Note: Descriptions are shown in the official language in which they were submitted.
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REINFORCING FOR CONCRETE PRODUCTS AND REINFORCED CONCRETE
PRODUCTS
THIS INVENTION relates to reinforced concrete
products.
Polymeric fibres, tapes and meshes are used as
reinforcing in hydraulic matrices (also referred to as
cementitous matrices). They are the conventional products of
the textile and plastics industries and are primarily
intended to be used for spinning and weaving, or have been
produced for other purposes. The problems that hydraulic
matrices, of which type 1 cement (Ordinary Portland Cement)
is an example, have in interfacing with them have not
hitherto been addressed to the best of Applicant's knowledge.
The creation of polymeric reinforcing fibre, tape
or mesh of high tenacity, involves a draw down or stretch
ratio. This can vary in the range 5:1 to 15:1 for extruded
tapes and spun multi-filaments and up to 50:1 for solvent/gel
spun multi-filaments. In both cases the fibre produced has a
smooth surface. Some polymers from which the fibres are made
' are hydrophobic. For an hydraulic matrix to achieve any
significant mechanical, frictional or chemical bond to fibres
or yarns made in this way is virtually impossible.
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According to one aspect of the present invention
there is provided a yarn for use in a cement mortar matrix, .
the yarn including a core and a multitude of staple fibres
forming a layer which envelopes the core and provides an
extended surface area and interstical spaces for infiltration
by cement fines and hydrates, the staple fibres being spun
around the core and attached to the core, the staple fibres
having sufficient freedom of radial movement to provide said
spaces and permit ingress of cement fines and the formation
of its hydrates in said spaces.
Preferably said core comprises two or more core
strands which are twisted together, portions of the staple
fibres being trapped between the core strands as the core
strands are twisted together thereby to form a mechanical
connection between the core strands and the staple fibres.
The strands of the core can have adhesive between them.
In one form said core and said staple fibres are
of synthetic plastics materials which weld to one another
upon being softened, the core and the fibres of the layer
being welded to one another at spaced locations along the ,
length of the yarn. In another form said fibres and said
core are adhered to one another at spaced locations.
Said layer can consist mainly of fibres with
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hydrophobic properties intermingled with some fibres which
have hydrophillic properties. It is also possible for said
layer to include soluble fibres containing additives for
enhancing the properties of the hydrate crystals during their
formation, Alternatively the core and/or the staple fibres
can have thereon a soluble coating containing additives for
enhancing the properties of the hydrate crystals during their
formation.
According to a further aspect of the present
invention there a.s provided a concrete article with yarn as
defined above therein as reinforcing.
The yarn can be used in the form in which it is
produced but cut into pieces, or can be woven to form a tape
or cloth which is embedded in the concrete matrix.
By using two or more strands in the core, the
leading ends of the staple fibres which are fed transversely
towards the core during the spinning process can be trapped
by the core strands. As the staple fibres are spun around
the core and the core strands are twisted, they become
mechanically locked together.
Additionally the core strands can be coated in-
' line with an adhesive of a type compatible with the materials
of which the core strands and the staple fibres are made.
The staple fibres then also form a barrier which prevents the
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adhesive from causing a length of the coated finished yarn
from sticking to an adjacent length of the yarn.
The function of the core strands is to provided
the reinforcing. The staple fibres are there to provide the
means for the hydraulic matrix to grip the core strands. The
staple fibres offer a surface area several orders of
magnitude greater than the surface area of the core strands.
Furthermore, their interstices provide a void space which can
be infiltrated by the hydraulic matrix, which as it
crystallises envelopes the staple fibres, thus forming a
composite interface between the reinforcing core and a
cementaceous matrix.
The staple fibres preferably consist mainly of
hydrophobic material so as not to interfere with the
water/cement ratio which significantly influences the
strength of the fully cured cement mortar, or concrete, in
which the fibre product is used.
The staple fibres can be a blend of fibres, a
small percentage of the total having hydrophillic properties,
enabling them to retain sufficient water to ensure that the
hydraulic matrix in contact with them is fully cured.
