Polymer Degradation and Stability 98 (2013) 1300e1310
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
Photochemical aging of an e-PTFE/NOMEXÒ membrane used in firefighter
protective clothing
Rachid El Aidani a, Phuong Nguyen-Tri a, *, Yassine Malajati a, Jaime Lara b, Toan Vu-Khanh a
a
b
Département de Génie Mécanique, École de Technologie Supérieure, 1100 Rue Notre-Dame Ouest Montréal, H3C 1K3 QC, Canada
Département de santé environnementale et santé au travail, Faculté de médecine, Université de Montréal, C.P. 6128, succ. Centre-ville, Montréal, QC H3C 3J7, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 January 2013
Received in revised form
21 March 2013
Accepted 2 April 2013
Available online 18 April 2013
The moisture barrier membrane is an important protective layer of fire protective clothing. This membrane usually consists of a coating of expanded polytetrafluorethylene (e-PTFE) laminated to a NomexÒ
fabric. In this study, the effects of accelerated photochemical aging on morphology, structure and performance of ePTFE/NomexÒ membrane were investigated. The mechanical properties and chemical
structural changes during the photochemical aging process were studied by using appropriate techniques including infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry
(DSC), scanning electron microscopy (SEM), atomic force microscopy (AFM) and permeability measurements. The results showed a significant reduction of mechanical properties of the membrane after
photochemical aging due to the degradation of the NomexÒ fibers. The dramatic decrease of vapor
permeability after photochemical aging was involved the closing of transpiration pores on the aged
membrane surface. These results provide useful information to better understand phenomena occurring
during photochemical aging of high-performance microporous laminates and may help to improve the
manufacturing process for fire protective clothing.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Photochemical aging
Degradation
NomexÒ membrane
Protective clothing
1. Introduction
Personal protective equipment such as gloves or clothing is
necessary for many professionals to protect against a variety of
occupational hazards which daily arise in their work environment.
In recent decades, considerable progress has been made in the
design and manufacture of protective clothing [1]. Predominant
among technologies currently used in manufacturing garments for
fire resistance are microporous barrier materials and synthetic
fibers of high performance. When firefighters are deployed to fight
against a fire, they may be subjected to a variety of dangerous and
severe constraints such as heat flux, hot water vapor and also high
energy radiation [2]. Due to the importance of these problems,
many studies on the degradation and durability of protective
clothing issues from textile materials have been undertaken [3e10].
The level of degradation of the textile material can be directly
related to visual indicators of degradation of the polymer structure
which may occur during their service life. However, the protective
functions of materials may be lost before that changes in
morphology could be observable by the naked eye [3e5]. Although
* Corresponding author. Tel.: þ1 5143968800.
E-mail address: phuong.nguyen-tri@etsmtl.ca (P. Nguyen-Tri).
0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymdegradstab.2013.04.002
it is not always reported, moisture from the body is usually present
inside the garment when it is worn [6]. During a firefighter training
exercise, it was noted that the humidity inside the garment quickly
reached values close to 100% as soon as they entered to a burning
building and remained relatively high (above 60%) after the fire was
extinguished [7].
Lawson [8] tried to establish the possible causes of first and
second degree burns suffered by a firefighter in the fight against
fires. He concluded that, in general, a firefighter can be burned even
before entering the room where the fire occurs. Lawson also noted
the importance of humidity outside and inside protective clothing.
He explained that the water vapor trapped between the skin and
clothing of a firefighter may cause burns, while moisture outside
may reduce the rate of heat transfer and, consequently the protective properties of the material. Another type of burn, due to
the presence of reflective trim attached to clothing, has also been
reported [9]. These burns occur apparently without significant
degradation of the outer coating. Mäkinen [10] studied the effects
of wear and laundering conditions on the properties of several
textile fabrics used in protective clothing. He discovered that the
combination of the double effects of wear and laundering were
much larger than the effects of single laundering. Various researchers have studied the effects of moisture transfer in protective
clothing in hazardous conditions [11,12]. The development of
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
1301
Fig. 1. SEM photographs of moisture membrane (a) e-PTFE side and (b) NomexÒ Side.
a special test to measure the protection and thermal comfort of
protective clothing for firefighters and other applications using a
specially designed cylinder, simulating a torso sweating has been
also reported [13].
