Preparation of Magnesium-Aluminum Hydrotalcite by Mechanochemical Method and Its Application As Heat Stabilizer in Poly(Vinyl Chloride)

materials

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

Yinan Jiang 1,2, Zhanhong Yang 1,* , Qingsong Su 1,2, Linlin Chen 1,2, Jian Wu 1,2 and Jinlei Meng 1,2

1 Institute of Chemical Power and Materials, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China; [email protected] (Y.J.); [email protected] (Q.S.); [email protected] (L.C.); [email protected] (J.W.); [email protected] (J.M.) 2 Institute of Chemical Power and Materials, Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central South University, Changsha 410083, China * Correspondence: [email protected]

Received: 16 October 2020; Accepted: 16 November 2020; Published: 19 November 2020

Abstract: 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.

Keywords: poly(vinyl chloride); magnesium-aluminum layered double hydroxide; carbon dioxide; mechanochemical method; thermal stability

1. 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,

Materials 2020, 13, 5223; doi:10.3390/ma13225223 www.mdpi.com/journal/materials Materials 2020, 13, 5223 2 of 15 there is an urgent need for a material that is not only inexpensive, non-toxic, and environmentally friendly, but also has excellent overall performance. Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, are a class of compounds characterized by a layered structure. It has the generic formula h ix+ 2+ 3+( ) ( n ) 2+ 3+ M1 xMx OH 2 A − x/n mH2O where x = 0.2~0.33, M is a divalent metal cation, M is − n · trivalent cation, and A − is non-framework charge compensating anion. Particularly, its structure may be positively charged by substituting a portion of divalent cations in the brucite lattice with trivalent cations, which the intercalation of anions between the layers balance the charge [9,10]. Due to its special layered structure, LDHs have controllability in the chemical composition of the cationic sheets, the type and number of the intercalated anions, and the size and distribution of the grains. Therefore, there are many kinds of hydrotalcites, which have broad application prospects in the fields of adsorption [11], medicine [12], catalysis [13], electrochemistry [14–16], flame-retardant [17,18], and photochemistry [19]. Moreover, it can also be used as a heat stabilizer for PVC [20]. In the 1980s, hydrotalcite was firstly found that when added to PVC materials had better thermal stability than PVC materials without adding hydrotalcite [10]. This set off a research upsurge concerning the application of hydrotalcite in PVC resin. Nowadays, as a typical layered double hydroxides, Mg-Al-CO3-LDHs has been widely used as a high-efficiency long-term stabilizer for calcium and zinc soap heat stabilization systems [21]. Generally, the synthesis methods of Mg-Al-hydrotalcite-based heat stabilizers include the co-precipitation method [22], ion exchange method [23], urea method [10], hydrothermal method [24], roasting recovery method [25], and nucleation/crystallization separation method [26]. However, these synthetic methods require a large amount of salt solution and alkaline solution, which results in the need to wash with a large amount of water in the later stage and produce low-value salt solutions and alkaline solutions. The production cost is too high and the raw material utilization rate is low. This runs counter to the goal of reducing production costs and environmental friendly. Nowadays, the world-population and energy consumption are continuous increasing, which will consume a large amount of fossil energy and release the greenhouse gas CO2, causing serious environmental problems such as global warming. For the sustainable development of human society, it is urgent to develop new green technologies to realize the conversion and utilization of CO2 resource [27]. In this work, traditional corresponding salts were substituted with Mg(OH)2 and Al(OH)3 and CO2 was used as carbon sources, Mg2Al-CO3 LDHs were prepared by mechanochemical methods, the possible synthesis mechanisms were discussed as well. Meanwhile, the prepared Mg2Al-CO3 LDHs were formulated with Zn-based soap initial heat stabilizers to make multicomponent complex formulation and added to PVC to evaluate the thermal stability performance.

2. Experimental

2.1. Materials

PVC resin (S-65) was industrial grade (Formosa Plastics Co. Ltd., Ningbo, China). Mg(OH)2, Al(OH)3 and CO2 were of A.R. grade and dioctyl-phthalate (DOP). The water used in the experiment was deionized water.

2.2. Preparation of Hydrotalcite

Briefly, 11.6g (0.2 mol) Mg(OH)2 and 7.8 g (0.1 mol) Al(OH)3 were mixed, the grinding ball and the mixture were put into the grinding jar according to the ball-to-powder weight ratio of 10:1. The rotation speed was set at 400 r/min. After 5 h of ball milling in a planetary ball mill, the solid powder was transferred into a three-necked flask, then 100 mL water was added, 2.2 g (0.05 mol) CO2 was passed into the solution, the pH was controlled at 11. After stirring in a 95 ◦C water bath for 30 h, the reactant was filtered, washed, dried in an 80 ◦C drying oven for 12 h, and ground for use. Materials 2020, 13, 5223 3 of 15

2.3. Material Characterization The X-ray diffraction (XRD) analysis was performed using a D8 ADVANCE diffractometer (Bruker, German) with Cu-Kα 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 1 600 ◦C at a constant rate of 5 ◦C min− . 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.

