I. INTRODUCTION

Among various preparation techniques, the sol-gel method offers an unprecedented process to prepare advanced materials because of its easy mechanism, low cost, short synthesis time, and low fabricating temperature.^1,2 In the past decade, the sol-gel approach has been extensively used for the preparation of nanoparticles,^3–5 hybrid materials,^6–9 and hybrid coatings.^10–12 It is well known that the sol-gel method operates via different steps.^2,13 Metal alkoxides and metal chlorides are the typical precursors in sol-gel reactions. Initially, the precursors undergo hydrolysis and partial condensation reactions to form a stable suspension of colloidal solid particles dispersed in a solvent. This colloidal solution is called sol. In the next step, gel is formed via polycondensation reaction. Simply, gel is defined as an inorganic three-dimensional, continuous solid network that expands through a liquid phase. Next, the drying process is done to remove the liquid phase from the gel. In the final step, the calcination process is performed to produce crystalline structures. The most important advantage associated with the sol-gel method is the formation of highly pure and uniformly dispersed nanostructure at low temperature. Furthermore, the sol-gel method influences the particle size, shape, and morphology of synthesized materials through control of reaction conditions.¹⁴ Thus, due to its versatility, sol-gel technology can be used for modern industrial purposes.

Because of their exceptional combination of mechanical properties, elastomers or rubbers are probably the most versatile and broadly recognized group of polymers. Historically, rubbers are the irreplaceable component in different industrial and engineering products related to car and aircraft tires, sports equipment, footwear, hoses, glues, belts, and gaskets. Now, pure rubber is not able to provide the properties required for its industrial applications. The vulcanization process is usually done to improve the processability and final performance of rubber.¹⁵ In this process, discrete rubber chains are chemically linked by crosslinking reaction to form a three-dimensional network structure with enhanced mechanical properties.¹⁵ Vulcanization is an important process for both natural and synthetic rubbers such as SBR, CR, and NBR. During vulcanization, several components are mixed with the pure rubber matrix. These components are usually known as compounding ingredients. The compounding ingredients are classified into activator, accelerator, crosslinking agent, antidegradant, and filler. The choice of compounding ingredients is the most crucial factor in the vulcanization process of rubber. Figure 1 schematically represents the conventional method for the preparation and characterization of rubber composites.

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In modern rubber technology, some noticeable problems arise from the use of conventional ingredients in rubber compounding. Zinc oxide (ZnO) plays a vital role in the vulcanization of natural and synthetic rubbers. Historically, ZnO has been the most widely applied cure activator in the sulfur vulcanization of rubber. The conventional quantity of ZnO is 5 parts per hundred of rubber (phr) during the vulcanization process.^16,17 But the excess use of ZnO in rubber technology has some limitations from the environmental point of view. At the end of the product life, excess ZnO is released into the lithosphere due to degradation of rubber.¹⁷ Also, via leaching, ZnO is released into landfill sites.¹⁶ Now, soluble zinc compounds are found to be toxic for aquatic organisms.^18,19 Furthermore, the European Union has stated ZnO is a hazardous material for the environment.²⁰ Thus, the application of ZnO in rubber technology should be minimized.

In rubber technology, another major problem is related to the application of carbon black as a common reinforcing filler for rubber composites. Carbon black is prepared from a petroleum source, and it contributes to environmental pollution during the processing of carbon black–filled rubber compounds.^21–24 In addition, carbon black generates a black coloration in rubber products.^21,23 To solve these problems, it is essential to use new alternative materials instead of ZnO and carbon black in rubber formulations.

In this review, sol-gel–synthesized nanoparticles are introduced to replace the conventional cure activator (i.e., ZnO) and conventional filler (i.e., carbon black) in high performance rubber compounds. The specific aim of the review is to describe the contribution of sol-gel technology on the recent development of rubber technology from both environmental and industrial points of view.

II. PREPARATION OF SOL-GEL–DERIVED MATERIALS-BASED RUBBER COMPOSITES

There have been a very limited number of studies focusing on the preparation of rubber composites based on sol-gel–derived nano ZnO, despite very interesting results. Roy et al.^16,25 reported the preparation and characterization of sol-gel–derived ZnO-based NR composites. Initially, ZnO was synthesized via a simple sol-gel approach following the method given by Khouzani et al.²⁶ In brief, Figure 2a describes the process for the sol-gel synthesis of ZnO nanocrystals by using zinc nitrate hexahydrate, citric acid, and ethylene glycol as starting materials. In the next step, sol-gel–synthesized nano ZnO was added as a cure activator into the NR matrix during preparation of rubber composites by conventional two-roll mixing mill process. During preparation of rubber composites, the quantity of externally added ZnO is expressed in phr. Continuing their research, Roy et al.^27,28 reported the preparation of sol-gel–synthesized nano magnesium oxide (MgO)-based rubber composites by conventional two-roll mixing mill process. In those publications, MgO was synthesized by sol-gel auto-combustion method.²⁹ In brief, Figure 2b describes the process for the sol-gel synthesis of MgO nanocrystals by using magnesium nitrate hexahydrate, citric acid, and ammonia (NH₃) as starting materials. In the next step, sol-gel–synthesized MgO was added as cure activator into the rubber matrix during preparation of rubber composites by using two-roll mixing mill.

