Study on the influence of environmentally friendly plasticizers on the properties of polyvinyl chloride

In this work, the torque rheology, mechanical properties, migration resistance, thermal deformation, and Vicat softening point temperature of PVC plasticized by four environmentally friendly plasticizers are investigated. The experimental results reveal that the plasticizer’s heavy metal content meets production requirements while falling below the limiting standard. Plasticizer melting points differ, as do the enthalpy and torque of the entire melting process, resulting in different energy consumption when melting plasticizers. Also, the mechanical properties of PVC are not different, but the mechanical properties of PVC plasticized by epoxy soybean oil are the most prominent. Besides, the Vicat softening point temperature of PVC plasticized by four environmentally friendly plasticizers is within the normal standard range, and the migration amount in different solvents is changed in a small range, which will not affect human health.


Introduction
Polyvinyl chloride (PVC), as one of the five general resins, has been widely applied in many fields in recent years due to its extraordinary properties in high impact resistance, flame retardancy, corrosion resistance, wear resistance, insulation, low processing cost, and so on [1][2][3][4] .Simultaneously, with the rapid development of national economic sustainability, the production of PVC continues to increase in China.As early as 2005, China had become the number one producer of PVC in the world [5] .In 2020, China had 50.16% of the global PVC production capacity [6] .As an indispensable part of people's daily lives, PVC products play an important role in economic development.At the same time, the PVC additive industry was also developing rapidly, especially the plasticizer, which was the largest addition to PVC [7] .It was well known that plasticizers play an important role in the production of PVC, which could make plastics easier to process, change the mechanical properties of plastics, and improve the plasticity and flexibility of PVC [8][9][10] .However, plasticizers would precipitate when PVC was used, which is an existing additive in PVC in some crucial fields, such as toys and biomedical devices.Migration of Di-2-ethylhexyl phthalate (DEHP) could be hazardous to human health, and environmental protection issues were not guaranteed, so migration of DEHP has caused a wide concern, and this problem must be solved [11][12][13] .Meanwhile, with the improvement of people's living standards, people paid attention to the safety of plastic products for human health while paying attention to prices.Therefore, the development of non-toxic and environmentally friendly plasticizers for PVC has become a necessary step for the development of the PVC industry.
To solve the above problem, the design of the formula, the preparation process, and the specimen preparation and test were introduced in this paper.The four kinds of environmentally friendly plasticizers were used to plasticize PVC.Its structure and properties were characterized by an X-ray fluorescence spectrometer (XRF), differential scanning calorimetry (DSC), torque rheometer, heat distortion, Vicat softening temperature apparatus, etc.The effects of four different plasticizers on the properties of PVC were analyzed in detail.This has been more and more widely used as an environmentally friendly plasticizer.

Materials
Polyvinyl chloride resin, denoted as (PVC), was used as the base resin and has been supplied by Xinjiang Zhongtai Chemical Co., Ltd, Xinjiang, China.Plasticizer 1 in the form of a solid, denoted as (S20), was supplied by A Company, China.Plasticizer 2 in the form of a solid, denoted as (E15), was supplied by B Company, China.Plasticizer 3 in the form of a solid, denoted as (P230), was supplied by C Company, China.Epoxy soybean oil, denoted as (ESO), was purchased from Guangzhou Xinjinlong Plastic Additives Co., Ltd, Guangdong, China.Calcium carbonate, denoted as (CaCO 3 ), was received from Guilin Jinshan New Material Co., Ltd, Guangxi, China.Ca-Zn was received from Zhejiang Transfar Whyyon Chemical Co., Ltd, Zhejiang, China.ACR was supplied by Shandong Ruifeng Chemical Co., Ltd, Shandong, China.Glacial acetic(AR grade), sulfuric acid(AR grade), and nitric acid(AR grade) were purchased from Guangdong Guanghua Sci-Tech Co., Ltd, Guangdong, China.

Apparatus and Equipment for Experiment
The experimental unit studied was the high-speed stirrer (

Preparation of Sample
According to the formula in Table 1, four samples of PVC were prepared, and the sample size was cut into a suitable size by the universal material sampling machine for performance testing.
Table 1.The formula of PVC plasticized by different plasticizers (g).Heavy metal detection can meet the demand for harmful element analysis in PVC.The content of harmful elements in the environmentally friendly plasticizer was detected by an X-ray fluorescence spectrometer (XRF).According to the relevant national standards, the results of heavy metal detection were compared with the national standard limits, and heavy metal pollution was evaluated.Epoxy soybean oil was liquid, so this test was not required.

