Meltblowing Process

• Meltblowing is a process for producing fibrous webs or articles directly from polymers or resins using high-velocity air or another appropriate force to attenuate the filaments.
• This one is a recent technique developed in nonwoven manufacturing.
• The advantage of this process is its ability to produce microfiber webs having diameters ranging from 0.1 mm to 15 mm, from which diameters 2-4 mm are popular.

Raw Materials

The meltblown process has no or little orientation after extrusion. Hence many polymers apart from Polypropylene and Polyester can be processed:

1)      High-density polyethylene (HDPE), Low-density polyethylene (LDPE), LLDPE

2)      Polyamides (PA 6, PA 6.6, PA10)

3)      Polystyrene (PS)

4)      Polytrifluorochloroethene (PCTFE)

5)      Polycarbonate (PC)

6)      Polyurethane (PUR)

 

Meltblown Process

Meltblowing is a single-stage process in which high-velocity air blows a molten thermoplastics resin an extruder die tip onto a conveyor or take-up screen to form a finely fibrous and self-bonding web as below:

 The components of meltblowing process are:

• Extruder: from the hopper feeder, the polymer chips are feds to the Archimedean screw, which rotates inside the cylinder; due to this rotation chips are forwarded to the hot walls of the cylinder, so the polymer melted by heat and frictional forces. The screw is divided into feed, transition, and metering zone.

ð  The feed zone preheats the polymer pellets.

ð  Transition zone compresses and homogenizes the melting polymers.

ð  Metering zone serves to generate maximum pressure for extrusion.

• Metering Pump:  It controls the delivery of the melt to the die assembly ensuring a consistent flow of polymer with the required pressure, temperature, and viscosity. It is designed with two intermeshing and counter-rotating toothed gears.
  
• Die Assembly:

ð  Feed Distribution: It distributes the flow evenly to all the spinnerets flow and ensures constant residence time across the width of the die in all instances. There are two types of feed distribution, one is T-type and another is coat hanger type (widely used).

ð  Air Manifolds: The die nosepiece is designed with slots through which the air manifolds supply the high-velocity hot air. Generally, the temperature of 230˚C to 360˚C and air velocities of 0.5-0.8% of the speed of sound.

ð  Die Nosepiece: The design of the die nosepiece is responsible for web uniformity. It is typically a hollow and tapered piece of metal having a linear arrangement of several hundred orifices along its width. The polymer melts extruded through these holes and the emerging filaments are quenched using hot air. The dimension of the nosepiece includes a diameter of 0.4 mm and the number of orifices per mm ranges from 1 to 4.

• Web Formation: Extruded polymers passed through the die holes, hot air streams of high velocity, and the die nosepiece to form microfibers. The hot air stream directs the microfibers to a collecting screen. On the way to the same screen, the secondary air cools and solidifies the fibers. Due to random deposition of fibers, they entangled themselves, and hence air turbulence forms a self-bonded nonwoven web. The collector speed and the distance of the die nosepiece from the collector screen are playing major roles in producing different types of meltblowns. The hot air lying on the fibers can be withdrawn by applying a vacuum inside the collector screen.
• Winding: The web is wound onto a cardboard core and processed according to the end-use requirement. 
• Bonding: To improve the fiber adhesion and web characteristics, the web may be subjected to additional bonding processes like thermal bonding. Either area bonding (overall) or pattern bonding (spot) techniques can be adapted, which improves the web strength and abrasion resistance.
• Finishing: This treatment is not mandatory, finishing treatments like calendaring, embossing can be performed at the end of the production line. 

Process Variables

• The process variables are divided into two categories such as operational/online and offline variables.     

Web Characteristics and Properties

• The distribution of fiber in the air stream and the vacuum settings below the collecting screen determine the web uniformity.
• Non-uniform distribution can be caused due to poor die design and improper airflow in the air stream.
• The vacuum under the collector should be capable of withdrawing the entire air stream through the perforations and randomly locking the fibers in place.
• As the distance between the die and the collecting screen increases, the web uniformity decreases.

Product characteristics

ð  Random fiber orientation in the web

ð  Lower to moderate web strength

ð  High opacity

ð  Low GSM material

ð  Fiber diameter ranges from 0.5 to 30m, but typically 2-7 m

ð  Basis weight ranges from 8 – 350 g/m², but typically 20 – 200 g/m²

ð  Good insulation and filtration characteristics

ð  Smooth and soft surface texture and are circular in cross-section

ð  Layered structure, the number of layers increases with basis weight.

Application

Medical fabrics	Disposable gown Drape market Sterilization Wrap segment Sanitary products etc.
Adsorbents	Sorbents to pick up oil from the surface of the water like, encountered in an accidental oil etc.
Filtration media	Filter media Cartridge media Cleanroom filters and others
Apparel	Thermal insulation Disposable industrial apparel Substrate for synthetic leather
Electronic specialities	Liner fabric in computer floppy disks Battery separators Insulation capacitors
Miscellaneous	Manufacture of tents Elastomeric non-woven fabrics etc.

References

• Karthik T., Prabha Karan C., and R. Rathinamoorthy (2016), Non-woven - Process, Structure, Properties and Applications, Woodhead Publication India, pp. 80-86.
• Giovanni Tanchis (2008), The nonwovens, ACIMIT.
• Lokesh K V (2013), Meltblown nonwoven, www.textilelearner.blogspot.com

Spun-bonding Process

     The spun-bonding process is specifically applied for spinning of synthetic filaments. Nonwoven formed by this process consists extrusion, drawing and laying of the filaments on a moving conveyor belt. 

