The multilayered distribution of intumescent flame retardants and its influence on the fire and mechanical properties of polypropylene

Composites Science and Technology 93 (2014) 54–60

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The multilayered distribution of intumescent flame retardants and its influence on the fire and mechanical properties of polypropylene Baoshu Chen, Wanli Gao, Jiabin Shen ⇑, Shaoyun Guo ⇑ The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

a r t i c l e

i n f o

Article history: Received 2 October 2013 Received in revised form 18 December 2013 Accepted 22 December 2013 Available online 3 January 2014 Keywords: A. Layered structures A. Polymer–matrix composites (PMCs) B. Mechanical properties E. Extrusion Flame-retarding properties

a b s t r a c t The multilayered distribution of intumescent flame retardants (IFRs) was obtained by fabricating a series of alternating multilayered composites consisting of polypropylene (PP) and IFR-filled PP (PPFR) layers through a layer-multiplying coextrusion technology. With increasing the layer numbers, the limiting oxygen index (LOI) and elongation at break (eb) of the multilayered composites were enhanced, accompanied with the gradually decreased values of the peak heat release rate and the total heat release. When the layer numbers beyond 16, the LOI and eb values were even larger than those of the PP/IFR conventional composite filled with the same content of IFRs. The layer interfaces and confined layer spaces in the multilayered system were considered to play a crucial role in retarding the spread of fire as well as the propagation of crazes. The significance of this work was providing an optimal route to fabricate flameretarding composites with excellent mechanical properties. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The addition of intumescent flame retardants (IFRs) has been regarded as one of the most environment-friendly and promising methods to reduce the flammability of polymeric materials. During combustion, IFRs tend to decompose into expanded carbonaceous chars, acting as an insulating barrier to reduce heat and fuel transfer between the heat source and the polymer and limit the diffusion of oxygen into the material. Hence, in order that enough char contents can be generated to inhibit the spread of fire, a large amount of filler loading is generally required which in turn causes deteriorated mechanical properties and processing issues [1,2]. Actually, it is recognized that the content of IFRs satisfying the flame-retarding requirement of the polymeric matrix is dependent on both the chemical structure and the distribution of fillers [3]. Structural modification is regarded as a direct and effective way to enhance the flame-retarding efficiency of IFRs. However, as the IFR type is chosen, the distribution would be a key factor in order to achieve the maximum performance of the whole composite system [4,5]. In most cases, IFRs are randomly distributed in polymeric matrix. A large reduction in flammability would be approached accompanied with the formation of a network-structured protective layer during burning [6]. Hence, there should be a critical distance (DC) between IFR particles. Only when the distance is less than DC, the expanded carbonaceous chars would contact each other acting as a flame-retarding layer. It can be imagined that ⇑ Corresponding authors. Fax: +86 28 85466077. E-mail addresses: [email protected] (J. Shen), [email protected] (S. Guo). 0266-3538/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.12.022

there are two kinds of basic ways for IFR distribution: uniform and multilayered distribution as schematically displayed in Fig. 1. A uniform distribution state is commonly pursued [7–10]. For example, Pack et al. [7] and Song et al. [8] improved the dispersion of IFRs by increasing their compatibility with polymeric matrix. Wang et al. [9] and Isitman et al. [10] introduced inorganic fillers (such as clays, fibers and nanotubes) into an IFR-filled polymeric system for breaking the aggregation of IFRs and making them uniformly dispersed in the matrix. Combustion results displayed that a uniform IFR distribution was indeed beneficial to the formation of even and compact char layers. However, it is recognized that the random distribution of IFRs would also promote the propagation of crazes (or cracks) in the matrix which may result in the deterioration of mechanical toughness. Compared to the uniform distribution, the multilayered distribution of IFRs is less reported and mainly applied in the layer-bylayer deposition system. Laufer et al. [11], Li et al. [12], and Carosio et al. [13] found that the multilayered coating effectively prolonged the time to ignition and reduced the heat release rate of polymeric substrates. However, few studies related to the influence of the multilayered distributing state of IFRs on the flame-retarding properties of the matrix are concerned, to the authors’ best knowledge. Besides, Sung et al. [14,15] studied the mechanical behaviors of alternating multilayered materials consisting of polycarbonate (PC) and styrene–acrylonitrile copolymer (SAN) layers. It was found that the crazes originated from SAN layers could be terminated by shear bands in adjacent PC layers through interfacial adhesion, leading to the increase of the strength and toughness of the whole material. Hence, it is imagined that the multilayered distribution of

