processes

Article Itaconate Based Elastomer as a Green Alternative to Styrene–Butadiene Rubber for Engineering Applications: Performance Comparison

Liwei Li, Haijun Ji, Hui Yang, Liqun Zhang, Xinxin Zhou * and Runguo Wang *

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic-Inorganic Composites, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] (L.L.); [email protected] (H.J.); [email protected] (H.Y.); [email protected] (L.Z.) * Correspondence: [email protected] (X.Z.); [email protected] (R.W.); Tel.: +86-1590-127-3417 (X.Z.); +86-1381-067-5108 (R.W.)


Received: 31 October 2020; Accepted: 22 November 2020; Published: 24 November 2020


Abstract: In response to increasingly stringent requirements for the sustainability and environmental friendliness of the rubber industry, the application and development of bio-based elastomers have received extensive attention. In this work, we prepared a new type of bio-based elastomer poly(dibutyl itaconate-butadiene) copolymer (PDBIB) nanocomposite using carbon black and non-petroleum-based silica with a coupling agent. Using dynamic thermodynamic analysis (DMTA) and scanning electron microscope (SEM), we studied the effects of feed ratio on dynamic mechanical properties, micro morphology, and filler dispersion of PDBIB composites. Among them, silica-reinforced PDBIB60 (weight ratio of dibutyl itaconate to butadiene 40/60) and carbon black-reinforced PDBIB70 (weight ratio of dibutyl itaconate to butadiene 30/70) both showed excellent performance, such as tensile strength higher than 18 MPa and an elongation break higher than 400%. Compared with the widely used ESBR, the results showed that PDBIB had better rolling resistance and heat generation than ESBR. In addition, considering the development of green tires, we compared it with the solution polymerized styrene–butadiene rubber with better comprehensive performance, and analyzed the advantages of PDBIB and the areas to be improved. In summary, PDBIB prepared from bio-based monomers had superior performance and is of great significance for achieving sustainable development, providing a direction for the development of high-performance green tire and holding great potential to replace petroleum-derived elastomers.

Keywords: bio-based elastomer; nanocomposites; carbon black; silica; styrene–butadiene rubber

1. Introduction Due to its unique high elasticity and large deformation capacity, rubber materials have become irreplaceable in many fields, such as the tire industry [1–3], sealing industry [4], as well as the damping and shock absorption industry [5–7]. The annual consumption of rubber worldwide exceeds 20 million tons. Thus far, the world has formed a relatively large-scale production of general synthetic rubber [8–10] such as styrene–butadiene rubber, nitrile rubber, neoprene rubber, butadiene rubber, ethylene–propylene rubber, butyl rubber, isoprene rubber, styrene block copolymer thermoplastic elastomers, and special rubber [11,12] represented by polyurethane, fluorine rubber, and silicone rubber. Furthermore, the research and production application systems of all kinds of synthetic rubbers are well developed. At present, synthetic rubber still uses fossil resources such as petroleum as raw materials, which may be sufficient to meet current application needs, but it is not a long-term solution.

Processes 2020, 8, 1527; doi:10.3390/pr8121527 www.mdpi.com/journal/processes Processes 2020, 8, 1527 2 of 17

Its huge consumption makes the environment and resources greatly threatened, especially in the automotive industry application (tires, hoses, etc.). In future development, it is urgent to develop new high-performance synthetic rubbers that use sustainable resources as raw materials [13,14]. Using renewable resources to prepare a new generation of bio-based elastomer materials is an innovative idea to keep sustainable development of the synthetic rubber industry. At present, two routes are mainly involved. One is to prepare isoprene [15,16], butadiene, and other monomers through biological fermentation processes, and then prepare bio-based isoprene rubbers and other bio-based materials. For example, the Goodyear company has successfully prepared bio-based isoprene rubber, and the LANXESS company has prepared bio-based butyl rubber and ethylene–propylene rubber. The other route is to use bio-based chemicals [13,17] such as propylene glycol, succinic acid, and itaconic acid. Bio-based elastomers are prepared via condensation polymerization [18] or emulsion polymerization [19,20]. Pure synthetic rubber cannot meet engineering applications in terms of mechanical properties and wear resistance, so it needs to be reinforced with nano-fillers. With the development of nanotechnology, a variety of nanoparticles have been developed to be combined with rubber, which not only strengthens the basic mechanical properties of rubber, but also gives the material many functionalities. The widely used nano-fillers mainly include carbon black [21], silica [22], layered silicate [23,24], graphene [25–27], and carbon nanotube [28,29]. Nano-fillers have a significant strengthening effect on rubber materials, which is mainly reflected in the improvement of modulus, tensile strength, and wear resistance. More than 92% of carbon black production in the world was used in rubber manufacturing, especially tire production, such as inner linings, sidewall carcasses, tread, air springs, belts, conveyor wheels, and some vibration isolation devices. At present, the global production of carbon black is about 8.1 million metric tons, ranking among the top 50 industrial chemical manufacturers in the world. However, CB causes pollution because of its origin from petroleum. Silica is a non-petroleum based filler with good prospects. For tires, silica can also reduce the rolling resistance of rubber without sacrificing the wet skid resistance of rubber, that is, it can improve fuel efficiency and reduce greenhouse gas emissions under the premise of ensuring the safe driving of cars. Therefore, silica has great application prospects in “green tire” manufacturing [30]. Due to the surface characteristics of silica, it has a poor affinity with the rubber matrix and is easy to form aggregates, so the key to preparing silica composites is to solve the problem of mixing silica and rubber and dispersion in the rubber matrix. The current methods mainly include in-situ modification technology [31,32], rubber matrix modification [33,34], and wet compounding technology [35]. The continuous development of these technologies has led to the rapid expansion of the silica-filled application. Therefore, the research on the these two types of universal nano-fillers nanocomposites of bio-based elastomers is meaningful for the development of nanofillers and bio-based elastomers. Bio-based itaconate elastomer (PDBIB) [14,36] involved in this article belongs to the second route. It is a new type of bio-based synthetic rubber prepared based on bulk bio-based chemicals, itaconic acid, n-butanol, and a small amount of diene. At present, the annual output of itaconic acid in China has reached 50,000 tons, and the price is 1662 Dollars/ton. Compared with traditional petrochemical-based chemicals, the cost is no longer a disadvantage. In addition, the static and dynamic mechanical properties of bio-based itaconate elastomers can be flexibly adjusted by adjusting the side groups [37], the copolymerization ratio [36], and the functionalization in the chain [23,38,39]. Moreover, the properties are becoming more perfect. Different from traditional petroleum-based engineering rubber, the molecular structure of bio-based elastomers designed in this paper includes weakly polar long-chain ester groups and flexible butadiene segments, which are quite different from traditional NR (natural rubber), SBR (styrene–butadiene rubber), and NBR (nitrile rubber), so the research about performance and interface interaction of silica-filled PDBIB and carbon black-filled PDBIB are meaningful. Meanwhile, a comparatively comprehensive comparison of the properties of bio-based elastomers nanocomposites and traditional bulk ESBR and SSBR nanocomposites are of great value for the true application of bio-based elastomers. Processes 2020, 8, 1527 3 of 17

2. Experimental Section

2.1. Materials Dibutyl itaconate (purity of 96%) was purchased from Sigma-Aldrich Company (Burlington, MA, USA). All of the other chemicals and solvents used in the polymerization, including butadiene (purity of 94%), rosin soap, phosphoric acid, potassium hydroxide, ethylene diamine tetraacetic acid (EDTA), sodium dinaphthyllmethanedisulfonate (TAMOL), ferric ethylene diamine tetraacetic acid salt (Fe-EDTA), sodium hydrosulfite (SHS), sodium hydroxymethanesulfinate (SFS), p-menthane hydroperoxide (PMH), hydroxylamine (HA), and ethanol, were kindly provided by Jilin Petroleum Company and used as received. Emlusion-polymerized styrene–butadiene rubber (ESBR 1502) was bought from Tianjin Changli Rubber Trade Co., Ltd. (Tianjin, China) and solution-polymerized styrene–butadiene rubber (SSBR2466) was provided by the TSRC Corporation. The precipitated silica (Ultrasil VN3) with a BET specific surface area of 175 m2/g was bought from Degussa Chemical. All of the rubber additives were industrial grade and commercially available. Deionized water was used for all polymerization runs.