Soluble fibres, or fibres that have a soluble
coating, can be included to release additives into the
hydraulic matrix that enhar~c;e the properties of the cement
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hydrate crystals as they form, without affecting the
properties of the bulk of the matrix. One example of a
performance enhancing additive is silica fume. This can
change the ratio of the hydrates produced during hydration in
an advantageous manner. Another additive is gypsum
anhydrate, which when in contact with cement hydrates, can
cause expansion. Other additives, and their effect, are
known to those skilled in the art.
While dosing via soluble fibres is a preferred
method, the additives can also be infiltrated into the
interstices of the staple fibres and retained there by the
use of a soluble coating. Sodium alginate is the preferred
coating.
The staple fibres are preferably applied to the
core yarns by a spinning process. An example of such a
process is friction spinning as developed by Feher AG of
Linz, Austria. Adhesive can be applied in-line immediately
prior to the spinning process. The staple fibres then also
serve to prevent the adhesively coated strands from sticking
to each other. This could be a problem were it not an in-
line process. The friction spinning process therefore has to
be customised to meet the needs of the method of production
of the yarn according to the invention. The use of multiple
adhesively coated strands that converge at the point where
the staple fibres are being introduced adds an adhesive bond
to the mechanical interlock that occurs between the core and
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the staple fibres.
The friction spun staple fibres can be more
loosely applied to the core strands if the core strands are
coated with an adhesive before the friction spinning process
takes place. This is of particular significance in the case
of high tech fibres, where high interlaminar shear forces
have to be transmitted through the interface layer of staple
fibres into the ultra strong reinforcing core. Such forces
can exceed 1 GPa.
A suitable adhesive can be made from a hot melt
adhesive by dissolving it in a suitable hot solvent and
allowing it to cool. A room temperature volatile gel is thus
produced. This can be coated onto the core strands. The
solvent volatilises leaving a thin layer of hot melt adhesive
gel. The solvent can be recovered and condensed for reuse.
The hot melt adhesive can also be formulated to become the
carrier of the matrix performance enhancing additives
mentioned above.
After the staple fibres have been friction spun
onto the surface of the adhesively coated core strands, the
composite core can be heated. This softens the adhesive thus
heat setting the friction spun fibres to the surface of the
core and at the same time creating a ridged surface of
adhesive on the core strands along which the staple fibres
will, once in the cement matrix, not be able to slide.
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Yarn made in accordance with this invention
provides interstitial spaces into which cement and its
hydrates can flow and mechanically interact with the staple
fibres. In some cases additives are included which
chemically interact with the cement and/or its hydrates to
create a preferred interface, selectively using the hydraulic
matrix in which the interactive strands, or products made
from them, are used to enhance the matrix where .it is to
become the interface with the interactive fibre strands.
Yarn made in accordance with this invention
comprises two or more components each with its own well
defined function.
The yarn can have:
A high tenacity core, to carry the load, the core
comprising one, two or more polymeric core strands or
alternatively multi-filaments;
one or more layers of staple fibres spun onto the core;
a mechanical locking system between the core and the
staple fibres;
an adhesive bonding layer between the core and the
staple fibres;
an adhesive that includes additives to react with the
hydraulic matrix;
hydrophillic fibres forming a portion of the staple
fibres;
fibres coated with a water soluble adhesive that
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dissolves releasing additives into the cement as it hydrates;
inclusions in the extended surface layer of the fibres
to react chemically with the hydrating cement in order to
create a preferred topical matrix.
The surface layer of staple fibres is preferably
applied to the high tenacity core by the process known as
friction spinning, for which Feher AG, of Linz, Austria
supplies suitable equipment.
A preferred embodiment uses a plurality of core
strands, the strands being fed in a cone to a nip in order to
catch the leading ends of the staple fibres as these are fed
transversely towards the nip. With the leading ends of the
staple fibres trapped between the strands of the core,
spinning serves to bind them in place. Twisting of the core
strands enhances the bond.
