The present invention relates to a filter medium for filtering impurities out of fluids and a method of manufacturing the filter medium.
A filter medium for filtering impurities having relatively large diameters is generally formed by a kind of sieve. However, in a fluid there are contained not only solid grains having large diameters, but also particles having very small diameters and fluids of different kinds. For instance, in an engine oil for use in motor cars there are included various kinds of particles having different sizes such as iron powders, carbon particles, pigments and liquids such as water. Particularly, fine particles and liquids could not be easily removed by the conventional filter medium. Therefore, absorbing agents such as active carbon and activated white clay(kaoline) are provided in the filter medium.
Heretofore, there has been proposed a charcoal filter in which absorbing agents are mixed in a filtering member or inserted between sheet-like filtering members such as filtering paper and nonwoven fabric. In these known filter medium, it is quite difficult to perform the desired filtering action due to the fact that the absorbing agents might move in the filtering members or flow out of the filtering members.
It has been further known that the hot active carbon particles are spread on an nonwoven fabric made of polypropylene resin and are fused onto the fabric. However, in such a filter medium, the absorbing agents, i.e. carbon particles are not firmly secured to the filtering member i.e. polypropylene unwover fabric and further the filterability of the fabric is largely lost at positions where the carbon particles are fused, so that the filtering efficiency is decreased. It has been also known to secure the absorbing agents to the filtering member by means of a bonding agent. In such a known filter medium, the filtering efficiency is also decreased. Further, the bonding agent might affect the absorbing function of the absorbing agents.
In an oil cleaner, fine magnetic particals such as iron particles produced by the friction of machine parts should be removed. Such magnetic particles may be absorbed by means of a filter medium including permanent magnet particles. However, it is almost impossible to spread uniformly the magnet particles or powder on the filtering member, because the magnet particles are attracted to each other.
Further, in case of filtering impurities out of an oil, it is necessary to remove water contained in the oil. However, in the known filter medium since the filtering member is clogged due to the water absorption by means of OH-radical, the filterability is decreased to a large extent.