UV curable high tensile strength resin composition

Disclosed is a resin composition which comprises a mixture of a polyester, an acrylate urethane, in about stoichiometric proportion with the polyester up to about 10% excess polyester over stoichiometric, and a liquid acrylate reactive diluent sufficient to give the resin composition the viscosity of less than 5,000 centipoise. The polyester resin is the reaction product of a polyhydroxy compound having at least three hydroxyl groups, and an aromatic compound that is at least difunctional in acid, anhydride, or ester groups, where the proportion of the polyhydroxy compound to the aromatic compound is such that the polyester has about 150 to about 250 mole % excess hydroxyl groups. The acrylate urethane compound is the reaction product of a hydroxy acrylate and an isocyanate having at least two isocyanate groups, where the hydroxy acrylate and a hydroxy acrylate having a single hydroxyl group are equimolar (when a diisocyanate is used) with the isocyanate .+-.10 mole %. Also disclosed is a method of making the resin composition.

BACKGROUND OF THE INVENTION 
Amorphous metal strip continuously wound into toroidal cores for use in 
distribution transformers dramatically reduces the magnetic loss compared 
to orientated steel coils. However, the amorphous metal strip wound cores 
are very fragile and are not self-supporting, so an external support 
system is required to protect the cores against mechanical forces during 
handling, winding, and assembly operations. This can be accomplished by 
coating or encapsulating the cores with a suitable resin. Resins for use 
in this application must have a very high tensile strength. It is also 
desirable that they cure rapidly without the evolution of polluting 
solvents. 
SUMMARY OF THE INVENTION 
We have discovered a high tensile strength UV curable resin composition 
which is ideally suited for use in coating and encapsulating amorphous 
metal strip cores. The resin compositions of this invention, without the 
addition of any filler, have tensile strengths as high as 7200 psi at room 
temperature and 850 psi at 100.degree. C. When a filler is added, of 
course, the tensile strength of the resulting filled resin is increased. 
In addition, the filled resin compositions of this invention exhibit 
extremely good dimensional stability, typically only 7% elongation at room 
temperature and 9% elongation at 100.degree. C. 
Other advantages of the resin compositions of this invention are that they 
can be cured very fast, usually in only a few seconds, without the 
evolution of any solvent. Moreover, coatings as thick as 20 mils or more 
can be formed from these resin compositions. 
DESCRIPTION OF THE INVENTION 
In the first step of the process of this invention, a polyester is prepared 
by reacting a polyhydroxy compound with an aromatic di- or polyfunctional 
acid, anhydride, or ester. The polyhydroxy compound is a compound that 
contains three or more hydroxyl groups, such as trimethylol propane, tris 
(2-hydroxyethyl) isocyanurate (THEIC), tris (2-hydroxyethyl) cyanurate, 
glycerol, trimethylol ethane, pentaerythritol, trimethylol propane, or 
erythritol. The preferred polyhydroxy compound is THEIC as that gives a 
better tensile strength and toughness. 
The aromatic di- or polyfunctional acid, anhydride, or ester is an aromatic 
compound that has two or more acid, anhydride, or ester groups, or 
mixtures of these groups. Difunctional compounds are preferred as there is 
less chance of a resin composition gelling with a difunctional compound. 
Esters are preferred for a smoother reaction, but compounds having acid 
groups are less expensive. Suitable compounds include terephthalic acid, 
isophthalic cid, trimellitic acid, trimellitic anhydride 
3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic 
dianhydride, polyazelaic polyanhydride, pyromellitic tetracarboxylic acid, 
3,3',4,4'-benzophenone tetracarboxylic acid, and alkyl (methyl, ethyl, 
propyl, etc.) esters of these acids. The preferred aromatic compound is 
terephthalic acid as it gives better tensile strength. 
The polyester reaction product of the polyhydroxy compound and the aromatic 
compound must have excess hydroxy groups in order to react with the 
acrylate urethane compound, which is also part of the resin composition. 
If insufficient hydroxyl groups are present on the polyester, the resin 
product will have poor tensile strength, and if too many hydroxyl groups 
are present on the polyester, the resin product will have poor shelf life. 
If less than 150 mole % excess hydroxyl groups are present, cure is slow, 
and if more than 250 mole % excess hydroxyl groups are present, unreactive 
polyhydroxy compound will be present and toughness and tensile strength 
will be lower. If about 200 mole % excess hydroxyl groups are present on 
the polyester, the resin product will have the maximum tensile strength. 
