Facile synthesis of di-mannitol adipate ester-based zinc metal alkoxide as bifunctional additives for PVC

A new di-mannitol adipate ester-based zinc metal alkoxide (DMAE-Zn) was synthesized as a bi-functional PVC thermal stabilizer for the first time. The materials were characterized with Fourier-transform Infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). Characterization results confirmed the formation of Zn-O bonds in DMAE-Zn; and that DMAE-Zn had a high decomposition temperature and a low melting point. The thermal stability of DMAE-Zn on PVC also was tested by conductivity test, thermal aging test, and UV-visible spectroscopy (UVVIS) test. PVC stabilized by DMAE-Zn had a good initial color and excellent longterm stability. UV-VIS also showed that the conjugated structure in PVC stabilized by DMAE-Zn was almost all of the triene, suggesting that the addition of DMAE-Zn would suppress the formation of conjugated structures above tetraene. Dynamic processing performance of PVC samples tested by torque rheometer indicated that, having a good compatibility with PVC chains in the amorphous regions, DMAE-Zn contributed good plasticizing effect to PVC. DMAE-Zn thus effectively demonstrates bi-functional roles, e.g., thermal stabilizers and plasticizers to PVC. Furthermore, FT∗ Corresponding author Tel: +86 18766957507, E-mail address: ldg@sdut.edu.cn (Degang Li); Email address: zhanglipeng@sdut.edu.cn (Lipeng Zhang) Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 1 April 2019 doi:10.20944/preprints201904.0006.v1 © 2019 by the author(s). Distributed under a Creative Commons CC BY license. Peer-reviewed version available at Polymers 2019, 11, 813; doi:10.3390/polym11050813

VIS) test.PVC stabilized by DMAE-Zn had a good initial color and excellent longterm stability.UV-VIS also showed that the conjugated structure in PVC stabilized by DMAE-Zn was almost all of the triene, suggesting that the addition of DMAE-Zn would suppress the formation of conjugated structures above tetraene.Dynamic processing performance of PVC samples tested by torque rheometer indicated that, having a good compatibility with PVC chains in the amorphous regions, DMAE-Zn contributed good plasticizing effect to PVC.DMAE-Zn thus effectively demonstrates bi-functional roles, e.g., thermal stabilizers and plasticizers to PVC.Furthermore, FT-

