Preparation and Characterization of Diene Rubbers/Silica Composites via Reactions of Hydroxyl Groups and Blocked Polyisocyanates

To improve the curing reaction rate and efficiency of sulfur-cured diene-based rubbers, the introduction of some chemical compounds as activators and accelerants is inevitably required, causing potential harm to humans and ecological systems. Moreover, silica is usually employed as a green filling material for rubber reinforcement, and a silane coupling agent is always required to improve its dispersion. Herein, we reported an effective method to cure hydroxyl-functionalized rubbers/silica composites with blocked polyisocyanates, avoiding the use of any other additives. The enhanced dispersion of silica by interaction with hydroxyl groups on molecular chains endowed the composites with high-mechanical performance. The mechanical properties and crosslinking kinetics of the resultant silica composites can be regulated by adjusting the content of hydroxyl groups in the rubber, as well as the amount of the blocked polyisocyanates. The dynamic heat build-up was related to the distance between crosslinking points. A SBROH/B-TDI/silica composite prepared with blocked toluene diisocyanatem (TDI) exhibited comparable tanδ (0.21 at 0 °C and 0.11 at 60 °C) to that of silica composites cured by sulfur with the help of a silane coupling agent (SBR/S/Si69/silica, 0.18 and 0.10), suggesting great applicable potential for new tire rubber compounds.


Introduction
Vulcanization involves crosslinking between polymer chains physically or chemically to generate three-dimensional network structures, which play a key role in achieving rubbery elasticity [1,2]. Even though various chemicals, such as peroxides [3], metal peroxides [4], amines [5] and resins [6], have been developed for the vulcanization of rubbers, the sulfur crosslinking system, first reported by Goodyear in 1839, is still most widely used in the industry. Due to the low crosslinking efficiency of using sulfur alone, activators and accelerants must be introduced to facilitate the crosslinking process and optimize the network performance [7,8]. However, these additives are usually toxic and will release harmful substances during the curing process [9]. For example, the activator zinc oxide is classified as toxic to aquatic organisms by the European Commission, and its application in rubber technology must be controlled by legislation [10]. Moreover, N-cyclohexyl-2benzothi-azolesulphonamide (accelerant CZ), dibenzothiazoledisulfide (accelerant DM), and 2,2 -dibenzothiazole disulphide (accelerant MBTS) are classified as R43 (may cause sensitization by skin contact) and R50/53 (very toxic to aquatic organisms) according to the dangerous substances directive of European Union laws (67/548/EEC) [11]. Therefore, it is an urgent demand to explore an efficient and green method to crosslink diene rubbers. This leads to many attempts for developing new crosslinking structures, such as a physical crosslinking network formed by hydrogen bonds [12], ionic interaction [13], epoxy-functionalized elastomers crosslinked by dibasic acids [14,15], hydroxylated rubbers rate that the mCPBA solution was added completely after 1 h. The reaction mixture was then further stirred for another 2 h. A solution of 4.7 mL of hydrochloric acid (0.056 mol of HCl) in 50 mL of THF was first prepared and transferred into a 100 mL dropping funnel; it was then added dropwise at room temperature (25 • C) into the epoxidized BR solution prepared previously. After the addition of the HCl solution, the reaction mixture was further stirred for another 2 h to complete the ring-opening reaction. The resultant solution was poured into H 2 O under stirring to precipitate the modified BR. The product was further dissolved in THF and precipitated by H 2 O repeatedly to fully remove the unreacted agents. Subsequently, the resulting polymer was dried in a vacuum oven at 40 • C to remove the residual solvent. The hydroxylated BR is denominated as BROH x , where x represents the molar percentage of the hydroxyl group. To investigate the effect of hydroxyl group content on cured rubber performance, BR with different contents of hydroxyl groups was synthesized according to the abovementioned procedures. Besides 3.0 mol%, other hydroxyl group contents were controlled to be 1.0 and 5.0 mol%. Additionally, SBR with different contents of hydroxyl groups (2.0, 5.0, and 7.0 mol%) were synthesized.

Preparation of Blocked Polyisocyanates
In a three-port bottle equipped with magnetic stirring and a condensation tube and protected by dry nitrogen gas, 8.7 g TDI (0.05 mol), 10.4 g butanone oxime (0.12 mol), and 20 mL ethyl acetate were added, This mixture was reacted for 5 h at 77 • C and then distilled under reduced pressure to remove ethyl acetate. The resulting product was dried in a vacuum oven at 40 • C to remove the residual solvent. Blocked hexamethylene diisocyanate (BHDI); blocked 1,4-Phenylene diisocyanate (BPPDI); and blocked tolylene-2,4diisocyanate (BTDI) were separately synthesized using butanone oxime as a blocking agent.

