A wet frictional material which functions in lubricant having both effects of cooling and lubrication is used in automatic transmission for automobiles, differential gear for four-wheel-drive vehicles, wet brake of tightly closed type, etc. With regard to an organic fiber material for the wet frictional material, there have been used acrylic fiber and fibrillated acrylic fiber which are less expensive as compared with aramid pulp and have excellent adhesive property to a bonding material where improvement in strength of the wet frictional material can be attempted and an appropriate porosity is achieved. For example, in an invention of the Japanese patent publication No. 05/45,808 B, there is proposed a wet frictional material where fibrillated acrylic fiber in an amount of 5 to 50% byweight to the total fiber is used and, in the Japanese patent publication No. 07/37,606 B, there is proposed a wet frictional material using a fiber base material by the joint use of acrylic pulp and aramid pulp. However, as a result of an increase in generating power of car engine and tendency of making efficiency and size of automatic transmission higher and smaller in recent years, sliding speed increases and load upon engagement becomes high resulting in high temperature of the wet frictional material whereupon durability of the wet frictional material has become a problem.
Thus, when acrylic fiber and fibrillated acrylic fiber which have been used as fibrous base materials for wet frictional materials are subjected to stress such as sliding and compression for a long period in a wet frictional material of high temperature, there is a disadvantage that elastic modulus extremely lowers and fiber is plastically deformed. As a result thereof, the wet frictional material is deteriorated, pores formed among the fibers are crushed and appropriate oil membrane is not prepared whereupon there is a problem that frictional characteristic lowers and that, finally, burning is resulted. Usually, a sulfur compound such as calcium sulfonate or zinc dialkyldithiophosphate is added to a lubricant as a cleansing dispersing agent and an anti-abrasion agent but such a compound produces acidic decomposition products by heat. They accelerate deterioration of not only jointly used cellulose pulp but also acrylic fiber and fibrillated acrylic fiber used conventionally for the wet frictional material whereupon durability lowers and that is another problem.