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Nonwovens technology primer.

Nonwovens are unconventional textile assemblies that are slowly replacing the use of traditional woven/knitted fabrics in household and industrial applications. Nonwovens are defined by INDA, Association of the Nonwovens Fabrics Industry[1], as "sheet or web structures made by bonding and/or interlocking fibers, yarns or filaments by mechanical, thermal, chemical or solvent means." These fabrics do not require the conversion of fibers to, yarns and are manufactured by processes other than spinning, weaving or knitting, hence the name "nonwovens." Thus the basic elements of a nonwoven structure are fibers that are either natural or synthetic. Today, various types of nonwovens are being produced that are aimed at meeting specific end uses. The ingenuity in fiber research is being exploited by the nonwovens industry to design functional/aesthetic fabrics ranging from baby diapers to automotive textiles.

The basic nonwoven types can be classified according to the way the webs are produced, as shown in Table 1. During the first stage, individual fibers are formed or acquired. A thin web of these fibers is produced in the second stage, which is very weak. A coherent bonded web is produced in the final stage by introducing fiber-to-fiber contacts with appropriate bonding. The differences between woven, paper and nonwoven structures are shown in Table 2[2]. The difference in the properties[3] (Table 3) between a conventional textile fabric and a nonwoven web is largely dependent on the way they are engineered. The properties of a nonwoven web are dependent on:

- fiber type, diameter and fineness

- fiber strength and elongation

- fiber-to-fiber frictional properties

- fiber geometry and orientation

- chemical properties (functional groups of fibers)

- thermal properties (heat conductivity, melting point, glass transition)

- web formation methods

- types of bonding

- number of bonds and the distance between bond areas

- number of pores, pore sizes and distribution of pores.

[TABULAR DATA OMITTED]

 Table 2
 DIFFERENCES BETWEEN WOVEN, PAPER,
 AND NONWOVEN STRUCTURES

Woven and Knitted Fabrics - based on yarns interlocking
in patterns
Nonwoven Fabrics - based on individual fibers in webs
Papers - based on webs of short cellulose fibers so
extensively bonded that most fibers lose their individual
identity

Source. "A simplified classification of nonwoven fabrics,"
Arthur Drelich, Nonwovens Workshop, Aug. 8-12,
1988, The University of Tennessee, Knoxville.

 Table 3
 COMPARISON OF PROPERTIES OF
 WOVEN AND NONWOVEN FABRICS

Properties Woven Nonwoven
Fiber arrangement Orthogonal Random
Properties Directional Nondirectional
Breaking Strength Higher Lower
Breaking Elongation Lower Higher
Initial Modulus Higher Lower
Tear Resistance Lower Higher
Openings Can be regular Irregular
Filtration Single layer Often multilayer
Porosity 35% to 45% 55% to 93%
Inplane flow Low Can be high
Edge May ravel Does not ravel

Source: Raumann, G., "Geotextiles: Construction materials
in evolution," p.10-15, proceedings of Second International
Conference on Geotextiles, Aug. 1-6, 1982, Las Vegas, NV.


Fibers For Nonwovens

The types of fibers used in the nonwovens industry are shown in Table 4. All natural[4] and synthetic fibers that are commercially available can be used to produce nonwoven webs. In practice, wood pulp, which is far shorter in length than conventional textile fibers, is the only natural fiber used in large amounts because of its high water absorbency, bulk and low cost. However, cotton has excellent inherent properties for nonwovens. Wool and linen are too expensive for nonwoven fabrics. Viscose rayon has been widely used ill the nonwovens industry in the area of disposables and sanitary products. Rayon fibers can be easily made into nonwoven webs and readily bonded.

 Table 4
 FIBERS USED IN NONWOVEN FABRICS

 Major Minor
Polyester Nylon
Polypropylene Cotton
Rayon Polyvinyl Alcohol
Wood Pulp Cellulose Acetate
 Polyvinyl Chloride
 Glass
 Acrylic
 Polyethylene

Source: "A simplified classification of nonwoven fabrics,"
Arthur Drelich, Nonwovens Workshop, Aug. 8-12
1988, The University of Tennessee, Knoxville.