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The preparation of sol-gel–derived in situ silica–based rubber composites is the most attractive approach for the progress of green elastomeric technology. The concept of the preparation of in situ silica–filled rubber composites via sol-gel reaction of tetraethoxysilane (TEOS) was introduced in the early 2 000s.^30–33 The stepwise synthesis of silica nanoparticles via sol-gel reaction of TEOS is shown in Figure 3.³⁴ In the past decade, considerable research has been related to the development of sol-gel–synthesized in situ silica–based rubber composites.^34–60 In those publications, the sol-gel approach has been successfully used to synthesize silica in different rubber matrices such as NR,^{35–39,41,42,44,45} methyl methacrylate–grafted NR (MMA-GNR),⁴⁰ styrene-grafted NR (ST-GNR),⁴³ SBR,^44–46 vinyltriethoxysilane-grafted SBR (VTES-SBR),^47–49 EPDM,^{34,44,50–53} hydrogenated NR (HNR),⁵³ NBR,⁵⁴ CR,⁵⁵ and epoxidized NR (ENR).⁵⁶ Furthermore, sol-gel science is very effective to produce in situ silica in different rubber blend systems such as NR and NBR^57,58 and NR and CR.^59,60Table I summarizes the key points during synthesis of in situ silica in some important rubber matrices. The alkoxide silica formation in a rubber matrix mainly occurs via two methods: soaking and solution. In the soaking method, swelling of rubber sheets in TEOS at ambient temperature, followed by immersion of the swollen sheets in the basic or acidic aqueous solution of catalyst, is done.^{35,40,44,53,54} The different steps of the soaking method during preparation of in situ silica–based rubber composites are shown in Figure 4a. In solution the method, dissolution of rubber in a solvent is by mechanical stirring, followed by the stepwise addition of TEOS, water, and catalyst in the rubber solution.^{34,36,40,46,50}Figure 4b outlines the key steps of the solution sol-gel method. Recently, n-butylamine and n-hexylamine are commonly used catalysts during the preparation of in situ silica–based rubber composites.

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Table I Key Points during Synthesis of In Situ Silica in Different Rubber Matrices

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In some studies, the latex solution method is used to prepare in situ silica in rubber latex.^{37–39,41,42} The procedure of latex solution method for in situ silica synthesis is slightly different from that of the normal solution method. Poompradub et al.³⁹ reported an easy approach for the synthesis of silica in NR latex via sol-gel reaction. They claimed that NH₃ present in NR latex can act as a catalyst during sol-gel reaction of TEOS. Thus, silica nanoparticles were generated in the NR matrix via sol-gel reaction in the absence of base catalyst.³⁹ However, in some cases, n-butylamine was used as a basic catalyst in the preparation of silica nanoparticles in NR matrix via the latex solution method.

During synthesis of metal oxide via non-hydrolytic sol-gel chemistry, the oxygen of the oxide can be derived from either metal alkoxide or solvents such as alcohol, ether, and ketone.^2,61,62 As shown in Figure 5, the crucial factor is the formation of metal oxide via several condensation steps such as alkyl halide elimination, ether elimination, ester elimination, and aldol-like condensation. In this context, Mokhothu et al.⁵¹ reported an interesting study for the synthesis of in situ silica in EPDM matrix via non-hydrolytic sol-gel method in the presence of silicon tetrachloride, tert-butanol, and tin(II) 2-ethylhexanoate (1:25:0.04 mole ratio). In this process, tert-butanol was used as not only an oxygen donor but also as a capping agent to control particle size and shape.

