Facile synthesis of an eco-friendly nitrogen-phosphorus ammonium salt to enhance the durability and flame retardancy of cotton

Article abstract of DOI:10.1039/c7ta01106g

An ammonium salt of ethylene glycol diphosphoric acid (AEGDP) containing nitrogen and phosphorus, a durable and eco-friendly flame retardant for cotton, was synthesized under moderate and solvent-free conditions. The AEGDP structure was characterized by IR, ¹³C NMR, ¹H NMR, and ³¹P NMR. Thermogravimetry (TG) and TG-infrared (TG-IR) spectroscopy indicate that phosphoric or polyphosphoric acid produced from AEGDP during combustion could catalyze the dehydration of cellulose to form char instead of decomposing it to form volatile flammable species, which include hydrocarbons, some carbonyl compounds, and ethers. The degradation mechanism of cotton fabrics treated with AEGDP was modified because of covalent bonds formed between cotton cellulose and AEGDP. Scanning electron microscopy (SEM) shows that no obvious materials were observed on the surface of the treated cotton, indicating that the -PO(O^-NH₄⁺)₂ groups of AEGDP may react with the -OH groups of cellulose to form P-O-C covalent bonds and/or penetrate into the inner space of cotton fibers. Vertical flammable tests imply that the control cotton burned completely, whereas the treated cotton showed neither after-flame nor after-glow. The limiting oxygen index (LOI) of cotton treated with 120 g L^-1 of AEGDP solution could reach 41.0%, which is significantly higher than that of the control cotton. After 50 laundering cycles, the LOI could be maintained at 28.4%, exhibiting the excellent durability of the treated cotton. Cone calorimetry at 35 and 50 kW m^-2 irradiative heat flux reveals that AEGDP causes a reduction in the peak heat release rate and total heat release relative to those of the control cotton. In addition, X-ray diffraction shows that AEGDP fails to substantially change the crystal structure of cotton cellulose. There is no significant change in the bending length of cotton before and after treatment. And the decreased surface tension of the treated cotton is beneficial for its self-cleaning ability.

Full text of DOI:10.1039/c7ta01106g

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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Facile synthesis of an eco-friendly nitrogen–phosphorus
ammonium salt to enhance durability and flame retardancy of
cotton
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
∗b
c
∗a
DOI: 10.1039/x0xx00000x
Yaling Jia, Yi Lu, Guangxian Zhang, Yaojia Liang and Fengxiu Zhang
www.rsc.org/
An ammonium salt of ethylene glycol diphosphoric acid (AEGDP) containing nitrogen and phosphorus, a durable and eco-
friendly flame retardant for cotton, was synthesized under moderate and solvent-free conditions. The AEGDP structure
1
3
1
31
was characterized by IR, C NMR, H NMR, and P NMR. Thermogravimetry (TG) and TG–infrared (TG–IR) spectroscopy
indicate that phosphorus or polyphosphoric acid produced from AEGDP during combustion could catalyze the dehydration
of cellulose to form char instead of decomposing to form volatile flammable species, which include hydrocarbons, some
carbonyl compounds, and ethers. The degradation mechanism of cotton fabrics treated with AEGDP was modified because
of covalent bonds formed between cotton cellulose and AEGDP. Scanning electron microscopy (SEM) shows that no
−
+
4 2
obvious materials were observed on the surface of the treated cotton, indicating that the -P=O(O NH ) groups of AEGDP
may react with the –OH groups of cellulose to form P–O–C covalent bonds and/or penetrate into the inner space of cotton
fibers. Vertical flammable tests imply that the control cotton burned completely, whereas the treated cotton showed
neither after-flame nor after-glow. The limiting oxygen index (LOI) of cotton treated with 120 g/L of AEGDP solution could
reach 41.0%, which is significantly higher than that of the control cotton. After 50 laundering cycles, the LOI could be
2
maintained at 28.4%, exhibiting the excellent durability of the treated cotton. Cone calorimetry at 35 and 50 kW/m
irradiative heat flux reveals that AEGDP causes a reduction in peak heat release rate and total heat release relative to
those of the control cotton. In addition, X-ray diffraction shows that AEGDP fails to substantially change the crystal
structure of cotton cellulose. There is no significant change in bending length of cotton before and after treatment. And
the decreased surface tension of treated cotton is beneficial for its self-cleaning ability.
3
efficient and widely used fire retardants (FRs) for cotton.
Introduction
However, they can generate toxic gases that could be
endocrine disruptors and environment pollutants. Studies have
Cotton fabrics comprise a large share in the textile market and
are widely used for apparel, home furnishings, and cushion
covers, among others, because of the properties of cotton,
such as comfort and breathability. However, the high
flammability of cotton presents a significant threat to humans
also found that these compounds accumulate and persist in
organisms and the environment. Thus, their application is
banned in European countries. As substitutes for halogen-
containing FRs, phosphorus-based FRs—tetrakis (hydroxyl-
methyl) phosphonium salt derivatives and N-methylol
dimethylphosphonopropionamide derivatives—which are
1
and their possessions. A number of serious burning injuries
associated with cotton fabrics occur annually worldwide.
