dipentaerythritol system

Materials and Design 181 (2019) 107913

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Synergistic barrier effect of aluminum phosphate on flame retardant polypropylene based on ammonium polyphosphate/ dipentaerythritol system Zhaolu Qin a, Rongjie Yang a,⁎, Wenchao Zhang a, Dinghua Li a, Qingjie Jiao b a b

National Engineering Research Center of Flame Retardant Materials, School of Materials, Beijing Institute of Technology, Beijing 100081, China School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Obvious improvement in flame retardancy was observed by the partial replacement of APP with AlPO4 in FRPP composites. • The evolution of ammonia and volatile P2O5 in APP was delayed by the crystalline transformation process of AlPO4. • The formation of crystallized aluminum metaphosphate on the surface of the char residue cotribute to the formation of a strong intumescent char for FR-PP during combustion.

a r t i c l e

i n f o

Article history: Received 25 March 2019 Received in revised form 31 May 2019 Accepted 3 June 2019 Available online 04 June 2019 Keywords: Intumescent flame-retardant polypropylene Aluminum phosphate Ammonium polyphosphate Char formation behavior

a b s t r a c t The synergistic barrier effect of aluminum phosphate (AlPO4) on the char-formation behavior of flame-retarded polypropylene (FR-PP) based on the ammonium polyphosphate (APP)/dipentaerythritol (DPER) system was investigated. The flame-retardant property and combustion behavior of FR-PP was studied by means of the limiting oxygen index, UL 94 vertical burning and cone-calorimeter tests. Results indicate that the partial replacement of APP with AlPO4 in the FR-PP composites showed significant synergistic effects during combustion. The interactions between AlPO4 and APP were examined by means of thermogravimetry and thermogravimetryFourier transform infrared (FTIR) analyses. FTIR spectra and X-ray diffraction spectra were obtained to detect residues of APP/AlPO4 mixtures at different temperatures. Scanning-electron microscopy and energy dispersive spectrometry was used to analyze the condensed-phase residues from cone-calorimeter tests. Scanningelectron microscopy showed that the crystallized aluminum metaphosphate that was derived from the crystal transition of AlPO4 sealed the char residue surface and contributed to the formation of a more uniform and compact char. Thus, surface char with excellent barrier properties will protect the underlying polymer matrix from further degradation and combustion. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

⁎ Corresponding author. E-mail address: [email protected] (R. Yang).

https://doi.org/10.1016/j.matdes.2019.107913 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Z. Qin et al. / Materials and Design 181 (2019) 107913

1. Introduction

Table 1 The formulation of FR-PP composites (wt%).

Halogen-free fire-retardant additives have been used extensively in modified polypropylene (PP) products for the past few years. Among these additives, intumescent flame retardant (IFR), provides a surfaceprotective coating during heating and insulates polymer materials from fire [1–4]. Additives, such as IFR, act with a condensed-phase mechanism that depends on the formation of a carbonaceous expanded layer. Therefore, an effective covering layer with a high temperature resistance and a structural compactness can obstruct the heat and mass transfer between the flame and the polymer during combustion, which leads to flame extinguishment [5–7]. Compared with traditional halogen flame retardants that releasing toxic gas and acidic fumes during combustion, the advantages of IFR as an additive result from it being environmentally benign and having a good dripping resistance. However, IFR has a lower flame-retardant efficiency compared with that of halogen-containing flame retardants. To deal with the shortcomings of the IFR, many individuals and groups have researched the synergistic effect between other additives and IFR [8–10]. Among them, metallic compounds, such as CeO2, ZnCl2, LaMnO3, La2O3, zinc hydroxystannate, aluminum hypophosphite, aluminum hydroxide and other transition-metal phosphide nanocrystallines show an excellent application perspective for improving the flame-retardant efficiency of IFR polymer materials [11–18]. Most investigations have shown that the incorporation of metallic compounds into IFR have led to a significant improvement in char formation of the PP composites. Because of its excellent high-temperature performance, aluminum phosphate (AlPO4) has been used extensively in the field of building materials, fireproof coatings and refractory materials. AlPO4 can be used as binders and matrices in ceramics, and may also be a promising, oxidation-resistant alternative material for coating fibers used in ceramic-matrix composites [19–21]. Many synthetic forms and crystal structures of AlPO4 are known. Among these crystal structures, berlinite (B-AlPO4) has a structure like silica with Si replaced by Al and P to form the AlO4 and PO4 tetrahedron alternate. Like quartz, phase transformation of AlPO4 occurs during heating [22,23], as shown in Scheme 1. In our previous work, synergistic effect in the condensed phase of aluminum hydroxide on IFR-PP has been discussed [17]. It turns out that aluminum hydroxide reacted with APP and generated aluminum metaphosphate, which improved the density and isolation effect of the char residue of IFR-PP. According to the strategies of Castrovinci, the formation of a thermally stable ceramic material can act as a shield to heat and mass transfer between the polymer and the flame [24]. More recently, Elbasuney investigated the synergism between aluminum hydroxide (ATH) and IFR. Results indicated that synergism between ATH and IFR is correlated to the catalytic reaction between Al2O3 resulted from ATH decomposition and phosphoric acid resulted from IFR decomposition to form AlPO4 which could add much strength the formed char barrier [25]. Considering the application of AlPO4 in ceramic-matrix composites, we thought it would be necessary to study the effect of AlPO4 on intumescent flame retardant system. Here, a partial replacement of APP with AlPO4 in FR-PP composites, which based on ammonium polyphosphate (APP)/dipentaerythritol (DPER) system was studied, with the aim to explore the synergistic barrier effect of AlPO4 on the flame-retardant efficiency and char-formation behavior of FR-PP by LOI, UL 94 vertical burning and cone-calorimeter tests. The interactions between AlPO4 and APP were examined by thermogravimetry (TG) and TG-Fourier transform infrared (TG-FTIR) analysis. FTIR spectra and X-ray diffraction (XRD) spectra were obtained to detect the residues of an APP/AlPO4 mixture at different