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Finally, in most cases, mixing of in situ silica–filled rubber sample with vulcanizing chemicals was done in a conventional two-roll mixing mill to prepare in situ silica–filled rubber composites. By contrast, very few reports are available regarding the preparation of rubber composites based on externally added sol-gel–synthesized silica nanoparticles.^63,64

There are some interesting articles that describe the preparation of rubber composites based on sol-gel–synthesized nano titania.^65–68 For example, Das et al.⁶⁵ reported controlled synthesis of in situ titania in NBR matrix via the sol-gel method. Figure 6 represents the stepwise generation of in situ titania in NBR matrix by using titanium(IV) n-butoxide (TNB) as precursor and n-heptylamine as catalyst. Furthermore, Das et al.⁶⁵ optimized the sol-gel reaction condition during synthesis of in situ titania in NBR matrix. It was found that tetrahydrofuran (THF) is a much better solvent than either xylene or toluene during in situ synthesis of titania in NBR matrix. As claimed by Das et al.,⁶⁵n-heptylamine was the most effective catalyst during conversion of TNB to in situ titania in NBR matrix, and the optimum concentration of catalyst was found to be 0.096 M with respect to 1 mole of TNB. In situ titania–filled NBR mixes were mixed with vulcanizing ingredients in conventional two-roll mixing mill. Paderni et al.⁶⁶ were the first to prepare in situ titania–based EPDM matrix via non-hydrolytic sol-gel method in presence of titanium(IV) chloride and tert-butanol as starting materials. Figure 7 summarizes the procedure for the generation of in situ titania in EPDM matrix. Dicumyl peroxide (DCP) was used as vulcanizing agent in EPDM composites filled with in situ titania. Roy et al.^67,68 also reported the preparation of rubber composites based on sol-gel–derived titania (TiO₂). In the both studies, titania was synthesized using titanium tetra-isopropoxide and acetic acid as starting materials.⁶⁹ Next, sol-gel–synthesized titania was externally added to the rubber matrix by using conventional two-roll milling process.^67,68

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In recent studies, Roy et al.⁷⁰ and Roy and Potiyaraj⁷¹ reported the development of rubber composites based on sol-gel–derived alumina. Initially, nano alumina was prepared from aluminium nitrate and citric acid following the sol-gel method given by Li et al.⁷² Next, sol-gel–synthesized alumina was externally added to maleated NR (MNR) and NR matrix by using conventional two-roll mixing mill process.^70,71 However, the preparation of in situ alumina in rubber matrix via sol-gel process is still rare.

Sun et al.^73,74 explored the preparation and use of sol-gel–based microcrystalline cellulose (MCC)/silica (SiO₂) hybrid materials in rubber compounds. In one work, Sun et al.⁷³ reported the preparation of sol-gel–based MCC/SiO₂ hybrid materials by using TEOS precursor via three different processes (Figure 8). In their next study, Sun et al.⁷⁴ followed a nearly similar procedure to prepare MCC/SiO₂ hybrid materials in the presence of different amount of TEOS. The rubber composites filled with MCC/SiO₂ hybrid materials were prepared in an internal mixer.^73,74 In another study, Liang et al.⁷⁵ reported a new technology for the preparation of MCC/ ZnO hybrid materials via sol-gel process. During the sol-gel process, MCC acts as template to prepare MCC/ZnO hybrid materials with different hetero-structures. The rubber composites based on MCC/ZnO hybrid materials were prepared by this research group in a torque rheometer. Roy et al.^76,77 reported an interesting approach for the development of rubber composites based on calcium carbonate (CaCO₃)/silica hybrid materials. In those studies, alkoxide silica was generated on the surface of nano CaCO₃ via sol-gel reaction of TEOS in the presence of NH₃.⁷⁸ The external mixing process of silica-coated CaCO₃ with rubber matrix was done by conventional two-roll milling process.

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III. FUNDAMENTAL DISCUSSION ABOUT THE KINETICS AND DIFFERENT FACTORS IN SOL-GEL SYNTHESIS

It is well known that catalyst has an important role during the hydrolysis and condensation steps of the sol-gel reaction. The mechanism of hydrolysis of silicon alkoxides in presence of both acidic and basic catalyst systems is shown in Figure 9. As shown in the mechanism, the stability of the transition state has an important role on the rate of the hydrolysis step. In addition, the stability of the transition state is closely related to the electron-withdrawing or electron-donating nature of –OH and –OR groups. The hydrolysis step is faster under basic conditions than acidic conditions.²Figure 10 represents the mechanism of condensation of silicon alkoxides in presence of both acidic and basic catalyst systems. In presence of basic catalyst, condensation starts after the completion of the hydrolysis step. Under this condition, the resulting product is (OH)₃Si–O–Si(OH)₃, which has six possible sites for successive condensation steps. As a result, multiple condensation steps occur to form highly branched sol and colloidal gel. In presence of acidic catalyst, the condensation step begins before the completion of the hydrolysis step. Under this condition, condensation occurs via terminal silanol groups, producing chain-like sol and network-like gel.²

