Nanostructured SiO2 material: synthesis advances and applications in rubber reinforcement

Silica is a commercially significant material due to its extensive use in widespread applications and products. Synthetic amorphous silica (SAS) is a form of SiO2 that is intentionally manufactured and has been produced and marketed for decades without significant changes in its physico-chemical properties. The industrial production of nanostructured SiO2 is nowadays challenged by the expensive raw material use and high energy consumption. The search for non-petroleum-based fillers such as nanostructured SiO2, which are environmentally friendly, cheap, abundant, renewable, and efficient, has been initiated nowadays. Therefore, a large number of research activities have been carried out so far for the preparation of SAS from potential alternate precursors, i.e., synthetic chemicals, biogenic, and mineral ore resources. Reinforcement of rubbers with nanostructured SiO2 fillers is a process of great practical and technological importance for improving their mechanical, dynamic, and thermal properties. The efficiencies of SiO2 reinforcement correlate with different factors such as filler structure, surface area, rubber–filler interactions, and filler–filler interactions with their effects. This review paper discusses the recent synthesis advances of nanostructured SiO2 from synthetic chemicals, biogenic and mineral ore resources, their physical characteristics, and applications in rubber reinforcement, overcoming challenges. Finally, summary and future work recommendations have been mentioned well for future researchers.


Introduction
With the rapid emergence of the eld of nanotechnology, regulations specic to nanomaterials are under development. One of the key issues hindering regulation is a lack of agreement on the denition of what constitutes a nanomaterial. Currently, the most comprehensive and internationally recognized denition of nanomaterials is presented by the International Standards Organization (ISO). The ISO denition distinguishes between two subgroups, nano-objects and nanostructured materials, and denes them as: 1 nano-objects are materials that exist in a dened singular form that have at least one dimension in the nano-scale (<100 nm), which includes nano-particles (3D in nano-scale), nanobers (2D in nanoscale), and nano-plates (1D in nano scale). Nanostructured materials are materials that have structural features on the nano-scale but whose particle size is typically greater than 100 nm. Examples of these are materials that primarily exist in aggregated and/or agglomerated forms.
Silica is a commercially signicant material due to its extensive use in widespread applications and products. The base unit of the structure of the macromolecular network nSiO 2 is the [SiO 4 ] 4À tetrahedron. Synthetic amorphous silica (SAS) is a form of SiO 2 that is intentionally manufactured and has been produced and marketed for decades without signicant changes in its physico-chemical properties. SAS is in the form of white dry powders or dispersions of these powders are used in a multitude of industrial applications, i.e., an additive in rubber. It is also approved to be used in consumer products, i.e., food, cosmetics, and pharmaceuticals. To dene, SAS in powder form is a nanostructured material according to the technical specication of ISO TS 80004-1. The aggregate is the smallest indivisible unit upon dispersion. There are three types of synthetic amorphous SiO 2 produced at the industrial scale: fumed SiO 2 , precipitated SiO 2 , and SiO 2 gel. Of these three types of amorphous SiO 2 , precipitated SiO 2 , whose production started in the 1940s, has the greatest commercial importance. Precipitated SiO 2 is a nely divided white powder sparingly soluble in water and composed of aggregates up to approximately 1 mm in diameter.
The current commercial precipitated SiO 2 is produced by a wet SiO 2 production route or sol-gel process, in which an aqueous alkali metal silicate solution is neutralized with acid (e.g., H 2 SO 4 ), releasing SiO 2 and nH 2 O in a reaction tank to produce a slurry of SiO 2 . The most commonly used aqueous alkali silicate is water glass (Na 2 O$nSiO 2 ; n ¼ 2-4), which is produced by melting quartz sand with soda at about 1300 C. 2 Reaction conditions are manipulated according to the particle size required. Hydrogen bonding among particles will form clusters or aggregates, and these aggregates may loosely bond as agglomerates. Fumed silica (SiO 2 ), also known as pyrogenic silica (SiO 2 ), is mainly produced by reacting any SiO 2 source with any carbon source in an electric arc furnace at approximately 1900 C undergoing carbothermal reduction to metallurgical grade silicon, Si (met), which is then treated with HCl to produce SiCl 4 . The SiCl 4 , in turn, is combusted in a hydrogenoxygen ame to produce fumed SiO 2 plus byproduct HCl. 3 Both synthetic amorphous silica commercial processes require expensive precursor use, corrosive, toxic and polluting side product generation, the need for tailoring properties of products, high-temperature and energy-intensive steps make production processes expensive in which further synthesis advancement has been the subject of much of the ongoing researches. Many researchers have devoted studies to replace the expensive source of synthetic silica with one that is cheaper and renewable, i.e., biogenic and mineral ore resources. In addition to this, researchers extensively worked on the promotion of widespread preparation methods with an emphasis on sol-gel synthesis parameter optimization to tailor the nanostructured SiO 2 product physicochemical properties for specic applications, particularly for rubber reinforcement.
