Note: Descriptions are shown in the official language in which they were submitted.
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FABRIC STRUCTURE
FIELD OF THE INVENTION
This invention relates to fabric structures based on silver wires, which may
comprise the whole or part of said structures.
BACKGROUND TO THE INVENTION
The literature on production of silver wire is relatively sparse. For example
US
Patent 6627149 (Tayama et al.) discloses the production of silver wire of
relatively
large diameter and high purity for use in recording or image transmission
applications.
The literature concerning woven structures based on silver is also sparse.
Such
woven structures have mainly been based on strips, strands or filaments
braided
together, see US-A-240096 (Crane), US-A-253587 (Crane) and US-A-5203182
(Wiriath). However, US-A-2708788 (Cassman et al) discloses silver mesh or foil
through which material is to be evaporated during the manufacture of
television tubes,
the mesh being tautened by depositing gold thereon and alloying the silver and
gold to
bring about shrinkage of the mesh. US-A-5122185 (I4oche1la) discloses mesh of
precious metal used as so-called "getters" in recovery of platinum from a gas
stream
from the oxidation of ammonia. The mesh is preferably of pure palladium, but
may also
be an alloy of palladium with one or more metals selected from nickel, cobalt,
platinum,
ruthenium, iridium, gold, silver and copper.
It is known to knit metal wires or fibres e.g. as in US-A-2274684 (Goodloe),
but
existing knitted metal fabrics are predominantly of ferrous alloys. US-A-
5188813
(Fairey et al; Johnson Matthey) discloses weft-knitted fabrics consisting
essentially of
interlocking loops of fibres of precious metal selected from platinum group
metals,
gold, silver and alloys thereof using circular or flat-bed knitting machines,
with
platinum or platinum alloys for use as catalyst gauzes being preferred. Fairey
et al
found that wires of platinum alloy or of metals with similar mechanical
properties could
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not be knitted effectively and that attempts to do so resulted in fibre
breakage and
machine jams because the tensile strength of the metal fibres was insufficient
to
withstand the frictional forces in the knitting process. The solution
disclosed was to feed
the metal fibre with a supplementary fibre that acted as a lubricant, the
supplementary
fibre preferably being in the form of a multi-strand rather than a
monofilament and the
strands surrounding the metal wire to minimise metal-to-metal contact. After
knitting,
the supplementary fibre may be removed by dissolving in a solvent or by
pyrolysis. WO
92/02301 (Heywood) discloses warp-knit fabric of platinum, palladium or
rhodium
wires e.g. using tricot, raschel or jacquard knitting to give catalyst gauzes
that are more
flexible or open than woven gauzes and that are less likely to warp under
thermal stress.
Knitting is facilitated either by lubricating the wire with a lubricant such
as starch or
wax or by feeding a supplementary fibre. A particular structure of fine mesh
warp-knit
fabric based on wires of noble metal and for use as a catalyst is disclosed in
US-A-
6089051 (Gorywoda et ao. None of the above references discloses or suggests
forming
knitted structures based on fine silver or on a silver alloy, and our
experience is that
standard Sterling silver has insufficient tensile strength for effective
machine knitting.
Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche)
discloses a novel silver alloy that maintains the properties of hardness and
lustre
inherent in Ag-Cu alloys while reducing problems resulting from the tendency
of the
copper content to oxidise. The alloys are ternary Ag-Cu-Ge alloys containing
at least
92.5 wt% Ag, 0.5-3 wt% Ge and the balance, apart from impurities, copper. The
alloys
are stainless in ambient air during conventional production, transformation
and finishing
operations, are easily deformable when cold, easily brazed and do not give
rise to
significant shrinkage on casting. They also exhibit superior ductility and
tensile
strength. Germanium is stated to exert a protective function that was
responsible for the
advantageous combination of properties exhibited by the new alloys, and is in
solid
solution in both the silver and the copper phases. The microstructure of the
alloy is
constituted by two phases, a solid solution of germanium and copper in silver
surrounded by a filamentous solid solution of germanium and silver in copper
which
itself contains a few intermetallic CuGe phase dispersoids. The germanium in
the
copper-rich phase was said to inhibit surface oxidation of that phase by
forming a thin
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GeO and/or Ge02 protective coating which prevented the appearance of firestain
during
brazing and flame annealing. Furthermore the development of tarnish was
appreciably
delayed by the addition of germanium, the surface turned slightly yellow
rather than
black and tarnish products were easily removed by ordinary tap water.