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Generally, the sol-gel synthesis of in situ silica in a rubber matrix depends on several factors such as the nature and concentration of the catalyst, the TEOS:water ratio, and the reaction time. Ikeda et al.³⁵ explained the effect of the nature and concentration of catalyst on the in situ silica content in uncrosslinked NR matrix. Both the polarity and basicity are important factors in the choice of catalyst during preparation of in situ silica in a rubber matrix. As reported by the Ikeda et al.,³⁵ the effect of different amines as catalyst during preparation of in situ silica in NR matrix is quite interesting. The amount of in situ silica in uncrosslinked NR matrix increased with increasing numbers of methylene unit in the primary amine. The in situ silica content in NR matrix was significantly higher in presence of n-hexylamine or n-octylamine than n-butylamine when the catalyst concentration was set at 0.096 mol/L. In this work, they also investigated the effect of n-hexylamine catalyst concentration on the generation of in situ silica in NR matrix. It was found that the amount of in situ–generated silica in NR matrix increases with an increase in the concentration of n-hexylamine as catalyst. Above a certain concentration of catalyst, TEOS in rubbery matrix has a tendency to migrate in the aqueous solution of catalyst. As claimed by Ikeda et al.,³⁵ the optimum concentration of n-hexylamine for the synthesis of in situ silica in NR matrix is 0.064 mol/L. The average particle size of in situ silica in the NR matrix was found to be 46 nm when the concentration of n-hexylamine was set at 0.064 mol/L. The average particle size of in situ silica increased continuously with an increase in the concentration of n-hexylamine, due to the higher conversion of TEOS (percent) or higher in situ silica content in presence of higher concentration of catalyst.³⁵ A very similar observation was found in electron beam crosslinking–based block copolymer/in situ silica composites.⁷⁹ In this study, the size of in situ silica was successfully controlled by changing the electron beam irradiation dosage. As suggested by the authors, irradiation-based crosslinking can inhibit the growth of silica via the sol-gel process. Thus, the volume percent of in situ silica showed a decreasing trend with increasing irradiation dosage. By contrast, the size of in situ silica showed reduction with decreasing volume percent of silica.

According to Poompradub et al.,³⁹ the in situ generation of silica in NR latex depends on reaction parameters, such as TEOS concentration, mole ratio of water:TEOS, reaction time, and reaction temperature. As suggested by the authors, high in situ silica content (54 phr) can be obtained in NR latex in 24 h reaction time at room temperature in the presence of 200 phr TEOS content and a water:TEOS mole ratio of 28.9:1.

IV. APPLICATIONS OF SOL-GEL–BASED MATERIALS AS CURE ACTIVATOR IN RUBBER TECHNOLOGY

a. Sol-gel–derived ZnO as cure activator

Roy et al.¹⁶ investigated the suitability of sol-gel–synthesized nano ZnO as cure activator in place of conventional ZnO for the reduction of ZnO level in the sulfur vulcanization of NR. The value of cure rate index (CRI) indicates the rate of rubber vulcanization in the presence of a particular cure activator system. Roy et al.¹⁶ claimed that only 0.5 phr sol-gel–synthesized ZnO was able to provide greater CRI values in NR composites than 5 phr conventional ZnO. The excellent cure efficiency of sol-gel–derived nano ZnO in NR compounds can be explained critically by considering the different steps during sulfur vulcanization of rubber, as shown in Figure 11.^16,80 From Figure 11, it is clear that the cure rate of sulfur vulcanization strongly depends on the formation of zinc accelerator complex via the reaction between ZnO and accelerator. According to this report, the specific surface area of sol-gel–synthesized ZnO was nearly eight times greater than that of the conventional ZnO. Thus, the formation of zinc accelerator complex was much faster in 0.5 phr nano ZnO-cured NR sample than in NR sample with 5 phr conventional ZnO. Consequently, significant improvement (∼60%) was attained in the CRI of NR composite due to the addition of 0.5 phr sol-gel–synthesized ZnO compared with NR composite containing 5 phr conventional ZnO as cure activator. As shown in Table II, the mechanical properties and thermal stability of nano ZnO–cured NR composite were superior to those of the conventional ZnO-cured NR composite. Above 0.5 phr loading level, nano ZnO had the tendency of agglomeration within the rubber matrix. Roy et al.¹⁶ confirmed that sol-gel–derived nano ZnO can substitute for conventional ZnO with a 10 times reduction of ZnO level in NR compounds.