Sample

PP

APP

DPER

AlPO4

1010

168

FR-PP-1 FR-PP-2 FR-PP-3 FR-PP-4 FR-PP-5

74.7 74.7 74.7 74.7 74.7

17.8 16.8 15.8 14.8 –

7.2 7.2 7.2 7.2 7.2

– 1 2 3 16.8

0.1 0.1 0.1 0.1 0.1

0.2 0.2 0.2 0.2 0.2

temperatures. Scanning-electron microscopy (SEM) and SEM-energydispersive spectroscopy (EDS) were used to investigate the condensed-phase residues of FR-PP from cone-calorimeter tests. 2. Experimental 2.1. Materials Ammonium polyphosphate (APP) particles with a mean size of 20 μm were from Hangzhou JLS Flame Retardants Chemical Co., Ltd. AlPO4 (amorphous, Al2O3 ≥ 54%) with a mean size of 30–50 μm was purchased from Beijing Tongguang Fine Chemicals Company. Dipentaerythritol (DPER) was provided by Shandong Pulisi Chemical Co., Ltd. Polypropylene (PP, S1003, melt flow rate = 3.2 g/10 min) was from Yanshan Petroleum Chemical Company. Antioxidant 1010 and antioxidant 168 were supplied by Ciba Specialty Chemicals Company. 2.2. Preparation of FR-PP composites The flame-retardant PP composites were prepared by using a twinscrew extruder (SJ-20). The processing temperature was 180 °C and the screw speed was 20 rpm. Samples for testing were prepared by using an injection-molding machine (HTF90X1) at 190 °C. The formulation of flame-retardant PP composites is given in Table 1. 2.3. Measurements Cone-calorimeter tests were conducted by using a fire-testingtechnology apparatus (FTT 0007) according to ISO 5660 protocol (50 kW/m2). Results from the cone calorimetry were an average of three measurements. The specimen size and shape for measurement was 100 mm × 100 mm × 3 mm. The limiting oxygen index (LOI) was measured by an oxygen-index instrument (Rheometric Scientific Ltd., British) in terms of ASTM D 2863. The specimen size and shape for measurement was 118 mm × 6.5 mm × 3 mm. A vertical burning test was performed according to the UL-94 standard. The dimensions of each sample were 125 mm × 12.5 mm × 3.2 mm. FTIR spectra were recorded by using a Nicolet 6700 Fourier infrared spectrometer. The spectra were collected at 32 scans from 400 to 4000 cm−1 with a spectral resolution of 4 cm−1. Thermogravimetric analysis (TGA) was carried out at 10 °C/min in a N2 atmosphere from 40 °C to 800 °C by using a Netzsch 209 F1 thermal analyzer. TGA coupled with TGA-FTIR was used to detect the gas species in the sample. Measurements were carried out at a N2 flow rate of 60 mL/min and a heating rate of 20 °C/min from 40 °C to 800 °C. XRD analysis was carried out over a 2θ range of 2–55° with an X'Pert PRO MPD diffractometer system (PANalytical, Tokyo, Japan). Cu–Kα1 radiation (λ = 0.154 nm) was used with a copper target.