Preparation of BROH x /BI/Silica and BR/S/Silica Composites
For the preparation of BROH x /BI/silica, 100 g of BROH x , 30 g of silica, and various amounts of BI (4, 6, 8 g) were mixed in a Haake internal mixer. The obtained compounds were then subjected to compression moulding at 160 • C for optimum time to produce BI-cured BROH x . The formed product was named BROH x BI z , where z is the content (phr, parts per hundreds of rubbers) of the crosslinker. Traditional sulfur-cured BR (BR/S) was prepared according to the abovementioned protocols based on the below formulation: BR 100 g; silica 30 g; bis[γ-(triethoxysilyl)propyl]tetrasulfide (Si69) 0 or 2.4 g; zinc oxide 5.0 g; stearic acid 1.0 g; N-Isopropyl-N -phenyl-4-phenylenediamin (4010NA) 1.0 g; 1,3-Diphenylguanidine (D) 0.5 g; N-cyclohexyl-2-benzothiazole sulphonamide (CZ) 1.5 g; and sulfur 1.5 g.

Preparation of SBROH x /BI/Silica and SBR/S/Silica Composites
For the preparation of SBROH x /BI/silica, 100 g of SBROH x ; 50 g of silica; and blocked polyisocyanates (6 g BI, 3.06 g BHDI, 2.99 g BPPDI and 3.11 g BTDI, respectively) were mixed in a Haake internal mixer. The obtained compounds were then subjected to compression moulding at 160 • C for optimum time to produce BI-cured SBROH x . The formed product was named SBROH x BI z , where z is the content (phr, parts per hundreds of rubbers) of the crosslinker. Traditional sulfur-cured SBR (SBR/S) was prepared according to the abovementioned protocols based on the below formulation: SBR 100 g; silica 50 g; bis[γ-(triethoxysilyl)propyl]tetrasulfide (Si69) 0 or 5 g; zinc oxide 3.0 g; stearic acid 2.0 g; N-Isopropyl-N -phenyl-4-phenylenediamin (4010NA) 1.0 g; N-tert-butylbenzothiazole-2sulphenamide (NS) 2 g; and sulfur 1.5 g.

Methods
The 1 H NMR spectra of products were recorded using a Bruker 500 spectrometer with CDCl 3 as a solvent and tetramethylsilane (TMS) as an internal standard. A Fouriertransform infrared spectroscopy (FTIR) analysis was conducted on a VERTEX 70 instrument in attenuated total reflectance (ATR) mode. The spectra were collected over the wavenum- ber of 4000−500 cm −1 with a 4 cm −1 resolution and 32 scans. The molecular weight (M n ) and molecular weight distribution (PDI, M w /M n ) of the polymers were measured by GPC equipped with an RI detector (GPC-RI) and an EcoSEC832 pump. The flow rate of tetrahydrofuran (THF) was 0.35 mL/min. Differential scanning calorimetry (DSC) was recorded on a DSC204F1 instrument (NETZSCH) and thermograms were recorded using a heating rate of 10 • C/min under nitrogen atmosphere from −80 • C to 40 • C. Crosslinking kinetics were determined at different temperatures on an MDR2000 (Alpha Technologies, Wilmington, DE, USA) vulcameter. The tensile and tear test was carried out by a Zwick/Roell Z005 instrument. Shore A hardness was performed on a durometer (GOTECH Testing Machines Company). Resilience was performed on a rubber rebound testing machine (GOTECH Testing Machines Company). Compression set retention rate was performed on a rubber compression permanent deformer at 25% compression for 72 h at room temperature. Crosslinking density was measured by the equilibrium swelling experiment (see Supporting Information). Dynamic mechanical analysis (DMA) was performed using a NETZSCH EPlexor 500N analyzer under the conditions of a frequency of 10 Hz at the speed of 3 • C/min from −120 • C to 80 • C. Filler networks were analyzed using an RPA 2000 at 60 • C (Alpha Technologies, Wilmington, DE, USA). The compounds were analyzed over the strain range of 0.28-400% at 1 Hz. Transmission electron microscopy (TEM) for the ultramicrotomed samples was performed on a JEM-2100 instrument (Tokyo, Japan).

Hydroxyl Functionalization of Butadiene Rubber
A series of pendant hydroxyl-group-functionalized cis-polybutadiene rubber (denominated as BROH x , where x represents the molar ratio of structural units containing hydroxyl groups) were synthesized according to Scheme 1. The commercial blocked polyisocyanates, i.e., B1358 (a closed ring aliphatic polyisocyanate that is based on the isophorone diisocyanate), referred to as BI, was used to cure the silica-filled hydroxyl functionalized rubbers. The formed product was named BROH x BI z , where z is the content (phr, parts per hundreds of rubbers) of the crosslinker. weight (Mn) and molecular weight distribution (PDI, Mw/Mn) of the polymers wer ured by GPC equipped with an RI detector (GPC-RI) and an EcoSEC832 pump. T rate of tetrahydrofuran (THF) was 0.35 mL/min. Differential scanning calorimetr was recorded on a DSC204F1 instrument (NETZSCH) and thermograms were r using a heating rate of 10 °C/min under nitrogen atmosphere from −80 °C to 40 °C linking kinetics were determined at different temperatures on an MDR2000 (Alph nologies, Wilmington, DE, USA) vulcameter. The tensile and tear test was carried a Zwick/Roell Z005 instrument. Shore A hardness was performed on a du (GOTECH Testing Machines Company). Resilience was performed on a rubber r testing machine (GOTECH Testing Machines Company). Compression set retent was performed on a rubber compression permanent deformer at 25% compressio h at room temperature. Crosslinking density was measured by the equilibrium s experiment (see Supporting Information). Dynamic mechanical analysis (DMA) w formed using a NETZSCH EPlexor 500N analyzer under the conditions of a frequ 10 Hz at the speed of 3 °C/min from −120 °C to 80 °C. Filler networks were analyze an RPA 2000 at 60 °C (Alpha Technologies, Wilmington, DE, USA). The compoun analyzed over the strain range of 0.28-400% at 1 Hz. Transmission electron mic (TEM) for the ultramicrotomed samples was performed on a JEM-2100 instrum kyo, Japan).

