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MANUFACTURING OF NONWOVENS FOR MEDICAL APPLICATIONS by Bhala

INTRODUCTION

Now woven plays a major role in the medical field. The manufacturing process of non woven involves three types. They are mechanical bonding, thermal bonding and binder bonding. Mechanical bonding includes needle punching and hydro entanglement. Thermal bonding includes hot air bonding and calendar roll bonding. Binder bonding is produced by means of coating techniques. Then all the process is taken to the finishing treatment. In the finishing treatment heat setting, printing and dyeing is carried out.

MANUFACTURING PROCESS

â€SPUN BONDING

SPINNING AND WEB FORMATION

Spunbond fabrics are produced by depositing extruded, spun filaments onto a collecting belt in a uniform random manner followed by bonding the fibers. The fibers are separated during the web laying process by air jets or electrostatic charges. The collecting surface is usually perforated to prevent the air stream from deflecting and carrying the fibers in an uncontrolled manner. Bonding imparts strength and integrity to the web by applying heated rolls or hot needles to partially melt the polymer and fuse the fibers together. Since molecular orientation increases the melting point, fibers that are not highly drawn can be used as thermal binding fibers. Polyethylene or random ethylene-propylene copolymers are used as low melting bonding sites. Spunbond products are employed in carpet backing, geotextiles, and disposable medical/hygiene products. Since the fabric production is combined with fiber production, the process is generally more economical than when using staple fiber to make nonwoven fabrics.

Spunbonding combines fiber spinning with web formation by placing the bonding device in line with spinning. In some arrangements the web is bonded in a separate step which, at first glance, appears to be less efficient. However, this arrangement is more flexible if more than one type of bonding is applied to the same web. The spinning process is similar to the production of continuous filament yarns and utilizes similar extruder conditions for a given polymer. Fibers are formed as the molten polymer exits the spinnerets and is quenched by cool air. The objective of the process is to produce a wide web and, therefore, many spinnerets are placed side by side to generate sufficient fibers across the total width. The grouping of spinnerets is often called a block or bank. In commercial production two or more blocks are used in tandem in order to increase the coverage of fibers.
Before deposition on a moving belt or screen, the output of a spinneret usually consists of a hundred or more individual filaments which must be attenuated to orient molecular chains within the fibers to increase fiber strength and decrease extensibility. This is accomplished by rapidly stretching the plastic fibers immediately after exiting the spinneret. In practice the fibers are accelerated either mechanically or pneumatically. In most processes the fibers are pneumatically accelerated in multiple filament bundles; however, other arrangements have been described where a linearly aligned row or rows of individual filaments is pneumatically accelerated.
In traditional textile spinning some orientation of fibers is achieved by winding the filaments at a rate of approximately 3,200 m/min to produce partially oriented yarns (POY). The POYs can be mechanically drawn in a separate step for enhancing strength. In spunbond production filament bundles are partially oriented by pneumatic acceleration speeds of 6,000 m/min or higher. Such high speeds result in partial orientation and high rates of web formation, particularly for lightweight structures (17 g/m2). The formation of wide webs at high speeds is a highly productive operation.
For many applications, partial orientation sufficiently increases strength and decreases extensibility to give a functional fabric (examples: diaper coverstock). However, some applications, such as primary carpet backing, require filaments with very high tensile strength and low degree of extension. For such application, the filaments are drawn over heated rolls with a typical draw ratio of 3.5:1. The filaments are then pneumatically accelerated onto a moving belt or screen. This process is slower, but gives stronger webs.
The web is formed by the pneumatic deposition of the filament bundles onto the moving belt. A pneumatic gun uses high-pressure air to move the filaments through a constricted area of lower pressure, but higher velocity as in a venturi tube. In order for the web to achieve maximum uniformity and cover, individual filaments must be separated before reaching the belt. This is accomplished by inducing an electrostatic charge onto the bundle while under tension and before deposition. The charge may be induced triboelectrically or by applying a high voltage charge. The former is a result of rubbing the filaments against a grounded, conductive surface. The electrostatic charge on the filaments must be at least 30,000 esu/ m2.

The belt is usually made of an electrically grounded conductive wire. Upon deposition, the belt discharges the filaments. This method is simple and reliable. Webs produced by spinning linearly arranged filaments through a so-called slot die eliminating the need for such bundle separating devices.
Filaments are also separated by mechanical or aerodynamic forces. The figure below illustrates a method that utilizes a rotating deflector plane to separate the filaments by depositing them in overlapping loops; suction holds the fiber mass in place.
For some applications, the filaments are laid down randomly with respect to the direction of the lay down belt. In order to achieve a particular characteristic in the final fabric, the directionality of the splayed filament is controlled by traversing the filament bundles mechanically or aerodynamically as they move toward the collecting belt. In the aerodynamic method, alternating pulses of air are supplied on either side of the filaments as they emerge from the pneumatic jet.
By proper arrangement of the spinneret blocks and the jets, lay down can be achieved predominantly in the desired direction. The production of a web with predominantly machine direction and cross-machine direction filament lay down is shown in the figure below. Highly ordered cross-lapped patterns can be generated by oscillating filament bundles, as shown.
If the lay down belt is moving and filaments are being rapidly traversed across this direction of motion, the filaments are being deposited in a zig-zag or sine-wave pattern on the surface of the moving belt. The effect of the traverse motion on the coverage and uniformity of the web has been treated mathematically. The result is that relationships between the collecting belt speed, period of traverse, and the width of filament curtain being traversed determine the appearance of the formed web. The following illustration shows the lay-down for a process where the collecting belt travels a distance equal to the width of the filament curtain x during one complete period of traverse across a belt width y. If the belt speed is Vb and the traverse speed is Vt, the number of layers deposited, z, is calculated by z = [x Vt/y Vb]. If the traverse speed is twice the belt speed and if x and y are equal, a double coverage occurs over all areas of the belt.

