Abstract:
The electrical resistivity of hindered polyol ester lubricants can be raised to very high levels by treating the lubricants with activated alumina, preferably after the acid values of the lubricants treated have already been reduced to low levels by conventional refining with alkali or other acid scavengers.

Description:
This application is a continuation of application Ser. No. 08/247,790 filed on May 23, 1994 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to processes for increasing the electrical resistivity of ester lubricants, particularly esters of hindered polyols, which are defined for this purpose as organic molecules containing at least five carbon atoms, at least 2 --OH groups, and no hydrogen atoms on any carbon atom directly attached to a carbon atom bearing an --OH group, more particularly such esters that are intended to be used to lubricate refrigerating compressors and similar machinery which will bring the lubricant esters into contact with, and thus form mixtures of them with, hydrofluorocarbon refrigerants. Substantially chlorine-free hydrofluorocarbon refrigerants (hereinafter often abbreviated as &#34;HFC&#39;s&#34;) are scheduled by international treaty and the laws and regulations of most industrially advanced countries to replace the most common previously used chlorofluorocarbon refrigerant heat transfer fluids (&#34;CFC&#39;s&#34;), in order to reduce the threat to the earth&#39;s ozone layer from the chlorine content of the emissions from imperfectly sealed and/or discarded refrigerators. 
     2. Statement of Related Art and Object of the Invention 
     CFC&#39;s and the mineral oil lubricants normally used with them generally have high electrical resistivities. This resistivity was relied on to provide an important part of the electrical insulation needed in many existing compressor designs. Commercial HFC&#39;s themselves are more polar than and have lower electrical resistance than commercial CFC&#39;s, and commercial examples of the ester type lubricants that are often needed for adequate miscibility with HFC&#39;s have often had considerably lower electrical resistance than commercial mineral oil lubricants. There is therefore a demand for lubricants with higher electrical resistivities that also are well adapted for use with HFC&#39;s. 
     Esters of hindered polyols have already been recognized in the art as high quality lubricant basestocks for almost any type of refrigeration machinery employing a fluorocarbon refrigerant, particularly one free from chlorine. However, these esters as practically made have often not had adequate electrical resistivity for the uses most demanding of this property. Providing such esters with higher electrical resistivity is a major object of this invention. 
     DESCRIPTION OF THE INVENTION 
     Except in the claims and the operating examples, or where otherwise expressly indicated, all-numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the term &#34;about&#34; in defining the broadest scope of the invention. Practice of the invention within the boundaries corresponding to the numerical quantities stated is usually preferable, however. Also, unless explicitly stated to the contrary, the description of more than one chemical compound or class of compounds as suitable or preferred for a particular purpose in connection with the invention shall be understood as implying that mixtures of any two or more of the entities so described individually as suitable or preferred are equally as suitable or preferred as the individual entities, and all descriptions of mixtures in terms of ratios, percentage, or parts shall be understood as ratios, percentages, or parts by weight or mass. 
     SUMMARY OF THE INVENTION 
     It has now been found that treatment with activated alumina is surprisingly effective in increasing the electrical resistivity of lubricant esters as commonly made commercially in the past, even when the esters have already been extensively refined to reduce their acid values. This treatment is particularly effective when applied to esters of the types described in copending U.S. patent application Ser. Nos. 08/027,628, 08/027,629, 08/028,856, 08/029,194, 08/029,196, and 08/029,204, all filed Mar. 10, 1993, and in U.S. Pat. No. 5,021,179 of Jun. 4, 1991 to Zehler et al., the complete specifications of all of which, except to any extent contrary to any explicit statement herein, are hereby incorporated herein by reference. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A process according to this invention at a minimum includes a step of contacting a liquid lubricant ester containing composition, also containing some impurities of unknown chemical character that are present in the starting materials and/or produced during the esterification reaction under practical conditions, with solid activated alumina, maintaining the contact for a selected period of time, and then separating the solids to produce a product liquid lubricant ester containing mixture with higher electrical resistivity. Preferably, contact between the solid activated alumina and the lubricant to be refined is assisted by mechanical agitation. 
     The temperature during contact between the lubricant and alumina preferably is, with increasing preference in the order given, at least 30°, 52°, 72°, 90°, 98°, 105°, 111°, 116°, or 119° C. and independently preferably is, with increasing preference in the order given, not more than 300°, 210°, 188°, 168°, 150°, 142°, 134°, 128°, 124°, or 121° C. 
     The time of contact between alumina and ester before separation preferably is, with increasing preference in the order given, not less that 1, 2, 4, 8, 16, 32, 40, 48, 52, or 56 minutes (hereinafter often abbreviated &#34;min&#34;) and independently preferably is, with increasing preference in the order given, not more than 480, 360, 300, 280, 265, 255, 250, or 244 min. 
     The ratio, expressed as percentage, of the mass of activated alumina used to the mass of liquid treated preferably is, with increasing preference in the order given, not less than 0.01, 0.02, 0.04, 0.08, 0.16, 0.26, 0.35, 0.39, 0.43, 0.46, or 0.48% and independently not more than 10, 5, 2.5, 1.8, 1.3, 0.80, 0.67, 0.62, 0.58, 0.55, or 0.52%. 