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Table II Properties of NR Composites in Presence of Conventional ZnO and Sol-Gel–Synthesized Nano ZnO as Cure Activator¹⁶

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In their next study, Roy et al.²⁵ reported the effect of surface-modified nano ZnO as a cure activator on the cure and mechanical properties of SBR composites. The surface of sol-gel–synthesized ZnO was successfully modified by two different modifying agents: stearic acid and bis[3-(triethoxysilyl)propyl]tetrasulfide (Si-69). As shown in Figure 12a, the value of maximum rheometric torque (R_∞) of SBR composites was considerably higher in the presence of Si-69–treated ZnO than either stearic acid–treated ZnO or untreated nano ZnO. This might be explained by considering improved interfacial interaction between Si-69–treated nano ZnO and SBR matrix, as shown in Figure 12b. Si-69–treated ZnO was also very effective for enhancing the mechanical properties such as tensile strength and modulus of SBR composites. According to this study, 1 phr Si-69–treated nano ZnO is more attractive choice than 1 phr unmodified nano ZnO for the reduction of ZnO level in SBR compounds. However, the main drawback of this concept is the reduction of CRI value due to addition of Si-69–treated nano ZnO as cure activator in SBR compounds.

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b. Sol-gel–derived MgO as cure activator

Roy et al.²⁷ were the first to study the effect of sol-gel–synthesized nano MgO as a cure activator in the sulfur vulcanization of NR. Zinc (N-benzyl piperazino) dithiocarbamate (ZBPDC)/2-mercaptobenzothiazyl disulfide (MBTS) binary accelerator system has been used in this investigation. The main purpose of this study was to prepare ZnO-free eco-friendly rubber composites. The typical cure curves of NR specimens containing both conventional ZnO and sol-gel–synthesized MgO are shown in Figure 13a. In this figure, NMgO indicates nano MgO, CZnO indicates conventional ZnO, and the number indicates the amount (phr) of cure activator. This figure clearly indicates that nano MgO–cured NR samples showed faster cure reaction than NR sample cured with conventional ZnO. Guzmán et al.^81,82 suggested that the MgO-cured NR system follows different mechanism during formation of active sulfurating agents than the NR system with ZnO as cure activator. In MgO-cured NR composite, the active sulfurating agents are claimed to form via quick sulfur insertion in active accelerator species. Thus, the strong affinity of MgO for sulfur might be responsible for the quick formation of active sulfurating agents in the NR system with nano MgO as a cure activator. Because of the quick formation of active sulfurating agent, the value of CRI showed a tremendous increment by 400% for 1 phr nano MgO–cured NR composite compared with 5 phr conventional ZnO-cured NR composite. As claimed by Roy et al.,²⁷ the mechanical properties and thermal stability of 1 phr nano MgO–cured NR composite were far better compared with those of NR composite containing 5 phr conventional ZnO as cure activator. According to this report, 1 phr sol-gel–synthesized nano MgO can successfully replace 5 phr conventional ZnO in sulfur vulcanization of NR with no loss of final mechanical properties. However, the proper explanation regarding the role of ZBPDC/MBTS binary accelerator system on the property advancement nano MgO–cured NR system was absent in this report. There was a chance of generation of in situ nano ZnO during vulcanization of nano MgO–cured NR sample in the presence of ZBPDC/MBTS binary accelerator system. Moreover, Roy et al.²⁷ suggested further mechanistic investigation of MgO-cured NR composites in the presence of various single and binary accelerator systems. Sol-gel–synthesized ZnO and MgO particles were also used to develop a suitable nanostructured cure activator system for CR composites with improved cure, mechanical, and thermal properties.²⁸ As shown in Figure 13b, a nanostructured cure activator system containing 2 phr nano ZnO along with 2 phr nano MgO had the potential to provide greater torque value than conventional cure activator system containing 5 phr conventional ZnO along with 4 phr conventional MgO in CR formulation.²⁸

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V. APPLICATIONS OF SOL-GEL–BASED MATERIALS AS FILLER IN RUBBER TECHNOLOGY