2.4. 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 cm2 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.

3. Results and Discussion

3.1. Characterization of Mg2Al-CO3 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 Figure1. It can be clearly seen from the figure that the sample a has obvious characteristic diffraction peaks at 2θ = 11.66◦, 23.46◦, 34.80◦, 39.38◦, 46.84◦, 60.72◦, and 62.13, respectively, corresponding to the 003, 006, 222, 225, 228, 600 and 603 crystal planes, which matches the standard card (Chao and Gault, 1997) of Mg Al (OH) CO 3H O 4 2 12 3· 2 (PDF# 51-1525). According to Bragg’s law, the d-values between standard card and prepared LDHs are indicated in Table1. As shown in Table1, 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 Mg2Al-CO3 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 Al (OH) CO 3H O. It indicates that there ◦ 4 2 12 3· 2 is almost no reaction of the raw materials in the sample, and only a small amount of amorphous LDH is Materials 20202020,, 1133,, xx FORFOR PEERPEER REVIEWREVIEW 44 of 1515 Materials 2020, 13, 5223 4 of 15 sample,sample, andand onlyonly aa smallsmall amounamountt ofof amorphousamorphous LDHLDH isis generated.generated. TheThe reasonreason forfor thisthis isis thatthat,, under mechanicalgenerated. Theforce reason effects, for such this is as that, impact under force, mechanical shear force, force pressure, effects, such etc., as the impact mixture force, sample shear force, will undergopressure, etc.,crystal the lattice mixture distortion sample will and undergo particle crystal amorphization, lattice distortion and its and structure particle amorphization, will form an amorphousand its structure layer will after form being an strongly amorphous damaged. layer after being strongly damaged.

Figure 1. XRD patterns of Mg Al-CO LDHs prepared with (a) CO and (b) noneas carbon sources. Figure 1. XRD patterns of Mg222Al--CO33 LDHs prepared with ((a)) CO222 and (b)) noneas carboncarbon sources.sources. Table 1. Crystal parameters of LDHs. Table 1. Crystal parameters of LDHs.. Sample d003/Å d006/Å d222/Å d225/Å d228/Å d600/Å d603/Å Sample d003003//Å d006006//Å d222222//Å d225225//Å d228228//Å d600600//Å d603603//Å Standard card 7.570 3.778 2.570 2.281 1.932 1.524 1.493 Standard card 7.570 3.778 2.570 2.281 1.932 1.524 1.493 a 7.586 3.789 2.576 2.286 1.938 1.524 1.493 a 7.586 3.789 2.576 2.286 1.938 1.524 1.493

InIn orderorder order toto to exploreexplore explore thethe the morphologymorphology morphology of of the the samplessamples samples,,, SEMSEM SEM microphotographsmicrophotographs microphotographs ofof thethe of thesamplessamples samples areare shownareshown shown inin Figure in Figure 2. It 2can. It be can clearly be clearly seen from seen the from imagesimages the imagesthatthat thethe that morphologmorpholog the morphologyy of sample of a sampleexhibits a a regularexhibitsregular ahexagonal hexagonal regular hexagonal structure, structure, structure, and and the the and sheets sheets the sheets are are stacked stacked are stacked together, together, together, which which which is is is a a a typical typical typical layered layered hydrotalcite structure. structure. The The crystal crystal size size of of sample sample a ais is 300 300~500~500 nm nm and and the the thickness thickness is about is about 30 30nm. nm. It showsItshows shows thatthat that samplesample sample aa has ahas has smallsmall small cryscrys crystaltaltal size,size, size, andand and itsits its sizesize size isis is uniform,uniform, uniform, thethe the crystalcrystal crystal morphologymorphology morphology isis is regular,regular, regular, which is consistent with the XRD pattern result. Sample b exhibits exhibits a a flaky ﬂaky structure with only a small amount of hexagonal structure and uneven crystal size, indicating that mechanical g grindingrindingrinding cannot cannot directly synthesize Mg 22AlAl-CO--CO33 LDHs,LDHs, and and it it needs needs to to be be crystallized crystallized for for a a certain certain time time under certain temperaturetemperature conditions. conditions.

Figure 2. SEM imagesimages of Mg22Al--CO33 LDHs prepared with ((a)) CO22and ((b)) noneascarboncarbon sources.sources. Figure 2. SEM images of Mg2Al-CO3 LDHs prepared with (a) CO2 and (b) noneascarbon sources.

