Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for the preparation of polyolefin fibers
Field of the invention
The present invention relates to a process for the preparation of polyolefin
fibers, preferably
nanofibers, in particular for the preparation of polyethylene fibers. The
present invention
also relates to polyolefin fibers obtained therewith.
Background of the invention
There is a growing need for very fine fibers and fibrous webs made from very
fine fibers.
These types of webs are useful for selective barrier end uses. The nanofiber
webs find use
in a wide range of applications such as filtration, membrane separation,
protective military
clothing, biosensors, wound dressings, and scaffolds for tissue engineering.
However,
despite the potential mentioned above, the application of nanofibers has been
limited due to
its poor mechanical properties.
Electrospinning and melt-blown spinning are the most widely used spinning
methods to
prepare polymeric fibers. Electrospinning is preferred for the nanosize fibers
but this technic
presents some drawbacks such as the requirement for a high voltage electrical
field, a low
production rate and the requirement for precise solution conductivity.
Unlike electrospinning, forcespinning doesn't require materials presenting
dielectric
properties for processing which limits the materials that can be produced into
fibers.
Forcespinning is a method where a spinneret is rotated at high speed.
Centrifugal force and
hydrostatic pressure are combined to eject jets of liquid material through
orifices. As a jet
spray of material exits an orifice, the aerodynamic environment and the
inertial force of the
rotating spinneret stretch the material into a nanoscale fiber.
To this day there is little or no success reported in making polymeric
nanofibers, especially
from polyolefin, such as polyethylene and polypropylene having good mechanical
properties.
In view of the foregoing, there is a need to develop other efficient processes
for the
preparation of polyolefin fibers and in particular polyolefin nanofibers
having improved
mechanical properties.
Summary of the invention
The present invention provides the solution to one or more of the
aforementioned needs.
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According to a first aspect of the present invention, a process for the
preparation of
polyolefin fibers having mean fiber diameters of less than 5000 nm is
provided, said process
comprising the steps of:
a) preparing a polyolefin solution in a solvent,
b) placing said polyolefin solution in a fiber producing device comprising a
body configured
to receive said polyolefin solution, said body comprising one or more
openings, and
c) rotating the fiber producing device, wherein rotation of the fiber
producing device causes
the polyolefin solution to be passed through said one or more openings to
produce
polyolefin fibers having mean fiber diameters of less than 5000 nm,
wherein said polyolefin is selected from the group comprising polyethylene
polymers and
copolymers having a weight average molecular weight Mw of at least 40 000
daltons, and
polypropylene polymers and copolymers, having a weight average molecular
weight Mw of
at least 120 000 daltons.
Preferably, the present invention provides a process for the preparation of
polyolefin fibers
having mean fiber diameters of less than 5000 nm, said process comprising the
steps of:
a) preparing a polyolefin solution in a solvent,
b) placing said polyolefin solution in a fiber producing device comprising a
body configured
to receive said polyolefin solution, said body comprising one or more
openings, and
c) rotating the fiber producing device, wherein rotation of the fiber
producing device causes
the polyolefin solution to be passed through said one or more openings to
produce
polyolefin fibers having mean fiber diameters of less than 5000 nm,
wherein said polyolefin is selected from the group comprising polyethylene
polymers and
copolymers having a weight average molecular weight Mw of at least 40 000
daltons, and
polypropylene polymers and copolymers, having a weight average molecular
weight Mw of
at least 120 000 daltons;
wherein said fiber producing device is rotated at a speed of at least 10 000
revolutions per
minute (RPM).
In a second aspect, the present invention also encompasses polyolefin fibers
having mean
fiber diameters of less than 5000 nm obtained by the process according to the
first aspect of
the invention.
In a third aspect, the present invention also encompasses polyolefin fibers
having mean
fiber diameters of less than 5000 nm, wherein said polyolefin is selected from
the group
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comprising polyethylene polymers and copolymers having a weight average
molecular
weight Mw of at least 40 000 daltons, and polypropylene polymers and
copolymers, having
a weight average molecular weight Mw of at least 120 000 daltons.
In a fourth aspect, the present invention also encompasses articles comprising
the
polyolefin fibers according to the second or third aspect of the invention, or
prepared
according to the process according to the first aspect of the invention.
It was surprisingly found that the process of the invention allows the
manufacturing of
polyolefin fibers having mean fiber diameters of less than 5000 nm, and
improved
mechanical properties (tensile strength, modulus and tenacity).
The present invention will now be further described. In the following
passages, different
aspects of the invention are defined in more detail. Each aspect so defined
may be
combined with any other aspect or aspects unless clearly indicated to the
contrary. In
particular, any feature or statement indicated as being preferred or
advantageous may be
combined with any other features or statements indicated as being preferred or
advantageous.
Brief description of the figures
Figure 1 represents a cross section schematic view of a fiber producing device
as used in
Examples 1 and 2.
Detailed description of the invention
Before the present process, fibers, and articles, encompassed by the invention
are
described, it is to be understood that this invention is not limited to
particular process, fibers,
and articles described, as such process, fibers, and articles may, of course,
vary. It is also
to be understood that the terminology used herein is not intended to be
limiting, since the
scope of the present invention will be limited only by the appended claims.
