non-destructive assessment of outer shell degradation for ... · final report by: r. bryan ormond,...

© 2017 Fire Protection Research Foundation

1 Batterymarch Park, Quincy, MA 02169-7417, USA Email: [email protected] | Web: nfpa.org/foundation

Non-Destructive Assessment of Outer Shell Degradation for Firefighter Turnouts FINAL REPORT BY:

R. Bryan Ormond, Ph.D., William J. Gabler, Ph.D. Textile Protection and Comfort Center North Carolina State University Raleigh, NC October 2017

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FOREWORD Current retirement criteria for firefighter protective clothing including outer shell materials

establishes a maximum service life in NFPA 1851, Standard on Selection, Care, and Maintenance

of Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting. However, this

maximum service life does not compensate for amount of usage and exposure time, i.e., a major

city fire department versus a rural fire department fire volume. A non-destructive method is

needed to evaluate the outer shell integrity to determine when a garment is no longer protecting

the wearer at peak efficiency.

The goal of this project was to determine whether Fourier Transform Infrared Spectroscopy (FTIR)

can be used as a non-destructive measurement technique to monitor the degradation of outer

shell fabrics of firefighter turnouts. This summary describes the 1) Outer shell materials evaluated

2) Methods used to expose the samples and to test them, and 3) Results from the material-level

testing.

The Fire Protection Research Foundation expresses gratitude to the report authors R. Bryan

Ormond, and William J. Gabler with Textile Protection and Comfort Center at North Carolina State

University located in Raleigh, NC. The Research Foundation appreciates the guidance provided

by the Project Technical Panelists, and all others that contributed to this research effort. Thanks

are expressed to the National Fire Protection Association (NFPA) for providing the project funding

through the NFPA Research Fund.

The content, opinions and conclusions contained in this report are solely those of the authors and

do not necessarily represent the views of the Fire Protection Research Foundation, NFPA,

Technical Panel or Sponsors. The Foundation makes no guaranty or warranty as to the accuracy

or completeness of any information published herein.

About the Fire Protection Research Foundation

The Fire Protection Research Foundation plans,

manages, and communicates research on a broad

range of fire safety issues in collaboration with

scientists and laboratories around the world. The Foundation is an affiliate of NFPA.

About the National Fire Protection Association (NFPA)

Founded in 1896, NFPA is a global, nonprofit organization devoted to

eliminating death, injury, property and economic loss due to fire, electrical and

related hazards. The association delivers information and knowledge through

more than 300 consensus codes and standards, research, training, education,

outreach and advocacy; and by partnering with others who share an interest in

furthering the NFPA mission.

All NFPA codes and standards can be viewed online for free.

http://www.nfpa.org/foundation

http://www.nfpa.org/codes-and-standards/free-access

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NFPA's membership totals more than 65,000 individuals around the world.

Keywords: turnout, outer shell, degradation, FTIR, trapezoidal tear strength, non-destructive measurement, meta-aramid, para-aramid, polybenzimidazole

Report number: FRPF-2017-13

http://www.nfpa.org/member-access

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PROJECT TECHNICAL PANEL

Jeffrey Stull, International Personnel Protection

Stephen King, Chair Structural and Proximity Fire Fighting Protective

Clothing and Equipment TC

William Haskell, National Institute for Occupational Safety & Health

Dave Trebisacci, NFPA

Steven Townsend, City of Carrollton Fire Rescue

Tim Tomlinson, Addison Fire Department

PROJECT SPONSORS

National Fire Protection Association

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EXECUTIVE SUMMARY

The goal of this project was to determine whether Fourier Transform Infrared Spectroscopy (FTIR)

could be used as a non-destructive measurement technique to monitor the degradation of outer

shell fabrics of firefighter turnouts. In this study, commonly fielded turnout outer shell materials

comprised of para-aramid, meta-aramid, and polybenzimidazole were subjected to ultra-violet

(UV) exposures, standardized laundering cycles, and repeated radiant heat exposures followed

by chemical surface analysis via FTIR and tear strength measurements.

For the UV exposures, the materials were exposed for 2.5, 5, 7.5, and 10 continuous days of UV

light in an Atlas Ci3000+ Weather-Ometer. The laundered samples were subjected to the NFPA

1971 (8.1.2) laundering procedure for 10, 26, and 50 cycles. For the radiant heat exposures, the

samples were exposed to a blackbody radiant heat source with a heat flux of 0.2 cal/cm2/s for 10,

25, and 50 exposures.

