Thermal Bonding [PDF]

Zitiervorschau

THERMAL BONDING - NON WOVEN TECHNOLOGY

DEVARAJA.H.M 9513870937 E-mail: [email protected]

INTRODUCTION TO THERMAL BONDING • It offers high production rates because bonding is accomplished at high speed with heated calendar rolls or ovens. • Thermal bonding process has been used successfully with a number of thermoplastic powder or fibers or Films. • The first thermally bonded nonwovens were produced in 1940s. • The thermal bonding process consumes less energy compared to foam bonding or hydro entanglement bonding. • Thermal bonding is the most popular method of bonding used in nonwovens manufacture.

PRINCIPLES OF THERMAL BONDING The formation of a bond during thermal bonding follows in sequence through three critical steps: 1. heating the web to partially melt the crystalline region, 2. heat is applied until the thermoplastic component becomes viscous or melts &repetition of the newly released chain segments across the fibre-fibre interface, and 3. subsequent cooling of the web to re-solidify it and to trap the chain segments that diffused across the fibre-fibre interface.

MWNT(Multiwall carbon Nano tube)

THERMAL BONDING PROCESS

• A process in which a web consisting of thermoplastic and nonthermoplastic fibers was made and then heated to the melting or softening temperature of the constituent thermoplastic fibers followed by cooling to solidify the bonding area. There two Materials are used in web bonding processes: 1. Base fiber : it is the web (it may be base fiber or Base fiber & binder fiber) 2. Binder : It may be binder fiber, binder powder Thermal bonding requires a thermoplastic component to be present in the form of a homofil fibre, powder, film, web, hot melt ores a sheath as part of a bicomponent fibre. The binder fibre component normally ranges from 5-50% on weight of' fibre depending on the physical property requirements of' the final product. Typically; a carded web from a blend of base fiber and binder fiber was produced and hot calendared followed by cooling to solidify and bond the web structure.

BASE FIBRES 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, and biodegradability amongst many other properties. • The commonly used base fibers include (a) Natural fibers (regenerated cellulosic fibers, bast fibers, vegetable and protein fibers such as wool), (b) 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 bi-component fiber, with the sheath component being the binder.

BINDER MATERIALS • Binder components are produced in many different forms including fiber or filament (homogeneous or bi-component 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 fiber matrix which has significant impact 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 minimize energy costs it is desirable that binder fibers have a high melting speed, a low melting shrinkage and a narrow melting point range. • The most widely used thermoplastic binder polymers are given in Table

Fibre

Melting Temperature (degree Celsius)

PET

245-265

PP

160-175

POLY AMIDE(PA)

210-230

PE

115-135

PE/PET

130/250

PE/PP

130/175

CoPET/PET

110/250

PVC

200

METHODS OF APPLICATION OF BINDERS Many materials that are used as a binder for thermally bonded nonwovens can be apply by following methods in production. 1. • Binding fibers 2. • Binding powder 3. • Binding web

1.

2. 3.

Powder has a binders: Powdered polymers are sometimes used in thermal bonding of nonwovens. The most prevalent use is powdered polyethylene. Powdered polymeric binders can be applied during web or batt formation or following web formation and pre-bonding Fiber as a Binders: Single-component and bicomponent fibers, as binder fibers, are most widely used in thermal bonding of nonwovens. Film as a Binders: A very open-structured, lowmelting-point thermoplastic fabric is placed between the webs and, during thermal bonding between the calendar rolls, the fabric melts completely bonding the webs together.

METHODS OF THERMAL BONDING In through-air bonding, a hot fluid, air, is forced through a preformed web. 1. Hot calendaring rollers 2. Belt calendaring 3. Through-air thermal bonding 4. Ultrasonic bonding 5. Radiant-heat bonding, etc.

1. HOT CALENDARING

• Thermal bonding relies on the use of heat energy to melt or soften one or more components of a web to achieve bonding. • There are different methods of applying heat energy to the web and the heal transfer mechanism can take different forms; conduction, convection and heat radiation. • Thermal calendar bonding is a process in which a fibrous web containing thermoplastic components (fibers, powders or webs) is passed continuously through a heated calendar nip that is created by two rolls pressed against each other. Multi-nip calendars are also employed depending on the web weight and degree of bonding required. • This causes the binder components of the web to become soft and tacky and induces polymer flow in and around the base fibers. The fluid polymer tends to collect at fiber crossover or contact points and bonding sites are formed. Cooling leads to solidification of the polymer and bonding.

1. HOT CALENDARING … • To increase the efficiency of the process, the web may be preheated immediately prior to calendar bonding sometimes by infra-red heaters. Commercially, light-weight webs of 25-30 g/m2 for medical and hygiene applications and mediumweight webs of l00 g/m2 for interlining and filtration applications are thermally bonded using calendar bonding. The degree of bonding depends on temperature, pressure and speed, which determines the contact or dwell time.

