Note: Descriptions are shown in the official language in which they were submitted.
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CURABLE COMPOSITION FOR ELECTRICAL MACHINE,
AND ASSOCIATED METHOD
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of US Patent Publication
No.
2015-0353788 Al, entitled "Composition for Bonding Winding or Core Laminates
in an
Electrical Machine, and Associated Method", filed June 06, 2014.
BACKGROUND
[0002] The invention generally relates to curable compositions including
cyanate
ester resins. More particularly, the invention relates to cyanate ester resin
compositions
for encapsulating components of an electrical machine.
[0003] Thermosetting resins are typically used as electrically insulating
encapsulating (e.g., potting) materials for electrical machines. Thermosetting
resins, such
as cyanate ester resins, have desirable mechanical properties, thermal
stability, and
chemical resistance. However, when used at high temperatures, the performance
of some
of these materials may be unsatisfactory and may result in significant thermal
degradation
even after short operating times. The conventional encapsulating materials may
further
generate cracks and produce excessive heat at the operating temperature of the
electrical
machines. So, it may be desirable to improve the thermal conductivity as well
as the
mechanical properties of the cyanate ester resins used for encapsulating
materials.
Improved materials for dissipating heat generated in electrical machines are
also
desirable, particularly for generators used in aircraft and other aerospace
applications.
[0004] Therefore, electrically insulating encapsulating materials that can
be used
at high temperatures, and are effective in dissipating heat from electrical
machines are
desirable. Further, improved methods for encapsulating components of an
electrical
machine are also desired.
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BRIEF DESCRIPTION OF THE INVENTION
[0005] Embodiments of the present invention are included to meet these and
other
needs. One embodiment is a curable composition for an electrical machine. The
curable
composition includes:
(A) about 10 weight percent to about 30 weight percent of a polyfunctional
cyanate ester having a structure (I)
NCO ¨ Arl ¨ R1-1Y ¨ RI Arl ¨ OCN
(I)
wherein "n" is an integer equal to or greater than 1, Y has a structure (i) or
(ii):
*R2¨Ar2¨ OCN
(i) , Or
*Ar2-0CN
Arl and Ar2 are independently at each occurrence a C5-C3o aromatic radical, R1
and R2 are independently at each occurrence a C1-C3 aliphatic radical or a C3-
C2o
cycloaliphatic radical, and * represents the bonding site;
(B) about 25 weight percent to about 60 weight percent of a first difunctional
cyanate ester having a structure (II), or a prepolymer thereof
(II) NCO ¨Ar3¨ R3¨Ar3¨ OCN
wherein Ar3 is a C5-C3o aromatic radical, R3 is a bond or a C1-C2 aliphatic
radical;
(C) about 10 weight percent to about 30 weight percent of a second
difunctional
cyanate ester having a structure (III), or a prepolymer thereof
(III) NCO ¨Ar4¨ R5¨ A r4¨ OCN
wherein Ar4 is a C5-C3o aromatic radical, and R5 is a C3-Clo aliphatic
radical; and
(D) about 5 weight percent to about 25 weight percent of a thermally
conductive
filler comprising boron nitride.
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[0006] One embodiment is a curable composition for encapsulating a
component
of an electrical machine. The curable composition includes:
(A) about 14 weight percent to about 18 weight percent of a polyfunctional
cyanate ester having a structure (I)
NCO ¨Arl ¨ ¨OCN
(I)
wherein "n" is an integer equal to or greater than 1, Y has a structure (i) or
(ii)
(i) *R2¨Ar2-0CN
, or
(ii) *Ar2-0CN
Arl and Ar2 are independently at each occurrence a C5-C30 aromatic radical, R1
and R2 are independently at each occurrence a Cl-C3 aliphatic radical or a C3-
C20
cycloaliphatic radical, and * represents the bonding site;
(B) about 32 weight percent to about 40 weight percent of a first difunctional
cyanate ester having a structure (II), or a prepolymer thereof
(II) NCO ¨Ar3¨R3¨Ar3-0CN
wherein Ar3 is a Cs-C3o aromatic radical, R3 is a bond or a C1-C2 aliphatic
radical;
(C) about 18 weight percent to about 22 weight percent of a second
difunctional
cyanate ester having a structure (III), or a prepolymer thereof
(III) NCO¨Ar4¨R5¨Ar4-0CN
wherein Ar4 is a C5-C3o aromatic radical, and R5 is a C3-C10 aliphatic
radical;
(D) about 18 weight percent to about 22 weight percent of a thermally
conductive filler comprising boron nitride; and
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(E) about 5 weight percent to about 15 weight percent of a toughener, wherein
the toughener comprises a a thermoplastic polymer, a reactive cyanate ester,
or a
combination thereof.
[0007] One
embodiment is a method of encapsulating a component of an
electrical machine, includes: contacting the component with an encapsulating
material
comprising a curable composition and curing the curable composition, wherein
the
curable composition comprises:
(A) about 10 weight percent to about 30 weight percent of a polyfunctional
cyanate ester having a structure (I)
¨ OCN
(I)
wherein "n" is an integer equal to or greater than 1, Y has a structure (i) or
(ii)
*R2¨Al2-0CN , or
*Ar2-0CN
AO and Ar2 are independently at each occurrence a C5-C3o aromatic radical, R1
and R2 are independently at each occurrence a Cl-C3 aliphatic radical or a C3-
C2o
cycloaliphatic radical, and * represents the bonding site;
(B) about 25 weight percent to about 60 weight percent of a first difunctional
cyanate ester having a structure (II), or a prepolymer thereof
(II) NCO ¨At-3¨ R3¨A13 ¨ OCN
wherein Ar3 is a C5-C30 aromatic radical, R3 is a bond or a C1-C2 aliphatic
radical;
(C) about 10 weight percent to about 30 weight percent of a second
difunctional
cyanate ester having a structure (III), or a prepolymer thereof
(III) NCO ¨Ar4¨R5¨Ar4-0CN
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wherein Ar4 is a C5-C3o aromatic radical, and R5 is a C3_Cio aliphatic
radical; and
(D) about 5 weight percent to about 25 weight percent of a thermally
conductive
filler comprising boron nitride.
