Melt Blown Polymers
Melt blowing 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. The melt-blown process is one of the newer and least developed nonwoven processes. This process is unique because it is used almost exclusively to produce microfibers rather than fibers the size of normal textile fibers. Melt-blown microfibers generally have diameters in the range of 2 to 4 mm, although they may be as small as 0.1mm and as large as 10 to 15mm. Differences between melt-blown nonwoven fabrics and other nonwoven fabrics, such as degree of softness, cover or opacity, and porosity can generally be traced to differences in filament size. The basic technology to produce these microfibers was first developed under U.S. government sponsorship in the early 1950s. The Naval Research Laboratory initiated this work to produce micro filters for the collection of radioactive particles in the upper atmosphere. The most commonly accepted and current definition for the 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 melt-blown process is similar to the spunbond process in that both convert resins to nonwoven fabrics in a single integrated process. The schematic of the melt blowing process is shown in Figure 1. A typical melt blowing process consists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.
Melt Blown System Process Flow Chart:
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 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, as shown in Figure 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
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 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).
|Fig: Metering and controlled reaction of EB 43-76 with PP in an extruder to form high quality melt blown resins.|
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.
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.
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 die's 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.
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 5. 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. The exits from the top and bottom sides of the die through narrow air gaps, as shown in Figure 5. Typical air temperatures range from 230oC to 360oC at velocities of 0.5 to 0.8 the speed of sound.
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 microfibres. As the hot air stream containing the microfibres progresses toward the collector screen, it draws in a large amount of surrounding air (also called secondary air) that cools and solidifies the fibers. 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 fabric-like.
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 calendaring, embossing, and flame retardance. Some of these treatments can be applied during production, while others must be applied in separate finishing operations.
Process variables can be classified into two categories: operational or on-line variables and off-line variables. The on line variables include: polymer and its throughput, polymer/die and air temperature, die-to-collector distance, and quench environments.
The off-line variable includes: hole size, die setback, air gap, air angle, web collection type, and polymer/air distribution.
The following represent some of the variables that must be controlled during melt blown production.
- Polymer type
- Polymer characteristics: molecular weight, melt viscosity, melt strength
- Extruder conditions: temperature, shear, polymer degradation
- Die tip geometry: hole diameter, air gap, die tip position
- Hot air conditions: volume, temperature, velocity
- Polymer conditions: temperature, flow rate, shear rate
- Die conditions: temperature profile, gas flow rate profile, polymer flow rate profile
- Ambient air conditions: temperature, lack of turbulence
- Distance from the die to the forming drum or belt
- Laydown conditions
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.
Melt-blown webs usually have a wide range of product characteristics. The main characteristics and properties of melt-blown webs are as follows:
- Random fiber orientation
- Lower to moderate web strength, deriving strength from mechanical entanglement and frictional forces
- Generally high opacity (having a high cover factor)
- Fiber diameter ranges from 0.5 to 30 : m, but typically from 2-7 : m
- Basis weight ranges from 8-350 g/m2 , but typically 20-200 g/m2
- Microfibers provide a high surface area for good insulation and filtration characteristics
- Fibers have a smooth surface texture and are circular in cross-section
- Most melt-blown webs are layered or shingled in structure, the number of layers increases with basis weight
Three of the major defects that occur in melt-blown production are roping, shot, and fly. Roping is caused by uncontrolled turbulence in the air-stream and by movement of fibers during and after lay down. The defect is observed as a narrow, elongated, thick streak in the web and resembles a slightly twisted "rope". Shot are small, spherical particles of polymer formed during the blowing operation. Shot are generally caused by excessively high temperatures or by too low a polymer molecular weight. Fly is a defect that does not go directly into the web, but instead contaminates the surrounding environment. Fly is composed of very short and very fine microfibres not trapped on the drum or belt during lay down. This can be caused by too violent blowing conditions.
The type of polymer or resin used will define the elasticity, softness, wetability, dyeability, chemical resistance and other related properties of formed fibers. One of the advantages of melt-blown technology is to handle many different polymers as well as mixture of polymers. Some polymers, which can be melt-blown, are listed below. However, the list is not complete.
- Polypropylene is easy to process and makes good web.
- Polyethylene is more difficult to melt-blow into fine fibrous webs than is polypropylene. Polyethylene 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.
- Poly (4-methyl pentene-1) blows well and produces very fluffy soft webs.
- Polystyrene produces an extremely soft, fluffy material with essentially no shot defects.
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.
The second largest melt blown market is in medical/surgical applications. The major segments are disposable gown and drape market and sterilization wrap segment.
Meltblown products are used in two types of sanitary protection products - feminine sanitary napkin and disposable adult incontinence absorbent products.
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 take advantage of microvoids in the structure filled with quiescent air, resulting in excellent thermal insulation.
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.
Interesting applications in this segment are manufacture of tents and elastomeric nonwoven fabrics, which have the same appearance as continuous filament spunbonded products.
Melt-blown Vs Spunbond:
The spunbond and melt-blown processes are somewhat identical from an equipment and operator's point of view. The two major differences between a typical melt-blown process and a spunbond process that uses air attenuation are: (1) the temperature and volume of the air used to attenuate the filaments and (2) the location where the filament draw or attenuation force is applied.
A melt-blown process uses large amounts of high-temperature air to attenuate the filaments. The air temperature is typically as high or higher than the temperature of the polymer. In contrast, the spunbond process generally uses a smaller volume of air close to ambient temperature to apply the attenuation force.
In the melt-blown process, the draw or attenuation force is applied at the die tip while the polymer is still in the molten state. Application of the force at this point is ideal for forming microfibers but does not allow for polymer orientation to build good physical properties. In the spunbond process, this force is applied at some distance from the die or spinneret, after the polymer has been cooled and solidified. Application of the force at this point provides the conditions necessary for polymer orientation and the resultant improved physical properties, but is not conductive to forming microfibers.
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.
The considerable work to modify the blowing step to something more akin to spraying is also going to have an impact on this technology and the products derived from it. So a strong and bright future be forecasted for this technology.
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Author of This Article:
DKTE’s Textile & Engineering
Institute, Ichalkaranji, India
(3rd yr Manmade textile technology)