Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Q~
_ECIFICATION
Backgxound and Description of the Invention
The present invention generally relates to electroless
nickel plating onto substrates in a manner to reduce the
internal tensile stress of the electroless deposition. More
particularly, the electroless nickel plating according to
this invention utilizes a bath that is sulfur-free and that
includes an unsaturated carboxylic acid tensile stress
reduction agent, which bath lays down a nickel phosphorus
deposit that has a reduced internal tensile stress, relative
to deposits formed from baths that are not in accordance with
this invention.
Electroless deposition of nickel onto metal substrates
has long been known to impart to the substrate enhanced
corrosion resistance, hardness and similax properties. When
electroless nickel deposits are made onto various substrates,
there tends to develop cracking, blistering, sur~ace distortion
and adhesion failure of the electroless deposit. It is
generally accepted that these undesirable properties axe -the
result of deposits that exhibit a high tensile stress and
that these problems can be substantially reduced by laying
down a deposit that is of exceedingly low tensile stress or
that has a compressive internal stress, the latter typically
being particularly effective for maintaining the integrity
of the electroless nickel deposit onto the substrate for
especially long time periods and/or under exceptionally
adverse conditions. It is, therefore, generally observed
that great advantages can be realized by electrolessly plating
from a bath thay lays down a deposit having reduced
tensile stress, it being understood that when used
herein, the term "reduced tensile stress" includes both
lowering the tensile stress (also known as positive or
contractile stress) to as low as zero and also reducing
the tensile stress to such an extent that the stress
becomes compressive (also known as negative or expansive
stress~. Tensile stress is sometimes referred to as
concave internal stress, while compressive stress is
correspondingly referred to as convex internal stress.
It is generally believed that the ~enacity of the
electroless nickel deposit and the advantageous protective
properties thereof with respect to substrates, especially
metal substrates, are enhanced and that the tensile stress
is decreased as the percentage of phosphorus in the electro
less nickel deposit is increased. Heretofore, in order to
reduce the internal tensile stress, it has been necessary
to increase the phosphorus content of an electroless nickel
deposit by reducing the pE~ of the bath to a level at which
the rate of deposition is severely slowed, with the result
that an electroless nickel deposit having especially -
high resistance to failure and low tensile stress had to be
a deposit having an exceptionally high phosphorus content
such as can be plated from a low pH bath exhibiting a slow
rate of deposition. It is of course desirable to form a
nickel-phosphorus deposit having reduced tensile stress and
enhanced deposit integrity within a bath that has a high
deposition rate and that does not have to be carried out
under conditions traditionally recoynized as needed for
reduced tensile stress and increased phosphorus content o
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the deposit.
Baths according to this invention accomplish
these desirable results; such baths are sulfur-free and
include a tensile stress reduction agent that is a bath
soluble unsaturated carboxylic acid and/or derivative thereoE,
the baths also including an electroless bath reducing agent
and a nickel source. The products of this invention exhibit
reduced tensile stress when compared with products plated
~rom baths that are not in accordance with this invention.
It is accordingly a general object of this invention
to improve electroless nickel plating.
Another object of the present invention is to
provide an improved method, bath, and product produced
thereby having reduced tensile or contractile stress.
Another object of this invention is improved
nickel plating onto metallic surfaces which characteristically
bring about high internal stresses, the improvement being a
substantial reduction of these high tensile stresses without
sacrificin~ the plating rate of the electroless bath.
Another object of the present invention is an
improved method of enhancing the stress properties and
corrosion resistance of circuit boards having an electroless
nickel deposit thereon.
Ano-ther object of this invention is irnproved, tensile
stress reduced electroless plating within a sulfur-free
electroless bath.
Another object of the present invention is an
improved nickel phosphorus electroless deposition procedure
by l,7hich maxirnum phosphorus contents are achieved at relatively
high pH values.
Another object of the present invention is an
improved method~ bath and plating product produced thereby
having a residual internal stress that has a negative value,
that is, compressive or expansive~
Another object of this invention is an improved
method, bath, and product pxoduced thereby, which product
is an electroless nickel plated high strength s-teel.
These and other objects of the present invention
will be apparent from the following detailed description
thereof.