DRAWINGS
[0008] These and other features, aspects, and advantages of the present
invention
will become better understood when the following detailed description is read
with
reference to the accompanying drawings, wherein:
[0009] Fig. 1 shows a stator including the curable composition, in
accordance
with some embodiments of the invention.
[0010] Fig. 2 is a graph illustrating change in thermal conductivity with
increasing concentration of boron nitride, in accordance with some embodiments
of the
invention.
DETAILED DESCRIPTION
[0011] As discussed in detail below, some of the embodiments of the
invention
relate to compositions for encapsulating components or bonding of windings in
an
electrical machine. More particularly, the invention relates to cyanate ester
resin
compositions for encapsulating components in an electrical machine.
[0012] Approximating language, as used herein throughout the specification
and
claims, may be applied to modify any quantitative representation that could
permissibly
vary without resulting in a change in the basic function to which it is
related.
Accordingly, a value modified by a term or terms, such as "about", and
"substantially" is
not to be limited to the precise value specified. In some instances, the
approximating
language may correspond to the precision of an instrument for measuring the
value. Here
and throughout the specification and claims, range limitations may be combined
and/or
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interchanged; such ranges are identified and include all the sub-ranges
contained therein
unless context or language indicates otherwise.
[0013] In the following specification and the claims, the singular forms
"a", "an"
and "the" include plural referents unless the context clearly dictates
otherwise. As used
herein, the term "or" is not meant to be exclusive and refers to at least one
of the
referenced components being present and includes instances in which a
combination of
the referenced components may be present, unless the context clearly dictates
otherwise.
[0014] As used herein, the term "aromatic radical" refers to an array of
atoms
having a valence of at least one comprising at least one aromatic group. The
array of
atoms having a valence of at least one comprising at least one aromatic group
may
include heteroatoms such as nitrogen, sulfur, selenium, silicon and oxygen, or
may be
composed exclusively of carbon and hydrogen. As used herein, the term
"aromatic
radical" includes but is not limited to phenyl, pyridyl, furanyl, thienyl,
naphthyl,
phenylene, and biphenyl radicals. As noted, the aromatic radical contains at
least one
aromatic group. The aromatic group is invariably a cyclic structure having
4n+2
"delocalized" electrons where "n" is an integer equal to 1 or greater, as
illustrated by
phenyl groups (n = 1), thienyl groups (n = 1), furanyl groups (n = 1),
naphthyl groups (n
= 2), azulenyl groups (n = 2), anthraceneyl groups (n = 3) and the like. The
aromatic
radical may also include nonaromatic components. For example, a benzyl group
is an
aromatic radical, which comprises a phenyl ring (the aromatic group) and a
methylene
group (the nonaromatic component). Similarly a tetrahydronaphthyl radical is
an aromatic
radical comprising an aromatic group (C6I-13) fused to a nonaromatic component
¨(CH2)4-.
For convenience, the term "aromatic radical" is defined herein to encompass a
wide range
of functional groups such as alkyl groups, alkenyl groups, alkynyl groups,
haloalkyl
groups, haloaromatic groups, conjugated dienyl groups, alcohol groups, ether
groups,
aldehyde groups, ketone groups, carboxylic acid groups, acyl groups (for
example
carboxylic acid derivatives such as esters and amides), amine groups, nitro
groups, and
the like. For example, the 4-methylphenyl radical is a C7 aromatic radical
comprising a
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methyl group, the methyl group being a functional group which is an alkyl
group.
Similarly, the 2-nitrophenyl group is a C6 aromatic radical comprising a nitro
group, the
nitro group being a functional group. Aromatic radicals include halogenated
aromatic
radicals such as 4-trifluoromethylphenyl, hexafluoroisopropylidenebis(4-phen-1-
yloxy)
(i.e., ¨0PhC(CF3)2Ph0-), 4-chloromethylphen-1-yl, 3-trifluoroviny1-2-thienyl,
3-
trichloromethylphen-l-yl (i.e., 3-CC13Ph-), 4-(3-bromoprop-1-yl)phen-1-y1
(i.e., 4-
BrCH2CH2CH2Ph-), and the like. Further examples of aromatic radicals include 4-
allyloxyphen-1-oxy, 4-aminophen-1-y1 (i.e., 4-H2NPh-), 3-aminocarbonylphen-1-
y1 (i.e.,
NH2COPh-), 4-benzoylphen-l-yl, dicyanomethylidenebis(4-phen-1-yloxy) (i.e., -
0PhC(CN)2Ph0-), 3-methylphen-1-yl, methylenebis(4-phen-1-yloxy) (i.e., ¨
0PhCH2Ph0-), 2-ethylphen-1-yl, phenylethenyl, 3-formy1-2-thienyl, 2-hexy1-5-
furanyl,
hexamethylene-1,6-bis(4-phen-1-yloxy) (i.e., ¨0Ph(CH2)6Ph0-), 4-
hydroxymethylphen-
1-y1 (i.e., 4-HOCH2Ph-), 4-mercaptomethylphen-l-y1 (i.e., 4-HSCH2Ph-), 4-
methylthiophen-1-y1 (i.e., 4-CH3SPh-), 3-methoxyphen-1-yl, 2-
methoxycarbonylphen-1-
yloxy (e.g., methyl salicyl), 2-nitromethylphen-1-y1 (i.e., 2-
NO2CH2Ph), 3-
trimethylsilylphen-1-yl, 4-t-butyldimethylsilylphen1-1-yl, 4-
vinylphen-l-yl,
vinylidenebis(phenyl), and the like. The term "a C3 ¨ Cm aromatic radical"
includes
aromatic radicals containing at least three but no more than 10 carbon atoms.
The
aromatic radical 1-imidazoly1 (C3H2N2-) represents a C3 aromatic radical. The
benzyl
radical (C7H7-) represents a C7 aromatic radical.
