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Polymer Testing 128 (2023) 108227
Available online 30 September 2023
0142-9418/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Review on the mechanical deterioration mechanism of aramid fabric under
harsh environmental conditions
Yaojie Xu
a
, Hong Zhang
a
,
b
,
*
, Guangyan Huang
a
,
b
,
**
a
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, China
b
Beijing Institute of Technology Chongqing Innovation Center, China
ARTICLE INFO
Keywords:
Aramid fabric
Harsh environment
Mechanical properties
Ballistic performance
Deterioration mechanism
ABSTRACT
Aramid fabrics have been widely used in ballistic protection elds such as body armour and protective con-
struction due to their excellent mechanical properties, low density, and exibility. The widespread use of aramid
fabric results in its inevitable exposure to harsh environmental factors, such as ultraviolet radiation (UV), high
and low temperatures, acids, alkalis, and humidity, which can signicantly affect the mechanical and ballistic
properties of the fabrics, limiting their application life. This review highlights the macroproperty changes and
microdeterioration mechanisms of aramid fabrics in harsh environments. Ultraviolet radiation has the greatest
effect on the mechanical and thermal properties of aramid bres, followed by high- and low-temperature, acidic,
alkaline, and moist environments. The mechanical deterioration mechanism of aramid fabric is attributed to
changes in the molecular weight, amide bond integrity, molecular orientation, water content, chemical bond
energy and aramid bre surface roughness. In this study, the metallic oxide and organic bre modication
methods for improving environmental adaptability are described in detail for the most harmful ultraviolet ra-
diation. This work provides a convenient reference for the comprehensive performance evaluation and modi-
cation optimization of aramid fabrics and proposes design methods and a maintenance guidance for the
application of aramid fabric products in harsh environments.
1. Introduction
Body armour can effectively protect the lives of combat personnel by
reducing the damage caused by standard projectiles and irregular frag-
ments. Due to the light weight and exibility of aramid fabric, it is
chosen as the main protective material for exible body armour. The
application of aramid fabric can reduce the load on combat personnel,
enhance the allocation of ammunition and electronic equipment, and
improve the mobility and comprehensive combat ability of soldiers.
Due to the diversity of operational mission requirements, combat on
plateaus and in tropical jungles, extremely cold regions, and coastal
areas are often unavoidable. Environmental conditions due to factors
such as ultraviolet radiation, humidity, high and low temperatures, and
acid and alkali corrosion (see Fig. 1) can affect the ballistic resistance
performance and service life of aramid protective products in various
ways [1–6]. For example, Davis et al. [7] found that the tear strength of
aramid fabrics decreased by 40% in a moist environment. Yuan et al. [8]
and Said et al. [9] found that the smooth bre surface was corroded and
fractured after ultraviolet radiation. Brown et al. [10] and Iqbal et al.
[11] found that aramid bres underwent photooxidation and photo-
degradation reactions, resulting in the breakage of the polymer chains
within the bres after UV irradiation. Feng et al. [12] identied the
initial mass loss temperature of aramid bres as 100 ◦C. Arrieta et al.
[13] found that the material thermal life of aramid fabric was reduced
by 50% after treatment at 190 ◦C. Liu et al. [14] found that the atmo-
sphere and temperature of the environment affected the thermal sta-
bility of aramid fabrics. Jia [15] dipped aramid fabric in 30%
phosphoric acid solution, and the results showed that the strength of the
aramid fabric to resist breaking was reduced by 7%. Zhao et al. [16]
conducted water immersion tests on mica-modied aramid fabrics, and
the results showed that the modied fabrics had good moisture
resistance.
The impact resistance of aramid bres has long been a research
hotspot, and many review papers have conducted comprehensive
research on this topic [17–21]. Nurazzi et al. [17] reviewed the ballistic
resistance performance of armour structure systems composed of aramid
* Corresponding author. State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, China.
** Corresponding author. State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, China.
E-mail addresses: hong.zhang@bit.edu.cn (H. Zhang), huanggy@bit.edu.cn (G. Huang).
Contents lists available at ScienceDirect
Polymer Testing
journal homepage: www.elsevier.com/locate/polytest
https://doi.org/10.1016/j.polymertesting.2023.108227
Received 10 July 2023; Received in revised form 31 August 2023; Accepted 25 September 2023
Polymer Testing 128 (2023) 108227
2
fabric. Tabiei et al. [19] reviewed the ballistic penetration resistance and
failure modes of aramid fabrics. Tan et al. [20] and Ribeiro et al. [21]
reviewed the impact resistance of shear thickening uid (STF)-modied
aramid fabrics at different impact velocities. Although there have been
many studies on the effect of a single environmental factor on the me-
chanical properties of aramid fabrics [22–27], there have been few
studies summarizing the comprehensive characterization of aramid
fabrics in different environments and revealing their micro deterioration
mechanisms. This article comprehensively describes the effect of envi-
ronmental conditions such as ultraviolet rays, temperature, moisture,
acid and alkali on the mechanical properties of aramid fabrics. The
mechanical deterioration mechanism of aramid fabrics is analyzed from
microscopic morphology, molecular structure and molecular dynamics.
Methods for improving the environmental adaptability of aramid fabric
products in daily application environments are described in detail.
The focus of this work is shown as a schematic in Fig. 2. Section 2
discusses the aramid fabric property changes under ultraviolet radiation,
the ultraviolet radiation deterioration mechanisms, and the corre-
sponding bre modication methods. Section 3 introduces the effect of
temperature on the mechanical and ballistic properties of aramid fab-
rics. Section 4 introduces the effect of environmental moisture on the
mechanical and thermal properties of aramid fabrics. Section 5 describes
the inuence of the pH value on the mechanical properties and micro-
scopic morphology of aramid fabrics, and Section 6 proposes a brief
summary and prospects for future work.
2. UV resistance of aramid fabric
2.1. Ultraviolet radiation environmental treatment
Ultraviolet radiation environmental treatment entails the aramid
fabric being xed on aluminium foil through clamping devices and then
placed in an ultraviolet light accelerated ageing testing machine for
radiation exposure [28–30]. The effects of ultraviolet radiation on
aramid fabrics in different environments are studied by controlling the
radiation time, light intensity, temperature, and humidity.
2.2. Inuence of UV radiation on the properties of aramid bres
2.2.1. Tensile properties
The aramid fabric tensile properties are signicantly reduced under
ultraviolet radiation. Luo et al. [31] found that the aramid bre tensile
strength and failure strain decreased by 40% and 79.9%, respectively,
after 144 h of ultraviolet radiation, as shown in Fig. 3a. Dong et al. [32]
and Said et al. [9] reached similar conclusions. Ultraviolet radiation in
the air was found to cause photodegradation of fabrics, which led to
chain breakage, cross-linking, and the formation of oxygen-containing
functional groups at the microscopic scale. Fourier transform infrared
spectra (FTIR) showed a decrease in peak strength at approximately
3300 cm-1, indicating that the hydrogen bond inside the aramid bre
had broken. New absorption peaks were formed near 1816, 1765, 1720,
and 1673 cm-1, as shown in Fig. 3a, indicating that the amide bonds
within the aramid bre were broken and oxidized to carbonyl groups.
Further analysis showed that the nal oxidation products of the amide
groups included carboxylic acids, aldehydes, and anhydrides. Ghosh
et al. [33] studied the effects of atomic oxygen and ultraviolet radiation
conditions on the mechanical properties of aramid fabrics. The reduc-
tion in tensile strength and the elastic modulus of the bres in the
combined environment was 16% and 12% higher than those of bres
exposed to atomic oxygen, as shown in Fig. 3b. When the aramid bres
were exposed to atomic oxygen, oxidation reactions occurred on the
surface of the bres, as shown in Fig. 3c. Atomic oxygen made contact
with the surface of the bre to undergo chemical absorption and formed
oxides (carbonyl and carboxyl). The oxidized surface of the aramid bre
further collided with atomic oxygen to generate CO or CO
2
. Ultraviolet
radiation promoted the gasication reactions of fabrics during exposure
to an atomic oxygen environment.
The decrease in the aramid fabric tensile properties was found to be
related to ultraviolet radiation time and exposure energy fraction. For
ultraviolet radiation time, Li et al. [34] found that when the radiation
time increased from 1 h to 3 h, the elastic modulus and fracture elon-
gation of the aramid fabric linearly decreased by approximately 10%.
Aidani et al. [35] found that as the radiation time increased from 3 h to
400 h, the tensile strength of polytetrauoroethylene (PTFE)/aramid
fabric decreased by approximately 50%, as shown in Fig. 3d. Gu [36]
and Davis et al. [7] also arrived at similar conclusions. For ultraviolet
exposure energy, Wakatsuki et al. [30,37] found that the retention
fraction of aramid fabric tensile strength was exponentially reduced by
80% as the UV exposure energy fraction increased to 10, as shown in
Fig. 3e.
The decline in the tensile properties of aramid bres was found to be
attributed to changes in the macro- and microstructures of the bres.