Unless otherwise defined, all terms used in disclosing the invention,
including technical and
scientific terms, have the meaning as commonly understood by one of ordinary
skill in the
art to which this invention belongs. By means of further guidance, definitions
for the terms
used in the description are included to better appreciate the teaching of the
present
invention. When describing the polymer resins, processes, articles, and uses
of the
invention, the terms used are to be construed in accordance with the following
definitions,
unless the context dictates otherwise.
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As used herein, the singular forms "a", "an", and "the" include both singular
and plural
referents unless the context clearly dictates otherwise. By way of example, "a
resin" means
one resin or more than one resin.
The terms "comprising", "comprises" and "comprised of" as used herein are
synonymous
with "including", "includes" or "containing", "contains", and are inclusive or
open-ended and
do not exclude additional, non-recited members, elements or method steps. The
terms
"comprising", "comprises" and "comprised of" also include the term "consisting
of".
The recitation of numerical ranges by endpoints includes all integer numbers
and, where
appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1,
2, 3, 4 when
referring to, for example, a number of elements, and can also include 1.5, 2,
2.75 and 3.80,
when referring to, for example, measurements). The recitation of end points
also includes
the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and
5.0). Any
numerical range recited herein is intended to include all sub-ranges subsumed
therein.
Reference throughout this specification to "one embodiment" or "an embodiment"
means
that a particular feature, structure or characteristic described in connection
with the
embodiment is included in at least one embodiment of the present invention.
Thus,
appearances of the phrases "in one embodiment" or "in an embodiment" in
various places
throughout this specification are not necessarily all referring to the same
embodiment, but
may. Furthermore, the particular features, structures or characteristics may
be combined in
any suitable manner, as would be apparent to a person skilled in the art from
this
disclosure, in one or more embodiments. Furthermore, while some embodiments
described
herein include some but not other features included in other embodiments,
combinations of
features of different embodiments are meant to be within the scope of the
invention, and
form different embodiments, as would be understood by those in the art. For
example, in the
following claims and statements, any of the embodiments can be used in any
combination.
Preferred statements (features) and embodiments of the polymer resins,
processes,
articles, and uses of this invention are set herein below. Each statement and
embodiment of
the invention so defined may be combined with any other statement and/or
embodiment,
unless clearly indicated to the contrary. In particular, any feature indicated
as being
preferred or advantageous may be combined with any other features or
statements
indicated as being preferred or advantageous. Hereto, the present invention is
in particular
captured by any one or any combination of one or more of the below numbered
aspects and
embodiments 1 to 22, with any other statement and/or embodiment.
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1. A process for the preparation of polyolefin fibers having mean fiber
diameters of less
than 5000 nm, comprising the steps of:
a) preparing a polyolefin solution in a solvent,
b) placing said polyolefin solution in a fiber producing device comprising a
body
5 configured to receive said polyolefin solution, said body comprising one
or more
openings, and
c) rotating the fiber producing device, wherein rotation of the fiber
producing device
causes the polyolefin solution to be passed through said one or more openings
to
produce polyolefin fibers having mean fiber diameters of less than 5000 nm,
wherein said polyolefin is selected from the group comprising polyethylene
polymers
and copolymers having a weight average molecular weight Mw of at least 40 000
daltons, and polypropylene polymers and copolymers, having a weight average
molecular weight Mw of at least 120 000 daltons.
2. A process for the preparation of polyolefin fibers having mean fiber
diameters of less
than 5000 nm, comprising the steps of:
a) preparing a polyolefin solution in a solvent,
b) placing said polyolefin solution in a fiber producing device comprising a
body
configured to receive said polyolefin solution, said body comprising one or
more
openings, and
c) rotating the fiber producing device, wherein rotation of the fiber
producing device
causes the polyolefin solution to be passed through said one or more openings
to
produce polyolefin fibers having mean fiber diameters of less than 5000 nm,
wherein said polyolefin is selected from the group comprising polyethylene
polymers
and copolymers having a weight average molecular weight Mw of at least 40 000
daltons, and polypropylene polymers and copolymers, having a weight average
molecular weight Mw of at least 120 000 daltons;
wherein said fiber producing device is rotated at a speed of at least 10 000
RPM.
3. The process according to any one of statements 1 or 2, wherein said fiber
producing
device is rotated at a speed of at least 10 000 RPM, preferably at least 15
000 RPM,
preferably at least 20 000 RPM, preferably at least 22 000 RPM.
4. The process according to any one of statements 1 to 3, wherein said
polyolefin is
selected from the group comprising polyethylene polymers and copolymers, and
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polypropylene polymers and copolymers, having a weight average molecular
weight Mw
of at least 120 000 daltons.