The present invention utilizes and incorporates
an electroless nickel bath that plates phosphorus as well
as nickel onto substrates, which bath includes a tensile
stress reduction agent that is an unsaturated carboxylic
acid of the formula: R(COO~)n and b~th soluble derivatives
thereof, wherein R is an unsaturated alkyl chain having
at least two carbon atoms and wherein n is a~ least one.
Further included within the bath is a source of nickel and
a reducing agent which may also be the phosphorus source~
The bath is also sulfur~free; that is, the bath
does not contain sulfur in a form or state th~t will inter-
fere with the stress reduction properties of the bath.
Typically, the bath will be free of sulfur except for sulfur
in its highest oxidation state, for example sulfur added
as nickel sulfate to supply the nickel to be plated by the
bath. Other typical electroless nickel bath additives may
also be included, provided they are sulfur-free and do not
otherwise adversely affect the advantageous properties of
the bath.
With more particular reference to the tensile
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stress reduction agent in accordance with this invention,
R represents an unsaturated alkyl group having a carbon
chain length short enough to obtain bath solubility when the
tensile stress reduction agent is either in its acid ~orm
or in the form of a bath soluble derivative thereof, the
carbon chain length typically being no greater than 20,
preferably no greater than 10, and most preferably no
greater than 6, and n is preferably 2 or more, most
preferably 2. Exemplary unsaturated acid tensile stress
reduction agents include aconitic acid, citraconic acidl
fumaric acid, itaconic acid, maleic acid, and their bath
soluble deriva-tives, ~hich will preferably be present within
the electroless nickel batll at a concentration of at least
about 1 gm/l, with the upper limit being a matter of
economics and bath solubility. There reaches a point,
typically at no more than 10 gm/l, based on the total bath,
at which added stabilizer no longer increases t~e percentage
of phosphoxus deposition.
Referring more particularly to the sulfur~free
characteristic or condition of these baths, it has been
discovexed that the inclusion in these baths of sulfur
that is in an oxidation state lower than its highest oxidation
state, such as that of the sulfate group, will substantially
offset the stress reduction properties imparted to the bath
by the tensile stress reduction agent. Baths according to
this invention avoid the sulfur-containing condition of many
electroless nickel baths that often include sulfur-containing
compounds, either as bath impurities or as an added constituent
for bath stabilizatlon or some other function. The sulfur-
free baths of this invention do not include divalent sulfur
containing compounds such as the organic sulfur-containing
compounds, the organic and inorganic thiocompounds, and the
inorganic sulfides.
Organic sulfur containing compounds include thiourea
and its derivatives, dithioglycol, thioglycolic acid,
2,2-thiodiethanol, 1,2~ethanedithiol, 2-mercaptobenzothiazole,
1,2~benziosothioazine, methionine, and the like, Thiocompounds
include the thiocyanate salts and the thiosulfate salts such
as sodium thiocyanate, potassium thiocyanate, potassium
dithionate, sodium thiosulfate, potassium thiosulfate, and
the like. Included within the organic sulfides are sodium
sulfide, potassium sulfide, sodium polysulfide, potassium
polysulfide, and the like.
A buffer is typically included within baths
according to this invention. Such buffers provide the proper
environment for the tensile stress reduction agent, While
traditional monocarboxylic acid derived buffering systems may
be incorporated in baths according to tnis in~ention, such as
acetic acid-sodium acetate systems, boric acid-borate systems,
and propionic acid-propionate systems, maximum efficiency
of these baths, especially in connection with the enhancement
of phosphorus deposition percentages without adversely
affecting the plating rate~ is attained when the buffer is a
saturated alkyl or aromatic polycarboxylic acid and/or bath
soluble derivative thereof, which may be exemplified by the
formula: R'~COOH)p, wherein R~ is a saturated carbon chain
of from 0 to 20 carbon atoms or an aromatic ring containing
a chain of not more than 20 carbon atoms, and p is at least
2, preferably 2. Preferably Rl is a carbon chain of not more
than 10 carbon atoms, more preferably of not more than 6 carbon
atoms. Especially preferred buffers are those defined
when R' is hetween 2 and 4 and when p is 2, and combinations
of such buffers.