Scheme 1. Phase transformation of AlPO4

Z. Qin et al. / Materials and Design 181 (2019) 107913

The structure and morphology of the char residue was characterized by S4800 field-emission–SEM. SEM-EDS was used to detect the elemental composition for the microstructural surface of the condensed-phase residues. Mechanical properties of FR-PP composites were measured according to GB/T 1040-2006 (equivalent to ISO 527-2012) with an electronic tensile testing machine (DXLL-5000) at a tensile rate of 20 mm/min. The dimensions of the dumbbell specimens were the same as those specified in GB/T 1040.2-2006. The samples were preconditioned at 23 °C and 50% RH for 24 h, and the results are the average of five measurements. 3. Results and discussion 3.1. Combustion behavior

FR-PP-1 FR-PP-2 FR-PP-3 FR-PP-4 FR-PP-5

2

HRR (kW/m )

Fig. 1 shows the heat release rate (HRR) curves of the FR-PP composites with different AlPO4 additions. Two significant peaks were visible in the HRR curves for all FR-PP samples, which show a typical combustion behavior of intumescent flame-retardant materials. The first peak is attributed to sample ignition and the formation of an intumescent char [26]. At this stage, the underlying polymer matrix is protected by the intumescent protecting shield from external thermal radiation. The time to the first peak is slightly earlier for samples with AlPO4, but the first peak is lower. The second peak is assigned to further destruction of the external protecting shield and the formation of a carbonaceous residue. For FRPP-1, the HRR curve exhibits a sharp peak at ~280 s, which implies rapid destruction of the external protecting shield. However, for samples with AlPO4, the time to the second peak is 330 s (FR-PP-2), 400 s (FR-PP-3) and 390 s (FR-PP-4), which shows that the external protecting shield of the samples with AlPO4 provides a longer protection time for the underlying polymer matrix from external thermal radiation during combustion. The incorporation of AlPO4 in FR-PP composites enhanced the protection efficiency of the surface intumescent shield. The second peak decreased significantly with an increase in concentration of AlPO4 in the FR-PP, which shows an improvement in surfaceprotection efficiency for FR-PP composites. Surprisingly, for FR-PP-5, when APP is fully replaced by AlPO4, the HRR curve of FR-PP-5 shows no features of intumescent flame-retardant materials. This can be explained by the fact that the crosslinking between APP and DPER for FR-PP-5 is interrupted and no carbonaceous expanded layer is produced during combustion. Detailed experimental data from cone calorimetry are summarized in Table 2. The TTI (time to ignition) of FR-PP composites is reduced after the incorporation of AlPO4. A shorter TTI is attributed to the partial

280s 330s 400s

390s

Time(s) Fig. 1. HRR curves of FR-PP composites.

3

Table 2 Cone calorimeter parameters of FR-PP composites. Samples

TTI (s)

PHRR (Kw/m2)

avCO (kg/kg)

avCO2 (kg/kg)

THR (MJ/m2)

EHC (MJ/Kg)

Residue (%)

