Polypropylene/Layered Double Hydroxide Nanocomposites: Influence of LDH Intralayer Metal Constituents on the Properties of Polypropylene.

Sonication-assisted delamination of layered double hydroxides (LDHs) resulted in smaller-sized LDH nanoparticles (∼50-200 nm). Such delaminated Co-Al LDH, Zn-Al LDH, and Co-Zn-Al LDH solutions were used for the preparation of highly dispersed isotactic polypropylene (iPP) nanocomposites. Transmission electron microscopy and wide-angle X-ray diffraction results revealed that the LDH nanoparticles were well dispersed within the iPP matrix. The intention of this study is to understand the influence of the intralayer metal composition of LDH on the various properties of iPP/LDH nanocomposites. The sonicated LDH nanoparticles showed a significant increase in the crystallization rate of iPP; however, not much difference in the crystallization rate of iPP was observed in the presence of different types of LDH. The dynamic mechanical analysis results indicated that the storage modulus of iPP was increased significantly with the addition of LDH. The incorporation of different types of LDH showed no influence on the storage modulus of iPP. But considerable differences were observed in the flame retardancy and thermal stability of iPP with the type of LDH used for the preparation of nanocomposites. The thermal stability (50% weight loss temperature (T 0.5)) of the iPP nanocomposite containing three-metal LDH (Co-Zn-Al LDH) is superior to that of the nanocomposites made of two-metal LDH (Co-Al LDH and Zn-Al LDH). Preliminary studies on the flame-retardant properties of iPP/LDH nanocomposites using microscale combustion calorimetry showed that the peak heat release rate was reduced by 39% in the iPP/Co-Zn-Al LDH nanocomposite containing 6 wt % LDH, which is higher than that of the two-metal LDH containing nanocomposites, iPP/Co-Al LDH (24%) and iPP/Zn-Al LDH (31%). These results demonstrated that the nanocomposites prepared using three-metal LDH showed better thermal and flame-retardant properties compared to the nanocomposites prepared using two-metal LDH. This difference might be due to the better char formation capability of three-metal LDH compared to that of two-metal LDH.

Introduction

Layered materials are an interesting class of compounds, consisting of a two-dimensional (2D) sheet-like structure with strong bindings within the individual layer and weak van der Waals forces between the layers.[1−4] The weak interaction between these layers allows them to be exfoliated or delaminated into ultrathin 2D nanosheets. These ultrathin nanomaterials have gained significant interest after the discovery of exfoliated graphene from graphite.[5] Dozens of 2D nanomaterials have been reported in the last few years, including transition metal oxides, transition metal dichalcogenides, hexagonal boron nitride, synthetic silicate clays, layered metal oxides, layered double hydroxides (LDHs), etc.[3,6−9] Because of the promising applications of these ultrathin 2D nanomaterials, a large number of synthetic methods, such as mechanical cleavage,[5,10] chemical vapor deposition,[11] ion-intercalation and exfoliation,[2] liquid exfoliation,[3,6] etc., have been developed.

Polymer matrices filled with nanosized particles have drawn widespread attention of researchers due to the promise of combining the superior thermal and mechanical properties of nanoparticles with the flexibility and processability of polymers.[7−9,12−16] The performance of the nanocomposites depends on many factors, such as the degree of dispersion of nanoparticles in the polymer matrix, compatibility between the nanoparticles and polymer, and the aspect ratio of the nanofillers used.[7−9,12−14] In recent years, a range of layered materials of varying dimensions have been used as nanofillers in the preparation of polymer nanocomposites due to their exciting properties.[7−9,12,14,15]

In recent times, LDHs, also known as anionic clays, are considered to be an emerging class of layered nanofillers for the preparation of polymer nanocomposites.[7,15−32] The flexibility in tuning the type of metal ions in inorganic layers, as well as balancing anions in the interlayer space (easily exchangeable with other anionic species), enables these materials to find many applications, including in catalysis, pharmaceuticals, UV absorbents, CO2 adsorbents, and flame-retardant additives.[7,17,18,33,34] The exfoliation of LDH in the polymer matrix is always a challenge because of the stronger interlayer electrostatic interactions. Recently, liquid exfoliation of LDH has drawn increasing attention as a process to produce ultrathin nanosheets with drastically improved surface activity.[4,18,21,28,35,36] This method is preferred over the other methods by polymer scientists, as the aggregation of exfoliated nanosheets can be prevented by polymers, which results in the formation of highly dispersed polymer/LDH nanocomposites by the solution blending method.[18,21,36,37] Moreover, this method does not require the prior modification of LDH with organic modifiers or surfactants. The degree of dispersion and the exfoliation of layered materials in polymer matrices have been shown to improve optical, thermal, rheological, flame retardancy, and mechanical properties of the base polymer.[7,18,22,23,31,37]

