Abstract:
Dielectric fluids having good electrical properties, low viscosities, low vapor pressures, low freezing points, high flash points and excellent biodegradability comprise a mixture of an alkylhalodiphenyl oxide and a chloro- or bromodiphenyl oxide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of my earlier application Ser. No. 377, 839 filed July 9, 1973, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The most widely used dielectric fluids heretofore have been highly chlorinated hydrocarbons, such as the polychlorinated biphenyls (PCB&#39;s). While these are functionally satisfactory, they are highly objectionable from an environmental and ecological standpoint because of their high toxicity and extreme resistance to biodegradation. For this reason, acceptable substitutes have been actively sought. 
     Polyhalogenated diphenyl oxides are known to be useful as dielectric fluids, either alone (U.S. Pat. No. 2,022,634) or in admixture with alkyldiphenyl oxides (U.S. Pat. No. 2,169,995). Such fluids are no longer acceptable, however, because the polyhalo component is not sufficiently biodegradable. 
     Alkylated diphenyl oxides have been recommended for use as dielectric fluids (U.S. Pat. No. 2,170,809) but have never found wide use because of their poor electrical properties, especially their low dielectric constants. 
     Lower alkylchlorodiphenyl oxides are known and have been suggested for use as dielectric fluids (U.S. Pat. No. 2,170,989) but have not been accepted in the industry because of high volatility, low flash point and relatively poor electrical properties. 
     SUMMARY OF THE INVENTION 
     Dielectric fluids having suitable physical, chemical, electrical and biological properties comprise a mixture of diphenyl oxide (DPO) compounds, at least one of which is an alkylhalo DPO and at least one is a monohalo DPO. The alkylhalo DPO has 1-3 alkyl groups of about 2-12 carbon atoms each, the total alkyl carbon content being about 2-16, and one halogen substituent. In another embodiment, the alkylhalo DPO has 1-2 alkyl groups of about 4-12 carbon atoms each, the total alkyl carbon content being about 4-16, and one halogen substituent. The halogen in both components of the fluid is chlorine or bromine. In one embodiment, the proportions of halo DPO and alkylhalo DPO are between about 30:70 and 70:30; i.e., the mixture should contain at least about 30% by weight of each component. The preferred proportions are about 50:50. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In its broadest aspect, the dielectric fluid of the invention comprises a mixture of (a) at least one monochloro- or monobromodiphenyl oxide and (b) at least one monochloro- or monobromoalkyldiphenyl oxide having 1-3 alkyl groups, each of which has 2-12 carbon atoms, said oxide having a total of about 2-16 alkyl carbon atoms. In another embodiment, (a) and (b) are each present in the mixture in an amount between about 5 and about 95 weight percent based upon the total weight of the mixture. 
     As indicated above, the monochloro- and monobromoalkyldiphenyl oxide component in the mixture can have 1 to 3 alkyl substituents. Thus, for example, in a specific embodiment, the dielectric fluid can comprise a mixture of (a) monochlorodiphenyl oxide and (b) a mixture of monochlorobutyldiphenyl oxide, monochlorodibutyldiphenyl oxide, and monochlorotributyldiphenyl oxide. In a more specific embodiment, in the mixture defined above, the monochlorodiphenyl oxide is present in an amount between about 5 and about 40 weight percent, the monochlorobutyldiphenyl oxide is present in an amount between about 30 and about 80 weight percent, the monochlorodibutyldiphenyl oxide is present in an amount between about 10 and about 50 weight percent, and the monochlorotributyldiphenyl oxide is present in an amount between about 2 and about 10 weight percent. 
     The alkylation and halogenation of DPO are well known in the art. It is recognized in the art that these reactions inherently produce a mixture of isomeric, homologous and/or analogous compounds wherein the number and arrangement of the substituents vary. Such mixtures, usually referred to as cogeneric mixtures, can be partially or completely resolved by fractional distillation and/or crystallization, or by other known means. For the present purpose, however, it has been found that such separation is not ordinarily necessary, provided the average values stay within the limits set forth herein. In particular, polyhalo components should not be present in more than very low concentrations because of their increased toxicity and resistance to biodegradability. For most applications, concentrations of dihalo compounds of up to about 5% can be tolerated. 
    
    
     SPECIFIC EMBODIMENTS OF THE INVENTION 
     Examples 1-11 
     The table below shows several important properties of some typical examples of the invention. 
     In the table the DPO compounds are identified by the substituents on the DPO nucleus. Thus, for instance, Cl--C 12  -- is monochlorododecyldiphenyl oxide. The hexyl and butyl substituents are represented by C 6  and C 4 , respectively. Except as indicated, the mixtures used in the examples were 50:50 by weight. 
     
