Preparation of Magnesium-Aluminum Hydrotalcite by Mechanochemical Method and Its Application as Heat Stabilizer in poly(vinyl chloride)

The traditional methods for preparing magnesium aluminum layered double hydrotalcite (Mg2Al-CO3LDHs) in industry include coprecipitation and hydrothermal methods. Both these methods have the disadvantages of high preparation cost and complicated water washing process. Using Mg(OH)2, Al(OH)3, and CO2 as raw materials in this work, the Mg2Al-CO3 LDHs are successfully prepared by mechanochemical method, which solves the shortcomings of traditional preparation method and realizes the conversion and utilization of CO2 resource. The prepared Mg2Al-CO3 LDHs are evaluated as a heat stabilizer in poly(vinyl chloride) (PVC). The result indicates that, when 2.4 phr Mg2Al-CO3 LDHs, 0.3 phr ZnSt2, and 0.3 phr of zinc acetylacetonate are added to the PVC, the thermal stability time of PVC can reach 190 min, which is better than PVC containing commercial Mg2Al-CO3 LDHs. Meanwhile, its processing performance is basically the same as the PVC containing commercial Mg2Al-CO3 LDHs.


Introduction
Heat stabilizer is one of the most important categories in plastic additives, which is in sync with the birth and development of poly(vinyl chloride) (PVC). There are many types of heat stabilizers, among which traditional heat stabilizers include organic tin, organic antimony, lead salts, metal soaps, etc. [1,2]. Organotin-based heat stabilizers have excellent thermal stability, weather resistance, initial colorability, nontoxicity, transparency, among other excellent properties, and are currently the most widely used, most effective and promising class of heat stabilizers. However, its expensive price limits its widespread application [3,4]. Organic antimony stabilizers have good thermal stability, low price, and low toxicity, but they have poor light stability, and lubricity [5]. Lead salt stabilizers were widely used in PVC products due to their low cost and excellent performance. However, lead is a heavy metal that is harmful to humans and the environment [6]. Metal soap heat stabilizers are mainly Ca-Zn composite heat stabilizers. However, they may also have a "zinc burn" phenomenon even though they are inexpensive and non-toxic [7]. So, they need to be formulated with another auxiliary heat stabilizer to make a multi component complex formulation when added to PVC [8]. In general,

Material Characterization
The X-ray diffraction (XRD) analysis was performed using a D8 ADVANCE diffractometer (Bruker, German) with Cu-K α targets (λ = 1.5406 Å) at a scanning rate of 8 • /min, with 10~80 • of the scanning range, and operated at a voltage of 40 kV and current of 40 mA.
The surface morphology of the samples were investigated via scanning electron microscope (SEM) study. The SEM measurements were carried out in Mira3 scanning electron microscope (Tescan, China). All samples were coated with a thin layer of gold prior to testing.
The fourier transform infrared spectroscopy (FTIR) study of the samples were carried out using a Nicolet iS5 (Thermo Fisher, Waltham, MA, USA) spectrometer at room temperature. The samples were crushed well and then examined in KBr pellets.
A TGA5500 thermogravimetric analysis instrument (Wakefield, MA, USA) was employed to analysis the sample stability, the sample was heated under an air flow from room temperature to 600 • C at a constant rate of 5 • C min −1 .
The sample size was analyzed with MS2000 laser particle size analyzer (Malvern, UK). The sample was dispersed in absolute ethanol. After ultrasonication for 1 h, it was placed in the sample cell for particle size testing.

