Influence of incident energy on the properties of arc rated para-aramid fabric

Abstract

To study the protection mechanism of para-aramid fabrics under an electric arc, the structural composition, surface morphology, and thermal properties of an untreated para-aramid fabric and that treated with different incident energy arcs were compared. The intensity of the N-H peak of the para-aramid cellulose amide decreased with increasing exposed arc energy. Moreover, intensities of the C=O peak of the amide bond type I carbonyl and the C-H deformation vibration absorption peak originating from the benzene ring gradually weakened. In contrast, the carbon content of the fabric increased. In the arc deflagration process, the fiber broke and then carbonized and embrittled. With an arc energy of 30.9 cal/cm², the carbonization degree of the front surface of the fabric increased and was highest at the float line. The initial combustion temperature remained unchanged, and the mass residual quantity gradually increased with increasing arc energy exposed to the fabric. The above results suggest that para-aramid fabric can protect the end user in an arc by carbonization, offering effective assistance in the research and development of personal protective equipment for arc flashes.

Keywords

Para-aramid arc protective fabric electric arc incident energy

An electric arc is a continuous plasma discharge resulting from current flowing through air, which is an abnormally non-conductive media.^1,2 An arc fault can cause hazardous thermal impacts involving instantaneous temperatures as high as 13,000ºC. It can also cause pressure waves, flying particles, noise, and toxic impacts. The main damage caused by an electric arc to the human body is severe burns because of thermal radiation.^3,4 In addition to the danger to operating personnel, common consequences of electric arc injuries can include substantial economic losses due to follow-up treatment problems of workers, equipment damage, and lengthy system interruptions.

There are several options available to limit the risks arising from arc faults. Some of the approaches emphasize arc prevention, while others focus on developed technologies for arc mitigation.^5,6 As noted by Doan et al.,⁷ a comprehensive, arc flash hazard management program that includes hazard analysis to identify and quantify hazardous exposures can mitigate risks. This is even more cost effective when combined with the suitable selection of personal protective equipment and clothing, the application of engineering design solutions to reduce the frequency and magnitude of arcing faults, and improved safety practices at work. Personal protective equipment is the last line of defense for the human body when an arc accident occurs, which can ensure that personnel have sufficient time to escape, avoid, or reduce arc heat damage, and save their lives. Several types of materials are commonly used in electric arc applications, such as flame-retardant-treated cotton, meta-aramid and its blends, modacrylic and modacrylic blends, Lenzing FR, para-aramid blends, and PBI/Kevlar ®blends.⁸ Among these materials, the glass transition temperature of para-aramid fibers is approximately 350°C. When exposed to 150°C for a long time, the shrinkage rate of the fiber is almost zero, and it does not decompose or melt at temperatures as high as 560°C. In the macromolecular chain of para-aramid fibers, the amide group is connected to the para-phenyl group, as shown in Figure 1. Therefore, compared to meta-aramid fibers, para-aramid fibers show higher heat resistance.^9,10

Figure 1.

Molecular structure of poly-p-phenylene terephthalamide.

Several existing studies have focused on evaluating the thermal properties of para-aramid fabrics. Li and Huang¹¹ used a novel high-resolution thermogravimetric analysis (TGA) technique to evaluate the thermal degradation and kinetics of Kevlar fibers in the temperature range of 25–900°C. Tomizuka et al.¹² investigated the thermal degradation behavior of Kevlar fibers under a high pressure of 60 kbar. Thermal degradation under high pressures led to a higher yield of carbonaceous products and the formation of a ‘new’ component, compared with thermal degradation under normal pressures, and this component exhibited higher atomic orientation and graphitization. Zhang¹³ used TGA, differential thermal analysis (DTA), and Fourier transform infrared (FTIR) spectroscopy to compare the thermal degradation processes of Kevlar®49 and Nomex®. These two aramid fibers have similar thermal stabilities, but their thermal degradation processes and temperatures are different. Kevlar®49 is a para-aramid copolymer having a high degradation temperature, and its initial decomposition temperature in air is 548.1°C. Cai and Yu¹⁴ used TGA/FTIR spectroscopy and pyrolysis–gas chromatography–mass spectrometry to determine the thermal degradation behavior of Kevlar®49, Kevlar 129 (polyterephthalamide), Nomex® (polymetaphenylene terephthalamide), and PBO (polyterephthalamide benzisoxazole) fibers. The gases released on heating in air are mainly CO₂, CO, H₂O, NO, and HCN, along with a small amount of NH₃. Brown and Ennis¹⁵ used DTA, TGA, and thermomechanical analysis (TMA) to evaluate the behavior of Kevlar®49 and Nomex® fibers at high temperatures. During thermal analysis of Kevlar®49, oxidation is more likely to occur than thermal decomposition.