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(a)

IFRs may provide one of optimal routes to fabricate flame-retarding composites with excellent mechanical properties. In this work, the multilayered distribution of IFRs was obtained by preparing a series of alternating multilayered composites consisting of pure polypropylene (PP) and IFR-filled polypropylene (PPFR) layers through a layer-multiplying coextrusion technology. The distributing state of IFRs was controlled by adjusting the number of layers or the thickness ratio of PPFR and PP layers. For comparison, the conventional composites with a uniform distribution of IFRs were also prepared by filling with the same content of IFRs as that of the corresponding multilayered system. The effects of the multilayered and uniform distribution of IFRs on the fire and mechanical properties of the PP matrix were compared and investigated schematically. 2. Experimental

(b)

30

The weight fraction of IFRs in the whole system, wt%

Fig. 1. Schematic of the uniform (left) and multilayered (right) distribution of IFRs in the matrix.

The content of IFRs in PPFR layers is maintained at 37.5 wt%

24.1

20.7

20 17.1

10

0 6.5:9.5

8:8

9.5:6.5

The thickness ratio of PPFR and PP layers Fig. 2. (a) Schematic of layer-multiplying coextrusion system: A, B—extruders; C— connector; D—coextrusion block; E—layer multiplying elements; F—die; G—the specimen with an alternating multilayered structure. (b) The corresponding relationship between the content of IFRs in the whole multilayered system and the thickness ratio of PPFR and PP layers.

2.1. Materials PP (T30S), with a density of 0.899 g/cm3 and a melt index of 3 g/ 10 min, was provided by Lan Zhou Petroleum Chemical Company Ltd. (China). Commercial IFRs (6000-1), with a mean size of about 1 lm, were supplied by Dongguan Doher Chemical Company Ltd. (China). 2.2. Specimen preparation Prior to the layer-multiplying coextrusion, PP/IFR pellets (denoted as PPFR) filled with 37.5 wt% IFRs were prepared using a twin-screw extruder. Subsequently, the PPFR/PP alternating multilayered composites were prepared using the layer-multiplying coextrusion system, the mechanism of which was described previously [16]. As illustrated in Fig. 2(a), the dried PPFR pellets and pure PP were extruded from different extruders (A and B) forming a 2-layer melt in the coextrusion block (D) and then went through an assembly of layer-multiplying elements (LMEs, E). The PPFR/PP alternating multilayered composites with 2, 4, 8, 16 and 32 layers were prepared by using 0, 1, 2, 3, and 4 LMEs, respectively. The thickness ratios of PPFR and PP layers (6.5:9.5, 8:8, and 9.5:6.5) were obtained by controlling the extruding rate of each extruder. The corresponding relationship between the content of IFRs in the whole multilayered system and the thickness ratio of PPFR and PP layers is shown in Fig. 2(b). It should be noted that the content of IFRs in PPFR layers was maintained at 37.5 wt%, irrespective of the number of layers and the thickness ratio of the PPFR and PP layers. The number of layers, the overall thickness, the extruding rate and the PP layer thickness of the PPFR/ PP multilayered composites are summarized in Table 1. For comparison, the PP/IFR conventional composites filled with 17.1, 20.7 and 24.1 wt% IFRs were prepared using only one extruder of the layer-multiplying system by applying 4 LMEs. In this paper, these PP/IFR composites were referred as single-layer samples where IFRs were dispersed evenly in PP matrix.

