A Review of Current and Emerging Strategies for Recycling Waste: Bicycle Tires and Inner Tubes

Abstract

Bicycle tires and inner tubes constitute a growing waste stream mainly composed of natural rubber, butyl rubber, synthetic elastomers, carbon black, and reinforcing materials. Their multi-material structure and highly crosslinked networks make their recycling challenging, yet efficient recovery is essential for advanced circular economy practices. This review summarizes the current and emerging strategies for recycling bicycle tires and inner tubes. It first outlines the materials and additives present in tire casings and butyl inner tubes, which determine their recycling behavior. Mechanical pre-processing methods, including shredding, grinding, and fiber/steel separation, are presented as essential feedstock preparation steps. Thermochemical approaches, such as pyrolysis and thermolysis, are discussed with emphasis on producing value-added fractions, including pyrolysis oil, recovered carbon black, and fuels. Solvent-based feedstock recycling and chemical dissolution are highlighted as promising routes for selective recovery of rubber polymers and additives. Physical, chemical, and biological devulcanization methods are also reviewed for their potential to restore partial processability to reuse reclaimed rubber. Finally, current and prospective applications of recycled materials are discussed, and key challenges with future research needs are identified, including improving devulcanization efficiency, expanding collection systems, and increasing the value of recovered products.

1. Introduction

The COVID-19 pandemic significantly reshaped urban travel habits. When governments encouraged people to limit the use of buses, trains, and other public transport systems, cycling quickly became a preferred alternative [1]. As a result, bicycle sales and rentals rose significantly. For example, the bike-share system in New York City expanded from 19,000 bicycles with an average of 1.7 million rides in 2020 to 30,000 bicycle (including electric models) and more than 2000 docking stations in 2024. During this period, ridership reached record levels, surpassing 4 million rides [2]. A similar growth was observed in Europe. In April 2024, Paris’s Vélib’ Métropole network operated nearly 1500 stations over 55 municipalities, offering about 19,000 bicycles covering an area of 400 km². The system supported around 409,000 annual subscribers and recorded an average of 136,783 daily trips, reflecting steady year-after-year growth [2].

Cycling is widely recognized as one of the cleanest and most resource-efficient ways to travel as it produces no direct carbon emissions [3], consumes minimal energy, helps relieve traffic congestion, generates far less noise than motor vehicles, and uses urban street space more effectively than cars or buses [2,4].

Although several cities now encourage cycling and manufacturers to produce large numbers of bicycles, the environmental impact of post-consumer bicycle waste is often overlooked. Discarded tires, inner tubes, and all other bicycle parts contribute substantially to solid waste streams. Millions of bikes are abandoned due to broken frames, outdated designs, or replacement cycles in bike-share programs [5]. An example is the rapid expansion and subsequent collapse of several dockless bike-sharing initiatives in China, which left millions of unwanted bicycles in massive “bike graveyards” (Figure 1), creating challenges to manage metal, rubber, and plastic waste [6].

A typical bicycle contains components such as steel or aluminum frames, rubber tires and tubes, plastics, and, especially for electric bicycles, electric/electronic systems. Each of these materials follows a different recycling process. Although research suggests that 95% of the materials in shared bicycles could theoretically be recovered, effective recycling programs require coordinated pathways for each material stream [6].

Metal components from discarded bicycles, such as steel or aluminum, can be shredded and reprocessed into new metal products. Plastics from saddles, grips, reflectors, and various casings may be mechanically recycled and converted into pellets to manufacture molded parts. Lithium-ion batteries from e-bikes typically require dedicated facilities to recover valuable metals such as lithium, cobalt, and nickel. Tires and inner tubes represent other major waste streams, and this review focuses on the end-of-life management and recycling of these rubber products.

The growing popularity of cycling, combined with insufficient recycling infrastructure, has turned the disposal of used bicycle tires and inner tubes into a major environmental concern. These components are typically manufactured from vulcanized natural rubber, synthetic elastomers, carbon black, and textile reinforcements, which are materials known for their durability and chemical resistance [7,8]. The vulcanization process forms a robust three-dimensional network of mono-, di-, and polysulfide crosslinks, giving tires long service life, but also making them slow to degrade [9]. Consequently, most end-of-life tires and inner tubes are still send to landfills or incineration, resulting in the loss of recoverable resources and introducing long-term soil and water risks [10].

Environmental impacts extend beyond slow decomposition. Rainwater accumulating in discarded rubber can leach toxic substances harmful to aquatic organisms, while the waste itself can create breeding grounds for rodents and mosquitoes. As rubber ages, it can release a variety of pollutants, including heavy metals and organic compounds such as the anti-ozonant 6PPD and its derivative 6PPD-quinone, which is associated with fish toxicity and has been linked to mortality events involving species such as Coho salmon [11]. Incineration presents additional hazards, producing emissions of dioxins, polycyclic aromatic hydrocarbons (PAH), and other toxic volatile compounds [12]. Ash residues and mobilized heavy metals from these processes can further contaminate soil and water bodies [13].

Statistics

Data from the UK-based recycling program Velorim indicate that roughly 30.5 million bicycle tires and 152.5 million inner tubes are discarded into landfills each year, highlighting a substantial waste challenge [14]. In the USA, an estimated 10 million used inner tubes are thrown away annually, according to Schwalbe North America [15]. Additional estimates for the UK suggest that more than 10,000 tons of bicycle tires and inner tubes end up in landfill sites every year [16]. Globally, the bicycle tire and tube market was valued at US $5 billion in 2017 which is projected to reach US $8.6 billion by 2026 representing a compound annual growth rate of 5.8% over 10 years [17].

A study by Market Research Future (MRF) estimates that the global tire materials sector reached a value of US $99.46 billion in 2024 [18]. Despite this sizable market, only 15–20% of end-of-life tires are currently routed into recycling or reuse pathways [19,20]. The majority (70–80%) are still being discarded in landfills, a disposal method known to cause environmental risks because tire waste can release heavy metals and other pollutants into surrounding soils and groundwater. Similar concerns apply to other rubber-based products, such as retired fire hoses, which pose comparable leachate hazards when landfilled [21].

But recycling systems dedicated especially for bicycle tires and inner tubes remain limited. In Canada, progress has been gradual:

• British Columbia introduced a collection model in 2011 where bike shops can return used tubes alongside other regional tire waste streams [22].
• New Brunswick expanded its waste-diversion regulations in 2018 to include bicycle tires and inner tubes [23]. In Québec, RECYC-QUÉBEC recovered more than 3.1 million used bicycle tires and inner tubes since the start of its program in 2007. To date, 50,000 of these items were repurposed by community organizations into products such as belts, juggling equipment, and keychains. The amounts collected each year are shown in Figure 2 [24]. When the program began in 2007, around 25,000 units were collected. By 2024, this number increased to 330,729 units, corresponding to a total of 120 tonnes (average mass of 360 g per unit).

In Europe, Schwalbe has operated one of the longest-running bicycle inner tube recycling programs. Since 2015, the company reports the processing of 10 million tubes (around 1760 metric tons), including two million tubes (320 tons) in 2023 alone. Currently, each standard Schwalbe inner tube contains about 20% recycled content [25]. Their bicycle tire recycling initiative, launched in 2022, has already diverted more than 650,000 tires from waste streams. Schwalbe’s updated projections estimate that by the end of 2025, they will have recycled about 20 million inner tubes and more than two million tires (Figure 3) [25].

In South Asia, AR Reclaim Rubber, a subsidiary of Anwer Rashid Industries (Pvt.) Ltd., operates a facility near Lahore (Pakistan). The company focuses on reclaiming materials from bicycle tires and inner tubes. The plant was established in 2019, but recycling output data have not been published yet [26].

Unlike existing reviews mainly focusing on automotive tires or general elastomer recycling, this review focuses on bicycle tires and inner tubes, a rapidly growing waste stream for which no dedicated review is currently available in the literature. This review examines the material composition of bicycle tires and inner tubes and shows that bicycle tires have compositions similar to those of passenger car tires, indicating that existing tire recycling technologies can potentially be adapted for bicycle applications. Furthermore, this review provides a material-specific perspective by systematically comparing the types of rubbers used in bicycle tires and inner tubes, their mechanical and chemical properties, and the suitability of different recycling technologies. By consolidating scattered information and highlighting application-driven reuse options, this work fills a clear knowledge gap and supports the development of more targeted and efficient recycling strategies for bicycle-related rubber waste.

The main objective of this review is to provide a comprehensive and critical overview of the recycling of bicycle tires and inner tubes. In particular, it aims to (i) identify and compare the different rubber materials used in bicycle tires and inner tubes, (ii) summarize their key physical and mechanical properties relevant to recycling, (iii) review current mechanical, chemical, and thermal recycling processes, and (iv) discuss potential reuse pathways and applications for recycled materials. By consolidating dispersed information from the rubber recycling and tire recovery literature, this review seeks to support researchers, recyclers, and policymakers by clarifying material-specific challenges and opportunities and by providing a practical guide for the development of sustainable recycling strategies for bicycle tires and inner tubes.

2. Material Composition of Bicycle Tires and Inner Tubes

2.1. Composition of Bicycle Tires

A typical bicycle tire is built from three main structural elements: the carcass, the bead core and the tread (Figure 4) [11]. The bead core establishes the tire’s diameter and locks it onto the rim. Conventional bicycle tires generally use bundles of steel wire for the bead, while folding models replace steel with aramid fiber hoops, allowing greater flexibility. The carcass forms the tire’s internal framework. It is usually constructed from polyamide (Nylon) textile cords coated with rubber. These cords are commonly cut at 45° and, when aligned in the rolling direction, provide a balance between strength, stability, and flexibility [11].

The tread and sidewall rubber consist of a compound formulated from several ingredients:

• natural and synthetic elastomers;
• reinforcing fillers such as carbon black, calcium carbonate, or silica;
• softeners (processing oils);
• anti-aging and antioxidant agents;
• sulfur and other vulcanizing components;
• vulcanization accelerators such as zinc-based compounds;
• pigments and colorants.

Although the exact formulations remain proprietary to manufacturers, general compositional ranges are known with typical values for rubbers (40–60%), fillers (15–30%), and additives (20–35%).

Table 1. Typical composition of a bicycle tire (by weight) [9].

Component	Content (wt.%)	Notes
Rubber compounds (NR, SBR, BR, IIR, etc.)	40–60	Includes tread, sidewalls and protective layers.
Carbon black/silica	20–30	Reinforcement and resistance to wear.
Textile cords (Nylon, polyester, cotton, aramid)	10–20	Carcass structure; determines flexibility (TPI).
Steel wires (edges, belts)	5–10	Fix the tire to the rim; folding tires use aramid (Kevlar).
Plasticizing oils, resins, waxes, antioxidants	3–5	Improves flexibility and resistance to aging.
Sulfur + activators (vulcanizing agents)	1–2	Necessary for vulcanization.
Pigments/other additives	<1	Colored stripes, etc.

presents a list of general components with their respective range of composition.

The report on Scrap Tire Recycling in Canada (2005) provides representative compositions for passenger-car tires [27]. When these values are compared with values for bicycle tires (Table 1), the overall proportions fall within similar ranges, especially the rubbers: around 41% for car tire vs. 40–60% for bicycle tires. The main distinction lies in the reinforcement as car tires use higher amounts of steel, while bicycle tires mainly rely on textile cords with minimal metal contents.

Bockstal et al. [28] and Hassan et al. [29] reported similar patterns across different tire categories. Their results indicate that car, truck, and off-road tires contain roughly 45–47% rubber, 21–22.5% carbon black/silica, and 2.5–3% vulcanization agents, while varying significantly in reinforcement materials with metals (12–23.5%) and textiles (1–10%), depending on the tire’s performance requirements. These comparisons further highlight that rubber and filler contents remain relatively consistent across tire types, while metal and textile contents are different to meet structural requirements.

A substantial proportion of any tire is made from a mixture of natural rubbers (polyisoprene, PI) and various synthetic elastomers, such as polybutadiene (PB), styrene–butadiene rubber (SBR), and (halo)butyl rubber. The specific polymer blend is selected to meet performance needs for the tread and sidewalls [30]. Following elastomers, carbon black and silica represent the next largest category of ingredients, acting as reinforcing agents improving abrasion resistance, mechanical durability, and overall lifespan [30].

Metals and textile fabrics serve as reinforcements [30]. Steel and metal alloys appear mainly in bead wires and belt regions, while rayon, polyamide, and polyester textiles are used for carcass reinforcement. Trucks tend to use more metal reinforcement and less textiles due to the higher mechanical loads, while passenger tires often incorporate textiles to reduce weight and enhance comfort [30].