Thermal Stability Testing of PVC-LDHs Composites
The static thermal aging test, Congo red test and thermal weight loss test were used to evaluate the thermal stability of PVC-LDHs composites. The specific steps of the static thermal aging test were as follow: 100 g PVC resin, 70 mL DOP and a certain of stabilizers were mixed evenly and then added in a SK-160B plastics mixing mill (Shanghai Light Industry Machinery Co. Ltd., Shanghai, China) for 5 min at 175 • C to form films. In addition, a group of sample without heat stabilizers was as comparison. After that, the films were cut into 2 × 2 cm 2 strips, which could be placed into a 180 • C oven for static thermal aging test. The color changes of the composites were recorded by a digital camera every 10 min. According to ISO standard 182-1:1990 (E), the specific steps of Congo red test were as follow: 2 g PVC strips were cut into pieces and added into a test tube, which a Congo red test paper was located at 3 cm above the sample. Then, the test tube was immersed in an oil bath at 190 • C to evaluate the thermal stability of the sample. The time required for the Congo red paper to change from red to blue was recorded and repeated three times to take the average. The specific steps of the thermal weight loss test were as follows: 10 g PVC resin, 7 mL DOP, and certain stabilizers were mixed evenly and then transferred into the porcelain boats. The porcelain boats were placed into an oven with a temperature of 190 • C and took out every certain time to weigh and calculate the weight loss.

Characterization of Mg 2 Al-CO 3 LDHs
In order to explore the crystal form of the prepared sample and the mixture after ball milling, XRD analyses of the samples are carried out and the result is shown in Figure 1. It can be clearly seen from the figure that the sample a has obvious characteristic diffraction peaks at 2θ = 11.66 • , 23 Table 1, the d-values of the standard card and the prepared LDH at the 003, 006 crystal plane are almost identical, which indicates that the sample a is typical hydrotalcite layered structure. The baseline is stable, the peak width is narrow and sharp, and there is no obvious impurity peak, indicating that the Mg 2 Al-CO 3 LDHs are successfully synthesized. Curve b has strong Mg(OH) 2 and Al(OH) 3 characteristic diffraction peaks, and there is a weak and wide diffraction peak at 2θ = 11.66 • , corresponding to the 003 crystal planes of Mg 4 Al 2 (OH) 12 CO 3 ·3H 2 O. It indicates that there is almost no reaction of the raw materials in the sample, and only a small amount of amorphous LDH is generated. The reason for this is that, under mechanical force effects, such as impact force, shear force, pressure, etc., the mixture sample will undergo crystal lattice distortion and particle amorphization, and its structure will form an amorphous layer after being strongly damaged. sample, and only a small amount of amorphous LDH is generated. The reason for this is that, under mechanical force effects, such as impact force, shear force, pressure, etc., the mixture sample will undergo crystal lattice distortion and particle amorphization, and its structure will form an amorphous layer after being strongly damaged.  In order to explore the morphology of the samples, SEM microphotographs of the samples are shown in Figure 2. It can be clearly seen from the images that the morphology of sample a exhibits a regular hexagonal structure, and the sheets are stacked together, which is a typical layered hydrotalcite structure. The crystal size of sample a is 300~500 nm and the thickness is about 30 nm. It shows that sample a has small crystal size, and its size is uniform, the crystal morphology is regular, which is consistent with the XRD pattern result. Sample b exhibits a flaky structure with only a small amount of hexagonal structure and uneven crystal size, indicating that mechanical grinding cannot directly synthesize Mg2Al-CO3 LDHs, and it needs to be crystallized for a certain time under certain temperature conditions.   In order to explore the morphology of the samples, SEM microphotographs of the samples are shown in Figure 2. It can be clearly seen from the images that the morphology of sample a exhibits a regular hexagonal structure, and the sheets are stacked together, which is a typical layered hydrotalcite structure. The crystal size of sample a is 300~500 nm and the thickness is about 30 nm. It shows that sample a has small crystal size, and its size is uniform, the crystal morphology is regular, which is consistent with the XRD pattern result. Sample b exhibits a flaky structure with only a small amount of hexagonal structure and uneven crystal size, indicating that mechanical grinding cannot directly synthesize Mg 2 Al-CO 3 LDHs, and it needs to be crystallized for a certain time under certain temperature conditions. sample, and only a small amount of amorphous LDH is generated. The reason for this is that, under mechanical force effects, such as impact force, shear force, pressure, etc., the mixture sample will undergo crystal lattice distortion and particle amorphization, and its structure will form an amorphous layer after being strongly damaged.  