Hydroxyl Functionalization of Butadiene Rubber
A series of pendant hydroxyl-group-functionalized cis-polybutadiene rub nominated as BROHx, where x represents the molar ratio of structural units con hydroxyl groups) were synthesized according to Scheme 1. The commercial block yisocyanates, i.e., B1358 (a closed ring aliphatic polyisocyanate that is based on phorone diisocyanate), referred to as BI, was used to cure the silica-filled hydrox tionalized rubbers. The formed product was named BROHxBIz, where z is the conte parts per hundreds of rubbers) of the crosslinker. The chemical structures of formed products were investigated by FTIR spec ure S1). The peak transmittance intensity was normalized to the peak at around 30 ascribed to C−H bending vibration and used to compare the changes of each moi intensity of peaks around 3575 cm −1 corresponding to the O−H bending increased uously while the peak intensity of around 2939 cm −1 representing =C−H weaken The chemical structures of formed products were investigated by FTIR spectra ( Figure S1). The peak transmittance intensity was normalized to the peak at around 3008 cm −1 ascribed to C−H bending vibration and used to compare the changes of each moiety. The intensity of peaks around 3575 cm −1 corresponding to the O−H bending increased continuously while the peak intensity of around 2939 cm −1 representing =C−H weakened with the increasing amount of m-chloroperoxybenzoic acid (mCPBA) and hydrochloric acid, indicating that the hydroxyl group content increased in the polybutadiene rubber (BR). The hydroxyl group contents were quantitatively determined by 1 H NMR spectra as listed in Table S1. The peak at 2.74 ppm ( Figure 1) assigned to epoxide groups, which was formed by the epoxidation of mCPBA, completely disappeared after the ring opening reaction in the presence of hydrochloric acid. The double peaks at 3.73 and 3.96 ppm corresponding to hydroxyl and chlorine groups were used to calculate the efficiency of ring opening reactions with epoxide and chlorine, respectively [34]. As shown in Table S1, both efficiencies were very high. The molecular weight (M W ) and molecular weight distribution (MWD) of BROH x before and after hydroxylation were determined by GPC ( Figure S2 and Table S2). The M W slightly increased as the content of hydroxyl groups increased, indicating the successful grafting of hydroxyl groups onto BR chains.
the increasing amount of m-chloroperoxybenzoic acid (mCPBA) and hydrochloric acid, indicating that the hydroxyl group content increased in the polybutadiene rubber (BR). The hydroxyl group contents were quantitatively determined by 1 H NMR spectra as listed in Table S1. The peak at 2.74 ppm ( Figure 1) assigned to epoxide groups, which was formed by the epoxidation of mCPBA, completely disappeared after the ring opening reaction in the presence of hydrochloric acid. The double peaks at 3.73 and 3.96 ppm corresponding to hydroxyl and chlorine groups were used to calculate the efficiency of ring opening reactions with epoxide and chlorine, respectively [34]. As shown in Table S1, both efficiencies were very high. The molecular weight (MW) and molecular weight distribution (MWD) of BROHx before and after hydroxylation were determined by GPC ( Figure S2 and Table S2). The MW slightly increased as the content of hydroxyl groups increased, indicating the successful grafting of hydroxyl groups onto BR chains.

Crosslinking of BROHx with Blocked Polyisocyanates
To explore the reaction mechanism between BROHx and blocked polyisocyanates, we monitored the FTIR spectra of BI, BROH3, and BROH3/BI6 compounds and BROH3/BI6 vulcanizate. As seen from Figure 2a, the intensity of peaks at 1739 cm −1 related to C=O stretching vibration in blocked polyisocyanates decreases after crosslinking, indicating the successful unblocking of blocked polyisocyanates [35]. Upon the addition of silica, the degree of association of hydroxyl groups increases, leading to peak broadening and red shift to a lower wavenumber. The decreased intensity of peaks at 3440 cm −1 assigned to hydroxyl groups after the curing process confirms clearly the partial consumption of hydroxyl groups in BROH3 by unblocked isocyanate groups. The curing behavior of the BROH3/BI/silica composite was examined to verify the effectiveness of crosslinking reactions. Figure 2b showed the curing profiles of BROH3/BI/silica at different temperatures. At temperatures above the unblocking temperature (130 °C) of BI, the curing process was accelerated as the curing temperature increased, and the maximum torques (MH) were nearly the same, indicating the completion of crosslinking reactions between newly released highly active isocyanate groups and hydroxyl groups. Additionally, the curing curve began to level off after 15 min at temperatures higher than 150 °C, similar to traditional sulphur-based curing systems. However, at temperatures below the unblocking temperature, the crosslinking reaction was rather slow since the reaction can only rely on a trace amount of free isocyanate groups. For the same reason, the low temperature blending process did not cause early crosslinking and can ensure the processing safely. The crosslinking chemistry based on BROHx and blocked polyisocyanates was summarized in Scheme 2.

