Article Using Epoxidized Solution Polymerized - Rubbers (ESSBRs) as Coupling Agents to Modify Silica without Volatile Organic Compounds

Chaohao Liu 1,2, Mingming Guo 2, Xiaobo Zhai 1,3, Xin Ye 1,3,* and Liqun Zhang 1,3,* 1 State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, P.O. Box 57, Beisanhuan East Road, Beijing 100029, China; [email protected] (C.L.); [email protected] (X.Z.) 2 SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China; [email protected] 3 Engineering Research Center of Materials on Energy Conservation and Resources, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China * Correspondence: [email protected] (X.Y.); [email protected] (L.Z.)

 Received: 9 April 2020; Accepted: 27 May 2020; Published: 30 May 2020 

Abstract: Rubber used in is usually strengthened by nanofiller, and the most popular nanofiller for tire tread rubber is nano silica, which can not only strengthen rubber but also lower the tire rolling resistance to reduce fuel consumption. However, silica particles are difficult to disperse in the rubber matrix because of the abundant silicon hydroxyl on their surface. Silane coupling agents are always used to modify silica and improve their dispersion, but a large number of volatile organic compounds (VOCs) are emitted during the manufacturing of the nanosilica/rubber composites because of the condensation reaction between silane coupling agents and silicon hydroxyl on the surface of silica. Those VOCs will do great harm to the environment and the workers’ health. In this work, epoxidized solution polymerized styrene-butadiene rubbers (ESSBR) with different epoxy degrees were prepared and used as macromolecular coupling agents aimed at fully eliminating VOCs. Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) analyses verified that the different ESSBRs were successfully synthesized from solution polymerized styrene-butadiene rubbers (SSBR). With the help of the reaction between epoxy groups and silicon hydroxyl without any VOC emission, nanosilica can be well dispersed in the rubber matrix when SSBR partially replaced by ESSBR which was proved by Payne effect and TEM analysis. Dynamic and static mechanical testing demonstrated that silica/ESSBR/SSBR/BR nanocomposites have better performance and no VOC emission compared with Bis-(γ-triethoxysilylpropyl)-disulfide (TESPD) modified silica/rubber nanocomposites. ESSBR is very hopeful to replace traditional coupling agent TESPD to get high properties silica/rubber nanocomposites with no VOCs emission.

Keywords: SSBR; epoxidation; silica; VOCs

1. Introduction In recent years, with the stricter requirements for tire performance, shortage of petroleum resources, and people’s attention to environmental protection, better wet-skid resistance property as well as lower rolling resistance are demanded when rubber is applied to tire tread [1–3]. In this context, the concept of “green ” was proposed by Michelin in 1992, which refers to a compound composed of SSBR, silica and some other reinforcing agents [4,5]. However, there is a problem when silica is used to strengthen the rubber. Silica is a general term for fine powdery or superfine particle precipitated silica. There is a mass of activated silicon hydroxyl

Polymers 2020, 12, 1257; doi:10.3390/polym12061257 www.mdpi.com/journal/polymers Polymers 2020, 12, 1257 2 of 15 groups on the surface of silica [6–8] which makes it hard to infiltrate and disperse in the organic rubber phase and easy to by itself [9,10]. In the 1970s, it was found the silica ability to be modified by silane coupling agents such as bis-(γ-triethoxysilylpropyl)-tetrasulfide (TESPT) or bis-(γ-triethoxysilylpropyl)-disulfide (TESPD) which can improve the compatibility between silica and rubber [11–13]. In this process, the silane coupling agent reacts with the hydroxyl on the surface of silica and makes the silica turn from hydrophilic to hydrophobic [14,15]. It usually requires a high temperature (150 ◦C) for this reaction when the mixing of rubber compounds begins. Inevitably, volatile organic compounds (VOCs) [16–19] like methanol and ethanol will be produced during this process [20,21], which is harmful to rubber performance and worker’s health [22,23]. To improve the performance of silica/rubber nanocomposites and lower the VOCs emission, as opposed to modifying the silica surface, the compatibility of silica and rubber can also be improved by the functionalization of the rubber molecular chain, e.g., introducing functional groups like carboxyl groups [24,25], hydroxyl groups [26–30], alkoxysilane groups, and epoxy groups [3,20,28] onto rubber chains during post- process. Additionally, these functional groups, especially alkoxysilane groups [31–34], can be introduced to the chain during the polymerization process, copolymerization, or termination. These groups provide the rubber with polarity or more reactivity with silicon hydroxyl on silica surface. However, some of alkoxysilane groups are likely to emit VOCs because the processing temperature of rubber and silica is always high (150 ◦C) and the alcohol products with low molecular weight will turn into gases. Therefore, the other groups are more environmentally friendly. Jacobi et al [35,36] have synthesized ESSBR and elaborated the relationship of epoxy degree with double bond content, hydrogen peroxide concentration, reaction temperature and time. However, the reaction between epoxy groups and hydroxyl groups was rarely reported [20,37], which is a ring opening reaction without any VOCs. Therefore, we plan to use the ESSBR as macromolecule coupling agents to modify the performance of silica/rubber nanocomposites and eliminate the VOCs emission. In this study, a series of ESSBR with different epoxy degrees (7%–25%) were prepared by using formic acid and hydrogen peroxide as oxidant, and silica/ESSBR/SSBR/BR nanocomposites were manufactured. SSBR matrix were partially replaced by ESSBR with different epoxy degrees (7%–25%) as macromolecule coupling agents. The performance of silica/ESSBR/SSBR/BR composites were examined by transmission electron microscopy (TEM), rubber process analyzer (RPA), tension tester, and dynamic mechanical thermal analysis (DMTA).

2. Experiment

2.1. Materials Solution polymerized styrene-butadiene rubber 2557 (SSBR2557) was supplied by SINOPEC Lanzhou Co. Ltd. (Lanzhou, China). Solution polymerized styrene-butadiene rubber 4526 (SSBR4526) and butadiene rubber (CB24) were purchased from LANXESS Corporation (Shanghai, China). Cyclohexane was obtained from Beijing Chemworks Company (Beijing, China). Hydrogen peroxide (30%) was obtained from Shanghai Chemical Reagent Company (Shanghai, China). Formic acid was obtained from Beijing TG fine chemicals Company (Beijing, China). Tween-80 was obtained from Beijing Yili Fine Chemicals Company (Beijing, China). All of the rubber additives, including zinc oxide, stearic acid, N-Isopropyl-N’-phenyl-4-phenylenediamin ( 4010NA), wax, N-cyclohexyl-2-benzothiazole sulfonamide (accelerator CZ), and sulfur, were industrial grade and commercially available.