Patents US-A-6168071 (Johns) and EP-B-0729398 (Johns) disclosed a
silver/germanium alloy which comprised a silver content of at least 77 wt %
and a
germanium content of between 0.4 and 7%, the remainder principally being
copper
apart from any impurities, which alloy contained elemental boron as a grain
refiner at a
concentration of greater than 0 ppm and less than 20ppm. The boron content of
the alloy
could be achieved by providing the boron in a master copper/boron alloy having
2 wt %
elemental boron. It was reported that such low concentrations of boron
surprisingly
provided excellent grain refining in a silver/germanium alloy, imparting
greater strength
and ductility to the alloy compared with a silver/germanium alloy without
boron.
Argentium (Trade Mark) sterling comprises Ag 92.5 wt % and Ge 1.2 wt %, the
balance
being copper and about 4 ppm boron as grain refiner. The Society of American
Silversmiths maintains a website for commercial embodiments of the above-
mentioned
alloys known as Argentium (Trade Mark) at the web address
http;//w_ww.silvqrsrnithinZcom/ 1 arcntium.htrn.
US-A-6726877 (Eccles) discloses inter alia an allegedly fire scale resistant,
work hardenable jewellery silver alloy composition comprising 81-95.409 wt %
Ag,
0.5-6 wt% Cu, 0.05-5 wt% Zn, 0.02-2 wt% Si, 0.01-2 wt % by weight B, 0.01-1.5
wt%
In and 0.01-no more than 2.0 wt% Ge. The germanium content is alleged to
result in
alloys having work hardening characteristics of a kind exhibited by
conventiona10.925
silver alloys, together with the firestain resistance of allegedly firestain
resistant alloys
known prior to June 1994. Amounts of Ge in the alloy of from about 0.04 to 2.0
wt%
are alleged to provide modified work hardening properties relative to alloys
of the
firestain resistant kind not including germanium, but the hardening
performance is not
linear with increasing germanium nor is the hardening linear with degree of
work. The
Zn content of the alloy has a bearing on the colour of the alloy as well as
functioning as
a reducing agent for silver and copper oxides and is preferably 2.0-4.0 wt%.
The Si
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content of the alloy is preferably adjusted relative to the proportion of Zn
used and is
preferably 0.15 to 0.2 wt%. Precipitation hardening following annealing is not
disclosed, and there is no disclosure or suggestion that the problems of
distortion and
damage to soldered joints in nearly finished work made of this alloy can be
avoided.
By way of background, US-A-4810308 (Eagar et al.; Leach & Gamer) discloses
a hardenable silver alloy comprising not less than 90% silver; not less than
2.0%
copper; and at least one metal selected from the group consisting of lithium,
tin and
antimony. The silver alloy can also contain up to 0.5% by weight of bismuth.
Preferably, the metals comprising the alloy are combined and heated to a
temperature
not less than 1250-1400 F (676-760 C) e.g. for about 2 hours to anneal the
alloy into a
solid solution, a temperature of 1350 (732 C) being used in the Examples. The
annealed alloy is then quickly cooled to ambient temperature by quenching. It
can then
be age hardened by reheating to 300-700 F (149-371 C) for a predetermined time
followed by cooling of the age hardened alloy to ambient temperature. The age-
hardened alloy demonstrates hardness substantially greater that that of
traditional
sterling silver, typically 100 HVN (Vickers Hardness Number), and can being
retumed
by elevated temperatures to a relatively soft state. The disclosure of US-A-
4869757
(Eagar et al.; Leach & Garner) is similar. In both cases the disclosed
annealing
temperature is higher than that of Argentium, and neither reference discloses
firestain or
tarnish-resistant alloys. The inventor is not aware of the process disclosed
in these
patents being used for commercial production, and again there is no disclosure
or
suggestion that hardening can be achieved in nearly finished work.
