What is Bonding Techniques in Nonwovens?
Bonding techniques in nonwovens are the process of joining loose fibers together to make a strong and stable fabric. Once the webs are formed, they have little strength and need to be held together. Different web bonding techniques are available depending upon the end product properties and cost. They can be bonded using one of three methods:
- Mechanical bonding
- Thermal bonding
- Chemical bonding
A. Mechanical Bonding:
In mechanical bonding process, fibrous sheet or web is bonded together through the application of liquid or air jets, punching needles and by stitching. Depending upon the selection of any type of mechanical media, nonwovens are classified as hydro entanglement, needle punching and stitch-bonded fabrics. Mechanical bonded nonwovens commonly used in geotextiles, carpets, wipes, padding, and insulation.
This bonding uses a number of methods:
1. Frictional Bonding: The fibers are rubbed together, which capitalizes on certain fibers’ properties holding them together e.g. scales of wool in felting.
In frictional bonding technique, fibers are held together mainly by mechanical interlocking and surface friction rather than adhesives, heat, or chemical agents. Frictional bonding in nonwovens is widely used in producing geotextiles for road construction, erosion control, and soil stabilization; automotive components such as insulation, carpet backing, and trunk liners; and filtration media for air, water, and oil.
2. Needling or Needle Punching: The fibers are held by a barbed needle that penetrates the web and pulls fibers throughout the web before carrying them over to produce self-made stitches and entangle the fibers more.
In needle punching technique, fibrous web is allowed to pass under a bar containing multiple needles. These needles pass in through the thickness direction of web and entangle the fibers to give strength to the fibrous sheet. Schematic representation of a needle punching is shown if Figure 1. Needle punched nonwovens are used in automotive, construction, home furnishing industries, geotextiles, shoe felts, blankets, filters and insulators.
3. Stitch Bonding: This method uses a needle with thread to form stitches that hold the fibers in rows along the length of the fabric. The method provides added strength to the fabric, and can often resemble corduroy.
Stitch bonded nonwoven fabrics are produced by stitching the fibrous web or sheet with other fibers or yarns. The performance properties of stitch bonded nonwoven fabric depends upon the stitching yarn type, stitch density, stitch length, stitching yarn tension and machine gauge. Stitched bonded fabrics may be of one side stitched, two sides stitched or one side stitched with the projection of pile on the other side of the fabric. In order to get flexibility in the fabric Lycra yarn is used and for higher strength fabric, high performance yarns are used for stitching purpose. Commercially, two stitch bonding systems: Maliwatt and Malivlies are available. Stitch bonded fabrics are used to produce vacuum bags, geotextiles, filters, and interlining, the biggest market is shaped by home furnishing industry.
4. Hydro-Entangling: Small, high-pressure jets of water push the fibers upward; the fibers then drop back into different places, which entangle them. This method is also called spun lacing.
In hydro entanglement method, fibrous sheet is allowed to pass under the liquid jets provided by multiple nozzles. Through the jet pressure web is fused, consolidated and provide strength to the sheet as shown in Figure 3. The major disadvantage of this technique is the drying of sheet after consolidation. Hydro entangled nonwoven fabrics are used in wipes and medical nonwoven industry because of their additive free, lint free, soft, strong, and cost effective characteristics.
B. Thermal Bonding:
Thermal bonding is only suitable for fibrous webs containing fibers (or particles) made from thermoplastic polymers such as polyethylene, polypropylene, polyester and polyamide. Common uses in producing hygiene products, filters, and lightweight fabrics.
Thermal bonding requires a thermoplastic component to be present in the form of a fiber, powder or as a sheath as part of a bicomponent fiber. The heat is applied until the thermoplastic component becomes viscous or melts. The polymer flows by surface tension and capillary action to fiber-to-fiber crossover points where bonding regions are formed. These bonding regions are fixed by subsequent cooling. No chemical reaction takes place between the binder and the base fiber at the bonding sites. Binder melt and flow into and around fiber crossover points, and into the surface crevices of fibers in the vicinity, and adhesive or mechanical bond is formed by subsequent cooling. Thermal bonded products are relatively soft and bulky depending upon the fibers composition.
Thermal bonding uses controlled heat to ensure that the fibers are not distorted or destroyed. If a thermoplastic fiber has not been used as part of the original web, a low-melt or bicomponent fiber is added in during the web’s production. Thermal bonding can be carried out in one of three ways: hot calendering, which can be patterned or smooth; through air bonding drums and blanket systems or sonic bonding.
In ultrasonic bonding, fiber fusing is achieved by the conversion of acoustic energy into heat at ultrasonic wave frequencies higher than 20,000 Hz, because of mechanical vibration of polymer molecules in the thermoplastic material. Other thermal bonding technologies include microwave, infrared and laser heating.
Thermal bonding process is economical, environment friendly and 100 % recycling of fibers components can be achieved. In thermal bonding technique generally hot calendar rollers are used to bind the fibrous sheet. Thermal bonding has further sub categories: point, area, infra-red, ultra-sonic and through air bonding.
C. Chemical Bonding:
The fibers are bonded together by means of chemical adhesives applied to the web in the form of liquid dispersions, polymer solutions, powders and particles.
In chemical bonding, uses different types of chemicals or binders – rubber (latex), synthetic rubber, acrylate polymer and copolymers, styrene-butadiene copolymers or vinyl acetate ethylene copolymers by impregnating, coating or spraying. Latex binder is most commonly used for nonwoven web bonding. This bonding brings the fibers together so they can be dried and cured. Sometimes, the resin is turned into foam to coat the web; the advantage to using this process is that less heat energy is required and the time to dry and cure the binder is reduced. This strengthens the fabric.
Also knows as saturation bonding the web is saturated using latex, which adheres the web together. After that, it is dried and cured.
During chemical bonding process chemicals are sprayed on the nonwoven web or web is allowed to pass through the chemical box. In chemical bonding different techniques are used for the web bonding. Most frequently used chemical bonding processes are spray adhesives, print bonding, saturation adhesives, discontinuous bonding and application of powders.
Chemical bonding is used in manufacturing of wipes, medical fabrics, coating base fabrics, and interlinings.
Conclusion
Bonding is the most important step in making nonwovens strong and useful. Bonding in nonwovens joins fibers by tangling, melting, or gluing, making fabrics fit for many uses like diapers, wipes, filters, and medical supplies. This choice shapes the fabric’s strength, softness, durability, and price. Choosing the right method is key to making fabric that fits the desired use, cost, and quality.
References
[1] Ashford, B. (2016). Fibers to Fabrics.
[2] Nawab, Y., & Shaker, K. (2023). Textile Engineering: An Introduction. Walter de Gruyter GmbH & Co KG.
[3] Sinclair, R. (2014). Textiles and fashion: Materials, Design and Technology. Woodhead Pub Limited.
[4] Albrecht, W., Fuchs, H., & Kittelmann, W. (2006). Nonwoven fabrics: Raw Materials, Manufacture, Applications, Characteristics, Testing Processes. John Wiley & Sons.
[5] Elise, R. (2020). Nonwoven fabric: Manufacturing and Applications. Nova Science Publishers.
[6] Karthik, T., C, P. K., & Rathinamoorthy, R. (2016). Nonwovens: Process, Structure, Properties and Applications. Woodhead Publishing India in T.
