Journal of Nanomaterials
Volume 2019 (2019), Article ID 6597494, 13 pages
https://doi.org/10.1155/2019/6597494

Research Article

Tailoring the Localization of Carbon Nanotubes and Ammonium Polyphosphate in Linear Low-Density Polyethylene/Nylon-6 Blends for Optimizing Their Flame Retardancy

Shuai Zhang, Chang Lu, Xi-ping Gao, Da-hu Yao, and Yu-xin He

Chemical Engineering & Pharmaceutics School, Henan University of Science and Technology, Luoyang 471003, China

Correspondence should be addressed to Chang Lu; luchang139@126.com

Received 27 February 2019; Accepted 18 August 2019; Published 30 September 2019

Guest Editor: Laijun Liu

Copyright © 2019 Shuai Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Carbon nanotubes (CNTs) and ammonium polyphosphate (APP) was used to improve the flame retardancy of linear low-density polyethylene/nylon-6 (LLDPE/PA6) blends. It was observed that APP or CNTs tended to be dispersed in the PA6 phase of the blends when all components were melt-blended together. CNTs dispersed in the PA6 phase caused the decrease of flame retardancy. Different processing methods were used to tailor the localization of APP and CNTs in the blends. The results showed that the localization of CNTs or APP strongly influenced the flame retardancy of blends. APP-incorporated CNTs had antagonism in blends with APP localized in the LLDPE phase and CNTs in the PA6 or LLDPE phases. A synergism between APP and CNTs was exhibited only in blend with the localization of APP in the PA6 phase and CNTs in the LLDPE phase. SEM observation showed that the residual char layer in blends with poor flame retardancy was either discontinuous or continuous but porous. A continuous and compact-residue char layer was observed in blends with excellent flame retardancy. Different morphologies of the residual char layer could be attributed to the difference of residual char mass and network structure.

1. Introduction

The application of carbon nanotubes (CNTs) in flame-retarded polymers was first reported by Kashiwagi et al. in 2002, attracting significant recent research interest because it affords the enhancement of thermal stability and reduction in heat release rate in a polymer matrix through the use of low loadings [1–6]. The use of CNTs as a flame retardant is also possible, primarily through the formation of a solid, jammed network structure consisting of CNTs and tangled polymer chains. Such a network can form a continuous protective layer during combustion to hinder the diffusion of oxygen and heat and slow the release of combustion products [7, 8].

The addition of CNTs into polymers primarily decreases their peak heat release rate (PHRR), which is a flame-retardant property obtained from a typical cone calorimeter test. CNTs did not exhibit consistent enhancement of flame retardancy when the limiting oxygen index (LOI) and standard UL 94 tests were used to evaluate the flame retardancy [9, 10]. Therefore, CNTs were always employed as a synergist of flame retardants to improve the flame retardancy of polymers or polymer blends.

Intumescent flame-retardant additives (IFR), with characters of nonhalogen and low poison, have become a hot point in a flame retardant area [11–14]. During combustion, IFR can form a foamy, charred layer on the surface of the polymer matrix to improve its flame retardancy [15–17]. One of the drawbacks of IFR is relatively low flame-retardant efficiency. The synergistic effects of CNTs with IFR are expected to optimize the flame-retardant efficiency of IFR because both function according to a condensed flame-retardant mechanism. Although synergistic effects between IFRs and CNTs were reported [18–21], Du and Fang found that the introduction of CNTs deteriorated flame retardancy [9]. The conflicting results indicate that the combined effect of IFR and CNTs on flame retardancy deserved further research.

Polymer blends are typically comprised of a two-phase structure. CNTs or IFR may localize in one of the two phases in polymer blends. Although the effect of IFR and CNT localization on blend flame retardancy has not been reported, some of the studies found that clay or IFR localization in polymer blends is critical in improving flame retardancy. Lu et al. [22, 23] and Pack et al. [24] found that clay tended to segregate into one of the phases and the blend morphology affected the flame retardancy greatly. Pack et al. [25, 26] found that the synergistic effects between flame retardant and clay in the blends with clay localization at the interface were more efficient than in the polymers. Moreover, it was reported that tailoring the localization of clay or IFR was used for optimizing the flame retardancy of polymer blends [27–30]. Lu et al. [27, 28] tailored clay localization from the PA6 phase to the interface and improved the flame retardancy of PS/PA6. In LLDPE/PA6 blends, tailoring the localization of clay from the PA6 phase to the LLDPE phase caused a remarkable improvement of flame retardancy. Jin et al. [29] found the selective dispersion of IFR in polypropylene blends and compatibilizer can be applied to adjust the dispersion of IFR in the blends, causing the improvement of flame retardancy.

The results that suitable localization of clay or IFR in polymer blends can improve their synergistic effects indicated that tailoring the localization of CNTs and IFR may be also used for optimizing their flame retardancy in polymer blends. Therefore, CNTs and ammonium polyphosphate (APP) were employed to improve the flame retardancy of LLDPE/PA6. The method [28] that we had used to tailor IFR and clay dispersion in polymer blends was also employed to prepare blends with different APP and CNT localization. The flame retardancy of the blends with different APP and CNT localization was investigated.