Following the exposures, the material surfaces were analyzed through non-destructive FTIR

measurements to determine if any noticeable changes had occurred in the chemical structures of

the materials. The exposed samples were then subjected to the trapezoidal tear strength test to

physically measure any degradation that may have occurred.

UV light caused significant changes in trapezoidal tear test for all samples at all levels. Only the

highest number of launderings (50) resulted in changes, for all three fabrics. This could be the

result of changes in friction and yarn structure, not necessarily fiber degradation; however, FTIR

signal intensity was found to be influenced by laundering. Only the polybenzimidazole (PBI)

material was affected by radiant exposure, though the change was small and inconsistent.

Interestingly, transmitted heat flux was not affected by the repeated radiant exposures.

For the para-aramid and meta-aramid materials, several FTIR peaks were found to have some

level of linear correlation with the changes in tear strength. The peak at 1730 cm-1, which is

generally characteristic of a carboxylic acid functional group (a known degradation product for

aramid fibers), produced a correlation value (R2) of 0.95285. Further statistical analysis should be

conducted in the future to determine if any of the data follow correlations other than linear trends.

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NC State University

Textile Protection and Comfort Center

1

Table of Contents List of Figures ............................................................................................................................................... 1

List of Tables ................................................................................................................................................ 2

Summary ....................................................................................................................................................... 3

Background ................................................................................................................................................... 3

Outer Shell Materials .................................................................................................................................... 3

Methods ........................................................................................................................................................ 4

Trapezoidal Tear Results .............................................................................................................................. 7

Transmitted Heat Flux Results .................................................................................................................... 13

FTIR Results ............................................................................................................................................... 13

Discussion and Conclusions ....................................................................................................................... 18

Recommendations ....................................................................................................................................... 18

References ................................................................................................................................................... 18

Appendix: FTIR Results ............................................................................................................................. 20

List of Figures

Figure 1. Photograph of NCSU’s Stored Energy Tester; Fabric specimen is being exposed to the

radiant source on the left. .............................................................................................................................. 5

Figure 2. Photograph of the fabric samples loaded into the Atlas Ci3000+ Weather-Ometer ........ 5

Figure 3. Photo of a trapezoidal tear test being performed .............................................................. 6

Figure 4. Photograph of the NCSU FTIR test instrument ................................................................ 7

Figure 5. Trapezoidal Tear Results from the UV Exposures for the Fabric B and Fabric C ........... 9

Figure 6. Trapezoidal Tear Results from the UV Exposures for the Fabric A ................................ 9

Figure 7. Trapezoidal Tear Results from Radiant Exposures for Fabric B and Fabric C .............. 10

Figure 8. Trapezoidal Tear Results from Radiant Exposures for the Fabric A .............................. 10

Figure 9. Trapezoidal Tear Results from the Laundering for Fabric B and Fabric C .................... 11

Figure 10. Trapezoidal Tear Results from the Laundering for Fabric A ....................................... 11

Figure 11. Trapezoidal Tear Results from UV Exposures Followed by one cycle of Laundering

(Wash/Dry) for Fabric B and Fabric C ....................................................................................................... 12


(Wash/Dry) for Fabric A ............................................................................................................................. 12

Figure 13. Average Total Transmitted Heat Flux passing through the fabric samples during the

first and fiftieth radiant heat exposure ........................................................................................................ 13

Figure 14. Control spectra of the three test fabrics ........................................................................ 15

Figure 15. Fabric A after Radiant exposure ................................................................................... 20

Figure 16. Fabric A after UV Exposures ....................................................................................... 21

Figure 17. Fabric A after Laundering ............................................................................................ 22

Figure 18. Fabric C after Radiant Heat .......................................................................................... 23

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Figure 19. Fabric C after UV Exposures........................................................................................ 24

Figure 20. Fabric C after Laundering ............................................................................................ 25

Figure 21. Fabric B after Radiant Heat .......................................................................................... 26

Figure 22. Fabric B after UV Exposures........................................................................................ 27

Figure 23. Fabric B after Laundering ............................................................................................ 28

Figure 24. Fabric A after combined UV and Laundering .............................................................. 29

Figure 25. Fabric C after Combine UV and Laundering ............................................................... 30

Figure 26. Fabric B after Combined UV and Laundering ............................................................. 31

List of Tables

Table 1. Details of the Test Materials .............................................................................................. 4