Elastomers or rubbers, whether natural or synthetic, are not usually used in their pure form due to insufficient practical physico-mechanical properties. Fillers are extensively used in the rubber industry to improve service efficiency and ease of processing, and their addition results in a fundamental change in the properties of rubber. According to the kinetic theory of elasticity, the rubber modulus (with no llers) increases with the rise of temperature; the addition of llers signicantly changes the temperature coefficient of modulus and may even alter the sign of the coefficient, resulting in a decrease of the modulus with increasing temperature.
Fillers are classied based on chemical composition and their inuence on rubber properties. Further, in rubber compounding, they can be divided primarily into three categories in accordance with their reinforcing effect: inactive, semi-active, and very active. The term active signies the degree of reinforcement, i.e., the inuence of the ller on the viscosity of the compound and the resulting mechanical properties. The main characteristics that determine the reinforcing effect of llers are their structure and surface properties. Active llers have a large relative surface area and high structure, providing strong physical and chemical interaction between the ller and polymer. However, a highly active ller surface leads to strong interparticle forces, which negatively inuence the processing behavior as a result of the agglomeration of ller particles during mixing and storage. 4 Common to composites' preparation and modication mechanisms, several factors inuence the property proles, which lie between those of pure rubber and ller or processability. These include volume fraction, particle dimension and geometry, dispersion quality, the interaction between rubber and llers or among llers, and, if applicable, the degree of orientation of llers for anisotropic properties. These factors, in turn, affect one another, and synergy is achieved if an optimum balance is reached. 5 The most common llers used in the rubber industry are the carbon family of materials (carbon black, carbon ber (CF), carbon nanotubes (CNTs), graphite and graphene), inorganic particles (nanoclays, polyhedral oligomeric silsesquioxane (POSS), SiO 2 , calcium carbonate (CaCO 3 ), talc, zinc oxide (ZnO), titanium oxide (TiO 2 ), alumina (Al 2 O 3 ), halloysite), and bio-llers (cellulose, husk, wood, coir). [5][6][7] However, the large volume applications of rubber, certainly those involving high elasticity and mechanical properties, including stiffness, strength, toughness, abrasion resistance, anti-scratching property, or friction behavior, etc., are reinforced by carbon black and precipitated SiO 2 as the dominant llers. Even if both carbon black and nanostructured silica are the dominant llers for rubbers, the search for non-petroleum based ller and a promising efficiency for reducing the rolling resistance of rubber nanocomposites in tire tread makes nanostructured silica an important and economical ller in the rubber industry. The potential of lowering the rolling resistance makes silica a promising reinforcement in the fabrication of green tire tread. 8 Modern fuel-saving tire treads are commonly reinforced by silica due to the fact that this leads to lower rolling resistance and higher wet grip compared to carbon black-lled alternatives, which may have the potential to improve tire performance further.
Very limited review works have been reported yet on the synthesis advances of nanostructured SiO 2 material from cheap, renewable potential precursors and characteristic features of SiO 2 ller for rubber reinforcement. This paper highlights the synthesis advancement of nanostructured silica from potential precursors with emphasis on so template based synthetic chemical precursors, biogenic, and mineral ore resources. The review also describes the reinforcement of rubber with nanostructured SiO 2 llers due to its mechanical, dynamic, and thermal properties and correlates with different factors like ller structure, surface area, ller-ller, and rubber-SiO 2 ller interactions effects on SiO 2 lled rubber nanocomposites with the overcoming challenges, which are reported in different works published so far.

Research advances for the synthesis of nanostructured SiO 2
The widespread preparation methods, e.g., hydrothermal, solgel, precipitation, etc., generally tend to yield spherical or slightly irregular nanostructures, which may or not be manipulated to design different physical shapes (e.g., elongated structures 9 and rods of different aspect ratios, etc.). SiO 2 nano-bers have been produced by electro spinning 10 or by chemical vapor deposition (CVD). 11 The properties of SAS are strongly dependent on the parameters of synthesis, such as the reactant concentration, reaction temperature, time of precipitation, pH range, type and addition of surfactants, solvents, and modes of washing and drying. These parameters inuence the development of the size and uniformity of the SiO 2 particles, their morphology, aggregation, pore dimensions and volume, and specic surface area.
These, in turn, inuence the properties of the designed structures and the consideration of the properties along with the high surface area enables their use for a wide range of applications. The preparation of amorphous SiO 2 nanostructures has been studied by various research groups using different categories of precursors: synthetic chemicals, biogenic, and naturally occurring mineral resources. The research efforts in these three categories are briey discussed in the following sections.

