Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROSPINNING PROCESS
FIELD OF THE INVENTION
[0001] The present invention relates to an electrospinning process, the
resulting electrospun fiber and polymers used in the electrospinning process.
BACKGROUND OF THE INVENTION
[0002] The process of electrospinning uses an electrical charge to form
fine fibers. The process consists of a spinneret with a dispensing needle, a
syringe pump, a power supply and a grounded collection device. Polymers in
solution or as melts are located in the syringe and driven to the needle tip
by
the syringe pump where they form a droplet. When voltage is applied to the
needle, a droplet is stretched to an electrified liquid jet. The jet is
elongated
continuously until it is deposited on the collector as a mat of fine fibers
usually
of nanometer-sized dimensions. The resultant fibers are useful in a wide
variety of applications such as protective clothing, wound dressing and as
supports or carriers for catalyst. To form a fiber, the polymeric melt or
solution must have a sufficient viscosity other-wise a drop rather than a
liquid
jet will form. Typically, thickeners are included in the poiymer solution or
melt
to provide the necessary viscosity. However, thickeners can adversely affect
the properties-of the resultant fibers and for this reason, their use should
be
minimized.
SUMMARY OF THE INVENTION
[0003] The present invention provides for a process of electrospinning
a fiber from an electrically conductive solution of a polymer in the presence
of
an electric field between a spinneret and a ground source. The polymer
undergoes a crosslinking reaction prior to and during the electrospinning
process resulting in a viscosity buildup of the polymer solution enabling
fiber
formation and minimizing the use of thickeners.
[0004] The invention also provides for the resultant electrospun fiber
that contains silane, preferably carboxyl and hydroxyl groups and optionally a
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nitrogen-containing group such as amine or amide groups. The silane groups
provide for crosslinking and viscosity buiid-up. The carboxyl, hydroxyl, amine
and amide groups provide for a hydrogen bonding and viscosity build-up. The
carboxyl group, in the form of carboxylic acid, and the nitrogen-containing
groups are good electrical charge carrying groups.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 depicts a basic electrospinning system.
[0006] FIG. 2 simulates a scanning electron microscopic (SCM) image
of a non-woven mat.
DETAILED DESCRIPTION OF THE INVENTION
[0007] For purposes of the following detailed description, it is to be
understood that the invention may assume various alternative variations and
step sequences, except where expressly specified to the contrary. Moreover,
other than in any operating examples, or where otherwise indicated, all
numbers expressing, for example, quantities of ingredients used in the
specification and claims are to be understood as being modified in all
instances by the term "about". Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the following specification and attached
claims are approximations that may vary depending upon the desired
properties to be obtained by the present invention. At the very least, and not
as an attempt to limit the application of the doctrine of equivalents to the
scope of the claims, each numerical parameter should at least be construed in
light of the number of reported significant digits and by applying ordinary
rounding techniques. Notwithstanding that the numerical ranges and
parameters setting forth the broad scope of the invention are approximations,
the numerical values set forth in the specific exampfes are reported as
precisely as possible. Any numerical value, however, inherently contains
certain errors necessarily resulting from the standard variation found in
their
respective testing measurements.
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[0008] Also, it should be understood that any numerical range recited
herein is intended to include all sub-ranges subsumed therein. For example,
a range of "1 to 10" is intended to include all sub-ranges between (and
including) the recited minimum value of 1 and the recited maximum vafue of
10, that is, having a minimum value equal to or greater than I and a maximum
value of equal to or less than 10.
[0009] In this application, the use of the singular includes the plural and
plural encompasses singular, unless specifically stated otherwise. In
addition,
in this application, the use of "or" means "and/or" unless specifically stated
otherwise, even though "and/or" may be explicitly used in certain instances.
[0010] The term "polymer" is afso meant to include copolymer and
oligiomer. The term "acrylic" is meant to include methacrylic and is depicted
by (meth)acrylic.
[0011] With reference to FIG. 1, the electrospinning system consists of
three major components, a power supply 1, a spinneret 3 and an electrically
grounded collector 4. Direct current or alternating current may be used in the
electrospinning process. The polymer sofution 5 is contained in a syringe 7.