2. Experimental

2.1. Materials

Linear low-density polyethylene (LL6201XR) was supplied by ExxonMobil Corp. Polyamide-6 (PA6) (33500) was supplied by Xinhui Meida-DSM Nylon Chips Co., Ltd. Ammonium polyphosphate ((NH₄PO₃)n, , ) was supplied by Zhejiang Longyou Gede Chemical Factory (China). The CNTs were obtained from Chengdu Organic Chemical Co., Ltd. They were multiwalled with hydroxyl groups providing surface functionality.

2.2. Preparation of Composites

A corotating twin-screw extruder was used to melt-blend the blends, and an injection-molding machine was employed to shape the blends. Before processing, the components and blends were dried at 80°C for 24 h to remove any moisture. We followed the methods [28] to control the localization of APP and CNTs in LLDPE/PA6 blends, as shown in Figure 1. The formula and code of the blends are shown in Table 1.

Figure 1: Four processing methods for controlling APP and CNT distribution in LLDPE/PA6 blends. Method 1: all the components were melt-blended at 230°C. The processing temperature was higher than the melt temperature of LLDPE and PA6. Method 2: the blends were prepared through two steps. LLDPE/PA6 blend was prepared at 230°C firstly, and then LLDPE/PA6 blend was blended with APP and CNTs at 160°C. In the second step, the processing temperature was lower than the melt temperature of PA6. Method 3: the blends were prepared through two steps. LLDPE/PA6/CNT blend was prepared at 230°C firstly, and then LLDPE/PA6/CNT blend was blended with APP at 160°C. In the second step, the processing temperature was lower than the melt temperature of PA6. Method 4: the blends were prepared through two steps. LLDPE/PA6/APP blend was prepared at 230°C firstly, and then LLDPE/PA6/APP blend was blended with CNTs at 160°C. In the second step, the processing temperature was lower than the melt temperature of PA6.

Table 1: Formulation of blends.

Sample code
LLDPE
PA6
APP
CNTs
Preparing method

Blends
80
20
0
0

Blends/CNTs
80
20
0
0.5

D1
60
15
25
0
Method 1

M1
60
15
25
0.5
Method 1

M2
60
15
25
0.5
Method 2

M3
60
15
25
0.5
Method 3

M4
60
15
25
0.5
Method 4

2.3. Measurement and Characterization

The values of the LOI were tested in accordance with the standard oxygen index test (ASTM D2863-77). The vertical burning grade was evaluated in accordance with the UL 94 test (ASTM D635-77).

A cone calorimeter (Fire Testing Technology Ltd, United Kingdom) was employed to test the flammability of samples (). The external heat flux was 35 kW/m².

The blend of LLDPE/PA6/CNTs (80/20/0.5) was cryogenically cut at -120°C to obtain the slice with the thickness of 70 nm. The slice was examined by TEM (FEI TECNAI-G20) to observe the dispersion of CNTs at an acceleration voltage of 300 kV.

The thermal degradation of the samples was characterized by thermogravimetric analysis (NETZSCH, STA409PC). The test was implemented using a heating rate of 20°C/min in nitrogen.

The morphology of the fractured specimen surface and char residue was observed by SEM (JEOL 6301F). The samples were immersed in liquid nitrogen for 2 h and then broken quickly to obtain the fractured specimen. Char residue was obtained from the samples after vertical flammability tests. The samples were coated with a gold layer before examination. The elemental compositions of fractured specimen surface were measured using an energy-dispersive spectrometry (EDS) analyzer.

The rheological performance of the samples was measured using an ARES rheometer (AR 1500ex) in dynamic mode, on a parallel-plate geometry with a diameter of 25 mm and a gap of ~1 mm. The measurement temperature was 225°C and frequencies ranged from 100 to 0.01 rad/s.

3. Results and Discussion

3.1. Localization of APP and CNTs in LLDPE/PA6 Blends

The blend of LLDPE/PA6/CNTs (80/20/0.5) was melt-blended at 230°C, and the localization of CNTs in the blend was investigated by TEM (Figure 2). From Figure 2(a), the CNTs were observed in the dispersed PA6 phase with no CNTs in the LLDPE matrix. The enlarged view of Figure 2(b) shows that the aggregation of CNTs occurred due to their large specific surface energy. In the blends, it has been known that CNTs tended to disperse in the phase with low viscosity [31]. As long as the viscosities of two polymers are comparable, affinity of CNTs for each component is the main factor determining uneven distribution of CNTs in polymer blends. The CNTs used in this research were multiwalled with hydroxyl groups providing surface functionality, indicating higher affinity of CNTs for PA6 than for LLDPE. Moreover, lower viscosity of PA6 than LLDPE was also benefited for the localization of CNTs in the PA6 phase. Therefore, the combined factors caused that CNTs were dispersed in the PA6 phase rather than in the LLDPE phase.

