The production of nonwoven fabric by non-woven machine such as meltblown machine can be divided into three stages, and in some cases, all 3 stages can be carried out simultaneously.
Web formation
The manufacturing of nonwoven fabric begins with arranging fibers in sheets or webs. The fibers are short fibers or long filaments extruded from molten polymer particles.
(1) Dry-laid process
Carding is the mechanical process that begins with the fiber batt. These fibers are "opened" and mixed, then transported to the carding machine via air. The machine combs them into a web, which can be a rotating drum or a series of toothed rollers covered with carding wire (fine strips). The precise configuration of the carding machine will depend on the type of fiber and the basis weight of the production. The web can be laid parallel, where most fibers are oriented along the length, or randomly. A typical parallel-laid carded web forms good tensile strength, low elongation, and low tear strength when it is oriented in the opposite direction in the longitudinal and transverse directions.
(2) Short-fiber airlaid process
In the short-fiber airlaid process, shorter fibers are fed into a forming head by air flow. The forming head ensures even mixing of all fibers. The controlled portion of the fiber mixture is released from the forming head again through air and deposited onto a moving belt, where a random orientation web is formed. Compared to the carded web, the airlaid web has lower density, higher softness, and no layered structure. The airlaid fiber web offers great versatility in terms of fibers and fiber blends that can be used.
(3) Wet-laid process
The wet-laid process is similar to papermaking. The difference lies in the amount of synthetic fiber present in the wet-laid nonwoven fabric. A slurry of water and fiber is deposited on a moving screen, and the water is drained away, leaving the fiber to form a web. The fiber web is then further dried and consolidated between rollers. In the later stages of the process, impregnation with adhesive is typically included.
The strength of a randomly oriented web is similar in all directions in the fabric plane. A variety of natural, mineral, synthetic, and man-made fibers of various lengths can be used.
(4) Spunbond
Spunbond is a general term that describes the process of making nonwoven fiber webs directly from thermoplastic polymers. It includes both spinning and meltblown processes, which are often combined.
Polymer particles are extruded into continuous filaments through so-called spinnerets. The continuous filaments are then drawn and rapidly cooled before being deposited onto a conveyor belt to form a uniform fiber web. Spunbond nonwoven fabric has higher strength compared to carding. The disadvantage is that the selection of raw materials is more limited. Co-extrusion of two components can form fibers that can both enhance the performance of the fiber web and bond it with air permeability.
(5) Meltblown
Meltblown is similar to spinning, in which low viscosity polymers are essentially extruded. However, when the filaments leave the spinning plate, they are not quenched but are weakened by hot air flow, so that the filaments remain partially molten. This results in finer filaments with lower tensile strength. The filaments hit a belt or conveyor belt to form a web.
(6) Sub-micron filament spinning
Meltblowing produces fine filaments with a minimum diameter of 1 micrometer. For over a decade, researchers have been attempting to develop nonwoven fabrics with even finer fibers, which can improve the efficiency of filtration media or enhance barrier performance. The market's current technologies include sub-micron meltblowing, centrifugal spinning, solution spinning, and electrospinning. Despite lower productivity compared to spinning melt, these methods enable nonwoven fabrics to exhibit unique properties.
Web Binding
After the web is formed, the initial strength of the web is limited (depending on various bonding mechanisms). Therefore, the web needs to be merged in one way or another. The choice of web consolidation method largely depends on the desired functional characteristics and the type of fiber used. There are three basic types of binding:
(1) Thermal bonding
This method utilizes the thermoplastic properties of some synthetic fibers to form a bond under controlled heating. In some cases, the web fibers themselves can be used, but more commonly, low-melting point fibers or bicomponent fibers are introduced during the web forming stage to perform the bonding function later in the process.
Several thermal bonding systems are in use: calender bonding uses heat and pressure applied through rolls to weld the fiber web together at high speed. Air through bonding increases the volume of the product by overall bonding of the web containing low-melting point fibers in a carefully controlled hot air stream. Drum and rubber blanket bonding applies pressure and heat to make an even volume product.
Ultrasonic bonding technology stimulates the fiber molecules through a "ultrasonic generator" to vibrate at high frequency under a patterned roller, which produces internal heating and fiber softening.
(2) Mechanical bonding
In mechanical bonding, the fiber web is strengthened by physical entanglement through friction between fibers. There are mainly two types of mechanical bonding:
Needle punching, which pushes specially designed needles into and out of the fiber web to entangle the fibers. Different types of fiber webs can be needle punched together to achieve performance levels that are difficult to achieve by other means. Needle punching can be used for most fiber types, but cannot be used for very fine fibers due to the nature of the process.
Hydroentanglement is typically used for carded webs or wet-laid webs and uses fine high-pressure water jets to interlock the fibers. The arrangement of the jets can also be used to create various aesthetically pleasing effects, and the water jet pressure used directly affects the strength of the web.
(3) Chemical bonding
Chemical bonding refers to the application of liquid-based adhesives to the web. Three types of materials are commonly used as adhesives - acrylic ester polymers and copolymers, styrene-butadiene copolymers, and vinyl acetate-ethylene copolymers. Water-based adhesive systems are the most widely used adhesives, and there are also powder adhesives, foam adhesives, and, in some cases, organic solvent solutions.
Adhesives can be applied in a variety of ways, such as immersion, coating, or spraying evenly or intermittently, as in print bonding.
(4). Sorting and Processing
The diversity of the industry and its products are highlighted through opportunities combining different raw materials and techniques. A series of post-treatments enhances the diversity of products further. Customizing non-woven materials or functionalizing them to meet specific performance requirements is made possible through sorting. Precision machining can be mechanical (stretching, perforation, curling, etc.) or chemical, with chemical methods altering the surface of fibers and non-woven materials, as well as their tactile or repellent properties.
Non-woven fabrics can be made into conductive, flame-retardant, hydrophobic, absorbent, porous, anti-static, and breathable materials. They can also be applied to coating, printing, flocking, dyeing, or laminating to other materials.
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