[0015] As used
herein, the term "cycloaliphatic radical" refers to a radical having
a valence of at least one, and comprising an array of atoms which is cyclic
but which is
not aromatic. As defined herein a "cycloaliphatic radical" does not contain an
aromatic
group. A "cycloaliphatic radical" may comprise one or more noncyclic
components. For
example, a cyclohexylmethyl group (C6HHCH2-) is a cycloaliphatic radical which
comprises a cyclohexyl ring (the array of atoms which is cyclic but which is
not
aromatic) and a methylene group (the noncyclic component). The cycloaliphatic
radical
may include heteroatoms such as nitrogen, sulfur, selenium, silicon and
oxygen, or may
be composed exclusively of carbon and hydrogen. For convenience, the term
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"cycloaliphatic radical" is defined herein to encompass a wide range of
functional groups
such as alkyl groups, alkenyl groups, alkynyl groups, haloalkyl groups,
conjugated dienyl
groups, alcohol groups, ether groups, aldehyde groups, ketone groups,
carboxylic acid
groups, acyl groups (for example carboxylic acid derivatives such as esters
and amides),
amine groups, nitro groups, and the like. For example, the 4-methylcyclopent-
1-y1
radical is a C6 cycloaliphatic radical comprising a methyl group, the methyl
group being a
functional group which is an alkyl group. Similarly, the 2-nitrocyclobut-1-y1
radical is a
C4 cycloaliphatic radical comprising a nitro group, the nitro group being a
functional
group. A cycloaliphatic radical may comprise one or more halogen atoms, which
may be
the same or different. Halogen atoms include, for example; fluorine, chlorine,
bromine,
and iodine. Cycloaliphatic radicals comprising one or more halogen atoms
include 2-
trifluoromethylcyclohex- 1-yl, 4-bromodifluoromethylcyclooct-l-yl, 2-
chlorodifluoromethylcyclohex-1-yl, hexafluoroisopropylidene-2,2-bis (cyclohex-
4-y1)
(i.e., -C6H10C(CF3)2 C61-110-), 2-chloromethylcyclohex-1-yl, 3-
difluoromethylenecyclohex-1-yl, 4-trichloromethylcyclohex-1-yloxy, 4-
bromodichloromethylcyclohex-1-ylthio, 2-bromoethylcyclopent-1-yl, 2-
bromopropylcyclohex-1-yloxy (e.g., CH3CHBrCH2C6Hi00-), and the like. Further
examples of cycloaliphatic radicals include 4-allyloxycyclohex-1-yl, 4-
aminocyclohex-1-
yl (i.e., H2NColli0-), 4-aminocarbonylcyclopent-1-y1 (i.e., NH2C0C5H8-), 4-
acetyloxycyclohex-1-yl, 2,2-dicyanoisopropylidenebis(cyclohex-4-yloxy) (i.e., -
0C6H1oC(CN)2C611100-), 3-methylcyclohex-1-yl, methylenebis(cyclohex-4-yloxy)
(i.e., -
0C6H10CH2C6H100-), 1-ethylcyclobut-1-yl,
cyclopropylethenyl, 3-formy1-2-
terahydrofuranyl, 2-hexy1-5-tetrahydrofuranyl, hexamethylene-1,6-bis(cyclohex-
4-yloxy)
(i.e., -0 C6H1o(CH2)6C61-1100-), 4-hydroxymethylcyclohex-1-y1 (i.e., 4-
HOCH2C6H10-),
4-mercaptomethylcyclohex-1-y1 (i.e., 4-HSCH2C61-110-), 4-methylthiocyclohex-1-
y1 (i.e.,
4-CH3SC6H10-), 4-methoxycyclohex-1-yl, 2-methoxycarbonylcyclohex-1-yloxy (2-
CH30C0C6H100-), 4-nitromethylcyclohex-1-y1 (i.e., NO2CH2C6H10-
), 3-
trimethy lsilylcyclohex- 1-yl, 2-t-butyldimethylsilylcyclopent-l-yl, 4-
trimethoxysilylethylcyclohex-1-y1 (e.g., (CH30)3SiCH2CH2C6Hio-), 4-
vinylcyclohexen-
l-yl, vinylidenebis(cyclohexyl), and the like. The term "a C3 - Cia
cycloaliphatic
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radical" includes cycloaliphatic radicals containing at least three but no
more than 10
carbon atoms. The cycloaliphatic radical 2-tetrahydrofuranyl (C41170-)
represents a C4
cycloaliphatic radical. The cyclohexylmethyl radical (C6H11CH2-) represents a
C7
cycloaliphatic radical.
[0016] As used
herein, the term "aliphatic radical" refers to an organic radical
having a valence of at least one consisting of a linear or branched array of
atoms which is
not cyclic. Aliphatic radicals are defined to comprise at least one carbon
atom. The array
of atoms comprising the aliphatic radical may include heteroatoms such as
nitrogen,
sulfur, silicon, selenium and oxygen or may be composed exclusively of carbon
and
hydrogen. For convenience, the term "aliphatic radical" is defined herein to
encompass,
as part of the "linear or branched array of atoms which is not cyclic" a wide
range of
functional groups such as alkyl groups, alkenyl groups, alkynyl groups,
haloalkyl groups,
conjugated dienyl groups, alcohol groups, ether groups, aldehyde groups,
ketone groups,
carboxylic acid groups, acyl groups (for example carboxylic acid derivatives
such as
esters and amides), amine groups, nitro groups, and the like. For example, the
4-
methylpent-1-y1 radical is a C6 aliphatic radical comprising a methyl group,
the methyl
group being a functional group which is an alkyl group. Similarly, the 4-
nitrobut-1-y1
group is a C4 aliphatic radical comprising a nitro group, the nitro group
being a functional
group. An aliphatic radical may be a haloalkyl group which comprises one or
more
halogen atoms which may be the same or different. Halogen atoms include, for
example;
fluorine, chlorine, bromine, and iodine. Aliphatic radicals comprising one or
more
halogen atoms include the alkyl halides trifluoromethyl, bromodifluoromethyl,
chlorodifluoromethyl, hexafluoroisopropylidene, chloromethyl,
difluorovinylidene,
trichloromethyl, bromodichloromethyl, bromoethyl, 2-bromotrimethylene (e.g., -
CH2CHBrCH2-), and the like. Further examples of aliphatic radicals include
allyl,
aminocarbonyl (i.e., ¨CONH2), carbonyl, 2,2-dicyanoisopropylidene (i.e., -
CH2C(CN)2CH2-), methyl (i.e., -CH3), methylene (i.e., ¨CH2-), ethyl, ethylene,
formyl
(i.e.,-CH0), hexyl, hexamethylene, hydroxymethyl (i.e.,-CH2OH), mercaptomethyl
(i.e.,
¨CH2SH), methylthio (i.e., ¨SCI-13), methylthiomethyl (i.e., ¨CH2SCH3),
methoxy,
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methoxycarbonyl (i.e., CH30C0-), nitromethyl (i.e., -CH2NO2), thiocarbonyl,
trimethylsilyl ( i.e., (CH3)3Si-), t-butyldimethylsilyl, 3-
trimethyoxysilylpropyl (i.e.,
(CH30)3SiCH2CH2CH2-), vinyl, vinylidene, and the like. By way of further
example, a
Ci ¨ Cm aliphatic radical contains at least one but no more than 10 carbon
atoms. A
methyl group (i.e., CH3-) is an example of a Ci aliphatic radical. A decyl
group (i.e.,
CH3(CH2)9-) is an example of a Cio aliphatic radical.
