Preparation of Halloysite NanotubesâSilica Hybrid Supported

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Preparation of halloysite nanotubes/silica hybrid supported vulcanization accelerator for enhancing interfacial and mechanical strength of rubber composites Bangchao Zhong, Huanhuan Dong, Jing Lin, Zhixin Jia, Yuanfang Luo, Demin Jia, and Fang Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02250 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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Industrial & Engineering Chemistry Research

Preparation of halloysite nanotubes/silica hybrid supported vulcanization accelerator for enhancing interfacial and mechanical strength of rubber composites Bangchao Zhong1, Huanhuan Dong1, Jing Lin, Zhixin Jia*, Yuanfang Luo, Demin Jia, Fang Liu School of Materials Science and Technology, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China *Corresponding author: Tel: +86-020-87114857; E-mail address: [email protected] 1

Bangchao Zhong & Huanhuan Dong contributed equally to this work.

Abstract: To enhance the interfacial interaction between rubber and hybrid nanofiller (HS) consisting of HNTs and silica, and simultaneously prevent rubber additives from migration and volatilization, commercial vulcanization accelerator N-cyclohexyl-2-benzothiazole sulfenamide (CZ) was chemically grafted onto HS to obtain functionalized nanofiller (HS–s–CZ). It was found that the grafted CZ molecules within an inorganic host structure have better efficiency to accelerate crosslinking reaction than free CZ molecules. Besides, HS–s–CZ could be evenly dispersed into styrene butadiene rubber (SBR) with further improved filler-rubber interaction compared with that between HS and SBR. Consequently, SBR/HS–s–CZ composites exhibited more excellent mechanical strength than SBR/HS composites containing equivalent accelerator component, showing 45% increase in tensile strength when the filler content was 40 phr. Potentially, this work offers a new design strategy for the surface modification of nanofiller and preparation of high-efficiency rubber additives, which may represent a new path to prepare high-performance elastomer composites. 1

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1. Introduction Rubbers have been acknowledged as irreplaceable materials in many fields due to their high elasticity. Nevertheless, most rubbers show weak mechanical strength and need to be reinforced by rigid inorganic fillers. It is well known that carbon black (CB) is the most commonly used filler for rubber reinforcement because of the strong interaction between rubber and CB surface1-4. As a particulate carbon product, CB is produced by the carbon black oil furnace process. However, with the rapid depletion of oil resource, non-oil-dependent fillers are urgently needed. Even worse, the monotonous black color of CB filled rubber composites is not accepted in some specific rubber products. Therefore, the use of other promising fillers for rubber reinforcement is of important significance5-9. Halloysite nanotubes (HNTs), with the molecular formula of Al2Si2O5(OH)4·nH2O, are natural nanomaterials formed by rolling up kaolinite sheets. The outer diameter, inner diameter and length of the purified HNTs are in the range of 50~70 nm, 10~30 nm and 0.5~2 µm, respectively. However, the geometric parameters of HNTs are also affected by the place where they are mined. Owing to the unique nanostructure, natural availability and low cost, HNTs have attracted extensive interest from academic and industrial perspective as reinforcing nanofiller in rubber composites5, 10-14. Actually, along with rubbers, a lot of polymer matrix can be mechanically reinforced by HNTs15-17. However, due to the one-dimensional nanostructure, the specific surface area of HNTs is much lower than those of spherical nanofillers, leading to the lower contacting area between HNTs and rubber and less adsorbed rubber chains on HNTs surface. Since the amount of adsorbed rubber chains on filler is a key factor in determining the mechanical strength of rubber composites, the lower specific surface area of HNTs should be an important reason for their 2