Other studies examined the effects of long-term exposure of
fabric and protective clothing to heat flux that might be experienced
during the fight against fire with respect to the water vapor permeability of different textiles used in manufacturing a moisturescreen [14]. These authors noted that the material permeability to
water vapor decreased with exposure time except for one hydrophilic material which showed no reduction in permeability. Similarly,
they demonstrated on SEM images that the samples were so
damaged that the permeability measurement could not be made.
Changes in permeability took place without any visual indication to
the naked eye. Other study on thermal aging of different materials
used in protective clothing on their mechanical properties has been
published [15]. It was found that all tested materials retained at least
90% of their tensile strength while a large decrease in tearing resistance up to a loss of 25% of the initial value was observed. It suggested
that the extent of the tear resistance is considered as a proxy for
assessing the effects of thermal aging on the mechanical properties of
protective clothing. Torvi and Thorpe [16] compared the results of
tensile strength of textiles materials exposed to heat flux according to
the criteria of NFPA 1971. The authors submitted samples dress
materials to fire at different heat flux and then measured their tensile
strength. By comparing the results, the authors showed that after
exposure to heat flow, the tissues tested no longer met the criteria of
tensile strength recommended by the standard.
Other researchers have studied the modification of mechanical
properties of protective clothing for firefighter [17]. A reduction in
tensile strength of 85% after 14 days of thermal treatment was
observed. Rossi and al. [18] also studied the effect of heat flux on the
thermal and mechanical properties of various materials used in
manufacturing of fire proof closing. The authors noted that heat
exposure had no influence on thermal protective properties because
all exposed textiles met the criteria of EN 469. Regarding mechanical
properties, the authors found that for some textiles, the effect of
heat treatment is not significant, while for others, a significant loss
of mechanical properties was observed after heat treatment.
Various studies have shown that the structure of aramid fibers
was closely linked to their method of synthesis and implementation [19,20]. In the literature, features of several structural models
have been proposed by various authors including molecular
structure, the structure of crystalline defects, the fibrillar structure,
the structure “pleated sheets” structure “skin-heart” and the
distribution of micro voids [21e23].
In the previous paper, we reported the effects of thermal aging
on mechanical and barrier properties of an e-PTFE/NomexÒ moisture membrane used in firefighters’ protective suits. This paper
provides additional results about the modification of the properties,
structure and morphology of an e-PTFE/NomexÒ moisture membrane used in the manufacturing of fire proof clothing following
photochemical aging. Mechanisms responsible for the degradation
that lead to the irreversible changes in their physico-chemical and
mechanical properties were also investigated and discussed.
2. Experimental
2.1. Materials
Le material used in this work is an expanded e-PTFE membrane
laminated to a flame-resistant meta-aramid fabric (NomexÒ). Their
trade name is CrosstechÒ (WL Gore & Associates, Maryland, USA)
provided by Innotex Inc. (Québec, Canada). The average pore sizes
of the e-PTFE membrane are about 1 micron (Fig. 1a) and the
average diameter of the fabric wires is about 15 microns (Fig. 1b).
2.2. Photochemical aging test
The aging by light irradiation was carried out in an Accelerated
Weathering Tester from Q-Lab Corporation, model QUV, at different
temperatures. The samples are positioned so that the NomexÒ side is
in contact directly with light source to simulate the actual conditions
of fire protective clothing. The tests are performed with several light
intensities and different aging times as shown in Table 1.
2.3. X-ray diffraction (XRD)
X-ray diffraction used in this study is a diffractometer Philips
Picowatt model 1710 using CuKa as anode tube with radiation
wavelength of 1.54 A and scintillation counter as detector at 40 kV
and 50 mA.
2.4. Water vapor permeability
Measurements of permeability to water vapor were carried out
on the MOCON Permatran W-Model 101K. A round sample with
Table 1
Photochemical aging program in QUV chamber.
Temperature T ( C)
Light intensity I (W/m2)
Wavelength (nm)
50
70
80
0.35; 0.68; 1 and 1.35
0.35; 0.68; 1 and 1.35
0.35; 0.68; 1 and 1.35
340
340
340
1302
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
(MTS) universal testing machine equipped with 1000N load cell
and operated at a cross-head speed of 300 mm/min according to
the ASTM D5035 standard test method. For each condition, five
replicates were measured.
