Research progress on sustainability of key tire materials

In recent years, countries worldwide have engaged actively in the research and development of green and sustainable materials in the face of the increasing depletion of petroleum resources, the need to reduce material waste, and the environmental pollution caused by the various types of waste. In the tire industry, the key materials for the various components of tires are mostly dependent on petroleum resources. Development of green tires and green processing technologies using sustainable materials is an important development direction for the future of the tire industry, and many tire‐manufacturing companies have proposed their visions for the development of eco‐friendly tires. Rubber, cord fabric, and additives are the main materials used in tire manufacturing. This article summarizes the research status of the green materials that can meet the requirements of environmental friendliness and sustainability, replace traditional materials, and reduce petroleum resource consumption in existing tire production. These materials mainly include natural rubber or bio‐based synthetic rubber, green renewable cord fabrics, and green processing additives. The prospects for the application of these new green materials in tire manufacturing are also discussed.

ever, greenhouse gases and other pollutant emissions have combined with the consumption of nonrenewable resources during tire manufacture and use to aggravate global warming and cause environmental degradation, and these problems are detrimental to the ecological balance of the environment.Studies have shown that the entire automotive industry needs to take responsibility for approximately 40% of global pollution, with tire production and use accounting for around 20%-30% of this figure. 1o reduce the industry's impact on the natural environment and achieve the ultimate goal of carbon neutrality, tire companies worldwide have taken related measures, which mainly include reducing raw material consumption, reducing tire weights, and increasing the use of natural and renewable resources.Green materials and technologies for use in tire production are the current research hotspots in tire manufacturing.The main raw materials used in tire production are rubber, steel cord, polymer cord fabric, and carbon black (CB); the steel cord and cord fabric are used to form the bead, the carcass, and the cushion layer, and CB serves as an important rubber reinforcement and filling material.In addition, other fillers and additives, such as silica, antiaging agents, rubber accelerators, and plasticizers are used.Traditional rubber materials are mostly derived from nonrenewable petrochemical resources, thus hindering the sustainable development of the tire industry.Therefore, it will be necessary to replace traditional materials either partially or completely with green and sustainable materials, for example, by developing green rubber materials, eco-friendly cord materials, green CB, and other components.
To date, researchers have developed many alternative rubbers, fibers, CBs, and other additives that have been derived from bio-based or waste polymers, and some of these materials have already been used in tire manufacturing.Kuraray used liquid-phase ethylene rubber (LFR), a bio-based diene monomer, in passenger tire production in 2017.In 2021, Continental launched the Conti GreenConcept tire, which was fabricated using dandelion rubber (DR), silica from rice husk ash, and renewable raw materials obtained from various vegetable oils and resins.In addition to processed steel and CB, Continental also used polyester from recycled polyethylene terephthalate (PET) bottles in the casing of a tire for the first time.The proportion of renewable raw materials used in the Conti GreenConcept tire was 35%. 2 In 2022, Goodyear announced the development of a demonstration tire that contained 90% sustainable materials. 3With the increasing attention being paid to environmental issues and sustainable development in countries around the world, the demand for natural rubber, sustainable fillers, and other green materials is likely to continue to increase in the future.Despite existing literatures providing some overviews of the tire industry's sustainable development from perspectives, such as industry policies, the progress of corporate sustainability, green materials applicable to tires, and future development trends, 1,4 there is a lack of comprehensive summaries regarding the sustainable advancements of the pivotal materials comprising tires from the viewpoints of material structure, fabrication, modification, and progress in industrialization.Based on the latest research and industrial data in the field of tires, this review summarizes the research progress made in the development of three key tire materials (Figure 1), focusing on green rubber, green cords, green additives, and their sustainability, and looks forward to the application prospects of these materials in tire fabrication.These green materials have been derived from bio-renewable resources or waste materials or are obtained through environmentally friendly production technologies, which will play an important role in the sustainable development of the tire manufacturing industry.

RUBBER
Depending on their source, the rubber materials used in tires can be classified as either natural rubber from Hevea brasiliensis trees or synthetic rubber derived from fossil-based raw materials.Currently, both natural and synthetic rubbers are facing resource scarcity issues.The global natural rubber industry is vulnerable to climate effects, pests and diseases, and geopolitical factors.The modern synthetic rubber industry, which consumes significant quantities of nonrenewable energy and resources, is also facing severe challenges.The European Union is considering legislation to increase the proportion of renewable resources that must be included in products to be imported into the EU. 5 Developed countries such as the United States and Japan are also establishing standards to regulate the scope of renewable resources, with tires being among the products that are receiving particular attention. 5urrently, many countries have taken action by seeking alternatives to traditional Hevea rubber while also planning their development direction for bio-based rubber and laying out an industrial framework for the use of bio-based rubber.In terms of natural rubber, the development of second-generation natural rubber, which is mainly based on DR, guayule rubber (GR), and Eucommia ulmoides gum (EUG) as alternative resources to Hevea rubber, has become a research hotspot in recent years.In terms of the bio-based synthetic rubbers, European countries and the USA have focused on the bio-based conversion of traditional rubber monomers, whereas China has mainly used biomass raw materials, including sugars, starch, and cellulose, which are fermented into bio-based monomers such as alcohols and acids that can then be converted directly into bio-based rubbers.[8]

2.1
Natural rubber

Hevea brasiliensis
H. brasiliensis, which is also known as the Pará rubber tree of Brazil, is the only natural rubber source that has seen large-scale industrialization (the abbreviation NR used in this article refers to Hevea rubber).The NR is processed by the latex tapped from the rubber vessels in the Pará rubber tree.The main component of NR is cis-1,4-polyisoprene (Figure 2), which has a high molecular weight. 9Because NR is a natural product, its actual molecular composition is highly complex.Related research has shown that nonrubber components, including proteins, lipids, inorganic salts, and water-soluble substances, in NR participate in the interactions between the cis-1,4-polyisoprene macromolecules, resulting in the presence of a gel phase in NR. 9 This phase gives NR unique and superior properties.When compared with other rubbers, NR has an excellent strain-induced crystallization (SIC) property and excellent metal adhesion and also shows modulus self-recovery after shear processes such as plasticizing and mixing. 10Therefore, the vulcanized rubber shows high mechanical strength, high resilience, and good fatigue resistance, making it suitable for use as tire tread rubber.In the tire industry, demand for NR accounts for more than 80%, and this rubber is irreplaceable.More than 70% of the NR is used in bias tires, passenger/commercial radial tires, and aircraft tires. 11lthough NR has excellent all-round properties, there is still room for development in areas of tire production: (1) Increased production.The main factors that limit NR production are the threat of South American leaf blight, manual labor dependence, high production costs, and the presence of allergenic proteins.To increase production, researchers have studied the principles of ethylene-induced latex production, providing a potential transgenic target for future molecular breeding of rubber trees, 12,13 and the selection of plants with disease-resistant genes to resist leaf blight, 8 thereby increasing production.
(2) Performance improvement.NR contains large numbers of unsaturated double bonds within its molecular chains, thus making it highly reactive, and its aging and oil resistance performances are poor.Accordingly, chemical modification methods, including halogenation modification, epoxidation modification, graft modification, and cyclization modification, can be used to alter NR's molecular composition and structure, thereby improving its performance.Among the resulting materials, epoxidized rubber (ENR) has good application prospects for highperformance tires.Sumitomo Rubber Industries of Japan uses ENR to produce tire treads, which reduced tire rolling resistance by 35% and improved wet grip significantly. 14n addition, NR is commonly used in combination with synthetic rubbers such as styrene-butadiene rubber (SBR), butyl rubber (IIR), and chloroprene rubber in tire manufacturing, and this requires the addition of reinforcing agents, including CB and silica.Currently, progress is being made in research into new reinforcement systems such as graphene and rice husk-derived nanocellulose. 15,16