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relatively low reinforcing efficiency compared to that of spherical silica, another commercial non-black nanofiller18. In our previous works, to enlarge the specific surface area of HNTs, a novel kind of nano hybrid consisting of HNTs and silica has been prepared by the methods of electrostatic self-assembly19 and in-situ growth of silica particles on HNTs surface20. It was found that the specific surface area of nano hybrid was dramatically enlarged by the nano-protrusions formed by the nanosilica particles on the surface of HNTs. For instance, the specific surface area of nano hybrid prepared by electrostatic self-assembly was increased to 151 m2/g−1 from 51 m2/g−1. So far, this nano hybrid has not been applied in rubber yet. On the other hand, the interfacial interaction between filler and rubber mainly affects the final performance of rubber composites. Unfortunately, hydrophilic inorganic nanofillers and hydrophobic rubbers are not compatible in nature, leading to weak interfacial interaction in rubber composites. Consequently, the stress can be hardly transferred to nanofillers, which obstructs the reinforcing efficiency of nanofillers. Surface modification of nanofiller has already proved to be an effective method to enhance interfacial interaction between nanofiller and polymer21-25. Therefore, developing new surface modifiers as required can bring benefits to both academy and industry. Recently, rubber antioxidants were also used as surface modifiers for nanofillers, such as silica and HNTs26-28. Results showed that the chemically linked antioxidants on the surface of nanofiller not only improved the interfacial interaction between nanofiller and rubber, but also endowed rubber composites with much better aging resistance than the corresponding antioxidants with low molecular weight owing, since the avoidance of physical loss of antioxidants caused by their migration and volatilization is significantly avoided. In fact, many low molecular rubber additives, such as vulcanization accelerator, suffer from migration 3



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and volatilization. The migration and volatilization reduce the efficiency of rubber additives. To make matter worse, the migrated and volatilized rubber additives even become organic pollutants in environment. Here, to enhance the interfacial interaction between rubber and hybrid nanofiller (HS, consisting of HNTs and silica) reported in our previous work20, and simultaneously elude issues related to physical loss of rubber additives, commercial vulcanization accelerator N-cyclohexyl-2-benzothiazole sulfenamide (CZ) was chemically bonded on the surface of silane modified HS, and then the effects of CZ functionalized HS (HS–s–CZ) on the vulcanization, interfacial and mechanical performance of rubber composites were systematically disclosed. 2. Experimental 2.1. Materials HNTs were mined from Hubei province, China, and purified according to the reported method22. The outer diameter, inner diameter and length of the purified HNTs are in the range of 50~70 nm, 10~30 nm and 0.5~2 µm, respectively. γ-(2,3-epoxypropoxy)propytrimethoxysilane (KH–560) was produced by Sinopharm chemical reagent Co., Ltd., China. Styrene butadiene rubber (SBR) prepared by radical emulsion polymerization was bought from Sinopec company, China. Tetraethoxysilane(TEOS) and absolute ethanol were analytical reagents supplied by Tianjin Damao chemical reagents company, China. CZ, ZnO, stearic acid, 2-mercaptobenzimidazole (antioxidant MB) and sulfur were industrial grade products and used as received. 2.2. Preparation of HS and HS–s–CZ 4


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HS was prepared by in-situ growth of nano silica particles on the surface of HNTs though sol-gel method according to our previous work 20. It has been demonstrated that silica particles with the diameter of 10–20 nm were chemically grafted on the surface of HNTs through Si–O bonds. The easy ring-opening reaction of epoxy group with amino group was introduced to synthetise HS–s–CZ 29. As illustrated by Fig. 1, 0.001 mol of CZ and 0.001 mol of KH–560 were dissolved in absolute ethanol. After mechanical stirring for 3 h at 90 oC under N2 atmosphere, 8g of HS was added into the solution. The suspension was continuously stirred for 12 h at the same temperature. The resultant suspension was filtered, successively washed with ethanol and dried to obtain HS–s–CZ.