[0017] As discussed in detail below, some embodiments of the invention are
directed to a curable composition for an electrical machine. The curable
composition
includes:
(A) about 10 weight percent to about 30 weight percent of a polyfunctional
cyanate ester having a structure (I)
NCO ¨Arl¨R1¨k ¨ OCN
wherein "n" is an integer equal to or greater than 1, Y has a structure (i) or
(ii)
*R2¨Ar2-0CN
(ii) *Ar2-0CN
Arl and Ar2 are independently at each occurrence a Cs-C30 aromatic radical, R1
and R2 are independently at each occurrence a Ci-C3 aliphatic radical or a C3-
C2o
cycloaliphatic radical, and * represents the bonding site;
(B) about 25 weight percent to about 60 weight percent of a first difunctional
cyanate ester having a structure (II), or a prepolymer thereof
(II) NCO ¨Ar3¨ R3¨ Ar-3¨ OCN
wherein Ar3 is a C5-C3o aromatic radical, R3 is a bond or a Ci-C2 aliphatic
radical;
(C) about 10 weight percent to about 30 weight percent of a second
difunctional
cyanate ester having a structure (III), or a prepolymer thereof
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(III) N CO ¨ Ar4 ¨R5¨ A r4¨ OCN
wherein Ar4 is a C5-C3o aromatic radical, and R5 is a C3_Cio aliphatic
radical; and
(D) about 5 weight percent to about 25 weight percent of a thermally
conductive
filler comprising boron nitride.
[0018] The term
"polyfunctional cyanate ester" as used herein, refers to a material
including three or more cyanate ester functional groups. The term
"difunctional cyanate
ester" as used herein refers to a material including two cyanate ester
functional groups.
[0019] Non-
limiting examples of suitable polyfunctional cyanate esters include
phenolic novolac cyanate ester, dicylopentadiene novolac cyanate ester, 1,2,3-
tris(4-
cyanatopheny1)-propane, or combinations thereof. In certain
embodiments, the
polyfunctional cyanate ester includes a structure having formulae (VII) or
(VIII):
OCN OCN OCN
(VII)
NCO
11101 OCN
(VIII) OCN
[0020] Non-
limiting examples of a suitable polyfunctional cyanate ester include
PrimasetTM PT30, PrimasetTM PT15, or combinations thereof, commercially
available
from Lonza.
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[0021] In some embodiments, the amount of polyfunctional cyanate ester in
the
curable composition is in a range from about 10 weight percent to about 30
weight
percent. In certain embodiments, the amount of polyfunctional cyanate ester in
the
curable composition is in a range from about 12 weight percent to about 22
weight
percent. In certain embodiments, the amount of polyfunctional cyanate ester in
the
curable composition is in a range from about 14 weight percent to about 18
weight
percent.
[0022] In some embodiments, the first difunctional cyanate ester includes
a
structure having a formula (IV):
NCO ¨ Ar3¨ CH ¨ Ar3-0 CN
1
(IV) CH3
,
wherein Ar3 is a C5-C30 aromatic radical. In certain embodiments, the first
difunctional
ester includes a structure having a formula (IX):
CH3 H
0 401
(IX) NCO OCN
[0023] Non-limiting example of a suitable first difunctional cyante ester
includes
PrimasetTM LECY, commercially available from Lonza.
[0024] The amount of the first difunctional cyanate ester in the curable
composition may be in a range from about 25 weight percent to about 60 weight
percent.
In some embodiments, the amount of the first difunctional cyanate ester in the
curable
composition is in a range from about 30 weight percent to about 50 weight
percent. In
certain embodiments, the amount of the first difunctional cyanate ester in the
curable
composition is in a range from about 32 weight percent to about 40 weight
percent.
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[0025] In some embodiments, the second difunctional cyanate ester includes
a
prepolymer of a difunctional cyanate ester having a formula (V):
(V) NCO ¨Ar4¨ R4¨ Ar4¨ OCN
wherein Ar4 is a Cs-C30 aromatic radical, and R4 is C3-C20 aliphatic radical.
The term
"prepolymer" as used herein refers to a monomer or a plurality of monomers
that have
been reacted to an intermediate molecular weight state. This material is
capable of further
polymerization by reactive groups to a high molecular weight state. As such,
mixtures of
reactive polymers with un-reacted monomers may also be referred to as pre-
polymers.
The term "prepolymer" and "polymer precursor" are sometimes used
interchangeably.
[0026] In some embodiments, the second difunctional cyanate ester includes
a
prepolymer of a difunctional cyanate ester having a formula (VI):
CH3
NCO ¨Ar4¨ C¨ Ar4 ¨ OCN
(VI) CH3
wherein Ar4 is a C5-C30 aromatic radical.