Recently, highly dispersed polymer/LDH nanocomposites have been prepared by the liquid exfoliation method. Wang et al. converted the hydrophilic LDH layers to hydrophobic ones by an aqueous miscible organic solvent treatment (AMOST) and prepared the stable dispersions of LDH in nonpolar solvents.[21,28,36,38] Subsequently, the exfoliated LDH nanosheets were used to prepare polypropylene/LDH nanocomposites by the solution blending method.[21,28] Using this method, it was shown that the PP nanocomposites prepared with Zn2Al-borate displayed better performance than the nanocomposites prepared with the equivalent Mg3Al-borate.[28] In another study, Wang and co-workers revealed that the gallery anions resulted in a significant difference in the properties of the polymer/LDH nanocomposites.[37] The synergistic effect of different LDH with the combination of an intumescent flame-retardant additive was studied by the melt blending method, and it was shown that the nanocomposite containing ternary LDH is a better flame-retardant additive.[39] Recently, we have reported highly dispersed isotactic polypropylene (iPP)/LDH nanocomposites using two different-sized (lateral size) LDH and showed that the lateral size of the LDH has a significant influence on the thermal stability and crystallization rate of iPP.[18]

Here, we prepared iPP/LDH nanocomposites filled with three different types of LDH (Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH) by the solvent mixing method, particularly using the sonicated LDH (fragmented LDH). Both LDH and iPP/LDH nanocomposites were carefully characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and wide-angle X-ray diffraction (WAXD). This study aims at unraveling the influence of the intralayer metal constituents of LDH on the crystallization rate, thermal stability, mechanical properties, and flame retardancy of iPP nanocomposites. It was found that iPP/LDH nanocomposites containing three-metal LDH showed better performance in thermal stability and flame retardancy compared to iPP/LDH nanocomposites containing two-metal LDH with the same loading.

Results and Discussion

Characterization of LDH and Their Exfoliated Nanosheets

Both two-metal LDH (Co–Al and Zn–Al LDH) and three-metal LDH (Co–Zn–Al LDH) were synthesized under similar conditions by the co-precipitation method. The powder X-ray diffraction (XRD) patterns of LDH are displayed in Figure a. The sharp and symmetric features of the X-ray reflections corresponding to the (00n) planes suggest that the produced LDH have well-organized 2D layer stacking. A slight difference in the peak positions of (00n) planes of different LDH may be due to the presence of either different anions or number of water molecules within the LDH layer.[37,40] Several other reflections assigned to the lattice were observed in the 2θ range 30–70° (Figure S1). Overall, the X-ray pattern of the Co–Zn–Al LDH was nearly identical to those of Co–Al LDH and Zn–Al LDH in terms of peak positions and intensities. No peaks other than the typical LDH were detected, indicating the high purity of the obtained products. In the literature, a similar observation was made in the X-ray patterns of Mg–Al LDH upon the substitution of Mg2+ with Co2+ to obtain the Mg–Co–Al LDH.[41]Figure b shows the Fourier transform infrared spectroscopy (FTIR) spectra of the various LDH studied in this work. All LDH exhibited the characteristic bands for interlayer carbonate (CO32–) and interlayer nitrate (NO3–) at 1356 and 1382 cm–1, respectively. The conditions favoring the formation of CO32– and NO3– were explained in our previous paper.[18] The presence of IR bands at 3440 cm–1 (ν (O-H)) and 1632 cm–1 (δ (H2O)) confirmed the presence of interlayer water molecules.

Figure 1

(a) Powder XRD patterns and (b) FTIR spectra of as-prepared Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH.

To obtain the information about the size and shape of the as-synthesized LDH (bulk), the scanning electron microscopy (SEM) and TEM images were taken and are shown in Figure . The as-prepared LDH platelets displayed a three-dimensional (3D) platelet-like morphology with a dark contrast, indicating the presence of several layers of platelets in a single particle. However, the sizes and shapes of these LDH were found to be different from each other. The Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH adopted different morphologies such as circular platelets, hexagonal platelets with rounded edges, and hexagonal platelets with sharp edges, respectively. This difference could be associated with the nucleation and growth mechanism of LDH in different reaction environments (metal salts) and conditions.[42] The chemical compositions of the different LDH were confirmed by energy-dispersive X-ray spectra (EDS) (Figure ). The dominant oxygen peak in EDS of different LDH indicates the presence of water molecules within the layers as well as hydroxyl groups associated with the layers of the as-synthesized LDH platelets.

Figure 2

SEM, TEM, and EDS analysis of the as-synthesized LDH: (a) Co–Al LDH, (b) Zn–Al LDH, and (c) Co–Zn–Al LDH.