The ATR-IR spectrum showed that the normalized peak strength of the
amide bond in the bre changed, indicating that the amide bond broke,
leading to a decrease in the molecular weight of the aramid bre, as
shown in Fig. 4a and b [31,32], and the tensile strength of the aramid
bre decreased accordingly. Compared with the inner core of the bre,
the outer skin of the bre had a higher molecular orientation, a more
Fig. 1. Various environmental factors affect the performance of aramid bre products.
Y. Xu et al.
Polymer Testing 128 (2023) 108227
3
randomly distributed chain, and a higher toughness along the bre axis.
The electron microscopy images showed that after ultraviolet radiation,
the surface of the bre became rough, cracks appeared, and the broken
end of the yarn became brittle. The cracks in the bre quickly extended
from the surface to the core, leading to continuous disintegration within
the bre, as shown in Fig. 4c. The equilibrium and stability of the inner
core and outer skin of the bre were disrupted. The mode and extent of
tensile damage was signicantly changed, leading to rapid loss of tensile
properties, especially in terms of fracture energy, tensile strength and
elastic modulus [39–42]. As the time and intensity of ultraviolet radia-
tion increased, the amide bond breakage and corrosion of the outer skin
and inner core of the aramid bres continued to increase, and the tensile
properties of the bres were further reduced.
2.2.2. Bending properties
Ultraviolet radiation was found to have a negative impact on the
bending fatigue life of aramid bres. Cai et al. [43] found that the
bending fatigue of aramid fabrics was reduced by approximately 35%
after ultraviolet radiation due to the increasing degradation of the
aramid bre. Silva et al. [44] found that the bending strength and
modulus of curaua/polyester resin/aramid composite fabric decreased
by 43.1% and 41.2%, respectively, after 300 h of ultraviolet radiation
exposure. Ultraviolet radiation exacerbated the pull-out effect of the
bres in the polymer matrix, as shown in Fig. 5. The macromolecular
chains within the aramid bres broke under ultraviolet radiation, and
the decrease in the molecular weight of the bres was found to lead to
obvious transverse fracture of the bre, further weakening the bending
performance of the aramid fabric.
2.2.3. Thermal properties
The thermal properties of aramid bres are weakened after ultravi-
olet radiation. Li et al. [34] found that after exposure to ultraviolet ra-
diation for 1 h, the initial decomposition temperature (T
di
) of aramid
fabrics decreased from 423.8 ◦C to 384.3 ◦C, as shown in Fig. 6a.
Thermal performance refers to the ability of the fabric to maintain its
integrity when heated. The greater the bond energy of aramid fabrics,
the more stable their thermal properties are [45]. T
di
generally repre-
sented the lower limit stability of the fabric when heated. The value of
T
di
depended on the bonds of the aramid fabrics with the weakest
thermal stability. After ultraviolet radiation, the oxidation, fracture, and
oxidation of amide bonds produced more polar groups, greatly weak-
ening the chemical bonds with the worst thermal stability. The value of
T
di
decreased accordingly.
Aidani et al. [37] studied the relationship between the glass transi-
tion temperature T
g
of aramid bres and ultraviolet radiation. For the
same ultraviolet radiation time, the T
g
of the bres gradually decreased
Fig. 2. Categorization of environmental effects on the properties of aramid bres.
Y. Xu et al.
Polymer Testing 128 (2023) 108227
4
with increasing light intensity, as shown in Fig. 6b. The glass transition
of the bre was linearly related to its molecular weight. According to the
Fox-Flory equation [37],
Tg(M) = Tg∞−(K
Mn)(1)
where Mn is the average molecular weight and T
g∞
and K are constants.
The increase in light radiation time and intensity led to an increase in the
degree of molecular chain breakage of aramid bres, further reducing
the molecular weight Mn of the bres, thereby decreasing the T
g
of the
aramid fabric.
2.2.4. Ballistic performance
The ballistic performance of aramid fabric is weakened after ultra-
violet radiation. Many studies have shown that the ballistic performance
of fabrics is strongly related to their tensile properties and yarn pull-out
performance [46–51]. As mentioned earlier, ultraviolet radiation can
cause a decline in the tensile properties of the aramid yarn, resulting in a
negative impact on the ballistic performance of aramid fabric. Silva et al.
[1] studied the ballistic properties of curaua and aramid composite
fabrics after 300 h of ultraviolet radiation. The ballistic energy absorp-
tion of the composite fabric after ultraviolet irradiation was 24.3% lower
than that of the nonirradiated fabric. From the FTIR spectrum, the
normalized peak strengths of amide I (1639 cm
−1
) and amide II (1537
cm
−1
) of the bres changed, which indicated that the amide bonds
broke, and the fracture of microscopic groups led to a decrease in the
molecular weight of the bres, reducing the tensile properties of the
bre and thereby weakening the ballistic resistance performance of the
composite fabric. Nascimento et al. [52] studied the ballistic perfor-
mance of 10-layer aramid fabric after 310 h of ultraviolet radiation. The
results showed that the fabric exposed to ultraviolet radiation pene-
trated 1 layer more than the unexposed fabric, and the corresponding
back clay dummy indentation depth was 3 mm more. The macroscopic
failure of the fabric is shown in Fig. 7a.
The deterioration mechanism of the aramid fabric ballistic perfor-
mance that was caused by ultraviolet radiation was as follows. Ultra-
violet radiation degraded the bre molecular chains, causing carbon
nitrogen bonds to break, forming carboxylic acids and other oxides,
which reduced the molecular weight and affected the crystallinity of the
bre. The surface of the aramid bre had a thin, highly crystalline
covering layer, with a low crystallinity at its core and various defects.
Ultraviolet radiation led to further deterioration of this part of the
defect, as shown in Fig. 7b. Microscopic defects of the bre caused
changes in macroscopic properties. After ultraviolet radiation, the ten-
sile strength of the aramid bre was decreased by approximately 20%,
and the ballistic resistance performance was correspondingly weakened.
2.2.5. Interfacial properties
Ultraviolet radiation has a positive effect on the interfacial properties
of aramid bres. The interface properties are mainly reected in the
bonding state and strength of composite materials. Wang et al. [22]
found that ultraviolet radiation accelerated the polymerization process
of catechol/polyamine (CPA)/aramid composite fabric, as shown in
Fig. 8a. The process of CPA deposition on the surface of composite bres
Fig. 3. (a) Tensile properties and FTIR spectra of UV-aged aramid bres [31]; (b) tensile strength and modulus of aramid bres under exposure to atomic oxygen and
ultraviolet radiation [33]; (c) XPS spectra of aramid bres under exposure to atomic oxygen and ultraviolet radiation [33]; (d) effect of radiation time on the tensile
properties of PTFE/aramid bres [35]; (e) effect of radiation exposure energy on the tensile properties of aramid bres [30,37].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
5
was found to be shortened to 1 h. Zhang et al. [53] studied the effect of
high-energy radiation on the interfacial shear strength (IFSS) of epoxy
resin/aramid composite fabric. The IFSS between the bre and matrix is
calculated by the following formula:
IFSS =F
π
DL (2)
where F is the maximum pull-out force, D is the average diameter of the
aramid bre, and L is the length of the bres embedded in the matrix.
The IFSS of the composite fabric was increased by 17.7% due to the
signicant improvement in bre surface roughness and wettability
caused by high-energy radiation, as shown in Fig. 8b. Xing et al. [54]
found that the interlaminar shear strength (ILSS) between aramid fabric
and epoxy resin was increased by 45.2%, as shown in Fig. 8c. The
Fig. 4. (a) Schematic diagram of degradation of aramid bres under ultraviolet radiation [32]; (b) ATR-IR spectra of aramid bre after 168 h UV irradiation [38]; (c)
SEM images of aramid macro bres with ultraviolet irradiation times of 0–120 h [31].
Fig. 5. Photographs of fracture surfaces of curaua/polyester resin/aramid composite fabric (a) without ultraviolet irradiation and (b) after ultraviolet irradia-
tion [44].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
6
dynamic contact angle analysis showed that the total surface free energy
and polar groups of the aramid bres were increased after ultraviolet
radiation, and the interface properties were improved accordingly.
2.3. Methods for improving the UV resistance of modied aramid fabrics
The surface of aramid bre lacks chemically active groups, and the
benzene ring structure produces a large steric hindrance effect, resulting
in a high level of chemical inertness, which is not conducive to the
combination of aramid bre and other materials. Hydrogen bonds are
formed between aramid polymer chains, resulting in a smooth surface
and poor surface wettability, leading to the formation of interfacial
defects between aramid bres and other materials and limiting the ap-
plications of composite materials. Therefore, the methods of surface
modication of aramid bres generally focus on two aspects: using
chemical reactions to improve the composition and structure of the bre
surface or using physical methods to coat other materials on the surface
of aramid fabrics.
The modication methods for enhancing UV resistance are mainly
based on three principles: the UV absorption principle, free radical
capture principle, and nonradiative transition principle.
2.3.1. Modication method based on UV absorption principle
The molecules in the material have a specic energy-level structure.