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Figure 3 is a comparison diagram of the particle size of the Mg(OH)2 and Al(OH)3 mixture Figure3 is a comparison diagram of the particle size of the Mg(OH) 2 and Al(OH)3 mixture samplesFigure before 3 is and a comparison after the ball diagram milling process. of the particle Figure 3a size is ofthe the particle Mg(OH) size2 diagram and Al(OH) of the3 mixture samples before and after the ball milling process. Figure3a is the particle size diagram of the mixture samplesbefore ball before milling, and itafter can the be seenball milling that the process. particle Figuresize distribution 3a is the particle of the mixturesize diagram sample of theis relatively mixture before ball milling, it can be seen that the particle size distribution of the mixture sample is relatively beforeconcentrated, ball milling, with itan can average be seen particle that the size particle of 1224 size nm. distribution Figure 3b ofis the mixtureparticle samplesize diagram is relatively of the concentrated, with an average particle size of 1224 nm. Figure3b is the particle size diagram of the concentrated,mixture after with ball milling.an average It can particle be seen size fromof 1224 the nm. figure Figure that 3 theb is particlethe particle size size distribution diagram of the mixture after ball milling. It can be seen from the ﬁgure that the particle size distribution of the mixture mixture is after also ball relatively milling. con Itcentrated, can be seen with from an average the figure particle that diameter the particle of 1041 size nm. distribution It is found of that the is also relatively concentrated, with an average particle diameter of 1041 nm. It is found that the mixturethe average is also particle relatively size of con thecentrated, sample after with ball an averagemilling processparticle isdiameter reduced of by 1041 14.95%. nm. DuringIt is found the thatball average particle size of the sample after ball milling process is reduced by 14.95%. During the ball themilling average process, particle the sizeparticles of the of sample the mixture after ball are milling continuously process subjected is reduced to by intense 14.95%. shearing, During friction,the ball milling process, the particles of the mixture are continuously subjected to intense shearing, friction, millingimpact, process,and grinding, the particles which reducesof the mixture the grain are size continuously of the mixture subjected sample. to The intense crystals shearing, in the friction,mixture impact, and grinding, which reduces the grain size of the mixture sample. The crystals in the mixture impact,undergo and plastic grinding, deformation, which reduces and dislocations the grain size multiply of the andmixture move. sample. Mechanical The crystals energy in is the converted mixture undergo plastic deformation, and dislocations multiply and move. Mechanical energy is converted undergointo chemical plastic energy deformation, and stored and in dislocations the crystal defects, multiply which and move.increases Mechanical the chemical energy reaction is converted activity into chemical energy and stored in the crystal defects, which increases the chemical reaction activity of intoof the chemical mixture energy and greatly and stored reduces in the reactioncrystal defects, activation which energy increases [28]. the chemical reaction activity ofthe the mixture mixture and and greatly greatly reduces reduces the the reaction reaction activation activation energy energy [28 [28]]. .

Figure 3. Comparison diragram of particle size (a) before and (b) after ball milling process. FigureFigure 3. 3. ComparisonComparison diragram diragram of of particle particle size size ( (aa)) before before and and ( (bb)) after after ball ball milling milling process process.. In order to further explore the relevant information of the types of interlayer anions, crystal In order to further explore the relevant information of the types of interlayer anions, crystal water waterIn and order lattice to furt oxygenher explore vibration the in relevant the prepared information sample, of infrared the types spectroscopy of interlayer analysis anions, is carriedcrystal and lattice oxygen vibration in the prepared sample, infrared spectroscopy analysis is carried out and waterout and and the lattice result oxygen is shown vibration in Figure in the 4.prepared It is found sample, that infrared the prepared spectroscopy sample analysis showes is acarried broad the result is shown in Figure4. It is found that the prepared sample showes a broad absorption peak out and the result is shown in −Figure1 4. It is found that the prepared sample showes a broad absorption peak around1 3460 cm , which is due to the stretching vibration of the hydroxyl attached around 3460 cm , which is due− to1 the stretching vibration of the hydroxyl attached to the metal ion absorptionto the metal peak ion and−around the interlayer3460 cm , waterwhich molecules. is due to the The stretching absorption vibration peaks of of the the prepared hydroxyl sample attached at and the interlayer water molecules. The absorption peaks of the prepared sample at 1362 cm 1 is the to the metal−1 ion and the interlayer water molecules. The absorption peaks of the prepared2− sample− at 1362 cm is the splitting peaks of the CO asymmetric stretching vibration2 in the CO3 group. It splitting−1 peaks of the CO asymmetric stretching vibration in the CO group. It indicates2− that the 1362indicates cm thatis the the splitting carbonate peaks anion of is th successfullye CO asymmetric inserted stretching between vibrationthe3 −layers. in The the peaks CO3 appearing group. It carbonate anion is successfully inserted between the layers. The peaks appearing in the low wave indicatesin the low that wave the number carbonate band anion represent is successfully lattice vibrational inserted between vibrations the of layers. Mg-O, The Al- O,peaks and appearing Al-O-Mg. number band represent lattice vibrational vibrations of Mg-O, Al-O, and Al-O-Mg. Among them, in the low wave number band represent lattice vibrational vibrations−1 of Mg-O, Al−1 -O, and Al-O-Mg. Among them, the stretching vibration of Al-O bond is1 near 785 cm 1 and 553 cm [22]. Amongthe stretching them, the vibration stretching of Al-O vibration bond isof nearAl-O 785 bond cm −is nearand 553785 cm −1 and[22]. 553 cm−1 [22].