2.2. Synthesis of Poly(dibutyl itaconate-co-butadiene) (PDBIB) As summarized in Table1, all of the chemicals except SHS and PMH, were added in the sealed reaction bottle, and the mixture in the bottle was pre-emulsified at 25 ◦C for 4 h to form a stable and homogeneous latex. Later, the SHS solution was injected into the reaction bottle to eliminate the residual oxygen in the pre-emulsion, followed by the PMH solution. The polymerization was allowed to proceed for 8 h at 5 ◦C, followed by adding hydroxylamine solution to terminate the polymerization, and the PDBIB latex was obtained. The PDBIB gum was obtained by coagulating the latex with ethanol and drying the product in a vacuum oven at 40 ◦C until a constant weight was obtained.

Table 1. Recipe for redox-initiated emulsion polymerization of PDBIB.

Ingredients (Concentration) Amount (g) Dibutyl itaconate [a] Various Butadiene [a] Various Deionized water 190.00 Disproportionated potassium rosinate 1.50 Sodium soap 1.50 Phosphoric acid 0.23 Potassium hydroxide 0.40 EDTA 0.03 TAMOL 0.14 SHS 0.04 Fe-EDTA 0.02 SFS 0.04 PMH 0.06 [a] The total dosage of the monomers was fixed at 100.00 g, as shown in Table1.

2.3. Preparation of the PDBIB/Silica and ESBR/Silica Nanocomposites The compounding formulation for PDBIB/silica is shown in Table2. First, silica with a coupling agent was mixed with neat PDBIB in a HAAKE Rheomix 600 OS internal mixer for 10 min at room temperature. Second, the mixture obtained was rotated in the internal mixer for 5 min at 150 ◦C to facilitate further reaction between PDBIB and silica, and it was then taken out and cooled down to room temperature. Third, the other additives were mixed with the PDBIB/silica. Finally, the PDBIB/silica compound was cured under 15 MPa at 150 ◦C for an optimum curing time as determined by a rotor-less rheometer. The ESBR/silica compound was also compounded and vulcanized in the same manner. Processes 2020, 8, 1527 4 of 17

Table 2. Recipe of PDBIB/silica and ESBR/silica nanocomposites.

Ingredients. Loading (phr) [a] PDBIB/ESBR 70.0 BR9000 30.0 Silica (VN3) 60.0 Si69 6.0 Zinc oxide 5.0 Stearic acid 2.0 Antioxidant 4020 [b] 1.0 Antioxidant RD [c] 1.0 Wax 1.0 Accelerator CZ [d] 1.0 Accelerator NS [e] 1.2 Sulfur 1.5 [a] phr is the abbreviation for weight parts per 100 parts rubber by weight. [b] N-Isopropyl-N’-phenyl-p-phenylene diamine. [c] Poly(1,2-dihydro-2,2,4-trimethyl-quinoline). [d] N-cyclohexylbenzothiazole-2-sulphenamide. [e] N-tert-butylbenzothiazole-2-sulphenamide.

2.4. Preparation of the PDBIB/CB and ESBR/CB Nanocomposites The PDBIB and additives were mixed by a 15.24 cm two-roll mill according to the formulation provided in Table3. The compound was cured in an XLB-D 350_350 hot press (Huzhou East Machinery, Huzhou, China) under a pressure of 15 MPa at 150 ◦C for its optimum cure time. ESBR/CB compound was also compounded and vulcanized in the same manner.

Table 3. Formulation for the PDBIB/CB and ESBR/CB nanocomposites.

Ingredient Loading (phr) PDBIB/ESBR 100 Carbon black 60 Zinc Oxide 3 Stearic acid 2 TDAE 10 Antioxidant 4020 1 Accelerator DM [a] 1.2 Accelerator D [b] 0.6 Sulfur 1.5 [a] [b] 2,20-dibenzothiazoledisulfde. 1,3-Diphenylguanidine.

2.5. Measurements and Characterization The average molecular weight of PDBIB and ESBR was measured using gel permeation chromatography (GPC) on a Waters Breeze instrument equipped with three water columns (Steerage HT3 HT5 HT6E) using tetrahydrofuran as the solvent (1.0 mL/min) and a Waters 2410 refractive index detector, and polystyrene standards were used for calibration. The FTIR spectra of PDBIB and ESBR were collected on a Tensor 27 spectrometer (Bruker Optic GmbH, Ettlingen, Germany) with the 1 attenuated total reflection (ATR) technique at 4 cm− resolution with 32 scans under air atmosphere. 1H NMR measurements of PDBIB and ESBR were conducted with a Bruker AV400 spectrometer (Bruker, Bremen, Germany) with CDCl3 as the solvent. The test frequency was 400 MHz and scanned 16 times. Differential scanning calorimetry (DSC) measurements were performed on a Mettler-Toledo DSC instrument under nitrogen. A sample of 5 mg was heated to 373 K (100 ◦C) and kept isothermal for 5 min to remove any previous thermal history. Then it was cooled to 173 K ( 100 C) and reheated − ◦ to 373 K (100 ◦C) at 10 K/min. The surface morphology of the PDBIB and ESBR nanocomposites were observed in Hitachi S-4800 scanning electron microscope. The samples were prepared by Processes 2020, 8, x FOR PEER REVIEW 5 of 18 for 5 min to remove any previous thermal history. Then it was cooled to 173 K (−100 °C) and reheated Processesto 373 K2020 (100, 8, °C) 1527 at 10 K/min. The surface morphology of the PDBIB and ESBR nanocomposites 5were of 17 observed in Hitachi S-4800 scanning electron microscope. The samples were prepared by fracturing fracturingthe composites the composites in liquid nitrogen in liquid and nitrogen were then and sputter were then coated sputter with coated gold. The with dynamic gold. The rheological dynamic rheologicalproperties of properties the PDBIB, of theESBR, PDBIB, and ESBR,SSBR andnanocomp SSBR nanocompositesosites were analyzed were analyzed using an usingRPA2000 an RPA2000 (Alpha Technologies Co., Hudson, Ohio, USA) at 333 K (60 °C). The strain amplitude was varied from 0.1% (Alpha Technologies Co., Hudson, Ohio, USA) at 333 K (60 ◦C). The strain amplitude was varied from 0.1%to 100% to 100%at the attest the frequency test frequency of 1 Hz. of The 1 Hz. dynamic The dynamic mechanical mechanical properties properties were determined were determined on a VA on3000 a VAdynamic 3000 dynamicmechanical mechanical thermal analyzer thermal (01 analyzer dB-Metravib (01 dB-Metravib Co., Rhone Co., Alpes, Rhone France) Alpes, in the France) tension in themode tension at a fixed mode frequency at a fixed of frequency 10 Hz (the of 10most Hz common (the most frequency common frequencyused to investigate used to investigate the dynamic the dynamicmechanical mechanical properties properties in rubber inindustry, rubber industry,especially especially in tire industry) in tire industry) and a strain and amplitude a strain amplitude of 0.3%. ofThe 0.3%. scanned The scannedtemperature temperature ranged from ranged 193K from (−80 193K °C) (to 80373C) K (100 to 373 °C), K (100and theC), heating and the rate heating was rateof 3 − ◦ ◦ wasK/min. of 3Tensile K/min. tests Tensile of teststhe PDBIB, of the PDBIB, ESBR, ESBR,and SS andBR SSBRnanocomposites nanocomposites were were conducted conducted according according to toASTM ASTM D412 D412 (dumbbell-shaped) (dumbbell-shaped) on a on LRX a LRX Plus Plustensile tensile tester tester made made by Lloyd by Lloyd Instruments, Instruments, Ltd., West Ltd., WestSussex, Sussex, UK. The UK. interval The interval between between vulcanization vulcanization and testing and testing was was24 h. 24 The h. Thetest tensile test tensile rate ratewas was500 500mm/min. mm/min. The Thenumber number of samples of samples in the in same the same group group waswas five. five.

3. Results and Discussion

3.1. Synthesis and Char Characterizationacterization of PDBIB A series series of of bio-based bio-based elastomers elastomers PDBIB PDBIB with with Bd(buta Bd(butadiene)diene) contents contents of 50 of wt.% 50 wt.% to 80 towt.% 80 wt.%were wereprepared prepared using usingthe redox-initiated the redox-initiated polymerization polymerization carried carried out under out under mild mildconditions. conditions. The Thepolymerization polymerization equationequation is illustrated is illustrated in Scheme in Scheme 1. The1. redox-initiated The redox-initiated polymerization polymerization can obtain can obtainhigh molecular high molecular weight weightpolymers polymers as it can as decr it canease decrease the possibility the possibility of chain of transfer chain transfer and chain and chaintermination. termination. As shown As shown in Table in Table 4, with4, with a variet a varietyy of of Bd(butadiene) Bd(butadiene) content, content, the number-averagenumber-average molecular weightsweights (Mn) (Mn) and and polydispersity polydispersity index(PDI) index(PDI) ranged ranged from from 33.8 33.8104 ×g /10mol4 g/mol to 45.0 to 45.0104 g ×/mol 104 × × andg/mol ranged and ranged from 2.97 from to 3.79,2.97 respectively.to 3.79, respectively. We compared We compared it with ESBR it with and ESBR found and that found the molecular that the weightmolecular of PDBIBweight was of PDBIB higher was than higher ESBR. than The ESBR. yields The of the yields polymerization of the polymerization were above were 65%. above 65%.

