Intercalation of organic and inorganic anions into layered double hydroxides for polymer flame retardancy

Applied Clay Science 187 (2020) 105481

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Review article

Intercalation of organic and inorganic anions into layered double hydroxides for polymer flame retardancy

T

⁎

Li Jin, Hong-Yan Zeng , Jin-Ze Du, Sheng Xu 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: Layered double hydroxide Polypropylene Flame retardancy Intercalation Stearic and MoO42− anions

A series of novel organic and/or inorganic-intercalated MgAl layered double hydroxides (SxMoy-LDH) were designed and prepared via calcination-reconstruction, where the organic anion was stearic (S) and inorganic anion was MoO42− (Mo). The intercalation of stearic and/or MoO42− anions into the interlayer galleries of the LDH host layers was controlled by adjusting the mass ratios (x/y) of stearic acid to (NH4)2MoO4, in which the stearic-intercalated LDH was obtained above the 1: 1 of the x/y mass ratio, and the MoO42−-intercalated LDH was synthesized below the 1: 1 of the x/y mass ratio. Especially at x/y mass ratio of 1: 1, stearic and MoO42− anions were co-intercalated into the interlayers of the S1Mo1-LDH to confer excellent flame-retardant properties. The morphology, chemical composition and structure of the as-prepared S1Mo1-LDH determined by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy and energy dispersive spectrometer (SEM/EDS) and thermogravimetry (TG) confirmed the co-intercalation of stearic and MoO42− anions into the interlayer galleries. After adding the SxMoy-LDH materials into polypropylene (PP) matrix, the flame retardant performance of the PP-composites was significantly enhanced compared with the neat PP. It was also noticed that different interlayer anions could provide different retardant performances, and the stearic-MoO42−co-intercalated LDH (S1Mo1-LDH) exhibited the highest thermostability and the best flame retardant performance owing to the synergistic effect of the interlayer stearic and MoO42− anions as well the LDH host layers. Therefore, it was possible to obtain a certain intercalation state of the organic and inorganic anions by modulating the mass ratios (x/y) of organic to inorganic anions.

1. Introduction In the recent times, fire safety and environmental compatibility had become a prime concern for selecting potent compounds to synthesize flame retardant polymers in order to safeguard life (Zhao et al., 2009; Mohapatra et al., 2012). Polypropylene (PP) has superior mechanical properties and is easily processed. It is widely demanded in many commodity as well as industrial applications such as car, furniture, electronic piece, electric shell, interior decoration, insulation, architectural material and so on (Gao et al., 2014a). However, in common with other engineering polymers, its application has been limited because of the inflammable nature of PP (Gao et al., 2014b). Hence, it is imperative to enhance the flame retardancy of PP by means of developing flame retardants. Today, inorganic flame retardants have attracted more and more attention as most promising candidates to substitute halogen-containing flame retardants due to its environmental-friendly and antidripping properties (Chen and Qu, 2003; Wang et al., 2011). Layered

⁎

double hydroxides (LDH), are to be widely used in many fields like adsorbent, catalyst, flame retardants and so on due to their typical chemical composition, unique layered structure, adjustable chemical composition and exchangeable interlayer anion (Du et al., 2007; Du et al., 2019a; Zeng et al., 2014). Especially, LDH are regarded as promising new type of environmentally friendly flame retardant additives for polymers due to high flame retardancy and smoke suppression properties (Ardanuy and Velasco, 2011; Wang et al., 2010). However, it is difficult to make the LDH to be compatible with PP matrix due to its hydrophilcity. In order to improve the compatibility with PP matrix and other properties, there are many efforts to enhance the dispersibility of the LDH in PP matrix. Modification of the LDH by the incorporation of various anionic surfactants can improve the compatibility with hydrophobic PP. For instance, the thermal stability of PP-composites was improved by 37–60 °C compared to the pristine PP by adding organic anion-intercalated LDH filler (Yang et al., 2015). The stearic-intercalated MgAl-LDH also increased the thermal stability and flame retardancy of the PP matrix, which PHRR reduction reached to about 60%

Corresponding author. E-mail address: [email protected] (H.-Y. Zeng).

https://doi.org/10.1016/j.clay.2020.105481 Received 20 October 2019; Received in revised form 20 January 2020; Accepted 26 January 2020 0169-1317/ © 2020 Elsevier B.V. All rights reserved.

Applied Clay Science 187 (2020) 105481

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15 min, where the mass ratio of PP to FR was 100: 20, namely the 20 g∙100 g−1 loading. 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 composites of PP with LDH, S-LDH, S4Mo1-LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7LDH, S1Mo4-LDH and Mo-LDH were denoted as PP/LDH, PP/S-LDH, PP/S4Mo1-LDH, PP/S7Mo3-LDH, PP/S1Mo1-LDH, PP/S3Mo7-LDH, PP/ S1Mo4-LDH, PP/Mo-LDH in sequence.

