All-round advantage of reclaimed rubber
By TA News Bureau
Reclaim rubber (RR) can be used in most commercial applications as a raw material by blending with fresh virgin rubber. Research by Dr Subhas Chandra Debnath of the Department of Chemistry at the University of Kalyani, India, has shown that RR’s Incorporation in fresh virgin rubber will be an important step towards achieving product quality, economy and market competition. This is also one of the most ideal forms for disposing end of life tyres (ELT) in an environment-friendly way. However, most manufacturers are still reluctant to increase the RR content in tyres
Amid the rising population of vehicles, governments are worried over the environment-friendly disposal of end of life tyres (ELT) that are piling up the world over. One way to address this issue is production of reclaim rubber (RR) from discarded tyres.
Dr Subhas Chandra Debnath of the Department of Chemistry at the University of Kalyani in India has been studying RR and its use. He has undertaken research projects on producing RR that can be blended with virgin rubber.
The disposal of ELTs is a major environmental problem, and it is a challenge for scientists, he told Tyre Asia in interview. “These polymeric materials are non-biodegradable or degrade very slowly. Hence, such materials are not eco-friendly. To save the environment, reclaiming or recycling is desirable.”
When asked how much RR could be reclaimed from an end-of-life car or truck tyre, he put the figure at 40 to 50 per cent.
However, he noted that most reputed tyre manufacturers are still reluctant to increase the proportion of styrene–butadiene rubber (SBR)/RR fearing dilution of brand equity. It is obvious that this reluctance stems from the fact that research on RR blending with virgin rubber for tyre manufacturing is still going on.
Currently most research on reclaiming ground rubber tyre (GRT) is confined to laboratory scale. Researchers continue to study the physical as well as ageing properties etc, but in the practical field, especially in the manufacturing of tyres, the progress is slow.
After the production of tyre using RR, it needs to be tested in the field to study many of its critical aspects, including safety and durability. These are not happening at a scale that is required for substitution of RR in tyres.
Reclaiming ELT not only protects the environment, but also saves the limited petroleum resources from which the raw material such as synthetic rubbers is made, he points out.
Referring to current research trends in devulcanisation that could be considered a ‘green step’ towards building a sustainable mobility sector, Dr Debnath said that many processes are being studied in producing RR on an economical scale.
“During the reclaiming process, the three-dimensional network is broken down at the crosslink sites or in the main chain bonds by using mechanochemical, thermomechanical, microwave, ultrasonic, microbial, and chemical reclaiming processes.”
Crosslink bond scission called devulcanization changes the vulcanizates back to its original form, while the main chain scission creates shorter molecular chains causing a deterioration of mechanical properties.
“For both good processability and mechanical properties of the RR, an ideal reclaiming process would be needed to selectively break the crosslink bonds, but leave the main chains intact,” Dr Debnath explained.
However, none of the present reclaiming methods can achieve this goal. Supercritical carbon dioxide (sCO2) with superior mass transferability has been introduced to reclaim vulcanized rubber to a high degree.
Some researchers have obtained a 90 per cent sol fraction (mass fraction of the dissolved or dispersed material resulting from a network-forming polymerization or crosslinking process). They achieved this by sulfurcured isoprene rubber (IR).
Some researchers had done reclamation of sulphur-vulcanized rubber using oil, water vapour and aryl disulfide (diphenyl disulfide (DD) as the reclaiming agent at 180 degree Celsius temperature and 230-260 psi for 3 – 4 hours.
Recently, the twin-screw extruder was introduced as processing equipment that has ensured that the reclaiming process is safer. It offers continuous operation and is environment-friendly.
Elaborating on his current research in mechanochemically reclaimed ground rubber tyre and its re-vulcanisation in combination with SBR, Dr Debnath said that in mechanical reclaiming of GRT, an open two-roll mixing mill was used at various concentrations of reclaiming agents at various milling times.
The extent of reclaiming was monitored by the measurement of percentages of sol and gel content, Mooney viscosity, inherent viscosity of sol rubber, swelling ratios and RR’s crosslink density.
After reclaiming by these milling procedures, small pieces of approximately 0.2 gram of samples were extracted and these were placed in 20 ml toluene for five days at room temperature for sol-gel determination.
The gel was then filtered, washed with toluene and dried in a hot air oven at 100 degree Celsius for 24 to 48 hours at constant weight. The swelling ratio was determined with about 0.2 g of reclaim. The sample was immersed in 20 ml pure toluene at room temperature for five days to allow the swelling to reach diffusion equilibrium.
At the end of this period, the test piece was taken out and the adhered liquid was rapidly removed by blotting paper. The sample was dried in vacuum to constant weight and the dry weight was taken.
Since the evaporation time of the solvent from the small-sized particle was critical, as it was directly dependent on its shape and size, it was assumed that the ground particles had the same crosslink density as in the sheet form.
It was found that the highest sol content was obtained at 50 minute milling while the maximum increase was observed up to 20 minute milling then the increasing trend had become slow. During milling, the vulcanized rubber sample had undergone tremendous mechanical shearing, resulting in random polymer chain breakdown.
It was discovered that the highest sol content was obtained at 50 minutes of milling. It was also noted that at 40 minute milling, the sol content was found to be comparable to every concentrations of reclaiming agent.
Dr Debnath found that the presence of TBzTD (Tetrabenzylthiuram disulfide) as a reclaim agent (environmentally safe) had definitely improved the sol content of RR. The inherent viscosity increased with increasing milling time.
For good quality RR the sol fraction and inherent viscosity of the sol fraction should be as high as possible. It is also very important to note that when 2 g TBzTD is used, the inherent viscosity is comparable with 4 g TBzTD at 50 min milling.
“Hence we can get moderate inherent viscosity by using minimum concentration of reclaiming agent with increasing milling time,” Dr Debnath said.
It was also found that gel fraction and crosslink density had decreased to an optimum value at 40 min milling and then decreased slowly. “It can be concluded that gel content and crosslink density are mostly dependent on milling time,” he explained.
The molecular weight between crosslink bonds had increased with milling time and reached an optimum value at 50 min milling for every concentration of reclaiming agent.
Dr Debnath said the maximum rheometric torque increased with the increase in RR content, because of the presence of cross-linked gel in it. Optimum cure time remained more or less same with increase in the carbon black loading in SBR/RR (80:20) vulcanizates but scorch time decreased with increase in carbon black loading.
The tensile value increased with the increase in the amount of RR content in the vulcanizates. In all SBR/RR blend vulcanizates, the moduli at 100 per cent and 200 per cent elongation increased with increase in RR content compared to that of the control formulation.
As crosslink density increased with increasing RR content in the rubber matrix, chain mobility decreased and more loads were required for 100 per cent and 200 per cent elongation.
The increase in value of tensile strength with RR content was probably due to the presence of carbon black left in RR. It is known that partial devulcanization of GRT facilitates the interface adhesion between the surface chains of RR particles and surrounding rubber matrix due to their co-crosslinking in the inter-phase layer.
Hardness is also higher for more RR vulcanizates, due to presence of higher gel within the RR. As higher amount of RR is incorporated with SBR, the vulcanizates become stiffer.
For both good processability and mechanical properties of RR, an ideal reclaiming process would selectively break the crosslink bonds, but leave the main chains intact.
But none of the present reclaiming methods can achieve this goal, Dr Debnath explained. Supercritical carbon dioxide with superior mass transfer ability has been introduced to reclaim vulcanized rubber to a high degree.