2.2. Preparation of Epoxidized Styrene-Butadiene Rubber (ESSBR) Cyclohexane was added to a three-necked flask. The SSBR was then added at a mass-to-volume ratio of 10 g/100 mL. The mixture was stirred until the rubber was dissolved. The temperature was set at 40 ◦C. Hydrogen peroxide was used in excess of the polymers double bond content (H2O2/C=C Polymers 2020, 12, x FOR PEER REVIEW 3 of 16

Cyclohexane was added to a three-necked flask. The SSBR was then added at a mass-to-volume ratio of 10 g/100mL. The mixture was stirred until the rubber was dissolved. The temperature was Polymers 2020, 12, x FOR PEER REVIEW 3 of 16 set at 40 °C. Hydrogen peroxide was used in excess of the polymers double bond content (H2O2/C=C 1.5/1), and formic acid was added at a ratio of the reactant mol (H2O2/HCOOH 3/1) to generate in situ PolymersCyclohexane2020, 12, 1257 was added to a three-necked flask. The SSBR was then added at a mass-to-volume3 of 15 performicratio of 10 acid. g/100mL. The mixing The mixture was done was at stirred 50 rpm until rotor the speed rubber with was a dissolved.mechanical The stirrer. temperature Formic acid was wasset atall 40 added °C. Hydrogen one time, peroxideand hydrogen was used peroxide in excess was ofadded the polymers by the rate double of 3 ml/min bond content with an (H addition2O2/C=C funnel.1.51.5/1),/1), and andAt the formic formic end acid ofacid the was was reaction, added added atthe at a a ratiomixture ratio of of the wasthe reactant reactant neutralized mol mol (H with(H2O2O2 a2//HCOOHHCOOH (5% w/v) 3 3/1)Na/1)2to COto generategenerate3 solutionin in andsitu situ washedperformicperformic with acid.acid. distilled TheThe mixingmixing water. was wasAfter donedone removing atat 5050 rpmrpm the rotoraqueousrotor speedspeed phase, withwith the aa mechanical mechanicalorganic phase stirrer.stirrer. was Formic Formiccoagulated acidacid inwaswas ethanol all all added added and one driedone time, time, under and and vacuum hydrogen hydrogen to peroxide aperoxide constant was was weight. added added The by by thereaction the rate rate of principleof 3 3 mL ml/min/min and withwith flow anan of additionaddition ESSBR preparationfunnel.funnel. AtAt thethe was endend described ofof thethe reaction,reaction, in Figure the the 1. mixture mixture was was neutralized neutralized with with a a (5% (5% w w/v)/v) Na Na2CO2CO33solution solutionand and washedwashedIn order withwith to distilleddistilled prepare water.water. ESSBR AfterAfter with removingremoving different the epoxythe aqueousaqueous degree, phase,phase, the amount thethe organicorganic of reactants phasephase was waswere coagulated coagulated constant: SSBRinin ethanol ethanol 100 g, andand hydrogen dried dried under under peroxide vacuum vacuum (30%) to to a 180a constant constant g, formic weight. weight. acid The The24.4 reactionreaction g. And principleprinciplethe reaction andand time flowflow ofchanged:of ESSBR ESSBR ESSBR7%preparationpreparation (28 was was min), described described ESSBR10% in in Figure Figure (35 min),1 .1. ESSBR15% (68 min), ESSBR20% (117 min), ESSBR25% (264 min). In order to prepare ESSBR with different epoxy degree, the amount of reactants were constant: SSBR 100 g, hydrogen peroxide (30%) 180 g, formic acid 24.4 g. And the reaction time changed: ESSBR7% (28 min), ESSBR10% (35 min), ESSBR15% (68 min), ESSBR20% (117 min), ESSBR25% (264 min).

Figure 1. ReactionReaction principle principle and flow flow chart. In order to prepare ESSBR with different epoxy degree, the amount of reactants were constant: 2.3. Preparation of Silica/SSBR/BR Composites SSBR 100 g, hydrogen peroxide (30%) 180 g, formic acid 24.4 g. And the reaction time changed: ESSBR7%In our (28 earlier min), research, ESSBR10% we (35Figure used min), pure1. ESSBR15% Reaction ESSBR principle (68to min),replace and ESSBR20% flowSSBR. chart. When (117 min),the epoxy ESSBR25% degree (264 of pure min). ESSBR is more than about 7%, a crushing phenomenon of rubber compounds appears during the mixing2.3.2.3. PreparationPreparation process. of ofWhen SilicaSilica/SSBR/BR /SSBRblends/BR of Composites CompositesSSBR and ESSBR are used, this issue can be solved. ESSBR with differentInIn ourour epoxy earlierearlier degrees research,research, (7%–25%) wewe usedused were purepure used ESSBRESSBR as macr totoomolecular replacereplace SSBR.SSBR. coupling WhenWhen agents thethe epoxyepoxy to modify degreedegree the ofof silica purepure surfaceESSBRESSBR isandis moremore improve thanthan about itabouts dispersion. 7%,7%, aa crushingcrushing Bis-(γ-triethoxysilylpropyl)-disulfide phenomenonphenomenon ofof rubberrubber compoundscompou (TESPD)nds appearsappears shown duringduring in Figure thethe 2,mixingmixing which process. process.is the most WhenWhen popular blendsblends silane ofof SSBRSSBRcoupling andand agent ESSBRESSBR used areare in used,used, tire factories, thisthis issueissue was cancan selected be be solved. solved. for comparison. ESSBRESSBR withwith Alldidifferentff erentthe silica/SSBR/BR epoxy epoxy degrees degrees compounds (7%–25%) (7%–25%) were wereare listed used used in as as Table macromolecular macr omolecular1. Firstly, silica, coupling coupling , agents agents to to modifywax, modify zinc the the oxide, silicasilica stearicsurfacesurface acid, and and improveand improve TESPD its its dispersion. ordispersion. ESSBR were Bis-( Bis-( γmixed-triethoxysilylpropyl)-disulfideγ-triethoxysilylpropyl)-disulfide with SSBR/BR in a Haake (TESPD) internal (TESPD) shownmixer shown at in 50in Figure °CFigure by2, thewhich2, which standard is theis the mostadding most popular popularsequence. silane silane Secondly, coupling coupling the agent agentcompounds used used in in tirewere tire factories, factories, masticated was was for selected selected 5 min forforat 150 comparison.comparison. °C, then theAllAll compounds thethe silicasilica/SSBR/BR/SSBR were/BR taken compoundscompounds out, cooled areare down listedlisted to inin r TableoomTable temperature,1 .1. Firstly, Firstly, silica, silica, and antioxidants, mixedantioxidants, with the wax, wax, vulcanization zinc zinc oxide, oxide, accelerators and sulfur on a 6-inch two-roll mill for 5 min at room temperature. Finally, the stearicstearic acid, acid, and and TESPD TESPD or or ESSBR ESSBR were were mixed mixed with with SSBR SSBR/BR/BR in ain Haake a Haake internal internal mixer mixer at 50 at◦ C50 by °C the by compounds were cured under 15 MPa at 150 °C to yield the vulcanized nanocomposite. standardthe standard adding adding sequence. sequence. Secondly, Secondly, the compoundsthe compounds were were masticated masticated for5 for min 5 min at 150 at ◦150C,then °C, then the compoundsthe compounds were were taken taken out, out, cooled cooled down down to roomto room temperature, temperature, and and mixed mixed with with the the vulcanization vulcanization acceleratorsaccelerators and and sulfur sulfur on aon 6-inch a 6-inch two-roll two-roll mill for mill 5 min for at 5 room min temperature.at room temperature. Finally, the compoundsFinally, the werecompounds cured under were 15cured MPa under at 150 15◦C MPa to yield at 150 the °C vulcanized to yield the nanocomposite. vulcanized nanocomposite.

Figure 2. The molecular structure of bis-(γ-triethoxysilylpropyl)-disulfide (TESPD).

Figure 2. The molecular structure of bis-(γ-triethoxysilylpropyl)-disulfide (TESPD). Polymers 2020, 12Figure, x; doi: FOR2. The PEER molecular REVIEW structure of bis-(γ-triethoxysilylpropyl)-disulfidewww.mdpi.com/journal/polymers (TESPD).

