Note: Descriptions are shown in the official language in which they were submitted.
CA 02298000 2000-O1-21
WO 99/OS358 PCTIGB98102168
~IND~IJSTRfAL FABRICS AND METHOD OF TREATMENT
This invention relates to industrial fabrics and to a method of treating
such fabrics during the manufacture thereof,to impart desired characteristics
to the fabric.
The invention relates for example to all forms of papermaking
machine fabrics including dryer fabrics, press felts, including extended nip
press belts, shoe press sleeves, corrugator machine press belts, and also to
conveyor belts, printing blankets, silicon wafer grinding belts and filter
cloths.
These fabrics may be treated for example to influence the wetting
characteristics of the fabric, or in a composite fabric of two or more layers,
of at least one layer of the fabric. Thus it is known e.g from US 5372876,
to provide e.g. a papermaking felt which includes a flow control layer
formed of a porous hydrophobic material. Conversely, EP-A-0761872
discloses a dryer fabric having a paper side contact surface which is
hydrophilic, to improve adhesion between the paper web and the dryer
fabric.
In both cases, the hydrophilic or hydrophobic properties of the layer
or surface are achieved by coating or chemical treatment by application of
the relevant compositions in a liquid medium.
As a method and means for treating substrates to influence their
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surface characteristics, the technology of plasma treatment is in the process
of being developed. A 'plasma' is a fluid state of matter wherein due to
excitation, the matter is present in the form of positive and negative ions.
Polar molecules may be dissociated into anionic and cationic radicals, whilst
free atoms lose electrons to form free electrons and positively charged
atoms - depending upon the degree of excitation, more than one electron
may be stripped from atomic shells. Plasmas can exist over an extremely
wide range of temperature and pressure; from over atmospheric to near
vacuum. Because matter in the plasma exists in the form of charged ions,
the plasma environment is extremely chemically energetic, and it has been
proposed to treat the surfaces of materials by exposure to plasmas of
selected composition.
It is an object of the present invention to provide fabrics for use in
industrial processes, such as in papermaking machines, conveyor belts,
printing blankets and filtration fabrics by way of non-limiting example, which
have had surface properties modified by plasma treatment, as an
economical and environmentally improved alternative to treatment or
coating with a liquid medium.
According to the invention papermaking or filter fabric comprises or
includes a layer or component of synthetic yarns or fibres which have been
subjected to plasma treatment.
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From another aspect the invention provides a method of making or
preparing papermaking or filter fabric including the step of subjecting at
least one surface of a layer or component of the fabric comprised of
synthetic yarns or fibres to plasma treatment.
The invention also includes apparatus for use in making or preparing
papermaking or filter fabric, including a plasma chamber to which material
containing synthetic yarns or fibres comprising or for use in making said
fabric may be introduced, and removed after treatment.
The purpose of the plasma treatment may be to modify the wetting
characteristics of the yarn or fibre surface being treated, i.e. to render the
surface hydrophobic or hydrophilic.
Alternatively the yarn or fibre surface is provided with activated sites
to improve subsequent coating or dyeing. This treatment may utilise glow
discharge dielectric barrier discharge or spray discharge plasma. Compared
with corona discharge treatment techniques the modified characteristics of
the treated surface are permanent and far more durable.
Other characteristics can be enhanced by appropriate choice of
composition of the plasma, e.g. improved softness in drying felts especially
for tissue making machines, crease resistance and antistatic properties.
In a preferred method of the invention, hydrophilic properties are
enhanced by subjecting the surface to a plasma comprising or containing
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oxygen, air or ammonia for example.
In another preferred method of the invention, hydrophobic properties
are enhanced by subjecting the surface to a plasma comprising or
containing a silane, e.g. Si(CH3)4, a siloxane, e.g. (Si(OCH3)4) or a
perfluorocarbon , e.g. 1-6C perfluoroalkane or tetrafluoroethylene or a
combination thereof.
Hard coatings of e.g. carbon may be formed by subjecting the surface
to a plasma comprising or containing hydrocarbon e.g methane. Heat
resistant coatings can be applied by using a plasma comprising or.containing
halogenated hydrocarbons or unsaturated amines (e.g. tetrachloroethylene;
1,1,2-trichloroethane; allylamine or trichloroethylene). The plasma may be
diluted with a diluent gas such as helium.
The synthetic yarn or fibre containing material may be treated in the
form of a ready made up layer or web or as fibres or yarns prior to weaving
or making up the layer.
The apparatus may be arranged so that only one surface of a fabric
is exposed to plasma treatment.
The apparatus of the invention may comprise means for continuously
introducing the material into the plasma chamber, moving the material
through the chamber during treatment, and causing it to leave the chamber
after treatment. Alternatively the material may be introduced to the plasma
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chamber in batches, which are subjected to treatment and then removed.
The operating pressure of the plasma chamber may be from in the
order of 0.1-3.0 mbar, up to and above 1 atmosphere.
A preferred embodiment of the invention will now be described by
way of example, and with reference to the accompanying drawing, which
is a diagrammatic view of a plasma treatment machine according to the
invention.
In the preferred embodiment, a fabric 10 e_g. a woven or nonwoven
web of synthetic yarns or fibres destined to form all or part of an industrial
fabric such as a paper machine press felt, dewatering drying or forming belt,
is fed from a dispensing station 11. This is shown simply as a feed roll, but
would in practice include guide and compensating devices in addition, as is
well known in the art. The fabric 10 is introduced through a self-sealing
entry slot 12 into a plasma chamber 13, and exits through a similar self-
sealing exit slot 14 after passing through the plasma chamber and being
subjected to plasma treatment within the chamber. The treated fabric 10
is then taken up on a winding station 15 which is shown as a simple take
up roll, but in practice would include such guide and compensating
apparatus as is required, as is well known in the art.