Conferring flame retardancy to cotton fabrics can reduce these
occurrences and contribute to saving lives and properties. By
delaying the burning processes, there is more time for people
to escape from fire places and for firefighters to extinguish
®
®
typified by Proban and Pyrovatex , were developed and are
4
still currently used. Nevertheless, these phosphorus-based
FRs containing hydroxymethyl can release formaldehyde,
which is carcinogenic to humans, in FR finishing and service
process. Durability is obtained by covalently linking FRs and
2
fires before significant damage is sustained.
6
cellulose C hydroxyl groups.
Halogen-based compounds such as polybrominated
diphenyl ethers and polybrominated biphenyls are the most
With the continuous search for novel, efficient, and
environmentally sustainable FRs, bio-macromolecules,
particularly DNA and proteins, are considered efficient
alternatives to traditional approaches. By direct comparison
with the aforementioned intumescent system, DNA contains
the 3 components of an intumescent material in a single
molecule: the phosphate groups, which could produce
phosphoric acid; the deoxyribose units, which act as carbon
sources and blowing agents; and the nitrogen-containing bases
a
College of Chemistry and Chemical Engineering, Southwest University, No.2
Tiansheng Street, Chongqing 400715, People’s Republic of China. E-mail:
zhangfx656472@sina.com.cn; Tel.: 86-02368251681, +86-15825921682
College of Textile and Garment, Southwest University, No. 2 Tiansheng Street,
b
Chongqing 400715, People’s Republic of China. E-mail: zgx656472@sina.com;
Tel.: +86-02368251228
High School Affiliated with Southwest University, No. 2 Tiansheng Street,
c
Chongqing 400715, People’s Republic of China.
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5–7
A, G, C, and T), which may release ammonia. Moreover, phonic acid and ammonium salt of hexamethylenediamine–
(
1
6, 17
DNA coupled with chitosan is employed via layer-by-layer (LbL) N,N,N',N'–tetra(methylphosphonic acid).
assembly to build green coatings on cotton and thus confer
efficient FR properties to cotton. The reason is that DNA could
Experimental
release phosphoric and polyphosphoric acid, promoting the
char formation of chitosan. The obtained thermally stable
Materials
chars protect the substrates from heat and oxygen, thereby
8
inhibiting the continuous combustion of cotton. In addition,
2
Scoured and bleached pure cotton fabrics (133.27 g/m ) were
purchased from Chaotianmen Market in Chongqing, China.
Ethylene glycol and urea were supplied by Chengdu Kelong
Chemical Reagent Co., Ltd. (Chengdu, China). Aqueous
ammonia and phosphoric acid (85%) were purchased from
Chongqing Chuandong Chemical Co., Ltd. (Chongqing, China).
Dicyandiamide was supplied by Aladdin Reagent Co. Ltd.
some phosphorus- and sulfur-rich proteins, such as caseins,
9
10
hydrophobins, and whey proteins derived from animal or
microbial sources, have been investigated as novel green FRs
by deposition on the surface of cotton fabrics. They can favor
the dehydration of cellulose to enhance the production of a
thermally stable carbonaceous structure (char), significantly
enhancing the flame retardancy of cotton fabrics. However, a
challenging issue regarding the durability of biomacro-
molecules-based coatings on cotton remains unsolved.
(
Shanghai, China). All chemicals were of reagent grade before
use without further purification.
Several innovative and environmentally friendly FR finishing
solutions have been adopted by researchers; these techniques
Synthesis and characterization of AEGDP
Ethylene glycol (
3.05 g, 0.200 mol) were added into a 150 mL beaker. The
mixture was maintained at 130 °C for 2 h under magnetic
stirring. Urea ( , 12.012 g, 0.200 mol) was then added into the
beaker under magnetic stirring, and the mixture was heated to
05 °C for 1 h. The crude product AEGDP was subsequently
1, 6.207 g, 0.100 mol) and phosphoric acid (2,
1
1
12
include nanoparticle adsorption, LbL assembly, and sol–gel
processes. Flame-retardant nanocomposite coatings, including
polyurethane (PU)/montmorillonite (MMT), PU/polyhedral
2
4
1
3
14
oligomeric silsesquioxane, and carbon nanotubes, have
been applied onto cotton fabrics. These PU coatings exhibit
good flame retardancy for cotton; however, their synthesis
involves complexity. Meanwhile, the high cost and the reduced
whiteness of carbon nanotubes likely limit their
commercialization. LbL assembly may be considered an
evolution of nanoparticle adsorption. The most common
method used for LbL assembly consists of alternately dipping a
substrate into positively and negatively charged subtrates,
diluting (< 1 wt % solids) aqueous solutions/mixtures of
polymers or particles, and building up multiple
positive/negative pairs of layers (also known as bilayers). This
approach takes advantage of the electrostatic attraction
between layers to hold the film assembly together. Cotton 13
fabric was treated with FR coatings composed of branched
1
obtained. The product was purified by precipitation using
ethanol, filtered, and dried in an oven at 100 °C. A white solid
was obtained with 98% yield. The synthesis route of AEGDP is
presented in Scheme 1. The characterizations of AEGDP were
also analyzed in the following:
-1
FT–IR (KBr) (cm ): 3193 and 3029 (vs, N–H, O–H), 2857 (vs, C-
H), 1338 (vs, P=O), 1093(vs, P–O–C), and 938 (vs, P–OH).