Fig. 1. Synthesis route of HS–s–CZ. 2.3. Preparation of SBR composites HS and HS–s–CZ were incorporated in SBR as reinforcing filler to prepare SBR composites (SBR/HS and SBR/HS–s–CZ) at different filler content. Unfilled SBR was also prepared as a control sample. The detailed components of rubber compounds were (in phr: part per hundred rubber): rubber, 100; stearic acid, 2.0; ZnO, 5.0; MB, 1.0; sulfur, 1.6. The filler content was varied from 0 to 40 phr. Besides, the accelerator (CZ) was fixed at 2 phr. It should be pointed out that the amount of 5



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bonded CZ on the surface of HS was determined by the residue weight of HS–s–CZ after being heated to 700 oC and calculated as part of the accelerator amount to ensure the equivalent accelerator component in each rubber compounds. To prepare SBR composites, rubber and other rubber additives were first mixed in an open mill at room temperature. Subsequently, the compounds were subjected to compression at 160 oC for the optimum curing time with a platen press. 2.4. Characterization Attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy was recorded by a Nicolet iS10 spectrometer from 400-4000 cm-1. The resolution of the instrument was 1 cm-1. X–ray photoelectron spectroscopy (XPS) spectra of HS and HS–s–CZ were collected using a Kratos Axis Ultra with an aluminum Kα source. The samples for ATR–FTIR and XPS testing were extracted with ethanol for 72 h to remove unreacted KH–560 and CZ. Thermogravimetric analysis (TGA) was conducted on a NETZSCH TG209F1 under N2 atmosphere from room temperature to 700 oC (10 °C/min). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted on a ZEISS Merlin and on a JEOL2100, respectively. The heat capacity jump (∆Cp) during glass transition of rubber composites were determined by a NETZSCH DSC 204 F. Rubber composites were heated from –80 oC to 20 oC at a heating rate of 5 oC/min under N2 flow. Heat capacity jump normalized to the rubber weight fraction (∆Cpn) and weight fraction of immobilized rubber ( χim)30 were calculated according to the following two equations: 6


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∆C pn = ∆C P /(1 − w)

(1)

χ im = (∆C p 0 − ∆C pn ) / ∆C p 0

(2)

Where w was the weight fraction of filler, while ∆Cp0 was the heat capacity jump during glass transition of the unfilled rubber. An UCAN UR–2030 rheometer was employed to get the vulcanization parameters of rubber compounds. Tensile strength of rubber composites was tested by an UCAN UT–2060 following ASTM D 412. The relationship between storage modulus and strain of SBR compounds was recorded by an Alpha rubber processability analyzer.

3. Results and discussion 3.1. Surface functionalization of nano hybrid with vulcanization accelerator ATR–FTIR spectra of CZ, HS and HS–s–CZ are shown in Fig. 2a. In the spectrum of HS, the strong bands around 3699 cm-1 and 3626 cm-1 are attributed to the stretching vibrations of the inner–surface hydroxyl groups of HNTs, while the vibrations centered at 1080 cm-1 and 1029 cm-1are assigned to the antisynnetric stretching vibration of Si-O-Si from silica and HNTs. In the spectrum of HS–s–CZ, besides the vibrations of CH2 around 2937 cm-1, 2865 cm-1, 1455 cm-1 and 1427 cm-1, the vibration of C–N is detected to 1308 cm-1 which is corresponding to the vibration at 1310 cm-1 in the spectrum of neat CZ. Moreover, taking the intensity of the strong bands between 3626 cm-1 and 3699 cm-1 as reference, it can be found that the peak intensity of hydroxyl group for HS–s–CZ around 3425 cm-1 is decreased compared to that for HS, because the hydroxyl groups from the surface of HNTs and silica were 7



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consumed by the reaction of HS with silane coupling agent. Considering the fact that scarcely any unreacted CZ molecules were left after being Soxhlet extracted sufficiently, it is demonstrated that CZ has been chemically grafted onto HS surface successfully with the aid of silane coupling agent. To further confirm the successful graft of CZ on the surface HS through the chemical linkage of silane coupling agent, XPS was conducted to detect the S 2p binding energy. As shown in Fig. 2b, no obvious S 2p peak is detected in HS. In contrast, the peak of S 2p is observed in the spectrum of HS–s–CZ and can be split into two peaks. Moreover, the area of the two peaks is nearly the same, corresponding to the ratio of the two kinds of chemical bonded S atoms in CZ structure. Combining the results of ATR–FTIR and XPS, it can be concluded that the CZ molecules have been chemically grafted onto the surface of HS with the aid of KH–560.