2.3. Characterization The polar light microscope (PLM) observation was performed using a PLM (XSZ-H, Chongqing Photoelectric Instrument Co., China) equipped with a video camera. A thin slice about 10 lm in thickness was obtained by a microtome from each sample along the extrusion direction. All thermogravimetric tests were performed on a thermogravimetric analyzer (TGA, TG-209F1, NETZSCH, Germany). Each sample, with a weight of 5–10 mg, was heated from 30 to 700 °C at a rate of 10 °C/min in air. Fourier transform infrared (FTIR) spectra were taken on a Nicolet-IS10 (Thermo Electron Co., USA) spectrometer from 4000 to 650 cm1 with a 4 cm1 resolution. Absorption bands between 3700 and 800 cm1 were analyzed. The limiting oxygen index (LOI) values were measured by an oxygen index meter (HC-2C, Jiangning Analytical Instrument Factory, China). The dimensions of each specimen are 130  6.5  1.4 mm3. An FTT cone calorimeter was used to evaluate the flammability of samples under an external heat flux of 35 kW/m2, with specimen dimensions of 100  100  2.8 mm3. Tensile tests were performed using an Instron 4302 tension machine (Canton, MA, USA) at a crosshead speed of 20 mm/min in accordance with ASTM D638. At least of five specimens for each sample were tested and the average value was calculated. 3. Result and discussion 3.1. Uniform and multilayered distribution of IFRs The cross-sectional morphologies of PPFR/PP multilayered and PP/IFR conventional composites were observed through PLM. Micrographs in Fig. 3(a–c) display the multilayered distribution

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Table 1 The summarization of the number of layers, the overall thickness, the extruding rate and the PP layer thickness of the PPFR/PP multilayered system. Layer numbers

Overall thickness (mm)

PPFR/PP = 6.5/9.5 Extruding rate (r/min)

PP layer thickness (lm)

Extruding rate (r/min)

PP layer thickness (lm)

Extruding rate (r/min)

PP layer thickness (lm)

2 4 8 16 32

1.4

PPFR extruder 260 PP extruder: 380

861 401 208 104 51

PPFR extruder 320 PP extruder: 320

725 338 175 33 43

PPFR extruder 380 PP extruder: 260

589 274 142 71 35

PPFR/PP = 8/8

of IFRs in 2-, 8- and 32-layer composites. The alternating dark and bright layers correspond to the IFR-filled PP and pure PP layers, respectively. With increasing the number of layers, there are more interfaces between PPFR and PP layers. For comparison, the micrograph of PP/IFR conventional composite is shown in Fig. 3(d). The dark visualization indicates that IFR fillers are uniformly dispersed in the matrix covering the PP crystallites.

3.2.1. LOI tests LOI test is one of the most useful tools to quantitatively evaluate whether the materials are flame-retarded in air. Generally, if the LOI value of an established plastic is less than 22, it should be associated to an inflammable material, and higher than 27 means flame-retardant; while the plastic with an LOI value between 22 and 27 belongs to flammable material [17]. Fig. 4 shows the effect of the number of layers on the LOI values of PPFR/PP multilayered composites. There are three different thickness ratios of PPFR and PP layers and the content of IFRs in PPFR layers is maintained at 37.5 wt%. When the PP and PPFR layers are combined into a 2-layer system, the LOI value is less than 22. However, with increasing the number of layers, the LOI value rises up obviously irrespective of the thickness ratio of PP and PPFR layers. Taking the system with a thickness ratio of 8:8 for example, the LOI value approaches to about 29 when the number of layers reaches 32, revealing that a transition from inflammable to flame-retardant state occurs. In halogen-free flame-retarded polymers, a linear relationship between LOI and char residue was found by Van Krevelen [18]. It

