Thermal Bonding of Nonwoven: Raw Materials, Methods and Applications

What is Thermal Bonding?
Thermal bonding is the conglutination of thermoplastic fibers with each other or with other fibers by means of thermoplastics. Thermoplastic binding fibers can be added during fiber production. Thermal activation is achieved using hot air (thermofusion) or a calender (thermobonding). The nonwoven can also be partially bonded ultrasonically. In all processes, the nonwovens are compacted using a calender. Typical area weights are below 100 g/m2. When a structured roller is used, the nonwoven obtains an imprint. 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 Bonded Nonwoven
Figure 1: Thermal Bonded Nonwoven

Thermal bonding is successfully employed in bonding dry-laid, polymerlaid and wet-laid webs as well as multi-layer materials. The basic concept of thermal bonding was introduced by Reed in 1942.

These fabrics are manufactured by applying the thermal or heat energy to the thermoplastic component present in the fibrous web and the polymer flows by means of surface tension and capillary action to form the required number of bonds at crossover regions of fibers. It is mainly classified into two categories: through-air bonding and calendaring. In through-air bonding, the fibrous web is passed in a heated air chamber (oven) for forming the bonds at the crossover positions of fibers. Calendaring involves the passage of the fibrous web through a heated pair of rollers that impart high pressure and temperature in order to melt the thermoplastic fibers.

Thermal bonding is a widely-used bonding technology in the nonwovens industry, especially in spunbond, meltblown, airlay and wetlay manufacturing, as well as with carded web-formation technologies. Considerable effort has been spent to optimize the web-formation processes, bonding processes, and feed-fiber properties to achieve the desired end-use properties while reducing the cost of manufacture.

The advancement of new raw materials, better web arrangement innovations and higher production speeds have made thermal bonding a reasonable process for the production of both durable and dispensable nonwovens.

There are three major types of bonding in nonwoven:

  1. Chemical Bonding
  2. Thermal Bonding
  3. Mechanical Bonding

Principle of Thermal Bonding:
Thermal bonding requires a thermoplastic component to be present in the form of a homofil fiber, powder, film, web, hot melt or as a sheath as part of a bicomponent fiber. In practice, 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. In this case, no chemical reaction takes place between the binder and the base fiber at the bonding sites. When binders melt and flow into and around fiber crossover points, and into the surface crevices of fibers in the vicinity, an adhesive or mechanical bond is formed by subsequent cooling. Such an adhesive bond is a physio-chemical bond at the interface of two dissimilar materials. In the thermal bonding context, a mechanical bond is formed as a result of thermal shrinkage of the bonding material, which while in the liquid state encapsulates the fiber crossover points. In contrast, if at the binderfiber interface both components soften or melt, inter-diffusion and interpenetration of the molecules across the interface can occur and the interface may disappear. This arises where compatible polymers are present with nearly comparable solubility parameters. Bonds formed in this way may be called cohesive bonds.

Raw Materials of Thermal Bonding:
Thermally bonded fabrics are produced both from entirely thermoplastic materials and from blends containing fibers that are not intended to soften or flow on heating. The non-binder component may be referred to as the base fiber component and commercially, a variety of base fiber types are used. The binder fiber component normally ranges from 5–50% on weight of fiber depending on the physical property requirements of the final product.

1. Base fibers types:
The base fiber contributes to key physical, chemical and mechanical properties of the fabric derived from the polymer from which it is constituted. This influences dyeing characteristics, flame resistance, tensile and attritional properties, hydrolytic resistance, biodegradability amongst many other properties. The commonly used base fibers include natural fibers (regenerated cellulosic fibers, bast, vegetable and protein fibers such as wool), synthetic fibers (polyester, polypropylene, acrylic, nylon, aramid and many others), mineral fibers (e.g., glass and silica) and metallic fibers. Sometimes the base fiber (carrier fiber) is the core of a bicomponent fiber, with the sheath component being the binder portion.

2. Binder materials:
Binder components are produced in many different forms including fiber or filament (homogeneous or bicomponent; sheath/core or side-by-side type melt-bonding fibers), powder, film, low melt webs, and hot melts. The physical form of the binder affects its distribution throughout the fiber matrix which has a significant impact on fabric properties. The amount of binder also plays an important role in determining the properties of the resultant nonwoven fabric. If the binder content is more than 50% of the total blend the fabric behaves like a reinforced plastic. At a binder content of 10% the fabric is a bulky, porous and flexible structure with relatively low strength. To minimise energy costs it is desirable that binder fibers have a high melting speed, a low melting shrinkage and a narrow melting point range.

