Preparation and Performance of cis-Polybutadiene Rubber Composite Materials Reinforced by Organic Modified Palygorskite Nanomaterials

The hydrophilic character of palygorskite has beenmodified by grafting organic group and controlling surface energy for improving compatibility of palygorskite in rubber matrix using palygorskite as cis-polybutadiene rubber fillers. The effects of coupling modification on the performance of cis-polybutadiene rubbermaterials filled with palygorskite were investigated, and the influence of coupling agent dosage on their mechanical properties was also studied.The results indicated that the mechanical performance of cis-polybutadiene rubbermaterials reinforced bymodified palygorskite could be improved significantly, and the tensile strength and tearing strength increased by 122.5% and 107.6% at the optimal dosage (15%) of coupling agent 3-mercaptopropyl trimethoxysilane. Moreover, the reinforcement mechanism of rubber composite materials as prepared was also analyzed.


Introduction
cis-Polybutadiene rubber materials have the characteristics of wear resistance, excellent elastic, age resistance and so forth, and thus they have become essential in many fields [1][2][3].Nevertheless, cis-polybutadiene rubber materials as organic materials could not form crystals at room temperature unless they are sufficiently stretched and have disadvantages of low compressive strength, high cost, and poor dimensional stability, and thus their stress-induced crystallization is obviously lower than that of natural rubber [4,5].Consequently, fillers have been widely used in rubber products, which could enhance the performance of rubber composites and reduce the production cost.The carbon black is the prime fillers in rubber industry, whose process exacerbates tensions in the energy supply and pollutes the environment.The studies on energy saving and environment-friendly fillers have focused on polymer composites along with the emphasis on the environmental protection.
Natural mineral materials such as sepiolite, montmorillonite, and kaolin have been applied as fillers of polymers for various composite materials with excellent performance [6][7][8][9][10][11], because applying natural mineral materials will solve the serious environmental problem caused by carbon black production and so forth.Palygorskite is typical natural fibrillar silicate clay with large reserves in South China.The chemical structure of palygorskite is Mg and its smallest structure unit is fibrillar single crystal with a diameter of 20-40 nm [12][13][14][15].Unlike the layerlayer interaction in layered silicates, the interaction between palygorskite single crystals is extremely weak due to similar line-line contact, and voluminous interstitial spaces are in these agglomerated single crystals [16,17].Palygorskite as a kind of filler has hydrophilic characteristics, which limit the dispersibility of mineral powder in rubber matrix and the application of mineral fillers in rubber industry.Surface modification is widely applied to overcoming this limitation of mineral fillers.The first method is performed based on polar functional oligomer.The second method is carried out through predispersing clay compatible with cispolybutadiene rubber.The main purpose of the above two methods is to improve interfacial interaction, and thus the properties of nanocomposites are strongly influenced by the nature of filler-matrix interface [18][19][20][21].However, there is only a few reports about cis-polybutadiene rubber nanocomposites containing modified palygorskite nanomaterials.
The objective of this work is to develop the cispolybutadiene rubber composite materials using palygorskite minerals as reinforced fillers.The strong interaction caused by the coupling reaction could improve the dispersity effect of the palygorskite in the cis-polybutadiene rubber matrix, and the corresponding properties of the resulting nanocomposites are also systematically investigated.
Under the condition of acid, silane coupling agent hydrolysis was performed by means of mixing silane coupling agent in the water solution of ethanol.Then, an appropriate amount of palygorskite was added to the mixture with vigorous stirring at 70 ∘ C. The products were filtered and washed several times with deionized water.The filter cake was taken out and then dried by airing.In order to remove water sufficiently, the modified palygorskite was ground using a mortar and a pestle and then dried at 110 ∘ C in oven until the weight was not changed.The rubber composites filled with palygorskite minerals were prepared.cis-Polybutadiene rubber, modified palygorskite minerals, and other compounding ingredients such as zinc oxide, stearic acid, zinc oxide, accelerator CZ, antioxidant RD, emollient, and sulphur were mixed using a laboratory-sized 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 above composites were vulcanized at platen press with 13 MPa pressure, based on the 90 values.
The 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.On the above basis, the surface free energy values were calculated using the contact angles of three kinds of liquid by Wu's equation [22,23].The microstructure of the samples was observed by scanning electron microscopy (Philips XL30) at 25.0 kV and 30 A.Dumb-bell 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 (CMT6104).