Among the synthetic fibers, polypropylene (PP) is widely used. Polypropylene is cheap and has very good rheological characteristics to form fine fibers. Polypropylene fibers are hydrophobic, voluminous;and thermoplastic in nature. PET (polyethylene terephthalate) is used where strength and mechanical properties are of prime importance. Nylon fibers are used for their excellent recovery (resiliency) properties. Cellulose acetate and polyvinyl alcohol fibers are used in nonwovens also. Bicomponent fibers with different fibers in the core and sheath are widely used in thermally bonded nonwovens.

Web Forming Methods

There are four basic web forming methods as shown in Table 1. The selection of a particular method is dependent on the expected end use of the web, availability and cost of fibers. The most widely used web forming method are wet laid, dry laid and spunbonded. Webs produced should be of superior quality, clean and without any defects (thick and thin places) irrespective of the methods chosen.

Wet Laid. Wet laid nonwovens[5] are produced by a modified paper making process that employs very short fibers (6-12 mm), typically wood pulp and short synthetic or regenerated fibers. There are three main stages in the manufacture of nonwoven bonded fabrics by the wet laid method:

(a) swelling and dispersion of fibers in water; transport of the suspension on a continuous traveling screen

(b) continuous web formation on the screen by filtration

(c) drying and bonding of the web.

Longer fibers[5] may not disperse completely and tend to entangle (flocculate) and form nonuniform sheets. In fact. papermaking machinery has been modified to deal with the problems of removing large amounts of water quickly - without rupturing the sheet as it forms - and controlling fiber orientation in the final product. Wet laid nonwovens are different from paper in the sense that there is no strong hydrogen bond formation as in paper products. High density hydrogen bonding also results in stiff structures, with little or no wet strength, which is not desirable in a textile material. Water disposal (through pressure, vacuum, heat) in the drying stage is the most important step in wet laid process.

Dry Laid. Dry laid webs[6] are produced from staple fibers that range from 1.2 to 20 cm long. The fibers from the bales are thoroughly opened and cleaned using conventional openers and beaters. The dry laid webs can be categorized into mechanically (oriented webs) and aerodynamically (random webs) formed webs.

Mechanically, formed webs are usually produced by carding machines that utilize opposed moving beds of closely spaced needles to pull and tease the clumps apart. When the clumps are caught between two moving surfaces whose surface velocities are quite different from each other, the result is a disentanglement of lumps and individualization of fibers. Modern cards with sophisticated suction systems for dust extraction that can be used to produce clean webs with little nep formation are currently available on the market. Carded webs are predominantly oriented in the machine direction and the strength of the web is typically higher in the machine direction (5:1) than in the cross direction.

To overcome this problem of imbalance in the properties, the cross-laying process can be employed. In this method, an oriented web is laid down at or near alternating 45 [degrees] onto another oriented web moving on a transporting belt. A composite web[7] is the one where webs are laid both longitudinally and cross-wise.

Air laying techniques are used to produce randomly oriented webs. These machines utilize a carded wet or a lap. The fibers are individualized by needles and then introduced into an air stream. Total randomization would avoid any preferential orientation when the fibers are collected on the screen. The length of fibers is normally between 1.9 to 6.4 cm. Production of air laid webs is slower and more expensive than that by carding.

Spunbonded. This falls into the category of direct conversion of polymers into nonwoven webs. A typical spunbonding process would consist of the following four steps[8]: extrusion of filaments, drawing, lay down and bonding. The first three operations are not different from conventional melt spinning process and the bonding step results in web consolidation. The filaments formed by this method are virtually endless.

A typical spunbonded line[8] consists of the following elements: an extruder, a metering pump, a die assembly, a filament spinning unit, a drawing and deposition system, a belt for collecting the filaments, a bonding zone and a winding unit. The drawing and deposition of spun filaments in spunbonding is achieved using specially designed aerodynamic devices. The process variables[8] in the process can be classified into operational and material variables. The operational variables can further be classified into on-line variables. such as polymer throughput, polymer/die temperature. quench air rate and temperature, takeup speed or bonding conditions and off-line variables such as spinneret hole size or spinneret-to-collector distance. The material variables include polymer type, molecular weight and molecular weight distribution. The following are some of the important characteristics[8] of a spunbonded web:

- random orientation of fibers (isotropic)

- basis weight ranges from 5-800 gsm

- fiber diameter from 1-50 [mu]m, preferably 15-35 [mu]m

- high specific strength (strength/weight) compared to other nonwovens.