Tear strength was also assessed since it has been reported to be
more sensitive to photochemical aging than tensile strength in a
study involving various laminate type fabrics. A sample similar to
the ASTM D 5587 standard test method but with slightly scaled
down dimensions was used. An isosceles trapezoid was drawn on
50.8 101.6 mm rectangular samples to mark the position of the
grips and a notch was cut in its smallest side. Samples were fixed in
the grips of the Alliance 2000 test machine by clamping it along the
oblique marks, leaving the side opposite to the notch wavy.
Grips were pulled apart at a rate of 200 mm/min until the sample
was completely torn apart. Tear strength is obtained from the
maximum value of the force. For each condition, five replicates
were measured.
2.6. Infrared spectroscopy analysis (FT-IR)
Fig. 2. Schematic representation of equipment used for the permeability measuring.
dimension of 50 50 mm was prepared for measuring the rate of
transmission (TR) of the water vapor through the membranes. The
test temperature is controlled at 23 C 2 C and the humidity
percentage is fixed at 60%. Fig. 2 presents the equipment used for
the permeability measuring.
2.5. Mechanical testing
Tensile properties were measured on 15 10 mm2 rectangular
specimens. Measurements were performed with an Alliance 2000
Changes in the structure of polymer membranes before and
after photochemical aging were observed by Thermo Nicolet
equipped with an ATR mono-reflexion. This instrument allowed
analysis of membrane surface with a depth of 3e5 mm. Wavelength
scanning ranges were taken from 400 to 4000 cm1 with 128
points and at 4 cm1 resolution.
2.7. Differential scanning calorimetry (DSC)
Differential scanning calorimetry analyses of membrane samples were performed using a thermal analyzer PerkineElmer Dsc-7.
All measurements were made under N2 flow (20 mL/min), maintaining constant heating and cooling rates of 10 C/min. The analysis sample weights are about 5 mg. In many cases, two tests were
carried out to obtain the reproducible results.
Fig. 3. IR-ATR spectrum of NomexÒ during the photochemical irradiations (T ¼ 70 C and I ¼ 0.68 W/m2).
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
1303
2.9. Atomic force microscopy (AFM)
In this study, we measured the roughness index (Rq) or
root mean square (RMS) value of the sample surface, which
represented the standard deviation of z values in a given area.
The roughness index (RMS) was calculated over the entire
test surface through the following mathematical equation
(Eq. (1)):
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
ðZi Zave Þ2
RMS ¼ Rq ¼
N
(1)
where Zave is average value of z, Zi is a considered value under and N
is total number of points. To obtain reproducible and comparable
values, the image size used was set at 1 1 mm and an average of at
least four images was used for each test.
The atomic force microscopy used in this study was a Nanoscope IIIA multimode with a piezoelectric. The apparatus was
placed on a marble table to avoid movement during the tests. For
preliminary screening of the sample area, the apparatus was
equipped also with an optical system and a camera having a field of
view up to 800 microns. This system allows visualizing the surface
and the tip.
Fig. 4. Proposition of the schematic representation of the possible mechanics of
photoaging of NomexÒ fibers.
3. Results and discussion
3.1. Structural changes
2.8. Scanning electron microscopy (SEM)
Scanning electron microscopy model Hitachi S570 was used to
investigate of potential changes of external morphology of
analyzed samples due to thermal aging process. Observations were
made on both sides of the membrane meaning on the e-PTFE
laminate and on the Nomex fabric.
The alteration of mechanical properties and modification of
morphology at macro or micro levels are often related to structural
changes in the material. It is well known that under UV irradiation
actions, polymer networks may undergo chemical modification
such as chain scission, formation of free radicals, functional groups
and photoinduced crosslinking. ATR-FTIR was used to follow the
Fig. 5. IR-ATR spectrum of NomexÒ fibers before (a) and after 504 h of thermal aging at 80 C without UV irradiation.
1304
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
Fig. 6. Evolution of the percent crystallinity of NomexÒ as a function of photoaging
time at different light intensities (0.35; 1.35 W/m2 and T ¼ 70 C).
structural changes of NomexÒ membrane during photochemical
aging. Fig. 3 shows the ATR-IR spectra of NomexÒ membrane before
and after photochemical aging at I ¼ 0.68 W/m2 and T ¼ 70 C for
different exposure times. It is interesting that a new absorption
band at 1725 cm1 is observed for the aged sample. This band can
be attributed to the presence of a carbonyl group C]O in carboxylic
acid group (eCOOH).