A syringe pump 9 forces the solution through the spinneret 3 at a controlled
rate. A drop of the solution forms at the tip of the needle 11. Upon
application of a voltage, typically from 5 to 30 kilovolts (kV), the drop
becomes
electrically charged. Consequently, the drop experiences electrostatic
repulsion between the surface charges and the forces exerted by the external
electric field. These electrical forces will distort the drop and wilf
eventually
overcome the surface tension of the polymer solution resulting in the ejection
of a liquid jet 13 from the tip of the needle 11. Because of its charge, the
jet is
drawn downward to the grounded collector 4. During its travel towards the
collector 4, the jet 13 undergoes a stretching action leading to the formation
of
a thin fiber. The charged fiber is deposited on the coilector 4 as a random
oriented non-woven mat as generally shown in FIG. 2.
[0012] The polymers of the present invention can be acrylic polymers.
As used herein, the term "acrylic" polymer refers to those polymers that are
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well known to those skilled in the art which results in the polymerization of
one
or more ethylenically unsaturated polymerizable materials. (Meth)acrylic
polymers suitable for use in the present invention can be made by any of a
wide variety of methods as will be understood by those skilled in the art. The
(meth)acrylic polymers can be made by addition polymerization of
unsaturated polymerizabie materials that contain silane groups, carboxyl
groups, hydroxyl groups and optionally a nitrogen-containing group.
Examples of silane groups include, without limitation, groups that have the
structure Si-Xõ (wherein n is an integer having a value ranging from 1 to 3
and
X is selected from chlorine, aikoxy esters, and/or acyloxy esters). Such
groups hydrolyze in the presence of water including moisture in the air to
form
silanol groups that condense to form -Si-O-Si- groups.
[0013] Examples of silane-containing ethylenically unsaturated
polymerizable materials, suitable for use in preparing such (meth)acrylic
polymers include, without limitation, ethylenically unsaturated alkoxy silanes
and ethylenically unsaturated acylaxy silanes, more specific examples of
which include viny[ silanes such as vinyl trimethoxysilane,
acryfatoalkoxysilanes, such as gamma-acryloxypropyl trimethoxysilane and
gamma-acryloxypropyl triethoxysilane, and methacrylatoalkoxysilanes, such
as gamma-methacryloxypropyl trimethoxysilane, gamma-methacryloxypropyl
triethoxysilane and gamma-methacryloxypropyl tris-(2-methoxyethoxy) silane;
acyloxysilanes, including, for example, acrylato acetoxysiianes, methacrylato
acetoxysilanes and ethylenically unsaturated acetoxysilanes, such as
acrylatopropyl triacetoxysilane and methacrylatopropyl triacetoxysilane. In
certain embodiments, it may be desirable to utilize monomers that, upon
addition polymerization, will result in a (meth)acrylic polymer in which the
Si
atoms of the resulting hydrolyzable silyl groups are separated by at least two
atoms from the backbone of the polymer. Preferred monomers are
(meth)acryloxyaikylpoiyaikoxy silane, particularly
(meth)acryloxyalkyltrialkoxy
silane in which the alkyl group contains from 2 to 3 carbon atoms and the
alkoxy groups contain from 1 to 2 carbon atoms.
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[0014] In certain embodiments, the amount of the silane-containing
ethylenically unsaturated polymerizable material used in the total monomer
mixture is chosen so as to result in the production of a (meth)acrylic polymer
comprising silane groups that contain from 0.2 to 20, preferably 5 to 10
percent by weight, silicon, based on the weight of the total monomer
combination used in preparing the (meth)acrylic polymer.
[0015] The (meth)acrylic polymer suitable for use in the present
invention can be the reaction product of one or more of the aforementioned
silane-containing ethylenically unsaturated polymerizable materials and
preferably an ethylenically unsaturated polymerizable material that comprises
carboxyl such as carboxylic acid groups or an anhydride thereof. Examples of
suitable ethylenically unsaturated acids and/or anhydrides thereof include,
without limitation, acrylic acid, methacrylic acid, itaconic acid, crotonic
acid,
maleic acid, maleic anhydride, citraconic anhydride, itaconic anhydride,
ethylenically unsaturated sulfonic acids and/or anhydrides such as sulfoethyl
methacrylate, and half esters of maleic and fumaric acids, such as butyl
hydrogen maleate and ethyl hydrogen fumarate in which one carboxyl group
is esterified with an alcohol,
[0016] Examples of other polymerizable ethylenically unsaturated
monomers to introduce carboxyl functionality are alkyl including cycloalkyl
and
aryl (meth)acrylates containing from 1 to 12 carbon atoms in the alkyl group
and from 6 to 12 carbon atoms in the ary( group. Specific examples of such
monomers include methyl methacrylate, n-butyl methacry[ate, n-butyl acrylate,
2-ethylhexyl methacrylate, cyclohexyl methacrylate and phenyl methacrylate.
