Durable Nonwovens
Technical textiles applications — have they come full circle?
Dr. Behnam Pourdeyhimi, Technical Editor
Historically, the nonwovens industry has presented a broad array of highly functionalized, yet
disposable, single-use engineered products driven by high speed, large volume and low cost. The
market segments most impacted by nonwovens — ranging from hygiene, medical, filtration, wipes and
consumer products to geo-nonwovens — require desired performance at reasonable costs, in contrast
to the more traditional textiles focused primarily on apparel. Interestingly, there has been little
or no overlap with technical textiles products because nonwovens are driven by large volumes.
Future expansions beyond the historical market segments will be quite different: We are now coming full-circle and focusing on smaller-volume, high-value products, which means that technical textiles and nonwovens will emerge as functional structures driven by performance. This is the battleground for durable products made from nonwovens.
Figure 1: This full-sized tent, delivered recently to Tyndall Air Force Base, United States, is made using a coated nonwoven substrate in which the islands-in-the-sea fibers have been fibrillated.
The term "durable" is in, but its meaning is not always clear in this context; nonwovens can be long-lasting or have a short life cycle. Most nonwovens currently are engineered to be single-use products, and function adequately in the applications for which they're designed. Automotive nonwovens, geosynthetic nonwovens and the like are intended to last for a long time, and are often called durable, but it is preferable to refer to these as long-life nonwovens.
There is also the multi-use nonwovens classification. For example, many commercial wipes used in Europe today can be used to wipe a surface and be washed/rinsed/cleaned and reused multiple times. From the perspective of functional clothing, the materials need to withstand multiple launderings without loss of functionality or appearance. One must make a distinction here: Long-life nonwovens are not necessarily launderable, although they can function for a very long time. Durable, launderable nonwovens are a different class altogether. There are not too many such products on the market — yet. But, functional nonwovens products in technical clothing applications are going to emerge a lot sooner than many imagine. The technology of choice depends on the assets in place, applications, functions required and other parameters.
Figure 2: The shape of Coolmax® fiber affects the way it packs in a yarn, creating more capillarity compared with a round fiber, and enhancing the yarn's moisture-transport function. Schematic courtesy of Invista
Durable Nonwoven Fabric Formation
Historically, there have been two major efforts in forming durable nonwoven fabrics. United States-based Polymer Group Inc.'s (PGI's) Miratec® fabrics were carded staple-fiber-based products that were hydroentangled using PGI's unique Apex™ technology that would create textures and structures equivalent to those of any textiles. Most were formed with blends of fibers, and their performance could be equal to or better than that of their woven counterparts. Most of these fabrics had additional binders to ensure that the fabrics would not unentangle during laundering. Consequently, they did not have the hand, feel or drape required; and their uses remained limited. Some groups are still pursuing staple-fiber-based products. For example, the U.S. Army Natick Soldier Systems Center, United States, has worked with a number of groups to develop staple-fiber-based nonwovens for soldiers' uniforms.
The other effort, initiated by Freudenberg Nonwovens, Germany, utilizes bicomponent spunbond technology coupled with hydroentangling. Spunbonding bicomponent extrusion technology involves the spinning of continuous filaments composed of two polymers deposited onto a forming belt followed by mechanical, thermal or chemical bonding. The fine-fiber spunbond process often is capable of producing only fibers larger than 10 to 15 microns. The key will be to form a structure composed of smaller fibers than usual. One key patent in this area was granted to Dr. Robert Groten and others at Freudenberg for detailing a continuous process for splitting segmented pie fibers. The fabric thus formed is now known as Evolon® and is the first commercial spunbond reusable, durable microdenier fabric. These structures are far superior to those made from staple fibers in terms of durability, strength and drape.
Figure 3: The Winged Fiber™, whose cross section is shown here, measures 15 by 10 microns. This particular fiber is made from polyester.
Figure 4: A cross section of a nonwoven fabric made with the Winged Fiber
The term "splittable" refers to bicomponent fibers that have one common interface and in which the two components are exposed to air on the surface of the fibers. Classic examples include: segmented pie; segmented ribbon; and side-by-side. Mechanical splitting requires the fiber components to have little affinity to one another; therefore, the selection of polymers and polymer ratios plays a key role in the ability and quality of the splitting.
Hydroentangling uses high-pressure water-jet curtains to mechanically move, wrap and entangle fibers. The water jets split the bicomponent segments, resulting in two different, wedge-shaped fibers. The fiber size after splitting is dependent on the diameter of the original fiber, the number of segments and the spinning parameters.
