an apparatus other than the tank is also useful without particular limitations so far as it is able to control the oxygen concentration. 

The kind of the storage tank is not specifically limited, and known metal-made storage tanks or those in which a lining of glass, a resin, or the like is applied to an inner surface thereof, glass-made or resin-made containers, and the like are useful. From the standpoint of strength or the like, metal-made tanks or those in which a lining is applied are preferably used. 

As a material of the metal-made tank, known materials are used. Specifically, examples thereof include carbon steels, ferrite based stainless steels, martensite based stainless steels such as SUS410, etc., austenite based stainless steels such as SUS310, SUS304, SUS316, etc., clad steels, cast iron, copper, copper alloys, aluminum, Inconel, Hastelloy, titanium, and the like. 

As to an oxygen concentration within the storage tank of the dicarboxylic acid, though a lower limit thereof is not particularly limited, in general, it is preferably 0.00001 % or more, and more preferably 0.01 % or more relative to the total volume of the storage tank. On the other hand, an upper limit thereof is preferably not more than 16%, more pref› erably not more than 14%, and still more preferably not more than 12%. 

When the oxygen concentration within the storage tank of the dicarboxylic acid is the foregoing lower limit or more, the matter that the equipment or control step becomes complicated is prevented, an aspect of which is thus eco› nomically advantageous. On the other hand, when the oxy- gen concentration within the storage tank of the dicarboxylic acid is not more than the foregoing upper limit, coloration of the produced polyurethane can be suppressed. 

As to a temperature within the storage tank of the dicar› boxylic acid, in general, a lower limit thereof is preferably -50° C. or higher, and more preferably 0° C. or higher. On the other hand, in general, an upper limit thereof is prefer› ably not higher than 200° C., more preferably not higher than 100° C., and still more preferably not higher than 50° C. For the reason that the temperature control is not neces› sary, a method of storage at room temperature is the most preferable. When the temperature is -50° C. or higher, an increase of the storage costs can be prevented. In addition, when the temperature is not higher than 200° C., concur› rence of a dehydration reaction of the carboxylic acid or the like can be prevented. 

As to a humidity within the storage tank of the dicarbox› ylic acid, in general, a lower limit thereof is preferably 0.0001 % or more, more preferably 0.001 % or more, still more preferably 0.01 % or more, and most preferably 0.1 % or more; and an upper limit thereof is preferably not more than 80%, more preferably not more than 60%, and still more preferably not more than 40%, relative to the total volume of the storage tank. 

When the humidity within the storage tank of the dicar› boxylic acid is 0.0001 % or more, the matter that the control step becomes complicated is prevented, an aspect of which is thus economically advantageous. In addition, when the humidity within the storage tank of the dicarboxylic acid is not more than 80%, attachment of the dicarboxylic acid onto the storage tank or piping and blocking of the dicarboxylic acid can be prevented, and in the case where the storage tank is made of a metal, corrosion of the tank or the like can be prevented. 

In general, a pressure within the storage tank of the dicarboxylic acid is preferably atmospheric pressure (ordi› nary pressure). 

In general, the dicarboxylic acid which is used in the present invention is preferably one with less coloration. As to the yellow index (YI value) of the dicarboxylic acid which is used in the present invention, in general, an upper limit thereof is preferably not more than 50, more preferably not more than 20, still more preferably not more than 10, yet still more preferably not more than 6, and especially preferably not more than 4. On the other hand, though a lower limit thereof is not particularly limited, in general, it is preferably -20 or more, more preferably -10 or more, still more preferably -5 or more, especially preferably -3 or more, and most preferably -1 or more. 

When a dicarboxylic acid having a YI value of not more than 50 is used, coloration of the produced polyurethane can be suppressed. On the other hand, when a dicarboxylic acid having a YI value of -20 or more is used, it is economically advantageous because not only extremely expensive invest› ment in plant and equipment is not required for the produc› tion, but a long production time is not required. In the present description, the YI value is a value measured by a method on the basis of JIS-K7105. 

(2) Aliphatic Dial: 

The aliphatic dial which is used in the present invention is not particularly limited so far as it is an aliphatic or alicyclic compound having two OH groups, and examples thereof include aliphatic dials in which a lower limit value of the carbon atom number is preferably 2 or more, and an upper limit value thereof is preferably not more than 10, and more preferably not more than 6. 

In addition, the dial unit as referred to herein is one derived from an aromatic dial and/or an aliphatic dial, and known compounds can be used. Of these, the use of an aliphatic dial is preferable. 

Specific examples of the aliphatic dial include ethylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, neopen› tyl glycol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,2- butanediol, 1,6-hexanediol, decamethylene glycol, 1,9- nonanediol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and the like. These may be used solely or in admixture of two or more kinds thereof. 

Of these, ethylene glycol, 1,4-butanediol, 1,3-propane› diol, 2-methyl-1,3-propanediol and 3-methyl-1,5-pen› tanediol are preferable. Above all, ethylene glycol, 1,4- butanediol, and a mixture thereof are preferable; and one containing, as a main component, 1,4-butanediol or 1,4- butanediol is especially preferable. 

It is meant by the terms "main component" as referred to herein that in general, the subject component accounts for preferably 50% by mole or more, more preferably 60% by mole or more, still more preferably 70% by mole or more, and especially preferably 90% by mole or more relative to the whole of dial units. These dials may be used solely, or may be used in admixture of two or more kinds thereof. 

The dial having a branched structure is especially pref- erably 2-methyl-1,3-propanediol or 3-methyl-1,5-pen› tanediol. 

When a dial having a methylene group between the hydroxyl groups and having an even carbon number is used, the mechanical strength of the resulting polyurethane increases, and when a dial having an odd carbon number or a branched structure is used, the handling properties of the polyester polyol are enhanced. 

In addition to the above, examples of a dial other than the aliphatic dial, which may be mixed, include aromatic dials. Though the aromatic dial is not particularly limited so far as it is an aromatic compound having two OH groups, examples thereof include aromatic dials in which a lower 

