US  10,619,000 B2  34  33  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 