In order to explore the morphology of the samples, SEM microphotographs of the samples are shown in Figure 2. It can be clearly seen from the images that the morphology of sample a exhibits a regular hexagonal structure, and the sheets are stacked together, which is a typical layered hydrotalcite structure. The crystal size of sample a is 300~500 nm and the thickness is about 30 nm. It shows that sample a has small crystal size, and its size is uniform, the crystal morphology is regular, which is consistent with the XRD pattern result. Sample b exhibits a flaky structure with only a small amount of hexagonal structure and uneven crystal size, indicating that mechanical grinding cannot directly synthesize Mg2Al-CO3 LDHs, and it needs to be crystallized for a certain time under certain temperature conditions.   Figure 3 is a comparison diagram of the particle size of the Mg(OH) 2 and Al(OH) 3 mixture samples before and after the ball milling process. Figure 3a is the particle size diagram of the mixture before ball milling, it can be seen that the particle size distribution of the mixture sample is relatively concentrated, with an average particle size of 1224 nm. Figure 3b is the particle size diagram of the mixture after ball milling. It can be seen from the figure that the particle size distribution of the mixture is also relatively concentrated, with an average particle diameter of 1041 nm. It is found that the average particle size of the sample after ball milling process is reduced by 14.95%. During the ball milling process, the particles of the mixture are continuously subjected to intense shearing, friction, impact, and grinding, which reduces the grain size of the mixture sample. The crystals in the mixture undergo plastic deformation, and dislocations multiply and move. Mechanical energy is converted into chemical energy and stored in the crystal defects, which increases the chemical reaction activity of the mixture and greatly reduces the reaction activation energy [28].
Materials 2020, 13, x FOR PEER REVIEW 5 of 15 Figure 3 is a comparison diagram of the particle size of the Mg(OH)2 and Al(OH)3 mixture samples before and after the ball milling process. Figure 3a is the particle size diagram of the mixture before ball milling, it can be seen that the particle size distribution of the mixture sample is relatively concentrated, with an average particle size of 1224 nm. Figure 3b is the particle size diagram of the mixture after ball milling. It can be seen from the figure that the particle size distribution of the mixture is also relatively concentrated, with an average particle diameter of 1041 nm. It is found that the average particle size of the sample after ball milling process is reduced by 14.95%. During the ball milling process, the particles of the mixture are continuously subjected to intense shearing, friction, impact, and grinding, which reduces the grain size of the mixture sample. The crystals in the mixture undergo plastic deformation, and dislocations multiply and move. Mechanical energy is converted into chemical energy and stored in the crystal defects, which increases the chemical reaction activity of the mixture and greatly reduces the reaction activation energy [28]. In order to further explore the relevant information of the types of interlayer anions, crystal water and lattice oxygen vibration in the prepared sample, infrared spectroscopy analysis is carried out and the result is shown in Figure 4. It is found that the prepared sample showes a broad absorption peak around 3460 cm −1 , which is due to the stretching vibration of the hydroxyl attached to the metal ion and the interlayer water molecules. The absorption peaks of the prepared sample at 1362 cm −1 is the splitting peaks of the CO asymmetric stretching vibration in the CO3 2− group. It indicates that the carbonate anion is successfully inserted between the layers. The peaks appearing in the low wave number band represent lattice vibrational vibrations of Mg-O, Al-O, and Al-O-Mg. Among them, the stretching vibration of Al-O bond is near 785 cm −1 and 553 cm −1 [22].  Figure 5 shows the thermogravimetric-differential thermal analysis (TG-DTA) curve of the prepared sample. It can be seen from the figure that the sample has two obvious stages of mass loss. In order to further explore the relevant information of the types of interlayer anions, crystal water and lattice oxygen vibration in the prepared sample, infrared spectroscopy analysis is carried out and the result is shown in Figure 4. It is found that the prepared sample showes a broad absorption peak around 3460 cm −1 , which is due to the stretching vibration of the hydroxyl attached to the metal ion and the interlayer water molecules. The absorption peaks of the prepared sample at 1362 cm −1 is the splitting peaks of the CO asymmetric stretching vibration in the CO 3 2− group. It indicates that the carbonate anion is successfully inserted between the layers. The peaks appearing in the low wave number band represent lattice vibrational vibrations of Mg-O, Al-O, and Al-O-Mg. Among them, the stretching vibration of Al-O bond is near 785 cm −1 and 553 cm −1 [22].
