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j.clay.2018.07.024

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Applied Clay Science 163 (2018) 196–203
Contents lists available at ScienceDirect
Applied Clay Science
journal homepage: www.elsevier.com/locate/clay
Research paper
Effect of P3O105− intercalated hydrotalcite on the flame retardant properties
and the degradation mechanism of a novel polypropylene/hydrotalcite
system
⁎
T
⁎
Sheng Xu , Si-Yu Li, Min Zhang, Hong-Yan Zeng , Jin-Ze Du, Chao-Rong Chen
College of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan, China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Polyphosphate hydrotalcite
Polypropylene
Composites
Flame retardant
P3O105− intercalated Mg/Al hydrotalcite (LDH-P) was successfully prepared by impregnation-reconstruction,
and its microstructure and surface chemical properties were characterized. It was found that P3O105− anion has
completely intercalated into the interlayer space of the LDH-P. And the LDH-P exhibited a higher dispersity and
weaker hydrophobicity than the CO32– intercalated Mg/Al hydrotalcite (LDH-C). The LDH-P was investigated as
a potential flame retardant for polypropylene (PP) matrix, then the LDH-P and PP composite (PP/LDH-P) was
characterized using X-ray diffraction (XRD) and thermogravimetric analysis (TGA) as well as the limited oxygen
index (LOI), vertical burning UL94 and mechanical test, the results revealed that the introduction of LDH-P into
PP not only increased the char residue, but also formed compact and folded morphology of char residue providing more effective protection for underlying materials against heat and oxygen compare with LDH-C. The
morphological structures and component observed by digital photos, scanning electron micrograph (SEM) and
Fourier transform infrared (FT-IR) of fire residues demonstrated that P-C vibration between LDH-P and PP was
generated by intercalating LDH-P with P3O105− anion. Compared with the LDH-C, the LDH-P promoted the
formation of a more continuous and compact char layer during the burning process. Thus, the LDH-P intercalating with P3O105− anion enhanced the flame retardancy of PP matrix. Promising developments for use of
LDH-P in flame retardant formulations were expected in future applications.
1. Introduction
Polypropylene (PP) is widely demanded in many commodity as well
as industrial applications, due to its low density, excellent process
ability, mechanical properties, good dimensional stability and impact
strength, low cost, etc. (Yin et al., 2016; Beuguel et al., 2017; Zhang
et al., 2017). Nevertheless, its application has been limited because of
the inflammable nature of PP (Ruan et al., 2014; Wang et al., 2017).
Hence, numerous investigations have been conducted in the field for
developing flame retardants for PP.
Generally, flame retardant include halogenated flame retardants
and halogen-free flame retardants. As we know, owning to their persistence in the environment, bioaccumulation and toxicity, halogenated
flame retardants are under scrutiny in many countries. (Liu et al.,
2014a, 2014b; Venier et al., 2015). Driven by the urgent need of the
human health and environmental protection, the development of halogen-free flame retardants have been regarded as green chemistry and
sustainable development since 1970s (Liu et al., 2014a, 2014b; Abbasi
⁎
et al., 2015). Up to now, environmental-friendly halogen-free flame
retardants such as clays, Al(OH)3 and Mg(OH)2 with high glass transition temperature and good mechanical properties, have been extensively used as fire retardant additives for polymers matrix (Camino
et al., 2001; Qiu et al., 2003; Lin et al., 2009; Gao et al., 2014a, 2014b).
However, the drawbacks Al(OH)3 or Mg(OH)2 originate from its high
loading (20–60 wt%) due to the poor efficiency and inherent incompatibility with organic polymers, which drastically decrease the
processibility and mechanical strengths of composites (Rothon and
Hornsby, 1996; Haurie et al., 2006).
Layered double hydroxides (LDH) reveal higher flame retardancy
and smoke suppression properties than Al(OH)3 or Mg(OH)2 at the
same loading level (Gao et al., 2014a, 2014b; Kamiyama et al., 2016).
Nowadays, LDH have elicited considerable interest in the field of environmentally friendly and highly efficient flame retardants for polymers, due to its tunable chemical compositions and layered structures
(Leroux and Besse, 2001; Rives, 2001; Wang and O'Hare, 2012; Wang
et al., 2015). A surprising amount of research have proven that both the
Corresponding authors at: College of Chemical Engineering, University of Xiangtan, Xiangtan 411105, Hunan, China.
