Preparing magnet wire having electron beam curable wire enamels

An electron beam curable magnet wire enamel comprises the reaction product of a polyester prepolymer having terminal functional groups reactive with an isocyanate, and a urethane-acrylate prepolymer formed by reacting a diisocyanate and a hydroxyalkylacrylate. The resulting polymer is extrudable at 100% solids onto a magnet wire substrate and electron beam curable.

DESCRIPTION 
1. Technical Field 
The field of art to which this invention pertains is electron beam curable 
polymers, and specifically electron beam curable magnet wire enamels. 
2. Background Art 
The magnet wire industry, although an old industry, is constantly in search 
of ways to improve both the insulation properties of the polymeric 
insulation applied to the magnet wire, and the methods of applying such 
polymers. Such efforts have included, for example, increasing the solids 
content of the polymer systems applied, to both reduce the amount of 
organic solvents utilized and increase the speed of application. And while 
radiation curing of such coating materials has been contemplated (note 
U.S. Pat. Nos. 4,000,362; 4,246,297; and 4,361,799) electron beam curable 
materials represent a relatively unchartered area in this art. 
Accordingly, what is needed in this art are polymer systems which are 
electron beam curable and usable on magnet wire substrates. 
DISCLOSURE OF INVENTION 
The present invention is directed to a polymer system which is not only 
extrudable at 100% solids onto a magnet wire substrate but is also curable 
by electron beam radiation. The polymer is the reaction product of a 
polyester prepolymer having terminal functional groups reactive with an 
isocyanate, and a urethane-acrylate prepolymer formed by reacting a 
diisocyanate and a hydroxyalkylacrylate. 
Another aspect of the invention is a method of making an insulated magnet 
wire by extrusion coating such polymer at 100% solids onto the magnet wire 
and curing with electron beam radiation. 
Another aspect of the invention is a magnet wire so produced. 
The foregoing and other features and advantages of the present invention 
will become more apparent from the following description. 
BEST MODE FOR CARRYING OUT THE INVENTION 
The polyester prepolymer can be any conventionally prepared or commercially 
available polyester with terminal functional groups reactive with an 
isocyanate, such as hydroxyl, carboxyl, etc. The prepolymer typically has 
the generic formula: 
##STR1## 
where R is a polyester prepolymer backbone such as prepared by the 
reaction of (a) a carboxyl containing component, e.g., 
##STR2## 
where R' and R" are the same or different and are selected from the group 
hydrogen and lower alkyl groups, such a methyl, ethyl, n-propyl, isopropyl 
and the like, and other aliphatic, cyclic and heterocyclic groups, and 
mixtures thereof, with (b) a hydroxyl compound. 
While dimethyl terephthalate ester is preferred as the carboxyl containing 
material, any of a number of suitable carboxyl constituents can be used 
including terephthalic acid, isophthalic acid, and the lower alkyl esters 
of each, such as the diethyl esters, dipropyl esters, dibutyl esters, and 
anhydrides and mixtures of the foregoing and the like. 
While ethylene glycol is preferred as the hydroxyl compound, any of a 
number of suitable diol or glycol constituents can be used including 
propanediols, butanediols, pentanediols, hexanediols, octanediols, and 
mixtures of the foregoing and the like. And while the dihydric alcohols 
are preferred, triols or higher polyols can be also be used such as 
trihydroxyethyl isocyanurate, glycerine, trimethylolethane, 
trimethyolpropane, 1,2,5 hexanetriol, polyether triol, mono-, di- and 
tri-pentaerythritol, and mixtures thereof. 
It should be noted that while this invention is described in terms of a 
polyester prepolymer, other prepolymers having groups reactive with an 
isocyanate group may also be used such as polyurethanes; polyesterimides; 
carboxyl or anhydride terminated polyamides, polyimides, or 
polyamideimides; etc. 
Once the polyester prepolymer is formed it is then reacted with the 
urethane-acrylate prepolymer. This prepolymer is formed by reacting a 
diisocyanate with an acrylate. 
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-b 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-xylylenediisocianate 
1,3-dimethyl-4,6-bis-(b-isocyanateoethyl)-benzene 
3-(a-isocyanatoethyl)-phenylisocyanate 
1-methyl-2,4-cyclohexylenediisocyanate 4,4'-biphenylenediisocyanate 
3,3'-dimethyl-4,4'-biphenylenediisocyanate 
3,3'-dimethoxy-4,4'-biphenylenediisocianate 
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-tolidone-4,4'-diisocyanato 
benzophenone-4,4'-diisocyanato 
biuret triisocyanates 
polymethylenepolyphenylene isocyanate 
The acrylic component should be sufficiently reactive to react with the 
isocyanate component without disturbing the ethylenic unsaturation 
provided by the acrylate. A particularly useful acrylate in this regard 
has the formula: 
##STR3## 
where R' and R" can be an alkyl group such as methyl, ethyl, propyl, 
isopropyl, etc., and R" can also be hydrogen. Hydroxypropyl acrylate and 
2-hydroxyethyl acrylate are particularly preferred. Typical formation of 
the urethane-acrylate prepolymer is shown as follows: 
##STR4## 
Sufficient stoichiometric amounts of acrylate should be reacted with the 
isocyanate to leave one isocyanate group unreacted. 
The two prepolymers are then reacted, preferably at temperatures below 
85.degree. C. to preserve the ethylenic unsaturation. The relative amounts 
of the materials can be varied depending upon the amount of unsaturation 
desired to be incorporated into the resulting polymer. A typical 
polyester-urethane-acrylate reaction product is shown below where R is the 
polyester prepolymer backbone: 
##STR5## 
This reaction product is extrudable using standard extrusion equipment at 
100% solids by heating to a temperature which provides an appropriate 
viscosity for extrusion. Typically this is a temperature below about 
120.degree. C. producing a viscosity under about 20,000 cps (centipoise) 
and preferably under 10,000 cps. 
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 30 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. The polymer coat can be applied in a single 
coat or multiple coats with curing between coats. The coating can be used 
as a sole insulation coat or part of a multicoat system in combination 
with other conventional polymer insulation, such as polyesters, 
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. 
The coatings can be applied using conventional extrusion equipment. 
Conventional, commercially available electron beam curing apparatus such 
as Electrocurtain.RTM. apparatus (Energy Science, Inc.) can be used with 
the dosage varied depending on thickness of coating, amount of 
cross-linking desired etc. Typically doses up to about 60 megarads are 
used and preferably about 1 megarad to about 20 megarads.