FR-PP-1 FR-PP-2 FR-PP-3 FR-PP-4 FR-PP-5

30 ± 2 28 ± 2 27 ± 2 27 ± 2 26 ± 2

396 318 275 262 831

0.073 0.072 0.071 0.071 0.069

2.56 2.48 2.45 2.47 2.43

104 95 92 97 112

44.46 43.81 43.42 43.01 44.48

14.07 15.33 17.16 22.19 14.68

replacement of APP with AlPO4. For PHRR and THR, the partial replacement of APP with AlPO4 in the FR-PP composites shows significant synergistic effect. For example, at 1 wt% concentration, AlPO4 (FR-PP-2) reduces the PHRR by 19.7% and the THR by 8.6%. The increase in concentration of AlPO4 from 1 wt% (FR-PP-2) to 3 wt% (FR-PP-4) decreased the PHRR further. The slight decrease on the release of CO and CO2 can be explained by the decrease in decomposition rate and higher char residue for FR-PP composites. Moreover, the EHC (effective heat of combustion) of FR-PP composites is also reduced except for FR-PP-5. The theory behind this result is that a more effective surface protective shield inhibited the oxidation and degradation of the underlying polymer matrix, and reduced the release of combustible products. 3.2. Flame retardancy of FR-PP composites LOI and UL 94 test results for the FR-PP composites are given in Table 3. When the loading amount of flame retardant was kept at 25 wt%, the replacement of 2 wt% APP with AlPO4 (FR-PP-3) could increase the LOI from 26.4% to 28.6%. Results from the UL 94 test show that the sample without AlPO4 (FR-PP-1) can only reach the V-1 level, whereas the sample with AlPO4 loadings of 2 wt% could achieve a V-0 rating in the UL 94 test. Samples were weighed after the UL 94 test to calculate the mass loss during combustion. Table 3 shows that the mass loss is smaller after AlPO4 incorporation into the FR-PP composites. The smallest weight loss was detected for samples that contained 2 wt% AlPO4 (FR-PP-3). As for the results in Fig. 2, the larger black areas of FR-PP-1 indicate a longer burning time in the UL 94 test. For FR-PP-3, the smaller black area implies a shorter burning time and a faster self-extinguish after second ignition. 3.3. TG and TG-FTIR analysis TGA on the APP, AlPO4 and APP/AlPO4 mixtures (mass ratio 1:1) was performed to analyze their interaction during heating, which could affect the combustion behavior of FR-PP composites. Fig. 3 shows the TG and DTG curves of the APP, AlPO4 and APP/AlPO4 mixture. The APP thermal-degradation progress is divided into two steps. The first step at 260–420 °C with ~20 wt% mass loss occurs because of ammonia and water evolution from APP to form a highly cross-linked polyphosphate acid [27–29]. The second step at 500–700 °C with 55 wt% mass loss occurs because of further decomposition of the polyphosphate network, which leads to the formation of volatile P2O5 and P4O10 above 500 °C [30,31]. For AlPO4, minor mass loss was detected at 250 °C, as indicated by the DTG curve, which can be explained by the elimination of water or impurities. Table 3 LOI value and UL 94 results of FR-PP composites. Samples

LOI (%)

UL 94 (3.2 mm)

t1 (s)

t2 (s)

FR-PP-1 FR-PP-2 FR-PP-3 FR-PP-4 FR-PP-5

26.4 27.9 28.6 28.5 24.1

V-1 V-1 V-0 V-1 NR

0.9 0.8 0.7 0.9 ~

8.7 7.5 5.3 15.7 ~

Flame to clamp

Dripping

Cotton ignited

Mass loss (%)

No No No No Y

Yes No No No Y

Yes No No No Y

13.7 8.8 3.9 11.2 0

4

Z. Qin et al. / Materials and Design 181 (2019) 107913

DTG/(%/min)

Fig. 2. Pictures of the residues of FR-PP composites after UL-94 test.

APP AlPO4 APP/AlPO4Experimental APP/AlPO4Caculated

APP AlPO4

[E-C]

APP/AlPO4Experimental

(a)

(b)

APP/AlPO4Caculated

Temperature

Temperature

Fig. 3. TG (a) and DTG (b) curve of APP, AlPO4 and APP/AlPO4 in N2 atmosphere.

For the APP/AlPO4 mixture, as is shown in Fig. 3(a), little difference results in experimental mass loss and the calculated TG curve between 300 and 500 °C. However, as the temperature continues to increase, the changing trends for the experimental and calculated TG curves of the APP/AlPO4 mixture are opposite, even when the experimental error is considered. The mass loss of the experimental TG curve is lower than that for the calculated TG curve. Therefore, AlPO4 affects the second degradation of APP. TG coupled with FTIR was used to detect gaseous products in the APP/AlPO4 mixture. Fig. 4 shows the ammonia and volatile P_O release of the APP and APP/AlPO4 mixture in a N2 atmosphere. As shown in

Fig. 4, ammonia evolution for the APP/AlPO4 (413 °C) mixture occurs at a higher temperature than for the APP (354 °C), which means that the first degradation stage of the APP is suppressed slightly by AlPO4. The maximum evolution of volatile P2O5 for APP appears at 650 °C. Conversely, the evolution of volatile P2O5 for the APP/AlPO4 mixture is delayed and the maximum evolution temperature is higher than 800 °C. It can be concluded that the degradation of APP in the APP/AlPO4 mixture is slowed significantly, which could be explained by the inhibiting effect of the endothermic and volumetric expansion during the crystalline transformation of AlPO4.

354

APP APP/AlPO4

APP APP/AlPO4

413 -1

-1

1279cm P=O

960cm NH3

100

Temperature

200

300

400 500 Temperature( )

Fig. 4. TG-FTIR with ammonia and volatile P2O5 of APP and APP/AlPO4 mixture in N2 atmosphere.