(a)

(b)

Figure 2: TEM of (a) LLDPE/PA6/CNTs prepared above the melt temperature of LLDPE and PA6 and (b) enlarged view.

Samples of LLDPE/PA6/APP (60/15/25) were prepared by two processing methods, in which LLDPE/PA6 blends were melt-blended with CNTs at 230°C or 160°C, respectively. The samples were characterized by SEM-EDS to investigate the dispersion of APP in the blends. The SEM-EDS results are shown in Figure 3. For the blends prepared at 160°C, the dispersed phase consisted of irregular particles and spherical voids (Figure 3(a)). The P content in the irregular particles was 20.36 wt%, indicating that the irregular particles were APP. For the blends prepared at 230°C, the spherical particles were observed, as shown in Figure 3(b). The EDS results of Figure 3(b) showed that the P content in the spherical particles or continuous phase was 2.27 wt% or 1.27 wt%, respectively. The results indicated that irregular APP particles were coated by the PA6 phase, causing the spherical particles observed and low P contents detected at the surface of spherical particles.

(a)

(b)

Figure 3: SEM-EDS of LLDPE/PA6/APP prepared by different methods. (a) APP blended with LLDPE/PA6 at 160°C, (b) APP blended with LLDPE/PA6 at 230°C.

The melt temperature of PA6 is about 220°C. Therefore, the PA6 phase was melted when APP was processed with LLDPE/PA6 at 230°C. Correspondingly, the PA6 phase did not melt when APP was processed with LLDPE/PA6 at 160°C. Therefore, SEM-EDS results indicated that APP was coated by the melted PA6 phase when APP were melt-blended with LLDPE/PA6 at 230°C. While APP were melt-blended with LLDPE/PA6 at 160°C, APP was obligated to localize at the LLDPE phase due to that the PA6 phase was not melted.

The spontaneous dispersion of APP or CNTs in the PA6 phase of the LLDPE/PA6 blends prepared at 230°C indicated that the localization of APP and CNTs in the blends can be tailored by the methods employed in this paper. The blends with the localization of APP and CNTs in the PA6 phase can be obtained when all the components were melt-blended at 230°C (method 1). For method 2, when LLDPE/PA6 blend was blended with APP and CNTs at 160°C, APP and CNTs were obligated to localize at the LLDPE phase due to that the PA6 phase was not melted. For method 3, CNTs were localized in the PA6 phase due to that CNTs were melt-blended with LLDPE and PA6 at 230°C in the first step. Meanwhile, APP was obligated to localize in the LLDPE phase because APP was melt-blended with LLDPE/PA6/CNTs at 160°C in the second step. In the method 4, LLDPE/PA6/APP blend was prepared at 230°C firstly, and then LLDPE/PA6/APP blend was blended with CNTs at 160°C. Therefore, the blends with the dispersion of APP in the PA6 phase and CNTs in the LLDPE phase can be prepared.

3.2. Flame Retardancy of LLDPE/PA6/APP/CNTs

The flame retardancy of the blends was investigated by LOI and horizontal burning rating (UL 94) tests, as shown in Table 2. The results showed that addition of CNTs, LOI value increased slightly, indicating that CNTs were not an effective flame retardant towards LLDPE/PA6. The presence of APP increased the LOI value of LLDPE/PA6 sharply, as the value was raised from 19.8 to 25.5.

Table 2: Flammability characteristics of blends.

Samples
Flammability
ΔLOI

LOI (%)
UL 94 rating

Blends
19.8
No rating

Blends/CNTs
20.4
No rating

D1
25.5
No rating

M1
24.8
No rating
-1.3

M2
25.5
No rating
-0.6

M3
24.8
No rating
-1.3

M4
28.3
V-1
2.2

The flame retardancy of blends with different CNTs and APP localization is also shown in Table 2. The LOI value of D1 was slightly higher than that of M1~3, and all of the samples failed the vertical UL 94 test by burning completely. The highest LOI value () was observed in M4, and the samples reached the UL 94 V-1 grade. The results show that the localization of CNTs and APP in the LLDPE/PA6/APP/CNT blends significantly affects flame retardancy. The localization of CNTs and APP in the PA6 phase, CNTs and APP in the LLDPE phase, or CNTs in the PA6 phase and APP in the LLDPE phase was unfavorable to improve flame retardancy. The localization of CNTs in the LLDPE phase and APP in the PA6 phase favorably improved flame retardancy.

The comparison of and can be employed to judge the synergism of two kinds of flame retardant according to the formula of . is the LOI value of the composites containing two kinds of flame retardant. or is the LOI value of the composites with flame retardant used alone, is the LOI value of polymer matrix. Two kinds of flame retardant have synergism only when the result of is exhibited. The result of indicates the antagonism of two kinds of flame retardant [29].

The ΔLOI () values of LLDPE/PA6/APP/CNT blends with different APP and CNT localization are also listed in Table 2. The results showed that in M4 and in M1~3, respectively. Therefore, APP-incorporated CNTs had synergism in blends with the localization of APP in the PA6 phase and CNTs in the LLDPE phase. An antagonism between APP and CNTs was exhibited in blends with other localizations of APP and CNTs.

