Electrospinning Process

Abstract

A method for electrospinning polymer fibers and the resultant electrospun fibers are disclosed. In the electrospinning method, the polymer undergoes a crosslinking reaction prior to and during the electrospinning process.

Description

FIELD OF THE INVENTION

The present invention relates to an electrospinning process, the resulting electrospun fiber and polymers used in the electrospinning process.

BACKGROUND OF THE INVENTION

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 otherwise a drop rather than a liquid jet will form. Typically, thickeners are included in the polymer 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

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.

The invention also provides for the resultant electrospun fiber that contains silane, preferably carboxyl and hydroxyl groups and optionally a nitrogen-containing group such as amine or amide groups. The silane groups provide for crosslinking and viscosity build-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

FIG. 1 depicts a basic electrospinning system.

FIG. 2 simulates a scanning electron microscopic (SCM) image of a non-woven mat.

DETAILED DESCRIPTION OF THE INVENTION

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 examples 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.

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 value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

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.

The term “polymer” is also meant to include copolymer and oligiomer. The term “acrylic” is meant to include methacrylic and is depicted by (meth)acrylic.

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 solution 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 will 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 collector 4 as a random oriented non-woven mat as generally shown in FIG. 2.

The polymers of the present invention can be acrylic polymers. As used herein, the term “acrylic” polymer refers to those polymers that are 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 polymerizable 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—X_n (wherein n is an integer having a value ranging from 1 to 3 and X is selected from chlorine, alkoxy 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.

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 acyloxy silanes, more specific examples of which include vinyl silanes such as vinyl trimethoxysilane, acrylatoalkoxysilanes, 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 acetoxysilanes, 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)acryloxyalkylpolyalkoxy 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.

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.

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.

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 aryl group. Specific examples of such monomers include methyl methacrylate, n-butyl methacrylate, n-butyl acrylate, 2-ethylhexyl methacrylate, cyclohexyl methacrylate and phenyl methacrylate.

The amount of the polymerizable carboxyl-containing 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.

The (meth)acrylic polymer used in the invention also preferably contains hydroxyl functionality typically achieved by using a hydroxyl functional ethylenically 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, hydroxypropyl(meth)acrylate and 4-hydroxybutyl (meth)acrylate. The amount of the hydroxy functional ethylenically unsaturated monomer is sufficient to provide a hydroxyl content of up to 6.5 such as 0.5 to 6.5, preferably 1 to 4 percent by weight based on the weight of the total monomer combination used to prepare the (meth)acrylic polymer.

The (meth)acrylic polymer optionally contains nitrogen functionality introduced from a nitrogen-containing ethylenically 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-butylaminoethyl(meth)acrylate, dimethylaminoethyl(meth)acrylate, diethylaminoethyl(meth)acrylate, dimethylaminopropyl(meth)acrylate and dimethylaminopropyl(meth)acrylamide; amido-functional ethylenically unsaturated materials that include acrylamide, methacrylamide, n-methyl acrylamide and n-ethyl(meth)acrylamide; urea functional ethylenically unsaturated monomers that include methacrylamidoethylethylene urea.

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.

Besides the polymerizable monomers mentioned above, other polymerizable ethylenically 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, trimethylolpropane 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 nitrites. 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)acrylic polymer.

The (meth)acrylic polymer is typically formed by solution polymerization of the ethylenically 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 mercaptoalkyl trialkoxysilanes such as 3-mercaptopropyl trimethoxysilane may be used for this purpose as well as other chain transfer agents such as cyclopentadiene, allyl acetate, allyl carbamate, and mercaptoethanol.

The polymerization reaction for the mixture of monomers to prepare the acrylic polymer can be carried out in an organic solvent medium utilizing conventional solution polymerization procedures which are well known in the addition polymer art as illustrated with particularity in, for example, U.S. Pat. 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 methyl isobutyl ketone; esters such as butyl acetate; and aromatic hydrocarbons such as xylene, toluene, and naphtha.

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.

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.

