Abstract
Introduction
Nano-barium sulfate is a white amorphous inorganic nano-powder. Compared to common barium sulfate, nano-barium sulfate is superfine and has many excellent characteristics, 1 such as strong anti-aging power, good adsorption, excellent optical properties, and high stability. It can be used as a filler in a variety of industries such as coatings, plastics, rubber, and paper. However, due to the small particle size, large specific surface area, and high cohesive energy of nano-barium sulfate, it is easy to produce hard agglomerates during the drying process, thus leading to it losing the advantages of nano-scale, which largely limits its application.
Traditional methods often use organic solvent washing, 2 azeotropic distillation, 3 the inorganic dispersant method, 4 wet surface modification, 5 and other process methods to modify the barium sulfate treatment, so as to improve the material properties. The key to modification is to achieve ultrafine dispersion of nano-barium sulfate in the polymer. The organic solvent washing method has the disadvantage of using too much organic solvent, which is not suitable for industrial scale-up and production applications. The azeotropic distillation method has the disadvantage of using too much azeotrope, which is not suitable for large-scale industrial applications. The inorganic dispersant method is only suitable for the preparation of nano-barium sulfate suspensions, which can improve the stability of the barium sulfate suspension.
The wet surface modification method involves chemical modification of the surface of barium sulfate by surfactants, aluminates, titanates, and other coupling agents, which improves the impact strength of the polymer filled with modified barium sulfate to a greater extent, making the toughening effect of the material matrix greatly improved. At the same time, the compatibility between the filler particles and the polymer matrix is poor and the interfacial bonds are weak. Therefore, an increase in the amount of added filler leads to a significant reduction in the tensile strength of the material. In order to improve the compatibility between the inorganic filler and the organic matrix, the surface of the inorganic filler needs to be modified by grafting with polymeric organic substances.6–10 In this study, the polymerization of methyl methacrylate is initiated by free radicals, and the surface of nano-barium sulfate has been grafted with polymethyl methacrylate (PMMA), thus effectively solving the problem of nano-barium sulfate surface modification. Only a few papers on this type of chemistry have been reported to date. 11
Results and discussion
Influence of the modification method on the modification effect
A suspension of nano-barium sulfate with a solid content of 56 g L−1 and a particle size of 30 nm was prepared, to which 1 mL of a silane coupling agent was added for alkylation. This was followed by addition of 0.2 g of the initiator potassium persulfate and 15 mL of the monomer methyl methacrylate (27% by mass of nano-barium sulfate). Under protection of nitrogen, nano-barium sulfate was modified by grafting polymerization at a temperature of 80 °C and a modification time of 2 h. After modification, the product was filtered, washed, and dried. Under the same conditions of stirring rate, modification time, and modification temperature, the effects of different modification methods on the modification effect were investigated. The results are shown in Table 1.
Effect of the modification method on the modification effect.
The wet modification method includes the wet modification method (ordinary stirrer) and the wet modification method (emulsifier). A suspension of nano-barium sulfate with a known solid content was added to a beaker, heated in a constant temperature water bath, and then the same wet modification agent was added. The modification temperature was 80 °C and the time was 30 min. After modification, the suspension was filtered using a vacuum pump and the filter cake was washed with water to remove the sodium chloride and unreacted modifier. The modified nano-barium sulfate sample formed as hard lumps and soft powders is obtained by oven-drying. The stirring rate was the key factor for the success of the modification of nano-barium sulfate. As can be seen from Table 1, nano-barium sulfate still had agglomerates and hard agglomerates after wet modification, and the modification effect obviously did not reach the standard of nano-scale, similar to the appearance of hard lumps of unmodified direct drying products, which is caused by the non-uniform mixing of the modifier and nano-barium sulfate during the reaction modification process. When an emulsifier was used instead of the common stirrer as the modifying instrument, the problem of uneven mixing of the reactants could be largely alleviated and the product had the appearance of a soft powder. However, the use of an emulsifier makes it difficult to achieve industrial production.
