Effect of Modified Sepiolite Nanofibers on Properties of cis-Polybutadiene Rubber Composite Nanomaterials

In order to use sepiolite nanofibers as cis-polybutadiene rubber fillers, the hydrophilic character of sepiolite nanofibers should be modified by grafting organic group and controlling surface free energy for improving compatibility of sepiolite nanofibers in rubber matrix. The relationship between the performance of the cis-polybutadiene rubber filled with sepiolite and the coupling modification was investigated, and the influence of coupling agent γ-(2,3-epoxypropoxy)propyltrimethoxysilane dosage onmechanical properties of cis-polybutadiene rubbermaterials was also studied.The results showed that themechanical properties could be improved obviously after reinforcement by modified sepiolite nanofibers. The optimum dosage of coupling agent γ(2,3-epoxypropoxy)propyltrimethoxysilane was 7%, and the tensile strength and tearing strength increased by 108.3% and 74.1%, respectively. On this basis, the reinforcement mechanism of the composite rubber materials was also discussed.


Introduction
Nowadays, cis-polybutadiene rubber materials play a critical role in many fields.Owing to the characteristics of cold tolerance, wear resistance, excellent elastic, age resistance, and so forth, cis-polybutadiene rubber materials have become a kind of useful product in the fields of rubber shoes, rubberized fabric, automotive sponge, and so forth [1][2][3].However, cispolybutadiene rubber materials belong to organic materials, which have disadvantages of low compressive strength, high cost, and poor dimensional stability, especially not being able to form crystals at room temperature unless it is sufficiently stretched (stress-induced crystallization), and thus the stressinduced crystallization of cis-polybutadiene rubber materials is obviously lower than that of natural rubber [4][5][6].Therefore, cis-polybutadiene rubber fillers have been developed in the recent years.The prime fillers in rubber industry such as carbon black production always make reinforced rubber deep color, which limits its applications in medical, sports, and domestic products.Moreover, the above fillers will consume large amounts of energy such as petroleum and natural gas and emit lots of heat and waste gas composed by the prime greenhouse gas of carbon dioxide and gaseous pollutants of sulfur dioxide and nitrogen oxide.This process would not only exacerbate tensions in the energy supply, but also pollute the environment [7][8][9][10].
Comparing with the former, the application of natural mineral materials as cis-polybutadiene rubber fillers will solve the serious environmental pollution problem.Sepiolite is a kind of porous mineral materials widely existing in nature in the form of fiber assemblies, leading to the limitation of usage [11][12][13][14][15].Although the defibering treatment of sepiolite as a simple and cheap way to obtain mineral nanofibers is available for practical use, it has been less investigated except for a few exploratory researches which involve the destruction of its fibrous micromorphology by ball milling and the use of expensive and sensitive agents [16][17][18][19][20]. Therefore, the recent studies on sepiolite minerals reinforced composite materials have not fully explored the potential of sepiolite minerals [21][22][23][24][25][26], which provides promising future for further research.In our previous research, sepiolite nanofibers have been prepared using dry method airflow defibering technique [12].
In this paper, the cis-polybutadiene rubber composite materials were prepared using sepiolite mineral nanofibers as reinforced fillers, which have been reported scarcely in the literature.And their mechanical properties were also systematically investigated.

