Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR INCREASING THE ELECTRICAL RESISTIVITY OF
HINDERED POLYOL ESTER REFRIGERANT LUBRICANTS
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to processes for increasing the electrical resistivity
of ester lubricants, particularly esters of hindered poiyols, which are
defined far
s 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
,o mixtures of them with, hydrofluorocarbon refrigerants. Substantially
chlorine-
free hydrofluorocarbon refrigerants (hereinafter often abbreviated as "HFC's")
are scheduled by international treaty and the laws and regulations of most in-
dustrially advanced countries to replace the most common previously used
chiorofluorocarbon refrigerant heat transfer fluids ("CFC's"), in order to
reduce
,s the threat to the earth's ozone layer from the chlorine content of the
emissions
from imperfectly sealed and/or discarded refrigerators.
Statement of Related Art and Obiect of the Invention
CFC's and the mineral oil lubricants normally used with them generally
have high electrical resistivities. This resistivity was relied on to provide
an
Zo important part of the electrical insulation needed in many existing
compressor
designs. Commercial HFC's themselves are more polar than and have lower
electrical resistance than commercial CFC's, and commercial examples of the
ester type lubricants that are often needed for adequate miscibility with
HFC's
have often .had considerably lower electrical resistance than commercial
zs mineral oil lubricants. There is therefore a demand for lubricants with
higher
electrical resistivities that also are well adapted for use with HFC's.
Esters of hindered poiyols 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.
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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 "about" 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
The present invention provides a process for increasing the electrical
resistivity of a liquid mixture consisting essentially of esters of hindered
polyols
selected from the group consisting of neopentyl glycol, trimethylolpropane and
pentaerythritol with organic carboxylic acids, which is used with chlorine-
free
hydrofluorocarbon refrigerants, said process comprising the steps of:
(1 ) pretreating the liquid mixture by alkali refining in order to reduce the
acid
value of the mixture;
(2) mixing the pretreated liquid mixture with solid activated alumina so as to
form
a solid-liquid mixture and maintaining mutual contact between the liquid and
the
alumina in said solid-liquid mixture at a temperature of at least 30°
C. for a period
of time;
(3) separating the solid material from the liquid in the solid-liquid mixture
formed
at the end of step (2) to produce a second liquid consisting essentially of
esters of
hindered polyols with organic carboxylic acids;
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(4) mixing the second liquid mixture with solid activated alumina so as to
form a
solid-liquid mixture and maintaining mutual contact between the liquid and the
alumina in said solid-liquid mixture at a temperature of at least 30°
C. for a period
of time; and
(5) separating the solid material from the liquid in the solid-liquid mixture
formed
at the end of step (4) to produce a third liquid consisting essentially of
esters of
hindered polyols with organic carboxylic acids and having an electrical
resistivity
that is higher than that of the first liquid mixture.
In another aspect, the invention provides a process for increasing the
electrical resistivity of a first liquid mixture consisting essentially of
esters of
hindered polyols with organic carboxylic acids selected from the group
consisting
of pentanoic acid and 2-ethyl-hexanoic acid, which is used with chlorine-free
hydrofluorocarbon refrigerants, said process comprising the steps of:
(1 ) pretreating the first liquid mixture by alkali refining in order to
reduce the acid
value of the mixture;
(2) mixing the pretreated liquid mixture with solid activated alumina so as to
form
a solid-liquid mixture and maintaining mutual contact between the liquid and
the
alumina in said solid-liquid mixture at a temperature of at least 30°
C. for a period
of time;
(3) separating the solid material from the liquid in the solid-liquid mixture
formed
at the end of step (2) to produce a second liquid mixture consisting
essentially of
esters of hindered polyols with organic carboxylic acids;
(4) mixing the second liquid mixture with solid activated alumina so as to
form a
solid-liquid mixture and maintaining mutual contact between the liquid and the
alumina in said solid-liquid mixture at a temperature of at least 30°
C. for a period
of time; and
(5) separating the solid material from the liquid in the solid-liquid mixture
formed
at the end of step (4) to produce a third liquid mixture consisting
essentially of
esters of hindered polyols with organic carboxylic acids and having an
electrical
resistivity that is higher than that of the first liquid mixture wherein the
esters of
said first liquid mixture.
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It should be noted that the standard definitions for the acronyms NPG,
TMP, PE, DTMP and DPE apply herein and these are:
NPG - neopentyl glycol
TMP - trimethylol propane
PE - pentaerythritol
DTMP- di-trimethylol propane
DPE - di-pentaerythritol
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 Canadian
Patent Applications Nos. 2,136,852, 2,136,853, 2,137,257, 2,137,263, 2,136,851
and 2,137,149, all filed May 27, 1993 and in U.S. Patent No. 5,021,179 of June
4,
1991 to Zehler et al.
