Friday, March 9, 2018

How to Test Heat Resistant Fabrics

In any hazardous work setting, a worker is exposed to a multitude of risk factors, hence the fire resistance clothing is supposed to cater to all the different risk scenarios. This is the reason that fire resistant fabric manufacturers run the fabric samples through multiple different tests to ensure complete and proper compliance with personal safety and protection standards.

How to Test Heat Resistant Fabrics:

Performance tests for FR fabrics:
The protective fabric or garment is tested for its flame and heat resistance, strength, and response on numerous scales. These performance tests and criteria not only determine the performance of the fabric or garment against a specific hazard but also predict the extent of burn injury in a situation. Let us first see what those different requirements are that the FR clothing must pass for.
Fire retardant fabric test
Fig: Fire retardant fabric test
FR fabric performance requirements:
Depending upon the workplace setting, climate, the type of machinery or processes involved, a selection of materials is exposed to different tests that scale test samples on safety criteria such as level of protection, durability, and fabric strength as well as comfort and ease of use for everyday wear. These various factors can be categorized as follows:
  • Safety Factors – Such as flame resistance, thermal protection, arc flash protection, metal splash
  • Resilience Factors – Durability, Abrasion resistance, shrinkage, breaking point
  • Comfort Factors – Thermal comfort, breathability, functional fit, freedom of motion etc.
Laboratory performance tests cater to the first two of these sets of factors i.e. safety and resilience.

Flame resistance test methods:

Thermal Protective Performance Test on Fabric Sample including Multiple Layers:
The thermal Protective Performance test is used to measure and determine the exposure time and thermal dose (TPP rating) to a fabric in an event of second degree burn injury. A heat flux of 83kw/m2 is applied to the outer surface of the fabric which causes a second-degree burn. The test collects data during this exposure to determine the amount of heat energy transmitted through the fabric via a sensor positioned at the innermost surface of the fabric sample.

This test method uses a combination of convective and radioactive heat flux to be applied to the test fabric for 30 seconds to measure the thermal dose or the transmitted energy per unit area, at skin location. For the fabric sample to pass the test, the criteria sanction a maximum thermal dose of 25 kj/m2 (6 cal/cm2) for spacing of a quarter inch and a thermal dose of 12 kj/m2 (3 cal/cm2) at the contact surface.

ASTM F1930 – (Standard Test Method for Evaluation of Flame Resistant fabrics for Protection against Fire Simulations)
This test method is used to measure and evaluate the heat resistance provided by varied materials, garments, products, and systems upon exposure to specified heat and flame under controlled conditions. However, the test does not measure the response for all factors required in a fire hazard or fire risk assessment of materials.

The aim of the test is to provide a measurement of predicted burn injury for the single layer of a specified garment or protective clothing ensemble on a stationary upright manikin of specified dimensions, and gauging the performance of the garment or ensemble against a laboratory fire simulation of an average heat flux of 84kw/m2 (2 cal/cm2) with durations of up to 20 seconds. The visual and physical damage to the single layer garment or ensemble is then recorded to help determine the overall performance of the test sample and the extent of predicted skin burn injury that may be caused in an actual situation.

However, ASTM F1930 is not a quality assurance test and does not include high radiant exposures to simulate arc flash exposures, liquid or solid fuel exposures or explosive nuclear exposures.

ASTM F1939 – (Standard Test Method for Radiant Heat Resistance of Flame Resistant Clothing Materials)
Workers in a hazardous work setting may use a single material or a combination of materials in order to attain a sustainable degree of protection against the risk situation. The objective of this test is to determine the overall heat-resistant performance of a material or a combination of materials or a comparison of different materials used in a protective clothing ensemble exposed to a continuous and constant radiant thermal hazard.

The test gauges the thermal resistance or insulating characteristics of FR clothing materials, keeping the specimen in a static, vertical position exposed to a heat flux of either 21 kW/m2 (0.5 cal/cm2) or 84 kW/m2(2 cal/cm2). The test may also use other optional specified conditions to represent an expectant hazard involved in a situation.

ASTM D6413 / D6413M – (Standard Test Method for Flame Resistance of Textiles)
The is a vertical flame resistance test for textiles with the scope to determine the response of a test sample to a standard ignition source and application time for both electric arc and flash fire hazards. The specimen is maintained in a static, draft-free, upright position to measure the after-flame time, after-glow time, and char length. It does not involve any sort of movement except whatever caused because of exposure.

This test method was adopted from Federal Test Standard No. 191A method 5903.1 and is limited to studying the response of materials, products or assemblies to heat and flame under controlled laboratory conditions without any representation of actual hazard conditions. Therefore, the laboratory precision conditions for this test have not been established.

ASTM F1959 / F1959M – (Standard Test Method for Determining the Arc Rating of Materials for Clothing)
This test method uses flat specimens to measure and describe the properties of FR materials, products, or assemblies in response to an electric arc (convective and radiant energy) under controlled laboratory conditions. The test records and measures the arc ratings for varied materials to be used for fire resistant clothing intended for workers exposed to electric arc hazards that would generate heat flux from 84 up to 25,120 kw/m2 (2 to 600 cal/cm2).

The materials used for the test will meet the following requirements for arc ratings: less than 150 mm [6 in.] char length and less than 2 seconds after-flame when tested. However, the test is not applicable to incidents of electrical contact or electric shock hazards.

Tips for Buyer:
When choosing the right work wear fabric, there are several other factors that come into play providing the best possible protection to the wearer. These factors must do with the material and production of the fabric.

Fiber blends – Obviously it counts what kind of fibers has been used to make up the fabric and garment. For instance, durable fibers such as nylon and aramids make for stronger fabrics that possess greater resistance to abrasion and higher tear strength.

Fabric Construction –The fabric construction affects the durability and quality of the final fabric in several ways. In fact, each step of the production process has something to add to the fabric. For instance, a twill weave is far more durable compared to a plain weave. Similarly, a fabric with a higher number of yarn counts would have higher tensile strength compared to the higher tear strength of less number of threads used, and so on and so forth.

Garment Construction – Another crucial step, garment construction is a combination of different processes such as garment design, fitting, and trims; all of which contribute towards durability and practicality of the FR garment.

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