| HOW NONWOVENS ARE MADE |
A basic concept used in making a nonwoven is to transform fibre-based materials into flat, flexible, porous, sheet structures with fabric characteristics. In practice, this concept is carried out in a number of different manners depending on the fibre material used and/or the fabric characteristics desired. Technologies used in three primary manufacturing industries - textiles, paper, and extrusion - and various combinations of established processes from one or more of these industries form the basis of the processes for manufacturing nonwovens. Accordingly, processes for manufacturing nonwoven fabric can be grouped into one of four general technology bases: textile, paper, extrusion, or hybrid (combination).
Textile-Based Processes
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Traditional textile fabrics are made by weaving, which is interlacing two or more yarn sets at right angles on a loom, or by knitting, which is the interlooping one or more yarns upon itself or themselves. Yarns are intermediate products to these fabric making processes and are composed of fibre or filament bundles held together by twist or entanglement. Nonwoven fabrics are similar to woven and knitted fabrics in that both are planar, inherently flexible, porous structures composed of relatively long fibres. The main difference between a nonwoven fabric and a woven or knitted fabric is the manner in which fibres or fibre-based materials are transformed into a planar configuration and interlocked to form a porous sheet with some degree of flexibility.
The use of textile technology to produce nonwovens involves adapting garneting, carding, and aerodynamic fibre handling methods to place textile fibres into preferentially-oriented webs. Fabrics produced by these systems are referred to as dry laid nonwovens and carry terms such as "garnetted'', "carded'', and ''air laid''. These fabrics or fibre-network structures are manufactured with machinery associated with staple fibre processing designed to manipulate preformed fibres in the dry state. Also included in this category are structures formed from tow and fabrics composed of staple fibres bonded by stitching filaments or yarns.
A fabric made by placing a predetermined number of textile fibres, filaments, or yarns in a planar array through the use of textile technology and subsequently interlocking or bonding them by mechanical, chemical. or thermal means is designated a dry laid nonwoven.
In the manufacture of dry laid nonwovens, discontinuous fibres are formed into parallel, two-dimensional layered, two-dimensional isotropic, or three-dimensional random orientations by mechanical or aerodynamic means and are subsequently consolidated (bonded) mechanically, chemically, or thermally. Mechanical web formation involves the utilization of textile carding or garnetting machinery or components to transform tufts of fibres or fibre blends into fibrous webs in which individual fibres are held by cohesion. As individual fibres are straightened by being pulled through the machinery, fibre orientation is mostly in the direction of flow through the machine, and the weight of the fibre web is generally lower than that required to form many fabrics. Different fibre orientations can be made by scrambling or randomizing the webs or by plaiting the webs through the use of a lapping device. Different web weights can be made by placing machines in tandem or by lapping. Lapping webs in the cross direction also provides a means of making wide batts from narrow webs. For optimum processability, fibres must have crimp in addition to flexibility, length uniformity, cross-sectional consistency, and a finish which provides lubricity and anti-static properties.
Staple-fibre air laid nonwoven webs are generally formed on single machines. In this technology, air currents and vacuum boxes are used to transport, mix, and collect fibres. Use of air in this manner allows for a wide variety of fibre geometries, properties, and fibre combinations to be handled. Air laid systems designed to process textile-length fibres employ mechanical fibre opening apparatus to prepare a loose batt of fibre tufts. The batt is mechanically fed through a feed-roll/feed-plate arrangement onto a metal-toothed lickerin roll which separates the fibre tufts and combines the fibres into a controlled air suspension and into a venturi zone where the fibres are tumbled while being transported to a collection screen. A basic fibre requirement for textile based nonwoven systems is thermal stability sufficient to withstand the heat encountered by impact with machine parts to avoid fusion or melting together of individual fibres. Web consolidation or bonding (the interlocking of fibres in adjacent horizontal layers or vertical zones) can be accomplished by mechanical, chemical, or thermal means.