(Qiu et al., 2018). By adding 15 wt% borate-intercalated Zn2Al-LDH into PP matrix, the peak heat release rate (PHRR) of the PP-composite could be decreased by 63.7% compared to pure PP (Wang et al., 2013). The composite of the MoS2 nanosheets and NiFe-LDH as flame retardant also showed excellent flame retardancy and smoke suppression properties (Zhou et al., 2017). And the Mo-decorated stearic-intercalated MgAl-LDH provided an excellent fire resistance towards PP polymer (Jin et al., 2020). Thus, to further improve the compatibility and flame retardancy of LDH, it was reasonable to believe that the intercalation of organic/inorganic anions into the interlayers of the MgAl-LDH was an effectively approach for polymer flame retardancy. Herein, a novel strategy was designed to prepare stearic-MoO42−co-intercalated LDH by adjusting the mass ratios (x/y) of stearic acid to (NH4)2MoO4 via calcination-reconstruction method. Then the as-prepared materials were characterized by XRD, FT-IR, SEM, TG-DTG, and contact angle measurement, in order to understand the effect of the x/y mass ratio on the anion-intercalation forms. To determine the flame retardant performances of the as-prepared materials, polypropylene (PP) was chosen as model polymers, where the as-prepared materials were blended into PP matrix to produce the PP-composites, which were investigated by TGA, LOI, UL94 and cone calorimeter (CC) tests.

2.5. Characterization 2.5.1. Characterization of the FR materials X-ray diffraction (XRD) patterns were collected on a Rigaku D/max2550PC (λ = 1.5406 Å) with Cu Kα radiation. The scan step was 0.0671°∙s−1 with a filament intensity of 30 mA and a voltage of 40 kV. Scanning electron microscopy (SEM, JEOL JSM-6700F) was used to observe the morphologies and microstructure of the samples, and energy dispersive spectrometer (EDS) analysis was performed by a Noran System Six instrument. Fourier transform infrared (FT-IR) was recorded on Perkin-Elmer Spectrum One B instrument using KBr pellet technique. Water contact angle (WCA) measurements for the FR particles were conducted using the sessile drop technique performed on a contact angle goniometer (CAM 2000, Finland) at 25 °C. Thermogravimetry and differential thermal gravity (TG-DTG) was carried out using Mettler ToledoO/1-1600HT equipment. 15.0 mg sample was heated at 10 °C·min−1 up to 800 °C, in the N2 condition.

2. Experimental 2.1. Materials Polypropylene (PP) particles (K8303, melt flow rate of 2.6 g∙10 min at 230 °C and 2.16 kg) with the particle size about 1 mm, were purchased from YanShan Petrochemical Industry Co., Ltd. (Beijing, China). All other chemicals were of analytical grade and used without further purifications. All the solutions were made with deionized water, where 0.1 mol·L−1 NaOH was used for pH adjustment. A pH electrode (Mettler Toledo S40K) was used for pH measurements. −1

2.5.2. Characterization of the PP/FR composites The morphology of the PP/FR composites were observed using SEM, where the specimen was prepared by cryogenic fracture in liquid nitrogen for SEM analysis. Thermal analysis was performed on a PerkinElmer Pyris-1 thermogravimertic analyzer (TG). 10 mg sample was loaded in an open ceramic crucible, and heated in air atmosphere at a heating rate of 10 °C∙min−1. The limiting oxygen index (LOI) was measured using a JF-3 instrument (Nanjing, China) on bars of 120 × 10 × 4 mm3 according to the standard oxygen index test on a GB/T 2406.2-2009. The UL94 test was carried out with 120 × 10 × 4 mm3 specimens based on the standard ANSI/UL-94-1985 and averaged over five measurements for each composition. The Cone calorimeter (CC) tests were carried out using cone calorimeter (JCZ-2, Jiangning Analytic Instrument Company, China) according to ISO 5660. The specimen with dimension of 100 × 100 × 4 mm3 was irradiated horizontally at heat flux of 50 kW∙m−2.

2.2. Preparation of the SxMoy-LDH The MgAl layered double hydroxide precursor with Mg/Al molar ratio of 3.0 was prepared by urea method according to our previous work (Zeng et al., 2008). Typically, 2.30 g Mg(NO3)2·6H2O, 1.13 g Al (NO3)3·9H2O and 4.86 g urea were dissolved in 150 mL deionized water. After ultrasonic for 30 min, the mixed solution were transferred to three-necked flask and heated at 105 °C for 24 h under vigorous stirring. The suspensions were centrifuged, washed thoroughly by the deionized water and ethanol, and then dried at 60 °C overnight, which was denoted as LDH. Part of the Mg/Al-LDH was calcined at 500 °C for 4 h, which was denoted as MgAlO.