Fig. 2. (a) FTIR spectra of CZ, HS and HS–s–CZ; (b) XPS spectra of HS and HS–s–CZ. Fig. 3 shows the TGA curves of HS and HS–s–CZ. It is found that HS loses more weight than HS–s–CZ in the temperature range 30–120 oC. This is because HS 8


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is hydrophilic before organic modification and has larger quantities of adsorbed water. Nevertheless, with the increase of temperature, HS–s–CZ gives worse thermal stability than HS. This behavior is attributed to the pyrolysis of silane and CZ on the surface of HS–s–CZ. According to the difference of weight loss for HS and HS–s–CZ from 120 oC to 700 oC, the loading of CZ on the surface of HS is ca. 1.6 wt%.

Fig. 3. TGA curves of HS and HS–s–CZ. Fig. 4 shows the morphologies of HS and HS–s–CZ. From Fig. 4a, it is observed that silica nanoparticles with the diameter of 10-20 nm are uniformly adsorbed on the surface HNTs to form nano hybrid HS. However, the morphology of HS–s–CZ (Fig. 4b) is quite different with that of HS and covered with a organic layer, which intuitively proves the successful modification of HS by CZ functionalized silane.

Fig. 4. TEM images of (a) HS; (b) HS–s–CZ. 3.2. Effect of HS–s–CZ on the vulcanization performance of SBR compounds 9



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Fig. 5a is the typical vulcanization curves of unfilled SBR and SBR compounds with the filler content of 40 phr. With the loading of filler, the torque of SBR compounds is significantly increased, while there is no obvious difference between SBR/HS and SBR/HS–s–CZ compounds. The vulcanization parameters of SBR compounds, scorch time (t10), optimum cure time (t90) and difference between them (t90-t10) are shown in Fig. 5b and Fig. 5c. The t10 values are monotonically decreased with the increasing filler content and SBR/HS–s–CZ compounds give lower t10 than SBR/HS compounds at any filler content. Compared with those of unfilled SBR, the t90 values of SBR/HS compounds are not affected by the loading of HS. However, at same filler content, the t90 value of SBR/HS compounds is higher than that of SBR/HS–s–CZ compounds. It is accepted that the value of t90-t10 can be used to evaluate the vulcanization rate, and lower t90-t10 represents higher vulcanization rate31. Though higher than that of unfilled SBR when the loading of filler is up to 20 phr, the t90-t10 values of SBR/HS–s–CZ compounds is much lower than those for SBR/HS compounds, indicating that the bonded CZ within an inorganic host structure (HS) has better efficiency to accelerate crosslinking reaction than free CZ. The improved resistance of migration and volatilization of grafted CZ should be responsible for the vulcanization performance of SBR/HS–s–CZ compounds27, 32.

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Fig. 5. Vulcanization parameters of SBR compounds: (a) vulcanization curves of unfilled and 40 phr of filler filled SBR compounds; (b) t10 and t90; (c) t90-t10. 3.3. Payne effect of SBR compounds The loading of rigid filler in rubber matrix leads to the significant change in dynamic properties of rubber compounds. Payne firstly found that the three dimensional structure network constructed by the aggregation of CB affects the dynamic viscoelasticity properties of rubbers. The effect of amplitude-dependence of the dynamic viscoelastic properties of filled rubbers is called Payne effect 33. It has been accepted that weaker Payne effect indicates better filler dispersion34-36. Fig. 6 shows the curves of strain–dependant storage modulus (G’) of unfilled SBR and SBR compounds with the filler loading of 20 phr and 40 phr. The values of G’ for unfilled SBR is much lower than those for SBR compounds with HS or HS–s–CZ and is almost unchanged with increasing stain from 0.5% to 40%. However, the values of G’ for filled SBR is decreased and the downward trend with increasing stain becomes 11



