Note: Descriptions are shown in the official language in which they were submitted.
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TWO-PART, LOW-VISCOSITY EPOXY RESIN COMPOSITION
Technical Field
The invention concerns a two-part, low-viscosity
epoxy resin potting composition which provides cured,
electrical-insulating products that are "semi-flexible",
that is, have a Shore D hardness of less than 90, and hence
have good resistance to thermal and mechanical shock.
Background Art
Epoxy resin compositions are known which have
sufficiently low viscosity to be used for potting elect
tribal components and cure to a tough, semi-flexible,
thermoses state providing good electrical-insulating
properties at temperatures as high as 155C, good
resistance to thermal and mechanical shock, and good flame
retardancy. However, it is believed that the cured
products of every prior potting composition having those
attributes gradually deteriorate from exposure to the sort
of hot, humid environment that prevails in the tropics.
United States MOLLY, Levi. G, requires retention of at
least ninety percent of the original Shore D hardness after
exposure to 71~C at 95% RHO for 120 days while retaining a
volume resistivity value greater than 1 x 1012 ohm-cm.
An epoxy resin potting composition which cures to
a tough, semi-flexible, therlrloset state providing good
electrical-insulating properties is disclosed in US.
Patent No. 3,523,143 (Queuing), Preferred compositions of
the Queuing patent are marketed in two parts. One part come
proses liquid polyglycidyl ether of bisphenol A. The other
part comprises a mixture of likelihood carboxyl-terminated polyp
ester of dicarboxylic acid and polyalkylene glycol, low-
melting cyclic dicarboxylic acid android, and a catalyst.
Cured products ox the working examples of the Queuing patent
would no-t meet the aforementioned MIX specification.
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If a cycloaliphatic android were substituted for the
tetrapropenyl succinic android used in most of the working
examples of the Queuing patent, the cured products might be
sufficiently resistant to a hot, humid environment to meet the MIX
specification. However, the android would tend to decarboxylate
gradually during storage at ordinary room temperatures, as
evidenced by evolving gas.
Some cycloaliphatic androids such as tetrahydro-
phthalic android by themselves have high melting points but
in admixture with other cyclic dicarboxylic acid androids can
provide low-melting android mixtures as taught in US. Patent
No. 3,078,235 (Bowman et at.).
Disclosure of Invention
The present invention concerns a two-part, low-viscosity
epoxy resin potting composition which cures to a tough, semi-
flexible, thermoses state providing
good electrical-insulating properties at 155C,
good resistance to thermal and mechanical shock,
and also
satisfactory resistance to deterioration upon
prolonged exposure to tropical environment.
As in the Queuing patent, one part of the novel composition comprises
liquid polyglycidy] ether of bisphenol A and the other
part is a hardener comprising a mixture of
liquid carboxyl-terminated polyester of dicarboxylic
acid and polypropylene glycol having no more than one ester group
per 500 average molecular weight,
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cyclic dicarboxylic acid android having a melting
point of less than 40C, and
a catalyst,
wherein the cyclic dicarboxylic acid android
primarily comprises low-melting cycloaliphatic android, and the
catalyst is an imidazole.
Also as in the Queuing patent, there are approximately 0.9 to 1.3
epoxy groups for each android group of the android plus each
carboxyl group of the polyester.
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The epoxy resin colnposition of the present invent
lion differs from that of the Queuing patent in that the
cyclic dicarboxylic acid android primarily comprises
low-melting cycloaliphatic android and the catalyst is an
imidazole.
Mixtures of low-mel~ing cycloaliphatic android
and imidazole, when stored at ordinary room temperature in
admixture with the carboxyl-terminated polyester, are
surprisingly resistant to decarboxylation and should remain
fully effective after being stored a year or more at
ordinary room temperatures.
Many uses for electrical-insulating potting come
positions require flame retardancy, even though known
flame-retarding additives tend to impair certain
electrical-insulating properties. Where lame retardancy
is important, the composition of the present invention
preferably also includes a bromina-ted compound in an amount
providing at least 5 parts, preferably at least 10 parts,
of bromide per 100 parts by weight of total interactive
ingredients of the novel potting composition. The
brominated compound may either be an inert filler material
such as decabromodiphenyl ether or a viscous delineate such
as pentabromodiphenyl ether. Preferably the novel potting
composition also contains up to one part of antimony
trioxides per part by weight of bromide.
