Melt-blown process is a one-step process in which high-velocity air blows a molten thermoplastic resin from an extruder die tip onto a conveyor or take-up screen to form a fine fibrous and self-bonding web. The fibres in the melt blown web are laid together by a combination of entanglement and cohesive sticking. The ability to form a web directly from a molten polymer without controlled stretching gives melt blown technology a distinct cost advantage over other systems. Melt blown webs offer a wide range of product characteristics such as random fibre orientation, low to moderate web strength. About 40% of melt blown material is used in the uncombined (monolithic) state. The remainder of melt blown materials are composites or laminates of melt blown webs with another material or nonwoven. The largest end-uses for monolithic melt blown materials are oilsorbents, air and liquid filtration media.
The melt blown process is a unique among nonwoven systems. Fundamentally it is a high tech version of the making of cotton candy at amusement parks. In making cotton candy, sugar is melted and pressure fed through small openings in a rotary spinning wheel. Upon exciting, the molten sugar congeals and is stretched into short fibres by centrifugal forces. In the melt blown system, molten polymers are forced through small slit openings and high temperature (230oC-390oC) air is impinged (300-500 miles/hr) at both sides of the exiting film. The fast-moving air streams effectively stretch or attenuate the molten polymer by multiple orders of magnitude and solidify it into a random array of discontinuous subdenier fibers. The fibers are then condensed (separated from the air stream) as a randomly entangled web and compressed between heated rolls. This paper deals with manufacturing technology, type of polymers used, web characteristics, properties and its various applications.
1. Process Description
A typical melt blowing process consists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.
The polymer pellets or granules are fed into the extruder hopper. Gravity feed supplies pellets to the screw, which rotates within the heated barrel. The pellets are conveyed forward along hot walls of the barrel between the flights of the screw, as shown in Figure 2. As the polymer moves along the barrel, it melts due to the heat and friction of viscous flow and the mechanical action between the screw and barrel. The screw is divided into feed, transition, and metering zones. The feed zone preheats the polymer pellets in a deep screw channel and conveys them to the transition zone. The transition zone has a decreasing depth channel in order to compress and homogenize the melting polymer. The molten polymer is discharged to the metering zone, which serves to generate maximum pressure for extrusion. The pressure of molten polymer is highest at this point and is controlled by the breaker plate with a screen pack placed near the screw discharge. The screen pack and breaker plate also filter out dirt and infused polymer lumps. The pressurized molten polymer is then conveyed to the metering pump.
1.2. Metering pump
The metering pump is a positive-displacement and constant-volume device for uniform melt delivery to the die assembly. It ensures consistent flow of clean polymer mix under process variations in viscosity, pressure, and temperature. The metering pump also provides polymer metering and the required process pressure. The metering pump typically has two intermeshing and counter-rotating toothed gears. The positive displacement is accomplished by filling each gear tooth with polymer on the suction side of the pump and carrying the polymer around to the pump discharge, as shown in Figure 2. The molten polymer from the gear pump goes to the feed distribution system to provide uniform flow to the die nosepiece in the die assembly (or fiber forming assembly).
1.3. Die Assembly
The die assembly is the most important element of the melt blown process. It has three distinct components: polymer-feed distribution, die nosepiece, and air manifolds.
1.3.1. Feed Distribution
The feed distribution in a melt-blown die is more critical than in a film or sheeting die for two reasons. First, the melt-blown die usually has no mechanical adjustments to compensate for variations in polymer flow across the die width. Second, the process is often operated in a temperature range where thermal breakdown of polymers proceeds rapidly. The feed distribution is usually designed in such a way that the polymer distribution is less dependent on the shear properties of the polymer. This feature allows the melt blowing of widely different polymeric materials with one distribution system. The feed distribution balances both the flow and the residence time across the width of the die. There are basically two types of feed distribution that have been employed in the melt-blown die: T-type (tapered and untapered) and coat hanger type. Presently, the coathanger type feed distribution is widely used because it gives both even polymer flow and even residence time across the full width of the die.
