Patent Application
20100178507
The Invention is in fibers having small or micro- and nano-scale, diameters and to methods of forming such fibers. These fibers have substantially improved properties. The fibers can be used in filtration applications.
Microfiber and nanofiber have been prepared from polyamides in the past. Depending on their applications, such fibers have had some success. Polyamides belong to a class of engineering polymers that have found a wide variety of successful thermoplastic commercial applications including in the synthetic fiber industry. In this class of materials, certain polyamide have been most widely used because of their desirable combination of physical properties which include high strength, toughness, flexibility, thermal resistance, and chemical resistance.
Polyamides are condensation polymers typically polymerized from a lactam and from a diacid and a diamine and processed in to useful forms. In certain types of processing where the solubility of the material is important, the poor solubility of these materials in common organic and environmentally friendly solvents limits their applications if they must be processed from solution. Strategies to dissolve these materials have included strong acids such as formic acid and sulfuric acid, fluorinated solvents such as 2,2,2-trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP), and mixtures of TFE and methylene chloride. These solvents are hazardous and can add significant processing costs and in some cases can be detrimental to the material. For example, acidic solvents have been shown to degrade aliphatic polyamides and fluorinated solvents are considerably more expensive than commonly used organic solvents.
The use of nylon-6 6 in combination with other materials to electrospin nanofibers for filtration applications has been described in U.S. Pat. No. 6,743,273. However, the ability to produce nanofibers from this material was limited by its poor solution stability in EtOH/H2O cosolvent mixtures and the formation of non-homogeneous nanofiber structures. Better nanofiber formation was only achieved after this material was blended with an alcohol soluble Nylon 6 Nylon 6 6 Nylon 6 10 polyamide copolymer. As will be discussed later, many of these solution blends still suffer from poor solution stability which limits their use in fiber forming processes.
Other approaches in improving the solubility of polyamides have involved N-alkylation and N-allylation in which amide hydrogens were deprotonated to form dimethyl sulfoxide (DMSO) soluble polyanions that were subsequently converted to N-alkylated and N-allylated products with heat or by reacting with allyl bromide.
A substantial need exists to show that improved materials can achieve, particularly in nanofiber sizes, increased environmental stability, increased processability including solubility in environmentally safe solvent systems, electrical conductivity for electrospinning and viscosity control. Lastly, when the fiber is used in a filter structure, the fiber must obtain effective filtration efficiency over an array of conditions including fluid type, type of particulate, particulate concentration, temperature and fluid velocity through the fiber mass.
The invention comprises a micro- or nano-fiber comprising a novel nylon polymer. The fibers can be made into a layer or layers comprising a distribution of micro- or nano-fibers. The polymer in the fibers in any one layer can be crosslinked.
The invention comprises a fiber comprising an N-modified nylon of the formula (I or II):
The fiber can consist of the nylon of the invention; wherein in I or II, CO represents a carbonyl, R1 is a lower alkyl, such as methyl, ethyl, propyl, a vinyl containing chain, such as ethylene, allyl, etc., a hydroxyl group, an alkoxy group, such as lower alkyl only groups, such as methoxy, ethoxy, butoxy, etc., m and n is independently an integer number from 2 to 12 and p+q is an integer number from about 10 to 5000, and the ratio of p to p+q is a number between 0.05 to 0.99. Formula I shows that a fraction of the —N— amide nitrogens are N-modified. In use, in this aspect, the N-modified nylon can be used as a single component or blended with another polymer in the fine fiber materials. A preferred aspect is a fiber consisting of a methoxy, ethoxy, N-methylol or N-methoxymethyl modified nylon-6 such as the nylon polymer derived from s-caprolactam.
We have also found that nanofibers from an N-modified or N-substituted (e.g.) an N-alkylol, N-alkoxy, or N-alkoxy alkyl nylon-X1, wherein X1 is an integer from 5 to 15, and the degree of substitution on the nitrogen is less than about 50% and is often about 2 to 40% can be formed into a fiber. These modified nylons include modified polymers made of Nylon 5, Nylon 6, Nylon 7 and Nylon 12. Preferred are N-methoxymethyl-nylon-6, or N-ethoxymethyl-nylon-6. These materials can surprisingly be crosslinked under heat in the absence of acid catalysts without any loss of solvent resistance characteristics. The high filtration efficiency of these structures demonstrates their use in a variety of particulate filtration applications that require material resistance to elevated temperatures, high humidity, and chemical exposure.