E-mail addresses: xusheng2016@xtu.edu.cn (S. Xu), hongyanzeng99@hotmail.com (H.-Y. Zeng).
https://doi.org/10.1016/j.clay.2018.07.024
Received 5 November 2017; Received in revised form 13 June 2018; Accepted 16 July 2018
Available online 25 July 2018
0169-1317/ © 2018 Published by Elsevier B.V.
Applied Clay Science 163 (2018) 196–203
S. Xu et al.
mechanical properties and flame retardant of a polymer matrix were
significantly improved by the addition of LDH with a new intercalated
anion (Ye and Qu, 2008; Wang et al., 2013; Edenharter and Breu, 2015;
Kaul et al., 2017). For instance, by adding 15 wt% Zn2Al-borate LDH
into PP, the peak heat release rate (PHRR) of composite could be decreased by 63.7% compared to pure PP (Wang et al., 2013). Nyambo
and Wilkie (2009) reported that the PHRR of LDH and EVA composite
was reduced to about 40% with only 3% by weight of the ZnAl-LDH
intercalated with borate anion was presented. Moreover, Zhang et al.
(2015) proved that the flame retardant of PP was greatly improved after
the addition of dihydrogen phosphate anion-intercalated LDH. Therefore, to further improve the flame retardancy of LDH, intercalating
flame retardant anions into its interlayer was an effectively approach.
Recently, phosphorus-containing compounds as efficient halogenfree flame retardants have been widely used as flame retardant in
polymers (Tang et al., 2013; Zhang et al., 2014; Qin et al., 2016, Wang
et al., 2017). Ye and Qu (2008) have demonstrated that the UL-94 of PP
could reach V-1 rating by adding 55 wt% of MgAl-PO4 LDH, and the LOI
values of EVA/MgAl-PO4 samples were 2% higher than that of the
corresponding EVA/MgAl-CO3 samples at the same loading level. In our
previous researches (Xu et al., 2015), Mg/Al LDH coated by P3O105− as
flame retardant for PP showed a high anti-flaming performances.
However, to the best of our knowledge, the flame-retardant mechanism
of the phosphorus-containing compound for PP matrix remains virtually unexplored.
In the present work, a functional group with flame retardant property, P3O105− intercalated into Mg/Al LDH (LDH-P) was prepared and
used as phosphorus containing flame retardant for PP matrix. The LDHP was characterized by XRD, FT-IR, particle size distribution, BET and
contact angle measurement, in order to understand the structure and
morphology as well as surface chemical property of the LDH-P particle.
Moreover, the PP/LDH-P composite and corresponding fire residues
was characterized to disclose flame-retardant mechanism of the LDH-P
for PP matrix.
2.3. Preparation of PP and LDH-P composite
PP/LDH-P composite was prepared by melting LDH-P into the PP
matrix in a GH-10A high-speed mixer (Beijing Plastic Machinery
Factory) at a rotor speed of 250 rpm at 230 °C for 15 min. The admixture was molded into bar (120 × 10 × 4 mm3) using a JK-WZM-I
micro injection molding machine with a twin-screw extruder (SHJ-30A)
(Beijing Heng Odd Instrument Co., Ltd.) for the testing. For convenience, the neat PP and polypropylene/LDH-C composite were designated as PP and PP/LDH-C, respectively. Besides, the polypropylene/
LDH-P and polypropylene/LDH-C composite were designated as PP/
LDH-P-n and PP/LDH-C-n, where the mass loading of the LDH-P or
LDH-C added was n wt%.
2.4. Characterization
2.4.1. Characterization of the LDH samples
XRD patterns were collected on a Rigaku D/max-2550PC with CuKα
radiation (λ = 1.5406 Å). The scan step was 0.02° (2θ) with a filament
intensity of 30 mA and a voltage of 40 kV. FT-IR was recorded on
Perkin-Elmer Spectrum One B instrument using KBr pellet technique.