30

28

26

LOI,%

3.2. Influence of the IFR distribution on combusting behaviors of PP

PPFR/PP = 9.5/6.5

24 PPFR/PP=6.5/9.5

22

PPFR/PP=8/8 PPFR/PP=9.5/6.5

20 0

4

8

12

16

20

24

28

32

Layer numbers Fig. 4. LOI values of PPFR/PP multilayered composites as a function of the number of layers.

is considered that the formation of chars can suppress the release of combustible carbon-containing gases, and decrease the exothermicity induced by pyrolysis reactions as well as the thermal conductivity of the burning material. Hence, a higher residue left should be followed with a higher LOI value. Fig. 5 shows the TGA trance of neat PP, PP/IFR composite filled with 37.5 wt% IFRs, and PPFR/PP multilayered composites with a thickness ratio of 8:8. The residues of the PPFR/PP multilayered composites at 700 °C

Fig. 3. Polar light micrographs of (a) 2-, (b) 8-, (c) 32-layer PPFR/PP composites and (d) PP/IFR conventional composite.

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are almost around 7.9%, intermediate between those of neat PP (0.01%) and PP/IFR composite (15.3%) and basically consistent with the theoretical value (7.7%) according to the additive principle. It can be noticed that the number of layers plays little role in the degradation of the multilayered specimens and their residues at 700 °C. The similar degradation behavior reveals that: (a) IFRs distributed in PPFR layers nearly have no influence on the degradation of adjacent PP layers although plenty of them tend to be located around interfaces with increasing the number of layers; (b) the high LOI value of the specimen with a high number of layers should not be ascribed to the amount of char residues. It is known that the reduction in flammability is related to the formation of network-structured chars which may act as an insulating barrier to reduce heat and fuel transfer between the heat source and the polymer during burning [6]. Hence, the maximum distance between two adjacent IFRs should be reduced below a critical level so that the chars can be contacted each other forming a network structure. In the conventional IFR-filled system, IFRs are evenly distributed in the matrix, thus a desirable network-structured chars can be obtained by increasing the content of fillers [3]. However, in the PPFR/PP multilayered system, IFRs are separated by PP layers. Hence, the maximum distance between IFRs is determined by the thickness of PP layers (hPP) which can be controlled by the number of layers as listed in Table 1. Assuming that both of the thickness of whole specimen and the thickness ratio of PPFR and PP layers are maintained constant, hPP would decline following the increase of the number of layers. Fig. 6 displays the dependence of LOI values on hPP and three stages are observed. Taking the system with the thickness ratio of 8:8 for example, the LOI maintains below 22% although the hPP decreases from 725 to 338 lm, indicating that the char layers separated by a large hPP are difficult to retard the spread of fire through PP layers. While further decreasing the hPP, the LOI is rapidly increased to 28% and tends to become stable as the hPP reaches 88 lm, which suggests that the maximum distance between chars is approached to a critical value leading to the formation of a network structure so that more oxygen is required during burning. The influence of hPP on the combusting behavior of the PPFR/PP multilayered system with a thickness ratio of 8:8 is further investigated by burning in air after being ignited for 10 s. The residual parts of the specimen with a hPP of 725 lm and the one with a hPP of 43 lm are displayed in Fig. 7(a). As hPP equals to 725 lm, the specimen is inflammable in air accompanied with an obvious

100

neat PP PP/IFR 2 layers

80

16 layers 32 layers

Mass, %

60

40

7.7%

20

0 0

100

200

300

400

500

600

700

800

o

Temperature, C Fig. 5. Thermogravimetric curves for pure PP, PP/IFR conventional composite filled with 37.5 wt% IFRs, 2-, 16- and 32-layer PPFR/PP composites with a thickness ratio of 8:8.

