SCOURING (Fundamentals)

 Introduction

• The loom state cotton fabric contains about 8-12% natural impurities of the total weight of the fibre.
• These impurities mainly consist of waxes, proteins, pectic substances and mineral matters. 
• In addition to this, the mechanically held impurities called ‘motes’ are present containing seed-coat fragments, aborted seeds and leave etc. that cling to the fibre.
• Apart from these, the loom-state fabric is also contaminated with adventitious oils such as machine oils, tars, greases etc.
• Scouring is a purifying treatment of textiles.
• The objective of scouring is to reduce the number of impurities sufficiently to obtain level and reproducible results in dyeing and finishing operations.
• Scouring agents can be generally classified into different groups.
• The appropriate type of scouring agent generally depends on the kind of fibre; fabric type i.e. woven or knitted, thick or thin; texturised or non-texturised and the extent of impurities present in the fibre.

 Classification of textile scouring agents 

• The selection of alkali is most important as free alkali can have a deleterious effect on certain fibres. Alkali neutralises the carboxyl group in cellulose and in pectin.
• The hydroxyl groups on the glucose units in cellulose are also weakly acidic. Owing to this preferential sorption, the concentration of alkali in the fibre is increased and thus attacks the impurities more intensively.

Mechanism of Removal of Impurities

• The action of an alkaline scouring agent is to saponify any residual oils, neutralise carboxylic acids, solubilise any sizing materials and cause dispersion of naturally occurring impurities in natural fibres.

               Techniques for Removing Natural Impurities of Cotton during Scouring

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