Synthesis of SAS using synthetic chemicals
Dispersed, amorphous, and uniform SiO 2 nanostructures are of interest due to their simple preparation and potential applications in various industries as they may be used as effective materials for improving the strength, exibility, durability, workability, etc. The most common precursors used at the laboratory scale are tetraethyl or tetra methyl orthosilicate (TEOS or TMOS) or inorganic sodium silicate. Different synthetic strategies have been applied to prepare these structures: the modied Stober method, 12-16 combustion techniques, 17 chemical vapor deposition, 18 aerosol spray, 19 and emulsion 20,21 methods. Various types of morphologies such as hexagonal, cubic, lamellar, and wormhole like mesostructures have been synthesized using these methods and TEOS or TMOS as precursors. It is well-known that spherical dispersed silica nanoparticles can be prepared using the sol gel method with TEOS or TMOS or sodium silicate precursors.
The main advantage of the sol-gel method is the control of the particle size at the nano level and morphology development of SiO 2 nanostructures by changing the concentration of reagents, type of catalyst, temperature, reaction conditions, etc. Many efforts have been made to control the particle size and morphology of SiO 2 nanoparticles through the use of different types of surfactants as templates. In the synthesis of mesoporous materials (such as SiO 2 ), organic surfactant molecules play an important role in generating porosity within the building blocks and thus act as the endotemplate or structure directing agent (SDA). In this regard, the so templating method has been the most successful pathway for the synthesis of ordered and disordered mesoporous matrices.
The SDAs can be of different types: (i) surfactants may be employed as SDA, wherein the essential feature is the coexistence of chemically bonded non-polar hydrocarbon 'tail' and a polar 'head' group in a molecule. These molecules have high molecular weight and form aggregates in the solvent to form self-assembled micelles, [22][23][24] (ii) non-surfactant single molecules, which may or not form self-assembly bearing hydrophobic-hydrophilic groups in a single molecule, but act as templates in the design of mesopores in a material; 25 and (iii) dendrimers or polymers (macromolecular single molecule having high molecular weight). 26,27 All three types of templates are so templates. Fig. 1 shows the classication of so templates, 28 some of which are used to study structure development in SiO 2 nanostructures. Different SDAs, depending on their structure and nature, have different critical micelle (CMC) values in water or solvent. 29,30 Beyond the CMC values, the self-assembly of micelles occurs to form a 3D spherical or 2D rod like array with further increasing concentration, and this self-assembly helps in pore generation (Fig. 2). These self-assembled micelles are formed by the association of individual amphiphilic templating molecules bonded through weak forces like van der Waals, hydrogen bonding, etc., but without covalent linkage between the amphiphiles. 31 Thus, SDA molecules are placeholders: what becomes the void space to produce a nanoporous material. They not only allow controlling the variation of pore size but also the shape of the pores, i.e., the total architecture of the template molecule whose size and shape are imprinted in the porous solid, as shown in Fig. 3. 32 The concentration of SDA also inuences the shape and size of the nanostructures, as shown by the example of using CTAB as SDA in Fig. 4. 33,34 The hydrothermal method, actually a sol-gel process, consists of a number of steps. Initially, the formation of surfactant self-assembly occurs via the true liquid crystal template (TLCT) mechanism to form a homogeneous surfactant solution in an aqueous solvent media. Then, the silicate precursor, TEOS or TMOS or sodium silicate, is added to the surfactant solution when it gets hydrolyzed under the acidic or basic conditions to form a silicate oligomer sol. These oligomers then condense with surfactant micelles via cooperative assembly and aggregation to form an inorganic-organic hybrid, which eventually precipitates in the form of a gel. The gel is treated hydrothermally for further condensation, solidication, and reorganization of the material to an ordered arrangement. 35,36 Aer a certain time of ageing, the resultant product is cooled, ltered, washed, and dried. The process is shown in Fig. 5. 34,37 Ordered mesostructured SiO 2 material is obtained from this as-synthesized solid aer the removal of surfactant through calcination or solvent extraction.
Some of the research efforts using different precursors and without and with SDAs are summarized in Table 1.

