Softening/melting temperature and flammability evaluation
Fiber-softening temperatures can be evaluated using the ASTM E 2347 standard. In this standard, a fiber specimen is heated in a thermo-mechanical analyzer until the specific modulus of the fiber specimen reaches 6.65 MPa. The temperature at which
Thermal Protective Clothing for Firefighters. http://dx.doi.org/10.1016/B978-0-08-101285-7.00005-8
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the specific modulus becomes 6.65 MPa is identified as an indentation-softening temperature. For some fibers, it may show their softening temperatures in a range; however, this test standard (ASTM E 2347) does not consider the range aspect while calculating the softening temperature. Additionally, this standard is applicable only when the conditions of time, temperature, and method of loading are similar to those specified in the test. This indicates that the data obtained from this test standard cannot predict the behavior of fibers at a higher temperature than the actual test temperature. The ASTM D 7138 standard is also available to determine the melting temperatures of fibers. Using this standard, a fiber specimen is positioned in a melting temperature evaluating device. The temperature of the device is raised until the fiber specimen melts as determined by visual observation. The minimum temperature at which the fiber specimen starts melting is inferred as the melting temperature. The limitation of the ASTM D 7138 is quite similar to the ASTM E 2347, because this standard also does not highlight the range of melting temperatures that might be applicable for some fibers. Also, this standard depends upon the visual observation of an experimenter. Hence, the ASTM D 7138 standard is limited to quality control and research purposes only [312,313].
For flammability evaluation, various standardized test methods have been developed [314,315]. Flammability is defined as how easily a fiber or fabric will burn or ignite, resulting in fire or combustion. It has been identified that various parameters need to be considered for the determination of flammability; these parameters include flame duration, the rate and extent of flame spread, ease of flame extinction, the temperature of burning fibers or fabrics, and the amount of heat during burning [316-321]. The selection of a particular parameter to evaluate flammability depends on the nature of the ignition source, orientation of the tested fibers or fabrics (top, bottom, edge, or face), location of the ignition source, and environmental conditions . As a consequence, various standardize test methods are available to evaluate the flammability of fibers or fabrics .
The flammability of apparel textiles can be measured according to the ASTM D 1230 (as well as ISO 15025) standard method; this test was developed to evaluate the flammability of fabrics made of natural or synthetic fibers. The key with this standard is that it can measure two aspects of fabric flammability: ease of ignition (how fast a fabric catches fire), and flame spread time (the time it takes for the flame to spread a certain distance). In this test, a fabric specimen (50 mm x 165 mm) is cut and brushed (if there is any raised fiber surface on the specimen). Then, this specimen is placed on a flammability tester at a 45 degree angle, and a flame is applied to the lower end of the fabric specimen for 1 s (Fig. 5.1). Then, the time required for the flame to proceed up to the specimen’s other end (a distance of 127 mm) is recorded. This standard applies only to flammable fabrics; thus, this standard may not be suitable for fire-retardant/resistant fabrics. However, this standard can easily, accurately, and reliably identify/distinguish between flammable fabrics.
The flammability of apparel textiles can also be evaluated by the ASTM D 3659 standard, which is also called the semirestraint method. This method was developed to evaluate the flammability of fabrics in a vertical configuration, in which the fabrics have a limited mobility from the vertical plane of suspension. The nature of this test
Fig. 5.1 Forty-five degree flammability tester described in ASTM D 1230.
standard is to simulate the flammability performance of an A-line type garment on a manikin. During the test, a fabric specimen is exposed to a vertical flame, and after exposure, the char-length, after-flame, and after-glow times are measured [323-326] (Fig. 5.2). The reliability of the individual test may vary significantly. Due to this limitation, this test standard has been temporarily withdrawn from the ASTM standard manual.
Fig. 5.2 Vertical flammability tester described in ASTM D 3659.
The fire-retardant/resistant property of a fiber or fabric can be expressed in terms of amount of oxygen required for its combustion, referred to as limiting oxygen index (LOI) as described in ASTM D 2863 [173,327-330]. This standard describes a procedure for measuring the minimum concentration of oxygen (expressed as a volume percentage) that will support the flaming/combustion of a fiber or fabric in a flowing mixture of oxygen and nitrogen. In this test, a small fiber or fabric specimen is supported vertically in a mixture of oxygen and nitrogen, which flow upward through a transparent chimney (Fig. 5.3). The upper end of the specimen is then ignited. The subsequent burning behavior of the specimen is observed in order to measure the time period for which burning continues, and/or the length of the specimen burnt. By testing a series of specimens in different oxygen concentrations, the minimum oxygen concentration is determined.
Fig. 5.3 Limiting oxygen index (LOI) tester by ASTM D 2863.