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Vulcanization Accelerator Functionalized Nanosilica: Effect on the Reinforcement Behavior of SSBR/BR.

INTRODUCTION

Along with the enforcements of more and more harsh regulations and laws for environmental protection, researchers have made enormous attempts to develop new generation of tires with improved energy conservation capability, fuel efficiency, weathering resistance, and abrasion resistance, thereby achieving energy saving, environmental protection, and atmospheric pollution control [1, 2]. To keep pace with such a trend and establish the raw material ground for "green tire", engineers need to develop high-performance rubber material possessing outstanding mechanical properties, good abrasion resistance, weathering resistance as well as low rolling resistance [3, 4]. Nanosilica, one kind of widely used reinforcing filler for rubber, could be of special significance, because it can improve the wet skid resistance of rubber materials and significantly reduce hysteresis loss, abrasion resistance, and rolling resistance [5,6]. However, silica particles exhibit poor process ability and dispersion as well as weak interactions with rubber matrices, which restricts their application in green tire treads [7].

The abovementioned drawbacks of silica nanoparticles, fortunately, could be overcome through the surface modification to reduce the amount of hydroxyl groups, thereby enhancing the uniform dispersion of silica nanoparticles in rubber matrix and improving its compatibility with the rubber matrix [8-11]. For such a purpose, silane coupling agent like bis-(triethoxysilylpropyl)-tetrasulfide (denoted as TESPT; also called Si69) is often used as the organic surface-modifying agent [12-16]. Moreover, in recent years, the rubber additives modified nanosilica have been researched, for example, ZnO nanoparticles anchored on silica surface are of particular significance, because they can improve the curing efficiency of rubber [17]. Silica and graphene oxide modified with N-cyclohexyl-2-benzothiazole sulfenamide are worth of special attention, because they exhibit reduced agglomeration and can improve the mechanical properties of solution polymerized styrene butadiene rubber (denoted as SSBR) [18, 19]. Moreover, rubber additives such as 2-mercaptobenzimidazole [20, 21], ethylenethiourea [22], and benzothiazole-2-thiol [23] are also suitable modifiers for silica filler. The results showed that they can improve the dispersion of the silica.

The abovementioned surface-modification methods that rubber additives as modifier not only can reduce the processing steps, but also can improve the dispersion of additives. In our previous work, the obtained silica-supported rubber accelerator CBS (DNS-CZ) enhanced the filler-rubber compatibility, thereby greatly improving the mechanical properties and abrasion resistance [24]. Also, because of the common usage of DPG in the formulations of tire tread compound, the DPG was used as a modifier agent. We intend to prepare a series of dispersible nanosilica modified by rubber accelerator diphenyl guanidine (denoted as DPG) through in situ liquid-phase surface modification in the present research. This article reports the preparation of DNS-DPG filler, which was characterized by FTIR, TGA, DLS, and TEM. In the meantime, we adopt the as-obtained DNS-DPG as a filler to reinforce SSBR/BR (BR refers to butadiene rubber) so as to improve the properties of the rubber. Additionally, the effects of DNS-DPG on cure characteristics, mechanical properties, dynamic rheological properties, abrasion resistance, and dynamic mechanical properties were investigated.

MATERIAL AND METHODS

Materials

SSBR (SSBR5025) and BR (BR-CB24) were purchased from Germany Lanxess Chemical Industry Co., Ltd. Dispersible nano-Si[O.sub.2] (DNS-DPG) was prepared at our laboratory (reference to 2.2). Silane coupling agent TESPT was purchased from Nanjing Pinning Coupling Agent Co., Ltd. Silane coupling agent [gamma]-(2, 3-epoxypropoxy) propytrimethoxysilane (KH560) was purchased from Juli Chemical Co., Ltd (Zhengzhou, China). Analytical reagent absolute ethanol and other additives were also commercially obtained.