Extraction and preparation of amorphous SiO 2 from biogenic sources
Much of the world's agricultural waste contains SiO 2 , and the search for a practical way to extract it stretches back almost 80 years. The extraction of amorphous SiO 2 from biogenic sources, especially the third category of biomass, i.e., true bio-waste, which means they are generated as a byproduct of another useful resource, is a hot area of research. Research focus has and is on approaches to nd the source of SiO 2 that is safe, cheap, and more environmentally friendly as an alternative to the commercial activities of production. Research efforts have involved the use of agricultural waste such as rice husk, rice straw, coffee, cane husk, etc., as precursors for the preparation of pure SiO 2 in either the amorphous or crystalline form. Table 2 shows the ash and SiO 2 content of some plants. 52,53 The major biogenic source of research is rice husk and its ash. The world paddy production in 2017 was 769.65 million tons (503.6 million tons, milled basis) and is produced in many regions of the world. 54 On average, 28% of the rice paddy is husk, giving an annual total global production in excess of 212.5 million tons. The rice husk (rice hull) is formed from hard materials, including SiO 2 and lignin, which protect the seed. Each kilogram of milled white rice results in roughly 0.28 kg of rice husk as a by-product of rice production during milling.  Table 1 Physical characteristics of nanostructured silica synthesized from synthetic chemical precursors Over the years, different preparation procedures have been investigated: the dry bio-digestion process involving the anaerobic digestion of rice straw to produce both bioenergy and SiO 2 enriched solids followed by calcination and washing to obtain high purity SiO 2 , 52,53 precipitation method (before and aer calcination of the precursor), 55 sol-gel process using rice husk ash as a low cost precursor and chitosan as template for the preparation of bimodal macroporous SiO 2 (BPS), 56 pre and post strong acid leaching and alkali treatment of rice husk followed by calcination to prepare relatively pure activated SiO 2 , were investigated, 57,58 SiO 2 gel from rice husk derived sodium silicate and neutralization by microwave heating, 59 combustion of rice husk in a uidized bed to obtain SiO 2 white higher in quality than that of GB-precipitated SiO 2 and approaches that of pyrogenic SiO 2 except for specic surface area and iron content 60 and high-purity SiO 2 from rice husks using a carboxylic acid and citric acid leaching process followed by calcination, 59,61 etc. Some representative research efforts utilizing different biogenic precursors for the extraction of amorphous SiO 2 are summarized in Table 3.

Extraction and preparation of SAS from mineral ores
The extraction of SAS from mineral ores is a thematic research area with efforts aimed at developing sustainable economic processes compared to the current commercial processes, which are energy intensive and considered less environmentally friendly. The motive of the research efforts is to develop extraction and transformation processes from suitable lower cost silicious mineral ore materials, which can signicantly reduce the costs of production. Silicates constitute approximately 95 percent of the Earth's crust and upper mantle, occurring as the major components of most igneous rocks and also in substantial quantities in sedimentary and metamorphic varieties. Of the nearly 600 known silicate minerals, only a few dozen-a group that includes the feldspars, amphiboles, pyroxenes, micas, olivine's, feldspathoids, and zeolites-are signicant in the rock formation. In these minerals, SiO 2 exists in both crystalline and amorphous forms. The crystalline form with limited applications due to low activity is more abundant in the Earth's crust compared to the amorphous form. Siliceous materials are either SiO 2 -rich or silico-aluminate rich materials  -Calcination derived primarily from geological sources. To ensure adequate levels of activity, these materials must be mainly amorphous. Given their geological origin, many amorphous SiO 2 sources contain small proportions of crystalline SiO 2 minerals (quartz and Cristobalite), which must be considered in the efforts to extract amorphous SiO 2 from them. The variety of mineral rock sources of amorphous SiO 2 materials results in a wide range of material properties and reactivities. The properties reect their diverse sourcing and generally involve several independent parameters that govern the extraction process. These parameters must be optimized to improve the efficiency of SiO 2 production as well as its purity for any meaningful technological and large-scale applications.
The global efforts to extract amorphous SiO 2 involve the use of indigenous mineral rock resources as precursors found in the respective regions. This aspect assumes importance as any selection of siliceous raw material for commercial production will be directly inuenced by a plethora of techno-commercial factors such as its abundance, consolidated availability, cost, transport, and the types and relative concentrations of impurities. In general, the wet process with due modication of individual process steps (ore dependent) is used for the extraction of SiO 2 . [69][70][71][72][73] The process steps make use of different lixiviants (e.g., acids/bases and their salts) to extract SiO 2 in the form of either H 4 O 4 Si or solutions of Na 2 SiO 3 or K 2 SiO 3 , which are then neutralized to produce amorphous SiO 2 . The process does not require excessively high-temperature reaction input, and hence, the energy demand is less. An elevated temperature conditioning step, post extraction of amorphous SiO 2 , is optional depending on the target application. Some representative research efforts utilizing different mineral ores as precursors for the extraction of amorphous SiO 2 are summarized in Table 4.

Classes of SiO 2 and rubber reinforcement
In the polymer industry, two classes of silica are generally used: fumed SiO 2 , whose main use is to reinforce silicone rubber since its cost limits more general application to rubbers and precipitated SiO 2 . A third class being viewed as a possible internal substitution for precipitated SiO 2 by manufacturers of SiO 2 is colloidal SiO 2 .