[0017] The amount of the polymerizable carboxylwcontaining
ethylenically unsaturated monomers is preferably sufficient to provide a
carboxyl content of up to 55, preferably 15.0 to 45.0 percent by weight based
on the weight of the total monomer combination used to prepare the
(meth)acrylic polymer. Preferably, at least a portion of the carboxyl groups
are derived from a carboxylic acid such that the acid value of the polymer is
within the range of 20 to 80, preferably 30 to 70, on a 100% resin solids
basis.
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[0018] The (meth)acrylic polymer used in the invention also preferably
contains hydroxyl functionality typically achieved by using a hydroxyl
functional ethyfenically unsaturated polymerizable monomer. Examples of
such materials include hydroxyalkyl esters of (meth)acrylic acids having from
2 to 4 carbon atoms in the hydroxyalkyl group. Specific examples include
hydroxyethyl (meth)acrylate, hydroxypropyi (meth)acrylate and 4-hydroxybutyl
(meth)acrylate. The amount of the hydroxy functional ethyienicaliy
unsaturated monomer is sufficient to provide a hydroxyl content of up to 6.5
such as 0.5 to 6.5, preferably I to 4 percent by weight based on the weight of
the total monomer combination used to prepare the (meth)acrylic polymer.
[0019] The (meth)acrylic polymer optionally contains nitrogen
functionality introduced from a nitrogen-containing ethylenicaily unsaturated
monomer. Examples of nitrogen functionality are amines, amides, ureas,
imidazoles and pyrrolidones. Examples of suitable N-containing ethylenically
unsaturated monomers are: amino-functional ethylenically unsaturated
polymerizable materials that include, without limitation, p-dimethylamino
ethyl
styrene, t-butyfaminoethyl (meth)acryiate, dimethylaminoethyl (meth)acrylate,
diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate and
dimethylaminopropyl (meth)acrylamide; amido-functional ethyienically
unsaturated materials that include acrylamide, methacrylamide, n-methyl
acrylamide and n-ethyl (meth)acrylamide; urea functional ethylenically
unsaturated monomers that include methacrylamidoethylethylene urea.
[0020] If used, the amount of the nitrogen-containing ethylenically
unsaturated monomer is sufficient to provide nitrogen content of up to 5 such
as from 0.2 to 5.0 , preferably from 0.4 to 2.5 percent by weight based on
weight of a total monomer combination used in preparing the (meth)acrylic
polymer.
[0021] Besides the polymerizable monomers mentioned above, other
polymerizab[e ethyfenicafly unsaturated monomers that may be used to
prepare the (meth)acrylic polymer. Examples of such monomers include
poly(meth)acrylates such as ethylene glycol di(meth)acrylate,
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trimethylofpropane tri(meth)acrylate, ditrimethylolpropane tetraacrylate;
aromatic vinyl monomers such as styrene, vinyl toluene and alpha-
methylstyrene; monoolefinic and diolefinic hydrocarbons, unsaturated esters
of organic and inorganic acids and esters of unsaturated acids and nitriles.
Examples of such monomers include 1,3-butadiene, acrylonitrile, vinyl
butyrate, vinyl acetate, allyl chloride, divinyl benzene, diallyl itaconate,
triallyl
cyanurate as well as mixtures thereof. The polyfunctional monomers, such as
the polyacrylates, if present, are typically used in amounts up to 20 percent
by
weight. The monfunctional monomers, if present, are used in amount up to
70 percent by weight; the percentage being based on weight of the total
monomer combination used to prepare the (meth)acryEic polymer.
[0022] The (meth)acrylic polymer is typically formed by solution
polymerization of the ethyfenically unsaturated polymerizable monomers in
the presence of a polymerization initiator such as azo compounds, such as
alpha, alpha'-azobis(isobutyronitrile), 2,2'-azobis (methylbutyronitrile) and
2,2'-
azobis(2,4-dimethylvaleronitrile); peroxides, such as benzoyl peroxide,
cumene hydroperoxide and t-amylperoxy-2-ethylhexanoate; tertiary butyl
peracetate; tertiary butyl perbenzoate; isopropyl percarbonate; butyl
isopropyl
peroxy carbonate; and similar compounds. The quantity of initiator employed
can be varied considerably; however, in most instances, it is desirable to
utilize from 0.1 to 10 percent by weight of initiator based on the total
weight of
copolymerizable monomers employed. A chain modifying agent or chain
transfer agent may be added to the polymerization mixture. The mercaptans,
such as dodecyl mercaptan, tertiary dodecyl mercaptan, octyl mercaptan,
hexyl mercaptan and the mercaptoalicyl trialkoxysilanes such as 3-
mercaptopropyl trimethoxysilane may be used for this purpose as well as
other chain transfer agents such as cyclopentadiene, allyl acetate, aflyl
carbamate, and mercaptoethanol.