As is typically the case for buffering systems,
these buffers may be provided as acids in combination with
salts or esters thereof. Exemplary buffers in accordance
with this invention include the acid and salt or ester forms
of adipic acid, glutaric acid, isophthalic acid, malonic acid,
oxalic acid, and succinic acid. These buffers are included
within the electroless nickel baths at a total concentration
o at least about 1 gm/l the concentration being varied
according to needs for maintaining pH control, which
concentration will usually be no more than about 40 gm/l
and often not more than about 20 gm/l.
It is also preferred within the baths utilized
and prepared according to this invention to include within
the bath, in combination with the unsaturated carboxylic
acid tensile stress reduction agent, and preferably in further
combination with the saturated alkyl or aromatic carboxylic
buffer systems, a hydroxy and/or amino substituted carboxylic
acid co~plexing agent having the general formula XR''~COOH~S,
wherein X is either or both a hydroxy group or an amino
yroup, including O~I, NH, NH2, +NH, +NH2, ~NH3, it being
especially preferred that the X group is in the alpha position
relative to at least one of the carboxylic groups; R" is
saturated alkyl, heterocyclic, or alkylaryl, and may be
substituted or unsubstituted, the carbon chain length being
between 1 and about 14, and preferably not greater than about
6, especially preferred compounds ha-ving an R" chain length
of not more than 4; and s may be between 1 and 4. The
40~
carboxylic acid group may be in the acid, anhydride, salt
or ester form, provided it is bath soluble.
Exemplary complexing agents include the amino acids
such as ~-alanine, aspartic acid, glutamic acid, glycine,
and the like, as well as citric acid, glycolic acid
(hydroxyacetic acid), iminoacetic acid, iminodiacetic acid,
lactic acid and malic acid. When lactic acid is incorporated
into the bath, the p;ating rate tends tc be enhanced when
compared with that achieved in baths using other complexing
agents, and citric acid has been found to be especially useul
in enhancing the highest possible percentage of phosphorus
deposit. These complexing agents are included within the
baths at a concentration of at least about 1 gm/l, with the
upper limit being dictated by economic considerations and
bath solubility limitations, with a typical upper limit being
no more than about 100 gm/l, and most often no more than
about 50 gm~l.
The bath must also çontain a reduci~g a~ent and a
source of phosphorus, and the well-established manner o~
accomplishing same is to utilize a reducin~ agent that is also
a source of phosphorus ions, such as the widely used reducing
agent sodium hypophosphite. The bath also, of course,
includes a source of nickel, which may be added as a bath-
soluble salt, such as the sulfates, chlorides, sulfamates,
or other anions compatible with these electroless systems.
Typically the baths will be operated at a te~perature of
between about 160 and 212F (about 71 t~ 100C).
Deposition baths prepared with formulations according
to this invention may, if desired, also contain conventional
bath ad~itives that are commonly employed in electroless
nickel deposition batns. Included are traditional buffers
su~h as acetic acid/sodium acetate, other compiexing ayents
and stabilizers, and the like, except for those that add
sulfur to the bath in a form other than the highest oxidation
state of sulfur, which is necessary in order that the bath
will be a sulfur-free bath.
In proceeding with the method according to this
invention, an electroless deposition bath is prepared to
include an unsaturated carboxylic acid compound R(COOH)n
as the tensile stress reduction agent previously defined
herein, a source of nickel, a reducing agent and a source
of phosphorus, said bath being a sulfur-free bath. Also
typically included is a saturated or aromatic polycarboxylic
acid compound R'(COOH)p as the buffer previously defined
herein, usually in combination with a hydroxy and/or amino
substituted carboxylic acid complexing agent of the formula
XR''(COOH)S as previously defined herein. The bath lays /7
down a deposit that is lower in tensile stress than those
laid down by baths which.are not sulfur-fr~e and/or do not include
the tensile stress reduction agent, which deposition
according to this invention lays down a nickel deposit having
a high phosphorus content while avoiding a substantial
slowing of the deposition rate by maintaining the pH at as
high a value as can be attained by the combination of bath
ingredients. ~ore particularly, the bath prepared according
to this invention has a pH above 4.0, which is the pH to
which known baths are often adjusted in order to lay down
an electroless nickel deposit having a high phosphorus conten~.
