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Effect of the particle size of expandable graphite on the thermal stability, flammability, and mechanical properties of high-density polyethylene/ethylene vinyl-acetate/expandable graphite composites.

INTRODUCTION

Polyethylene (PE) is one of the most widely used polymeric materials owing to its low cost, balanced mechanical properties, good processability, and so on [1-3]. However, there are some disadvantages for PE, such as low environmental stress-cracking resistance, poor compatibility with various additives, and so on, which have restricted its applications in some important fields [4, 5]. Therefore, many attempts have been made to ameliorate such limitations and improve the compatibility of PE by blending it with different fillers. Among the polyolefin elastomers, poly(ethylene-co-vinyl acetate) (EVA) is a suitable candidate for blending with PE to improve its flexibility, toughness, environmental stress-cracking resistance, and transparency [6, 7]. In addition, EVA can be well miscible with PE because of the presence of polar ester groups in the EVA chemical structure [1],

As blending of PE with EVA has attracted great attention for use as wire- and cable-insulating materials, the flame retardancy of PE/EVA blends is becoming very important. There are a number of publications dealing with the various characteristics of PE/EVA blends. For example, Llias et al. [8] studied the thermal property, morphology, and viscoelastic behavior of high-density polyethylene (HDPE)/EVA/CaC03 ternary blends. It was found that the expected heterogeneous nucleating effect of CaC03 was hindered owing to the presence of EVA. In addition, from the rheological point of view, Faker et al. [9] reported that the interfacial interactions of PE/ EVA blends were enhanced with increasing PE content, which led to a finer and well-distributed morphology in PE-rich blends. However, both HDPE and EVA are flammable materials, and hence it is worthwhile to investigate the flame retardancy of HDPE/EVA blends. Liu et al. [10] investigated the thermal stability and flame retardancy of HDPE/EVA/fullerene ([C.sub.60]) nanocomposites, and the larger the [C.sub.60] loading level, the better the flame retardancy of HDPE/EVA/[C.sub.60] nanocomposites.

In recent years, intumescent flame retardants (IFRs) have aroused a great attention because they are not only more environmentally friendly than the halogen-containing flame retardants but also have higher flame retardant efficiency than inorganic fillers [11-13]. Expandable graphite (EG) is a typical example of IFRs, which is a flake graphite intercalation compound. EG consists of layers of graphite with a blowing agent being incorporated between the layers [14], When heated up in a fire, the blowing agent evaporates and induces the graphite layers to expand. The expanded graphite forms worm-like structures that act as a thermal barrier for underlying materials. In general, EG is used in a growing number of flame-retardant applications as a blowing agent and a smoke suppressor [14-16]. EG has been successfully used as a flame retardant for polymers giving satisfactory properties. For example, Cai et al. [17] prepared a flame-retardant shape-stabilized phase change material made of paraffin, HDPE, EG, and ammonium polyphosphate (APP), and zinc borate (ZB). The results indicated that the synergistic effect between EG and APP contributed to the higher thermal stability; however, the anti-synergistic effect between EG and ZB may occur, leading to the lower thermal stability. Svoboda et al. [18] has studied the effect of different weight ratios of EG on the flame retardancy of ethylene-octene copolymer composites. In addition, the [H.sub.2][O.sub.2]-hydrothermal method was used to prepare EG called as [H.sub.2][O.sub.2]-EG, and its flame retardancy of the corresponding composites was investigated by Kuan et al.[19]. It was found that the [H.sub.2][O.sub.2]-hydrothermal method was more efficient for expanded volume of EG, and the HDPE/[H.sub.2][O.sub.2]-HEG composites possessed excellent thermal degradation behavior and flame-retardant property.

In this study, the two different particle sizes of EG1 (45 [micro]m) and EG2 (150 pm) were applied to reduce the flammability of the HDPE/EVA-based composites owing to their intumescent efficiency, special layer structure, and low cost. The effects of two different particle sizes of EG on the thermal stability, flammability, char morphology, and mechanical properties of HDPE/EVA/EG composites were investigated by thermogravimetric analysis (TGA), cone calorimeter test (CCT), tensile test, and scanning electron microscopy (SEM). This main aim of this study is to develop a low smoke and halogen-free flame-retardant polymeric material.

EXPERIMENTAL

Materials

High-density PE (HDPE, 5000S, melt flow rate [MFR] = 0.923 g/10 min) was obtained from Daqing Petrochemical, China Petroleum. EVA copolymer (7240 M, VA = 15 wt%, MFR= 1.5 g/10 min) was purchased from TAISOX, Taiwan, China. EG, with average particle sizes of 45 [micro]m (EG1) and 150 pm (EG2), is supplied by Qingdao Kangboer Graphite.