Scheme 1. Polymerization reaction of PDBIB.

Table 4. Molecular weight and yield of PDBIB and ESBR.

4 4 Sample Mn (10 )Mw (10 ) PolydispersityPolydispersity Index Yield (%) Sample Mn (104) Mw (104) Yield (%) PDBIB50 45.0 170.5 3.79Index 67 PDBIB60 40.0 133.9 3.36 70 PDBIB50PDBIB70 38.345.0 129.6170.5 3.383.79 68 67 PDBIB60PDBIB80 33.840.0 100.2133.9 2.973.36 65 70 PDBIB70ESBR 11.538.3 44.0129.6 3.833.38 —— 68 PDBIB80 33.8 100.2 2.97 65 3.2. StructureESBR Characteristic Comparison11.5 of Raw PDBIB44.0 and ESBR 3.83 —— Figure1 displays the FTIR spectra of raw PDBIB polymers with di fferent butadiene contents and 3.2. Structure Characteristic Comparison of Raw PDBIB and1 ESBR ESBR. The absorption peaks at 3017, 2915, and 2840 cm− were attributed to the stretching vibrations of -CHFigure3, -CH 1 2displays, and -CH the groups FTIR spectra in the ESBR,of raw respectively.PDBIB polymers The with peak different position butadiene of PDBIB contents here slightly and 1 shifted.ESBR. The The absorption absorption peaks at 1640 at 3017, cm− 2915,belonged and 2840 to the cm C=−1 Cwere stretching attributed vibration to the ofstretching the butadiene vibrations unit. 1 1 Theof -CH diff3,erence -CH2, inand the -CH spectra groups was in the the absorption ESBR, respectively. at 1728 cm −Theand peak 1175 position cm− , whichof PDBIB corresponded here slightly to theshifted. C=O The and absorption C-O-C stretching at 1640 cm vibration−1 belonged of the to DBI the (dibutylC=C stretching itaconate) vibration unit. Asof the butadiene monomer ratiounit. 1 changedThe difference (i.e., butadiene in the spectra gradually was the increased), absorption we at clearly 1728 cm saw−1 and that 1175 the peakcm−1, intensitieswhich corresponded of 1728 cm −to 1 and 1175 cm− gradually decreased, while the corresponding peak positions of butadiene, such as 1 1 1640 cm− and 969 cm− , gradually increased. Processes 2020, 8, x FOR PEER REVIEW 6 of 18 Processes 2020, 8, x FOR PEER REVIEW 6 of 18 the C=O and C-O-C stretching vibration of the DBI (dibutyl itaconate) unit. As the monomer ratio the C=O and C-O-C stretching vibration of the DBI (dibutyl itaconate) unit. As the monomer ratio changed (i.e., butadiene gradually increased), we clearly saw that the peak intensities of 1728 cm−1 changed (i.e., butadiene gradually increased), we clearly saw that the peak intensities of 1728 cm−1 and 1175 cm−1 gradually decreased, while the corresponding peak positions of butadiene, such as Processesand 11752020 cm, 8,− 15271 gradually decreased, while the corresponding peak positions of butadiene, such6 of as 17 1640 cm−1 and 969 cm−1, gradually increased. 1640 cm−1 and 969 cm−1, gradually increased.

FigureFigure 1. 1. FTIRFTIR spectra spectra of of PDBIB PDBIB with with differe differentnt butadiene butadiene contents contents and ESBR. Figure 1. FTIR spectra of PDBIB with different butadiene contents and ESBR. The structural difference of PDBIB and ESBR were further confirmed via 1H NMR. The spectra The structural difference of PDBIB and ESBR were further confirmed via 1H NMR. The spectra The structural difference of PDBIB and ESBR were further confirmed via 1H NMR. The spectra andand the the detailed assignments of each peakpeak toto thethe molecularmolecularstructure structure comparison comparison are are shown shown in in Figure Figure2. and the detailed assignments of each peak to the molecular structure comparison are shown in Figure 2.The The proton proton shifts shifts of butadiene of butadiene in their in copolymerstheir copolymers are similar. are similar. The obvious The obvious peaks of peaks dibutyl of itaconate dibutyl 2. The proton shifts of butadiene in their copolymers are similar. The obvious peaks of dibutyl itaconateat 0.94 ppm at corresponded0.94 ppm corresponded to the methyl to the protons methyl (6) ofprotons the side (6) groups of the andside thegrou signalsps and at the 4.03 signals ppm was at itaconate at 0.94 ppm corresponded to the methyl protons (6) of the side groups and the signals at 4.03from ppm the methylene was from protons the methylene (3) of the sideprotons groups. (3) of Moreover, the side 7.0–7.3 groups. ppm Moreover, are attributed 7.0–7.3 to theppm proton are 4.03 ppm was from the methylene protons (3) of the side groups. Moreover, 7.0–7.3 ppm are attributedof the benzene to the ring. proton of the benzene ring. attributed to the proton of the benzene ring.

Figure 2. 1H NMR spectra of PDBIB60 and ESBR. Figure 2. 1H NMR spectra of PDBIB60 and ESBR. Figure 2. 1H NMR spectra of PDBIB60 and ESBR. The elastomeric properties of materials are influenced by the glass transition temperature (Tg), The elastomeric properties of materials are influenced by the glass transition temperature (Tg), whichThe is elastomeric generally below properties room of temperature materials are for infl elastomers.uenced by Neitherthe glass of transition the five curvestemperature show (T anyg), which is generally below room temperature for elastomers. Neither of the five curves show any crystallizationwhich is generally peak, below indicating room that temperature the PDBIB andfor ESBRelastomers. were all Neither amorphous of the copolymers, five curves as show shown any in crystallization peak, indicating that the PDBIB and ESBR were all amorphous copolymers, as shown Figurecrystallization3. The T peak,g values indicating of PDBIB that with the increased PDBIB and butadiene ESBR were and all ESBR amorphous were 57.9, copolymers,63.9, 66.2, as shown70.2, g − − − − inand Figure53.9 3. TheC, respectively. T values of PDBIB The reason with increased is that the butadiene Tg of styrene and homopolymerESBR were −57.9, is much−63.9, − higher66.2, −70.2, than in Figure− 3.◦ The Tg values of PDBIB with increased butadiene and ESBR were −57.9, −63.9, −66.2, −70.2, the dibutyl itaconate homopolymer. Simultaneously, it can be found that PDBIB with high butadiene content has better low temperature resistance than ESBR. Processes 2020, 8, x FOR PEER REVIEW 7 of 18 and −53.9 °C, respectively. The reason is that the Tg of styrene homopolymer is much higher than the dibutyl itaconate homopolymer. Simultaneously, it can be found that PDBIB with high butadiene content has better low temperature resistance than ESBR. Processes 2020, 8, x FOR PEER REVIEW 7 of 18 and −53.9 °C, respectively. The reason is that the Tg of styrene homopolymer is much higher than the Processesdibutyl2020 itaconate, 8, 1527 homopolymer. Simultaneously, it can be found that PDBIB with high butadiene7 of 17 content has better low temperature resistance than ESBR.

Figure 3. DSC thermograms of the synthesized PDBIB and ESBR.

