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Nonwovens / Technical Textiles

Fueling Nonwovens Growth

Research and technology advancements continue to add to the variety and complexity of nonwovens products.<b>By Richard G. Mansfield, Technical Editor</b>

Richard G. Mansfield, Technical Editor

Fueling Nonwovens Growth Research and technology advancements continue to add to the variety and complexity of nonwovens products.At the Association of the Nonwoven Fabrics Industry-Technical Association for the Worldwide Pulp, Paper and Converting Industry (INDA-TAPPI) International Nonwovens Technical Conference, held recently in Baltimore, the growing diversity and sophistication of the technology used for producing nonwovens was evident. As the nonwovens industry has grown, there has been greater involvement from academia in the major processes and materials used to produce nonwovens. At the conference, papers were presented by members of 10 universities, in addition to papers from industry and government organizations. Under the leadership of Ted Wirtz, INDAs retiring president, membership in the organization has more than doubled since 1996. In his industry forecast presented at the conference, Wirtz emphasized the important role technology plays in the growth of the nonwovens industry throughout the world. He cited the automotive industry as an example: The industry now uses more than 33 square meters of nonwovens per vehicle in a growing number of under-the-hood and interior applications. On a worldwide basis, INDA estimates the nonwovens business grew at an annual rate of about 8 percent from 1991 to 2001. The fastest growth in nonwovens production is in the Middle East, and the Asian Pacific and Latin American regions. The growing middle class in these regions is stimulating the demand for products made with nonwovens. The United States is still a net exporter of nonwovens. From 1997 to 2001, imports rose by about $100 million to $367 million annually, while exports remained at about $700 million.