Polymers 2020, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2020, 12, 1257 4 of 15

Table 1. Formulation of silica/ solution polymerized styrene-butadiene rubber (SSBR)/ butadiene rubber (BR) compounds.

A1 A2 B3 C3 D3 E3 F1 F2 F3 PS(pure silica)- TS(TESPD-silica)- E7-SSBR/ E10-SSBR/ E15-SSBR/ E20-SSBR/ E25a-SSBR/ E25b-SSBR/ E25c-SSBR/ Materials SSBR/BR/phr a SSBR/BR/phr a BR/phr a BR/phr a BR/phr a BR/phr a BR/phr a BR/phr a BR/phr a SSBR 74 74 44.4 44.4 44.4 44.4 59.2 51.8 44.4 BR 26 26 26 26 26 26 26 26 26 Silica 60 60 60 60 60 60 60 60 60 ESSBR7% c 0 0 29.6c 0 0 0 0 0 0 ESSBR10% c 0 0 0 29.6c 0 0 0 0 0 ESSBR15% c 0 0 0 0 29.6c 0 0 0 0 ESSBR20% c 0 0 0 0 0 29.6c 0 0 0 ESSBR25% c 0 0 0 0 0 0 14.8 22.2 29.6c TESPD 0 6 0 0 0 0 0 0 0 Other additives 1# b 1# b 1# b 1# b 1# b 1# b 1# b 1# b 1# b a Parts per hundred of rubber. b 1# stearic acid 2.0, zinc oxide 3.0, N-Isopropyl-N’-phenyl-1,4-phenylenediamine 2.0, N-Cyclohexyl-2-beozothiazole sulfenamide 1.5, 1,3-Diphenylguanidine 2.0 and sulfur 1.5. c 7%–25% indicates epoxy degree of ESSBR. c the ratio of SSBR replaced by ESSBR is 40%. Polymers 2020, 12, 1257 5 of 15

2.4. Characterizations Fourier transform infrared (FT-IR, Bruker Tensor-27 FT-IR Spectrometer, Bruker Optik Gmbh Co., Ettlingen, Germany) measurement was used to identify the groups in ESSBR, using attenuated total 1 reflection(ATR) mode under a wave ranging from 400 to 4000 cm− with 32 scans. 1 The H NMR spectroscopy were recorded on a Bruker AV400 spectrometer and CDCl3 was the solvent, using a concentration of 7–20 mg polymer/mL. Differential scanning calorimetry (DSC) measurements were conducted using a STARe system DSC instrument from 100 to 100 C with a heating rate of 10 C min 1 under nitrogen. − ◦ ◦ − The molecular weights of rubbers were obtained by gel permeation chromatography (GPC) on an Agilent 1260 Infinity instrument equipped with a G1362A refractive index detector. Toluene was the 1 mobile phase (1.0 mL min− ), and standards were used for calibration. The curing behavior of silica/SSBR/BR composites were measured by a MR-C3 rotorless rubber vulcanizing machine at 150 ◦C and 1.67 Hz. The silica dispersion was observed under a Tecnai G220 TEM (FEI Co., Hillsboro, OR, USA) with an accelerating voltage of 200 kV. The thin sections of silica/SSBR/BR nanocomposites were cut for TEM observations using a microtome at 100 C and collected on copper grids. − ◦ The bound rubber contents of the silica/SSBR/BR composites were measured based on the previously reported method [38]. A Bruker AVANCE III 400 WB solid-state NMR spectrometer was used to characterize the crosslink density of silica/SSBR/BR composites. The sample was packed into a 10 mm diameter NMR tube and then moved to the heating zone of the nuclear magnetic instrument with the same temperature as the oven. The sample was stabilized for 5 min and then scanned at 90 ◦C. The dynamic rheological properties of the silica/SSBR/BR composites were analyzed by RPA 2000 (Alpha Technologies Co., Bellingham, WA, USA) at 60 ◦C and 1 Hz (mainly to get tanδ@60 ◦C). For compounds, the strain amplitude was varied from 0.1% to 450%. For cured composites, the strain amplitude was varied from 0.1% to 100%. The thermo-mechanical properties of the nanocomposites were analyzed by a 01dB-Metravib VA 3000 dynamic mechanical thermal analyzer (DMTA) at 10 Hz in the tension mode with a strain amplitude of 0.1%. The test temperature ranged from 80 to 80 C with a heating rate of 5 C min 1. − ◦ ◦ − (mainly to get tanδ@0 ◦C). The mechanical properties of the silica/SSBR/BR composites were investigated according to ASTM D638 specifications using a CMT4104 electrical tensile tester (Shenzhen SANS Test Machine Co., Shenzhen, China) at across head speed of 500 mm/min. The abrasion loss properties of the silica/SSBR/BR composites were measured due to GB/1689–1998 with a MZ-4061 Akron abrasion machine (Jiangsu Mingzhu Experimental Machinery Co. LTD., Yangzhou, CN).

3. Results and Discussion

3.1. Chemical Structure of the Epoxidized Styrene-Butadiene Rubber (ESSBR)

1 Figure3a shows the FT-IR spectra of the synthesized product. Peaks at 760 cm − are related to 1 1 1,4-cis, those at 966 cm− to 1,4-trans, and those at 911 cm− to 1,2-vinyl double bonds, respectively. 1 The new peaks at 1260 and 801 cm− , which refer to the symmetrical stretching deformation absorption peak of C–O–C and the asymmetric extension deformation vibration absorption peak of C–O–C, respectively, can be observed from the ESSBR curve, indicating that the epoxy groups have been introduced into the molecular chain of the SSBR. Polymers 2020, 12, 1257 6 of 15 Polymers 2020, 12, x FOR PEER REVIEW 6 of 16

FigureFigure 3.3. StructureStructure characterizationcharacterization ofof SSBRSSBR andand epoxidizedepoxidized solutionsolution polymerizedpolymerized styrene-butadienestyrene-butadiene 1 rubbersrubbers ESSBRs:ESSBRs: ((aa)) FT-IRFT-IR spectraspectra ((bb)) 1HH NMRNMR spectraspectra andand ((cc)) DSC.DSC.

