Influences of a glycerin co-solvent on the compatibility of MgAl hydrotalcites with a polypropylene matrix

Abstract

Hydrotalcites as flame retardants for industrial applications require good compatibility with polymers. Glycerin co-solvent was employed during the precipitation of MgAl hydrotalcite (Mg/Al-HT) particles and the dispersion of the Mg/Al-HT particles in a polypropylene (PP) matrix was studied. The microstructures, textural and surface properties of the hydrotalcites were contrastively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, laser particle size analysis, Barrett–Joyner–Hallender/Brunauer–Emmett–Teller methods (BJH/BET), and thermogravimetric and differential thermal analysis (TG-DTA) as well as pH_zpc analyses. The results suggested that the interactions between the co-solvent and the Mg/Al-HT affected the nucleation, resulting in the variation of crystallinity. The employment of glycerin co-solvents during the nucleation was conducive to the combination of crystal water with the brucite sheets. The hydrotalcite (mix-MHT) obtained by adding glycerin co-solvent during the nucleation possessed the smallest particle sizes with the narrowest size distribution and the most hydrophobic particle surfaces, which made the mix-MHT particles disperse uniformly throughout the PP matrix due to their good compatibility with PP. The improvement of the compatibility between the particles and the polymer was mainly caused by the decrease of the hydrophilicity on the surface of the particles due to the presence of glycerin in the interlayer spaces of the mix-MHT particles. The insertion of the mix-MHT particles into the PP matrix could also significantly enhance the thermal stability and maintain the mechanical properties of PP, and the mix-MHT had the best performance as a flame retardant with the PP matrix.

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1. Introduction

Due to the great demand for polymeric materials today, non-flammable or flame-retardant techniques are particularly craved, because most polymeric materials are easily burned.^1,2 Inorganic materials such as clays, Al(OH)₃ and Mg(OH)₂ have shown to improve the flame resistance, thermal, and mechanical properties of polymers.^3–5 Two inorganic materials, Al(OH)₃ and Mg(OH)₂, are now widely used as flame retardants, which have poor compatibility with polymer materials and need a very high loading (50–70 wt%) to impart a good flame retardancy to polymer materials.⁶ Of these clays, layered double hydroxides (LDH) have received much attention because of their environmental friendliness, low cost, low toxicity and low smoke production,^7,8 especially Mg/Al hydrotalcites (Mg/Al-HT) which have a greater flame retardant effect than Al(OH)₃ or Mg(OH)₂ at the same loading level.^9,10 As compared to conventional materials, polymer/Mg/Al-HTs composites have better mechanical properties, barrier, thermal stability and flame resistance properties. However, their flame retardant efficiency is still low due to high loading, as the compatibility between the nonpolar polymers and Mg/Al-HT is poor and the Mg/Al-HT particles are large.^8,11,12 The challenges are the preparation of HT particles with smaller particle sizes, the homogeneous dispersion of inorganic material into the polymeric matrix and the compatibility of polymer/inorganic blends.

If the size of the crystallites and the Mg/Al-HT particles can be controlled during their synthesis, the prepared Mg/Al-HT will be more successfully applied in the field. Some properties of the particles are modulated by the organic solvents, an effect which induces subtle changes in the microstructures and surface chemical properties of gel materials.¹³ Four kinds of polyhydric alcohols including glycerin are employed as co-solvents to synthesize Mg/Al-HT, and it is found that polyhydric alcohols have an important influence on the microstructure and thermal stability of Mg/Al-HT synthesized using a hydrothermal method.¹⁴ The polyhydric alcohols can modify the surface and porosity properties of Mg/Al-HT, and synthetic routes affect the properties, too.¹⁵ All in all, the polyhydric alcohol co-solvents have a great influence on the preparation of the Mg/Al-HT, which affects the nucleation and subsequent crystal growth of the HT resulting in the variation of crystallinity and thermal stability.^14,15

To make Mg/Al-HT particles smaller in size, with a narrower size distribution as well as better compatibility, the present work demonstrates a new method to prepare the ultrafine Mg/Al-HT particles using glycerin as a co-solvent to control the nucleation and growth of the crystallites. The microstructure, morphology and surface properties of the Mg/Al-HT particles were investigated by XRD, SEM, FT-IR spectroscopy, laser particle size analysis, BJH/BET, DTA and pH_zpc analyses. The Mg/Al-HT particles were incorporated into polypropylene (PP) to obtain PP/Mg/Al-HT composites in order to improve the flame retardant properties of the PP matrix.

