U.S. patent application number 16/593885 was filed with the patent office on 2020-03-05 for electrically conductive textile element and method of producing same.
This patent application is currently assigned to EPRO DEVELOPMENT LIMITED. The applicant listed for this patent is EPRO DEVELOPMENT LIMITED. Invention is credited to Lee Cheung LAU, Casey YAN, Zijian ZHENG.
Application Number | 20200071877 16/593885 |
Document ID | / |
Family ID | 56849253 |
Filed Date | 2020-03-05 |
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United States Patent
Application |
20200071877 |
Kind Code |
A1 |
ZHENG; Zijian ; et
al. |
March 5, 2020 |
ELECTRICALLY CONDUCTIVE TEXTILE ELEMENT AND METHOD OF PRODUCING
SAME
Abstract
A method of producing an electrically conductive textile element
that includes the steps of modifying a surface of a textile element
with a negatively-charged polyelectrolyte; and coating the modified
surface of the textile element with metal particles.
Inventors: |
ZHENG; Zijian; (Hung Hom,
HK) ; YAN; Casey; (Hung Hom, HK) ; LAU; Lee
Cheung; (Hung Hom, HK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EPRO DEVELOPMENT LIMITED |
Hung Hom |
|
HK |
|
|
Assignee: |
EPRO DEVELOPMENT LIMITED
Hung Hom
HK
|
Family ID: |
56849253 |
Appl. No.: |
16/593885 |
Filed: |
October 4, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15554695 |
Aug 30, 2017 |
|
|
|
PCT/IB2016/000132 |
Feb 16, 2016 |
|
|
|
16593885 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D06M 14/16 20130101;
D06M 2101/06 20130101; C23C 18/38 20130101; D06M 15/263 20130101;
C23C 18/1662 20130101; D06M 14/14 20130101; C23C 18/32 20130101;
D06M 14/04 20130101; D06M 11/83 20130101; D06M 13/513 20130101;
D06M 14/06 20130101 |
International
Class: |
D06M 14/04 20060101
D06M014/04; C23C 18/38 20060101 C23C018/38; C23C 18/32 20060101
C23C018/32; C23C 18/16 20060101 C23C018/16; D06M 11/83 20060101
D06M011/83; D06M 14/06 20060101 D06M014/06; D06M 14/14 20060101
D06M014/14; D06M 14/16 20060101 D06M014/16; D06M 15/263 20060101
D06M015/263; D06M 13/513 20060101 D06M013/513 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2015 |
HK |
15102150.5 |
Claims
1. A method of producing an electrically conductive textile
including the steps of: silanising a surface of the textile to
provide a silanised surface; grafting a negatively-charged
polyelectrolyte onto the silanised surface by in-situ free radical
polymerisation; adding metal ions into the polyelectrolyte by ion
exchange; reducing the metal ions to elemental metal; and coating
the textile with metal particles.
2. The method of claim 1, wherein the negatively-charged
polyelectrolyte includes poly(methacrylic acid) or a salt thereof,
or poly(acrylic acid) or a salt thereof.
3. The method of claim 2, wherein the negatively-charged
polyelectrolyte includes poly(methacrylic acid) sodium salt, or
poly(acrylic acid) sodium salt.
4. The method of claim 1, wherein the metal ions are copper
ions.
5. The method of claim 1, wherein coating the textile with metal
particles is performed by electroless metal deposition.
6. The method of claim 1, wherein the metal particles are nickel or
copper.
7. The method of claim 1, wherein the textile includes yarn or
fibers made of cotton, nylon, silk or polyester.
8. The method of claim 7, wherein the textile includes cotton yarn
or fibers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/554,695, filed Aug. 30, 2017, which is
incorporated herein by reference in its entirety for all
purposes.
TECHNICAL FIELD
[0002] The present invention relates to the field of electrically
conductive textile elements and methods of producing same.