Finally, the compound includes several additional components, such as softeners, antioxidants, vulcanizing chemicals (sulfur), accelerators, and pigments. Their proportions fall within the same additive fraction (20–35%) as shown in Table 1.

In summary, bicycle tires share a chemical composition similar to that of automotive tires, dominated by elastomers and reinforcing fillers. However, bicycle tire construction differs notably in the reinforcement architecture, relying more heavily on textile materials rather than high steel content. These distinctions influence material recovery strategies and recycling approaches for both tire categories.

Table 2. Typical composition (wt.%) of passenger and truck tires in North America [27,29].

Composition	Passenger Tire	Truck Tire
Rubbers	41–48	20–28
Carbon black	21–28	20–28
Steel	13–16	20–27
Textiles	4–6	0–10
Additives	10–12	7–10

presents a comparison between car and truck tire composition to compare with Table 1 for bicycle.

2.2. Composition of Bicycle Inner Tubes

Bicycle inner tubes are mainly manufactured from butyl rubber, a synthetic elastomer valued for its excellent gas impermeability and high mechanical elasticity. Similar to tire compounds, the rubber in tubes is blended with fillers and other ingredients, the final performance depending on the exact formulation used by the manufacturer [31].

Carolin [32] used thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) to show that commercial inner tubes typically contain 50–60% butyl rubber (IIR/CIIR). The primary reinforcing phase is carbon black, which usually makes up 25–35%, while zinc oxide (3–7%) functions as a key activator in the vulcanization system. Zheng et al. [33] further examined the thermal breakdown of sulfur-crosslinked elastomers, showing that organic polymers degrade over a wide range (300–485 °C), while carbon black oxidizes only at much higher temperature (560–800 °C). In their samples, the inner tubes contained around 60 wt.% of organic rubber (

Table 3. Main constituents of inner tubes.

Component	Material	Content (wt.%)	Function
Base polymers	Butyl rubber (IIR) or halobutyl rubber (CIIR/BIIR)	62–65	Provide air impermeability, elasticity, and chemical resistance.
Fillers	Carbon black	30–33	Increase strength, wear resistance, and UV stability.
Plasticizers/oils, vulcanizing agents, antioxidants/antiozonants, pigments, etc.	Mineral oil, paraffinic oil, sulfur, zinc oxide, stearic acid, etc.	3.5–6.5	Improve flexibility and processability. Create crosslinks for elasticity and durability, prevents degradation by oxygen and ozone, improves color, etc.

) [33].

Although butyl rubber is the dominant material, inner tubes can also be produced from latex [31], or thermoplastic polyurethane (TPU) [34]. Latex versions have higher elasticity than butyl inner tubes, giving them significantly lower rolling resistance and strong puncture-resistant behavior because the material can stretch substantially before failure. On the other hand, TPU inner tubes are made from thermoplastic elastomers combining rubber-like flexibility with low weight and excellent dimensional stability.

Table 4. Comparison between the composition of bicycle inner tube materials.

Property	Butyl Rubber	Latex	TPU (Thermoplastic Polyurethane)
Material type	Synthetic rubber (isobutylene–isoprene)	Natural rubber (latex)	Thermoplastic polyurethane
Rolling resistance	Moderate–high	Lowest (best performance)	Low (better than butyl, but not as low as latex)
Comfort/ride feel	Firm, less flexible	Most flexible and comfortable	Moderate (between butyl and latex)
Air retention	Excellent (slow air loss)	Poor (requires frequent inflation)	Good (better than latex)
Weight	80–120 g (standard), 50–70 g (lightweight)	60–80 g (lighter than butyl)	22–50 g (lightest available)
Puncture resistance	Good	Fair	Good to very good (depends on brand)
Durability/Aging	Excellent	Moderate (sensitive to ozone and UV)	Good (resistant to temperature and aging)
Cost	Low (most affordable)	Moderate–high	Highest (brands like RideNow cheaper)
Best use	Everyday riding, commuting	Racing, performance riding	Hill climbing, weight-conscious racing
Typical brands/models	Continental, Schwalbe, Kenda	Vittoria, Challenge	Tubolito, RideNow, Pirelli SmartTube
Remarks	Reliable and low maintenance	Requires frequent inflation; best performance	Extremely light; more expensive but compact

summarizes the key differences among these inner tube types, while Figure 5 presents typical mass ranges for standard butyl, lightweight butyl, latex, and TPU versions [34].

Beyond the base polymer and reinforcing fillers, inner tube compounds also incorporate several minor additives. Plasticizers, often mineral or paraffinic oils, improve flexibility and support processing during extrusion. The vulcanization system (usually sulfur) together with activators (zinc oxide and stearic acid) create the crosslinked network giving the tube strength, elasticity, and resistance to deformation [35].

Latex and TPU tubes differ from butyl tubes not only in terms of polymer type, but also in filler and additive requirements. Latex tubes, made largely from natural rubber, exhibit high elasticity and partial self-sealing behavior. TPU tubes rely on thermoplastic polyurethane, which needs fewer traditional fillers and vulcanizing chemicals, but may include specialized stabilizers for puncture resistance, UV durability, and abrasion protection [27].

Overall, the formulation of a bicycle inner tube represents a balance between air retention, elastic recovery, rolling performance, puncture protection, and long-term durability. Differences in polymer chemistry, filler content, and additive packages allow manufacturers to tailor the tube’s behavior for various cycling conditions and performance goals.

3. Recycling and Recovery Technologies

3.1. Bicycle Tire Recycling

The rubber materials used in tire production can be divided in two categories. General-purpose elastomers include natural rubber (NR), styrene–butadiene rubber (SBR), and butadiene rubber (BR). On the other hand, special-purpose rubbers, such as ethylene–propylene elastomers (EPM/EPDM), butyl rubber (IIR), halobutyl rubber (HIIR), chloroprene rubber, and nitrile rubber, are used where specific performance properties are required [36]. For bicycle, tire compounds are typically based on NR, SBR, and BR, while butyl rubber (IIR) remains the dominant material for inner tubes.

Recycling end-of-life tires (ELT), including bicycle tires, remains a global concern due to both their extremely high production volume (around 1.5 billion units per year) and their resistance to degradation [37]. Current approaches dealing with waste tires include the following [37]:

• Direct reuse;
• Landfilling;
• Incineration;
• Retreading;
• Reclaimed-rubber production;
• Feedstock recycling via chemical dissolution;
• Pyrolysis.

Currently, waste tires are commonly managed through direct use, landfill, direct incineration, retreading, reclaiming, and pyrolysis [38].

As noted earlier, landfilling and incineration present serious environmental drawbacks. Direct reuse, such as incorporation into playground flooring or small consumer items via rubber crumbs (particles of different sizes), addresses only a small portion of the annual waste stream [39]. Retreading is effective, but limited to tires with structurally sound casings and can only be performed a finite number of times [39]. Figure 6 presents a classification of the different methods to manage bicycle tire wastes which are discussed next.

Reclaimed rubber is recycled rubber from used products that has been treated to regain some flexibility and processability for reuse. Reclaimed rubber is another option, but the industry faces several barriers including low economic margins, high energy requirements, and pollutant emissions during processing [40]. Feedstock recycling via chemical dissolution can convert rubber into reusable chemicals, but the industrial processing capacity of this method remains limited.

Consequently, the most viable large-scale pathways for bicycle tire recycling are reclaiming and pyrolysis. Among these, pyrolysis has gained significant attention because it enables the recovery of multiple high-value products, while reducing environmental impacts. It is widely seen as one of the most comprehensive methods to convert waste bicycle tires (WBT) into reusable products/resources [41]. A practical example is the German company Schwalbe, which operates a bicycle-tire recycling system based on pyrolysis. In 2023, the program processed about 500,000 waste bicycle tires.

Figure 7 presents a simplified scheme of the Schwabe plant configuration [31]. Waste bicycle tires are collected (1) from various bicycle shops and transported (2) to the recycling facility. Before entering the pyrolysis reactor (4), the waste bicycle tires are shredded (3), and metallic and textile components are removed. The pyrolysis process then enables the recovery of pyrolysis oil and carbon black powder (5), which can be further processed for subsequent applications.

3.1.1. Mechanical Recycling (Pre-Process Treatment)

As previously discussed, an essential first step in recycling waste bicycle tires (WBT) is the removal of the steel and textile reinforcements embedded in the tire structure (Figure 8).

WBT collected by bicycle shops are transported to processing facilities, where they undergo size-reduction operations. The tires are cut or mechanically shredded into smaller fragments, after which magnetic separators extract the steel wires. The recovered steel can then be redirected to metallurgical recycling streams [42]. Following steel removal, the remaining mixture is processed using air-classification systems or vibratory screening to separate textile fibers (typically polyester or Nylon) from the rubber fraction. Historically, this blend of recycled tire fibers (RTF) and small rubber particles has often been incinerated for energy recovery or disposed in landfills due to its challenging processing characteristics [43]. However, recent studies have explored new uses for RTF, such as reinforcing agents in composites based on recycled polyethylene [43,44]. Effective separation of steel and textile components ensures that the rubber feedstock entering downstream recycling technologies is clean (no contamination) and uniform (homogeneous), which improves both the processing efficiency and material quality of the resulting recycled materials.

To convert WBT into fine rubber granules or powders, several grinding technologies are available, including the following [45,46]:

• Ambient grinding;
• Cryogenic grinding;
• Wet grinding.

Each technique operates under different conditions and yields distinct particle characteristics.

Table 5. Comparison between the different grinding processes.

Process	Description	Advantages	Disadvantages
Ambient grinding	Mechanical size reduction of vulcanized rubber at ambient temperature using grinding mills.	Simple process: particle size can be controlled by the number of grinding steps and mill type.	Generates significant heat leading to oxidation and degradation; high cost for very fine particles.
Cryogenic grinding	Rubber is frozen with liquid nitrogen below its Tg and then crushed to produce fine crumbs (powders).	Produces smaller, cleaner crumbs with smoother surfaces; faster process.	Requires liquid nitrogen; higher operational cost.
Wet grinding	Rubber is ground with water as a lubricant and cooling agent to produce very fine crumbs.	Produces very fine crumbs (10–20 μm); high purity; low energy consumption; water can be recycled.	Product requires drying; process involves handling water systems.

summarizes the main differences, such as achievable particle size, energy requirements, operating temperature, and overall product quality. Additional considerations include potential thermal degradation during processing, cost factors, and purity of the final crumb rubber output. Ambient grinding is generally the most practical option due to its lower cost and industrial availability. Cryogenic grinding offers superior particle quality but at higher energy and operational costs. Wet grinding provides fine particle sizes but involves additional water treatment and drying steps.

3.1.2. Pyrolysis Process Recycling

Pyrolysis is a thermal conversion process in which waste tires are exposed to high temperatures in an oxygen-limited environment, causing the long-chain polymers within the rubber to break down into smaller molecules. One commercial example of this technology is Pyrum Innovations AG, a German company operating industrial thermolysis facilities. The company supplies its recovered carbon black (rCB) to Continental for incorporation into new tire compounds, while the pyrolysis oil is delivered to BASF as a feedstock in the production of new plastics [47]. A simplified overview of the Pyrum’s plant configuration is presented in Figure 9.

The pyrolysis of waste bicycle tires typically generates three main product streams:

• Gas phase: mainly hydrogen, methane, and carbon oxides;
• Liquid phase: oils, tars, and condensed organic compounds;
• Solid residue (char): fixed carbon along with ash, metals, oxides, and inert materials.

These are often referred to pyrolysis gas, pyrolysis oil, and pyrolytic carbon black (char) [38].

The exact yields and chemical compositions of each fraction depend strongly on a range of operational variables, including temperature, heating rate, pressure, residence time, particle size (granulometry), and the condensation conditions used during vapor recovery. Because pyrolysis can handle large volumes of waste rubber while generating materials of commercial value, such as reusable oils, feedstock chemicals, carbon black, and energy-rich gases, it is considered one of the most promising technological routes for waste tire recycling [48]. Successful industrial deployment of waste tire pyrolysis requires the production of high-quality outputs with minimal energy demand. Both reaction temperature and the use of catalysts strongly influence the process efficiency and the physical/chemical properties of the resulting products. By controlling these parameters, the final properties of pyrolysis oil, gas, and char can be obtained. After suitable upgrading or purification, each of these product streams can be integrated into a variety of industrial applications (oil, carbon black, etc.).