2. Experimental

2.1. Materials

Polypropylene particles (K8303, melt flow rate: 2.6 g 10 min⁻¹ at 230 °C and 2.16 kg) with a particle size of about 1 mm, were purchased from Yanshan Petrochemical I Co., Ltd (Beijing, China). All chemicals were of analytical grade, and were purchased from Sinopharm Chemical Reagent Co. Ltd, China. All other reagents used in the experiments were of analytical grade, and all the solutions were made with deionized water.

2.2. Preparation of Mg/Al-HT

The hydrotalcite (Mg/Al-HT) with a Mg/Al molar ratio of 3.0 was prepared using the urea method (urea/NO₃⁻ molar ratio of 3.0).¹⁶ The precipitation of the Mg/Al-HT particles could be divided into two steps: nucleation and crystallization. Firstly (nucleation), the mixed salt solution containing Mg(NO₃)₂·6H₂O (0.12 mol L⁻¹), Al(NO₃)₃·9H₂O (0.04 mol L⁻¹) and urea was placed into a three-neck flask. The solution was maintained at 105 °C for 10 h under stirring (300 rpm). Secondly (crystallization), it was then crystallized statically at 80 °C for another 6 h. The solid was collected by filtration and washed until neutral using deionized water, subsequently dried at 90 °C for 24 h, and denoted as AHT.

2.3. Preparation of Mg/Al-HT in a co-solvent system

The preparation procedure by adding glycerin co-solvent was basically the same as above. The co-solvent solution was obtained by the addition of glycerin into the mixed salt solution with a volume ratio of 15 vol%. When glycerin was added into the mixed salt solution before the nucleation, the obtained solid sample was denoted as mix-MHT. When glycerin was added into the mixed salt solution before crystallization, the obtained solid sample was denoted as bef-MHT. After crystallization, the solid was collected by filtration and washed until neutral using deionized water. The washed solid was again added into the co-solvent solution containing glycerin at 30 °C for 90 min under stirring (300 rpm), subsequently dried, and denoted as aft-MHT.

2.4. Preparation of PP/Mg/Al-HT composites

The polypropylene/mix-MHT (PP/mix-MHT) composite was prepared by melting, and then mixing the mix-MHT with a PP matrix in a GH-10A high-speed mixer (Beijing Plastic Machinery Factory) with a rotor speed of 250 rpm at 230 °C for 15 min. The mass loading of mix-MHT added (corresponding to pure PP) was 10 wt%. The admixtures were molded into bars (120 × 10 × 4 mm³) using a JK-WZM-I micro injection molding machine with a twin screw extruder (SHJ-30A) (Beijing Heng Odd Instrument Co., Ltd) for testing. The polypropylene/AHT (PP/AHT) composite was used as a comparison.

2.5. Characterization

2.5.1. Characterization of Mg/Al-HT particles. X-ray diffraction (XRD) patterns were collected on a Rigaku D/max-2550PC (λ = 1.5406 Å) with Cu Kα radiation. The scan step was 0.0671° s⁻¹ with a filament intensity of 30 mA and a voltage of 40 kV. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-6700F instrument at an accelerating voltage of 10 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer Spectrum One B instrument using the KBr pellet technique. The particle size distribution was determined using a Malvern Mastersizer 2000 laser particle size analyzer. The pore size distribution was calculated from the desorption isotherm using the Barrett–Joyner–Hallender (BJH) method, and the specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method based on the N₂ adsorption isotherm from a Quantachrome NOVA-2200e instrument. Thermogravimetric and differential thermal analysis (TG-DTA) was carried out in a nitrogen atmosphere with a Seiko 6300 TG-DTA instrument with a heating rate of 10 °C min⁻¹ under a He stream flowing at 60 mL min⁻¹.