1.3.2. Die Nosepiece
From the feed distribution channel the polymer melt goes directly to the die nosepiece. The web uniformity hinges largely on the design and fabrication of the nosepiece. Therefore, the die nosepiece in the melt blowing process requires very tight tolerances, which have made their fabrication very costly. The die nosepiece is a wide, hollow, and tapered piece of metal having several hundred orifices or holes across the width. The polymer melt is extruded from these holes to form filament strands which are subsequently attenuated by hot air to form fine fibers. In a dies nosepiece, smaller orifices are usually employed compared to those generally used in either fiber spinning or spunbond processes. A typical die nosepiece has approximately 0.4-mm diameter orifices spaced at 1 to 4 per millimeters (25 to 100 per inch). There are two types of die nosepiece used: capillary type and drilled hole type. For the capillary type, the individual orifices are actually slots that are milled into a flat surface and then matched with identical slots milled into a mating surface. The two halves are then matched and carefully aligned to form a row of openings or holes as shown in Figure 3. By using the capillary type, the problems associated with precise drilling of very small holes are avoided. In addition, the capillary tubes can be precisely aligned so that the holes follow a straight line accurately. The drilled-hole type has very small holes drilled by mechanical drilling or electric discharge matching (EDM) in a single block of metal, as shown in Figure 3. During processing, the whole die assembly is heated section-wise using external heaters to attain desired processing temperatures. It is important to monitor the die temperatures closely in order to produce uniform webs. Typical die temperatures range from 2l5oC to 340OC.
1.3.3. Air Manifolds
The air manifolds supply the high velocity hot air (also called as primary air) through the slots on the top and bottom sides of the die nosepiece, as shown in Figure 4. The high velocity air is generated using an air compressor. The compressed air is passed through a heat exchange unit such as an electrical or gas heated furnace, to heat the air to desired processing temperatures. They exits from the top and bottom sides of the die through narrow air gaps, as shown in Figure 4. Typical air temperatures range from 230oC to 360oC at velocities of 0.5 to 0.8 the speed of sound.
1.4. Web Formation
As soon as the molten polymer is extruded from the die holes, high velocity hot air streams (exiting from the top and bottom sides of the die nosepiece) attenuate the polymer streams to form microfibers. As the hot air stream containing the microfibers progresses toward the collector screen, it draws in a large amount of surrounding air (also called secondary air) that cools and solidifies the fibers, as shown in Figure 4. The solidified fibers subsequently get laid randomly onto the collecting screen, forming a self-bonded nonwoven web. The fibers are generally laid randomly (and also highly entangled) because of the turbulence in the air stream, but there is a small bias in the machine direction due to some directionality imparted by the moving collector. The collector speed and the collector distance from the die nosepiece can be varied to produce a variety of melt-blown webs. Usually, a vacuum is applied to the inside of the collector screen to withdraw the hot air and enhance the fiber laying process.
The melt-blown web is usually wound onto a cardboard core and processed further according to the end-use requirement. The combination of fiber entanglement and fiber-to-fiber bonding generally produce enough web cohesion so that the web can be readily used without further bonding. However, additional bonding and
finishing processes may further be applied to these melt-blown webs.
Additional bonding, over the fiber adhesion and fiber entanglement that occurs at lay down, is employed to alter web characteristics. Thermal bonding is the most commonly used technique. The bonding can be either overall (area bonding) or spot (pattern bonding). Bonding is usually used to increase web strength and abrasion resistance. As the bonding level increases, the web becomes stiffer and less fabriclike.
Although most nonwovens are considered finished when they are rolled up at the end of the production line, many receive additional chemical or physical treatment such as calendering, embossing, and flame retardance. Some of these treatments can be applied during production, while others must be applied in separate finishing operations.
Other Processes for Melt blown Production of Non-woven.
2. Process Variables
The process variables can be divided into two categories : a) operational variables and b)material variables. By manipulating operational and material variables one can produce a variety of meltblown fabrics with desired properties. Each of these variables play a significant role in process economics and product reliability.
2.1. Operational variables
This can be classified as: a) on-line and b) off-line variables. The on-line variables are those which can be changed according to requirements during production. The polymer throughput rate, air throughput rate, polymer/die temperatures, air temperature and die-to-collector distance are the five basic on-line operational variables. These variables are easy to change and dictate the major fabric properties.
The polymer and air throughput rates basically control the final fiber diameter, fiber entanglements and the extent of zone of attenuation.
The polymer/die and air temperatures in conjunction with air flow rate affect the appearance and hand of the fabric, fabric uniformity and fabric defects in the production.