Pyrolysis Mechanism

The thermal degradation of waste bicycle tires is governed by the breakdown of their two principal organic components: natural rubber (NR) and synthetic rubbers (SR). NR begins to undergo noticeable decomposition around 326 °C, where the polymer chains start to split into smaller fragments, yielding dimers, trimers, and other low-molecular-weight species. As the temperature increases, a larger proportion of isoprene monomers is liberated, along with various cyclic products, including aromatic species such as xylene. At higher temperatures, additional secondary reactions take place, such as cracking, rearrangement, and aromatization, resulting in a more complex mixture of volatile compounds [49]. A schematic representation of the pyrolysis pathways of natural rubber (NR) is provided in Figure 10.

Pyrolysis of Synthetic Rubber (SR)

The synthetic rubber (SR) portion of waste bicycle tires is composed mainly of butadiene rubber (BR) and styrene–butadiene rubber (SBR). Their decomposition behaviors have been investigated in detail using analytical techniques, such as TG-FTIR/MS (thermogravimetry/mass spectrometry coupled with Fourier transform infrared spectroscopy and/or mass spectrometry) and Py-GC-TOF/MS (pyrolysis gas chromatography–time-of-flight mass spectrometry) [50].

Pyrolysis of Butadiene Rubber (BR)

When BR is heated, the polymer chains generate free radicals, which initiate several concurrent reaction pathways. Four dominant transformation routes have been identified:

• Chain cleavage and dehydrogenation, producing 1,3-butadiene;
• Radical rearrangement followed by cyclization, yielding 4-vinyl-1-cyclohexene;
• Dehydrogenation and subsequent ring-forming reactions, generating 1,3-cyclopentadiene;
• Cyclization processes leading to 1,4-cycloheptadiene.

These pathways collectively describe the primary thermal-cracking behavior of BR, as illustrated in Figure 11a.

Pyrolysis of Styrene–Butadiene Rubber (SBR)

SBR decomposes over a relatively wide temperature interval (180–500 °C). Free radicals originating from the _C4 segments of the polymer mainly form 1,3-butadiene, while radicals derived from the aromatic (styrene) units mainly yield styrene (monomer). At elevated temperatures, these benzene-based radicals undergo secondary reactions, including further cracking, recombination, and aromatic growth, producing a range of more complex aromatic hydrocarbons including ethylbenzene, toluene, xylene isomers, indene, naphthalene, methyl-naphthalenes, biphenyl, and other polycyclic aromatic hydrocarbons (PAH). The main degradation pathways of SBR under high-temperature, oxygen-free conditions are summarized in Figure 11b.

Xu et al. [51] studied the pyrolysis behavior of waste bicycle tires using TG-FTIR and identified two distinct decomposition stages. Stage I (285–531 °C) corresponds to primary chain scission. At the lower end of this temperature range, the cleavage of rubber macromolecules produces small alkenes and cycloalkenes. But as temperature increases, these species undergo additional transformations, such as Diels–Alder reactions, cyclization, and aromatization, resulting in the formation of benzene and related aromatic compounds. During this stage, ring-opening and other secondary reactions also proceed in parallel. In Stage II (above 531 °C), benzene and its derivatives continue to decompose, generating a more diverse group of aromatic hydrocarbons.

In a separate study, Seidelt et al. [52] analyzed the thermal decomposition of SBR, NR, and BR using GC/MS. Their findings showed that NR predominantly produced xylene and isoprene, while SBR mainly decomposed into ethylbenzene, styrene, and cumene. Unfortunately, no detailed BR product distribution was reported.

Overall, pyrolysis involves a combination of chemical and physical transformations, including chain scission, isomerization, and radical-driven recombination. The thermal stability of the polymers depends on the strengths of the bonds within the rubber matrix as stronger bonds degrade at higher temperatures, while weaker bonds break more readily. The composition and yield of the resulting pyrolysis products are governed by factors such as polymer type, reaction temperature, heating rate, and the presence of catalysts. A clear understanding of NR and SR degradation pathways is essential to optimize product yield and distribution via process control.

Key Factors Influencing Pyrolysis

Pyrolysis converts the polymeric components of waste tires into gas, liquid oil, and solid carbonaceous residues through thermal decomposition under oxygen-free or inert conditions. The performance of this process is controlled by a range of operational and material parameters, including the type and design of the reactor, composition and particle size of the feedstock, as well as temperature, pressure, heating rate, residence time, and catalytic conditions used during treatment. Among these variables, the reaction temperature and presence of catalysts have the highest impact on product distribution and overall efficiency [53]. Generally, non-catalytic systems operate at higher temperatures (≈350–600 °C), while catalytic systems allow pyrolysis to proceed at lower temperatures (≈300–450 °C). The effect of each parameter is described next.

A.

Temperature

Thermogravimetric analysis (TGA), often combined with mass spectrometry (MS), is generally used to evaluate how waste tire rubbers decompose upon heating. The main elastomers present are natural rubber (NR), styrene–butadiene rubber (SBR), and butadiene rubber (BR), each degrading within specific temperature ranges.

NR begins to break down around 326 °C, exhibits its maximum mass loss rate near 375 °C, and is almost completely decomposed by 455 °C. For SBR, the initial onset of degradation is about 286 °C, with a peak decomposition rate near 452 °C. The reaction is mainly completed by 491 °C. On the other hand, BR shows an initial degradation temperature near 374 °C, reaches its highest decomposition rate around 483 °C, and is almost fully decomposed by 497 °C [54].

These temperature trends indicate that, within mixed rubber systems, NR degrades first, followed by SBR, and finally BR, which begins its major decomposition at the highest temperatures.

Similarly, Han et al. [55] used combined MS and TGA results to divide waste tire (WT) pyrolysis into four temperature-dependent stages:

• <320 °C: evaporation of moisture and volatilization of plasticizers;
• 320–400 °C: primary decomposition of natural rubber (NR);
• 400–520 °C: breakdown of styrene–butadiene rubber (SBR) and butadiene rubber (BR);
• 520 °C: minimal additional mass loss.

On the other hand, Kan et al. [56] proposed a similar, but slightly simplified classification as the following:

• 200–350 °C: volatilization of oils, plasticizers, and other additives;
• 300–450 °C: degradation of NR;
• 400–500 °C: decomposition of SBR and BR.

Similarly, Islam et al. [57] reported overlapping temperature windows, with additives degrading between 150 and 350 °C, NR between 330 and 400 °C, and SBR/BR between 400 and 480 °C.

Taken together, these studies indicate a consistent sequence:

• Additives and low-boiling species are removed starting around 200 °C;
• NR degradation follows around 300 °C;
• SBR and BR begin to decompose near 400 °C;
• The pyrolysis of most tire rubbers is essentially finished by 500 °C.

Despite these common patterns, the results can vary widely between studies because of tire formulations, reactor designs, operating conditions, and catalysts used. Even when identical waste rubber materials are heated to the same temperature, the product distributions may not match as several studies reported that higher temperatures tend to increase gas yield, while decreasing oil yield [58,59]. Other works reported that oil production peaks, or reaches a minimum, at a specific temperature (599 °C) [60], while some studies observed opposite trends, i.e., the oil yield increased with temperature [61]. Overall, the reaction temperature is one of the dominant factors controlling oil production in waste-tire pyrolysis. A comparison of oil yields at different temperatures is provided in

Table 6. Effect of temperature on the yields of each compound fraction [59].

	Yields (wt.%)
Temperature (°C)	C1–C4 Fraction	C5–C10 Fraction	Tar or C11 Fraction	Total	Char
425	1.81	54.96	9.30	66.07	33.93
500	4.24	52.41	9.24	65.89	34.11
600	8.26	44.46	11.48	64.20	35.80

.

López et al. [59] examined the pyrolysis behavior of waste tires (WT) composed primarily of natural rubber (SMR 5CV, 29.59 wt.%), styrene–butadiene rubber (SBR 1507, 29.59 wt.%), carbon black (ISAF N220, 29.59 wt.%), along with additives. The tire material was cryogenically ground into particles smaller than 1 mm using a Retsch ZM 100 mill. Pyrolysis experiments were carried out in a continuous conical spouted-bed reactor equipped with a cylindrical extension at the top. Nitrogen was introduced at a flow rate of 9.5 L/min, and the system was operated continuously by feeding 3 g/min of shredded tire material for runs lasting about 30 min. The distribution of products obtained at different temperatures is summarized in Table 6. As the reaction temperature increased, the overall yield of volatile products showed a gradual decline, while the proportion of char (solid residue) experienced a slight upward trend.

Kaminsky et al. [58] studied waste tire pyrolysis using the Hamburg Pyrolysis Process, carrying out experiments in a laboratory-scale fluidized bed reactor with a processing capacity of 1 kg/h. The tests were conducted in the 500–600 °C range without catalysts. The feed mixture contained 50% SBR 1500, 27% carbon black N220, and smaller amounts of additional additives. As summarized in

Table 7. Typical product distribution (wt.%) as a function of pyrolysis temperature [58,61].

Temperature (°C)	Oil	Carbon Black (Char)	Gases	Ref.
500	65	30	5	[58]
550	57	34	9.2
600	51	40	9.1
350	30	50	20	[61]
450	33	40	27
550	38	33	29
550	38.1	49.1	2.39	[62]
600	33.0	47.4	8.16
680	32.8	48.9	10.75
450	50.47	36.47	13.06	[63]
500	51.98	36.09	11.92
550	52.61	35.69	11.70
600	54.10	36.30	9.61

, increasing the reaction temperature resulted in a lower oil production, while the yield of carbon black increased. Gas formation increased sharply between 500 and 550 °C, followed by a slight drop at 600 °C.

In another study, Diez et al. [61] evaluated the pyrolysis of shredded tire material at 350, 450, and 550 °C using a horizontal furnace equipped with a quartz reactor tube. Each experiment used 50 g of tire particles, and helium was supplied as the carrier gas at 200 mL/min. Only one experiment was run per temperature, with a residence time of 15 min. As reported in Table 7, the oil yields increased steadily (30 to 38%) with increasing temperature (350 to 550 °C).

Although temperature is widely recognized as the most influential variable governing waste tire pyrolysis behavior, the product distribution is also shaped by numerous other factors, such as system pressure, reactor configuration, heating rate, type of catalyst, and residence time. Depending on these conditions, the oil production may rise or fall at the same nominal temperature. Therefore, further systematic experimental studies under well-controlled and comparable conditions are required to establish clear quantitative trends for process optimization. Furthermore, using tailored catalysts can significantly shifts product selectivity, promoting either liquid or gaseous fractions.

B.

Catalysts

Catalysts play an important role in modifying the behavior of waste tire pyrolysis. By lowering the activation energy and accelerating reaction pathways, they reduce the thermal energy required for decomposition and can be reused because they remain chemically unchanged during the process. In comparison with purely thermal pyrolysis, catalytic operation typically promotes higher gas production, reduces liquid-oil yield, and has only a limited influence on the amount of solid carbonaceous residues formed.

Pyrolysis may proceed either thermally, relying solely on high temperatures, or catalytically where catalysts assist the breakdown of polymer chains. Thermal pyrolysis generally needs relatively high temperatures and extended residence times, often yielding oils of lower quality [64]. On the other hand, catalytic pyrolysis enhances product selectivity, improves oil quality, and enables operation at temperatures just above the decomposition threshold. By lowering both the temperature and reaction time required, catalysts can improve the overall process efficiency and reduce the formation of unwanted polycyclic aromatic hydrocarbons (PAH), contributing to a cleaner environment [38].

A wide range of catalytic materials has been explored to enhance the pyrolysis of waste tires. Some examples include CaC₂, MgO, ZSM-5, Cu/HBETA, MgCl₂, Ca(OH)₂, various zeolites, Al-MCM-41, Al-SBA-15, Al₂O₃, SiO₂, and AlCl₃ [64]. Among these, zeolite-based catalysts have drawn particular attention. Their porous structure and high surface activity often lead to higher gas and char yields while reducing the amount of liquid tire pyrolysis oil (TPO) produced [36]. Comparative data from the literature on zeolite catalysts and non-catalytic conditions are summarized in

Table 8. Summary of zeolite catalyst effects in waste tire catalytic pyrolysis.