3. Results and discussion 2.3. Preparation of the SxMoy-LDH 3.1. Characterization of the FR materials The stearic and MoO42− intercalated LDH was prepared by calcination-reconstruction method. 1.0 g MgAlO and 3.0 g of stearic acid (x) and (NH4)2MoO4 (y) with different x/y mass ratios were suspended in 500 mL CO2-free deionized water in initial pH 7.0 with magnetic stirring at 80 °C under a N2 atmosphere for 12 h. And then the suspensions were centrifuged, washed to neutrality with hot deionized water, and dried at 60 °C for 12 h, and the resulting products were donated as SxMoy-LDH. For the convenience, the SxMoy-LDH with the x/y mass ratios (x/y = 1: 0, 4: 1, 7: 3, 1: 1, 3: 7, 1: 4 and 0: 1) separately were designated as S-LDH, S4Mo1-LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7LDH, S1Mo4-LDH and Mo-LDH.

To identify the structure change of the SxMoy-LDH, the XRD patterns of the LDH, MgAlO, S-LDH, S4Mo1-LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7-LDH, S1Mo4-LDH, and Mo-LDH are shown in Fig. 1. As seen in Fig. 1A, the LDH had the typical layered double hydroxide structure corresponding to (003), (006), (009), (015), (110) and (113) lattice planes, while the interlayer distance (d003 0.76 nm) of the LDH were typical of CO32−-intercalated layered double hydroxide (Du et al., 2019a). In Fig. 1A, the MgAlO exhibited the (200) and (220) reflections attributing to the typical feature of a mixed oxide MgAlO type (Zhang et al., 2014; Jin et al., 2020). After the intercalation of the stearic and/ or MoO42− into the interlayer galleries of the LDH by calcination-reconstruction, the XRD patterns of the SxMoy-LDH samples exhibited a basically similar crystalline phase to the LDH, indicating a restoration of the corresponding as-prepared LDH (Fig. 1B). Furthermore, the d003 value with 2.88 nm for the S-LDH indicated the stearic was intercalated into the interlayer galleries of the host LDH layers, while the d003 value

2.4. Preparation of the PP/FR composites The composites (PP/FR) of PP and flame retardant (FR) were prepared by melting the FR into PP matrix in a GH-10A high-speed mixer (Beijing Plastic Machinery Factory) under 250 rpm stirring at 230 °C for 2

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S-LDH

3448

A

MgAlO

S4Mo1-LDH

MgAl-LDH

40

50

Transmittance (a.u.)

60 (003)

2835

2915

70

3500

88 =2. 3

814

Mo-LDH LDH

2 (°) d 00

S1Mo4-LDH

(113)

(110)

(018)

(009)

30

S3Mo7-LDH

3000

2500

2000

Wavenumber (cm

B

S-LDH

1

1386

20

S1Mo1-LDH

1635

10

(015)

76 =0. d 003

(006)

(003)

(220)

Intensity (a.u.)

(200)

S7Mo3-LDH

1500

1000

500

)

Fig. 2. FT-IR spectra of the LDH and SxMoy-LDH samples.

2

9 = 2.

S4Mo1-LDH

Intensity (a.u.)

d 003 d 00

=2. 3

d 00

1386 cm−1 corresponded to the CO32– in the interlayer galleries of the LDH host layers (Zhang et al., 2014). Especially in the SxMoy-LDH, the band intensities at 1386 cm−1 were significantly decreased, implying that a small amount of CO32– anion still remained in the interlayer galleries. For the S1Mo1-LDH, S3Mo7-LDH, S1Mo4-LDH and Mo-LDH, the new band at 814 cm−1 attributable to −Mo = O stretching of a tetrahedrally coordinated Mo species was observed, demonstrating the presence of MoO42− anions. At the same time, there were the two new bands at 2915 and 2835 cm−1 assigning to the –C-H– stretching vibration in the stearic molecules appeared in the S-LDH, S4Mo1-LDH, S7Mo3-LDH and S1Mo1-LDH, revealing the stearic anions were intercalated into the interlayer galleries (Saber and Tagaya, 2008). It was worth mentioning that the two new bands to the –C-H– groups was also observed in the S3Mo7-LDH and S1Mo4-LDH owing to the residual stearic molecules adsorbed on the surfaces of the materials, though their intensities were very weak. But no band at 814 cm−1 (−Mo = O stretching) could be observed in the S4Mo1-LDH and S7Mo3-LDH because the interlayer MoO42− anion was low to be detected. The results were agreed with the XRD observations, confirming that the S1Mo1LDH possessed co-intercalated stearic and MoO42− anions and the S4Mo1-LDH and S7Mo3-LDH had few MoO42− anions into the interlayer galleries, which was consistent with the observations in XRD. The microstructure and morphology of the LDH, S-LDH, S4Mo1LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7-LDH, S1Mo4-LDH and Mo-LDH were determined by SEM, and the results are shown in the Fig. 3. All the samples indicated layered platelets, which confirmed the assumption from XRD analyses that the samples had typical LDH structure. The LDH and S-LDH particles were made up of individual flat platelets with irregular edges, while the S4Mo1-LDH, S7Mo3-LDH also showed monodisperse thin flat platelets and slightly accumulated in all space direction. These indicated that the stearic-intercalated LDH still kept the individual layer structure like the CO32−-intercalated LDH precursor (Jiang et al., 2013). With the x/y mass ratio was further decreased, the MoO42− anions were intercalated into the interlayer galleries and the flat platelets were aggregate in a clumpy manner because the intercalated stearic anions into the interlayer galleries decreased dramatically. Comparing to the S3Mo7-LDH, S1Mo4-LDH and Mo-LDH, the S1Mo1-LDH particles only exhibited some tendency for platelets to aggregate due to the co-intercalation of stearic and MoO42− anions into the interlayer galleries of the LDH host layers. An indirect method to determine the charge of stearic and MoO42− anions into the interlayer galleries of the LDH was to perform an elemental chemical analysis. So, the semi-quantitative analysis by EDS was used to detect the elements of the LDH, S4Mo1-LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7-LDH and S1Mo4-LDH, and the results are shown in