Preparation of DNS-DPG

Dispersible nano-Si[O.sub.2] was prepared with sodium silicate as the precursor through liquid phase in situ surface chemical modification [25]. DNS-DPG was prepared by classical ring-opening reaction of the epoxy group of KH560 with amino group of rubber accelerator DPG. Figure 1 schematically showed the synthesis route. Briefly, a proper amount of rubber vulcanization accelerator DPG was added into the mixture of KH560 and industrial ethanol. The mixture was stirred at 50[degrees]C in a reflux condenser for 2 h under nitrogen atmosphere to obtain modified DPG (denoted as m-DPG). In the meantime, 9.43 kg of sodium silicate and 28 kg of distilled water were added into anotherreactor; and the resultant mixture was stirred at 50[degrees]C for 0.5 h, followed by the addition of 4.2 kg of dilute sulfuric acid and stirring for 2 h at 80[degrees]C to obtain nanosilica suspension. Finally, m-DPG was added into the nanosilica suspension and stirred at 80[degrees]C for 2 h; and the as-obtained solid was fully washed with distilled water to remove residual inorganic ions and spray dried to afford rubber additive DNS-DPG. A series of DNS-DPG additives, denoted as DNSDPG-0, DNS-DPG-1, DNS-DPG-2, DNS-DPG-3 and DNS-DPG4 (the numeral suffixes refer to different dosages of the surface-modifiers; see Table 1), respectively, were prepared by properly adjusting the dosage of the surface-modifiers.

Preparation of SSBR/BR/DNS-DPG Composites

The formulations for preparing various SSBR/BR/DNS-DPG composites were listed in Table 2. Briefly, SSBR/BR matrix was pulverized with a two-roll mill (Shanghai Light Industry Machinery Co., Ltd); and DNS-DPG, TESPT, stearic acid, zinc oxide, rubber vulcanization accelerators N-cyclohexyl-2-beozothiazole sulfenamide (CBS) and DPG, paraffin wax, antioxidant N-l,3-dimethylbutyl-N'-phenylp-phenylenediamine (6PPD) and sulfur were sequentially and directly compounded with SSBR/BR matrix at room temperature. The total amount of accelerators DPG grafted and ungrafted was fixed at 2phr. It should be pointed out that the content of grafted DPG determined by the loss weight after being heated to 800[degrees]C in thermogravimetric analysis (TGA) for DNS-DPG was calculated as part of the accelerator amount to make sure that all of the compounds contained equivalent accelerator component. The optimum cure time of the composites at 180[degrees]C was determined with a No-Rotor Vulkameter GOTECH M-3000 (prior to test, the composites were stored at room temperature for 8-12 h). The vulcanizates were cut into specimens with desired sizes according to ASTM D412 (USA) and ASTM D624 (USA), respectively.

Characterizations

Characterizations of as-Obtained DNS-DPG. FTIR analysis was conducted with a Nicolet 170sx FTIR spectrometer (Thermo Nicolet Co., USA) to investigate whether the rubber vulcanization accelerator DPG is grafted to the surface of silica. The samples for FTIR analysis were pressed into thin KBr tablets and tested in the wavenumber range of 400-4,000 [cm.sup.-1].

Thermogravimetric analysis (TGA) of DNS-DPG under air atmosphere was conducted with a TGA/SDTA851e thermal balance (METTLER TOLEDO, Switzerland) at a heating rate of 10[degrees]C/min in the temperature range of 25~800[degrees]C, with which 3-5 mg of the as-prepared composites was tested.

The morphology and particle size of DNS-DPG were investigated with a JEM 2010plus transmission electron microscope (TEM; Japan Electronics Co., Ltd., Japan). The to-be-tested composites were dispersed in absolute ethanol and dropped to copper grids before the TEM observations.

Cure Test of SSBR/BR/DNS-DPG Composites. The optimum cure time [Tc.sub.90] and the scorch time [Tc.sub.10] of the as-prepared SSBR/BR/DNS-DPG composites at 180[degrees]C were determined with a GOTECH M-3000 rotorless curemeter (Gotech Testing Machines, Inc., Taiwan, China).

Scanning Electron Microscopy of SSBR/BR/DNS-DPG Composites.