Materials 2020, 13, x FOR PEER REVIEW 5 of 15 Figure 3 is a comparison diagram of the particle size of the Mg(OH)2 and Al(OH)3 mixture samples before and after the ball milling process. Figure 3a is the particle size diagram of the mixture before ball milling, it can be seen that the particle size distribution of the mixture sample is relatively concentrated, with an average particle size of 1224 nm. Figure 3b is the particle size diagram of the mixture after ball milling. It can be seen from the figure that the particle size distribution of the mixture is also relatively concentrated, with an average particle diameter of 1041 nm. It is found that the average particle size of the sample after ball milling process is reduced by 14.95%. During the ball milling process, the particles of the mixture are continuously subjected to intense shearing, friction, impact, and grinding, which reduces the grain size of the mixture sample. The crystals in the mixture undergo plastic deformation, and dislocations multiply and move. Mechanical energy is converted into chemical energy and stored in the crystal defects, which increases the chemical reaction activity of the mixture and greatly reduces the reaction activation energy [28]. In order to further explore the relevant information of the types of interlayer anions, crystal water and lattice oxygen vibration in the prepared sample, infrared spectroscopy analysis is carried out and the result is shown in Figure 4. It is found that the prepared sample showes a broad absorption peak around 3460 cm −1 , which is due to the stretching vibration of the hydroxyl attached to the metal ion and the interlayer water molecules. The absorption peaks of the prepared sample at 1362 cm −1 is the splitting peaks of the CO asymmetric stretching vibration in the CO3 2− group. It indicates that the carbonate anion is successfully inserted between the layers. The peaks appearing in the low wave number band represent lattice vibrational vibrations of Mg-O, Al-O, and Al-O-Mg. Among them, the stretching vibration of Al-O bond is near 785 cm −1 and 553 cm −1 [22].  Figure 5 shows the thermogravimetric-differential thermal analysis (TG-DTA) curve of the prepared sample. It can be seen from the figure that the sample has two obvious stages of mass loss.  Figure 5 shows the thermogravimetric-differential thermal analysis (TG-DTA) curve of the prepared sample. It can be seen from the figure that the sample has two obvious stages of mass loss. The first stage (~14%) is between 30 • C and 235 • C, mainly due to the loss of surface absorbed and intercalated water molecules. It can be expressed by the Equation (1).
The second stage (~28%) is between 235 • C and 450 • C, mainly due to the decompose of hydroxyl in the layer and carbonate ion between the layers [29]. It can be expressed by the Equation (2).
The first endothermic peak at 225 • C corresponds to the removal of crystallization water. The second and third peaks at 334 • C and 399 • C corresponds to the dehydroxylation of aluminum hydroxide and magnesium hydroxide, respectively [22].
Materials 2020, 13, x FOR PEER REVIEW 6 of 15 The first stage (~14%) is between 30 °C and 235 °C, mainly due to the loss of surface absorbed and intercalated water molecules. It can be expressed by the Equation (1).
The second stage (28%) is between 235°C and 450°C, mainly due to the decompose of hydroxyl in the layer and carbonate ion between the layers [29]. It can be expressed by the Equation (2).
The first endothermic peak at 225 °C corresponds to the removal of crystallization water. The second and third peaks at 334 °C and 399 °C corresponds to the dehydroxylation of aluminum hydroxide and magnesium hydroxide, respectively [22]. As shown in Figure 6, the weighed mixture is added to a ball mill pot, and the LDH precursor is generated under the action of mechanical force [30,31], which can be combined with carbonate ions in an alkaline solution to form Mg2Al-CO3 LDHs after stirring 30 h. According to previous studies, the formation mechanism of Mg2Al-CO3 LDHs prepared by Mg(OH)2, Al(OH)3 is similar to the formation mechanism of hydrotalcite prepared by co-precipitation. A possible formation mechanism of Mg2Al-CO3 LDHs prepared by cleaning method occurred according to the following reaction.