Thermal Stability Test.
The thermal stability of the environmentally friendly plasticizer was studied by differential scanning calorimetry (DSC).Three kinds of plasticizers with 10 mg each were weighed and put into aluminum crucibles, respectively, and then put into differential scanning calorimetry (DSC) for a thermal stability test.The parameters of DSC were set as follows: the gas atmosphere was nitrogen, the gas flow rate was 20 mL•min -1 , the initial temperature was 20°C, the heating rate was 10°C•min -1 , the temperature was heated to 170°C for 15 min, and then cooled from 170°C to 30°C at a rate of -10°C•min -1 , waiting for the computer to automatically display the relevant data and record the relevant data.Epoxy soybean oil was liquid, so this test was not required.

Rheological Property Test.
The plastic behavior of the sample was investigated using a torque rheometer.First, the parameters of the torque rheometer were set, such as reaction temperature, test time, and torque on the computer software.Then, after the computer was turned on and showed that it was ready, the raw materials were weighed according to the formula in Table 1.After the materials were mixed and stirred evenly, 75 g of the mixed material was put into the feed tank for fusion, and the computer would automatically record the whole process data.Finally, after the reaction was completed, the sample was taken out and the data was saved.

Mechanical Properties Test.
According to GB/T 1004-2006, tensile strength and elongation at break were tested with tensile stress at 100%.In addition, the tear strength was measured according to GB/T 529-2008 and hardness was tested according to GB/T 2411-2008.

Migration Resistance Test.
From Table 1, four environmentally friendly plasticizers were used to plasticize PVC, respectively.The sample was cut into small discs with an area of 4 square centimeters, and the weight of the small discs was weighed with an analytical balance and the original data was recorded.The data was recorded to the four digits after the decimal point, and the environment that PVC was exposed to in real life was simulated.The samples were immersed in 200 mL of water, ethanol, nitric acid, and glacial acetic acid, respectively, and placed at room temperature for 24 h, 48 h, and 96 h.After taken out, it was put in the oven and blast dried for 48 h at a set temperature of 30°C.Afterward, the quality was accurately weighed and the quality loss was calculated.

The Heat Distortion Vicat
Softening Point Temperature Test.Heat distortion and Vicat softening temperature apparatus were mainly used for the determination of the thermal deformation of plastics and the softening temperature of the Vicat.The main method of measurement was to place the sample horizontally under the unloaded pressure needle, and the pressure needle was not less than 3 mm from the upper surface of the sample, and then it was put into the heating device.The machine was started, the 50 N weight was added to the load board and the parameters were zeroed.The initial temperature of the equipment was 16.5°C and the heating rate was 50°C•h -1 .When the depth of the needle was more than 1 mm ± 0.01 mm, the temperature was recorded as the Vicat softening point temperature, and the deformation of the sample was observed.2 to 4, it is clear that after 180 s of detection time of the three plasticizers, the content of heavy metal elements was below the standard content, whereas the Sb, Ba, and Sn elements had relatively large content and high spectral intensity.In addition, Compared with the type and content of heavy metal elements in the three plasticizers, the plasticizer (S20) was the most, followed by the plasticizer (P230), and the plasticizer (E15) was the least.Therefore, it is necessary to strictly select plasticizers according to the requirements of PVC products for the types and contents of heavy metal elements.