Introduction
Poly (vinyl chloride) (PVC) is one of the five general plastics, has been widely used in infrastructure construction and home supplies because of its excellent lowcost, high strength, corrosion resistance, and self-extinguishing, etc. [1] In the applications the poor thermal stability is the biggest weakness of PVC.When the temperature reaches 130 o C or higher, PVC starts to break down and release hydrogen chloride (HCl), which can further aggravate this degradation process [2] .The color of PVC will change significantly from white to brown, and finally black, meanwhile mechanical properties also decline rapidly [3] .Therefore, thermal stabilizers must be added during the PVC processing to inhibit the production of HCl and remove the free HCl [4] .
At present, lead salts, metal soap, and organotin are the mainstream thermal stabilizers.Although lead salts have the best thermal stability for PVC, the toxicity restricts their application.Metal soap, such as calcium stearate (CaSt2), and zinc stearate (ZnSt2), has no toxicity, but the thermal stabilizing effect on PVC is not ideal, especially the "zinc-burning" will lead to rapid degradation of PVC [5][6][7][8][9][10] .Organotin has an excellent thermal stability on PVC, but the cost of is too high [11] .It is then necessary to develop new non-toxic and efficient thermal stabilizer of PVC [12][13][14] .
A number of new types of thermal stabilizers, for example, zinc and calcium salts of 11-maleimideoundecanoic acid, liquid Ca/Zn thermal stabilizer synthesized, zinc and calcium oxolinic complexes, ricinoleic acid-based Ca/Zn stabilizer, have been reported [11,13,[15][16][17][18][19] .Some researchers also indicated that polyols can serve as auxiliary thermal stabilizers.Having many hydroxyl groups, polyols are able to chelate ZnCl2 to inhibit the "zinc burning" phenomenon [3,20] .Jenneskens et al. found that the natural polyols can significantly improve the thermal stability of PVC [21,22] .Guo et al. reported that the addition of pentaerythritol combined with CaSt2/ZnSt2 would improve the thermal and color stability of PVC [23] .
Recently, we have synthesized a series of polyols-based metal alkoxides as PVC thermal stabilizers, and showed that they were efficient in inhibiting the degradation of PVC [24,25] .There is still a challenge for these polyols-based metal alkoxides to be used as PVC thermal stabilizers, because their melting point (about 220 o C) [1,2] are significantly higher than the processing temperature of PVC, affecting their consistency with PVC [26] .In order to reduce the melting point of the polyol-based metal alkoxides, Shentu et al. employed mannitol with a low melting point of 166 o C to synthesize mannitol-zinc metal alkoxides, which was then used to improve PVC thermal stability [2] .New strategy has been applied by our group to overcome this shortcoming by esterification.For example, we synthesized the pentaerythritol stearate ester-based zinc alkoxides (PSE-Zn) [26] and cis-1,2-cyclohexanedicarboxylic acid di-mannitol ester-based zinc metal alkoxides [27] .We observed that the new esterbased alkoxides have a lower melting point and good thermal stabilization performance on PVC.Generally speaking, plasticizer, one of the most important PVC processing aids, is to be added to adjust the mechanical and thermal properties of PVC.Phthalate esters are the most commonly used plasticizers in PVC processing.However, most of the phthalate plasticizers are well-known to be toxic [28] .More and more researchers are committed to developing new non-toxic PVC plasticizers.It is recommended to use sustainable alternative instead of phthalate esters.Epoxidized vegetable oils are one of the alternatives [28,29] .
In this study, a new kind of thermal stabilizers, di-mannitol adipate ester-based zinc metal alkoxide (DMAE-Zn), were synthesized, and characterized by Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis.Their thermal stability performances on PVC also were tested through oven aging method, conductivity test, UV-VIS spectroscopy test, and torque rheometer test.It was found that DMAE-Zn can effectively play bi-functional roles of being both a thermal stabilizer and a plasticizer to PVC.DMAE-Zn was synthesized through the method of alcoholysis.Zinc acetate and DMAE in a molar ratio of 1:1 were mixed in a three-necked flask with a mechanical stirring.Excess absolute ethanol was added and the reactor was then heated to 160 ± 1 o C for 4 h.After the solvent was evaporated, the samples were washed with ethanol for three times to obtain the products, DMAE-Zn.

Material characterization
Samples were characterized by Fourier transform infrared (FT-IR) spectroscopy (Nicolet 5700) with the KBr disc method.Spectral range was 400-4000 cm -1 , with a scan rate 128 min -1 and a resolution of 4 cm -1 .The blank spectrum was tested before each measurement to eliminate the spectrum subtraction resulting from carbon dioxide and water in the air.TGA and DTG of the samples were performed using a Q500 analyzer (TA instruments, United States) at a 10 o C min -1 heating rate from 25 to 700 o C in nitrogen atmosphere.The results were used to investigate the decomposition of the materials [26] .

PVC sample preparation
About 100.0 g of PVC, 5 mL of dioctyl phthalate (DOP), 4.0 g of TiO2, 9.0 g of chlorinated polyethylene (CPE), 2.0 g of acrylics copolymer (ACR), 20.0 g of CaCO3, 1.6 g of stearic acid and 4 g of thermal stabilizers were added into a blender to undergo sufficient mixing.Then the mixture was milled using an open twin-roller (PX-GY-150, Shenzhen Pengxiang Yunda Machinery Technology Co., China) at 180 o C for 5 min to produce the PVC sheets with a thickness of about 1.0 mm.
2.5.Thermal stability test 2.5.1.Conductivity measurement PVC sheets were cut into small square pieces of 0.5 mm × 0.5 mm with a total weight of 2.0 g, and put into a home-made reaction vessel.The vessel was placed in an oil bath and heated to 180 o C. Then 60 mL of deionized water was added in a beaker of 100 mL for testing its conductivity.The HCl gas produced during the thermal decomposition of PVC was introduced into the measuring beaker by the nitrogen stream (about 7 L/h).The HCl was absorbed by the deionized water in the measuring beaker so that the conductivity of water gradually changed with time.
Therefore, the decomposition rate of PVC can be recorded through measuring the change of the conductivity of the deionized water.A conductivity meter (DDS-307, Shanghai Instrument Electric Scientific Instrument Co., Ltd) was used to measure the conductivity.