3.3. Curing Characteristics of PDBIB and ESBR Compounds As vulcanization is a critical step for elastomeric products, the effect of filler (silica-VN3 and carbon black-N330) Figureon the 3. curingDSC thermograms characteristics of the was synthesized studied. PDBIB As the and modulus ESBR. of the elastomer Figure 3. DSC thermograms of the synthesized PDBIB and ESBR. increased dramatically during curing, it was used to evaluate the curing process. The curing 3.3. Curing Characteristics of PDBIB and ESBR Compounds characteristics3.3. Curing Characteristics of PDBIB and of PDBIB ESBR compoundsand ESBR Compounds are shown in Figure 4. As there were many hydroxyl groupsAs on vulcanization the silica surfaces, is a critical polar step organic for elastomeric molecules products, such as theaccelerator effect of molecules filler (silica-VN3 were prone and carbon to be adsorbedblack-N330)As vulcanization on onthem. the Therefore, curing is a critical characteristics compared step for with was elastomeric studied.PDBIB/CB products, As or the ESBR/CB modulus the effect compounds, of theof filler elastomer (silica-VN3PDBIB/silica increased and or ESBR/silicadramaticallycarbon black-N330) compounds during on curing, requiredthe curing it was a longer usedcharacteristics tocuring evaluate time was the. In studied. the curing silica process.As system, the modulus The PDBIB/silica curing of characteristicsthe had elastomer a longer scorchofincreased PDBIB time anddramatically and ESBR curing compounds duringtime than curing, are ESBR/silica shown it was in Figureusedbecause 4to. Asevaluatethe there polar werethe groups curing many in hydroxylprocess.PDBIB alsoThe groups hadcuring onan adsorptionthecharacteristics silica surfaces, effect of PDBIBon polar the accelerator. organicand ESBR molecules compounds However, such thare asis shown acceleratorphenomenon in Figure molecules did 4. not As wereappearthere pronewere in the many to carbon be adsorbedhydroxyl black system.ongroups them. on Therefore, the silica surfaces, compared polar with organic PDBIB molecules/CB or ESBR such/CB as compounds, accelerator molecules PDBIB/silica were or ESBRprone/ silicato be compoundsadsorbedThe vulcanization on required them. Therefore, a longercharacteristics curing compared time. of PDBIB with In the PDBIB/CB and silica ESBR system, orcompound ESBR/CB PDBIB/silica arecompounds, shown had a longerin TablesPDBIB/silica scorch 5 and time or6. TheandESBR/silica curingtorque timecompoundsdifference than ESBR became required/silica larger, becausea longer which thecuring polarwas time ca groupsused. In theby in silicathe PDBIB increase system, also had in PDBIB/silica butadiene an adsorption content.had eaff longerect The on resultsthescorch accelerator. indicatedtime and However, thecuring higher time this degree phenomenonthan of ESBR/silica crosslinking. did notbecause appear the in thepolar carbon groups black in system.PDBIB also had an adsorption effect on the accelerator. However, this phenomenon did not appear in the carbon black system. The vulcanization characteristics of PDBIB and ESBR compound are shown in Tables 5 and 6. The torque difference became larger, which was caused by the increase in butadiene content. The results indicated the higher degree of crosslinking.

(a) (b)

Figure 4. Torque of PDBIB and ESBR compounds as a function of curing time (a) PDBIBPDBIB/silica/silica and ESBR/silica;ESBR/silica; (b) PDBIBPDBIB/CB/CB and ESBRESBR/CB./CB.

The vulcanization characteristics of PDBIB and ESBR compound are shown in Tables5 and6. The torque difference became(a larger,) which was caused by the increase in butadiene(b) content. The results indicatedFigure the 4. higherTorque degreeof PDBIB of and crosslinking. ESBR compounds as a function of curing time (a) PDBIB/silica and ESBR/silica; (b) PDBIB/CB and ESBR/CB.

Processes 2020, 8, x FOR PEER REVIEW 8 of 18

Processes 2020, 8, 1527 8 of 17 Table 5. Curing for the PDBIB/silica and ESBR/silica compound rubber.

Sample t10/min t90/min MH-ML (dN·m) Table 5. Curing for the PDBIB/silica and ESBR/silica compound rubber. PDBIB50/silica 7:03 20:39 26.3

PDBIB60/silicaSample t6:2710/min t9021:36/min MH-ML (dN 38.6m) · PDBIB70/silicaPDBIB50/silica 7:20 7:03 20:3921:02 26.3 41.8 PDBIB80/silicaPDBIB60/silica 6:43 6:27 21:3623:35 38.6 45.7 ESBR/silicaPDBIB70 /silica7:05 7:20 21:0224:19 41.8 27.6 PDBIB80/silica 6:43 23:35 45.7 ESBR/silica 7:05 24:19 27.6 Table 6. Curing for the PDBIB/CB and ESBR/CB compound rubber.

SampleTable 6. Curing for thet10/min PDBIB /CB andt90/min ESBR /CB compoundMH-M rubber.L (dN·m) PDBIB50/CB 2:47 7:31 21.9 Sample t10/min t90/min MH-ML (dN m) PDBIB60/CB 2:49 9:51 26.2· PDBIB70/CBPDBIB50 /CB2:37 2:47 11:15 7:31 21.9 30.6 PDBIB80/CBPDBIB60 /CB2:29 2:49 14:12 9:51 26.2 35.3 PDBIB70/CB 2:37 11:15 30.6 ESBR/CBPDBIB80 /CB3:11 2:29 9:45 14:12 35.3 28.7 ESBR/CB 3:11 9:45 28.7 3.4. Mechanical Properties of PDBIB and ESBR Nanocomposites 3.4. MechanicalMechanical Properties properties of PDBIB of PDBIB and ESBRand ESBR Nanocomposites nanocomposites are displayed in Figure 5 and summarized in Table 7. In Figure 5a, mechanical properties of PDBIB with various butadiene contents Mechanical properties of PDBIB and ESBR nanocomposites are displayed in Figure5 and were excellent and comparable to that of ESBR nanocomposites, such as the tensile strength and summarized in Table7. In Figure5a, mechanical properties of PDBIB with various butadiene contents elongation at break of the PDBIB/silica nanocomposites, all of which exceeded 19 MPa and 450%, were excellent and comparable to that of ESBR nanocomposites, such as the tensile strength and respectively. The PDBIB70/silica nanocomposite exhibited the best mechanical properties with a elongation at break of the PDBIB/silica nanocomposites, all of which exceeded 19 MPa and 450%, tensile strength of 25.3 MPa and an elongation at break of 536%. The tensile stress at 300% strain and respectively. The PDBIB70/silica nanocomposite exhibited the best mechanical properties with a tensile the permanent set are also shown in Table 7. In Figure 5b, since the ester group imparts the polarity strength of 25.3 MPa and an elongation at break of 536%. The tensile stress at 300% strain and the of PDBIB, the compatibility with non-polar filler carbon black was not good. The content of ester permanent set are also shown in Table7. In Figure5b, since the ester group imparts the polarity of group was high and carbon was difficult as a reinforcing role, resulting in limited tensile strength. PDBIB, the compatibility with non-polar filler carbon black was not good. The content of ester group When the introduced butadiene content was higher than 60%, the tensile strength was above 18 MPa, was high and carbon was difficult as a reinforcing role, resulting in limited tensile strength. When the and the elongation at break was above 350%. Compared with ESBR/CB, the tensile strength was not introduced butadiene content was higher than 60%, the tensile strength was above 18 MPa, and the as good as that, but the stress at 300% was obviously higher. In summary, the mechanical properties elongation at break was above 350%. Compared with ESBR/CB, the tensile strength was not as good as of PDBIB/silica and PDBIB/CB nanocomposites all could meet most requirements of engineering that, but the stress at 300% was obviously higher. In summary, the mechanical properties of PDBIB/silica applications. and PDBIB/CB nanocomposites all could meet most requirements of engineering applications.

Figure 5. Stress–strain curves for PDBIB and ESBR nanocomposites (a) PDBIB/silica and ESBR/silica; (Figureb) PDBIB 5. Stress–strain/CB and ESBR curves/CB. for PDBIB and ESBR nanocomposites (a) PDBIB/silica and ESBR/silica; (b) PDBIB/CB and ESBR/CB.

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Table 7. Mechanical performance of the PDBIB and ESBR nanocomposites.