O’Hare and co-workers have recently reported an AMOST method for the preparation of stable dispersions of LDH in nonpolar solvents.[21,28] Recently, we have reported a slightly modified method to obtain highly delaminated sheets of LDH by sonication.[18] The advantage of this sonication process is simultaneous delamination and the lateral fragmentation of LDH. It is also worth mentioning here that ultrasonic vibration has been extensively used in the liquid exfoliation of 2D layered materials such as graphene, transition metal oxides, and transition metal dichalcogenides.[3,43−45] In this study, we used the same process to delaminate the different LDH in xylene. The SEM images of different LDH (Figure S2) show the broken platelets on a large scale, and the lateral size of LDH reduced to a few tens of nanometers. The ultrathin sheets of LDH were directly observed by TEM, as shown in Figure . It is evident that LDH platelets are broken into small pieces and exhibit a faint contrast compared to their bulk counterparts. Furthermore, high-resolution TEM was used to obtain more detailed information on the exfoliated LDH (Figure ). These images revealed that the exfoliated LDH sheets are fairly clean and highly crystalline. The uniform atomic orientation and lattice spacing reveal that an individual layer consists of a single-crystal domain. The lattice fringes with the same d-spacings of ∼0.25 nm corresponding to the (012) crystal plane were observed in all LDH samples, and these values are consistent with the d-spacing estimated from the XRD results.[46] The EDS spectra of exfoliated LDH nanosheets are almost comparable to those of their bulk counterparts other than the oxygen/carbon peak intensity ratio. The intensity of the oxygen peak decreased drastically, indicating the removal of water molecules in the exfoliated samples. The removal of water molecules in the exfoliated samples was further confirmed by FTIR spectra (data not shown here).

Figure 3

Low- and high-resolution TEM images and the corresponding energy-dispersive spectra of exfoliated LDH nanosheets: (a) Co–Al LDH, (b) Zn–Al LDH, and (c) Co–Zn–Al LDH.

The thickness of Co–Zn–Al LDH (both in bulk and exfoliated form) was evaluated by AFM. Figure shows the typical AFM height images and the corresponding height profiles of Co–Zn–Al LDH before and after exfoliation. It is evident from Figure a that the as-synthesized sample shows a 3D platelet-like morphology with lateral size ∼5 μm and thickness around a few hundreds of nanometers. On the other hand, the exfoliated nanosheets (Figure b) show that the thickness of the fragmented LDH is less than 1.0 nm. This value is in good agreement with the reported value for a single layer of LDH.[2,40] From these results, we may say that the as-synthesized sample contains a few hundreds of stacked layers of LDH. The lateral sizes of the exfoliated LDH range from 50 nm to a few hundreds of nanometers and are consistent with the TEM results.

Figure 4

AFM images and height profiles of (a) as-synthesized and (b) exfoliated single-layer Co–Zn–Al LDH nanosheets.

Synthesis and Characterization of iPP/LDH Nanocomposites

Schematic representation of the methodology used for the preparation of highly dispersed iPP/LDH nanocomposites is shown in Figure . The as-synthesized LDH was washed with acetone to remove interlayer H2O molecules from the stacked LDH layers. This process helps in converting the hydrophilic LDH to hydrophobic, and this facilitates the better dispersibility of LDH within the iPP matrix.[25,28] Subsequently, such washed LDH was sonicated for four days in xylene. The advantage of this step is the simultaneous surface modification and fragmentation of the LDH. Most importantly, this method produces 2D layered materials with lateral sizes and thickness in the nanometer range. The resultant LDH solution was directly added to the iPP solution to obtain highly dispersed nanocomposites. In this study, iPP nanocomposites were prepared using three different LDH (Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH) by adjusting the amount of LDH to 6 wt %. In addition to this, to understand the influence of LDH loading, in one case, iPP/LDH nanocomposites were prepared with 10 wt % of Co–Zn–Al LDH.

Figure 5

Schematic representation of the methodology used for the preparation of highly dispersed polymer nanocomposites based on iPP and LDHs.

Such prepared iPP/LDH nanocomposites were crystallized isothermally at 130 °C after melting at 200 °C under strictly controlled conditions and analyzed by XRD. Figure a shows the XRD patterns of various nanocomposites along with pristine iPP and Co–Zn–Al LDH. The XRD patterns of nanocomposites are almost similar to those of pristine iPP. Both iPP and iPP/LDH nanocomposites show reflections corresponding to the monoclinic α form.[47] At the same time, no reflections corresponding to the (00n) planes of LDH were observed in nanocomposites, indicating the loss of 2D layer stacking of LDH within the iPP matrix. These results suggest that exfoliated LDH platelets have been successfully dispersed into the polymer matrix without much agglomeration during solution blending. The state of LDH dispersion within the iPP matrix was further confirmed by TEM measurements. Figure b shows a cross-sectional TEM image of the nanocomposite containing 10 wt % of Co–Zn–Al LDH. The nanosized LDH platelets are homogeneously dispersed in the polymer matrix with sizes ranging from 50 nm to a few hundreds of nanometers. This suggests that in the solution blending method LDH platelets are successfully transformed from solution to the solid state with minimum agglomeration.