600

700

800

Z. Qin et al. / Materials and Design 181 (2019) 107913

APP 25 AlPO4 25 APP/AlPO4 25 APP 800 AlPO4800

APP/AlPO4 800

-1

Wavenumbers(cm ) Fig. 5. FTIR spectra of APP, AlPO4 and APP/AlPO4 mixture after heat treatment at 25 °C and 800 °C.

FR-PP-1

FR-PP-3

-1

1308 cm

-1

1274 cm

-1

-1 1137 cm 731 cm

4000

3500

3000

2500 2000 1500 -1 Wavenumbers(cm )

1000

500

5

heat treatment at 800 °C, no significant absorbance peaks can be found, which indicates the full degradation of APP. For AlPO4, the variation of FTIR spectra means that the crystal form of AlPO4 is transformed from the amorphous phase to aluminum metaphosphate [32]. For the APP/AlPO4 mixture, absorption peaks at 1430 cm−1 and 850 cm−1 mean that some of the undecomposed APP still exists in the APP/ AlPO4 mixture after heat treatment at 800 °C. Therefore, the degradation of APP is delayed by the crystalline transformation process of AlPO4. Furthermore, the chemical structure of the residue of FR-PP composites from cone calorimeter test was also investigated by FTIR spectra, as shown in Fig. 6. Comparing the two spectrums, the main difference is the existence of a band at 1308 cm−1, 1274 cm−1, 1107 cm−1 and 931 cm−1, reflecting the characteristic absorption band of crystalline aluminum metaphosphate. The test results indicate that the surface enrichment of crystalline aluminum metaphosphate in the char residue of FR-PP-3. The crystalline aluminum metaphosphate can help to form an insulating barrier to prevent the heat transfer between the flame zone and the underlying polymer matrix and protect the substrate from heat and fire. The XRD analysis of the APP, AlPO4 and APP/AlPO4 mixtures at different temperatures is presented in Fig. 7. In agreement with the result FTIR, the XRD analysis carried out on APP at 25 °C and 800 °C indicates the formation of a vitreous crosslinked ultraphosphate (Fig. 7b) from linear crystalline APP (Fig. 7a). The XRD analysis that was carried out on AlPO4 at 25 °C and 800 °C indicates a transformation of the amorphous phase (Fig. 7a) into a crystalline aluminum metaphosphate (Fig. 7b). The XRD analysis that was carried out on the APP/AlPO4 mixture at 800 °C shows that the evolution of volatile P2O5 in APP may be hindered, which occurs because of the effect of crystal transformation of the AlPO4. Fig. 8 shows the XRD patterns of the residue of the FR-PP composites from cone calorimeter test. As can be seen from the figure, for FR-PP-3, char residue of sample appears the characteristic diffraction peaks of crystalline aluminum metaphosphate which indicating the crystalline transformation of amorphous AlPO4 during combustion, and thus improve the effective expansion of the char structure to increase flame retardant properties. 3.5. Morphological analysis of char residue of FR-PP composites

Fig. 6. FTIR spectra of the residue of FR-PP composites from cone calorimeter test.

3.4. FTIR and XRD analysis The FTIR spectra of the APP, AlPO4 and APP/AlPO4 mixtures after heat treatment at 25 and 800 °C are shown in Fig. 5. For APP, after

Digital photographs of the char residue of the FR-PP composites from cone calorimeter are shown in Fig. 9. The surface of the char residue of the FR-PP-3 composite (Fig. 9d) is complete and uniform. For a sample without AlPO4 (Fig. 9a), many gaps are distributed on the char-residue surface. A multilayered and rigid intumescent char

25

(a)

800 Al(PO3)3 [PDF 13-0430]

(b)

APP

[PDF 44-0739]

cps

cps

AlPO4

APP APP/AlPO4

AlPO4 5

10

15

20

25

30

35

2 theta

40

45

50

APP 55

10

15

20

25

30

35

40

45

2 theta

Fig. 7. XRD of APP, AlPO4 and APP/AlPO4 mixture: (a) diffraction spectrum at 25 °C; (b) diffraction spectrum after heating to 800 °C.