The formation of this new absorption band at 1725 cm1 is
assigned to the appearance of a photooxidation product. From these
results and by reference to the work of previous research workers
[24e27], we proposed a possible mechanism for photochemical
aging of NomexÒ fibers (Fig. 4). The main products of the reaction
are carboxylic acids and Photo-Fries rearrangement [24e27]. This
mechanism can be linked to the change in mechanical properties of
the e-PTFE/NomexÒ membrane. The photochemical degradation of
the structure of NomexÒ probably involves an oxidation process
and rearrangement of radicals formed during the scission of the
amide bond. The formation of intermediate peroxides was also
observed in other aromatic polyamides presumably by photo-Fries
rearrangement involving reactions with oxygen [28].
To examine the contribution of the thermal effect alone on the
oxidation process, the same samples were thermally aged in an
oxygenated environment at the same temperatures as those used in
the study of photochemical aging (50, 70 and 80 C). The ATR-FTIR
spectra of Nomex membranes before and after photoaging are
shown in Fig. 5. It shows that there are no oxidation products in
aged membrane even after 504 h of thermal aging (3 weeks). This
indicates that the phenomena observed and reported in the previous paragraph are due to photochemical effects. Temperature
plays an essential role in accelerating the formation of photooxidation products.
3.2. Crystallinity percentage analysis
The crystallinity percentage (Xt) of a semi crystalline polymer
plays a key rule to properties and structure of polymer materials. In
the case of photoaging, it noted that the crystalline areas are very
sensitive to photooxidation. We used the XRD technique to calculate the crystallinity of the NomexÒ sample before and after aging
to investigate the effect of aging condition to crystallinity. Fig. 6
shows the degree of crystallinity as a function of light intensity
and photoaging time. It demonstrates that the crystallinity
Fig. 7. SEM photographs of NomexÒ membrane surface before (a) and after different aging time (72 h-b, 240 h-c,d) at light intensity I ¼ 0.68 W/m2.
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
1305
Fig. 8. High resolution of SEM photographs of NomexÒ membrane surface before (a) and different aging time (72 h-b, 240 h-c,d) at light intensity I ¼ 0.68 W/m.2.
percentage of NomexÒ increases with UV irradiation time. This
increase is initially low and then accelerated with aging time.
In the literature, several authors ascribed these changes to the
scission of macromolecular chains due to UV irradiation [29].
Indeed, in the amorphous region, the amorphous chains become
more mobile and flexible due to the scission which decreases the
length of the macromolecular chains [30] and thus favors to secondary crystallization [31]. This contributed to increasing degree of
crystallinity. However, the extent of the degree of crystallinity in
the case of photochemical and thermal aging cannot explain the
loss of tear strength and tensile strength (see next section) of
polymers because the increase of percentage of crystallinity
generally leads to improved mechanical properties [32]. The
decrease of mechanical properties with an increase of the degree of
crystallinity seems unlikely in our case. It may be attributed to the
Fig. 9. AFM images of NomexÒ surface before (a) and after photoaging during 168 h
with I ¼ 0.68 W/m2 and T ¼ 80 C (b).
Fig. 10. Roughness index of NomexÒ membrane as function of photochemical aging
time (I ¼ 0.68 W/m2 and T ¼ 80 C).
1306
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
resolution (Fig. 8). It can be clearly seen that UV irradiation leads to
extensive damage in the fibers at the surface of NomexÒ. This is
particularly marked by longitudinal cracks which lead to the formation of peeling, transverse cracks that generate fiber breaking
and micro- porosities in the membrane structure.
The atomic force microscopy (AFM) is also used to study the
morphological modification and roughness of NomexÒ fibers
before and after photochemical aging (Fig. 9). The roughness index
of the membrane surface can be calculated from the images obtained by AFM by the use of equation 1 (eq. (1)) as mentioned in
experimental section.
The evolution of the roughness versus aging time is shown in
Fig. 10. Two different stages of photodegradation can be identified.
In the first stage the increase of the surface roughness is extremely
rapid before 168 h of exposure and the second stage which corresponds to a slow increase of roughness index with the exposure
time. This morphology change is very complex and may be attributed to various phenomena such as the development of microcracks in the material and the occurrence of several oxidation
products of NomexÒ.
Fig. 11. Evolution of the glass transition temperature as a function of photoaging time
at different light intensities (0.35; 1; 1.35 W/m2 and T ¼ 70 C).