In this study, we treated pure para-aramid fabric with different incident arc energies, investigated the differences in fiber structure changes and thermal degradation processes via FTIR/TGA, and observed the surface morphologies of the samples after arc deflagration using optical microscopy (OM)/scanning electron microscopy (SEM) to explore the reaction mechanism of para-aramid fabrics under an electric arc.

Materials and methods

Materials

The para-aramid fabrics used were manufactured by Yantai Taipron Advanced Manufacturing Technology Co., Ltd, China, with a warp and weft yarn density of 315 pieces/10 cm × 228 pieces/10 cm, a fabric structure of 2/1 twill, and a weight of 267 g/m².

Test methods

We laundered the required amount of test material for the test specimens, allowing for fabric shrinkage, in a laundering procedure using the AATCC Test Method 135, Procedure 3, IV, A, and iii, for three laundry cycles.

The fabrics were evaluated according to the ASTM F1959/F1959M-14 standard test method for determining the arc rating of materials for clothing,¹⁶ commissioned by the High Current Laboratory of Kinectrics, Canada. The experimental process used for one sample is shown in Figure 2. Incident energy is a concept defined as the amount of energy generated on a surface at a certain distance from the source when an arc event occurs.¹⁷ The sample dimensions were Ф660 mm × 300 mm, and the incident arc energies were 7.1, 10.0, 13.6, 22.8, and 30.9 cal/cm², respectively.

Figure 2.

Field process of electric arc exposure.

Characterization

Fourier transform infrared spectrometric analysis

FTIR spectra of the original and amphoteric samples were recorded over a frequency range of 400–4000 cm⁻¹ using a Thermo Fisher Nicolet iS50 spectrometer, Thermo Fisher Scientific, USA.

Elemental analysis

The amounts of C, H, N, and S in the samples were determined and analyzed using a Vario EL Cube elemental analyzer, Elementar Trading (Shanghai) Co., Ltd, Germany.

OM

The appearance of the fiber surface was studied using a Leica MC190 HD optical microscope, Leica Microsystems Ltd, Germany.

SEM

Morphologies of the para-aramid fibers sprayed with platinum film were studied using a Zeiss Supra5 scanning electron microscope, Carl Zeiss AG, Germany.

Synchronous thermal analysis

Thermal properties of the samples were determined and analyzed under N₂ protection at a heating rate of 20°C/min, from 26°C to 1000°C, using a Netzsch STA 449 F5 synchronous thermal analyzer, Netzsch-Gerätebau GmbH, Selb, Germany.

Results and discussion

FTIR characterization of the para-aramid fabric

Figure 3 shows the infrared spectra of the para-aramid fiber before and after arcing. Upon examination of the structural elements of PPTA (poly-p-phenylene terephthalamide), the characteristic absorption peaks of the para-aramid fibers showed an amide NH stretching vibration (wavenumber = 3292 cm⁻¹) and an amide type I single absorption peak (wavenumber = 1639 cm⁻¹, carbonyl C=O stretching vibration). They also included amide II and amide type III double absorption peaks (1540, 1508 cm⁻¹) that appeared near 1540 and 1300 cm⁻¹ (O=CN, NH deformation coupling vibration). Amide type III (O=CN deformation coupled vibration) also features, as does the benzene ring CH deformation vibration with a wavenumber of 820 cm⁻¹ (para-disubstituted) and amide type III at a wavenumber of 650 cm⁻¹ (O=CN in-plane bending vibration).^18,19

Figure 3.

