Effect of modified layered double hydroxide on the flammability and mechanical properties of polypropylene

2.1 Materials

All materials were used as received without any further purification. Anhydrous aluminum chloride (AlCl₃) sodium chloride (NaCl), and methanol (CH₃OH) were purchased from Alfa Aesar Co. Sodium stearate (C₁₈H₃₅O₂Na) and sodium hydroxide (NaOH) were received from Merck Aldrich (Germany), whereas nickel chloride (Co. NiCl₂.6H₂O) and copper chloride (CuCl₂.2H₂O) were supplied by Sdfine. Chem Limited, Co (UK).

2.2 Preparation of nanosized copper aluminum LDH and nickel-aluminum LDH

Nanosized LDHs were synthesized by using the co-precipitation method [21]. Anhydrous aluminum chloride (13.33 g) was dissolved in 200 ml distilled water in the presence of 19.99 g (0.342 m) of NaCl in a three-neck round-bottom flask under magnetic stirrer (800 rpm). Copper chloride dehydrate (34.52 g) was separately dissolved in 200 ml distilled water. Copper solution was mixed with aluminum solution for 30 min at 800 rpm and at room temperature. The pH of the resulting copper and aluminum salt mixture solution was around 3.95. Sodium hydroxide (50 wt%) solution was added drop-wise to the salt mixture solution with vigorous stirring to reach pH of 10.28. The resulting slurry was aged in the mother liquid for 24 h at 70°C. LDH precipitate was collected by filtration using a Buchner funnel. The collected LDH precipitate was washed several times by using distilled water and dried at 70°C in an oven for 24 h. Ni-Al LDH was typically synthesized and processed as Cu-Al LDH. The resulting LDHs were marked as Cu-LDH and Ni-LDH for Cu and Ni ion, respectively.

2.3 Preparation of nanosized modified copper-aluminum LDH and nickel- aluminum LDH by sodium stearate

Cu-LDH (15 g) or Ni-LDH (15 g) was dispersed in 400 ml of sodium stearate solution (0.008 m) and then vigorously stirred for 48 h at 100°C. The resulting organic modified LDHs were collected by filtration, washed several times using distilled water, and then dried at 70°C in an oven for 24 h. The resulting modified LDHs were labeled as m.Cu-LDH and m.Ni-LDH.

2.4 Processing of PP/LDH nanocomposites

Different percentages (0.5, 1, 1.5 and 2 wt%) of LDHs without further additives were mixed with polypropylene powder. The PP/LDHs mixture was fed into an extruder (at 220°C and 36 rpm). The PP/LDH composites were shredded using a crusher and injected using injection molding. The composition of the PP/LDH composites is listed in Table 1. Organic m.Ni-LDH with three different loadings (0.5, 1, and 1.5 wt%) are illustrated in Figure 1. The critical limit of Ni-LDHs and Cu-LDHs in the PP matrix was found to be 1.5%, at which LDHs aggregated at 2% and were no longer completely spattered by the PP matrix loading. Therefore, in this study, different percentages of LDHs were dispersed and processed in neat PP, namely, 0, 0.5, 1.0 and 1.5 (wt./wt)%.

Figure 1:

PP/m.Ni-LDHs (H1:0.5 wt%, H2:1 wt%, and H3:1.5 wt%).

2.5 Characterization

3.1 X-ray diffraction (XRD)

The XRD patterns of nanosized Ni-LDH, m.Ni-LDH, Cu-LDH, and m.Cu-LDH, are shown in Figure 2(A) and (B). The peaks of Ni-LDH are recorded at 2θ=32°, 46°, and 56°. The sharp peaks of Cu-LDH are recorded at 2θ=15°, 32°, 40°, and 46°. The peaks of m.Ni-LDH and m.Cu-LDH are broad, and the peaks at 46° and 56° could not be detected for the m.Ni-LDH sample. Meanwhile, the peak at 2θ=46° is not found in m.Cu-LDH. These broad and non-detectable peaks in the m-LDH sample can be attributed to the intercalation of modified hydroxide layers in the presence of stearate. The extension between the layers of double hydroxide was achieved as a result of the intercalation of stearate chains onto the layered double hydroxide [22]. The shifting and broadening of peaks due to an increase of basal spacing) and the increase of inter layer space depended on the intercalation nature of the functional group, the size of the surfactant, and the alkyl chain length of sodium stearate [23].

