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Review

Research Progress of Natural Rubber Wet Mixing Technology

by
Qinghan Zhao
,
Fangyan Niu
,
Junyu Liu
and
Haishan Yin
*
College of Electromechanical and Engineering, Qingdao University of Science and Technology, Qingdao 266100, China
*
Author to whom correspondence should be addressed.
Polymers 2024, 16(13), 1899; https://doi.org/10.3390/polym16131899
Submission received: 31 May 2024 / Revised: 25 June 2024 / Accepted: 28 June 2024 / Published: 2 July 2024
(This article belongs to the Special Issue Advances in Functional Rubber and Elastomer Composites II)
Browse Figures
Versions Notes

Abstract

:
The performance of natural rubber (NR), a naturally occurring and sustainable material, can be greatly enhanced by adding different fillers to the NR matrix. The homogeneous dispersion of fillers in the NR matrix is a key factor in their ability to reinforce. As a novel method, wet mixing technology may effectively provide good filler dispersion in the NR matrix while overcoming the drawbacks of conventional dry mixing. This study examines the literature on wet mixing fillers, such as graphene, carbon nanotubes, silica, carbon black, and others, to prepare natural rubber composites. It also focuses on the wet preparation techniques and key characteristics of these fillers. Furthermore, the mechanism of filler reinforcement is also examined. To give guidance for the future development of wet mixing technology, this study also highlights the shortcomings of the current system and the urgent need to address them.

Graphical Abstract

1. Introduction

Extracted from the Brazilian rubber tree, natural rubber (NR) is a sustainable and renewable resource. Because of its exceptional physical qualities, including its exceptional elasticity, resilience, high tensile strength, and raw strength, it is one of the elastomers that is utilized in tire production the most. Because of its low heat buildup, resistance to fracture formation, and low hysteresis, NR is favored for tire production [1]. In most cases, the qualities of pure rubber are insufficient for practical uses; therefore, reinforcing fillers must be added to improve the properties of rubber [2]. The failure behavior of rubber composites is greatly influenced by the reinforcing fillers’ particle size, form factor, surface activity, number of filler components, and dispersion condition in the rubber matrix [3].
Traditional mixing, sometimes referred to as dry mixing, is mostly accomplished by alternating or continuously mixing fillers with granular or solid rubber ingredients. Reinforcing fillers, such as carbon black, tend to clump when traditional dry mixing techniques are applied. This leads to inadequate filler dispersion in the rubber matrix, and the resulting rubber composite does not show the desired qualities.
The wet mixing technique is a novel approach to chemical solidification that involves combining filler solution and polymer latex (solution) following conventional dry mixing. Wet mixing can address issues with dust pollution, energy consumption, and other issues associated with traditional dry mixing since it uses a liquid phase to complete the mixing and dispersion of the filler and rubber. Wet mixing offers notable benefits, particularly for heavily packed rubber composites.
According to contemporary basic research on rubber composites, in order to manufacture rubber composites with outstanding overall performance, a thorough investigation of the issue of particle binding at the microscopic level is required. As illustrated in Figure 1, the three layers that make up natural latex’s rubber particles are a viscous sol-gel layer made of rubber hydrocarbons with a small degree of polymerization in the innermost layer, a gel layer made of rubber hydrocarbons with a large degree of polymerization in the middle layer, and a protective layer made of proteins and lipids in the outermost layer. Of these, the protective layer can help the rubber particles stay evenly distributed in the natural latex; however, because of its presence, it prevents the filler particles from coming into direct contact with the rubber particles in the natural latex, which in turn impacts the filler reinforcement’s effectiveness.
The filler particles must first have a high dispersibility to effectuate the reinforcement of natural latex by reinforcing filler in an emulsion condition and to obtain an outstanding reinforcing effect. To produce a uniform distribution of fillers in the natural latex film and improve the reinforcing effect, co-precipitation of the natural latex particles and reinforcing filler particles is required when coalescence occurs. Rubber hydrocarbon and reinforcing filler need to come into direct contact with one another; the greater their mutual contact area, the better and more excellent the reinforcing effect.
This research examines the wet fabrication and application of graphene (GO)/NR, carbon nanotubes (CNTs)/NR, silica (SiO2)/NR, and carbon black (CB)/NR composites. While there are a lot of reviews on natural rubber composites, there are not many on the wet preparation of NR composites. This review aims to close that gap by offering a thorough analysis of the body of research as well as a forecast for the promising future possibilities in this area.

2. Wet Preparation of NR Composites

The two primary challenges of agglomeration and filler–matrix interaction must be overcome to produce NR composites with homogenous filler dispersion. Stronger filler–matrix contact and high filler dispersion are necessary for the composites to have a reinforcing effect. The three main wet preparation techniques for NR composites are sol-gel, latex, and solution mixing.

2.1. Latex Mixing Method

This approach uses natural latex as the matrix and stabilizes the reinforcing particles through a series of techniques that culminate in a homogenous mixture and agglomeration. Zhao et al. [4] prepared GO/NR nanocomposites by using the latex blending method. It was discovered that the latex mixing approach may be used to accomplish the homogenous dispersion of GO in the NR matrix. Without compromising the final strength, the uniformly distributed GO greatly raised the tensile strength and energy storage modulus of NR at a reduced filling percentage. This process is less harmful to the environment than the solution mixing method since it does not require costly and highly toxic organic solvents [5].

2.2. Solution Mixing Method

By combining the rubber solution and nanofiller dispersion and then draining the solvent, rubber composites were produced using this technique [6]. Yang et al. [7] prepared SiO2/NR composites by accomplishing the mixing between silica and rubber molecular chains in an organic phase solution. The silica/natural rubber composites made via the solution approach presented superior filler dispersion when compared to those made by the dry mixing method. The main disadvantages of this technique, however, are its high cost, the need for a lot of solvents during the solvent removal process, and the environmental issues related to disposing of solvents [8]. The final properties of the rubber composites are greatly influenced by the solvent used during the preparation process [5].