Spunbond microfibers are also formed by the removal of one of the components in a bicomponent structure using caustic and other solvents. The most common cross section used is the islands-in-the-sea (I/S), in which the sea is removed, leaving the islands behind. As the number of islands increases, the size of the resulting fibers decreases. Because this method requires removal of a component, there are often environmental concerns along with additional costs due to the removal process and waste of the sea polymer. An additional challenge is that the islands tend to stay as bundles.
Microfiber nonwovens are used in suede and leather products, durable wipes, and automotive components such as headliners, but they have made little headway in durable, launderable, technical clothing applications. This is partly due to the fact that microdenier fabrics thus far have lacked adequate drape and stretch, and are difficult to dye.
Figure 5: When islands-in-the-sea fibers are fibrillated through mechanical action such as shear or hydroentangling, the fractured and fibrillated sea elements wrap the fibers and can act as a binder when and if melted
Emerging Durable Nonwoven Fabrics
There have been numerous attempts to overcome the shortcomings of the existing microdenier and staple-fiber durable nonwovens. These efforts have resulted in new developments that will likely appear as products in the near future. Following is a glimpse into the future and what some new technologies may offer in a durable nonwoven structure.
Structures with super moisture transport: A new structure known as the Winged Fiber™ was developed by Allasso Industries Inc., United States, for use in a spunbond nonwoven. The fiber can easily be utilized in critical moisture-transport applications in which United States-based Invista's Coolmax® and other similar structures are used. The filaments are formed as a bicomponent fiber in which the winged component is wrapped by a sacrificial sheath. The shape is controlled through the spinpack design, and fibers as little as 1 denier or less are possible. The fibers are formed into the final product, and a finishing step removes the sheath, releasing the winged fibers. Therefore, the fibers do not interdigitate but, rather, stay apart, providing for higher permeability and capillarity. The fibers can reach a specific surface area of 20 square meters per gram (m2/g), compared with 0.2 m2/g for a round fiber of the same size and between 0.3 and 0.4 m2/g for Coolmax.
The nonwovens version of this structure is durable and drapable, and will be an interesting component in activewear. Whether in a knitted, woven or nonwoven fabric, the high surface area will translate to much faster drying of the fabric. Therefore, for next-to-skin applications requiring moisture management, this structure can offer unrivaled performance.
Structures made with fibers such as the Winged Fiber also can be used to form durable wipes, filters, suede and leather products.
High-strength micro and nanofiber structures: It was recently discovered that through mechanical action including shear and hydroentangling, I/S fibers can be fibrillated. If the sea component is fractured and fibrillated, it remains in the structure, making the process more economical and environmentally friendly, as the fractured sea elements wrap the fibers and can act as a binder when and if melted.
This fibrillation allows the formation of structures composed of sub-micron fibers that are superior in terms of tear and tensile and abrasion properties, and offer properties that are not easily achievable. As a coated substrate, the structures can be formed into shelters, tents, awnings and other structures.
Durable structures with super drape and hand: A modification in the typical tipped trilobal bicomponent fiber can lead to a flexible, drapable, durable fabric. Here, the cross section is modified in one of two ways: The structure accommodates a core within the trilobal configuration; or the tips become the majority component, allowing for easy splitting. With the addition of a core, once split, there will be three tip fibers and one sheath core. The sheath is normally a different polymer and melts at lower temperatures, allowing it to be used as a binder fiber to further strengthen the structure.
The core-modified structure offers interesting possibilities in that the polymer B component can be polymers that are not easily spinnable. Recently, the combination of elastomers with other polymers such as nylon and polyester has led to the introduction of durable nonwovens with stretch and recovery. The fibers in these structures appear to be self-crimping, which consequently results in much better hand.
These configurations result in a microfiber nonwoven with superior mechanical properties over other cross sections with similar fiber diameters; however, it is limited in possible polymer-to-polymer and polymer-to-additive ratios.
Figure 6 (top): Tipped trilobal fibers are easily split by hydroentangling. Examples include (left to right): a tipped trilobal in which both core and tips are exposed on the surface; and a modified tipped trilobal in which the core is wrapped by the tips.
Figure 7(bottom): The core-modified structure offers interesting possibilities, as shown here.
Conclusions
New durable nonwovens developments outlined above can emerge as the next generation of technical textiles in many critical applications. These structures are strong and possess significantly higher surface area than existing fabrics. Some of the developments may equally impact wovens and knits as well in that the fibers developed for these nonwovens can readily be spun into filaments and staples and can form the basis for the next generation of technical clothing fabrics.
The emerging nonwovens, however, will be different from nonwovens in use today. The future promises to be interesting and potentially very rewarding.
