Nonwoven fabrics are flexible, breathable materials made up of one or more layers of fibers. These fibers can either be aligned in a particular direction or randomly arranged. They are bonded together through chemical, thermal, or mechanical processes to form textile products. Nonwovens are typically flat structures. This emerging segment of the textile industry saw rapid growth after World War II, thanks to high production efficiency and cost-effectiveness.
The origins of modern nonwoven fabrics date back to the early 1930s, when a few textile companies began exploring the use of bonded materials to repurpose cotton waste. The first large-scale production of what we now know as nonwovens started in 1942 in the United States, focusing on directly producing fabric from fibers. The demand for nonwoven products has since skyrocketed, and the market continues to show significant growth potential.
Nonwovens can be classified as either disposable or durable. Disposable nonwovens, which are designed for single use, include products like diapers, medical dressings, household cleaning wipes, and disposable protective wear. On the other hand, durable nonwovens are found in long-lasting goods like apparel linings, car headliners, roadbed underlayments, and carpets.
Manufacturing Process:
The steps involved in producing modern nonwoven fabrics typically include fiber preparation, web formation, web bonding, drying, curing, and finishing. In the case of films, the chemical solution is prepared, then extruded or cast into film form.
Raw Materials:
Polyester is the most commonly used fiber in the United States, while olefin and nylon are chosen for their strength. Cotton and rayon are preferred for their absorbency. Other materials like acrylic, acetate, and vinyon are also utilized in smaller amounts. The choice of fibers depends on their properties and how well they will perform in the final product. New, high-quality fibers are usually preferred over recycled or repurposed fibers. Both staple and filament fibers are used, and they can be blended based on length and type. Fiber selection is driven by the product’s intended purpose, care requirements, and desired durability. Like all textile production, the cost of the fibers plays a major role in determining the final product’s price.
Web Formation:
Web formation refers to the creation of a loosely assembled sheet of fibers using various techniques, such as dry-laid, wet-laid webs, spunbonding, or film casting. Both staple and filament fibers are utilized in this process, although the web formed at this stage is typically too fragile to be used without further processing.
1. Dry-Laid Web:
The dry-laid web is formed through a process similar to that of producing a card sliver in spinning. Fibers are either separated and suspended in an air stream, then blown onto a conveyor belt, or arranged using a mechanical card to create a uniform web on a moving belt. In a random web, the fibers may lie somewhat parallel, or they can be arranged perpendicularly by layering fibers at right angles. Alternatively, the web can have a parallel fiber structure, resembling that of a combed yarn. Bonding of the fibers is achieved either by applying binders or adhesives, or by incorporating heat-sensitive fibers into the blend. These fibers soften and fuse with others, securing the fabric’s structure.
A wet-laid web is created by depositing a water-based suspension of fibers onto a screen belt or perforated drum. This method allows manufacturers to utilize very short fibers, even those under half an inch in length. Wet-laid webs are produced quickly and can be more cost-effective than other nonwoven processes, especially when using recycled fibers.
The process resembles paper manufacturing, where the fibers are suspended in water to create a uniform mixture. This mixture is then spread over a moving screen. As the water drains, the fibers are left behind, forming a wet web. The remaining water is pressed out, and the fabric is dried. Bonding can be further enhanced using rollers. A bonding agent, such as heat-sensitive fibers or adhesive, may be mixed into the suspension to help bind the web as it dries. Alternatively, the web can be sprayed with a binder after formation, which locks the fibers together when dry.
III. Direct-Laid Web:
In the direct-laid process, webs are formed directly from fibers that are spun from molten polymers. The two main methods of direct-laid web production are spunbonding and melt blowing.
a. Spunbonding:
Spunbonding is a technique used to create fabrics from man-made filament fibers that melt under heat, such as polyester. This process involves producing fabric directly from thermoplastic polymers like polyester, nylon, polypropylene, or polyethylene. The molten polymer is extruded through a spinneret, where it cools slightly in the air and is then laid onto a moving conveyor to form a continuous web. As the web cools, the fibers naturally bond together.