Table 2. Steel-Dwass Results for UV Exposure .............................................................................. 8

Table 3. Steel-Dwass Results for Radiant Exposure ........................................................................ 8

Table 4. Steel-Dwass Results for Laundering .................................................................................. 8

Table 5. Molecular structures of the polymers .............................................................................. 14

Table 6. Assigned Bands for Meta-Aramid and Para-Aramid ....................................................... 14

Table 7. Linear Coefficients of Determination between FTIR intensity and Tear Strength .......... 16

Table 8. Test results comparing the UV and UV-Wash FTIR intensities ...................................... 17

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Summary

The goal of this project is to determine whether Fourier Transform Infrared Spectroscopy (FTIR) can be

used as a non-destructive measurement technique to monitor the degradation of outer shell fabrics of

firefighter turnouts. This summary describes the 1) Outer shell materials evaluated, 2) Methods used to

expose the samples and to test them, and 3) Results from the material-level testing.

Background

Previous studies have used spectroscopic methods to investigate changes in high performance fibers and

other materials commonly used in firefighter clothing. A study on the combined hydrolytic and

photochemical ageing of a para-aramid/polybenzimidazole (PBI) blend found that humidity and

photochemical ageing caused reductions in breaking strength. FTIR analysis could relate this change to a

rise in [-COOH] bonds, which were modeled by temperature-dependent reaction kinetics.1 A similar

approach was used to monitor photochemical ageing of an e-PTFE/meta-aramid composite, finding that

UV irradiation had a negative impact on meta-aramid strength and membrane permeability. This study

claimed that the [-COOH] bond was the only consistent change in the aramid under FTIR analysis.2 A

combination of ATR-FTIR and confocal microscopy found similar results that UV light was the primary

source of damage to para-aramid/polybenzimidazole and meta-aramid/para-aramid blend fabrics. Visual

analysis observed pitting of the fibers and spectroscopic analysis observed changes in several absorbance

bands. The results were used to recommend service lifetimes and storage guidelines for turnout ensembles.3

Other studies have explored the use of raman spectroscopy and NMR analysis, especially for PBI, which

has not shown significant changes in FTIR spectra as a result of environmental stresses.4 Raman

spectroscopy was used to measure changes in the microstructure and molecular structure of para-aramid

with a focus on changes caused by thermal exposure.5 These studies suggest that thermal, UV, and moisture

effects can cause changes in the physical properties of firefighter turnout ensemble materials and that

spectroscopic methods can be used to investigate these changes. However, none of the approaches thus far

have explored the predictive capability of such an approach for whole-fabric samples with the inclusion of

radiant heat and laundering effects.

Outer Shell Materials

All three of the materials used are commonly available outer shells and were donated by manufacturers for

use in this project. The identification and details for each material are provided in Table 1. Fabric A was a

70% polybenzimidazole/30% para-aramid fabric, Fabric B was a 50% para-aramid/50% meta-aramid blend,

and Fabric C was mostly a meta-aramid fabric (~95%) with some (~5%) para-aramid fibers added for

stability.

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Table 1. Details of the Test Materials

Fabric A:

-70% polybenzimidazole/para-aramid

spun yarns

-30% para-aramid filament

-Weight = 7.0 oz/sq yd

-Color: Gold

Fabric B:

-50% para-aramid

-50% meta-aramid


-Color = Black

Fabric C:

-95% meta-aramid

-5% para-aramid


-Color = Red

Methods

Three exposure stimuli were chosen to represent stressors that would be typical to a firefighter turnout in

the field. These were: radiant heat (lower level), laundering (wash and dry cycle), and sunlight (ultraviolet

radiation). After exposure, the fabric samples were tested using FTIR and the trapezoidal tear method. The

results from each set of exposures were compared to “unexposed” materials, and the results can be seen in

the following sections. All reported values of tear strength and FTIR spectra are the mean of six

measurements for each fabric.

The radiant heat exposures were performed using the Stored Energy Testing platform (Figure 1) without

compression, where the heat is provided by a temperature controlled ceramic blackbody radiant heat source

set to 0.2 cal/cm2/s. The 6”x6” fabric specimen is loaded into the pneumatically driven holder that moves

the fabric in front of the radiant heat source. A heat flux sensor is located behind the fabric specimen and

measures heat flux transmitted through the fabric. The heat flux sensor and its surrounding plate are water

cooled to 35°C to provide more accurate long duration heat flux and to better replicate the isothermal nature

of human skin. For this project, the outer shells were exposed for 60-second intervals at 0.2 cal/cm2/s and

allowed to cool back down to room temperature between each test. Each of the three sample fabrics were

tested using this method for 10, 25, and 50 radiant exposures.