Mechanical consolidation methods include stitchbonding, needlepunching, and hydroentangling. Fibre-to-fibre bonding in this instance is friction dependent and does not recover upon appreciable stretching. Chemical bonding methods include air or airless spraying, saturation, printing, and stable or semi-stable foam bonding. Fibre-to-fibre bonding in this instance is highly dependent on binder surface tension and fibre and surface energy compatibility. Also, binder properties often mask or override fibre properties. Thermal bonding methods employ radiant, convection, conductive, or sonic energy sources; fibre-to-fibre bonding in this instance is achieved through thermal fusion and (as in chemical bonding) is set or stabilized upon cooling. |
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Paper-Based Processes
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The classic papermaking process consists of suspending relatively short fibres in water and pumping the mixture onto a fine mesh screen through which the water flows and upon which the fibres are deposited in the form of a wet continuous mat. The mat is then pressed between rolls to mechanically remove the water held on and between the fibres, heated to dry out the water within the fibres and provide for hydrogen bonding, and wound into rolls. The chemical composition of papermaking fibres is cellulose in the form of wood pulp or chopped plant fibres such as hemp, cotton, sisal, abaca, or flax. When wetted and mechanically treated, cellulosic fibres split into a network of fine fibrils, which are drawn into intimate contact with other fibres by surface tension forces during drying, and interlock by the mechanism of hydrogen bonding. A nonwoven fabric can also be made by suspending fibres in water or some other fluid (including air); controlling the way by which the fibres and suspending media are separated; and then mechanically, chemically, or thermally interlocking the fibres together.
The use of paper technology base to produce nonwovens includes dry laid pulp and modified wet laid paper systems designed to accommodate fibres longer than wood pulps and different from cellulose. Fabric produced by these systems are referred to as ''shortfibre airlaid'' and "wet laid'' nonwovens. These fabrics are manufactured with machinery associated with pulp fibreizing (i.e. hammer mills) and paper forming (i.e. slurry pumping onto continuous screens) designed to manipulate short fibres suspended in a fluid.
A fabric made by suspending short fibres in a fluid, depositing the fibres on a porous surface to separate the fibres from the fluid, and interlocking or bonding them by mechanical, chemical, or thermal means is designated a fluid laid nonwoven.
In the wet laid or wet forming process, fibres are suspended in water, brought to a forming unit where the water is drained off through a screen and the fibres deposited on the wire, and then picked off the wire to be dried. Processing long fibres, synthetic fibres, or inorganic fibres in slurry form creates interesting challenges. As a general rule, these fibres do not wetout readily, are difficult to disperse, and tend to tangle with one another. Consequently, very high water dilutions are necessary to keep the fibres apart in the water suspension. If not handled properly, the fibres will tangle and poor sheet formation will result. The use of fibres other than wood pulp and the need for high ratios of water to fibre in order to process these fibres are two primary ways that wet laid nonwovens differ from traditional papers.
Another basic difference between conventional papermaking and wet lay nonwoven technology is the mechanism of bonding employed. Most manufactured fibres used in nonwovens do not selfbond as readily as natural cellulose. Thus, some external bonding method must be employed. A number of binder types and application methods are used, each engineered to yield specific fabric properties. Binders can be applied either before web formation or afterwards. Binders are applied after web formation by saturation, spraying, printing, foaming, or a combination thereof.
Web drying and binder activation are usually accomplished with steam heated cans. High synthetic fibre content webs frequently bag or stretch during drying on multiple steam cans. Ovens or other air drying devices, including the use of infrared, are employed for specialty nonwoven production. At the end of the processing line, calender or creping rolls are often placed to densify, smooth, and soften the fabric. Air laid systems designed to handle pulp-length fibres employ mechanical defibrators such as pin mills, disc refiners, and hammer mills housed in close proximity to a perforated screen to disperse the fibres. When the fibres have been sufficiently dispersed, they pass through the screen into a controlled air stream and onto a forming wire. For these systems, fibre rigidity is required to avoid fibre tangling by air currents. Arrangements for the addition of textile-length fibres enable air laid pulp systems to produce fabric structures with increased durability. |
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Extrusion-Based Processes
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The film extrusion process consists of passing molten polymer molten polymer through a narrow slit, solidifying the polymer in flat form, stretching the solid sheet to align the polymer molecules, and winding the sheet onto rolls. Synthetic fibre manufacturing is also a form of polymer extrusion. In this instance, materials such as poltethylene, polypropoylene, polyester, and nylon are heated, forced through small holes, solidified by cooling, stretched to align polymer molecules, and appropriately packaged for further processing. Nonwoven fabrics can also be made by extending the fibre extrusion process to include interlocking the fibrous material concurrent with its extrusion, modifying the porosity of a formed film by perforating it, or modifying the film manufacturing process to incorporate the forming of a porous films concurrent with its extrusion.
The extrusion technology base includes spunbond, meltblown, and porous film systems.
Fabrics produced by these systems are referred to individually as "spunbond", "meltblown", and "textured" or "apertured film'' nonwovens; or, generically, as "polymer laid'' nonwovens. These fabrics are produced with machinery associated with polymer extrusion (i.e. manufactured fibre spinning, film casting, extrusion coating). In polymer laid systems fibre structures are simultaneously formed and manipulated.