50

55

6

Z. Qin et al. / Materials and Design 181 (2019) 107913

CPS

Al(PO3)3 [PDF 13-0430]

FR-PP-3 can inhibit the release of flammable gases during combustion more effectively. In conclusion, the addition of AlPO4 contributes to form a more effective protective shield for FR-PP composites during combustion. The formation of a compact and uniform char residue can slow down the release of flammable gas, prevent heat transfer and provide a longer protection time for the underlying polymer matrix from further degradation. 3.6. Mechanical properties

FR-PP-3 FR-PP-1 10

15

20

25

30 35 2 theta

40

45

50

55

Fig. 8. XRD spectra of the residue of FR-PP composites from cone calorimeter test.

(Fig. 9e) was achieved after AlPO4 incorporation into the FR-PP composites. From the side, the height of the char residue increased apparently (Fig. 9c and f). The char-formation behavior of the FR-PP composites was improved significantly by AlPO4 incorporation. Fig. 10 presents SEM micrographs of different magnification on the outside and inside surface of the char residue, which were derived from the cone-calorimeter test of the FR-PP composites. For the inside surface, numerous holes for the char residue exist, as left by FR-PP-1 (Fig. 10c), which means an easy permeation of flammable gas during combustion. For the outside surface, the residue (Fig. 10b) that was left by FR-PP-3 was more compact and continuous compared with that of FR-PP-1 (Fig. 10a). Therefore, the char residue that was left by

Effects of AlPO4 on the mechanical properties of FR-PP composites are illustrated in Fig. 11(a). The representative stress-strain curves of PP control and FR-PP composites are illustrated in Fig. 11(b). It is wellknown that the presence of flame retardant additive yields a significantly decrease in the mechanical properties of polymers. The mechanical properties of the FR-PP composites are dramatically worsened due to the poor compatibility of flame retardant filler with the PP matrix. For FR-PP-5, when APP is fully replaced by AlPO4, worse mechanical properties are attained. The tensile strength and the elongation at break of the FR-PP composites both decreased with increasing loading of AlPO4 in FR-PP composites. This can be explained by the worse compatibility between AlPO4 and PP matrix. 3.7. SEM-EDS analysis of char residue of FR-PP composites Figs. 12 and 13 present the SEM-EDS results of the outer and inner surface of the char residue for the FR-PP composites. Elemental compositions of the outside and inside surface of the char residue for the FR-PP composites are shown in Table 4. For the FR-PP-3, Al was detected on the outer (2.7%) and inner (0.9%) surface of the char residue. The Al content of the outer surface was higher than the inner surface, which means that most crystalline aluminum metaphosphate was concentrated on the outer surface of the char residue.

Fig. 9. Digital photographs of the char residue of FR-PP composites from cone calorimeter test.

Z. Qin et al. / Materials and Design 181 (2019) 107913

7

Fig. 10. SEM micrographs of outside (×5000) and inside surface of the char (×300).

The mechanism for the synergistic barrier effect of AlPO4 on FRPP composites is summarized in Fig. 14. APP first degrades with the losses of water and ammonia, generating polyphosphoric acid. Then polyphosphoric acid crosslinks with the DPER to yield intumescent char. In this process, the decomposition rate of partial APP is delayed. Meanwhile, the aluminum metaphosphate is formed by the

crystal transformation of AlPO4. Because the melting point of AlPO4 is as high as 1500 °C, which is much higher than the fire temperature of the materials, aluminum metaphosphate exists as a solid powder during combustion. The content of aluminum metaphosphate increases gradually due to thermo-oxidation of the sample surface. Besides, the higher surface tension and bubble effects may also lead to

30

FR-PP-1

25

FR-PP-4

FR-PP-3

FR-PP-5

20

Stress (MPa)

Elongation at Break (%)

Tensile Strength (Mpa)

PP Control FR-PP-2

(b)

(a)

15

PP Control FR-PP-1 FR-PP-2 FR-PP-3 FR-PP-4 FR-PP-5

10 5 0 0

10

20

Fig. 11. Mechanical properties (a) and stress-strain curves (b) of FR-PP composites.

30

Strain (%)

40

50

60

8

Z. Qin et al. / Materials and Design 181 (2019) 107913

Fig. 12. SEM-EDX results for the exterior surface of the char residue.

Fig. 13. SEM-EDX results for the interior surface of the char residue.

