積層ゴム支承被覆ゴムの長期劣化による化学特性変化

 Durability or weather resistance is one of the biggest concerns about isolation rubber bearings. Since rubber material is thought to be less durable than other structural materials such as concrete and steel, several investigations about the durability of rubber materials actually used for structural members of bridges and buildings for several decades to one century have been conducted. Those results show that mechanical properties of surface part of the rubber materials have been changed to a certain degree, by being exposed to some degradation factors such as oxygen, ozone, ultraviolet rays and water, and those of inner part are assumed to keep almost the same as those in the initial condition. Hence, covering rubber or surface rubber has an important role for rubber bearings to keep their properties less changed for an expected long life span.<br> In this paper, aging effects of oxygen, water / saltwater and the combination of oxygen and water / saltwater on chemical properties of two types of covering rubbers actually used for rubber bearings were experimentally studied. Rubber A (natural rubber, NR) and Rubber B (complex of NR and Ethylene-Propylene-Diene terpolymer, EPDM) were selected for test materials. Accelerated aging test corresponding to 80 years, water / saltwater cyclic spray test for 30 / 60 days, and the serial testing of the above two tests were conducted for the both rubber materials. Considering the configuration of rubber bearings and the environmental condition of isolation levels, it is evident that degradation factors affect only in one direction from the surface to the inside of rubber bearings. Therefore, rubber blocks, not thin pieces such as dumbbell specimens, were chosen and exposed to the above degradation factors. After the degradation, rubber blocks were sliced and cut into small specimens, disposing the peripheral part and the back side, to remove the degradation effects from the side and the back of the rubber blocks. Finally, some chemical properties, such as chlorine penetration depth, chemical constitution, crosslinking density, glass transition temperature and antioxidant quantity, were evaluated for each degraded specimens, and compared to the initial values.<br> Test results showed that oxidative degradation affected the chemical properties of the both materials much greater than water / saltwater degradation. Also it became clear that oxidative degradation had a complicated effect on the changes in the chemical constitution of Rubber A, due to the counterbalance of the depolymerization characteristics of NR and the lasting vulcanization, while Rubber B simply became harder and denser with the progress of oxidation, due to the synergistic effect of the cross linking characteristics of EPDM and the lasting vulcanization. Although antioxidants contained in the both rubber materials were apparently decreased by the consumption and the leakage, some sufficient amount were still remained even after the oxidative degradation corresponding to 80 years. Thus, it might be concluded that covering rubbers were able to protect rubber bearings from degradation factors such as oxygen and water / saltwater for an expected life span of isolation buildings.