Inert Miller may comprise at least lo by weight
of the total composition in order to provide optimum
resistance to thermal and mechanical shock. At least half
of the inert filler should be inorganic because this
economically enhances flame retardancy and also has a
mechanical reinforcing effect. Preferred inorganic fillers
include calcium and magnesium silicates, calcium carbonate,
talc, silica, and mica.
To provide a composition having both 10% or more
filler content and desirably low viscosity, each of the
liquid polyglycidyl ether arid the liquid carboxyl-
terminated polyester Schloss have a viscosity not exceeding
~224595
20,000 cups at 25C oven lower viscosities are preferred
and, to this end, the dicarboxylic acid used in making the
polyester preferably is aliphatic or cycloaliphatic and
desirably has ethylenic unsaturation. Malefic android is
especially useful. However, any dicarboxylic acid may be
used as long as the viscosity of the polyester does not
exceed 20,000 cups at 25C.
The low-melting cycloaliphatic android may be
provided either by a single cycloaliphatic android which
by itself has a melting point below 40C or by a mixture of
one or more cycloaliphatic androids and one or more other
cyclic dicarboxylic acid androids, some of which by
themselves have melting points above 40C, but which
together have a melting point below 40C as taught in Us
Patent No. 3,07~,235 (cowman et at.). useful cycle-
aliphatic androids include hexahydrophthalic android,
MOP. 35C; methylhexahydrophthalic android, MOP. <-15C;
tetrahydrophthalic android, MOP. 10~C; methyltetrahydro-
phthalic android, MOP. 63C; endomethylene-tetrahydro-
phthalic android ("Nadir" android) MOP. 120C; andmethylenclomethylenetetrahydropllthalic android ("Methyl
Nadir" anhydricle) Pi <-15C. Other cyclic dicarboxylic
acid androids which may be blended with one or more of
the cycloaliphatic androids in compositions of the pro-
sent invention include phtllalic android, MOP. 130C; but
the anhyclride mixture shallowly be primarily cycloaliuhatic.
If the low-melting cycloaliphatic android or
its mixture with other cyclic dicarboxylic acid androids
were to have a melting point much higher than 40C, it
would tend to crystallize and then need to be heated to
provide a homogeneous liquid before being blended with the
polyglycidyl ether. To insure against crystallization, the
melting point of the low-melting cycloalipha-tic android
or android mixture is desirably less than 20C.
For ease of Mooney, the viscosities of the
polyglycidyl ether portion and the hardener portion of the
two-part epoxy resin potting composition of the invention
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should be approximately the same, and inert filler should
comprise no more than 70% by weight of each part. Usually inert
filler comprises no more than 50% by weight of the total compost
it ion since its viscosity might otherwise be too high to impreg-
Nate minute interstices such as the spaces between the wires of an
electrical coil. For good impregnation, the individual filler
particles preferably are quite fine (e.g., less than 30 micrometers)
and have a cubic or equi-axial shape, as opposed to needle-like
or plate-like particles. Because exceedingly fine particles tend
to produce unduly increased viscosities, a preferred particle size
range is from 5 to 20 micrometers.
Preferred imidazole catalysts include imidazole,
2-ethylimidazole, and 2-ethyl-4-methylimidazole. The imidazole
catalyst preferably is used in an amount from 0.2 to I by weight
of the polyglycidyl ether. At lesser amounts, the curing rate
tends to be too slow. At greater amounts, the composition
obtained by mixing the hardener and polyglycidyl ether parts might
have undesirably short pot life.
Although the epoxy resin composition of the invention
is primarily useful for potting electrical components, it may be
used as a dipping composition, and for such use, it should include
a thixotropic filler.
When used for potting electrical components, the
process involves preparation of the subject composition, and,
after mixing, pouring same onto the electrical device to impreg-
Nate and cover substantial portions thereof, and heating the
device and the mixture with which it is impregnated and covered
to cure the mixture to a tough, semi-flexible, thermoses state.
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In the following examples, all parts are by weight.
EXAMPLE 1
A two-part liquid epoxy resin composition was prepared
as follows:
Resin Part A
195 grams liquid polyglycidyl ether of bisphenol
A having an epoxy equivalent of 175-210 and
a viscosity of 11,000 - 15,000 centipoises at 25C.
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104.8 grams calcium carbonate having an average
particle size of 10 micrometers ("Gamma-Sperse"
255 from Georgia warble Co.)