Figure 6

(a) Powder XRDs of iPP and its nanocomposites containing different types of LDH (for the purpose of comparison, the powder XRD pattern of LDH is shown) and (b) cross-sectional TEM image of the iPP/Co–Zn–Al LDH (10 wt %) nanocomposite.

Crystallization of iPP/LDH Nanocomposites

Tailoring the crystallinity and crystallization rate of semicrystalline polymers is of great importance to the polymer processing industry. The incorporation of nanofillers in polymer matrices is known to alter the crystallization behavior and the degree of crystallinity of the polymer matrix, which in turn controls the physical properties of the polymers.[18,48] To understand the effect of different LDH on the crystallization rate of iPP, the melt crystallization temperature (Tmc) was measured upon cooling the polymer melt. The Tmc values have often been used to measure the crystallization rate of the polymer. The higher the Tmc, the higher is the crystallization rate of the polymer.[49]Figure a shows differential scanning calorimetry (DSC) cooling curves of pristine iPP and its nanocomposites at 10 °C/min. Pristine iPP shows a Tmc at around 108 °C; however, the Tmc values shift to 121 ± 1 °C for nanocomposites irrespective of the type of LDH. The higher Tmc of the nanocomposites compared to that of iPP clearly indicates the faster crystallization of iPP in the presence of LDH; however, no change in the crystallization rate was observed with the type of LDH used for the preparation of nanocomposites.

Figure 7

(a) DSC cooling thermograms of pristine iPP and its nanocomposites crystallized nonisothermally after melting at temperature 190 °C for 1 min. (b) Crystallization isotherms obtained at 132 °C for pristine iPP and its nanocomposites.

To further confirm the effect of different types of LDH on the crystallization rate of iPP, the crystallization half-time (T1/2) was obtained at an isothermal crystallization temperature (Tc). T1/2 is usually defined as the time at which 50% of the crystallinity is developed. Figure b shows the representative DSC isothermal curves of pristine iPP and various nanocomposites crystallized at 132 °C. T1/2 of pristine iPP was 33.3 min, and it decreased significantly for all nanocomposites irrespective of the type of LDH (T1/2 ∼ 4 ± 0.2 min). Usually, the crystallization rate G is the reciprocal of T1/2, that is, G = 1/T1/2. Figure shows the temperature dependence of 1/T1/2 for pristine iPP and nanocomposites containing different types of LDH. It is observed that the crystallization rate decreases with increasing Tc for all of the samples, suggesting that the overall isothermal crystallization rate decreases with increasing Tc because of the low degree of supercooling (ΔT = Tm° – Tc, where Tm° is the equilibrium melting temperature) at higher Tc.[50] It was also observed that the crystallization rate of nanocomposites (0.53–0.05 min–1) is higher than that of the pristine polymer (0.17–0.01 min–1). These results suggested that the presence of LDH enhanced the crystallization process of iPP significantly, indicating that the fragmented LDH are effective nucleating agents for iPP. In our previous work, detailed crystallization kinetics has been carried out using different-sized LDH particles, and we showed that the lateral size of the LDH particles has a significant role in controlling the crystallization rate of iPP.[18] This study revealed that the type of LDH does not affect the crystallization rate of iPP.

Figure 8

Temperature dependence of crystallization rate (G = 1/T1/2) for pristine iPP and its nanocomposites using different types of LDH.

Dynamic Mechanical Analysis (DMA)

Inorganic fillers dispersed in a polymer matrix are known to influence the viscoelastic properties of the polymer.[51−53] The effect of the different types of LDH particles on the dynamic mechanical performance of iPP/LDH nanocomposites was investigated by DMA. Figure shows the temperature dependence of the storage modulus (E′) and loss tangent (tan δ) of iPP and its nanocomposites containing different types and amounts of LDH. Table summarizes the DMA data extracted from Figure for various samples. The storage modulus of pristine iPP is 2.5 GPa at −30 °C, and it decreases over the entire temperature range. The plateau region observed in the temperature range of −10 to 40 °C is associated with the relaxation of the amorphous region. The tan δ curve, which measures the energy dissipation (damping) ability of the material, shows peaks at 9.7 °C and another broad peak around ∼112 °C for pristine iPP. The first peak at 9.7 °C corresponds to the glass transition temperature (Tg), and the second one is attributed to the damping within the crystalline lamellae.[51−53] Around 150 °C, a sharp decrease in the storage modulus and a sudden increase in tan δ are due to the onset of melting of iPP crystals.

Figure 9

Temperature-dependent (a) storage modulus (E′) and (b) loss factor (tan δ) measured in the heating process for iPP and its nanocomposites containing different LDH.

