Development of New FKM O-Rings with Superior Fuel-Oil Resistance and Low-Temperature Properties

摘要

O-rings are used as rubber sealing parts for fixation in various fields of application because they provide many advantages in space-savings and ease of handling. The sealing theory of an O-ring is that when an O-ring is installed in a groove of a given size, it can produce sealability against a fluid by means of a compression stress that is produced by crushing at a predetermined compressive rate. For this reason, an O-ring is subjected to constant stain when in service. In addition, there is a need to select the materials to be used for O-rings in match with the fluid to be sealed against. For selection of materials, one of the critical properties is oil resistance. There is a method of determining a degree of swelling in rubber (the rate of change in volume) as one of the indicators that represent oil resistance. O-rings made of fluorine rubber materials are often selected for use in fuels for motor vehicles because of excellent fuel-oil resistance and less swelling as compared with other types of rubber materials. On the other hand, with the increasing globalization of automobile industries, the international standardization of functional parts is called for. Fuels for motor vehicles, used as sealing fluids, are different in the methods of refinery and the types and quantities of additives from one geographical region to another. Moreover, as a countermeasure for environmental problems in recent years, there has been a growing trend toward a shift from conventional gasoline fuels to alcohol-contained fuels. The use of alcohol-contained fuels provides advantages in that alcohol can be produced from grains or woods, and that the effective use can be made of alcohol as an alternative fuel in light of the availability of limited petroleum resources. Alcohol also offers various advantages in terms of the emission of exhaust gas, including the significant reduction of CO{sub}2 emissions, the reduction of sulfur oxides SO{sub}x, and the mitigation of soot and black smoke. However, it is shown that an alcohol-contained fuel causes fluorine rubber to be subjected to swelling as compared with gasoline. If a fuel in service is shifted from conventional gasoline to an alcohol-contained fuel, the amount of swelling in an O-ring increases, whereby internal strain increases, and the amount of crushing also increases seemingly. If internal strain increases to a higher level beyond a material strength limit, an O-ring may be subjected to compression cracks (damage). In contrast, if an O-ring is developed with product design that assumes a given compression rate after swelling, possible damage may be prevented, but the amount of crushing may decrease during an initial stage or at the time of drying, thus resulting in a loss of sealability. As a countermeasure, there is a need to develop an O-ring made of fluorine rubber materials, showing a small amount of swelling in an alcohol-contained fuel. On the other hand, standardization is called for in the low-temperature properties of O-rings so that O-rings can be used in all and every geographical regions. However, it was difficult to improve both the low-temperature properties and alcohol resistance of conventional fluorine rubber. In general, alcohol-fuel resistance of fluorine rubber can be improved by increasing the fluorine contents in a polymer. However, there is a conflict between an increase in fluorine contents and a deterioration in the low-temperature properties of rubber materials. Fluorine rubber materials having excellent resistance to alcohol-contained fuels cannot be put into service where low temperatures are required. To establish such a conflicting relationship, there is a need to develop a fluorine rubber material having excellent resistance to alcohol-contained fuels and excellent low-temperature properties. In this study, a low-temperature sealing test and a compression cracks test were performed on O-rings to verify the effectiveness of newly developed fluorine rubber mat