The primary particles of reinforcing llers, such as carbon black and SiO 2 are spherical and tend to aggregate or agglomerate due to self-association of the active functionalities present on their surface during production. 81 Similar to carbon black, precipitated SiO 2 exists as aggregates, but unlike fumed SiO 2 , the aggregates tend to be more highly clustered, with some having the appearance of fragments of silica gel. A primary SiO 2 particle ranges in cross-sectional dimensions from 5-100 nm, but aggregates formed by chemical and physical interactions of these particles range from 100-500 nm. The aggregates are quantied according to the specic surface area of primary particles, their geometrical arrangement, and the number of primary particles. The combination of these three gives rise to the structure (a general measure of the aggregate) of SiO 2 . Aggregates condense into agglomerates in the range of 1-40 mm.
During compounding, they more or less disintegrate to the size of aggregates or even primary particles (rare) (Fig. 6). The efficiency of the distribution, dispersion, and disintegration of agglomerates into aggregates depends on the degree of shear afforded by the mixing equipment: internal mixers are the most efficient (and are the work horse of the rubber industry) compared to two roll mills. This higher structure gives a greater reinforcing effect than carbon black, but the higher specic component of the surface energy of the SiO 2 ller results in difficulty in dispersion in rubbers and even re-agglomeration post mixing. Nonetheless, excellent properties have been obtained in rubber compounds especially tires with precipitated SiO 2 , by improving the bonding to rubber, either by activating the SiO 2 or by the addition of coupling agents. 82,83 Grades of precipitated SiO 2 are generally classied as semireinforcing or reinforcing, similar to carbon black. Further, due to the considerable diversity in the types of SiO 2 , because of the different production processes and in-process variations, these llers are of various types, as shown in Table 5. 85 Colloidal SiO 2 is an alternative to carbon black, although typically, the polarity difference between SiO 2 and common rubbers gives decient reinforcing properties unless coupling agents are employed.
3.1.1. Aggregate size of SiO 2 . In determining the physical properties of rubber compounds, e.g., rubber retread, the microstructure of the SiO 2 ller plays an important role. The anisometry of these llers may be described in terms of the structure, which includes aggregate shape, density, and size. The formation of aggregates of the primary SiO 2 particles with typical dimensions of 100-200 nm is considered the real reinforcing species in rubber compounds.
As for the structure, conventional silica has a typical dibutylphthalate (DBP) adsorption value of 175/100 g while for highly dispersible (HD) SiO 2 , it is 200/100 g or even more. These higher values show that the structure of HD SiO 2 is less fragile than conventional silica. Improvement in the dispersion behavior of HD SiO 2 is explained by their high aggregate porosity, surviving for longer times than conventional SiO 2 under high shear during compounding. The result is that the polymer has more space and time to penetrate into the voids present in HD SiO 2 compared to conventional SiO 2 with more compact structures. The aggregates of HD SiO 2 have a more branched structure; with three to four major branches on average. 86 These branched structures also improve the dispersion characteristics of the HD SiO 2 during the mixing process   6 Reinforcing filler in rubber after mixing in an internal mixer. 84 Fig. 7 Classifications of filler reinforcing effect according to the particle size. 33 and at the same time, reect a bimodal distribution of the aggregates. Compared to conventional SiO 2 , the amount of small aggregates is relatively high in HD SiO 2 . Bulky particulate materials like calcium carbonate can be employed just to reduce the cost of the nal material without any improvement of properties. In this case, they can be simply called extenders or non-reinforcing llers. On the other hand, there are llers like precipitated silica, as observed in Fig. 7, with a reduced nano size scale, which can reinforce the rubber, known as active llers. 33 3.1.2. Specic surface area of SiO 2 . Primary particle sizes of HD precipitated amorphous SiO 2 are in the range of 10-50 nm, inclusive of micropores, which are less than 0.2 nm in size. No polymer molecules are able to penetrate into the micropores but only low molecular weight compounds such as vulcanizing and coupling agents may. This results in a loss of active components in a polymer formulation. In general, the BET surface varies between 50 and 300 m 2 g À1 , the more common between 50 and 200 m 2 g À1 . Exclusion of the micropores in measurement, the external surface area of the ultimate primary particles typically ranges between 100 and 200 m 2 g À1 . The micropore area excluded values strongly correlate with the physical properties of lled rubbers. 87 3.1.3. SiO 2 surface chemistry and characteristics. Silica particles generated from the polymerization of silicic acid form complex amorphous polycyclic ring structures. Because of internal defects in these ring structures, silanol groups (^Si-OH) and siloxane groups are generated on the surface of the particles. The surface is mainly characterized by the (i) number of silanol groups, (ii) degree of hydration, (iii) amount of adsorbed water, and (iv) its surface acidity. The silanols are classied into three categories depending on the precipitation conditions shown in Fig. 8. 86,88 These groups display a strong affinity for water molecules, especially the geminal type of silanol groups. In HD SiO 2 , the germinal content is less than 20%. Silanol groups on the silica surface also improve the bond ability of new rubber compounds with older rubber, e.g., retread compound with old rubber. The polar component of SiO 2 is relatively high due to the presence of a large number of polar groups on the ller surface and the difference in the solubility parameters of the polymers and the ller is responsible for the degree of wetting of llers by polymers. 89,90 The Hildebrand solubility parameters of some polymers and silica are shown in Table 6. 84 From Table 7, it may be seen that in comparison to the polymers listed and carbon black, the high solubility parameter of SiO 2 leads to difficulty in blending SiO 2 llers with polymers. Based on the studies of the interactions of SiO 2 surfaces with low molecular weight analogs of elastomers, the level of interaction has been classied as: 89 NBR > SBR > NR $ BR > HV-BR > EPR > IIR