The proportions of polyhydroxy compound to aromatic compound are adjusted 
to give a mole % of excess hydroxyl groups within this range. The reaction 
of the polyhydroxy compound with the aromatic compound proceeds without 
the presence of a solvent, but a suitable amount (about 0.05 to about 0.50 
parts per 100 parts resin (phr)) of an esterification catalyst is needed. 
Any esterification catalyst, such as dibutyl tin oxide, tetraisopropyl 
titanate, dihydroxy butyl tin chloride, triphenyl tin chloride, or 
triphenyl tin acetate can be used. Dibutyl tin oxide is preferred as it is 
an inexpensive, readily available, and works well. The composition of the 
polyhydroxy compound and the aromatic compound is mixed thoroughly and is 
heated in 15.degree. C. increments over about a one hour period to a 
temperature of about 210.degree. to about 250.degree. C., or until no 
further condensate is emitted. 
The reaction of the polyhydroxy compound with the aromatic compound 
produces an ester oligomer having excess hydroxyl groups: 
##STR1## 
In the next step in the process of this invention, an acrylate urethane is 
produced by the reaction of a hydroxy acrylate with an isocyanate. The 
hydroxy acrylate is a compound having at least one hydroxyl group and at 
least one acrylate group. Preferably, the hydroxy acrylate includes only a 
single hydroxyl group because polyhydroxy groups will form a high 
molecular weight polymer with isocyanate instead of the adduct of 
hydroxyethyl acrylate and isocyanate. Also, the hydroxy acrylate 
preferably includes only a single acrylate group because the polyacrylate 
group will increase the undesired properties such as brittleness and high 
shrinkage. Suitable hydroxy acrylates include hydroxyethyl acrylate, 
hydroxyethyl-.beta.-carboxyethyl acrylate, and 3-hydroxyethyl acrylate. 
Hydroxyethyl acrylate is preferred as it is readily available and produces 
a tougher product. Aromatic hydroxy acrylates are preferred for thermal 
stability and toughness; however, they are not commercially available at 
this time. 
The isocyanate compound used in preparing the acrylate urethane must have 
two or more isocyanate groups. Preferably, the isocyanate compound is an 
aromatic compound as they produce tougher resin products. Suitable 
isocyanate compounds include meta-phenylene diisocyanate, toluylene 
diisocyanate, hexamethylene diisocyanate, metaxylyene diisocyanate, 
4,4'-diisocyanato diphenyl sulfone, 4,4'-diisocyanato diphenyl ether, and 
4,4'-diisocyanato diphenyl methane. The preferred isocyanate is toluylene 
diisocyanate as it is readily available and gives a tougher product. 
In reacting the hydroxy acrylate compound with the isocyanate compound, 
equimolar (if a diisocyanate and a hydroxy acrylate having a single 
hydroxyl group are used) amounts are used, though either component may be 
present in up to about 10 mole % excess of equimolar (if a diisocyanate 
and a hydroxy acrylate having a single hydroxyl group are used). No 
solvent or catalyst is required. The mixture of the hydroxy acrylate and 
the isocyanate compound reacts exothermically and is preferably cooled to 
under 70.degree. C. during the reaction because a higher reaction 
temperature will give a premature gellation. After about one hour the 
reaction is usually complete. The reaction of hydroxy acrylate with the 
isocyanate compound produces an acrylate urethane, containing one urethane 
group and one nonreacted isocyanate group. 
##STR2## 
In the next step of the process of this invention, the polyester is reacted 
with the acrylate urethane. These two components are reacted, 
stoichiometrically, although up to about 10 mole % excess polyester may be 
present. Excess acrylate urethane, however, should not be present as it is 
toxic in the product. A stoichiometric reaction would typically mean that 
about 4 moles of acrylate urethane are present for each mole of polyester 
that is present. While no solvent is required for this reaction, it is 
necessary to include a reactive diluent to solubilize the polyester and 
the acrylate urethane. Unlike a solvent, a reactive diluent is reacted 
into the resin product and is not volatilized during resin cure. The 
reactive diluent is a liquid acrylate which has a viscosity of less than 
1,000 centipoises (cps). The acrylate preferably has two or more acrylate 
groups as this gives greater tensile strength in the product. Suitable 
examples of acrylates which can be used as reactive diluents include 
tetraethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 
1,4-butanediol diacrylate, diethyleneglycol diacrylate, 1,6-hexanediol 
diacrylate and neopentyl glycol diacrylate. The preferred reactive diluent 
is tetraethylene glycol diacrylate because it has a low viscosity and 
gives a high tensile strength to the product. Sufficient reactive diluent 
should be used so that the resin composition has a viscosity of less than 
5,000 cps. If the resin composition viscosity is greater, the resin 
composition will not flow readily and will not fill all of the interstices 
of the substrate. Typically, this means that the reactive diluent 
constitutes about 20 to about 60% by weight of the total weight of the 
resin composition. 