[0023] The polymerization reaction for the mixture of monomers to
prepare the acrylic polymer can be carried out in an organic sofvent medium
utilizing conventional solution polymerization procedures which are well
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known in the addition polymer art as illustrated with particularity in, for
example, United States Patent Nos. 2,978,437; 3,079,434 and 3,307,963.
Organic solvents that may be utilized in the polymerization of the monomers
include virtually any of the organic solvents often employed in preparing
acrylic or vinyl polymers such as, for example, alcohols, ketones, aromatic
hydrocarbons or mixtures thereof. Illustrative of organic solvents of the
above
type which may be employed are alcohols such as lower alkanols containing 2
to 4 carbon atoms including ethanol, propanol, isopropanol, and butanol; ether
alcohols such as ethylene glycol monoethyl ether, ethylene glycol monobutyl
ether, propylene glycol monomethyl ether, and dipropylene glycol monoethyl
ether; ketones such as methyl ethyl ketone, methyl N-butyl ketone, and
methy[ isobutyl ketone; esters such as butyl acetate; and aromatic
hydrocarbons such as xylene, toluene, and naphtha.
[0024] In certain embodiments, the polymerization of the ethylenically
unsaturated components is conducted at from 0 C to 150 C, such as from
50 C to 150 C, or, in some cases, from 80 C to 120 C.
[0025] The polymer prepared as described above is usually dissolved
in solvent and typically has a resin solids content of about 15 to 80,
preferably
20 to 60 percent by weight based on total solution weight. The molecular
weight of the polymer typically ranges between 3,000 to 1,000,000, preferably
5,000 to 100,000 as determined by gel permeation chromatography using a
polystyrene standard.
[0026] For the electrospinning application, the polymer solution such as
described above can be mixed with water to initiate the crosslinking reaction
and to build viscosity necessary for fiber formation. Typically about 5 to 20,
preferably 10 to 15 percent by weight water is added to the polymer solution
with the percentage by weight being based on total weight of the polymer
solution and the water. Preferably a base such as a water-soluble organic
amine is added to the water-polymer solution to catalyze the crosslinking
reaction. Optionally a thickener such as polyvinyl pyrrolidone, polyvinyl
alcohol, polyvinyl acetate, polyamides and/or a cellulosic thickener can be
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added to the electrospinning formulation to better control its viscoelastic
behavior. If used, the thickener is present in amounts no greater than 20
percent by weight, typically from I to 6 percent by weight based on weight of
the polymer solution.
[0027] The electrospinning formulation prepared as described above is
then stored to permit the viscosity to build to the crosslinking reaction.
When
the viscosity is sufficiently high but short of gelation, the formulation is
subjected to the electrospinning process as described above.
[0028] Typically, the viscosity is at least 5 and less than 2,000, usually
less than 1,000, such as preferably within the range of 50 to 250 centistokes
for the electrospinning process. A Bubble Viscometer according to ASTM D-
1544 determines the viscosity. The time for storing the electrospinning
formulation will depend on a number of factors such as temperature,
crosslinking functionaiity and catalyst. Typically, the electrospinning
formulation will be stored for as low as one minute up to two hours.
[0029] When subject to the electrospinning process, the formulations
described above typically produce fibers having a diameter of up to 5,000,
such as from 5 to 5,000 nanometers, more typically within the range of 50 to
1,200 nanometers, such as 50 to 700 nanometers. The fibers also can have
a ribbon configuration and in this case diameter is intended to mean the
largest dimension of the fiber. Typically the width of the ribbon shaped
fibers
is up to 5000 such as 500 to 5000 nanometers and the thickness up to 200
such as 5 to 200 nanometers.
[0030] The following examples are presented to demonstrate the
general principles of the invention. However, the invention shoufd not be
considered as limited to the specific examples presented. All parts are by
weight unless otherwise indicated.
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EXAMPLES A, B and C
Synthesis of Acrylic Silane Polymers
[00311 For each of Exampfes A to C in Table 1 below, a reaction flask
was equipped with a stirrer, thermocouple, nitrogen inlet and a condenser.
