Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROPLATING CLEANED GRAPHITE
FIBERS WITH METAL
The present invention relates to an improved process
for the production of bundles of fibers having conductive graphite
cores coated with thin, uniform, adherent layers of electro-
deposited metals. The process improvement comprises providing the
cores free of adsorbed organic compounds prior to and during
electrodeposition of the metal. Adsorbed organic compounds may
interfere with the electrodeposition of uniform, adherent metal
coatings.
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BACKGROUND OF THE INVENTION
Bundles of high strength fibers Gf non-metals
and semi-metals, such as carbon, boron, silicon carbide,
and the like, in the form of filaments, mats, cloths
and chopped strands are known to be useful in reinforc-
ing metals and organic polymeric materials. Articles
comprising metals or plastics reinforced with such
fibers find wide-spread use in replacing heavier com-
ponents made of lower strength conventional materialssuch as aluminum, steel, titanium, vinyl polymers,
nylons, polyesters, etc., in aircraft, automobiles,
office equipment, sporting goods, and in many other
fields.
High strength carbon fibers are made by heating
polymeric fiber, e.g., acrylonitrile polymers or copoly-
mers, in two stages, one to remove volatiles and carbon-
ize and other to convert amorphous carbon into crystal-
line carbon. During such procedure, it is known thatcarbon changes from amorphous to single crystal then
orients into fibrils. If the fibers are stretched
during the graphitization, then high strength fibers are
formed. This is critical to the formation of the
boundary layer, because as the crystals grow, there are
formed high surface energies, as e~emplified by incomplete
bonds, edge-to-edge stresses, differences in morphology,
and the like. It is also known that the new carbon
fibrils in this form can scavenge organic compounds,
because the surfaces behave like activated carbon, to
produce non-carbon surface layers which are firmly
adsorbed thereto.
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Numerous unsuccessful attempts have been reported to
provide such filaments, especially carbon filaments, in a form
uniquely suitable for reinforcing metals and plastics. Many have
involved depositing layers of metals, especially nickel and copper,
as thin surface layers on the filaments. Such a coated fiber was
then to be used in a plastic or metal matrix. The metals in the
prior art procedures have been vacuum deposited, electrolessly
deposited, and electrolytically deposited, but the resulting
coatings were poorly bonded resulting in poor translation of
physical properties from the fiber to the matrix.
It is suggested in Canadian Application 457,193 (see also
United States Patent 4,661,403) that if electroplating is selected,
and if operating conditions are chosen to require a very high order
of external voltage, then uniform, continuous adherent, thin metal
coatings can be provided to fibers, especially carbon fibers.
High voltage is used in the present invention. Its use is
believed to provide energy sufficient to transport metal ions
through the boundary layer to provide uniform nucleation of the
metal on the fibers directly. Yarns or tows comprising the thin
metal coatings on the fibers, woven cloth, yarns, and the like,
according to the present invention can be knotted and folded
without the metal flaking off. The coated fibers are distinguish-
able from any of the prior art because they can be sharply bent
without disrupting the metal-carbon bond, as observed with
electroless metal or vacuum deposited coated fibers. The fibers
are characterized by bond strengths which provide that even if the
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coating cracks, it does not peel off. In other words, coated
fibers of the present invention are distinguishable from those of
the prior art because (i) they are con-
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tinuous; (iil the majority of the fibers are uniformly metal
coated; and (Iii) the bond strength (metal~to-core) on the maj-
ority of the fibers is very high. When scaling up production of
the electoplated fiber bundles to higher production rates, and
when using a variety of sources of graphite fibers, production
rates are increased and fiber quality is improved with careful
attention to the presence or absence of adsorbed organic compounds.
It was discovered that providing and maintaining the fibers free
of organic materials produced uniform plating and increased pro-
duction rates.
BRIEF DESCRIPTIOM OF THE DRAWINGS
The invention may be more readily understood by refer-
ence to the accompanying drawings in which;
Figure 1 is a transverse cross sectional view of a metal
coated fiber produced by the improved process of this invention.