The staple fibres reinforce the cement interface
and transfer the load on the concrete product into the core
strands. A load usually results from the bending or flexing
of the cement matrix or concrete product in which the yarn is
used.
The staple fibres, when wound onto the core,
result in a permeable layer of fibres. This makes the yarn
suitable for gas treatment, an example being fluorination,
and equally suitable for treatment by irradiation. The
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latter can additionally be used to modify the properties of
the adhesive by the process known as cross linking. The
fluorination process is known to create a polar surface on
certain polymers that can improve its adhesion to hydraulic
matrices,
Failure of the presently used polymeric fibres is
usually by "pull out" from the concrete. Under load, they
increase in length and reduce in diameter thereby freeing
themselves from the largely frictional grip of the concrete.
The yarns of this invention are gripped
mechanically, and in many cases chemically, by the hydraulic
matrix. If the final product fails it is because the matrix,
the interface or the fibres themselves have failed under load
as pull out of the interactive fibre strands under load is
not possible.
During hydration of the cement, the crystals of
hydration become the matrix in and around the staple fibres
and are modified by additives in or around those staple
fibres, so as to become an enhanced matrix. Products made
using the methods taught in this application can be
internally reinforced and also reinforced to the outer
surfaces of the product.
Replacing steel reinforcing systems in chosen
applications by yarn according to this invention enables
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thinner, lighter cement based products to be made.
Accelerators can be used that would cause the corrosion of
i
steel, enabling moulds to be more productively utilised.
Y
Bulk concrete products according to the invention
are less prone to cracking. Furthermore because the
physical properties of the cement matrix interface to the
yarn is enhanced, the toughness of the bulk matrix is
improved and the deflection under load with respect to steel
reinforced concrete is reduced.
The deflection of a conventionally reinforced
beam under load leads to cracking of the tensile, or
flexural, face of the beam. The greater the deflection the
wider the cracks. Cracks that are uniform and fine can self
heal. The yarn in accordance with this invention
redistributes stress resulting in more but finer cracks.
This leads to more durable concrete.
Yarn, tapes and cloths made in accordance with
the teachings of this application are more desirable than
steel for the purpose of reinforcing cellular or lightweight
aggregate cement based products because steel reinforcing is
generally incompatible with the significantly reduced
compressive strength of lightweight concrete.
The reinforcing core of the yarn can be man made
synthetic textile yarns or natural textile yarns. Examples
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are rayon, nylon, polyester, polyethylene, polypropylene,
TM
carbon, Kevlar, gel spun polyethylene or zirconia glass high
tech fibres.
All of these fibres can be characterised as
having a surface requiring chemical, gas, corona discharge or
irradiation treatment to create a surface to which a chemical
bond can be achieved by an adhesive matrix. Epoxy or
polyester resins are examples of adhesives that will bond
after such treatment. However, these treatments do not yield
a surface to which a water based matrix such as cement, or
its hydrates, can either interlock mechanically or
significantly bond chemically. Further these process are not
normally used in the textile industry. This leads to
multiple handling, increasing the cost of the end product.
Such fibres cannot therefore be used as reinforcing unless
they form part of composite yarns as described herein.
Cement and similar hydraulic matrices are by
their nature used in bulk as low cost matrices. The cost of
any required additive or reinforcing is a factor in
determining whether or not they would be used. This does
not, of course, eliminate such treatments from being used
when there is a commercially or technically valid reason to
do so.
In some cases it is desirable to apply two layers
of friction spun staple fibres, the two layers being spun
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with opposite helixes, ie by being applied from the opposite
ends of the core. This makes it possible for the grip of
these fibres to the core to be enhanced.
The extended surface area of the fibres of the
composite yarn provides a fibrous surface. This acts
partially as a filter allowing only the finer more reactive
cement particles and the hydrate gels to enter the
interstices of the friction spun fibre layer.