5. The process according to any one of statements 1 to 4, wherein said
polyolefin is
selected from the group comprising polyethylene polymers and copolymers, and
polypropylene polymers and copolymers, having a weight average molecular
weight Mw
of at least 200 000 daltons, preferably at least 300 000 daltons, preferably
at least 400
000 daltons, preferably at least 500 000 daltons, preferably at least 600 000
daltons,
preferably at least 700 000 daltons, preferably at least 800 000 daltons,
preferably at
least 900 000 daltons, preferably at least 1 000 000 daltons.
6. The process according to any one of statements 1 to 5, wherein the
polyethylene or
polypropylene polymers used in the process of the invention are homopolymers.
7. The process according to any one of statements 1 to 6, wherein the
polyethylene or
polypropylene polymers are linear polymers.
8. The process according to any one of statements 1 to 7, wherein the
polyethylene or
polypropylene polymer has a molecular weight distribution of at most 10.
9. The process according to any one of statements 1 to 8, wherein the
polyethylene or
polypropylene polymer has a molecular weight distribution of at least 5.
10. The process according to any one of statements 1 to 9, wherein the
polyethylene or
polypropylene polymer has a molecular weight distribution of at least 5 and of
at most
10, preferably of at least 6 and of at most 10; preferably of at least 7 and
of at most 9.
11. The process according to any one of statements 1 to 10, wherein the
polyolefin is
polyethylene.
12. The process according to any one of statements 1 to 11, wherein the
polyolefin is ultra-
high molecular weight polyethylene (UHMWPE).
13. The process according to any one of statements 1 to 12, wherein the
polyolefin fibers
produced have mean fiber diameters of less than 2000 nm, preferably less than
1000
nm.
14. The process according to any one of statements 1 to 13, further comprising
heating the
fiber producing device, preferably at a temperature of at least 40 C, more
preferably at a
temperature of at least 100 C and at most 200 C.
15. The process according to any one of statements 1 to 14, further comprising
cooling the
fibers.
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16. The process according to any one of statements 1 to 15, further comprising
collecting
the fibers onto a collector to form a fibrous web.
17. The process according to any one of statements 1 to 16, further comprising
removing
the solvent from the fibers.
18. The process according to any one of statements 1 to 17, wherein the
polyolefin solution
comprises at least 1 % and at most 50 % by weight of the polyolefin based on
the total
weight of the polyolefin solution, preferably at least 5 % and at most 20 % by
weight.
19. The process according to any one of statements 1 to 18, wherein said
solvent is
selected from the group comprising 06-016 alcohols, fully saturated white
mineral oil,
vegetable oils, 04-020 carboxylic acids, aliphatic and alicyclic hydrocarbons,
petroleum
fractions, mineral oil, kerosene, aromatic hydrocarbons including hydrogenated
derivatives thereof, halogenated hydrocarbons, cycloalkanes, cycloalkenes, and
terpenes.
20. The process according to any one of statements 1 to 19, wherein the one or
more
openings have at least one diameter of at least 2 mm, preferably at least 1
mm,
preferably of at least 0.5 mm, preferably of at least 0.1 mm.
21. Polyolefin fibers having mean fiber diameters of less than 5000 nm
obtained by the
process according to any one of statements 1 to 20.
22. Polyolefin fibers having mean fiber diameters of less than 5000 nm,
wherein said
polyolefin is selected from the group comprising polyethylene polymers and
copolymers
having a weight average molecular weight Mw of at least 40 000 daltons, and
polypropylene polymers and copolymers, having a weight average molecular
weight Mw
of at least 120 000 daltons.
23. The polyolefin fibers according to statement 21 having mean fiber
diameters of less than
5000 nm, wherein said polyolefin is selected from the group comprising
polyethylene
polymers and copolymers having a weight average molecular weight Mw of at
least 40
000 daltons, and polypropylene polymers and copolymers, having a weight
average
molecular weight Mw of at least 120 000 daltons.
24. The polyolefin fibers according to any one of statements 21 to 23, in the
form of a
nonwoven web.
25. The polyolefin fibers according to any one of statements 21 to 24, wherein
the
polyethylene or polypropylene polymers are homopolymers.
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26. The polyolefin fibers according to any one of statements 21 to 25, wherein
the
polyethylene or polypropylene polymers are linear polymers.
27. The polyolefin fibers according to any one of statements 21 to 26, wherein
the
polyethylene or polypropylene polymer has a molecular weight distribution of
at most 10.
28. The polyolefin according to any one of statements 21 to 27, wherein the
polyethylene or
polypropylene polymer has a molecular weight distribution of at least 5.
29. The polyolefin according to any one of statements 21 to 28, wherein the
polyethylene or
polypropylene polymer has a molecular weight distribution of at least 5 and of
at most
10; preferably of at least 6 and of at most 10; preferably of at least 7 and
of at most 9.
30. The polyolefin fibers according to any one of statements 21 to 29, wherein
the polyolefin
is polyethylene.
31. The polyolefin fibers according to any one of statements 21 to 30, wherein
the polyolefin
is UHMWPE.
32. An article comprising the polyolefin fibers according to any one of
statements 21 to 31,
or prepared according to the process according to any one of statements 1 to
20.
As used herein, the term "fiber" generally refers to an elongate structure
that either has a
definite length or is substantially continuous in nature.