The fiber pattern and arrangement in spunbonded fabrics can be modified in several ways. For instance, the spinneret can rotate to deliver filaments in various patterns; a controlled air stream can be applied to entangle the filaments; the speed of the conveyor can be adjusted to collect varying amounts of filaments at different points; and electrical charges can be used to make the fibers loop and crimp.
Spunbonded fabrics are highly durable due to the continuous filament fibers, making them resistant to tearing. These fabrics are used in numerous products, including apparel interlinings, carpet backings, furniture and bedding materials, and packaging. Additionally, spunbonded fabrics are utilized in geotextiles for erosion control and road construction. Some spunbonded materials made from olefins serve as strong, long-lasting alternatives to paper in items like wall coverings, maps, tags, and charts.
Spunbonding Process: Fabrics created through spunbonding utilize olefin materials to produce a robust and exceptionally durable alternative to paper, ideal for applications like wall coverings, charts, maps, and tags. Notable spunbonded fabrics include Mirafi 140 (composed of nylon and polypropylene), Celestra polypropylene, Reemay polyester, Tyvek polyethylene, Typar polypropylene, Bondtex polyester, Cerex nylon, and Bidim polyester.
Melt Blowing: The melt-blowing technique also generates fabrics directly from fibers but differs from spunbonding. In this method, molten polymer filaments are attenuated and fragmented into shorter lengths as they exit the spinnerettes. The molten polymer is pushed through a spinnerette into a high-speed air stream, where the force of the air breaks the filaments into short fibers. These fibers accumulate on a moving belt to form a web, while cool air helps distribute them evenly onto a screen. As the fibers cool, they bond together, creating a white, opaque web of fine fibers. The fine nature of these fibers makes meltblown fabrics excellent for filtration applications. Specialty products can be engineered by layering spunbonded and meltblown fabrics or by embedding absorbent materials within the meltblown structure.
Film Fibrillation or Extrusion Process: An alternative method of fabric production begins with a plastic film rather than fibers. In this process, a melted polymer is extruded through a slotted die to create a film instead of fiber filaments. The film undergoes embossing and is subsequently stretched in two directions, resulting in a netting of fibers. The mechanical embossing weakens specific areas of the film, allowing it to open into a nonwoven net during the stretching process. The design of the netting is determined by the embossing pattern. Hercules Incorporated produces a net fabric known as Delnet.
Web Bonding: Once the web is created, it needs to be strengthened and stabilized through various bonding techniques, including:
Chemical Bonding: This involves applying an adhesive to the web and setting it, effectively “gluing” the fibers together. Latex adhesives, which are water-based, are commonly used. The web is soaked in a latex bath and dried afterward. While this method makes the fabric stiffer, it also imparts the properties of the adhesive to the surface. To mitigate this, adhesives can be selectively printed onto the web, allowing for better drapability and a more pleasant texture. Alternatively, fibers can be solution-bonded by spraying a mixture of chemicals and water, which dissolve a small portion of the fibers at the intersections, creating bonds when the fibers resolidify.
Thermal Bonding: Heat-sensitive thermoplastic fibers can be bonded using heat. By applying heat, these fibers fuse together, securing them effectively. This technique can also be employed selectively to provide durability while maintaining flexibility and softness in the final product. Nonwovens can incorporate a small percentage of binder fibers with a lower melting point, which melt during heating to provide adhesion. Two common thermal bonding methods are calendaring, where the web passes between heated cylinders, and air heating, which uses hot air to bond fibers.
Mechanical Bonding: This is the oldest method for creating nonwovens, relying on the entanglement of fibers to provide strength. The most prevalent mechanical methods are needle punching, spunlacing (or hydroentangling), and stitch bonding.