Catalyst/Condition	Key Findings	Main Effect on Products	Ref.
General acid catalysts	Reduce liquid yield and increase gas yield	↓ Oil, ↑ Gas	-
USY and HZSM-5	Both increase gas yield	↑ Gas	[65,66]
USY with SiO2/Al2O3 = 5.3	Favors aromatic hydrocarbon formation	↑ Aromatics	[67]
USY with SiO2/Al2O3 = 11.5	Favors olefin formation	↑ Olefins	[67]
HZSM-5 (increasing amount)	Promotes aromatics, especially BTX	↑ Benzene, ↑ Toluene, ↑ Xylene; ↓ Non-aromatics	[68]
USY vs. HZSM-5	USY has larger pore size → produces more aromatics	USY: ↑ Aromatics	[69]
Y-type zeolites	Increase saturated and monoaromatics; reduce diaromatics/polyaromatics	↑ Saturates, ↑ Monoaromatics; ↓ Heavy aromatics	[70]
USY and HZSM-5	Produce high concentrations of BTX; reduce oil and increase gas yield	↑ BTX; ↓ Oil; ↑ Gas	[63,71]
USY	High BTX; reduced limonene due to larger pore size and lower acidity	↑ BTX; ↓ Limonene	[72]
SAPO-11 and HZSM-22	Gas yield order: SAPO-11 (10.45) > USY (9.97) > Hβ (8.24) > HZSM-5 (6.49) > HZSM-22 (6.17) > non-catalyst (4.5 wt.%)	SAPO-11 gives highest gas yield	[73]
HZSM-5	Produces highest oil yield: 56% (non-catalyst 55.5 wt.%)	↑ Oil, the other catalyst 53–55 wt.%.	[73]
SAPO-11	Highest gas production: 10%; lowest carbonization: 34% (non-catalyst 40%)	↑ Gas; ↓ Char	[73]

.

Overall, the studies showed that acidic catalysts significantly shape product distributions during waste-tire pyrolysis:

• USY and HZSM-5 typically enhance gas formation and favor the production of aromatic hydrocarbons, such as benzene, toluene, and xylene (BTX).
• The behavior of USY is influenced by its SiO₂/Al₂O₃ ratio: lower ratios tend to increase aromatic formation, while higher ratios tend towards olefin production.
• Larger-pore zeolites (USY) often generate more aromatics than HZSM-5 and also limit the formation of compounds such as limonene.
• Y-type zeolites increase saturated hydrocarbons and monoaromatic compounds, while decreasing heavy aromatics.
• SAPO-11 has been reported to give the highest gas yield with minimal carbon deposition, while HZSM-5 tends to produce the highest fraction of liquid oil.

These results collectively highlight that both the structure and acidity of the catalyst strongly affect the pyrolysis pathways of tire-derived polymers, ultimately determining the proportions and chemical nature of gases, liquids, and solids obtained.

Metal- and metal-oxide-modified catalysts have been shown to significantly improve the performance of catalytic waste tire pyrolysis. Precious metals, such as Pd, Pt, and Ru, tend to increase gas formation, promote hydrogenation pathways, and shift the liquid product toward lighter hydrocarbon fractions. Incorporating metal dopants generally enhances catalyst textural properties, such as surface area and pore volume, thereby improving overall catalytic activity.

Mesoporous materials, such as MCM-41 and MCM-48, preferentially generate light olefins and monoaromatic hydrocarbons, while suppressing the formation of heavier polyaromatic species and reducing sulfur-containing compounds in the product oil. Furthermore, metal-doped zeolites, for example Cu- or Ce-modified structures, were shown to enhance oil quality and limit coke deposition, underscoring their potential to upgrade pyrolysis-derived products.

A comparative summary of metal/metal-oxide-doped catalysts and mesoporous zeolite systems relative to non-catalytic conditions is provided in

Table 9. Summary of metal/metal-oxide-doped catalysts in waste tire pyrolysis.

Catalyst/Modification	Key Findings	Main Effect on Products	Ref.
Hβ and Pd/Hβ	Gas yield: non-catalytic 20%, Hβ 28%, Pd/Hβ 37%	↑ Gas; oil shifts to C9–C13 (gasoline–naphtha range)	[74]
TiO2, Pd/TiO2, Pt/Pd/TiO2, Bi2O3/SiO2	Gas yield: non-cat 20%, TiO2 27%, Pd/TiO2 41%, Pd–Pt/TiO2 40%	↑ Gas; improved catalytic activity due to morphology changes	[75]
Ru-doped zeolite	Increased activity; lower pyrolysis temperature; ↑ hydrogen production	↑ H2; ↑ catalytic efficiency	[76]
Metal-doped zeolite	Promotes hydrogenation; removes sulfur and oxygen	↑ Desulfurization; ↑ quality of oil	[77]
Cu-doped zeolite	Strong acid sites help reduce sulfur in products	↓ Sulfur content; ↑ oil quality	[78]
MCM-41 and Ru/MCM-41	↑ Gas; ↓ liquid; lighter oil; Ru/MCM-41 gives 4× light olefins vs. non-catalytic	↑ Light olefins; lighter fractions	[79]
MCM-41 (mesoporous)	Inhibits polyaromatics; promotes monoaromatics and saturates	↑ Monoaromatics; ↓ PAH	[79]
MCM-41, ZSM-5, and zeolite Y	Gas yield increased and oil yield decreased, Coke is lowest (2.5%), ZSM-5 and zeolite Y of 2.7 and 11.7%.	↑ Gas, ↓ Oil coke	[80]
Ru/MCM-48 (mesoporous, cubic)	Light olefins doubled; ↑ light oil; ↓ sulfur in aromatics	↑ Light olefins; ↑ light oil; ↓ sulfur	[81]

.

3.1.3. Feedstock Recycling (Chemical Dissolution)

Chemical dissolution is a form of feedstock recycling in which specific solvents are used to disassemble the rubber matrix of ELT, allowing the separate recovery of high-quality polymers and fillers [33]. Unlike devulcanization, which focuses primarily on breaking sulfur crosslinks to restore processability, dissolution targets a broader breakdown of the network structure. This method disrupts or bypasses the crosslinked network, while avoiding extensive degradation of the polymer backbone, making it a viable approach to extract relatively intact polymer chains from vulcanized rubber.

Three representative strategies are reported in the literature: decalin dissolution [82], gas-oil dissolution with clay separation [83], and reductive silylation (hydrosilane + B(C₆F₅)₃) [33], illustrating the versatility and chemical selectivity of dissolution-based recycling.

Decalin Dissolution Method

Sultan et al. [82] proposed a small-scale technique using decalin as the solvent. About 10 g of ground tire material was immersed in decalin at 50 °C for 30 days, during which the rubber was slowly swelled and dissolved. The swollen material was then heated under reflux at 198 °C for 3 h, followed by centrifugation to separate carbon black and other insoluble fillers. The dissolved polymer was then precipitated using methanol and dried to obtain a rubbery solid. Although this process yields relatively clean polymer, it is inherently slow, limited to a small sample, and difficult to scale-up for commercial/industrial applications.

Gas-Oil Dissolution with Clay-Assisted Separation

Abdulrahman et al. [83] introduced a method using gas oil as the dissolving medium, supported by 4-hydroxy-TEMPO to improve solution homogeneity and inhibit unwanted side reactions during heating. In this system, gas oil dissolves the polymeric fraction, while carbon black and inorganic additives remain insoluble. Montmorillonite clay is added after dissolution to adsorb fillers, allowing the rubber to be recovered as a viscous polymer fraction. From 5 g of tire waste, the process yielded 0.955 g (19%) of rubber and 0.3 g (6%) of carbon black, both showing chemical properties similar to their virgin counterparts. More importantly, the gas-oil solvent and clay can be regenerated, indicating good potential for repeated use (circularity).

Reductive Silylation Method

Zheng et al. [33] developed a reductive silylation process involving a hydrosilane reductant combined with a Lewis acid catalyst B(C₆F₅)₃. This system selectively cleaves sulfur-based crosslinks (S–S and polysulfides), while preserving the polymer backbone, converting the vulcanized rubber network into soluble silylated polymer oils. High conversion efficiencies were obtained: 92–93% for butyl inner tubes and 88% for mixed tire crumb after repeated treatments. In particular, the recovered polymer oils can be re-crosslinked to form new elastomeric materials, suggesting potential for closed-loop recycling. However, the method is currently limited to very small samples and is not yet applicable to industrial-scale processing.

An overall assessment includes the following:

• Chemical dissolution offers a promising route to recover high-quality polymers from end-of-life tires.
• Decalin dissolution relies mainly on long-term swelling and solvent extraction.
• Gas-oil + clay separation enables more targeted separation of rubber and fillers with recyclable reagents.
• Reductive silylation provides the most efficient and selective dissolution, producing high-purity polymer oils with the possibility to form new elastomers.

While these methods confirmed the advantages of selective dissolution compared to mechanical processing or pyrolysis, they face challenges including low throughput and high solvent cost, as well as environmental and operational difficulties associated with solvent handling and recovery. Overcoming these limitations is essential before such techniques can be implemented at larger scale.

3.1.4. Energy Recovery (Thermal Recycling)

Energy recovery refers to the combustion of shredded waste tires as tire-derived fuel (TDF) in industrial facilities such as cement kilns, power plants, and industrial boilers [84]. In these controlled high-temperature systems, TDF can partially replace conventional fuels, reducing disposal volume by up to 90%, and in some cases allowing recovery of metal residues from the remaining ash. TDF is characterized by a high calorific value (33–37 MJ/kg, comparable to coal), making it a dense, consistent energy source and an economically attractive supplemental fuel for industrial combustion systems [85].

Despite these advantages, thermal recycling presents several drawbacks. The combustion of tires leads to gas emissions, including CO₂ and regulated pollutants, and produces ash containing hazardous substances that must be managed under strict environmental controls. Furthermore, using tires as fuel eliminates valuable rubber, fiber, and filler materials that could otherwise be recovered, potentially diverting waste away from more sustainable recycling options.

3.1.5. Rubber Devulcanization (Reclaiming)

Vulcanized rubber consists of a three-dimensional network of sulfur-based crosslinks (C–S and S–S), imparting durability but severely limit reprocessability [46]. Devulcanization seeks to selectively cleave these sulfur bonds while largely preserving the carbon–carbon backbone, typically achieving a 50–70% reduction in crosslink density [86]. This can be achieved using mechanical/physical, chemical, thermo-chemical, or biological methods, as summarized in Figure 12 [87]. Regardless of the approach, process efficiency is commonly evaluated through parameters such as the degree of bond cleavage, soluble fraction, and residual crosslink density, which collectively determine the suitability of the reclaimed rubber for reuse [87].

Burelo et al. [88] examined elastomer degradation processes, mainly for natural and synthetic rubbers. Thermal, mechanical, and physical degradation pathways were discussed at a general level, whereas chemical degradation (e.g., depolymerization via metathesis) and biological degradation were treated in greater detail, with proposed mechanistic pathways. As thermoset materials, rubbers are inherently resistant to conventional mechanical recycling. However, chemical degradation routes can cleave polymer backbones and/or crosslinks, producing low-molecular-weight species that may be used as feedstocks for new materials. From a sustainability standpoint, such approaches enable the recovery of monomers or functional oligomers for repolymerization or upcycling, contributing to circular material flows.

In contrast to degradation, devulcanization is designed to preferentially break sulfur crosslinks while minimizing backbone scission, thereby allowing material reuse with partial retention of mechanical properties. Degradation, by comparison, involves extensive main-chain scission and results in irreversible deterioration of physical, mechanical, thermal, and chemical properties. In practice, especially for vulcanized rubbers, devulcanization and degradation often occur simultaneously, making fully selective crosslink cleavage without some degree of backbone damage difficult to achieve.

Physical Devulcanization

Physical devulcanization methods aim to selectively cleave the C–S and S–S bonds in waste tire rubber while limiting damage to the main polymer backbone. This selectivity is crucial to preserve mechanical strength in reclaimed materials. Several forms of physical energy can be applied to achieve this goal including thermal, mechanical, thermo-mechanical, microwave, ultrasonic, and thermosonic techniques [87]. Mechanical size-reduction processes, such as shredding, granulating, and grinding, are often the first step to reduce the particle sizes (easier handling) and increase the surface area (improved heat/mass transfer). This step produces crumb rubber, which can then be used in products such as composite materials, flooring, and asphalt modifiers [28]. In some cases, partial devulcanization is performed to restore a degree of elasticity and improve compatibility with fresh rubber (improved chain mobility). Although these physical approaches are generally low cost and relatively simple to implement, the properties of the recovered material usually remain lower than those of virgin rubber.

A.