2.5.2. Characterization of PP/Mg/Al-HT composites. The phase morphologies of the PP/AHT and PP/mix-MHT composites were observed using SEM with an accelerating voltage of 25 kV. The specimen for the SEM observation was prepared by cryogenic fracturing in liquid nitrogen, and the fracture surface was coated with a thin layer of gold before measurement. The TG analysis was performed using a Perkin-Elmer Pyris-1 TG-DTA instrument. 10 mg of the samples (PP, PP/AHT and PP/mix-MHT) were loaded into an open ceramic crucible, and heated in an air atmosphere at a heating rate of 10° min⁻¹.

The impact strength was measured with a simple beam impact testing machine (XJJ-22) at room temperature based on the standard GB/T1043-1993 with a 45° V-shaped notch and a notch-tip radius of 0.2 mm. Three specimens were tested, and the values were averaged in order to ensure that the results were reproducible. The other mechanical properties were measured using an electronic tensile test machine (RGD-5) with a crosshead speed of 30 mm min⁻¹. Tensile strength, fracture elongation were determined based on the standard GB/T1042-1992, GB/T1042-1992 and GB/T9341-2000, respectively. At least three specimens were tested to determine the average values in order to ensure that the results were reproducible.

2.6. pH point of zero charge

The determination of the pH point of zero charge (pH_zpc) of the Mg/Al-HT particles was carried out using the potentiometric titration (PT) method described by Li et al.¹⁷ The pH at pH_zpc was determined in NaCl solutions (inert electrolytes) with different concentrations. The experiments were carried out in a shaker at 150 rpm and 25 °C for 200 min. After the experiments, the pH in the solution was measured while a 0.1 mol L⁻¹ NaOH solution was added. The adsorption amount of H⁺ (Γ_H⁺) and OH⁻ (Γ_OH⁻) was calculated. Finally, PT curves were obtained by plotting (Γ_OH⁻ − Γ_H⁺) versus pH in NaCl solutions with different concentrations, and the crossover point of these curves was pH_zpc, which was electrically neutral. The permanent charge density (σ_p) at pH_zpc was as follows,¹⁸

σ_p = F(Γ_OH⁻ − Γ_H⁺)_zpc/S_BET

where S_BET and F were the specific surface area of the Mg/Al-HT particles and the Faraday constant (96 485 C m⁻²), respectively.

3. Results and discussion

3.1. Characterization of Mg/Al-HT particles

3.1.1. XRD analyses. The powder XRD patterns for the AHT, mix-MHT, bef-MHT and aft-MHT samples are shown in Fig. 1. There is a typical layered double hydroxide structure with sharp and intense (003), (006), (009), (110) and (113) reflections and broadened (012) and (015) reflections in the samples. Further analysis of the XRD patterns reveals some differences in the cell parameters among the samples. The interlayer distances (d₀₀₃ ≈ 0.76 nm) (Table 1) in all the XRD patterns of the Mg/Al-HT samples are typical of carbonated hydrotalcites.¹⁹ No other crystalline phases are observed in the XRD patterns of any of the samples, indicating that the samples are highly crystalline hydroxide structures. Importantly, the d₀₀₃ value (0.761 nm) of the mix-MHT is 0.006 nm higher than that of the AHT (0.755), which may be due to the intercalation of glycerin. The interlayer distances of the bef-MHT (0.758 nm) and aft-MHT (0.753 nm) are basically the same as that of the AHT, which may be because the content of the glycerin molecules in the interlayer spaces is too small to detect using XRD. The results show that the mix-MHT has the maximum interlayer distance due to having the largest amount of glycerin molecules in the interlayer spaces. The parameter a, the average cation–cation distance in the brucite sheets, is calculated from the (110) XRD reflection in Table 1. The similarity in the a value among the samples indicates that the addition of glycerin does not change the microstructure of the brucite sheets. The crystallite size in the a direction (S_a) of the mix-MHT is smaller than that of the other three samples, and the crystallite size in the c direction (S_c) follows a similar trend, implying that the mix-MHT possesses the smallest crystallite size. These results reveal that glycerin co-solvent has little effect on the crystallinity of the Mg/Al-HT particles. However, glycerin added during nucleation has an impact on the crystallite size, which causes the crystallite size to be smaller.