Thermal aging test
PVC sheets were cut into small samples of 15 mm × 15 mm size.These sheets were heated to 180 o C in an oven.The sheets were taken out after every 10 minutes.The thermal stability of PVC was evaluated by monitoring the color change.

UV-VIS spectroscopy test
It is known that PVC begins to decompose and produce conjugated double bonds when the temperature reaches 130 o C. PVC samples were first dissolved by tetrahydrofuran (THF).Then the concentration of conjugated double bonds was measured by UV-visible spectrometer (UV-VIS).The UV-VIS spectra were recorded at 25± 5 o C by using a UV-2450PC spectrometer with the slit width set at 2 nm over the wavelength range of 200-500 nm.

Torque rheometer test
The effect of DMAE-Zn on the dynamic rheological properties of PVC was tested by a torque rheometer (RM-200C, Harbin Harp Electric Technology Co.).Operating temperature was set at 180 o C while keeping the rotor with a speed of 35 rpm.

The capacity for neutralizing HCl
The capacity of stabilizers to neutralize HCl was investigated byh conductometric titration experiments.6.00 mL of 0.1 M HCl standard solution was diluted with 20.00 mL of ethanol and 10.00 mL of deionized water.0.0500 g of thermal stabilizer, such as DMAE-Zn, lead salts, ZnSt2, CaSt2, and so on was dissolved in this solution with a   Fig. 1 is the FT-IR spectra of the synthesized DMAE and DMAE-Zn.Fig. 1 shows that both the DMAE and DMAE-Zn have strong peaks at 3400-3500 cm -1 which are the characteristic absorbance band of -OH, as a result of the stretching vibration.The peak at about 2900 cm -1 was the stretching vibration of the -C-H groups.The absorption peak at about 1738 cm -1 can be assigned to the ester carbonyl bonds (-C=O).There were no absorption peaks at 1786 cm -1 of carboxylic acid carbonyl bonds in DMAE and DMAE-Zn, indicating that the esterification of stearic acid with D-mannitol was complete.The absorption bands between 650-550 cm -1 in Fig. 1b were attributed to the -O-Zn bonds [24].The appearance of -O-Zn bands indicated that the synthesized compound contained metal alkoxides.

Results and discussion
There are six hydroxyl groups in a D-mannitol molecule.All of these hydroxyl groups can be esterified using adipic acid.Therefore, DMAE may contain more than twelve esters.Its derivative, DMAE-Zn is expected to have a more complex structure.
Scheme 1 shows a simplified reaction pathway for the synthesis of DMAE-Zn.Two carboxyl groups of adipic acid react with two alcohols hydroxy of the di-mannitol to form esters.Then, through alcoholysis reaction, the ethyoxyl of ethanol zinc will exchange with the hydroxy of polyol ester.After completely evaporating the ethanol, the targeted DMAE-Zn can be obtained.Fig. 2 shows the TGA, DTA and DTG curves of the targeted DMAE-Zn.There was one weight-loss step within the temperature range from 200 to 500 o C in the TG curve of the DMAE-Zn, which was accompanied by a maximum weight-loss peak.The DTG curve of Fig. 2 shows that there are two pyrolysis temperatures for DMAE-Zn, 265.8 and 342.5 o C, respectively.All the pyrolytic temperatures were far more than 180 o C which is the processing temperature of PVC, meaning that the synthesized DMAE-Zn did not break down during PVC processing and suitable to be used as thermal stabilizers of PVC.TG curve in Fig. 2 shows that the residue is 20.57%, which is slightly higher than the result of elemental analysis.The residue might be composed of ZnO and carbon.DTA curve in Fig. 2 shows that the melting point of the targeted DMAE-Zn can be estimated from the first endothermic peak where the weight changes slightly.Thus, the melting point of DMAE-Zn can be estimated to be 154.4o C, lower than PVC processing temperature (180 o C).Therefore, the targeted metal alkoxides had a good compatibility with PVC.In the Conductivity vs.Time plot, the introduction time (Ti) refers to the time from the start of the heating to when the conductivity starts to rise, while the stability time (Ts) refers to the time where the conductivity of the water reaches 50 μS cm -1 [30] ,