[0027] In some embodiments, the second difunctional cyanate ester includes
a
prepolymer of a difunctional cyanate ester having a formula (X):
CH3 CH3
(X) NCO OCN
[0028] Non-limiting example of a suitable second difunctional cyanate
ester
includes PrimasetTM BA3000, commercially available from Lonza.
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[0029] In some embodiments, the amount of second difunctional cyanate
ester in
the curable composition is in a range from about 10 weight percent to about 30
weight
percent. Further, the amount of second difunctional cyanate ester in the
curable
composition may be in a range from about 15 weight percent to about 25 weight
percent.
In certain embodiments, the amount of second difunctional cyanate ester in the
curable
composition is in a range from about 18 weight percent to about 22 weight
percent.
[0030] The curable composition further includes a thermally conductive
filler.
The thermally conducting filler includes boron nitride. In some embodiments,
the
thermally conductive filler is added at various loading levels depending on
several
requirements of the cured composition (such as thermal conductivity, viscosity
or
toughness).
[0031] In some embodiments, the thermally conductive filler is present in
the
curable composition in an amount in a range from about 5 weight percent to
about 25
weight percent. In some embodiments, the thermally conductive filler is
present in an
amount in a range from about 15 weight percent to about 25 weight percent. In
certain
embodiments, the amount of the thermally conductive filler in the curable
composition is
in a range from about 18 weight percent to about 22 weight percent.
[0032] The curable composition may further comprise a toughener, wherein
the
toughener includes a reactive cyanate ester, a thermoplastic polymer, or a
combination
thereof. The toughener may be present in an amount in a range from about 5
weight
percent to about 15 weight percent. The term "reactive cyanate ester" as used
herein
refers to a material including cyanate ester moieties, that is capable of
reacting with one
or more of the polyfunctional cyanate ester, the first difunctional cyanate
ester, and the
second difunctional cyanate ester. Non-limiting example of a suitable reactive
cyanate
ester includes PrimasetTM HTL 300, commercially available from Lonza.
[0033] In some embodiments, the thermoplastic polymer includes a
polyimide. In
certain embodiments, the polyimide includes structural units having a formula
(XI):
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N7C I. N-R6
11 r1/
0 0 - (XI)
wherein R6 is a C3-Cio aliphatic radical, a C5-C30 aromatic radical, or
combinations
thereof. Non-limiting example of a suitable polyimide includes P84',
commercially
available from Evonik.
[0034] The cross-linking reaction of the cyanate ester resins at elevated
temperatures may be controlled by selection of appropriate catalysts. In some
embodiments, the curable composition may further include a suitable catalyst.
Without
being bound by any theory it is believed that in the absence of a sufficient
amount of
catalyst, the curable composition may react quickly on heating to high
temperatures, and
the corresponding reaction rate may be higher than the heat dissipation rate
resulting in
local hot spots and thermal runaway. In some embodiments, the reaction
temperature
may be reduced and the reaction rate may be controlled by choosing the
appropriate
catalyst chemistry and by controlling the amount of catalyst used.
[0035] Non-limiting examples of suitable catalysts include transition
metal
carboxylates or chelates. In some embodiments, non-limiting examples of
suitable
catalyst include acetylacetonates of zinc, copper, cobalt, manganese, iron,
aluminum, or
combinations of thereof. In certain embodiments, the catalyst includes
copper
acetylaceton ate, cobalt acetylacetonate, aluminum acetylacetonate or
combinations
thereof. In some embodiments, co-catalysts that provide an active hydrogen
source (for
example, alkylphenols) may also be used. In some embodiments, the catalyst may
be
present in the curable composition in a range from about 25 parts per million
(ppm) to
about 150 ppm.
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[0036] In some embodiments, the curable composition may further include
additional additives, such as, stabilizing agents, additional toughening
agents and the like.
[0037] The curable composition may be cured using any suitable method. In
some embodiments, the curable composition may be cured by heating the
composition.
In some embodiments, a temperature in a range from about 18 C to about 400 C
may be
used for curing. In certain embodiments, a temperature in a range from about
100 C to
about 300 C may be used for curing. The time required for curing may differ
depending
on the end application, for example, it may depend upon the thickness of the
molded
article or laminate. In some embodiments, a time period sufficient for curing
the
composition may be in a range of from about 2 hour to about 12 hours. In
embodiments
wherein the cured composition is used as molded articles (such as those
produced via
resin transfer molding, compression molding, or injection molding), laminates,
bonded
articles, or as encapsulating materials, it may be desirable to apply pressure
during the
heat curing step. In some other embodiments, microwave, radio frequency,
ionizing
radiation, electron beams, or combinations thereof may be used to effect the
curing step.
[0038] Without being bound by any theory, it is believed that the
polyfunctional
cyanate ester after cross-linking provides the desired thermal stability and
thermal
properties. By blending the first difunctional cyanate ester with the
polyfunctional
cyanate ester, a thermally stable resin may be achieved, wherein the resin has
a glass
transition temperature above 280 C. Further, the first difunctional cyanate
ester when
blended with the polyfunctional cyanate ester provides the desired viscosity
for
processing. However, the cured composition formed after cross-linking of
the
polyfunctional cyanate ester and the first difunctional cyanate ester is
typically rigid, and
may be susceptible to crack formations during thermal cycling. Toughening
materials,
such as, the second difunctional cyanate ester are added to improve the
mechanical
properties of the composition, while maintaining the viscosity of the curable
composition
and desired thermal stability. The toughening material may react with the
cyanate resins
and form a homogeneous cross-linked structure with longer chains, to provide
heat
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resistance and thermal stability. In some embodiments, as mentioned earlier,
additional
tougheners may be further added to the composition to improve the toughness,
wherein
the toughener includes a reactive cyanate ester, a thermoplastic polymer, or a
combination thereof.
[0039] Thermal conductivity of the cured composition may further be
increased
to provide a high temperature, thermally conductive, and mechanically tough
encapsulation (e.g., potting) composition for electric machines. In some
embodiments,
boron nitride is added to increase thermal conductivity of the cured
composition.
