Post-consumer tires back into new tires: de-vulcanization and re-utilization of passenger car tires

Enabling recycling loops for used passenger car tires is a challenge and an opportunity. The challenge lies in the presence of SBR as the main elastomer in this type of tires, which makes this material difficult to reclaim due to the tendency of the elastomer chain fragments to re-combine. The opportunity lies in the wide availability of the material and in the fact that passenger car tires form a huge potential market for recycled rubber.\ud The aim of this study is to develop a de-vulcanization process for passenger car tire rubber, as sensitive to tire compound compositions. The research described in this thesis is focused on the influence of material composition on the de-vulcanization efficiency and material properties, on choosing an appropriate de-vulcanization aid, on optimizing the de-vulcanization process conditions, and finally on an application study. Polymer chain scission mainly occurs during physical treatment, therefore a shift of the balance between physical and chemical breakdown in a thermo-chemical de-vulcanization process will lead to a higher ratio of crosslink scission to polymer chain breakage, the precondition for efficient de-vulcanization. The first objective within this study is to define the pathway for the most efficient de-vulcanization process of SBR. Secondly, a best compromise for the de-vulcanization process conditions for all tire rubbers needs to be found. Finally, the intention is to investigate the productivity of the process for whole passenger car tire material and its properties in new tire products

Comparative investigation of the de-vulcanization parameters of tire rubbers

The optimal process conditions for a high ratio of de-vulcanization to\ud polymer degradation have been investigated for tire rubbers: SBR, BR, NR and CIIR. These polymers all show their own particular breakdown characteristics. The temperature dependence of the breakdown mechanism was investigated by measuring sol fractions and crosslink densities. For SBR and BR, the highest reduction in crosslink density was found at a temperature of 220°C, together with a moderate increase in sol content. According to the Horikx theory, which correlates sol fraction and decrease in crosslink density, this is the result of a high degree of crosslink scission. Higher process temperatures result in a lower decrease in crosslink density due to recombination of active chain fragments. NR and CIIR show different behaviour. Breakdown of NR in this temperature range results in an almost complete destruction of the polymer network; crosslink density is reduced to almost zero and the sol fraction is close to 100%. The same result is found for CIIR at higher temperatures.\ud Although different rubbers react via other de-vulcanization mechanisms, the best devulcanization conditions for whole passenger car tire material are optimized

Best Practice for the Devulcanization of Sulfur-cured SBR Rubber

In the present paper, special attention will be devoted to thermo-chemical devulcanization of sulfur-cured styrene butadiene rubber (SBR) using diphenyldisulfide (DPDS) as devulcanization aid. SBR is the main component in whole passenger car tire rubber and, at the same time, the most critical one in terms of devulcanization. This study is the first step to realize an effective devulcanization process for whole passenger car tire rubber. Diphenyldisulfide was found to be an effective agent for reclaiming of mainly natural rubber based sulfur-cured materials: DPDS acts as a radical scavenger and helps in preventing broken rubber chains to recombine. In this study, the effect of DPDS on the ratio of crosslink to polymer scission is investigated. The most important process parameter is the temperature, at which the polymer network is broken down. A reduction of the crosslink density of the devulcanizate is observed with increasing devulcanization temperature from 180C to 220C. However, above a temperature threshold of 220C, the crosslink density rises again. This is due to intra-molecular rearrangements of chain fragments of butadiene moieties from uncontrolled degradation and oxidation effects. Oxidation stabilizers are added during the devulcanization process in order to reduce the degradation and interrupt the oxidation cycles. Above a temperature threshold of 220C, a further decrease in crosslink density without creating more sol fraction can be achieved this way. The combination of DPDS and oxidation stabilizers significantly enhances the devulcanization efficiency of SBR versus the one obtained when DPDS is used alone. The results are interpreted in terms of mechanisms of main chain and sulfur bridge scissions and the degradative cycles triggered by the presence of oxyge

Degradation behavior during mixing of silica-reinforced Natural Rubber: Changes of the dynamic responses

High shearing forces and temperature are applied during mixing of silica filled natural rubber (NR) for tire tread applications, in order to achieve the best possible filler-rubber interactions and a sufficient silanization reaction of the silica. Both thermal and mechanical conditions in the mixing process can lead to polymer degradation. The present work investigates NR degradation during mixing via monitoring changes of its viscoelastic behavior. Silica-filled NR compounds prepared by using various dump temperatures were investigated taking pure NR and gum or unfilled NR compounds as references. Chain scission and chain recombination as two competitive reactions affect the molecular weight and chain architecture. Chain scissions most likely contribute more to the viscous response whereas chain recombination and interactions contribute to the elastic component of viscoelasticity. Increasing viscous responses of masticated pure NR and gum compounds are observed with higher dump temperature as measured by Mooney stress relaxation rates, changes of storage and loss moduli, resp. tan delta with frequency, and large amplitude oscillatory shear (LAOS). Chain scission causes a lower molecular weight, but a broader molecular weight distribution and more branching. For silica-filled NR compounds, the elastic response rises at high dump temperatures above 150 degree C due to crosslinking and/or branching. The long-chain branching index (LCB) rises with increasing dump temperature. Furthermore, deterioration of tensile properties of the corresponding vulcanizates are observed which is attributed to degradation and chain modifications

Making rubber – even more – sustainable by introducing circularity

Elastomers are widely used in a broad range of products from tiny sealings requiring outstanding dynamic performance to very large products like roofing foils, in mass products like tires and specialties such as flood defense. The polymers are either bio-based like natural rubber, or oil-based like EPDM, SBR, BR etc. Both these resources for elastomers have their own risks: monocultures and diseases in the case of NR, the depletion of mineral oil resources for synthetic rubber. In order to economize on these resources, elastomers should be recycled and reused. For most elastomer products, the service life of the product is much shorter than the lifetime of the material. An example are tires: when a tire is worn out, the rubber is still a valuable material. The challenge for recycling of elastomers is to re-plasticize the vulcanized rubber and make it reprocessable without damaging the polymer. Such a process is de-vulcanization: the reversion of the network formation during curing of elastomers. The devulcanization reaction of sulfur cured elastomers requires a certain temperature level, shearing forces and a chemical additive to break sulfur bonds. As there will always be a balance between polymer and crosslink scission, the polymer radicals have to be quenched by the additive as well. The process conditions for the devulcanization as well as the nature of the additive have to be optimized for different elastomers. Besides, the type of network determines the efficiency of network breakdown, as does the type of fillers and polymers. In this presentation, the devulcanization process, its efficiency as well as the properties of the final recycled rubber for different types of elastomers will be elucidated. This includes different types of polymers, fillers as well as crosslink networks