It can also be seen that nano-barium sulfate is a powdered solid after modification by the grafting modification method using a common stirrer at the same stirring rate, and the modification effect is obvious. This indicates that the grafting polymerization reaction of the MMA monomer is initiated on the surface of nano-barium sulfate. The organic polymer, poly(methyl methacrylate), is successfully grafted on the surface of nano-barium sulfate, which largely prevented agglomeration of nano-barium sulfate.
Oil absorption value, also known as resin absorption capacity, is an index that indicates the amount of resin absorbed by a filler. Oil absorption values are usually expressed as the amount of linseed oil (mL 100 g−1) required for 100 g of filler to achieve complete wetting. Nano-barium sulfate is commonly used as a filler due to its small particle size, large surface area, and narrow distribution, resulting in high oil absorption values. Higher oil absorption values for fillers result in increased resin consumption, which effectively increases costs. To reduce the moisture absorption of filler powders and increase their usage, the powders are typically subjected to surface modification, by grafting modification with organic polymers, to form a layer on the surface of the nano-barium sulfate. Thus, the oil absorption values of modified nano-barium sulfate become lower. The lower the oil absorption values, the better the modification effect.
Nano-barium sulfate is hydrophilic when precipitated, but after modification with organic polymers through grafting reactions, the surface of the powder becomes hydrophobic. Characterization using activation degree determination shows that modified nano-barium sulfate floats on the surface of water, while unmodified nano-barium sulfate powder sinks below the water surface. The percentage of activated grafted nano-barium sulfate mass over nano-barium sulfate mass reflects the activation degree. The higher the activation degree, the better the modification effect.
Under the same common stirrer conditions, the activation degree and oil absorption values of nano-barium sulfate modified by the grafting modifier method are compared with those of the wet modification method in which several modifiers are more effective, as shown in Table 2.
Determination of the activation degree and oil absorption values.
As can be seen from Table 2, under the same general stirring conditions, PMMA graft polymerization is the best, with a product activation degree of 76.86% and an oil absorption value of 77.41%. Compared with other types of surface modifiers such as surfactants, water-soluble polymers, and coupling agents, PMMA polymerization is the best. Because the polarity of PMMA grafted on the surface of barium sulfate is very close to that of the PVC substrate, its long chain segments and PVC molecular chains are intertwined, which improves the compatibility of barium sulfate with the PVC substrate. The use of organic polymers with similar polarity to the filler matrix to graft the surface of the inorganic filler enhances the compatibility between the filler matrix and the inorganic filler.
Influence of alkylation on the effect of polymerization modification
A suspension of nano-barium sulfate with a solid content of 56 g L−1 and a particle size of 30 nm was leaded 0.2 g of the initiator potassium persulfate and 15 mL of the monomer methyl methacrylate (27% by mass of nano-barium sulfate). The temperature was 80 °C and the modification time was 2 h. After modification, the product was filtered, washed with water, and dried. Under the same conditions of stirring rate, modification time, and modification temperature, compared to alkylation of silane coupling agent before grafting reaction (1 mL of silane coupling agent dropwise), see Table 3.
Determination of the activation degree and oil absorption values.
As can be seen from Table 3, direct alkylation of the surface of the inorganic particles under the same common stirrer conditions did not achieve a good modification effect, and the activation degree of the modified nano-barium sulfate was only 16.90%. However, the modified nano-barium sulfate with an activation degree of 8.25% is somewhat better compared to that of unalkylated before the grafting reaction. The reason for this is that the silane coupling agent is not used in the grafting reaction. The MMA molecule is not amphiphilic and is not as effective as it could be if used alone. The pre-grafting alkylation involves a combination of the silane coupling agent and the MMA molecule, which has a significant effect on modification of the nano-barium sulfate.