The die-to-collector distance generally affects the openness of the fabric and fiber-to-fiber thermal bonding. The fiber diameter increases slightly with increase in die-to-collector distance.
The offline processing variables are those variables which can only be changed when production line is not in operation, such as die hole size, die set-back, air gap, air angle, web collection type and polymer/air distribution. Most of the offline process variables are set for particular product line.
The die hole size, die design parameters and die set-back are believed to affect the fiber size. The die hole size should be large enough to pass the polymer melt without plugging. The back pressure generated by the polymer feed distribution typeand the ratio of orifice length to diameter should be high enough to give good
distribution and controllable flow of the polymer melt.
The air gap affects the air exit pressure and is also believed to affect the degree of fiber breakage.
2.2. Material Variables
The material variables include the polymer type, molecular weight, molecularweight weight distribution, polymer additives, polymer degradation and polymer forms.
Basically, any fiber-forming polymer that can give an acceptably low-melt viscosity at a suitably processing temperature and can solidify before landing on the collector screen ca be melt blown into fine-fibered webs. Some of the processed polymers are :
Polypropylene is easy to process and makes good web.
Polyethylene is more difficult to melt-blow into fine fibrous webs than is polypropylene. Polyethylen is difficult to draw because of its melt elasticity.
PBT processes easily and produces very soft, fine-fibered webs.
Nylon 6 is easy to process and makes good webs.
Nylon 11 melt-blows well into webs that have very unusual leather like feel.
Polycarbonate produces very soft-fiber webs.
Polystyrene produces an extremely soft, fluffy material with essentially no shot defects.
Usually polymer in pellet form is used for processing, but the present trend is to use polymer granules. It is beloved that the granules melt faster and give a more even heat distribution.
The melt blown process requires low molecular weight and narrow molecular weight distribution resins to produce uniform fine-fibered webs.
3. Web Characteristics and Properties
The uniformity of the web is controlled by two important parameters: uniform distribution of fiber in the air stream and proper adjustment of the vacuum level under the forming wire or belt. Non-uniform distribution of fiber in the air stream can result from poor die design and from non-uniform ambient airflow into the air stream. The vacuum under the forming media should be adjusted to pull the total air stream through the media and lock the fibers in place. Generally, the closer the die is to the forming drum or belt, the better the web uniformity.
5.1. Filtration media
This market segment continues to be the largest single application. The best known application is the surgical face mask filter media. The applications include both liquid filtration and gaseous filtration. Some of them are found in cartridge filters, clean room filters and others.
5.2. Medical fabrics
The second largest meltblown market is in medical/surgical applications. The major segments are disposable gown and drape market and sterilization wrap segment.
5.3. Sanitary products
Meltblown products are used in two types of sanitary protection products feminine sanitary napkin and disposable adult incontinence absorbent products.
5.4. Oil adsorbents
Melt blown materials in variety of physical forms are designed to pick up oily materials. The best known application is the use of sorbents to pick up oil from the surface of water, such as encountered in an accidental oil spill.
The apparel applications of melt-blown products fall into three market segments: thermal insulation, disposable industrial apparel and substrate for synthetic leather. The thermal insulation applications takes advantage of microvoids in the structure filled with quiescent air, resulting in excellent thermal insulation.
5.6. Hot-melt adhesives
The melt-blown process has a special feature: it can handle almost any type of thermoplastic material. Thus, the task of formulating a hot-melt adhesive to provide specific properties can be greatly simplified by using the melt blown system to form the final uniform adhesive web.
5.7. Electronic specialties
Two major applications exist in the electronics specialties market for melt blown webs. One is as the liner fabric in computer floppy disks and the other as battery separators and as insulation in capacitors.
5.8. Miscellaneous applications
Interesting applications in this segment are manufacture of tents and elastomeric nonwoven fabrics which have the same appearance as continuous filament spunbonded products.
The melt blown technique for making nonwoven products has been forecast in recent years as one of the fastest-growing in the nonwovens industry. With the current expansion and interest, it cannot be questioned that meltblown is well on its way to becoming one of the major nonwoven technologies. Technical developments are also on the horizon that will increase the scope and utility of this technology. The application of speciality polymer structures will no doubt offer new nonwoven materials unobtainable by other competitive technologies. So a strong and bright future be forecasted for this technology.
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