57

Fig. 6. The dependence of LOI values of the PPFR/PP multilayered composites on hPP.

melt drip. However, when the hPP decreases to 43 lm, the fire is extinguished after 3 s as the specimen is ignited, and numerous chars are produced eventually. Fig. 7(b) and (c) exhibit the SEM images and corresponding FTIR spectra of these residues. The residue of the specimen with a thinner hPP shows a coarser surface with numerous cavities, and nearly no characteristic peaks of PP phase located around 2900 and 1400 cm1 are observed in the FTIR spectrum, which further indicates that the chars are produced in the burning process. The different burning behaviors of above two specimens are also schematically illustrated in Fig. 7(d). When the whole specimen is ignited, numerous chars are produced in PPFR layers and separated by combustible PP layers. Hence, a large hPP may promote the spread of fire along PP layers and bring a distinct melt drip with the increase of temperature, which is like the burning behavior of a pure PP specimen. While with the decrease of the hPP, the produced chars may expand towards PP layers forming a network-structured char layer on the ignited side acting as an insulating barrier to reduce heat and fuel transfer between the heat source and the specimen. Accordingly, the reduced hPP should be the main reason that the combustion of the whole multilayered system can be retarded accompanied with the formation of network-structured chars. According to the results obtained from PLM (Fig. 3), further decreasing hPP, the multilayered distribution of IFRs tends to be regarded as a uniform distribution state. It can be deduced that with the decrease of hPP, the combusting behavior of the PPFR/PP multilayered composite should get close to that of the conventional PP/ IFR system. However, the LOI results compared in Fig. 8 display that the LOI values of the 32-layer specimens with three different thickness ratios of PPFR and PP layers are larger than those of PP/ IFR composites filled with the same content of IFRs. This suggests that the distribution of flame retardants plays a crucial role in the combustion of the polymeric matrix. According to the structural observation, IFRs are randomly dispersed in the PP/IFR conventional composite system. As the oxygen concentration beyond the LOI value, the fire may ignite the whole specimen instantly. While for the PPFR/PP multilayered composite filled with the same content of IFRs, IFRs are only distributed in PPFR layers. Hence, the content of IFRs in PPFR layers is much higher than that in PP/IFR conventional composite, indicating that more oxygen is required to ignite PPFR layers. When the whole multilayered system is ignited, numerous chars are produced in PPFR layers acting as insulating barriers and forming confined burning space for adjacent PP layers. Numerous studies have shown that the confined burning space can effectively retard the combustion of the

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Fig. 7. (a) The residual parts of the PPFR/PP multilayered specimens with the hPP of 725 and 43 lm; (b) SEM images and (c) FTIR spectra of residual parts; (d) schematic of the burning mechanism of the PPFR/PP multilayered system with different hPP.

30

pure PP burns very fast after ignition and one sharp HRR peak appears with a HRR peak (PHRR) of 2134 kW m2, while the HRR curves of PP/IFR composites show a plateau-like behavior with a PHRR of about 130 kW m2. It is considered that with the addition of IFRs, numerous chars are produced during burning and prevent heat and oxygen from transferring into the matrix interior. The HRR curves of the multilayered system are located between those of pure PP and the PP/IFR composite and distinctly influenced by the number of layers. It should be mentioned that the layer numbers of the multilayered specimen are even. The PP layer, as one of surfaces, is always toward upside and ignited firstly in this work. If the insulating effect of each layer is ignored, the amount of the heat release of the whole multilayered specimen (HRRm) can be described by the following equation:

32-layer PPFR/PP composites PP/IFR conventional composites

LOI, %

27

24

21 17.1

20.7

24.1

Weight fraction of IFRs in the whole composite, wt% Fig. 8. LOI values of 32-layer PPFR/PP and PP/IFR conventional composites as a function of the weight fraction of IFRs.

whole material, because there are less surface area contacting with the oxygen [19]. 3.2.2. Cone calorimeter tests The influence of the multilayered distribution of IFRs on the combustion of PP is also investigated through the cone calorimeter tests which is regarded as an efficient way to evaluate the flammability characteristics of materials. The heat release rate (HRR) is believed to have the greatest influence on the fire hazard. The measured HRR curves for pure PP, the 2-, 16- and 32-layer PPFR/PP specimens with a thickness ratio of 8:8, the PP/IFR conventional composite filled with the same content of IFRs as the multilayered system, and the PP/IFR conventional composite filled with 37.5 wt% IFRs are displayed in Fig. 9(a) and (b) for comparison. The results show a large difference in combusting behavior between pure PP and the PP/IFR composite. The

