Technological breakthroughs

Against the backdrop of climate change and the depletion of fossil resources becoming global concerns, the rubber industry is undergoing a revolutionary transformation. From alternative feedstocks to advanced recycling processes, green solutions are gradually replacing environmentally harmful traditional methods, ushering in a new era of sustainable rubber production.

An Overview of the Global Rubber Industry

The modern rubber industry plays an indispensable role in the global economy, with total production reaching 29.6 million tonnes in 2022, where the ratio between natural rubber and synthetic rubber was nearly balanced at 49:51. The Asia–Pacific region continues to lead global production of both rubber types. From the automotive industry, producing millions of tyres annually, to the medical sector with gloves and specialized equipment, from footwear and furniture manufacturing to aerospace applications, rubber has become an irreplaceable material across nearly all industrial sectors.

However, this rapid growth has also created serious challenges. The traditional rubber industry relies heavily on petroleum-based materials, including synthetic rubber, fillers such as carbon black, vulcanization agents, processing oils, and numerous additives. The production and use of these materials not only consume large quantities of depleting fossil resources but also generate significant CO₂ emissions and persistent waste, contributing directly to global climate change. Notably, with an estimated 1.2 billion end-of-life tyres projected to be generated annually by 2030, rubber waste management has emerged as one of the most pressing environmental challenges worldwide.

Natural Rubber Sources: Diversification for a Secure Future

Currently, natural rubber is predominantly derived from Hevea brasiliensis, a species native to the Amazon basin but now widely cultivated across Southeast Asia. Despite being the primary source of natural rubber, this crop faces growing concerns. Latex allergy has become increasingly prevalent, with up to 6% of the population estimated to suffer allergic reactions to proteins present in natural latex. Furthermore, the fungal pathogen Microcyclus ulei, responsible for South American leaf blight, poses a significant threat to industrial-scale rubber production in Central and South America, with fears of potential spread to Asian plantations. Breeding disease-resistant Hevea varieties requires approximately 25 years, limiting the industry’s ability to respond swiftly to such threats.

These challenges have driven intensive research into alternative natural rubber sources. Among approximately 2,500 plant species capable of producing natural rubber, two have attracted particular scientific and industrial interest: guayule (Parthenium argentatum) and Russian dandelion (Taraxacum kok-saghyz).

Guayule, native to the Chihuahuan Desert of Mexico and the southern United States, produces rubber in its bark and roots. Crucially, guayule rubber is completely free of allergenic proteins, making it an ideal candidate for medical gloves and other latex products. The plant exhibits exceptional drought tolerance and requires significantly less water than conventional rubber trees, rendering it suitable for arid and semi-arid regions. Major industrial players, such as Bridgestone Americas, are leading efforts to develop guayule-based rubber extraction technologies through pilot plantations in Arizona, Mexico, and southern Spain.

Russian dandelion, a perennial herbaceous plant belonging to the Asteraceae family and native to the Tian Shan mountains, produces rubber in its roots at concentrations reaching up to 20% of dry weight. Cultivated experimentally in the Soviet Union since the 1930s, this species has regained attention due to its short life cycle and high adaptability. In Germany, pilot-scale cultivation of Taraxacum kok-saghyz is underway, and the first prototype tyres made from TKS rubber have been publicly introduced, marking a significant milestone in the commercialization of this alternative rubber source.

Research efforts are also expanding to other species such as lettuce (Lactuca sativa), tall goldenrod (Solidago altissima), and additional rubber-producing plants. Decoding the genomes of these species could significantly accelerate breeding programs aimed at improving yield and rubber quality. Each species offers unique characteristics that may contribute to diversifying natural rubber sources, a critical step toward ensuring global supply stability and reducing dependence on traditional rubber plantations.

Rubber Recycling: From Waste to Valuable Resources

Alongside the search for new feedstocks, the rubber industry has made notable progress in recycling and reuse. In the United States alone, 9.2 million tonnes of rubber and leather products were generated in 2018, yet only 1.7 million tonnes were recycled. Recycling trends are increasing due to advancements in technology and environmental policies rooted in circular economy principles.