The dispersion of DNS-DPG in the rubber matrix was tested by a Nova Nano 450 field emission scanning electron microscope (FEI Company). The vulcanized rubber was placed in liquid nitrogen and fractured. The surface of sample was covered with gold to exam.

Mechanical Properties of SSBR/BR/DNS-DPG Vulcanizates. The

mechanical properties of the rubber vulcanizates, including stress-strain, tensile strength, elongation at break, tear strength, and reinforcing index, were measured with a GOTECH TCS-2000 universal testing machine (Gotech Testing Machines, Inc., Taiwan, China) at a speed of 500 mm/min according to ASTM D412 and ASTM D624, respectively. Five repeat measurements were conducted for each sample, and the averages of the repeat measurements are reported in this article.

Dynamic Rheological Properties of SSBR/BR/DNS-DPG Composites.

The dynamic rheological behavior of SSBR/BR/DNS-DPG was evaluated with a DISCOVERY-HR2 hybrid rheometer (TA DHR-2) at a temperature of 60[degrees]C in the shear oscillation mode with parallel plates: the diameter of the parallel plate is 8 mm and the axial force is 20 N. The frequency sweep tests of SSBR/BR/DNS-DPG were performed in the strain amplitude of frequency sweep range from 0.01% to 10% under an angular frequency of 10 rad/s.

Abrasion Resistance Measurements. The abrasion resistance of SSBR/BR/DNS-DPG composites was determined with an Akron abrasion tester MZ-4061(Jiangsu Mingzhu Testing Machinery Co., Ltd., Jiangsu, China) according to the Akron method. Three repeat measurements were conducted for each DNS-DPG additive, and the average of the repeat tests is reported in this article. The volume loss is calculated as:

V = [m.sub.1] - [m.sub.2]/[rho] x [W.sub.1] x ([W.sub.1] - [W.sub.2])

Where V is the volume loss ([cm.sup.3]) of the tested composite; mand m2 are the mass (g) of the composites before and after test; p is the density of water (g/[cm.sup.3] ); [W.sub.1] and [W.sub.2] are the mass (g) of the composite specimens in air and in water, respectively.

Dynamic Mechanical Properties (DMA) of SSBR/BR/DNS-DPG Composites. The DMA of SSBR/BR/DNS-DPG composites (rectangular shape, size 30 x 6 x 2 [mm.sup.3]) were measured with a DISCOVERY-HR2 hybrid rheometer (TA DHR-2) in the tension mode. The measurements were conducted in the temperature range of -100 ~ 100[degrees]C at a heating rate of 5[degrees]C/min and an angular frequency of 10 rad/s.

Bound Rubber of SSBR/BR/DNS-DPG Compounds. The bound rubber of SSBR/BR/DNS-DPG compounds was tested at room temperature. About 0.5 g of each compound was weighed ([m.sub.1]), wrapped in filter paper and outside wrapped with copper grids, which was covered with 100 mL toluene for 7 days and replaced with solvent every 2 days. Then the sample was covered with 100 mL acetone solvent for 2 days to remove toluene. And last the samples were dried in an oven at 50[degrees]C to a constant weight ([m.sub.2]). The bound rubber ([W.sub.B]) of the samples could be calculated by

[W.sub.B] = ([m.sub.2] - [m.sub.1] * [W.sub.a])/[m.sub.1] * [W.sub.b]

Where, [W.sub.B] is the bound rubber content, [W.sub.a] is the mass fraction of the filler phase in the compound, and [W.sub.b] is the mass fraction the rubber phase in the compound.

RESULTS AND DISCUSSION

Structure Characterization of DNS-DPG by FTIR and TGA

Figure 2 showed the FTIR spectra of the unmodified silica DNS-DPG-0 and modified silica DNS-DPG-4 (the FTIR spectra of other DNS-DPG series additives were similar, and they were not shown here). The characteristic absorption peaks at 3435 and 1,075 [cm.sup.-1] corresponded to the stretching modes of Si-OH, adsorbed water and Si-O-Si. Compared with DNS-DPG-0, DNS-DPG-4 showed the absorbance peaks 1,563 [cm.sup.-1] (assigned to the bending vibrations of C-H) as well as the absorbance peak at 1384 [cm.sup.-1] (assigned to the stretching vibration of C-N), which indicated that the rubber vulcanization accelerator DPG was successfully bonded to the surface of silica through chemical reaction.