HRRm ¼ HRRPP  xPP þ HRRPPFR  xPPFR

ð1Þ

where xPP and xPPFR represent the weight fraction of pure PP and PPFR layers filled with 37.5 wt% IFRs, respectively. The curve plotted in Fig. 9(a) shows that the HRR of 2-layer specimen is basically consistent to that of the theoretically calculated result, which indicates that the PP layer in 2-layer specimen burns like pure PP without being influenced by the PPFR layer. With the increase of the number of layers, the experimental curve tends to deviate from the theoretical one and shows a plateau-like behavior. As the number of layers approaches to 32, the HRR curve almost overlaps that of the PP/IFR composite filled with the same content of IFRs. The total heat release (THR), the integral of the HRR curve over the duration of the experiment, of each specimen is also measured and displayed in Fig. 10. At the end of burning, pure PP releases a total heat of 213 MJ m2, while the value is reduced with the addition of IFRs and is distinctly dependent on their distribution in the matrix. It can be found that the THR of 2-layer PPFR/PP composite is halved compared with that of pure PP. Further increasing the number of layers to 32, the value is reduced to 63 MJ m2 and gets closer to that of PP/IFR composite filled with the same content of IFRs (39 MJ m2), which is in accordance with the tendency of HRR.

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Pure PP 2 layers Predicted curve

2000

(b) 300

1500 1000 500

16 layers 32 layers PP/IFR filled with 20.7 wt% IFRs PP/IFR filled with 37.5 wt% IFRs

250

HRR, kW/m 2

HRR, kW/m2

(a) 2500

200 150 100 50

0

0 0

200

400

600

0

200

Time, s

400

600

800

1000

1200

Time, s

Fig. 9. HRR curves of (a) pure PP, 2-layer PPFR/PP composite with a thickness ratio of 8:8, the predicted curve obtained according to Eq. (1); and (b) 16-, 32-layer PPFR/IFR composites with a thickness ratio of 8:8, the PP/IFR conventional composites filled with 20.7 and 37.5 wt% IFRs.

250 213

2

200

THR, MJ/m

It is known that the heat radiation on a specimen would first result in the degradation of surface layer leading to its ignition, and then penetrate toward sublayers until all specimen is burnt. In the PPFR/PP multilayered system, PPFR and PP layers are distributed alternately vertical to the penetrating direction. Numerous chars are produced in PPFR layers, which can act as insulating barriers to inhibit the release of flammable decomposition volatiles, as well as to restrain the heat transmission into the underlying PP matrix. With the increase of the number of layers, the thickness of the surface layer (pure PP) tends to be reduced so that more PP matrix would be sandwiched between PPFR layers as displayed in Fig. 3. When the whole specimen is burnt, the char layers produced in PPFR layers would effectively prevent heat and oxygen from transferring into the matrix interior making the decrease in HRR and THR.