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Figure 1. Photograph of NCSU’s Stored Energy Tester; Fabric specimen is being exposed to the

radiant source on the left.

Laundering was performed using the wash and dry cycles specified in NFPA 1971 8.1.2. This is Machine

Cycle 1, Wash Temperature V, and Drying Procedure Ai of AATCC 135 Dimensional Changes in

Automatic Home Laundering of Woven and Knit Fabrics. For this project “1 Laundering” is defined as both

the wash and dry cycle, which were performed 10, 26, and 50 times.

The UV exposures were performed using the Atlas Ci3000+ Weather-Ometer shown in Figure 2. This

device has a solar lamp located in the center, and the fabric specimens are loaded in a circle facing inward

towards the light. The instrument also regulates the temperature and humidity of the environment to more

closely resemble outside weather. For this project, the instrument was set to represent a summer’s day in

Phoenix, Arizona (highest UV setting), with UV radiation of 350 MJ/m2, temperature of 39°C, and relative

humidity of 32%. The outer shells were exposed to 2.5, 5, 7.5, and 10 continuous days of sun and tested

after each interval.

Figure 2. Photograph of the fabric samples loaded into the Atlas Ci3000+ Weather-Ometer

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Unexposed and exposed samples were scanned using an FTIR (non-destructive) instrument followed by

the trapezoidal tear test (destructive) and compared. From the recommendation of the technical panel,

trapezoidal tear is a primary indicator of outer shell failure in a turnout suit. The trapezoidal tear resistance

was performed according to NFPA 1971 (2013) Section 8.12. The fabric sample is held in place by two

grips, a small slit is cut into the side of the fabric (Figure 3), and a pulling force is applied until the fabric

rips. This test is typically run in both the warp and weft directions, but due to limited fabric amounts, the

samples were only torn in the weft direction, which represented the “worst-case scenario.” However, the

three fabrics were relatively balanced and performed similarly in warp and weft directions. For NFPA 1971,

woven fabrics must have a tear strength of not less than 100N. Due to the high strength of these materials,

the highest level grips and load cell were used for the instrument and occasional “slip” still occurred.

Figure 3. Photo of a trapezoidal tear test being performed

FTIR spectroscopy was performed using a bench-size instrument for this project (Figure 4), but handheld

devices exist. FTIR is a technique where a controlled beam of electromagnetic energy in the infrared

spectrum is applied to the fabric sample and the amount of reflectance/absorbance is quantified. This is

performed for all wavenumbers of the infrared spectrum (500-4,000 cm-1) and the presence of “peaks”

represent the abundance of particular bonds within the fabric specimen. Of particular interest for the aramid-

based materials is the peak located at the wavenumber of 1730 cm-1, which represents the formation of a

carboxylic acid (-COOH) bond as a result of chain scission between the benzene rings in the aramids. This

bond is typically the first to break down when exposed to mechanical or energy stimuli.

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Figure 4. Photograph of the NCSU FTIR test instrument

Trapezoidal Tear Results

Fabrics B and C (para- and meta-aramid blends) performed similarly for the trapezoidal tear, and their data

are included on the same graph. The tear strengths for Fabric A were significantly higher due to the

incorporation of the polybenzimidazole fiber, so the results were included on separate graphs. The data

were analyzed first using the Median Test to detect if the median of each level of treatment was significantly

different from the group, then using the Steel-Dwass method to test for differences in the tear strength

resulting from the various levels of treatment for all pairs. The significance values from the test are

summarized in Table 2-Table 4, with a p<0.05 indicating a significant difference. The mean trapezoidal

tear strengths are given in Figure 5-Figure 12.

The UV exposure had the most drastic impact on the turnout material trapezoidal tear strength, where a

significant drop was seen after just 2.5 days (60 hrs) for all the fabrics (Figure 5 and Figure 6). The majority

of the drop in performance can be seen in the first 2.5 days of UV exposure and the performance drop for

each subsequent 2.5 day interval is not as dramatic. The 10 days of UV exposure resulted in an average

drop in trapezoidal tear strength of 68%, 62%, and 81% for the Fabric A, Fabric B, and Fabric C,

respectively. Both Fabric B and Fabric C dropped below the NFPA performance standard of 100N after

just 2.5 days of constant UV exposure. All of the results for the UV exposures on Fabric A were different

from the control, but were not significantly different from each other.