The term "nanofibers" as used herein refers to fibers having a number average
diameter (or
similar cross-sectional dimension for non-circular shapes) of less than about
1000 nm. In
the case of non-round cross-sectional nanofibers, the term "diameter" as used
herein refers
to the greatest cross-sectional dimension.
The present invention employs a fiber forming device that uses centrifugal
spinning
techniques, also referred herein as force spinning techniques.
The processes and equipment for forcespinning are known to persons skilled in
the art by
virtue of various known teachings, as well by virtue of commercial equipment
suppliers such
as FibeRio Technology Corporation, McAllen, Texas, USA, which supplies a line
of
forcespinning equipment (see http://fiberiotech.com/products/forcespinning-
products/).
Therefore, a detailed description of forcespinning is unnecessary, and only a
brief
description will be provided herein.
The fibers are formed by a process that includes the ejection of a polyolefin
solution from a
fiber forming device that comprises a body (e.g., a spinneret or spin disc)
that propels the
polymer solution by centrifugal force into the form of fibers. The polyolefin
fibers can be
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produced using forcespinning of the polyolefin solution through one or more
openings
provided in the body.
According to the invention the present process comprises the steps of:
a) preparing a polyolefin solution in a solvent,
b) placing said polyolefin solution in a fiber producing device comprising a
body configured
to receive said polyolefin solution, said body comprising one or more
openings, and
c) rotating the fiber producing device, wherein rotation of the fiber
producing device causes
the polyolefin solution to be passed through the one or more openings to
produce polyolefin
fibers having mean fiber diameters of less than 5000 nm,
wherein said polyolefin is selected from the group comprising polyethylene
polymers and
copolymers having a weight average molecular weight Mw of at least 40 000
daltons, and
polypropylene polymers and copolymers, having a weight average molecular
weight Mw of
at least 120 000 daltons. Preferably, said fiber producing device is rotated
at a speed of at
least 10 000 RPM.
The polyethylene suitable for use in the present invention may be any ethylene
homopolymer or any copolymer of ethylene and one or more comonomers having a
weight
average molecular weight Mw of at least 40 000 daltons. The comonomer is
different from
ethylene and chosen such that it is suited for copolymerization with the
olefin. The
comonomer may be a 03-020 alpha-olefin, such as propylene, 1-butene, 1-
pentene, 1-
hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-
hexadecene, 1-octadecene or 1-eicosene. In a preferred embodiment, said
polyethylene is
a homopolymer.
For example, polyethylene polymers and copolymers for use in the present
invention can
have a melt flow index MI2 of at most 34 g/10 min as measured according to ISO
1133
Procedure B, condition Data temperature of 190 C and a load of 2.16 kg, for
example at
most 30 g/10min, preferably at most 20 g/10min, preferably at most 10 g/10min,
preferably
at most 1 g/10min, preferably at most 0.1 g/10min.
Polyethylene polymers and copolymers for use in the invention can be produced
by
polymerizing ethylene and optionally one or more comonomers, such as ethylene,
in the
presence of a catalyst system and optionally in the presence of hydrogen. As
used herein,
the term "catalyst" refers to a substance that causes a change in the rate of
a
polymerization reaction. In the present invention, it is especially applicable
to catalysts
suitable for the polymerization of propylene to polypropylene. In some
embodiments, the
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catalyst can be a chromium, a Ziegler-Natta or a metallocene catalyst system.
In a preferred
embodiment, said catalyst is a Ziegler-Natta catalyst.
Preferably, the polyethylene polymers used herein is a homopolymer, preferably
a
homopolymer with a low long chain branching content.
5 Preferably, the polyethylene is ultra-high molecular weight polyethylene
(UHMWPE) having
a molecular weight distribution of at most 10; preferably the UHMWPE has a
molecular
weight distribution of at least 5; preferably the UHMWPE has a molecular
weight distribution
of at least 5 and of at most 10; preferably, the UHMWPE has a molecular weight
distribution
of at least 5 and of at most 9; UHMWPE has a molecular weight distribution of
at least 6
10 and of at most 9.
The polypropylene suitable for use in the present invention may be any
propylene
homopolymer or any copolymer of propylene and one or more comonomers, having a
weight average molecular weight Mw of at least 120 000 daltons.
The polypropylene can be a random copolymer. The one or more comonomers are
preferably selected from the group consisting of ethylene and 04-010 alpha-
olefins, such
as for example 1-butene, 1-pentene, 1-hexene, 1-octene, or 4-methyl-1-pentene.
Ethylene
and 1-butene are the preferred comonomers. Ethylene is the most preferred
comonomer.
The polypropylene can be a propylene homopolymer.
For example, the polypropylene polymers and copolymers can have a melt flow
index of at
most 32 g/10 min as measured according to ISO 1133, condition M, at 230 C and
under a
load of 2.16 kg, for example at most 30 g/10min, preferably at most 20
g/10min, preferably
at most 10 g/10min, preferably at most 1 g/10min, preferably at most 0.1
g/10min.