When the molecule is irradiated with ultraviolet rays, energy can pro-
mote the transition of electrons in the molecule from low to high energy
levels. This electronic transition leads to ultraviolet absorption. Metallic
oxides such as ZnO and TiO
2
have excellent ultraviolet absorption and
scattering capabilities, and grafting corresponding metal oxides onto the
surface of aramid bres is an effective way to improve the ultraviolet
radiation resistance of aramid fabrics. The preparation process of ZnO-
modied fabric is as follows. To improve the surface properties of
aramid bres, pretreatment is carried out on the fabric, including
alkaline washing, acid washing, and deionized water washing. The
peptide bond between the amide and carbonyl functional groups is
cleaved through hydrolysis reactions. Subsequently, the planting of ZnO
Fig. 6. (a) Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of aramid bres after ultraviolet radiation [34]; (b) evolution of the glass
transition temperature as a function of ageing time at different light intensities [37].
Fig. 7. (a) Macroscopic surface and projectile penetration of aramid bres before and after ultraviolet radiation [52]; (b) SEM images of aramid bres before and
after UV degradation [1,52].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
7
Fig. 8. (a) SEM images of aramid bres modied by CPA solution after UV radiation [22]; (b) IFSS of aramid/epoxy microcomposites [53]; (c) ILSS of aramid/epoxy
composites after high-energy radiation [54].
Fig. 9. Methods for modifying aramid bres with ZnO. (a) Schematic of the preparation of the ZnO-modied aramid bre by (a) KH602 [55]; (b)
L-DOPA/ZnO [56];
(c) Sc-CO
2
atmosphere [57]; (d) ALD methods [58].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
8
is usually achieved through silane coupling agent treatment and atmo-
sphere treatment methods.
For coupling agent treatment, Ma et al. [55] washed aramid bres
with dimethylformamide and SOCl
2
solution to obtain acyl chloride
functionalized bres, as shown in Fig. 9a. The bres were immersed in a
tetrahydrofuran solution containing KH602, allowing the –NH
2
group to
react with the COCl
2
group of the functionalized bres, bonding Si–OH
to the bre surface. Then, ZnO nanoparticles were grafted onto the bre
surface through a chemical reaction to grow ZnO nanowires. Zhang et al.
[56] coated L-3,4-dihydroxyphenylalanine (L-DOPA) on the surface of
aramid bre and grafted the active functional group –COO– onto the
surface of the bre through plasma oxygen treatment, as shown in
Fig. 9b. A coordination interaction between ZnO and –COO– was formed
and grafted onto the surface of the aramid bre. The growth process of
ZnO was similar to that reported in Ma et al. [55].
For atmosphere treatment, Zhang et al. [57] dried the aramid fabric
taken from the ZnO culture solution via a Sc-CO
2
atmosphere, removing
impurities and liquids from the bre surface during the drying process
and promoting the formation of coordination binding between ZnO and
the bre surface, as shown in Fig. 9c. Azpitarte et al. [58] achieved the
combination of ZnO and an aramid bre surface through atomic layer
deposition (ALD) and multiple pulsed inltration (MPI), as shown in
Fig. 9d.
The planting of ZnO can signicantly improve the mechanical
properties of the aramid fabrics after exposure to ultraviolet radiation.
ZnO mainly grows into nanowires and nanoparticles on the bre surface,
as shown in Fig. 10a. Azpitarte et al. [58] found that the toughness
modulus of ZnO-modied fabrics was increased by 80% compared with
neat fabrics after 24 h of ultraviolet radiation, as shown in Fig. 10b. SEM
images showed that the surface of the bre was corroded by ultraviolet
light and produced vertical cracks, resulting in cracking of the bre from
the outside to the inside. However, the ZnO-modied bre basically
maintained its initial state, and the bres with ZnO growing on the
surface were basically undamaged. ZnO mainly absorbed ultraviolet
radiation by converting it into heat through electronic excitation and
recombination. The shading and absorption effects were found to pre-
vent the breakage of amide bonds and reduce the generation of surface
cracks on the bres, thereby improving the UV resistance of the modi-
ed fabrics. From microscopy images, it was found that ZnO particles
protected the outer surface of aramid bres from ultraviolet radiation
corrosion, as shown in Fig. 10c. The mechanical properties of the aramid
bres were improved after ultraviolet radiation, as shown in Fig. 10d.
Notably, the thickness of the ZnO nanolayers was found to signi-
cantly affect the UV resistance of the modied bre. Conventional metal
coatings require a thickness of at least 100 nm to achieve UV resistance
[58], as shown in Fig. 11a. Azpitarte et al. [59] found that when the
thickness of ZnO nanoparticles was less than 50 nm, their improvement
of the UV resistance of bres was minimal. TEM images of cross sections
of the ZnO coating are shown in Fig. 11b. The improvement was minimal
because the ZnO coating with a thickness of 50 nm did not completely
block and absorb ultraviolet radiation. The ZnO coating close to the bre
acted as a photocatalyst and came into contact with water in the air to
produce oxygen-containing free radicals that were degraded the
framework of the aramid bres. This photocatalytic side reaction was
Fig. 10. (a) SEM images of ZnO nanoparticle/nanowire-modied aramid bres [55,56]; (b) toughness modulus of the aramid bres after UV irradiation [58]; (c)
SEM micrographs of ZnO-modied aramid bres and neat bres after UV irradiation [55]; (d) reductions in tensile property retention of irradiated aramid bres [57].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
9
found to degrade the aramid bres near the ZnO particles, affecting the
UV resistance of the modied bres. Therefore, a thickness of at least
100 nm was required for the ZnO coating to ensure that the
oxygen-containing radicals generated by the catalytic reaction did not
come into contact with the surface of the aramid bre, thereby pre-
venting such adverse effects.
TThe ZnO nanoparticle size was found to also affect the UV ab-
sorption performance of the modied bre. Patterson et al. [60] found
that the existence of a bandgap in ZnO nanoparticles exhibited a
shielding effect, converting UV radiation into heat through electronic
excitation and recombination. As the size of the ZnO nanoparticles
decreased, their frequency gap widened, and their UV absorption per-
formance changed with the particle size. When the diameter of ZnO
nanoparticles was adjusted to approximately 12.6 nm, the ZnO coating
was found to absorb a signicant amount of ultraviolet radiation and to
reduce UV damage to the aramid fabric.
Unlike the preparation method for achieving chemical bonds of ZnO-
modied fabrics, TiO
2
particles are mainly combined with aramid bres
by physical precipitation [61,62]. To improve the binding rate between
TiO
2
particles and aramid bres, Zhao et al. [63] added a KH550 silane
coupling agent to make TiO
2
nanoparticles rich in amino groups, as
shown in Fig. 12a. Nanobrillated cellulose (NFC) was also added to
reduce the TiO
2
nanoparticle aggregation phenomenon and improve
coating uniformity. Moreover, the ammonia groups on the surface of
TiO
2
particles were found to interact with the NFC hydroxyl groups to
form a stable core-shell structure, improving the stability of physical
adhesion. In this process, the aramid bres needed to be washed with
phosphoric acid and deionized water to introduce hydroxyl and carboxyl
groups to the surface. Zheng et al. [3] used a centrifugal grinding
method to prepare pyrite and TiO
2
sol and dipped aramid fabric into the
sol to achieve the physical attachment of TiO
2
nanoparticles, as shown in
Fig. 12b. Zhai et al. [64] used the atomic layer deposition (ALD) method
to attach TiO
2
particles on the bre surface, and two oxygen-containing
active sites were provided to destroy the amide bond on the aramid bre
surface through surface chemical treatment, which improved the aramid
bre surface activity, as shown in Fig. 12c. TiO
2
particles went through
the pulse, exposure, purging, pulse, exposure, and purging steps in turn,
and the adhesion thickness of TiO
2
particles on the aramid bre layer
was accurately controlled by controlling the number of ALD cycles.
TiO
2
coatings can signicantly improve the UV resistance of aramid
bres. Zhai et al. [64] evaluated the UV resistance of TiO
2
-modied
fabrics through metrics such as the UV protection coefcient, colour
strength, and mechanical properties. The ultraviolet protection factor
(UPF) value of TiO
2
-modied aramid bre was 75.9, which was
approximately 4 times that of neat bre, as shown in Fig. 13a. Deng et al.
[65] and Dong et al. [32] also found an increase in the UPF value of
TiO
2
-modied bres. For colour strength, the shade depth of the fabric
was determined from the K/S values. When the UV radiation time
increased, the K/S values of the aramid bres showed an increasing
trend, as shown in Fig. 13b. The K/S value of TiO
2
-modied aramid -
bres was always lower than that of neat bres, indicating that the TiO
2
coating improved the UV resistance of the aramid bres. To analyse the
UV resistance of TiO
2-
modied bres from the angle of decomposition
products, the strength of the stable benzene ring C]C tensile vibration
peak was selected as the benchmark. By comparing the peak size of
carboxylic acid groups, the UV resistance of TiO
2
-modied bres can be
clearly explained. Zhai et al. [64] found that the peak height ratio of
COOH/C]C in the modied bres after UV radiation was 3.96, which
was much lower than that of neat bres (14.87), indicating that the
presence of TiO
2
minimized the transition of amide bonds to carboxylic
acid groups in aramid bres and signicantly protected the aramid -
bres from ultraviolet radiation, as shown in Fig. 13c.