- Needle Punching: In this technique, barbed needles penetrate the web vertically, hooking and entangling fiber tufts. Needle-punched nonwovens, which resemble felt, can be produced from a variety of fibers other than wool and are characterized by their high density and bulk. The process involves two main steps: preparing the fiber web through carding, garneting, or air-laying, and then passing it through a machine with specialized needles to entangle the fibers. The web’s strength is influenced by the arrangement of the fibers; parallel placement offers strength in one direction, while random arrangements provide equal strength across all directions. This method produces fabrics used in various applications, including carpeting, wall coverings, blankets, insulation, and automotive materials.
Hydroentangling (Spun Lacing):
In the hydroentangling process, also known as spun lacing, a fibrous web is exposed to high-speed water jets that intertwine the fibers, causing them to curl and knot together. This method does not utilize binders, resulting in lightweight, soft, and drapable fabrics. A prime example of this category is the Nexus fabric from Burlington, which comes in various patterns that can be dyed or printed. Some fabrics are washable, while others require dry cleaning. The weight of these fabrics varies from 0.7 to 2.2 ounces per square yard, with thicknesses ranging from 3.5 to 25 mils. Common applications include quilt backing, mattress pad ticking, substrates for various coated fabrics, interlinings, curtains, tablecloths, and specific apparel items. Although most spun-laced fabrics are predominantly made from polyester, other fibers can also be utilized. Examples of polyester spun-laced products include Nexus, Sontara, and Polyspun. Additionally, spun-lacing is frequently employed in the production of kitchen wipes, which feature a consistent pattern of holes surrounded by tightly intertwined fibers. Many of these wipes are also bonded with adhesives to enhance their wet strength.Stitch Bonding:
The stitch bonding technique involves stitching fiber webs together to secure the fibers. Fabrics like Maliwatt consist of stitched fiber webs and find applications as linings, furnishing materials, insulation, base fabrics for tufted goods, and in industrial and geotextile applications. Malivlies fabrics are created by forming stitches from the fibers within the web itself, eliminating the need for additional yarns. Other notable methods include Kunit and Multiknit processes. Kunit fabrics are produced by introducing a fiber web into a machine that employs a special type of compound knitting needle to create stitches from the web, resulting in a fabric that may have a plush, furry, or flat appearance. These fabrics are utilized for linings in clothing and shoes, plush toys, automotive interiors, as well as for acoustical and thermal insulation, packaging materials, and as base fabrics for coatings. The Multiknit process combines two Kunit-formed fabrics into a double-sided, multilayered material, using a compound needle with a sharp point that penetrates both fabrics. Applications for Multiknit include insulation materials, garment interlinings, base materials for molded textile composites, and as foam alternatives in car and furniture upholstery.Finishing Processes:
The final stage in nonwoven fabric production involves finishing steps such as drying, curing, embossing, printing, and dyeing. Various methods like hot-air ovens, infrared lights, heated rollers, or high-frequency electrical equipment are employed to dry the fabrics, which also eliminates any residual solvents. Many finishing techniques mirror those used for traditional woven or knitted textiles.Care Instructions:
The care requirements for nonwoven fabrics depend on several factors, including the type of fiber used, the thickness and arrangement of the fibers in the web, the adhesive system, and the applied finishes and colors. Naturally, the care procedures for durable items differ from those for disposable products. For example, the disposal methods for items like diapers and medical products are now regulated in certain municipalities.Economic Benefits:
The production of nonwoven fabrics offers several economic advantages, including:- Elimination of weaving or knitting processes and their preparatory stages.
- Absence of mechanical spinning processes, as nonwovens are made from carded fiber webs, pneumatically produced fiber webs, or those formed on paper machines.
- Very high production rates, with dry-laid nonwovens exceeding ten meters per minute and wet-laid nonwovens reaching several hundred meters per minute.
- Reduced labor and machinery costs.
- Production of affordable disposable items such as diapers, briefs, towels, napkins, aprons, blankets, and industrial clothing.