AsAs shownshown inin FigureFigure3 b,3b, H H atoms atoms in in SSBR SSBR and and ESSBR ESSBR are are marked marked as as a–k a–k according according to to their their local local chemicalchemical environment.environment. TheThe peakspeaks shownshown reasonablyreasonably correspondcorrespond toto thethe protonsprotons ofof styrene,styrene, butadienebutadiene 1 (including(including 1,4-addition 1,4-addition and and 1,2-addition), 1,2-addition), and theand epoxy the epoxy group. group. In the HIn NMRthe 1 spectrumH NMR ofspectrum SSBR2557, of thereSSBR2557, are two there peaks are at two 4.90 peaks (f,l) and at 4.90 5.25 (d.e)ppm,(f,l) and 5.25 attributed (d.e)ppm, to theattributed protons to of the the protons double bonds of the 1,2double and 1 1,4bonds butadiene 1,2 and units 1,4 butadiene (cis and trans) units in (cis the and polymer. trans) Inin thethe polymer.H-NMR spectrumIn the 1H-NMR of ESSBR, spectrum three new of ESSBR, peaks appearthree new at 2.55 peaks (j,k), appear 2.80 (h,i), at 2.55 and (j,k), 3.64 ppm2.80 (h,i), (m,). and The 3.64 peaks ppm at 2.55 (m,). (j,k) The and peaks 2.80 at (h,i) 2.55 ppm (j,k) correspond and 2.80 (h,i) to theppm methine correspond resonance to the of methine the epoxy resonance groups of in the trans epoxy and cisgroups position, in trans respectively, and cis position, which demonstraterespectively, thatwhich SSBR demonstrate was successfully that SSBR epoxidized. was successfully The peaks at epox 3.64idized. (m,) derive The frompeaks hydroxyl at 3.64 groups(m,) derive [29] which from 1 formedhydroxyl during groups ring-opening [29] which reactionsformed during after epoxidation ring-opening and reactions the wide after bands epoxidation in 3100–3600 and cm the− fromwide FT-IRbands also in 3100–3600 indicates the cm presence−1 from FT-IR of hydroxyl also indicates groups, the but presence the amount of hydroxyl of hydroxyl groups, groups but was thefew amount and theof hydroxyl epoxy degree groups was was little few aff andected. the As epoxy the reaction degree wa progresses,s little affected. the epoxy As the degree reaction increases, progresses, resulting the inepoxy an increase degree ofincreases, the signals resulting at 2.55 (j,k)in an and increase 2.80 (h,i) of the ppm signals (trans at and 2.55 cis (j,k) epoxy) and and2.80 a (h,i) decrease ppm of(trans the peakand cis at 5.25epoxy) (d.e) and ppm a decrease (unsaturated of the 1,4-polybutadiene peak at 5.25 (d.e) protons). ppm (unsaturated The peak at1,4-polybutadiene 4.9 (f,l) ppm (vinyl protons). group) remainsThe peak practically at 4.9 (f,l) ppm constant. (vinyl These group) findings remains indicated practically that constant. the reactivity These findings of trans indicated and cis 1,4 that units the isreactivity higher than of trans that and of vinyl cis 1,4 1,2 units units. is higher the epoxy than that degree, of vinylX, for 1,2 SSBR units. has the been epoxy calculated degree, X using, for SSBR the following Equation: has been calculated using the following Equation :   Aepox AHest  =  −  X%    100 100 Aepox AH + A,+A, × − est 1,4 1,2 where Aepox is the normalized proton area intensities for the epoxide peaks at 2.55 and 2.80 ppm, AHest where Aepox is the normalized proton area intensities for the epoxide peaks at 2.55 and 2.80 ppm, AHest is the normalized area intensities for methylene bonded to the styrene ring at 2.55 ppm, A1,4 and A1,2 is the normalized area intensities for methylene bonded to the styrene ring at 2.55 ppm, A and A are the normalized area for the unsaturated 1,4 polybutadiene peak at 5.25 ppm and unsaturated1,4 1,2 are the normalized area for the unsaturated 1,4 polybutadiene peak at 5.25 ppm and unsaturated 1,2 polybutadiene peak at 4.9 ppm. polybutadiene peak at 4.9 ppm. As shown in Figure 3c, when the epoxy degree increases, the polarity of macromolecules increases, and the Tg ( temperature) increases which results in tanδ@0 °C. This is helpful for improving the wet-skid resistance of rubber composites below.

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As shown in Figure3c, when the epoxy degree increases, the polarity of macromolecules increases, and the Tg (glass transition temperature) increases which results in tanδ@0 ◦C. This is helpful for improving the wet-skid resistance of rubber composites below. ESSBR with the epoxy degree from 7% to 25% were prepared and got ready to use as macromolecule coupling agents, because the epoxy groups can react with the silicon hydroxyl on silica surface, and ESSBR has good compatibility with the SSBR/BR matrix, the residual double bonds on ESSBR can also crosslink with SSBR/BR matrix by vulcanizing agent. Polymers 2020, 12, x FOR PEER REVIEW 8 of 16 3.2. Compositions and Molecular Weights of Rubbers 3.3. Application of ESSBR in Rubber Composites From Table2, it can be seen that the ESSBRs have a little increase about Mn and Mw and similar 3.3.1.PDI (Polymer Payne Effect dispersity of Silica/SSBR/BR index) with Compounds SSBR2557 after epoxidation, which means they have consistent molecular weight distribution, the side reaction of epoxidation is less and the reaction is controllable. The strain amplitude dependence of the storage modulus (G’) of silica/SSBR/BR compounds are shown in TableFigure 2. 4.Compositions The filler’s andnetwork molecular and weightsits situation of SSBR2557, of agglomeration SSBR4526, CB24,make and a great ESSBRs. influence on the modulus of rubber compounds. The G’ decreases rapidly with the increase of strain amplitude, Composition (%) M 10 5 M 10 5 namedSample the Payne effect [39,40], which is closely related to then breakdown− ofw the− filler networkPDI (Da)× (Da)× structure in rubberBound matrix Styrene when the 1,4-unit deformation ra 1,2-unitte of specimen increases. The difference between theSSBR2557 maximum and the 27 minimum G’ in 44 the curve names 56 the ΔG’ value, 3.96 which is 9.06 usually negatively 2.28 correlatedSSBR4526 to the dispersion 26 of filler. The 55 lower the ΔG’ 45 value, the better 1.75 the filler 5.4disperses. 3.09 AsCB24 shown in Figure - 4a, PS-SSBR/BR(A1) - has a significant Payne 1.45 effect because 4.05 there is not 2.79 any ESSBR7% - - 4.35 10.11 2.32 modifier for polar silica, therefore, silica particles couldn’t be well dispersed in the nonpolar rubber ESSBR10% - - 4.48 10.53 2.35 matrixESSBR15% and so the silica - particles agglomerated - by themselves. With 4.58 part of the 10.68 SSBR replaced 2.33 by ESSBR,ESSBR20% the epoxy groups - on the macromolecular - chains could react 4.31 with silicon 10.42hydroxyl groups 2.42 on theESSBR25% silica surface and reduce - the polarity - of silica particles, which 4.46 could greatly 10.71 improve the 2.40 silica dispersion in rubber matrix [41,42]. That means that the silica–silica direct contact networks reduced and3.3. Applicationmore silica of–rubber ESSBR networks in Rubber were Composites established and then the ΔG’ decreased. It is clear that the Payne effects of ESSBR-silica samples and TESPD-silica sample are significantly less than that of the pure3.3.1. silica Payne sample. Effect ofWith Silica the/SSBR epoxy/BR degree Compounds increase, the Payne effect of the composites decrease (B3 to F3).The When strain the amplitude epoxy degree dependence is fixed,of ΔG’ the decreased storage modulus as the amount (G’) of silicaof ESSBR/SSBR increased/BR compounds (F1 to F3). are Allshown the inresults Figure show4. The that filler’s the introd networkuction and of its epoxy situation groups of agglomeration on SSBR is very make beneficial a great influence to improve on thethe silicamodulus dispersion. of rubber Δ compounds.G’ of E20-SSBR/BR The G’ decreases (E3) and rapidly E25-SSBR/BR with the (F3) increase is even of strain less amplitude,than that namedof TS- SSBR/BR(A2).the Payne effect This [39 result,40], which indicates is closely that ESSBR related used to theas a breakdownmodifier has of similar the filler or even network better structure effects on in improvingrubber matrix the when dispersion the deformation of silica ratein silica/ of specimenSSBR/BR increases. compounds The diwhenfference comparing between thewith maximum TESPD, becauseand the minimumthe reaction G’ inbetween the curve epox namesy groups the ∆ onG’ value,ESSBR which and silicon is usually hydroxyl negatively groups correlated on the tosilica the surfacedispersion can of reduce filler. Thethe lowerhydrophilicity the ∆G’ value, of silica the and better improve the filler the disperses. compatibility between silica and rubber.