Experimental Section
The raw sepiolite samples which are one of natural fibrillar minerals were supplied by LB Nano-Material Technology Co., Ltd.The titanate coupling agent, NDZ-201 (isopropyl tri(dioctylpyrophosphate)titanate), and silane coupling agents, SG-Si998 (bis[-(triethoxysilyl)propyl]disulfide), KH560 (-(2,3-epoxypropoxy)propyltrimethoxysilane), KH570 (3-(Methacryloyloxy)propyltrimethoxysilane), and KH590 (3-mercaptopropyltrimethoxysilane) were supplied by Shuguang Chemical Group Co., Ltd.The cis-polybutadiene rubber was supplied by Yanshan Petrochemical Co., Ltd.The compounding ingredients were purchased from chemical stores.500.00g of natural sepiolite samples were added into 3500 mL of water solution containing 175 mL of hydrochloric acid.Secondly, the mixture was stirred at speed of 2000 rpm for 2 h by high-speed dispersing machine, and then the sepiolite solid was separated from the admixture by vacuum filtering and washed several times with distilled water until chlorine ion was not tested with silver nitrate, dried at 60 ∘ C, and then the dry cake was crushed and ground into powders using the 20 mesh sieves.Then the above acid purified sepiolite samples were superfined by jet mill grinding.The grinding room air pressure is controlled between 0.7 and 0.85 MPa, the bearing protection air pressure is 0.2 MPa, the wash air pressure is 0.15 MPa, and the feeding rate is 6 kg/h [12].Contact angle of powder was measured using capillary penetration measurements.In this work, the powder was placed in a glass tube with a filter on the bottom.The tube was attached to an electrobalance (DataPhysics DCAT21), which could record the weight gain as a function of time when the bottom of the tube touched the testing solvent.The contact angle was calculated using the following equation: where  lv is the liquid-vapor interfacial tension,  is the viscosity of the liquid,  is the density of the liquid,  is the contact angle which needs to be measured, and  is the weight of the liquid which penetrates into the capillary. is a parameter contributed by capillary and is calculated by the  of n-Hexane whose contact angle can be considered as 0 ∘ .The surface energy (SE), dispersive interaction (Disp.), and polar interaction (Polar) were calculated using the contact angles of three kinds of liquid by Wu's equation: where   is the liquid interfacial tension,    and    are the dispersive interaction and polar interaction of solid, respectively,    and    are the dispersive interaction and polar interaction of liquid, respectively, and  is the contact angle which is measured before [27][28][29].The rubber composites filled with sepiolite mineral nanofibers were prepared.cispolybutadiene rubber, modified sepiolite mineral nanofibers, and other compounding ingredients such as zinc oxide, stearic acid, zinc oxide, accelerator CZ, antioxidant RD, emollient, and sulphur were mixed using a laboratorysized two-roll mill.The optimum cure time (90) which is the time for the completion of cure was determined at 145 ∘ C using a curometer.The composites were vulcanized at platen press with 13 MPa pressure, based on the 90 values.The microstructure of the samples was observed by scanning electron microscopy (Philips-XL30) at 25.0 kV and 30 A.Dumbbell shaped specimens were punched from the moulded sheets by a tensile specimen cutter.Tensile strength and elongation at break were measured following GB/T 528-1998 using a universal tensile testing machine (CMT-6104).