Preparation of Samples

HDPE/EVA/EG composites were prepared according to the following steps. First, HDPE, EVA, and EG were dried at 50[degrees]C for 10 h. Second, the flame-retardant HDPE/EVA composites with the desired amounts of EG were compounded on a twin-screw extruder (Type HFB-150/3300, made in Nanjing, China). The temperature range of the twin-screw extruder was 80-150[degrees]C and screw speed was 150 rpm. Then, the extrudate was cut into pellets. After being dried, all the samples were prepared by using the compression molded of sheets at about 150[degrees]C under 8 MPa for 10 min. The formulations of the samples are summarized in Table 1.

Measurements and Characterization

Thermogravimetric Analysis. The thermal properties of the composites were examined by TGA using NETZSCH TG209F1 Thermal Analyzer. In each case, about 8-10 mg specimens were heated from room temperature to 700[degrees]C at a linear heating rate of 20[degrees]C/min under [N.sub.2] atmosphere.

Cone Calorimeter Test. The cone calorimeter (Stanton Redcroft, UK) tests were carried out according to ISO 5660 standard procedures. Each specimen of dimensions 100 x 100 x 3 [mm.sup.3] was wrapped in aluminum foil and exposed horizontally to an external heat flux of 35 kW/[m.sup.2].

Tensile Test. The mechanical properties of the composites were investigated by tensile tests using sheets with 1 mm of thickness. Tensile tests were performed using AG S-J manufactured by Shimadzu, China, at a tensile speed of 5 mm/min by following the ISO 527 procedure at room temperature. The same measurements were repeated at least five times and the results were averaged.

Morphology Observation. SEM observations of the fractured surfaces were performed by using a field emission scanning electron microscopy (FE-SEM, JEOL JSM-7500F). The samples were cryogenically fractured in liquid nitrogen and all the fractured surfaces were sputter-coated with a thin gold layer before SEM investigation.

RESULTS AND DISCUSSION

Thermal Properties

TGA and derivate thermogravimetric (DTG) curves of HDPE/EVA, HDPE/EVA/EG1, and HDPE/EVA/EG2 under a flow of nitrogen at a heating rate of 20[degrees]C/min are shoen in Fig. 1. As shown in Fig. 1, the TGA curves of three samples displayed a two-step thermal degradation process. The first stage occurring between 300 and 400[degrees]C was related to the deacylation of the EVA phase through radical and ionic [beta]-elimination mechanism [20, 21], The second stage which took place in the temperature range of 400-530[degrees]C involved the degradation of the HDPE phase accompanied with the decomposition of the polyacetylene-ethylene chains formed in the first stage [21]. There was not any residue remaining after degradation for HDPE/EVA, suggesting that the degradation of HDPE/EVA was complete. However, 14.4 and 15.1% of the residue was left after degradation for HDPE/EVA/ EG1 and HDPE/EVA/EG2 at the high temperature of 700[degrees]C, respectively (Table 2). This possible reason is that the stable and thick expanded char is formed between the fire and the matrix to prohibit polymer materials to further decompose.

The onset degradation temperature ([T.sub.onset]), the maximum weight loss temperature ([T.sub.max]), as well as the maximal weight loss rate and its corresponding temperature ([D.sub.pk] and [T.sub.pk]) were determined from the TGA and DTG curves and listed in Table 2. Tonset of HDPE/EVA/EG1 and HDPE/EVA/EG2 were lower than that of HDPE/ EVA by 31 and 42[degrees]C, respectively. When EG was exposed to a heat source of >200[degrees]C, it started to decompose because of the presence of sulfuric acid or nitric acid, which resulted in the low [T.sub.onset] for HDPE/EVA/ EG1 and HDPE/EVA/EG2 [22], On the other hand, the [T.sub.pk] values of HDPE/EVA/EG1 and HDPE/EVA/EG2 increased by 18 and 7[degrees]C, respectively, compared with HDPE/EVA. At the same time, it could be seen that the [D.sub.pk] of HDPE/EVA/EG 1 and HDPE/EVA/EG2 decreased to 37 and 42 from 53.3% per min, respectively, and the degradation temperature was shifting to a higher temperature with the addition of EG, indicating that the degradation rate of the flame-retardant HDPE/EVA was slower than that of HDPE/EVA, and the degradation rate of HDPE/EVA/EG2 was faster than that of HDPE/EVA/EG1 in the temperature range of 400-530[degrees]C. All results indicated that the thermal stabilities of HDPE/EVA/EG were improved compared with HDPE/EVA, and the thermal property of the flame-retardant composites was increased with decreasing particle size of EG.