Tensile Stress at Stress at Elongation Permanent Shore A Sample Strength 100% 300% at Break (%) Set (%) Hardness (MPa) (MPa) (MPa)

ProcessesPDBIB50/silica2020, 8, 1527 19.6 ± 0.4 2.2 ± 0.1 9.4 ± 0.3 527 ± 35 12 66.19 of 17 PDBIB60/silica 22.0 ± 0.6 2.2 ± 0.1 9.4 ± 0.4 537 ± 23 10 66.8

PDBIB70/silica Table25.3 7.± 0.5Mechanical 2.6 performance± 0.1 10.5 of± the0.6 PDBIB536 and ± ESBR13 nanocomposites.8 68.1 Tensile Stress at Stress at Elongation Permanent Shore A PDBIB80/silicaSample 24.2 ± 0.4 2.6 ± 0.1 10.6 ± 0.3 498 ± 23 6 69.5 Strength (MPa) 100% (MPa) 300% (MPa) at Break (%) Set (%) Hardness PDBIB50ESBR/silica/silica 24.319.6 ± 0.10.4 2.1 ± 2.2 0.1 0.18.5 ± 0.5 9.4 0.3584 ± 16 527 3514 1265.2 66.1 ± ± ± ± PDBIB60/silica 22.0 0.6 2.2 0.1 9.4 0.4 537 23 10 66.8 ± ± ± ± PDBIB70PDBIB50/CB/silica 13.825.3 ± 0.80.5 1.7 ± 2.6 0.1 0.18.6 ± 0.6 10.5 0.6424 ± 29 536 1316 857.2 68.1 ± ± ± ± PDBIB80/silica 24.2 0.4 2.6 0.1 10.6 0.3 498 23 6 69.5 PDBIB60/CB 16.5 ±± 0.4 1.8 ± 0.1± 10.4 ± 0.3 ± 414 ± 25 ± 6 60.6 ESBR/silica 24.3 0.1 2.1 0.1 8.5 0.5 584 16 14 65.2 ± ± ± ± PDBIB70/CBPDBIB50/CB 18.5 13.8 ± 0.20.8 2.0 ± 1.7 0.1 0.111.4 ± 0.4 8.6 0.6416 ± 4247 294 1660.9 57.2 ± ± ± ± PDBIB60/CB 16.5 0.4 1.8 0.1 10.4 0.3 414 25 6 60.6 ± ± ± ± PDBIB80/CBPDBIB70/CB 18.9 18.5 ± 0.80.2 2.3 ± 2.0 0.1 0.113.5 ± 11.40.3 0.4372 ± 45 416 72 462.1 60.9 ± ± ± ± PDBIB80/CB 18.9 0.8 2.3 0.1 13.5 0.3 372 45 2 62.1 ± ± ± ± ESBRESBR/CB/CB 21.2 21.2 ± 0.50.5 2.1 ± 2.1 0.1 0.19.4 ± 0.5 9.4 0.5508 ± 22 508 228 864.1 64.1 ± ± ± ±

3.5.3.5. Filler–Filler and Filler–Polymer InteractionsInteractions in PDBIB andand ESBRESBR CompoundsCompounds InIn orderorder toto analyzeanalyze thethe didifferentfferent tensiletensile strengthstrength responsesresponses ofof thethe abovementionedabovementioned PDBIBPDBIB/silica/silica andand PDBIBPDBIB/CB/CB withwith thethe samesame fillingfilling amount,amount, thethe fillerfiller dispersion andand its interaction with PDBIB were analyzedanalyzed by combiningcombining RPA andand SEM.SEM. AsAs aa supplementarysupplementary notenote inin previousprevious research,research, itit waswas foundfound thatthat therethere waswas aa hydrogenhydrogen bondbond betweenbetween itaconateitaconate elastomerelastomer andand silicasilica [[19]19] asas FigureFigure6 6,, which which had had a a positivepositive eeffectffect onon thethe dispersiondispersion ofof silica,silica, butbut therethere waswas nono suchsuchforce force between between ESBR ESBR and and silica. silica.

Figure 6. Figure 6. SchematicSchematic diagram diagram of ofthe the hydrogen hydrogen bonding bonding interaction interaction between between itaconate itaconate elastomer elastomer and and silica. silica. The rubber process analyzer (RPA) allows for the investigation of the strength of a filler–filler The rubber process analyzer (RPA) allows for the investigation of the strength of a filler–filler network and filler–polymer interactions in a filled elastomeric compound in a wide range of shear network and filler–polymer interactions in a filled elastomeric compound in a wide range of shear strain amplitudes. Figure7 shows the RPA curves of PDBIB /silica (a) and PDBIB/CB (b) compounds. strain amplitudes. Figure 7 shows the RPA curves of PDBIB/silica (a) and PDBIB/CB (b) compounds. The storage modulus decreased nonlinearly with increasing applied dynamic strain, which is referred The storage modulus decreased nonlinearly with increasing applied dynamic strain, which is to as the Payne effect [40,41]. As the butadiene moieties of the PDBIB/silica and PDBIB/CB compounds referred to as the Payne effect [40,41]. As the butadiene moieties of the PDBIB/silica and PDBIB/CB increased, the initial modulus increased, which was caused by the formation of filler–filler interactions. compounds increased, the initial modulus increased, which was caused by the formation of filler– In the Figure7a, we found that the initial storage modulus of PDBIB was lower than that of ESBR, and the filler interactions. In the Figure 7a, we found that the initial storage modulus of PDBIB was lower downward trend was gentler. The reasons were as follows: Compared with the benzene ring in the than that of ESBR, and the downward trend was gentler. The reasons were as follows: Compared molecular chain structure of ESBR, PDBIB contains polar ester groups. Hydrogen bonding interactions could be formed between silanols on the silica surfaces and ester groups of the macromolecule chains according to our previous studies [42]. The interaction of PDBIB and silica could prevent silica from aggregating to a filler network, resulting in a homogenous dispersion of silica. Meanwhile, the number of ester groups per unit volume decreased with increasing butadiene content in the PDBIB elastomer, which led to the increased initial storage modulus. Therefore, the interaction of PDBIB and silica became weakened, which implied an increased probability for the formation of silica network, resulting in Processes 2020, 8, x FOR PEER REVIEW 10 of 18 Processes 2020, 8, x FOR PEER REVIEW 10 of 18 with the benzene ring in the molecular chain structure of ESBR, PDBIB contains polar ester groups. with the benzene ring in the molecular chain structure of ESBR, PDBIB contains polar ester groups. Hydrogen bonding interactions could be formed between silanols on the silica surfaces and ester Hydrogen bonding interactions could be formed between silanols on the silica surfaces and ester groups of the macromolecule chains according to our previous studies [42]. The interaction of PDBIB groups of the macromolecule chains according to our previous studies [42]. The interaction of PDBIB and silica could prevent silica from aggregating to a filler network, resulting in a homogenous and silica could prevent silica from aggregating to a filler network, resulting in a homogenous dispersion of silica. Meanwhile, the number of ester groups per unit volume decreased with dispersion of silica. Meanwhile, the number of ester groups per unit volume decreased with increasing butadiene content in the PDBIB elastomer, which led to the increased initial storage increasing butadiene content in the PDBIB elastomer, which led to the increased initial storage Processesmodulus.2020 ,Therefore,8, 1527 the interaction of PDBIB and silica became weakened, which implied10 of 17an modulus. Therefore, the interaction of PDBIB and silica became weakened, which implied an increased probability for the formation of silica network, resulting in an increase in the Payne effect increased probability for the formation of silica network, resulting in an increase in the Payne effect and inhomogeneous dispersion of silica. In the Figure 7b, in addition to PDBIB50m which had a more anand increase inhomogeneous in the Payne dispersion effect and of silica. inhomogeneous In the Figure dispersion 7b, in addition of silica. to PDBIB50m In the Figure which7b, in had addition a more obvious Payne effect, the initial storage modulus and change trend of PDBIB and ESBR were toobvious PDBIB50m Payne which effect, had the a moreinitial obvious storage Payne modulus effect, and the change initial storage trend modulusof PDBIB and and change ESBR trendwere relatively close. Compared with the benzene ring, the ester group has no obvious advantage in the ofrelatively PDBIB andclose. ESBR Compared were relatively with the close.benzene Compared ring, the with ester the group benzene has no ring, obvious the ester advantage group has in the no dispersion of carbon black, and even the high content of ester group will affect the dispersion of obviousdispersion advantage of carbon in black, the dispersion and even of the carbon high black,content and of evenester thegroup high will content affect of the ester dispersion group will of carbon black. Moreover, the G′ of the PDBIB filled with carbon black was higher than that of them acarbonffect the black. dispersion Moreover, of carbon the G′ black.of the PDBIB Moreover, filled the with G ofcarbon the PDBIB black was filled higher with carbonthan that black of them was filled with silica. 0 higherfilled with than silica. thatof them filled with silica.