93

=3. 3

S7Mo3-LDH 04

S1Mo1-LDH 8

8 =0.

d 003

S3Mo7-LDH 6

8 =0.

d 003

S1Mo4-LDH

5 0. 8

= d 003

10

Mo-LDH

20

30

40

2 (°)

50

60

70

Fig. 1. XRD patterns of the LDH, MgAlO and SxMoy-LDH samples.

of the Mo-LDH was 0.85 nm corresponding a typical of MoO42−-intercalated layered double hydroxide (Zhang et al., 2013; Sels et al., 2001; Jin et al., 2020). The S4Mo1-LDH and S7Mo3-LDH had a similar d003 values with that of the S-LDH, indicating that the stearic-intercalated layered double hydroxide structure. As explained in the literature (Shkirskiy et al., 2015), the further heightening in d003 value from about 2.90 to 3.04 nm was mainly the partial replacement of the interlayer stearic anions by the MoO42− anions, and so the interlayer galleries of the S1Mo1-LDH possessed co-intercalated stearic and MoO42− anions. At the same time, the interlayer stearic anion of the S4Mo1-LDH and S7Mo3-LDH was mainly stearic anion along with few MoO42− anion because their interlayer distances were about 2.93 nm. There were not only one phase for S3Mo7-LDH, indicating that a small percentage of stearic was inserted into the interlayers. When the mass ratio (x/y) of stearic acid to (NH4)2MoO4 was further decreased, the d003 value dropped to about 0.87 nm corresponding to the intercalating MoO42− anions, suggesting that the S1Mo4-LDH only had MoO42− anions into the interlayer galleries. The results demonstrated that the MoO42− and stearic were successfully co-intercalated into the interlayer galleries between the LDH host layers when the x/y mass ratio was 1: 1. The FT-IR spectra of the LDH, S-LDH, S4Mo1-LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7-LDH, S1Mo4-LDH and Mo-LDH in the region 400–4000 cm−1 are indicated in Fig. 2, which were basically similar except for some minor differences. For all the samples, the bands at around 3448 cm−1 (structural –OH groups) and 1635 cm−1 (bending vibrations of water molecules) were observed, and the band at 3

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L. Jin, et al.

Fig. 3. SEM images of the LDH and SxMoy-LDH samples.

into the LDH interlayer galleries and the interlayer stearic anion of the S4Mo1-LDH and S7Mo3-LDH was mainly stearic anion along with few MoO42− anion. For the practical application of flame retardants, the water-repellent property of the FR is a very important parameter. The wettability of the LDH and SxMoy-LDH samples were also tested using WCA measurement, and the results are shown in Fig. 4. As seen in Fig. 4, the water contact angles of the LDH, S-LDH, S4Mo1-LDH, S7Mo3-LDH, S1Mo1LDH, S3Mo7-LDH, S1Mo4-LDH and Mo-LDH were 37, 90, 71, 70, 64, 41,

Fig, S1, where the LDH precursor had C, O, Mg and Al elements. Mo element was detected in the S4Mo1-LDH, S7Mo3-LDH, S1Mo1-LDH, S3Mo7-LDH and S1Mo4-LDH, showing that the MoO42− anions were successfully intercalated into the interlayer galleries of the samples. The appearance of the trace C element in the S1Mo1-LDH, S3Mo7-LDH and S1Mo4-LDH was due to trace residual interlayer CO32– anion and residual stearic molecules adsorbed on the surface of the materials as described in FT-IR. Accordingly, it proved the deduction from the XRD and FT-IR that the S1Mo1-LDH had both the stearic and MoO42− anions 4

Applied Clay Science 187 (2020) 105481

L. Jin, et al.

Fig. 4. WCA images of the LDH and SxMoy-LDH samples.