Fourier transform infrared spectra of para-aramid fabric under different incident energies.

Comparing a and the spectral curves b–f after the arc action in Figure 3, it can be seen that with increasing arc energy, the intensity of the NH peak of the amide with a wavenumber of 3292 cm⁻¹ gradually weakened, and the wavenumber became 1639 cm⁻¹ as the fiber was damaged. The C=O peak intensity of the amide bond type I carbonyl group gradually weakened, and the CH deformation vibration absorption peak on the benzene ring with a wavenumber of 820 cm⁻¹ also gradually weakened, indicating that the para-aramid fiber polymer gradually degraded under arc deflagration.

Elemental analysis of para-aramid fabric

To further determine the change in the carbon content of the para-aramid fiber after arc deflagration with different energies, elemental analysis was performed on the samples before and after arc action.

The content of C, H, N, and S elements and the C/H and C/N atomic ratios in each sample were compared. Table 1 shows that C element content of the para-aramid fiber without arc action was 66.29%; as the arc energy increased, the C element content in the fabric gradually increased. When the arc energy was 30.9 cal/cm², the C element content was the highest, reaching 70.21%. The N and H element contents of all samples were similar.

Table 1.

Elemental composition of samples

	Element mass percentage/%				Element mass ratio
Ei/(cal·cm^–2)	C	H	N	S	C/H	C/N
0	66.29	4.506	11.09	0.513	14.712	5.977
7.1	67.91	4.388	11.36	0.403	15.476	5.978
10.0	68.58	4.341	11.46	0.357	15.798	5.984
13.6	69.05	4.307	11.51	0.32	16.032	5.999
22.8	69.59	4.231	11.59	0.254	16.448	6.004
30.9	70.21	3.975	11.27	0.213	17.663	6.230

The atomic ratios of C/H and C/N were calculated and compared to more clearly reflect the differences of each sample. The results show that as the arc energy increases, the C/H and C/N atomic ratios of the fiber gradually increase.

Surface topography analysis

The appearance of the para-aramid fabric after arc deflagration of different energies is shown in Figure 4.

Figure 4.

Appearance of para-aramid fabric before and after electric arc exposure.

Figure 4(i) shows that the fabric that has not been subjected to the arc has a bright color and clear hairiness; with increasing exposed arc energy, the para-aramid fiber appears blackened and carbonized at the floating line. The degree is always higher than that at other positions, and the area of black charring on the front surface of the fabric gradually increases, while the color gradually darkens. Figure 4(ii) shows that with increasing arc energy, the color change of the back of the fabric is initially small and the material is not significantly affected, indicating that the fabric has some resistance to arcs with low energy values. When the arc energy reached 22.8 cal/cm², the color of the back of the fabric changed slightly from light yellow to dark yellow. When the arc energy reached 30.9 cal/cm², the fibers on the back of the fabric clearly broke, generating a large number of holes at the same time. The color deepened and blackened, and the degree of carbonization increased. The cross-section of the fabric in Figure 4(iii) shows that as the arc energy increased, the carbonization intensity also gradually increased.

Thickness measurement

Figure 5 shows the variation in thickness of the para-aramid fiber under different incident arc energies. During the arc action, the thickness of the para-aramid fabric first decreases with increasing arc energy and then increases. At low arc energies, the molecular structure of the para-aramid fabric disintegrates and the thickness decreases. When the arc energy is 13.6 cal/cm², the thickness decreased by 9.4%. As the energy further increased, the degree of carbonization of the fabric gradually increased, and the formed carbon adhered to the surface, thereby increasing the thickness. When the arc energy was 30.9 cal/cm², the thickness increased by 10%.

Figure 5.

Sample thicknesses compared under different incident energies.