We have demonstrated that the solutions of these improved nylon materials have significantly improved solution stability in EtOH/H2O cosolvent mixtures and that homogenous nanofiber structures were produced from homopolymer solutions of N-methoxymethyl-nylon-6 and solution blends with nylon-6,66,PACM 6. The molecular weights of the polymers are:
Because of their small diameter and high surface area, polymer nanofibers are highly susceptible to mechanical damage and chemical or thermal degradation under high temperature, high humidity, and chemical exposure. Under these conditions, crosslinking the polymer matrix helps to stabilize the fiber and to retain the filtration characteristics of the nanofiber structures.
The materials of the invention are derived from the generic polymer class consisting of the polyamides and copolyamides including linear homopolyamides and copolyamides which are prepared in a known manner from cyclic lactams, lactams or suitable derivatives of these compounds. Useful monomers are those used in polymerization of such polyamides and copolyamides as nylon 3, 4, 5, 6, 8, 11, 12, 13, 6 6, 6 10 or 6 13; or a polyamide obtained from cyclic lactams and from other monomers including metaxylylenediamine and adipic acid or from trimethylhexamethylenediamine or isophoronediamine and adipic acid; nylon 6, 6 6, 6 10 or nylon 6, 6 6, 6 12; or a polyamide of ε-caprolactam/adipic acid/hexamethylenediamine/bis(4-aminocyclohexyl)methane, which are produced by copolymerizing equal amounts of adipic acid, caprolactam and hexamethylene diamine comonomers with bis(4-aminocyclohexyl)methane. The materials of this invention also include N-modified derivatives of all these homopolyamides and copolyamides which include N-alkyl, an N-vinyl containing chain, such as ethylene, allyl, etc., an N-methylol group, or an N-alkoxymethyl group.
The invention provides a range of improved polymeric materials. These polymers have improved physical and chemical stability. The polymer fine fiber (microfiber and nanofiber) can be fashioned into useful product formats. Nanofiber is a fiber with diameter less than 200 nanometer or 0.2 micron. Microfiber is a fiber with diameter larger than 0.2 micron, but not larger than 5 microns. This fine fiber can be made and then made into the form of an improved layered or multi-layer microfiltration media structure. The fine fiber layers of the invention comprise a random distribution of fine fibers which can be bonded to form an interlocking net. Such layers or nets can be formed on a filter substrate layer. Such layers are cellulosic, synthetic or mixed cellulosic/synthetic. Filtration performance is obtained largely as a result of the cooperation between the fine fiber barrier to the passage of particulate and contribution of the of filter substrate barrier. Structural properties of stiffness, strength, pleatability are provided by the substrate to which the fine fiber adhered. The fine fiber interlocking fiber networks provide important characteristics to a fiber layer. Fine fiber layers consist of relatively small spaces between the fibers and form pores in the layer at pore sizes that are useful in filter applications. Such spaces typically range, between fibers, of about 0.01 to about 25 microns or often about 0.1 to about 10 microns. The filter products comprising a fine fiber layer are combined with a choice of appropriate substrate. The fine fiber adds less than few microns and often less than a micron in thickness to the overall fine fiber layer on the substrate filter media. In service, the filters can stop incident particulate from passing through the fine fiber layer and can attain substantial surface loadings of trapped particles. The particles comprising dust or other incident particulates can form a dust cake on the fine fiber surface and maintain high initial and overall efficiency of particulate removal. Even with relatively fine contaminants having a particle size of about 0.01 to about 1 micron, the filter media comprising the fine fiber has a very high dust capacity.
The polymer materials as disclosed herein have substantially improved resistance to the undesirable effects of heat, humidity, high flow rates, reverse pulse cleaning, operational abrasion, submicron particulate penetration, cleaning of filters in use and other demanding conditions. The improved microfiber and nanofiber performance is a result of the improved character of the polymeric materials forming the microfiber or nanofiber. Further, the filter media of the invention using the improved polymeric materials of the invention provides a number of advantageous features including higher efficiency, lower flow restriction, high durability (stress related or environmentally related) in the presence of abrasive particulates and a smooth outer surface free of loose fibers or fibrils. The overall structure of the filter materials provides an overall thinner media allowing improved media area per unit volume, reduced velocity through the media, improved media efficiency and reduced flow restrictions.
A particularly preferred material of the invention comprises a small diameter fiber material having a dimension of about 5 to 0.005 microns, about 2 to 0.01 micron or between 0.8 to 0.05 micron. Such fibers with the preferred size provide excellent filter activity, ease of back or reverse pulse cleaning and other aspects.