Figure 2:

XRD patterns of (A) nanosized Ni-LDH and m.Ni-LDH and (B) Cu-LDH and m.Cu-LDH.

3.2 Thermogravimetric analysis (TGA)

Figure 3(A) shows that the thermal stability of PP can be significantly improved in the presence of m.Ni-LDH compared with PP, PP/Ni-LDH, and PP/Cu-LDH. This can be ascribed to the enhanced interfacial force between PP and Ni-LDH in the presence of stearate. PP and PP/LDH composites exhibit a one-step degradation process. The thermal degradation of neat PP starts dramatically at 425°C. The degradation stage of neat polypropylene shifts from 421.85°C to 426.6°C, 450°C and 421.85°C for H3, H9, and H6, respectively. The highest thermal stability of the PP/m.Ni-LDH composite is observed when the weight decreased significantly at 437.9°C. The values of onset decomposition temperature values are presented in Table 2. The temperature values at weight loss (10%) and (50%) are presented as (T₁₀) and (T₅₀), respectively.

Figure 3:

(A) Thermogravimetric analysis (TGA) and (B) derivative TGA of the PP/LDH composites containing different LDH contents.

3.3 Scanning electron microscopy (SEM)

Figure 4(A) shows the SEM images of Ni-LDH. As can be seen, the unit cell of the LDH is in the nanoscale range of <50 nm. Figure 4(B) shows that the LDH size is <100 nm in the Cu-LDH. These results are similar with those reported by Choi et al. [24].

Figure 4:

(A) SEM of nanosized Ni-LDH, (B) SEM of nanosized Cu-LDH.

3.4 Mechanical properties

The results of tensile modulus, as presented in Figure 5, suggest that LDHs are able to impart stiffness to the composites. The addition of stearate to PP does not seem to contribute to the significant improvement of the tensile strength of the composites. The mechanical properties of PP and the PP/LDH nanocomposites were tabulated in Table 3. Bassyouni [23] suggests that stiffness does not primarily depend on the particle-matrix interface, but more likely on the entire particle’s contents in the tensile loading direction, because the modulus of elasticity is defined as a tangent modulus at low strain values (off set 0.05%–0.25%). According to Abalov et al. and Zoromba et al. [25], [26], [27], the incorporation of inorganic fillers into a thermoplastic matrix can increase or decrease the tensile strength of the resulting nanocomposites. A slight improvement in tensile strength has been detected in presence of stearate (PP/m.Cu-LDH and PP/m.Ni-LDH). For the H3 composite, the tensile strength improved to 33.7 [N/mm²] and elongation at break decreased to 31%. For the H6 composite, the elongation at break was enhanced to 62%; its modulus of elasticity also improved by 18% with 489 [N/mm²] compared with neat PP.

Figure 5:

Stress–strain curve for the PP and PP/LDH nanocomposites.

