Heat transfer fluids having low freeze points

Heat transfer fluids consisting essentially of about, by weight: PA1 A. 20 to about 40 percent diphenyl ether; PA1 B. 40 to about 60 percent of a 2-biphenylylphenyl ether (2-bippe) and 4-biphenylylphenyl ether (4-bippe) mixture at a 2-bippe:4-bippe weight ratio of at least about 2:1; and PA1 C. 12 to about 25 percent of a polyphenylphenol mixture Exhibit freeze points of -18.degree. C or less. These heat transfer fluids have excellent fluidity over a broad temperature range and display unexpectedly good thermal stability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to heat transfer fluids. In one aspect, this 
invention relates to aryl ether heat transfer fluids. In another aspect, 
this invention relates to heat transfer fluids comprising aryl ethers and 
polyphenylphenols. 
2. Description of the Prior Art 
The components of the present invention are known. Jackson et al. teach in 
U.S. Pat. Nos. 3,888,777, 3,907,696 and 3,966,626, various heat transfer 
fluids comprising certain of the components used herein, particularly 
diphenyl ether, 2-biphenylylphenyl ether and 4-biphenylylphenyl ether. 
However, these heat transfer fluids also comprise such components as 
biphenyls and polyphenyl ethers which are not present in the instant 
invention. Moreover, these heat transfer fluids do not have the unusually 
good fluidity of the instant invention at low temperatures (0.degree. C. 
and below). 
Copending application by Watson et al., entitled "HEAT TRANSFER FLUIDS 
HAVING A LOW FREEZE POINT" and filed even date herewith, teaches a heat 
transfer fluid comprising diphenyl ether, 2-, 3- and 4-biphenylylphenyl 
ether, and polyphenylphenols in various proportions. The present invention 
differs from the Watson et al. invention by the relative amounts of 
polyphenylphenol and 3-biphenylylphenyl ether. 
SUMMARY OF THE INVENTION 
Heat transfer fluids having freeze points of about -18.degree. C or less 
consist essentially of about, by weight: 
A. 20 to about 40 percent diphenyl ether; 
B. 40 to about 60 percent of a 2-biphenylylphenyl ether (2-bippe) and 
4-biphenylylphenyl ether (4-bippe) mixture at a 2-bippe:4-bippe weight 
ratio of at least about 2:1; and 
C. 12 to about 25 percent of a polyphenylphenol mixture. 
This invention demonstrates remarkable fluidity at low temperatures while 
maintaining exceptional thermal stability. The latter is surprising since 
it is known that most phenolic materials decrease the thermal stability of 
a heat transfer fluid. 
DETAILED DESCRIPTION OF THE INVENTION 
All the components of this invention are commercially available and require 
little description. Diphenyl ether has the structural formula 
##STR1## 
The biphenylylphenyl ethers here used have the structural formulae 
##STR2## 
The preparation of biphenylylphenyl ether generally results in a mixture 
of the above isomers. The mixture typically contains about 2 percent by 
weight of the 3-isomer, the remainder being the 2- and 4-isomers in a 
ratio of about 2:1. This ether mixture is quite suitable for use in the 
instant invention without modification. 
The polyphenylphenol mixture here used comprises both diphenylphenols of 
the formulae 
##STR3## 
and various other isomers thereof. While the practice of this invention 
can include any suitable di- and tri-phenylphenol mixture, the 
polyphenylphenol mixture consisting primarily (about 80 weight percent) of 
2,6- and 2,4-diphenylphenol with lesser amounts (about 20 weight percent) 
of various triphenylphenol and other diphenylphenol isomers is typically 
used. 
Minor amounts, typically less than about 7 weight percent, of various 
impurities can be present in the instant invention without adversely 
affecting its properties (low freeze point and thermal stability). These 
impurities are usually a result of the manufacturing processes employed to 
prepare this invention's various components and include such compounds as 
o- and p-phenylphenol, o- and m-terphenyl, phenylnaphthalene, 
methylphenylnaphthalene and the like. Of course, it is preferred to keep 
these impurities at the lowest practical level. 
The heat transfer fluids of the instant invention are, of course, mixtures 
of the above components. At a minimum, these mixtures consist essentially 
of about: 
a. 20 percent, and preferably about 25 percent, diphenyl ether; 
b. 40 percent, and preferably about 45 percent, of a 2-biphenylylphenyl 
ether (2-bippe) and 4-biphenylylphenyl ether (4-bippe) mixture at a 
2-bippe:4-bippe weight ratio of at least about 2:1; and 
c. 12 percent, and preferably about 15 percent, of a polyphenylphenol 
mixture. 
At a maximum, these mixtures consist essentially of about: 
a. 40 percent, and preferably about 35 percent, diphenyl ether; 
b. 60 percent, and preferably about 55 percent, of a 2-biphenylylphenyl 
ether (2-bippe) and 4-biphenylylphenyl ether (4-bippe) mixture at a 
2-bippe:4-bippe weight ratio of at least about 2:1; and 
c. 25 percent, and preferably about 20 percent, of a polyphenylphenol 
mixture. 
The heat transfer fluid consisting essentially of about: 
a. 30 percent diphenyl ether; 
b. 50 percent of a 2-biphenylylphenyl ether (2-bippe) and 
4-biphenylylphenyl ether (4-bippe) mixture at a 2-bippe:4-bippe weight 
ratio of at least about 2:1; and 
c. 20 percent of a polyphenylphenol mixture 
is especially preferred. 
The heat transfer fluids of the present invention are used in the same 
manner as known fluids. However, because of their exceptional fluidity at 
low temperatures, they can be used under more adverse conditions than the 
known fluids without the need for special precautions. Illustratively, the 
present fluids can be employed in outside conduits year-round in the 
northern climes. Moreover, these fluids exhibit high boiling points and 
the characteristically lower vapor pressure of such fluids. Consequently, 
pressure in the heating units wherein these fluids are employed is reduced 
and accordingly, the work required for maintenance and leak prevention is 
reduced.