The Meltblowing Process

The concept of meltblowing was first demonstrated in 1954 by Van A. Wente, of the Naval Research Laboratories, who was interested in developing fine fibers to collect radioactive particles in the upper atmosphere to monitor worldwide testing of nuclear weapons. In this process, an extruder forced a molten polymer through a row of fine orifices directly into two converging high-velocity streams of heated air or other gas. It was claimed that fibers as small as 0.1 to 1 |im can be formed by this method [39,40]. In the late 1960s and early 1970s, Exxon Research launched the first semiworks line, licensing the technology and providing the name for a new process: "meltblowing" Thus, Exxon became the first to demonstrate, patent, publicize, and license the use of Wente's concept as a very practical one-step process to produce unique types of nonwoven webs. Early successful licensees included Kimberly-Clark, Johnson & Johnson, James River, Web Dynamics, and Ergon Nonwovens, followed by many other companies [41].

Among the meltblowing equipment developers, Accurate Products Co. was the first to successfully build a 40-in meltblowing die [42], and Reifenhauser was the first to significantly improve the meltblow-ing die design [43] 3M built equipment to produce high-temperature stable nonwoven webs based on multilayer blown microfibers [44]. Biax FiberFilm has designed meltblowing equipment that uses multiple rows of orifices to provide higher productivity [45,46], and Kimberly-Clark has patented a slot die for meltblowing to minimize orifice plugging [47]. Chisso Corporation developed the equipment to produce conjugate meltblown I/S [48] and side-by-side [49] web types. Kimberly-Clark patented the process to produce an in-line perturbation of the flow of the fluid (air) to make crimped or uncrimped fibers at reduced energy costs [50], and Mitsui Petrochemical Industries obtained patents on the use of capillary meltblowing dies [51,52]. Fiber Web North America Inc. patented a process to produce subdenier and micro-denier fibers utilizing multicomponent dies to produce continuous, easily splittable hollow fibers of low orientation [53]. This became an alternative process to make microfibers. The use of modular dies to produce a mixture of fibers in a range 0.5 to 1 |im was patented in 2000 by Fabbricante et al. [16].

Considerable efforts have been made in the last 30 years on the process study. For example, the study of the influence of the airflow rate on the fiber diameter showed that significantly smaller fiber diameter is observed at higher flow rates under the same polymer throughput [15,54]. It was also shown that the fiber attenuation occurred mostly in the first 5 cm from the die [15]. The model of a steady-state meltblowing process, which showed a rapid decrease of the filament temperature and rapid increase of filament elongational viscosity within the first 5 cm from the die, was developed as well [55]. However, the most advanced development is a new bicomponent meltblown technology [14,15,48,49,56,57].

The initial application of the meltblown web was as a battery separator [58-61]. Other uses include face masks, respirators, and filter media. The replacement of glass fibers with meltblown fibers in face masks and respirators was initiated by Johnson & Johnson in the early 1970s, and it is now essentially complete [41]. One of the earliest filtration markets targeted for the meltblown webs was cigarette filters, and a sheath-core bicomponent web was developed for such use [62]. However, despite these early activities, commercial success of the meltblown webs for cigarette filters has not been achieved to date [41]. Meltblowing technology is successfully used for air and liquid filtration applications [63-67]. FiberWeb North America Inc. patented the production of fine web as filtration media for disposable medical products [68]. Kimberly-Clark was one of the first companies to meltblow elastomeric materials for potential use in medical products [41]. More recently, the meltblowing process has been used to form meltblown adhesive filaments for bonding substrates in the production of a variety of bodily fluid absorbing hygienic articles, such as disposable diapers and incontinence pads, sanitary napkins, patient underlays, wipes, and surgical dressings [69]. Meltblown web is used for artificial leather application as well [41].

Meltblowing is an extrusion technology that produces fiber webs directly from a polymer. The schematic of the meltblowing process is presented in Figure 21.1.