SEM characterization of para-aramid fabric

Figure 6 shows the SEM images of the para-aramid fiber before and after arc burning (the left- and right-hand images are at 100× and 700× magnification). After the fabric was washed with water, the surface fibers floated, and the stem feathers were clear. After arc deflagration, the fiber surface exhibited clear fractures and embrittlement, but did not shrink and deform. This indicated that the thermal expansion coefficient of the para-aramid fiber was small, with good dimensional stability. For an arc energy of 7.1 cal/cm², the fibers on the surface of the fabric had obvious fractures and severe local embrittlement. When the arc energy was equal to or exceeded 13.6 cal/cm², the inner fibers of the fabric also distinctly became carbonized and embrittled.

Figure 6.

Scanning electron microscopy photographs of para-aramid fabric surfaces under different incident energies.

Thermogravimetric analysis

Figure 7 shows the thermogravimetry–differential thermogravimetry (TG–DTG) curve of the para-aramid fiber before arcing. The thermal decomposition process of aramid 1414 fabric consisted of three stages. The first stage is a micromass loss stage from 26°C to 135°C, with a mass loss rate of 1.6%, which is mainly the process of losing intermolecular bound water. The second stage is the thermal decomposition of para-aramid fibers in the temperature range of 387–645°C. Since para-aramid fiber is a para-linked benzamide, its amide bond forms a large π bond conjugate structure with the benzene ring group, and the internal rotation potential is exceedingly high. Under these conditions, the molecular structure is highly stable, and the thermal decomposition temperature is very high. When the temperature reached 587°C, the para-aramid fiber molecular chain started breaking down and decomposed randomly, which led to complex reactions, such as violent degradation, carbonization, and possible cross-linking, resulting in significant quality loss. The third stage is the carbonization stabilization process in the temperature range of 645–1000°C. The para-aramid fibers have now been degraded and carbonized. As the temperature continued to increase, the mass loss of the residue slowly decreased, which shows that the DTG curve tends to peak at 1000°C, and the mass loss rate was 37.6%.

Figure 7.

Thermogravimetry–differential thermogravimetry (TG–DTG) curves of untreated para-aramid fabric.

Figure 8 shows the TG curves of the para-aramid fiber before and after arc deflagration with different incident energies. The initial combustion temperature of the TG curves is essentially the same, the maximum decomposition rate was reached at approximately 587°C, and the quality decline trend line gradually flattened. As the exposed arc energy increased, the residual amount of fabric quality gradually increased.

Figure 8.

Thermogravimetry curves of para-aramid fabric under exposure to different incident energies.

Table 2 lists the thermogravimetric parameters of the para-aramid fabric before and after arc treatment. It is evident from the TGA indexes that the initial decomposition temperature, corresponding to a specific mass loss, reaches the maximum decomposition temperature. The termination decomposition temperature is close. Figure 8 shows that the six TG curves have a high degree of coincidence and are almost overlapping. When the incident energy is 30.9 cal/cm², the maximum decomposition rate temperature of the para-aramid fiber is significantly reduced. This may be due to the high degree of carbonization of the para-aramid fiber under the action of high arc energy. The maximum decomposition rate can be reached at a low temperature. In addition, the residual mass of the six samples at 1000°C increased with increasing incident energy. When the incident energy was 30.9 cal/cm², the residual mass of the para-aramid fabric at 1000°C reached 51.9%. Thus, the arc affects the thermal stability and component properties of the para-aramid fiber.

Table 2.

Thermogravimetric analysis indexes of para-aramid fabric before and after arc treatment

Ei/(cal·cm^–2)	Indicators in the process of quality loss/°C			Residual mass at 1000°C/%
Ei/(cal·cm^–2)	Initial decomposition temperature	Maximum decomposition rate temperature	Final decomposition temperature	Residual mass at 1000°C/%
0	401.5	587.5	645.6	37.6
7.1	405.6	587.6	645.3	37.2
10	405.7	587.8	647.6	37.5
13.6	405.9	588.2	645.3	38.0
22.8	406.0	592.1	644.8	40.2
30.9	406.2	583.6	644.5	51.9

Conclusion

Para-aramid fabrics with ATPV (arc thermal performance value) of 8.9 cal/cm² were exposed to different incident energies, and their behavior was characterized by different analytical methods. In addition, their behavior in practical use was investigated.