BACKGROUND OF THE INVENTION
[0003] With the rapid advancement of flexible and wearable
electronic devices there has been a demand for conductors as
interconnects, contacts, electrodes and metal wires which can be
integrated into conductive textiles/garments. Accordingly, methods
for synthesizing fabricated high performance electrically
conductive textiles have been developed including synthesizing of
yarns by or incorporated with metal wires, metal oxide,
intrinsically conducting polymers (ICPs), and carbon nanotubes
(CNTs). However, conductive textiles fabricated in accordance with
these existing methods are not ideal due to their inflexibility,
chemical instability, cost of production, hazards posed to the
human body, and most significantly, the difficulties associated
with large scale production with compatible technology in the
current textile and garment industry.
[0004] Another approach to synthesizing conductive textiles
involves depositing metal coatings on to textile substrate surfaces
utilising various metal particle deposition techniques. However,
there are also limitations associated with this approach in terms
of the relative amount of investment in technology, advanced
instrumentation and specialized workforce expertise involved, as
well the relatively precise control parameters required which limit
industrialization of this process commercially. Furthermore,
adhesion of the deposited metal on the textile surface remains
another major concern on the durability and conductivity of such
conductive textiles.
[0005] Further processes have been developed which involve
modifying the surface architecture of textile substrates by
grafting of functionalized polymer brushes thereon. In particular,
polyelectrolytes that covalently tether one end on a textile
substrate surface may not only provide modified functional groups
on the textile substrate surface, but also increase the amount of
functional groups to be utilized in subsequent chemical reactions.
By way of example, Azzaroni et al., demonstrated the grating of
positively-charged poly[2-(methacryloyloxy)ethyl]trimethylammonium
chloride (PMETAC) polyelectrolytes on to a substrate surface. With
the loading of catalytic moieties tetrachloropalladtae(II) anion
([PdCl4]2-) for subsequent metal electroless deposition (ELD), a
robust metal layer is able to be selectively deposited with
suitable adhesion properties. In 2010, Liu et al. reported a
versatile approach to prepare durable conductive cotton yarns also
by growing PMETAC brushes on cotton fiber surfaces using
surface-initiated atom transfer radical polymerization (SI-ATRP),
which was the first ever demonstration on grafting of PMETAC
brushes on natural textile fibers. Subsequent metal ELD yielded
conductive cotton yarns with high electrical stability that is able
to withstand multiple bending, stretching, rubbing and even washing
cycles. However, the feasibility of scale production of the SI-ATRP
method taught by Liu et al. suffers from various problems. For
instance, SI-ATRP is not able to be suitable performed under
ambient conditions and requires nitrogen protection. Furthermore,
the SI-ATRP reaction involves a relatively long period of time
(.about.24 hours) which is undesirable and not cost-effective for
mass production. Thus, there is a need to modify the synthesizing
process to allow for high throughput conductive textile
production.
[0006] Other attempts have been made to modify the synthesizing
approach by preparing electrically conductive fibers, yarns and
fabrics by deposition of metals onto various textile substrates
which are prior-modified with the same positively-charged
polyelectrolytes PMETAC using in-situ free radical polymerization.
In-situ free radical polymerization may increase the throughput of
the polymerization of polyelectrolytes. Generally, the reaction
only takes up .about.1-3 hours to complete and can be carried out
under ambient conditions, which is highly advantageous over other
polymerization methods such as previously mentioned SI-ATRP.
However, this modified approach suffers from a drawback in that as
the selection of catalytic moieties highly depends on the
properties and nature of polyelectrolyte brush that grafted on the
textile surface, cationic PMETAC is restricted to couple with
anionic [PdCl4]2- moieties for subsequent electroless metal
deposition. Furthermore, the [PdCl4]2- moieties used are relatively
expensive (USD159.5 per 2 grams for 97% ammonium
tetrachloropalladate(II)). Even though the anionic [PdCl4]2-
moieties can be reused, it is still not economical if it is used in
the mass production.
SUMMARY OF THE INVENTION
[0007] The present invention seeks to alleviate at least one of the
above-described problems.
[0008] The present invention may involve several broad forms.
Embodiments of the present invention may include one or any
combination of the different broad forms herein described.
[0009] In a first broad form, the present invention provides a
method of producing an electrically conductive textile element
including the steps of:
[0010] (i) modifying a surface of a textile element with a
negatively-charged polyelectrolyte; and
[0011] (ii) coating the modified surface of the textile element
with metal particles.