a. Sol-gel–synthesized alkoxide silica nanoparticles as filler

Historically, sol-gel–synthesized in situ silica is a well-known reinforcing filler in rubber composites.^30,31,35 The excellent reinforcing ability of in situ silica is mainly due to the low hydrophilic nature of in situ silica surface.³⁰ Also, fewer filler–filler interactions is another reason for good performances in the sol-gel–synthesized in situ silica–filled rubber compounds.³⁰ Recently, many research groups have reported the suitable use of sol-gel–synthesized in situ silica nanoparticles as filler on the development of advanced NR composites.^39,40,83 Poompradub et al.³⁹ investigated the effect of in situ silica as filler on the overall performance of NR compounds. The mechanical and thermal properties of NR composites with in situ alkoxide silica were more superior to those of NR composites filled with ex situ silica.³⁹ In another study, Watcharakul et al.⁴⁰ reported the preparation and properties of sol-gel–derived in situ silica–reinforced MMA-GNR composites by the use of both solid rubber and rubber latex. In a very similar way, at same amount of filler loading, the mechanical properties were higher for MMA-GNR/NR composites containing in situ silica than for MMA-GNR/NR composites containing commercial silica.⁴⁰ The mechanical properties of in situ silica–filled NR composites greatly depends on the filler precursors³⁷ and the shape of filler.³⁸ The reinforcing effect of in situ silica in NR vulcanizates was found to be good agreement with the Guth–Gold equation by using an assumed shape factor of 2.53.³⁶

Sol-gel–synthesized in situ silica is also effective to improve the properties of different synthetic rubbers such as NBR,^54,63 CR,⁵⁵ EPDM,^34,51,84 SBR,^46,48,85 BR,⁸⁶ and silicone rubber.⁸⁷ Kapgate et al.⁵⁴ reported the preparation and properties of NBR/silica composites in the presence of γ-mercaptopropyltrimethoxysilane (γ-MPS) as a silane coupling agent. Excellent interaction between in situ–synthesized silanized silica and NBR matrix was confirmed from mechanical, dynamic mechanical, and thermal studies.⁵⁴ In another study, Kapgate et al.⁶³ investigated the reinforcing effect of γ-MPS–treated Stöber silica for NBR composites. Stöber silica was synthesized separately by simple sol-gel method and then it was modified by different amount of γ-MPS. The mechanical properties such as modulus and tensile strength of NBR composites were greatly improved in presence of γ-MPS–treated Stöber silica as filler.⁶³ The mechanical properties of γ-MPS–treated Stöber silica–filled NBR compounds are shown in Figure 14. It is possible to generate in situ silica inside the CR matrix via sol-gel reaction of TEOS.⁵⁵ γ-Aminopropyltrimethoxysilane–treated in situ silica was found to be very useful to improve mechanical and dynamic mechanical properties of CR composites.⁵⁵ The tensile strength and crosslink density values of CR/in situ silica composites are summarized in Figure 15.

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Mokhothu et al. discussed the reinforcement of EPDM rubber composites by in situ–generated silica by using both hydrolytic³⁴ and non-hydrolytic⁵¹ sol-gel methods. In both cases, bis-[-3-(triethoxysilyl)-propyl]-tetrasulfide (TESPT) was used as silane coupling agent to increase the rubber–filler interaction between hydrophilic silica and hydrophobic EPDM matrix.^34,51 The thermal stability of EPDM composites containing 20 and 30% in situ silica was far better than that of the unfilled EPDM system.³⁴ The values of tensile strength, tensile modulus, and storage modulus (at 50 °C) increased significantly with increasing filler content in EPDM/silica composites.³⁴ In an advanced study, Raman et al.⁴⁶ reported the development of high performance solution SBR (SSBR) composites by controlling the amount and size of the sol-gel–synthesized in situ silica.

On the other hand, very few studies have reported the effect of sol-gel–derived in situ silica on the mechanical properties of dissimilar rubber blend systems.^57,59,60 Kapgate et al.⁵⁷ examined the reinforcing effect of in situ silica in NR/NBR blend system in terms of mechanical and thermal properties. Among the different NR/NBR blend composites, the value of tensile strength was found to be optimum in the NR/NBR blend at a 40/60 blend ratio.⁵⁷ In addition, several authors reported some interesting findings for sol-gel–derived in situ silica–filled NR/CR blends.^59,60 Among the different NR/CR blend systems, the best mechanical properties were observed in the NR/CR blend at a 40/60 blend ratio.⁵⁹ Of note, sol-gel–synthesized in situ silica can enhance the compatibility of immiscible NR/CR blend. Thus, sol-gel–synthesized in situ silica can act as both a reinforcing filler and a compatibilizer in immiscible NR/CR blend systems.⁵⁹ Externally added silica was not able to enhance the compatibility of rubber blend composed of polar and nonpolar rubber constituents.⁵⁹

b. Sol-gel–synthesized titania nanoparticles as filler

In an advanced study, Das et al.⁶⁵ reported improved performance of NBR composites in presence of sol-gel–derived in situ titania as filler. At a 35 phr loading level, TESPT-treated titania was very effective in increasing mechanical properties such as hardness, tensile strength, and modulus of NBR composites. In this study, TESPT was successfully used as a surface modifier to improve the rubber–filler interaction in titania-filled NBR composites. The role of TESPT in sol-gel–derived in situ titania–filled NBR composites is shown in Figure 16. In addition, Messori et al.⁶⁶ discussed the reinforcement of EPDM composites by in situ–generated titania by using non-hydrolytic sol-gel method. It was found that in situ–generated titania can act as rigid nano filler to improve stiffness and stress at break of EPDM matrix.