Without being bound by any theory, it is believed that although boron nitride
filler
increases the thermal conductivity of the cured composition, the toughness of
the
resulting composition may be reduced in some instances. In some such
instances, to-
improve the cured composition toughness, an additional toughener (e.g., a
reactive
cyanate ester or a thermoplastic polymer) may be added. The additional
toughener may
improve the fracture resistance. In some embodiments of the present invention,
for
preparing a cured composition with the desired properties, five primary
materials may be
blended, which include a polyfunctional cyanate ester, a first difunctional
cyanate ester, a
second difunctional cyanate ester, a thermally conductive filler (such as
boron nitride)
and a toughening material (such as thermoplastic polymer or a reactive cyanate
ester).
[0040] Without being bound by any theory, it is believed that by
controlling the
relative amounts of polyfunctional cyanate ester, the first difunctional
cyanate ester, the
second difunctional cyanate ester, a thermally conductive filler (such as
boron nitride),
and a toughening material (such as thermoplastic polymer or a reactive cyanate
ester), the
desired combination of properties (such as, thermal stability, thermal
conductivity,
flexural strength, fracture toughness, and viscosity) may be achieved. For
example, as
described earlier, a certain minimum amount of the polyfunctional cyanate
ester is
desired to achieve the desired thermal stability. Similarly, the amount of the
first
difunctional cyanate ester may be controlled in the curable composition such
that the
curable composition has viscosity sufficiently low for processing. In some
embodiments,
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the curable composition may be substantially solvent-free and the first
difunctional
cyanate ester may provide the desired viscosity characteristics. Further, the
amount of
the second difunctional cyanate ester may be controlled in the curable
composition to
provide the desired mechanical properties in the cured composition. As noted
earlier, an
appropriate amount of thermally conductive filler may be added to further
increase the
thermal conductivity of the composition. However, the addition of boron
nitride may
decrease the toughness of the composition, which may be regained by addition
of a
controlled amount of a toughening material, such as, a thermoplastic polymer
or a
reactive cyanate ester.
[0041] In some embodiments, the curable composition has a viscosity, and
the
related cured composition has thermal and mechanical properties suitable for
the end-use
application (for example, potting material for a generator stator). The term
"cured
composition" as used herein includes both partially and completely cured
compositions.
[0042] In some embodiments, the curable composition has a viscosity less
than
about 5000 centiPoise (cP) at a temperature of about 100 C. In some
embodiments, the
curable composition has a viscosity less than about 4000 centiPoise (cP) at a
temperature
of about 100 C. In some embodiments, the curable composition has a viscosity
less than
about 3000 centiPoise (cP) at a temperature of about 100 C.
[0043] To facilitate improvement in thermal stability, it is desirable
that the cured
composition has a glass transition temperature at least about 280 C, in some
embodiments. In some embodiments, the cured composition of the curable
composition
has a glass transition temperature of at least about 300 C.
[0044] As noted, thermally conductive filler material (such as boron
nitride) is
added to increase the thermal conductivity of the cured composition. In some
embodiments, the cured composition has a thermal conductivity in a range from
about
0.75 W/m.K to about 1.2 W/m.K. In some other embodiments, the cured
composition
has a thermal conductivity greater than 1.0 W/m.K.
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[0045] The cured composition after thermal cycling may also be
characterized by
a flexural strength and fracture toughness. In some embodiments, the cured
composition
has a flexural strength greater than 8000 psi. In some embodiments, the cured
composition has a flexural strength in a range from about 8500 psi to about
11000 psi. In
some embodiments, the cured composition has a fracture toughness in a range
from about
1.2 MPa.m1/2.to about 1.6 MPa.m1/2.
[0046] Non-limiting examples of suitable applications for the curable
composition include encapsulation, bonding, insulation, lamination, or
combinations
thereof. In some embodiments, the curable composition may be used for
encapsulating
components of an electrical machine. Non¨limiting examples of suitable
components
include stators, windings, core-laminates, slot liners, slot wedges, or
combinations
thereof. The electrical machine may be selected from the group consisting of a
motor, a
generator, a transformer, a toroid, an inductor, and combinations thereof. In
some
embodiments, a stator of a generator includes the cured composition of the
curable
composition. FIG. 1 illustrates a schematic of a stator 100 impregnated with
the curable
composition 120. As illustrated in FIG. 1, the curable composition 120 and the
resulting
cured composition may fill all the gaps and spaces present between the stator
windings
110.
[0047] In some embodiments, an electrically insulating encapsulating
material
including a cured composition of the curable composition is also presented.
The
encapsulating material in some instances may provide: (1) desired heat
stability at a
temperature greater than 250 C by using the polyfunctional cyanate ester and a
thermally
conductive filler (boron nitride); and (2) processability for the impregnation
process by
using the first difunctional cyanate ester to maintain viscosity, and by
appropriate catalyst
selection to minimize exothermic heat resulting in thermal run ways.
[0048] Further, in some embodiments, the encapsulating material may be
thermally stable and may be effective at dissipating heat from the electrical
machines
operating at high temperatures. The high temperature stability and heat
dissipation
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property may be useful, for example, in generator applications. The generator
requires
heat reduction while maximizing the output; specifically when used in aircraft
and other
aerospace applications. The encapsulating material may also be suitable for
usage in
motors and transformers. In some such instances, the encapsulating material
may provide
the desired resistance to shock and vibration, and exclusion of moisture and
corrosive
agents under high speed vibration.
[0049] A method of encapsulating a component of an electrical machine is
also
presented. The method includes contacting the component with an encapsulating
material including a curable composition as described earlier. The term
"contacting" as
used herein refers to impregnating one or more components of the electrical
machine with
the curable composition using a suitable technique, for example, pressure
impregnation or
vacuum impregnation. For example, in some embodiments, the method may include
placing a stator in a mold. An encapsulating composition may be impregnated
into the
cavity of the stator by applying pressure such that the encapsulating
composition fills all
the spaces and gaps present within the stator components (such as gaps between
stator
windings). The method further includes heating the component to cure the
curable
composition in the presence or absence of a vacuum.
