accept that laboratory tests will be a simplified or idealized representation of the end use, and then understand the basis of design for these tests. Testing methodologies and instruments are continuously evolving to better represent the property or performance metric needed.

By thoughtful and deliberate application of TPP, flame manikin, sweating hotplate, and sweating thermal manikin testing, garment designers and users can gain valuable insight into the component properties and functional performance of flame-resistant garments and ensembles.