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In a different way, Roy et al.^67,68 conducted two studies on the properties of rubber composites filled with sol-gel–synthesized nano titania. During preparation of rubber composites, sol-gel–synthesized titania was externally added into the rubber matrix.^67,68 Because of its hydrophilic nature, sol-gel–synthesized titania was not able to distribute homogeneously within the hydrophobic rubber matrix.^67,68 To increase the hydrophobicity, the surface of titania was modified by the use of different surfactants such as polyethylene glycol (PEG), polypropylene glycol, and cetyltrimethylammonium bromide (CTAB).^67,68 The surfactant-modified nano titania was found to be much more effective to increase mechanical and thermal properties of rubber composites than unmodified nano titania.^67,68 Roy et al.⁶⁷ confirmed that PEG-treated nano titania is more effective reinforcing filler than unmodified nano titania for NR composites. Continuing their research, Roy et al.⁶⁸ suggested the use of CTAB-treated titania as efficient reinforcing filler for SBR composites. The mechanical and thermal properties of sol-gel–synthesized titania filled rubber composites are described in Table III.

Table III Properties of Rubber Composites in Presence of Sol-Gel–Synthesized Nano Titania as Filler⁶⁷,⁶⁸

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c. Sol-gel–synthesized alumina nanoparticles as filler

Recently, Roy et al.⁷⁰ suggested a fresh approach for the development of high performance MNR composites filled with sol-gel–synthesized alumina nanoparticles. MNR is a type of modified NR that is prepared by graft copolymerization of maleic anhydride with NR.^88–90 It was found that 2 phr is the optimum amount of sol-gel–synthesized alumina to improve cure rate and mechanical properties of MNR composites. Crosslinking degree measurement confirmed that the interfacial interaction between MNR and alumina is more suitable than that between NR and alumina.⁷⁰ Thus, the mechanical and thermal properties of MNR/alumina nanocomposite were greater than those of NR/ alumina nanocomposite. A probable mechanism was also reported by Roy et al.⁷⁰ to describe outstanding interfacial interaction between MNR and sol-gel–derived alumina. Actually, the hydrogen bonding interaction between succinic anhydride groups of MNR and surface hydroxyl group of alumina is the key parameter that determines the final properties of MNR composites.⁷⁰ The reinforcing effect of sol-gel–synthesized alumina on the mechanical properties MNR composites is summarized in Figure 17.

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d. Sol-gel–synthesized zirconia nanoparticles as filler

Until now, only a single research paper had been published based on the application of sol-gel–synthesized zirconia as non-black filler in rubber composites.⁹¹ The mechanical properties of unmodified and surface-modified zirconia-filled NBR composites are shown in Figure 18. It was found that zirconia nanoparticles can provide moderate reinforcement in the mechanical and dynamic mechanical properties of NBR composites. In this study, rubber–filler interfacial interaction greatly depends on the hydrogen bonding interaction between polar nitrile (–CN) groups of NBR and the surface hydroxyl groups of zirconia. The mechanical properties of NBR/ZrO₂ composites can be further improved by in presence of organosilane (TESPT)-treated ZrO₂. This result is due to the ability of TESPT to make a chemical bridging between NBR and ZrO₂. The excellent reinforcing effects of TESPT-treated ZrO₂ in NBR composites are clarified in Figure 18. More interestingly, TESPT-treated ZrO₂ was found to be very useful in increasing the thermal stability of NBR composites. NBR composites with surface-treated ZrO₂ showed noticeably higher onset decomposition temperature than unfilled NBR composites. The refractoriness of ZrO₂ is an important factor regarding the thermal stability of NBR/ZrO₂ composites.

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e. Sol-gel–synthesized hybrid materials as filler

Some researchers tried to use sol-gel–based MCC/SiO₂ hybrid materials as an effective filler in SBR compounds.^73,74 The mechanical properties of MCC/SiO₂-filled SBR composites were comparable with those of silica only–filled SBR composites. By contrast, rolling resistance is the most important term for the development of environmentally friendly green tire technology.^92,93 In fact, rolling resistance is closely connected to the fuel consumption efficiency of tire tread composites.^92,94 Simply, lower value of rolling resistance is the basic criteria for the fuel benefit of the consumer.⁹⁴ It was found that MCC/SiO₂-filled SBR composite showed lower rolling resistance than that of only silica-filled SBR composite.⁷³ This is the most interesting result regarding the practical use of MCC/SiO₂ hybrid materials as an alternative filler in the rubber industry. However, the crack growth resistance of MCC/SiO₂-filled SBR composite has not been clarified. But in situ silica–filled SBR composite showed good crack growth resistance after silane treatment.⁹⁵ Thus, in situ silica is still a more suitable filler than MCC/SiO₂ for SBR composites. According to Liang et al.,⁷⁵ the effect of MCC/ZnO hybrid materials was similar to that of MCC/SiO₂ hybrid filler system in SBR composites.