Mechanical grinding and shredding remain the dominant recycling method, accounting for 87.5% of all recycling processes according to the European Tyre and Rubber

Manufacturers’ Association. This process begins with sorting discarded rubber products by material type, a critical step given the compositional differences between passenger car tyres and truck tyres. The materials are then size-reduced and separated into reusable components. Depending on the technology employed, ground rubber exhibits varying particle sizes and surface properties. Ambient grinding produces irregular particles with porous surfaces and high surface area, whereas cryogenic grinding using liquid nitrogen yields more uniform and smoother particles.

Ground tire rubber (GTR) is widely used in applications such as anti-corrosion coatings, playground surfaces, sports facilities, concrete and asphalt mixtures, and construction foundations. However, its use in high-performance products such as seals, shoe soles, and tyres remains limited, primarily due to challenges in integrating recycled rubber with polymer matrices. To improve adhesion and compatibility, GTR surfaces can be modified using mechanical, thermo-mechanical, chemical, biological, or ultrasonic treatments.
Particularly promising is devulcanization technology, which reverses the vulcanization process and enables high-quality reuse of vulcanized rubber. Unlike conventional reclaimed rubber, devulcanized material can be employed in demanding applications comparable to virgin rubber compounds. This process selectively breaks sulfur cross-links while preserving the polymer backbone, yielding materials with properties closely resembling those of the original rubber.

Devulcanization methods include mechanical, chemical, and biological approaches. Chemical devulcanization employs agents such as thiourea, amines, or peroxides, offering high efficiency and uniform sulfur bond cleavage, albeit with moderate cost and environmental concerns due to chemical toxicity. Mechanical methods rely on shear energy in grinders or extruders, offering moderate efficiency, lower cost, and improved environmental performance. Notably, biological devulcanization is emerging as a promising field, utilizing microorganisms or enzymes capable of cleaving sulfur bonds.

Recent studies indicate that sulfur-oxidizing bacteria such as Thiobacillus spp. and sulfur-reducing archaea like Pyrococcus furiosus can selectively break sulfide bridges in vulcanized rubber while preserving polymer integrity. Additionally, Gram-positive bacteria from genera such as Nocardia, Streptomyces, Gordonia, Rhodococcus, and Bacillus, along with certain fungi, have demonstrated the ability to degrade unvulcanized poly(cis-1,4-isoprene). These organisms utilize enzymes including rubber oxygenase, laccase, and peroxidase to cleave double bonds in the polyisoprene chain. Despite their environmental advantages, biological methods currently face limitations in processing speed and efficiency.

Bio-Based Synthetic Rubber: Advancing Sustainable Manufacturing

Beyond natural and recycled rubber, bio-based synthetic rubber is emerging as a highly promising solution. These materials are produced from renewable feedstocks or through processes designed to enhance sustainability. A notable example is EPDM rubber synthesized from bio-olefins. Bioethylene can be derived from renewable resources and waste using microbial processes, although yields remain relatively low. More efficient methods include catalytic cracking of bio-oils and dehydration of bioethanol. Biopropylene can be produced via hydrothermal cracking of 3-hydroxybutyric acid, dehydration-dimerization of bio-alcohols, biodiesel dehydrogenation, and catalytic thermal decomposition of biomass.

The production of biobutadiene from renewable sources such as bioethanol or biodiols is part of a broader trend toward sustainable materials manufacturing. Biobutadiene is a key monomer used in synthetic rubbers such as styrene-butadiene rubber (SBR), butadiene rubber (BR), and nitrile-butadiene rubber (NBR). Large-scale biobutadiene production is becoming increasingly feasible. Synthos S.A., a major chemical producer and key player in the global synthetic rubber market, plans to construct a plant with an annual capacity of 40,000 tonnes of biobutadiene derived from bioethanol. This technology offers a low carbon footprint and has the potential to establish new industry standards.