Dandelion rubber (DR)
DR is mainly found in the latex tubes in the roots of Taraxacum kok-saghyz (TKS), with rubber contents ranging from 5 to 24 wt.%.This rubber can be extracted using methods, such as solvent, dry grinding, wet grinding, enzyme digestion, or acid-base neutralization, to obtain solid DR. [17][18][19] Because of the wider distribution and its advantages, which include a short growth cycle, high yield, unrestricted planting conditions, and climate requirements, and the absence of allergenic proteins in the extracted rubber, DR has the potential to replace NR in tire manufacturing.DR has a cis-polyisoprene structure, and its relative molecular weight is similar to that of NR.Studies suggest that the nonrubber components of DR have a high-order physical polymer structure (Figure 3), 20 and these nonrub-ber components are more prone to melting or softening at high temperatures, thus inhibiting the sulfur crosslinking process, making DR less thermally stable, and giving the material a lower stress-induced crystallization degree.Researchers have developed a method for the continuous and efficient extraction of DR and inulin from TKS using the principle of water-oil separation, and the extracted liquid is separated from the DR by water vapor distillation. 21s part of the trend for green sustainable development, the US Excellence Program (2007-2011-2020) and the EU's Pearl Program (2008-2012) have been implemented successively with the aim of using DR as a substitute for NR to meet local demand for natural rubber materials and reduce dependence on NR. 5 The Continental Group's Taraxagum project aims to achieve industrial application of DR in tires by studying the entire process of DR breeding, planting, harvesting, root processing, and rubber extraction.The Urban Taraxagum bicycle tire, which was the first to be made from DR, was launched in 2019 and has been produced at the Korbach tire factory in Germany for three years. 8In 2014, Shandong Linglong exhibited three DR tire samples at the International Rubber Conference. 22Shandong Linglong behind these tires has now built a 1-t solvent extraction pilot plant capable of stable DR production. 23][27][28][29][30] Researchers have shown confidence in ridge cropping as a means to boost yield, and future breeding research is suggested to concentrate on optimizing DR cultivation production and management. 31Regarding the molecular synthesis mechanism, a comprehensive understanding of the specific functions and interactions of individual genes or proteins in DR has not yet been fully achieved.Therefore, further in-depth studies are needed to explore the molecular characteristics and mechanisms of interaction with other factors, as well as the signaling pathways related to rubber synthesis in latex cells. 32The existing solid DR extraction process faces significant challenges, primarily due to high extraction cost and low purity, which have hindered its industrialization and ability to meet market demands. 33Although the preparation of DR through latex is a viable alternative, it is necessary to address the problems from equipment, storage, and transportation concerns to make the process more practical and feasible. 34n light of the global emphasis on nonfossil resources and sustainable practices, DR is expected to gain increasing attention and promotion in the future.It holds great potential to contribute to greening initiatives and meet the growing demand for eco-friendly alternatives.

Guayule rubber (GR)
GR is a rubber-producing plant from the Asteraceae family that produces latex within its thin-walled bark cells, with a rubber content of 8-12 wt.%.The basic cis-1,4-polyisoprene composition of GR is similar to that of NR, but the protein, the low-molecular-weight substances, and the impurities vary, which results in performance variations.GR is resistant to pests and diseases, can be grown in relatively dry soil, can be harvested mechanically, and has the potential to replace traditional H. brasiliensis. 35esearchers found through wide-angle X-ray diffraction testing that cross-linked GR has larger oriented amorphous components and larger parallel-to-stretchingdirection microcrystals than both NR and DR, which result in GR showing larger crystal volumes and smaller orientation fluctuations. 36Therefore, GR has superior SIC characteristics relative to NR (Figure 4A-C).GR has mechanical properties similar to those of NR and, like DR, has a short growth cycle, is not limited by regional or climatic conditions, and does not contain allergenic proteins, thus demonstrating great potential to replace current passenger car tire materials.In 2015, Bridgestone established a GR tire research and development (R&D) center and a GR extraction factory before successfully producing the first 100% GR tire.The rolling resistance coefficient of GR tires is low, which can help to reduce fuel consumption, and studies have shown that the energy consumption per tire life cycle for GR tires is 13.7 GJ/tire, which is significantly lower than the 16.4 GJ/tire figure for traditional tires. 37ooper Tire Company in the United States has launched a concept tire using GR as the raw material for multiple components.The test results show that the performance of the GR concept tire is at the same level of a standard tire. 38Future avenues for GR tire improvement mainly include (1) further rolling resistance coefficient reduction through structural design, (2) improving the rubber yield by using other agricultural methods, for example, drip irrigation and flood irrigation, (3) the optimization of the use of by-products, and (4) further research into its carbon sequestration capacity.