3.4. Evolution of glass transition temperature
fact that the degradation of fibrils [33], which can then recrystallize
on the surface of pre-existing crystallites.
3.3. Morphology changes
To gain a better understanding of the damage at micro level
under UV irradiation, the SEM technique was used to observe the
NomexÒ side of the membrane. Fig. 7 shows the SEM photographs
of NomexÒ sample before and after photochemical aging at
I ¼ 1.35 W/m2. NomexÒ un-aged fibers sample show a very smooth
surface (Fig. 7a), whereas several cracks, holes and even broken
fibers (Fig. 7b,c,d) were observed on the surface of photochemical
aged fibers. These morphological changes help in understanding
the reduction of mechanical properties due to the chemical
degradation induced by UV irradiation that will be discussed in the
next section. Further surface analyses were also made at higher
The glass transition is one of the most important properties of
polymers and it varies linearly with the molar mass according to
the FoxeFlory equation
Tg M ¼ TgN K
Mn
where Mn is number average of molecular weight, TgN and K are
constants.
The measurement of Tg changes allows supporting our previous
assumptions about the chain scission of NomexÒ fiber because
the Tg are linked to molecular weight of polymer. The evolution of
Tg of aged samples during photoaging at different light intensities is
shown in Fig. 11. A significant and progressive change of Tg is
observed for all aging conditions. For a given aging time, the
decrease of Tg becomes even more important by increasing the light
Fig. 12. Example of forceedisplacement curves of tearing test of e-PTFE/NomexÒ membrane at different aging times (I ¼ 0.68 W/m2 and T ¼ 80 C).
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
1307
Fig. 13. Evolution of tearing force of membrane as a function of light intensities and aging time (T ¼ 70 C).
3.5. Evolution of mechanical properties
intensity. On the basis of these observations, it appears that the
decrease of Tg can be related to a reduction in molecular weight
caused by the splitting of the macromolecular chains of NomexÒ.
Therefore, the displacement of Tg to low temperatures may be taken
as an indication of the chain scission process during the photoaging. The contribution of NomexÒ to mechanical properties of the
membrane is more important than that of the e-PTFE. Therefore,
the scission of NomexÒ chains has a major effect on the loss of
mechanical properties of the membrane. This result confirms the
findings of the chemical analysis by FTIR which indicated that the
photoaging produces a chemical degradation of the structure of
NomexÒ.
3.5.1. Tearing resistance
Tearing resistance is an important parameter of textile materials, particularly in the case of protective clothing. Fig. 12 shows
examples of forceedisplacement curves of tearing the membrane
e-PTFE/NomexÒ representing different aging times at I ¼ 0.68 W/
m2 and T ¼ 80 C. Data for unaged samples are also inserted for
comparing the results. A reduction of both the tearing force and
displacement is observed as aging time increases.
Fig. 13 shows that whatever the value of the light intensity, the
tear strength of the membrane decreases during aging until
Fig. 14. Evolution of tearing force of membrane as a function of light intensities and
aging time (T ¼ 50 C).
Fig. 15. Evolution of tearing force of membrane as a function of light intensities and
aging time (T ¼ 80 C).
1308
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
Fig. 16. Example of stressestrain curves for aged sample at I ¼ 0.68 W/m2 and
T ¼ 80 C.
Fig. 18. Evolution of elongation at break of e-PTFE/NomexÒ as a function of aging time
and temperature (I ¼ 0.68 W/m2).
reaching a plateau region. The tearing force rapidly decreases
during the first 200 h of UV exposure and then remains almost
stable over time. In the first region, the reduction of properties
seems to be controlled by the rate of the initiation step of the
oxidation reaction. This rate depends on the extent of photolysis
occurring in the samples. After 200 h of exposure, the fall in tear
resistance of the membrane varies between 58% and 65% for
different intensities and then it is reduced more slowly for all light
intensities.
Figs. 14 and 15 show the changes of tearing force as a function of
photochemical aging time at 50 and 80 C respectively. Similar
trends were observed. This implies that there is a rapid decrease of
tearing energy in the first step of aging (before 200 h of exposure)
subsequently this reduction seems to be stable after 200 h of aging.
Clearly light intensity plays an important role in the alteration of
tearing force. The degradation of material is significant as light
intensities increase from 0.35 to 1.35 W/m2.