Flammability

CCT based on the oxygen consumption principle has been widely used to evaluate the flammability characteristics of polymeric materials [23-25], Some parameters can be obtained from cone calorimeter including the time to ignition (TTI), the heat release rate (HRR), the peak heat release rate (PHRR), the total heat release (THR), mass loss, smoke production rate (SPR), C[O.sub.2] and CO production rate of which the most important parameters are HRR and PHRR.

Figure 2 shows the HRR curves versus time for HDPE/EVA, HDPE/EVA/EG 1, and HDPE/EVA/EG2. The detailed data obtained by the cone calorimeter from the aforementioned series of samples are summarized in Table 3. It was clearly observed that HDPE/EVA burned very fast after ignition and a sharp peak appeared with a PHRR as high as 275.5 kW/[m.sup.2], and then decreased dramatically. It was interesting to observe that the HRR of HDPE/EVA had two peaks, and the possible reason for this might be that the degradation of EVA and HDPE phase occurs at different temperatures. The two-step pyrolysis behaviors were pronounced for this sample, and the phenomenon was also proved by the previous study [21] and TGA curves (Fig. 1). With the addition of EG, the flammability of the flame-retardant composites was obviously restrained. The PHRR values of HDPE/EVA/ EG1 and HDPE/EVA/EG2 dramatically decreased to about 55.1 and 87.9 kW/[m.sup.2], respectively, which were only 20.0 and 31.9% of that of HDPE/EVA. This indicates that EG acting as a fire retardant can form the efficient char layer which can separate the inside from the heat and oxygen effectively. In addition, the HRR of HDPE/EVA/EG 1 was lower than that of HDPE/EVA/ EG2 during the whole process, and showed a better fire resistance. The results illustrated that the small particle size of EG1 had better flame retardancy than the big one (EG2), which may be attributed to EG1, had a much more stable interfacial structure, and had a better dispersion in the polymer matrix [26].

Fire performance index (FPI), the ratio of TTI and PHRR, is generally used to design the escape time for firefighters in a real fire [27]. The data from Table 3 summarize that FPI of HDPE/EVA was 0.0044; however, FPI of HDPE/EVA/EG1 and HDPE/EVA/EG2 increased to 0.6371 and 0.4435, which were about 145 and 101 times of that of HDPE/EVA, respectively. Generally, the higher FPI and lower PHRR, the better is the fire resistance to reduce the loss and casualty in a real fire [27]. This indicated that the flame-retardant properties of HDPE/EVA/EG were improved greatly by the addition of EG. At the same time, the FPI of HDPE/EVA/EG 1 was higher than that of HDPE/EVA/EG2, suggesting that better flame retardancy was obtained for the composites with EG1.

The THR values for the above samples are shown in Fig. 3. The addition of EG significantly reduced the THR values of polymer materials, and the THR values of HDPE/EVA/EG 1 and HDPE/EVA/EG2 dramatically decreased to about 16.5 and 17.5 MJ/[m.sup.2] compared to HDPE/EVA (34.7 MJ/[m.sup.2]), respectively. The phenomenon could be explained that an intumescent char was formed on the surface of the polymer matrix, which made a thermal insulation, provoked the extinguishment of the flame, prevented combustible gases from feeding the flame, and separated oxygen from burning materials [28], In addition, HDPE/EVA/EG 1 had lower THR values than HDPE/EVA/EG2, which indicated that small size EG1 could form a more stable char than large size EG2 in the high temperature.

Figure 4 shows mass curves for the above samples. During combustion, a compact char may occur on the surface of the burning materials, creating a physical protective barrier. It could be seen that HDPE/EVA lost its mass faster than HDPE/EVA/EG1 and HDPE/EVA/EG2. The mass of HDPE/EVA quickly decreased to 9.2% in a short time, whereas the mass loss of HDPE/EVA/EG1 and HDPE/EVA/EG2 was lower than that of HDPE/EVA, and HDPE/EVA/EG1 presented a much lower mass loss rate and left the biggest residual mass at the end of burning. This was attributed to char formation and its morphological structure on the surface of the polymer matrix [29].

Polymer materials can release a large amount of toxic gases and smoke during combustion process, and the emission of smoke, C[O.sub.2], and CO along with HRR also plays a critical role in fire conditions. Figure 5 shows the SPR curves for HDPE/EVA, HDPE/EVA/EG1, and HDPE/EVA/EG2. It could be found that the SPR values of HDPE/EVA/EG1 and HDPE/EVA/EG2 were much lower than that of HDPE/EVA, and this trend was the same as that of the HRR curves in the cone calorimeter. The intumescent layers would act as a protective barrier to limit smoke diffusion to the substrate and retard the volatilization of the flammable decomposition products.