Thermal devulcanization

Thermal devulcanization is the earliest known method to reclaim rubber, having been introduced about 150 years ago. It relies on applied heat to disrupt the crosslinked structure of vulcanized materials [89]. Achieving selective cleavage requires an understanding of bond dissociation energies, which are 614, 347, 273, and 227 kJ/mol to break C=C, C–C, C–S, and S–S bonds, respectively [90]. Although sulfur-based bonds require less energy to cleave, their values are still close to that of C–C bonds, making highly selective devulcanization difficult. As a result, both C–S and S–S bonds can be broken, but backbone scission may also occur.

In the practical thermal devulcanization of waste tire rubber (WTR), steel and fiber are first removed, after which the rubber is processed into ground tire rubber (GTR). The material is then treated in an autoclave with steam, commonly for around 12 h at 180–260 °C and 15 bar, followed by drying (Figure 13). Although this procedure effectively decreases the crosslink density, it lacks selectivity and often produces reclaimed rubber with lower mechanical performance [87]. As reported by De et al. [89], this approach is more effective for natural rubber (NR) than for styrene-butadiene rubber (SBR) due to the higher thermal stability of SBR, whose crosslink structure is more resistant to heat-induced cleavage.

B.

Mechanical and thermo-mechanical devulcanization

Mechanical devulcanization depends solely on shear/elongational forces, such as during two-roll milling or grinding, to induce limited but clean cleavage of sulfur-based crosslinks. One example is the patented High Shear Mixing (HSM) process, which devulcanizes rubber manufacturing scrap entirely through mechanical action [92]. Because sulfur bonds have lower bond energies than carbon–carbon bonds (see previous section), applying sufficiently high stresses can preferentially break these crosslinks. The technology, reported to be suitable for different categories of rubber waste, operates by repeatedly subjecting the material to alternating high stress and relaxation zones. A built-in cooling system maintains the temperatures below 80 °C throughout mixing to avoid thermal degradation [92]. In principle, the rubber can be devulcanized solely through stress at relatively low temperatures, but most practical processes in this category combine mechanical forces with heat.

Thermo-mechanical devulcanization is the predominant industrial approach because rubber crumbs are simultaneously exposed to strong shear and elevated temperatures, typically in the range of 150–250 °C, generated by internal friction and external heating. Various solvents, such as water, oils, hexane, or supercritical fluids (N₂ and CO₂), may be introduced before or during grinding to improve swelling, lower viscosity, and help solubilize short polymer fragments, thereby facilitating crosslink breakdown [28]. Over the last decades, this method has been widely investigated because it can achieve high degrees of devulcanization. For example, at constant screw speed (120 rpm), the devulcanization rate was higher at 220 °C than 280 °C, reaching 88% and 85%, respectively [28]. Studies on waste tires consistently showed that thermo-mechanical techniques yield reclaimed rubber with superior tensile properties and reduced gel content when compared with processes relying strictly on mechanical shear.

Mouri et al. [93] proposed a thermo-mechanical devulcanization mechanism as illustrated in Figure 14. In their model, the thermal energy first cleaves the weaker polysulfidic bridges, while the stronger monosulfidic linkages require additional shear stress to break. During this process, some backbone scission may occur, leading to the formation of compounds such as carbon disulfide (CS₂), sulfur dioxide (SO₂), and hydrogen sulfide (H₂S). Owing to these mechanistic features, most thermo-mechanical technologies make use of both elevated temperature and mechanical shear.

Common mechanical and thermo-mechanical devulcanization systems include batch mixers (open mills and internal mixers) and continuous devices (single- or twin-screw extruders), although numerous alternative designs have also been evaluated in the literature. Batch mixer devulcanization is simple, economical, and environmentally benign because it generally operates without added chemicals or external heating [87]. In these mixers, the crumb rubber is exposed to intense shear for several minutes, during which internal friction can raise the temperature up to 250 °C [94]. To mitigate overheating and minimize molecular degradation, a water-cooled two-roll mill has recently been applied to devulcanize carbon black-filled NR [95], but this configuration yielded only a modest devulcanization efficiency (20–37.8%).

Among continuous processes, extruder-based technologies (especially Ficker-type devices and single- or twin-screw extruders) are the most widely used because of their industrial availability and ability to handle high throughput. Devulcanization behavior is governed primarily by barrel temperature, screw geometry, and screw speed. The temperature must remain above 50 °C to ensure shear-softening, yet below 400 °C to prevent extensive thermal degradation [96]. Numerous studies showed that moderate processing temperatures (180–220 °C), combined with optimized screw speeds (80–150 rpm), promote effective crosslink cleavage while minimizing main-chain scission, consistent with Horikx-type analyses [97]. For example, the devulcanization of GTR in a twin-screw extruder with a L/D ratio of 40 was optimized at 220 °C and 120 rpm. High-shear or plasticizing screw designs further enhance the performance at reduced temperatures, while additives (waste engine oil) can improve sulfidic selectivity and lower gel fraction [98]. However, excessive sol-fraction development indicates undesirable backbone degradation negatively affecting the quality of reclaimed rubber [99]. Several studies consistently showed that optimized thermo-mechanical extrusion can achieve high degrees of devulcanization (50–70%), even without chemical agents, producing reclaimed rubber with properties close to those of virgin compounds [97,100,101].

Other thermo-mechanical concepts have been explored, including High-Pressure High-Temperature Sintering (HPHTS) [102]. In this method, vulcanized rubber powder is compressed at 0.5–26 MPa and heated between 80 and 240 °C. The applied pressure enhances particle-particle contact, while the elevated temperature induces limited crosslink and chain cleavage. These conditions promote the formation of new covalent bonds at the particle interfaces, creating a consolidated material whose mechanical properties can be similar to conventionally produced rubber.

C.

Microwave devulcanization

Microwave devulcanization uses microwave radiation to generate rapid, volumetric heating within the rubber matrix, promoting selective cleavage of S–S and C–S crosslinks while ideally avoiding disruption of the stronger C–C backbone bonds [103]. When the alternating electromagnetic field interacts with dipolar molecules or ionic species in the rubber, the material can heat very quickly, often reaching 250–350 °C. This makes the technique highly suitable for polar rubbers and materials containing microwave-absorbing fillers such as carbon black [87]. Because heating occurs uniformly throughout the volume rather than from the surface inward, microwave processing can yield high degrees of devulcanization (up to 95%) within short exposure times (15–60 s) without the addition of chemical reagents [104]. The concept was first patented by Goodyear [105], showing efficient devulcanization at frequencies between 915 and 2450 MHz and energy inputs of 325–1404 kJ/kg, producing reclaimed rubber that could be revulcanized with relatively minor property losses. Subsequent work focused on the importance of carefully controlling both temperature and irradiation time since inadequate control can trigger thermo-oxidative degradation or excessive C–C scission, especially with NR. A study by García et al. [106] indicated that the final temperature achieved during irradiation highly determines which sulfur bonds were cleaved and how selective the devulcanization can be.

Spectroscopic evidence, such as reduced S–S and C–S vibrational bands in FTIR spectra, confirmed that microwave treatment effectively broke sulfur-containing crosslinks [104]. The reclaimed rubber obtained from this method was successfully incorporated into various polymer composites and often provides better mechanical reinforcement than untreated GTR. However, the efficiency of the process strongly depends on the carbon-black content: high CB level rubbers devulcanize more readily, while low CB SBR can even exhibit increased crosslink density after irradiation [107]. Overall, microwave devulcanization is one of the most efficient physical recycling strategies because it is fast with uniform heating, leading to high devulcanization yield and minimal use of chemical additives.

D.

Ultrasonic devulcanization

Ultrasonic devulcanization uses high frequency mechanical vibrations to generate alternating tensile–compressive stresses and cavitation within the rubber, leading to selective cleavage of S–S and C–S crosslinks while leaving most of the backbone intact. The method proceeds rapidly, does not require solvents, and enables good control over the final properties of the reclaimed rubber. However, widespread industrial use remains limited due to the specialized, high-power ultrasonic equipment and electricity costs [103].

The first description of ultrasonic devulcanization was provided in Pelofsky’s 1973 patent [108], which proposed that solid rubber immersed in a liquid medium could be broken down by intense ultrasonic waves. Later work by Isayev and co-workers [109] applied this approach to SBR and GTR, finding that higher devulcanization generally produced improved mechanical behavior, while excessive degradation had the opposite effect. Typical properties for devulcanized waste tire rubber included tensile strength (1.5–10.5 MPa), elongation at break (130–230%), and tensile modulus (1.7–3.5 MPa), with performance governed by the degree of crosslink breakdown rather than the specific processing conditions [109].

Yun et al. [110] investigated ultrasonic devulcanization of carbon-black-filled EPDM in a grooved-barrel reactor. The initial low devulcanization degree was attributed to ultrasonic disruption of carbon-black-rubber interactions, but higher ultrasound amplitude substantially increased the extent of crosslink cleavage. Subsequent studies showed that variables, such as initial molecular weight [111], particle size [112], and ultrasonic amplitude [113], strongly influenced the devulcanization outcome. Compared with coaxial and barrel-type ultrasonic reactors, Yun et al. [110] reported that the shear stresses generated in the barrel reactor produced reclaimed rubber with superior mechanical properties (tensile strength ≈ 8.7 MPa, elongation ≈ 217%, modulus ≈ 2.6 MPa), while limiting chain scission. Additional research confirmed that applying shear in combination with ultrasonics promotes crosslink rupture, lowers gel fraction and viscosity, but NR-rich GTR devulcanizes more readily than SBR-rich material. Nevertheless, additives such as alkyl-phenol polysulfide can further enhance the process efficiency and mechanical recovery [87].

Chemical Devulcanization

Chemical devulcanization uses specific reagents that selectively cleave sulfur-based crosslinks (S–S and C–S bonds), while leaving the stronger C–C backbone mainly intact. By breaking these specific bonds, the three-dimensional network structure is partially opened, making the rubber suitable for further processing. Based on the nature of the active agent involved, chemical devulcanization is generally divided into four major categories (see Figure 11): catalytic systems [108,114], nucleophilic agents [115], radical-based agents [116,117], and miscellaneous approaches that do not fit into other groups. Together, these categories represent the principal chemical strategies to decease crosslink density with minimal damage to the polymer chains. Because the reagents show high selectivity toward sulfur-containing bonds, chemical devulcanization can be performed at relatively low temperatures (100–200 °C) while limiting backbone scission. Nonetheless, challenges exist including reagent cost and environmental and safety considerations, as well as the need for strict reaction control to avoid unintended degradation or insufficient crosslink cleavage [28]. A wide range of chemical agents has been reported, including sulfides, mercaptans, amine-based compounds, various inorganic reagents (propane thiol/piperidine, PPh₃, trialkyl phosphites, LiAlH₄, methyl iodide), and different ionic liquids (IL) [87].

A.

Catalytic devulcanization

Catalytic devulcanization involves the use of catalysts accelerating the selective scission of S–S and C–S crosslinks while minimizing damage to the C–C polymer backbone [87]. Reported catalyst families include organosulfur species (thiols, disulfides), silane- or organosilicon-based catalysts, organoboron compounds, and various transition-metal complexes [118,119]. Representative examples include propane thiol/piperidine systems, Grubbs catalysts, PPh₃, trialkyl phosphites, LiAlH₄, methyl iodide, DBU, and benzoyl peroxide (BPO). Some examples of available studies are the following:

• Gutierrez et al. [120] showed that natural rubber (NR) can undergo cross-metathesis with α-pinene using a ruthenium-based second-generation Grubbs catalyst, achieving 80–90% yield under solvent-free conditions. The molecular weight of the product could be tuned by adjusting the NR/α-pinene ratio.
• Smith et al. [121] reported catalytic depolymerization of polybutadiene and styrene-butadiene rubber using first- and second-generation Grubbs catalysts. The reactions proceeded at room temperature over 2–3 h with minimal chain degradation, highlighting the mild conditions enabled by these catalysts.
• Sodium desulfurization systems showed the possibility to selectively cleave mono-, di-, and polysulfidic crosslinks in swollen vulcanizates under high-temperature, oxygen-free conditions [122]. The resulting reclaimed rubber retained molecular weights similar to virgin material. However, the method is constrained by its use of organic solvents.

Advantages include the following: Catalytic approaches offer high selectivity toward sulfur-containing bonds, promote retention of the polymer backbone, enable shorter reaction times, and typically operate under relatively mild thermal conditions. As a result, the reclaimed rubber often exhibits superior mechanical properties compared with products obtained through non-selective methods.

Limitations include the following: The main drawbacks are the cost and sensitivity of many catalysts, the potential need for organic solvents, and the requirement for controlled reaction environment factors limiting both economic feasibility and environmental sustainability.