Figure 4.FigureStrain 4. amplitudeStrain amplitude dependence dependence of the of storage the storage modulus modulus (G’) of (G’) silica of /silica/SSBR/BRSSBR/BR compounds: (acompounds:(a)samples) samples with different with ESSBRs(epoxy different ESSBRs(epoxy degree), (b) samples degree),(b) with samples different with content different of E25. content of E25 As shown in Figure4a, PS-SSBR /BR(A1) has a significant Payne effect because there is not any 3.3.2.modifier TEM for Images polar silica,of silica/SSBR/BR therefore, silica Composites particles couldn’t be well dispersed in the nonpolar rubber Figure 5 are the TEM images which show the dispersion of silica in rubber matrix. The darker phase represents the silica particles. Silica particles are obviously agglomerated in the PS-SSBR/BR composite and forms a lot of clumps. Moreover, the phase interface between silica particles and the rubber matrix looks obviously clear. As is known to all, there is a layer of silicon hydroxyl distributed evenly on the surface of silica particles leading to silica agglomerate. With part of the SSBR replaced by ESSBR, the dispersion of silica particles was obviously improved. It can be seen from Figure 5 that the silica in E25-SSBR/BR (F3) has the best dispersion, even dispersing better than silica in TS-SSBR/BR(A2). The dispersion of silica in E15-SSBR/BR (D3) and E20-SSBR/BR (E3) is comparable to that of TS-SSBR/BR(A2). Previous studies [43–45] have indicated

Polymers 2020, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2020, 12, 1257 8 of 15 matrix and so the silica particles agglomerated by themselves. With part of the SSBR replaced by ESSBR, the epoxy groups on the macromolecular chains could react with silicon hydroxyl groups on the silica surface and reduce the polarity of silica particles, which could greatly improve the silica dispersion in rubber matrix [41,42]. That means that the silica–silica direct contact networks reduced and more silica–rubber networks were established and then the ∆G’ decreased. It is clear that the Payne effects of ESSBR-silica samples and TESPD-silica sample are significantly less than that of the pure silica sample. With the epoxy degree increase, the Payne effect of the composites decrease (B3 to F3). When the epoxy degree is fixed, ∆G’ decreased as the amount of ESSBR increased (F1 to F3). All the results show that the introduction of epoxy groups on SSBR is very beneficial to improve the silica dispersion. ∆G’ of E20-SSBR/BR (E3) and E25-SSBR/BR (F3) is even less than that of TS-SSBR/BR(A2). This result indicates that ESSBR used as a modifier has similar or even better effects on improving the dispersion of silica in silica/SSBR/BR compounds when comparing with TESPD, because the reaction betweenPolymers epoxy 2020, 12 groups, x FOR PEER on ESSBRREVIEW and silicon hydroxyl groups on the silica surface can reduce9 of 16 the hydrophilicity of silica and improve the compatibility between silica and rubber. that the chemical interaction between silica and rubber can make its dispersion in the rubber matrix 3.3.2.more TEM stable, Images as silica of Silica is not/SSBR easy/ BRto aggregate Composites again because of the stability of the chemical interaction between silica and rubber. TESPD has the ability to create the chemical interaction between the silica Figure5 are the TEM images which show the dispersion of silica in rubber matrix. The darker surface and rubber. On the one hand, ESSBR can make chemical interactions with silica, on the other phasehand, represents it can be the well silica mixed particles. with rubber Silica particlesmatrix. Similar are obviously to TESPD, agglomerated ESSBR can also in improve the PS-SSBR the /BR compositedispersion and of forms silica ain lot the of rubber clumps. matrix Moreover, by forming the a phasechemical interface interaction between between silica silica particles and rubber. and the rubber matrixAs shown looks in obviously Figure 5, when clear. the As ratio is known of SSBR to all,replaced there by is aESSBR layer is of 40%(B3,C3,D3,D3,F3), silicon hydroxyl distributed the evenlydispersion on the surface of silica of improves silica particles with the leading increase to of silica the epoxy agglomerate. degree of WithESSBR. part Additionally, of the SSBR when replaced the by ESSBR,epoxy the degree dispersion of ESSBR of silica used particles remains wasat 25%(F1,F2,F3) obviously improved., the dispersion of silica improved with the increase of the ratio of ESSBR/SSBR.

FigureFigure 5. TEM 5. TEM images images of PS-SSBR/BR( PS-SSBR/BR(A1A1), TS-SSBR/BR(), TS-SSBRA2/BR(), E7-SSBR/BR(A2), E7-SSBRB3),./BR( E10-SSBR/BR(B3), E10-SSBRC3), E15-/BR(C3), E15-SSBRSSBR/BR(/BR(D3),), E20-SSBR/BR( E20-SSBR/BR(E3E3), E25a-SSBR/BR(), E25a-SSBR/F1BR(), F1E25b-SSBR/BR(), E25b-SSBRF2/BR() andF2 E25c-SSBR/BR() and E25c-SSBRF3). /BR(F3).

3.3.3. Bound Rubber of Silica/SSBR/BR Composites Bound rubber, which is the adsorbed rubber on the filler surface, is influenced by the interfacial interaction between the filler and rubber. The bound rubber contents of the silica/SSBR/BR compounds were measured, as shown in Figure 6. When the ratio of SSBR replaced by ESSBR is 40%, it can be seen that the bound rubber content increases with the increase of the epoxy degree of ESSBR. As more epoxy groups react with silica [41,42], the hydrophilicity of silica decreases and the interaction between silica and rubber improves. It also can be seen that when the epoxy degree of ESSBR used remains at 25%, with the increase of the ratio of ESSBR/SSBR, the bound rubber content increases.

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It can be seen from Figure5 that the silica in E25-SSBR /BR (F3) has the best dispersion, even dispersing better than silica in TS-SSBR/BR(A2). The dispersion of silica in E15-SSBR/BR (D3) and E20-SSBR/BR (E3) is comparable to that of TS-SSBR/BR(A2). Previous studies [43–45] have indicated that the chemical interaction between silica and rubber can make its dispersion in the rubber matrix more stable, as silica is not easy to aggregate again because of the stability of the chemical interaction between silica and rubber. TESPD has the ability to create the chemical interaction between the silica surface and rubber. On the one hand, ESSBR can make chemical interactions with silica, on the other hand, it can be well mixed with rubber matrix. Similar to TESPD, ESSBR can also improve the dispersion of silica in the rubber matrix by forming a chemical interaction between silica and rubber. As shown in Figure5, when the ratio of SSBR replaced by ESSBR is 40% (B3,C3,D3,D3,F3), the dispersion of silica improves with the increase of the epoxy degree of ESSBR. Additionally, when the epoxy degree of ESSBR used remains at 25% (F1,F2,F3), the dispersion of silica improved with the increase of the ratio of ESSBR/SSBR.