To observe the extreme effect of light intensity on the mechanical
properties of TiO
2
-modied aramid bres, aramid bres were exposed
to 4260 W/m
2
ultraviolet radiation for 360 min (approximately equiv-
alent to continuous exposure to strong sunlight for 7.5 years). The me-
chanical properties of the aramid bres were signicantly decreased,
with tensile strength and failure strain were only 34% and 10% of those
before radiation. The tensile strength and failure strain of TiO
2
-modied
aramid bres was found to be maintained at 54% and 29%, respectively,
and were greatly improved compared with neat fabrics. The decline in
the mechanical properties of the aramid bres was found to be attrib-
uted to the fracture of amide bonds and the formation of carboxylic acid
groups [66], as shown in Fig. 13d and e.
In addition to absorbing ultraviolet radiation, the interface reection
performance of the TiO
2
coating has been found to improve the UV
resistance of aramid bres. Moreover, the dense TiO
2
layer also plays an
important role in preventing oxygen from seeping into the interior of the
bre. Because the refractive index of the amorphous TiO
2
layer was
found to be much higher than that of the aramid bre, when UV light
was irradiated on the surface of the aramid bre, some of the light was
reected by the aramid bre, as shown in Fig. 13f. As a result, the
proportion of UV light absorbed by the amorphous TiO
2
layer further
decreased, which was found to help improve the UV resistance of the
aramid bre [64,67].
The thickness of TiO
2
has a signicant inuence on the UV resistance
of the modied bres. This is because ultraviolet radiation can cause
TiO
2
to undergo oxidation‒reduction reactions, leading to strong
oxidation holes in the valence band and reducing electrons in the con-
duction band. These electrons react with water and oxygen in the air to
generate strong oxidizing substances that penetrate the TiO
2
protective
layer, causing oxidation and degradation on the surface of the aramid
bres. The amide bonds of the bres break, and the mechanical prop-
erties of the bres are affected. Zhai et al. [64] found that when the
thickness of the TiO
2
layer reached 100 nm, the negative effects of the
oxidation reactions were effectively mitigated. Therefore, when pre-
paring TiO
2
-modied aramid fabrics, it was necessary to ensure that the
thickness of the TiO
2
coating was greater than 100 nm, which
Fig. 11. (a) UV absorbance spectra of traditional metallic oxide-modied bres with different adhesion layers after UV exposure [58]; (b) TEM images of cross
sections of ZnO-modied aramid bres [59].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
10
corresponded to the ZnO-modied aramid fabric mentioned earlier.
2.3.2. Modication method based on free radical capture principle
The highly active free radicals generated by the photocatalytic re-
action are captured by the middle coating to improve the UV resistance
of the fabric (free radical capture principle). Stacking other metal layers
outside the TiO
2
coating was also an effective method to avoid the
negative effects of oxidation reactions. Chen et al. [68] and Xiao et al.
[69] proposed adding an additional layer of material outside the TiO
2
coating to further enhance the UV resistance of the modied fabric in
response to the issue of the redox reaction of TiO
2
under ultraviolet
radiation, as shown in Fig. 14a and b. The ALD method was used to cover
the aramid bre with Al
2
O
3
and TiO
2
coatings. After 90 min of exposure
to 4260 W/m
2
UV radiation, the tensile strength and elastic modulus of
the modied aramid bres were only reduced by 17.07% and 0.85%,
respectively, as shown in Fig. 14c. Introducing an ultrathin Al
2
O
3
layer
with a thickness of less than 37.4 nm can effectively avoid charge
transfer at the interface between the aramid bres. When the total
coating thickness was limited to 100 nm, it was found that the modied
aramid bre with a thickness of 20 nm Al
2
O
3
coating and 80 nm TiO
2
Fig. 12. (a) Schematic illustration of the preparation of NFC/TiO
2
/aramid fabric [63]; (b) schematic diagram of the modied fabric preparation and the photo-
catalytic mechanism [3]; (c) schematic of preparing highly modied aramid fabric by using the ALD method [64].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
11
coating had the best UV resistance, as shown in Fig. 14d. The
Al
2
O
3
–TiO
2
interlayer was found to prevent the active free radicals and
electrons generated by the redox reaction from reaching the surface of
the aramid bre by forming an energy barrier. Moreover, this ultrathin
Al
2
O
3
layer also hindered the penetration of external materials, further
improving the antioxidant properties and UV resistance of the bres.
Fig. 13. (a) UPF values of TiO
2
-modied aramid fabric [64]; (b) K/S curves of TiO
2
-modied aramid fabric after UV irradiation [64]; (c) –COOH/C]C peak height
ratio of TiO
2
-modied aramid fabric after UV radiation [64]; (d) Stress‒strain curves of TiO
2
-modied aramid bre after UV irradiation [64]; (e) Tensile properties of
TiO
2
-modied aramid bre after UV irradiation [66]; (f) Schematic diagram of the mechanism of the modied bre resistance to ultraviolet radiation [64].
Fig. 14. (a) The reaction mechanism of trimethylaluminium (Al(CH
3
)
3
, TMA) on the framework of aramid bre polymer chains [68]; (b) TEM image of a
cross-sectional view of the interfacial region of Al
2
O
3
/TiO
2-
and TiO
2
-modied aramid bres [64,68]; (c) stress–strain curves of Al
2
O
3
/TiO
2-
and TiO
2
-modied
aramid bres after UV irradiation [68]; (d) FTIR spectrum and –COOH/C]C peak height ratio of TiO
2
-modied aramid bre [68].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
12
Although TiO
2
and ZnO particles have excellent UV absorption
properties, the oxidation catalytic effect caused by a thin coating can
weaken the improvement in the UV resistance of the modied bres. To
enhance the UV resistance of thin-coated modied bres, methods for
modifying aramid fabrics with composite metallic oxides have been
proposed, as shown in Fig. 15.
The UV resistance of the mixed metallic oxide-modied aramid
fabric was found to be signicantly improved. Zhou et al. [70] prepared
SiO
2
and MgAlFe layered double hydroxide (LDH)-modied aramid -
bres with the layer-by-layer (LBL) method. The thickness of the coating
was controlled by changing the number of self-assembly cycles. The
X-ray diffraction (XRD) pattern showed that the diffraction peak shape
and position of the modied bres were the same as those of neat fab-
rics, which indicated that the modication process only affected the
chemical morphology of the bre surface and did not affect the crystal
structure. Compared with neat bre, modied aramid bre under an
irradiation intensity of 1.55 W/m
2
for 168 h had an increase in tensile
strength, elastic modulus, and fracture strain of 25.3%, 11.5%, and
17.3%, respectively, as shown in Fig. 16a. This modication method
mainly protected the aramid bre by shielding it from ultraviolet
radiation.
Cheng et al. [71] used the principle of uorescence emission
enhancement to modify aramid bres with Fe
3+
. It was found that Fe3+
and H
+
had a strong coordination ability and uorescence efciency and
exhibited strong uorescence effects when combined. Compared with
neat fabric, the modied bre tensile strength and fracture strain after
96 h of exposure to a 3 kW ultraviolet radiation lamp increased by 31.5%
and 26.9%, respectively, as shown in Fig. 16b. Fe
3+
converted most of
the ultraviolet radiation energy into photons through uorescence
emission, reducing the heat release caused by molecular rotation and
vibration, thereby reducing the breakage of the bre amide bonds.
Zhu et al. [72,73] treated aramid bre with coupling agents to obtain
methoxy groups on the bre surface and then grafted the bre with CeO
2
and CaO particles to obtain Ce
0.8
Ca
0.2
O
1.8
-modied aramid bre. Under
the radiation of a 3 kW ultraviolet lamp for 168 h, the tensile strength
and fracture strain of the modied aramid bres were increased by
29.6% and 21%, respectively, compared with neat bre, as shown in
Fig. 16c and d. The ultraviolet radiation absorption of Ce was mainly
through charge transfer between the O 2p and 4f empty orbitals. The
ultraviolet radiation absorption strength of Ce
0.8
Ca
0.2
O
1.8
was higher
than that of Ce due to its smaller size and wider bandgap. When Ca
2+
was mixed with Ce
4+
, oxygen vacancies formed within the lattice to
maintain electrical balance, making ultraviolet radiation more easily
absorbed. Later, they added CaO to reduce the oxidation and photo-
catalytic activity of CeO
2
, thereby reducing radiation damage to the -
bres. At the same time, defects such as oxygen vacancies and lattice
distortion inside the crystal were conducive to light scattering.
Ce
0.8
Ca
0.2
O
1.8
’s absorption and scattering of ultraviolet radiation can
better protect aramid bres.
Similarly, to optimize the UV radiation resistance of CeO
2
-modied
bres, Cai et al. [74] prepared modied bres by covering the surface of
CeO
2
particles with boron nitride (tBN) and polydopamine (PDA).
Compared with neat bre, the modied bre tensile strength and frac-
ture strain were increased by 18.1% and 17.9%, respectively. The tBN
and PDA coatings isolated the contact between ultraviolet light and
CeO
2
, thereby weakening the photooxidation reaction and reducing the
generation of photoelectrons. The CeO
2
coating was found to reduce the
contact between reactive oxygen species generated by ultraviolet
catalysis and bres, weakening the corrosion effect of ultraviolet light
on aramid bres.