A silver alloy called Steralite is said to be covered by US-A-5817195
(Davitz),
5882441 (Davitz) and to exhibit high tarnish and corrosion resistance. The
alloy of US-
A-5817195 (Davitz) contains 90-92.5 wt % Ag, 5.75-5.5 wt % Zn, 0.25 to less
than 1 wt
% Cu, 0.25-0.5 wt % Ni, 0.1-0.25 wt % Si and 0.0-0.5 wt % In. The alloy of US-
A-
5882441 (Davitz) contains 90-94 wt % Ag, 3.5-7.35 wt % Zn, 1-3 wt % Cu and 0.1-
2.5
wt % Si. A similar high zinc low copper alloy is disclosed in US-A-4973446
(Bernhard
et al) and is said to exhibit reduced firestain, reduced porosity and reduced
grain scale.
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SUMMARY OF THE INVENTION
It has now been found that silver wire can be machine-formed into fabric
structures by processes such as weaving, knitting or braiding and that
sufficient strength
5 can be imparted to the wire for machine-forming if the wire is work hardened
from its
fully annealed state prior to fabric forming, while permitting the further
work-hardening
that takes place in the fabric-forming process and still permitting the
development of
further hardness by precipitation hardening. Argentium wire and other
silver/copper/germanium alloy wires, in particular, have a particularly
desirable
combination of physical properties that permits them to be knitted or
otherwise formed
into fabric or cable structures or into braided cord structures.
The invention provides a fabric structure comprising wires of silver alloy
which
may be knitted, woven, braided, crocheted or otherwise formed and which may
comprise wholly, predominantly or partially silver fibres.
The invention also provides a process for making a fabric structure as
aforesaid
which comprises providing silver wire having a temper of more than fully soft
but less
than half hardness, forming said wire into the fabric structure, and heating
the structure
to precipitation harden the wire.
In a further aspect, the invention provides a fabric structure (e.g. a
structure
formed by knitting, crocheting or otherwise assembling interlocking loops of
wire)
comprising (as the totality of the filaments or yarns in said structure or as
some of the
filaments or yams in said structure) wires of silver alloy having a grain
structure refined
by incorporation into molten silver alloy from which said wire is formed of a
decomposable boron compound.
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DESCRIPTION OF PREFERRED FEATURES
Alloys for forming wire
The wire used to form the present structures may be any work and precipitation-
hardenable grade of silver, but is preferably an alloy of silver, copper and
germanium
e.g. an alloy that consists, apart from impurities and any grain refiner, of
80-96% silver,
0.1-5% germanium and 1-19.9% copper, by weight of the alloy. Sterling grade
alloys of
the above type may comprise apart from impurities and grain refiner, 92.5-98%
silver,
0.3-3% germanium, and 1-7.2% copper, by weight of the alloy, together with 1-
200
ppm e.g. 1-40 ppm boron as grain refiner. A particularly preferred group of
such alloys
consists, apart from impurities and grain refiner, of 92.5-96% silver, 0.5-2%
germanium, and 1-7% copper, by weight of the alloy, together with 1-40 ppm
boron as
grain refiner. The alloy may further comprise zinc, preferably in a ratio, by
weight, to
the copper of no more than 1:1. Thus the alloy may comprise 81-95.49 wt % Ag,
0.5-6
wt% Cu, 0.05-5 wt% Zn, 0.02-2 wt% Si, 0.01-2 wt % by weight B, optionally 0.01-
1.5
wt% In, optionally 0.25-6 wt% Sn and 0.01-no more than 2.0 wt% Ge.
