Note: Descriptions are shown in the official language in which they were submitted.
WO 92/16589
Pc~rius92io2~aa
1
Title
Electroless Plated Aramid Surfaces
Iiackgrou I of the Invention
Field of the Invention
This invention relates to electroless metal plating of aramid
surfaces wherein the metal is strongly adhered to the aramid substrate and
provides a highly conductive surface. The aramid includes a small amount of
polyvinyl pyrrolidone as an additive polymer which improves the plating
quality,
1o the adhesion, and, consequently, the conductivity of the metal coating.
Description of the Prior Art
Electroless plating is the deposition of a metal film by interaction
of a metal salt and a chemical reducing agent. Electroless plating, in a
general
way, is well known. One of the difficulties in achieving successful
electroless
plating has resided in obtaining good adhesion between the plating substrate
and
the plated metal. While mere encapsulation may suffice for some applications
and some articles, good adhesion of the plated metal is essential for fibers
because the plated metal coating must withstand the forces of further
processing.
United States Patent No. 4,614,684 issued September 30, 1986 on
2 o the application of Ebneth et al. discloses that a fabric of polyp-
phenylene
terephthalamide) (PPD-T) fibers can be nickel plated by a rather complicated
process which includes activating the fabric in butadiene-palladium chloride
and
immersing the fabric in a nickel chloride bath with citric acid and
dimethylamine
borane.
German Published Application 2,820,502 published November 15,
1979 in the name of Ebneth discloses electroless deposition of nickel on
fabrics
of PPD-T and poly(m-phenylene isophthalamide) (M-PDI) by pretreatment of
the fabric with a colloidal solution of palladium in aqueous HCI.
European Patent Publication number 401,740, published
December 12,1990 in the name of Lee, discloses fibers which include PPD-T
and an additive amount of polyvinyl pyrrolidone (PVP) and a process for making
such fibers. It is the ~tbers of that application which are used in practice
of the
present invention.
International Patent Publication WO 90/00634, published January
3 5 25, 1990, discloses a process for plating a polymer wherein metal cations
are
WO 92/16589 ~ 1 U ~ ~ ~l PCT/US92/0214f)
,..
~electrostatically bonded to the polymer in a non-aqueous system and are,
then,
reduced to metal.
Summary of the Inven~Zn,
The present invention provides a pracess for plating an aramid
surface with a strongly adherent metal coating wherein the material which
forms
the aramid surface is a substantially homogeneous composition including from
70 to 98, weight percent, aramid and from 2 to 30, weight percent
polyvinylpyrrolidone (PVP). The process of the invention comprises the steps
of: a) contacting the surface of an aramid structure with an aqueous solution
of
Zo activating metal cations to, thereby, adhere activating metal cations to
the
surface of the aramid structure; b) rinsing the surface of the aramid
structure to
remove nonadherent activating metal catians; c) immersing the rinsed aramid
surface in azz aqueous solution of metal rations to be plated; and d) adding a
reducing agent to the aqueous solution of metal rations; whereby metal rations
15 are reduced to metal and are plated on the aramid surface.
The present invention includes the plated aramid product of this
process. The product is an article comprising the above-mentioned
substantially
homogeneous combination of aramid and PVP having a metal coating strongly I
adhered to a surface of the article.
2 o Detailed Description of the Invention
There has long been a need for conductive aramid fibers which
have durable metallic coatings and that need is especially acute for fibers
which
exhibit high strength and modulus.
Fibers, of aramids, particularly para-aramids, have been difficult to
2 5 plate with a strongly adherent metal coating. Aramid fiber surface
treatments
and pretreatments have, generally, not been effective.
This invention provides a process for electrolessly plating fibers of
aramids in a way that yields a plated fiber product of maintained strength and
modulus and a metal coating which is highly conductive and strongly adherent.
3 o The fibers used in this invention include an additive amount of
polyvinyl pyrrolidone (PVP) distributed throughout the fiber structure. It is
believed that the presence of the PVP assists, in some way, to provide sites
for
adherence of metal in the electroless plating process of the fibers. The
reason
for improvement of plating adhesion is not fully understood.
WO 92/16589 , ~ ~ ~ ~ ~ ~ ~ YCf/US92/02140
3
By "aramid" is meant a polyamide wherein at least 85% of the
amide (-CO-NH-) linkages are attached directly to two aromatic rings. Suitable
aramid fibers are described in Man-Made Fibers - Science and Technology,
Volume 2, Section titled Fiber-Forming Aromatic Polyamides, page 297, yV.