Table 1

Summary of the Degree of Crystallinity Measured from WAXD and Temperature-Dependent Dynamic Mechanical Properties

samples	Xc (%)a (±1)	E′ at −30 °C (GPa) (±0.1)	E′ at 30 °C (GPa) (±0.1)	E′ at 100 °C (GPa) (±0.1)	tan δmax(°C) (±1)
iPP	65.1	2.5	1.04	0.25	9.7
iPP/Co–Al LDH (6 wt %)	65.8	4.5	2.02	0.57	9.2
iPP/Zn–Al LDH (6 wt %)	65.8	4.6	2.46	0.70	9.6
iPP/Co–Zn–Al LDH (6 wt %)	64.6	4.65	2.2	0.64	10.3
iPP/Co–Zn–Al LDH (10 wt %)	63.3	6.580	3.25	1.08	9.8

The degree of crystallinity (Xc) was calculated as the ratio of the area under the crystalline peaks to the total area under the X-ray scattering curve.

On addition of 6 wt % of different types of LDH, the storage modulus at −30 °C increased to 4.5, 4.6, and 4.65 GPa for nanocomposites containing Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH, respectively, which is almost 1.8-fold increment compared to that of pristine iPP. The iPP/LDH nanocomposites exhibit a higher storage modulus across the measured temperature range. The average storage modulus measured at room temperature is almost two-fold higher for nanocomposites than that of pristine iPP. Increasing the LDH content to 10 wt % further increases the storage modulus to 6.6 and 3.2 GPa at −30 °C and room temperature, respectively, for iPP/Co–Zn–Al LDH nanocomposites. Generally, the increase in the storage modulus could be due to the increase in the percent crystallinity; however, in this present study, the percent of crystallinity (see Table ) measured for various samples using the WAXD data shown in Figure was almost the same for pristine iPP and iPP/LDH nanocomposites. Moreover, it is clearly observed that the increase in the LDH content increases the storage modulus of iPP. On the basis of these observations, the increase in the storage modulus could be attributed to the reinforcing effect of LDH, that is, the homogeneous dispersion of LDH within the polymer matrix without agglomerates. It has to be noted that a little or no difference in the storage modulus was observed with the type of LDH chosen (6 wt % loading) for the preparation of nanocomposites. The addition of LDH caused no change in the peak position of the tan δ curve near 9.5 °C, indicating that Tg of iPP did not change in the presence of the LDH particles. It is worth mentioning here that the Tg value of iPP is in the range −10 to 10 °C, depending on the grade and molecular weight.[51,54−56] However, the height of the tan δ peak decreases with the addition of LDH, which indicates the good wettability between the LDH and iPP.[51] In a few reports, it has been reported that Tg of the polymer was reduced, where the organically modified fillers were used for the preparation of nanocomposites due to the plasticization effect.[57] In some other cases, Tg of the polymer was increased due to the restricted mobility of the polymer chains in the presence of fillers.[51] However, in the present study, no change in Tg of iPP was observed in the presence of surfactant-free LDH particles. The magnitude of the broad peak around ∼112 °C, which was attributed to the damping within the crystalline lamellae, decreased for the nanocomposites. In nanocomposites, the LDH particles are expected to accumulate within the amorphous region of the polymer matrix. These particles influence the relaxation of iPP chains within the crystalline lamellae and crystalline–amorphous interfaces.

Flammability and Thermal Properties

Microscale combustion calorimetry (MCC) is a relatively new technique used for the evaluation of flammability of materials using small quantities of the sample, and this works on the principle of oxygen combustion. This technique was successfully used as a preliminary test for the evaluation of flame-retardant properties of polymer/LDH nanocomposites.[19,29−31] The heat release rate (HRR) is considered to be the most important parameter to estimate the flammability behavior of the polymer materials. Figure presents the HRR plots for pristine iPP and its nanocomposites using different types and quantities of LDH. Several parameters such as the specific HRR, heat release capacity (HRC), and total heat release (THR) are summarized in Table along with the 50% degradation temperature estimated from thermogravimetric analysis (TGA). Unlike the crystallization rate and storage modulus, the nanocomposites containing different types of LDH showed different HRR values. For pristine iPP, the HRR value is around 1435 W/g. With the addition of 6 wt % of different types of LDH, the HRR values were reduced to 1080, 990, and 880 W/g for Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH, respectively. These results clearly suggested that different LDH have different flame-retardant efficiencies. Three-metal LDH showed 38% reduction in the HRR value, which is better than that of the other two-metal LDH with the same LDH loadings. Matusinovic et al. showed that the dispersion of LDH in the polymer matrix is one of the key parameters in achieving good flame retardancy.[58] As discussed in the preceding section, highly dispersed nanocomposites were obtained using all types of LDH, so the dispersion of LDH may not be the key reason for the difference in the flame-retardant behavior. It was also demonstrated that different gallery anions have different flame-retardant efficiencies.[29,59,60] However, in the present study, the different LDH used have the same anions (both CO32– and NO3–) because of the same protocol used for the synthesis of different LDH. It has been shown that the char formation can slow the HRR upon the ignition of polymers, and it can inhibit the flame spreading.[22,39,61,62] To understand the difference in the flame-retardant efficiency of different LDH, the amount of char formed upon the degradation of the polymer matrix was analyzed by TGA. Figure a shows the TGA thermograms of pristine iPP and its nanocomposites containing 6 wt % of different types of LDH. The thermal stability of the polymer is discussed in a later section. As seen from Figure a, the pristine polymer shows almost no char, whereas the samples containing different types of LDH show different char amounts ranging from 4.1 to 4.6 wt %. To further understand the reason for the difference in the char yield, TGA thermograms of different LDH were taken and are shown in Figure b. By considering the char yields of different LDH, the expected char residues were estimated for the nanocomposites prepared using different types of LDH and are shown in Table S1. It is quite clear that different LDH give different char yields, which follow the order Co–Zn–Al LDH > Zn–Al LDH > Co–Al LDH. Similar to char yields, the HRR also follows the same order. On the basis of these results, the lowest HRR value of the nanocomposite prepared using three-metal LDH (Co–Zn–Al LDH) can probably be attributed to the better char formation of three-metal LDH, which reduces the heat and mass transfer between the gas and condensed phases. It is worth mentioning here that LDH acts like both endothermic flame retardant and char forming flame retardant.[62] Under fire conditions, the LDH filler endothermically decomposes into water, carbon dioxide, metal hydroxides, and other gases, depending on the gallery anions. Furthermore, the decomposition products of the LDH are nonflammable, and so the residue left behind by the thermal decomposition (usually a metal oxide) dilutes the total amount of polymer fuel availability (condensed phase).[22] This process promotes the formation of char and protects the bulk polymer exposure to air. This char helps in reducing the HRR during the combustion and suppresses the smoke production. From the TGA results of LDH, it is clear that the char formation is effective in three-metal LDH compared to that of two-metal LDH.

