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Additional recommended knowledge
Sulfron 3000, 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 affects 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:
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:
Tear and flex-fatigue properties:
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sulfron". A list of authors is available in Wikipedia.|