3.3.3. Bound Rubber of Silica/SSBR/BR Composites Bound rubber, which is the adsorbed rubber on the filler surface, is influenced by the interfacial interaction between the filler and rubber. The bound rubber contents of the silica/SSBR/BR compounds were measured, as shown in Figure6. When the ratio of SSBR replaced by ESSBR is 40%, it can be seen that the bound rubber content increases with the increase of the epoxy degree of ESSBR. As more epoxy groups react with silica [41,42], the hydrophilicity of silica decreases and the interaction between silica and rubber improves. It also can be seen that when the epoxy degree of ESSBR used remains at Polymers 2020, 12, x FOR PEER REVIEW 10 of 16 25%, with the increase of the ratio of ESSBR/SSBR, the bound rubber content increases.

Figure 6. Bound rubber content of PS-SSBR/BR, ESSBR-SSBR/BR and TS-SSBR/BR composites. Figure 6. Bound rubber content of PS-SSBR/BR, ESSBR-SSBR/BR and TS-SSBR/BR composites. 3.3.4. Dynamic Mechanical Properties of Silica/SSBR/BR Composites 3.3.4. Dynamic Mechanical Properties of Silica/SSBR/BR Composites When rubber nanocomposite is applied to tire treads, the anti-wet skid performance is generally When rubber nanocomposite is applied to tire treads, the anti-wet skid performance is generally correlated with the tanδ values at 0 ◦C[46,47]. The higher the tanδ value at 0 ◦C is, the better the correlatedanti-skid performance with the tanδ is. values Additionally, at 0 °C [46,47]. the rolling The resistancehigher theof tan tiresδ value is correlated at 0 °C is, with the better the tan theδ values anti- skid performance is. Additionally, the rolling resistance of tires is correlated with the tanδ values at at 60 ◦C[48,49], the lower the tanδ value at 60 ◦C is, the lower the rolling resistance is. 60 °CFrom [48,49], Figure the lower7a, when the tan theδ ratiovalue ofat SSBR60 °C replacedis, the lower by the ESSBR rolling is 40%, resistance it can is. be seen that with the increaseFrom Figure of the 7a, epoxy when degreethe ratio of of ESSBR, SSBR replaced the glass by transition ESSBR is temperature40%, it can be of seen rubber that gradually with the increaseincreases of [50 the,51 epoxy]. As thedegree rigidity of ESSBR, of the molecularthe glass tr chainansition enhances temperature after the of increaserubber gradually of epoxy groupsincreases on [50,51].the rubber As molecule,the rigidity the of internal the molecular rotation chain hindrance enhances of the after molecular the increase chain increasesof epoxy andgroups the activityon the rubberdecreases. molecule, It can bethe seen internal from rotation Table3 that hindrance with the of increase the molecular of the epoxychain degreeincreases of ESSBR,and the theactivity tan δ decreases. It can be seen from Table 3 that with the increase of the epoxy degree of ESSBR, the tanδ value at 0 °C increases, indicating that the anti-wet skid performance increases. The tanδ values at 0 °C of E20-SSBR/BR (E3) and E25-SSBR/BR (F3) exceeds that of TS-SSBR/BR. This result indicates that ESSBR is very useful to improve the wet-skid resistance performance of the tire tread and its effect is better than TESPD when the epoxy degree is up to 20%. As shown in Figure 7b, the tanδ values at 60 °C of ESSBR-SSBR/BR composites and TS-SSBR/BR are both lower than that of PS-SSBR/BR. The rolling resistance of PS-SSBR/BR is significantly higher than that of ESSBR-SSBR/BR and TS-SSBR/BR. The above situation happens mainly because there exists strong mutual friction between silica particles under cyclic reversed loading in PS-SSBR/BR. In contrast, for ESSBR-SSBR/BR and TS-SSBR/BR composites, this friction loss between silica particles decreased due to the chemical interaction between silica and rubber molecular and less silica-silica interaction. It can be seen from Table 3 that with the increase of epoxy degree of ESSBR, tanδ values at 60 °C decreases, indicating that the rolling resistance of tires decreases. The tanδ values at 60 °C of E25-SSBR/BR (F3) is lower than that of TS-SSBR/BR and tanδ values at 60 °C of E20-SSBR/BR (E3) is similar to that of TS-SSBR/BR. This result indicates that ESSBR is beneficial for lowering the rolling resistance of tire tread and its effect is better than TESPD when the epoxy degree is up to 20%.

Table 3. tanδ at 0 °C, 60 °C and Tg (glass transition temperature) of PS-SSBR/BR, ESSBR-SSBR/BR and TS-SSBR/BR composites.

A1 A2 B3 C3 D3 E3 F3

tanδ@0 ℃ 0.213 0.433 0.246 0.324 0.366 0.467 0.605

tanδ@60 ℃ 0.167 0.105 0.127 0.119 0.108 0.103 0.079

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value at 0 ◦C increases, indicating that the anti-wet skid performance increases. The tanδ values at 0 ◦C of E20-SSBR/BR (E3) and E25-SSBR/BR (F3) exceeds that of TS-SSBR/BR. This result indicates that ESSBR is very useful to improve the wet-skid resistance performance of the tire tread and its effect is betterPolymers than 2020 TESPD, 12, x FOR when PEER REVIEW the epoxy degree is up to 20%. 11 of 16

FigureFigure 7. 7.Dynamic Dynamic properties of of PS-SSBR/BR, PS-SSBR/BR, ESSB ESSBR-SSBRR-SSBR/BR and/BR TS-SSB and TS-SSBRR/BR composites:/BR composites: (a) the (a) the relationrelation of of the the tantanδδ with temperature temperature and and (b) ( bthe) the relation relation of the of tan theδ tanwithδ withstrainstrain amplitude. amplitude.

TableTable 3. 4tan andδ at Figure 0 ◦C, 608a◦ Care and theT gdynamic(glass transition properties temperature) of E25-SSBR/BR of PS-SSBR composites/BR, ESSBR-SSBR with different/BR and ratiosTS-SSBR of ESSBR/SSBR./BR composites. It can be seen that when the epoxy degree of ESSBR used remains up to 25%, with the increase of ratio of ESSBR/SSBR, the glass transition temperature of rubber increases. The tanδ values at 60 °C ofA1 E25-SSBR/BR A2 composites B3decreases with C3 the increase D3of ratio of ESSBR/SSBR, E3 F3 indicatingtanδ@0 ◦C that the rolling 0.213 resistance 0.433 of tires decreases. 0.246 0.324 0.366 0.467 0.605 tanδ@60 ◦C 0.167 0.105 0.127 0.119 0.108 0.103 0.079 Table 4. tanδ at 0 °C and 60 °C of E25-SSBR/BR composites with different ratio of ESSBR/SSBR.

As shown in Figure7b, the tan δ values atF1 60 ◦C ofF2 ESSBR-SSBR F3 /BR composites and TS-SSBR/BR are both lower than that of PS-SSBR/BR. The rolling resistance of PS-SSBR/BR is significantly higher than that of ESSBR-SSBR/BRtan andδ@0 TS-SSBR ℃ / 0.537BR.The above0.568 situation0.627 happens mainly because there exists strong mutual friction between silica particles under cyclic reversed loading in PS-SSBR/BR. In contrast, for ESSBR-SSBR/BRtan andδ@60 TS-SSBR ℃ / 0.096BR composites, 0.088 this0.083 friction loss between silica particles decreased due to the chemical interaction between silica and rubber molecular and less silica-silica interaction. It can be seen from Table3 that with the increase of epoxy degree of ESSBR, tan δ values at 60 ◦C decreases, indicating that the rolling resistance of tires decreases. The tanδ values at 60 ◦C of E25-SSBR/BR (F3) is lower than that of TS-SSBR/BR and tanδ values at 60 ◦C of E20-SSBR/BR (E3) is similar to that of TS-SSBR/BR. This result indicates that ESSBR is beneficial for lowering the rolling resistance of tire tread and its effect is better than TESPD when the epoxy degree is up to 20%. Table4 and Figure8a are the dynamic properties of E25-SSBR /BR composites with different ratios of ESSBR/SSBR. It can be seen that when the epoxy degree of ESSBR used remains up to 25%, with the increase of ratio of ESSBR/SSBR, the glass transition temperature of rubber increases. The tanδ values at 60 ◦C of E25-SSBR/BR composites decreases with the increase of ratio of ESSBR/SSBR, indicating that the rolling resistance of tires decreases.