3. Bicomponent binder fibers:
Bicomponent fibers and filaments, which are also referred to as conjugate fibers, particularly in Asia, are composed of at least two different polymer components. They have been commercially available for years; one of the earliest was a side-by-side fiber called Cantrece developed by DuPont in the mid-1960s followed by Monsanto’s Monvel, which was a self-crimping bicomponent fiber used by the hosiery industry during the 1970s. Neither of these fibers was commercially successful because of complex and expensive manufacturing processes. Later in 1986, commercially successful bicomponent spinning equipment was developed by Neumag, a producer of synthetic fiber machinery. Use of bicomponent fibers accelerated dramatically in the early 1990s partly because of the need to uniformly bond the entire thickness of nonwoven fabrics, which in heavy weight per unit area structures could not be satisfactorily accomplished by chemical bonding. More recently the market for bicomponent fibers has been greatly developed by Japan and Korea.

Bicomponent fibers are commonly classified by the structure of their cross-section as side-by-side, sheath-core, island in the sea or segmented pie. Of these, the side-by-side and sheath-core arrangements are relevant for thermal bonding applications.

Methods of Thermal Bonding:

  1. Calender bonding
  2. Through-air bonding
  3. Powder bonding
  4. Ultrasonic bonding
  5. Radiant-heat bonding, etc.

1. Calender bonding:
The web passes between the nip of two large heated (calender) rollers that operate under pressure, compressing the fibrous assembly and conducting heat into the fibers, causing them to soften and melt. Provided that the web is not too heavy in mass per unit area, the heating is very rapid and the process can be carried out at high linear speed (> 350 m min−1). The design of calender rollers for this purpose has become highly developed; they can extend to more than 5 m width and can be heated to produce less than 1°C temperature variation across the rollers. Also systems have been developed to ensure that uniform pressure is applied all the way across the rollers because rollers of this width have a tendency to deflect. Most commonly, at least one engraved roller is used in calender bonding systems because the area bonding resulting from the use of two plain rollers produces fabrics that are too stiff and impermeable for practical use.

Calender rollers for point bonding are engraved with a pattern that limits the degree of contact between the rollers to roughly 5–25% of the total area to maintain the permeability and flexibility of the fabric. The size, shape, and geometry of the pattern influence not only the appearance of the resulting fabric but also its physical properties. Thermal bonding is mostly confined to those raised or embossed points on the rollers where they touch and compress the web and leaves the rest effectively unbonded. Fabrics made in this way are flexible and relatively soft because of the unbonded areas. At the same time, fabrics maintain reasonable strength, especially in the case of spunlaid fabrics. These fabrics have many uses, for example, as a substrate for tufted carpets, in geosynthetics, in filtration media, in protective/disposable clothing, as coating substrates and as hygiene cover stock. Another use of calendering is to melt fibers on just one side of a fabric in a process known as “skinning” the surface to increase mechanical stability. This can be done by passing the fabric between a set of plain calender rollers, only one of which is heated to a temperature near the melting point of the polymer.

Types of calender bonding:
There are three main types of calender bonding.

  • Area bonding
  • Point bonding
  • Embossing

2. Through-air bonding:
Through-air thermal bonding includes the utilization of hot air to the surface of the nonwoven fabric. Webs pass through a hot air oven supported on an air-permeable conveyor in the form of a belt or drum. The aim is to rapidly heat the web to the melting point of the constituent fibers by drawing hot air through the web so that all the constituent fibers are heated uniformly. The air temperature, air velocity, and dwell time of the web in the oven all influence the bond strength developed in the fabric during thermal bonding.

In some through-air processes, an upper perforated surface is provided in the oven to slightly compress the web to the required thickness as well as to control shrinkage. As the web leaves the oven, its final thickness can also be controlled by passing it between two calender rollers, which can be set to a specific gauge as required to bring the web to the required thickness. Chill rollers are also employed to cool the thermally bonded fabric before winding or further processing to prevent layers from sticking. Through-air bonding is particularly suitable for the production of high-loft or low-density fabrics because there is minimal compression of the web during the process. It is also an effective means of bonding heavyweight webs uniformly through their thickness, which can be difficult to achieve using direct contact methods such as calender bonding.

The application of through-air (thru-air) bonding has been growing in producing high bulk and heavy-weight thermally bonded nonwoven fabrics. Despite competition from other technologies, through-air bonding continues to penetrate in different markets due to its versatility and ability to produce webs with good softness, drape, re-wet and high bulk.

3. Powder bonding:
In some applications, thermoplastic powders are applied to webs for the purpose of bonding. The powder can be mingled with the fiber during web formation, which is particularly convenient in some airlaying processes, after web formation, or can be applied to the surface of prebonded nonwoven fabrics. One of the challenges is to uniformly apply the powder to the web. Particularly when the powder is applied after web formation penetration into the internal structure can be quite limited. Products made by powder bonding are characterized by softness, low density, and flexibility and in general they have relatively low strength. Again there is a very wide range of uses covering particularly interlinings, shoe fabric components, and floor coverings.

4. Ultrasonic bonding:
This process involves the application of rapidly alternating compressive forces to localized areas of fibers in the web. The stress created by these compressive forces is converted to thermal energy, which softens the fibers as they are pressed against each other (Figure 2). Upon removal from the source of ultrasonic vibration, the softened fibers cool, solidifying the bond points. This method is frequently used for spot or patterned bonding of mechanically bonded materials.