B.

Radical and nucleophilic devulcanization

Chemical devulcanizing agents, such as disulfides, polysulfides, TESPT, and various amines, can selectively cleave sulfur-containing crosslinks through either radical or nucleophilic pathways [123]:

• Radical-based mechanism [116]: In this route, sulfur bonds (mono-, di-, and polysulfidic links) undergo homolytic cleavage, generating macromolecular free radicals. Stabilizing reagents, such as diphenyl disulfide (DPDS), are often incorporated to inhibit radical recombination and favor controlled crosslink scission. Common radical initiators include DPDS, dithiobis(benzothiazole), tetramethylthiuram disulfide (TMTD), diallyl disulfide, and di(2-aminophenyl) disulfide.
• Nucleophilic mechanism [115]: Nucleophilic agents react directly with sulfur atoms in the crosslinks through substitution pathways, breaking S–S or C–S bonds without forming free radicals. Representative examples include 2-mercaptobenzothiazole, thiosalicylic acid, 1-hexadecylamine, and N-cyclohexyl-2-benzothiazolesulfenamide.

Some examples of available studies are the following:

• De et al. [124] applied TMTD as a devulcanizing agent for GTR using a two-roll mill. The resulting reclaimed rubber was then blended with virgin NR, and the mechanical performance of the revulcanized blends was evaluated.
• Sutanto et al. [96] and Walvekar et al. [125] reported effective devulcanization of waste tire rubber using amine-based chemicals, showing selective sulfur-bond cleavage and enhanced reactivity upon revulcanization.

The selection of a devulcanizing agent and mechanism is influenced by the structure of the rubber, the targeted degree of crosslink reduction, and the need to preserve polymer backbone integrity. Radical pathways are often favored for materials with high crosslink density, while nucleophilic or amine-based approaches offer more controlled de-crosslinking, making them suitable for rubber systems that are more sensitive to degradation.

C.

Other chemical devulcanization

• Periodic acid (HIO₄) acts as a strong oxidizing reagent that cleaves the polymer backbone through oxidative scission. Significant reductions in molecular weight are achieved but do not selectively target sulfur-based crosslinks [126].
• Benzoyl peroxide (BPO) and nitric acid (HNO₃) have also been examined for their ability to oxidize sulfur crosslinks, offering an alternative route for selective S–S and C–S bond disruption [127,128].
• Additional strategies involve swelling-assisted devulcanization, in which additives, such as ionic liquids (IL) [129], supercritical carbon dioxide (scCO₂) [130], or natural oils (soybean) [131], are used to swell the rubber network. Swelling increases the penetration of chemical agents, improves accessibility to sulfur bonds, and enhances overall devulcanization efficiency as chemical bonds under tension are easier to break.

These alternative approaches complement catalytic, radical, and nucleophilic methods, expanding the toolbox available to reclaim vulcanized rubber and provide greater flexibility in designing processes for recycling or upcycling waste rubber into new materials.

Table 10. Summary of the chemical devulcanization processes.

Category	Mechanism	Representative Agents/Examples	Ref.
Catalytic	Catalysts selectively cleave S–S and C–S bonds	Grubbs catalysts (NR, SBR); PPh3; trialkyl phosphites; LiAlH4; methyl iodide	[120,121]
Radical-based	Homolytic cleavage of sulfur crosslinks	DPDS; TMTD; diallyl disulfide; dithiobis(benzothiazole)	[123,124]
Nucleophilic	Nucleophilic substitution at sulfur atoms	Amines; mercaptobenzothiazole; thiosalicylic acid	[115,123]
Oxidative/others	Oxidative or non-selective bond cleavage	Periodic acid (HIO4); nitric acid (HNO3); benzoyl peroxide (BPO)	[127,128]
Swelling-assisted	Swelling improves access to sulfur bonds	Ionic liquids; scCO2; soybean oil	[129,130,131]

compares the different methods in terms of their mechanism, agents used, and specific conditions.

Biological Devulcanization

Emerging devulcanization technologies, including microwave-assisted processes, biological degradation pathways, and supercritical fluid treatments, offer more environmentally sustainable alternatives by enabling selective crosslink cleavage under relatively mild conditions [132]. Although these techniques show considerable promise, most of them remain in early stage research or pilot-scale testing and require further development before they can be implemented on an industrial scale. Among them, biological desulfurization is a green and cost-effective approach because of its inherent selectivity toward sulfur-containing bonds. However, its practical adoption is still constrained by slower reaction rates compared to physical or chemical processes [132].

Biological devulcanization relies on microorganisms or isolated enzymes able of selectively attacking sulfur-based bonds inside vulcanized rubber. Bacterial desulfurization uses sulfur-metabolizing species, such as Rhodococcus and Thiobacillus, which can modify or cleave C–S and S–S bonds. Fungal pathways involve fungi, such as Aspergillus and Penicillium, producing oxidative enzymes that soften or disrupt the crosslinked rubber network. On the other hand, enzymatic desulfurization uses purified enzymes, including laccases, peroxidases, and sulfur oxygenases, to target sulfur bonds directly under controlled, low-temperature conditions. Collectively, these biological strategies provide highly selective, energy-efficient routes to devulcanize rubber, with potential for future scaling as bioprocess optimization advances.

A.

Bacterial desulfurization

Although vulcanized rubber is typically resistant to microbial degradation, numerous studies have shown that biological devulcanization can be achieved when specific bacterial strains able of targeting sulfur crosslinks are used [103,133]. Bacterial desulfurization has been shown for various rubber substrates, including GTR, natural rubber (NR), and latex, using microorganisms such as Sphingomonas sp. [134], Alicyclobacillus sp., Gordonia desulfuricans, Nocardia, Rhodococcus sp., and Bacillus cereus [87]. Depending on the bacterial strain, the process may proceed under aerobic conditions through sulfur oxidation or under anaerobic conditions via sulfur-reducing pathways.

Typical treatment conditions involve temperatures around 30 °C and exposure times ranging from 1 to 30 days. Reported sulfur reductions generally fall within 8–30%, resulting in partially devulcanized rubber that can be revulcanized to produce materials with mechanical performance approaching that of virgin compounds.

B.

Fungal desulfurization

Fungal desulfurization leverages the oxidative capabilities of certain fungi, whose extracellular enzymes can selectively cleave sulfur-containing crosslinks in vulcanized rubber. Bredberg et al. evaluated three white-rot fungi (Pleurotus sajor-caju, Trametes versicolor, and Ricinicum bicolor) using the model polymer Poly-R478. They found that R. bicolor was the most effective, with its growth rate correlating strongly with sulfur oxidation within the rubber matrix [135]. In another study, Ceriporiopsis subvermispora was shown to degrade vulcanized natural rubber sheets placed on a wood substrate, achieving a 29% reduction in sulfur content after 200 days of incubation at 28 °C [136].

The devulcanization activity of fungi was attributed to ligninolytic enzymes, such as laccases, manganese peroxidases, and lignin peroxidases, promoting highly selective, low-temperature cleavage of sulfur bonds. Although fungal processes are slower than mechanical or chemical methods, they offer an environmentally benign and energy-efficient route for rubber recycling. Current research focuses on accelerating the reaction rates and expanding the applicability of fungal systems to a wider range of rubber formulations.

C.

Enzymatic desulfurization

Enzymatic desulfurization involves the application of isolated enzymes that selectively target sulfur-containing crosslinks in vulcanized rubber. Although this approach has been less investigated than bacterial or fungal pathways, several oxidative enzymes (laccases, peroxidases, and sulfur-metabolizing oxygenases) showed the ability to cleave or modify S–S and C–S bonds under mild, environmentally friendly conditions. Some examples of available studies are the following:

Sato et al. [137] reported that oxidative enzymes, including manganese peroxidase (MnP), horseradish peroxidase (HRP), and laccase (in the presence of a mediator), were able of degrading both vulcanized and non-vulcanized polyisoprene. The enzymes produced free radicals from unsaturated lipids within the rubber, initiating lipid peroxidation and consequent disruption of the polymer network. Pyrolysis GC–MS detected isoprenoid fragments, confirming enzymatic cleavage of rubber components.

Ferreira et al. [138] investigated four enzymes of the microbial 4S desulfurization pathway (DszA–D), showing their potential for selective sulfur removal, but also highlighting the need for substantial rate enhancement (around 500 fold) to reach industrial viability. Their mechanistic analysis revealed that DszA uses a unique flavin monooxygenase mechanism involving a N5OOH intermediate stabilized within a hydrophobic O₂-binding pocket. This insight provides a foundation to design faster and more efficient enzyme variants.

These enzymatic systems promote oxidative transformation of sulfur links with minimal damage to the polymer backbone, offering a controlled and highly selective devulcanization route. However, practical limitations, including poor enzyme stability in hydrophobic rubber environments, inhibition by carbon black, and the high cost of enzyme production and purification, currently restrict their broader application. As a result, enzymatic devulcanization remains at the proof-of-concept stage, although providing valuable mechanistic understanding that complements broader biological devulcanization research.

Biological devulcanization can be carried out using bacteria, fungi, or isolated enzymes, each with distinct strengths and challenges. Bacterial methods are selective and operate under mild conditions but are slow, surface-limited, and vulnerable to contamination. Fungal strategies offer strong selectivity and sustainability but suffer from extremely long processing times and dependence on specific growth conditions. Enzymatic approaches provide precise sulfur-bond targeting yet face hurdles in stability, cost, and scalability (see

Table 11. Comparison between bacterial, fungal, and enzymatic biological devulcanization methods, highlighting their main advantages and disadvantages.

Method	Advantages	Disadvantages
Bacterial	Selective sulfur bond cleavage; mild conditions; applicable to NR, GTR, latex; re-vulcanizable	Slow (days–weeks); surface-limited; contamination risk; scale-up challenges
Fungal	Highly selective via ligninolytic enzymes; mild and sustainable; effective on various rubbers	Very slow (weeks–months); surface-limited; specific growth media needed; scale-up difficult
Enzymatic	Targeted bond cleavage; high selectivity; controllable mechanism	Enzyme stability issues; inhibited by additives; costly; limited industrial adoption

). Collectively, these biological routes represent environmentally friendly alternatives to traditional devulcanization methods, with ongoing efforts aimed at improving efficiency, rate, and feasibility for industrial rubber recycling.

3.2. Inner Tube Recycling

3.2.1. Butyl Recycling

Most bicycle inner tubes are made from butyl rubber, a synthetic elastomer prized for its low gas permeability with high durability and chemical resistance. Butyl rubber can be recycled through several established pathways. Mechanical recycling, including shredding or cryogenic grinding, converts waste inner tubes into fine powders that can serve as fillers or performance modifiers in new rubber compounds. Thermal recycling, such as pyrolysis, decomposes vulcanized butyl rubber at elevated temperatures to yield liquid hydrocarbons, gases, and solid residues. Chemical or catalytic devulcanization selectively cleaves sulfur crosslinks, often using siloxanes, oils, or specialized catalysts, to regenerate flexible rubber able of revulcanization. Biological and enzymatic devulcanization, although slower, offer selective sulfur-bond cleavage under mild, environmentally friendly conditions.

Because bicycle inner tubes are relatively clean and straightforward to collect, they provide an excellent feedstock for recycling. The same recycling processes applied to waste bicycle tires, including mechanical grinding, devulcanization (thermal, chemical, or biological), or energy recovery, can be effectively adapted to butyl inner tubes. The optimal method depends on the intended end use, processing scale, and environmental considerations. Recycled butyl rubber can be incorporated into new elastomeric products, blended with other polymers, or used in composite materials, extending its service life and reducing waste.

Zheng et al. [33] proposed an efficient hydrosilicone-based recycling method for bicycle inner tubes (Chaoyang 700 × 38/45C, China). The tubes contained 61.1 wt.% organic components (isobutene–isoprene rubber, IIR), 33 wt.% carbon black, and 5 wt.% inorganic solids. After cryogenic grinding to produce homogeneous rubber powder, 300 mg of material was dispersed in 12 mL of dry toluene, followed by the addition of tetramethyldisiloxane (MH–MH, 8.5 mmol). A catalyst solution (600 μL, 50 mg mL⁻¹ in toluene; 10 wt.% catalyst relative to rubber) initiated the reaction. Heating the suspension in a 100 °C oil bath for 30 min yielded polymer recovery efficiencies between 56% (mixed crumb) and 93% (pure bicycle butyl rubber), with 87% recovery under typical conditions. After filtration to remove solid residues, the recovered polymeric oils could be crosslinked, either radically or oxidatively, to form new elastomers, optionally reinforced with the reclaimed solids. This mild and efficient hydrosilicone reduction process provided a practical strategy to reuse polymeric components from sulfur-crosslinked rubbers, including tires and inner tubes [30].

