| Generally protective wax is consisted of paraffin wax and microcrystalline wax and could delay the ozone aging of unsaturated rubber. So it is widely used in tires and other rubber products. This paper respectively applied paraffin waxes, microcrystalline waxes, Fischer-Tropsch waxes with different melting point and their mixed waxes to NR/BR tire rubber. The migrating rate and characters of bloomed wax film in vulcanizates were studied, including chemical composition, thermal properties, molecular branching degree, crystal morphology, crystal structure and microstructure. Ozone resistance and protection principle of wax were discussed.The results showed raw waxes had little effect on the processing, curing and mechanical properties of vulcanized rubber. GC, DSC, and NMR tests showed that with the melting point increasing, carbon number of alkanes and content of iso alkanes for raw waxes increased. The carbon number distribution and the range of melting peak became wider. The peak of solid-solid transition gradually weakened and even disappeared. The molecular branching degree increased. While under 25 ℃ all bloomed waxes of paraffin waxes and microcrystalline waxes had similar composition of alkanes. The carbon number of alkanes with maximum migrating rate was from C27 to C31. The content of normal alkanes for bloomed waxes significantly increased. But the transition temperature, melting range and molecular branching degree were alike. Polarizing microscope and XRD tests showed that paraffin waxes got large lamellar crystals, while microcrystalline waxes had tiny needle crystals. The crystallization degree of raw waxes declined as the melting point increased. However, all bloomed waxes had large lamellar crystals and the crystals were relatively perfect. Migration quantity test of bloomed wax indicated that with the melting point of raw waxes increasing, the thickness of bloomed waxes on rubber surface decreased. SEM test showed that only 58 # and 62 # paraffin waxes clould form relatively dense wax film and had better ozone resistance under 25℃.For mixed waxes of 58# paraffin wax and 70# microcrystalline wax, the carbon number of alkane with maximum content, the content of iso alkanes and melting point increased with the ratio of wax 70# rising. The melting range became wider and the peak of solid-solid transition gradually weakened and even disappeared. The crystal morphology of raw waxes changed gradually from large lamellar crystals to tiny needle crystals and the degree of crystallization decreased. Under 25℃ all bloomed waxes had similar composition of alkanes, similar melting point, similar crystal morphology and silimar crystal structure, which were closer to 58# paraffin wax. As the ratio of 70# wax increased, the migration quantity of mixed waxes declined. The bloomed wax with the ratio of 50/50 had relatively dense wax film and better ozone resistance.For mixed waxes of 70# Fischer-Tropsch wax and 70# microcrystalline wax, the carbon number of alkane with maximum content, the content of iso alkanes both increased with the ratio of microcrystalline wax getting up. But the melting point got lower. All mixed waxes only had solid-liquid transitions, tiny needle crystals and similar crystal structure. At 40℃ all bloomed waxes had similar carbon number distribution of alkanes, among which the alkanes from C34 to C37 had the maximum rate of migration. All bloomed waxes had much more normal alkanes than mixed waxes. The melting point and melting enthalpy of all mixed were nearly the same. They all got large lamellar crystals and tiny needle crystals coexisted. Their crystal structure were close to mixed waxes. As the ratio of microcrystalline wax increased, the migration quantity of mixed waxes declined. But all bloomed wax had dense wax films and excellent ozone resistance. |
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