Bioisoprene production from renewable feedstocks such as plant sugars is a cornerstone of emerging biotechnology and bioeconomy sectors. Bioisoprene is produced via microbial fermentation, using substrates such as glucose from corn, sucrose from sugar beet or sugarcane, and cellulose. A critical step involves microorganisms such as Escherichia coli or Saccharomyces cerevisiae, which convert sugar metabolites into bioisoprene.

Revolutionizing Vulcanization with Green Agents

Traditional vulcanization processes in the rubber industry, which rely on sulfur and chemical additives such as zinc oxide, have long raised environmental concerns. These methods are energy-intensive and generate hazardous waste and toxic emissions. Zinc oxide can leach into the environment and accumulate in soil and water, negatively affecting aquatic life. Many accelerators and their degradation products are carcinogenic or cause skin, eye, and respiratory irritation. Vulcanization also releases volatile organic compounds (VOCs) and other toxins.

As ecological concerns intensify and environmental regulations tighten, demand for environmentally friendly alternatives to conventional crosslinking agents is growing. Research by Masek et al. investigated amino acid-based compounds, along with the flavonoid quercetin and dodecanoic acid, as potential crosslinkers for biocomposites based on epoxidized natural rubber and polycaprolactone. The best crosslinking performance was achieved with quercetin and phenylalanine, forming optimal network structures with elongation at break exceeding 400% and tensile strength approaching 20 MPa. The optimal concentration of natural crosslinkers ranged from 0.75 to 1 phr.

Quercetin exhibited high thermal stability, maintaining its chemical structure up to approximately 200°C, ensuring functionality under typical vulcanization or melt-mixing conditions. Amino acids such as phenylalanine also demonstrated sufficient thermal resistance to participate effectively in crosslinking reactions. Moreover, due to their inherent antioxidant properties, these natural crosslinkers enhance the aging resistance of rubber composites. Quercetin acts as a free-radical scavenger, reducing oxidative degradation during thermal aging and stabilizing mechanical properties over time.

Vegetable oils such as soybean oil and linseed oil, as well as natural resins, have been explored as crosslinking agents to reduce reliance on petroleum-based additives. Zhang et al. developed a novel elastomer crosslinking system in which epoxidized soybean oil served as the crosslinker in carboxylated nitrile rubber. This system relies on epoxy-acid reactions to form interchain bonds. However, in the absence of zinc oxide, optimal curing times exceeded 25 minutes, while the addition of 2 phr zinc oxide reduced curing time to under 10 minutes.

Regarding eco-friendly accelerators, Kamoun et al. developed a bio-based dual-component accelerator system consisting of garlic powder and cystine, both sulfur-containing biomolecules. Results showed a significant improvement in crosslinking efficiency, with the addition of 1 phr garlic powder increasing crosslink density by nearly 250%. Another example is starch-supported sodium isobutyl xanthate (SSX). Kuncai et al. modified starch with sodium isobutyl xanthate, significantly improving thermal stability. SSX effectively accelerated natural rubber vulcanization at 145°C, increasing tensile strength by 22.4% compared to conventional accelerators.

Bio-Based Fillers: Sustainable Alternatives to Carbon Black and Silica

Fillers play a critical role in rubber composites, influencing their mechanical and chemical properties. Conventional fillers such as carbon black and silica present environmental drawbacks, prompting a shift toward bio-based alternatives such as cellulose and biochar. While cellulose and natural fibers have been widely studied, recent research has focused on less common fillers such as biochar and biogenic silica.

Biochar is a carbon-rich material similar to carbon black, produced via biomass pyrolysis under oxygen-free conditions. Pyrolysis parameters, including temperature, residence time, and heating rate, determine the resulting biochar properties. Unlike petroleum-derived carbon black, biochar properties vary widely due to biomass heterogeneity.