The polypropylene polymers and copolymers for use in the present invention can
be
produced by polymerizing propylene and optionally one or more co-monomers,
such as
ethylene, in the presence of a catalyst system and optionally in the presence
of hydrogen.
In some embodiments, the catalyst can be a chromium, a Ziegler-Natta or a
metallocene
catalyst system.
Preferably, the polyethylene or polypropylene polymers used in the process of
the invention
is ultra-high molecular weight (UHMW), i.e. having an intrinsic viscosity (IV)
as measured on
solution in decalin (decahydronaphthalene) at 135 C, according to ISO 1628-3,
of at least 5
dl/g, preferably at least 10 dl/g, more preferably at least 15 dl/g.
Preferably, the IV is at most
dl/g, more preferably at most 30 dl/g.
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The UHMW polyolefin solution is preferably prepared with a concentration of at
least 1 % by
weight. The UHMW polyolefin solution, preferably, has a concentration of at
most 50 % by
weight, more preferably at most 30 % by weight, even more preferably at most
25 % by
weight, most preferably at most 20 % by weight.
To prepare the polyolefin solution (or gel), any of the known solvents
suitable for forming a
polyolefin gel may be used. In some embodiments, said solvent can be selected
from the
group comprising 06-016 alcohols; fully saturated white mineral oil; vegetable
oils, such as
vegetable oil selected from the group comprising olive oil, peanut oil, palm
oil, and coconut
oil; 04-020 carboxylic acids, such as 04-020 carboxylic acids selected from
the group
comprising butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid,
octanoic acid,
nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic
acid,
tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid,
octadecanoic acid, nonadecanoic acid, and eicosanoic acid; aliphatic and
alicyclic
hydrocarbons such as aliphatic and alicyclic hydrocarbons selected from the
group
comprising octane, nonane, decane and paraffins, including isomers thereof;
petroleum
fractions; mineral oil; kerosene; aromatic hydrocarbons, such as aromatic
hydrocarbon
selected from the group comprising toluene, xylene, and naphthalene, including
hydrogenated derivatives thereof such as decalin and tetralin; halogenated
hydrocarbons
such as monochlorobenzene; cycloalkanes such as methylcyclopentane;
cycloalkenes; and
terpenes such as camphene, p-menthane-3,8-diol, limonene and dipentene. Also
combinations of the above enumerated solvents may be used, the combination of
solvents
being also referred to for simplicity as solvent.
In the most preferred embodiment the solvent of choice is white mineral oil,
paraffins,
decalin, nonan-2-ol (CAS 628-99-9), 09-011 alcohols such as (CAS 66455-17-2)
or 010-16
alcohols such as (CAS 67762-41-8).
Preferably, when analyzing the solution (gel) by DSC in comparison with the
analysis of the
pure polymer, the crystallization temperature must decreased by at least 1 C.
To determine
such crystallization temperature decrease, the following procedure is used:
- after calibration of the temperature using a Indium sample (see e.g. thermal
analysis of
polymers, fundamentals and applications, edited by J.D. Menczel and R.B.
Prime, John
Wiley & Sons, Hoboken, New Jersey (2009)), a polymer sample (between 2 and 10
mg) is
introduced in the DSC apparatus (e.g. DSC1 of Mettler Toledo). The following
thermal
history is imposed: stabilization of the sample at 20 C during 4 minutes,
heating at 20 C/min
up to 220 C, stabilization at 220 C during 3 minutes, cooling at -20 C/min up
to -20 C,
stabilization of the sample at 20 C during 3 minutes and heating at 20 C/min
up to 220 C.
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The crystallization temperature is determined during the above described
cooling step. After
substraction of the baseline (a linear line drawn between the onset of the
crystallization
peak, so close to 130 C, and 20 C), the crystallization temperature is
assimilated to the
temperature of the extreme of the peak;
- the above procedure is repeated with the gel. The difference between the
crystallization
temperatures of the pure polymer and the one of the polymer in the solution
(gel) could then
be calculated.
Step b) of the present process comprises placing said polyolefin solution in a
fiber
producing device comprising a body configured to receive said polyolefin
solution, said body
comprising one or more openings, and step c) comprises rotating the fiber
producing
device, wherein rotation of the fiber producing device causes the polyolefin
solution to be
passed through the one or more openings to produce polyolefin fibers having
mean fiber
diameters of less than 5000 nm.
In some embodiments, the fiber forming device can include a spinneret having a
reservoir
configured to contain the polyolefin solution. During operation, the spinneret
is rotated
centrifugally on an axis at high revolutions per minute creating hydrostatic
and centrifugal
forces. As the spinneret rotates, the hydrostatic and centrifugal forces push
the solution to
an outer wall having at least one opening (orifice) located therein. The
polyolefin solution
enters the one or more openings and is released therefrom. The centrifugal and
hydrostatic
forces combine to initiate a jet of the solution that impinges against a fiber
collector to
produce the fibers.