BONDING

Many methods can be used to bond the fibers in the spun web. Although most procedures were developed for nonwoven staple fibers, they have been successfully adapted for continuous filaments. These include mechanical needling, thermal bonding, and chemical bonding. The last two may bond large regions (area bonding) or small regions (point bonding) of the web by fusion or adhesion of fibers. Point bonding results in the fusion of fibers at points, with fibers between the point bonds remaining relatively free. Other methods used with staple fiber webs, but not routinely with continuous filament webs include stitch bonding, ultrasonic fusing, and hydraulic entanglement. The last method has the potential to produce very different continuous filament structures, but is more complex and expensive. The choice of a particular bonding technique is dictated mainly by the ultimate fabric applications; occasionally a combination of two or more techniques is employed to achieve bonding. Some few spun bond process systems used with appropriate modification are mentioned below:
1.â€DOCAN SYSTEMâ€
2.â€REICOFIL SYSTEMâ€
3.â€LUTRAVIL SYSTEMâ€

â€MELTBLOWING
Melt blowing (MB) 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 MB process is one of the newer and least developed nonwoven processes. This process is unique because it is used almost exclusively to produce micro fibers rather than fibers the size of normal textile fibers. MB micro fibers generally have diameters in the range of 2 to 4 m, although they may be as small as 0.1 m and as large as 10 to 15 m. Differences between MB 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.

PROCESSING

The most commonly accepted and current definition for the MB 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 fiberous and self-bonding web.
The MB process is similar to the spunbond (SB) process which converts resins to nonwoven fabrics in a single integrated process. The schematic of the process is shown MB in figure. A typical MB process consists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.

EXTRUDER
The extruder is one of the important elements in all polymer processing. It consists of a heated barrel with a rotating screw inside as shown in figure. Its main function is to melt the polymer pellets or granules and feed them to the next step/element. The forward movement of the pellets in the extruder is along the hot walls of the barrel between the flights of the screw. The melting of the pellets in the extruder is due to the heat and friction of the viscous flow and the mechanical action between the screw and the walls of the barrel. There are four different heaters in the extruder, which are set in incremental order. The extruder is divided in to three different zones:

Feed Zone: In the feed zone the polymer pellets are preheated and pushed to the next zone.

Transition Zone: The transition zone has a decreasing depth channel in order to compress and homogenize the melted polymer.

i)Metering Zone: This is the last zone in the extruder whose main purpose is to generate maximum pressure in order to pump the molten polymer in the forward direction. At this point the breaker plate controls the pressure generated with a screen pack placed near to the screw discharge. The breaker plate also filters out any impurities such as dirt, foreign particle metal particles and melted polymer lumps.

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).

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.

i) 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.

ii) 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 die's nosepiece, smaller orifices are usually employed compared to those generally used in either fiber spinning or spun bond 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 4. 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.

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 2l5C to 340C.

iii) 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 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 are shown in Figure 5. Typical air temperatures range from 230C to 360C at velocities of 0.5 to 0.8 the speed of sound.

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. 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.

WINDING

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.

BONDING

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.

â€SPUNLACING

The oldest technique for consolidating a web is mechanical bonding, which entangles the fibers to give strength to the web. Under mechanical bonding, the two most widely used methods are needlepunching and spunlacing(hydroentanglement) .
Hydroentanglement uses high-speed jets of water to strike a web so that the fibers knot about one another. As a result, nonwoven fabrics made by this method have specific properties, as soft handle and drapability. Japan is the major producer of hydroentangled nonwovens in the world. In 1995,the output of spunlaced fabrics containing cotton was 3,700 metric tons and a significant growth in production can still be seen The biggest producers of spunlaced fabrics in the U.S. are DuPont, Chicopee and Kendall corporations.
This technology was officially introduced by DuPont in 1973 (Sontara) and is a result of considerable work done by DuPont and Chicopee (DuPont obtained five patents on spunlaced nonwovens within the 1963-1970 period. Since the 1990's, the technology has been made more efficient and affordable for more manufacturers. Though a vast majority of hydroentangled fabrics have incorporated dry-laid webs (carded or air-laid webs as precursors), this has changed very recently with an increase in wet-laid precursor webs. This is because of Dexter making use of Unicharm's technology to make spunlaced fabrics using wet-laid fabrics as precursors.

TYPES OF SPUNLACING PROCESS
1.Flat Lapping Spunlace
2.Cross Lapping Spunlace
3.Flat Style Spunlace
4.Mesh Style Spunlace
5.One Colour Printing Spunlace
6.Multicolour Printing Spunlace
7.Embossing Spunlace
8.Embroidery Spunlace
9.Coating Spunlace
10.Micro Fiber Spunlace
11.Woodpulp Bond Spunlace

â€THERMAL BONDING

Thermo bond nonwovens have a process that heating, ausform, melt, fl

Article Source: http://www.earticlesonline.com/Article/MANUFACTURING-OF-NONWOVENS-FOR-MEDICAL-APPLICATIONS/420955

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