2.1.4
Eucommia ulmoides gum (EUG) EUG mainly exists in various parts of the E. ulmoides Oliv, including its roots, bark, leaves, and seed coat. 39The gum content in the leaves is 1%-3%, that in the seeds is 10%-12%, and that in the bark is 6%-10%.EUG's molecular chain structure mainly consists of trans-1,4-polyisoprene, which is an isomer of natural rubber.The three main characteristics of the molecular chain are chain flexibility, unsaturated double bonds, and a trans-chain structure (Figure 5A,B). 40,41Therefore, when compared with NR, EUG offers three unique properties: a dual rubber-plastic nature, ease of chemical modification, and ease of crystallization. 42Initially, EUG was used as a substitute for plastics until the 1980s, when Yan and others successfully developed the trans-polyisoprene sulfide rubber production process, which transformed traditional hard EUG into an elastic material. 43Since then, EUG has become an optional material for use in tire manufacturing.EUG can be used as a raw material for tire production mainly because: (1) EUG offers the flexibility of a large molecular chain, good elasticity, and self-adhesiveness; (2) the main chain molecule contains a double bond structure, which can be vulcanized using either a sulfur vulcanization system or a peroxide vulcanization system; (3) the large molecular chain has a trans structure, which shows obvious regularity, reduces the internal friction force between the molecules, and reduces heat generation during compression; (4) the high regularity and flexibility of EUG mean that it can easily form small microcrystals after tire forming, which can absorb energy under dynamic loading and improve the tire's fatigue resistance and wear resistance; (5) EUG has a thermodynamic compatibility index similar to that of NR and the materials can thus be used together. 44Studies have shown that the internal stress caused by the large numbers of cross-linking points inside EUG transforms it into a random elastic network state, which hinders the crystallization process and is the main reason for the reduced tire rolling resistance. 45EUG is prone to crystallization at room temperature and cannot be made into tires directly.At present, researchers usually blend EUG with NR and use nanofillers such as CB and carbon nanotubes (CNTs) to reinforce the material.The resulting composite material shows high tensile strength and tear strength and offers good all-round performance.After EUG addition, the Deutsche Industrie Normen (DIN) abrader of the composite material is reduced significantly, and it has better wear resistance.As the EUG content increases, the heat generated by compression decreases in tandem. 7Therefore, the addition of a certain proportion of EUG to tires will lead to desired characteristics that include high elasticity, low heat generation, wear resistance, tear resistance, and puncture resistance.
China is one of the native sources of E. ulmoides Oliv, and its EUG resources are abundant.In 2017, China applied EUG in the tread rubber of aviation tires and all-steel radial tires successfully. 46,47Because the gum content of EUG is relatively low, it will be necessary to solve three major problems, specifically, biosynthesis regulation, the high cost of the material, and crystallization at room temperature, to achieve wider application of EUG in tire manufacturing in future. 41In the future, it is preferable to use green and bio-based solvents for the extraction of EUG.Importantly, the refined characterization of the molecular structure of EUG and the study on certain aspects of mechanism for EUG materials should be developed.

Traditional commercial rubber based on bio-based monomers
In recent years, developed countries in Europe and the USA have focused on the bio-based transformation of traditional rubber synthesis monomers. 48Companies, such as Genencor, Amyris, Lanxess, and Goodyear, have obtained bio-based monomers through biomass fermentation, including isoprene-based phosphoric acid esters, ethanol, propanol, isobutanol, and butanediol; these monomers were then converted further into traditional monomers, including isoprene, ethylene, propylene, isobutylene, and butadiene, and were synthesized into bio-based traditional rubbers, including polyisoprene rubber, ethylene-propylene rubber, IIR, and SBR; finally, the materials were applied to prepare corresponding bio-based rubber tires. 49myris has achieved commercial-scale production of a 15-carbon molecule (farnesene) that can also be used to convert sugars into isoprene, [50][51][52][53] which is then used to synthesize polyisoprene rubber (Figure 6A).The production principle of bio-based ethylene-propylene-diene monomer (EPDM) begins by fermenting renewable resources such as sugarcane to obtain ethanol and acetone; then, the ethanol is dehydrated to obtain ethylene, and the acetone is hydrogenated to obtain isopropanol, which is then dehydrated by catalysis to obtain propylene; finally, the ethylene and propylene are synthesized into EPDM in a traditional manner (Figure 6B). 54In addition, the industrial production of bio-based IIR (Figure 6C) and biobased SBR (Figure 6D) is also being realized gradually. 55,56he researchers explored the application of a new biobased polybutadiene liquid rubber PM4 (LPB PM4) in the winter tread formula of passenger cars.The results showed that when 10 parts of LPB PM4 replaced the petroleumbased rubber in the original formula, the rolling resistance of the finished tire in the low-temperature environment was reduced, and the ground grip and wet skid resistance were effectively improved. 57lthough the cost of obtaining traditional rubber through the use of bio-based monomers is higher than traditional methods and the industrial production process is still immature, this method also offers additional advantages.For example, the bio-based β-laurene in laurel or citronella can replace butadiene to synthesize solution-polymerized styrene-myrcene-butadiene rubber (S-SMBR), which can improve both CB dispersibility and the wet skid resistance of the rubber by introducing a nonpolar sub-isopropyl long side chain without affecting the rubber's rolling resistance. 58However, the large-scale production of bio-based olefins remains challenging, and in the future, the development of new technologies and the optimization of the existing technologies will be used to improve production yields and reduce costs in efforts to achieve large-scale bio-based rubber synthesis and fabricate bio-based tires.

Bio-based itaconate elastomers
Bio-based polyester elastomers are varieties of polar rubber synthesized from bio-based monomers.Among these materials, the molecular weight of the itaconate elastomer obtained by using itaconic acid as a monomer is greater than 200 000, and it has excellent physical-mechanical properties and dynamic mechanical properties.This elastomer interacts with polar fillers such as silica through hydrogen bonding; the silica is dispersed well within the rubber and can thus meet the material requirements for tire manufacture.The itaconic acid monomer can be obtained through bio-fermentation, and it has achieved large-scale production. 59he itaconic acid molecule contains two carboxyl groups and one C=C double bond and cannot construct a full carbon chain elastomer via free radical polymerization.Therefore, monohydric alcohol is used to esterify the itaconic acid to endcap it, producing itaconic acid diisopropyl ester, which is then copolymerized with conjugated dienes (e.g., isoprene and butadiene) to obtain bio-based itaconate elastomer (Figure 7A,B). 60Tires prepared using the poly(dibutyl itaconate-butadiene) (PDBIB)/silica composite has a rolling resistance coefficient of 7.7 kg/t, which is lower than that of traditional tire materials. 61oly(diethyl itaconate-co-butyl acrylate-co-ethyl acrylateco-glycidyl methacrylate) (PDEBEG) (Figure 7C) has excellent mechanical properties, and its tensile strength and elongation at breaking point can reach 14.5 MPa and 305%, respectively.After immersion in Standard Oil 3# for ASTM standard oil for 72 h at 150 • C, the mechanical properties of the PDEBEG/CB composite were maintained at 50%-80% of their original values, which is equivalent to the performance of the commercially available acrylic rubber AR72LS. 62n 2020, China established the world's first 1000-t-level itaconate elastomer demonstration production line.The mechanical properties of the itaconate elastomer produced on this line are similar to those of traditional butadiene rubber, and its dynamic and static mechanical properties and wear resistance are good.The rubber is suitable for low rolling resistance tire tread applications, and it is expected to become a supplement to existing petroleum-based rubber tire tread materials.When compared with traditional petroleum-based synthetic rubber, it is estimated that each ton of itaconate elastomer used in tire production can reduce carbon emissions by 1.44 t, which can provide positive support for sustainable development in the rubber industry. 63