150 102

100

96 63

50

39

0 PP

16 layers

32 layers

PP/IFR

Sample name

3.3. Influence of the IFR distribution on mechanical properties of PP It is generally recognized that although IFRs can effectively retard the combustion of polymers, they may also promote the propagation of crazes (or cracks) in the matrix resulting in the deterioration of the strength and toughness of the whole composite. Hence, the mechanical properties of IFR-filled system are attached much attention in the actual application [20–22]. In this work, tensile tests were performed to investigate the effect of the distribution of IFRs on the tensile strength (Ts) and elongation at break (eb) of the PP-based composites. Fig. 11(a) displays the dependence of the Ts of the PPFR/PP multilayered system on the number of layers. In the stretching process, all specimens deform in a ductile manner with necking and have approximately the same Ts around 24 MPa, irrespective of the number of layers. The facture behavior of the multilayered system is characterized through eb. As shown in Fig. 11(b), eb is enhanced obviously with increasing the number of layers, which is in accordance with the result of the carbon black-filled multilayered system reported in previous work [23]. Specially taking the system with a thickness ratio of 6.5:9.5 for example, eb is only 17% for the 2-layer specimen, while the value reaches 361% when the number of layers approaches to 32. Similarly, eb can also be increased about 4 times by thickening the PPFR layers utill the thickness ratio to its adjacent PP layer reaches 9.5:6.5. Actually, Sung et al. [14,15] have made a lot of studies on tensile behaviors of the multilayered materials combining with brittle and ductile layers. It has been reported that the crazes (or cracks) originated from the brittle layers would be terminated by adjacent ductile layers through interfacial adhesion. In this work, the surface of a stretched specimen was also observed through PLM along tensile direction. The micrograph inserted in Fig. 11(b) demonstrates that the crack produced from PPFR layer is indeedly terminated by PP

2 layers

Fig. 10. THR values of pure PP, the 2-, 16-, 32-layer PPFR/IFR composites with a thickness ratio of 8:8, and the PP/IFR composite filled with 20.7 wt% IFRs.

layer. Hence, it can be supposed that increasing the number of layers may produce more interfaces to inhibit the propagation of crazes (or cracks) so that the whole material can experience a larger elongation without being broken. However, when IFRs are evenly distributed in the matrix, crazes (or cracks) originated from those solid particles tend to propagate throughout the whole specimen, so that it is commonly difficult to balance the mechanical and flame-retarding properties of the conventional composites. In this work, the tensile properties of the 32-layer PPFR/PP and PP/IFR composites are compared in Fig. 11(c) and (d). It can be found that when the content of IFRs is maintained at 17.1 wt%, the Ts and eb of the multilayered specimen are increased about 22% and 484%, respectively, compared with those of the conventional composite. Besides, the specimens filled with 20.7 and 24.1 wt% IFRs also display a similar tendency. This indicates that the multilayered distribution of IFRs plays a significant role in retarding the propagation of crazes as well as the spread of fire, which may provide an optimal route to fabricate flame-retarding composites with excellent mechanical properties.

4. Conclusions The influence of the multilayered distribution of IFRs on the combusting and mechanical properties of PP matrix was investigated and compared with that of the PP/IFR conventional composites with a uniform distribution of IFRs. The multilayered distribution of IFRs was obtained by fabricating a series of alternat-

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Fig. 11. (a) Tensile strength and (b) elongation at break of PPFR/PP multilayered composites as a function of the number of layers; (c) tensile strength and (d) elongation at break of 32-layer PPFR/PP and PP/IFR conventional composites as a function of the weight fraction of IFRs.

ing multilayered composites consisting of pure PP and IFR-filled polypropylene layers through a layer-multiplying coextrusion technology. Flammability properties measured by LOI and cone calorimeter tests revealed that the increase of the number of layers significantly enhanced the LOI values and reduced the PHRR and THR leading to a distinct transition from inflammable to flameretardant state. Moreover, the specimen with a large number of layers even had a higher LOI value than the PP/IFR conventional composite filled with the same content of IFRs. The layer interfaces and confined layer spaces in the multilayered system were considered to play a crucial role in retarding the spread of fire. The influence of the multilayered distribution of IFRs on the mechanical properties of matrix was investigated through tensile tests. Results displayed that the increased number of layers induced a distinct enhancement of eb. The crazes originated from the PPFR layers were considered to be terminated by adjacent PP layers through interfaces. While when IFRs were evenly distributed in the matrix, crazes tended to propagate throughout the whole specimen causing a lower Ts and eb.

Acknowledgment The authors are grateful to the National Natural Science Foundation of China (51203097, 51227802 and 51073099) and the Applied Basic Research Foundation of Sichuan Province for financial support of this work.

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