TYPES OF HOT CALENDERING There are three main types of hot calendaring. I. Area bond Calendaring II. Point bond Calendaring III. Embossing Calendaring

1.I. AREA BONDING Area-bond hot calendaring is influenced by five factors: Heat: Bonding occurs at the surface of the metal roll, which obtains its heat by conduction from heated oil circulated through its center or from restrictive heating. Pressure: Bonding occurs through simultaneous application of heat and pressure. The heat causes the fiber binder to become thermoplastic. The pressure enhances mechanical bonding by forcing the binder polymer to flow in and around the carrier fibers. Speed: The speed at which the nonwoven passes through the calendar, combined with heat and pressure conditions, determines the degree of bonding in the nonwoven.. Roll combination: The only practical roll combination for area bonding is a metal roll-felt roll. The metal roll applies the heat Cooling: The product is warm and thermoplastic as it leaves the calendar nip. A set of two cooling rolls placed immediately after the calendaring stage eliminates these unwanted side effects.

1.II. POINT BONDING • Point-bond hot calendaring is the main method of thermally bonding in disposables as diaper, sanitary products, and medical products. • This method involves the use of a two-roll nip consisting of a heated patterned metal roll and a smooth or patterned metal roll. • Second roll may or may not be heated, depending on the application. • In a typical production line, the web is fed by an apron leading to a calendar nip and the fiber temperature is raised to the point at which tackiness and melting cause fiber segments caught between the tips of engraved points and the smooth roll to adhere together.

1.III. EMBOSSING OR NOVONETTE SYSTEM • This method is a figured or sculptured area-bond hot calendaring. In this case, though, the area bonding is threedimensional. • The calendar roll combination has a male patterned heated metal roll and a matching female patterned felt roll.

• • • • •

2.BELT CALENDERING Belt calendaring is a modified form of roller calendaring with two main differences i.e. time in the nip and the degree of applied pressure. In roller calendaring the heating time is measured in milliseconds whereas in belt bonding, time in the nip is extended to 1-10 seconds. Pressure in a calendar roll nip is in the range 35-260 N/mm while in belt bonding the pressure does not normally exceed 9 N/mm. The belt bonder consists of a heated roller and a rubber blanket. The roller diameter ranges from 40-250 cm and is usually coated with PTFE to increase its life. The resilient, heat-resistant (up to a temperature of 250 °C), silicone rubber blanket wraps around the heated roller, covering up to 90% of the roller surface. The nonwoven fabric is bonded by running in between the roller and the blanket when the heat and pressure are applied simultaneously.

3. THROUGH-AIR BONDING • Through-air thermal bonding involves the application of hot air to the surface of the nonwoven fabric. • The hot air flows through holes in a plenum positioned just above the nonwoven. However, the air is not pushed through the nonwoven, as in common hot air ovens, but negative pressure or suction, pulls the air through the open conveyor apron that supports the nonwoven as it passes thorough the oven. • Pulling the air through the nonwoven fabric allows much more rapid and even transmission of heat and minimizes fabric distortion.

3. THROUGH-AIR BONDING… Through-air bonding can take different forms: a. The perforated drum or rotary system and b. The perforated conveyor or flat bed system. c. Impingement bonding (air jetting system)

3. a. PERFORATED DRUM-THROUGH-AIR BONDING The drum system has the following advantages compared with the belt method: • Compact design (i.e., perforated drums, fans and radiators are installed in an insulated housing). • Reduced energy consumption (no thermal losses by conveying device, i.e. the drum remains inside the insulated chamber). • Through-air bonding provides automatic heat recovery from the material, unless the line is combined with a calibrating unit.

3. a. PERFORATED DRUM-THROUGH-AIR BONDING • Perforated drums are available in diameters from 1000 to 3500 mm and working widths from 400 to 7000 mm. Generally, onedrum or two-drum units are in use; however there are some multi drum lines. • Parameters to control the efficiency of Bonding: Perforation, Hot air, Speed Etc.,. • Production speeds of more than 1000 m/min and working widths of up to 10 m are available.

3.b. PERFORATED FLAT CONVEYOR-THROUGH-AIR BONDING • In flat conveyor systems, the web is carried without the need for control by suction draught. A uniform air flow and temperature distribution across the working width is essential to avoid irregular thermal shrinkage and bonding in the fabric. Generally, the perforated belt system is particularly suitable for bulky, low-density webs.

IMPINGEMENT BONDING (AIR JETTING SYSTEM) • In air jetting systems, hot air is blown onto the web from one or both sides by means of a nozzle system at a speed of up to approximately 40 m/s. • In the case of double-sided air jetting the web is not pressed against the belt, but rather it floats on the bottom air flow and both sides of the web are bonded. Fibers within the web structure are less effectively heated by the hot air and therefore limited bonding occurs within the • This technique is preferred for products where a pile is to be raised by adjusting the top and bottom air flows. It is mostly used in perforated belt systems. Figure below illustrates the arrangement of typical one- and doublesided air jetting units.

4. ULTRASONIC BONDING

• The stress created by the ultrasonic compressive forces is converted to thermal energy, which softens the fibers as they are pressed against each other. 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. • No binder is necessary when synthetic fibers are used since these are selfbonding. To bond natural fibers, some amount of synthetic fiber 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 takes place by exposing the web or mat to a source of radiant energy in the infrared range. The web, increasing its temperature, absorbs the electromagnetic energy radiated from the source. They also can be reactivated by heat for use in the manufacture of laminated composites.

APPLICATIONS OF THERMAL BNDED NONWOVENS

REFERENCES • • • •

https://www.sonimak.com/ https://www.asianonwovens.org/ https://www.edana.org/ www.inda.org