Table 4 Elemental compositions of FR-PP char from cone calorimeter test. Elemental composition FR-PP-1 (wt/%) C N Exterior surface Interior surface

FR-PP-3 O

P

54.8 7.2 16.8 21.2 58.2 9.8 14.1 17.9

Al C 0 0

N

O

P

Al

41.3 8.2 15.5 32.3 2.7 32.2 10.1 19.7 37.2 0.9

the migration of aluminum metaphosphate to the surface of the samples. Thus, the surface of residue is sealed by aluminum metaphosphate. The compactness and uniformity of the char residue was improved by the crystalline aluminum metaphosphate. Consequently, the isolation effect of the char residue was increased and the transfer of oxygen, flammable gas and heat during combustion was prevented.

Z. Qin et al. / Materials and Design 181 (2019) 107913

9

Fig. 14. Proposed mechanism for the AlPO4 sealed the char surface during combustion.

4. Conclusions We studied the effects of partial replacement of APP with AlPO4 in FR-PP composites based on an APP/DPER system. Significant improvement in the flame-retardant efficiency was observed. At a 1 wt% concentration AlPO4 (FR-PP-2), the PHRR was reduced by 19.7% and the THR decreased by 8.6%. An increase in the AlPO4 concentration from 1 wt% (FR-PP-2) to 3 wt% (FR-PP-4) decreased the PHRR. Studies on the interactions between APP and AlPO4 showed that the degradation of APP was suppressed by AlPO4. SEM and EDS results gave positive evidence that the formation of crystallized aluminum metaphosphate showed good penetration into the char-residue surface and filled structural defects, such as pores, cracks and gaps, which lead to the formation of a high-efficiency protecting shield for FR-PP during combustion. CRediT authorship contribution statement Zhaolu Qin: Methodology, Conceptualization; Data curation; Formal analysis; Investigation, Writing-original draft. Wenchao Zhang: Conceptualization, Methodology, Investigation, Writing - review & editing. Dinghua Li: Methodology, Project administration, Conceptualization, Investigation, Review-editing. Rongjie Yang: Supervision, Investigation, Resources, Funding acquisition. Qingjie Jiao: Investigation, Funding acquisition. Acknowledgements This work was funded by grants from the China Postdoctoral Science Foundation funded project (Grant No. 2018M631353), National Program on Key Research Project (Grant No. 2016YFB0302101) and National Science Foundation for Young Scholars of China (Grant No. 51803008). References [1] M. Lewin, Physical and chemical mechanisms of flame retarding of polymers, Fire Retardancy of Polymers 1998, pp. 3–32. [2] M. Lebras, S. Bourbigot, Fire retarded intumescent thermoplastics formulations, synergy and synergistic agent-a review, Fire Retardancy of Polymers 1998, pp. 64–75. [3] G. Camino, L. Costa, G. Martinasso, Intumescent fire-retardant systems, Polym. Degrad. Stab. 23 (1989) 359–376. [4] A.R. Horrocks, Developments in flame retardants for heat and fir resistant textilesthe role of char formation and intumescence, Polym. Degrad. Stab. 54 (1996) 143–154. [5] M.C. Yew, N.H. Ramli. Sulong, Fire-resistive performance of intumescent flameretardant coatings for steel, Mater. Des. 34 (2012) 719–724.