Also from Table 3, it can be seen whether or not alkylation before grafting had a significant influence on the effect of grafting modification, the appearance state, the activation degree, and the oil absorption value of nano-barium sulfate after modification. The appearance of nano-barium sulfate after modification without alkylation before grafting was a hard block, and the activation degree of nano-barium sulfate after modification was 8.25%, while the appearance of nano-barium sulfate after modification by alkylation before grafting was that of a soft powder, and the activation degree of nano-barium sulfate after modification was 76.86%.
Effect of modification time on the modification effect
A suspension of nano-barium sulfate with a solid content of 56 g L−1 and a particle size of 30 nm was used. Alkylation was achieved by adding the silane coupling agent (1 mL), followed by 0.2 g of the initiator potassium persulfate and 15 mL of the monomer methyl methacrylate (27% by mass of nano-barium sulfate) under nitrogen protection. Thus, the nano-barium sulfate was modified by graft polymerization, and the reaction temperature was fixed at 80 °C; only the modification time was varied: 60, 90, 120, 150, and 180 min. The activation degree and oil absorption value of the resulting products were measured (Figure 1).
Effect of the modification time on the activation degree and the oil absorption value.
As can be seen from Figure 1, the activation degree of the modified nano-barium sulfate powder gradually increased on increasing the modification time. When the modification time was 120 min, the activation degree reached 76.86%, which is not much different from the effect of a modification time of 150 min. An emulsification time of 120 min was chosen to save costs in industrial production. The oil absorption value decreases on increasing the modification time, and the oil absorption value was 77.41% after a modification time of 120 min.
Effect of modifier dosage on modification effect
A suspension of nano-barium sulfate with a solid content of 56 g L−1 and a particle size of 30 nm was used. Alkylation was achieved by adding the silane coupling agent (1 mL), the reaction temperature was fixed at 80 °C, and the modification time was 120 min under nitrogen protection. Only the amount of modifier methyl methacrylate and the corresponding initiation agent varied. The dosage values were 6.75%, 13.5%, 27%, and 54% of the mass of nano-barium sulfate, respectively. The activation degree and oil absorption values of the resulting products were determined and are shown in Figure 2.
Effect of the modifier dosage on the activation degree and oil absorption value.
From Figure 2, the activation degree of the modified nano-barium sulfate tends to increase and then decrease on increasing the modifier dosage. Too much modifier will increase the agglomeration and adhesion of the powder. The oil absorption value first decreases and then increases on increasing the modifier dosage. Therefore, the most suitable modifier dosage is 27%, with the lowest oil absorption value and the highest activation degree.
Effect of modification temperature on the modification effect
A suspension of nano-barium sulfate with a solid content of 56 g L−1 and a particle size of 30 nm was used. Alkylation was achieved by adding the silane coupling agent (1 mL), followed by 0.2 g of the initiator potassium persulfate and 15 mL of the monomer methyl methacrylate (27% by mass of nano-barium sulfate). The nano-barium sulfate was modified by graft polymerization under nitrogen protection, varying only the modification temperatures, which were 60 °C, 70 °C, 80 °C, 90 °C, and 100 °C, respectively. The activation degree and oil absorption values of the resulting products were determined and are shown in Figure 3.
Effect of the modification temperature on the activation degree and the oil absorption value.
From Figure 3, the activation degree of the modified nano-barium sulfate increases with increasing the modification temperature below 80 °C. When the modification temperature reaches 80 °C, the activation degree reaches 76.86%. However, when the modification temperature exceeds 80 °C, the activation degree of the modified nano-barium sulfate is no longer sensitive to temperature factors. Considering the industrial production, energy saving, and consumption reduction, prolonging the service life of equipment, and reducing the production cost of enterprises, the optimal modification temperature is 80 °C. The oil absorption value decreases on increasing the modification temperature. When the modification temperature is 80 °C, its oil absorption value is 77.41%.
Characterization
Infrared spectroscopic and XPS analysis of modified nano-barium sulfate
The characteristic peaks in the IR spectrum of PMMA occur at 2996 cm−1 and 2951 cm−1 for the stretching vibration of CH3, at 1731 cm−1 for the characteristic vibration peak of C=O, and at 1388 cm−1 and 1449 cm−1 for the deformation vibrations of CH3. As can be seen from Figure 4, the strong PMMA peak in the IR spectrum indicates that the PMMA has been firmly grafted onto the surface of the nano-barium sulfate particles.