Bound rubber model of SiO 2 -lled rubber
During the processing of SiO 2 -lled rubbers, various chemical reactions occur, and generally, there exist two components: (i) Fig. 8 Types of silanol groups on the SiO 2 surface. 86,88  occluded rubber in SiO 2 aggregates and (ii) cross linked polymer due to polymer chain scission and recoupling reactions. These two phenomena are responsible for the bound rubber of SiO 2 -lled rubber 91 and a model of the SiO 2 /silane reinforcement based on the hydrodynamic-occlusion-interaction theory 86,92,93 is shown in Fig. 9. The large polarity difference between the SiO 2 ller and the rubber matrix leads to the ease of formation of a ller-ller network, with part of the matrix occluded in the ller network. This occluded rubber is both physically and chemically immobilized within the ller network. Under high deformation, the ller network partially breaks down, and with the increase in deformation of the rubber, the occluded rubber within the ller network reduces, followed by a deformation of the matrix. Due to the chemical bonding via a silane coupling agent, the occluded rubber and rubber on the SiO 2 surface remain immobilized and still contribute to the modulus even at high deformations. The chemically immobilized rubber is the inrubber structure.
The mechanisms of components of bound rubbers directly contacted to silica surface are in terms of multiple contact rubber chain, insert rubber chain, chemically bonded rubber chain, and single contact rubber chain, as shown in Fig. 10. 94

Effects of ller-ller and SiO 2 ller-rubber interactions
The ller-rubber interaction depends on the particle size and shape, surface characteristics of the ller, as well as the chemical nature of the polymer. It increases with the ller dispersion and with the extent of the organic/inorganic interface. Many studies have also demonstrated that the ller-rubber interaction induces the formation of a polymer layer around ller particles (the so-called bound rubber) with a consequent slowdown of the dynamics of the rubber chains interacting with the particle surface. The stronger the interaction, the more tied the polymer layer. 95 Filler-ller interaction is due to the strong tendency of silica particles embedded in rubber to interact with each other by surface silanol condensation. This favors a strong inter-particle aggregation, which contrasts the ller-rubber interaction and lowers the homogeneous distribution of the ller. This drawback can be ridden out by using suitable coupling agents (e.g., silanes), which form chemical bonds between the polymer chains and the oxide, improving the compatibility with the rubber matrix. 96 The control of both ller-ller and ller-rubber interactions plays a key role in determining the ller reinforcing action. In fact, the reinforcement due to ller-rubber interaction is almost independent of the strain amplitude. On the other hand, the ller-ller interaction enhances the modulus at low strain, but breakdowns at high strain. This causes a drop in the modulus (Payne effect), which is associated with an energy dissipative process and Mullen's effect as well. 97 3.3.1. Payne effect. Payne effect and its associated weak overshoot are of importance for understanding and regulating the soening of rubber nanocomposites under large amplitude oscillations. Rubber nanocomposites widely used in the industry demonstrate the Payne effect, also named as Fletcher-Gent effect, under large amplitude oscillations, depicting soening and dissipation behavior is usually accompanied by a weak overshoot of loss modulus (G 00 ) depending on the dispersity of the ller and its weak forces like H-bonds and van der Waals interactions with the rubber matrix. The Payne effect and its associated weak overshoot are usually attributed to the breakdown of the ller network and the desorption of bound chains from the ller surface, and strain soening phenomenon is known as the Payne effect, and the components of reinforcement are readily separated, as shown in Fig. 11, while recent studies show that the ller amplied viscoelastic deformation of the matrix is crucial. [97][98][99] The vulcanized nanocomposites of diverse compositions demonstrate the Payne effect and weak overshoot behavior being dominated by the viscoelastic deformation of the rubber matrix with defective crosslinking network. The ller amplies the microscopic strain of the matrix, which is more signicant for the silicious than the carbonaceous llers. While both the silicious and carbonaceous llers tend to lower the intensities of the weak overshoot, and the serious agglomeration of silica may cause interparticle friction to intensify the overshoot at high ller loadings. On the other hand, the interaction between silica and rubber based on covalent bonding from silanization tends to intensify the strain amplication effect and llerrubber interfacial friction. Payne effect at small strain shear deformation and other strain independent contributions (strong ller-rubber interaction) on reinforcement degree of rubber by SiO 2 ller can be reduced by surface modication of either the ller or polymer due to covalent bonds existing at the polymer-particle interface (through coupling agents), and in the in-rubber structure (sulfur crosslinks). 98 3.3.2. Tensile tests and Mullins effect. From the macroscopic point, the interface stability (associated with llerrubber interactions) can also be evaluated by mechanical measurements that explore higher strain regimes than the Payne effect: tensile tests and Mullins effect. In tensile tests, the material is stretched at strain values 20-50 times higher than for the Payne effect, and the high-strain effects can be attributed to ller-rubber interactions that play a complex but important role in tuning the breakage mechanism. The Mullins effect, in turn, refers to a particular aspect of the mechanical response in lled rubbers in which the stress-strain curve depends on the maximum loading previously encountered. The Mullins effect can be evaluated by a series of tensile traction and retraction tests until the break, in which a lag time between cycles allows to restore reversible deformations, while pseudo-permanent deformations cannot be restored.