Dr. Behnam Pourdeyhimi is Associate Dean for Industry Research and Extension, and William A. Klopman Distinguished Professor of Textile Materials at North Carolina State University's (NCSU's) College of Textiles, Raleigh, N.C. (United States); and Executive Director of The Nonwovens Institute at NCSU. A bibliography supporting the topics discussed is included with a longer online version of this article posted on Textile World Asia's sister website located at www.TextileWorld.com.
July/August/September 2011
Future expansions beyond the historical market segments will be quite different: We are now coming full-circle and focusing on smaller-volume, high-value products, which means that technical textiles and nonwovens will emerge as functional structures driven by performance. This is the battleground for durable products made from nonwovens.
Figure 1: This full-sized tent, delivered recently to Tyndall Air Force Base, United States, is made using a coated nonwoven substrate in which the islands-in-the-sea fibers have been fibrillated.
The term "durable" is in, but its meaning is not always clear in this context; nonwovens can be long-lasting or have a short life cycle. Most nonwovens currently are engineered to be single-use products, and function adequately in the applications for which they're designed. Automotive nonwovens, geosynthetic nonwovens and the like are intended to last for a long time, and are often called durable, but it is preferable to refer to these as long-life nonwovens.
There is also the multi-use nonwovens classification. For example, many commercial wipes used in Europe today can be used to wipe a surface and be washed/rinsed/cleaned and reused multiple times. From the perspective of functional clothing, the materials need to withstand multiple launderings without loss of functionality or appearance. One must make a distinction here: Long-life nonwovens are not necessarily launderable, although they can function for a very long time. Durable, launderable nonwovens are a different class altogether. There are not too many such products on the market — yet. But, functional nonwovens products in technical clothing applications are going to emerge a lot sooner than many imagine. The technology of choice depends on the assets in place, applications, functions required and other parameters.
Figure 2: The shape of Coolmax® fiber affects the way it packs in a yarn, creating more capillarity compared with a round fiber, and enhancing the yarn's moisture-transport function. Schematic courtesy of Invista
Durable Nonwoven Fabric Formation
Historically, there have been two major efforts in forming durable nonwoven fabrics. United States-based Polymer Group Inc.'s (PGI's) Miratec® fabrics were carded staple-fiber-based products that were hydroentangled using PGI's unique Apex™ technology that would create textures and structures equivalent to those of any textiles. Most were formed with blends of fibers, and their performance could be equal to or better than that of their woven counterparts. Most of these fabrics had additional binders to ensure that the fabrics would not unentangle during laundering. Consequently, they did not have the hand, feel or drape required; and their uses remained limited. Some groups are still pursuing staple-fiber-based products. For example, the U.S. Army Natick Soldier Systems Center, United States, has worked with a number of groups to develop staple-fiber-based nonwovens for soldiers' uniforms.
The other effort, initiated by Freudenberg Nonwovens, Germany, utilizes bicomponent spunbond technology coupled with hydroentangling. Spunbonding bicomponent extrusion technology involves the spinning of continuous filaments composed of two polymers deposited onto a forming belt followed by mechanical, thermal or chemical bonding. The fine-fiber spunbond process often is capable of producing only fibers larger than 10 to 15 microns. The key will be to form a structure composed of smaller fibers than usual. One key patent in this area was granted to Dr. Robert Groten and others at Freudenberg for detailing a continuous process for splitting segmented pie fibers. The fabric thus formed is now known as Evolon® and is the first commercial spunbond reusable, durable microdenier fabric. These structures are far superior to those made from staple fibers in terms of durability, strength and drape.
Figure 3: The Winged Fiber™, whose cross section is shown here, measures 15 by 10 microns. This particular fiber is made from polyester.
Figure 4: A cross section of a nonwoven fabric made with the Winged Fiber
The term "splittable" refers to bicomponent fibers that have one common interface and in which the two components are exposed to air on the surface of the fibers. Classic examples include: segmented pie; segmented ribbon; and side-by-side. Mechanical splitting requires the fiber components to have little affinity to one another; therefore, the selection of polymers and polymer ratios plays a key role in the ability and quality of the splitting.
Hydroentangling uses high-pressure water-jet curtains to mechanically move, wrap and entangle fibers. The water jets split the bicomponent segments, resulting in two different, wedge-shaped fibers. The fiber size after splitting is dependent on the diameter of the original fiber, the number of segments and the spinning parameters.
Spunbond microfibers are also formed by the removal of one of the components in a bicomponent structure using caustic and other solvents. The most common cross section used is the islands-in-the-sea (I/S), in which the sea is removed, leaving the islands behind. As the number of islands increases, the size of the resulting fibers decreases. Because this method requires removal of a component, there are often environmental concerns along with additional costs due to the removal process and waste of the sea polymer. An additional challenge is that the islands tend to stay as bundles.