3.3. Morphology of Residual Char

An intumescent flame-retardant primarily plays a role in the condensed phase by forming porous carbonaceous char, which helps to provide effective fire retardation [13, 32]. The characterization of the morphologies of residue char enables an understanding of the relationship between APP and CNT localization and flame retardancy. Photographs of the residual char after cone calorimeter tests are shown in Figure 4. APP localization in the blends exhibits a remarkable influence on the appearance of the residual char. In the samples where APP was localized in the PA6 phase (D1, M1, and M4), the sample holders were covered by a continuous residual char layer. In samples where APP was localized in the LLDPE phases (M2, M3), aluminum foil covering the sample holder was exposed, and residual char accumulated around its inner wall, indicating that discontinuous residual char formed in these samples. Discontinuous residual char provided poor protection on the substrate, causing poor flame retardancy shown for M2 and M3.

(a)

(b)

(c)

(d)

(e)

Figure 4: The photos of the aspect of the crust of blends after a cone calorimeter test.

For blends with continuous residual char, the flame-retardant properties of M1 and D1 were lower than that of M4. In order to understand this, SEM was used to investigate the microstructure of the residual char, as shown in Figure 5. The morphologies of the residual chars in D1, M1, and M4 are entirely different. A continuous residual char with abundant voids is observed in D1. Residual chars in M1 were made of spherical cokes and contained abundant voids. Meanwhile, a tight and compact intumescent char was observed in M4. Compared with a char residue with many voids, a compact residue char was more effective in heat insulation and impeding the release of flammable volatiles, which were benefited for the improvement of flame retardancy. Therefore, good flame retardancy in M4 can be attributed to the formation of compact residue char.

(a)

(b)

(c)

Figure 5: SEM of intumescent char residue for LLDPE/PA6/APP and LLDPE/PA6/APP/CNTs after vertical flammability tests.

3.4. Residual Char Mass

TGA and cone calorimetry were used to investigate the residual char mass of LLDPE/PA6/APP/CNT blends. The residual char mass curves from TGA and cone calorimetry are corresponding to oxygen-free thermal degradation and oxygen combustion, respectively. The curves of residual char mass obtained from the cone calorimeter test are shown in Figure 6. The residual char mass of LLDPE/PA6/APP/CNT blends with APP localized in the PA6 phase (M1 and M4) was higher than that when APP was localized in the LLDPE phase (M2 and M3), indicating that localization of APP in PA6 was favored for improving the residual char mass. For blends with the same APP localization, residual char mass in blends with CNTs localized in the LLDPE phase was higher than that of CNTs in the PA6 phase. For example, the residual char mass of M4 was higher than that of M1, and M2 higher than M3. The results obtained from cone calorimetry show that the localization of APP in the PA6 phase and CNTs in the LLDPE phase can promote charring during combustion, resulting in a high residue char mass.

Figure 6: Residual mass curves of LLDPE/PA6/APP and LLDPE/PA6/APP/CNTs prepared by different processing methods.

The TGA curves for LLDPE/PA6/APP and LLDPE/PA6/APP/CNTs are shown in Figure 7. They show two significant slope changes, proving a two-step process of the degradation. The degradation occurred within the temperature range of 280–360°C was APP thermal degradation and its reaction with PA6 [33, 34]. In this step, the reaction of APP and PA6 produced the intumescent char. It was observed that the degradation rate of LLDPE/PA6/APP/CNT was higher than that of LLDPE/PA6/APP. The results suggested that the intumescent char of LLDPE/PA6/APP was more conductive to delay thermal degradation, resulting in improved thermal stability.

Figure 7: TGA curves of LLDPE/PA6/APP and LLDPE/PA6/APP/CNTs prepared by different processing methods.

The second step began at approximately 360°C, and degradation curves of different blends were overlapped, indicating similar thermal degradation process. As above, the thermal stability shows that the APP and CNT dispersion in blends had a little influence on the degradation of the LLDPE and PA6 at this step. However, APP and CNT dispersion in blends did have a major impact on char residue mass. The char residue mass in blends with APP dispersed in the PA6 phase (M1, M4) was higher than the blends with APP dispersed in the LLDPE phase (M2, M3). The comparison between M1 and M3 or between M2 and M4 shows that the CNT localization in the LLDPE phase is more advantageous to increasing residual char mass than in the PA6 phase.

The results of TGA and cone calorimetry show that APP localization in the PA6 phase or CNTs in the LLDPE phase are both beneficial in promoting charring. Compared to blends with APP dispersed in the PA6 phase, a lower residual char mass was exhibited in blends with APP dispersed in the LLDPE phase regardless of where the CNTs were localized. These results indicate that APP localization in the PA6 phase was more conductive to improve residual char mass than the localization of CNTs in the LLDPE phase. The reaction of APP and PA6 produced the intumescent char. For blends with APP localized in the PA6 phase, APP and PA6 can contact each other, benefitting their reaction and causing the improvement of char residue mass. For blends with APP dispersed in the LLDPE phase, APP and PA6 were insulated from contact by the LLDPE. The isolation between APP and PA6 in blends prevented their interaction, causing a low char residue mass.