and S7Mo3-LDH, which arose from the co-intercalation of stearic and MoO42− anions. The results further determined the observations from XRD, FT-IR and SEM/EDS analyses. Thermogravimetric analysis (TG) is one of the thermal analysis techniques used to measure physical chemistry properties of a material as a function of temperature or mass loss. The effect of the organic stearic and MoO42− anions on the SxMoy-LDH samples was investigated by TG-DTG in N2 atmosphere, where the TG-DTG curves show some differences (Fig. 5). The LDH demonstrated three mass loss steps with

32 and 30°, respectively. Combining with XRD and FT-IR analyses, the angle of the Mo-LDH was minimum in the samples, and the angles of the S3Mo7-LDH and S1Mo4-LDH increased slightly probably due to the residual stearic molecules adsorbed on the surface of the materials. Similarly, the decrease in the water contact angles for the S4Mo1-LDH and S7Mo3-LDH compared with that of the S-LDH was due to the presence of the interlayer MoO42− anions (Iyi et al., 2008). Especially, the water contact angle of the S1Mo1-LDH was higher than those of the S3Mo7-LDH and S1Mo4-LDH, but lower than those of the S4Mo1-LDH 5

Applied Clay Science 187 (2020) 105481

L. Jin, et al.

100

100

S-LDH

S4Mo1-LDH

10.5 %

9.5 %

Wight loss (%)

90

90

646 C 84.0 C 317 C

80

27.3%

70

655.3 C 90.3 C

80

317.5 C

70

29.5 %

60

60 12.4 % 220 C

50 40

4.8 % Total 55.0%

100

200

300

400

500

600

700

100

40

800

4.6 % Total 54.8%

100

200

300

400

500

600

700

100

S7Mo3-LDH

12.0 %

90

662.3 C

800

S1Mo1-LDH

10.0 %

90

11.2 %

220.4 C

50

633.2 C

Wight loss (%)

86.0 C 315.3 C

80 70

80

219.0 C

200

300

400

500

600

700

40

800

S3Mo7-LDH

Wight loss (%)

200

300

400

500

600

700

800

S1Mo4-LDH

90 18.7 % 10.1 %

80

80

126.6°C

331.1 C

70

14.8 %

70

16.1 %

117.6 C

328.2°C 235.6°C

60

Total 34.8%

Total 38.1%

50

50 100

200

300

400

500

600

100

700

40

800

Mo-LDH

100

200

300

400

500

600

700

800

LDH

100 15.5 %

90

90

22.2%

Wight loss (%)

100

100

90

80

80

321.2 C 11.7%

70 60

40

2.6 % Total 49.6%

Total 54.5%

13.2 %

50

10.0 %

50

3.5 %

100

223.5 C

12.5 %

100

40

25.0 %

60

50

60

49.6 wt.% total

70

28.5 %

60

40

321.5 C

90.7 C

70 Total 33.9%

154.7 C

60

33.3 %

47.7 C

50

100

200

300 400 500 Temperature ( C)

600

700

800

40

Total 49.0%

256.8 C

100

200

300 400 500 Temperature ( C)

600

700

800

Fig. 5. TG/DTG curves of the LDH and SxMoy-LDH samples.

total mass loss of 49%, where the first stage (30–211.5 °C) was due to the loss of the surface and interlayer water and the second stage (212–550 °C) with 33.3% corresponding to the dehydroxylation of the LDH host layers (Du et al., 2019b; Du et al., 2006). For the S-LDH, there were four mass loss steps with total mass loss of 55%, i.e., 25–160 °C, 160–280 °C, 280–470 °C and 600–800 °C, where the first mass loss was

the loss of the surface water, the second mass loss corresponded to the dehydroxylation of the LDH host layers, the third mass loss was assigned to decomposition of interlayer organic stearic anions, and the fourth mass loss was due to the complete decomposition of interlayer stearic anions (Zhang et al., 2013; Xu and Braterman, 2010; Bujdosó et al., 2009). At the same time, the S4Mo1-LDH, S7Mo3-LDH and S1Mo16

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L. Jin, et al.