During the hydration of cement, a solution of
hydrate gels forms. This solution is composed of water and
products leached from the cement by the water. Calcium
hydroxide and calcium silicate hydrate are two examples. The
latter is the preferred matrix or binder.
The staple fibres are wound in a spiral semi-hoop
wise fashion. The volume between the core and the spirally
aligned fibres, the adhesive, or specially manufactured
fibres, can be used to carry additives that can enhance one
or more properties of the cement hydrates. Examples of such
additives are sodium and calcium silicate, gypsum,
ettringite, rapid hardening cement or pozzalans such as
pulverised fuel ash, or silica fume. Other additives will be ,
known to those skilled in the art.
One or more of these additives can be used to
cause an interaction with the hydrating cement. As an
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example, the formation of calcium hydroxide can be suppressed
in favour of the formation of calcium silica hydrate. Silica
fume is known to be a suitable additive in this regard.
Further in the presence of, for example, gypsum anhydrate the
cement hydrates can be caused to expand.
The expansion that takes place does so within the
confines of the annular space between the inside faces of the
staple fibres and the core and has no effect on the bulk of
the concrete within which the fibres are being utilised. The
additives can be present in soluble coatings on the fibres or
in the adhesives used to hold the fibres in place on the
core, or as particulate matter infiltrated into the
interstices of the fibres and if necessary held in place by a
soluble material.
During the expansion of the cement interface
matrix, the annularly aligned fibres constrain radially
outward expansion, causing the crystals of hydration to press
against the reinforcing core of the composite yarn. This
leads to an enhanced grip of the core strands by the matrix.
The concept of selectively dosing part of a
cement matrix by the incorporation of fibres as the carrier
for an enhancement additive has many practical applications,
not necessarily limited to the use of such fibres for
reinforcing. In this way a direct bond can be established
between the hydrates and the extended surface area of the
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staple fibres, between the hydrates and the core and between
the fibrous-cement hydrate composite layer and the
cementaceous matrix of the product in which the cement forms
the binder or matrix.
By positioning preferential additives so that
they only enhance the matrix in contact with the fibres, the
full benefit of adding the additives is gained, without
having to add the additives to the bulk matrix. There are
two reasons to prefer this. Firstly, only the matrix in
contact with the fibres is the subject of enhancement. The
bulk matrix is not necessarily enhanced by such additives as
the fibres are typically a small fraction by volume of the
total mix. To modify the bulk mix, for an effect only
benefiting the fibre interface, is both counter productive
and expensive. Using the extended surface of the reinforcing
yarn to selectively 'alter the characteristics of a bulk
cement mix where it contacts the fibres, restricts the
reaction between the additive used and the cement to where it
is of the most benefit in so far as the reinforcing effect of
the fibres is concerned.
Interactive fibre strands such as .are described
in this application adsorb water, due to reduced surface
tension, before the cement hydration process commences. With
the exception of natural or hydrophillic fibres they will not
absorb water and so will have little or no effect on the
critical water/cement ratio.
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During consolidation of the cement matrix either
by vibration or by vacuum table or other techniques, water
carrying with it cement particles penetrates into the
friction spun layer of fibres. Intermixing of the particles
of cement with the additives contained in this area takes
place and the chosen hydrates form.
The calcium silicate hydrate gel from the cement
particles can be encouraged to form within the cross-section
defined by the outside of the core and the outer extremity of
the friction spun layer of staple fibres, namely. in the area
partially filled with the prechosen additives. The normal
hydrate ratio is of the order of 60~ calcium silicate to 40~
calcium hydroxide. The ratio can be altered to, for example,
80:20.
The calcium silicate hydrate that forms within
the layer of fibres crystallises as calcium silicate.
Calcium silicate comprises fine, strong but brittle crystals
that, as they continue to crystallise, impinge against each
other and fuse together. During the crystallisation stage
they occupy the interstitial spaces of the staple fibres
forming a calcium silicate-fibre composite interface.
