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Waxes compounded into rubber migrate to the surface and form a protection film on the rubber surface. In general, antiozonants were used with wax to protect ozonation of rubber. Influence of wax barrier formed on the surface of a rubber vulcanizate on migration of antiozonants was studied using natural rubber (NR) vulcanizates containing various type waxes. IPPD (N-isopropyl-N'-phenyl-p-phenylenediamine), HPPD (N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine), SBPPD (N,N-di(sec-butyl)-p-phenylenediamine), and DMPPD (N,N'-di(1,4-dimethylpentyl)-p-phenylenediamine) were employed as antiozonants. Migration experiments were performed at constant temperatures of 60 and 80℃ for 10, 20, 30 days using a convection oven. The migration rates of the antiozonants in the vulcanizate without wax are faster than those in the vulcanizates containing waxes. The antiozonants migrate slower in the vulcanizate containing wax with a high molecular weight distribution than in the vulcanizate with a low one. The migration rates of DMPPD and SBPPD are faster than those of HPPD and IPPD.
Retreading of tire is closely related with adhesion between precured and uncured rubber sheets. Adhesion specimens between precured and uncured rubber sheets of BR and NR were prepared and thermally aged at 90 ℃ for 3 days. Organic materials remained in the adhesion sheets were extracted with THF and analyzed using gas chromatography/mass spectrometry (GC/MS). Elemental sulfur (S8), mercaptobenzothiazole (MBT), antidegradants, and fatty acids were detected as major organicmaterials remained in the adhesion sheets. Amounts of the organic materials were reduced by the thermal aging and varied with the adhesion specimens of BR/BR, NR/NR, BR/NR, and NR/BR as the precured and uncured rubber sheets. Decrements of the S8, MBT, and fatty acids in the NR sheet were larger than in the BR one due to the faster cure characteristics of NR. Changes of the adhesion forces and crosslink densities by the thermal aging were measured. Adhesion force of only the precured BR/uncured NR specimen by the thermal aging was enhanced whereas those of the other specimens were reduced. This was explained with the crosslink density changes and migration of the organic materials through the interface of the adhesion specimen.
The recovery behavior of vulcanized rubber composed of a chloroprene rubber (CR) and natural rubber (NR) blend was investigated using a circular deformation technique. A linear sample of vulcanized rubber was circularly deformed and thermally aged. The tensile and compressive strains, and stress distributions, in the circularly deformed state were evaluated using finite element analysis (FEA). The stress and strain varied uniformly across the thickness of the sample, and the degree of recovery increased with the measurement time after the removal of deformation and decreased by increasing the thermal aging time. Instantaneous recoveries were obtained by extrapolation of the degree of recovery as a function of aging time. The crosslink density in the compressed interior of the sample was greater than that of the elongated exterior component.
Influence of the acrylonitrile content of NBR on the water swelling behaviors of silica-filled NBR composites at room temperature and 90℃ was studied. Variation of the water swelling behaviors with the silica contentwas also investigated. The water swelling ratios at 90 8C were on the whole higher than those at room temperature. The water swelling ratios of the specimen with higher silica content were larger than those with lower one except for the water swelling at 90℃ for long time. When factors influencing the water swelling such as the crosslink density and silica content were corrected, the water swelling ratio of the silica-filled NBR vulcanizate increased as the acrylonitrile content increased.
A simple and efficient whitening test method of a carbon black-filled sulfur-cured EPDM composite by formation of calcium stearate was established using Ca2+ solution and convenient analytical techniques. The sample was aged in 0.1 M CaCl2 solution at 30–90 8C for 1 and 3 days. ATR-FTIR spectra of the aged sample surfaces displayed unique calcium stearate peaks. Level of the whitening of a rubber article can be determined by analysis of the cross section of the aged sample using an image analyzer and by analysis of calcium stearate on the aged sample surface using an ATR-FTIR.
Thermoplastic elastomers (TPEs) composed of maleic anhydride-grafted ethylene–propylene–diene terpolymer (MAH-g-EPDM) and ZnO were characterized using IR, TGA, solvent extraction, crosslink density, and physical properties. The degree of the ionomer formation increased with increasing the zinc oxide content, and it was also enhanced by adding stearic acid or zinc stearate. The crosslink densities were measured using a solvent swelling method using toluene, xylene, and 1,2-dichlorobenzene. The crosslink density increased when increasing the zinc oxide content. By adding stearic acid or zinc stearate, heteroionic bonds were formed and the crosslink density notably enhanced. The physical properties were dependent on the crosslink densities.
SBR and NBR composites were thermally aged and the crosslink density changes were investigated. The crosslink density changes of the SBR composite after thermal aging were larger than those of the NBR one. This could be explained by the allylic proton contents and the role of phenyl or nitrile group as a ligand in a zinc complex. The activation energies of the SBR composite were slightly lower than those of the NBR one. The lifetime to reach 50 and 100% increase of crosslink density of the NBR sample was shorter than that of the SBR one. SBR and NBR composites were thermally aged and the crosslink density changes were investigated. The crosslink density changes of the SBR composite after thermal aging were larger than those of the NBR one. This could be explained by the allylic proton contents and the role of phenyl or nitrile group as a ligand in a zinc complex. The activation energies of the SBR composite were slightly lower than those of the NBR one. The lifetime to reach 50 and 100% increase of crosslink density of the NBR sample was shorter than that of the SBR one.
Water swelling behaviors of silica-filled NBR composites in deionized water and NaCl solution were investigated by changing the swelling medium. Variation of the water swelling behaviors with the silica content was also investigated. The water swelling ratios increased with increasing the silica content. The water swelling ratios in deionized water were higher than those in NaCl solution. By changing the swelling medium from the deionized water to the salt solution, the water swelling ratio decreased or slightly increased at the point of swellingmedium change. For the swellingmedium change from the salt solution to the deionized water, the water swelling ratio notably increased when the swelling medium was changed. Low water swelling ratio in the salt solution was explained with the hydrated ions and ionic atmospheres.
NR composite and PET cord-inserted NR composite were prepared to investigate the contribution level for permanent deformation of fabric cord-inserted rubber composite by thermal aging. The degree of deformation of the PET cord-inserted NR composite was much larger than that of the NR composite without cord. The level of the contribution of the PET cord part on the permanent deformation was much larger than that of the rubber part for the thermal aging below 70 8C. The experimental results were explained by the increased crosslink density change of the rubber part and increased crystallinity of the PET cord part.
We introduced a new test method, the circular deformation method which changing a linear sample to a circular form by fixing the both ends with a pin, to investigate recovery behaviors and level of the permanent set of a rubber vulcanizate by thermal aging. The pin was removed after the thermal aging, and the gap distance between both ends of the sample was measured. The recovery increased as the measuring time elapsed. The instantaneous recovery less than 0.1 s could be obtained from the extrapolation method. Influence of the 1,2-unit content on the recovery behaviors of silica-filled SBR vulcanizates was also investigated. The SBR specimen with the higher 1,2-unit content had faster recovery behaviors than that with lower one. The silica-filled SBR vulcanizates containing the silane coupling agent had higher recoveries than those without the coupling agent irrespective of the 1,2-unit contents. The experimental results were explained with the crosslink density and the filler-polymer interactions. The instantaneous recovery at 0.09 s became larger as the crosslink density of the sample increased.