The alloy from which the present wire is formed may contain one or more
incidental ingredients known per se in the production of silver alloys in
amounts (e.g. in
total up to 0.5 wt%) that are not detrimental to the mechanical strength,
tarnish
resistance and other properties of the material. Cadmium may also be added in
similar
amounts although its use is presently not preferred. Tin may be beneficial,
typically in
an amount of 0.5 wt%. Indium may be added in small quantities e.g. as a grain
refiner
and to improve the wetability of the alloy. Other possible incidental
ingredient elements
selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Si, Ti, V, Y, Yb
and Zr,
provided the effect of germanium in terms of providing firestain and tarnish
resistance
is not unduly affected.
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Grain refmement of the alloys
Boron may be incorporated into silver alloys used to make wire for the present
purposes as a grain refiner. It may be added e.g. to molten silver alloy as
copper/boron
master alloy, 2 wt% B. However, it has recently been found that alloys having
improved
mechanical properties (including e.g. tensile strength) may be made by
introducing the
boron into the alloy as a boron compound selected from alkyl boron compounds,
boron
hydrides, boron halides, boron-containing metal hydrides, boron-containing
metal
halides and mixtures thereof. The use of wire made from molten silver treated
with
decomposable boron compounds as aforesaid is advantageous for the present
invention
insofar as the mechanical properties thereof are more consistent and the
strength may be
higher both prior to formation of the fabric structure and after oven-heating
of the fabric
structure to effect hardening. In some embodiments, silver grain-refined by
means of a
decomposable boron compound is detectable e.g. on electron micrograph
examination
because of its fine grain structure.
The boron compound may be introduced into molten silver alloy in the gas
phase, advantageously in admixture with a carrier gas which assists in
creating a stirring
action in the molten alloy and dispersing the boron content of the gas mixture
into said
alloy. Suitable carrier gases include, for example, hydrogen, nitrogen and
argon. The
gaseous boron compound and the carrier gas may be introduced from above into a
vessel containing molten silver e.g. a crucible in a silver-melting furnace, a
casting ladle
or a tundish using a metallurgical lance which may be a elongated tubular body
of
refractory material e.g. graphite or may be a metal tube clad in refractory
material and is
immersed at its lower end in the molten metal. The lance is preferably of
sufficient
length to permit injection of the gaseous boron compound and carrier gas deep
into the
molten silver alloy. Alternatively the boron-containing gas may be introduced
into the
molten silver from the side or from below e.g. using a gas-permeable bubbling
plug or a
submerged injection nozzle. For example, Rautomead International of Dundee,
Scotland
manufacture horizontal continuous casting machines in the RMK series for the
continuous casting of semi-fmished products in silver. The alloy to be heated
is placed
in a solid graphite crucible, protected by an inert gas atmosphere which may
for
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example be oxygen-free nitrogen containing < 5 ppm oxygen and < 2 ppm moisture
and
is heated by electrical resistance heating using graphite blocks. Such
furnaces have a
built-in facility for bubbling inert gas through the melt. Addition of small
quantities of
thermally decomposable boron-containing gas to the inert gas being bubbled
through
the melt readily provides a desired few ppm or few tens of ppm boron content
The
introduction of the boron compound into the alloy as a dilute gas stream over
an period
of time, the carrier gas of the gas stream serving to stir the molten metal or
alloy, rather
than in one or more relatively large quantities is believed to be favourable
from the
standpoint of avoiding development in the metal or alloy of boron hard spots.
Compounds which may be introduced into molten silver or alloys thereof in this
way
include boron trifluoride, diborane or trimethylboron which are available in
pressurised
cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being
preferred
because apart from the boron, the only other element is introduced into the
alloy is
hydrogen. A yet further possibility is to bubble carrier gas through the
molten silver to
effect stirring thereof and to add a solid boron compound e.g. NaBH4 or NaBF4
into the
fluidized gas stream as a finely divided powder which forms an aerosol.
