Abstract:
The well-known heat transfer fluid consisting essentially of the eutectic mixture of diphenyl oxide and biphenyl is improved by the addition thereto of monomethyl- or monoethylbiphenyl. The resulting fluids have very nearly the same boiling point and heat stability as the binary eutectic but have substantially lower freezing points.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of our copending application Ser. No. 139,882, filed May 3, 1971. 
    
    
     BACKGROUND OF THE INVENTION 
     The eutectic mixture of diphenyl oxide and biphenyl has been widely used as a high temperature heat transfer fluid for more than forty years (Grebe, U.S. Pat. No. 1,882,809). Among its more critical properties are its high boiling point, its thermal stability at and above the boiling point in both the liquid and the vapor phase, and the fact that the liquid and vapor phases have substantially the same composition. A primary drawback of this binary eutectic mixture is its relatively high freezing point (54°F.). This complicates its use in many outdoor applications. 
     In order to improve its utility, many additives have been proposed for the mixture. Among these are naphthalene, aniline, diphenylmethane and chlorodiphenyl (Rittler et al., U.S. Pat. No. 2,149,789). These have not been satisfactory, for one reason or another. Some raise the freezing point. Others are unstable at high temperatures or in the vapor phase. Still others have boiling points so different from that of the binary eutectic that the vapor composition differs widely from that of the liquid. 
     The alkylated diphenyls have been known for almost as long as the above binary eutectic and have been suggested for use as heat transfer fluids (Krase, U.S. Pat. No. 2,172,391) but have not been combined with the eutectic to improve their respective properties. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a heat transfer fluid having substantially the desirable high temperature properties of the binary eutectic mixture of diphenyl oxide and biphenyl while having a substantially lower freezing point is obtained by adding methyl- or ethylbiphenyl to said eutectic mixture. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The binary eutectic mixture of diphenyl oxide and biphenyl (DPO and BP respectively) is widely sold and extensively used under the trade name Dowtherm A fluid (hereinafter DTA). While this fluid contains the two components in the preferred proportions (73.6% DPO, 26.4% BP) for use in the present invention, it is not necessary to use precisely these proportions. Almost equally good results can be obtained with slightly different proportions (e.g., 70-80% BP) provided the proportion of methylbiphenyl (MBP) or ethylbiphenyl (EBP) is somewhat increased. Accordingly, these variations are to be understood as included wherever reference is made to DTA or the eutectic mixture. Also, MBP and EBP may be used separately or in combination as a mixture in any proportions for combining with DTA to produce the fluids of the invention. In fact, such mixtures are usually slightly more effective than either component alone in lowering the freezing point of DTA without substantially altering its high-temperature properties. 
     The proportion of MBP or EBP that should be incorporated into DTA depends primarily on the desired freezing point of the resulting fluid. For minimizing the differences other than freezing point between DTA and the new fluid, it is desirable to keep the proportion as low as possible while attaining the desired lowering of the freezing point. As little as 5-10% produces a significant lowering while less than 50% will ordinarily be sufficient to give an adequately low freezing point. The mixtures are made, of course, by merely blending together the components. 
     SPECIFIC EMBODIMENTS OF THE INVENTION 
     The following table shows the freezing points and boiling points of the components of the fluids of the invention and of several typical examples of formulated fluids. The MBP and EBP used therein were the unfractionated mixed isomers unless otherwise specified and consisted mostly of the meta and para isomers. Such mixtures are the equal of, or even superior to, the individual isomers; hence, it is usually preferred to use a mixture rather than a single isomer. When using such mixtures of isomers, it is preferred that the meta isomer constitute at least about 50% of the mixture and that the meta and para isomers together constitute at least about 90% of the isomer mixture. The MBP contained about 70% by volume of the meta isomer and about 23% of the para isomer, the balance being the ortho isomer, unsubstituted biphenyl and polymethylated biphenyl. 
     
                                           TABLE I__________________________________________________________________________   Fluid Composition, % by Vol.                       Freezing                              BoilingExample No.   DPO BP  DTA MBP EBP Pt., °F.                              Pt., °F.__________________________________________________________________________   100                 77     495       100             158    493           100         54     495               100     -4     515                   100 -44    5361               67  33      16     5012               50  50      -29    5053               33  67      -25    5084               70      30  10     5065               50      50  &lt;-58   5126               30      70  -50    522__________________________________________________________________________ 
    
     To show the effect of individual isomers of MBP as compared to a mixture thereof, the meta and para isomers were separated and used separately. The ortho isomer was not included because it is present in the crude mixture in only ver low proportions (&lt;5%) and is therefore quite difficult to isolate in useful quantities. Its F.P. is 32°F. 
     
                       TABLE II______________________________________  Composition, % by Vol.                     FreezingEx. No.  DTA      p-MBP     m-MBP   Pt., °F.______________________________________             100               120                       100     41 7       91        9                41 8       82       18                26 9       74       26                2110       68       32                2311       62       38                2312       91                  9      4613       82                 18      3514       70       30        1915       60                 40      -316       50                 50      -22______________________________________ 
    
     The fluids of the above examples have high-temperature properties substantially equivalent to those of DTA; hence, these fluids can be used in most applications as a direct replacement of DTA. Moreover, they are compatible with DTA and can be mixed therewith without harm.