A typical pH value according to this invention is at least
about 4.5, usually on the order of 5.0, including a pH o~
5.0 + 0.5, preferably a p~ of 5.0 + 0.3, and most preferably
a pH of 5.0 ~ 0.2.
With the bath thus prepared, a substrate is immersed
therein to form a deposit of nickel and phosphorus having
an especially low tensile stress condition for a bath at
such a relatively high pH and that exhibits a rate of
deposition that is rapid for a bath that lays down a deposit
having a high phosphorus content. The method is most
advantageously employed when the substrate upon which the
deposit is made is one that results in a nickel phosphorus
deposit onto the substrate that has a high tensile stress
condition when plating from a bath that is not in accordance
; with this invention. The method according to this invention
results in a deposit having a low internal tensile stress,
which irlcludes substantially zero internal stress as well
as an internal stress in the compressive or negative range.
Although a conventional bath at a pH on the order
of 4.0 will provide nickel deposits having high phosphorus
contents in excess of 10 weight per cent, the plating rate
thereof is on the order of 0,2 mil/hr, while baths according
to this invention~ which have a pH on the order of 5.0, attain
plating rates more on the order of 0.4 through 0.8 mil/hr
while providing a nic~el deposit having the same high phosphorus
content as such a conventional bath. Accordingly, the method ?-~
according to this invention has a plating rate from 2 to 4
times faster than that of con~entional baths which form
nickel phosphorus deposits having a high phosphorus content.
Loadings of baths according to this invention are between
hbOUt 0. 25 and 1.0 square foot per gallon.
Products produced according to this invention have
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a low tensile stress nickel phosphorus ~eposit over a
substrate, including substrates that are known to be
characterized by having nickel phosphorus deposits thereon
which exhibit a high internal tensile stress condition.
Products according to this invention have deposits of a
low tensile stress to thereby enhance the integrity of the
plating onto the metal substrate in order to increase the
~seful life of the product and to reduce the susceptibility
of the product to exhibit metal fatigue leading to
catastrophic metal failure. Such products also resist
cracking, blistering, surface distortion and adhesion
fallure while providing substantial corrosion protection
of the underlying metal substrate.
The invention finds special application for products
of nickel plated high strength steel that are utilized in
highly fatigue inducing situations such as aircraft parts,
turbine blades and the like as well as for nic~el plated
cireuit boards and the like~ The advantageous reduced tensile
stress condition of the products accordin~ to this invention
typically has the greatest advantage when the substrate of
the product is titanium or a ferrous alloy sueh as nickel
alloy steels, nickel-cobalt alloy steel, stainless steel,
or the like. Other substrates that may be advantageously
included within these products are copper, copper alloys,;
beryllium and its alloys, especially beryllium-nickel alloys,
cast iron, magnesium and non-eonductive materials.
Product having the nickel-phosphorus deposits onto
these substrates preferably have a phosphorus eontent of at
least 10 per cent, with the maximum phosphorus content being
limited only by the maximum phosphorus deposition capabiliites
of the -total bath, such maximum amount typically approaching
not more than about 15 per cent phospAorus. The thickness
or the quantity of the nickel phosphorus deposit varies,
of course, with the plating rate and the leng~h of time
that the metal substrate is immersed within the bath,
varying anywhere between a flash deposit and a heavily
built-up plating of several mils. Typical hardness values
for the deposits are between 500 and 600 VHNloo and between
800 and 950 VHNloo after heat treatment at 400C for one
hour.
The following examples are offered to illustrate
the present invention.
EXArlPLE 1
Various sulfur-free baths were formulated in
accordance with this invention, and steel panels were
electrolessly plated, after which the plated steel panels
were subjected to internal stress measurements made with a
Spiral Contractometer. The bath pH was between 4.8 and 5.0
for these several baths, which were maintained at temperatures
between about 190 and 195F. Various unsaturated poly-
carboxylic acid tensile stress reduction agents were adcled
at varying concentrations, and the results of the stress
measurements were as follows, a positive stress value
indicating internal tensile stress, and a negative stress
value indicating internal compressive stress.