Mastersizer 2000 laser particle size analyzer was from the United
Kingdom Malvern company. BET specific surface area (SSA) was determined by N2 adsorption-desorption at −196 °C with a NOVA 1000e
from the Quantachrome Instruments. Contact angle measurement was
measured by means of sessile drop method with uncompacted powder
stuck on a fixed glass support using a drop shape analysis system (DSA
100, Kruss Co., Germany). Scanning electron micrograph (SEM) was
obtained with a JEOL JSM-6700F instrument and energy dispersive xray spectroscopy (EDS) analysis was performed by a Noran SystemSix
instrument. EDS was used to determine the contents of P and Al elements in the samples. Thermogravimetric analysis was carried out in a
nitrogen atmosphere with a Seiko TG-DTA 6300. The nitrogen gas flow
rate for 50 cm3·min−1, and the heating rate was 10 °C·min−1.
2. Experimental
2.4.2. Characterization of the PP and LDH composites
TGA was performed using a Perkin-Elmer Pyris-1. 6.0–10.0 mg of
the sample (PP or composites) was loaded in an open ceramic crucible,
and heated within the temperature range from ambient to 800 °C. The
specimen for TGA was prepared by cryogenic fracturing in liquid nitrogen. The heating rate was 10 °C·min−1 in this study. A high purity
nitrogen stream (99.5% nitrogen, 0.5% oxygen content) was continuously passed into the furnace at a flow rate of 50 cm3·min−1 at
room temperature and atmospheric pressure. Before starting each run,
nitrogen was used to purge the furnace for 30 min to establish an inert
environment in order to prevent any unwanted oxidative decomposition. SEM with a JEOL JSM-6700F instrument was obtained to examine
the morphology of the char residue obtained after burning. XRD patterns and FT-IR were characterization as mentioned in Section 2.4.1.
LOI was measured using a JF-3 instrument (Nanjing, China) on bars
120 × 10 × 4 mm3 according to the standard oxygen index test on a
GB/T 2406.2-2009. The vertical burning UL94 V test was carried out
with 120 × 10 × 4 mm3 specimens based on the standard ANSI/UL-941985 and averaged over five measurements for each composites.
The impact strength was measured with a simple Beam Impact
Testing Machine (XJJ-22) at room temperature based on the standard
GB/T1043-1993 with 45° V-shaped notch and a notch-tip radius of
0.2 mm. Five specimens were repeated, and the average values in order
to obtain reproducible results. And the other mechanical properties
were measured using Electronic Tensile Test Machine (RGD-5) with a
crosshead speed of 30 mm·min−1. Tensile strength, fracture elongation
and elongation at break based on the standard GB/T1042-1992, GB/
T1042-1992 and GB/T9341-2000, respectively. Three specimens at
least were repeated to determine the average values in order to obtain
reproducible results.
2.1. Materials
Polypropylene particles (K8303, melt flow rate: 2.6 g∙10 min−1 at
230 °C and 2.16 kg) with the particle size about 1 mm, were purchased
from Yanshan Petrochemical I Co., Ltd. (Beijing, China). All chemicals
were of analytical grade. Sodium tripolyphosphate (Na5P3O10) was
purchased from Sigma (St Louis, MO, USA). These materials were used
as received without further purification. 0.1 mol·L−1 NaOH solution
was used for pH adjustment. All solutions were prepared with deionized
water or boiled deionized water.
2.2. Preparation of the LDH-P
Based on our previous works (Zeng et al., 2014; Xu et al., 2015), the
precursor hydrotalcite with Mg/Al molar ratio of 3.0 was prepared by
the high-gravity equipment (RPB) with high gravity field using a selfdesigned devices. The resulting slurry was filtrated, deionized water
washing to neutral, and the drying at 100 °C for 10 h, which was designated as LDH-C. Part of the LDH-C was calcined at 5 °C·min−1 followed by keeping at 500 °C for 4 h in a muffle furnace, which was denoted as LDO. And then 100 mg of the LDO was suspended in 20 mL of
Na5P3O10 (0.1 mol·L−1) CO2-free solution at initial pH of 10 (by adding
small amounts of NaOH solution), and kept under magnetic stirring at
50 °C under a nitrogen atmosphere for 5 h. The product was centrifuged, washed to neutrality, and then dried at 100 °C for 24 h, which
was designated as LDH-P. For convenience, the LDH-C and LDH-P be
collectively called LDH.
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Applied Clay Science 163 (2018) 196–203
S. Xu et al.
Fig. 1. X-ray diffraction patterns of the LDH-C and LDH-P.