2.3.3. Modication method based on non-radiative transition principle
Nonradiative transitions indicate that atoms are not accompanied by
photon launch or absorption at different levels of energy transitions but
instead pass excess energy to other atoms or absorb other atoms to their
energy. Thin metallic coatings can act as oxidation catalysts under
Fig. 15. (a) Preparation process of SiO
2-
and MgAlFe-modied aramid fabric [70]; (b) grafting method of CeO
2-
modied aramid fabric [74]; (c) schematic of the
preparation of hyperbranched polysiloxane (HSi)–Ce
0.8
Ca
0.2
O
1.8-
modied aramid fabric [72]; (d) mechanism for preparing Ce
0.8
Ca
0.2
O
1.8
-modied aramid fab-
ric [73].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
13
ultraviolet radiation to weaken the UV resistance improvement of the
modied bres. To avoid the negative effects of photooxidation re-
actions, the concept of organic modied aramid bres has been pro-
posed, as shown in Fig. 17.
The mechanical properties of the organic-modied bres are greatly
improved under ultraviolet radiation. Zhu et al. [75] obtained poly-
dopamine (PDA)-graphene oxide (GO)-modied aramid bre by
immersing aramid bre into dopamine (DA) and GO solution for reac-
tive culture, as shown in Fig. 17a. DA had strong adhesion, which
enabled it to evenly wrap over the surface of the aramid bre and form a
PDA coating. Furthermore, the GO coating was formed through an
esterication reaction with PDA and
π
-
π
stacking. The tensile strength,
elastic modulus and fracture strain of the modied bre were increased
by 21.6%, 8.3% and 20.4%, respectively, compared with those of the
neat bre after being irradiated for 168 h, as shown in Fig. 18a. Ultra-
violet radiation promoted the conversion of bre laments to microl-
aments, and the presence of microlaments weakened the overall tensile
properties of the bre. GO had a strong absorption peak near the ul-
traviolet wavelength, and the GO coating had a strong absorption and
insulation effect on ultraviolet light by uniformly wrapping aramid -
bres, thus improving the UV resistance of the modied bres.
Zhang et al. [76] prepared hyperbranched polysiloxane-modied
aramid bres through the hydrolysis and condensation reactions of
γ-methacryloxy-propyltrimethoxysilane (MPS) and γ-glycidylox-
ypropyltrimethoxysilane (GPTMS) on the surface of aramid bres, as
shown in Fig. 17b. After being exposed to a 3 kW ultraviolet radiation
lamp for 168 h, the retention of tensile strength, elastic modulus, and
fracture strain of the modied bre were increased by 22.7%, 13.1%,
and 14.1%, respectively, compared with those of neat bre, as shown in
Fig. 18b. The energy of a –Si–O– bond (446 kJ/mol) was found to be
signicantly higher than UV radiation power (314–419 kJ/mol), while
the energy of a –C–C– bond was only 358 kJ/mol. The chemical stability
of -Si-O- under ultraviolet radiation is signicantly better than that of
–C–C–. Therefore, the preparation of coatings using hyperbranched
polysiloxanes composed of –Si–O– bonds was found to improve the UV
resistance of aramid bres. Moreover, compared with C–C, C]C conju-
gated bonds had a higher bond energy and better ability to absorb ul-
traviolet radiation. Therefore, MPS containing C]C bonds was used to
prepare hyperbranched polysiloxanes, which contained many double
bonds and had better UV resistance, as shown in Fig. 18c.
Lu et al. [77] prepared polypyrrole (PPy)-modied aramid fabric
using the impregnation method, as shown in Fig. 17c. After being
treated in a 50 J ultraviolet ageing device for 6 h, the retention of tensile
strength in the modied aramid bre was increased by 23.1% compared
with that of neat bre, as shown in Fig. 18d. This was because the black
PPy coating was found to absorb most of the ultraviolet radiation,
reducing the breakage of amide bonds in the bres caused by ultraviolet
radiation. Xia et al. [78] prepared cellulose-modied aramid fabric by
the hydrogel method, as shown in Fig. 18d, and found that the ultravi-
olet resistance of the modied bre was improved.
Fig. 16. Tensile properties of (a) SiO
2
/MgAlFe [70]; (b) Fe
3+
[71]; (c) HSi–Ce
0.8
Ca
0.2
O
1.8
[72]; (d) Ce
0.8
Ca
0.2
O
1.8
[73] modied aramid fabric; (e) Infrared (IR)
spectra of SiO
2
/MgAlFe modied aramid fabric [70] and attenuated total reection (ATR) spectra of Fe
3+
modied aramid fabric [71]; (f) UV–Vis absorption spectra
[72] and ATR-IR spectra [73] of Ce
0.8
Ca
0.2
O
1.8
modied aramid fabric after UV irradiation.
Y. Xu et al.
Polymer Testing 128 (2023) 108227
14
Fig. 17. The mechanism for preparing (a) polydopamine/graphene oxide [75]; (b) hyperbranched polysiloxane [76]; (c) polypyrrole (PPy) [77] (d) cellulose [78]
modied aramid fabric.
Fig. 18. Retentions of tensile properties of (a) PDA/GO [75]; (b) hyperbranched polysiloxane [76]-modied aramid fabric; (c) C 1s core-level spectra of hyper-
branched polysiloxane-modied aramid fabric [76]; tensile properties of PPy-modied aramid fabric after UV irradiation [77].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
15
3. High and low temperature resistance of aramid fabric
3.1. High and low temperature environmental treatment
There are usually two testing methods for exploring the effect of
temperature on the performance of aramid fabrics. The rst method is to
perform heat treatment on fabrics. The fabric is placed in a thermal
mechanical analysis testing chamber, and the effect of temperature on
the aramid bres is tested by controlling the temperature inside the
chamber. Notably, when the testing temperature is above 100 ◦C, the
bre sample needs to be suspended in the chamber space and without
coming into contact with the surface of the chamber to prevent bre
burning. During the heating process, the temperature inside the test
chamber needs to change steadily over time. The test of the treated bre
properties is carried out at room temperature [79,80].
The second method is to control the temperature of the testing
environment through a constant temperature box. To maintain a stable
ambient temperature, the uid media equipped with low-temperature
and high-temperature thermostats are anhydrous ethanol and silicone
oil. A uid medium ows into the constant temperature chamber
through a pipeline and then returns to the constant temperature bath to
achieve constant temperature circulation. The edges of the incubator are
sealed with silicone to prevent heat exchange with the outside air. When
the ambient temperature stabilizes and reaches the test temperature,
corresponding property testing begins [81,82].
3.2. Inuence of environmental temperature on the properties of aramid
bres
3.2.1. Tensile properties
As the temperature increases, the tensile strength of the aramid bre
gradually decreases. Yue et al. [83] studied the mechanical properties of
the aramid bres at temperatures ranging from 100 ◦C to 300 ◦C under
vacuum and air conditions. In the air atmosphere, the tensile strength of
aramid bres was barely weakened at 100 ◦C. During this process, the
aramid bres experienced slight mass loss due to the loss of
Fig. 19. (a) TGA curve for aramid fabric in an air environment [92]; (b) tensile strength of heat-treated aramid fabric [83]; (c) schematic of the structural changes
within the outermost layer bres exposed to thermal conditions [93]; (d) aramid fabric tensile strength after various levels of thermal exposure [92]; (e) crystallinity
of aramid fabric with different thermal exposure times and temperatures [91]; (f) T
g
of aramid fabric with different thermal exposure times and temperatures [91].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
16
intermolecular bound water, but the surface morphology and chemical
structure of the bres remained unchanged [84–87]. At 200 and 300 ◦C,
the tensile strength of the aramid bre decreased by approximately 20%
and 30%, respectively, as shown in Fig. 19a and b.
The tensile strength of a bre is mainly determined by its molecular
orientation [83]. Generally, the molecular orientation of the surface
layer of the bre is higher than that of the inner core. When the bre was
subjected to stable external forces, the surface area with a higher mo-
lecular orientation under the same strain would prioritize the failure of
the inner core, inducing the failure of the whole bre. The occurrence of
surface cracks in bres at high temperatures was attributed to changes in
microstructure. High temperature induced an increase in bre
Fig. 20. (a) Temperature of the outermost layer of aramid fabrics under different thermal conditions [93]; (b) the aramid fabric tensile properties under different
thermal conditions [93]; (c) mechanical parameters of aramid bres at different temperatures [96]; (d) number of hydrogen bonds at different temperatures for
aramid bre molecules [96]; (e) MSD time curves for amorphous regions of aramid bres at different temperatures [97]; (f) hydrogen bonding network in the
crystalline regions of meta-aramid bres [97]; (g) model of the amorphous area of aramid bres [97].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
17
microcrystalline size, which in turn created micropores on the bre
surface, and the surface roughness of the bres was increased. Changes
in the microstructure of the surface and core was found to lead to an
increase in orientation differences between the surface and core, as
described in Fig. 19c. Therefore, the tensile strength of the bres at high
temperatures was signicantly affected.