2.3. Sol-Gel Method

Precursors like siloxanes or metal salts are added to the rubber matrix in the sol-gel process to facilitate hydrolysis and condensation processes, which produce uniformly distributed nanoparticles in situ. Tetraethoxysilane (TEOS) is a widely utilized precursor for the preparation of SiO2/NR composites. Poompradub et al. [9] used ethyl orthosilicate (TEOS) as a silica precursor to create SiO2/NR composites utilizing the sol-gel process. When compared to commercial (ectopic formation) SiO2/NR vulcanizates, the mechanical and thermal characteristics of SiO2/NR composites, including silica produced in situ using the sol-gel method, were greatly enhanced. While the non-in situ generated commercial silica agglomerated in the NR matrix, the in situ-created silica particles were evenly dispersed throughout the matrix.

3. Progress in the Application of Wet Mixing Process in Natural Rubber Composites

3.1. Carbon Black (CB)/Natural Rubber Composites

3.1.1. Mechanism of Carbon Black Reinforced Rubber

Carbon black is thought to be the most efficient filler material additive since it makes rubber-based polymers stronger and harder [10]. The binding effect between the carbon black particles and the rubber matrix is the primary mechanism by which carbon black is reinforced with rubber. Due to the small particle size of carbon black, its large contact area with the rubber matrix, and its ease of infiltration by rubber molecules, physical bonding mostly manifests as van der Waals force. Simultaneously, the presence of active sites on the surface of carbon black particles, which have a higher surface energy and are easier to firmly connect with the rubber matrix, is the primary cause of chemical adsorption. Numerous academics and researchers have investigated the workings of carbon black reinforcing rubber in great detail, and they have put forth the following models of reinforcement.
According to the principle of volumetric effect, scattered carbon black in the rubber matrix creates a solvent effect that raises the rubber’s viscosity and has a strong reinforcing impact. The majority of the mechanical characteristics of carbon black rubber composites are associated with rubber’s tendency to increase viscosity. Rubber’s mechanical strength is increased because the molecular chains adsorbed on the surface of carbon black particles exhibit an orientated state. When there is a tensile tension applied to the rubber, if the carbon black adsorption on the rubber molecular chain is weak, the “gap phenomenon” will occur [11]. Rubber can more easily penetrate areas with higher carbon black particle surface adsorption energies. The degree of surface structure of carbon black can be improved by actively treating the particles’ surfaces, which increases the material’s ability to permeate the rubber matrix and distribute itself evenly.
However, the surface structure theory [12] contends that the active filler’s surface is not smooth and that the filler’s size range and surface roughness would affect the rubber’s characteristics. Donnet et al. [13] first saw the surface of carbon black particles using a scanning tunneling electron microscope. They discovered that the surface of the carbon black is uneven and rough, with sharp edges separating the particles. Based on these observations, they proposed a model theory for carbon black. Later, Donnet et al. discovered numerous localized crystal structures on carbon black’s surface and put forth the notion of the polymer adsorption carbon black surface model.
A molecular chain sliding model was proposed by Dannenberg et al. [14] in which diverse adsorption activities are exhibited by carbon black particles and rubber macromolecules gliding over the surface. Under tension, the rubber chains adsorbed on the carbon black surface lengthen and slide. Hysteresis energy loss into thermal energy occurs during the molecular chain sliding process, preventing the rubber substance from being destroyed. Most molecular chains do not require sliding when stretched back to their initial length, demonstrating the phenomena of stress relaxation.
The interaction between the filler and the rubber is crucial for the reinforcing of filled rubbers, as several investigations have demonstrated. Generally speaking, the quantity of binding gel that forms between the filler and the rubber is how much of an interaction there is. The inclusive rubber model, the rubber-shell model, and the glassy rubber-shell model are three of the most developed theoretical models of binding gel that have been put forth.
The inclusive rubber model, as described by Medalia [15] and Kraus [16], states that a certain amount of rubber is contained in the spaces of the filler network, whereas aggregates or agglomerates of active fillers can create a network of fillers due to their adsorption (Figure 2a). Due to its decreased mobility, the encapsulated rubber no longer contributes to the rubber’s elastic behavior and joins the packing, increasing the packing’s effective volume.
Rubber macromolecular chains may be chemisorbed by the filler’s surface active sites as a result of chemisorption around the filler particles, as demonstrated in Figure 2b, according to the rubber shell model put out by Smit [17] and Pliskin [18]. Using Figure 2c as support, O’Brien [19] further suggested that the rubber molecular chains adsorbed around the filler particles are practically glassy and have no motility. In this regard, LeBlanc [20] conducted numerous experimental investigations utilizing H1-NMR, and he suggested that rubber adsorbed in the immediate area of the filler particles as a layer that was strongly linked. The external force field is in a glassy state and does not affect the macromolecular chains, which interact significantly with the filler particles. When the adsorption force weakens, rubber molecule chains become more mobile, and the binding layer becomes comparatively free. As seen in Figure 3, the rubber’s solvent can dissolve the opposing outer layer, or free rubber, which is not constrained by the filler particles.
An interfacial model based on the interaction of fillers and rubber macromolecules is called the binding rubber model. Based on this binding rubber model, Fukahori [21] suggested a new interfacial structure model based on the stress analysis results, accounting for the fact that the rubber’s volume expands in a large deformation state. As seen in Figure 4a,b, the bonded rubber bilayer structure model is the core of this model. According to the model, the structure of the binding rubber adsorbed at the carbon black’s periphery is made up of two parts: rubber layers with distinct moduli and an uncrosslinked structure. The inner structure is the glassy hard layer (GH) of the polymer in its glassy state, which has a thickness of around 2 nm, and the outer structure is the sticky hard layer (SH) of the polymer, which has a thickness of between 3 and 8 nm. The outermost layer, which has a thickness of 3–8 nm, is a sticky hard layer with limited polymer macromolecule mobility. The combined thickness of the two layers is around 5 nm for carbon black particles with a smooth surface and 10 nm for carbon black particles with a high degree of roughness on their outer layer.
The GH layer does not affect the sharp increase in stress under large magnitudes because the effect of the polymer glassy hard layer is limited to increasing the effective diameter of the carbon black particles, and its contribution to stress remains constant regardless of the amount of strain applied to it. While the GH layer cannot undergo orientation due to the poor mobility of the molecular chains, it does not contribute to the increase in the modulus of the rubber. At the same time, the viscous hard layer behaves similarly to the rubber matrix at small strains and contributes less to the modulus (Figure 4d). When the strain gradually increases, the SH layer undergoes orientation, a phenomenon that plays an important role in the increase in the rubber’s modulus. Furthermore, at higher carbon black contents, the SH layers separating various binding rubber particles may overlap, creating a super-network structure (Figure 4c). This super-network structure changes orientation and hardens under high strains, which adds to the modulus. Little holes will grow between the bundles of rubber macromolecular chains as they are assembled, and these holes can absorb some energy, postponing the rubber molecular chains’ eventual disintegration (Figure 4e). Simultaneously, the polymer’s volume expands under tension due to the existence of these microscopic pores.
Haghgoo et al. [22] investigated how multiscale fillers affected the electrical conductivity and resistivity of multiscale nanocomposites made of reinforced polymer and CB/CF. The outcomes demonstrated that the conductivity qualities and interoperability in the direction of electron flow were enhanced by the multiscale fillers. These enhancements were in good agreement with the microstructure’s multiscale filler modifications. Multiscale fillers can be added to polymer matrix composites’ microstructure to improve their electrical characteristics and reduce percolation thresholds.
Drawing on the micro-level analysis of the reinforcing model discussed above, we can employ several techniques to enhance the surface quality and surface activity of carbon black, resulting in NR composites with consistent filler dispersion. To create high-performance natural rubber composites, we can simultaneously lessen the force between carbon black particles, increase the force between carbon black and the NR matrix, and encourage the combination of carbon black and latex particles.

