Low coefficient of friction magnet wire enamels

An improved thermally stable, lubricious magnet wire enamel is disclosed which is particularly suitable for use as an outermost insulating, lubricating layer on a magnet wire substrate. The enamel comprises the reaction product of a tri-basic acid anhydride, a diisocyanate and a polyfunctional organosiloxane. The coating has a coefficient of friction, after application to magnet wire, of less than 0.10. It is also substantially all polyfunctional organosiloxane reacted, i.e. substantially no polyfunctional organosiloxane is capable of being extracted from the cured coating with organic solvents.

DESCRIPTION 
1. Technical Field 
The field of art to which this invention pertains is silicon containing 
polymers and specifically low coefficient of friction magnet wire enamels. 
2. Background Art 
In the manufacture of electrical motors, the more magnet wire which can be 
inserted into a stator core, the more efficient the motor performance. In 
addition to motor efficiency considerations, motor manufacturers are also 
interested in manufacturing efficiency. Accordingly, such coils where 
possible are inserted automatically, generally by two methods: either a 
gun-winding method or a slot insertion method. In the older gun-winding 
method, the winding is done by carrying the wire into the stator slot by 
means of a hollow winding needle. Turns are made by a circular path of the 
gun to accommodate the individual coil slots. As described in Cal Towne's 
paper entitled "Motor Winding Insertion" presented at the 
Electrical/Electronics Insulation Conference, Boston, Mass. in September, 
1979, in the more preferred slot insertion method, coils are first wound 
on a form, placed on a transfer tool and then pressed off the transfer 
tool into the stator core slots through insertion guides or blades. In 
order to accommodate these automated insertion methods, wire manufacturers 
have responded by producing magnet wires with insulating coatings with low 
coefficient of friction. Note, for example, U.S. Pat. Nos. 3,413,148; 
3,446,660; 3,583,885; 3,632,440; 3,681,282; 3,686,030; 3,775,175; 
3,779,991; 3,856,566; 4,002,797; 4,216,263; 4,348,460; 4,350,737; 
4,350,738; 4,385,436; 4,385,437; 4,390,590; 4,400,430; 4,404,331; 
4,410,592; and published European Patent Application Number 0-033-244, 
published Aug. 5, 1981 (Bulletin 8/31). 
Typically, approaches of the past have included the blending of various 
lubricants into wire enamels and/or topically applying a lubricant to the 
finished magnet wire. The materials blended have included such things as 
silicone fluids, polyethylene, fatty acids, and esters of fatty acids, and 
alcohols. The topical applied lubricants have generally included light 
oils or paraffin wax solutions. None of these approaches have been totally 
successful in eliminating magnet wire windability and insertion problems. 
Blending has resulted in nonhomogeneous systems due to low solubility of 
lubricants in the enamels or, in the case of silicone fluids, an 
undesirable migration of the silicone fluid out of the magnet wire coating 
and onto other surfaces. Topically applied lubricants have also been 
extremely difficult to apply uniformly to magnet wire under production 
conditions and are subject to removal by necessary wiping and handling of 
the wire during processing. 
Accordingly, what is needed in this art is an improved magnet wire enamel 
with a low coefficient of friction without the deficiencies of prior 
systems. 
DISCLOSURE OF INVENTION 
The present invention is directed to magnet wire enamel comprising a 
polyamideimide formed by the reaction of a tri-basic acid anhydride, a 
diisocyanate, and a polyfunctional organosiloxane. The resulting enamel, 
after applying to a magnet wire substrate, has a coefficient of friction 
less than 0.10. Furthermore, the polyfunctional organosiloxane is 
substantially completely reacted in the polymer, resulting in 
substantially no extraction of the polyfunctional organosiloxane from the 
polymer when treated with organic solvent. 
Another aspect of the invention is magnet wire coated with above-described 
magnet wire enamel. 
The foregoing, and other features and advantages of the present invention 
will become more apparent from the following description. 