Singh et al. [139] applied pyrolysis to waste automobile inner tubes (85 g batches), obtaining a product composed of 64% liquid hydrocarbons, 11.64% light gases, and 24.36% solid residues. Each sample was placed in a glass reactor inside a muffle furnace, with individual runs lasting 3–4 h. Temperatures between 340 °C and 375 °C produced liquid products suitable as environmentally friendly petrochemical substitutes derived from waste rubber.

Jiang et al. [130] investigated the devulcanization of sulfur-cured butyl rubber (Polysar 301) in supercritical CO₂ using diphenyl disulfide (DD) as a devulcanizing agent. DD selectively cleaved sulfur crosslinks, while tetramethylthiuram disulfide (TMTD) served as a vulcanization accelerator to enhance reaction efficiency. By optimizing parameters, such as temperature, pressure, and reagent dosage, the authors achieved near-complete devulcanization, yielding a sol fraction up to 98.5%. These results highlight the efficiency of DD-assisted scCO₂ processes to regenerate highly processable reclaimed butyl rubber.

Finally, Schwalbe operates one of the most advanced large-scale recycling systems for bicycle inner tubes, collecting used tubes across Europe and North America, and processing them via their proprietary devulcanization technology at its facility in Indonesia [25]. This process recovers high-quality butyl rubber while reducing energy consumption by 80% compared with the production of virgin butyl. The reclaimed material is reintegrated into new Schwalbe inner tubes, currently containing around 20% recycled content, creating a successful closed-loop system showing the industrial feasibility and environmental advantages of inner-tube recycling (see Figure 15).

3.2.2. Latex Recycling

Waste natural rubber latex can be recycled through several different routes, including mechanical, chemical, and thermal processes. Mechanical recycling converts latex scrap into fine particles by shredding or grinding, allowing blending with virgin rubber into new formulations. Chemical devulcanization, often carried out using disulfides, amines, or related reagents, selectively cleaves sulfur crosslinks and produces a softened rubber phase that can be revulcanized into new products. Thermal recycling, such as pyrolysis, degrades latex waste at high temperatures to yield liquid hydrocarbons, gases, and solid char, providing an additional pathway for energy recovery [140].

In particular, the Rubber Research Institute of India (RRII) described a reclaiming process for waste latex products involving mechanical size reduction, oil-assisted compounding, and thermal treatment [141]. This study showed that reclaimed latex rubber can be obtained with good processability. However, it also highlights the limitations in reuse, as only small amounts (25%) of reclaimed material can be blended with virgin natural rubber without significant deterioration of mechanical properties.

3.2.3. TPU Recycling

Thermoplastic polyurethane (TPU) can be recycled using both mechanical and chemical strategies. Mechanical recycling relies on shredding followed by remelting, enabling TPU to be reprocessed into extruded or molded components. Chemical recycling techniques, such as glycolysis, hydrolysis, or depolymerization in supercritical fluids, break TPU down into its monomeric building blocks, including polyols and di-isocyanates, which can then be repolymerized into new TPU materials. These approaches promote material circularity and reduce environmental impacts associated with TPU waste [142]. Nevertheless, TPU is a thermoplastic material which can generally be recycled through conventional melt reprocessing routes.

4. Applications of Recycled Rubber

As described above, waste bicycle tires (WBT) and end-of-life tires (ELT) are composed of rubber, carbon black, steel, and textile fibers. These materials can be separated after shredding and mechanical processing. In most recycling systems, the main output is rubber granules, which typically account for roughly 70% of the recovered mass, while steel represents 5–30%, and textile fibers make up as much as 15% of the total [143]. Unlike rubber and steel, which have well-established reuse pathways, textile fibers are often classified as special waste and therefore require disposal or use in energy recovery. For example, Ecopneus reported in 2013 that about 60% of the contaminated textile fraction was used as fuel in cement kilns, 25% was used for electricity generation, and the remaining 15% was landfilled [144]. Steel is generally reintegrated into metallurgical processes, while textile fibers can sometimes be repurposed as reinforcement in recycled polymer composites [45].

According to the Ecopneus Green Economy Sustainability Report 2023, ELT shredding plants operating within the Ecopneus system processed a total of 177,955 tons of material in 2023. The dominant product stream was vulcanized rubber, including shreds, chips, SCCS (steel-cleaned cut shreds), granules, and powders, which accounted for 84.4 wt.% of the total output. Steel represented 9.7 wt.%, textile fibers 5.7 wt.%, and production scraps only 0.1 wt.% [145]. These recent data confirm that coarse rubber fractions remain the primary recovered material, with finer granules, steel, and textiles forming the remaining fractions.

4.1. Application of Vulcanized Rubber

The rubber fraction recovered from ELT is commonly processed into vulcanized rubber granules (VRG), which serve a wide array of applications such as sports surfaces, rubber-modified asphalts, vibration-damping systems, and acoustic insulation products. Because bicycle tires share similar material compositions with ELT (as described in Section 2), their recycled rubber can follow similar application pathways.

VRG is widely used in sports flooring, including athletics tracks, panel and tennis courts, basketball and volleyball courts, gym flooring, and playground surfacing. Its elasticity, resilience, weather stability, and shock-absorbing behavior help reduce impact forces, while enhancing safety and comfort for users [145]. Before use, VRG is typically ground into crumb-sized rubber particles (0.5–2 mm).

VRG also plays an important role in rubberized asphalt, where it is mixed with hot asphalt binders to reduce road noise, improve pavement durability, and enhance resistance to cracking [29,146]. Additional uses include equestrian surfaces, water-sports structures, construction materials, agricultural mats, urban infrastructure components, automotive materials, and other composite systems incorporating recycled rubber.

For material fractions that cannot be reintegrated into new products, energy recovery, including co-incineration in cement kilns and waste-to-energy facilities, continues to serve as a practical end-of-life route.

Table 12. Output from ELT shredding in 2023 [145].

Materials	Amount for 2023 (ton)	Relative Amount (%)
End-of-waste VRG	47,762	26.8
Shreds + chips + SCCS	102,516	57.6
Steel	17,347	9.7
Textile fibers	10,222	5.7
Production scraps	107	0.2
Total recuperation	177,955	100

summarizes the distribution of materials exiting ELT shredding plants within the Ecopneus system for 2023.

4.2. Applications of Devulcanized Rubber

Devulcanized rubber is commonly incorporated into polymeric blends, allowing waste rubber to be converted into new materials for applications in construction, automotive components, and consumer products. Because devulcanization breaks sulfur crosslinks in vulcanized rubber, the material becomes more flexible and can be reprocessed.

4.2.1. Blending Devulcanized Rubber with Other Polymers

A.

Blends with Natural Rubber (NR)

Devulcanized rubber (DR) can be combined with natural rubber to tailor mechanical properties and reduce formulation costs. Mangili et al. [147] compared DR obtained from three devulcanization techniques (supercritical CO₂, ultrasonic treatment, and biological processing) and incorporated each into NR at 10 phr. Their results showed that ultrasonic and supercritical CO₂ devulcanization produced higher levels of network breakdown, yielding NR blends with superior mechanical performance. Biological devulcanization was more selective but only affected the surface of GTR, resulting in blends with properties similar to those containing untreated GTR. Overall, ultrasonic-treated material provided the most significant improvements, although performance gains plateaued beyond a certain ultrasonic amplitude (7.2 µm).

B.

Blends based on synthetic rubbers

Devulcanized rubber is also frequently blended with various synthetic elastomers. Valentini et al. [148] investigated mixtures of virgin EPDM with both devulcanized (DR) and non-devulcanized (NDR) truck tire rubber, produced using azodicarbonamide as the foaming agent. DR particles dispersed more effectively within the EPDM matrix and formed stronger interfacial bonding, likely due to the presence of reactive sites created during devulcanization, promoting new crosslinks upon re-vulcanization.

Other studies incorporated microwave-devulcanized GTR and SBR crumbs into virgin SBR formulations [149]. Across these systems, devulcanization consistently improved composite performance. For example, Karabork et al. [149] reported that SBR composites containing devulcanized GTR achieved a strain at break of 445%, compared to 217% for blends containing untreated GTR.

C.

Blends with thermoplastic elastomer (TPE)

Devulcanized rubber (DR) can also be incorporated into various thermoplastics, such as polypropylene (PP) and high-density polyethylene (HDPE), to create thermoplastic elastomers (TPE). When added to these matrices, DR can partially restore elastic behavior and tensile performance, allowing the resulting blends to be used in more demanding applications, including flexible seals and impact-resistant components.

Beyond elastomer–elastomer systems, DR has been explored extensively in rubber–thermoplastic composites. Studies reported its blending with PP [150], HDPE [151,152], copolyester (COPE) [153], and polystyrene (PS) [153], illustrating its versatility and value as a functional recycled material in thermoplastic formulations.

Several investigations highlighted the performance benefits of DR-modified thermoplastics. For example, waste tire rubber subjected to mechano-chemical devulcanization was blended with PP at various ratios (0/100; 25/75; 50/50; 75/25; 100/0), and the thermo-mechanical behavior was examined as a function of both DR content and γ-irradiation dose [150].

Jiang et al. [151] developed GTR/HDPE composites by combining chemical surface devulcanization using tetraethylenepentamine (TEPA) with in situ grafting of HDPE using styrene and glycidyl methacrylate. This two-step compatibilization strategy significantly improved dispersion and mechanical performance, and the final properties could be tailored by adjusting the initiator-to-monomer ratio. The resulting DR-modified HDPE blends also exhibited consistent processing behavior during reprocessing cycles.

DR has also been introduced into inherently brittle matrices such as PS. Valentini et al. [148] melt-compounded devulcanized (DR) and non-devulcanized (NDR) truck-tire rubber with PS. DR generated smaller and more uniformly distributed rubber domains with stronger interfacial adhesion, producing better mechanical performance than NDR. However, both DR and NDR reduced surface hardness, tensile modulus, and tensile strength due to the limited compatibility between PS and rubber, as well as the relatively large, dispersed domains. At higher rubber contents, the composites became more ductile, resulting in increased elongation at break with improved impact resistance.

Hittini et al. [154] further evaluated DR powder as a filler for PS-based thermal-insulating composites. Formulations containing 0–50 wt.% were studied. Samples below 40 wt.% DR exhibited the most balanced properties, with thermal conductivities of 0.050–0.071 W/m·K, densities of 463–482 kg/m³, compressive strengths of 7.5–11.7 MPa, and flexural strengths of 19.3–40.4 MPa. These mechanical properties improved further when DR underwent an alkaline treatment, increasing interfacial bonding with the PS matrix.

4.2.2. Blend with Asphalt and Road Materials

Devulcanized rubber (DR) is increasingly incorporated into asphalt binders, where it enhances properties such as flexibility, fatigue resistance, rutting resistance, and low-temperature performance. Compared with untreated crumb rubber (CR), DR shows better dispersion inside the bitumen matrix, resulting in more stable, homogeneous, and consistent rubber–asphalt binders [155].

Experimental studies indicate that the addition of rubber influences the density and air-void structure of asphalt mixtures. In particular, gap-graded mixtures are more sensitive to changes in binder content when modified with rubber. Overall, rubber-modified asphalt provides modest improvements in mechanical behavior, but achieving optimal performance requires careful control of binder dosage to manage void levels and deformation characteristics [155].

Across natural rubber, synthetic rubber, thermoplastic systems, and asphalt binders, devulcanized rubber consistently enhances compatibility and mechanical performance compared with untreated crumb rubber. Its ability to disperse more uniformly and form stronger interfacial interactions enables the creation of higher-quality recycled materials. As a result, DR represents a promising path toward more efficient, value-added recycling of waste bicycle and automotive tires.

Table 13. Application of devulcanized rubber (DR) in blend with polymer systems.