Figure la is a longitudinal cross sectional view of a metal
coated fiber produced by the improved process of this invention.
Figure 2 and 2a are transverse and longitudinal cross sect-
ional views of, respectively, of a core fiber coated with metal
not according to this invention, illustrating the non-uniform
plating resulting from organic compound contamination.
Figure 3 is a view showing an apparatus for carrying out the
process of the present invention.
Figure 4 is a magnified photographic view of a metal coated
fiber according to this invention which has a uniform coating
because a core fiber free of organic compound contamination was
provided.
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Figure 5 is a magnified coating of a fiber not according to
this invention which has a non uniform coating because a core
fiber contaminated with adsorbed organic compounds- was provided.
All the drawings repres-ent schematics of the articles
des-cribed.
SUMMARY OF THE INVENTIO~
According to the present invention, in a process for
the production of coated fibers, said process comprising:
(,a), providing a continuous length of a plurality of elect-
rically conductive graphite core fibers,
(b), continuously immersing at least a portion of the lengthof s~aid fibers in a solution capable of electrolytically depositing
at least one metal,
(c) providing a quantity of electricity while applying an
external voltage between the fibers and an electrode immersed in
the solution, which voltage is in excess of what is normally re-
quired to cause metal deposition, whereby (i) the metal is re-
duced on the surface of the fibers, (,ii) the metal nucleates sub-
stantially uniformly onto the surface of the fibers and (iii) there
is provided a substantially uniform, firmly adherent layer of
metal on said core,
the improvement comprises providing graphite core fibers
whose surfaces are substantially free of adsorbed organic compounds
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61109-7298
which interfere with the electrodeposition of the metal, wherein
said adsorbed organic compounds are removed by solvent cleaning,
by chemical cleaning or by high temperature cleaning.
. .
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The term "substantially free" as used herein and in
the appended claims means avery lowamount of adsorbed organic
compounds, i.e., an amount which does not interfere with the
plating, and contribute to a non-uniform coating. It is very
difficult to remove or avoid all traces, but usually an amount
covering up to about five percent of the surface area of a short
length can be tolerated, without serious detriment.
In one preferred embodiment, a surface substantially
free of adsorbed organic compounds is provided by removing
substantially all of any such compounds by contacting the graphite
core fibers with a solvent for such compounds. A useful solvent
is l,l,l-trichloroethane. This can be used in a conventional
degreaser. Because of the high purity requirements of the cleaned
fiber, the solvent must be changed or purified frequently.
In another preferred embodiment, the surface free of
adsorbed organic compounds is provided by removing substantially
all of any such compounds by contacting the graphite fibers with
an aqueous solution of an inorganic cleaning agent, preferably
an inorganic phosphorus-containing compound including but not
limited to phosphates and polyphosphates, and especially preferably
trisodium phosphate. Typically a solution in water of 60 g./l. of
trisodium phosphate can be used and heating to a temperature of
about 140F. to boiling facilitates removal of any organic
compounds when the bundles of fibers are passed through the
solution. Because the inorganic cleaning agents are, in most
cases, alkaline when dissolved in water, and the subsequent plating
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solutions are generally acidic, it is a preferred feature to
neutralize the pretreated yarns or tows with a dilute inorganic
acid prior to immersion in the electroplating solution.
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110-022
~ydrochloric acid or other mineral acids are suitable
in aqueous solution.
In its other primary aspect, the present
invention includes maintaining the graphite surface free
of organic compounds by excluding them from the electro-
plating solution, or permitting the presence therein of
only organic compounds which are capable of reduction to
free sulfur at the cathodic surface (the clean graphite
fibers themselves). This improvement comprises in
essence, rigorously excluding conventional electroplating
additives which comprise or include organic compounds --
such as wetting agents, like sodium lauryl sulfate,
chelating agents, and brighteners which include organic
components. Saccharin , for example, is suitable as an
organic grain modifying agent Ibrightener) because, on
the cathode, saccharin has the rather unique ability to
reduce to elemental sulfur.
