Every type of fabric in every garment comes with physical property numbers — a veritable scorecard for that fabric’s performance in various categories. But deciphering these numbers can be confusing.

To choose the right fabrics and garments for your needs, follow these steps:

Step 1: Identify your hazard and the performance standard that applies.

Step 2: Compare physical property numbers side by side and eliminate any fabrics that don’t meet applicable standard(s).

Step 3: Identify/rank your most important properties.

Step 4: Find the fabric with the closest match to your needs, recognizing the trade-offs.

Step 5: Choose reputable suppliers that use your fabric of choice and do wear trials.

Step 6: Make your final choice.

With the above process in place, it’s time to understand some of the common physical properties assigned to FR fabrics, assess the tests used to produce the performance numbers, and interpret the results. Physical properties can be grouped in the following categories:

• Fabric FR capabilities
• Fabric strength
• Fabric appearance

Fabric FR capabilities

• Vertical Flammability (ASTM D6413) – Fabric specimens in both the warp and fill directions are exposed to a regulated flame for a specified time. Char length is measured and the after-flame is noted. There will be a maximum acceptable char length and after-flame required by the specific standard for your application. Look for low numbers: The lower the numbers, the more flame resistant the fabric.
• Thermal Protective Performance (NFPA 2112) – Fabric samples are exposed to a specified heat source on one side with a thermal sensor on the other side. The sensor heat flux reading over time is compared to the Stoll skin burn model that predicts the onset of second-degree burn. The rating is the product of the heat flux measured and the time to sustain a second-degree burn. NFPA 2112 requires a minimum TPP of 6 cal/cm2 with a spacer between the fabric and heat source and 3 cal/cm2 without a spacer. Look for high TPP numbers: The higher the TPP, the more the fabric provides thermal protection.
• Flash Fire Exposure (ASTM F1930) – A standardized coverall of the fabric being tested goes onto an instrumented manikin which is exposed to a specified flame source with thermal sensors measuring the heat transfer through the fabric. The thermal value over time is compared to the same skin burn model as the TPP test. Results provide the predicted percentage of body burn at various exposure times. NFPA 2112 has a maximum allowable body burn percentage of 50 percent after 3 seconds of exposure. Some users focus on the body burn percentage at time intervals of 4 and 5 seconds to see how the protection holds up at longer exposures.
• Arc Thermal Performance Value (ASTM F1959) – Fabric specimens are exposed to a series of electric arc flashes of increasing energy levels on one side with a thermal sensor on the other side. The arc rating results from the exposure energy level that produces a 50 percent probability of a second-degree burn as determined by the Stoll skin burn model. The higher the ATPV number, the greater the arc exposure protection, so look for numbers that meet or exceed your exposure potential.

When examining FR properties, the standard relevant to your workplace, industry and/or job assignment will provide acceptable performance values. Remember, standards only provide minimum or maximum numbers; you may want a fabric that exceeds these marks.

Fabric strength

• Tensile Strength (ASTM D1682) – A strip of woven fabric in both the warp and fill direction is pulled until it has yarn breakage. The force required to cause breakage is the tensile strength. The higher the number, the stronger the fabric. There is not pass/fail criteria, but it can provide a relative strength comparison of various fabrics.
• Elmendorf Tear Strength (ASTM D1424) – Fabric specimens with a small cut in both the warp and fill directions are subjected to a calibrated pendulum, causing a tear. The force required to propagate the tear is measured. The higher the number, the more tear-resistant the fabric. Like tensile strength, this number is also used for comparisons.
• Dimensional Stability (AAATCC 135) – A square of fabric is marked at prescribed measurements before laundering. After laundering, it is re-measured to determine the percentage of shrinkage or growth. Standards often have a maximum allowable shrinkage, or the number can be used for comparison. Lower numbers mean less shrinkage.

Fabric appearance

• Colorfastness to Washing (AATCC 61-3A) – A fabric specimen is washed according to the specified method and number of washings. After washing, the sample is compared to a standardized color scale, and a rating is given. The rating is from 1 to 5 with 5 being no visible change. This rating is somewhat subjective. It’s used to simply compare different fabrics’ ability to hold their color.
• Colorfastness to Xenon light (AATCC 16) – A fabric specimen is exposed to a Xenon light for typically 20 hours. After exposure, the sample is compared to the original fabric, and a rating is given. The rating is from 1 to 5 with 5 being no visible change. Like the wash test, this rating is also subjective and typically used for comparison when light exposure may be an issue.
• Fabric Weight – Although there is no test for fabric weight, it may be another factor to consider in the decision-making process. Typically, lighter weight fabrics are more comfortable in hot conditions. But other factors like wicking and breathability can also affect comfort. Wear testing can help determine comfort levels.

Five out of the six strength and appearance properties are not typically part of any FR fabric or garment standard but are commonly used as comparative measures. The exception is the dimensional stability number, which is sometimes included in various standards.

Final analysis

Before comparing physical properties of various fabrics, identify the hazard and then find the performance standard that applies to that hazard. Once you eliminate any fabrics that don’t comply, compare the numbers of appropriate fabric choices. Often you will find that no one fabric is superior in all the physical properties, making final selection a matter of trade-offs.

For example: A lighter-weight fabric may have better vertical flame, manikin percentage, tensile strength, and dimensional stability numbers, but lower ATPV and lower colorfastness ratings. Another choice may offer a higher ATPV, and higher tensile and tear strength, but lower TPP, higher manikin percentage, and lower colorfastness ratings. You must determine which properties are most important for your workers. It’s also a good idea to have your workers wear test garments before you buy. Always buy garments from a reputable manufacturer, because service before and after the sale can be as important as some of the physical properties.