Polymer/Matrix	Type of DR Used	Devulcanization Method(s)	Key Findings/Effect on Properties	Ref.
Natural rubber (NR)	DR from GTR	Supercritical CO2, ultrasonic, biological	- scCO2 and ultrasonic DR produce higher devulcanization and better mechanical properties (tensile, elasticity).	[147]
			- Biological DR affects mainly surface; blends behave similarly to untreated GTR.
EPDM (synthetic rubber)	DR and NDR truck-tire rubber	Mechanical/chemical devulcanization	- DR shows better dispersion and bonding in EPDM.	[148]
			- Improved network formation during re-vulcanization.
SBR (synthetic rubber)	Microwave-devulcanized GTR; DR-modified SBR	Microwave devulcanization	- DR significantly improves elongation at break (up to 445%).	[149]
			- Better stress–strain response than untreated GTR blends.
Polypropylene (PP)	Mechano-chemically devulcanized WTR	Mechano-chemical devulcanization, γ-irradiation studies	- DR enhances thermo-mechanical behavior depending on DR content.	[150]
			- γ-irradiation further modifies performance.
High-density polyethylene (HDPE)	Surface-devulcanized GTR; DR-modified HDPE	TEPA surface devulcanization + in situ grafting (styrene, GMA)	- Strong compatibility and dispersion.	[151,152]
			- Mechanical performance tunable by monomer/initiator ratio.
			- Good reprocessing stability.
Copolyester elastomer (COPE)	DR blended with COPE	Not specified (DR used as modifier)	- Show viability of DR for TPE formulations.	[153]
Polystyrene (PS)	DR and NDR truck tire rubber; alkaline-treated DR	Mechanical or chemical DR; alkaline surface treatment	- DR forms smaller, uniform domains with improved adhesion.	[148,154]
			- Enhances ductility and impact strength at higher DR contents.
			- Alkaline-treated DR improves thermal-insulating composites.
Asphalt/bitumen	Devulcanized rubber powder or granules	Various devulcanization pathways (thermal, mechanical, chemical)	- Improved flexibility, rutting resistance, and low-temperature performance.	[155]
			- DR disperses more uniformly than untreated crumb rubber.

presents an overview of the system studied.

4.3. Application for Pyrolysis Products

Pyrolysis of waste tires yields three principal products, pyrolysis oil, recovered carbon black (rCB), and pyrolysis gas, each with established industrial value as described in Figure 16.

Tire Pyrolysis Oil (TPO)

Tire pyrolysis oil is a hydrocarbon-rich liquid fraction (C₅–C₂₀) with a high heating value. It can be used as a substitute fuel in diesel engines, industrial furnaces, and power-generation systems, offering a means to reduce reliance on conventional petroleum-derived fuels and potentially lowering operating costs [12]. Its energetic properties make it attractive as a renewable fuel option in sectors requiring steady, high-temperature heat.

Recovered Carbon Black (rCB)

Recovered carbon black is the solid residue left after pyrolyzing tires, comprising carbon and inorganic components such as silica and zinc compounds [156].

rCB has several applications:

• Rubber reinforcement: used in products such as gaskets, hoses, belts, and some tire formulations (typically blended with virgin carbon black).
• Pigments: used in inks, paints, and coatings due to its coloring capacity and stability.
• Plastics: added to polymer matrices to enhance UV resistance, electrical conductivity, and color properties.
• Construction materials: incorporated into asphalt to improve pavement durability or used in mortars and concrete mainly as a coloring agent or functional filler, with limited structural contribution [156].
• Because of its multifunctionality, rCB supports both material circularity and cost reduction in manufacturing.

Pyrolysis Gas

The gas fraction consists of non-condensable, high-calorific-value gases that are typically reused on-site. Its main applications include the following:

• Fueling the pyrolysis reactor, enabling partial or full-energy self-sufficiency;
• Generating electricity for plant operations;
• Potentially being supplied to local or national energy grids, improving the overall economic performance of the process [157].

Using pyrolysis gas internally reduces external energy demands and enhances both the environmental and financial sustainability of tire pyrolysis systems.

Pyrolysis of waste tires yields tire pyrolysis oil, recovered carbon black, and pyrolysis gas, all of which can be used as fuels, additives, or reinforcing materials in various industrial applications. Together, these products support energy recovery and material circularity, enhancing the economic and environmental value of tire recycling.

4.4. Application of Recycled Bicycle Inner Tubes

Bicycle inner tubes, which are predominantly composed of butyl rubber, can be recycled through several material-recovery pathways. Devulcanized rubber obtained from used inner tubes can be reintegrated into new tube manufacturing. For example, Schwalbe reports that its recycling program incorporates about 20% reclaimed butyl rubber into new inner tube formulations [25]. Mechanical processing routes, such as shredding or grinding, produce granules or powders that can be used as fillers or modifiers in products, such as rubber mats, footwear components, gaskets, hoses, and can also be blended into asphalt to enhance pavement flexibility and resistance to cracking.

In polymer blend applications, shredded inner tube material can be combined with thermoplastics, such as polypropylene (PP) or polyethylene (PE), to create thermoplastic elastomer (TPE) products with good durability and flexibility. Inner tubes may also enter energy recovery processes, including controlled combustion or pyrolysis, where they produce oil, gas, and recovered carbon black which can be used as industrial fuels or additives.

On the consumer side, recycled inner tubes are commonly repurposed into items, such as garden accessories, anti-slip mats, playground surfacing, and craft materials, taking advantage of their elasticity, toughness, and water resistance. Collectively, these recycling and reuse pathways help reduce waste generation and support more sustainable use of rubber resources.

The recycling route directly determines the chemical structure and physical properties of the recovered materials, which in turn govern their possible end-use applications. Vulcanized rubber recycling routes preserve the crosslinked network, leading to materials with high elasticity and durability that are mainly reused in low-value applications such as mats, flooring, and civil engineering products. In contrast, devulcanization partially breaks sulfur crosslinks, restoring processability and enabling reuse in higher-value rubber compounds where improved compatibility and mechanical performance are required. Pyrolysis converts rubbers into oils, gases, and carbonaceous solids with properties suitable for energy recovery, chemical feedstocks, or filler substitution, depending on process conditions and upgrading steps. Finally, recycled bicycle inner tubes, whether made from vulcanized rubbers or thermoplastic elastomers, can be reprocessed into flexible products or secondary materials, with applications dictated by their retained mechanical integrity and thermal behavior. Overall, the choice of recycling route governs material properties and thus defines the spectrum of suitable applications.

5. Outlook and Future Directions for the Recycling of Bicycle Tires and Inner Tubes

The future of bicycle tire and inner tube recycling is increasingly aligned with the principles of a circular economy, moving beyond traditional downcycling toward higher-value material recovery, advanced processing technologies, and product redesign. Although several technical and structural barriers remain, emerging recycling methods and growing engagement from manufacturers signal promising pathways toward more sustainable cycling systems.

Current recycling challenges

At present, most bicycle tire and inner tube recycling rely on small scale, voluntary collection schemes, limiting consistency and large-scale impact. Several core challenges still restrict progress, such as the following:

• Complex material structure: Bicycle tires incorporate vulcanized rubber, steel wire, textile reinforcements, and additives, making disassembly and purification technically demanding and costly.
• Predominance of downcycling: Mechanical processing (shredding and grinding) mainly produces low-value crumb rubber, which is typically used in mats or playground surfaces and are unsuitable to manufacture new, high-performance tires.
• Collection and logistics constraints: Unlike automotive tires, bicycle tires are collected in much smaller volumes, making efficient nationwide collection systems and cost-effective transportation difficult to implement.
• Unstable end markets: Recycling systems depend on a reliable demand for secondary materials. When market demand fluctuates, large quantities of stored rubber can accumulate, creating environmental, storage, and fire risk concerns.
• Several gaps remain in the specialized literature: These include (i) a lack of material-specific recycling studies focusing on bicycle tires and inner tubes; (ii) limited comparative data on recycling technologies applied to small-sized, textile-rich rubber products; (iii) insufficient mechanistic understanding of devulcanization and pyrolysis processes applied to butyl-based inner tubes; and (iv) the absence of standard methods to evaluate the performance of recycled materials in bicycle-related applications.

5.1. Advanced Recycling Technologies

Emerging technologies are enabling the recovery of higher-quality secondary materials and the development of more valuable, closed-loop products from the following:

• Enhanced devulcanization processes: Modern devulcanization methods target sulfur crosslinks in vulcanized rubber, allowing the recovered material to be reprocessed. Companies such as Schwalbe already incorporate substantial quantities of devulcanized butyl rubber into new inner tubes (20%), demonstrating the feasibility of circular manufacturing. Current research aims to increase process efficiency, protect the polymer backbone during treatment, and boost recycled content in final products.
• Pyrolysis for material recovery: Pyrolysis converts waste tires into fractions such as recovered carbon black (rCB), oils, and gases under oxygen-free conditions. Partnerships between Schwalbe and Pyrum Innovations AG have shown that rCB can be reintegrated into new bicycle tires. For example, the “Green Marathon” tire is produced with 100% recycled carbon black, highlighting the potential for high-quality circular materials.

5.2. Incorporation of Circular Design Principles

Manufacturers are increasingly embracing design strategies simplifying future recycling and extending product lifetimes by the following:

• Using renewable or more recyclable materials: Research is progressing toward the adoption of biobased elastomers from crops such as dandelion and guayule. In parallel, the use of thermoplastic composites reinforced with recycled carbon fibers is being explored to improve durability while enhancing recyclability [158].
• Improving repairability and extended service life: Designing tires and inner tubes with features, such as retreadable treads, self-sealing layers, or self-healing polymers, can significantly reduce material consumption and waste generation over a product’s lifetime.

5.3. Industry-Wide Collaboration and Strengthened Responsibility Frameworks

Moving beyond isolated initiatives, the bicycle industry is beginning to recognize the importance of coordinated, systemic solutions via the following:

• Shared recycling systems: Joint programs in which manufacturers participate collectively in the collection, processing, and recycling of waste tires can increase efficiency and reduce fragmentation across the sector.
• Extended producer responsibility (EPR): EPR models allocate responsibility for end-of-life management directly to manufacturers or retailers. Such frameworks stimulate improved product design, support large scale recycling infrastructure, and ensure long term accountability for material recovery.

The long-term development of bicycle tires and inner-tube recycling will depend on the industry’s ability to transition from isolated initiatives to a fully integrated circular system. Several key directions are likely to shape this evolution, such as the following:

• Progressing from downcycling to upcycling: Demonstrated successes, such as producing new inner tubes from recycled butyl rubber and using pyrolysis-derived recovered carbon black in new tire formulations, show that higher-value recycling is achievable. These innovations provide a model for future upcycling pathways keeping material quality rather than degrading it.
• Embedding circularity into product design and system infrastructure: Achieving true circularity will require manufacturers to prioritize designs facilitating disassembly, use more sustainable and recyclable materials, and actively participate in coordinated collection and recycling schemes. Such systemic integration enables more efficient material recovery and reduces reliance on low-value end uses.
• Empowering consumers through awareness and behavior: As cyclists become increasingly conscious of sustainability, their purchasing choices and participation in collection programs will play a crucial role. Actions such as returning used tubes to designated drop-off points or selecting bicycles and components designed with circular principles can significantly accelerate industry-wide adoption of circular practices.

Recent review articles have already comprehensively discussed the environmental and life cycle aspects of waste tire rubber recycling. While life cycle assessment (LCA) has been applied in related contexts, such as recycled rubber crumbs used as drainage layers in extensive green roofs [159], thermo-mechanical devulcanization of EPDM waste using co-rotating twin-screw extruders [160], and the incorporation of waste tire rubber in concrete [161], its application to bicycle tires and inner tubes remains limited. In particular, comprehensive LCA-based comparisons of the environmental impacts of major recycling technologies directly targeting bicycle tires and inner tubes are still lacking.

The future of bicycle tire and inner tube recycling is moving toward a fully circular system driven by advancements in devulcanization, pyrolysis, and product ecodesign. Achieving large-scale circularity will require improved technologies, coordinated industry action, and increased consumer participation to enable higher-value material recovery and sustainable product lifecycles.

6. Conclusions

Recycling bicycle tires and inner tubes is becoming increasingly important as cycling grows and concerns about rubber waste intensify. Advances in pyrolysis, devulcanization, and material separation are expanding the potential for higher value recovery, enabling recycled rubber to be used in applications ranging from polymer blends and road materials to closed-loop tire production. However, challenges such as fragmented collection systems, complex material structures, and unstable end-markets continue to limit large-scale progress. Moving toward a truly circular cycling economy will require improved technologies, better product design, coordinated industry action, and active consumer participation. With continuous innovation and collaboration, the environmental and economic benefits of circular rubber use in the cycling sector can be more fully realized. Strengthening policy frameworks, such as extended producer responsibility (EPR), will also be essential in driving consistent collection and recycling efforts. As the market for sustainable cycling products grows, increased demand may help accelerate investment in advanced recycling technologies and encourage manufacturers to adopt circular material strategies.