The modication of the microstructure in the thermal environment
was found to be related to the oxidation of bres [83,88]. The tensile
strength of the aramid bre decreased in an air atmosphere but
remained unchanged in vacuum. Therefore, when modifying bres
through heat treatment, vacuum heating should be used as much as
possible to maximize the mechanical properties of the fabric. Dolez et al.
[89] and Cai et al. [90] also reached a similar conclusion.
The heat treatment time also has been found to have a signicant
impact on the tensile strength of aramid bres. As the heat treatment
time increased, the tensile strength of the aramid bre decreased, but
the degree of reduction depended on the strength of the heat treatment.
Arrieta et al. [91] and Rezazadeh et al. [92] studied the effect of heat
treatment time on the tensile strength of aramid bre. When the heat
treatment strength was 10k W/m
2
and the heat treatment time increased
from 600 s to 2400 s, the tensile strength of the bre decreased by 5%.
The heating time had little effect on the bres because the degradation
degree of the fabric was relatively small at this temperature. When the
heat treatment strength was 20 kW/m
2
, the heat treatment time
increased from 30 s to 300 s, and the tensile strength of the bre
decreased by approximately 20%. Under higher heat treatment strength,
the tensile strength of the bre decreased signicantly in the early stage
of heat treatment compared with the later stage. When the heat treat-
ment strength was 30k W/m
2
and the heat treatment time increased
from 15 s to 30 s, the tensile strength of the bre decreased by 55%. If
the bre continued to be exposed at this temperature for 30 s, its tensile
strength decreased by only 10% because the bre had been completely
degraded, and further exposure time had little impact on the tensile
property of the aramid bre, as shown in Fig. 19d–f.
Unlike tensile strength, the elastic modulus of aramid bre rst in-
creases and then decreases with the temperature of heat treatment.
Maurya et al. [93] and Parimala et al. [94] found that the elastic
modulus of aramid bre was increased by 20 MPa, 64 MPa, and 229 MPa
compared with that of neat fabrics under heat treatment at 220 ◦C,
330 ◦C, and 440 ◦C for 60 s, as shown in Fig. 20a and b. However, as the
temperature continued to rise, the elastic modulus of the aramid bre
decreased due to the degradation of the aramid bre [95].
As mentioned earlier, when the heat treatment temperature excee-
ded 100 ◦C, the water content of the aramid bres decreased, causing an
increase in the elastic modulus of the bre. Molecular dynamics are
usually used to explain the effect of water content on the elastic modulus
of aramid bres. Yin et al. [96] studied the effect of water content on the
elastic modulus of aramid bre using molecular dynamics simulation.
The elastic modulus of the bre with a moisture content of 2% was
41.12% lower than that of the bre in an anhydrous state, as shown in
Fig. 20c and d. It was found that water molecules and meta-aramid bres
formed many hydrogen bonds, as shown in Fig. 20f, and the number of
hydrogen bonds increased with increasing water content. Moisture
affected the position of molecular chains between the bres. As the
water content increased, the hydrogen bonding network of the bres
was disrupted, and the interaction between bres weakened, resulting in
a decrease in their elastic modulus. This was found to also explain why
the elastic modulus of the bres increased with the heat treatment
temperature.
To investigate why the elastic modulus of aramid bres decreased
when the heat treatment reached a degradation temperature, Tang et al.
[97] discussed the structure of the crystalline and amorphous regions of
aramid bres, as shown in Fig. 20e–g. The mean square displacement
(MSD) reected the centre mass displacement of a molecular chain
within a single period, which was found to effectively describe the
motion characteristics of the molecular chain. The MSD expression was
dened as
MSD =〈|ri
→(t) − ri
→(0)|2〉(3)
where ri
→(t)is the position of atom i in the system at time T and ri
→(0)is
the initial position of the atom. The molecular chain motion in the
amorphous region of aramid bres signicantly increased with tem-
perature. For the crystalline and amorphous regions of aramid bres,
their properties were also affected by temperature, and the amorphous
region was more sensitive to temperature than the crystalline region.
When the temperature reached the degradation temperature of the
aramid bre, the hydrogen bond structure in the crystalline zone of the
aramid bre was destroyed [98], and the mechanical properties of the
bre decreased. This was the reason for the decrease in the mechanical
properties of the aramid bre after the heat treatment temperature
reached the degradation temperature.
In addition to conducting military operations in high-temperature
environments, soldiers often carry out combat duties in icy environ-
ments while wearing aramid body armour. To investigate the effect of
low temperature on the mechanical properties of the aramid bres,
Islam et al. [99] placed aramid bres in a liquid nitrogen environment at
−196 ◦C for 6 h. The elastic modulus and tensile strength of the aramid
bres decreased by 4.5% and 1.7%, respectively. Bang et al. [5] found
that when the temperature decreased from 20 ◦C to −20 ◦C, the elastic
modulus of glass/aramid composite bre increased by 32.8%. When the
temperature decreased slightly, the water content inside the bre
decreased, and the hardness and stiffness were improved. When the
temperature dropped from −20 to −70 ◦C, as shown in Fig. 21a and b,
the elastic modulus of the composite bre decreased due to the
embrittlement of the adhesive between the bres.
The surface bre partially froze at low temperature, leading to an
increase in the strength and brittle fracture sensitivity of the surface
yarns, as shown in Fig. 21c. When the bre bundles began to fracture,
the remaining intact bre bundles withstood the load. The subsequent
failure behaviour of the yarn depended on the strength of the remaining
unbroken bres. When the surface bres broke, the remaining bres in
the inner core were found to withstand the load at low temperature, and
there was a limit to the elongation of the bres. Therefore, the fracture
performance of the entire yarn at low temperature was not greatly
affected [5].
3.2.2. Yarn pull-out properties
At room temperature, as the temperature increases, the maximum
yarn pullout force continuously decreases. As the temperature de-
creases, the maximum yarn pullout force rst increases and then de-
creases. Qin et al. [82] found that when the temperature increased from
10 ◦C to 50 ◦C, the maximum aramid yarn pullout force decreased by
approximately 20%. When the temperature dropped to −50 ◦C, the
maximum pullout force rst increased and then decreased, as shown in
Fig. 22a and b. As the temperature increased, the lubrication between
yarn strands increased, and the friction between them decreased. When
the temperature dropped to −30 ◦C, the water content of the aramid
bres decreased, the elastic modulus of the bres increased, and the
resistance of the yarn to being pulled out from the fabric increased.
When the temperature dropped to an extremely low temperature of
−50 ◦C, the hardness of the yarn signicantly increased, the deforma-
tion ability of the yarn was weakened, and the contact area of the yarn
decreased, such that the friction between the yarn was reduced [100].
Li et al. [101] studied the effect of temperature on the pull-out
performance of shear thickening uid (STF)-modied aramid fabrics.
When the ambient temperature increased from 20 ◦C to 50 ◦C, the
maximum yarn pull-out force of the modied bres decreased by 13.6%,
as shown in Fig. 22c. At room temperature, the repulsive force between
particles was easily overcome, promoting more SiO
2
particle aggrega-
tion. The aggregation effect of SiO
2
particles increased the resistance of
yarn to being pulled out. However, as the temperature increased, the
Y. Xu et al.
Polymer Testing 128 (2023) 108227
18
Fig. 21. (a) Tensile chord modulus and tensile strength of aramid bre under a cryogenic environment [99]; (b) temperature-dependent elastic modulus of aramid
bre [5]; (c) SEM images of aramid bre at room temperature and after cryogenic exposure [99].
Fig. 22. (a) Load versus displacement for the aramid fabric in the single yarn pull-out test [82]; (b) the maximum pull-out force of single aramid fabric with a
temperature variation from 50
◦C to −50 ◦C [82]; (c) pull-out force versus displacement for aramid fabric at 20 ◦C and 50 ◦C [101].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
19
repulsive force between particles increased through Brownian motion,
which was not conducive to the formation of clusters, resulting in a
decrease in the yarn pulling force of STF-modied fabrics.
3.2.3. Friction and wear performance
The friction coefcient of aramid bres rst decreases and then in-
creases with increasing temperature, and the bre wear rate continues
to increase. Hu et al. [102] found that the friction coefcient of a pol-
ytetrauoroethylene (PTFE)/Kevlar structure at room temperature was
0.26, which decreased to 0.22 when the temperature increased to 50 ◦C
and increased to 0.31 when the temperature increased to 200 ◦C, as
shown in Fig. 23a. At room temperature, the amount of bre pulled out
and crushed continuously increased with the friction process, which
formed a lubricating friction contact lm at the friction interface, and
the friction coefcient of the bre decreased, as shown in Fig. 23b and c.
In addition, the shear strength of the bre decreased with the friction
process, which also led to a further reduction in the friction coefcient
[103]. With increasing environmental temperature and load, the friction
heat at the bre interface rapidly accumulated, and the friction heat
generated on the bre surface continuously increased. The friction pair
underwent thermal expansion, increasing contact pressure [104,105].