Besides the tensile test, it is possible to evaluate the energy losses localized at different strain intervals, allowing the material to restore between one cycle and the other, separating permanent (purely viscous) losses from reversible (viscoelastic) losses with the hysteresis measured in each cycle considered as an indicator of these losses. 100 The dissipative effect at different strain levels has been related to different physical phenomena, as shown in Fig. 12. 33,101 3.3.3. Failure properties of SiO 2 lled rubber. Particulate llers improve the strength and fatigue resistance of rubber. This is illustrated in Fig. 13, showing substantial increases in tensile strength, even for a ller such as calcium carbonate that provides minimal actual reinforcement. The strength of rubber increases with ller content, attaining a broad maximum at high concentrations. The majority of applications of rubber, certainly those involving mechanical properties, require reinforcement with carbon black and precipitated silica, the dominant llers. Reinforcing llers increase the viscosity and modulus by a hydrodynamic effect, augmented by rubber occluded within the ller. The ultimate properties (tensile strength, fatigue life, abrasive wear, etc.) are an important aspect of reinforcement, although the magnitude of the improvements and the relevant mechanisms are not always well understood. 102 The mechanical properties of SSBR composites lled with SiO 2 /CB exhibited a ''synergistic effect.'' Among these, the composite with SiO 2 /CB (20/50) showed good ller dispersion with low heat generation, as shown in Table 7. 103

Overcoming difficulties in the use of SiO 2 llers
Mixing SiO 2 ller into rubber is a challenge. The dispersive part, a component of surface free energy, is low, i.e., weak interaction between the ller particles and rubber, resulting in a low reinforcing effect. However, the specic part, the second component of surface free energy, is high, i.e., a strong ller-ller interaction leading to a substantial increase in the viscosity of SiO 2 -rubber compounds. The interparticle forces between ller particles must be overcome during mixing so that a satisfactory dispersion of ller particles can take place and a link between the SiO 2 particles and rubber may be established. For a homogeneous distribution of ller aggregates within a matrix, intensive contact between ller particles and rubber is a necessary criterion for efficient mixing. The two most important characteristics that determine the contact between the surface of the ller and the rubber matrix are the surface area of the ller and its wettability. 104,105 Wettability is signicant when using SiO 2 llers as there exists a mismatch between the solubility parameters of the polar SiO 2 ller and nonpolar rubber. This mismatch affects both its compatibility with non-polar elastomers and the cure characteristics of the compounds. The high structure of SiO 2 ller (aggregates and agglomerates) is characterized by a high surface area and a high percentage of the void volume within the structure. Since a direct interaction between the ller and polymer is low, polymer chains are physically entrapped in the voids of the ller structure. To enhance the polymer-SiO 2 ller interaction, the lled structure has to be broken to an optimum level during mixing. 106 From an industrial perspective, it has been observed that reinforcing by carbon black was more efficient than SiO 2 until silane coupling agents were introduced to reduce the polarity differences between the polar ller and nonpolar rubbers. These agents are able to react with both the SiO 2 surface and the polymer forming a chemical linkage between the ller particles and rubber, thus overcoming the problems of both increased viscosity and dispersion. The result is the enhancement of the properties of the nal product. 107 They may be premixed or prereacted with the SiO 2 particles or may be added to the rubber mix during mixing with consideration that the coupling agents then combine in situ with SiO 2 . 108 Different types of coupling agents have been developed over the years to improve the compatibility and dispersion of SiO 2 Fig. 13 Tensile strength of SBR unfilled (line), with 0.13 volume fraction (45 phr) calcium carbonate (squares) and with 0.13 volume fraction (30 phr) HAF carbon black and precipitated SiO 2 , as a function of crosslink density. 102 llers in different polymer matrices. Some commonly used silane coupling agents include, e.g., bis(triethoxysilylpropyl) tetrasulde (TESPT): the polysulde part of which reacts with the polymer, and the ethoxysilyl groups on the silicon atom react with the hydroxyl groups present on the surface of SiO 2 . 109 The average sulfur rank of the polysulde is 3.86 and is unstable at high shear or high temperature. During rubber processing, the TESPT molecules split and release reactive sulfur moieties in the compounds, thus performing as a sulfur donor. 