Figure 4. Infrared spectra of Mg Al-CO LDHs prepared by CO as carbon sources. Figure 4. Infrared spectra of Mg2Al-CO3 LDHs prepared by CO22 as carbon sources. Figure 4. Infrared spectra of Mg2Al-CO3 LDHs prepared by CO2 as carbon sources. Figure 5 shows the thermogravimetric-differential thermal analysis (TG-DTA) curve of the preparedFigure sample 5 shows. It can the be t hermogravimetricseen from the figure-differential that the sample thermal has a nalysistwo obvious (TG- DTAstages) curveof mass of loss. the prepared sample. It can be seen from the figure that the sample has two obvious stages of mass loss.

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Materials 2020, 13, x FOR PEER REVIEW 6 of 15 Figure5 shows the thermogravimetric-di fferential thermal analysis (TG-DTA) curve of the The first preparedstage (~14 sample.%) is Itbetween can be seen 30 from °C and the figure 235 ° thatC, mainly the sample due has to two the obvious loss of stages surface of mass absorbed loss. and The first stage (~14%) is between 30 ◦C and 235 ◦C, mainly due to the loss of surface absorbed and intercalatedintercalated water molecules. water molecules. It can It canbe beexpressed expressed by by thethe EquationEquation (1). (1). Mg Al (OH) CO ∙ 3H O → Mg Al (OH) CO + 3H O (1) Mg4 2Al (OH12) CO3 3H2O Mg 4Al 2(OH) 12CO +3 3H O2 (1) 4 2 12 3· 2 → 4 2 12 3 2 The second stage (28%) is between 235°C and 450°C, mainly due to the decompose of hydroxyl The second stage (~28%) is between 235 ◦C and 450 ◦C, mainly due to the decompose of hydroxyl in the layerin the and layer carbonate and carbonate ion between ion between the the layers layers [29]. [29]. ItIt cancan be be expressed expressed by the by Equation the Equation (2). (2). Mg Al (OH) CO → 3MgO + MgAl O + 6H O + CO ↑ (2) Mg4 Al2 (OH12) CO3 3MgO + MgAl 2O 4+ 6H O2 + CO 2 (2) 4 2 12 3 → 2 4 2 2 ↑ The first endothermic peak at 225 °C corresponds to the removal of crystallization water. The The first endothermic peak at 225 ◦C corresponds to the removal of crystallization water. The second second and third peaks at 334 °C and 399 °C corresponds to the dehydroxylation of aluminum and third peaks at 334 ◦C and 399 ◦C corresponds to the dehydroxylation of aluminum hydroxide and hydroxidemagnesium and magnes hydroxide,ium hydroxide, respectively [respectively22]. [22].

Figure 5. TG-DTA curve of Mg2Al-CO3 LDHs prepared by basic magnesium carbonate as carbon source. Figure 5. TG-DTA curve of Mg2Al-CO3 LDHs prepared by basic magnesium carbonate as carbon As shown in Figure6, the weighed mixture is added to a ball mill pot, and the LDH precursor is source. 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, As shownthe formation in Figure mechanism 6, the weighed of Mg2Al-CO mixture3 LDHs is prepared added to by a Mg(OH) ball mill2, Al(OH) pot, and3 is similarthe LDH to the precursor is generatedformation under mechanism the action of of hydrotalcite mechanical prepared force by [30,31 co-precipitation.], which can A possiblebe combined formation with mechanism carbonate ions in an alkalineof Mg 2solutionAl-CO3 LDHs to form prepared Mg by2Al cleaning-CO3 LDHs method after occurred stirring according 30 h.to According the following to reaction. previous studies, the formation mechanism of Mg2Al-CO3 LDHs prepared2 by Mg(OH)2, Al(OH)3 is similar to the CO2 + 2OH− CO − + H2O (3) formation mechanism of hydrotalcite prepared by→ co-precipitation.3 A possible formation mechanism of Mg2Al-CO3 LDHs prepared byAl( cleaningOH)3 + OH method− AlO occurred2− + 2H2O accordingAl(OH)4− to the following reaction.(4) x+ Mg(OH)2 + xAl(OH)4− Mg1 −xAlx(OH2)−2 + xMg(OH)2 + 2xOH− (5) CO2 →+ 2OH− → CO3 + H2O (3) x+ 2 Mg1 xAlx(OH)2 + 1/2xCO3− + mH2O Mg1 xAlx(OH)2(CO3)1/2x mH2O (6) − − − → − − · Al(OH)3 + OH ⇌ AlO2 + 2H2O ⇌ Al2(OH)4 (4) In Equation (3), CO2 is absorbed by lye and converted into CO3 −. 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 − x+ 3+ − octahedral voidsMg(OH of OH)2 +− accumulationxAl(OH)4 → inMg brucite.1−xAl Amongx(OH)2 them,+ xMg Al (entersOH)2 into + 2xO the octahedralH void, (5) substituting a portion of Mg2+ in the brucite lattice, forming a coordination structure. This octahedral x+ 2− structureMg unit1−xAl formsx(OH a) network2 + 1 structure/2xCO3 through+ mH2 stackingO → Mg and1−x diAlﬀxerent(OH) connection2(CO3)1/2x methods.∙ mH2O During (6)

In Equation (3), CO2 is absorbed by lye and converted into CO32−. 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, Al3+ enters into the octahedral void, substituting a portion of Mg2+ 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 2− (6), in order to balance the positive charge carried by the laminate, CO3 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].