At the same time, the friction heat had a signicant impact on the sur-
face lubrication layer and lubrication effect of the bre and wear
self-healing characteristics of the bre. When the ambient temperature
further increased, the tensile strength and microstructure of the bres
deteriorated, leading to fracture and detachment of bres under
macroscopic characterization, as described in Fig. 23d. The local lubri-
cation layer on the friction surface peeled off, and a large amount of
internal bres were exposed [106,107], increasing the roughness of the
friction interface and leading to an increase in the bre friction coef-
cient and wear rate.
3.2.4. Stab resistance performance
At room temperature, as the temperature increases, the stab resis-
tance of the fabric decreases. As the temperature decreases, the stab
resistance rst increases and then remains stable. The stab resistance
performance of fabrics shows a similar trend to the yarn pull-out per-
formance, and many studies have shown that the maximum pull-out
force of yarn is one of the most important factors in evaluating the
stab resistance performance of aramid fabrics [108–111]. Qin et al. [82]
studied the stab resistance performance of aramid fabric from −50 ◦C to
50 ◦C. When the temperature increased from 10 ◦C to 50 ◦C, the
maximum load of fabric under knife penetration was slightly decreased.
When the temperature dropped to −50 ◦C, the maximum load rst
increased and then remained stable, as shown in Fig. 24a. During the
process of fabric being punctured by a cutting tool, due to local defor-
mation and fracture of bres, the puncture load uctuated with the
puncture displacement, and the puncture energy was gradually absor-
bed by the fabric. The pull-out force of the yarn was the accumulation of
friction between the warp and weft yarn at the interweaving point. The
increased friction between bres was found to better resist external
forces and generate greater resistance to sharp objects, as described in
Fig. 24b and c. Therefore, the puncture load and yarn pull-out force
showed a similar trend with temperature changes. Moreover, the in-
uence of temperature on the modulus of the Kevlar lament was an
important reason for the change in its stab resistance performance. As
Fig. 23. (a) Friction coefcient of aramid fabric under different sliding conditions [105]; (b) illustration of the textile structure of the broken bres [103]; (c)
introduction of the wear mechanism changes with temperature [102]; (d) stereo microscope images of aramid fabric at temperatures of 25
◦C and 200 ◦C [102].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
20
mentioned earlier, in low-temperature environments, the elastic
modulus of aramid bres increased with decreasing temperature, which
also increased the extrusion resistance of the bres during the tool
penetration process [82,112]. When the temperature dropped to
−50 ◦C, the positive effect of temperature on the inter-yarn friction and
elastic modulus yarn gradually faded away, and the stab resistance of
the fabric tended to stabilize.
3.2.5. Ballistic performance
The ballistic performance of the aramid fabric at room temperature is
the best, and it decreases to some extent as the temperature increases or
decreases. The ballistic limit velocity of aramid fabrics was tted using
the Recht-Ipson [113] formula:
vr=
α
(vp
i−vp
bl)1
p(4)
where v
bl
is the ballistic limit velocity.
α
and p were parameters that
controlled the shape of the curve. To evaluate the energy absorption of
the projectile by the fabric, the kinetic energy change of the projectile,
ΔE
k
, was selected as the evaluation parameter.
ΔEk=1
2mv2
i−1
2mv2
r(5)
where m is the mass of the projectile. Xie et al. [114] studied the ballistic
performance of STF-modied aramid fabric at −50 ◦C–100 ◦C. At a room
temperature of 20 ◦C, the maximum ballistic limit velocity of the fabric
was 67.7 m/s. As the temperature increased to 100 ◦C or decreased to
−50 ◦C, the ballistic limit velocity of the fabric decreased by 15.8% and
37.4%, respectively, as shown in Fig. 25a. The average energy absorp-
tion of the fabric at 20 ◦C was 244.66 J, which was 4.1% and 8.7%
higher than that at 100 ◦C and −50 ◦C, respectively, as shown in
Fig. 25b. When the temperature increased, as mentioned earlier, the
inter-yarn friction and tensile strength of the fabric decreased. The
ballistic resistance of the main yarn area driving the overall protection of
the fabric through mutual friction was reduced. The bres were more
prone to fracture when impacted by a projectile [115,116]. Moreover, as
the temperature increased, the yarn underwent signicant brillation.
When the temperature reached 100 ◦C, the fracture of the bres became
greatly chaotic, and the ballistic resistance performance of the fabric
was further weakened. The ballistic limit velocity of the STF-modied
aramid fabric also decreased in low-temperature environments. The
shear resistance of the STF-modied fabric and the elongation at break
of the yarn signicantly decreased at low temperatures, with a great
decline in the tensile strength of the bres due to changes in the surface
structure of the bres, as shown in Fig. 25c [114]. The fabric was found
to not fully exert its energy absorption effect on the projectile through
large deformation, resulting in a weakened ballistic performance.
Fig. 24. (a) Load and absorbed energy versus displacement for the aramid fabric in the process of knife penetration at −50
◦C–50 ◦C; (b) macroscopic and
microscopic failure characteristics of aramid fabrics; (c) macroscopic failure of the front and back of aramid fabric [82].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
21
4. Moisture resistance of aramid fabric
4.1. Moist environmental treatment
Aramid bres with the same length and cross-sectional area were
selected. To minimize the possible temperature-induced changes in
molecular recombination on mechanical properties, the bres are dried
in a vacuum environment at 50 ◦C for 2 days to maintain bres with the
same initial conditions [117,118]. Subsequently, the bres are wrapped
around the Teon cylinder, which is covered with a cotton cloth to
ensure that the surface of the bres remains immersed in water. It is
necessary to ensure that bres are evenly and gently wrapped to avoid
bre wear as much as possible, and then the bres are exposed to water
without stress. Heating coils and thermostats are used to keep the bre
sample in a constant temperature liquid environment. Vents need to be
installed on the surface of the water container to maintain a balance of
internal and external air pressure and minimize liquid evaporation.
4.2. Inuence of moist environment on the properties of aramid bres
4.2.1. Tensile properties
The tensile properties of aramid bers are slightly decreased in moist
environments. Derombise et al. [119] studied the effect of immersion
environment on the mechanical properties of aramid ber. The results
showed that the tensile strength of immersed ber was only decreased
by 2% compared with neat ber at room temperature. When the im-
mersion temperature was raised to 80 ◦C, the tensile strength was only
decreased by 5%, but the elastic modulus of immersed bers remained
almost unchanged. Li et al. [120] also reached a similar conclusion. For
aramid bers, at room temperature, the molecules mainly underwent
rotational motion of the free diamine ring, diffusing small polar mole-
cules to the defect area inside the crystal [121–123]. Due to the presence
of amide groups, aramid bers were susceptible to water diffusion, and
the interaction between water molecules and amide groups could lead to
aramid ber slightly aging, resulting in a slight decrease in tensile
strength [124,125]. The FTIR spectrum showed that the moist envi-
ronment did not signicantly cause chemical degradation and oxidation
of the bers. The absorption band strength at 1745 cm
−1
did not
decrease, indicating that the ester bonds was not chemically eroded by
water, and no signicant changes in ber quality was observed in the
linear density measurement of the bers. As was well known, the elastic
modulus of high-performance bers was inuenced by chain orientation
[126,127]. The chain breakage that occurs during the hydrolysis aging
process did not affect the orientation of the chains, and the ber elastic
modulus was not affected. Muralidharan et al. [128] studied the effect of
immersion time on the mechanical properties of aramid bers, and the
results showed that the tensile strength and fracture strain of aramid
bers were gradually decreased with the increase of immersion time.
This was because with the increase of immersion time, the degree of
hydrolysis on the ber surface increased. Fibers were easier to pull out
and break, and the tensile strength and fracture strain of bers were
decreased.
4.2.2. Thermal properties
The thermal properties of aramid bers are almost unaffected by
moist and hot environments. Derombise et al. [119] found that the T
di
of
aramid bers was 474 ◦C, and the T
di
of aramid bers treated in 80 ◦C
water for 549 days was only changed by 0.4% compared with neat -
bers, indicating that the impact of moist and hot environment on the
thermal properties of aramid bers was minimal. As mentioned earlier,
the thermal T
di
of aramid bers depended on the chemical bond with the
weakest thermal stability. Moist environment treatment did not damage
the amide bonds inside the bers, and the molecular chains of the bers
remained intact, and the thermal stability of aramid ber was almost
unchanged.
Fig. 25. (a) Ballistic limit velocity of STF-modied aramid fabric at different temperatures; (b) the effect of temperature on the energy absorption of fabric; (c)
macroscopic and microscopic failure of fabrics subjected to ballistic impact at different temperatures [114].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
22
5. Acid and alkali resistance of aramid fabric
5.1. Acid and alkali environmental treatment
To conduct acid or alkali environmental treatment, the surface of
clean aramid bres is immersed in a constant pH acid or alkali solution
for a period, and the bre surface is repeatedly washed with deionized
water to remove the acid or alkali solution from the bre surface until
the washing solution becomes neutral. To minimize the impact of
environmental moisture on bre performance, the bres are vacuum
dried at 80 ◦C [15].