Most cementaceous interfaces are brittle and
under shock or impact loads fail. A characteristic of the
fibre-calcium silicate composite interface is that it is a
composite with a mechanical bond both to the reinforcing core
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and to the cement matrix. Any crystals that form inside the
spirally bound hoop like fibres, expand and impinge against
the reinforcing core. This effect is enhanced by the use of
an additive such as gypsum anhydrate that can be within the
friction spun fibre layer.
The fibres in the composite interface reduce the
brittleness of the calcium silicate, creating a pseudo
ductile interface layer.
The interactive composite yarn described can be
used as produced ie in yarn form, or woven into a tape or
cloth suitable for use in beam or sheet type applications.
The yarn, when is used as it is produced, can be cut into
pieces of some chosen length. Longer lengths are required
for use in large aggregate mixes and shorter lengths for use
Z5 in grout type mixes. The shorter the yarn is cut the more
its friction spun staple fibres will benefit from the
adhesive bond to the core of the composite yarn.
Interactive yarn, cloth etc can be cut using a
laser or hot air gun. This also serves to fuse the ends of
the staple fibres together and, in the case of a
thermoplastics core, to the core. This is beneficial in the
absence of an adhesive bond between the fibre outer layer and
the core.
During mixing, the fibres can be uniformly
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dispersed throughout the mix. They serve to prevent de-
mixing during pumping and placing and segregation under
vibration. Because the mix remains mixed it is easier to
., obtain site test results that compare with those obtained in
the laboratory.
The reduced surface tension around the strands
causes free water to be adsorbed from the concrete mix thus
preventing the formation of surface puddles which, when they
evaporate, reduce the water-cement ratio. The water adsorbed
by the strands remains available to the cement throughout the
hydration process.
Reinforcing interactive composite yarns made with
a high elongation core can be pre-stressed by, for example,
20~. At practical diameters this results in a reduction of
about 10~ in the diameter of the reinforcing core. Once the
fibrous cement mix has hydrated and the prestressing load is
removed the staple fibres and the cement hydrates prevent the
core from recovering its original length. The tension in the
yarn is therefore converted into a compressive force in the
concrete. The fibres in the mix help to contain the force
following the release of the pre-stressing force even if, for
example, the product is cut through between the places at
which the yarn is anchored. Tapes and even woven cloth can
be prestressed in this way, enabling load bearing beams and
floor panels to be precast. Tapes and cloth have the
advantage that they can be cut to fit and maintain the load
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bearing properties of the whole beam.
Tape and woven cloth made from the interactive
yarns described can be dipped into a fluidised bed containing
a cementitous powder. The powder infiltrates the friction
spun layer of stable fibres. As the tape or cloth leave the
fluidised bed, they can be wrapped in a layer of impermeable
material such as polyethylene film to keep the dry cement
mixture in place. Alternatively a non-woven finely textured
tissue can be used. The later can remain in place as a
component of the end product.
The subsequent use of such preimpregnated
materials need only involve their being wetted and allowed to
drain, prior to being used in a mould or against a former for
moulding. Pre-impregnated materials are suitable for hand or
machine lay up into sheet-like structures. Alternatively
they can form the surfaces of a cellular or normal density
sandwich panel.
Interactive fibre strands can be used to create
satin weave or knitted cloths enabling articles with complex
curvatures to be made using tt.-0~ae techniques. Polymeric
fibres have the advantage of being non aggressive and are
therefore not harmful either to the hands of the user, or to
a
the environment in which they are used.
Cloth made from interactive composite yarn acts
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as a filter. When used to line shuttering it provides a mesh
which prevents large aggregate particles from reaching the
surface of a shutter during casting. Fines from the concrete
mix penetrate the mesh and, flow through the mesh in a
controlled manner. The space bounded by the shutter fills
from the bottom up, the cement fines displacing air as the
space against the shutter face is filled.
If the cloth is displaced away from the shutter
by the denser cement and fine sand particles, this gives the
face of the pour a fines rich reinforced surface. This
reinforced surface is better able to resist impact damage and
is less permeable making it ideal for marine use and for use
in corrosive environments in general.
