Premise of Circular Tire Materials
The term “circular tire” is often used to mean a tire that can be completely recycled. However, in real material design such a simple objective does not hold. Current rubber compounds are extremely complex systems combining multiple polymers, various reinforcing fillers, and different resins and oils, and it is difficult to completely separate them and reuse them after use.
Therefore, the realistic objective of circular design is not full recovery. Instead, the formulation is adjusted so that the initial performance does not drop significantly while the recovery yield after use is improved.
The idea is roughly as follows.
• New product performance
Maintain about 95 to 98
• After recycled material is used
Maintain about 85 to 90
In this way, the design is not based only on absolute performance when new, but includes the allowable capacity of the material in its second cycle. This is also consistent with the current quality level of rCB and recycled polymers.
Center of Gravity of Material Design
In circular design, the evaluation axis changes from conventional compound development. In addition to normal evaluation items, the suitability for re-compounding after grinding must be examined.
Main viewpoints
• ash residue
• Mooney during re-mixing
• tanδ after re-vulcanization
• Payne effect
• TGA residue
Not only the performance when new, but also the behavior of the material in its second cycle becomes the center of compound design.
Three Directions of Compound Design
There are three important elements for increasing circular compatibility.
Reduce Sources of Ash
The main factor that deteriorates rCB quality after pyrolysis is ash. When ZnO, silica, and calcium-based fillers are mixed together, the purity of recovered carbon decreases.
Therefore, circular design favors the following directions.
• minimize ZnO
• do not increase the number of inorganic reinforcing materials
• reduce metal-based additives
Simplifying the reinforcing system is also advantageous for devulcanization and re-mixing processes.
Reorganizing Polymer Composition
Current tires often contain mixtures such as
NR
BR
SSBR
ESBR
In addition, resins and multiple oils are added, making the material structure quite complex.
In circular design it is better to organize the polymer architecture.
Examples
• NR-dominant system
• SSBR / BR dominant system
Thus, the material system is organized according to application. The direction is not to increase materials, but to simplify the overall material design.
Exchangeable Crosslinks
The most important element is the crosslink structure.
Conventional sulfur crosslinks are strongly irreversible and chain scission tends to occur during recycling. In contrast, networks partially introducing exchangeable bonds may maintain the network during use while rearranging under heating conditions.
Research examples
• disulfide exchange
• imine bond networks
• elastomer vitrimer
These are not yet mass-production materials, but research is progressing on reprocessable rubber networks.
A Realistic Compound Proposal
It is difficult to apply circular design to high-load tread compounds from the beginning. A realistic entry point is the following components.
• sidewall
• base tread
• bead filler area
These areas do not experience dynamic demand as high as the tread surface.
Circular-Compatible Sidewall Compound
Polymer
NR 50 phr
BR 30 phr
ENR 20 phr
Reinforcement system
rCB 20 phr
N550 20 phr
Silica 10 phr
Plasticization
Bio-based oil 6 to 8 phr
Crosslinking
Sulfur 0.8 to 1.2 phr
CBS or TBBS
ZnO 1.5 to 2 phr
Stearic acid 1.5 phr
Network support
Disulfide-type additive 2 to 4 phr
The aim of this formulation is polarity introduction by ENR and formation of a platform for disulfide exchange. It improves compatibility with rCB while securing reprocessability.
New performance is assumed to maintain about 95 percent of a normal sidewall, and after use it returns as recycled material of about 20 to 30 phr.
Circular-Compatible Base Tread
Polymer
SSBR 45 phr
BR 35 phr
ENR 20 phr
Reinforcement
Silica 35 phr
rCB 15 phr
N660 10 phr
Crosslinking
Low sulfur design
ZnO 1.5 phr
Here the target is not the outer tread but the layer beneath the cap or the base tread. It allows verification of recyclability while maintaining the balance between rolling resistance and durability.
Circular tire materials are unlikely to progress by changing the entire tire at once. The approach is to verify recyclability component by component and update material design step by step. Sidewalls and base tread are relatively realistic entry points for this process.