studied by TG, and the TG curves for neat PP and PP-composites under an air atmosphere at 20 g∙100 g−1 loading of FR filler are shown in Fig. 7, and corresponding parameters are summarized in Table 1. As seen in Fig. 7, the TGA curves of the PP/FR composites were similar with that of the neat PP with one sharp mass loss, implying a similar degradation process. As listed in Table 1, the onset decomposition temperature (T0.1) and midpoint decomposition temperature (T0.5) of the PP/FR composites were shifted to higher temperatures compared to those of neat PP, indicating that the addition of the FR into PP could improve the thermal stability of PP polymer and the FR were not catalytic activities during the thermal decomposition. It was noteworthy that the PP/S1Mo1-LDH exhibited the highest thermal stability corresponding to the largest T0.1 and T0.5 values (Fig. 7 inset), suggesting that a moderate x/y mass ratio could greatly improve the thermal stability of the PP-composites due to the co-intercalation of stearic and MoO42− anions into the interlayer galleries of the LDH host layers. Furthermore, the char residue amount of the PP/S1Mo1-LDH composite with 9.3% at 550 °C was the highest, implying the best flame retardant properties on the PP polymer. As a result, it had reason to suggest that the S1Mo1-LDH offered an effect against the flammability of PP owing to the strong interface interaction between PP and S1Mo1-LDH, in which the shield of the LDH host layers and the synergy of the interlayer stearic and MoO42− anions enhanced the heat resistant and thermal stability. The flammability and fire safety of polymer materials were evaluated by the LOI and UL-94 tests, and the LOI and UL-94 values of the neat PP, PP/LDH, PP/S-LDH, PP/S4Mo1-LDH, PP/S7Mo3-LDH, PP/ S1Mo1-LDH, PP/S3Mo7-LDH, PP/S1Mo4-LDH and PP/Mo-LDH are presented in Table 1. It was showed that the LOI value of neat PP was 17.6%, and the LOI values of the PP-composites increased significantly after the incorporation of the FR fillers. In the UL94 tests, the PP/ S1Mo1-LDH and PP/S7Mo3-LDH composites displayed a self-extinguishing behavior corresponding to an appreciable fire retardant behavior (V-0), followed by the PP/S4Mo1-LDH, PP/S1Mo4-LDH, PP/ S3Mo7-LDH and PP/S-LDH with V-1, and then PP/LDH with a V-2 in UL94 ratings, whereas the neat PP could not pass any UL94 ratings. The results were in keeping with those from the TG analyses, revealing high flame retardant properties came from high thermal stability and larger amount of char residue of the PP-composites. Especially, the LOI value of the PP/S1Mo1-LDH composite reached to the maximum of 28.2% with V-0 in UL94 ratings. So, it was easy to conclude that 1: 1 was the optimal x/y mass ratio to gain the stearic-MoO42−-co-intercalated LDH, with which both the flame-retardant performance and the thermal stability of the PP/FR composites were good. The combustion properties of the PP/FR composites were further characterized through the cone calorimeter (CC) tests, and the heat release rate (HRR) plots at 50 kW∙m−2 heat flux are shown in Fig. 8 and the corresponding data are listed in Table 1. The pure PP burned rapidly after ignition, and burned out within 322 s, which PHRR value was 1228.8 kW∙m−2. At the same time, the PHRR was significantly reduced (> 60%) in the PP-composites with stearic and/or MoO42− as the interlayer anion in the LDH, while it was reduced by only about 36.5% in the PP/LDH composite with interlayer CO32−-pillared MgAlLDH. Simultaneously, the THR values of the PP-composites were significantly reduced compared to the neat PP (Table 1), meaning that the addition of the FR fillers could greatly improve the thermal and combustion properties of PP polymer. In particularly, the PP/S1Mo1-LDH exhibited the best flame retardancy with the lowest PHRR value of 335.1 kW∙m−2, approximately 72.7% reduction relative to the neat PP. The synergy of interlayer stearic and MoO42− anions as well as the parent LDH host layer might be the reason for the observed enhanced thermal and combustion properties, which could promote the formation of a char layer to prevent the PP matrix from further combustion. The mechanical properties of the polymer composites are very important in practical applications. To investigate the effects of the FR fillers on the mechanical properties of PP-composites, the elongation at