Infrared spectra of directly dried unmodified and modified nano-barium sulfate with particle sizes of 20–40 nm: (a) unmodified and (b) grafting modified.
In addition, we conducted XPS analysis on unmodified nano-barium sulfate and modified nano-barium sulfate. The atomic ratio on the sample surface can be calculated from the atomic relative content. For modified nano-barium sulfate, the atomic ratio of Ba:S:O is 1:1.2:4.4, which is close to the atomic ratio of BaSO4. However, for modified nano-barium sulfate, the surface atomic ratio of Ba:S:O:C is 1:1.2:7.7:3.5, indicating a higher presence of C and O compared to BaSO4, suggesting the presence of compounds containing C and O on the surface, indicating that this compound is PMMA.
Furthermore, as observed in Figure 5, characteristic peaks due to S, O, and Ba were detected in unmodified nano-barium sulfate, while characteristic peaks due to C, O, S, and Ba were detected in modified nano-barium sulfate. The binding energy of O1s in unmodified nano-barium sulfate is 532.81 eV, whereas the binding energy of O1s in modified nano-barium sulfate is 533.08 eV, which indicates an increase in binding energy of 0.27 eV. This change is due to the altered chemical environment surrounding oxygen, resulting in a change in the binding energy of oxygen. Therefore, PMMA is grafted onto the surface by interacting with the O atoms on the BaSO4 surface.
XPS spectra of directly dried unmodified and modified nano-barium sulfate particles with a size range of 20–40 nm.
Thermogravimetric analysis of modified nano-barium sulfate
The thermogravimetric analysis in Figure 6 shows that the mass of modified nano-barium sulfate decreases by 36.5% in the temperature range of 100 °C–800 °C, while the mass of unmodified barium sulfate only decreases by 1.894% in this temperature range, which indicates that both contain a certain amount of bound water (which cannot be removed by drying) and the modification effect is more obvious.
Thermogravimetric analysis of directly dried unmodified and modified nano-barium sulfate with particle sizes of 20–40 nm: (a) unmodified and (b) grafting modified.
The difference between the two thermogravimetric analysis curves in Figure 6 is obvious. As nano-barium sulfate does not decompose below 800 °C and PMMA depolymerizes above 270 °C, the weight loss of modified nano-barium sulfate is due to the formation of a polymer. Figure 6 shows that the PMMA content of the modified nano-barium sulfate is approximately 34.6%.
In addition, the modified nano-barium sulfate was subjected to toluene extraction. The unreacted MMA, silane coupling agent, and PMMA dissolve in toluene and are filtered after 24 h of extraction and dried at 80 °C until the mass did not change, indicating that all the soluble components had been extracted. The mass fraction of the soluble fraction of the modified nano-barium sulfate is calculated to be 4.7%, which is lower than the weight loss of 34.6% in the thermogravimetric analysis. This indicates that the surface of the modified nano-barium sulfate particles is covered by PMMA and that the PMMA cannot be completely extracted by the toluene due to the fact that PMMA is firmly grafted onto the surface of the nano-barium sulfate by chemical bonding.