3.1.2. CB/NR Composites Wet Mixing Preparation Process

Due to their hydrophobicity and/or high elemental carbon content (90~99%), carbon black particles agglomerate and produce unstable dispersions when dissolved in water at a pH close to the isoelectric point (IEP). Consequently, it is essential to regulate the surface charge of carbon black and limit its particle size to produce extremely stable carbon black aqueous dispersions [23]. An old issue in materials science is the creation of stable carbon black dispersions in water, either for use as pigments or as reinforcing filler particles in polymers. The application properties of carbon black are weakened due to their tendency to form agglomerates due to their tiny primary particle size and high inter-particle forces. Table 1 [24] illustrates the three primary categories into which the techniques employed in the literature to characterize the dispersion of carbon black can be separated.
As was previously noted, carbon black was dispersed in water using conventional surfactants or water-dispersible amphiphilic polymer architectures. Fresh natural latex was combined and agitated with a carbon black slurry that had been made by Alex et al. [25] while surfactants (alkali metal salts of fatty acids) were present. To create carbon black wet master gum, drying and acid flocculation were applied in the end. The results demonstrated that protein displacement and high adsorption onto the rubber particles were caused by adding fatty acid soap-based surfactants to the carbon black slurry. This process converts the protein-stabilized latex into a system that is stabilized by surfactants. The adsorbed anions combine with the acid in the surfactant enclosing the latex to generate undissociated surfactants, which rob the latex particles of their stabilizer. In addition, the latex’s Menni viscosity is low, and the surfactant might sensitize it to promote quick solidification.
Martínez-Pedrero et al. [26] studied the rheological properties of binary colloidal mixtures. During the sonication step, carbon black’s dispersion state was improved with the addition of sodium dodecyl sulfate (SDS). It was then agitated and combined with natural latex to produce a gel. The findings of the experiment demonstrated that the carbon black particles experienced a structural transformation in the binary colloidal mixes, bridging the natural rubber particles together. This phenomenon of bridging allows a generative network of fractal clusters to emerge, which in turn forms an elastic solid. The interparticle attraction energy between CB particles and NR droplets, which is impacted by the surfactant concentration, is primarily responsible for controlling this transition. The homogeneity of the dispersion is controlled before it loses stability by the adsorption of surfactant molecules on the carbon black surface, which modifies the interaction between the two particles. Applying shear can overcome the energy barrier preventing the bridging effect in the more stable scenario.
Using a similar strategy, Dong et al. [27] were able to effectively diminish the interaction between carbon blacks and improve the interaction between carbon blacks and rubber molecule chains, hence improving the dispersion of carbon blacks. Furthermore, it was shown that while the hardness and elongation at break showed the opposite pattern, the mechanical characteristics increased first and then dropped as the size of the carbon black particles increased. This is because more active sites are present in carbon black particles with smaller particle sizes since they have bigger specific surface areas. As a result, there is a greater degree of chemical bonding and physical adsorption between the rubber molecular chain and the surface of carbon black, resulting in improved bonding. The performance of vulcanized rubber made by the latex co-sinking process is still superior to that of conventional dry mixing, even though too-small carbon black particles are easy to agglomerate in the rubber matrix and reduce its mechanical qualities.
As previously shown, surfactants or polymers grafted with carbon black have improved dispersion stability. Carbon black can also employ its hydroxyl group and other active sites as low molecules for cationic, anionic, and free radical polymerization processes. The inter-particle resistive effect of the carbon black is enhanced by the grafting of the polymer onto its surface, which can successfully stop the carbon black from clum** together in the aqueous phase. The production of new or modified polymers has made extensive use of ultrasonic, high-energy electron beam (EB), and C-ray radiation in recent years as effective and ecologically acceptable free radical polymerization techniques.
By grafting acrylic acid (PAA) onto the CB surface with high-energy electron beam irradiation (EB), Jiang et al. [28] created water-dispersible CB. When compared to C-ray irradiation, EB is more efficient [29]. In addition to increasing the CB’s hydrophilicity and lowering its surface energy, grafting also improves the CB’s average aggregated particle size and dispersity when compared to unmodified CB. Fu et al. [30] grafted polyethylene glycol 400 onto the surface of carbon black using an in situ liquid phase grafting approach. Better dispersion of the modified carbon black in the rubber system increases the mechanical characteristics and the interaction between the modified filler and rubber. The dispersibility of carbon black can be effectively increased by the polymer graft modification method; however, because linear polymers are susceptible to molecular chain entanglement, carbon black particles may reaggregate and form new aggregates.
Because of their distinct structure, high number of reactive functional groups, good solubility, low viscosity, non-entangled molecular chains, and simplicity of synthesis, hyperbranched polymers are of interest [31,32]. On the surface of carbon black, Han et al. [33] grafted end-carboxylated hyperbranched poly(2-hydroxypropane-1,2,3-tricarboxylic acid) (Figure 5). The modified CB greatly enhanced the dispersibility and wettability of the composite; its average particle size was significantly smaller, and its particle size distribution was narrower, according to the results. This could be explained by the hyperbranched polymer development on the CB surface, which raises the spatial site resistance and electrostatic repulsion between CB particles. This, in turn, increases the repulsive force between CB particles, making it more challenging for CB to assemble [34,35]. A significant amount of carboxyl groups, which are hydrophilic oxygen-containing functional groups, are present in the topmost layer of the hyperbranched polymer layer and may help to improve the dispersion of carbon black in water.
