The XRD patterns of Zn-Al-LDHs with different Zn-Al molar ratios are exhibited in Figure 1. The strong peaks at 11.2°, 23.5°, 35.1°, 61.8°are characteristic of a layered structure. The sharp and symmetrical diffraction peaks with a low and stable baseline indicate the well-crystallized layered structure of the Zn-Al-LDHs. It can be found that the XRD patterns of Zn-Al-LDHs with different Zn/Al molar ratios are similar. The LDHs can be successfully synthesized with three different Zn/Al ratios of 3:1, 2:1 and 1:1. According to a = 2d₁₁₀, the crystal structure of Zn-Al-LDHs with different molar ratios are the same. The charge number of Al³⁺ is greater than that of Zn²⁺, hence, the distance of the layers increases and the coulomb repulsion between metal ions in the adjacent hexagonal unit cells decreases with increasing the Zn/Al ratio, making the lattice parameter a and c (=3d₀₀₃) increase, especially c. This is based on the fact that parameter a is a function of the average radius of the metal cations while parameter c is a function of the average charge of the metal cations [14]. From Figure 1 the diffraction peak of curve b is more sharp than that of a and c, indicating that the Zn-Al-LDHs has high crystallinity and integrated crystalline structures with molar ratio of 2:1.

FT-IR spectra of the LDHs samples are shown in Figure 2. As shown in Figure 2, a band near 3,420 cm⁻¹ corresponds to the vibration bands of hydroxyls (νOH). The peak moves to a higher wavenumber with increasing Zn/Al ratio. The possible reason is that the ionic radius of Al³⁺ is smaller than that of Zn²⁺ and the charge number of Al³⁺ is greater than that of Zn²⁺, the electrostatic force between Al³⁺ and OH⁻ is stronger than that of Zn²⁺ and OH⁻ with increasing the Zn/Al ratio. The bending mode of water molecules appears at 1,620 cm⁻¹ and the intensity increases slightly with increasing Zn/Al ratio. The sharp intense band at 1,364 cm⁻¹ is the antisymmetric stretching of interlayer carbonate with high values of Zn/Al ratio. Compared with the peak of CO₃²⁻ in CaCO3 (1,430 cm⁻¹), a large shift toward a lower wavenumber indicates that the CO₃²⁻ intercalating into the interlayer is not free, and is interlayered with water molecules between with hydrogen bonds are formed [16]. The interlayer charge density and the coulomb repulsion between interlayer anions increases with decreasing Zn/Al ratio, which results in a stronger peak of interlayer CO₃²⁻ and a lower wave number (1,357 cm⁻¹). The band at 794 cm⁻¹ is the flexural vibration of CO₃²⁻. The bands at 430 cm⁻¹ and 560 cm⁻¹ can be attributed to the lattice vibration modes, M-O and O-M-O vibration, respectively. The structural parameters of FT-IR are listed in Table 1.

Figure 3 shows typical SEM micrographs of Zn-Al-LDHs with a Zn-Al molar ratio of 3:1. It can be seen that the existing lamellar particles have a rounded hexagonal shape, which is the typical structure of the hydrotalcite-like materials.

Figure 4 shows the particle granularity analysis pattern of Zn-Al-LDHs with a Zn-Al molar ratio of 3:1. The range of the particle granularity was relatively narrow and the particle size showed normal distribution, which indicates the particle size distribution of powders was homogeneous and narrow.

The specific area of Zn-Al-LDHs with Zn-Al ratio of 3:1 was analyzed by adsorption apparatus. The specific area of the particles is 43.48 m²/g and the pore volume is 0.2613 cm³/g.

As shown in Table 2, PVC mixture without Zn-Al-LDHs (serial number 1) became completely black within just 10 min at 180 ± 1 °C. When 0.04 g LDHs (serial number 2) was added to the PVC, the stability time increases to 70 min, indicating that the thermal stability is significantly enhanced. The reaction between Zn-Al-CO₃-LDHs and HCl occurs in a two-step process. First of all, the counter ions OH⁻¹ and CO₃²⁻ between the LDH layers react with the HCl gas. Then, the LDH itself reacts with HCl with complete destruction of the LDH structure and formation of metal chlorides. The Zn-Al-CO₃-LDHs can react with labile chlorine atoms (such as allyl chloride), so it is easy to restrain the initial coloring and prevent the growth of “Polyolefin Structure” when it is used alone. As the amount of LDHs increased from 0 to 0.1 g, there is a prominent increase in thermal stability of PVC. However, further addition of LDHs in PVC cannot enhance the thermal stability more, and weakens the stability instead. This phenomenon can be illustrated as the quantities of Zn increasing with the increment of LDHs, leading to the formation of ZnCl₂ which can catalyze the dehydrochlorination. In Zn-Al-LDHs, hydroxyl can absorb HCl; furthermore, the existance of Zn can increase the initial stability, which makes good-initial coloring. However, the long-term stability is impaired due to the “zinc burning” as adding excess Zn-Al-LDHs. Compared with Mg-Al-LDHs, the PVC samples with Zn-Al-LDHs are light yellow after 110 min in the aging test in this work, while the samples with Mg-Al-LDHs turned yellow inclining to black during the initial 20 min [17]. The optimum amount of LDHs is determined to be 0.1 g from Table 2. Xu et al. [18] investigated the thermal stability of Zn-Al-CO₃-LDH-incorporated PVC at high temperatures and found that LDHs facilitated dehydrochlorination at a lower temperature but LDHs can reduce the subsequent dehydrocarbonation and help to form more char-like carbonaceous materials. Unlike that investigation, the CaSt₂ and β-diketone added here cooperated with LDHs to effectively reduce the formation of ZnCl_2, then the dehydrochlorination of HCl was impaired even at initialization. The photos for the color changes of PVC with 0.1 g LDHs are displayed in Figure 5.

Table 3 shows the color changes of PVC with various amounts of CaSt₂ added. It is clear that the short-term stabilization of PVC is enhanced with increasing quantities of CaSt₂. The CaSt₂ not only absorbs HCl, but also exchanges ions with ZnCl₂ in the PVC-Zn-Al-LDHs systems, which could be expressed as following: ZnCl 2   +   Ca ( OOCR ) 2   →   Zn ( OOCR ) 2   +   CaCl 2

The active ZnSt₂ is generated and the undesirable ZnCl₂ is consumed. The product ZnSt₂ can scavenge HCl and react with the labile chlorine (allylic chlorine) atoms on the PVC chain. From Table 3, the optimum amount of CaSt₂ is thought to be 0.15 g. The photos for the color changes of PVC with 0.15 g CaSt₂ are presented in Figure 6.

The color stability of PVC with β-Diketone is shown in Table 4. For PVC mixtures without any β-Diketone or with a low amount of β-Diketone (series 4 and 5), the discoloration time is only 10 min. The later color change from yellow to black is sudden. When the quantity of β-diketone is 0.25 g, the initial color is stable to some degree, as well as the later color, which means that the optimum amount of β-Diketone is 0.25 g. The β-diketone behaves as a short-term co-stabilizer and increases initial color stability. The acetylacetonates substitute allylic chlorides inhibiting the formation of long polyenes and resulting in no discoloration of the polymer [19].The reaction of β-Diketone with polyvinyl chloride is presented in Figure 7.

The photos for the color changes of PVC without β-diketone are shown in Figure 8.