If the resin composition is not to be used soon after it is prepared, it is 
desirable to include about 0.005 to about 0.05% by weight (based on total 
resin composition weight) of an inhibitor to add to its shelf life. If 
less inhibitor is used the resin composition may start to gel during 
storage, and if more is used it may be difficult to gel the composition 
when it is used. Suitable inhibitors include benzoquinone, 
methyl-p-benzoquinone, hydroquinone, hydroquinone monoethyl ether, and 
2,3,5,6-tetrachlorobenzoquinone. The preferred inhibitor is benzoquinone 
because it is readily available and it works well. 
A photoinitiator sensitive to ultraviolet (UV) light is required to cure 
the composition, but the photoinitiator need not be added to the 
composition until it is ready to be used. Particularly stable 
photoinitiators can, however, be mixed into the resin composition when it 
is stored. Examples of suitable photoinitiators include isopropyl benzoin 
ether, diethoxy phenyl acetophenone, 2,2-dimethoxy-2-phenyl acetophenone, 
isobutyl benzoin ether, and 2-hydroxy-2-methyl-1-phenyl-propan-1-one. The 
preferred photoinitiator is isobutyl benzoin ether because it is 
inexpensive, it is in easily applied liquid form, and has been found to 
work very well. Isobutyl benzoin ether is normally not added to the resin 
composition until the composition is to be applied and cured. Typically 
about 1 to about 5 phr of the photoinitiator is satisfactory. 
The tensile strength of the cured resin composition can be further 
increased by the addition of up to about 50% by weight (based on total 
composition weight) of a filler. The filler should be transparent to 
ultraviolet light so that it does not absorb the ultraviolet light and 
prevent the resin composition from curing. Suitable filler include fibers, 
roving, mat, or cloth of glass, polyethylene terephthalate, mica, quartz, 
silica, talc, calcium carbonate, and boron. The preferred filler is glass 
cloth as it has a very high tensile strength and has been found to work 
well. 
When it is desired to use the resin composition, it is applied to a surface 
and is exposed to ultraviolet light. Any type of ultraviolet light is 
effective and a complete cure will normally occur in only a few seconds by 
a free radical curing mechanism. 
The resin compositions of this invention are primarily intended for use in 
bonding amorphous metal cores. In that application, a primer resin is 
first applied to the core to insure good adhesion of the resin composition 
to the core. After the primer composition is applied, a filler is pressed 
into the primer coating, then the primer coating is cured. An example of a 
suitable primer composition can be found in U.S. Pat. No. 4,481,258, which 
discloses an acrylated epoxy resin, herein incorporated by reference. The 
resin composition of this invention is then applied over the primer 
coating, filler is pressed into the resin coating, and the resin coating 
is cured with UV light. Any number of layers of the resin coating and 
filler can be applied to build up the total thickness of the coating. It 
is preferable to post bake the coatings after UV curing to insure that the 
resin compositions have been completely cured. This can be accomplished by 
heating for 2 to 5 hours at about 100.degree. to 150.degree. C. 
The resin compositions of this invention can also be used as adhesives, 
encapsulants, or as coatings in other applications. 
The following example further illustrates this invention.

EXAMPLE 
A polyester resin was made in a three liter four-neck reaction flask 
equipped with stirrer, thermometer, nitrogen sparge, and downward 
distillation column. The flask was charged with 218.6 grams of tris 
(2-hydroxyethyl) isocyanurate, 69.8 grams of terephthalic acid, and 3.5 
grams of deionized water. The reaction mixture was heated to 140.degree. 
C., and 0.8 grams of dibutyl tin oxide was added. The mixture was then 
reacted at 210.degree. to 250.degree. C. in 15.degree. C. increments per 
hour until 17 ml of water was collected. The temperature was reduced to 
120.degree. C. and the resin was cut with 517 grams tetraethylene glycol 
diacrylate (TEGDA). 