In one aspect the process for increasing the electrical resistivity of a first
liquid mixture consisting essentially of esters of hindered polyols with
organic
carboxylic acids, said process comprising the steps of:
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( 1 ) mixing the first liquid mixture with solid activated alumina so as to
form a solid - liquid mixture and maintaining mutual contact between the
liquid
mixture and the alumina in said solid - liquid mixture at a temperature of at
least
30 ' C for a time of at least one minute;
(2) separating the solid material from the liquid in the solid - liquid
mixture formed at the end of step (1) to produce a second liquid consisting
essentially of esters of hindered polyols with organic carboxylic acids;
(3) mixing the second liquid mixture with solid activated alumina so as to
form a solid - liquid mixture and maintaining mutual contact between the
liquid
and the alurnina in said solid - liquid mixture at a temperature of at least
30' C for
a time of at least one minute; and
(4) separating the solid material from the liquid in the solid - liquid
mixture formed at the end of step (3) to produce a third liquid consisting
essentially of esters of hindered polyols with organic carboxylic acids and
having
an electrical resistivity that is higher than that of the first liquid mixture
and an
acid value of not greater than 0.10.
In a more preferred aspect the invention provides the process for increasing
the electrical resistivity of a first liquid mixture consisting essentially of
esters of
hindered polyols with organic carboxylic acids, said process comprising the
steps
of
( 1 ) pretreating the first liquid mixture by alkali refining in order to
reduce
the acid value of the mixture;
(2) mixing the first liquid mixture with solid activated alumina so as to
form a solid - liquid mixture=and maintaining mutual contact between the
liquid
and the alumina in said solid - liquid mixture at a temperature of at least
30' C for
a time of at least 1 minute;
(3) separating the solid material from the liquid in the solid - liquid
mixture formed at the end of step (2) to produce a second liquid consisting
essentially of esters of hindered polyols with organic carboxylic acids;
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(4) mixing the second liquid mixture with solid activated alumina so as to
form a solid - liquid mixture and maintaining mutual contact between the
liquid
and the alumina on said solid - liquid mixture at a temperature of at least
30~C
for a time of at least one minute; and
(5) separating the solid material from the liquid in the solid - liquid
mixture formed at the end of step (4) to produce a third liquid consisting
essentially of esters of hindered polyols with organic carboxylic acids and
having
an electrical resistivity that is high than that of the first liquid mixture
and an acid
value of not greater than 0.10.
More particularly, the invention provides the process for increasing the
electrical resistivity of a liquid mixture consisting essentially of esters of
hindered
polyols with organic carboxylic acids, which is used with chlorine-free
hydrofluorocarbon refrigerants, said process comprising the steps of:
(1) pretreating the liquid mixture by alkali refining in order to reduce
the acid value of the mixture;
(2) mixing the pretreated liquid mixture with solid activated alumina so
as to form a solid - liquid mixture and maintaining mutual contact between the
liquid and the alumina in said solid - liquid mixture at a temperature of at
least
30°C for a period of time;
(3) separating the solid material from the liquid in the solid - liquid
mixture formed at the end of step (2) to produce a second liquid consisting
essentially of esters of hindered polyols with organic carboxylic acids;
(4) mixing the second liquid mixture with solid activated alumina so as
to form a solid - liquid mixture and maintaining mutual contact between the
liquid
and the alumina in said solid - liquid mixture at a temperature of at least
30°C for
a period of time; and
(5) separating the solid material from the liquid in the solid - liquid
mixture formed at the end of step (4) to produce a third liquid consisting
essentially of esters of hindered polyols with organic carboxylic acids and
having
an electrical resistivity that is higher than that of the first liquid
mixture.
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Descriation 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 aiumina 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 "min") 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 aiumina
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 °/a.
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
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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 specifrc surface area of the aiumina used, measured by
the conventional Brunauer-Emmett-Teller ("BET') 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 "m2lg").
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 % aiumina 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 "AV"). The AV of the lubricant mixture to be
treated before beginning treatment according to the invention preferably is,
with
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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 torn.
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", 2.0~10",
4.0~10", 5.9~10", 6.8~10", 8.0~10", 9.0~10", or 10.0~10" ohm centimeters.
A process according to the invention is particularly advantageously
applied to esters made by reacting hindered polyois, preferably neopentyl
glycol
("NPG"), trimethyiolpropane ("TMP"), or pentaerythritol ("PE"), 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. Patent 5,021,179
column 8 lines 9 - 62, using sodium hydroxide to remove residual acidity in
the
manner generally known in the art as "alkali refining". 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 T"" with a particle size of -100 mesh, commercially
obtainable from Alcoa Industrial Chemicals Division (Vidalia, Louisiana), for
1 - 2
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hours per treatment step at 120° C. In some cases, as indicated in
Table 1
below, more than one step of alkali refining andlor aiumina 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
Ester Type "Number of Acid Teraohm
Re etitions
of:
and Lot No. Alumina Alkali Value Cm
1 1 0.004 1196
2 1 0.006 2286
1.1 4 1 0.006 232
1 4 0.004 162
4 4 0.004 238
2.1 0 1 0.006 643
2.2 1 4 0.029 58
It is apparent from the results in Table 1 that alumina treatment is more
effective than alkali refining in increasing electrical volume resistivity.