Thermogravimetric analysis
which can be considered as the maximum acceptable conductivity value in PVC degradation.stability of PVC stabilized with ZnSt2/CaSt2 was worse.This is because that ZnCl2 would be produced when ZnSt2 absorbed HCl produced during PVC degradation.
ZnCl2 was a strong Lewis acid, which would catalyze the degradation of the PVC.
When the amount of ZnCl2 was accumulated to a certain extent, it would suddenly accelerate the degradation of PVC, leading to rapid aging of PVC.This phenomenon was called "zinc-burning" [31] .It was worth noting that the Ti and Ts of PVC stabilized by DMAE-Zn was extended to 31.1 min and 67.9 min, respectively, indicating this PVC samples had an excellent color stability and long-term stability.This might be due to the fact that DMAE-Zn had many hydroxyl groups, which might complex with the produced ZnCl2 to inhibit the catalytic degradation of PVC.Especially, after having been heated for 60 min, the Cdb of PVC stabilized with CaSt2/ZnSt2 became the biggest one, even exceeded that of pure PVC.It was because that "zinc-burning" phenomenon happened in PVC stabilized by CaSt2/ZnSt2.The results of oven thermal aging tests (Fig. 4) also confirmed this conclusion, in which the color of PVC sheet CaSt2/ZnSt2 already became dark gray after being heated for 60 min.The Cdb of PVC stabilized by the synthesized DMAE-Zn did not increase evidently, just increased from 0.26 to 0.32, indicating that DMAE-Zn had the highest efficiency in suppressing the production of conjugated double bonds.

UV-VIS spectroscopy test
The position of absorption peaks should also be noted.Compared Fig. 5 A and B, it can be seen that the position of these main absorption peaks do not shift, showing that most of the PVC thermal degradation products are the conjugated triene structure.
However, the absorbance curve of pure PVC (see Fig. 5Aa) shows that there is weak absorption at the range of 300-330 nm which belongs to tetraenes and pentaene.PVC samples stabilized by the two thermal stabilizers have no absorption at the same range.As for the system of ZnSt2/DMAE-Zn, Fig. 6 shows that, when the ZnSt2 is used alone as a PVC thermal stabilizer, PVC samples turn black quickly within 10 min due to the "zinc-burning" phenomenon.With the dosage of DMAE-Zn increasing, the initial color and long-term stability of PVC can be improved obviously.For example, color of PVC stabilized by ZnSt2/DMAE-Zn with the mass ratio of 1:3 remained unchanged in 70 min, and did not turned black in 120 min.Therefore, there existed excellent synergistic effect between ZnSt2 and DMAE-Zn on PVC thermal stability.
was probably because that there were a large number of hydroxyl groups in DMAE-Zn which could complex with ZnCl2 to avoid catalyzing PVC thermal degradation.The torque rheology test was according to ASTM D 2538-02 [32] , operated at 180 o C as was described in Section 2.6.4.Fig. 7 shows the results for pure PVC and PVC stabilized by DMAE-Zn.Both samples had two distinct peaks which were the feed peak and plasticized peak of PVC resin, respectively.The first peaks for both samples come out at almost the same time.The first peak height of PVC stabilized by DMAE-Zn was 50 Nm higher than that of pure PVC, indicating that the addition of DMAE-  [33] .Therefore, we can draw such a conclusion that the reason why DMAE-Zn had a good plasticizing effect on PVC is that DMAE-Zn has a good compatibility with PVC chains in the amorphous regions.

Torque rheology test of DMAE-Zn
It is also worth noting that the balance torque of PVC stabilized by DMAE-Zn was slightly lower than that of pure PVC.Considering that DMAE-Zn had no lubricity to PVC, lower balance torque of PVC stabilized by DMAE-Zn indicated that the addition of DMAE-Zn could decrease friction between PVC molecules through solvation.

The thermal stabilizing mechanism of DMAE-Zn
In our previous work, we found that, due to its high electronegativity, the alkyl oxygen of metal alkoxides had a tendency to attack the carbon atoms (given its high positive charge) attached to allyl chloride in PVC chains [15] .At the same time, allyl chloride with a high electronegativity would attack the zinc atom (having high positive charge) of the metal alkoxides.Scheme 2 illustrates the reaction mechanism.
This reaction mechanism also applies to DMAE-Zn since it contains metal alkoxides, which could explain the fact that PVC stabilized by DMAE-Zn had a better initial color (i.e., white) than pure PVC (Refer to magnetic stirring at 40 o C. The excess HCl was back-titration with 6.00 mL of 0.1 M NaOH standard solution.The conductivity of the solution was measured by a conductivity meter (DDS-307, same as 2.5.1.).The volume of NaOH solution corresponding to the minimum conductivity of the solution was the titration endpoint, and the capacity for neutralizing HCl was calculated by the volume of NaOH solution used.