3.5. Rheological Behavior

CNTs can form a network structure to enhance the thermal stability of polymer matrix, causing the reduction of heat release rate and the improvement of flame retardancy [5, 7, 8]. A high residual char mass in blends with CNTs dispersed in LLDPE rather than in PA6 may be related to a CNT network structure, and so a rheological test was used to investigate it.

The relationship between storage modulus () and frequency () in LLDPE/PA6/APP/CNTs with different APP and CNT localization is shown in Figure 8. For the blends with CNTs localized in the PA6 phase (M1, M3), was decreased as the reduction of at low frequencies. The results indicated that the blends exhibited Newtonian liquid behavior at low frequencies, suggesting that CNTs cannot form networks. However, in blends with CNT localization in the LLDPE phase (M2, M3), an ideal Hookean solid was observed due to that became nearly constant as the reduction of at low frequencies, indicating the network structure of CNTs formed in these blends.

Figure 8: Storage modulus () of LLDPE/PA6/APP/CNTs prepared by different processing methods versus frequency ().

The results of Figure 8 indicate that localization of CNTs in LLDPE was more favorable to the forming network structure than that in the PA6 phase. SEM results showed that the PA6 phase was the dispersed phase and the LLDPE phase was the continuous phase. The heterogeneous dispersion of CNTs in blends caused CNTs to form a network structure only in the PA6 or the LLDPE phase, respectively. For blends with CNTs dispersed in LLDPE, CNTs can form networks in the continuous LLDPE phase, causing a network structure itself to be continuous and therefore allow it to be detected in blends. For blends with CNTs dispersed in the PA6 phase, the network structure was continuous in the PA6 phase but discontinuous in whole samples. As a result, the network structure in the matrix could not be detected. It can be observed that the residual char mass in continuous blends was higher than in blends with discontinuous network structure. Therefore, it can be concluded that continuous network structure benefitted from improved residual char mass.

The effect of CNT and APP localization in the LLDPE/PA6/APP/CNT blends on complex viscosities () is shown in Figure 9. The increase of caused the decrease of , indicating shear thinning behaviors. Although the components’ proportion in blends was kept consistent, the at low frequencies was quite different. High complex viscosities were observed in blends with CNTs localized in the continuous LLDPE phase (M2, M4). In general, the formation of a CNT network structure can increase melt viscosity. Therefore, high complex viscosity in blends of M2 and M4 could be attributed to a continuous network structure. In the blends with CNTs localized instead in the PA6 phase (M1, M3), the network structure was discontinuous in the matrix even though the CNTs also formed a network structure in the dispersed PA6 phase, with the CNTs’ network structure showing a weak influence on melt viscosity.

Figure 9: Complex viscosity () of LLDPE/PA6/APP/CNTs prepared by different processing methods versus frequency ().

High viscosity of blends can hinder the penetration of O₂ and combustible gas [16], enhances the antidripping property, causing the improvement of flame retardancy. Therefore, the synergistic effect between CNTs and APP in blends with APP localization in the PA6 phase may be related to its high melt viscosity.

3.6. Cone Calorimeter

The cone calorimeter was employed to investigate the combustion process of LLDPE/PA6/APP and LLDPE/PA6/APP/CNTs, as shown in Figure 10 and Table 3. Compared with LLDPE/PA6 composites, the time to ignition (TTi) increases after CNTs were added. This is due to that the good thermal conductivity of carbon nanotubes can effectively transfer heat to the interior of the composites, causing the increase of TTi. The lowest values of peak HHR and maximum of the average rate of heat emission (MAHRE) were exhibited in M4, compared with D1, M1, M2, and M3. The difference of the THR values between D1 and M4 was small and lower than that of M1, M2, and M3. The results indicated the best flame retardancy was exhibited in M4, which was in accordance with the results of LOI and UL 94.

(a)

(b)

Figure 10: Heat release rate curves (a) and smoke production rate curves (b) of LLDPE/PA6/APP and LLDPE/PA6/APP/CNTs prepared by different processing methods.

Table 3: Cone calorimeter data of blends.

