Module 4: Polymer-extrusion based Technologies

The key process factors of spunbond nonwoven technology are polymer throughput rate, polymer melting temperature, quench air temperature, quench air velocity, and lay-down velocity. These process factors play important roles in deciding the morphology and diameter of the filaments which are the building block of any spunbond nonwovens. The bonding parameters are also important and their effects are already discussed earlier.

The polymer throughput rate determines the morphology and diameter of the filaments. The morphology of the filaments spun at lower throughput rate is better developed than those at higher throughput rate. Because the rhelogical conditions are more favorable for crystallinity and orientation of the filaments spun at lower throughput rate. The filaments spun at lower throughput rate are thus more stable than those spun at higher throughput rate. The filament diameter increases with increasing throughput rate.

The polymer melting temperature influences on the drawing of the filaments through the spinneret that in turn decides the diameter of the filaments. The lower polymer melting temperature results in increase in melt viscosity of the polymer that leads to difficulty in drawing of the filaments. On the other hand, the higher melting temperature results in decrease in the melt viscosity of the polymer that makes drawing easier. Too high polymer melting temperature can cause polymer degradation leading to filament breakages.

There is a great debate going on the effect of quench air temperature on the diameter and morphology of the filaments. One group of researcher argues that the lower quench air temperature results in increase of viscosity that leads to slower draw-down which finally resulting in higher filament diameter. As the draw-down takes place slowly, an increase in crystallinity and orientation is observed. The other group argues that lower quench air temperature is helpful in generating higher spinline stress that leads to reduction in filament diameter. As the draw-down takes places under higher stress, an increase in crystallinity and orientation is observed.

The quench air pressure has a role to decide filament diameter. Higher quench air pressure increases spinline draw ratio that in turn reduces filament diameter. The pressure drop is known to be proportional to air velocity.

The spunbond nonwovens are finding applications in a variety of end uses. Today they are used both for durable and disposable applications. The main applications for spunbond nonwovens are in automobiles, civil engineering, hygiene, medical, packaging, and agriculture.

2. 
   Meltblown technology
   

Polypropylene has been the most widely used polymer for meltblown technology. Besides, a variety of different polymers including polyamide, polyester, and polyethylene are used. It is known that polyethylene is more difficult to meltblow into fine fibre webs than polypropylene, but polyamide 6 is easier to process and has less tendency to make shot (particles of polymers that are larger than fibres) than polypropylene. In general, the requirements of polymers for meltblown technology are high MFR or MFI (300-1500 gm/10 min), low molecular weight, and narrow molecular weight distribution.

Meltblown technology converts polymeric resin to fine fibered nonwoven fabric. The schematic diagram of this technology is shown in Figure 4.4. It works as per the following sequence.

• 
  Prepares polymers for extrusion
  
• 
  Extrude low viscosity polymer melt through fine capillaries
  
• 
  Blow high velocity hot air to the molten polymer and attenuate the polymer melt
  
• 
  Cool the molten polymer by turbulent ambient air to form fine fiber
  
• 
  Deposit the fibers onto a collecting device to form useful articles like fabric.
  

Figure 4.4

he extruder. Extruder throat should be cooled to assist feeding and prevent melting when the extruder is shutdown. The design of the extruder screw must be such that a deeper feed section should be used for better feeding and it should have ability to receive granule and pellets. A shallower metering section is required for higher shear and better pumping. The compression ratio must be greater than 3.5. The screws with chrome platting with normal flight tip hardening are preferable. In the transition/melting section, the barrier flight can improve melting rate and melt quality.

For melt filtration, a screen changer down stream of extruder is must. Fine mesh screen (325 mesh screen) is recommended to remove undispersed pigment, carbonized materials, etc. A metering pump is needed to maintain a constant output rate, otherwise the extruder may not be able to provide sufficient pressure due to low melt viscosity. The pump inlet pressure is lower than the typical fiber spinning/melt blown process. This is important for maintaining product quality. A static melt mixer may be used at the entrance to the die Maintain good melt temperature homogeneity.

The compressed air has been the major cost component of the process. The lower the process pressure requirement, the lower the energy cost. The requirement for air is typically 500-1500 m³/h/m die width. Air handling around the process area is very important. The handling of the make up air and the exhaust air are important.

The die system is known to be one of the important components of meltblown technology. There are generally two die systems used. The “exxon” die system was developed in early 60’s by Exxon Chemical Company. It was a coat hanger die feeding a single row of capillaries and one piece die tip construction. There were 25-35 capillaries per inch of die width. The advantage of this system is that higher quality web can be produced, but the disadvantage is that the output per unit die width may be limited. The “biax-fibrefilm” die system has multiple rows of spinning nozzles and concentric air holes. There are around 200 capillaries per inch of die width up to 12 rows of capillaries. The advantage of this system is that higher output per unit die width may be obtained (higher hole density), but the disadvantage is that it is more challenging to maintain uniformity at each hole (air and polymer flow rate and temperature) and it results in broader fiber size distribution.