Crosslinking of BROH x with Blocked Polyisocyanates
To explore the reaction mechanism between BROH x and blocked polyisocyanates, we monitored the FTIR spectra of BI, BROH 3 , and BROH 3 /BI 6 compounds and BROH 3 /BI 6 vulcanizate. As seen from Figure 2a, the intensity of peaks at 1739 cm −1 related to C=O stretching vibration in blocked polyisocyanates decreases after crosslinking, indicating the successful unblocking of blocked polyisocyanates [35]. Upon the addition of silica, the degree of association of hydroxyl groups increases, leading to peak broadening and red shift to a lower wavenumber. The decreased intensity of peaks at 3440 cm −1 assigned to hydroxyl groups after the curing process confirms clearly the partial consumption of hydroxyl groups in BROH 3 by unblocked isocyanate groups. The curing behavior of the BROH 3 /BI/silica composite was examined to verify the effectiveness of crosslinking reactions. Figure 2b showed the curing profiles of BROH 3 /BI/silica at different temperatures. At temperatures above the unblocking temperature (130 • C) of BI, the curing process was accelerated as the curing temperature increased, and the maximum torques (MH) were nearly the same, indicating the completion of crosslinking reactions between newly released highly active isocyanate groups and hydroxyl groups. Additionally, the curing curve began to level off after 15 min at temperatures higher than 150 • C, similar to traditional sulphur-based curing systems. However, at temperatures below the unblocking temperature, the crosslinking reaction was rather slow since the reaction can only rely on a trace amount of free isocyanate groups. For the same reason, the low temperature blending process did not cause early crosslinking and can ensure the processing safely. The crosslinking chemistry based on BROH x and blocked polyisocyanates was summarized in Scheme 2.

Mechanical Properties of BROHx/BI/Silica Composites
The mechanical properties of formed rubber composites can be manipulated justing the BI content. Figure 3a was the crosslinking density plot of BROHx/BI/sili posite vs. BI amount. The crosslinking density increased gradually with the increa amount due to the increase in reaction points. With the increase in BI dosage from phr, the crosslinking density enhanced from 6.63 to 8 ×10 −5 mol/cm 3 . Figure 3b p the curing curves of BROH3 using different BI doses. With the increase in BI cont crosslinking density and MH value increased monotonously. Moreover, the cro content had little effect on the curing time. The stress−strain curves of BROHx/ composites areshown in Figure 3c. With the increase in BI amount, the tensile s and modulus of BROHx/BI/silica composites increased obviously, and the breakin gation decreased because of increasing crosslinking density.

Mechanical Properties of BROHx/BI/Silica Composites
The mechanical properties of formed rubber composites can be manipulated by ad justing the BI content. Figure 3a was the crosslinking density plot of BROHx/BI/silica com posite vs. BI amount. The crosslinking density increased gradually with the increase in B amount due to the increase in reaction points. With the increase in BI dosage from 4 to 8 phr, the crosslinking density enhanced from 6.63 to 8 ×10 −5 mol/cm 3 . Figure 3b presents the curing curves of BROH3 using different BI doses. With the increase in BI content, the crosslinking density and MH value increased monotonously. Moreover, the crosslinker content had little effect on the curing time. The stress−strain curves of BROHx/BI/silica composites areshown in Figure 3c. With the increase in BI amount, the tensile strength and modulus of BROHx/BI/silica composites increased obviously, and the breaking elon gation decreased because of increasing crosslinking density. Scheme 2. The unblocking of blocked polyisocyanates and the vulcanization mechanism of polyisocyanates-cured BROH x /BI/silica composite.