2.2.3
Bio-based thermoplastic elastomers (TPEs) Thermoplastic elastomers (TPEs) are composed of soft segments with a relatively low glass transition temperature (T g ) and hard segments with a T g or a melting temperature (T m ) that is higher than room temperature, allowing these materials to display rubberlike elasticity at room temperature but to become plasticized and moldable at higher temperatures. 64Depending on the types of polymers used to form the hard and soft segments, TPEs can be classified as materials, such as thermoplastic polyester elastomers (TPEEs), polyurethane elastomers (TPUs), and polyamide elastomers (TPAEs).The sulfur-cross-linked rubber that is used in traditional tires is difficult to recycle and the nondegradable microplastics, and other chemicals produced by tire wear can cause environmental damage.Therefore, bio-based TPEs with excellent biodegradability can be prepared from renewable resources to replace crosslinked rubber in tire manufacturing and thus reduce the environmental impact of vulcanized rubber waste. 65,66PEEs use polyester as their hard segment and polyether or polyester as their soft segment, and they exhibit high strength, high elasticity, excellent dynamic properties, and high creep resistance.Existing bio-based TPEEs can mainly be divided into bio-based hard segment TPEEs and fully bio-based TPEEs.Researchers have fabricated rubber-based biodegradable polymers with low T g using β-methyl-δ-valerolactone (βMδVL) and propylene carbonate. 67By controlling the molar mass, the structure, and the end-block segments, materials with mechanical properties comparable to those of commercial styrene block copolymers can be obtained.][70] TPUs use isocyanates as their hard segment and polyester or polyether as their soft segment.Researchers have prepared castor oil-based TPUs with branched network structures using castor oil, poly(tetrahydrofuran) polyol, and poly(dimethyl siloxane) as the soft segment (Figure 8A,B).The maximum bio-based content can reach 14.1 wt.%, and the synthesized TPU demonstrated excellent rebound resilience. 71However, the cost of this TPU is high, and its mechanical properties must be improved.Progress has also been made in the use of bio-based fatty acid diisocyanates obtained from fatty acids, oleic acid, carbohydrates, or amino acids to prepare bio-based TPUs. 72,73enerally speaking, the mechanical properties of biobased hard segment TPUs are good, but they are expensive and difficult to obtain, while the mechanical properties of bio-based soft segment TPUs require further improvement.In the future, the production scale of bio-based soft segment TPUs should be increased to improve the performance of these materials with the aim of achieving their use in tire formulations.
TPAEs are composed of polyamide hard segments and polyether soft segments and offer excellent material properties, including low density, good wear resistance, good chemical resistance, good thermal stability, high tensile strength, and high impact strength at low temperature. 74he bio-based polyamide hard segments are obtained by polymerizing monomers, such as bio-based dicarboxylic acids, diamines, and bio-based lactams.Existing bio-based dicarboxylic acids include succinic acid, adipic acid, and sebacic acid, [75][76][77] of which succinic acid and adipic acid have been industrialized.9][80] Lactams can also be obtained from bio-based compounds such as lysine and 5-hydroxymethylfurfural. 81,82 Some companies have already achieved the commercial production of bio-based TPAEs, including Arkema, which has produced bio-based TPAEs (Pebax) by copolymerizing bio-based polyamide 11 obtained from castor oil with polyether.The resulting TPAE shows good elasticity and low-temperature resistance, is lightweight, and has been commercialized. 83,84n addition, researchers have synthesized a new type of long-chain rigid segment/bio-based soft segment TPAE via a two-step melt condensation reaction process using low-molecular-weight oligomers composed of long-chain polyamide 1212 (PA1212) and 100% renewable polytrimethylene glycol (PPDO) (Figure 8C).This new type of bio-based TPAE is lightweight as well and has good mechanical properties, including low-temperature elasticity, high rebound elasticity, and good thermal stability. 85In general, existing bio-based TPAEs have high production costs and present high technical barriers, and the commercialization of these TPAEs will require production process optimization, reduction of the technical barriers, and overall cost reduction.
2][73] Many bio-based TPEs that are still at the laboratory research stage have demonstrated outstanding physical and mechanical properties, with many characteristics similar to those of commercially available traditional rubber, and these TPEs are expected to be used in the tire industry in future.Furthermore, some companies have already realized large-scale production of bio-based TPEs using renewable resources.For example, Arkema produces bio-based TPEs with a bio-based content of more than 90%, 86 and DSM produces bio-based TPEs with a bio-based content of 50%. 87However, the production costs of these bio-based TPEs are high, the technical threshold for production is also high, the production yield is low, and the mechanical properties of those TPE materials still need to be improved, which limits their applications.There is tremendous potential for the application of bio-based TPEs in the tire industry, but there are still some technical challenges and the production costs are high.

TIRE CORDS
As the skeleton materials of tires, tire cords play an essential role in supporting the strength of tires, bearing the load of the vehicle on the tires, and restricting the deformation of the tires during use.Depending on the arrangement direction used for the tire cords, tires can be divided into bias-ply tires and radial-ply tires. 88Radial-ply tires have demonstrated good wear resistance and can meet highspeed driving requirements for automobiles.Currently, all passenger car tires are radialized.Traditional tire cord configurations include steel wire tire cords, polyester tire cords, polyamide tire cords, adhesive tire cords, and aramid tire cords. 75Steel wire cords have the highest production and consumption figures, and polyester cords are ranked second in terms of consumption.The increase in the rate of the radialization of tires has led to a slight reduction in the production of nylon fabrics, whereas the application of adhesive tire cords and aramid/nylon, aramid/polyester, and other mixed tire cords has increased. 89lthough the production and application techniques for traditional tire cords are mature, most of the materials are based on nonrenewable polymers and are heavily reliant on petrochemical resources.Although adhesive fibers are regenerated cellulose fibers, the production process for these fibers causes severe pollution.With the increasing scarcity of petrochemical resources, it is inevitable that renewable and environmentally friendly green tire cord materials are being sought.The development of new green fiber-based tire cords that perform well and are easy to produce will be highly significant for the green and sustainable development of the tire industry.To date, companies, including Teijin Fibers, Michelin, Continental, and Goodyear, have begun the use of recycled PET (r-PET) tire cords in tire manufacturing. 90,91