Sample code
Peak HRR (kW/m²)
THR (MJ/m²)
MAHRE (kW/m²)
Time to ignition (s)

D1
222.15
114.78
132.37
84

M1
242.00
110.92
144.65
116

M2
245.29
111.99
160.15
117

M3
284.77
122.70
182.32
128

M4
206.80
116.26
123.97
158

In Figure 10, the HRR curves show a double PHRR and double peaks are also exhibited in the smoke production rate (SPR) curves, indicating two processes of combustion. According to other studies [12, 32, 35], the first peak or the second peak is ascribed to the ignition and the formation of an intumescent shield and is assigned to the destruction of the intumescent shield and the formation of a carbonaceous residue. It was observed that the first PHRR value of LLDPE/PA6/APP was lower than that of LLDPE/PA6/APP/CNT blends prepared by different methods. TG results also showed that the expanded protective shield of LLDPE/PA6/APP/CNTs provided weaker protection on matrix than that of LLDPE/PA6/APP in the initial degradation period. The results indicated that CNTs went against the formation of expanded protective shield. High HHR in LLDPE/PA6/APP/CNTs suggested that the samples burned more violently, causing high SPR. The second PHHR value of D1 was lower than that of M2 and M3 and higher than that of M1 and M4. Meanwhile, the SPR values of M1 and M4 were lower than that of D1 after 600 s. The char residue of M1 and M4 was higher than that of D1, as shown in Figures 6 and 7. Therefore, the results indicated that CNTs can improve the carbonaceous residue only when APP is localized in the PA6 phase of blends, causing lower PHHR and SPR.

For the LLDPE/PA6/APP blend (D1), dispersed PA6 particles acted as “flame-retardant particles” because of the flame-retardant properties of dispersed APP in the PA6 phase. In the first combustion process, the decomposition of APP and interaction of APP and PA6 transformed “flame-retardant particles” into charred particles. NH₃ produced by the decomposition of APP expanded charred particles until they joined each other to form a continuous residual char. With the development of the combustion process (the second process), the accumulation of pyrolysis gas in the interior of the material caused the rupture of the superficial intumescent char, resulting in the formation of residual char with abundant voids, as shown in Figure 5(a).

When APP and CNTs were both localized in the PA6 phase (M1), the first or second PHRR and SPR values were higher or lower than that of LLDPE/PA6/APP, respectively. In this blend, dispersed PA6 particles also acted as “flame-retardant particles” due to their localization. CNTs sharply increased the melt viscosity of the PA6 particles due to the formation of a network structure in the PA6 phase. In the first combustion process, “flame-retardant particles” were transformed into charred particles. However, high melt viscosity of the PA6 phase restrained the coalescence of intumescent charred particles to form a continuous char layer, resulting in a high PHRR value in the first combustion process. The morphology of spherical cokes formed in the residual char also indicates the inhibition of CNTs in aggregating into charred particles. Spherical cokes cannot provide effective protection for the matrix during the combustion, resulting in poor flame retardancy exhibited in this blend.

Good flame retardancy was exhibited in the blend with APP localized in the PA6 phase and with CNTs in the LLDPE phase (M4). The results attributed the network structure of CNTs formed in the continuous LLDPE phase and the localization of APP in the PA6 phase. In the initial combustion process, interaction of APP and PA6 formed charred particles. Network structure increased the melt viscosity of the LLDPE and delayed the aggregation of charred particles, resulting in its initial PHRR values being higher than that of LLDPE/PA6/APP in its initial stage. As combustion developed, CNTs would perform three beneficial roles. The localization of the CNTs in LLDPE improved the residual char mass, causing a thick char layer. Meanwhile, the network structure of the CNTs reinforced that char layer. The high strength of the intumescent char layer was difficult to rupture during combustion, causing the formation of a tight and compact intumescent char layer. Moreover, CNTs can enhance the antidripping property due to that CNTs improved melt viscosity. As a result, the synergistic effect between CNTs and APP was exhibited.

For blends where APP was dispersed in the LLDPE phase (M2 and M3), the first or second PHRR values were both higher than that of LLDPE/PA6/APP, indicating an antagonistic effect between the CNTs and APP with regard to flame retardancy. The results of TGA and the cone calorimeter tests both indicate that the localization of APP in blends played a key role in the continuity of the residual char layer. The separation of APP and PA6 did not benefit their interaction, causing low residual char and the formation of a discontinuous char layer. Clearly, this kind of char layer provided poor protection for the substrate, causing poor flame retardancy.

4. Conclusion

The influence of localization of CNTs and APP in LLDPE/PA6/APP/CNT blends on flame retardancy was investigated. The different localization of CNTs and APP in blends formed different morphologies of a residual char layer, causing remarkable differences in flame retardancy. For blends where APP was localized in the LLDPE phase and CNTs were localized in the PA6 or LLDPE phases, APP localization in the LLDPE phase contributed to a decrease in the interaction of APP and PA6, causing a low residual char mass and a discontinuous char layer to appear, resulting in poor flame retardancy. For blends with APP and CNT localization in the PA6 phase, even though the APP localization in the PA6 phase promoted the formation of a continuous residual char layer, the CNT network structure in the PA6 phase restrained the swell and coalescence of the char layer, causing a continuous residual char layer to appear that consisted of microscopic spherical cokes. As a result, poor flame retardancy was exhibited in this blend. For blends where APP and CNTs were localized in the LLDPE phase, the localization of APP in the PA6 phase benefitted the formation and appearance of a continuous char layer. In addition to reinforcing the residual char layer, the CNT network structure in the LLDPE phase showed poor influence on the swell and coalescence of the char layer. As a result, a tight and compact intumescent char layer was formed, causing good flame retardancy.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge the financial support of this work by the National Natural Science Foundation of China (Contract Number: 51673059), the Natural Science Foundation of Education Department of Henan Province (Contract Number: 17A150009), the Natural Science Foundation of Henan Province (Contract Number: 112300410208), and the Project National United Engineering Laboratory for Advanced Bearing Tribology of Henan University of Science and Technology (Contract Number: 201813).