                                           TABLE I__________________________________________________________________________                                  DissipationEx.   DPO   Flash Pt.,            Fire Pt.,                 Pour Pt.,                      Boiling Pt.,                            Dielectric                                  FactorNo.   Cpds. ° F.            ° F.                 ° C.                      ° C.                            Const.                                  10.sup.3 HZ                                       10.sup.4 HZ__________________________________________________________________________  Cl-- 290  340   -27 271   4.7   0.003 Br--  350  530   -40 305   4.3   .003 Cl--C.sub.12 --       450  480  0    &gt;400  3.9   .0001 Cl--C.sub.12       325  370   -37 &gt;300  4.3   .002 Cl Cl--C.sub.12 --       360  470   -29 350   4.2   .0032 Br Cl--C.sub.6 --       390  415   -30 340   4.3        .0002 C1--C.sub.6 --3 Cl--  315  345   -27 290   4.4        .0002 Cl--C.sub.6 --       360  440   -35 310   4.3        .00034 Br-- Cl--C.sub.4 --       335  375  &lt;-40 310   4.5        .0002 Cl--C.sub.4 --5 Cl--  300  335  &lt;-40 280   4.7        .0002 Cl--C.sub.4 --6 Br--  340  385  &lt;-40 305   4.5        .0002 Cl--C.sub.12       310  340  &lt;-20 270-460                            4.6        0.0002  7.sup.(a) Cl Cl--C.sub.12       355  385  &lt;-20 270-460                            4.2        0.0002  8.sup.(b) Cl Cl--C.sub.3       330  370  &lt;-45 --    4.6        0.0002  9.sup.(c) Cl Cl--C.sub.4       350  375  &lt;-45 290-410                            4.5        0.0001  10.sup.(d) Cl Cl--C.sub.6       340  360  &lt;-45 300-410                            4.3        0.0001  11.sup.(e) Cl__________________________________________________________________________ .sup.(a) 70% by weight Cl DPO and 30% by weight Cl--C.sub.12 .sup.(b) 70% by weight Cl--C.sub.12 and 30% by weight Cl .sup.(c) 79% by weight Cl--C.sub.3 and 21% by weight Cl DPO .sup.(d) 83% by weight Cl--C.sub.4 and 17% by weight Cl DPO .sup.(e) 64% by weight Cl--C.sub.6 and 36% by weight Cl DPO 
    
     All of the above materials are readily biodegradable, as are the homologous mono- and dialkylchloro- and -bromo-DPO compounds. 
     Example 12 
     The toxicity to fish of the dielectric fluids was determined by a 96 hour static water test using Fathead minnows. In these runs, the toxicity of monochlorodiphenyl oxide, monochlorobutyldiphenyl oxide, and mixtures thereof was determined. Table II below sets forth the results which were obtained. Ten fish were employed in each run. The LC 50  is defined as the concentration of the oxides in the water in milligrams per liter at which 50 percent or five of the fish remain unaffected (the five remaining fish were either dead or demonstrated evidence of toxicity). 
     
                       TABLE II______________________________________Wt. %C.sub.4 C1--DPO          0     70    75  80  85  90  95   100Wt. %Cl-- DPO      100    30    25  20  15  10   5    0______________________________________LC.sub.50 Expected           5.7 6.8 8.5 11  17  34LC.sub.50 Observed    1.7    7   7   7   20  27  NT   NT______________________________________ NT means not toxic at the solubility in water 
    
     As can be seen from the data in Table II, the EC 50  for the monochlorodiphenyl oxide by itself is only 1.7 mg/liter of water. The presence of the monochloroalkyldiphenyl oxide apparently diminishes the toxicity of the Cl DPO because the actual EC 50  is much greater than expected for mixtures containing up to 15 percent Cl DPO. 
     Other alkylhalo DPO compounds that may be mixed with Cl-DPO and/or Br-DPO, in proportions of 30-70% for example, to produce dielectric fluids according to the invention include the ethyl-, propyl-, hexyl-, octyl-, decyl-, dihexyl-, dioctyl-, hexyl octyl-, heptyl nonyl-, and hexyl decyl-Cl-DPO and the corresponding alkyl-Br-DPO compounds. In these compounds, the positions of the substituents on the DPO nucleus is of little significance in the present invention. Likewise, the configuration of the alkyl groups is relatively unimportant, though those having a high degree of branching are less readily biodegradable than those having straight chains.