3.5 Flammability properties of PP/Ni-LDH and PP/m.Ni-LDH nanocomposites

The flammability properties of the PP/Ni-LDH and PP/m.Ni-LDH nanocomposites were evaluated using a cone calorimeter. The cone calorimeter data are listed in Table 4. The neat PP (H0) burns very fast after ignition, and a sharp PHHR appears at 1831.96 kW/m². For the PP/Ni-LDH samples (H1, H2, and H3 for 0.5%, 1.0%, and 1.5% Ni, respectively), all composites showed a reduction in PHHR compared with neat PP. The reduction rates are 10.72%, 21.90%, and 30.80% for H1, H2, and H3, respectively, indicating that, with the increase in Ni-LDH loading to PP, the PHHR decreases, as shown in Figure 6. The same behavior reduction of PHHR is detected (39.06%, 43.95%, and 31.59%) for the modified composites PP/m.Ni-LDH (H4, H5, and H6, respectively, as shown in Figure 7. The PHHR values increase in relation to PP/Ni-LDH and PP/m/Ni-LDH nanocomposites by increasing the percentage of Ni-LDH or m.Ni-LDH. The increasing PHHR values in case of PP/m/Ni-LDH is lower than in the case of PP/Ni-LDH. This can be attributed to the existence of stearate in the modified LDH. The H4 and H5 composites show THR of 70.2 MJ/m² (36.64% reduction) and 81.24 MJ/m² (26.67% reduction), respectively, in comparison to neat PP, which has a THR of 110.8 MJ/m². Meanwhile, the H4 and H5 composites showed reduced EHC and SEA values compared with H0, as shown in Table 4.

Figure 6:

Heat release rate curves of neat PP (H0) and the PP/Ni-LDH nanocomposites.

Figure 7:

Heat release rate curves of neat PP (H0) and the PP/m.Ni-LDH nanocomposites.

The fire growth rate (FIGRA) was calculated as PHRR/time to PHRR. This can serve as the basis of predicting fire spread rate and the size of a fire [28]. The FIGRA values for the PP/Ni-LDH and PP/m.Ni-LDH composites show a positive sign in terms of the contribution to the fire growth of materials, as listed in Table 4. The decrease in FIGRA value presents better cone calorimeter performance. Table 4 shows that all PP/Ni-LDH and PP/m.Ni-LDH composites have decreased FIGRA values when the content of Ni-LDH loading increased in the following order H1>H2>H3 for PP/Ni-LDH and H4>H6>H7 for PP/m.Ni-LDH compared with neat PP (H0). Furthermore, the fire performance index (FPI) was calculated as the ratio between TTI to PHRR, as given in Equation (1) below.

This FPI value gives interesting information about all PP/Ni-LDH and PP/m.Ni-LDH compositions. These results are used in this study as a reference for the degree of fire hazard involved [29], [30], [31]. The properties of the final masses of the samples are presented in Table 4. The highest final masses corresponding to the PP show the highest FPI values. Hence, all PP/Ni-LDH and PP/m.Ni-LDH composites (H1, H3, H4, and H5) are completely effective.

3.6 Flammability properties of PP/Cu-LDH and PP/m.Cu-LDH nanocomposites

PP/Cu-LDH composites (H7, H8, and H9), show a reduction in PHHR compared with neat PP (H0). The reductions are detected by 43.94% for H7, 30.32% for H8, and 20.85% for H9, as shown in Figure 8. These values refer to the increase in PHHR with increasing Cu-LDH % loadings, as shown in Figure 9. The modified composites (H11, H12, and H13) show a reduction in PHHR values by 46.18%, 35.81%, and 26.57%, respectively, as shown in Figure 9. With increasing percentage of m.Cu-LDH, the PHHR also increases with respect to PP/Cu-LDHs. The H7 sample shows THR of 81.2 MJ/m² (36.64% reduction), whereas all samples have a THR value greater than that of neat PP. All PP/Cu-LDH and PP/m.Cu-LDH composites showed slightly changed EHC and SEA values compared with neat PP.

Figure 8:

Heat release rate curves of neat PP and the PP/Cu-LDH nanocomposites.

Figure 9:

Heat release rate curves of neat PP and the PP/m.Cu-LDH nanocomposites.

The fire growth rates (FIGRA) of all PP/Cu-LDH and PP/m.Cu-LDH composites are shown in Table 5. As can be seen, FIGRA decreases with the increase in the unmodified Cu-LDH loadings in the following order: H7>H9>H8 for PP/Cu-LDH. The FPI values of all PP/Cu-LDH and PP/m.Cu-LDH are presented in Table 5.