3. Results and discussion
3.1. Characterization of the LDH samples
3.1.1. XRD analysis
The powder XRD patterns for the LDH-C and LDH-P were shown in
Fig. 1. The LDH-C and LDH-P samples had the typical layered double
hydroxide structures with sharp and intense (003), (006), (009), (110)
and (113) reflections and broadened (015) and (018) reflections. In
each case no other crystalline phases were observed in the XRD patterns, pointing out that both the samples were highly crystalline hydrotalcite structures (Cavani et al., 1991). Further analyses of the XRD
patterns revealed some differences in the cell parameters between the
two samples. For the CO32– intercalated hydrotalcite, the interlayer
space distance d003-value was supposed to be in the range
0.760–0.780 nm (Zeng et al., 2009), and for the P3O105−, it was about
0.983 nm (Badreddine et al., 1999). The distance of the LDH-C was
0.765 nm, indicating the intercalated anions were CO32−. The distances
of the LDH-P increased to 1.083 nm, possibly attribute to the intercalation of P3O105− anions. The results indicating that the P3O105−
anions were intercalated into the interlayer space.
Fig. 2. SEM images of the LDH-C and LDH-P, ×10,000.
3.1.2. SEM/EDS analysis
In order to investigate the morphology and P/Al molar ratio, samples observed by SEM analyses are shown in Fig. 2 (EDS figure no
shown). Thin flat crystals indicating the layered structure were observed for the two samples in line with the typical hydrotalcite morphology. All particles of the LDH-C showed well-developed hexagonal
plates with narrow size distribution (about 1–2 μm). And particles of
the LDH-P revealed platelets with collapsed layer structure and broad
size distribution (0.5–2.5 μm).The results implied that the impregnation-reconstruction may break the edges of brucite sheets. Besides, the
semi-quantitative analysis of EDS was used to detect the elements of P
and Al in the LDH-P sample, revealing that the P/Al molar ratio of the
LDH-P was about 0.59.
4000
3.1.3. FT-IR analysis
Fig. 3 displays the FT-IR spectra of the LDH-C and LDH-P in the
region 400–4000 cm−1, demonstrating typical layered double hydroxide structures. The reflections at around 3450 cm−1 (structural eOH
groups stretching vibrations) and around 1620 cm−1 (water bending
vibrations) were observed in the two samples (Kloprogge and Frost,
1999; Dos Reis et al., 2004). The reflections around between 500 and
1200 cm−1 can be attributed to the metal-oxygen vibrations (BlanchRaga et al., 2013). The LDH-P spectrum was generally similar to the
precursor LDH-C except for some minor differences. For the LDH-C
sample, three bands appeared at 1357, 786 and 685 cm−1 were assigned to the asymmetrical and symmetrical stretching vibration of
786
685
554
1357
1620
3451
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
582
903
1215
1120
1556
LDH-P
3457
Absorbance (a.u.)
LDH-C
500
Fig. 3. FT-IR spectra of the LDH-C and LDH-P.
CO32– (Cavani et al., 1991; Othman et al., 2006). However, for the LDHP sample, the reflections assigning to CO32– basically disappear, and
there were new reflections at around 1215, 1120, 903 and 582 cm−1,
which were assigned to the vibration of P3O105−. The new reflections at
1215 and 903 cm−1 were assigned to asymmetric valence vibration
νas(OePeO), the reflection at 1120 cm−1 corresponded to asymmetric
valence vibration of PO3 groups and one at 582 cm−1 belonged to angular deformation vibration of external PO3 groups δ(OePeO)
(Badreddine et al., 1999; Hull et al., 2005). In the LDH-P sample, a new
198
Applied Clay Science 163 (2018) 196–203
S. Xu et al.
Fig. 5. Images of water wetting on the LDH-C and LDH-P surface at equilibrium.
Fig. 4. Particle size distributions of the LDH-C and LDH-P.
reflection observed at 1556 cm−1 was due to the overlapping of
structural eOH groups and νas(OePeO), where the shift to lower wavenumber from 1620 to 1556 cm−1 was due to some interaction between eOH groups and P3O105− anions. The results further confirmed
that P3O105− anions were intercalated into the interlayer space of the
LDH-P.