Table 4. tanδ at 0 ◦C and 60 ◦C of E25-SSBR/BR composites with different ratio of ESSBR/SSBR.

F1 F2 F3

Figure 8. Dynamictanδ@0 properties◦C of E25-SSBR/BR 0.537 composites with 0.568 different ratio of ESSBR/SSBR: 0.627 (a) the relation of thetan tanδδ@60 with◦C temperature and 0.096 (b) the relation of the 0.088 tanδ with strain amplitude. 0.083

3.3.5. Curing Behavior of Silica/SSBR/BR Composites When the ratio of SSBR replaced by ESSBR is 40%, it can be seen from Table 5 that with the increase of the epoxy degree of ESSBR, T10 (the time when the torque reaches 10% of the maximum

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Figure 7. Dynamic properties of PS-SSBR/BR, ESSBR-SSBR/BR and TS-SSBR/BR composites: (a) the relation of the tanδ with temperature and (b) the relation of the tanδ with strain amplitude.

Table 4 and Figure 8a are the dynamic properties of E25-SSBR/BR composites with different ratios of ESSBR/SSBR. It can be seen that when the epoxy degree of ESSBR used remains up to 25%, with the increase of ratio of ESSBR/SSBR, the glass transition temperature of rubber increases. The tanδ values at 60 °C of E25-SSBR/BR composites decreases with the increase of ratio of ESSBR/SSBR, indicating that the rolling resistance of tires decreases.

Table 4. tanδ at 0 °C and 60 °C of E25-SSBR/BR composites with different ratio of ESSBR/SSBR.

F1 F2 F3

tanδ@0 ℃ 0.537 0.568 0.627

Polymers 2020, 12, 1257 tanδ@60 ℃ 0.096 0.088 0.083 11 of 15

Figure 8. Dynamic properties of E25-SSBR/BR composites with different ratio of ESSBR/SSBR: (a) the Figurerelation 8. of Dynamic the tanδ propertieswith temperature of E25-SSBR/BR and (b) the composit relationes of with the different tanδ with ratio strain of amplitude.ESSBR/SSBR: (a) the relation of the tanδ with temperature and (b) the relation of the tanδ with strain amplitude. 3.3.5. Curing Behavior of Silica/SSBR/BR Composites 3.3.5. WhenCuring the Behavior ratio of SSBRof Silica/SSBR/BR replaced by ESSBR Composites is 40%, it can be seen from Table5 that with the increase of the epoxy degree of ESSBR, T (the time when the torque reaches 10% of the maximum torque) When the ratio of SSBR replaced10 by ESSBR is 40%, it can be seen from Table 5 that with the increases. It was speculated that the epoxy group reacted with the promoter during the mixing process, increase of the epoxy degree of ESSBR, T10 (the time when the torque reaches 10% of the maximum which weakened the effect of the promoter, T90 (the time when the torque reaches 90% of the maximum Polymerstorque) 2020 tends, 12, tox; doi: decrease FOR PEER slightly. REVIEW This is possibly because during the vulcanizationwww.mdpi.com/journal/polymers process the epoxy group increases the reactivity of adjacent double bonds, thus the crosslinking time had been shortened. ML (the minimum torque) is related to filler-filler network, with the increase of the epoxy degree of ESSBR, filler-filler network is weakened and ML decreases. MH (the maximum torque) is influenced by filler–filler network, filler–rubber network, and rubber–rubber network, and there is no obvious rule to its changing. An optimal crosslink density is very important to achieve rubber with good mechanical properties, with the increase of the epoxy degree of ESSBR, crosslink density tends to increase.

Table 5. Curing behavior of silica/SSBR/BR composites.

Crosslink Density T10 (min) T90 (min) ML (dNm) MH (dNm) ∆M (dNm) 4 3 (10− mol/cm ) A1 1.5 53.4 38.9 56.3 17.4 1.12 A2 3.1 29.0 28.4 65.4 37 1.54 B3 3.7 53.0 34.6 66.3 31.7 1.53 C3 4.4 53.5 33.1 68.1 35 1.55 D3 4.8 51.2 31.7 65.6 33.9 1.58 E3 4.5 46.7 30.9 74.8 43.9 1.62 F1 6.6 47.3 31.2 76.1 44.9 1.59 F2 6.7 43.4 28.6 72.3 43.7 1.58 F3 6.2 44.6 26.7 69.7 43 1.61

3.3.6. Static Mechanical Properties of Silica/SSBR/BR Composites When the ratio of SSBR replaced by ESSBR is 40%, it can be seen from Figure9 and Table6 that with the increase of the epoxy degree of ESSBR, the modulus at 100% and 300% strain, the tensile stress of vulcanized rubber increases, and the elongation at break decreases. The tensile strength is related to the crosslinking density and the interaction between filler and rubber matrix. The higher the epoxy degree is, the better the binding force of silica with the molecular chain is. This binding force limits the movement of the molecular chain, thereby the tensile strength and the modulus of constant elongation increases, and the elongation at break decreases. Polymers 2020, 12, x FOR PEER REVIEW 12 of 16

torque) increases. It was speculated that the epoxy group reacted with the promoter during the mixing process, which weakened the effect of the promoter, T90 (the time when the torque reaches 90% of the maximum torque) tends to decrease slightly. This is possibly because during the vulcanization process the epoxy group increases the reactivity of adjacent double bonds, thus the crosslinking time had been shortened. ML (the minimum torque) is related to filler-filler network, with the increase of the epoxy degree of ESSBR, filler-filler network is weakened and ML decreases. MH (the maximum torque) is influenced by filler–filler network, filler–rubber network, and rubber– rubber network, and there is no obvious rule to its changing. An optimal crosslink density is very important to achieve rubber with good mechanical properties, with the increase of the epoxy degree of ESSBR, crosslink density tends to increase.

Table 5. Curing behavior of silica/SSBR/BR composites.