Figures 6 and 7 show the C[O.sub.2] and CO production rate curves versus time for the above samples. It was clearly observed that the trend of C[O.sub.2] and CO emission was similar to those of heat and smoke release. When HDPE/ EVA was filled with EG, C[O.sub.2] and CO emissions decreased significantly, indicating that the toxic gases of the combustion products of HDPE/EVA/EG1 and HDPE/ EVA/EG2 were decreased. This was because EG could form the efficient char layer that covered the materials to stop its further decomposition. EG1 performed lower smoke, toxic, and inflammable gases release than EG2, which made it have a better flame-retardant effect.

Morphology of Char Residues After Flammability

To elucidate how the formation of intumescent char affects the combustion of the flame-retardant composites, the char residues left after burning tests were characterizered for changes in char appearance by taking photographs with a digital camera. The char morphologies of HDPE/EVA/EG composites with two different particle sizes of EG after combustion are shown in Fig. 8. As shown in Fig. 8A, the specimen for HDPE/EVA did not have any char residue, indicating that the combustion of HDPE/EVA was very acute and complete. However, the swollen char residues were formed for HDPE/EVA/EGI and HDPE/EVA/EG2 as shown in Fig. 8B and C. In addition, the char layer structure of HDPE/EVA/EGI on the burnt surface showed more continuous and denser than that of HDPE/EVA/EG2. Although the expansion ratio of EG1 was smaller than that of EG2, there were more intumescent char residues left for HDPE/EVA/EGI in the combustion process. Such a char layer could construct a more effective barrier to heat, oxygen, and volatile fuel, and accordingly possessed better fire resistance in the condensed phase as compared to HDPE/EVA. This indicated that the trend of flame retardancy was HDPE/EVA/ EG1>HDPE/EVA/EG2>HDPE/EVA.

Mechanical Properties

To investigate the effect of particle sizes of EG on the mechanical properties of HDPE/EVA/EG composites, the injection-molded standard specimens are characterized with a crosshead speed of 5 mm/min. The typical stress-strain curves of HDPE/EVA, HDPE/EVA/EGI, and HDPE/EVA/EG2 are shown in Fig. 9, and the data of tensile properties of the studied samples are summarized in Table 4. The elongation at break of HDPE/EVA blend was up to 517.5%, but it decreased to 315.5 and 232.5% while filled with EG1 and EG2, respectively. This fact was mainly owing to poor compatibility between filler and polymer matrix, which probably could be improved by the use of filler surface treatment [23, 30], But, it could be seen that the elongation at break of the flame-retardant composites remained relatively high values when filled with 20 phr of EG. The tensile strengths of HDPE/EVA/EGI and HDPE/EVA/EG2 decreased to 10.1 and 9.2 MPa, respectively, compared to 15.6 MPa of HDPE/EVA. The tensile modulus was measured from the initial region of tensile deformation and indicative of the composites value of the constituent stiffness. EG-filled HDPE/EVA composites exhibited higher tensile modulus than that of HDPE/EVA blend. Comparing the two different particle sizes of EG-filled systems, EG2 system showed higher stiffness. The improvement in the tensile modulus was mainly owing to stiffness of the composites in the presence of EG. For example, the tensile modulus of HDPE/EVA/EG2 increased from 281 MPa of HDPE/ EVA to 346 MPa, which is increased by 22.8%.

The mechanical properties are influenced by the shape, particle size, and particle size distribution of the filler particles, the filler content, and the physical properties of filler and polymer. In addition, the interfacial interaction between the matrix and the filler is one of the most important factors [31]. Figure 10 shows the SEM images of fractured surfaces of HDPE/EVA, HDPE/EVA/EGI, and HDPE/EVA/EG2. One observed a leaf-like structure at HDPE/EVA blend and the surface was very rough. Comparing SEM picture of HDPE/EVA, the leaf-like structure disappeared for HDPE/EVA/EGI and HDPE/ EVA/EG2 owing to the addition of EG. The lamellar structures of graphite could be observed easily. HDPE/ EVA/EG1 presented a rough surface and enhanced interfacial adhesion between polymer matrix and inorganic particles, whereas the fracture surface was very smooth and the loose interfacial structure was clearly seen in the HDPE/EVA/EG2, suggesting the poor interfacial adhesion between the matrix and the EG2 particles. This was responsible for the poor mechanical properties. This was consistent with the results of the stress-strain curves.