[0012] Preferably, the step (i) may include modifying the surface
of the textile element with a negatively-charged polyelectrolyte by
in-situ free radical polymerisation.
[0013] Preferably, the negatively-charged polyelectrolyte may
includes at least one of poly(methacrylic acid sodium salt) and
poly(acrylic acid sodium salt).
[0014] Preferably, the step (i) may include modifying a silanized
surface of a textile element with a negatively-charged
polyelectrolyte.
[0015] Preferably, the step (ii) may include coating the modified
surface of the textile element with metal particles by electroless
metal deposition.
[0016] Preferably, the metal particles may include at least one of
copper and nickel particles.
[0017] Preferably, the textile element may include at least one of
a yarn and a fiber configured for being formed in to a fabric.
[0018] Preferably, the textile element may include at least one of
a polyester, nylon, cotton and silk yarn or fiber.
[0019] In a further broad form, the present invention provides an
apparatus for producing an electrically conductive textile element
including:
[0020] an apparatus for modifying a surface of a textile element
with a negatively-charged polyelectrolyte; and
[0021] a coating apparatus for coating the modified surface of the
textile element with metal particles.
[0022] Preferably, the apparatus for modifying the surface of the
textile element with the negatively-charged polyelectrolyte may be
configured to modify the surface of the textile element with a
negatively-charged polyelectrolyte by in-situ free radical
polymerisation.
[0023] Preferably, the negatively-charged polyelectrolyte may
include at least one of poly(methacrylic acid sodium salt) and
poly(acrylic acid sodium salt).
[0024] Preferably, the apparatus for modifying the surface of the
textile element with the negatively-charged polyelectrolyte may be
configured to modify a silanized surface of a textile element with
a negatively-charged polyelectrolyte.
[0025] Preferably, the coating apparatus may be configured to coat
the modified surface of the textile element with metal particles by
electroless metal deposition.
[0026] Preferably, the metal particles may include at least one of
copper and nickel particles.
[0027] Preferably, the textile element may include at least one of
a yarn and a fiber configured for being formed in to a fabric.
[0028] Preferably, the textile element may include at least one of
a polyester, nylon, cotton and silk yarn or fiber.
[0029] In a further broad form, the present invention provides an
electrically conductive textile element produced in accordance with
the method steps of the first broad form of the present
invention.
[0030] In a further broad form, the present invention provides a
fabric formed from at least one textile element wherein the at
least one textile element is produced in accordance with the method
steps of the first broad form of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The present invention will become more fully understood from
the following detailed description of a preferred but non-limiting
embodiment thereof, described in connection with the accompanying
drawings, wherein:
[0032] FIG. 1 is a schematic illustration of a process of preparing
conductive cotton yarns via in-situ free radical polymerization in
accordance with an embodiment of the present invention;
[0033] FIG. 2 depicts an exemplary copper-coated cotton yarn
fabricated in accordance with the method depicted in FIG. 1;
[0034] FIG. 3 depicts a representation of Fourier transform
infrared spectroscopy (FTIR) spectra data in respect of pristine
cotton yarns, silane-modified cotton, and PMANa-modified cotton
yarns formed in accordance with an embodiment of the present
invention;
[0035] FIG. 4 depicts a representation of EDX spectrum of
PMANa-modified cotton produced in accordance with an embodiment of
the present invention;
[0036] FIG. 5 depicts SEM images representing surface morphologies
of cotton fibers with different modifications including (A)
pristine cotton; (B) silane-modified cotton; (C) PMANa-coated
cotton; (D-F) copper-coated cotton in accordance with an embodiment
of the present invention;
[0037] FIG. 6 depicts data representing (A) linear resistance of
the as-synthesized copper-coated cotton yarns and (B) Tensile
strength of the cotton yarns produced in accordance with an
embodiment of the present invention;
[0038] FIG. 7 depicts process steps for fabrication of a woven
fabric formed from copper-coated yarns produced in accordance with
an embodiment of the present invention;
[0039] FIG. 8 depicts sheet resistance data of fabrics woven from
copper-coated yarns produced in accordance with an embodiment of
the present invention;
[0040] FIG. 9 depicts SEM images of cotton yarns unraveled from
washed fabrics under different washing times, the cotton yarns
being produced in accordance with an embodiment of the present
invention;
[0041] FIG. 10 depicts a PMANa-assisted nickel-coated cotton fabric
produced in accordance with an embodiment of the present
invention;
[0042] FIG. 11A depicts an exemplary PAANa-assisted copper-coated
yarn formed in accordance with an embodiment of the present
invention;
[0043] FIG. 11B depicts an exemplary PAANa-assisted nickel-coated
silk yarn formed in accordance with an embodiment of the present
invention;
[0044] FIG. 12A depicts PAANa-assisted copper-coated nylon yarn
produced in accordance with an embodiment of the present invention;
and
[0045] FIG. 12B depicts a polyester fabric formed from
PAANa-assisted copper-coated nylon yarn produced in accordance with
an embodiment of the present invention.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0046] Exemplary embodiments of the present invention will now be
described with referenced to the FIGS. 1 to 12B.