Roy et al.^76,77 reported an exceptional way to use silica-coated CaCO₃ as a filler, specifically for polar rubbers such as NBR and CR. The sol-gel–synthesized silica coated at the surface of CaCO₃ can able to form a linkage between nano CaCO₃ and polar rubber matrix. Thus, the interfacial interaction between silica-coated CaCO₃ and polar rubber matrix was far better than that between unmodified CaCO₃ and polar rubber matrix. As a result, sol-gel–modified CaCO₃ was more effective filler to improve the mechanical and thermal properties of polar rubber composites than unmodified nano CaCO₃. However, above a certain loading level, both unmodified and sol-gel–modified CaCO₃ had the tendency of agglomeration within the rubber matrix. The mechanical and thermal properties of NBR and CR composites in presence of unmodified and sol-gel–modified CaCO₃ are shown in Table IV.

Table IV Properties of Rubber Composites in Presence of Unmodified and Sol-Gel–Modified Nano CaCO₃ as Filler⁷⁶,⁷⁷

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VI. CHALLENGES AND FUTURE PROSPECTS

The challenges with engineering rubber compounds for industrial adaptation are complex and diverse in nature. The viscoelastic nature and unique dynamic properties of rubber products offer ample scope to explore and understand the fundamentals of structure–property mechanisms. From miniature to bulk, design and development of rubber products demand high-yield and low-cost methods for commercial sensitization. Accountability in regard to fly loss of filler powder while compounding and adversities caused in handling fluffy filler in industrial scales compelled scientists to brainstorm on exploiting more efficient and easy techniques for filler incorporation in rubber matrix. In this aspect, sol-gel is an acceptable process in the areas of handling, processing, and commercial viability. However, there are still challenges for the direct application of sol-gel science in the rubber industry. Most researchers reported the development of sol-gel–synthesized nanoparticles-based rubber compounds in laboratory scale; however, little is known about adopting industrial-scale application of sol-gel–synthesized nanoparticles in rubber compounds. In this review, we presented that sol-gel–synthesized nanoparticles can enhance the performance of rubber compounds up to a certain loading level. First, for industrial-scale production, it is necessary to optimize the optimum loading level of particular nanofiller for a specific rubber matrix. Second, in most cases, the interfacial interaction between sol-gel–synthesized polar filler and nonpolar rubber matrix is very weak. Therefore, most of the researchers used Si-69 or TESPT as a coupling agent to improve the dispersion of sol-gel–synthesized nanoparticles within the rubber matrix. More fruitful and improvised research is needed to discover low-cost and effective surface modifiers compared with Si-69 for large-scale production of sol-gel–derived nanoparticles-based rubber compounds. Upscaling sol-gel technology in manufacturing of in situ–developed filler-reinforced rubber compounds is a challenge that needs to be addressed with utmost importance by industry.

VII. CONCLUSIONS

The main purpose of this review was to discover promising methods for the use of sol-gel science on the advancement of rubber technology. Although the industrial application sol-gel–synthesized nanoparticles in rubber compounds is still in the preliminary stages, recent research suggests that development of sol-gel–synthesized nanoparticles-based rubber composites with improved performance can be easily achieved. Sol-gel–synthesized nano ZnO can simply replace conventional ZnO as cure activator with the significant reduction of ZnO level in NR compounds. In addition, sol-gel–synthesized nano MgO can be used to prepare ZnO-free environmentally friendly NR composites in the presence of a ZBPDC/MBTS binary accelerator system. By contrast, sol-gel–synthesized in situ silica has proved to be ideal filler for the replacement of carbon black in green rubber technology. In addition, in situ silica can compatibilize dissimilar rubber blends in rubber compounds composed of polar and nonpolar rubber constituent. Some other sol-gel–derived nanoparticles such as nano titania and nano alumina have the potential to improve the mechanical and thermal properties of different rubber composites. By considering the above-mentioned promising results, sol-gel–synthesized nanoparticles-based rubber composites can be selected as a new type of environmentally friendly and low-cost composite material for future industrial applications.