Figure 7. Strain dependence of storage modulus (a) PDBIB/silica and ESBR/silica, (b) PDBIB/CB, and Figure 7.7. Strain dependence of storage modulus (a) PDBIBPDBIB/silica/silica and ESBRESBR/silica,/silica, (b)) PDBIBPDBIB/CB,/CB, and ESBR/CB compounds. ESBRESBR/CB/CB compounds.compounds. TheThe strainstrain amplitudeamplitude dependencedependence ofof tantan δδ inin thethe PDBIBPDBIB andand ESBRESBR compoundscompounds areare shownshown inin The strain amplitude dependence of tan δ in the PDBIB and ESBR compounds are shown in FigureFigure8 .8. DestructionDestruction ofof thethe strongstrong filler–fillerfiller–filler network network increased increased filler–filler filler–filler friction, friction, and and weak weak Figure 8. Destruction of the strong filler–filler network increased filler–filler friction, and weak interfacialinterfacial interaction increases increases filler–rubber filler–rubber friction, friction, which which resulted resulted in high in high tan δ tan at highδ at strain. high strain. From interfacial interaction increases filler–rubber friction, which resulted in high tan δ at high strain. From FromFigure Figure 8a, the8a, ester the ester groups groups of PDBIB of PDBIB helped helped weaken weaken the thefiller–filler filler–filler network network and and improved improved the Figure 8a, the ester groups of PDBIB helped weaken the filler–filler network and improved the theinterfacial interfacial interaction. interaction. Therefore, Therefore, the thetan tan δ valuesδ values of of PDBIBs/silica PDBIBs/silica we werere all all lower lower than thatthat ofof interfacial interaction. Therefore, the tan δ values of PDBIBs/silica were all lower than that of ESBRESBR/silica./silica. TheThe resultsresults showshow thatthat PDBIBPDBIB elastomerselastomers possessedpossessed aa greatgreat potentialpotential forfor futurefuture greengreen tiretire ESBR/silica. The results show that PDBIB elastomers possessed a great potential for future green tire treadtread applications.applications. InIn thethe FigureFigure8 8b, b, it it can can also also be be seen seen that that the the tan tanδ δvalues values ofof PDBIBsPDBIBs/CB/CB areare lowerlower tread applications. In the Figure 8 b, it can also be seen that the tan δ values of PDBIBs/CB are lower thanthan thatthat ofof ESBRESBR/CB,/CB, especially especially PDBIB70 PDBIB70 and and PDBIB80. PDBIB80. than that of ESBR/CB, especially PDBIB70 and PDBIB80.

δ FigureFigure 8.8. StrainStrain dependencedependence ofof lossloss tangenttangent (tan(tan δ)() (aa)) PDBIBPDBIB/silica/silica andand ESBRESBR/silica,/silica, ((bb)) PDBIBPDBIB/CB/CBand and Figure 8. Strain dependence of loss tangent (tan δ) (a) PDBIB/silica and ESBR/silica, (b) PDBIB/CB and ESBRESBR/CB/CB compounds. compounds. ESBR/CB compounds. 3.6. Dispersion State of Silica and Carbon Black in the PDBIB and ESBR Nanocomposites

To further compare PDBIB and ESBR nanocomposites, SEM was used to investigate the state of dispersion of fillers (silica or carbon black) in the PDBIB and ESBR nanocomposites. The dark and light parts of the SEM images of Figure9 represent the PDBIB or ESBR matrix and fillers particles, respectively. The state of dispersion of fillers depended strongly on the filler–polymer interactions. As discussed above, the hydrogen bonding interaction could be formed between silanols and ester Processes 2020, 8, x FOR PEER REVIEW 11 of 18

3.6. Dispersion State of Silica and Carbon Black in the PDBIB and ESBR Nanocomposites To further compare PDBIB and ESBR nanocomposites, SEM was used to investigate the state of dispersion of fillers (silica or carbon black) in the PDBIB and ESBR nanocomposites. The dark and Processeslight parts2020 ,of8, 1527the SEM images of Figure 9 represent the PDBIB or ESBR matrix and fillers particles,11 of 17 respectively. The state of dispersion of fillers depended strongly on the filler–polymer interactions. As discussed above, the hydrogen bonding interaction could be formed between silanols and ester groups ofof PDBIB60 PDBIB60/silica/silica nanocomposites, nanocomposites, resulting resulting in more in homogenousmore homogenous dispersion dispersion of silica compared of silica withcompared ESBR /withsilica, ESBR/silica, as shown inas Figureshown9 a,b.in Figure Figure 9a,b.9c,d Figure showed 9c,d that showed the dispersion that the dispersion of carbon of black carbon in theblack PDBIB70 in the PDBIB70/CB/CB is more homogenousis more homogenous than that than of ESBR that/ CB.of ESBR/CB.

Figure 9. SEM micrographs of (a) PDBIB60/silica, (b) ESBR/silica, (c) PDBIB70/CB, and (d) ESBRFigure/CB 9. nanocomposites.SEM micrographs of (a) PDBIB60/silica, (b) ESBR/silica, (c) PDBIB70/CB, and (d) ESBR/CB nanocomposites. 3.7. Dynamic Mechanical Properties of PDBIB and ESBR Nanocomposites 3.7. Dynamic Mechanical Properties of PDBIB and ESBR Nanocomposites Dynamic viscoelastic properties were investigated for PDBIB/silica and PDBIB/CB nanocomposites sinceDynamic a majority viscoelastic of engineering properties elastomers were are usedinve understigated dynamic for PDBIB/silica loading. The valueand ofPDBIB/CB tan δ at 60nanocomposites◦C was used assince a criterion a majority for of the engineering rolling resistance, elastomers while are that used at under 0 ◦C was dynamic used asloading. a criterion The forvalue the of wet tan grip δ at resistance60 °C was [used34]. Anas a ideal criterion material for the for rolling production resistance, of high-performance while that at 0 °C tires was should used haveas a criterion a low tan forδ thevalue wet at grip 60 ◦resistanceC and a high [34]. tanAn δidealvalue material at 0 ◦C. for Figure production 10 shows of high-performance the temperature dependencetires should have of the a tanlowδ tanvalues δ value of the at PDBIB60 °C and/silica a high and ESBRtan δ/ silicavalue nanocomposites,at 0 °C. Figure 10 and shows Table the8 gatherstemperature most dependence relevant data. of Fromthe tan PDBIB50 δ values/silica of the to PDBIB/silica PDBIB80/silica, and theESBR/silica rolling resistance nanocomposites, (60 ◦C tan andδ) ofTable PDBIBs 8 gathers/silica most decreased, relevant the data. wet From skid resistancePDBIB50/silica (0◦ C to tan PDBIB80/silica,δ) also decreased. the rolling The tanresistanceδ value (60 of PDBIB80°C tan δ)/ silicaof PDBIBs/silica nanocomposites decreased, was the the lowest, wet skid indicating resistance that (0° it hadC tan excellent δ) also lowdecreased. rolling resistanceThe tan δ andvalue energy of PDBIB80/silica saving characteristics. nanocomposites However, was consideringthe lowest, indicating the safety, that we haveit had taken excellent into accountlow rolling the wetresistance resistance, and soenergy we chose saving PDBIB60 characteristics./silica, which Howe it hadver, a considering lower tan δ value the safety, at 60 ◦ Cwe and have a comparable taken into tanaccountδ values the wet at 0 ◦resistance,C. Compared so we with chose ESBR PDBIB60/silica,/silica, the dynamic which mechanical it had a lower properties tan δ value of PDBIB60 at 60 °C/silica and wasa comparable more excellent. tan δ values This is at related 0 °C. toCompared the macromolecule with ESBR/silica, structure the of dynamic PDBIB and mechanical ESBR that properties (1) stronger of interfacialPDBIB60/silica interaction was more between excellent. polar This ester is grouprelated and to the interface; macromolecule (2) better structure dispersion of ofPDBIB silica. and ESBR that (1) stronger interfacial interaction between polar ester group and interface; (2) better dispersion of silica.