Nanotechnology And NanofibersIn September 2001, Scientific American published a special issue titled Nanotech The Science of the Small Gets Down to Business. In that issue, author Gary Stix wrote an article titled Little Big Science, which had the subhead: Nanotechnology is all the rage. But will it meet its ambitious goals And what the heck is itStix wrote: The definition is indeed slippery. Some of nanotechnology isnt nano, dealing instead with structures on the micron scale (millionth of a meter), 1,000 or more times larger than a nanometer [nm].Mihail C. Roco, an official at the National Science Foundation who oversees its nanotechnology initiative, offers a more restrictive definition: The emerging field new versus old nanotech deals with materials and systems having these key properties: they have at least one dimension of about 1 to 100 nanometers; they are designed through processes that exhibit fundamental control over the physical and chemical attributes of molecular-scale structures; and they can be combined to form larger structures.One danger of all of the publicity and hype that has been generated concerning nanotechnology is that it is being oversold to the public by some futurists as a pathway to utopia, with the promise of prolonging the human lifespan and producing low-cost self-replicating robots.At the INDA-TAPPI conference, 10 papers were presented on nanofibers. Fiber Charging Effects on Target Covering in Electrospinning by Peter P. Tsai, Ph.D., Textiles and Nonwovens Development Center (TANDEC), University of Tennessee, Knoxville; and Heidi L. Schreuder-Gibson, US Army Research Development and Engineering Command, Natick, Mass. describes an electrospinning process that is used to make nanofibers: Electrospinning (ES) is a process that spins nanofibers of diameters ranging from 10 nm to 10 µm [micrometers] from polymer solution or melt by the application of an electrostatic drawing force. A wide variety of polymer types, ranging from polar polymers in polar solvents to nonpolar polymer/solvent combinations have been electrospun over the last 70 years. Some electrospun polymers such as nylon and polycarbonate have been commercialized in high-performance filter media.Tsai and Schreuder-Gibson also refer to other work that has been done to combine electrospun fibers with a meltblown nonwoven fabric to enhance a fabrics resistance to 2-µm aerosol penetration. Fabrics with this property are of interest for chemical warfare uniforms and for chemical protectivewear. The experimental apparatus for ES at the University of Tennessee is described in the paper: Different types of polymers including polyethylene oxide (PEO), Estane® thermoplastic polyurethane (PU), nylon (PA), polycarbonate (PC), polycaprolactone (PCL), polyacrylonitrile (PAN) and polystyrene (PS) with polar and nonpolar properties were investigated in this study. Power supplies (manufactured by SIMCO®) of positive and negative polarities with 50 kV [kilovolts] and 2 mA [milliamps] were employed as the voltage source for the ES process. A SIMCO ionizing blower was applied to neutralize the charges in the fibers between the nozzle and the collector. A microsyringe pump from the Orion Company was used to control the throughput of the polymer solution. Stationary plates (18 by 24 inches) and a 12-inch-diameter rotating drum with a 26-inch width were used as fiber collectors. Plastic syringes fitted with metal needles were used as the polymer solution reservoir and the ES nozzle. The electrode from the power supply was attached to the metal needle. Figure 2 shows a sketch of the ES process.
Donaldson And NanofibersPapers also were presented by technologists from the Donaldson Co., a Minneapolis-based manufacturer of filters and filter systems. Donaldson is one of the leaders in using nanofibers in composite structures for specialized filtration applications. The first paper presented by Donaldson, Incorporation of Electrospun Nanofibers into Functional Structures by Kristine Graham and Mark Gogins, Donaldson Co.; and Heidi L. Schreuder-Gibson describes the work Donaldson and the US Army Soldier Systems Center are doing to improve the performance of the Battle Dress Overgarment (BDO) that protects military personnel from chemical warfare agents:Critical performance requirements for chemical protection suits include:chemical protection from a variety of liquid and vapor-phase chemical contaminants;resistance to liquid (rain) intrusion;air permeability to maintain wearer comfort;tear strength and fabric weight requirements; anddurability launderings and hours of wear.Current materials systems incorporate a variety of layers to meet the requirements: the inner layer of activated carbon-based chemical vapor filtration material is supported by a fabric and protected by a top layer of adhesively-bonded nonwoven scrim. This inner layer is covered by a durable outer shell fabric treated with a water repellent to provide resistance to liquid intrusion. There is an opportunity to improve the performance of this system through the incorporation of polymeric nanofiber webs. First, a polymeric nanofiber web can provide enhanced protection against aerosols (e.g. chemical agent micro-droplets, biological aerosols, radioactive dusts, etc.) without adding weight or thickness and while maintaining adequate permeability for wearer comfort. Second, the polymeric nanofiber web can be used as a carrier for active chemistry that may allow for improvements in chemical protective properties (and/or permeability, and/or weight).In designing the nanofiber composites, spunbonded fabrics were chosen as the substrates:The following design approaches were evaluated for the composite layered material design:Nanofiber constructions: nanofibers were applied to a 0.6 osy [ounces per square yard] nylon spunbond material and to a 1.0 osy nylon spunbond material. The nylon spunbond materials were chosen because it was thought that the adhesion and durability of the polyamide nanofibers would be improved by choosing a carrier of like material.Nanofiber composites: nanofibers were applied to the surface of the spunbond material and incorporated into the final fabric architecture. Additionally, some samples were made where lighter layers of nanofibers were applied to the spunbond materials, then two layers of spunbond/nanofiber composite were laminated together in a face-to-face configuration, i.e. a structure of spunbond-nanofiber-lamination-nanofiber-spunbond. It was thought that this configuration would protect the nanofiber layers from surface scuffing. Figure 3 shows the Final Fabric Architecture Options.