3.5.2. Tensile strength
Fig. 16 shows an example of stressestrain curves of neat and
aged e-PTFE membranes at different exposure times at I ¼ 0.68 W/
m2 and T ¼ 80 C. This contrasts with our observations in the case of
thermal aging [34], where the initial region of pretension increased
with increasing aging time. In this case, the nonlinear part of the
curve gradually disappears. Tensile behavior becomes completely
elastic up to the point of failure for the aged sample during 17 days
(I ¼ 0.68 and T ¼ 80 C). This indicates that the degradation
mechanisms of thermal and photochemical processes for aging are
different. In the case of thermal aging [34] a significant decrease of
the module of pretension and increase of elongation at break were
observed due to misalignment of polymer chains. In the case of
photochemical aging, the module of pretension seems not to be
affected by UV aging conditions. However, a remarkable decrease of
elongation at break with increasing of aging time was observed.
This behavior may be related to chemical degradation of the fiber
structure during UV irradiation conditions.
Fig. 17. Evolution of tensile strength of e-PTFE/NomexÒ as a function of aging time and
temperature (I ¼ 0.68 W/m2).
Fig. 19. Evolution of the permeability to water vapor of the membrane e-PTFE/NomexÒ
as a function of the light intensities and the aging time.
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
1309
The loss of tearing resistance and tensile properties after a
certain aging time as noted in the previous paragraphs can be
attributed to a deceleration of oxidation rate induced by the
accumulation of photo-Fries by-products in the fiber surface. These
products (2-aminobenzophenone) may act as stabilizers by absorbing
the energy of the UV radiation and prevent cleavage of the amide
bonds. A similar decrease in oxidation rate after extended UV irradiation of aliphatic and semi-aromatic polyamides has been reported
in several studies [35,36].
3.6. Water vapor permeability of membrane
Fig. 19 represents the permeability to water vapor of the membrane e-PTFE/NomexÒ as function of the aging time at three light
intensities 0.35, 1 and 1.35 W/m2 at 70 C. It shows that photooxidation of the membrane leads to a significant reduction in water
vapor permeability and it was found that the higher light intensity is,
the greater the drop in permeability. Indeed, after 250 h of aging,
permeability decreases about 25% compared to virgin samples at
a light intensity of 1.35 W/m2, while it decreases about 10% at
I ¼ 1 W/m2 between 96 and 200 h. However, at 0.35 W/m2, loss of
permeability is negligible, even after 300 h of light exposure.
To examine the possible causes of these changes of water vapor
permeability, we analyzed the morphology of the microporous
membrane before and after photochemical aging by using the SEM.
Fig. 20 shows examples of SEM images obtained from e-PTFE
membrane for aged samples at light intensity of 1.35 W/m2 for 48
and 96 h. We compared them with the image of the unaged sample
(Fig. 20a), it could be clearly seen that a gradual closure of the
membrane pores e-PTFE is observed as a function of photochemical
aging time. The reduction of the pores size of membrane leads to
the decrease of diffusion capacity of water vapor through the
membrane and thus decreases the water vapor permeability due to
aging at light intensity of 1 and 1.35 W/m2.
4. Conclusion
Fig. 20. SEM images of e-PTFE unaged (a) and aged at light intensity of 1.35 W/m2 and
temperature of 70 C during 48 h (b) and 96 h (c).
Fig. 17 shows the effect of UV radiations on the tensile strength
of the membrane e-PTFE/NomexÒ. The degradation curve trend is
similar to that of the tearing force: after 168 h of UV irradiation,
tensile strengths were also decreased rapidly and resulted in a loss
its performance in the range from 30 to 50% compared to initial
values depending on treatment condition and then the tensile
strength decreases are less pronounced until arriving at a stable
state.
Fig. 18 presents the changes of elongation at break as a function
of the aging time. The results are totally different compared to
those obtained for the thermal aging where an increase of elongation at break with thermal aging time is observed. In this case,
the elongation of the e-PTFE/NomexÒ membrane rapidly decreases
during photochemical aging. At 163 h of UV aging, a reduction of
elongation at break about 46 and 53% was observed at T ¼ 70 and
80 C (I ¼ 0.68 W/m2) respectively. It seems that a plateau region
was appeared from 300 h of aging.
These significant decreases of elongation at break with
increasing photochemical aging time demonstrate that chemical
degradation of the membrane under UV irradiation at high temperature is possible as discussed in section 3. 1.