LDH showed the same TG-DTG behavior as the S-LDH, suggesting that they were stearic-intercalated MgAl-LDH. It was worth stressing that, the mass loss peaks of the S1Mo1-LDH shifted towards a high temperature and the total mass loss decreased to 49.6% due to the presence of MoO42− anions, which demonstrated that the co-intercalation of stearic and MoO42− anions between the LDH host layers could improve the thermal stability (Sels et al., 2001; Nhlapo et al., 2008; Zhu et al., 2008). The Mo-LDH showed two mass loss steps with total mass loss of 33.9%, namely the mass loss of the surface and interlayer water with 22.3% occurred between 35 and 235 °C, and the second mass loss (11.6%) between 235 and 525 °C from the dehydroxylation, in which MgMoO4 was formed by transformations (Frost et al., 2005; Carriazo et al., 2006). The increase in temperature of the first stage revealed the strengthening interaction, making the removal of water molecules more difficult due to the stronger hydrogen bonding interactions between MoO42− anions and some water molecules. As seen in Fig. 5, the TGDTG curve of the S1Mo4-LDH was similar to the Mo-LDH curve, indicating a similar structure and composition, i. e. MoO42−-intercalated MgAl-LDH, which total mass losses were slightly higher than that of the Mo-LDH due to the presence of few stearic molecules adsorbed on the surface of the materials. For the S3Mo7-LDH, its TG curve was similar to that of the S1Mo1-LDH, but the third mass loss centered at about 322 °C increased from 10.0% to 14.8% and the fourth mass loss at about 633 °C disappeared. The enhancement in the third mass loss step as due to transformations from interlayer MoO42− anion to MgMoO4 and decomposition of organic stearic molecule adsorbed on the surface of the S3Mo7-LDH, which was consistent with the results in XRD, FT-IR and SEM/EDS analyses. The results further verified that stearic and MoO42− anions could be intercalated directly into the interlayer galleries simultaneously at a moderate x/y mass ratio (1: 1) of stearic acid to (NH4)2MoO4 using calcination-reconstruction method. 3.2. Characterization of the PP/FR composites The fractured surface of the PP/FR composites with 20 g∙100 g−1 loading of FR fillers was determined by SEM, and results are shown in Fig. 6. The LDH and Mo-LDH fillers in the composites were distinctly separated from PP matrix, agglomerated and dispersed uniformly throughout the PP matrix, suggesting that the interfacial interaction between the FR and PP matrix were much weaker and lack of sufficient adhesion. With the adding of stearic molecules, the particle sizes of the FR fillers obviously decreased and the particles were better distributed, which suggested that the incorporation of stearic anions could promote the compatibility with the PP matrix leading to homogeneous dispersion of FR fillers into PP polymer. Especially, when the x/y mass ratio increased to 1: 1, the FR particles could be evenly distributed in the PP matrix due to the co-intercalation of stearic and MoO42− anions into the interlayer galleries. Apparently, the appearance of the PP/FR composites was also one of the most important parameters in commercial application, and the photographs of the PP/FR specimens are shown in Fig. S2. There were different colors in the specimens of the PP/FR composites, where the neat PP was transparent, the PP/LDH bright gray, the PP/S-LDH ivory, and Mo-LDH pale brown. For the stearic-intercalated MgAl-LDH, S4Mo1-LDH and S7Mo3-LDH exhibited an ivory green, while the S3Mo7LDH and S1Mo4-LDH as MoO42−-intercalated MgAl-LDH as well as the S1Mo1-LDH as stearic-MoO42−-co-intercalated LDH showed a light fawn. At the same time, it was clearly that there were some agglomerations remained in the PP/LDH and PP/Mo-LDH specimens, but the other FR fillers had a more homogeneous aspect and no agglomeration was found. 3.3. Combustion behavior and mechanical properties of the PP/FR composites The effect of the FR on the thermal stability of PP polymer was 7

Applied Clay Science 187 (2020) 105481

L. Jin, et al.

Fig. 6. SEM images of the PP/FR composites (white arrow pointing to the FR particles).

S/Mo

80

PP/S1Mo1-LDH PP/S3Mo7-LDH

Temperature ( C)

375

T0.1

Mass loss (%)

10 g·100 g−1), and then the ε decreased above the loading (10–25 g·100 g−1). It was the same as the ε, the PP/S-LDH and PP/ S1Mo1-LDH composites exhibited a weaker effect on the PP-composite compared with the PP/LDH and PP/Mo-LDH composites. This was because the interfacial adhesion between the FR filler and PP was not strong enough to stand up to large mechanical forces. The results showed that the little effect of the S1Mo1-LDH filler on the mechanical properties of the PP-composite was due to high compatibility between FR and PP polymer. Based on the results presented above, it was confirmed that the stearic and MoO42− anions intercalated into the interlayer galleries of the LDH host layers could effectively improve the fireretardant properties of the LDH filler, and the S1Mo1-LDH was very efficient as a flame retardant for PP polymer.

380

100

365 360 355

PP/S1Mo4-LDH

60

370

350

4/1

7/3

1/1

3/7

1/4

S/Mo

PP/S7Mo3-LDH

T0.5

PP/S4Mo1-LDH

40

PP/S-LDH PP/Mo-LDH

20

PP/LDH PP

0 150

200

250

300

350

Temperature ( C)

400

450

4. Conclusions

500

In this study, stearic-MoO42−-co-intercalated LDH was successfully prepared by adjusting the mass ratios (x/y) of stearic acid to (NH4)2MoO4 via calcination-reconstruction method. Based on XRD, FTIR, SEM/EDS and TG-DTG analyses, the both stearic and MoO42− anions could be inserted into the interlayer galleries of the LDH in single or combining form, resulting in new intercalated SxMoy-LDH materials. After incorporating the SxMoy-LDH fillers into PP matrix, the thermostability and flame retardancy of the PP-composites was improved significantly. In particular, the stearic-MoO42−-co-intercalated LDH (S1Mo1-LDH) showed the highest fire-retardant properties towards PP polymer at 20 g∙100 g−1 loading of FR filler in PP, which not only had a high LOI value of 28.2% and UL-94 V-0 rating, but also had low PHRR (335.1 kW∙m−2) and high PHRR reduction 72.7% relative to pure PP