Ultrasonic bonding process
Figure 2: Ultrasonic bonding process

No binder is necessary when synthetic fibers (thermoplastic) are used since these are self-bonding. To bond natural fibers, some amount of synthetic fiber (thermoplastic) must be blended with the natural fiber. Fabrics produced by this technique are soft, breathable, absorbent, and strong. This bonding method is used to make patterned composites and laminates, such as quilts and outdoor jackets.

5. Radiant heat bonding:
Radiant heat bonding happens by uncovering the web or mat to a source of radiant energy in the infrared extent. The electromagnetic energy transmitted from the source is consumed by the web, expanding its temperature. The use of radiant heat is controlled so it softens the binder without influencing the carrier fiber. Bonding occurs when the binder re-solidifies upon removal of the source of radiant heat.

Lower energy and shipping expenses make this a favored strategy for handling powder-bonded nonwovens. Adaptability and lower delivery expenses are additionally considers. Post-calendered rolls can be sent in slight, compacted shape and re-bulked by reapplication of heat, without weight or restrictions, to the wanted state at the season of utilization. Powder bonded items made in this way are delicate, open, and permeable with low-tomedium quality. They likewise can be reactivated by warmth for utilization in the assembling of laminated composites.

Advantages of Thermal Bonding:
Some of the main advantages of thermal bonding are as follows:

  1. Products can be relatively soft and textile-like depending on blend composition and bond area.
  2. Good economic efficiency compared to chemical bonding involving lower thermal energy requirements and less expensive machinery.
  3. High bulk products can be bonded uniformly throughout the web cross-section.
  4. 100% recycling of fiber components can be achieved.
  5. Environmentally friendly since no latex binders are required.

Advantages of Thermal Bonding Compare to Others Bonding in Nonwoven:
Thermal bonding is increasingly used at the expense of chemical bonding for a number of reasons. Thermal bonding can be run at high speed, whereas the speed of chemical bonding is limited by the drying and curing stage. Thermal bonding takes up little space compared with drying and curing ovens. Also thermal bonding requires less heat compared with the heat required to evaporate water from the binder, so it is more energy efficient.

The rising cost of energy and greater awareness of the environmental impact of latex bonding led to a change in direction. A comparison of energy consumption by various web-bonding processes which shows a considerable energy saving for the thermal bonding process. The high production rates possible with thermal bonding and the significant energy savings as compared to chemical bonding, due to the absence of significant water evaporation during bonding, makes the process economically attractive. In contrast to chemical bonding, the environmental impact of the process is also significantly reduced.

Thermal bonding can use three types of fibrous raw material, each of which may be suitable in some applications but not in others. First, the fibers may be all of the same type, with the same melting point. This is satisfactory if the heat is applied at localised spots, but if overall bonding is used it is possible that all the fibers will melt into a plastic sheet with little or no value. Second, a blend of fusible fiber with either a fiber with a higher melting point or a non-thermoplastic fiber can be used. This is satisfactory in most conditions except where the fusible fiber melts completely, losing its fibrous nature and causing the batt to collapse in thickness. Finally, the fusible fiber may be a bicomponent fiber, that is, a fiber extruded with a core of high melting point polymer surrounded by a sheath of lower melting point polymer. This is an ideal material to process because the core of the fiber does not melt but supports the sheath in its fibrous state. Thermal bonding is used with all the methods of batt production except the wet-laid method, but it is worth pointing out that the spun-laid process and point bonding complement each other so well that they are often thought of as the standard process.

Thermal bonding is applicable to webs made by virtually all formation methods, but there are variations in the heat transfer mechanisms that are used to bond the fibers.

Applications of Thermal Bonded Nonwoven:

  1. Cover stock, medical and sanitary webs
  2. Interlining
  3. Filtration webs
  4. High-loft webs or needled webs
  5. Geotextiles
  6. Carpet backing
  7. Industrial textiles
  8. Coating substrates
  9. Protective material
  10. Insulating material
  11. Decorative webs
  12. Nonwoven covers
  13. Webs for upholstery industry
  14. Nonwoven wall coverings
  15. Padding material fiber fill webs
  16. Wiping cloths
  17. Waste-fiber webs for various applications
  18. Needle-punched carpets
  19. Roofing felts
  20. Light-weight webs
  21. Tea bag paper
  22. Technical products etc.

References:

  1. Handbook of Nonwovens Edited by S. J. Russell
  2. Introduction to Nonwovens Technology By Subhash K. Batra, Behnam Pourdeyhimi
  3. Nonwovens: Process, Structure, Properties and Applications By T. Karthik, R. Rathinamoorthy and C. Praba Karan
  4. Handbook of Technical Textiles – Second Edition; Volume 1: Technical Textile Processes Edited by A Richard Horrocks, Subhash C. Anand
  5. Textile Technology: An Introduction, Second Edition by Thomas Gries, Dieter Veit, and Burkhard Wulfhorst
  6. Textile Engineering – An Introduction Edited by Yasir Nawab

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