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more apparent when the filler content is up to 40 phr, which is consistent with the so called Payne effect. Due to the surface modification of HS, the Payne effect of SBR/HS–s–CZ compounds with the filler content of 40 phr is much weaker than SBR/HS compounds with the same filler content, revealing the better dispersion of HS–s–CZ in SBR matrix37. The inapparent Payne effect of SBR composites with the filler content of 20 phr when compared to that of SBR composites with the filler content of 40 phr should be attributed to the low filler content in rubber matrix, which limits the formation of three dimensional structure filler network.

Fig. 6. Strain dependence of storage modulus (G’) of the unvulcanizated SBR compounds on strain: (a) unfilled SBR and filled SBR with the filler content of 20 phr; (b) unfilled SBR and filled SBR with the filler content of 40 phr. 3.4. Morphologies of SBR composites Fig. 7 compares the morphologies of SBR/HS and SBR/HS–s–CZ composites with the filler content of 20 phr and 40 phr. When the filler content is 20 phr, as can be seen in Fig. 7a, though HS is relatively uniformly dispersed38, small agglomerates can also be found in SBR matrix. Interestingly, HS–s–CZ is evenly dispersed as individuals into SBR (Fig. 7b). From the insets of Fig. 7a and 7b, the surfaces for HS and HS–s–CZ are still rough as their original morphologies shown in Fig. 4, implying that the mechanical force during the preparation process of rubber composites cannot 12


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destroy the hybrid structure of HS–s–CZ. More importantly, most HS is semi-exposed on the surface SBR matrix, indicating the poor interfacial interaction between HS and SBR. However, HS–s–CZ is totally embedded in the SBR matrix, providing preliminary evidence that HS–s–CZ is more compatible with SBR and strong interfacial interaction is formed between HS–s–CZ and SBR. When the filler content is up to 40 phr, large size of HS aggregate can be found from Fig. 7c. However, HS–s–CZ is still uniformly dispersed in the rubber matrix as shown in Fig. 7d, further confirming the easier dispersion of HS–s–CZ than that of HS in rubber.

Fig. 7. SEM images of SBR composites with different contents of filler: (a) SBR/HS (20 phr); (b) SBR/HS–s–CZ (20 phr); (c) SBR/HS (40 phr); (d) SBR/HS–s–CZ (40 phr). 3.5. Immobilized rubber approaching the filler surface To further investigate the interfacial interactions in SBR composites, the immobilized rubber approaching filler surface is calculated by DSC. The immobilized 13



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polymer layer spans the interfaces between regions of filler and rubber and can be used to reflect the interfacial strength in rubber composites39-42. Fig. 8 compiles the DSC curves of unfilled SBR and SBR composites with the filler content of 40 phr. It is generally known that ∆Cpn at glass transition is closely related to the internal degree of freedom of molecular motion. As shown also in Fig. 8, the ∆Cpn of SBR/HS–s–CZ composites is lower than that of SBR/HS composites, suggesting that more rubber chains are immobilized surrounding HS–s–CZ than those surrounding HS. This is also reflect by the results of the value of χim. The χim of SBR/HS–s–CZ composites is increased by 15% compared with that of SBR/HS composites, demonstrating the stronger interfacial interaction between HS–s–CZ and rubber.

Fig. 8. DSC curves of the glass transition region of unfilled SBR and SBR composites. Proverbially, vulcanization accelerator can participate in the vulcanization reaction of rubber chains. Therefore, the improvement of interfacial interactions in SBR/HS–s–CZ composites may be mainly explained by the chemical bonding between HS–s–CZ and rubber chains. As shown in Fig. 9, the grafted CZ molecules firstly react with ZnO to form highly active zinc complexes and then react with sulfur to form active sulphating agents43. The sulphating agents can react with the allylic 14


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hydrogen atoms of unsaturated rubber chains through the rearrangement and breakage of sulfur bonds. Consequently, HS–s–CZ is chemically grafted to rubber chains, significantly enhancing the interfacial interaction in rubber composites. Besides, the improved compatibility between HS–s–CZ and rubber due to the organic surface modification, together with the physical entanglements between silane molecules and rubber chains also contributes to the superior interfacial interaction44, 45.