[0047] Referring firstly to FIG. 1, a procedure for preparing PMANa
polyelectrolytes on textile substrates such as cotton yarn is
illustrated schematically. The embodiment involves an in-situ free
radical polymerization method which may be performed upon cotton
yarns by way of example to prepare poly(methacrylic acid sodium
salt) (PMANa)-coated cotton yarns. Subsequent ion exchange, ion
reduction and electroless deposition of metal particles onto the
PMANa-coated cotton yarns may then be performed in order to yield
electrically conductive cotton yarns of suitable quality for
production on a commercial scale. It should be noted that this
embodiment may also be applicable to the preparation of PAANa
polyelectrolytes on textile substrates.
[0048] In performing the process, cotton yarns are first immersed
in a solution of 5-20% (v/v) C.dbd.C bond bearing silane for
approximately 30 minutes so as to allow the hydroxyl groups of
cellulose to suitably react with the silane molecules. The cotton
yarns are then rinsed thoroughly with fresh deionized (DI) water so
as to remove any excess physical adsorbed silane and by-product
molecules. This step of silanisation is represented by (100) in
FIG. 1.
[0049] The rinsed cotton yarns are then placed into an oven at
100-120.degree. C. for between approximately 15-30 minutes to
complete the condensation reaction. Subsequently, the
silane-modified cotton yarns are immersed into approximately 50 mL
aqueous solution comprising of 3-7 g of MANa powder and 35-75 mg of
K2S2O8 (similarly, AANa powder may be used in respect of PAANa
polyelectrolytes). The whole solution mixture with cotton yarns is
heated at 60-80.degree. C. in an oven for 0.5-1 hour in order to
carry out the free radical polymerization. In the free radical
polymerization process, the double bond of silane can be opened by
the free radicals resulting in the growth of PMANa polyelectrolyte
onto the cotton fiber surface. This step of free radical
polymerisation is represented by (110) in FIG. 1.
[0050] Thereafter, the PMANa-coated cotton yarns are immersed into
a 39 g/L copper(II) sulphate pentahydrate solution for 0.5-1 hour,
where the Cu2+ ions are immobilized onto the polymer by ion
exchange. Followed by reduction in 0.1-1.0 M sodium borohydride
solution, Cu2+ will be reduced to Cu particles which act as
nucleation sites for the growth of Cu in the subsequent electroless
deposition of Cu. This step of ion exchange and reduction is
represented by (120) in FIG. 1.
[0051] The polymer-coated cotton after reduction in sodium
borohydride solution is immersed in a copper electroless plating
bath consisting of 12 g/L sodium hydroxide, 13 g/L copper(II)
sulphate pentahydrate, 29 g/L potassium sodium tartrate, and 9.5
mL/L formaldehyde in water for 60-180 minutes. The as-synthesized
Cu-coated yarns are rinsed with deionized (DI) water and blown dry.
The step of performing electroless metal deposition is represented
by (130) in FIG. 1 and an exemplary Cu-coated cotton yarn produced
in accordance with the methods steps of this first embodiment is
represented by (200) in FIG. 2.