A boron compound may also be introduced into the molten silver alloy in the
liquid phase, either as such or in an inert organic solvent. Compounds which
may be
introduced in this way include alkylboranes or alkoxy-alkyl boranes such as
triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane
which for
safe handling may be dissolved in hexane or THF. The liquid boron compound may
be
filled and sealed into containers of silver or of copper foil resembling a
capsule or
sachet using known liquid/capsule or liquid/sachet filling machinery and using
a
protective atmosphere to give filled capsules sachets or other small
containers typically
of capacity 0.5-5 ml, more typically about 1-1.5 ml. The filled capsules or
sachets in
appropriate number may then be plunged individually or as one or more groups
into the
molten silver or alloy thereof. A yet further possibility is to atomize the
liquid boron-
containing compound into a stream of carrier gas which is used to stir the
molten silver
as described above. The droplets may take the form of an aerosol in the
carrier gas
stream, or they may become vaporised therein.
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Preferably the boron compound is introduced into the molten silver alloy in
the
solid phase, e.g. using a solid borane e.g. decaborane B10H14 (m.p. 100 C,
b.p. 213 C).
However, the boron is preferably added in the form of either a boron-
containing metal
hydride or a boron-containing metal fluoride. When a boron-containing metal
hydride
is used, suitable metals include sodium, lithium, potassium, calcium, zinc and
mixtures
thereof. When a boron-containing metal fluoride is used, sodium is the
preferred metal.
Most preferred is sodium borohydride, NaBH4 which has a molecular weight of
37.85
and contains 28.75% boron.
Boron can be added to molten silver alloy both on first melting and at
intervals
during storage of the alloy in the molten state and subsequent to make up for
boron loss
if the alloy is held in the molten state for a period of time, as in a
continuous casting
process for grain.
It has surprisingly been found that when adding a decomposable boron
compound such as a borane or borohydride that more than 20 ppm can be
incorporated
into a silver alloy without the development of boron hard spots. This is
advantageous
because boron is rapidly lost from molten silver: according to one experiment
the
content of boron in molten silver decaying with a half-life of about 2
minutes. The
mechanism for this decay is not clear, but it may be an oxidative process. It
is therefore
desirable to incorporate more than 20 ppm boron into an alloy as first cast,
and amounts
of e.g. up to 50 ppm, typically up to 80 ppm, and in some instances up to 800
or even
1000 ppm may be incorporated. Thus there could be produced silver casting
grain
containing about 40 ppm boron. Owing to boron loss during subsequent re-
melting and
formation of wire, the boron content of the finished wire may be closer to 1-
20 ppm, but
the ability to achieve relatively high initial boron concentrations means that
improved
consistency and improved mechanical properties may be achieved.
Forming wire from the alloys
Forming germanium-containing silver into wire for forming into fabric
according to the invention of may be carried out using conventional wire-
manufacturing
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processes. In certain embodiments of the invention the metal is cast to form
ingots
which are rolled in a roiling mill to form wire rod. The resulting rod is
drawn
successively through a series of dies of progressively reducing diameter to
give the
required size. Drawing may be in single block machines, or the wire may be
drawn on
5 continuous wire-drawing machines having a series of guides through which the
wire
passes in a continuous manner. Lubrication may be provided as necessary.
At the final step, and as required at intermediate steps, the wire may be
annealed
to restore ductility. Preferably this step is carried out in an atmosphere
which is not too
10 reducing or is mildly oxidizing. The corrosion resistance of the present
AgCuCe alloys
depends on the presence of oxide films, and these are reduced by e.g. an
atmosphere of
50% hydrogen, 50% nitrogen with some loss of tarnish resistance. At each
stage, it is
desirable that the annealing atmosphere should be inert gas, generally
nitrogen, with
less than 10% of hydrogen, typically 3-10%, preferably about 3-5%. If the
furnace
atmosphere is cracked ammonia, it is preferred that the hydrogen content
should be not
more than the above indicated range.