Fig. 9. Interfacial reaction in SBR/HS–s–CZ composites during vulcanization. 3.6. Mechanical properties of SBR composites The improved dispersion of HS–s–CZ and its strong interaction with rubber chains should have a positive influence on the mechanical strength of rubber composites. As shown in Fig. 10a, the tensile strength of SBR composites is monotonously increased with the increasing of filler content from 0 to 40 phr, while the elongation at break is only continually increased as the filler content increases from 0 to 20 phr, and then begins to level off due to the decreased extensibility of rubber composites at high filler content. As expected, the tensile strength and modulus (estimated by the stress at 300%, which is reflected by the true stress-strain curves in Fig. 10b) of SBR/HS–s–CZ composites are further improved compared with those for SBR/HS composites and the difference is particularly magnified with the increment in filler loading. For instance, at the filler content of 40 phr, the tensile strength of 15



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SBR/HS–s–CZ composites shows an increase of 45% compared with that of SBR/HS composites. On the other hand, the enhanced interfacial interaction between HS–s–CZ and rubber chains restricts the slippage of rubber chains on the surface of HS–s–CZ during stretch, which leads to the lower elongations at break of SBR/HS–s–CZ composites than those of SBR/HS composites. The stress-strain curves are simulated according to the Mooney-Rivlin equation ∗ −2 −1 as follows: σ = σ / (λ − λ ) = 2C1 + 2C2 λ (3), where σ and σ* are the normal stress

and reduced stress, respectively, while C1 and C2 are constants independent of extension ratio λ. Fig. 10c and 10d compare the Mooney-Rivlin plots of unfilled SBR and SBR composites with the filler content of 30 and 40 phr. Due to the limited extensibility of rubber chains46, upturn appears at high strain in the Mooney-Rivlin plots of all samples and is more prominent in the case of filled SBR composites. The upturn points of SBR/HS–s–CZ composites are at the lower strain than those of SBR/HS composites, indicating that the orientation of SBR molecular chains among the surface of HS–s–CZ particles is easier. As the ability of orientation for rubber chains under tensile loading is critical to the rubber reinforcement47, the excellent mechanical strength of SBR/HS–s–CZ composites are mainly originated from the combination of enhanced interfacial interaction and promoted chains orientation.

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Fig. 10. Mechanical properties of the unfilled SBR and SBR composites (a, b); Mooney-Rivlin plots of the unfilled SBR and SBR composites (c, d). 4. Conclusion Vulcanization accelerator (CZ) was chemically bonded onto a nano hybrid (HS) consisting of

halloysite nanotubes and silica to obtain surface functionalized

nanofiller (HS–s–CZ) for rubber. HS–s–CZ could be evenly dispersed into styrene-butadiene rubber (SBR) with further improved filler-rubber interaction compared with that between HS and SBR. The bonded CZ molecules within an inorganic host structure have better efficiency to accelerate crosslinking reaction than free CZ molecules due to the improved resistance of migration and volatilization of the bonded CZ. Moreover, SBR/HS–s–CZ composites exhibited more excellent mechanical strength than SBR/HS composites containing equivalent accelerator component, which may be a valuable inspiration for reinforcing rubber composites. Acknowledgments 17



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This work was supported by the 973 Program, Grant No.2015CB654700 (2015654703), Fundamental Research Funds for the Central Universities (2017BQ033) and China Postdoctoral Science Foundation (2017M612658). References 1.

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Preparation of Halloysite NanotubesâSilica Hybrid Supported

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