[6] J.W. Gu, G.C. Zhang, S.L. Dong, Study on preparation and fire-retardant mechanism analysis of intumescent flame-retardant coatings, Surf. Coat. Technol. 201 (2007) 7835–7841. [7] M.M. Souza, S.C. Sa, A.V. Zmozinski, Biomass as the carbon source in intumescent coatings for steel protection against fire, Ind. Eng. Chem. Res. 55 (2016) 11961–11969. [8] M. Lewin, M. Endo, Catalysis of intumescent flame retardancy of polypropylene by metallic compounds, Polym. Adv. Technol. 14 (2003) 3–11. [9] X.C. Chen, Y.P. Ding, T. Tang, Synergistic effect of nickel formate on the thermal and flame-retardant properties of polypropylene, Polym. Int. 54 (2005) 904–908. [10] F. Samyn, S. Bourbigot, S. Dequesne, R. Dolobel, Effect of zinc borate on the thermal degradation of ammonium polyphosphate, Thermochim. Acta 456 (2007) 134–144. [11] C.M. Feng, M.Y. Liang, J.L. Jiang, Synergism effect of CeO2 on the flame retardant performance of intumescent flame retardant polypropylene composites and its mechanism, J. Anal. Appl. Pyrolysis 122 (2016) 405–414. [12] L. Ye, Y.J. Zhang, S.H. Wang, Synergistic effects and mechanism of ZnCl2 on intumescent flame-retardant polypropylene, J. Therm. Anal. Calorim. 115 (2014) 1065–1071. [13] Y. Sheng, Y.H. Chen, Y.Z. Bai, Catalytically synergistic effects of novel LaMnO3 composite metal oxide in intumescent flame-retardant polypropylene system, Polym. Compos. 35 (2014) 2390–2400. [14] C.M. Feng, Y. Zhang, S.W. Liu, Synergistic effect of La2O3 on the flame retardant properties and the degradation mechanism of a novel PP/IFR system, Polym. Degrad. Stab. 97 (2012) 707–714. [15] X.Q. Su, Y.W. Yi, J. Tao, Synergistic effect of zinc hydroxystannate with intumescent flame-retardants on fire retardancy and thermal behavior of polypropylene, Polym. Degrad. Stab. 97 (2012) 2128–2135. [16] M.J. Xu, J. Wang, Y.H. Ding, Synergistic effects of aluminum hypophosphite on intumescent flame retardant polypropylene system, Chin. J. Polym. Sci. 33 (2015) 318–328. [17] Z.L. Qin, D.H. Li, Q. Li, Effect of nano-aluminum hydroxide on mechanical properties, flame retardancy and combustion behavior of intumescent flame retarded polypropylene, Mater. Des. 89 (2016) 988–995. [18] K.Q. Zhou, S.H. Jiang, B.B. Wang, BB. Combined effect of transition metal phosphide (MxPy, M = Ni, Co, and Cu) and intumescent flame retardant system on polypropylene, Polym. Adv. Technol. 25 (2014) 701–710. [19] C.R. Maier, L.E. Jones, The influence of aluminum phosphates on graphite oxidation, Carbon 43 (2005) 2272–2276. [20] S. Stockel, S. Ebert, M. Bottcher, W.A. Goedel, Coating of alumina fibres with aluminium phosphate by a continuous chemical vapour deposition process, Chem. Vap. Depos. 20 (2014) (388-298). [21] Y.F. Li, X.H. Hong, Study on the effects of toughness modification on the properties of glass fibric reinforced phenoic composites, Fiber Reinf. Plast. Compos. 11 (2015) 71–74. [22] D.E.C. Corbridge, Phosphorous 2000: Chemistry, Biochemistry & Technology. New York, vol. 185, 2000. [23] X.P. Wang, S. Tian, The Fifth Pacific Rim International Conference on Advanced Materials and Processing, 1197, Materials Science Forum, Switzerland, 2004 475–479. [24] A. Castrovinci, G. Camino, C. Drevelle, S. Duquesne, C. Magniez, M. Vouters, Ammonium polyphosphate–aluminum trihydroxide antagonism in fire retarded butadiene–styrene block copolymer, Eur. Polym. J. 41 (2005) 2023–2033. [25] S. Elbasuney, Novel multi-component flame retardant system based on nanoscopic aluminium-trihydroxide (ATH), Powder Technol. 305 (2017) 538–545. [26] S. Bourbigot, M. Lebras, S. Duquesne, Recent advances for intumescent polymers, Macromol. Mater. Eng. 289 (2004) 499–511. [27] S.V. Levchik, G. Camino, L. Costa, Mechanism of action of phosphorous-based flame retardants in nylon 6. I: ammonium polyphosphate, Fire Mater. 19 (1995) 1–10.

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Z. Qin et al. / Materials and Design 181 (2019) 107913

[28] G. Camino, L. Costa, L. Trossarelli, Study of the mechanism of intumescence in fire retardant polymer: part I-thermal degradation of ammonium polyphosphatepentaerythritol composites, Polym. Degrad. Stab. 6 (1984) 243–252. [29] G. Camino, L. Costa, L. Trossarelli, Study of the mechanism of intumescence in fire retardant polymer: part V-mechanism of formation of gaseous products in the thermal degradation of ammonium polyphosphate, Polym. Degrad. Stab. 12 (1985) 203–211.

[30] G. Gamino, L. Costa, L. Martinasso, Intumescent fire-retardant systems, Polym. Degrad. Stab. 23 (1989) 359–376. [31] G. Camino, L. Costa, L. Trossarelli, Study of the mechanism of intumescence in fire retardant polymers: part II-mechanism of action in polypropylene-ammonium polyphosphate-pentaerythritol composites, Polym. Degrad. Stab. 7 (1984) 25–31. [32] M. Vippola, S. Ahmaniemi, J. Keranen, et al., Aluminum phosphate sealed alumina coating: characterisation of microstructure, Mater. Sci. Eng. A 323 (2002) 1–8.