Module : Polymer-EXTRUSION based Technologies
The meltblown technology is based on meltblowing process, where, usually, a thermoplastic fiber forming polymer is extruded through a linear die containing several hundred small orifices. Convergent streams of hot air (exiting from the top and bottom sides of the die nosepiece) rapidly attenuate the extruded polymer streams to form extremely fine diameter fibers (1–5 micrometer). The attenuated fibers are subsequently blown by high-velocity air onto a collector conveyor, thus forming a fine fibered self-bonded meltblown nonwoven fabric.（Click to share to LinkedIn）
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 fiber webs than polypropylene, but polyamide 6 is easier to process and has less tendency to make shot (particles of polymers that are larger than fibers) 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 Fiber nonwoven fabric. The schematic diagram of this technology is shown in Figure 1. It works as per the following sequence.（Click to share to Facebook）
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.
The preparation of the polymers for extrusion in meltblown technology is the same as that in spunbond technology. The e xtruder for melt blown technology is longer L/D (30+) so that more external heating surface is available. The energy for melting comes mostly from barrel heating and practically no viscous shear heating when high MFR resins are used. Also, the longer extruder can achieve a higher output rate and better melt homogeneity than the shorter extruder. Further, longer extruder offers good barrel support and allowance for thermal expansion due to high screw speed and high barrel temperature. The extruder should be able to provide heating and cooling. Air cooling for barrel zones is usually sufficient for melt blown technology. High watt density heaters are desirable, especially at the first half of the 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.（try Suntech ST-AMB Meltblown machine）
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 m 3/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-fiber film” 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 entrapment 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 w ater 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 b etter 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.
Owing to the smaller fibers and larger surface area occupied by the fibers the meltblown nonwovens offer enhanced filtration efficiency, good barrier property, and good wicking property. They are finding applications in filtration, insulation, and liquid absorption.
Spunbond versus Meltblown
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 compressed hot air. The meltblown nonwoven is generally found to be costlier than the spubnnond nonwoven.
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