Black et al., lnterscience Publishers, 1968. Aramid fibers are, also,
disclosed in
U.S. Patents 4,172,938; 3,869,429; 3,819,587; 3,673,143; 3,354,127; and
3,094,511.
Additives can be used with the aramid and it has been found that
up to as much as 10 percent, by weight, of other polymeric material can be
blended with the aramid or that copolymers can be used having as much as 10
1 o percent of other diamine substituted for the diamine of the aramid or as
much as
percent of other diacid chloride substituted for the diacid chloride or the
aramid.
Para-aramids are the primary polymers in fibers of this invention
and poly (p-phenylene terephthalamide)(PPD-T) is the preferred para-aramid.
By PPD-T is meant the homopolymer resulting from mole-for-mole
polymerization of p-phenylene diamine and terephthaloyl chloride and, also,
copolymers resulting from incorporation of small amounts of other diamines
with the p-phenylene diamine and of small amounts of other diacid chlorides
with the terephthaloyl chloride. As a general rule, other diamines and other
2 o diacid chlorides can be used in amounts up to as much as about 10 mole
percent
of the p-phenylene diamine or the terephthaloyl chloride, or perhaps slightly
higher, provided only that the other diamines and diacid chlorides have no
reactive groups which interfere with the polymerization reaction. PPD-T, also,
means copolymers resulting from incorporation of other aromatic diamines and
2 5 other aromatic diacid chlorides such as, for example, 2,6-naphthaloyl
chloride or
chloro- or dichloroterephthaloyl chloride; provided, only that the other
aromatic
diamines and aromatic diacid chlorides be present in amounts which permit
preparation of anisotropic spin dopes. Preparation of PPD-T is described in
United States Patents No. 3,869,429; 4,308,374; and 4,698,414.
3 0 PVP is the additive polymer in fibers of this invention. By PVP is
meant the polymer which results from linear polymerization of monomer units
of N-vinyl-2-pyrrolidone with or without the inclusion of small amounts of
comonomers which may be present in concentrations below those which do not
interfere with interaction of the PVP with the PPD-T or with metal cations.
WO 92/16589 PCT/US92/02140
i
_ ~
It has been determined that PVI' of nearly any molecular weight
can be used in practice of this invention. PVP of molecular weights ranging
from as little as about 5000 to as much as about SU0,000 can be used, and all
will
result in the benefits of this invention to some extent. PVP with a molecular
weight of about 10,000 to about 40,000 is preferred, and 10,000 to 24,000 is
most
preferred. PVP with a molecular weight below about 5,000 does not appear to
make a strong complex with the para-aramid PVP combination and is extracted
easily from the fiber. PVP with a molecular weight above about 100,000, causes
an increase in metal demand for a minimum conductivity level. The reason for
1 o that increased metal demand is not understood.
Fibers used in the present invention can be spun using the process
of European Patent Publication No. 401,740, published December 12,1990
wherein an agitated anisotropic mixture of acid solvent, para-aramid, and PVP
is
heated and extruded through a spinneret, into and through a non-coagulating
layer, and into and through an aqueous coagulating bath. Elements of that
process, using PPD-T, alone, are taught in United States Patent No. 3,767,756,
issued October 23, 1973. Fibers used in this invention can be used as-spun or
heat treated.
The electrical qualities of the plated fiber of this invention are
2 o controlled or altered by the amount of PVP which is included in the
fibers. As
stated, PVP can be present in amounts from 2 to 30 weight percent, based on
weight of the PPD-T. The lower limit is a limit of practicality because,
although
the benefits of the invention will be present at any concentration of PVP, the
benefits are difficult to measure at concentrations of less than about 2
weight
2 5 percent.
The upper limit represents the concentration at which some
qualities of the fiber begin to deteriorate due to the presence of excess PVP.
It
should be recognized that PVP is not known to be an outstanding or even
impressive fiber forming material; and that, even though its presence in
fibers in
3 o combination with PPD-T yields fibers of excellent and entirely unexpected
improvements, there is a concentration for the PVP above which some qualities
of the fibers are not acceptable. It is believed that above about 30 weight
percent of PVP, based on PPD-T, the PVP is irreversibly leached from the fiber
into the coagulation bath during manufacture.
WO 92116589 PGT/US92/02140
21068'~~'~~-
Fibers of para-aramid/PVP combination can be plated by the
process of this invention in accordance with the following general process.
An acqueous activation bath is prepared using appropriate
activating cations, among which stannous is preferred. Para-aramid fibers to
be
5 plated are immersed in the bath and agitated to promote the activation. The
fibers are removed from the activation bath and rinsed until there is
substantially na activating ration in the rinse water.