1
H NMR (D
2
O, 600 MHz) δ (ppm): 3.96 (s, 2CH
+
2
, H
1 2
, H ). No
signal peaks of the –OH groups and –NH
4
ions were detected
because of the replacement of 2 types of reactive hydrogen by
deuterium.
C NMR (D
2
O, 600 MHz) δ (ppm): 64.85 (1C, C
1
), 67.36 (1C,
C
2
).
polyethylenimine (BPEI) and sodium MMT clay prepared via 31
P NMR (D
2
O) δ (ppm): 18.81.
LbL assembly. In the present study, MMT was deposited with
BPEI to generate nanocomposite assemblies on cotton fibers.
The thickness and weight composition of these films were
modified by changing the pH of the polymer solution and the
1
5
concentration of the clay mixture.
In the current study, AEGDP as an efficient and
environmentally friendly FR for cotton was synthesized under
moderate and solvent-free conditions. It covers the 3 main
requirements, including excellent flame retardancy, durability,
and nontoxicity to the environment and humans. The LOI of
the cotton fabric treated with 120 g/L of AEGDP could reach
Scheme 1 Synthesis of AEGDP
4
1.0%. No sustained combustion was observed in the vertical
®
®
flammability tests. Compared with Proban and Pyrovatex ,
AEGDP contains no halogen and would not release
formaldehyde. AEGDP also addressed the durability issue for
biomacromolecules. Moreover, no formaldehyde was used in
the synthesis of AEGDP, compared with the synthesis of the
ammonium salt of ethylenediamine tetramethylenephos-
Scheme 2 Grafting reaction between AEGDP and cotton cellulose
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Impregnation of cotton fabrics with AEGDP solutions
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wide-angle diffractometer (Beijing Purkinje General Instru-
ment Co. Ltd., Beijing, China) at 36 kV and 20 mA. The
diffraction angle ranged from 5° to 50° with a step size of 0.02°
AEGDP solutions (60, 90, and 120 g/L) were prepared by
dissolving AEGDP in deionized water. Dicyandiamide as a
catalyst was also dissolved in the solution. Cotton fabrics were
impregnated with AEGDP solutions for 1 h at 70 °C, with a bath
ratio of 1:20 (the bath ratio is the cotton weight versus the
volume of the solution). Subsequently, the cotton fabric after
soaking was padded through a nip to obtain a wet pickup of
about 100%. The cotton fabrics were then cured using an
automatic continuous baking machine at 190 °C for 5 min,
following Scheme 2. The cotton fabrics were finally rinsed with
running water and dried in an oven at 60 °C and the procedure
(λ = 0.154 nm).
The chemical structures of AEGDP, original, and AEGDP-
treated cotton fibers were evaluated by attenuated total
reflectance (ATR)–FTIR spectroscopy. ATR–FTIR spectra in the
-
1
-1
000–500 cm range with a resolution of 4 cm were recorded
4
at room temperature by using a Nicolet is50 FT–IR/ATR
spectrophotometer (Thermo Scientific Co., Germany)
equipped with a diamond crystal.
The surface tension of the AEGDP-treated cotton fabric was
calculated indirectly by measuring the water contact angle on
an OCA 15EC instrument (Dataphysics, Germany). The contact
angle measurements were obtained when a 3.0 uL distilled
water drop tended to be stationary on the surface of the fabric
for nearly 1 min. Each sample was measured at 5 different
sites, and their average values were recorded.
was repeated once
.
The weight gain of the cotton fabric samples (WG, wt%) was
determined by weighing each sample before (W ) and after FR
i
finishing (W ). The weight gain of the cotton fabric samples were
f
calculated according to the following equation (1):
WG (%) = [ (W − W )/ W ] × 100
(1)
f
i
i
The stiffness of the fabric samples was measured using an
LLY-01-type computer-controlled stiffness tester (Shaoxing
Yuanmore electromechanical equipment Co. Ltd, Zhejiang,
China) in accordance with the ASTM D 1388-96 (2002)
standard.