Analysis of the dispersion behavior of modified nano-barium sulfate in PVC
The PVC resin and unmodified nano-barium sulfate, and PVC resin and modified nano-barium sulfate were added into the high-speed mixer to mix, respectively, at a certain temperature. The mixture was plasticized on a two-roller refiner at 175 °C for 10 min and then formed into flakes. A mold was then preheated at (180 ± 5) °C for 10 min and the sample then pressed in the mold at 14.5 MPa for 4 min to form a sheet. The mold was then transferred with the pressed material to an unheated press of the same size while still hot and rapidly pressed to the pressure of the hot press for cooling and shaping under pressure. Scanning electron microscopy was used to observe the dispersion of nano-barium sulfate in the PVC matrix after unmodified and PMMA grafting modification. On analyzing the scanning electron microscope (SEM) photographs in Figure 7, it can be seen that the particle size after polymerization is larger than that before polymerization and that the edges of the particles become blurred compared to before polymerization, but the dispersion in the PVC matrix is significantly improved. It is tentatively suggested that this change is due to the coating of PMMA on the surface of nano-barium sulfate after polymerization. The untreated nano-barium sulfate in the PVC matrix is more heavily agglomerated and has a larger particle size, which is due to the lower interfacial bonding between the unmodified particles and the PVC matrix, whereas the modified nano-barium sulfate particles are uniformly dispersed in the PVC matrix after grafting on the PMMA surface and are basically free of agglomeration. The modified particles are not obviously contoured in the PVC matrix.
Scanning electron microscopy of nano-barium sulfate with particle sizes of 20–40 nm after direct drying of unmodified and grafting modification: (a) unmodified and (b) grafting modified.
The polarity of the PMMA grafted with nano-barium sulfate is very similar to that of the PVC matrix. Also the intertwining of the PMMA long chain segments and the PVC molecular chains improves the compatibility between nano-barium sulfate and the matrix. Although agglomerates are present before polymerization, the monomer is permeable such that the particles inside the agglomerates can also be grafted and wrapped sufficiently to open the agglomerates effectively. Nano-barium sulfate particles have good interfacial bonding with the PVC matrix after the grafting modification.12–16
Grafting modification mechanism
The molecular structure of the silane coupling agent is usually Y-R-Si(OR)3 (where SiOR is a silicone alkoxy group and Y is an organic functional group). The silane oxygen group can be hydrolyzed to produce an Si-OH group, which can react with inorganic substances such as nano-barium sulfate; the organic functional group Y has compatibility or reactivity with organic substances such as PMMA. Therefore, when the coupling agent between the organic and inorganic interface, between the interface of organic and inorganic substances to set up a “molecular bridge,” can form inorganic substrates—silane coupling agent–organic substrate bonding layer. It serves to link two materials with very different properties, namely PMMA and nano-barium sulfate, together. 17 Please refer to Figure 8.
The grafting modification schematic diagram.
In this paper, the surface of nano-barium sulfate is modified by “branched” surface grafting, that is, by first forming an active site in the middle of the molecule and then triggering the polymerization of another monomer to grow a branched chain. Nano-barium sulfate does not have an active site on its surface, so a two-step process is used to graft the surface of nano-barium sulfate. The surface of nano-barium sulfate is first coated with a double-bonded silane coupling agent, which acts like an emulsifier and is based on the principle of “similar compatibility.” Under the principle of “similar compatibility,” the surface of nano-barium sulfate can adsorb the monomer MMA. 18 Under the action of stirring, the free radicals generated by the initiator rapidly diffuse onto the surface of nano-barium sulfate. After grafting of the organic polymer on the surface of nano-barium sulfate, as the hydrophilicity of the monomer is much higher than that of the polymer, the rest of the monomer is between the polymer and the water interface, and the polymerization reaction between the emulsions takes place within this monomer layer. The result is a grafted nano-barium sulfate particle with nano-barium sulfate as the core and PMMA as the cladding layer.
Conclusion
Compared to wet surface modification agents, the polymerization of methyl methacrylate initiated by free radicals and grafting of PMMA through the surface of nano-barium sulfate can greatly improve the surface modification effect of nano-barium sulfate.
The optimum modification conditions for the modification of nano-barium sulfate by the grafting method are as follows: the modification time is 120 min, the modification temperature is 80 °C, and the amount of modifier is 27%, resulting in well-dispersed, hydrophobic powdered nano-barium sulfate with an activation degree of 76.86% and an oil absorption value of 77.41%.
