Phosphate esters and other phosphate esters such as phosphate ester-amides have excellent flame retardancy and self-extinguishing properties resulting from phosphorus atoms and are therefore widely used as flame retardants for various resins (for example, Patent Literatures 1 to 3).
As the flame retardant applications diversify, the phosphate esters are required to have higher performance. More specifically, in order to reduce influences on the physical properties and functions of substances to which the phosphate esters are added, the phosphate esters are required to exhibit their flame retardant effect with a smaller amount of addition and to have chemical and thermal stability so that the phosphate esters are less likely to degenerate even in high-temperature conditions.
One particular problem of the phosphate esters is that they easily undergo hydrolysis because of their chemical stability. When phosphate esters undergo hydrolysis, problems occur in that flame retardancy deteriorates or disappears and that acidity derived from phosphoric acid may exert an influence on the performance and durability of substances to which the phosphate esters are added. To address these problems, phosphate esters with various structures have been proposed (for example, Patent Literatures 4 and 5) to improve hydrolysis resistance. However, the task of improving flame retardancy so that the flame retardant effect is achieved with a small amount of addition still remains.
Among the phosphate esters, relatively low-molecular weight compounds that are in liquid form at room temperature are used not only as flame retardants for resins but also for various applications such as a flame retardant for an electrolyte for a lithium ion secondary battery (Patent Literature 6), a hydraulic fluid (Patent Literature 7), an extractant (Patent Literature 8), and a solvent for organic synthesis (Patent Literature 9). Trimethyl phosphate, tributyl phosphate, etc. are used as the above phosphate esters. However, their resistance to hydrolysis is insufficient, and they are flammable materials with flash points, so that their flame retardant effect is insufficient.
Phosphate esters having fluorine atoms in their ester side chains are known to have high flame retardancy due to the synergistic effect of the phosphorus and fluorine atoms and to exhibit the flame retardant effect even with a smaller amount of addition (Non-Patent Literature 1). The fluorine-containing phosphate esters also have a feature in that resistance to oxidation is high and are therefore contemplated to be used mainly as flame retardants for an electrolyte for a lithium ion secondary battery that is required to have particularly high safety and oxidation resistance (Non-Patent Literature 1, Patent Literature 10).
However, the hydrolysis resistance of the fluorine-containing phosphate esters is rather low because of the influence of the electron-withdrawing fluorine atoms. In addition, since a phosphate diester, which is a hydrolysis product, exhibits strong acidity (Non-Patent Literature 2), hydrolysis is likely to be accelerated with the phosphate diester acting as a catalyst. Therefore, the fluorine-containing phosphate esters have a problem with duration of flame retardancy when used as flame retardants for resins and also have a problem in that use conditions are limited when they are used as extractants or solvents for organic synthesis. Also in the secondary battery electrolyte application, the influence of moisture introduced from various components is not negligible, and the generation of a phosphate diester by hydrolysis causes an adverse effect on battery performance.
Among the fluorine-containing phosphate esters, fluorine-containing phosphate ester-amides having a P—N bond in their molecule are known (Non-Patent Literature 3, Patent Literature 11). With such a fluorine-containing phosphate ester-amide, an amine is generated when the P—N bond is broken by hydrolysis, and therefore the phosphate diester is neutralized. Therefore, the influence of the above-described acid catalytic action is reduced, and it is expected that the fluorine-containing phosphate ester-amide has improved hydrolysis resistance. Unsubstituted alkyl groups and linear alkyl groups have been reported as substituents for the amide moiety of the fluorine-containing phosphate ester-amide.
Perfluoroalkyl compounds such as perfluorohexane, perfluoroalkyl ethers, and perfluoroalkyl amines are known as compounds that have no flash point and are substantially not degraded by hydrolysis. These compounds are used mainly as dewatering agents for an industrial parts washing process on the basis of low latent heat of vaporization of these compounds. However, since the compatibility of these perfluoroalkyl compounds with various organic compounds is not sufficient, it is difficult to use the perfluoroalkyl compounds in a wide range of applications such as flame retardants for resins, solvents for organic synthesis, flame retardants for an electrolyte for a secondary battery, and extractants.