Figure 10

HRR vs temperature curves for pristine iPP and its nanocomposites containing Co–Al LDH (6 wt %), Zn–Al LDH (6 wt %), and Co–Zn–Al LDH (6 and 10 wt %).

Table 2

Summary of Thermal and Flammability Properties for iPP and Its Nanocomposites

samples	HRR (W/g) (±5)	HRR reduction (%)	THR (kJ/g) (±1)	HRC (J/g K) (±5)	50% weight loss temperature (°C) (±2)
iPP	1435		54.0	1460	410
iPP/Co–Al LDH (6 wt %)	1080	25	49.9	1050	420
iPP/Zn–Al LDH (6 wt %)	990	31	48.7	985	430
iPP/Co–Zn–Al LDH (6 wt %)	880	39	48.17	905	443
iPP/Co–Zn–Al LDH (10 wt %)	640	55	43.9	690	435

Figure 11

TGA thermograms for (a) pristine iPP and its nanocomposites containing 6 wt % of Co–Al LDH, Zn–Al LDH, and Co–Zn–Al LDH and for (b) pure LDH powders.

To further verify the effect of LDH loading, 10 wt % of Co–Zn–Al LDH was added to iPP, and it was observed that the HRR value was further reduced by 55%. Wang and co-workers showed the reduction in the HRR value by 54% using 4 times higher LDH loadings (i.e., 40 wt %) in high-density polyethylene/LDH nanocomposites using MCC.[29] The HRC values estimated from the HRR curve (see Table ) also show a similar trend like the HRR values, indicating that the three-metal LDH are efficient flame-retardant fillers compared to the two-metal LDH.

The THR calculated from the area under the HRR curve is an important parameter to understand the fire hazards of the material.[29,30] It has been shown that for an efficient flame-retardant filler, the THR value should reduce effectively when it is incorporated into the polymer.[29,30] It is obvious from Table that the THR value is considerably reduced with the addition of 6 wt % of LDH and further reduced with increasing LDH loading to 10 wt %. A small difference in the THR values was observed with the type of LDH used for the preparation of nanocomposites. Three-metal LDH showed a lower THR value compared to that of the other two-metal LDH, indicating that the three-metal LDH is a better flame-retardant nanofiller for iPP.