Raw Materials 

Polypropylene, nylon, polyester, polyethylene and polyurethane are the best suitable raw material. 

• Polypropylene is the most widely used fiber, due to its low density. It is also available in various forms such as virgin fiber, dope dyed fiber, recycled fiber. Stability of the fiber to UV is low, creep resistance is low, and melting point is also low (160 °C).   
• Polyester is applicable where higher strength and UV resistance required. It has low resistance to alkali.
• Nylon 6 and 66 have higher moisture regain (4% at 65 RH and 70 deg.F) compare to other synthetic fibers, which makes nylon suitable for the spun-bonding. For nylon processing, cost of energy is high.
• Polyethylene is also used due to its low cost. It is limited due to its lower strength and low melting point (110 °C).
• Bicomponent fibers (Core/Sheath and side by side) can also be used for the process. These fibers can be produced by combining low melt polymer on the surface and high melt fiber in the core of the fiber. Eco-friendly fiber using polylactic acid (PLA) in core and polypropylene on the surface can be manufactured    

Batt manufacturing process

    Sequence for spun-bonding process is as follow:

        

All spinning technique can be applied for spun-bonding, like melt, dry or wet extrusion. But prominently melt spinning is applied. 

• Polymer chips are melted in extruder fed by the hopper.
• This molten polymer is filtered and extruded through the spinnerets. 
• Extruded fiber then quenched by the supply of cold air and oriented mechanically or pneumatically to increase the strength.

Polymer Melting: 

• Pellets, granules or chips of polymer are added in extruder hopper.
• By the means of gravity, polymer entered in heated barrel where rotating screw is also attached, from the hopper.
• Temperature of barrel melts the polymer, which generate the viscous form of polymer.
• The screw designed in three sections: feed, transition, and metering zones.
• In feed zone, polymer is preheated and transferred to the transition zone.
• In transition zone, polymer melts completely.
• This molten polymer is conveyed to the metering zone.

Metering of the melt:

• For proper filament extrusion, process parameters such as viscosity, pressure, and temperature are essential for a uniform flow of molten polymer.
• The same purpose is fulfilled by using a positive displacement volume metering device to deliver the polymer to the die assembly. 

Die assembly:

• The die assembly utilized for spun-bonding has two identical sections: the polymer feed distribution section and the spinneret section.
• Feed distribution section: 
• The polymer delivered by the metering device is supplied to all the spinneret through feed distribution section. 
• T-type and the coat hanger type system can be applied.
• Molten polymer directed into the spinneret from the same section.
• Spinneret:
• Spinnerets placed by side by side to produce a wide web (up to about 5 m) in commercial practice.
• The grouped spinnerets are also known as bank of block.   

Filament spinning, drawing and deposition:

• Filaments are drawn, entangled and deposited on to conveyer belt or collector.
• Proper filaments deposition based on aerodynamic principles for fanning and entangler.
• To increase cross-directional integrity of the web, fanning unit crosses or translates adjacent filaments.

Variants:

• In batt formation process, two variants are there, viz. Partial orientation and Full orientation.
• Partial orientation is suitable for most of the regular used products like cover stock for diapers and hygiene materials. It also provide high production rates and sufficient for required strength.
• Full orientation is applied for geotextiles, carpet backing, roofing and industrial products. 
• Full orientation can be achieved by drawing the filaments over heated godet rollers, with draw ratio of 1:3 or 4 with pneumatic acceleration.
• Then filaments are passed through a pneumatic air gun, where high velocity air is force with low pressure through a constricted area.
• During the drawing process, filament entanglements are avoided by applying electrostatic charges.
• Finally, filaments are deposited randomly or uniformly on moving conveyer belt.
• End use decides the lay down direction in both orientation variants.

The Production rate of spunbonding machine per meter working width in kg/hr

The mass per unit area of the web created on the perforated belt

Filament bonding and Staple fiber bonding:

Filament bonding		Staple fiber bonding
1.	Strength is high.	1.	Strength is low.
2.	Elongation is low.	2.	Elongation is high.
3.	GSM and thickness uniformity is higher.	3.	GSM and thickness uniformity is lower.
4.	Feel and textile character are lacking	4.	Feel and textile character are good.
5.	Tear strength is high.	5.	Tear strength is low.
6.	Product range GSM is 20-250	6.	Product range GSM is 20-1500
7.	Flexibility is lower in choice of raw materials.	7.	All types of raw material can be processed in the same line.
8.	Production is high.	8.	Production is low to medium.
9.	Investment is high at initiation step.	9.	Investment is low to medium at initiation step.
10.	Manufacturing process consist two steps.	10.	Manufacturing process consist single step.

Applications: 

• Industrial products
• Filters
• Cover stock for diapers and hygiene products
• Carpet backing
• Surgical Materials
• Bedding and furniture
• Geotextiles 
• Roof materials and other construction materials

References:

• Hosul Lim (2010), A review of spunbond process, J Text Apparel Technol Manage 6(3): pp 1-13
• Fourne F (1992), New process for spunbond fabric production. In spunbond technology today 2: onstream in the 90's (pp. 169-174), San Francisco.
• Gilmor TF (1992), Spunbond web formation process: A critical review. In spunbond technology today 2: onstream in the 90's (pp. 139-145), San Francisco.
• Karthik T., Prabha Karan C., and R. Rathinamoorthy (2016), Non-woven - Process, Structure, Properties and Applications, Woodhead Publication India, pp. 75-80.