We have found that it is possible to have mildly oxidising conditions during
annelaing, i.e. temperatures and oxygen partial pressures, which allow the Ag-
Cu-(Zn)-
Ge alloys to be processed such that Ge will react to formGe02 without Cu
forming
Cu20. However, restrictions on the maximum processing temperature and time at
temperature arise from the normal commercial annealing temperature and time
used for
producing silver-copper alloys such as Sterling silver, typically about 625 C
or 650 C.
We have established that Ag-Cu-(Zn)-Ge alloys can be processed even at
annealing
temperatures such as 625 C and 650 C to selectively oxidise Ge toGeO2, by
using a
controlled atmosphere. Preferably, the annealing atmosphere is a wet
selectively
oxidizing atmosphere. By 'wet' in this context is meant an atmosphere
containing
moisture (H20), such that the atmosphere exhibits a dew point of at least +1
C,
preferably at least +25 C, more preferably at least +40 C. Preferably, the dew
point
falls within the range from +1 C to +80 C, more preferably in the range from
+2 C
to+50 C. The dew point may be defined as the temperature to which an
atmosphere
containing water vapour must be cooled in order for saturation to occur,
whereby
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further cooling below the dew point temperature results in the formation of
dew. A
more comprehensive definition is given in "Handbook of Chemistry and Physics",
65th
Ed. (1985-85), CRC Press Inc., USA, page F-75. We prefer that the selectively
oxidizing atmosphere comprises hydrogen and moisture, for example an
atmosphere of
nitrogen, hydrogen and water vapour, such as a 95% nitrogen/5% hydrogen gas
mixture
(v/v) containing water vapour, or a furnace atmosphere of nitrogen, hydrogen,
carbon
monoxide, carbon dioxide, methane, and water vapour.
In practice, it is preferred to produce the wet selectively oxidizing
annealing
atmosphere by controlling the addition of water vapour to a substantially dry
inert or
dry reducing furnace atmosphere, for example to a furnace atmosphere of
predominantly nitrogen or nitrogen and hydrogen, and typically comprising
nitrogen,
hydrogen, carbon monoxide, carbon dioxide and methane. The dew point in the
furnace
can be measured by conventional means such as a dew point meter or probe in
the
furnace, and the gas mixing ratios adjusted accordingly in order to control
the
selectively oxidizing atmosphere.
As explained above, in some embodiments of the invention, the annealing of
the wire is carried out under the selectively oxidizing atmosphere. If, as is
usual, the
annealing is carried out as successive annealing steps, for example with
intervening
drawing steps, then at least the final annealing step should be carried out
under the
selectively oxidizing atmosphere. In further embodiments of the invention, one
or more
of the annealing steps preceding the final annealing step is conducted under a
reducing
atmosphere. However, in other embodiments of the invention, all of the
annealing steps
are carried out under a selectively oxidizing atmosphere.
In embodiments of the invention, the annealing of the wire is carried out at a
temperature in the range from 400 C to750 C, typically in the range from 400 C
to
700 C, preferably in the range from 500 C to 675 C, more preferably in the
range from
600 C to 650 C, and in particular at about 625 C. In embodiments of the
invention, the
annealing is carried out for a total period in the range of from 5 minutes, at
the higher
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annealing temperatures, to 5 hours, at the lower annealing temperatures, and
preferably
in the range from 15 minutes to 2 hours.
A further improvement in tarnish resistance may be obtained by heating the
wire post production, i.e. after the alloy has been drawn and annealed to
provide a
finished wire. Heating may be in an air or steam atmosphere at a temperature
in the
range from 40 C to 220 C, preferably in the range from 50 C to 200 C, more
preferably
in the range from 60 C to 180 C. Preferably, the post-production heat
treatment is
carried out for a period in the range from 1 minute to 24 hours, preferably in
the range
from 10 minutes to 4 hours. Thus, the germanium oxide protective coating may
be
further developed within the surface of the alloy. Advantageously, this post-
production
treatment further enhances the alloy protection against tarnishing, which is
particularly
important for fine wire because of its high surface area relative to its mass.