BEST MOST FOR CARRYING OUT THE INVENTION 
The tri-basic acid anhydride useful with the present invention has the 
general formula 
##STR1## 
where R is at least trivalent and includes such things as: trimellitic 
anhydride; 2,6,7-naphthalene tricarboxylic anhydride; 3,3'4-diphenyl 
tricarboxylic anhydride; 3,3'4-benzophenone tricarboxylic anhydride; 
1,3,4-diphenyl tricarboxylic anhydride; diphenyl sulfone 
3,3'4-tricarboxylic anhydride; 3,4,10-perylene tricarboxylic anhydride; 
3,4-dicarboxyphenyl 3-carboxyphenyl ether anhydride; ethylene 
tricarboxylic anhydride; 1,2,5-naphthalene tricarboxylic anhydride, etc. 
As the isocyanate component, any polyisocyanate with at least 2 isocyanate 
groups having the generic formula 
EQU O.dbd.C.dbd.N--R--N.dbd.C.dbd.O 
where R is an organic radical, may be used, such as: 
tetramethylenediisocyanate 
hexamethylenediisocyanate 
1,4-phenylenediisocyanate 
1,3-phenylenediisocyanate 
1,4-cyclohexylenediisocyanate 
2,4-tolylenediisocyanate 
2,5-tolylenediisocyanate 
2,6-tolylenediisocyanate 
3,5-tolylenediisocyanate 
4-chloro-1,3-phenylenediisocyanate 
1-methoxy-2,4-phenylenediisocyanate 
1-methyl-3,5-diethyl-2,6-phenylenediisocyanate 
1,3,5-triethyl-2,4-phenylenediisocyanate 
1-methyl-3,5-diethyl-2,4-phenylenediisocyanate 
1-methyl-3,5-diethyl-6-chloro-2,4-phenylenediisocyanate 
6-methyl-2,4-diethyl-5-nitro-1,3-phenylenediisocyanate 
p-xylylenediisocyanate 
m-xylylenediisocyanate 
4,6-dimethyl-1,3-xylylenediisocyanate 
1,3-dimethyl-4,6-bis-(b-isocyanatoethyl)-benzene 
3-(a-isocyanatoethyl)-phenylisocyanate 
1-methyl-2,4-cyclohexylenediisocyanate-4,4'-biphenylenediisocyanate 
3,3'-dimethyl-4,4'-biphenylenediisocyanate 
3,3'-dimethoxy-4,4'-biphenylenediisocyanate 
3,3'diethoxy-4,4'-biphenylenediisocyanate 
1,1-bis-(4-isocyanatophenyl)cyclohexane 
4,4'-diisocyanato-diphenylether 
4,4'-diisocyanato-dicyclohexylmethane 
4,4'-diisocyanato-diphenylmethane 
4,4'-diisocyanato-3,3'-dimethyldiphenylmethane 
4,4'-diisocyanato-3,3'-dichlorodiphenylmethane 
4,4'-diisocyanato-diphenyldimethylmethane 
1,5-naphthylenediisocyanate 
1,4-naphthylenediisocyanate 
4,4',4"-triisocyanato-triphenylmethane 
2,4,4'-triisocyanato-diphenylether 
2,4,6-triisocyanato-1-methyl-3,5-diethylbenzene 
o-tolidine-4,4'-diisocyanato 
m-tolidine-4,4'-diisocyanato 
benzophenone-4,4'-diisocyanato 
biuret triisocyanates 
polymethylenepolyphenylene isocyanate 
Polyfunctional organosiloxanes which may be used according to the present 
invention have the generic formula 
##STR2## 
where n is greater than 1 and A and A' are terminal functional groups that 
can be reacted into the polyamide-imide backbone chain, are the same or 
different and are typically groups such as --NH.sub.2, --OH, --COOH, 
##STR3## 
--NCO, --CH.dbd.CH.sub.2, --R"--OH, etc., where R,R', and R" are the same 
or different and are aliphatic, aromatic, branched aliphatic, etc., 
typically methyl, ethyl or phenyl. Such R groups may also contain groups 
reactive with the polyamide-imide backbone. If trifunctionality is desired 
either R or R' can contain a group the same as A or A'. Some examples of 
such organosiloxanes are 
##STR4## 
(where the n's are the same or different and R and R' are alkyl groups) 
The enamels of the present invention are typically formed by first reacting 
the anhydride component with the isocyanate component (note, for example, 
commonly assigned U.S. Pat. No. 4,374,221, the disclosure of which is 
incorporated by reference) followed by reaction with the organosiloxane. 