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 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 1 to 6 percent by weight based on weight of the polymer solution.

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.

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 functionality and catalyst. Typically, the electrospinning formulation will be stored for as low as one minute up to two hours.

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.

The following examples are presented to demonstrate the general principles of the invention. However, the invention should not be considered as limited to the specific examples presented. All parts are by weight unless otherwise indicated.

EXAMPLES A, B and C

Synthesis of Acrylic Silane Polymers

For each of Examples 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 simultaneously 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.

	TABLE 1
	Example 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	0.3	0.64
Acrylamide	7.2	—	—
Sartomer SR 3553	—	30.3	—
Ethanol SDA 40B	155.7	325.5	622.6
Charge C (weight in grams)
Vazo 674	6.1	12.8	24.5
Ethanol SDA 40B	76.7	160.4	306.8
Charge D (weight in grams)
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	51.96	45.64	45.03
(100% resin solids)
Mn	—	30215	5810
1Denatured ethyl alcohol, 200 proof, available from Archer Daniel Midland Co.
2gamma-methacryloxypropyltrimethoxysilane, available from GE silicones.
3Di-trimethylolpropane tetraacrylate, available from Sartomer Company Inc.
42,2′-azo bis(2-methyl butyronitrile), available from E.I. duPont de Nemours & Co., Inc.
5Mn of soluble portion; the polymer is not completely soluble in tetrahydrofuran.

EXAMPLES 1, 2 AND 3

Acrylic-Silane Nanofibers

Example 1

The acrylic-silane resin solution from Example C (8.5 grams) was blended with polyvinylpyrrolidone (0.2 grams) and water (1.5 grams). 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 0.010-inch internal diameter). This tube was connected to a grounding 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

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 ml 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

The acrylic-silane resin from Example A (8.5 grams) was blended with polyvinylpyrrolidone (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 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.

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.

Claims

1. A method for electrospinning a fiber from an electrically conducting solution of polymer in the presence of an electric field between a spinneret and a ground source, the polymer undergoing a crosslinking reaction prior to and during electrospinning.

2. The method of claim 1 in which the polymer contains crosslinkable groups along the polymer backbone.

3. The method of claim 2 in which the crosslinkable groups are reactive with moisture.

4. The method of claim 3 in which the crosslinkable groups are silane groups.

5. The method of claim 2 in which the polymer is a (meth)acrylic polymer.

6. The method of claim 2 in which the polymer is a (meth)acrylic polymer containing silane groups.

7. The method of claim 2 in which the polymer, besides containing crosslinkable groups, also contains groups selected from carboxyl and hydroxyl.

8. The method of claim 2 in which the polymer contains silane groups, carboxyl groups, hydroxyl groups and nitrogen-containing groups.

9. The method of claim 2 in which the silane groups are present in the polymer in amounts of 0.2 to 20 percent by weight silicon based on total polymer weight.

10. The method of claim 8 in which the polymer contains from: the percentages by weight being based on total polymer weight.

(a) 0.2 to 20 percent silane group measured as silicon,

(b) 1 to 45 percent carboxyl groups,

(c) 0.5 to 6.5 percent hydroxyl groups, and

(d) 0.2 to 5.0 percent nitrogen groups;

11. The method of claim 1 in which the solution contains a thickener.

12. The method of claim 11 in which the thickener is polyvinyl pyrrolidone.

13. The method of claim 12 in which the polyvinyl pyrrolidone is present in amounts of no greater than 20 percent by weight based on total weight of solution.

14. An electrospun fiber comprising a polymer that has been crosslinked prior to and during the electrospinning process.

15. The electrospun fiber of claim 14 having a diameter of from 5 to 5,000 nanometers.

16. The electrospun fiber of claim 14 having —Si—O—Si— crosslinks.

17. The electrospun fiber of claim 14 being a crosslinked (meth)acrylic polymer.

18. The electrospun fiber of claim 14 being a (meth)acrylic polymer with —Si—O—Si— crosslinks.