The effect of photoaging e-PTFE/NomexÒ membranes on
permeability, morphology, mechanical properties, and structure
has been investigated. It was found that UV light irradiation has a
significant effect on the water vapor permeability, mechanical and
physico-chemical properties of the membrane. A decrease of
permeability to water vapor of the membrane e-PTFE/NomexÒ with
the photoaging process was observed. According to SEM images,
the reduction of permeability is due to the gradual decrease of size
and number of pores of the membrane.
The alteration of the mechanical properties of the membrane
e-PTFE/NomexÒ during photoaging was also observed. This may be
related to the degradation of structure of the material as FTIR
analysis suggests the formation of photo-products of low molecular
mass. The relation between the mechanical properties and
morphology has been also investigated. The responsibility for the
loss of tensile strength, tearing resistance could be related to the
modification of surface roughness index and the molecular weight
decrease of the NomexÒ membrane. These results were also
confirmed by DSC analysis which revealed that a reduction of glass
transition temperature occurred during aging.
Photoaging also has a significant effect on the morphology of the
fibers NomexÒ. SEM images showed several cracks, holes and
broken fibrils after UV irradiation. The AFM observations showed
an increase of the surface roughness index of NomexÒ membrane
and this may be linked to the accumulation of oxidation products of
low mass in the polymer matrix.
These results provide useful information to improved understanding of phenomena occurring during aging of high-performance
1310
R. El Aidani et al. / Polymer Degradation and Stability 98 (2013) 1300e1310
microporous laminates and should help to improve manufacturing
procedures for fire protective clothing. The outdoor aging test of this
material for predicting of the service lifetime in exterior conditions
should be made in the next step of this project.
Acknowledgments
[15]
[16]
[17]
[18]
The authors thank Innotex, for providing the materials used in
the study. We are also very grateful to Dr. Mélina Hamdine from the
Ecole Polytechnique de Montreal for water vapor permeability
measurements and thank to Dr. Jack R. Plimmer (retired), US
Department of Agriculture, Beltsville for his helpful discussions
during this work.
[20]
References
[22]
[1] Horrocks AR. Textiles for protection. C. Press. Boca Raton, FL: Scott, Richard;
2005, p. 398e440.
[2] Loren L. Guide d’entretien des vêtements de protection (Bunkers) 2006.
[Toronto].
[3] Slater K. Progressive deterioration of textile materials e 1: characteristics of
degradation. Guelph: ON. University of Guelph. Dept. of Consumer; 1985.
[Studies].
[4] Slater K. Progressive deterioration of textile materials. Part I: characteristics of
degradation. The Textile Institute 1986;77(2):76e87.
[5] Slater K. Progressive deterioration of textile materials. Part II: a comparison of
abrasion testers. The Textile Institute 1987;78(1):13e25.
[6] Keiser C, Becker C, Rossi RM. Moisture transport and absorption in multilayer
protective clothing fabrics. Textile Research Journal 2008;78:604e13.
[7] Rossi R. Fire fighting and its influence on the body. Ergonomics 2003;46(10):
1017e33.
[8] Lawson JR. Fire fighters’ protective clothing and thermal environments of
structural fire fighting. Gaithersburg, MD: National Institute of Standards and
Technology; 1996, p. 26.
[9] Jason A, Steven C, Dean C, Doug D, Rich D, Pat F, et al. Thermal capacity of fire
fighting protective clothing. NIOSH National Personal Protective Technology
Laboratory (NPPTL), National Institute of Standards and Testing; 2008.
[10] Mäkinen H. The effect of wear and laundering on flame-retardant fabrics. In:
Performance of protective clothing: 4th volume, ASTM STP 1133. West Conshohocken: J. P. McBriarty and N. W. Henry; 1992. p. 754e64.
[11] Zimmerli T. Contact heat testing of dry and wet fire-fighters gloves. In: Proceedings of the fourth Scandinavian Symposium on Quality and Usage of
Protective Clothing. Nokobetef IV 1992.
[12] Rossi RM, Zimmerli T. Influence of humidity on the radiant convective and
contact heat transmission through protective materials. In: Performance of
protective clothing: 5th volume, ASTM STP 1237. West Cgnshohocken: J. S.
Johnson and S. Z. Mansdorf; 1996. p. 269e80.
[13] Zimmerli T, Weder M. Protection and comfort a sweating torso for the
simultaneous measurement of protective and comfort properties of PPE. In:
Performance of protective clothing: 6th volume, ASTM STP 1273. West
Cgnshohocken: J. O. Stull and A. D. Schwope; 1997.