Melt blown. Microfine fibers from bulk polymers can be produced by a Exxon patented one-step process known as a melt blowing process. The molten polymer[4] is forced through very fine holes in a special die into a high velocity air stream where the polymer is formed into very fine, although irregular, filaments of indeterminate lengths. Virtually all thermoplastic polymers can be used to produce melt blown webs. However, polypropylene and polyethylene terephthalate are the most widely used.

Polymer throughput rate[8], air throughput rate, polymer/die temperature, air temperature and die-to-collector distance are important operational variables. The material variables include polymer type, molecular weight, molecular weight distribution and degradation. Some important characteristics[8] of melt blown webs are:

- random fiber orientation

- high cover factor

- fiber diameter of 2-7 [mu]m

- basis weight of 20-200 gsm

- high surface area suitable for insulator and filter characteristics.

Bonding Methods

The bonding of fibers gives strength to the web and influences other web properties. There are three major bonding methods that are widely employed to produce webs with sufficient integrity. They are chemical (adhesive based), thermal (using thermoplastic fibers) and mechanical (needling to cause entanglement). Other techniques include spunlacing (using high velocity water jets), ultrasonic bonding (high frequency), stitchbonding and powder bonding.

Chemical bonding. In the chemical/adhesive bonding[8] method, a film forming polymer latex is deposited in and around the fibrous structure and then cured (heated) to activate bonding. The bonding agent is usually sprayed or saturated in the web. Until recently, the most widely used bonding agents have been synthetic latices such as polyacrylates, polyacetates, polychlorides, polyacrylonitriles and copolymers. The advantages are the ease of handling and low cost. Latices are effective for hydrophilic fibers, but not useful for synthetic fibers like polypropylene or polyethylene terepthalate. To be a good binder[9] the polymeric material should have high strength, good adhesion to fibers, good flexibility, good elastic recovery, very good resistance to washing, dry cleaning, and aging, low weight and color retention.

Thermal Bonding. Thermal bonding[10] of nonwovens involves the use of heat energy to activate an adhesive process for the purpose of interlocking fibers and consequently consolidating or providing dimensional stability to fiber webs. The adhesive component is subjected to heat. As the adhesive approaches its melting point, its surface softens and the area of contact with more stable fibers expands to form potential bonding sites. Upon melting, the liquefied adhesive is attached to a network fiber and flows along it to a crossing of two or more fibers. Once the nonwoven material is cooled, the adhesive solidifies and forms a bond or thermal fusion at each fiber contact.

In thermal bonding, fiber surfaces are fused to each other by melting the fiber surfaces or by melting fusible additives in the form of a powder, granules or fibers. Calendering is a method of bonding in which the web is pulled between heated rollers so that the web is exposed to heat and pressure necessary to bond the constituent fibers and layers together. The calender rolls[11] may be smooth, embossed or a combination of both. Two smooth rolls lead to the bonding over the entire surface (called "area bonding") of the web, making the end product stiff and less drapable. The use of one embossed roll with a raised pattern (at the top) and one smooth roll (at the bottom) leads to only a portion of the web being exposed to the heat and pressure of rolls to be bonded. This is called as "point bonding." The resultant product has good drape, hand and strength.

Polypropylene, polyester, rayon and wood pulp are the fibers used to make webs by thermal bonding. The most commonly used fiber is polypropylene. Even though polyester fiber is used, its high melting temperature is seen as a limiting factor[2]. The thermal bonding process has been widely accepted in the nonwovens industry because of its simplicity and its many advantages over traditional chemical bonding methods. Low raw material and energy costs, product versatility, lesser space requirements, cleanliness of the process, better product quality characteristics and increased production rates are the features that have been the reasons for its widely gained acceptance. Since the binder powders or fibers are mixed into the web, thermal bonding processes are rapidly, adaptable to the manufacturing and designing of composite structures.

Needlepunching. Needlepunched[6] nonwovens are created by mechanically interlocking the fibers of a crosslapped carded Web or a spunbonded web. This mechanical interlocking is achieved with thousands of barbed felting needles repeatedly passing into and out of the web, creating fiber transfer. The compression of the web by needles causes increased interfiber friction.