Characterization Techniques
Vertical flammability tests were conducted using a YG815B
vertical fabric FR tester (Nantong Sansi Electromechanical
Science & Technology Co. Ltd., China) in accordance with the
ASTM D6413-99 standard. LOI measurement and vertical flame
test were conducted to evaluate whether the flame retardancy
Results and discussion
of the treated cotton fabric was enhanced. LOI values were Flammability
measured in accordance with the ASTM D2863-2000 standard
test method on an M606B digital oxygen index apparatus The flammable behaviors of the control cotton and the cotton
(
Qingdao Shanfang Instrument Co., Ltd., Shandong, China). The fabric treated with 120 g/L of AEGDP solution were
washing resistance of the AEGDP-treated cotton fabrics was investigated by cone calorimetry under irradiative flux of 35
2
evaluated using a soaping fastness tester in accordance with and 50 kW/m . Fig. 1 presents the curves of HRR and THR at 2
levels of heat flux versus time, and the involved main
the AATCC 61-2006 standard (Roaches Co., England).
The surface morphologies of the untreated and the AEGDP- parameters are listed in Table 1. At 35 kW/m , the control
treated cotton fabrics were observed using a Hitachi S-4800 cotton fabric burned rapidly; PHRR and THR were 195.1 kW/m
2
2
2
and 2.804 MJ/m , respectively. After AEGDP was grafted on
scanning electron microscope (imaging beam voltage: 20 kV).
The thermal stabilities of the cotton fabric samples were the cotton fabric, PHRR and THR decreased significantly to
evaluated by TG analysis, using a Pyris 1 thermogravimetric 7.366 kW/m and 0.601 MJ/m , respectively, which were
2
2
analyzer (PerkinElmer, USA) from 40 °C to 800 °C with a reduced by 96.2% and 78.6%, respectively, compared with
2
those of the control cotton fabric. At 50 kW/m , the HRR
heating rate of 20 K/min under nitrogen and air.
TG–IR was conducted on a Netzsch STA 409PC thermal curves of the treated cotton and the control cotton show a
2
analyzer coupled with a Bruker Tensor 27 FTIR spectrometer similar trend with those at 35 kW/m . The PHRR value of the
via a polytetrafluoroethylene pipe. Samples (8.0 mg) were control cotton is also significantly higher than that of the
placed under a nitrogen gas flow of 30 mL/min at a heating treated cotton. These findings indicate that the treated cotton
rate of 20 K/min.
Cone calorimetry was employed to investigate the and enhanced fire resistance at 35 kW/m than at 50 kW/m .
combustion behavior of square cotton samples (100 mm
100 However, the THR curves of the control cotton, treated cotton,
mm) in a horizontal configuration in accordance with the and treated cotton after 50 laundering cycles (Lcs) at 50
under both irradiative fluxes exhibit excellent flame retardancy
2
2 18
×
2
2
2
ASTM E 1354 standard under 35 kW/m and ISO 5660 under 50 kW/m present a different trend from those at 35 kW/m . The
2
kW/m of irradiative heat flux by using cone calorimeters (FTT THR of the treated cotton is weaker than that of the control
0
007, UK) and Govmark (CC-2-2128, US). The following cotton until 440 s; however, combustion continues, and THR
2
parameters were recorded: time to sustained ignition (TSI, s), finally reaches 6.1 MJ/m . The THR trend of the treated cotton
2
peak of heat release rate (pkHRR, kW/m ), total heat release after 50 Lcs is similar to that of the treated cotton but reached
2
(
THR, MJ/m ), ratio of carbon dioxide to carbon monoxide the THR of the control cotton early (295.2 s). The final THR (4.5
2
(
CO
The X-ray diffraction spectra of the control cotton fabric and lower than that of the treated cotton after 50 Lcs. To some
the treated cotton fabric were obtained with a Rigaku XD-3 degree, it is thought that insulating barrier of carbonaceous
2
/CO, kg/kg), and final residue (%).
MJ/m ) of the control cotton at the end of combustion was
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2
Fig. 1 (a) Heat release rate (HRR) and (b) total heat release (THR) as functions of time at 35 kW/m ; (c) HRR and (d) THR curves at 50
2
kW/m .
2
Table 1 Collected data of the untreated cotton and the treated cotton fabrics by cone calorimetry at 35 and 50 kW/m
Radiation
t
ig
PHRR
Time to PHRR
(s)
FGR
THR
CO
2
/CO
Residue
(%)
Sample
2
2
2
(s)
(kW/m )
(kW/(m s))
(MJ/m )
(kg/kg)
flux
Untreated sample
Treated sample
29
-
195.1
7.366
142.8
16.1
22
28
8.868
0.263
7.140
0.152
2.804
0.601
4.2
86.18
25.44
78.01
5.36
1.31
24.9
0.02
0.09
3
5
2
kW/m
Untreated sample
Treated sample
5
-
20
5
0
106
6.1
2
kW/m
Treated sample after
4
23.4
38
0.616
4.5
2.81
0.08
5
0 Lcs
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Fig. 2. Digital photographs of the residues of (a, b) the control cotton and (c, d) the AEGDP-treated cotton. Photographs (a, c) are taken in a
vertical orientation, whereas photographs (b, d) are taken in a the horizontal orientation.
structure could be broken at high level of heat flux of 50
2
kW/m .