86.2 grams decabromodiphenyl ether
14 grams antimony trioxides
To the polyglycidyl ether in a vessel with a turbine type
mixer, the calcium carbonate was mixed in, followed by the
brominated compound and the antimony trioxides After being
uniformly mixed, this material was set aside as Resin Part A.
Hardener Part B
128.4 grams acid-terminated divester of two
moles of rnaleic android and one mole of
polypropylene ylycol having an average molecular
weight of 1025 (Example III of So 3,523,143)
110.~ cruelness rnethylhexahydrophthalic android
1.1 grams 2-ethyl-4-me~hyl imidazole
0.02 gram silicone anti-foam agent (DB-100, Dow Corning)
358.1 grams calcium carbonate ("Gamma-Sperse"~255)
The acid-terminated divester was charged to a mixing vessel
into which the android was mixed, followed by the
imidazole and the silicone fluid. Finally the calcium
carbonate ways aided an-l us orlrlly mixed into the batch to
provide Hardener Part B.
Two parts of Resin Part A and three parts of
Hardener Part B when blended had a viscosity of 20,000 cups
at 25C and a gel time of 17 minutes at 121C. After
decoration, the blended material was poured into molds and
cured for four hours in a 120C oven. After cooling, the
cured specimens were tested.
*I trade Clark
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Property Test Method Value
Specific
Gravity 1.71
Thermal 1/8" (3.2 mm) Olyphant Pass 10 cycles
shock insert, test temp. cycle
130C to -55C
[M. Olyphant, Jr.; National Conference
on the Application of Electrical Insular
lion sponsored jointly by NEMO and Am.
10 Inst. of Electrical Engineers, 1958,
Paper No. 85; pup 1118-35.]
Flame United States EDITED Pass
Retardancy Method 2021 1/8" (3.2 mm) (non-burning)
thickness
US Subject 94 V-O rating
1/4" (6.4 mm) thickness
Moisture united States FED-STD-406 0.0125%
Absorption Method 7031 1" by 3" by 1/8"
(2.5 by 7.5 by 0.3 cm)
specimens, 24 hour immersion
Hydrolytic 168 his. in scaled contailler
Stability a 120C and 17 psi (117 IcPa)
water vapor, 5" by 1/2" by 1/4"
(13 by 1.3 by 0.6 cm)
specimens
Hardness, before 81 Shore D
Hardness, after 79 Shore D
Volume Resistivity, before >1015 ohm-cm
Volume Resistivity, after 1012 ohm-cm
dielectric 1/8" (3.2 mm) specimen 340 volts/mil
Strength 500 v/sec rate of rise (13.6kV/mm)
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The above Hydrolytic Stability test method is believed to
be more rigorous than is the above-cited US. MOLLY,
Rev. G.
another cured specimen 2" by 2" by 1" (5 by 5 by
2.5 cm) was immersed in water and heated in a sealed vessel
at 125C or 28 days. This caused surface blistering, and
the Shore D hardness was reduced from an initial value of
85 to 55. These test results indicate that cured products
of this example have extraordinarily good hydrolytic
stability for a semi-flexible cured product of an epoxy
resin composition.
Cured specimens of Example 1 were also tested for
dissipation factor and dielectric constant. The following
results (averages of two tests) show that dissipation
factor when measured at low frequencies is poor at high
temperatures, a common characteristic of cured epoxy resin
products which have good flame retardancy.
dissipation Factor Dielectric Constant
(%)
Tampa loo 1 10 100 100 1 10 100
( C) tlzkHz kHz ktlz Ho kHzkHz kHz
24 2.22.2 2.1 2.1 5.14.84,7 4.6
106 >20 Lo 5.9 I 8.76.86.3 6.0
130 >20 >20 8.0 4.4 NT 7.26.5 6.1
25155 >20,>20 14 4.1 NT 8.36.5 6.0
NT = not tested
Examples 2-5
Four two-part liquid epoxy resin compositions
were Prepared silnilar to aye ox example I except that all
fillers were omitted. In three of the your, other
cycloaliphatic androids were substituted for the
methylhexahydropllthalic android. Each composition was
cured and tested for hydrolytic stability as in Example 1.
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Example Android Shore D
2 Methylhexahydroph~halic Before, 78-79
After, 76
3 Hexahydrophthalic Before, 76-77
After, 73-74
4 Mixture of 95 parts of Before, 77-78
methyltetrahydrophthalic After, 77
android and 5 ports of
phthalic android
Methylendomethylenetetra- Before, 79
hydrophthalic After, 74-75