Other preferred features include carrying out
the process in an electrolytic bath which is recycled
into contact with the fibers immediately prior to
immersion in the bath so as to provide increased current
carrying capacity to the fibers and replenishment of the
electrolyte on the surface of the fibers. In preferred
embodiments, coating metals will be nickel, silver,
zinc, copper, lead, cadmium, tin, cobalt, gold, indium,
iron, palladium, platinum, tellurium, and alloys of
the foregoing, without limitation.
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DETAILED DESCRIPTION OF THE INVENTION
Referring to Figs. 1 and la continous yarns and
tows for use in the core 2 according to the present
invention are available from a number of sources
commercially. For example, suitable carbon fiber yarns
are available from Hercules Company, Hitco, Great Lakes
Carbon, Union Carbide Companyand similar sources in the
United States, and overseas. All are made, in general,
10 by procedures described in U.S. 3,677,705. The fibers
can be long and continuous or they can be short, e.g.,
1 to 15 cm. in length. As mentioned above, all such
carbon fibers may have a thin layer of adsorbed materials,
such as organics, like oils, waxes, etc. that are on
their surface. These layers may be continuous or non-
continuous.
Referring to Figs. 2 and 2a, adsorbed organic
compounds 3 are shown to be adsorbed to the surface of
fiber 2- Plating occurs at bare spots on the carbon and
grows upwardly and outwardly, somewhat like a mushroom,
as shown, to produce non-uniform deposits.
Figs. 4 and 5 are electron photomicrographs
showing, respectively, uniform and non-uniform plated
coatings, the latter being bumpy, while the former is
smooth.
Removing adsorbed organic compounds can be
carried out in any known way. Preferably, it will be
done either with solvents, in a degreaser, or in washing
baths with aqueous inorganic cleaning agents and
optional neutralization.
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110-022
.
10 _
Referring to Fig. 3, shown schematically is an
arrangement of elements in a continous yarn or tow
plating processing apparatus in which yarns or tow 24 is
removed from storage reel 40 and sent through hot solvent
degreaser 36 (or hot inorganic cleaning agent solution
bath 36). Rinsing conduits and baths (not shown) can be
included and, if the yarn or tow retain alkaline inorganic
cleaner residues, they can be neutralized in one more
optionalneutralizers 34 which can contain an inorganic
acid, such as 10 percent aqueous hydrochloric acid. The
cleaned fibers can be held in protected storage, free of
organic contaminants, or sent directly to electroplating.
Referring again to ~ig. 1, metal layer 4 will
be of any electrodepositable metal, and it will be
electrically continuous. Two layers, or even more, of
metal can be applied and metal can be the same or differ-
ent, as will be shown in the working examples. In any
case, the innermost layer will be so firmly bonded to
core 2 that sharp bending may cause the coating to
fracture, but it will not peel off. The metal will sub-
stantially not flake off when the fiber is twisted or knotted,
which is a problem in fibers metal coated according to
the prior art.
Formation of the metal coating layer by the
electrodeposition process used in this invention can be
carried out in a number of ways. For example, a plurality
of core fibers can be immersed in an electrolytic bath
and through suitable electrical connections the high
external voltage used herein can be applied. The pro-
cedure may be carried out in a continuous fashion on a
moving tow of fibers.
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To overcome the problem oE fiber burnout because of
the high currents or power, to keep them cool enough outside the
bath, one can pour water on the fibers, for example, but it is
preferred to operate in an apparatus s~lown schematically in
Figure 3. Electrolytic bath solution 8 is maintained in tank 10.