19
Thermal and thermo-oxidative stability
On the basis of the HRR results, the fire growth rate (FGR)
was determined to assess the fire hazard of the composite by
The thermal and thermo-oxidative stabilities of AEGDP, the
untreated sample, and the cotton sample treated with 120 g/L of
AEGDP solution were evaluated by TG and DTG in nitrogen and air,
respectively. Their TG and DTG curves are presented in Fig. 3. The
thermal degradation of cotton in nitrogen generally proceeds in one
step, which includes 2 alternative pathways: decomposition of the
glycosyl units to char at reduced temperatures and
depolymerization of such units to volatile products containing
2
0, 21
the following equation:
FGR = PHRR/tPHRR
where tPHRR is the time to reach PHRR. Generally, a lower FGR
2
2
2
indicates that the time to flashover is delayed. At 35 kW/m ,
2
for the AEGDP-treated cotton, the FGR was 0.263 kW/(m s),
which was significantly decreased by 97% relative to that of
2
the control cotton. At 50 kW/m , the FGR values of the treated
5
levoglucosan at increased temperatures. As shown in Figs. 3a and
cotton and the treated cotton after 50 Lcs were 0.152 and
2
.616 kW/(m s), respectively, which were also markedly lower
3
c, AEGDP undergoes intense decomposition at temperatures
0
o
ranging from 160.6 to 244.7 C, with a residue of 45.15%. The
untreated cotton is promptly decomposed at temperatures ranging
2
than that of the control cotton at 7.14 kW/(m s).
Fig. 2 shows the digital photographs of residues for the
control cotton and the AEGDP-treated cotton after the cone
calorimetry test. The control cotton left a small quantity of
residues. The area of char formed from the treated cotton in
Figs. 2a and 2c was enlarged compared with that of the control
cotton in Figs. 2b and 2d. The intumescent char layer could
generally delay heat and mass transfer between gas and
condensed phases, as well as protect underlying materials
from further burning, thereby achieving enhanced flame
retardancy.
from 313.8 to 394.8 °C, with
a residue of 14.92% after
decomposition. Meanwhile, severe decomposition of the AEGDP-
treated cotton occurs at significantly lower temperatures ranging
from 252.9 to 296.8 °C, with 42.62% more residue than that of the
untreated cotton. By comparison, the marked decrease in
decomposition temperature and increase in residue demonstrate
24
the excellent flame retardancy of the treated cotton. These
effects may be attributed to the relatively higher thermal stability
of small-molecule AEGDP and phosphorus or/and polyphosphoric
acid formed during decomposition, which catalyzes the dehydration
The ratio of CO
2
and CO production can be considered as an
25
of cotton cellulose, causing a lower decomposition temperature
indicator of combustion efficiency. In fact, a high ratio suggests
efficient combustion, whereas a low ratio indicates inefficient
and more residue of the AEGDP-treated cotton.
Thermo-oxidative stability was also evaluated , as shown in Figs.
23
2
and hindered combustion. As listed in Table 1, the CO /CO
3
b and 3d. The pyrolysis of cotton in air is generally characterized
ratio for the AEGDP-treated cotton is considerably lower than
that of the control cotton whether at the irradiative flux of 35
by 3 stages. Stage I (300 °C to 400 °C) involves 2 competing
pathways: dehydration of cellulose to form aliphatic char and
decomposition reaction to produce volatiles. In stage II (400 °C to
2
2
kW/m or at 50 kW/m . These results verify the efficiency of
AEGDP in hindering combustion and favoring the formation of
char. Thus, the amount of residue of the AEGDP-treated cotton
was markedly increased (see residue values in Table 1)
.
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Fig. 3. TG and DTG curves of the untreated cotton and the treated cotton in nitrogen (a, c) and air (b, d).
Table 2 Crystal structure-related parameters of the cotton samples
Crystalline Dimension (nm)
Cotton sample
Crystalline Index (CrI)
Degree of crystallinity (%)
1
01
101’
6.79
6.67
002
5.33
5.57
040
4.97
4.81
Control cotton
Treated cotton
77.3
70.9
61
56
4.03
4.11
8
00 °C), some aliphatic chars in stage I are converted to an aromatic cotton had a residue of 14.92% at 600 °C in nitrogen. This
form, and both chars are partly further oxidized into CO and CO . In result could be attributed to the further oxidation of unstable
stage III (above 800 °C), the remaining chars and any remaining char in the presence of O . In addition, from Figs. 3a and 3b,
2
2
2
6
hydrocarbon species are mainly oxidized to CO and CO. As shown under both nitrogen and air atmosphere, a slight weight loss of
2
in Figs. 3b and 3d, AEGDP undergoes severe decomposition at 2 cotton samples before and after treatment occurs within the
o
temperature ranges: 161–272 °C and 602–739 °C, with a final 40–100 C temperature range, which corresponds to the loss
residue of 11.98%. The untreated cotton rapidly decomposed of water due to dehydration between hydroxyl groups,
within the 299.5–364.9 °C temperature range without residues at resulting from the broken hydrogen bonds in cellulose
2
7
5
00 °C. Meanwhile, the AEGDP-treated cotton rapidly decomposed molecules.
within the 237.9–289.1 °C temperature range, with a residue of
2
7.94% at 600 °C.