[0050] One example of such a method is the vacuum pressure impregnation
method, in which an entire component of an electrical machine assembly is
placed in a
pressure vessel under a high vacuum that draws out entrapped air and other
gases. The
curable composition is introduced to the pressure vessel and the entire tank
is pressurized,
typically to at least 90 psi or higher to achieve a total penetration of the
machine
components. The assembly may be baked at elevated temperatures to cure the
curable
composition. In some embodiments, the curable composition is cured at a
temperature in
a range from about 120 C to about 250 C.
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EXAMPLES
[0051] Materials: BA3000, PT-30, LECY, and Primaset HTL-300 were obtained
from Lonza. P84 NT2 polyimide was obtained from Evonik, and PTX60P boron
nitride
was obtained from Momentive. All mixing of resins were done in plastic cups
(resin cup)
designed for use in high speed planetary centrifugal mixer equipped with
vacuum. The
boron nitride filler and thermoplastic tougheners were dried in vacuum oven at
150 C for
at least 24 hours, and removed from oven prior to additions.
Method for preparing catalyst solution:
[0052] Copper (II) acetylacetonate solution was prepared by adding 0.300 g
of Cu
(acac)2 to 10.0 g of nonylphenol. The resulting copper (II) acetylacetonate
solution was
added to an oversized vial and sonicated at 50 C for 60 minutes.
Curing process in detail:
[0053] Curing schedule: A variety of formulations contained in various
molds and
metallic pans were cured in programmable oven according to the following
protocol:
1) Heat from room temperature to 120 C and hold at 120 C for 3 hours.
2) Heat to 165 C and hold for 2 hours
3) Heat to 250 C and hold for 4-6 hours
[0054] In an alternate process, the first step was skipped, and the sample
was
heated directly to 150 C to165 C, in order to fully gel the material and
effect most of the
polymerization before final cure at high temperature.
[0055] The following ASTM methods were used for determining: 1) Flexural
Strength (D790); 2) Flexural Strain (D790); 3) Fracture Toughness (D5045); 4)
glass
transition temperature or Tg by DMA (E1640); and 5) thermal conductivity
(E1540)
measured at 250 C.
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Viscosity measurements were performed as follows:
[0056] The viscosity of the resin formulation was tested using a
Brookfield DV-
11+ programmable viscometer with Thermosel temperature controller. A
disposable
sample chamber (model #HT-2DB) was charged with 17.5 grams (+/- 0.5 grams) of
pre-
warmed resin formulation (50 C) that was gently mixed after warming to insure
homogeneity. A SC4 series Thermoset spindle was placed inside the sample
chamber
that was already inserted into the Thermosel temperature controller. Viscosity
was
measured at a spin rate of 6 rpm's and temperature of 100 C. After 15 minutes
of
equilibration under these conditions, the viscosity was recorded.
EXAMPLE 1 Blends of PT30, LECY, BA3000, and boron nitride (BN)
[0057] BA3000 was placed in a 75 C oven for approximately 1 hour in order
to
facilitate transfer to a resin cup. The resin cup was charged with BA3000,
LECY and
PT-30 and heated for about 15-30 minutes in oven. The components were then
mixed for
4 minutes, followed by hand mixing to ensure any undissolved BA3000 was
distributed
throughout the cup, and further mixed for 4 minutes. After cooling the resin
mixture on
bench for 15-20 minutes, copper acetylacetonate in nonylphenol (50 ppm of
copper
acetylacetonate relative to resins) was added to the resin mixture and mixed
for an
additional 30 seconds. The dry boron nitride was added to the above mixture of
resin and
catalyst and mixed for 30 seconds. The mixing was continued for additional 3.5
minutes
under vacuum to complete mixing, and the homogeneous mixture was degassed.
Immediately after mixing, samples for thermal conductivity and mechanical
testing were
prepared by pouring the mixture into molds and/or simple disposable pans that
had been
pretreated with Frekote. The remaining resin was stored at 4 C prior to
viscosity
measurements and as reserve for any additional tests. Table 1 lists the
different blend
samples and their compositions. In Table 1, sample no. 1 is the comparative
example
without PT30 or any toughener. Table 2 shows the data for samples 2-12 (blends
with
BN filler) versus the comparative sample 1 (blend without BN filler).
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Table 1: Compositions of different blends
Sample No. BA 3000 LECY PT 30 BN P84
Wt% Wt% Wt% Wt% Wt%
1 38.7 38.7- 22.5 -
2 19.4 38.7 19.4 22.5 -
3 31.0 31.0 15.5 22.5 -
4 19.4 38.7 15.5 22.5 3.9
19.4 34.8 15.5 22.5 7.7
6 20.6 41.2 20.6 17.5 -
7 20.6 37.1 16.5 17.5 8.2
8 20.6 41.2 20.6 17.5 -
9 20.0 36.0 16.0 20.0 8.0
20.0 39.9 20.0 20.0 -
11 21.8 39.3 17.5 12.5 8.7
12 21.2 38.2 17.0 15.0 8.5
Table 2: Properties of different blends
Sample No. Viscosity Tg Flexural Strain FT TC
cp@100 ( C) strength (Mps.m1/2) (W/m.K)
(psi)
1 - 292
2 1625 312 8542 1.6 1.241 1.08
3 - 308 8699 1.61 1.287
4 - - 8594 1.65 1.349
5 5000 - 9412 1.83 1.483
6 2150 - 8940 1.83 1.23
7 1200 - 9563 1.99 1.266 0.7
8 600 - 9300 2.05 1.074 0.56
9 2400 311 9697 2.25 1.433 0.82
10 1100 314 9110 1.90 1.234 0.77
11 450 - - - - 0.55
12 700 - - - - 0.62
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EXAMPLE 3 Blends of PT30, LECY, BA3000, boron nitride, and thermoplastic
polyimide (P84)
[0058] BA3000 was placed in a 75 C oven for approximately 1 hour in order
to
facilitate transfer to a resin cup. The resin cup was charged with of BA3000,
LECY and
PT-30 and heated for 15-30 minutes in oven. These components were then mixed
for 4
minutes, followed by hand mixing to ensure any undissolved BA3000 was
distributed
throughout the cup, and further mixed for 4 minutes. The P-84 thermoplastic
was then
added to the above resin-mixture and mixed for 30 seconds. After cooling, the
resin-
mixture and the toughener P-84 on bench for 15-20 minutes, copper
acetylacetonate in
nonylphenol (50 ppm of copper acetylacetonate relative to reactive resins) was
added and
mixed for an additional 30 seconds. The dry boron nitride was added to the
mixture and
mixed for 30 seconds. The mixing was continued for additional 3.5 minutes
under
vacuum to complete mixing, and the homogeneous mixture was degassed.