5.2. Inuence of acid and alkali environment on the properties of aramid
bres
5.2.1. Tensile properties
As the acidity or alkalinity of the solution increases, the tensile
strength, elastic modulus, and fracture strain of the aramid bres de-
creases to a small extent. Li et al. [120] found that when the pH value of
the HCl solution was decreased from 7 to 1, the tensile strength, elastic
modulus, and fracture strain of the bres decreased by only 1.4%, 1.6%,
and 2.6%, respectively. Jia et al. [15] and Deng et al. [129] treated the
bres with phosphoric acid and dilute sulfuric acid, and the tensile
strength of the bres decreased by less than 7%, as shown in Fig. 26a.
The X-ray diffraction patterns showed that the bres in acidic environ-
ments did not exhibit additional diffraction peaks compared with the
original bres, as shown in Fig. 26b, indicating that no new structures
were generated within the bres and that the chemical properties of the
bres did not change. The slight decrease in mechanical properties was
caused by crystal structure damage on the bre surface, as shown in
Fig. 26c. Lin et al. [45] immersed aramid bres in 60% concentrated
sulfuric acid solution for 100 min, resulting in a 58.3% decrease in
tensile strength, which was signicantly lower than that in other types of
acid solution environments. Concentrated sulfuric acid had strong oxi-
dization, and the bre surface was corroded. The amide bond was
broken in the concentrated sulfuric acid environment, which led to the
degradation of the aramid bre and seriously damaged the structural
stability of the aramid bre. The tensile strength of the aramid bre was
greatly reduced.
Derobise et al. [119] and Lin et al. [45] studied the changes in the
mechanical properties of the aramid bres in sodium carbonate solution
and sodium hydroxide solution. The results showed that the tensile
strength and elastic modulus of the bres treated in an alkaline envi-
ronment were decreased by less than 5% compared with those of neat
bres. The modulus of bres was mainly inuenced by chain orientation
[119]. The alkaline environment only caused slight corrosion on the
surface of the aramid bres, and the tensile strength of the bre
decreased slightly. Moreover, there was no signicant change in the
orientation of the bre chains during this process, so the elastic modulus
of the aramid bre did not change signicantly.
5.2.2. Thermal properties
After being treated with acidic or alkaline solutions, the thermal
properties of the aramid bres are slightly decreased. Li et al. [120]
found that the thermal weight loss of aramid bres increased with
decreasing pH in acidic environments, as shown in Fig. 27. Derombise
et al. [119] studied the effect of sodium carbonate solution with pH 11
on the thermal T
di
of aramid bres. The results showed that after alka-
line solution treatment, the thermal T
di
of aramid bres decreased by
11 ◦C. The aromatic bre underwent cracking and cross-linking re-
actions at high temperature. The thermogravimetric analysis of the bre
treated with acid and alkali solutions mainly involved the chemical
fracture and thermal fracture of the bre molecular chain. The greater
the bond energy of the bre chemical bond is, the better its thermal
stability. The treatment of acid and alkaline solutions was found to lead
to a slight fracture of the bond inside the bre. The degree of
cross-linking of the molecules inside the bre was slightly reduced, and
the bond energy of its chemical bond was reduced accordingly [45].
Therefore, the thermal stability of aramid bre was reduced after
treatment with acidic and alkaline solutions.
Fig. 26. (a) Tensile strength of aramid bres treated with sulfuric acid [129]; (b) X-ray diffraction pattern of aramid bres treated with sulfuric acid [129]; (c) SEM
images of aramid bres treated with different acid and alkali environments [15,45,119,129].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
23
6. Summary
This review provides a detailed introduction to the effect of harsh
environments on the mechanical and ballistic properties of the aramid
bres. A combination of macroscopic and microscopic analysis is used to
elucidate the failure characteristics and deterioration mechanisms of
aramid bres under ultraviolet radiation, high and low temperatures,
humidity, and acid‒base environments. The tensile, thermal, friction
and yarn pullout properties of the aramid fabrics are mainly related to
the integrity of amide bonds, molecular weight, molecular orientation,
water content, chemical bond energy and surface roughness of bres.
The ultraviolet radiation environment has the greatest effect on the
mechanical and thermal properties of the aramid bres. Ultraviolet ra-
diation destroys the amide bond of the aramid bre, reducing the mo-
lecular weight and chemical bond energy. The skin of the bre is
degraded, and cracks occur. The cracks rapidly extend from the surface
to the core, leading to the continuous disintegration of the interior of the
bre. The balance between the internal and external molecular orien-
tation of the bre is destroyed. The tensile, thermal and ballistic prop-
erties of fabrics decreased accordingly. To effectively reduce the damage
of ultraviolet radiation to aramid fabrics, the bre modication mea-
sures of metallic oxide and organic modication are described in detail.
The effect of high- and low-temperature environments on the me-
chanical and thermal properties of bres is secondary. High temperature
leads to an increase in bre microcrystalline size, which in turn forms
micropores on the bre surface and increases surface roughness. The
balance of molecular orientation between the bre skin and inner core is
disrupted, triggering bre thermal degradation. Moreover, high tem-
peratures can affect the hydrogen bonding network and bre in-
teractions, which causes negative effects on the tensile and friction
properties of the aramid bres. The ballistic resistance performance of
the fabric is weakened accordingly. Low temperatures can also cause a
decline in the tensile strength of aramid bres. However, bres can
maintain structural integrity at moderate low temperatures, and their
internal water content is appropriately reduced, resulting in an increase
in elastic modulus and pull-out resistance.
The effect of humid heat and an acid‒base environment on the
mechanical and thermal properties of the aramid bres is minimal. The
surface of the aramid bre is slightly corroded in humid, hot and acid‒
base environments. A small part of the crystal structure is damaged, and
the tensile strength of the bre is slightly decreased. Most of the internal
amide bonds and chemical structures remain in the initial state. The
molecular weight and chemical bond remain stable. The bre maintains
good structural stability and molecular orientation, and therefore, the
elastic modulus and thermal properties of the aramid bre are almost
unchanged.
Furthermore, in the worst-case scenario, aramid fabrics will be
subjected to the synergistic effects of the four environments mentioned
above. UV radiation will rst cause the amide bonds to break, and the
bre skin will crack and continuously develop towards the inner core.
Damage to the bre skin will cause temperature to quickly transmit to
the inner core of the bres. The micropores generated by the tempera-
ture will not only be limited to the surface but also inside the bres,
which greatly weakens the structural stability of the bre, allowing acid,
alkali, and water to corrode the fragile inner core of the bre. Acids and
alkalis diffuse to various layers of the bre along the horizontal and
vertical directions. The instability of the structure and the imbalance of
the molecular orientation inside and outside the bre result in a sig-
nicant decrease in the elastic modulus and tensile strength of the bre.
The weakening of the molecular weight and chemical bond energy leads
to a decrease in thermal performance. The ballistic resistance perfor-
mance of the fabric is weakened, increasing the risk of soldiers’ injury on
the battleeld. The degradation of bres caused by the four environ-
mental factors originates from the damage of ultraviolet radiation to the
bre skin. Therefore, improving the UV resistance of bres can effec-
tively protect the structural integrity of bres and minimize the corro-
sion of bres caused by the other three environmental factors.
7. Future outlook
Overall, aramid bres have good application prospects in body ar-
mour and protective construction due to their excellent mechanical
properties. This paper provides useful design ideas and maintenance
guidelines for the application of aramid body armour in different
battleeld environments. For future work, due to the complexity of the
battleeld environment, the superposition effect of a variety of envi-
ronmental factors on the mechanical and ballistic resistance perfor-
mance of the aramid fabric may become a future research hotspot.
Analysis of environmental factors on the mechanical performance of
bre by numerical simulation may become a potential direction. To
date, the numerical simulation of yarns is limited to the mesoscopic
stage, and the simulation of microscopic yarns faces problems such as
modeling difculties, small mesh sizes, and easy mesh distortion. The
numerical simulation of the effect of the environment on yarns at the
microscopic scale may become a research focus. By numerically simu-
lating the environmental adaptability of the yarn at the microscopic
scale, the microscopic failure of the yarn can be digitally described with
mechanical parameters. The action mode of environmental factors on
yarn and its deterioration mechanism can be fully revealed. Meanwhile,
the multi-scale effects of environmental factors on yarns can be effec-
tively explained from the mechanical simulation of microscopic yarns to
the ballistic simulation of macroscopic fabrics. Extensive environmental
adaptability and excellent ballistic resistance performance are the future
development directions for modied aramid fabrics. The modication of
woven aramid fabrics or the original bre before weaving will become
an effective way to achieve the improved performance. Chemical
grafting through covalent bonding and physical attachment through
adhesive bonding will become the main methods to achieve ber
modication. Realizing the stable combination of modied substances
and fabrics, controlling the weight addition of aramid fabrics, and
achieving signicant improvement in ballistic performance will be
challenges for future research.
Author statement
Yaojie Xu: Investigation, Methodology, Formal Analysis, Writing-
Original Draft; Hong Zhang: Methodology, Data Curation, Validation;
Guangyan Huang: Supervision, Project Administration, Writing - Review
& Editing.
Fig. 27. Thermogravimetric analysis (TGA) curves and corresponding HCl
content of samples [120].
Y. Xu et al.
Polymer Testing 128 (2023) 108227
24
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgement
This research was sponsored by the National Key Research and
Development Program of China [Grant number 2022YFC3320500].
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