The highly preferred polymer systems of the invention have adhering characteristic such that when fibers are contacted with a cellulosic or other synthetic or mixed cellulosic/synthetic substrate, they adhere to the substrate with sufficient strength such that they are securely bonded to the substrate and can resist the delaminating effects of a reverse pulse cleaning technique and other mechanical stresses. In such a mode, the polymer material must stay attached to the substrate while undergoing a pulse clean input that is substantially equal to the typical filtration conditions in a reverse direction across the filter structure. Such adhesion can arise from solvent effects of fiber formation as the fiber is contacted with the substrate or the post treatment of the fiber on the substrate with heat or pressure. However, polymer characteristics appear to play an important role in determining adhesion, such as specific chemical interactions like hydrogen bonding, contact between polymer and substrate occurring above or below Tg, and the polymer formulation such as conductivity stability and viscosity. Polymers plasticized with solvent or steam at the time of adhesion can have increased adhesion.
We have found that additive materials can improve the properties of certain of the copolymer materials in the form of a fine fiber. The resistance to the effects of heat, humidity, impact, mechanical stress and other negative environmental effect can be substantially improved by the presence of additive materials. We have found that while processing the microfiber materials of the invention, that the additive materials can improve the oleophobic character, the hydrophobic character and can appear to aid in improving the chemical stability of the materials. We believe that the fine fibers of the invention in the form of a microfiber are improved by the presence of these oleophobic and hydrophobic additives as these additives form a protective layer coating, ablative surface or penetrate the surface to some depth to improve the nature of the polymeric material. We believe the important characteristics of these materials are the presence of a strongly hydrophobic group that can preferably also have oleophobic character. Strongly hydrophobic groups include fluorocarbon groups, hydrophobic hydrocarbon surfactants or blocks and substantially hydrocarbon oligomeric compositions. These materials are manufactured in compositions that have a portion of the molecule that tends to be compatible with the polymer material affording typically a physical bond or association with the polymer while the strongly hydrophobic or oleophobic group, as a result of the association of the additive with the polymer, forms a protective surface layer that resides on the surface or becomes alloyed with or mixed with the polymer surface layers. Additive layers can range form 10 to 200 angstroms.
An important aspect of the invention is the utility of such microfiber or nanofiber materials formed into a filter structure. In such a structure, the fine fiber materials of the invention can consist of stand alone fiber layers or the polymer fiber material can be formed onto and adhered to a filter substrate. Natural fiber and synthetic fiber substrates, like spun bonded fabrics, non-woven fabrics of synthetic fiber and non-wovens made from the blends of cellulosics, synthetic and glass fibers, non-woven and woven glass fabrics, plastic screen like materials both extruded and hole punched, UF and MF membranes of organic polymers can be used. Sheet-like substrate or cellulosic non-woven web can then be formed into a filter structure that is placed in a fluid stream including an air stream or liquid stream for the purpose of removing suspended or entrained particulate from that stream. The shape and structure of the filter material is up to the design engineer. One important parameter of the filter elements after formation is its resistance to the effects of heat, humidity or both. One aspect of the filter media of the invention is a test of the ability of the filter media to survive immersion in warm water for a significant period of time. The immersion test can provide valuable information regarding the ability of the fine fiber to survive hot humid conditions and to survive the cleaning of the filter element in aqueous solutions that can contain substantial proportions of strong cleaning surfactants and strong alkalinity materials. Preferably, the fine fiber materials of the invention can survive immersion in hot water while retaining at least 50% of the fine fiber formed on the surface of the substrate. Retention of at least 50% of the fine fiber can maintain substantial fiber efficiency without loss of filtration capacity or increased back pressure, most preferably retaining at least 75% of the fiber for filtration purposes.
All of these materials and admixtures of materials can be crosslinked using appropriate crosslinking agents, processes or mechanisms. Nylons can be crosslinked using crosslinking agents that are reactive with the nitrogen atom in the amide linkage. Such reactive materials include monoaldehydes, such as formaldehyde, ureas, melamine-formaldehyde resin and its analogues, boric acids and other inorganic compounds. dialdehydes, diacids, urethanes, epoxies and other known crosslinking agents. Crosslinking can be accomplished using radiation source to bond adjacent polymer chains. Simple heating processes can act to crosslink. A preferred crosslinking agent for polyamide materials is p-toluene sulfonic acid (p-TSA). Crosslinking technology is a well known and understood phenomenon in which a crosslinking reagent reacts and forms covalent bonds between polymer chains to substantially improve molecular weight, chemical resistance, overall strength and resistance to mechanical degradation.