Charge A was then added and stirred with heat to reflux temperature (75 C-
80 C) under nitrogen atmosphere. To the refluxing ethanol, charge B and
charge C were simu[taneously added over three hours. The reaction mixture
was held at reflux condition for two hours. Charge D was then added over a
period of 30 minutes. The reaction mixture was held at reflux condition for
two hours and subsequently cooled to 30 C.
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TABLE, 1
Exnmple A Example B Example C
Charge A (weight in grams)
Ethanol SDA 40B1 360.1 752.8 1440.2
Charge B (weight in grams)
Methyl Methacrylate 12.8 41.8 137.9
Acrylic acid 8.7 18.1 34.6
Silquest A-1742 101.4 211.9 405.4
2-hydroxylethylmethacrylate 14.5 0.3 0.64
n-Butyl acrylate 0,2 4.3 0,64
Acrylamide 7.2 - -
Sartomer SR 3553 - 30.3 -
Ethanol SDA 40B 155.7 325.5 622.6
Charge C (weight in arams)
Vazo 67Q 6.1 12.8 24.5
Ethanol SDA 40B 76.7 160.4 306,8
Charge C (weight in rams
Vazo 67 1.5 2.1 6.1
Ethanol SDA 40B 9.1 18.9 36.2
% Solids 17.9 19.5 19.1
Acid value (100% resin solids) 51.96 45.64 45.03
Mn -- 3021' 5810
Denatured ethyl alcohol, 200 proof, available from Archer Daniel Midland Co.
2 gamma-methacryloxypropyltrimethoxysilane, available from GE silicones.
3 Di-trimethylolpropane tetraacrylate, available from Sartomer Company Inc.
a 2,2'-azo bis(2-methyl butyronitrile), available from E.I. duPont de Nemours
& Co., Inc.
$ Mn of soluble portion; the polymer is not completely soluble in
tetrahydrofuran.
EXAMPLES 1, 2 AND 3
Acryiic-Siiane Nanofibers
Example 1
[0032] The acrylic-silane resin solution from Example C (8.5 grams)
was blended with poiyvinylpyrrolidone (0.2 grams) and water (1.5 grams).
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The formulation was stored at room temperature for 215 minutes. A portion of
the resulting formulation was loaded into a 10 ml syringe and delivered via a
syringe pump at a rate of 1.6 milliliters per hour to a spinneret (stainless
steel
tube 1/16-inch outer diameter and 0.01 0-inch internal diameter). This tube
was connected to a groundng aluminum collector via a high voltage source to
which about 21 kV potential was applied. The delivery tube and collector
were encased in a box that allowed nitrogen purging to maintain a relative
humidity of less than 25%. Ribbon shaped nanofibers having a thickness of
about 100-200 nanometers and a width of 500-700 nanometers were
collected on the grounded aluminum panels and were characterized by optical
microscopy and scanning electron microscopy.
Example 2
[0033] The acrylic-silane resin solution from Example B (8.5 grams)
was blended with polyvinylpyrrolidone (0.1 grams) and water (1.5 grams).
The formulation was stored at room temperature for 210 minutes. A portion of
the resulting solution was loaded into a 10 mi syringe and delivered via a
syringe pump at a rate of 0.2 milliliters per hour to the spinneret of Example
1.
The conditions for electrospinning were as described in Example 1. Ribbon
shaped nanofibers having a thickness of 100-200 nanometers and a width of
900-1200 nanometers were collected on grounded aluminum foil and were
characterized by optical microscopy and scanning electron microscopy.
Example 3
[0034] The acrylic-silane resin from Example A (8.5 grams) was
blended with polyvinyfpyrrolidone (0.1 grams) and water (1.5 grams). The
formulation was stored at room temperature for 225 minutes. A portion of the
resulting solution was loaded into a 10 ml syringe and delivered via a syringe
pump at a rate of 1.6 milliliters per hour to the spinneret as described in
Example 1. The conditions for electrospinning were as described in Example
1. Ribbon shaped nanofibers having a thickness of 100-200 nanometers and
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a width of 1200-5000 nanometers were collected on grounded aluminum foil
and were characterized by optical microscopy and scanning electron
microscopy. A sample of the nanofibers was dried in an oven at 110 C for
two hours. No measurable weight loss was observed. This indicates the
nanofibers were completely crosslinked.
[0035] Whereas particular embodiments of this invention have been
described above for purposes of illustration, it will be evident to those
skilled
in the art that numerous variations of the details of the present invention
may
be made without departing from the invention as defined in the appended
claims.
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