CONCLUSIONS

In this study, the thermal stability, fire behavior, char layer morphology, and mechanical properties of HDPE/ EVA containing two different particle sizes of EG were investigated by TGA, CCT, tensile test, and SEM. Great increase in the thermal stability was achieved with the addition of EG to HDPE/EVA. In addition, the degradation temperature of the flame-retardant products increased with decreasing the size of EG. Owing to the addition of EG into HDPE/EVA, the values of HRR, PHRR, and THR significantly decreased. For example, the PHRR value of HDPE/EVA was 227.1 kW/[m.sup.2]; however, the PHRR values of HDPE/EVA/EG1 and HDPE/EVA/EG2 decreased to 54.9 and 87.9 kW/[m.sup.2], respectively. This showed that the addition of EG dramatically decreased the flammability of HDPE/EVA. At the same time, the addition of EG resulted in a significant increase of the char residue. In addition, the small particle size of EG dispersed more homogeneously and had strong interfacial adhesion with polymer matrix, and then HDPE/EVA/EG1 had better mechanical properties than HDPE/EVA/EG2.

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Zhidan Sun, (1,2) Yonghong Ma, (1) Yang Xu, (1) Xiaolang Chen, (1) Man Chen, (1) Jie Yu, (3) Shuchun Hu, (1) Zhibin Zhang (2)

(1) Key Laboratory of Advanced Materials Technology Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, People's Republic of China

(2) School of Life Science and Engineering, Southwest Jiaotong University, Chengdu 610031, People's Republic of China

(3) National Engineering Research Center for Compounding and Modification of Polymer Materials, Guiyang 550014, People's Republic of China

Correspondence to: Xiaolang Chen; e-mail: chenxl6l2@sina.com

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 51003088; contract grant sponsor: Specialized Research Fund for the Doctoral Program of Higher Education; contract grant number: 20100184120006; contract grant sponsor: National Science and Technology Supporting Project Foundation of China; contract grant number: 2007BAB08B05; contract grant sponsor: Fundamental Research Funds for the Central Universities; contract grant numbers: SWJTU12CX009, SWJTU11ZT10; contract grant sponsor: Sishi Star Foundations of Southwest Jiaotong University (2011).

DOI 10.1002/pen.23659

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Formulations of HDPE/EVA blend and HDPE/EVA/EG
composites.

Samples         (HDPE/EVA) (phr)    EG1 (phr)   EG2 (phr)

HDPE/EVA               100              0           0
HDPE/EVA/EG1           100             20           0
HDPE/EVA/EG2           100              0          20

HDPE/EVA = 50/50 (wt/wt).

TABLE 2. Detailed data from TGA and DTG of several samples.

Samples    [T.sub.onset]   [T.sub.max]     [T.sub.pk]
           ([degrees]C)    ([degrees]C)   ([degrees]C)

A               334            488            469
B               301            506            479
C               292            495            471

Samples     [D.sub.pk]        Residue at
             (%/min)      700[degrees]C (wt%)

A              53.3               0.0
B              37.3              14.4
C              42.0              15.1

TABLE 3. Cone calorimeter data of HDPE/EVA/EG composites.

Samples    TTI (a)       PHRR (b)          AHRRC
             (s)      (kW/[m.sup.2])   (kW/[m.sup.2])

A             1           275.5            110.0
B             37           55.1             45.6
C             39           87.9             51.8

Samples         THR              FPI (d)         Residues
           (MJ/[m.sup.2])   ([m.sup.2] x s/kW)      (%)

A               34.7              0.0044            9.2
B               16.5              0.6731           46.3
C               17.5              0.4435           42.3

(a) Time to ignition.

(b) Peak of heat release rate, expressing the intensity of a fire.

(c) Average HRR within the front 300 s from heat radiation.

(d) Fire performance index, the ratio of TTI and PHRR.

TABLE 4. Tensile properties of HDPE/EVA/EG composites.

                Tensile             Tensile           Elongation at
Samples      modulus (Mpa)       strength (Mpa)         break (%)

A          281 [+ or -] 3.5    15.6 [+ or -] 0.8    517.5 [+ or -] 7.3
B          300 [+ or -] 2.8    10.1 [+ or -] 0.4    315.5 [+ or -] 5.6
C          345 [+ or -] 3.2     9.2 [+ or -] 0.5    232.5 [+ or -] 3.8
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Author:Sun, Zhidan; Ma, Yonghong; Xu, Yang; Chen, Xiaolang; Chen, Man; Yu, Jie; Hu, Shuchun; Zhang, Zhibin
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:May 1, 2014
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