The rinsed and activated fibers are placed in another aqueous
bath which will become the plating bath and which may include a surfactant to
1 o assist in complete wetting. The surfactant is preferred but not necessary
and, if
used, should be nonionic and should be used at a concentration of 1 to 5 grams
per liter of bath.
A metal complex solution is added to the fibers to form the plating
bath. The metal complex solution is made by dissolving the appropriate amount
of metal salt, for example silver nitrate, in water followed by addition of
ammonia until the solution is a light straw color and has a pH of 8-9.5,
preferably, 9. The appropriate amount of metal salt is that amount which will
provide the desired weight of reduced metal as a function of the fiber
material to
be plated. For example, if it is desired to make a "40 percent bath", there
must
2 o be enough silver nitrate to provide reduced silver in a weight of 40% of
the
weight of the fiber to be plated in the bath.
Baths having a wide range of metal concentrations can be used in
practice of this invention. As will be seen in the Examples, the preferred
plating
baths are from about 30 to 45 percent silver. In tests by the inventors
herein,
2 5 baths of 35 to 40 percent silver are most effective and most preferred.
The total volume of the plating bath should be such that the
concentration of the silver nitrate is less than about 10 grams per liter. It
has
been found that metal complex solutions which are too concentrated in the
metal rations may yield free metal granules rather than a strongly adherent
3 0 metal coating.
The plating bath, with immersed fibers, is moderately agitated for
10 to 20 minutes to assure near equilibrium of the system; and then
formaldehyde is added to the bath as a reducing agent. The formaldehyde is
generally added as a 37, weight percent, aqueous solution; and is added in an
35 amount to constitute a mole ratio with the silver of 1.1/1 to 2/1, or more.
The
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WO 92/16589 PCT/US92/02140
~.~~~f;.: d~
formaldehyde can be added all at once or in increments over a period of time.
For example, it has been found to be useful to add the formaldehyde in 10%
increments aver the course of an hour, or so. The agitatian is preferably
continued until an analysis of the plating bath reveals silver residuum of
less
than about 5 parts per million. The plated material can then be rinsed and
dried.
All of the above steps can be conducted with the various baths at
temperatures of 10 to 60°C, and preferably 20-40°C.
Instead of the silver complex solutions, other solutions can be used
1o to plate other metals. Nickel, copper, gold, and the like can be plated by
these
processes. If desired or required, a bath having a low concentration (2 to S%)
of
silver can be used for additional surface activation and then complex
solutions of
other plating metals can be used. A nickel bath can include 0.2 molar nickel
chloride and 0.2 molar sodium hypophosphite adjusted to a pH of 9-9.5 with
1s ammonia. A copper bath can include 30 grams per liter copper sulfate, 100
grams per liter Rochelle salt (potassium sodium tartrate), and 50 grams per
liter
37% formaldehyde solution adjusted to a pH of 11-12 with sodium hydroxide.
Test Methods
Resistivity
2 o Resistivity of plated fibers can be determined as ohms per inch of
fiber or, in the case of woven or knitted fibers, as ohms per square. Absolute
values for either determination are a matter of fiber size and fabric weave in
addition to amount of plated metal and continuity of the plating.
'f'o determine ohms per inch on a fiber, the dry fiber is placed on a
2 5 flat surface and the probes of an ohmmeter are pressed firmly on the fiber
at a
one-inch spacing.
To determine ohms per square on a woven fabric, ohmmeter
probes which are one inch long are pressed firmly on the fabric parallel and
at a
one inch spacing.
3 o In each case, the resistivity is the ohms indicated by the ohmmeter
reading.
Metal Adhesion
Adhesion of the plated metal is measured on fabrics of plated
fibers by rubbing the plated surface to determine the ease by which the metal
is
3 5 removed from the substrate. Degree of adhesion can be measured in a
W~ 92/16589 ~ ~ PG'I'/US92/02140
7 , . ; ~, ;
subjective way by rubbing a plated article by hand to estimate the dagree of
metal rubbed off as compared with a control material.
Objective means for measuring adhesion are not well developed,
The inventors have elected to use a so-called "Crocking Test" developed and
certified by the American Association of Textile Chemists and Colorists
(AATCC) for determination of color fastness in dyes. In the Crocking Test,
identified as AATCC Test Method 116-198, a Rotary Vertical Crockmeter is
used to perform a controlled, rubbing on a fabric to be tested. Results of the
Crocking Test are presented on a scale of 1 to S with S representing no
transfer
s o and 1 representing transfer of a very high degree.