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the formation of the network structure, defects will inevitably be formed, such as holes and incomplete Materialscoordination, 2020, 13, x FOR resulting PEER REVIEW in the crystal layer being positively charged. Then, in Equation (6), in order7 to of 15 2 balance the positive charge carried by the laminate, CO3− in the solution intercalates into the interlayer through electrostatic interaction, hydrogen bonding and van der Waals force to balance the positive charge on the4Mg laminate(OH) and2 + gradually2Al(OH)3 forms+ CO2 the+ hydrotalc2H2O → Mg structure4Al2(OH [32)12,33C].O3 ∙ 3H2O (7) Equation (7) is the reaction equation, in which the crystal water in hydrotalcite are all from the 4Mg(OH) + 2Al(OH) + CO2 + 2H2O Mg Al2(OH) CO3 3H2O (7) solution. It can be seen from the2 Equation3 (7) that the reactant→ raw4 materials12 are· fully utilized. 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.

FigureFigure 6. Schematic 6. Schematic diagram diagram of mechanochemical of mechanochemical preparation preparation methodmethod of of Mg Mg2Al-CO2Al-CO3LDHs.3LDHs.

3.2. Thermal Stability of Mg2Al-CO3 LDHs on PVC 3.2. Thermal Stability of Mg2Al-CO3 LDHs on PVC In order to explore the synergistic thermal stability of Mg2Al-CO3 LDHs with other stabilizers Inon order PVC, to the explore thermal the stability synergistic of the thermal samples stability have been of tested. Mg2Al The-CO result3 LDHs of Congowith other red teststabilizers of PVC on PVC, samples the thermal are shown stability in Table of 2the, and samples the results have of the been static tested. oven thermal The result aging of test Congo and thermal red test weight of PVC samplesloss are test shown are shown in Table in Figures 2, and7 and the8 results. When of the the Congo static red oven time thermal is longer aging and the test quality and thermal loss of PVC weight loss testin theare staticshown oven in Figure experiments 7 and is less, 8. When indicating the Congo that the red thermal time stabilityis longer of and PVC the is better. quality loss of PVC in the static oven experiment is less, indicating that the thermal stability of PVC is better. Table 2. The result of Congo red test of PVC samples. Table 2. The result of Congo red test of PVC samples. Label PVC/g LDHs/phr ZnSt2/phr Thermal Stability Time of PVC Samples (min) a 100 0 0 8 Label PVC/g LDHs/phr ZnSt2/phr Thermal Stability Time of PVC Samples (min) b 100 3.0 0 30 a 100c 1000 00 0.6 128 b 100d 1003.0 2.40 0.6 3630 c 100 0 0.6 12 d 100 2.4 0.6 36

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FigureFigureFigure 7. 7.7. Thermal Thermal stability stabilitystability of ofof ( (a(aa)) )pure purepure PVC PVCPVC and andand composites compositescomposites of ofof ( ( b(b)) )PVC PVC PVC + + LDHs,LDHs, LDHs, ( (c(c)c) )PVC PVC PVC + + ZnSt ZnStZnSt22 2and andand ((d(dd)) )PVC PVCPVC + ++ LDHs LDHs + + ZnSt ZnStZnSt2.2 2. .

Figure 8. Thermal weight loss rate of (a) pure PVC and composites of (b) PVC + LDHs, (c) PVC + FigureFigure 8. 8. Thermal Thermal weight weight loss loss rate rate of of ( a(a) )pure pure PVC PVC and and composites composites of of ( b(b) )PVC PVC + + LDH LDHs,s, ( c()c )PVC PVC + + ZnSt2 and (d) PVC + LDHs + ZnSt2. ZnStZnSt2 2and and ( d(d) )PVC PVC + + LDHs LDHs + + ZnSt ZnSt2.2 .

AsAs shown shown in in Figure Figures s7a 7a and and 8, 8, the the sample sample without without any any addition addition of of heat heat stabilizer stabilizer has has been been completelycompletely turned turned black black at at 20 20 min, min, its its thermal thermal stability stability time time is is only only 8 8 min min according according to to Congo Congo red red test,test, and and it it has has a a w weighteight retention retention rate rate of of 56.92 56.92%% at at 1440 1440 min min under under 190 190 ° C°C condition. condition. Sample Sample with with

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As shown in Figures7a and8, 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 Mg2Al-CO3 LDHs addition amount of 3 phr begin to color at 20 min and completely turn black at 80 min as shown in Figure7b. 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 Mg2Al-CO3 LDHs belongs to a long-term thermal stabilizer, so it is necessary to add an initial thermal stabilizer into it. Here, zinc stearate (ZnSt2) is used as an initial heat stabilizer. As shown in Figure7c, the sample added with ZnSt2 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 Figure7d, 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 ZnSt2. 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).