The TGA results of various DNS-DPG nanoparticles were shown in Fig. 3 and Table 3. It can be seen that all the nanosilica displayed the weight loss at 30-120[degrees]C, which was attributed to the removal of adsorbed water. At 120-800[degrees]C, the weight loss of silica was due to the degradation of the organic functional groups. The weight loss of modified silica was higher than the unmodified silica DNS-DPG-0, and the weight loss of the DNS-DPG increased with increasing amount of vulcanization accelerator DPG. This also proved that the vulcanization accelerator DPG was successfully grafted onto the surface of silica.

Size Distributions of DNS-DPG Determined

Figure 4 showed the TEM images of various DNS-DPG composites, where the TEM image of DNS-DPG-0 nanoparticle had a particle size of 30-40 nm and showed signs of agglomeration. The DNS-DPG-3, with a higher dosage of DPG, exhibited a particle size of 20-30 nm and seemed to be free of aggregates (Fig. 4b). The size distributions of DNS-DPG composites determined by DLS are shown in Supporting Information Fig. SI.The other DNS-DPG exhibited a smaller particle size and a narrower size distribution. Moreover, with the increase of the DPG content, the peaks of particle size shifted toward small size side. DNS-DPG-4 nanoparticle had the smallest particle size of 50 nm. This indicated that rubber vulcanization accelerator DPG as the surface-modifier can effectively prevent nanosilica from agglomeration, which should be favorable for DNS-DPG fillers to improve the reinforcing efficacy for rubber.

Cure Behavior of SSBR/BR/DNS-DPG Vulcanizates

The cure parameters and MDR graph of SSBR/BR/DNS-DPG nanocomposites were listed in Table 4 and Supporting Information Fig. S2. It can be seen that the optimum cure time [Tc.sub.90], the scorch time [Tc.sub.10], and the difference ([Tc.sub.90]-[Tc.sub.10]) between [Tc.sub.90] and [Tc.sub.10] of SSBR/BR/DNS-DPG nanocomposites tend to decrease with increasing fraction of DPG in DNS-DPG filler, indicating that the introduction of the DNS-DPG filler with a higher content was beneficial to increasing the vulcanization rate of the rubber nanocomposites. A shorter [Tc.sub.10] often refers to a reduced processing safety to some content. That is to say, after being grafted to the surface of silica, DPG was still able to accelerate the crosslinking reaction and its dispersion in rubber matrix rises, thereby increasing the rate of vulcanization.

The difference between maximum torque (MH) and minimum torque {ML), that is, MH-ML can reflect the rubber-filler interactions [22, 26]. SSBR/BR/DNS-DPG exhibited a high MH-ML, which indicates that the SSBR/BR/DNS-DPG have strong rubber-filler interaction. Particularly, SSBR/BR/DNS-DPG-3 exhibited the highest MHML value among all the tested rubber-matrix composites, which meant that the rubber-filler interaction was the strongest in SSBR/BR/DNSDPG-3 nanocomposite.

This was because of the abundant silanol groups on the unmodified DNS-DPG-0 surface that may absorb the curing agents and reduced the effective amount of vulcanization agent, leading to a retarded vulcanization reaction [8]. After being grafted to the surface of silica, DPG still continue to have active group of guanidine accelerators to accelerate the vulcanization reaction, also could improve the dispersion, thereby increasing the rate of vulcanization.