The trapezoidal tear results for laundering and radiant heat applied in these samples in general did not differ

from the unexposed fabric samples or show inconsistent results. After 50 washes, Fabric A showed a

reduction in trapezoidal tear strength of by approximately 46% (Figure 10), while Fabric B reduced by

approximately 29% (Figure 9). Only the Fabric B sample exposed to 10 radiant exposures showed a

significant drop in tear strength as a result of radiant exposure.

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The addition of one laundering after the UV exposures did not appear to influence the trapezoidal tear

strength. Some of the Fabric A samples exhibited “slip” of the grips during testing, which resulted in high

outlier samples and higher variability.

Table 2. Steel-Dwass Results for UV Exposure

UV p-values

Level Level Fabric A Fabric B Fabric C

7.5 0 0.0261 0.0261 0.0261

7.5 2.5 0.9998 0.0261 0.0261

7.5 5 0.747 0.3335 0.0261

5 0 0.0405 0.0261 0.0261

5 2.5 0.9684 0.0261 0.0261

2.5 0 0.0411 0.0261 0.0261

Table 3. Steel-Dwass Results for Radiant Exposure

Radiant p-values


50 0 0.2539 0.9998 0.8889

50 10 0.9998 0.9783 0.0931

50 25 0.8149 0.9951 0.9951

25 0 0.0931 0.9783 0.4247

25 10 0.6262 1.000 0.0261

10 0 0.2539 0.9437 0.0261

Table 4. Steel-Dwass Results for Laundering

Wash p-values


50 0 0.0261 0.0261 0.4247

50 10 0.0261 0.0261 0.7254

50 26 0.0261 0.134 0.8889

26 0 0.9783 0.0261 0.9998

26 10 0.1873 0.0411 1.000

10 0 0.3335 0.9783 0.9783

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Figure 5. Trapezoidal Tear Results from the UV Exposures for the Fabric B and Fabric C

Figure 6. Trapezoidal Tear Results from the UV Exposures for the Fabric A

140.7

91.168.7 61.9 53.7

169.9

68.846.2 37.0 32.0

0.0

50.0

100.0

150.0

200.0

250.0

Unexposed 2.5 Days UV 5 Days UV 7.5 Days UV 10 Days UV

New

ton

sEffect of UV Light - Fabric B and Fabric C

UV Exposure for Continuous Days

959.5

411.5 412.8 374.9309.5

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

Unexposed 2.5 Days UV 5 Days UV 7.5 Days UV 10 Days UV

New

ton

s

Effect of UV Light - Fabric A

UV Exposure for Continuous Days

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Figure 7. Trapezoidal Tear Results from Radiant Exposures for Fabric B and Fabric C

Figure 8. Trapezoidal Tear Results from Radiant Exposures for the Fabric A

140.7 136.2 138.0 137.8

169.9149.1

162.1 165.1

0.0

50.0

100.0

150.0

200.0

250.0

Unexposed 10 Radiant Exposures 25 Radiant Exposures 50 Radiant Exposures

New

ton

sEffect of Radiant Exposures - Fabric B and Fabric C

Exposures @ 0.2 Cal/cm2/s for 60 seconds

959.5

757.8 703.1769.7

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

Unexposed 10 Radiant Exposures 25 Radiant Exposures 50 Radiant Exposures

New

ton

s

Effect of Radiant Exposures - Fabric A

Exposures @ 0.2 Cal/cm2/s for 60 seconds

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Figure 9. Trapezoidal Tear Results from the Laundering for Fabric B and Fabric C

Figure 10. Trapezoidal Tear Results from the Laundering for Fabric A

140.7 139.1110.8 100.8

169.9 171.8 168.2 174.4

0.0

50.0

100.0

150.0

200.0

250.0

Unexposed 10 Launderings 26 Launderings 50 Launderings

New

ton

sEffect of Laundering - Fabric B and Fabric C

Wash and Dry Cycles - NFPA 1971

959.5

749.2

910.5

518.5

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

Unexposed 10 Launderings 26 Launderings 50 Launderings

New

ton

s

Effect of Laundering - Fabric A

Wash and Dry Cycles - NFPA 1971

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(Wash/Dry) for Fabric B and Fabric C