Description of the Prefern~l Embodiments
1n the Examples which follow, all parts are by weight unless
specifically stated to be otherwise.
Exam,~le 1
Ten foot skeins of several yarns, as identified below, were each
soaked for 20 minutes in a solution of 45.6 grams of stannous chloride in 400
milliliters of water having the pH adjusted to about 1.5 with HCI. The skeins
were each rinsed with water and then placed in a solution of 6.8 grams of
silver
nitrate, 3 milliliters of nitric acid solution, and 5.5 grams of 3?%
formaldehyde,
2 o all in 400 milliliters of water, to which ammonium hydroxide was added to
reach
a pH of about 9.
Silver spontaneously plated onto each yarn and the yarns were
rinsed and inspected. The yarns in the test were all 1000 filament - 1500
denier
yarns. One of the yarns was made from filaments 85% PPD-T and 15% PVP
2 5 (40,000) and one of the yarns was made from filaments 7S % PPD-T and 25 %
PVP. A control yarn was 100% PPD-T.
While all of the yarns exhibited some degree of electrical
conductivity, the plating was poorly adhered to the control (it was easily
rubbed
off) and the plating was strongly adhered to the fibers containing PVP (it was
s o difficult to rub off).
The process which was used to electrolessly plate aramid surfaces
for the following Examples, in accordance with this invention, is as follows:
Yarn of the kind described in the particular example was knitted
into knit/deknit tubing. S00 grams of that tubing were soaked for about six
3 5 minutes in an aqueous 0.1 molar solution of stannous chloride with the pH
WO 92/16589 . 1'(.'T/US92/U2140
210G~77
_ 8
about 9. Enough formaldehyde was used to provide a molar excess based on the
silver.
The mole ratio of formaldehyde to silver can range from 1.1 to 2 or higher,
Example 2
Yarn of 85% polyp-phenylene terephthalamide) (PPD-T~ and 15% PVP
s (40,000 MW) having 1000 filaments and 1500 denier, and in the farm of a
knit/deknit
tubing, was plated, by the process of this invention, in a 40% silver bath, as
described
above. The fibers were spun as taught in Example 1 of previously mentioned
European
Patent Publication No. 401,740.
As comparisons, yarn of 100% PPD-T having 1000 filaments and 1500
to denier and in the form of a knit/deknit tubing was plated using the same
procedure as
described above; and, also, was plated using a process from the prior art. All
comparison plating tests used a 40% silver bath.
PPD-T yarn was, also, plated in accordance with the procedure set out in
German Patent No. 1,197,720 naming Ebneth as the inventor. The PPD-T surface
was
15 activated by immersion for about 3 minutes in a bath of colloidal
palladium, rinsed,
soaked fax about 2 minutes in a 5% solution of sodium hydroxide, and rinsed.
The
fibers were then plated in a 40% silver bath using the same procedure as
described
above. The bath of colloidal palladium was made by pouring together equal
volumes of
a 0.2 molar solution of stannous chloride and a 0.2 molar solution of
palladium chloride
2o and, then, adjusting the pH to less than 1 using HCI.
The fabric of the plated yarns were tested for resistivity and plating
adhesion. The plating adhesion test was the "Crocking Test", as described
above.
T 12
Material Resistivity Adhesion
Oh~L~Sluarel in
This Invention 0.2 3.5
PPD-T Only 2.4 1
[Ebnetha 0.8 1
example 3
As a further test of the improved adhesion between fiber substrates and
plated metal, yarn of this invention, as plated in Example 2 above, and yarn
of 100%
PPD-T were plated by the procedure as described above and the plated yarns
were
conducted through a device wherein the fibers are drawn through an extrusion
die to be
encased by a thermoplastic matrix material and then the encased fibers were
chopped
35 to provide pellets for later molding processes. An example of such a device
is described
in U.S. Patent 4,312,917 and the process is commonly referred to as
"poltrusion".
CA 02106877 2001-03-O1
WO 92/16589 PCT/US92/02140
9
The thermoplastic matrix material was nylon 6,6; the processing
temperature range was 26s-295°C, and the machine settings were such
that the
encased product would be about 35 weight percent fiber. Processing of the yarn
plated
by this process went smoothly and the encased product was cut into pellets
about 0.25
s inch long. Processing of the PPD-T only yarn was difficult because the
plated metal
kept flaking off of the fibers and interfering with extrusion of the matrix
resin. Some
encased product was, however, collected and tested, as a comparison, with
pellets of the
encased fibers of this invention
To test the degree of plated metal loss from the fibers due to the
io extrusion process, an amount of each of the pellet products was dissolved
in a solvent of
equal volume parts of methylene chloride and trifluoroacetic anhydride. The
fibers in
each sample were separated from the solvent, were rinsed with acetone, and
were
pressed into a mat. The resistivity of the fibers of this invention was
determined to be
ohms/square and the resistivity for the PPD-T only fibers was determined to be
L5 greater than 500,000 ohms/square; thus, indicating that the adhesion of
metal on the
fibers of this invention is much improved over that of the control materials.