Results and Discussion
Figure 1 shows the variation of surface free energy of sepiolite nanofibers after modification with different coupling agent addition (-(2,3-epoxypropoxy)propyltrimethoxysilane) amounts.
From Figure 1, it can be seen that the sepiolite nanofibers after modification with the coupling agent (-(2,3-epoxypropoxy)propyltrimethoxysilane) addition amount of 5% have the lowest surface free energy.When powders are dispersed into media, the lower the surface free energy, the better their dispersion [30][31][32].According to the above analysis, 5% is selected as the coupling agent (-(2,3-epoxypropoxy)propyltrimethoxysilane) addition amount for the sepiolite nanofibers in order to obtain the best modification effect of sepiolite nanofibers and the optimal properties of composite materials.To study the influence of coupling agent for sepiolite nanofibers on mechanical properties of cis-polybutadiene rubber composite materials, five kinds of coupling agents for sepiolite nanofibers are chosen based on the above optimal coupling agent (-(2,3-epoxypropoxy)propyltrimethoxysilane) addition amount.The tearing strength results of rubber composite materials are shown in Figure 2.
From Figure 2, we can see that the tearing strength values of rubber composite materials filled with sepiolite nanofibers modified with different kinds of coupling agent are close to each other.Among them, rubber composite materials filled with isopropyl tri(dioctylpyrophosphate)titanate modified sepiolite nanofibers have the best tearing strength.The reason for the phenomenon is mainly that isopropyl tri(dioctylpyrophosphate)titanate belongs to lipid coupling agent, which has relatively long chain and good toughness.Figure 3 shows the tensile strength results of cispolybutadiene rubber composite materials.
From Figure 3, it can be seen that the tensile strength of rubber composite materials filled by different kinds of coupling agent modified sepiolite nanofibers are different obviously.Among them, rubber composite materials filled with isopropyl tri(dioctylpyrophosphate)titanate modified sepiolite nanofibers have the lowest tensile strength.The reason for the phenomenon is mainly that isopropyl tri(dioctylpyrophosphate)titanate belongs to lipid coupling agent, which has physical winding rather than chemical bond [33,34].The above results indicate that coupling agent modified sepiolite nanofibers could enhance the mechanical properties of rubber composite materials effectively.Moreover, rubber composite materials filled with sepiolite nanofibers modified by -(2,3-epoxypropoxy) propyltrimethoxysilane have the highest tensile strength.As shown in Figure 2, the rubber composite materials filled with -(2,3-epoxypropoxy)propyltrimethoxysilane and isopropyl tri(dioctylpyrophosphate)titanate modified sepiolite nanofibers have similar tearing strength.Therefore, -(2,3epoxypropoxy)propyltrimethoxysilane is chosen as the optimal coupling agent.Figure 4 shows the variation of tearing strength of cis-polybutadiene rubber composite materials reinforced by sepiolite nanofibers modified by different coupling agent (-(2,3-epoxypropoxy)propyltrimethoxysilane) addition amounts.
From Figure 4, we can see that the tearing strength values of rubber composite materials filled with different coupling agent addition amounts are different obviously.Among them, rubber composite materials filled with 6% -(2,3-epoxypropoxy)propyltrimethoxysilane modified sepiolite nanofibers have the best tearing strength.Figure 5 shows the tensile strength results of cis-polybutadiene rubber composite materials.From Figure 5, we can see that the tensile strength values of rubber composite materials filled with different coupling agent addition amounts are different obviously.Among them, rubber composite materials filled with 7% -(2,3-epoxypropoxy)propyltrimethoxysilane modified sepiolite nanofibers have the best tensile strength.As shown in Figure 4, the rubber composite materials filled with 6% and 7% -(2,3-epoxypropoxy)propyltrimethoxysilane modified sepiolite nanofibers have similar tearing strength.Therefore, 7% -(2,3-epoxypropoxy)propyltrimethoxysilane is chosen as the optimal coupling agent amount.The main reason for this phenomenon lies in the good dispersion of modified sepiolite nanofibers in the cis-polybutadiene rubber matrix at a nanometer scale and the strong interaction between sepiolite nanofibers and cis-polybutadiene rubber, which exhibits nanometer effect and physical crosslink of sepiolite nanofibers.Due to transferring stress and limiting the cracks expansion of sepiolite nanofibers, the modified sepiolite nanofibers can improve the mechanical properties of cispolybutadiene rubber.
It is known that the dispersion of the fillers in the polymer matrix has significant effect on the mechanical properties of the composites [35,36].A good dispersion can be achieved by surface modification of the filler particles and appropriate processing conditions.As shown in Figure 6, the dispersibility of modified sepiolite nanofibers in the cis-polybutadiene rubber matrix is improved obviously compared with the unmodified ones (Figure 6(a)), and the average diameter of modified sepiolite nanofibers is less than 100 nm.For the cis-polybutadiene rubber nanocomposites containing 7% -(2,3-epoxypropoxy)propyltrimethoxysilane modified sepiolite nanofibers shown in Figure 6(b), most sepiolite nanofibers aggregates are broken down to primary particles, which should maximize the interfacial interaction between the sepiolite nanofibers and the polymer matrix.

Conclusions
The hydrophilic character of sepiolite nanofibers should be modified by grafting organic group and controlling surface energy for improving compatibility of sepiolite nanofibers in rubber matrix, in order to use sepiolite nanofibers as cis-polybutadiene rubber fillers.It can be concluded that the mechanical properties can be improved obviously after reinforcement by modified sepiolite nanofibers.The tensile strength and tearing strength increased by 108.3% and 74.1%, respectively, when the optimum dosage of coupling agent -(2,3-epoxypropoxy)propyltrimethoxysilane was 7%.Microstructure of cis-polybutadiene rubber composite materials fracture surface exhibits nanometer effect and physical crosslink of sepiolite nanofibers, which should maximize the interfacial interaction between the sepiolite nanofibers and the polymer matrix.

Figure 5 :
Figure 5: Variation of tensile strength of cis-polybutadiene rubber composite materials reinforced by sepiolite nanofibers modified by different coupling agent (-(2,3-epoxypropoxy)propyltrimethoxysilane) addition amounts (errors in tensile strength values (-axis) are designated as the vertical error bars).