A fabric made by any of these variations of the extrusion process is designated a polymer laid nonwoven.
Extrusion-technology based (or polymer laid) nonwoven manufacturing systems
transform polymer solutions, melts, or sheets into fabric in one continuous operation. Three generic fabric types are produced by extrusion methods: spunbonds, meltblowns, and textured or apertured films.
Spunbond nonwovens are composed of continuous filaments which have been extruded
(spun) in web form onto a collection belt and subsequently bonded by mechanical, chemical, or thermal means. Four simultaneous operations are involved: (1) filament extrusion, (2) filament drawing, (3) filament layering (web formation), and (4) bonding. Most spunbond processes utilize melt extruders. Upon extrusion, individual filaments are separated and/or oriented aerodynamically, electrostatically, or mechanically and patterned into a web on an apron or collection conveyor. The web is then consolidated by one or a combination of the following methods: (a) mechanical entanglement using needle looms; (b) adhesive bonding using latexes; (c) inherent bonding using acids, solvents, or gases to etch the filament surfaces and calender rolls to interlock the structure; (d) thermal bonding using heated calender rolls. Following consolidation, further mechanical and/or chemical fabric finishing operations can be incorporated. An inherent characteristic of spunbond fabrics is a high ratio of strength to unit weight. This property is due mainly to continuous filament networks being interlocked throughout the fabric structure.
Meltblown nonwoven manufacturing processes are similar to spunbonds in that melt
extrusion is used. Upon passage through the extrusion orifice, the molten polymer (melt) is accelerated (blown) by high-velocity, high-temperature jets of air. The air streams attenuate the molten polymer which solidifies into microdenier fibres. The fibres are then separated from the air and collected in the form of a loosely entangled web. Subsequently, the web is compressed and bonded by embossed or smooth calender rolls. An inherent characteristic of meltblown nonwovens is a high ratio of surface area to unit weight. This property is due to the combined effect of fine fibre diameters, random entanglement, and close fibre packing which brings about a fabric structure with a large number of fibres and a large number of small pores.
Porous film nonwoven systems employ both slit and annular die extrusion technology. A
number of methods are used. One method is to extrude molten polymer sheets onto
engraved drums and subsequently stretch the then semi-solid film along its length and across its width. As the structure is drawn, fibrillization occurs in the patterned areas and a net-like fabric results. Uniform-thickness, partially-oriented films can be heated on an apertured screen, vacuumed to create surface texture, set on chill rolls, and abraded to establish controlled porosity. Alternatively, molten polymer can be cast onto an apertured vacuum drum. An inherent characteristic of porous film nonwovens is uniformity of weight and porosity throughout the fabric structure. |
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Finishing
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Nonwoven web forming and bonding processes produce fabric in continuous lengths
at widths greater than most product applications require. In tandem with these primary
processes or “off-line” as separate “finishing” treatments, the fabric may be subjected to other operations to bring about or improve inherent properties. For logistics reasons, most nonwoven fabrics are handled in roll form. Roll dimensions are specified to accommodate end-use application or subsequent conversion processes.. Roll width is determined at the slitting operation, and roll length is determined at the winding operation. Slitting and winding are packaging processes common to all nonwoven manufacturing methods
Surface treatments adapted or borrowed directly from traditional textile, paper, or plastic
finishing technologies are used to enhance fabric performance or aesthetic properties.
Performance properties include functional characteristics such as moisture transport,
absorbency, or repellency; flame retardancy; electrical conductivity or static propensity;
abrasion resistance; and frictional behavior. Aesthetic properties include coloration, surface texture, and fragrance.
Generically, fabric finishing processes can be categorized as being either chemical,
mechanical, or thermo-mechanical. Chemical finishing involves the application of dyestuffs, pigments, or chemical coatings to fibres as well as the impregnation of fabrics with chemical additives or fillers. Mechanical finishing processes alter fabric surface texture by physically repositioning and/or trimming fibres on or near the fabric surface. Thermo-mechanical finishing involves altering fabric dimensions or physical properties through the use of heat and/or pressure.
Finishing may also be viewed as an efficient means for providing nonwovens with additional application-dependent chemical and/or physical properties. Finishing processes bring about value-added fabrics with technically sophisticated properties for specific end-use applications. |
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Nonwoven Converting and Product Fabrication Processes
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Nonwovens can be transformed into end-use products much more quickly and with a
greater degree of flexibility than other sheet materials. This characteristic is due to the facts that nonwovens can be cut in virtually all directions without fraying or curling and can be joined by methods other than traditional sewing, namely, by adhesives or thermal bonding. The processes used to transform nonwoven roll goods into forms or shapes for use as a nonwoven product or for further processing as a sub-component of a nonwoven-based product has been generically termed converting. Converted nonwoven configurations include premeasured rolls with perforations for dispensing fabric in specific sheet dimensions, continuous rolls for wrapping, individual or interfolded sheets, and various diecut or molded geometries. Converting processes also include arrangements of machine elements to achieve continuous fabrication or assembly of multi-component products such as diapers and garments.