EaamDle 4
A series of fibers made from PPD-T with varying amounts of PVP (40,000
MVO was plated by the process of this invention, as described above, and
tested for
2o resistivity. The plating bath for this example included 25% silver.
The plated fibers were tested for resistivity. The results are shown in
Table 3.
The fibers were spun to have PVP concentrations as noted in Table 3. As
a control comparison, fibers made from PPD-T only, with no PVP, were, also,
plated
25 under the same conditions.
Table 3
PVP Resistivity
(,WP,~ght %)~ (ohms/inchl
0 (Control) Off Scale'
30 9 1,800,000
12 45,000
1,400,000
' "Off Scale" is greater than 200,000,000.
This example provides indication that there is a range of PVP
3s concentrations useful for obtaining the plated fibers of this invention.
While it may
appear that fibers made using 12% PVP yield remarkably lower resistance
CA 02106877 2001-03-O1
WO 92/16589 PCT/US92/02140
than do fibers made using higher and lower PVP concentrations, such results
are
believed to vary considerably with change in other parameters of the process.
The benefits of this invention are realized over a broad range of PVP
concentrations, from about 3 to 30 percent; and the preferred range of
5 concentrations is about 9 to 15 percent.
As a subjective test of silver plating adhesion, the fibers of this
example were rubbed by hand and the results were noted. The silver was
difficult to rub off of the fibers which had PVP content and the silver rubbed
off
very easily from the Control fibers with no PVP content.
In accordance with the procedure set out above, PPD-T fibers,
including 15% PVP (40,000 MW), were plated with silver in baths having a
series of varying silver concentrations.
~5 The plated fibers were tested for resistivity. The results are
shown in Table 4.
. , able 4
Silver in Bath Resistivity
l%a- lohms,/~uarel"
Off Scale
25.1
2.9
0.95
37 0.44
0.48
SO 4.37
60 Off Scale.
"Off Scale" is greater than 200,000,000.
Fibers of the highest conductivity were obtained using baths which
2 o contained silver in concentrations from 25 to about 55%. At bath
concentrations
of less than 20% and greater than 60%, the plating was ineffective to provide
conductive coatings. Fibers of the highest conductivity had silver coatings
which
were most difficult to rub off, thus, indicating the greatest silver coating
adhesion
to the fiber substrate.
CA 02106877 2001-03-O1
WO 92/16589 PCT/US92/02140
11
In accordance with the procedure set out above, PPD-T fibers,
including 15% of PVP of various molecular weights, were plated in baths of
50% silver concentration.
The plated fibers were tested for resisdvity. The results are shown
in Table 5.
Table 5
PVP Resistivitv
_~MWI ~ohms/inl ~ohms,/~uare)
40000 27 6.8
24000 5.9 1.6
10000 1.4 0.1
There is a wide range of acceptable molecular weights for the PVP
used in practice of this invention. For reasons of solubility limits -- that
is,
10' excessive solubility at very low molecular weights and excessive solution
viscosity
at very high molecular weights -- the practical molecular weight range for PVP
appears to be about 5,000 to 500,000. The preferred range is about 10,000 to
24,000.
The silver was very difficult to rub off of the plated fibers in this
example.
w0 92/16589 YCf/1JS92/0214()
2106~7'~
Example 6
In accordance with the procedure set out above, PPD-T fibers,
including 15% of PVP of various molecular weights, were plated in baths of
50% silver concentration.
The plated fibers were tested for resistivity. The results are shown
in Table S.
Table 5
P~ R i ivi~y
~(~~. ~h1 In (ohmsf a ~a_rel
40000 27 6.8
24000 5.9 1.6
10000 1.4 0.1
There is a wide range of acceptable molecular weights for the PVP
used in practice of this invention. For reasons of solubility limits -- that
is,
1 o excessive solubility at very low molecular weights and excessive solution
viscosity
at very high molecular weights -- the practical molecular weight range for PVP
appears to be about 5,000 to 500,000. The preferred range is about 10,000 to
24,000.
The silver was very difficult to rub nff of the plated fibers in this
example.
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