Fig. 7. TG curves of the neat PP and PP/FR composites.

break (ε) and tensile strength (σT) were evaluated, and the results are reported in Fig. 9. Fig. 9A shows the elongation at break (ε) property of the PP/LDH, PP/S-LDH, PP/S1Mo1-LDH and PP/Mo-LDH composites at different loading of FR filler. The σT was decreased with increasing the loading of the FR filler, and the σT values of the PP/S-LDH and PP/ S1Mo1-LDH composites were obviously higher than those of the PP/ LDH and PP/Mo-LDH composites. As seen in Fig. 9B, it was found that no obvious effect on the ε of the PP-composites at low loading of the FR filler (≤ 8

Applied Clay Science 187 (2020) 105481

L. Jin, et al.

Table 1 Flammability and Mechanical Properties of the PP/FR composites at 20 g∙100 g−1 loading of FR filler. Thermal stability

PP PP/LDH PP/Mo-LDH PP/S-LDH PP/S4Mo1-LDH PP/S1Mo4-LDH PP/S7Mo3-LDH PP/S3Mo7-LDH PP/S1Mo1-LDH

Combustion

CC test

T0.1 (°C)

T0.5 (°C)

Residues at 500 °C(%)

UL94

LOI (%)

PHRR (KW·m

283.6 300.5 288.1 296.3 298.1 289.0 299.2 314.2 313.6

338.9 349.6 352.4 356.1 361.2 364.3 361.7 366.1 370.2

1.3 2.7 3.1 2.9 3.2 4.7 5.1 8.5 9.3

– V-2 V-2 V-1 V-1 V-1 V-1 V-0 V-0

17.6 22.3 25.7 26.2 26.4 26.5 26.9 27.4 28.2

1228.8 779.2 486.4 477.4 451.6 467.3 434.1 420.6 335.1

143.2 122.7 108.7 104.3 97.3 103.3 92.8 87.4 73.4

)

Reduction (%)

σT (MPa)

ε (%)

NA 36.5 60.4 61.1 63.2 61.9 64.6 65.7 72.7

23.9 21.9 22.2 22.6 – – – – 22.7

37.7 14.8 15.1 16.4 – – – – 16.7

A

35

PP/LDH PP/Mo-LDH

1000

PP/S3Mo7-LDH

PP/S-LDH PP/S1Mo1-LDH

30

PP/LDH PP/Mo-LDH

PP/S1Mo1-LDH

800

(%)

2

THR (MJ·m

PP

1200

HRR (kW·m )

)

−2

40

1400

PP/S7Mo3-LDH

600

25

PP/S1Mo4-LDH

20

PP/S4Mo1-LDH

400

PP/S-LDH

15

200 0

Mechanical properties −2

0

100

200

300

400

10

500

24.0

Time (s) Fig. 8. Curves of the HRR versus time for the neat PP and PP/FR composites.

0

5

10

15

20 B

23.5

(1228.8 kW∙m−2) value. It was clear that the S1Mo1-LDH was a very promising candidate for the protection and functionalization of polymers.

25

T

(MPa)

23.0

Declaration of Competing Interest We would like to submit the enclosed manuscript entitled “Intercalation of organic and inorganic anions into layered double hydroxides for Polymer Flame Retardancy”, which we wish to be considered for publication in “Applied Clay Science”. No conflict of interest exists in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

22.5 22.0

PP/S-LDH PP/S1Mo1-LDH

21.5

PP/LDH PP/Mo-LDH

21.0 0

5

10

15

20

1

25

Loaing of the FR filler (g 100g ) Fig. 9. Elongation at break (ε) and tensile strength (σT) of the PP/FR composites at different loading of FR filler.

References Acknowledgments Ardanuy, M., Velasco, J., 2011. Mg-Al Layered double hydroxide nanoparticles: Evaluation of the thermal stability in polypropylene matrix. Appl. Clay Sci. 51, 341–347. Bujdosó, T., Patzkó, Á., Galbács, Z., Dékány, I., 2009. Structural characterization of arsenate ion exchanged MgAl-layered double hydroxide. Appl. Clay Sci. 44, 75–82. Carriazo, D., Domingo, C., Martin, C., Rives, V., 2006. Structural and texture evolution with temperature of layered double hydroxides intercalated with paramolybdate anions. Inorg. Chem. 45, 1243–1251. Chen, W., Qu, B., 2003. Structural characteristics and thermal properties of PE-g-MA/ MgAl-LDH exfoliation nanocomposites synthesized by solution intercalation. Chem. Mater. 15, 3208–3213. Du, L., Qu, B., Meng, Y., Zhu, Q., 2006. Structural characterization and thermal and mechanical properties of poly propylene carbonate/MgAl-LDH exfoliation

This work was financially supported by Key Research and Development Program of Hunan Province, China (2018SK20110) and Natural Science Foundation of Hunan Province, China (2019JJ50613).

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2020.105481. 9

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