Mechanical Properties of BROH x /BI/Silica Composites
The mechanical properties of formed rubber composites can be manipulated by adjusting the BI content. Figure 3a was the crosslinking density plot of BROH x /BI/silica composite vs. BI amount. The crosslinking density increased gradually with the increase in BI amount due to the increase in reaction points. With the increase in BI dosage from 4 to 8 phr, the crosslinking density enhanced from 6.63 to 8 ×10 −5 mol/cm 3 . Figure 3b presents the curing curves of BROH 3 using different BI doses. With the increase in BI content, the crosslinking density and MH value increased monotonously. Moreover, the crosslinker content had little effect on the curing time. The stress−strain curves of BROH x /BI/silica composites areshown in Figure 3c. With the increase in BI amount, the tensile strength and modulus of BROH x /BI/silica composites increased obviously, and the breaking elongation decreased because of increasing crosslinking density.  The effect of hydroxyl group content on the performance of cured rubber was furt investigated. As shown in Figure 4a, at a constant dosage of BI (6 phr), the crosslink density of BROHx increased with the increasing hydroxyl group content because of creasing amount of curing sites. The crosslinking density exhibited an abrupt incre from 4.46 to 7.34 × 10 −5 mol/cm 3 when the hydroxyl group content increased from 1 t mol%. In contrast, a further increase in the hydroxyl content from 3 to 5 mol%, only led a slight increase in crosslinking density. The possible reason was that effective crossli ing can only be realized when at least two of the three isocyanate groups in each BI re with the rubber chains. When the BROHx hydroxyl content was low (e.g., BROH1), effective crosslinking could be formed because of the reduced possibility of the limi hydroxyl groups to react with multiple isocyanate groups in each BI. At a higher cont of hydroxyl groups (e.g., BROH5), more hydroxyl groups were accessible to isocyan groups of BI for effective crosslinking. Figure 4b depicts the curing curves BROHx/BI/silica composites with the hydroxyl group contents. High hydroxyl group c tent led to a shortened curing time and an increased MH due to a higher crosslink density. The stress−strain curves of BROHx/BI/silica composites are shown in Figure  Both the tensile strength and modulus were enhanced with the increase in hydroxyl gro content. The elongation at break depended also remarkably on the hydroxyl conte which was attributed to the increment of the crosslinking density, as well. Notably, the modulus of the BROH3/BI6/silica composite was as high as 4.4 MPa strain of 300%, close to the value of the BR silica composite with Si69 added (Tabl BR/S/Si69/silica), much higher than those of BR composites with similar crosslinking d sity prepared using the traditional sulphur-cured BR/S/silica composite ( Figure  BR/S/silica). Additionally, the resilience and the permanent deformation retention in co pression were all enhanced with the increase in hydroxyl content. The tensile strength a The effect of hydroxyl group content on the performance of cured rubber was further investigated. As shown in Figure 4a, at a constant dosage of BI (6 phr), the crosslinking density of BROH x increased with the increasing hydroxyl group content because of increasing amount of curing sites. The crosslinking density exhibited an abrupt increase from 4.46 to 7.34 × 10 −5 mol/cm 3 when the hydroxyl group content increased from 1 to 3 mol%. In contrast, a further increase in the hydroxyl content from 3 to 5 mol%, only led to a slight increase in crosslinking density. The possible reason was that effective crosslinking can only be realized when at least two of the three isocyanate groups in each BI react with the rubber chains. When the BROH x hydroxyl content was low (e.g., BROH 1 ), no effective crosslinking could be formed because of the reduced possibility of the limited hydroxyl groups to react with multiple isocyanate groups in each BI. At a higher content of hydroxyl groups (e.g., BROH 5 ), more hydroxyl groups were accessible to isocyanate groups of BI for effective crosslinking. Figure 4b depicts the curing curves of BROH x /BI/silica composites with the hydroxyl group contents. High hydroxyl group content led to a shortened curing time and an increased MH due to a higher crosslinking density. The stress−strain curves of BROH x /BI/silica composites are shown in Figure 4c. Both the tensile strength and modulus were enhanced with the increase in hydroxyl group content. The elongation at break depended also remarkably on the hydroxyl content, which was attributed to the increment of the crosslinking density, as well. The effect of hydroxyl group content on the performance of cured rubber was furt investigated. As shown in Figure 4a, at a constant dosage of BI (6 phr), the crosslink density of BROHx increased with the increasing hydroxyl group content because of creasing amount of curing sites. The crosslinking density exhibited an abrupt incre from 4.46 to 7.34 × 10 −5 mol/cm 3 when the hydroxyl group content increased from 1 t mol%. In contrast, a further increase in the hydroxyl content from 3 to 5 mol%, only led a slight increase in crosslinking density. The possible reason was that effective crossli ing can only be realized when at least two of the three isocyanate groups in each BI re with the rubber chains. When the BROHx hydroxyl content was low (e.g., BROH1), effective crosslinking could be formed because of the reduced possibility of the limi hydroxyl groups to react with multiple isocyanate groups in each BI. At a higher cont of hydroxyl groups (e.g., BROH5), more hydroxyl groups were accessible to isocyan groups of BI for effective crosslinking. Figure 4b depicts the curing curves BROHx/BI/silica composites with the hydroxyl group contents. High hydroxyl group c tent led to a shortened curing time and an increased MH due to a higher crosslink density. The stress−strain curves of BROHx/BI/silica composites are shown in Figure  Both the tensile strength and modulus were enhanced with the increase in hydroxyl gro content. The elongation at break depended also remarkably on the hydroxyl conte which was attributed to the increment of the crosslinking density, as well. Notably, the modulus of the BROH3/BI6/silica composite was as high as 4.4 MPa a strain of 300%, close to the value of the BR silica composite with Si69 added (Table BR/S/Si69/silica), much higher than those of BR composites with similar crosslinking d sity prepared using the traditional sulphur-cured BR/S/silica composite (Figure BR/S/silica). Additionally, the resilience and the permanent deformation retention in co pression were all enhanced with the increase in hydroxyl content. The tensile strength a modulus of BROH5/BI6 were even superior to the traditional BR/S/Si69/silica compos but the values of permanent set, resilience, and compression set were higher than tha the BR/S/Si69 composite at different hydroxyl contents. Notably, the modulus of the BROH 3 /BI 6 /silica composite was as high as 4.4 MPa at a strain of 300%, close to the value of the BR silica composite with Si69 added (Table 1, BR/S/Si69/silica), much higher than those of BR composites with similar crosslinking density prepared using the traditional sulphur-cured BR/S/silica composite (Figure 4a, BR/S/silica). Additionally, the resilience and the permanent deformation retention in compression were all enhanced with the increase in hydroxyl content. The tensile strength and modulus of BROH 5 /BI 6 were even superior to the traditional BR/S/Si69/silica composite, but the values of permanent set, resilience, and compression set were higher than that of the BR/S/Si69 composite at different hydroxyl contents.