Several significant technical advances were made during this project: Aerosol barrier and permeability modeling and testing confirmed the performance advantages of nanofibers for aerosol barrier materials.Advancements in design of a durable composite structure were made. The best durability performance was achieved using two layers of nanofibers captured between two layers of a very strong, smooth and flexible spunbond material. This combination protected the nanofibers from direct abrasion and provided a dimensionally stable supporting structure.Some of the same attributes that make nanofibers attractive as aerosol barriers also lead to challenges for durable structures. Since the nanofiber layer effectively removes particles yet allows flow through high permeability, dirt and detergent particles are caught and retained, leading to reduced durability as these particles grate against the nanofiber layer and ultimately create holes in the structure.This project primarily focused on using nanofibers as an add-on layer within an existing garment system. Re-engineering the entire system with the capabilities and limitations of the nanofibers in mind is more likely to result in success. New fabric layers, redistribution of the nanofibers and repositioning of the components, etc., could resolve many of the durability issues.Incorporation of catalysts directly into the electrospun nanofibers enhances the reactivity of the catalysts, rather than diluting their effectiveness within the fiber bulk.ES nanofibers already are in commercial use for very specialized filtration applications. Donaldson is making and marketing filter media that incorporate nanospun nylon fibers for gas turbines, compressors and generators. Hills' Nanofiber ApproachMulti-component fiber technology has been used for more than 50 years in the fiber and textile industries, but usage has been limited because of the high cost of producing specialized spinnerets and polymer distribution systems. Hills Inc., West Melbourne, Fla., has been a pioneer in developing lower-cost systems for making multi-component fibers. As a result of the work by Hills, such systems now are widely used throughout the world for fibers, and spunbonded and meltblown nonwovens. Hills paper, Multi-Component Fiber Technology For Filtration and Other Uses, presented by John F. Hagewood, describes some common fiber types:By far the most common type of multi-component fibers are bicomponent fibers (consisting of two polymer components). The major types include:Sheath/core fibers, most commonly used to make binder fibers.Side by side fibers, most commonly used to produce bulky, self-crimping fibers.Tipped products, most commonly used in special filtration products.Segmented products, wherein by chemical, thermal and/or mechanical methods, the segments are split into small individual fibers.Islands-in-a-Sea products, wherein the sea is normally dissolved away to leave only the very small islands.Various mixes of two or more fiber types to make such specialized products as yarns or fabrics having multiple cross-sections. By using the newer islands-in-a-sea techniques, Hills has been able to make fibers with up to 1,000 islands at normal spinning rates. By this method, with 600 islands from a 1-denier fiber, each island is approximately 300 nm in diameter. A major advantage of Hills method of producing nanofibers is that the production rate is much higher than the ES method and should result in lower-cost products.
Cross-section images of Hills' multi-component fibers (left to right): islands-in-a-sea; bicomponent sheath-core; and segmented productsThermal Point BondingThermal point bonding of nonwovens is one of the key technologies that has aided the growth of nonwovens in a wide range of consumer and industrial products. The work done by Stephen Michielson, Ph.D., and his associates provides a comprehensive analysis of the theory of this process, and also offers practical guidelines to optimize thermal point bonding for the production of spunbond, air-laid, wet-laid and carded web nonwovens. Review of Thermally Point Bonded Nonwovens: Materials, Processes and Properties, presented by Michielson, Georgia Institute of Technology, Atlanta; Behnam Pourdeyhimi, Ph.D., Nonwovens Cooperative Research Center, North Carolina State University, Raleigh, N.C.; and Prashant Desai, Ph.D., FiberVisions Inc., Covington, Ga., reached a number of conclusions:Thermalbonding of nonwoven webs occurs through three steps 1) heating the fibers in the web, 2) forming a bond through reptation of the polymer chains across the fiber-fiber interface, 3) cooling and resolidifying the fibers. In [calender] bonding, step 1 must occur while the web is in the nip. Step 2 must begin while the web is in the nip to tie the structure together, but it can finish during the initial portion of step 3. There is excellent agreement between the required times for heating and forming the bond and commercial bonding times. The processes described above show that the observed failure mechanisms can readily be understood. In under-bonded webs, there are too few polymer chains diffusing across the fiber-fiber interface. During tensile testing, these bonds simply disintegrate. In well-bonded webs there is sufficient reptation of the chains across the interface to form a strong bond, but only a moderate loss of mechanical properties of the bridging fibers at the bond periphery. Hence, there is a trade off between the strength of the bond and the strength of the fibers at the bond periphery. In over-bonded webs, there is sufficient reptation of the chains across the interface to form a strong bond, but there is a large loss of mechanical properties of the bridging fibers at the bond periphery. During tensile testing, the fibers break at the bond periphery.As academic and industry involvement in research in materials and for nonwovens production processes grows, the level of sophistication and diversity of processes available to the nonwovens industry also will grow. Editor's Note: Extracts from papers delivered at the INDA-TAPPI 2003 conference reprinted by permission of INDA. Copies of the papers can be purchased by contacting INDA, (919) 233-1210; fax (919) 233-1282.

November 2003