References

S. Bourbigot, F. Samyn, T. Turf, and S. Duquesne, “Nanomorphology and reaction to fire of polyurethane and polyamide nanocomposites containing flame retardants,” Polymer Degradation and Stability, vol. 95, no. 3, pp. 320–326, 2010.

J. B. Dahiya, S. Rathi, H. Bockhorn, M. Haußmann, and B. K. Kandola, “The combined effect of organic phoshphinate/ammonium polyphosphate and pentaerythritol on thermal and fire properties of polyamide 6-clay nanocomposites,” Polymer Degradation and Stability, vol. 97, no. 8, pp. 1458–1465, 2012.

J. Alongi, M. Poskovic, P. M. Visakh, A. Frache, and G. Malucelli, “Cyclodextrin nanosponges as novel green flame retardants for PP, LLDPE and PA6,” Carbohydrate Polymers, vol. 88, no. 4, pp. 1387–1394, 2012.

T. Kashiwagi, E. Grulke, J. Hilding, R. Harris, W. Awad, and J. Douglas, “Thermal degradation and flammability properties of poly(propylene)/carbon nanotube composites,” Macromolecular Rapid Communications, vol. 23, no. 13, pp. 761–765, 2002.

S. Peeterbroeck, F. Laoutid, B. Swoboda et al., “How carbon nanotube crushing can improve flame retardant behaviour in polymer nanocomposites?” Macromolecular Rapid Communications, vol. 28, no. 3, pp. 260–264, 2007.

H. Y. Ma, P. A. Song, and Z. P. Fang, “Flame retarded polymer nanocomposites: development, trend and future perspective,” Science China Chemistry, vol. 54, no. 2, pp. 302–313, 2011.

B. Schartel, P. Potschke, U. Knoll, and M. Abdel-Goad, “Fire behaviour of polyamide 6/multiwall carbon nanotube nanocomposites,” European Polymer Journal, vol. 41, no. 5, pp. 1061–1070, 2005.

T. Kashiwagi, E. Grulke, J. Hilding et al., “Thermal and flammability properties of polypropylene/carbon nanotube nanocomposites,” Polymer, vol. 45, no. 12, pp. 4227–4239, 2004.

B. Du and Z. Fang, “Effects of carbon nanotubes on the thermal stability and flame retardancy of intumescent flame-retarded polypropylene,” Polymer Degradation and Stability, vol. 96, no. 10, pp. 1725–1731, 2011.

S. Bourbigot, S. Duquesne, G. Fontaine, S. Bellayer, T. Turf, and F. Samyn, “Characterization and reaction to fire of polymer nanocomposites with and without conventional flame retardants,” Molecular Crystals and Liquid Crystals, vol. 486, no. 1, pp. 325/[1367]–339/[1381], 2008.

M. Chen, Y. Xu, X. Chen et al., “Thermal stability and combustion behavior of flame-retardant polypropylene with thermoplastic polyurethane-microencapsulated ammonium polyphosphate,” High Performance Polymers, vol. 26, no. 4, pp. 445–454, 2014.

S. Bourbigot, B. M. Le, C. I. Siat, and M. Lewin, Recent advances in flame retardancy of polymeric materials, BCC Pub, Norwalk, 1997.

X. Wang, Y. Li, W. Liao, J. Gu, and D. Li, “A new intumescent flame-retardant: preparation, surface modification, and its application in polypropylene,” Polymers for Advanced Technologies, vol. 19, no. 8, pp. 1055–1061, 2008.

R. Delobel, M. L. Bras, N. Ouassou, and F. Alistiqsa, “Thermal behaviours of ammonium polyphosphate-pentaerythritol and ammonium pyrophosphate-pentaerythritol intumescent additives in polypropylene formulations,” Journal of Fire Sciences, vol. 8, no. 2, pp. 85–108, 1990.

K. S. Lim, S. T. Bee, L. T. Sin et al., “A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites,” Composites Part B: Engineering, vol. 84, pp. 155–174, 2016.

S. H. Chiu and W. K. Wang, “Dynamic flame retardancy of polypropylene filled with ammonium polyphosphate, pentaerythritol and melamine additives,” Polymer, vol. 39, no. 10, pp. 1951–1955, 1998.

X. Almeras, M. Le Bras, P. Hornsby et al., “Effect of fillers on the fire retardancy of intumescent polypropylene compounds,” Polymer Degradation and Stability, vol. 82, no. 2, pp. 325–331, 2003.