The development of converting technologies has provided products which have paralleled or led major changes in the American lifestyle. Convenience items such as premoistened wipes, laundry softener sheets, household scrubbing devices, skin cleansers, and absorbent devices for medical and personal hygiene are disposable product examples.
These and durable and multi-use products such as auto interior trim, hood and trunkliners, roadbeds and erosion control members, furniture and bedding components, and highperformance filters that prolong life by keeping our air, water, food, bodies, and homes free from contaminates are only a few among literally hundreds of nonwoven products that were virtually nonexistent just a generation ago. |
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Manufacturing Systems Summary
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| Nonwoven manufacturing methods derived from primary textile, paper, and extrusion technologies are collectively termed Basic Nonwoven Fabric Manufacturing Systems. These systems are or can be made to be continuous processes. Common to each of these systems are four principal elements or phases of manufacturing: fibre selection and preparation, web formation, web consolidation (bonding), and finishing. Fibre selection and preparation involves choosing the fibre or fibre material for a specific application and treating it so that the process will yield a fabric with properties sufficient to perform its intended function. Web formation is the process by which individual fibres or fibrous materials are arranged in order to bring about the physical properties desired in the fabric structure. Web consolidation or bonding is the process by which the fibres or fibrous materials are interlocked in order to provide the integrity or strength desired in the fabric structure. Finishing includes slitting the fabric to the width desired, winding the fabric in roll form or cutting it to the length desired, and treating the fabric surface chemically or mechanically to bring about enhanced functional or aesthetic properties. An outline of the three basic nonwoven manufacturing systems according, to parent technology, and the principal elements or manufacturing phases common to each is given in Table I. |
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Nonwoven Hybrids
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The hybrid technology base includes (1) fabric/sheet combining systems, (2)
combination systems, and (3) composite systems. Combining systems employ lamination technology or at least one basic nonwoven web formation or consolidation technology to join two or more fabric substrates. Combination systems utilize at least one basic nonwoven web formation element to enhance at least one fabric substrate. Composite systems integrate two or more basic nonwoven web formation technologies to produce web structures.
From the manufacturing flow matrix given in Table I, the routes for producing most
nonwovens can be traced and additional fundamental points regarding the general nature of nonwovens can be inferred, namely: (1) nonwovens are fibre-material dependent; (2) individual fibres or fibrous materials are arranged in two or three dimensional networks; (3) fibre networks are interlocked to yield flat, flexible, porous sheet structures in the form of rolls; and (4) rolls can be provided engineered properties and are prepared for conversion to end-use items.
Each of the basic technology systems has its specific advantages and weaknesses.
Nonwoven hybrids are a means to minimize these weaknesses and provide product
synergism. Hybrid manufacturing systems provide means of incorporating the advantages of two or more nonwoven manufacturing systems to produce specialized nonwoven structures with properties unattainable by any single nonwoven process.
These systems also allow for the selective addition of non-fibrous materials, such as powders or granules, into the fibre matrix. Fabric materials from the three basic nonwoven hybrid groupings can be distinguished from each other by attempting to separate their components. A fabric made by fabric/sheet combining can be delaminated into the distinct substrates which were joined together. A fabric made as a combination hybrid may delaminate, but the various components will not readily separate from each other. The components of a fabric made by integrated web formation technologies cannot be readily separated into its various constituents.
Nonwoven hybrids can be made by hydroentangling a tissue with carded rayon webs to
produce wipes with large and small capillaries, or by thermoembossing a film to a highloft to yield a moisture-impermeable insulator, or by air-forming pulps and textile fibres to yield enhanced filter media. Hybrids can also be made by laminating porous films and needlepunched nonwovens to produce a fluid transport/entrapment structure. One of the more popular nonwoven hybrids is made by laminating spunbonds and meltblowns to make enhanced strength/surface area fabrics. Successful coformed nonwovens include meltblown/pulp fabrics for water and oil absorption, meltblown/polyester staple fabrics for apparel insulation, and meltblown/staple fibre/charcoal granule fabrics for respiratory masks. A recent innovation involves hydrorntangling splittable spunlaid/drylaid web combinations for use as microdenier apparel applications. |
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