A fine fiber filter structure includes a bi-layer or multi-layer structure wherein the filter contains one or more fine fiber layers. Such layers can be used as is or can be combined with or separated by one or more synthetic, cellulosic or blended substrate webs. Another preferred motif is a structure including fine fiber in a matrix or blend of other fibers.
Electrospinning can be achieved in apparatus that includes a reservoir of fine fiber forming polymer solution in contact with an emitter. An emitter can be immersed into a reservoir of polymer. A droplet of the solution from the emitter is accelerated by an applied electrostatic field toward the collecting media. Facing the emitter, but spaced apart therefrom, is a substantially planar grid upon which the collecting media, substrate or combined substrate is positioned. The collecting media is passed over the grid at a rate to form the fiber in a continuous layer. Air can be drawn through the grid. A high voltage electrostatic potential is maintained between emitter and grid. In use, the electrostatic potential between grid and emitter imparts a charge to the polymer solution which causes liquid droplets to be emitted therefrom as thin fibers. Solvent is evaporated off the fibers during their flight. The fine fibers are directed to and bond to the substrate fibers as they form. Electrostatic field strength is selected to ensure that the acceleration of the polymer material is sufficient to render the material into a very thin microfiber or nanofiber structure as it is accelerated from the emitter to the collecting media. Increasing or slowing the advance rate of the collecting media can deposit more or less emitted fibers on the forming media, thereby allowing control of the thickness of each layer deposited thereon. Fibers smaller than 1 micron are best made from polymer solution. As the polymer mass is drawn down to smaller diameter, solvent evaporates and contributes to the reduction of fiber size. Electrostatic spinning can be done at a polymer solution flow rate of 0.001 to 5 ml/min per emitter, a target distance of 1 to 20 cm, and an emitter voltage of 1 to 60 kV.
The fine fiber materials of the invention can be used in a variety of filter applications including pulse clean and non-pulse cleaned filters for dust collection, gas turbines and engine air intake or induction systems; gas turbine intake or induction systems, heavy duty engine intake or induction systems, light vehicle engine intake or induction systems; “Z” filter; vehicle cabin air; off road vehicle cabin air, disk drive air, photocopier-toner removal; HVAC filters in both commercial or residential filtration applications.
Various filter designs are shown in patents disclosing and claiming various aspects of filter structure and structures used with the filter materials. Engel et al., U.S. Pat. No. 4,720,292, disclose a radial seal design for a filter assembly having a generally cylindrical filter element design, the filter element being sealed by a relatively soft, rubber-like end cap having a cylindrical, radially inwardly facing surface. Kahlbaugh et al., U.S. Pat. No. 5,082,476, disclose a filter design using a depth media comprising a foam substrate with pleated components combined with the microfiber materials of the invention. Stifelman et al., U.S. Pat. No. 5,104,537, relate to a filter structure useful for filtering liquid media. Liquid is entrained into the filter housing, passes through the exterior of the filter into an interior annular core and then returns to active use in the structure. Such filters are highly useful for filtering hydraulic fluids. Engel et al., U.S. Pat. No. 5,613,992, show a typical diesel engine air intake filter structure. The structure obtains air from the external aspect of the housing that may or may not contain entrained moisture. The air passes through the filter while the moisture can pass to the bottom of the housing and can drain from the housing. Gillingham et al., U.S. Pat. No. 5,820,646, disclose a Z filter structure that uses a specific pleated filter design involving plugged passages that require a fluid stream to pass through at least one layer of filter media in a “Z” shaped path to obtain proper filtering performance. The filter media formed into the pleated Z shaped format can contain the fine fiber media of the invention. Glen et al., U.S. Pat. No. 5,853,442, disclose a bag house structure having filter elements that can contain the fine fiber structures of the invention. Berkhoel et al., U.S. Pat. No. 5,954,849, show a dust collector structure useful in processing typically air having large dust loads to filter dust from an air stream after processing a workpiece generates a significant dust load in an environmental air. Lastly, Gillingham, U.S. Design Pat. No. 425,189, discloses a panel filter using the Z filter design. A general understanding of some of the basic principles and problems of air filter design can be understood by consideration of the following of filter media types including surface loading media and, depth media. Each of these types of media has been well studied, and each has been widely utilized. Certain principles relating to them are described, for example, in U.S. Pat. Nos. 5,082,476; 5,238,474; and 5,364,456. The complete disclosures of these three patents are incorporated herein by reference.
One hundred and fifty grams of a methoxy methyl N-modified nylon 6 with a molecular weight of about 20,000, were added to 743.5 grams of Ethanol with 106.5 grams of distilled water and 4.5 ml of a 40% solution of para-toluene sulfonic acid in isopropanol.