The relative thermal stability of iPP was evaluated in the presence of different types of LDH. The 50% weight loss temperatures (T0.5) measured for various samples from Figure a are summarized in Table . It can be seen that pristine iPP degraded completely with T0.5 at 410 °C. The thermal decomposition temperature of iPP containing different types of LDH shifted to higher temperatures compared to pristine iPP. However, significant differences are seen in the thermal stability of nanocomposites containing different types of LDH. The T0.5 values of iPP containing Co–Al LDH (6 wt %), Zn–Al LDH (6 wt %), and Co–Zn–Al LDH (6 wt %) increased to 420, 430, and 443 °C, respectively. It has been shown that the enhanced thermal stability is due to the homogeneous dispersion of the nanosized LDH in the iPP matrix, where the dispersed nanoparticles act as trapping sites for the radicals generated during the degradation of the polymer.[18,21,23] However, in the present study, the dispersion of the LDH particles in the iPP matrix is more or less the same in different nanocomposites. One of the possibilities for the difference in the degradation temperature is the catalytic ability of the metal constituents used for the preparation of LDH. Typically, cobalt is known for the catalytic degradation of polyolefins.[63] However, in the present study, the cobalt containing three-metal LDH (Co–Zn–Al LDH) showed better thermal stability than the other two LDH. On the basis of the present results, we speculate that the difference in the thermal stability of iPP containing different types of LDH might be due to the thermally stable char formed by the degradation of LDH, which prevents further degradation of iPP. As mentioned in the preceding section, the char formation in different LDH follows the order Co–Zn–Al LDH > Zn–Al LDH > Co–Al LDH. Similar to this, the thermal stability and flame retardancy of the nanocomposites also follow the same order. On the other hand, the nanocomposite with 10 wt % of Co–Zn–Al LDH shows a T0.5 value of ∼435 °C, which is less than the T0.5 value of the nanocomposite containing 6 wt % of Co–Zn–Al LDH. This might be due to the agglomeration of LDH in higher loadings. Similar kinds of observations were reported in the literature.[18,23,28] It was shown that 2.5 wt % of sonicated LDH was the optimal loading for the effective thermal stability of iPP due to its better dispersion in the iPP matrix.[18] In this way, the intralayer metal constituents of LDH play an important role in determining the properties of the nanocomposites.

Conclusions

Two-metal LDH and three-metal LDH were successfully synthesized by the co-precipitation method and delaminated by ultrasonication in xylene. The sonication step resulted in both delamination and fragmentation of LDH. TEM and AFM analysis confirmed the delamination of LDH. Subsequently, highly dispersed iPP/LDH nanocomposites were prepared by the solution blending method. The dispersion of LDH within the iPP matrix was further confirmed by TEM and WAXD. We have found that the incorporation of either two-metal LDH or three-metal LDH dramatically improves the crystallization rate and storage modulus of iPP. However, not much difference is observed in these properties with the type of LDH used for the preparation of nanocomposites. On the other hand, the kind of LDH could influence the thermal stability and flame-retardant properties of iPP. Keeping the filler loading at 6 wt %, the 50% weight loss temperature (T0.5) of the iPP nanocomposite containing three-metal LDH (Co–Zn–Al LDH) is higher than that of the corresponding nanocomposites prepared using the two-metal LDH (Co–Al LDH and Zn–Al LDH). Preliminary data on the flame-retardant properties showed better reduction in the HRR value compared to that of two-metal LDH containing nanocomposites. These differences might be due to the better char formation capability of three-metal LDH compared to that of two-metal LDH. These results demonstrated that the proper selection of metal constituents of LDH is imperative in the preparation of polymer/LDH nanocomposites with desirable properties.

Experimental Section

Materials

iPP pellets (Mw 120 000, Mw/Mn 4.5) were kindly supplied by Sumitomo Chemicals Co. Ltd., Japan. Metal salts such as Al(NO3)3·9H2O, Co(NO3)2·6H2O, and Zn(NO3)2·6H2O were purchased from Sigma-Aldrich Co. Ltd. Other chemicals such as urea, xylene, ethanol, and acetone were obtained from Merck, India.

Synthesis of LDH

Different types of LDH used in this study were synthesized by a conventional co-precipitation method using urea as the base. Three-metal LDH (Co–Zn–Al LDH) was prepared by dissolving three metal salts (Co(NO3)2·6H2O, Zn(NO3)2·6H2O, and Al(NO3)3·9H2O), and urea in Millipore water with a ratio of 1:1:1:7. The total mixture was then heated to the refluxing temperature (about 100 °C) under continuous stirring for 24 h in an ambient atmospheric environment. The resultant light pink color precipitate was rapidly quenched in cold water and filtered, and then the precipitate was repeatedly washed with hot Millipore water to remove the unreacted reactants if any. The thus-obtained LDH powder was repeatedly washed with acetone. The end product was dispersed in xylene and sonicated for 4 days in an ultrasonication bath at room temperature. The resulting suspension was stable at room temperature for a few hours. Two-metal LDH (Co–Al LDH and Zn–Al LDH) were synthesized in the same way, with the exception that the Co2+ and Al3+, Zn2+ and Al3+ metal nitrates were taken in 2:1 ratio.

Preparation of iPP/LDH Nanocomposites

Three different LDH (Co–Zn–Al LDH, Co–Al LDH, and Zn–Al LDH) were used as reinforcement fillers for the preparation of iPP nanocomposites. The synthetic procedure is as follows: for example, to prepare 6 wt % of iPP/LDH nanocomposite, 0.06 g of LDH powder was dispersed in 150 mL of xylene in an ultrasonic bath for four days at 45 °C to yield a uniform suspension. Then, 0.94 g of iPP was dissolved in the suspension and the sonication was continued for 10 min. The resultant solution was refluxed at 150 °C with continuous magnetic stirring for 24 h under an argon atmosphere. Finally, the polymer solution was reprecipitated out in ethanol. The precipitate was filtrated, washed with ethanol, and vacuum dried at 100 °C for 24 h. The type and amount of LDH were adjusted to prepare different nanocomposites with various LDH loadings (0, 6, and 10%). The actual loading of the LDH in iPP was estimated using TGA and the details are given in Table S1.