In Equation (3), CO2 is absorbed by lye and converted into CO3 2− . In Equation (4), in an alkaline system, Al(OH)3 reacts with OH − to produce Al(OH) 4 − . In Equation (5) As shown in Figure 6, the weighed mixture is added to a ball mill pot, and the LDH precursor is generated under the action of mechanical force [30,31], which can be combined with carbonate ions in an alkaline solution to form Mg 2 Al-CO 3 LDHs after stirring 30 h. According to previous studies, the formation mechanism of Mg 2 Al-CO 3 LDHs prepared by Mg(OH) 2 , Al(OH) 3 is similar to the formation mechanism of hydrotalcite prepared by co-precipitation. A possible formation mechanism of Mg 2 Al-CO 3 LDHs prepared by cleaning method occurred according to the following reaction.
In Equation (3), CO 2 is absorbed by lye and converted into CO 3 2− . In Equation (4), in an alkaline system, Al(OH) 3 reacts with OH − to produce Al(OH) − 4 . In Equation (5), Al(OH) − 4 diffuses into the octahedral voids of OH − accumulation in brucite. Among them, Al 3+ enters into the octahedral void, substituting a portion of Mg 2+ in the brucite lattice, forming a coordination structure. This octahedral structure unit forms a network structure through stacking and different connection methods. During the formation of the network structure, defects will inevitably be formed, such as holes and incomplete coordination, resulting in the crystal layer being positively charged. Then, in Equation (6), in order to balance the positive charge carried by the laminate, CO 2− 3 in the solution intercalates into the interlayer through electrostatic interaction, hydrogen bonding and van der Waals force to balance the positive charge on the laminate and gradually forms the hydrotalc structure [32,33].
Equation (7) is the reaction equation, in which the crystal water in hydrotalcite are all from the solution. It can be seen from the Equation (7) Equation (7) is the reaction equation, in which the crystal water in hydrotalcite are all from the solution. It can be seen from the Equation (7) that the reactant raw materials are fully utilized.

Thermal Stability of Mg2Al-CO3 LDHs on PVC
In order to explore the synergistic thermal stability of Mg2Al-CO3 LDHs with other stabilizers on PVC, the thermal stability of the samples have been tested. The result of Congo red test of PVC samples are shown in Table 2, and the results of the static oven thermal aging test and thermal weight loss test are shown in Figures 7 and 8. When the Congo red time is longer and the quality loss of PVC in the static oven experiment is less, indicating that the thermal stability of PVC is better.

Thermal Stability of Mg 2 Al-CO 3 LDHs on PVC
In order to explore the synergistic thermal stability of Mg 2 Al-CO 3 LDHs with other stabilizers on PVC, the thermal stability of the samples have been tested. The result of Congo red test of PVC samples are shown in Table 2, and the results of the static oven thermal aging test and thermal weight loss test are shown in Figures 7 and 8. When the Congo red time is longer and the quality loss of PVC in the static oven experiment is less, indicating that the thermal stability of PVC is better.   As shown in Figures 7a and 8, the sample without any addition of heat stabilizer has been completely turned black at 20 min, its thermal stability time is only 8 min according to Congo red test, and it has a weight retention rate of 56.92% at 1440 min under 190 °C condition. Sample with  As shown in Figures 7a and 8, the sample without any addition of heat stabilizer has been completely turned black at 20 min, its thermal stability time is only 8 min according to Congo red test, and it has a weight retention rate of 56.92% at 1440 min under 190 °C condition. Sample with As shown in Figures 7a and 8, the sample without any addition of heat stabilizer has been completely turned black at 20 min, its thermal stability time is only 8 min according to Congo red test, and it has a weight retention rate of 56.92% at 1440 min under 190 • C condition. Sample with Mg 2 Al-CO 3 LDHs addition amount of 3 phr begin to color at 20 min and completely turn black at 80 min as shown in Figure 7b. And it has the weight retention rate of 85.37% at 1440 min. Meanwhile, its thermal stability time is 30 min, which