3.1.4. Particle size and SSA
The particle size distribution and the average particle sizes for the
LDH-C and LDH-P samples were illustrated in Fig. 4. The most probable
sizes of the LDH-C and LDH-P were approximately 18.5 and 3.1 μm,
respectively. The most probable size distribution for the LDH-C with
90% of the particles was found in the range of 6.5–35.4 μm, whereas it
was found in the range of 1.9–10.4 μm for the LDH-P. The results revealed that the LDH-P had more uniform and smaller particle size
comparing with the LDH-C sample due to the intercalation of P3O105−
anions. On the other hand, the LDH-P had the highest SSA
(158 m2·g−1), followed by the LDH-C with 103 m2·g−1. The higher SSA
for the LDH-P was due to the smallest particle size and narrowest size
distribution with high dispersity.
3.1.5. Water wetting
The water wetting images on LDH-C and LDH-P samples surface at
equilibrium are shown in Fig. 5. The water contact angles at equilibrium for the LDH-C and LDH-P samples were 21° and 34°, respectively.
As expected, water wettability decreased after the modification by the
intercalation of P3O105− anions, which further confirmed that the
modification had indeed occurred for LDH-P, which resulted in a
weaker hydrophobicity of the LDH-P surface.
Fig. 6. TGA curves of the LDH-C and LDH-P.
199
Applied Clay Science 163 (2018) 196–203
S. Xu et al.
Fig. 7. X-ray diffraction patterns of the neat PP, PP/LDH-C and PP/LDH-P.
3.1.6. TGA analysis
The TGA curves for LDH-C and LDH-P are shown in Fig. 6. Both the
samples showed two distinct steps in their thermal decomposition behaviors. For LDH-C, the first decomposition with 17.9% weight loss
occurred at 166.7 °C due to the loss of the surface and interlayer water.
The second peak with a weight loss of 13.5% at 269.7 °C corresponding
to the decomposition of CO32– and dehydroxylation in layers. In the
case of LDH-P, the first weight loss (17.5%) corresponding to the loss of
the surface and interlayer water was at 224.7 °C, the second (20.6%) at
393.7 °C. The results shown that the thermal decomposition peak of
LDH-P were both higher than these of LDH-C, implying that the intercalation of P3O105− anions could increase the thermostability of the
LDH-P.
3.2. Characterization of the PP and composites samples
3.2.1. XRD analysis
The powder XRD patterns for the neat PP (control), PP/LDH-C-20
and PP/LDH-P-20 were shown in Fig. 7. The PP sample had the α-PP
crystals with sharp and intense (110), (040), (130), (111), and (041)
reflection, which were observed at 2θ = 14.2, 17.1, 18.6, 21.1 and
21.8°, respectively (De Rosa and Corradini, 1993; Auriemma and De
Rosa, 2006). For the PP/LDH-C-20 sample, (003) and (006) reflections
were observed in the XRD patterns, pointing out that hydrotalcite
structures exists in the PP/LDH-C-20 sample (Cavani et al., 1991). In
the PP/LDH-P-20 sample, the characteristic peaks of LDH-P were
completely disappeared in the composite, indicating that the initial
structure of LDH-P layers has been destroyed by the delamination and
the dispersion within the copolymer matrix to form the nanocomposite
(Matusinović et al., 2009). Moreover, a new peak was observed at about
2θ = 16°, corresponding to the (300) plane of the β form of PP, which
meant a trigonal structure (Mead, 1994). The results indicated that
LDH-P was found to induce an attritive interaction with PP chains and
to generate β-crystalline phase after melt blending. As reported by
Obadal et al. (2005) the presence of β-crystalline phase could elevate
the impact strength for PP matrix, which agreed with the results of
mechanical test (Fig. 11).
Fig. 8. SEM images of the PP/LDH-C-20 (A) and PP/LDH-P-20 (B) composites,
×1000.
particles in the PP/LDH-P-20 composites. A similar result was also
obtained in XRD characterization (Fig. 7). Thus, intercalation of
P3O105− anions promoted the LDH-P-20 particles to disperse homogeneously in the PP/LDH-P-20 composites, namely improving compatibility.