During rotation, the polyolefin solution is ejected as a jet of material from
one or more
openings (orifices) into the surrounding atmosphere. The one or more openings
and
associated channel feeding can be configured with a size and shape to cause a
fine jet of
the solution to form on exit from the openings. As used herein, an opening
means an exit
orifice plus any associated channel or passage feeding the opening and serving
to define
the nature of the expelled jet of fiber-forming solution. These openings may
be of a variety
of shapes (e.g., circular, elliptical, rectangular, square) and of a variety
of diameter sizes.
When multiple openings are employed, not every opening need to be identical to
another
opening, but in certain embodiments, every opening is of the same
configuration.
Preferably, each opening has a diameter of at most 2 mm, preferably at most 1
mm, yet
more preferably at most 0.5 mm, for example at most 0.1 mm. The diameter of
the opening
is herein meant to be the effective diameter, i.e. for non-circular or
irregularly shaped
openings, the largest distance between the outer boundaries of the openings.
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The ejected material can solidify as a superfine fiber that has a diameter
significantly less
than the inner diameter of the outlet port.
The fiber producing device may rotate at a speed of, for example, at least 10
000
revolutions per minute (RPM), in some embodiments it is at least 15 000 RPM.
In other
embodiments, it is at least 20 000 RPM. In other embodiments, it is at least
22 000 RPM.
The speed of the fiber producing device may be fixed while the fiber producing
device is
spinning, or may be adjusted while the fiber producing device is spinning.
If desired, the temperature of the rotating body may also be controlled during
fiber spinning.
For example, the rotating body temperature may range from about 40 C,
preferably from
about 100 C to about 200 C.
During centrifugal spinning, the fibers are distributed radially away from the
rotating member
onto a collection surface. As used herein "collecting" of fibers refers to
fibers coming to rest
against a fiber collection device or collector. After the fibers are
collected, the fibers may be
removed from a fiber collection device by a human, robot, a conveyor belt, by
gravity or
other technics. A variety of methods and fiber (e.g., nanofiber) collection
devices may be
used to collect fibers.
For example, the fibers could be ejected from the spinneret onto a surface
disposed below
the spinneret or on a wall across from outlet ports on the spinneret. The
collection surface
may vary as desired, and can be either stationary or rotated during collection
of the fibers.
In one embodiment, for example, the collection surface may be provided on a
collection wall
that surrounds the rotating member.
The collected fiber material can form a web of two- or three-dimensional
entangled fibers
that can be worked to a desired surface area and thickness, depending on the
amount of
time fibers continue to be expelled onto a collector, and control over the
surface area of the
collector.
Preferably the polyolefin fibers are then cooled. In some embodiments, the
temperature to
which the polyolefin fibers are cooled is at most 100 C, more preferably at
most 80 C, most
preferably at most 60 C. Preferably, the temperature to which the polyolefin
fibers are
cooled is at least 1 C, more preferably at least 5 C, even more preferably at
least 10 C,
most preferably at least 15 C.
Regardless of the particular technique employed, the solvent may be removed
from the
fibers during and/or after spinning. The fibers (or a web containing the
fibers) may simply be
washed, dried and/or heated to remove the solvent. The solvent may therefore
be removed
by evaporation, washing or any other technics.
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Subsequently to forming the polyolefin fibers, said polyolefin fibers can be
subjected to a
solvent removal step wherein the solvent is at least partly removed from the
polyolefin fibers
to form solid polyolefin fibers.
The solvent removal process may be performed by known methods, for example by
evaporation when a relatively volatile solvent, e.g. decaline, is used to
prepare the
polyolefin solution or by using an extraction liquid like cyclohexane, e.g.
when mineral oils
are used, or by a combination of both methods. Suitable extraction liquids are
solvent
dependent. They are preferably liquids that do not cause significant changes
to the
polyolefin network structure of the polyolefin gel fibers, for example
cyclohexane, ethanol,
ether, acetone, cyclohexanone, 2-methylpentanone, n-hexane, dichloromethane,
trichlorotrifluoroethane, diethyl ether and dioxane or mixtures thereof.
Preferably, the
extraction liquid is chosen such that the solvent can be separated from the
extraction liquid
for recycling.
In a preferred embodiment, the residual solvent left in the polyolefin fiber
of the invention is
removed by placing said fiber in a vacuumed oven at a temperature of
preferably at most
148 C, more preferably of at most 145 C, most preferably of at most 135 C.
Preferably, the
oven is kept at a temperature of at least 20 C, more preferably of at least 50
C. More
preferably, the removal of the residual solvent is carried out while keeping
the fiber taut, i.e.
the fiber is prevented from slackening.
The amount of residual solvent, left in the solid polyolefin fibers after the
extraction step
may vary within large limits but lowest amount of residuals solvent are
preferred. Preferably
the residual solvent is, in a mass percent, of at most 15 % of the initial
amount of solvent in
the polyolefin solution, more preferably in a mass percent of at most 10 %,
most preferably
in a mass percent of at most 5 %, even more preferably in a mass percent of at
most 1 %.
Preferably, the polyolefin fiber at the end of the solvent removal step
comprises solvent in
an amount below 800 ppm by mass. More preferably said amount of the solvent is
below
600 ppm, even more preferably below 300 ppm, most preferably below 100 ppm by
mass.