TG–IR analysis of volatile pyrolysis products
As observed, the initial decomposition temperature of the
cotton fabrics in air was lower than that in nitrogen On the basis of the aforementioned TG and cone calorimetry test
atmosphere. In air, the untreated cotton completely results, AEGDP is found to catalyze the dehydration of cellulose to
decomposed with no residues at 500 °C, whereas the treated form char. Char prevents heat and unburned cellulose from flame,
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DOI: 10.1039/C7TA01106G
ARTICLE
Fig. 4 a TG–IR spectra of the total pyrolysis products of the control cotton and the treated cotton during the entire pyrolysis
process; Intensity of characteristic peaks for the specific pyrolysis products of the control cotton and the treated cotton CO
O, hydrocarbons, carbonyl compounds, and ethers.
b
2
, c
H
2
d
e
f
thus reducing the decomposition temperature, PHRR, and THR in
Fig. 4a shows the TG–IR spectra of the pyrolysis products of
thermal pyrolysis. TG–IR analysis is conducted to evaluate the the control cotton and those of the cotton treated with 120 g/L of
amount of volatile species and to compare the toxic levels of the AEGDP solution during the entire pyrolysis process. AEGDP does not
products released from the AEGDP-treated cotton during significantly change the position of the peaks and no new peaks
combustion with those of the control cotton.
appeared, indicating the similar pyrolysis products of the control
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cotton and the AEGDP-treated cotton. This finding suggests that no Notably, the diffraction pattern of the treated cotton fibers is
more toxic species were produced. Some pyrolysis products are similar to that of the control cotton fibers, suggesting that AEGDP
unambiguously identified by characteristic TG–IR signals: the peak does not significantly affect the crystal structure of cellulose fibers.
-1
at 3559 cm is attributed to the vibration absorption of the -OH The intensities of the diffraction peaks of the AEGDP-treated cotton
2
8
-1
o
group derived from water vapor; the peak at 2817 cm is assigned fibers at 15.21°, 16.76°, 22.68°, and 34.28 are marginally weakened
2
8
relative to those of the control cotton fibers. Furthermore, the
degree of crystallinity, crystalline index and crystalline size of cotton
cellulose before and after treatment were also obtained according
to the aliphatic C-H bond derived from diverse alkanes; the peak
-1
29
at 2356 cm is attributed to the absorbance of CO
2
;
the intense
-1
peak at 1742 cm is attributed to the absorbance of the stretching
3
4
3
0
-1
to the following equations and the results are listed in Table 2.
vibration of the C=O group; the adsorption peak at 1111 cm is
ꢀ
ꢁꢂꢃꢄꢀꢅꢆ
ꢇ 100 (2)
ꢀꢅꢆ
3
1
CrI =
caused by the stretching vibration of the C-O-C bond from ethers.
Figs. 4b–4f present the absorption intensity versus time of
selected pyrolysis products: CO , H O, hydrocarbons, carbonyl noncrystal when 2
where CrI is the crystalline index; Iam is the diffraction intensity of
o
θ
is 18 for cellulose-I.
2
2
ꢈ
ꢉ
35, 36
compounds, and ethers. The pyrolysis products were
investigated by dividing them into 2 categories: one is the
nonflammable species, such as H O and CO ; the other is the
Cr(%)=
ꢇ 100% (3)
ꢊꢋꢌꢈ
ꢆ
ꢈ
o m
where Cr represents the degree of crystallinity; S and S are the
2
2
crystallization area and amorphous region area, respectively.
flammable species, such as hydrocarbons, carbonyl
ꢍλ
37
L
hkl
=
(4)
3
2
β ꢎꢏꢐθ
compounds, and ethers. In Figs. 4b–4f, the time of maximum
absorbance intensities for all pyrolysis products of the AEGDP-
treated cotton was reached earlier compared with that of the
control cotton. This result indicates that the decomposition
temperature for the AEGDP-treated cotton is reduced, which
agrees with the TG results. In addition, the maximum
absorption intensities for the AEGDP-treated cotton are
considerably lower than those of the control cotton. These
results indicate that the quantities of the releasing flammable
volatile species are significantly reduced, causing decrease in
PHRR and THR.
where Lhkl denotes the crystalline size; K is the Scherrer constant
equal to 0.9, and is the line width at half-maximum height and is
β
used in the form of a radian.