As was previously indicated, the emulsion coalescence process can be used to create composites by adding coating resin to a mixed system of carbon black and latex to form a powder system [36]. With the use of the emulsifier AEO-9, Lin et al. [37] generated the CB emulsion, which they subsequently mixed and combined with NR latex to create a powder system. After being heated in a water bath, carbon black and natural rubber thermally aggregate. Afterward, flocculation and drying were accomplished using a CaCl2 solution. According to the findings, natural rubber composites filled with carbon black and powdered had superior mechanical and dynamic properties than those made by traditional dry blending. The improved composite properties were also attributed to the good dispersion of CB in the rubber matrix and the improvement in rubber–filler interactions.
Process approaches can also be used to obtain a homogenous mixing of carbon black with natural latex in addition to carbon black modification. The earliest method to homogeneously disperse carbon black in latex was developed by Cabot [38,39]. It involved using high-pressure jetting technology to quickly mix and flocculate natural latex with carbon black slurry, but the equipment and process requirements are complex. Afterward, Han et al. [40] used high-speed impact jet processing to directly combine natural rubber latex (NRL) with carbon black (CB) to create rubber composites. The filler dispersion and latex were combined using a side-mounted jet mixer to create a combination that was then baked in an oven to create master rubber. The findings demonstrated that, in comparison to the traditional dry procedure, the jet compounding technique allowed for a more uniform dispersion of CB into natural rubber. Furthermore, a comparison of the jet Reynolds number mixing forms revealed that the carbon black was more likely to agglomerate and have bigger particle sizes in the matrix and that the particles in laminar flow mixing were subjected to less shear stress. Turbulent jet mixing, on the other hand, successfully prevents particle agglomeration. It improves the interaction between rubber and fillers, diminishes the network of fillers, and improves filler dispersion in the rubber matrix, all of which increase load transfer efficiency.
Using ultrasonic technology to pretreat natural latex can boost the reinforcing effect by increasing the contact area between rubber hydrocarbon particles and filler particles and rupturing the protective barrier. Simultaneously, filler particle size can be decreased via ultrasonic treatment, improving filler dispersion uniformity. Galinovskiy et al. [41] explored the effect of ultrasound and ultrasonic jet treatment methods on the dispersion of nanosuspensions. The experimental results indicate that ultrasonic jet dispersion is a potential method for particle deagglomeration and a solution to the issue of achieving the required degree of dispersion in suspensions. Using a novel technique called field emission scanning electron microscopy (FESEM), Cattinari et al. [42] examined how the nanoscale structure of NR–CB concretions changed as the solvent evaporated. Osmium vapor was used to specifically chemically immobilize the sample, preventing it from submerging in the immobilizer-containing liquid solution while maintaining the colloidal structure of the NR particles. We looked into and examined the impact of external physical stressors (shear and ultrasonic) on the coagulation of NR latex and CB slurry. It was discovered that combining the two parts in a colloidal suspension while subjected to sonication kept the CB filler in the form of tiny aggregates (20–200 nm), with excellent filler homogeneity surrounding the NR pellets (Figure 6). It was determined through experimentation that the NR’s sonication did not affect the structure (Figure 6g,h). The NR and CB were evenly distributed, and heterogeneity was mostly responsible for the clots’ aggregation (Figure 6a,b). Some of the aggregates between the NR are highlighted in images captured at high magnification (Figure 6c,d). Some of the aggregates have sunk into the sphere, as shown by the red arrows in Figure 6f, and these aggregates formed a distinct contact on the sphere’s surface (Figure 6e).
Using the grinding balls’ inherent gravity and the impact of the balls colliding, ball milling technology breaks up aggregates and helps refine particle size. Yamamoto et al. [43] used a ball mill under various ball milling conditions to study the dispersion characteristics of carbon black in solvents. The results of the trials demonstrated that the dispersion performance rose as the stirrer speed and bead loading ratio increased and that there was a strong correlation between the two parameters and the impact energy of the beads. To prepare silica and carbon black dispersions for the creation of natural rubber composites, Hamran et al. [44] used ball milling sonication. The dispersion treated with ultrasonication followed by ball milling demonstrated the best dispersing ability, and the results indicated that this combination of treatment methods was the optimum way to minimize the particle size of silica and carbon black.
Sui et al. [45] suggested a completely formulated wet mixing approach for carbon black. Using all of the formulation’s fillers, they ball-milled the pretreatment to create an aqueous dispersion slurry, which was then combined physically, via stirring, with the latex. Ultimately, a twin-screw was used to accomplish the flocculation and dewatering steps, and an oven was used to dry and further treat the rubber masterbatch. The generated rubber composites containing carbon black exhibited a homogeneous dispersion and distribution, tiny particle size, and no visible aggregation phenomenon due to the completely formulated wet continuous mixing procedure that combines wet and continuous mixing methods. The carbon black dispersion was enhanced in comparison to the conventional dry mixing, and the composites’ tensile strength, elongation at break, and cut resistance all increased by 9.4%, 9.6%, and 35%, respectively. On the other hand, constant wet mixing permits the molecular chains in natural rubber to be evenly distributed throughout the rubber by minimizing chain breaking. Consequently, the fully formulated wet blend’s Menni viscosity is higher than the dry blend’s.
In summary, dispersant or surfactant can be used to modify carbon black, even though it is dense, hydrophobic, and settles a lot in aqueous dispersion. Furthermore, the modified carbon black frequently cannot match the latex’s settling speed due to its differing polarity and density, which may have an impact on the carbon black filler’s stability and dispersion in the rubber matrix. Since the rubber phase in the mixed product is not cross-linked and tends to aggregate due to the thermodynamics of carbon black, we should try to take steps to dehydrate and dry the product quickly to reduce its viscosity and lengthen its survival period. To minimize wastewater, waste gas, and solid waste, shorten the production cycle, reduce the three wastes, and improve product quality and performance, wet mixing technology’s benefits for the economy and environment should also be carefully taken into account during the production process. This is because there is an antagonistic relationship between the search for wet mixing technology’s advantages and the development of low-carbon, energy-saving, and environmentally friendly directions.