Dispersion of Silica in SSBR/BR Vulcanizates

The dispersion of silica in rubber vulcanizates was determined by SEM. Figure 5 showed the SEM images of SSBR/BR nano-composites filled with DNS-DPG-0, m-DPG and DNS-DPG-3, respectively. In Fig. 5a, the DNS-DPG-0 was unevenly dispersed in SSBR/BR vulcanizates. A large number of agglomerate was observed. This was because the surface of unmodified silica (DNS-DPG-0) existed abundant silanol groups and the formation of hydrogen bonds between silanol groups [22]. It can be seen from Fig. 5b that the dispersion of m-DPG was not improved obviously in comparison to DNS-DPG-0. For DNS-DPG-3 (Fig. 5c), the silica particles were more uniformly dispersed in SSBR/BR. This can be attributed that the silanol groups were reacted and the accelerator DPG was grafted on the silica surface, thus the agglomerates were reduced and changed the surface of DNS-DPG-3 from hydrophilic to hydrophobic, which made the compatibility between DNS-DPG-3 and rubber matrix improve [21, 24].

Bound Rubber of SSBR/BR/DNS-DPG Compounds

The bound rubber refers to the part of rubber in compound that cannot be dissolved by its good solvent. In essence, the bound rubber is the rubber adsorbed on the surface of the filler, that is, the rubber in the interfacial layer of the filler and the rubber, has a similar glassy character. The more bound rubber, the stronger the reinforcement, so the bound rubber is a measure of the reinforcing ability of the filler. The bound rubber of SSBR/BR/DNS-DPG compounds was displayed in Fig. 6. From the figure, it can be seen that after modified with DPG, the bound rubber was more, indicating that the rubber/silica interactions were stronger, the reinforcing ability of the silica was stronger. When filled with DNS-DPG-3, the bound rubber was most. The reason may be that after modification, the particle size of silica decreased, and the interface area with rubber increased. It was also possible that hydrogen bond was formed between the amino group of accelerator DPG and the hydroxyl group on the silica surface, which may not break under the action of the solvent, thus increasing the bound rubber.

Mechanical Properties of SSBR/BR/DNS-DPG Vulcanizates

The stress-strain curves of SSBR/BR/DNS-DPG vulcanizates were displayed in Fig. 7. The mechanical properties (tensile strength, elongation at break, tear strength, and the reinforcing index) of SSBR/BR/DNS-DPG vulcanizates were shown in Supporting Information Fig. S3. It can be seen that, with the increase of the modifier DPG, the tensile strength, elongation at break and tear strength overall showed the first increase and then decrease. Compared to the SSBR/BR/DNS-DPG-0 nanocomposite, the other SSBR/BR/DNSDPG nanocomposites exhibited the better tensile strength, elongation at break and tear strength. Besides, among them, SSBR/BR/DNSDPG-3 nanocomposite had the maximum tensile strength (Supporting Information Figs. S3a, S3b, and S3c). This could be closely related to the improved rubber-filler compatibility with varying fraction of DPG in the DNS-DPG filler (as showed in Table 4).

As reported elsewhere, the reinforcing index (the ratio of the modulus at 300% to modulus at 100%) can reflect the reinforcing effect of fillers for rubber material: a higher reinforcing index corresponds to a stronger reinforcing effect [27-29]. Supporting Information Fig. S3d showed the reinforcing index of various fillers. It can be seen that DNS-DPG-0 had a larger reinforcing index, which means they had better enhancement effect for the rubber. Besides, DNS-DPG-3 and DNS-DPG-4 exhibited slightly higher reinforcing indexes than DNS-DPG-0, which corresponded to their better reinforcing effect than DNS-DPG-0. Therefore, DPG-modified silica as the filler can effectively improve the mechanical properties of SSBR/BR rubber. Particularly, the DNSDPG with a DPG fraction of 135.25 mmol/kg was most effective in improving the mechanical properties of SSBR/BR, which was because this DNS-DPG filler exhibited the strongest interactions with the SSBR/BR matrix (as Table 4).