Table 2 shows that compared with the control cotton, CrI and Cr
slightly declined because of the –OH group in cellulose being
substituted by grafting of AEGDP. The decrease of the –OH group
reduces the density of hydrogen bonds and thus extend the
3
3
amorphous region , leading to reduced CrI and Cr for the treated
cotton cellulose. No remarkable change is observed in the Lhkl of the
cotton cellulose before and after flame retardant finishing, which
indicates that AEGDP does not significantly affect the crystalline size
of the crystalline region for cotton cellulose.
XRD analysis
The crystal structures of the control cotton and the cotton treated
with 120 g/L of AEGDP solution were investigated by XRD analysis.
Fig. 5 shows the XRD spectra of the control cotton and the AEGDP-
Flame retardant behavior of the cotton fabrics
Vertical flammability tests were employed to evaluate the FR
efficiency of the control cotton fabric and the cotton fabrics
treated with 60, 90, and 120 g/L of AEGDP solutions. Fig. 6
presents these cotton fabric samples after exposure to direct
flame for 12 s. Data relating to the flammability test and
weight gain of the treated cotton samples are presented in
Table 2. The untreated cotton fabric vigorously burned and
was completely consumed during the test, with 12 s of after-
flame time and 10 s of afterglow time; meanwhile, all treated
samples completely stopped the flame from propagating
farther almost as soon as the flame was removed. The weight
gain values of the cotton fabrics treated with 60, 90, and 120
g/L of AEGDP solutions were 16.1%, 20.1%, and 23.7%,
respectively. The cotton fabric treated with 120 g/L of AEGDP
solution after 50 laundering cycles left a char of 83 mm after
the vertical flammability test.
treated cotton. Both samples show diffraction peaks at 15.21° and
o
1
6.76° for the (101) plane, 22.68° for the (002) plane, and 34.28 for
33
(
040) plane, which are typical of the cellulose-I crystalline form.
Flame resistance efficiency and durability
The flame resistance efficiency of the treated samples after
several laundering cycles (Lcs) was assessed using the LOI
values listed in Table 3. The LOI (35.1%) of the cotton treated
with 60 g/L of AEGDP solution is significantly higher than that
3
8
of the control cotton (18.4%). The LOI of the cotton treated
with 90 g/L of AEGDP solution was reduced from 37.8% to
Fig. 5 XRD analysis for the control and the AEGDP-treated cotton.
2
8.1%
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA01106G
Fig. 6 Vertical flammability test results of the control cotton fabric a and the cotton fabrics treated with b 60, c 90, d 120 g/L of AEGDP
solutions and e after 50 Lcs treated with 120 g/L of AEGDP solution.
Table 2 Vertical flammability test data for the control cotton fabric and the AEGDP-treated cotton fabrics.
Samples
Weight gain (%)
After-flame time (s) After-glow time (s)
Char length (mm)
Control sample
0
12
0
10
0
0
6
9
0 g/L
0 g/L
16.1
20.1
23.7
14.8
63
52
43
83
0
0
1
20 g/L
0
0
1
20 g/L after 50 Lcs
0
0
Table 3 LOI values of cotton fabrics treated with different concentrations of AEGDP solution and after different Lcs
LOI values (%)
Concentrations of AEGDP (g/L)
0
Lcs
10 Lcs
32.6
20 Lcs
29.0
30 Lcs
25.7
40 Lcs
23.7
50 Lcs
20.9
6
9
0
0
35.1
37.8
41.0
32.8
30.5
28.1
27.2
25.9
1
20
36.2
35.0
32.4
30.0
28.4
after 30 laundering cycles. Thus, the cotton treated with 90 g/L
of AEGDP solution could be used as a semi-durable FR cotton
Surface morphology
fabric. The LOI of the cotton treated with 120 g/L of AEGDP The surface morphology of the control cotton and the cotton
solution decreased from 41.0% to 28.4% after 50 laundering treated with 120 g/L AEGDP solution were observed by SEM.
cycles. Thus, the cotton treated with 120 g/L of AEGDP Fig. 7 shows the SEM images of the control cotton and the
solution could be used as durable FR cotton fabric.
AEGDP-treated cotton. As shown in Fig. 7a and 7b, the control
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Fig. 7 SEM photographs of the control cotton (a, b), the AEGDP-treated cotton (c, d), and the AEGDP-treated cotton after burning (e, f);
Scale bars are 20 μm in a, c, and e and 10 μm in b, d, and f.
cotton fibers exhibit irregular shapes with rough surfaces and
curly edges. As for the AEGDP-treated cotton (in Figs. 7c and
ATR spectra
7d) no significant change in morphology is observed relative to
To verify the bonds formed between AEGDP and cotton
cellulose, the chemical structures of the control and the cotton
fibers treated with 120 g/L AEGDP solution were investigated
by ATR spectra. Fig. 8 shows the ATR spectra of the cotton
samples. The spectrum of untreated cotton exhibits some
characteristic signals of cellulose. It contains the signal peak at
that of the control cotton. No coating layer is observed on the
surface of the AEGDP-treated cotton, indicating that AEGDP
may penetrate the inner space of the cotton instead of being
deposited on its surface. Figs. 7e and 7f present the SEM
images of the AEGDP-treated cotton after burning. The
residual carbonized frame (char) remains the original
morphology of the control cotton. Many protuberances appear
on the surface of the AEGDP-treated cotton after burning.