110,111 Another silane that releases sulfur during processing is bis (triethoxysilylpropyl) disulde (TESPD: Si266/Si75), which comprises mixtures of polysuldes and has an average sulfur rank close to 2. This silane is more stable at high shear or high temperatures compared to TESPT, and therefore, rubber compounds are less sensitive to scorch. But, due to its lower sulfur content, additional elemental sulfur needs to be incorporated into rubber to achieve comparable reinforcement to TESPT. 92,112 There are also other types of silane coupling agents commercially available and are used based on the type of the polymer matrix and target application of the SiO 2 -rubber compound. They are used to reduce the emission of volatile organic compounds such as ethanol during the mixing process or product lifetime, e.g., tires. [113][114][115][116][117][118] The reaction mechanism of silanization is exemplied by that of TESPT in Fig. 14(a) and (b). In the primary step, the reaction of the rst alkoxy group of the silane with silanol groups on the SiO 2 surface takes place via two possible mechanisms ( Fig. 14(a)): a direct reaction of the silanol groups on the silicon with the alkoxy group of TESPT and hydrolysis of the alkoxy group to form a reactive silanol with the release of ethanol.
These reactions occur slowly on the SiO 2 surface in the presence of water. 119 The rate constant of hydrolysis increases with increasing temperature in the presence of a catalytic agent such as an acidic or alkaline medium. Post hydrolysis, the activated silane is capable of reacting with silanol groups on SiO 2 . The rate constant of this reaction is relatively faster than hydrolysis, i.e., the hydrolysis reaction is the rate determining step for silanization. 114 Just aer the primary reaction, a secondary reaction (Fig. 14(b)), and an intermolecular condensation between the silanes on the SiO 2 surface caused by the unreacted ethoxy groups of the silanes occur. The rate of reaction is, however, slower than the primary reaction and may be accelerated by water accompanied by an increase in temperature. For an optimal level of reinforcement, it is preferred to have a low degree of intermolecular condensation. 91,120 The degree of silanization can be signicantly improved and accelerated by 1,3-diphenylguanidine (DPG) or by accelerators such as amines, enamines (-RC ¼ C-NR 2 ), and aldimines (R-CH ¼ N-R) in combination with DPG. 121,122

Summary and future recommendations
The search for nonpetroleum-based llers, which are environmentally friendly, cheap, abundant, and renewable, has been raised nowadays. Literature reports published so far evidenced the improving physicochemical properties of nanostructured SiO 2 and reduction of the cost of production by searching for potential precursors, and low energy consuming methods is an important research area. The main bottleneck problem for the application of sol-gel processes at a larger scale is the use of expensive alkoxide precursors in the liquid state. Therefore, in the near future, alternative precursors from local materials are an urgent call to researchers.
Other parameters that play a crucial role in the performance of rubber materials are size, surface chemistry, surface area, and aspect ratio of nanollers. Owing to their large availability, low cost, high surface area, green and possible sol-gel preparation from sodium silicate precursors, nowadays, nanosilica structures with different physicochemical characteristics and aspect ratios have attracted huge interest as reinforcing agents for rubber materials. Despite the fact that rare studies have been conducted so far on the effect of nanosilica structure on rubber nanocomposites, a larger scope exists in the development of novel rubber products using different nanostructures of silica synthesized from natural sources, i.e., biogenic and mineral ore resources.
Nanollers, particularly anisotropic silica, show great promising results, and their commercial potential is signicant. However, the dispersion problem of SiO 2 nanostructures, encountered in the use of all reinforcing llers, remains a major obstacle to the utilization of nanoparticles in the rubber industry. Recent works showed that the surface modication of silica with silane coupling agent made it competitively applied to rubber industries. Recently, researchers tried some investigations on the effect of anisotropic silica on rubber reinforcement, Payne and Mullen's effects reduction, and parameter optimized sol-gel synthesized spherical and rode shaped silica becomes a promoted rubber reinforcement ller but very rarely investigated yet. Therefore further studies in all aspects of nanostructured SiO 2 material for rubber reinforcement efficiency is a hot research area of study.

Conflicts of interest
The authors declared that they have no conict of interest.