The design of air plate is also important. The air gap and set-back are adjustable. Typical air gap is around 0.5-1.0 mm. A smaller air gap results in higher air velocity, but lower air volume. A larger air gap results in lower air velocity, but higher air volume. The en trapment of secondary air for cooling is also required. Typical set-back is around 0.8-1.2 mm

Ambient air is often used for cooling. Auxiliary cooling devices are also used. Water spray or cold air is used. In water spray, multiple nozzles create a fine water mist. This has been an excellent way of quenching fibers. It can be used to incorporate hydrophilic agent. Its position must be as close to die tip as practical. The water must be sprayed uniformly across the whole width. It is however difficult to keep all nozzles clean and functioning and used mostly in sorbent product where high output is a must. The cold air works in the same principle as that of water spray. It is however less effective as compared to water spray.

While forming webs, the fibers are distributed (spread) on a moving belt or rotating drum. The suction underneath the forming web removes drawing air and holds the fiber to the web. The distance to forming web (die to collector distance) affects the web properties. The belt collector provides good fiber support and retention as well as good web release. It should have minimum wire mark onto the web and proper air flow (maximum air flow with minimum openings). The most of the belts are constructed of 100% polyester strand materials. The drum collector is generally used in small lines, less critical applications. Its advantages are simpler, lower cost, easier to operate, less space requirement, etc. But, its disadvantages are lying in the difficulty to dissipate heat (metal screen).

Key process factors

The key process parameters in meltblown technology are polymer melt temperature, polymer throughput rate, process (primary) air temperature, process (primary) air flow rate, and die-to-collector distance. The aforesaid process variables play important roles in deciding the morphology and diameter of the fibers which are the building block of the meltblown nonwovens.

Melt temperature controls the melt viscosity of polymer at die. To increase melt temperature, increase the die temperature, the extruder barrel temperatures (last 2-3 zones) and all zones between die and extruder. Melt temperature decreases with increasing screw speed /output rate, this needs to compensate for lower melt temperature by using a higher barrel temperature at high screw speed/output rate. Higher melt temperature results into finer fiber, more tendency to produce “shots”, higher energy cost (heating and cooling), shorter die tip life (degradation of pigment, polymer, etc.) The PP with 1500 MFR has a very low melt viscosity (< 10 pa-sec at normal processing temperature). The viscosity stays fairly constant over a wide range of shear rate (close to Newtonian fluid)

The polymer throughput rate can be increased by increasing the screw speed. Typical throughput rate is 0.2-0.8 g/hole/min for most of the applications (e.g. battery separator, filtration media, etc.), but for some other applications (e.g., wipe, oil sorbent, etc.), it varies from 0.8 g/hole/min to 3 g/hole/min. The output rate affects fiber size, the higher is the output rate, the more is the size of the fibers. With higher output rate, it is more difficult to achieve good quality web.

The process air temperature offers limited range. It is typically kept to be the same or slightly higher than die temperature (depending on thermocouple location). The lower air temperature results in better fiber cooling, less shots, whereas higher air temperature results in finer fiber diameter and more energy cost.

Primary airflow affects fiber entanglement significantly. Increasing primary airflow rate reduces fiber entanglement, especially when DCD is shorter. Fine fiber bundles are affected more than coarse fiber bundles when the primary airflow is changed. The influence of primary airflow rate on fiber entanglement is reduced at larger airflow rates. Increasing primary airflow rate generally increased global orientation of fibers in the machine direction. Increasing primary airflow rate reduces pore cover in the webs substantially. This is thought to occur because the increased airflow decreases fiber entanglement and reduces fiber diameter.

The die to collector distance plays an important role on the quality of meltblown nonwoven. The higher distance results in higher fiber entangling, bulkier and softer web, better fiber cooling, less tendency to disturb fiber lay down, less web uniformity, and is used for heavy basis weight fabric (sorbent products, etc.). The lower distance results in less fiber entangling, more compact/stiffer web, balance of process air and suction capability, more uniformed web with better barrier properties, and is used for light basis weight fabric, especially light weight spunmelt composites.

Applications

Owing to the smaller fibres and larger surface area occupied by the fibres the meltblown nonwovens offer enhanced filtration efficiency, good barrier property, and good wicking property. They are finding applications in filtration, insulation, and liquid absorption.

It is interesting to note the differences between the spunbond and meltblown technologies and products thereof. The meltblown technology requires polymers with considerably lower melt viscosity as compared to the spunbond technology. The initial investment for spunbond technology is three to four times higher than that for meltblown technology. The meltblown technology consumes more energy than the spunbond technology because of the usage of compressd hot air. The meltblown nonwoven is generally found to be costlier than the spubnnond nonwoven.

1. 
   Russel, S. J., Handbook of nonwovens, Woodhead Publishing Ltd., UK, 2007.
   
2. 
   Mastubara, Y., Polymer Engineering and Science, Volume 19, No. 3, pp. 169–172, 1979.
   
3. 
   Mastubara, Y., Polymer Engineering and Science, Volume 20, No. 3, pp. 212–214, 1980.
   
4. 
   Mastubara, Y., Polymer Engineering and Science, Volume 20, No. 11, pp. 716–719, 1980.
   
5. 
   Mastubara, Y., Polymer Engineering and Science, Volume 23, No., pp. 17–19, 1983.
   
6. 
   Hartmann, L., Textile Manufacturer, Volume 101, September, pp. 26, 29, and 30, 1974.