is 22 min longer than the sample without any addition of heat stabilizer but it still has a poor initial thermal stability. That is because Mg 2 Al-CO 3 LDHs belongs to a long-term thermal stabilizer, so it is necessary to add an initial thermal stabilizer into it. Here, zinc stearate (ZnSt 2 ) is used as an initial heat stabilizer. As shown in Figure 7c, the sample added with ZnSt 2 completely turned black at 30 min, has a "zinc burn" phenomenon, its thermal stability time is 12 min. Meanwhile, it has the weight retention rate of 66.74%, which is higher than pure PVC. In Figure 7d, the sample added with LDHs and ZnSt 2 does not start coloring until 100 min, and has been completely turned black at 160 min. Its thermal stability time is 36 min, which depends on the synergistic effect of LDHs and ZnSt 2 . This is because the fatty acid radicals in zinc stearate can react with allyl chloride on the PVC polymer chain, which can partially eliminate the unstable structure in the long chain of PVC and reduce the initial thermal decomposition rate of PVC. In addition, zinc stearate can absorb HCl gas in the system, and the specific reaction can be expressed by Equations (8) and (9) At the same time, it has a weight retention rate of 69.91%, the reason for the decrease in weight retention rate after adding ZnSt 2 is that the decomposition product ZnCl 2 is a strong Lewis acid, which will catalyze the degradation of PVC [7].
In order to compare the thermal stability of PVC containing commercial Mg 2 Al-CO 3 LDHs with the experimentally prepared Mg 2 Al-CO 3 LDHs, different content of the auxiliary heat stabilizer zinc acetylacetonate compound with experimentally prepared Mg 2 Al-CO 3 LDHs and zinc stearate, then are added to PVC. The result of Congo red test of PVC samplesare shown in Table 3. The results of the static oven thermal aging test and thermal weight loss test are shown in Figures 9 and 10.   The sample containing commercial Mg2Al-CO3 LDHs does not start coloring until 110 min, and completely turn black at 170 min, which is 10 min longer than the sample containing experimentally prepared Mg2Al-CO3 LDHs as shown in Figure 9a,b. The reason is that small amounts of additivesuch as stearic acid is added to commercial Mg2Al-CO3 LDHs, which can slightly improve its thermal   The sample containing commercial Mg2Al-CO3 LDHs does not start coloring until 110 min, and completely turn black at 170 min, which is 10 min longer than the sample containing experimentally prepared Mg2Al-CO3 LDHs as shown in Figure 9a,b. The reason is that small amounts of additivesuch as stearic acid is added to commercial Mg2Al-CO3 LDHs, which can slightly improve its thermal The sample containing commercial Mg 2 Al-CO 3 LDHs does not start coloring until 110 min, and completely turn black at 170 min, which is 10 min longer than the sample containing experimentally prepared Mg 2 Al-CO 3 LDHs as shown in Figure 9a,b. The reason is that small amounts of additivesuch as stearic acid is added to commercial Mg 2 Al-CO 3 LDHs, which can slightly improve its thermal performance. As shown in Figure 9b-f, the samples added with zinc acetylacetonate effectively improved the whiteness in the initial and slightly improved the long-term thermal stability of PVC.
As the content of zinc acetylacetonate increases, the initial and long-term thermal stability of the PVC samples gradually improve, especially the sampled with 0.2 phr zinc acetylacetonate added, which begins to color at 130 min, and completely turn black at 190 min, which is 20 min longer than the sample containing commercial Mg 2 Al-CO 3 LDHs.Zinc acetylacetonate belongs to β-diketone, which is a kind of auxiliary heat stabilizer. The mechanism of action of zinc acetylacetonate as auxiliary heat stabilizer can be represented in Figure 11. As shown in Figure 11a, zinc acetylacetonate can effectively absorb the HCl gas generated by the thermal degradation of PVC, and the conversion product is acetylacetone. Figure 11b shows that acetylacetone can replace the active allyl Cl atoms on the PVC polymer chain under the catalytic action of ZnCl 2 , reducing the active sites on the PVC polymer chain. Meanwhile, acetylacetone can also be cross-linked with PVC polymer chains to form a stable structure [34].