[0052] The silane-modified cotton and PMANa-grafted cotton are able
to be characterized by Fourier transform infrared spectroscopy
(FTIR). As shown in FIG. 3, the presence of additional peaks
located at 1602 and 1410 cm-1 represent C.dbd.C bonds in the silane
molecules. Another distinctive peak located at 769 cm-1 is
attributed to Si--O--Si symmetric stretching, indicating that the
silane molecules are successfully cross-linked with each other on
the cotton fiber surface. For the PMANa-modified cotton sample, a
new peak located at 1549 cm-1 standing for carboxylate salt
asymmetrical stretching vibrations confirm the PMANa grafting.
Other peaks located at 1455 and 1411 cm-1 are both attributed to
carboxylate salt symmetrical stretching vibrations from the
PMANa.
[0053] The PMANa-grafted cotton is also able to be characterized by
energy-dispersive X-ray spectroscopy (EDX). It is shown in FIG. 4
that polymerization of MANa leaves the cotton sample with a sodium
element which indicates the presence of PMANa. Referring further to
the FIG. 5 scanning electron microscopy (SEM) image, no obvious
difference between the morphology on the surfaces of silanized
cotton fiber surface and the raw cotton fiber surfaces may be
visibly evident. However, after polymerization of PMANa upon the
silanized cotton fiber surface, it is notable that a layer of
coating had been wrapped on the cotton fiber surface. FIGS. 5D-F
show that the copper metal particles are deposited relatively
evenly, without any signs of cracks.
[0054] The conductivity of the copper-coated cotton yarns is able
to be characterized by a two-probe electrical testing method. In
this regard, linear resistance of the copper-coated yarns in the
fabrication is found to be .about.1.4 .OMEGA./cm as shown in FIG.
6A, and with superior tensile properties compared to the untreated
cotton yarns, with both increase in tensile extension (+33.6%) and
maximum load (+27.3%) as shown in FIG. 6B. The increase in tensile
extension and maximum load is perceived to be due to the
reinforcement on the strength of cotton yarns by a layer of
copper.
[0055] To further test the adhesion of the copper on the cotton
yarn surface and the washing durability, the copper-coated cotton
yarns are first woven into a fabric first. As-synthesized
copper-coated cotton yarns shown in FIG. 7A are firstly wound upon
a cone as shown in FIG. 7B by use of an industrial yarn winder.
Thereafter, the cone is transferred to a CCI weaving machine as
shown in FIG. 7C whereby the copper-coated yarns are woven into a
fabric. In the weaving setting, the copper-coated cotton yarns are
configured to form the wefts of the fabric while the warps of the
fabric are formed by the untreated cotton yarns as shown in the
inset image of FIG. 7D which are initially mounted on the weaving
machine. No problems or defects are found in the weaving process.
After weaving, the fabric is cut into pieces of 5 cm.times.15 cm
and overlocked at the four edges as shown in FIG. 7D, and
subsequently, subjected to a series of washing cycles according to
the testing standard AATCC Test Method 61-Test No. 2A:
Colorfastness to Laundering, Home and Commercial: Accelerated
(Machine Wash) (FIG. 7E) under following washing conditions:
TABLE-US-00001 Washing Temperature 49 .+-. 2.degree. C. Volume of
DI Water 150 mL No. of Steel Balls Added 50 pcs Time of Washing 45
minutes
[0056] It should be noted that according to the testing standard, 1
washing cycle is equivalent to approximately 5 commercial machine
laundering cycles. In total, 6 washing cycles are conducted, which
accordingly, is considered to equate to approximately 30 commercial
machine laundering cycles. Changes in the electrical resistance of
the washed fabrics are able to be evaluated using a four-probe
method whereby the sheet resistances of the fabrics produced in
accordance with this embodiment are measured to be 0.9.+-.0.2
ohm/sq (unwashed), and 73.8.+-.13.4 ohm/sq after the fourth wash
which is equivalent to approximately 20 commercial machine
laundering cycles as shown in FIG. 8.
[0057] The surface morphology of the washed copper-coated cotton
yarns are able to be characterized by unraveled the washed
copper-coated cotton yarns from the fabric and examined under an
SEM. As shown in the SEM images of FIG. 9, it is visibly evident
that the copper metal particles are retained on the surface of the
cotton fibers. One perceived reason for the increase in sheet
resistance is due to the loosened structure of the cotton fibers
arising from repeated washing cycles.