2RCH = CHCH Cl + Zn(C H COO) 2R + ZnCl +2C H COOCH (8) 2 17 35 2 → 2 17 35 2 Zn(C H COO) +2HCl ZnCl + 2C H COOH (9) 17 35 2 → 2 17 35 At the same time, it has a weight retention rate of 69.91%, the reason for the decrease in weight retention rate after adding ZnSt2 is that the decomposition product ZnCl2 is a strong Lewis acid, which will catalyze the degradation of PVC [7]. In order to compare the thermal stability of PVC containing commercial Mg2Al-CO3 LDHs with the experimentally prepared Mg2Al-CO3 LDHs, diﬀerent content of the auxiliary heat stabilizer zinc acetylacetonate compound with experimentally prepared Mg2Al-CO3 LDHs and zinc stearate, then are added to PVC. The result of Congo red test of PVC samplesare shown in Table3. The results of the static oven thermal aging test and thermal weight loss test are shown in Figures9 and 10.

Table 3. The result of Congo red test of zinc acetylacetonate added to the PVC samples.

Thermal Stability LDHs Zinc Label PVC/g LDHs/phr ZnSt /phr Time of PVC Category 2 Acetylacetonate/phr Samples (min) a commercial 100 2.4 0.6 0 39 b experimental 100 2.4 0.6 0 36 c experimental 100 2.4 0.5 0.1 38 d experimental 100 2.4 0.4 0.2 45 e experimental 100 2.4 0.3 0.3 46 f experimental 100 2.4 0.2 0.4 41 MaterialsMaterials 20202020,, 1313,, 5223x FOR PEER REVIEW 1010 ofof 1515 Materials 2020, 13, x FOR PEER REVIEW 10 of 15

Figure 9. Thermal stability of (a) PVC + commercial LDHs + ZnSt2, and composite PVC +

Figureexperimentally 9. Thermal prepared stability LDHs of (a +) ZnSt PVC2 + (commercialb) 0, (c) 0.1, LDHs (d) 0.2, + ZnSt (e) 2 0.3, and and compo (f) 0.4site phr PVC Zinc + Figure 9. Thermal stability of (a) PVC + commercial LDHs + ZnSt2, and composite PVC + experimentally experimentallyacetylacetonate. prepared LDHs + ZnSt2 + (b) 0, (c) 0.1, (d) 0.2, (e) 0.3 and (f) 0.4 phr Zinc prepared LDHs + ZnSt2 + (b) 0, (c) 0.1, (d) 0.2, (e) 0.3 and (f) 0.4 phr Zinc acetylacetonate. acetylacetonate.

FigureFigure 10.10. ThermalThermal weightweight lossloss raterate ofof ((aa)) PVCPVC+ + commercialcommercial LDHsLDHs ++ ZnStZnSt22, andand compositecomposite PVCPVC ++ Figureexperimentallyexperimentally 10. Thermal prepared prepared weight LDHs LDHsloss+ rateZnSt + ZnStof2 + (a(b)2 )PVC+ 0, ( bc ))+ 0.1, 0,commercial ( (dc) 0.2, 0.1, ( e () dLDHs 0.3) 0.2, and + ( (feZnSt)) 0.4 0.32 phr, and and Zinc composite (f acetylacetonate.) 0.4 phr PVC Zinc + experimentallyacetylacetonate. prepared LDHs + ZnSt2 + (b) 0, (c) 0.1, (d) 0.2, (e) 0.3 and (f) 0.4 phr Zinc acetylacetonate.The sample containing commercial Mg2Al-CO3 LDHs does not start coloring until 110 min, and completelyThe sample turn containing black at 170commercial min, which Mg is2Al 10- minCO3 longerLDHs thandoes thenot sample start coloring containing until experimentally 110 min, and preparedcompletelyThe sample Mg turn2Al-CO blackcontaining3 LDHs at 170 ascommercial min, shown which in FigureMg is 102Al min9-a,b.CO longer3 TheLDHs reason than does the is not that sample start small coloring containing amounts until of experiment additivesuch110 min, andally completelyasprepared stearic Mg acid turn2Al is black-CO added3 LDHs at to170 commercialas min, shown which in MgFigureis 102Al-CO min 9a,b longer3. TheLDHs, reason than which the is thatsample can small slightly containing amounts improve experimentof additivesuch its thermalally 2 3 preparedas stearic Mg acidAl is-CO addedLDHs to as commercial shown in Figure Mg2Al 9a-CO,b. 3The LDHs reason, which is that can small slightly amounts improve of additivesuch its thermal as stearic acid is added to commercial Mg2Al-CO3 LDHs, which can slightly improve its thermal