The structures of the invention may consist wholly or principally of silver
wire,
or silver wire may be a minor component e.g. when incorporated into bandages
to take
advantage of the antibacterial properties of the silver. Wire is a solid
section other than
strip, and may be furnished in a coil on a spool or reel. The wire used to
make the
present woven structures may be of circular cross section, but other sections
may be
employed, e.g. oval, polygonal, strip or flat wire depending on the appearance
desired
for the finished chain. The wire will typically be of circular section. It may
be of
diameter or size 0.05 - 2.0 mm, typically 0.1- 1 mm. The wire may be single
stranded
or may comprise a plurality of strands twisted together.
Wire hardness for forming fabric structures
Prior to formation of the present structures, the wire of the invention should
preferably be more than fully soft but less than half hard. These expressions
have well-
understood meanings in the jewellery trade. In jewellery wire, hardness or
malleability
is graded soft or dead soft, quarter hard, half-hard, hard, and spring hard.
Numbers
instead of names can also designate wire hardness. The numbering system, which
goes
from zero to 10 or more, is based on the number of times wire has been drawn
though
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progressively smaller holes in a drawplate. Each increment in the number
designates a
doubling of the preceding number. Soft or dead soft wire is as annealed, has
not
subsequently been drawn through a plate and has a number of zero. It is
malleable and
can be bent easily by hand into a myriad of shapes but does not hold its shape
under
stress. Quarter-hard wire has been drawn through a single plate, half-hard
wire has been
drawn twice and hard wire has been drawn through four times. The wire used to
form
the present structures is preferably quarter hard, which imparts the necessary
bending
and breaking strength for machine weaving or machine knitting but leaves
enough
material in solid solution for both work hardening during weaving or knitting
and for
subsequent precipitation hardening.
Structures that can be formed from the wire
The wire may be weft-knit on a flat-bed or circular knitting machine to
produce
e.g. a single layer tricot stitch structure, or double-layer structures, or
more open net-
like structures which may be tubular or may be flat sheets. In particular,
single-layer
tubular cable-like structures based on a single layer or on two layers may be
used as a
substitute for conventional chains in the manufacture of jewellery such as
bracelets and
necklaces and has the advantage of attractive appearance and lightness. The
wire may
also be warp-knit. The wire may further be formed into braided cable
structures e.g. by
twisting together a plurality of single filaments of silver to form plied yams
which are
then braided, see e.g. US-A-4170921 and US-A6070434 (Fig. 6) e.g. to form a
jacket of
braided silver surrounding core which may be of silver, another metal or e.g.
plastics
filaments. A further possibility is to form the wire into a crocheted
structure "Crochet"
as used herein means a process of making needlework comprising looped stitches
formed from a single thread or filament e.g. of silver/copper/germanium alloy
using a
hooked needle and includes both formation of a foundation row which may be
useful
per se as a jewellery chain and making plain or open-work fabric structure
from
successive rows of stitching. Lace and band-type structures can be made.
Embodiments of the invention for knitting or crocheting further employ a
sacrificial thread placed substantially parallel and adjacent to the silver
alloy wire
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14
during the operations involved in knitting or crocheting and fed
simultaneously with it.