Fig. 1 XRD patterns of the AHT, mix-MHT, bef-MHT and aft-MHT particles.

Table 1 Crystallographic parameters of the samples^a

Parameter	mix-MHT	bef-MHT	aft-MHT	AHT
a FW: half-width of diffraction peak; Sa: crystallite size in the a axis direction; Sc: crystallite size in the c axis direction.
d003 (nm)	0.761	0.758	0.753	0.755
d006 (nm)	0.380	0.377	0.377	0.376
d009 (nm)	0.256	0.256	0.262	0.255
d110 (nm)	0.152	0.152	0.152	0.152
FW003 (rad)	0.763	0.564	0.559	0.608
FW110 (rad)	0.349	0.406	0.384	0.383
a (nm)	0.304	0.304	0.303	0.303
c (nm)	2.292	2.280	2.293	2.268
Sc (nm)	10.36	14.02	14.15	13.99
Sa (nm)	21.68	22.48	22.59	22.53

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3.1.2. SEM analyses. In order to investigate the morphology, the AHT, mix-MHT, bef-MHT and aft-MHT samples were observed using SEM as shown in Fig. 2. For all the samples, thin flat crystals indicating the layered structure are found in line with the typical hydrotalcite morphology with irregular edges. The mix-MHT is made up of individual platelet particles and there are little platelets to stack, while the particles of the AHT, bef-MHT and aft-MHT samples are slightly stacked in all space directions forming some aggregates. The improvement show that the reunion of the mix-MHT particles may be explained by the fact that glycerin is added during the nucleation as a co-solvent rendering a decrease in the agglomeration. On the other hand, the particle sizes of the mix-MHT are significantly smaller than those of the other three samples, whereas there are no significant differences in particle sizes among the AHT, bef-MHT and aft-MHT samples. The results indicate that glycerin added during nucleation makes the mix-MHT particles more diffuse, and at the same time the particles are smaller, which is consistent with the inference provided by the XRD analyses (Fig. 1).

Fig. 2 SEM images of the AHT, mix-MHT, bef-MHT and aft-MHT particles, ×10 000.

3.1.3. FT-IR analyses. The FT-IR spectra of the AHT, mix-MHT, bef-MHT and aft-MHT samples in the region 400–4000 cm⁻¹ are displayed in Fig. 3, where the FT-IR spectra of the samples are typical of pure hydrotalcite structures and generally similar except for some minor differences. The absorption band at about 3446 cm⁻¹ is attributed to the stretching vibrations (ν₁–OH) of structural hydroxyl groups in the brucite sheets,⁸ where the increase in intensity and shifts to lower wavenumber indicate an increase in the number of –OH groups due to the addition of the glycerin.¹⁴ There is a similarity of band width and shift (4 cm⁻¹) between the bef-MHT and aft-MHT, though it is wider than the band width of the AHT, implying that the bef-MHT and aft-MHT had glycerin molecules. The widest characteristic band with the highest shift (7 cm⁻¹) appeared for the mix-MHT indicating that the mix-MHT particles contain the most glycerin molecules. A weak band appeared around 2956 cm⁻¹, attributed to the asymmetric stretching vibration of –CH₂,^14,20 and can be observed in the mix-MHT, bef-MHT and aft-MHT, where the intensity of the broad band for the mix-MHT is obviously the greatest. The result shows that glycerin is present in the three samples and the amount of glycerin in the mix-MHT is more than that in the bef-MHT or aft-MHT. The band at around 1386 cm⁻¹ results from the asymmetrical stretching vibration of CO₃²⁻,¹¹ and the bands in the mix-MHT, bef-MHT and aft-MHT split into two bands which is likely due to the existence of –CH₂.²¹ The appearance of these bands suggests that glycerin molecules exist in the mix-MHT, bef-MHT and aft-MHT, and the mix-MHT has the highest amount of glycerin, which is in agreement with the results of the XRD. The sharp absorption band at 1630 cm⁻¹ is usually assigned to the bending vibration of the interlayer water or physically adsorbed water.¹¹ The bands at 780 and 553 cm⁻¹ (Al–OH), 447 cm⁻¹ ([AlO₆]³⁻, or Al–OH) and 686 cm⁻¹ (Mg–OH) are clearly observed in the spectra of all the samples, too.