Materials 2020, 13, 5223 11 of 15 Materials 2020, 13, x FOR PEER REVIEW 11 of 15 performance. As As shown shown in in Figure Figure 99bb–f,–f, the the samples samples added added with with zinc zinc acetylacetonate acetylacetonate e effectivelyﬀectively improved thethe whiteness inin thethe initialinitial andand slightlyslightly improvedimproved thethe long-termlong-term thermalthermal stabilitystability ofof PVC.PVC. As the content content of of zinc zinc acetylacetonate acetylacetonate increases, increases, the the initial initial and and long long-term-term thermal thermal stability stability of the of thePVC PVC samples samples graduall graduallyy improve, improve, especially especially the the sampled sampled with with 0.2 0.2 phr phr zinc zinc acetylacetonate acetylacetonate added, added, which beginsbegins toto colorcolor at at 130 130 min, min, and and completely completely turn turn black black at at 190 190 min, min, which which is 20is min20 min longer longer than than the samplethe sample containing containing commercial commercial Mg2Al-CO Mg2Al3 LDHs.Zinc-CO3 LDHs acetylacetonate.Zinc acetylacetonate belongs belongs to β-diketone, to β-diketone, which is awhich kind of is auxiliary a kind of heat auxiliary stabilizer. heat The stabilizer. mechanism The of mechanism action of zinc of action acetylacetonate of zinc acetylacetonate as auxiliary heat as stabilizerauxiliary heat can bestabilizer represented can be in represented Figure 11. As in shownFigure in11. Figure As shown 11a, zincin Figure acetylacetonate 11a, zinc acetylacetonate can eﬀectively absorbcan effectively the HCl absorb gas generated the HCl gas by generated the thermal by degradationthe thermal degradation of PVC, and of the PVC, conversion and the conversion product is acetylacetone.product is acetylacetone Figure 11.b Figure shows 11b that shows acetylacetone that acetylacetone can replace can the replace active the allyl active Cl atoms allyl Cl on atoms the PVC on polymerthe PVC chain polymer under chain the under catalytic the action catalytic of action ZnCl2 , of reducing ZnCl2, reducing the active the sites active on the sites PVC on polymerthe PVC chain.polymer Meanwhile, chain. Meanwhile, acetylacetone acetylacetone can also can be cross-linked also be cross with-linked PVC with polymer PVC polymer chains to chains form ato stable form structurea stable structure [34]. [34].

(a)

(b)

Figure 11. MechanismMechanism of of action action of of zinc zinc acetylacetonate acetylacetonate as asauxiliary auxiliary heat heat stabilizer. stabilizer. (a) absorb (a) absorb acid acid;; (b) (PVCb) PVC polymer polymer chains chains crosslinking crosslinking..

However, as the content of zinc acetylacetonate continued to increase, the initial and long-termlong-term thermal stability of the PVC sample decreased when the addition amount is 0.4 phr as shown in Figure9 9f.f. ThisThis isis becausebecause asas thethe contentcontent ofof zinczinc acetylacetonateacetylacetonate increased,increased, more more ZnCl ZnCl22 is produced in the system,system, which will catalyze the thermal degradation of of PVC PVC due due to to its strong Lewis acidity [7], [7], the initial and long-termlong-term thermalthermal stabilitystability performanceperformance willwill decrease.decrease. As shown shown in in Table Table3 and 3 Figureand Figu 10,re the 10, thermal the thermal stability stability time of PVC time sample of PVC containing sample commercial containing LDHscommercial is 38 LDHs min, which is 38 min, is 3 which min longer is 3 min than longer experimentally than experimentally prepared prepared LDHs, and LDHs, it has and a weightit has a retentionweight retention rate of 74.63% rate of at 74.63% 1440 min. at 1440 However, min. However, as the content as the of content zinc acetylacetonate of zinc acetylacetonate increased, theincreas thermaled, the stability thermal time stability and weight time and retention weight rate retention of PVC rate sample of PVC increases sample ﬁrst increases and then first decreases, and then whendecreases, 0.3 phr when zinc 0.3 acetylacetonate phr zinc acetylacetonate is added, the is PVC added, sample the PVChas a sample maximum has thermal a maximum stability thermal time ofstability 46 min, time which of 46 is 7min, min which longer is than 7 min commercial longer than LDHs. commercial Meanwhile, LDHs it has. Meanwhile, weight retention it has weight rate of 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.