Mechanical Properties of SBROH x /BI/Silica Composites
It should be noted that this BI-cured system was generally applicable to other dienebased rubbers, such as styrene butadiene rubber (SBR). A series of pendant hydroxylgroup-functionalized SBR (denominated as SBROH x , FTIR, 1 H NMR, and GPC, which are shown in Figures S3 and S4 and Table S3) were synthesized in the same way. Additionally, the effect of hydroxyl content on glass transition temperature (T g ) was characterized by a DSC test. As shown in Figure 5, with the increase in content of hydroxyl groups, T g gradually increased from −52 to −35.1 • C, which may be attributed to the enhancement of chain polarity and hydrogen bonding interactions. There were some changes in the DSC curve of SBR at about −20 and 10 • C. It is proposed that these changes may be attributed to some unknown thermodynamic transformations originating from the impurities and additives in commercial SBR.

Mechanical Properties of SBROHx/BI/Silica Composites
It should be noted that this BI-cured system was generally applicable to othe based rubbers, such as styrene butadiene rubber (SBR). A series of pendant hy group-functionalized SBR (denominated as SBROHx, FTIR, 1 H NMR, and GPC, w shown in Figures S3 and S4 and Table S3) were synthesized in the same way. Addit the effect of hydroxyl content on glass transition temperature (Tg) was characteriz DSC test. As shown in Figure 5, with the increase in content of hydroxyl groups, T ually increased from −52 to −35.1 °C, which may be attributed to the enhancement polarity and hydrogen bonding interactions. There were some changes in the DS of SBR at about −20 and 10 °C. It is proposed that these changes may be attributed unknown thermodynamic transformations originating from the impurities and ad in commercial SBR. The effect of hydroxyl content on the properties of SBROHx/BI/silica compos investigated with a fixed dosage (6 phr) of BI. As shown in Figure 6, as with BROH ica composites, the crosslinking density increased with the increase in hydroxyl The MH also increased with the increase in crosslinking density. As shown in Fi the elongation at break of SBROH2 was close to that of the SBR/S/silica composite by sulfur), while the modulus and tensile strength were higher. The modulus for S can be further improved close to or even higher than that of the SBR/S/Si69/silica site (cured by sulfur with the help of Si69). However, the elongation at break w decreased with contents of hydroxyl groups (see Table 2). The effect of hydroxyl content on the properties of SBROH x /BI/silica composites was investigated with a fixed dosage (6 phr) of BI. As shown in Figure 6, as with BROHx/BI/Silica composites, the crosslinking density increased with the increase in hydroxyl content. The MH also increased with the increase in crosslinking density. As shown in Figure 6c, the elongation at break of SBROH 2 was close to that of the SBR/S/silica composite (cured by sulfur), while the modulus and tensile strength were higher. The modulus for SBROH x can be further improved close to or even higher than that of the SBR/S/Si69/silica composite (cured by sulfur with the help of Si69). However, the elongation at break would be decreased with contents of hydroxyl groups (see Table 2).

Dispersion of Silica in the Rubber Matrix
We recall that the dispersion of filler is key for deciding the performance of rub [26,27]. It was found that the introduction of hydroxyl groups facilitates the dispersion silica (TEM image Figure 7b) because of the enhanced interaction of rubber chains w silica. An RPA analysis was performed on rubbers before and after hydroxylation tre ment. As shown in Figure 7a, typical Payne effects were observed for SBR/S/silica com sites: the initial very high modulus decreases rapidly with the increase in strain [36] contrast, the SBROH/BI/silica composites exhibited a much lower initial storage modu and the Payne effects were weaker with higher hydroxyl contents, even equivalent to t of te SBR/S/Si69/Silica composite at a hydroxyl content of 7 mol%, suggesting a good d persion of silica. Similarly, Payne effects with BROH/BI/Silica composites ( Figure S5) w weakened significantly with the increasing content of hydroxyl groups.

Dispersion of Silica in the Rubber Matrix
We recall that the dispersion of filler is key for deciding the performance of rubber [26,27]. It was found that the introduction of hydroxyl groups facilitates the dispersion of silica (TEM image Figure 7b) because of the enhanced interaction of rubber chains with silica. An RPA analysis was performed on rubbers before and after hydroxylation treatment. As shown in Figure 7a, typical Payne effects were observed for SBR/S/silica composites: the initial very high modulus decreases rapidly with the increase in strain [36]. In contrast, the SBROH/BI/silica composites exhibited a much lower initial storage modulus, and the Payne effects were weaker with higher hydroxyl contents, even equivalent to that of te SBR/S/Si69/Silica composite at a hydroxyl content of 7 mol%, suggesting a good dispersion of silica. Similarly, Payne effects with BROH/BI/Silica composites ( Figure S5) were weakened significantly with the increasing content of hydroxyl groups.