Key factors influencing biochar quality include biomass type, lignocellulosic composition, and pyrolysis conditions. Higher pyrolysis temperatures generally increase carbon content, ash content, aromaticity, and pH, while reducing polarity and functional group accessibility. Elevated temperatures enhance hydrophobicity but may reduce surface functionality.

Current research focuses on optimizing the mechanical, thermal, and environmental properties of biochar-reinforced composites. Biochar has been produced from diverse biomass sources, including nut shells, cereal husks, corn, bamboo, sawdust, and coconut waste. The specific plant component used significantly affects composite performance. Studies show that coconut-derived biochar substantially enhances natural rubber properties, achieving a 47% increase in Shore A hardness at 40 phr, maximizing tensile strength at 20 phr, and exhibiting higher crosslink density than other biochars.

However, biochar tends to agglomerate within elastomers, complicating uniform dispersion and negatively affecting mechanical properties. Advanced mixing techniques or surface modification are required to improve performance. Peterson and Thomas employed chemical surface modification using gas treatment at 300°C with air or carbon dioxide, followed by lauric acid coating. Biochar pretreated with carbon dioxide and coated with lauric acid showed notable improvements, including a 19% increase in tensile strength and a 48% increase in fracture toughness.

Alongside biochar, biogenic silica—particularly rice husk ash (RHA)—has demonstrated significant potential as an eco-friendly reinforcing filler. Silicon is the second most abundant element in soil, accounting for approximately 32% of its mass. Consequently, plants inherently contain silicon in their tissues. Common sources of biogenic silica include grasses and herbaceous plants, especially monocots from the Poaceae, Cyperaceae, and Equisetaceae families, with silicon content ranging from 0.1% to 15%.

Rice husk ash is the most extensively studied biogenic silica source due to its exceptionally high silica content. Raw rice husk contains approximately 20% ash, of which up to 94% is silica. By comparison, corn cob ash contains about 60% silica, bamboo leaf ash around 80%, and elephant grass ash approximately 50%. Incorporation of RHA into rubber composites significantly enhances thermal stability. The high amorphous silica content acts as an effective thermal barrier, limiting diffusion of degradation products and delaying thermal decomposition.

Studies confirm that chemically purified RHA can serve as a sustainable substitute for conventional fillers such as carbon black and silica in tyre rubber. Wet-mixing techniques facilitate effective dispersion of biogenic silica in natural rubber latex, producing composites with improved abrasion resistance and reduced rolling resistance compared to those prepared via dry mixing. This approach is more environmentally friendly and enhances both performance and durability.

Bio-Based Additives: Optimizing Processing and Performance

Bio-based additives in rubber technology refer to environmentally friendly substances derived from renewable natural resources, used to enhance material properties and performance. These additives play critical roles in improving processing, vulcanization, and mechanical and chemical properties.

Bio-based coupling agents represent a promising avenue. Silane coupling agents act as chemical bridges between organic and inorganic materials, enhancing adhesion and mechanical properties. Novel bio-based silane coupling agents have been synthesized from eugenol, an inexpensive and environmentally friendly terpene, via hydrosilylation with triethoxysilane. A method was developed to synthesize 25 silane coupling agents with diverse functional groups. Selected agents were used in silica-filled rubber composites, resulting in significant improvements in elongation at break, dry tensile properties, and rebound resilience.

Non-silane coupling agents are also under investigation. Sorbitol and sorbic acid—both inexpensive, readily available, and ecologically safe—have been introduced as alternatives to bis(3-triethoxysilylpropyl) disulfide (TESPD). Sorbitol achieved most desired tyre properties, including rolling resistance, wet grip, and heat dissipation, except abrasion resistance. Other non-silane agents improving filler-elastomer adhesion include tannins, chitosan, stearic acid, and other fatty acids.

Vegetable oils serve as natural plasticizers, offering biodegradable and renewable alternatives to petroleum-based oils. These oils enhance rubber elasticity and processability. Elburg et al. evaluated sunflower oil, coconut oil, and cardanol alongside squalane in simplified tyre formulations. Linseed oil has also proven effective as a plasticizer in natural rubber and expandable graphite vulcanizates, offering superior mechanical properties and thermal stability compared to naphthenic oil-based composites.