FTIR spectroscopy showed that the intensities of the amide NH peak, C=O peak of the amide bond type I carbonyl group, and CH deformation peak originating from the benzene ring weakened. However, no new substances were generated during the arc deflagration process, proving that the arc explosion is mainly a physical explosion, caused by the rapid conversion of electrical energy into heat. With increasing arc energy, the degree of carbonization of the para-aramid fabric increased; however, it still showed excellent dimensional stability, which is reflected in the fact that the fiber does not undergo any shrinkage. TG showed weight loss of the tested materials but not pyrolysis in particular. Visualization of the residues of the combusted materials by SEM confirmed the theory of different decomposition mechanisms during heat exposure.

The responses of different materials in an actual arc event differ based on many variables during the event. This study characterizes how para-aramid fabrics protect the user in case of carbonization from an arc. From a scientific perspective, the study provides a good understanding about how fabrics respond for protection in an arc event and offers effective assistance in the research and development of personal protective equipment.

Footnotes

Declaration of conflicting interests

The authors have no conflicts of interest to declare.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Key R&D Project, China (2018YFC0810302), and the Natural Science Research Project of Jiangsu Province, China (18KJB540002).

ORCID iD

Hong Tang

References

Fedoroff Basil

T (eds).

Encyclopedia of explosives and related items. Dover, NJ: Picatinny Arsenal, 1960–1983.

Parikn

Allcock

Luna

, et al. A novel approach for arc-flash detection and mitigation: at the speed of light and sound. IEEE Trans Ind Appl 2014; 50:1496–1502.

Laverty

Protective performance of arc protective clothing. Power Saf Technol 2008; 10: 64–68.

Short

TA.

Arc-flash analysis approaches for medium-voltage distribution. IEEE Trans Ind Appl 2011; 47: 1902–1909.

Kumpulainen

Hussain

Rival

, et al. Aspects of arc-flash protection and prediction. Electr Pow Syst Res 2014; 116: 77–86.

Floyd HL Doan

, et al . Arc flash hazards and electrical safety program implementation. In: Louie JP (ed.) Industry Applications Conference, Hong Kong, China, 2--6 October 2005, pp.1919--1923. Vol.3, no.54. Piscataway: IEEE Operations Center.

Doan

Floyd

Neal

TE.

Comparison of methods for selecting personal protective equipment for arc flash hazards. IEEE Trans Ind Appl 2004; 40: 963–969.

Hoagland

Flame resistant textiles for electric arc flash hazards.

Handbk Fire Resist Text 2013; 20: 549–580.

Kondo

Honshida

Amano

Flame retardant adhesive sheet, coverlay film and flexible copper-clad laminate using same. Patent 782074[P], USA, 2010.

10.

Song

LZ.

New high-efficiency arc protective clothing––meta-aramid protective clothing. Techn Text 1997; 06: 29–32.

11.

Huang

MR.

Thermal degradation of Kevlar fiber by high-resolution thermogravimetry. J Appl Polym Sci 2015; 71: 565–571.

12.

Tomizuka

Ogawa

Tanaka

High pressure thermal degradation of Kevlar fiber. Tanso 1983; 115: 155–159.

13.

Zhang

HT.

Analysis of thermal degradation process of aramid fibers. Shanghai Text Sci Technol 2012; 40: 8–10.

14.

Cai

WD.

Study on the thermal degradation of high performance fibers by TG/FTIR and Py-GC/MS. J Therm Anal Calorim 2011; 104: 757–763.

15.

Brown

Ennis

BC.

Thermal analysis of Nomex® and Kevlar® fibers. Text Res J 2016; 47: 62–66.

16.

ASTM F1959:1999. Standard test method for determining the arc thermal performance value of materials for clothing.

17.

IEEE Std 1584TM:2002. IEEE guide for performing arc-flash hazard calculations.

18.

Zhu

Ren

, et al. Analysis of fabrics made by PPTA. J Zhongyuan Univ Technol 2011; 22: 22–24.

19.

Chen

GQ.

Analysis of molecular structure of domestic para-aramid fiber. Shandong Text Sci Technol 2015; 56: 49–52.