Lyocell fiber
Lyocell fiber is a type of cellulose fiber made by spinning cellulose pulp dissolved in a water solution of Nmethylmorpholine-N-oxide (NMMO).This fiber belongs to the same category of fibers as viscose fibers. 92Lyocell fibers offer high strength and a high modulus, especially under wet conditions, along with good dimensional stability and heat resistance.The lyocell fiber production process is environmentally friendly.However, although viscose and cotton fibers are pleated and flattened, respectively, the lyocell fibers are rounded. 93Lyocell fibers have a unique skin-core structure (Figure 9A).The skin layer is composed of amorphous cellulose molecules, and the core layer is composed of highly oriented, parallel-aligned, large-scale primary fibers.Because the fibril is connected by the cellulose molecules, the bonding force is weak, and thus the radial strength of this fiber is low.Therefore, under the action of external forces such as mechanical friction in the wet state, the fibrils will be separated from the fiber surfaces, resulting in fibrillation.This fibrillation of lyocell fibers has been used in both battery separators and filter membranes, 94,95 but the damage-caused strength decrease of the lyocell fibers by primary fibrillation must be reduced to realize the application of these fibers in tire cords.Researchers conducted a study on the impact of various draw ratios on the multilayered structure and mechanical properties of lyocell fibers.The results (Figure 9B-D) revealed that increasing the draw ratio within a specific range enhances the mechanical properties of the fibers.However, when excessive drawing occurs, fiber slip actually leads to the decrease of the modulus and yield strength. 96yocell fiber products, including TENCEL from Austria's Lenzing AG, Newcel from Akzo Nobel, and lyocell from the Lanzhou Chemical Company, are all based on the NMMO/H 2 O dissolution system.In 2004, Kumho Tires first used lyocell as a new type of tire reinforcement material in its ECSTA ultrahigh-performance tires.In 2007, the first batch of tires using lyocell tire cords was launched in Europe.In 2005, Hyosung Advanced Materials developed an environmentally friendly lyocell fiber suitable for use in tire cords and mechanical rubber products with a tensile strength of up to 6.0 g/day. 97Since 2007, Kolon Industries has also been developing a series of lyocell-based tire cord products, including several types of long lyocell filaments that have excellent crystallization properties after wet and dry processing under hightemperature and high-pressure conditions, along with tire cords made from these fibers that show good dimensional stability. 98,99n summary, the lyocell fiber is an environmentally friendly fiber with excellent all-round performance.Use of its unique features along with the existing spinning technology to produce green cellulose fibers for tire cords will have significant practical application value.If the mechanical properties of lyocell fibers can be improved further, then lyocell tire cords may replace viscose tire cords or even other tire cord types to enable the development of tires with improved performance.

Recycled PET fiber
Currently, more than 90% of r-PET fibers are sourced from r-PET bottles (Figure 10A). 100r-PET fibers can also be obtained by classifying and recycling PET waste fabric (Figure 10B), 101,102 but the resulting fibers may not meet the mechanical performance requirements of tire cord materials.PET bottles are typically physically processed into fibers or other products via melt extrusion.Alternatively, waste PET bottles can initially be depolymerized into monomers, then polymerized into r-PET, and finally melt-spun into r-PET filaments.Researchers used five types of waste PET materials to prepare r-PET fibers (Figure 10C) and found that PET-B is the most suitable for melt spinning, and the resulting fibers have ideal toughness, excellent elongation at break, and excellent fiber fineness. 103Several companies have already developed polyester tire cord fabrics made from waste PET bottles, and related tire products have also been launched in the market. 102Teijin's Eco Circle fiber made from r-PET is the world's first recycled polyester fiber to be used in

Bio-based polyamide fiber
The raw materials used for traditional polyamide production are petroleum-based and are obtained through the condensation of dicarboxylic acid/diamine monomers and the condensation/polymerization of amino acids to form lactams.To reduce the consumption of petrochemical resources, environmentally friendly biomass is first converted into bio-based monomers through a biotechnologybased approach, and these monomers are then polymerized to produce polyamide, thus providing a path for the development of environmentally friendly and sustainable polyamide tire cords.In 2022, Toray Industries of Japan announced the development of a technology that uses sugar extracted from inedible biomass such as crop straw to produce 100% bio-based adipic acid, which can then be used as a raw material to produce nylon 66.The company combined microbial fermentation technology with separation membrane chemical purification technology to convert crop residues and other inedible plant resources into sugars and, for the first time, discovered that certain microorganisms can produce adipic acid intermediates from these sugars.Toray used genetic engineering technology to increase the production efficiency of these microorganisms and realized a more than 1000fold increase in the yield of the adipic acid intermediates synthesized by these microorganisms since their initial discovery. 104io-based PA56 is synthesized by the polymerization of pentanediamine and adipic acid, which can both be prepared using renewable resources (Figure 11A-C).Researchers have found that semi-bio-based PA56 based on bio-based pentanediamine and petroleum-based adipic acid has thermal, mechanical, and processing properties comparable to those of petroleum-based PA66.The static room temperature and high-temperature performance, dynamic fatigue performance, heat resistance, and adhesion performance of this impregnated bio-based PA56 cord fabric were comparable to those of the impregnated petroleum-based PA66 cord fabric, 105 and the highspeed durability, noise reduction, and driving comfort are superior to petroleum-based PA66 cord fabric. 106The various properties of PA56 meet the production requirements for cord fabric, and PA56 can thus be used as a skeleton material for tire crown plies.At present, companies, including Toray Group of Japan, Hyosung Corporation of South Korea, and BASF of Germany, are all at the laboratory development stage for bio-based PA56 and have not achieved large-scale production to date.Kasei Corporation of China has conducted comprehensive and systematic research into and development of bio-based 1,5-pentanediamine and PA56 materials, achieving mass production on the scale of thousands of tons.Current research into the application of bio-based PA56 is mainly focused on improving the material's moisture absorption and dyeing properties and other performance aspects.To achieve the goal of replacing traditional tire cords with PA56-based cords, the production of bio-based PA56 still requires further upscaling and stabilization. 107

Hybrid curtain fabric
The term hybrid curtain fabric refers to a new type of fabric that is produced by twisting two different materials together in a specific manner to realize a fabric that has entirely new properties.Hybrid curtain fabric can combine the advantages of multiple materials and also overcome the performance shortcomings of certain single materials. 108Researchers have studied the physical and mechanical properties of several differently structured polyester/aromatic polyamide composite curtain fabrics and have found that a higher proportion of aromatic polyamide produced greater toughness 109 ; in addition, higher PET density produced a greater shrinkage force, and when the twist was higher, the density of the aromatic polyamide was lower, the effect of temperature on the fiber is more obvious.Wang invented a composite curtain fabric composed of aromatic polyamide fibers as the core yarn with polyamide and lyocell fibers forming the covering yarn. 110The renewable lyocell fibers partially replace the aromatic polyamide fibers, thus reducing the dependence of the curtain fabric on the use of petrochemical resources.Mahdavipou and others studied the thermodynamic properties of PA66/PET hybrid curtain fabrics, 111 and their experimental results showed that the residual shrinkage rate and shrinkage force will increase with increasing load at any heat treatment temperature.To date, PET/nylon hybrid curtain fabrics, aromatic polyamide/nylon or polyester hybrid curtain fabrics, and other fabrics have been studied and applied as skeleton materials for the crown belts of radial tires.In the future, hybrid curtain fabrics fabricated from r-PET fibers, lyocell fibers, and high-performance synthetic fibers may also be considered.These fabrics can satisfy tire material performance requirements while reducing both the use of chemically synthesized fibers and the reliance on petrochemical resources.