(Wash/Dry) for Fabric A

140.7

90.367.7 61.1

169.9

56.242.7 37.0

0.0

50.0

100.0

150.0

200.0

250.0

Unexposed 2.5 Days UV+ Wash 5 Days UV+ Wash 7.5 Days UV+ Wash

New

ton

sEffect of Laundering and UV Light - Fabric B and Fabric C

UV Exposure then Wash/Dry Cycle

959.5

465.2331.0 298.3

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

Unexposed 2.5 Days UV+ Wash 5 Days UV+ Wash 7.5 Days UV+ Wash

New

ton

s

Effect of Laundering and UV Light - Fabric A

UV Exposure then Wash/Dry Cycle

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Transmitted Heat Flux Results

Another finding from the project is related to the thermal protective performance of the outer shell samples.

After 50 exposures under the radiant heat protocol, the fabric samples did not show statistically significant

differences in their protective performance (Figure 13). The mean values of the samples that had undergone

repeated radiant heat exposures did have consistently lower mean heat fluxes, implying there may have

been a small improvement in protective performance for these samples.

Figure 13. Average Total Transmitted Heat Flux passing through the fabric samples during the

first and fiftieth radiant heat exposure

FTIR Results

The averaged FTIR spectra for all the test scenarios are displayed in the Appendix (Figure 15-Figure 26).

The intensity of the peaks are related to their abundance in the chemical structure of the fibers. Figure 14

shows the spectra of the controls. Table 5 shows the molecular structures of the polymers and Table 6

identifies several peaks of interest for each polymer based on the cited literature.

135

140

145

150

155

160

165

170

175

180

Average Total Transmitted Heat Flux (kJ/m2)

1st Exposure 50th Exposure

Fabric B

Para-Aramid BlendFabric A

PBI/Para-Aramid Blend

Fabric C

Meta-Aramid Blend

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Table 5. Molecular structures of the polymers

Fiber type Molecular structure

Para-Aramid1

Meta-Aramid

Polybenzimidazole1

Table 6. Assigned Bands for Meta-Aramid and Para-Aramid

Wavenumber (cm-1) Band assignments6,7

3300 N—H stretchin in secondary amide

1730 -COOH formed from degredation

1645 C=O stretching on amide

1600 C=C stretching in aromatic ring

1540 N—H bending and C—N stretching of the C—N—H group

1305 C—N aromatic stretching

1240 C—N stretching, N—H in plane bending, and C—C stretching

Unidentified wavenumbers that show changes in this study: 2915, 2845, 2358, 1405, 1043

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Figure 14. Control spectra of the three test fabrics

The purpose of the FTIR analysis was to test the hypothesis that changes in peak intensities could be

correlated with changes in tear strength caused by the environmental treatments. An initial analysis of the

relation between FTIR intensities and tear strength was performed using linear regression analysis for each

wavenumber. The 108 combinations of fiber, treatment, and wavenumber analyzed and sorted by the

coefficient of determination value, R2 (an ideal or perfect correlation would produce an R2 value of 1.0000).

Thirteen (12%) of the 108 combinations resulted in R2>0.8 (Table 7), though the values dropped off quickly,

with 75% of the combinations showing an R2 of less than 0.3. These were all for wavenumbers from the

meta-aramid and para-aramid samples exposed to UV. All of these fits had positive slopes with the

exception of the 1730cm-1 wavenumber, which had a negative regression line between peak intensity and

tear strength. This supports previous findings that this wavenumber represents the formation of a

degradation product of the aramid polymer.

None of the Fabric A samples showed high correlation (R2) values with a wavenumber. The highest was

R2=0.5 for the 1240cm-1 of the laundered samples. It should be noted that Fabric A did contain 30% para-

aramid fibers blended with the polybenzimidazole fibers. A previous study on the FTIR spectrum of a para-

aramid/PBI yarn blend found that para-aramid dominated the FTIR spectra and few characteristic peaks

were attributable to the PBI polymers.6 The 1240cm-1 band should be shared by all three polymers and was

found to be a highly influencing wavenumber. Interestingly, several of the unidentified bands, notably 2358,

were found to be significant.