Today[12] there are many different fibers used in making needled fabrics, excluding the high performance fibers like ceramic, glass, steel and carbon. The density of the needle-punched fabric can be controlled by fiber selection, web formation and needling density. Surface character can be controlled by the selection of needle loom and needle design. The diameter of the needles (900 commercially available) ranges from 0.015 to 0.9 inch. The needles generally vary in length, cross section, barb design and configuration. Some of the important physical properties[13] of needled nonwovens are:

- elongation in the X, Y and Z directions (useful for molding applications)

- ability to attach layers of different type fiber webs (composites)

- ability to achieve extremely high densities

- high strength (geotextiles)

- superior filtration.

Typical web weight ranges from a minimum of 200 gsm to a maximum of 1200 gsm.

Spunlacing. Spunlacing is another of the mechanical bonding methods. It is a process[2,14] of entangling a web of loose fibers on a porous belt or moving (perforated or patterned) screen to form a sheet structure by subjecting the fibers to multiple rows of fine high pressure jets of water. The impinging of the water jets on the web causes the entanglement of fibers. The end product takes on an aesthetically appealing pattern depending on the design of the moving screen on which it is supported and carried. Water is first passed through a perforated cylinder in order to convert it into a pattern of jets. Web strength is achieved by the uniform transfer of energy from water jets. Pressures as high as 2200 psi (150 bars) are used to direct the water jets onto the web.

Hydroentanglement can be done on dry laid (carded or air laid), wet laid or composite webs. The web to be used is dependent on the end use properties desired it, the final product. Cellulosic, polyester, polyamide and acrylic fibers can be spunlaced. Glass and carbon fibers made into wet formed webs are spunlaced and targeted for industrial filtration, composites and high performance engineering requirement,[15]. "Kevlar" and "Nomex" fibers are being used by DuPont to produce spunlaced fabrics that can be used in specialty applications such as automotive, aircraft and filtration[15].

Fibers having lower bending modulus are easily entangled[16]. Flat fibers are seen to more easily entangle than round fibers, although round fibers are easier to work with. On a relative scale, round fibers are easier to work with than trilobal fibers. Thus it is seen that cotton and rayon are easier to hydroentangle than polyester fibers, which are easier to process than aramid fibers. Spunlaced fabrics have good dimensional stability, which also[17] accounts for the drape, softness and strength/weight properties of the fabric as well as the pilling and abrasion behavior.

Hydroentanglement is considered to be a highly versatile process[18] because it can be used to produce nonwovens with significant differences in end use properties. These differences are achieved as a result of a wide range of fibers that are available and also because of the wide range of possible adjustments of the process parameters. The versatility of the hydroentanglement process is seen as an advantage in that this process can be used to combine conventionally formed webs with melt blowns, spunbonds, paper, textiles and scrims in order to get a combination of properties that cannot be obtained by the use of a single web. Spunlaced nonwovens have wide acceptance in medical applications, apparel, wipes and in home furnishings.

Ultrasonic bonding. Ultrasonic bonding is one of the thermal bonding methods. It is the application of ultrahigh frequency vibrations to produce heat by mechanical impact to small areas to cause localized fusion and bonding of fibers. Ultrasonic energy is simply mechanical vibratory energy operating at frequencies greater than 18,000 Hz[19]. A frequency of 20,000 Hz is used to bond nonwoven fabrics. Pressure and vibration, which cause intermolecular stress, are applied to the area to be bonded. The thermal energy released causes softening to occur at points of limited contact where stress is at a maximum. Exposure time is controlled to ensure that adequate energy is delivered into the material to achieve bonding.

The advantages of this process are that it does not require binders or needles, energy is used at precise locations and the area of bond and heat energy is not conducted through the fiber to be bonded but is generated within the fibers themselves, thus minimizing degradation of materials through excessive heat. The "Pinsonic" process (the oldest application of the ultrasonic technique of bonding) is used for laminating webs of nonwoven fabrics, fiberfill and woven shell fabric in the formation of mattress pads and bedspreads.

Stitchbonding. Stitchbonding is one of the oldest techniques of mechanical bonding (which enmeshes or entangles fibers to give strength to the webs). In this technique, a filament is used to sew the web in a pattern. It is used in applications like quilting and not very commonly used ill nonwovens[1].