OTHER ADDITIVES
The formulation of the tire tread rubber is the most important factor in determining the tire's lifespan and performance. 1124][115][116] Typical tire formulation ingredients include rubber, fillers, anti-degradation agents, adhesion promoters, activators, vulcanization systems, processing oils, and special additives.In addition to the rubber materials described above, the other additives are typically nonrenewable and are dependent on the use of petrochemical resources.These additives can also produce pollutants.Therefore, it is essential to use bio-based/environmentally friendly raw materials to prepare these traditional additives while also meeting tire performance requirements.

Fillers
Traditional fillers such as CB affect the ecological balance and are detrimental to daily life. 117Use of agricultural waste, including cellulose, clay, plant fibers, biomass, rice husks, straw, and lignin, to prepare new types of fillers can minimize the damage to the environment.Biologically derived fillers (including fillers based on cellulose, lignin, cellulose nanocrystals, cellulose nanofibers, starch, eggshells, and pistachio shells) have been explored for use as partial substitutes for CB to obtain material properties, such as low rolling resistance, improved durability, lightweight, and ease of manufacture. 118

Carbon black (CB)
CB is one of the most commonly used fillers in tire formulations and can enhance the wear resistance, heat resistance, and oil resistance of the rubber.However, CB is produced by burning crude oil and is dependent on petrochemical resources.From the perspective of green tire tread development, current trends involve the use of recycled CB rather than industrial CB and the development of new green fillers to replace CB.Waste tires can be pyrolyzed in an inert atmosphere to produce black powdered pyrolysis carbon, which can then be processed and used as a reinforcing agent and filler in rubber products. 119esearchers have found that the partial replacement of the reinforcing CB N550 with an appropriate amount of pyrolysis carbon reduced the tire sidewall's rubber wear factor and cost while maintaining similar physical properties to the conventional tire.However, when the pyrolysis carbon content was increased, the wear resistance and the utility performance both decreased, indicating that the appropriate amount of pyrolysis carbon should be used according to specific tire performance requirements. 120,121esearchers have also compared pyrolysis carbon with CB N220, N330, N660, and N774.Because of the wide particle size distribution and the high ash content of pyrolysis carbon, the addition of this carbon to natural rubber compounds can effectively improve the tensile and tear strength of the rubber, but the material's wear resistance and flex fatigue resistance are both poor. 122Goodyear's 90% sustainable materials demonstration tire uses four different types of CB made from raw materials, including methane, carbon dioxide, vegetable oil, and scrap tire pyrolysis oil. 123This technique can reduce carbon emissions and carbon cycling while also achieving the use of bio-based carbons and maintaining good performance. 115owever, the reinforcement performance of pyrolytic carbon without deashing treatment cannot meet the needs of commercialization. 124,125Therefore, pyrolysis carbon is mainly used as a general filler for the production of lowquality rubber and plastics at present, and issues such as process costs must still be considered to enable its application to tires.Besides, biochar has been used in research as a full or partial replacement of traditional CB in rubber composites to varying degrees of success.Researchers have created and characterized biochar from two different low-ash nutshell feedstocks (hazelnut and walnut) and two high-silica grain husk feedstocks (oat and rice).The results demonstrated that the nut shell biochars provided high reinforcement properties that met or exceeded those of CB and that slurry-based activation of biochar improved mechanical properties in the filled composites. 126

Silica
Silica, which exists in abundance in nature, has considerable potential for use as a replacement for CB.
Michelin was the first tire maker to use silica widely; silica gives tires low-rolling resistance for improved fuel efficiency and improved grip for performance. 127One green source of SiO 2 is agricultural waste, specifically rice husk ash (Figure 12A). 128Sun studied the effects of silica and its surface modification on the physical properties of tire tread formulations in detail. 129They developed amino-functionalized solution-polymerized SBR composites (FSSBRs) that aid in the nanoscale dispersion of silica in SSBR (Figure 12B).The static and dynamic mechanical properties of the SSBR-SiO 2 composites are both strong (Figure 12C), and the composites exhibit excellent wet skid resistance, which is a prerequisite for application to green tires.Some researchers partially replaced CB with highly dispersed silica (HDS) nanofillers extracted from agricultural waste products such as rice husk ash and found that this approach reduced the carbon footprint of the material significantly.In the tread rubber formulation used for passenger car tires, HDS from rice husk ash has a lower Mooney viscosity than conventional silica, and it also exhibits better dispersion.Furthermore, HDS from rice husk ash shows a higher enhancement factor (M300/M100), higher tensile strength, and greater elongation at the breaking point. 130

Other fillers
Graphite, CNTs, and clay extracted from renewable resources also have the potential to replace traditional CB and enhance the properties of the tire tread rubber.
Weili investigated the effects of corn flour graphene (CGE) obtained from renewable resources on the mechanical and physical properties of natural rubber (Figure 13A). 131rya studied the synergistic enhancement effects of nanofillers such as silicon carbide nanofibers, CNTs, and graphite fibers (partially replacing CB) in tire tread compositions through a latex-phase mixing process, finding that CB and nanofibers formed a dual hybrid reinforcement system in the NR matrix (Figure 13B). 132Lots of clay/rubber nanocomposites have been used in tire treads, base, and liner components. 133Bao incorporated organomontmorillonite clay into model tire tread formulations; the results showed that organo-montmorillonite filler provided effective reinforcement in the elastomer matrix, and tread compounds using higher organoclay loadings displayed preferred ice traction, wet traction, and dry handling. 134The soluble proteins allow the formation of a much stronger interaction between polar momtmorillonite platelets and the nonpolar rubber network (Figure 13C). 135hese findings indicate that clay has more potential for tire reinforcement application.