Materials 2020, 13, x FOR PEER REVIEW 11 of 15 performance. As shown in Figure 9b-f, the samples added with zinc acetylacetonate effectively improved the whiteness in the initial and slightly improved the long-term thermal stability of PVC.
As the content of zinc acetylacetonate increases, the initial and long-term thermal stability of the PVC samples gradually improve, especially the sampled with 0.2 phr zinc acetylacetonate added, which begins to color at 130 min, and completely turn black at 190 min, which is 20 min longer than the sample containing commercial Mg2Al-CO3 LDHs.Zinc acetylacetonate belongs to β-diketone, which is a kind of auxiliary heat stabilizer. The mechanism of action of zinc acetylacetonate as auxiliary heat stabilizer can be represented in Figure 11. As shown in Figure 11a, zinc acetylacetonate can effectively absorb the HCl gas generated by the thermal degradation of PVC, and the conversion product is acetylacetone. Figure 11b shows that acetylacetone can replace the active allyl Cl atoms on the PVC polymer chain under the catalytic action of ZnCl2, reducing the active sites on the PVC polymer chain. Meanwhile, acetylacetone can also be cross-linked with PVC polymer chains to form a stable structure [34]. However, as the content of zinc acetylacetonate continued to increase, the initial and long-term thermal stability of the PVC sample decreased when the addition amount is 0.4 phr as shown in Figure 9f. This is because as the content of zinc acetylacetonate increased, more ZnCl2 is produced in the system, which will catalyze the thermal degradation of PVC due to its strong Lewis acidity [7], the initial and long-term thermal stability performance will decrease.
As shown in Table 3 and Figure 10, the thermal stability time of PVC sample containing commercial LDHs is 38 min, which is 3 min longer than experimentally prepared LDHs, and it has a weight retention rate of 74.63% at 1440 min. However, as the content of zinc acetylacetonate increased, the thermal stability time and weight retention rate of PVC sample increases first and then decreases, when 0.3 phr zinc acetylacetonate is added, the PVC sample has a maximum thermal stability time of 46 min, which is 7 min longer than commercial LDHs. Meanwhile, it has weight retention rate of 73.53% at 1440 min. The Congo red test and thermal weight loss test demonstrate the same conclusion as the static oven thermal aging test. However, as the content of zinc acetylacetonate continued to increase, the initial and long-term thermal stability of the PVC sample decreased when the addition amount is 0.4 phr as shown in Figure 9f. This is because as the content of zinc acetylacetonate increased, more ZnCl 2 is produced in the system, which will catalyze the thermal degradation of PVC due to its strong Lewis acidity [7], the initial and long-term thermal stability performance will decrease.
As shown in Table 3 and Figure 10, the thermal stability time of PVC sample containing commercial LDHs is 38 min, which is 3 min longer than experimentally prepared LDHs, and it has a weight retention rate of 74.63% at 1440 min. However, as the content of zinc acetylacetonate increased, the thermal stability time and weight retention rate of PVC sample increases first and then decreases, when 0.3 phr zinc acetylacetonate is added, the PVC sample has a maximum thermal stability time of 46 min, which is 7 min longer than commercial LDHs. Meanwhile, it has weight retention rate of 73.53% at 1440 min. The Congo red test and thermal weight loss test demonstrate the same conclusion as the static oven thermal aging test.
Therefore, when 0.3 phr of zinc acetylacetonate, 2.4 phr Mg 2 Al-CO 3 LDHs, and 0.6 phr ZnSt 2 are added to the PVC, the thermal stability of PVC is the best.

Processing Performance Test of Compound Heat Stabilizer Added to PVC
In order to explore the difference in processing performance between the experimentally prepared Mg 2 Al-CO 3 LDHs and commercial Mg 2 Al-CO 3 LDHs, a torque rheometer is carried out to test them. The balanceable torque and the mixing process energy were carried out by a Torque Rheometer ZJL-200 (Chang Chun Intelligent Apparatus Co. Ltd., Changchun, China) at 190 • C at a speed of 40 r/min for 10 min.