Dispersion of Silica in the Rubber Matrix
We recall that the dispersion of filler is key for deciding the performance of rubbe [26,27]. It was found that the introduction of hydroxyl groups facilitates the dispersion o silica (TEM image Figure 7b) because of the enhanced interaction of rubber chains with silica. An RPA analysis was performed on rubbers before and after hydroxylation treat ment. As shown in Figure 7a, typical Payne effects were observed for SBR/S/silica compo sites: the initial very high modulus decreases rapidly with the increase in strain [36]. In contrast, the SBROH/BI/silica composites exhibited a much lower initial storage modulus and the Payne effects were weaker with higher hydroxyl contents, even equivalent to tha of te SBR/S/Si69/Silica composite at a hydroxyl content of 7 mol%, suggesting a good dis persion of silica. Similarly, Payne effects with BROH/BI/Silica composites ( Figure S5) wer weakened significantly with the increasing content of hydroxyl groups.

Dynamic Mechanical Properties of SBROH 5 /Silica Composites Cured with Different Polyisocyanates
A trade-off was the decrease in resilience and the permanent deformation retention in compression for BROH/BI/silica composites and SBROH/BI/silica composites (see Tables 1 and 2). The effect of hydroxyl groups content on the dynamic behavior of the SBROH/BI/silica composites is shown in Figure 8. The T g increased with the increase in hydroxyl groups content, coherent with the observation in DSC tests. However, compared to the SBR/S/Si69/silica composite, the tanδ at 60 • C for SBROH/BI/silica composites were all clearly higher, which is possibly because of a bigger inter crosslink distance. Although some damping materials required higher tanδ, low tanδ and dynamic heat build-up is more desirable for more applications, such as tread compound. Therefore, the effect of the isocyanate structure was further studied by varying the distance between crosslinking points.

Dynamic Mechanical Properties of SBROH5/Silica Composites Cured with Different Polyisocyanates
A trade-off was the decrease in resilience and the permanent deformation ret in compression for BROH/BI/silica composites and SBROH/BI/silica composites (s bles 1 and 2). The effect of hydroxyl groups content on the dynamic behavior SBROH/BI/silica composites is shown in Figure 8. The Tg increased with the incre hydroxyl groups content, coherent with the observation in DSC tests. However, com to the SBR/S/Si69/silica composite, the tanδ at 60 °C for SBROH/BI/silica composites all clearly higher, which is possibly because of a bigger inter crosslink distance. Alth some damping materials required higher tanδ, low tanδ and dynamic heat build more desirable for more applications, such as tread compound. Therefore, the effect isocyanate structure was further studied by varying the distance between crossli points. Blocked hexamethylene diisocyanate (B-HDI); 1,4-Phenylene diisocyanate (B-P and toluene-2,4-diisocyanate (B-TDI) were synthesized (Scheme 3) using butanone o The distance between the two NCO groups for these three diisocyanates increas quentially. The blocking reaction was complete, as the peak at 2270 cm −1 assigned to groups in TDI disappeared after the reaction (FTIR spectra, Figure S6). Firstly, the SBROH5/Silica composites were vulcanized under the same molar r NCO/OH (6 phr BI, 3.06 phr BHDI, 2.99 phr BPPDI, and 3.11 phr BTDI). The curing c are shown in Figure S7, and all three samples were cured effectively. As shown in F 9a, the tensile strengths for these three blocked diisocyanates were similar, but the m lus increased gradually in the order of B-HDI, B-PPDI, and B-TDI (Table 3). Further as shown in tanδ-T curves (Figure 9b): (1) the structure of blocked diisocyanate had effect on Tg; (2) surprisingly, tanδ values at 60 °C for B-HDI, B-PPDI, and B-TDI w significantly smaller than that of BI and decreased in the order of HDI, PPDI, and Blocked hexamethylene diisocyanate (B-HDI); 1,4-Phenylene diisocyanate (B-PPDI); and toluene-2,4-diisocyanate (B-TDI) were synthesized (Scheme 3) using butanone oxime. The distance between the two NCO groups for these three diisocyanates increased sequentially. The blocking reaction was complete, as the peak at 2270 cm −1 assigned to NCO groups in TDI disappeared after the reaction (FTIR spectra, Figure S6).