Synthetic oils
In tire formulations, synthetic oils, including aromatic oils, cycloalkane oils, and paraffin oils, are used as processing aids to enhance the dispersion of the fillers in the polymer matrix. 136However, these synthetic oils, which are derived from petroleum, have carcinogenic toxicity.Biodiesels obtained from fats from plant or animal sources also have the potential to be used as rubber plasticizers and lubricants, with the prominent advantages of being fabricated from renewable raw materials and having good biodegradability. 1377][138] Some industrially important plant oils (Figure 14A,B) have the potential to be bio-based plasticizers for use in tire tread rubber.Goodyear has demonstrated the use of soybean oil as a plasticizer in its prototype tires, thus reducing the company's dependence on petroleum products.Eight of its product lines and some of its racing tires already use soybean oil as an additive. 123Liquid natural rubber (LNR) has been tested as a bio-based renewable processing aid. 139The tensile properties of NR composites filled with silica were improved by adding LNR up to 10% by weight into NR latex, but processability measurements and comparisons with traditional plasticizers were not reported. 140

Promoters and activators
In tire formulations, promoters and activators are added to activate the curing agent, accelerate the cross-linking reaction, reduce the vulcanization time, and reduce the vulcanization temperature.Traditional promoters include sulfenamide, thiazole, sulfonylurea, guanidine, thiourea, and dithiocarbamate compounds. 141One major drawback of these chemical additives is their carcinogenicity, and some of these compounds may release toxic substances such as aniline at high temperatures.Use of alkyl amines with long alkyl chains can avoid this hazard.Researchers have investigated the potential of alkyl amine promoters including hexylamine (HEX), octadecylamine (OCT), cyclohexylamine (CYC), and dicyclohexylamine (DIC) for use in natural rubber-based tires.The results showed that these alkyl amine promoters increased the rate constant of the primary silanization reaction and improved the curing performance, whereas the shielding effect of the long alkyl chains promoted both hydrophobicity and interfacial compatibility, 142 thus demonstrating the potential of these materials to replace traditional promoters.Zinc oxide (Figure 15A) and stearic acid are commonly used as activator systems in tire formulations. 143To address ongoing global environmental issues, these heavy metal compounds are being replaced with zinc-free processing aids (ZFA).Kim synthesized a ZFA (Figure 15B). 144ddition of ZFA to an SBR rubber matrix improved the material's modulus, tensile strength, elongation at breaking point, and dynamic performance significantly.Tires made from these ZFA/SBR composites showed significant improvements in dry and wet braking, rolling resistance, and high-speed durability, thus demonstrating the effectiveness of the ZFAs in rubber composite production.Wu used environmentally friendly amine-functionalized carbon dots (CDs) as green activators, which reduced the amount of ZnO that was used significantly. 145The addition of CDs also produced a major reduction in the activation energy of the sulfur cross-linking reaction and promoted the formation of sulfur-based networks in the SBR composites, and when 60 wt.% of the ZnO was replaced with CDs, the cross-linking rate increased significantly without affecting the mechanical properties or the cross-linking density of the samples.This study thus provides a green use for CDs and is expected to reduce ZnO pollution in the rubber industry.

CONCLUSION AND PERSPECTIVES
Due to the increasingly severe environmental issues caused by the production of harmful products, byproducts, and waste materials from applications based on synthetic materials, sustainable development has become the development goal for many industries, particularly for responsible sectors such as the tire industry with high carbon emissions.The development of sustainable tires has become the objective for numerous tire companies worldwide in recent years.Several measures to achieve sustainability include reducing raw material consumption and tire weight, improving tire performance, enhancing energy efficiency, reducing carbon footprint, increasing the use of natural and renewable resources, and decreasing reliance on petroleum resources.This article reviews the current state of sustainable research on key tire materials, such as rubber, cord fabrics, and additives, and analyzes the challenges and prospects in terms of material struc-ture, performance, and commercialization.The current development of sustainable materials in tires primarily involves the use of renewable materials or environmentally friendly modifications to partially or fully replace traditional materials.At present, the development of rubber and cord fabric materials for tires is progressing rapidly.Many natural rubbers or bio-based synthetic rubbers have been used in tire manufacturing, and cord lines based on waste polyester bottles and semi-bio-based PA56 cord lines have been used for tire reinforcement.In addition, more potentially promising rubber and cord fabric materials are still under development, with some conceptual products already emerging (Table 1).Fillers from nonpetroleum sources (such as silica and clay) and biological sources (such as rice husk ash, cornflower starch, and bagasse), as well as processing aids from nonpetroleum sources, such as soybean oil, orange oil, rapeseed oil, sunflower oil, and Azadirachta oil, are also under the studies for applications in tires.In our view, despite the many achievements mentioned above, there are still many challenges in the field of sustainable materials for tires as follows: (1) Although natural rubbers, such as DR, GR, and EUG, have been developed and produced on a large scale, the natural rubber represented by Hevea rubber (the traditional NR) stands out due to its remarkable attributes, including high elasticity, rebound resilience, dynamic performance, elevated tensile strength, excellent wear resistance, low electrical conductivity, and exceptional heat dispersion.With its outstanding comprehensive properties and abundant availability, NR is poised to maintain its dominant position in the tire market in the future.DR exhibits a lower degree of stress-induced crystallization and inferior mechanical strength compared to NR, necessitating improvements in latex collection processes to elevate latex quality.GR and NR have the same chemical structure and similar mechanical properties, making it the most promising alternative to NR in tire  production.EUG tends to crystallize at ambient temperatures, and thus, it is unsuitable for direct tire fabrication.However, it is commonly used in combination with NR, cis-butadiene rubber, and other rubbers as it could impart features like high elasticity, low heat generation, wear resistance, tear resistance, and puncture resistance to tires.In the future, the explo-ration of new rubber tree varieties and the pursuit of other yield-enhancing measures will remain the focal point, ensuring the continuity of contemporary rubber production while safeguarding and enriching the supply of natural rubber for the future, underscoring the ongoing developmental emphasis within this field.
(2) Bio-based synthetic rubbers currently lack the capacity to match the key performance attributes of NR.Polybutadiene based on bio-alkenes is anticipated to replace the current petroleum-based polybutadiene, but the challenge of scaling up the production of bioalkenes must be addressed in the future.The mechanical properties of the itaconate elastomer in the biobased polyester elastomer are similar to those of traditional styrene-butadiene, exhibiting good dynamic and static mechanical properties as well as wear resistance.This makes it suitable for the treads having low rolling resistance, serving as a potential complement to the existing petroleum-based rubber tread materials.Bio-based TPEs pose high technical barriers and yield low production rates, whereas their mechanical properties still require improvement.Much of the related research is still in the laboratory stage.Nevertheless, with the future optimization of the process and enhanced performance leading to large-scale industrial production, they also hold the potential to supplement existing rubber tread materials.
(3) The reliance on petrochemical resources and the emission of toxic substances during the production process of traditional tire cord materials has necessitated the pursuit of renewable and environmentally friendly green tire cord materials.Although new types of green fibers, such as lyocell fiber, r-PET fiber, and biobased PA56, have been developed and applied in tire manufacturing, overall, there is still a certain gap in mechanical performance between these existing green cords and traditional cords, and the green cords usually need to be combined with other types to achieve reinforcement effects.In the future, it is necessary to further develop new green fiber tire cords with excellent performance and easy production.(4) In the field of fillers and additives, although environmentally friendly substitutes, such as silicon carbide nanofibers, carbon nanotubes (CNTs), graphite fibers, and biomass-derived silica, have been developed to replace conventional CB, and bio-oils, such as tung oil, soybean oil, cashew nut oil, and castor oil, can replace aromatic oils, cycloalkane oils, and paraffin oils for rubber plasticization, the variety of alternatives remains relatively limited.Additional additives such as long-chain alkylamines and zinc-free activators are still under research and development.Furthermore, the utilization of recycled CB derived from waste tires, due to its complex composition and limited reinforcement properties, has not yet achieved widespread application.Further advancements in tire pyrolysis process equipment and technology are required to enhance the yield and quality of recycled CB.
(5) The inability of waste tires to undergo biodegradation is one of the primary factors contributing to environmental pollution and the expansion of landfill areas.By simply adding bio-based ingredients to the formula, similar biodegradation enhancement effects cannot be expected in every type of synthetic rubber.Beyond the existing waste tire recycling process through the tire pyrolysis and grinding that produces waste rubber powder, from perspectives of sustainable-material introduction and formulation design, enhancing the biodegradation rate of waste tires represents a pivotal concern for the sustainable development of the tire industry.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Summarization of sustainable materials currently used in tires.