Unsaturated Acid
Polycarboxylic Concentration
Acid _ _ (In Bath~ _ _ Stress
- - 13,200 p~i
Aconitic Acid 2 gm/l 6,400 psi
30 Aconitic Acid 5 gm/l 3,800 psi
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Aconitic Acid10 gm/l 3,900 psi
Citraconlc Acid5 gm/l 6,600 psi
Citraconic Acid10 gm/l 4,600 psi
Citraconic Acid15 gm/l 1,500 psi
Itaconic Acid5 gm/l 5,400 psi
Itaconic Acid10 gm/l -4,100 psi
Itaconic Acid15 gm/l -7,800 psi
Maleic Acid 1 gm/l 11,000 psi
Maleic Acid 5 gm/l -4,500 psi
It is observed that the addition of the unsaturated
polycarboxylic acids substantially lowered the tensile stress
of the plated panels even to the extent that, with respect
to certain of the panels, the tensile stress was removed
: completely, and the stress was moved into the compressive
range, which enhanced the fatigue resistance of these panels.
EXAMPLE II
Sulfur-free baths were prepared to include 27 gm/l
of malic acid, 9 gm/l of citric acid, a total of g gm/1
saturated alkyl dicarboxylic acid buffers, 6 gm/l of aconitic
acid, 37 gm/l of sodium hypophosphite, 27 gm/l of sodium
hydroxide, and enough nickel salt to provide 6 gm/l of nickel
as nickel metal. Nine 1010 steel Q panels were electrolessly
nickel phosphorus plated in the bath, three of the panels
having been plated to a thickness of 0.5 mil, three to a
plating thickness of 1 mil r and three were immersed in the
bath until the plating thickness was 2 mils. All nine of -the
samples were exposed to salt spray, 5~, for one thousand hours
in accordance with ASTM B-117 wherein failure was defined as
pitting and/or red rust in three or more locations on the
panel. The testing chamber was open at 24 hour intervals on
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~ ~ ~ 5L~ 4
weekdays, and each panel was examined after the first 3~0
hours of exposure, after whlch the panels were examined
after 72 hour intervals on weekdavs~ All nine of the panels
passed the tests in that there was no pitting or rusting
except for minor occurrences originating at panel edges,
and there was some tarnish on most panels. The internal
stress of the panels was slightly compressive, and they
passed the 180 bend adhesion test.
EXAMPLE III
A sulfur-free bath including 30 gm/l lactic acid,
10 gm/l succinic acid buffer, a 15 gm~l acetic acid and
15 gm/l sodium acetate buffer system, 5 gm/l aconitic acid,
30 gm/l sodium hypophosphite and enough liquid nickel
sulfate to provide 6 gm/l of nickel as nickel metal,
balance being deionized water, the pH of this system being 5.2.
High strength steel panels were plated in this bath
to thicknesses of 0.5 mil, 1.0 mil and 2.0 mils, after which
they were subjected to salt spray for one thousand hours
under the conditions specified in ASTM B-117. These panels
were inspected at the same intervals and to the same extent
as those of Example II, and all nine of these panels passed
the salt spray test.
EXAMPLE IV
Sulfur-free baths generally in accordance with ;
Example II were prepared and successfully plated onto steel
panels. These baths, which had pH values of 4~7, 4.6, 4.8,
5.0, 4.9, 4.9 and 5.0, had plating rates between about 0.4
and 0.5 mil per hour at a tank loading of about 0.25 square
foot p~r gallon and at a temperature between about 190 and
195F. The deposit appearance was hazy bright. A brightener
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was added to some of the baths, and deposit brightness was
found to be enhanced.
E~MPLE V
A sulfur-free bath was prepared to include about
36 gm/l of a combination of citric acid and malic acid
complexing agent, 36 gm/l of sodium hypophosphite, 10 gm/l
of a blend of saturated alkyl dicarboxylic acids, 5 gmfl of
aconitic acid, and enough nickel salt to provide 6 gm/l of
nickel as nickel metal. This bath had a pH of ~.9, the
temperature was maintained between 190 and 195F, and its
plating rate was estimated at 0.33 mil/hr when plating
steel panels. Panels having a plating thickness of 0.5
and 0.6 mil were tested according to ASTM B-117 for one
thousand hours of 5~ salt spray, after which no spots were
observed. Another panel plated in this bath -to 0.5 mil
was subjected to heat treatment at ~00C for two hours, and
again no spots were observed. Four other panels having a
O.S mil deposit from this bath were subjected to heat treat-
ment at 260C for either 4 or 12 hours, and again no spots
were observed after one thousand hours of salt spray.
