Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIBROUS MATERIAL COMPRISING FIBERS MADE FROM LINEAR,
ISOTACTIC POLYMERS
FIELD OF THE INVENTION
The present invention relates to fibrous web materials such as nonwoven
materials used for example in hygienic articles. Specifically, the present
invention
relates to elastic fibrous web materials.
BACKG ROU N D
Fibrous materials and in particular nonwoven web materials comprising olefinic
polymers are well known in the art and enjoy widespread usage throughout the
industry. Typical areas of application of such fibrous web material include
for
example hygienic articles and in particular disposable absorbent articles.
Fibrous web materials made from commonly used polyolefins such as PP, PE, PS,
PIB have a number of useful properties. They are bio-compatible and food
compatible, chemically stabile, inert, non toxic materials. However, most of
them
have poor mechanical properties including insufficient strength/tear
resistance,
insufficient stretchability/elasticity and the like.
Several approaches have been proposed in the prior art to provide elastic
properties to such fibrous web materials. The most commonly used approach -
besides providing laminates of fibrous web material with for example elastic
films
- is based on changing the chemical structure of the polymer by introducing
hinged joints/moieties into the main chain of the polymer. These hinges
provide
more flexibility to the polymeric backbone preventing crystallization of
polymer,
lowering the glass transition temperature (Tg) and improving the elasticity of
the
resulting material. Usually, the hinge groups contain heteroatoms providing
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flexibility such as oxygen, nitrogen or chlorine placed into the main chain or
into
bulky side groups. Another approach is mastication of the polymer by blending
with special plasticizing agents. Both approaches, however, require
heteroatoms
to be introduced into the molecule or into the bulk of the coating material.
The third approach proposed in the prior art to provide elastic properties to
such
fibrous web materials, which is more close to the present invention, is to
exploit
the formation of hetero-phases which reinforce the bulk material by forming a
physical net. To do this the block-co-polymerization of two or more different
monomers has been used leading to polymeric backbones comprising blocks with
different Tg. This results in micro-phase separation in the bulk with
formation of
reinforcing crystalline domains of one co-polymer linked with each other by
flexible chains of the second co-polymer.
In essence, conventional polymeric web materials carry a wide variety of
inherent
disadvantages including but not being limited to insufficient strength/tear
resistance, insufficient stretchability/elasticity, not being bio-compatible,
not
being food compatible, comprising heteroatoms such as chlorine and hence
leading to toxic residues when burnt, and the like.
It is an object of the present invention to provide fibrous web materials
which
overcome the disadvantages of the prior art fibrous web materials.
It is an further object of the present invention to provide articles which
comprise
fibrous web materials.
It is an further object of the present invention to provide a method for
manufacturing fibrous web material of the present invention.
It is a further object of the present invention to provide a method processing
a
fibrous web material of the present invention.
SUMMARY OF THE INVENTION
The present invention provides a fibrous web material comprising a plurality
of
fibers. The fibrous web material is characterized in that the fibers comprise
a
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polyolefinic homopolymer having a an isotacticity of less than 60% of [mmmm]
pentad concentration.
The present invention further provides a method for manufacturing fibers from
the aforementioned polymeric material comprising a step of processing the
polymeric material selected from the group of wet spinning, dry spinning, melt
spinning, semi dry spinning (solvent evaporation or sedimentation), and
combinations thereof.
The present invention further provides a method for manufacturing a fibrous
web
material comprising the steps of providing fibers of the aforementioned
polymeric
material and of combining the fibers into a web material.
The present invention further provides a method for stabilizing a fibrous web
material according to the present invention comprising the steps of providing
a
fibrous web material and of stabilizing step the fibrous web material.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides fibrous web materials comprising polyolefinic
homopolymers.
The term ~~polyolefinic homopolymer" as used herein refers to those
polyolefins
which comprise only one phase of molecules all of which exhibiting a similar
stereochemical configuration. For example, blends of atactic and isotactic
polymers where the two phases have polymerized simultaneously are excluded
when this term is used. The term homopolymer includes copolymers where all
molecules exhibit a similar stereochemical configuration.
The polyolefinic homopolymer of the present invention may comprise linear
isotactic polymers having a structure of one or several C3 to Czo olefinic
monomers, having an isotacticity of less than 60%, preferably less than 55%,
more preferably less than 50%, and most preferably less than 45% of [mmmm]
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pentad concentration, and having an isotacticity of more 15%, preferably more
than 20%, more preferably more than 25%, and most preferably more than of
[mmmm] pentad concentration. Preferably, the polyolefinic homopolymer is
polypropylene.
The isotacticity of the homopolymers may be reduced compared to the isotactic
polypropylenes of the prior art due to a statistic distribution of
stereoscopic errors
in the polymer chain. The term °stereoscopic error" refers to a
stereoscopic
sequence characterized by a [mrrm] pentad. In this case, the central monomer
has a stereo configuration opposed to the other four monomers in this pentad.