T10 T90 ML MH ΔM Crosslink Density (10−4

(min) (min) (dNm) (dNm) (dNm) mol/cm3) A1 1.5 53.4 38.9 56.3 17.4 1.12 A2 3.1 29.0 28.4 65.4 37 1.54 B3 3.7 53.0 34.6 66.3 31.7 1.53 C3 4.4 53.5 33.1 68.1 35 1.55 D3 4.8 51.2 31.7 65.6 33.9 1.58 E3 4.5 46.7 30.9 74.8 43.9 1.62 F1 6.6 47.3 31.2 76.1 44.9 1.59 F2 6.7 43.4 28.6 72.3 43.7 1.58 F3 6.2 44.6 26.7 69.7 43 1.61

3.3.6. Static Mechanical Properties of Silica/SSBR/BR Composites When the ratio of SSBR replaced by ESSBR is 40%, it can be seen from Figure 9 and Table 6 that with the increase of the epoxy degree of ESSBR, the modulus at 100% and 300% strain, the tensile stress of vulcanized rubber increases, and the elongation at break decreases. The tensile strength is related to the crosslinking density and the interaction between filler and rubber matrix. The higher the epoxy degree is, the better the binding force of silica with the molecular chain is. This binding force limits the movement of the molecular chain, thereby the tensile strength and the modulus of constant elongation increases, and the elongation at break decreases. Polymers 2020It also, 12, 1257can be seen from Figure 9 and Table 6 that when the epoxy degree of ESSBR used remains12 of 15 up to 25%, with the increase of ratio of ESSBR/SSBR, the Modulus at 100% and 300% strain of vulcanized rubber increases, the elongation at break decreases.

Figure 9. MechanicalFigure 9. Mechanical properties properties of silica /ofSSBR silica/SSBR/BR/BR composites: composites: (a) samples (a)samples with with diff erentdifferent ESSBRs(epoxy degree), (b) samples withESSBRs(epoxy different degree),(b) content of samples E25. with different content of E25

Table 6. Mechanical properties of silica/SSBR/BR composites.

Polymers 2020, 12, x; doi:Elongationat FOR PEER REVIEWModulus at Modulus at Tensilewww.mdpi.com/journal/polymers Stress Shore A Sample Break (%) 100% (MPa) 300% (MPa) (MPa) A1 437 39 2.6 0.1 8.2 0.3 16.5 1.4 64 ± ± ± ± A2 386 3 4.8 0.1 13.6 0.3 21.2 0.3 63 Polymers 2020, 12, x FOR± PEER REVIEW ± ± ± 13 of 16 B3 377 7 3.9 0.2 13.1 0.4 18.2 2.4 63 ± ± ± ± C3 355 11 4.7 0.1 13.8 0.1 19.7 2.5 62 ± Table 6. Mechanical± properties of silica/SSBR/BR± composites. ± D3 336 27 4.9 0.2 16.5 0.4 20.3 0.8 61 ± ± ± ± E3 338 34 5.8 0.4 17.4 0.7Tensile 21.6 2.5 64 Elongationat± Modulus± at Modulus± at ± Shore A F1Sample 346 29 5.5 0.4 16.5 0.4Stress 21.9 2.6 61 Break± (%) 100%± (MPa) 300% (MPa)± ± Hardness F2 309 16 6.1 0.5 20.3 0.5(MPa) 20.6 1.5 64 ± ± ± ± F3A1 317 437 26± 39 6.6 2.6 ± 0.1 8.2 22.6 ± 0.3 0.3 16.5 ± 23.91.4 1.2 64 65 ± ± ± ± A2 386 ± 3 4.8 ± 0.1 13.6 ± 0.3 21.2 ± 0.3 63 B3 377 ± 7 3.9 ± 0.2 13.1 ± 0.4 18.2 ± 2.4 63 It also canC3 be seen from 355 ± Figure11 9 and 4.7 Table ± 0.16 that when 13.8 ± the 0.1 epoxy 19.7 degree ± 2.5 of ESSBR 62 used remains up to 25%, with theD3 increase 336 of ± ratio27 of ESSBR 4.9 ±/SSBR, 0.2 the Modulus 16.5 ± 0.4 at 100% 20.3 ± and 0.8 300% strain 61 of vulcanized rubber increases,E3 the elongation 338 ± 34 at break 5.8 decreases. ± 0.4 17.4 ± 0.7 21.6 ± 2.5 64 F1 346 ± 29 5.5 ± 0.4 16.5 ± 0.4 21.9 ± 2.6 61 F2 309 ± 16 6.1 ± 0.5 20.3 ± 0.5 20.6 ± 1.5 64 3.3.7. Abrasion Loss Properties of Silica/SSBR/BR Composites F3 317 ± 26 6.6 ± 0.1 22.6 ± 0.3 23.9 ± 1.2 65 When the ratio of SSBR replaced by ESSBR is 40%, it can be seen from Figure 10, with the increase 3.3.7. Abrasion loss properties of silica/SSBR/BR composites of the epoxy degree of ESSBR, the abrasion volume decreases gradually, due to more epoxy groups reacting withWhen silica the and ratio creating of SSBR replaced more by rubber–filler ESSBR is 40%, chemicalit can be seen interaction, from Figure 10, which with the may increase be helpful to of the epoxy degree of ESSBR, the abrasion volume decreases gradually, due to more epoxy groups improve wear-resistingreacting with silica properties. and creating Itmore also rubber can– befiller seen chemical that interaction, when the which epoxy may degree be helpful of ESSBRto used remains upimprove to 25%, wear-resisting with the increaseproperties. of It ratioalso can of ESSBRbe seen /thatSSBR, when the the abrasion epoxy degree volume of ESSBR decreases. used remains up to 25%, with the increase of ratio of ESSBR/SSBR, the abrasion volume decreases.

FigureFigure 10. Abrasion 10. Abrasion loss loss properties properties of of silica/SSBR/BR silica/SSBR composites./BR composites.

4. Conclusion In this research, SSBR was epoxidized to ESSBR with different epoxy degrees and then used as a macromolecular coupling agent to modify silica/rubber nanocomposites. Due to the ring- opening reaction between epoxy groups and the silicon hydroxyl without any VOCs emission, silica/rubber nanocomposite for tire tread can be made with no VOC emission. Additionally, as ESSBR has good compatibility with SSBR/BR matrix and can be crosslinked with the rubber matrix, silica–ESSBR– rubber matrix chemical bonds can be formed. As a result, ESSBR as a macromolecular coupling agent is beneficial for silica/SSBR/BR nanocomposites used for green tire treads to get better wet-skid resistance and lower rolling resistance, with no VOC emission. It is a hopeful candidate to replace the traditional coupling agent, TESPD, which has VOCs emission.

Author Contributions: L.Z, M.G., X.Y. and C. L. conceived and designed the experiments; C. L. performed the experiments; M. G., X.Y., C.L. and L.Z. analyzed the data; X.Z. contributed reagents/materials/analysis tools and

Polymers 2020, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2020, 12, 1257 13 of 15

4. Conclusions In this research, SSBR was epoxidized to ESSBR with different epoxy degrees and then used as a macromolecular coupling agent to modify silica/rubber nanocomposites. Due to the ring- opening reaction between epoxy groups and the silicon hydroxyl without any VOCs emission, silica/rubber nanocomposite for tire tread can be made with no VOC emission. Additionally, as ESSBR has good compatibility with SSBR/BR matrix and can be crosslinked with the rubber matrix, silica–ESSBR–rubber matrix chemical bonds can be formed. As a result, ESSBR as a macromolecular coupling agent is beneficial for silica/SSBR/BR nanocomposites used for green tire treads to get better wet-skid resistance and lower rolling resistance, with no VOC emission. It is a hopeful candidate to replace the traditional coupling agent, TESPD, which has VOCs emission.

Author Contributions: L.Z., M.G., X.Y. and C.L. conceived and designed the experiments; C.L. performed the experiments; M.G., X.Y., C.L. and L.Z. analyzed the data; X.Z. contributed reagents/materials/analysis tools and search literatures; X.Y. and C.L. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (51790501) and the APC was funded by Beijing University of Chemical Technology. Conflicts of Interest: The authors declare no conflict of interest.

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