As a conventional art relating to a gland packing which is to be used in a shaft seal part of a fluid apparatus such as a pump or a valve, and a yarn used in the packing, well known is a technique in which expanded graphite is used as disclosed in Patent Literature 1. Expanded graphite has advantages that it is excellent in flexibility and chemical resistance, high in adhesiveness, and durable at high temperature, but also has drawbacks that it has a very low tensile strength, and is brittle and easy to break.
Therefore, a yarn formed by fibers which are high in strength and wear resistance, and a gland packing in which such a yarn is twisted or braided into a cord-like shape are produced. For example, fiber products are known which are disclosed in Patent Literatures 2 and 3, and in which acrylic fiber are used. In addition to acrylic fibers and modacrylic fibers, other various fibers such as rayon, cellulose, hemp, cotton, and polyvinyl alcohol may be used as organic fibers which are useful in such fiber products.
Organic fibers have various features depending on the material, such as high strength and wear resistance, low thermal resistance and hygroscopicity, excellent resistance to high temperature and chemical resistance, low resistance to ultra violet light, and high power of resistance. Therefore, it is requested to select fibers made of a material suitable for the purpose.
Namely, fibers (organic fibers) forming a yarn are requested to reduce to some extent friction (friction coefficient) which is critical in sliding between a gland packing and a shaft of a fluid apparatus such as a pump, or to have flexibility which is important in compatibility with a gland portion or a shaft.
As a factor indicating the characteristics of fibers which are requested to have various characteristics, there is the limiting oxygen index (LOI), as disclosed in Patent Literature 4. The limiting oxygen index is one of methods of evaluating the ease of burning of a material such as plastics or fibers. While changing the ratio of “nitrogen molecules/oxygen molecules” of the air, the oxygen concentration of the limit of causing ignition, i.e., the limiting oxygen index (LOI) is obtained.