Fig. 3
Fig.3is the Conductivity vs.Time plots for pure PVC and PVC stabilized by 4 phr of ZnSt2/CaSt2 (1:1), and DMAE-Zn, respectively.Generally speaking, the shorter the Ti, the worse the color stability of PVC.It can be seen from Fig.3that the Ti and Ts of pure PVC are 9.6 min and 21.8 min, respectively, indicating that the initial color stability of pure PVC is quite short and, after having been heated for 21.8 min, pure PVC might degrade completely.As for the PVC samples stabilized by ZnSt2/CaSt2, Fig.3bshows that the Ti and Ts of PVC stabilized by ZnSt2/CaSt2 are 16.1 min and 24.3 min, respectively.By comparing Fig.3aand Fig.3b, although the Ti of PVC stabilized by ZnSt2/CaSt2 is longer than that of pure PVC, the Ts values of these two PVC samples are relatively close.This means that the initial stability of PVC stabilized with ZnSt2/CaSt2 was better than that of pure PVC.However, the long-term

Fig. 4
Fig.4 Color evolution of PVC samples with different thermal stabilizers at 180℃

Fig. 5
Fig. 5 UV-VIS spectrum of pure PVC and PVC stabilized with different thermal stabilities heated for 0 min (A) and 60 min (B) at 180 o C. a: Pure PVC; b: CaSt2/ZnSt2; c: DMAE-Zn

Fig. 5B shows 3 . 2 . 4 .Fig. 6
Fig.5Bshows that, after having been heated for 60 min, the absorbance curve of pure PVC increased obviously at the range of 300-330 nm.This suggests that the Cdb of tetraenes and pentaene in pure PVC has further increased.The absorbance curve of PVC stabilized by DMAE-Zn increased slightly at the range of 300-330 nm.What's surprising was that the absorbance curve of PVC stabilized by CaSt2/ZnSt2 still did not increase, indicating that ZnCl2 produced from the reaction between ZnSt2 and HCl intends to catalyze the PVC degradation to only form conjugated triene.

Fig. 7
Fig. 7 Torque rheology test results obtained at 180 o C on pure PVC (a) and PVC stabilized by DMAE-Zn (b)

Fig. 9
Fig. 9 Color evolution of PVC samples with different ratio of ZnCl2 and DMAE-Zn heated at 180 o C for different time

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 1 April 2019 doi:10.20944/preprints201904.0006.v1
Peer-reviewed version available at Polymers 2019, 11, 813; doi:10.3390/polym11050813Zn did not improve the lubricating property of PVC.The second peaks of pure PVC and PVC stabilized by DMAE-Zn however appeared at different time points.Pure PVC required approximately 111 s to complete the plasticization.In PVC stabilized by DMAE-Zn case, however, plasticization was completed in 55 s.The significantly reduced plasticizing time obtained by incorporating DMAE-Zn as compared to pure PVC confirmed the excellent plasticizing effect of DMAE-Zn on PVC.It is well known that plasticizers must have a good compatibility with PVC.For example, Gilbert et al. reported that plasticizers acted as solvents for amorphous regions of PVC, and the PVC chains in the amorphous regions might become solvated at elevated temperatures during processing

Table 2
The HCl absorption capacity of different thermal stabilizers for PVC Preprints (www.

preprints.org) | NOT PEER-REVIEWED | Posted: 1 April 2019 doi:10.20944/preprints201904.0006.v1
Peer-reviewed version available at Polymers 2019, 11, 813; doi:10.3390/polym11050813Conductivitytitrationexperimentwascarriedout to verify the ability of DMAE-Zn to absorb HCl.The experimental method was described in Section 2.5.5.The results are shown in Table2.Table2shows that lead salts has the highest capacity to absorb HCl, reaching 281.1 mg/g.HCl absorption capacity of DMAE-Zn was 131.4 mg/g, which was larger than that of ZnSt2 and CaSt2, showing that DMAE-Zn could improve the long-term thermal stability of PVC.