3.2.3. TGA analysis
The TGA curves of the neat PP (control), PP/LDH-C-20 and PP/LDHP-20 were shown in Fig. 9 and the results listed in Table 1. Neat PP
exhibited one sharp weight loss in the temperature range of 271–412 °C,
and the thermal curve of the PP/LDH composite were similar to that of
neat PP, except for that the onset decomposition temperature (T10%)
and decomposition temperature (T50%) were higher than the net PP
(Table 1). As reported in Table 1, the onset decomposition temperature
(T0.1) and midpoint decomposition temperature (T0.5) of the PP/LDH-C20 and PP/LDH-P-20 composites were shifted to higher temperatures
compared to those of neat PP, implying that the addition of the LDH-C
(LDH-P) into PP improved the thermal stability of PP matrix. In the
Table 1, the T10% and T50% values of the PP/LDH-P-20 were 79.8 °C and
61.2 °C higher than the corresponding PP/LDH-C-20, indicating that the
thermal stability of the PP/LDH-P-20 was much higher than that of the
PP/LDH-C-20. Comparing with the LDH-C, the LDH-P has a higher
dispersity, larger specific surface area and weaker hydrophobicity
leading to higher thermal capacity for releasing large amount of water
and absorbing heat. The higher thermal capacity increased the thermal
stability and flame retardant properties of the PP/LDH-P composites.
3.2.2. SEM analysis
The effect of compounding and the quality of the LDH (20 wt%)
dispersion into PP matrix was evaluated from SEM images by cryofractured surfaces of PP/LDH-C-20 and PP/LDH-P-20 composites, seen
in Fig. 8. The LDH-P particles were dispersed uniformly throughout the
PP matrix as shown in Fig. 6B. However, the LDH-C-20 particles in the
PP/LDH-C-20 were badly agglomerated as white platelets pointed by
the arrow in Fig. 6A. The result showed that the intercalation of
P3O105− anions improved the homogeneous dispersion of the LDH-P-20
200
Applied Clay Science 163 (2018) 196–203
S. Xu et al.
Table 2
UL-94 results of the PP/LDH composites at different loadings of the LDH.
Samples
PP/LDH-C
PP/LDH-P
Table 1
Thermal decomposition parameters of the PP/LDH-P-20 and PP/LDH-C-20.
T10% (°C)
T50% (°C)
Char Residue rate at 600 °C (%)
0
3
5
10
20
30
40
Not pass
Not pass
Not pass
Not pass
HB
V-2
V-2
V-1
V-1
V-0
V-0
V-0
V-0
V-0
properties such as elongation at break (εmax), flexural strength (Fstr),
impact strength (Istr) and tensile strength (σT) were evaluated and result
was shown in Fig. 11. It was found that no obvious effect on the mechanical properties of the PP matrix by adding low content
(≤10 g·100 g−1) of the LDH-C (LDH-P). Upon the LDH-C (LDH-P)
loading (10 g·100 g−1–40 g·100 g−1), the εmax and σT of the PP/LDH-C
and PP/LDH-P composites apparently exhibits an weakening trend. And
more importantly, compared with the PP/LDH-C, PP/LDH-P composites
have shown a positive effect on decreasing the mechanical properties of
the PP by filling of LDH-P in PP matrix.
Fig. 9. TGA curves of the neat PP, PP/LDH-C-20 and PP/LDH-P-20 in air atmosphere.
Parameters
Loadings of the LDH (g·100 g−1)
3.3. Characterization of fire residues
Samples
Neat PP
PP/LDH-C-20
PP/LDH-P-20
314.6
365.8
0
340.1
401.4
5.9
419.9
462.6
14.5
3.3.1. Morphology of the final char
The fire residues from neat PP (control), PP/LDH-C-20 and PP/LDHP-20 were compared in the photographs provided in Supplementary
material (Scheme 1). Apparently, there was no residue in neat PP
sample as shown in Scheme 1(a), indicating that the neat PP was
completely burned without any residue. As shown in Scheme 1(b) and
(c), some residue remained in PP/LDH-C-20 and PP/LDH-P-20 indicating that the addition of the LDH-C (LDH-P) into PP could improve
the combustion performance of PP matrix. Comparing with the PP/
LDH-C-20, the PP/LDH-P-20 had a more stable and intact char layer as
a result of a more effective thermal barrier providing by the LDH-P.
In order to investigate the relationship between the microstructure
of fire residues and flame retardant properties of the composites, the
fire residues of PP/LDH-C-20 and PP/LDH-P-20 are imaged by SEM and
magnified by 1000 times, as shown in Fig. 12. PP/LDH-C-20 sample
produced a thin but discontinuous layer with major voids and cracks in
Fig. 12(A), while Fig. 12(B) shows that a thick, continuous and crackfree nanoparticle network char layer with some small hole on the surface was formed for the PP/LDH-P-20 system. The results suggested
that LDH-P promoted the formation of crosslinking network, rendering
to a more compact char layer with better mechanical performance, and
consequently improved flame retardant properties (Table 2 and
Fig. 10).