3.2. NR/Silica (SiO2) Composites

3.2.1. Mechanism of Silica Reinforced Rubber

Silica’s reinforcing process is quite similar to that of carbon black. When compared to carbon black, silica’s biggest distinguishing attribute is its surface activity. Silica is an amorphous structure of silicon dioxide, having silicon atoms in the center and oxygen atoms at the apex, forming an irregular tetrahedral shape. The silicon atoms on the surface are unevenly organized, and the hydroxyl groups that connect them change depending on the chemical reaction. The numerous hydroxyl groups on the surface of silica can be classified into three types: double hydroxyl, isolated hydroxyl, and neighboring hydroxyl.
The adsorption–slip theory, which states that fillers strengthen rubber primarily through molecular chain slip when the composite is stretched by external pressures, is now generally accepted. The addition of silica filler to rubber results in the binding of several silica particles to the rubber molecular chain, which in turn binds each silica particle to numerous rubber molecular chains. When an external force is applied, the rubber molecules will align themselves in the force’s direction. This is because the silica particles are separated by several rubber molecular chains, each of which has a different length. The longer chains are forced to slip on the silica surface when the shorter chain segments restrict the orientation of the larger chains after they have finished their tensile orientation. The molecular chains connecting the silica particles are completely stretched as the orientation proceeds. To achieve a greater modulus, each rubber molecular chain segment is currently subjected to a more uniform external force thanks to the silica-reinforcing effect of the rubber. This reinforcing structure will be destroyed, the slip orientation will cease, and the composite material will fracture when the external force exceeds a particular critical amount.
The silica exhibits a high specific surface area and strong surface adsorption. It is typically presented in an aggregated state, which allows for a larger contact area with rubber and promotes the formation of a bond between the two materials. The crystallization effect of the particles causes the adsorption layer to rise, resulting in a smaller particle diameter than the spacing between them, which plays the role of physical reinforcing [46]. Chemical reinforcement can be created when the double bond of the rubber molecular chain reacts with the hydroxyl group present on the surface of silica [47]. Bonded rubber is created when the alkaline reactive groups of rubber react with the acidic hydroxyl groups on the surface of silica [48]. This reaction between the two groups creates silica-reinforced rubber.