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Processes 2020, 8, 1527 12 of 17 Processes 2020, 8, x FOR PEER REVIEW 12 of 18

Figure 10. Temperature dependence of loss tangent (tan δ) for PDBIB/silica and ESBR/silica nanocomposites. Figure 10. TemperatureTemperature dependence dependence of of loss loss tangent (tan δ)) forfor PDBIBPDBIB/silica/silica andand ESBRESBR/silica/silica nanocomposites.Table 8. Dynamic mechanical thermal analysis of PDBIB/silica and ESBR/silica nanocomposites.

tan δ Table 8. Dynamic mechanical thermal analysis of PDBIB/silica and ESBR/silica nanocomposites. Table 8. Dynamic mechanicalSample thermal analysisTg (°C) of PDBIB/silica and ESBR/silica nanocomposites. 0 °C 60 °C Max tantan δ δ SamplePDBIB50/silicaSample T g (T◦−C)g 39.2(°C) 0.230 0.131 0.680 00◦ °CC 60 60 °C◦ CMax Max PDBIB60/silica −43.3 0.189 0.105 0.685 PDBIB50/silica 39.2 0.230 0.131 0.680 PDBIB50/silica − −39.2 0.230 0.131 0.680 PDBIB60/silica 43.3 0.189 0.105 0.685 PDBIB70/silica − −49.3 0.165 0.097 0.669 PDBIB70PDBIB60/silica/silica 49.3−43.3 0.1650.189 0.105 0.097 0.685 0.669 − PDBIB80PDBIB80/silica/silica 56.4−56.4 0.1460.146 0.088 0.088 0.637 0.637 − ESBRPDBIB70/silica/silica 48.1−49.3 0.1470.165 0.097 0.100 0.669 0.633 ESBR/silica − −48.1 0.147 0.100 0.633 PDBIB80/silica −56.4 0.146 0.088 0.637 From Figure 11, the Tg of the PDBIB/CB nanocomposites decreased with increasing butadiene From Figure 11, the Tg ESBR/silicaof the PDBIB/CB −48.1 nanocomposites 0.147 0.100 dec reased0.633 with increasing butadiene content in the PDBIB, resulting in a decrease in tan δ at both 60 C and 0 C for the PDBIB/CB content in the PDBIB, resulting in a decrease in tan δ at both 60 ◦°C and 0 ◦°C for the PDBIB/CB nanocomposite. From Table9, the tan δ value of PDBIB80/CB nanocomposite was the lowest at 60 ◦C, nanocomposite.From Figure From 11, the Table Tg of9, thethe tanPDBIB/CB δ value ofnanocomposites PDBIB80/CB nanocomposite decreased with was increasing the lowest butadiene at 60 °C, indicating the lowest rolling resistance compared to the other PDBIB/CB nanocomposites. Based on contentindicating in thethe lowestPDBIB, rolling resulting resistance in a decrease compared in totan the δ atother both PDBIB/CB 60 °C and nanocomposites. 0 °C for the PDBIB/CB Based on the mechanical properties, the wet grip resistance, and the rolling resistance, PDBIB70/CB had the best nanocomposite.the mechanical properties,From Table the9, the wet tan grip δ value resistance of PDBIB80/CB, and the rollingnanocomposite resistance, was PDBIB70/CB the lowest athad 60 °C,the balanced properties (compared with ESBR/CB, it had a comparable tan δ values at 0 ◦C and a lower indicatingbest balanced the lowestproperties rolling (compared resistance wi comparedth ESBR/CB, to theit had other a comparable PDBIB/CB nanocomposites.tan δ values at 0 Based°C and on a tan δ value at 60 ◦C). This emerged as a potential elastomer to produce high-performance materials thelower mechanical tan δ value properties, at 60 °C). the This wet emerged grip resistance as a potential, and the elastomer rolling resistance, to produce PDBIB70/CB high-performance had the instead of ESBR in tire application. bestmaterials balanced instead properties of ESBR (compared in tire application. with ESBR/CB, it had a comparable tan δ values at 0 °C and a lower tan δ value at 60 °C). This emerged as a potential elastomer to produce high-performance materials instead of ESBR in tire application.

Figure 11. Temperature dependence of loss tangent (tan δ) for PDBIB/CB and ESBR/CB nanocomposites. Figure 11. Temperature dependence of loss tangent (tan δ) for PDBIB/CB and ESBR/CB nanocomposites. Figure 11. Temperature dependence of loss tangent (tan δ) for PDBIB/CB and ESBR/CB nanocomposites.

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Table 9. Dynamic mechanical thermal analysis of PDBIB/silica nanocomposites.

Processes 2020, 8, 1527 13 of 17 tan δ sample Tg (°C) 0 °C 60 °C Max Table 9. Dynamic mechanical thermal analysis of PDBIB/silica nanocomposites. PDBIB50/CB −36.5 0.284 0.231 0.652 tan δ Sample Tg (◦C) PDBIB60/CB −41.1 0.2430 ◦C 0.190 60 ◦C 0.648 Max PDBIB50/CB 36.5 0.284 0.231 0.652 PDBIB70/CB − −45.5 0.211 0.155 0.661 PDBIB60/CB 41.1 0.243 0.190 0.648 − PDBIB70PDBIB80/CB/CB 45.5−50.7 0.211 0.184 0.125 0.155 0.717 0.661 − PDBIB80/CB 50.7 0.184 0.125 0.717 − ESBRESBR/CB/CB 34.4−34.4 0.211 0.211 0.206 0.206 0.571 0.571 −

3.8. Heat Build-Up Test an andd Abrasion Resistance of PDBIBPDBIB and ESBR Nanocomposites In tiretire applications, applications, apart apart from from dynamic dynamic mechanical mechanical properties, properties, the heat the build-up heat build-up test and abrasiontest and resistanceabrasion resistance are key indicators, are key indicators, and are commonly and are commonly used to characterize used to characterize tire performance. tire performance. In Figure 12 Ina, theFigure heat 12a, build-up the heat are build-up arranged are from arranged high to lowfrom in high the followingto low in order: the following ESBR/silica, order: PDBIB50 ESBR/silica,/silica, PDBIB80PDBIB50/silica,/silica, PDBIB60PDBIB80/silica,/silica, andPDBIB60/silica, PDBIB70/silica. and ThePDBIB70/silica. result aligns The with re tansultδ measuredaligns with by tan RPA, δ exceptmeasured for PDBIB80by RPA, /silicaexcept (Figure for PDBIB80/silica7a). In Figure (Figure12b, we 7a). observed In Figure that 12b, the heat we observed build-up ofthat PDBIB the heat/CB decreasedbuild-up of with PDBIB/CB increasing decreased butadiene with content increasing in the PDBIB.butadiene Moreover, content thein the heat PDBIB. build-up Moreover, of all PDBIBs the heat/CB werebuild-up lower of thanall PDBIBs/CB ESBR/CB. were lower than ESBR/CB.

Figure 12. Heat build-up ( a,b) of PDBIB and ESBR nanocomposites.

In Figure 1313a,a, as thethe butadienebutadiene moieties increased,increased, the volume loss of abrasionabrasion of PDBIBPDBIB/silica/silica nanocomposites decreased. Among Among them, them, abrasion abrasion resistance of PDBIB80/ PDBIB80/silica was excellent and closest to ESBRESBR/silica./silica. In Figure 13b,13b, abrasion resistance of PDBIB70/CB PDBIB70/CB and PDBIB80/CB PDBIB80/CB were better than ESBR/CB. Therefore, bio-based elastomers PDBIB had a great potential for future engineering Processesthan ESBR/CB. 2020, 8, x FORTherefore, PEER REVIEW bio-based elastomers PDBIB had a great potential for future engineering14 of 18 applications, such as car and truck tires.tires.

Figure 13. Akron abrasion loss (a,b) of PDBIB and ESBR nanocomposites. Figure 13. Akron abrasion loss (a,b) of PDBIB and ESBR nanocomposites.

3.9. Performance of PDBIB/Silica Nanocomposite Compared with SSBR/Silica Nanocomposite As mentioned above, PDBIB has a great potential in the tire field, and SSBR is an ideal material for the production of green tires in the modern tire industry. Here, PDBIB60 and SSBR2466 were used to prepare silica-nanocomposites used by the same method described in Section 2.3. Their dynamic and static mechanical properties are shown in Figure 14a,b. The tensile strength and elongation at break of the PDBIB60/silica nanocomposites were higher than the SSBR2466/silica nanocomposites, while its moduli was lower than the SSBR2466/silica nanocomposites. Although the PDBIB60/silica nanocomposites exhibited lower moduli than SSBR2466/silica nanocomposites, they showed mechanical properties that satisfactorily met basic tire requirements. Dynamic properties of PDBIB60/silica and SSBR2466/silica nanocomposites, especially loss tangent (tan δ) values at 60 °C and at 0 °C, were investigated. The tan δ value of the PDBIB60/silica nanocomposite at 60 °C was comparable to the SSBR2466/silica nanocomposite, but it was lower at 0 °C. This is an urgent problem for PDBIB, and we will improve it through functional modification and other means in the future. From Figure 14c,d, we can see that the initial storage modulus of PDBIB60/silica was obviously lower than the SSBR2466/silica, indicating better silica dispersion in PDBIB. Usually, the tan δ value at 7% strain is closely related to the tire rolling resistance [34]. The result showed that the tan δ values at 7% strain in the PDBIB60/silica nanocomposites was a little bit higher than the SSBR2466/silica. Overall, compared with SSBR, the static mechanical properties and filler dispersion of PDBIB are better, but dynamic mechanical properties still need to be improved.