Y. Ma, P. Ma, Y. Ma, D. Xu, P. Wang, and R. Yang, “Synergistic effect of multiwalled carbon nanotubes and an intumescent flame retardant: toward an ideal electromagnetic interference shielding material with excellent flame retardancy,” Journal of Applied Polymer Science, vol. 134, no. 31, 2017.

Q. Zhao, Y. Hu, and X. Wang, “Mechanical performance and flame retardancy of polypropylene composites containing zeolite and multiwalled carbon nanotubes,” Journal of Applied Polymer Science, vol. 133, no. 3, article 42875, 2016.

G. Huang, S. Wang, P.’. Song, C. Wu, S. Chen, and X. Wang, “Combination effect of carbon nanotubes with graphene on intumescent flame-retardant polypropylene nanocomposites,” Composites Part A: Applied Science and Manufacturing, vol. 59, pp. 18–25, 2014.

H. Y. Ma, L. F. Tong, Z. B. Xu, and Z. P. Fang, “Functionalizing carbon nanotubes by grafting on intumescent flame retardant: nanocomposite synthesis, morphology, rheology, and flammability,” Advanced Functional Materials, vol. 18, no. 3, pp. 414–421, 2008.

C. Lu, D. Yang, Q. Q. Cao, X. H. Huang, J. C. Liu, and Y. Q. Zhang, “Influence of morphology on the flame retardancy of polystyrene/nylon-6/ammonium polyphosphate/clay blends,” High Performance Polymers, vol. 26, no. 5, pp. 507–516, 2014.

C. Lu, Q. Q. Cao, X. N. Hu, C. Y. Liu, X. H. Huang, and Y. Q. Zhang, “Influence of morphology and ammonium polyphosphate dispersion on the flame retardancy of polystyrene/nylon-6 blends,” Fire and Materials, vol. 38, no. 8, pp. 765–776, 2014.

M. Entezam, H. A. Khonakdar, and A. A. Yousefi, “On the flame resistance behavior of PP/PET blends in the presence of nanoclay and a halogen-free flame retardant,” Macromolecular Materials and Engineering, vol. 298, no. 10, pp. 1074–1084, 2013.

S. Pack, M. Si, J. Koo et al., “Mode-of-action of self-extinguishing polymer blends containing organoclays,” Polymer Degradation and Stability, vol. 94, no. 3, pp. 306–326, 2009.

S. Pack, T. Kashiwagi, D. Stemp et al., “Segregation of carbon nanotubes/organoclays rendering polymer blends self-extinguishing,” Macromolecules, vol. 42, no. 17, pp. 6698–6709, 2009.

C. Lu, X. P. Gao, D. Yang et al., “Flame retardancy of polystyrene/nylon-6 blends with dispersion of clay at the interface,” Polymer Degradation and Stability, vol. 107, pp. 10–20, 2014.

C. Lu, X. P. Gao, D. H. Yao, C. L. Cao, and Y. J. Luo, “Improving flame retardancy of linear low-density polyethylene/nylon 6 blends via controlling localization of clay and intumescent flame-retardant,” Polymer Degradation and Stability, vol. 153, pp. 75–87, 2018.

J. Jin, H. Wang, Z. Shu, and L. Lu, “Impact of selective dispersion of intumescent flame retardant on properties of polypropylene blends,” Journal of Materials Science, vol. 52, no. 6, pp. 3269–3280, 2017.

Y. Chen, Z. Guo, and Z. Fang, “Relationship between the distribution of organo-montmorillonite and the flammability of flame retardant polypropylene,” Polymer Engineering & Science, vol. 52, no. 2, pp. 390–398, 2012.

R. A. Khare, A. R. Bhattacharyya, A. R. Kulkarni, M. Saroop, and A. Biswas, “Influence of multiwall carbon nanotubes on morphology and electrical conductivity of PP/ABS blends,” Journal of Polymer Science Part B: Polymer Physics, vol. 46, no. 21, pp. 2286–2295, 2008.

S. Bourbigot, M. Le Bras, and R. Delobel, “Fire degradation of an intumescent flame retardant polypropylene using the cone calorimeter,” Journal of Fire Sciences, vol. 13, no. 1, pp. 3–22, 1995.

S. V. Levchik, L. Costa, and G. Camino, “Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides: part II—polyamide 6,” Polymer Degradation and Stability, vol. 36, no. 3, pp. 229–237, 1992.

S. V. Levchik, G. F. Levchik, A. I. Balabanovich, G. Camino, and L. Costa, “Mechanistic study of combustion performance and thermal decomposition behaviour of nylon 6 with added halogen-free fire retardants,” Polymer Degradation and Stability, vol. 54, no. 2-3, pp. 217–222, 1996.

S. Bourbigot, M. Le Bras, R. Delobel, P. Bréant, and J. M. Tremillon, “4A zeolite synergistic agent in new flame retardant intumescent formulations of polyethylenic polymers — Study of the effect of the constituent monomers,” Polymer Degradation and Stability, vol. 54, no. 2-3, pp. 275–287, 1996.