[14] Rossi R, Zimmerli T. Breathability and protection aspects of heat protective
clothing after thermal aging. In: Performance of protective clothing: 6th
[23]
[19]
[21]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
volume ASTM STP 1273. West Conshohocken: J. O. Stull and A. D. Schwope;
1997. p. 238e47.
An SK. Flammability measurement and thermal aging of chemical protective
suit materials. Sen’i Gakkaishi 1999;55(10):464e72.
Thorpe PA, Torvi DA. Global needs and emerging markets. In: 8th Symposium
ASTM, Tampa, Florida, USAvol. 8. p. 74e88.
Fries AM, Eichinger H. In: 3rd European Conference on protective clothing,
Gdynia, Pologne 2006.
Rossi RM, Walter B, Roft S. Thermal and mechanical performance of firefighters’ protective clothing after heat exposure. International Journal of
Occupational Safety and Ergonomics 2008;14(1):55e60.
Weyland HG. The effect of anisotropy in wet spinning poly(p-phenyleneterephthalamide). Polymer Bulletin 1980;3(6e7):331e7.
Northolt MG, Den Decker P, Picken SJ, Baltussen JJM, Schlatmann R.
The tensile strength of polymer fibres. Advances in Polymer Science
2005;178:1e108.
Li LS, Allard LF, Bigelow WC. On the morphology of aromatic polyamide fibers
(Kevlar, Kevlar-49, and PRD-49). Journal of Macromolecular Science Physics
1983;B22(2):269e90.
Dobb MG, Robson RM. Structural characteristics of aramid fibre variants.
Journal of Materials Science 1990;25(1 B):459e64.
Panar M, Avakian P, Blume RC, Gardner KH, Gierke TD, Yang HH. Morphology
of poly(p-phenylene terephthalamide) fibers. Journal of Polymer Science,
Polymer Physics Edition 1983;21(10):1955e69.
Hamilton LE, Gatewood BM, Sherwood PMA. Photodegradation of high performance fibers. Textile Chemist and Colorist 1994;26(12):39e45.
Carlsson DJ, Gan LH. Photodegradation of aramids e 1. Irradiation in the
Absence of Oxygen 1978;16(9):2353e63.
Powell SC, Kiefer RL, Pate PL. The effects of atomic oxygen and ultraviolet
radiation on two aramid materials. Polymer Preprints 1991;32(1):122e3.
Hamilton LE, Sherwood PMA, Reagan BM. X-ray photoelectron spectroscopy
studies of photochemical changes in high-performance fibers. Applied Spectroscopy 1993;47(2):139.
Wypych G. Handbook of material weathering. Toronto: ChemTec Publishing; 2008.
Yang R, Yu J, Liu Y, Wang K. Effects of inorganic fillers on the natural photooxidation of high-density polyethylene. Polymer Degradation and Stability
2005;88(2):333e40.
Guadagno L, Naddeo C, Vittoria V, Camino G, Cagnani. Chemical and
morphological modifications of irradiated linear low density polyethylene
(LLDPE). Polymer Degradation and Stability 2001;72(1):175e86.
Craig IH, White JR, Shyichuk AV, Syrotynska I. Photo-induced scission and
crosslinking in LDPE, LLDPE, and HDPE. Polymer Engineering and Science
2005;45(4):579e87.
Gnanou Y, Fontanille M. Structure moléculaire et morphologie des polymères.
A3042.Techniques Ingénieur 1994.
Young RJ, Lu D, Day RJ, Knoff WF, Davis HA. Relationship between structure and
mechanical properties for aramid fibres. Journal of Materials Science
1992;27(20):5431e40.
El Aidani R, Dolez P, Vu-Khanh T. Effect of thermal aging on the mechanical
and barrier properties of an e-PTFE/NomexÒ moisture membrane used in
firefighters’ protective suits. Journal of Applied Polymer Science 2011;121:
3101e10.
Arnaud R, Fanton E, Gardette JL. Photochemical behaviour of semi-aromatic
polyamides. Polymer Degradation and Stability 1994;45(3):361e9.
Ivanov VB, Barashkova II, Selikhov W, Vysotsky VN, Yakovlev YY,
Sadekova RA, et al. Photooxidation and photodestruction of modified polyamides. Polymer Degradation and Stability 1992;35(3):267e76.