F I G U R E 2
Schematic diagram of Hevea rubber (NR) macromolecular structure.

F I G U R E 3
Physical polymerization diagram of nonrubber components in guayule rubber, Hevea rubber, and dandelion rubber.Source: Reproduced with permission from Ref.[20].Copyright 2017, RSC.

F I G U R E 5
Eucommia ulmoides gum (EUG) in the various tissues and its molecular structure: (A) filaments of EUG in the tissues of bark, root, stem, leaf, and fruit; (B) molecular structure of EUG and Hevea rubber NR.Source: Reproduced with permission from Ref.[41].Copyright 2021, ACS.

F
I G U R E 7 Itaconicate-based elastomers obtained by the reaction of itaconic acid with (A) isoprene, (B) itaconic acid and butadiene, and (C) diethyl itaconate-co-butyl acrylate-co-ethyl acrylate-co-glycidyl methacrylate.

E 9
Special structure of lyocell fibers and the evolution of structural properties during stretching/drawing: (A) skin-core structure of lyocell fibers; (B) schematic diagram to depict the evolution of lyocell fiber microstructure with different draw ratios; (C) the effect of draw ratios on fibrillation of lyocell fibers; (D) diameter and mechanical properties of lyocell fibers with different draw ratios.Source: (B-D) Reproduced with permission from Ref.[96].Copyright 2022, Springer.
tire cords and was introduced to the European market in 2009 for use in specialized eco-friendly passenger car tires.In recent years, Michelin has successfully recycled various plastics and textiles (including bottles, trays, and polyester clothing) containing PET using the Carbios enzyme recycling process and manufacture high-performance PET fibers that meet the requirements.In 2021, Continental Group introduced ContiRe.Tex technology, a new recycling process that is free from chemical decomposition, to sort bottles, remove their caps, and clean, mechanically grind, melt, and granulate the bottles, then after solidstate polymerization and modified spinning process, r-PET fibers were obtained from 40 to 60 PET bottles and were then introduced as tire reinforcement materials in automotive production.By June 2022, Continental Group had already launched tires made from r-PET bottles throughout Europe.90At the end of 2022, Goodyear launched a demonstration tire that contained 90% sustainable materials, with the tire cords made from r-PET fibers obtained by chemically depolymerizing and repolymerizing waste PET bottles and/or other waste plastics.

F I G U R E 1 1
The reaction mechanism of PA56: (A) preparation of glutaraldehyde by decarboxylation of lysine; (B) preparation of adipic acid from glucose; (C) synthesis of PA56.

F I G U R E 1 2
Schematic diagram of the microstructure of green SiO 2 and its interaction mechanism with solution-polymerized SBR composites (SSBR): (A) structural model of SiO 2 nanoparticles and formation mechanism of the porous SiO 2 skeleton; (B) size distribution statistics of SSBR/SiO 2 composites; (C) schematic diagram of the interaction between SiO 2 and the functionalized SSBR (FSSBR) hydrogen bonds.Source: (A) Reproduced with permission from Ref. [128].Copyright 2012, ACS.(B and C) Reproduced with permission from Ref. [129].Copyright 2019, ACS.

F I G U R E 1 3 F I G U R E 1 4
Application of renewable graphite and carbon nanotubes in rubber reinforcement: (A) preparation process for KH590-corn flour graphene (CGE)/Hevea rubber (NR) composites; (B) development of hybrid microstructure and its effect on the failure resistance of the composites; (C) schematic illustration of MMT and rubber latex mixing and the co-coagulation processes.Source: (A) Reproduced with permission from Ref. [131].Copyright 2020, ACS.(B) Reproduced with permission from Ref. [132].Copyright 2021, John Wiley and Sons.(C) Reproduced with permission from Ref. [135].Copyright 2017, Elsevier.Some important industrial vegetable oils: (A) chemical structure of triglyceride; (B) typical compositions of industrially important plant oils in % (R (x:y) = composition of the fatty acids; x = chain length in carbon atoms; y = number of double bonds).(Important note: numbers do not add up to 100%; R≠R′.)Source: (A and B) Reproduced with permission from Ref. [136].Copyright 2007, RSC.

F I G U R E 1 5
Several common ZnO microstructures and their substitutes are zinc-free processing aid (ZFA): (A) SEM images of ZnO particles; (B) schematic diagram of the preparation route for ZFA.Source: (A) Reproduced with permission from Ref. [143].Copyright 2021, Springer.(B) Reproduced with permission from Ref. [144].Copyright 2019, Hindawi.
This work was financially supported by the National Natural Science Foundation of China, the Basic Science Center Program (51988102), the Beijing Nova Program (20220484213), and the Innovation Teambuilding Program of the Beijing Institute of Fashion Technology (BIFTTD201904).
Summary of the sustainable progress of rubber and tire cord materials in current tires.