The [mrrm] pentad concentration of this polymer therefore is above the
statistical probability of p2 (1-p)Z where p=[m] and hence 1-p=[r] and
p4=[mmmm]. Preferably, the pentad concentration is at least [p (1-p)]q p (1-p)
with q being 0.8, more preferably q being 0.6, yet more preferably q being
0.4,
yet more preferably q being 0.2, most preferably q being 0.1.
In some embodiments of the homopolymer and in particular in those
embodiments where the crystallinity is reduced by means of single stereo
errors,
a low content of atactic sequences has proven beneficial to the properties of
the
fiber of the present invention. Preferably, the [rmrm] pentad concentration is
below 6%, more preferably below 5%, yet more preferably below 4%, yet more
preferably below 3%, most preferably below 2.5%.
In some embodiments of the homopolymer and in particular in those
embodiments where the crystallinity is reduced by means of single stereo
errors,
a low content of syndiotactic sequences has proven beneficial to the
properties of
the fiber of the present invention. Preferably, the [rrrr] pentad
concentration is
below 6%, more preferably below 5%, yet more preferably below 4%, yet more
preferably below 3%, most preferably below 2.5%.
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Alternatively, the homopolymer of the present invention may include sequences
of atactic and isotactic blocks of polymer.
Preferably, the mean molecular weight MW of the polymer is above 100000 g/mol,
more preferably above 200000 g/mol, yet more preferably above 250000 g/mol,
yet more preferably more than 300000 g/mol, most preferably more than 350000
g/mol.
The glass temperature T9 is between -50 and +30 °C. Preferably the
glass
temperature is below 10°C, more preferably below 5°C, yet more
preferably
below 0°C, most preferably below -6°C. The melt temperature of
the polymer is
obtained after heating the sample 150°C and subsequently cooling the
polymer
to -50°C.
Without wishing to be bound by this theory, the polyolefinic polymers exhibit
a
semi-crystalline structure. The structure contains elastic amorphous areas of
nano-scale-size reinforced with self arranged crystalline domains of nano-
crystals.
The formation of brittle macro-crystalline material from the polymer is
achieved
by introducing the defects into the polymeric backbone. Isolated monomer units
with opposite stereo configuration have been used as the defects, i.e. single
stereo errors.
Suitable polymers and a process for manufacturing such polymers are described
in PCT patent application EP99/02379 incorporated herein by reference. A
catalyst combination suitable for the preparation of such polymers is
described in
PCT patent application EP99/02378 incorporated herein by reference.
Preferably,
the process of PCT patent application EP99/02378 is carried out by
temperatures
of less than 30°C, more preferably less than 25°C, yet more
preferably less than
20°C, most preferably less than 15°C to increase the molecular
weight of the
resulting polymer. In order to increase the molecular weight, the
polymerization
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is preferably carried out in liquid monomer such as in liquid propene. In
order to
increase the molecular weight, the catalyst is preferably used in combination
with
the boron activators mentioned in PCT patent application EP99/02378.
Other suitable polymers and a process for manufacturing such polymers is
described in W099/20664 incorporated herein by reference.
It is preferred to use homopolymers for the fibers of the present invention
since
during manufacture of homopolymers the batch to batch variability is greatly
reduced in comparison to multi phase polymers where the phases are
polymerized in a single reaction.
Preferably, the polymers used in manufacturing the fiber materials of the
present
invention have a distinctive rubber-elastic plateau in their stress strain
curves.
The polymers used for the fiber of the present invention are bio-compatible
may
be burnt without toxic residues since they contain no heteroatoms such as
chlorine. The further do not contain toxic monomer residues.
The fiber materials of the present invention have been found to be able
exhibit
superior softness. Preferably, the fiber material has a Shore hardness on the
A
scale of less than 30, more preferably, of less than 25, yet more preferably
of
less than 20, yet more preferably of less than 15, most preferably of less
than 10.
The softness of the fiber material of the present invention can be increased
by
manufacturing the fiber by reducing the isotacticity ([mmmm] pentad
concentration).
The fiber material has been found to exhibit increased temperature stability
compared to prior art fiber materials. This is partly due to the fact that for
the
fibers of the present invention a homopolymer is used and is partly due to the
high molecular weight of the homopolymer. Preferably, the fiber material of
the
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present invention has a melting point of at least 100°C, more
preferably of at
least 110°C, more preferably of at least 120°C, most preferably
of at least 130°C.
The melt temperature of the polymer is obtained after heating the sample
150°C
and subsequently cooling the polymer to -50°C. Higher melting point may
be
achieved my blending the homopolymer for example with conventional isotactic
polymer such as polypropylene.