Immediately
after mixing, samples for thermal conductivity and mechanical testing were
prepared by
pouring the mixture into molds and/or simple disposable pans that had been
pretreated
with Frekote. The remaining resin was stored at 4 C prior to viscosity
measurements and
as reserve for any additional tests. Table 1 lists the different blend samples
and their
compositions. Table 2 shows the data for samples 4, 5, 7, 9, 11, and 12
(blends with P84
formulations) versus the comparative samples 2, 3, 6, 8, and 10 (blends which
are devoid
of P84 toughener).
[0059] Different blends including PT30, LECY, BA3000, P84, with varying
concentration of BN were compared in Table 2. In Table 1, samples 11, 12, 7
and 9 show
low to high concentration (such as 12.5, 15, 17.5 and 20 wt %) of BN in the
blend with
approximately same amount of P84 toughener. As the BN concentration increases
from
12.5 to 20 wt %, the concentration of other components including P84 was
reduced
proportionately. The thermal conductivity (0.55, 0.62, 0.7, 0.82) and
viscosity (450, 700,
1200, 2400 cP at 100 C) of the blend increased with increased concentration of
BN. The
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data is also presented in FIG. 2, which shows improved thermal conductivity of
the blend
with increased concentration of boron nitride.
[0060] Use of toughener, such as thermoplastic polyimide in the blend
(e.g., blend
of PT30, LECY, BA3000, EN and P84- samples 4, 5, 7, 9, 11 and 12) increased
the
toughening property compared to the blend without a toughener (e.g., a blend
of PT30,
LECY, BA3000, and BN- samples 2, 3, 6, 8, and 10). A comparison of samples
showed
that the toughener P84 increased the mechanical strength of the material;
especially the
fracture toughness (FT) (Table 2). The fracture toughness increased from 1.07
(sample
no. 8) to 1.27 Mps.m1/2 (sample no. 7) at 17.5 wt% BN, and by increasing the
concentration of P84 of the samples from 0 to 8.2 wt %. Similarly the fracture
toughness
(FT) increased from 1.23 (sample no. 10) to 1.43 (sample no. 9) Mps.m1/2 for
20 wt%
BN, by increasing the concentration of P84 of the samples from 0 to 8.0 wt %.
EXAMPLE 4 Blends of PT30, LECY, BA3000, BN and Primaset HTL-300:
[0061] BA3000 was placed in 75 C oven for approximately 1 hour in order to
facilitate transfer of the sample to a resin cup. The resin cup was charged
with BA3000,
LECY, of PT-30 and HTL-300 (15 wt%) and heated for 15-30 minutes in oven. The
components were then mixed for 4 minutes, followed by hand mixing to ensure
any
undissolved BA3000 was distributed throughout the cup, and further mixed for 4
minutes. After cooling on the bench for 15-20 minutes, copper acetylacetonate
in
nonylphenol (50 ppm of copper acetylacetonate relative to reactive resins) was
added and
mixed for an additional 30 seconds. The dry boron nitride was added and mixed
for 30
seconds. Vacuum was then applied to the mixer; mixing continued for additional
3.5
minutes to complete mixing and the homogeneous mixture was degassed.
Immediately
after mixing, samples for thermal conductivity and mechanical testing were
prepared by
pouring into molds and/or simple disposable pans that had been pretreated with
Frekote.
The remaining resin was stored at 4 C prior to viscosity measurements and as
reserve for
any additional tests.
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[0062] The result of mechanical testing for blends with HTL-300 is
presented in
Table 3. Viscosity increased (from 1900 cP to 6500 cP at 100 C) for the
formulations of
HTL-300 as shown in Table 3, such as for sample no.s 13, 14, 15, 18, 16, and
17.
Viscosity increased substantially when the levels of HTL-300 were high enough
to
improve the various mechanical properties. Therefore, HTL-300 based blend may
be
used in applications where low viscosity values are not essential.
Table3: Various properties of a blend with HTL-300
Sample BA LECY PT BN HTL Viscosity Tg Flexural Strain FT
No. 300 30 Wt% -300 ( C) strength (mps.m1/2)
0 cp@l00
13 19.4 38.7 15.5 22.5 3.9 1900 314 8350 1.55
14 19.4 34.8 15.5 22.5 7.7 2300 318 7718 1.65 1.272
15 19.4 31.0 15.5 22.5 11.6 2500 318 8028 1.85 1.305
16 19.4 27.1 15.5 22.5 15.5 4300 - 8597 1.82 1.299
17 19.4 23.2 15.5 22.5 19.4 6500 - 7905 1.62 1.319
18 19.4 31.0 15.5 22.5 11.6 2400 - 9033 1.94 1.198
[0063] The appended claims are intended to claim the invention as broadly
as it
has been conceived and the examples herein presented are illustrative of
selected
embodiments from a manifold of all possible embodiments. Accordingly, it is
the
Applicants' intention that the appended claims are not to be limited by the
choice of
examples utilized to illustrate features of the present invention. As used in
the claims, the
word "comprises" and its grammatical variants logically also subtend and
include phrases
of varying and differing extent such as for example, but not limited thereto,
"consisting
essentially of" and "consisting of." Where necessary, ranges have been
supplied; those
ranges are inclusive of all sub-ranges there between. It is to be expected
that variations in
these ranges will suggest themselves to a practitioner having ordinary skill
in the art and
where not already dedicated to the public, those variations should where
possible be
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construed to be covered by the appended claims. It is also anticipated that
advances in
science and technology will make equivalents and substitutions possible that
are not now
contemplated by reason of the imprecision of language and these variations
should also
be construed where possible to be covered by the appended claims.
27