Dynamic Mechanical Properties of SBROH5/Silica Composites Cured with Different Polyisocyanates
A trade-off was the decrease in resilience and the permanent deformation r in compression for BROH/BI/silica composites and SBROH/BI/silica composites bles 1 and 2). The effect of hydroxyl groups content on the dynamic behavio SBROH/BI/silica composites is shown in Figure 8. The Tg increased with the inc hydroxyl groups content, coherent with the observation in DSC tests. However, co to the SBR/S/Si69/silica composite, the tanδ at 60 °C for SBROH/BI/silica composi all clearly higher, which is possibly because of a bigger inter crosslink distance. A some damping materials required higher tanδ, low tanδ and dynamic heat bui more desirable for more applications, such as tread compound. Therefore, the effe isocyanate structure was further studied by varying the distance between cros points. Blocked hexamethylene diisocyanate (B-HDI); 1,4-Phenylene diisocyanate (B and toluene-2,4-diisocyanate (B-TDI) were synthesized (Scheme 3) using butanon The distance between the two NCO groups for these three diisocyanates incre quentially. The blocking reaction was complete, as the peak at 2270 cm −1 assigned groups in TDI disappeared after the reaction (FTIR spectra, Figure S6). Firstly, the SBROH5/Silica composites were vulcanized under the same molar NCO/OH (6 phr BI, 3.06 phr BHDI, 2.99 phr BPPDI, and 3.11 phr BTDI). The curin are shown in Figure S7, and all three samples were cured effectively. As shown i 9a, the tensile strengths for these three blocked diisocyanates were similar, but th lus increased gradually in the order of B-HDI, B-PPDI, and B-TDI (Table 3). Furth as shown in tanδ-T curves (Figure 9b): (1) the structure of blocked diisocyanate h effect on Tg; (2) surprisingly, tanδ values at 60 °C for B-HDI, B-PPDI, and B-TDI significantly smaller than that of BI and decreased in the order of HDI, PPDI, a Therefore, the dynamic heat build-up was relevant to the chemical structures of d Firstly, the SBROH 5 /Silica composites were vulcanized under the same molar ratio of NCO/OH (6 phr BI, 3.06 phr BHDI, 2.99 phr BPPDI, and 3.11 phr BTDI). The curing curves are shown in Figure S7, and all three samples were cured effectively. As shown in Figure 9a, the tensile strengths for these three blocked diisocyanates were similar, but the modulus increased gradually in the order of B-HDI, B-PPDI, and B-TDI (Table 3). Furthermore, as shown in tanδ-T curves (Figure 9b): (1) the structure of blocked diisocyanate had little effect on T g ; (2) surprisingly, tanδ values at 60 • C for B-HDI, B-PPDI, and B-TDI were all significantly smaller than that of BI and decreased in the order of HDI, PPDI, and TDI. Therefore, the dynamic heat build-up was relevant to the chemical structures of diisocyanates. The tanδ values with TDI were close to those of the SBR/S/Si69/silica and SBROH x /silica composites, as shown in Table 4. Moreover, the resilience properties increased, and the compression set decreased, superior to the SBR/S/Si69/silica composite. It also should be noted that the higher tanδ values at 0 • C (0.21) (Table 4) suggested improved wet skid resistance in tire compound application. mers 2022, 14, x FOR PEER REVIEW 11 o SBROHx/silica composites, as shown in Table 4. Moreover, the resilience properties creased, and the compression set decreased, superior to the SBR/S/Si69/silica compos It also should be noted that the higher tanδ values at 0 °C (0.21) (Table 4) suggested i proved wet skid resistance in tire compound application.

Conclusions
In summary, high crosslinking efficiency can be achieved without the introduct of other toxic activators and accelerants via sulfurization chemistry based on the react of hydroxyl groups with blocked polyisocyanates. In addition, the hydroxyl groups rubber chains promoted the dispersion of silica filler. A good dispersion of silica was tained even without the usage of the silane coupling agent. The mechanical properties a crosslinking density of the samples can be readily adjusted by varying the content of h droxyl groups and the amount of blocked polyisocyanates. Compared to the sulph

Conclusions
In summary, high crosslinking efficiency can be achieved without the introduction of other toxic activators and accelerants via sulfurization chemistry based on the reaction of hydroxyl groups with blocked polyisocyanates. In addition, the hydroxyl groups in rubber chains promoted the dispersion of silica filler. A good dispersion of silica was attained even without the usage of the silane coupling agent. The mechanical properties and crosslinking density of the samples can be readily adjusted by varying the content of hydroxyl groups and the amount of blocked polyisocyanates. Compared to the sulphurcured counterpart of the SBR/S/Si69/silica composite with a similar crosslinking density, the blocked polyisocyanates-cured SBROH 5 /silica composite exhibited a higher tensile modulus. In addition, the dynamic heat build-up was affected by the distance between the two NCO groups in the diisocyanate. The SBROH 5 /B-TDI/silica composite showed superior resilience properties and compression set, as well as a low tanδ value at 0 • C, which was very close to that of the SBR/S/Si69/silica composite. Overall, this work provides a new and efficient pathway to fabricate rubbers with novel structures and high mechanical and dynamic heat build-up properties. Many other properties and the applications for this strategy are currently being explored in our laboratory.
Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/polym14030461/s1: Crosslinking density; Figure S1: Evolution of FTIR spectra of BROH x with increasing mCPBA and HCl dose; Figure S2: The molecular weight distribution (MWD) of BROH x ; Figure S3: Evolution of FTIR spectra of SBROH x ; Figure S4: 1 H NMR spectra of SBROH 5 ; Figure S5: RPA curves of BROH x cured by 6 phr BI; Figure S6: Evolution of FTIR spectra of TDI and blocked TDI; Figure S7: Curing curves of SBROH 5 cured by different blocked diisocyanates; Table S1: Degree of hydroxylation under different conditions; Table S2: Molecular weight distribution of BR after hydroxylation; Table S3: Molecular weight distribution of SBR after hydroxylation.