Nanofibers were electrospun from solutions described in Example 1 by applying a voltage of 10.7 to 16.6 kV to polymer solutions eluted from syringe needles at a flow rate of 0.10 mL/min. The distance from the emitter to collector substrate was fixed at 3 inches. Nanofibers were electrospun onto a cellulose substrate (product no. FF6168; Hollingworth and Vose Company) and thermally crosslinked by annealing in a 150° C. oven for 10 min.
Polymer thermal transitions were obtained with a DSC 2920 instrument (TA Instruments). Scans were obtained at a rate of 10° C./min over a temperature range of 0-300° C. The sample chamber was purged with N2 during analysis. Data acquisition and analysis was performed with TA Universal Analysis software (TA Instruments). Melting peak transitions were;
Tm=185° C. for nylon-6,66,PACM6,
Proton (1H-NMR) NMR analysis was used to determine the degree of N-substitution in N-substituted polyamide (nylon 6) [N-PA-6].
[—CH2—NH(C═O)—], and the peak at 2.55 ppm was consistent with methylene protons attached alpha to amide carbonyl atoms [—CH2—(C═O)—NH—]. The peaks in the higher field region ˜1.6-2 ppm were consistent with methylene protons that were further removed from the electron withdrawing amide nitrogen and amide carbonyl groups. Consistent with previous 1H-NMR reports on N-modified nylon-6,
where Ies and Ie are integration intensities of the protons alpha to the carbonyl group in the substituted and unsubstituted carbonamide groups, respectively.
DSC analysis of N-PA-6 shown in
Scanning electron microscopy was performed on samples that were first mounted onto aluminum stubs with carbon tape and then sputtered with gold. Images were obtained with a JEOL JSM-5900LV scanning electron microscope.
Nanofibers were prepared by electrospinning solutions of Example I and blends with nylon-6,66,PACM6 (see copending U.S. application Ser. No. 12/558,499) in EtOH/H2O.
The LEFS measurement were done according to ASTM1215-89. Particle capture efficiency was measured on a LEFS bench using 0.80 μm latex spheres with velocity of 20 ft/min as a test challenge contaminant. Measurements were made on substrate and nanofiber composite samples that were annealed at 150° C. for 10 min. Measurements were also made on substrate and nanofiber composite samples that were annealed at 150° C. for 10 min and soaked in EtOH for 10 min. Samples soaked in EtOH were air dried for at least 3 h prior to measurement. Filter efficiency (LEFS) measurements (Table 1) of nanofibers from Example 1 showed high particle capture efficiency. LEFS measurements of samples that were immersed in EtOH for 10 min showed a substantial efficiency retention. The efficiency retained from nanofibers alone (Fr) in EtOH immersed samples was calculated using equation 2:
where Fx is the post EtOH soak nanofiber efficiency, and Fi is the initial nanofiber layer efficiency. Fi and Fx are expressed by equations 3 and 4:
F
i=1−eln(1-E
F
x=1−eln(1-E
where Ei=initial composite efficiency, Ex=post EtOH soak composite efficiency, Eis=initial substrate efficiency, and Exs=post EtOH soak efficiency.
The solution of claim 1 was electrospun at four different web speeds −5, 10, 15, and 20 fpm and was tested for efficiency.
This application claims priority from U.S. Pat. App. No. 61,096,513, filed Sep. 12, 2008, and to U.S. patent application Ser. No. 12/008,919, filed Jan. 14, 2008, which is a continuation of U.S. patent application Ser. No. 11/398,788, filed Apr. 6, 2006, now U.S. Pat. No. 7,318,852; which is a continuation of U.S. patent application Ser. No. 10/894,848, filed Jul. 19, 2004, now U.S. Pat. No. 7,179,317; which is a divisional of U.S. patent application Ser. No. 10/676,189, filed Sep. 30, 2003, now U.S. Pat. No. 6,924,028; which is a divisional of U.S. patent application Ser. No. 09/871,583, filed May 31, 2001, now U.S. Pat. No. 6,743,273, which claims benefit of U.S. Pat. App. No. 60/230,138, filed Sep. 5, 2000, which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61096513 | Sep 2008 | US | |
| 60230138 | Sep 2000 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10676189 | Sep 2003 | US |
| Child | 10894848 | US | |
| Parent | 09871583 | May 2001 | US |
| Child | 10676189 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12008919 | Jan 2008 | US |
| Child | 12558496 | US | |
| Parent | 11398788 | Apr 2006 | US |
| Child | 12008919 | US | |
| Parent | 10894848 | Jul 2004 | US |
| Child | 11398788 | US |