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Processes 2020, 8, 1527 14 of 17 Figure 13. Akron abrasion loss (a,b) of PDBIB and ESBR nanocomposites.

3.9.3.9. Performance Performance of of PDBIB/Silica PDBIB/Silica Nanocomposit Nanocompositee Compared with SSBR /Silica/Silica Nanocomposite AsAs mentioned mentioned above, PDBIBPDBIB hashas aa greatgreat potential potential in in the the tire tire field, field, and and SSBR SSBR is anis an ideal ideal material material for forthe the production production of greenof green tires tires in thein the modern modern tire tire industry. industry. Here, Here, PDBIB60 PDBIB60 and and SSBR2466 SSBR2466 were were used used to toprepare prepare silica-nanocomposites silica-nanocomposites used used by by the the same same method method described described in Section in Section 2.3. 2.3. Their Their dynamic dynamic and andstatic static mechanical mechanical properties properties are shown are shown in Figure in 14 Figua,b.re The 14a,b. tensile The strength tensile andstrength elongation and elongation at break of theat break of the PDBIB60/silica nanocomposites were higher than the SSBR2466/silica nanocomposites, PDBIB60/silica nanocomposites were higher than the SSBR2466/silica nanocomposites, while its moduli while its moduli was lower than the SSBR2466/silica nanocomposites. Although the PDBIB60/silica was lower than the SSBR2466/silica nanocomposites. Although the PDBIB60/silica nanocomposites nanocomposites exhibited lower moduli than SSBR2466/silica nanocomposites, they showed exhibited lower moduli than SSBR2466/silica nanocomposites, they showed mechanical properties that mechanical properties that satisfactorily met basic tire requirements. Dynamic properties of satisfactorily met basic tire requirements. Dynamic properties of PDBIB60/silica and SSBR2466/silica PDBIB60/silica and SSBR2466/silica nanocomposites, especially loss tangent (tan δ) values at 60 °C nanocomposites, especially loss tangent (tan δ) values at 60 ◦C and at 0 ◦C, were investigated. and at 0 °C, were investigated. The tan δ value of the PDBIB60/silica nanocomposite at 60 °C was The tan δ value of the PDBIB60/silica nanocomposite at 60 ◦C was comparable to the SSBR2466/silica comparable to the SSBR2466/silica nanocomposite, but it was lower at 0 °C. This is an urgent problem nanocomposite, but it was lower at 0 ◦C. This is an urgent problem for PDBIB, and we will improve for PDBIB, and we will improve it through functional modification and other means in the future. it through functional modification and other means in the future. From Figure 14c,d, we can see From Figure 14c,d, we can see that the initial storage modulus of PDBIB60/silica was obviously lower that the initial storage modulus of PDBIB60/silica was obviously lower than the SSBR2466/silica, than the SSBR2466/silica, indicating better silica dispersion in PDBIB. Usually, the tan δ value at 7% indicating better silica dispersion in PDBIB. Usually, the tan δ value at 7% strain is closely related to the strain is closely related to the tire rolling resistance [34]. The result showed that the tan δ values at tire rolling resistance [34]. The result showed that the tan δ values at 7% strain in the PDBIB60/silica 7% strain in the PDBIB60/silica nanocomposites was a little bit higher than the SSBR2466/silica. nanocomposites was a little bit higher than the SSBR2466/silica. Overall, compared with SSBR, the static Overall, compared with SSBR, the static mechanical properties and filler dispersion of PDBIB are mechanical properties and filler dispersion of PDBIB are better, but dynamic mechanical properties better, but dynamic mechanical properties still need to be improved. still need to be improved.

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FigureFigure 14.14. Performance comparison of PDBIB and SSBR: (a)) mechanicalmechanical properties,properties, ((bb)) dynamicdynamic δ mechanicalmechanical properties,properties, andand ((cc,,dd)) strainstrain dependencedependence ofof storagestorage modulusmodulus andand lossloss tangenttangent (tan(tan δ).). 4. Conclusions 4. Conclusions Bio-based cross-linkable poly(dibutyl itaconate-co-butadiene) (PDBIB) elastomers nanocomposites Bio-based cross-linkable poly(dibutyl itaconate-co-butadiene) (PDBIB) elastomers were prepared using silica (VN3) and carbon black (N330). Mechanical properties of PDBIB with nanocomposites were prepared using silica (VN3) and carbon black (N330). Mechanical properties of different butadiene contents were excellent. The tensile strength and elongation at break of the PDBIB with different butadiene contents were excellent. The tensile strength and elongation at break of the PDBIB/silica nanocomposites exceeded 19 MPa and 450%, as did those of the PDBIB70/CB and PDBIB80/CB nanocomposites, exceeding above 18 MPa and 350%. Compared with ESBR, due to the difference of macromolecule structure, polar ester groups in PDBIB could prevent silica from aggregating to a filler network, resulting in a homogenous dispersion of silica. However, the ester group had no obvious advantage in the dispersion of carbon black, and even the high content of ester group could worsen the dispersion of carbon black. In silica-filled and carbon black-filled system, PDBIB60/silica and PDBIB70/CB had the best balanced properties. Moreover, their comprehensive performance were significantly better than the ESBR/filler nanocomposite. Compared with the SSBR, mechanical properties and filler dispersion of PDBIB are better, and dynamic mechanical properties still had space for improvement. On the basis of our results, PDBIBs showed great potential to be a strong and useful supplement for commercial elastomers in engineering applications, especially in the tire industry.

Author Contributions: conceptualization, X.Z., L.Z., and R.W.; formal analysis, L.L. and X.Z.; funding acquisition, R.W.; investigation, L.L., H.J., and H.Y.; project administration, L.Z.; supervision, R.W.; validation, R.W.; writing—original draft, L.L.; writing—review and editing, X.Z. and R.W. All authors have read and agreed to the published version of the manuscript.

Funding: The authors would like to thank the National Key Research and Development Program of China (2017YFB0306903), the National Natural Science Foundation of China (51988102, 51503010), and the China– France Cooperation Program of PHC CAI YUANPEI (CHINA SCHOLARSHIP COUNCIL, No. 201504490120) for their financial support.

Acknowledgments: The authors would also like to thank Jilin Petrochemical Research Institute for its great help.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Bockstal, L.; Berchem, T.; Schmetz, Q.; Richel, A. Devulcanisation and reclaiming of tires and rubber by physical and chemical processes: A review. J. Clean. Prod. 2019, 236, 117574. 2. Gao, W.; Lu, J.; Song, W.; Hu, J.; Han, B. Solution Mechanochemical Approach for Preparing High- Dispersion SiO2-g-SSBR and the Performance of Modified Silica/SSBR Composites. Ind. Eng. Chem. Res. 2019, 58, 7146–7155.

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PDBIB/silica nanocomposites exceeded 19 MPa and 450%, as did those of the PDBIB70/CB and PDBIB80/CB nanocomposites, exceeding above 18 MPa and 350%. Compared with ESBR, due to the difference of macromolecule structure, polar ester groups in PDBIB could prevent silica from aggregating to a filler network, resulting in a homogenous dispersion of silica. However, the ester group had no obvious advantage in the dispersion of carbon black, and even the high content of ester group could worsen the dispersion of carbon black. In silica-filled and carbon black-filled system, PDBIB60/silica and PDBIB70/CB had the best balanced properties. Moreover, their comprehensive performance were significantly better than the ESBR/filler nanocomposite. Compared with the SSBR, mechanical properties and filler dispersion of PDBIB are better, and dynamic mechanical properties still had space for improvement. On the basis of our results, PDBIBs showed great potential to be a strong and useful supplement for commercial elastomers in engineering applications, especially in the tire industry.

Author Contributions: Conceptualization, X.Z., L.Z., and R.W.; formal analysis, L.L. and X.Z.; funding acquisition, R.W.; investigation, L.L., H.J., and H.Y.; project administration, L.Z.; supervision, R.W.; validation, R.W.; writing—original draft, L.L.; writing—review and editing, X.Z. and R.W. All authors have read and agreed to the published version of the manuscript. Funding: The authors would like to thank the National Key Research and Development Program of China (2017YFB0306903), the National Natural Science Foundation of China (51988102, 51503010), and the China–France Cooperation Program of PHC CAI YUANPEI (CHINA SCHOLARSHIP COUNCIL, No. 201504490120) for their financial support. Acknowledgments: The authors would also like to thank Jilin Petrochemical Research Institute for its great help. Conflicts of Interest: The authors declare no conflict of interest.

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