A thermoplastic fiber-forming polymer is extruded through a linear die containing closely arranged small orifices. The extruded filaments are attenuated by two convergent streams of high-velocity hot air to form fine fibers. After the polymer threads are attenuated by hot air, the resulting fibers are expanded into the free air of room temperature. Due to the mixture of high-speed hot air and fibers with ambient

Air knife

Hot air

Air knife

Air knife

Room air

Fibers

Conveyor belt

FIGURE 21.1 Schematic representation of the meltblowing process.

air, the fiber bundles start their movements forward and backward (Figure 21.1). These movements help to stretch the filaments even more due to the so-called "form drag." This form drag appears with every change in fiber direction and leads to a variability in the meltblown fiber diameter.

Generally, fiber attenuation is achieved by three different forces: aerodynamic drag near the die, aerodynamic drag near the collector, and the fiber elongation due to fiber vibration movements along spinline. However most of the attenuation is appearing near the die, as it has been reported by Bresee and Ko [70]. An additional function of hot air streams is to transport the fibers to a collector (conveyor belt), where they self-bond at the contact points [14,70].

Filaments produced by the meltblowing process have generally low or no molecular orientation. Also fibers do not often crystallize until reaching the collector, as it has been reported by Bresee and Ko [70]. The processing conditions influencing the final properties of the meltblown fibers and webs include: the melt temperature, the throughput, the die geometry, the airflow rate and its temperature, the die-to-collector distance, and collector speed [15,54]. By varying any of these input parameters final properties of fibers, such as the cross-sectional shape, the diameter, morphology, and the web structure can be changed.

There are three distinct regimes for the meltblowing process [41]. The most common is a very-high airflow rate regime that allows production of fibers in the range from 2 to 5 ||m. This regime is in current commercial use. An ultra-high flow rate regime allows production of ultrafine fibers with a diameter of less than 1 |im. Although fibers as small as 0.1 |im can be produced via this regime, it is still under development [71]. A low airflow rate regime produces 1 denier (approximately 10 |im) and larger fibers [41]. The effort to produce sub-micron fibers by using splittable cross-sectional fiber morphology in the meltblowing process was made; however, the smallest achieved fiber diameter was generally in the range of 1 to 2 |lm [53,72].

The fine fibers of the conventional meltblowing process result in a soft, self-bonded fabric having excellent covering power and opacity. Because of the fineness and tremendous number of fibers comprising these webs, the meltblown nonwovens can develop significant bonding strength through fiber entanglements. Also, meltblown fiberwebs are characterized by their high surface area per unit weight and fine porosity [41]. Nevertheless, meltblowing has few drawbacks. Only low viscosity materials could be spun into meltblown webs to avoid excessive polymer swelling upon the exit of the spinneret. It is estimated that over 90% of all meltblown nonwovens are made of PP, with the melt flow rate ranging from 1000 to 1500 g/10 min [56]. The inability of using different polymers limits many potential applications of the meltblown webs. Another disadvantage is low strength of the meltblown fibers, caused by little or no fiber molecular orientation and low molecular weight of the used polymers [70]. Like electrospun nanofibers, meltblown fibers typically need a supporting structure and are generally employed in a composite structure [7]. This allows for the meltblown web to optimize its filtration properties; it, however, becomes an expensive way to meet the customer's needs and adds complexity to the manufacturing process. The brittleness of meltblown nonwovens causes difficulties with their downstream processing as well. These fabrics are difficult to dye and incorporate into other nonwoven filter media structures, such as carded, air-laid, needle-punched, or wet-laid composites. Finally, meltblown webs typically exhibit broad fiber diameter distributions that can be inappropriate for some applications [54].

Even though fibers with diameters less than 1 | m can be made through the meltblowing process, the mean diameters of meltblown fibers are much larger than those of electrospun fibers. They are typically in the range of from 2 to 10 |m [7,15,73]. There have been various attempts to reduce the diameter of the meltblown fibers. One example of such attempts includes reducing the polymer throughput or the orifice diameter. However, this direct controlling approach limits the productivity of the process extremely. A few years ago, another method of production of polymeric fine fibers was introduced. In this technique, fibers are created by meltblowing with a modular die [16]. This approach allows the production of a mixture of fibers in the range of 0.5 to 1 |m. Nevertheless, from the abovementioned, it follows that the meltblowing process is applicable for the production of microfibers rather than nanofibers.

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