0

0.01

0.02

0.03

0.04

0.05

5001000150020002500300035004000

Abso

rban

ce

Wavenumber (cm -1)

Control Samples (Unwashed and Unexposed to UV or Radiant Heat)

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Table 7. Linear Coefficients of Determination between FTIR intensity and Tear Strength

Fabric Treatment Wavenumber

(cm-1) R2

Fabric C - meta-aramid UV 1240 0.82490


Fabric B - para-aramid UV 1540 0.88038











The influence of combined factors and treatments was examined next. FTIR is a surface measurement and

it is possible that a treatment may not cause a linear change in FTIR signal. Changes to the molecular

structure that are located deeper inside the fiber or fabric structure may not be detected and an

environmental treatment could exhibit a “saturated” surface signal. Additionally, interference or

counteracting effects could result from surface soiling or the combination of treatments such as radiation

and laundering.

The most likely combined effect was decided to be laundering and UV exposure, so a separate set of

samples were subjected to the UV exposure regime with the addition of a laundering immediately after each

UV exposure. The FTIR readings taken before and after each laundering were compared using the Kruskal-

Wallis test, a nonparametric test with a null hypothesis that the mean ranks of data from the two groups are

the same (i.e. whether the readings before and after laundering come from indistinguishable distributions).

The 108 combinations of data (12 wavenumbers for 3 fibers at 4 levels of UV exposure) were tested, and

28 (26%) of the results indicated a significant difference (p<0.05 for the 2-sample normal approximation),

meaning the laundering caused a significant change in the FTIR readings. The values are presented in Table

8, sorted by wavenumber. A positive Z value indicates that the laundering caused an increase in FTIR peak

intensity and a negative Z value indicates a decrease. The most significant finding is that the laundering

was shown to decreases the 1730 cm-1 signal for all treatment levels of Fabric C and some levels of Fabric

B and the 1540cm-1 signal was increased. This means that laundering counteracted the trends seen in the

linear fits in the previous section. 2845cm-1 and 2358cm-1 were also affected.

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Table 8. Test results comparing the UV and UV-Wash FTIR intensities

Wavenumber Fabric Intensity Z Prob>|Z|

1043 B 5 2.32186 0.0202

1043 B 2.5 2.00160 0.0453

1405 A 7.5 2.16173 0.0306

1540 A 7.5 2.64211 0.0082

1540 C 2.5 2.32186 0.0202

1540 C 5 2.32186 0.0202

1540 B 2.5 2.00160 0.0453

1645 A 7.5 2.48199 0.0131

1645 A 5 2.16173 0.0306

1730 C 2.5 -2.80224 0.0051

1730 C 5 -2.80224 0.0051

1730 B 7.5 -2.80224 0.0051

1730 C 7.5 -2.80224 0.0051

1730 B 5 -2.64211 0.0082

1730 A 5 -2.16173 0.0306

2358 B 2.5 -2.80224 0.0051

2358 C 7.5 -2.80224 0.0051

2358 C 5 2.64211 0.0082

2358 A 5 -2.64211 0.0082

2358 C 2.5 2.16552 0.0303

2358 A 2.5 -2.16173 0.0306

2845 C 2.5 2.64211 0.0082

2845 A 7.5 2.32186 0.0202

2845 C 5 2.16173 0.0306

2915 C 7.5 -2.16173 0.0306

2915 C 2.5 2.0016 0.0453

3300 A 7.5 2.16173 0.0306

Three of the wavenumbers, 1200, 1305, and 1600, were not found to be impacted by laundering and could

potentially be more reliable wavenumbers to monitor.

The future step in evaluating the relation between FTIR intensity and tear strength will be to form a multiple

regression fit model that incorporates several wavenumbers to see if a combination of the signals gathered

can provide better predictive capability than the single wavenumber data above.

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Discussion and Conclusions

This work is unique in that it considers the possibility of correlating FTIR surface measurements of whole

fabric samples with changes in physical properties of aramid fibers caused by environmental exposure.

Fabrics used in commercially available firefighter turnout shells were conditioned to UV light, laundering,

and radiant heat exposure. UV light caused significant changes in trapezoidal tear test for all samples at all

levels. Only the highest number of launderings (50) resulted in changes, for all three fabrics. This could be

the result of changes in friction and yarn structure, not necessarily fiber degradation; however, FTIR signal

intensity was found to be influenced by laundering. Only Fabric A was affected by radiant exposure, though

the change was small and inconsistent. Interestingly, transmitted heat flux was not affected by the repeated

radiant exposures.

Several FTIR peaks were found to change linearly with changes in trapezoidal tear strength; however, none

of the signals were applicable for Fabric A and the changes were only as a result of UV exposure in the

other fibers/fabrics. Further, several of these wavenumber correlations were found to be counteracted by

laundering, meaning that samples would have to be tested before laundering, or the effect would have to be

corrected for in an applied field method. While some effects on outer shell material strength can be

ascertained on some materials using FTIR, at the current time a significant amount of further work would

be needed to develop a procedure that may provide means of non-destructively assessing shell strength.