The recipe of the samples are shown in Table 4, the rheological curves are shown in Figure 12 and their rheological data are shown in Table 5.

Processing Performance Test of Compound Heat Stabilizer Added to PVC
In order to explore the difference in processing performance between the experimentally prepared Mg2Al-CO3 LDHs and commercial Mg2Al-CO3 LDHs, a torque rheometer is carried out to test them. The balanceable torque and the mixing process energy were carried out by a Torque Rheometer ZJL-200 (Chang Chun Intelligent Apparatus Co. Ltd., Changchun, China) at 190°C at a speed of 40 r/min for 10 min.
The recipe of the samples are shown in Table 4, the rheological curves are shown in Figure 12 and their rheological data are shown in Table 5.   It can be seen from Figure 12 and Table 5 that the order of maximum torque of the three samples is b > c >a, the balance torque is c > a > b, the melting time is a > c > b, melting temperature is a > c > b, and the balance temperature is c > a > b. The comparison of the three samples show that the maximum torque of sample a is the smallest, which is 20.68 Nm, indicating that its melt viscosity is low, the melt fluidity is the best, which is the easiest to process. Moreover, it has the longest melting time 32 s, and the highest melting temperature 160.3 °C, indicating that sample a is the most difficult to plasticize. Compared with the sample containing commercial Mg2Al-CO3 LDHs, the maximum torque of the sample containing experimentally prepared Mg2Al-CO3 LDHs is larger, and the melting  It can be seen from Figure 12 and Table 5 that the order of maximum torque of the three samples is b > c >a, the balance torque is c > a > b, the melting time is a > c > b, melting temperature is a > c > b, and the balance temperature is c > a > b. The comparison of the three samples show that the maximum torque of sample a is the smallest, which is 20.68 Nm, indicating that its melt viscosity is low, the melt fluidity is the best, which is the easiest to process. Moreover, it has the longest melting time 32 s, and the highest melting temperature 160.3 • C, indicating that sample a is the most difficult to plasticize. Compared with the sample containing commercial Mg 2 Al-CO 3 LDHs, the maximum torque of the sample containing experimentally prepared Mg 2 Al-CO 3 LDHs is larger, and the melting time is shorter, indicating that its plasticization is faster, the viscosity of the melt is larger, is harder to process, and the external lubricity of PVC composite is poorer [35]. It is necessary to improve the external lubricity and balance the internal and external lubrication. Both the balance torque and the balance temperature of sample b are low, indicating that PVC compounds consume the least energy during processing. Comparing the sample adding zinc acetylacetonate with the sample without zinc acetylacetonate, the maximum torque is reduced at 21.55 Nm, and the balance torque, balance temperature, the melting time and melting temperature are slightly increased, indicating that the addition of the auxiliary thermal stabilizer zinc acetylacetonate increases the external lubricity of the PVC composite and improves the processing flow properties of the PVC composite. In summary, their processing performance is basically the same, as the processing performance and thermal stability of the prepared samples have reached the commercial level.

Conclusions
Through mechanochemical methods, the Mg 2 Al-CO 3 LDHs were successfully prepared by Mg(OH) 2 , Al(OH) 3 , and CO 2 . This method is simple in process, low in cost, high in materials utilization rate, and suitable for large-scale production.
The samples Mg 2 Al-CO 3 LDHs were used as a heat stabilizer in PVC. The result shows that when 2.4 phr Mg 2 Al-CO 3 LDHs, 0.3 phr ZnSt 2 and 0.3 phr of zinc acetylacetonate are added to the PVC, the thermal stability time of PVC can reach 190 min under 180 • C oven test conditions, which is 20 min longer than the PVC composite containing 2.4 phr commercial Mg 2 Al-CO 3 LDHs and 0.6 phr ZnSt 2 . Meanwhile, the result of Congo red test shows that its thermal stability time is 46 min, which is 7 min longer than the time of commercial Mg 2 Al-CO 3 LDHs. Moreover, its processing performance is basically the same as the PVC containing commercial Mg 2 Al-CO 3 LDHs.