The fiber of the present invention have been found to be stretchable as well
as
elastic. The stretchability of the fiber versus its elastic behavior can be
adjusted
by means of the tacticity of the homopolymer of the present invention. The
fiber
material of the present invention has been found to be stretchable without
tearing to at least 500% of its original length, more preferably 1000% of its
original length, yet more preferably to at least 1500% of its original length,
most
preferably to at least 2000% of its original length. In addition, the fiber
material
of the present invention preferably recovers within 10 minutes after being
stretched and held for 1 minute to 500% of its original length back to less
than
300% its original length, preferably less than 200% its original length, most
preferably less than 150% of its original length. In addition, the fiber of
the
present invention has been found to exhibit a low compressive set. The fiber
of
the present invention recovers within 10 minutes after a compression to 50% of
its original thickness for 1 minute to at least 60% of its original thickness,
more
preferably at least 70% of its original thickness, yet more preferably to at
least
80% of its original thickness, yet more preferably to at least 90% of its
original
thickness, most preferably to at least 95% of its original thickness. The
compressibility of the fiber of the present invention can be adjusted by
increasing
the tacticity of the homopolymer or by blending the low tacticity homopolymer
with conventional isotactic polymer such as polypropylene.
The fiber of the present invention has been found to exhibit a relative low
tackiness at room temperature due to the high molecular weight of the polymer.
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Various additives may be added to the homopolymer of the present invention to
change the properties of the polymer such as is well known in the art.
There are known in the art a wide variety of suitable methods to manufacture
and /or processing fibers from the polymer of the present invention including
but
not being limited to wet spinning, dry spinning, melt spinning, semi dry
spinning
(solvent evaporation or sedimentation), crazing, and combinations thereof.
Fibers
suitable for the web materials of the present invention may be mono fibers or
the
may comprise filaments.
For at least some of the spinning processes, it has been found that the amount
of the low isotacticity homopolymer of the present invention present in the
fiber
of the present invention needs to be reduced in order to accelerate
crystallization
of the fibers after spinning. Preferably, the fiber of the present invention
comprises less than 80% of the low isotacticity homopolymer, more preferably
less than 60%, yet more preferably less than 40%, most preferably less than
30%. Alternatively, a high isotacticity polymer having a broader molecular
weight
distribution may be used in order to accelerate crystallization times.
Compared to spinning of conventional, isotactic polypropylene, the addition of
the low isotacticity homopolymer of the present invention reduces the requires
forces, pressures, or torques respectively to process the polymer.
There are known in the art a wide variety of suitable methods to manufacture
fibrous web material according to the present invention from fibers including
but
not being limited to meltblowing, spunbonding, carding, air laying, wet
laying,
weaving, knitting, bailing, and the like. There are further known in the prior
art a
wide variety of suitable methods for optional stabilization of the fibrous web
material of the present invention including but not being limited to
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hydroentangling, thermo bonding, pressure bonding, air through bonding,
needling, resin bonding, combinations thereof, and the like.
It is a further aspect of the present invention to provide an article
comprising a
fibrous web material according to the present invention.
The article according to the present invention may be a hygienic article. The
term
"hygienic article" as used herein refers to articles which are intended to be
used
in contact with or in proximity to the body of a living being. Such hygienic
articles
may absorbent or non-absorbent. Such hygienic articles may be disposable or
intended for multiple or prolonged use. Such hygienic articles include but are
not
limited to disposable absorbent article (diapers, sanitary napkins, adult
incontinence devices such as briefs, bed mats, wound plasters, underarm sweat
pads, and the like), medical supply items (coverings, gowns, drapes, face
masks,
bandages, body implants, and the like), and other hygienic articles such as
toys,
bed covers, and the like. Having regard to the specific advantages of the
polymers used for the articles of the present invention, it will be readily
apparent
to the skilled practitioner to apply the fibrous web materials according to
the
present invention in the above and similar hygienic articles.
The article of the present invention may also be a clothing article or a
household
article including but not being limited to bed covers, underwear, tights,
socks,
gloves, sport clothing, outdoor clothing, low temperature clothing, shoes and
show covers, protective clothing such as for motor biking, blankets, covers,
bags,
items of furniture, and the like. Having regard to the specific advantages of
the
polymers used for the articles of the present invention, it will be readily
apparent
to the skilled practitioner to apply and to optionally modify the fibrous web
materials according to the present invention in the above and similar
articles.
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The fibrous web material according to the present invention may also be used
as
a construction element in an article. Thereby, the functionalities of the
fibrous
web material includes but is not limited to supporting, carrying, fixing,
protecting
other elements of the article and the like. Such articles include but are not
limited
to adhesive tapes, protective wraps, complex constructions such as buildings
(floor coverings, house wraps, and the like), cars, household appliances,
horticultural and agricultural constructions (geotextiles), and the like.
Having
regard to the specific advantages of the polymers used for the articles of the
present invention, it will be readily apparent to the skilled practitioner to
apply
and to optionally modify the fibrous web materials according to the present
invention as construction elements in the above and similar articles.
The article of the present invention may further a membrane such as in
filters,
car batteries, and the like. Having regard to the specific advantages of the
polymers used for the articles of the present invention, it will be readily
apparent
to the skilled practitioner to apply and to optionally modify the fibrous web
materials according to the present invention in the above and similar
articles.