3.2.2. SiO2/NR Composites Wet Mixing Preparation Process

Silica (SiO2) is the preferred filler for tire tread applications because it has lower rolling resistance and higher wet traction than carbon black. Utilizing silica as much as possible instead of traditional carbon black while preparing NR-based green tire rubber compounds is crucial for the automotive sector, given the current state of the world and environmental concerns [49]. Generally speaking, rubber’s mechanical qualities are enhanced by silica addition. However, silica is more polar due to the abundance of hydroxyl groups on its surface, which reduces the interaction between silica and rubber. Furthermore, silica dispersion is typically poor in carbon black-filled compounds made using the traditional mixing method. The viscosity rises dramatically with substantial additions of silica, which complicates processing and puts undue strain on processing machinery. The strong contact between silica particles is responsible for the viscosity increase. Two approaches can be taken to address this issue: either the hydrophilic silica surface is modified, or the rubber’s non-polar structure is functionalized to make it more polar [50]. Table 2 and Table 3 include a list of papers on the wet filling of natural latex with silica.
Silane coupling agents (SCAs) are used to alter the silica surface. Amphoteric surface modifiers and interfacial compatibilizers are silane coupling agents (SCA). Chemically speaking, SCA has at least one alkoxy group that, by dehydration condensation, can form stable Si-O-Si structures with SiO2 particles. Reactive groups, particularly reactive sulfur atoms, are also present in SCA and can react chemically with diene rubber macromolecules.
The most practical and technologically advantageous silane coupling agents for silicone-based NR compounds are sulfide–alkoxy silane coupling agents, according to research by Kaewsakul et al. [62] on silane coupling agents with various particular functionalities. By comparatively offering higher filler–rubber contacts than single alkoxy or sulfide-based silanes, these chemicals can effectively lower the viscosity and filler–filler interactions of the compounds, leading to a notable increase in mechanical characteristics.
By altering SiO2 with 3-mercaptopropylethoxybis(tridecylpentamethoxy)silane (Si-747), a long-arm silane coupling agent, Zheng et al. [63] created NR/SiO2 composites. Si747 is a type of SCA with two long arms that can form abundant and concentrated hydrogen bonds with SiO2, and the creation of an NR/NR masterbatch will be more satisfying when the total amount of Si747 is sufficient to reduce the electronegativity of SiO2. When the total concentration of Si747 is sufficient to counteract the electronegativity of SiO2, a more suitable silica/NR masterbatch will be prepared. Some papers on wet filling of natural latex with silica are listed in Table 2 and Table 3.
The hydroxyl groups on the surface of silica react with conventional silane coupling agents to produce a lot of volatile organic compounds (VOCs), which degrade rubber composite performance and contaminate the environment [64]. By reacting fatty alcohol polyoxyethylene ether (AEO-X) with varying polyether lengths with bis[γ-(triethoxysilyl)propyl]-tetrasulphide TESPT (Si69), Li et al. [65] created a range of novel coupling agents with low VOC emission, which they then integrated into silica/natural rubber nanocomposites. The findings demonstrated that the new coupling agents could properly balance the tires’ “magic triangle” performance and greatly enhance silica dispersion in the rubber matrix. SiO2/NR composites showed improvements in wet slip resistance of 14.2%, abrasion resistance of 20.3%, and rolling resistance of 15.8%.
A set of low volatile organic compound (VOC) Mx-Si69 couplers (Figure 7) was applied to SiO2/NR nanocomposites by Zhai et al. [66] These couplers correspond to the number of ethoxyl groups in bis-(γtriethoxysilylpropyl)-tetrasulphide (Si69) substituted by aliphatic polyether chains (x = 1, 2, 3, 4, 5, 6). Excellent performing tire treads were ready. First off, M1-Si69 is the optimal option for both Si69 and Mx-Si69. In addition to giving the tires exceptionally good anti-slip performance and very low energy loss, applying the recently developed M1-Si69 coupling agent to SiO2/NR nanocomposites can also drastically cut down on VOC gas emissions.
Different ratios of TWEEN-20 and TESPT were chosen by **. The GO/NRL nanocomposites prepared in one step (103.7 Fg−1) and the nanocomposites prepared in two steps (32.6 Fg−1) had a lower specific capacitance. This method involved preparing the GO/NRL nanocomposites concurrently with the GO and NRL mixing product. Consequently, the synthesized GO/NRL nanocomposites may find use in energy storage devices as supercapacitors.
Sodium poly 4-styrene sulfonate was utilized by Li et al. [118] as a stabilizer for the chemical reduction of graphite oxide. The resulting graphene was then combined with NR using a latex process. It was demonstrated that adding surfactants to the graphene surface causes NR molecules to experience both spatial site resistance and electrostatic repulsion. Using the latex composite approach, high-performance NR/graphene nanocomposites were created in conformity with the morphological findings, and the generated nanocomposites’ energy storage modulus was greatly increased.
Following the solidification and drying of rGO/NR latex mixtures made by ultrasound-assisted latex mixing and in situ reduction, Wang et al. [119] developed rGO/CB/NR composites. Both rGO and CB were evenly distributed throughout the rubber composites, as demonstrated by morphological observations. The inclusion of rGO significantly increased the composites’ hardness, heat conductivity, and resistance to aging. When compared to normal latex mixing, the rGO/CB/NR composites made with ultrasonic assistance had superior mechanical characteristics.
Mao et al. [120] created GO/NR composites by using the latex co-coagulation process. By ultrasonically removing graphite oxide from water, combining the aqueous GO dispersion with the latex, and co-coagulating the mixes with the addition of a flocculant, an aqueous GO dispersion stabilized by electrostatic repulsion was produced. The outcomes demonstrated the strong interfacial contacts between GO and NR and the fine dispersion of the highly exfoliated GO flakes in the NR rubber matrix. The mechanical characteristics of the GO/NR composites were further assessed by the researchers. With a GO concentration of less than 2 phr, the results demonstrated a considerable improvement in the tensile strength, rip strength, and modulus. In particular, because of the stress-induced crystallization effect of NR, GO shows a unique strengthening mechanism in NR.
Using the latex co-precipitation approach, Li et al. [121] created CB/GO/NR composites with various crosslinked networks. In the meantime, research was done on how various crosslinking techniques affected fatigue life and crack extension resistance in various vulcanization systems. The findings demonstrated that, in the conventional vulcanization (CV) system, the polysulfone-based CB/GO/NR composites had the lowest crack extension rate (64.1 nm/cycle) and the highest tear strength (71.6 KN/m). The improvement in crack extension resistance was primarily attributed to the CV system’s well-developed crosslinking network and polysulfone-based crosslinking structure.
A wide range of graphene can be modified to improve its dispersion in a rubber matrix or water. With the emergence of several of the aforementioned surfactants and processes, graphene dispersion acts as a potent aid, and when compared to conventional dry mixing, wet mixing can better capture the graphene’s reinforcing properties.

4. Expectations and Conclusions

Due to its distinct benefits over dry mixing, wet mixing technology has drawn a lot of attention and has recently been a hot topic for research. The review above indicates that the primary issue with CB/NR composites is the high density and hydrophobicity of carbon black, which settles a lot in the aqueous dispersion and makes it difficult to co-settlement well with the latex. This requires modification with a dispersant or surfactant. The effective interface between particular active sites on the surface of carbon black and the rubber macromolecular chains is always reduced or hindered by the surface’s preferential contact with and adsorption of surfactants.
A significant amount of research has also been conducted on wet mixing technology for SiO2/NR composites. However, because of the conflict between silanization and the rubber material’s characteristics and the high viscosity of the prepared masterbatch resulting from the presence of SiO2, overloading the mixing equipment can easily occur, impeding appropriate mixing and processing. Wet mixing methods for graphene/NR and NR/CNT composites have been extensively studied, and related research is still ongoing.
In summary, weakening filler–filler interactions and strengthening filler–matrix interactions are necessary to greatly improve the characteristics of NR nanocomposites. However, the broad use of NR composite nanomaterials is restricted by the weaker interactions between nonpolar NR and other nanofillers. As this review points out, better dispersion mixing techniques and filler surface functionalization can help with this. In summary, the wet mixing method makes up for the drawbacks of conventional dry mixing and is anticipated to become a widely used emergent technology.