Dynamic Rheological Properties of SSBR/BR/DNS-DPG Vulcanizates

Dynamic rheological properties of the nanocomposites can reflect the filler networking. Figure 8 showed the dependence of the storage modulus (G') of compounds and vulcanizates filled with various DNS-DPG fillers on strain. In general, the G' values of all the compounds and vulcanizates sharply decrease with increasing strain. This phenomenon is called Payne effect, and it is mainly related to the collapse of the filler network and release of the trapped rubber in the filler network [30-32]. From Fig. 8, both compounds and vulcanizates showed obvious Payne effect. Particularly, the uncured and vulcanizates of SSBR/BR/DNS-DPG-3 exhibited the weakest Payne effect. Moreover, when the content of DPG in the DNS-DPG was below 135.25 mmol/kg, the Payne effect of the nanocomposites decreased with the increasing DPG content; and when the content of DPG was above 135.25 mmol/kg, the Payne effect increased with the increasing DPG content. The reason lies in that DPG-modified DNS exhibited improved dispersion in the rubber matrix. Combined with SEM images, with the increase of modification contents, the dispersion of nanosilica shift toward uniformly at the modified amounts below 135.25 mmol/kg, but the shift reverses direction at the modified amounts above 135.25 mmol/kg. This was because after the DPG grafted on the silica surface, hydrogen bonds were formed between the amino groups in the DPG. When the amount of DPG was excessive, a large amount of hydrogen bonds were formed, leading to agglomeration. This was why the load amount of DNS-DPG-4 was larger, but the performance was inferior.

Abrasion Resistance of SSBR/BR/DNS-DPG Vulcanizates

Generally, the wear resistance of rubber material directly determines the service life of tire, and the rubber particles generated by tire abrasion have significant correlation with the formation of atmospheric particulate matter (PM2.5) [33]. Figure 9 showed the Akron abrasion loss of SSBR/BR/DNS-DPG nanocomposites. It showed that the Akron abrasion loss of SSBR/BR/DNS-DPG nanocomposites decreased with increasing amount of DPG. Particularly, SSBR/BR/DNS-DPG-3 nanocomposite had the minimum Akron abrasion loss, which was consistent with similar to the aforementioned improvements in the tensile strength, tear strength, and elongation at break of the rubber nanocomposites. This was also related to the homogeneous dispersion of the DNSDPG-3 in the rubber matrix [34]. And so, the higher Akron abrasion losses of SSBR/BR/DNS-DPG-0 and SSBR/BR/m-DPG were attributed to the uneven dispersion of DNS-DPG-0 and mDPG fillers in the rubber matrix.

Dynamic Mechanical Properties

To examine the reinforcing efficiency of for SSBR/BR/DNS-DPG nanocomposites, we measured the dynamic mechanical properties of the nanocomposites. The loss factor (tan[delta]) of rubber-matrix composites, the ratio of the viscous part to the elastic part (energy loss/energy stored), was a measurement of how efficiently the material loses energy owing to molecular rearrangements and internal friction [35]. The tan<5 versus temperature curves of SSBR/BR nanocomposites were depicted in Fig. 10. From the DMA curves, it can be clearly seen that an obvious peak which correspond to glass transition region emerges (Fig. 10a) [36]. Besides, the glass transition temperature ([T.sub.g]) of SSBR/BR/DNSDPG-1, SSBR/BR/DNS-DPG-3, and SSBR/BR/DNS-DPG4, nanocomposites obviously shifted toward high temperature. Namely, these three nanocomposites exhibited a higher tan[delta] at [T.sub.g] than the other nanocomposites, which was because the these three kinds of DNS-DPG homogeneously dispersed in the rubber, contributing to enhancing the filler network structure and rubber chains trapped in the filler network.

Usually, the tan[delta] values at 0[degrees]C and 60[degrees]C, representing wet skid resistance and rolling resistance, respectively, are two important indexes for high-performance tires. So high performance rubber materials should have a high tan[delta] at 0[degrees]C and a low tan[delta] at 60[degrees]C simultaneously [37]. As shown in Fig. 10b, the SSBR/BR/DNS-DPG-1 nanocomposite exhibited the maximum tan[delta] value at 0[degrees]C, which meant it possessed the best wet skid resistance among all the samples. However, SSBR/BR/ DNS-DPG-1 vulcanizates exhibited relatively high tan[delta] values at 60[degrees] C (Fig. 10c). Compared to SSBR/BR/DNS-DPG-0, the nanocomposites filled with silica which modified with DPG exhibited higher tan[delta] value at 0[degrees]C except for SSBR/BR/m-DPG. This meant that the rubber vulcanization accelerator DPG grafted onto the surface of nanosilica had an effect on enhancing the wet skid resistance of SSBR/BR. From Fig. 10c, SSBR/BR/m-DPG nanocomposite exhibited the lowest tan[delta] value at 60[degrees]C, which indicated it possessed lowest rolling resistance. When the dispersion of silica was very bad, tan 5 at 60[degrees] C of the nanocomposites was low because there was less filler agglomeration/de-agglomeration. Therefore, SSBR/BR/DNS-DPG nanocomposites possessed significantly improved mechanical properties and could find promising application as a potential green tire material.