The sacrificial thread can be formed of any suitable material which can be
removed
after the knitted structure is formed. For example, suitable materials for the
sacrificial
thread can include cotton, readily soluble metal and natural or synthetic
polymers,
including polyamides, polyesters, cellulosic fibres, acrylic styrene polymers,
PVA and
other vinyl polymers, aliginate, and the like. Multistrand fibres or threads
and
monofilament fibres or threads may be used. One of the advantages of a
sacrificial
thread is to provide a spacer to control spacing in the knitted fibre
structure. Thus, the
thickness of the sacrificial thread can be used as one way to increase or
decrease the
volume of space between adjacent portions of the knitted wire. Typically, the
sacrificial
thread can have a diameter which is about the same as the wire. As mentioned
above, it
may be desirable to decompose or dissolve the sacrificial thread, and the
selection of
sacrificial thread is conveniently made to permit easy decomposition or
dissolution after
the fabric structure has been formed. Most organic fibres, for example, may be
pyrolysed and/or oxidised to leave little or no residue, or a strong acid such
as sulphuric
or nitric acid may be used.. In addition or as an alternative to a sacrificial
thread there
may be used a lubricant e.g. starch to reduce the friction in the knitting or
crocheting
process.
After formation of a knitted, braided, crocheted or woven structure, it may be
subjected to a precipitation hardening treatment by heating in a furnace to
e.g. about
300 C for about 30-45 minutes followed by gradual cooling. A surprising
difference in
properties exists between conventional Sterling silver alloys and other Ag-Cu
binary
alloys on the one hand and Ag-Cu-Ge silver alloys on the other hand, in which
gradual
cooling of the binary Sterling-type alloys results in coarse precipitates and
little
precipitation hardening, whereas gradual cooling of Ag-Cu-Ge alloys results in
fine
precipitates and useful precipitation hardening, particularly where the silver
alloy
contains an effective amount of grain refiner. Furthermore, the addition of
germanium
to sterling silver changes the thermal conductivity of the silver alloy,
compared to
standard sterling silver. The International Annealed Copper Scale (IACS) is a
measure
of conductivity in metals. On this scale the value of copper is 100%, pure
silver is
106%, and standard sterling silver 96%, while a sterling alloy containing 1.1%
CA 02587350 2007-05-10
WO 2006/051338 PCT/GB2005/050202
germanium has a conductivity of 56%. The significance of this is that the
Argentium
sterling and other germanium-containing silver alloys do not dissipate heat as
quickly as
standard sterling silver or their non-germanium-containing equivalents, a
piece will take
longer to cool, and precipitation hardening to a commercially useful
Ieve1(preferably to
5 Vickers hardness 110 or above, more preferably to 115 or above) can take
place during
natural air cooling or during slow controlled air cooling. A number of Ag-Cu-
Ge-Zn
alloys grain refined with boron using copper boron master alloy or using a
decomposable boron compound also exhibit precipitation hardening under the
conditions indicated above.
The present structures can be used for making wearable articles e.g. chain,
bracelets, necklaces, earrings, key-chains and the like. Silver wire may be
incorporated,
in embodiments of the invention, into a variety of additional structures e.g.
for use in
catalysis or water treatment. Thus it may be incorporated into backing
material e.g. for
carpets, as a minor component into woven or knitted garments e.g. for
protective
clothing or into fashion garments, into circular or flat knitted general
textile fabrics,
warp knitted fabrics, sleeves, tapes, needle punched or other felts, and
twisted or
braided cordage or ropes. Silver wire either alone or in admixture with other
metallic or
natural or synthetic organic fibres or filaments may be formed into porous
media e.g.
three-dimensional non-woven structures e.g. for filtration (e.g. of water
where the anti-
bacterial qualities of silver may be an advantage) or catalyst support
applications. It
may be incorporated as a component of bandaging on account of its
antibacterial
properties. In additional embodiments, silver wire may be formed into a non-
woven
high porosity matrix of sintered metal fibres which exhibits high gas
permeability, or
into a layer which may be pleated. The sintered metal fibres may be formed
into media
having a plurality of layers e.g. 1-3 layers optionally with an internal or
superficial
support mesh or scrim for a variety of filtration and other applications
including
catalysts, gas-solid and/or gas/liquid filtration and/or odour removal and
liquid/solid
filtration. Because of the high porosity achievable, filter media made using
fibres
according to the invention may exhibit a relatively low pressure drop. They
may be used
as such or incorporated as minor components into textile products e.g. into
bandaging to
provide antibacterial properties.