Microfiber nonwovens are used in suede and leather products, durable wipes, and automotive components such as headliners, but they have made little headway in durable, launderable, technical clothing applications. This is partly due to the fact that microdenier fabrics thus far have lacked adequate drape and stretch, and are difficult to dye.
Figure 5: When islands-in-the-sea fibers are fibrillated through mechanical action such as shear or hydroentangling, the fractured and fibrillated sea elements wrap the fibers and can act as a binder when and if melted
Emerging Durable Nonwoven Fabrics
There have been numerous attempts to overcome the shortcomings of the existing microdenier and staple-fiber durable nonwovens. These efforts have resulted in new developments that will likely appear as products in the near future. Following is a glimpse into the future and what some new technologies may offer in a durable nonwoven structure.
Structures with super moisture transport: A new structure known as the Winged Fiber™ was developed by Allasso Industries Inc., United States, for use in a spunbond nonwoven. The fiber can easily be utilized in critical moisture-transport applications in which United States-based Invista's Coolmax® and other similar structures are used. The filaments are formed as a bicomponent fiber in which the winged component is wrapped by a sacrificial sheath. The shape is controlled through the spinpack design, and fibers as little as 1 denier or less are possible. The fibers are formed into the final product, and a finishing step removes the sheath, releasing the winged fibers. Therefore, the fibers do not interdigitate but, rather, stay apart, providing for higher permeability and capillarity. The fibers can reach a specific surface area of 20 square meters per gram (m2/g), compared with 0.2 m2/g for a round fiber of the same size and between 0.3 and 0.4 m2/g for Coolmax.
The nonwovens version of this structure is durable and drapable, and will be an interesting component in activewear. Whether in a knitted, woven or nonwoven fabric, the high surface area will translate to much faster drying of the fabric. Therefore, for next-to-skin applications requiring moisture management, this structure can offer unrivaled performance.
Structures made with fibers such as the Winged Fiber also can be used to form durable wipes, filters, suede and leather products.
High-strength micro and nanofiber structures: It was recently discovered that through mechanical action including shear and hydroentangling, I/S fibers can be fibrillated. If the sea component is fractured and fibrillated, it remains in the structure, making the process more economical and environmentally friendly, as the fractured sea elements wrap the fibers and can act as a binder when and if melted.
This fibrillation allows the formation of structures composed of sub-micron fibers that are superior in terms of tear and tensile and abrasion properties, and offer properties that are not easily achievable. As a coated substrate, the structures can be formed into shelters, tents, awnings and other structures.
Durable structures with super drape and hand: A modification in the typical tipped trilobal bicomponent fiber can lead to a flexible, drapable, durable fabric. Here, the cross section is modified in one of two ways: The structure accommodates a core within the trilobal configuration; or the tips become the majority component, allowing for easy splitting. With the addition of a core, once split, there will be three tip fibers and one sheath core. The sheath is normally a different polymer and melts at lower temperatures, allowing it to be used as a binder fiber to further strengthen the structure.
The core-modified structure offers interesting possibilities in that the polymer B component can be polymers that are not easily spinnable. Recently, the combination of elastomers with other polymers such as nylon and polyester has led to the introduction of durable nonwovens with stretch and recovery. The fibers in these structures appear to be self-crimping, which consequently results in much better hand.
These configurations result in a microfiber nonwoven with superior mechanical properties over other cross sections with similar fiber diameters; however, it is limited in possible polymer-to-polymer and polymer-to-additive ratios.
Figure 6 (top): Tipped trilobal fibers are easily split by hydroentangling. Examples include (left to right): a tipped trilobal in which both core and tips are exposed on the surface; and a modified tipped trilobal in which the core is wrapped by the tips.
Figure 7(bottom): The core-modified structure offers interesting possibilities, as shown here.
New durable nonwovens developments outlined above can emerge as the next generation of technical textiles in many critical applications. These structures are strong and possess significantly higher surface area than existing fabrics. Some of the developments may equally impact wovens and knits as well in that the fibers developed for these nonwovens can readily be spun into filaments and staples and can form the basis for the next generation of technical clothing fabrics.
The emerging nonwovens, however, will be different from nonwovens in use today. The future promises to be interesting and potentially very rewarding.
Dr. Behnam Pourdeyhimi is Associate Dean for Industry Research and Extension, and William A. Klopman Distinguished Professor of Textile Materials at North Carolina State University's (NCSU's) College of Textiles, Raleigh, N.C. (United States); and Executive Director of The Nonwovens Institute at NCSU. A bibliography supporting the topics discussed is included with a longer online version of this article posted on Textile World Asia's sister website located at www.TextileWorld.com.
July/August/September 2011
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