Phosphorous and nitrogen in AEGDP is attributed to catalyzing
-1
3
332 cm assigned to O-H absorption and C–H absorption
-1 40
peaks at 1315 cm . Compared with these characteristic
signal intensities of the control cotton, they were slightly
reduced probably because of the substitution of the –OH
group in cellulose by AEGDP and the formation of a new bond
P-O-C between cellulose and AEGDP. Beyond that, new signals
appear in the spectrum of the treated cotton. Strong peaks at
3
9
dehydration to form char.
-
1
1
235 and 848 cm are attributed to P=O and P-O-H vibration
41
absorption, respectively. Meanwhile, the absorption at 1050
42
cm is due to the P-O-C bonds , which may be an indication of
the formed
-1
bond between cellulose and AEGDP. In addition, a weak
-1
absorption signal peak at 1702 cm is attributed to the C=O
bond, which probably results from the subtle oxidation of
1
7
cellulose.
Mechanical and physical property
The surface tension data of the treated samples were
calculated based on their water contact angles according to
4
3
Equation (5):
ꢑꢕꢓ
Fig. 8 ATR spectra of the control cotton and the AEGDP-treated
cotton
ꢖ
ꢡ
ꢑꢒꢓ
ꢔ
ꢗꢘꢙ ꢚ ꢛꢒꢜꢝ ꢞꢟ ꢋꢚ ꢠꢡꢒ ꢞꢢ ꢋꢋꢋꢋꢋꢋꢣ ꢤ ꢥ ꢦ ꢙꢧꢣ ꢋꢋꢋꢋꢋꢛꢨꢟ
ꢖ
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Table 4 Contact angle and surface tension data of the control cotton fabric and the cotton fabrics treated with different concentrations of
AEGDP
Concentrations of AEGDP (g/L)
Physical property
Contact angel
Control cotton
6
0
90
120
-
106.8
39.87
114.9
33.82
127.3
24.45
2
Surface tension(mN/m )
200
Table 5 Stiffness test data of the control cotton fabric and the treated cotton fabrics before and after 50 Lcs
Concentrations of AEGDP solution (g/L)
Control cotton fabric
6
0
90
120
120 after 50 Lcs
44.3
Bending Length(mm)
34.2
43.1
45.2
46.9
4
1.0 % and remains at 28.4% after 50 laundering cycles. No
As shown in Table 4, compared with the original cotton, the ignition was observed in vertical flammable tests for all of the
surface tension of the treated samples exhibits an decreasing AEGDP-treated cotton fabrics. Cone calorimetry reveals that
2
trend. This behaviour is attributed to the substitution of some – the PHRR and THR values at both 35 and 50 kW/m heat flux
OH groups of cellulose in the reaction between AEGDP and present a significant reduction compared with those of the
3
3
cellulose , which is verified by the decrease in –OH absorption control cotton. TG results indicate that the AEGDP-treated
intensity of the treated cotton in the ATR FT-IR spectra. The cotton decomposed at
lower temperature and left
decrease in the –OH group as hydrophilic group and the considerably more residue compared with those of the control
a
a
increase in AEGDP with the hydrophobic group lead to the cotton. The TG–IR spectra of the control cotton and the
increase in surface tension for the treated cotton after AEGDP-treated cotton verified that AEGDP could catalyze
finishing. Overall, AEGDP enhanced the hydrophobicity of the cotton dehydration to form char instead of volatile flammable
treated cotton, which contributed to the self-cleaning property species, thus inhibiting flame spread. XRD spectra demonstrate
4
4
of the treated cotton. Meanwhile, this flame retardant that the crystal structure of the AEGDP-treated cotton only
finishing exerts no significant effect on the serviceability of slightly changed, compared with that of the control cotton.
cotton fabrics.
The surface tension data indicates that the self-cleaning
The stiffness of the cotton samples is related to their
“hand.” property of the treated cotton was improved. The treated
To compare the stiffness of the cotton samples before and cotton remains soft without influencing its serviceability.
after flame retardant finishing, the stiffness test was conducted
by measuring their bending lengths. The specific bending lengths of
the control cotton and the treated cotton fabric before and after 50 Acknowledgements
Lcs are presented in Table 5. The bending length of the treated
This study was supported by Chongqing Postgraduate
samples increased steadily with increasing concentration of the
Education, the Teaching Reform Major Project in China (No:
AEGDP solution. Nevertheless, even for cotton fabric treated with
yjg152022).
the maximum concentration of the AEGDP solution in this study,
the bending length increased by 37.1% compared with that of the
control cotton. After 50 Lcs, its stiffness slightly recovers, showing a Notes and references
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finishing remains soft.
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4
5x21mm (300 x 300 DPI)