[0058] It is also noted that during application of the standard
washing cycle to the produced fabric, 50 pieces of steel balls are
added into the washing canisters in seeking to simulate vigorous
rubbing and stretching forces of a laundering machine. The abrasion
of the steel balls on the fabric impacts substantially upon the
fiber structure. As the copper-coated cotton fibers are no longer
held in a tightened manner it is perceived that they lose contact
with each other so as to reduce conductive pathways available for
the movement of electrons. Accordingly, the sheet resistance
increases upon repeated washing cycles notwithstanding, the SEM
images in FIG. 9 which confirm the relatively strong adhesion of
copper metal particles on the cotton fiber surface.
[0059] In alternate embodiments of the present invention, rather
than coating the cotton fibers with copper particles, nickel metal
particles may instead be electrolessly plated on to the textile
surface by using the same approach described above. Same
experimental procedures and testing may be conducted however the
source of nickel that may be utilised is 120 g/L nickel(II)
sulphate solution in the ion exchange procedure. Subsequently an
electroless nickel plating bath is utilised consisting of 40 g/L
nickel sulphate hexahydrate, 20 g/L sodium citrate, 10 g/L lactic
acid, and 1 g/L dimethylamine borane (DMAB) in water for 60-180
minutes. The sheet resistance of the resulting nickel-coated cotton
fabric is found to exhibit substantially similar results as that of
the copper coated fiber yarns as shown in FIG. 8. Turning to FIG.
10, an exemplary nickel-coated cotton fabric is represented by
(300) which exhibits a high degree of evenness of nickel metal,
with bulk resistance measured as 3.2.OMEGA..
[0060] It will be appreciated that other embodiments of the present
invention may involve the use of substrates other than cotton and
could be suitably applied to various textile materials such as
silk, nylon and polyester. In this regard, an exemplary
PAANa-assisted copper-coated yarn produced in accordance with an
embodiment of the present invention is shown represented by (400)
in FIG. 11A, an exemplary PAANa-assisted nickel-coated silk yarn
produced in accordance with an embodiment of the present invention
is shown represented by (500) in FIG. 11B, an exemplary
PAANa-assisted copper-coated nylon yarn produced in accordance with
an embodiment of the present invention is shown represented by
(600) in FIG. 12A, and, an exemplary polyester fabric formed from
PAANa-assisted copper-coated nylon yarn produced in accordance with
an embodiment of the present invention is represented by (700) in
FIG. 12B.
[0061] It will be appreciated from the preceding summary of the
broad forms of the invention that various advantages may be
conveniently provided including electrically conductive textile
elements may be produced which may be suitably flexible, wearable,
durable and/or washable for integration into a textile/fabric.
Moreover, such high performance electrically conductive textile
elements (fibers, yarns and fabrics) may be produced utilising
relatively low-cost technology cost-effectively on a mass scale
based upon the chemical reaction of in-situ free radical
polymerization to grow negatively-charged polyelectrolytes such as
PMANa or PAANa on textile substrates which may conveniently provide
an improved negatively-charged polyelectrolyte layer bridging the
electrolessly deposited metal and textile elements and substrates.
Notably, the adhesion of conductive metal to textile substrates may
be greatly improved by such surface modification of a layer of
negatively-charged polyelectrolyte PMANa or PAANa, in which the
electrical performance of such conductive textiles may be more
reliable, robust and durable under repeated cycles of rubbing,
stretching, and washing. Also, the in-situ free radical
polymerization method used to prepare the negatively-charged
polyelectrolyte may be performed under ambient and aqueous
conditions without using any strong chemicals.
[0062] Those skilled in the art will appreciate that the invention
described herein is susceptible to variations and modifications
other than those specifically described without departing from the
scope of the invention. All such variations and modification which
become apparent to persons skilled in the art, should be considered
to fall within the spirit and scope of the invention as broadly
hereinbefore described. It is to be understood that the invention
includes all such variations and modifications. The invention also
includes all of the steps and features, referred or indicated in
the specification, individually or collectively, and any and all
combinations of any two or more of said steps or features.
[0063] The reference to any prior art in this specification is not,
and should not be taken as, an acknowledgment or any form of
suggestion that that prior art forms part of the common general
knowledge.
* * * * *