EXAMPLE 
An isocyanate-acrylate prepolymer was prepared as follows: 
______________________________________ 
Equiva- Weight 
Material Moles lents Grams Percent 
______________________________________ 
Toluene diisocyanate 
4 8 696 60.00 
2-hydroxyethyl acrylate 
4 4 464 39.998 
phenothiazene 0.06 0.002 
(double bond stabilizer) 
______________________________________ 
The toluene diisocyanate (TDI) and the phenothiazene were combined and 
heated up to 66.degree. C. The 2-hydroxyalkylacrylate (HEA) was slowly 
added. The heat was turned off and the exothermic reaction was kept at 
70.degree. C. with a water bath. After all the HEA was added the 
temperature was allowed to rise to 75.degree. C. The reaction temperature 
was maintained with external heating at 75.degree. C. for approximately 
two and one-half hours. The viscosity (measured at 30.degree. C.) 
eventually stabilized at X 1/2. 
A polyester prepolymer was prepared as folows: 
______________________________________ 
Equiva- Weight 
Material Moles lents Grams Percent 
______________________________________ 
Phthalic anhydride 
2.5 5.00 370 51.11 
Trimethylol propane 
1.8 5.40 241 33.29 
Ethylene glycol 
1.8 3.60 112 15.47 
Tetrabutyl titanate 1 0.13 
(catalyst) 
______________________________________ 
##STR6## 
The above ingredients were combined and heated under nitrogen gas 
over-pressure. The temperature rose gradually from 175.degree. to 
225.degree. C. and water distillate collected until an acid number of 6 
was obtained. Heating was discontinued and with a reaction kettle 
temperature of 190.degree. C., a light vacuum was applied to strip off any 
residual water. The viscosity, at 60% solids in cresylic acid at 
30.degree. C. was Z13/4. 
______________________________________ 
Equiva- Weight 
Material Moles lents Grams Percent 
______________________________________ 
Isocyanate-acrylate 
2 2 338 36.8 
prepolymer 
Polyester prepolymer 
2 2 580 63.2 
Phenothiazene 0.04 &lt;0.1 
______________________________________ 
The above materials were charged to a reaction vessel and heated to 
40.degree. C. under low nitrogen gas purge. The mixture became homogeneous 
with mixing. The temperature was raised to 65.degree. C. and the heat 
source turned off. At this point the reaction showed signs of being 
exothermic with increasing viscosity. With the heat off, the temperature 
continued to rise to about 70.degree. C. As the viscosity continued to 
rise and the temperature rose to 86.degree. C., 2.1 grams (0.3% by weight) 
of hydroquinone was added to stabilize the doubled bonds with the increase 
in temperature. External temperature was added to decrease the viscosity 
allowing for better mixing. The temperature was allowed to rise to 
95.degree.. With the temperature at 106.degree. C. a sample was taken 
which indicated that 98.6% of the isocyanate had reacted with the hydroxyl 
groups on the polyester. The resin material was then extruded through 
conventional extrusion dies at 100% solids and 100.degree. C. onto an 18 
AWG magnet wire substrate in a thickness of about 3 mils. The coated wire 
was then subjected to about 20 megarads of electron beam radiation. The 
resulting wire had good magnet wire insulating properties although did 
suffer somewhat in flex. (This could be altered by adjusting film build or 
applying supplemental base or topcoats such as thermoplastic material.) 
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 the claimed invention.