Analysis of Thermal Stability
Differential scanning calorimetry (DSC) was a test method to study the physical and chemical changes of polymer materials and was widely used in the research of plastic composites.In this paper, the properties of plasticizers were accurately obtained at a reasonable cost.These properties were transition temperature and transition enthalpy.The results of the differential scanning calorimetry (DSC) for plasticizers are shown in Figure 4 to Figure 6.
Figures 4 to 6 results revealed that there were differences in the melting points of different plasticizers.Among these, the melting point of plasticizer (S20) was the highest, at 66.80°C; the melting point of plasticizer (E15) was the second highest, at 66.60°C; plasticizer (P230) had the lowest melting point of 54.60°C.Also, the enthalpy change of the plasticizer in the whole melting process was different.The enthalpy change of plasticizer (P230) was the highest, which was 166.40 J•g -1 , followed by plasticizer (S20), with the enthalpy change of 146.13 J•g -1 ; plasticizer (E15) had the lowest enthalpy change of 123.38 J•g -1 .
The above analysis shows that the plasticizer (P230) consumed more energy during its melting.Therefore, the appropriate plasticizer could be selected according to the actual production and application fields of PVC products.The torque-time curves of the samples were shown in Figure 7, in which the green curve was Sample 1, the black curve was Sample 2, the red curve was Sample 3, and the purple curve was Sample 4. It could be seen from the curve that the overall torque of the four samples was not much different, but it could still be seen that the torque of Sample 2 and Sample 3 was lower than that of the other samples.It could be seen that the smaller the torsion is, the smaller the friction there is on the double stick, and the smaller the frictional heat is generated, resulting in less energy lost.In addition, the energy consumption of Sample 4 in the whole plasticizing process was higher than that of the other three plasticizers, indicating that Sample 4 had high viscosity, poor fluidity, and high power requirements for processing.Moreover, the plasticizing time of Sample 4 was the fastest, but it was not conducive to molding in the actual production process.

Analysis of Mechanical Properties
The experimental results show that the tensile strength, tensile stress at 100%, and elongation at break and tear strength of Sample 4 were higher than those of Sample 1, Sample 2, and Sample 3. The reason was that the epoxy soybean oil was liquid and the macromolecular chains of the epoxy soybean oil were entangled, which led to more adequate dissolution with other additives and increased the adhesion and toughness of the sample.Therefore, the tensile strength, tensile stress at 100%, elongation at break, and tear strength of Sample 4 were improved.On the Shore A hardness, the difference between the four samples was small, Sample 2 was slightly larger, and Sample 4 was the smallest.The reason was that the plasticizer (E15) has a large polarity, so it has a large polar interaction with PVC, which limits the movement of PVC.
In conclusion, according to the mechanical properties of the four samples, the tensile strength, tensile stress at 100%, and elongation at break and tear strength of Sample 4 were excellent, which was related to its molecular structure and compatibility with PVC.

Analysis of Migration Resistance
The leaching and migration of plasticizer molecules in PVC not only affects the service life of the material but also has an impact on human health, so the migration of plasticizers has received extensive attention.In this experiment, the migration of four plasticized PVC in different solvents was measured.It could be seen that the quality of the samples did not change after soaking in deionized water for 24 h, 48 h, and 96 h.Therefore, the plasticizers were almost insoluble in deionized water.These plasticizers contain a large number of hydrophobic groups that are insoluble in water.However, the migration of samples in alcohol and nitric acid immersion was relatively large.At the same time, the migration of PVC samples in different solvents was obvious in a short time and changed slowly after a long time.Therefore, the migration of four plasticized PVC changes in a small range in different solvents, which would not affect human health.

Analysis of the Heat Distortion Vicat Softening Point Temperature
From Table 5, it was found that the Vicat softening temperature of each sample was a little different.The Vicat softening temperature of Sample 4 was the lowest, which was 74.1°C, and the Vicat softening temperature of Sample 3 was the highest, which was 78.4°C.According to the use standard of a company, if the Vicat softening temperature was greater than 74°C, then it could be used normally.Therefore, the Vicat softening temperatures of the samples obtained from the test were within the normal range of use.

Conclusions
(1) The types and contents of heavy metals in plasticizers are different, but all are below the limited standards.If heavy metal content is strictly required, it is necessary to strictly select plasticizers according to the types and content requirements of heavy metal elements in PVC products.
(2) By comparing the difference in plasticizer melting point, there is a difference between the enthalpy and torque of the whole melting process, and the energy consumption is different when the plasticizer is melted.Therefore, it is necessary to select the appropriate plasticizer according to the actual production and application fields of PVC.
(3) The mechanical properties of PVC plasticized by four kinds of plasticizers are not much different, and all of them are excellent.Among them, the mechanical properties of PVC plasticized by epoxy soybean oil are the most prominent.
(4) The Vicat softening temperature of PVC plasticized by four plasticizers is not significant, which is within the standard range.At the same time, the migration amount of PVC in different solvents changed within a small range, which will not affect human health.

Figure 7 .
Figure 7.The torque and time curve of samples.

Table 4 .
Content of heavy metal elements in plasticizer (P230).

Table 5 .
The heat distortion Vicat softening point temperature of the samples.