The fabric 10 enters and leaves the chamber 13 through self-sealing
slots 12 and 14 as the plasma usually operates at a considerable under-
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pressure, often extremely rarified, e.g. 0.1-3.0 mbar; and sometimes
contains components which must not be allowed into the ambient.
To achieve plasma conditions, the chamber 13 is evacuated to the
required pressure by pump means acting under pressure control 16. The
material to form the plasma is introduced at 17, and excitation proceeds by
the creation of an intense electrical field between electrodes 18, 7 9 subject
to a controller 20. The plasma is created by ionising the molecules or atoms
of the plasma forming material to form ionised radicals, or to provide free
electrons and positively charged atoms.
Treatment is achieved by exposing one or both surfaces of the fabric
to the plasma within chamber 13.
The composition of the plasma will be selected in accordance with
the objective of the treatment. For example, to improve wettability
(hydrophilic) properties of a fabric, the plasma may be created from ordinary
air, oxygen, ammonia or a mixture of these.
To provide a water-repellant finish (hydrophobic) the plasma may be
created from a siloXane or perfluorocarbon compound.
Examples of other properties which may be enhanced by such plasma
treatment are softness (oxygen plasma), crease resistance (dipped in DMSO,
then exposed to nitrogen plasma); anti-static finish - chloro (chloromethyl)
dimethylsiiane plasma; oil repellent finishes, improvement of capillarity, dye
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reception, dyeing depth, bleaching, UV-protection and flame retardancy may
also be provided for.
The fabric may instead of being fed continuously as illustrated, be
placed in the machine in batches, treated and then removed. The fabric
may be placed on a conveyor to be fed through the chamber, or associated
with a backing layer when only one surface is to be treated.
The material to be treated may be in the form of synthetic fibres or
yarns prior to making up into a woven or nonwoven fabric and such are
particularly suited to a conveyor feed, or to batch treatment e.g in mesh
cages which can be introduced to and removed from the plasma chamber.
A hydrophobic (water repellant) fabric may be used as a flow control
layer in a papermaking felt of the kind described in US, 5,372,876, whilst
a hydrophilic (with enhanced wettability) fabric may be used as a dryer
fabric such as described in EP-A-0761872 mentioned above.
The term 'fabric' as used above should be interpreted to cover
sintered felts, coatings of coated papermaking fabrics, nonwovens and
films, spiral link structures, membranes or polymer matrix material and their
components prior to making up where appropriate e.g. sintered particles.
IThe speed of material being fed into the chamber in the illustrated
version may be 5-100 metres per minute. At present working widths are
limited to about 2.7 metres. The development of larger machines capable
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of treating greater web widths is hindered by the practical difficulties
associated in maintaining a sufficient vacuum in a larger chamber. It is thus
often more convenient to treat yarns and fibres or other fabric constituents
rather than finished fabrics.
A hydrophilic surface for a dryer fabric, as in EP-A-0761872 obtains
improved adhesion between the dryer fabric and the paper web. A
hydrophobic surface may be desirable for certain dryer fabrics for
contaminant release, whilst forming fabrics may be rendered hydrophobic
for the same reasons or may be hydrophilic to improve drainage. It is
believed that a very thin film of water is attracted to the surface of the
yarns in the fabric providing "lubrication" for water flowing through the
interstices of the fabric. Batt staple fibres in press felts can be rendered
hydrophilic to make the felts absorb water better initially, improving the
conditioning of the felt and thereby reducing start up times. However, a
layer of hydrophobic batt staple fibres (or base cloth yarns) within the felt
can form an anti-rewetting or flow control layer, which hinders the return
flow to the web of water which has initially been urged into the felt from
the paper (as in US 5372876).
A further possible use of the invention is by plasma treatment of
yarns, fibres, films or membranes used to reinforce polymer matrices to
form belts, such as extended nip press belts, shoe press sleeves, press belts
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for corrugator machines, conveyor belts, printing blankets, silicon wafer
grinding belts, or carcass stripping belts. The activated groups formed on
the reinforcing substrate surfaces improve bonding with the matrix polymer.
This is particularly useful for reinforcing a polyurethane matrix with
polyester structural members such as yarns, fibres or fabrics (woven,
nonwoven spiral link or membrane), the latter having excellent dimensional
stability but poor adhesion to polyurethane. Adhesion may be improved by
a factor of 2-3. For this purpose ammonia or nitrogen plasmas are
preferred. Yarns, fibres, films, membranes or finished fabrics may be
treated by plasma according to the invention to provide hydrophobic
coatings for filter cloths (especially those used in dust filtration) to
improve
cake release.
tn some cases, only one surface of the fabric is exposed to plasma
treatment. This may be used e.g. for a forming fabric which may be
rendered hydrophilic on the machine side. This will reduce rewetting of the
web between a couch roll and felt pick-up at the end of a forming section
by increasing solids content in the web by as much as 1-3%. This can
entail a significant cost saving for the papermaker.
Plasma treatment as outlined above has the advantages that the
process is solvent free, (including no use of water), very small amounts of
the raw materials are required (e.g. 30-100 mg per m2 fabric); energy
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consumption is low, as no water is used which absorbs heat; labour costs
are negligible, as the process can be automated, and the overall cost per
unit area of fabric can be very low.
The invention is applicable to all industrial fabrics, including conveyor
belts and filter cloths, but is mainly directed to all forms of paper machine
clothing.