An acrylate urethane was prepared in a one liter four-neck reaction flask 
equipped with stirrer, thermometer, nitrogen sparge, and water bath for 
cooling. The flask was charged with 194.4 grams 2-hydroxyethyl acrylate 
and 0.1 grams benzoquinone. To the mixture was gradually added 291.6 grams 
toluylene diisocyanate and the reaction mixture was kept cooled to a 
temperature just below 60.degree. C. After all the toluylene diisocyanate 
was added, the mixture was reacted at 60.degree. to 70.degree. C. for one 
hour. The acrylate urethane was poured under nitrogen into an air-tight 
tared container. 
A resin composition was prepared by mixing 486 grams of the acrylate 
urethane with the entire amount of the polyester resin described above. 
The mixture was reacted 120.degree. C. for one hour and was cut with 621.0 
grams TEGDA, then poured into a tared container. The viscosity of the 
resin composition was in the range of 3,000 to 5,000 cps at 25.degree. C. 
The resin was sensitized by the addition of 4 phr isobutyl benzoin ether 
sold by Stauffer Chemical Company under a trade designation "V-10." This 
composition is identified as composition A. 
Identical compositions B, C, and D were prepared in the same manner except 
that the 60% TEGDA was replaced with 26.7% phenoxyethyl acrylate (PEA) and 
33.3% TEGDA, or 60% PEA, or 26.7% TEGDA and 33.3% trimethyl propane 
triacrylate (TMPTA), respectively. 
Transformer oil compatibility tests (ASTMD-3455-78) were carried out using 
pieces of amorphous metal strip 2 inches wide by 5 inches long by 0.005 
inches thick. The specimens were coated with a primer composition which 
consisted of 56.70% (by weight) acrylated epoxy, 6.45% acrylated urethane, 
24.10% phenoxy ethyl acrylate, 7.98% tetra ethylene glycol diacrylate and 
3.85% isobutyl benzoin ether. A fiberglass cloth 4 inches by 12 inches was 
pressed into the primer composition coating and it was cured with UV 
light. Compositions A, B, C, and D were then coated onto other specimens, 
a similar glass cloth was pressed into these coatings, and they were cured 
with UV light. The specimens were all post baked for four hours at 
150.degree. C. The results of the ASTM test indicated that the cured resin 
compositions do not contaminate transformer oil. 
A 25 pound rectangular amorphous metal coil was encapsulated in the same 
manner and was then thermally cycled between -35.degree. C. and 
110.degree. C., nine times in air and nine times in transformer oil. The 
coating showed no visible damage after the cycling. 
In other tests, coatings of the resins were applied to the mold of tensile 
test specimens (ASTM 0638-77a) until a layer 13 mils thick was obtained. 
Some of the coatings were post baked for four hours at 130.degree. C. and 
some were not. The coatings were then tested for tensile strength and 
elongation at room temperature and at 100.degree. C. The following table 
gives the results. 
__________________________________________________________________________ 
Test At 
Test At 
No. of Layers Room Temp. 
100.degree. C. 
to Obtain 
Post Baking 
TSB EB TSB EB 
Composition 
13 mil. Thick. 
4 hrs, 130.degree. C. 
(psi) 
(%) 
(psi) 
(%) 
Comments 
__________________________________________________________________________ 
A 1 No 7200 
7 850 9 Excellent tensile strength 
1 Yes -- -- 900 7 properties and dimensional 
stability. 
B 1 No 10000 
5 323 7 The tensile strength of this 
1 Yes -- -- 450 9 material at 100.degree. C. was not 
good enough. 
C 3 No 6346 
9 -- -- The tensile strength of this 
6 to 7 No 4835 
3 107 7 material at 100.degree. C. was low. 
6 to 7 Yes -- -- 128 9 
D 1 Yes -- -- 1400 
-- This material has too much 
shrinkage after curing. 
Primer 4 No 1159 
25 22 13 The tensile strength of this 
material at 100.degree. C. was too 
low. 
4 Yes 2250 
33 87 23 The cure rate was slow. 
__________________________________________________________________________ 
TSB = Tensile Strength at Break. 
EB = Elongation at Break. 
The above table shows that the compositions that contained tetraethylene 
glycol diacrylate had superior tensile strength compared to the 
compositions that instead contained a monofunctional acrylate. Composition 
D, while it had high tensile strength, shrunk a great deal after curing. 
This may adversely affect the magnetic properties of a metal substrate, 
and may make it unsuitable for use in coating magnetic cores.