Author Contributions

Conceptualization, H.Y.; writing—original draft preparation, Q.Z.; writing—review and editing, F.N. and J.L.; supervision, H.Y.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic structure of natural latex rubber particles.
Figure 1. Schematic structure of natural latex rubber particles.
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Figure 2. Binding gel theory model diagram.
Figure 2. Binding gel theory model diagram.
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Figure 3. Theoretical diagram of rubber layer distribution around carbon black particles [20].
Figure 3. Theoretical diagram of rubber layer distribution around carbon black particles [20].
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Figure 4. Diagrammatic representation of the binding gel bilayer model (a) A new double-layer interface model consisting of a GH layer and a SH layer. (b) Detailed molecular structures in the GH and SH layers as vulcanized. (c) Overlapped SH layers in triaxial expansion along extension direction at ε 0 = 10 % and ϕ = 0.2 . (d) Molecular movements within the SH layer under large extension. (e) Supernetwork structure of carbon particles interconnected by strands of oriented molecules. (f) Buckling of extended and oriented molecular bundles [21].
Figure 4. Diagrammatic representation of the binding gel bilayer model (a) A new double-layer interface model consisting of a GH layer and a SH layer. (b) Detailed molecular structures in the GH and SH layers as vulcanized. (c) Overlapped SH layers in triaxial expansion along extension direction at ε 0 = 10 % and ϕ = 0.2 . (d) Molecular movements within the SH layer under large extension. (e) Supernetwork structure of carbon particles interconnected by strands of oriented molecules. (f) Buckling of extended and oriented molecular bundles [21].
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Figure 5. Schematic diagram illustrating the preparation route (A) Reaction to prepare CB-COOH. (B) Reaction preparation of CB-g-CTHBP. (C) Preparation of CB-g-CTHBP dispersions (D) Wet mixing of dispersion with latex. (E,F) Compounding to make rubber compounds [33].
Figure 5. Schematic diagram illustrating the preparation route (A) Reaction to prepare CB-COOH. (B) Reaction preparation of CB-g-CTHBP. (C) Preparation of CB-g-CTHBP dispersions (D) Wet mixing of dispersion with latex. (E,F) Compounding to make rubber compounds [33].
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Figure 6. FESEM micrographs of the NR–CB heterocoagulum obtained with sonication during mixing: Mass ratio in the liquid suspension = 40% CB/rubber and the structure was fixed at t = 1 h. (a) Large view SE and (b) BSE images of the same region showing the homogeneous distribution of NRs and CB filler (magnification: ×5000). (c) Magnified SE and (d) BSE views corresponding to the yellow squared region in (a,b) (magnification: ×10,000), highlighting the partial coalescence between the NR globules that are surrounded by several small CB aggregates. (e) Magnified SE of the red boxed region in (c) (magnification: ×80,000). (f) Magnified SE and BSE views of the blue boxed region shown in (c,b) (magnification: ×40,000), showing that the CB filler partially sinks toward the inner part of the NR globules. (g) Magnified SE and (h) BSE views corresponding to the control sample composed only of sonicated NR globules [42].
Figure 6. FESEM micrographs of the NR–CB heterocoagulum obtained with sonication during mixing: Mass ratio in the liquid suspension = 40% CB/rubber and the structure was fixed at t = 1 h. (a) Large view SE and (b) BSE images of the same region showing the homogeneous distribution of NRs and CB filler (magnification: ×5000). (c) Magnified SE and (d) BSE views corresponding to the yellow squared region in (a,b) (magnification: ×10,000), highlighting the partial coalescence between the NR globules that are surrounded by several small CB aggregates. (e) Magnified SE of the red boxed region in (c) (magnification: ×80,000). (f) Magnified SE and BSE views of the blue boxed region shown in (c,b) (magnification: ×40,000), showing that the CB filler partially sinks toward the inner part of the NR globules. (g) Magnified SE and (h) BSE views corresponding to the control sample composed only of sonicated NR globules [42].
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Figure 7. Schematic diagram of silica modified with Mx-Si69 induced by NRCs [66].
Figure 7. Schematic diagram of silica modified with Mx-Si69 induced by NRCs [66].
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Figure 8. A wet compounding process combined with ultrasonically assisted latex mixing (WCL) method for the preparation of reduced graphene oxide (rGO)/silica/natural rubber (NR) composites [54].
Figure 8. A wet compounding process combined with ultrasonically assisted latex mixing (WCL) method for the preparation of reduced graphene oxide (rGO)/silica/natural rubber (NR) composites [54].
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Figure 9. (a) Schematic of the synthetic pathway for the formation of 3D self-assembled NR-SiO2 nanocomposite and its SEM images. (b) Depiction of nanoscopic aggregation in NR-SiO2 nanocomposite in the presence of 100 mM Mg2+ [55].
Figure 9. (a) Schematic of the synthetic pathway for the formation of 3D self-assembled NR-SiO2 nanocomposite and its SEM images. (b) Depiction of nanoscopic aggregation in NR-SiO2 nanocomposite in the presence of 100 mM Mg2+ [55].
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Figure 10. SEM micrographs of silica suspension; (a) before grinding and (b) after grinding [61].
Figure 10. SEM micrographs of silica suspension; (a) before grinding and (b) after grinding [61].
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Figure 11. Silicon dioxide-loaded 1,3 diphenylguanidine (DPG) combined with self-flocculation technology process [82].