However, other methods of adding the organosiloxane to the polyamide-imide 
backbone such as reacting the organosiloxane with the anhydride prior to 
reaction with the isocyanate can also be used. After cooling, the reaction 
mixture is typically diluted with conventional magnet wire solvents to a 
solids content in the order of 30% by weight. 
The organosiloxane typically constitutes about 0.5% to about 5.0% and 
preferably about 2% to 4% by weight of the enamel system, although this 
can be varied depending on the particular polyamide-imide and its ultimate 
use.

EXAMPLE 1 
Using a refluxing technique water was removed from the NMP-xylene mixture. 
After refluxing, this mixture was cooled to 95.degree. C. and TMA was 
added. The solution was continued to allow to cool until a temperature of 
55.degree. C. was obtained at which point the MDI was added. The 
temperature was increased 25.degree. C. per hour and held at 105.degree. 
C. until the percent COOH was 3.75%. The organopolysiloxane and benzyl 
alcohol were then added to the solution in the order specified in Table 1. 
After 20 minutes at 105.degree. C., the temperature of the solution was 
raised to 130.degree. C. and held there for two hours. The temperature was 
then raised 15.degree. C. per 30 minutes and held at 155.degree. C. to 
160.degree. C. for 90 minutes. When the percent COOH reached 1.4%, the 
reaction mixture was cut with the NMP and xylene mixture. The enamel was 
then cooled to 90.degree. C. and cut again with a mixture of NMP and 
n-butyl alcohol. Using the Gardner-Holt scale, an enamel viscosity greater 
than Z-6 was measured. A further cut with 85:15-NMP:xylene was added until 
an enamel viscosity of Z-3-1/2 was obtained. The enamel was filtered at 
80.degree. C. The order of addition and relative amounts of materials are 
as set forth in Table 1. 
TABLE 1 
______________________________________ 
Material Equivalents 
WT. % 
______________________________________ 
NMP (N--methylpyrrolidone) 
-- 44.97 
Xylene -- 11.25 
TMA (trimellitic anhydride) 
3.0 14.24 
MDI (methylenediisocyanate) 
3.0 18.55 
Dow Corning 1248 .01 .99 
Organopolysiloxane 
Benzyl Alcohol .09 .49 
NMP -- 2.30 
Xylene -- 3.21 
NMP -- 2.0 
n-butyl alcohol -- 2.0 
Final Solids = 29.1 
Gardner-Holt Viscosity = Z-31/2 
Brookfield Viscosity = 5370 
Final % COOH = 1.39 
Effective Solids = 27.8 
Equivalent weight of organopolysiloxane = 
2000 
Equivalent % TMA = 49.17 
Equivalent % MDI = 49.17 
Equivalent % Benzyl--OH = 1.49 
Equivalent % organopolysiloxane = 
0.17 
______________________________________ 
EXAMPLE 2 
An enamel composition was prepared in the same manner as in Example 1 with 
the following modifications. The organopolysiloxane was added to the 
reaction mixture when the percent COOH reached 9.76% as opposed to the 
3.75% of Example 1. The organopolysiloxane (2.0% based on polymer weight) 
was reacted for 10 minutes before the benzyl alcohol was added. See Table 
2 for the order of reactant addition and relative amounts. 