Recommendations

This work opens the possibility for additional phases of study to further evaluate non-destructive

measurements of turnout gear materials. Building directly off this research, the next step would be to apply

more sophisticated data correlation models to assess the correlation between the measured tear strength and

the FTIR spectra. Instead of taking only a single peak in the spectrum to assess correlations, it should be

possible to assess multiple peaks simultaneously with the appropriate models.

Several additional questions could be addressed such as:

What is the variability of the FTIR test?

Do surface characteristics of the fabrics affect response?

Do the treatment methods affect those characteristics?

What is the influence of soiling and laundering on analysis of worn samples?

What other non-destructive measurement techniques are feasible for evaluation?

References

1. Arrieta, C., David, É., Dolez, P. & Vu-Khanh, T. Hydrolytic and photochemical aging studies of a

Kevlar®-PBI blend. Polym. Degrad. Stab. 96, 1411–1419 (2011).

2. El Aidani, R., Nguyen-Tri, P., Malajati, Y., Lara, J. & Vu-Khanh, T. Photochemical aging of an e-

PTFE/NOMEX® membrane used in firefighter protective clothing. Polym. Degrad. Stab. 98,

1300–1310 (2013).

3. Davis, R., Chin, J., Lin, C.-C. & Petit, S. Accelerated weathering of polyaramid and

polybenzimidazole firefighter protective clothing fabrics. Polym. Degrad. Stab. 95, 1642–1654

(2010).

4. Guenther, J., Wong, M., Sue, H.-J., Bremner, T. & Blümel, J. High-temperature steam-treatment

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of PBI, PEKK, and a PEKK-PBI Blend: A solid-state NMR and IR spectroscopic study. J. Appl.

Polym. Sci. 128, 4395–4404 (2013).

5. Ahmed, D. et al. Structural Changes Of Kevlar Fibers In The Process Of Heat Treatment Using

Raman Spectroscopy. Int. J. Adv. Sci. Tech. Res. 4, 11–16 (2013).

6. Arrieta, C., David, E., Dolez, P. & Vu-Khanh, T. Thermal aging of a blend of high-performance

fibers. J. Appl. Polym. Sci. 115, 3031–3039 (2010).

7. Villar-Rodil, S., Paredes, J. I., Martínez-Alonso, A. & Tascón, J. M. D. Atomic Force Microscopy

and Infrared Spectroscopy Studies of the Thermal Degradation of Nomex Aramid Fibers.

doi:10.1021/cm001219f

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Appendix: FTIR Results

Figure 15. Fabric A after Radiant exposure

-Unexposed

- After 10 Radiant Exposures



Fabric A

Radiant Exposures

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Figure 16. Fabric A after UV Exposures

-Unexposed

- After 2.5 Hours UV

- After 5 Hours UV


Fabric A

UV Exposures

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Figure 17. Fabric A after Laundering

-Not Laundered

- After 10 Launderings



Fabric A

Laundering Cycles

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Figure 18. Fabric C after Radiant Heat

-Unexposed




Fabric C

Radiant Exposures

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Figure 19. Fabric C after UV Exposures

-Unexposed


- After 5 Hours UV


Fabric C

UV Exposures

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Figure 20. Fabric C after Laundering

-Not Laundered




Fabric C

Laundering Cycles

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Figure 21. Fabric B after Radiant Heat

-Unexposed




Fabric B

Radiant Exposures

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Figure 22. Fabric B after UV Exposures

-Unexposed


- After 5 Hours UV


Fabric B

UV Exposures

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Figure 23. Fabric B after Laundering

-Not Laundered




Fabric B

Laundering Cycles

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Figure 24. Fabric A after combined UV and Laundering

-Unexposed, Not Laundered

- 2.5 Hours UV, Before Laundering

- 2.5 Hours UV, After Laundering

- 5 Hours UV, Before Laundering

- 5 Hours UV, After Laundering



Fabric A

Combined UV and

Laundering

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Figure 25. Fabric C after Combine UV and Laundering








Fabric C

Combined UV and

Laundering

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Figure 26. Fabric B after Combined UV and Laundering








Fabric B

Combined UV and

Laundering

non-destructive assessment of outer shell degradation for ... · final report by: r. bryan ormond,...

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