Figure 11. Silicon dioxide-loaded 1,3 diphenylguanidine (DPG) combined with self-flocculation technology process [82].
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Figure 12. Schematic diagram of the cellulation model formed in CNTs/elastomer composites. (a) Percolation, (b) partial cellular structure, and (c) three-dimensional cellular structure [85].
Figure 12. Schematic diagram of the cellulation model formed in CNTs/elastomer composites. (a) Percolation, (b) partial cellular structure, and (c) three-dimensional cellular structure [85].
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Figure 13. TEM images of NR/CNTs composites prepared by slurry blending method and latex blending method, (a) slurry blending method, (b) latex blending method. (c) The SEM image of NR/CNTs composites prepared by slurry blending method [104].
Figure 13. TEM images of NR/CNTs composites prepared by slurry blending method and latex blending method, (a) slurry blending method, (b) latex blending method. (c) The SEM image of NR/CNTs composites prepared by slurry blending method [104].
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Figure 14. The general route for nanocomposite preparation [102].
Figure 14. The general route for nanocomposite preparation [102].
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Figure 15. TEM and HRTEM images of aqueous dispersions of (a,c) pristine MWNTs and (b,d) SLS-functionalized MWNTs [108].
Figure 15. TEM and HRTEM images of aqueous dispersions of (a,c) pristine MWNTs and (b,d) SLS-functionalized MWNTs [108].
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Figure 16. Schematic representation of the EBT model, (a) GE/NR (b) GO/NR (c) NR [109].
Figure 16. Schematic representation of the EBT model, (a) GE/NR (b) GO/NR (c) NR [109].
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Table 1. Carbon black water dispersion main method.
Table 1. Carbon black water dispersion main method.
FormMethodologies
1Using water-dispersible amphiphilic polymer structures or traditional surfactants to disperse carbon black in water
2Carbon black suspension in water, precipitation encapsulation, and emulsion polymerization
3Methods like using the right oligomers to ball mill carbon black powder in the solid state
Table 2. Summary of modified silica/natural rubber composite methods improved in this paper.
Table 2. Summary of modified silica/natural rubber composite methods improved in this paper.
NR CompositesModified Treatment Method/ReagentPreparation MethodYear/Reference
NR/SiO2Methyltriethoxysilane, vinyltriethoxysilane, γ-aminopropyltrimethoxysilaneSol-gel method2014
[51]
NR/SiO2/GEElectron-beam irradiationLatex mixing2019
[52]
NR/SiO2Deep eutectic solvents (DES) 3phrMechanical mixing2022
[53]
NR/SiO2Cystamine dihydrochloride (CDHC)Latex mixing2020
[54]
NR/SiO2AEO-9, KH-590Solution compounding2022
[6]
Table 3. Summary of wet preparation methods for silica/rubber composites.
Table 3. Summary of wet preparation methods for silica/rubber composites.
NR CompositesSynthesisFlocculants/MethodsYear/Reference
NR/SiO2Latex compounding
self-assembling techniques		MgSO42019
[55]
NR/SiO2Latex co-coagulation method3 wt% acetic acid2020
[56]
NR/SiO2Latex co-coagulation methodCalcium chloride/acetic acid2020
[57]
NR/ENR/SiO2Wet masterbatch techniqueEthanol2016
[58]
NR/SiO2Solution compoundingSolvent removal2022
[6]
NR/SiO2Latex co-coagulation methodCalcium chloride2011
[59,60]
NR/SiO2Latex co-coagulation methodAcetic acid2022
[61]
Table 4. Summary of methods mentioned in this paper on wet preparation of NR Carbon Nanotube composites.
Table 4. Summary of methods mentioned in this paper on wet preparation of NR Carbon Nanotube composites.
NR CompositesSynthesis MethodSurfactants/ModifiersYear/Reference
NR/MWCNTLatex co-coagulation methodTCl42014
[97]
NR/CNTLatex co-coagulation methodSDS2016
[98]
NR/MWCNTsLatex co-coagulation methodVulcastab VL2017
[99]
NR/MWCNTsLatex co-coagulation method1-ethyl3-methylimidazolium bromide and 1-hexyl-3-methylimidazolium bromide2018
[100]
NR/MWNTsLatex co-coagulation methodK2FeO42024
[101]
NR/MWCNTsLatex co-coagulation methodSDBS/AOTPh/TCPh2015
[102]
NR/SWNTsLatex co-coagulation methodNaDDBS2009
[102]
NR/MWCNTsLatex co-coagulation methodSDS2010
[103]
NR/CNTsSlurry blending methodSDS2019
[104]
NR/CB/CNTsLatex co-coagulation methodEmulsifier OP2011
[105]
Table 5. Summary of methods mentioned in this paper on wet preparation of NR/graphene composites.
Table 5. Summary of methods mentioned in this paper on wet preparation of NR/graphene composites.
NR CompositesDispersal MethodsImprovement of PerformanceYear/Reference
NR/GEUltrasonic dispersionMechanical properties2011
[113]
NR/GEUltrasonic irradiationElectrical conductivity2012
[114]
NR/grapheneUltrasonic irradiationMechanical properties2013
[115]
NR/GOVibrateMechanical properties2014
[116]
NR/GOMechanical stirring and bathElectrical conductivity2015
[117]
NR/rGOMechanical stirringElectrical conductivity
Thermal conductivity		2017
[118]
rGO/NR/CBUltrasonic dispersionMechanical properties2018
[119]
NR/GOUltrasonic dispersionMechanical properties2020
[120]
CB/GO/NRUltrasonic dispersionMechanical properties2023
[121]
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Zhao, Q.; Niu, F.; Liu, J.; Yin, H. Research Progress of Natural Rubber Wet Mixing Technology. Polymers 2024, 16, 1899. https://doi.org/10.3390/polym16131899

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Zhao Q, Niu F, Liu J, Yin H. Research Progress of Natural Rubber Wet Mixing Technology. Polymers. 2024; 16(13):1899. https://doi.org/10.3390/polym16131899

Chicago/Turabian Style

Zhao, Qinghan, Fangyan Niu, Junyu Liu, and Haishan Yin. 2024. "Research Progress of Natural Rubber Wet Mixing Technology" Polymers 16, no. 13: 1899. https://doi.org/10.3390/polym16131899

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