TABLE 2 
______________________________________ 
Material Equivalents 
WT. % 
______________________________________ 
NMP -- 45.17 
Xylene -- 11.30 
TMA 3.0 14.31 
MDI 3.0 18.63 
Dow Corning 1248 .0056 .55 
Organopolysiloxane 
Benzyl Alcohol .09 .50 
NMP -- 2.31 
Xylene -- 3.23 
NMP -- 2.01 
n-butyl alcohol -- 2.01 
Final Solids = 33.9 
Gardner-Holt Viscosity = Z-22/3 
Brookfield Viscosity = 4430 
Final % COOH = 1.68 
Effective Solids = 30.4 
Equivalent weight of organopolysiloxane = 
2000 
Equivalent % TMA = 49.19 
Equivalent % MDI = 49.19 
Equivalent % Benzyl--OH = 1.52 
Equivalent % organopolysiloxane = 
0.10 
______________________________________ 
EXAMPLE 3 
Using the same refluxing procedure as with Example 1 water was removed from 
the NMP-xylene mixture which also included the TMA, and the 
organopolysiloxane. They were all initially charged to a three liter 
flask. The solution was cooled down to 65.degree. C. and the MDI was 
added. After 20 minutes the temperature was increased 15.degree. C. per 45 
minutes until temperature of 105.degree. C. was reached. The percent COOH 
was 9.84% after 40 minutes. The butyl alcohol was then added. After 10 
minutes the temperature was again increased 15.degree. C. per 35 minutes 
and held at 160.degree. C. After 90 minutes the percent COOH was 1.69% and 
the enamel was then cut with a solvent mixture of NMP and xylene. After 
cooling the enamel to 90.degree. C., a NMP and n-butyl alcohol mixture was 
added. The enamel was filtered at 80.degree. C. 
TABLE 3 
______________________________________ 
Material Equivalents 
WT. % 
______________________________________ 
NMP -- 45.17 
Xylene -- 11.28 
TMA 3.0 14.31 
SWS F-801 organopolysiloxane 
.0013 .55 
MDI 3.0 18.64 
Benzyl Alcohol .09 .50 
NMP -- 2.31 
Xylene -- 3.23 
NMP -- 2.01 
n-butyl Alcohol -- 2.01 
Final Solids = 34.1 
Brookfield Viscosity = 3300 cps 
Final % COOH = 1.69 
Effective Solids = 30.5 
Equivalent Weight of 
organopolysiloxane = 8333 
Equivalent % of TMA = 49.23 
Equivalent % MDI = 49.23 
Equivalent % Benzyl--OH = 1.52 
Equivalent % organopolysiloxane = 
.02 
______________________________________ 
EXAMPLE 4 
This enamel was prepared in the same manner as Example 3 except that the 
reactive organopolysiloxane component constituted 3.6% of the polymer 
weight. 
TABLE 4 
______________________________________ 
Material Equivalents 
WT. % 
______________________________________ 
NMP -- 44.97 
Xylene -- 11.25 
TMA 3.0 14.24 
SWS F-801 organopolysiloxane 
.0024 .99 
MDI 3.0 18.55 
Benzyl Alcohol .09 .49 
NMP -- 2.30 
Xylene -- 3.21 
NMP -- 2.00 
n-butyl alcohol -- 2.00 
Final Solids = 35.2 
Gardner-Holt Viscosity = Z-3 
Brookfield Viscosity = 4700 cps 
Final % COOH = 1.64 
Effective Solids = 31.0 
Equivalent Weight of 
organopolysiloxane = 8333 
Equivalent % TMA = 49.24 
Equivalent % MDI = 49.23 
Equivalent % Benzyl--OH = 1.50 
Equivalent % organopolysiloxane 
0.03 
______________________________________ 
EXAMPLE 5 
The procedure followed in Example 1 was also performed here except for the 
exclusion of the reactive organopolysiloxane (control sample). 