Characterization

WAXD

XEUSS 2D SAXS/WAXS system with a Genix microsource from Xenocs was used for the WAXD measurements. The X-ray system was operated at 50 kV and 0.6 mA. The Cu Kα radiation (λ = 1.54 Å) was collimated with a FOX2D mirror and two pairs of scatterless slits. The measurements were carried out in the transmission geometry. The fiber diagrams were recorded on a Mar345 image plate and processed using the Fit2D software. The sample to the detector distance was 214.5 mm. Samples were crystallized under controlled conditions using a differential scanning calorimeter, and the prepared samples were used for the XRD measurements.

FTIR

A Perkin Elmer Series FTIR spectrum-2 was used to measure the infrared spectrum over the wavenumber range 4000–400 cm–1. The dried LDH powder was mixed with KBr and pressed in the form of pellets for FTIR measurements. The spectra were collected with 32 scans and a resolution of 1 cm–1.

SEM

The shape and size of the LDH were investigated by SEM (Zeiss EVO 18 cryo-SEM) with an accelerating voltage of 20 kV. LDH suspension was directly drop-cast onto a carbon-coated grid and imaged after natural drying at room temperature. Before imaging, a thin layer of gold was evaporated onto the sample to avoid charging and to improve contrast.

TEM

A JEOL 2010 transmission electron microscope operating at 300 kV was used to investigate the LDH morphology. LDH suspension was directly drop-cast on a carbon-coated copper grid and allowed for natural drying for two days in a dust-free environment. The prepared samples were then used for both imaging and elemental composition analysis using an energy-dispersive spectrometer (Technai G2 30LaB6, ST with EDS). To view the dispersion of LDH in the polymer matrix, ultrathin sections of the nanocomposite samples (∼ 60 nm thickness) were sliced with a diamond knife (35° knife angle; DIATOME, Switzerland) using ultramicrotome EM UC/FC 6, Leica (Austria) at −140 °C. The sections were floated with a dimethyl sulfoxide/water mixture on a carbon-filmed TEM copper grid. These thin slices of the samples were analyzed using a TEM LIBRA 120, Carl Zeiss Microscopy GmbH (Oberkochen, Germany), with an acceleration voltage of 120 kV.

AFM

The surface morphology and thickness of the LDH sheets were analyzed using a Bruker Multimode atomic force microscope, Germany, in the tapping mode. The as-synthesized LDH in water and the sonicated LDH suspension in xylene were coated on silicon wafers.

TGA

TGA thermograms were collected in the heating process at 10 °C/min using TA Q50. All measurements were carried out under a nitrogen atmosphere (nitrogen gas flow rate of 60 mL/min for the furnace and 40 mL/min for the balance).

DSC

DSC measurements were performed on a Perkin Elmer Pyris 6 DSC apparatus under nitrogen gas flow. The temperature and heat flow were calibrated using high-purity indium. The crystallization half-time (T1/2) was measured to evaluate the crystallization rate of iPP and its nanocomposites. Molten samples were rapidly cooled to the desired crystallization temperature (Tc) (132 °C) at a rate of 100 °C/min, and the samples were allowed to crystallize at that temperature. These samples were reheated to 190 °C to measure the melting temperature.

MCC

The MCC-1 (FTT) was used as a preliminary test to evaluate the combustion behavior of iPP and iPP/LDH nanocomposites. In the MCC system, approximately 5 mg of the sample was heated to 700 °C at a heating rate of 1 °C/s in a stream of N2 flow at 80 cm3/min. The resulting volatile anaerobic thermal degradation products in the nitrogen gas stream are mixed with 80 cm3/min carrying gas (nitrogen of 80 mL/min; oxygen of 20 mL/min) and subsequently burned at 900 °C in a combustion furnace. The flame-retardant parameters were measured from this test, and are the HRR in W/g, % of reduction in HRR, THR rate in KJ/g, temperature maxima (T-max) in °C, and HRC in J/g K.

DMA

DMA (TA Instruments Model Q800) was used to study the temperature-dependent dynamic mechanical properties of the samples. The samples with dimensions 25 × 6 × 0.4 mm3 were prepared using the hot press. All samples were annealed at 130 °C for 5 h. The prepared samples were used to measure the temperature-dependent storage modulus (E′), loss modulus (E″), and their ratio (tan δ) at a constant frequency (ω) of 6.28 rad/s with a strain amplitude of 0.05%. The measurements were carried out in a temperature range of −50 to 160 °C, with a heating rate of 2 °C/min.