Hiroyuki Kaidou

Effects of secondary structure of fillers on the mechanical properties of silica filled rubber systems

Abstract A study was carried out on the mechanical properties of silica filled styrene–butadiene rubber systems in relation to the secondary structure formed by silica particles in the systems, which was controlled by a surface chemistry of silica particles and by a strain applied to the samples. The breakdown of the secondary structure was successfully detected by transmission electron microscopy (TEM) observations when the strain was applied to silica filled vulcanizates. The degree of breakdown was more prominent in the larger agglomerates of which the size was controlled by the number of silanol group per unit surface area of silica particles. The existence of the entrapped rubber within the agglomerate was suggested by the TEM image analysis. A part of the entrapped rubber might be released when the filler network was broken by a strain. The initial modulus of the filled rubber systems increased with the increase in the size of agglomerate. At a larger strain, the modulus decreased with the increase in the strain. The decrease was more prominent in the filled rubber, which had larger agglomerates. The reduction of modulus by the strain was closely related to the reduction of entrapped rubber phase within the agglomerates and to the breakdown of secondary structure of silica particles.

Relationship between Viscoelastic Properties and Characteristics of Filler-Gel in Filled Rubber System

Abstract The wide-line NMR and viscoelastic measurements were carried out for the filled rubber systems. NMR results revealed that the filler-gel in the filled rubber composites had a multi-component system depending on the mobility of rubber molecules. The fraction and the spin-spin relaxation time of each component were well correlated with the viscoelastic properties such as dynamic storage modulus (E′) and loss-tangent (tan δ) of the filled rubber systems. Such findings turned out to be useful in designing new tread materials with well-balanced wet traction and rolling resistance in tires.

Aging of Tire Parts during Service. I. Types of Aging in Heavy-Duty Tires

Abstract Three types of aging were found to exist. The factors determining the types were the temperature and the air-supply conditions irrespective of where the rubber was aged, in the tires or in the laboratory. This means that the aging characteristics of a tire part in the field can be properly predicted if these factors in a tire are taken into account in the laboratory evaluation. In the first type, Type I, the aging yields increased M100 and reduced λb closely following the reference relation, Equation (3), which holds for the rubbers crosslinked with increasing the amount of curatives. This type of aging was found at temperatures below about 80°C, under either oxidative or anaerobic conditions. The extents of the changes in λb and M100 were large under the former conditions and small under the latter. As for the aging in the heavy-duty tires, the sidewall and the belt showed this type, with the changes to considerable extents suggesting an oxidative aging. The oxidative condition for the sidewall ...

Aging of Tire Parts during Service. II. Aging of Belt-Skim Rubbers in Passenger Tires

Abstract The belt-skim rubber of a passenger-car tire has changes in its tensile properties, M100 and λb, during service in the field. The changes are larger in higher temperature fields with an equivalent duration. It can be interpreted that the changes are caused simply by oxidative crosslinking similar to that which takes place on aging rubber sheets in an air oven at relatively low temperatures, below 100°C. The belt-skim compound showing better aging resistance in the laboratory also shows better aging resistance when used in a tire. Therefore, the aging characteristics of the rubber part in a tire can be satisfactorily predicted. The Arrhenius equations with the same activation energy can be used for the aging of a rubber in the laboratory and in the tires by introducing a factor to the tire equation. The factor is added to the atmospheric temperature to correct for a difference; however, it was found to be slight in the present case.

Reactions of radicals in filled rubber compounds : II. Generation and subsequent reactions of radicals upon deformation

Effect of stretching on oxidative crosslinking of a carbon black filled IR vulcanizate at a normal temperature was studied by means of ESR and crosslink density measurements. Upon deformation, ESR intensity increased and crosslink density decreased, indicating scission of load supporting bonds generating free radicals. Bond scission increased with increasing strain. Resting after deformation, the ESR intensity decayed steadily following simple combination of first order and second order kinetics. The decay was overwhelmingly caused by first order reactions, which were considered to be irrelevant to crosslinking. On the other hand, crosslink density increased during resting. The results strongly suggested that ESR observed the radicals at the surface of the carbon black filled rubber, while crosslink density evaluated the bulk property. The crosslink density increase during resting outweighed the reduction due to deformation. Deformation accompanied by radical generation apparently accelerated oxidative crosslinking.

Reactions of radicals in filled rubber compounds : I. Detection of free radicals in rubbers by ESR

ESR measurements were made with vulcanized rubbers of a gum, three carbon black filled and a silica filled compounds. The radicals in the filled rubbers were found to have long lives unlike the gum rubber. Carbon black was found to reduce the sensitivity of ESR due to shielding effect in the magnetic field. The degree of sensitivity reduction appeared to depend on the inter-aggregate distance. Upon exposure to toluene vapor, the ESR intensity of black filled rubbers increased for a short period as swelling widened the inter-aggregate distance and then decayed steadily with further exposure. Such a variation was not found with the gum and silica filled vulcanizates. The decay was faster with higher oxygen content of the environment. In a given environment, however, the decay could be dealt by simple kinetics. Diffusion of oxygen did not seem to play any part. It was inferred that ESR observes radicals only in a thin surface layer of the black filled rubber. The kinetic zero-time radical concentration, obtained by toluene vapor exposure in a nitrogen environment, was considered to be the surface radical concentration in the absence of the shielding effect. Among the rubbers of different black loadings, it showed an exponential relation with the value obtained by direct observation due to the difference in the shielding effect arising from difference in loadings. With the rubber of the same formulation, however, the radical concentration obtained by direct observation showed a linear relation with the kinetic zero-time value, suggesting possibility of quantitative comparison by ESR among the rubbers with the same carbon black content.