Also included are anode baskets 12 and idler rollers 14 near the
~ottom of tank 10. Two electrical contact rollers 16 are located
above the tank. Stripped and cleaned tow 24 is pulled by means
not shown off feed roll 26, over first contact roller 16 down
into the bath under idler rollers 14, up through the bath, over
second contact roller 16 and into take up roller 28. Optional,
but very much preferred, is a simple loop comprising pump 18,
conduit 20, and feed head 22. This permits recirculating the
plating solution at a large enough flow rate to sufficiently cool
the tow and pumping the solution onto contact rollers 16. Dis-
charged just above the roller, the sections of tow 24 entering and
leaving the solution are totally immersed and thus cooled. At the
high current carried by the tow, the heat generated in some cases
might destroy them before they reach or after they leave the bath
surface without such cooling. The flow of the electrolyte fac-
ilitates electrical contact between the rollers 16 and all filaments
in the tow. More than one plating bath can be used in series, and
is preferred, and 3 to 4 are used for reasons of economy, and the
fibers can be rinsed free of electrolyte solution, dried, chopped,
woven into fabric, all in accordance with conventional procedures .
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following Examples illustrate the present
invention, but are not intended to limit it.
EXAMPLE 1
Four tows (fiber bundles) of 12,000 strands
each of 7 micron graphite fibers were continuously drawn
through a solution of 8 oz./gal. of trisodium phosphate
heated at 180-200F., then through two rinse tanks. The
effluent from the second rinse tank was returned to the
first. The tows were then passed through a neutralizing
tank containing 10% aqueous solution of C.P. hydrochloric
acid (35%), then through two rinse tanks. The effluent
from the second of these two rinse tanks was returned to
the first, and the waste from all tanks was combined for
self-neutralization.
In a continuous electroplating system, a plat-
ing solution was provided having the following composi-
tion:
INGREDIENTS AMOUNT
nickel sulfate (NiSO4.6H20) 40 oz. /gallon
nickel chloride (NiCl2.6H2O) 12-20 oz./gallon
boric acid (H3BO3)5-8 oz./gallon
The bath was heated to 140-160F., and had a pH of
3.8-4.2 .
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O
The anode baskets were kept filled with el~ctro-
lytic nickel pellets and four tows (fiber bundles) of
12,000 strands each of 7 micron carhon fibers stripped
of organic contaminants as described above were continu-
ously drawn through the bath at a tow speed of 5 feet/min. and with 120 amps. current, adjusted to give 5
ampere-minutes per 1,000 strands total. The voltage
drop from anode to cathode was 30 volts. ~t the same
time, electrolytic solution was recycled through a loop
into contact with the entering and leaving paths of the
tow. The tow was next passed continuously through an
identical bath, at a tow speed of 5.0 ft./min. with
180 amps. current. The final product was a tow of
high strength coated fibers according to this invention
comprising a 7 micron fiber core and about 50% by weight
of the coated fiber of crystalline electrodeposited
nickel adhered firmly to the core. The metal coating
was uniform and free from bare spots.
Whei~ a length of the fiber was sharply bent, then
examined, there was no circumferential cracking on the
metal coating in the tension side of the bend. The tow
could be twisted and knotted without causing the coating
to flake or come off as a powder.
E~AMPLE 2
When the procedure of Example 1 was repeated,
substituting graphite tows which had been passed through
hot li~uid and condensinq vapors of l,l,l-trichloroethane
to strip them free from organics, uniform metal coatings
were ultimately produced.
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To illustrate the adverse effects of organic compounds,
when the concentration of organic impurities in the degreaser of
Example 2 was allowed to build up, the organics recontaminated the
graphite fibers and the nickel coating became non-uniform. When
the trisodium phosphate cleaning bath in Example 1 was replaced with
wetting agents containing organics (Wyandotte BN* cleaner, Oakite
190* cleaner of Ivory Liquid* detergent) ultimately the nickel
plating was non-uniform.
Many variations of the present invention will suggest
themselves to those skilled in this art in light of the above,
detailed description. For example, instead of nickel plating
solutions, those capable of depositing silver, zinc, copper, lead
and gold can be substituted. All such variations are within the
full intended scope of the invention as defined in the appended
claims.
* Trade mark
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