To demonstrate the improvement in coated wires according to the present 
invention the following tests were conducted. Standard 18 AWG copper wires 
were coated with a conventional THEIC polyester basecoat followed by 
application of the polyamide-imide topcoats of the above examples. The 
basecoat to topcoat ratios of the total enamel build on the wire ranged 
from 75-80:25-20. The enamels were cured by passing through a standard 20 
foot gas fired oven with bottom and top zone temperatures of 620.degree. 
F. and 804.degree. F. respectively. A comparison of properties as shown in 
Table 5. Coefficient of friction entries in the Tables marked with an 
asterisk also contained a thin layer (e.g. less than 0.1 mil) of external 
lubricant (a mixture of paraffin wax, beeswax and vaseline in roughly 
equal amounts applied out of conventional enamel solvents recited herein). 
Coefficient of friction values were determined using weights ranging from 
2 to 22 pounds. 
TABLE 5 
______________________________________ 
Dissipation 
Example Factor at Scrape Coefficient 
No. Smoothness 
240.degree. C. 
grams/mil 
of Friction 
______________________________________ 
1 -9/-9* .25 597 .025*-.031* 
2 -9/-9 .33 617 .035-.066 
3 -9/-9 .06 600 .067-.10 
4 -9/-9 .26 596 .043-.058 
5 -9/-9 .13 588 0.20 
&gt;2 lbs. 
oscillation 
______________________________________ 
*A -9 rating = good 
While the polymers according to the present invention can be used on any 
electrical conductor, they are preferably used on wires and specifically 
magnet wires. The wires are generally copper or aluminum. And wires 
ranging anywhere from 4 AWG to 42 AWG (American Wire Gauge) in diameter 
are coated, with 18 AWG being the most commonly coated wire. Wire coatings 
can be anywhere from 0.2-5 mils or any thickness desired, and preferably 
about 3.2 mils on 18 AWG wire when applied in 6 coatings of equal 
thickness with curing between coats. The coatings can be used as a sole 
insulation coat or part of a multicoat system in combination with other 
conventional polymer insulation, such as polyester, polyurethanes, 
polyvinyl formal, polyimides, etc., and combinations thereof. The polymer 
coatings of the present invention can also contain lubricants either 
externally on the coating, internally in the coating, or both. If a 
multicoat coating system is used, polyester basecoats are preferred and 
THEIC (tris-hydroxyethylisocyanurate)polyester basecoats particularly 
preferred. Note U.S. Pat. Nos. 3,342,780 and 3,249,578, the disclosures of 
which are incorporated by reference. 
The enamels made according to the present invention can be applied by any 
conventional means such as coating dies, roller or felt application with 
viscosity adjustments made accordingly. Viscosity adjustments can be made 
by dilution with appropriate enamel solvents or diluents for any coating 
method. As the enamel solvents, any conventionally used, relatively inert, 
polar solvents such as N-methyl pyrrolidone, N,N-dimethyl or N,N-diethyl 
formamide, and N,N-diethyl acetamide can be used, and similarly any 
conventional hydrocarbon diluent such as xylene, Solvesso 100 (Exxon) or 
D59 hydrocarbon (Drake Petroleum Co.). 
Conventional curing ovens can be used to heat treat the coated magnet wire. 
Inlet oven temperatures of the order of about 500.degree.-700.degree. F. 
(260.degree. C.-571.degree. C.), preferably about 580.degree. F. 
(304.degree. C.), and outlet oven temperatures of about 
800.degree.-1100.degree. F. (427.degree.-593.degree. C.), and preferably 
about 900.degree. F. (482.degree. C.) are used for drying and curing. 
While this invention has been described in terms of magnet wire insulation, 
this invention includes the use of this material as a free standing film, 
e.g. for such uses as phase insulation, coil wrapping, etc., and as 
varnishes for uses other than magnet wire insulation. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and scope of a claimed invention.