Another panel having a 0.5 mil deposit laid down by this
bath was subjected to heat treatment at 400~C for one hour,
and six spots were observed after one thousand hours of salt
spray, while another substantially identical panel failed
after 168 hours. Two panels having a 0.5 mil deposit from
this bath were subjected to two hours of heat treatment
at 600C; six small spots appeared after one thousand hours
of salt spray on one of them and the other exhibited some
blistering and nine small spots after one thousand hours.
This ba-tll was also used to plate a 1 mil nickel
phosphorus electroless deposit onto zincate pretreated
aluminum panels. One of them, which was subjected to heat
treatment at 200C for two hours, developed blistering
after 88 hours of salt spray testing while the other one
that was not heat treated exhibited no spots after one
thousand hours of salt spray testing according to ASTM B-1170
EXAMPLE VI
A deposit of 89% nickel and 11% phosphorus
was plated at a rate of deposition of 0.6 mil/hr, the deposit
having an internal stress of 1,000 psi, compressive, this
deposit having been on high strength steel from a sulfur-free
bath at a pH of 5.2 including 30 gmfl lactic acid, 10 gm/l
succinic acid, 5 gm/l aconitic acid, 30 gm/l sodium
hypophosphite and 6 gm/l of nickel.
EXAMPLE VII
A sulfur-free bath was prepared to include 3 gm/l
of aconitic acid, 9 gm/l of citric acid, 27 gm/l of malic
acid, 36 gm/l of sodium hypophosphite, 10 ~m~l of a mixed
dicarboxylic acids buffer system, and enough liquid nickel
sulfate to provide 6 gm/l of nickel as nickel metal. This
bath had a pH of 4.8 and a rate of deposition of 0.4 mil/hr
and plated a nickel phosphorus deposit having 10.5% phosphorus.
Analysis on a Spiral Contractometer showed an internal stress
of zero.
EXAMPLE VIII
A sulfur-free bath having a pH of 4.8 was prepared
to include 5 gm/l of aconitic acid, with the rest of the
bath being substantially identical with the bath of Example VIID
The nickel phosphorus deposit included about 11.5~ phosphorus,
and the plated product had an internal stress of 3,000 psi
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in the negative or compressive range.
EX~MPLE IX
Another sulfur-free bath similar to Example VII
was prepared, except this one included about 7 gm~l of
aconi~ic acid and deposited 12% phosphorus to provide a
plated steel product having an internal stress of 3,000 psi,
compressive.
EXAMPLE ~
A sulfur-free bath similar to that of Example VI,
but having a pH of 5, was founa to have a plating rate of
0.8 mil/hr onto steel plates to form deposits thereon
exhibiting low tensile stress and good corrosion resistance.
The plated product, when observed in photomicrographs, was
found to have a particularly homogeneous appearance.
EXAMPLE XI
A sulfur-free bath including aconitic acid as the
tensile stress r~duction agent, citric acid and malic acid
complexing agents, and saturated dicarboxylic acid buffers
according to this invention, together with sodium hypophosphite
reducing agent and an appropriate source of nickel provided
a nickel phosphorus deposit of ll~ phosphorus to form a
plated product having an internal compressive~ or negative
stress of 2,000 psi. When substantially the same bath was
modified to be sulfur-containing rather than sulfur-free ~y
adding a thiourea stabilizer thereto, this bath still formed
a deposit having 11% phosphorus, but the internal stress of
the plated product was ~,000 psi in the tensile, or positive,
range; that is, the sulfur-containing bath had an internal
tensile stress that was 8,000 psi greater than the sulfur-free
ba~h.
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While in the foregoing specification certain
embodiments and examples of this invention have been described
in detail~ it will be appreciated that modifications and
variations therefrom will be apparent to those skilled in
this art in that this invention is to be limited only by the
scope of the appended claims.
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