Materials 2020, 13, 5223 12 of 15

73.53% at 1440 min. The Congo red test and thermal weight loss test demonstrate the same conclusion Materials 2020, 13, x FOR PEER REVIEW 12 of 15 as the static oven thermal aging test. Therefore, when 0.3 phr of zinc acetylacetonate, 2.4 phr Mg Al-CO LDHs, and 0.6 phr ZnSt are Therefore, when 0.3 phr of zinc acetylacetonate, 2.4 phr Mg22Al-CO33 LDHs, and 0.6 phr ZnSt2 are addedadded to to the the PVC, PVC, the thermal stability of PVC is the best. 3.3. Processing Performance Test of Compound Heat Stabilizer Added to PVC 3.3. Processing Performance Test of Compound Heat Stabilizer Added to PVC In order to explore the diﬀerence in processing performance between the experimentally prepared In order to explore the difference in processing performance between the experimentally Mg2Al-CO3 LDHs and commercial Mg2Al-CO3 LDHs, a torque rheometer is carried out to test them. prepared Mg2Al-CO3 LDHs and commercial Mg2Al-CO3 LDHs, a torque rheometer is carried out to The balanceable torque and the mixing process energy were carried out by a Torque Rheometer ZJL-200 test them. The balanceable torque and the mixing process energy were carried out by a Torque (Chang Chun Intelligent Apparatus Co. Ltd., Changchun, China) at 190 C at a speed of 40 r/min for Rheometer ZJL-200 (Chang Chun Intelligent Apparatus Co. Ltd., Changchun◦ , China) at 190°C at a 10 min. speed of 40 r/min for 10 min. The recipe of the samples are shown in Table4, the rheological curves are shown in Figure 12 and The recipe of the samples are shown in Table 4, the rheological curves are shown in Figure 12 their rheological data are shown in Table5. and their rheological data are shown in Table 5. Table 4. Rheology test recipe of the samples. Table 4. Rheology test recipe of the samples. LDHs zinc Label PVC/g DOP/g CaCO3/g StearicStearic Acid /g LDHs LDHs/phr ZnSt2/phr zinc Label PVC/g DOP/g CaCO3/g Category LDHs/phr ZnSt2/phr Acetylacetonate/phr Acid/g Category Acetylacetonate/phr a a100 10010 1030 30 0.5 0.5 commercialcommercial 2.42.4 0.6 0 0 b b100 10010 1030 30 0.5 0.5 experimentalexperimental 2.42.4 0.6 0 0 c 100 10 30 0.5 experimental 2.4 0.3 0.3 c 100 10 30 0.5 experimental 2.4 0.3 0.3

FigureFigure 12. 12.Rheological Rheological curvesof curvesof composite composite of PVC of+ PVCZnSt 2 ++ ( ZnSta) commercial2 + (a) commercial LDHs, or (b ) LDHs,experimentally or (b) experimentallyprepared LDHs, prepared or (c) experimentally LDHs, or (c) experimentally prepared LDHs prepared+ Zinc acetylacetonate. LDHs + Zinc acetylacetonate. Table 5. Rheological data of the samples. Table 5. Rheological data of the samples. Maximum Balance Melting Melting Balance LabelMaximum Balance Melting Melting Balance Label Torque/(Nm) Torque/(Nm) Time/s Temperature/◦C Temperature/◦C Torque/(Nm) Torque/(Nm) Time/s Temperature/°C Temperature/°C A 20.68 9.73 32 160.3 193.0 A B20.68 22.099.73 9.3532 26 159.8160.3 192.7193.0 B C22.09 21.559.35 10.9126 27 160.1159.8 193.5192.7 C 21.55 10.91 27 160.1 193.5 It can be seen from Figure 12 and Table5 that the order of maximum torque of the three samples 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 is b > c >a, the balance torque is c > a > b, the melting time is a > c > b, melting temperature is a > c > a > c > b, and the balance temperature is c > a > b. The comparison of the three samples show that the 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 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

Materials 2020, 13, 5223 13 of 15 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 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.

4. Conclusions

Through mechanochemical methods, the Mg2Al-CO3 LDHs were successfully prepared by Mg(OH)2, Al(OH)3, and CO2. This method is simple in process, low in cost, high in materials utilization rate, and suitable for large-scale production. The samples Mg2Al-CO3 LDHs were used as a heat stabilizer in PVC. The result shows 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 under 180 ◦C oven test conditions, which is 20 min longer than the PVC composite containing 2.4 phr commercial Mg2Al-CO3 LDHs and 0.6 phr ZnSt2. 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 Mg2Al-CO3 LDHs. Moreover, its processing performance is basically the same as the PVC containing commercial Mg2Al-CO3 LDHs.

Author Contributions: Conceptualization, Z.Y.; methodology, Z.Y. and Y.J.; software, Y.J. and L.C.; validation, Z.Y. and Y.J.; formal analysis, Z.Y. and Y.J.; investigation, Y.J. and Q.S.; resources, Z.Y.; data curation, Y.J., J.W. and J.M.; writing-original draft preparation, Y.J. and Q.S.; writing-review and editing, Z.Y., Y.J. and L.C.; visualization, Y.J. and Q.S.; supervision, Z.Y.; project administration, Z.Y.; funding acquisition, Z.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by [Nature Science foundation of Hunan Provincial] grant number [2 020JJ4734]. Conflicts of Interest: To the best of our knowledge, the named authors have no conflict of interest, financial or otherwise.

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