Material made in this way can be used after the
fashion of papier mache to create strong thin fibre cement
mouldings, such as garden ornaments, floor tiles, roofing
sheets and boat hulls.
The methods described enable cement matrixed
mouldings with more than 20~ of fibres by volume to be made.
This results in thin, light strong.finished products.
Because proportionately less matrix is used, additives can be
incorporated into the cement matrix, without having a major
impact on the cost of the final product. This enables
products made using hydraulic matrices to compete for market
share with those made from solvent based, or catalytic resin
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systems.
Reinforcing yarn with a friction spun surface is
particularly suitable for use in cellular cement and low
density aggregate cement mixes where traditional reinforcing,
such as steel bars or meshes, are less effective. The
cellular/lightweight aggregate mixes develop insufficient
strength to be able to grip steel reinforcing. The larger
surface area of the described yarn etc is more suitable.
Cellular fibre or lightweight aggregate cement
mixes used in conjunction with woven cloth or tape made in
accordance with the methods disclosed in this application are
suitable as an alternative to wooden joists or plywood. The
cement composite fibre ply is suitable for use where marine
ply would otherwise be specified and is particularly suitable
for use as lost formwork. Such a formwork remains in place
as the finished surface of the concrete.
A specific example of application is waffle or
trough type floors as it avoids the problem of having to
strip, clean and store large mouldings or shutters. A
further example of use is as highway barriers. These can
stack for ease of transport, can be quickly positioned and ,
bolted together without the need for lifting equipment. The
hollow core can be used to hold a plastics bag filled with
water. A vent valve can be provided on the bag to allow the
water to escape at a controlled rate on impact.
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Alternatively, the units can be back filled with soil, sand,
or concrete. In the latter case they can be used with a weak
mix as left in situ moulds, or with a strong mix as re-usable
r moulds.
Figure 1 illustrates the preferred method of
producing a yarn in accordance with the present invention.
The two strands designated 18 and 20 together constitute a
core designated 22. The strands 18 and 20 are fed on a
converging path to a nip. At the nip the staple fibres 26
are presented to the strands and their leading ends are
trapped between the strands. In accordance with the present
invention the strands preferably have an adhesive coating 24
applied thereto just before they reach the nip. The adhesive
coating secures the strands 18, 20 to one another and also
assists in binding the staple fibres. The staple fibres 26
themselves form a cover for the adhesive coating 24. This
prevents adhesion between turns of the yarn when it is wound
onto a bobbin or the like.
The yarn produced by the process described has a
central core and a fluffy sheath of staple fibres. Each
fibre has the end thereof which was presented to the nip
trapped between the strands and the remainder of the fibre is
wound in a helical manner around the core. Because each
staple fibres is overlapped by a multitude of other staple
fibres, the end result is that the core is entirely sheathed
by a layer of staple fibres.
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The staple fibres of the sheath are secured to
the core at intervals along the length of the yarn. This can
r
be achieved by passing the yarn through heated rollers which
make contact with the yarn at intervals of, for example, 5mm.
As each individual staple fibre extends for about 40mm along
the core, it is thus attached to the core at six to nine
locations.
The resulting yarn, as shown in Figure 2, has
spaced locations 30 at which the fibres of the sheath are
secured to the sheath. Between these locations the fibres
sre spaced outwardly from the sheath leaving an annular gap
between the core and the staple fibres. It is these gaps
that the cement fines and hydrates enter when the yarn is
used for reinforcing purposes.
Figure 3 is a diagrammatic cross section which
shows the strands 18, 20. It also shows the fibres 26.
Reference numeral 28 designates crystals that have formed
within the fibrous cover constituted by the staple fibres 26.
As explained above the product can include an additive which
promotes formation of the requisite crystals.
It is also possible to use staple fibres of
different types. Simply by way of example, 90~ of the staple
fibres in a product can be hydrophobic, 5$ can be
hydrophillic and 5$ can be of resorbable material.
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