Fig. 3 FT-IR spectra of the AHT, mix-MHT, bef-MHT and aft-MHT particles.

3.1.4. Particle size, BJH and BET analyses. The particle size distribution and the average particle sizes for all the samples are illustrated in Fig. 4. The most probable sizes of the mix-MHT, bef-MHT, aft-MHT and AHT particles are approximately 0.45, 0.98, 1.87 and 5.9 μm, respectively. The most probable size distribution, with 90% of the particles, is found in the range of 0.08–1.45 μm for the mix-MHT, whereas it is found in the range of 0.8–11.5 μm for the AHT. The size distribution is the same for the bef-MHT and aft-MHT, and is in the range of 0.31–5.7 μm. The mix-MHT has the most uniform and smallest particle sizes compared with the other three samples, while the bef-MHT and aft-MHT samples possess a narrower size distribution and smaller particle sizes than the AHT. It can be speculated that the reduction in the particle agglomeration is due to the presence of glycerin. Mg/Al-HT particles with the most probable size distribution of 2–20 or 2–8 μm have been obtained using a urea method,^22,23 while the size distribution and sizes in the range of 1–120 μm are obtained using co-precipitation.²⁴ The most probable size distribution and particle sizes of the mix-MHT are obviously narrower and smaller than those reported in the literature, respectively. The results indicate that the use of glycerin as a co-solvent can prepare ultrafine hydrotalcites, and the mix-MHT has the most uniform and smallest particle sizes due to the largest amount of glycerin, for the addition of glycerin as a co-solvent before nucleation could give hydrotalcite particles more glycerin molecules.

Fig. 4 Particle size distributions of the AHT, mix-MHT, bef-MHT and aft-MHT particles.

The specific surface area, average pore diameter and pore volume of the samples were also investigated, and the results are shown in Table 2. There are no significant differences or changes in the average pore diameters and pore volumes among all the samples, implying that the use of glycerin co-solvent cannot impact the textural structure of the Mg/Al-HT particles. The phenomenon further verified the result of XRD that the structure of the brucite sheets do not change when using glycerin co-solvent (Table 1). The mix-MHT has the highest specific surface area (S_BET, 96.71 m² g⁻¹) and lowest permanent charge density (σ_p, 1.12 C m⁻²), followed by the bef-MHT and aft-MHT with permanent charge densities σ_p of 1.87 and 1.51 C m⁻², respectively. The AHT possesses the lowest specific surface area, and exhibits the highest σ_p of 2.95 C m⁻². Thus, there is reason to believe that glycerin molecules can increase the specific surface area by decreasing the particle size, which is in agreement with the deduction from XRD and SEM analyses that the more glycerin molecules in the particles, the smaller the particle size.