CONCLUSIONS

A series of dispersible nanosilica modified by rubber vulcanization accelerator DPG were synthesized through liquid phase in situ surface chemical modification in order to improve the dispersion of nanosilica and accelerator DPG in rubber and better exert its reinforcing effect for SSBR/BR-matrix composites. FTIR and TGA proved that the vulcanization accelerator DPG was grafted onto the surface of nanosilica, thereby effectively preventing nanosilica from agglomeration and significantly reducing its average particle size. The resultant DNS-DPG exhibited homogeneous dispersion in SSBR/BR matrix, which contributed to enhancing the filler-rubber compatibility and improving the mechanical properties of the rubber-matrix nanocomposites. SSBR/BR/ DNS-DPG nanocomposites improved wet skid resistance and rolling resistance. Particularly, SSBR/BR/DNS-DPG-3 nanocomposite exhibited the best mechanical properties, while it integrated high abrasion resistance. Therefore, the nanosilica modified with accelerator DPG was given both functional characteristics of the reinforcing agent nanosilica and the effect of accelerating the crosslinking reaction of accelerator DPG at the same time. Silica modified with DNS-DPG nanocomposites showed promising application in green tire tread.

ACKNOWLEDGMENTS

This researchis financially supported by the Ministry of Science and Technology of China (in the name of National Basic Research Program, grant No. 2015CB654703) and Program for Innovative Research Team from the University of Henan Province under Grant 17IRTSTHN004.

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Yimei Mao, (1) Qingfeng Tian, (2) Chunhua Zhang, (1) Yuan Tang, (1) Yanpeng Wang, (1,3) Xiaohong Li, (2) Tao Ding (1,3)

(1) College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, People's Republic of China

(2) Engineering Research Center for Nanomaterials, Henan University, Kaifeng, 475004, People's Republic of China

(3) Engineering Laboratory for Flame Retardant and Functional Materials of Henan Province, Henan University, Kaifeng, 475004, People's Republic of China

Additional Supporting Information may be found in the online version of this article.

Correspondence to: T. Ding; e-mail: 10150002@vip.henu.edu.cn and X. Li; e-mail: xiaohonglihenu@163.com

Yimei Mao and Qingfeng Tian contributed equally to this work.

Contract grant sponsor: National Basic Research Program, Ministry of Science and Technology of China; contract grant number: No. 2015CB654703. contract grant sponsor: Program for Innovative Research Team from the University of Henan Province, China; contract grant number: NO. 17IRTSTHN004.

DOI 10.1002/pen.25110

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Liquid phase in situ surface chemical modification route for the synthesis of DNS-DPG.

Caption: FIG. 2. FTIR spectra of unmodified DNS-DPG-0 and modified DNS-DPG4. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. TGA curves of a series of DNS-DPG additives. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. TEM images of (a) DNS-DPG-0 and (b) DNS-DPG-3.

Caption: FIG. 5. SEM images of SSBRBR nanocomposites (a) SSBRBRDNS-DPG-O, (b) SSBRBRm-DPG, and (c) SSBRBRDNS-DPG-3.

