, modified aramid
derived from Twaron
, is a rubber ingredient which improves hysteresis
and heat generation in sulfur vulcanized rubber
compounds. Sulfron is produced and sold by Teijin Aramid
In practice, sulfur vulcanized rubber compounds suffer from the adverse effect of reversion leading to high hysteresis and subsequently higher heat generation. Heat generation causes a burning problem in compounds for large truck and off-the-road (OTR) tires as the surface layers are subjected to longer curing times than is optimal. This is unavoidable due to the minimum cure requirements of the inner section of the tire. A similar situation may occur during the curing of passenger tires if productivity gains are sought through the use of higher curing temperatures.
Degradation may also occur during the service life of a tire operating under high temperatures. This process is self-perpetuating since the decline in physical properties leads to an increased rate of heat build up which in turn accelerates the degradation process. The result is a shortened service life and perhaps catastrophic failure.
To date the means of combating the detrimental effects of reversion center on the use of semi-efficient (SEV) or efficient (EV) vulcanization systems. These curing systems employ high accelerator/sulfur ratios or sulfur donors to reduce the formation of polysulfide crosslinks. Reversion resistance is improved since the resulting network is based on more thermally stable di- and monosulfide crosslinks. However, this improvement in reversion resistance is achieved at the expense of a reduction in scorch safety, flex-fatigue life and other strength related properties. Additionally, compounds based on SEV and EV curing systems bond poorly to brass coated steel cord and fabric due to low sulfur levels and high cure rates.
Ideally, a system to address hysteresis and heat generation should not affect the desired compound properties and should maintain these under conditions that would otherwise lead to their decline. The scorch time and cure rate of the compound should also not be affected.
Sulfron provides various solutions for the tire industry:
- Rolling resistance is improved by 20%, which means a reduction of 5% in fuel consumption.
- Durability of the tire is improved by at least 15%, i.e. tires last longer. Indoor truck tire tests have shown 25% improvement in running distance of the tire when Sulfron 3000 is used.
- Tires with Sulfron show less from cuts, chipping and chunking, especially for off-the-road (OTR) tire tread.
Mechanism of action
Sulfron 3000 maintains compound properties by introducing crosslinks by means of sulfur crosslinks present in Sulfron 3000. As a result, crosslink density, and thus compound properties, is maintained following the onset of vulcanization. In addition, the scorch time and cure rate are unaffected. In practice, this allows the material to be added to in-line compounds with marginal change in formulation or processing conditions.
The effect of Sulfron 3000 on the stabilization of total crosslink density has been shown in a truck tire during service. Original truck tires, size 295/R22.5, were retreaded with a compound containing 3.0 phr Sulfron 3000; retreaded tires using the control tread were also produced. The retread cure conditions were 50 minutes at 150°C. Crosslink densities were determined on the vulcanized treads before and after 150.000 km of service. The beneficial effect of Sulfron 3000 was observed both immediately following curing and after use in service for 150.000 km. Following curing the control compound suffers a loss in crosslink density due to overcure experience in the tread compound. Polysulfide crosslinks are also lost in the test compound, but this is significantly less than in the control formulation. As a result, a high level of total crosslink density is maintained. A further loss in crosslink density was observed in the control compound after the tire had been in service. This is due to the combined effects of heat and flexing generated during tire service. The test compound containing Sulfron 3000, however, maintains total crosslink density almost to the level of the non-run tire.
Performance in tires
Heat build up:
- Goodrich flexometer testing has shown that the addition of Sulfron 3000 reduces heat build up significantly, increasing blow out times dramatically. This concept is particularly suited to heavy duty tires – truck, OTR, aircraft, solid – where the reduction of heat build up translates into increased durability and reduced rolling resistance. Significant tread wear improvements can be expected by employing higher reinforcing blacks and/or increased black levels. The increased heat build up that would normally result from such compound modifications can be prevented by the use of Sulfron 3000.