     The average particle size of the activated alumina used in the process preferably is chosen to be sufficiently large to separate easily when desired, by some well established and economical process such as filtration or centrifugation, and sufficiently small and/or porous to have a high surface area, inasmuch as it is believed that adsorption may contribute significantly to the increases in electrical resistivity achieved by a process according to the invention. More particularly, the alumina used preferably is, with increasing preference in the order given, fine enough to pass through screens with standard mesh sizes of 24, 40, 50, 60, 80, or 100 mesh and independently preferably is, with increasing preference in the order given, not fine enough to pass through screens or cloths with standard mesh sizes of 1000, 600, 400, or 320. Independently the specific surface area of the alumina used, measured by the conventional Brunauer-Emmett-Teller (&#34;BET&#34;) nitrogen gas adsorption technique, is at least, with increasing preference in the order given, 100, 150, 200, 225, or 250 square meters per gram (hereinafter abbreviated as &#34;m 2  /g&#34;). 
     Under some conditions it is advantageous to reduce the pressure over the mixture of activated alumina and lubricant being treated according to the invention. When such a partial vacuum is desired, the pressure of gas over the mixture of alumina and esters being treated preferably is, with increasing preference in the order given, not more than 100, 50, 25, 15, 10, 8, 7, 6, or 5 torr during at least half of the total contact time. 
     The conditions above have been described for a single application of a process according to the invention. In practice, however, it has been found advantageous in many cases to repeat one or all of the steps of the process outlined above at least once, and sometimes more than once, as indicated in the working examples below. A somewhat less preferred, but nevertheless often useful, alternative to repeating all the steps outlined above is to use amounts of alumina nearer to the upper limits of the preferred values in a single step, rather than amounts nearer to the most preferred values. For example, instead of using 0.5% alumina three times, 1.5% alumina might be used in a single step. 
     The effectiveness of activated alumina in raising electrical resistivity is substantially reduced if the lubricant ester containing mixture being treated contains free acids of any sort. Therefore, the lubricant mixture to be treated according to the invention preferably is first treated if necessary by alkali refining as generally known in the art in order to reduce its acid value (hereinafter often abbreviated as &#34;AV&#34;). The AV of the lubricant mixture to be treated before beginning treatment according to the invention preferably is, with increasing preference in the order given, not greater than 0.10, 0.085, 0.065, 0.050, 0.040, 0.032, 0.028, 0.026, 0.024, 0.022, 0.020, 0.018, 0.016, 0.014, 0.012, or 0.010. 
     The effectiveness of the alumina can also be reduced by water present in the liquid being treated. Therefore, the liquid to be treated preferably is dried by partial vacuum and heat and/or by contact with desiccant before treatment according to this invention. One suitable method of drying is to maintain the ester mixture at 120° C. for about 2 hours under a partial vacuum, with a pressure of not more than 2 torr, or preferably not more than 0.2 torr. However, other drying methods known in the art may also be used. 
     The volume resistivity, measured according to ASTM method D 1169-80, of a lubricant product from a process according to this invention preferably is, with increasing preference in the order given, at least 1.0·10 14 , 2.0·10 14 , 4.0·10 14 , 5.9·10 14 , 6.8·10 14 , 8.0·10 14 , 9.0·10 14 , or 10.0·10 14  ohm centimeters. 
     A process according to the invention is particularly advantageously applied to esters made by reacting hindered polyols, preferably neopentyl glycol (&#34;NPG&#34;), trimethylolpropane (&#34;TMP&#34;), or pentaerythritol (&#34;PE&#34;), with pentanoic acid or 2-ethylhexanoic acid. 
     The practice of the invention may be further appreciated by consideration of the following examples and comparative examples. 
     Ester base stocks 1 and 2 were prepared by reacting TMP for #1 and PE for #2 with pentanoic acid in the manner described in U.S. Pat. No. 5,021,179 column 8 lines 9-62, using sodium hydroxide to remove residual acidity in the manner generally known in the art as &#34;alkali refining&#34;. Different lots of each type of ester base stock were made at different times, using the same nominal directions but producing slightly different results, probably because of variations in the raw materials and/or process conditions within the ranges of permissible variation and practical control of these factors. The refined and dried ester lubricants were then treated according to the present invention with 0.5% of their mass of Alumina F-1 with a particle size of -100 mesh, commercially obtainable from Alcoa Industrial Chemicals Division (Vidalia, La.), for 1-2 hours per treatment step at 120° C. In some cases, as indicated in Table 1 below, more than one step of alkali refining and/or alumina treatment were used. The electrical resistance values were measured according to ASTM D 1169-80, using a test voltage of 500 volts per mm at a temperature of 25° C. 
     
                       TABLE 1______________________________________      Number of                  Tera-Ester Type Repetitions of:     Acid   ohm.and Lot No.      Alumina  Alkali     Value  Cm______________________________________1.1        1        1          0.004  1196      2        1          0.006  2286      4        1          0.006   232.sub.0      1        4          0.004   162.sub.0      4        4          0.004   238.sub.02.1        0        1          0.006   6432.2        1        4          0.029   58.sub.0______________________________________ 
    
     It is apparent from the results in Table 1 that alumina treatment is more effective than alkali refining in increasing electrical volume resistivity.