Regarding flame retardants, Lin et al. developed a novel system based on tannic acid—a natural phenolic compound—combined with poly(ammonium phosphate) and functionalized graphene. This system demonstrated reduced flammability and improved mechanical properties by altering degradation pathways and forming a stable, carbon- and phosphorus-rich protective char layer.

Green Antioxidants: Naturally Protecting Rubber

Rubbers such as natural rubber, styrene-butadiene rubber, butadiene rubber, and nitrile-butadiene rubber contain unsaturated structures susceptible to oxidative and ozonolytic degradation. Antioxidants and antiozonants are essential to ensure stability, yet many conventional antioxidants pose toxicity and environmental concerns.

Green antioxidants, including polyphenols, tocopherols, and plant extracts, offer advantages over traditional synthetic antioxidants such as IPPD and TMQ. Although synthetic antioxidants often provide superior thermal stability, natural antioxidants exhibit reduced migration within the rubber matrix, minimizing blooming and ensuring prolonged protection. Moreover, natural antioxidants display favorable toxicological profiles, with low cytotoxicity, minimal irritation or allergenic potential, and enhanced biodegradability.

Green stabilizers used in rubber composites include green tea, spent coffee grounds, oak bark, stinging nettle, mint, catechin hydrate, eugenol, flavones, henna, and others. Flavonoid-based antioxidants such as quercetin and catechin effectively neutralize free radicals, interrupting oxidative chain reactions. Terpenoids, carotenoids, tannins, and tocopherols further enhance oxidative stability by chelating metal ions and scavenging reactive oxygen species.

Plant-derived anti-aging compounds play a vital role in protecting against oxidative stress and environmental degradation. Available as powders or extracts from roots, leaves, and seeds, these compounds provide sustainable alternatives for rubber stabilization, significantly enhancing durability and resistance to UV radiation, ozone exposure, and high temperatures.

Outlook and Challenges Ahead

The journey toward green and sustainable rubber is complex and fraught with challenges. Despite demonstrated potential, widespread industrial adoption remains constrained by several key barriers.

Performance limitations persist, as some bio-based alternatives do not yet match conventional materials in demanding applications. Biochar agglomeration and vegetable oil plasticizer effects on wet grip exemplify such challenges, necessitating further formulation and processing optimization.

Quality control and consistency pose additional challenges due to natural variability in bio-based materials. Production cost remains a hurdle, as many green technologies lack economic competitiveness at scale. Infrastructure and supply chain development further complicate the transition.

Nevertheless, prospects remain highly promising. Growing environmental awareness, stricter regulations, and corporate commitments to carbon neutrality and circular economy principles are driving adoption. Advances in biotechnology, materials science, and chemical engineering continue to improve performance and scalability.

Conclusion: Toward a Sustainable Rubber Future

The green revolution in the rubber industry is not a temporary trend but a fundamental and inevitable transformation. From alternative natural rubber sources such as guayule and Russian dandelion, to advanced recycling technologies like biological devulcanization, from bio-based fillers to plant-derived additives, each innovation contributes to a sustainable and environmentally responsible rubber industry.

Research confirms that natural and bio-based components are not only viable alternatives but can outperform conventional materials in certain applications. Improved recyclability and circularity, particularly through devulcanization and biological processes, pave the way for a truly circular rubber economy.

For Vietnam, and particularly the Vietnamese chemical industry, this represents a strategic opportunity. With abundant agricultural biomass, potential for cultivating alternative rubber crops, and a long tradition in rubber production, Vietnam is well positioned to become a pioneer in green rubber manufacturing. Investment in research, infrastructure, and workforce development will not only protect the environment but also create sustainable competitive advantages in an increasingly globalized market.

The future of the rubber industry is green—where high performance no longer comes at the expense of the environment, where waste becomes valuable resources, and where continuous innovation unlocks new pathways to sustainable development.