- Minimum vulcanization times in a production environment are determined by the need to achieve adequate curing throughout the section of a component. In doing so, the surface layers of the compound often attain a cure state that is far beyond the optimum required, resulting in reversion and consequently a degradation of desired properties. It is this phenomenon that often limits the maximum curing temperatures that can be applied in practice, which in turn limits productivity. The incorporation of Sulfron 3000 prevents degradation during high temperature vulcanization thus maintaining the required physical properties. Laboratory studies indicate that comparable unaged physical properties can be achieved in a NR based vulcanizate when cured at 160°C compared to a normal cure of 150°C. Moreover, aged physical properties, dynamic mechanical properties and Goodrich heat build up characteristics are improved in the compound containing Sulfron 3000, despite the increased cure temperature. The use of Sulfron 3000, therefore, provides a means of increasing productivity by adopting higher curing temperatures thus shortening cure cycles.
Tear and flex-fatigue properties:
- The crosslink density of a vulcanizate is governed largely by the level of sulfur and accelerator in the formulation. The distribution of crosslink type, however, is determined by the ratio of sulfur to accelerator. A high proportion of polysulfide crosslinks predominates in ‘Conventional Vulcanization’ systems based on high sulfur to accelerator ratios. These crosslinks cleave and reform readily under the application of stress and thus are associated with high mechanical strength such as tear and flex-fatigue resistance. On the other hand, polysulfide crosslinks have relatively poor thermal stability, decomposing under the action of heat giving cyclic sulfide and conjugated diene/triene structures along the polymer backbone. It is this process that is responsible for the phenomenon of reversion. ‘Efficient Vulcanization’ and ‘Semi-Efficient Vulcanization’ systems were developed in order to overcome the problem of reversion. These features reduced sulfur to accelerator ratios. Improved reversion resistance is obtained due to the presence of mono- and disulfide crosslinks, but at the expense of tear, flex and other strength related properties. A network processing the reversion resistance imparted by an EV cure system combined with the tear and flex/fatigue resistance of a CV cure is often the goal of rubber compounders. This is now feasible with the use of Sulfron 3000. Tear and flex/fatigue properties can be improved whilst maintaining compound properties by reformulating from an EV or SEV to a CV cure system containing Sulfron 3000.
- Although tearing, and chipping or chunking are most commonly associated with off-the-road tires, this phenomenon is also observed with high way truck as well as all-season passenger tires. Reduced tire life is a consequence of tearing, and chipping or chunking of tread rubber due to repeated contact with sharp stones, curbs, etc. It was demonstrated by laboratory and field testing that the chip and chunk resistance can be significantly improved by addition of Sulfron 3000.
- Degradation effects are damaging to the in-service performance of tires, which are subjected to dynamic loading. Tire energy losses and performance can be accurately predicted for a range of operating conditions from the known visco-elastic properties of the tire components. This enables tires to be evaluated in the laboratory at the design stage, thus minimizing development time and costs incurred for manufacturing and testing numerous experimental tires. It was demonstrated that the tan δ at rolling conditions (60°C, 10Hz and 2% strain) of a truck tread compound reduced by 30% upon addition of Sulfron 3000, which indicates a significantly decreased rolling resistance.
Sulfron 3000 is not reactive during the initial stages of vulcanization. Scorch and cure time are therefore not affected. This means that it can be added to existing formulations with only slight modifications: optimize the stearic acid dosage based on applied Sulfron dosage (for example when you use 3,0 phr of Sulfron 3000, reduce the stearic acid from 2phr to 0,5 to 1,0 phr); in order to maintain similar modulus/hardness, reduce carbon black dosage with equal amount as Sulfron dosage.
The recommended dosing level of Sulfron 3000 depends largely on the sulfur/accelerator level being employed. A network containing a high proportion of polysulfide crosslinks, for example a CV cured system, is more prone to degradation than a more efficiently cured system and requires therefore a larger amount of Sulfron 3000. This means that the dosage level of Sulfron 3000 needs to be optimized based on the expected degree of degradation. As a starting point it is recommended to use the following levels for evaluation of NR-based compounds: 0.5 to 1.0 phr for EV, 1.0 phr for SEV and 1.0 – 2.0 phr for a CV curing system. It is recommended to add Sulfron 3000 to the rubber compound during the first, non-productive, mix stage together with the fillers.