Under the optimal modification conditions, the products have been characterized. From the scanning electron micrographs, it can be seen that the dispersion of nano-barium sulfate is significantly improved after modification. The modified nano-barium sulfate shows deformation vibrations of CH3 at 1388 cm−1 and 1449 cm−1, and the characteristic vibration peak of C=O at 1731 cm−1, indicating that PMMA has been firmly adsorbed on the surface of nano-barium sulfate particles; the mass of modified nano-barium sulfate decreases by 36.5% from 100 °C to 800 °C on thermogravimetric analysis. On the other hand, the unmodified barium sulfate only reduces its mass by 1.894% in this temperature range. This indicates that the PMMA is significantly modified with an encapsulation of 34.6%.
Experimental
Materials
The reagents used in the experiments are shown in Table 4.
List of experimental reagents.
The experimental equipment used in the experiments are shown in Table 5.
Experimental equipment.
Apparatus and experimental procedure
Nano-barium sulfate with particle size of 30 nm was prepared using an impingement flow reactor with a feed cross-sectional area of 1.56 mm2, a reactant concentration of 1.2 mol L−1, a reactant feed flow rate of 10.42 m s−1, and an impingement angle of 60°.19,20 A suspension of nano-barium sulfate of known solid content was poured into a four-necked flask. A constant temperature water bath was adjusted to the experimental temperature and heated. The initiator potassium persulfate was added to the four-necked flask and the flask was purged with nitrogen and evacuated (the above procedure was repeated several times). A balloon was used to maintain a positive pressure of nitrogen and methyl methacrylate was added dropwise using a constant pressure dropping funnel, after which the temperature and time of the grafting modification were controlled. The modified suspension was filtered, and sodium chloride was removed by aqueous washing. After drying the water and extracting with toluene, the grafted modified nano-barium sulfate powder can be obtained. The experimental setup is shown in Figure 9.
Grafting modification of nano-barium sulfate and the experimental setup.
Product characterization
Determination of the oil absorption value: 1 g of modified nano-barium sulfate powder was placed on a glass plate and dioctyl phthalate (DOP) was added dropwise. During the dropping process, fully press with a metal sheet and constantly turn and grind to make DOP fully contact with the modified nano-barium sulfate powder. When the barium sulfate powder and DOP had bonded into a lump and did not fall apart when shoveled up with a metal sheet, the end point of drip addition had been reached. The smaller the oil absorption value of the product, the better the modification effect and the better its dispersibility. 21
Determination of degree of activation: 1 g of the modified nano-barium sulfate powder was placed in a partition funnel (size: 125 mL) containing 100 mL of water. The funnel was shaken at a certain rate for 1 min and then left aside for 30 min. When the state of the solution no longer changed, the sinking barium sulfate powder was placed in the crucible. The water evaporated and the sample dried to a constant weight. The percentage of floating material in the original mass was calculated. The higher the degree of activation, the better the modification effect and the better the dispersion. 21
Thermogravimetric-differential thermal analysis: an SDT-Q600 thermogravimetric-differential thermal analyzer (TA, USA) was used to analyze the decomposition of the main components of modified and unmodified nano-barium sulfate by weighing 200 mg of the sample and increasing the temperature from room temperature to 800 °C under an air atmosphere at a rate of 10 °C min−1.
Infrared spectroscopy: A PerkinElmer Frontier infrared spectrometer with a wavenumber range of 15,800–400 cm−1 and a spectral resolution of 0.4–64 cm−1 was used to analyze the surface functional groups of modified and unmodified nano-barium sulfate.
Scanning electron microscope analysis: An S-4800-1 scanning electron microscope (Hitachi, Japan) with an electron current intensity of 20 keV, a magnification of 20–800,000, and an electron acceleration voltage of 0.5–30 kV was used to analyze the dispersion of modified and unmodified nano-barium sulfate and its compatibility with the matrix PVC.
X-ray photoelectron spectroscopy analysis: XPS spectra of modified and unmodified nano-barium sulfate were obtained using a PHI 5300 X-ray photoelectron spectrometer (PERKIN-ELMER Physic Electronics, USA).
Footnotes
Declaration of conflicting interests
Funding
ORCID iDs