Results and Discussion
Figure 1 shows the variation of surface free energy of palygorskite after modification with different coupling agent (3mercaptopropyl trimethoxysilane) addition amounts.From Figure 1, it can be seen that the palygorskite after modification with the coupling agent addition amount of 15% has the lowest surface free energy.When powders are dispersed into media, the lower surface free energy has positive influence on their dispersion [24][25][26].According to the above analysis, 15% is selected as the coupling agent addition amount for the palygorskite in order to obtain the best modification and dispersion effect of palygorskite in the composite materials.
To study the influence of coupling agent for palygorskite on mechanical properties of cis-polybutadiene rubber composite materials, five kinds of coupling agent for palygorskite are chosen based on the above optimal coupling agent (3-mercaptopropyl trimethoxysilane) 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 different kinds of coupling agent modified palygorskite are close to each other.Among them, rubber composite materials filled with isopropyl tri(dioctylpyrophosphate) titanate modified palygorskite 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 [27,28].
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 with different kinds of coupling agent modified palygorskite is different obviously.Among them, rubber composite materials filled with isopropyl tri(dioctylpyrophosphate) titanate modified palygorskite 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.The above results indicate that coupling agent modified palygorskite could enhance the mechanical properties of rubber composite materials effectively.Moreover, rubber composite materials filled with 3-mercaptopropyl trimethoxysilane modified palygorskite have the highest tensile strength.As shown in Figure 2, the rubber composite materials filled with isopropyl tri(dioctylpyrophosphate) titanate and isopropyl tri(dioctylpyrophosphate) titanate modified palygorskite have similar tearing strength.Therefore, 3-mercaptopropyl trimethoxysilane is chosen as the optimal coupling agent.Figure 4 shows the variation of tearing strength of cispolybutadiene rubber composite materials reinforced by palygorskite modified by different coupling agent 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 17% 3mercaptopropyl trimethoxysilane modified palygorskite have the best tearing strength.
Figure 5 shows the tensile strength results of cispolybutadiene 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 of modified palygorskite are different obviously.Among them, rubber composite materials filled with 15% 3mercaptopropyl trimethoxysilane palygorskite have the best tensile strength.As shown in Figure 4, the rubber composite materials filled with 15% and 17% 3-mercaptopropyl trimethoxysilane modified palygorskite have similar tearing strength.Therefore, 15% 3-mercaptopropyl trimethoxysilane is chosen as the optimal coupling agent amount.
It is known that the dispersion of a filler in the polymer matrix can have a significant effect on the mechanical properties of the composites, and good dispersion can be achieved by surface modification of the filler particles and appropriate processing conditions [29][30][31].The dispersibility of modified palygorskite in the cis-polybutadiene rubber matrix is confirmed by SEM shown in Figure 6, the dispersibility of modified palygorskite in the cis-polybutadiene rubber matrix is improved obviously compared with that of the unmodified ones (Figure 6(a)), and the average diameter of modified palygorskite is much less than 100 nm.
In the case of nanocomposites containing 15% 3-mercaptopropyl trimethoxysilane modified palygorskite, most palygorskite nanofibers aggregates are broken down to primary particles, which could maximize the interfacial interaction between the palygorskite and the polymer matrix.Accordingly, the main reason for the obvious enhancement of mechanical performance lies in the good dispersion of modified palygorskite nanofibers in the cis-polybutadiene rubber matrix at a nanometer scale and the strong interaction between palygorskite and cis-polybutadiene rubber, which exhibits nanometer effect and physical cross-link of palygorskite.Due to the transferring stress and limiting the expansion of palygorskite cracks, the modified palygorskite nanomaterials could improve the mechanical properties of cis-polybutadiene rubber.

Conclusions
In this paper, the hydrophilic character of palygorskite was modified by grafting organic group and controlling surface energy for improving compatibility of palygorskite in rubber matrix, and the palygorskite minerals as prepared were used as cis-polybutadiene rubber fillers.The results showed that the mechanical properties of cis-polybutadiene rubber composite materials reinforced by modified palygorskite could be improved obviously.When the optimum dosage of coupling agent 3-mercaptopropyl trimethoxysilane was 15%, the tensile strength and tearing strength increased by 122.5% and 107.6%, respectively.The reason for the above phenomenon was that nanometer effect and physical crosslink of palygorskite nanofibers shown in the microstructure of cis-polybutadiene rubber composite materials fracture surface could maximize the interfacial interaction between polymer matrix and palygorskite.

Figure 4 :
Figure 4: Variation of tearing strength of cis-polybutadiene rubber composite materials reinforced by palygorskite modified by different coupling agent (3-mercaptopropyl trimethoxysilane) addition amounts (errors in tearing strength values (-axis) are designated as the vertical error bars).

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