Regarding the fibers that are collected, in certain embodiments, at least some
of the fibers
that are collected are continuous, discontinuous, mat, woven, nonwoven or a
mixture of
these configurations. The fibers may be formed into two- or three-dimensional
webs, i.e.,
mats, films or membranes.
The fibers produced using any of the devices and methods described herein may
be used in
a variety of applications. Some general fields of use include, but are not
limited to: food,
materials, electrical, defense, tissue engineering, biotechnology, medical
devices, energy,
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alternative energy (e.g., solar, wind, nuclear, and hydroelectric energy),
therapeutic
medicine, drug delivery (e.g., drug solubility improvement, drug
encapsulation, etc.),
textiles/fabrics, nonwoven materials, filtration (e.g., air, water, fuel,
semiconductor,
biomedical, etc.), automotive, sports, aeronautics, space, energy
transmission, papers,
5 substrates, hygiene, cosmetics, construction, apparel, packaging,
geotextiles, thermal and
acoustic insulation.
Some products that may be formed using the polyolefin fibers include but are
not limited to:
filters; wound dressings; cell growth substrates or scaffolds; battery
separators; sutures;
chemical sensors; textiles/fabrics that are water & stain resistant, odor
resistant, insulating,
10 self-cleaning, penetration resistant, anti-microbial, porous/breathing,
tear resistant, and
wear resistant; force energy absorbing for personal body protection armor;
construction
reinforcement materials; tissue engineering substrates; tissue engineering
Petri dishes;
filters used in pharmaceutical manufacturing; filters for deep filter
functionality; hydrophobic
materials such as textiles; building products that enhance durability,
flexibility, air tightness;
15 adhesives; tapes; epoxies; glues; adsorptive materials; diaper media;
mattress covers;
acoustic materials; and liquid, gas, chemical, or air filters.
The present process has the advantage of not having the usual drawbacks of
electrospinning. For instance, there is less constraint on the solvent used as
the polar
aspect of the gel is not necessarily required. Polyolefin fibers having mean
fiber diameters
of less than 5000 nm are obtained.
Examples of mechanical properties construed in the light of the present
invention are tensile
strength, elastic modulus, breaking force, elongation at break and the like.
The following examples serve to merely illustrate the invention and should not
be construed
as limiting its scope in any way. While the invention has been shown in only
some of its
forms, it should be apparent to those skilled in the art that it is not so
limited, but is
susceptible to various changes and modifications without departing from the
scope of the
invention.
Examples
Test methods
In the description above and in the non-limiting examples that follow, the
following test
methods were employed to determine various reported characteristics and
properties.
Except for polymers presenting too high mass and therefore solubility issue
(like
UHMWPE), the molecular weights (Mn (number average molecular weight), Mw
(weight
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average molecular weight), Mz (z average molecular weight)) can be determined
by size
exclusion chromatography (SEC) and in particular by gel permeation
chromatography
(GPO). Briefly, a GPO-1R5 from Polymer Char is used: 10 mg polyethylene sample
is
dissolved at 160 C in 10 ml of trichlorobenzene for 1 hour. Injection volume:
about 400p1,
automatic sample preparation and injection temperature: 160 C. Column
temperature:
145 C. Detector temperature: 160 C. Two Shodex AT-806MS (Showa Denko) and one
Styragel HT6E (Waters) columns are used with a flow rate of 1 ml/min.
Detector: Infrared
detector (2800-3000 cm-1). Calibration: narrow standards of polystyrene (PS)
(commercially
available). Calculation of molecular weight M, of each fraction i of eluted
polyethylene is
based on the Mark-Houwink relation (logio(MpE) = 0.965909 x logio(Mps) ¨
0.28264) (cut off
on the low molecular weight end at MpE = 1000).
The molecular weight averages used in establishing molecular weight/property
relationships
are the number average (Ma), weight average (Mw) and z average (Mz) molecular
weight.
These averages are defined by the following expressions and are determined
form the
calculated M,:
EiNiMi EiWi Ei hi
Mn =
Ei __________________________________________ = EiWii Eihii
EiNiW EiWiMi Ei hiMi
M
WNMEiWi Ei hi
Ei NiMi3 EiWi Ei hi
Mz =
Ei 11/1i EiWiMi EihiMi
Here, N, and W, are the number and weight, respectively, of molecules having
molecular
weight Mi. The third representation in each case (farthest right) defines how
one obtains
these averages from SEC chromatograms. Here, h, is the height (from baseline)
of the SEC
curve at the ith elution fraction and M, is the molecular weight of species
eluting at this
increment.
Measurement of molecular weight distribution of UHMWPE. For UHMWPE the
molecular
weight distribution was measured from the quantification of the transition
zone between the
Newtonian viscosity and the power-law domain. Such transition is known to be
related to
both the molecular weight distribution and to the long chain branching content
in the
polymer. As the long chain branching content in the polyethylene grade used in
the example
(UHMWPE GURO 4113) is zero or at least very low, such transition can be
related to the
molecular weight distribution.