3.2.4. Combustion performance
The combustion tests had been performed on PP/LDH-C and PP/
LDH-P composites samples containing different amounts of LDH-C
(LDH-P) filler (Fig. 10), and the results showed that the ratio of fillers to
PP had to be at least 10: 100 in order to avoid dripping of burning melts
in terms of the vertical burning test (UL-94) (Table 2). Besides, the LOI
value of the PP/LDH-P composites was found to slightly improved with
the addition of LDH-P filler. The reason of the cleavage of P-C bonds
during combustion was due to their low bond energy for LDH-P sample.
It can be concluded that by adding LDH-P as the flame-retardant filler
significantly improved the inflammability of PP, although this effect
should be more logically if more detail analyzed by other techniques as
the cone calorimeter.
3.2.5. Mechanical properties
In order to investigate the effect of LDH-C (LDH-P) on the mechanical properties of PP/LDH-C (PP/ LDH-P) composites, mechanical
3.3.2. FT-IR of the final char
Fig. 13 shows the FT-IR spectra of fire residues from PP/LDH-C-20
and PP/LDH-P-20. The PP/LDH-P-20 spectrum was generally similar to
that of PP/LDH-C-20 except for some minor differences. Especially for
the PP/LDH-P-20, the three new bands at 1239, 1078 and 1019 cm−1
were P]O symmetric stretch, P]O antisymmetric stretch and PeC
vibration, respectively, (Badreddine et al., 1999; Beekes et al., 2007)
owning to the crosslinking between P from P3O105− and PP. The new
bands appear in fire residues from PP/LDH-P-20, suggesting that with
the addition of LDH-P, P remained in the char surface to improve the
strength of char. Combining the analysis results from SEM, LOI and
UL94, it could be concluded that the LDH-P indeed promots the flame
retardant properties of PP matrix.
4. Conclusions
P3O105− intercalated Mg/Al hydrotalcite (LDH-P) as a functional
fire-retarding filler was successfully prepared by impregnation-reconstruction, where the LDH-P was used to prepare polypropylene (PP)
Fig. 10. Effect of LDH loading on the LOI of the PP/LDH composites.
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S. Xu et al.
Fig. 11. Effect of LDH loading on the elongation at break, tensile strength, bending strength and impact strength of the PP/LDH composites.
Fig. 13. FT-IR spectra and photographs of fire residues of PP/LDH-C-20 and
PP/LDH-P-20.
and LDH-P composite (PP/LDH-P). The LDH-P and LDH-C were characterized by XRD, SEM/EDS, FT-IR, particle size, SSA, contact angle
measurement and TGA. The results indicate that P3O105− anions were
intercalated into the interlayer space of LDH-P, and the LDH-P has
shown a higher dispersity and weaker hydrophobicity than that of LDHC demonstrating that the LDH-P could act as an efficient fire-retarding
filler for PP.
LDH-P are found to surpass LDH-C in terms of XRD,TGA, LOI and
UL94 analyses of PP/LDH-P and PP/LDH-C when used in a potential
flame-retardant additive in PP. The results revealed that LDH-P could
induce an attritive interaction with PP chains and to generate β-crystalline phase after melt blending leading which elevate the impact
strength for PP matrix. Moreover, the thermal stability of the PP/LDH-P
sample has significantly enhanced by filling LDH-P into PP matrix.
Fig. 12. SEM photographs of fire residues of PP/LDH.
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Applied Clay Science 163 (2018) 196–203
S. Xu et al.
Comparing with the PP/LDH-C, the analysis of fire residues indicate
that LDH-P as a more effective thermal barrier remain P to enhance the
strength of char and improve the flame retardant properties of PP/LDHP.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.clay.2018.07.024.
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Acknowledgements
This work is supported by Hunan 2011 Collaborative Innovation
Center of Chemical Engineering & Technology with Environmental
Benignity and Effective Resource Utilization, PhD Startup Foundation
of Xiangtan University (17QDZ05), General project of Hunan Provincial
Education Department (17C1526) and Hunan Provincial Innovation
Foundation For Postgraduate (CX2018B043).
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