Table 2 Particle size distribution, textural properties and σ_p of the samples

Samples	SBET (m^2 g^−1)	Pore volume (mL g^−1)	Average pore diameter (nm)	σp (C m^−2)
AHT	75.16	0.296	3.821	2.95
Mix-MHT	96.71	0.294	3.775	1.12
Bef-MHT	87.81	0.244	3.774	1.87
Aft-MHT	80.32	0.192	3.828	1.51

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3.1.5. pH point of zero charge analyses. The point of zero charge (pH_zpc) was used in the determination of the surface charge properties of the materials. As seen in Fig. 5, the pH_zpc value of the AHT is the highest at 2.42, followed by that of the bef-MHT and aft-MHT, and the pH_zpc value of the mix-MHT is the lowest at 1.84. The decrease in pH_zpc and σ_p demonstrates that the surface of the Mg/Al-HT particles becomes more negative, leading to a higher electrostatic repulsion between the particles.

Fig. 5 PT curves of the AHT, mix-MHT, bef-MHT and aft-MHT particles.

3.1.6. TG-DTA analyses. Fig. 6 shows the TG-DTA curves of the AHT, mix-MHT, bef-MHT and aft-MHT particles. The TG-DTA curves of the three samples (mix-MHT, bef-MHT and aft-MHT) are basically similar, but there are significant differences between the three samples and AHT. The DTA curve of the AHT shows two endothermic peaks, where the first peak at 193 °C is due to the loss of the surface and interlayer water, and the second peak is at 393 °C corresponding to the decomposition of CO₃²⁻ and dehydroxylation in layers.²⁵ However, the curves of the mix-MHT, bef-MHT and aft-MHT show three endothermic peaks. The first peaks of the AHT, mix-MHT, bef-MHT and aft-MHT are at 193, 245, 230 and 218 °C, respectively. The increase in temperature of the first endothermic peak reveals the strengthening interaction, and the removal of water molecules becomes more difficult. So, for the AHT particles prepared in pure water, the hydration level is the lowest, and the crystal water is easy to release. For the mix-MHT, bef-MHT and aft-MHT particles, especially for the mix-MHT, the release of the crystal water becomes more difficult due to the stronger hydrogen bonding interactions. A similar trend in the temperature change of the second endothermic peaks (393 °C) is also found. The increase in the decomposition temperature of CO₃²⁻ and –OH at 393 °C is due to the presence of glycerin molecules, too. For the mix-MHT, bef-MHT and aft-MHT, the new weak endothermic peak at 320 °C may be due to the effect of glycerin, which is associated with some complex processes such as the decomposition of CO₃²⁻ in the interlayers and –OH in the brucite sheets held with different strengths due to intercalation of glycerin molecules. The weight losses at the new peak for the mix-MHT, bef-MHT and aft-MHT are 23.6%, 22.5% and 21.6%, respectively, implying that the mix-MHT contains the most glycerin molecules. The result confirms that the mix-MHT has the maximum quantity of glycerin molecules in the interlayer spaces, which is already demonstrated by the XRD and FT-IR (Fig. 1 and 3). This may be because more glycerin molecules are intercalated into the interlayer spaces accompanying water molecules and CO₃²⁻ during nucleation, while only a small amount of glycerin molecules can insert into the spaces before or after the crystallization.

Fig. 6 TG-DTA profiles of the AHT, mix-MHT, bef-MHT and aft-MHT particles.

According to the above analysis, it is evident that glycerin co-solvent limits the growth of the HT particles leading to a reduction in the particle size and the size distribution, and improves the hydrophobicity of the particle surface, so that the particles are repelled from each other due to electrostatic forces. In particular, the employment of glycerin during nucleation improved the interaction between the crystal water and brucite sheets. The more glycerin molecules that enter the interlayer spaces, and the higher the hydration level, the smaller and more hydrophobic the Mg/Al-HT particles become. Based on the results of the experiments, the mix-MHT sample was chosen to investigate the dispersion of hydrotalcite particles into the PP matrix.