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- Rheological measurements were carried out on ARES equipment
(manufactured by TA
Instruments) at frequencies 0) ranging between 0.05 and 50 rad/s (3
frequencies were
analyzed per frequency decade) at 230 C, imposing a 0.5 % strain (to remain
in the
linear viscoelasticity domain);
- As soon as the temperature (230 C) was reached, a stabilization time as
long as 3
hours was imposed before starting the measurement (due to the very long
required
stabilization time, when preparing the polymer disk to be used in the ARES
equipment,
5000 ppm of lrganox B215 were previously mixed in the UHMWPE fluff)
- Rheological data were then fitted using the Carreau-Yasuda equation
(see below). In
this equation, "n" has been imposed to be "0" and "a" is the parameter
quantifying the
transition zone between the Newtonian viscosity and the power-law domain. The
higher
is "a", the narrower is the relaxation spectrum (so the narrower is the
molecular weight
170
17= 1-n
distribution if no long chain branching is present in the polymer)
[1+(Atu)a a
- The fitted parameters for UHMWPE GURO 4113 were:
no = 3.52 * 10^9 Pa*s
= 2629 s
a = 0.166.
For UHMW polyethylene, the molecular weight was determined by measurement of
the
intrinsic viscosity (IV ¨ unit : dl/g) in decalin at 135 C, which allows
calculation of the
viscosity molecular weight average (My) via Margolies' equation : My (kdalton)
= 53.7 *
OW 49.
Measurement of the fiber diameter was performed using the optical microscopy
approach if
the diameter of the fiber was higher than 4 pm; and for lower diameter, the
Scanning
Electronic microscopy (SEM) approach was used:
- Optical microscopy determination of the fiber diameter: 5 fiber segments
were fixed
on a flat glass and introduced in a Leica DMLP microscope, connected to a JVC
camera. A "X40" lens was used. Images of the 5 fibers were then registered and
analyzed using the IM500 software from Leica: the fiber diameter was
determined by
comparison with the image of a reference (the image of a certificated
graduated
glass previously recorded using the same lens). The reported diameter was the
average value of the 5 determinations.
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-
For SEM microscopy measurement, a small bundle containing several fibres was
vertically introduced in a capsule and embedded into an epoxy resin. After 24
hours
hardening of the resin, the bundle+epoxy sample was cut (using a milling
machine
equipped with a diamond) in a direction transverse to the fibre axis. Doing
so, a
surface was available, which was treated (metallization) with carbon.
The SEM images were taken from retrodiffused electrons providing a chemical
contrast between fibres and the resins used for the embedding procedure.
Measurements were then performed using an image treatment software.
References (books) for SEM measurements: Preparation des echantillons pour
MEB et Microanalyse - Philippe Jonnard (GNMEBA) ¨ EDP Sciences and in
Polymer Microscopy >> - Linda C.Sawyer, David T.Grubb and Gregory F. Meyers ¨
Ed. Chaoman and Hall.
Equipment used
Figure 1 represents a cross section schematic view of a fiber producing device
as used in
Examples 1 and 2. The fiber producing device comprises a spinneret 1. The
spinneret 1
was mechanically coupled to a motor 2, which rotates the spinneret 1 in a
circular motion
via a shaft 5. The speed of the motor 2 is adjustable by increment of 2 000
RPM from 10
000 to 22 000 RPM. The motor 2 was fixed on a strong frame 3, a circular
collector 4 of 56
cm diameter was centered from the axis of the spinneret 1. The spinneret 1
comprised a
stainless steel cell 7 closed with a screwed lid 6. In the bottom of the cell
7, two orifices
diametrically opposite were equipped of screws with calibrated bore 8 of 0.5
mm diameter.
The shaft 5 was screwed on the top of the lid 6 to allow attachment at the
mandrel of the
motor 2. The external dimensions of the cell 7 were 29.96 mm diameter and
35.40 mm
height. The internal dimensions of the cell 7 were 20.80 mm diameter and 11.13
mm height.
Example 1
0.52 g of a UHMWPE (commercialized by Ticona under the name GURO 4113; a
linear
polyethylene in powder form with a molecular weight of approximately 3.9 MM
g/mol
calculated using Margolies' equation (M (kdalton) = 53.7 /V1 49 ), an
intrinsic viscosity (IV
in dl/g) of about 17.90 dl/g as measured according to ISO 1628-3, and a
molecular weight
distribution ranging from 7 to 9 measured as described above) were mixed with
10 g nonan-
2-ol. The mixture was placed in an oven at 180 C and regularly mixed. A gel
was
progressively formed.
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Once a homogenous gel was formed, a part of the hot gel was rapidly placed in
the fiber
producing device as described above and represented in Figure 1. Then rotation
of the
spinneret was imposed at 16 000 RPM. The fibers were then collected on the
collector 4.
Mean measured diameter of the produced fibers was 3.5 pm.
Example 2
The procedure described in example 1 has been reproduced but the imposed
rotation
speed of the spinneret was 22 000 RPM. The mean measured diameter of the
produced
fibers was 0.9 pm.