Caption: FIG. 6. The bound rubber of SSBRBRDNS-DPG compounds. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. The stress-strain curves of SSBRBRDNS-DPGvulcanizates. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Dependence of storage modulus G' on strain of SSBRBRDNS-DPG (a) compounds and (b) vulcanizates. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Akron abrasion loss of SSBRBRDNS-DPG vulcanizates. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Tan8 of SSBRBRDNS-DPG nanocomposites as a function temperature.tif. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Surface chemical composition of a
series of as-prepared DNS-DPG additives.

              Amount of modifiers
             (mmol/kg Si[O.sub.2])

Code of     Accelerator DPG   KH560   Industrial
additives                             ethanol (g)

DNS-DPG-0          0            0         400
DNS-DPG-1        67.63        40.60       400
DNS-DPG-2       108.19        64.90       400
DNS-DPG-3       135.25        73.00       400
DNS-DPG-4       162.31        81.10       400

TABLE 2. Formulations of SSBR/BR/DNS-DPG composites.

                     Composition (wt. parts)

Components      DNS- DPG-0   m-DPG (a)   DNS-DPG-1

SSBR              96.25        96.25       96.25
BR                  30          30          30
DNS-DPG-0           70          70
m-DPG                          2.58
DNS-DPG-1                                   70
DNS-DPG-2
DNS-DPG-3
DNS-DPG-4
TESPT               7            7           7
Zinc oxide          3            3           3
Stearic acid        1            1           1
6PPD               1.5          1.5         1.5
Paraffin wax        1            1           1
CZ                 1.5          1.5         1.5
DPG (b)             2          0.77        1.38
Sulfur             1.4          1.4         1.4

                Composition (wt. parts)

Components      DNS-DPG-2     DNS-DPG-3     DNS-DPG-4

SSBR              96.25         96.25         96.25
BR                  30            30           30
DNS-DPG-0
m-DPG
DNS-DPG-1
DNS-DPG-2           70
DNS-DPG-3                         70
DNS-DPG-4                                      70
TESPT               7             7             7
Zinc oxide          3             3             3
Stearic acid        1             1             1
6PPD               1.5           1.5           1.5
Paraffin wax        1             1             1
CZ                 1.5           1.5           1.5
DPG (b)            1.25          0.77         0.38
Sulfur             1.4           1.4           1.4

(a) m-DPG is the product of condensation of DPG with KH560,
contains no silica. The content of bound DPG is 1.23 g.

(b) An additional free DPG was added to maintain
the constant total concentration of DPG
(free and bound to silica).

TABLE 3. Weight loss of DNS-DPG additives with varying
amounts of vulcanization accelerator DPG.

             Theoretical value    Weight loss at     Actual load
Samples          (mmol/kg)       800[degrees]C (%)    (mmol/kg)

DNS-DPG-0          0.00                6.07             0.00
DNS-DPG-1          67.63               7.96             41.88
DNS-DPG-2         108.19               8.34             50.45
DNS-DPG-3         135.25               9.78             82.47
DNS-DPG-4         162.31               10.97           108.93

TABLE 4. Cure behavior of SSBR/BR/DNS-DPG nanocomposites.

             Cure behavior

Samples       ML      MH     MH-ML

DNS-DPG-0    8.36    25.49   17.13
m-DPG       11.44    31.15   18.71
DNS-DPG-1    9.33    26.51   17.18
DNS-DPG-2    4.78    24.31   19.53
DNS-DPG-3    4.04    24.95   20.91
DNS-DPG-4    5.51    24.75   19.24

                          Cure behavior

Samples      [Tc.sub.10]   [Tc.sub.90]   [Tc.sub.10]/
                                         [Tc.sub.90]

DNS-DPG-0       1.93          26.60         24.67
m-DPG           1.52          28.12         26.60
DNS-DPG-1       1.42          17.07         15.65
DNS-DPG-2       1.37          12.15         10.98
DNS-DPG-3       1.25          11.28          9.92
DNS-DPG-4       1.17           9.82          8.57
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Title Annotation:solution polymerized styrene butadiene rubber/butadiene rubber
Author:Mao, Yimei; Tian, Qingfeng; Zhang, Chunhua; Tang, Yuan; Wang, Yanpeng; Li, Xiaohong; Ding, Tao
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jun 1, 2019
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