3.2. Characterization of PP/Mg/Al-HT

3.2.1. SEM analysis of PP/Mg/Al-HT composites. The effect of blending the mix-MHT (10 wt%) with the PP matrix was evaluated from SEM images using cryo-fractured surfaces of PP/mix-MHT composites, which are presented in Fig. 7. The mix-MHT particles, as pointed to by the white arrows, are dispersed uniformly throughout the PP matrix. However, the AHT particles in the PP/AHT composite are badly agglomerated as white platelets, as pointed to by the white arrows. The particle sizes of the AHT are much greater than those of the mix-MHT. The result shows that the employment of glycerin during nucleation can improve the homogeneous dispersion of the Mg/Al-HT particles in the PP matrix. Thus, the glycerin added during nucleation can act as a co-solvent and promote the homogeneous dispersion of the mix-MHT particles in the PP matrix, namely improving compatibility.

Fig. 7 SEM images of the PP/AHT and PP/mix-MHT composites, ×500. White arrows point to the Mg/Al-HT particles in the composites.

3.2.2. Thermal behavior of PP/Mg/Al-HT composites. The thermal decomposition temperatures of the PP/AHT and PP/mix-MHT composites are shown by the TG curves (Fig. 8). Compared to PP, the addition of Mg/Al-HT particles increased the thermal stability, but did not affect the degradation steps of the PP matrix. The effect of Mg/Al-HT on the thermal stability of the PP matrix can be compared using the two temperatures, namely the onset decomposition temperature (T_0.1) and the decomposition temperature (T_0.5), which are significantly increased with the presence of the Mg/Al-HT particles (Table 3). The PP/mix-MHT exhibits higher decomposition temperatures than the PP/AHT due to higher compatibility with the PP matrix, indicating that the PP/mix-MHT possesses higher thermal stability. The incorporation of glycerin during nucleation can enhance the thermal stability of the PP/mix-MHT. Compared with the AHT, the mix-MHT has smaller particle sizes, a larger specific surface area and higher hydrophobicity, which means that the PP/mix-MHT composite is more thermally stable and flame retardant.

Fig. 8 TG curves of the PP/AHT and PP/mix-MHT composites.

Table 3 Thermal stability and mechanical properties of PP/Mg/Al-HT composites

Samples	PP	PP/AHT	PP/mix-MHT
T0.1 (°C)	314.6	352.3	339.1
T0.5 (°C)	365.8	379.4	400.2
Impact strength (kJ m^−2)	5.646	5.821	5.925
Tensile strength (MPa)	23.87	23.21	25.87
Flexural strength (MPa)	46.08	48.50	50.01
Fracture elongation (%)	37.69	31.43	34.43

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3.2.3. Mechanical properties. The mechanical properties of the PP/AHT and PP/mix-MHT composites are revealed in Table 3. The inclusion of Mg/Al-HT particles in the PP matrix has a small effect on the mechanical properties of PP. Among them, the PP/mix-MHT demonstrates the highest flexural strength, impact strength and tensile strength. The reinforced mechanical properties of the PP/mix-MHT may be caused by higher compatibility with and dispersion within the polymer from the more ultrafine particles as well as higher hydrophobicity outside compared with the PP/AHT.

4. Conclusions

In order to obtain better dispersion and compatibility of hydrotalcites with the PP matrix, the Mg/Al-HT were prepared using glycerin co-solvent by a urea method. The samples were characterized by XRD, SEM, FT-IR, laser particle size analysis, BJH/BET, DTA and pH_zpc analyses. It was found that the hydrophilic nature of the Mg/Al-HT particles was reduced, and the particle sizes decreased, consequently promoting diffusion of the particles in the PP matrix. Specifically, the incorporation of glycerin during nucleation was found to be an effective method to obtain smaller and more hydrophobic Mg/Al-HT particles (the mix-MHT) that could be evenly dispersed into the PP/mix-MHT composites to improve thermal stability and keep the mechanical properties of the PP matrix.

Acknowledgements

This work is supported by the Key Project of Hunan Provincial Natural Science Foundation of China (12JJ2008), Open Project of Hunan Provincial University Innovation Platform (12K048), and Xiangtan University Graduate Innovation Project (XJCX201405).

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