Worldwide Nonwovens Volume. The estimate worldwide volume of nonwovens in 1991 was 3.5 billion pounds. North America accounts for just below 50% of this output, 29% is the share of western Europe and the rest of the world accounts for the remaining 20%. The estimated value of the global nonwoven roll goods consumption is in the range of $6-10 billion and the value of the converted disposable nonwoven products is estimated to be about $25-30 billion[20]. When the consumption of nonwovens is broken down based on the process used to make it, it is seen that the largest nonwoven type is the staple-fiber based (carded resin and thermal bonded) fabrics accounting for nearly two thirds of the U.S. total of 1.6 billion pounds in 1990, spunbonded processes at about one third of total consumption and the remainder by processes such as melt blown, air laid, spunlaced and wet laid[20].

The versatility of raw materials, processes and equipment used to manufacture nonwovens has been one of the principal reasons for the entry of nonwovens into hitherto unfamiliar territories. This versatility has led to products that have the properties necessary to be used in a variety of end use markets.

References

[1.] Guide to Nonwoven Fabrics, INDA, Association of the Nonwoven Fabrics Industry, New York, 1976. [2.] Drelich, Arthur. "A simplified classification of nonwoven fabrics." Sixth Annual Nonwovens Workshop. The University of Tennessee, Knoxville, August 8-12, 1988. [3.] Raumann, G. "Geotextiles: Construction materials in evolution." Proceedings of the Second International Conference on Geotextiles, August 1-6, 1982, Las Vegas. [4.] Drelich, Arthur. "Nonwoven Fabrics." Reprint from the Encyclopedia of Polymer Science and Engineering, Vol. 10, 1987, pp. 204-226. [5.] Williamson, E. James, "Wet laid nonwovens: A survey of the fundamentals of making specialty fabrics on papermaking machinery," Nonwovens Workshop, The University of Tennessee, Knoxville, August 8-12, 1988. [6.] Lunenschloss, J. Nonwoven Bonded Fabrics, 1st edition, published by Ellis Horwood Limited. [7.] Gordon, B. Harvey & Wood, Dennis E. "Web formation for nonwoven manufacture," paper presented in Nonwoven '71, The Textile Trade Press, Manchester, England. [8.] Malkan, Sanjiv. R. and Wadsworth, Larry C. "Polymer Laid Systems," Nonwovens Fundamentals. TAPPI Press. [9.] Meazey, A. E. Binders used in bonded fiber fabric production, Nonwovens '71. [10.] Rattner, David. Thermal Bonding Principles and Applications. Nonwoven Fabrics Forum, 1987. [11.] Christopher, David B. Calender vs. Through-hot-air bonding of bicomponent fibers, 1990. [12.] Thomas M. Holliday, "Needlepunched Nonwovens," Nonwovens Industry, March 1984, Vol. 15, No. 3, p. 54. [13.] John Foster, "Needlepunching, retaining a sharp image in nonwovens." Nonwovens Industry, Oct. 1990, Vol. 21, No. 10. p. 20. [14.] White, Colin F. Hydroentanglement technology applied to wet formed and other precursor webs, TAPPI Nonwovens Conference, 1990, pp. 177-187. [15.] White, Colin F. A review of hydroentanglement technology - development of future products and markets, Eighth Annual Nonwovens Conference, The University of Tennessee, Knoxville, 1990. [16.] Wilharm, Madelyn. Spunlaced Nonwovens, Nonwovens Industry, December 1991, Vol. 22, No. 12, pp. 48-49. [17.] Vaughn, Ed. Spunlaced fabrics, Canadian Textile Journal, October 1978, pp. 31-36. [18.] Information brochure for Hydroentanglement technology from Valmet Paper Machinery, Honeycomb Systems Inc. [19.] Flood, Gary. "Bonding Technologies: Ultrasonic Bonding." Nonwovens Industry, October 1992, Vol. 23, No. 10, pp. 36-44. [20.] Starr, John R. Prospects for key nonwoven processes in the U.S. Nonwoven Industry, June 1992, Vol. 23, No. 6, pp. 37-41.
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Author:Narayanan, Vasanthakumar; Gosavi, Nataraj; Duckett, Kermit
Publication:Nonwovens Industry
Date:Mar 1, 1994
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